A review of the authors' work on orthogonally polarized lasers and their applications in self-sensing metrology is given. Zhang's group at Tsinghua University has been studying orthogonally polarized lasers, their phenomena and applications in metrology. Two kinds of dual frequency HeNe laser, which outputs a beam with two orthogonally polarized components, have been developed including (1) birefringence HeNe lasers (B-lasers) with 40 MHz to 1 GHz frequency difference and (2) birefringence-Zeeman HeNe lasers (BZ-lasers) with ~0–1 GHz frequency difference. A large number of laser tuning characteristics and feedback effects in HeNe laser and Nd:YAG laser are discovered, which are the basis for utilizing the lasers to realize new sensors for measurement of displacement and phase retardation of optical elements. This paper introduces some original metrology instruments including (1) HeNe laser nanometer rulers, (2) frequency splitting wave-plate measurement system, (3) laser feedback wave-plate measurement system, (4) quasi-common-path Nd:YAG laser feedback interferometer and (5) microchip Nd:YAG laser feedback sensor with birefringence external cavity. These instruments are able to supplement or replace traditional sensors.

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Orthogonally polarized dual frequency lasers and applications in self-sensing metrology

We present a measuring method for any wave plate retardation with fairly high precision that utilizes a laser frequency splitting technique. To avoid strong mode competition in measuring half and full wave plates, we use two separate methods: comparing adjacent longitudinal mode spacing, and phase offset with an additional quarter wave plate. Therefore any wave plate can be characterized by a single instrument, and no complicated experimental arrangement or data analysis is required. The performance of the system is demonstrated by determining the phase retardation of several samples to a precision and repeatability better than lambda/10(4); moreover, an error analysis is proposed.

Method for the measurement of phase retardation of any wave plate with high precision

Optical-phase-retardation elements are widely used in many fields. Accurate measurement of their phase retardation is crucial to the practical effect of the element’s processing and application. The development and present situation of the methods for optical phase retardation measurement are reviewed, with the wave plate, the most typical phase-retardation element, as an example. The latest research progress in this field is introduced; the principles and characteristics of individual measurement method are summarized and discussed. Three new methods based on laser frequency splitting or laser feedback are presented in detail, in which the laser is not only regarded as a light source but also plays a role of sensor. Moreover, no standard wave plates are needed and arbitrary phase retardation can be measured. Traceability, high precision and high repeatability are achieved as well.

Methods for optical phase retardation measurement: A review

Differential variation of laser longitudinal mode spacing induced by small intra-cavity phase anisotropies

We propose a laser frequency splitting method to determine the relative stress-optic coefficient and internal stresses in Nd:YAG crystals with high resolution. In this method a mirrored Nd:YAG crystal in a circular disk shape is made into an all-internal cavity laser. Once diametral compression force is applied, due to intracavity photoelastic effect, the single laser frequency is split in two. Through measuring the beat frequency of the split frequencies, the relative stress-optic coefficient and internal stresses in the Nd:YAG crystal can be determined according to the proportional relationship. In our experiment the measured value of the stress-optic coefficient, C111=1.27×10−12 m2/N, is close to the theoretical value; the principal stress difference at the center of the Nd:YAG crystal was also determined. The optical path retardation resolution is approximately 1.5×10−8 λ, and correspondingly the principal stress difference resolution is approximately 5 Pa. The resolution is approximately 5-8 orders of magnitude higher than those of conventional methods, therefore even very small residual stresses can be easily determined with this method. Although this method is a kind of point-by-point method instead of a full-field method, it is a promising novel photomechanics method with attractive advantages such as extremely high resolution.

Laser frequency splitting method for high-resolution determination of relative stress-optic coefficient and internal stresses in Nd:YAG crystals

A measurement system utilizing laser frequency splitting is presented for the precise measurement of retardations of arbitrary wave plates. The wave plate to be measured is inserted into a laser resonance cavity, and its retardation can be determined by measuring the difference of split frequencies and longitudinal mode spacing of the laser. In the system, mode spacing measurement is performed under multimode oscillation according to the relation between polarization directions of modes. To solve the problem of strong mode competition in measuring half-wave plates, a transverse magnetic field is applied to weaken the competition so as to make the system applicable even when the frequency difference is less than the threshold for strong mode competition. The error of the system is less than 5 arcmin, and standard deviations of about 1 arcmin are obtained from repeated experiments samples of quarter- and half-wave plates. No element rotation is required during measurement, and no mechanical quantity, such as angle, needs to be measured.

Xiao-bin Zong

Papers

Measurement of retardations of arbitrary wave plates by laser frequency splitting