Detonation velocity measurements with uniform fibre Bragg gratings.

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.

Detonation Velocity Measurements Using Rare-Earth Doped Fibres.

In this paper, a simple detonation velocity measurement scheme is presented, which exploits the length-dependent amplified spontaneous emission (ASE) power emitted by off-the-shelf Er-doped fibres. This measurement scheme is first calibrated using cutback tests, so that minimal processing is required between data collection and velocity readout. We then demonstrate the use of this method in an explosive cylinder test and achieve a spatial resolution of approximately ±2 mm, owing to its implementation in a helical geometry. Alongside the standard Er fibres, a specially made, high-concentration Er/Yb-doped fibre is also calibrated, which demonstrates a potential spatial resolution approaching ±20 µ m.

Optimised Chirped Fibre Bragg Gratings for Detonation Velocity Measurements.

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-chirp-rate, reflectivity and apodisation-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.

Simple technique of determining the fibre diameter during etching.

We present a technique to measure, in situ, the diameter of an optical fibre during etching using a fibre Bragg grating (FBG). Differential shifts between the fundamental mode, and the higher-order Bragg resonances generated by the etching process are used to determine the diameter of a standard optical fibre (SMF28) with a precision of ~200nm. Numerical simulations are also carried out to investigate the overlap of the evanescent field of the fundamental mode and higher-order modes (LP11, LP02, LP21 and LP12). These simulations were used to find and calibrate the diameter of the etched-cladding fibre. Subsequently, the technique was used to experimentally determine the refractive index of two buffered hydrofluoric (BHF) acid solutions, (20:1) and (7:1), to be ~1.360 ± 0.005 and ~1.370 ± 0.005 respectively @ ~1550nm. The refractive index of both BHF solutions is calibrated against known indices of liquids and solvents, including deionised water, methanol, acetone, ethanol, isopropanol, and ethylene glycol. The numerical simulations and experimental results are in very good agreement. We believe the approach presented in this work provides a controlled technique to achieve precise target diameter of the etched fibres in real time.

Birefringent Bragg Grating in C-Shaped Optical Fiber as a Temperature-Insensitive Refractometer.

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.

Functionalized Fiber End Superstructure Fiber Bragg Grating Refractive Index Sensor for Heavy Metal Ion Detection.

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.

Compensation-free broadband entangled photon pair sources.

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.

Detailed study of four-wave mixing in Raman DFB fiber lasers.

We both experimentally and numerically studied the ultra-compact wavelength conversion by using the four-wave mixing (FWM) process in Raman distributed-feedback (R-DFB) fiber lasers. The R-DFB fiber laser is formed in a 30 cm-long commercially available Ge/Si standard optical fiber. The internal generated R-DFB signal acts as the pump wave for the FWM process and is in the normal dispersion range of the fiber. Utilizing a tunable laser source as a probe wave, FWM frequency conversion up to ~40 THz has been demonstrated with conversion efficiency > -40 dB. The principle of such a wide bandwidth and high conversion efficiency in such a short R-DFB cavity has been theoretically analyzed. The simulation results match well with the experimental data.

All-fiber frequency-doubled visible laser.

All-fiber ns-pulsed visible laser at λ=521 nm is realized by frequency doubling an Yb-doped fiber laser with a periodically poled silica fiber. A 50-mW second-harmonic (SH) output power is produced that is over 6-orders of magnitude greater than previous results obtained with poled fibers in the visible spectral range. The normalized conversion efficiency of 0.3%/W is to date the largest demonstrated with poled fiber technology. Furthermore, 21% conversion efficiency is achieved for the doubling of 8-ps pulses from a neodymium-doped yttrium vanadate solid-state laser. The advances are made possible by the precision and flexibility offered by using the continuous periodic UV erasure, as opposite to photolithographic methods, for the fabrication of over 20-cm-long χ(2)-gratings for quasi-phase matched SH generation.

104 MHz rate single-shot recording with subpicosecond resolution using temporal imaging.

We demonstrate temporal imaging for the measurement and characterization of optical arbitrary waveforms and events. The system measures single-shot 200 ps frames at a rate of 104 MHz, where each frame is time magnified by a factor of -42.4x. Impulse response tests show that the system enables 783 fs resolution when placed at the front end of a 20 GHz oscilloscope. Modulated pulse trains characterize the system's impulse response, jitter, and frame-to-frame variation.

Ultrawide-range four-wave mixing in Raman distributed-feedback fiber lasers.

We report ultrawide-range and highly efficient wavelength conversion by exploiting four-wave mixing (FWM) in Raman distributed-feedback (R-DFB) fiber lasers. The lasers are 30 cm long center π phase-shifted DFB gratings UV written in commercially available germano-silica (Ge/Si) single-mode fibers (PS980 from Fibercore Ltd., and UHNA4 from Nufern). The R-DFB lasing signal acts as a pump wave for the FWM process within the DFB cavity, and the obtained FWM conversion efficiency is around -25 dB with a maximum wavelength conversion range of 112 nm.

Highly efficient Raman distributed feedback fibre lasers.

We demonstrate highly efficient Raman distributed feedback (DFB) fibre lasers for the first time with up to 1.6 W of continuous wave (CW) output power. The DFB Bragg gratings are written directly into two types of commercially available passive germano-silica fibres. Two lasers of 30 cm length are pumped with up to 15 W of CW power at 1068 nm. The threshold power is ~2 W for a Raman-DFB (R-DFB) laser written in standard low-NA fibre, and only ~1 W for a laser written in a high-NA fibre, both of which oscillate in a narrow linewidth of <0.01 nm at ~1117 nm and ~1109 nm, respectively. The slope efficiencies are ~74% and ~93% with respect to absorbed pump power in the low-NA fibre and high-NA fibre respectively. Such high conversion efficiency suggests that very little energy is lost in the form of heat through inefficient energy transfer. Our results are supported by numerical simulations, and furthermore open up for the possibility of having narrow linewidth all-fibre laser sources in wavelength bands not traditionally covered by rare-earth doped silica fibres. Simulations also imply that this technology has the potential to produce even shorter R-DFB laser devices at the centimetre-level and with mW-level thresholds, if Bragg gratings formed in fibre materials with higher intrinsic Raman gain coefficient than silica are used. These materials include for example tellurite or chalcogenide glasses. Using glasses like these would also open up the possibility of having narrow linewidth fibre sources with DFB laser oscillating much further into the IR than what currently is possible with rare-earth doped silica glasses.

Laser-induced crystalline optical waveguide in glass fiber format.

We report on the first fabrication of a glass fiber based laser-induced crystalline waveguide. The glass and crystal are based on the stoichiometric composition of (La,Yb)BGeO(5). A laser induced waveguide has been fabricated on the surface of a ribbon glass fiber using milliwatt-level continuous wave UV laser radiation at a fast scanning speed. Evidence of crystallinity in the created structure was observed using micro-Raman spectroscopy and scanning electron microscopy. Preliminary investigations on the waveguiding behavior and the nonlinear performance in the crystalline waveguide are reported.

Sub-watt threshold, kilohertz-linewidth Raman distributed-feedback fiber laser.

We report a low-threshold, narrow linewidth Raman distributed-feedback (R-DFB) fiber laser at 1109.54 nm based on a 30 cm long center π phase-shifted Bragg grating written directly in a commercially available germano-silica (Ge/Si) fiber. The R-DFB was pumped by a continuous-wave (CW) linearly polarized fiber source at 1064 nm, and the threshold power, full width at half-maximum (FWHM) linewidth and slope efficiency with respect to the incident pump power are measured to be 440 mW, <2.5 kHz (measured with a 29.75 km fiber delay line) and 13.5%, respectively. An rf spectrum analyzer confirms that the R-DFB fiber laser exhibits single-frequency performance.

Direct generation of polarization-entangled photon pairs in a poled fiber.

We experimentally demonstrate the direct generation of polarization-entangled photon pairs in an optical fiber at room temperature by exploiting type-II phase-matched spontaneous parametric down-conversion. A second-order nonlinearity is artificially induced in the 17-cm-long weakly birefringent step-index fiber through the process of thermal poling, and quasi-phase-matching allows for the generation of entangled photons in the 1.5-micron telecom band when the fiber is pumped at 775 nm. A greater-than 80:1 coincidence-to-accidental ratio is achieved, limited mainly by multiphoton pair generation. Without the need to subtract accidentals or to compensate for walk-off, the raw two-photon interference visibility is found to be better than 95%, and violation of Bell's inequality is observed by more than 18 standard deviations. This makes for a truly alignment-free, plug-and-play source of polarization-entangled photon pairs.

OTDM to WDM format conversion based on quadratic cascading in a periodically poled lithium niobate waveguide.

We propose and demonstrate error-free conversion of a 40 Gbit/s optical time division multiplexed signal to 4 x 10 Gbit/s wavelength division multiplexed channels based on cascaded second harmonic and difference frequency generation in a periodically poled lithium niobate waveguide. The technique relies on the generation of spectrally (and temporally) flat linearly chirped pulses which are then optically switched with short data pulses in the nonlinear waveguide. Error-free operation was obtained for all channels with a power penalty below 2dB.

Multi-watts narrow-linewidth all fiber Yb-doped laser operating at 1179 nm.

An all-fiber, narrow-linewidth, high power Yb-doped silica fiber laser at 1179 nm has been demonstrated. More than 12 W output power has been obtained, corresponding to a slope efficiency of 43% with respect to launched pump power, by core-pumping at 1090 nm. In order to increase the pump absorption, the Yb-doped fiber was heated up to 125 degrees C. At the maximum output power, the suppression of amplified spontaneous emission was more than 50 dB. Furthermore, theoretical work confirms that the proposed laser architecture can be easily scaled to higher power.

Aperiodically poled silica fibers for bandwidth control of quasi-phase-matched second-harmonic generation.

Precise control of the bandwidth of quasi-phase-matched second-harmonic generation in silica fibers is realized through chirped-period poling. The bandwidth is expanded by a factor of 33 over a uniform-period poled fiber of the same interaction length.

Measurement of chi((2)) symmetry in a poled fiber.

We measure the values of individual chi((2)) tensor components in a birefringent periodically poled silica fiber through spectrally separated type I and type II second-harmonic generation. We demonstrate that the chi((2)) tensor symmetry is consistent with that of chi((3)) in silica and thereby provide experimental evidence that this chi((2)) originates from a chi((3)) process.

High-average-power second-harmonic generation from periodically poled silica fibers.

The generation of 236 mW of second-harmonic power in a 32-cm-long periodically poled silica fiber, corresponding to an average conversion efficiency of 15.2+/-0.5%, is reported. This represents the highest normalized second-harmonic conversion and the highest average second-harmonic power ever reported for a periodically poled silica fiber, to our knowledge. The enhancement is attributed to an improved design of the specialty twin-hole fiber and the extension of the nonlinear interaction length.