Difference Frequency Spectrometer

While generation of sum frequencies yields tunable ultraviolet radiation by mixing the output from two lasers in the visible range, the phase-matched generation of difference frequencies allows one to construct tunable coherent infrared sources. One example is the difference frequency spectrometer of Pine [5.255] which has proved to be very useful for high-resolution infrared spectroscopy. 

Two collinear CW beams from a stable single-mode argon laser and a tunable single-mode dye laser are mixed in a LiNb03 crystal (Fig.5.111). For 90 phase-matching of collinear beams the phase-matching condition 

The spectral linewidth of the infrared radiation is determined by that of the two pump lasers. With frequency stabilization of the pump lasers, a linewidth of a few MHz has been reached for the difference frequency spectrometer. In combination with a multiplexing scheme, devised for calibration, monitoring, drift compensation, and absolute stabilization of the difference spectrometer, a continuous scan of 7.5 cm has been achieved and a reproducibility of better than 10 MHz [5.256]. 

The search for new nonlinear materials will certainly enhance the spectroscopic capabilities in the whole infrared region [5.259]. 

The whole spectral range of the difference spectrometer from 2.2 to 4.2 ym can be continuously covered by tuning the dye laser and the phase-matching temperature of the LiNbO crystal (-0.12°C/cm ). The infrared power is, according to (7.15) and (7.23), proportional to the product of the incident laser powers and to the square of the coherence length. For typical operating powers of 100 mW (argon laser) and 10 mW (dye laser) a few yW of in- 

Pulsed difference frequency generation from the outputs of a ruby laser and a dye laser mixed in LiNbO has achieved 6 KW of infrared power tunable between 3.1 and 4.5 ym [7.89], Spectral narrowing of the dye laser output reduces the bandwidth to less than 1 cm , and peak infrared powers of several hundred watts with repetition rates up to 30 s have been generated with a long-term frequency stability of better than 1 GHz [7.90]. 

Of particular interest are tunable sources in the far infrared region where no microwave generators are available and incoherent sources are very weak. With selected crystals, such as proustite (AgAs2S2) or HgS, phase matching for difference frequency generation can be achieved for the middle infrared. The search for new nonlinear materials will certainly enhance the spectroscopic capabilities in the whole infrared region. 

A very useful frequency-mixing device is the MIM diode (Sect. 4.5.2), which allows the realization of continuously tunable FIR radiation covering the difference-frequency range from the microwave region (GHz) to the 

This is 10 to 10 times higher than the noise equivalent input power of standard IR detectors. 

A simple and portable DFG-spectrometer for in-field trace gas analysis was constructed by R Hering and his group [5.296]. 

More information on the generation of VUV radiation by nonlinear mixing techniques can be found in [574-590]. 

The spectral linewidth of the infrared radiation is determined by that of the two pump lasers. With frequency stabilization of the pump lasers, a linewidth of a few megahertz has been reached for the difference-frequency spectrometer. In 

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Fig. 5.117. Difference-frequency spectrometer based on mixing a cw Ti sapphire ring laser with a single-frequency III-V diode laser in the nonlinear crystal AgGaS2 [5.280J...

Figure 6.23 Difference-frequency spectrometer with diode lasers [569]...

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