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Here we present a fully fiberized Bi-doped ring laser incorporating a fiber Bragg grating (FBG) operating at a wavelength of 1340nm, which provides 172mW of continuous-wave output power with a slope efficiency of 35%. In addition, we demonstrate a wavelength-tunable version of the laser, which can be tuned from 1320nm to 1370nm by employing a home-made mechanically tunable FBG. Within the whole 50nm tuning band the laser maintains the output power of more than 100mW and the slope efficiency of more than 25%. A maximum output power of ~165mW is achieved over the wavelength range of 1330-1340nm. The linewidth of tunable laser is 0.04nm.

We present a Bi-doped fiber laser (BDFL) at 1340nm with >170mW output power and 35% slope efficiency. A wavelength-tunable version of the BDFL facilitated by an FBG is demonstrated from 1315-1340nm with >120mW output power.

We have studied second-harmonic generation (SHG) of a cw single-frequency ytterbium-doped fibre laser, using a periodically poled silica fibre as a nonlinear medium for frequency conversion. All-fibre external cavity SHG has been investigated for the first time. A twofold increase in second-harmonic power in a fibre ring cavity has been demonstrated and possibilities of further optimising the fibre scheme have been analysed.

Over the last decade, the use of chirped fibre Bragg gratings (CFBGs) in detonation velocity experiments has been steadily increasing. In this paper, we show how CFBG design parameters&mdash, chirp-rate, reflectivity and apodisation&mdash, affect linearity in detonation velocity tests. It is found that the optimal CFBG detonation velocity probe should have a high chirp-rate, a low reflectivity and no apodisation. As a further demonstration of these findings, we measure detonation velocity with a 24 cm optimised CFBG, the longest CFBG test of this kind so far.

Research into optical fiber refractometers yielded remarkable results over the past decade. Numerous sensing schemes were proposed and demonstrated, which possessed different advantages while facing unique limitations. On top of their obvious applications in measuring refractive index changes of the ambient environment, several studies reported advanced applications of such sensors in heavy metal ion detection by means of surface coating of the refractometers with heavy metal ion sensitive materials. This paper surveys the effort these optical fiber metal ion sensors based on surface coated optical fiber refractometer, discusses different technologies and methods involved, and highlights recent notable advancements. Published version

We demonstrate a simple-to-fabricate refractometer based on the inscription of fiber Bragg gratings in a special C-shaped optical fiber. The C-shaped fiber was drawn into shape using a quarter cladding removed preform of a commercial standard single-mode fiber by simple machining. The sensor did not suffer from cross-sensitivity of the refractive index with ambient temperature fluctuations, commonly occurring with many optical fiber refractometers. A refractive index sensitivity of 1300 pm per refractive index unit (RIU) was achieved without employing any additional sensitization techniques such as tapering or etching. Published version

We present a tunable thulium-doped fiber laser (TDFL) incorporating a Tm/Ge co-doped fiber capable of accessing the U-band wavelength region (1620-1660nm). These results represents by far the shortest laser wavelengths so far for a TDFL.

Quantum sources that provide broadband biphotons entangled in both polarization and time-energy degrees of freedom are a rich quantum resource that finds many applications in quantum communication, sensing, and metrology. Creating such a source while maintaining high entanglement quality over a broad spectral range is a challenge, which conventionally requires various compensation steps to erase temporal, spectral, or spatial distinguishabilities. Here, we point out that in fact compensation is not always necessary. The key to generate broadband polarization-entangled biphotons via type-II spontaneous parametric downcoversion (SPDC) without compensation is to use nonlinear materials with sufficiently low group birefringence that the biphoton bandwidth becomes dispersion-limited. Most nonlinear crystals or waveguides cannot meet this condition, but it is easily met in fiber-based systems. We reveal the interplay of group birefringence and dispersion on SPDC bandwidth and polarization entanglement quality. We show that periodically poled silica fiber (PPSF) is an ideal medium to generate high-concurrence (>0.977) polarization-entangled photons over a broad spectral range (>77nm), directly and without compensation. This is the highest polarization-entanglement concurrence reported that is maintained over a broad spectral range from a compensation-free source.

Paper-based microfluidics is a rapidly progressing inter-disciplinary technology driven by the need for low-cost alternatives to conventional point-of-care diagnostic tools. For transport of reagents/analytes, such devices often consist of interconnected hydrophilic fluid-flow channels that are demarcated by hydrophobic barrier walls that extend through the thickness of the paper. Here, we present a laser-based fabrication procedure that uses polymerisation of a photopolymer to produce the required fluidic channels in paper. Experimental results showed that the structures successfully guide the flow of fluids and allow containment of fluids in wells, and hence the technique is suitable for fabrication of paper-based microfluidic devices. The minimum width for the hydrophobic barriers that successfully prevented fluid leakage was ~120 μm and the minimum width for the fluidic channels that can be formed was ~80 μm, the smallest reported so far for paper-based fluidic patterns.

In this paper we demonstrate a simple and new fibre optic technique for measuring detonation velocities using uniform fibre Bragg gratings. We compare this new system with chirped fibre Bragg grating diagnostics and show how coherent source illumination can yield spatial uncertainties below ±10 μm - a percentage error that is an order of magnitude lower than the broadband ASE methods we have tested.

We demonstrate water-salinity sensing based on the relative shifts of the higher-order Bragg-resonances in a cladding-etched fibre Bragg grating, and achieve sensitivities of 2.9nm/RIU(LP01) and 3.4nm/RIU(LP11) atλ=1560nm for the two sets of modes

The development of super continuum sources is driven by the requirements of a wide range of emerging applications. This paper points out how non-silica fibers are of benefit not only because their broad mid-IR transparency enables continuum generation in the 2–5µm region but also since the high intrinsic nonlinearity of the glasses reduces the power-threshold for devices at wavelengths below 2µm. For these glasses, the material zero-dispersion wavelength is typically shifted to long wavelengths compared to silica so dispersion tailoring is key to creating sources based on practical, near-IR, solid state pump lasers. We show how modeling work has produced fiber designs that provide flattened dispersion profiles with high nonlinearity coefficients and zero-dispersion wavelengths in the near-IR. Building on this flexibility, modeling of the pulse dynamics then demonstrates how coherent mid-IR supercontinuum sources could be developed. We also show the importance of the second zero-dispersion wavelength using bismuth fibers as an example. Nonlinear mode-coupling is shown to be a factor in larger core fibers for high-power applications. Demonstrations of supercontinuum in microstructured tellurite fibers, all-solid lead–silicate (SF57) fibers and in bismuth fibers and tapers are then reported to show what has been achieved experimentally using a range of materials and fiber geometries.

We investigate efficient broadband infrared supercontinuum generation in meter-length single-mode small-core tellurite holey fiber. The fiber is pumped by 1.06µm picosecond pulses in the normal dispersion region. The high Raman gain coefficient and the broad Raman gain bands of the tellurite glass are exploited to generate a cascade of Raman Stokes orders, which initiate in the highly normal dispersion region and quickly extend to longer wavelengths across the zero dispersion wavelength with increasing pump power. A broadband supercontinuum from 1.06µm to beyond 1.70µm is generated. The effects of the pump power and of the fiber length on the spectrum and on the power conversion efficiency from the pump to the supercontinuum are discussed. Power scaling indicates that using this viable normal dispersion pumping scheme, 9.5 W average output power of infrared supercontinuum and more than 60% conversion efficiency can be obtained from a 1 m long tellurite fiber with a large mode area of 500µm2.

We present a novel superstructure fiber Bragg grating fiber end sensor capable of detecting variations in refractive index (RI) of liquids and potentially that of gases, and demonstrated an application in the detection of heavy metal ions in water. The sensor is capable of sensing RI variations in the range of 1.333 to 1.470 with good sensitivity of up to 230 dB/RIU achieved for the RI range of 1.370 to 1.390. The sensor is capable of simultaneously measuring variations in ambient temperature along with RI. A simple chemical coating was employed as a chelating agent for heavy metal ion detection at the fiber end to demonstrate an possible application of the sensor. The coated fiber sensor can conclusively detect the presence of heavy metal ions with concentrations upwards of 100 ppm. RI sensing capability of the sensor is neither affected by temperature nor strain and is both robust and easily reproducible. Published version

An experimental demonstration of a two-channel OCDMA system with detection performed using standard linear detection or a TPA-based nonlinear detector is presented. These results show an improvement in the extinction ratio of the decoded signal by ∼5 dB using TPA detection. A simulation model of the TPA detector used during the experiments was created and used in a four-channel OCDMA system simulation using both linear and nonlinear detection methods. The simulation results show that error-free performance is achievable for a 4-user system using the nonlinear TPA detector while the OCDMA system employing linear detection is severely limited by the effects of noise generated by adjacent optical channels (multiple access interference).

In this paper, we analyze the optical conversion efficiency of a multimode pump source into a diffraction-limited beam in cladding-pumped Raman fiber lasers and amplifiers. We theoretically derive a relation of the cladding/core area ratio for efficient energy transfer from the pump into the first Stokes while avoiding the second Stokes generation that impedes the conversion efficiency of the first Stokes. In addition, we investigate experimentally the limits on the conversion efficiency in a pulse-pumped double-clad Raman fiber. We obtained a peak power conversion efficiency into the first Stokes in excess of 75% and a pulse energy conversion of 60%. We found that spatial pump mode depletion is the main limiting factor in this fiber, with concentric cladding and core. We discuss various solutions to overcome these limitations and improve on the current results.

Udgivelsesdato: 2008 The study aims to establish the predictive value of a diagnosis of depression among elderly according to the 10th revision of the International Statistical Classification of Diseases (ICD-10) by measuring morbidity, medication usage, health service utilization and mortality during an 8-year follow-up of depressed elderly inpatients (n=76) and community-living depressed patients (n=38) compared with controls (n=116). The data were taken from GPs' medical records and health statistics registers. At baseline, no significant differences were observed between the two cohorts of depressed patients and the controls in terms of prevalence of cardiovascular, respiratory or cerebrovascular morbidity. During follow-up, both cohorts of depressed patients had significantly increased rates of recurrent depressions, consumption of antidepressants, psychiatric in- and outpatient admissions, and home visits; inpatients used more psychiatric hospital days. Health service utilization in somatic hospitals and somatic diagnoses was not significantly increased. Inpatients used significantly fewer GP office-hour services but more out-of-hours services than the control group. Community-living depressed patients experienced no significant increase in use of GP services. Survival was unaffected in both cohorts. In agreement with other studies, especially inpatient depression predicted increased rates of recurrent depressions and increased use of psychiatric hospital services, indicating poor long-term outcome. Inpatients consumed fewer GP office-hour services but more out-of-hours services, possibly due to less office-hour contact. Contrasting with other studies, ICD-10 depression among elderly predicted no increase in the use of somatic hospital facilities.

We demonstrate a reconfigurable 16-channel optical code-division multiple access (OCDMA)/dense wavelength division multiplexing (DWDM) system (4 OCDMA times 4 DWDM times 625 Mb/s) based on novel 31-chip, 40 Gchip/s quaternary phase coding gratings operating at a channel spacing of just 50 GHz. The system performance is studied for cases of both fixed and code-reconfigurable decoders. Error-free performance is achieved in both cases and for all 16 channels.

We investigate the benefits of employing a saturated semiconductor optical amplifier (SOA) to reduce the optical noise in an incoherent light optical code division multiple access (OCDMA) system. In the context of spectrum slicing, SOA-based noise suppression has shown significant potential in enhancing the signal quality of noisy light. In this paper, we evaluate the viability of the technique for spectral amplitude coded OCDMA and show that the benefits of SOA-based noise suppression do not extend readily to this application due to post-SOA optical-filtering effects at the receiver. However, appreciable performance improvements can in principle be realized through optimized system and decoder design

Ultra-short wavelength operation of a thulium fibre laser is investigated. Through use of core pumping and high feedback efficiency wavelength selection, a continuously-tunable fibre laser source operating from 1660 nm to 1720 nm is demonstrated in a silica host. We discuss the range of applications within this important wavelength band such as polymer materials processing and medical applications targeting characteristic C-H bond resonance peaks. As a demonstration of the power scalability of thulium fibre lasers in this band, fixed wavelength operation at 1726 nm with output power up 12.6 W and with slope efficiency > 60% is also shown.

In this talk we will review some of the latest advances in the use of fibre Bragg gratings and periodically poled fibre devices to access and generate high efficiency light sources in new wavelength bands.

By monitoring the wavelength-shift of Bragg-resonances in a fibre grating during cladding-etching, we measure the refractive-index of buffered-HF acid (20:1) as 1.36±0.005 @ 1.55μm, and use that to demonstrate precise control the fibre etch-diameter.

A time-division add-drop multiplexer capable of high-extinction-ratio operation is presented both theoretically and experimentally. The approach used is based on time-to-frequency domain conversion of optical signals and relies upon the switching of linearly chirped optical pulses. By converting a 40-Gb/s optical time-division multiplexing (OTDM) signal to 4 /spl times/ 10-Gb/s wavelength-division multiplexing (WDM) channels and using fiber Bragg gratings for frequency-domain add-drop multiplexing, a timeslot suppression ratio in excess of 30 dB and error-free operation for the dropped, through, and added channels were achieved. A further stage of WDM-to-TDM signal conversion was used to map the resulting signal back into the time domain. Moreover, it is shown that it is straightforward to simultaneously operate on multiple channels by simply cascading gratings to make more complex filtering functions without the requirement for any further synchronization of the tributary channels.

In this paper, we present an experimental and numerical study of semiconductor optical amplifier (SOA)-based noise suppression and its relevance to high-channel-density spectrum sliced wavelength-division-multiplexed systems. We show that the improvement in signal quality is accompanied by spectral distortion, which renders it susceptible to deterioration in the presence of subsequent optical filtering. This phenomenon originates from the loss of intensity correlation between spectral components of the SOA output when the signal spectrum is altered. As a consequence, a design tradeoff is introduced between intensity noise and crosstalk in high-channel-density systems. These adverse effects can be overcome by optimized SOA design, resulting in a significant improvement in signal quality.

A novel high-spectral-efficiency modulation scheme using square time pulses in orthogonal wavelength-division multiplexing is proposed. Experimental results demonstrate the significant reduction of the interchannel linear crosstalk-induced penalty compared with Gaussian return-to-zero (RZ) modulation. The proposed technique allows a maximum spectral efficiency of 1 b/s/Hz. Simulation studies and experimental work confirming the expected performance are presented.

We demonstrate the use of fiber Bragg grating based pulse-shaping technology to provide timing jitter tolerant data demultiplexing in an 80 Gb/s all-optical time division multiplexing (OTDM) system. Error-free demultiplexing operation is achieved with /spl sim/6 ps timing jitter tolerance using superstructured fiber Bragg grating based 1.7 ps soliton to 10 ps rectangular pulse conversion at the switching pulse input to a nonlinear optical loop mirror (NOLM) demultiplexer comprising highly nonlinear dispersion shifted fiber (HNLF). A 2-dB power-penalty improvement is obtained compared to demultiplexing without the pulse-shaping grating.

This paper presents a new enhanced design of fibre Bragg grating for flattening the gain profile of an erbium-doped fibre amplifier in transmission configuration. This grating has modulation of refractive index change and constant chirp rate, wherein both profiles of the parameters are properly apodized. The design utilizes an alternative approach to the renowned inverse scattering method, yet produces a similar spectral response quality and conceptually simpler. Moreover, it offers precise control over all parameters of the grating structure. It is shown that the design approach could produce gain-flattening filters of satisfactory quality through simulations using the accurate transfer matrix method.

We report on a high-power single-frequency mode-hop-free fiber distributed Bragg reflector (DBR) laser fabricated from Tm-doped photosensitive alumino-silicate fiber and in-band pumped by an Er/Yb fiber laser at 1565 nm. The fiber DBR laser yielded up to 580 mW of single-frequency single-ended output at 1943 nm for ~ 5 W of absorbed pump power. Further scaling of the DBR laser output power was achieved with the aid of a simple Tm-doped fiber amplifier stage spliced directly to the DBR fiber without the need of an optical isolator. The maximum output power from the DBR laser and fiber amplifier was 2.9 W for a combined absorbed pump power of 9 W. The merits of this DBR laser architecture compared to conventional DBR and distributed-feedback laser configurations are discussed.

We report a record multi-watt level and up to 45% efficient all-fiber frequency doubling in periodically poled silica fibers. Modulation instability is identified as the primary factor preventing further growth of the second-harmonic power.

We present a technique for the shaping of short pulses based on the use of superstructured fiber Bragg gratings (SSFBGs). We apply this technique to demonstrate the generation of 20-ps rectangular pulses by phase and amplitude profiling of 2.5-ps soliton pulses. Numerical calculations validate our experimental findings.

We experimentally demonstrate nondegenerate phase-sensitive amplification of a 40-Gb/s differential phase-shift-keying data signal in both single- and double-pumped parametric configurations. Careful optimization of the system has allowed a uniform phase-sensitive gain and attenuation to be achieved over at least 20 nm of bandwidth.

An experimental and theoretical investigation of how the polarization mode competition and beat frequency of Er-doped fiber distributed-feedback lasers depend on perturbations such as localized transverse forces, back reflections, or changes in pump polarization is reported. Good agreement between the experiments and a comprehensive theoretical model is obtained. Use of a dual-polarization laser as a transverse force sensor with a resolution on the order of 1-100 nN//spl radic/Hz above 20 Hz is also discussed.

We experimentally demonstrate the use of saw-tooth optical pulses, generated using a superstructured fiber Bragg grating, to achieve high performance and efficient all-optical wavelength conversion in a scheme based on cross-phase modulation in a highly nonlinear fiber, with subsequent offset filtering. Compared to the use of more conventional waveforms, such as Gaussian pulses, the purpose-shaped saw-tooth pulses allow an improvement in the optical signal-to-noise ratio of the wavelength-converted signal of around 15 dB as well as a more than 6-dB improvement in receiver sensitivity.

We experimentally demonstrate the generation of picosecond triangular optical pulses using a superstructured fiber Bragg grating and show experimentally that use of this pulse shape can provide a three-fold improvement in conversion efficiency relative to the use of Gaussian pulses with similar pulse widths when used in a wavelength conversion scheme based on self-phase-modulation and subsequent offset filtering.

We experimentally demonstrate an all-optical all-fiberized scheme that can eliminate linear perturbations in the time domain. The technique relies on performing optical Fourier transforms on distorted signals to move their distortions from the time to the frequency domain. A near-complete transformation is obtained by using cross-phase modulation with parabolic pulses as a time lens. Mitigation of second- and third-order dispersed pulses is demonstrated as well as good transform behaviour.

We use a linear frequency-resolved optical gating technique based on electrooptic modulation to fully characterize for the first time the highly chirped pulses from a 1.06-m Fabry-Perot laser diode and design a chirped fiber Bragg grating to provide high-quality pulse compression. With this grating, we achieved 18-ps pulses at a repetition rate of 1.35 GHz with a time-bandwidth product of 0.7.

We propose and demonstrate new optical code-division multiple-access (OCDMA) encoder-decoders having a continuous phase-distribution. With the same spatial refractive index distribution as the reconfigurable optical phase encoder-decoders, they are inherently suitable for the application in reconfigurable OCDMA systems. Furthermore, compared with conventional discrete-phase devices, they also have additional advantages of being more tolerant to input pulse width and, therefore, have the potential of bandwidth saving.

We demonstrate the capacity for fast dynamic reconfiguration of optical code-division multiple access (OCDMA) phase en/decoders based on fiber Bragg gratings and a thermal phase-tuning technique. The tuning time between two different phase codes is measured to be less than 2 s. An OCDMA system using tunable-phase decoders is compared with a system using fixed-phase decoders and, although the system using fixed-phase decoders exhibits a shorter output autocorrelation pulsewidth and lower sidelobes, the system using tunable-phase decoders has advantages of flexibility and a more relaxed requirement on the input pulsewidth.

We investigate experimentally and theoretically the linewidth of an Er-Yb-doped distributed feedback fiber laser. We find that two noise sources are mainly responsible for the observed behavior, fundamental thermal noise at low pump power levels and temperature fluctuations induced by pump intensity noise at higher powers. Potential techniques to overcome these limitations are discussed.

We experimentally demonstrate a novel all-optical all-fiberized pulse retiming scheme incorporating parabolic pulses generated in a linear fashion through pulse shaping in a superstructured fiber Bragg grating. The scheme relies on chirping the signal to be retimed using cross-phase modulation with the broader parabolic clock pulses, and subsequently retiming it through linear propagation in a dispersive medium. We demonstrate the cancellation of up to 4-ps root-mean-square timing jitter for /spl sim/2-ps data.

We report the first experimental demonstration of the optical pushbroom - a novel type of all-optical pulse compression. In the optical pushbroom high intensity pump pulses, tuned well away from the resonance of a Bragg grating, modify the transmission of a weak probe tuned near to the grating's photonic bandgap. The clarity of the experimental results and their close agreement with numerical simulations highlight the tremendous potential of the fiber environment for the detailed study and practical application of nonlinear Bragg grating effects.

We demonstrate an in-band-pumped continuous-wave single-frequency distributed-feedback (DFB) fiber laser emitting at a wavelength of 1836 nm with an output power of /spl sim/5 mW. The laser is subsequently amplified to 345 mW using a master-oscillator power amplifier configuration. To the best of the authors' knowledge, this is the first report of a Tm-co-doped single-frequency DFB fiber laser that is in-band pumped at 1565 nm. The laser is confirmed to operate in a single mode, using a scanning Fabry-Pe/spl acute/rot interferometer.

An experimental investigation of the anomalous linewidth behavior in a distributed feedback fiber laser is presented. It is found that not only does the linewidth deviate from the Schawlow-Townes linewidth formula by increasing with pump and laser power, but it also varies significantly with the pumping configuration. These observations have potentially important implications for the design and operation of such fiber lasers.

We present a compact high-power continuous-wave tunable neodymium-doped double cladding fiber laser operating on three-level 4F3/2 - 4I9/2 transition with a maximum output power up to 810 mW. At 926.7 nm, it has a maximum slope efficiency of 49.3% against absorbed 808-nm pump. By compressing the fiber Bragg grating, 15-nm tuning range is achieved.