Design considerations for the structural optimization of a single-mode fiber

Design considerations are made for the structural optimization of single-mode fibers used in high-bit-rate and long-haul transmission systems in the long-wavelength region. As the basic fiber parameters, a combination of the spot size W0and the effective cutoff wavelength λceis newly chosen, because the combination is found to suitably describe various actual index profiles which deviate from an ideal step-index profile. A procedure to specify the usable range of W0and λceis established, whereby the overall transmission-line loss in one repeater section is calculated using simple expressions for fiber intrinsic loss, excess loss in the cabling process, and splice loss, etc. The optimum values for a 400 Mbit/s transmission system operating at 1.3 μm with a repeater spacing of 20 km are obtained asW_{0} = 5.0 pm 0.5mum and 1.1 μmleqlambda_{ce}leq 1.28 mum taking into consideration the additional requirement for the possible use atlambda=1.55 mum     

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.     

Analysis on splice, microbending, macrobending, and Rayleigh losses in GeO2-doped dispersion-shifted single-mode fibers

Independent of the actual index profile of a single-cladded dispersion-shifted single-mode fiber, it is shown that the loss components given in the headline at the zero dispersion wavelength λ0accurately can be obtained by simple expressions that only depend on the Laplace spot size value at λ0. Withlambda_{0} = 1.55 mum, these relations are used to estimate and minimize the total loss in single-cladded dispersion-shifted fibers with arbitrary core-index profiles.     

Transmission characteristics and reliability of pure-silica-core single-mode fibers

Transmission characteristics and reliability for pure-silica-core single-mode fiber with matched cladding are presented. On account of the "pure" silica core, without any additives, the fiber features the low attenuation and improved chemical stability under the existence of hydrogen and γ-ray radiation. High mechanical reliability and good splicing behavior of the fibers were also confirmed. More than 2000 km of pure-silica-core fiber have been fabricated, exhibiting median attenuation of 0.35 dB/km at 1.3 μm and 0.21 dB/ km at 1.55 μm. The achieved minimum attenuation was 0.154 dB/km at1.55-1.56 mum, which is the lowest attenuation ever reported.     

A universal fiber-optic (UFO) measurement system based on a near-IR fiber Raman laser

A universal fiber-optic measurement system, which is useful for measuring loss and dispersion in the1.06-1.6 mum wavelength region, is described. The source is a silica fiber Raman laser pumped by a mode-locked andQ-switched Nd:YAG laser at 1.06 μm. Subnanosecond multiple-Stokes pulses in the1.1-1.6 mum 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.     

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.     

Optical fiber tapers--A novel approach to self-aligned beam expansion and single-mode hardware

We investigate the performance characteristics of single-mode optical fiber tapers. These devices have a standard single-mode fiber geometry at one end and gradually increase in cross section so that the size of the core at the other end is comparable or greater to that of a multimode fiber. These tapers effectively expand the single-mode spot size and are envisioned as basic building blocks in a multitude of optical components. Analytical and experimental studies, atlambda = 0.63 mum, show that the dominant mode is preserved while traveling through the taper, from either direction. The excess coupling loss between two tapers is less than 0.1 dB. The sensitivity of the excess loss to lateral and axial displacements for two coupled tapered sections is greatly reduced compared to that between two single-mode fibers. The sensitivity to angular displacement is increased but is within practical limits. For example, for an excess loss of 0.5 dB, the maximum allowed lateral displacement is 3.1 μm for taper coupling, while only 0.73 μm is allowed in the case of fiber coupling. An axial displacement of 291 μm for taper coupling produces 0.5 dB loss while a displacement of only 16.5 μm produces a 0.5 dB loss for fiber coupling. For the same loss, angular displacements of 0.42° for the tapers and 1.77° for the fiber are allowed.     

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.     

Repeater spacing of 280 Mbit/s single-mode fiber-optic transmission system using 1.55 µm laser diode source

This paper describes an attainable repeater spacing for a high bit-rate single-mode fiber-optic transmission in the 1.55 μm wavelength region where laser mode partition noise comes to be significant. An expression for evaluating mode partition noise is given as the form involving the influence of laser spectral fluctuations under high bit-rate modulation, together with the intersymbol interference and the equalized pulse shape in tile optical receiver. After the validity of its numerical results is confirmed experimentally, the resulting evaluation of laser mode partition noise is connected to a systematic discussion on the attainable repeater spacing of a 280 Mbit/s fiber-optic transmission system operating at 1.55 μm, along with fiber loss versus dispersion tradeoffs. This discussion permits the attainable repeater spacing to be 60-70 km for the combination of a laser diode with 1.5-2.0 nm spectrum broadening and a fiber with the loss of 0.5 dB/km and the dispersion of 4-6 ps/km - nm.     

Long-span single-mode fiber transmission characteristics in long wavelength regions

Experimental and analytical results on high-speed optical pulse transmission characteristics for long-span single-mode fibers by using InGaAsP lasers, emitting at 1.1, 1.3, and 1.5 μm, as well as a Ge-APD are reported. At 1.1 μm, 400 Mbit/s transmission experiments were successfully carried out with 20 km repeater spacing. At 1.3 μm, where single-mode fiber dispersions approach zero, error rate characteristics showed that optical power penalties at 100 Mbits/s and 1.2 Gbits/s are negligible even after 30 and 23 km fiber transmission, respectively. It was confirmed that a 1.6 Gbit/s transmission system has 15 km repeater spacing. At 1.5 μm, where silica fibers have ultimately minimum loss, single-mode fiber transmission experiments were carried out at 100 Mbits/s with about 30 km repeater spacing. 400 Mbit/s transmission characteristics using 20 km fibers were also studied. Fiber bandwidths, measured by optical pulse broadenings after 20 km transmission, were 24, 140, and 37 GHz . km . nm at 1.1, 1.3, and 1.5 μm, respectively. Progress in lasers, fibers, and optical delay equalizers at 1.5μm will bring about large-capacity transmission systems having about 150 km repeater spacing. These results reveal fiber dispersion characteristics in the long wavelength region essential to high data rate single-mode fiber transmission system design.     

Characteristics of a hemispherical microlens for coupling between a semiconductor laser and single-mode fiber

A hemispherical microlens is fabricated on the end of a single-mode fiber by an electric arc discharge technique. It improves coupling efficiency between InGaAsP lasers with buried heterostructure geometry and single-mode fiber. The lowest coupling loss of 2.9 dB is achieved with the optimum lens radius of 8.5 μm. This loss is 4.4 dB lower than that with a butt joint. Experimental results of coupling efficiency and alignment tolerances in coupling with different lens radii in the range of3.5-17 mum are discussed in detail. The results are in good agreement with theoretical values derived by Gaussian beam and paraxial ray approximations.     

InGaAsP/InP double-heterostructure lasers: Simple expressions for wave confinement, beamwidth, and threshold current over wide ranges in wavelength (1.1-1.65 µm)

For InGaAsP/InP devices emitting in the wavelength range of1.1-1.65 mum, we present novel and accurate analytical approximations of the crucial parameters in laser design: the radiation confinement factorGamma; the effective refractive index Neff; and the transverse beamwidththeta_{perp}. It is found thatGammais independent of wavelength or step-index difference andtheta_{perp}becomes independent of wavelength as the active layer thicknessesddecreases below 0.15 μm. An explicit analytical expression is derived for the threshold current density Jth. The new linear gain-current relationshipg(cm^-1) = 28.5(J/d) - 50is deduced from a critical assessment of recent experimental data. A theoretical framework is provided to explain experimental observations such as: the Jthversusdcurve is independent of wavelength, the threshold has a negligible variation over the0.1-0.3 mum active-layer thickness range, and the normalized threshold current density (J_{th}/d, ford geq 0.5 mum) varies strongly (3-5 kA/cm²/μm) with cavity length.     

Highly accurate long-span chromatic dispersion measurement system by a new phase-shift technique

A highly accurate long span chromatic dispersion measurement system, which is based on a wavelength-division-multiplexing phase-shift technique and utilizes six laser diodes in1.2 sim 1.6 mum spectral region, has been developed. It is intrinsically free from error due to the fiber length variation caused by temperature changes under the measurement. The measurement accuracies of dispersion and Zero-dispersion wavelength are extremely good and within ±0.02 ps/km . nm and ±0.1 nm in 1250 ∼ 1450 nm spectral region in the case of a 10.5-km single-mode fiber measurement. The dynamic range is over 50 dB excluding system theS/Nmargin of 5 dB. Using this system, chromatic dispersion measurements of a 101.9-km pure-silica-core single-mode fiber and a 100.7 km concatenated dispersion-shifted single-mode fiber have been successfully carried out. The measured result has coincided with the arithmetical mean of those of constituent fibers.     

Ga0.47In0.53As: A ternary semiconductor for photodetector applications

Ga0.47In0.53As has been used to make fast (rt < 1 ns), photodiodes with low dark current (i_{D} < 10^{-8}A) and good quantum efficiency (ηQext > 50 percent over the entire1.0-1.7 mum region of the optical spectrum). The physical properties related to the crystal growth and carrier transport are discussed in this paper in terms of both the design and the operating characteristics of detectors fabricated from this ternary alloy. The results of our work show that Ga0.47In0.53As is a material well-suited to several important semiconductor device applications. A comparison to other semiconductor photodiodes shows that Ga0.47In0.53As is one of the most sensitive detectors available in the1.0-1.7 mum wavelength region. One can expect repeater-free transmission in excess of 150 km at 100 Mbits . s^-1using these detectors in a digital optical fiber link at the 1.55 μm low-loss (alpha < 0.3dB . km^-1) low-dispersion transmission window.     

Effective refractive index and first-order-mode cutoff conditions in InGaAsP/InP DH laser structures (λ = 1.2-1.6 µm)

The virtual wavelength-invariance of the ratio between bandgap wavelength and the square root of the dielectric-constant step in InGaAsP/InP DH structures [i.e., (lambda_{g}/sqrt{Deltaepsilon}) = 0.95 pm 0.03for 1.2 μm< lambda_{g} < 1.6 mum] and an analytical approximation for the transverse propagation constantb, allow the derivation of an accurate, closed-form expression for the effective refractive index Neffof InGaAsP/InP planar DH lasers emitting in the1.2-1.6 mum range. Then, Neffis only a function of two readily measurable parameters: emission wavelength and active-layer thickness. Furthermore, the mode cutoff conditions for various lateral waveguides: buried-rectangular, buried-crescent, and ridge-guide, become wavelength-independent analytical expressions. First-order-mode cutoff conditions for these lateral waveguides are derived, plotted and compared to experimental data from mode-stabilized 1.3 and 1.55 μm DH lasers.     

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.     

Receiver performance evaluation of various digital optical modulation-demodulation systems in the 0.5-10 µm wavelength region

Error rate characteristics of various digital optical modulation-demodulation schemes are studied. The main concern is whether we can improve receiving power levels to achieve a prescribed error rate by employing a coherent optical transmission system in place of the presently available amplitude-shift-keyed (ASK) baseband direct detection system. The receiving power level reduction in various modulation-demodulation schemes is calculated by taking into account the optical carrier wavelength, data rate, photodetector performance, local oscillator power level, and number of levels in multilevel codes. The phase-shift-keyed (PSK) homodyne detection system requires the least receiving power. The improvement in the receiving power level compared to the conventional ASK baseband direct detection system is expected to be 16-22 dB at the carrier wavelength oflambda_{c} = 0.5-3 mum, 31-36 dB atlambda_{c} = 3-5 mum, and 35-40 dB atlambda_{c} = 5-10 mum.     

High-speed traveling-wave directional coupler switch/modulator for λ = 1.32 µm

We have designed and fabricated a Ti:LiNbO3waveguide traveling-wave directional coupler switch/modulator operating atlambda = 1.32 mum that exhibits a 3 dB modulation bandwidth of 7.2 GHz, a switching voltage of 4.5 V, and power per unit bandwidth of 7.6 mw/ GHz. Using short drive pulses, optical pulsewidths as short as 58 ps have been generated and directly measured with a high-speed InGaAs/InP p-i-n photodiode. Thick (sim 2.8 mum) electroplated gold has been used to produce a small gap (5 μm) and low-loss coplanar strip electrode. The 1.5 cm long, 15 μm wide electrode has a dc resistance of 7 Ω and a total microwave power loss of ∼4dB at 5 GHz.     

Optical transmission in dispersion-shifted single mode spliced fibers and cables

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 mum). 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.     

Fabrication of dispersion-free VAD single-mode fibers in the 1.5-µm wavelength region

Low-loss and low-dispersion single-mode fibers in the 1.5- μm wavelength region were fabricated by the VAD method. Causes for loss increase in these fibers were investigated. By improving uniformities in the refractive index, both in core section and along the core axis, minimum loss of 0.35 dB/km at 1.55 μm was obtained. Bending loss of the 1.5-μm optimized single-mode fiber was also discussed.