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10

Chapter 3. Operation and Optimisation

Spring plunger

λ adjust

λ lock screw

Filter notch

Filter assembly (spindle)

Spindle-clamp

 lock screw

Filter assembly (clamp)

Figure 3.1:

Filter angle adjustment, showing the primary wavelength adjustment

screw and counter-acting spring plunger.

3.3

Mode-hops

Mode-hops are a frequent occurrence with external cavity diode lasers. A

mode-hop is a discontinuity when tuning or scanning the laser wavelength.

As the laser wavelength is varied, usually by changing the cavity length

with a piezo, competition between the wavelength determined by the dif-

ferent wavelength-dependent cavity elements can lead to a

mode hop

:

a

jump in laser wavelength to a different external cavity mode. Wavelength-

dependent elements include the external cavity, the laser diode internal

cavity between the rear and front facets of the diode, the filter, and the

gain bandwidth of the laser diode.

The different wavelength-dependent characteristics are shown schemati-

Summary of Contents for CEF

Page 1: ...External Cavity Diode Laser CEL CEX and CEF Cateye Revision 1 21 ...

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 2014 2022 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: ... the MOGLabs DLC external cavity diode laser controller the CEL cateye laser described here provides a robust stable acoustically inert low linewidth and highly tunable laser system We hope that the MOGLabs CEL works well for your application Please let us know if you have any suggestions for improvement in the laser or in this document so that we can make life in the laser lab easier for all and ...

Page 4: ...ii ...

Page 5: ...at 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 the laser Please Avoid direct exposure to the beam Avoid looking directly into the beam Note the safety labels examples shown in figure below and heed their warnings When the laser is switched on th...

Page 6: ...onents are fragile In particular the filter piezo actuator and output coupler are very easily damaged Please take care of these items when working inside the laser The filter and output coupler are hard coated and can be cleaned but great care is needed as with any intracavity laser optics NOTE MOGLabs products are designed for use in scientific research labora tories They should not be used for c...

Page 7: ...d JULY 2019 Complies with 21 CFR 1040 10 and 1040 11 except for deviations pursuant to Laser Notice No 50 dated 24 June 2007 MOG Laboratories Pty Ltd 49 University St Carlton VIC 3053 AUSTRALIA IEC 60825 1 2007 AS NZS 2211 5 2006 INVISIBLE LASER RADIATION AVOID EXPOSURE TO BEAM CLASS 3B LASER PRODUCT Wavelength 750 790 nm Max Power 200 mW Figure 1 Warning advisory and US FDA compliance labels ...

Page 8: ...ured JULY 2019 Complies with 21 CFR 1040 10 and 1040 11 except for deviations pursuant to Laser Notice No 50 dated 24 June 2007 MOG Laboratories Pty Ltd 49 University St Carlton VIC 3053 AUSTRALIA Emission indicator Model number CEL Serial number A31907001 Manufactured JULY 2019 Complies with21 CFR 1040 10 and1040 11 exceptfor deviations pursuant to Laser Notice No 50 dated24 June2007 MOG Laborato...

Page 9: ...Y 2019 Complies with 21 CFR 1040 10 and 1040 11 except for deviations pursuant to Laser Notice No 50 dated 24 June 2007 MOG Laboratories Pty Ltd 49 University St Carlton VIC 3053 AUSTRALIA Emission indicator Modelnumber CEL Serialnumber A31907001 Manufactured JULY2019 Complieswith21CFR1040 10 and1040 11exceptfor deviationspursuanttoLaserNoticeNo 50 dated24June2007 MOG LaboratoriesPtyLtd 49Universi...

Page 10: ...ission indicator The MOGLabs controller will illuminate the emission warning indi cator LED immediately when the laser is switched on There will then be a delay of at least 2 seconds before actual laser emission Interlock It is assumed that the laser power supply is keyed and interlocked for safety The laser head board also provides connection for an interlock see appendix B if used with a power s...

Page 11: ... RoHS Directive 2002 95 EC and is not subject to compliance in accordance with DIREC TIVE 2002 95 EC Out of Scope Electronics 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 ix ...

Page 12: ...uick startup the next day But the laser diode current and piezo voltage will be zero extending their operating life In the morning switch back on 1 Switch from STANDBY to RUN 2 Switch the laser diode toggle down on You don t need to adjust the current just wait a few minutes for the diode temperature to equilibrate You should switch your MOGLabs DLC into STANDBY mode at nights and weekends and whe...

Page 13: ... 2 1 2 Piezo electric frequency control 3 1 3 Temperature and current 3 2 First light 5 2 1 Standby Run 5 2 2 Current 5 2 3 Temperature 6 3 Operation and Optimisation 9 3 1 Power 9 3 2 Wavelength 9 3 3 Mode hops 10 3 4 Scanning 11 3 5 Faraday isolator 12 3 5 1 Faraday isolator alignment 13 4 Troubleshooting 17 4 1 Scanning adjustment 17 4 1 1 BIAS optimisation 18 xi ...

Page 14: ...g a visual fault locator 25 4 4 2 Mirror adjustment 25 A Specifications 29 A 1 CEL mechanical Rev8 31 A 2 CEL mechanical Rev9 32 A 3 CEF mechanical 33 B Laser head board 35 B 1 B1045 1046 headboard 36 B 1 1 RF coupling 36 B 2 B1047 B1240 headboards 38 B 2 1 SMA input 39 B 3 Headboard connection to controller 40 References 44 ...

Page 15: ...y The MOGLabs CEL see Fig 1 1 is machined from a solid aluminium block so that the laser is stable robust and insensitive to acoustic disturbances The cavity is hermetically sealed for additional suppression of environmen tal fluctuations and drift The MOGLabs CEL is a cat eye design see Fig 1 2 in which an ex ternal cavity is formed between the rear reflecting surface of the semi conductor diode ...

Page 16: ... laser frequency One longitudinal cavity mode is selected by an ultranarrow intracavity bandpass filter A cateye reflector is formed by the output coupler and intracavity lens 1 The output beam from a laser diode is collimated with a high numerical aperture NA lens and incident on the filter The filter transmission wave length depends on the rotation angle Transmitted light is back reflected by th...

Page 17: ...GHz over the full 150 V range of the piezo The bandwidth is limited by mechanical resonances typically 25 kHz 1 3 Temperature and current The laser frequency is also dependent on temperature and injection cur rent the sensitivities are typically 3 MHz µA and 30 GHz K 5 Thus low noise stable electronics such as the MOGLabs DLC external cavity diode laser controller are essential see Ref 6 to achiev...

Page 18: ...4 Chapter 1 Introduction ...

Page 19: ... and home movie or still cameras are also good options although they often have infra red filters which may need to be removed 2 1 Standby Run Please first check that the MOGLabs DLC has been set to the correct mains supply voltage by inspecting the red voltage selector above the rear panel IEC power inlet Then turn the main power switch on Make sure that the laser diode current supply CURRENT kno...

Page 20: ...tion 4 3 Above threshold the laser power vs injection current is well approximated by a linear curve function see Fig 2 1 Initially the current should be set above threshold but well below the maximum operating current until the laser is fully aligned with your experiment 2 3 Temperature The optimum temperature has been set by MOGLabs and should not re quire adjustment Once the diode current is se...

Page 21: ...ate Figure 2 1 Sample laser diode power current characteristic curves with and without an external cavity The output power of a diode with an anti reflection coating is typically low The steps in the power curve are caused my variation in the frequency of the external cavity relative to the diode intrinsic cavity mode and or to the filter transmission peak ...

Page 22: ...8 Chapter 2 First light ...

Page 23: ... measured wavelength is within 0 1 nm of the desired wavelength the cur rent and piezo FREQUENCY can be used to make small changes If the precise wavelength cannot be reached with current and piezo adjustments then the wavelength should be adjusted using the λ adjustment screw see figure3 1 Ensure that the spring plunger is not locked against the arm of the filter spindle For larger wavelength cha...

Page 24: ...ntinuity when tuning or scanning the laser wavelength As the laser wavelength is varied usually by changing the cavity length with a piezo competition between the wavelength determined by the dif ferent wavelength dependent cavity elements can lead to a mode hop a jump in laser wavelength to a different external cavity mode Wavelength dependent elements include the external cavity the laser diode ...

Page 25: ... 384 1 384 2 384 3 384 4 Frequency THz External cavity Diode cavity Diode gain COMBINED Filter Figure 3 2 Schematic representation for the various frequency dependent factors of an ECDL adapted from Ref 4 for wavelength λ 780 nm and external cavity length Lext 15 mm 3 4 Scanning The external cavity length is controlled by a piezo actuator moving the output coupler The cavity length changes with pi...

Page 26: ...the external cavity mode relative to the filter frequency will cause the laser to jump to another external cavity mode where the net gain is higher The mode hop free scan range MHFR can be optimised by careful ad justment of the injection current which affects the refractive index of the diode and hence the frequency of the cavity mode 3 5 Faraday isolator The laser can be supplied in a very compa...

Page 27: ...an have a significant effect on the laser frequency At least 30 dB of isolation is needed that is the optical feedback into the ECDL should be less than 0 1 of the output power Double stage isolators provide 60 dB or more of isolation which is necessary if locking to a high finesse optical cavity The extended chassis version of a MOGLabs laser allows internal mount ing of a Faraday isolator see fi...

Page 28: ...e end cap of the isolator see figure 3 7 On older EOT isolators the waveplate is in the final silver coloured metal element of the retarder The waveplate angle may need adjustment for example to vary the power ratio for the two beams exiting the PBS or to align the polarisation to a more convenient horizontal or vertical axis for experiments or to align to a polarisation preserving fibre To adjust...

Page 29: ...ator Each can be supplied with exit λ 2 waveplate inside one end cap The waveplate can be rotated to rotate the plane of polarisation of the exit beam for example to optimise coupling into polarisation maintaining fibre or to adjust the ratio of exit beams for lasers fitted with a polarising beamsplitter instead of mirror M2 ...

Page 30: ...16 Chapter 3 Operation and Optimisation ...

Page 31: ...AN while monitoring the frequency change with a wavemeter 2 If there is no change in scan range then disable feed forward current bias using DIP switch 4 in the DLC If there is no evidence that the laser is scanning with bias disabled then the piezo has failed please contact MOGLabs for assistance 3 If the laser scans with bias disabled adjustment of CURRENT and BIAS can be used to increase the sc...

Page 32: ...can range is observed Small changes to the injection current optimise the scan range near the nominal centre frequency A fast Fizeau wavemeter an atomic absorption spectroscopy signal or a Fabry Perot cavity can be used to monitor the laser frequency while varying the different control parameters 1 Ensure that BIAS is enabled DIP switch 4 2 With SPAN set to max monitor CHAN AFreq and CHAN BCurrent...

Page 33: ...s Increase SPAN and adjust CURRENT and FREQUENCY until the mode hops cannot be suppressed 10 Adjust the BIAS trimpot to suppress the mode hops Repeat the steps above increase SPAN adjust CURRENT FREQUENCY and BIAS and repeat until no further improvements can be made 11 If the MHFR is substantially less than expected refer to the factory test report it may be helpful to change to a different intrin...

Page 34: ...e full turn 3 Rotate the filter assembly spindle using an allen key or hex ball driver in the hex socket in the centre of the shaft making sure that the notch does not align at 0 to the laser body 4 Lock the spindle clamp For adjustments of less than 1nm 5 Check that the spring plunger is engaged but not locked against the brass arm of the spindle clamp That is the pin should be visibly protruding...

Page 35: ...ensitivity to rotation of the fine tangential wavelength adjustment screw is about 0 5 nm to 1 nm per turn 4 3 Threshold optimisation The lasing threshold at which the overall gain exceeds losses should be as low as possible to maximise the output power and also the scan range and frequency stability The lowest threshold is achieved by optimising the focus of the laser diode collimation lens and t...

Page 36: ...the spindle but is still attached see fig 4 2 Figure 4 2 Removing the intracavity filter assembly 4 Undo the wavelength locking screw a few turns and undo the spring plunger and wavelength adjust screws until they are clear of the bearing below the brass filter adjust clamp 5 Rotate the filter such that the filter alignment mark the mark on the aluminium spindle not the mark on the brass clamp is ...

Page 37: ... screws a little to hold the diode tube in place Do the same for the cateye output coupler as sembly use a ball driver to gently slide the wires away from the filter assembly see fig 4 3 then resecure the set screws to hold the output coupler in place The filter assembly should then pull free Restore the diode lens tube and cateye output coupler tubes back to their original positions fully inserte...

Page 38: ... figure 3 6 The arrangement can also be configured to allow splitting the output into two beams using a PBS as the first reflector Given the 8 Fresnel loss from entrance and exit facets of the fibre the maximum theoretical efficiency for single mode fibre coupling is 92 The stainless steel kinematic mirror mounts are stable and easy to use and coupling efficiency of over 70 is easily attained at 7...

Page 39: ...le laser beam into fibre which allows basic alignment and mode matching Aligning the MOGLabs laser beam to the fibre is then simply a matter of adjusting the mirrors so that the MOGLabs laser beam and the visual fault locator beam overlap inside the laser 4 4 2 Mirror adjustment To maximise the fibre coupling efficiency the incident angle and location of the laser beam at the fibre coupler must be...

Page 40: ...able transmitted beam should be evident at the fibre exit 9 Locate a power meter sensor to monitor the output power exiting from the optical fibre Ensure that the reading is not affected by background light 10 For the horizontal axis first find the maximum output power by ad justing mirror M1 closest to the fibre furthest from the isolator and record the output power 11 Adjust the horizontal axis ...

Page 41: ...s at https www sukhamburg com documents Adjustment_SMS pdf Focus adjustment is not normally needed unless severe shock has moved the lens or if a new diode has been installed in the laser leading to change of beam waist location 17 Once optimised record the input power to the fibre coupler maximum output power and the laser current 18 Increase the laser current to the desired operating current and...

Page 42: ...28 Chapter 4 Troubleshooting ...

Page 43: ...ilability Linewidth Typically 100 kHz Filter 0 2 to 0 4 nm bandpass Tuning range Diode dependent 5 nm to 50 nm Sweep scan Scan range 20 to 30 GHz Mode hop free 5 to 30 GHz Piezo 3 µm 150 V 100 nF typical Cavity length 35 mm Optical Beam 3 mm 1 2 mm 1 e2 typical Polarisation Vertical linear 100 1 typical can be rotated 29 ...

Page 44: ...ional 4 mm diam quick fit water cooling connections Electronics Protection Diode short circuit relay cover interlock connection reverse diode Indicator Laser ON OFF LED Connector MOGLabs Diode Laser Controller single ca ble connect Modulation input Active AC and DC coupled or RF bias tee Mechanical power Dimensions 108 70 83 mm L W H 1 kg Beam height 58 mm Shipping 420 360 260 mm L W H 3 1 kg ...

Page 45: ...A 1 CEL mechanical Rev8 31 A 1 CEL mechanical Rev8 70 86 70 58 108 6 15 74 50 52 50 52 53 4 x M6 clearance 4 x M4 tapped 58 Figure A 1 Dimensions of CEL laser head Rev8 ...

Page 46: ...32 Appendix A Specifications A 2 CEL mechanical Rev9 70 58 86 7 108 50 5 50 5 13 7 9 7 4 x M6 clearance Figure A 2 Dimensions of CEL laser head Rev9 ...

Page 47: ...A 3 CEF mechanical 33 A 3 CEF mechanical 58 20 20 92 6 240 95 75 0 76 2 74 2 75 0 125 200 201 2 7 5 27 5 27 5 Figure A 3 Dimensions of CEF laser head ...

Page 48: ...34 Appendix A Specifications ...

Page 49: ...vides high bandwidth active current modulation for wide bandwidth frequency stabilisation and linewidth narrowing for example using a high finesse optical cavity or polarisation spectroscopy Higher bandwidth is provided by the B1240 headboard which further increases bandwidth and reduces phase delay to allow sub Hz linewidth narrowing The B1240 is limited to low compliance voltage laser diodes red...

Page 50: ...B 1 MOGLabs B1045 and B1046 laser head boards showing connectors for laser diode piezo actuator temperature sensors TEC and head enclosure interlock Connectors are Hirose DF59 B 1 1 RF coupling For the B1045 1046 headboard the SMA connector allows high frequency current modulation via a bias tee The RF input is AC coupled with low and high frequency limits of about 30 kHz and 2 5 GHz see fig B 2 C...

Page 51: ...he laser diode Exces sive power can destroy the diode which is separated from the head board relay by an inductor Thus the relay does not provide protection from high frequency signals RBW 30 kHz VBW 10 MHz SWT 17 s Att 50 dB TG 30 dBm Ref 20 dBm Center 1 5 GHz Span 3 GHz 300 MHz 70 65 60 55 50 45 40 35 30 25 20 Figure B 2 RF response SMA input on laser headboard to diode SMA output ...

Page 52: ...wavelength below 600 nm Note that connection to the SMA input will reduce the diode current even if the control voltage is at zero C 5 C 4 R 4 R 7 C 6 R 5 R 6 C 11 R 12 D1 R 10 C 8 R 14 U 3 R 1 C 3 Q 1 R 2 C 1 C 2 R 3 R 11 P2 U1 P5 U2 R 13 C 7 R 9 SMA P3 P4 P6 L1 L2 P1 AC DC LED Thermistor Piezo1 Laser diode TEC Piezo2 C 1 C 6 C 7 R 15 R 3 R 4 R 6 R 2 D1 R 5 C5 R 16 C2 R 1 R 14 C8 R 17 C 4 U 1 R 7...

Page 53: ... 1 6 mA B1047 to 2 5 mA B1240 with zero input voltage B1047 B1240 Input range 2 0 V max 2 0 V max Input coupling AC DC DC direct AC DC buffered AC time constant 15 µs 10 kHz 15 µs 10 kHz Phase delay 40 ns 20 ns direct 30 ns buffered Gain bandwidth 3 dB 3 MHz 20 MHz Input impedance 5 kΩ AC buffered 1 kΩ at 10 kHz DC buffered 1 kΩ Direct 1 kΩ Current gain 1 mA V 1 mA V Laser diode voltage 10 V max 2...

Page 54: ...t 24 NTC 1 8 17 24 1 8 17 24 LASER Socket DVI D DL Dual Link Plug DVI D SL Single Link Do not use SL cables Missing pins Figure B 4 Headboard connector Note that the pinout is different to that of the matching connector on the rear of the DLC controller A 10 k thermistor should be connected to NTC and NTC but an AD590 or AD592 temperature sensor can be instead be connected to Tsense Pin 15 should ...

Page 55: ...a P 3 1 5 e l g n i S 4 1 6 e l g n i S 5 1 4 e l g n i S 6 1 0 r i a P 7 1 0 r i a P 8 1 d l e i h S 5 0 P 9 1 5 r i a P 0 2 5 r i a P 1 2 d l e i h S 6 P 2 2 6 r i a P 3 2 6 r i a P 4 2 e l o H t n u o M 1 D 1 R 10k 4 R R 3 4 I N D r o s n e s e v i t c A r o s n e s e v i t c A g i S d n G 2 R 9 9 k 4 4 C 47nF or 100pF V 0 5 2 1 L 3 3uH 1 9A v 5 P v 5 P h t r a E s i s s a h C d l e i h S d l e...

Page 56: ...42 Appendix B Laser head board ...

Page 57: ...nd R E Scholten Mode stability of external cavity diode lasers Appl Opt 48 35 6692 2009 3 11 5 H Talvitie A Pietila inen H Ludvigsen and E Ikonen Passive fre quency and intensity stabilization of extended cavity diode lasers Rev Sci Inst 68 1 1 1997 3 6 S D Saliba and R E Scholten Linewidths below 100 khz with external cavity diode lasers Appl Opt 48 36 6961 2009 3 7 P J Fox R E Scholten M R Walki...

Page 58: ...cation to a commercial dye laser with an external stabilizer J Opt Soc Am B 10 802 1993 12 M Prevedelli T Freegarde and T W Ha nsch Phase locking of grating tuned diode lasers Appl Phys B 60 241 1995 13 P Feng and T Walker Inexpensive diode laser microwave modulation for atom trapping Am J Phys 63 10 905 908 1995 14 C J Myatt N R Newbury and C E Wieman Simplified atom trap by using direct microwav...

Page 59: ......

Page 60: ...Laboratories Pty Ltd 49 University St Carlton VIC 3053 Australia Tel 61 3 9939 0677 info moglabs com 2014 2022 Product specifications and descriptions in this docu ment are subject to change without notice ...

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