Simultaneous measurement of strain and temperature using single tilted fibre Bragg grating

Tilted fibre Bragg grating (TFBG) with grating planes tilted at an angle of 3deg corresponding to the fibre axis shows core mode and a large number of cladding-mode resonances in its transmission. Discriminating temperature and strain simultaneously using a single tilted fibre Bragg grating is proposed and demonstrated experimentally. The wavelength shifts of the TFBG in response to temperature and strain are investigated. The results show that the cladding modes and core mode have different strain sensitivities while the temperature sensitivities are equal. So, it is possible to monitor the core-mode resonance and the cladding-mode resonances of the TFBG spectrum, allowing the separation of the temperature and strain induced wavelength shifts.     

Analysis of core-mode to radiation-mode coupling in fiber Bragggratings with finite cladding radius

Fiber Bragg gratings (FBGs) are often surrounded with dielectric material whose refractive index is higher than that of cladding. The purpose is to prevent cladding modes because coupling to cladding modes may cause undesirable dips on the short wavelength side of the main Bragg band in the core-mode transmission spectrum. In such FBGs, core-mode to radiation-mode coupling can cause a loss band continuum. Until now, however, reported analyses of this core-mode to radiation-mode coupling have been limited to hypothetical FBGs with an infinite cladding radius. This paper presents, to the best of our knowledge, the first analysis of core-mode to radiation-mode coupling in real FBGs with a finite cladding radius. We derive a formula to calculate the core-mode transmission loss caused by the radiation-mode coupling. We show calculated core-mode transmission spectra that exhibit such fringes as have already been observed experimentally but not yet theoretically estimated     

Numerical analysis of core-mode to radiation-mode coupling inlong-period fiber gratings

This letter presents, to the best of our knowledge, the first analysis of core-mode to radiation-mode coupling in long-period fiber gratings (LPGs) that are surrounded with dielectric material whose refractive index is higher than that of the cladding. We calculated core-mode transmission spectra through LPGs by integrating numerically the coupled-mode equations between the core mode and a continuum of radiation modes. We show calculated core-mode transmission spectra that exhibit such loss band continuums as have already been observed experimentally but not yet theoretically estimated     

Saturable absorber modelocked polarisation maintaining erbium-doped fibre laser

Using a polarisation-maintaining erbium-doped fibre near the zero dispersion wavelength and a bulk InGaAsP saturable absorber, the authors passively modelock a fibre laser to generate nearly transformer-limited 320 fs pulses with 40 pJ energy. The laser produces a stable state of linear polarisation and the semiconductor, which acts both as a slow and fast saturable absorber, self-starts the laser.<>     

High repetition rate passively Q-switched Nd/sup 3+/:Cr/sup 4+/ all-fibre laser

A passively Q-switched fibre laser comprising an Nd/sup 3+/-doped gain fibre spliced to a new Cr/sup 4+/-doped saturable absorber fibre delivers 0.3 nm-bandwidth continuous pulsed laser beam at 1084 nm, with 38 to 84 kHz repetition rate, and 3.0 to 6.4 /spl mu/s pulse duration.     

Narrow-linewidth, singlemode erbium-doped fibre laser withintracavity wave mixing in saturable absorber

Narrow-linewidth, single longitudinal mode operation is demonstrated in an erbium-doped fibre laser with an intracavity nonlinear absorptive wave mixing of the counter propagating beams. An unpumped length of erbium-doped fibre serves as the saturable absorber. Linewidths of the order of a few kilohertz are obtained     

Three-layer wave absorber using common building material for wireless LAN

A wave absorber with three layers using only common building material for wireless LAN is introduced. The design method is shown, and a suitable structure by fibre reinforced cement board is presented. Based on this, the validity of the experimental absorber produced by this method is confirmed by comparison with calculated results.     

Short-cavity, passively modelocked fibre laser oscillator at 1.5 /spl mu/m with 550 MHz repetition rate and high average power

A short-cavity fibre laser oscillator emitting 12 ps-long pulses at the fundamental cavity frequency of 550 MHz is reported. The simple, stable and ultra-compact laser cavity consists of a short active fibre section which is spliced to a narrowband fibre Bragg grating, and a butt-coupled semiconductor saturable absorber mirror. Only 8 cm of the heavily Er/Yb co-doped phosphate-glass active fibre is sufficient to produce as much as 775 mW of average output power at 1.5 /spl mu/m directly from the oscillator.     

Tunability of erbium-doped fibre ring laser based on bending loss of active fibre

A single frequency depressed-cladding erbium-doped fibre ring laser which can be tuned by exploiting the fibre bending losses has been proposed and experimentally demonstrated. A tunability range of about 44 nm, between 1491 and 1535 nm, has been measured. A saturable absorber formed by an unpumped erbium-doped fibre has been employed to guarantee single frequency operation     

Tunable modelocked bismuth-doped soliton fibre laser

A modelocked bismuth-doped soliton fibre laser is reported. A stable passively modelocked operation in an anomalous dispersion regime is achieved using a dilute nitride (GaInNAs)-based semiconductor saturable absorber mirror and a transmission grating pair for dispersion compensation. The laser generated 0.9 ps soliton pulses with a repetition rate of 7.5 MHz and wavelength tunable from 1153? 1170 nm. These are the shortest pulses obtained so far from Bidoped fibre lasers.     

Saturable absorption in a tapered single-mode optical fibre

A novel all-fibre saturable absorber based on the interaction between the evanescent field of a tapered single-mode fibre and a surrounding solution of a laser dye is described. Results are presented which demonstrate saturable absorption at pump powers of only tens of milliwatts.<>     

Polarisation-maintaining, harmonically modelocked soliton fibrelaser with repetition rate stabilisation using optical pumping ofsaturable Bragg reflector

Well-organised harmonic modelocking with up to 12 pulses in a 1.5 μm polarisation-maintaining erbium soliton fibre laser is achieved. With eight pulses in the cavity, repetition rate timing jitter is suppressed by 7.3 dB through optical pumping above the absorber bandgap. This stabilised laser operates at 463 MHz with nearly transform-limited, 660 fs, 1 pJ pulses     

50 GHz InGaAs edge-coupled PIN photodetector

An edge-coupled PIN photodetector with an InGaAs absorber layer has been designed and fabricated. Values of bandwidth at 1530 nm, with and without bias, have been measured as 50 GHz and 20 GHz, respectively. The external quantum efficiency is as high as 40% using a standard 12 mu m radius lens-ended single-mode fibre.<>     

Ultra-low repetition rate linear-cavity erbium-doped fibre laser modelocked with semiconductor saturable absorber mirror

Reported is an ultra-long linear-cavity erbium-doped fibre laser modelocked by a high modulation depth hybrid mirror-based semiconductor saturable absorber mirror with ultra-low repetition rate and highlychirped output pulses of high energy. With a repetition rate of 250.7 kHz, the laser delivers the single pulse energy of 14 nJ at a pump power of 430 mW. The minimum achievable repetition rate was as low as 25.4 kHz.     

Passive and hybrid modelockings in a multi-electrode DBR laser withtwo gain sections

Passive and hybrid modelockings are achieved in a multi-electrode distributed Bragg reflector laser with two gain sections. Nonuniform injection in the two gain sections is found to play an important role in suppressing the saturated gain level in a long gain section and in achieving passive modelocking. By applying electrical modulation to the absorber, hybrid modelocking is realised. Nearly transform-limited picosecond pulses are obtained after fibre compression with a low timing jitter and a low duty ratio     

Acousto-optic-induced cladding-mode reflection in a blazed-superstructure fiber grating

The coupling phenomenon between the cladding mode and the core mode in a blazed-superstructure fiber grating is experimentally demonstrated in this letter by applying both transverse and longitudinal acoustic waves. This is based on the difference between the cladding-mode wave vector and the core-mode wave vector, which is equivalent to the acoustic wave vector. By this means, a reflectivity-tunable multiwavelength switchable comb filter would be developed for the applications in fiber optics.     

Reflectivity-tunable fiber Bragg grating reflectors

We report a novel device that modulates the Bragg reflectivity of a fiber grating by exciting the transverse vibration of the fiber through an acoustic wave. The excitation of the transverse vibration, leading to fiber microbending, induces the coupling of the fiber core mode into cladding modes. This leads to the reduction of core-mode power and hence that of Bragg reflection. This mechanism provides us a means to control the reflectivity after a fiber Bragg grating is fabricated. The numerical results based on a simple micro-bending model agree well in trend with the experimental data     

Pure Bending Characteristic of Tilted Fiber Bragg Grating

a novel structure of the pure macro- bending sensor based on the tilted fiber Bragg grating （TFBG） is proposed. The TFBG located in the half circle with the different diameters is bent at a constant angle with respect to the tilted grating planes. With the variations of the curvature, the core-mode resonance is unchanged and the transmission power of cladding modes detected by the photodiodes varies linearly with curvature, while the ghost mode changes by the form of two-order polynomial. So we can use the transmission power of ghost mode or other cladding modes to detect bending curvature as shape sensor. From a practical point of view, the sensor proposed here is simple, low cost and easy to implement. Moreover, it is possible to make a temperature-insensitive shape sensor due to the same temperature characteristic between the core mode and the cladding modes.     

Theory of mode-locked lasers containing a reverse saturable absorber

An analytic solution is presented for mode-locking a laser using a saturable absorber and reverse saturable absorber with long relaxation times compared to the temporal pulse widths. A reverse saturable absorber is a material with an excited-state absorption cross section larger than the ground-state absorption cross section, where increasing the incident light intensity increases the absorption. The reverse saturable absorber plays an important role in mode-locking laser materials where individual pulses cannot saturate the gain. A numerical example of mode-locking a CW alexandrite laser with a reverse saturable absorber and a saturable absorber is presented.     

Chebyshev multilevel absorber design concept

Pyramidal- and wedge-absorber materials are used extensively in anechoic measurement chambers to attenuate stray signals. Typical absorber layouts result in large absorber walls in which the absorber tips and bases are roughly aligned in the same plane. Such a quasi-periodic configuration produces a strong coherent specular reflection which dominates the absorber scattered field. Based on the multisection impedance transformer concept, one can divide absorber elements into different levels (layers) so that this coherence can be destroyed to reduce the specular absorber scattering level. The synthesis of this desired behavior can be implemented by the Chebyshev transformer technique, which provides the largest bandwidth given a passband ripple threshold. The resulting reflected field is then the product of the original absorber response times the Chebyshev reduction factor, which is independent of polarization and absorber properties. Various measured results are used to show that more than a 10-dB improvement can be achieved at the critical low end of the frequency band using this approach. This improvement cannot be achieved using conventional design concepts unless the absorber size is doubled