Laser diode cavity ring-down spectroscopy using acousto-optic modulator stabilization B. A. Paldus, J. S. Harris, J. Martin, J. Xie, and R. N. Zare Citation: J. Appl. Phys. 82, 3199 (1997); doi: 10.1063/1.365688 View online: http://dx.doi.org/10.1063/1.365688 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v82/i7 Published by the American Institute of Physics. Additional information on J. Appl. Phys. Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors Downloaded 01 Jun 2013 to 171.64.124.91. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions Laser diode cavity ring-down spectroscopy using acousto-optic modulator stabilization B. A. Paldus and J. S. Harris, Jr. Department of Electrical Engineering, Stanford University, Stanford, California 94305-4070 J. Martin,a) J. Xie,b) and R. N. Zarec) Department of Chemistry, Stanford University, Stanford, California 94305-5080 ~Received 28 April 1997; accepted for publication 23 June 1997! By using an acousto-optic modulator, we have stabilized a free-running continuous wave ~CW! laser diode in the presence of strong reflections from a high finesse Fabry–Perot resonator. The laser diode linewidth can be stabilized from several MHz, for high resolution spectroscopy of species at low pressures, to several hundred MHz, for lower resolution spectroscopy of species at atmospheric pressures. We demonstrated CW cavity ring-down spectroscopy of water vapor at both 1 atm and 5 Torr. We achieved ring-down repetition rates of 10–50 kHz, and a noise level of 231028 cm21. © 1997 American Institute of Physics. @S0021-8979~97!01219-X# I. INTRODUCTION of light inside the ring-down cavity, then switched off the light with an acousto-optic modulator ~AOM! and measured Cavity ring-down Spectroscopy ~CRDS! is a high sensi- the intensity decay. Acquisition rates of several hundred Hz tivity absorption technique with potential for absolute con- were thereby demonstrated, in addition to excellent light 1–7 centration measurements of trace gases and impurities. throughput, and reduced baseline noise leading to very high CRDS is usually practiced by coupling a pulsed laser source sensitivities. Romanini et al.11 were also able to apply CW- into a high-finesse optical resonator ~Fabry–Perot cavity! CRDS to an external cavity diode laser ~ECDL!. that encloses the sample of interest, and detecting the decay Much of current ring-down spectroscopy1–6 still relies of light in the resonator. Under many conditions, the decay is on fairly costly laser sources. As solid state lasers ~e.g., exponential and a plot of the ring-down decay rate versus Ti:Sapphire lasers, Nd:Yag-pumped OPOs, and ECDLs! frequency gives the absorption spectrum. The ring-down de- have gained in reliability, tuning range, and output power, cay rate is controlled by the resonator finesse, and changes they have started to replace the more traditional tunable dye wherever the sample absorbs. This approach is limited in lasers, although they are no less expensive. Simultaneously, spectral resolution by the laser linewidth (.100 MHz for semiconductor laser diodes ~LDs! have also been improving pulsed sources!,8 in data acquisition rates by the pulse rep- etition rate of the laser ~ca. 10–100 Hz!, and in sensitivity by in power, wavelength coverage, and reliability. The rapid the inherent trade off between resonator finesse and light growth of the communications industry in recent years has throughput. resulted in the availability of tunable near-infrared LDs at a Recently, efforts have been made to overcome these rapidly diminishing cost (,$500!. In fact, owing to their limitations of pulsed CRDS by the use of narrow linewidth compactness, low cost, durability, high wallplug efficiency, (,10 MHz! continuous wave ~CW! lasers. Lehmann,7 and compatibility with both fiber and silicon technologies, Meijer,9 and Romanini10 were the first to propose and imple- infrared laser diodes seem to be an ideal light source for ment the use of narrowband laser sources in CRDS. realizing practical CRDS systems. Lehmann7 first proposed coupling narrowband CW diode la- Early attempts demonstrated difficulties in applying LD ser sources to ring-down cavities, in order to build up enough sources to CRDS:7,12 whenever a LD beam is reflected di- energy in the cavity to perform shot noise limited measure- rectly back into the laser, as occurs in a linear cavity con- ments. Meijer et al.9 measured phase shifts ~PSS-CRDS!, figuration, even under optical isolation, the optical feedback rather than intensity decay of an intensity-modulated CW may result in phase fluctuations and mode hopping of the light beam. They demonstrated for the first time, that acqui- LD. In fact, at higher feedback levels, as would be typical for sition rates of kHz were compatible with high-resolution most practical setups, a wide variety of effects ranging from CRDS, and that narrowband laser throughput, could be im- linewidth broadening to complete ‘‘coherence collapse’’ proved by using stable resonators with a dense transverse ~linewidth .10 GHz! is often observed13 and is illustrated in 10 mode structure. In an alternative approach, Romanini et al. Fig. 1~a!. The inherent problem is the formation of ‘‘external swept one resonant mode of the cavity through the CW laser cavities’’ by reflective optics with the output facet @which linewidth ~CW-CRDS!, thereby allowing significant buildup has either a low reflectivity or is antireflection ~AR! coated to increase output power# of the LD. The external cavity a!SL Microtest, Wildenbruchstrasse 15, 07745 Jena, Germany. affects both the gain and phase relations of the LD.14–16 b! State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Thus, whenever back reflection is allowed, the lasing char- Chemical Physics, CAS, P.O. Box 110, Dalian 116023, Peoples Republic of China. acteristics of the LD ~frequency, number of excited modes, c!Electronic mail: [email protected] output power, etc.! become highly dependent on uncontrol- J. Appl. Phys. 82 (7), 1 October 1997 0021-8979/97/82(7)/3199/6/$10.00 © 1997 American Institute of Physics 3199 Downloaded 01 Jun 2013 to 171.64.124.91. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://jap.aip.org/about/rights_and_permissions and a frequency selective element!, that allows only one spe- cific mode to exist, and that reduces ~but does not completely eliminate! the sensitivity of the LD to feedback;11 the feed- back from a linear cavity configuration can be completely eliminated by using a ring resonator structure, as will be discussed elsewhere;17 or, the external cavity effect can be controlled by placing an AOM inside the external cavity, thereby stabilizing the time-averaged behavior of the LD. The last approach, first demonstrated by Martin et al.13 as a useful scheme for stabilizing LDs in the presence of direct back reflections, and its application to CW-CRDS, is the primary focus of this article. II. AOM STABILIZATION OF LASER DIODES In an AOM device, a pressure transducer creates a sound wave that modulates the index of refraction in an active non- linear crystal, thereby producing a Bragg diffraction grating that disperses an incoming light beam into multiple orders.18 By switching the radio frequency ~rf! power that drives the pressure transducer, light can be rapidly deflected by the AOM into and out of the first order, so that AOMs can be used as high-speed (,(10 ns risefall time! CW light modu- lators. The resulting diffracted beam is also frequency shifted by an amount equal to the acoustic wave frequency.18 Hence, by placing an AOM into the external cavity formed by the LD back facet and the ring-down resonator input mirror, any light fed back from the first order becomes frequency shifted by twice the rf driver frequency. By considering the Lang and Kobayashi equations19 that model the effects of optical feedback on LD behavior, it becomes clear that the fre- quency shift of the AOM forces the diode to cycle through all values of phase with a period equal to the reciprocal of the frequency shift.13 Therefore, in order to achieve adequate stabilization, the AOM frequency must exceed the ring-down decay rate. Typically, a driver frequency greater than several MHz is sufficient for good quality ring-down reflectors ~R .99.99%!, and will eliminate any changes in phase shift associated with changes in length of the external cavity. Un- der these conditions, the effective LD linewidth becomes in- sensitive to both thermal ~seconds time scale! and mechani- cal ~milliseconds time scale! perturbations of the external cavity length. By placing an AOM between the laser diode and the input mirror of the ring-down cavity, the AOM can be used not only to switch the CW beam into and out off the first order diffraction, but simultaneously control LD linewidth. The AOM driving power determines the diffraction effi- FIG. 1. ~a! Linewidth for a free-running LD ~solid! and for a LD under feedback ~36 dB feedback! from a ring-down cavity ~dashed!. ~b! LD line- ciency and hence, the amount of feedback to the LD. The width as a function of external cavity length for only first order feedback: external cavity length fixes the maximum achievable line- Lext5215 cm ~solid!, Lext5100 cm ~dashed!, and Lext555 cm ~dashed- width for each feedback level @cf. Fig. 1~b!#. The first order dotted!. The feedback level is 50 dB in all three cases.