The Berkeley Tunable Far Infrared Laser Spectrometers G
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The Berkeley tunable far infrared laser spectrometers G. A. Blake,‘) K. B. Laughlin,b, R C. Cohen, K. L. Busarow, D.-H. Gwo,@ C. A. Schmutienmaer, D. W. Steiert, and R. J. Saykally Department of Chemistry, University of California, and Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 (Received 24 September 1990; acceptedfor publication 10 March 1991) A detailed description is presentedfor a tunable far infrared laser spectrometerbased on frequency mixing of an optically pumped molecular gas laser with tunable microwave radiation in a Schottky point contact diode. The system has been operated on over 30 laser lines in the range 10-100 cm- i and exhibits a maximum absorption sensitivity near one part in 106.Each laser line can be tuned by f 110 GHz with first-order sidebands.Applications of this instrument are detailed in the preceding paper. 1. THE BERKELEY TUNABLE FAR-INFRARED LASER beam, which circulates between the FIR laser end mirrors SPECTROMETER after expanding through a 4 mm hole in the input coupler. The 2.5 m cavity of the FIR laser is of the dielectric wave- A. General description guide design, with planar gold-coated copper end mirrors. Tunable far-infrared (FIR) lasers have become pow- FIR power is coupled out through a lo-mm-diam hole in erful tools for investigating the structures of ions, radicals, the end mirror, which is backed by a hybrid quartz/ and clusters, and for probing intermolecular forces dielectric mirror to reflect the pump beam, while transmit- through measurement of FIR spectra of van der Waals ting the FIR output. complexes. In the preceding paper we have described the The output beam of the FIR laser then enters a rapid evolution of FIR laser spectroscopyand some recent Martin-Puplett polarizing diplexer, which couples the la- applications. In this article we present a detailed descrip- ser radiation onto the (Schottky diode) comer cube mixer, tion of the tunable FIR laser spectrometerscurrently used while simultaneously extracting the tunable sidebands.The at Berkeley. We begin with a relatively generaloverview of first polarizer (the analyzer) is rotated so the FIR laser the design, and then proceed to the details of construction beam (either horizontally or vertically polarized) is re- and operation. It is our hope that this article will serve as flected completely. Any residual cross polarization of the a useful guide to those who seek to construct similar sys- laser output beam is transmitted, thus purifying the polar- tems. ization. The beamsplitting polarizer is set at a 45” angle The design of the tunable FIR laser systems used at projected onto the plane perpendicular to the propagation Berkeley is similar to that of Farhoomand et al.’ In the of the beam, so that the power splits equally into both arms following, we first present a general description of this de- of the diplexer. The two beams are reflected using retrore- sign, in sufficient detail to afford all readers a reasonable flectors mounted with one face horizontal. These devices understanding of the underlying principles and function. rotate 45” incident polarization by 90” so that the beam We then proceedto describeeach component of the system initially transmitted through the beamsplitter reflects upon in sufficient detail to effectively guide those actually seek- recombination, and vice-versa.Thus, the recombinedbeam ing to construct a similar apparatus. is always transmitted towards the comer cube, and tuning The overall experimental design is diagrammed in Fig. the movable mirror cycles the polarization from vertical to 1. The complete spectrometer is built on a 5 ft. x 12 ft. horizontal, with elliptical polarization being produced at vibration isolation honeycomb table. A COz.laser provides intermediate positions. an intense mid-infrared beam (maximum power > 150W) The comer cube preferentially couples to radiation po- that is used to pump a molecular gas FIR laser. The larized in the plane of the whisker antenna (see below), CO2 laser is line tunable over some 100 different vibration- which is horizontal for our design. With the diplexer set to rotation transitions between 9.1 and 11.O pm using a pre- couple maximum laser power onto the diode, the reradi- cision grating in first-order autocollimation. The output ated, horizontally polarized beam at exactly the laser wave- frequency is fine-tuned over the 65 MHz free spectral range length will be reflected back into the laser by the analyzing of the cavity (limited by its 2.3 m length) using a piezo- polarizer by reversibility. Unless the comer cube is per- electric transducer (PZT), and the zeroth-order beam re- fectly aligned, however, some reradiated laser power is ver- flected from the grating is focused into a CO2 spectrum tically polarized. Hence some of the sideband power is analyzer to identify the laser line. horizontally polarized. The latter can then couple into the The FIR laser is pumped coaxially by the COz laser laser cavity, causing severebaseline fluctuations when fre- ‘)Division of Geological and Planetary Sciences,California Institute of Technology, MS-170-25-, Pasadena,CA 91125. b)ResaarchLaboratories, Rohm & Haas Company, 727 Norristown Road, Spring House, PA 19477. ‘IDepartment of Physics, Hansen Laboratory (GP-B, MS-4085), Stanford University, Stanford, CA 94035. 1701 Rev. Sci. lnstrum. 62 (7), July 1991 0034-6740/91/0717Ql-16802.00 0 1991 American Institute of Physics 1701 Downloaded 13 Sep 2006 to 128.32.220.140. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp O-1000pm) Tunable Sidebands FIG. 1. Schematic diagram of the Berkeley tunable FIR laser spec- trometer. quency modulation is used. A Fabry-Perot cavity has laser beam. Sloping the ground plane has a rather small therefore been installed betweenthe laser and the analyz- effect on the coupling and reradiation efficiencies,2but re- ing polarizer to help eliminate feedback from the side- duces the separationefficiency required of the diplexer. bands. The whisker that contacts the diode servesas the an- Because the sidebands have a different wavelength tenna for receiving and transmitting the FIR beams,and than the laser, there existsa movableretroreffector position carries the necessarydc bias, as well as the microwave at Amicrowave/4 where both of the sidebandsare polarized radiation, when coaxial coupling is used. It entersfrom the 90” with respect to the laser, and are thus transmitted front of the corner reflector, angling down over the diode through the analyzing polarizer, provided the microwave with appropriate length. The diode chip is soldered to a frequencyis low (2%) compare:dto that of the laser. For post, which is mounted to a differential micrometer for higher microwave frequencieseach sidebandmust be cou- precise vertical control during contacting. pled out individually to obtain optimum power. The ana- Microwave radiation may be coupled onto the diode lyzer can be easily rotated to reflect a FIR laser beam of either by a coaxial cable or with a waveguide. For the either vertical or horizontal polarization, facilitating con- application of microwave power to the diode at frequencies version between different laser lines, which are essentially that are too high for effective transmissions with coaxial totally linearly polarized with a dielectric waveguidecav- cable ( > 60 GHz) the diode post passesthrough a wave- ity. guide section embeddedin the body of the comer cube, The laser and microwave frequenciesare mixed in a with an adjustablebackshort to optimize the coupling. The GaAs Schottky barrier diode, which is contacted by a coupling is optimized at frequencies where microwave metal whisker and mounted at the apex of a corner cube power is limited, and the backshort is detuned to reduce reflector. Frequency mixing results from the nonlinear re- baselinedrift when sufficient power is available. sponseof the diode to the electric fields of the laser and The fundamental microwave source is a digital sweep microwave radiation. The FIR laser beam is focused at a oscillator, providing approximately 10 mW of microwave right angle by an electroformed off-axis parabolic mirror. radiation from 2-26.5 GHz. This power is usually sufli- A rotation stage allows the corner cube to be adjusted to cient to saturate production of the first-order sidebands, the optimum angle with respectto the incident beam, and The frequency range of this phase-locked source is ex- the entire assembly is mounted on an XYZ translation tended to 110 GHz with fixed-tuned millimeter wave mul- stage. Under optimum conditions for coupling the laser tipliers and, when necessary,with the use of microwave beam onto the diode, sidebandsare reradiated away from amplifiers. the comer cube with the same radiation pattern as the After the polarizing diplexer selects out the tunable incoming laser beam; the focusing mirror thus collimates FIR sidebands,they are then directed to a sample region. the outgoing beam as well. Transmissionthrough the sampleregion is monitored with The Berkeley comer cube designallows in situ optimi- a liquid helium cooled detector. At wavelengths longer zation of the antenna-to-comerdistance using a microme- than 300pm, an InSb hot electron bolometer is used,while ter that translates the back reflector. The baseof the cube at shorter wavelengthsthe detectorsof choice are either a is sloped to avoid creating an efficient retroreflector for the Putley mode (cyclotron resonanceassisted) InSb detector 1702 Rev. Sci. Instrum., Vol. 62, No. 7, Juiy 1991 Laser spectrometers 1702 Downloaded 13 Sep 2006 to 128.32.220.140. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp or a Ga-doped Ge photoconductor. Detector signals are which can enhance the laser oscillation if the totally re- demodulated using one of several possible modulation flecting gold surface surrounding the hole is coated directly schemes.The signal is then collected and displayed with a onto the dielectric mirror in order to maintain a well-de- PDP 1l/53 computer, which also controls the microwave fined phase front.