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In this paper, we report the first experimental demonstration, to the best of our knowledge, of low-energy distortion-free femtosecond laser inscription in a free-standing fiber without the need for oil immersion. The methodology proposed features radial-plane beam shaping to eliminate the focal-point distortion induced by the cylindrical geometry of the fiber, thus achieving optimal focusing and spot-like inscription within the fiber. Based on the proposed methodology, a strong, first-order fiber Bragg grating is achieved for the first time, to the best of our knowledge, in a coated free-standing single-mode fiber by using the point-by-point inscription technique with only 50 nJ laser pulse energy.

Facet reflections at the waveguide-air interface for strongly-guiding waveguides with sub-wavelength scale dimensions do not follow the usual Snell's law. Significant amount of reflected power can be channeled into higher order modes as well as radiation modes. This paper shows for the first time how the exact analytical solution of the facet reflection can be obtained by using a new technique based on Fourier analysis and perturbative series summation without the need for approximation or iteration. The proposed analysis enables the distribution of power reflected into various guided and radiation modes to be readily computed. Through this technique, a spectral overlapping criterion and a coupling matrix are derived that analyze effectively the power distribution among all the strongly and weakly-coupled radiation modes in an end-facet reflection. Accurate pre-determination of the number of radiation modes for efficient computation without compromising resultant accuracy is achieved. More importantly, the anomalous wave reflection behaviors at the facet of a strongly-guiding waveguide are presented. These include anomalous high radiation modes coupling as a function of cladding refractive index not reported before. This paper further includes an exemplary illustration of the analysis based on a symmetric planar nano-waveguide with high refractive index contrast for both TE and TM polarization under fundamental incident mode.

Optically-pumped microsquare cavity laser on InP-based multiple-quantum-wells (MQW) material platform is demonstrated. Continuous wave operation is achieved for microsquare cavity with footprint as small as 4×4μm 2 . Numerical mode analysis and experimental characterization of the microsquare laser are conducted, and the results are compared with the commonly used microdisk cavity laser fabricated on the same platform. The microsquare laser shows a lower lasing threshold and infers a higher differential efficiency than the microdisk counterpart. The microsquare cavity laser has sufficiently high quality factor, and higher pumping injection efficiency due to the more evenly distributed field profile as compared to that of the microdisk. Experimental result also shows that the microsquare laser has better temperature stability than the microdisk. These results promise a potential alternative laser structure for onchip light source applications.

A temperature distribution sensing system based on the reflection spectrum of a 70 mm apodized chirped fiber Bragg grating has been demonstrated. Reflection variations created by the Fabry–Perot effect due to the intra-grating wavelength shift are presented. By fitting a parametric transfer matrix model of the grating response to measured spectra, the temperature change, position and width of a localized temperature change are obtained. This system is particularly attractive of its simplicity, high spatial resolution and ability in measuring a non-monotonic distribution. Experiments to measure two localized heating regions along the grating indicate that the technique is promising for measuring more complex temperature profiles.

A fully distributed temperature sensor consisting of a chirped fibre Bragg grating has been demonstrated. By fitting a numerical model of the grating response including temperature change, position and width of localized heating applied to the grating, we achieve measurements of these parameters to within 2.2 K, 149 µm and 306 µm of applied values, respectively. Assuming that deviation from linearity is accounted for in making measurement, much higher precision is achievable and the standard deviations for these measurements are 0.6 K, 28.5 µm and 56.0 µm, respectively.

We have investigated numerically and experimentally a fiber Bragg grating (FBG) sensor interrogation scheme utilizing a linear chirped grating-based Sagnac loop as a wavelength-dependent receiver. The scheme is suitable for both static and dynamic sensor interrogation with advantages of stable and linear readout response and easily-adjustable sensing resolution and dynamic range. Static and dynamic strain resolutions as high as /spl plusmn/ 4.2 /spl mu//spl epsiv/ and 0.406 /spl mu//spl epsiv///spl radic/ Hz have been demonstrated using this scheme.

We present an experimental and numerical study of transversely loaded uniform fibre-Bragg gratings. A novel loading configuration is described, producing pressure-induced spectral holes in an initially strong uniform grating. The birefringence properties of these gratings are analysed. It is shown that the frequency splitting of the two spectral holes, corresponding to two orthogonal polarisation states, can be adjusted precisely using this loading configuration. We finally demonstrate a new and simple scheme to induce multiple spectral holes in the stop-band. © 2003 Elsevier Science B.V. All rights reserved.

A pitch-by-pitch grating inscription method that allows both high laser intensity and long-exposure dosage when inscribing each pitch is demonstrated. Based on slit beam-profiling, proposed scheme does not require oil-immersion-based beam distortion compensation techniques for inscription into free-standing single-mode fiber. High grating fringe visibility inscription is achieved for 1.07μm-period structure in single-mode fiber even though each grating pitch are individually defined by 50msec-long laser exposure.

In this Letter, we report for the first time to our knowledge that all-optical switching gain can be achieved with a dual-wavelength control versus pump beam scheme in a single semiconductor waveguide structure. That means a weak optical beam can switch a strong optical beam. Moreover, a high switching speed of 10–100 Gb/s can be achieved. The all-optical switching is simulated numerically via a multilevel multielectron (MLME) FDTD program capable of modeling complex semiconductor band properties. It is shown that a weak control/input-signal beam at a longer wavelength is able to switch the transmission of a strong pump/output-signal beam at a shorter wavelength. A 50 Gbps and 0.5 pJ per bit switching operation with switching gain of around 10 is shown for a 40 μm-long waveguide with pump beam power around 20 mW based on bulk InGaAsP material and a 300 nm×300 nm waveguide (the control beam power is 1/10 of that for the pump).

A high spatial resolution photodetector is designed and realized using a newly developed two-sided nanoscale three-dimensional fabrication technique. Two types of nanophotodetectors with slab and channel structures are fabricated and characterized. By comparison, nanophotodetectors with channel structures show a 10 times lower dark current, higher detection resolution, and signal-to-noise ratio than nanophotodetectors with slab structures. The smallest nanophotodetector fabricated is as small as 100 nm wide. A responsivity of 0.19 A/W at 1310-nm wavelength and 3-V bias for a channelized nanophotodetector is registered, with a photocurrent of 135 nA over a dark current of 1.25 nA. In addition, using a modified near-field scanning optical microscopy-based photocurrent mapping system, we demonstrated a high spatial resolution photodetection of 400 nm using a nanophotodetector with channel structure fabricated.

Selective infiltration of air holes in photonic crystal fibers (PCFs) with functional materials can equip optical fiber devices with advanced functions. We propose a new method which enables precise selective infiltration of dual-core PCF (DC-PCF) by using femtosecond laser micromachining. Through this proposed technique, we have successfully selective infiltrated single air hole with a diameter of 2.5μm inside a DC-PCF. This method is simple, precise and repeatable, and it provides great potential for fabricating advanced fiber devices using selective infiltration technique.

A bidirectional nonreciprocal wavelength-interleaving filter based on an optically coherent high birefringence fiber transversal filter structure is demonstrated. Stable, low loss, low dispersion, and high isolation operation is demonstrated with reconfigurable transfer characteristics for interleaved channel spacing of 0.8 nm.

Low-loss, high reflectivity, first-order fiber Bragg gratings are achieved by pitch-bypitch fabrication methodology in a truly free-standing fiber. Slit beam-profiling adopted attain transversely-extended index modification for each grating period defined by single laser pulse. Fiber Bragg grating of 0.52µm period with >27dB transmission stopband and

Low-energy, distortion-free femtosecond pulse laser inscription in coated freestanding fiber is demonstrated for the first time. Methodology features radial plane beamshaping to achieve optimal focusing and spot-like inscription within the fiber. Strong, first- order fiber Bragg grating is achieved in coated single-mode fiber using only 50nJ pulse energy.

Spot-like femtosecond pulse laser inscription with large inscription depth in a freestanding fiber is demonstrated for the first time. Methodology uses axial plane slit beamprofiling to improve inscription uniformity near the core to enable spatially-large in-fiber structural inscription.

A simple efficient method for stabilizing a harmonically mode-locked fiber ring laser is proposed. In this method, a linear optical filter and a nonlinear Fabry–Pérot filter in which the refractive index is optical intensity dependent are located in the laser cavity. The linear filter is used to select a fixed lasing wavelength, and the Fabry–Pérot filter introduces a negative all-optical feedback mechanism that is able to suppress pulse-to-pulse amplitude fluctuations in the laser cavity. The scheme was experimentally demonstrated using a fiber Bragg grating as the linear filter and a laser diode biased below threshold as the nonlinear Fabry–Pérot, and stable harmonically mode-locked pulses with a supermode noise suppression ratio >55 dB were obtained.

A low cost and CMOS-process compatible solution capable of passive-alignment coupling of singlemode optical fiber (SMF) to silicon-on-insulator (SOI) based photonic waveguide (size: sub-μm) is critical for realization and acceptance of fiber-pigtailed SOI-based electronics/photonics integrated circuits (EPIC) chips for mass applications in optical interconnects, optical information processing and sensing. In this work, we propose and demonstrate monolithic integration of an ultra-compact Si/SiO 2 multilayer super-high numerical-aperture (NA) graded-index (MLS-GRIN) lens mode-size converter on SOI-based waveguide platform. The MLS-GRIN lens, with longitudinal focal-length of 20μm, total stack height of 8μm, and simulated NA better than 3, converts optical mode size vertically from SMF (core-φ ∼9μm) to 300nm-thick SOI waveguide by an engineered near-parabolic refractive index profile formed by varying the thickness ratio of alternating Si/SiO 2 layers, while keeping the thicknesses of Si and SiO 2 less than quarter of optical wavelength in the medium. The horizontal mode size conversion is performed by lateral up-taper of Si-waveguide prior to MLS-GRIN lens on the chip.

Vertical optical mode-size transformation from 260nm-thick Si-nanowaveguide to 10∼12µm matching the single-mode-fibre-core has been demonstrated using compact multilayer Si/SiO 2 asymmetric GRIN lens (length:∼24µm). GRIN lens to single-mode-fibre practical coupling loss of −3.45 dB was achieved.

We propose a novel technique based on Fourier analysis and perturbation theory to describe completely the step discontinuity in a slab waveguide of any index contrast without the need for analytical approximations or assumptions. Through this technique, we derive a spectral overlapping criterion and a coupling matrix that analyzes effectively the roles of the radiation modes in the resultant computation accuracy. A symmetric nano-waveguide with 55.9% index contrast is investigated under normal incident light condition along TE polarization. The total reflectance and the guided power reflectivity are computed at the waveguide-air interface. Results obtained are consistent with the FDTD simulation and only requires very short computational time.

The influence of the fiber geometry on the point-by-point inscription of fiber Bragg gratings using a femtosecond laser is highlighted. Fiber Bragg gratings with high spectral quality and strong first-order Bragg resonances within the C-band are achieved by optimizing the inscription process. Large birefringence (1.2x10-4) and high degree of polarizationdependent index modulation are observed in these gratings. Potential applications of these gratings in resonators are further illustrated. © 2007 Optical Society of America.

The capabilities of the Dynamic Thermal Electron Quantum Medium FDTD (DTEQM-FDTD) model in simulating semiconductor media and devices are illustrated. Both transient and steady-state carrier dynamics, along with associated physical phenomena are obtained. Using the same computational model, the operational dynamics of a Fabry-Perot laser cavity are further illustrated.

The dynamic thermal electron quantum medium FDTD (DTEQM-FDTD) model is used to demonstrate ultrafast carrier dynamics in saturable gain and absorber media. The same model is then used to simulate a passive mode locked laser.

A series of symmetric and asymmetric LPGs were inscribed in photonic crystal fibre by a low repetition rate femtosecond laser system. The asymmetric LPGs were found to be spectrally sensitive to bend orientation, with some of the attenuation bands producing both red and blue wavelength shifts, whilst the symmetric devices produced only a unidirectional wavelength shift. Both sets of devices displayed strong polarisation dependence.

A 1.2 microm (height) x 125 microm (depth) x 500 microm (length) microslot along a fiber Bragg grating was engraved across the optical fiber by femtosecond laser patterning and chemical etching. By filling epoxy in the slot and subsequent UV curing, a hybrid waveguide grating structure with a polymer core and glass cladding was fabricated. The obtained device is highly thermally responsive with linear coefficient of 211 pm/ degrees C.

A series of LPGs with the same period was inscribed by femtosecond laser into photonic crystal fibre with various powers. All suffered post-fabrication spectral evolution at low temperatures, apparently related to inscription power.

A 1.2(height)times125(depth)times500(length) micro-slot was engraved along a fiber Bragg grating by chemically assisted femtosecond laser processing. By filling epoxy and UV-curing, waveguide with plastic-core and silica-cladding was created, presenting high thermal responding coefficient of 211pm/degC.

The interactions of the core-propagating light with an intersecting microslit within a conventional single-mode fiber are investigated. Orientation-dependent out-coupling of core light was utilized to create side-detection, miniature fiber rotation sensors.

Point-by-point inscription of sub-µm period fiber Bragg gratings with good spectral quality, first order Bragg resonances within the C-band is achieved. Distinct polarization characteristics are further observed in these fiber gratings.

The use of high intensity femtosecond laser sources for inscribing fibre gratings has attained significant interest. The principal advantage of high-energy pulses is their ability for grating inscription in any material type without preprocessing or special core doping - the inscription process is controlled multi-photon absorption, void generation and subsequent local refractive index changes. The formation of grating structures in photonics crystal fibre has proven difficult, as the presence of holes within the fibre that allow wave-guidance impair and scatter the femtosecond inscription beam. Here we report on the consistent manufacture of long period gratings in endlessly single mode microstructure fibre and on their characterisation to external perturbations. Long period gratings are currently the subject of considerable research interest due to their potential applications as filters and as sensing devices, responsive to strain, temperature, bending and refractive index. Compared to the more mature fibre Bragg grating sensors, LPGs have more complex spectra, usually with broader spectral features. On the other hand they are intrinsically sensitive to bending and refractive index. Perhaps more importantly, the fibre design and choice of grating period can have a considerable influence over the sensitivity to the various parameters, for example allowing the creation of a bend sensor with minimal temperature cross-sensitivity. This control is not possible with FBG sensors. Here we compare the effects of symmetric and asymmetric femtosecond laser inscription.

Microfluidic devices composed of microslits in step-index optical fibers are thoroughly investigated. Numerical simulations are performed to explain scattering and power loss in such devices. Experimental results based on microslits fabricated by femtosecond laser processing corroborate theoretical data. Dependency of the device performance on the refractive index of fluid in the slit is further utilized to construct a refractive index sensor and an in-fiber attenuator.

A liquid core waveguide as a refractometer is proposed. Microtunnels were created in standard optical fiber using tightly focused femtoscond laser inscription and chemical etching. A 1.2(h)×l25(d) ×500(1) μm micro-slot engraved along a fiber Bragg grating (FBG) was used to construct liquid core waveguide by filling the slot with index matching oils. The device was used to measure refractive index and sensitivity up to 10-6/pm was obtained. © 2007 Optical Society of America.

We report for the first time the fabrication of long-period gratings in near-single-mode GeO2glass fiber by 800nm-femtosecond-laser inscription, revealing grating resonances strongly polarisation dependent and their temperature sensitivities varying significantly from 96pm/°C to 11pm/°C.

A 1.2X500μm slot was engraved across a fiber Bragg grating (FBG) using femtosecond laser patterning and chemical etching. liquid core FBGs were constructed and their sensitivity to refractive index of up to 10-6/pm was measured.

A novel architecture for microwave/millimeter-wave signal generation and data modulation using a fiber-grating-based distributed feedback laser has been proposed in this letter. For demonstration, a 155.52-Mb/s data stream on a 16.9-GHz subcarrier has been transmitted and recovered successfully. It has been proved that this technology would be of benefit to future microwave data transmission systems. © 2006 IEEE.

A femtosecond laser inscription technique is proposed for the fabrication of a fiber grating laser directly into a nonphotosensitive gain fiber. A distributed Bragg reflector fiber grating laser is realized in a conventional, untreated Er:Yb-codoped fiber as an illustration. Robust, single-mode, single-polarization laser operation at temperatures in excess of 600 degrees C is further achieved without compromising performance. A nonlinear, thermally induced wavelength shift is also observed at elevated temperatures.

We present recent results on experimental micro-fabrication and numerical modeling of advanced photonic devices by means of direct writing by femtosecond laser. Transverse inscription geometry was routinely used to inscribe and modify photonic devices based on waveguiding structures. Typically, standard commercially available fibers were used as a template with a pre-fabricated waveguide. Using a direct, point-by-point inscription by infrared femtosecond laser, a range of fiber-based photonic devices was fabricated including Fiber Bragg Gratings (FBG) and Long Period Gratings (LPG). Waveguides with a core of a couple of microns, periodic structures, and couplers have been also fabricated in planar geometry using the same method.

Micro-channels are fabricated into conventional single-mode fibers using focused femtosecond laser processing and chemical etching. A microfluidic fiber device consisting of a micro-channel intersecting the fiber core for refractive index sensing is demonstrated.

The use of high birefringence fiber interrogating interferometer for optical sensing applications was discussed. The method is of low cost and permits simple adjustment of the optical path difference and has much lower sensitivity to environmental perturbation. The polarization-maintaining (PM) fiber interferometer adopted a heterodyne approach using interferometric wavelength shift detection. The study showed that the inclusion of power amplifier driving a multi-element piezoelectric stack will enable the bandwidth to be pushed up into the kHz regime.

A novel compact, wavelength-switchable, single-mode DBR fibre laser configuration is demonstrated. Stable single-line or wavelength-switching operations are achieved, by controlling the cavities' resonance mode conditions through a simple strain mechanism, without pump power limitations. (2 pages)

A novel superimposed fiber Bragg grating (FBG) structure with polarization-discriminating characteristics is demonstrated. By superimposing the FBGs onto a tilted fiber grating, simultaneous wavelength- and polarization-filtering are achieved and are tailored purely by grating parameters.

A bidirectional nonreciprocal wavelength-interleaving filter based on an optically coherent high birefringence fiber transversal filter structure is demonstrated. Stable, low loss operation is achieved with reconfigurable transfer characteristics for interleaved channel spacing of 0.8 nm with >30 dB isolation and ultra-low chromatic dispersion.

A simple technique based on superimposed cavities structure for direct real-time assessment of a DFB fiber laser mode condition during operation is demonstrated and used to provide a cavity mode conditioning feedback mechanism to optimize output performance. Significant improvements to the output performance and robustness are achieved over the entire pump power range without ambient isolation.

Optical millimeter wave generation is realized using dual polarization modes operation from a co-located dual distributed feedback fiber laser configuration. A narrow linewidth optical millimeter wave signal at 32.5 GHz is demonstrated without using complex control mechanism.

We demonstrate a phase-shifted fiber grating transmission filter remotely tunable using optical pump-induced thermal effects in Er/Yb codoped fiber sections. A wavelength-independent induced phase shift of 0.1 /spl pi//mW and a transmission extinction ratio of > 21dB are achieved.

We propose and demonstrate single- and multiple-passband fiber grating transmission filters that are remotely tunable by exploitation of the optical pump-induced thermal effects in Er/Yb-codoped fiber sections. A repeatable, wavelength-independent induced phase shift of 0.1 pi/mW is obtained without hysteresis and anisotropic effects. A transmission extinction ratio of23 dB with a 3-mW change in pump power is achieved.

A novel dual complementary output optical fiber transversal filter is realized for DWDM applications. Stable, simultaneous complementary filter responses with flattened passbands and large sidelobe suppressions are achieved with a single-line cascaded Hi-Bi fiber structure.

Differential group delay measurement of narrowband fiber devices using a fiber polarization scrambler with a modulation phase shift technique is demonstrated. Accurate measurement is realized with high wavelength and delay resolution and immunity to environmental perturbation.

Optical generation of a millimetre wave signal based on a co-located, dual distributed feedback fibre laser structure is proposed and demonstrated. Through the use of high birefringence fibre grating for polarization-discriminating self-injection feedback, the fibre laser produces stable dual polarization mode operation. Due to high common-mode suppression of phase noise, a millimetre wave beat signal of high spectral purity is obtained. Experimental result illustrates a heterodyne signal measured at /spl sim/32.5 GHz with a linewidth of