Fiber fuse phenomenon in step-index single-mode optical fibers

The unsteady-state thermal conduction process in step-index single-mode (SM) optical fiber was studied theoretically with the explicit finite-difference method. We considered a high-temperature loss-increase mechanism, which includes two factors that bring about an increase in the absorption coefficients: 1) electronic conductivity due to the thermal ionization of a Ge-doped silica core and 2) thermochemical SiO production in silica glass. The core-center temperature changed suddenly and reached over 4/spl times/10/sup 5/ K when a 1.064-/spl mu/m laser power of 2 W was input into the core layer heated at 2723 K. This rapid heating of the core initiated the "fiber fuse" phenomenon. The high-temperature core areas were enlarged and propagated toward the light source. The propagation rates of the fiber fuse, estimated at 1.064 and 1.48 /spl mu/m, were in fair agreement with the experimentally determined values. We found that the threshold power for initiating the fiber fuse increases from 0.98 to 1.26 W when the input laser wavelength is increased from 1.06 to 1.55 /spl mu/m.     

Evaluation of high-temperature absorption coefficients of optical fibers

The large absorption coefficient /spl alpha/ of single-mode optical fiber at high temperatures is closely related to the generation of the fiber fuse phenomenon. We propose a high-temperature loss-increase mechanism, which includes three factors that bring about an increase in the /spl alpha/ values: 1) point-defect (Ge E' center) formation; 2) electronic conductivity due to the thermal ionization of a Ge-doped silica core; and 3) thermochemical SiO production in the silica glass. Ge E'-center formation and electronic conductivity in the Ge-doped silica core are dominant factors at temperatures near the critical temperature of 1323 K. However, these factors cannot explain the large /spl alpha/ values observed at high temperatures above 2273 K. By contrast, SiO gas and/or solid, which are thermochemically produced in the silica glass, exhibit large /spl alpha/ values of the order of 10/sup 4/ m/sup -1/ at 2293 K. The /spl alpha/ value estimated at 0.5 /spl mu/m is close to the experimental value reported in the fiber fuse experiments.     

Simulation of fiber fuse phenomenon in single-mode optical fibers

The unsteady-state thermal conduction process in single-mode (SM) optical fiber was studied theoretically with the explicit finite-difference method (FDM). In the numerical calculation it was assumed that the electrical conductivity of the core layer increased rapidly above 1323 K. The core-center temperature changed suddenly and reached over 3/spl times/10/sup 5/ K when an optical power of 1 W was input into the core layer heated at 1373 K. This rapid heating of the core initiated the "fiber fuse" phenomenon. The high-temperature core areas, whose radiation spectrum extended over a wide range from the infrared to the ultraviolet regions, were enlarged and propagated toward the light source at a rate of about 0.7 m s/sup -1/. This rate was in fair agreement with the experimentally determined value.     

High-speed photography, spectra, and temperature of optical discharge in silica-based fibers

The dynamics of fiber fuse effect including process of bubble formation in fiber core was investigated. Bubbles in the core were observed not later than 20/spl divide/70 /spl mu/s after the plasma forepart passed. For the first time, the temperature of optical discharge was measured for a wide range of laser radiation powers. For 40-W laser power, the temperature reached 10/sup 4/ K, the velocity of discharge propagation having increased to 10 m/s.     

Germania-based core optical fibers

Germania-glass-based core silica glass cladding single-mode fibers (/spl Delta/n up to 0.143) with a minimum loss of 20 dB/km at 1.9 /spl mu/m were fabricated by the modified chemical vapor deposition (MCVD) method. The fibers exhibit strong photorefractivity with the type-IIa-induced refractive-index modulation of 2/spl times/10/sup -3/. The Raman gain of 300 to 59 dB/(km/spl middot/W) was determined at 1.07 to 1.6 /spl mu/m, respectively, in a 75 mol.% GeO/sub 2/ core fiber. Only 3 m of such fibers are enough for the creation of a 10-W Raman laser at 1.12 /spl mu/m with a 13-W pump at 1.07 /spl mu/m. Raman generation in optical fiber at a wavelength of 2.2 /spl mu/m was obtained for the first time.     

Propagation-mode-controlled fabrication of self-written waveguide in photosensitive polyimide for single-mode operation

We have developed a new fabrication method of single-mode self-written waveguide by controlling the propagation mode in an optical fiber. This method is very appropriate for repeatable fabrication of the single-mode self-written waveguide. Since a Gaussian-like near-field pattern is required for the fabrication of a tiny and uniform waveguide core, the propagation mode in a conventional optical communication fiber was controlled by coupling with an optical fiber having 3-/spl mu/m core, which shows a single-mode operation at visible wavelength region. Single-mode propagation at optical communication wavelength was confirmed for the fabricated self-written waveguide. The evaluated core diameter of the self-written waveguide was /spl sim/9.5 /spl mu/m.     

Silica-clad neodymium-doped lanthanum phosphate fibers and fiber lasers

We have fabricated a neodymium-doped phosphate glass fiber with a silica cladding and used it to form a fiber laser. Phosphate and silicate glasses have considerably different glass transition temperatures and softening points making it hard to draw a fiber from these two glasses. A bulk phosphate glass of composition (Nd/sub 2/O/sub 3/)/sub 0.011/(La/sub 2/O/sub 3/)/sub 0.259/(P/sub 2/O/sub 5/)/sub 0.725/(Al/sub 2/O/sub 3/)/sub 0.005/ was prepared and the resultant material was transparent, free from bubbles and visibly homogeneous. The bulk phosphate glass was drawn to a fiber while being jacketed with silica and the resultant structure was of good optical quality, free from air bubbles and major defects. The attenuation at a wavelength of 1.06 /spl mu/m was 0.05 dB/cm and the refractive index of the core and cladding at the pump wavelength of 488 nm was 1.56 and 1.46, respectively. The fibers were mechanically strong enough to allow for ease of handling and could be spliced to conventional silica fiber. The fibers were used to demonstrate lasing at the /sup 4/F/sub 3/2/-/sup 4/I/sub 11/2/ (1.06 /spl mu/m) transition. Our work demonstrates the potential to form silica clad optical fibers with phosphate cores doped with very high levels of rare-earth ions (27-mol % rare-earth oxide).     

Laser link cutting for memory chip repair

Link processing with individual laser pulses has become an industry standard process in IC memory chip manufacturing. It is gaining wide acceptance in analog chip reprogramming and tuning as well. Traditional laser processing, using the standard output of Nd:YAG at 1.064-/spl mu/m and Nd:YLF at 1.047-/spl mu/m laser wavelengths, works well for polysilicon links but is not satisfactory for metal links. This paper describes the physics modeling and computer simulation of the laser link process and a new technique of using 1.3-/spl mu/m laser wavelength for the process. While light absorption of link materials at 1.064-, 1.047-, and 1.3-/spl mu/m wavelengths are relatively the same, the absorption of a Si substrate at 1.3 /spl mu/m is considerably less. The improved absorption contrast between the link material and silicon substrate at 1.3-/spl mu/m delivers a much wider laser process window. Both simulation and experimental results are given and discussed. A brief introduction of another new technique, which uses UV laser pulses for link processing, is given. This UV laser process delivers a laser beam spot size much smaller than 1.5 /spl mu/m.     

Two- and three-photon absorption based optical limiting and stabilization using a liquid dye

It is experimentally shown that both two- and three-photon absorption in a high-molar-concentration chromophore system can be more efficiently utilized to accomplish optical power limiting and stabilization at laser wavelengths of 1.064 and /spl sim/1.3 /spl mu/m, respectively. The nonlinear absorbing medium is a novel liquid dye system, trans-4-[4-(dihexylamino)styryl]-N-(2-{2-[2-(2-hydroxy-ethoxy) -ethoxy]-ethoxy}ethyl)-pyridini um p-tosylate (abbreviated as ASEPT), consisting of chromophore molecules capable of multiphoton absorption in the near IR range. The nonlinear transmission property and output/input characteristics have been studied based on this liquid dye system, using nanoseconds 1.064-/spl mu/m laser pulses for two-photon excitation and /spl sim/1.3-/spl mu/m subpicoseconds laser pulses for three-photon excitation. A fairly good optical stabilization capability of this new material has been demonstrated at both laser wavelengths. The relative intensity fluctuation of the laser pulses can be remarkably reduced by simply passing through this multiphoton absorbing medium.     

1.47 /spl mu/m Tm/sup 3+/:ZBLAN fibre laser pumped at 1.064 /spl mu/m

A large core area (1257 /spl mu/m/sup 2/) Tm/sup 3+/-doped ZBLAN fibre laser operated at 1.47 /spl mu/m is demonstrated. The pump source is a Nd:YAG laser operated at 1.064 /spl mu/m. A laser output power of 1.56 W continuous wave was obtained for 5.2 W of launched pump power. The slope efficiency with respect to the launched pump power was measured to be 33%.     

Waveguide parameter design of graded-index plastic optical fibers for bending-loss reduction

The waveguide structure of graded-index plastic optical fibers (GI POFs), such as index profile, numerical aperture (NA), and core diameter, is appropriately designed for eliminating bending losses, even under a severe bending condition. The bending loss of GI POFs under a severe bending condition is drastically reduced when the core diameter is smaller than 200 /spl mu/m and when the NA is higher than 0.25. The bending loss of GI POFs even under a severe bending condition vanishes with a core diameter of 200 /spl mu/m and an NA of 0.24. It is experimentally confirmed for the first time that the mode coupling due to the bending induces the bending loss. The mode coupling strength before the fiber is bent affects the bending loss seriously. Moreover, the mode-coupling strength is evaluated by the propagation constant difference /spl Delta//spl beta/ between the adjacent modes. The /spl Delta//spl beta/ value, which is a function of the core diameter and NA, affects the bending loss. Therefore, based on the calculation of the /spl Delta//spl beta/, we propose a guideline for the appropriate design of waveguide parameters for GI POF, in order to suppress the bending loss.     

Chalcogenide glass thin films through inverted deposition and high velocity spinning

The fabrication of optical chalcogenide glass thin films through inverted deposition and high velocity spinning is presented. Good quality films up to 275 /spl mu/m thick of this infrared transmitting material, with propagation loss of 0.3 dB cm/sup -1/ at 1.064 /spl mu/m, were engineered using this technique.     

PCB-compatible optical interconnection using 45/spl deg/-ended connection rods and via-holed waveguides

In this paper, a new architecture for a chip-to-chip optical interconnection system is demonstrated that can be applied in a waveguide-embedded optical printed circuit board (PCB). The experiment used 45/spl deg/-ended optical connection rods as a medium to guide light paths perpendicularly between vertical-cavity surface-emitting lasers (VCSELs), or photodiodes (PDs) and a waveguide. A polymer film of multimode waveguides with cores of 100/spl times/65 /spl mu/m was sandwiched between conventional PCBs. Via holes were made with a diameter of about 140 /spl mu/m by CO/sub 2/-laser drilling through the PCB and the waveguide. Optical connection rods were made of a multimode silica fiber ribbon segment with a core diameter of 62.5 and 100 /spl mu/m. One end of the fiber segment was cut 45/spl deg/ and the other end 90/spl deg/ by a mechanical polishing method. These fiber rods were inserted into the via holes formed in the PCB, adjusting the insertion depth to locate the 45/spl deg/ end of rods near the waveguide cores. From this interconnection system, a total coupling efficiency of about -8 dB was achieved between VCSELs and PDs through connection rods and a 2.5 Gb/s /spl times/ 12-ch data link demonstrated through waveguides with a channel pitch of 250 /spl mu/m in the optical PCB.     

35-dB gain Tm-doped ZBLYAN fiber amplifier operating at 1.65 /spl mu/m

We demonstrate the first high gain rare-earth-doped fiber amplifier operating at 1.65 /spl mu/m. It consists of ZBLYAN fiber with a Tm/sup 3+/-doped core and Tb/sup 3+/-doped cladding, pumped by 1.22 /spl mu/m laser diodes. It is possible to achieve efficient amplification with Tm/sup 3+/ ions if their amplified spontaneous emission (ASE) in the 1.75 to 2.0 /spl mu/m wavelength region is suppressed by doping Tb/sup 3+/ ions in the cladding. A two-stage-type fiber amplifier is constructed and a signal gain of 35 dB is achieved for a pump power of 140 mW. A gain over 25 dB is realized in the 1.65 /spl mu/m to 1.67 /spl mu/m wavelength region.     

Low-threshold continuous-wave 1.5-/spl mu/m GaInNAsSb lasers grown on GaAs

We present the first continuous-wave (CW) edge-emitting lasers at 1.5 /spl mu/m grown on GaAs by molecular beam epitaxy (MBE). These single quantum well (QW) devices show dramatic improvement in all areas of device performance as compared to previous reports. CW output powers as high as 140 mW (both facets) were obtained from 20 /spl mu/m /spl times/ 2450 /spl mu/m ridge-waveguide lasers possessing a threshold current density of 1.06 kA/cm/sup 2/, external quantum efficiency of 31%, and characteristic temperature T/sub 0/ of 139 K from 10/spl deg/C-60/spl deg/C. The lasing wavelength shifted 0.58 nm/K, resulting in CW laser action at 1.52 /spl mu/m at 70/spl deg/C. This is the first report of CW GaAs-based laser operation beyond 1.5 /spl mu/m. Evidence of Auger recombination and intervalence band absorption was found over the range of operation and prevented CW operation above 70/spl deg/C. Maximum CW output power was limited by insufficient thermal heatsinking; however, devices with a highly reflective (HR) coating applied to one facet produced 707 mW of pulsed output power limited by the laser driver. Similar CW output powers are expected with more sophisticated packaging and further optimization of the gain region. It is expected that such lasers will find application in next-generation optical networks as pump lasers for Raman amplifiers or doped fiber amplifiers, and could displace InP-based lasers for applications from 1.2 to 1.6 /spl mu/m.     

A surface micromachined optical scanner array using photoresist lenses fabricated by a thermal reflow process

This paper presents the design, fabrication, and operation of a newly developed micromechanical optical scanner array using a translating microlens. We have used photoresist reflow technique to form a microlens on a surface micromachined XY-stage of the scratch-drive actuation mechanism. The lens scanner is placed at the focal length from an incident optical fiber to collimate the transmitting light. The collimated beam is steered two-dimensionally by the XY-motion of the microlens with respect to the incident fiber. We also have developed a theoretical model to predict appropriate initial resist thickness and diameter for the scanning lens. An optical scanning angle of /spl plusmn/7/spl deg/ has been demonstrated by sliding a microlens of 670-/spl mu/m focal length at a physical stroke of /spl plusmn/67 /spl mu/m. Typical angular positioning resolution has been estimated to be 0.018/spl deg/.     

Long-term reliability of single-mode fibers when exposed to high-power laser light

The reliability of a pure silica-core single-mode fiber (SCF) and a germanium-doped fiber [dispersion-shifted fiber (DSF)] with long-term exposure to continuous-wave (CW) high-power laser light is reported. It was found that the optical loss in the 0.6-1.6-/spl mu/m region and the Raman spectra of SCF and DSF remained unchanged after 1900 h of exposure to an 8-W CW laser light and 2000 h of exposure to an 8.5-W CW laser light at 1.48-/spl mu/m, respectively. The nonbridging oxygen hole center (NBOHC) concentration generated in SCF by 1900 h of exposure is estimated to be below the detection limit of 2/spl times/10/sup 12/ cm/sup -3/. SCF and DSF can be expected to exhibit long-term (> 20 years) reliability when exposed to several watts of laser power in the S, C, and L bands in terms of optical degradation due to defect formation via multiphoton absorption.     

Fabrication of semiconductor optical switch module using laser welding technique

The 4/spl times/4, 1/spl times/2, and 1/spl times/4 semiconductor optic-switch modules for 1550 nm optical communication systems were fabricated by using the laser welding technique based on the 30-pin butterfly package. For better coupling efficiency between a switch chip and an optical fiber, tapered fibers of 10-15 /spl mu/m lens radius were used to provide the coupling efficiency up to 60%. The lens to lens distance of the assembled tapered fiber array was controlled within /spl plusmn/1.0 /spl mu/m. A laser hammering technique was introduced to adjust the radial shift, which was critical to obtain comparable optical coupling efficiencies from all the channels at the same time. The fabricated optical switch modules showed good thermal stability, with less than 5% degradation after a 200 thermal cycling. The transmission characteristics of the 4/spl times/4 switch module showed good sensitivities, providing error free transmissions below -30 dBm for all the switching paths. The dynamic ranges for the 4/spl times/4 and 1/spl times/2 switch modules were about 8 dB for a 3 dB penalty and about 17 dB for a 2 dB penalty, respectively.     

Fiber-Optical Transmission Between 0.8 and 1.4 /spl mu/m

The present state of the art and expected development in discrete components for Fiber-optic transmission systems are reviewed. Predicted performance of fiber systems in the 0.85, 1.06, and 1.27 /spl mu/m regions is presented, and the advantages of longer wavelength operation quantified. Itisconcluded that operation near 1.27 /spl mu/m is particularly attractive for a) moderate data rate systems employing LED's and multimode fibers whose chromatic dispersion and attenuation are greatly reduced compared with 0.85 and 1.06 /spl mu/m, and b) high data rate systems employing lasers and monomode fibers. In systems employing lasers and graded index multimode fibers, the advantage of 1.27/spl mu/m versus 1.06 /spl mu/m operation is not as pronounced, although transmission distances at both of these longer wavelengths are significantly increased from those at 0.85 /spl mu/m.     

Optical-transmission characteristics of optical-fiber cables and installed optical-fiber cable networks for WDM systems

Many cables containing 1.3-/spl mu/m zero-dispersion single-mode (SM) optical fibers are installed in trunk and access networks. Recently, there have been a number of studies on wavelength-division-multiplexing (WDM) systems designed to increase transmission capacity and flexibility. If we can construct WDM systems using SM optical-fiber cable networks designed to transmit using wavelengths in the 1.3-/spl mu/m window (O-band), this will prove very effective in reducing construction costs. It is therefore important to examine the wavelength dependence of the transmission characteristics of SM optical-fiber cables and networks that have already been installed and in which several optical fibers are joined. In this paper, we describe the measured optical characteristics of SM optical-fiber cables and installed optical-fiber cable networks at various wavelengths. The optical characteristics were stable in the 1.46 to 1.625-/spl mu/m wavelength range and we confirmed that the installed SM optical-fiber cable networks could be used for WDM system applications.