Simple and accurate wavemeter implemented with a polarization interferometer

Abstract

A simple and accurate wavemeter for measuring the wavelength of monochromatic light is described. The device uses the wavelength-dependent phase lag between principal polarization states of a length of birefringent material (retarder) as the basis for the measurement of the optical wavelength. The retarder is sandwiched between a polarizer and a polarizing beam splitter and is oriented such that its principal axes are 45 deg to the axis of the polarizer and the principal axes of the beam splitter. As a result of the disparity in propagation velocities between the principal polarization states of the retarder, the ratio of the optical power exiting the two ports of the polarizing beam splitter is wavelength dependent. If the input wavelength is known to be within a specified range, the measurement of the power ratio uniquely determines the input wavelength. The device offers the advantage of trading wavelength coverage for increased resolution simply through the choice of the retarder length. Implementations of the device employing both bulk-optic components and fiber-optic components are described, and the results of a laboratory test of a fiber-optic prototype are presented. The prototype had a wavelength accuracy of +/-0.03 nm.

Compensation of output error due to the change of input intensity in an optical wavemeter based on a polarization interferometer

“…To analyze the changed polarization state after passing through the retarder, the light is divided into x and y polarization components by the polarization beam splitter. The phase difference is calculated from the ratio of the split intensities, and the optical wavelength of the input light can be known [9]. Ideally, the split intensities by the polarization beam splitter can be represented [10] as…”

“…G can be inferred from the wavemeter output within a range between singularities. The measurement bandwidth can be expressed with [9] Dl ¼ l 2 2LDn…”

“…The wavemeter bandwidth is inversely proportional to DnL, and the resolution is proportional to DnL. Therefore, a trade-off relation exists between resolution and bandwidth [9]. Fig.…”

“…For realizing inexpensive and portable wavemeters, several techniques have been investigated, including a wavemeter based on photo diodes with different spectral response curves [7], fiber couplers [8], and polarization interferometers [9]. Among the techniques, the optical wavemeter based on a polarization interferometer is more accurate, simple, and stable in the field.…”

“…If an optical signal is modulated or its intensity through a polarizer is fluctuated by changing a polarization state, a measurement error of the wavemeter is increased and a resolution and bandwidth are limited [9]. In fiber optic systems, an optical signal is modulated and its polarization state is randomly fluctuated because of a birefringence of a fiber.…”