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Non Linear Effects in Optical Fiber

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Kamal Jeet SinghRoll No. 141610ME Modular ECE 2014 BatchNITTTR Chandigarh25-Feb-1511Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20142ContentsIntroductionSRS Stimulated Raman ScatteringSBS Stimulated Brillouin ScatteringSPM Self Phase ModulationCPM Cross Phase ModulationFWM Four Wave MixingConclusion2Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20143Non Linear Effects in Optical Fibre

Inelastic Scattering Effects Non Linear Refractive Index Effects

SRS SBS SPM CPM FWM

Table - 13Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20144

Linear and Non Linear InteractionsThe terms linear and nonlinear (Figure 1), in optics, mean intensity independent and intensity-dependent phenomena respectivelyFigure 14Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20145Nonlinear effects in optical fibers (Table 1) occur due to:

Change in the refractive index of the medium with optical intensity andInelastic scattering phenomenon

The power dependence of the refractive index is responsible for the Kerr-effect

5Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20146Depending upon the type of input signal, the Kerr-nonlinearity manifests itself in three different effects such as:Self Phase Modulation (SPM)Cross Phase Modulation (CPM)Four Wave Mixing (FWM)

At high power level, the inelastic scattering phenomenon can induce stimulated effects such asStimulated Brillouin-Scattering (SBS)Stimulated Raman-Scattering (SRS)

6Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20147Except for SPM and CPM, all nonlinear effects provide gains to some channel at the expense of depleting power from other channels

SPM and CPM affect only the phase of signals and can cause spectral broadening, which leads to increased dispersion7Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20148Inelastic Scattering Effects

Stimulated Raman Scattering (SRS)Stimulated Brillouin Scattering (SBS)8Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20149Stimulated Raman Scattering (SRS)SRS is an interaction between light waves and the vibrational modes of silica molecules.

When photons are scattered from an atom or molecule, most photons are elastically scattered (Rayleigh scattering), such that the scattered photons have the same energy (frequency and wavelength) as the incident photons

If a photon with energy hv1 is incident on a molecule having a vibrational frequency Vm, the molecule can absorb some energy from the photon.9Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201410In this interaction, the photon is scattered, thereby attaining a lower frequency V2 and a corresponding lower energy hv2.

The modified photon is called Stokes Photon or Pump Wave, since it supplies power for the generated wave.

This process generates scattered light at a wavelength longer than that of the incident light.

If another signal is present in this longer wavelength, the SRS light will amplify it and pump wavelength signal will decrease in power.

The figure in the next slide is illustrating the SRS effect.

10Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201411

Figure: SRS transfers optical power from shorter wavelengths to longer wavelengths11Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201412Stokes and anti-Stokes scattering

The Raman interaction leads to two possible outcomes:

The material absorbs energy and the emitted photon has a lower energy than the absorbed photon. This outcome is labeled Stokes Raman scattering.

The material loses energy and the emitted photon has a higher energy than the absorbed photon. This outcome is labeled anti-Stokes Raman scattering.

12Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201413SRS ApplicationsRaman scattering is used as a tool to detect high-frequency phonon and magnon excitations.Raman lidar is used in atmospheric physics to measure the atmospheric extinction coefficient and the water vapour vertical distribution.Stimulated Raman transitions are also widely used for manipulating a trapped ion's energy levels, and thus basis qubit states.Raman spectroscopy can be used to determine the force constant and bond length for molecules that do not have an infrared absorption spectrum.Raman amplification is used in optical amplifiers.

13Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201414Stimulated Brillouin Scattering (SBS)

SBS arises when a strong optical signal generates an acoustic wave that produces variations in the refractive index.

These index variations cause lightwaves to scatter in the backward direction toward the transmitter.

This backscattered light experiences gain from the forward propagating signals, which leads to depletion of the signal power.

The result of the interaction between the light-wave and the carrier-deformation-wave is that a fraction from the passing-through.

14Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201415light-wave changes its momentum (thus its frequency and energy).

The frequency of the scattered light experiences a Doppler shift

The frequency is given by:

VB=2nVs/

Where

n= Index of refractionVs = Velocity of sound in material15Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20141616Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20141717Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20141818Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201419Non Linear Refractive Index Effects(Kerr Effect)

Self Phase Modulation (SPM)Cross Phase Modulation (CPM)Four Wave Mixing (FWM)

19Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201420The Kerr Effect: The Kerr effect, also called the quadratic electro-optic effect (QEO effect), is a change in the refractive index of a material in response to an applied electric field.

Two special cases of the Kerr effect are normally considered, these being the Kerr electro-optic effect, or DC Kerr effect, and the optical Kerr effect, or AC Kerr effect.20Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201421Self Phase Modulation (SPM)

The higher intensity portions of an optical pulse encounter a higher refractive index of the fiber compared with the lower intensity portions while it travels through the fiber

The time varying signal intensity produces a time varying refractive index in a medium that has an intensity-dependant refractive index

The leading edge will experience a positive refractive index gradient (dn/dt) and trailing edge a negative refractive index gradient (dn/dt)21Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201422This temporally varying index change results in a temporally varying phase change as shown in (Figure-2)

The optical phase changes with time in exactly the same way as the optical signal

This Non Linear Phase modulation is self induced, therefore it is called Self Phase Modulation (SPM).

(Figure-2) Spectral Broadening of Pulse dueto SPM22Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201423Different parts of the pulse undergo different phase shift because of intensity dependence of phase fluctuations.

This results in frequency chirping.

The rising edge of the pulse finds frequency shift in upper side whereas the trailing edge experiences shift in lower side.

Hence primary effect of SPM is to broaden the spectrum of the pulse, keeping the temporal shape unaltered.23Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201424 The SPM effects are more pronounced in systems with high-transmitted power because the chirping effect is proportional to transmitted signal power.The phase () introduced by a field E over a fiber length L isgiven by

= 2nL/

where is wavelength of optical pulse propagating in fiber of refractive index n, and nL is known as optical path length

24Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201425SPM Induced Chirp: For Pulse Width = 90ps and Fibre Length = 100m

Non Linear Phase ShiftFrequency Spectra25Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201426SPM: Benefits and Losses

SPM-induced spectral broadening can degrade performance of a lightwave system.Modulation instability often enhances system noise.

On the positive side . . .

Modulation instability can be used to produce ultra short pulses at high repetition rates .SPM can be used for fast optical switching.It has been used for passive mode locking.Responsible for the formation of optical solitons.26Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201427Applications of SPM:

Solitons

Pulse Compression

Optically Tuneable Delays

Optical 40Gb/s 3R Regenerator27Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201428Cross Phase Modulation (CPM)Cross-phase modulation (CPM) is a nonlinear optical effect where one wavelength of light can affect the phase of another wavelength of light through the optical Kerr effect.

When two or more optical pulses propagate simultaneously, the cross-phase modulation is always accompanied by SPM

The nonlinear refractive index seen by an optical beam depends not only on the intensity of that beam but also on the intensity of the other copropagating beams which results CPM.28Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201429The efficiency of Cross Phase Modulation (CPM) depends on :

The fiber chromatic dispersionThe SOP of the different channelsChannel spacingChannel power

CPM converts power fluctuations in a particular wavelength channel to phase fluctuations in other copropagating channels. 29Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201430Advantages of CPM:

Non linear pulse compressionPassive mode lockingUltra fast optical switchingDemultiplexing of OTDMWavelength Conversion of WDMChannels

Disadvantages of CPM:

CPM leads to interchannel crosstalk in WDM systemsIt can produce amplitude and timing jitter30Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201431Applications of CPM:

Cross-phase modulation can be used as a technique for adding information to a light stream by modifying the phase of a coherent optical beam with another beam through interactions in an appropriate non-linear medium. This technique is applied to fiber optic communications.31Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201432Four Wave Mixing (FWM)Four-wave mixing (FWM) is an intermodulation phenomenon in non-linear optics, whereby interactions between two wavelengths produce two extra wavelengths in the signal.

FWM is a nonlinear process that transfers energy of pumps to signal and idler waves.

Mechanism:When three frequencies (say f1, f2, and f3) interact in a nonlinear medium, they give rise to a fourth wavelength (f4) which is formed by the scattering of the incident photons, producing the fourth photon.32Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201433The Relationship between the frequencies is as shown on left side. f4 =f1f2f3

Where f1, f2, f3 are the frequencies of three optical fields and f4 is the frequency of generated electric field due to f1, f2 and f3.33Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201434When this new frequency falls in the transmission window of the original frequencies, it can cause severe crosstalk.

Figure shows a simple example for two waves at frequencies V1 and V2.As these waves copropogate along a fiber, they mix and generate sidebands at 2V1 V2 and 2V2 V1.

Similarly, three copropogating waves will create nine new optical sideband waves at different frequencies.

34Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201435The efficiency of FWM depends on fibre dispersion and the channel spacing.

Since the dispersion varies with wavelength, the signal waves and the generated waves have different group velocities.

This destroys the phase matching of the interacting waves and lowers the efficiency at which power is transferred to newly generated frequencies.

The higher the group velocity mismatches and the wider the channel spacings, the lower the Four Wave Mixing.

35Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-20143636Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201437Example: Consider a 75km link of dispersion shifted single mode fiber carrying two wavelengths at 1540.0 nm and 1540.5 nm. The new frequencies generated due to FWM are at

V112 = 2V1-V2 = 2(1540.0 nm) 1540.5 nm = 1539.5 nmand V221 = 2V2-V1 = 2(1540.5 nm) 1540.0 nm = 1541.0 nm

Assume the fiber has an attenuation of =0.20 dB/km = 0.0461/km, a refractive index of 1.48, and a 9.0 m core diameter, so that Leff =22 km37Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201438and Aeff = 6.4x10-11 m2. Consider 5 percent for a 62 GHz (0.5 nm) channel spacing. If each channel has an input power of 1 mW, then, using the values 1111 = 6x10-16 cm3/erg = 6x10-15 m3/(W.s) and D = 3, we find

38Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201439FWM: Good or Bad?FWM leads to interchannel cross talk in WDM systems.It generates additional noise and degrades system performance.

On the other hand . . .FWM can be used beneficially for Parametric amplificationOptical phase conjugationDemultiplexing of OTDM channelsWavelength conversion of WDM channelsSuper continuum generation39Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201440ConclusionOptical fibers exhibit a variety of nonlinear effects.Fiber nonlinearities are feared by telecom system designers because they can affect system performance adverselyFiber nonlinearities can be managed through proper system design.Nonlinear effects are useful for many device and system applications: optical switching, soliton formation, wavelength conversion, broadband amplification, demultiplexing, etc.New kinds of fibers have been developed for enhancing nonlinear effects.Supercontinuum generation in such fibers is likely to found new applications.40Non Linear Optical Effects25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201441References

Fiber-Optic Communication Systems by Govind P. Agrawal (4th Edition)Optic-Fiber Communication by John M. Senior (3rd Edition)Optic-Fiber Communication by Gerd Keiser (4th Edition)http://ieeexplore.ieee.org/http://en.wikipedia.org/http://www.optiwave.com

41Junction Breakdown25-Feb-15Kamal Jeet Singh, Roll No. 141610, ME-MOD-ECE-201442Thanks42