technical reference
35Synrad Firestar ti-Series operator’s manual
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 SYNRAD UC-2000 Universal
Laser Controller to provide PWM Command signals to the ti-Series laser. If using an alternate method of
laser control, thoroughly review this section, Controlling laser power, as well as the following section, User
I/O 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.
Tickle pulse
Firestar lasers incorporate a built-in tickle generator, freeing customers from the need to supply tickle
pulses between lasing commands. In cases where a 5 kHz, 1 microsecond (µs) tickle pulse is still sent to the
laser via a UC-2000 or other PWM controller, it is ignored until pulse width exceeds approximately 4 µs,
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 Firestar’s RF ampli-
fiers, 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 ~75 µs; however, the laser cannot precisely follow
PWM input signals if the “On” pulse is less than 75 µs in duration. At a constant 50% duty cycle, ti-Series
lasers typically reach 90–100% of full optical output when operated at a frequency of 5 kHz. The percent-
age of optical output increases as duty cycle increases (at a constant PWM frequency) or as PWM fre-
quency decreases (at a constant duty cycle). Figure 3-3 on the following page shows representative ti-Series
optical output waveforms at two different duty cycles with the same PWM frequency.
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.
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