The status of single-mode optical fiber measurements and standardization is critically reviewed with a discussion the possible evolutions. Both advanced refinements of already established methods and new techniques called by the most recent evolutions in the field (e.g., for active fiber characterization) are considered.

The cut-back method is the standard test method to check the attenuation of optical fibers. The advantages of this method are low uncertainty, good reproducibility, and an applicability in a broad spectral range. But for some applications, especially in the field service, the optical time domain reflectometry seems to be more useful because (1) this method is non-destructive, (2) measurements can be made from one end of the fiber, and (3) the back-scattered signal contains information about the longitudinal homogeneity of the fiber or the fiber system. For an approval of this technique as a second standard test method, an uncertainty of 0.01 dB/km of the attenuation coefficient measurement is required. This small uncertainty demands a calibration of the loss scale of the optical time domain reflectometer (OTDR) used. Therefore, a calibration procedure is proposed using a standard fiber as a scale unit. The specification of this fiber, the preparation as a standard and its calibration in an accredited calibration laboratory, are discussed. An uncertainty of about 0.005 dB should be achievable in attenuation measurement of the standard. The calibration of the power scale of the OTDR with the aid of transfer standard, lead-in fiber and/or attenuator, and a proposal for linearizing the scale of power response are presented.

The technique of attenuation measurement using an optical time domain reflectometer (OTDR) was introduced in 1976. Over the years the features, size, and performances of OTDRs have steadily improved. Today the OTDR is a well-established, mature, and ubiquitous test set. The success of the OTDR is due to its unique ability to characterize key parameters of an optical- fiber link, such as the fiber attenuation coefficient, fiber length, and splice loss and location. These measurements can be made having access to one end of the fiber only. Despite a widespread reliance upon OTDR measurements, instrument performance, specification, and calibration remain unstandardized. Standardized OTDR calibration procedures are still missing even for most fundamental parameters. This lack of standardization confuses the OTDR users community when selecting an OTDR, interpreting measurement results, and comparing measurements made with different OTDRs. In 1988 the International Electrotechnical Commission (IEC) Technical Committee 86 (TC 86) Working group 4 (WG 4): Metrology, began work on OTDR calibration techniques concerning attenuation and distance measurements. As one of the basic laboratories of the Bureau National de Metrologie (BNM) the Laboratoire Central des Industries Electriques (LCIE) is designated to work in the field of fiber optics. As a national representative it develops and/or sets new methods for standardization in collaboration with national and international partners. This paper presents the techniques related to research leading to a French proposal for IEC/TC 86/WG 4 OTDR calibration. The methods worked out by the French group are passive techniques. Their aim is to permit OTDR calibration in order to reduce resulting effects of uncertainties arising from fiber characteristics. Two complementary methods are presented: (1) A coupler method proposed by the Centre National d'Etudes des Telecommunications (CNET). (2) A cavity method proposed by the Centre d'Etudes et de Recherches de Toulouse (CERT).

The absolute spectral responsivity, the linearity, the response uniformity across the sensitive surface, and the angular response are the main radiometric characteristics of any detector measuring optical power. These characteristics have been determined for two Ge photodiodes at the wavelengths of 1300 and 1550 nm. The measurement techniques used, the results and their uncertainties are presented. Based on them, the measurement conditions that have to be met to perform accurate measurements of the power leaving a fiber are established.

Since research began on optical fiber sensors, claims have been made for their safety in hazardous environments without objective evidence. Research has, however, now commenced which will establish quantitatively the safe operating power levels for employing light-based techniques in such environments. This paper describes the current state of knowledge in this field in relation to one potentially hazardous mechanism: a particle being heated by incident radiation to the point where an explosion is triggered. The relative lack of knowledge has led to the establishment of a new European R&D project, involving participants in France, Germany, and the UK. This program is described and placed in context as an enabling step to encourage wider industrial use of optical measurement techniques.

Measurement needs, some very novel, arise at all stages of the development, manufacture, commercial exploitation and use of optical fibers. Measurement standards for fiber parameters enable users and manufacturers to verify the accuracy of their results and, hence, have confidence in their measurements. The facilities developed at the National Physical Laboratory to provide measurement standards for the physical and transmission properties of optical fibers are described and the sources of error are discussed.

Special fibers are mainly prepared by material system modification or structure change of the conventional fused silica fiber with the UV-curable resin coating. Material system modification of the fibers is reviewed, and special structures introduced in the fiber cross section and fiber longitudinal direction are also explained. Special fiber are tolerable to more influence by the physical parameters such as the temperature, strain, pressure, rotation, electric field, magnetic field, and moisture. Measurement of each physical parameter with special fibers are explained.

Modal performances of multimode optical passive components are measured by use of a selective excitation technique. Pure LPm 1 modes up to LP15 1 mode are generated in a Ti:Sapphire laser cavity and launched in the input fiber of the component under test. Power measurements and modal analysis are operated at the output. Azimuthal dependency of LP1 1 mode excitation coefficient is used in an accurate centering technique of monomode and multimode splices or connectors.

The importance of measuring multiphoton processes in photosensitive glasses is discussed.

The results of extensive studies of the attenuation and Raman spectra of all silica (AS), plastic clad silica (PCS), and hard clad silica (HCS and HCR) fibers are presented. A broadband fluorescence centered around 658 nm was observed in the unirradiated fibers and HCR fibers exposed to a dose of 35 rad. The intensity of this band was found to decrease and broaden with irradiation. Another band around 629 nm was observed to increase in intensity with irradiation. The 1350 cm-1 peak to the Raman spectra, observed by Lan et. al. in both boron- and phosphorous-doped fused silica fibers was originally assigned to boron and phosphorous-doped fused silica fibers was originally assigned to boron and phosphorous dopants even though there is some evidence that it is not present in glasses containing a high concentration of these dopants. Results suggest that this peak might be due to the interaction of chlorine with boron or phosphorous dopants. The presence of chlorine in some fibers is not now thought to be responsible for the 1350 cm-1 peak, which is absent from the Raman spectra of high-purity silica fibers with high-chlorine concentrations (HCR type fiber). The analysis of attenuation spectra of 'pure' silica fibers indicated that the spectral differences are unlikely to be due to differing OH concentrations alone and that the different manufacturing processes also play a part. A broadband absorption around 625 nm was observed in a high OH-content fiber receiving 105 Rad dose and is thought to originate from non-bridging oxygen hole center (NBOHC) defects.

The temperature and wavelength dependence of bending loss in monomode optical fibers is reported. The bend-induced optical loss in a monomode optical fiber has been studied as a continuous function of wavelength in the range 1.2 - 1.6 micrometers , and as a function of temperature in the range 20 - 60 degree(s)C. The loss was observed to be an oscillatory function of wavelength for a fixed temperature, and to be a oscillatory function of temperature for a fixed wavelength. These observations were explained by using a model based on the interference between the core-guided mode and a 'whispering-gallery' mode ejected from the core and propagating in the cladding and buffer of the fiber. The wavelengths of the minima in the oscillatory function for fixed temperature and the temperature spacing for fixed wavelength were accurately predicted by the model for a whispering-gallery mode propagating in the buffer of the fiber.

A novel time-domain method with which the dynamic phase modulation index can be directly measured from the waveform of the photocurrent, which corresponds to the interferometer output, is described. Its potential advantage for the measurement of large dynamic-phase deviations in optical or fiberoptic interferometers is also discussed.

A simple theory was developed to study the effects of Kerr nonlinearity on the electromagnetic-field coherence. A symmetric nonlinear Mach-Zehnder optical fiber interferometer is analyzed. The field is perturbed in phase and amplitude. The statistical properties of the output intensity are calculated. The effects of nonlinearity are shown as positive or negative depending on the phase difference between the interferometer arms.

Adiabatically tapered single-mode fibers provide a novel and effective method of gaining access to the optical field inside an optical fiber. The fabrication of low-loss tapered fibers with waist diameters of less than one micron is described. By surrounding a tapered fiber with laser-dye solution, compact very efficient amplifiers, saturable absorbers, and super- fluorescent sources have been demonstrated.

Conventional low-coherence sources used in 'white light' interferometry have previously limited the optical system resolution, due to the inefficient coupling of light into a single-mode fiber. The theoretical and experimental operation of sensors illuminated by a multimode laser diode source are presented. The principle advantages are discussed.

In Japan the study of a fiberoptic gyroscope (FOG) was started in 1980. The FOG has been vigorously developed from both system architecture point of view and components. Now, interferometer-type FOGs (I-FOG) in Japan have reached the stage of field test. This paper presents the current status of the I-FOGs in Japan.

The design, construction, operation, and preliminary evaluation of miniature fiber Fabry-Perot (FFP) interferometers used as heat transfer gauges is described. These gauges are being developed for a particular application where heat transfer data is currently obtained using conventional platinum thin-film resistance thermometers. The specifications that the sensors must exceed are: (1) temperature resolution of 25 mK over a 50 K range; (2) temporal response of 10 microsecond(s) ; (3) an ability to operate as a calorimetric heat-transfer gauge. The sensor consists of a short length of single-mode optical fiber (approximately equals 3 mm) to which low- reflectivity coatings have been applied at each end. It is illuminated and interrogated by an arbitrary length of addressing fiber. A laser diode is used as the source and the authors have exploited the facility to frequency modulate the diode in a novel signal processing scheme. To determine the performance of the sensor, short duration heat pulses derived from a pulsed Nd:YAG laser were applied to one end of the FFP. The response time was found to be 8 microsecond(s) and the sensor operation as a calorimeter was verified.

A novel optical-fiber version of Young's interferometer using a low coherent light source and a linear CCD detector is described. With its unique, simple structure, this interferometer greatly reduces the spatial coherence mismatch from which other electronically scanned 'white-light' interferometers suffer. Experimental results are presented for the use of this interferometer as a strain or temperature sensor with large dynamic range.

Economical and high-performance two-wavelength multiplexers-demultiplexers can be made from correctly cut optical fibers coated at their ends with multidielectric filters and then put together. The authors modelized the filters' performances by assimilating the inhomogeneous guided wave to an open beam whose characteristics depend upon the sort of optical fibers used in the connection and, for this purpose, introduced the notion of equivalent aperture for the guided beam. The validity of the prevision given by this model has been checked for a dichroic filter when the coating separates 850 nm and 1300 nm. Nowadays, the development of telecommunication through optical fibers induces conceivers to consider the use of closer and closer wavelengths. This implies the use of multicavity Fabry-Perot filters. The authors have made multiplexers-demultiplexers for wavelengths as close as 750 nm and 900 nm, set on optical fibers 85/125 with slight insert losses, and an optical crosstalk better than 30 dB. It is also shown that this type of coating can be used to devise multiplexers-demultiplexers working on 1.3 micrometers and 1.5 micrometers wavelengths. The experimental results prove the validity of the model used, and show that this research work can be generalized to different types of optical fibers.

Fiber-optic sensors based on the amplitude modulation of the light guided by the fiber are considered. These can be sketched as an electro-optic module connected to the sensing tip by means of a multimode fiberoptic link. The problems related to the optical system and light propagation and those related to the referencing system and probe manufacturing are discussed and different solutions, depending on the working principle of the sensing tip, are proposed. Particular attention is given to the fiberoptic sensors designed and tested at IROE-CNR, which have been developed for the measurement of physical (temperature, proximity, vibration) and chemical (pH, bile refluxes) parameters.

A fiberoptic electronic speckle pattern interferometer illuminated with a diode-laser source has led to a novel system that allows shape measurement. The technique utilizes the modulation of the diode-laser wavelength to produce height contours of the object under study. This technology has been combined with a phase-stepping algorithm to produce a contour map of the object surface and, therefore, a representation of its shape. Results from a profiling study of a gas-turbine compressor blade using contours from 0.9 to 8 mm are presented.

A remote fiberoptic-based temperature sensor capable of operating above 1000 degree(s)C, based upon a hemispherical solid etalon, is described. Low-coherence techniques are used to interrogate the sensor, significant improvements in the signal-to-noise ratio have been achieved using a multimode laser rather than a conventional low-coherence source.

A fiberoptic interferometric sensor illuminated by a laser source requires a minimum path imbalance in order to implement one of the standard signal-processing techniques. In this work, a modulation technique is described in conjunction with a fiber interferometric sensor with a typical size of 2-3 mm. This is a passive technique which makes use of modal properties of highly birefringent fiber to generate quadrature-output components. The quadrature outputs are then used to generate a fixed-frequency carrier where the phase information is affected by the temperature change.

A novel optical configuration for a current sensor utilizing the Faraday effect is described. A common-path optical-fiber heterodyne interferometer is used to greatly reduce noise produced by environmental effects. A miniature sensing element at the distal end of the fiber allows remote magnetic field or current measurements to be made.

Fiberoptic systems based upon two different forms of miniature Fabry-Perot interferometer have been developed to measure the oscillatory motion generated in a commercially available vortex shedder flow meter. The results of a comparative study of the performance of the optical systems with the previously used piezoelectric detector are given for a range of water and air flows.

A high-speed heterodyne polarimeter at 1.55 micrometers wavelength with an intermediate frequency of 1 GHz is demonstrated. To verify the good performance of the instrument, birefringence measurements on a polarization maintaining fiber with a submillimetric beat- length have been performed. The repeatability of the measurement is excellent, and the precision ranges around 0.3%.

A very simple sensor for the measurement of rotation angles is presented. The sensor is based on the wavelength variation observed in the focal plane of a system conceived with a diffraction grating and a lens. White light is launched in the input fiber. If the grating undergoes a rotation, the wavelength received by the output fiber is altered.

Two slit and bar grids and optical fibers for lighting and reading are specially conceived to resist at high temperature (some hundreds of degrees Celsius) and constitute a fully optical moire setup, allowing measurement of displacement with a sensitivity of 0.25 micrometers . Automatic compensation of temperature is possible.

The excess loss caused by random core radius fluctuation for communication fibers, taking the presence of the core into account, is researched through the phenomenon of coupling between the dominant mode and cladding modes. The presence of the core has an effect on the field shape of cladding modes. In addition, the dependence of excess loss on the number of cladding modes is given.

An interference system for the measurement of flatness and the perpendicularity of the end face of an optical fiber is shown. The system is based on a microscope interferometric objective and resolve differences of flatness of 0.3 micrometers . The resolution in the measurement of face angle has been estimated as 0.063 deg for optical fibers of 125 micrometers in diameter.

The precise far-field scanning measurement is used for determination of a spot-size wavelength dependence of real single-mode (SM) fibers. Both Gauss's and Petermann's formulas for mode field radius (MFR) calculation are used and compared. The results of spot- size measurement by transverse-offset (TO), knife-edge-scan (KE) in far field, and far-field pattern (FFP) techniques are compared.

The main characteristics of polarization-maintaining single-mode fibers (PMSMF) were computed and measured for different fiber structures. The stress-induced linear birefringence profile was computed. The propagation constant and cutoff for each mode were determined numerically. An experimental setup for fiber beat-length measurement was developed. Group linear birefringence, modal coupling parameters, and dichroism have been achieved for fibers prepared at the Institute of Glass Chemistry and Ceramic Materials.

Problems may arise when electrical power cannot be transferred through conventional cables due to safety requirements or other reasons. A typical example may be those coal mines where the danger of methane explosion exists and an accidental break of electrical wire can cause a spark. In such a case, an alternative way for the electrical energy transfer may be the transfer of sufficient amount of light energy via optical fibers and its conversion into electrical energy at the place where it is needed. There are some reports of the possibility of such conversion. The most important advantages of such a system are intrinsic safety, sparkless work, and maintenance-free operation. In this work, the authors analytically show, using the example of the real-silicon photocells, quartz optical fiber, and halogen lamp as a light source, that such a system may be characterized with quite satisfactory overall efficiency and may be used for the energy transfer on distances as long as a few kilometers.

Nowadays spectral attenuation measurements on optical fibers are performed using high- precision, commercially available measurement equipment and the cut-back technique. Most instruments use a combination of halogen lamp and monochromator and operate with spectral widths of 5-10 nm. Since the spectral width of the absorption peaks of Si-O-H bonds in standard silica fibers is of the same order of magnitude, a significant error is introduced into these measurements. Simulations show considerable distortion of the measured data. Basically, the measured curve is a convolution of the true attenuation curve and the source spectral distribution. When measured curves are to be interpreted, the inverse problem, deconvolution of the respective curves, must be dealt with. The authors developed an algorithm to compute the true attenuation values from the measured data by numerically deconvoluting the measured curve with the source spectrum. It is show how the mathematical problem can be reduced to solving a system of linear equations. The algorithm is very unstable with respect to measurement errors, as a Monte-Carlo simulation shows. The necessary stability for practical purposes is achieved by fitting the data with a smooth function prior to the deconvolution. A procedure to deal with practical curves is derived: The measured data are first fitted with a special set of Gauss functions describing the OH-absorption peak at 1.4 micrometers . Then the numerical deconvolution is carried out with the source spectrum and the curve-fit performed earlier is repeated. By comparing the two fits and their parameter values, the influence of the spectral width is discussed.

Plastic fibres are considered suitable for applications in short distance datalinks, local area networks, and automotive applications. Components such as Tcouplers are essential in these applications. This paper describes results obtained fromanovel T-coupler formed by profiling the ends of the 1mm plastic fibres used, this profiling being produced by a simple polishing technique. Excess losses of less than 2.5dB have been obtained.

This paper summarizes the principles of distributed optical fiber sensing (DOFS), gives examples of systems which have been studied, and gives some indication as to what is in store for the future.

The free-electron laser (FEL) amplification mechanism is discussed, including the influence of the electron beam quality (energy speed, emittance, stability) on the FEL performance. Considerations regarding the choice of the electron accelerator are presented and some characteristic properties of the laser radiation are discussed. An overview of FEL facilities for the infrared spectral region, which are being designed and built worldwide, is given.