Novanta v40, User Manual

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CONTROLLING LASER POWER PAGE | 51
UC-2000 or other PWM controller, is ignored until the pulse width exceeds approximately 4
microseconds (
μ
s), at which point lasing occurs.
Tickle pulses pre-ionize the laser gas to just below the lasing threshold so that a further increase in pulse
width adds enough energy to the plasma to cause laser emission. Tickle pulses cause the laser to
respond predictably and almost instantaneously to PWM Command signals, even when there is
considerable delay (laser off time) between applied Command signals.
Pulse Width Modulation (PWM)
Pulse Width Modulation, or PWM, controls laser power by varying the duty cycle of ’s RF amplifiers,
which in turn control the time-averaged RF power applied to the laser. Typically, laser output follows the
PWM input with a rise and fall time constant of ~100 μ s; however, the laser cannot precisely follow PWM
input signals if the “On” pulse is less than 100 μ s in duration. At a constant 50% duty cycle, v-Series
lasers typically reach 90–100% of full optical output when operated at a frequency of 5 kHz and reach
65–80% optical output at 7 kHz. The percentage of optical output increases as duty cycle increases (at a
constant PWM frequency) or as PWM frequency decreases (at a constant duty cycle). Following figure
shows representative v40 optical output waveforms at two different PWM frequencies.
Controlling laser power
Figure 4-3 Representative v40 waveforms
v40 lasers are designed to operate at Command signal base frequencies up to 25 kHz; however, the
choice of PWM frequency depends on the user’s specific application. In the majority of laser applications,
Warning: Serious Personal Injury
Because of phase differences, external tickle pulses may combine with the internally generated tickle
signal causing the LASE LED to flicker during the transition from tickle to lasing. Laser output may
occur if the Lase LED flickers.