MOGlabs FSC100 Bedienungsanleitung
Fast servo controller
FSC100
Version 0.1.2, Rev 2–4 hardware
Limitation of Liability
MOG Laboratories Pty Ltd (MOGLabs) does not assume any liabi-
lity arising out of the use of the information contained within this
manual. This document may contain or reference information and
products protected by copyrights or patents and does not convey
any license under the patent rights of MOGLabs, nor the rights of
others. MOGLabs will not be liable for any defect in hardware or
software or loss or inadequacy of data of any kind, or for any direct,
indirect, incidental, or consequential damages in connections with
or arising out of the performance or use of any of its products. The
foregoing limitation of liability shall be equally applicable to any
service provided by MOGLabs.
Copyright
Copyright c
MOG Laboratories Pty Ltd (MOGLabs) 2017 – 2019.
No part of this publication may be reproduced, stored in a retrieval
system, or transmitted, in any form or by any means, electronic,
mechanical, photocopying or otherwise, without the prior written
permission of MOGLabs.
Contact
For further information, please contact:
MOG Laboratories P/L
49 University St
Carlton VIC 3053
AUSTRALIA
+61 3 9939 0677
MOGLabs USA LLC
419 14th St
Huntingdon PA 16652
USA
+1 814 251 4363
www.moglabs.com
Contents
1 Introduction 1
1.1 Schematics......................... 5
2 Connections and controls 11
2.1 Front panel controls . . . . . . . . . . . . . . . . . . . 11
2.2 Rear panel controls and connections . . . . . . . . . . 15
2.3 Internal DIP switches . . . . . . . . . . . . . . . . . . 17
3 Operation 19
3.1 Laser and controller configuration . . . . . . . . . . . 20
3.2 Achieving an initial lock . . . . . . . . . . . . . . . . . 22
3.3 Optimisation........................ 23
A Specifications 27
B 115/230 V conversion 29
B.1 Fuse ............................ 29
B.2 120/240 V conversion . . . . . . . . . . . . . . . . . . . 29
References 34
i
ii Contents
1. Introduction
The MOGLabs FSC provides the critical elements of a high-bandwidth
low-latency servo controller, primarily intended for laser frequency
stabilisation and linewidth narrowing. The FSC can also be used for
amplitude control, for example to create a “noise-eater” that stabi-
lises the optical power of a laser, but in this manual we assume the
more common application of frequency stabilisation.
Feedback frequency stabilisation of lasers can be complicated. We
encourage readers to review control theory textbooks [1, 2] and lite-
rature on laser frequency stabilisation [3].
The concept of feedback control is shown schematically in figure 1.1.
The frequency of the laser is measured with a frequency discrimi-
nator which generates an error signal that is proportional to the
difference between the instantaneous laser frequency and the de-
sired or setpoint frequency. Common discriminators include optical
cavities and Pound-Drever-Hall (PDH) [4] or H¨ansch-Couillaud [5]
detection; offset locking [6]; or many variations of atomic absorption
spectroscopy [7–10].
+
–
0Servo Laser
Frequency
discriminator
Error
signal
Control
signal
dV/df
Figure 1.1: Simplified block diagram of the FSC.
The key common feature is that the error signal should reverse sign
as the laser frequency shifts above or below the setpoint, as in
figure 1.2. From the error signal, a feedback servo or compensator
generates a control signal for a transducer in the laser, such that
the laser frequency is driven towards the desired setpoint.
1
2Chapter 1. Introduction
Error Error
Frequency fFrequency f
f0
ERROR OFFSET
∆f
Figure 1.2: A theoretical dispersive error signal, proportional to the dif-
ference between a laser frequency and a setpoint frequency. An offset on
the error signal shifts the lock point (right).
Note the distinction between an error signal and a control signal.
An error signal is a measure of the difference between the actual
and desired laser frequency, which in principle is instantaneous and
noise-free. A control signal is generated from the error signal by a
feedback servo or compensator. The control signal drives an actuator
such as a piezo-electric transducer, the injection current of a laser
diode, or an acousto-optic or electro-optic modulator, such that the
laser frequency returns to the setpoint. Actuators have complica-
ted response functions, with finite phase lags, frequency-dependent
gain, and resonances. A compensator should optimise the control
response to reduce the error to the minimum possible.
The operation of feedback servos is usually described in terms of
the Fourier frequency response; that is, the gain of the feedback
as a function of the frequency of a disturbance. For example, a
common disturbance fmis mains frequency, fm= 50 Hz or 60 Hz.
That disturbance will alter the laser frequency fby some amount,
at a rate of 50 or 60 Hz. The effect of the disturbance on the laser
might be small (e.g. f=f0±1 kHz where f0is the undisturbed
laser frequency) or large (f=f0±1 MHz). Regardless, the Fourier
frequency of the disturbance is either 50 or 60 Hz.
3
To suppress that disturbance, a feedback servo should have high gain
at 50 and 60 Hz. Gain has a low-frequency limit usually defined by
the gain-bandwidth limit of the opamps used in the servo controller.
The gain must also fall below unity gain (0 dB) at higher frequencies
to avoid oscillations such as the familiar high-pitched squeal of audio
systems (commonly called “audio feedback”) for frequencies above
the reciprocal of the minimum propagation delay of the combined
laser, frequency discriminator, servo and actuator system. Typically
that limit is dominated by the response time of the actuator and for
laser piezos that is usually of order kHz.
Figure 1.3 is a conceptual plot of gain against Fourier frequency
for the FSC. To minimise the laser frequency uncertainty, the area
under the gain plot should be maximised. PID (proportional integral
and differential) servo controllers are a common approach, where
the control signal is the sum of three components derived from the
one input error signal. The proportional feedback (P) attempts to
promptly compensate for disturbances, whereas integrator feedback
(I) provides high gain for offsets and slow drifts, and differential
feedback (D) adds extra gain for sudden changes.
When using a single integrator, the gain decreases at 20 dB per
decade of Fourier frequency change, indicating a stronger response
at lower frequencies. Adding a second integrator increases this to
40 dB per decade, reducing the long-term offset between actual and
setpoint frequencies. Increasing the gain too far however, results
in oscillation as the controller “overreacts” to changes in the error
signal. For this reason it is sometimes beneficial to restrict the gain
at low frequencies, such as in the fast servo loop, where a large
response can cause a laser mode-hop.
The differentiator compensates for the finite response time of the
system and has gain that increases at 20 dB per decade. To prevent
oscillation and limit the influence of high-frequency noise, there
is an adjustable gain limit that restricts the differentiator at high
frequencies.
4Chapter 1. Introduction
Gain (dB)
20
0
40
60
–20 102
101103104105106107
Fourier frequency [ Hz]
Integrator
Double integrator
Proportional
High freq. cuto
Dierentiator
Integrator
108
FAST LF GAIN (limit)
FAST
GAIN
DIFF GAIN (limit)
FAST INT
FAST DIFF
SLOW INT
Filter
Figure 1.3: Conceptual Bode plot showing action of the fast (red) and
slow (blue) controllers. The slow controller is either a single or double
integrator with adjustable corner frequency. The fast controller is PID
with adjustable corner frequencies and gain limits at the low and high
frequencies. Optionally the differentiator can be disabled and replaced
with a low-pass filter.
Alternatively, applications that do not require a differentiator may
benefit from low-pass filtering of the fast servo response to further
reduce the influence of noise. This can be achieved by switching on
the “filter” mode, which causes the servo response to roll-off at the
specified frequency.
1.1 Schematics 5
1.1 Schematics
The FSC has two parallel feedback channels that can drive two ac-
tuators simultaneously: a “slow” actuator with large range (usually
a piezo-electric transducer), and a second “fast” actuator (such as
the injection current of a diode laser, or an electro-optic modulator).
The FSC provides precise control of each stage, a gain limit at low
frequency, offsets, a sweep (ramp) generator, and convenient signal
monitoring.
Double integrator [2]
Slow error
SLOW SERVO
Gain
SLOW GAIN
0v
∫#1
∫#2
SLOW OUT
+
LF sweep
SLOW INT
SLOW INT
B IN
0v
+
–
CHB
+
+
–
VREF
A IN
ERR OFFSET
FAST SIGN
SLOW SIGN
INPUT
DC block
0v
0v
INPUT
B
A
MODULATION & SWEEP
MOD IN
+
0v
0v
Bias [3] FAST OUT
TRIG
BIAS
0v
LF sweep
0v
SPAN
0v
0v
Fixed oset [5]
+
OFFSET
SWEEP IN
RATE Ramp
Slope [6]
INT/EXT
–
External gain [1]
FAST SERVO
FAST GAIN
P
I
D
NESTED
0v
+
GAIN IN
++
0v
Mod [4]
LOCK IN (FAST)
FAST = LOCK
LOCK IN (FAST)
LOCK IN (SLOW)
0v
0v
FAST = LOCK
SLOW = LOCK
+
Figure 1.4: Schematic of the MOGLabs FSC. Green labels refer to controls
on the front-panel and inputs on the back-panel, brown are internal DIPs
witches, and purple are outputs on the back-panel.
6Chapter 1. Introduction
1.1.1 Input stage
The input stage of the FSC (figure 1.5) generates an error signal as
VERR =VA−VB−VOFFSET.VAis taken from the “A IN”SMA con-
nector, and VBis set using the CHB selector switch, which chooses
between the “B IN”SMA connector, VB= 0 or VB=VREF as set by
the adjacent trimpot.
The controller acts to servo the error signal towards zero, which
defines the lock point. Some applications may benefit from small
adjustments to the DC level to adjust this lock point, which can be
achieved with the 10-turn knob ERR OFFSET for up to ±0.1 V shift,
provided the INPUT selector is set to “offset” mode (∆). Larger offsets
can be achieved with the REF trimpot.
B IN
0v
+
–
CHB
+
+
–
VREF
A IN
ERR OFFSET
FAST SIGN
SLOW SIGN
INPUT
FE
SE SLOW ERR
Fast error
Slow error
FAST ERR
DC block
0v
0v
INPUT
B
A
Figure 1.5: Schematic of the FSC input stage, showing coupling, offset
and polarity controls. Hexagons are monitored signals available via the
front-panel selector switches.
Inhaltsverzeichnis
Andere MOGlabs Controller Handbücher
