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Index of refraction profiles of optical fibers are obtained by a new interference technique closely related to Mach-Zehnder microscope interferometry. Data are obtained from specially prepared samples using a conventional reflected light microscope. Excellent agreement is obtained with the results using the Mach-Zehnder method but at a considerable reduction in the cost and complexity of instrumentation.

Measurements have been made of Rayleigh scattering in CCl(4) and C(2)Cl(4) as a function of wavelength between 6000 A and 10,600 A. The scattering loss rate is 25 dB/km for CCl(4) and 68 dB/km for C(2)C(4) at 6328 A. The scattering loss rate follows a (wavelength)(-4) dependence over the entire spectrum observed. The Rayleigh scattering was measured by placing the liquid sample in a hollow fused-quartz fiber and measuring the light scattered by the liquid through the fiber wall. It appears that this is the first time that the expected (wavelength)(-4) dependence of Rayleigh scattering has actually been observed in liquids over a wide spectral range. This technique is simple and accurate and may be applied to the measurement of the scattering coefficient of any liquid whose index of refraction exceeds the cladding index of refraction (for fused quartz n(D) = 1.458).

Narrowband optical filters were made by direct deposition of thin films of TiO/sub 2/ and SiO/sub 2/ on the ends of optical fibers. Single-cavity etalons with bandwidths of 1.1 nm, insertion loss of 1 dB, and 2.25 nm, insertion loss of 0.5 dB, were made. It is necessary to use fibers with mode fields larger than standard single mode fiber. Thermal expansion was used to diffuse the core dopant and produce tapered-core fibers with mode-field diameters of 22 mu m. >

We have used an interferometric technique to measure first- and second-order dispersion of a particular type of dispersion-shifted fiber. The values of beta and beta near the dispersion minimum are substantially the same as those of a conventional fiber except for being shifted from the 1.3-microm to the 1.5-microm wavelength range. We discuss the implications of this observation on the bandwidth of dispersion-shifted fibers.

Time-averaged and time-resolved spectra of single-mode, optically pumped, ultrashort-cavity semiconductor film lasers have been studied in detail. The time-averaged spectra are broad (3-17 nm). Time-resolved spectra show that the lasing wavelength increases (chirps) during the picosecond-time-scale duration of the pulse. The chirp is due to free-carrier plasma effects on the refractive index of the highly excited semiconductor (10(20) carriers/ cm(3)). Film-laser pulses are compressed by a factor of 2 in an 8-m length of single-mode optical fiber because of the linear group-velocity dispersion of the fiber, producing a pulse of 3.7-psec duration at 1.264 microm.

A new Raman-gain medium consisting of molecular hydrogen dissolved at high pressure in low-loss solid-silica optical fibers is described. Stimulated Raman gain at 0.88-microm wavelength was measured in a half-kilometer length of this H(2)-diffused fiber by using a krypton-laser pump and an injection-laser probe. The gain coefficient determined from these experiments at 0.88 microm is 5 x 10(-12) cm/W. The overtone absorption spectrum of high-pressure hydrogen in silica was observed.

A low-loss triangular-profile single-mode fiber is reported. The fiber loss at 1.3-1.55microm wavelengths is below 0.4 dB/km, and its zero-chromatic-dispersion wavelength is 1.402 microm.

Ultrashort-cavity, thin-film lasers from In(1-x)Ga(x)As(y)P(1-y) of five different compositions, including InP and In(0.53)Ga(0.47)As, have been made to lase between 0.83 and 1.59 microm. The multitude of lasing wavelengths observed had line-to-line separations of less than 10 nm. The lasers were pumped with 1-psec pulses from a mode-locked dye laser. An output pulse of 6-psec duration was measured at a wavelength of 1.16 microm.

A key component of Wavelength Division Multiplexed (WDM) networks is a tunable broadband wavelength demultiplexer, with good selectivity and high tuning speed. In this communication we report low waveguide loss InGaAsP Fabry-Perot filters, grown by Metal Organic Vapor Phase Epitaxy, with tuning ranges up to 3.07 nm (380 GHz). These filters have waveguides with bandgap wavelengths between 1.3 and 1.48 /spl mu/m so that they are transparent at zero bias and at low tuning currents ( >

The authors report an 8-Gb/s lightwave transmission system operating at 1.53- mu m wavelength through 130 km of single-mode fiber having a chromatic dispersion of zero at 1.3- mu m wavelength. A receiver sensitivity of -34.2 dBm (365 photons/bit) was achieved using an erbium-doped fiber preamplifier pumped at 980-nm wavelength. A sensitivity of -31.9 dBm (620 photons/bit) was reached by a preamplifier pumped at a 1480-nm wavelength. By employing a Ti:LiNbO/sub 3/ external modulator and a fiber Gires-Tournois interferometer to equalize fiber chromatic dispersion, the penalty for 130 km transmission was held to only 0.9 dB. >

FiEnd (fiber end) etalon filters have been made by multilayer deposition directly on the beveled end facets of fibers. Both standard depressed-cladding single-mode fibers and beam-expanded fibers made by thermal diffusion of the core dopant of standard fibers were used. The fiber end facets were polished at appropriate bevel angles to eliminate back reflection. It was found that, if a standard fiber is used, the bandpass of the FiEnd etalon filter is asymmetric and skewed toward shorter wavelengths. It was also shown that to make reflection free FiEnd etalon filters on beveled fiber and facets with bandpasses smaller than about 5 nm, it is necessary to use fibers with mode field diameters larger than those of standard fibers. >

A fiber Fabry-Perot resonator containing a Faraday-active medium and measured nonreciprocal resonance-enhanced transmission for circularly polarized light was built. The results show that an 18- mu m length of material producing only 2.1 degrees of Faraday rotation could achieve an optical isolation of 10 dB. The isolation would be larger for interferometers with higher finesse or more single-pass Faraday rotation. The device could also serve as a nonreciprocal wavelength filter transmitting different sets of wavelengths in the two directions. >

A 5-Gb/s lightwave transmission experiment demonstrates the effectiveness of an all-pass-type fiber Fabry-Perot optical equalizer for eliminating an error floor caused by laser chirp and chromatic dispersion. >

Recently, we proposed a spectrum-sliced fiber amplifier light source for multi-channel WDM applications [1]. The high-power ASE from an EDFA could be efficiently divided into many channels using an integrated optic WDM demultiplexer [2]. Thus, this "spectrum-sliced" ASE could be used as light sources for WDM systems rather than several wavelength-selected DFB lasers. However, these incoherent light sources would suffer from excess noise caused by the beating of the various Fourier components within the broad spectrum. In particular, this excess noise becomes dominant over the electrical noise when the optical bandwidth per channel is significantly reduced, as in the proposed WDM light source. Thus, the SNR of the spectrum-sliced ASE source at the receiver is given.

A temperature-insensitive, nontunable fibre Fabry-Perot etalon is demonstrated. This compensation has been obtained for an etalon with a cavity length of 740 μm (free spectral range = 135 GHz), and the principle may be applied for etalons of arbitrary length. This device may prove useful in setting frequencies for WDM applications.

Single-mode-fibre output couplers for diode lasers have been fabricated to have long working distances. The fibres have (i) tapered cores expanded at one end to 25 μm diameter by thermal diffusion of the core dopant and (ii) outside diameters etched down to 30–60 μm. Hemispherical lenses were made by melting the ends. The resulting working distances are 35–40 μm, and coupling losses, as low as 3dB, are mostly between 3.5 and 5.5dB.

1.2 ps transform-limited pulses at 1557 nm have been produced by passively mode locking an Er-doped fibre laser. An InGaAs/GaAs-on-GaAs superlattice was used as a fast saturable absorber. The all-fibre laser is designed to be in a ring configuration. The low pump power requirement allowed pumping by a semiconductor laser diode.

An all fibre, diode-pumped, electrically tunable ring laser is reported. Gain is provided by an erbium-doped fibre and tuning by a fibre Fabry-Perot etalon. The threshold at l.566 μm is 2.9mW, the slope efficiency is 0.15 and the output power is 4.2 mW with 32 mW of pump power. The output wavelength can be tuned from 1.525 to 1.586 μm with a variation in power of less than 3.5 dB.

The performance of fibre Fabry–Perot interferometers has been improved by using different mirror coatings. Finesse of 1000 with insertion loss of 4dB has been demonstrated.

A diode-pumped, broadly tunable, single frequency erbium-doped fibre ring laser is reported. Tuning and single longitudinal mode selection are accomplished by use of two fibre Fabry-Perot etalons. The threshold for 1.48 μm pumping is 8.8 mW and the slope efficiency is 11.5%. The laser can be tuned to wavelengths between 1.530 and 1.575 μm and operates singlemode throughout this range. Its linewidth is measured at 1.56 μm to be less than 5.5 kHz.

The effective reflectance of multilayer high reflectance mirrors applied to fiber ends is determined by a shuttle pulse method. Light from a pulsed laser diode at 1.53-μm wavelength is injected into a fiber with mirrors at both ends, and the emergent pulse train amplitude decrement is used to obtain the reflectance.

A method is described for measuring transmittance, reflectance, and loss spectra in thin optical films. The method is applied to measure the properties of multilayer coatings of Si and SiO(2) used to make mirrors and antireflection coating in the 1.0-1. 7-microm wavelength region. Mirrors with reflectance up to 99.5% with nine quarter-wavelength layers and two-layer antireflection coatings with reflectance of0.2% have been made.

Multilayer thin-film filters have been deposited on the end faces of optical fibres. Single and double cavity bandpass filters and low-ripple long-pass filters have been made with good optical quality. Reflections are reduced by depositing on bevelled end faces.

The effects of inherent fiber birefringence, packaging stresses, and stress tuning on the behavior of a fiber Fabry-Perot interferometer are examined. A polarization doublet due to birefringence is observed. For birefringence of 1 ps/km, the doublet spacing is 40 MHz, at 1.5- mu m wavelength, independent of the FFP finesse or free spectral range, and is due mainly to the inherent fiber birefringence. It was found that annealing depressed-cladding single-mode fiber reduced the fiber birefringence, for example, from 40-80 MHz to 15-25 MHz. Where the bandpass of the FFP is greater than the birefringence doublet, this device should be polarization insensitive for most applications. >

We have recorded quantitative optical absorption and Raman spectra of H 2 and D 2 diffused into silica optical fibers at a pressure of 500 atm. The absorption spectra recorded were fundamental through the third overtone for H 2 (0.6 through 2.5 μm) and the first through fourth overtone for D 2 (0.7 through 1.7 μm). The observed spectra show fine structure similar to what has been seen in gas-phase Raman spectra. By considering all our absorption and Raman spectra for both H 2 and D 2 , including temperature dependence and anharmonicities, and comparing the spectra to gas-phase Raman spectra, we have made unambiguous assignments for all observed spectral lines. As a result of these assignments, we conclude that H 2 and D 2 are trapped singly in interstitial sites in silica and are free to rotate. The absorption spectra provide information on the added loss due to H 2 saturation of silica optical fibers. For example, extrapolating the experimental results to the case of a fiber saturated under 1 atm of H 2 , the added loss due to the intense first overtone Q-branch line is 14 dB/km at 1.2445 /im and 0.14 dB/km at 1.30 μm. High loss due to the fundamental absorption of H 2 occurs beyond 1.5 μm; at 1.55 μm it is 0.8 dB/km for 1 atm of H 2 . There is, however, a transmission window in H 2 -saturated fibers between 1.32 and 1.45 μm.

Two-electrode distributed-feedback (DFB) lasers show promise for combining high speed and frequency tunability for frequency-division-multiplexed-frequency-shift-keyed (FDM-FSK) networks. The FM and FSK responses of such lasers have been measured up to modulation frequencies of approximately 1 GHz. Using these lasers in a noncoherent detection system in which a fiber Fabry-Perot tunable optical filter converts an FSK signal into an amplitude-shift-keyed (ASK) format, a 10/sup -9/ BER was observed up to 1 Gb/s. Nonuniform FM response and consequent tone broadening of the optical FSK spectra can lead to system power penalties due to optical-filtering effects. Thus, for a given FM response, the behavior of these lasers in FSK optical systems can be projected. >

Fiber Fabry-Perots have been built with a finesse of 500 and throughput of 30-40 percent. The devices are piezoelectrically scanned and operate at 1.52μm, and have free spectral ranges of 1000, 500, 333, and 250 MHz, with linewidth resolution as small as 0.5 MHz. The sources of loss which limit the attainable finesse have been identified and analyzed.

An air-gap Fabry-Perot resonator with plane mirrors between closely spaced fiber ends may yield low throughput because of the poor match between the modes of typical single-mode fibers and the resonant mode in the air-gap cavity. The throughput can be improved by confining the resonant mode by a hollow dielectric tube placed inside the resonator. Short fiber-coupled Fabry-Perot resonators with and without an inserted hollow dielectric waveguide and expressions for their transmission losses are derived. It is shown that the throughput of both types of resonator can be improved significantly by using fiber with large mode size to couple to the resonator. The special fiber is then spliced to a conventional single-mode fiber. It is concluded that the resonator with an inserted hollow dielectric waveguide offers increased throughput for resonators with high finesse. >

This paper reviews all the interactions of hydrogen and deuterium with silica-based optical fibers. Included in the discussion are H 2 overtone absorption, OH formation, increased UV and IR absorption, radiation effects on H 2 -treated fibers, the gas-in-glass laser and applications, the reduction of OH absorption by isotope exchange OH → OD and other isotope-exchange related effects, and applications of isotope exchange and substitution for diagnostic purposes. Also included is a description of common sources of H 2 in materials used in fiber and cable manufacture, and countermeasures against hydrogen generation and intrusion into the fiber.

An N-channel optical frequency-division multiplexing (FDM) network can be realized using frequency-shift keying (FSK) modulation and a tunable filter to select and detect the channels. A tandem arrangement of two fiber Fabry-Perot (FFP) filters is demonstrated. For FFPs with individual finesse (F) of 57 an effective finesse of 675 is obtained in a tandem arrangement. This has a capacity of N=F/3=225 channels in a frequency-division multiaccess (FDMA)-FSK network. >

We have observed Raman scattering from N 2 in a silica optical fiber which had been pressurized in an N 2 atmosphere. The Raman shift is 2190 cm −1 and the half-width of the line is 90 cm −1 (fwhm).

Picosecond pulses from optically pumped, ultrashort cavity semiconductor film lasers have a large positive chirp that is not linear in time. Average chirp values are about 1 nm/psec for InGaAsP film lasers. The chirp is caused by transient change in the refractive index of the semiconductor film during the pulse due to variation of the free carrier concentration. The transient carrier concentrations of 1020 cm-3 caused by the intense picosecond pump pulses is sufficient to account for the observed chirp. For film lasers, the dominant mechanism by which the carriers affect the index of refraction is free carrier plasma refraction. The positively chirped pulses have been compressed by the negative linear group velocity dispersion of 8m of single-mode optical fiber. Pulse compression factors as large as 3.4 have been obtained. Pulses as short as 2.5 psec at 1.23 μm and 3.4 ps at 1.42 μm have been generated by this compression scheme.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical communication systems operating at rates in the gigabit per second range are currently being explored. To study temporal limits on components and systems, it is necessary to use a source that produces short optical pulses at wavelengths throughout the near-IR spectral range. In this paper we will describe the use of picosecond pulses from optically pumped, ultrashort cavity, semiconductor film lasers to study system components for ultrahigh-speed optical-fiber communication.

A Mach-Zehnder fibre interferometer and a fibre Raman laser are used to measure modal group delays with subpicosecond resolution. Chromatic dispersion spectra, in several metres of fibre, are obtained within the 1.06?1.62 ?m wavelength region.

Frequency confinement of FDM optical signals to a comb of equally spaced frequencies has been demonstrated. The result was achieved by locking the frequencies of three optical sources to three different resonances of a fibre Fabry-Perot resonator. The frequency spacing between the frequency-locked signals was adjusted to be 500 MHz, the spacing between adjacent resonances, or a multiple of this value. Implementation of the frequency stabilisation circuit requires only a fibre Fabry-Perot resonator, a photodetector and simple electronics. Such a simple circuit provides the means to frequency-stabilise a large number of FDM optical signals originating from the same location.

In this paper a new type of magnetooptic modulator having a bounce-cavity structure is described. Direct amplitude modulation is achieved by utilizing the Faraday effect in conjunction with the differential reflectivity of dielectric mirrors for polarizations parallel to (TM) or perpendicular to (TE) the plane of incidence. We show that the transmission through the modulator is determined by the number of reflections, the reflectivity of the mirrors, and the difference in the phase shift for TE and TM reflected waves. The operation of this type of modulator has been successfully demonstrated with a preliminary device fabricated from YIG.

We show that fibre-coupled open resonator Fabry-Perot interferometers can be made with reduced insertion loss due to diffraction by using fibre with slightly larger mode size. For a 50% increase in mode size, the insertion loss is reduced by 3–4dB for a 7μm-long resonator at 1.5μm wavelength.

Room-temperature c.w. laser action at 1.3 μm wavelength has been obtained using single-crystal glass-clad Nd:y.a.g. fibres end pumped with a single high-radiance l.e.d. The fibres were 0.5 cm long, 50 μm in diameter and had a cladding 3 μm thick. Threshold in a nonoptimum configuration was less than twice that at 1.06 μm. Application of a transparent, adherent glass cladding to the crystalline fibre is described.

Light from two InGaAsP injection lasers of different wavelengths was multiplexed into a 15 km-long single-mode fibre. Owing to the nonlinear refractive index, the phase of light in one channel was changed by power changes in the other channel. A 1 mW power charge in one channel produced a 1.4° phase shift in the other channel.

We have made prototypes of three kinds of fibre Fabry-Perot (FFP) tunable filters, each of which can cover a separate free spectral range interval as determined by its resonator length. Free spectral ranges from hundreds of angstroms to tens of megahertz can be realised from the appropriate type. We have built FFPs with free spectral ranges of >1000 A, 25 GHz, 5 GHz and 2.5 GHz. Finesse up to 200 has been demonstrated. The devices are piezoelectrically tuned and are coupled to fibres in-line using standard rotary splices or ST connectors.

A stimulated-Raman-gain medium at 1.56-microm wavelength, consisting of molecular deuterium dissolved in low-loss silica optical fiber and pumped by 1.06-microm Nd:YAG laser light, is described. In a 100-m-long fiber with pump power of 130 W, InGaAsP injection-laser probe light at 1.56 microm was amplified by a factor of 4000 to an output power of 150 mW. The measured stimulated-Raman-gain coefficient at 1.56 microm is 7 x 10(-12) cm/W, about equal to the Raman gain of SiO(2) at the same wavelength.

The effect of deuteration (OH - OD exchange) of the MCVD substrate tube on the absorption loss in fibres was studied using tubes of moderate and low OH content. The OH absorption at 1.39 μm was reduced from 4 dB/km and 0.9 dB/km to 1 dB/km and 0.7 dB/km, respectively, in fibres made without a barrier layer using deuterated tubes.

The effects of ray bending due to graded refractive-index profiles have been studied in relation to the measurement of optical-fiber refractive-index profiles by microscopic interferometric techniques. In particular, the wavefront curvature produced by a simple parabolic profile has been calculated analytically. It is concluded that profile measurements by currently used methods require samples for which the fiber thickness (length) is much less than the effective focal length of the fiber. For parabolic profile fibers with diameter d greater, similar 100 microm, this poses no problem; however, for small-core versions of this fiber, sample thicknesses of 10-15 microm may be required and an alternate measurement technique is described. Interferograms made on a Leitz transmitted-light interference microscope are included in illustrations.

An isotopic exchange process is used to convert the OH in optical fibre preforms into OD. The OH first overtone absorption is reduced from 80 dB/km to 1.5 dB/km. The OD second overtone absorption at 1.26 μm is less than 0.5 dB/km.

The impulse responses of very-high-speed photodiodes are determined using 5?15 ps pulses from optically pumped ultrashort-cavity film lasers operating between wavelengths of 1.15 ?m and 1.54 ?m. The measured response time (FWHM) for a back-illuminated InGaAs/InP punch-through PIN photodiode is 30 Ps.