Study of optical fibre structures by atomic force microscopy

A novel method using atomic force microscopy (AFM) to study optical fibre structures at the fibre end-face has been successfully developed. The doping concentration profiles of fibres revealed by differential etching speeds in a saturated solution of ammonium bifluoride at room temperature (25°C) were obtained from AFM topographic images. The superior spatial resolution of AFM made it possible to resolve concentric structures a hundred times smaller than the feature, due to the difference in the known refractive index (n) of 1×10^-3. Fibres with small core diameters and anisotropic structures, such as polarization-maintaining fibres, were studied with ease.     

A self-frequency-locked multi-wavelength erbium-doped fibre ring laser

A self-frequency-locked laser in a 1.55 m band is demonstrated by adding a segment of samarium-doped fibre (SDF) into an erbium-doped fibre (EDF) ring laser cavity. The narrow band output signal, with a 3 dB down line width of less than 0.15 nm and a side-mode suppression ratio larger than 30 dB, is nearly polarization independent. A multiwavelength EDF ring laser source, using a 1×n synchronous optical switch and n segments of SDF with different lengths, is proposed.     

Operation of Tm3+ fiber laser at λ = 2.3 μm coupled to a silica fiber Bragg grating

A wavelength tunable all fibre laser which utilizes Tm³⁺-doped fluoride fibre as a gain mechanism emitting around a wavelength of = 2.3 m is reported. Tm³⁺-doped fluoride fibre was coupled to a fibre Bragg grating inscribed in silica. This laser was evaluated by employing it as a wavelength tunable source in a methane gas optical sensor. A minimum gas detectivity of 100 ppm m limited by the ability to prepare low concentrations of gases was achieved. Emission wavelength control was implemented by thermally tuning the grating, although this method may prove to be too slow and cumbersome for practical use. A model describing the coupling between the silica fibre Bragg grating and the Tm³⁺-doped fibre was developed and integrated with a fibre laser model. This theoretical framework was used to examine the optimum silica and fluoride fibre parameters to achieve the maximum coupling between the fibres, with the aim of further reducing the fibre laser threshold.     

High power diode-pumped triply doped fibre laser

A triply doped fibre laser is demonstrated for the first time. Using the 805 nm and 975 nm output from two diode laser sources to simultaneously pump a 2.1 m Yb³⁺, Tm³⁺, Ho³⁺-doped silica fibre laser, a maximum output power of 4.3 W was generated at slope efficiencies of 21% and 11% with respect to the launched 805 nm and 975 nm pump light, respectively. Whilst a number of energy transfer upconversion processes at the current dopant concentration levels appear to have impacted on the overall efficiency of the laser, the use of two separately pumped sensitiser ions to excite a single lasant ion has been shown to be a successful approach. PACS 42.55.Wd; 42.55.Xi; 42.60.Lh; 42.60.Pk     

Spectrum-sliced channel transmission over 94 km of dispersion-shifted fibre at 1.7 Gb s-1 using a double-stage fibre amplifier light source

Using a double-stage spectrum-sliced fibre amplifier light source pumped by a single 1.48 m laser diode, we have transmitted 1.7 Gb s^-1 bit rate spectrum-sliced channels over 94 km of dispersion-shifted fibre. The dispersion penalties were less than 0.1 dB even for the channel 12 nm away from the zero dispersion wavelength.Y. G. Chung is with the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea.     

Stimulated Raman scattering of barium laser output in a silica fibre

Strong stimulated Raman emission has been observed on three Stokes' lines at 1.608, 1.744 and 1.896m from 12.6 km of single-mode silica fibre pumped at 1.50m with the output of a pulsed barium vapour laser. A study has been made of the spectral distribution of fibre output as a function of peak input power to the fibre (up to 47 MW cm^–2), and the results are compared to theoretical predictions. Incomplete conversion of the pump to first Stokes', and first to second Stokes', lines is observed despite the length of the fibre.     

High-power diode-cladding-pumped Tm<Superscript>3+</Superscript>, Ho<Superscript>3+</Superscript>-doped silica fibre laser

The fundamental characteristics of a continuous-wave high-power diode-pumped Tm³⁺, Ho³⁺-doped double-clad silica fibre laser are presented. A maximum output power of 5.2 W was measured and was generated at a slope efficiency of 42 (44)% with respect to the launched (absorbed) pump power. At the optimum length of 7 m (_effL=2.9, where _eff is the effective absorption coefficient of the fibre and L is the fibre length), the fibre laser output was measured to have a centre wavelength of 2105 nm and a line width of 20 nm. The centre wavelength of the emission was tunable over a 32-nm extent when 0.68<_effL<3.28 or for a 6.2-m change in L. PACS 42.55.Wd; 42.55.Xi; 42.60.Lh; 42.60.Pk     

Fibre swelling during laser drilling of carbon fibre composites

Fine holes have been laser-drilled in bundles of carbon fibres, using a pulsed Nd:YAG laser (). Examination of the drilled fibres showed that some were significantly swollen, with diameter increases up to 60%. Scanning electron microscopy (SEM) and Raman spectroscopy have been used to investigate the effects of fibre type on the extent of laser-induced fibre swelling. It has been established that extensive swelling only occurred with low-modulus, poly acrylo-nitrile (PAN)-based fibres. Based on the Raman spectra obtained from both as-received and laser-drilled fibres, the swelling mechanism is attributed to simultaneous structural rearrangement and rapid, heating-induced volatilisation of impurities. A prior heat treatment was found to reduce the laser-induced swelling in low-modulus fibres. A 2D numerical heat flow model has been used to investigate the thermal fields generated during laser drilling of carbon fibres, and this information is correlated with the observed effects and the proposed swelling mechanism.     

Formaldehyde sensor using interband cascade laser based quartz-enhanced photoacoustic spectroscopy

A novel continuous-wave mid-infrared distributed feedback interband cascade laser was utilized to detect and quantify formaldehyde (H₂CO) using quartz-enhanced photoacoustic spectroscopy. The laser was operated at liquid-nitrogen temperatures and provided single-mode output powers of up to 12 mW at 3.53 m (2832.5 cm^-1). The noise equivalent (1) detection sensitivity of the sensor was measured to be 2.2×10^-8 cm^-1W(Hz)^-1/2 for H₂CO in ambient air, which corresponds to a detection limit of 0.6 parts in 10⁶ by volume (ppmv) for a 10 s sensor time constant and 3.4 mW laser power delivered to the sensor module. PACS 42.62.Fi; 72.50.+b     

A simple fibre Fabry-Pérot sensor

A simple fibre Fabry-Pérot sensor is described in which the uncoated cleaved ends of a lenght of single mode fibre are employed as the mirror surfaces to produce high visibility near perfect cosine-squared fringes in the back-reflected light. The experimentally observed signal-to-noise performance of the interferometer illuminated by a HeNe gas laser and a semiconductor diode laser is reported. In an accelerometer configuration the device displayed a below resonance sensitivity of ≥200 rad g^-1.     

Simple dispersion measurement of a long-span,single-mode fibre using the longitudinal mode spacing of a semiconductor laser

A simple method for measuring dispersion in long-span single-mode fibre, by using directly modulated multi-longitudinal mode semiconductor lasers, is proposed. Experimental values for single-mode fibre dispersion over a 44.3km span at 1.1 and 1.5m are 19 and 17 ps km^–1 nm^–1, respectively, and are in good agreement with Gloge's theory. The zero-dispersion wavelength estimated from the experimental results is about 1.28m, 8 nm shorter than the theoretical value.     

Phase conjugation in an optical fibre

We report experimental studies of phase conjugation by stimulated Mandelstam Brillouin scattering in multimode fibres. A dramatic decrease in the threshold power as compared to that (10⁴ to 10⁵ W) typically observed in conventional media was achieved. A ruby pulsed pump with 500-ns pulses was used. For the 7-m-long fibre, the threshold power was 50 W, the degree of phase conjugation was almost one, the pump-to-Stokes conversion efficiency was about 80%. For the 130-m-long fibre, the threshold power decreased to 10 W and the pump-to-Stokes conversion efficiency reached 100%. Polarization scrambling was observed in the 130-m-long fibre, but the Stokes wave with polarization coinciding with that of the pump wave was a phase-conjugate replica of the pump, as before.     

A sensitivity comparison of three photoacoustic cells containing a single microphone,a differential dual microphone or a cantilever pressure sensor

Three photoacoustic (PA) cells designed for trace-gas sensing were compared by measuring absorption by the P(15) rotational line of the vibrational combination band of acetylene using a distributed-feedback diode laser. Normalised sensitivities were determined for each cell by analysis of the signal to noise ratios of acquired spectra. The first cell was constructed in-house, and contained a single electret microphone held in a stainless steel tube. The second cell was a differential PA cell that contained two microphones housed in identical flow tubes, with one microphone to detect the PA signal and the other to determine background noise levels. The third cell contained a novel cantilever pressure sensor, movement of which was measured by a compact laser interferometer. Normalised sensitivities (2σ) of 3.1×10^-7, 1.7×10^-7 and 2.2×10^-9 cm^-1 W Hz^-1/2, respectively, were obtained. An erbium-doped fibre amplifier was used to amplify the laser power, and a detection limit of 9.8×10¹⁰ molecule cm^-3 was obtained using the cantilever pressure sensor PA cell, with a laser power of 1.17 W. This detection limit corresponded to a mixing ratio of 14.5 parts per billion by volume at 277 mbar. PACS 82.80.Kq; 42.62.Fi; 82.80.Gk     

Lead-free BNBT-6 piezoelectric ceramic fibre/epoxy 1-3 composites for ultrasonic transducer applications

Barium-modified bismuth sodium titanate, 0.94 ×(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ (BNBT-6), fine-scale piezoelectric fibres were fabricated using a viscous suspension spinning process (VSSP). The sintered BNBT-6 fibres with diameters of 300 m were fabricated into 1-3 composites with fibre volume fraction v_f of 0.2–0.5. Piezoelectric and dielectric properties of the 1-3 composites were measured. The electromechanical coupling coefficient k_t of a v_f=0.40 composite is 0.52. Properties of the VSSP fibres were calculated using the measured properties of the 1-3 composites. A v_f=0.40 composite was thinned down to 213-m thickness and constructed into an ultrasonic transducer. The pulse-echo response, bandwidth and insertion loss of the transducers were studied. The VSSP fibre composite transducer with v_f=0.40 has a centre frequency of 7 MHz with a bandwidth of 88%. The good performance indicated that the BNBT-6/epoxy 1-3 fibre composite transducer has potential for medical imaging applications. PACS 77.84.Dy; 77.84.Lf; 43.38.+n     

Mid-infrared trace-gas sensing with a quasi- continuous-wave Peltier-cooled distributed feedback quantum cascade laser

A recently developed distributed feedback quantum cascade laser (QCL) capable of thermoelectric-cooled (TEC) continuous-wave (cw) operation and emitting at 9 m is used to perform laser chemical sensing by tunable infrared spectroscopy. A quasi-continuous-wave mode of operation relying on long current pulses (5 Hz, 50% duty cycle) is utilized rather than pure cw operation in order to extend the continuous frequency tuning range of the quantum cascade laser. Sulfur dioxide and ammonia were selected as convenient target molecules to evaluate the performance of the cw TEC QCL based sensor. Direct absorption spectroscopy and wavelength-modulation spectroscopy were performed to demonstrate chemical sensing applications with this novel type of quantum cascade laser. For ammonia detection, a 18-ppm noise-equivalent sensitivity (1 ) was achieved for a 1-m absorption path length and a 25-ms data-acquisition time using direct absorption spectroscopy. The use of second-harmonic-detection wavelength-modulation spectroscopy instead of direct absorption increased the sensitivity by a factor of three, achieving a normalized noise-equivalent sensitivity of 82 ppbHz^-1/2 for a 1-m absorption path length, which corresponds to 2×10^-7 cm^-1Hz^-1/2. PACS 42.55.Px; 42.62.Fi; 07.88.+y     

3.5 μm optical communication experiment at 100 Mbits−1 using a PbCdS diode laser

A mid-infrared optical communication experiment using a lead-salt diode laser emitting at 3.5 m is reported. Bit-error rate measurements have been performed for pulse code modulated data at 100 Mbits^–1 using a fluoride glass fibre as a transmission channel. A bit-error rate better than 10^–10 was achieved at a signal-to-noise ratio of 20 dB.S. T. Eng has a joint appointment with the Jet Propulsion Laboratory, Pasadena, California 91109, USA.     

Absolute frequency measurement in the 28-THz spectral region with a femtosecond laser comb and a long-distance optical link to a primary standard

A new frequency chain was demonstrated to measure an optical frequency standard based on a rovibrational molecular transition in the 28-THz spectral region accessible to a CO₂ laser. It uses a femtosecond-laser frequency comb generator and two laser diodes at 852 nm and 788 nm as intermediate oscillators, with their frequency difference phase locked to the CO₂ laser. The rf repetition rate of the femtosecond laser was compared with a 100-MHz signal from a hydrogen maser, located at BNM-SYRTE. The 100-MHz signal is transmitted by amplitude modulation of a 1.55-m laser diode through a 43-km telecommunication optical fibre. As a first example, the absolute measurement of a saturation line of OsO₄ in the vicinity of the P(16) laser line of CO₂ is reported with a relative uncertainty of 10^-12, limited by the CO₂/OsO₄ frequency day-to-day reproducibility. The current limit on the stability of the frequency measurement is 4×10^-13 at 1 s. PACS 06.20.-f; 42.62.Eh; 06.30.Ft     

Ultrasensitive gas detection by quartz-enhanced photoacoustic spectroscopy in the fundamental molecular absorption bands region

A trace gas sensor based on quartz-enhanced photoacoustic spectroscopy with a quantum cascade laser operating at 4.55 m as an excitation source was developed. The sensor performance was evaluated for the detection of N₂O and CO. A noise-equivalent (1) sensitivity of 4 ppbv N₂O with 3 s response time to (1-1/e) of the steady-state level was demonstrated. The influence of the relevant energy transfer processes on the detection limits was analyzed. Approaches to improve the current sensor performance are also discussed. PACS 42.62.Fi; 43.35.Sx