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Using a YVO4/Nd:YVO4/YVO4 composite crystal end pumped by laser diode, we demonstrate the simultaneously Q-switched and mode-locked self-Raman laser at the firststokes wavelength of 1176.07 nm. Its corresponding linewidth was measured to approximately be 0.14 nm. At the pump power of 38 W and the pulse repetition frequency of 50 kHz, the maximum average output power at 1176 nm was obtained to be 1.34 W with the corresponding optical conversion efficiency of 3.6%. The highest pulse energy and the highest peak power were obtained to be 35 μJ and 10.5 kW, respectively. The shortest mode-locked pulse width of the laser was obtained to be ~300 ps with the corresponding repetition rate of mode-locked laser pulse is ~ 1.11 GHz

A 45° fiber cantilever beam for high temperature measurement was fabricated by femtosecond laser in single mode fiber. The temperature sensitivity is obtained to be about 17 pm/℃ at 800 ℃ and 1560 nm. The 45° fiber cantilever beam possesses good repeatability and stability in high temperature. The sensor is small in size, cheap and good stability.

This paper demonstrate an intracavity Raman laser at 2291 nm by using YVO4 crystal as Raman crystal to achieve efficient Raman conversion in a compact diode-end-pumped actively Q-switched Tm:YLF (Tm3+-doped lithium yttrium fluoride) laser with an L-shaped cavity. With a pump source of fiber-coupled continuous-wave diode laser operating at 793 nm, the Tm:YLF crystal served as gain material in the Raman laser. We made experimental research on the characteristics of first-Stokes Raman laser output the different pulse repetition rates (PRFs). The maximum average output power, pulse width and the corresponding peak power were 175.2 mW, 21.41 ns and 1.63 kW respectively at a pluse repetition frequency of 5 kHz and an incident pump power of 21W.

The thermo-optic coefficient of standard single mode fiber (SMF) is researched in the temperature range from 20 to 1000 °C by using a fiber-optic intrinsic Fabry-Perot interferometer. Polynomial fit of the interference fringe shift indicates that the thermo-optic coefficient increases with temperature, and is estimated to reach its peak at ~1144 °C. The error of the temperature measurements is also analyzed when taking the thermo-optic coefficient value as a constant.

A modified fiber-optic Sagnac interferometer with a stable Œ/2 phase bias is demonstrated for the demodulation of diaphragm-based acoustic sensors. The phase bias is obtained by introducing a nonreciprocal frequency shift between the counter-propagating waves. The system is passive and without complicated servo-control for phase stabilization. Interrogated by the proposed demodulation system, a 100 nm-thick graphite diaphragm-based acoustic sensor demonstrated a minimum detectable pressure level of ~ 450 µPa/Hz 1/2 at the frequency of 5 kHz and the output signal stability is less than 0.35 dB over an 8-hour period. Keywords: Optical fiber sensor, Sagnac interferomete r, acoustic sensor, graphite diaphragm 1. INTRODUCTION Diaphragm based fiber Fabry-Perot interferometers (FPIs) with a deflectable diaphragm attach ed to a hollow cavity have been investigated extensively for acoustic detection [1-4]. These FPI acoustic sensors are conventionally demodulated by using a narrow-band laser with the wavelength tuned to a quadrature point (Q-point) of the interference spectrum. However, the dependence of the Q-point of a FPI on the cavity-length and the difficulty of the fabrication of fiber FPIs with exactly the same cavity length mean that, for different sensors, the laser wavelength need to be tuned to different values to ensure optimal phase to intensity conversion. Furthermore, the Q-point of the sensors may drift due to fluctuation in ambient temperature and background pressure during operation, causing instability of the demodulation system. Methods such as electrical feedba ck and diffractive grating control have been employed to follow the drift and maintain the Q-point in real time but the active control increases the complexity of the system [5, 6]. In this paper, we report a stable phase demodulation system for diaphragm-based reflection-type acoustic sensors. It is based on a modified Sagnac interferometer which the sensor is incorporated into and functions as part of the Sagnac loop by using an additional fiber directional coupler. Stable phase bias is achieved by inducing a nonreciprocal frequency shift between the two counter-propagating waves in the modified Sagnac interferometer. The system could be used for universal phase demodulation of diaphragm-based acoustic sensors as well as other reflection-type sensors.

A novel and compact fiber-probe pressure sensor was demonstrated based on micro Fabry-Perot interferometer (FPI). The device was fabricated by splicing both ends of a short section simplified hollow-core photonic crystal fiber (SHCPCF) with single mode fibers (SMFs), and then a micro channel was drilled by femtosecond laser micromachining in the SHC-PCF to significantly enhance the pressure sensitivity. The pressure sensing characteristics based on micro-FPI have been investigated by measuring the signals through the demodulation of phase since the external signal imposing on the interferometer will induce the phase change of interference signal. Then a FBG was cascaded to demodulate the signal. A micro FPI demonstrates a maximum pressure sensitivity of 32 dB/MPa, while a low temperature cross-sensitivity of 0.27 KPa/°C. Hence it may have potential for pressure applications in harsh environment.

The subway is a representative form of the rail transit, and its catenary suspension system is a very important aspect to the safety of the whole system. The safety monitoring of the subway catenary suspension system is studied in this paper. A demonstrate model is set up in the laboratory, and some fiber Bragg grating (FBG) sensors including strain sensors and displacement sensors were utilized in the demonstrate system. It is shown that the used sensors could indicate the safety information of the system effectively. Especially, the designed displacement sensor that is packaged by athermal technique can abandon the influence of the environment temperature in a certain degree. Its engineering applicability is greatly improved.

We propose an ultrasensitive temperature sensor based on an in-line liquid-filled photonic crystal fiber (PCF) Mach- Zehnder interferometer. It consists of a small piece of index-guiding PCF fully infiltrated by fluid and two standard single-mode fibers offset spliced with the PCF. Two core modes of LP01 mode and LP11 mode are conveniently utilized as optical arms to form an in-line Mach-Zehnder-type interferometer. Experimental and theoretical investigation of its response to temperature confirms that and a high temperature sensitivity up to -0.244nm/(oC-mm) could be realized by such a compact inline liquid-filled PCF Mach-Zehnder interferometer. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

In this paper, we present a novel optical fiber fluorescent temperature sensor based on photonic crystal fiber(PCF) and its theory of forward stimulated fluorescence emission in PCF. It is realized by liquid filling in hollow-core PCF, the liquid is of higher temperature coefficient of refractive index than that of silicon, and is mixed with fluorescent material. Stimulating light and fluorescence propagating in the fiber are in the same direction. Because the band gaps of PCF are modulated by temperature, besides the sensitivity of fluorescence, the stimulating light passing through the photonic crystal fiber is also sensitive to temperature. Experiment results and theory modulation show that when PCF length is shorter than an optimum length, stimulating light in fiber will increase and fluorescence will decline gradually with the increase of temperature. © (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

In this letter, LPFGs in standard telecommunication fibers without hydrogen loading were fabricated in air using laser direct writing method, by femtosecond laser pulses with pulse duration of 200 fs and output wavelength of 800 nm. The loss peak of 1430 nm, the transmission loss of 22.86 dB and the FWHM of 6.6 nm were obtained. Temperature dependence of wavelength shift in air was measured by placing the LPFG in a temperature chamber that is temperature controlled in the range of 70 -150 °C. The temperature sensitivities (uλ/uT) are estimated by using linearly regression fits, which was 43.2 pm/°C. The linearity of the temperature sensitivities is high and the R-squared values for uλ /uT is larger than 0.9979.

Two kinds of supercontinuum (SC) sources are successfully generated by propagating 200-fs unamplified and amplified pulses through a 0.85-m long nonlinear photonic crystal fiber (PCF), respectively. The spectra bandwidth of amplified-femtosecond-pumped SC is about 870 nm spanning 480 nm to 1350 nm, which is flat to 1.2dB over 100 nm spreading from 550nm to 650 nm. With the same PCF, SC spectra pumped by unamplified-femtosecond-pulses are narrower and unevener. A detailed simulation is carried out to help us understand the mechanism of supercontinuum evolution. For pump wavelength located at the zero dispersion wavelength of the PCF, spectra are broadened by the interaction between SPM and higher-order dispersion at early stage. With increasing the pump power, the spectra are broadened by fission of higher solitons and parametric four-wave mixing (FWM). When the peak power is up to mega-watt, FWM plays a notable role in flattening and further broadening the supercontinuum spectra in short wavelength side. An effective way to generate a flat SC laser source pumped by femtosecond pulses is also demonstrated.

1 Shenzhen Key Laboratory of Laser Engineering, College of Electronic Science and Technolog, Shenzhen University, Shenzhen, Guangdong 518060, P.R.China 2 College of Physical Science and Technology, Shenzhen University, Guangdong 518060, P.R.China Abstract We have demonstrated a low threshold all fiber laser based on a 9.68 m-long small core Erbium doped photonic crystal fiber (PCF) and also presented results from two separate device experiments. The threshold and maximum output power of the laser are 6.72 mW and 82.6 mW, respectively. Fiber loop mirrors is used as resonato r mirrors. The overall system is all-fibered and with low threshold. The PCF we used as gain medium with 9.68 m long, 3.34 m mode diameter, 0.143 numerical aperture, 1000 ppm doped concentration, 545 dB/km transmission attenuation coefficient to 1310 nm laser. A single-mode diode laser, centered at 980 nm, was used as pump source and the maximum power was 400 mW. Keywords : fiber laser, Erbium-doped Photonic Crystal Fiber, fiber loop mirrors, all fiber system, low threshold

Femtosecond laser pulses with ultrashort time duration and ultrahigh peak power can cause the refractive index change in transparent materials and micron scale machining precision. Long period fiber gratings (LPFGs) with different grating lengths in the standard single mode fiber are fabricated. Using laser direct writing method, Femtosecond laser pulses with pulse width of 200 fs at a center wavelength of 800 nm in air is applied as laser source. The transmission spectra are studied in the range of 1510 nm to 1610 nm. The loss peak of 1552 nm, the transmission loss of 16 dB and the FWHM of 20 nm are obtained with the period of LPFG of 400 μm. According to the theory of mode field coupling for long period grating, transmission spectra of LPFGs are numerically simulated depending on the grating length.

Microchip-laser-pumped supercontinuum (SC) is successfully generated through a 30-m long photonic crystal fiber (PCF). The spectra bandwidth of SC is 850 nm spanning from 550 nm to 1300 nm. The -15dB-flat bandwidth is 400 nm spanning from 600 nm to 1000 nm. It is easy to achieve a much more flat SC with the sacrifice of spectra bandwidth. A detailed simulation is carried out to help us understand more about the supercontinuum generation process. The primary mechanism of spectral broadening is identified as parametric four-wave mixing (FWM) combined with stimulated Raman scattering (SRS). These findings also demonstrate an effective way to generate a flat supercontinuum laser source.

Femtosecond laser micromachining of solid materials such as Si, GaAs, polymethyl methacrylate (PMMA) and Fused Silica has been performed in air. Femtosecond (fs) laser pulses with pulse duration of 180 fs and output wavelength of 800 nm is applied for a light source. The relationship between pulse energy and sub-micron scale crater on the surface of Si, GaAs, PMMA and Fused Silica induced by fs laser pulses has been figured out. The sub-micron void can be produced in the focal point of fs laser beam. Two kinds of gratings have been fabricated on the surface of Fused Silica by femtosecond laser machining (FLM). The parameters of one grating such as the length of 20 mm, the width of 1.5 mm and the period of 2 μm have been obtained. A 20-mm-long and 0.45-mm-wide grating with period of 11 μm has been also fabricated. The diffraction efficiency of each order has been measured. An elliptical cross section waveguide has been obtained inside the bulk of Fused Silica. The distance beneath the surface was measured to be 2 mm. The diffraction rings can be observed in the far field image of the light output from the waveguide.

Temporal coherence property of supercontinuum (SC) generated in a polarization-maintaining photonic crystal fiber (PM-PCF) pumped by Ti:sapphire fs laser was experimentally studied by using a modified Michelson interferometer. The coherence length of supercontinuum light was measured to characterize the temporal coherence property. The coherence lengths of supercontinuum and pump laser were measured to be 6.5 μm and 59.14 μm, respectively. The shortened coherence length of supercontinuum output from the PM-PCF was due to the broadened spectrum. Moreover, the interferogram evolution versus the supercontinuum spectra was also investigated. It was indicated that the flatter the supercontinuum was, the better the interferogram was.

To our best knowledge, the most supercontinuum sources generated by use of cw pump lasers were obtained with fiber interaction lengths of the order of 100 m to a few kilometers to enhance the optical nonlinearity. However, the material loss was accumulated and the price cost was boosted in the long fibers. In this paper, CW supercontinuum generation by use of a 1480-nm fiber-Raman laser (FRL) in a 100-m high-nonlinear photonic crystal fiber (PCF) together with a 76-km single mode fiber (SMF) was demonstrated. The broadband supercontinuum was obtained with 10-dB bandwidth of 124 nm spanning from 1476 to 1600 nm. It was shown that the principal mechanism of the broadening could be assigned to stimulated Raman scattering, four-wave mixing (FWM) and modulation instability (MI).

With an acousto-optical Q-switch and Co:LaMgAl 11 O 19 (Co:LMA) crystal as the saturable absorber, diode-end-pumped actively and passively Q-switched Nd:GdVO 4 lasers at 1.34 P m were demonstrated, respectiv ely. For acous to-optical Q-switched operation, the maximum average output power, the highest pulse energy, the shortest pulse width and the highest peak power were obtained to be 4.54 W, 223 P J, 19 ns and 11.75 kW, respectively. For passively Q-switched operation with a 0.3-mm-long Co:LMA crystal as the saturable absorber, the maximum average output power, the highest pulse energy, the shortest pulse width and the highest peak power were obtained to be 1.43 W, 112 P J, 55 ns and 1.95 kW, respectively. Keywords: Nd:GdVO4 crystal, diode-end-pumped, Q-switched, Co:LaMgAl 11 O 19 crystal, 1.34 P m 1. INSTRUCTION High-power diode-pumped solid-state lasers (DPSSL) operating in the 1.3- P m spectral region have wide applications in the fields of medical treatment, optical fiber communication, and efficient production of red radiation by frequency doubling. Recently, we report a 13.3-W laser-diode-array end-pumped 0.3 at.% Nd:GdVO

Supercontinuum was generated in a polarization-maintaining (PM) PCF. The main advantage with the PM-PCF is the preservation of the state of polarization of the field propagating. For several applications, it is essential that the output has a well-defined polarization. In addition, the preserved polarization enhances the nonlinear interactions so that less power is required to generate the SC. The field patterns, birefringence and nonlinearity in the PM-PCF were simulated using a full-vector finite element method. Spectral broadening was achieved pumped by the optical parameter amplifier system (OPA). It was concluded that the SC generation was depended on the pump wavelength, and the difference group velocities in the two sides of the zero dispersive wavelength of the PCF and the OH absorption at about 1.4μm in the PCF affected the flatness of the generated supercontinuum. When the pump wavelength lied further away from the zero dispersive wavelength, the flatness was reduced, even in the visible region the blue-shift peaks were separated and didn't come into being supercontinuum. Supercontinuum with a broad bandwidth of 300 nm to 1350 nm was generated when the pump wavelength of 1200 nm, and the generated spectral bandwidth was beyond two octaves.