提升光学系统性能：光电二极管详解与应用指南

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在《光电检测与测量 - 光电二极管性能优化在光学系统中的应用》这本书中，作者深入探讨了光电二极管这一主题，特别是针对那些希望在实际电路中有效利用这种光敏元件的研究人员和小信号测量专家。本书首先从基础出发，提供了一个简单的模型来理解光电二极管的主要特性和限制。章节一"光电检测基础"部分，首先简要介绍了光电二极管，尽管其原理和特性在其他许多书籍和文献中已有详尽阐述，但这里着重强调的是如何在实际应用中有效地运用。模型化的核心是将光电二极管视为一个硅PN结二极管，这个模型并不完全等同于现代工艺制造的实际组件，但它提供了理解和设计的基础。 1.1 节中，作者讨论了硅作为半导体材料的基本性质，每个硅原子通常有四个化学键，但由于掺杂了五价元素（如磷或砷），下部区域的电子数量被改变，形成了P型半导体。这个过程导致了光电效应，即当光照射到掺杂的硅表面时，电子从价带跃迁到导带，形成电流。了解如何确定二极管输出极性、预测输出电流水平以及掌握检测速度与其负载的关系至关重要。 1.2 节进一步深入探讨了光电二极管的工作原理，包括PN结的形成和其在光电检测中的作用。图1.1展示了两个硅片区块，上部为本征硅（n型），下部为掺杂的P型硅，它们通过PN结连接。当光子击中PN结，会引发电子-空穴对的产生，这些载流子在外部电路中形成光电流。这部分内容还可能涉及光敏二极管的响应特性（如量子效率、暗电流、响应时间等）及其在不同波长和光照强度下的行为。此外，书中还可能涵盖了光电二极管的类型，例如PIN光电二极管（具有额外的本征层来减小暗电流）、雪崩光电二极管（APD，用于提高灵敏度和动态范围）和特殊的多层结构设计。设计者需要了解如何选择适当的光电二极管以满足特定的应用需求，比如线性或非线性响应、宽动态范围或高量子效率。放大电路的设计也是一个关键部分，它涉及到如何将光电二极管的小信号放大，以便进行精确测量。这可能包括考虑噪声性能、增益稳定性和输入/输出阻抗匹配等因素。书中可能还包含了一些实用的电路设计技巧和注意事项，以帮助读者在实际应用中实现最佳性能。《光电检测与测量 - 光电二极管性能优化在光学系统中的应用》是一本实用指南，不仅涵盖了光电二极管的理论基础知识，还提供了设计和应用中需要的关键技术，对于从事光电探测和测量工作的专业人士来说，是不可或缺的参考资料。

the junction bandgap energy (Sze 1969). Different processing parameters

during manufacture can have a large effect on C

. In Chaps. 2 and 3 we will see

how important is C

to the speed and noise performance of an ampliﬁed

photodetector.

1.7.6 Leakage resistance

All photodiodes act as though they have a resistance shunting the diode (R

Fig. 1.7). Its value depends on the processes used to fabricate the diode sub-

strate and junction and decreases with increasing area. A high quality 1-mm

silicon device (e.g., PIN040 from UDT Sensors Inc.) speciﬁes a leakage resist-

ance of typically 1GW and not less than 200MW at room temperature. Tem-

perature increase reduces R

. Other manufacturers publish instead a leakage

current or dark current, which is just the current that will ﬂow through the

shunt resistance for a speciﬁed terminal voltage. If the diode voltage in the

actual circuit is zero, then no current will ﬂow and the effects of R

will be

minimized. We will see later that R

can have a signiﬁcant effect on the detec-

tion signal-to-noise ratio. For low values of R

even a few millivolts offset from

an operational ampliﬁer can lead to signiﬁcant leakage current.

Leakage can be greatly modiﬁed by process parameters, although signiﬁcant

reduction is usually at the expense of other diode characteristics. For example,

the 7.6-mm

detection area BPW34 photodiode manufactured by Siemens is

speciﬁed with a leakage of 2nA at 10-V reverse bias and capacitance of 72 pF at

zero bias. The very similar BPW33 exhibits only 20-pA dark current, but the

parasitic capacitance increases to 750pF under the same conditions.

1.7.7 Series resistance

Series resistance comes from the bulk resistance of the photodiode substrate,

from the ohmic contact diffusions, and from the leads. It is probably the least

troubling diode characteristic for normal laboratory detection needs but

becomes important in high bandwidth systems and in power generators using

solar cells.

1.7.8 Table of representative photodiodes

Table 1.2 shows a few representative photodiodes. The BPW34 is a common

silicon device in a clear plastic package and has been available for 30 years. The

BPW33 looks similar, with the same active area. However, as noted, the dark

leakage current has been reduced signiﬁcantly, at the expense of a tenfold

increase in junction capacitance. Items 3 and 4 are modern design silicon devices

and show the change in dark current and capacitance incurred by changing the

active area. These also offer very low temperature coefﬁcients of responsivity

at less than 100 ppm/°C. Items 5 and 6 are InGaAs detectors. The ﬁrst has an

area of 1 mm

and a capacitance of 90pF. Item 6 is similar, except that a spheri-

cal lens has been used to accept about 1mm

of beam area but focus it onto a

16 Chapter One

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Photodetection Basics

much smaller diode. In the process the capacitance has dropped to 1 pF. Item 9

is another device manufactured for a very small surface area and for use with

single-mode ﬁbers. Subpicofarad capacitance has been achieved in this way.

Item 10 is a small active area and low capacitance chip device offered for 10-

Gbps telecomms applications. The last column gives the noise equivalent power

(NEP) at the wavelength of peak responsivity. This is the lowest power that

could be detected under optimum conditions in a 1-Hz bandwidth, limited by

the dark current noise.

1.8 APDs, Photomultipliers, Photon-Counting

Avalanche photodiodes (APDs) are similar to conventional pin photodiodes but

are designed to produce and withstand a high internal electric ﬁeld, which can

accelerate photoelectrons generated near the depletion region. As they are accel-

erated, additional photoelectrons are generated and a cascade process produces

10 to 1000 photoelectrons per incident photon. This high internal gain can be

of great use in receiver design. The gain is produced through application of

a higher than normal reverse terminal voltage. For example, the low-bias

Hamamatsu S2383 is a 1-mm-diameter silicon device that operates at 150 to

200V for a gain of 100. Some very large APDs also are now becoming available.

Advanced Photonix Inc. offers, for example, the 16-mm diameter 630-70-72-

Photodetection Basics 17

TABLE 1.2 Selection of Representative Photodiodes

Active Peak Respons. Capacitance NEP

Type Dimensions (A/W) Dark Current (pF) (W/Hz

)

1 Inﬁneon BPW34 2.75 ¥ 2.75 mm 0.6 @ 0.85mm2 (<30 nA) @ 72pF @ 0 V 4.2 ¥ 10

-14

Silicon 10V

2 Inﬁneon BPW33 2.75 ¥ 2.75 mm 0.6 @ 0.85mm 20 pA @ 1V 750 pF @ 0V 5.3 ¥ 10

-15

Silicon

3 Hamamatsu 1.1 ¥ 1.1mm 0.5 @ 0.95 mm 20 pA @ 10 mV 20pF @ 0 V 5.7 ¥ 10

-15

S1336-18 Silicon

4 Hamamatsu 10 ¥ 10mm 0.5 @ 0.95 mm 200 pA @ 10mV 1100pF @ 0 V 1.8 ¥ 10

-14

S1337-1010 Silicon

5 Hamamatsu 1-mm diam. 0.95 @ 1.55mm 5 nA @ 5V 90pF @ 5 V 2 ¥ 10

-14

G8370-01 InGaAs

6 Hamamatsu (Spherical lens) 0.95 @ 1.55 mm 80pA @ 5 V 1 pF @ 5V 2 ¥ 10

-15

G6854-01 InGaAs

7 AME-UDT 1 ¥ 1 mm 0.55 @ 0.90 mm 500pA @ 20 V 3 pF @ 20 V 2.3 ¥ 10

-14

BPX65 Silicon

8 AME-UDT 100-mm diam. 0.95 @ 1.55 mm 50pA @ 10 V 1.5pF —

InGaAs-100L InGaAs

9 Hamamatsu 40-mm diam. 0.95 @ 1.55mm 60 pA @ 5V 0.6 pF @ 2V 2 ¥ 10

-14

G8198-01 InGaAs

10 OSI Fibercomm 25-mm diam. 0.95 @ 1.55mm 100 pA @ 5V 0.2 pF @ 5V —

FCI-InGaAs-25C InGaAs

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Photodetection Basics

5X1, providing a responsivity of 93A/W at 750nm with a bias voltage in the

range of 1700 to 2000V. APDs in InGaAs are also available for long wavelength

(1.55mm) use.

Note that these detectors are not inherently more sensitive nor do they offer

lower noise than conventional pin-diodes. They all offer about the same wave-

length response, capacitance, and quantum efﬁciency and so have essentially

the same limitations. APDs are also inherently noisier because of the presence

of excess noise caused by ﬂuctuations in the gain mechanism. However, where

the provision of sufﬁcient external electronic gain and bandwidth is problem-

atic, leading to limitations in receiver performance, the internal gain of the APD

can be very useful. This is generally the case only at higher speeds (say 1MHz

and above). They are rarely superior for low light level detection at low fre-

quencies. They are also much more difﬁcult to use than pin diodes, owing to

the precise regulation of the high bias voltage that is needed, which is itself

temperature-dependent. However, complete detection modules are available

from Hamamatsu and others to ease the design difﬁculties.

Similar tradeoffs occur with photomultipliers (PM tubes), which generate

photoelectrons in an inorganic semiconductor phosphor. Like the APDs, these

use an internal cascade multiplication process to achieve very high external

responsivities, typically 10

to 10

A/W. However, their intrinsic sensitivity is

rather low, as their photocathodes exhibit quantum efﬁciencies much less than

peak semiconductor detector values, for example, 0.1A/W. They also require a

stable, low noise high voltage supply at the kilovolt level. Advantages of PM

tubes are their wide range of sensitivity obtained through a choice of photo-

cathode materials, large detection areas compared with semiconductor photo-

diodes, and low dark currents. They are also relatively insensitive to ionizing

radiation. Both APDs and PM tubes are available with low enough dark cur-

rents to be used in photon-counting mode. Photon counting is a sensitive detec-

tion method for use at low incident powers (typically <10

photoelectrons/

second), where instead of averaging photoelectrons into a continuous current,

individual electron charges are counted. This brings the advantage of a kind of

separation between thermal noise and photoevents through the use of carefully

set up threshold discriminators. Large gains in performance are sometimes pos-

sible using this mode of detection.

1.9 Summary

In this chapter we have discussed the basics of photodetection using junction

photodiodes and shown how they can be represented by a simple current-

generator model. This is adequate in most cases to predict performance in a

wide variety of externally connected circuits. Different materials may offer

advantages in special situations, but the majority of laboratory and instru-

mentation applications are dealt with using silicon and InGaAs devices. With

this information, we will now investigate how to use photodiodes in practice to

perform useful measurements.

18 Chapter One

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Photodetection Basics

Chapter

Ampliﬁed Detection Circuitry

2.1 Introduction

In this chapter we progress from photodetectors to photodetection, simple elec-

tronic circuits that allow observation and measurement of static and varying

optical signals with a voltmeter, on an oscilloscope, or as part of an optoelec-

tronic product. We saw in Chap. 1 that the signal output from a photodiode is

strongly inﬂuenced by the circuitry connected to it. In particular, unless the

load voltage is much smaller than a few tens of millivolts, simple connection to

a resistive load leads to a logarithmic or at least highly nonlinear response,

while reverse biasing into the third quadrant of the diode’s IV characteristic

generally leads to a current-output response that is linear over many orders of

magnitude of incident power. This reverse bias connection is the basis of much

optical measurement technology. In fact, the simple circuit seen in Fig. 1.8 can

be called our ﬁrst “instrument.”

2.2 The Bias Box

Figure 2.1 shows a photodiode reverse biased by a small battery, with a series

load resistor whose imposed voltage can be read with a high impedance volt-

meter or oscilloscope. With the addition of a multiway switch to change load

resistors over a wide range, say from 100W to 1 MW, and perhaps a changeover

switch to swap the detector polarity, this “equipment” should be in every optical

researcher’s kit bag. I try to keep several available. A 3.3-V lithium cell or 6-V

camera battery is unlikely to damage the majority of silicon photodiodes

through reverse breakdown, but check the photodiode’s voltage ratings ﬁrst to

be sure. However, it is possible to pass excessive current if the photodiode is

brightly illuminated on the low load resistance settings, so connect it ﬁrst,

monitor the voltage, and then illuminate. The bias box can be used with almost

any detector that is handy. It has an easily varied sensitivity and responds lin-

early to power changes. It never oscillates, and the battery voltage is almost

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Source: Photodetection and Measurement

always sufﬁcient. The battery should last for months if the detector is not left

in the sunlight.

However, this circuit has limitations. In using it with an oscilloscope you will

ﬁnd that the most sensitive range (1 MW) gives less output voltage than

expected, because of shunting of the bias box load with a second 1-MW resistor

in the scope. You may also have problems with voltage offsets, even in total

darkness, caused by leakage currents driven by the reverse voltage. There may

also be pickup of electrical interference around the photodiode’s ﬂoating BNC

socket. You can connect the photodiode with a length of coaxial cable, but this

is not recommended. The cable is ﬂoating too, so touching the cable screen

against the output BNC screen under incorrect forward bias may destroy the

diode through excess current. Pickup also will be worse, and the cable capaci-

tance appears directly across the diode, limiting high-speed response. The bias

box is extremely useful for the initial investigations, but once the approximate

optical signal power level is known and the performance requirements are

better deﬁned, using an optimized ampliﬁed detection circuit is preferable.

Several combinations of photodiodes with electronic ampliﬁers are useful for

your arsenal.

2.3 Voltage Follower

For many applications with low light levels a 1-MW load resistor is too small to

optimize the signal to noise. To overcome the input impedance limitation of the

scope, a simple impedance buffer or voltage follower may be used. This can

easily be conﬁgured with an operational ampliﬁer (opamp) as shown in Fig. 2.2.

Connected like this the opamp has a voltage gain of unity and a high input

impedance limited by its input bias current. If a ﬁeld effect transistor (FET)

ampliﬁer is used, the effective impedance can be several gigohms at DC. For a

linear dependence of the output voltage on light intensity, reverse bias of the

photodiode will still be needed, just as in the bias box. Bias voltage must be

20 Chapter Two

= I

Output to scope,

voltmeter, etc.

Switched load resistors R

100, 1k, 10k, 100k ...

Polarity

switch

3–9V

Battery

1µF

Insulated

BNC

Photodiode

Figure 2.1 The bias box uses a small battery to reverse bias a

photodiode and pass its photocurrent through a choice of load

resistors. It is a simple, useful detector. Changing the resistor

allows a trade-off between output voltage and detection speed.

Amplified Detection Circuitry

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