Real-time displacement measurements with a Fabry-Perot cavity and a diode laser

Design of a folded, multi-pass Fabry–Perot cavity for displacement metrology

“…However, the 40 mm long cavities, both traditional and modified, have frequency shifts of 11.85 MHz. This is because the frequency shift is inversely proportional to the nominal length of the traditional cavity from equation (2) and the length change is multiplied in the modified cavity. From this, the sensitivity of the FP interferometer with the modified cavity is four times better than that of the typical FP interferometer using the long cavity.…”

“…The optical frequency of a tunable laser can be locked to a resonance peak of the FP cavity and the locked frequency is shifted when one cavity mirror is moved [1][2][3][4]. From this, the displacement can be calculated using the nominal cavity length and the calibrated optical frequency.…”

“…An external cavity laser diode (ECLD) with 635 nm wavelength is an attractive, tunable source due to its wide tunable range (∼10 nm) and its narrow linewidth. The frequency shift can be measured with an iodine stabilized He-Ne laser up to a few tens of GHz, which is dependent on the bandwidth of the photodetector (PD) used, so the displacements can be measured continuously without any fly back states in the range of a few micrometers [2]. However, the use of an ECLD and high-speed PD is limited in some industrial and research applications due to its high cost.…”

“…of the FP cavity should be narrower than the gain bandwidth of the He-Ne laser for the locking and unlocking technique to measure long displacements [1]. Therefore, this leads to using long cavities because of the narrow bandwidth, which are undesirable because they decrease the stability of the system caused by mechanical vibrations, laser frequency instabilities and refractive index changes [2].…”

High-precision positioning stage using optical zooming laser interferometer for linear encoder calibration

“…[7][8][9] The laser-frequencybased technology using a Fabry-Perot cavity is a distinguished technique because it has no cyclic error in principle. [10][11][12] However, it has the disadvantage that the dynamic range is small because of the limit of the laser frequency change.…”