Exact analysis of intermodal dispersion compensation in spliced graded-index optical fibers

Intermodal dispersion compensation in spliced graded-index optical fiber is investigated. Group delay times of modes in individual and spliced fibers are exactly analyzed using the Wentzel-Kramers-Brillouin (WKB) method. It is found that in actual fibers whose refractive index profiles are not represented by the power-law function, the compensation effect is less remarkable than that predicted from the power-law approximation. The baseband bandwidth of spliced fiber, composed of under- and over-compensated index profile fibers, is calculated based on the measured index profile. As a result, the measured bandwidth is found to be well predicted with an error less than 5 percent.     

Mode coupling at arc-fusion splices in graded-index fibers

Mode coupling at an arc-fusion splice has been investigated theoretically and experimentally. It has been certified experimentally that fiber parameters change at an arc-fusion splice. A mode transfer matrix has been derived which describes mode coupling at splices with fiber parameter changes along the fiber axis. The mode-coupling effects on the frequency response of spliced graded-index fibers have been investigated. Length dependence of 3 dB bandwidth has been measured for 10 km long graded-index fiber both with and without a splice. It has been clarified that the mode-coupling effect at the splice broadens the 3 dB bandwidth by 10 percent in the long fiber link in comparison with that for the fiber without a splice.     

Minimum pulse broadening in multimode fibers with index imperfections

Minimum pulse broadening of multimode graded-index fibers is investigated theoretically. Exact solutions of the rms pulse-width σ for a fiber with a power-law index profile is obtained by using the integral expression of group delay time based the multilayer approximation. It is shown that the minimum value of σ and the optimum α for the power-law index profile calculated by the multi-layer approximation in the steady-state modal power distribution are nearly equal to the well-known WKB solution. Next,sigma_{min}and the corresponding αoptare evaluated for certain index imperfections such as a central index dip and sinusoidal index ripple along a radial direction. The results offer the possibility to avoid the bandwidth degradation due to an inevitably generated index tipple in an MCVD graded-index fiber.     

Length dependence of bandwidth for fibers with random-axial profile fluctuations

Length dependence of bandwidth for graded-index optical fibers with random-axial profile fluctuations is investigated theoretically in connection with optical equalization in long spliced fibers. Based on the statistical approach, a simple analytical formula is derived on the assumption that the profile fluctuation does not cause mode coupling. At a short distance, the bandwidth has been found to be inversely proportional to square root of distance due to the profile fluctuation. The length dependence presents a remarkable contrast to that of the mode-coupling case showing the same dependence at a long distance.     

High-silica multimode channel waveguide structure for minimizing fiber-waveguide-fiber coupling loss

The optimum structure for high-silica channel waveguides which are connected to input and output graded-index fibers having a 50-μm core diameter and a 1.0-percent refractive-index difference is studied theoretically and experimentally. Theoretical optimum waveguide structure is obtained on the basis of ray optic analysis. For the step-index waveguide with 1.0-percent refractive-index difference, the theoretical minimum coupling loss (input coupling loss + output coupling loss) is 1.7 dB for an optimum core of 38 μm both in depth and width. For the graded-index waveguide with parabolic-index profile in the depth direction and step-index profile in the width direction, the theoretical coupling loss is 1.1 dB for an optimum core of 38 μm in width and 50 μm in depth. These estimations were in good agreement with the experimental results, which were 1.8 dB for the step-index waveguide and 1.3 dB for the graded-index waveguide.     

Fusion mass-splices for optical fibers using high-frequency discharge

This report describes a high-frequency discharge massheating technique and a fiber aligning mechanism for fusion mass-splicing of silica fibers, and a reinforcement technique for the spliced fibers. Splice loss, operation time, and loss stability of reinforced mass-splices were evaluated by using fiber ribbon units comprising five graded-index fibers with 50-μm core and 125-μm outer diameters. They proved the excellence of the method.     

Impulse response prediction based on experimental mode coupling coefficient in a 10-km long graded-index fiber

Mode coupling coefficients and impulse responses in a multimode graded-index fiber are investigated both experimentally and theoretically. Mode coupling coefficient measurements at 1.27 μm are made for a fiber before and after nylon coating. The fiber is 10 km in length and has no splicing point. The mode coupling coefficient change due to nylon coating is found to be relatively small. In the nylon-coated fiber, the coefficient increases monotonically as increasing a principal mode number, and the tendency is in good agreement with the theoretical prediction considering random bends along the fiber axis. With the increase in mode coupling coefficient and 0.01 dB/km of excess loss by nylon coating, 2.7 percent of increase in 3-dB bandwidth is observed. This increase in 3-dB bandwidth is explained by the theoretical calculation. The length dependence of 3-dB bandwidth exhibits a small mode mixing effect in the fiber. By the theoretical predictions based on the experimental mode coupling coefficients, the coupling length is estimated to be 25 km. It is also clarified that the length dependence of 3-dB bandwidth shows the L^-0.5characteristic forL > 200km.     

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.     

Optimum profile parameter on graded-index optical fibre at 1.27 ?m wavelength

The optimum profile parameter on germanium phosphosilicate graded-index optical fibres at 1.27 ?m wavelength is examined experimentally and is found to be 1.88 ± 0.01. Graded-index optical fibres that simultaneously possess high transmission bandwidth (in excess of 1 GHz km) and attenuation below 1 dB/km are obtained.     

Single and multimode fiber bandwidth measurements with single and multilongitudinal mode lasers operating at 0.8-, 1.3-, and 1.5-µm wavelength

The bandwidth characteristics of single and multimode optical fibers have been investigated with single and multilongitudinal mode laser sources operating at 0.8, 1.3, and 1.5 μm. It is shown that single-mode fiber with a cutoff wavelength of 1.3 μm can support 1 Gb/s transmission over at least 7.5 km with a 0.8-μm laser source.     

Optimum index profile of the perfluorinated polymer-based GIpolymer optical fiber and its dispersion properties

The significant advantages in bandwidth and low material dispersion of perfluorinated (PF) polymer-based graded-index polymer optical fiber (GI POF) are theoretically and experimentally reported for the first time. It is confirmed that the low attenuation and low material dispersion of the PF polymer enables 1 Gb/s km and 10 Gb/s km transmission at 0.85-μm and 1.3-μm wavelengths, respectively. The PF polymer-based CI POF has very low material dispersion (0.0055 ns/nm·km at 0.85 μm), compared with those of the conventional PMMA-based POF and of multimode silica fiber (0.0084 ns/nm km at 0.85 μm). Since the PF polymer-based GI POF has low attenuation from the visible to near infrared region, not only the 0.65-μm wavelength which is in the low attenuation window of the PMMA-based GI POF, but other wavelengths such as 0.85-μm or 1.3-μm etc. can be adopted for the transmission wavelength. It is clarified in this paper that the wavelength dependence of the optimum index profile shape of the PF polymer-based GI POF is very small, compared to the optimum index profile shape of the silica-based multimode fiber. As a result, the PF polymer-based GI POF has greater tolerance in index profile variation for higher speed transmission than multimode silica fiber. The impulse response function of the PF polymer-based GI POF was accurately analyzed from the measured refractive index profile using a Wentzel, Kramers, Brillouin (WKB) numerical computation method. By considering all dispersion factors involving the profile dispersion, predicted bandwidth characteristic of the PF polymer-based GI POF agreed well with that experimentally measured     

Multimode and single-mode fiber connectors technology

Various optical fiber connectors using a graded-index fiber have been fabricated and good performances have been obtained during field trial tests. The average connection loss of aC-type, plug-in type, andU-link connector has been found to be 0.4, 0.34, and 0.62 dB, respectively. A new field assembly (FA) type connector using a precision made ceramic capillary has also been developed. This FA type connector is fabricated for use with single-mode fibers, and has a connection loss of 0.56 and 0.52 dB at 0.85 and 1.3 μm, respectively. By using coupling experiments between laser diodes and single-mode fibers, a laser module with a coupling loss of 1.8 dB under optimum alignment conditions has been fabricated.     

Performance and reliability of high radiance InGaAsP/InP DH LED́s operating in the 1.15-1.5 µm wavelength region

The performance and reliability of InGaAsP/InP double heterostructure (DH) light emitting diodes (LED's) have been investigated over a wide range of emission wavelengths1.15-1.5 mum. High radiance performance has been achieved by growing optimized DH structures and adopting a method of coupling a monolithic-lensed LED to a spherical-ended fiber. Interface instability previously encountered in the liquid phase epitaxy (LPE) to grow a DH for 1.5 μm wavelength has been eliminated by a new growth method using a reduced temperature. LED's exhibiting typical output powers of 50, 44, and 21 μW at 100 mA, measured at the end of a 50 μm core 0.20 NA graded-index fiber with a length of 1 m, have been realized, respectively, at wavelengths of 1.15, 1.27, and 1.5 μm. The spectral width, coupled power and its temperature dependence, and the cutoff frequency have been analyzed in terms of the emission wavelength. Operating lives of LED's at three different wavelengths have been estimated from the result of accelerated aging carried out at the ambient temperature of 200°C and at a constant current of 100 mA. It has been found that the degradation rate exhibits no dependence on the wavelength confirming the same value of half-lives in excess of 10⁹h for the 60°C operation.     

Transmission bandwidth of the two-mode fiber link

The transmission bandwidth of the two-mode fiber link is investigated both experimentally and theoretically. It is shown that the experimental bandwidth as a function of wavelength is accurately predicted by using the individual fiber transfer functions and the mode conversion matrix at a splice. The bandwidth is maximized at the wavelength where the overall group delay time difference between the LP01and LP11modes becomes zero, showing the so-called modal equalization effect. It is experimentally certified that the 3 dB bandwidth over 2 GHz is obtained in the wide two-mode-propagation spectral region around 1.3 μm for 5.2 km long two-mode fiber link for the maximum output power launching condition, due to the relatively large attenuation of the LP11mode after long propagation.     

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.     

Speckle Noise Reduction in Fiber Optic Analog Video Transmission Using Semiconductor Laser Diodes

This paper investigates the effects of speckle noise on analog video transmission systems using semiconductor laser diodes. The system linearity degradation due to speckle noise is examined using different fiber types. Then this paper proposes a new modulation technique employing a superimposed pulse scheme to reduce speckle noise. It is experimentally confirmed that the proposed modulation method is effective in reducing speckle noise. Based on the above investigations, analog video transmission experiments are performed using single-mode fibers, step-index multimode fibers, and graded-index multimode fibers in the 0.8 and 1.3 μm wavelength regions. The results of the transmission tests have confirmed the feasibility of analog video transmission using semiconductor laser diodes.     

Parameter optimization in graded-index dispersion-shifted single-mode fibers

The characteristics of graded-index single-mode nonsegmented-core fibers with a single cladding region, in which the wavelength of zero dispersion is shifted to 1.55 μm, are studied analytically. It is found that for a given relative index difference above a certain value, there are two core sizes at which this zero dispersion shifting is realized. The larger core has certain advantages and has been invariably used in practice. For fibers in which the core is Ge-doped and the index of refraction has a triangular or a parabolic profile, we calculate the rate of change of dispersion with wavelength, the sensitivity of the zero dispersion wavelength to small changes in the core radius and in the refractive index difference, and the outer radius of the cladding needed to limit microbending losses in the cabled fiber. There is a doping level at which the wavelength of zero dispersion is not sensitive to the exact level of doping. The factors involved in choosing a doping level are expounded.     

High-Speed CMI Optical Intraoffice Transmission System: Design and Performance

This paper presents the design and performance features of a successfully developed optical intraoffice transmission system operating at 100-400 Mbits/s. The keys to the commercial realization of this simple, highly reliable, and low-cost system are the employment of the 1.3 μm LED and graded-index multimode fiber. Additionally important, the system makes use of coded mark inversion (CMI) coding to ensure bit sequence independence (BSI) and good error-monitoring capability. Experimental results have clarified the optimum bandwidth of the low-pass filter at the receiver end and the commercially attainable transmission distance. Furthermore, an available system gain of 15.4 dB is demonstrated through 400 Mbit/s transmission experiments. This value enables transmission over distances in excess of 4 km through multimode fiber (900 MHzcdotpkm, 0.8 dB/km).     

Transmission characteristics of 116.3 km and 65.1 km graded-index v.a.d. fibres at 1.55 ?m and 1.3 ?m

Seven long pieces of fibre with low loss and wide bandwidth properties made by the v.a.d. method were spliced with V-grooves into an 116.3 km graded-index fibre, and three long pieces of fibre into a 65.1 km fibre. Transmission characteristics of these graded-index fibres were measured with a swept-frequency method using InGaAsP/InP b.h. laser diodes operating at 1.55 ?m (made by low temperature liquid-phase epitaxial growth) and 1.3 ?m, respectively. The overall transmission losses and 6 dB down electrical bandwidths at the two wavelengths were 37 dB and 16.5 MHz, 35.2 dB and 36.5 MHz, respectively.     

Mode conversion at splices in multimode graded-index fibers

Mode conversions occurring at a splice in multimode graded-index fibers are investigated theoretically, and their effects on impulse responses are verified experimentally. The relation describing the mode behavior at a splice in existence of a geometrical offset and fiber parameter mismatches is derived by taking a skew ray as well as meridional ray into consideration. Then the mode transfer matrix is obtained to determine the variations in mode power distribution and impulse response due to the mode conversions occuring at a splice. The measured mean delay time difference between lower and higher mode pulse responses and baseband frequency response for spliced graded-index fibers are compared with the theory obtained from the transfer functions of individual fibers and the mode transfer matrix which describes the mode conversions at a splice due to a transverse displacement and fiber parameter mismatches.