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.     

Characteristics of Gbit/s optical receiver sensitivity and long-span single-mode fiber transmission at 1.3 µm

Sensitivity of a 1.3 μm Ge APD receiver was measured at data rates ranging from 100 Mbits/s to 2 Gbits/s, using a high-speed GaAs FET RZ driver, low-noise Si bipolar transistor (BIT) receiver amplifier, and a highly sensitive TD comparator. The required received optical level at a 10^-9error rate was -31.9 dBm for 2 Gbits/s with a Ge APD/Si BIT front end having a 50 Ω input impedance. A Ge APD/ GaAs FET front end, with a 500 Ω input impedance, brought about 2 dB improvement at 100 Mbits/s, as compared with a Ge APD/Si BIT (50 Ω) front end. A coupling loss of 4 dB, achieved by a hemispherical microlens tipped on a single-mode fiber, and a low fiber loss of 0.57 dB/km, including splice loss, enabled 44.3 km single-mode fiber transmission at 2 Gbits/s. The 1.3 μm transmission system has a data rate repeater-spacing product of 88.6 (Gbit/s)km. Prospects of Gbit/s receiver sensitivity and the 2 Gbit/s transmission system, with more than 50 km repeater spacing, are also discussed.     

Single-mode optical fibre tapers for self-aligned beam expansion

We demonstrate that optical fibre tapers can be utilised as a new means for achieving self-aligned beam expansion in single-mode fibres. These devices, which have a standard single-mode geometry at one end and gradually increase to a core size in the order of 100 ?m at the other end, have greatly reduced sensitivities to lateral and axial displacements and an excess coupling loss between two tapers of less than 0.1 dB.     

Low-current 1.3-µm edge-emitting LED for single-mode fiber subscriber loop applications

Light-emitting diodes coupled to single-mode fiber can offer an economical and reliable alternative to the use of diode lasers in many single-mode fiber systems. This paper describes index-guided InGaAsP edge-emitting LED's (ELED's) which couple 7 μW into single-mode fiber at a drive current of only 20 mA. The devices exhibit rise times of less than 1.5 ns and can be modulated at data rates in excess of 432 Mbit/s. Operating characteristics of these low-current ELED's are well-suited for single-mode fiber subscriber loop applications.     

Low-threshold InGaAsP ridge waveguide lasers at 1.3 µm

The ridge waveguide configuration is shown to provide reliable low-threshold fundamental-transverse-mode lasers that are readily fabricated. Two variants are described: in the simple ridge laser, the 1.3 μm bandgap active layer is sandwiched between InP layers and in the cladded ridge, the active layer is surrounded by 1.1 μm bandgap InGa AsP. Thresholds as low as 34 mA and efficiencies as high as 66 percent are observed. Output power is linear to more than 12 mW. Several lasers have been operated at 30°C for over 1500 h without measurable degradation. Selected lasers exhibit stabilized longitudinal mode behavior over extended temperature and current ranges. The potential manufacturability of this device is its most attractive feature.     

Low-loss polarization-independent electrooptical switches at λ = 1.3µm

Polarization-independent single-mode optical cross points with low loss have been fabricated forlambda = 1.3-mum wavelength. Waveguide design, coupling strength equalization for the two polarizations, fabrication tolerances, and the performance of several Ti:LiNbO3directional-coupler switches are presented. Two 2 × 2 switches have been made with fiber pigtails. The switches have an insertion loss of ∼3- andsim-14-dB crosstalk isolation with a 70-V operating voltage. A 4 × 3 switch had similar performance but higher insertion loss.     

Lateral confinement InGaAsP superluminescent diode at 1.3 µm

A superluminescent diode (SLD) has properties, based on the degree of coherence, that are bounded by those of the light emitting diode and the laser diode. The SLD can be designed to meet a wide range of optical system needs. By introducing ridge-waveguide lateral confinement, a good anti-reflection coating at one end and a high reflectivity mirror at the other, we have demonstrated an SLD that allows 30 percent coupling efficiency into a lensed 0.23 NA, 50 μm diameter graded index core fiber. The power in the fiber is 550 μW at 250 mA and 20°C. It is possible to maintain a constant power level in the fiber greater than 250 μW over the temperature range 0 to 35°C by adjusting the current. The spectral width is 300 Å and the modulation bandwidth 350 MHz. PCM with 400 Mbit/s rate has been observed. These devices are relatively easy to fabricate from ridge-waveguide lasers or any other lateral confinement laser.     

Gigabit single-mode fiber transmission using 1.3-µm edge-emitting LEDs for broad-band subscriber loops

This paper describes gigabit single-mode fiber transmission using 1.3-μm edge-emitting LED's for broad-band subscriber loops, focusing on a method of calculation for maximum transmission distance and 1.2-Gbit/s and 600-Mbit/s transmission experiments. Gigabit single-mode fiber transmission is necessary for subscriber loops, especially in broad-band ISDN and optical CATV systems. Edgeemitting LED's are excellent light sources because of their high power launched into the fiber compared with surface-emitting LED's, and currently lower cost and higher reliability than laser diodes. The maximum transmission distance is carefully estimated by taking into account the wavelength dependence for both chromatic dispersion and loss of the single-mode fiber, and the possibility of gigabit transmission near the dispersion free wavelength 1.3 μm, is confirmed. Encouraged by the above results, we demonstrate 1.2-Gbi,t/s 10-km and 600-Mbit/s 20-km transmission experiments using a newly developed 1.3-μm edge-emitting LED and a new driver circuit with a simple response compensation circuit. These results show the proposed calculation method and the LED response compensation circuit to be powerful tools for the realization of low-cost gigabit single-mode fiber transmission using edge-emitting LED's.     

High-order soliton pulse compression and splitting at 1.32 µm in a single-mode optical fiber

The optically compressed pulses from a CW mode-locked Nd:YAG laser operating at 1.32 μm have been further compressed through a high-order soliton generation effect in a 40 m length of single-mode dispersion-shifted optical fiber. Two- and three-fold splitting of the soliton pulses have been demonstrated. Experimental compression factors and spectra forN = 3, 5, 7, 9, 11, and 13 solitons have been shown to agree qualitatively with theory. In shorter fiber lengths pulses as short as 33 fs have been generated, corresponding to an over-all compression factor of X2700, the largest reported so far.     

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.     

Transmission characteristics of a single-mode fiber in the 1.3-µm wavelength region

Transmission characteristics of 1.3-μm-band single-mode optical fibers were studied experimentally and theoretically. OH-ion content dependence of optical loss in the 1.3-μm region was investigated on single-mode fibers fabricated by the VAD method. The optical loss was evaluated for OH-ion content by calculating correlation factors between the optical loss and 1.39-μm OH-peak absorption loss. It was clarified that the optical loss calculated by correlation factors agreed approximately with the loss given by Lorentzian absorption in case of 0.0253-μm absorption half width, Next, the relationship between chromatic dispersion and fiber parameters was studied for step-index single-mode fibers. It was found that the zero-dispersion wavelength and the incline at the wavelength was expressed as simple functions of effective cutoff wavelength and relative index difference. Experimentally obtained zero-dispersion wavelengths on single-mode fibers fabricated by the VAD method were found to be in excellent agreement with the calculated values.     

A polarization-independent optical circulator for 1.3 µm

A compact polarization-independent optical circulator for 1.3μm is described. The device utilizes birefringent plates (rutile) for beam splitting, YIG plus a single-order quartz half-wave plate for polarization rotation, and a slotted 45° glass prism for beam steering. The fiber-to-fiber insertion loss is 2.0 or 2.5 dB and the isolation is 25 or 35 dB, depending upon the ports in question. The use of birefringent plates for beam splitting and combining eliminates the need for multiple dielectric coatings for polarization beam splitting, requires only optical plate processing, virtually eliminates sensitivity to translational misalignment of individual beam-splitting/combining components, and reduces the required number of optical-quality surfaces.     

Optical single sideband generation at 10.6 µm

The indicatrix ellipsoid of an electrooptic crystal with a threefold rotation axis under the influence of an applied microwave field circularly polarized in a plane normal to the axis intersects this plane in a rotating ellipse of constant shape. A circularly polarized optical field in this plane produces an electric polarization at one sideband frequency only, upshifted if the optical field rotates in the opposite direction the indicatrix rotates, downshifted if the rotations are the same. Ideally, 100 percent conversion of the optical incident field into the up- or downshifted field may be achieved. A square waveguide containing a CdTe crystal is investigated as the structure appropriate for phase-matched single sideband conversion. Experiments are reported yielding a 67 percent conversion efficiency for shifting 10.6 μm laser radiation by 17 GHz. The efficiency was limited by crystal length and/or microwave breakdown. For longer crystals conversion efficiencies approaching 100 percent are extrapolated. Scaling laws for other microwave and laser frequencies are presented.     

Distributed feedback laser emitting at 1.3 µm for gigabit communication systems

A distributed feedback (DFB) laser emitting at 1.3 μm for gigabit lightwave communication systems has been developed. The distributed feedback structure has been introduced in a newly developed buried heterostructure and designs for stable single-mode operation, high speed modulation, and low noise have been done. Threshold current of 10-15 mA, differential efficiency of around 0.28 mW/mA, low noise, small signal modulation bandwidth of 13.9 GHz, and satisfactory modulation waveform at 5-Gbit/s NRZ modulation have been attained with high single-mode operation yield.     

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.     

Characteristics of dispersion free single-mode fiber in the 1.5 µm wavelength region

Characteristics of dispersion free single-mode fibers in the wavelength regions 1.5 and 1.3 μm are compared experimentally and theoretically. We consider the influence of the refractive index profile on dispersion, the tolerance limits of structure parameters for minimum dispersion, attainable fiber bandwidth, and transmission loss including splicing and bending losses. For a fiber designed for minimum dispersion at 1.5 μm, the measured fiber loss was less than 1 dB/km and bandwidth was 250 GHz. km. nm. The achievable minimum loss estimation shows the advantage of dispersion free fibers at the 1.5 μm wavelength over dispersion free fibers at 1.3 μm.     

Trapezoidal index single-mode fibers for the 1.55 µm band

An analytical study is made of single-mode fibers with a trapezoidal index of refraction profile for low dispersion operation in the 1.55-μm band using Ge doped silica core and pure silica cladding. It is found that certain trapezoid proportions give a fiber with a combination of less Ge doping, larger core diameter, faster decay of the field in the radial direction within the cladding, and smaller rate of change of dispersion with wavelength than is simultaneously possible with either the step index or triangular profiles.     

1.3 µm high-power BH laser on p-InP substrates

In our fabrication of a 1.3 μm band high-power BH laser on a p-type InP substrate, 79 mW CW laser output was obtained, and the spectrum width was 10 nm at 50 mW; it also obtained a high-power pulse output of more than 200 mW at 30 ns pulse width. It shows high-speed pulse response at 2 Gbits/s. These CW and pulse lasing characteristics are reported in this paper, and we also show the output and threshold current distribution of about 1000 samples from six wafers. This high-power laser is very useful for light sources of measuring instruments.