Yidong Tan

Response of microchip solid-state laser to external frequency-shifted feedback and its applications

The response of the microchip solid-state Nd:YAG laser, which is subjected to external frequency-shifted feedback, is experimentally and theoretically analysed. The continuous weak response of the laser to the phase and amplitude of the feedback light is achieved by controlling the feedback power level, and this system can be used to achieve contact-free measurement of displacement, vibration, liquid evaporation and thermal expansion with nanometre accuracy in common room conditions without precise environmental control. Furthermore, a strong response, including chaotic harmonic and parametric oscillation, is observed, and the spectrum of this response, as examined by a frequency-stabilised Nd:YAG laser, indicates laser spectral linewidth broadening.

Laser confocal feedback tomography and nano-step height measurement

A promising method for tomography and step height measurement is proposed, which combines the high sensitivity of the frequency-shifted feedback laser and the axial positioning ability of confocal microscopy. By demodulating the feedback-induced intensity modulation signals, the obtained amplitude and phase information are used to respectively determine the coarse and fine measurement of the samples. Imaging the micro devices and biological samples by the demodulated amplitude, this approach is proved to be able to achieve the cross-sectional image in highly scattered mediums. And then the successful height measurement of nano-step on a glass-substrate grating by combination of both amplitude and phase information indicates its axial high resolution (better than 2 nm) in a non-ambiguous range of about ten microns.

Laser feedback interferometry based on phase difference of orthogonally polarized lights in external birefringence cavity.

A simple and effective displacement sensor based on external birefringent feedback in Nd:YAG lasers is demonstrated. The measurement is based on the principle that, when linearly polarized light passes through the birefringent external cavity and then is fed back into laser resonator by external object, a phase difference is generated between laser sinusoidal-modulated intensities in the two orthogonal directions. These two sinusoidal intensities with lambda/2 period can be subdivided to lambda/8 after 4-fold evaluation. Moreover, the directional discrimination can be easily obtained according to the phase relationship between them. The chief advantages of the sensor are that it is compact, small size, flexible, low cost, and robust. Experimental results have shown that the standard deviation of displacement measurement is 0.093 microm in a 7 mm range and 0.34 microm in a 20mm range.

Self-mixing interference effects with a folding feedback cavity in Zeeman-birefringence dual frequency laser

The self-mixing interference effects with a folding feedback cavity in a Zeeman-birefringence dual frequency laser have been investigated theoretically and experimentally. The fringe frequency of the self-mixing interference system can be doubled due to the hollow cube corner prism, with which a folding cavity is formed. The intensities of the two frequencies are changed periodically in the modulation of the external cavity length. When the phase difference between the two frequencies equals pi/2, the intensity modulation curves can be divided into four zones with equal width in a period. Each zone corresponds to one polarization state. Based on the experimental results, a novel displacement sensor with a high resolution of lambda/16, as well as functions of direction discrimination, is discussed.

Phase difference in modulated signals of two orthogonally polarized outputs of a Nd:YAG microchip laser with anisotropic optical feedback

We present an experimental observation of the output responses of a Nd:YAG microchip laser with an anisotropic external cavity under weak optical feedback. The feedback mirror is stationary during the experiments. A pair of acousto-optic modulators is used to produce a frequency shift in the feedback light with respect to the initial light. The laser output is a beat signal with 40 kHz modulation frequency and is separated into two orthogonal directions by a Wollaston prism. Phase differences between the two intensity curves are observed as the laser works in two orthogonal modes, and vary with the external birefringence element and the pump power. Theoretical analyses are given, and the simulated results are consistent with the experimental phenomena.

Power spectral characteristic of a microchip Nd:YAG laser subjected to frequency-shifted optical feedback

We systematically investigated the power spectrum of a Nd:YAG laser with external frequency-shifted feedback and identified three factors dominating the spectrum, namely, the feedback level, the pumping level of the laser diode (LD), and the shifted frequency introduced in the external cavity. For very weak feedback, the laser power spectrum presents two peaks at frequencies of Ω and ωr, which are the shifted frequency and relaxation oscillation frequency, respectively. When the feedback level is increased to an intermediate level, the laser presents a series of nonlinear effects to cause harmonic and parametric oscillation; for strong feedback, only harmonic peaks are observed in the spectrum. The impact of the pumping levels and the amount of frequency shifting are also experimentally investigated. Especially, even at a weak feedback level, when the frequency Ω approaches ωr, strong nonlinear effects still appear in laser dynamics to make the laser power present only harmonic oscillation. A theoretical analysis is provided which agrees well with the experimental results.

The structure measurement of micro-electro-mechanical system devices by the optical feedback tomography technology

We describe a potential way to obtain the images of the surface and internal structure of micro-electro-mechanical system devices by optical feedback tomography technology. By using different materials and various structures of micro devices as the samples, this approach is proved to be able to measure the geometric structure of the micro device, including the internal structure, which makes it possible to detect whether the micro-device tested meets its fabrication requirements.

Generation of phase difference between self-mixing signals in a-cut Nd:YVO₄ laser with a waveplate in the external cavity.

We present a novel method using Nd:YVO4 laser with a waveplate in the external cavity to generate two orthogonally polarized signals with stable and adjustable phase difference. The phase difference is observed in the presence of external interference, and it is determined by the phase retardation of the waveplate. A model based on birefringent external-interference effect is proposed to theoretically explain the phase difference phenomenon, and the arithmetic solution of the relation between the phase difference and the phase retardation of waveplate is given. The simulated results accord with the experimental phenomena. This Letter provides the possibility for the measurement of phase retardation and also offers guidance to the design of interferometers based on fringe counting technique.

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.

Note: High-performance HeNe laser feedback interferometer with birefringence feedback cavity scanned by piezoelectric transducer

We demonstrate a high-performance HeNe laser feedback interferometer (HLFI) based on a birefringence feedback cavity scanned by a piezoelectric transducer (PZT). The PZT scanning technique leads to a significant improvement in the HLFIs anti-jamming capability. The null drift under general laboratory conditions is approximately ±32 nm, corresponding to ±2 pulses. The phase difference between the two measurement signals does not vary when the feedback cavity length changes. This ensures a large measurement range for the HLFI. The HLFI presented here features a large measurement range (>1 m), nanometer-scale resolution (15.82 nm), a compact configuration, and low cost.