## 4 Experimental Setup and Method of Measurement  
The experimental setup consisted of the following main components (see Figure 2): (a) a cryostat containing a FP cavity with mirrors mounted on the ends of a cylindrical bar PM with a central tunnel; (b) an optical node containing the laser of the EM-pump together with elements of a guide light and a photo detector plus the monitor, located on a separate desk; and (c) control electronics unit. The configuration of the optoelectronic scheme and the principle of measurement are shown in Figure 2.  
The laser beam passes through a Faraday cell and an electrooptical modulator, and acquires a phase modulation with a frequency of 8 MHz. It is then directed to the studied FP-cavity. The beam reflected from the FP-cavity falls on the photo-detector, whose signal is fed to the monitor and to a spectrum analyzer. The photo-detector is formed from spherical and flat (input) mirrors.  
In our experiment, we used a single mode tunable Nd:YAG laser with \(\lambda=1.064\) um and maximum power up to P\(\sim\)1 W, which was developed in the Institute of Laser Physics SB RAS, Novosibirsk. Construction was based on the principle of a running wave ring laser [29]. Laser tuning in the low frequency region 0-5 kHz was provided by piezo-ceramic stacks (slow and fast), which were attached to mirrors of the laser resonator.  
In the experiment, two main integral characteristics of the FP cavity at the optical resonance were measured: finesse (\(F\)) (or sharpness of the resonance) and contrast (C). By adjusting the laser frequency through the voltage applied to piezo-ceramics, one can tune the optical resonance of the FP cavity and measure its characteristics using the absorption curve in reflected light.  
This curve appears on the oscilloscope when synchronizing the sweep speed with the rate of change of the laser frequency. However, the possibility of observing it depends on the laser's frequency stability and the width of the optical resonance. The laser device with pre-stabilization that we used [29] has a radiation line width of \(\sim\)10 kHz, which is less than (but close to) the width of the FP resonance of \(\sim\)40 kHz and with finesse \(F\sim\) 2000. Therefore, we applied a calibration method to estimate the resonance width. The scale along the frequency axis (sweep axis) is set by the position  
Figure 1: Mounting of mirror holder to the body of pilot model: (1) pilot model body; (2) holder lower part; (3) holder upper part; (4) mirror; (5) screws; and (6) temperature sensor.  
of side modulation component in respect of the resonance frequency (their separation just equal the modulation frequency). The sweep was chosen so that the carrier and side-band were visible on the screen at the same time. The sharpness (finesse) was calculated as the ratio of the intermode interval (or free spectral range) \(f_{FSR}\) to the width of the resonance.  
The contrast parameter \(C\) gives the fraction of light involved in the interference (i.e., in optical resonance) and significantly affects the sensitivity of the FP standard in precision measurements.  
According to the theory of a FP cavity, under absence of losses, the intensity of reflected light equals zero at resonance frequency (i.e., the wave coming out from the cavity is compensated by the anti-phase wave reflected from the input mirror). This situation corresponds to the contrast parameter \(C=1\)[30].  
In reality, the complete compensation does not exist due to losses (i.e., there is no zero intensity in reflected light on resonance). The measure of contrast is estimated by a relative depth of the dip in the absorption curve. Usually, an average value of the contrast under a satisfactory FP cavity tuning is on the order of 20-30%. The contrast essentially depends not only on the parameters of the mirrors themselves but also on the mode matching with the radiation structure (i.e., from the delicate accuracy of the resonance tuning). Temperature distortions of the mirrors obviously destroy the contrast.