一 Make a choice1) In graded-index optical fiber, the numerical aperture NA can be expressed as C. A.21n n - B.∆2a C.∆2n 1D.21n n a-2) In practical SMFs, the core diameter is just below the cutoff of the first higher-order mode; that is, for V slightly A.A. <2.4B. > 2.4C. =3D. =3.53) When the phase difference is an integral multiple of ______, the two modes will beat and the input polarization state will be reproduced. A. 2π B. π C. 1800 D. π/2 4) It is well known that the total dispersion in the single-mode regime is composed of two components: C.A. mode-partition noise, inter- symbol InterferenceB. frequency chirp , modal dispersionC. material dispersion , waveguide dispersionD. modal dispersion , waveguide dispersion 5) Which of the following codes cannot be transmitted in fibers B.A. CMIB. HDB3C. 5B6BD. 8B1H6) Dispersion-shifted fiber (DSF) is a type of single-mode fiber designed to have zero dispersion near A nm.A. 1550B. 850C. 1310D. 15107) To make sure that the APD photo detector works properly, a sufficiently D is applied across the p-n junction.A. high forward-bias voltageB. low forward-bias voltageC. low reverse-bias voltageD. high reverse-bias voltage 8) A single mode fiber usually has a core diameter of A.A. 10mB. 62.5nmC. 125nmD. 50mm二Blank filling1) Each SDH frame comprising three areas: a section overhead area （SOH ）, a pointer area, a payload area including the data to be transmitted.2) List three applications of FBG(fiber Bragg grating): filter 、Optical Add Drop Multiplexer(OADM) and dispersion compensator.3)According to whether there is electric or magnetic field in the direction of propagation or not, transverse modes of light waves are classified into different types: TEM modes, TE modes, TM modes and hybrid modes.4) Transmission of information in an optical format is carried out not by frequency modulation of the carrier, but by varying the intensity of the optical power.5) Largely due to attenuation and dispersion, the optical signals undergo waveform distortion and decreased amplitude.6) Material dispersion occurs because the index of refraction varies as a function of the optical wavelength.7) ZDSF is a dispersion shifted single mode fiber that has the zero dispersion wavelength near the 1550 nm window, but outside the window actually used to transmit signals.8)A laser is constructed from three principal parts: a pump source, a gain medium, and an optical resonator.9)An optical fiber comprises at least two layers, a core and a cladding.10)Optical transmitter consists of optical source, a modulator and a channel coupler.11)Fiber refractive index profiles classify fibers as step-index fibers and graded-index fibers.12)BER (The bit error rate) performance and jitter are two important indicators in a opticaldigital communication system.13)LASER is a mechanism for emitting light within the electromagnetic radiation region of thespectrum, via the process of stimulated emission.14)There are two kinds of SOA：Fabry-Perot Amplifier (FPA) and Traveling-Wave Amplifier(TWA) .15)The principal noises associated with photo detectors that have no internal gain are quantumnoise, dark-current noise generated in the bulk material of the photodiode, and surface leakage current noise.16)In a step-index fiber, the refractive index of the core is uniform and undergoes an abruptchange at the core-cladding boundary. In graded-index fibers, the refractive index of the core varies gradually as a function of radial distance from the fiber center.17)If the input pulse excites both polarization components, it becomes broader as the twocomponents disperse along the fiber because of their different group velocities. This phenomenon is called the PMD.18)The most common semiconductor photo detector is the pin photodiode.19)The main cause of intrinsic absorption in the infrared region is the characteristic vibrationfrequency of atomic bonds.20)In SDH transmission structures, a TU (Tributary Unit) includes a low level VC and a TUPTR.21)Extrinsic absorption is caused by impurities introduced into the fiber material.22)Intramodal dispersion is a result of the group velocity being a function of the wavelength.23)The optical amplifiers is divided into 3 groups: semiconductor optical amplifier (SOA),optical fiber amplifier (OPA) and Raman amplifier (FRA).24)There are two kinds of directional couplers: a prism-fibre and a fibre-fibre lapped coupler.25)EDFA has three pumping structures: ①the forward pumping structure; ②the backwardpumping structure; ③ the double pumping structure.26) A laser consists of a gain medium inside a highly reflective optical cavity, as well as a meansto supply energy to the gain medium.27)Attenuation in an optical fiber is caused by absorption losses, scattering losses, and bendinglosses.28)STM-1 frames provide a transmission bit rate of 155 Mbit/s.29)From the point of view of the wave theory, light wave could be described as anelectromagnetic wave.30)Intermodal dispersion is a result of each mode having a different value of the group velocityat a single frequency.31) A typical optical digital communication system usually comprises three parts: transmitter,optical fiber channel, and receiver.32)The pin Photo detector structure consists of p and n regions separated by a very lightlyn-doped intrinsic (i) region.33)Please list three steps of SDH Multiplexing: mapping，aligning ，multiplexing.34) There are three variations of WDM that are commonly used: Broad WDM, Coarse WDM, and Dense WDM.35) The STM-1 frame is the basic transmission format for SDH. The frame lasts for 125 microseconds; therefore there are 8000 frames per second.36) In SDH frame structure, the SOH is made up of a regenerator section overhead (RSOH) and a multiplexing section overhead (MSOH).37) The sensitivity of a photo detector in an optical fiber communication system is describable in terms of the minimum detectable optical power.38) Please list three applications of EDFA in optical fiber communication system: as preamplifier of receiver 、as power amplifier of transmitter and as the optical repeater.39)An optical isolator (optical diode), is an optical component which allows the transmission of light in only one direction.三Interpretation of terms and phrases1) AON (all-optical network) 2) DBR (distributed Bragg reflector) 3）FDDI (fiber distributed data interface) 4）HFC (hybrid fiber-coaxial) 5）ISDN (integrated services digital network) 6）RA (raman amplifier) 7）OC (optical carrier) 8）OTDM (optical time-division multiplexing) 9）PON (passive optical network) 10）SCM (subcarrier multiplexing) 11）SDH (synchronous digital hierarchy) 12）SPM (self-phase modulation) 13）STS (synchronous transpor signal) 14）TW (traveling wave)15）WGA (waveguide-grating router) 16）IMD (intermodulation disortion) 17）AWG (arrayed-waveguide grating) 18）AOTF (acousto-optic tunable filter) 四 画图题1) Draw the element block of a Distributed forward Raman amplifier2) Draw an element diagram of a Double pump EDFA3) Draw a block diagram of a typical optical digital communication system and briefly describe the functions of each part.OutputInputEDFWDMIsolatorPump LaserIsolatorPump LaserWDMPump LaseroutputfiberinputAn optical communication system consists of a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal. The optical repeater is to extend the transmission distance of optical signal.4) Draw the element diagram of the application of optical amplifier.四 简答题1) Dispersion: Any phenomenon in which the velocity of propagation of any electromagnetic wave is wavelength dependent. 2) Stimulated EmissionsIf a photon of energy hv 12 impinges on the system while the electron is still in its excited state, the electron is immediately stimulated to drop to the ground state and give off a photon of energy hv 12.3) There are 3 dispersion types in the optical fibers in general: 1- Material Dispersion 2- Waveguide Dispersion3- Polarization-Mode Dispersion4) Polarization mode dispersion (PMD) is due to slightly different velocity for each polarization mode because of the lack of perfectly symmetric & anisotropic of the fiber5) Laser is an optical oscillator. It comprises a resonant optical amplifier whose output is fed back into its input with matching phase. Any oscillator contains: 1. An amplifier with a gain-saturated mechanism 2. A feedback system3. A frequency selection mechanism4. An output coupling schemeOptical transmitterRepeaterOptical receiverfiberfiber6) In thermal equilibrium the stimulated emission is essentially negligible, since the density of electrons in the excited state is very small, and optical emission is mainly because of the spontaneous emission. Stimulated emission will exceed absorption only if the population of the excited states is greater than that of the ground state. This condition is known as Population Inversion. Population inversion is achieved by various pumping techniques.7) Turn on DelayWhen the driving current suddenly jumps from low (I1 < Ith) to high (I2 > Ith) , (step input), there is a finite time before the laser will turn on8) The Quantum LimitFor an ideal photo-detector having unity quantum efficiency and producing no dark current, it is possible to find the minimum received optical power required for a specific BER performance in a digital system. This minimum received power level is known as the quantum limit.9) Gain flatness: The difference between the biggest gain and the smallest gain of the different frequency signal.10) The advantage of Raman amplifier: Simple fabricationLow noise, because amplifying action take place inside the ordinarily fiber.The wavelength can be selected in the low loss waveband.Very wide gain bandwidth.11) Micro bending Loss: microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables. The power is dissipated through the micro bended fiber, because of the repetitive coupling of energy between guided modes & the leaky or radiation modes in the fiber.12) Gain saturation: when near saturation, the gain is nonlinear; saturation, the signal cannot be amplified.13) The disadvantage of Raman amplifier:Need large output power pump laser. As Raman Scattering, the energy is transferred from high frequency to low frequency. Cross talk will affect signal.14) The principal noises associated with photo detectors are:1- Quantum (Shot) noise: arises from statistical nature of the production and collection of photo-generated electrons upon optical illumination. It has been shown that the statistics follow a Poisson process.2- Dark current noise: is the current that continues to flow through the bias circuit in the absence of the light. This is the combination of bulk dark current, which is due to thermally generated e and h in the pn junction, and the surface dark current, due to surface defects, bias voltage and surface area.15) List the advantages of fiber-optic communications over other types of communication technologies.The advantage of optical fiber communication:1. Weight and Size2. Material cost (SiO2 is plentiful)3. Information Capacity4. No electromagnetic interference5. No electrical connection6. Distance between repeaters7. Better security 8. Low crosstalk16) The fabrication of amplifierOptical isolator ，Optical multiplexer, EDF, Pump laser17) What are the advantages and disadvantages of SDH system as compared to PDH system? The main limitations of PDH are:Inability to identify individual channels in a higher-order bit stream; Insufficient capacity for network management; Most PDH network management is proprietary;There is no standardized definition of PDH bit rates greater than 140 Mbit/s; and,There are different hierarchies in use around the world. Specialized interface equipment is required to interwork the two hierarchies.18) List the types of fiber attenuation and dispersion. Absorbing\scattering and bending loss Material/ mode/ waveguide dispersion. 19) The avalanche effect.The created carriers are accelerated by the high electric field, gaining enough energy to cause further impact ionization. 20) Dynamic range:System dynamic range is the maximum optical power range to which any detector must be able to respond.21) Differentiate between step index and graded index optical fiber.Step index fiber has a core of one index of refraction; graded index fiber has a core in which the outside edge starts with a low index of refraction that gradually increases towards the center. 五 计算题1) Suppose two graded index fibers are misaligned with an axial offset of d=0.3a. Try to calculate the fraction of optical power coupled from the first fiber into the second fiber. （Parameter a is the core radius ）The fraction of optical power coupled in the fiber ：122222arcco s()152262T P d d d d P a a a a π⎧⎫⎡⎤⎛⎫⎪⎪⎛⎫=---⎢⎥⎨⎬ ⎪ ⎪⎝⎭⎢⎥⎝⎭⎪⎪⎣⎦⎩⎭()()21220.320.15arcco s(0.15)10.15532π⎧⎫⎛⎫⎪⎪⎡⎤=--- ⎪⎨⎬⎣⎦ ⎪⎪⎪⎝⎭⎩⎭0.748=Turn it into d B , obtain 10lo g1.27T P d B P=-2) A GaAs laser operating at800nm has a 500-µm length and a refractive index n=3.7.What arethe frequency and wavelength spacing?From 2c L nν∆=，22L nλλ∆=obtain:863108122500103.7c G H z L nν-⨯∆===⨯⨯⨯，3) In a 100-ns pulse, 6×106 photons at a wavelength of 1300nm fall on an In GaAs photo detector. On the average, 5.4×106 electron-hole (e-h) pairs are generated.Please calculate the quantum efficiency. The quantum efficiencyNumber of e-h pairs generated = ----------------------------------------- Number of incident photons =665.410610⨯⨯0.90=4) Consider a graded-index optical fiber, core index n1=1.50 and the core cladding index difference Δ=0.01.Try to calculate: 1. The cladding index n22. The numerical aperture NA解：已知：n 1 =1.50，∆=0.01， 根据(1)(2)由（1）式，可知2 n 12∆= n 12- n 22 n 22= n 12（1-2∆） n 2= n 1∆-21 将n 1、∆代入上式，可得n 2==1.5002.01-=1.5098.0⨯=1.50⨯0.98995=1.48491将n 1、∆代入（2）式，可得NA = n 1∆2=1.5002.0=1.50⨯0.14142=0.21213 5) The numerical aperture NA is NA=∆21n =22.001.02560.1=⨯⨯6) The normalized frequencyV=∆221λπna =01.0231.15056.11416.3⨯⨯⨯=26.454>V C =2.4057) Consider a 30-km long optical fiber that has an attenuation of 0.8dB/km at 1300 nm. If 200µW of optical power is launched into the fiber, try to calculate the optical output power P out .First we turn the input signal power unit from mW into dBm63()20010()10lo g 10lo g 7.01110in in P W W P d B m d B m m W W --⎡⎤⨯⎡⎤===-⎢⎥⎢⎥⨯⎣⎦⎣⎦From()10(0)l g()P d Bo kmzP z α=, as z=30k,the output power is ： ()()()10lo g10lo g 11o u t in P WP W Pd B m z o u tm W m W α⎡⎤⎡⎤==-⎢⎥⎢⎥⎣⎦⎣⎦7.0(0.8/)(30)31.0d B m d B km km d B m=--=-Also31.0/103(30)10(1)0.79100.79P km m W m W W μ--==⨯=8) A photodiode is constructed of GaAs, which has a band-gap energy of 1.43eV at 300K.What is the cutoff wavelength? The cutoff wavelength is ()()()()348196.62510310/8691.43 1.610/c gJ s m s h c n m E eV J eVλ--⨯⋅⨯===⨯。