Experimental verification of modal dispersion free characteristics in a two-mode optical fiber

Group delay time difference between LP01and LP11modes has been measured in a step-index type optical fiber over a two-mode region. The measured results have revealed that there exists a wavelength at which the group delay times of the two modes are coincident.     

Length dependence of effective cutoff wavelength in single-mode fibers

Transmission characteristics of single-mode fiber operated in the two-mode (LP01and LP11) propagation region is investigated experimentally and theoretically. The effective cutoff wavelength of the LP11mode λceis commonly used to assure practically the single-mode operation of the LP01mode above λceup to certain fiber length. The experimental λcedecreases linearly in a logarithmic scale of fiber length as increasing fiber length, and eventually becomes constant after a few kilometers' propagation. Theoretical λceis obtained from a coupled power equation by taking into account the mode coupling as well as the losses of the two modes. To calculate λce, the losses of the two modes are individually measured by a new measurement technique. It is clarified from the theory that the nonlinear property of the length dependence of λcein the logarithmic scale is due to a relatively small mode coupling between the LP01and LP11modes.     

Effective cutoff wavelength of the LP11mode in single-mode fiber cables

The effective cutoff wavelength of the LP11mode is investigated for nylon-coated step-index type single-mode fibers stranded in a cable. It is found from the pulse propagation experiments that the single-mode operation is effectively attained in the two-mode region up to theV-value of 3.0. It is numerically estimated that splice loss, bending loss, and absorption loss due to the OH-ion in cladding are reduced by choosing the operatingV-value as large as 3.0.     

Study of vectorial properties of optical-fiber modes using a fiber resonator

Each eigenmode in optical fibers is experimentally analyzed individually by changing the resonance condition of the eigenmode in optical-fiber resonators. When a stress is applied to fibers, it is found that eigenmodes in a single-mode fiber and a two-mode fiber are LP modes, and eigenmodes in a fiber Faraday rotator are elliptically polarized modes. A significant result for the two-mode fiber is that the directions of polarization and orientation of the LP11modes are those of a transverse pressure and principal axes of the elliptical core, respectively. A coupled-mode theory is also presented to explain these experimental results.     

Asymptotic effective cutoff condition in single-mode optical fibers

Recently some experimental investigations have shown that the effective cutoff wavelength of the LP11mode in a single-mode optical fiber decreases with increased distance from the optical source. In this paper such a phenomenon is theoretically investigated confirming that asymptotically the effective cutoff wavelength reaches a minimum value, dependent on few key parameters. Furthermore, attention is focused onto the excess attenuation which characterizes this behavior, leading to a remarkable penalty in the design of the optical link. The wavelength dependence of this parameter seems to be very useful in order to improve the cable manufacturing process.     

Simple approximations for chromatic dispersion in single- mode fibers with various index profiles

Simple approximate formulas of chromatic dispersion and zero-chromatic dispersion wavelength are empirically derived for single-mode fibers with various index profiles having the deformations such as a central dip, hump, and/or tailing at a core cladding interface. These profiles are typical which are commonly obtained in current single-mode fibers. The approximation is based on the exact solution of vector-wave equation. These accuracies are better than 0.3 and 0.03 percent for the chromatic dispersion and zero-chromatic dispersion wavelength, respectively. With the aid of the simple approximations, the properties of chromaric dispersion and zero-chromatic dispersion wavelength for practical single-mode fibers are discussed. The relation between the chromatic dispersion and basic fiber parameters of single-mode fibers, effective cutoff wavelength of the LP11mode, and the spot-size of the LP01mode, are clarified.     

Practical upper limits to cutoff wavelength for different single-mode fiber designs

A practical upper limit to cutoff wavelength in single-mode fiber is investigated. Based on the relationship between the attenuation of the LP11mode and the length dependence of cutoff wavelength, a formula is developed to predict this limit for four fiber designs commonly seen in commercially available fibers. It is found that, depending on fiber design, the upper limit of factory-measured fiber cutoff wavelength is anywhere from 20 to 60 nm above the system operating wavelength, even for a worst case "straight fiber" layout. Under actual field layout conditions, however, the limits converge to approximately 65 nm above the system wavelength for all four fiber designs. The practical upper limit predicted here is confirmed through an experimental evaluation of modal noise effects in a 1.2-Gbit/s single-mode fiber system.     

Characterization of single-mode coated fibers at low temperatures using periodic distortion of the fiber axis

Microbending effects on cabled single-mode optical fibers are analyzed both theoretically and experimentally over the temperature range -30°C-25°C. The loss in single-mode fiber is attributed to the coupling between the guided and radiative modes caused by micro-bending effects. The primary coupling between the guided LP01and the radiative LP11modes is studied by imposing periodically controlled perturbations on the fiber axis at different temperatures.     

Optical fiber-based dispersion compensation using higher ordermodes near cutoff

Higher order spatial modes in optical fibers exhibit large, negative chromatic dispersion when operated near their cutoff wavelength. By using a spatial mode-converter to selectively excite a higher order mode in specially designed multimode fiber, this dispersion can be used to compensate the positive dispersion in conventional single-mode fiber spans. In this paper, issues related to compensating fiber and mode-converter design are explored. Experimental measurements in specially designed two-mode fibers operated in LP₁₁ mode show negative dispersion as large as -70 ps/nm·km at 1555 nm. Pulse propagation and system experiments employing spatial mode-converters to excite LP₁₁ mode in a two-mode fiber demonstrate the feasibility of this technique for dispersion compensation in lightwave systems     

High performance fused-type mode-selective coupler using ellipticalcore two-mode fiber at 1550 nm

In this letter, we demonstrate a high performance fused-type mode selective coupler that couples the LP₁₁ mode in one fiber and the LP₀₁ mode in another using highly elliptical core two-mode fibers. The phase-matching condition was achieved by etching and prepulling portions of two-mode fibers. The coupling efficiency and the mode extinction ratio of 56% to 80% and 22-32 dB, respectively, were achieved with high temperature stability over 1515- to 1595-nm wavelength range     

High-nonlinearity fiber optical parametric amplifier with periodicdispersion compensation

Theory shows that the maximum gain and bandwidth of one-pump fiber optical parametric amplifiers made from high-nonlinearity fiber, operated with a pump wavelength λ_p far from the fiber zero-dispersion wavelength λ₀ can greatly be improved by periodic dispersion compensation. We have performed experiments and obtained good agreement with theory: for λ_p=1542 and λ₀=1591 nm, we have increased the bandwidth from 7 to 28 nm, and the maximum gain from 15 to 20 dB, by splicing three pieces of standard fiber at regular intervals in a 40-m long nonlinear fiber     

Characterization of single-mode fibers from wavelength dependence of modal field and far field

We present direct methods for determination of equivalent-step-index (ESI) parameters and file modal field by measurement of the wavelength dependence of the far-field intensity in single-mode fibers. A comparison is made with the spot-size method commonly used for characterization of such fibers. A new technique for measuring the LP11mode cutoff wavelength is also presented.     

All-fiber wavelength-tunable acoustooptic switches based on intermodal coupling in fibers

In this paper, we demonstrate a novel all-fiber wavelength-tunable acoustooptic switch utilizing intermodal coupling in a two-mode fiber (TMF). Its all-fiber configuration consisting of a fiber acoustooptic tunable filter and a mode-selective coupler results in the low loss (<2 dB) operation. The operating bandwidth >50 nm, the switching time of 40 /spl mu/sec, and the crosstalk of 20 dB were achieved. By controlling the design parameters of the two-mode fiber, the 3-dB bandwidth of the switched signal was varied from 2.5 nm to >35 nm. A novel all-fiber dynamic optical add-drop multiplexer is also demonstrated using two acoustooptic switches in series.     

Increase of the SBS threshold in a short highly nonlinear fiber byapplying a temperature distribution

We evaluate numerically and experimentally the stimulated Brillouin scattering (SBS) threshold increase for a short, highly nonlinear GeO₂-doped fiber by applying different temperature distributions along the fiber. The temperature coefficient for the Brillouin frequency downshift is measured to 1.2 MHz/°C. A threefold SBS threshold increase is obtained for a 100-m long highly nonlinear fiber with a 140°C temperature gradient. The proposed scheme is implemented in a wavelength converter based on fiber optical four-wave mixing (FWM). The SBS suppression scheme shows negligible influence on the FWM efficiency as well as the wavelength conversion bandwidth. The temperature coefficient for the zero dispersion wavelength is measured to 0.062 nm/°C     

Low-loss 1.3-μm MQW electroabsorption modulators forhigh-linearity analog optical links

Low-loss InAsP-GaInP multiquantum-well electroabsorption waveguide modulators have been developed for transmitting microwaves as subcarriers over optical fibers. The fiber-to-fiber insertion loss is only 5 dB at 1.32-μm wavelength. The electrooptic slope efficiency of an 185-μm-long 11-GHz bandwidth device is equivalent to a Mach-Zehnder modulator with a V_π of 2.2 V. The linearity performance was characterized for a test link without any form of amplification. A RF-to-RF link efficiency of -25.5 dB, noise figure of 27 dB and suboctave spurious-free dynamic range of 114 dB.Hz^4/5 have been achieved with 16 mW input optical carrier power. The measured 3-dB electrical bandwidth exceeds 20 GHz for a 90-μm-long device     

A single-polarization optical fiber of hollow pit type with zerototal dispersion at wavelength of 1.55 μm

This paper gives a design method for the single-polarization optical fiber which satisfies simultaneously the wide single-polarization bandwidth, the large modal birefringence, and the zero total dispersion at the wavelength of 1.55 μm. As a type of single-polarization fiber, the optical fiber with two hollow circular pits across the core-clad interface is proposed and designed. The normalized power (=Δ the power in the core/the total power) in the core is also evaluated for the designed fibers. It was found from the numerical analysis that when the zero total dispersion is satisfied at the wavelength of 1.55 μm, the maximum modal birefringence of 1.133×10^-3 and the maximum single-polarization bandwidth of 100.6 nm are attained for the relative index difference of 1.6%. Then the normalized power in the core is 0.859     

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.     

Analysis of dynamic spectral width of dynamic-single-mode (DSM) lasers and related transmission bandwidth of single-mode fibers

A simple formula of the dynamic spectral width of a directly modulated dynamic-single-mode (DSM) laser, and the related maximum transmission bandwidth of a single-mode fiber limited by chromatic dispersion are theoretically given. The dynamic spectral width of a DSM laser is determined by the modulated optical shape and the linewidth enhancement factor α. The spectral width caused by the dynamic wavelength shift is shown to be larger by (1 + alpha^{2})^1/2than that caused by the sideband of the signal of the intensity modulation. Furthermore, the maximum transmission bandwidth of a conventional single-mode fiber with a DSM laser is expressed by using the parameter α and the chromatic dispersion of the fiber. The product of the maximum bit rate and the square root of the fiber length at the wavelength of 1.55 μm is estimated to be about 25 Gbit/s . km^1/2.     

Gigabyte/s parallel fiber-optic links based on edge emitting laserdiode arrays

We present a ten-channel parallel fiber optic link consisting of a transmitter based on an edge emitting laser diode array operating nominally at 1 μm wavelength and a complementary receiver based on an InGaAs pin photodetector array. We demonstrate fiber optic link performance up to data rates of 1 Gb/s per channel with low skew at measurement time limited bit error rates lower than 10^-11 over 100 m of multimode fiber ribbon cable. The transmitter is operational, with very clear eye opening, up to baseplate temperatures of 105°C     

An electroabsorption modulator module for digital and analogapplications

This paper presents an electroabsorption modulator (EAM) module for digital and analog (D/A) applications. Optically broad-band operation of the EAM module is studied for such digital application as wavelength division multiplexing (WDM) systems. Utilizing anisotropic electroabsorption of a multiple quantum-well (MQW) EAM, 40- and 100-nm bandwidth operations in 2.5-Gb/s digital signal transmission over 200-km standard fiber are confirmed by the experiments and the simulations, respectively. For analog applications, low distortion and high link gain characteristics of the EAM module are investigated at the wavelength of 1535 nm. High spurious-free dynamic range of 123 dB·Hz^4/5 and high link gains of -10.3 dB with matching circuit and -20.6 dB without matching circuit are obtained using the EAM module