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MOGlabs DLC-202 Manual

External cavity diode laser controller
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External Cavity Diode Laser Controller
Models DLC-202, DLC-252, DLC-502
Revision 6.00

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  • Page 1 External Cavity Diode Laser Controller Models DLC-202, DLC-252, DLC-502 Revision 6.00...
  • Page 2 Contact For further information, please contact: MOG Laboratories Pty Ltd 420 Victoria St Brunswick VIC 3056 AUSTRALIA Tel: +61 3 9940 1427 Fax: +61 3 9381 0700 Email: info@moglabs.com Web: www.moglabs.com...
  • Page 3 Robert Scholten and Alex Slavec labs, Melbourne, Australia www.moglabs.com...

  • Page 5: Safety Precautions

    Safety Precautions Please note several specific and unusual cautionary notes before using the labs , 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.

  • Page 6: Protection Features

    Protection Features labs 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...
  • Page 7 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 Bright white 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...

  • Page 8: Rohs Certification Of Conformance

    RoHS Certification of Conformance MOG Laboratories Pty Ltd certifies that the labs 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;...

  • Page 9: Table Of Contents

    Contents Preface Safety Precautions Protection Features RoHS Certification of Conformance 1 Introduction Simplest configuration ....Passive frequency control ....locking to an atomic transition .
  • Page 10 viii Contents Noise spectra ..... . . A Specifications response ......A.2 Sweep saturation and trigger .

  • Page 11: Introduction

    Peltier cooler ( ), and a temperature sensor. All connections between Figure 1.1: The MOGlabs is read- labs and the ily connected to a laser diode, temperature sensor and thermo-electric cooler via the laser head are via a single provided laser head board.

  • Page 12: Passive Frequency Control

    F r e t r o r Figure 1.2: MOGlabs 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 current;...

  • Page 13: Dc Locking To An Atomic Transition

    locking to an atomic transition STACK 120V SPAN SPAN FREQUENCY TRIG time 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.

  • Page 14: Ac Locking To An Atomic Transition

    Chapter 1. Introduction λ/4 λ/4 Vapour cell Offsets ECDL Servo Figure 1.4: Schematic setup for locking to an atomic transition. PD is the photodetector. BS beamsplitter, M mirror, λ/4 a quarter-wave retarder. controls. Feedback can be via one or both piezo LOCK OFFSET actuators, or the diode injection current, or all three.
  • Page 15 locking to an atomic transition Vapour cell + coil λ/4 λ/4 250kHz ECDL Servo Lock-in Figure 1.5: Schematic setup for locking to an atomic transition. PD is the photodetector. BS beamsplitter, M mirror, λ/4 a 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.
  • Page 16 Chapter 1. Introduction...

  • Page 17: Connections And Controls

    2. Connections and controls 2.1 Front panel controls l o r r e l a t l e r r r r u TEC voltage C ° e r r e r r l i F r e t r o r mode, the maintains the laser temperature, but STANDBY...
  • Page 18 Chapter 2. Connections and controls Diode injection current, 0 to 200 mA (DLC-202) or 500 mA (DLC-502). CURRENT 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.
  • Page 19 2.1 Front panel controls Modulator enable, to switch on the coil driver, diode current dither, OFF/MOD or external modulator. Offset of the frequency error lock signal. The will lock such LOCK OFFSET that the error signal plus is zero, allowing for small LOCK OFFSET adjustment of the lock frequency.

  • Page 20: Front Panel Display/Monitor

    Chapter 2. Connections and controls 2.2 Front panel display/monitor Display selector labs includes a high-precision 4.5 digit display with four unit annunciators and 8-channel selector switch. Actual diode current (mA) Current Current limit (mA) Curr max ( ) sign indicates limit rather than actual current Diode voltage (V) Voltage ◦...
  • Page 21 2.2 Front panel display/monitor CHAN B Photodetector [30 mV/µW] Input Feedback error Error Diode current (to monitor bias) [10 V/A] Current Modulator output current [1 V/A] ◦ Temperature error [10 V/ Temp...

  • Page 22: Rear Panel Controls And Connections

    Chapter 2. Connections and controls 2.3 Rear panel controls and connections l o r r e l o l r a r t a i l : l e r o t a i r The unit should be preset for the appropriate voltage for your coun- IEC power in/out try.
  • Page 23 2.3 Rear panel controls and connections Oscilloscope trigger, -level. Connect to external trigger input on TRIG oscilloscope. Set oscilloscope triggering for external, rising edge. I max Diode current limit. The current limit can be set approximately with the display selector set to Curr max. For more precise adjustment, the laser current should be set above the desired maximum (using a dummy load, e.g.

  • Page 24: Internal Switches And Adjustments

    Chapter 2. Connections and controls 2.4 Internal switches and adjustments See appendix H for the location of relevant internal components. CAUTION The cover of the controller should be left on, even loosely, to ensure proper airflow and cooling. Link LK1 (rear right of main board) can be shorted internally to Interlock avoid the requirement for an external interlock, if permitted by local safety regulations.
  • Page 25 2.4 Internal switches and adjustments on, and fast if switch is off. It is assumed that the actuator is STACK a wide-range but slow device, such as an stack, NEC-Tokin AE0203D04 while the actuator is faster but with smaller range, such as a DISC piezoelectric disc [1, 2].
  • Page 26 Chapter 2. Connections and controls If this switch is , the rear-panel signal drives the fast DIP 6 EXT ERROR feedback signal only, bypassing the internal servo shaping filters to allow for maximum bandwidth. The fast gain adjustment knob and fast toggle switches are active.
  • Page 27 2.4 Internal switches and adjustments Note The feedback to the actuator reverses when flipped, so STACK DIP 1 should also be flipped when is flipped. DIP 10 DIP 1 is on, is added to the internally generated DIP 13 DIP 13 EXT SWEEP STACK signal, independent of the state of...
  • Page 28 Chapter 2. Connections and controls Digital control HD12 is a 10-pin header which provides access to several control signals for locking and for sample-and-hold of the lock-point. HD12 is located near the switches, slightly towards the front and left- hand side of the unit (see appendix H). The pinout of the header is described in section G.4.
  • Page 29 2.4 Internal switches and adjustments Internal trimpots Current dither amplitude limit Phase lead RT12 Ambient temp for active sensors (AD590, AD592) RT13 current limit RT15 locking, either the laser frequency or the external reference must be modulated at the dither frequency, 250 kHz. An external modulator (see appendix D) is normally used, but the laser injection current can be modulated directly.
  • Page 30 Chapter 2. Connections and controls...

  • Page 31: Operation

    3. Operation 3.1 Simplest configuration In the simplest application, the labs will be used to control the diode injection current, and temperature. Thus the must be connected to the diode, a thermoelectric Peltier cooler ( ), and a temperature sensor (fig. 1.1). All connections are via a single cable.

  • Page 32: Laser Frequency Control

    Chapter 3. Operation diode injection current supply and piezo drivers. 5. If the controller is switched back to , all electronics STANDBY will be powered down, except for the temperature controller, which will continue to operate normally. 6. Temperature control can be optimised by adjustment of the integrator gain, rear-panel trimpot T .
  • Page 33 3.2 Laser frequency control One, two, or three piezo elements can be controlled. Typically, only a single “stack” actuator, such as the Tokin (available from AE0203D04 Thorlabs, www.thorlabs.com), will be required. The single stack ac- tuator allows frequency scanning and frequency offset selection, and active slow (up to ≈...

  • Page 34: External Scan Control

    Chapter 3. Operation switch should be on SCAN LOCK SCAN LOCK SCAN Sets the mid-point voltage of the ramp. FREQUENCY Sets the height of the ramp (limited at 0 and 120 V SPAN or 150 V); see fig. 3.1. front-panel trimpot controls a feed- BIAS BIAS forward bias injection current which follows the...
  • Page 35 3.4 Locking to an atomic transition: Saturated absorption spectrum for natural Rb −0.2 −4 −2 Frequency (GHz) Figure 3.2: A saturated absorption spectrum of rubidium using a standard uncoated laser diode and low diffraction efficiency grating in Littrow con- figuration (upper trace). The entire 780 nm rubidium hyperfine structure can be scanned, for both naturally occurring isotopes (nearly 10 GHz), by ramping both the external cavity length and simultaneously the injection current, with appropriate adjustment of the feed-forward...
  • Page 36 Chapter 3. Operation λ/4 λ/4 Vapour cell Offsets ECDL Servo Figure 3.3: Schematic setup for locking to an atomic transition. PD is photodetector. BS beamsplitter, M mirror, λ/4 retarder. Ch1 100mV Ch2 100mV 20.0ms Ch1 100mV Ch2 100mV 20.0ms Figure 3.4: Examples of spectra for locking, for wide and narrow spans (lower traces) and error signals (upper traces).

  • Page 37: Locking To An Atomic Transition: Ac

    3.5 Locking to an atomic transition: 7. Find an appropriate spectral feature. 8. Adjust front-panel to obtain INPUT OFFSET LOCK OFFSET a zero-crossing signal at the desired frequency. The ERROR slope should normally be positive (depending on switches ); it can be inverted by coarsely adjusting the 10, 11 PHASE control.
  • Page 38 Chapter 3. Operation below, for wide and narrow spans. These traces were obtained with an 8 cm long Rb vapour cell at room temperature, using a Zeeman modulation coil as described in appendix C. To operate in locking configuration: 1. Select locking by setting internal switch 2.
  • Page 39 3.5 Locking to an atomic transition: 4. Adjust the such that saturated absorption trace INPUT OFFSET is near zero. 5. Switch the modulation on with OFF/MOD 6. Find an appropriate spectral peak and observe the dispersive error signal with set to CHAN B ERROR 7.

  • Page 40: Locking Using An External Signal

    Chapter 3. Operation Ch1 100mV Ch2 100mV 20.0ms Ch1 100mV Ch2 100mV 20.0ms Figure 3.7: Examples of spectra for locking, for wide and narrow spans (lower traces), with error signals (upper traces). 13. Increase gains to minimise the error signal, SLOW FAST ideally using an external audio spectrum analyser (see chapter...

  • Page 41: External Control Of Lock Frequency Setpoint

    3.7 External control of lock frequency setpoint ternal input. 2. Select the external locking signal by setting internal switch 3. Follow the procedure above for locking. Note that actually the external error signal is added to the inter- nal error signal, so it may be advisable to switch off the internal modulator.
  • Page 42 Chapter 3. Operation...

  • Page 43: Optimisation

    4. Optimisation Laser frequency stabilisation is a complex and ongoing research topic. A thorough treatment would require extensive discussion of control theory, actuator response, mechanical design, laser-atom in- teractions and electronics. Here we consider the problem from a pragmatic perspective. The laser is assumed to be moderately stable, operating close to the desired frequency, with a linewidth of a few MHz averaged over a typical measurement time of about one second.
  • Page 44 Chapter 4. Optimisation produce clear (low-noise) dispersive error signals as shown in the upper trace of fig. 3.7. Note that the error signal should be about 0.5 to 1.0 V p-p. While the signal looks cleaner at larger ampli- tude relative to background oscilloscope noise, in fact the overall performance will deteriorate.

  • Page 45: Noise Spectra

    4.2 Noise spectra Unlocked Piezo locked -100 Piezo+current locked Off resonance Dark -120 -140 Frequency (Hz) Figure 4.1: Error signal spectra, with laser unlocked, locked with SLOW (piezo) feedback only, and with (piezo+current) feedback. SLOW FAST The off-resonance and dark noise spectra provide information on the effec- tive noise floor.
  • Page 46 Chapter 4. Optimisation onances, outside a Doppler absorption peak. The Off resonance spectrum gives the frequency discriminator noise floor: it is mean- ingless to try to reduce the laser frequency noise below this level. With feedback enabled, the noise for low Fourier frequencies SLOW is drastically reduced.

  • Page 47: A Specifications

    A. Specifications Specification Parameter Current regulator 0 to 200/250/500 mA (DLC-202/252/502) Output current 3.2 V at 200 mA; 6 V at 100 mA Max diode voltage DLC-202: Higher compliance optional ±0 01 mA Display resolution < 10 nA rms (10 Hz – 1 MHz)
  • Page 48 Appendix A. Specifications Specification Parameter Piezos 0 to 120 V for (default) FREQUENCY STACK 0 to 150 V optional 100 ± 16 4 V feedback DISC 4 to 70 Hz Scan rate Note The default maximum piezo voltage is 120 V but can be increased to 150 V by a simple resistor change.
  • Page 49 Feedback system 250 kHz, ±8 V, ±500 mA MOD OUT Current output (1 Ω sense) Control via rear-panel trimpot I set ◦ 0 to 360 (min) PHASE 10 V to +10 V INPUT OFFSET ±0 5 V LOCK OFFSET ±20 dB MASTER ±20 dB GAIN...
  • Page 50 Appendix A. Specifications mains off, or fault condition DARK detected ( failure, polar- ity reversed, open-circuit, ca- ble unplugged, missing sen- sor, temperature out of range) STANDBY RUN LED mains power on Standby ORANGE (temperature controller on) Fully operational GREEN (piezo, current, ramp) Start sequence error or fault (Either...

  • Page 51: Rf Response

    response response RBW 30 kHz -30 dBm VBW 10 MHz Ref -20 dBm 50 dB SWT 17 s Center 1.5 GHz 300 MHz/ Span 3 GHz Figure A.1: response, input on laser headboard to diode SMA output. A.2 Sweep saturation and trigger In normal scanning mode, a sawtooth is supplied to the stack piezo (or other laser frequency actuator), at a frequency of 1 to 70 Hz;...
  • Page 52 Appendix A. Specifications STACK 120V SPAN TRIG time Figure A.2: output voltage and trigger pulse, when STACK FREQUENCY is set near the midpoint (upper) or moved closer to 0 V (lower), where the output voltage exceeds the maximum range.

  • Page 53: B Troubleshooting

    B. Troubleshooting labs detects a wide range of fault conditions and deac- tivates related circuitry accordingly. The front-panel provide LEDs indication of the state of these functions. indicator STANDBY Status Colour Temperature controller off. DARK Reset via keyswitch, → → STANDBY Possible faults: •...

  • Page 54: Diode Off / On Indicator

    Appendix B. Troubleshooting B.2 Diode indicator Status Colour Fault Reset via switch → → Possible faults: • switch not up ( SCAN LOCK SCAN • switch not up ( LOCK • Rear interlock disconnected • Laser head interlock disconnected • Laser head cable disconnected •...

  • Page 55: Locking

    B.3 Locking B.3 Locking labs controller provides feedback via three channels each with a complex servo loop function. A few common problems are addressed here; for more difficult problems, labs will be happy to work with you to find the best possible solution. B.3.1 does not lock SLOW...
  • Page 56 Appendix B. Troubleshooting B.3.3 does not lock FAST • feedback has wrong polarity. Try reversing the polarity FAST with the front-panel switch. • If the laser frequency is close to a mode hop (i.e. intrinsic diode cavity resonance is half way between two external-cavity longitudnal modes), the current response can be opposite to normal.

  • Page 57: C Modulation Coils

    Figure C.1: Vapour cell, Zeeman coil, and primary excitation coil, mounted (available from MOGlabs). C.1 Field requirements Ideally the Zeeman dither coil should produce a frequency shift of about half the peak width, typically a few MHz.

  • Page 58: Coil Impedance

    Appendix C. Modulation coils where is the number of turns per unit length and the current. For wire diameter 0.4 mm, = 2500 m , and the current requirement is only 22 mA/MHz. C.2 Coil impedance However, driving an oscillating current through a coil is problematic because the impedance grows with the frequency.

  • Page 59: Impedance Matching

    C.3 Impedance matching labs operates at ω = 250 kHz. For a cell of length 8 cm, 0.4 mm wire, and 20 mA, we find L Wheeler ≈ 650 µH, and = 20 V, and the maximum slew rate is 32 V/µs. labs does not have that direct output capability.

  • Page 60: Tuning

    Appendix C. Modulation coils equals the inductive impedance. That is, ωL = (C.3.7) ωC ω Using the long-solenoid equation for inductance, (C.3.8) ω µ although in practice we find that the inductance is about half the long-solenoid prediction and hence the capacitance should be dou- bled, typically about 1 to 5 nF.

  • Page 61: Shielding

    C.5 Shielding RBW 1 kHz Marker 1 [T1 ] -30 dBm VBW 30 kHz -41.37 dBm Ref -26 dBm 5 dB SWT 2.5 s 250.000000000 kHz -100 -110 -120 Center 250 kHz 50 kHz/ Span 500 kHz Figure C.3: Coil response acquired using a spectrum analyser with track- ing generator.
  • Page 62 Appendix C. Modulation coils...

  • Page 63: D External Modulators And Injection Current Modulation

    D. External modulators and injection current modulation labs is designed for locking a laser to an external reference such as an atomic resonance or an optical cavity. In many cases it is convenient to use the internal modulator driver, and Zee- man modulation of an atomic transition, as described in appendix C.

  • Page 64: Injection Current Modulation

    Appendix D. External modulators and injection current modulation former. Primary and secondary were wound with 10 turns of PVC- insulated hookup wire around a ferrite bead approximately 15 mm diameter. A 500 Ω potentiometer allows control of the modulation amplitude, and a 9 V battery and 100 kΩ potentiometer provide a shift to set the centre modulator frequency.
  • Page 65 75 kHz/ Span 750 kHz Figure D.3: beatnote from two MOGlabs -locked lasers. The 3 dB peak width was 750 Hz with a spectrum analyser RBW setting of 300 Hz. For a 20 s average, the width was about 4 kHz.
  • Page 66 Appendix D. External modulators and injection current modulation...

  • Page 67: E Photodetector

    M4 and 8-32 threaded holes allow mounting in different configurations to minimise the footprint on an optical bench (see figure E.2). Figure E.1: MOGlabs balanced differential photodetector. Figure E.2: M4 mounting holes are marked with a circular ring; others...

  • Page 68: Photodiodes

    Appendix E. Photodetector E.1 Photodiodes The standard photodetector uses Si-PIN photodiodes encapsulated in a coloured plastic which transmits in the near-infrared and blocks most room light. The diodes include a lens to reduce the acceptance ◦ angle to ±10 . Unfiltered diodes, and wider acceptance angles, are also available.

  • Page 69: F Laser Head Board

    The laser head board can be mounted to the supplied laser head panel. Figure F.1: MOGlabs laser head board showing headers for con- nection of laser diode, piezo actuators, temperature sensor, and head enclosure interlock.

  • Page 70: Grating Pivot Point Compensation

    Appendix F. Laser head board Note only one temperature sensor should be connected. For high bandwidth modulation (see below), the diode should be con- nected to the connector ( ) rather than to the MOLEX Another very small circuit board, to connect directly to the diode, is also available from labs, with connectors.

  • Page 71: Rf Coupling

    F.3 RF coupling F.3 RF coupling connector on the laser head board allows high-frequency current modulation. The input is coupled, with low- and high- frequency limits of about 160 kHz and 2.5 GHz (see fig. A.1). Capac- itor , normally 10 nF, can be changed to adjust the low-frequency cutoff.
  • Page 72 Appendix F. Laser head board Figure F.3: MOGlabs laser head board schematic. The modulation low-pass cutoff frequency is determined by C4 and the diode impedance (∼ 50Ω).

  • Page 73: G Connector Pinouts

    G. Connector pinouts G.1 Photodetector The photodetector is connected via standard 6-pin (FireWire) IEEE-1394 connectors. Signal Differential Signal – Signal + +12 V 12 V Figure G.1: PHOTODETECTOR connector on rear panel. Differential output is enabled if pin 2 is TTL high (+5 V). G.2 Laser WARNING: The connector on the rear panel is a standard...

  • Page 74: Interlock

    Appendix G. Connector pinouts Signal Signal Signal TEC – DIODE – DISC + TEC + DIODE + DISC – Shield Shield Shield TEC – DIODE – STACK + TEC + DIODE + STACK – AD590/592 – Relay GND AD590/592 + Relay +5V NTC –...

  • Page 75: Digital Control

    G.4 Digital control G.4 Digital control HD12 is a 10-pin header which provides access to several impor- tant control signals for locking and for sample-and-hold of the lock- point, as described in section 2.4. The signals are standard compatible, > 2 4 V and <...
  • Page 76 Appendix G. Connector pinouts...

  • Page 77: H Pcb Layout

    H. PCB layout...
  • Page 78 Appendix H. PCB layout...
  • Page 79 Bibliography [1] C. J. Hawthorn, K. P. Weber, and R. E. Scholten. Littrow config- uration tunable external cavity diode laser with fixed direction output beam. Rev. Sci. Inst., 72(2):4477, 2001. i, 15 [2] L. Ricci, M. Weidem¨ u ller, T. Esslinger, A. Hemmerich, C. Zim- mermann, V.

  • Page 80: References

    [10] S. C. Bell, D. M. Heywood, J. D. White, and R. E. Scholten. Laser frequency offset locking using electromagnetically in- duced transparency. Appl. Phys. Lett., 90:171120, 2007. 54 [11] G. C. Bjorklund. Frequency-modulation spectroscopy: new method for measuring weak absorptions and dispersions. Opt.
  • Page 82 MOG Laboratories Pty Ltd c 2010 420 Victoria St, Brunswick VIC 3056, Australia Product specifications and descriptions in this doc- Tel: +61 3 9940 1427 Fax: +61 3 9381 0700 info@moglabs.com ument are subject to change without notice.

This manual is also suitable for:

Dlc-252Dlc-502

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