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Background Sepsis in children is one of the main causes of death in pediatric intensive care units (PICUs); however, the pathogenesis of sepsis is not fully clear. Previous studies revealed that many genetic variations were related to sepsis susceptibility. A long non-coding RNA SOX2 overlapping transcript (SOX2OT) may play a role in mitochondrial homeostasis and antioxidative activity, but the relationship between the lncRNA SOX2OT polymorphism and sepsis susceptibility has not been reported. Methods In this study, 474 pediatric sepsis patients and 678 healthy controls were recruited from southern China. After genotyping, the strength of the associations was evaluated through odds ratios (ORs) and 95% confidence intervals (CIs). Results The SOX2OT rs9839776 T allele was associated with decreased susceptibility to sepsis in southern Chinese children (TT/CT vs CC adjusted OR = 0.778, 95% CI = 0.610-0.992; P = 0.0431). Moreover, the difference in susceptibility was greater in children of age >60 months (adjusted OR = 0.458, 95% CI = 0.234-0.896; P = 0.0225), survivors (adjusted OR = 0.758, 95% CI = 0.585-0.972; P = 0.0358), males (adjusted OR = 0.655, 95% CI = 0.479-0.894; P = 0.0077) and the sepsis subgroup (adjusted OR = 0.548, 95% CI = 0.343-0.876; P = 0.0120). Conclusion The rs9839776 T allele may contribute to decreased sepsis risk in Chinese children. Future studies with a larger sample size are needed to verify these results.

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.

A simple Mach–Zehnder interferometer (MZI) was presented for high temperature measurement. The device was composed of a short segment of suspended-core fiber (SCF) sandwiched between two standard single mode fibers (SMFs) with a misaligned splicing joint. Lower temperature measurements were firstly carried out from room temperature to 190 °C, and the obtained sensitivity at the wavelength of 1465 nm was 25.82 pm/°C. However, a nonlinear response was further achieved for high-temperature sensing up to 1000 °C. Between 20 and 200 °C, the sensitivity was just 24.24 pm/°C at the wavelength of 1457 nm, whereas a higher sensitivity of 53.87 pm/°C was acquired in the range of 200–1000 °C. In addition, the strain cross-sensitivity was extremely low. As a result, the device can be developed as a strain-independent temperature sensor and become an excellent candidate for high temperature measurement.

Quantum dots (QDs)-based multiplexed fiber-optic temperature sensors are proposed for multi-point sensing, which are composed of hollow-core microstructured optical fibers filled with aqueous QDs solutions of different fluorescent wavelengths. A parallel reflective configuration is adopted to avoid the crosstalk of fluorescent emissions, and to construct practical probes. Temperature experiments show that the fluorescent peak wavelength and the self-referenced intensity of two sensors both change with temperature linearly in the range from −10 °C to 120 °C with favorable reversibility. At last, the crosstalk of both the fluorescent peak wavelength and the self-referenced intensity of two sensors is tested for in the range of the temperature measurements.

In this letter, we proposed and experimentally demonstrated a novel fiber sensing head based on 45° tilted fiber end for high-temperature measurement. The sensing head was fabricated by femtosecond laser using a line-scanning method. The light is mainly split into two beams at the 45° tilted fiber end, which forms a Michelson interference with a fringe visibility of $\sim 20$ dB. The experimental consequences show that the sensing head can withstand high temperature up to 800 °C and the temperature sensitivity is 19.7 pm/°C at 600 °C and 1605 nm. The merits of the sensing head are extremely compact, low cost, simple in configuration, easy for batch fabrication, and available to detect the temperature at precise location in microsize space.

We present an approach for manufacturing liquid-core microstructured optical fiber (MOF) with an all-in-fiber configuration. The MOF is first fusion-spliced with conventional fiber pigtails, with channels left open at the spliced interfaces to allow filling. After liquid filling, the channels are sealed by adhesives to prevent evaporation of the liquid. To verify the efficacy of this method, we filled a simplified hollow-core MOF (SHC-MOF) with a solution of aqueous quantum dots, and the temperature characteristics of the filled SHC-MOF were measured. The fluorescent peak wavelength and intensity changed reversibly over 48 h of repeated temperature cycling, indicating that the all-in-fiber configuration of the integrated liquid-core SHC-MOF has long-term stability and that the evaporation of the filling solution is minimal.

A simple and compact optical fiber directional bending vector sensor with simultaneous measurement of temperature based on the Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. The device consists of a piece of photonic crystal fiber (PCF) sandwiched between two single mode fibers (SMFs) with a lateral offset splicing. It shows the capacity for recognizing positive and negative directions. Within a curvature range of -7.13 m-1 to 7.13 m-1, the bending sensitivities of two resonant dips with opposite fiber orientations are obtained to be 0.484 nm/m-1 and 0.246 nm/m-1, respectively. This simple MZI is formed by invoking interference between LP01 and LP21 core modes, which leads to that the sensor is not sensitive to ambient refractive index (ARI). The temperature sensitivity has also been investigated. Two dips have obviously different sensitivities on the temperature and bending, so two parameters of both curvature and temperature can be distinguished and measured simultaneously by constructing a matrix and using one simple model interferometer.

Fiber-optic surface plasmon resonance sensors with multi metal layers (Ti/Ag/Au) and single silver or gold layer are presented. In comparison with widely used single-layer sensors, the multi-layer sensor shows a wider refractive index measurement range (in a range of 1.3344 to 1.4111 RIU) and a better resolution in the high refractive index region (refractive index over 1.3697 RIU). In addition, the sensitivity of multi-layer sensor is between those of sensors with silver and gold layers, the chemical stability, and the film adhesion capability of multi-layer sensor are much better than those of silver layer sensor.

We demonstrate a novel and compact fiber-probe pressure sensor based on a micro-Fabry–Perot interferometer (FPI). The device is fabricated by splicing both ends of a short-section simplified hollow-core photonic crystal fiber (SHC-PCF) with single-mode fibers. Then, a microchannel is drilled by a femtosecond laser micromachining in the SHC-PCF to allow air to diffuse in. The pressure sensing mechanism is based on the dependence of the air refractive index on pressure. We use both theory and experiment to investigate the sensing characteristics. A micro-FPI with a length of 272 μm demonstrates a pressure sensitivity of 4.071 nm/MPa at 1580 nm and a low-temperature sensitivity of 1.1 pm/°C at atmospheric pressure. We further study the temperature cross sensitivity of the sensor under different pressures. The sensor also shows strong stability and good reversibility, and may be potentially used in pressure sensing applications.

In this letter, we reported long period fiber gratings (LPFGs) fabricated by femtosecond laser pulses direct writing in grapefruit microstructured fiber. The mode field distribution and dispersion property of this fiber was analyzed by finite-element method. Two LPFGs were fabricated with different periods of 230 and $250~\mu \text{m}$ , the resonant dips appeared at the wavelengths of 1325.8 and 1449.4 nm, respectively. In addition, sensing characteristics of the LPFG with the period of $250~\mu \text{m}$ were investigated in theory and experiment. The experimental results show that the strain and temperature sensitivities are 1.88 pm/ $\mu \varepsilon $ and 0.267 pm/°C, respectively. It indicates that the temperature cross sensitivity is as low as $0.142~\mu \varepsilon $ /°C.

A novel and simple sensor based on fiber-optic modal interferometer fabricated by a segment of low elliptical hollow-core photonic bandgap fiber for simultaneous temperature and twist measurements is demonstrated. Meanwhile the sensor can also measure the twist angle and determine the twist direction simultaneously. The mode distribution of EHC-PBGF is demonstrated both in theory and experiments. There is an obvious difference of two transmission dips on the temperature and twist. The twist sensitivities of Dip 1 and Dip 2 are obtained to be −31.95 and −585.8 pm/(rad/m), respectively. The temperature sensitivities are 12.99 pm/°C for Dip 1 and 5.09 pm/°C for Dip 2, respectively. Two parameters of twist and temperature can be distinguished and measured simultaneously by using a sensing matrix. Meanwhile the structure is found to be weakly sensitive to the axial strain. It has the advantage of avoiding the crosstalk of strain in the applications.

A core-mode-cladding-supermode modal interferometer with all-solid photonic bandgap fiber (AS-PBF) is constructed, and a reflective Michelson-type high-temperature sensor is fabricated. Such a fiber sensor is constituted by a small segment of AS-PBF and a leading single-mode fiber. The splice region of the two fibers is weakly tapered to excite the cladding supermode. Both the interference spectra and the near-field infrared CCD images verify that the LP01 cladding supermode is effectively excited and interferes with the LP01 core mode, which agrees well with theoretical results. Benefiting from a large effective thermooptic coefficient between the two modes, temperature sensitivity up to 0.111 nm/°C at 500 °C is obtained in experiment. The proposed sensor is compact and easy to fabricate, which makes it very attractive for high-temperature sensing applications.

A diode-end-pumped composite YVO4/Nd:YVO4/YVO4 crystal self-Raman laser at the second-Stokes wavelength of 1,764 nm is demonstrated. The maximum average output power of second-Stokes radiation was up to 0.99 W at a pump power of 34 W and a pulse repetition frequency of 20 kHz, corresponding to an optical conversion efficiency of 2.9 %. The highest peak power and the shortest pulse duration were 21.5 kW and 1.92 ns, respectively.

Mach–Zehnder interferometer strain sensors made from pieces of photonic crystal fiber spliced to standard single mode fibers are fabricated with different lengths. High strain sensitivity of 2.1 pm/μe at 1550 nm is achieved with the length of 45 mm. The interference is induced by the core mode and the high order core mode due to the special air hole structure of PCF, which is independent of the surrounding refractive index. The temperature sensitivity (∼13.24 pm/°C) is relatively low. This structure is good in avoiding cross sensitivity in strain measurement and is fabricated with a simple process and low cost.

We report a diode-end-pumped acousto–optically Q-switched composite YVO4/Nd:YVO4/YVO4 crystal self-Raman yellow–green laser, based on the intracavity sum-frequency generation of the fundamental and first-Stokes fields in LiB3O5 crystal. Maximum average output power at 559 nm is 3.55 W at the incident pump power of 32 W and pulse repetition frequency (PRF) of 30 kHz, with an optical conversion efficiency of 11%. The highest pulse energy and peak power are 129 μJ and 10.5 kW at 20 kHz PRF, respectively. The power stability of yellow–green radiation is better than 2% for 40 min at an average output power around 3 W.

An all-fiber high-sensitivity temperature fiber sensor based on a Mach-Zehnder interferometer in standard single-mode fibers (SMFs) is described. The interferometer consists of two concatenated waist-enlarged fusion bitapers which are fabricated simply by cleaving and fusion splicing. It is demonstrated that such an all-fiber Mach-Zehnder interferometer incorporates intermodal interference between the LP01 mode and a high-order cladding mode of LP07 mode. Its response to temperature is investigated and a high sensitivity of 0.070 nm/°C is obtained by a 7.5 mm interferometer. This simple, low-cost and easy-to-fabricate core-cladding modal interferometer with entire SMF-based structure also has great potential in diverse sensing applications.

In this paper, a cascaded Mach–Zehnder interferometer based on a photonic crystal fiber is reported. It is demonstrated that by applying a small dose of extra arc discharge to the photonic crystal fiber sensing part which is spliced to lead-in and lead-out single-mode fibers with core offset, the in-line Mach–Zehnder interferometer could be simply cascaded. From the analysis of Fourier transformed spatial spectra and the response tests as a sensor, it is shown that such a cascaded interferometer could effectively broaden the utilizable spectral shift range for a measured value.

A passive all-solid photonic bandgap fiber (PBGF) was fabricated for ytterbium ASE controllable filtering.By direct adjustment of the fiber out-diameter in drawing process, we can tune the stop-band of ytterbium ASE to the desirable wavelength region. Our simulation results show a good agreement with measured bandgaps. The fiber is single mode and has a steep “cutting edge” in favor of ASE suppression. The measured transmission loss is 0.41 dB/km at wavelength of 1.55 μm within the longer wavelength bandgap, which is believed the lowest loss level of all-solid PBGF reported to the best of our knowledge. Its “stop-band” lies just in the range of 1.03–1.14 μm and shows a perfect ASE suppression by using only 0.2 m length of fiber. It provides a costless and feasible way to build the gain-controllable laser with the nowadays mature conventional ytterbium-doped fiber and such passive PBGF fiber. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:2629–2632, 2010; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25578

We have demonstrated a continuous-wave (CW) all fiber laser operation at 1558.4 nm of a diode-pumped erbium-doped PCF laser based on 9.6 m erbium-doped PCF. The maximum output power and the threshold of the fiber laser are 49.4 mW and 6.67 mW, respectively. We show that it is possible to achieve a high stability and beam quality laser, which has a great application potential in optical communication field in future.

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 periods and different grating lengths in the standard single mode fiber were fabricated, using laser direct writing method, by femtosecond laser pulses with pulse width of 200 fs at a center wavelength of 800 nm in air. The transmission spectra were studied in the range of 1510 nm to 1610 nm. Two LPFGs with period of 400 μm, and 550 μm, respectively fabricated with same irradiation power of 275 mW, were shown. The loss peak of 1552 nm, the transmission loss of 16 dB and the FWHM of 20 nm were obtained when the period of LPFG was 400 μm, while the loss peak of 1588 nm, the transmission loss of 20 dB and the FWHM of 25 nm were achieved when the period of LPFG was 550 μm. According to the theory of mode field coupling for long period grating, it was indicated that the modulation of refractive index Δn was in the level of 10-2.

A stable phase demodulation system for diaphragm-based acoustic sensors is reported. The system is based on a modified fiber-optic Sagnac interferometer with a stable quadrature phase bias, which is independent of the parameters of the sensor head. The phase bias is achieved passively by introducing a nonreciprocal frequency shift between the counter-propagating waves, avoiding the use of complicated active servo-control. A 100 nm-thick graphite diaphragm-based acoustic sensor interrogated by the proposed demodulation system demonstrated a minimum detectable pressure level of ~450 µPa/Hz(1/2) and an output signal stability of less than 0.35 dB over an 8-hour period. The system may be useful as a universal phase demodulation unit for diaphragm-based acoustic sensors as well as other sensors operating in a reflection mode.

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.

A compact fiber sensing head based on 45° tilted fiber end in thin core fiber (TCF) for high temperature sensing is fabricated by femtosecond laser using line-scanning method.

A long-period fiber grating (LPFG) was inscribed in suspended-core fiber by femtosecond laser. The LPFG was insensitive to surrounding refractive index, and the sensitivities to temperature and strain were 4.8 pm/°C and -1.65 pm/μϵ, respectively.

Ga0.6In0.4P/(Al0.8Ga0.2)0.4In0.6P strain-compensated multiple quantum wells (SCMQW) and Ga0.5In0.5P/(Al0.4Ga0.6)0.5In0.5P unstrained MQW were grown by MOVPE to investigate the shift of photoluminescence (PL) peak wavelength and the carrier lifetime caused by strain effect in SCMQW. The PL peaks of SCMQW and unstrained MQW were 608 and 628 nm, respectively. From the rocking curve of SCMQW, it indicated that the active layers of SCMQW were not zero net strain. The residual strain caused the emission intensity of SCMQW to be about four times weaker than that of unstrained MQW. The carrier lifetimes of SCMQW and unstrained MQW were measured to be 3 and 6.02 ns, respectively. It was conjectured that the residual strain in SCMQW induced a decrease in the carrier lifetime by a factor of two with respect to that of unstrained MQW. # 2002 Elsevier Science B.V. All rights reserved.

In the present paper, strain-compensated multi-quantum wells (SCMQW) InGaAs/GaAsP grown by low-pressure metal organic chemical vapour deposition (MOCVD) were used as the active layers of near infrared LED. The single nude LED chip was put down on the copper header p-side down, using a GaAs transparent substrate and AuBe alloy mirror reflector. The forward voltage under a bias current of 20 mA was measured to be 1.2 V while the reverse voltage at 10 μA was 5–6 V. The output power of 10.5 mW and 18% quantum efficiency were obtained under a bias current of 50 mA.

We report continuous-wave (CW) and actively Q-switched laser performance achieved with an uncoated mixed laser crystal Nd:Lu 0.99 La 0.01 VO 4 at 1065.655 nm end-pumped by a laser-diode-array. The maximum CW output power of 6.26 W was obtained with the optical conversion efficiency of 31.3%. For the Q-switching operation, a maximum average output power of 6.06 W was obtained at the pulse repetition frequency (PRF) of 90 kHz, yielding an optical conversion efficiency of 30.3%. The highest pulse energy and highest peak power obtained were 954 μJ and 63.8 kW at the PRF of 5 kHz, respectively.

By 532 nm laser pumped PL spectral and Xe pumped fluorescence spectral, characterized InGaAsP/In0.5( GaxAl1-x)0.5P quantum well on substrates 2°, 15° tilted to (110) the optical property dependence of substrates tilt. The PL wavelength varies from 755 nm of the quantum well on substrate of 2° to the 780 nm of the quantum well on substrate of 15°. From fluorescence excitation spectral to excite about 760 nm wavelength of quantum well, the absorption light peak wavelength is 508 nm in common, and has another absorption peak wavelength of 622 nm. Excited by 508 nm and 622 nm wavelength, the fluorescence spectral intensity of the 15° quantum well is about 300 and 104accordingly. Lifetime of charge recombination is about 1.26 ns for this material.

We demonstrate a passively mode-locked erbium-doped fiber laser (EDFL) by using the smallest single-walled carbon nanotubes (SWNTs) with a diameter of 0.3 nm as the saturable absorber. These ultrasmall SWNTs are fabricated in the elliptical nanochannels of a ZnAPO4-11 (AEL) single crystal. By placing an AEL crystal into an EDFL cavity pumped by a 980 nm laser diode, stable passive mode-locking is achieved for a threshold pump power of 280 mW, and 73 ps pulses at 1563.2 nm with a repetition rate of 26.79 MHz.

LPFGs in all-solid photonic bandgap fiber are inscribed by femtosecond laser pulses. The resonance peaks appear in the forbidden band of AS-PBGF, which LP01-like supermode can be guided. Mode coupling between the guided LP01-like supermodes can be obtained.

A long period fiber grating (LPFG) with resonance wavelength of 1335.76 nm was investigated, a twist sensitivity of 43.33 pm/(rad/m) against wavelength was obtained and its twist direction can be detected as well.

We report an intracavity frequency-doubled Q-switched self-Raman yellow laser at 587 nm. A composite Nd:YVO 4 crystal was utilized as self-Raman gain medium. The maximum average output power of yellow light obtained was 1.5 W at the incident pump power of 30 W and at a repetition rate of 50 kHz, corresponding to the optical conversion efficiency of 5%. The shortest pulse width, the maximum pulse energy and the highest peak power were measured to be 5.8 ns, 46.7 μJ and 5.9 kW, respectively.

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.

In this paper, an in-line comb filter with flat-top spectral response is proposed and constructed based on a cascaded all-solid photonic bandgap fiber modal interferometer. It consists of two short pieces of all-solid photonic bandgap fiber and two standard single-mode fibers as lead fibers with core-offset splices between them. The theoretical and experimental results demonstrated that by employing a cut and resplice process on the central position of all-solid photonic bandgap fiber, the interference spectra are well tailored and flat-top spectral profiles could be realized by the controllable offset amount of the resplice. The channel position also could be tuned by applying longitudinal torsion with up to 4 nm tuning range. Such a flat-top fiber comb filter is easy-to-fabricate and with a designable passband width and flat-top profile.

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, core-cladding modal beating in a short piece of all-solid photonic bandgap fiber (AS-PBF) is observed in longitudinal propagation direction. It is demonstrated that at the stopband range of AS-PBF, the power could transfer back and forth between the fiber core and the first layer of high-index rods. Both experimental results and the theoretical analysis from transverse coupled mode theory confirm that the 3-dB width of the sharp stopband could be significantly narrowed by multicycles of such core-cladding modal couplings, which is of great benefit to the high-resolution sensing applications. Based on such a guiding regime, a high-temperature sensor head is also made and its response to temperature is tested to be of 59.9 pm/°C.

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.

A wavelength-encoded interferometric high-temperature sensor based on an all-solid photonic bandgap fiber (AS-PBF) is reported. It consists of a small piece of AS-PBF spliced core offset with standard single-mode fibers. Two core modes LP(01) and LP(11) are conveniently utilized as optical arms to form Mach-Zehnder-type interference at both the first and the second photonic bandgaps, and the maximum extinction ratio exceeds 25 dB. Experimental and theoretical investigation of its response to temperature confirms that high temperatures up to 700 °C can be effectively sensed using such an AS-PBF interferometer, and benefiting from a large effective thermo-optic coefficient of fiber structure, the sensitivity can be significantly enhanced (71.5 pm/°C at 600 °C).

Long period fiber gratings (LPFGs) with different periods in the standard single mode fiber were fabricated, using laser direct writing method, by femtosecond laser pulses with pulse width of 200 fs and the repetition rate of 250 kHz at a center wavelength of 800 nm in air. Comparative with bare LPFG in temperature sensor, LPFG had been encapsulated using large coefficient of thermal expansion of epoxy polymer and Aluminum to enhance the temperature sensitivity. The results showed that the temperature sensitivity of encapsulated LPFG was 2 times than that of bare LPFG. In addition, we also researched the relationship between resonant wavelength and surrounding refractive index (SRI) when LPFG immersed in refractive index of solution of different index at 20 degree Celsius.

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.