[show abstract][hide abstract] ABSTRACT: The up to date progress of fiber sensing technologies in Tianjin University are proposed in this paper. Fiber-optic temperature
sensor based on the interference of selective higher-order modes in circular optical fiber is developed. Parallel demodulation
for extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors is realized based on white light interference.
Gas concentration detection is realized based on intra-cavity fiber laser spectroscopy. Polarization maintaining fiber (PMF)
is used for distributed position or displacement sensing. Based on the before work and results, we gained National Basic Research
Program of China on optical fiber sensing technology and will develop further investigation in this area.
KeywordsFiber sensing–high temperature–parallel demodulation–gas detection–distributed sensing
[show abstract][hide abstract] ABSTRACT: We demonstrate a terahertz-wave parametric oscillator (TPO) with a corner-cube resonator consisting of a corner-cube prism (CCP) and a flat mirror. By using the cavity configuration proposed in this Letter, the generation of tunable monochromatic terahertz (THz) waves can be achieved just by rotating the flat mirror instead of rotating the TPO cavity relative to the pump beam. The THz-wave output intensity and pulse width can be controlled periodically by rotating the CCP around the cavity axis. The TPO stability against cavity misalignment is significantly improved by at least 1 to 2 orders of magnitude compared with the conventional plane-parallel resonator configuration.
[show abstract][hide abstract] ABSTRACT: A novel frequency tuning scheme for terahertz-wave parametric oscillators (TPOs) is proposed. We demonstrate that the generation of the tunable terahertz wave can be realized by continuously varying the incident pump wavelength at a fixed angle of incidence of pump relative to the TPO resonator axis, based on the variation of noncollinear phase-matching conditions between the pump and Stokes/THz waves in the process of simulated polariton scattering. Combined with the angle-tuning method, this potential pump-wavelength tuning technique can further extend the TPO tuning range, especially in low-frequency THz region. However, the stability of the output direction of THz waves from the Si prism under this tuning method has yet to be further improved. The characteristics of THz-wave parametric gain involved in this frequency tuning scheme are also studied.
[show abstract][hide abstract] ABSTRACT: In this paper, the characteristics of bent large-mode-area photonic crystal fibers are investigated comprehensively by using a finite difference mode solver with perfectly matched layers. Numerical results show that in bent large-mode-area photonic crystal fibers, the mode-field distributions are deformed and the effective mode area and the width of the guided region are reduced. The propagation character of bent photonic crystal fibers is determined by normalized propagation number and normalized bend radius. Meanwhile, the bending loss oscillation is observed and the position of the first loss peak is obtained in bent large-mode-area photonic crystal fibers.
Journal of Optics A Pure and Applied Optics 06/2008; 10(8):085303. · 1.92 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper presents a generalized analysis model for fringe pattern profilometry. We mathematically derived a new analysis model that gives a more general expression of the relationship between projected and deformed fringe patterns. Meanwhile, based on the proposed generalized model, a new algorithm is presented to retrieve 3-D surfaces from nonlinearly distorted fringes. Without any prior knowledge about the projection system, we still can obtain very accurate measurement results by using a generalized analysis model and a proposed algorithm. Computer simulation and experimental results show that the generalized model and the proposed algorithm can significantly improve the 3-D reconstruction precision, especially when the projected fringe pattern is nonlinearly distorted.
IEEE Transactions on Instrumentation and Measurement 02/2008; · 1.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: A high-repetition-rate dual-signal intracavity optical parametric generator (IOPG) inside a diode-end-pumped acousto-optically (AO) Q-switched Nd:YVO4 laser is presented. The nonlinear material is a periodically-phase-reversal periodically poled MgO-doped lithium niobate (ppr-PPMgLN). At an incident diode pump power of 6.1 W and a Q-switch repetition rate of 20 kHz, an average dual-signal output power of 0.44 W is achieved. The dual-signal wavelengths of 1477 and 1491 nm are obtained at a crystal temperature of 40 °C. The measurements of the beam quality factor of 1.4 and the pulse duration of 77 ns show good spatial and temporal overlaps for the dual-signal radiation.
Journal of Optics A Pure and Applied Optics 08/2007; 9(10):797. · 1.92 Impact Factor
[show abstract][hide abstract] ABSTRACT: A tunable dual-signal optical parametric generator (OPG) based on multi-grating periodically poled lithium niobate (PPLN) is presented. An acousto-optic device (AOD) was used as a beam splitter outside the cavity of a high-repetition-rate, Q-switched 1064 nm Nd:YAG laser. A synchronous and widely tunable dual-signal OPG was obtained by coupling the split beams into two parts with different poled periods in the same PPLN crystal and by tuning the temperature of the crystal. A total conversion efficiency of 27.9% was achieved when the system operated at 1687 and 1864 nm.
Journal of Optics A Pure and Applied Optics 02/2007; 9(3):235. · 1.92 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper, we introduce a concept of phase matching bandwidth of broadband sources by using the retracing behavior of collinear quasi-phase-matching (CQPM) optical parametric generation (OPG) in periodically poled lithium niobate (PPLN). By comparing various pump wavelengths we derive the pumping condition and parameters of PPLN to realize a broadband source near 1550 nm in the CQPM-OPG. We predict the optimum pump wavelength and the maximum ideal bandwidth range to be 940.75 nm and 1475-1681 nm respectively. Experimentally we have demonstrated a 946 nm Nd:YAG laser which serves as the pumping source of CQPM-OPG.
[show abstract][hide abstract] ABSTRACT: This paper reports on a high-repetition-rate dual signal-wave (DSW) optical parametric oscillator (OPO) operating at the 1.5μm band with tunable wavelength intervals from 2.5nm to 69.1nm. Two periodically poled crystals, a periodically poled lithium niobate (PPLN) with multiple gratings and a single grating MgO-doped PPLN (PPMgOLN), are cascaded in the same OPO cavity to generate dual signal-waves by using quasi-phase-matched (QPM) technique. The pump source was a Q-switched diode-pumped Nd:YVO4 laser operating at 50kHz. At an incident pump power of 3W, an average output power of 169.6mW at 1489.2nm and 1558.3nm has been achieved.
[show abstract][hide abstract] ABSTRACT: What is believed to be a new analysis algorithm to carry out profile measurement with low computational complexity and less noise sensitivity is presented. First, a discrete cosine transform (DCT)-based representation method is introduced to express the height distribution of a 3D surface. Then a novel shift estimation algorithm, called the DCT-based shift estimation (DCT-SE), is presented to reconstruct 3D object surfaces by using the proposed expression and the generalized analysis model. The advantage of DCT-SE is that without loss of measurement precision it provides lower computational complexity to implement 3D reconstruction from nonlinearly distorted fringe patterns and, at the same time, survives the random noise. Simulations and experiments show that the proposed DCT-SE is a fast, accurate, and efficient reconstruction algorithm for digital projection- based fringe pattern profilometry techniques.
[show abstract][hide abstract] ABSTRACT: A high-conversion-efficiency, low-threshold, quasi-continuous-wave optical parametric generator (OPG) based on a periodically poled lithium niobate (PPLN) crystal is presented. Pumped by an acousto-optically Q-switched 1064 nm Nd:YAG laser with a power output of 848 mW, the OPG generated an output power of 452 mW for the signal and the idle waves, achieving an internal conversion efficiency of 62.7% and a slope efficiency of 75.6%. To the best of our knowledge, this is the highest efficiency ever reported for single-pass, quasi-continuous-wave OPGs by using periodically poled crystals.
[show abstract][hide abstract] ABSTRACT: A high-power continuous-wave (cw) Nd:YAG laser operating at 946 nm by utilizing a quasi-three-level transition is reported. The laser consists of a composite Nd:YAG rod end pumped by a fiber-coupled diode laser and a simple plane-concave cavity. At an incident pump power of 40.2 W, a maximum cw output of 15.2 W at 946 nm is obtained, achieving a slope efficiency of 45%. To the best of our knowledge, this is the highest output at 946 nm ever generated by diode-pumped Nd:YAG lasers. In addition, at an incident pump power of 15.2 W, a 1.25 W blue output at 473 nm is achieved with a simple compact three-element cavity and a type-I lithium triborate (LiB(3)O(5)) crystal as a frequency doubler.
[show abstract][hide abstract] ABSTRACT: An efficient continuous-wave (CW) simultaneous dual-wavelength lasing (SDWL) of an LD end-pumped Nd:YAG laser utilizing a quasi-three-level transition at 946 nm and a four-level transition at 1064 nm is reported. A theoretical model has been introduced to determine the threshold conditions for SDWL. The temperature distributions of a Nd:YAG crystal under different pump powers have been analyzed. In the experiments, a CW SDWL output power of 5.12 W at a temperature of 273 K has been achieved with a pump power of 17 W, giving a slope efficiency of 16.36%.
[show abstract][hide abstract] ABSTRACT: This paper presents a new approach to fringe pattern profilometry. In this paper, a generalized model describing the relationship between the projected fringe pattern and the deformed fringe pattern is derived, in which the projected fringe pattern can be arbitrary rather than being limited to being sinusoidal, as are those for the conventional approaches. Based on this model, what is believed to be a new approach is proposed to reconstruct the three-dimensional object surface by estimating the shift between the projected and deformed fringe patterns. Additionally, theoretical analysis, computer simulation, and experimental results are presented, which show how the proposed approach can significantly improve the measurement accuracy, especially when the fringe patterns are distorted by unknown factors.
[show abstract][hide abstract] ABSTRACT: In this paper, we proposed a new algorithm, referred to as inverse function analysis (IFA) method based on the derived mathematical model to reconstruct 3-D surfaces using fringe pattern profilometry (FPP) technique. Compared with traditional methods, our algorithm has neither the requirement for the structure of projected fringe patterns, nor the prior knowledge of the characteristics of projection systems. The correctness of inverse function analysis (IFA) method has been confirmed by simulation results. It can be seen that the measurement accuracy has been significantly improved by inverse function analysis (IFA) method, especially when the expected sinusoidal fringe patterns are distorted by unknown nonlinear factors.
Intelligent Signal Processing and Communication Systems, 2005. ISPACS 2005. Proceedings of 2005 International Symposium on; 01/2006
[show abstract][hide abstract] ABSTRACT: We experimentally demonstrate a dual-wavelength continuous wave (CW) Nd:YVO4 laser simultaneously operating at 1342 and 1386nm. A total output power of 439mW is achieved when the laser is diode-end-pumped at an incident pump power of 2.4W, indicating a slope efficiency of 35.6%. To the best of our knowledge, it is the first time that simultaneous dual-wavelength CW operation at 1342 and 1386nm is reported.
[show abstract][hide abstract] ABSTRACT: Photonic crystals (PCs) are synthetic micro-structures which have periodic refractive index variations and produce photonic bandgaps similar to electronic bandgaps produced by the crystal potentials of semiconductors. Different methods have been proposed and demonstrated to fabricate two- or three-dimensional photonic crystal structures. Among them, the holographic lithography method, in which multi-beam interference is employed, offers a number of advantages. These include the ability to create a large volume of uniform periodic structures through an irradiation process, and more degrees of freedom to control the structures. In this study, a model is presented for predicting the multi-dimensional photonic crystal structures formed using multi-beam interference. Various parameters, including beam propagation and polarization directions, beam intensities, and phase shifts are considered. Calculations have been carried out to simulate two four-beam configurations which have been popularly used in the fabrication of photonic crystals. It has been demonstrated that the contours of the interference pattern are related to the polarization states, the intensity ratios among the four beams, and the phase delays. Therefore, by controlling the beam intensities, polarization directions, and phase delays, different structures can be obtained. The results presented in this study provide a useful guide for choosing various optical parameters and selecting suitable photoresists to fabricate 2D and 3D photonic crystal structures.
Smart Materials and Structures 01/2006; 15(1). · 2.02 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper reports a diode-end-pumped continuous-wave (CW) Nd:YAG laser operating at 946-nm by utilizing the 4F3/2-4I9/2 transition. We demonstrated that at an incident pump power of 27.7 W, an output power of 8.3-W could be achieved with a slope efficiency of 33.5%. To the best of our knowledge, this is the highest CW output power at 946 nm generated by LD end-pumped Nd:YAG lasers. By using intracavity frequency doubling with an LBO crystal, we further obtained a 473-nm blue laser with an output power of 1.2 W, achieving an optical-to-optical conversion efficiency of 7.1% at a pump power of 16.9 W. The short-term power instability of the blue laser was less than 1 %.
[show abstract][hide abstract] ABSTRACT: The electro-optic effect of aperiodically poled lithium niobate (APLN) has been theoretically investigated and proposed to use as a Q-switch in a simultaneous dual-wavelength laser. Our analysis shows that the polarization planes of the z-polarized (or y-polarized) dual-wavelength beams can be simultaneously rotated by 90° through a well-constructed APLN with an external electric field applied along the y-axis, which enables Q-switch function in a dual-wavelength laser cavity. Using a Nd:YVO4 laser operating at 1.0643μm and 1.3419μm as an example, we present a design method of APLN by using the so-called simulated annealing algorithm. The influence of the domain errors in fabricating an APLN device is also studied. The results show that the device is not susceptible to the fabrication errors.