Heterodyne signal processing

The subject of phase and phase retrieval with pulsed optical signals, although it is textbook material and involves well-known signal processing concepts [64, 65], has impacted on molecular spectroscopy only recently [66] through consideration of optical control experiments. As we shall see the phase is a consideration in heterodyne laser experiments because it influences the mixing of fields incident on a square-law detector. It is well known that a quadratic phase alters the spectrum, the time envelope and the time-frequency bandwidth of a pulse. Consider a pulse ... 

Fig. 4. Pressure dependence of rf resonance linewidths (HWHM) of AM- and FM-Raman heterodyne signals for xenon and helium collision partners. Experimental points are given by crosses the full curves correspond to fits that are based on a theory including velocity diffusion processes. Dashed lines, initial slopes for the AM-RHS pressure broadening giving YvcC dash-dotted lines, asymptotic slopes for FM-RHS linewidths giving Yc-...

Intermediate frequency (IF) In a heterodyne process, the sum of difference frequency at the output of a mixer stage which will be used for further signal processing. 

The difference between optical nutation and free induction decay should be clear. While the optical nutation occurs at the Rabi frequency which depends on the product of laser field intensity and transition moment, the free induction decay is monitored as a heterodyne signal at the beat frequency 0) 2 which depends on the Stark shift. The importance of these coherent transient phenomena for time-resolved sub-Doppler spectroscopy is discussed in the next section. Its application to the study of collision processes is treated in Chap.12. For more detailed information the excellent reviews of BREWER [11.43,48] are recommended. 

After briefly reviewing conventional optical and infrared heterodyne detection, we examine the behavior of a multiphoton absorption heterodyne receiver. Expressions are obtained for the detector response, signal-to-noise ratio, and minimum detectable power for a number of cases of interest. Receiver performance is found to depend on the higher-order correlation functions of the radiation field and on the local oscillator irradiance. This technique may be useful in regions of the spectrum where high quantum efficiency detectors are not available since performance similar to that of the conventional unity quantum efficiency heterodyne receiver can theoretically be achieved. Practical problems which may make this difficult are discussed. A physical interpretation of the process in terms of the absorption of monochromatic and nonmonochromatic photons is given. The double-quantum case is treated in particular detail the results of a preliminary experiment are presented and... 

Attenuator "A" attenuates the laser-beam power to the appropriate level which is allowed by the MCT detector (<0,5 mW). The optical signal scattered back from the target is collected by the same telescope that was transmittin the laser beam. This weak optical signal is introduced into the laser by the same mirrors and the same "S" beamsplitter, that were used in the transmission process. Inside the laser cavity this low intensity radiation is amplified in the same cavity mode in which the laser is operating. For this reason the mode of the received radiation is identical with local beam mode. This identity secures the best mixiflg efficiency for the heterodyne detection. This mode identity is the reason that we called our system self aligning The lens in front of the detector serves to fit the spot size of the local and amplified (received) beams to the detector size. 

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