MOGlabs FSC100 User manual

Fast servo controller
FSC100
Version 0.1.2, Rev 2–4 hardware

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[phone]
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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
Dierentiator
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 oset [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.

1.1 Schematics 7
1.1.2 Slow servo
Figure 1.6 shows the slow feedback configuration of the FSC. A va-
riable gain stage is controlled with the front-panel SLOW GAIN knob.
The action of the controller is either a single- or double-integrator
depending on whether DIP2 is enabled. The slow integrator time
constant is controlled from the front-panel SLOW INT knob, which is
labelled in terms of the associated corner frequency.
Double integrator [2]
Slow error
SLOW SERVO
Gain
SLOW GAIN
0v
∫#1
∫#2
SLOW OUT
+
LF sweep
LF
SLOW
Integrators
SLOW INT
SLOW INT
Figure 1.6: Schematic of slow feedback I/I2servo. Hexagons are monitored
signals available via the front-panel selector switches.
The purpose of the slow servo is to compensate for long-term drifts
and acoustic perturbations that are undesirable for the fast servo to
respond to. For example, if the fast servo is modulating the laser
current, then such drifts and perturbations can induce mode-hops,
after which the laser cannot be brought back to the lock-point by
the servo. Using a double-integrator ensures that the slow servo
has the dominant response at low frequency.

8Chapter 1. Introduction
1.1.3 Fast servo
The fast feedback servo (figure 1.7) is a PID-loop with a variable
gain P-stage controlled with the front-panel FAST GAIN knob, or an
external control signal through the rear-panel GAIN IN connector.
The P, I and D components can be individually adjusted via front-
panel selector switches, and a low-frequency gain limit is applied
to prevent mode-hops caused by external perturbations.
The fast servo has three modes of operation: SCAN,SCAN+P and
LOCK. When set to SCAN, the feedback is disabled and only the bias
is applied to the fast output (if enabled). When set to SCAN+P,
the proportional feedback is applied, which allows for determination
of the fast servo sign and gain while the laser frequency is still
scanning, simplifying the locking and tuning procedure (see §3.2).
In LOCK mode, the scan is halted and full PID-controller is engaged.
External gain [1]
Fast error
FAST SERVO
FAST GAIN
P
I
D
NESTED
0v
+
Slow
control
Fast
control
GAIN IN
LOCK IN (FAST)
++
0v
FAST = LOCK
Figure 1.7: Schematic of fast feedback servo PID controller.
For applications that do not include a separate slow actuator, the
slow control signal (single or double integrated error) can be added
to the fast by setting the SLOW switch to “NESTED”. In this mode it
is recommended that the double-integrator in the slow channel be
disabled with DIP2 to prevent triple-integration.

1.1 Schematics 9
1.1.4 Modulation and scanning
Laser scanning is controlled by either an internal sweep generator
or an external sweep signal. The internal sweep is a sawtooth with
variable period as set by an internal four-position range switch and
a single-turn trimpot marked “RATE” on the front-panel.
The fast and slow servo loops can be individually engaged with
either a TTL input to the rear-panel, or the associated front-panel
switches. Setting either loop to LOCK stops the sweep and activates
stabilisation.
Fast control
MODULATION & SWEEP
MOD IN
+
0v
0v
Bias [3] FAST OUT
TRIG
HF
FAST
BS
BIAS
RA
RAMP
BIAS
0v
LF sweep
0v
SPAN
0v
0v
Fixed oset [5]
+
OFFSET
SWEEP IN
+
RATE Ramp
Slope [6]
INT/EXT
LOCK IN (FAST)
LOCK IN (SLOW)
0v
–
Mod [4]
0v
FAST = LOCK
SLOW = LOCK
Figure 1.8: Sweep, external modulation, and feedforward current bias.
The ramp can also be added to the fast output by enabling DIP3 and
adjusting the BIAS trimpot, which may be useful for direct current
modulation of the laser. Note that many laser controllers (such as
the MOGLabs DLC) will generate the necessary bias current using
the slow servo signal and it is unnecessary to also generate it within
the FSC.

10 Chapter 1. Introduction

2. Connections and controls
2.1 Front panel controls
The front panel of the FSC has a large number of configuration op-
tions that allow the servo behaviour to be tuned and optimised.
Please note that switches and options may vary between hardware
revisions, please consult the manual for your specific device as in-
dicated by the serial number.
INT
SCAN
SCAN+P
LOCK
NESTED
SCAN
LOCK
RATE
BIAS
INPUT
AC
DC
Δ
FAST SIGN
SLOW GAIN FAST GAIN DIFF GAIN
GAIN LIMIT
0
10
20 30
60
40 50
MAX
FAST DIFF/FILTER
SPAN
FREQ OFFSET ERR OFFSET
FAST INTSLOW INT
EXT
SLOW SIGN
FAST
SLOW
CHA
CHB
FAST ERR
SLOW ERR RAMP
BIAS
FAST
SLOW
MON1
MON2
Fast Servo Controller
CHB
PD
0
REF
+
–
+
–
100k
200k
OFF
10M
1M
750k
500k
5M 2.5M
50k
25k 100k
250k
OFF
10k
20k
50k
500k
1M
2M
100k 200k
OFF
25
50 75 500
750
1k
100 250
CHA
CHB
FAST ERR
SLOW ERR RAMP
BIAS
FAST
SLOW
STATUS
0
6 18
24
12
2.1.1 Configuration
INPUT Selects error signal coupling mode; see figure 1.5.
AC Fast error signal is AC-coupled, slow error is DC coupled.
DC Both fast and slow error signals are DC-coupled.
∆Signals are DC-coupled, and the front-panel ERROR OFFSET is
applied for control of the lock point.
CHB Selects input for channel B: photodetector, ground, or a variable
reference adjustable with the adjacent trimpot.
FAST SIGN Sign of the fast feedback.
SLOW SIGN Sign of the slow feedback.
11

12 Chapter 2. Connections and controls
2.1.2 Ramp control
The internal ramp generator provides a sweep function for scanning
the laser frequency through a piezo actuator, diode injection current,
or both. A trigger output synchronised to the ramp is provided on
the rear panel (TRIG).
INT/EXT Internal or external sweep mode.
RATE Trimpot to adjust internal sweep rate.
BIAS When DIP3 is enabled, the slow output is added to the fast out-
put as scaled by this trimpot. This bias feed-forward is typically
required when adjusting the piezo actuator of an ECDL to prevent
mode-hopping. However, this functionality is already provided by
some laser controllers (such as the MOGLabs DLC) and should only
be used when not provided elsewhere.
SPAN Adjusts the ramp height, and thus the extent of the frequency sweep.
FREQ OFFSET Adjusts the DC offset on the slow output, effectively providing a
static shift of the laser frequency.
2.1.3 Loop variables
The gain of each proportional, integrator and differentiator stage
can be adjusted. For integrators and differentiator stages, the gain
is presented in terms of the unit gain frequency, sometimes referred
to as the corner frequency.
SLOW INT Corner frequency of the slow servo integrator; can be disabled or
adjusted from 25 Hz to 1 kHz.
SLOW GAIN Additional single-turn slow servo gain; from −20 dB to +20 dB.
FAST INT Corner frequency of the fast servo integrator; off or adjustable from
10 kHz to 2 MHz.

2.1 Front panel controls 13
FAST GAIN Ten-turn fast servo proportional gain; from −10 dB to +50 dB.
FAST DIFF/FILTER Controls the high-frequency servo response. When set to “OFF”, the
servo response remains proportional. When turned clockwise, the
differentiator is enabled with the associated corner frequency. Note
that decreasing the corner frequency increases the action of the
differentiator. When set to an underlined value, the differentiator is
disabled and instead a low-pass filter is applied to the servo output.
This causes the response to roll-off above the specified frequency.
DIFF GAIN High-frequency gain limit on the fast servo; each increment changes
the maximum gain by 6 dB. Has no effect unless the differentiator is
enabled.
2.1.4 Lock controls
GAIN LIMIT Low-frequency gain limit on the fast servo, in dB. MAX represents
the maximum available gain.
ERROR OFFSET DC offset applied to the error signals when INPUT mode is set to ∆.
Useful for precise tuning of the locking point or compensating for
drift in the error signal. The adjacent trimpot is for adjusting the
error offset of the slow servo relative to the fast servo, and may be
adjusted to ensure the fast and slow servos drive towards the same
exact frequency.
SLOW Engages the slow servo by changing SCAN to LOCK. When set to
NESTED, the slow control voltage is fed into the fast error signal
for very high gain at low frequencies in the absence of an actuator
connected to the slow output.
FAST Controls the fast servo. When set to SCAN+P, the proportional feed-
back is fed into the fast output while the laser is scanning, allowing
the feedback to be calibrated. Changing to LOCK engages the full
PID control action.

14 Chapter 2. Connections and controls
STATUS Multi-colour indicator displaying status of the lock.
Green Power on, lock disabled.
Orange Lock engaged but error signal goes out of range, indicating the
lock has failed.
Blue Lock engaged and error signal is within limits.
2.1.5 Signal monitoring
Two rotary encoders select which of the specified signals is routed
through to the rear-panel MONITOR 1 and MONITOR 2 outputs. The
TRIG output is a TTL compatible output that switches from low to high
at the centre of the sweep. The table below defines the signals.
CHA Channel A input
CHB Channel B input
FAST ERR Error signal used by the fast servo
SLOW ERR Error signal used by the slow servo
RAMP Ramp as applied to SLOW OUT
BIAS Ramp as applied to FAST OUT when DIP3 enabled
FAST FAST OUT control signal
SLOW SLOW OUT control signal

2.2 Rear panel controls and connections 15
2.2 Rear panel controls and connections
Serial: TRIG FAST OUT SLOW OUT MOD IN POWER B POWER A
MONITOR 1MONITOR 2 SWEEP IN GAIN IN B IN A IN
LOCK IN
All connectors are SMA, except as noted. All inputs are over-voltage
protected to ±15 V.
IEC power in The unit should be preset to the appropriate voltage for your country.
Please see appendix B for instructions on changing the power supply
voltage if needed.
A IN, B IN Error signal inputs for channels Aand B, typically photodetectors.
High impedance, nominal range ±2.5 V. Channel Bis unused unless
the CHB switch on the front-panel is set to PD.
POWER A, B Low-noise DC power for photodetectors; ±12 V, 125 mA, supplied
through an M8 connector (TE Connectivity part number 2-2172067-
2, Digikey A121939-ND, 3-way male). To be used with standard M8
cables, for example Digikey 277-4264-ND. Ensure that photodetectors
are switched off when being connected to the power supplies to
prevent their outputs railing.
3
1
4
1 +12 V
3−12 V
4 0 V
Figure 2.1: M8 connector pinout for POWER A, B.

16 Chapter 2. Connections and controls
GAIN IN Voltage-controlled proportional gain of fast servo, ±1V, correspon-
ding to the full-range of the front-panel knob. Replaces front-panel
FAST GAIN control when DIP1 is enabled.
SWEEP IN External ramp input allows for arbitrary frequency scanning, 0 to
2.5 V. Signal must cross 1.25 V, which defines the centre of the sweep
and the approximate lock point.
MOD IN High-bandwidth modulation input, added directly to fast output,
±1V. Requires DIP4 to be enabled1.
SLOW OUT Slow control signal output, 0 V to 2.5 V. Normally connected to a
piezo driver or other slow actuator.
FAST OUT Fast control signal output, ±2.5 V. Normally connected to diode in-
jection current, acousto- or electro-optic modulator, or other fast
actuator.
MONITOR 1, 2 Selected signal output for monitoring.
TRIG Low to high TTL output at sweep centre.
LOCK IN TTL scan/lock control; 3.5 mm stereo connector, left/right (pins 2, 3)
for slow/fast lock; low (ground) is active (enable lock). Front-panel
scan/lock switch must be on SCAN for LOCK IN to have effect. Digikey
cable CP-2207-ND provides a 3.5 mm plug with wire ends; red for slow
lock, thin black for fast lock, and thick black for ground.
123
1 Ground
2 Fast lock
3 Slow lock
Figure 2.2: 3.5 mm stereo connector pinout for TTL scan/lock control.
1When DIP4 is enabled, MOD IN must be terminated when not in use.