【導(dǎo)讀】相位檢測器將一個周期輸入信號的相位與壓控振蕩器的相位進(jìn)行比較。的輸出是它兩個輸入信號之間相位差的度量。差值電壓由環(huán)路濾波后，再加到壓控振蕩。的一個顯而易見的應(yīng)用是自動頻率控制。用這種方法可以獲得完美的頻率控制，而傳統(tǒng)的自動頻率控制技術(shù)不可避免地存在某些頻率誤差。環(huán)路是在有一些相位誤差條件下工作的。不過實際上對于一個設(shè)計良好的環(huán)路這種誤差。作用是重建原信號而盡可能地去除噪聲。抑制噪聲，誤差在一定的時間間隔內(nèi)被平均，將此平均值用于建立振蕩器的頻率。信息可通過長時間的平均得到，從而消除可能很強的噪聲。傳輸信號并抑制噪聲的濾波器。自動跟蹤和窄帶的特點說明了鎖相接收機(jī)的主要用途。噪聲，難怪鎖相環(huán)路常用來恢復(fù)深深地淹沒在噪聲中的信號。為了正常工作，必須調(diào)節(jié)振蕩器。頻器輸出含有解調(diào)出來的，由信號邊帶攜帶的信息。干擾與本地振蕩器不同步，因此由

　　

【正文】 ly pensated by propagation at a higher velocity. Hence the broadening of a pulse must be much lower in a graded index fiber as pared to a 16 homogeneous core fiber. In fact this is indeed the case, and for highbandwidth applications, graded index fibers are more suitable than homogeneous core fibers. It may be mentioned here that in optical fibers having very small core radii and small index difference between the core and cladding, it can be so arranged that only one mode of propagation exists in the fiber. Such fibers are therefore referred to as singlemode fibers. Because of the presence of just one mode, the dispersion in these fibers is very small and is only due to intramodal broadening. Such fibers are indeed expected to be used in future super high bandwidth systems. In addition to the extremely large informationcarrying capacity of a system using lightwaves, munication or transmission through optical fibers has several other additional advantages over the conventional metallic systems like the coaxial cable, etc. (i) Because of the extremely low transmission loss of practically available fibers, one can have much greater distance between repeater stations, resulting in substantial cost savings. (ii) Optical fibers are typically about 100 mm in diameter and are basically made of silica or glass. This results in a heavy reduction in weight and volume of space required, which is an important consideration for laying in already crowded available conduits. This saving in weight and volume is also important for shipboard applications and data handling using optical fibers in aircrafts. (iii) Optical fibers are immune to electromagic interference and there is no cross talk. This is an important consideration for secure munications in defense. (iv) Optical fibers can be used in explosive as well as highvoltage environments due to the absence of any hazard due to short circuits, etc. In addition to the main application in telemunications, optical fibers are also expected to play an important role in puter links, space vehicles, industrial automation and process control, etc. In fact, recently optical fibers have been used to carry data and control information within big fusion lasers at Lawrence Livermore Laboratory and Los Alamos Scientific Laboratory and also for monitoring underground nuclear explosions at the Nevada test site. The additional advantages of using optical fibers include lower cost and immunity from noise. 17 Modulators and Detectors What we have discussed above is just one of the ponents of a lightwave munication system. In addition to this, one requires modulators, which would code the information into the lightwave, and detectors, which could detect the pulses of light at the receiver and decode the information. We will discuss briefly the principle behind modulators and detectors. Light sources can either be modulated directly by varying some source input parameter like input current, or the light output can be modulated externally by passing it through devices known as modulators. The most promising source to be used in optical fiber munication systems, namely, the semiconductor laser sources, can be modulated easily by varying the input current. In fact, in digital systems, the source has to be onoff modulated, and practically, the semiconductor sources can be onoff modulated at high speeds with rise times of less than a nanosecond. The onoff modulation is done by biasing the laser diode slightly below the threshold value, which is typically ~100 mA. At this stage, the laser diode operates as an LED and emits incoherent light at a low optical output power. An additional current (~20 mA) is added by a highspeed driver, which switches the laser diode from incoherent emission state to a coherent emission with large output optical power. By keeping the “off state” slightly below threshold, the delay between the applied electrical pulse and the resulting optical output pulse is minimized。 this delay must indeed not be more than the bit interval so that the optical pulses accurately reproduce the input signal. An important factor to be taken care of is the temperature sensitivity of the output optical power. In the abovementioned scheme of operation, this fact can be taken care of by varying the DC bias through an optical feedback circuit so as to take care of both slow changes in ambient temperature and the gradual aging of the laser itself. The monitoring of the output power is usually done by collecting the light emanating from the backside of the laser, the light from the front surface being coupled into the fiber itself. For nonsemiconductor laser sources, an external modulator is used for modulation. The external modulators make use of various properties possessed by different materials. Thus, certain crystals have a birefringence which changes in the presence of an applied electric field. 18 Thus, the state of polarization of a beam can be changed by passing it through such a crystal. If the crystal is placed between crossed polarizers, one would have an intensity modulation. Similarly, acoustooptic modulators are based on the interaction of an acoustic beam with the light wave. The propagating acoustic wave creates a refractive index grating which in turn diffracts the optical wave. At the receiving terminals or at repeater stations, one requires optical detectors which receive the input optical signal and convert it into electrical signals. The three important detector types that find use in lightwave munication are the photomultiplier, the PIN photodiode, and the avalanche photodiode. Even though photomultipliers possess large gains, the latter two are expected to find more widespread application because they are less bulky, do not need high bias voltages, and are much cheaper. The simpl