NASA/TP-2000-210096

Characterization of Advanced Avalanche

Photodiodes for Water Vapor Lidar

Receivers

Tamer F. Refaat

Old Dominion University, Norfolk, Virginia

Gary E. Halama and Russell J. DeYoung

Langley Research Center, Hampton, Virginia

July 2000
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NASA/TP-2000-210096

Characterization of Advanced Avalanche

Photodiodes for Water Vapor Lidar

Receivers

Tamer F. Refaat

Old Dominion University, Norfolk, Virginia

Gary E. Halama and Russell J. DeYoung

Langley Research Center, Hampton, Virginia

July 2000
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(301) 621-0390 (703) 605-6000
iii

Contents

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1. Water Vapor Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2. DIAL Technique for Measuring Water Vapor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1. Lidar Atmospheric Sensing Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2.2. DIAL Detection System Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3. Avalanche Photodiodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1. APD Structure and Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.2. APD Characteristics and Applied Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4. Responsivity Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4.1. Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.2. Experimental Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.3. Data Analysis and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5. Temperature Dependent Responsivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5.1. Experimental Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5.2. Data Analysis and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

6. Bias Voltage Dependent Responsivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6.1. Experimental Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6.2. Data Analysis and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

7. Responsivity Uniformity Scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

7.1. Experimental Setup and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

7.2. Data Analysis and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8. Noise Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

8.1. Types of APD Inherent Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

8.2. APD Noise Measurement and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

9. Frequency Response. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

9.1. Experiment Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

9.2. Experimental Procedure and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

10. Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Appendix A—APD Manufacturer’s Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Appendix B—Characterization Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Appendix C—APD Test Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Appendix D—MATLAB Data Analysis Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Abstract

Development of advanced differential absorption lidar (DIAL)

receivers is very important to increase the accuracy of atmospheric

water vapor measurements. A major component of such receivers is

the optical detector. In the near-infrared wavelength range avalanche

photodiodes (APD’s) are the best choice for higher signal-to-noise

ratio, where there are many water vapor absorption lines. In this

study, characterization experiments were performed to evaluate a

group of silicon-based APD’s. The APD’s have different structures

representative of different manufacturers. The experiments include setups to calibrate these devices, as well as characterization of the effects

of voltage bias and temperature on the responsivity, surface scans,

noise measurements, and frequency response measurements. For each

experiment, the setup, procedure, data analysis, and results are given

and discussed. This research was done to choose a suitable APD

detector for the development of an advanced atmospheric water vapor

differential absorption lidar detection system operating either at

720, 820, or 940 nm. The results point out the benefits of using the

super low ionization ratio (SLIK) structure APD for its lower

noise-equivalent power, which was found to be on the order of 2 to

4 fW/Hz1/2, with an appropriate optical system and electronics. The

water vapor detection systems signal-to-noise ratio will increase by a

factor of 10.

1. Introduction

Water vapor is an important molecular species in the Earth’s atmosphere, which is primarily located

in the troposphere (part of the atmosphere extending from the surface of the Earth to an altitude of about

18 km). Although the distribution of atmospheric water vapor is highly variable in both time and location, its measurement is very important for understanding the Earth’s water cycle, greenhouse effect,

and weather phenomena (refs. 1 and 2).

The water cycle involves interactions among the Earth’s global systems; the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere. Water is considered the main media for energy transfer

between most of these systems. Although the amount of atmospheric water vapor represents only a

small percent of the Earth’s water reservoir, it is very dynamic and its latent heat transformation is considered the main energy source that maintains the atmospheric general circulation (refs. 1 and 2).

Water vapor and clouds affect the incident solar radiation by reflecting solar radiation back to space

and also absorbing some of this energy within the atmosphere, which substantially moderates the

Earth’s climate. On the other hand, water vapor and clouds affect infrared radiation released by the

Earth’s surface. Some of this radiation is reflected back to the surface and some is absorbed and reemitted at a lower temperature which contributes to the global warming problem or greenhouse effect

(ref. 2).

Water vapor has a direct role in most weather phenomena and natural disasters such as hurricanes.

The latent heat of water vapor was found to be the main energy source for hurricanes (ref. 3). The measurement of water vapor flow into a hurricane associated with other observations aids in estimating the
2

hurricane direction and strength (ref. 3). Therefore, the interest in measuring atmospheric water vapor

has increased and leads to developing various techniques to accurately measure its density.

1.1. Water Vapor Measurement

Several techniques are used to measure atmospheric water vapor such as balloon radiosondes, aircraft in situ, and ground- or aircraft-based laser remote sensing. For global measurement of the distribution of water vapor, the most effective method is space-based laser remote sensing. A future goal is to

apply laser remote sensing on a space-based platform to continuously measure the water vapor density

of the Earth (ref. 4).

Laser remote sensing is a technique used for measuring a molecular density without any physical

contact between the sensing device and the atmospheric molecule under observation. It is basically used

for applications where direct measurement is very difficult to achieve because of large distances. The

technique usually is reliable and fast and does not disturb the measured quantity. Two main classes of

laser remote sensing systems are routinely used. One uses the Raman technique and the other uses differential absorption lidar (DIAL) (ref. 4). In the Raman technique, the laser radiation is scattered inelastically from the observed molecule with a frequency shift characteristic of the molecule. The

disadvantage of this technique is the complexity of the Raman spectrum. The DIAL technique is more

typically used because it is a relatively simpler measurement with higher accuracy (refs. 4, 5, and 6).

1.2. DIAL Technique for Measuring Water Vapor

Light detection and ranging (lidar) is an active remote sensing technique which uses a pulsed laser

and a colocated receiver to measure the density of atmospheric gases and aerosols as a function of

range. In the DIAL technique, two laser pulses at slightly different wavelengths are transmitted into the

atmosphere. The transmitted laser pulses are subjected to scattering and absorption because of the molecules and particles in the atmosphere; therefore, the light backscattered to a telescope receiver contains

some information about these molecules and particles which can be evaluated by using the lidar equation (refs. 4, 6, 7, and 8).

When looking at the lidar signals in terms of the received power and if the transmitted laser pulse

has an initial optical power Po, the backscattered received power from a range r is given by (refs. 7

and 8)

 (1)

where c is the speed of light, τ is the laser pulse duration, A is the receiver area, η is the receiver efficiency, β(r) is the atmospheric volume backscatter coefficient, σ is the absorption cross section of the

molecular species of interest, n(r) is the number density profile of the gas, and k(r) is the atmospheric

extinction coefficient resulting from all other attenuation processes.

Because the absorption profile of a gas is a unique function of the wavelength, the DIAL technique

can distinguish and monitor different atmospheric gases including water vapor. In a water vapor DIAL

system, two laser pulses are transmitted through the atmosphere. The wavelength of the first pulse is

tuned to the peak of a water vapor absorption line and is called the on-line pulse, whereas the other

pulse wavelength is adjusted to a wavelength which corresponds to no absorption and is called the offline pulse as shown conceptually in figure 1. Because the two wavelengths are so close, the scattering

P r( ) Po

cτAηβ( )r

2r

2 ------------------------ 2 [ ] σn r( ) + k r( ) dr

0

r

∫ –

      = exp
3

by atmospheric molecules and particles is essentially equal for both wavelengths, and the difference in

the attenuation with range of the return signal is entirely due to the absorption by the water vapor molecules. Thus, the ratio of the backscatter signal at the two wavelengths as a function of range can be used

to calculate the water vapor concentration profile (refs. 4, 7, and 8).

Using equation (1) to form a ratio of the on- and off-power returns allows the ability to measure

water vapor as a function of range. If the wavelength difference between the on-line and the off-line

signals is less than 0.1 nm, βon(r) = βoff(r) and kon(r) = koff (r) can be assumed, and the number density

profile of water vapor can be solved as

 (2)

where r2 − r1 is the range cell for the average concentration, σon − σoff is the differential absorption

cross section for the two wavelengths, Pon is the power received from range r for the on-line wavelength, and Poff is the power received from range r for the off-line wavelength. One can then convert the

number density profile to a mass mixing ratio by dividing the gas number density by the ambient

atmospheric number density (refs. 4, 7, and 8).

2. Background

A critical component in any remote sensing technique is the detector. Water vapor DIAL detection

systems typically use avalanche photodiodes (APD’s). Compared with a photomultiplier tube (PMT),

an APD is much more compact, lightweight, and mechanically rugged and has a lower bias voltage

which is suitable for compact size detection systems. Also, an APD has higher quantum efficiency close

to 90 percent at wavelengths of 720, 820, and 940 nm, which are water vapor DIAL absorption bands of

interest where PMT quantum efficiency is usually very low. APD’s typically use a lower bias voltage

(hundred of volts) than is required for a PMT (kilovolt range) (refs. 9 and 10).

Compared with p-i-n photodiodes, APD’s include an internal gain mechanism which increases their

signal-to-noise ratio (SNR). APD’s have excellent linearity with respect to incident light intensity. With

some structures, an APD can have very low noise in the range of a few fW/Hz1/2. For these reasons,

water vapor DIAL detection systems use APD’s to measure the backscattered light signals (refs. 10

to 13).

2.1. Lidar Atmospheric Sensing Experiment

The lidar atmospheric sensing experiment (LASE) is shown in the block diagram of figure 2. LASE

is an instrument that detects water vapor by using the 815-nm water vapor absorption line. LASE measurements were found to have an accuracy better than 6 percent or 0.01 g/kg, whichever is greater

across the entire troposphere (refs. 14 and 15).

The LASE transmitter was designed to generate two laser pulses of 30 ns duration each separated by

400 μs at a frequency of 5 Hz with a pulse energy output of 150 mJ. A Ti:sapphire (Ti:Al2O3) power

oscillator was constructed with an Nd:YAG laser as the pump source. Narrow line width and wavelength tuning of the Ti:Al2O3 laser is achieved by a continuous-wave 100-mW single mode diode laser

performing as an injection seed source. This injected seeding allows control of the spectral line width to

within 1 pm and provides wavelength tuning stability to ±0.25 pm. The diode laser wavelength is locked

n r( ) 1

2 r2 r1 ( )σ – on σoff ( ) – ----------------------------------------------------- Poff r2 ( ) Pon r1 ( )

Poff r1 ( ) Pon r2 ( ) = ln -----------------------------------------
4

onto the selected water absorption line and tuned by passing a fraction of its frequency modulated light

through a reference cell filled with water vapor and detecting the cell transmission. The tunable diode

laser seeds the pulsed laser alternately between the on-line wavelength, located at the center of the water

vapor absorption line, and the off-line wavelength, typically located 20 to 80 pm away from the on-line

wavelength (ref. 14).

The LASE receiver has a Cassegrain telescope with a 40-cm-diameter collecting area. The received

light is split into two channels. The two data channels use silicon APD’s to increase the measurement

dynamic range. The first low-gain channel uses 12 percent of the collected light for measuring strong

return signals, whereas the second high-gain APD channel uses 88 percent for low signal level detection. The APD detectors for both channels were EG&G C30955E RT’s (reach-through structures) with

a 1.5-mm-diameter active area (ref. 14).

The APD detector output signal is applied to a transimpedance amplifier stage which limits the

signal bandwidth to 2.5 MHz. A 1.5-MHz low-pass filter is used to set the signal bandwidth, then the

signals are applied to 12-bit, 10-MHz digitizers. Both the amplifier and the digitizer stages are mounted

in a CAMAC (computer automated measurements and control) crate which communicates with an

onboard computer for data recording. The whole system is synchronized by a time-based generated

trigger signal (ref. 14).

Although LASE has performed well, several improvements especially in the detection system could

be made to reduce system mass and improve performance. Eliminating the CAMAC crate would reduce

the system mass substantially and using advanced APD technology could reduce the system noise; thus,

the measurement range would be improved (ref. 16).

2.2. DIAL Detection System Development

The main goal of this research is to increase the signal-to-noise ratio of the water vapor DIAL detection system by a factor of 10 compared with the current LASE detection system. Also, the system must

be compact in size suitable for placing it directly on the receiver telescope. These goals are achievable

with state-of-the-art electronic components, a newly evaluated very low noise APD detector, and the

construction of a 14-bit, 10-MHz waveform digitizer, which will be placed as close as possible to this

detector (refs. 17 to 20). The new DIAL receiver system is shown schematically in figure 3. The analog

circuit will condition the APD output signal and also control its bias voltage and temperature; the digital

circuit will convert the analog signal into a digital form. The basis for the APD selection is discussed.

3. Avalanche Photodiodes

APD’s are solid state quantum detectors suitable for low light detection in the visible and near infrared regions. These devices are commercially available from many manufacturers and are fabricated with

different solid state structures. They have several applications including backscatter lidar, DIAL, and

fiber-optic communication. Although they are widely used, few papers in the literature have discussed

their characterization.

3.1. APD Structure and Theory of Operation

The basic structures of a p-i-n diode and three other APD’s and their electric field distribution are

shown in figure 4. The p-i-n diode shown in figure 4(a) is a p-n junction with an intrinsic or lightly

doped layer sandwiched between the p and n layers. This structure serves to extend the width of the
5

depletion region which increases the volume available for absorbing the incident photons. Also it

reduces the junction capacitance; thereby, the RC time constant is reduced, which results in higher

detection bandwidth. Thus the response time of these devices is in the range of tens of picoseconds,

which corresponds to bandwidths of gigahertz (refs. 10 to 13). A disadvantage of p-i-n detectors is that

they have no internal gain mechanism. The APD structure, similar to the p-i-n diode structure, has a

gain mechanism within the device called the impact ionization process in which the photoelectric

charge carriers, because of their high energy, can impact and cause ionization of lattice atoms leading to

an avalanche breakdown or internal gain (refs. 10 to 13 and 21).

As shown in figure 4(b) the beveled-edge APD has the simplest structure of the ones used. It consists of a p+-n junction with a high resistivity n layer which increases the breakdown voltage of the

device into the range of kilovolts. Breakdown at the edges is prevented by beveling and making the

junction very deep in the range of 50 μm. Therefore, the dead part of the p+ layer is usually etched away

to reduce the device depth. Because the n layer is much deeper than the p+ layer, electrons produced

there are more likely to be multiplied than holes. This reduces the dark-current noise which is mostly

generated by hole current. The disadvantage, however, is that only light absorbed in the p layer leads to

effective multiplication, and this layer has the lowest electric field. Therefore, charge accumulates

slowly, which leads to a longer response time typically in the range of tens of nanoseconds (ref. 21).

The reach-through structure APD with its electric field distribution is shown in figure 4(c). This

structure consists of an absorption region and, separated from it, a multiplication region. In the absorption region, the p+ layer at the active surface is followed by an intrinsic wide layer, which increases the

photon absorption depth. The emitted photoelectrons drift and reach a constant velocity. In the multiplication region, the p-n+ layers form a thin junction with a high internal field, which enhances the impact

ionization process (refs. 21, 22, and 23). Recently, an enhancement of the reach-through structure,

known as the “super low ionization ratio κ” (SLIK) geometry, has become available and is shown in

figure 4(d) (ref. 24). The ability of electrons and holes to “impact ionize” to generate additional charge

carriers is characterized by the ionization coefficients α and β for electrons and holes, respectively. The

ionization coefficient represents the ionization probabilities per unit length. An important parameter is

the ionization ratio κ given by

 (3)

In APD’s, it is desirable to reduce κ as much as possible so that the multiplication process is only due to

electrons, since hole current leads to high dark-current noise (ref. 10). This reduction was achieved in

the SLIK structure by increasing the absorption region electric field and merging this region with the

multiplication region (ref. 24). Typically, the value of κ is in the range of 0.02 for the reach-through

structure, whereas it is 0.002 for the SLIK structure (ref. 24). The structure types and the manufacturer’s

data for the characterized APD’s are given in appendix A.

3.2. APD Characteristics and Applied Circuit

An APD with no incident light has V-I characteristics similar to a rectifier diode as shown theoretically in figure 5(a). However, when the device absorbs photons due to incident light, the characteristics

will be shifted downward. Fortunately, this shift is highly linear with the intensity of the absorbed light.

κ β

α = ---
6

Furthermore, the new shifted curves are parallel with the original curves as shown in figure 5(b). The

V-I relation of the device can be given by (refs. 10 and 11)

 (4)

where I is the current through the device, is the saturation current, q is the electron charge, V is the

applied voltage, k is the Boltzmann constant, and T is the temperature. Equation (4) resembles the diode

equation with the first term representing the dark current and the additional term representing the

photocurrent given by (refs. 10 and 11)

 (5)

where η is the quantum efficiency, G is the APD internal gain, h is Planck constant, c is the light speed

in vacuum, λ is the incident light wavelength, and P is the incident optical power. By knowing the APD

sensitive area A, the optical power can be related to the light intensity I (W/m2

) by

(6)

By definition the detector responsivity (A/W) can be obtained from equation (5) and is defined by

(refs. 10 and 11)

 (7)

Because APD’s can source current through the internal photoeffect, they may operate without the

need of an external power source. However, speed of response and gain can be improved by using an

externally applied bias voltage. Thus, in most applications an external reverse voltage bias is applied to

the APD. The APD current variation, representing the change in the light intensity, is then converted to

a voltage variation by a current-to-voltage converter or transimpedance amplifier (TIA). This configuration is shown in figure 6 where is the amplifier feedback resistance, and R and C act as a low-pass

filter which eliminates any bias voltage ripple noise. A disadvantage of this technique is the additional

noise associated with the TIA as well as the limitation in the frequency response (refs. 10 and 25).

4. Responsivity Calibration

The APD spectral response was measured over a wavelength range from 600 to 1100 nm by comparison with a reference detector calibrated by the NIST (National Institute of Standards and Technology. (See appendix B.) Each detector was placed in the same uniform light field at the same position;

this allowed the APD’s to be calibrated at certain wavelengths in the specified range. Because the APD

response is dependent on its bias voltage and temperature, both dependencies were characterized relative to the reference detector and were maintained constant during the calibration. At each wavelength,

the test APD responsivity was calculated by substituting equation (6) into (7) to obtain

 (8)

I Is e

qV/kT – 1     Id = –

Is

Id

Id ηG q

hc = ----- λP

P = IA

R

R

Id

P

---- ηG q

hc = = ----- λ

R f

Rd

Rd

Id

IAd

= --------
7

where and are the test APD output current and sensitive area, respectively, and I is the intensity

of the uniform field. The light intensity can be obtained from

 (9)

where is the reference detector output current, is the sensitive area, and is the responsivity at

the same wavelength. Combining equations (8) and (9) gives

 (10)

Furthermore, the test and reference detector output currents were determined by dividing the feedback resistance into the measured voltage output of a TIA. Substituting this result into equation (10)

results in

 (11)

where and are the feedback resistances for the reference and test detectors, respectively, and

and are the TIA output voltages for the reference and test detectors, respectively.

As indicated from equation (4), the APD output signal is associated with a dc offset due to its dark

current. Another offset is also added due to the background and thermal radiation. A good practice in

such experiments is to use an optical chopper to modulate the input light signal to the detector. The dc

offsets can then be eliminated with a high-pass filter at the APD output. This technique allows one to

measure very small photocurrents in a range of nanoamperes with sensitive instruments such as lock-in

amplifiers. In this case, the chopping factor CF must be considered for correcting the APD output reading obtained from this instrument Vlock in to its peak-to-peak value Vp-p which is the true indication of

the light intensity. Thus, the chopping factor is given by

 (12)

Because of ac coupling, the operating frequency of the chopper is chosen to maximize the output

signal. The typical chopped output response and that of the ideal square wave response are shown in

figure 7(a). A good practice is to obtain the variation of the chopping factor against the chopping

frequency as shown in figure 7(b). For this particular case, the chopping factor was maximized at

frequencies above 700 Hz.

If CFr and CFd are the chopping factors for the reference and test detectors, respectively, equation (11) can be modified to

(13)

Id Ad

I

Ir

RrAr

= -------------

Ir Ar Rr

Rd

Id

Ir

---- Ar

Ad

------ Rr =

Rd

Vd

Vr

------ Rr

Rd

------ Ar

Ad

------ Rr =

Rr Rd Vr

Vd

CF

Vlock in

V p- p

= -----------------

Rd

Vd ( )lock in

Vr ( )lock in

------------------------- CFr

CFd

--------- Rr

Rd

------ Ar

Ad

------ Rr =
8

Equation (13) was used for absolute calibration of the tested APD’s in terms of the calibrated reference

detector.

4.1. Experimental Setup

The experimental setup for the APD responsivity calibration is shown in figure 8. The light source

was a halogen lamp supplied by a stabilized power supply to ensure a stable spectrum and intensity. The

lamp output was filtered by a 600-nm high-pass filter to prevent higher order dispersion of shorter

wavelengths from being collected in first-order dispersion in the range of 600 to 1100 nm. The chopper

was used to modulate the optical signal. For most of the APD’s, a 200-Hz chopping frequency was sufficient to optimize the chopping factor. The monochromator was used to separate the light input into its

spectral components. An integrating sphere was used to diffuse the exiting light to ensure intensity uniformity at the detector, which is especially important for large area detectors. A disadvantage of using

the integrating sphere is the considerable reduction in light intensity. For small area detectors, where a

higher intensity was required, the integrating sphere was replaced by a diffuser; or in some cases, the

light was applied directly to the detector. For these cases, the field intensity was measured to determine

its uniformity.

The APD output was filtered by a high-pass filter to eliminate any dc offsets, as discussed in

section 2. An oscilloscope was used to check the detected signal and to obtain its peak-to-peak value,

and a lock-in amplifier was used to measure the signal root-mean-square (rms) value.

The test detector was placed on an electrical board and put inside a cooling chamber (appendix C).

The chamber was located on a three-axis translation stage for alignment purposes. A temperature controller and a thermoelectric cooler (TEC) were used to fix the temperature of the APD under test. This

temperature was measured with a temperature sensor and a digital voltmeter. A stable high-voltage supply biased the APD, and a ±15-V power supply was used to bias the TIA.

The chopper controller adjusted the chopping frequency and supplied synchronization signals for

the other instruments. A personal computer sent commands to the monochromator to adjust the grating

position that sets the wavelength for the spectral scan, and it acquired the lock-in amplifier and the temperature readings with a GPIB data acquisition card. Appendix B gives the model numbers, manufactures, and descriptions of the instruments used in this setup.

4.2. Experimental Procedure

The reference and test detectors required accurate positioning because the output of the integrating

sphere, diffuser, and the light source have intensities that decrease by the inverse of the square of the

distance between the source and the detector. The distance between the light outlet and the detector

active area was 75 to 150 mm. A microscope with a depth of focus measured to be 200 μm was used to

position both detectors as shown in figure 9. Applying the inverted square function, the worst case deviation of the intensity at the detector was ±0.53 percent. Errors due to positioning of the detectors will

cause absolute calibration uncertainty of less than 1 percent. The microscope was placed on a kinematic

mount (1 μm placement precision) so that it could be removed from the optical path and precisely

replaced in the path for detector positioning.

The monochromator was set at 690 nm, because the halogen spectral output was a maximum at this

value, and the chopping factor and measurement system range were determined for both the reference

and test detectors. The slits of the monochromator were adjusted to have a wavelength band pass of

10 nm.
9

4.3. Data Analysis and Results

The results from the measurements were analyzed with the Mathworks MATLAB software. The

MATLAB software uses vector and array processing that simplifies the analysis of large repetitive data

sets. (See appendix D.) By referring to equation (13), we can define a normalized calibration vector

{cal} given by

 (14)

This vector was calculated for each APD and used for converting its output voltage variation, measured

by the lock-in amplifier, into a responsivity variation with respect to wavelength.

The spectral response of the test detectors is shown in figure 10. To compare the results with the

manufacturer data sheets, room temperature and manufacturer-specified bias voltage were used as indicated in the figure. The responsivity at wavelengths of interest to water vapor DIAL measurements are

also given. In some detectors with a built-in TIA, the value of the feedback resistance was unknown,

and the responsivity was given in V/W.

According to equation (7), the APD responsivity is directly proportional to the wavelength of the

incident light; this is true, as indicated in figure 10, for wavelengths starting at 600 nm up to the point

where the response begins to roll off. Ideally, the roll-off point would be sharp and correspond to the

energy bandgap of silicon which is 1000 nm. At this cutoff wavelength, the responsivity decreases

sharply because of insufficient energy in the incident photons for the generated electrons to overcome

the energy bandgap; this results in a reduction of the APD quantum efficiency. The deviation from the

ideal cutoff found in our characterized APD’s was mainly due to charge collection inefficiency of photons outside the depletion region of the APD, which was dependent on the type and level of the doping

materials used to manufacture the device (refs. 10, 21, and 23).

5. Temperature Dependent Responsivity

At fixed bias voltage and wavelength, the responsivity of an APD detector increases with decreasing temperature. Low temperature operation of an APD leads to an increased output signal due to the

increase in the responsivity and also a decreased noise level due to the reduction of the dark current.

This low temperature operation results in an increased signal-to-noise ratio (refs. 10, 11, 26, and 27).

This experiment investigates the effect of temperature on the APD spectral response. Empirical

relationships for the responsivity versus temperature were determined at the water vapor absorption

lines near wavelengths of 720, 820, and 940 nm. Parasitic heat load from the electronics and a nitrogen

gas purge allowed the APD operating temperature to be adjusted from near 0°C up to room temperature

(appendix C). The nitrogen gas flow was used to avoid condensation and icing inside the cooling chamber. Remember that a lower operating temperature causes the detector breakdown voltage to also

decrease. Therefore, this bias must be chosen carefully while performing this test to avoid destroying

the APD.

5.1. Experimental Procedure

The responsivity calibration setup, shown in figure 8, was used in this experiment but only for the

test detectors. During the experiment, the detector bias voltage was kept constant to ensure that the

{ } cal

Rr { } ( ) λ

Vr ( )lock in { } ( ) λ --------------------------------------- Rr

Rd

------ Ar

Ad

------ CFr =
10

spectral response variation is only due to changes in temperature. To avoid detector saturation, the bias

voltage was maintained at 5 percent lower than the manufacturer-specified value. Using the alignment

setup shown in figure 9, the detector position with respect to the light source was adjusted to the same

value used for the responsivity calibration experiment to ensure the same input light intensity.

The temperature controller shown in figure 8 fixed the operating point for each spectral scan. For

each temperature set point, the system was allowed to reach equilibrium before proceeding with the

experiment. This whole procedure was repeated for each APD under investigation.

5.2. Data Analysis and Results

For each APD, the following characterization results are presented:

1. The APD detector output voltage variation with wavelength {Vn(λ)}

2. The APD temperature variation with wavelength {Tn(λ)}

Ideally, the temperature should be constant with respect to wavelength. This relationship was not true in

our investigation because of some deficiencies in the temperature controller used in the experiment.

Therefore, the temperature had to be recorded for each wavelength increment {Tn(λ)}. To obtain the

APD temperature Tn, this data set was averaged according to

(15)

At the same temperature Tn and using equation (14), the detector output voltage variation was converted to a responsivity variation with respect to wavelength according to the relation

(16)

This procedure was repeated for every temperature setting giving the spectral response variation with

temperature shown in figure 11 for all APD’s.

Again we used MATLAB software to analyze the results because of its ability to efficiently handle

vector and matrix operations. A responsivity vector and a temperature vector {T} were constructed, at a certain wavelength λx, as shown in the following equations:

(17)

(18)

When applying a polynomial curve fit, the responsivity variation with temperature at λx took the form

 (19)

Tn Tn = { } ( ) λ

Rn { } ( ) λ

Vn { } ( ) λ

CFn

= --------------------- { } cal

{ } R( ) T

{ }R R( ) T 1 λx ( )R2 λx ( )...Rn λx ( )...RN λx = { } ( )

{ } T T1 T2 ... Tn ... TN = { }

R( ) T λx

amTm

m=0

M

= ∑
11

where M is the curve fit order and N in equation (18) is the index for maximum temperature. The

responsivity versus temperature for all APD’s at the water vapor DIAL wavelengths of interest is shown

in figure 11. Table 1 gives the curve fitting results and conditions for each APD.

The responsivity with respect to the temperature relationship allowed us to determine the optimum

APD operating temperature for the future DIAL detection system. It also determined the control temperature stability requirements. As an example, let us assume APD12 is the chosen detector, and it will

operate at 820 nm with 336 V and 0°C. The responsivity derivative (table 1) with respect to temperature

is given by

 (20)

Thus, a temperature deviation of ∆T = 0.1°C, near T = 0°C, results in a responsivity deviation of

∆R = −0.823 A/W with a relative error ∆R/R = −0.72 percent. Thus, the temperature controller and

heat sinking requirements were established for the design of the final detection system.

6. Bias Voltage Dependent Responsivity

At fixed temperature and wavelength, the responsivity of an APD increases with increasing bias

voltage up to the breakdown voltage. This increase in responsivity is due to the increase of the internal

electric field leading to the transfer of more energy to the photocharge carriers, which enhances the

impact ionization process. Bias voltages above breakdown cause rapid charge carrier generation which

usually leads to device damage if applied for long periods of time (refs. 21, 23, and 24). This experiment

investigates the effect of the APD bias voltage variation on the spectral response of interest and obtains

responsivity versus bias voltage relations at the water vapor DIAL wavelengths.

6.1. Experimental Procedure

Similar to the previous experiment, the responsivity calibration setup was used only for the test

APD’s. During the experiment, the APD temperature was kept constant at room temperature to ensure

that the spectral response variation is only due to the bias voltage. The detector position with respect to

the light source was adjusted to the same value as for the responsivity calibration experiment to ensure

the same light intensity input.

With the high-voltage supply shown in figure 8, the detector bias is set to a certain value, and the

temperature was stabilized by the controller to room temperature. The output peak-to-peak voltage was

then measured with the oscilloscope, and its rms value was measured with the lock-in amplifier in order

to calculate the chopping factor. Next, the wavelength was scanned from 600 to 1100 nm with a step

increment of 10 nm. For each step increment, the wavelength, output voltage, and temperature were

recorded with the computer. Finally, the experiment was repeated for a set of different bias voltages.

This procedure was repeated for each APD under investigation.

6.2. Data Analysis and Results

At the end of this experiment, each APD had a group of data files describing its output voltage variation with wavelength at a given bias voltage. The data analysis was performed once again by using

MATLAB software. The value of the bias voltage was measured directly from the high-voltage

supply. For a certain APD at a given bias voltage, the available data were as follows:

∂R( ) T

∂T ---------------- λx=820

– 0.82 8.23 T 23.7 10–3T2 = + – ×

VBn
12

The APD detector output voltage variation with wavelength {Vn(λ)}

The APD temperature variation with wavelength {Tn(λ)}

Similarly, the temperature data were averaged by using equation (15), and the detector output voltage

variation was converted to a responsivity variation with respect to wavelength according to equation (16). This procedure was repeated for every bias voltage setting which produced the spectral

response variation with bias voltage.

Now, to obtain the responsivity variation with bias voltage at a certain wavelength λx, the spectral

response variation with bias voltage is used to form two vectors: a responsivity vector {R(VB)} and a

voltage bias vector {VB}, which are

(21)

(22)

By using a polynomial curve fit, the responsivity variation with bias voltage is given by

 (23)

where M is the curve fit order and N is the index for maximum voltage bias. This analysis was applied to

each APD at wavelengths of 720, 820, and 940 nm.

The experimental results are shown in figure 12. For each detector, the spectral response variation

with bias voltage is shown on the set of curves to the left, and the responsivity variation with bias

voltage is shown to the right. Table 2 shows the curve fit coefficients and the test conditions for each

detector.

The responsivity–bias voltage relation can be used for determining the APD operating bias in the

final DIAL detection system, and it gives an error estimate which helps in evaluating the system accuracy. As an example, assume APD12 is the chosen detector, and it operates at 820 nm with a bias voltage of 336 V and a temperature of 25°C. When referring to figure 12(b), we find that any small variation

in the bias voltage around 336 V will cause a small variation in its responsivity relative to higher voltage

bias, but the value of the responsivity is relatively stable. On the other hand, table 2 shows that the

responsivity derivative with respect to bias voltage is given by

 (24)

Thus, a bias voltage deviation of ∆V = 1 V near the bias of 336 V bias results in a responsivity deviation

of ∆R = 1.8034 A/W which will lead to a relative error of ∆R/R = 4 percent.

R VB { }R ( ) 1 λx ( )R2 λx ( )...Rn λx ( )...RN λx = { } ( )

VB { } V1 V2 ... Vn ... VN = { }

R( ) V λx

amVm

m=0

M

= ∑

∂R( ) V

∂V ----------------- λx=820

2.27 105 – 2 10 ×

3

V 6.12V2× – 6 10–3V3 = + + ×
13

7. Responsivity Uniformity Scan

The APD sensitive area can be considered a group of point detectors distributed along the surface.

Ideally, this distribution is uniform with each of these detectors having the same responsivity for similar

operating conditions, and therefore, the responsivity distribution is constant along the APD surface.

Practically, this condition is not true because of defects developed in the APD manufacturing processes.

In this experiment, we investigated the uniformity of APD responsivity across its surface and determined its active area. This measurement required a relatively small-spot-size light source and the ability

to scan it across the detector area. Measuring the APD output voltage as a function of light spot position

results in a responsivity map of the APD area.

7.1. Experimental Setup and Procedure

The small-spot-size light source was achieved with the setup shown in figure 13. A 633-nm He:Ne

laser output was focused by using a microscope objective. The position of the detector was adjusted

with a computer-controlled two-dimensional translation stage. The motion of the detector was adjusted

so that the focused laser beam spot and the APD sensitive area remained in the same plane. A neutral

density filter was used to avoid APD saturation.

The laser focusing optical system was calibrated to determine the displacement between the laser

focus at its minimum waist and the visual focus of the microscope system shown in figure 14. In order

to obtain this calibration, a pinhole was mounted on a three-dimensional translation stage, as shown in

figure 15. The laser focus was determined by positioning the pinhole such that the maximum laser output was observed on the detector. The laser focus was measured with the micrometer on the translation

stage. The visual focus was determined by viewing the best focus of the pinhole through the eyepiece.

The test detectors were positioned by finding the visual focus of the detector surface and then translating to the laser focus by the calibrated displacement described previously. During the scan sequence the

detector moves in a two-dimensional raster scanning sequence with a fixed step size. The data are then

plotted and analyzed with MATLAB.

7.2. Data Analysis and Results

The focused laser spot intensity profile was characterized by scanning a 10-μm-diameter hole in the

plane of the laser focus. The results of this scan are displayed in figure 16, and the full width half maximum (FWHM) of the laser spot was 8 μm. Table 3 lists the results of each APD active area, percentage

nonuniformity, scanned array step format, and the step size. The active area was defined by the FWHM

points in the uniformity scans. The nonuniformity is defined by

(25)

where Std and Mean are the standard deviation and the mean of the surface scan data, respectively.

The normalized surface scan results for the tested APD’s (three-dimensional plots shown to the left)

and the surface images (shown to the right) are shown in figure 17. In the surface images, darker areas

represent lower responsivity, whereas brighter areas represent higher responsivity.

Nonuniformity Std

Mean = -------------100
14

For example, figures 17(c) and (d) demonstrate the response variation over the area of APD10. In

some DIAL systems, the light intensity is focused on a small region of the detector. Therefore, these

surface scan data can be used to calculate the responsivity correction factor because the calibration of

the APD, as discussed in section 2, considers the average responsivity over the entire detector active

area.

On the other hand, the surface scan data can be used to determine the APD hot spots, which are

points on the APD surface with very high responsivity relative to the average as shown, for example, in

figure 17(i) for APD7. The hot spots may cause a problem since at high light intensities, the concentration of charge carrier will increase at this position. This concentration leads to an increased power dissipation at this spot, which will possibly result in local damage of the APD area.

8. Noise Measurement

Noise signals can be divided into two kinds: systematic noise and natural device noise. Systematic

noise is mainly due to conduction and interference from the wiring associated with the detector and the

surrounding instruments (such as power supplies and pulse generators), and it is independent of the

detection process. It can be successfully reduced by proper shielding of the connecting wires, grounding, differential measurement technique, or measuring it in dark condition and subtracting it from the

detector output with light applied. The natural device noise is fundamental in nature and is mainly due

to the operation of the optical detector itself. It is due to random processes and, thus, can be reduced by

averaging. The dominant types of natural device noise are the signal-induced shot noise, the dark

current shot noise, and the Johnson noise. In this section, we will discuss these dominant natural noise

sources associated with APD operation and its TIA circuit and how it can be measured (refs. 10, 11,

and 21).

8.1. Types of APD Inherent Noise

The first type of APD inherent noise is signal-induced shot noise. It is generated by the randomness

in photon arrival times which leads to fluctuations in the detector output signal. Because of the internal

gain in the APD, an additional noise is added to the optical shot noise; this is known as the multiplication gain noise, which is mainly due to the randomness in the impact ionization process. The total shot

noise current in this case is given by (refs. 10, 11, 12, 28, and 29)

(26)

where B is the effective bandwidth of the detector and G is the device internal gain.

The second type of APD noise is the dark current shot noise. The APD dark current is given by the

first term in equation (4) which contributes a dc offset to the detected current signal. This dc offset can

be reduced by proper filtering or by adding another offset with opposite polarity. The problem with dark

current lies in the shot noise it creates which is independent of the operating frequency. The dark current

noise can be obtained from (refs. 10, 11, and 12)

(27)

i

n ( )shot

2 2ηq

2

BG2 P

hc = ----- λ

i

n ( )dark

2 2qi = darkGB
15

The noise sources described above are independent; therefore the noise powers can be added

together. The resulting equivalent noise current In of the APD is given by combining the noise currents

of each noise source obtained from equations (26) and (27) as

(28)

and the signal-to-noise ratio is given by

(29)

The noise and signal as a function of the input optical power are shown in figure 18. Only the optical

shot noise depends on the input power. The power independent noise is plotted as a straight horizontal

line and represents the dark current. The photon shot noise increases as the square root of the optical

power. This shot noise power term adds directly to the other noise power of the system. The signal current increases linearly with input power as shown in figure 18. The power at which the signal current

equals the total noise current is called the noise equivalent power (NEP), which is the minimum power

required to achieve a unity signal-to-noise ratio used to define the detector minimum detectable signal.

The NEP (in W/Hz1/2) can be evaluated from

(30)

where In is the noise current spectral density in A/Hz1/2 and R in A/W. Another figure of merit useful in

comparing APD’s in terms of noise is called the detectivity D* (in cm-Hz1/2/W). This figure of merit is

independent of the detector area A and is given by

(31)

An additional noise source introduced to the APD output photocurrent is due to the current-tovoltage conversion process of the TIA. The operational amplifier noise at its output may be considered a

combination of the effects of several independent noise sources at its input as shown in figure 19.

The first noise source is the amplifier equivalent noise voltage generator with spectral density

ev (V/Hz1/2). The second noise source is the noise voltage generated by the amplifier input current noise

flowing through the feedback resistor ei

 (V/Hz1/2). Usually values for these two noise sources are given

in the amplifier data sheet. The last noise source is the Johnson noise er (in V/Hz1/2) due to the feedback

resistance Rf and is given by

(32)

where kB is Boltzmann constant and T is the temperature in Kelvins. The total noise at the amplifier output is then given by the sum of the individual noise powers plus the APD noise itself as shown in the

following equation (ref. 25):

(33)

In i

n ( )shot

2 i

n ( )dark

2 = +

S

N

---- Id

2

In

2 = ----

NEP

In

R = ----

D* A

NEP = ----------

er

4kBT

R f

= -------------

En ev

2

ei

2

er

2 InR f ( )2 = +++
16

8.2. APD Noise Measurement and Results

The APD noise measurements were performed by using a spectrum analyzer with a 1-Hz normalized spectrum at a spectral resolution bandwidth of 10 kHz. Appropriate care was taken to ensure that

the detector dark current and preamplifier noise were the dominant noise sources. The measured power

spectral noise n (in dBm) was converted to the APD noise current spectral density In by the equation

(34)

where RL is the APD load resistance and GA is a preamplifier gain. This quantity was observed to be

constant over the APD bias voltage and temperature operating range. Table 4 gives the noise current

spectral density, NEP, and D* for the tested detectors at their responsivity calibration bias voltage and

temperature.

The results of this experiment are very important since they directly indicated that APD12 and

APD11 were the best detectors for the water vapor DIAL detection system; APD2 represents the detector that is currently used in the LASE detection system.

The APD excess noise factor F as a function of its gain G and ionization ratio κ is given by

(refs. 29, 30, and 31)

(35)

For SLIK APD’s, APD12 and APD11, the gain is in the range of 500, whereas for a reach-through

structure, such as APD2, the gain is 150. Substituting the gain and the ionization factor values given in

section 1.2, we find that the SLIK structure has a 40-percent reduction in the excess noise factor over

the reach-through structure. A quick comparison (table 4) agrees with this statement and indicates the

lower noise content of APD12 and APD11, which helps in achieving our goal of increasing the signalto-noise ratio of the DIAL detection system by a factor of 10 over the current detection system (APD2).

9. Frequency Response

The frequency response of an APD is determined by the device time response. The time response of

an APD is the time interval between the event of photon absorption and the event of photocurrent generation at its output. This time is dependent on the transient time spread, the diffusion time, the RC time

constant, and the avalanche buildup time.

Transient time spread is the time interval between photocharge carrier generation and its detection.

The charge delivered to the external circuit, contributing to the photocurrent, by carrier motion in the

APD material is not provided instantaneously but consumes a time interval because of the drift of the

carrier. This time interval is known as the transient time spread. Since holes are much slower than

electrons, the transient time spread is dominated by the hole mobility.

Carriers generated outside the depletion region, but sufficiently close to it, take time to diffuse into

it. These carriers will contribute to the photocurrent. But since the diffusion is a slow process relative to

the drift, it increases the device response time.

In

100.1nRL 10–3×

GAR f

= ---------------------------------------

F κG ( ) 1 – κ 2 1

G– ---     = +
17

The RC time constant in an APD is formed by the equivalent resistance and equivalent capacitance

of the device and its circuitry. These elements serve to integrate the output photocurrent of the device

leading to increased response time.

The generation of additional charge carriers by the impact ionization process in the multiplication

region consumes time, specially if the resulting carriers themselves are causing the generation of more

carriers by the same process. This time interval is known as the avalanche buildup time. Since the

multiplication is a random process, the avalanche buildup time interval is random and difficult to

characterize.

This experiment investigated the responsivity variation of APD12 and APD11 with respect to the

frequency of the input signal and the cutoff frequency of the devices to check the manufacturer’s stated

bandwidth. APD’s have a very wide bandwidth on the order of gigahertz, but because of the existence

of the built-in TIA in APD12 and APD11, their frequency response is limited by the bandwidth of the

internal amplifier which is on the order of megahertz.

9.1. Experiment Setup

The experimental setup for the APD frequency response is shown in figure 20. It consists of a

720-nm laser diode controlled by a pulse generator. The power supply is used to bias the laser diode

driver circuit consisting of a buffer amplifier and a voltage-to-current converter. The output laser beam

was split to apply part of the optical signal to the reference detector, whereas the other part was applied

to the test APD. The reference detector was used to measure the input signal applied to the APD under

test. Next, the output of both detectors was measured by a digital oscilloscope and a spectrum analyzer.

The function of the oscilloscope was to check the magnitude of the output signal while the spectrum

analyzer measured the frequency spectrum. When necessary, a neutral density filter was used to reduce

the light intensity to avoid saturating the test APD. The electronic high-pass filter was used to eliminate

dc offsets in the detector output.

9.2. Experimental Procedure and Results

With the setup discussed in section 9.1, the APD bias voltage was set to its manufacturer-specified

value for room temperature operation. Then, an optical signal was applied to the detectors, after choosing a suitable neutral density filter and checking their output with the oscilloscope. Next, setting the frequency of the input optical signal to a certain value fi

 and using the pulse generator, the amplitude of the

fundamental frequency components of the APD input and the output signals Ri and Ci

, respectively, are

recorded in dBm by using the spectrum analyzer. This corresponds to one data point in the frequency

domain. By changing the frequency setting of the pulse generator and repeating the same procedures,

we obtained the complete frequency scan. Starting from 100 kHz to 1 MHz, we incremented the frequency in 100-kHz steps or from 1 MHz to 20 MHz, a 1-MHz step was used. The spectrum analyzer

was set to a frequency range starting at 50 kHz and ending at 20 MHz with each data point averaged

30 times.

Finally, after collecting the data in the frequency range of interest, the analysis was done by using

MATLAB. First, the data were stored in three vectors {f}, {C}, and {R} equivalent to the frequency

increments and the corresponding outputs and inputs, respectively. Then, the gain vector {G}, in dBm,

was calculated for each frequency setting from

G (36) i Ci Ri = –
18

The gain-frequency relation was obtained by applying a polynomial curve fit to the gain vector with

respect to the frequency vector. By plotting this relation, we can obtain the cutoff frequency from the

intersection between the curve and the −3-dB line drawn from the low-frequency gain.

The previous experiment was used to obtain the frequency response for APD12 and APD11 as mentioned. The actual data and the curve fit of the frequency response of APD12 as well as the frequency

response of its output and input are shown in figure 21. A similar set of curves is shown in figure 22 for

APD11. The measured cutoff frequency of APD12 was 12.75 MHz and for APD11 was 7.8 MHz.

10. Concluding Remarks

In this report, we discussed the characterization of a group of 10 avalanche photodiodes (APD)

obtained from three different manufacturers. Each APD was evaluated by calibrating its spectral

response, temperature dependent responsivity, voltage dependent responsivity, responsivity uniformity,

noise measurements, and frequency response. The results were compared so that a suitable APD detector can be found for advanced water vapor DIAL detection systems. The characterization was focused

on two main APD structures, the reach-through structure, which is currently used in the LASE detection

system (APD2), and the newer SLIK structure. The SLIK structure APD has better performance

because of higher gain and lower noise which leads to an increase in the signal-to-noise ratio of the

detection system by at least a factor of 10 over the current system. The SLIK APD package has

1. Built-in, low-noise, transimpedance amplifier

2. Built-in thermoelectric cooler and thermistor for APD temperature control

3. The transimpedance amplifier feedback resistance is cooled along with the detector reducing the

Johnson noise

This APD has a small area (0.24 mm2) compared with the LASE APD (1.7 mm2), which might produce alignment difficulties. The summary of results for APD11 and APD12 is given in the following

table:

Symbols used in the table are defined as follows:

A area

BW bandwidth

D* detectivity

APD

Manufacturer,

model,

serial

λ,

nm

d,

mm

A,

mm2

Area

nonuniformity,

percent

VB,

V

T,

°C

NEP,

fW/Hz1/2

R,

A/W

D*,

cm-Hz1/2/W

BW

(3 dB),

MHz

EG&G, 720 1.8 127.3 2.77 × 1013

11 C30649E, 820 0.556 0.242 14.5 415 0 1.7 133.8 2.90 × 1013 7.8

148 940 3.1 72.3 1.57 × 1013

EG&G, 720 1.9 113.3 2.87 × 1013

12 C30649E, 820 0.5 0.24 336 0 1.9 111.2 3.05 × 1013 12.8

147 940 3.9 55.9 1.63 × 1013
19

d diameter of sensitive area

NEP noise equivalent power

R detector responsivity

T temperature

VB bias voltage

λ wavelength

These characterization experiments provided the basis for the design of an advanced 14-bit atmospheric water vapor detection system, with the APD and its operating parameters indicated in the above

table. These detectors represent the best available APD’s for the near infrared detection of atmospheric

water vapor even with their smaller sensitive area.
20

Appendix A

APD Manufacturer’s Data

The manufacturer’s data for the APD’s investigated are given in the following table.
21

Photodiode Manufacturer Model, serial Type (a) d,b mm T,c °C Rd λ,e nm VB,f V VBD,g V In/Vn (h) Id

nA 

APD12 EG&G C30649E,

147

SLIK 0.5 25 50 MV/W 820 349 356 150 nV/Hz1/2 7 

APD11 EG&G C30649E,

148

SLIK 0.5 25 50 MV/W 820 431 440 150 nV/Hz1/2 8 

APD10 Electron Tubes,

Inc.

SSO-AD2500,

#8-12

RTS 2.52 21.5 45 A/W 880 110 161.7 30 

APD9 Electron Tubes,

Inc.

SSO-ADH500,

#10

RTS 0.5 21.5 45 A/W 880 150 1.

APD8 EG&G C30646E,

CD2507

RTS 0.5 22 20 MV/W 850 217 228 78 nV/Hz1/2 

APD7 EG&G C30646E,

CD2508

RTS 1.5 22 6 MV/W 1064 384 443 120 nV/Hz1/2 

APD6 EG&G C30659E,

CD2472

RTS 0.15 50 MV/W 830 420.4 427 110 nV/Hz1/2 36 

APD5 Advanced

Photonix, Inc.

197-70-71-

520

BE 5 22 108A/W 840 2450 2 pA/Hz1/2 16

APD4 EG&G C30950E RTS 0.8 25 0.52 MV/W 830 275 425 15 nV/Hz1/2 

APD2 EG&G C30955E,

1647

RTS 1.5 70 A/W 900 275 390 2 pA/Hz1/2 20

aSLIK, super low κ structure; RTS, reach-through structure; BE, beveled-edge structure.bDiameter of sensitive area.cTesting temperature.dAPD responsivity.eTesting wavelength.fBias voltage.gBreakdown voltage.hSpectral noise current (nA/Hz1/2) and voltage density (nV/Hz1/2).iDark current.jBandwidth.kNoise equivalent power.
22

Appendix B

Characterization Instruments

The reference detector was an ORIEL K7034 detector. It was a p-i-n diode, with a sensitive area of

100 mm2

. The TIA feedback resistor associated with the detector was set to 106 Ω. Figure B1 gives the

detector spectral response which was used to calibrate all the test APD’s.

The test instruments were as follows:

Light source: ORIEL 66181

Optical chopper: ORIEL 75152

Integrating sphere: ORIEL 70451

Temperature controller: Amherst Scientific 7600

High voltage supply: Stanford Research System PS350

Power supply: Hewlet Packard 6234A

Lock-in amplifier: Scitec Instruments

Digital oscilloscope: Hewlett Packard 54601A

Personal computer: Northgate Computer System Inc.

Pulse generator: Hewlett Packard 8116A

Spectrum analyzer: Hewlett Packard 6112A

Laser diode: SHARP LT020MC0

Automatic translation stages: Klinger Scientific MDC3

Monochromator: ORIEL 77200

Figure B1. Reference detector spectral response.

200 400 600 800 1000 1200

.05

.10

.15

.20

.25

.30

.35

.40

.45

Wavelength, nm

Responsivity, A/W
23

Appendix C

APD Test Chamber

A cross section of the APD chamber is shown in figure C1. The chamber is designed to control the

environment for detector operation. A thermoelectric cooler is used to cool the detector heat sink. Water

circulation is applied to the hot side of the cooler in order to dissipate the heat. The N2 flow prevents

water vapor condensation on the detector window at low temperatures and also avoids dust accumulation inside the chamber.

The detector heat sink is connected mechanically to the cold side of the thermoelectric cooler. The

APD’s can have different areas; therefore, a mounting ring is used to provide the maximum heat transfer

from the detector can to the heat sink, as shown in figure C2. The heat sink design holds the detector as

well as a small electronics card (shown in fig. 6). A temperature sensor is located on the heat sink as

close as possible to the APD. An experiment was carried out to check the accuracy of the sensor reading

with respect to the actual temperature of the detector can, which is assumed to equal the detector temperature itself, by placing another temperature sensor in the detector position as shown in figure C3. The

temperature was varied by using the temperature controller for the experimental temperature range. The

correction relation between the temperature sensor reading and the APD package temperature is given

by

The temperature sensor, Texas Instruments model LM35, was a semiconductor sensor. Figure C4 shows

the temperature sensor circuit. The output voltage reading in mV was calibrated to give the temperature

reading directly.

Another test was performed to examine the transmittance of the chamber window; this was

achieved by using APD8 during a responsivity calibration experimental run. The detector output voltage

was measured twice with and without the chamber window at the same bias voltage and temperature.

The window transmittance is given by

The measurement results shown in figure C5 indicate a 0.9844 average widow transmittance across

the wavelength range of interest.

Temperature at APD location 0.8622 Temperature sensor reading = × ( ) + 3.2771

Transmittance Output with chamber window

Output without chamber window = -------------------------------------------------------------------------------
24

Figure C1. Schematic of APD chamber.

Figure C2. Schematic diagram of APD heat sink.

Water

circulation

N2 flow

Thermoelectric

cooler

Window

APD

Heat

sink

Detector chamber

Electrical

connections

Mounting

ring APD APD card

Component

side

Heat sink

TEC fixer

Temperature

sensor

Side view Elevation
25

Figure C3. Relation between temperature sensor reading and temperature at APD location.

Figure C4. Temperature sensor circuit.

Figure C5. Chamber window transmittance as function of wavelength. Mean window transmittance = 0.9844.

Temperature

sensor

Sensor at

APD location

APD heat sink

–10 –5 0 5 10 15 20 25

–5

0

5

10

15

20

25

Temperature sensor reading, °C

Temperature at APD, °C

Temperature

sensor

|–Vs

|

R =

50 μA

–Vs

Vs

Output

600 700 800 900 1000 1100

1.10

1.05

1.00

.95

.90

.85

Wavelength, nm

Window transmittance
26

Appendix D

MATLAB Data Analysis Programs

D1. Read Data File Program

The raw data obtained from the APD characterization experiments were in the ASCII format. The

Read Data File Program was used to read the raw data files (filename) and convert it to a vector format

(vector) and is given as follows:

Function[vector] = read(filename)

fid = fopen(filename,'rt');

if fid < 0

ret = fid;

return;

end

vector = zeros([1,41]);

for I = 1:41;

[a,c] = fscanf(fid,'%f',4);

vector(I) = a(1);

if (c ~= 4);

ret = -1111;

return;

end

status = fseek(fid,0,0);

if (status < 0)

if (I == 41)

fclose(fid);

ret = 0;

return

end

ret = status;

return;

end

end

ret = 0;

fclose(fid);

D2. Calibrate Program

The Calibrate Program was used to obtain the calibration results for a certain APD. It reads the

wavelength (wavelength file), test APD output voltage (voltage file), reference detector output voltage

(reference file), and temperature (temperature file) raw data files. The program generates the APD spectral response waveform, the responsivity at the water vapor DIAL wavelengths, and the {cal} vector.

[w] = read(wavelength file);

[vd] = read(voltage file);

[vr] = read(reference file);

[T] = read(temperature file);
27

sd = APD output voltage sensitivity

Rd = APD feedback resistance

Ad = APD area

CFd = APD chopping factor

sr = reference detector output voltage sensitivity

Rr = reference detector feedback resistance

Ar = reference detector area

CFr = reference detector chopping factor

load Si_resp.ref; %Reference Detector Responsivity

for I = 60:110;

Res_r(i-59) = Si_resp(10*I-199,2);

end

m = Rr*Ar*CFr / (Rd*Ad*CFd);

for I = 1:51;

cal(i) = m*Res_r(i)/(vr(i)*sr);

end

vd = sd * vd;

Res_d = cal .* vd;

plot(w,Res_d)

xlabel('Wavelength in nm')

ylabel('Responsivity in A/W')

title('APDxx Spectral Response)

gtext(w(12) Res_d(12))

gtext(w(22) Res_d(22))

gtext(w(34) Res_d(34))

Temperature = mean(T)

D3. Temperature Response Program

The Temperature Response Program was used to obtain the spectral response variation with temperature and the responsivity versus temperature variation at the water vapor DIAL wavelengths. It reads

the wavelength (wavelength file), test APD output voltage (voltage file), and temperature (temperature

file) raw data files.

[w] = read(wavelength file);

[v1] = read(voltage file 1);

[v2] = read(voltage file 2);

[vn] = read(voltage file n);

[T1] = read(temperature file 1);

[T2] = read(temperature file 2);

[Tn] = read(temperature file n);

R1 = sensitivity1 * v1 .* cal;

R2 = sensitivity2 * v2 .* cal;

Rn = sensitivityn * vn .* cal;

figure(1)

plot(w,R1,w,R2,...,w,Rn)

.

.

.

.

.

.

.

.

.
28

xlabel('Wavelength in nm')

ylabel('Responsivity in A/W')

temp = [mean(T1) mean(T2) ... mean(Tn)];

Res720 = [R1(13) R2(13) ... Rn(13) ];

Res820 = [R1(23) R2(23) ... Rn(23) ];

Res940 = [R1(35) R2(35) ... Rn(35) ];

[ttemp,rres720,p720,eror720]=fitt(temp,res720,4);

figure(2)

plot(temp,res720,'o',ttemp,rres720)

title(' Temperature Fit at 720nm')

[ttemp,rres820,p820,eror820]=fitt(temp,res820,4);

figure(3)

plot(temp,res820,'o',ttemp,rres820)

title(' Temperature Fit at 820nm')

[ttemp,rres940,p940,eror940]=fitt(temp,res940,4);

figure(4)

plot(temp,res940,'o',ttemp,rres940)

title('Temperature Fit at 940nm')

D4. Voltage Bias Response Program

The Voltage Bias Response Program was used to obtain the spectral response variation with voltage

bias and the responsivity versus bias variation at the water vapor DIAL wavelengths. It reads the wavelength (wavelength file), test APD output voltages (voltage file), and temperature (temperature file) raw

data files.

[w] = read(wavelength file);

[v1] = read(voltage file 1);

[v2] = read(voltage file 2);

[vn] = read(voltage file n);

[T] = read(temperature file);

R1 = sensitivity1 * v1 .* cal;

R2 = sensitivity2 * v2 .* cal;

Rn = sensitivityn * vn .* cal;

figure(1)

plot(w,R1,w,R2,...,w,Rn)

xlabel('Wavelength in nm')

ylabel('Responsivity in A/W')

bias = [bias1 bias2 ... biasn];

Res720 = [R1(13) R2(13) ... Rn(13) ];

Res820 = [R1(23) R2(23) ... Rn(23) ];

Res940 = [R1(35) R2(35) ... Rn(35) ];

[vBias,rres720,p720,eror720] = fitt(bias,res720,4);

figure(2)

plot(bias,res720,'o', vBias,rres720)

title('Voltage Bias Fit at 720nm')

[vBias,rres820,p820,eror820] = fitt(bias,res820,4);

.

.

.

.

.

.
29

figure(3)

plot(bias,res820,'o', vBias,rres820)

title(' Voltage Bias Fit at 820nm')

[vBias,rres940,p940,eror940]=fitt(bias,res940,4);

figure(4)

plot(bias,res940,'o', vBias,rres940)

title(' Voltage Bias Fit at 940nm')

D5. Surface Scan Program

The Surface Scan Program was used to analyze the surface scan data. The inputs to this program

include the APD output voltage data (file), the background dark current voltage output (Vbkgd), the scan

step increment array in x-direction (x), the scan step increment array in y-direction (y), and the translation stage step size for each increment (step_size). The program outputs include the position arrays

(xaxis) and (yaxis), the value of the active area (act_area), and the responsivity variation along the APD

surface.

Function [xaxis, yaxis, act_area]= uniform( file, Vbkgd, x, y, step_size);

[dat1] = read(file);

if ( ret < 0 )

ret = -1;

sprintf('Input file not read')

return

end

[I] = mk1dto2d(dat1, x,y);

i1 = min(dat1);

i2 = max(dat1);

d_int = (i2-i1)/256;

if ( I(x / 2,y / 2) > Vbkgd)

norm=(I - i1)/(i2 - i1);

end

if ( I(x / 2,y / 2) < Vbkgd)

norm=(i2 - I)/(i2 - i1);

end

figure(1);

colormap(gray(255));

act_area = 255 * norm;

sel = (act_area > 128);

cnt = 0;

for l = 1:x;

for m=1:y;

if(sel(m,l) == 1)

cnt = cnt + 1;

lin_area(cnt) = act_area(m,l);

end;

end;

end;

if ( cnt == 0)

ret = -100;

return;

end;

area = cnt*step_size^2/1e6;

area_ave = mean(lin_area);
30

area_std = std(lin_area);

per_area = area_std*100/area_ave;

xaxis = step_size*(1:x);

yaxis = step_size*(1:y);

v = act_area*d_int+i1;

maxv = max(max(v));

minv = min(min(v));

for I = 1:x;

for j = 1:y;

v(i,j) = v(i,j)-minv;

v(i,j) = (100/(maxv-minv))*v(i,j);

end

end

mesh(xaxis,yaxis,v);

title([file, ': Active Area Response Uniformity ',num2str(per_area),' STD

Percent']);

zlabel('Relative Responsivity');

xlabel('Y-Position in microns');

ylabel('X-Position in microns');

colormap('default');

CM = colormap;

TiffFile = 'name_num.ext';

TiffFile = strrep( TiffFile, 'name', file);

TiffFile = strrep( TiffFile, 'num', '1');

TiffFile = strrep( TiffFile, 'ext', 'tif');

figure(2);

colormap(gray(255));

u = act_area;

maxu = max(max(u));

minu = min(min(u));

image(xaxis,yaxis,u);

title([file, ': Active Area ', num2str(area),' mm^2']);

xlabel('Y-Position in microns');

ylabel('X-Position in microns');

CM = colormap;

axis('square');

TiffFile = 'name_num.ext';

TiffFile = strrep( TiffFile, 'name', file);

TiffFile = strrep( TiffFile, 'num', '2');

TiffFile = strrep( TiffFile, 'ext', 'tif');

D6. Frequency Response Program

The Frequency Response Program was used to analyze the frequency response data. For an APD,

the raw data are located in three files: the frequency increment (frequency file), the reference detector

output voltage (reference file), and the test APD output voltage (test file) files. The output consists of

the frequency Bode plots for the APD input, output, and gain.

[f] = read(frequency file);

[r] = read(reference file);

[t] = read(test file);

[p1,s] = polyfit(f,r,2);

rr = polyval(p1,f);
31

figure(1)

semilogx(f,r,'o',f,rr)

grid

xlabel('Frequency in Hz')

ylabel('Input in dB')

[p2,s] = polyfit(f,t,2);

tt = polyval(p2,f);

figure(2)

semilogx(f,t,'o',f,tt)

grid

xlabel('Frequency in Hz')

ylabel('Output in dB')

for I = 1:35;

g(I) = t(I)-r(I);

end

g=g';

figure(3)

[p3,s] = polyfit(f,g,2)

gg = polyval(p3,f);

semilogx(f,g,'o',f,gg)

grid

xlabel('Frequency in Hz')

ylabel('Gain in dB')
32

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34

Table 1. Curve Fit of Responsivity Variation With Temperature for Tested APD’s

Wavelength,

nm Curve fit equation Unit

APD12; bias voltage, 336 V; −3°C < T < 25°C

720 116.3 − 8.69T + 0.43T 2 − (8.3 × 10−3)T 3 A/W

820 113.8 − 8.23T + 0.41T 2 − (7.9 × 10−3)T 3 A/W

940 57.2 − 3.65T + 0.18T 2 − (3.6 × 10−3)T 3 A/W

APD11; bias voltage, 415 V; −3°C < T < 25°C

720 130.02 − 9.63T + 0.45T 2 − (8.3 × 10−3)T 3 A/W

820 136.19 − 9.85T + 0.46T 2 − (8.6 × 10−3)T 3 A/W

940 73.14 − 4.99T + 0.26T 2 − (4.4 × 10−3)T 3 A/W

APD10; bias voltage, 110 V; 6.3°C < T < 22.3°C

720 (3.85 × 103) − (9.65 × 102)T + 90.53T 2 − 3.71T3 + 0.0557T4 A/W

820 (4.22 × 103) − (10.62 × 102)T + 99.88T 2 − 4.09T3 + 0.0617T4 A/W

940 (2.17 × 103) − (5.47 × 102)T + 51.55T 2 − 2.12T3 + 0.0319T4 A/W

APD9; bias voltage, 150 V; 10.4°C < T < 21.9°C

720 58.9 − 2.92T + (8.82 × 10−2)T 2 − (1.33 × 10−3)T 3 A/W

820 66.8 − 3.53T + (9.96 × 10−2)T 2 − (1.15 × 10−3)T 3 A/W

940 35.2 − 1.96T + (5.75 × 10−2)T 2 − (6.35 × 10−4)T 3 A/W

APD8; bias voltage, 190 V; 5°C < T < 23°C

720 (1.5 × 107) − (4.9 × 105)T + (7.6 × 103)T 2 V/W

820 (1.6 × 107) − (4.9 × 105)T + (7.58 × 103)T 2 V/W

940 (8.95 × 107) − (2.6 × 105)T + (4.2 × 103)T 2 V/W

APD7; bias voltage, 380 V; 13.3°C < T < 24.5°C

720 (2.24 × 1010) − (3.26 × 109)T + (1.63 × 108)T 2 − (2.77 × 106)T 3 V/W

820 (3.06 × 1010) − (4.46 × 109)T + (2.24 × 108)T 2 − (3.79 × 106)T 3 V/W

940 (3.88 × 1010) − (5.67 × 109)T + (2.86 × 108)T 2 − (4.85 × 106)T 3 V/W

APD6; bias voltage, 410 V; 1.1°C < T < 21.9°C

720 (2.91 × 108) − (4.63 × 107)T + (3.07 × 106)T 2 − (6.8 × 104)T 3 V/W

820 (2.81 × 108) − (4.53 × 107)T + (3.04 × 106)T 2 − (6.79 × 104)T 3 V/W

940 (1.78 × 108) − (2.89 × 107)T + (1.96 × 106)T 2 − (4.4 × 104)T 3 V/W

APD2; bias voltage, 317 V; 5.7°C < T < 23.1°C

720 928.5 − 189.9T + 17.1T 2 − 0.6985T 3 + 0.011T 4 A/W

820 1043.2 − 192.3T + 15.8T 2 − 0.6005T 3 + 0.0086T 4 A/W

940 1093.9 − 185.9T + 14.1T 2 − 0.4956T 3 + 0.0066T 4 A/W
35

Table 2. Curve Fit of Responsivity Variation With Bias Voltage for Tested APD’s

Wavelength,

nm Curve fit equation Unit

APD12; temperature, 25.0°C; 320 < V < 351 V

720 (1.88 × 107) − (2.25 × 105)V + (1 × 103)V 2 − 2.02V 3 + (2 × 10−3)V 4 A/W

820 (1.89 × 107) − (2.27 × 105)V + (1 × 103)V 2 − 2.04V 3 + (1.5 × 10−3)V 4 A/W

940 (1.01 × 107) − (1.22 × 105)V + (5.47 × 102)V 2 − 1.09V 3 + (8.2 × 10−4)V 4 A/W

APD11; temperature, 23.0°C; 390 < V < 430 V

720 −(2.76 × 105) + (2.06 × 103)V − 5.12V 2 + (4.25 × 10−3)V 3 A/W

820 −(2.91 × 105) + (2.17 × 103)V − 5.41V 2 + (4.49 × 10−3)V 3 A/W

940 −(1.65 × 105) + (1.23 × 103)V − 3.07V 2 + (2.54 × 10−3)V 3 A/W

APD10; temperature, 20.9°C; 80 < V < 115 V

720 −(9.1 × 103) + 296.7V − 3.19V 2 + (1.14 × 10−2

)V 3 A/W

820 −(9.5 × 103) + 308.6V − 3.32V 2 + (1.19 × 10−2)V 3 A/W

940 −(5.2 × 103) + 167.7V − 1.81V 2 + (6.46 × 10−3)V 3 A/W

APD9; temperature, 22.0°C; 125 < V < 150 V

720 −(3.2 × 103) + 73.4V − 0.55V 2 + (1.4 × 10−3)V 3 A/W

820 −(3.6 × 103) + 80.5V − 0.61V 2 + (1.5 × 10−3)V 3 A/W

940 −(1.9 × 103) + 42.6V − 0.32V 2 + (8 × 10−4)V 3 A/W

APD8; temperature, 19.9°C; 100 < V < 215 V

720 −(2.1 × 109) + (3.98 × 107)V − (2.5 × 105)V 2 + (5 × 102)V 3 V/W

820 −(2.25 × 109) + (4.2 × 107)V − (2.6 × 105)V 2 + (5.35 × 102)V 3 V/W

940 −(1.4 × 109) + (2.69 × 107)V − (1.66 × 105)V 2 + (3.4 × 102)V 3 V/W

APD7; temperature, 22.1°C; 100 < V < 380 V

720 (3.81 × 107) − (9.19 × 106)V + (7.83 × 103)V 2 − 27.1V 3 + (3.33 × 10−2)V 4 V/W

820 (5.07 × 107) − (1.22 × 106)V + (1.04 × 104)V 2 − 36.1V 3 + (4.45 × 10−2)V 4 V/W

940 (6.28 × 107) − (1.52 × 106)V + (1.29 × 104)V 2 − 44.7V 3 + (5.52 × 10−2)V 4 V/W

APD6; temperature, 22.6°C; 100 < V < 350 V

720 (4.02 × 108) − (8.77 × 106)V + (6.80 × 104)V 2 − 222.5V 3 + 0.263V 4 V/W

820 (4.61 × 108) − (10.02 × 106)V + (7.75 × 104)V 2 − 253.2V 3 + 0.299V 4 V/W

940 (2.96 × 108) − (6.35 × 106)V + (4.89 × 104)V 2 − 159.4V 3 + 0.188V 4 V/W

APD5; temperature, 20.0°C; 2050 < V < 2350 V

720 −(1.73 × 104) + 24.54V − 0.012V 2 + (0.184 × 10−5)V 3 A/W

820 −(1.75 × 104) + 24.79V − 0.012V 2 + (0.186 × 10−5)V 3 A/W

940 −(0.94 × 104) + 13.25V − 0.006V 2 + (0.099 × 10−5)V 3 A/W

APD2; temperature, 23.1°C; 310 < V < 340 V

720 −(1.71 × 104) + (1.68 × 102)V − 0.551V 2 + (6.05 × 10−4)V 3 A/W

820 −(3.28 × 104) + (3.16 × 102)V − 1.018V 2 + (1.095 × 10−3)V 3 A/W

940 −(4.44 × 104) + (4.25 × 102)V − 1.36V 2 + (1.45 × 10−3)V 3 A/W
36

Table 3. APD Responsivity Uniformity Scan Measurement Results

APD Active area,

mm2

Area

nonuniformity,

percent

Scan

array size,

μm

Step,

μm

APD11 0.2376 14.96 41 × 41 20

APD10 4.9550 18.76 61 × 61 50

APD9 0.1800 11.27 31 × 31 20

APD8 0.1820 16.62 31 × 31 20

APD7 1.5970 15.50 51 × 51 40

APD6 0.0148 17.64 31 × 31 10

APD5 18.88 7.40 41 × 41 150

APD4 0.5464 6.02 36 × 36 28

APD2 1.6920 6.10 41 × 41 47

Table 4. APD Noise Measurement Results

APD λ, nm In

NEP,

fW/Hz1/2

D*,

cm-Hz1/2W

720 1.9 2.87 × 1013

APD12 820 2.2 × 10−13 A/Hz1/2 1.9 3.05 × 1013

940 3.9 1.63 × 1013

720 1.8 2.77 × 1013

APD11 820 2.3 × 10−13 A/Hz1/2 1.7 2.90 × 1013

940 3.1 1.57 × 1013

720 74.3 2.99 × 1012

APD10 820 2.5 × 10−12 A/Hz1/2 69.3 3.21 × 1012

940 129.4 1.72 × 1012

720 80.7 5.26 × 1011

APD9 820 1.9 × 10−12 A/Hz1/2 74.9 5.67 × 1011

940 141.8 2.99 × 1011

720 11.8 3.62 × 1012

APD8 820 9.3 × 10−8 V/Hz1/2 11.0 3.88 × 1012

940 17.7 2.41 × 1012

720 28.0 4.75 × 1012

APD2 820 1.7 × 10−12 A/Hz1/2 22.2 5.99 × 1012

940 19.4 6.84 × 1012
37

Figure 1. DIAL on-line and off-line wavelengths relative to water vapor absorption line.

Figure 2. LASE system block diagram.

Wavelength

Water vapor absorption

Water vapor

absorption line Off-line laser

On-line laser

≈100 pm

Ti:sapphire

power oscillator

Laser

diode

Nd:YAG

pump

On/off

seeds

Tunable laser

subsystem Optics

and

filters

A

B

C

D

Digitizer

Digitizer

Digitizer

Gain content

APD

APD

88%

12%

High

Medium

Low

Signal processor

Microcontroller

Recorder

Time base Fiber optic

Laser Return signal

output

Tunable laser Receiver Signal processor Control and

data acquisition
38

Figure 3. Developed DIAL receiver system.

(a) p-i-n diode structure and electric field distribution. (b) Beveled-edge or deep diffusion APD structure and

electric field distribution.

(c) Reach-through APD structure and electric field

distribution.

(d) SLIK APD structure and electric field distribution.

Figure 4. p-i-n diode and different APD structures with electric field distributions.

Optical

system

APD

detector

Analog

circuit Return

signal

Receiver card

Detected

signal

Analog

signal

Optical

signal

Power supply Personal computer 10-MHz clock

Digital

data

Digital

circuit

Interface

Electric field

p n i

Electric field

p n +

Electric field

p i p + n+

Electric field

p i p + n+
39

(a) Theoretical, showing breakdown voltage and dark

current.

(b) Experimental, measured in dark and bright conditions

for APD2.

Figure 5. APD V-I characteristics.

Figure 6. APD package circuit.

(a) Ideal and actual signals. (b) Chopping factor versus chopping frequency.

Figure 7. Output of chopper experiment.

V

I

Dark current

Increasing

light intensity

Breakdown

voltage

–500 –400 –300 –200 –100 0 100 200

–200

–150

–100

–50

0

50

100

150

200

250

APD applied voltage, mV

APD current, nA

Dark characteristic

Bright characteristic

High voltage

+

–

3

2

6

R

C

Rf

APD

Light input

Output signal

TIA

Output signal

Time

Light off Light on

Ideal output

Actual output

Offset

Light Off

0 200 400 600 800 1000

.10

.15

.20

.25

.30

.35

.40

.45

.50

Frequency, Hz

Chopping factor
40

Figure 8. Optical and electrical setup to determine APD spectral response.

Figure 9. APD alignment using microscope.

Light

source 600-nm

Monochromator optical filter

Integrating

sphere

Detector

chamber

Output signal

Optical

chopper

Test APD

Oscilloscope

Lock-in amplifier

Voltage bias

and power supplies

Temperature

controller Digital voltmeter

RC highpass filter

Computer

control

Chopper

controller

Temperature signal

Synchronizing signal

Monochromator control

Light

source

Eye piece

5× objective

lens

Auxiliary

light source

Optical axis

Detector

x

y

z x

Intensity adjustment

Microscope

Reflector
41

(a) APD12 at 336 V and 25°C. (b) APD11 at 415 V and 23°C.

(c) APD10 at 110 V and 22.3°C. (d) APD9 at 150 V and 21.9°C.

(e) APD8 at 190 V and 23°C. (f) APD7 at 380 V and 23.6°C.

Figure 10. APD spectral response calibration.

600 700

38.3

720 820

39.8

940

23.4

800 900 1000 1100

5

10

15

20

25

30

35

40

45

Wavelength, nm

Responsivity, A/W

600 700 800 900 1000 1100

5

10

15

20

25

30

35

40

45

50

55

Wavelength, nm

Responsivity, A/W

720

45.6

820

50.1

940

30.7

600 700 800 900 1000 1100

0

5

10

15

20

25

30

35

40

45

Wavelength, nm

Responsivity, A/W

720

37.1

820 940

39.8

21.3

600 700 800 900 1000 1100

0

5

10

15

20

25

30

Wavelength, nm

Responsivity, A/W

720 820 940

23.2

25.1

13.2

0

1

2

3

4

5

6

7

8

9

Responsivity, MV/W

600 700 800 900 1000 1100

Wavelength, nm

720 820 940

7.8

8.4

5.2

5

10

15

20

25

30

Responsivity, MV/W

600 700 800 900 1000 1100

Wavelength, nm

720 820 940

17.6

23.5

29.1
42

(g) APD6 at 410 V and 22.2°C. (h) APD5 at 317 V and 2350 V.

(i) APD2 at 317 V and 23.1°C.

Figure 10. Concluded.

600 650 700

720 820 940

40.4

750 800

40.3

850 900

26.9

950 1000

15

20

25

30

35

40

45

Wavelength, nm

Responsivity, MV/W

600 650 700 750 800 850 900 950 1000

10

15

20

25

30

35

40

45

50

55

60

Wavelength, nm

Responsivity, A/W

720 820 940

52.9 54.3

32.6

600 700 800 900 1000 1100

20

30

40

50

60

70

80

90

Wavelength, nm

Responsivity, A/W

720

60.0

820

75.6

940

86.4
43

(a) APD12 at 336-V bias voltage for various

temperatures.

(b) APD12 at 336-V bias voltage for various

wavelengths.

(c) APD11 at 415-V bias voltage for various

temperatures.

(d) APD11 at 415-V bias voltage for various

wavelengths.

(e) APD10 at 110-V bias voltage for various

temperatures.

(f) APD10 at 110-V bias voltage for various

wavelengths.

Figure 11. Responsivity variation of wavelength λ and temperature T.

600 700 800 900 1000 1100 0

20

40

60

80

100

120

140

160

Wavelength, nm

Responsivity, A/W

–3

0

2

5

8

11

14

17

20

23

25

T, °C

–5 0 5 10 15 20 25 30 20

40

60

80

100

120

140

160

Temperature, °C

λ, nm

Responsivity, A/W

720

820

940

600 700 800 900 1000 1100

0

20

40

60

80

100

120

140

160

180

Wavelength, nm

Responsivity, A/W

14

17

20

23

25

–3

0

2

5

8

11

T, °C

–5 0 5 10 15 20 25 30

20

40

60

80

100

120

140

160

180

Temperature, °C

Responsivity, A/W

λ, nm

720

820

940

600 700 800 900 1000 1100

0

100

200

300

400

500

600

700

Wavelength, nm

Responsivity, A/W

6.3

9.7

13.2

15.6

17.8

20.1

22.3

T, °C

5 10 15 20 25

0

100

200

300

400

500

600

700

Temperature, °C

Responsivity, A/W

λ, nm

720

820

940
44

(g) APD9 at 150-V bias voltage for various

temperatures.

(h) APD9 at 150-V bias voltage for various

wavelengths.

(i) APD8 at 190-V bias voltage for various

temperatures.

(j) APD8 at 190-V bias voltage for various

wavelengths.

(k) APD7 at 380-V bias voltage for various

temperatures.

(l) APD7 at 380-V bias voltage for various

wavelengths.

Figure 11. Continued.

600 700 800 900 1000 1100

0

5

10

15

20

25

30

35

40

Wavelength, nm

Responsivity, A/W

12.2

14.3

16.4

18.2

20.7

21.9

T, °C

10 12 14 16 18 20 22

10

15

20

25

30

35

40

Temperature, °C

Responsivity, A/W

λ, nm

720

820

940

600 700 800 900 1000 1100

0

2

4

6

8

10

12

14

Wavelength, nm

Responsivity, MV/W

5

8.9

13

16.2

18.5

19.6

23

T, °C

0 5 10 15 20 25

5

6

7

8

9

10

11

12

13

14

Temperature, °C

Responsivity, MV/W

λ, nm

720

820

940

20.9

23.1

24.5

600 650 700 750 800 850 900 950 1000

0

.5

1.0

1.5

2.0

2.5

3.0

Wavelength, nm

Responsivity, GV/W

13.3

15.1

17.1

20.1

T, °C

12 14 16 18 20 22 24 26

0

.5

1.0

1.5

2.0

2.5

3.0

Temperature, °C

Responsivity, GV/W

λ, nm

720

820

940
45

(m) APD6 at 410-V bias voltage for various

temperatures.

(n) APD6 at 410-V bias voltage for various

wavelengths.

(o) APD2 at 317-V bias voltage for various

temperatures.

(p) APD2 at 317-V bias voltage for various

wavelengths.

Figure 11. Concluded.

15.2

20.2

21.9

600 650 700 750 800 850 900 950 1000

0

.2

.4

.6

.8

1.0

1.2

1.4

1.6

1.8

2.0

Wavelength, nm

Responsivity, GV/W

T, °C

1.1

3.9

9.5

Responsivity, GV/W

0 5 10 15 20 25

.2

.4

.6

.8

1.0

1.2

1.4

1.6

1.8

Temperature, °C

λ, nm

720

820

940

600 700 800 900 1000 1100

0

50

100

150

200

250

300

350

400

450

Wavelength, nm

Responsivity, A/W

16.9

20.1

23.1

5.7

7.1

9.4

11.4

13.9

T, °C

5 10 15 20 25

50

100

150

200

250

300

350

400

450

Temperature, °C

Responsivity, A/W

λ, nm

720

820

940
46

(a) APD12 at 25°C for various bias voltages. (b) APD12 at 25°C for various wavelengths.

(c) APD11 at 23°C for various bias voltages. (d) APD11 at 23°C for various wavelengths.

(e) APD10 at 20.9°C for various bias voltages. (f) APD10 at 20.9°C for various wavelengths.

Figure 12. Responsivity variation of wavelength λ and voltage V.

600 700 800 900 1000 1100

0

20

40

60

80

100

120

140

160

180

200

Wavelength, nm

Responsivity, A/W

351

350

349

348

347

345

343

340

336

330

320

V, V

320 325 330 335 340 345 350 355

0

20

40

60

80

100

120

140

160

180

Bias voltage, V

Responsivity, A/W

λ, nm

720

820

940

600 700 800 900 1000 1100

0

20

40

60

80

100

120

140

160

Wavelength, nm

Responsivity, A/W

390

V, V

395

400

405

410

415

420

423 425

427 428 430

390 395 400 405 410 415 420 425 430

0

50

100

150

Bias voltage, V

Responsivity, A/W

λ, nm

720

820

940

600 700 800 900 1000 1100

0

20

40

60

80

100

120

140

Wavelength, nm

Responsivity, A/W

80

V, V

90

100

105

110112 115

80 85 90 95 100 105 110 115

–20

0

20

40

60

80

100

120

140

Bias voltage, V

Responsivity, A/W

λ, nm

720

820

940
47

(g) APD9 at 22°C for various bias voltages. (h) APD9 at 25°C for various wavelengths.

(i) APD8 at 19.9°C for various bias voltages. (j) APD8 at 19.9°C for various wavelengths.

(k) APD7 at 22.1°C for various bias voltages. (l) APD7 at 22.1°C for various wavelengths.

Figure 12. Continued.

600 700 800 900 1000 1100

0

5

10

15

20

25

30

35

40

Wavelength, nm

Responsivity, A/W

125

130

135

140

145

150

152

154

155

V, V

125 130 135 140 145 150 155

0

5

10

15

20

25

30

35

40

Bias voltage, V

Responsivity, A/W

λ, nm

720

820

940

600 700 800 900 1000 1100

0

10

20

30

40

60

70

Wavelength, nm

Responsivity, MV/W50

V, V

100

130

160 175 190 205 215

100 120 140 160 180 200 220

–10

0

10

20

30

40

50

60

70

Bias voltage, V

Responsivity, MV/W

λ, nm

720

820

940

600 650 700 750 800 850 900 950 1000

0

2

4

6

8

10

12

14

16

18

Wavelength, nm

Responsivity, GV/W

380

V, V

350

300

250

200

150 100

100 120 140 160 180 200 220

–2

0

2

4

6

8

10

12

14

16

18

Bias voltage, V

Responsivity, GV/W

λ, nm

720

820

940
48

(m) APD6 at 22.6°C for various bias voltages. (n) APD6 at 22.6°C for various wavelengths.

(o) APD5 at 20°C for various bias voltages. (p) APD5 at 20°C for various wavelengths.

(q) APD2 at 23.1°C for various bias voltages. (r) APD2 at 23.1°C for various wavelengths.

Figure 12. Concluded.

V, V

600 650 700 750 800 850 900 950 1000 0

10

20

30

40

50

60

70

80

Wavelength, nm

Responsivity, MV/W

350

250 300 200 100

100 150 200 250 300 350

–10

0

10

20

30

40

50

60

70

80

Bias voltage, V

Responsivity, A/W

λ, nm

720

820

940

V, V

600 650 700 750 800 850 900 950 1000

0

10

20

30

40

50

60

Wavelength, nm

Responsivity, A/W

2350

2300

2250

2200

2150

2100 2050

2050 2100 2150 2200 2250 2300 2350

–10

0

10

20

30

40

50

60

Bias voltage, V

Responsivity, A/W

λ, nm

720

820

940

600 700 800 900 1000 1100

20

40

60

80

100

120

140

160

Wavelength, nm

Responsivity, A/W

310 317

320

325

330

338 335

340

V, V

310 315 320 325 330 335 340

40

60

80

100

120

140

160

Bias voltage, V

Responsivity, A/W

λ, nm

720

820

940
49

Figure 13. Responsivity uniformity scan setup.

Figure 14. Laser focal point evaluation setup.

Objective

lens

APD

Neutral density detector

filter

Scan

mount Optical axis

He:Ne

light

source

Detector

Pin hole

Neutral density

filter

Reflector

Eye piece

Objective

lens

Microscope

He:Ne

light

source
50

Figure 15. Laser focal point evaluation using pin hole.

Figure 16. Pin hole uniformity scan.

Pin hole

Light

spot Optical axis

Laser beam

Objective lens

Laser focal

point

0 10 20 30 40 50

5

0

10

15

20

25

30

35

40

45

y, μm

x, μ

m

Relative output

100

80

60

40

20

0

60

40

20

0 0

20

40

60

y, μm x, μm
51

(a) APD11 responsivity uniformity scan. (b) APD11 active area.

(c) APD10 responsivity uniformity scan. (d) APD10 active area.

(e) APD9 responsivity uniformity scan. (f) APD9 active area.

Figure 17. APD active area scan results.

Relative responsivity

100

80

60

40

20

0

1000

500

0 0

500

1000

y, μm x, μm 0 400 600 800 200

100

200

0

300

400

500

600

700

800

y, μm

x, μ

m

Relative responsivity

100

80

60

40

20

0

4000

2000

3000

1000

0 0

2000

3000

1000

4000

y, μm x, μm 0 1500 2000 2500 3000 500 1000

500

1000

0

1500

2000

2500

3000

y, μm

x, μ

m

Relative responsivity

100

80

60

40

20

0

800

400

200

600

0 0

400

200

600

800

y, μm x, μm 0 300 400 500 600 100 200

100

200

0

300

400

500

600

y, μm

x, μ

m
52

(g) APD8 responsivity uniformity scan. (h) APD8 active area.

(i) APD7 responsivity uniformity scan. (j) APD7 active area.

(k) APD6 responsivity uniformity scan. (l) APD6 active area.

Figure 17. Continued.

Relative responsivity

100

80

60

40

20

0

800

400

200

600

0 0

400

200

600

800

y, μm x, μm 0 200 300 400 500 600 100

100

0

200

300

400

500

600

y, μm

x, μ

m

Relative responsivity

100

80

60

40

20

0

3000

1000

2000

0 0

1000

2000

3000

y, μm x, μm 0 1000 1500 2000 500

200

0

400

600

800

1000

1200

1400

1600

1800

2000

y, μm

x, μ

m

Relative responsivity

100

80

60

40

20

0

400

200

300

100

0 0

200

300

100

400

y, μm x, μm 0 100 150 200 250 300 50

50

0

100

150

200

250

300

y, μm

x, μ

m
53

(m) APD5 responsivity uniformity scan. (n) APD5 active area.

(o) APD4 responsivity uniformity scan. (p) APD4 active area.

(q) APD2 responsivity uniformity scan. (r) APD2 active area.

Figure 17. Concluded.

Relative responsivity

100

80

60

40

20

0

8000

4000

6000

2000

0 0

4000

6000

2000

8000

y, μm x, μm 0 2000 3000 4000 5000 6000 1000

1000

0

2000

3000

4000

5000

6000

y, μm

x, μ

m

Relative responsivity

100

80

60

40

20

0

1500

500

1000

0 0

500

1000

1500

y, μm x, μm 0 400 600 800 1000 200

100

0

200

300

400

500

600

700

800

900

1000

y, μm

x, μ

m

Relative responsivity

100

80

60

40

20

0

2000

1000

500

1500

0 0

1000

500

1500

2000

y, μm x, μm 0 1000 2000 500 1500

200

0

400

600

800

1000

1200

1400

1600

2000

1800

y, μm

x, μ

m
54

Figure 18. Noise as function of optical power incident on detector.

Figure 19. Separation of TIA noise sources.

Figure 20. Frequency response experimental setup.

Incident power

Detector current

Photon shot noise

Dark current noise

Signal Total noise

NEP

+

–

3

2

6

Rf

+

-

3

2

6

Rf

e

v

e

i

e

r

I

n

En

Noisy amplifier Noiseless amplifier

Oscilloscope

Spectrum analyzer

Temperature

controller

RC highpass filter Synchronizing signal

Pulse generator

Voltage bias

and power supplies

Power supply

Reference detector Beam splitter Laser diode

Neutral

density filter

APD

chamber

Input signal

Output signal
55

(a) Frequency response.

(b) Input. (c) Output.

Figure 21. APD12 frequency spectrum at bias voltage of 336 V and temperature of 23°C. Spectrum in (a) was

derived from input in (b) and output in (c).

105 106 107 108 12

14

16

18

20

22

24

Frequency, Hz

Gain, dB

–50

–48

–46

–44

–42

–40

–38

105 106 107 108

Frequency, Hz

Input, dB

–40

–35

–30

–25

–20

105 106 107 108

Frequency, Hz

Output, dB
56

(a) Frequency response.

(b) Input. (c) Output.

Figure 22. APD11 frequency spectrum at bias voltage of 415 V and temperature of 23°C. Spectrum in (a) was

derived from input in (b) and output in (c).

–20

–10

0

10

20

30

105 106 107 108

Frequency, Hz

Gain, dB

–46

–44

–42

–40

–38

–36

105 106 107 108

Frequency, Hz

Input, dB

–60

–50

–40

–30

–20

–10

105 106 107 108

Frequency, Hz

Output, dB
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July 2000 Technical Publication

Characterization of Advanced Avalanche Photodiodes for Water Vapor Lidar

Receivers WU 622-63-13-70

Tamer F. Refaat, Gary E. Halama, and Russell J. DeYoung

L-17936

NASA/TP-2000-210096

Refaat: Old Dominion University, Norfolk, VA; Halama and DeYoung: Langley Research Center, Hampton, VA.

Development of advanced differential absorption lidar (DIAL) receivers is very important to increase the accuracy

of atmospheric water vapor measurements. A major component of such receivers is the optical detector. In the nearinfrared wavelength range avalanche photodiodes (APD’s) are the best choice for higher signal-to-noise ratio,

where there are many water vapor absorption lines. In this study, characterization experiments were performed to

evaluate a group of silicon-based APD’s. The APD’s have different structures representative of different manufacturers. The experiments include setups to calibrate these devices, as well as characterization of the effects of

voltage bias and temperature on the responsivity, surface scans, noise measurements, and frequency response

measurements. For each experiment, the setup, procedure, data analysis, and results are given and discussed. This

research was done to choose a suitable APD detector for the development of an advanced atmospheric water vapor

differential absorption lidar detection system operating either at 720, 820, or 940 nm. The results point out the

benefits of using the super low ionization ratio (SLIK) structure APD for its lower noise-equivalent power, which

was found to be on the order of 2 to 4 fW/Hz1/2, with an appropriate optical system and electronics. The water

vapor detection systems signal-to-noise ratio will increase by a factor of 10.

APD; Reach-through structure; SLIK structure; Beveled-edge structure 62

A04

NASA Langley Research Center

Hampton, VA 23681-2199

National Aeronautics and Space Administration

Washington, DC 20546-0001

Unclassified–Unlimited

Subject Category 33 Distribution: Standard

Availability: NASA CASI (301) 621-0390

Unclassified Unclassified Unclassified UL