Li Wei

Phase-shifted Bragg grating filters with symmetrical structures

A comprehensive theoretical analysis of the compound phase-shifted uniform fiber Bragg grating (FBG) filter is presented. The transmission characteristics are examined by a transfer matrix formulation. It is shown that the transmission spectrum of such a filter is significantly controlled by the ratio of the grating lengths. By suitably choosing the ratio, a quasi-flat-top spectrum may be obtained. The general condition for the optimum case is derived. This characteristic makes the compound phase-shifted uniform FBG filter particularly advantageous for applications in wavelength-multiplexed optic-fiber communication systems.

Compound ring resonator with double couplers

This paper analyzes a double-coupler double-ring resonator which has properties very suitable for the construction of lasers. The optimum design relationships are obtained. The outputs and circulating intensities of the resonator are investigated. Among the other properties examined are the effects on the resonance of the coupling coefficients, losses and resonant numbers.

Tunable graphene Q-switched erbium-doped fiber laser with suppressed self-mode locking effect

Self-mode locking effect in a wideband tunable graphene-based passively Q-switched erbium-doped fiber laser has been observed experimentally. Q-switching is achieved by using graphene as a saturable absorber, while a tunable bandpass filter with a narrow bandwidth is used to obtain wideband tunability. We propose to suppress the modulation on each pulse from self-mode locking by introducing three subring resonators constructed with three 3 dB couplers into the laser ring cavity. Moreover, the laser output characteristics with respect to pump power are studied in detail. A stable Q-switched erbium-doped fiber laser with a tunable range from 1522 nm to 1568 nm is demonstrated experimentally.

Compound ring resonator with an external reflector for lasers

Abstract We proposed and analyzed a compound ring resonator with an external reflector, which could be used to build single-mode travelling-wave doped fiber laser without using optical isolators. The theoretical results show that the loss difference depends greatly on the coupling coefficients and k 1 b − k 2 s is the better choice because of the lower insertion loss. The mode suppression could be achieved by choosing the proper ratio of the cavity lengths of the two rings. Such a resonator is particularly suitable for doped fiber lasers.

Propagation characteristics of hybrid guided modes in a 5-layer metal/dielectric/metal waveguide

We presented the propagation characteristics of the hybrid guided mode in a 5-layer metal/low-index/high-index/low-index/metal waveguide based on our derived analytical equations. The optical properties of the dielectric layers have played an important role on the optical performance of the hybrid mode. In this study, we investigated the effect of the refractive index of the high index dielectric core layer on the propagation length enhancement.

Compact passively Q-switched Er/Yb-codoped cladding pumped fiber laser

An all-fiber passively Q-switched erbium/ytterbium co-doped cladding pumped fiber laser is presented. A section of erbium/ytterbium co-doped fiber is used as both gain medium and saturable absorber, which makes the laser configuration simple and compact. By properly choosing the splitting ratio of the laser output coupler, stable Q-switching could be achieved. It has been found that the splitting ratio of the output coupler is very important for generating stable Q-switching pulses. The operation range of stable Q-switching is different for different lengths of the erbium/ytterbium co-doped fiber. Self-mode-locking effect is also observed accompanying with the Q-switched pulses. The characteristics of the laser output against the pump power and the splitting ratio of the output coupler are studied in detail.

Temperature-independent strain sensor based on a core-offset multimode fiber interferometer

A novel temperature-independent multi-mode fiber (MMF) lateral strain sensor based on a core-offset interferometer is presented and demonstrated experimentally. Slightly misaligning a splice between an MMF and a single-mode fiber (SMF), high extinction ratio of the interferometer based on SMF-MMF-SMF structure can be obtained. When the lateral strain is applied to a short section of the MMF, the extinction ratio of the interferometer will decrease accordingly while the interference phase remains almost constant. Temperature variation only leads to shift in the transmission power spectrum of the interferometer and does not affect the extinction ratio. Experimental results show that there is a good quadratic relationship between the lateral strain and the extinction ratio. The proposed strain sensor has the advantages of temperature-independency, high extinction ratio sensitivity, good repeatability, low cost, and simplicity in structure.

Low-cost lateral force sensor based on a core-offset multimode fiber interferometer with intensity based interrogation technique

A novel lateral force sensor based on a core-offset multimode fiber interferometer with intensity-based interrogation technique is reported. An offset between the cores of the single-mode fiber and multimode fiber is made to produce high extinction ratio. When a lateral force is applied to a short section of the multimode fiber, the extinction ratio decreases with the interference phase almost unchanged. In addition to serving as a sensing head, the multimode fiber can also act as a filter to realize lateral force measurement by determining the power change from a power meter. Experimental results show that the power ratio change has a linear relationship with respect to the applied lateral force, and the resolution of the sensor configuration is about 0.01 N.

Simultaneous Temperature and Strain Measurement Using Fiber Bragg Gratings and Multi-Mode Fibers

We present an alternative solution for simultaneous measurement of temperature and strain. The sensing head is formed by a fiber Bragg grating combined with a section of multi-mode fiber for temperature and strain discrimination.

Traveling-wave single-mode Er-doped fiber laser

An erbium-doped fiber ring laser, with a single- longitudinal-mode, operating at 1548.5 nm, has achieved traveling wave operation without the use of an optical isolator. The unidirectional operation is guaranteed by a combination of an output coupler and an external fiber Bragg grating. The laser can provide up to 19.6 mW output power. The linewidth of the laser was measured to be < 24 kHz.