wavelength converterSARBJEET SINGHECE-11 NITTTR

ObjectivesConcept of wavelength converterVarious type of conversionClasses of wavelength converterApplicationsAdvantages and Disadvantages..

ConceptConcept

It refer to as wavelength conversion, shifting, translating of wavelength .

Other name-VIFO (variable input-fixed output)Strategy used is nonlinearity of optical

medium which can be either active or passive, resulting to different non linear effect.

WAVELENGTH CONVERTER AND OPTICAL NETWORKSCONVERSIONWavelength conversion is a process by which the wavelength is changed without altering the data carried by the signal. The device that performs this function is usually called a wavelength converter but it is also referred to as a wavelength(or frequency) changer, shifter or translator. Therefore the input/output (I/O) ports should have capability of a variable input–fixed output Converter and the majority of the optical switching networksuse this type of device

The processor section may also contain an electronic regenerator circuit that reconstitutes the electrical signal from the received data thereby rectifying any errorsinduced by transmission through the fiber . The electrical data is thenconverted back to an optical signal in an electrical-to-optical (E/O) section by modulating an optical transmitter that emits a compliant wavelength at the output port (i.e. converted wavelength, λconv). This method can be modular and hence flexible, but the transmission rate tends to be restricted due to limitations of the O/E/O interfaces

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COHERENT WAVELENGTH CONVERTERS

Depends on principle of 4 Wave Wixing (FWM) and Difference freq generation(DFG) in non linear optical medium.

Interaction of two optical signals, input data and CW probe signal result to two new signal component .

Property of phase conjugation in conversion help us to overcome noise problem.

Four wave mixingArises from optical

nonlinearity. Property of phase

conjugation/spectral inversion in conversion help us to overcome noise problem.

Converted signal ωconv = 2ωCW − ωinSatellite signal ωs = 2ωin − ωCW, result as

cross talk effect Can be used for multiple

wavelength conversion by sending more than one CW probe signal.

This property is used in optical switching networks.

Difference frequency Generation(DFG)Arises from second order

non linerity of optical medium.

Produces single output signal which is determined from diff in freq of interacting optical signals.

No satellite signal as side bands, so it is preferred over FWM, no cross talk happens.

Realizing it practically is difficult due to complicated phase-matching of interacting waves.

Cross-modulation Wavelength Converter

Optical control signal experiences the changes produced due to intensity variation of intensity-modulated input signal in active cavity.

Definition- process of imposing the nonlinear response of medium onto control signal is cross modulation scheme.

This scheme is dependent on : # property of nonlinear

medium- optical gain &

material absorption # modulation on optical

Signal- XGM, XPM, XAM,

DPM.

Cross-gain modulation Cross-gain modulation wavelength converterwavelength converter

Principle- intensity modulated data on one signal wavelength (pump signal),produces variation in carrier density within SOA medium—inverted gain modulation. Which imprint gain modulation onto continuous wave (CW) signal called probe signal.

Probe signal have inverse copy of data and when wavelength of probe signal is diff from pump signal, wavelength conversion obtained.

Thus data is shifted from pump signal wavelength to a probe signal wavelength.

Its transfer function, it is result of slow gain recovery of SOA which gives gradual decrease in its response.

Cross-phase modulation Cross-phase modulation wavelength converterwavelength converter

Relies on dependency of refractive index on the carrier density in active region of the SOA.

Signal beam modulates the gain of the SOA by depleting the carriers, when CW beam encounter the modulated gain and refractive, its phase is changed .

PM-AM create an interferometer structure.

Probe signal wavelength λ CW propagates through the optical filter acquiring the refractive-index-induced phase modulation in the active cavity of the SOA while the input signal at λin is blocked by this optical filter.

• Hence the CW probe signal combines with the phase-modulated probe signal in the PM–AM element when the interference is constructive.

Cross-phase modulation Cross-phase modulation wavelength converterwavelength converter

Mach–Zehnder interferometer (MZI) configuration is used in the lower path to increases the gain of the converted probe signal.

Phase to amplitude modulation can be obtained when a relative phase difference is introduced between the interferometer paths because of different path lengths where probe signal is recombined to produce constructive interference.

Cross-phase modulation wavelength Cross-phase modulation wavelength converterconverter

Frequency/wavelength chirp- It is deviation in the emission frequency with respect to time when a laser is driven by a time-varying current source (i.e. intensity-modulated digital signal). Since refractive index is related to frequency then a variation in the refractive index produces a variation in frequency at each instant.

This chirp is commonly observed in the intensity-modulated converted probe signal affect of the pulses when the refractive index varies suddenly due to an instantaneous change in amplitude pulse .

When frequency chirp shifts the optical frequency towards the shorter wavelength it is known as blue shift, and for longer wavelength is called red shift.

The sign of the chirp is said to be positive when the leading edge of the pulse is red shifted in relation to the central wavelength and the trailing edge is blue shifted, and when the shifts are opposite to the foregoing then the sign of chirp will be negative.

•The relationship between the frequency chirp Δf and the phase of the converted signal φis given by

•Above eq.shows the influence of phase variations on the frequency chirp, the phase of the converted probe signal is dependent upon the intensity-modulated signalpower. If Pin(t) is the input signal power then the phase variation can be written as

•term α represents the line width enhancement factor In the context of the material properties of the nonlinear medium the α-parameter has a strong dependence on the signal wavelength and may be defined in terms of gain and refractive index variation as

where λ is the signal wavelength, dnr/dn represents the differential refractive index and dg/dn is the differential gain which defines the change in gain with respect to change in carrier density.

Interferometer configuration

Delay interferometric wavelength conversion

Michelson interferometer

Nonlinear optical loop mirrors showing Segnatec interferometer in the inset

Cross absorption modulation Cross absorption modulation wavelength converterwavelength converter

Basic principle of an XAM wavelength converter is that CW probe signal at wavelength λcw and the intensity-modulated input signal at wavelength λin are fed into the EAM and the converted output at signal wavelength λconv is obtained at the output of an optical filter.

EAM possesses faster absorption characteristics in comparison with the slow gain recovery of an SOA, XAM-based wavelength converters are faster than wavelength converters based on SOA technology.

WINTERTemplate• In distributing the network control and

management into smaller sub networks and allows flexible wavelength assignments within the sub network.

01 OPTICAL FIBER CO

MM

UN

ICATION

APPLICATION

• Ultra fast wavelength conversion by incorporating additional fiber Bragg grating elements.

• Used in 3R (optical regeneration), for wavelength reallocation or reassignment.

• Used in various optical switching and routing technique for avoiding any conflict among wavelength present in the network.

02ADVANTAGES

03Disadvantages

http://www.optical-network.com/terminology.php?letter=all&id=4

http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-23-11242

BOOKS:GERD KEISERJOHN M SENIOR

www.ece.ucsb.edu/courses/ECE228/228B.../Lecture11_228B_S11.pdf

Sarbjeet Singh12/11/11

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