Liqun Sun

Simultaneous measurement of refractive-index and thickness for optical materials by laser feedback interferometry

The refractive index measurement by ordinary interferometers cannot avoid the air disturbances in the optical path. A novel approach is presented in this paper based on the Nd:YAG microchip laser feedback interferometry (MLFI) with 1064 nm wavelength. For eliminating the air flow and electric-heating influence the heterodyne modulation and quasi-common path in the MLFI are used. The simultaneous measurement with high accuracy of the refractive index and thickness is realized. The measurement results for three kinds of materials are presented including N-SF57 glass with high index up to 1.81057. The measurement uncertainty of refractive index is better than 0.00002 and of thickness is better than 0.0006 mm.

Dual-sideband heterodyne of dispersion spectroscopy based on phase-sensitive detection

A methane sensor based on dispersion spectroscopy is presented in this paper. A standard Mach-Zehnder modulator working in carrier suppression mode is adopted to generate a spectrum of a carrier and two sidebands. We aim at detecting the phase shift of the beatnote generated by the two sidebands in a methane concentration evaluation process. We put forward an analytical model to describe the dual-sideband heterodyne scheme and carry out experiments to demonstrate the model. Long-term tests show that the sensor has a minimum detection limit of 0.4  ppm·mHz-0.5 at an average time of 1 s. And in the condition of 1 atm and room temperature, a linear measurement range from 0.4 to 44955  ppm·m is achieved.

Generalized method for calculating astigmatism of the unit-magnification multipass system

A generalized method to accurately calculate astigmatism of the unit-magnification multipass system (UMS) is proposed. A practical coaxial optical transmission model is developed for the UMS. Astigmatism analysis is then made convenient by a 4 by 4 general transfer matrix. Astigmatism correction is significantly promoted, and hence further improvement in imaging quality can be expected. Good agreement between numerical simulations and Zemax ray tracing results verifies the effectiveness of this method. The resulted RMS spot size of this method is only 25% to 64% of other previous methods based on the golden section search for minimum astigmatism in real design cases. This method is helpful for the optical design of the UMS.

Self-mixing interferometry with mutual independent orthogonal polarized light.

A self-mixing interferometry with mutual independent orthogonal polarized light is introduced. Its most important feature is that two mutual independent orthogonal lights are used as measuring and reference light. Frequency shifting and polarization multiplexing technologies are used in the proposed optical system. Phase variation of the two orthogonal polarized beams is simultaneously measured through heterodyne demodulation with a lock-in amplifier. The phase difference of the orthogonal polarized light accurately reflects the target displacement. The target in this system is a non-cooperative object which is different from a traditional Michelson interferometer. The primary experimental results show that this kind of self-mixing interferometry is very feasible. Under typical room conditions, the systems short-term resolution is better than 2 nm.

Depth selectivity for the assessment of microstructure by polarization studies

A polarimetric imaging system capable of continuously selecting imaging depth in a turbid media is demonstrated. The proposed system is based on the orthogonal polarization spectral (OPS) technique, and is able to detect microstructure and microvessel. First, we compare the performance of four polarization imaging channels on a biological phantom, and find that there is a linear relation between the degrees of ellipticity and image contrast in co-linear/co-elliptical channels. In addition, the cross-linear channel has the best image contrast. We then prove the performance of depth selectivity of microvessel in a mouse ear.

A microchip laser source with stable intensity and frequency used for self-mixing interferometry

We present a stable 40 × 40 × 30 mm(3) Laser-diode (LD)-pumped-microchip laser (ML) laser source used for self-mixing interferometry which can measure non-cooperative targets. We simplify the coupling process of pump light in order to make its polarization and intensity robust against environmental disturbance. Thermal frequency stabilization technology is used to stabilize the laser frequency of both LD and ML. Frequency stability of about 1 × 10(-7) and short-term intensity fluctuation of 0.1% are achieved. The theoretical long-term displacement accuracy limited by frequency and intensity fluctuation is about 10 nm when the measuring range is 0.1 m. The line-width of this laser is about 25 kHz corresponding to 12 km coherent length and 6 km measurement range for self-mixing interference. The laser source has been equipped to a self-mixing interferometer, and it works very well.

Refractive Index Measurement of Liquids by Double-Beam Laser Frequency-Shift Feedback

The liquid refractive index measurement has become an important process in many research and industrial applications. However, the measurement results using traditional methods cannot be calibrated, and the corresponding accuracy is limited, since those methods cannot trace to laser wavelength. This letter describes a novel method to measure the liquid refractive index based on the double-beam laser frequency-shift feedback. The two beams are incident on the liquid surface and the mirror under liquid, respectively. Utilizing the high sensitivity of the frequency-shift feedback of solid lasers and high stability of heterodyne interference, the height variation of the liquid surface and the depth variation of the mirror under liquid are measured. Addressing these two variations, the traceable measurement of liquid refractive index is realized. The measurement results of five different liquid samples and NaCl solution with various concentrations are presented, which proved the system has high repeatability of better than 0.00005.

Spectrum Broadening in Optical Frequency-Shifted Feedback of Microchip Laser

The influence of feedback level and shifted frequency on laser spectrum and power spectrum is investigated experimentally and theoretically. Due to optical frequency-shifted feedback, the spectrum of Nd:YVO4 microchip laser is expanded into one with multi equal-spaced sidebands around its initial central frequency. The detail of expanded spectrum depends on both the shifted frequency and relaxation oscillation frequency. Optical feedback level decides the possible maximum spectrum range. Nevertheless, the actual spectrum range depends on both the frequency shift and the feedback level. The power spectrum turns from simple to complex with the increase of feedback level when the shifted frequency is a constant. A rate-equation model with optical frequency-shifted feedback is built to interpret the experimental phenomena. The theoretical results accord with the experiments very well. This letter can help choose appropriate parameters for feedback interferometry systems and promote the accomplishment of optical frequency-shifted feedback diagram, which could be a guide for feedback interferometry applications.

Study on the influence of the response characteristics of a temperature sensor on the measurement accuracy of a water-absorption-based high-energy laser energy meter

When using water as a cooling or absorption medium for an energy meter, a temperature sensor is limited by response characteristics and cannot reflect the real-time temperature changes in the water flow. In order to improve the accuracy of measurement, we should ensure that the corresponding value of the temperature integral will be substantially independent of the effects of the sensor response time. According to the analysis of the interaction process between temperature sensor and water flow temperature field, we have established a hot physical model of the whole measurement process, and decomposed it into a superposition of a slowly varying process and a transient process, then simplified the model. Finally, a quantitative relationship between the sensor response characteristics and measurement accuracy of the high energy laser energy meter is derived. With the mandatory heat exchange method, the frequency characteristics of the temperature field meet the requirements of the frequency characteristics of a temperature sensor; as a result, the impact on measurement accuracy is eliminated. The experimental results show that this method has good effects, and it can help to improve the measurement accuracy of a high-energy laser energy meter.

Approximate analytic astigmatism of unit-magnification multipass system.

We develop a way to estimate the approximate analytic astigmatism with a high accuracy for any unit-magnification multipass system (UMS). The coaxial optical transmission model for UMS is simplified based on the systems features. Furthermore, astigmatism is derived as a distinct form of vector addition and, thus, feasible analytic astigmatism can be obtained. The effectiveness of our method is verified by simulations for a Bernstein-Herzberg White cell. In our cases, the relative error of optimization for astigmatism correction by our method is smaller than 5 per thousand, which is only one-tenth of that by Kohns method. Our method significantly improves the efficiency for astigmatism correction, and further benefits the optical design of a UMS.