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MOGlabs FSC100 Manual

Fast servo controller
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Fast servo controller
FSC100
Version 0.1.2, Rev 2–4 hardware

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Summary of Contents for MOGlabs FSC100

  • Page 1 Fast servo controller FSC100 Version 0.1.2, Rev 2–4 hardware...
  • Page 2 MOGL Contact For further information, please contact: MOG Laboratories P/L MOGLabs USA LLC 49 University St 419 14th St Carlton VIC 3053 Huntingdon PA 16652 AUSTRALIA +61 3 9939 0677 +1 814 251 4363 info@moglabs.com...

  • Page 3: Table Of Contents

    Contents 1 Introduction 1.1 Schematics ......2 Connections and controls 2.1 Front panel controls ....2.2 Rear panel controls and connections .
  • Page 4 Contents...

  • Page 5: Introduction

    1. Introduction provides the critical elements of a high-bandwidth MOGL low-latency servo controller, primarily intended for laser frequency stabilisation and linewidth narrowing. The can also be used for amplitude control, for example to create a “noise-eater” that stabi- lises the optical power of a laser, but in this manual we assume the more common application of frequency stabilisation.
  • Page 6 Chapter 1. Introduction Error Error Frequency f ∆f Frequency f ERROR OFFSET Figure 1.2: A theoretical dispersive error signal, proportional to the dif- ference between a laser frequency and a setpoint frequency. An offset on the error signal shifts the lock point (right). Note the distinction between an error signal and a control signal.
  • Page 7 To suppress that disturbance, a feedback servo should have high gain at 50 and 60 Hz. Gain 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 oscillations such as the familiar high-pitched squeal of audio systems (commonly called “audio feedback”) for frequencies above the reciprocal of the minimum propagation delay of the combined...
  • Page 8 Chapter 1. Introduction Proportional 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 with adjustable corner frequencies and gain limits at the low and high frequencies.

  • Page 9: Schematics

    ∫ #1 Gain SLOW GAIN Double integrator [2] Figure 1.4: Schematic of the MOGLabs . Green labels refer to controls on the front-panel and inputs on the back-panel, brown are internal DIPs witches, and purple are outputs on the back-panel.
  • Page 10 Chapter 1. Introduction 1.1.1 Input stage The input stage of the (figure 1.5) generates an error signal as is taken from the “ ” con- A IN OFFSET nector, and V is set using the selector switch, which chooses between the “ ”...
  • Page 11 1.1 Schematics 1.1.2 Slow servo Figure 1.6 shows the slow feedback configuration of the . A va- riable gain stage is controlled with the front-panel knob. SLOW GAIN The action of the controller is either a single- or double-integrator depending on whether is enabled.
  • Page 12 Chapter 1. Introduction 1.1.3 Fast servo The fast feedback servo (figure 1.7) is a -loop with a variable gain P-stage controlled with the front-panel knob, or an FAST GAIN external control signal through the rear-panel connector. GAIN IN The P, I and D components can be individually adjusted via front- panel selector switches, and a low-frequency gain limit is applied to prevent mode-hops caused by external perturbations.
  • Page 13 1.1 Schematics 1.1.4 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 and a single-turn trimpot marked “ ”...
  • Page 14 Chapter 1. Introduction...

  • Page 15: Connections And Controls

    2. Connections and controls 2.1 Front panel controls The front panel of the has a large number of configuration op- tions that allow the servo behaviour to be tuned and optimised. Please note that switches and options may vary between hardware revisions, please consult the manual for your specific device as in- dicated by the serial number.
  • Page 16 Chapter 2. Connections and controls 2.1.2 Ramp control The internal ramp generator provides a sweep function for scanning the laser frequency through a piezo actuator, diode injection current, or both. A trigger output synchronised to the ramp is provided on the rear panel ( TRIG Internal or external sweep mode.
  • Page 17 2.1 Front panel controls Ten-turn fast servo proportional gain; from 10 dB to +50 dB. FAST GAIN Controls the high-frequency servo response. When set to “ ”, the FAST DIFF/FILTER 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.
  • Page 18 Chapter 2. Connections and controls Multi-colour indicator displaying status of the lock. STATUS Power on, lock disabled. Green Lock engaged but error signal goes out of range, indicating the Orange lock has failed. Lock engaged and error signal is within limits. Blue 2.1.5 Signal monitoring Two rotary encoders select which of the specified signals is routed...

  • Page 19: Rear Panel Controls And Connections

    2.2 Rear panel controls and connections 2.2 Rear panel controls and connections MONITOR 2 MONITOR 1 SWEEP IN GAIN IN B IN A IN LOCK IN Serial: TRIG FAST OUT SLOW OUT MOD IN POWER B POWER A All connectors are , except as noted.
  • Page 20 Chapter 2. Connections and controls Voltage-controlled proportional gain of fast servo, ±1 V , correspon- GAIN IN ding to the full-range of the front-panel knob. Replaces front-panel control when is enabled. FAST GAIN DIP1 External ramp input allows for arbitrary frequency scanning, 0 to SWEEP IN 2.5 V.

  • Page 21: Internal Dip Switches

    2.3 Internal DIP switches 2.3 Internal DIP switches There are several internal switches that provide additional op- tions, all set to by default. WARNING There is potential for exposure to high voltages inside the , es- pecially around the power supply. Fast gain Front-panel knob External signal...
  • Page 22 Chapter 2. Connections and controls...

  • Page 23: Operation

    3. Operation A typical application of the is to frequency-lock a laser to an optical cavity using the technique, as shown in figure 3.1. The cavity acts as a frequency discriminator, and the keeps the la- ser on resonance with the cavity by controlling the laser piezo and current through its outputs, reducing the laser line- SLOW...

  • Page 24: Laser And Controller Configuration

    Chapter 3. Operation 3.1 Laser and controller configuration is compatible with a variety of lasers and controllers, provi- ded they are correctly configured for the desired mode of operation. When driving an (such as the lasers), the MOGL ECDL requirements for the laser and controller are as follows: •...
  • Page 25 3.1 Laser and controller configuration 3.1.2 DLC configuration Although the can be configured for either internal or external sweep, it is significantly simpler to use the internal sweep mode and set the as a slave device as follows: • Connect the “ ”...

  • Page 26: Achieving An Initial Lock

    Chapter 3. Operation 3.2 Achieving an initial lock controls of the can be used to tune the SPAN OFFSET laser to sweep across the desired lock point (e.g. cavity resonance) and zoom in on it. The following steps are illustrative of the process required to achieve a stable lock.

  • Page 27: Optimisation

    3.3 Optimisation Figure 3.2: 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. •...
  • Page 28 Chapter 3. Operation Ideally the spectrum analyser would be used with an independent frequency discriminator that is insensitive to laser power fluctua- tions [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.
  • Page 29 3.3 Optimisation When optimising the , it is recommended to first optimise the fast servo through analysis of the error signal, and then the slow servo to reduce sensitivity to external perturbations. In particular, mode provides a convenient way to get the feedback sign SCAN+P and gain approximately correct.
  • Page 30 Chapter 3. Operation laser mode. It is therefore preferable that these (low-frequency) fluctuations are compensated in the piezo instead. Adjusting the will not necessarily produce SLOW GAIN SLOW INT an improvement in the error signal spectrum, but when optimised will reduce the sensitivity to acoustic perturbations and prolong the lifetime of the lock.

  • Page 31: A Specifications

    A. Specifications Specification Parameter Timing > 35 MHz Gain bandwidth ( 3 dB) < 28 ns Propagation delay External modulation > 35 MHz bandwidth ( 3 dB) Input SMA, 1 MΩ, ±2 5 V A IN, B IN SMA, 1 MΩ, 0 to +2 5 V SWEEP IN SMA, 1 MΩ, ±2 5 V GAIN IN...
  • Page 32 Appendix A. Specifications Specification Parameter Output SMA, 50 Ω, BW 20 kHz SLOW OUT SMA, 50 Ω, BW > 20 MHz FAST OUT SMA, 50 Ω, BW > 20 MHz MONITOR 1, 2 SMA, 0 to +5 V TRIG M8 female connector, ±12 V, 125 mA POWER A, B All outputs are limited to ±5 V.

  • Page 33: B 115/230 V Conversion

    B. 115/230 V conversion B.1 Fuse The fuse is a ceramic antisurge, 2.5A, 5x20mm, for example Littlefuse 021502.5MXP. The fuse holder is a red cartridge just above the IEC power inlet and main switch on the rear of the unit (Fig. B.1). Figure B.1: Fuse catridge, showing fuse placement for operation at 230 V.
  • Page 34 Appendix B. 115/230 V conversion Figure B.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;...
  • Page 35 B.2 120/240 V conversion Figure B.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 B.5: 115 V bridge (left) and fuse (right).
  • Page 36 Appendix B. 115/230 V conversion...
  • Page 37 Bibliography [1] Alex Abramovici and Jake Chapsky. Feedback Control Sys- tems: A Fast-Track Guide for Scientists and Engineers. Sprin- ger Science & Business Media, 2012. 1 [2] Boris Lurie and Paul Enright. Classical Feedback Control: With MATLAB and Simulink . CRC Press, 2011. 1 [3] Richard W.

  • Page 38: References

    [9] S. C. Bell, D. M. Heywood, J. D. White, and R. E. Scholten. La- ser frequency offset locking using electromagnetically induced transparency. Appl. Phys. Lett., 90:171120, 2007. 1 [10] W. Demtr¨ o der. Laser Spectroscopy, Basic Concepts and Instru- mentation.
  • Page 40 MOG Laboratories Pty Ltd c 2017 – 2019 49 University St, Carlton VIC 3053, Australia Product specifications and descriptions in this do- Tel: +61 3 9939 0677 info@moglabs.com cument are subject to change without notice.

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