MOGlabs DLC202 User manual

External Cavity Diode Laser Controller
DLC102, DLC202, DLC252, DLC502
Revision 9.09

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) 2014. 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
18 Boase St
Brunswick VIC 3056
AUSTRALIA
[phone]
[email protected]
www.moglabs.com
MOGLabs USA LLC
419 14th St
Huntingdon PA 16652
USA
[phone]
[email protected]
www.moglabsusa.com
MOGLabs Europe
Goethepark 9
10627 Berlin
Germany
[phone]
[email protected]

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.
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.
MOGLabs www.moglabs.com
i

ii

Safety Precautions
Safe and effective use of this product is very important. Please read
the following safety information before attempting to operate your
laser. Also 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
instrumentation.
CAUTION – USE OF CONTROLS OR ADJUSTMENTS OR
PERFORMANCE OF PROCEDURES OTHER THAN THOSE
SPECIFIED HEREIN MAY RESULT IN HAZARDOUS
RADIATION EXPOSURE
Laser output can be dangerous. Please ensure that you implement
the appropriate hazard minimisations for your environment, such as
laser safety goggles, beam blocks, and door interlocks. MOGLabs
takes no responsibility for safe configuration and use of your laser.
Please:
•Avoid direct exposure to the beam.
•Avoid looking directly into the beam.
•Note the safety labels and heed their warnings.
•When the laser is switched on, there will be a short delay of
two seconds before the emission of laser radiation, mandated
by European laser safety regulations (IEC 60825-1).
•The STANDBY/RUN keyswitch must be turned to RUN before
the laser can be switched on. The laser will not operate if
the keyswitch is in the STANDBY position. The key cannot be
iii

iv
removed from the controller when it is in the clockwise (RUN)
position.
•To completely shut off power to the unit, turn the keyswitch
anti-clockwise (STANDBY position), switch the mains power
switch at rear of unit to OFF, and unplug the unit.
•When the STANDBY/RUN keyswitch is on STANDBY, there can-
not be power to the laser diode, but power is still being sup-
plied to the laser head for temperature control.
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.
CAUTION To ensure correct cooling airflow, the unit should not be oper-
ated with cover removed.
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 cover removed.
NOTE The MOGLabs DLC is designed for use in scientific research
laboratories. It should not be used for consumer or medical
applications.

Protection Features
The MOGLabs DLC includes a number of features to protect you and
your laser.
Softstart A time delay (3 s) followed by linearly ramping the diode cur-
rent (3 s max).
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.
v

vi
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.
Emission indicator The MOGLabs controller will illuminate the emission warn-
ing indicator LED immediately when the laser is switched on.
There will then be a delay of at least 2 seconds before actual
laser emission.
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. The key cannot be removed
from the controller when it is in the clockwise (RUN) position.
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.

Contents
Preface i
Safety Precautions iii
Protection Features v
1 Introduction 1
1.1 Basicoperation...................... 1
1.2 Passive frequency control . . . . . . . . . . . . . . . . 2
1.3 DC locking to an atomic transition . . . . . . . . . . . 4
1.4 AC locking to an atomic transition . . . . . . . . . . . 5
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 . . . . . . . . . . . 15
2.5 Feedback configurations . . . . . . . . . . . . . . . . . 20
2.6 Digitalcontrol....................... 23
2.7 Internal trimpots . . . . . . . . . . . . . . . . . . . . . 24
3 Operation 25
3.1 Simplest configuration . . . . . . . . . . . . . . . . . . 25
3.2 Laser frequency control . . . . . . . . . . . . . . . . . 26
3.3 External scan control . . . . . . . . . . . . . . . . . . . 27
3.4 Locking to an atomic transition: DC .......... 28
3.5 Locking to an atomic transition: AC .......... 31
3.6 Externalsweep ...................... 34
3.7 Locking using an external signal . . . . . . . . . . . . 34
vii

viii Contents
3.8 External control of lock frequency setpoint . . . . . . 36
4 Optimisation 37
4.1 Frequency reference . . . . . . . . . . . . . . . . . . . 37
4.2 Noisespectra....................... 39
A Specifications 41
A.1 RF response ........................ 45
A.2 Sweep saturation and trigger . . . . . . . . . . . . . . 45
B Troubleshooting 47
B.1 STANDBY/RUN indicator ................. 47
B.2 Diode OFF/ON indicator................. 48
B.3 250 kHz modulation . . . . . . . . . . . . . . . . . . . 49
B.4 Locking........................... 51
C Using DBR/DFB diodes 53
C.1 Fine current control . . . . . . . . . . . . . . . . . . . 53
C.2 DC current feedback . . . . . . . . . . . . . . . . . . . 53
C.3 Slow current feedback . . . . . . . . . . . . . . . . . . 54
C.4 Locksaturation ...................... 54
C.5 Specialoptions...................... 54
D Modulation coils 55
D.1 Field requirements . . . . . . . . . . . . . . . . . . . . 55
D.2 Coilimpedance ...................... 56
D.3 Impedance matching . . . . . . . . . . . . . . . . . . . 57
D.4 Tuning ........................... 58
D.5 Shielding ......................... 59
E External modulators and injection current modulation 61
E.1 Coupling circuit . . . . . . . . . . . . . . . . . . . . . . 61
E.2 Injection current modulation . . . . . . . . . . . . . . . 62
F Photodetector 65
F.1 Photodiodes........................ 66
G Laser head board 67
G.1 Headboard connectors . . . . . . . . . . . . . . . . . . 67
G.2 Dual piezo operation . . . . . . . . . . . . . . . . . . . 68

Contents ix
G.3 RFcoupling........................ 69
H Feedback overview 71
I Connector pinouts 75
I.1 Laser............................ 75
I.2 Photodetector....................... 76
I.3 Interlock .......................... 76
I.4 Digitalcontrol....................... 77
J PCB layout 79
K 115/230 V conversion 81
K.1 Fuse ............................ 81
K.2 120/240 V conversion . . . . . . . . . . . . . . . . . . . 81
References 86

xContents

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 Basic operation
In the simplest configuration, the MOGLabs DLC will be used to con-
trol the diode injection current, and temperature. All connections
are via a single cable to the MOGLabs laser. If using with a non-
MOGLabs laser, please see appendix G for information on connect-
ing the diode, thermoelectric Peltier cooler (TEC), and temperature
sensor via the laser head interface board which is provided. For
operation with DBR/DFB diodes, please see appendix C.
The front-panel display and selector switch can be used to monitor
the diode current, current limit, diode dropout voltage, temperature,
temperature setpoint, and TEC current; see figure 1.1.
tnerruC
doM
pmeT
rorrE
tupnI
Am
A
V
C°
FFO
DOM
NURYBDNATS
TT
E
S
NACS
YCNEUQERF NAPS
TUPNI RORRE
TESFFO
KCABDEEF
ROTINOM
ESAHP
B LENNAHC A
NIAG TSAFWOLS
EDOID
TNERRUC
FFO
KCOL
FFO
tnerruC
xam rruC
egatloV
tes pmeT erutarepmeT
tnerrucCET
TEC voltage
ycneuqerF
NO
NACS
KCOL
rellortnoC resaL edoiD
S
A
IB
qerF
wolS
tsaF
retliF
tupnI
e
Figure 1.1: MOGLabs DLC front panel layout.
1

2Chapter 1. Introduction
0V
0V
120V
time
TRIG
STACK
5V
FREQUENCY SPANSPAN
Figure 1.2: 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.
1.2 Passive frequency control
The MOGLabs DLC controls the laser frequency via the diode current,
and piezo electric actuators to control the cavity length of an ECDL.
In normal (SCAN) mode, a sawtooth is supplied to the main (STACK)
actuator to linearly sweep the laser frequency at a rate determined
by the rear-panel trimpot, fsweep, from 4 to 70 sweeps per second;
see figure 1.2.
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
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.
Figure 1.3 is an example of what is seen on the oscilloscope in
a simple scanning configuration. The laser beam transmitted by an
atomic vapour cell is detected on the photodetector provided with the
controller, as the laser frequency sweeps through atomic resonances,
thus showing the atomic absorption spectrum.
The FREQUENCY knob controls the offset to the piezo-electric actu-

1.2 Passive frequency control 3
C1
C2
Ch1 100mV Ch2 100mV 5.0ms
Figure 1.3: A simple absorption spectrum of rubidium with the controller
in simple frequency scanning mode.
ator (STACK) and thus the mid-point frequency of the sweep. As the
external cavity frequency changes, the laser may “mode-hop” due
to competition between the external cavity and the internal cavity
defined by the rear and front facets of the diode saemiconductor
chip itself. The internal frequency of the diode can be adjusted
by changing the diode current, either manually as the FREQUENCY
offset is adjusted when modehops are observed. The current can
also be automatically biased during the frequency sweep, if BIAS is
enabled via the internal DIP switch 4. Note that adjusting the fre-
quency offset (FREQUENCY knob) will affect the diode current if BIAS
is enabled, but it may still be necessary to adjust the diode current
as FREQUENCY is adjusted, to avoid modehops.
The extent of the frequency sweep is controlled with the SPAN con-
trol. The maximum range is typically 10 −100 GHz. Depending on
the offset, the span may be limited by the minimum and maximum
voltage that can be applied to the actuator, as described in detail
in section A.2.

4Chapter 1. Introduction
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
configuration of §1.2 is extended with the MOGLabs DLC photode-
tector (see appendix F), and an atomic vapour absorption cell. Al-
ternately, a Fabry-Perot optical cavity or other reference could be
used.
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/4 a quarter-wave
retarder.
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
and ERROR 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 5
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 D.
Figures 1.5, 3.5, 3.6 show examples of AC locking arrangements,
using a coil to Zeeman-modulate the atomic reference, or an acousto-
optic modulator (AOM) for modulating the frequency of the beam
passing through the vapour cell. If preferred, the modulator oscillator
can be set to dither the diode current (see §2.4). Feedback can again
be via one or both piezo actuators, the diode current, or all three.
BS
PD
250kHz
MM
BS
Lock-inECDL
BS Servo
Vapour cell + coil
AOM
λ/4 λ/4
f ~ 150mm
f ~ −25mm
Figure 1.5: Setup for AC locking to an atomic transition. PD DLC pho-
todetector, BS beamsplitter, M mirror, λ/4 quarter-wave retarder. See also
Figs. 3.5, 3.6.

6Chapter 1. Introduction

2. Connections and controls
2.1 Front panel controls
tnerruC
doM
pmeT
rorrE
tupnI
Am
A
V
C°
FFO
DOM
NURYBDNATS
TT
E
S
NACS
YCNEUQERF NAPS
TUPNI RORRE
TESFFO
KCABDEEF
ROTINOM
ESAHP
B LENNAHC A
NIAG TSAFWOLS
EDOID
TNERRUC
FFO
KCOL
FFO
tnerruC
xam rruC
egatloV
tes pmeT erutarepmeT
tnerrucCET
TEC voltage
ycneuqerF
NO
NACS
KCOL
rellortnoC resaL edoiD
S
A
IB
qerF
wolS
tsaF
retliF
tupnI
e
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 100/200/250/500 mA (DLC102 to DLC502).
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 (or 150 V; see LK2, p. 15). For
DFB/DBR diodes, the frequency control feedback signal can control
the diode current rather than the stack; see §2.4, DIP switch 16.
Note The FREQUENCY control will also affect the diode current, if BIAS
(DIP switch 4) is enabled.
SPAN Frequency scan range, from 0 to 120 V (or 150 V; see LK2, p. 15).
The span may be limited by the minimum and maximum voltage that
can be applied to the actuator; 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 ±25 mA
for the full frequency span.