External Cavity Diode Laser

Transcription

1 External Cavty Dode Laser CEL and CEF Cateye Revson 1.12

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 Dode lasers can be wonderful thngs: they are effcent, compact, low cost, hgh power, low nose, tunable, and cover a large range of wavelengths. They can also be obstreperous, senstve, and temperamental, partcularly external cavty dode lasers (ECDLs). In combnaton wth advanced electroncs such as the MOGLabs DLC external cavty dode laser controller, the CEL cateye laser descrbed here provdes a robust, stable, acoustcally nert, low lnewdth and hghly tunable laser system. We hope that the MOGLabs CEL works well for your applcaton. 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 CEL 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 CEL s desgned and prced wth the expectaton that the end-user can replace the dode and change the algnment, some components are fragle. In partcular the flter, pezo actuator, and output coupler are very easly damaged. Please take care of these tems when workng nsde the laser. The flter and output coupler are hard-coated and can be cleaned but great care s needed as wth any ntracavty laser optcs. 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 CEL laser and large-chasss CEF verson.

7 v 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 US FDA complance AVOID EXPOSURE 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 200 mw IEC :2007 AS/NZS :2006 Warnng and advsory label Class 3B Fgure 1: Warnng advsory and US FDA complance labels.

8 v INVISIBLE LASER RADIATION AVOID EXPOSURE TO BEAM CLASS 3B LASER PRODUCT Wavelength Max Power nm 200 mw IEC :2007 AS/NZS :2006 Emsson ndcator INVISIBLE LASER RADIATION AVOID EXPOSURE TO BEAM CLASS 3B LASER PRODUCT 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 Wavelength nm Max Power 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 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 CEL laser near the ext aperture; warnng advsory label on the rear and complance label on sde.

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 the cateye 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. The laser head board also 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 CEL to seed an amplfer, frst turn off the amplfer. 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 Pezo-electrc frequency control Temperature and current Frst lght Temperature Current Operaton Wavelength Scannng, mode-hops and BIAS Scannng BIAS optmsaton Algnment Pre-algnment of lens tube and dode Intal dode test Orentaton and polarsaton of the output beam Cateye reflector CEF extended chasss v x x x

14 x Contents Faraday solator algnment Fbre algnment Reverse beam: usng a vsual fault locator Mrror adjustment A Specfcatons 27 A.1 CEL mechancal A.2 CEF mechancal B Laser head board 31 B.1 B1045/1046 headboard B.1.1 RF couplng B.2 B1047/B1240 headboards B.2.1 SMA nput B.3 Laser connecton References 40

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 CEL (see Fg. 1.1) s machned from a sold alumnum block, so that the laser s stable, robust, and nsenstve to acoustc dsturbances. The cavty s hermetcally sealed for addtonal suppresson of envronmental fluctuatons and drft. Fgure 1.1: The MOGLabs CEL cateye laser. 1

16 2 Chapter 1. Introducton The MOGLabs CEL s a cat-eye desgn (see Fg. 1.2), n whch an external cavty s formed between the rear reflectng surface of the semconductor dode, and a cat-eye reflector at several centmetres from the dode [1 3]. Rather than the customary dffracton gratng of Lttrowconfguraton ECDLs, a hgh effcency ultranarrow flter s used to select a sngle external cavty mode. Wthout the need for llumnatng a large area of a gratng for feedback, a cat-eye retroreflector and partally transmttng output coupler can be used to form the external cavty. The cateye reflector s nherently self-algnng, so that the laser s extremely nsenstve to mechancal dsturbance, and also ensures hgh feedback couplng effcency and consequently narrow lnewdth. LENS PZT DIODE FILTER LENS OC LENS Fgure 1.2: Schematc of a cateye external cavty dode laser (ECDL). The external cavty, formed by the rear facet of the laser dode and the output coupler, determnes the laser frequency. One longtudnal cavty mode s selected by an ultranarrow ntracavty bandpass flter. A cateye reflector s formed by the output coupler (OC) and ntracavty lens, and the lght s recollmated by the extracavty output lens. The output beam from a laser dode s collmated wth a hgh numercal aperture (NA) lens and ncdent on the flter. The flter transmsson wavelength depends on the rotaton angle. Transmtted lght s back-reflected by the cateye lens/output-coupler combnaton whch effcently couples lght back nto the laser dode. More detals can be found n the references [1 3].

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 cateye and the dode rear facet, and when the external feedback s greater than that of the front facet, the external cavty determnes the lasng wavelength. The external cavty s typcally around 40 mm long from rear facet of semconductor to output coupler, gvng a cavty mode spacng (FSR) of c/2l = 3 to 4 GHz. The laser dode and collmatng lens are held rgdly n a focusng tube. The flter s fxed to a bearng-mounted rotaton assembly wth fne actuator screws to adjust the angle. The sprng-loaded screws operate n a pushpull arrangement whch can be locked aganst each other to further reduce the effects of mechancal vbraton. Varaton of the flter angle s used for coarse selecton of the wavelength, wthn the gan bandwdth of the laser dode. 1.2 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 CEL, the output coupler s mounted to a multlayer pezoelectrc stack. The cavty length varaton s of order 10 nm per volt, producng a frequency shft of 70 MHz/V wth a range of 10 GHz for 150 V drve voltage. The bandwdth s lmted by mechancal resonances, typcally 25 khz. 1.3 Temperature and current The laser frequency s also dependent on temperature and njecton current; the senstvtes are typcally 3 MHz/µA and 30 GHz/K [5]. Thus, low-nose stable electroncs, such as the MOGLabs DLC external cavty dode laser controller, are essental (see Ref. [6]) to acheve sub-mhz lnewdth and stablty.

18 4 Chapter 1. Introducton 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 [4]. 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 CEL 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 Once the laser s mounted approprately, the laser can be swtched on. For longer wavelength lasers, an nfra-red upconverson card or CCD 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 often have nfra red flters whch may need to be removed. 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. 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 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 18 C. 2.2 Current The output of semconductor laser dodes follow a lnear power vs. current 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 5

20 6 Chapter 2. Frst lght the nomnal maxmum operatng current, untl the laser s fully algned Bare 150mW dode nm Extracavty estmate Power (mw) Injecton current (ma) Fgure 2.1: Sample laser dode power-current characterstc curves, wth and wthout an external cavty. The output for a dode wth good ant-reflecton coatng s neglgble. The steps show that for hgher currents, some of the lght from the dode s not transmtted by the flter, typcally because the external cavty mode frequency s not perfectly algned wth the flter transmsson frequency.

21 3. Operaton Normal operaton of the laser s usually a matter of adjustng the flter rotaton angle to select the correct wavelength, and adjustng the pezo offset, dode njecton current and bas to acheve the maxmum possble mode-hop free scan. 3.1 Wavelength The prmary control of wavelength s the flter rotaton angle, whch can be adjusted whle the laser s operatonal. A wavemeter, hgh-resoluton spectrometer, or smlar s almost essental, although wth patence t s Sprng plunger Flter notch Spndle-clamp lock screw Flter assembly (clamp) Flter assembly (spndle) λ adjust λ lock screw Fgure 3.1: Flter angle adjustment, showng the prmary wavelength adjustment screw and counter-actng sprng plunger. 7

22 8 Chapter 3. Operaton possble to fnd an atomc resonance by carefully adjustng the flter angle whle scannng the laser. To change the wavelength: 1. Unlock the flter so that t can rotate, by turnng the spndle-clamp lock screw ant-clockwse, for example one full turn. The lock screw has a sprng-loaded ball plunger, so even released, t wll retan some pressure on the flter rotaton cam. 2. Set the laser current so that the output power s suffcent, takng care to ensure that the nternal cavty power s below the maxmum rated for the dode (see Fg. 2.1). 3. Adjust the flter angle usng the fne thread λ screw, actng aganst the sprng-loaded ball plunger of the lock screw. 4. The laser wll hop between external cavty modes as the wavelength s adjusted, through cycles of dm and brght output. 5. It may be necessary to adjust the sprng plunger, so that pressure s mantaned on the flter adjustment cam wthout lockng completely. 6. Adjust the angle to one of the brght modes nearest the optmum wavelength, and then adjust the laser current and the pezo voltage to acheve the exact wavelength requred. 7. Adjust tenson on the spndle-clamp and sprng-plunger to ensure stablty. Fully lockng the screws s useful f the laser s to be moved to a dfferent ste, but not necessarly best for normal laboratory use. The flter transmsson wavelength shfts wth rotaton accordng to ( ) sn(θ) 2 λ(θ) = λ 0 1 (3.1.1) where θ s the angle of ncdence, λ 0 s the flter wavelength at normal ncdence and n eff s an effectve refractve ndex; n eff = 1.7 s a good startng guess. The senstvty to rotaton of the fne tangental wavelength adjustment screw s about 0.5 nm to 1 nm per turn. n eff

23 3.2 Scannng, mode-hops and BIAS Scannng, mode-hops and BIAS Mode-hops are dscontnutes n laser wavelength when tunng an external cavty dode lasers. 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. Wavelength-dependent 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. [4] for a detaled dscusson. 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. [4], for wavelength λ = 780 nm and external cavty length L ext = 15 mm.

24 10 Chapter 3. Operaton 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. 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. 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.

25 3.3 Scannng 11 Fgure 3.4: Frequency of a laser scannng properly (left) and wth a mode-hop at one edge (rght) 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 current 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). 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.

26 12 Chapter 3. Operaton 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 (.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

27 3.3 Scannng 13 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.

28 14 Chapter 3. Operaton

29 4. Algnment The cateye reflector arrangement s self-algnng, and should not requre adjustment. If the laser dode s replaced, or the laser has been mshandled n shppng, the dode collmaton and cateye lenses may requre some focus adjustment. To assst wth algnment, an nfra-red upconverson card or CCD 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 nfrared flter whch should 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 thedode s not algned wth one of the algnment screws. 2. Add the retanng threaded rng, and tghten gently, just enough such that the dode does not 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 good. 15

30 16 Chapter 4. Algnment 5.6mm dode Retanng rng Lens 9mm dode Fgure 4.1: Lens tube assembly wth 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 four 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 dode remans algned. 11. Focus the collmaton lens such that the laser focuses to a spot at 4 m dstance. Then rotate the lens clockwse about 1/8 to 1/4 turn so that the beam s slghtly dvergng. Laser stablty and modehop free range are generally slghtly better f the beam s not perfectly collmated [6]. 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, normally TE polarsed; that s, wth polarsaton parallel to the short (mnor) axs of the ellpse. The flter performance s typcally better n p-polarsaton; that s, wth polarsaton n the plane of reflecton from the flter. In that

32 18 Chapter 4. Algnment Flter E From dode θ Fgure 4.3: Orentaton of the dode laser beam ellpse wth respect to the flter rotaton, for TM polarsed dode, orented wth p-plane polarsaton. case, for the MOGLabs CEL, the polarsaton should be horzontal and the ellpse should be wth long axs vertcal. Some dodes, partcularly around 750 to 820 nm, are TM polarsed, wth polarsaton parallel to the long axs of the ellpse, as shown n Fg For these dodes, the ellpse should be horzontal. The flter dependence on polarsaton s weak and n most cases any rotaton of the dode wll work acceptably well. 4.4 Cateye reflector Lght reflected from a cateye lens/mrror combnaton wll be parallel to the ncdent radaton, regardless of the ncdent angle. Thus the cateye reflector s self-algnng: the lght s always reflected back to the dode, even f the beam s not well collmated. The effect reles on the lens-mrror dstance matchng the focal length of the lens. There are several methods for achevng optmum focus; probably the easest s to adjust the focus so as to mnmse the lasng threshold. Mount the dode collmaton lens tube and cateye reflector assembly at

33 4.4 Cateye reflector 19 25mm Lens tube Cateye Recollmator Fgure 4.4: Arrangement of lens tube and cateye reflector for adjustment of focus of cateye. about 25 mm apart, wthout flter (see Fg. 4.4). Set the dode current just below threshold, and then adjust the cateye lens focus untl the output suddenly flashes brghtly, ndcatng effectve feedback whch tends to lower the overall ECDL gan threshold. Repeat untl the mnmum s obtaned. The sequence s as follows: 1. Mount the lens tube and cateye reflector about 25 mm apart. 2. Montor the output beam on a pece of black card at a short dstance from the cateye assembly. It s helpful to recollmate the output after the cateye; any lens can be used, at approxmately the focal length of that lens from the output coupler. Montor the beam spot usng a securty camera or webcam. 3. Adjust the njecton current to just below threshold. 4. Adjust the cateye lens focus untl a brght flash (.e. lasng) s observed. 5. Iterate reducton of the njecton current, followng by focus of the cateye, untl the mnmum threshold s acheved. 6. Reassemble the laser and adjust the flter angle to acheve the desred wavelength.

34 20 Chapter 4. Algnment 7. 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, you should see a reasonable fracton of the absorpton spectrum, wth one or more mode-hops. A Fabry-Perot etalon or a fast hgh-resoluton wavemeter (MOGLabs MWM) can also be used to optmse the mode-hop-free range. 8. Adjust the flter angle, and the njecton current, to optmse the scans so that you see the maxmum number of repeats and the deepest sgnals. 9. Check that the saturated absorpton traces are clean. Nosy spectra ndcate mult-mode operaton, or hgh lnewdth, whch may be due to weak feedback. The lasng threshold s a good dagnostc: lower threshold ndcates better feedback and consequently lower lnewdth. A scannng Fabry-Perot or a MOGLabs MWM wavemeter s a very useful dagnostc tool to check for sngle-mode operaton. 10. Measure the laser output power as a functon of dode njecton current, and plot the power/current response as n Fg Compare aganst the orgnal data provded wth your laser and f concerned about dscrepances, contact MOGLabs. 11. 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 mode-selected 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.

35 4.5 CEF extended chasss CEF extended chasss The CEL 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 CEL wth extended chasss CEF opton 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 the transmsson. Dependng on wavelength, the transmsson vares from

36 22 Chapter 4. Algnment M1 Fbre coupler Faraday solator M2 Fgure 4.6: Schematc of the extended chasss laser 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. 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). 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

37 4.6 Fbre algnment 23 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.

38 24 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

39 4.6 Fbre algnment 25 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.

40 26 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.

41 A. Specfcatons Parameter Specfcaton Wavelength/frequency nm Dode dependent. Please contact MOGLabs for avalablty. Lnewdth Flter Tunng range Typcally 100 khz 0.2 to 0.4 nm bandpass Typcally 10 nm for sngle dode Sweep/scan Scan range Mode-hop free Pezo stack Cavty length 5 to 20 GHz dependng on pezo 5 to 20 GHz V, 100 nf (typcal) 35 mm Optcal Beam Polarsaton 3 mm 1.2 mm (1/e 2 ) typcal Vertcal lnear 100:1 typcal (can be rotated) 27

42 28 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) Optonal: 4 mm dam quck-ft water coolng connectons Electroncs Protecton Indcator Connector Modulaton nput Dode short-crcut relay; cover nterlock connecton; reverse dode Laser ON/OFF (LED) MOGLabs Dode Laser Controller sngle cable connect Actve (AC and DC coupled) or RF bas tee Mechancal & power Dmensons mm (L W H), 1 kg Beam heght 58 mm Shppng mm (L W H), 3.1 kg

43 A.1 CEL mechancal 29 A.1 CEL mechancal x M4 tapped 4 x M6 clearance Fgure A.1: Dmensons of CEL laser head.

44 30 Appendx A. Specfcatons A.2 CEF mechancal Fgure A.2: Dmensons of CEF laser head.

45 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. Recent lasers have shpped wth the B1047 headboard whch 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 ncreases bandwdth and reduces phase delay, easly achevng sub-hz lnewdth narrowng. For RF modulaton, a 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. 31

46 32 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.

47 B.1 B1045/1046 headboard 33 The nput mpedance s 10 k. The senstvty depends on the dode mpedance but s typcally around 1 ma/v. WARNING: The RF nput s a drect connecton to the laser dode. Excessve power can destroy the dode, whch s separated from the head board relay by an nductor. Thus the relay does not provde protecton from hgh frequency sgnals. Ref -20 dbm -20 TG -30 dbm * Att 50 db * RBW 30 khz * VBW 10 MHz SWT 17 s Center 1.5 GHz 300 MHz/ Span 3 GHz Fgure B.2: RF response, SMA nput on laser headboard to dode SMA output.

48 34 Appendx B. Laser head board B.2 B1047/B1240 headboards The B1047 and B1240 provde hgh-speed actve modulaton of the dode current. They use 500 MHz opamps and very low latency crcutry to reduce phase delay to around 12 ns for the B1240. The B1047 allows for closed-loop bandwdth of about 1.2 MHz whle the B1240 can acheve about 4 MHz (n both cases, wthout phase advance). The latter makes t partcularly easy to acheve sub-hz lnewdth reducton by lockng to a hgh-fnesse optcal cavty. The B1240 also allows drect-ground connecton or buffered; the latter s about 10% slower but reduces problems wth ground-loop nose. The B1240 s not sutable for dodes wth hgh complance voltage, typcally dodes wth wavelength below 600 nm. DC AC SMA C 8 R 14 R 12 C 7 R 13 C 5 U 3 R 11 C 6 R 9 R 10 R 7 C 11 C 4 U2 L2 L1 R 6 D1 + P5 Laser dode + P6 Pezo2 U1 C 3 R 4 R 3 C 2 R 2 C 1 R 5 Q 1 + P4 Pezo1 + R 1 P2 LED + P1 TEC P3 Thermstor Fgure B.3: B1047 enhanced laser head board. Jumpers at top left can be confgured for AC or DC couplng. Modulaton nput va SMA connector, senstvty 2.5 ma/v. The B1240 s almost dentcal but has an addtonal jumper for drect or dfferental ground couplng adjacent to U2.

49 B.2 B1047/B1240 headboards 35 B.2.1 SMA nput The B1047/B1240 SMA nput provdes AC or DC couplng to an actve modulaton crcut. Note that connecton to the SMA nput wll reduce the dode current by about 1.6 ma (B1047) to 2.5 ma (B1240), wth zero nput voltage. B1047 B1240 Input range ±2.0 V max ±2.0 V max Input couplng AC/DC DC (drect) AC/DC (buffered) Phase delay 40 ns < 20 ns (drect) < 30 ns (buffered) Gan bandwdth ( 3 db) 3 MHz 20 MHz Input mpedance 5 k 1 k Current gan 1 ma/v 1 ma/v Laser dode voltage 10 V max 2.5 V max

50 36 Appendx B. Laser head board B.3 Laser connecton The MOGLabs cable can be replaced wth a standard dgtal DVI-D Dual cable. There s a bewlderng assortment of apparently smlar cables avalable; only hgh qualty dual-lnk dgtal DVI-D cables should be used. WARNING: The LASER connector s a standard DVI-D Dual Lnk socket as used for consumer dgtal dsplay devces. It should only be connected to the correspondng MOGLabs DLC controller. It supples the hgh-voltage sgnals to drve the laser pezoelectrc actuators. The pezo drvers wll be dsabled f the cable s dsconnected, but nevertheless consderable care should be taken to ensure that non-moglabs devces are not connected va ths connector. Pn Sgnal Pn Sgnal Pn Sgnal 1 TEC 9 DIODE 17 DISC + 2 TEC + 10 DIODE + 18 DISC 3 Sheld 11 Sheld 19 Sheld 4 TEC 12 DIODE 20 STACK + 5 TEC + 13 DIODE + 21 STACK 6 AD590/ Relay GND 22 7 AD590/ Relay +5V 23 NTC 8 16 Interlock +5V 24 NTC Fgure B.4: LASER connector.

51 B.3 Laser connecton 37 Laser r 2 r 2 a a P P d e P 2 4 S h r 4 r 4 a a P P S S n g n g S S n g n g r 1 r 1 a a P P d e P 1 3 S h r 3 r 3 a a P P S S n g n g r 0 r 0 a a P P d e P 0 5 S h r 5 r 5 a a P P P P P 6 S h e r 6 r 6 d a a e 1 e 2 e 3 e 4 e 5 e n p u m m d r + r - o o T T S h h e e r - r + e e P P L L k P k P e e e s s z z o + o - e e c c a a S S c P c P o + o - z z e e s s D D e e r - r + a s a s R R e e a a y - y + t I A e n R F L F e r C m a u e S M a s e e s e s n s o n s o r - r + e e A A c c v v R 2 4 k S P g G n d M A - 5 P S R R P 5 v 1 2 C V 47nF or 100pF R1 10k R4 43R D 2 L E H D 4 F y n g l e a d s DNI D1 D Laser Interlock Ld poston nterlock Voltage Free contact that closes when box n poston / / / l t t l t l t r r HD1 t t l l l l l l l l l l l t t l t P3 Sg l RF hgh bandwdth connecton to dode SMA Gnd Chasss Earth e d h S e d h S e d h S P 5 v 8 U 1 A 1 N C U 1 B 3 Gnd h P 5 v N C 4 5 R R 3 L1 3.3uH 1.9A P 5 v r r l l l l Mount Hole Fgure B.5: MOGLabs DLC laser head board schematc (B1040/1045). The RF modulaton low-pass cutoff frequency s determned by C4 and the dode mpedance ( 50Ω).

52 38 Appendx B. Laser head board

53 Bblography [1] Danel J. Thompson and Robert E. Scholten. Narrow lnewdth tunable external cavty dode laser usng wde bandwdth flter. Revew of Scentfc Instruments, 83(2):, [2] X. Ballard, A. Gauguet, S. Bze, P. Lemonde, Ph. Laurent, A. Claron, and P. Rosenbusch. Interference-flter-stablzed external-cavty dode lasers. Opt. Communc., 266:609, [3] M. Glowsk, Ch. Schubert, M. Zaser, W. Herr, T. WÃijbbena, T. Wendrch, T. MÃijller, E.M. Rasel, and W. Ertmer. Narrow bandwdth nterference flter-stablzed dode laser systems for the manpulaton of neutral atoms. Optcs Communcatons, 280(2): , [4] S. D. Salba, M. Junker, L. D. Turner, and R. E. Scholten. Mode stablty of external cavty dode lasers. Appl. Opt., 48(35):6692, , 9 [5] H. Talvte, A. Petlänen, H. Ludvgsen, and E. Ikonen. Passve frequency and ntensty stablzaton of extended cavty dode lasers. Rev. Sc. Inst., 68(1):1, [6] S. D. Salba and R. E. Scholten. Lnewdths below 100 khz wth external cavty dode lasers. Appl. Opt., 48(36):6961, , 17 [7] P. J. Fox, R. E. Scholten, M. R. Walkewcz, and R. E. Drullnger. A relable, compact, and low-cost mchelson wavemeter for laser wavelength measurement. Am. J. Phys., 67(7): , [8] S. C. Bell, D. M. Heywood, J. D. Whte, and R. E. Scholten. Laser frequency offset lockng usng electromagnetcally nduced transparency. Appl. Phys. Lett., 90:171120, [9] G. C. Bjorklund. Frequency-modulaton spectroscopy: a new method for measurng weak absorptons and dspersons. Opt. Lett., 5:15,