Semiconductor devices and opto-electronicsMeint SmitLeon KaufmannXaveer Leijtens

Opto-Electronic Devices GroupEindhoven University of Technology

Course informationOpto-electronics:Book:Gerd Keiser, Optical Fiber Communications 3rd edition, McGraw-Hill, obligatory!Contact:Xaveer Leijtens x.j.m.leijtens@tue.nl 040 247 5112Electronic devices:Book:Linda Edwards-Shea, The Essence of Solid-State Electronics, Prentice Hall, obligatory!Contact:Leon Kaufmann l.m.f.kaufmann@tue.nl 040 247 5801Website: http://oed.ele.tue.nl (education)

Course overview

Contents semiconductor devicesRecapitulation: electrons in atoms, introduction to quantum mechanicsSolid state materials: crystal structures, energy band diagrams of insulators, metals and (un)doped semiconductorsSemiconductors and carrier transportPrinciple of operation of pn junction diodesFundamentals of MOSFETsCMOS technology (incl. video demonstration)

OGO3.2Free space optical communicationKickoff Meeting Dec 1 in MA1.41 13:30h

Contents Opto-Electronics

ExaminationClosed-book examination, formula sheet will be providedElectronic devices: Edwards-Shea, chapter 1-8Opto-electronics: Keiser

Optical communicationFIBRE

Electromagnetic spectrumTV - radioI.R.U.V.X-rayg -radiationcosmic radiationf (Hz)l (m)Radar

Electromagnetic spectrumOptical communication wavelength: = 1500 nm corresponds to = c/ 200 THz = 200.000 GHz1% = 2 THz = 2000 GHzEDFA-bandwidth 30 nm 4 THz

Standard Single-Mode (SM) Fiber

Optical source+TRANSMITTERFIBERPerformance

Modulation speedFiber-coupled power

Light Emitting Diode (LED)Typical performance data

Power in MM-fiber:100 mWPower in SM-fiber:1mWDirect Modulation Bandwidth:100MHz+

LaserTypical performance

Power (in fiber):5-10mWMax:100-300mWDirect Modulation Bandwidth:1-10 GHz

Photodiode detectorTypical performance data

Responsivity:~1 mA / mWBandwidth:1-20 GHz

Optical communication systemsAttenuation2 dB/cm

WDM-transmission

Erbium-Doped Fiber Amplifier (EDFA)

Synchronous Digital Hierarchy EuropeSDH: SynchronousDigital Hierarchy

STM: SynchronousTransport Module

US & JapanSONET: SynchronousOptical Network

OC:OpticalCarriers

Data rateSDHEuropeSONETUS & Japan52 Mb/sOC-1155 Mb/sSTM-1OC-3622 Mb/sSTM-4OC-122.5 Gb/sSTM-16OC-4810 Gb/sSTM-64OC-19240 Gb/sSTM-256OC-768

Trunk transmission capacityWDM experimentsSi electronicsETDMinstalled(10x / 6 yrs)

Trunk transmission capacity

Undersea cables

Undersea cable

Optical Transport NetworkCourtesy: A.M.J. Koonen

Integrated optical cross-connectDimensions: 8x12 mm2

Fibre propagationn1n2

Fiber performance

Optical attenuation in glass

Fiber attenuation (SiO2)

A note on dB and dBmdBoptical signals: electrical signals:

dBmabsolute power value (with 1 mW as reference) power level in dBm: electrical dB = 2 x optical dB

Reflection & refraction

Numerical Aperture

Dispersion (intermodal)

Bandwidth and bit rate

Fiber typesrefractiveindex

Fiber classification (1)Core diameter50 - 400 mCladding125 (500) m2nd coating250 - 1000 mNA0.16 - 0.5Attenuation1 - 4 dB/kmBandwidth6 - 25 MHz.kmApplicationShort distance, low costlimited bandwidthMM-SI: Multi Mode - Step Index fiber

Fiber classification (2)Core diameter50 m standardCladding125 m2nd coating200-1000 mNA0.2 - 0.3Attenuation1 dB/km (1300 nm)Bandwidth150 MHz.km - 2 GHz.kmApplicationMedium distance communicationLED/Laser sourcesMM-GI: Multi Mode - Graded Index fiber

Fiber classification (3)Core diameter3-10 mCladding50-125 m2nd coating200-1000 mNA~0.1 (not used)Attenuation0.20@1550 - 0.4@1300 dB/kmBandwidth>> 500 MHz.kmApplicationLong distance communicationLasers, standard fiberSM-SI: Single Mode - Step Index fiber

The wave equationPlane wave:Spherical wave:Solutions to Maxwells equations:phase fronts

Wave vector and decomposition

Interference

The metallic waveguide

Modes & Rays

Optical waveguide modes

Mode intensity profilesOptical modes:

Excitation of modes:

Planar:

Single-mode if V Fiber:

Single-mode if V 2.405

V-parameterV number: determines how many modes a fiber supports Lowest order mode HE11 has no cut-offSingle-mode fiber:

Number of modesNumber of modes in step-index fiber Optical power in the claddingfor large values of V

Step index fiber modes (1)LP01 (lowest order)LP11 (first set of higher order modes)HE11TE01TM01HE21Meridional raysSkew rays

Step index fiber modes (2)Effective index b/k as a function of Single-mode fiber: V 2.405

BirefringenceHE11:

Birefringence: difference in effective refractive indices between two polarization modes

Fiber beat length: phase difference between the two polarization modes is Horizontal modeVertical mode

Polarization maintaining fibers Modal birefringence Little mode coupling Maintenance of linear polarizationnon-cylindricalgeometry

and/or

stress inducedanisotropyB - SiO2Ge - SiO2Elliptic core Bow-tie

Silica fibers preform fabricationGases inO2, HeSiCl4GeCl4BBr3POCl3SilicatubeHeating ringGases outDepositpreformfurnaceDiameter controlPolymer coating solutionPolymer curingPulling driveTake-up reelModified chemical vapor deposition for preform fabricationPulling machine

Fiber materialsSilica glass fiberstarting material: pure silica (SiO2) in the form of fused quartz (amorphous)modification of refractive index by addition of impuritieslowering refractive index : B2O3, Fraising refractive index : P2O5, GeO2Polymer optical fiber (POF)large core (multimode)large refractive index difference between core and claddingeasy handlingrelatively high losses

Losses in polymer optical fiberAbsorption loss in POF >>> Absorption loss in Silica fiber search for low loss polymersPMMA (Poly Methyl Metacrylate)PS (Polystyrene)FA (Fluoro acrylate)Typical absorption levels: 100 dB/kmLow loss windows: several windows in the range 500-800 nmNew material development: perfluorinated polymer 50 dB/km from visible to 1600 nmCore typeStep indexGraded index

Advantages of Optical communication Huge bandwidth Small and light Low loss Electrical isolation No EMI (Lightning, interference) Security (no tapping) Reliability Low cost per bit

Men bemerkt dat het menselijk oog slechts gevoelig is voor een klein gebied van dit spectrum, namelijk voor golflengtes van 380 tot 760 nm. Dit zijn gemiddelde waarden die afhankelijk zijn van de waarnemer en van de verlichtingssterkte. In de optica is men vooral genteresseerd in het infrarode, zichtbare en ultraviolette gebied, dit wil zeggen van een golflengte van 10 nm tot 105 nm.2. Propagation of light in fibers2.4. AttenuationThe concept of working with dBs is often a cause for confusion. An optical power ratio (e.g. ratio between output and input power, or, ratio between powers in two systems) expressed in dB means that one takes 10 log of the ratio. For example : an optical fibre has 6 dB loss means : Pout/Pin = 0.25 (formally one should say : the fibre has - 6dB transmission).A similar definition is used for electrical signals. Depending on whether one works with ratios of voltages or currents on one hand or with ratios of electrical power on the other, one has to work with either 20 log or 10 log (to get the same number in dB).A problem arises when an optical signal is converted into an electrical signal (or vice versa). For most types of conversion devices (photodiodes, laser diodes), this conversion is a linear conversion between optical power and electrical current. Therefore, when comparing two signals, the optical power ratio in dB(opt) will translate into an electrical power ratio in dB(el) which is twice the dB(opt)-value.To express absolute power levels, one uses dBm. This is the power ratio (in dB) as compared to 1mW. Hence a power level of 1 mW = 0 dBm, 10 mW = 10 dBm, 100 mW = 20 dBm etc.Try to remember the following (approximate) conversion between dB and power ratio. 0 dB=1+1 dB=+25%;-1 dB=-20%+3 dB=+100% (or x2);-3 dB=-50% (or :2)+6 dB=x4;-6 dB=:4+10 dB=x10;-10 dB=:10+20 dB=x100;-20 dB=:100

2. Propagation of light in fibers2.2. Wave Propagation ModelThe number of modes (including all degenerate modes) is given by the expressions shown here, which are approximately valid for a large number of modes. It is very interesting to see that this number of modes can be written as twice the solid angle x area product divided by wavelength square (this ratio is one for a diffraction limited source). This also implies that the radiance of the light emitted from a fiber end is inversely proportional to the number of modes for given intensity. In a multimode fiber with a typical core diameter of 50m and Dn of 0.01 the number of modes is of the order of 500.The entire modal discussion so far was focused on the Step Index case. In the GI-case everything is conceptually similar (but quantitatively different)In the parabolic graded index fiber (a-profile with a=2) the number of modes is smaller than for step index fiber for the same refractive index contrast. In a single mode fiber the number of modes is 2 (2 degenerate modes). Light emitted from a single mode fiber is in very good approximation a diffraction limited source. 2. Propagation of light in fibers2.2. Wave Propagation ModelIt turns out that the HE11 mode is the lowest order mode. It has one field maximum on the core axis. It exists in 2 variants with orthogonal polarisations. For reasons of rotational symmetry these modes are degenerate (same ) and therefore any (complex) linear combination is also a mode.The next higher order mode is actually a cluster of 3 mode types : the TE01, TM01 and HE21 mode. The latter is twice degenerate ( similar to the HE11 mode). All of these have a zero-field on the core axis (meaning that all field components change sign) and have a dough-nut shaped intensity profile.2. Propagation of light in fibers2.2. Wave Propagation ModelAs in the case of a slab waveguide the fiber is characterised by a V-number and the effective index can be drawn as a function of V for all modes. From this figure one can see that the HE11 is indeed the lowest order mode in a step-index fiber. Then follows the cluster of first order modes and so on. One can see that a step-index fiber will be single-moded if V2.405. A simple calculation shows that for n =0.001 and for a wavelength of 1.5 m the fiber core should have a diameter below 13 m to be single-moded. A typical value for standard single mode fibers is 9 m. This means that this standard single mode fiber will no longer be single mode for wavelengths below (about) 1 m . This wavelength (where the fiber becomes multi-moded) is called the cut-off wavelength.2. Propagation of light in fibers2.4. AttenuationCertain applications require single mode fibers that maintain a linear polarisation state. There are two types of fiber that can do this. The first is the true single mode fiber, in which a mode with only one polarisation state can propagate. The more important type is the polarisation maintaining fiber (PMF). In this type the core has a non-circular shape or alternatively, the fiber material is anisotropic because of elastic stress. This leads to modal birefringence, which means that the propagation constants of both modes differ significantly. Because of this the modal coupling is weak and therefore maintenance of a linear polarisation is obtained.PMF-fibers generally have higher losses than standard fibers.PMF obviously necessitates the use of special connectors which ensure angular alignment between the two connected fibers.1. Types of fiber and fabrication technologyThere are a variety of methods to produce Silica fiber. One can start from liquid materials or from gaseous materials. An important further distinction has to be made between continuous production and preform production. In the first case the fiber is directly made out of the starting materials.More important nowadays is the preform method in which a thick (e.g. 2cm diameter and 50 cm length) cylindrical rod is first made, being a longitudinally compressed version of the fiber to be made. A pulling machine then converts the rod into a fiber which can be tens of km long (for a single rod).The most widely used method to make the preform is the so-called Modified Chemical Vapour Deposition (MCVD) techniqueIn the MCVD technique one starts form a cylindrical Silica tube, which is mounted in a rotatable frame. The source gases are passed through the tube. At the same time an annular heating ring (generally a gas torch) heats part of the tube and moves slowly back and forth. A deposit will then form at the inner side of the tube by thermal oxidation. By controlling the gas flows an arbitrary index profile can be obtained. At the end of the process the tube is collapsed into a rod by passing once more the heating ring at very high temperature along the tube.The next step is to mount the preform in a pulling tower. There the preform is heated again to deform the preform into a (more than 100 times) thinner fiber. This is done with extreme precision. In the same machine a primary polymer coating is applied to protect the silica surface against microcracks and then the fiber is wound on a reel.1. Types of fiber and fabrication technologyThe most important optical fiber is based on amorphous Silica (a type of glass) which has a refractive index around 1.5. In order to produce an index difference between core and cladding, impurities are added to the material during fabrication. B2O3 and F are used for the cladding and P2O5 and GeO2 are used for the core.Silica fibers are not the only type of glass fibers. Some other glass types are also used, in particular fluoride fibers (with Fluor-bonds rather than Oxygen-bonds). They hold the promise of extremely low losses but are presently mainly important for fiber amplifiers. The most important fluoride material is ZBLAN (ZrF4 - BaF2 - LaF3 - AlF3 - NaF).In applications of very short distances fibers made of polymer can be used (Polymer Optical Fiber or POF) . They are cheap and can be handled more easily than silica fibers, when it comes to preparing a high quality fiber end. They often have a core diameter close to the cladding diameter (cladding is very thin) and are therefore always multimode. The cladding diameter is often larger than for Silica fiber and ranges from 125mm to a few millimeters. The refractive index difference between core and cladding is generally higher than in Silica fibers. The losses are much higher than in Silica fiber.2. Propagation of light in fibers2.4. AttenuationPolymer Optical Fibers (POF) have losses which are typically 3 orders of magnitude larger than in glass fibre. A lot of effort has been spent to find low loss polymers. Most common POF-materials are PMMA, PS (Polystyrene) and various types of fluorinated acrylates. The best absorption levels are of the order of 100 dB/km. These levels only occur in narrow spectral windows in the range of 500-800 nm.The main cause for the relatively high absoption levels is the C-H chemical bond. In recent years new types of polymer, so-called perfluorinated polymers, have been developed in which the C-H bond is replaced by a C-F bond. This allows to obtain lower loss (of the order of 50 dB/km) over a very wide wavelength range from visible to 1600 nm.While almost all POFs so far were Step Index fibers, Graded Index POFs are commercially emerging in 2001.Whereas PMMA SI-POFs were limited in use to optical links with a length of less than 100 m and a bandwidth-length product of less than 10 MHz. km, the new perfluorinated GI-fiber will allow links of several 100 m capable of Gb/s operation.