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CONTROLLING LASER POWER PAGE | 47
Controlling laser power
The Controlling laser power section includes subsections:
• Control signals
• Operating modes
Control signals
Much of the information provided in this section describes the use of a NOVANTA UC-2000 Universal
Laser Controller to provide tickle and PWM Command signals to the v30 laser. If using an alternate
method of laser control, thoroughly review this section, controlling laser power, as well as the following
section, Interface connections, for an understanding of the signal requirements necessary to control
Firestar lasers. For more information about the UC-2000, please consult the UC-2000 Laser Controller
Operator’s Manual.
Important Note: OEM v30 lasers do not provide internal tickle; therefore, users should provide 1
μ
s duration tickle pulses at a frequency of 5 kHz between applied PWM Command signals. For
optimum laser performance, a tickle pulse must be sent at the end of every 200- μ s interval in
which a PWM Command signal was not applied.
Tickle pulse
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. On cold starts, provide five to ten
seconds of tickle before sending PWM Commands to the laser.
The externally generated tickle pulse requirement is a square wave with an amplitude ranging from logic
low (typically 0 V) to logic high (typically 5 V) at a frequency of 5 kHz. A single pulse of 1
μ
s duration
must be applied at the end of every 200-
μ
s interval in which a PWM Command signal was not applied.
See Table 3-9 for complete voltage and current specifications.
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. 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,
v30 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). see
the following figure or a representative v30 optical output waveforms at two different PWM frequencies.