Laser Energy

The laser consists of two main amplifiers - a) the regenerative amplifier and b) the linear amplifier(s).

For alignment and most diagnostic work, the output of the RGA is sufficient in itself. The flash lamps that pump the linear amplifiers must be kept on though as they cause thermal lensing which is an integrated part of the system. Therefore without the flash lamps on, the thermal lensing isn't present and the output beam has a different divergence.

To allow a measure of output energy control whilst still having the flash lamps on, the time they are fired is delayed from the best time relative to the pockel's cell that kicks the laser pulse out of the RGA. By increasing the delay and mis-timing the flash lamps, the gain seen by the pulse in the linear amplifiers is lower. To characterise this, the output energy as a function of flash lamp delay was recorded.

The laser has an output that rivals the damage threshold of the energy meter. Therefore, an attenuator supplied by Gentec, the manufacturer of the energy meters, was used. This however, is supplied uncalibrated. The energy of the laser was measured and averaged over 60 pulses for various energies with the attenuator both present and absent. Table 1 shows this data.

Time (us)	Energy Unattenuated (mJ)	Energy Attenuated (mJ)	Ratio
160	46.1 5.15	9.12 1.32	5.05 0.92
170	27.8 2.95	5.78 0.71	4.81 0.78
180	18.4 2.34	3.55 0.55	5.18 1.04
		Average	5.02 1.47

Using this scaling factor, the data recorded over various flash lamp delays was scaled and is shown in Fig. 1.

Figure 1 : Graph of laser energy as a function of flash lamp delay.

This data trend is to be expected. The output of the RGA was recorded to be 9.31 1.69mJ. For large flash lamp delays, there is no gain for the incoming pulses and they're absorbed. As the delay is decreased, a threshold is overcome and the amplifier operates in a linear regime. What is not seen is the 'tail off' where the limit of the flash lamp input is reached.

The output of the RGA is lower than previous measurements of ~18 - 20mJ. These figures will therefore provide as a guide until the RGA is tuned up properly. This was avoided because of the proximity to a shift.