Higher Order Mode Fibers Research Articles

Soliton Self-Frequency Shift: Experimental Demonstrations and Applications

Soliton self-frequency shift (SSFS), a consequence of Raman self-pumping that continuously red-shifts a soliton pulse, has been widely studied recently for applications to fiber-based sources and signal processing. In this paper, the fundamentals of SSFS are reviewed. Various fiber platforms for SSFS (single-mode fiber, microstructured fiber, and higher order mode fiber) are presented and experimental SSFS demonstrations in these fibers are discussed. Observation of Cerenkov radiation in fibers exhibiting SSFS is also presented. A number of interesting applications of SSFS, such as wavelength-agile lasers, analog-to-digital conversion, and slow light, are briefly discussed.

Novel approach for polarization-sensitive measurements of transverse modes in few-mode optical fibers

We report for the first time on the polarization analysis of higher-order modes in a few-mode optical fiber using a three-mirror ring resonator. Higher-order fiber modes were generated by carefully squeezing the fiber. The power fraction of the dominating modes was measured, and we determined the polarization state which is different for each mode due to transverse variations of birefringence in the fiber.

Frequency-domain modal delay measurement for higher-order mode fiber based on stretched pulse interference

We propose and demonstrate a novel modal delay measurement technique for a higher-order mode fiber (HOF) based on optical frequency-domain reflectometry (OFDR) using an extremely simple, entirely passive, and ultrafast wavelength sweeping mechanism, namely, dispersion-induced optical pulse stretching. We obtained a high temporal resolution of approximately 1.12 ps, which was sufficient for discerning the four excited modes in an HOF with a length of only approximately 5 m. The results from our measurements were very consistent with those obtained by using a traditional time-domain measurement method and a conventional OFDR measurement based on a tunable CW laser. Our proposed technique can be also easily adapted to perform conventional time-domain modal delay measurements for very long HOFs.

Sub-60-fs ytterbium-doped fiber laser with a fiber-based dispersion compensation

We present a mode-locked ytterbium fiber laser with a higher-order mode fiber compensating the group-velocity dispersion and partially the third-order dispersion of the single-mode fiber at a wavelength of 1 microm. The generated pulses had an energy of 0.5 nJ and could be dechirped externally to a pulse duration of less than 60 fs. The power spectrum shows a spectral full width at half-maximum of 57 nm.

Calculation of optimal fiber radius and whispering-gallery mode spectra for a fiber-coupled microsphere

Coupled-mode theory is used to numerically calculate the coupling coefficients between the modes of a tapered fiber and those of a fused-silica microsphere. In the visible and near-infrared wavelength ranges, typical tapered fibers are multimode. To maximize the photon tunneling from the fundamental fiber mode to the microsphere whispering-gallery mode and to minimize the tunneling from the sphere to higher-order fiber modes, the optimal fiber radius is found to be somewhat smaller than that for phase matching. Also calculated are whispering-gallery mode spectra that take into account the eccentricity of the microsphere and the fact that in an experiment the tapered fiber is not necessarily perfectly aligned with the equatorial plane of the microsphere.

Propagation of femtosecond pulses in large-mode-area, higher-order-mode fiber

We demonstrate propagation of 14 nJ femtosecond pulses through a large-mode-area, higher-order-mode (HOM) fiber with an effective area of 2100 microm2. The pulses propagate stably in the LP07 mode of the fiber through lengths as long as 12 m. The strongly chirped pulses exiting the amplifier fiber are dechirped by the high-order-mode fiber, resulting in pulses with a peak power of 61 kW after propagation in 5 m of the positive-dispersion fiber. A small amount of self-phase modulation is observed in the compressed pulses and is described well by a nonlinear Schrödinger equation model that takes into account the measured effective area and dispersion of the HOM fiber.

Static and tunable dispersion management with higher order mode fibers

Static and tunable dispersion management with higher order mode fibers

Widely tunable long-period fiber grating with nm-thick higher refractive index film overlay

By introducing a four-layer step-index waveguide modeling, the characteristics of long-period fiber grating (LPFG) with an nm-thick film overlay, which has higher refractive index than that of fiber cladding are investigated in detail. The influence of both the overlay thickness and refractive index on the tuning ability of LPFG is analyzed. The numerical results demonstrate that spectral response of LPFG is divided into three distinct regions as the overlay deposition increases and the shift of resonant wavelength is drastic in some special thickness range. In conjunction with higher-order cladding mode coupling and fiber cladding etching, the sensitivity of LPFG to the overlay refractive index is enhanced significantly and over 120 nm resonant wavelength tunable range is achieved.

High-energy (nanojoule) femtosecond pulse delivery with record dispersion higher-order mode fiber

Delivery of high peak-power femtosecond pulses with fibers is constrained by nonlinear distortions accumulated during pulse propagation. We address this problem with a novel, to our knowledge, fiber schematic, where the pulse propagates in a small Aeff (18 microm2) but highly dispersive (record value of approximately -900 ps/nm km) medium, enabled by transmission in the LP02 mode of a few-mode fiber. The novel fiber yields a low dispersion-to-nonlinear-length ratio (due to its large dispersion) despite its small Aeff, hence enabling mitigation of nonlinearities. This enables fiber delivery of distortion-free <150 fs, approximately 1 nJ, and 840 nm pulses--an order-of-magnitude improvement over single-mode fibers of similar Aeff.

Optimization of Coupling Between Photonic Crystal Resonator and Curved Microfiber

The evanescent coupling from a photonic crystal resonator to a micron-thick optical fiber is investigated in detail by using a three-dimensional finite-difference time-domain (3D-FDTD) method. Properly designed photonic crystal cavity and taper structures are proposed, and optimal operating conditions are found to enhance the coupling strength while suppressing other cavity losses including the coupling to the slab propagating mode and to the higher-order fiber mode. In simulation, the coupling into the fundamental fiber mode is discriminated from other cavity losses by spatial and parity filtering of the FDTD results. The coupling efficiency of more than 80% into the fundamental fiber mode together with a total Q factor of 5200 is achieved for the fiber diameter of 1.0 /spl mu/m and the air gap of 200 nm between the fiber and the cavity.

Electrical spectrum measurements of dispersion in higher order mode fibers

We present a novel technique for measuring the chromatic dispersion of the higher order mode of an optical fiber. The measurement technique is simple, accurate, and capable of measuring the dispersion of kilometer lengths of fiber without the need for mode converters. The dispersion of the LP/sub 02/ mode is measured for several different fibers, and accuracies of better than 1%, compared with a measurement with the modulation phase shift method, are achieved.

Investigation of nonlinear distortion in 40-Gb/s transmission with higher order mode fiber dispersion compensators

The performance of 40-Gb/s transmission systems is limited by the nonlinearity of the transmission and dispersion compensating fibers, and hence, optimized dispersion map designs and signal formats are required to extend error-free transmission distances. In this paper, we describe systematic experiments and simulations of return-to-zero (RZ) and nonreturn-to-zero (NRZ) 40-Gb/s transmission over nonzero dispersion-shifted fiber using higher order mode fiber dispersion management devices. We compare the single channel transmission limits due to nonlinear distortion with dispersion compensators positioned at the start and end of each amplifier span (pre- and postcompensation) and show that the transmission distance of NRZ signals can be extended with the use of precompensation due to the reduced self-phase modulation in the transmission fiber. In contrast, the combination of precompensation and RZ format was found to give poor performance, and analysis shows that this is a result of intrachannel four-wave mixing. The best performance was obtained using an RZ signal format and postcompensated spans. The performance of wavelength-division multiplexing (WDM) signal transmission over the link was then investigated. The simultaneous transmission of multiple 40-Gb/s channels over long-distances was achieved, demonstrating broad-band compensation of the dispersion and dispersion-slope using higher order mode fiber. Results of experiments assessing cross-phase modulation between 40-Gb/s WDM channels spaced by 100 GHz are presented, and the penalties due to inter- and intrachannel nonlinear effects are compared.

Stimulated Brillouin scattering in high-order mode fibers employed in dispersion management modules

Dispersion management modules based on large effective area, relatively short high-order mode fibers are experimentally shown to have high input power threshold to stimulated Brillouin scattering. Theoretically, they behave similarly to the LP/sub 01/ mode of dispersion compensation fibers but with a larger effective area and shorter effective length. The resulting /spl sim/5-dB higher threshold further strengthens the viability of full dispersion and slope compensation without significant input power limitation.