Chapter 3-Double Frequency Interferometry.pdfVIP

Chapter Ⅲ Double Frequency Interferometry 3.1 principle of double-frequency interferometry 1. Optical simulation a) Heterodyne system Two high frequencies are mixed, produce a low beat frequency. Radio: high frequency signal from antenna is mixed with vibration frequency from radio set. Produce (f1-f2) b) Optical simulation (Moire system) Two gratings G1 G2 are overlapped, G1 is fixed, G2 moving, produce fringes. The separation is correspondent to the difference between grating period number. Such as G1 10mm 100cycle G1 10mm 101cycle Grating constant 0.1mm. (f1≠f2) (vernier Moire) Moire fringe (101-100=1) 10mm ! in10mm interval there is one scale, which has a Moire fringe. Grating moves 0.1mm, Moire fringe moves 10mm ??? 100 1.0 10 M 2. Optical heterodyne system The frequency of light is too high (≈1015/s), avalanche diode detector 108/s. If beat frequency f1- f2≤10 8, can be detected. Example: red light f=5×1014/s ? =6×10-7m Detector Df? =2×10 -8/s ?? =2.4×10-15m Wavelength difference is very small. Beat frequency: fff ??? 21 Beat wavelength: 21 21 ?? ?? ? ? ? Characters: two 1? 2? interfere, produce moving fringes, interference field is at beam splitter (before or behind), cannot see it. 3. ifference between mono-frequency and double frequency: mono-frequency double frequency ① I ? ?cos Special function I ? t?cos Time function ② Stable fringes Moving fringes ③ Detect intensity (I) Beat frequency f? ④ Sensitive to vibration Non sensitive to vibration 4. Condition 1) Two frequency f1 f2 must be radiated at same time (simultaneously). 2) Two frequency f1 f2 must coherent 3) Parameter of f1 f2 must be different. (polarization; direction; wavelength) 4) Beat frequency f? can be detected. Resolve: frequency modulation 3.2 modulation technique 1. Acoustic-optical modulation Principle: Acoustic-optical effect: light beam is diffracted by ultrasonic wave in acoustic optical medium Ultra