Selective excitation of parabolic-index optical fibers by Gaussian beams

Higher-order mode suppression technique for multimode sapphire fiber external Fabry-Perot interferometers

A signal picked-up technique to improve the demodulation stability and accuracy of sapphire fiber external Fabry-Perot interferometer is proposed and demonstrated. Through fusion splicing four pieces of multimode fiber in sequence with different core diameters, the in-step change of the core diameter is found to introduce a sufficient fliting effect on the transmitted higher-order guided modes in the sapphire fiber and further reduce their influence on the fundamental mode interference demodulation. Experimental results show that the proposed multi-stage coupling technique can suppress by five-fold the additional phase imposed on the fundamental mode demodulation when compared with the conventional single-stage coupling approach in which single-mode fiber is spliced with only one piece of multimode fiber. The standard deviation of the demodulated optical phase and cavity length can also be reduced by more than two times. The proposed technique provides a simple yet sufficient solution for the long-standing difficulty of multimode sapphire fiber Fabry-Perot interferometer demodulation.

Multispot launching for single-mode excitation in multimode fibers for high-speed applications

Several launching methods have been employed in multimode fibers (MMFs) to reduce the intermodal dispersion. Selective excitation of the fundamental mode alone in an MMF can eliminate intermodal dispersion completely, provided the mode coupling along the fiber is minimum. In this paper, we present the design of a two-core fiber taper that results in the generation of a mode-field diameter-matched twin-spot beam, which can effectively excite only the fundamental mode (LP₀₁) in an MMF. We also propose a new launching method, viz. dual twin-spot launching, which excites the LP₀₁ mode alone in an MMF for a wide range of input beam spot sizes as compared to the other existing methods. The design of a four-core fiber taper for generating the proposed dual twin-spot beam is also presented. Misalignment tolerance of the dual twin-spot beam launching is compared with mode-field diameter-matched center launch (MFDM-CL) and mode-field diameter-matched twin-spot launch (MFDM-TSL). We show that the dual twin-spot launching method has higher alignment tolerances.

Modified twin-spot launching: an improved launching technique for enhancing data rates in multimode fiber

Several launching techniques, such as center launching, offset launching, mode-field diameter-matched center launching, twin-spot launching, etc., have been employed in the past to reduce modal dispersion in multimode fibers (MMFs). Unfortunately, all these methods require stringent alignment accuracy of an input beam with the fiber axis. In this paper, we propose a new optical launching method, viz., the mode-field diameter-matched twin-spot launching technique, which combines the desirable features of two earlier techniques so as to excite only the fundamental mode in an MMF with much higher misalignment tolerance compared to existing launching techniques. Also, we derive for the first time an analytical expression for power coupling coefficients of the excited modes in an infinite parabolic refractive index profiled MMF under twin-spot launching. The proposed technique can excite the fundamental mode with more than 9 dB extinction ratio at 1300 nm, even though the two beams are at a lateral offset position of 5 μm from the axis (i.e., 10 μm apart). Additionally, it is tolerant of variations in the mode-field diameter, relative positions, power, and phase of the input beams. The comprehensive analysis shows cancellation of the odd/even symmetrical mode groups in an MMF depending on the relative phase of the input beams that results in reduced group-delay differences of the excited mode groups.

Modal content of living human cone photoreceptors

Decades of experimental and theoretical investigations have established that photoreceptors capture light based on the principles of optical waveguiding. Yet considerable uncertainty remains, even for the most basic prediction as to whether photoreceptors support more than a single waveguide mode. To test for modal behavior in human cone photoreceptors in the near infrared, we took advantage of adaptive-optics optical coherence tomography (AO-OCT, λc = 785 nm) to noninvasively image in three dimensions the reflectance profile of cones. Modal content of reflections generated at the cone inner segment and outer segment junction (IS/OS) and cone outer segment tip (COST) was examined over a range of cone diameters in 1,802 cones from 0.6° to 10° retinal eccentricity. Second moment analysis in conjunction with theoretical predictions indicate cone IS and OS have optical properties consistent of waveguides, which depend on segment diameter and refractive index. Cone IS was found to support a single mode near the fovea (≤3°) and multiple modes further away (>4°). In contrast, no evidence of multiple modes was found in the cone OSs. The IS/OS and COST reflections share a common optical aperture, are most circular near the fovea, show no orientation preference, and are temporally stable. We tested mode predictions of a conventional step-index fiber model and found that in order to fit our AO-OCT results required a lower estimate of the IS refractive index and introduction of an IS focusing/tapering effect.

Microbend-induced mode coupling in a graded-index multimode fiber

We present a theoretical analysis of the mode coupling effects at a microbend along a graded-index (GI) multimode fiber (MMF). By matching the incident and excited mode fields at the microbend, we obtain the coupling coefficients among the guided modes at different microbending conditions. The theoretical results compare well with the experimental results from near-field measurements on a GI MMF subject to microbending. The usefulness of our theory is demonstrated with two applications: (i) an explanation of the operation principle of a wavelength-switchable fiber laser that contains a fiber Bragg grating in a GI MMF and (ii) quantification of the function of a microbend-based mode scrambler.

Excitation of higher order modes in optical fibers with parabolic index profile

A large number of modes can be supported by multimode fibers. There are applications where higher order modes are preferred. Microbend intensity sensors are good examples. The sensitivity of these sensors is greatly increased if higher order modes are excited. In this work, a simple method to excite higher order modes preferentially is suggested. It consists of thin-film gratings deposited directly onto the fiber end. By controlling the film thickness or transparency of the grating structure, a desired transmission coefficient T(r,Phi) is synthesized. The desired mode can be excited preferentially by incident Gaussian beams without the aid of additional optical components. Binary intensity and binary phase gratings have been studied. Numerical investigation reveals that the phase gratings are more effective for the preferential excitation of higher order modes than the intensity gratings. In fact, by using binary phase gratings and in optimal excitation conditions as much as 81.1, 76.9, 74.6, 73.3, and 72.3% of the power in the incoming, linearly polarized, fundamental Gaussian beam can be converted to LP(02), LP(03), LP(04), LP(05), and LP(06) modes, respectively, excluding Fresnel loss.