PID Fast Servo Controller
Product Information
Specifications
- Model: MOGLabs FSC
- Type: Servo Controller
- Intended Use: Laser frequency stabilisation and linewidth
narrowing - Primary Application: High-bandwidth low-latency servo
control
Product Usage Instructions
1. Introduction
The MOGLabs FSC is designed to provide high-bandwidth
low-latency servo control for laser frequency stabilisation and
linewidth narrowing.
1.1 Basic Feedback Control Theory
Feedback frequency stabilisation of lasers can be complex. It is
recommended to review control theory textbooks and literature on
laser frequency stabilisation for a better understanding.
2. Connections and Controls
2.1 Front Panel Controls
The front panel controls are used for immediate adjustments and
monitoring. These controls are essential for real-time adjustments
during operation.
2.2 Rear Panel Controls and Connections
The rear panel controls and connections provide interfaces for
external devices and peripherals. Properly connecting these ensures
smooth operation and compatibility with external systems.
2.3 Internal DIP Switches
The internal DIP switches offer additional configuration
options. Understanding and correctly setting these switches are
crucial for customizing the controller’s behavior.
FAQ
Q: What should I do if the laser frequency is not
scanning?
A: Refer to the troubleshooting section in the manual,
specifically under “Laser frequency not scanning” for detailed
steps to address this issue.
a santec company
Fast servo controller
Version 1.0.9, Rev 24 hardware
Limitation of Liability
MOG Laboratories Pty Ltd (MOGLabs) does not assume any liability 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 © MOG Laboratories Pty Ltd (MOGLabs) 2017 2025. 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 info@moglabs.com www.moglabs.com
Santec LIS Corporation 5823 Ohkusa-Nenjozaka, Komaki Aichi 485-0802 JAPAN +81 568 79 3535 www.santec.com
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 Input stage . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.2 Slow servo loop . . . . . . . . . . . . . . . . . . . . . . 14
3.3 Fast servo loop . . . . . . . . . . . . . . . . . . . . . . 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
to a large voltage . . . . . . . . . . . . . . . . . . . . . 36
B.3 Large positive error signals . . . . . . . . . . . . . . . 36
B.4 Fast output rails at ±0.625 V . . . . . . . . . . . . . . 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 stabilises 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 literature 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 discriminator which generates an error signal that is proportional to the difference between the instantaneous laser frequency and the desired or setpoint frequency. Common discriminators include optical cavities and Pound-Drever-Hall (PDH) [4] or Ha¨nsch-Couillaud [5] detection; offset locking [6]; or many variations of atomic absorption spectroscopy [710].
0
+
Error signal
Servo
Control signal
Laser
dV/df Frequency discriminator
Figure 1.1: Simplified block diagram of a feedback control loop.
1
2
Chapter 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 towards 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
f
0
Frequency f
f Frequency f
ERROR OFFSET
Figure 1.2: A theoretical dispersive error signal, proportional to the difference 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 complicated response functions, with finite phase lags, frequencydependent 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 is mains frequency, = 50 Hz or 60 Hz. That disturbance will alter the laser frequency by some amount, at a rate of 50 or 60 Hz. The effect of the disturbance on the laser might be small (e.g. = 0 ± 1 kHz where 0 is the undisturbed laser frequency) or large ( = 0 ± 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 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.
4
Chapter 1. Introduction
Gain (dB)
High freq. cutoff Double integrator
60
FAST INT FAST GAIN
FAST DIFF DIFF GAIN (limit)
40
20
Integrator
0
FAST LF GAIN (limit)
Integrator
Proportional
Differentiator
Filter
SLOW INT
20101
102
103
104
105
106
107
108
Fourier frequency [Hz]
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 options 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 indicated by the serial number.
Fast Servo Controller
AC DC
INPUT
PD 0
REF
CHB
+
FAST SIGN
+
SLOW SIGN
INT
75 100 250
50k 100k 200k
10M 5M 2.5M
50
500
20k
500k OFF
1M
25
750 10k
1M 200k
750k
OFF
1k OFF
2M 100k
500k
EXT
50k
250k
25k
100k
SPAN
RATE
SLOW INT
FAST INT
FAST DIFF/FILTER
12
6
18
0
24
BIAS
FREQ OFFSET
SLOW GAIN
FAST GAIN
DIFF GAIN
30 20 10
0
40
50
NESTED
60
SCAN
MAX LOCK
SLOW
GAIN LIMIT
SCAN SCAN+P
LOCK
FAST
ERR OFFSET
STATUS
SLOW ERR
RAMP
FAST ERR
BIAS
CHB
FAST
CHA
SLOW
MON1
SLOW ERR
RAMP
FAST ERR
BIAS
CHB
FAST
CHA
SLOW
MON2
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
6
Chapter 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 provided on the rear panel (TRIG, 1M ).
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 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 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 feedback 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.
8
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 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 (1M ) that switches from low to high at the centre of the sweep. The table below defines the signals.
CHA CHB FAST ERR SLOW ERR RAMP BIAS FAST SLOW
Channel A input Channel B input Error signal used by the fast servo Error signal used by the slow servo Ramp as applied to SLOW OUT Ramp as applied to FAST OUT when DIP3 enabled FAST OUT control signal SLOW OUT control signal
2.2 Rear panel controls and connections
9
2.2 Rear panel controls and connections
MONITOR 2 LOCK IN
MONITOR 1
SWEEP IN
GAIN IN
B IN
A IN
Serial:
TRIG
FAST OUT SLOW OUT
MOD IN
POWER B
POWER A
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 A and B, typically photodetectors. High impedance, nominal range ±2 5 V. Channel B is 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, ±1 V , corresponding 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 4
1 +12 V
1
3 -12 V
4 0V
Figure 2.1: M8 connector pinout for POWER A, B.
MOD IN High-bandwidth modulation input, added directly to fast output, ±1 V if 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 injection 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, 1M .
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.
321
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 options, all set to OFF by default.
WARNING There is potential for exposure to high voltages inside the FSC, especially around the power supply.
OFF
1 Fast gain
Front-panel knob
2 Slow feedback Single integrator
3 Bias
Ramp to slow only
4 External MOD Disabled
5 Offset
Normal
6 Sweep
Positive
7 Fast coupling DC
8 Fast offset
0
ON External signal Double integrator Ramp to fast and slow Enabled Fixed at midpoint Negative AC -1 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 -1 V offset is added to the fast output. DIP8 should be off when the FSC is used with MOGLabs lasers.
3. Feedback control loops
The FSC has two parallel feedback channels that can drive two actuators 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.
INPUT
INPUT
+
AC
ERR OFFSET
DC
A IN
A
0v
+
B
B IN
0v +
VREF
0v
CHB
FAST SIGN Fast AC [7] DC block
SLOW SIGN
MODULATION & SWEEP
RATE
Ramp
INT/EXT
Slope [6] SWEEP IN
SPAN
0v
+
OFFSET
MOD IN
0v
Mod [4]
0v
Fixed offset [5]
0v
TRIG
0v 0v
+
BIAS
0v 0v
Bias [3]
LOCK IN (FAST) LOCK IN (SLOW) FAST = LOCK SLOW = LOCK
LF sweep
FAST OUT +
FAST SERVO
GAIN IN FAST GAIN
External gain [1] P
+
I
+
0v
NESTED
FAST = LOCK LOCK IN (FAST)
D
0v
SLOW SERVO
Slow error Gain SLOW GAIN
SLOW INT
#1
LF sweep
SLOW INT
+
#2
0v
Double integrator [2]
SLOW OUT
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. VA is taken from the “A IN” SMA connector, and VB is 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.
INPUT
INPUT
+ AC
ERR OFFSET
DC
A IN
A
0v
+
B
B IN
FAST SIGN Fast AC [7] FE FAST ERR
DC block
Fast error
0v +
VREF
0v
CHB
SLOW SIGN
Slow error SE SLOW ERR
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 variable 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.
SLOW SERVO
Slow error Gain SLOW GAIN
Integrators
SLOW INT
#1
LF sweep
SLOW INT
+
#2
0v
Double integrator [2]
SLOW OUT
LF SLOW
Figure 3.3: Schematic of slow feedback I/I2 servo. Hexagons are monitored signals available via the front-panel selector switches.
With a single integrator, the gain increases with lower Fourier frequency, with slope of 20 dB per decade. Adding a second integrator increases the slope to 40 dB per decade, reducing the long-term offset 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 restrict 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. Using 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 oscilloscope.
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 250 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.
3.2 Slow servo loop
17
3.2.2 Slow output voltage swing (only for FSC serials A04… and below)
The output of the slow servo control loop is configured for a range of 0 to 2.5 V for compatibility with a MOGLabs DLC. The DLC SWEEP piezo control input has a voltage gain of 48 so that the maximum input of 2.5 V results in 120 V on the piezo. When the slow servo loop is engaged, the slow output will only swing by ±25 mV relative to its value prior to engagement. This limitation is intentional, to avoid laser mode hops. When the slow output of the FSC is used with a MOGLabs DLC, a 50 mV swing in the output of the slow channel of the FSC corresponds to a 2.4 V swing in the piezo voltage which corresponds to a change in laser frequency of around 0.5 to 1 GHz, comparable to the free spectral range of a typical reference cavity.
For use with different laser controllers, a larger change in the locked slow output of the FSC can be enabled via a simple resistor change. The gain on the output of the slow feedback loop is defined by R82/R87, the ratio of resistors R82 (500 ) and R87 (100 k). To increase the slow output, increase R82/R87, most easily accomplished by reducing R87 by piggybacking another resistor in parallel (SMD package, size 0402). For example, adding a 30 k resistor in parallel with the existing 100 k resistor would give an effective resistance of 23 k providing an increase in the slow output swing from ±25 mV to ±125 mV. Figure 3.4 shows the layout of the FSC PCB around opamp U16.
R329
U16
C36
C362 R85 R331 C44 R87
C71
C35
R81 R82
Figure 3.4: The FSC PCB layout around the final slow gain opamp U16, with gain setting resistors R82 and R87 (circled); size 0402.
18
Chapter 3. Feedback control loops
3.3 Fast servo loop
The fast feedback servo (figure 3.5) is a PID-loop which provides precise control over each of the proportional (P), integral (I) and differential (D) feedback components, as well as the overall gain of the entire system. The fast output of the FSC can swing from -2.5 V to 2.5 V which, when configured with a MOGLabs external cavity diode laser, can provide a swing in current of ±2.5 mA.
FAST SERVO
GAIN IN
External gain [1]
FAST GAIN
Fast error
Slow control
0v
+ NESTED
FAST = LOCK LOCK IN (FAST)
P I
D
0v
+
Fast control
Figure 3.5: Schematic of fast feedback servo PID controller.
Figure 3.6 shows a conceptual plot of the action of both the fast and slow servo loops. At low frequencies, the fast integrator (I) loop dominates. To prevent the fast servo loop over-reacting to low frequency (acoustic) external perturbations, a low-frequency gain limit is applied controlled by the GAIN LIMIT knob.
At mid-range frequencies (10 kHz1 MHz) the proportional (P) feedback dominates. The unity gain corner frequency at which the proportional feedback exceeds the integrated response is controlled by the FAST INT knob. The overall gain of the P loop is set by he FAST GAIN trimpot, or via an external control signal through the rear-panel GAIN IN connector.
3.3 Fast servo loop
19
60
Gain (dB)
High freq. cutoff Double integrator
FAST INT FAST GAIN
FAST DIFF DIFF GAIN (limit)
40
20
Integrator
0
FAST LF GAIN (limit)
Integrator
Proportional
Differentiator
Filter
SLOW INT
20101
102
103
104
105
106
107
108
Fourier frequency [Hz]
Figure 3.6: 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 a PID compensator 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.
High frequencies (1 MHz) typically require the differentiator loop to dominate for improved locking. The differentiator provides phaselead compensation for the finite response time of the system and has gain that increases at 20 dB per decade. The corner frequency of the differential loop can be adjusted via the FAST DIFF/FILTER knob to control the frequency at which differential feedback dominates. If the FAST DIFF/FILTER is set to OFF, then the differential loop is disabled and the feedback remains proportional at higher frequencies. To prevent oscillation and limit the influence of high-frequency noise when the differential feedback loop is engaged, there is an adjustable gain limit, DIFF GAIN, that restricts the differentiator at high frequencies.
A differentiator is often not required, and the compensator may instead benefit from low-pass filtering of the fast servo response to further reduce the influence of noise. Rotate the FAST DIFF/FILTER
20
Chapter 3. Feedback control loops
knob anti-clockwise from the OFF position to set the roll-off frequency for filtering mode.
The fast servo has three modes of operation: SCAN, SCAN+P and LOCK. When set to SCAN, feedback is disabled and only the bias is applied to the fast output. When set to SCAN+P, 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 §4.2). In LOCK mode, the scan is halted and full PID feedback is engaged.
3.3.1 Measuring the fast servo response
The following two sections describe measurement of proportional and differential feedback to changes in the error signal. Use a function generator to simulate an error signal, and an oscilloscope to measure the response.
1. Connect MONITOR 1, 2 to an oscilloscope, and set the selectors to FAST ERR and FAST .
2. Set INPUT to (offset mode) and CHB to 0.
3. Connect the function generator to CHA input.
4. Configure the function generator to produce a 100 Hz sine wave of 20 mV peak to peak.
5. Adjust the ERR OFFSET knob such that the sinusoidal error signal, as seen on the FAST ERR monitor, is centred about zero.
3.3.2 Measuring the proportional response · Reduce the span to zero by turning the SPAN knob fully anticlockwise.
· Set FAST to SCAN+P to engage the proportional feedback loop.
3.3 Fast servo loop
21
· On the oscilloscope, the FAST output of the FSC should show a 100 Hz sine wave.
· Adjust the FAST GAIN knob to vary the proportional gain of the fast servo until the output is the same amplitude as the input.
· To measure the proportional feedback frequency response, adjust the frequency of the function generator and monitor the amplitude of the FAST output response. For example, increase the frequency until the amplitude is halved, to find the -3 dB gain frequency.
3.3.3 Measuring the differential response
1. Set FAST INT to OFF to switch off the integrator loop.
2. Set the FAST GAIN to unity using the steps described in the section above.
3. Set the DIFF GAIN to 0 dB.
4. Set FAST DIFF/FILTER to 100 kHz.
5. Sweep the frequency of the function generator from 100 kHz to 3 MHz and monitor the FAST output.
6. As you sweep the error signal frequency, you should see unity gain at all frequencies.
7. Set the DIFF GAIN to 24 dB.
8. Now as you sweep the error signal frequency, you should notice a 20 dB per decade slope increase after 100 kHz that will start to roll off at 1 MHz, showing the opamp bandwidth limitations.
The gain of the fast output can be altered by changing resistor values, but the circuit is more complicated than for slow feedback (§3.2.2). Contact MOGLabs for further information if required.
22
Chapter 3. Feedback control loops
3.4 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 (App. C), and a single-turn trimpot RATE on the front-panel.
The fast and slow servo loops can be individually engaged via TTL signals to the rear-panel associated front-panel switches. Setting either loop to LOCK stops the sweep and activates stabilisation.
MODULATION & SWEEP
INT/EXT
TRIG
RATE
Ramp
Slope [6] SWEEP IN
SPAN
0v
+
OFFSET
0v
0v
Fixed offset [5]
Fast control MOD IN
Mod [4]
0v
0v 0v
+
BIAS
0v 0v
Bias [3]
LOCK IN (FAST)
LOCK IN (SLOW)
FAST = LOCK SLOW = LOCK
RAMP RA
LF sweep
BIAS BS
FAST OUT +
HF FAST
Figure 3.7: 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, but many laser controllers (such as the MOGLabs DLC) will generate the necessary bias current based on the slow servo signal, in which case it is unnecessary to also generate it within the FSC.
4. Application example: Pound-Drever Hall locking
A typical application of the FSC is to frequency-lock a laser to an optical cavity using the PDH technique (fig. 4.1). The cavity acts as a frequency discriminator, and the FSC keeps the laser on resonance with the cavity by controlling the laser piezo and current through its SLOW and FAST outputs respectively, reducing the laser linewidth. A separate application note (AN002) is available that provides detailed practical advice on implementing a PDH apparatus.
Oscilloscope
TRIG
CH1
CH2
Laser
Current mod Piezo SMA
EOM
PBS
PD
DLC controller
PZT MOD
AC
Cavity LPF
MONITOR 2 MONITOR 1 LOCK IN
SWEEP IN GAIN IN
B IN
A IN
Serial:
TRIG
FAST OUT SLOW OUT MOD IN
POWER B POWER A
Figure 4.1: Simplified schematic for PDH-cavity locking using the FSC. An electro-optic modulator (EOM) generates sidebands, which interact with the cavity, generating reflections that are measured on the photodetector (PD). Demodulating the photodetector signal produces a PDH error signal.
A variety of other methods can be used to generate error signals, which will not be discussed here. The rest of this chapter describes how to achieve a lock once an error signal has been generated.
23
24
Chapter 4. Application example: Pound-Drever Hall locking
4.1 Laser and controller configuration
The FSC is compatible with a variety of lasers and controllers, provided they are correctly configured for the desired mode of operation. When driving an ECDL (such as the MOGLabs CEL or LDL lasers), the requirements for the laser and controller are as follows:
· High-bandwidth modulation directly into the laser headboard or intra-cavity phase modulator.
· High-voltage piezo control from an external control signal.
· Feed-forward (“bias current”) generation for lasers that require a bias of 1 mA across their scan range. The FSC is capable of generating a bias current internally but the range might be limited by headboard electronics or phase modulator saturation, so it may be necessary to use bias provided by the laser controller.
MOGLabs laser controllers and headboards can be easily configured to achieve the required behaviour, as explained below.
4.1.1 Headboard configuration
MOGLabs lasers include an internal headboard that interfaces the components with the controller. A headboard that includes fast current modulation via an SMA connector is required for operation with the FSC. The headboard should be connected directly to the FSC FAST OUT.
The B1240 headboard is strongly recommended for maximum modulation bandwidth, although the B1040 and B1047 are acceptable substitutes for lasers that are incompatible with the B1240. The headboard has a number of jumper switches which must be configured for DC coupled and buffered (BUF) input, where applicable.
4.2 Achieving an initial lock
25
4.1.2 DLC configuration
Although the FSC can be configured for either internal or external sweep, it is significantly simpler to use the internal sweep mode and set the DLC as a slave device as follows:
1. Connect SLOW OUT to SWEEP / PZT MOD on the DLC.
2. Enable DIP9 (External sweep) on the DLC. Ensure that DIP13 and DIP14 are off.
3. Disable DIP3 (Bias generation) of the FSC. The DLC automatically generates the current feed-forward bias from the sweep input, so it is not necessary to generate a bias within the FSC.
4. Set SPAN on the DLC to maximum (fully clockwise).
5. Set FREQUENCY on the DLC to zero using the LCD display to show Frequency.
6. Ensure that SWEEP on the FSC is INT.
7. Set FREQ OFFSET to mid-range and SPAN to full on the FSC and observe the laser scan.
8. If the scan is in the wrong direction, invert DIP4 of the FSC or DIP11 of the DLC.
It is important that the SPAN knob of the DLC is not adjusted once set as above, as it will impact the feedback loop and may prevent the FSC from locking. The FSC controls should be used to adjust the sweep.
4.2 Achieving an initial lock
The SPAN and OFFSET controls of the FSC can be used to tune the laser to sweep across the desired lock point (e.g. cavity resonance) and to zoom into a smaller scan around the resonance. The following
26
Chapter 4. Application example: Pound-Drever Hall locking
steps are illustrative of the process required to achieve a stable lock. Values listed are indicative, and will need to be adjusted for specific applications. Further advice on optimising the lock is provided in §4.3.
4.2.1 Locking with fast feedback
1. Connect the error signal to the A IN input on the back-panel.
2. Ensure that the error signal is of order 10 mVpp.
3. Set INPUT to (offset mode) and CHB to 0.
4. Set MONITOR 1 to FAST ERR and observe on an oscilloscope. Adjust the ERR OFFSET knob until the DC level shown is zero. If there is no need to use the ERROR OFFSET knob to adjust the DC level of the error signal, the INPUT switch can be set to DC and the ERROR OFFSET knob will have no effect, preventing accidental adjustment.
5. Reduce the FAST GAIN to zero.
6. Set FAST to SCAN+P, set SLOW to SCAN, and locate the resonance using the sweep controls.
7. Increase FAST GAIN until the error signal is seen to “stretch out” as shown in figure 4.2. If this is not observed, invert the FAST SIGN switch and try again.
8. Set FAST DIFF to OFF and GAIN LIMIT to 40. Reduce FAST INT to 100 kHz.
9. Set FAST mode to LOCK and the controller will lock to the zero-crossing of the error signal. It may be necessary to make small adjustments to FREQ OFFSET to lock the laser.
10. Optimise the lock by adjusting the FAST GAIN and FAST INT while observing the error signal. It may be necessary to relock the servo after adjusting the integrator.
4.2 Achieving an initial lock
27
Figure 4.2: Scanning the laser with P-only feedback on the fast output while scanning the slow output causes the error signal (orange) to become extended when the sign and gain are correct (right). In a PDH application, the cavity transmission (blue) will also become extended.
11. Some applications may benefit by increasing FAST DIFF to improve loop response, but this is typically not needed to achieve an initial lock.
4.2.2 Locking with slow feedback
Once lock is achieved with the fast proportional and integrator feedback, the slow feedback should then be engaged to account for slow drifts and sensitivity to low frequency acoustic perturbations.
1. Set SLOW GAIN to mid-range and SLOW INT to 100 Hz.
2. Set FAST mode to SCAN+P to unlock the laser, and adjust SPAN and OFFSET so that you can see the zero crossing.
3. Set MONITOR 2 to SLOW ERR and observe on an oscilloscope. Adjust the trimpot beside ERR OFFSET to bring the slow error signal to zero. Adjusting this trimpot will only affect the DC level of the slow error signal, not the fast error signal.
4. Relock the laser by setting FAST mode to LOCK and make any necessary small adjustments to FREQ OFFSET to lock the laser.
28
Chapter 4. Application example: Pound-Drever Hall locking
5. Set SLOW mode to LOCK and observe the slow error signal. If the slow servo locks, the DC level of the slow error may change. If this occurs, note the new value of the error signal, set SLOW back to SCAN and use the error offset trimpot to bring the slow unlocked error signal closer to the locked value and try relocking the slow lock.
6. Iterate the previous step of slow locking the laser, observing the DC change in the slow error, and adjusting the error offset trimpot until engaging the slow lock does not produce a measurable change in the slow locked versus fast locked error signal value.
The error offset trimpot adjusts for small (mV) differences in the fast and slow error signal offsets. Adjusting the trimpot ensures that both the fast and slow error compensator circuits lock the laser to the same frequency.
7. If the servo unlocks immediately upon engaging the slow lock, try inverting the SLOW SIGN.
8. If the slow servo still unlocks immediately, reduce the slow gain and try again.
9. Once a stable slow lock is achieved with the ERR OFFSET trimpot correctly set, adjust SLOW GAIN and SLOW INT for improved lock stability.
4.3 Optimisation
The purpose of the servo is to lock the laser to the zero-crossing of the error signal, which ideally would be identically zero when locked. Noise in the error signal is therefore a measure of lock quality. Spectrum analysis of the error signal is a powerful tool for understanding and optimising the feedback. RF spectrum analysers can be used but are comparatively expensive and have limited dynamic range. A good sound card (24-bit 192 kHz, e.g. Lynx L22)
4.3 Optimisation
29
provides noise analysis up to a Fourier frequency of 96 kHz with 140 dB dynamic range.
Ideally the spectrum analyser would be used with an independent frequency discriminator that is insensitive to laser power fluctuations [11]. Good results can be achieved by monitoring the in-loop error signal but an out-of-loop measurement is preferable, such as measuring the cavity transmission in a PDH application. To analyse the error signal, connect the spectrum analyser to one of the MONITOR outputs set to FAST ERR.
High-bandwidth locking typically involves first achieving a stable lock using only the fast servo, and then using the slow servo to improve the long-term lock stability. The slow servo is required to compensate for thermal drift and acoustic perturbations, which would result in a mode-hop if compensated with current alone. In contrast, simple locking techniques such as saturated absorption spectroscopy are typically achieved via first achieving a stable lock with the slow servo, and then using the fast servo to compensate for higher-frequency fluctuations only. It may be beneficial to consult the Bode plot (figure 4.3) when interpreting the error signal spectrum.
When optimising the FSC, it is recommended to first optimise the fast servo through analysis of the error signal (or transmission through the cavity), and then the slow servo to reduce sensitivity to external perturbations. In particular, SCAN+P mode provides a convenient way to get the feedback sign and gain approximately correct.
Note that achieving the most stable frequency lock requires careful optimisation of many aspects of the apparatus, not just the parameters of the FSC. For example, residual amplitude modulation (RAM) in a PDH apparatus results in drift in the error signal, which the servo is unable to compensate for. Similarly, poor signal-to-noise ratio (SNR) will feed noise directly into the laser.
In particular, the high gain of the integrators means that the lock can be sensitive to ground loops in the signal-processing chain, and
30
Chapter 4. Application example: Pound-Drever Hall locking
care should be taken to eliminate or mitigate these. The earth of the FSC should be as close as possible to both the laser controller and any electronics involved in generating the error signal.
One procedure for optimising the fast servo is to set FAST DIFF to OFF and adjust FAST GAIN, FAST INT and GAIN LIMIT to reduce the noise level as far as possible. Then optimise the FAST DIFF and DIFF GAIN to reduce the high-frequency noise components as observed on a spectrum analyser. Note that changes to FAST GAIN and FAST INT may be required to optimise the lock once the differentiator has been introduced.
In some applications, the error signal is bandwidth-limited and only contains uncorrelated noise at high frequencies. In such scenarios it is desirable to limit the action of the servo at high frequencies to prevent coupling this noise back into the control signal. A filter option is provided to reduce the fast servo response above a specific frequency. This option is mutually-exclusive to the differentiator, and should be tried if enabling the differentiator is seen to increase
60
Gain (dB)
High freq. cutoff Double integrator
FAST INT FAST GAIN
FAST DIFF DIFF GAIN (limit)
40
20
Integrator
0
FAST LF GAIN (limit)
Integrator
Proportional
Differentiator
Filter
SLOW INT
20101
102
103
104
105
106
107
108
Fourier frequency [Hz]
Figure 4.3: Conceptual Bode plot showing action of the fast (red) and slow (blue) controllers. The corner frequencies and gain limits are adjusted with the front-panel knobs as labelled.
4.3 Optimisation
31
the measured noise.
The slow servo can then be optimised to minimise the over-reaction to external perturbations. Without the slow servo loop the high gain limit means that the fast servo will respond to external perturbations (e.g. acoustic coupling) and the resulting change in current can induce mode-hops in the laser. It is therefore preferable that these (low-frequency) fluctuations are compensated in the piezo instead.
Adjusting the SLOW GAIN and SLOW INT will not necessarily produce an improvement in the error signal spectrum, but when optimised will reduce the sensitivity to acoustic perturbations and prolong the lifetime of the lock.
Similarly, activating the double-integrator (DIP2) may improve stability by ensuring that the overall gain of the slow servo system is higher than the fast servo at these lower frequencies. However, this may cause the slow servo to overreact to low-frequency perturbations and the double-integrator is only recommended if long-term drifts in current are destabilising the lock.
32
Chapter 4. Application example: Pound-Drever Hall locking
A. Specifications
Parameter
Specification
Timing Gain bandwidth (-3 dB) Propagation delay External modulation bandwidth (-3 dB)
> 35 MHz < 40 ns
> 35 MHz
Input A IN, B IN SWEEP IN GAIN IN MOD IN LOCK IN
SMA, 1 M, ±2 5 V SMA, 1 M, 0 to +2 5 V SMA, 1 M, ±2 5 V SMA, 1 M, ±2 5 V 3.5 mm female audio connector, TTL
Analogue inputs are over-voltage protected up to ±10 V. TTL inputs take < 1 0 V as low, > 2 0 V as high. LOCK IN inputs are -0 5 V to 7 V, active low, drawing ±1 µA.
33
34
Appendix A. Specifications
Parameter
Output SLOW OUT FAST OUT MONITOR 1, 2 TRIG POWER A, B
Specification
SMA, 50 , 0 to +2 5 V, BW 20 kHz SMA, 50 , ±2 5 V, BW > 20 MHz SMA, 50 , BW > 20 MHz SMA, 1M , 0 to +5 V M8 female connector, ±12 V, 125 mA
All outputs are limited to ±5 V. 50 outputs 50 mA max (125 mW, +21 dBm).
Mechanical & power
IEC input
110 to 130V at 60Hz or 220 to 260V at 50Hz
Fuse
5x20mm anti-surge ceramic 230 V/0.25 A or 115 V/0.63 A
Dimensions
W×H×D = 250 × 79 × 292 mm
Weight
2 kg
Power usage
< 10 W
B. Troubleshooting
B.1 Laser frequency not scanning
A MOGLabs DLC with external piezo control signal requires that the external signal must cross 1.25 V. If you are sure your external control signal crosses 1.25 V confirm the following:
· DLC span is fully clockwise. · FREQUENCY on the DLC is zero (using the LCD display to set
Frequency). · DIP9 (External sweep) of the DLC is on. · DIP13 and DIP14 of the DLC are off. · The lock toggle switch on the DLC is set to SCAN. · SLOW OUT of the FSC is connected to the SWEEP / PZT MOD
input of the DLC. · SWEEP on the FSC is INT. · FSC span is fully clockwise. · Connect the FSC MONITOR 1 to an oscilloscope, set the MONI-
TOR 1 knob to RAMP and adjust FREQ OFFSET until the ramp is centred about 1.25 V.
If the above checks have not solved your problem, disconnect the FSC from the DLC and ensure that the laser scans when controlled with the DLC. Contact MOGLabs for assistance if not successful.
35
36
Appendix B. Troubleshooting
B.2 When using modulation input, the fast output floats to a large voltage
When using the MOD IN functionality of the FSC (DIP 4 enabled) the fast output will typically float to the positive voltage rail, around 4V. Ensure MOD IN is shorted when not in use.
B.3 Large positive error signals
In some applications, the error signal generated by the application may be strictly positive (or negative) and large. In this case the REF trimpot and ERR OFFSET may not provide sufficient DC shift to ensure the desired lockpoint coincides with 0 V. In this case both CH A and CH B can be used with the INPUT toggle set to , CH B set to PD and with a DC voltage applied to CH B to generate the offset needed to centre the lock point. As an example, if the error signal is between 0 V and 5 V and the lock point was 2.5 V, then connect the error signal to CH A and apply 2.5 V to CH B. With the appropriate setting the error signal will then be between -2 5 V to +2 5 V.
B.4 Fast output rails at ±0.625 V
For most MOGLabs ECDLs, a voltage swing of ±0.625 V on the fast output (corresponding to ±0.625 mA injected into the laser diode) is more than required for locking to an optical cavity. In some applications a larger range on the fast output is required. This limit can be increased by a simple resistor change. Please contact MOGLabs for more information if required.
B.5 Feedback needs to change sign
If the fast feedback polarity changes, it is typically because the laser has drifted into into a multi-mode state (two external cavity modes oscillating simultaneously). Adjust the laser current to obtain singlemode operation, rather than reversing the feedback polarity.
B.6 Monitor outputs wrong signal
37
B.6 Monitor outputs wrong signal
During factory testing, the output of each of the MONITOR knobs is verified. However, over time the set screws that hold the knob in position can relax and the knob may slip, causing the knob to indicate the wrong signal. To check:
· Connect the output of the MONITOR to an oscilloscope.
· Turn the SPAN knob fully clockwise.
· Turn the MONITOR to RAMP. You should now observe a ramping signal on the order of 1 volt; if you do not then the knob position is incorrect.
· Even if you do observe a ramping signal, the knob position may still be wrong, turn the knob one position more clockwise.
· You should now have a small signal near 0 V, and perhaps can see a small ramp on the oscilloscope on the order of tens of mV. Adjust the BIAS trimpot and you should see the amplitude of this ramp change.
· If the signal on the oscilloscope changes as you adjust the BIAS trimpot your MONITOR knob position is correct; if not, then the MONITOR knob position needs to be adjusted.
To correct the MONITOR knob position, the output signals must first be identified using a similar procedure to above, and the knob position can then be rotated by loosening the two set screws that hold the knob in place, with a 1.5 mm allen key or ball driver.
B.7 Laser undergoes slow mode hops
Slow mode hops can be caused by optical feedback from optical elements between the laser and the cavity, for example fibre couplers, or from the optical cavity itself. Symptoms include frequency
38
Appendix B. Troubleshooting
jumps of the free-running laser on slow timescales, of the order of 30 s where the laser frequency jumps by 10 to 100 MHz. Ensure the laser has sufficient optical isolation, installing another isolator if necessary, and block any beam paths that are unused.
C. PCB layout
C39
C59
R30
C76
C116
C166
C3
C2
P1
P2
C1
C9
C7
C6
C4
C5
P3
R1 C8 C10
R2
R338 D1
C378
R24
R337
R27
C15
R7
R28
R8
R66 R34
R340 C379
R33
R10
D4
R11 C60 R35
R342
R37
R343 D6
C380
R3 C16 R12
R4
C366 R58 R59 C31 R336
P4
R5 D8
C365 R347 R345
R49
R77 R40
R50 D3
C368 R344 R346
R75
C29 R15 R38 R47 R48
C62 R36 R46 C28
C11 C26
R339
R31 C23
C25
C54 C22 C24 R9
R74 C57
C33
C66 C40
U13
U3
U9
U10
U14
U4
U5
U6
U15
R80 R70 C27
C55 R42
C65 R32
R29 R65
R57 R78 R69
R71 R72
R79 R84
C67
R73
C68
C56
R76
R333
C42 C69
C367 R6
R334 C369
C13
R335
C43 C372 R14 R13
C373 C17
U1
R60 R17 R329
U16
R81 R82
C35
C362 R85 R331 C44 R87
C70
U25 C124
R180 C131
C140 R145
U42
R197 R184 C186 C185
MH2
C165 C194 C167 R186 R187 C183 C195 R200
C126 R325 R324
R168 C162 C184
C157 R148 R147
C163 C168
C158 R170
R95 C85 R166 R99 C84
C86
C75 R97 R96 C87
R83 C83
U26
U27 C92
R100 R101 R102 R106
R104 R105
C88 R98 R86
R341 C95 R107 C94
U38
C90 R109
R103 U28
C128 C89
C141
R140 R143
R108
U48
R146 C127
R185
U50 R326
U49
R332
R201
R191
R199 C202
R198 R190
C216
P8
U57
C221
C234
C222 R210 C217
C169 R192 R202
R195 C170
R171
U51
R203
R211
U58
C257
R213 C223 R212
R214 C203 C204 C205
C172 R194 C199
R327 C171 C160 R188 R172 R173
C93 R111 C96 C102 R144 R117
R110 R112
C98 C91
R115 R114
U31
C101
FB1
C148
FB2
C159
C109 C129
C149
C130
U29
C138
U32
C150
C112 R113
C100
C105 C99 C103 C152 C110
U33
C104 C111 C153
C133
R118 R124
R119 R122
R123
U34 R130 R120 R121
C161
C134
R169 U43
C132
C182 R157 C197
C189 R155 C201
C181 R156
C173
U56
C198 R193
C206
R189
C174
C196
U52
R196 R154 R151 R152 R153
R204 C187 C176 C179
U53
C180 C188 C190
C178
C200
C207
U54
C209
U55 C191
C192
C208 R205
U62 C210
R217 C177
C227 C241 C243 C242 R221
R223 C263
C232
C231
C225
U59
C226
C259
C237
C238
C240 C239
R206
U60
C261
R207 C260 R215
R218
R216
U61 C262
U66 R219
U68 R222
U67 R220
C258 C235 C236
C273
SW1
R225 R224
C266
C265
R228
U69
C269
R231 R229
U70
C270
U71
R234
C272
R226
U72
C71
C36
R16 R18
C14
C114
R131
C115
C58 R93
C46
C371
C370
R43 C45
R44
U11
R330 R92
R90 R89 R88 R91
R20
U7
R19
R39 C34
C72
R61
C73
C19
R45 C47
C41 C78
P5
R23
U8
R22
C375
C374 R41 R21
C37
C38
C30
C20
R52 C48 R51
C49
U2
C50
U17
U18
R55 R53 R62 R54
C63
R63 C52 R26
U12 R25
P6
C377 C376
R64 R56 C51
MH1
C53
C79
C74
C18
C113 R174 R175 R176 R177
C120
R128
R126 C106
R127 R125
U35 R132 U39
R141 C117 R129 R158
R142
C136 R134 R133 R138 R137
C135
C139 R161 R162 R163
C118
C119 R159
C121
U41 C137
R160 C147
C164
U40 C146
C193
R164 C123
C122
R139 R165
U44
C107
U45
C142
C144 R135 C145
R182
R178 R167
R181
RT1
C155 R149
C21 C12
U47
U46
U30 C108
U21 C77 U23 C82
U24 C64 U22 C81
U19 C61
R68 R67 U20 C32
P7
C97 R116
C80 R94
U36 C143
C151
R179
R150 C156
R183
R136 C154
C175
C252
C220
C228 C229 C230
U63
C248
C247
C211
C212 C213 C214
U64
C251
C250
C215
C219
R208 R209 C224
C218 C253
U65
C256
C255 C254
C249 C233
C246 C245
C274
C244
C264
C268 R230
C276
C271
C267
C275
R238 R237 R236 R235 R240 R239
R328
REF1 R257
C285 R246
C286 C284
R242
U73
R247
C281 R243
C280
U74
C287
R248
C289 R251 R252
R233 R227 R232
C282 R244 R245
U75
R269
C288 R250 R249
R253 R255
C290
R241
R254
U76
R272
C291
R256
U77
C294 C296
C283
C277
MH5
C292
C293
C279 C278
U37 C125
MH3
C295
C307 R265
Q1
C309
C303 R267 R268
C305
C301
MH6
R282
C312
R274 R283 R284
C322
C298
C300
R264 C297 R262
U78
R273 C311
C299
R263
C302
R261 R258 R259 R260
U79
C306
U80
C315
C313
R266
U81
R278 R275 R276
C304
R277
C316
R271 C308
R270
U82
C314
C318
U83
R280 R279 C321
C310
U84
R285 C317
C320
R281
C319
R290 R291
D11
D12
D13
D14
R287 R286
SW2
R297 R296
R289 R288
C334 C328 C364
R299 C330
R293 R292
C324
C331
R300
R298 C329
C333 C332
U85
C335
C323
C325
D15
R303
D16
C336
R301 R302 C342 C341
C337
U86
C343
C339
C346
R310 R307
R309
R308
MH8
C347 R305 R306
R315
R321
C345
P10
C344 C348
MH9
C349 R318 C350 R319 R317 R316
C352
P11
C351
C354
U87
MH10
C353
U88
C338
C340
R294
C363
MH4 P9
XF1
C358
R295
C326
C327
D17
R304
D18
U89
C355 C356
U91
U90
C361 R323
C357
C359
P12
C360
MH7
R313 R314 R320 R311 R312 R322
39
40
Appendix C. PCB layout
D. 115/230 V conversion
D.1 Fuse
The fuse is a ceramic antisurge, 0.25A (230V) or 0.63A (115V), 5x20mm, for example Littlefuse 0215.250MXP or 0215.630MXP. The fuse holder is a red cartridge just above the IEC power inlet and main switch on the rear of the unit (Fig. D.1).
Figure D.1: Fuse catridge, showing fuse placement for operation at 230 V.
D.2 120/240 V conversion
The controller can be powered from AC at 50 to 60 Hz, 110 to 120 V (100 V in Japan), or 220 to 240 V. To convert between 115 V and 230 V, the fuse cartridge should be removed, and re-inserted such that the correct voltage shows through the cover window and the correct fuse (as above) is installed.
41
42
Appendix D. 115/230 V conversion
Figure D.2: To change fuse or voltage, open the fuse cartridge cover with a screwdriver inserted into a small slot at the left edge of the cover, just to the left of the red voltage indicator.
When removing the fuse catridge, insert a screwdriver into the recess at the left of the cartridge; do not try to extract using a screwdriver at the sides of the fuseholder (see figures).
WRONG!
CORRECT
Figure D.3: To extract the fuse cartridge, insert a screwdriver into a recess at the left of the cartridge.
When changing the voltage, the fuse and a bridging clip must be swapped from one side to the other, so that the bridging clip is always on the bottom and the fuse always on the top; see figures below.
D.2 120/240 V conversion
43
Figure D.4: 230 V bridge (left) and fuse (right). Swap the bridge and fuse when changing voltage, so that the fuse remains uppermost when inserted.
Figure D.5: 115 V bridge (left) and fuse (right).
44
Appendix D. 115/230 V conversion
Bibliography
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[2] Boris Lurie and Paul Enright. Classical Feedback Control: With MATLAB® and Simulink®. CRC Press, 2011. 1
[3] Richard W. Fox, Chris W. Oates, and Leo W. Hollberg. Stabilizing diode lasers to high-finesse cavities. Experimental methods in the physical sciences, 40:146, 2003. 1
[4] R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward. Laser phase and frequency stabilization using an optical resonator. Appl. Phys. B, 31:97 105, 1983. 1
[5] T. W. Ha¨nsch and B. Couillaud. Laser frequency stabilization by polarization spectroscopy of a reflecting reference cavity. Optics communications, 35(3):441444, 1980. 1
[6] M. Zhu and J. L. Hall. Stabilization of optical phase/frequency of a laser system: application to a commercial dye laser with an external stabilizer. J. Opt. Soc. Am. B, 10:802, 1993. 1
[7] G. C. Bjorklund. Frequency-modulation spectroscopy: a new method for measuring weak absorptions and dispersions. Opt. Lett., 5:15, 1980. 1
[8] Joshua S Torrance, Ben M Sparkes, Lincoln D Turner, and Robert E Scholten. Sub-kilohertz laser linewidth narrowing using polarization spectroscopy. Optics express, 24(11):11396 11406, 2016. 1
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[10] W. Demtr¨oder. Laser Spectroscopy, Basic Concepts and Instrumentation. Springer, Berlin, 2e edition, 1996. 1
[11] L. D. Turner, K. P. Weber, C. J. Hawthorn, and R. E. Scholten. Frequency noise characterization of narrow linewith diode lasers. Opt. Communic., 201:391, 2002. 29
46
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