Novanta 32-1 Laser, Руководство пользователя

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CONTROL SIGNALS PAGE | 33
The UC-2000 does not produce tickle pulses continuously but generates them only when the PWM signal
is low. Tickle pulses are sent one tickle period, 200
μ
s, after the falling edge of a PWM signal pulse. Figure
below illustrates tickle pulse parameters.
Figure 4-3 Tickle pulse waveform.
32-1 lasers are designed to operate at a tickle frequency of 5 kHz, which allows the laser to meet published
specifications. Tickle frequencies lower than 4.5 kHz may compromise laser performance, particularly
optical rise times, and stress the RF electronics thereby reducing long term reliability while tickle
frequencies greater than 5 kHz may cause unintended lasing. Special care must be taken to maintain
plasma ionization without lasing at tickle frequencies greater than 5 kHz. When sending 1
μ
s tickle pulses
at 5 kHz, PWM signals can be sent at an independent, higher frequency but must go to near zero (< 1%)
duty cycle to ensure laser turn-off.
Pulse width modulation (PWM)
Pulse Width Modulation, or PWM, controls laser power by varying the duty cycle of the laser’s RF
amplifiers, which in turn control the time-averaged RF power applied to the laser. Because laser output
follows PWM input with a rise and fall time constant of ~100
μ
s, the laser cannot precisely follow PWM
signal frequencies over 5 kHz with duty cycles greater than 50%. Typically, the depth of modulation for a
50% duty cycle is 90 to 100% at 2 kHz and 60 to 80% at 5 kHz. The following figure shows 32-1 optical
waveforms at two different modulation frequencies.