Qu Ronghui

Digital coherent detection research on Brillouin optical time domain reflectometry with simplex pulse codes

The digital coherent detection technique has been investigated without any frequency-scanning device in the Brillouin optical time domain reflectometry (BOTDR), where the simplex pulse codes are applied in the sensing system. The time domain signal of every code sequence is collected by the data acquisition card (DAQ). A shift-averaging technique is applied in the frequency domain for the reason that the local oscillator (LO) in the coherent detection is fix-frequency deviated from the primary source. With the 31-bit simplex code, the signal-to-noise ratio (SNR) has 3.5-dB enhancement with the same single pulse traces, accordant with the theoretical analysis. The frequency fluctuation for simplex codes is 14.01 MHz less than that for a single pulse as to 4-m spatial resolution. The results are believed to be beneficial for the BOTDR performance improvement.

Influence of laser linewidth on performance of Brillouin optical time domain reflectometry

The effects of optical sources with different laser linewidths on Brillouin optical time domain reflectometry (BOTDR) are investigated numerically and experimentally. Simulation results show that the spectral linewidth of spontaneous Brillouin scattering remains almost constant when the laser linewidth is less than 1 MHz at the same pulse width; otherwise, it increases sharply. A comparison between a fiber laser (FL) with 4-kHz linewidth at 3 dB and a distributed feedback (DFB) laser with 3-MHz linewidth is made experimentally. When a constant laser power is launched into the sensing fiber, the fitting linewidths of the beat signals (backscattered Brillouin light and local oscillator (LO)) is about 5 MHz wider for the DFB laser than for the FL and the intensity of the beat signal is about a half. Furthermore, the frequency fluctuation in the long sensing fiber is lower for the FL source, yielding about 2 MHz less than that of the DFB laser, indicating higher temperature/strain resolution. The experimental results are in good agreement with the numerical simulations.

Composite second-order performance improvement in optical fibre CATV transmission system using chirped fibre grating

Theoretically, we analyse the dispersion compensation characteristics of the chirped fibre grating (CFG) in an optical fibre cable television (CATV) system and obtain the analytic expression of the composite second-order (CSO) distortion using the time-domain form of the field envelope wave equation. The obtained result is in good agreement with the numerical simulation result. Experimentally, we verify the result by making use of the tunable characteristics of CFG to change the dispersion compensation amount and obtain an optimal CSO performance in a 125km fibre transmission link. Both the theoretical and experimental results show that the CSO performance can be improved by properly choosing the dispersion compensation amount for a certain fibre transmission link.

Thermal Tunable Narrow Linewidth External Cavity Laser With Thermal Enhanced FBG

We demonstrated a mode-hop-free external cavity laser (ECL) design incorporating a semiconductor gain chip and a fiber Bragg grating (FBG) with enhanced thermal sensitivity. The compact ECL features a narrow linewidth of 35 kHz and a high linear thermal tuning velocity of 65 pm/°C (8.125 GHz/°C), which is six times higher than that of normal FBG-based ECLs.

785Nm external cavity Laser Diode with volume Bragg grating used in Raman spectroscopy

A 785nm external cavity laser diode with volume Bragg grating (VBG) used in Raman spectroscopy was presented. In order to improve the performance of Raman signal, the line-width of Laser diode (LD) is narrowed by external cavity technology and VBG, and the beam is also shaped by aspheric lens and fast-axis-collimator.

Study on wavelength distribution of high power laser diode array

Measured lateral wavelength distribution of LDA is presented, showing typically a V-type feature. Analyses indicate it is induced by bonding stress. A linear distributed stress model is proposed to explain the experimental results.

Experimental study of the wavelength locked of the high power laser diode bar

An external cavity laser diode was built by using a high power laser diode bar (LDB, 10% duty cycle, 100 A current), collimated column Len for fast axis beam collimation and a volume Bragg grating (VBG) as a feedback element to get LDBs wavelength locked to the Bragg wavelength of the VBG.