R.B. Dennis

Tuning and mode characteristics of the C.W. InSb spin-flip Raman laser determined by use of a Fabry-Perot interferometer

Abstract A simple variable length (10–50 cm) air-spaced Fabry-Perot interferometer has been used to calibrate the tuning characteristics of a c.w. InSb spin-flip Raman laser (SFRL). The SFRL output frequency is determined by competition between the spontaneous spin-flip resonance line and the cavity axial modes of the InSb crystal. Over several cavity modes the SRFL is found to tune linearly within ±250 MHz and inside a single mode to ±50 MHz. The tuning rate over several modes is found to be 2.35 cm -1 /kgauss and within a single mode the tuning rate is 1.80 cm -1 /kgauss. Frequency jumps of ≈500 MHz between modes are inferred. Used in conjunction with the observation of known molecular absorption lines as frequency standards, the interferometer provides a calibration of the SFRL suitable for infra-red Doppler limited spectroscopy. of gases. The observed frequency asymmetry in the SFRL output power and the non-linear tuning rate (variable from ≈70-40 MHz/gauss) over a single axial mode show plane-wave cavity theory to be inadequate. These effects can be explained by considering gain controlled refraction induced by a pump beam of gaussian profile. The near resonant dispersion of the refractive index of InSb is inferred from a measurement of the spacing between adjacent peaks in the SFRL output.

Stimulated spin-flip Raman scattering: a magnetically tunable infrared laser. I

A microscopic theory of spin-flip Raman scattering from conduction electrons in semiconductors has been set out both in terms of appropriate general scattering theory and of detailed calculation of the electron band states of a solid in a magnetic field. From the latter, matrix elements determining the scattering cross-section have been evaluated as a function of frequency and magnetic field for the various ‘anharmonic’ processes involved in indium antimonide. New experimental results on magnetic field dependence of gain and loss, of direct relevance to comparison with microscopic theory, are presented and discussed. These results, together with earlier work, confirm the basic theoretical model and clearly demonstrate the potential of this continuously tunable infrared laser.

A study of factors affecting the InSb c.w. spin-flip Raman laser

Abstract The behaviour of the c.w. spin-flip Raman laser is quantified by comparing its operation in the regimes of pump depletion and spin-saturation using InSb samples of different electron concentrations. The use of such a system for wide range spectroscopic applications, complementary to its narrow linewidth characteristic, is indicated.

Spectroscopy between 5 and 6 μm with a tunable spin-flip Raman source

Abstract We describe the application of a tunable, infrared, spin-flip Raman source to the measurement of the absorption spectra of gases in the spectral region 5–6 μm. Continuous tuning was obtained by using a low finesse Raman crystal cavity to eliminate mode pulling and hopping effects. The effects of atmospheric absorption, and pulse-to-pulse and magnetic field dependent intensity variations were cancelled by using a double beam arrangement. As an illustration of the operation of such a Raman spectrometer we obtain the P and Q branch absorption spectra of nitric oxide at various gas pressures in the spectral region 1890-1835 cm -1 . Studies at low gas pressure and with the tunable source operating close to the threshold for stimulated emission show a resolution of ≈ 0.08 cm -1 , determined by the linewidth of the Raman scattered radiation. This linewidth is shown to be in agreement with a mechanism of stimulated superradiant narrowing of the spontaneously scattered lineshape.

Intensity dependent Faraday rotation in indium antimonide

Abstract Intensity dependent interband Faraday rotation has been observed in InSb using a low power cw CO laser. The dependence of induced rotation on incident power and frequency shows saturation and resonance enhancement effects. The induced rotation is qualitatively attributed to the dispersion associated with saturable absorption of a valence to conduction band magneto-optical transition and includes an acceptor impurity effect. Induced rotations of >10 3 degrees/W cm were observed.

Lock-in stabilised externak cavity cw spin-flip Raman laser

Abstract A lock-in stabilised cw external-cavity spin-flip Raman laser has been operated in which a cavity mode resonant frequency is servo-controlled to the peak of the spontaneous gain as the magnetic field is swept. The design of the optical system eliminated additional undesired cavity interactions.

Near room temperature cw CO laser with a high-power single-frequency output

Abstract A relatively simple semi-sealed-off CO laser which operates near room temperature is described. An intra-cavity grating is used to select single- line output powers of about 2W.

Investigation of parameters controlling stimulated Raman scattering in InSb around 5 μm

Abstract Raman-laser light with a peak power of 2 W and continously tunable with magnetic field from 5.24 to 5.62 μm has been obtained using a Q-sqitched CO2 laser as a primary source, the frequency being first doubled by phase matched second harmonic generation in tellurium and then shifted by the stimulated spin-flip process in InSb. The dependence of the output on input power, magnetic field, sample temperature and input focussing parameters has been investigated and theoretically interpreted. The Raman laser has been used to obtain high-resolution absorption spectra of atmospheric water vapour, over a range of ≈ 20 resolvable lines.

Investigation of the spectral content of a cw CO laser output using a high-resolution scanning Fabry-Perot interferometer

Abstract The use of a scanning Fabry-Perot interferometer of variable length (variable resolution) for investigating the output of a cw CO laser is reported. Under experimental conditions required to achieve maximum power it was found that the laser output consisted of two or more simultaneously occuring transitions due to the overlapping nature of the vibration- rotation bands. The frequency differences between these transitions were measured to an accuracy of ≲ 0.01 cm -1 . Operation of the CO laser on a single vibration-rotation transition was obtained by controlling the laser gain with the experimental variables of discharge pressure, gas temperature, and optical alignment. An alternative approach of obtaining monochromatic radiation, by using a Fabry-Perot interferometer as a frequency selective filter external to the CO laser, is reported.

Fundamental loss mechanisms of the spin-flip Raman laser

The authors have observed striking minima in the spectrum of the output power against magnetic field of the InSb spin-flip Raman laser near 10.6 mu m. These minima, which occur in the Stokes, anit-Stokes and double Stokes emissions, are attributed to harmonic cyclotron resonance absorption of the Stokes scattered radiation in the InSb crystal. Independent studies of the transmission of the sample as a function of magnetic field support this interpretation.