External Cavity Diode Laser

Transcription

1 External Cavty Dode Laser LDL Lttrow Revson 2.02

2 Lmtaton of Lablty MOG Laboratores Pty Ltd (MOGLabs) does not assume any lablty arsng out of the use of the nformaton contaned wthn ths manual. Ths document may contan or reference nformaton and products protected by copyrghts or patents and does not convey any lcense under the patent rghts of MOGLabs, nor the rghts of others. MOGLabs wll not be lable for any defect n hardware or software or loss or nadequacy of data of any knd, or for any drect, ndrect, ncdental, or consequental damages n connectons wth or arsng out of the performance or use of any of ts products. The foregong lmtaton of lablty shall be equally applcable to any servce provded by MOGLabs. Copyrght Copyrght c MOG Laboratores Pty Ltd (MOGLabs) No part of ths publcaton may be reproduced, stored n a retreval system, or transmtted, n any form or by any means, electronc, mechancal, photocopyng or otherwse, wthout the pror wrtten permsson of MOGLabs. Contact For further nformaton, please contact: MOG Laboratores P/L 49 Unversty St Carlton VIC 3053 AUSTRALIA nfo@moglabs.com MOGLabs USA LLC th St Huntngdon PA USA nfo@moglabsusa.com MOGLabs Europe Goethepark Berln Germany nfo@moglabs.eu

3 Preface Semconductor laser dodes can provde an energy-effcent, compact, low cost, hgh power, low nose, tunable source of coherent lght over a large range of wavelengths. Wth wavelength-dependent feedback from an external cavty, they can be very narrow n lnewdth, but also very senstve to vbraton and frequency drft caused by envronmental changes. The MOGLabs LDL Lttrow desgn offers excellent passve stablty wth low senstvty to vbraton by avodng common ECDL weaknesses, n partcular sprngs and flexures. We hope that the MOGLabs LDL works well for you. Please let us know f you have any suggestons for mprovement n the laser or n ths document, so that we can make lfe n the laser lab easer for all, and check our webste from tme to tme for updated nformaton. MOGLabs, Melbourne, Australa

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5 Safety Precautons Safe and effectve use of ths product s very mportant. Please read the followng laser safety nformaton before attemptng to operate the laser. Also please note several specfc and unusual cautonary notes before usng MOGLabs lasers, n addton to the safety precautons that are standard for any electronc equpment or for laser-related nstrumentaton. CAUTION USE OF CONTROLS OR ADJUSTMENTS OR PERFORMANCE OF PROCEDURES OTHER THAN THOSE SPECIFIED HEREIN MAY RESULT IN HAZARDOUS RADIATION EXPOSURE Laser output from the LDL can be dangerous. Please ensure that you mplement the approprate hazard mnmsatons for your envronment, such as laser safety goggles, beam blocks, and door nterlocks. MOGLabs takes no responsblty for safe confguraton and use of the laser. Please: Avod drect exposure to the beam. Avod lookng drectly nto the beam. Note the safety labels (examples shown n fgure below) and heed ther warnngs. When the laser s swtched on, there wll be a short delay of two seconds before the emsson of laser radaton, mandated by European laser safety regulatons (IEC ). The STANDBY/RUN keyswtch must be turned to RUN before the laser can be swtched on. The laser wll not operate f the keyswtch s n the STANDBY poston. The key cannot be removed from the controller when t s n the clockwse (RUN) poston.

6 v To completely shut off power to the unt, turn the keyswtch antclockwse (STANDBY poston), swtch the mans power swtch at rear of unt to OFF, and unplug the unt. When the STANDBY/RUN keyswtch s on STANDBY, there cannot be power to the laser dode, but power s stll beng suppled to the laser head for temperature control. WARNING The nternal crcut board and pezoelectrc transducers are at hgh voltage durng operaton. The unt should not be operated wth covers removed. CAUTION Although the LDL s desgned and prced wth the expectaton that the end-user wll tweak the algnment, some components are fragle. In partcular the pezo actuator behnd the gratng, and the gratng tself, are very easly damaged. Please take care of these tems when workng nsde the laser. Do not attempt to clean the dffracton gratng. Fnger prnts and blemshes usually do not mpact the laser performance. NOTE MOGLabs products are desgned for use n scentfc research laboratores. They should not be used for consumer or medcal applcatons. Label dentfcaton The Internatonal Electrotechncal Commsson laser safety standard IEC :2007 mandates warnng labels that provde nformaton on the wavelength and power of emtted laser radaton, and whch show the aperture where laser radaton s emtted. Fgure 1 shows examples of these labels and fgures 2 and 3 show ther locaton on the LDL laser and large-chasss CEF verson.

7 v Model number: LDL Seral number: A Manufactured: AUGUST 2017 Comples wth 21 CFR , and except for devatons pursuant to Laser Notce No.50, dated 24 June 2007 MOG Laboratores Pty Ltd, 49 Unversty St Carlton VIC 3053, AUSTRALIA US FDA complance AVOID EXPOSURE VISIBLE AND INVISIBLE LASER RADIATION IS EMITTED FROM THIS APERTURE Aperture label engravng INVISIBLE LASER RADIATION AVOID EXPOSURE TO BEAM CLASS 3B LASER PRODUCT Wavelength nm Max Power 100 mw IEC :2007 AS/NZS :2006 Warnng and advsory label Class 3B INVISIBLE LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION CLASS 4 LASER PRODUCT Wavelength nm Max Power 150 mw IEC :2007 AS/NZS :2006 Warnng and advsory label Class 4 Fgure 1: Warnng advsory and US FDA complance labels.

8 INVISIBLE LASER RADIATION AVOID EXPOSURE TO BEAM CLASS 3B LASER PRODUCT AS/NZS :2006 v Model number: LDL Seral number: A Manufactured: AUGUSUT 2017 Comples wth 21 CFR , and except for devatons pursuant to Laser Notce No.50, dated 24 June 2007 MOG Laboratores Pty Ltd, 49 Unversty St Carlton VIC 3056, AUSTRALIA Model number: LDL Seral number: A Manufactured: AUGUST 2017 Comples wth 21 CFR , and except for devatons pursuant to Laser Notce No.50, dated 24 June 2007 MOG Laboratores Pty Ltd, 49 Unversty St Carlton VIC 3053, AUSTRALIA Wavelength nm Max Power 100 mw IEC :2007 Emsson ndcator INVISIBLE LASER RADIATION AVOID EXPOSURE TO BEAM CLASS 3B LASER PRODUCT Wavelength nm Max Power 100 mw IEC :2007 AS/NZS :2006 Fgure 2: Schematc showng locaton of laser warnng labels complant wth Internatonal Electrotechncal Commsson standard IEC :2007, and US FDA complance label. Aperture label engraved on the front of the laser near the ext aperture; warnng advsory label on the rear and complance label beneath.

9 v INVISIBLE LASER RADIATION AVOID EXPOSURE TO BEAM CLASS 3B LASER PRODUCT Wavelength nm Max Power 200 mw IEC :2007 AS/NZS :2006 INVISIBLE LASER RADIATION AVOID EXPOSURE TO BEAM CLASS 3B LASER PRODUCT Wavelength Max Power nm 200 mw IEC :2007 AS/NZS :2006 Model number: CEL002 Seral number: A Manufactured: APRIL 2015 Comples wth 21 CFR , and except for devatons pursuant to Laser Notce No.50, dated 24 June 2007 MOG Laboratores Pty Ltd, 18 Boase St Brunswck VIC 3056, AUSTRALIA Model number: CEL002 Seral number: A Manufactured: APRIL 2015 Comples wth 21 CFR , and except for devatons pursuant to Laser Notce No.50, dated 24 June 2007 MOG Laboratores Pty Ltd, 18 Boase St Brunswck VIC 3056, AUSTRALIA Emsson ndcator Fgure 3: Schematc showng locaton of laser warnng labels for the largechasss CEF verson of a MOGLabs laser.

10 Protecton Features MOGLabs lasers ncludes a number of features to protect you and your laser. Protecton relay When the power s off, or f the laser s off, the laser dode s shorted va a normally-closed sold-state relay at the laser head board. Emsson ndcator The MOGLabs controller wll llumnate the emsson warnng ndcator LED mmedately when the laser s swtched on. There wll then be a delay of at least 2 seconds before actual laser emsson. Interlock It s assumed that the laser power supply s keyed and nterlocked for safety. In some cases, the laser head board provdes connecton for an nterlock (see appendx B), f used wth a power supply whch does not nclude such an nterlock. v

11 RoHS Certfcaton of Conformance MOG Laboratores Pty Ltd certfes that the MOGLabs External Cavty Dode Laser does not fall under the scope defned n RoHS Drectve 2002/95/EC, and s not subject to complance, n accordance wth DIREC- TIVE 2002/95/EC Out of Scope; Electroncs related; Intended applcaton s for Montorng and Control or Medcal Instrumentaton. MOG Laboratores Pty Ltd makes no clams or nferences of the complance status of ts products f used other than for ther ntended purpose. x

12 Extendng laser dode and pezo lfetme At nght, swtch to standby: 1. If usng the LDL to seed an amplfer, frst turn the amplfer off. 2. Swtch the laser dode current off. If usng a MOGLabs DLC controller, don t adjust the current, just swtch the toggle up (off). 3. Swtch from RUN to STANDBY. For a MOGLabs DLC controller n standby mode, the temperature controller wll contnue to operate, so the laser s ready for quck startup the next day. But the laser dode current and pezo voltage wll be zero, extendng ther operatng lfe. In the mornng, swtch back on: 1. Swtch from STANDBY to RUN. 2. Swtch the laser dode toggle down (on). You don t need to adjust the current, just wat a few mnutes for the dode temperature to equlbrate. You should swtch your MOGLabs DLC nto STANDBY mode at nghts and weekends and whenever the laser s not beng used for more than a few hours. Most lasers need to operate only 40 hours durng a 168 hour week; thus swtchng to standby mode can extend the dode and pezo lfetme by a factor of four. x

13 Contents Preface Safety Protecton Features RoHS Certfcaton of Conformance Extendng laser dode & pezo lfetme 1 Introducton External cavty Mode competton Pezo-electrc frequency control Temperature and current Frst lght Temperature Current Operaton Wavelength Scannng, mode-hops, and bas Scannng Algnment Pre-algnment of lens tube and dode Intal dode test Orentaton and polarsaton of the output beam Algnment CEF extended chasss v x x x

14 x Contents 4.6 Fbre algnment A Specfcatons 29 A.1 LDL mechancal A.2 CEF mechancal B Laser head board 33 B.1 B1045/1046 headboard B.2 B1047/B1240 headboards B.3 Laser connecton References 38

15 1. Introducton Semconductor laser dodes are compact, effcent and low-cost, but usually have poor wavelength control, lnewdth and stablty. The addton of an external frequency-selectve cavty allows control of the operatng wavelength over a few nm range, wth sub-mhz lnewdth and stablty. The MOGLabs LDL (see Fg. 1.1) s machned from a sold alumnum block, so that the laser s stable, robust, and nsenstve to vbraton. The cavty s hermetcally sealed for addtonal suppresson of envronmental fluctuatons and drft. The MOGLabs LDL s a Lttrow desgn (see Fg. 1.2) n whch an external cavty s formed between the rear reflectng surface of the semconductor dode, and a dffracton gratng at several centmetres from the dode. Many references descrbe desgns and desgn consderatons [1 6]. The output beam from a laser dode s collmated wth a hgh numercal aperture (NA) lens and ncdent on a dffracton gratng. The gratng s angled such that the frst order reflecton s drected back nto the laser Laser dode Monolthc block Puck Mrror Gratng Vertcal algnment Laser beam λ adjust Fgure 1.1: Sketch of MOGLabs LDL monolthc block external cavty dode laser. 1

16 2 Chapter 1. Introducton Laser dode Lens θ Gratng Fgure 1.2: Schematc of a Lttrow confguraton external cavty dode laser (ECDL). The external cavty, formed by the rear facet of the laser dode and the gratng, determnes the laser wavelength. One longtudnal cavty mode s selected by dspersve feedback from the gratng. dode. Ths feedback has a wavelength centered around λ = 2d sn θ where d s the gratng lne spacng and θ s the angle wth respect to the the gratng normal. Collmaton lens Gratng puck Gratng Pezo Laser dode Mrror Fgure 1.3: Cross-secton of MOGLabs LDL Lttrow laser, showng arrangement of laser dode, collmaton lens, gratng, pezoelectrc transducer and fold mrror. The gratng angle and hence wavelength s adjusted by rotatng the central cylnder (the gratng puck) whch forms a rgd mount for gratng and mrror.

17 1.1 External cavty External cavty Semconductor laser dodes normally have a hgh reflectvty rear facet and a front facet wth reflectvty of only a few percent. The dode cavty s called the ntrnsc or nternal cavty. The external cavty s formed by the dffracton gratng and the dode rear facet, and because the feedback from the gratng s generally greater than that of the front facet, the external cavty determnes the lasng wavelength. The external cavty s typcally around 20 mm long, wth cavty mode spacng (FSR) of c/2l = 7.5 GHz. The laser dode and collmatng lens are held rgdly n a focusng tube. The gratng (usually snusodal holographc) s fxed to a precson mechancal mount whch can be adjusted to optmse feedback of the frst order dffracton back nto the laser dode, and the zeroth order (drect reflecton) becomes the laser output beam. An optonal fold mrror cancels angular devaton of the output beam as the laser wavelength s tuned [3]. Varaton of the gratng angle s used for coarse selecton of the wavelength, wthn the gan bandwdth of the laser dode. 1.2 Mode competton As the wavelength s vared, competton between the frequency determned by the nternal and external cavtes, and the dsperson of the gratng dffracton, leads to mode hops. From fgures 3.2 and 3.3, the net gan (combned product of semconductor gan, dffracton dsperson, nternal and external cavty nterference) can be very smlar at adjacent external cavty modes. A small change n the nternal cavty mode, or the gratng angle, can lead to the overall gan beng greater at a mode adjacent to the mode n whch the laser s oscllatng, and the laser then hops to that hgher-gan mode. See Ref. [1] for a detaled dscusson. 1.3 Pezo-electrc frequency control Small changes to the laser frequency are acheved by controllng the external cavty length wth a pezo electrc actuator. For the MOGLabs LDL, the gratng s mounted to a multlayer pezoelectrc stack. The cavty length

18 4 Chapter 1. Introducton Dode cavty External cavtes Gratng Dode gan COMBINED Frequency (THz) Fgure 1.4: Schematc representaton for the varous frequency-dependent factors of an ECDL, adapted from Ref. [1], for wavelength λ = 780 nm and external cavty length L ext = 15 mm. varaton s about 10 nm per volt, producng a frequency shft of 250 MHz/V wth a range of 25 GHz for 100 V drve voltage. The bandwdth s lmted by mechancal resonances, typcally at a few khz. 1.4 Temperature and current The laser frequency also depends on temperature and current; the senstvtes are typcally 3 MHz/µA and 30 GHz/K [7]. Thus, low-nose stable electroncs, such as the MOGLabs DLC external cavty dode laser controller, are essental [2] to acheve sub-mhz lnewdth and stablty. An mportant aspect of an ECDL s temperature control of the cavty, snce the laser frequency depends on the cavty length and hence on the thermal expanson coeffcent of the cavty materal [1]. The cavty can be machned from materals wth low thermal expanson coeffcent but even then the passve stablty s nadequate for research applcatons. Actve feedback of the cavty temperature combned wth cavty length control provde a flexble and stable approach. The MOGLabs LDL uses a negatve temperature coeffcent (NTC) thermstor to sense the cavty temperature and Pelter thermoelectrc cooler (TEC) to heat and cool the cavty materal.

19 2. Frst lght Intal nstallaton of the laser s typcally a matter of mountng t to an optcal table and connectng to a MOGLabs controller. Mountng holes can be accessed by removng the cover, so that the M6x16 socket head cap screws provded can attach the laser to the optcal table. The hole spacng also allows drect mountng to mperal tables for non-metrc countres (Burma, Lbera and the USA). The laser ncludes a water coolng channel for laser operaton at unusually hgh or low temperatures, or n laboratores wth hgh or unstable ar temperature. For most applcatons, water coolng s not requred; dsspaton to the ar and/or optcal table s suffcent. The performance of an external cavty dode laser s strongly dependent on the external envronment, and n partcular acoustc vbratons. Very small changes n the external cavty length have a large effect on the laser frequency, typcally 25 MHz per nanometre length change. The monolthc block constructon of the MOGLabs LDL reduces the nfluence of vbratons on the cavty length, but some elastcty remans. The LDL s hermetcally sealed to substantally reduce the effects of acoustc dsturbances n the cavty ar gap. Actve feedback to the laser frequency, va pezo translaton and current modulaton, reduces external nfluences, but some smple measures to mnmse couplng to envronmental varatons and vbraton sources may be warranted. For example, a surroundng box to reduce ar movement and accdental bumpng of the laser, mountng the laser to a heavy support, and solaton from the optcal table wth an ntermedary breadboard whch s separated from the man optcal table wth vscoelastc polymer (e.g. Sorbothane TM ). Once the laser s mounted approprately, the laser can be powered on. It s assumed that a MOGLabs DLC controller has been provded wth your laser and that the temperature and current lmt have been set approprately. 5

20 6 Chapter 2. Frst lght If an alternatve supply s used, please set a current lmt accordng to the maxmum safe operatng current stpulated n the test data provded wth your laser. Also note that +5 V must be provded on pn 15 of the headboard connector to open the protectve relay; see appendx B for connecton detals. 2.1 Temperature The preferred dode temperature wll depend on the dode, the requred wavelength, and the ambent room temperature. For example, typcal AlGaAs dodes used for data storage applcatons (CD-R burners) have a nomnal wavelength of λ = 784 nm at 25 C, wth a dλ/dt slope of 0.3 nm/ C, mplyng an optmum temperature of about 12. Dependng on the humdty, low temperatures may nduce condensaton on the dode and collmaton lens. The gratng feedback wll determne the fnal wavelength, and the feedback s generally suffcent to pull the wavelength by ±5 nm, and thus n ths example a sensble set temperature would be about 17 to 18 C. 2.2 Current The output of semconductor laser dodes follow a nomnally lnear power vs. current (PI) relatonshp, once the current s above a devce-specfc threshold (see Fg. 2.1). Intally the current should be set above threshold, but well below the nomnal maxmum operatng current, untl the laser s fully algned.

21 2.2 Current nm 150mW dode Optcal output power (mw) Bare dode (no cavty) Wth external cavty feedback Injecton current (ma) Fgure 2.1: Sample laser dode power-current PI characterstc curves, wth and wthout an external cavty. The external cavty feedback reduces the threshold current, and also the apparent power/current slope because the measured power wth feedback s not the power from the bare dode, but the output beam reflected from the gratng. The slope wth feedback n ths example s 75% of the bare dode output slope, consstent wth the gratng drect reflectvty.

22 8 Chapter 2. Frst lght

23 3. Operaton Fgure 3.1 s a schematc of the laser. Normal operaton of the laser s usually a matter of selectng the correct wavelength, and adjustng the parameters to acheve the maxmum possble mode-hop free scan. Sprng plunger Collmaton lens Gratng puck Gratng Pezo Laser dode Mrror λ adjust Fgure 3.1: Cross-secton sketch of the MOGLabs LDL, showng the cylndrcal gratng mount (puck), pezo-mounted gratng, fold mrror, and wavelength adjustment. The gratng mount can be rotated usng the tangental λ fne adjustment screw. A counteractng sprng-plunger should be released to allow rotaton. 3.1 Wavelength The prmary control of wavelength s the gratng angle, whch can be adjusted whle the laser s operatonal. A wavemeter [8], hgh-resoluton spectrometer, or smlar s almost essental, although wth patence t s possble to fnd an atomc reference by carefully adjustng the gratng angle whle scannng the laser. Note that the wavelength s qute senstve to gratng angle. For example, 9

24 10 Chapter 3. Operaton wth the standard λ = 780 nm, 1/d = 1800 l/mm gratng, the angular dependence s about 14 nm per degree of gratng angle. Wth the LDL, that s 8 nm per full turn of the wavelength adjustment screw. Set the laser current so that the output power s suffcent, ensurng that the nternal cavty power s below the maxmum rated for the dode (see Fg. 2.1). Then change the gratng angle to adjust the wavelength. The laser wll hop between external cavty modes, as the wavelength s adjusted, through cycles of dm and brght output. Adjust the angle to one of the brght modes nearest the optmum wavelength, and then adjust the laser current and pezo voltage to acheve the exact wavelength requred. It may then be necessary to adjust the vertcal algnment slghtly; follow the flash procedure outlned prevously. That s, set the njecton current just below threshold, and adjust the vertcal algnment untl the laser flashes, and repeat untl the threshold current s mnmsed. 3.2 Scannng, mode-hops, and bas Mode-hops are a frequent occurrence wth external cavty dode lasers. A mode-hop s a dscontnuty when tunng or scannng the laser wavelength. As the laser wavelength s vared, usually by changng the cavty length wth a pezo, competton between the wavelength determned by the dfferent wavelength-dependent cavty elements can lead to a mode hop: a jump n laser wavelength to a dfferent external cavty mode. Wavelengthdependent elements nclude the external cavty, the laser dode nternal cavty between the rear and front facets of the dode, the flter transmsson, and the gan bandwdth of the laser dode. The dfferent wavelength-dependent characterstcs are shown schematcally n fgure 3.2. The net gan s the combned product of semconductor gan, flter transmsson, nternal and external cavty nterference. The net gan can be very smlar at adjacent external cavty modes. A small change n the nternal cavty mode, or the flter angle, can lead to the overall gan beng greater at a mode adjacent to the mode n whch the laser s oscllatng, and the laser then hops to that hgher-gan mode. See Ref. [1] for a detaled dscusson.

25 3.3 Scannng 11 Dode cavty External cavty Flter Dode gan COMBINED Frequency (THz) Fgure 3.2: Schematc representaton for the varous frequency-dependent factors of an ECDL, adapted from Ref. [1], for wavelength λ = 780 nm and external cavty length L ext = 15 mm. 3.3 Scannng The external cavty length s usually controlled by pezo actuators movng the output coupler. The cavty length changes wth pezo voltage, and for a large change, the laser wll usually hop to a neghbourng cavty mode. Fgure 3.3 s a schematc of the net gan varaton wth laser frequency, showng two adjacent modes of very smlar gan. Fgure 3.4 s a measurement of the frequency of a laser scannng properly, and wth a mode-hop at one edge of the scan. The mode-hop-free scan range (MHFR) can be optmsed by careful adjustment of the njecton current, whch affects the refractve ndex of the dode and hence the frequency of the cavty mode BIAS optmsaton Ths shft of cavty mode frequency allows for compensaton of the msmatch of tunng responses. The dode njecton current can be automatcally adjusted as the laser frequency s changed, usng a feed-forward or cur-

26 12 Chapter 3. Operaton 1 Relatve Gan Frequency (GHz) Fgure 3.3: Combned gan for an external cavty dode laser, ncludng the nternal and external modes, the dode laser gan, and the flter response. The broad feature s the frequency selectvty of the flter, and the smaller peaks are the external cavty modes (see fg. 3.2). A small relatve shft of the external cavty mode relatve to the flter frequency wll cause the laser to jump to another external cavty mode where the net gan s hgher. rent bas whch changes as the pezo voltage s changed. Feed-forward current bas adjustment s a feature of MOGLabs DLC controllers. Adjustment s straghtforward. Wth the laser frequency scannng, the current bas control s adjusted untl the maxmum mode-hop-free scan range s observed. Small changes to the njecton current optmse the scan range near the nomnal centre frequency. Detaled nstructons follow. A Fzeau wavemeter, an atomc absorpton spectroscopy sgnal, or a Fabry-Perot cavty s requred, to montor the actual laser frequency whle varyng the dfferent control parameters. 1. Make sure that BIAS s enabled (DIP swtch 4). 2. Set the FREQUENCY knob to approxmately 0V (use montor dsplay Frequency on the 8-poston selector swtch).

27 3.3 Scannng 13 Fgure 3.4: Frequency of a laser scannng properly (left) and wth a mode-hop at one edge (rght). 3. Wth SPAN set to max, adjust the BIAS trmpot to zero ampltude as see on montor CHAN B Current output. 4. Adjust the laser dode CURRENT so that the laser wavelength and power are correct. Use the values provded n the orgnal factory test report as a gude. 5. If the wavelength s close but not qute correct, small adjustments of ether CURRENT or FREQUENCY may be requred to fnd a better lasng mode. If more sgnfcant wavelength adjustment s requred, ether mechancally rotate the flter of the laser, or for changes of less than 0.2 nm, adjust the temperature set-pont by 0.2 to 0.5 C. Note that the response to adjustment of the temperature setpont s slow, and you should wat several mnutes for the temperature to equlbrate. 6. If the wavelength s wthn a few pm (GHz) of your target, ncrease the SPAN whle observng the Fzeau wavemeter Long Term measurement (or spectroscopy scan or FP cavty transmsson on an osclloscope), as shown n fg As the SPAN s ncreased, you wll at some pont observe a mode hop. For spectroscopy scans t s easer to observe mode hops usng the AC error sgnal from the MOGLabs DLC, f current modulaton s enabled. The mode hop should be at one edge of the scan; f so, adjust the FREQUENCY so that the scan no longer clps ths mode hop

28 14 Chapter 3. Operaton (.e. the scan s free of mode hops), and contnue adjustng n the same drecton untl a mode hop s observed on the other edge of the scan. 8. Adjust the FREQUENCY to the md-pont between the two extremes. 9. Increase SPAN further, untl a mode hop s agan apparent, and readjust the FREQUENCY to the md-pont. 10. Repeat untl mode hops are observed at both edges of the scan. 11. Adjust the dode CURRENT by small amounts to try to remove at least one of these mode hops, then attempt to ncrease the SPAN further. 12. If the mode hops are at both edges of the scan and cannot be removed by FREQUENCY or CURRENT adjustments, turn the BIAS trmpot ether clockwse or counterclockwse to remove one of both of the mode hops. If one trmpot drecton only makes the mode hops worse, try the other trmpot drecton. If both mode hops are removed, repeat the steps above (ncreasng SPAN) untl no further mprovements can be made to the MHFR. 13. If the MHFR s substantally less than expected (refer to the factory test report), t may be advsable to change mode by ncreasng or decreasng the CURRENT to fnd a nearby sngle-mode current, to rotate the flter slghtly to alter the net gan so that one cavty mode has hgher gan than those adjacent.

29 4. Algnment Algnment of the laser may be requred, for example f the laser dode s replaced, or perhaps ntally after shppng f the laser has been mshandled, or after makng sgnfcant changes to the laser wavelength. The process s straghtforward and normally takes only a few mnutes. For long-wavelength lasers, an nfra-red upconverson card or vdeo camera can be very helpful. Common low-cost securty cameras, computer USB cameras, and home move or stll cameras are also good optons, although they may have an nfra red blockng flter whch must be removed. Dodes are very senstve to electrostatc dscharge. Please make sure you are electrcally grounded, deally wth a wrst ground strap. If you do not have a proper wrst ground strap, at least be sure you are not wearng woolen clothng, and touch somethng grounded from tme to tme (e.g. a solderng ron tp, the earth of a power supply, the MOGLabs DLC controller). 4.1 Pre-algnment of lens tube and dode 1. Insert the laser dode nto the lens tube (see fg. 4.1). Ensure that the V-notch n the base flange of the dode can s not algned wth one of the radal algnment screws. 2. Add the retanng threaded rng, and tghten gently, enough such that the dode does not move but not so much that t cannot move. 3. Approxmately centre the dode usng the algnment adjustment screws and two 0.9 mm hex keys. 4. Insert the collmaton lens, takng care to ensure that the lens does not contact the dode. Also ensure the lens s tght; f not, use PTFE tape on the lens threads. One or two layers of thck tape (90 µm as used for gas plumbng) s usually suffcent. 15

30 16 Chapter 4. Algnment 5.6mm dode Retanng rng Lens 9mm dode Fgure 4.1: Lens tube assembly, showng dode, lens, and mountng hardware. The same tube can be used for 5.6 mm and 9 mm dodes. Fgure 4.2: Image showng collmaton tubes wth algnment adjustment screws. 5. Mount the lens tube n a holder or mount that allows rotaton of the entre assembly around the long axs. 6. Apply power to the dode, above threshold but well below the maxmum permssble current. 7. Approxmately focus at several metres dstance. It may be helpful to reflect t from a mrror and back so that you can adjust the algnment and see the effect nearby. You should adjust focus untl you see a clean symmetrc ellpse at ths dstance. 8. Rotate the collmaton assembly and adjust the algnment screws untl the beam remans reasonably well on-axs.

31 4.2 Intal dode test Adjust the algnment to optmse the laser beam spatal profle even at the expense of mantanng concentrc algnment. The profle should be a symmetrc ellpse wth Gaussan profle along each axs. 10. Tghten the retanng rng (hard) and re-check that the beam profle remans unform and symmetrc. 11. Focus the collmaton lens such that the laser focuses to a spot at some sgnfcant dstance, more than 4 m. The laser stablty and modehop free range can be better f the laser output s weakly convergng [2]. 4.2 Intal dode test 1. Inspect the beam profle for dffracton frnges. If the lens has been screwed n too far and made contact wth the dode (partcularly for 5.6 mm dodes), the lens can become scratched or stressed, leadng to poor performance. Frnges can be an ndcaton of such scratches (or an ndcaton of a poor dode). 2. On the MOGLabs DLC controller, make sure DIP swtch 4 (Bas) s OFF, the span s set to zero (fully ant-clockwse), and the frequency knob s at zero (mddle of range; set the dsplay selector to Frequency and adjust to zero volts). 3. Measure the power/current (PI) curve for the bare collmated dode. Ths provdes a useful benchmark for comparson when optmsng the threshold lowerng wth feedback. 4.3 Orentaton and polarsaton of the output beam The output from the dode s a wdely dvergng ellptcal beam. The gratng dsperson (.e. frequency selectvty) ncreases wth the number of rulngs llumnated by the lght: λ/λ 1/N where N s the number of gratng lnes llumnated. The ellpse s therefore typcally orented wth the major axs perpendcular to the gratng rulngs. For the MOGLabs LDL, the gratng rulngs are vertcal and so the ellptcal beam should have the

32 18 Chapter 4. Algnment Gratng E From dode Output beam Fgure 4.3: Orentaton of the dode laser beam ellpse wth respect to the dffracton gratng. major axs horzontal (see Fg. 4.3), for most laser dodes whch operate n TE mode. The dode laser polarsaton s usually parallel to the short (mnor) axs of the ellpse. Thus, for the orentaton descrbed above, the polarsaton s parallel to the gratng rulngs. However, the gratng feedback effcency s larger when the polarsaton of the ncdent lght s perpendcular to the gratng rulngs, so for the arrangement shown, the dffracton effcency s small (typcally around 15%). Whle low effcency mght seem undesrable, 15% s usually suffcent for sngle-mode operaton of the laser, and the hgh percentage of non-dffracted lght s drectly reflected to provde the maxmum possble power n the output laser beam. 4.4 Algnment The horzontal and vertcal angles of the gratng, and the lens focus, must be adjusted so that the dffracted beam propagates back nto the ext facet of the dode, so that the external cavty domnates the optcal feedback. When algned, the external cavty feedback overrdes the feedback from the front facet of the dode tself, so that the laser frequency s determned by the external cavty. The feedback algnment s optmsed by settng

33 4.4 Algnment 19 the dode current just below threshold, and then adjustng the vertcal algnment untl the output suddenly flashes brghtly, ndcatng effectve feedback whch tends to lower the overall gan threshold. The feedback s optmsed by algnng the beam from the laser dode, to the gratng and back to the dode. The vertcal angle of the beam s adjusted to mnmse the lasng threshold current. The laser dode tube s mounted nto a rotatable cylndrcal mount (see fg. 4.4). A fne adjustment screw s used to adjust the vertcal tlt angle of the laser dode, aganst a sprng plunger that pushes the tlt arm downwards. Two lockng screws at the rear clamp the cylnder; these can be released slghtly to make adjustments, and tghtened when satsfactory algnment s acheved. Sprng plunger Up/down Sprng Up/down vertcal tlt λ adjust 2 x lockng screws Fgure 4.4: Sketch showng locaton of vertcal algnment adjustments: a fne adjustment screw to push the laser dode beam upwards, and a sprng plunger that pushes downwards.

34 20 Chapter 4. Algnment The sequence s as follows: 1. Insert the lens tube nto the laser cavty. 2. Project the output beam onto a pece of black card at a dstance of about 30 cm from the laser. Montor ths beam spot usng a vdeo camera such as a webcam. 3. Rotate the lens tube so that the ellptcal profle of the output beam s horzontal (for TE polarsed laser dodes). 4. Adjust the dode current well above threshold, and search for a secondary output beam caused by the dffracted lght propagatng back nto the dode and then reflectng from the rear of the dode back to the screen. Try adjustng the vertcal algnment to see a spot movng up and down faster than the man output beam. 5. Algn the reflecton of the return beam so that the two spots are centred horzontally, but dsplaced vertcally. 6. Adjust the vertcal algnment untl the secondary beam s colnear wth the prmary. The laser should sgnfcantly brghten or flash when the gratng feedback s algned back nto the dode. 7. Adjust the njecton current to just below threshold. 8. Adjust the vertcal algnment and gratng angle (wavelength) untl flash (.e. lasng). 9. Iterate reducton of the njecton current, followng by algnment untl lasng occurs, untl the mnmum threshold s acheved. The gratng wavelength (horzontal angle) should then match the dode free-runnng wavelength. 10. If the threshold s not sgnfcantly lower (at least a few ma), remove the lens tube and adjust the focus of the collmaton lens, beng careful not to touch the surface of the lens. Usually the lens should be moved slghtly closer to the dode, clockwse when vewed from

35 4.4 Algnment 21 the lens sde, f the lens was prevously set to focus some dstance from the laser. 11. Iterate untl threshold lowerng s sgnfcant. Note that there s a compromse here. At mnmum threshold, feedback s optmsed gvng the narrowest lnewdth. However, then the overlap of the back-reflected beam wth the laser output facet s qute crtcal, whch can reduce the mode-hop-free scan range and make the laser more senstve to acoustc vbratons. It s generally easer to have a weakly focusng beam. 12. Increase the current to well above threshold, check the laser wavelength, and adjust the gratng angle f requred. The wavelength adjustment s about 0.1 turns per nm, and clockwse to ncrease wavelength. Note that f the wavelength of maxmum gan s far from the desred wavelength, t may be a good dea to change the operatng temperature to reduce that gap, before proceedng. 13. Adjust the vertcal algnment to mnmse the threshold. 14. If possble, scan the laser through an atomc resonance and vew the absorpton on an osclloscope. Wth current bas dsabled (DIP 4 on a MOGLabs controller) and full span, the pattern should repeat several tmes as the laser scans over a short range and then modehops. A Fabry-Perot etalon or a fast hgh-resoluton wavemeter (e.g. MOGLabs MWM) can also be used to optmse the mode-hopfree range. 15. Adjust the algnment and gratng angle, and the njecton current, to optmse the scans so that you see the maxmum number of repeats and the deepest sgnals. 16. Check that there s only one sgnfcant output beam spatally and, f avalable, use a Fabry-Perot to check for sngle frequency. 17. Check that the saturated absorpton traces are clean. Nosy spectra ndcate mult-mode operaton, or hgh lnewdth, whch may be due

36 22 Chapter 4. Algnment to weak feedback. The feedback depends on the collmaton lens focus. The lasng threshold s a good dagnostc: lower threshold ndcates better feedback and consequently lower lnewdth, at the expense of senstvty. A nosy spectrum can also be due to extreme senstvty to acoustc dsturbance, or to external feedback. A scannng Fabry-Perot s a very useful dagnostc tool to check for sngle-mode operaton. 18. Measure the laser output power as a functon of dode njecton current, and plot the power/current response as n Fg Swtch the current bas (DIP swtch 4) back on, and adjust the bas to optmse the mode-hop-free scan range. The laser should now be operatng wth gratng controlled feedback near the desred wavelength of the dode. The threshold current should be sgnfcantly lower than wthout feedback (2 to 5 ma for uncoated 780 nm dodes). Record the output power and threshold characterstcs for subsequent reference.

37 4.5 CEF extended chasss CEF extended chasss The LDL can be suppled n a very compact form, or wth optonal extended chasss (opton CEF) whch allows nternal mountng of Faraday solator, and also the addton of a fbre coupler (see fg. 4.5). Fgure 4.5: The MOGLabs extended chasss CEF opton, shown here wth CEL cateye nstalled Faraday solator algnment Faraday solators are almost always requred for external cavty dode lasers. The very hgh gan of the semconductor dode and low optcal feedback of the external cavty mean that even very low power external feedback can have a sgnfcant effect on the laser frequency stablty. Generally 30 db of solaton s needed; that s, the optcal feedback nto the ECDL should be less than 0.1% of the output power. The extended chasss verson of a MOGLabs laser allows nternal mountng of a Faraday solator (see fgure 4.6). Algnment s straghtforward: the solator should be concentrc wth the ext beam of the laser, and rotated axally so that the frst polarsers s parallel to the polarsaton of the laser beam. The power of the laser should be measured before nsertng the solator, and then the solator poston and rotaton adjusted to maxmse

38 24 Chapter 4. Algnment M1 Fbre coupler Faraday solator M2 Fgure 4.6: Schematc of the extended chasss laser (shown here wth a cateye laser barrel) showng Faraday solator, and two mrrors used for algnng the beam to a sngle-mode fbre. λ/2 waveplate or 0.9mm hex key Fgure 4.7: Faraday solator and ext λ/2 waveplate. The waveplate can be rotated to rotate the plane of polarsaton of the ext beam, for example to optmse couplng nto polarsaton mantanng fbre, or to adjust the rato of ext beams for lasers ftted wth a polarsng beamspltter nstead of mrror M2. the transmsson. Dependng on wavelength, the transmsson vares from about 70% to 95%, wth 90 to 92% typcal at 780 nm. The solator rotates the polarsaton of the laser beam by 45. For lasers ordered wth fbre couplng, or wth dual beam output (usng a PBS polarsng beamspltter cube), the solator can n most cases be ordered wth nternal half-wave retardaton waveplate. The waveplate s mounted nsde the fnal slver-coloured metal element of the retarder (see fgure 4.7).

39 4.6 Fbre algnment 25 The waveplate angle may need adjustment, for example to vary the power rato for the two beams extng the PBS or to algn the polarsaton to a more convenent horzontal or vertcal axs for experments, or to algn to a polarsaton preservng fbre. To adjust the waveplate angle, loosen the radal set screw holdng the waveplate usng a or 0.9mm hex key, rotate, and restore set screw tenson. A second waveplate holder s avalable, whch mounts nsde the ext face of the laser (see fgure 2). A standard 25.4 mm waveplate can be nserted, to algn the polarsaton of the reflected ext beam to match to an external experment or to a polarsaton preservng fbre, ndependent of the drect output beam. 4.6 Fbre algnment The extended chasss s most often used when couplng to a sngle-mode otpcal fbre. Two mrrors are used to algn the beam to the fbre coupler: a common and famlar arrangement for optcal scentsts (see fgure 4.6). The arrangement convenently allows splttng the output nto two beams, usng a PBS as the frst reflector. Gven the 8% Fresnel loss from entrance and ext facets of the fbre, the maxmum theoretcal effcency for sngle-mode fbre couplng s 92%. The stanless steel knematc mrror mounts are stable and easy to use, and couplng effcency of over 70% s easly attaned at 780 nm. Algnment requres frst adjustng the mrrors so that the beam exts the laser chasss n the centre of the fbre couplng port, and parallel to the long axs of the chasss. The fbre coupler can then be nstalled, wthout fbre, and the mrrors adjusted so that the beam s clearly transmtted by the coupler (see below for detaled nstructons). For Schäfter-Krchhoff 60FC fbre couplers, detaled nstructons on optmsng the couplng effcency are provded va ther webste: CouplngSMS.pdf. An eccentrc key s provded for adjustng the lens focus.

40 26 Chapter 4. Algnment Reverse beam: usng a vsual fault locator A vsual fault locator s a very useful devce for quckly achevng ntal couplng of the laser beam to the fbre. A vsual fault locator (see fg. 4.8) s a low-power red laser that njects a beam nto the ext end of the fbre patchcord, thus propagatng vsble lght backwards along the fbre and then nto free space, formng a beam back nto the laser cavty. These devces are very low n cost (search on ebay for vsual fault locator; they are typcally less than $20). Fgure 4.8: Fbre laser pen, or vsual fault locator. Injects vsble laser beam nto fbre, whch allows basc algnment and mode matchng. Algnng the MOGLabs laser beam to the fbre s then smply a matter of adjustng the mrrors so that the MOGLabs laser beam and the vsual fault locator beam overlap nsde the laser. It wll be easer f the Faraday solator s temporarly removed Mrror adjustment To maxmse the fbre couplng effcency, the angle and locaton of the laser beam at the fbre coupler must be optmsed by walkng the mrrors. Let M1 be the mrror closest to the fbre coupler, and M2 be closest to the laser (see fgure 4.6). 1. Adjust the laser current so that the output power s around 5 to 10 mw. 2. If some power s detected extng from the fbre, skp to step 9 below. 3. If the fbre coupler s not yet nstalled, frst coarsely adjust the mrrors so that the beam exts through the centre of the fbre coupler

41 4.6 Fbre algnment 27 mount, and parallel to the long axs of the laser chasss. Then nstall the coupler. 4. If some power s detected extng from the fbre, skp to step 9 below. 5. Wth fbre patchcord removed, adjust the mrrors so that the beam exts from the fbre coupler cleanly. You should be able to observe a brght beam centred n the crcle of a shadow of the fbre coupler. 6. Measure the power just before the fbre coupler and record the power meter readng. 7. If not already nstalled, connect the fbre. 8. If a vsual fault locator s avalable, use that to nject a backwardspropagatng beam, and adjust the mrrors so that the MOGLabs laser and vsual fault locator beams are concdent along ther paths. The vsual fault locator can then be removed: a measurable transmtted beam should be evdent at the fbre ext. 9. Fx the power meter to montor the output power extng from the optcal fbre. Make sure background lght s not affectng the readng. 10. For the horzontal axs frst, fnd the maxmum output power by adjustng the mrror M1, closest to the fbre (furthest from the solator), and record the output power. 11. Adjust the horzontal axs of mrror M2 furthest from the fbre (closest to the solator) clockwse such that the output power drops by no more than 25%. If the effcency s over 50%, drop the power by only 5 to 10% or less. Take note of roughly how many degrees rotaton were requred, so you can easly return to the orgnal poston. 12. Adjust the horzontal axs of mrror M1 and maxmse for output power. Compare the new maxmum output power to the output power obtaned at step Repeat steps 10 to 12 f the power s ncreasng, or repeat but wth reversed drecton of adjustment f the power s decreasng.

42 28 Chapter 4. Algnment 14. Once horzontal algnment s optmsed, repeat the procedure but usng vertcal adjustments. 15. Iterate horzontal and vertcal algnment untl power s fully optmsed. As optmum couplng s approached, the adjustments should be reduced at each step. 16. If the couplng effcency s less than expected, focus adjustment may be requred (see nstructons from Schäfter and Krchhoff). Focus adjustment s not normally needed unless severe shock has moved the lens, or f a new dode has been nstalled n the laser, leadng to change of beam wast locaton. 17. Once optmsed, record the nput power to the fbre coupler, maxmum output power, and the laser current. 18. Increase the laser current to the desred operatng current and optmse f needed. 19. Use the factory test results for your laser as reference. Degradaton may ndcate facet damage on the fbre patchcord. Reversng or replacng the fbre patchcord may be helpful.

43 A. Specfcatons Parameter Specfcaton Wavelength/frequency nm Dode dependent. Please contact MOGLabs for avalablty. Lnewdth Gratng Tunng range Typcally < 200 khz FWHM Standard: 1800 l/mm holographc Au Up to 100 nm, dependng on dode Sweep/scan Scan range Mode-hop free Pezo stack Cavty length 40 GHz typcal > 10 GHz; up to 40 GHz (780 nm, uncoated dode) V, 100 nf typcal mm Optcal Beam Polarsaton 3 mm 1.2 mm (1/e 2 ) typcal Vertcal lnear 100:1 typcal 29

44 30 Appendx A. Specfcatons Parameter Specfcaton Thermal TEC Sensor Stablty at base Coolng ±14.5 V 3.3 A Q = 23 W standard NTC 10 kω standard; AD590, 592 optonal ±1 mk (controller dependent) 4 mm dam quck-ft connectors Electroncs Protecton Indcator Modulaton nput Connector Dode short-crcut relay; cover nterlock connecton; reverse dode Laser ON/OFF (LED) Actve (AC and DC coupled) or RF bas tee MOGLabs DLC Dode Laser Controller sngle cable connect Mechancal & power Dmensons mm (L W H), 1 kg Beam heght 58 mm Shppng mm (L W H), 3.1 kg

45 A.1 LDL mechancal 31 A.1 LDL mechancal x M4x0.7 4 x M6 clearance Fgure A.1: Dmensons of LDL laser head.

46 32 Appendx A. Specfcatons A.2 CEF mechancal Fgure A.2: Dmensons of CEF laser head.

47 B. Laser head board The laser head nterface board provdes connecton breakout to the laser dode, TEC, sensor, pezo actuators, and laser head nterlock. It also ncludes a sold-state protecton relay and passve protecton flters, a laser-on LED ndcator, and an SMA connecton for drect dode current modulaton. The connectons are made wth Hrose DF59 swng-lock wre-to-board connectors. Several versons of the laser headboard are avalable. The B1047 headboard provdes hgh bandwdth actve current modulaton for wde bandwdth frequency stablsaton and lnewdth narrowng, for example usng a hgh fnesse optcal cavty or polarsaton spectroscopy. Hgher bandwdth s provded by the B1240 headboard whch further ncreases bandwdth and reduces phase delay, easly achevng sub-hz lnewdth narrowng. The B1240 s lmted to low complance voltage laser dodes (red and nfrared); the B1047 must be used for blue dodes. For RF modulaton, the B1045 s avalable. The B1045 ncludes an RF bas tee allowng modulaton up to 2.5 GHz, for example to add sdebands for repumpng, or to add nose for coherence control. For hgh bandwdth RF modulaton the dode can be drectly soldered to a specal nterconnect assembly avalable from MOGLabs. In all cases, there s no provson for the nternal photodode n many consumer-grade laser dodes. 33

48 34 Appendx B. Laser head board B.1 B1045/1046 headboard The B1045 and B1046 provde connecton to one or two pezos (slow hghrange mult-layer stack and fast dsc), and ether passve NTC thermstor or actve AD590/592 actve temperature sensor. Note only one temperature sensor should be connected, not both. They provde an SMA nput for drect dode modulaton va an RF bas tee (see B.1.1 below). Dsc Fgure B.1: MOGLabs B1045 and B1046 laser head boards showng connectors for laser dode, pezo actuator, temperature sensors, TEC and head enclosure nterlock. B.1.1 RF couplng For the B1045/1046 headboard, the SMA connector allows hgh-frequency current modulaton va a bas-tee. The RF nput s AC coupled, wth lowand hgh- frequency lmts of about 30 khz and 2.5 GHz (see fg. B.2). Capactor C4, ether 47 nf or 100 pf, can be changed to adjust the lowfrequency cutoff. For hgher bandwdths, use an external bas-tee such as the Mn-Crcuts ZFBT-4R2GW-FT between the head board and the dode.