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Reconfigurable Laser Servo
Product Manuals - https://www.vescent.com/manuals/
Gain Sign (two-position switch)
The GAIN SIGN reverses the sign of the signal input from ERROR INPUT and should be used if the
desired lock-point has the wrong slope (loop is providing positive feedback instead of negative
feedback). Switching GAIN SIGN will invert the pattern seen at DC ERROR. When triggering an O-
scope with the RAMP TTL signal on a positive edge, the Laser Servo locks to a zero crossing with a
positive slope.
Laser State (three-position switch)
The lock switch has three positions. The lowest is the RAMP, which connects the internal ramp to the
SERVO OUTPUT causing the laser to sweep. The amplitude of the sweep is controlled with RAMP AMP
knob. In the center position (UNLOCK) the ramp is disconnected and zero volts is output to SERVO
OUTPUT. In the top position (LOCK) the loop filter is engaged.
DC Offset (10-turn potentiometer)
The DC OFFSET knob adds a DC offset to the DC ERROR MONITOR signal. Since the servo locks to the
point where the DC ERROR MONITOR reads 0V, adjusting the DC OFFSET changes the lock point.
Coarse Gain (seven-position switch)
The COARSE GAIN sets the overall proportional gain of the circuit without changing the location of any
corners or poles in the loop filter transfer function. Relative to the DC ERROR MONITOR, the Coarse
Gain adjusts the gain from 0dB to -60dB.
The overall loop gain (controlled by both the COARSE GAIN and the FINE GAIN) should be set around
the point that minimizes the RMS noise on the DC ERROR MONITOR. This can sometimes result in
setting the gain too high because the DC ERROR MONITOR filters high frequencies and thus hides
some of the gain peaking with high gain. To precisely set the gain, look at the noise with a spectrum
analyzer through the AC ERROR MONITOR.
Fine Gain (1-turn knob)
The FINE GAIN control adjusts the proportional gain by +6 to -6 dB.
Servo Output
When connecting Servo Output to your
plant, always make sure to discharge any
residual voltages, for instance from a PZT.
The SERVO OUTPUT is a voltage output to control the frequency of the laser. When the Laser Servo is
used with the D2-105 Laser Controller, the SERVO OUTPUT is connected to the CURRENT SERVO
INPUT on the Laser Controller. The SERVO OUTPUT is the output from the loop filter when in LOCK
mode, zero volts when in UNLOCK mode, and a DC balanced triangle wave when in RAMP mode. The
ramp can be shifted with respect to 0V using the ramp centering functionality.
Auxiliary Servo Output
