Small Bandwidth OTDR (Optical Time Domain Reflectometer) for reflection measurement of DWDM systems used in the Antares project Pieter N.J.M. Jansen et al. January 2004 [email protected][email protected]Abstract: This document reflects the research of a modified OTDR (2) for reflection measurement of DWDM (3) systems in the Antares (15) telescope. - Part I contains basic optical fiber technology information. - Part II presents the findings and conclusion for an SB-OTDR (4) design. The report describes the occurring problem when using an OTDR for monitoring DWDM systems. It also gives a solution for OTDR measurement of DWDM systems.
Transcript
Small Bandwidth OTDR (Optical Time Domain Reflectometer)
for reflection measurement of DWDM systems used in the Antares project
Abstract: This document reflects the research of a modified OTDR(2) for reflection measurement of DWDM(3) systems in the Antares(15) telescope.
- Part I contains basic optical fiber technology information. - Part II presents the findings and conclusion for an SB-OTDR(4) design.
The report describes the occurring problem when using an OTDR for monitoring DWDM systems. It also gives a solution for OTDR measurement of DWDM systems.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
1
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
Part I General information _____________________________________________________ 4 1. Optical fiber theory _______________________________________________________ 5
1.1 Internal reflection in optical fibers __________________________________________ 5 1.2 Multimode and single-mode fiber __________________________________________ 6 1.3 Mode field diameter _____________________________________________________ 7 1.4 Spectral windows _______________________________________________________ 7 1.5 Transmission challenges__________________________________________________ 7
Part II Small Bandwidth – OTDR Research for Antares ____________________________ 27 Introduction _____________________________________________________________ 28 Connection monitoring_____________________________________________________ 28 OTDR Measurement of DWDM-systems ______________________________________ 29 Calculations _____________________________________________________________ 31 Options for Small Bandwidth OTDR creation ___________________________________ 33 Conclusion ______________________________________________________________ 35
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
3
Foreword This report has been written as a result of my first industrial placement for the university in ‘s-Hertogenbosch. The industrial placement took place at the NIKHEF(1) institute in Amsterdam from September 2003 until January 2004. During this 5-month period I have learned much about optical fiber technology and I have had the possibility to further develop my personal skills such as writing reports and giving presentations. For all these possibilities I would like to thank the following people: My company tutor - Ing J.J. Hogenbirk, BA My school tutor - Ir H.F.G.M. Huijnen, MsC I would also like to thank the NIKHEF and especially the Antares-team for providing the assignment. Indirectly - Ir M. v.d. Hoek, MsC, Baas research Introduction During my industrial placement, I have done research on the possibility to modify an optical measuring instrument so that DWDM systems can be measured without any problem. At first I had to study the basics of fiber technology before I could start with the actual assignment. A short summary of all necessary information is given in Part I of the report. All basic topics are discussed in order to understand optical fiber based systems. Part II contains information about the research, first a short introduction of the project involved, a clarification of the occurring problem, and information for finding the right solution.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
4
Part I
General information
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
5
1. Optical fiber theory Optical fibers are composed of fine threads of pure glass, silica, which can transmit light at about two-thirds the speed of light(5) in a vacuum. The transmitting medium of an optical fiber consists of two different layers, called the core and the cladding (figure 3). A protective coating surrounds the core to prevent damaging of the glass threads. The two layers are differently mixed with dopants (impurities), to adjust their refractive indices. The greater the index of refraction, the more the material is optically dense. In the following text the transmission of light through optical fiber will be explained using a simplified model. In a uniform medium, where the index of refraction is constant, light travels in straight lines. When the medium is not uniform due to any kind of variation or discontinuity, light undergoes refraction and reflection. When a ray of light is incident on the interface between two materials differing in refractive indices, it is generally split into one refracted ray and one reflected ray of light (figure 1). The angle of reflection is always equal to that of incidence. As the angle of incidence increases, the angle of refraction increases too. The angle of incidence at which the angle of refraction is 90o and the refracted ray emerges parallel to the interface
Figure 1 Reflection and reflection
between the media (figure 2) is known as the critical angle. If the angle of incidence is greater than the critical angle, there is no longer transmission into the second medium. In this case, all light is reflected back into the medium of incidence. This phenomenon is known as total internal reflection
Figure 2 Critical angle
1.1 Internal reflection in optical fibers The core of an optical fiber has a higher index of refraction than the cladding. The cladding completely surrounds the core and ensures light propagation within the core via total internal reflection. Light injected into the core at one end, and striking the core-to-cladding interface at a greater angle than the critical angle,
Figure 3 Optical fiber cross-section reflects back into the core (figure 3). Light will continue to be totally internally reflected and will zigzag down to the other fiber end.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
6
1.2 Multimode and single-mode fiber A mode is a physical concept describing how the light propagates through media. A mode can be thought of as the path that a light signal follows inside a fiber. In general there are two types of fibers, multimode fibers and singlemode fibers. A multimode fiber allows propagation along several thousand light-modes, whereas a singlemode fiber allows propagation along only one mode (figure 4). There are two types of multimode fibers available: step-index and graded-index fibers.
Figure 4
Multimode step index In step-index fibers (figure 5), the core and cladding have constant (but different) refractive index(6) values, resulting in a sharp step of the light signal at the core-cladding interface. Two modes can travel different distances to arrive at their destinations. This difference in arrival times between several propagating modes spreads out the signal, and is known as modal dispersion,
Figure 5 Multimode step index fiber
Multimode graded index To compensate modal dispersion in step-index fiber, graded-index fiber was invented (figure 6). The core of graded-index fiber gradually decreases from the center of the core outward. The higher refraction at the center of the core slows the speed of some light rays, allowing all the rays to reach their destination almost at the same time, reducing modal dispersion.
Figure 6 Multimode graded index fiber
Multimode fiber has a large core diameter of 50 µm or 62.5 µm with a typical cladding of 125 µm. However, bandwidth capacity and loss characteristics are not optimal. They suffer from inherent pulse spreading (modal dispersion) which limits the usable bandwidth. Multimode fibers have been very popular and widely used in short links. Single mode fiber Singlemode fiber has been developed to eliminate modal dispersion in long-haul links. With a core diameter of 5 to 10 µm, singlemode fiber allows propagation of only one mode. Single mode fiber is able to transmit a much higher capacity of data because it exhibits no dispersion caused by multiple modes. Singlemode fiber also enjoys lower fiber attenuation than multimode fiber.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
7
Singlemode fiber has some disadvantages. The very small core diameter makes coupling light into the core more difficult. The tolerances for singlemode connectors and splices(7) are also much more demanding. Development through the years has lead to three basic classes of singlemode fiber. The oldest type is the non-dispersion-shifted fiber (NDSF). The fiber moved the non-dispersion point within range of the used wavelength, the 1310 nm window. However, in combination with lasers in the new window at 1550 nm, NDSF showed high dispersion. A new type was developed, dispersion-shifted fiber (DSF), which moved the zero-dispersion point to the 1550 nm region. Again, this type of fiber showed problems with later technology. DSF in combination with multiple small bandwidth DWDM lasers showed serious nonlinearities. Non zero-dispersion-shifted fiber (NZ-DSF) was developed to improve these characteristics. This fiber is rapidly becoming the fiber of choice in new fiber deployment. 1.3 Mode field diameter Another major difference between singlemode and multimode fiber is that, in singlemode fiber, some optical energy travels through the cladding. The diameter of the energy distribution is known as the mode field diameter. In multimode fibers, energy is confined to the core, so the mode field diameter is not a relevant concept. Singlemode fiber cladding must, therefore, be a more efficient transmitter than the multimode fiber cladding, where poor transmission performances can be tolerated. 1.4 Spectral windows Optical fiber transmission is bound to specific regions on the optical spectrum where optical attenuation is low. These regions, called windows, lie between areas of high absorption. The earliest systems were developed to operate around 850 nm. This became the first window in silica-based optical fiber. A second window (S band), at 1310 nm, soon proved to be superior because of its lower attenuation, followed by a third window (C band) at 1550 nm with even lower attenuation than the second window. Today, a fourth window (L band) near 1625 nm is under development. Singlemode fibers commonly use the two wavelength(8) windows centered on 1310 nm or 1550 nm. The four windows are shown relative to the electromagnetic spectrum in figure 7.
Figure 7 Spectral windows
1.5 Transmission challenges Optical data transmission through optical fibers presents several challenges. The three main categories are discussed below.
- Attenuation, known as optical power loss caused by the fiber - Dispersion, spreading of light pulses as they travel down the fiber - Nonlinearities, cumulative effects from the interaction of light with the material through
which it travels, resulting in changes in the light wave and interactions between light waves.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
8
Attenuation Attenuation in optical fiber is caused by internal factors, primarily scattering and absorption, and external factors, from the environment and physical bending. The scattering phenomenon is most important for the OTDR workings. Internal factors The highest contribution to scattering is Rayleigh scattering, caused by small variations in the density of glass as it cools during manufacturing. Scattering especially affects short wavelengths and limits the use of wavelengths below 800 nm. The optical loss in decibel (dB) for each wavelength is shown in figure 8. Other forms of scattering are Brillouin, Raman and Fresnel scattering.
- Brillioun scattering is caused by the interaction of light with metal ions inside the fibers. This reflection is not so high compared to Rayleigh scattering, but plays a part during the whole fiber.
- Raman scatter is caused when photons(14) interact with particles whose diameter is much smaller than the light its wavelength. While most of this scattered light has the same wavelength as the incident light, some light is scattered at different wavelengths. This scattered light with a different wavelength is called Raman scatter. It affects only shorter wavelengths.
- Fresnel scattering is caused by differences in refractive indices. These differences can be located at connections or damaged spots inside a fiber. Fresnel reflections occur locally and are generally very high, causing high losses.
Attenuation due to absorption is caused by imperfections and atomic defects inside the fiber. These impurities absorb the optical energy. Absorption especially affects the longer wavelengths from 1700 nm. The primary factors affecting attenuation in optical fibers are the length of the fiber and the wavelength of the light.
Figure 8 Attenuation per wavelength
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
9
Dispersion Dispersion is the spreading of light pulses as they travel down optical fiber (figure 9).
Figure 9 Dispersion There are two general types of dispersion that affect DWDM systems:
- Chromatic dispersion, a linear effect - Polarisation mode dispersion (PMD), a non-linear effect
Chromatic dispersion Chromatic dispersion occurs when different wavelengths propagate at different speeds. In single-mode fiber, chromatic dispersion has two components, material dispersion and waveguide dispersion. Material dispersion occurs when wavelengths travel at different speeds through the material. Waveguide dispersion occurs because of the different refractive indices of the core and the cladding of fiber. The longer the wavelength is the more it will partially travel through the cladding, causing part of the signal to travel at lower speed. This gives time delay between the various wavelengths. The effect of chromatic dispersion increases as the square of the bit rate. When using higher data rates, dispersion will become a greater issue. Zero-dispersion-shifted fiber can reduce these effects. Polarisation mode dispersion Polarisation mode dispersion (PMD) is caused by the oval shape of the fiber core as the result of the manufacturing process. Single-mode fibers support two perpendicular polarizations of the original transmitted signal (figure 10).
Figure 10 Two perpendicular polarisations
If a fiber were perfectly round and free from all stresses, both polarization modes would propagate at exactly the same speed, resulting in zero PMD. However, practical fibers are not perfect, thus, the two perpendicular polarizations may travel at different speeds and, consequently, arrive at the end of the fiber at different times. Because the stress changes during manufacturing, PMD is nonlinear. Below certain speeds PMD will not be of any trouble. The difference in arrival times, normalized with length, is known as PMD (ps/km0.5). Other nonlinear effects Nonlinear effects cannot be compensated and play part when optical power gets high. The most important type of nonlinear effect in DWDM system is four-wave mixing. Four-wave mixing is caused by the nonlinear interactions among different DWDM channels, which creates sidebands. This result in cross-talk(9) and signal to noise ratio(10) (SNR) degradation. Four-wave mixing cannot be filtered out, either optical and electronically. It increases with the length of the fiber. Non-zero-dispersion-shifted-fiber can compensate the cross-talk effect.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
10
External factors Fiber bending and environmental influences on fiber contribute most to attenuation caused by external factors. The careless handling of the user often damages the fiber, causing high attenuation due to external factors. People sometimes forget, optical fibers are made of glass and are very fragile. The slightest damage could break the fiber or at least cause excessive loss. The most common problems are discussed below. An important specification for optical fibers is the minimum-bending radius. A too small bending radius causes light to escape from the core of the fiber, resulting in signal loss (figure 11). Also, insertion of high optical power may permanently damage the fiber. The minimum-bending radius for the Antares fiber is 40 mm.
Figure 11 Other causes could be: - Tension on the fiber. Light escapes from the fiber.
Figure 12
- Micro bending. Permanent damage to the fiber. Light is escaping from the fiber.
Figure 13
- Awkward treatment or poor stripping of primary coating damages the core. Little cracks can become larger due to vibrations and will eventually break the fiber.
Figure 14
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
11
2. Receivers (DWDM) DWDM receivers are used for the conversion of an optical signal into an electrical signal. The basic principle of an optical receiver is a light dependant current regulator. Such regulators are available as photoconductive diodes that change their current flow proportional with the incident light. The two most generally used photo diodes are, the photo intrinsic negative (PIN) diode and the avalanche photo diode (APD). 2.1 Photo Intrinsic Negative diode A PIN-diode is a photoconductive diode that consists out of three different layers (figure 15). Each layer has its own characteristics created by the addition of doping (impurities). The layers are placed in the following arrangement. First comes the negative, N-type layer. Next is the intrinsic (undoped) I-type layer. The final layer is the positive P-type layer. This composition is just like a normal diode except for the intrinsic layer in the middle. When photons fall onto this layer the P- and N-junction become conductive resulting in a current flow that is proportional with the incident light. A PIN-diode does not amplify the signal. The typical supply voltage of a PIN diode is 5 Volt.
Figure 15 PIN diode cross-section
Biasing PIN diode characteristics can be improved using voltage potentials connected to both sides of the diode, known as biasing. PIN diodes can operate with or without bias voltage. For low frequency signals it is best not to use bias voltage. The PIN diode will have better noise equivalent power11 (NEP) and less trouble with other types of noise. For higher frequency signals biasing can improve some important characteristics of a PIN diode. There are two ways of biasing, reverse biased and forward biased.
- Reverse biased means the P-side is connected to the negative electrode and the n-side is connected to the positive electrode of the VCC. A PIN diode used in reverse bias mode improves response time and reduces junction capacitance. This operation mode is used for pulsed detectors.
- Forward biased means the P-side is connected to the positive electrode and the n-side is connected to the negative electrode of the VCC. For this operation mode the PIN diode will increase the sensitivity of the detector.
In figure 16 the spectral responsivity(12) and the quantum efficiency(13) of a PIN diode is shown. An ideal PIN diode would have a responsivity of 1 A/W for all frequencies. Practically this is not feasible. All PIN diodes will be less sensitive especially in the blue region (short wavelengths 400 nm). PIN diodes are not very sensitive to external influences such as temperature changes, shock or vibrations. However it is important to keep the surface of the detector clean.
Figure 16 Silicon PIN diode characteristic
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
12
2.2 Avalanche Photo Diode The second, more sophisticated, photo diode is the Avalanche photo diode (APD). An APD is a photoconductive diode that amplifies the signal. The APD layer schematic is shown in figure 17. The main difference between a PIN diode and an APD is, the bias voltage that is significantly higher (approx. 50-70 V) compared to the PIN diode. This way the photogenerated carriers have enough energy to start an avalanche process. This contributes to amplification of the signal. An APD has a higher responsivity than a PIN diode, but also produces higher noise due to the avalanche process.
Figure 17 APD Cross-section
APD schematics are more complex because of the high voltage circuits. That is why APDs are usually avoided if possible. A few incident photons result in many carriers and appreciable external current. In fact an APD is the opposite component of a laser. Carriers accelerated due to a collision with a primary carrier, are called secondary carriers. This process of second carrier creation is called collision ionisation. Second carriers can accelerate third carriers and so on. The whole process is called photomultiplication. Every APD has a multiplication factor, which is an average value. The variation of this factor is one source of noise that limits the sensitivity of a detector using an APD. At lower voltages the APD operates as a PIN diode and exhibits no internal gain. The avalanche breakdown voltage of an APD is the voltage at which collision ionisation begins. The APD is often biased just below the breakdown point, so any optical power will create a fast response and strong output. The tradeoffs are that dark current and noise increases. The avalanche process is temperature sensitive, and most APDs will require temperature stabilisation. Because of these difficulties an APD is not as commonly used in datacom applications as the PIN diode.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
13
3. Optical transmitters 3.1 Laser principle Atoms can be in different states of excitation, containing different amounts of energy. If we apply a certain amount of energy to an atom, it can leave what is called the ground-state energy level and go to an excited level. An atom can absorb energy in the form of heat, light or electricity. Once an electron is excited and moves to a higher energy level, it eventually will return to the ground state level. To reach the ground state level it can release its energy as a photon (figure 18). An electron can also release its energy without creating a photon. This happens only exceptionally and degrades performance of a laser. A laser is a device that controls the way that energized atoms release photons. The word “Laser” is an acronym for light amplification by stimulated emission of radiation.
Figure 18 Atom excitement
All lasers have the same basic components, a lasing medium (an optical amplifier) and a resonator that recirculates the laser light through the lasing medium (figure 19). Lasing medium The lasing medium is pumped to get the atoms into an excited state. To reach the ground state an atom can release its energy in the form of a photon. The photon emitted has a specific wavelength (colour) that depends on the state of the electron’s energy when the photon is released. Two identical atoms in identical states will release photons with identical wavelengths.
Figure 19 Lasing medium cross-section
Stimulated emission In stimulated emission, photon emission is organized. If a photon, with a certain wavelength, encounters another atom that has an electron in the same excited state as the photon, stimulated emission can occur. The first photon can stimulate atomic emission such that the second emitted photon vibrates with the same frequency and direction as the incoming photon. The first and second emitted photons can stimulate a third photon and so forth. Resonator The second fundamental component of a laser is a pair of mirrors, the resonator, one at each end of the lasing medium (figure 19). The emitted photons reflect off the mirrors and travel back and forth through the lasing medium. In this process, they stimulate other electrons and can cause the emission of more photons of the same wavelength and phase. A cascade effect occurs. The mirror at one end of the laser is semi-transparent. So an amount of light reflects and an amount of light passes through the mirror out of the laser. Laser light is very different from normal light. The following characteristics are achieved by stimulated emission. Laser light has the following properties. Laser light is:
- monochromatic. It contains one specific wavelength of light determined by the amount of energy released.
- coherent. Each photon has the same wavelength and phase. - directional. The laser light has a tight, concentrated beam.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
14
3.2 Laser types There are many types of lasers. Below are the most common types listed.
- Solid-state lasers. The lasing material is distributed in a solid matrix. Wavelengths around 1.064 nm. Solid-state lasers are high power lasers (10 W). They are often used in power gain applications.
- Gas lasers have a primary output of visible red light. Helium and helium-neon, HeNe, are
the most common gas lasers. Gas lasers are used for cutting hard materials.
- Excimer lasers use reactive gases, such as chlorine and fluorine, mixed with inert gases such as argon, krypton or xenon. They are used in scientific, medical and industrial applications. Eximer lasers can reach precision rates of several micrometers (µm)
- Dye lasers use complex organic dyes, in liquid solution or suspension as lasing media.
They are tuneable over a broad range of wavelengths. Dye lasers are used for commonly used for medical applications, like medical surgery, or interferometry and pollution detection.
- Semiconductor lasers are very small and have low power consumption. They are
sometimes called diode lasers. They can be build into larger array’s, such as the writing source in some laser printers or CD players.
3.3 Laser classifications Lasers can be very dangerous because of the high concentration of invisible light. For that reason lasers are classified into four categories.
Figure 20 Warning sign - Class I – These lasers cannot emit laser radiation at known hazard levels. - Class I.A. – This is a special designation that applies only to lasers that are “not intended
for viewing,” such as a supermarket laser scanner. The upper power limit of Class I.A. is 4.0 mW.
- Class II – These are low-power visible lasers (0.4 to 0.7 nm) that emit at a power level less then or equal to 1 mW. Although one should never stare into a laser, the concept is that the human aversion response to bright light will protect a person.
- Class IIA – These lasers emit visible light that won’t produce hazardous radiation for periods not exceeding 1000 seconds.
- Class IIIA – These are intermediate-power lasers (1 to 5 mW), which are hazardous only for intrabeam viewing. Most pen-like pointing lasers are in this class.
- Class IIIB – These are moderate-power lasers (Average power < 500 mW). - Class IV – These are high-power lasers (Average power > 500 mW,), which are
hazardous to view under any condition (directly or diffusely scattered), and are a potential fire hazard and a skin hazard. Significant controls are required of Class IV laser facilities.
3.4 Lasers used for telecommunication Because of their small size and low power consumption, mostly semiconductor lasers are used for telecommunication systems. Only the semiconductor laser will be discussed in detail in this report.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
15
Semiconductor laser principle The lasing medium in a semiconductor laser is a forward-biased PN junction (figure 21). Electrons are injected from the N layer and holes are injected from the P layer. Between the P and N layers is the active layer where the photons are created. The PN junction is qualitatively the same for every semiconductor laser. The only differences can be the choices of materials, dopants or layer thickness, which affect the wavelength, efficiency or optical power and so forth.
Figure 21 Semiconductor laser cross-section
The resonator continuously recirculates the photons created and is responsible for the high level of coherence, both spatial (focusable to a very small spot) and spectral (consisting of a narrow range of frequencies). The resonator is usually formed by cleaved facets at each end of the wave-guide. A resonator composed of two plane-parallel mirrors is called a Fabry-Perot resonator; most laser diodes are of that type. The Fabry-Perot laser has a large frequency spectrum and is commonly used in OTDRs because of their low cost but good performance. One facet is coated with a high-reflective coating (99%), while the other facet is coated with a semi-reflective coating. Semiconductor lasers can reach efficiency rates of up to 50 % There are two major families of semiconductor lasers, the GaAs-based (Gallium-Arsenic) lasers and the InP-based (Indium-Phosphorus) lasers. The GaAs-based lasers have wavelengths from about 630 to about 1100 nm. The InP-based lasers range from 1100 to 2000 nm. The lasers with wavelengths of 1310 and 1550 are mostly used for telecommunication systems. Especially the semiconductor laser, called the distributed feedback (DFB) laser, is commonly used in telecommunication systems, because of its good performance. DFB laser The distributed feedback laser is a laser source with low noise and an extremely narrow frequency spectrum (0.2 nm) compared to common diode lasers (54 nm). The resonator of the DFB laser is made of a grating, a corrugated structure, embedded within the laser structure (figure 22). Each ripple in the grating provides a minute amount of feedback. The device operates only at the single frequency for which the feedback from the grating adds up to the phase. This single-frequency operation also eliminates a major source of noise in semiconductor lasers, called partition noise, which results from competition between longitudinal modes in multi- frequency lasers. These lasers can have intensity noise levels of less than –160 dB/Hz. The wavelength of DFB lasers range from 633 nm to 2000 nm.
Figure 22 DFB laser structure
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
16
DFB lasers are temperature dependant. To maintain the extremely narrow frequency spectrum, precise temperature control is necessary. This is achieved with a temperature controller, which uses a Peltier element. Vertical Cavity Surface Emitting Laser (VCSEL) The VCSEL is different from all other lasers. In the VCSEL, light travels perpendicular to the wafer and is emitted from the middle of the device (figure 23). The mirrors are stacked upon each other with the active layer in the middle. VCSELs are more difficult to construct than edge-emitting lasers. Despite this fact they have several advantages. Because they have no cleaved facets, they can be completely fabricated and tested at the wafer level, which results in lower production costs. They have low power requirements and a low divergence, nearly circular beam.
Figure 23 VCSEL structure
Because of their low cost but good performance VCSELs are commonly used in multimode fiber links 3.5 ITU (International Telecommunications Union) grid The ITU grid (appendix I) is the world standard for available wavelengths of DFB lasers, often used in DWDM systems. While this grid defines a standard, users are free to use any other wavelength from the spectrum. It is only meant for worldwide compatibility of laser systems. The grid has its center wavelength at 1553.52 nm and a typical spacing of 100 GHz between each wavelength. 3.6 Modulation techniques A modulated laser source functions as the carrier of the data signal. Laser modulation can be divided in two categories, internal and external modulation. Internal modulation Internal modulation changes the current according to the data input signal. Internal modulation is a bit similar to Amplitude modulation. The laser’s frequency does not change and the laser is never turned off because of switch on symptoms. Internal modulation of a semiconductor laser is a very fast technique. It can reach data rates beyond a several GHz per second. External modulation External modulation uses a modulator outside the physical lasing device, usually located in the same package. With this technique the laser current remains equal while the laser light is chopped or redirected from the receiving photo diode. It is like the laser is turned on and off in a modulated pattern. External modulation is not suitable for high frequent systems. Several external modulator techniques can be used. They are not being discussed in detail.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
17
4. Dense Wavelength Division Multiplexing 4.1 DWDM Principle A Wavelength Division Multiplex (WDM) system combines several optical signals, each at its own wavelength, into one optical signal with a compound wavelength (figure 24). The compound signal is transmitted over one single fiber. At the end of the link, the optical signal is separated back into the original input signals. WDM increases the capacity of existing networks without the need for expensive re-cabling and significantly reduces the cost of upgrading a network. The increasing need for larger capacity of the optical fibers caused an increase of the number of wavelengths used for WDM channels. Therefore a new kind of WDM was developed with very narrow channel spacing, known as Dense Wavelength Multiplexing (DWDM). DWDM uses the spectral band centered at 1550 nm.
Figure 24
The working of a DWDM system is best described according to the following 5 steps:
- Generation of the signals. The source is a DWDM laser. The optical source must provide a monochromatic light wave with a stable wavelength within a specific narrow bandwidth.
- Combining the signals. An optical multiplexer combines the several optical data input channels and gives at its output channel a light wave that consists out of al wavelengths given at the input.
- Transmission of the signal via an optical fiber. The link must have little loss, and little trouble with dispersion and other (side) effects of high-speed data transmission. Loss may occur due to large fiber length or caused by optical components. An optical amplifier can be used to boost the signal to a proper power level.
- Separation of the signals. An optical demultiplexer separates the composite light wave and directs the separated light waves into the corresponding output channel. A demultiplexer has been given some extra attention for unwanted addition of noise caused by channel interference. The channels are extremely well isolated and interfere very little.
- Receiving the signals. Each individual channel ends at a photodiode that receives the data signal and converts the optical signal into an electrical signal.
4.2 Components for DWDM system The following components are used in a DWDM system:
- Transmitters, lasers with a very narrow bandwidth and stable wavelength. - DWDM filters, for the combination and separation of the several optical data channels - Optical fiber, single mode fiber has the best characteristics for data transmission - Receivers, photodiodes for the conversion of optical to electric signals
These and other optical data transmission components are discussed in the following text.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
18
DWDM Transmitters DFB lasers are the preferred light source used in DWDM systems. Usually an external modulator (not to be mistaken by externally modulated) modulates the laser light. In the future transmitters will be internally equipped with the basic components such as the continuous wave (CW) laser, modulator and an optical amplifier. Multiplexers and Demultiplexers (Filters) A very important component is the DWDM multiplexer (Mux) or demultiplexer (Demux), commonly known as a DWDM filter. A DWDM filter is responsible for the combination respectively separation of the optical data signal. Most filter techniques are based on light refraction. Because Figure 25 Multiplexing principle light refraction is done passively, most DWDM filters are bi-directional. Still, a filter works best for the direction that it is designed for and is given by the manufacturer. A Mux and a Demux differ from each other. A Demux has a better channel isolation to prevent channel interference at the end of the DWDM connection. Because of this special precaution a Demux is more expensive than a Mux. There are several different filter techniques. The most essential techniques are being discussed.
Figure 26
Prism refraction demultiplexing (figure 26) Combining and separating light can be done using a prism. The incident light is focused onto a prism. Each component wavelength is refracted differently. In the output light, each wavelength is separated from the next by an angle. A lens focuses each wavelength into its corresponding fiber.
Waveguide grating diffraction (figure 27) This technology is based on diffracting using a grating. When incident light reaches the grating, each wavelength is diffracted with a different angle. The diffracted light rays are focussed into separate fibers.
Figure 27
Figure 28
Arrayed waveguide grating (figure 28) This technique is also based on diffraction. It consists of an array of curved-channel waveguides with a fixed difference in the path length between the channels. When the light enters the input cavity, it is diffracted and enters the waveguide array. There the optical length-difference of each waveguide causes phase delays in the output cavity, where an array of fibers is coupled. Arrayed waveguide filters are used in the Antares DWDM system.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
19
Multilayer interference filters (figure 29) These filters are composed of thin film filters. Each filter transmits one wavelength while reflecting the others. By cascading multiple thin film filters, many wavelengths can be demultiplexed. Behind each filter a fiber is connected, transmitting the light signal.
Figure 29
Figure 30
Optical Add/Drop Multiplexers (OADMs) This device is able to remove or insert one or more wavelengths while passing others on. The schematic is shown in figure 30. Fixed OADMs and tuneable OADMs are available.
DWDM receivers The DWDM receiver converts optical signals to electrical signals. The receiver must be compatible with the transmitter, both in primary wavelength and modulation characteristics. Usually an APD is used for high-speed datacom DWDM systems. Optical amplifiers Long distance connections suffer attenuation due to fiber length. The optical signal needs to be amplified. There are several types of optical amplifiers and some are still being further developed. Two types are discussed below. Semiconductor amplifiers Semiconductor optical amplifiers (SOAs) are essentially laser diodes, without end mirrors, which have fiber attached to both ends. They amplify the optical signal that comes from either fiber and transmit an amplified version of the signal out through the second fiber. SOAs are typically constructed in a small package, and they work for 1310 nm and 1550 nm systems. They transmit bi-directionally, making the reduced size of the device an advantage
Figure 31 Semiconductor
amplifier over other optical amplifying devices. However, the drawbacks to SOAs include high-coupling loss, polarization dependence, and a higher noise figure. Figure 31 illustrates the basics of a Semiconductor optical amplifier.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
20
Erbium Doped Fiber Amplifier Inside an erbium doped fiber amplifier (EDFA), a fiber is placed that is doped with erbium, a rare earth element, which has the appropriate energy levels in their atomic structures for amplifying light. EDFAs are designed to amplify light at 1550 nm. The device contains a pump laser to inject energy into the doped fiber. When a weak optical signal enters the fiber, the light stimulates the rare earth atoms to release their stored energy as additional light. This process continues as the signal passes down the fiber, growing stronger and stronger. Attenuators Attenuators are used to manually correct the optical power level. In large distance systems, high power lasers are used to overcome the need for optical amplification. Attenuation may be used for specific regions in the system so other optical components, near a high power laser source, do not damage due to exceeding optical power. Switches Switches are used for the routing of optical signals through a system. Recent developments have proved the possibility of all-optical switches without any electrical needs. Optical couplers Couplers distribute light from one fiber into one or more fibers. They are also called multiport couplers. Some coupler types are shown in figure 32. A splitter splits one signal in two. A combiner does the opposite. A splitter can separate a signal with various splitting ratios: 50:50, 30:70, 1:99 etc. A four-port coupler is also used for coupling and separating optical signals
Figure 32 Various couplers
using certain coupling ratios. A star coupler is generally used for distributing one single optical signal over multiple fibers.
Figure 33 Circulator schematic
Circulators Figure 33 shows the schematic representation of a circulator. Channel 2 is both an input and output channel. - Incoming light from channel 1 is directed to channel 2. - Incoming light from channel 2 is directed to channel 3. A circulator is like a directional coupler.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
21
5. Optical measurement Just like electrical designs, optical designs need to be tested and monitored to maintain good performance. For these tests special optical measurement instruments are required. These optical instruments mainly have the same functionality as electrical instruments. The instruments used for my research are an optical spectrum analyser (OSA), optical power meter (OPM), and an optical time domain reflectometer (OTDR). Especially the OTDR is important for my research. This instrument will be discussed in detail. The other instruments will be discussed shortly. 5.1 Optical Time Domain Reflectometer Light reflection in a fiber An optical time domain reflectometer is also known as an OTDR. This device makes use of light reflection within the fiber. As mentioned before in this report Rayleigh scattering contributes the most to this reflection. Rayleigh scattering is the effect that causes light to partially reflect by impurities in a fiber (figure 34). The amount of light that is reflected is defined by the backscatter coefficient. The backscatter coefficient for the fibers used in Antares is approximately –52 dB.
Figure 34 Light reflection through an optical fiber
Light pulse propagation Figure 34 shows a schematic overview of a light pulse propagating through a fiber. When laser pulse Pin is launched into the fiber, the reflection of Pin at l(x) is detected at the receiver when the travelled length through the fiber is two times l(x). So the light pulse has to travel twice the sample distance and undergoes twice the same amount of loss. For the pulse power at 50 km the pulse has to travel a total distance of 100 km to reach the photo diode. How does a commercial OTDR work? An OTDR works according to the following steps. First the OTDR fires a laser pulse into the fiber using a standard laser. The pulse length can be adjusted according to the fiber length to be measured. For long distance you will need a more powerful pulse because of the increasing loss per kilometer. This can be achieved by increasing the amplitude or pulse width. With the commercial OTDR’s only the pulse width can be adjusted because this is easier and less expensive than increasing the pulse amplitude. The fiber length should be about 20 times the pulse width. For a fiber length of 50 km a pulse width of 10 µs should be used. This pulse width is also used in further calculations.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
22
Figure 35 Commercial OTDR
The laser pulse is directed via the coupler into the fiber being measured. The reflected light from the pulse is directed via the coupler to the photo diode. The photo diode measures the optical power. Sampling and averaging these results creates a power loss characteristic,
with the power loss at the y-axis and distance at the x-axis (figure 36). OTDR Characteristic From OTDR characteristics some valuable data can be acquired. Figure 36 shows a typical OTDR characteristic. You can see reflection peaks that indicate connectors, a small drop that indicates a bend and another drop that indicate a fusion of two different fibers. A bend can be distinguished from a fusion by measuring the fiber two times using a different wavelength. Wavelengths of 1310 nm and 1550 nm can be used. A bend measured with a 1310 nm laser pulse shows lower loss than a measurement with a 1550nm laser pulse. A fusion shows in both cases the same amount of loss. However, a fusion can cause a remarkable result, which looks like a gain in the loss characteristic. When two fibers with different refractive indices or core diameters are fused together, a different amount of light is reflected. This amount can be higher causing more light to reflect. This explains the rising edge. OTDR specifications There are a lot of specifications given for an OTDR. The most important are discussed below.
Figure 36 OTDR characteristic
Dead zone and Resolution Event and attenuation dead zones result from the temporary blinding of the OTDR detector as it receives a high level of reflected light due to a Fresnel reflection from the fiber. The detector becomes saturated and needs time to recover. An OTDR event dead zone is the distance between the front panel connection (distance zero) and the point where the Fresnel peak drops by 1.5 dB (figure 37). Attenuation dead zone represents the minimum distance from the front panel connection where an event can be detected and its loss measured. Dead zones are functions of the pulse width.
Figure 37 Dead zones A practical solution is an additional fiber of 1 km to give the APD time to recover.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
23
Dynamic range Dynamic range is the difference (in dB) between the extrapolated backscatter level at distance zero and the noise floor. Noise floor can be defined in different ways. The two most popular methods are the 98% Telcordia method and the SNR=1 method. The 98% Telcordia method defines noise floor as the reference level where 98% of the noise data points are below the reference. The SNR=1 method defines noise floor as the reference level where the signal-to-noise ratio equals 1. the SNR=1 method typically results in a noise floor of 1.5 dB below the 98% Telcordia. The specification “measurement range” tends to replace dynamic range because it provides a better evaluation of the overall capability of the OTDR. It is the maximum attenuation that can be placed between an OTDR and a specified event to be measured and for which the OTDR will still be able to measure the event within acceptable accuracy limits. The most commonly used method is the non-reflective event measurement method. It represents the amount of attenuation (in dB) between the OTDR and a 0.5 dB splice, which must be detected and measured with an accuracy of +- 0.1 dB. This specification. Sampling resolution Sampling resolution is an important specification for determining the distance between data points on an acquisition. Longer ranges result in less precision. The OTDR software provides the user with the ability to set the distance range to concentrate all acquisition points within a specific section of fiber. All data points are acquired with minimal distance separating them. Accuracy Accuracy is often confused with resolution. Accuracy refers to the true value of a measurement, not to the preciseness of distance or value of a measurement. Accuracy represents the reliability of the measured value with respect to true loss and distance measurement values. 0.05 dB/dB specification means the true value should be between 0.95 and 1.05 dB. Accuracy depends mainly on the following three factors: calibration, clock stability, and fiber refractive index uncertainty. 5.2 Optical power meter Optical power represents the light intensity of an optical signal in the core of the fiber. It is measured with an optical power meter (OPM). Before starting a measurement an OPM needs to set a reference level. After this level is set the optical power meter measures the transmitter power and compares it with the reference level, the receiver sensitivity. The difference is called the power budget. Two types of measurements exist: relative power measurement and absolute power measurement. Relative measurements are used to determine return loss, attenuation, or gain in an optical system. Absolute measurements are required when dealing directly with sources, amplifiers, and receivers.
Figure 38 Optical
power meter
Relative measurements are expressed in decibels (dB) and absolute power measurements in dBm. 5.3 Optical Spectrum analyser An optical spectrum analyser (OSA) is a device that analyses the light spectrum of an optical signal. It shows a detailed overview of a, by the user defined, region of the light spectrum. Figure 39 shows a test result of a DWDM laser spectrum analysis.
Figure 39 DWDM laser spectrum
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
24
6. Connectors The goal of a connector is joining a fiber to an optical component or another fiber with the lowest loss of optical power across the connection. The critical factor in a fiber optic connector is the alignment. Unfortunately, fibers and connectors are produced with a certain precision tolerance resulting in less than perfect alignment. Another cause of optical power loss occurs when light passes from one index of refraction to another. This phenomenon is called Fresnel reflection. At this junction a small portion of light is reflected backward. Figure 40 shows some typical causes of loss.
Figure 40 Fiber connection errors
There are methods where fibers and connectors are manufactured to the same extremely precise tolerances, which helps in aligning fibers mechanically. The most common method is manufacturing a precisely placed hole in a precisely toleranced ferrule. 6.1 Splices A splice is a permanent connection. There are two sorts of splices, fusion splices and mechanical splices. The most popular type is the fusion splice, where the fiber cores are aligned and melted until they fuse. These offer the lowest losses, common loss of 0.02 dB. Unfortunately equipment costs make them only attractive for long-haul telecommunication systems with long fiber lengths and where the need for minimal losses justifies cost. Mechanical splices are little boxes with grooves to guide two fiber ends and align both cores and hold them together only by little pressure. This way the light “jumps” from one fiber to the other.
Figure 41 Fusion splicer
Inside a mechanical splice there could easily be an air gap or misalignment between both fibers causing optical power loss. Mechanical splices are used for temporary or permanent repair and restoration work. They offer a fast, cheap and easy alternative.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
25
6.2 Parameters Connectors have many performance characteristics to be considered. Some are similar to electronic components such as temperature range or shock and vibration influences. Insertion loss and return loss are the characteristics important to fiber optic systems only. Insertion loss expresses the reduction in optical power across the junction caused by applying a connector. Insertion loss can be measured by the following method. First the optical power through a length of fiber is measured. Then the fiber is cut and the connectors are applied, followed by a re-measurement of the optical power loss over the fiber. The difference between both measurements is the insertion loss of the connector. Losses range from 0.01 for permanent fusion splices to 0.2-0.5 dB for connectors for glass fiber to 3 dB for connectors for plastic fibers. It is not desirable to have light reflect upstream towards the source. Even small reflections can modulate the laser source. Return loss refers to the reduction in reflected energy compared with the incident energy. Greater loss of the reflected light is desired to minimize or eliminate the effects caused by reflected energy. Because Fresnel reflections can be a significant source of reflected energy, one important way to reduce them is to ensure that the fibers mate as good as possible. An option is creation of a slight radius on the ferrule (figure 42). The type of end finish on the fiber/ferrule is called a physical contact, or PC, end finish. Angling the end face (angled PC or APC finish) can further increase return loss.
Figure 42 Optical fiber end-faces
Both the PC and APC end finishes also work to prevent any reflected light from being guided back toward the source. Typical values of return loss are:
- PC (physical contact) >-30 dB - SPC (super PC) >-40 dB - UPC (ultra PC) >-50 dB - APC (angled PC) >-60 dB
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
26
6.3 Connector types Below are the most common used connector types listed. SC Connectors: (Subscriber Connector) SC connectors have a 2.5 mm ferrule, push-pull lock mechanism and pull-proof design that prevents a slight pull on the cable from pulling the ferrule out of optical contact.
Figure 43
ST Connectors: (Straight Tip) Originally developed AT&T. ST is a trademark of AT&T. They use quick-release bayonet couplings. A key ensures consistent, repeatable mating with the coupling bushing. ST connectors are the most widely used connector type. STII Connectors: Features a Non-Optical Disconnection (NOD) design
Figure 44
FC/PC Connectors: (Fiber Connector) Used mainly by the telecommunications industry. They use threaded couplings and 2.5 mm ferrules. Some variations of the connector use tuneable keying to achieve the lowest loss. Tuning allows one ferrule to be rotated in relation to the other to optimise the connection. The connector is keyed so that the connectors will always mate in the same tuned position.
Figure 45
FDDI MIC Connectors: This duplex connector uses a side-latching mechanism and two 2.5 mm ferrules.
Figure 46
Multi-Channel Connectors: Connectors with several optical fiber connections. Available in different numbers of connections:
Figure 47
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
27
Part II
Small Bandwidth – OTDR Research
for Antares
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
28
Introduction In Antares 600 Mb/sec is transmitted between the telescope and the shore station. The telescope resides 42 km offshore at a maximum depth of about 2.4 km below sea level. This makes physical access for service to the telescope very difficult. For full data acquisition (DAQ) of the complete telescope 144 optical fibers would be needed. This number was decreased to 24 fibers by use of dense wavelength division multiplexing, DWDM, in our multiple Gigabit Ethernet from shore to the detector and visa versa. Per string there are twelve optical fibers (channels) for data transport, six fibers for uploading data and six fibers for downloading data. Per six fibers, five are used for transmission of the data from the sensor modules and one fiber for the string control data. Each optical channel has been given its own wavelength. λ1- λ6 in figure 48 and 49.
Figure 48 DWDM filter / (de)-multiplexer
The data of these lines are wavelength multiplexed using a DWDM multiplexer, also known as a DWDM filter. The output signal from the multiplexer is transmitted through a single fiber to the de-multiplexer where the signal is separated into the six original wavelengths, each guided into their own channel. One string uses two optical fibers, one fiber for uploading data and one fiber for downloading data. A minimum of 24 fibers is needed for full DAQ of the complete telescope.
Figure 49 Schematic representation of the Antares telescope
Connection monitoring Measurement of the fiber connections can give much information about the status and the quality of the fiber connection. There are several measurement methods for optical fibers. An example of a simple measurement could be, using a laser source that sends out light through the fiber while measuring the output at the other end of the fiber. The difference between input and output power is the loss over the fiber. When the measured loss is excessive, one of the following errors could be the cause:
- Too small bending radius, causes light to escape from the core of the fiber. Results in signal loss. Occurring only possible during deployment
- Bad connections, damaged connector or not properly aligned connectors results in high attenuation and signal loss. Can occur any time
- Permanent damages, little cracks inside the fiber. These cracks might get bigger by vibrations and eventually break the fiber. Result is total signal loss. Can occur any time
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
29
It is very useful to measure the quality of the fibers and connections, not only when the telescope is fully assembled but also during the moment of assembly so problems can be detected immediately. There is however an extra difficulty of monitoring the fibers used in Antares. The fibers are only accessible from one end (at the shore side) for any monitoring operations. Fortunately there is a measurement method that can be done using only one end of the fiber. An Optical Time Domain Reflectometer, known as an OTDR, performs this method. OTDR Measurement of DWDM-systems An OTDR can be used to measure the quality of an optical fiber. However, an OTDR in combination with a DWDM system gives unusable data for fiber-lengths after a DWDM filter. The question why an OTDR measurement of a DWDM system results in unusable data had to be clarified with this research. For the Antares telescope it is very important to have the ability to monitor every single fiber connection, especially the smaller fibers behind the second DWDM filter. Figure 50 shows the schematic representation of all fiber connections for uploading or downloading data of one string.
Figure 50 Schematic representation of the Antares telescope optical fiber connections
The shortest fiber is connected at the bottom of the string. This fiber connection is used for string control data. The other 5 fibers are connections for the data from the sensor modules. The fibers with a length of 42 km, between both DWDM filters, are individually direct accessible from shore and can be measured without problems (figure 50). However the fibers, marked as the dashed red fibers, directly connected to the telescope, give problems because of the DWDM filter that is between the OTDR and the fiber to be measured. In this paragraph the arising problem is clarified.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
30
Bi-directionality of the filters First theory was that the filters where blocking the reflected signal. This would be peculiar because DWDM filters are passive devices based on refraction of light so both ways should guide the light the same way. A test had to make sure the DWDM filters where bi-directional.
Figure 51 Channel 1 frequency response of a DWDM filter (Mux)
Figure 51 shows the frequency response of channel 1 from a DWDM-multiplexer. The dashed blue line is the multiplexer direction and the pink line is the demultiplexer direction. It confirms that the filter is bi-directional with only very little difference between the multiplexer and demultiplexer direction. Clearly seen is that the multiplexer direction results in the highest efficiency. This is correct according to the specifications. Conclusion, this was not the cause of the unusable data from the OTDR measurement of a DWDM system. Pulse power segmentation An OTDR sends out a standard laser pulse with an optical spectrum of 54 nm. Figure 52 is an overview of the OTDR laser pulse spectrum compared with the 8 windows (channels) of the DWDM filter, each with a channel bandwidth of 1,6 nm. From the figure can be concluded that half of the OTDR laser pulse power is outside the range of the DWDM-filter windows, which means half of the pulse power is automatically gone. The other half is distributed over the 8 individual channels.
Figure 52 Spectrum of an OTDR laser pulse and filter windows
The remaining power per channel is about 1/16th of the usual power. When we take a closer look at the filter windows we see that per channel the remaining pulse power is different. Channel 1 has more remaining power than channel 8. Still the remaining power of channel 1 is much to low. The reflection of this signal cannot be measured by the OTDR photo diode. This shows the main problem of an OTDR measurement used for DWDM-systems, pulse power segmentation.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
31
Figure 53A shows the result of an OTDR measurement of a 50 km fiber. Figure 53B show the result of the same 50 km fiber in combination with a DWDM-filter. Clearly seen from figure 53B is the low optical power level of the reflected signal from the fiber in combination with the filter. At the end of the trace is a small peak. This is the reflection at the end-face of the fiber. This reflection indicates that a small amount of power from the laser pulse is reflected. This confirms the bi-directionality of the filters. Almost all of the reflected signal power resides beyond the dynamic range of the OTDR. Apparently an OTDR is needed which has some sort of tuneable DWDM laser. Such an OTDR would be called a Small Bandwidth OTDR (SB-OTDR). Calculations
Figure 53 OTDR measurement of a DWDM system
Now the problem is clear the search for the best solution can begin. First some typical values should be known like, minimum power levels, optical losses, OTDR output power, reflected power, several coefficients and other information. Below are calculations and typical values for OTDR measurements. Minimum signal power according to the Antares DAQ board BER test For Antares data transmission converter boards are created that converts electrical to optical Gigabit Ethernet data signals and visa versa. A bit error rate (BER) test has been done for the Antares converter boards. This test gives a value of the minimum signal power that can be properly detected when using an APD from the Antares converter boards. The test showed a signal power of –35 dBm. For dBm to power conversion see appendix III. The BER test was done at room temperature (23oC). Because the minimum signal power of the photo diode used in the OTDR is unknown, the minimum signal power for the APD from the Antares converter boards is used as a directive value for further calculations. For the following calculations a minimum signal power of –36 dBm is used. Minimum pulse power calculation GbE has a frequency of 1250 MHz with a pulse duration of 1.6 ns. Because the signal is “return to zero” (RZ) one pulse only emits pulse power during 800 ps (figure 54). A BER of –36 dBm means a total pulse power of 250 nW during 1.6 ns. Because the total power is an average over the 1.6 ns pulse period, the power amplitude is 500nW during 800 ps. That gives a pulse power of 400e-18 joule per 800 ps. Figure 54 GbE signal One photon with a wavelength of 1550 nm has a power of 0.129e-18 joule. Approximately 3100 photons are needed for a pulse power of 400e-18 joule/800 ps. When using a pulse width of 10 µs, the APD needs a minimum pulse power of 5 pW.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
32
Optical power loss caused by natural fiber loss A useful calculation is the theoretical loss occurring only due to the characteristics of the fiber itself. This loss is called natural loss and is caused mostly by backscattering and absorption. Figure 55 shows a schematic representation of the loss calculation only due to natural causes. In this calculation a standard OTDR laser pulse with a pulse width of 10 µs is used, which has a pulse power is 41 mW.
- The fiber-loss is 0.182 dB per kilometer. - The backscatter coefficient is approximately –52 dB.
Figure 55 Natural fiber loss calculation
A standard OTDR laser pulse is sent into the fiber. At zero distance (0km) the reflected power is 258nW (calc 1).
Wee 91052
3 2581041 −−
− =• (calc 1) After 50 km the loss is dB1.9182.050 −=• . The remaining signal is reflected at 50 km with a percentage of –52 dB. Then the pulse travels 50 km back, again with a loss of –9.1 dB. That gives a total loss of -70.2 dB. The remaining reflected power is 3.9 nW (calc 2) for the maximum distance of 50 km. In appendix III the accompanying loss-characteristic is given.
Wee 9102.70
3 9.31041 −−
− =• (calc 2) Dynamic range An important specification for an OTDR is the dynamic range. We can determine the dynamic range of an OTDR with an Antares APD, using the results of the previous calculations
- a pulse power of 5 pW is needed for minimum signal detection. - The reflected signal power after 0 km is 258 nW. - The reflected signal power after 50 km is 3.9 nW.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
33
In case of a perfect fiber, where loss is only caused by natural loss, a total loss occurs of –18.2 dB (calc 3).
dBee 2.18
2589.3log10 9
9
=
−
−
(calc 3)
Additional loss may occur caused by external influences like bends, bad connections or damaged fibers. The minimum detectable power level is 5 pW per 10 µs pulse. Thus the range for extra attenuation is 14.46 dB (calc 4).
dBe
e 46.149.3
5log10 9
12
=
−
−
(calc 4)
A total dynamic range of 34 dB is possible (figure 56). Averaging the results achieves a more reliable result.
Figure 56 Dynamic range of an OTDR using
an APD used for Antares Options for Small Bandwidth OTDR creation Generally there are two main options to create a small-bandwidth OTDR. One is by using an external module that replaces the OTDR laser pulse by a small-bandwidth laser pulse. The other option is to develop a whole new standalone SB-OTDR device. External Small Bandwidth laser module An external Small-Bandwidth laser module replaces the OTDR standard laser pulse by a small bandwidth tuneable laser pulse that fits in one of the 6 DWDM filter channels used in Antares.
Figure 57 External Small Bandwidth laser module
When the OTDR sends out a laser pulse, it is directed to a photo diode. This gives a signal to a trigger mechanism, which uses a build in delay to synchronise with the upcoming OTDR pulse. The best solution would be a direct connection from the OTDR trigger signal to the modulator. It would also be less expensive but the OTDR needs to be modified to tap the trigger signal.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
34
The pulse pattern from the OTDR laser for a 0-50 km fiber measurement is shown in Figure 58. This pattern is especially important for the trigger mechanism when a photo diode is used for the trigger mechanism. Precise measurement is needed to determine the precise clock cycle, because light travels 200 m in 1 µs, the smallest timing error results in unreliable measurement data.
Figure 58 OTDR pulse pattern After this delay, the trigger mechanism gives a pulse to the tuneable laser, which sends out a small bandwidth laser pulse that can be customised to each individual filter channel. Important is the amplifier to adjust the laser pulse power. A DWDM laser is a low power laser compared to the standard OTDR laser, it needs to be amplified to obtain a power level that’s high enough for to return to the OTDR. The power level of the reflected tuned laser pulse has to contain the same amount of power as the returned power of the OTDR standard laser pulse. The amplifier can be tuned when the linear schematic loss, caused by the insertion loss of the circulators and possible connector loss, is known. The circulator used in this example has an insertion loss of 1.1 dB. The total schematic loss from the example is 3.3 dB. To reach the same amount of reflected power back to the OTDR, the tuneable laser has to send a pulse with a power of approximately 68 mW. The OTDR laser pulse and the tuneable laser pulse are shown in figure 59.
Figure 59 Left the OTDR pulse power, Right the necessary
small bandwidth pulse power After the pulse is amplified it is sent into the fiber and redirected via two circulators back into the OTDR. Standalone Small Bandwidth OTDR The standalone SB-OTDR is just like a normal OTDR except for the laser. The standard laser is replaced by a small-bandwidth tuneable laser. The same difficulty occurs as with the external module, a DWDM laser is a low power laser compared to the standard OTDR laser. It needs to be amplified to obtain a power level that’s high enough for proper detection. It’s more difficult to develop than an external module but it’s surely possible. The total construction and development costs will be much higher than the external module.
Figure 60 Standalone Small Bandwidth OTDR
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
35
Conclusion It has become clear that the external module has the most advantages compared to the standalone SB-OTDR. The most important thing to keep in mind is that the external module needs to be tested for compatibility with the existing OTDR. The idea of replacing the OTDR laser pulse by a small bandwidth laser pulse has not yet been tested. The fiber might become more influential because of the high amplification of a small bandwidth laser signal. This might introduce physical side effects, such as creation of warmth inside the core because of the high intensity of the laser light. The triggering also needs special attention. The best solution is a direct connection from the trigger signal from the OTDR to the external module trigger circuit. Despite the serious testing, the external module will be a lot easier to develop and construct than a standalone SB-OTDR. It uses an already existing complicated device and most important, its already existing software, which otherwise should be redeveloped. Because of the simplicity of the external module, compared to the standalone SB-OTDR, the costs will be lower. Less expensive components will be used and a smaller amount of time will be needed for the testing and development. According to these results my conclusion is that the external module has the biggest chance for success.
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
36
Notes 1NIKHEF National Institute for Nuclear physics and High Energy Physics in
Amsterdam, Holland http://www.nikhef.nl
2OTDR Optical Time Domain Reflectometer, optical measuring instrument 3DWDM system Dense Wavelength Division Multiplexing system 4SB-OTDR Small Bandwidth OTDR, modified OTDR 5Lightspeed in vacuum 3*10^8 m/s 6Refractive index A property of light guiding materials that relates to the speed of light in
the material versus the speed of light in a vacuum. 7Splice Permanent fiber connection achieved by melting both cores together 8Wavelength For optical data transmission, wavelengths are used instead of
frequencies. f1
=λ
9Cross-talk Undesired coupling from one channel to another 10SNR Signal to Noise Ratio is the ratio of the signal level compared to the
noise level. An indication for signal quality
11NEP Noise Equivalent Power, The noise caused by optical receivers 12Spectral responsivity A specification for the optical to electrical response of a photo diode
[A/W] 13Quantum efficiency The ratio of how many incident photons created electrons. QE of 50%
means half of the photons created electrons 14Photon A particle of light 15Antares Telescope near Toulon in the south of France, to study neutrinos
from space For detailed information about the project visit the website: http://antares.in2p3.fr/
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
37
Bibliography Books: EXFO Lightwave test and measurement reference guide 2001 2nd edition http://www.exfo.com The photonics design & applications handbook 1999 Book 3 http://www.photonicsnet.com Handbook of fiber optic data communication ISBN: 0-12-437162-0 EXFO Guide to WDM technology & testing 2000 ISBN: 1-55342-000-4 Optical fiber communications principles and practice ISBN: 0-13-635426-2 Web Sites: Masstron http://www.masstron.com/products/connector.html How stuff works http://science.howstuffworks.com/laser.htm/printable Fundamentals of DWDM Technology (PDF) http://www.cisco.com/univercd/cc/td/doc/product/mels/dwdm/dwdm_ovr.pdf Fiber optics: http://www.fiber-optics.info/default.htm
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
38
Appendix
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
39
Appendix I
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
40
Appendix II
Small Bandwidth – OTDR Pieter N.J.M. Jansen et al. January 2004
