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Add-drop filters are demonstrated using silica-on-silicon optical waveguide technology. This device consists of a full directional coupler subdivided by Mach-Zehnder sections. Tapering of the coupling coefficients is experimentally shown to dramatically reduce the filters' sidelobes. Only one photolithographic step is required using the silica planar waveguide technology, yielding accurate wavelength control of the filters. Excellent agreement between measurement and design was achieved. >

Similarities between fiber bandwidth spectra and loss spectra are found to occur. Wavelength-dependent far-field radiation patterns were used to deduce that high OH ion concentrations, near the center of the core, caused differential attenuation of low-order modes. This resulted in bandwidth peaks at the same wavelengths where water absorption loss peaked.

Dispersive differences between B(2)O(3) and SiCO(2) constituents make nonparabolic profiles optimal equalizers of intermodal group delays in fibers with graded B(2)O(3)-SiO(2) cores and uniform B(2)O(3)-SiO(2) cladding. Pulse dispersion measurements were correlated with profile shapes in a systematic study of multimode fibers with near power law gradients. Far field spatial ray filters were used to diagnose impulse response shapes so that new fibers could be fabricated with closer-to-optimal profile gradients. One of the fibers had an alpha approximately 1.77 power law exponent that was nearly optimal for lambda = 907.5-nm wavelength and caused 2sigma = 0.26-nsec/km full rms output pulse spreading. When expected material dispersion effects were deconvolved from the output pulse spreading, the resultant pulse width was approximately 75 times less than the result expected for a comparable step-index fiber. This is the largest pulse width reduction reported yet.

The loss spectra, refractive-index profiles, numerical apertures, and pulse dispersion characteristics of three multimode optical fibers made by Corning Glass Works have been measured at Bell Laboratories. The lowest transmission loss, 4.3-6.8 dB/km, was observed at 1.06-microm wavelength; in the 0.8-0.9-microm spectral region the losses ranged from 6.6 dB/km to 11.6 dB/km. The numerical apertures of the fibers, designated as Nos. 1, 2, and 3, were calculated from the measured refractive-index differences to be 0.133, 0.157, and 0.121, respectively. Pulse dispersion due to multimode effects in fiber No. 1 (1 km long) was 8.4 nsec at 0.9-microm wavelength; the values were 6.0 nsec for fiber No. 2 (0.363 km long) and 2.6 nsec for fiber No. 3 (0.290Km).

Microscopic lenses, fabricated on optical fiber surfaces, have quadrupled the efficiency for coupling astigmatic beams from GaAs junction lasers into 4-microm cores of single-mode fibers. A novel photolithographic technique was used to make hemispherical and hemicylindrical microlenses, with diameters between 4 microm and 10 microm, from commercially available negative type photoresist that is transparent at ir laser wavelengths. Geometrical profiles of photoresist lenses, documented with scanning electron photomicrographs, were remarkably smooth even though their dimensions were more than an order of magnitude smaller than other known lenses.

The feasibility of using a variety of rare-earth doped optical fibers for measuring spatially averaged temperatures from approximately 0 to approximately 100 degrees C over distances of 10 to 20 m is discussed. Such distributed temperature sensors would be particularly well-suited for building climate control systems and industrial processing applications. The temperature-dependent absorption spectra of 6 MCVD processed fibers containing different concentrations of Nd/sup 3+/, Pr/sup 3+/, and Yb/sup 3+/ rare-earth ions were characterized and used to determine thermally active dopant species, optimal dopant concentrations, and most sensitive operating wavelengths for use as dual wavelength distributed temperature sensors. >

We present a theoretical analysis of highly elliptical optical fibers by expanding the vector fields as a series of orthogonal functions. Higher-order cutoff values of the normalized frequencies (V numbers) are evaluated for step-index fibers of varying ellipticities. The effect of different refractive-index profiles on the birefringence of the fiber is presented. For fibers operated in the two-mode regime, we discuss the effect of ellipticity and refractive-index profile on the differential propagation constant (Deltab) related to the scalar equivalent of LP(0l) and LP(11)(even)modes.

Transmission of 50-psec solitons in a 75-km recirculating loop of dispersion-shifted fiber (D = 1.38 psec/nm/km at lambda(s)), using low-gain erbium amplifiers spaced 25 km apart, displays jitter in pulse arrival times consistent with low error rates for transmission over 9000 km and for bit rates ?4 Gbits/sec. Furthermore, a study of soliton pair propagation in the same loop shows no significant interaction over 9000 km for pair spacings/=5tau.

A numerical parametric study is used to gain insight into how the shapes of dispersion and bandwidth spectra are influenced by dimensions and index differences of light-guide structures with two claddings. Computer-simulated fibers are demonstrated to have bandwidths greater than 25 GHz-km across the entire 1.3-1.55-microm wavelength region.

The fiber for the sensor is formed by cladding fused silica during drawing with polydimethyl siloxane into which an organic fluorescent dye, 9, 10-diphenylanthracene, has been dissolved. Upon side illumination at a wavelength within the excitation range of the dye, the cladding fluoresces; some of this fluorescence is coupled into guided modes in the fiber core through the evanescent fields of these modes. In the presence of oxygen, fluorescent emission by the dye is diminished. For the sensor described, the rubbery liquidlike nature of the polydimethyl siloxane cladding allows rapid diffusion of gases, and the intensity of the guided fluorescence is observed to drop by 30% in less than 5 s when the ambient atmosphere changes from pure nitrogen to pure oxygen. The advantages of this sensing technique, and some of the possibilities for new sensors based on this principle, are discussed. >

An attempt to relate polarization dispersion directly to some physical property and then use this as a means by which to characterize polarization-mode dispersion (PMD) is reported. A new diagnostic procedure has been developed and tested for screening dispersion-shifted (DS) fibers with PMD due to core ellipticity. Measurements of far-field radiation profiles across major and minor core axes are used to characterize polarization-mode dispersion. This technique is a relatively simple and quick method of screening dispersion-shifted fibers for polarization-mode dispersion greater than 5 ps/km. Differences in these far-field pattern widths were then correlated with direct core ovality measurements performed using the near-field refraction technique. The authors outline the test procedure in detail, analyze sources of error, and discuss detection limitations. >

A fiber optic chlorine sensor having its entire length as the sensing element is reported here. The fiber consists of a silica core and a chlorine-sensitive cladding. Upon exposure to chlorine, the cladding very rapidly changes color resulting in attenuation of the light throughput of the fiber. A two-meter portion of sensor fiber responds to 10-ppm chlorine in milliseconds and to 1 ppm in several seconds. Furthermore, response to 100 ppb chlorine is realized in minutes. The high sensitivity suggests that the propagating modes of the light interact strongly with the cladding, and that these interactions are massively increased (Beers Law) due to the extended sensor length. The sensitivity to 1 ppm chlorine gas as a function of the length of fiber exposed between 0.3-30 meters is presented. The sensitivity to concentrations of chlorine from 0.1ppm-10ppm has been determined for a fixed 2 meter length of fiber. Pre-exposure fiber attenuation measures 70 dB/km (@ 633 nm) making it possible to detect chlorine on a continuous length of fiber on the scale of one hundred meters or more using standard detection methods (e.g. laser and photodetectors). This will replace the need of having a collection of point-detectors to cover large areas.

The Letter shows that a commercial optical time domain reflectometer can be modified to measure the statistical nature of polarisation mode dispersion along the length of a communications grade fibre with short-length PMDs > 4 ps/km.

The “resonant coupler” (RC) add-drop filter is a 2x2 device that diverts one of N input wavelengths into one output while the remaining N-l wavelengths couple into the second output. Specifically at off-resonance wavelengths, light is transmitted in the “bar state” from the input to the corresponding output. However at the resonant wavelength coupling occurs, and light is transmitted from the input to the “cross state” located at the opposite output: hence the name resonant coupler. The RC functions like Bomholdt et. al.’s ‘meander coupler’ll], however our device requires only one mask level and has more accurate control of the resonant wavelength. An example of a RC is shown in Fig. 1. expanded 50x in the vertical. The RC consists of a series of parallel waveguide couplers connected by waveguides of unequal arm lengths. In the case of only two couplers, the device reduces to a MZI. Resonance is achieved by making the path length difference the same for all pairs of arms. Furthermore, the coupling strengths can be tapered to substantially suppress the passband sidelobes. To separate the waveguide coupling from the path length difference we have introduced identical curved paths in both waveguides in to and out of the couplers such that the path length difference, while the waveguides are coupled, is negligible.

Polarization mode dispersion (PMD) in dispersion-shifted fibers presents an inherent limitation in ultralong-distance communications systems. The advent of multigigabit-per-second systems and CATV applications makes it imperative that a good measure of PMD be obtained during fiber manufacture. Several techniques for the measurement of short-length PMD in fibers are currently available.1 A typical spool of fiber, however, has PMD values that vary along the length owing to minor perturbations during fabrication. As a result, a short-length PMD measurement on a specific section of a fiber made by using any of the techniques does not provide a complete knowledge of the statistical spread of PMD over the entire length. In this paper we show that by modifying a commercial optical time-domain re- flectometer (OTDR) with a polarizer/analyzer pair, one can measure short-length PMD’s in fibers.

Using an optical fiber whose transmission loss depends in a linear way on temperature, a thermometric system can be designed that will allow direct measurement of the temperature averaged along the length of the fiber. This paper describes such a system used to test the feasibility of using rare earth-doped fibers in practical distributed measurements of average temperature.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Using an exact waveguide analysis and measured stress profiles, we show that a delicate interplay between form-induced and stress-induced contributions to polarization mode dispersion in dispersion-shifted fibers can significantly reduce intrinsic short-length polarization mode dispersions. Our observations also indicate that the ovality of fibers is not a true indicator of long-length polarization mode dispersion in production-quality cabled fibers.

We describe the performance of an experimental 1.5-μm lightwave transmission system operating at 8 Gbit/s over 68.3 km of single-mode fiber. The dispersion penalty is limited to 1 dB through the use of external modulation and is attributable to the intrinsic information bandwidth.

A newly developed technique for directly measuring fiber dispersion in the frequency domain as a function of wavelength is described. Spectrally filtered white light from a xenon arc lamp is sinusoidally modulated in the range 0 to 1 GHz by an electrooptic modulator and injected into a fiber. The procedure is to vary the modulation frequency and measure the corresponding sideband output power with a photomultiplier and spectrum analyzer. Ratio measurements between the test fiber and a short reference fiber give the baseband frequency response. A number of germanium- and boron-doped fibers have been examined. The least dispersive borosilicate graded-index fiber has a 1 dB bandwidth of 1 GHz, after 1.07 km of propagation at λ = 908 nm. The width broadens gradually with increasing wavelengths up to λ = 1100 nm.

Silica tubing to be used as substrate tubes for MCVD has was demonstrated for a fiber having only one-fourth the deposited clad-merical tubing in that it is both dehydrated and fluorinated during processing to yield a silica which is dry and whose refractive index is lower than that of silica. These tubes are advantageously used to prepare single-mode fibers having fluorinated-silica cladding which controls dispersion and reduces scattering losses. In this case, the refractive index of the deposited cladding can be matched to that of the outer tube thus avoiding leaky-mode losses commonly encountered in depressed-index clad fibers. Using such an approach, low loss (0.28 dB/km at 1.55 μm) was demonstrated for a fiber having only one-fourth the deposited cladding required when a commercial silica tube is used. Such tubes could provide ecomomies to the processing of and the performance of complex cladding single-mode structures and impact multimode fiber designs.

Intermodal dispersion properties are compared for high silica fibers with borosilicate (B 2 O 3 -SiO 2 ) and germania borosilicate (GeO 2 -B 2 O 3 -SiO 2 ) graded-index profiles. Pulse transmission measurements were systematically correlated with profile shapes so that new fibers could be fabricated with closer-to-optimal profile gradients at a wavelength of 907.5 nanometers. Germania borosilicate fibers with power law profile exponents (α ≈ 2.03) lowered intermodal dispersion 50 times from the result expected for comparable step-index fibers with N.A. ≈ 0.19. By contrast, borosilicate fibers with α ≈ 1.78 caused a 100-fold pulse width reduction in fibers with N.A. ≈ 0.14, corresponding to a 2σ = 0.13 ns/km pulse-broadening rate.

This paper describes state-of-the-art measurement techniques for the characterization of fiber loss and dispersion. Special emphasis is placed on the achievement of high measurement accuracy, and on novel techniques to determine the loss and dispersion of single-mode and graded-index fibers in the 0.8 to 1.7-µm wavelength region where fiber loss and material dispersion are both small.

Using a multiplexer and demultiplexer built from discrete GaAs FET's, a directly modulated distributed feedback laser, and a low capacitance avalanche photodiode receiver, we have achieved transmission at 4 Gbit/s over 103 km of single-mode optical fiber at 1.54 μm.

Heavy metal halide glass fibers have the potential of optical loss between 0.001 and 0.01 dB/km in the 2-10 \mu m region. We have evaluated some of the system aspects of these fibers in order to determine the ultimate performance limits and to assist in defining waveguide design and fiber processing techniques. Extrinsic waveguide-related losses and limitations including microdeformation, optical nonlinearities, dispersion characteristics, and source and detector capabilities become more significant as the intrinsic losses decrease. Two representative halide glass systems are discussed: a heavy metal fluoride operating at \simeq 2 \mu m and a heavy metal chloride glass at \simeq 6 \mu m. The results indicate that repeater spacings ≳ 1200 and 3600 km at \lsim 1 Gbit/s may be possible for chlorides and fluorides, respectively.

A universal fiber-optic measurement system, which is useful for measuring loss and dispersion in the 1.06-1.6 \mu m wavelength region, is described. The source is a silica fiber Raman laser pumped by a mode-locked and Q -switched Nd:YAG laser at 1.06 μm. Subnanosecond multiple-Stokes pulses in the 1.1-1.6 \mu m wavelength region are generated in a low-loss single-mode silica fiber. The use of this near-infrared fiber Raman laser for characterizing various transmission properties of single and multimode test fibers is demonstrated. Loss spectra, intramodal dispersion, and intermodal dispersion data are obtained in the wavelength region of minimum loss and minimum material dispersion for silica fibers.

Single-mode lightwave has completely replaced multimode as the technology of choice for long-haul, metropolitan interoffice, and subscriber loop systems. This article discusses the design principles and performance of four kinds of single-mode fibers developed by ATT dispersion-flattened fiber that has high bandwidth over a wide wavelength range; and polarization-maintaining fiber that can be used with phase-sensitive receivers.

Bandwidth spectra of single-mode fibers are calculated from experimentally obtained chromatic-dispersion-versus-wavelength curves. Results include second-order effects on bandwidth which depend on the curvature of the dispersion curves. Examples illustrate how bandwidth spectra change as a function of source power spectra. They also show how small changes in fiber dimensions and refractive-index differences can cause significant bandwidth changes at 1.3 μm wavelength.

We consider the range of validity of a Hilbert-transform approach in which the measured magnitude of the modulation-transfer-function of an optical fiber is used to compute the fiber's impulse response. It is argued that a key “minimum-phase assumption” can fail to be satisfied in important cases, and a few closely related experimental and analytical results are presented.

Next-generation single-mode lightguides may be used in the 1.55-μm wavelength region where losses are a minimum for silica-based fibers. This paper discusses the basic fiber requirements of these new systems and provides design parameters for three types of dispersion-shifted fibers with various graded-index core profiles. The generic structural differences are that one type has a continuous cladding; the second has a depressed-cladding index relative to an outer substrate layer; and the third type has up to four cladding layers with different index values. The latter type includes a new lightguide structure that has a narrow depressed-index trench within its cladding. Results show how to choose lightguide parameters in order to tailor dispersion and mode-confinement properties.

The low-loss and low-dispersion properties of single-mode fibers make them obvious choices for wide bandwidth system applications with very long repeater spans. This brief describes the fabrication procedure and transmission properties of a double-clad single-mode fiber which is capable of wide bandwidth (greater than 100 GHz-km for laser sources with 4-nm emission bandwidths) transmission over the widest wavelength range (1.45 μm to 1.73 μm) thus far reported in the literature. This range completely covers the lowest-loss wavelength window for fused-silica optical fibers. Double-clad lightguides1–4 were formed by using an inner cladding to form an index well between the core and a pure silica outer cladding. A computer-aided analytical procedure was used to choose the proper fiber diameter so that waveguide dispersion effects could be used to cancel material dispersion at predetermined wavelengths.5

Transmission bandwidths of optical fibers would be maximized if their refractive index profiles were optimally graded. However, dispersive differences between fiber material constituents make the optimal power law profile exponent α depend on wavelength. This profile dispersion effect is significant for germanium borosilicate fibers and makes their observed transmission bandwidths change by more than 300 percent within a 650-1050 nm wavelength range. Measurements are made in spectrally filtered white light from a xenon arc lamp that is sinusoidally modulated by an electrooptic crystal. Reduction of sine wave envelope intensity due to transmission in a fiber gives its baseband frequency response. The functional dependence of bandwidth on wavelength is used to diagnose whether α is larger or smaller than the optimal value which minimizes intermodal dispersion at particular wavelengths.

Three hundred kilometers of single-mode fiber exhibiting median optical losses of 0.19 dB/km at 1.57 μm have been fabricated from preforms made by a high-rate Modified Chemical Vapor Deposition (MCVD) process. A new fiber design [1] was utilized which minimizes Rayleigh scattering loss by reducing the amount of dopants in the core. Milestone systems experiments incorporating this fiber have already demonstrated 420-Mbit transmission through 203 km [2], 2-Gbit transmission through 130 km [3], 1.37 Tbit km/s using 10 wavelength division multiplexed lasers [4], 4-Gbit through 102 km using a novel electronic multiplexer/demultiplexer [5], and 4 Gbit through 117 km using a Ti:LiNbO 3 external modulator [6]. Additionally, very low induced losses from hydrogen and radiation are reported.

Chromatic dispersion effects on LED-based optical fiber systems are calculated as a function of transmission length and the LED central wavelength. The computer analysis includes the filtering effects of water absorption which reduces the spectral width of LED signal pulses. We show that the basebandwidth of multimode fiber systems can be significantly increased by offsetting the LED central wavelength above the wavelength at which minimum chromatic dispersion occurs in the fiber medium. Additional bandwidth degradation due to modal dispersion effects are not considered. However, the combined effects of optical loss and chromatic dispersion are used to calculate upper limits for repeater spacings in possible multimode wideband systems.

We describe a novel technique for measuring dispersion in low-loss single-mode fibers in the 1.1-1.3 \mu m spectral region. The spatial equivalent of the pulse delay time as a function of wavelength is obtained in the measurement. The technique makes use of the wavelength-dependent pulse synchronization condition in a pulse-pumped neat IR fiber Raman oscillator. Experimental results on three single-mode germanium-doped silica fibers are compared, and the importance of matching the minimum dispersion wavelength with the local loss minimum in fiber design is discussed.

Dispersion-shifted single mode fibers have been fabricated with losses as low as 0.21 dB/km in the zero-dispersion region ( \lambda_{0} \approx 1.5-1.6 \mu m). Low-loss (average = 0.06-dB) fusion splices have been made with the chlorine-hydrogen flame fusion technique. The recoated splices show strengths in excess of 300 kpsi. The fibers have been incorporated into a cable structure with negligible change in loss.

Low-loss glass fiber waveguides are found to be excellent media for Raman lasers and amplifiers in the near-infrared region of the spectrum. Multiwavelength emission in the 1–1.3 μm range is readily obtained by efficient stimulated Raman scattering in single-mode silica fibers. With a 1.064 μm pulsed pump of 250 W in a 175-m, 6-μm diameter single-mode silica fiber we observed four orders of Stokes radiation at 1.12 μm, 1.18 μm, 1.23 μm and 1.3 μm, respectively. Our results imply that pulsed tunable stimulated Raman emission in this wavelength region is possible using kW tunable infrared dye lasers near 1 μm as pumps. These sources are useful for studying the dispersion of glass fibers as well as for other spectroscopic applications.

A newly developed technique for directly measuring fiber dispersion in the frequency domain as a function of wavelength is described. A number of germanium‐ and boron‐doped fibers have been examined. The least dispersive borosilicate graded‐index fiber has a 1‐dB bandwidth of 1 GHz, after 1.07 km of propagation at λ=908 nm. Frequency domain measurements were inverted into the time domain after assuming that the phase of a power transfer function could be calculated from its amplitude spectrum.

Pulse dispersion, refractive−index profile, and loss measurements were made on a low−loss graded−index optical fiber fabricated by a chemical vapor deposition technique. The transmission loss spectra had a minimum value of 3.8 dB/km at 1.06−μm wavelength. The core refractive index had a power−law profile with an exponential coefficient of 2.2±0.1. Mode−mixing effects and the near−parabolic−index profile in a 5.6−km extrapolated fiber length (actual length =1.12 km) reduces modal dispersion by a factor of 17 from the result expected for an ungraded step−index fiber.

A tunable 1.1‐μm fiber Raman oscillator is reported. The oscillator is pumped by a cw mode‐locked Nd : YAG laser at 1.064 μm. Wavelength tuning is obtained by using group velocity dispersion in a 600‐m‐long single‐mode borosilicate fiber and subnanosecond pulses tunable from 1.101 to 1.125 μm are generated. The slope of the experimental tuning curve gives a group dispersion of 31 ps/nm km at 1.12 μm for the Raman fiber, in good agreement with the calculated value of combined material and waveguide dispersion.

There are many applications in which small amounts of light need to be tapped out of an optical fiber. Linear tapped busses can be attractive if light from a common optical waveguide can be distributed to several spatially separated nodes. A useful arrangement might be to tap a small amount of power (0.1 dB ≈ 2%) at discrete points so that more than ten nodes can be interconnected before a significant amount of power is lost [Fig. 1(a)]. A commercial version is available but expensive. It is fabricated from a silicacore fiber that has a fluorinated polymer cladding. A small portion of the cladding is removed, and a launch or tap fiber is attached with an index-matched adhesive.1 The tapping fraction is controlled by carefully controlling the angle between the tap and bus fibers.

The incidence and pathology of liposarcoma is reviewed. A case is presented which is said to be one of primary liposarcoma of bone—an extremely rare condition. Arteriography, which has apparently never before been carried out in a case of liposarcoma, was performed, and features, not pathognomonic, are described. A diagnosis of liposarcoma was made from the arteriographic investigation but it was thought by the author to be periosteal rather than intramedullary in origin.

The capacity of loop transmission systems, such as local area networks (LAN), can be increased by using closed loops of high-bandwidth optical fibers. Since fiber loop lengths are relatively short, it is not crucial to use low-loss fibers. However, the ability to couple information into and out of the fiber loop at arbitrary points along the loop poses a major problem. Current procedures use fiber-optic directional couplers which have to be spliced into the loop,1,2 requiring breaking the loop at prearranged locations and increasing the downtime of the LAN.

Single-mode fibers can propagate light with two orthogonal polarizations. Although transmission fibers are nominally circular, a small-core ellipticity can make the propagation constants and group velocities slightly different for the two polarizations, causing polarization mode dispersion (PMD). This can be the major limitation for high-bit-rate systems operating near the wavelength of zero chromatic dispersion. We describe a new technique for screening out fibers with PMD values that can be large enough to degrade system performance. It involves locating the major and minor axes of the fiber’s elliptical core and measuring the difference between the widths of the corresponding orthogonal far-field radiation patterns.

The normalized scalar wave equation is solved for propagation parameters which depend on only the normalized frequency V and an arbitrary refractive-index profile. Analytic expressions relate these parameters to the optical fiber diameter, materials, and wavelengths of interest. These expressions are used to find dispersion and bandwidth. In this way it is possible to determine fiber parameters so that minimum dispersion occurs at preferred operating wavelengths. Then refractive-index profiles may be modified to improve bandwidth over desirable ranges.© (1982) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A fluorescent coating (phosphor) applied to the tip of an optical fiber converts incident radiation to longer wavelengths, and can considerably improve fiber-based UV spectrometry. Longer wavelengths are more readily transmitted by the fiber, are less apt to cause UV-induced damage in the fiber, and are more easily detected. Alignment of the fiber is much less critical for fluorescent-tipped fibers and the use of inorganic phosphors which have “peaked” excitation curves can help increase wavelength selectivity. This paper presents a method for evaluating the effectiveness of different combinations of fiber, phosphor, source collimation state, and detector, and describes two different methods for applying durable phosphor coatings to optical fibers.

Although recent fiber-optic transmission experiments have shown impressive results in unrepeatered transmission distance at high data rates,1,2 only a small fraction of the information carrying capacity of the optical fiber has been utilized. To utilize fully the fiber bandwidth, it is desirable to use many closely spaced wavelength division multiplexed (WDM) channels in the minimum loss region ~1.5 μm. We report here the results of a ten-channel WDM system operating at 1.5-μm wavelength. The system was tested with 68.3 km of fiber and with a channel spacing and data rate of 1.35 nm and 2 Gbit/sec, respectively. The experimental setup is shown in Fig. 1. The lasers employed in this experiment were HRO distributed feedback (DFB) lasers.3 The pure single-longitudinal-mode operation, even under high-speed modulation, of these lasers was essential for achieving the narrow channel spacing, low cross talk, and error-free (

We have conducted an experiment which demonstrates 4 Gb/s operation over 103 km of single-mode optical fiber. Previous transmission experiments have been limited to about 2 Gb/s [1,2,3], in part because this is the highest bit rate for which commercial error-rate measuring equipment is available. We have used a multiplexer and demultiplexer built with discrete GaAs FETs to generate a 4 Gb/s NRZ tat pattern from a 2 Gb/s pseudorandom sequence, and to recover 2 Gb/s for error-rate measurements from the 4 Gb/s receiver output. The 4 Gb/s pattern was used to directly modulate a 154 μm vapor phase transported buried heterostructure (VPTBH) distributed feedback laser [4].

The evanescent field of light propagating in an optical fiber core extends into the cladding; this can cause the optical absorbance of the cladding to affect the transmission loss of the fiber itself. The fact that the evanescent fields of propagating modes in the fiber extend into the cladding also means that the inverse effect is possible: light generated in the cladding can be coupled to guided modes and transmitted over long distances. This work describes a fluorescence-based distributed oxygen sensor which makes use of this process.

Single-mode fibers can have bandwidths of the order of 100-1000 GHz km nm, which correspond to several psec/km broadening of semiconductor laser pulses. This is too small to measure in kilometer lengths of fiber because direct detection receivers cannot resolve pulse widths narrower than 50 psec. Practical measurement techniques make use of group velocity dispersion effects which cause light pulses, at simultaneously launched wavelengths. to arrive at different times to the fiber output end. The purpose of this paper is to compare the accuracy, limitations, and ease of implementation of the major dispersion measurement techniques for single-mode fibers.