Loss reduction in fluorine-doped SM- and high N.A.-PCVD fibers

SM and GI fibers have been prepared by the low-pressure PCVD process using fluorine as the main dopant and small amounts of GeO2as codopant. Compared to pure fluorine doping, these fibers showed markedly improved optical properties. High N.A. (delta approx 2percent) GI fibers with Rayleigh scattering losses as low as 1.0 dB μm⁴/ km as well as "deeply depressed cladding" SM fibers with scattering coefficients of 0.85 dB μm⁴/km have been prepared. Simultaneously, a reduced sensitivity of the optical losses on the drawing conditions and improved mechanical properties have been observed.     

Transmission-loss characteristics of Al2O3-doped silica fibers

Transmission-loss characteristics of Al2O3-doped silica fibers have been investigated to develop an Al2O3alternative dopant for VAD silica-based optical fibers. Optical properties of Al2O3-doped silica bulk glass and fibers were measured in the0.11-30-mum spectral region. From the experimental results, the intrinsic loss due to glass material was estimated for Al2O3-doped silica fibers. The calculated intrinsic loss minima for Al2O3-doped silica fibers with 0.2- and 1.0- percent relative refractive-index differences were 0.17 dB/km at 1.548 μm and 0.28 dB/km at 1.565 μm, respectively.     

Fabrication of long lengths of low-loss IR transmitting As40S(60-x)Sex glass fibers

Teflon clad and As₄₀S₆₀ glass clad As₄₀ S₅₅Se₅ fibers transmitting in the 1-6 μm wavelength region have been fabricated in lengths of about 50 m and with minimum losses of 0.098 and 0.65 dB/m, respectively. Short lengths of the Teflon clad fiber possessed a minimum loss of 0.047 dB/m. While current fiber losses are dominated by extrinsic scattering and absorption, the calculated theoretical minimum loss is estimated to be 3.6 dB/km at 5.3 μm and is limited by the contribution from the weak absorption tail. Improvements in the purification and processing of the glasses into the optical fibers are required to reduce the losses further     

Low-loss 1.3-μm MQW electroabsorption modulators forhigh-linearity analog optical links

Low-loss InAsP-GaInP multiquantum-well electroabsorption waveguide modulators have been developed for transmitting microwaves as subcarriers over optical fibers. The fiber-to-fiber insertion loss is only 5 dB at 1.32-μm wavelength. The electrooptic slope efficiency of an 185-μm-long 11-GHz bandwidth device is equivalent to a Mach-Zehnder modulator with a V_π of 2.2 V. The linearity performance was characterized for a test link without any form of amplification. A RF-to-RF link efficiency of -25.5 dB, noise figure of 27 dB and suboctave spurious-free dynamic range of 114 dB.Hz^4/5 have been achieved with 16 mW input optical carrier power. The measured 3-dB electrical bandwidth exceeds 20 GHz for a 90-μm-long device     

An acoustooptical directional coupler for an optical time-domain reflectometer

An optical directional coupler in which a TeO2acoustooptical deflector (AOD) is employed has been designed and developed for use in an optical time-domain reflectometer (OTDR). A low round-trip insertion loss of 4.6 dB and low crosstalk of below -48 dB were attained for the fabricated coupler. Signals of large amplitude, such as Fresnel reflection, could be suppressed to less than - 33 dB by switching of the optical directional coupler. Amplitude fluctuation of the back-scattered signal due to polarization fluctuation in single-mode fibers is reduced to a negligible extent.     

Transmission loss of a 125-μm diameter PANDA fiber with circularstress-applying parts

The transmission loss of a 125-μm diameter polarization-maintaining and absorption-reducing optical fiber (the PANDA fiber) with circular stress-applying parts (SAPs) has been investigated. The loss is calculated at a 1.55- μm wavelength for SAPs consisting of B₂O₃ doped silica glasses. The experimental measurements show the validity of the analysis. It is shown that, when the half distance between SAPs is more than 2.2 times the core radius, the additional transmission loss due to B₂O₃ absorption is less than 0.05 dB/km with normalized frequency of 2.2 to 2.4 and a B₂O₃ dopant concentration of 20 mol.%. As a result, it has been confirmed that transmission losses of 125-μm diameter PANDA fibers can be comparable to those of commercially available single-mode optical fibers     

Chalcogenide glass fibers for mid-infrared transmission

Chalcogenide glass fibers for mid-infrared transmission have been fabricated in As-S, As-Ge-Se, and Ge-S glass systems using high purity materials. The preparation of unclad, Teflon FEP clad, and chalcogenide glass clad fibers and their transmission loss characteristics are reported. It is found that appropriate glass compositions for drawing low-loss fibers are limited to the narrow ranges in the glass-forming regions. The minimum losses obtained are 35 dB/km at 2.44μm for As40S60unclad fiber, 182 dB/km at 2.12 μm for As38Ge5Se57unclad fiber, and 148 dB/km at 1.68 μm for Ge20S80unclad fiber. It is shown that hydrogen impurity absorptions and short-wavelength weak absorption tails seriously enhance loss in the fibers. It is also suggested that ultralow loss cannot be achieved due to the existence of the weak absorption tail. However, it is expected that the chalcogenide glass fibers can be used in short fiber-length applications such as in the remote monitoring and delivery of CO laser radiation. This is due to their wide operating wavelength ranges of0.9-6mum for As-S,1.3-9mum for As-Ge-Se, and0.8-5mum for Ge-S, in which losses can be reduced to below 1 dB/m.     

Measurements at millimeter and micrometer wavelengths

Insertion loss measurements of waveguide components have been made with an accuracy 0.2 dB at the 10-dB level and 2.8 dB at the 30-dB level. Attenuation measurements of 60-mm TE10circular waveguide have been made over the frequency range 33 to 110 GHz with total uncertainty of 0.4 dB. Intercomparison between calorimeters developed by national laboratories at 100 GHz resulted in differences less than 0.5 percent. In measuring optical fibers calorimetry was used to measure loss and power with an accuracy of 1 to 2 percent. Loss measurements by comparison with a standard have resulted in accuracies of 0.5 dB in losses of 40 dB/km in the 1.0- to 1.6-µm range. Shuttle pulse measurement of pulse spreading indicates that pulsewidths of 0.4 ns are increased to 4.0 ns by passing through 2 km of fiber at a wavelength of 0.9 µm. Interferometer techniques were developed for determination of the complex permittivity of liquids and solids over a wide temperature range in the frequency range from 10.0 GHz to 18 THz. Complex permittivities have been measured at 94 GHz by transmission through a dielectric slab. Errors reported in relative permittivity and loss tangent are 0.2 and 2.5 percent, respectively.     

Fabrication of high-concentration rare-earth doped optical fibersusing chelates

A process which uses rare-earth complexes of 2,2,6,6,-tetramethyl-3,5-heptanedione for uniform incorporation of rare earths in the cores of optical fibers is demonstrated. It is a vapor-phase transport process capable of producing fibers having a rare-earth content of at least 11 wt.% and easily lends itself to incorporation of multiple rare-earth dopants. The process gives high rare-earth concentration with low loss and is readily adaptable to multiple rare-earth doping. High-concentration fibers, 1.0 wt.% Nd₂ O₃, necessary for the double-clad fiber laser configuration with low loss, less than 10 dB/km, have been fabricated and shown to have efficiencies approaching quantum limits when pumping well above threshold     

Infrared optical fibers with vapor-deposited cladding layer

Core-cladding structures constructed for KRS-5 polycrystalline and As₂S₃ glass infrared fibers by means of vapor deposition are discussed. In order to obtain high-numerical-aperture fibers, KRS-6 and AsS₃, respectively, have been used as cladding materials. A CO₂ laser beam of 15-20 W has been successfully transmitted through the KRS-5 fiber. For the As₂S₃ fiber an absorption loss due to Teflon coating has been reduced, and a loss of 0.13 dB/m has been achieved     

Gigabit/s optical receiver sensitivity and zero-dispersion single-mode fiber transmission at 1.55 µm

High-speed pulse response and receiver sensitivity at 1.55 μm were measured at data rates ranging from 400 Mbits/s to 2 Gbits/s, in order to elucidate characteristics of a reach-through p+nn- Ge APD. The p+nn- Ge APD receiver provided a 2 Gbit/s received optical power level of -32.0 dBm at 1.55μm and a 10^-9error rate, which was 4 dB better than the receiving level with a p+n Ge APD. Detector performance at 1.3μm was also studied for comparison with performance at 1.55μm. Single-mode fibers, which have 0.54 dB/km loss and zero dispersion at 1.55μm, and an optical transmitter-receiver, whose repeater gain is 29.2 dB, have enabled 51.5 km fiber transmission at 2 Gbits/s. The transmission system used in this study has a data rate repeater-spacing product of 103 (Gbits/s) . km at 1.55μm. Optical pulse broadening and fiber dispersion were also studied, using 1.55 and 1.3μm dispersion-free fibers. Future repeater spacing prospects for PCM-IM single-mode fiber transmission systems are discussed based on these experimental results.     

Fabrication of low-loss IR-transmitting Ge30As10Se30Te30 glass fibers

Improved purification and processing techniques have been utilized to fabricate Ge₃₀As₁₀Se₃₀Te₃₀ glass fibers with a minimum loss of O.11 dB/m at 6.6 μm. This is the lowest loss reported for any telluride glass fiber in the infrared region. Furthermore, the fibers exhibit less than 1 dB/m loss between 5.25 and 9.5 μm     

Research toward optical-fiber transmission systems

The fundementals of optical fiber transmission systems including the fiber transmission medium, sources suitable for use as a carrier, modulation and detection techniques, and some system design considerations are reviewed. The advent of low-loss optical fibers brings new dimensions to optical communication prospects. Fibers may soon be used much as wire pairs of coaxial cable are now used in communication systems. Transmission losses as low as 2 dB/km have been achieved. Experimental repeaters for fiber systems with 10^-9error rate at about 300-Mb/s pulse rate have been reported. Fiber cabling and splicing are among the problems requiring new ideas in order to make feasible an operable system.     

Heavy metal halide glass fiber lightwave systems

Heavy metal halide glass fibers have the potential of optical loss between 0.001 and 0.01 dB/km in the2-10 mum 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 atsimeq 2 mum and a heavy metal chloride glass atsimeq 6 mum. The results indicate that repeater spacings ≳ 1200 and 3600 km atlsim 1Gbit/s may be possible for chlorides and fluorides, respectively.     

Microoptic grating multiplexers and optical isolators for fiber-optic communications

As new optical devices for increasing further the utility of and to expand the application of fiber-optic communications, grating multiplexers and isolators have been developed for 0.8 μm band employing microoptic approach. The development of these devices is the subject of this paper. The devices have desirable features of small size, compactness, high optical performances, and high reliability. The grating multiplexer consists of a graded-index rod, a blazed reflection grating replicated onto the graded-index rod slanting facet or a wedge facet, and an input-output fiber array. Simple calculations have been done to determine necessary element parameters for a given channel spacing. Experimental results are presented for five-channel multiplexers devised using a SELFOC^®lens. Around 3 dB insertion loss and less than -30 dB crosstalk have been obtained for about 35 nm channel spacing in overall device size of18 times 13 times 50mm. Faraday rotation optical isolators for 0.8 μm band have been miniaturized by employing an efficient paramagnetic glass Faraday rotator, a magnet with a through hole and a folded optics in the Faraday rotator. The path number in the folded optics has been optimized in terms of trading-off between the magnet size and the insertion loss. A 0.9 dB insertion loss including fiber coupling loss and 36 dB isolation have been obtained in overall device size of24 times 24.5 times 42mm. Results on the temperature and wavelength dependence of the isolation are also presented. In addition, fundamental properties of optical circulators for 0.8 μm band and optical isolators and circulators both for 1.3 μm band, developed as extended modifications of the optical isolators for 0.8 μm band, are briefly described.     

High-speed optical pulse transmission at 1.29-µm wavelength using low-loss single-mode fibers

Optical-fiber transmission experiments in the 1.3-μm wavelength region are reported. GaInAsP/InP double-heterostructure semiconductor laser emitting at 1.293 μm is modulated directly in nonreturn-to-zero (NRZ) codes at digit rates tanging from 100 Mbit/s to 1.2 Gbit/s. Its output is transmitted through low-loss GeO2-doped single-mode silica fibers in 11-km lengths. Transmitted optical signals are detected by a high-speed Ge avalanche photodiode. Overall loss of the 11-km optical fibers, including 11 splices, is 15.5 dB at 1.3 μm. Average received optical power levels necessary for 10^-9error rate are -39.9 dBm at 100 Mbit/s and -29.1 dBm at 1.2 Gbit/s. In the present system configuration, the repeater spacing is limited by loss rather than dispersion. It seems feasible that a more than 30 km repeater spacing at 100 Mbit/s and a more than 20 km even at 1.2 Gbit/s can be realized with low-loss silica fiber cables, whose loss is less than 1 dB/km. Distinctive features and problems associated with this experimental system and constituent devices are discussed.     

Fiber optic communication - A technology coming of age

In a little over ten years, lightwave communication using optical fibers has progressed from a laboratory proposal to a near commercial reality. Losses in optical fiber waveguides have been reduced from hundreds of dB/Km in the early seventies to less than 1 dB/km at some wavelengths today. Bandwidths of multimode fibers can now exceed 1 GHz in km lengths. Strengths in kilometer length fibers have been increased to hundreds of KPSI (more than steel). Cables containing hundreds of fibers, multiple fiber splices, and single fiber connectors have been developed. Lasers which had lifetimes measured in minutes or hours in the early seventies now have extrapolated lifetimes of over a million hours. New material systems which can use the lower loss longer wavelength regions of the optical spectrum are evolving. Meanwhile, prototype systems carrying voice, data, and video services have been placed in service for commercial telephone and military applications. We can anticipate the widespread use of optical fibers on a routine basis beginning in the early eighties. Along with this will come reduced costs for existing services and the introduction of new services made more economical by this new transmission medium.     

Low-loss optical waveguides with pure fused SiO2cores

Optical waveguide fibers have been produced by a chemical-vapor-deposition technique with optical attenuations as low as 1.1 dB/km at 1.02 µm. The application of this technique to the fabrication of graded index fibers with losses below 2 dB/km is also reported.     

Circular core polarization-maintaining optical fibers withelliptical stress-induced cladding

The reproducible technology for producing high-birefringence fibers with stress-induced elliptical cladding and circular core is described. The authors have obtained fibers that have a birefringence of about (1-3) 10^-4, a mode coupling parameter of about (2-7) 10 ^-5 m^-1, and loss of less than 0.5 dB/km at 1.6 μm. The authors have found effects restricting the capability of test fibers to maintain the state of linear polarization     

High-sensitivity and wide-dynamic-range 10 Gbit/s APD/preamplifieroptical receiver module

InAlAs avalanche photodiodes (APD) and SiGe-HBT preamplifier 10 Gbit/s optical receiver modules have been developed. The measured back-to-back minimum sensitivity and the optical overload with a pseudorandom binary sequence of 2³¹ - 1 at a bit error rate of 10^-9 are -29.5 and +0.4 dBm, respectively. The dynamic range is 29.9 dB. These results show the highest sensitivity and the widest dynamic range yet reported for 10 Gbit/s APD receivers