A method is described for control of optical path length over a range of many optical wavelengths. The operational concept utilizes phase error signals from a pair of selected wavelengths of a multiline laser to generate unambiguous feedback control signals. A problem encountered in single wavelength phase control results with phase disturbances greater than the optical wavelength. The two wavelength technique provides synthetic long wave length error signals generated by beating a pair of single line error signals. The resulting long wavelength error signals in conjunction with single wavelength error signals are employed for control over an extended phase range of many wavelengths. The theory of two wavelength phase control is first presented for multidither and phase conjugate configurations. The latter is shown to have distinct advantages with regard to ease of implementation and performance potential. An operational two wavelength phase control system employing heterodyne phase measurement is then described along with experimental results of open and closed loop performance. The results verify theoretical predictions and indicate extended range phase control with fast response.
Gary E. Palma,
Robert K. Elkow,
Albert W. Angelbeck,
"Two-Wavelength Phase Control", Proc. SPIE 0179, Adaptive Optical Components II, (11 July 1979); doi: 10.1117/12.957301; https://doi.org/10.1117/12.957301