Low crosstalk 4 × 4 TiLiNbO3optical switch with permanently attached polarization maintaining fiber array

Several low crosstalk 4 × 4 crossbar optical switch arrays have been fabricated for use atlambda = 1.3 mum. Each array consists of 16 independently functioning directional coupler switches. We describe the typical device performance characteristics. The average insertion loss is 5.2 dB. Crosstalk levels routinely measured < -35 dB. The voltage required to operate the device is ≈ 13 V. The inputs to one of the device arrays were permanently attached to four laser transmitters using lensed polarization maintaining fiber at the laser end and an array of polarization maintaining fibers at the device end.     

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     

Self-alignment technique for fiber attachment to guided wave devices

We propose and demonstrate a new multiple fiber-waveguide alignment technique suitable for single-mode and multimode guided wave devices. The method uses an overlap between a precision-etched silicon substrate and the top surface of the waveguide substrate to align all but one of the six degrees of freedom automatically. We have attached six arrays, each with twelve single mode fibers, and have measured an average excess loss of 0.9 dB atlambda = 1.3 mum. The lowest loss array had an average excess loss of 0.4 dB.     

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.     

Design and performance of ultra-low-loss single-mode fiber cable in 1.5-µm wavelength region

The structural optimization of single-mode fiber for use in the 1.5-μm wavelength region is made with the aim of minimizing the total transmission-line loss over a repeater section. For example, the optimum ranges of the mode-field radius w0and effective cutoff wavelength λceare determined as 5.5 μmleq w_{0} leq 6.5 mum and 1.35μmleq lambda_{ce} leq 1.53 mum for high bit-rate transmission systems with a repeater spacing of 80 km. Based upon the design, ultra-low-loss single-mode fiber cables are fabricated. The average loss of 108 fibers in the cables is 0.19 dB/km at 1.55 μm. The total loss of a 216-km-long fiber link containing 107 splice points was 46.3 dB. Good loss stability at high temperatures as well as during the cable manufacturing processes, are achieved by the appropriate choices of coating materials and optimized fiber parameters.     

Lasing characteristics of 1.5 - 1.6 µm GaInAsP/InP integrated twin-guide lasers with first-order distributed Bragg reflectors

Lasing characteristics of1.5-1.6 mum GaInAsP/InP integrated twin-guide lasers with first-order distributed Bragg reflectors (DBR-ITG lasers) are given theoretically and experimentally. At this wavelength region the fiber loss is ultimately low, but the effect of the material dispersion is serious. It is theoretically found that single longitudinal mode operation over twice the threshold current is obtainable with high quantum efficiency by optimizing the coupling properties and configurational parameters of the DBR-ITG laser. The experimental results are also demonstrated. Single longitudinal mode operation of1.5-1.6mum DBR-ITG lasers under high-speed direct modulation was achieved with a pulsewidth of 1.5 ns.     

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.     

1.064- and 1.32-µm Nd:YAG single crystal fiber lasers

We report the development of CW 1.06- and 1.32-μm fiber lasers made of short sections of Nd:YAG single crystal fibers optically end-pumped either with an Argon-ion laser (lambda = 0.5145mum) or a single laser diode (lambda = 0.817mum). High conversion efficiencies, a few milliwatt thresholds and 10-20 mW output powers are reported, as well as a good coupling efficiency to a standard single-mode fiber. Loss mechanisms and means of reduction are also investigated for a variety of fiber lasers in either guided or unguided configurations.     

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.     

Efficient single-mode fiber to titanium diffused lithium niobate waveguide coupling for λ = 1.32 µm

We report detailed results on the achievement of very high optical throughput for titanium diffused lithium niobate waveguides coupled between input and output single-mode fibers. By determining appropriate diffusion parameters to obtain excellent dimensional match between the fiber and waveguide modes and simultaneously low propagation loss, we have achieved total measured fiber-waveguide-fiber insertion loss as low as 1 dB for a 1 cm long waveguide atlambda = 1.32 mum. The relative contributions of coupling and propagation loss are determined. Very good correlation is found between the coupling loss and the match between the fiber and waveguide mode dimensions. Design data for diffusion parameters to obtain good mode match for arbitrary fiber dimension are presented.     

Measuring dispersion in single-mode fibers in the 1.1-1.3 µm spectral region--A pulse synchronization technique

We describe a novel technique for measuring dispersion in low-loss single-mode fibers in the1.1-1.3 mum 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.     

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.     

Electrooptical effects in silicon

A numerical Kramers-Kronig analysis is used to predict the refractive-index perturbations produced in crystalline silicon by applied electric fields or by charge carriers. Results are obtained over the1.0-2.0 mum optical wavelength range. The analysis makes use of experimental electroabsorption spectra and impurity-doping spectra taken from the literature. For electrorefraction at the indirect gap, we findDelta n = 1.3 times 10^{5}atlambda = 1.07 mum whenE = 10^{5}V/cm, while the Kerr effect givesDelta n = 10^{-6}at that field strength. The charge-carrier effects are larger, and a depletion or injection of 10¹⁸carriers/cm³produces an index change ofpm1.5 times 10^{-3}atlambda = 1.3 mum.     

Low-threshold synchronously pumped all-fiber ring Raman laser

Experimental results on a synchronously pumped, all single-mode fiber Raman laser with a low threshold of 740 mW are reported. Suppression of gain fluctuations due to pump phase noise has been achieved by implementing a fiber coupler of high multiplexing effect. Multistokes generation of low-noise optical pulse trains atlambda = 1.12 mum andlambda = 1.18 mum has also been obtained with a subwatt pump threshold.     

Twelve-channel individually addressable InGaAs/InP p-i-n photodiode and InGaAsP/InP LED arrays in a compact package

We report on monolithically integrated 1 × 12 arrays of In0.53Ga0.47As p-i-n detectors and InGaAsP LED's for use in long-wavelength optical communication system applications. The detectors are sensitive in the wavelength region of0.95-1.65 mum, and the LED's emit at 1.30 μm. The devices utilize Si V-block fiber array connectors with one end polished at a 45° angle with respect to the fiber axis such that the optical path is bent by 90° to afford coupling laterally to the photonic device. The resultant structures are built in a 14-pin dual-in-line (DIP) compact metal package, and are capable of operation at least up to 200 Mbit/s.     

Influence of temperature and initial titanium dimensions of fiber-Ti:LiNbO3waveguide insertion loss at λ = 1.3 µm

This paper describes the influence of fabrication parameters on fiber-Ti:LiNbO3waveguide-fiber insertion loss measured atlambda = 1.32 mum inc-cut LiNbO3. We present a systematic study of the influence of titanium thickness on insertion loss. Within the range examined, diffusion of 950 Å thick 6 μm wide strip of Ti at 1050°C for 6 h produced the minimum loss of 2.0 dB in a 2 cm long waveguide for both TE and TM modes. An analysis of fiber-waveguide coupling for the particular case of an asymmetric diffused waveguide is presented. In general, we find that coupling loss due to modal mismatch between the fiber and waveguide and propagation loss in the waveguide contribute approximately equally to total insertion loss.     

Spectral properties of strongly coupled 1.5 µm DFB laser diodes

Spectral measurements of strongly coupled DFB lasers operating at 1.5 μm are presented. The magnitude of the coupling coefficientkin these devices was determined to be 80 cm^-1for lasers withlambda = 1.12 mum cladding layers and 160 cm^-1for devices withlambda = 1.3 mum cladding layers. These values forkare believed to be the largest reported for 1.5 μm DFB lasers. CW spectral linewidths as low as 10 MHz at 15 mW output power were obtained, and the linewidth was observed to vary approximately as the inverse of the device length cubed. Spectral measurements performed under 2 Gbit/s direct modulation exhibited a side mode suppression ratio of >38 dB. The effects of transient wavelength chirping were also investigated in detail and the maximum wavelength deviation was found to be ≃1.5 Å.     

Analysis and design of coupled-cavity lasers--Part I: Threshold gain analysis and design guidelines

The analysis and design of two-section and multisection coupled-cavity lasers are treated in two parts. In this first part, the focus is on two-section laser design and control using threshold gains. In the second part numerical analysis of the transient behavior is given. The present treatment begins by using the poles of a linear transfer function for the coupled-cavity laser to obtain mode wavelengths and wavelength dependent threshold gains. A general wave scattering matrix describes the intercavity coupling, so that simple waveguide discontinuities or complex arrays of discontinuities can be modeled. Numerical examples are given. Design relationships obtained primarily from the scattering analysis, together with numerical examples, show the dependence and interdependence of the laser mode wavelengths and threshold gain minima on various parameters such as cavity and coupler lengths, indexes of refraction, and gains. These relationships show optimum or preferred coupling gap lengths and cavity length ratios for various design criteria. For example, for optimum mode and spurious rejection stability a short (lsim40 mum) cavity can be coupled via a gap approximately an integer number of half wavelengths long to a second longer cavity (sim 100- 250 mum). Conversely, for maximum tunability (or sensitivity to input changes) together with good mode selectivity, two medium length cavities (sim 100-250 mum) differing in length by a small amount (lsim40 mum) should be chosen.     

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.