On construction of laser acousto-optical heterodyne interferometers with push-pull measurement of product motions

Features of an acousto-optical two-channel laser displacement interferometer with multi-frequency photodetectors with subpycometer resolution

“…It shows: laser 1, AO modulator 2, optical circuit 3, movable trippel prism 4, two microlenses 5 and 6 for two Y-shaped fiber-optic (FO) splitters 7 and 8, HF photodetector 9, LF photodetector 10, phase interpolator (in futureinterpolator) 11, PLL 12, adder 13, phase meter 14. The PLL 12 is described in [18][19][20] and has a standard circuit in the form of a ring from a serial circuit of a phase detector (with an additional digital output N pd ), a low-pass filter, a voltage-controlled generator (VCG) based on a highly stable quartz oscillator, the frequency output signal of which is fed to the AO modulator 2. The cutoff frequency of the low-pass filter is controlled by a frequency detector depending on the Doppler frequency rise, the effect of which is discussed further (not shown in figure 1).…”

“…The photodetector 9, the interpolator 11 and the adder 13 form a "fast inaccurate" channel of AO LDI, and the photodetector 10, the PLL system 12 and the phase meter 14 are "slow accurate". In the process of operation, laser radiation 1, following through the AO modulator 2, forms the first E(+1) and zero E(0) diffraction orders of different frequencies that pass through the optical system 3, are spatially separated in it and then follow the routes, respectively: -movable trippel-prism The detailed operation of the channel wallpaper is described in [18][19][20]. The possibility and conditions of using optical fibers in the lip, make it possible to make the scheme of AO LDI more compact.…”

“…The operation of the "fast inaccurate" channel is based on the operation of a phase interpolator and is carried out by calculating the fractions of the orders of interference with the resolution Δl fi , which is determined by the expression [18]: , a k int and N int are the interpolation coefficient and the bit depth of the interpolator, where λ is the laser wavelength. Thus, at Δl res =λ/64≈0.01 µm, 16 measurement channels are used [21].…”

“…The calculation of the phase error of the Δφ jit caused by the VCG's jitter can be carried out using the formula [18]:…”

“…The current trend of their improvement is to increase the resolution to the level of subpicometric values, less than 1 ρm [7][8][9]. LDI with acousto-optic (AO) modulation of the optical flow can be considered one of the most promising [11][12][13][14][15][16][17][18][19][20]. To increase the resolution of AO LDI, it was proposed to use two measuring channels (in future -channel) with the same stages for measuring cyclic start-stop movements: "fast inaccurate" and "slow accurate".…”