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Optica Publishing Group
  • CLEO/Europe and EQEC 2011 Conference Digest
  • OSA Technical Digest (CD) (Optica Publishing Group, 2011),
  • paper CA_P33

Strategies to run an ophthalmological CW Self-Raman laser in Microsecond pulsed laser regime

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Abstract

Micro-second pulsed laser exposure is a new procedure for retina disease treatments. This technique consists in applying laser power sequences of 200µs pulses (1000 times less than traditional treatments). Short laser exposure duration avoids photoreceptor lesions on the retina, providing a better final vision [1]. The laser cavity in 586nm and 4W CW output power is based on Self-Raman conversion in Nd:GdVO4 [2],[3]. Initially, it was developed to continuous mode, but now it is switched to 500Hz, with minimum pulse duration of up to 100µs. This regime is called fast pulse mode. This operation mode makes the cavity PID control system velocity vital and challenging [4]. The energy delivery should be very fast for the pulse to reach the desired level before its termination, and the control feedback system has to correct any deviation of output power to keep it stable and constant. The strategy found was to build a duplicated current driver, one plugged to the laser head and the second plugged to a fake laser cavity. Each one works in synchronism to keep the drained current constant while the laser pulse in turned on and off during treatment procedure. Consequently, it eliminates the power supply time response dependency which can take around 1ms. Fig. 1 shows how the current is switched between both loads against time, fake laser current (I2) and pump diode laser current (I1), and constant drained current from power supply. The output power control is performed by a high priority software to minimize any delay in power reading. The laser head showed good performance, without any stability issues. The cavity pulse time response was of 30µs (worst case) to reach a stable mode. Fig. 2 shows the control loop effort to make output power stable by changing laser diode current. It takes 50ms to achieve the regime state. Thermal lens effects are also investigated. It causes performance differences between CW and micro-pulse mode operation, and laser cavity alignment pulse regime is responsible to favor one mode. This work resulted in a commercial ophthalmic laser.

© 2011 Optical Society of America

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