Fig. 14 shows the basic block schematic of TE-module-based temperature control circuit. Key building blocks of the drive and control circuit include temperature sensor, error amplifier, error signal processor and a bipolar output drive circuit. Bipolar output driver provides the required power to drive the TE module. The output stage in most cases is designed to allow flow of drive current in either direction through the TE module. This enables both cooling and heating of the device to maintain its temperature at the specified value, regardless of ambient temperature being higher or lower.
Negative temperature coefficient (NTC) thermistor is the most commonly used temperature sensor due to its high sensitivity. RTD and semiconductor sensors have better linearity but suffer from poor sensitivity. Semiconductor sensors are also available in integrated circuit form producing a current linearly related to absolute temperature. AD 590 is one such sensor. Balanced bridge circuit configuration that generates a differential output is generally used. Error amplifier is configured around an op-amp. Error-signal processor could be anything from simple on-off controller to a proportional controller or proportional-integral (PI) or even fully digital PI-differential (PID) controller.
On-off controller is never used in practice. In proportional control, the drive signal to the TE module is proportional to the difference between the actual and desired temperatures. In a proportional controller, there is always a residual error even after the controller has settled to the final state. This error is proportional to the difference between the desired and the actual temperature and is inversely proportional to the gain of the control loop.
The problem of residual error of a proportional controller can be overcome by the addition of an integrator in the control loop. The result is a PI controller. One disadvantage of PI control is that it would be slow to respond to large residual errors. PID overcomes this problem encountered in PI controllers. Addition of derivative term improves the loop’s transient response. This type of controller is mainly used in applications where large thermal loads must be controlled rapidly and accurately. Fig. 15 shows a typical PID controller interfaced with other building blocks, including temperature sensor, reference generating circuit and error amplifier.
The output stage provides necessary drive power to the TE module. In most cases, electronic systems are designed to operate from a single positive DC voltage supply. Also, in most applications, TE modules need to be driven in bipolar mode to cater for both heating and cooling operations. A commonly used circuit topology to provide bipolar drive to TE modules while operating from a single DC voltage is the half-bridge circuit topology (Fig. 16). The circuit is usually driven at the input by a driver amplifier stage with a differential output. Transistors Q1-Q4 and Q2-Q3 conduct alternately to provide bidirectional operation of TE module. A cascade connection of two half-bridge circuits may be used to enhance voltage and current drive capability of the output stage.
TE drive and control modules for laser diode temperature control and stabilisation are commercially available, both as general-purpose benchtop equipment as well as for OEM applications. Even integrated-laser-diode drive and temperature-control modules are available for OEM applications. Fig. 17 shows a representative photograph.
Present-day military laser rangefinders and target designators are largely configured around diode-pumped pulsed solid-state lasers. We have discussed in the preceding paragraphs the building blocks of laser-diode drive and temperature-control electronics. Circuit topologies of laser-diode drivers for different modes of operation of laser diodes, including CW, quasi-CW and pulsed modes and also laser-diode temperature drive and stabilisation, have been discussed. Gas laser electronics is in focus in the concluding part of the article.
To be continued in the fourth part
Dr Anil Kumar Maini is a senior scientist, currently the director of Laser Science and Technology Centre, a premier laser and optoelectronics research and development laboratory of Defence Research and Development Organisation of Ministry of Defence. Nakul Maini is a technical editor with Wiley India Pvt Ltd