Reduced Power Laser Designation Systems

December 1, 2008

Naval Surface Warfare Center

This work contributes to the Micropulse Laser Designation (MPLD) project to develop a six-pound eye-safe micro-pulse laser system to locate, identify, range, mark, and designate stationary and moving targets. MPLD uses laser pulses of much lower energy and higher repetition rates than in existing laser designation systems. Because of this, MPLD presents a range of new circuit design and signal processing problems.

Work to date has involved investigating techniques for increasing photodiode amplifier bandwidth and reducing photodiode amplifier noise. A basic calculation of visibility angles for urban environments has also been made. A photodiode can be modeled as a current source in parallel with a capacitance. The photodiode envisaged for the present system has capacitance CD = 225 pF. The consideration is to minimize the noise of the circuit; unfortunately, photodiode amplifiers have very complex noise response.

The appropriate amplifier for a photodiode is a current-to-voltage amplifier (transconductance amplifier). Because of the virtual ground at the op-amp's inverting input pin, the voltage across the diode is very small. This helps to ensure a large amplifier bandwidth by reducing currents through the diode capacitance CD, thereby ensuring that almost all of the diode current passes through the feedback resistor rather than through CD. Bandwidth can be further increased by decreasing the closed-loop gain of the op-amp connected to the diode. If this is done, the overall gain can be brought up to the desired value by using more than one stage of amplification.

When used as a photodiode amplifier, the transconductance amplifier exhibits a complex noise behavior, in spite of the apparent simplicity of the basic circuit. The noise behavior of the photodiode amplifier has the following two undesirable properties that distinguish it from most other op-amp circuits:

There is "noise gain peaking" at high frequencies due to the interaction of parasitic capacitances within the amplifier. Above a certain frequency, the noise gain increases before eventually leveling off and then decreasing again once the op-amp's open-loop gain roll-off is reached.
The circuit amplifies the signal with a lower bandwidth than that with which the noise is amplified.

This project is investigating several approaches to increasing the signal bandwidth of the amplifier, and limiting the adverse effects of noise caused by noise gain peaking and excessive noise-gain bandwidth.

This work was done by Barry G. Sherlock of the University of North Carolina at Charlotte for the Naval Surface Warfare Center.

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