We have investigated the application of an electronic feedback technique recently reported by Repasky, et.al. [Appl. Opt. 45, 9013 (2006)] to an injection-locked semiconductor diode laser. We find that without electronic feedback, the injection-locked slave laser will only follow the master for less than 1 GHz, but once the electronic feedback is applied, the slave laser is capable of following for more than 20 GHz, corresponding to the full scan range of the master laser

Ranjit, G.

Shaffer, M. K.

Sukenik, C. I.

English

Old Dominion University

Old Dominion UniversityODU Digital CommonsPhysics Faculty Publications Physics4-2008Extended Tuning of an Injection-Locked DiodeLaserM. K. ShafferOld Dominion UniversityG. RanjitOld Dominion UniversityC. I. SukenikOld Dominion University, csukenik@odu.eduFollow this and additional works at: https://digitalcommons.odu.edu/physics_fac_pubsPart of the Atomic, Molecular and Optical Physics CommonsThis Article is brought to you for free and open access by the Physics at ODU Digital Commons. It has been accepted for inclusion in Physics FacultyPublications by an authorized administrator of ODU Digital Commons. For more information, please contact digitalcommons@odu.edu.Repository CitationShaffer, M. K.; Ranjit, G.; and Sukenik, C. I., "Extended Tuning of an Injection-Locked Diode Laser" (2008). Physics FacultyPublications. 5.https://digitalcommons.odu.edu/physics_fac_pubs/5Original Publication CitationShaffer, M.K., Ranjit, G., & Sukenik, C.I. (2008). Extended tuning of an injection-locked diode laser. Review of Scientific Instruments,79(4), 046102. doi: 10.1063/1.2906224Extended tuning of an injection-locked diode laserM. K. Shaffer, G. Ranjit, and C. I. SukenikOld Dominion University, Department of Physics, Norfolk, Virginia 23529, USAReceived 24 September 2007; accepted 16 March 2008; published online 10 April 2008We have investigated the application of an electronic feedback technique recently reported byRepasky et al. Appl. Opt. 45, 9013 2006 to an injection-locked semiconductor diode laser. Wefind that without electronic feedback, the injection-locked slave laser will only follow the master forless than 1 GHz, but once the electronic feedback is applied, the slave laser is capable of followingfor more than 20 GHz, corresponding to the full scan range of the master laser. © 2008 AmericanInstitute of Physics. DOI: 10.1063/1.2906224Semiconductor diode lasers are used in both commercialapplications and experimental physics as a result of their lowcost, high efficiency, and broad wavelength coverage. Thishas been especially true in atomic and molecular physicsexperiments.1,2 Because both precise tunability and fre-quency stability are often important, it is common to con-struct an external cavity diode laser ECDL by using afrequency-selective optical element such as a diffractiongrating in the cavity to control the frequency of the lightwhich is fed back to the laser diode. If the front facet of thelaser diode has sufficiently low reflectivity, the laser outputfrequency is then determined by the external cavity. Tuningis achieved by rotating the grating or a mirror, depending onthe cavity design.3,4 If the laser is to operate at a fixed wave-length, then placement of the tuning element in the cavity isusually not too important; however, if frequency scanning isrequired—as in spectroscopy experiments, for example—then cavity geometry is critical. If the pivot point of thetuning element in the cavity is not precisely positioned inorder to keep the cavity length commensurate with the newwavelength during tuning, the laser will intermittently modehop or run multimode as it is scanned.5,6 Correct positioningof the pivot point, however, requires micron level accuracyand is not trivial. Several authors have demonstrated tech-niques for increasing the scan range of an ECDL by applyingfeed-forward adjustments to the laser diode current as theexternal cavity is mechanically adjusted.7–11 Recently, Re-pasky et al.12 described an electronic feedback technique toenable very long mode hop-free scans of an ECDL.In this note, we describe our investigation of the appli-cation of the technique in Ref. 12 to an injection-locked di-ode laser system. In injection locking, light from a “master”laser is sent directly into a “slave” laser causing it to lasewith the spectral properties of the master, but usually provid-ing higher power than one would obtain from the masteralone. If the laser is to be scanned, it is critical that the slavelaser smoothly follow the master during scanning. We havefound that an injection-locked diode laser will typically fol-low the master for only 500 MHz before it begins to op-erate multimode, thereby limiting the utility of the long scancapability of the ECDL.In Ref. 12, a small modulation was placed on the lasercurrent and phase-sensitive detection was used to monitorthe laser power. If the laser cavity does not maintain theresonance condition during scanning, sideband suppressionis reduced and the amplitude and phase of the modulatedpower change, thereby providing an “error signal” which canbe applied to the laser current in order to bring the cavityback into resonance. Using this approach, Repasky et al.were able to extend the mode-hop free tuning of a Littman–Metcalf ECDL, operating at 758 nm, from 1 to more than65 GHz. We have implemented this technique to aninjection-locked diode laser system and find that althoughthe amplitude and phase characteristics of the modulatedpower of the slave laser differ from those of the ECDL, asuitable error signal—used to send feedback to the slave cur-rent and allow the slave laser to stay in resonance with themaster laser—is still produced. We have investigated twoseparate injection-locked laser systems, one with an ECDL inLittman–Metcalf configuration and the second operating inLittrow configuration. In both setups, no attempt was madeto correctly position the pivot point of the tuning element.We found the same performance in both setups. Here, wereport details of the apparatus using the Littman–MetcalfECDL.A schematic of the experimental arrangement is shownin Fig. 1. An 808 nm 150 mW diode laser Sanyo DL-LS2031 was collimated Thorlabs L110P-B and placed in a7 cm long external cavity containing a 1200 lines /mm dif-fraction grating Edmund Scientific K43-848. The laser wastuned by applying a voltage to a piezoelectric transducerPZT placed behind the horizontal adjust of the cavity mir-ror. The PZT Thorlabs AE0203D08F has an extension of6.1 m at 100 V. The master laser was temperature stabi-lized to 10 ppm with a homemade temperature controllerand operated at 125 mA with a homemade current controller.At this current, the ECDL output is 26 mW. With no elec-tronic feedback, the laser scans 1.5 GHz before mode hop-ing. We monitor the laser output with light reflected off anuncoated glass beam splitter and detected on an external pho-todiode, as shown in Fig. 1. We modulate our laser current at8 kHz. Output of the photodiode is directed to a current tovoltage converter and then sent to the input of a PrincetonApplied Research PAR 120 lock-in amplifier. The lock-inamplifier also provides the reference signal used to modulatethe laser current. The lock-in amplifier demodulates the pho-todiode signal. If the amplitude and/or phase characteristicsof the photodiode signal at the modulation frequencyREVIEW OF SCIENTIFIC INSTRUMENTS 79, 046102 20080034-6748/2008/794/046102/3/$23.00 © 2008 American Institute of Physics79, 046102-1 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  128.82.253.131 On: Wed, 24 Aug2016 15:16:42change, the voltage output of the lock-in amplifier alsochanges. This output voltage can be used as an error signal toprovide feedback to the diode laser current. We use the timeconstant setting of the PAR 120 to provide integration andthe zero-adjust feature dc offset to subtract a fixed voltagefrom the output and lock the laser to the zero crossing of theoutput signal. The output of the lock-in amplifier is sent tothe simple amplifier circuit shown in Fig. 2 to provide gainand polarity control of the integrated error signal and sum itwith the modulation needed for the phase-sensitive detectionof the laser power. The output of the lock circuit is then sentto the current controller where it is summed with the setpoint voltage which controls the laser current. When thefeedback technique of Ref. 12 is applied, we are able to scanthe ECDL by 23 GHz, limited by the full extension of thePZT element.Light from the ECDL is injected into a collimated slavelaser Sanyo DL-LS2031, Thorlabs L230P-B. The laser cur-rent is 168 mA. Only tens of microwatts of light is requiredto injection lock the slave when the two polarizations are thesame. We find it convenient to direct the ECDL light into theslave via a polarizing beam splitter cube. In this configura-tion, the injected light has a polarization nearly perpendicularto the slave light and it typically requires a few milliwatts topull the slave. Though the ECDL is now capable of scanning23 GHz, the slave laser without electronic feedback willonly be pulled single mode for a small range of 600 MHzand will then run multimode before switching to its free-running wavelength as shown in Fig. 3, where the laser fre-quency monitored by a Burleigh WA-1000 wavemeter isplotted versus ECDL PZT voltage. Additional laser diagnos-tics are provided by a low finesse 300 MHz Fabry–Pérotcavity, a low finesse 10 GHz solid etalon, and a high finesse2 GHz optical spectrum analyzer. A saturated absorption0.850.900.951.001.05-1000 -500 0 500 1000-180-90090180-20.00-10.000.0010.0020.00(a)(b)(c)(d)0.0000.3000.6000.9001.200Master Laser Frequency (MHz)Power(normalized)ErrorSignal(V)φ mod(degrees)Amod(normalized)FIG. 4. For master laser squares and slave laser triangles: a normalizedAmod vs laser frequency, b mod vs laser frequency, c output of the lock-inamplifier used as the error signal vs laser frequency, and d normalizedtotal laser power vs laser frequency.811nmMaster811nmMaster811nmSlave811nmSlave300 MHz Fabry-Perot cavity10 GHz solid etalonPDPD2 GHz Burleighspectrum analyzer Fiber to BurleighWavemeterFiber to saturatedabsorption spectrometerFiber toexperiment~37 dB OI~80 dB OICurrent controllerTemperature controllerMaster laser controlSlave laser controlPZT voltagecontrollerFrom PD I to V converterLock-in amplifierIn Ref OutTo laserlock circuit(Fig. 2)Laser control (identical for master or slave)WPPBSBSFIG. 1. Schematic of experimental setup. PBS: polarizing beamsplittercube; BS: beamsplitter; WP: waveplate; OI: optical isolator; PD: photodi-ode; dashed optical path indicates master and slave laser beam overlap.100K10K+-+-+-to Currentcontrollerfrom lock-inamplifier “Out”modulation fromlock-in amplifier “Ref”lock gainoptional inverter summing amplifier10K100K100K100K10K100K0-50K50K100KFIG. 2. Amplifier circuit with gain and polarity control for providing com-bined modulation and error signal to the diode laser current controller. Alloperational amplifiers are 1 /4 TL074.369555369560369565369570371535371540371545PZT Voltage (V)Frequency(GHz)0 20 40 60 80 100 120FIG. 3. Output frequency of slave laser without electronic feedback vsmaster laser PZT voltage corresponding to master laser frequency. Boxedregion indicates multimode behavior.046102-2 Shaffer, Ranjit, and Sukenik Rev. Sci. Instrum. 79, 046102 2008 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  128.82.253.131 On: Wed, 24 Aug2016 15:16:42spectrometer is used to provide an absolute wavelengthreference.In an attempt to extend the scan range of the slave, wehave applied the same technique used on the master laser tothe slave laser. Here, the current of the slave laser diode ismodulated at 12 kHz so as to not interfere with the masterECDL lock circuitry and the modulated power is againmonitored on a photodiode. In an alternate arrangement, wehave also used the internal photodiode of the laser diode tomonitor the power. As the frequency of the master laser isscanned, the amplitude and phase characteristics of themodulated power of the slave laser change resulting in achange to the output voltage of the lock-in amplifier. Thepresence of the modulation added to the current can be ob-served as an ac component of the total output power of thelaser, which we will refer to as Pmod, having amplitude Amodand phase mod relative to the reference modulation signal.Figures 4a and 4b show Amod and mod, respectively, forboth the master and slave laser as the master laser is scanned,with no feedback applied to either laser. Amod is normalizedto its maximum value during the scan. Figure 4c shows theresulting error signal with dc offset applied produced bythe lock-in amplifier. As seen in Figs. 4a and 4b, theamplitude and phase characteristics of the modulated powerof the slave laser do, in fact, differ from those of the master.What matters for locking the slave laser, however, is that thedemodulated signal the output of the lock-in amplifier has aform which can be used as an error signal. Indeed, as can beseen in Fig. 4c, where the output of the lock-in amplifierwith dc offset applied is shown, a perfectly suitable errorsignal is produced. Finally, in Fig. 4d, we show the normal-ized total output power of each laser as it is scanned withoutelectronic feedback. When the error signal is integrated, sentto the locking circuitry of Fig. 2, and forwarded to the slavelaser current controller, the slave laser is now capable offollowing the master ECDL over the full 23 GHz scan rangeas shown in Fig. 5, where the wavemeter output and trans-mission through a 10 GHz solid etalon are displayed.In conclusion, we have investigated the application of anelectronic feedback technique to an injection-locked diodelaser. We find that without the electronic feedback, the slavelaser will only follow the master ECDL for 600 MHz, butonce locked the slave laser is capable of following the fullscan range of the master laser. We expect that the scanningrange can be extended even further by application of a pi-ezoelectric element with greater displacement capability.We gratefully acknowledge support from the NationalScience Foundation. We thank K.S. Repasky for helpful cor-respondence.1 K. B. MacAdam, A. Steinbach, and C. Wieman, Am. J. Phys. 60, 10981992.2 C. E. Wieman and L. Hollberg, Rev. Sci. Instrum. 62, 1 1991.3 T. Hansch, Appl. Opt. 4, 895 1972.4 M. G. Littman and H. J. Metcalf, Appl. Opt. 17, 2224 1978.5 L. Nilse, H. J. Davies, and C. S. Adams, Appl. Opt. 38, 548 1999.6 M. de Labachelerie and G. Passedat, Appl. Opt. 32, 269 1993.7 V. V. Vassiliev, S. A. Zibrov, and V. L. Velichansky, Rev. Sci. Instrum. 77,013102 2006.8 C. Petridis, I. D. Lindsay, D. J. M. Stothard, and M. Ebrahimzadeh, Rev.Sci. Instrum. 72, 3811 2001.9 A. Andalkar, S. K. Lamoreaux, and R. B. Warrington, Rev. Sci. Instrum.71, 4029 2000.10 V. P. Gerginov, Y. V. Dancheva, M. A. Taslakov, and S. S. Cartaleva, Opt.Commun. 149, 162 1998.11 T. Nayuki, T. Fujii, K. Nemoto, M. Kozuma, M. Kourogi, and M. Ohtsu,Opt. Rev. 5, 267 1998.12 K. S. Repasky, A. R. Nehrir, J. T. Hawthorne, G. W. Switzer, and J. L.Carlsten, Appl. Opt. 45, 9013 2006.369555369560369565369570369575369580Frequency(GHz)PZT Voltage (V)0 20 40 60 80 100 120(a)(b)Tetalon(arb)FIG. 5. a Output frequency of slave laser with electronic feedback vsmaster laser PZT voltage b Transmission of slave laser light through a10 GHz solid etalon vs master laser PZT voltage.046102-3 Notes Rev. Sci. Instrum. 79, 046102 2008 Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP:  128.82.253.131 On: Wed, 24 Aug2016 15:16:42

Extended Tuning of an Injection-Locked Diode Laser

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Extended Tuning of an Injection-Locked Diode Laser

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