1. MODULE 4

2. INTRODUCTION  Even though Optical communication system has low noise and attenuation the signal will be degraded as it travels more than 100 kms so we use power boosters in between Rxr & Txr. They are:  Regenerative Repeaters  Optical Amplifiers

3. REGENRATIVE REPEATERS:  It detects an optical signal and then retransmits it as a new signal using an internal laser.  Needs Conversion of signal from optical domain to electrical and vice versa so it is time consuming.  Loss is high.  Restricts operating BW.  Low Signal Quality.  Bit rate specific and modulation format specific.  It requires source, detector, pulse slicing, shaping and timing circuits.

4. OPTICAL AMPLIFIERS:  It is basically a laser with low feedback (medium gain) whose excited carriers amplify an incident signal but do not generate their own coherent signal.  They are used to compensate signal attenuation.  They give gain but introduce some noise also.  The amplifier requires a power source also

5. ...................  The power given is an optical source will excite the electrons to higher levels which will decay and emit a photon at input signal frequency.  Regarding an OA, its location, input parameters, its noise factor and its design are of importance.

6. Advantages of OA:  It is insensitive to bit rate, modulation formats, power and wavelength of the signal passing through the amplifier.  If amplifier is linear, then WDM is allowed.  No conversion.  Cheaper.  More reliable.  Bidirectional.

7. Disadvantages of OA:  There are only finite number of excited carriers. So output power is limited before gain diminishes.  Gain spectrum is not flat over the entire range of signal transmitted.  Additive noise-degradation in receiver sensitivity.  Fiber dispersion and non linear effects accumulate.

8. Incorporation of OA :  3 ways in which we can use an OA  POST AMPLIFIERS- an amplifier is used soon after the transmitter to boost the signal level such that the signal is still above the thermal noise level in the receiver after attenuation. Noise introduced by the amplifier is also attenuated. It will maximize power and sometimes no gain can be attained.

9. ...................  IN-LINE AMPLIFIERS-there may be more than one and their spacing is a consideration. Used to increase the distance between the transmitter and receiver.  PRE AMPLIFIERS-They are high gain, low noise amplifiers which will boost the already attenuated signal.

10. Gain Profiles of OA:  If wide spectral bandwidth is there, then they can be used for WDM.  But in the case of Brillouins, it is narrow spectral bandwidth and hence cant be used for wide band applications.  But they are good for channel selection and amplification of a particular channel without boosting the nearby channels.

11. Types of OA: Raman Amplifier Fiber Amplifier (FA) Easier in line connectivity Semiconductor Optical Amplifier (SOA)/ Semiconductor Laser Amplifier (SLA) Useful for single mode appln High gain Low power constraints Brillouin Amplifier Fabry Perot Fiber Amplifier (FPA ) Traveling Wave SLA (TWSLA) (TWSOA ) Erbium Doped Fiber Amplifier (EDFA) Doped Fiber Amplifier (DFA)

12. SEMICONDUCTOR OPTICAL AMPLIFIER: SEMICONDUCTOR OPTICAL AMPLIFIER: It is like a laser with 30 to 35 percent output facet reflectivity's.

13. ............  Current pump produces population inversion.  Electrons are transferred from valence band to conduction band.  Then spontaneous emission takes place.  Stimulated emission happens when an external signal source is present.  This gives gain.

14. ..........  Spontaneous emission is also amplified and is incoherent producing an randomly fluctuating noise called amplified spontaneous emission-ASE.  Reflections effect the spontaneous and stimulated emission rates. So optical isolators are used which allow light only in one direction preventing reflection back into the amplifier.

15. TWA AND FPA  They differ in their reflectivities and an FPA has a larger reflectivity.  Due to this reflection, an FPA will form a resonant type of amplifier and has a narrow passband.  The narrow spectral bandwidth of FPA leads to an inherent filtering property. Due to this filtering property, an FPA is highly sensitive to fluctuations in bias current, temperature and signal polarization.

16. FPA Pass band

17. ........  A TWA needs a higher bias current compared to FPA. Due to reduced reflectivities in the TWA, we have a wide spectrum bandwidth.  Antireflection coatings are used. So it works in single pass amplification mode and resonance is suppressed.. This increases the spectral BW and reduces fluctuation dependencies. In this context TWA is advantageous over FPA.  TWA has less noise compared to FPA.  Signal gain is higher

18. TYPES OF AMPLIFIERS:  Wideband amplifier will have BW around 10s of nm. Ex: FPA, Brillouins amplifier.  Narrowband amplifier will have BW less than 0.1 nm.Ex: TWA,SOA,EDFA and Raman amplifier

19. Performance Characteristics:  Residual reflectivity in TWA introduces an effect of backward gain. Gain of the backward travelling signal G b is defined as the ratio of power in backward travelling signal P b to input power Pin into the amplifier.  G b =P b /P in

20. Backward gain vs Reflectivity:

21. ........  As reflectivity increase, backward gain approaches forward gain. Even at low reflectivity, backward gain is significant. So optical isolators are always needed.  In an SLA the noise decides the max number of devices that can be cascaded with the OCS  Overall noise in SLA includes spontaneous beat noise, spontaneous spontaneous noise, spontaneous emission shot noise and amplified signal shot noise

22. .........  Spontaneous beat noise is due to the interaction between signal and spontaneously emitted photon. It is dominant in high power region where the repeater operates.  If interaction is between spontaneously emitted photons then it is called spontaneous spontaneous noise.

23. ......  TWA and FPA have fluctuation dependencies and it is higher in FPA.  As temperature is increasing, gain will decrease. But if temperature is decreased, with an increase in gain, the passband ripple also increases. This is due to residual reflectivity.  In FPA there is an additional burden of wavelength shift as refractive index varies with temperature.

24. .......  Gain depends on polarization of input signal because the TE and TM modes have different single pass gains. In FPA due to resonant cavity structure, the effect is more pronounced. Hence we need polarization controllers in FPA. Gain difference between modes is less in TWA.

25. AMPLIFIER NOISE AND SNR :  Noise is due to random, incoherent spontaneous emission of excited carriers. It is called ASE. It occurs over the entire gain bandwidth.  ASE is given by P sp =n sp (G-1)h ν ( δν ) n sp =N 2 /(N 2 -N 1 ) δν = optical bandwidth of active medium  SNR = P sig G/P sp

26. SIGNAL PASSING OA and BEING DETECTED

27. Multichannel IM And Saturation Induced Crosstalk:  This phenomenon is seen in semiconductor amplifiers but negligible in FA due to its long carrier lifetime.  Usually an amplifier provides gain to all channels equally in an ideal case.  But with wavelength division muxed channels there are some non linear effects like IMD and cross talks.

28. Condition For NON LINEAR Effects:  If two channels are incident on a closed amplifier system and their combined power are near the amplifier saturation power, non linear effects occur generating beat frequency at the cross product of two optical carrier frequencies.

29. .......  These new signals interfere causing gain modulation. These modulations occur at various beat frequencies and they are called IMD or four wave mixing.  Gain modulation will modulate both gain and amplitude.  IMD produced signals are quite weak but there may be many which will be in optical BW of the desired signal. They pass through filters at receiver and produce amplitude fluctuations.

30. Cross Talks:  Cross talk occurs in gain saturated amplifiers.  As signal level increases, the amplifier saturates and cant produce any more output. Therefore the gain reduces. This is gain saturation or compression.  As input intensity drops, gain increases to its unsaturated value. Therefore gain and input are inversely proportional.

31. ......  Suppose we use a homogeneous amplifier that equally saturates across entire wavelength and assume we have a two channel system. In one channel, the input intensity increases beyond saturation level, then this decreases the gain in both channels. This causes cross talk in second channel. This produces signal distortion and power penalties.

32. Overcoming CROSS TALK:  Increase saturation output power of amplifier  Decreasing carrier lifetime.

33. Application of SLA:  Long distance communication at 1.3 micrometer has zero dispersion loss but long distance needs periodic amplification.  SLA is small compared to FA. Also cost is less and integrability to the chip containing many optoelectronic devices is easier.  SA can be used as high speed switching element in photonic system and semiconductor will amplify if pumped and absorb if unpumped. FA has high lifetime.

34. .......  SLA can be used as wavelength shifters and filters.  SLA can be used to tap optical energy from a signal passing through an amplifier. In an unpumped SLA material will absorb part or all of incoming light and generate e hole pairs and hence current. This partial detection can be used to tap off some control info in data packets.

35. FIBER AMPLIFIERS:  It consists of a length of glass fiber doped with rare earth metal ions like erbium.  Ions will act as an active medium which will experience population inversion.  Pump is a light source whose wavelength is absorbed by ions.  Pump and signal are combined by a coupler and they propagate inside the fiber.  Light is absorbed at pump frequency and produces a gain for signal at a different frequency.

36. FIBER AMPLIFIER

37. ......  In fiber amplifiers, BW and center frequency are defined by the atomic structure. So temperature dependency and variations in pump power are less significant compared to SLA.

38. RARE EARTH DOPED FA:  We may use rare earth metals like erbium or neodymium.  Erbium is mainly used for single mode fibers.  It has increased gain, reduced noise and lasing is at a larger wavelength of interest.  Erbium ions will emit light in loss minimum wavelength of 1.55 micrometer.

39. Working Of EDFA:

40. Difference between EDFA and SLA:  SLA is a two energy level system whereas EDFA is a three energy level system – metastable state.  In SLA the pump is a current source whereas in EDFA it is an optical source.  Length of EDFA is in meters and in SLA it is in mm ie approx 1mm. So a uniform inversion can be assumed only in SLA.

41. .......  EDFA is circular and hence eliminates attenuation when coupled in OFC and also polarization dependency in gain is reduced. SLA is rectangular.  Carrier lifetime in erbium ions is in ms and semiconductor carriers is in nm So in EDFA IMD and crosstalk is reduced.

42. Excited State Absorption:  Gain of an EDFA is reduced due to ESA.  Erbium has a three level lasing system.  In normal cases, electrons are excited from E1 to E3 from where it decays to E2. From E2 they are emitted spontaneously and also by stimulation.  But sometimes, from upper excited state, the electrons move to still highr=er excited state. Then from there it non radiatively decays to E3.

43. ......

44. .......  Only now it can come to metastable state and cause stimulated emission.  This reduces pumping efficiency of the device.  Hence we require higher pump power to obtain a specific gain.

45. Avoiding ESA:  Co doping of erbium silica FA with materials like phosphorous pent oxide.  Pump the FA at wavelength at which it does not cause population of an excited state.  Use flurozirconate glass instead of silica.

46. Applications of EDFA:  Long haul terrestrial and transoceanic communication.  As preamplifiers and power amplifiers due to their good sensitivity.  Storing optical data- optical memeory since losses are less in EDFA. Memory is nothing but a loop of fiber.  As narrow linewidth source.

47. Disadvantages of EDFA:  devices are large, there is gain saturation and presence of amplified spontaneous emission (ASE) and ESA( excited state absorption ).

48. RAMAN and BRILLOUIN’S AMPLIFIERS:  Compared to other OA, the Raman and Brillouin’s Amplifiers are less efficient, has more pump power and also we need fiber lengths in the range of km.

49. ......  Fundamental principle of Brillouin's is the interaction of photon with molecules of fiber causing the molecules to vibrate.  This produces a phonon (an accoustic wave) and a photon at lower frequency ie Stokes frequency shifted by around 10 GHz.  Pump signal and input signal must be counter propagating.

50. Comparing Raman and Brillouins:  Compared to Raman, Brillouins is efficent providing high gain at modest pupm power  It is a narrow band process so it can be used for channel selection  Brillouins is used for low speed communication due to its low spectral BW. BW can be improved by doping with Ge.  Limitation of Brillouins is cross talk.

51. Fundamental principle of Raman:  Pump photon is absorbed and sets the fiber molecules to mechanical vibration radiating a photon at Stokes frequency. Pump and signal may co propagate or counter propagate.It is less efficient compared to Brillouins.  Ramans has a broader spectral BW and hence can be used for WDM applications  They require higher pump powers.

52. ......  Raman gain depends on fiber length, fiber attenuation and fiber core diameter. G R =exp(g R P p L eff /A eff k) g R is the power Raman gain coefficient L eff and A eff are the effective fiber length and area respectively P p is the optical pump power k is factor representing polarization

53. ......  A eff = π r eff 2  L eff = (1 - exp( -α p L))/ α p  α p = fiber transmission loss at pupm wavelength  L is the actual length of the fiber

54. Raman gain dependence on fiber length and pump loss( α p ):

55. GAIN AND NOISE DEPENDENCIES:  In amplifiers absorption and emission depends on signal wavelength. Hence gain of the fiber also depends on the wavelength.  For a given pump power, the fiber exhibits gain for wavelength greater than a specific value and attenuation for a wavelength less than a specific value.  At a particular wavelength, the net gain is zero. As the pump power increases, the wavelength at which the net gain is zero is moving towards shorter wavelength.

56. .......  For a given pump power, net gain amplification happens at longer wavelength and net attenuation happens at shorter wavelength.  Apart from the peak gain at a particular pump power, gain is almost uniform over the gain bandwidth. This is the principle of the application of WDM.

57. ......  In any amplifier we have ASE. This noise will limit the receiver sensitivity. Noise outside the signal spectrum can be removed using filters but those that are in the signal spectrum interfere with detection and results in noise of the amplifier.  Noise figure is the ratio of the SNR are in the input to the SNR at the output.  F=SNR i /SNR o F=2(N 2 /(N 2 -N 1 )) F is minimum when N2>>N1 and then F=2.

58. Noise Figure And Pump Power:  After a limit of pump power, F reaches its theoretical limit of 3dB. As pump power increases, inversion increases and N 2 /(N 2 - N 1 ) decreases.

59. ......  To increase the capacity of OFC system either the bit rate has to be increased using sophisticated TDM or WDM has to be used. WDM involves multiple wavelengths in different channels transmitted together.  But due to wavelength dependent gain and noise characteristics, different wavelengths in signal have different gain and noise chara. So there will be imbalanced SNRs. So for WDM, gain and noise chara of various wavelengths have to be same – gain flattening.

60. .......  gain flattening can be done by  Al co doping  Pre emphasis of signal levels to compensate for different gains  Using filters WDM is mainly effected by four wave mixing-FWM