LASTEC Delhi in a joint collaborative activity with LEOS, Bangalore is developing a space qualified diode array
pumped Nd:YAG laser transmitter delivering 30 mJ @ 10 pps of 10 ns duration. For space applications laser diodes are
preferred because of their excellent reliability with lifetimes exceeding 100,000 hours. However, they are extremely
sensitive to electro-static discharge, excessive current levels, and current spikes and transients. Small variations in bias
voltage may produce large fluctuations in the current causing instability and damage to the device. Hence instead of the
traditional power supplies a current controlled laser diode driver is required. This paper presents the design of a Q-CW
laser diode driver based on closed loop current regulator, capable of driving 24 QCW laser diode bars each with 75W
peak power at 70 A. The driver can generate up to 100 Amp peak current and 200μsec pulse width operating at 10 Hz.
The current source design includes special circuits for low noise operation, slow turn-on and turn-off, circuits for over
voltage and transient current protection; and good regulation. Space qualified and radiation hardened components are
required to be used to sustain stringent space environment requirements during mission life of two years.
Differential Absorption Lidar (DIAL) Systems are advantageously used to detect and measure very small
concentrations of trace gases in the atmosphere. There is a requirement to interrogate and search for the presence of
one or more of toxic agents out of a large number (about 20 or so) of possible agents at distances up to several
kilometers with the help of a ground-based multi-wavelength DIAL system employing pulsed, tunable laser sources
in the wavelength bands of 2-5 micron and 9.2-10.8 micron.
The Laser beams from the two sources are directed in the atmosphere with a predefined divergence to scan the
atmosphere. Two methodologies can be implemented to provide the beam steering, one is to mount the entire
telescope of transmitting and receiving channel on to a motorized gimbal platform and second is to keep the optical
telescope stationary and use a slewing mirror to steer the beam in required direction. The first scheme is named as
mass control and second scheme is called mirror control. Both the schemes have relative advantages and
disadvantages and in the present DIAL application second scheme is being adopted. The present opto-mechanical
configuration of DIAL system employs a 700 x 500 mm<sup>2 </sup> (Elliptical) steering mirror for transmitting the collimated
beams in a required direction and receiving the reflected beam as well. In the receiving channel a Telescope is used
which collects the return beam and focuses the same on to a detector.
The slewing mirror is housed in a gimbal mount having a sufficient FOR (Field of Regard) in Azimuth and elevation
plane. The paper describes the modeling and simulation of Opto-mechanical and servo-mechanical subsystems of
precision gimbaled mirror and also discusses the issues related to design of control system. The requirement
specifications in regard to field of regard, slew rates 5°/s, scanning rates 1°/s are to be met with stringent beam
pointing and scanning accuracies. The design of this system is categorized as multidisciplinary problem. The design
parameters obtained from opto-mechanical analysis forms the input for control system design. The design of
control system is carried out using conventional design methodologies.