光纤通信-英文教材4new

1、H64OCN, H64OCA, H64OCP Optical Communications (+)Lecture Notes 2008200910-09101 Dr A J PhillipsGain non-flatnessEDFAs, unless properly designed, will have a gain that varies significantly over say 40 ITU-T channels separated by 100 GHz (approx. 30 nm). Even when well designed EDFAs will have a gain

2、(non-)flatness specification which states how much the gain varies from the overall gain at individual channel wavelengths. If, e.g., 0.5 dB is quoted for this then the gain for a particular channel may be up to 0.5 dB more or less than the overall gain. Unless the spec. specifically prohibits it th

3、ere is nothing to stop this variation happening for adjacent wavelength channels, though perhaps unlikely.The overall gain referred to typically is calculated from all the device output power divided by all the device input power. In practice it will deviate from the nominal gain of the amplifier by

4、 an amount governed by the control loop and measurement of the input and output powers.So the gain for a particular wavelength in a system would be in the range GnomdB GctrldB GflatdBStrictly, polarisation independent gain (PDG) should also be considered.SOAs have a gain profile that is often approx

5、imated by a parabola in the region of interest.Other amplifier classifications Amplifiers, partic. EDFAs, can be of 3 main types:o Controlled to have constant gain (typical)o Controlled to have constant output powero Allowed to respond according to saturation characteristic Amplifiers may also be ch

6、aracterized by the ITU-T wavelength bands in which they operate.o EDFAs are possible in the C-band (conventional, 1530 1565 nm) and the L-band (long, 1565 1625 nm).These are generally different devices as the L-band EDFA requires more doping and/or fibre length and/or pump power, and also does not r

7、equire as much gain flattening filtering.o The other wavelength bands are: O, original, 1260-1360 nm; E, extended, 1360-1460 nm; S, short, 1460-1530 nm; U, ultra-long, 1625-1675 nm. o SOA wavelength design more flexible than EDFAs. Raman amplifiers also exist. These make use of the nonlinear effect

8、stimulated Raman scattering (SRS) to transfer energy from a pump to a signal, within the communication fibre itself. More detail will be given later in the course.o Typically provides distributed gain, often with a counter-propagating pump arrangemento Distributed EDFAs also possible but require new

9、 (erbium doped) communication fibre to be installed1.4.9 Optical RegeneratorsAs we know, optical amplifiers directly boost the signal power in the optical domain i.e. without the need for the signal to be photo-detected, electronically processed and optically re-transmitted.So optical amplifiers are

10、 a way of significantly reducing the use of electronic repeaters.However electronic repeaters also can re-shape and re-time the signal, compensating for dispersion and for jitter. as well as re-transmitting at higher power (i.e. amplifying) optical amplifiers dont have re-shaping/re-timing abilities

11、.It is therefore desirable to be able to provide re-shaping and re-timing function, in addition to re-amplification, in the optical domain i.e. without the need for the information content of the signal to enter the electronic domain.NOTE: The reshaping function generally considered within an optica

12、l regenerator often has a strong focus on reshaping of the noise statistics on 1 and 0, and extinction ratio improvement. Compensation for residual dispersion is also possible, but much dispersion compensation can be provided passively, away from the regenerator.FIGURE: 1R, 2R and 3R optical regener

13、ation (OR). Regeneration is 1R if (re-)amplification only, 2R if re-amplification and re-shaping and 3R if re-amplification, re-shaping and re-timing Note the regen. not necessarily performed in separate stages, or in this order.Technology to provide OR is typically based on optical amplification. T

14、wo examples:2R: SOA followed by saturable absorber (SA) saturable absorber contributes some re-shaping. It passes high powers strongly but attenuates low powers3R: terahertz optical asymmetric demultiplexer (TOAD) in regenerator configuration fibre Sagnac interferometer, SOA asymmetrically placedFIG

15、URE: Operation of a TOAD in regenerator configuration. (similar devices: semiconductor laser amplifier loop mirror (SLALOM) (but slower), and nonlinear optical loop mirror (NOLM) uses non-linearity of fibre rather than SOA)There are many other 2R/3R possibilities/variants including synchronous modul

16、ation, schemes based on electroabsorption modulators (EAM) and other interferometric schemes. Too numerous and too complex to detail now.1.4.10 Optical transmittersFunction: to provide light that has been modulated (impressed with the data to be communicated)o modulation may be direct or external to

17、 input it to the communication channel (the optical fibre)o at sufficient power level, and with sufficient signal quality, to allow it to traverse the channel and have data recovered at the receiver.Lasers are the most important light sources in optical fibre communication systems, though light emit

18、ting diodes (LEDs) may be used where low data rates and short distances only are required. lasers being too expensive for such an application As well as being used for data high power semiconductor laser diodes (SLDs) are used as EDFA pumps (typically 980 nm or 1480 nm). An LED is a semiconductor di

19、ode that emits incoherent light (i.e. waves that have no phase relationships - random), through spontaneous emission, when forward biased i.e. when a current is passed through it.We are concerned almost entirely with SLDs. Typical material system is InGaAsP (active)/InP (substrate/confining layers)

20、for DWDM sources. Basic construction similar to SOAs (p.35), except facets highly reflective. Many variants though.What is it about lasers that make them so good (for comms)? (e.g. better than LEDs) emit coherent light o different parts of the laser output beam are related in phase (within the coher

21、ence length, which is long) near-monochromatic (if single longitudinal mode) highly directional & collimated (narrow beam)o due to reflections between parallel mirrors, to which light must be perpendicular (see below) in comparison LEDs are incoherent, have a wide spectrum and emit into a wider soli

22、d angleLasers also typically have a faster frequency response than LEDs.Some desirable laser characteristics: high output power narrow spectral width stable wavelength good dispersion performanceThe specific quality that marks out laser light is that it is coherent. The narrowness of its linewidth g

23、ives an indication of how coherent the laser is it is broader the more frequently random fluctuations of phase occur. The laser will have a coherence length and a coherence time, which indicate the distance and time over which a phase correlation existsSo what IS a laser? LASER: Light Amplification

24、by Stimulated Emission of Radiation An optical gain medium (an optical amplifier with the three processes of absorption, spontaneous emission and particularly stimulated emission) is placed within a resonant optical cavityo an SLD is forward biased, like SOA. Light output results from electron-hole

25、recombination. Simplest example uses the Fabry-Perot cavityo Essentially the gain medium is between two highly reflective mirrors (planar and parallel)o For a Fabry-Perot SLD the reflectivity would be provided in reality by the end facets of the device Fabry-Perot cavity has resonant wavelengthso th

26、is makes it a fundamental technology for making optical filters mentioned earlierConsider for a moment the operation of the cavity without the gain medium, and with an input signal incident upon it (i.e. the filter operation)FIGURE: Fabry-Perot filter operation (simplified) Some light goes straight

27、through (if the right facet reflected all light 100% we would not get any output at our desired facet). And some light is reflected by the right facet. And some of that is reflected again by the left facet. And some of that is transmitted. Etc.In other words some is transmitted (to the right) with 0

28、 reflections, some with 2 reflections (one extra round trip), some with 4 reflections (two extra round trips) etc. Some light may in theory leave the device in the opposite direction, depending on the properties of the input facet. Generally undesirable.Key point:At certain wavelengths transmitted l

29、ight of different numbers of round trips are in phase and add constructively. Other wavelengths do not add in phase so little/no output.If cavity length is an integer multiple m of half the wavelength in the cavity, so round trip is integer multiple of wavelength then transmitted light adds in phase

30、 (so ). Resonant wavelengths of cavity longitudinal modesNow consider the cavity with the optical gain medium within it. Analysis is simplified from Yariv, Optical Electronics, p. 174-176, 4th edition, 1991FIGURE: Model for Fabry-Perot laser theory of operation. t and r refer to the ratio of transmi

31、tted and reflected fields at the facets (1=left, 2=right). Optical gain medium taken to have complex propagation constant kEi and Et (see below) are complex electric field amplitudes.Background mathematics: complex function formalism (as per Yariv, Optical Electronics, p. 1-2):Consider cosine wave t

32、hen complex amplitude of a(t) is . This is basically what we do with phasors, except that the sqrt(2) that generally is used (to bring in the rms value) is not present here.Then Often, as a shorthand, a(t) you will see written simply as which is not really true but we are meant to know that we mean

33、the real part. Care is needed as there are situations when using is fine and situations when it will cause problems. You check the maths!Also needed to appreciate the following analysis properly, is that k is the (complex) propagation constant of the gain medium (k accounts for the refraction, gain

34、(inversion), and loss).The transmitted (complex) electric field amplitude iswhich leads to (from sum of an infinite geometric progression)If then Et/Ei infinite. So transmission can occur with no external optical input!This is the condition for laser oscillation.But so we requirei.e. but where g is

35、gain coeff. and a loss coeff. so we get (1)and, i.e. , m=1,2,3 (2)(1) is the amplitude condition with g (the gain coeff.) directly related to the inversion of the medium and thus the pumping. This condition ensures a wave making a round trip returns to the same point with the same amplitude If mediu

36、m gain and facet reflectivities are large enough to meet the amplitude condition the device will produce laser light even when no light is input. This happens as spontaneous emission occurs across the gain medium bandwidth, starting the process off. At least the same amplitude: the amplitude conditi

37、on (1) gives the threshold value required for lasing action (oscillation) to occur. Clearly laser action is also obtained if , i.e. if we invert the population further with more pumping.(2) is the phase condition waves making a round trip return to the same point with the same phase (except for a 2p

38、m phase difference) The phase condition cCan be re-stated as saying that cavity length l must be an integer multiple of the half the wavelength in the cavity (i.e. taking into account the refractive index). Such Wwavelengths that satisfy this condition form the longitudinal modes of the cavity/laser

39、 (dont confuse with the spatial modes of a fibre, see later in module).Below lasing we increase population inversion and single pass gain by increasing the pumping (current). Once a mode starts to lase the gain coefficient and inversion are clamped at threshold values. We modulate the laser directly

40、 by increasing the current beyond the value for threshold which increases the stimulated emission rate (it increases the light intensity in the lasing mode) without changing gain.The FP laser will typically provides output in all of these modes that also satisfy the amplitude condition i.e. has mult

41、iple longitudinal modes.But in a homogeneously broadened medium (wavelengths over a wide bandwidth saturate collectively) like semiconductors in principle this should only be one mode (due to the clamping). However spontaneous emission can coherently couple into modes whose gains just fall short to

42、give an effective round trip gain of unity.If we want single longitudinal mode operation (minimize dispersion in fibre) additional filtering mechanisms must be provided. Ability to select one mode is a reason why DFB lasers typically preferred to FP lasers for high rate/long distance applications. S

43、ee below.If medium gain and facet reflectivities are large enough then the device will produce light even when no light is input. This can happen as spontaneous emission will occur across the gain medium bandwidth and will then be amplified and experience positive feedback, leading to output.Fabry-P

44、erot based lasers provide the simplest example but variations on the theme have led to more interesting devices, prime among them the distributed feedback (DFB) laser.The distributed feedback is in contrast to the (localised) feedback from one place (the facet mirror) in the FP laser case.A periodic

45、 variation (grating) in the cavity width, by corrugating it, provides for reflections based on the Bragg effect, the strongest of which have wavelength double the period. The DFB laser is characterized by having the feedback region and the gain region combined. In contrast the distributed Bragg refl

46、ector (DBR) laser places Bragg reflectors at either end of the gain medium.FIGURE: Distributed feedback (DFB) and Distributed Bragg reflector (DBR) lasers.Through careful design DFB and DBR lasers can be made so that only one wavelength oscillates, and that is the one determined by the periodic vari

47、ation.DFB lasers are particularly popular in current optical fibre systems despite being more expensive than FP lasers they are essential for high speed/long distance systems. Other interesting laser types include: external cavity lasers (two cavities gain cavity and external cavity) can suppress un

48、wanted longitudinal modes vertical cavity surface emitter lasers (VCSEL)o as opposed to edge-emitting lasers (EEL)o cavity very short can be single modeo gain medium thin so needs very high reflectivities mode locked lasers narrow pulses in periodic train tunable lasers o various technologieso help

49、avoid inventory and sparing issues (keeping lots of fixed wavelength lasers in the warehouse or in redundant backup systems)o provide reconfigurabilityo typically have a tuning time overhead (temperature, mechanical, current injection tuning methods)ModulationWhen we intend to use light to carry dat

50、a we have to modulate the light. At its most obvious the light is either on (1) or off (0) and we have on-off keying (OOK) OOK is simplest form of amplitude shift keying (ASK) Intensity modulation (IM) is another term usedTypically we may use non-return-to-zero (NRZ) or return-to-zero (RZ) signallin

51、g schemes. The latter may be characterized by the duty cycle.FIGURE: RZ and NRZ signalling. Extinction ratio r and duty cycle a shown also (a=1 for NRZ).A range of modulation schemes, with varying strengths and weaknesses, have been proposed (DPSK, ODQPSK, optical duobinary, PPM, CRZ, FSK/PSK (coher

52、ent detection) etc. are just some alternatives) PPM trades bandwidth for receiver sensitivity Optical duobinary has good dispersion performance Due to standardisation work by the Optical Interworking Forum (OIF) 100 Gb/s systems probably most likely to use a PSK variant: polarization multiplexed quadrature phase shift keying (PM-QPSK) with coherent det