Mode coupling in spiral fibre gratings

The coupling properties of spiral fibre gratings are presented. It is shown that LP₀₁ modes in spiral fibre gratings couple to both LP₁₁ even and odd modes     

Mode-coupling characteristics of UV-written Bragg gratings indepressed-cladding fibre

Additional grating resonances are observed near the fundamental-mode Bragg wavelength in depressed-cladding fibre. They occur above and below the LP₁₁ cutoff. Images of the reflected and transmitted fields and calculations of the phase-matching conditions show that they are due to coupling between LP₀₁ and bound, or slightly leaky, LP₁₁ modes     

Cladding-mode coupling characteristics of Bragg gratings indepressed-cladding fibre

Experimental transmission spectra of UV-written Bragg gratings in depressed-cladding fibre are characterised. The fine-structure features on the short-wavelength side of the fundamental Bragg line are shown to be due to coupling between the LP₀₁ and higher-order LP_0n and LP_1n modes     

Bend loss in buffered over-moded optical fibre: LP11mode and `whispering gallery' mode interaction

Bend loss in over-moded fibre has been measured as a function of temperature. For the LP₁₁ mode, an oscillatory loss dependence on temperature was observed, caused by coupling between the LP₁₁ mode and a `whispering gallery' mode. These interactions could have a significant effect on singlemode cutoff wavelength measurements     

Optical bistability in reflective fiber gratings

Optical bistability in a nonlinearly reflective fiber grating through the mode coupling between the LP₀₁ and counter propagation LP₀₂ modes (i,e., the reflective LP₀₁&rlhar2;LP₀₂ mode converter) is investigated by using the coupled-mode theory. Both the transmissive and the reflective properties of this nonlinear device are analyzed, which show that the output-versus-input relation exhibits the optical bistability. The switching power and the bistable hysteresis performances are also discussed. In addition, the comparison between the nonlinearly reflective mode converter (LP₀₁&rlhar2;LP₀₂) and the nonlinear fiber Bragg grating reflector (i,e., the LP₀₁ and counter-propagation LP₀₁ modes) is also presented. It is shown that the former has much lower switching power than the latter     

Dispersion characteristics of bimodal heavy metal fluoride fibersat 1.3 μm

The design of heavy metal fluoride fibers having segmented-core refractive index profiles and operating a bimodal regime, with small modal dispersion or good equalization, between the LP₀₁ and LP₁₁ modes in the 1.3-μm wavelength region is discussed. The concept of effective cutoff of the LP₀₂ mode has been used in order to maximize the operating V value. A practical bimodal equalized design example which has negligible extrinsic losses is provided in which equalization between the modes is within ±25 ps/km over a wavelength interval of 160 nm (~1.2-1.36 μm). Hence, the predicted bandwidth of the bimodal fiber is high over a broad wavelength interval     

Polarization-selective mode coupling in two-mode Hi-Bi fibers

We propose and demonstrate a mode coupler which converts either of the LP₀₁ polarization states in a two-mode high-birefringence (Hi-Bi) fiber to the LP₁₁ mode with the same polarization. We use coupled-mode theory to develop design rules based on the polarization splitting of the beat length between the two lower-order modes. The device can be operated either as a narrow-band device in a region with large intermodal group delay difference or as a broadband device in a region with zero group delay difference. We use this novel device as a key component in a two-mode Hi-Bi fiber polarizer. In this configuration either of the polarization eigenstates can be selected and transmitted with an adjustable extinction ratio which can be as large as 30 dB. The coupling loss in the transmitted state of polarization can be less than 0.2 dB. In the broadband polarizer we demonstrate -20 dB extinction over 42 nm with a potential for considerable improvement. We also propose and experimentally investigate an increased differential group delay obtained by propagating one polarization state in the LP₁₁ mode instead of in the LP₀₁ mode. The largest differential group delay measured in this configuration is 14.5 ps/m which is seven times larger than the differential group delay between the polarization modes. We discuss several possible uses in fiber sensors and measure the transmission of a proposed two-coupler configuration     

Design optimization for efficient erbium-doped fiber amplifiers

The gain and pumping efficiency of aluminosilicate erbium-doped fiber amplifiers (EDFAs) are analyzed as a function of guiding parameters and Er-doping profile for two pump wavelengths of λ _p=980 nm and λ_p=1.47 μm. Three designs of fiber-amplifier waveguides are considered: one with the same mode size as standard 1.5-μm communication fibers (type 1); one with the same mode size as standard 1.5-μm dispersion-shifted fibers (type 2); and one with mode size smaller than those of communication fibers (type 3). For the 1.47-μm pump, fundamental LP₀₁ mode excitation is assumed, while for the λ_p=980-nm pump, concurrent excitation of LP₁₁ modes is considered. It is shown that excitation of higher-order pump modes at 980 nm does not significantly affect the amplifier gain performance. The effect of concentrating the Er³⁺ doping near the center of the fiber core is shown to increase the amplifier gain coefficients by a factor of 1.5 to 2     

Spatial investigation of transverse mode turn-on dynamics in VCSELs

Data transmission experiments with single-mode as well as multimode 850-nm vertical-cavity surface-emitting lasers (VCSELs) are carried out from a near-field point of view. Special attention is paid to important quantities like on/off-ratio and bit error rate (BER). A single-mode VCSEL oscillating on the fundamental LP₀₁ mode shows no change in eye opening, on/off-ratio, and BER at different lateral fiber coupling positions. In the case of a multimode VCSEL oscillating both on the LP₀₁ mode and LP₁₁ donut mode, we observe a significant lateral change in the on/off-ratio, which plays an important role in BER measurements     

Attenuation coefficient and effective cutoff wavelength of the LP11 modes in curved optical fibers

The measured attenuation coefficient of the LP₁₁ modes in curved optical fibers for various radii of curvature in the wavelength region below the cutoff wavelength for LP₁₁ modes is discussed. The experimental results indicate that the attenuation coefficient may be strongly enhanced at certain wavelengths due to phase-matched coupling between the LP₁₁ modes of the fiber core and whispering gallery modes of cladding. Implications for measurements of the effective cutoff wavelength in optical fibers are discussed     

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     

A novel fiber-optic Fabry-Perot resonator with two mode-conversionmirrors: proposal and application

A novel fiber-optic Fabry-Perot resonator composed of a piece of fiber with two LP₀₁&rlarr2;LP₀₂ mode-conversion reflectors is proposed and theoretically investigated. The reflective fiber-optic LP₀₁&rlarr2;LP₀₂ mode converter is analyzed based on the coupled-mode theory. The resonance characteristics including the transmission, reflection, and circulation of this novel fiber resonator are examined. In addition, we also describe the application of this device as an optical filter which can be used for FDM/WDM communications, etc. In contrast to a conventional Fabry-Perot optical filter with a periodic output spectrum, our proposed optical filter has only one peak in output spectrum, while the bandwidth of the filter spectrum is very narrow, with several Megahertz determined mainly by the cavity length of the fiber resonator. Finally, it is demonstrated that the characteristic of this optical frequency filter can be greatly improved by introducing a gain mechanism using a rare-earth-doped fiber. The tunable performances of this optical filter are also presented     

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     

Structural considerations for bimodal nonlinear optical switching

Based on parameters that arise in both scalar and vector coupled-mode formalisms, the authors consider the effect of circular and planar multilayer guide structure and the related choice of modes in the design of bimodal all-optical switches. Finite-cladding fiber LP₀₁ -LP₀₂ and LP₀₁-LP₁₁ modal interferometers and MQW-sandwich couplers are included as application examples. It is shown that the approach presented provides further insight into complex propagation characteristics of bimodal nonlinear waveguiding structures. This approach is expected to be useful in the design of all-photonic switching devices     

Single-mode optical fibers in the visible range

The fabrication and characterization of single-mode optical fibers in the visible range are discussed. The small core size is obtained by etching the core layers with a flux of freon during the collapse of the preform. The modifications of the index profile are attributed to diffusion of germanium and to incorporation of freon during collapse. From the index profile, the authors have calculated the cutoff frequencies of the LP₀₁ and LP₁₁ modes by assuming a Chebyshev power series of the modal field. The results are in good agreement with theoretical previsions as well as the cutoff wavelength and the losses values     

Optimum design of Er3+-Yb3+ codoped fibersfor large-signal high-pump-power applications

A comprehensive numerical fiber amplifier model has been used to optimize Er³⁺-Yb³⁺ codoped active fiber for maximum gain and quantum conversion efficiency (QCE) at large signal operation. The optimum cutoff wavelength of the LP₁₁ mode has been found to increase from 800 mm at low pump powers (≈50 mW) to 1400 mn at pump powers higher than 500 mW. While at low pump powers fibers with higher numerical aperture give higher QCE, at high pump levels better large signal performance is achieved with fibers having lower numerical aperture     

Acoustooptic Coherent Mode Coupling in Polarization-Maintaining Fiber and Its Application as a Variable-Polarization-Dependent Loss Element

In this letter, we investigate coherent acoustooptic coherent mode coupling from the LP₀₁ core mode to LP_1m cladding mode of a polarization-maintaining fiber (PMF) induced by two acoustic gratings. We show narrowband variable attenuation along either birefringent axis of the PMF by varying the relative phase between the two acoustic gratings. By launching two acoustic gratings at different frequencies, variable attenuation and low crosstalk along either birefringent axis within the acoustooptic coupling bandwidth are demonstrated     

Acoustically scanned low-coherence interrogated simultaneousmeasurement of absolute strain and temperature using highly birefringentfibers

We have utilized a traveling acoustic pulse in a two-mode optical fiber to create a moving beam splitter coupling light from LP₀₁ to LP₁₁ mode. As these optical modes have different group velocities, a variable intermodal delay is generated as a function of acoustic pulse position in the fiber. The device can be used as a receiving interferometer in low-coherence interferometry to scan time delay for carrying out fast extended range absolute measurement using simple analog electronic circuits. With this scanning technique we demonstrate measurement of absolute strain over 1600 με in the temperature range from 20 to 60°C with resolutions of the order of 40 με and 0.7°C, respectively, employing highly birefringent fibers as sensing elements     

Modeling and analysis of fiber-optic mode transducers: single fiberwith periodic perturbations

The fiber-optic LP₀₁&lrarr2;LP₁₁ mode transducers are analyzed by a scalar coupled-mode theory with vector correction. The authors deal with fiber-optic mode transducers made of a single fiber with periodic perturbations due to microbends, acoustic waves, or a photoinduced index grating. Both the couplings caused by the index perturbations and by the vector property of the fields (polarization effect) are taken into account in the analysis. Approximate analytical solutions to the coupled-mode equations are obtained. The power exchange among the modes along the fiber and spectral properties of the mode transducers are examined. The functions of the mode transducers used as wavelength filters and polarizers are studied     

Broadband four-wave mixing generation in short optical fibres

Several advantages of using a short length of fibre to generate broadband four-wave mixing are proposed and verified using wavelength conversion experiments for different lengths of a previously developed high-nonlinearity fibre with a nonlinear coefficient of 13.8 W^-1 km^-1. Consequently, 91.3 nm broadband simultaneous wavelength conversion is demonstrated using a 100 m long section of the developed fibre