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7/28/2019 Optim of APD

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An overview of avalanche photodiodes and pulsed lasers as they are usedin 3D laser radar type applications

Bruno DionCMC Electronics, Inc.

Montreal, Quebec, Canada

Nick BertoneOEC

Montreal, Quebec, Canada

ABSTRACT

This paper will examine how Avalanche Photodiodes (APD) and Infrared Pulsed lasers (PL) areused and optimized to provide the intelligence to smart weapons. The basics of APDs and PLwill be covered and the principle time of flight ranging which is the underlining principle of 3Dlaser radar will be illustrated. The time of flight principle is used for range finding, lidar, 3D laserradar and speed measurements this information can then be used to provide intelligence to the

smart weapon. Examples of such systems are discussed and illustrated, for example: Clusterbombs, Proximity fuses, and how laser range finding systems can be incorporated with GPS to

produce effective and lethal weapons. The APDs that are discussed include silicon APDs forcost effective weapons, and 1550nm APDs for eye-safe systems. An overview of the differentPLs will be outlined, but the focus will be on 905nm laser pulsars for cost effective laserweapons.

Introduction

Laser profiling, laser ranging, laser designation and laser proximity fuses, allhave one thing in common they make a dumb bomb or missile smart. This

paper will examine the enabling Opto-Electronic (OE) component technology,which provides the intelligence to the weapons. The key OE components are theInfrared detector and pulsed laser. Solid-state lasers, which are used in manymilitary applications including laser designation, will not be discussed in thispaper. Instead the focus will be on semiconductor pulsed lasers for applicationsrequiring; low cost, low power consumption, small size and light weight. Thedetectors examined will also be semiconductors; in this case the focus will be on

Avalanche photodiodes providing increased sensitivities.

The underlying principle that allows for laser ranging, laser profiling and proximityfuses is the time-of-flight principle. The principle will be illustrated and it will be

shown how this can be used to provide ranging, profiling and proximity fuses.

The components this paper will focus on is the Si APD receiver module, theInGaAs APD receiver module and the 905nm laser pulsar. It will be shown howthese components are optimized for range-finding, laser-profiling and proximityfuses. Examples of these components being used in smart weapon systems andpotential weapon systems will also be presented. The details of the working ofthese systems or principles will not be discussed in this paper.


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Fundamentals of the Time-of-Flight

The principle as illustrated in figure 1 is as follows:

- The pulsed laser sends a short pulse of light starting the clock.- The detector detects the reflected pulse and stops the clock.- The elapsed time which is referred to the time-of-flight is used to

determine the distance.

Figure 1: Time of flight principle, courtesy of LETI DSYS CEA Grenoble.

The distance that can be measured depends on several factors: the peak powerof the laser, the laser beam divergence, optics and air transmittance, targetreflectivity, and the sensitivity of the detector. Transmittance and reflectance

parameters are usually imposed by the application. The designer degrees offreedom reside mainly in the selection of the laser source (power) and thereceiver (sensitivity). In essence, the higher the peak power and the higher theSNR (single-to-noise ratio) and the longer the distance can be ranged.

The accuracy of time-of-flight measurements depends on the pulse width of thelaser, the speed of the detector and the timing resolution. The shorter the pulseand the faster the detector, the better the accuracy of the measurement is.


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Pulsed lasers with fast A/D converters have been proven to have similaraccuracy[1] as the commonly used phase technique.

Enabling Components:

The application will dictate the components that will be used. For example,applications requiring ultra-long range and eye-safety, will most likely use a solid-state diode pump laser as the source, and an InGaAs APD as the detector.These types of lasers have extremely high peak powers and produce shortpulses. However, they are bulky in size, and have high power consumption whencompared to semiconductor pulsed lasers. Therefore, they are ideal forapplications where size and power consumption are not a primary concern - likean eye-safe rangefinder for the M-1 tank or other fighting vehicles. For a smartmissile which is disposable, these types of sources may not be the idealsource because of their size, power consumption and cost.

The table below briefly summarizes the different types of lasers and detectorsthat are available:

Component Type Advantage Disadvantage Application

Solid State Lasers

Ultra-high power, short pulses,

excellent beam quality

Cost, weight and power consumption Long distance range finding and

profiling. Weight, size and power

consumption not an issue

Semiconductor Pulsed Lasers

Low cost, small size, low power

consumption and medium peak

power >120W and short pulses < 6ns

Power and beam quality beam

divergence

Range finding and laser profiling.

Weight, size and power consumption

is of primary importance

PIN detectors

Ultra low cost, small size and low

power consumption (300V)

Long distance range finding and

profiling

Table 1: Summary of detectors and Lasers available

Smart munitions weapon and portable systems require small, lightweight and lowpower consumption modules, for which semiconductor pulsed laser module and

APD receiver modules are ideally suitable. Improved performance such as higherpower laser, and better sensitivity detector modules are being anticipated.


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Semiconductor Pulsed Lasers

We report here the performance of a new family of 905nm pulsed lasers. Thesemiconductor laser is an InAlGaP, with a 200m wide and 5um pitch triple-

micro stack as shown in Figure 2. For test purposes, the laser was assembledon a small pulser hybrid (Figure-3). The pulser is a RC discharge circuit with anavalanche transistor. The avalanche transistors generate a rise time (Figure-4 &5) of some 2-3 ns followed by the RC discharge (820pF, 3.9 plus intrinsic).

Figure 2: Laser front facet Figure 3: laser with integral pulsar

Figure 4: I Mon output Figure 5: Optical Output

This 3-layer micro-stack laser produces an extremely high peak power for asemiconductor laser; Peak power >125W, with a pulse width of less than 10ns.

This laser hybrid which is lightweight, small form factor (TO-package 75%, generates high peak power and short pulses,makes it ideal for smart munitions requiring laser profiling and/or laser proximityfuses.


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The table below, summarizes the results of the sample tests:

able 2: Summary of output power tests

APD Receivers

he use of APDs is the most efficient way to increase receiver sensitivity, the

T

Part Number Peak Power

(W)

Pulse Width

(ns)Beam Divergence

x

Amperature Size

um

CMC-730, Sample 1 112 < 10ns 11o

x 25o

200x10

CMC-730, Sample 2 121 " " "

CMC-730, Sample 3 117 " " "CMC-730, Sample 4 126 " " "

CMC-730, Sample 5 123 " " "

Tgain of the APD is increased until the SNR becomes shot noise limited. APDsare only required when the noise of the receiver is amplifier-noise limited,

meaning that the noise contribution of the amplifier is larger than the noise of thedetector, which only happens at high speeds. In this case, the transimpedancegain is limited by the RC time constant, by introducing an APD with internalgain, the SNR of APD + Amplifier will increase relative to a module with PIN+Amplifier. The SNR equation[1] for an APD receiver is given by:

( )

( )( )[ ] 2200

2

00

2

2/

nadbbdsn

s

iBFMIRPPIq

MmRP

i

i

N

S

++++=

= Eq. (1)

ith P0 the average optical power,

vity,

nic charge,t ,

wer,

en, rf)e

, Where k = ionization factor Eq. (2)

he optimum gain can be calculated by solving the above equations. Figure 6

wm the modulation depth,R0 the unity gain responsiM the gain,q the electroIds the surface dark currenPb the optical background poIdb the bulk dark current,B the noise bandwidth,ina

2 the amplifier noise (in,F the excess noise factor wher

k)-1/M).(1-(2+kM=F

Tand 7, were generated using the FENA software to determine the optimum gainof the APD receiver with various excess noise values for the APD. It is seen thatthe lower the excess noise value, the higher the optimum gain and the higher the


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SNR of the receiver. InGaAs APDs are currently available with high QE buthave excess noise values of k=0.45, which as can be seen in figure 7, have theoptimum gain at M=10. However, new InGaAs APDs are expected to beintroduced, will have lower k values and high QE thus enabling receivermodules with much higher SNRs.

igure 8 shows a Silicon APD receiver module optimized for 1060nm, figure 9

gure 8 Figure 9 Figure 10

ummary of results for APD modules tested:

Figure 6: Background steps (Pb or Ibd) Figure 7: k steps (0.45, 0.22, 0.02, 0.005)

Fshows an InGaAs APD optimized for 1550nm and figure 10 shows a fiberpigtailed receiver module. In all the these cases, discrete components wereselected over ASICs, due to the fact that better sensitivity is achieved withdiscrete components when using large area detectors.

Fi

S

Table 3: Results of APD modules tested

type (V/W) (MHz) (nV/Hz)Si APD, 905nm 2500 50 0.011 0.02

Si APD, 1060nm R0= 0.24 - - -

InGaAs APD, 1550nm 450 50 0.06 0.22

Receiver module Responsivity Bandwidth NEP Excess Noise


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Applications

aser Profiling:

flight principle in conjunction with optical scanning, profiling andapping is achieved. The principle is demonstrated in Figure 11 and is as

The scanning optic is timed so that each pulse provides a distancemeasurement at one point and the scanning optics allows for scanning

-

ning optics scans in both the x-y plane and time of

e noted that the same results can be obtained using an array of detectors [3]. In

L

Using the time-of-mfollows:

-

across the object.3D images are obtained by adding a 3rd dimension to the scanningoptics, ie: the scanflight provides the 3rd dimension.

Figure 11: Laser profiling (mapping) configuration.

Profiling or mapping with this technique only requires one detector, but it should

Pulsed Laser

Scanning Optics

DetectorPulsed Laser

Scanning Optics

Detector

bthe case of the detector array, the resolution is determined by the pixel arraysize. In the case of the single detector, the resolution is determined by the step-increments of the scanning optics.

ensor Fused Weapon:

developed by Textron System contains 4 skeetarheads. Once the bomb is released from its dispenser and in a vertical

SThe sensor fuzed bombw

position, a rocket motor is fired in the submunition imparting spin. Once reachinga proper spin rate, 4 heat seeking and laser profiling Skeets are released at rightangles. The dual mode active laser and passive sensors perform target profiling.When a valid target is detected, the explosive formed penetrator (EFP) is fired,which defeats the target from the top. A simulation can be viewed using thefollowing link (Courtesy of Textron Systems):http://www.systems.textron.com/index.html?topframe/top.html&leftframe/productsmenu.html&mainframe/products/precisionstrike/sfw.html


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Eye-Safe Range Finding:

A laser range finder operating at 1550nm and longer wavelengths has thedvantage of being eye-safe (Higher laser power can be used compared to a YAG

ire and Forget Missiles:

ontinue to track its target and detonate at a locationnd time, which will cause the most damage. These missiles use laser detection,

a1060nm or 905nm pulsed laser before being un-safe). These 1550nm eye-safe range

finders can be produced using semiconductor IR pulsed lasers and InGaAsavalanche photodiode, making them lightweight, low power consumption andcompact. Compact eye-safe range finders with GPS receiver can be used torange the target to establish its coordinates, and relay the coordinates fordestruction using a GPS type bomb.

FMissiles that once fired will ca

laser ranging and target recognition algorithms which enable it to be truly Fireand Forget missile. The following link provides a detailed explanation of how thisis enabled: http://www.dtic.mil/ndia/2002missiles/buzzard.pdf , courtesy of ThalesMissile Electronics Limited.

Conclusion

New 905nm pulsed lasers with h w power consumption and smallrm factor are demonstrated. These lasers low cost, high power and small form

ts with InGaAs APDs with low excess noise factors - will enableceiver modules with higher sensitivities than with whats currently available. Si

igh power, lofofactor make them ideal for smart missiles requiring laser profiling and/or laserproximity fuses.

New developmenre

APD modules optimized for 905nm and 1064nm are also demonstrated.


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REFERENCES

[1]. R. J. McIntyre and al., nche Photodiodes, RCA Review,

Vol. 35 June 1974.

sense 2000 [4035-26].t

Properties of Avala

[2]. Vincent Delaye and al., High-resolution eye safe time of flight laser range

finding, SPIE Aero[3]. Heikki Ailisto and al., Scannerless Imaging pulsed laser rangfinding, J. Op

A.: Pure Appl. Opt. 4 (2002) S337-346

[4]. Gary Buzzard., Developments in laser target detectors, Thales Missiledefense Electronics Limited.

[5]. Geolas website Imaging Laser Altimetry

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