In Case Study: Medical Laser System; Part 1: Stabilizing a Laser Feedback Control Loop, the nonlinear flashlamp in the laser control loop had a parabolic transfer function which was gain-compensated with a square-root circuit. This resulted in constant loop gain over the output power range. In this sequel, more of the feedback loop design is addressed, notably the photodiode amplifier in the feedback path.
In Part 1, attention was on the forward path of the loop and the linearization of the square-law laser-flashlamp subsystem. The laser output reflected off a mirror, and a photodiode behind the mirror sensed it. A small but fixed fraction of laser light would transmit through the mirror. The fraction was different for different polarizations of light, and that caused an apparent "drift" in output power as polarization changed. Before this fact was discovered, it was thought that power drift was caused by drift in the photodiode amplifier (PDA).
The original PDA design is shown below. The redesign goal was to make it less noisy, simpler, less costly, and extended at the low end of the linear dynamic range to zero volts.
The full-scale photodiode current is 15 μA and the zero-scale value is 0.15 μA. The 100 to 1 range corresponds to a laser output power from 50 mW to 5 W. The original PDA has a split-supply (+V/2) biasing of the detector input and uses the photodiode in the fast but noisy voltage mode. Although +V/2 is common-mode-rejected from the input of U1, CMRR is not infinite and allows some supply noise to be amplified by the PDA. The OP-07 has good input characteristics, including low noise, and would not contribute appreciable drift. It is also costlier than “commodity” op-amps. The wide range of variability of the gain was necessary to cover the variations in mirror transmittance and photodiode sensitivity. The PDA output is scaled to 500 mV/W of output power.
The photodiode was a critical design choice. As the laser output sensor, its scale-factor stability would directly impact accuracy. The photodiode chosen for the application was an EG&G Judson J16 series germanium (Ge) diode with standard p-n structure, suitable for use in the 100 Hz to 100 MHz range. The active detector is circular, with a 1 mm diameter. The responsivity of the diode is typically 0.65 A/W at a 1.3 μm wavelength of input light, and peaks at 1.55 μm at 25oC, slightly above the 1.32 μm NdYAG laser output. The temperature coefficient (TC) of the responsivity (which no amount of feedback could correct) crosses 0 %/oC at 1.55 μm of wavelength and is flat below that, at a TC of –0.1 %/oC. Above 1.50 μm, the curve increases quickly to almost +3 %/oC at 1.8 μm.
The diode also has an equivalent shunt resistance at 25oC of at least 100 kΩ at a reverse voltage of 10 mV, a maximum reverse voltage, VR, of 5 V, and a dark current at this voltage of typically 2 μA and no more than 5 μA. Its capacitance at VR = 0 V is 1 nF.
A plot of J16 output current versus incident power density increases linearity as observed in the specifications. The product specification required that output power not vary more than +/-5 % over a four-hour interval. The conclusion drawn from these specs is that this diode easily meets these requirements.