MOGlabs DLC202 User manual

External Cavity Diode Laser Controller
Models DLC-202, DLC-252, DLC-502
Revision 6.00

Limitation of Liability
MOG Laboratories Pty Ltd (MOGlabs) does not assume any liabil-
ity arising out of the use of the information contained within this
manual. This document may contain or reference information and
products protected by copyrights or patents and does not convey
any license under the patent rights of MOGlabs, nor the rights of
others. MOGlabs will not be liable for any defect in hardware or
software or loss or inadequacy of data of any kind, or for any direct,
indirect, incidental, or consequential damages in connections with
or arising out of the performance or use of any of its products. The
foregoing limitation of liability shall be equally applicable to any
service provided by MOGlabs.
Copyright
Copyright c MOG Laboratories Pty Ltd (MOGlabs) 2010. 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 Pty Ltd
420 Victoria St
Brunswick VIC 3056
AUSTRALIA
Tel: [phone]
Fax: [phone]
Email: [email protected]
Web: www.moglabs.com

Preface
Diode lasers can be wonderful things: they are efficient, compact,
low cost, high power, low noise, tunable, and cover a large range
of wavelengths. They can also be obstreperous, sensitive, and tem-
peramental, particularly external cavity diode lasers (ECDLs). The
mechanics and optics needed to turn a simple $10 120 mW AlGaAs
diode laser into a research-quality narrow-linewidth tunable laser
are fairly straightforward [1, 2, 3, 4], but the electronics is demanding
– and, until now, not available commercially from a single supplier,
let alone in a single unit.
The MOGlabs range of ECDL controllers change that. With each DLC
unit, we provide everything you need to run your ECDL, and lock it
to an atomic transition. In addition to current and temperature con-
trollers, we provide piezo drivers, sweep ramp generator, modulator
for AC locking, lock-in amplifier, feedback servo system, laser-head
electronics protection board, even a high-speed low-noise balanced
photodetector.
We would like to thank the many people that have contributed their
hard work, ideas, and inspiration, especially Lincoln Turner, Karl
Weber, and Jamie White, as well as those involved in previous con-
troller designs, in particular Mirek Walkiewicz and Phillip Fox.
We hope that you enjoy using the DLC as much as we do. Please let
us know if you have any suggestions for improvement in the DLC or
in this document, so that we can make life in the laser lab easier for
all, and check our website from time to time for updated information.
Robert Scholten and Alex Slavec
MOGlabs, Melbourne, Australia
www.moglabs.com
i

ii

Safety Precautions
Please note several specific and unusual cautionary notes before
using the MOGlabs DLC, in addition to the safety precautions that
are standard for any electronic equipment or for laser-related in-
strumentation.
WARNING The rear-panel connector for the laser is similar to standard
DVI (Digital Video Interface) plugs as used for consumer digi-
tal display devices. The pins on this connector can be at high
potential (up to 150 V). These can be hazardous to life and
should be protected by connection of the correct cable to the
laser. Under no circumstances should a standard DVI device
such as an LCD display be connected to this socket!
CAUTION Please ensure that the unit is configured for the correct voltage
for your AC mains supply before connecting. The supply must
include a good ground connection.
WARNING The internal circuit boards and many of the mounted compo-
nents are at high voltage, with exposed conductors, in partic-
ular the high-voltage piezo driver circuitry. The unit should
not be operated with covers removed.
WARNING If using a Zeeman coil modulator as described in appendix C,
the secondary potential can easily be hundreds of volts. Please
ensure that your coil and balance capacitor do not have ex-
posed connections, and that all components have sufficient
voltage rating.
NOTE The MOGlabs DLC is designed for use in scientific research
laboratories. It should not be used for consumer or medical
applications.
iii

Protection Features
The MOGlabs DLC includes a number of features to protect you and
your laser.
Softstart A time delay followed by linearly ramping the diode current;
total of 2.5 s.
Circuit shutdown Many areas of the circuitry are powered down when not in use.
The high voltage supply and piezo drivers, the diode current
supplies, the coil driver, and others are without power when
the unit is in standby mode, if an interlock is open, or a fault
condition is detected.
Current limit Sets a maximum possible diode injection current, for all op-
erating modes. Note that current supplied through the RF
connector on the laser headboard is not limited.
Cable continuity If the laser is disconnected, the system will switch to standby
and disable all laser and piezo power supplies. If the laser
diode, TEC or temperature sensor fail and become open-circuit,
they will be disabled accordingly.
Short circuit If the laser diode, TEC or temperature sensor fail and become
short-circuit, or if the TEC polarity is reversed, they will be
disabled accordingly.
Temperature If the detected temperature is below −5◦C or above 35◦C, the
temperature controller is disabled.
Internal supplies If any of the internal DC power supplies (+5,±10,±12 V) is
1 V or more below its nominal value, the respective components
(temperature controller, diode current supply) are disabled.
iv

v
Protection relay When the power is off, or if the laser is off, the laser diode
is shorted via a normally-closed solid-state relay at the laser
head board.
Laser LED Bright white LED illuminates when laser is switched on.
Mains filter Protection against mains transients.
Key-operated The laser cannot be powered unless the key-operated STANDBY
switch is in the RUN position, to enable protection against
unauthorised or accidental use.
Interlocks Both the main unit and the laser head board have interlocks,
to allow disabling of the laser via a remote switch, or a switch
on the laser cover.

RoHS Certification of
Conformance
MOG Laboratories Pty Ltd certifies that the MOGlabs Diode Laser
Controller (Revision 3) is RoHS-5 compliant. MOG Laboratories
notes, however, that the product does not fall under the scope defined
in RoHS Directive 2002/95/EC, and is not subject to compliance, in
accordance with DIRECTIVE 2002/95/EC Out of Scope; Electron-
ics related; Intended application is for Monitoring and Control or
Medical Instrumentation.
MOG Laboratories Pty Ltd makes no claims or inferences of the
compliance status of its products if used other than for their intended
purpose.
vi

Contents
Preface i
Safety Precautions iii
Protection Features iv
RoHS Certification of Conformance vi
1 Introduction 1
1.1 Simplest configuration . . . . . . . . . . . . . . . . . . 1
1.2 Passive frequency control . . . . . . . . . . . . . . . . 2
1.3 DC locking to an atomic transition . . . . . . . . . . . 3
1.4 AC locking to an atomic transition . . . . . . . . . . . 4
2 Connections and controls 7
2.1 Front panel controls . . . . . . . . . . . . . . . . . . . 7
2.2 Front panel display/monitor . . . . . . . . . . . . . . . 10
2.3 Rear panel controls and connections . . . . . . . . . . 12
2.4 Internal switches and adjustments . . . . . . . . . . . 14
3 Operation 21
3.1 Simplest configuration . . . . . . . . . . . . . . . . . . 21
3.2 Laser frequency control . . . . . . . . . . . . . . . . . 22
3.3 External scan control . . . . . . . . . . . . . . . . . . . 24
3.4 Locking to an atomic transition: DC .......... 24
3.5 Locking to an atomic transition: AC .......... 27
3.6 Locking using an external signal . . . . . . . . . . . . 30
3.7 External control of lock frequency setpoint . . . . . . 31
4 Optimisation 33
4.1 Frequency reference . . . . . . . . . . . . . . . . . . . 33
vii

viii Contents
4.2 Noisespectra....................... 35
A Specifications 37
A.1 RF response ........................ 41
A.2 Sweep saturation and trigger . . . . . . . . . . . . . . 41
B Troubleshooting 43
B.1 STANDBY/RUN indicator ................. 43
B.2 Diode OFF/ON indicator................. 44
B.3 Locking........................... 45
C Modulation coils 47
C.1 Field requirements . . . . . . . . . . . . . . . . . . . . 47
C.2 Coilimpedance ...................... 48
C.3 Impedance matching . . . . . . . . . . . . . . . . . . . 49
C.4 Tuning ........................... 50
C.5 Shielding ......................... 51
D External modulators and injection current modulation 53
D.1 Coupling circuit . . . . . . . . . . . . . . . . . . . . . . 53
D.2 Injection current modulation . . . . . . . . . . . . . . . 54
E Photodetector 57
E.1 Photodiodes........................ 58
F Laser head board 59
F.1 Headboard connectors . . . . . . . . . . . . . . . . . . 59
F.2 Grating pivot point compensation . . . . . . . . . . . 60
F.3 RFcoupling........................ 61
G Connector pinouts 63
G.1 Photodetector....................... 63
G.2 Laser............................ 63
G.3 Interlock .......................... 64
G.4 Digitalcontrol....................... 65
H PCB layout 67
References 70

1. Introduction
The MOGlabs DLC can be used in various configurations, including
simple current/temperature controller, passive frequency controller
with internal or external sweep/scan, and as a complete system for
active frequency stabilisation with AC,DC or external locking signal.
Here is a quick outline of some modes of operation, so that you
can connect and go as quickly as possible. Details are provided in
chapter 3.
1.1 Simplest configuration
Figure 1.1: The MOGlabs DLC is read-
ily connected to a laser diode, temperature
sensor and thermo-electric cooler via the
provided laser head board.
In the simplest applica-
tion, the DLC will con-
trol the diode current and
temperature. Thus the
DLC must be connected to
the diode, a thermoelectric
Peltier cooler (TEC), and a
temperature sensor.
All connections between
the MOGlabs DLC and the
laser head are via a single
cable. An interface break-
out board, located as close
as possible to the laser it-
self, includes protection relay and passive protection filters, a laser-
on indicator, and MOLEX connectors for the diode, TEC and sensor
(10 k NTC thermistor, AD590, or AD592). See appendix F for details.
1

2Chapter 1. Introduction
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Figure 1.2: MOGlabs DLC front panel layout.
The front-panel display selector switch can be used to monitor the
diode current, current limit, diode dropout voltage, temperature and
temperature setpoint, and TEC current; see figure 1.2.
1.2 Passive frequency control
The MOGlabs DLC can control the laser frequency via the diode cur-
rent, or with high voltage (120 V) outputs for piezo electric actuators,
to control the cavity length of an ECDL. The actuators should be con-
nected to the laser head board via the provided MOLEX connectors.
One or two piezo elements can be controlled. Typically, only a
single “stack” actuator, such as the Tokin AE0203D04 (available from
Thorlabs, www.thorlabs.com), will be required. The single stack
actuator allows frequency scanning and frequency offset selection,
and active slow feedback (up to ≈100 Hz). A second piezo actuator,
typically a disc, can be added for faster active feedback control (see
below).
In normal (SCAN) mode, a sawtooth is supplied to the stack (or diode
current bias), at frequency of 4 to 70 Hz; for example as in figure
1.3. At the midpoint of the sweep, a trigger (low to high) pulse is
output via the rear panel TRIG connection, for synchronising to an
oscilloscope or external experiment.
Critical DLC signals can be monitored using the CHANNEL A and
CHANNEL B outputs on the rear panel, synchronised to the TRIG
trigger output, which should be connected to the equivalent inputs

1.3 DC locking to an atomic transition 3
0V
0V
120V
time
TRIG
STACK
5V
FREQUENCY SPANSPAN
Figure 1.3: Stack (or current bias) output and trigger pulse, when scan-
ning. Note that the ramp slope can be inverted. Details of the ramp
behaviour are described in section A.2.
on a two-channel oscilloscope. The particular signals are selected
from the front-panel CHAN A and CHAN B selector switches. The
signals are described in detail in the following chapter.
1.3 DC locking to an atomic transition
Figure 1.4 shows one possible configuration in which a MOGlabs DLC
is used to lock an ECDL to an atomic transition. Locking is to the side
of an absorption peak in a vapour cell; see for example Demtr¨ oder
[5] for more information on spectroscopy. The passive configura-
tion of §1.2 is extended with the MOGlabs DLC photodetector (see
appendix E), and an atomic vapour absorption cell. Alternately, a
Fabry-Perot optical cavity or other reference could be used.
The schematic shows a saturated absorption spectroscopy arrange-
ment, but often simply locking to the side of a Doppler-broadened
absorption peak will be adequate. The photodetector can be used
in single channel mode (default) or with balanced differential in-
puts, for example to subtract a Doppler background from a saturated
absorption spectrum.
The lock frequency is determined by the zero-crossing point of the
photosignal. The photosignal offset is adjusted via the INPUT OFFSET

4Chapter 1. Introduction
BS
PD
MM
BS
ECDL
BS Servo
Vapour cell
Offsets
λ/4 λ/4
Figure 1.4: Schematic setup for DC locking to an atomic transition. PD
is the DLC photodetector. BS beamsplitter, M mirror, λ/4a quarter-wave
retarder.
and LOCK OFFSET controls. Feedback can be via one or both piezo
actuators, or the diode injection current, or all three.
1.4 AC locking to an atomic transition
With AC locking (FM demodulation or “lock-in amplifier” detection),
the laser frequency can be locked to a peak centre. The AC ap-
proach offers the advantage of inherently lower detected noise and
thus the potential for improved laser frequency stability. The setup
is similar to that for DC locking, but modulation of the laser fre-
quency, or the reference frequency, is required. The MOGlabs DLC
provides an internal 250 kHz oscillator which can directly dither the
diode current, or drive an external modulator. In particular, it is
designed to drive a Zeeman-shift modulation coil surrounding the
atomic reference vapour cell; see appendix C.
Figure 1.5 shows a simplified AC locking setup, using a coil to
Zeeman-modulate the atomic reference, or an acousto-optic mod-
ulator (AOM) for modulating the frequency of the beam through the
vapour cell. If preferred, the modulator oscillator can be set to dither

1.4 AC locking to an atomic transition 5
BS
PD
250kHz
MM
BS
Lock-inECDL
BS Servo
Vapour cell + coil
AOM
λ/4 λ/4
Figure 1.5: Schematic setup for AC locking to an atomic transition. PD
is the DLC photodetector. BS beamsplitter, M mirror, λ/4a quarter-wave
retarder.
the diode current (see §2.4). Feedback can again be via one or both
piezo actuators, the diode current, or all three.

6Chapter 1. Introduction

2. Connections and controls
2.1 Front panel controls
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STANDBY/RUN In STANDBY mode, the DLC maintains the laser temperature, but
powers down all other components including the high-voltage piezo
power, and the main on-board low-voltage power.
In RUN mode, the DLC activates all circuits, including the laser cur-
rent driver and piezo drivers. The diode current is disabled, and the
STACK is on but not scanning, until the laser enable switch is ON.
On first power-up, the STANDBY indicator will be red; this is normal
and indicates there has been a power failure since last switched
to RUN. The unit should then be set to RUN to initiate temperature
control, and back to STANDBY if further operation is not desired.
If the unit fails to switch to RUN mode (indicator does not show
green), see appendix B.
OFF/ON Diode injection current enable. Also activates the STACK ramp and
current bias (if DIP switch 4in ON). The STANDBY/RUN key switch
must first be on RUN and the associated indicator must be green.
If the unit fails to switch to RUN mode (indicator does not show
green), see appendix B.
7

8Chapter 2. Connections and controls
CURRENT Diode injection current, 0 to 200 mA (DLC-202) or 500 mA (DLC-502).
The response is not linear; that is, the change in current varies for
a given rotation of the knob. The mid-range sensitivity is reduced
to allow greater precision at normal operating currents.
FREQUENCY The laser frequency will normally be controlled via a multilayer
piezo-electric actuator (STACK). This knob controls the offset voltage
applied to that actuator, 0 to 120 V. If BIAS is enabled (DIP switch 4),
the diode current will also be affected by the FREQUENCY setting.
For DFB/DBR diodes, the lock feedback signal to the piezo actuator
can instead control the diode current; see §2.4, DIP switch 16.
SPAN Frequency scan range, from 0 to 120 V. The span may be limited
by the minimum and maximum voltage that can be applied to the
actuator, 0 and 120 V; see detailed description in section A.2.
PHASE When AC locking, the controller demodulates the error signal from
the detected light intensity. PHASE adjusts the relative phase be-
tween the internal reference modulator and the detected signal, from
0 to 360◦. When DC locking, the sign of the error signal can be
flipped by rotating the PHASE control.
GAIN Overall error signal gain, 0 to 40 dB.
SLOW Gain for feedback to the slow (piezo) actuator, 0 to 40 dB.
FAST Gain for fast feedback to the diode current, 0 to 40 dB.
Tset Temperature set point, 0 – 30◦standard; extended range optional.
BIAS Feed-forward bias current. If DIP switch 4is ON, changes in laser fre-
quency, usually via the STACK actuator, will simultaneously change
the current. This trimpot controls the slope dI/df of current with
frequency. It can be positive or negative, with a range of ±10 mA
for the full frequency span.
INPUT OFFSET Offset of input light intensity signal, 0 to −10 V. This can be ad-
justed to bring the photodetector light signal close to zero on the
oscilloscope, and to shift the zero frequency lockpoint for DC locking.

2.1 Front panel controls 9
OFF/MOD Modulator enable, to switch on the coil driver, diode current dither,
or external modulator.
LOCK OFFSET Offset of the frequency error lock signal. The DLC will lock such
that the error signal plus LOCK OFFSET is zero, allowing for small
adjustment of the lock frequency.
SCAN/LOCK Switch between scanning mode and lock mode. When switching
from scan to lock, the controller will first reset the scanning actuator
(usually STACK) to the offset voltage at the trigger point, and then
lock to the nearest frequency at which the error signal is zero.
+/−Sign of fast (current) feedback. The sign of the slow feedback can
be changed with the PHASE control, for both AC and DC locking.
OFF/LOCK Enable fast (current) feedback. The laser can be locked with slow
(piezo) locking or fast (current) locking alone. Best performance is
usually obtained with both channels of feedback; see chapter 4 for
feedback optimisation.

10 Chapter 2. Connections and controls
2.2 Front panel display/monitor
Display selector
The MOGlabs DLC includes a high-precision 4.5 digit LED display
with four unit annunciators and 8-channel selector switch.
Current Actual diode current (mA)
Curr max Current limit (mA)
(−)sign indicates limit rather than actual current
Voltage Diode voltage (V)
Temp set Temperature set point (◦C)
Temperature Actual temperature (◦C)
TEC current Current to thermoelectric (Peltier) cooler (A)
TEC voltage Voltage on thermoelectric (Peltier) cooler (V)
Frequency Frequency actuator offset, usually slow piezo
(normalised to a range of ±1)
CHAN A
Several important signals can also be monitored externally with an
oscilloscope via the rear connectors CHANNEL A,CHANNEL B and
TRIG. The outputs to these can be selected with the CHAN A and
CHAN B selectors.
Input Photodetector [30 mV/µW]
Filter Filtered photodetector, 40 kHz low pass
Freq Frequency-scanning actuator (STACK) [1 V/48 V]
Slow Slow feedback STACK [1 V/0.24 V] DISC [1 V/4.8 V]
Fast Current feedback [1 V/100 µA]