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20

Chapter 3. Feedback control loops

knob anti-clockwise from the

OFF

position to set the roll-off fre-

quency 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 anti-

clockwise.

• Set

FAST

to

SCAN+P

to engage the proportional feedback loop.

Summary of Contents for FSC

Page 1: ...Fast servo controller Version 1 0 4 Rev 2 4 hardware ...

Page 2: ...quential 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 2021 No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechan...

Page 3: ...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...

Page 4: ...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 ...

Page 5: ... readers to review control theory textbooks 1 2 and lit erature 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 discrimi nator which generates an error signal that is proportional to the difference between the instantaneous laser frequency and the de sired or setpoint frequency Common di...

Page 6: ... 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 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...

Page 7: ...w 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 t...

Page 8: ...ST GAIN DIFF GAIN limit FAST INT FAST DIFF SLOW INT Filter 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...

Page 9: ...LOW CHA CHB FAST ERR SLOW ERR RAMP BIAS FAST SLOW MON1 MON2 Fast Servo Controller CHB PD 0 REF 100k 200k OFF 10M 1M 750k 500k 5M 2 5M 50k 25k 100k 250k OFF 10k 20k 50k 500k 1M 2M 100k 200k OFF 25 50 75 500 750 1k 100 250 CHA CHB FAST ERR SLOW ERR RAMP BIAS FAST SLOW STATUS 0 6 18 24 12 2 1 1 Configuration INPUT Selects error signal coupling mode see figure 3 2 AC Fast error signal is AC coupled sl...

Page 10: ...dy 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 differentia...

Page 11: ...ue 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...

Page 12: ...elect 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 that switches from low to high at the centre of the sweep The table below defines the signals CHA Channel A input CHB Channel B input FAST ERR Error signal used by the fast servo SLOW ERR Error signal used by the slow servo RAMP Ramp as applied to SLOW OUT BIA...

Page 13: ...tch 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 ...

Page 14: ...ormally connected to diode in jection 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 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 Digi...

Page 15: ...ntial 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 ...

Page 16: ...IP 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 2 5 V offset is added to the fast output suitable for direct connection to MOGLabs B1047 B1240 headboards ...

Page 17: ...IN 0v 1 2 SLOW OUT LF sweep SLOW INT SLOW INT MODULATION SWEEP MOD IN 0v 0v Bias 3 FAST OUT TRIG BIAS 0v LF sweep 0v SPAN 0v 0v Fixed offset 5 OFFSET SWEEP IN RATE Ramp Slope 6 INT EXT External gain 1 FAST SERVO FAST GAIN P I D NESTED 0v GAIN IN 0v Mod 4 LOCK IN FAST FAST LOCK LOCK IN FAST LOCK IN SLOW 0v 0v FAST LOCK SLOW LOCK B IN 0v CHB VREF A IN ERR OFFSET FAST SIGN SLOW SIGN INPUT DC block 0v...

Page 18: ...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 B IN 0v CHB VREF A IN ERR OFFSET FAST SIGN SLOW SIGN INPUT FE SE SLOW ERR Fast error Slow error FAST ERR DC block 0v 0v INPUT B A Fast AC 7 AC Δ DC Figure 3 2 Schematic of the FSC input stage showing coupling offset and polarity contr...

Page 19: ...int 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 re strict 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...

Page 20: ... 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...

Page 21: ...Hz 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 Ω and R87 100 kΩ To in crease the slow output increase R82 R87 most easily accomplished b...

Page 22: ...N FAST 0v FAST LOCK 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...

Page 23: ...re the differentiator loop to dominate for improved locking The differentiator provides phase lead 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 t...

Page 24: ...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 inp...

Page 25: ...ch 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 th...

Page 26: ...ng either loop to LOCK stops the sweep and activates stabilisation Fast control MODULATION SWEEP MOD IN 0v 0v Bias 3 FAST OUT TRIG HF FAST BS BIAS RA RAMP BIAS 0v LF sweep 0v SPAN 0v 0v Fixed offset 5 OFFSET SWEEP IN RATE Ramp Slope 6 INT EXT LOCK IN FAST LOCK IN SLOW 0v Mod 4 0v FAST LOCK SLOW LOCK Figure 3 7 Sweep external modulation and feedforward current bias The ramp can also be added to the...

Page 27: ...paratus TRIG CH1 Serial TRIG FAST OUT SLOW OUT MOD IN POWER B POWER A MONITOR 1 MONITOR 2 SWEEP IN GAIN IN B IN LOCK IN Laser EOM Cavity PBS LPF PD Piezo Current mod Oscilloscope AC CH2 PZT MOD DLC controller SMA A IN 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...

Page 28: ... 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 headbo...

Page 29: ... 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...

Page 30: ...ust 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 reso nance 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 ...

Page 31: ... with the fast proportional and integrator feed back 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 A...

Page 32: ...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 trim pot correctly set...

Page 33: ...n 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 con sult 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 transmis...

Page 34: ... 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...

Page 35: ...ed 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 sta bility by ensuring that the overall gain of the slow servo system is higher than the fast se...

Page 36: ...32 Chapter 4 Application example Pound Drever Hall locking ...

Page 37: ...ndwidth 3 dB 35 MHz Input A IN B IN SMA 1 MΩ 2 5 V SWEEP IN SMA 1 MΩ 0 to 2 5 V GAIN IN SMA 1 MΩ 2 5 V MOD IN SMA 1 MΩ 2 5 V LOCK IN 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 ...

Page 38: ...z MONITOR 1 2 SMA 50 Ω BW 20 MHz TRIG SMA 0 to 5 V POWER A B M8 female connector 12 V 125 mA All outputs are limited to 5 V 50 Ω outputs 20 mA max 20 mW 13 dBm Mechanical power IEC input 110 to 130V at 60Hz or 220 to 260V at 50Hz Fuse 5x20mm anti surge ceramic 250 V 2 5 A Dimensions W H D 250 79 292 mm Weight 2 kg Power usage 10 W ...

Page 39: ... 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 you...

Page 40: ...ock 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...

Page 41: ...uld 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...

Page 42: ...he 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 ...

Page 43: ...304 Q1 R228 C237 C226 C174 FB1 C56 C314 R270 R269 U75 R245 R244 C282 R227 U70 R229 R231 U60 R153 R152 C179 R154 R151 C187 R196 C138 U29 C130 R76 R74 R347 R345 C365 R306 C347 R280 R279 C321 R272 U76 R255 R254 U71 U61 R217 C177 C178 C190 C188 C180 C150 U32 R77 D3 D8 U77 R256 U72 R219 U66 C153 U33 C104 R113 C112 R333 R344 R346 C368 C345 C320 R220 R218 R122 R119 R118 C100 R13 C373 R129 C134 C17 R300 R...

Page 44: ...40 Appendix C PCB layout ...

Page 45: ...n 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 41 ...

Page 46: ...rt 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 ...

Page 47: ...20 240 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 ...

Page 48: ...44 Appendix D 115 230 V conversion ...

Page 49: ...tabilization 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 3 441 444 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...

Page 50: ...in duced transparency Appl Phys Lett 90 171120 2007 1 10 W Demtro der Laser Spectroscopy Basic Concepts and Instru mentation 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 ...

Page 51: ......

Page 52: ...Laboratories Pty Ltd 49 University St Carlton VIC 3053 Australia Tel 61 3 9939 0677 info moglabs com 2017 2021 Product specifications and descriptions in this doc ument are subject to change without notice ...

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