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We propose a novel method of tunable compensation for dispersion and dispersion slope based on a uniform fiber Bragg gratings (FBGs). A specially designed mechanical rotator and a metal beam with a thickness gradient can induce a nonlinear strain gradient along the length of the gratings so as to control the dispersion of the gratings and their dispersion slope as well. By way of changing rotation angle, the dispersion value was controlled from -894.6ps/nm to -365.2ps/nm while the dispersion slope from 1049.2ps/nm 2 to 134.8 ps/nm 2 . The centre wavelength was measured to shift by less than 0.07nm. In simulated 80Gb/s NRZ transmission system, we showed to compensate dispersion and dispersion slope with system power penalty of about 2.5 dB by using the proposed method.

We propose an analysis technique of thermally tunable LPFG Filter with multiple cladding mode using multiport lattice filter model. Using LPFG with different cladding modes, we could verify the validity of the proposed model experimentally.

We experimentally demonstrate a 320 Gbit/s (32Chs times 10 Gibt/s) WDM signal transmission of bending insensitive holey fibers of less than 0.04 dB. Error free operation is achieved for all 32 channels.

We propose a novel fabrication method of holey fibers with different size, pitch, and shape of air holes. By expanding a Ge-doped preform with air holes at high temperature, the size and shape of air holes can be readily modified.

We propose a simple and flexible multiwavelength Raman fiber laser based on phase-shifted fiber Bragg gratings (FBGs) and a tunable chirped FBG. Using a simple multiwavelength Raman laser configuration with a single phase-shifted FBG and a tunable chirped FBG, we readily achieved simultaneous two-channel sensing probes with a high extinction ratio of more than 50 dB over a 50-km distance. The multiwavelength output was stable and the peak fluctuation was less than 0.5 dB. The temperature and strain sensitivities are estimated to be 10.1 pm/degrees C and 7.5 pm/microstrain, respectively.

A novel, Raman amplifier-based long-distance sensing system using a combined sensing probe of an erbium-doped fiber (EDF) and a fiber Bragg grating is proposed for simultaneous measurement of temperature and strain. By recycling residual Raman pump power for generation of amplified spontaneous emission in the EDF after distributed Raman amplification in the transmission fiber, the overall system configuration was significantly simplified without requiring any additional broadband light source. A remote sensing operation of simultaneous temperature and strain measurement is obtained at a location of 50 km. High quality of sensing signals with a ~ 11 dB signal-to-noise ratio (SNR) is readily achieved even after the 50 km transmission with distributed Raman amplification.

We investigate a long distance temperature sensor with multichannel remote sensing functionality at a 50 km distance using Raman fiber laser based on multiple laser cavities based on FBG's and a tunable chirped FBG.

We experimentally investigate the simultaneous measurement of bending and temperature using the phase-shifted long-period fiber gratings (LPFGs) based on the resonant wavelength shift and wavelength spacing change by the bending and temperature change, respectively. The phase-shifted LPFGs can be fabricated by UV post-exposure on a half of the grating region. The UV post-exposure has the effect of inducing a positive phase shift for the core mode while decreasing the coupling strength between the core and cladding modes. The spectral evolution of the transmission thus depends on the initial coupling strength of the grating. We applied the phase-shifted LPFG fabricated with the second saturated LPFG to the simultaneous measurement of bending and temperature. The left and right resonant peaks shifted into the longer wavelength due to the variation of the grating period as the bending curvature increased and the wavelength spacing between two peaks was not changed by the bending. However, since the phase-shifted LPFG has two different sections with different photo-induced average indices, the wavelength spacing between left and right peaks decreases as the temperature increases. Based Consequently, it is possible to discriminate two effects between bending and temperature based on the proposed schematics.

We experimentally demonstrate a simple scheme for tunable repetition rate multiplication based on the fractional Talbot effect using a linearly tunable, chirped fiber Bragg grating. The key component in this scheme is our linearly tunable, chirped FBG without its center wavelength shift, which was fabricated with the S-bending method using a uniform FBG. By simply tuning the GVD of the chirped FBG, we readily multiply an original 8.5 ps, 10 GHz soliton pulse train by a factor of 2 ~ 5 to obtain high quality pulses at repetition rates of 20 ~ 50 GHz without significant system configuration change.

We experimentally demonstrate a flexibly tunable multiwavelength fiber laser with multiple functionalities like the wide tuning range of the wavelength spacing and the lasing wavelength based on the hybrid gain medium.

Summary form only given. Optical short pulse sources with a high repetition rate are important in high speed optical switching and optical time-division-multiplexing (OTDM). In this paper, a 10 GHz actively mode-locked fiber-integrated semiconductor ring laser is implemented, in which good short-term amplitude is shown and total cavity length is considerably shortened minimizing the impact of environmental perturbation compared with the erbium doped fiber laser (EDFL).

An optical short pulse source with a high repetition rate is important in high speed optical switching and optical time-division-multiplexing (OTDM). Among various potential techniques of generating optical short pulses, active modelocking, especially harmonically mode-locked erbium-doped fiber laser (EDFL) is one of the most attractive methods for gigahertz repetition rate pulses. However, EDFL requires the configuration of long cavity length to obtain sufficient gain from the EDFA. Thus, inevitable instability of the output pulses is induced from even a small environmental perturbation, such as thermal fluctuations and acoustic vibrations that affect the fiber laser. In the paper, 10 GHz actively mode-locked fiber-integrated semiconductor ring laser is implemented, which shows good short-term amplitude and total cavity length is considerably shortened minimizing the impact of an environmental perturbation compared with the EDFL.

We fabricate the piecewise-uniform LPFG using the synthesis method for the erbium gain equalization. The multiport lattice filter model is applied to the synthesis of LPFG using the simulated annealing and steepest descent methods.

In order to increase inhaled corticosteroid (ICS) use and to reduce hospitalization, emergency department visits and ultimately the economic burden of asthma, "Korean Asthma Management Guideline for Adults 2007" was developed. To assess the guideline effects on physician's ICS prescription for asthma, we conducted segmented regression and multilevel logistic regression using National Health Insurance claims database of outpatient visits from 2003 to 2010. We set each quarter of a year as a time unit and compared ICS prescription between before and after guideline dissemination. A total of 624,309 quarterly visits for asthma was observed. The ICS prescription rate before and after guideline dissemination was 13.3% and 16.4% respectively (P < 0.001). In the segmented regression, there was no significant guideline effect on overall ICS prescription rate. In multilevel logistic regression analyses, the effect of guideline on overall ICS prescription was not significant (odds ratio, 1.03; 95% CI, 1.00-1.06). In subgroup analysis, ICS prescription increased in secondary care hospitals (odds ratio, 1.15; 95% CI, 1.02-1.30) and in general hospitals (odds ratio, 1.10; 95% CI, 1.04-1.16). However, in primary clinics, which covered 81.7% of asthma cases, there was no significant change (odds ratio, 0.98; 95% CI, 0.94-1.02). From the in-depth interview, we could identify that the reimbursement criteria of the Health Insurance Review and Assessment Service and patient's preference for oral drug were barriers for the ICS prescription. The domestic asthma clinical guideline have no significant effect on ICS prescription, especially in primary clinics. Graphical Abstract

The electro-optic properties of the reflected light in a reflective mode, 45°-twisted nematic liquid crystal (TNLC) cell were investigated in the voltage regions near and away from the Freedericksz transition threshold. The measured reflectivity away from the threshold voltage (Vth) could not be described by the model which assumes a constant tilt angle as well as a linearized distribution of twist angle across the cell, although the data are well fitted near Vth. We found that in the voltage region away from Vth, the model considering the distributions of the tilt angle and the twist angle should be applied for the calculation of the reflectivity. The director-axis distributions were obtained from the numerical integration of the Euler–Lagrange equation.

We propose and experimentally demonstrate a novel method for tunable dispersion slope compensation. We use two uniform fiber Bragg gratings (FBGs), a spatially designed S-bending stage and 4-port circulator. Two FBGs are mounted on each surface of a metal plate along the calculated quadratic curve. The dispersion slope (DS) can be tuned by adjusting a nonlinear strain along two uniform FBGs without changing second order dispersion as well as the central wavelength. In the experiment, a DS tuning range from -13.9 to -54.8 ps/nm(2) is achieved with the bandwidth of larger than 2.0 nm.

We propose a semi-empirical model for the complete analysis (spectrum, bandwidth, and wavelength/phase shifts) of a temperature-tuned long-period fiber grating (LPFG) filter. By applying the multi-port lattice model to LPFGs, while deriving and utilizing the empirically determined temperature-dependence of core-to-cladding intermodal dispersions, we achieve a precise, practical means of spectrum analysis. Excellent agreement of the model with the experimental results was obtained over wide spectral ranges.

We propose a new method for synthesizing piecewise-uniform long-period fiber gratings (LPFGs) by using an extended fundamental matrix model with thermal changes. The proposed synthesis method is then applied to the design of the LPFG tuned by the thermal changes for erbium gain equalization by using the simulated-annealing and steepest-descent optimization techniques. We describe how a piecewise-uniform LPFG can be constructed by utilizing the inverted gain spectrum of erbium-doped fiber amplifiers (EDFAs), from the thermal change parameters' search process. A sensitivity analysis also is done to study the tolerance of our approach against possible error sources, such as the temperature controller, the fabrication of the LPFG, and the EDFA spectrum, by using Monte Carlo simulations. To verify the validity of the proposed synthesis method experimentally, we manufactured the piecewise-uniform LPFG with thermal changes by using a divided coil heater. We observe that the spectrum designed by the proposed synthesis method is close to the corresponding measured spectrum in the wavelength band of interest. We also compare the performance of the proposed method with traditional approaches, such as Newton-like methods.

We propose and experimentally demonstrate long-distance remote simultaneous measurement of strain and temperature based on a Raman fiber laser with a single fiber Bragg grating embedded on a quartz plate. A Raman fiber laser output with a high extinction ratio of more than approximately 50 dB is achieved by the fiber grating cavity. In the sensing probe, half of the fiber Bragg grating is fixed steadily on the quartz plate to respond to temperature only, while the other half of the grating is free to respond to both temperature and strain. Based on the proposed scheme, lasing wavelength shift and separation occur with temperature and strain change, respectively. Efficient simultaneous measurement of strain and temperature for long-distance sensing applications of more than 50 km is readily obtained.

We propose a simple and flexible multiwavelength Raman-fiber-laser-based long-distance remote-sensing scheme for simultaneous measurement of strain and temperature by use of fiber Bragg gratings. By combining two uniform fiber Bragg gratings with a tunable chirped fiber grating, we readily achieve simultaneous two-channel sensing probes with a high extinction ratio of more than approximately 50 dB over a 50-km distance. When strain and temperature are applied, lasing wavelength separation and shift occur, respectively, since the two uniform fiber Bragg gratings have identical material composition and different cladding diameters. This allows simultaneous measurement of strain and temperature for long-distance sensing applications of more than 50 km.

The effects of cladding diameter on cladding-mode coupling in fiber Bragg gratings are investigated both theoretically and experimentally. The peak depth induced by the cladding-mode coupling increased with decreasing cladding diameter whilst the number of cladding-modes coupled from the core mode decreased. The wavelength spacing between the main peak and the first-order cladding-mode increased as cladding diameter decreased.

We experimentally investigate transient effects of our proposed single pump, dispersion-compensating Raman/EDFA hybrid amplifier recycling residual Raman pump in a cascaded EDF under the situation of multi-channel add/drop, and then demonstrate the use of a FBG based all-optical gain-clamping technique to efficiently suppress the output transients. The results show that the proposed hybrid amplifier has a significantly long transient response time of ~ 2 ms compared to the previous reported value of the conventional separate pump, Raman/EDFA hybrid amplifiers due to both the low-pumping regime operation of 14XX nm pumped EDFA section and the additional pump transit time through the 12.6 km long dispersion compensating fiber to reach the EDF. However, using a simple gain-clamping method we obtain an almost, transient-free operation.

A novel wavelength-selective optical switching device based on multiply cascaded long-period fiber gratings is proposed and experimentally demonstrated. The on and off states of each channel in the optical switching device can be effectively switched by voltage-controllable coil heaters. The device has advantages of multichannel operation, multiwavelength selectivity, and bandwidth controllability. It can be useful for applications in multiwavelength operational signal gating, optical switching devices, routers, and multiplexers in optical communication systems.

A prescaled 10 GHz clock recovery from a 40 Gbps time-division-multiplexed (TDM) optical signal has been demonstrated with a low-frequency-oscillator-free phase-locked loop (PLL) circuit based on four-wave-mixing in a semiconductor optical amplifier. 40 GHz optical PLL using a 10 GHz mode-locked fiber ring laser as a clock pulse was stable over 5 h within 10 KHz lock-in frequency detuning of 40 GHz TDM optical signal.

Two reflective-mode, π/4-twisted-nematic liquid crystal cells were prepared using E7 and ZLI 4801-000 liquid crystals and their electrooptic responses were measured in the voltage regions close to and distant from the Freedericksz transition threshold. Our reflectivity data distant from the threshold voltage (V th) could not be described by the model that assumed a linearized distribution of the twist angle across the cell and considered the voltage dependency of the tilt angle to follow the equation suggested by Lu and Saleh [Appl. Opt. 30 (1991) 2354], although the fit is good near V th. We determined that the reflectivity in the voltage region distant from V th should be calculated using the model that considers the distributions of the tilt angle and the twist angle. The director-axis distributions were obtained from the numerical integration of the Ericksen-Leslie equation.

We measured the temperature dependence of the zero-dispersion wavelength of dispersion-shifted fibers using a four-wave mixing process in the range of 22°C–59°C. From the experimental results, we found that the zero-dispersion wavelength increases linearly as the temperature around dispersion-shifted fiber(DSF) increases, and its slope is 0.032 nm/°C.

Gain control in an erbium-doped fibre amplifier (EDFA) using two fibre Bragg gratings and a fibre-stretcher is demonstrated. Various stabilised gain levels can be achieved by tuning the centre wavelength of a grating, which changes the optical cavity loss of the EDFA in lasing.

Polarization splitters that use an asymmetric branching waveguide and can be fabricated along the Z axis as well as the X axis of LiNbO(3) are proposed and demonstrated. In these devices, two polarization states are converted to two different spatial modes, and these modes are split by the asymmetric branching waveguide. The extinction ratios of the two polarization splitters are shown to be less than -12 dB.