MOGlabs FSC User manual

Fast servo controller
Version 1.0.4, Rev 2–4 hardware

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Contents
1 Introduction 1
1.1 Basic feedback control theory . . . . . . . . . . . . . 1
2 Connections and controls 5
2.1 Front panel controls . . . . . . . . . . . . . . . . . . . 5
2.2 Rear panel controls and connections . . . . . . . . . . 9
2.3 Internal DIP switches . . . . . . . . . . . . . . . . . . 11
3 Feedback control loops 13
3.1 Inputstage......................... 14
3.2 Slowservoloop...................... 14
3.3 Fastservoloop ...................... 18
3.4 Modulation and scanning . . . . . . . . . . . . . . . . 22
4 Application example: Pound-Drever Hall locking 23
4.1 Laser and controller configuration . . . . . . . . . . . 24
4.2 Achieving an initial lock . . . . . . . . . . . . . . . . . 25
4.3 Optimisation........................ 28
A Specifications 33
B Troubleshooting 35
B.1 Laser frequency not scanning . . . . . . . . . . . . . . 35
B.2 When using modulation input, the fast output floats
toalargevoltage..................... 36
B.3 Large positive error signals . . . . . . . . . . . . . . . 36
B.4 Fast output rails at ±0.625V.............. 36
B.5 Feedback needs to change sign . . . . . . . . . . . . 36
B.6 Monitor outputs wrong signal . . . . . . . . . . . . . . 37
i

ii Contents
B.7 Laser undergoes slow mode hops . . . . . . . . . . . . 37
C PCB layout 39
D 115/230 V conversion 41
D.1 Fuse ............................ 41
D.2 120/240 V conversion . . . . . . . . . . . . . . . . . . . 41
References 46

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 sta-
bilises the optical power of a laser, but in this manual we assume
the more common application of frequency stabilisation.
1.1 Basic feedback control theory
Feedback frequency stabilisation of lasers can be complicated. We
encourage readers to review control theory textbooks [1, 2] and lit-
erature 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 a feedback control loop.
1

2Chapter 1. Introduction
1.1.1 Error signals
The key common feature of feedback control is that the error signal
used for control 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 to-
wards the desired setpoint. Critically, this control signal will change
sign as the error signal changes sign, ensuring the laser frequency
always gets pushed towards the setpoint, rather than away from it.
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 ac-
tuator 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
complicated 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.

1.1 Basic feedback control theory 3
1.1.2 Frequency response of a feedback servo
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 of the size of this
disturbance, the Fourier frequency of the disturbance is either at
50 or 60 Hz. To suppress that disturbance, a feedback servo should
have high gain at 50 and 60 Hz to be able to compensate.
The gain of a servo controller typically 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 inducing oscillations in the control
output, such as the familiar high-pitched squeal of audio systems
(commonly called “audio feedback”). These oscillations occur for
frequencies above the reciprocal of the minimum propagation delay
of the combined laser, frequency discriminator, servo and actuator
system. Typically this limit is dominated by the response time of
the actuator. For the piezos used in external cavity diode lasers, the
limit is typically a few kHz, and for the current modulation response
of the laser diode, the limit is around 100 to 300kHz.
Figure 1.3 is a conceptual plot of gain against Fourier frequency for
the FSC. To minimise the laser frequency error, the area under the
gain plot should be maximised. PID (proportional integral and differ-
ential) 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) pro-
vides high gain for offsets and slow drifts, and differential feedback
(D) adds extra gain for sudden changes.

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.

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 3.2.
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 0
to 2.5 V reference set with the adjacent trimpot.
FAST SIGN Sign of the fast feedback.
SLOW SIGN Sign of the slow feedback.
5

6Chapter 2. Connections and controls
2.1.2 Ramp control
The internal ramp generator provides a sweep function for scanning
the laser frequency typically via a piezo actuator, diode injection
current, or both. A trigger output synchronised to the ramp is pro-
vided on the rear panel (TRIG).
INT/EXT Internal or external ramp for frequency scanning.
RATE Trimpot to adjust internal sweep rate.
BIAS When DIP3 is enabled, the slow output, scaled by this trimpot, is
added to the fast output. This bias feed-forward is typically re-
quired 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 loop variables allow the gain of the proportional, integrator
and differentiator stages to be adjusted. For the integrator 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 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 7
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; that is, unless FAST DIFF is set to a value that is not
underlined.
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 stops the scan and
engages full PID control.

8Chapter 2. Connections and controls
STATUS Multi-colour indicator displaying status of the lock.
Green Power on, lock disabled.
Orange Lock engaged but error signal 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 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 9
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 D 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). Compatible with MOGLabs PDA
and Thorlabs photodetectors. 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.
GAIN IN Voltage-controlled proportional gain of fast servo, ±1V, correspond-
ing 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.

10 Chapter 2. Connections and controls
3
1
41 +12 V
3−12 V
40V
Figure 2.1: M8 connector pinout for POWER A, B.
MOD IN High-bandwidth modulation input, added directly to fast output,
±1Vif DIP4 is on. Note that if DIP4 is on, MOD IN should be
connected to a supply, or properly terminated.
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.

2.3 Internal DIP switches 11
2.3 Internal DIP switches
There are several internal DIP switches that provide additional op-
tions, all set to OFF by default.
WARNING There is potential for exposure to high voltages inside the FSC, es-
pecially around the power supply.
OFF ON
1Fast gain Front-panel knob External signal
2Slow feedback Single integrator Double integrator
3Bias Ramp to slow only Ramp to fast and slow
4External MOD Disabled Enabled
5Offset Normal Fixed at midpoint
6Sweep Positive Negative
7Fast coupling DC AC
8Fast offset 0 +2.5 V
DIP 1 If ON, fast servo gain is determined by the potential applied to the
rear-panel GAIN IN connector instead of the front-panel FAST GAIN
knob.
DIP 2 Slow servo is a single (OFF) or double (ON) integrator. Should be
OFF if using “nested” slow and fast servo operation mode.
DIP 3 If ON, generate a bias current in proportion to the slow servo output
to prevent mode-hops. Only enable if not already provided by the
laser controller. Should be OFF when the FSC is used in combination
with a MOGLabs DLC.
DIP 4 If ON, enables external modulation through the MOD IN connector
on the rear panel. The modulation is added directly to FAST OUT.
When enabled but not in use, the MOD IN input must be terminated
to prevent undesired behaviour.
DIP 5 If ON, disables the front-panel offset knob and fixes the offset to
the mid-point. Useful in external sweep mode, to avoid accidentally

12 Chapter 2. Connections and controls
changing the laser frequency by bumping the offset knob.
DIP 6 Reverses the direction of the sweep.
DIP 7 Fast AC. Should normally be ON, so that the fast error signal is AC
coupled to the feedback servos, with time constant of 40 ms (25 Hz).
DIP 8 If ON, a 2.5 V offset is added to the fast output, suitable for direct
connection to MOGLabs B1047, B1240 headboards.

3. Feedback control loops
The FSC has two parallel feedback channels that can drive two ac-
tuators simultaneously: a “slow” actuator, typically used to change
the laser frequency by a large amount on slow timescales, and a
second “fast” actuator. The FSC provides precise control of each
stage of the servo loop, as well as 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
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
+
B IN
0v
+
–
CHB
+
+
–
VREF
A IN
ERR OFFSET
FAST SIGN
SLOW SIGN
INPUT
DC block
0v
0v
INPUT
B
A
Fast AC [7]
∫
AC
Δ
DC
Figure 3.1: Schematic of the MOGLabs FSC. Green labels refer to controls
on the front-panel and inputs on the back-panel, brown are internal DIP
switches, and purple are outputs on the back-panel.
13

14 Chapter 3. Feedback control loops
3.1 Input stage
The input stage of the FSC (figure 3.2) 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
Fast AC [7]
∫
AC
Δ
DC
Figure 3.2: Schematic of the FSC input stage showing coupling, offset
and polarity controls. Hexagons are monitored signals available via the
front-panel monitor selector switches.
3.2 Slow servo loop
Figure 3.3 shows the slow feedback configuration of the FSC. A vari-
able gain stage is controlled with the front-panel SLOW GAIN knob.
The action of the controller is either a single- or double-integrator

3.2 Slow servo loop 15
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 3.3: Schematic of slow feedback I/I2servo. Hexagons are monitored
signals available via the front-panel selector switches.
With a single integrator, the gain increases with lower Fourier fre-
quency, with slope of 20 dB per decade. Adding a second integrator
increases the slope to 40 dB per decade, reducing the long-term off-
set between actual and setpoint frequencies. Increasing the gain too
far results in oscillation as the controller “overreacts” to changes in
the error signal. For this reason it is sometimes beneficial to re-
strict the gain of the control loop at low frequencies, where a large
response can cause a laser mode-hop.
The slow servo provides large range to compensate for long-term
drifts and acoustic perturbations, and the fast actuator has small
range but high bandwidth to compensate for rapid disturbances. Us-
ing a double-integrator ensures that the slow servo has the dominant
response at low frequency.
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.

16 Chapter 3. Feedback control loops
3.2.1 Measuring the slow servo response
The slow servo loop is designed for slow drift compensation. To
observe the slow loop response:
1. Set MONITOR 1 to SLOW ERR and connect the output to an
oscilloscope.
2. Set MONITOR 2 to SLOW and connect the output to an oscillo-
scope.
3. Set INPUT to ∆ (offset mode) and CHB to 0.
4. Adjust the ERR OFFSET knob until the DC level shown on the
SLOW ERR monitor is close to zero.
5. Adjust the FREQ OFFSET knob until the DC level shown on the
SLOW monitor is close to zero.
6. Set the volts per division on the oscilloscope to 10mV per
division for both channels.
7. Engage the slow servo loop by setting SLOW mode to LOCK.
8. Slowly adjust the ERR OFFSET knob such that the DC level
shown on the SLOW ERR monitor moves above and below zero
by 10 mV.
9. As the integrated error signal changes sign, you will observe
the slow output change by 50 mV.
Note that the response time for the slow servo to drift to its limit
depends on a number of factors including the slow gain, the slow
integrator time constant, single or double integration, and the size
of the error signal.