Phase-modulated mirror

The v sible wavelength beam splitter and compensating plate are both Yo (parallel to one second of arc) with 5 cm full aperture (Pyramid Optical Corp.). The beam splitter is located in a 45 optical mount (Burleigh) which is supported by a Star-Gimbal mount for adjustment. The fixed mirror is also the phase modulating mirror. In order to accomplish the phase modulation the mirror is mounted in a PZAT-90 (Burleigh) piezoelectric mount. There is sufficient linear motion available ( 10 micrometers/ 1000 V) from this mount to drive several orders of visible fringes. 

Drapcho et al. [14] demonstrated a very elegant way of performing multifrequency PA measurements. Rather than using a PM waveform composed of the sum of several sinusoidal waves, they drove the phase-modulation mirrors with a square wave, as shown schematically in Figure 20.14. A square wave with frequency / hertz is composed of the odd harmonics of / ... 

For many applications, there may be some advantage in employing phase modulation 54,85) instead of the usual amphtude modulation. In the latter technique the path of the radiation from the source to the detector is blocked and opened periodically by a chopper (cf. Fig. 20 and Section 4.3). For phase modulation, the chopper is removed from the spectrometer and the fixed mirror of the Michelson interferometer is moved back and forth about its mean position with a certain frequency. In contrast to the interference modulation (see Section 4.2), the amplitude of the mirror motion is small, being a quarter of the wavelength of the light. For the analogue Fourier transform or interference modulation, the amplitude of the mirror has to have many wavelengths in order to achieve a reasonable resolution... 

In Eq. (4.9), s is not expressed as a function of time since the motion of the movable mirror and hence the variation of s in time must be slow in comparison to the modulation a t). From Eq. 4.9 we gather further that 2nva t) in the argument of the cosine function is an oscillating phase, so this procedure is called phase modulation. In the narrow-band amplifier, only the a.c. component of 7 (s,i) with frequency /q = I/Tq is amplified to which frequency the amplifier is timed. All... 

In practice, a sinusoidal motion of the oscillating mirror is to be preferred to the square-wave motion, and in most applications, the sinusoidal motion is used. The effects of phase modulation are the same in this case, the only difference being that the modulation factor in Eqs. (4.11) to (4.13) will be the first Bessel function 7l(2 i (ro) instead of sin(25i [Pg.116]

This setup provides a flat dispersion by the mirrors and dispersion compensation by the prisms. The curved mirrors focus the pump beam into the laser crystal X. The glass plate P in the second focus causes self-phase modulation which results in a significantly wider spectrum of the laser emission and thus a shorter pulse [695]. 

However, for the same data collection time, interferometry is more sensitive to multiplicative noise (i.e., noise proportional to the signal) than continuous-scan interferometry [591]. To eliminate the multiplicative and 1// noise, phase modulation (at 400 Hz) of IR radiation in conjunction with LIA demodulation is used [591]. Since the LIA and some IR detectors need the IR signal to be modulated at a single carrier frequency, a mechanical chopper, phase modulation (when at each position the fixed mirror is dithered at a fixed frequency), or modulation of absorption of the sample is used to produce a carrier frequency. In this case, the TR measurement is referred to as a synchronous multiple-modulation experiment. Multiple modulation is unnecessary if the so-called dc coupled detector which does not require a varying signal is used. 

The polarization measurements in the mid-infrared (IR) and near-infrared (NIR) were performed on a Bruker IFS 88 FTIR/FTNIR spectrometer utilizing a wire-grid polarizer on KRS5-substrate which could be rotated pneumatically parallel or perpendicular to the selected reference direction. For the experiments in the step-scan mode a mercury-cadmium-telluride (MCT) detector with a DC-coupled preamplifier was used. This allowed an absolute intensity at each mirror position to be recorded and the use of phase-modulation-demodulation techniques, which are often applied for the step-scan mode [28-30] to be avoided. Further specific instrumental details of the individual applications are given in the corresponding sections. 

Any interferometer in which the mirror moves at a constant velocity and the beam is not modulated by an external chopper or by phase modulation (see Section 20.3)... 

Ideally, time-resolved and imaging measurements require that the OPD be held precisely constant at each sampling point. On the other hand, for photoacoustic and most sample modulation measurements, the position of one of the interferometer mirrors is varied sinusoidally with an amplitude of one, two, or more wavelengths of the HeNe laser by a piezoelectric transducer while the average position is held constant at each sampling point. This mode of operation is called phase modulation. 

We saw in Chapter 20 that phase modulation could be achieved by dithering the interferometer mirror with a waveform that was the sum of several sinusoids. 

Step-scanning interferometers can enhance the photoacoustic technique, because the variable modulation frequency of the scanning interferometer is decoupled. Thus, a constant modulation frequency is applied to all wavelengths in the spectral range. One can modulate the IR beam via phase modulation in which an interferometer mirror dithers to oscillate the retardation about the set point of each interferometer step. With this approach, all of the frequencies are modulated synchronously, and lock-in modulation can be used to extract the phase-modulation [53]. When phase-... 

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