Instrumentation for making localized, precise relative reflection measurements of metals and superconductors in the terahertz frequency range is demonstrated. The results can be used to determine the surface resistivity of these materials which is of particular importance to development of high-temperature superconductors. Emission from a commerically available 1000 "C blackbody source was reflected from the materials under test and a reference reflector. The reflected signals were detected by a Schottky diode heterodyne receiver using a CO,-laser-pumped 214.5 pm CH,F, laser as the local oscillator. Double-sideband bandwidth and receiver noise temperature were 2 GHz and 30 000 K, respectively. The use of a broadband incoherent diagnostic source minimizes instrumentation sensitivity to coherent interference effects. The heterodyne receiver provides for sensitive detection with good spatial and frequency resolution. Unoptimized spatial resolution of four times the diffraction limit was achieved. The potential for relative reflectivity measurement accuracy of better than 0.1% was demonstrated with 30 min measurement times though systematic errors limited actual measurement accuracy to about 0.3%.
A double heterodyne interferometer using a muhimode laser diode with a synthetic wavelength of,., I rom has been established and the stability of the synthetic wavelength has been investigated. !, INmOPUCTIQNSeveral kinds of lasers have been used in multiwavelength interferometry, e. g. a multiwavelength HeNe!, a monomode laser diode with acustooptical modulator (AOM), or several independent emitting lasersl.l. If the application in question is to perfonn a measurement 00 the synthetic wavelengtb at relatively large distances rather than making measurements, which are perfonned on one wavelength, unambiguous around the point of equal optical paths, the main task is to create a synthetic wavelength of sufficient stability. In principle the stability of the effective wavelength whicb is generated by two modes ofa multimode laser is bener than that of several mooomode lasers~; provided the stability of a single modes oftbe multimode laser equals the stability of the modes of the two free running lasers. To overcome the stability problem of two free running laser expensive ele<;tronic andlor optical coupling schemes are needed, which makes the usage especially of cheap multimode laser diodes (MMLD) attractive. In this paper we report a two-wavelength double heterodyne interferometer (DHO using a MMLO. 2, PRINCIPLES OF OPERATIONA double heterodyne interferometer consists ofmo independent heterodyne interferometers working at different wavelengths AI and A2 and different heterodyne frequencieS/I and/2 . Two wavelengths can be obtained from the multimode laser diode. After amplitude demodulation and band·pass filtering of the superimposed interference signals we get the low frequency signal jet) " jet) a: cos(27t(f, -/2)1 + (, -<1>2»The interference phase difference <1>1 -4>2 that depends on the wanted synthetic wavelength corresponds now to the target distancez.(2) Eq. (2) indicates the relationship among phase diljerence, the target distance and the synthetic wavelength. The distance sensitivity thus is reduced and the range of unambiguity for interferometric measurement is extended. Particularly, the fluctuation M_fd of the synthetic wavelength caused by fluctuation of the Wiving current and the temperature is smaller than the fluctuation MUd of the synthetic wavelength generated by two monomode laser diodes" because the two wavelengths of the multimode laser diode are amplified by the same cavity.(3)One of the main advantages of using a multirnode laser diode in the double heterodyne interferometer is indicated by eq. (3).
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