Huan Huan Liu

Ultrahigh Sensitivity Refractive Index Sensor Based on Optical Microfiber

We demonstrate a nonadiabatic microfiber sensor with a taper diameter of few micrometers. The modal interference caused by the abrupt taper results in a sinusoidal spectral response. The wavelength shift arising because of the changes in the external refractive index is found to be significant, achieving a maximum sensitivity of 18681.82 nm/RIU. The measured results show good agreement with the theoretical predictions. The high sensitivity and the simplicity offer the sensor the potential for many real applications.

Operation-Switchable Bidirectional Pulsed Fiber Laser Incorporating Carbon-Nanotube-Based Saturable Absorber

We present an operation-switchable bidirectional ring-cavity pulsed fiber laser incorporating a carbon-nanotube-based saturable absorber and a polarization-dependent four-port circulator. By manipulating the intra-cavity polarization state of light, two sets of mode-locked pulses in terms of fundamental repetition rate and pulse width can be achieved separately from the same laser cavity. In clockwise direction, the laser generates mode-locked output pulse train with a pulse width of 600 fs and a fundamental repetition rate of 12.68 MHz. While in counter-clockwise direction, the laser generates mode-locked pulses with a pulse width of 480 fs and a fundamental repetition rate of 16.46 MHz. Furthermore, the same laser cavity can produce bidirectional Q-switched output pulse trains with synchronized repetition rate. Such synchronized repetition rate is linearly proportional to pump power. The results show a multi-functional pulsed fiber laser with selectable output pulse trains.

Enhanced stability of dispersion-managed mode-locked fiber lasers with near-zero net cavity dispersion by high-contrast saturable absorbers

We experimentally investigate the stability of dispersion-managed mode-locked fiber lasers using carbon-nanotube-based saturable absorbers (SAs) with different modulation depths. An unstable operation region of the mode-locked fiber laser with near-zero net cavity dispersion is observed, where the laser produces random pulse burst rather than stable pulse train. Through the implementation of high-contrast SAs in the laser, the unstable region is found to be shrunk by ~31.3% when the modulation depth of the SAs increases from 6.4% to 12.5%. The numerical simulation is consistent with the experimental observation.

Enhancement of thermal damage threshold of carbon-nanotube-based saturable absorber by evanescent-field interaction on fiber end.

We present a scheme of fiber-connector-type carbon-nanotube-based saturable absorber (CNT-SA) with enhanced thermal damage threshold, in which the CNTs are deposited on the fiber connector end in a ring pattern for evanescent-field interaction instead of direct interaction. The thermal damage threshold of such CNT-SA is found to be increased by around 130% compared with an evenly deposited one. An all-fiber Fabry-Perot (FP) linear cavity passively mode-locked laser is further constructed incorporating the prepared CNT-SA, where the optical power is confined in a relatively short laser cavity to investigate the thermal damage threshold and the performance of the CNT-SA. Stable output pulses with a fundamental repetition rate of 211.84 MHz and a pulse width of 680 fs are generated from the fiber laser. The mode-locking operation can be maintained an intra-cavity average power of 30 mW, indicating that the CNT-SA can withstand a relatively high optical power without performance degradation.

Carbon-nanotube-based passively mode-locked erbium-doped fiber laser for broadband supercontinuum generation

We present broadband supercontinuum (SC) generation in highly nonlinear fiber (HNLF) using carbon-nanotube (CNT)-based passively mode-locked erbium-doped fiber laser seed pulses. The passively mode-locked fiber laser incorporating CNT-based saturable absorber has achieved a pulse width of 550 fs with a repetition rate of 18.5 MHz and a pulse energy of 16.686 pJ. An SC covering an optical spectrum from 1000 nm to beyond 1750 nm has been successfully generated by launching the amplified seed pulses through the HNLF.

Design and analysis of VCSEL based two-dimension wavelength converter

A novel two-dimensional vertical cavity surface emission laser (VCSEL) based wavelength converter is proposed. We developed a twodimensional transmission line laser model (TLLM) to analyze the proposed wavelength converter. This model takes into account Bragg reflectors by using the modified connecting matrix. Therefore, accurate and efficient modeling of the VCSEL structure is achieved. Extinction ratio of the output signal is investigated by considering input signal power, wavelength, facet reflectivity and cavity diameter.

A novel method for creating linearly and nonlinearly chirped fiber Bragg gratings

We propose a novel and flexible method for controlling the chirp rate of a linearly chirped fiber Bragg grating (FBG) and nonlinearly chirped FBG by adhering a uniform FBG onto a plastic plate with a pre-calculated curvature and applying an axial force on the plate. The center wavelength shift was measured to be only 0.1 nm, which is small compared with the previously reported techniques. The dispersions of the linearly chirped FBG and the nonlinearly chirped FBG were measured to be about 1200 ps/nm and 125 ps/nm/nm, respectively, and the bandwidth of the chirped FBG was about 0.5 nm.

Optically-driven deposition of carbon nanotubes on erbium-doped fibers for short-cavity passively mode-locked fiber lasers

We demonstrate optically-driven deposition of carbon nanotubes (CNTs) on erbium-doped fiber (EDF) facets for passively mode-locked fiber laser applications. A 21-dB difference in the reflectometry is observed showing successful deposition of CNTs. A passively mode-locked fiber laser incorporating the EDF with facet deposited with CNTs is constructed.

Amplified spontaneous emission pulses for high power supercontinuum generation

We demonstrate an incoherent light source based on a reflective semiconductor optical amplifier as pump for high power supercontinuum generation. The obtained power level is about 160 mW and 20-dB spectral bandwidth is around 170 nm.

A low-loss carbon-nanotube-based linear cavity fiber laser for high energy pulse generation

We demonstrate a low-loss linear cavity fiber laser incorporating carbon-nanotubes for high energy pulse generation. A pump efficiency of 47% for the intra-cavity power is obtained with an output pulse energy of 114 nJ.