光电信息专业英语

⏹Light detectors(光探测器）
⏹Light can be detected by the eye. The eye is not suitable for modern fiber
眼睛可以探测到光。

但是眼睛不适合用在现在的光线通信上因为它的反应太慢了。

communications because its response is too slow, its sensitivity to low-level signals is
它的敏感度对于低频信号来说太不足了，而且对于电子接收器进行调幅解码还有其
他信号处理也不是很简单。

inadequate, and it is not easily connected to electronic receivers for amplification,
decoding, or other signal processing. Furthermore, the spectral response of the eye is
而且眼睛的光谱响应仅限于0.4和0.7UM 之间的波长，而这也正是光损失最多的波长。

limited to wavelengths between 0.4 and 0.7 u m, where fibers have high loss.
Nonetheless, the eye is very useful when fibers are tested with visible light. Break and
虽然如此，眼睛在用光纤探测可见光时是非常有用的。

终止和打断能够通过观察散
射的光观察到
discontinuities can be observed by viewing the scattered light.
⏹System, such as couplers and connectors, can be visually aligned with the visible
source before the infrared emitter is attached. The remainder of this chapter is confined to an investigation of devices that directly convert optic radiation to electrical signals (either current or voltage) and that respond quickly to changes in the optic power level.
⏹Principles of Photodetection
⏹We will look at two distinct photodetection mechanisms. The first is the external
photoelectric effect, in which electrons are freed from the surface of a metal by the energy absorbed from an incident stream of photons. The vacuum photodiode and the photomultiplier tube are based on this effect. A second group of detectors are semiconductor junction devices in which free charge carriers (electrons and holes) are generated by absorption of incoming photons. This mechanism is sometimes called the internal photoelectric effect.
⏹hree common devices using this phenomenon are the pn junction photodiode, the
最常用的应用这个现象的手段是pn节光电二极管，pin二极管，还有雪崩二极管
PIN photodiode, and the avalanche photodiode.
⏹Important detector properties are responsivity, spectral response, and rise time. The
重要的探测要素有敏感度，光谱响应，还有回升时间。

灵敏度P是探测器的电流输出与光输入的比。

它的定义式为。

它的单位是安培每瓦。

responsiveρis the ratio of the output current of the detector to its optic input power. In equation form it is ρ= i / P, the units of responsivity are amperes per watt. In some detector configurations
在有些探测器的构造中电学输出是电压。

这种情况下，敏感度的是以伏特每瓦特。

the electrical output is a voltage. In this case, the responsivity is given in units of volts per watt of incident power.
⏹The spectral response refers to the curve of detector responsivity as a function of
对探测器敏感度的光谱效应曲线是波长的函数。

由于波长响应的快速变化，不同的探测器都工作在光纤损失较低的视觉光谱金属带范围中。

wavelength. Because of the rapid change in responsivity with wavelength, different detectors must be used in the windows of the optic spectrum where fiber losses are low. Within any of the
在这些金属带范围内。

windows, the responsivity at the specific wavelength emitted by the source must be used when 在设计接收器时运用到光源发出的特殊波长的灵敏度
designing the receiver.
⏹The rise time t r is the time for the detector output current to change from 10 to 90%
上升时间是当光学输入功率变化为阶跃时探测器的输出电流由最终值的10%变化为90%的时间。

这符合第六章给出的光源的上升时间的定义。

of its final value when the optic input power variation is a step. This is consistent with the
探测器的上升时间如图。

definition of the rise time of an optic source given in Chapter 6. Detector rise time is illustrated in 探测器的调制带宽是。

Fig. 7-1. The 3-dB modulation bandwidth of the detector is f3-dB = 0.35 / t r .
At this frequency the electrical signal power in the receiver is half of that obtained at very low
在接收器中这个频率的电信号功率是在非常低的调制频率时接收到的一半。

modulation frequencies, assuming the same amount of optic signal power incident on the detector
如果在这两种情况下同样的光信号功率入射到探测器。

in both cases.
⏹PIN Photodiode
⏹PIN photodiodes are most common detectors in fiber systems. The PIN diode has a
Pin 光电二极管在光纤系统中是最常用的探测器。

Pin二极管在p极和n极之间有一个较宽的固有半导体层，如图。

wide intrinsic semiconductor layer between the p and n regions, as illustrated in Fig. 7-5. The intrinsic layer has no free charges, so its resistance is high, most of the diode voltage appears
这个层没有自由电荷，所以它的电阻很高，二极管的大部分电压出现在这里。

而且这里的电力非常强。

因为这层非常宽，所以在进入的光子在这里被吸收的概率大于在P节或N节被吸收的概率。

across it, and the electrical forces are strong within it. Because the intrinsic layer is so wide, there is a high probability that incoming photons will be absorbed in it rather than in the thin p or n regions. This improves the efficiency and the speed relative to the pn photodiode
这提高了效率和速度相对于PN光电二极管来说。

Cutoff wavelength截至波长
To create an electron-hole pair, an incoming photon must have enough energy to raise an electron 为了产生一个电子空穴对，一个刚进入的光子必须有足够的能量能让一个电子穿越能带隙。

这就需要。

产生一个截至波长。

across the bandgap. This requirement, hf ≥ w g, leads to a cutoff wavelength
λ=1.24 / w g
where λ is in u m and w g is the bandgap energy in electron volts. This is just like Eq.
(7-4)
其中。

以um为单位，wg是带宽能量以电子伏特为单位。

就如式7-4光电发射体。

for photoemitters.
⏹Examination
⏹Semiconductor junction photodiodes are small, light, sensitive, fast, and can
半导体节点光电二极管小巧，轻便，敏感，迅速而且还能够在非常小的偏置电压下运作。

它们差不多是理想的光纤系统。

我们将调查这种装置的三种形式：pn，pin，雪崩光电二极管。

operate with just a few bias volts. They are almost ideal for fiber systems. We will investigate three forms of these devices: the pn, PIN, and avalanche photodiodes. The simple pn photodiode, drawn in Fig.7-4, illustrates the basic detection mechanism of a junction
最简单的pn光电二极管，如图7-4，说明了节点探测器的最基础探测机制。

detector. When reverse biased, the potential energy barrier between the p and n regions 当反向偏压，p极和n极之间的势能势垒将增加。

increases. Free electrons (which normally reside in the n region) and free holes (which 自由电子（存在于n极中）和自由空穴（存在于p极中）不能够越过势垒，所以没有电流流过。

normally in the p region) cannot climb the barrier, so no current flows.
⏹Couplers and Connectors耦合器和连接器
⏹Connections are normally quite simple in metallic systems. Wires can be spliced
连接器是一个非常简单而且常见的在金属系统中。

金属丝能够非常简单的用焊接固定。

这种固定仅仅通过融化焊锡就能够松动。

very easily by soldering. The splice can even be undone, merely by melting the solder. The losses in a solder joint are so small that they are not usually considered in the system
这种损失在焊锡的连接处是非常小的以至于它们常常在系统设计中没被发现
design. Removable connectors for wires are also simple, easy to attach, reliable,
economical, and virtually lossless. The favorable attributes of wire connectors are not shared by their fiber counterparts. We shall see what the problems are in splicing and connecting fibers and how these problems can be overcome with sufficient care.
⏹Fiber-to-fiber connections are needed for a variety of reasons. Several fiber must be
spliced together for links of more than a few kilometers because only limited continuous lengths of fiber are normally available from manufactures. Moderate lengths of fiber are easier to pull through ducts than very long cables are, and moderate lengths simplify direct burial or aerial installations. Coupling of light from a source into a fiber can be very inefficient. We evaluate source-coupling losses and describe techniques for reducing them.
⏹At the receiver, light is coupled from the fiber onto the detector surface. This
surface can be chosen to be larger than the fiber core, resulting in very efficient coupling. A small loss due to reflections at the fiber-to-air and air-to-detector interfaces does occur. It can be removed by filling the air gap with index-matching material or by antireflection-coating the detector surface. In any case, detector coupling is not difficult and need not be discussed further.
⏹Connection principles
⏹Losses in fiber-to-fiber connections arise in a number of ways. Core misalignments
and imperfections, illustrated in Fig. 8-1, are major factors. A perfect joint would require lateral (or axial) alignment, angular alignment (parallel fiber axes), contacting ends (no gap), and smooth, parallel ends. Coupling efficiency may be reduced when fibers that have different numerical apertures or core diameters are connected. More loss is present when cores having elliptical (rather than circular) cross sections are attached with their major axes unaligned.
⏹If the core is not centered in the cladding, and if the outside of the cladding is used
as the reference for aligning the joint, then more loss occurs. With care, these problems can be minimized, producing splices with losses of the order of 0.1dB and reusable connectors with losses less than 1dB.
⏹Splices
⏹Splices are generally permanent fiber joint. (Connectors can be mated and unmated
repeatedly and rather easily.) Basic splicing techniques include fusing the two fibers or bonding them together in an alignment structure. The bond may be provided by an adhesive, by mechanical pressure, or by a combination of the two. Fusion splices are produced by welding two glass fibers, as sketched in Fig.8-20. Commercial fusion machines use an electric arc to soften the fiber ends. The ends are prepared by the scribe-and-break method.
Alignment is obtained by adjusting micromanipulator attached to the fibers.
⏹Connectors
⏹Rematable attachment have tested the ingenuity of connector designers and the
pocketbooks of fiber users. The stringent mechanical tolerances required for efficient coupling make quality connectors difficult to design and expensive to build.
⏹Requirements for a good connector include the following:
a)Low loss. The connector assembly must ensure that misalignments are minimized
automatically when connectors are mated.
b)Repeatability. The coupling efficiency should not change much with repeated matings.
c)Predictability. The same efficiency should be obtained if the same combinations of
connectors and fibers are used. That is the loss should be relatively insensitive to the skill of the assembler.
d)Long life. Repeated matings should not degrade the efficiency or strength of the
connection. The loss of a mated connector should not change with time.
e)High strength. The connection should not degrade owing to forces on the connector body
or tension on the fiber cables.
f)Compatibility with the environment. The connection may have to withstand large
temperature variations, moisture, chemical attack, dirt, high pressures, and vibrations.
g)Ease of assembly. Preparing the fiber and attaching it to a ferrule should not be difficult
or time consuming.
h)Ease of use. Mating and unmating the connection should be simple.
i)Economy. Precision connectors are expensive. Cheaper connectors, normally plastic, may
not perform as well
⏹Most connectors are designed to produce a butt joint, placing the fiber ends as
closed together as possible. Butt designs include the straight-sleeve, tapered-sleeve, and overlap connectors. A lensed connector is an alternative to the butt configuration. The
connector assemblies to be described in the remainder of this section are meant to illustrate the general approaches that have been successful in joining fibers. The descriptions do not give complete details of specific commercial connectors but include features found in many of them.
⏹Fiber Bragg grating
⏹Fiber gratings are a periodic variation in the refractive index of the core as
measured along its axis. A sketch of the grating appears in Fig. 9-45. For an input wavelength equal to one-half the repetition period Λ, the waves reflec ted at each periodic refractive index change add up in phase. The grating acts as a reflector as all the reflected beams add up in phase with each other. This is the same phenomenon we noted in describing the distributed-feedback laser diode in Section 6-6.
⏹As in that section the reflected wavelength obeys Bragg laws, restated here as
Λ=λ/2 (9-19) where λ is measured in the fiber core and we consider only the strongest reflection, that of the first order. We may say that the grating is resonant at the wavelength which satisfies Bragg’s law. The length of the grating L (indicated on the figure) and the depth of the reflective-index change are the two other most important design parameters of the device.
⏹Wavelength that do not obey Bragg’s laws are unaffected; they are transmitted past
the grating. Basically, then, the fiber Bragg grating acts as a filter. We will describe the applications of this device in a bit. Next, however, we will consider construction of the grating. The grating is produced by exposing the core to high-power ultraviolet light. The UV light first passes through a phase mask, producing an interference pattern which creates a periodic structural change in the fiber’s core. A permanent and stable variation of the core’s refractive index is the result.
⏹Modulation
⏹LED modulation
⏹It would be impractical to describe even a small fraction of the circuits utilized, or proposed, for modulation of LEDs. Instead, we present basic modulation requirements and strategies and illustrate them with a few specific circuits.
⏹Analog modulation
⏹Figure 6-7 illustrates the basic requirements for analog modulation of a LED. The total modulating current and the resulting optic power, illustrated in Fig.10-1, are given by
i = I dc + I SP sinωt(10-1)
⏹In this equation the first term is the dc bias and the second represents the information signal. We will use a sinusoidal waveform (represented either by the sine or cosine function) to determine the network performance. The modulation factor m’ is the peak current excursion relative to the average current, divided by the average current. That is
m’ = I SP / I dc(10-3a)
⏹Since the total peak and minimum currents are I SP + I dc and I SP - I dc , respectively, the signal amplitude I SP can have its largest value if the dc bias is half the maximum permissible diode current. Setting I SP = I dc for this case produces a peak current of 2 I SP, a minimum current of zero, and a unity modulation factor.
⏹We define the optic modulation factor in term of the optic power, thus
m = P SP / P dc (10-4b)
⏹Allowing us to write the optic power as
P = P dc ( 1 + m sin ωt ) (10-4)
⏹Laser diode modulation
⏹Laser diode present more problems to the circuit designer than LEDs. The trouble arise from
a)The existence of a threshold current
b)The threshold current’s age dependence
c)The threshold current’s temperature dependence
d)The emission wavelength’s temper ature dependence
⏹Digital systems generally operate just below threshold in the off state. The dc current is
I dc≈ I TH, as illustrated in Fig.6-23. Operation near threshold (rather than at zero current) minimizes turn-on delay. Analog systems require a bias current in addition to the threshold current to achieve linear operation, as indicated in Fig. 6-24. The increase in threshold current owing to aging or a temperature rise cause a decrease in the output power if the current remains fixed.
⏹Carrier wavelength changes are of the order of 0.2 nm /℃. This corresponds to a frequency shift of 89 GHz /℃at 0.82 u m. For some applications the shift is insignificant, and for others it may be very important. For links operating near the minimum dispersion wavelength, a shift away from the optimum wavelength decrease the system’s bandwidth. Wavelength-multiplexed systems also require a high degree of carrier-wavelength stability to minimize crosstalk between adjacent channels.
⏹The laser’s temperature dependence can be overcome by cooling the diode. Strategies include adequate heat-sinking and thermoelectric cooling (described briefly in section 6-5). Threshold variations can be corrected by increasing (or decreasing ) the dc current to compensate for the temperature- or age-induced changes in the laser’s characteristics. This last solution, accomplished automatically by feedback control, does not solve the temperature dependent wavelength shift, however.
⏹Laser-diode frequency modulation
⏹The oscillation frequency of a single-mode laser diode depends on the instantaneous amplitude of the injected current. We can explain this result as follows: the current determines both the carrier density and the temperature in the semiconductor’s active layer. In turn, these two factor s determine the layer’s refractive index. As shown in earlier by Eq.(3-24), the resonant frequency of a cavity depends on its refractive index. Thus, the resonant frequency (which is also the output frequency) changes when the current does. In this way, modulation of the drive current produces frequency modulation of the emitting diode.
⏹Heterodyne receivers
⏹As we know from the discussion in Chapter 7, photodetectors produce currents proportional to the incident optic power. Detectors respond to fluctuations in the light intensity, a characteristic independent of the light’s phase or frequency. Thus optic detectors do not reproduce variations in the frequency of phase of the oscillating lightwave. Because of this, frequency modulation of an optic source is ineffective for communications using the direct detection
methods described so far. However, optic frequency-modulation systems are possible using heterodyne detection.
⏹Noise and detection
⏹In this chapter we investigate the major sources of noise and show how to compute the noise power. The signal quality, given by its signal-to-noise ratio, can then be calculated. For digital communications, noise increases the probability of errors. For these systems, we also calculate the error rates.
⏹Toward the end of this chapter we describe a few basic receiver circuit designs.
⏹Two major causes of signal degradation occurring during reception are thermal noise and shot noise
⏹Thermal and Shot Noise
⏹Thermal noise(also called Johnson noise and Nyquist noise) originates within the photodetector’s load resistor R L. Electrons within any resistor never remain stationary. Because of their thermal energy, they continually move, even with no voltage applied. The electron motion is random, so the net flow of charge could be toward one electrode or the other at any instant. Thus, a randomly varying current exists in the resistor, as pictured in Fig.11-1. The average value of the thermal noise current is zero.
⏹The discrete nature of electrons cause a signal disturbance called shot noise.In photodetectors, either photoemissive tubes or semiconductor junction devices, incoming optic signals generate discrete charge carriers. Each carrier contributes a single pulse to the total current. We illustrate this for the vacuum photodiode in Fig. 11-4. The pulse starts when the electron escapes from the cathode and ends when the electron strikes the anode (where it disappears by recombining with a positive charge). Thus, the pulse duration equals the electron’s transit time (the time it takes the electron to travel from cathode to the anode).
⏹There are several important types of noise we have not yet described: modal noise, mode-partition noise, amplifier noise, and laser noise. We will see how they arise and how their effects can be minimized. In addition to noise, jitter in a digital link can increase the error rate. We will discuss jitter and its measurement.
⏹Modal Noise
⏹Modal noise is a random variation in optic power occurring in multimode fiber. If the light source is highly coherent (say, a good laser diode), then the fiber modes interfere with one another and form a speckle pattern consisting of bright and dark spots. Figure 11-14 illustrates speckle. The spot are bright where the net mode interference is additive (in-phase modal fields) and dark where the net interference is subtractive (out-of-phase modal field). Because of its wide linewidth, a noncoherent source (such as LED) will not form a speckle pattern.
⏹Mode-Partition Noise
⏹We noted in Section 6-5 that typical laser diodes emit a multimode spectrum due to the multiple resonances of the laser cavity. The total laser power is the sum of the powers present in the various longitudinal modes. While the total power remains constant, the power associated with the individual modes varies randomly. In fact, the fluctuation in the power of any one mode can be quite large. In a single mode fiber with high dispersion, each mode has a unique propagation velocity. The modal delay, combined with the randomly changing amplitude of each mode, result in a total power of the pulse at any instant of time which is not always the same constant.
⏹Electronic-Amplifier Noise
⏹An electronic amplifier normally follows the photodetector to boost the receiver signal to a useful level. In an ideal situation, both signal and noise powers would be multiplied by the amplifier’s power gain G. Then, the signal-to-noise ratio at amplifier output would equal that at input. Unfortunately, real amplifiers not only multiply the input noise but also produce noise of their own. This reduces the SNR. The effect of amplifier noise can be minimized by designing amplifiers with low noise figure.
⏹Optical Amplifier Noise
⏹It is now apparent that optical amplifiers, such as the erbium-doped amplifier discussed in Section 6-7, will be included in some networks where the transmission path is long or where the light power is distributed to many receiving terminals. In such applications cascaded amplifier chains may be required. For the optical amplifier we will start with the definition of noise figure as given by Eq.(11-41). The amplifier chain illustrated in Fig. 11-17 consist of N amplifiers, each having a power gain of G k and a noise figure of F k.
⏹Laser Noise
⏹Laser noise is an undesirable random fluctuation in the output of a laser diode that occurs even when the driving current is constant. It is a characteristic associated with poor lasers but is present to some extent in all of them. Laser noise reaches a peak when modulating a diode at its resonant frequency (typically a few gigahertz). For this reason, laser diode is more significant for high-frequency links than for lower-frequency one. Well-constructed laser diode contribute only small amount of noise to systems operating well below the diode’s resonance.
⏹Jitter
⏹As we have seen, digital light pulses are distorted in a number of ways. These include the distortion caused by noise and the distortion caused by pulse spreading. Pulse spreading is basically caused by the limited bandwidth of the system, including the transmitter, the fiber, and the receiver. In addition the system introduces timing errors, a phenomenon referred to as jitter. All these distortion reduce the ability of receiver to correctly identify the presence of binary ones and zeros.
⏹1/f noise
⏹Semiconductor device produce a slowly fluctuating current called current noise or 1/f noise. It is limited to low frequencies, varying as 1/f below 1Hz. Current noise can be minimized by passing the amplified signals through filters that severely attenuate frequencies below about 10 Hz. In an atmospheric optic communication system, light can enter the photodetector from sources other than the desired one. Energy from sunlight, street lamps, or car lights can be detected, increasing the receiver’s dc current and, consequently, i ncreasing the shot noise. This background noise is easily eliminated from fiber links because they are normally completely enclosed.
⏹For the purpose of this article a machine may be further defined as a device consisting of two or more resistant, relatively constrained parts that may serve to transmit and modify force and motion in order to do work. The requirement that the parts of a machine be resistant implies that they be capable of carrying imposed load without failure or loss of function. Although most machine parts are solid metallic bodies of suitable proportions, nonmetallic materials, springs, fluid pressure organs, and tension organs such as belts are also employed.
⏹The material compositions of core and cladding, as well as their dimensions and index profiles, determine the optical attenuation and signal dispersion characteristics of the fiber. These properties are also dependent on the wavelength used. Nowadays 850nm is mostly used, but in the
near future, 1300nm will probably be referred because of the demonstrated lower dispersion and attenuation values.
Computer viruses are a problem that exists within the personal computer community. A computer virus is a program (or instructions hidden within a program) that infects other programs by modifying them without your knowledge. Like any other programs, the virus can do anything it is programmed to do. Some viruses are practical jokes, causing unusual or erratic screen behavior. Others are destructive, erasing or damaging files or overloading memory and communication networks.。