Cai Haiwen

Sampled FBG based optical interleaver

In this text, we propose a novel optical interleaver using sampled fiber Bragg grating. According to coupling-mode theory, the interleaver based on sampled fiber Bragg grating with low sampling duty cycle may realize separate odd and even channel into two groups with power equalization, low insertion loss, and low dispersion.

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.

Study on the protection of Er-doped phosphate glass waveguide surface in ion-exchange processing

A novel method, sputtering K9 glass film, is proposed to solve the surface corrosion of Er-doped phosphate glass during ion-exchange processing for optical waveguide fabrication. The corrosion causes are analyzed to be the intrinsically weak stabilization of phosphate glass structure, hydrophile and weakly acidic property of phosphate radical. Experimental results show that the K9 glass film could not only protect the Er-doped phosphate glass surface from being corroded but also give no influence on the waveguide fabrication. The effect of thickness of K9 glass film on the optical property of waveguide is also investigated and the optimal thickness is found to be 60–80 nm. It provides a good base for further fabrication of active phosphate glass optical waveguide devices.

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.

Brillouin Optical Time Domain Reflectometry with Sub-Meter Spatial Resolution Based on Double-Pulse Detection

In conventional Brillouin optical time domain reflectometry (BOTDR),mutual restraint between the spatial resolution and the width of spontaneous Brillouin scattering spectrum can not be avoided. To solve this problem, we ameliorated the approach using double ultra-short pulses as probe light. By detecting spectral envelope, the spatial resolution of BOTDR can be extracted. This method improves the spatial resolution of the BOTDR system and meanwhile avoids the influence of Brillouin gain spectrum broadening caused by narrowing pulse on measurement accuracy. The experimental results show that with this novel BOTDR system, the temperature measurement can be realized at a 0.5 m spatial resolution, and the large broadening of the spontaneous Brillouin scattering spectrum can be avoided.