The rotation of the polarization in low birefringence monomode optical fibres due to geometric effects

Monomode optical fibres of very low intrinsic linear and circular birefringence have been developed for use in polarimetric optical fibre sensors, most notably the fibre optic current sensor. The polarization of light travelling along such fibres is known to be modified by external stress applied to the fibre. In this paper it is shown that the polarization state may also be rotated if the path of the fibre is bent in a nonplanar curve. A theory is developed which enables the effect to be quantified and the results are shown to agree with some measurements on a fibre bent into a helix.     

Extrinsic fibre-optic sensors for remote measurement: Part two

The impact of incorporating fibre optic components into well established optical techniques such as laser Doppler velocimetry, non-contact vibration measurement, holography and light scattering, is discussed in the two sections of this paper. The possibilities range from simply including a fibre ‘optic link’ in a conventional system to facilitate measurements in difficult or remote areas, to the realization of entirely new optical configurations in which virtually all of the conventional optical components have been replaced by their fibre-optic equivalents. In part two laser velocimetry, vibration measurement, holography and scattering are discussed.     

Transfer function of a multimode step-index fibre: a geometrical optical investigation

The transfer function of a multimode step-index fibre has been calculated by means of geometrical optics taking into account mode coupling and leaky rays. The light source may have axial symmetry (light emitting diode) or be line shaped (semiconductor laser). Radial displacement of the line source relative to the fibre axis is allowed. Calculations performed for a large numerical aperture fibre (N.A.=0.475) with a coupling length of 2 km show that leaky rays and radial displacement of the line source are significant for fibre lengths shorter than 200m. The influence of the width of the entrance beam is significant for lengths up to 5 km.Varying coupling length and steady state angle for a fibre length of 150 m calculations show that the bandwidth varies between 15 and 95 MHz for penalty losses between 8 and 18 dB.Calculations have been compared with direct baseband frequency measurements for a high-loss fibre with a large numerical aperture and excellent agreement has been found.     

Semiconductor cw laser packages with full hermeticity integral power monitoring and direct fibre optic launching

In recent years a large number of papers have been presented describing both the physics of semiconductor lasers and the design of fibre optic systems. The interface between these two areas of technology however has largely been ignored. This paper aims to describe a high launched power, hermetic cw laser package with integral photodetector for feedback control, designed to meet the needs of the fibre optic market. It is believed that this cw packaged device bridges the gap by providing the system designer with a component which can be incorporated with ease and confidence in his fibre optic system. The laser source is described with the design concepts of the package, its construction and performance.     

Temperature and strain discrimination using a single tilted fibre Bragg grating

A fibre optic sensor capable of discriminating between temperature and strain, using a single fibre Bragg grating, is presented. The technique exploits the core-cladding mode coupling of a tilted fibre Bragg grating (TFBG). The core and cladding modes exhibit different thermal sensivities, while the strain sensivities are approximately equal. Monitoring the core-core mode coupling resonance and the core-cladding mode coupling resonance of the TFBG spectrum allows the separation of the temperature and strain induced wavelength shifts.     

GHz optical communication experiments using lead-salt laser diodes in the 3.5–5.5μm wavelength region

The modulation performance of lead-salt laser diodes suitable for mid-infrared fibre optic communication systems has been investigated. The electron and photon lifetime for a PbCdS laser have been determined to 3.1 ns and 3.3 ps, respectively. Moreover, an experiment using short lengths of fluoride fibres shows the feasibility of GHz fibre optic communication links in the mid-infrared region using lead-salt lasers if the potential ultra-low loss of mid-infrared optical fibres can be realized.     

Single Mode Er-Doped π-Phase-Shifted Distributed Feedback Fibre Grating Laser

A novel method incorporating the shielded method and the post-processing method has been proposed to fabricate the π-phase-shifted fibre grating. Then an Er-doped π-phase-shifted distributed feedback fibre grating laser has been fabricated using the grating. The laser threshold is 20mW. When pumped with 90mW light at 980nm, the laser gives an output of 1.1mW. Its signal-to-noise ratio is better than 60dB. It is demonstrated that the laser is single mode operation by means of a Fabry--Perot scanning interferometer.     

Extrinsic fibre-optic sensors for remote measurement: Part one

The impact of incorporating fibre-optic components into well established optical techniques such as laser Doppler velocimetry, non-contact vibration measurement, holography and light scattering, is discussed in the two sections of this paper. The possibilities range from simply including a fibre ‘optic link’ in a conventional system to facilitate measurements in difficult or remote areas to the realization of entirely new optical configurations in which virtually all of the conventional optical components have been replaced by their fibre-optic equivalents. In part one, fibre properties, optical sources, directional couplers and modulators are discussed.     

Port wine spectronephelometry

The main purpose of this research was to create a portable equipment capable of measuring colour and turbidity during production of port wine. Actually, human, chemical and colour analysis are performed in key production stages for quality control. Several wine transformation processes are done without any kind of control. By using a fibre optic illuminator with a tungsten/halogen light source and a small spectrometer with fibre optic input, it was possible to perform angular measurements in wine samples. We have adapted these optical devices with a specially designed mechanical equipment which allows the passage of the wine during production. The spectral measurements were acquired in four different angular positions in a bypass of the mechanical support. This way we could determine colour coordinates and spectral transmittance and turbidity for three stages of a Ruby production. The entire equipment assembly had been previously calibrated in terms of colour and turbidity determination. The obtained results are in agreement with wine transformations between measurements, showing a small variation in colour and a high diminishing in spectral turbidity when filtration was performed. Transmittance has also increased after filtration turning the wine more limpid as it should be for human appreciation.     

Hand-held laser welding of metals using fibre optics

A fibre optic delivery system has for the first time been used in laser welding of metals. The optical fibre is flexible and has a length of 140 cm. It can transmit pulsed energies from a Nd-YAG laser in excess of 25 J with an efficiency of 85%. Potential applications in industry and in dentistry are discussed.     

The measurement of sperm motility by the fibre optic Doppler anemometer as a prediction of bovine fertility

The fibre optic Doppler anemometer (FODA) has been used to develop an accurate quantitative method of routinely assessing bull fertility. This method is of importance to the artificial insemination industry because the present qualitative estimation, performed by viewing semen using a microscope, can only set broad limits of quality.Laser light from the FODA was directed into diluted semen samples and the back scattered light was measured. A digital correlator was used to calculate the signal correlation of the back scattered light. The resultant data curves were interpreted in terms of the collective motility and swimming speed of the spermatozoa using a microcomputer. These two parameters are accepted as being indicative of fertility. The accuracy of this method is demonstrated by examination of results obtained in an experiment where enzymes, thought to alter fertility, were added to semen. The effect of the enzymes on the swimming speed and motility was clearly demonstrated.     

86-km optical link with a resolution of 2 × 10<Superscript>-18</Superscript> for RF frequency transfer

RF frequency transfer over an urban 86 km fibre has been demonstrated with a resolution of 2×10^-18 at one day measuring time using an optical compensator. This result is obtained with a reference carrier frequency of 1 GHz, and a rapid scrambling of the polarisation state of the input light in order to reduce the sensitivity to the polarisation mode dispersion in the fibre. The limitation due to the fibre chromatic dispersion associated with the laser frequency fluctuations is highlighted and analyzed. A preliminary test of an extended compensated link over 186 km using optical amplifiers gives a resolution below 10^-17 at 1 day.     

Solid state and fibre optic laser doppler velocimeters

Laser Doppler velocimetry (LDV) has become established as an important technique for the measurement of velocities of macroscopic objects and fluids: the dynamic range is large (～10^-6-10⁵ ms^-1) and the measurement is absolute and non-invasive. However, the size and cost of LDV has restricted its use in some areas. This paper presents two separate approaches to reduce these problems: we describe a compact LDV system incorporating a solid state laser diode and also an investigation of the feasibility of a fibre optic LDV system in which the conventional optical components are replaced by fibre optics.The experimental arrangement used for the solid state LDV system was of the Doppler difference type; i.e. a system of parallel interference fringes is focused in the measurement volume, so that a particle passing through this volume produces a scattered light signal which is intensity modulated. In its simplest form, the technique cannot determine the direction of motion of the particle, but this difficulty may be overcome by causing the fringes to ‘move’ within the measurement volume with known velocity. In the present experiments, the laser output frequency was modulated by modulating its drive current; since the path lengths of the two beams interfering in the measurement volume were unequal, fringe motion was achieved.The fibre optic LDV experiment was also of the Doppler difference type, and it was demonstrated that the necessary stabilised interference fringe system could be projected using a fibre optic system. An electronic servo was devised to compensate for the random differential thermal drifts in the fibres which would otherwise have produced unacceptable drifts in the fringe pattern.     

Fibre optic target reflectivity sensor

This paper reports the principle of operation, the design aspects, experimentation and performance of a fibre optic target reflectivity sensor to examine the correlation between the detector output, variation in material type and the reflectivity properties of the materials tested. The device consists of a fibre optic transmitter, a fibre optic probe, target and a photodiode detector. The fibre optic probe consists of two well-polished PMMA (polymethyl methacrylate) fibres cemented together along some distance over the length. The principle of fibre optic lever displacement sensors is applied. Material effects are examined by preparing a variety of samples namely gold coated mirror, copper, brass, aluminium, steel and galvanized iron using the same polishing techniques. It is found that the response of the sensor changes with change of target surface. The results show that the fibre optic probe is capable of discriminating between materials. With the use of commercially available fibre, source and detector, the set-up proves to be simple, highly sensitive, low cost and versatile one, which can be adopted for on-line measurement or inspection of test components.     

Development of fibre optic sensing systems—A review

A novel type of sensing system, in which we utilise optical fibres as sensing elements or transmission lines, is currently being developed. Some fibre optic sensing systems have already been put to practical use. Their advantages are outlined, and the principles, performance, and problems of some typical fibre optic sensing systems are described in detail.     

The limits to incoherent optical power launching into lightguides

Large diameter optical fibres and fibre bundles are currently used in a growing number of fields for low and medium range transmission of light. This paper presents a compilation and an analysis of theoretical limits and possible practical techniques in the endeavour towards an optimum launching of light emitted by incoherent sources into large diameter (geometric optic) lightguides. The maximum attainable incoherent optical power flows in lightguide transmission are estimated for various high intensity light sources.     

Squeezed light from microstructured fibres: towards free-space quantum cryptography

Amplitude-squeezed pulsed light has been produced using a microstructured silica fibre. By spectrally filtering after the non-linear propagation in the fibre a squeezing value of -1.7 dB has been measured. A quantum key distribution scheme based on squeezed light from such microstructured fibres is proposed. Received: 9 July 2001 / Published online: 23 November 2001     

Coupled metallic ring gap waveguide

Employing the surface plasmon polaritons (SPPs), a kind of coupled metallic squareness ring waveguide structure is presented. Its properties has been analyzed with the finite different time domain method and the coupling length has been derived from the coupled mode theory. It is demonstrated that the SPPs excited by the light with different wavelength will come out from different output port due to different coupling length. By appropriately designing the structure, it can be utilized to realize some optics devices such as multiple-wavelength sorter and beam splitter. This will break through the diffraction limit of traditional optical devices.     

Random distributed feedback fibre lasers

The concept of random lasers exploiting multiple scattering of photons in an amplifying disordered medium in order to generate coherent light without a traditional laser resonator has attracted a great deal of attention in recent years. This research area lies at the interface of the fundamental theory of disordered systems and laser science. The idea was originally proposed in the context of astrophysics in the 1960s by V.S. Letokhov, who studied scattering with “negative absorption” of the interstellar molecular clouds. Research on random lasers has since developed into a mature experimental and theoretical field. A simple design of such lasers would be promising for potential applications. However, in traditional random lasers the properties of the output radiation are typically characterized by complex features in the spatial, spectral and time domains, making them less attractive than standard laser systems in terms of practical applications. Recently, an interesting and novel type of one-dimensional random laser that operates in a conventional telecommunication fibre without any pre-designed resonator mirrors–random distributed feedback fibre laser–was demonstrated. The positive feedback required for laser generation in random fibre lasers is provided by the Rayleigh scattering from the inhomogeneities of the refractive index that are naturally present in silica glass. In the proposed laser concept, the randomly backscattered light is amplified through the Raman effect, providing distributed gain over distances up to 100 km. Although an effective reflection due to the Rayleigh scattering is extremely small (∼0.1%), the lasing threshold may be exceeded when a sufficiently large distributed Raman gain is provided. Such a random distributed feedback fibre laser has a number of interesting and attractive features. The fibre waveguide geometry provides transverse confinement, and effectively one-dimensional random distributed feedback leads to the generation of a stationary near-Gaussian beam with a narrow spectrum. A random distributed feedback fibre laser has efficiency and performance that are comparable to and even exceed those of similar conventional fibre lasers. The key features of the generated radiation of random distributed feedback fibre lasers include: a stationary narrow-band continuous modeless spectrum that is free of mode competition, nonlinear power broadening, and an output beam with a Gaussian profile in the fundamental transverse mode (generated both in single mode and multi-mode fibres).     

内腔自适应光学系统校正激光器畸变

固体激光器中,增益介质由于热沉积产生的热畸变严重影响了激光器的稳定性、输出功率和光束质量。研究了一种内腔自适应光学系统校正激光器腔内畸变的方法,利用微机电变形反射镜作为固体激光器的内腔全反射镜,通过控制变形镜的面型改善激光器输出光束的模式及功率。从腔外引入的一束信标光通过激光器内腔后反射出腔外,用波前探测器可测得激光器工作时腔内畸变对此信标光的影响,并通过搭建的自适应光学系统平台可闭环校正此畸变。实验结果表明,闭环校正后,激光器的输出功率提高了近3倍,且激光光束质量得到了明显的改善。