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APD receiver design for Laser rangefinder

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Newbie level 2
Mar 28, 2015
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Dear Members and Colleagues
I am having some trouble designing the APD receiver in a laser rangefinder. My maximum range is only 100 feet while i need at least 500 feet target range. The setup is described below. Please do correct me if i am wrong or if i am missing something.

Transmitted power & Optics
I am using a pair of orthogonal plano-cylindrical lenses to collimate the beam of SPL PL 90_3 diode. Our objective is to get minimum divergence and circular beam output. Based on my research, using an aspheric lens and beam expanders is too costly (and results in an elliptical beam) so that is not an option. To increase the range we need more signal power and less noise (High SNR, minimum ~10). Currently i am operating the diode at its peak output (75 watt, 30 Amps) using a 15 nsec laser driver (PCO-7110-50-15) from IXYS Colorado. I cannot increase the power further without damaging the diode. So increasing the signal power is not an option.

Receiver Optics
I am using a 2 inch, 100 mm focal length lens to focus the reflected pulse on the APD along with 900nm CWL, 50 nm FWHM bandpass filter (>90% transmission). This seems to be okay as well

APD Receiver
Next comes the APD receiver and i think here is where the problem lies. I have custom designed the APD receiver using First Sensor's AD1500-9 APD and Texas instruments OPA656 transimpedance amplifier. My TIA receiver has 20 Mhz bandwidth and 20Kohm gain which seems reasonable for rangefinding application. However, I am not able to operate the APD at maximum gain (M=100). The maximum APD gain i can achieve is M=40 and i think that is where the problem lies. In the biasing circuit for the APD ( which is just a series resistor of 2 Megaohm) on increasing the High voltage input, there is large voltage drop across the series resistor which results in APD bias of ~ 140 V (M=40) whereas we need 180 V as per the datasheet to get a gain M=100. Increasing the High Voltage input further (upto 300 V) does not increase the APD bias voltage.

Since i am not well versed with electro-optical design i am struggling to figure out how to increase the range and troubleshoot the APD receiver.Any help/suggestion/comments on this will be greatly appreciated. Please let me know if you have any questions

Thank You!
Abhishek SinghalView attachment ad1500-9-501208.pdfView attachment ad1500-9-501208.pdf

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