Thermally stable optical fibre Bragg grating wavelength reference

The inherent temperature dependence of optical fibre Bragg gratings is caused mainly by the positive thermo-optic effect of the fibre core material, and it results in an increase of Bragg wavelength with temperature. Special mounting techniques may be designed in order to compensate this thermo-optic effect by counter-acting effects: decreasing mechanical strain, as well as a decreasing effective refractive index for the guided light wave by evanescent field interaction with a liquid of negative thermo-optic effect. Because of the non-linear interaction characteristics, exact temperature compensation is obtained for a certain temperature, which depends on the design parameters. Such stabilised fibre Bragg gratings find application as wavelength references in Bragg grating sensor networks.     

Stimulated Brillouin scattering as the referencing mechanism of an optical fibre intensity sensor

An optical fibre intensity sensor referenced by stimulated Brillouin scattering is presented. The optical sensor uses Fresnel reflection signal at the sensor fibre end and employs an adequate relationship between Brillouin and Rayleigh scattering and Fresnel reflection to have a referenced optical fibre intensity sensor addressed in reflection.     

Enhancement of the wavelength selectivity of a volume hologram by use of multimode optical fiber referencing

Wavelength multiplexing of a volume hologram was combined with multimode fiber referencing. A photorefractive volume hologram was recorded and probed by reference light diffracted from a multimode optical fiber. A tunable laser diode was used as a light source that was launched into the multimode fiber. As the wavelength of the source laser shifted, the speckle patterns from the multimode fiber changed. The wavelength selectivity of the volume hologram was enhanced by the combined effect of wavelength change and speckle change.     

Reflective fibre optical displacement sensors for the inspection of tilted objects

Modulation functions of fibre optical displacement sensors, based on the principle of reflective intensity modulation, are calculated. Effects of surface features, such as reflectivity slope and inclination of samples, on the sensor response are studied. These studies have been carried out for two types of samples: perfectly smooth ones, that only reflect a probe beam specularly; and samples considered as Lambertian objects, that diffuse the probe beam in all directions. Applications of both theoretical models for practical measurements are discussed, particularly for a dual-channel sensor, using two receiving fibres, that is able to compensate for the effects of sample surface features on the modulation functions.     

Dual wavelength lasers

Dual wavelength lasers are discussed, covering fundamental aspects on the spectroscopy and laser dynamics of these systems. Results on Tm:Ho:Er:YAG dual wavelength laser action (Ho at 2.1 μm and Er at 2.9 μm) as well as Nd:YAG (1.06 and 1.3 μm) are presented as examples of such dual wavelength systems. Dual wavelength lasers are not common, but there are criteria that govern their behavior. Based on experimental studies demonstrating simultaneous dual wavelength lasing, some general conclusions regarding the successful operation of multi-wavelength lasers can be made.     

Single LED based dual wavelength referenced optical fibre sensor system using intensity modulation

An intensity-modulated optical fibre sensor system is described which employs a single LED source to provide the measurand and the reference signals with two separate wavelength bands. The allocated wavelength bands are selected to minimize any differential intensity effects that may arise from LED thermal variations. The sensor system, comprising a transceiver unit connected to an optical displacement sensor using a single optical fibre, provides an output fully referenced for all major common-mode variations that are likely to occur. Performance characteristics for the prototype system are reported which show a linear displacement range of over 20 mm.     

Polarimetric fibre laser sensors using Er-doped fibre

A polarimetric fibre laser sensor with narrow polarization mode beat frequency bandwidth is demonstrated using a polarization-maintaining Er-doped fibre. The slope coefficients of 124kHz°C^-1 cm^-1 and 137kHz m^-1 are obtained for temperature change and longitudinal strain measurements, respectively. A new configuration of polarimetric fibre laser sensor is also demonstrated using a conventional Er-doped fibre as a gain medium and a short length of polarization-maintaining fibre as a sensing part.     

Temperature sensors and optical sensors

The dependence of temperature on the fluorescence lifetime and fluorescence intensity ratio using Stark sublevels and thermally coupled (close lying) levels in triply ionized rare earth ions, doped into a variety of glasses and fibers, have been reviewed. Also, it is claimed that not only for the two closely lying levels of triply ionized rare earth ion of the same kind, but also for two different triply ionized rare earth ions, having their excited levels very close to each other, may be used to monitor the temperature. PACS 42.70.Ce     

Refractive index design and validation for evanescent field power maximization in optical fibre sensors

The viability and operation of evanescent field-based optical fibre sensors is largely determined by the fraction of the total supported modal field power in the evanescent field. As this fraction is highly dependent on the refractive index profile of the fibre, one design strategy for fibre sensors is to maximize this field power fraction over the class of all refractive index profiles. This paper documents this design strategy for circular geometry optical fibres, where the goal is to maximize the evanescent field power fraction for a particular mode via the selection of an optimal refractive index profile. The axially symmetric profiles obtained approximate “holey” annular structures, the performance of which can be validated using existing waveguide analysis techniques.     

Dual wavelength Q-switched cascade laser

A diode-cladding-pumped dual wavelength Q-switched Ho3+-doped fluoride cascade fiber laser operating in the mid-infrared is demonstrated. Stable pulse trains from the (5)I6 → (5)I7 and (5)I7 → (5)I8 laser transitions were produced, and the μs-level time delay between the pulses from each transition was dependent on the pump power. At maximum pump power and at an acousto-optic modulator repetition rate of 25 kHz, the (5)I6 → (5)I7 transition pulse operated at 3.005 μm, a pulse energy of 29 μJ, and a pulse width of 380 ns; the (5)I7 → (5)I8 transition pulse correspondingly produced 7 μJ pulse energy and 260 ns pulse width at 2.074 μm. To the best of our knowledge, this is the first demonstration of a Q-switched fiber laser operating beyond 3 μm.     

Technique for referencing of fiber-optic intensity-modulated sensors by use of counterpropagating signals

We present a new all-optical fiber-referencing scheme for intensity-modulated sensors. It consists of a closed loop traversed by sensing and reference optical signals in opposite directions. With the proposed scheme the noise induced by power fluctuations of the optical source and mechanical perturbations can be greatly reduced. We experimentally demonstrate the efficiency of the scheme and discuss its use in a sensor array.     

Simultaneous generation and wavelength conversion of a pulse train from multi-wave mixing in an optical fibre

The insertion of three continuous-wave optical frequencies in a low-dispersion optical fibre resulted in the generation of a high-repetition-rate train of ultra-short pulses and its simultaneous wavelength conversion. Two of the frequencies were spaced by ∼0.17 THz and multi-wave mixing generated a frequency comb to which is associated a train of 1.6 ps pulses. Wave-mixing between the generated comb and the third input optical frequency then converts the pulse train into different wavelengths. The Split-Step Fourier method was applied to numerically simulate the generation/wavelength conversion and results are in good agreement with experiments.     

Liquid-core optical ring-resonator sensors

We have demonstrated a novel sensor architecture based on a liquid-core optical ring-resonator (LCORR) in which a fused silica capillary is utilized to carry the aqueous sample and to act as the ring resonator. The wall thickness of the LCORR is controlled to a few micrometers to expose the whispering gallery mode to the aqueous core. Optical characterization with a water-ethanol mixture shows that the spectral sensitivity of the LCORR sensor is approximately 2.6 nm per refractive index unit. A model based on Mie theory is established to explain the experimental results. The LCORR takes advantage of the high sensitivity, small footprint, and low sample consumption with the ring resonator, as well as the efficient fluidic sample delivery with the capillary, and will open an avenue to future multiplexed sensor array development.     

On the feasibility of optical fibre sensors for strain monitoring in thermoplastic composites under fatigue loading conditions

This study investigates the possibility of using optical fibres with Bragg gratings for measurements in thermoplastic composites under fatigue loading conditions. Two setups are considered: (i) the fibre is embedded in the composite and (ii) the grating is bonded externally. Detailed information is given on the principle of optical fibre measurements and the data acquisition for both setups. To verify the strain derived from the optical fibre, the strain is compared with extensometer measurements. A special design of the blades of the extensometer is presented, since the standard blades suffer from a loss of grip on the surface of the specimen. The material used for this study was a carbon fibre-reinforced polyphenylene sulphide.It can be concluded for both setups that the optical fibre survives over half a million loading cycles, without de-bonding of the fibre. The advantage of the external fibre over the embedded one is that it can be mounted after manufacturing of the plate, but it has a higher risk of being damaged during working conditions of the component.     

Dual wavelength dye laser incorporating distributed feedback

A dual wavelength dye laser has been constructed in which one wavelength is selected by distributed feedback and the other by a conventional external cavity tuned by a grazing incidence/Littrow grating pair. Both wavelengths are emitted collinearly and simultaneously, show the same temporal profile (10 ns pulse), have comparable linewidths (0.1/0.2 Å), are of comparable energies (0.5 mJ), and are independently tunable. The laser exhibits the low amplified spontaneous emission properties normally associated with distributed feedback lasers. Gain competition effects have been examined.     

Polymer optical fibre Bragg gratings based fibre laser

For the first time the fibre laser constructed from a polymer optical fibre Bragg grating is reported. The single frequency laser with the peak power of −5 dBm and signal to noise ratio greater than 45 dB has been achieved. Further examination demonstrates the excellent characteristics of the fibre laser. First, the fibre laser can be easily tuned over 35 nm by the simple axial tension method. Second, the fibre laser has the high strain sensitivity of 1.48 pm/με with the dynamic measurement range as large as 2.37%.     

Giga-bit optical receiver for plastic optical fibre

An optical receiver with high sensitivity and linearity specially designed for Giga-bit communications over small-bandwidth high-attenuation multimode plastic optical fiber is presented. An automatic gain control transimpedance amplifier and linear post amplifiers are used to maintain a good performance with multilevel modulation. Using multilevel signaling and large-diameter integrated photodiodes make the presented optical receiver suitable for large core plastic optical fiber. For a wavelength of 675 nm, a sensitivity of −26.3 dB m (BER = 10⁻⁹) at 500 Mb/s is presented by a binary signal. A data rate of 1 Gb/s and a sensitivity of −19.8 dB m (BER = 10⁻⁹) are achieved with four-level pulse amplitude modulation.     

Effective wavelength scaling for optical antennas

In antenna theory, antenna parameters are directly related to the wavelength lambda of incident radiation, but this scaling fails at optical frequencies where metals behave as strongly coupled plasmas. In this Letter we show that antenna designs can be transferred to the optical frequency regime by replacing lambda by a linearly scaled effective wavelength lambda(eff)=n(1)+n(2)lambda/lambda(p), with lambda(p) being the plasma wavelength and n(1), n(2) being coefficients that depend on geometry and material properties. It is assumed that the antenna is made of linear segments with radii R < lambda. Optical antennas hold great promise for increasing the efficiency of photovoltaics, light-emitting devices, and optical sensors.