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In this paper, we demonstrate the refractive index (RI) sensing properties of long-period fiber gratings (LPFGs) at 2 μm. The RI sensing properties of the resonant dips of LP03, LP04, LP05, and LP06 cladding modes operating at 2 μm have been studied, respectively. The sensitivity of resonant dip of LP06 mode operating at 2 μm could reach -670 nm/RIU and - 15483.9 nm/RIU in the RI region of 1.3500-1.4500 and 1.4500-1.4624, respectively, which is higher than that of the resonant dip of lower-order cladding modes. Furthermore, the RI sensitivity can be achieved higher with the longer resonance wavelength. The proposed LPFGs-based RI sensor has potential for the next generation optical fiber sensing systems.

We demonstrate the fabrication of long-period fiber gratings (LPFGs) by using focused carbon dioxide laser. The mode coupling and characteristics of the LPFGs written in the few mode fibers were investigated experimentally. The generation and conversion of the orbital angular momentum (OAM) modes were achieved by the special designed gratings. The LPFGs could have promising application as high sensitivity optical sensors and all fiber mode converters.

A fiber grating prism (FGP) consisting of one single-mode fiber delay line and four tunable chirped grating delay lines is proposed and demonstrated. This FGP can provide five-channel time-delayed signals that can be fed to an antenna array to achieve continuous beam scanning. The proposed true-time-delay unit with 5-element phased array steerer is suitable for beamforming at frequencies up to 18 GHz.

A helical long-period grating (HLPG) was fabricated by twisting a conventional single-mode fiber when CO 2 laser beam was sweeping along the fiber axis. A grating with a contrast of 20 dB can be written in the fiber with a length of 1 cm. The spectral and sensing characteristics were investigated experimentally. The maximum sensitivity to surrounding refractive index and temperature measurement was measured to be 995 nm/RIU and 53 pm/°C, respectively.

We demonstrate the fiber Bragg grating (FBG) vibration sensors working at a frequency up to 900 kHz. The FBGs were surface-mounted on the piezoelectric (PZT) ceramic, which is used as the vibration sensor head. A nonlinear response was measured with a periodically strong response at the frequencies of 1 kHz, 5 kHz, 12 kHz, 40 kHz, 70 kHz and 400 kHz. Four kind of polymer were used to package the FBG on the PZT plate. The gratings have similar pattern of vibration response with different deviation on the output voltage. The FBGs packaged with the polymer 705B and EPO-TEK 353ND were found to have a better response at lower frequency, while the FBGs packaged with the polymer T120-023-C2 and TRA-BOND F112 have a better response at higher frequency. The sensors could be developed for the real-time monitoring of the large infrastructure.

We demonstrate the fabrication of the long-period fiber gratings (LPFGs) in the thinned cladding fiber (TCF) using CO 2 laser. The sensing response of the gratings to surrounding refractive index has been investigated experimentally. The LPFGs written in the TCF could be used as the high sensitive refractive index sensors.

This paper investigates the transmission spectral properties of tilted long period fiber gratings (TLPFGs) written in fiber tapers with different radiuses. We found that the tilted angle, the radius of fiber and the external refractive index both have influences on the transmission spectral by simulations. With the decrease of the radius of the fiber taper and the increase of refractive index, the resonant wavelength of the tilted LPFG was found to shift towards shorter wavelength. Experimental results show the agreement with the simulation predictions. The TLPFG inscribed in fiber taper also brings the high sensitivity of external refractive index as 566.7 nm/RIU.

We demonstrated experimentally the fabrication of tilted long-period fiber gratings (TLPFGs) with by CO2 laser. The writing efficiency can be dramatically improved by increasing the tilt angle. We fabricated the non-titled long-period fiber gratings with equivalent grating period in the same fiber. The experimental results show that the transmission spectra of non-titled equivalent LPFGs are similar with that of the TLPFGs. The TLPFG is found to have higher refractive index sensitivity than the corresponding equivalent LPFG.

In this paper, effects of diameter on characteristics of a LPFG with numerical simulations adopting a three-layer model and experimental demonstrations are presented, including the normalized coupling coefficient, the effective refractive index, the resonant wavelength, the shape of the attenuation dip and the refractive index sensitivity. The couplings between the core mode and the EH cladding modes increase faster than that of the HE cladding modes as the fiber diameter decreases. Moreover, the depth and bandwidth of the attenuation dip vary faster in thinner fiber due to the increase of the coupling coefficient, comparing with LPFGs in common fiber. The resonant wavelengths of lower order cladding modes move mainly toward the shorter wavelength, while that of higher order cladding modes move mainly toward the longer wavelength as a function of the exponential shape as the fiber radius reducing and the external refractive index increasing. The refractive index sensitivity is greatly enhanced when reducing the fiber diameter and using the taper structure. Therefore, we can utilize the higher sensitivity of different cladding mode at a suitable wavelength by controlling the diameter of the fiber taper waist. Furthermore, the sensing resolution and the cladding mode can also be selected by controlling the fiber diameter, which can be greatly used in many fields.

Recently, specialty fibers with various functional material doping have attracted significant attention. In this paper, we fabricated Nb-doped silica fiber and measured its resonant optical nonlinearity with long-period fiber gratings (LPFG) interferometer. The Nb-doped fiber was made with a combined MCVD and ALD technology. Then, we fabricated a pair of LPFGs and cascaded them as a Mach-Zehnder interferometer (MZI).By measuring the wavelength shifts of the interference fringe with the 532nm laser pump power, the resonant nonlinear refractive index of Nb-doped silica fiber around 1537 nm was estimated to be 8.12×10 -16 m 2 /W.

A sensing configuration of the fiber taper intrinsic Fabry-Perot interferometer (FTIFPI) was presented and developed. Direct inscription method of intrinsic Fabry-Perot interferometers in optical fiber tapers with a femtosecond laser has been fabricated experimentally. Fabry-Perot interferometers are firstly realized in the waist of fiber tapers. The experimental results indicate that the change in wavelength has a good linear relationship with temperature, while the fringe visibility keeps the same approximately. In the range of 0°C~100°C, the interference spectrum sensitivity is 12pm/°C. The fringe visibility decreases linearly with the liquid refractive index increasing, while extremes keep the same approximately.

Many methods have been proposed to analyze the characteristics of FBG-based cavity, as a novel method for analyzing reflection spectra of FBG-based Fabry-Perot (F-P) cavity, V-I transmission matrix combines the exactitude of traditional effective index method and the speed of coupled-mode method. In this paper, we demonstrate the analysis of F-P cavity based on non-matching fiber Bragg gratings using the V-I matrices method. The evolution of F-P cavity spectra were numerical analyzed with different grating length and reflectivity. Simulation results show that difference between length and reflectivity of two gratings should not be too large, and if the length and reflectivity of two gratings are both different, it would be better to make the longer grating's reflectivity larger.

We report a highly tunable liquid-filled photonic bandgap fiber (LF-PBGF) based on both bend and temperature change. By bending the LF-PBGF and changing its temperature, the blue shift of the red edge of the bandgap resulting from bend loss and temperature increasing is speeded up, and higher bandwidth tunability is achieved. Numerical and experimental results are presented, and 177-nm bandwidth tunability is achieved by tuning the temperature of the LF-PBGF with bend radius of 5 mm from 40°C to 60°C, and the average compression rate is 8.85 nm/°C.

We demonstrate the unique bending characteristics of the long-period fiber gratings (LPFGs) written in a panda polarization-maintaining (PM) fiber using CO 2 laser. The bending sensitivity of the grating is not only sensitive to the direction of the fiber bending, but also dependent on the irradi ation direction of the laser writing beam with respect to the principal axes of the PM fiber. The high sensitivity directiona l bending effect can be attributed to the asymmetric index distribution of the fiber cladding induced by both stress changes in the stress-applying parts of the PM fiber and high absorption of the CO 2 laser at the exposure side, which may find promising application in fiber grating based bending sensors. Keywords: long-period fiber grating (LPFG), CO 2 laser, polarization-maintaining (PM) fiber, bending 1. INTRODUCTION Long period fiber grating (LPFG) is a passive mode-coupling device which couples light from the guided core mode to the cladding modes of a single-mode fiber (SMF) and provides distinct attenuation bands in the transmission spectrum [l]. The unique characteristics of LPFG make it attractive for applications in sensing strain, temperature, bending and refractive index of surrounding medium, et al [2]. Among the methods reported for the fabrication of LPFGs, the CO

We studied experimentally the bending sensitivity of long-period fiber gratings (LPFGs) written by CO 2 laser under external tension. Not only higher sensitivity was observed, but also the LPFG was sensitive to the bending direction, which can be attributed to the laser-induced asymmetric index modification across the fiber section.

Wavelength Switching and multi-wavelength picosecond pulses are generated from a self-seeded Fabry-Perot laser diode and a tilted multimode fiber Bragg grating. 9 discrete wavelengths are obtained by changing the modal distribution in the grating. The sidemode suppression ratio is better than 25dB and pulsewidth is no more than 42ps.

A method to reconstruct a chirped quasi-phase matching grating by using a discrete layer-peeling algorithm is proposed. It shows an efficient approach of quality inspection on quasi-phase-matching grating.

In this paper, a new method was proposed to fabricate tapered optical fiber which has strong evanescent field in the tapered region. Our system relies on a scanned high-frequency pulsed carbon dioxide laser (CO 2 laser) beam across the taper region. The optimal heating and stretching parameters were determined through a series experiments. An effective laser scanning pattern was designed to obtain a relatively uniform temperature field. Symmetrical fiber tapers with taper waist diameters of ~10-20μm, overall lengths of ~10-17mm and transmission losses of ~0.8-3dB at 1550nm were obtained. The taper profile was measured which presents a good fit with the "decaying-exponential" model. The "selfregulating" law is demonstrated, which shows the desired taper waist diameter as a function of the laser power. A refractive index sensing experiment using the tapered optical fiber is also studied to show the potential application in refractive index sensor.

An in-fiber Michelson interferometer is proposed based on a double-cladding (DC) special fiber. With the DC special fiber, light wave can be partially coupled into outer cladding. The in-fiber Michelson interferometer can be constructed by splicing a length of DC fiber into standard single mode fiber (SMF). The interferometer is very sensitive to ambient refractive index change because fiber cladding is as one of interference arms. A sensitivity of 36nm/RIU has been achieved in the range of 1.33~1.40 in this work. The proposed technique has the dominant advantage of simple fabrication process, which can be expected to have wide applications in biosensors and chemical sensors.

A fiber-optic refractive index (RI) sensor is proposed based on a double-cladding special fiber. The double-cladding fiber (DCF) consists of core, inner cladding and outer cladding. And refractive index of core and outer cladding is higher than that of inner cladding. Through evanescent wave coupling, cladding mode can be excited resonantly at phase-matched wavelength. Because the cladding mode transmission is sensitive to ambient refractive index variation, the DCF can be used to solution refractive index sensor. By splicing a section DCF into standard single mode fiber (SMF), SMF-DCFSMF sensor was constructed and studied for refractive index sensing. A resonant wavelength shift of 65nm was achieved by changing ambient refractive index within the range of 1~1.4525.

The paper reviews the recent progress on CO 2 -laser writing of long-period fiber gratings (LPFGs) in different kinds of fibers, including conventional single-mode fiber (SMF), boron-doped SMF, polarization-maintaining fiber, photonic crystal fiber, and polarization-maintaining photonic crystal fiber. In particular, we report the writing dynamics for the understanding of the physical mechanisms involved in the writing process and show that the CO 2 -laser pulses can not only relax the internal stress in the fiber core but also induce a frozen-in stress in the cladding of the fiber under tension. The applications of CO 2 -laser written LPFGs, especially for the realization of broadband optical couplers, are also discussed.

We propose a low-cost, general-purpose fiber Bragg grating (FBG) cavity sensing technique, where optical picosecond pulses generated from a self-seeded Fabry-Perot laser diode are used to interrogate a fiber cavity formed with two chirped FBGs and the optical loss in the cavity is determined from the peak power ratio of the first two pulses reflected from the cavity. This technique does not require intensity referencing and the sensor output is insensitive to drifts in the center wavelengths of the FBGs. We demonstrate this technique experimentally with a long-period fiber grating (LPFG) placed in the fiber cavity as a refractive-index sensor. The results from the cavity-loss measurement are consistent with the wavelength-shift measurement of the LPFG in response to a change in the surrounding index. This technique is applicable to general intensity-based sensors and has the potential to be further developed for remote sensing of a wide range of physical or chemical parameters. It also provides a new way of converting the wavelength shift of an LPFG sensor into an intensity-based signal for easy processing.

We present a simple technique for the interrogation of fiber Bragg grating (FBG) sensors using a self-seeded Fabry-Perot laser diode (FP-LD). The FBG sensor is configured in such a way that its reflection spectrum consists of two peaks, whose separation depends on the physical parameter to be measured. When the modulating frequency of the FP-LD is set properly, stable short pulses at the two wavelengths selected from the same FP-LD mode with the FBG are generated simultaneously. The wavelength separation of the dual-wavelength pulses is converted into a time difference by transmitting the pulses through a dispersive fiber. In this way, accurate wavelength measurement is replaced by the less expensive time measurement. To multiplex similar sensors that are sufficiently far apart spectrally, it is only necessary to change the modulating frequency of the FP-LD. To balance the pulse intensities at the two wavelengths for a particular sensor, the temperature of the FP-LD can be adjusted. Using this technique, we demonstrated experimentally a number of FBG-based sensors, including displacement/bending sensors, temperature-compensated current sensors, and transverse force sensors. This technique provides an intensity-referencing-free demodulation scheme, an easy way to interrogate a series of sensors, and an effective means to overcome drifts.

We demonstrate a self-seeded gain-switched Fabry-Perot laser diode (FP-LD) for simultaneous generation of stable picosecond pulses at five close wavelengths. The principle of the laser is based on filtering the spectrum of one of the FP-LD modes with a tunable spectrum-split fiber Bragg grating (FBG). In addition to two wavelengths produced from the reflection end of the FBG, three more wavelengths can emerge from the transmission end of the FBG when the laser is adjusted properly. The pulse characteristics can be adjusted by controlling the gain-switching frequency and the temperature of the FP-LD and the separation of the reflection peaks of the FBG. The tradeoffs in balancing pulse intensities, optimizing side-mode-suppression ratios, and equalizing wavelength separations are studied. We also demonstrate the scalability of the laser by cascading another FBG to select a different mode of the FP-LD, which defines another set of five wavelengths. To switch from one set of wavelengths to the other, we only need to adjust the gain-switching frequency and the temperature of the FP-LD. This laser is perhaps the most cost-effective one demonstrated so far for the generation of multiwavelength picosecond pulses with close wavelength separations.

A fiber grating prism consisting of a chirped grating delay line and three delay lines with thirteen discrete fiber Bragg gratings is fabricated and experimented. The system comprises a chirped grating for short time delays and three discrete Bragg grating arrays for long time delays. The introduction of the chirped grating delay line enables the system to work for beamforming at higher microwave frequencies. The experimental results of the time delay measurements agree well with the theoretical calculations. Based on the experimental data, the radiation patterns of a 4-elements array are calculated and analyzed. This fiber grating prism can provide four-channel time-delayed signals that can be fed to an antenna array to achieve beam scanning at different radiation angle. The beampointing direction of the system is determined by the grating spacing and independent of the microwave frequency. The proposed true- time delay unit with 4 X 13 phased array steerer is suitable for beamforming at frequencies up to 6 GHz.

Long period fiber gratings (LPFGs) have important applications in fiber sensors because of their high sensitivity to temperature, strain, and refractive index of surrounding medium, et al. In this paper, we demonstrate the bending-induced resonance mode splitting of the LPFG in three different photosensitive fibers (QPS-T)fiber, O/E Land fiber and Fiber Core fiber). For the QPS-T photosensitive fiber, when the grating is bent, the resonant dip shifts to a longer wavelength with a reduction in strength and a new dip emerges at a shorter wavelength. The wavelength separation between the new dip and the original one is linear with the deflection of the grating. The similar results are also observed in two other LPFGs. But when the grating is bent, the original dip of grating shifts to a shorter wavelength and the new dip shift to longer wavelength. The different bending properties of the LPFGs are observed in the different photosensitive fiber, which is attributed to the different fiber parameters. For the LPFG written in the QPS fiber, the bending and temperature properties of the dip corresponding to the LP 09 mode is studied experimentally. Based on the property that the new dip induced by the bending of fiber has the same temperature sensitivity with the original one, a temperature-insensitive curvature sensor is proposed by measuring the wavelength separation of the two dips.

A wideband wavelength tunable Erbium-doped fiber laser using a unidirectional traveling-wave fiber ring combined with a novel switchable fiber Bragg grating (FBG) array is presented. The switchable FBG array consists of a 1 by 3 optical switch and three parallel discrete FBGs. The central reflection wavelengths of these three discrete FBGs are 1534 nm, 1549.15 nm and 1559.4 nm, respectively. The corresponding reflectivities are 90 percent, 99 percent and 99.8 percent, respectively. The wavelength of each grating can be continuously tuned in a range of over 16 nm by stretching (7 nm) and compressing (8 nm) the FBG. Therefore a fiber ring laser with a very large wavelength tuning range has been successfully demonstrated. In our experiment, the wavelength tuning range of the Erbium-doped fiber ring laser is 38 nm, from 1527 nm to 1565 nm. Within this tuning range, the total output power is about 7-8 dBm, the 3-dB linewidth is 0.01 nm, and the side mode suppression ratio is more than 48 dB. This tunable fiber ring laser can also work in the mode-locking mode when an amplitude modulator is inserted in the cavity. By applying the driving frequency matching the round-trip frequency or its multiples to the modulator, active mode-locking is realized. The highest harmonic order achieved is 125, the repetition rate and pulse-width of the mode-locked pulses are 1.00475 GHz and 539 ps, respectively.

We present a novel technique to produce adjustable chirped fiber gratings that allows the spectral width tuning without center wavelength shift. The chirp rate can be precisely controlled by bending the uniform fiber Bragg grating (FBG) surface-mounted on one side of a simply supported beam. This technique allows the dynamic control of the spectral width of the chirped grating. It has a simple structure with a linear strain gradient. Both compression and tension are simultaneously applied to the beam, so center wavelength shift is avoided. In the experiment, a maximum spectral width of 18.3 nm is obtained, which corresponds to a chirp rate of 3.05 nm/cm. To the best of our knowledge, this is the largest chirp rate produced on a uniform FBG by using strain-gradient beam tuning technique. The proposed technique can also be used to tune the chirp rate of a linearly chirped grating.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We demonstrate a high-sensitivity, relatively simple and inexpensive technique for interrogation of fiber-Bragg- grating sensors, where the signals from the sensor are resolved with a fiber-Bragg-grating filter tuned by a cantilever beam. We achieve a wavelength resolution of 0.002nm, which corresponds to a strain resolution of 1.7(mu) (epsilon) or a temperature resolution of 0.03 degrees C.

A small volume fiber Bragg grating (FBG) sensor head is designed by using especial package technique with polymer material to measure the pressure and temperature simultaneously, which can solve the temperature cross- sensitivity of FBG effectively. We measure the pressure and temperature simultaneously using only one grating by measuring the wavelength of the two reflective peaks with different pressure and temperature sensitivity coefficients through the process of packaging FBG. Due to the wavelength resolution of 0.002 nm, the resolution of the pressure and temperature measurements are 0.36 MPa and 0.30 degree(s)C for grating A, 0.14 MPa and 0.12 degree(s)C for grating B.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We propose a novel structure of Er-doped fiber ring laser, in which a fiber Bragg grating and an all-fiber M-Z interferometer are used to select wavelength. Output laser with line-width narrower than 0.1 nm at 1.53 micrometers waveband is obtained.

A new luminescent copolymer containing three functional moieties was prepared. It possesses high thermal stability (decomposition temperature = 456 °C), good electrochemical reversibility, and good thin film-forming ability. Electrical characterization of a two-layer diode device based on the structure of ITO/CuPc/TPDPQ-FPQ/AI showed excellent electroluminescence performance (bright yellow emission, a rectification ratio greater than iO and a low turn-on voltage of3.0 V).Keywords: polyquinolines, tetraphenyldiaminobiphenyl, electroluminescence, light emitting diodes. 1. INTRODUCTION Since the first report on electroluminescence (EL) in polymeric light emitting diode (LED) by Cambridge group in 1990', asintensive research has been directed to the development of new polymers, and much progress was achieved.26 A highperformance polymer should possess the combination of desirable properties, such as high EL efficiency, good thermal,chemical, electrochemical and optical stability. This is often not the case for most of the polymers studied so far, therefore,the search for such a new polymer remains to be a frontier subject in this field. In addition, high quantum efficiency ELdevices can only be obtained by both efficient charge injection and balanced mobility of both charge carriers inside the ELmaterials. In most of the EL polymers, it is rather more difficult to inject electrons than holes and therefore the majoritycharge carriers are holes. In such cases, hole charge injection and transport are favored and the recombination of holes andelectrons takes place near the interface between EL material and cathode, which leads to the quenching of the excitons. On theother hand, a part of holes goes through the EL material to the cathode without recombination, and the Joule heat produceddamages the devices. Recently, a few research groups79 and our previous reported the results of light-emittingpolymers containing both electron and hole transporting moieties to achieve efficient device performance. Thus, it isdesirable to design and synthesize a high performance polymer that possesses high EL efficiency, high stability, excellentprocessibility, as well as a bipolar ability in order to meet the requirements for efficient LEDs. Polyquinolines (PQ) haveoutstanding thermal stability, low dielectric constants, low moisture absorption, and good film-forming properties.'214Recently, blue EL device using a fluorinated polyquinoline (PQ-lOO) as the emitting layer has been However, thisdevice possessed a very high turn-on voltage (50

We design a kind of all-fiber high-sensitivity temperature sensor based on intensity measurement, by using long period fiber grating (LPG) as sensor element and using tunable erbium-doped fiber ring laser as optical source. In the temperature range of 74 degrees Celsius, the logarithm of the transmissive intensity of LPG is linear with the measured temperature and the linearity of sensing measurement can be repeated well. The resolution of temperature measurement is 0.1 degrees Celsius.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We demonstrate the higher pressure and temperature sensitivity of a fiber Bragg grating (FBG) sensor when it is packaged with a polymer jacket. The Bragg wavelength shift due to pressure and temperature can be enhanced by the polymer packaged by about 31.5 times for pressure and 7.7 times for temperature. The experiment results are consistent with the theoretical analysis. The sensitivity enhancement is due to the driving effect of the polymer substance. The 'structured' grating exhibits no significant chirp.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In recent years, light-emitting diodes fabricated with organic molecules and conjugated polymers have attracted much attention due to their potential application in full color flat panel display. Among the conjugated polymers ever used in polymer light-emitting diodes, poly(p-phenylene vinylene)s (PPV), poly(N-vinylcarbazole) (PVK) and poly(3- alkylthiophene)s (P3AT) have been investigated most widely. PPV, PVK are good emissive materials as well as hole- transportation materials. P3AT, with their good solubility and chemical stability, are also promising candidate materials for the emitting layer. Furthermore, the properties of P3AT can be easily changed by the size of side chain groups. In this paper, a new kind of P3AT was synthesized, and multilayer light-emitting diodes based on the blended polymer of P3AT and PVK were fabricated. The I-V characteristics and electroluminescent spectroscopy were measured and discussed.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.