Optical interference and pulse propagation in multimode fibers

The role played by optical interference in determining the propagation of pulses in multimode fibers is examined under conditions of mode independence and mode coupling. Single pulses and pulse-pairs generated by a source that has arbitrary temporal and spatial coherence are considered, and the role played by the spatial integration at the detector, which effectively is a form of spatial incoherence, is investigated. It is shown that three forms of optical interference may result under different conditions of source spatiotemporal coherence—intramodal, intermodal, and interpulse interference. These effects change the profile of received pulses in a, in general, nonlinear way.     

Optical interference and pulse propagation in multimode fibers

The role played by optical interference in determining the propagation of pulses in multimode fibers is examined under conditions of mode independence and mode coupling. Single pulses and pulse-pairs generated by a source that has arbitrary temporal and spatial coherence are considered, and the role played by the spatial integration at the detector, which effectively is a form of spatial incoherence, is investigated. It is shown that three forms of optical interference may result under different conditions of source spatiotemporal coherence—intramodal, intermodal, and interpulse interference. These effects change the profile of received pulses in a, in general, nonlinear way.     

All-fiber multimode interference bandpass filter

A novel design for an all-fiber bandpass filter based on a multimode interference reimaging phenomenon is presented. The filter has achieved low insertion loss with adequate bandwidth and isolation for coarse wavelength-division multiplexing. The filter can easily be made with any central wavelength that is compatible with the single-mode fiber used for its construction. The measured filter performance matches the theoretical predictions well. The filter can have broad applications in fiber-optic telecommunications, spectroscopy, and sensing.     

Self-guiding multimode interference threshold switch

We propose a new passive optical thresholding device that combines the principles of multimode interference (MMI) with self-guiding. The multimode region is composed of a nonlinear optical material that will support a self-guided beam (i.e., a material with a positive Kerr nonlinearity). The device operates by switching between the MMI mode of operation and the self-guiding mode of operation, depending on the input light intensity. We describe the basic principles of a self-guiding MMI device, simulate the device, and discuss design issues associated with these optically controlled optical switches.     

Two-dimensional overlapping-image multimode interference couplers

The overlapping-imaging effect of one-dimensional (1D) multimode interference (MMI) coupler is widened to study the two-dimensional (2D) MMI coupler. 2D overlapping-image MMI couplers permit uniform and nonuniform 2D power splitting. Analytical formulas are derived for the intensities and phases of the overlapping-images at the end of MMI section. The overlapping-imaging properties in 2D MMI couplers are also concluded. And the guided-mode propagation analysis method is used to confirm the analytical results.     

Compact multimode interference coupler with tapered waveguide geometry

In this paper, a novel MMI coupler, based on general interference, with tapered waveguide geometry has been proposed for reduction of coupling length. The coupling characteristics and power imbalance of the proposed structure are compared with conventional MMI structures by using a mathematical model based on sinusoidal modes. It is seen that the beat length for tapered MMI coupler with angle of taper ∼1.05° is reduced by ∼24% of that of conventional MMI coupler and the coupling characteristics obtained with the mathematical model, match well with those obtained by more sophisticated BPM computer aided design software. The power imbalance for tapered 3 dB MMI coupler is more sensitive to the wavelength than that for conventional 3 dB MMI coupler and variation of power imbalance with fabrication tolerance for both the MMI coupler is almost same.     

Pulse propagation in multimode fibres

The Gloge model for mode-coupled pulse propagation in a step index fibre has been re-examined and a dynamic model proposed to explain the significance of the pulse-broadening regime in which the pulse width varies asL. This leads to the conclusion that the critical factor controlling such propagation is the existence of a particular distribution of power which is a function of space, time and angle.     

A study of multimode interference power splitter with a truncated-and-raised interference region

Multimode interference (MMI) power splitters with various truncated-and-raised interference regions near the input edge are proposed. Simulation results show that ridge width decreasing is more effective for the reduction of non-uniformity and device length, whereas the ridge height increasing is more advantageous for the increase of the total transmitted power. Based on these results, MMI power splitter with a truncated-and-raised interference region is proposed. Simulation results show that the proposed power splitter can have both negligible non-uniformity and larger transmitted power.     

Pulse propagation through multimode fibres

To evaluate their communication potential, measurements have been made on multimode glass fibres. Detector limited pulses of 180 psec risetime from a Ga-As laser were transmitted through various lengths of fibre. After a 120 m length the risetime was still not greater than 450 psec. Measurements with a mode locked He-Ne laser, done on shorter lengths of fibre, agreed with the above results.     

Wave propagation in low-loss acrylic resin films

Acrylic resin is presented as a well suited material for waveguides. The losses due to scattering and absorption in the film are less than 0.13 dB cm^?1. The mode-structure has been investigated as a function of the angle of incidence and the film thickness, both by experiment and numerical calculation. It is demonstrated that acrylic resin is an appropriate material for forming integrated optical components for beam refraction, reflection and splitting.     

Thermo-optic multimode interference switches with air and silicon trenches

A novel thermo-optic multimode interference (MMI) switch with air and silicon trenches was proposed, and the performance of the switch was simulated. In the design, one heating electrode is used to alter the refractive index at a spot image which changes the phase of this image to realize the switching function. The simulation results clearly indicate that the MMI switch can satisfy −39 dB crosstalk at two states. The electric power consumption for the MMI switch with these trenches is less than half of that of a conventional MMI switch.     

Optimization of non-ideal multimode interference devices

Multimode interference devices are examined outside of the step-index, paraxial regime. To determine the optimum length of these devices, we maximize the projection of the propagated field in the multimode section onto the desired field profile. By consideration of the orthogonality among the guided modes, this reduces to a criterion of minimum weighted phase errors at the imaging plane, with no dependence on the actual mode field profiles. The results are confirmed by comparison with mode propagation analysis simulations. This analysis is immediately applicable to the design of multimode interference devices in weakly-guiding geometries such as shallow ridge waveguides, weakly-guiding materials systems such as photosensitive glasses and polymers, and waveguides with graded claddings and/or weak index gradients in the core, such as those formed through diffusive processes. The procedure is also used to explain the presence of the recently observed pseudo-self-image, which is not predicted by the standard theory of multimode interference.     

Aerosol lenses propagation model

We propose a model based on the properties of cascading lenses modulation transfer function (MTF) to reproduce the irradiance of a screen illuminated through a dense aerosol cloud. In this model, the aerosol cloud is broken into multiple thin layers considered as individual lenses. The screen irradiance generated by these individual layers is equivalent to the point-spread function (PSF) of each aerosol lens. Taking the Fourier transform of the PSF as a MTF, we cascade the lenses MTF to find the cloud MTF. The screen irradiance is found with the Fourier transform of this MTF. We show the derivation of the model and we compare the results with the Undique Monte Carlo simulator for four aerosols at three optical depths. The model is in agreement with the Monte Carlo for all the cases tested.     

Study on self-imaging properties for line-tapered multimode interference couplers

The line-tapered multimode interference (MMI) couplers have advantage of compact dimension compared with conventional straight MMI couplers and then are more suitable for integrated optical components. In this paper, the self-imaging properties including general self-image and overlapping-image properties for the line-tapered MMI couplers are discussed thoroughly. Based on the width equation we defined, compact equations for the positions, amplitudes, phases of general images and overlapping images are deduced. Three disciplines for general self-imaging and four disciplines for overlapping-imaging are summarized and discussed. In addition, the overlapping-image properties are further studied by matrix analytic method and an inductive reasoning method of constructing phase and intensity matrix is developed based on it. Finally, all the theoretical results are compared with simulations results obtained by the finite-difference beam propagation method (FD-BPM). Both theoretical and simulation results are shown in this paper and demonstrated to be coincided with each other to a great extent.     

Quantum interference of multimode two-photon pairs with a Michelson interferometer

We perform the second-order quantum interference experiment with the multimode photon pairs produced via an optical parametric oscillator far below threshold in a Michelson interferometer, measure the second-order correlation function in different cases. We find when the interferometer is highly unbalanced, the shape of the second-order correlation function is clearly dependent on the path length difference between two interfering beams. On the contrary, when the interferometer is nearly balanced, beside its height, the shape of the second-order correlation function is independent on the small path length difference. The second-order correlation function shows a multipeaked structure in both cases. All experimental results agree very well with the theoretical predictions.     

Fiber-optic beam shaper based on multimode interference

A new method of fiber-optic based beam shaping is investigated both numerically and experimentally. A cylindrically symmetric method of lines (MoLs) is developed to simulate the device. The device is fabricated by fusion splicing a predetermined length of multimode fiber (MMF) to a single-mode fiber. The multimode interference (MMI) effects create ring-shaped field profiles at certain positions inside the MMF. The shaped beam can be used in medical applications requiring particular irradiation patterns.     

Wavelength dependent propagation in optical multimode fibers

Under various assumptions concerning the wavelength dependence of the refractive index of a fiber whose radial dependence is not an “a priori” known function of r, a method is described to obtain a closed form expression for the flight time of a ray in a multimode fiber. Consequently, the refractive index n(r) is shown to be the solution of a differential equation. Thus, for these selected profiles, we avoid the problem of the high precision required in the calculation of the flight time of a ray, a problem that arises frequently if the impulse response is to be reconstructed from a large number of rays, and transpose it to the determination of a single function of r, namely the refractive index.     

Polymeric bidirectional wavelength filter based on multimode interference

Optical and Quantum Electronics - We proposed and demonstrated a polymer-based wavelength division multiplex filter based on multimode interference for 1.31/1.55-μm bidirectional operations. A...     

Silicon photonic current sensor based on multimode interference

We propose and demonstrate an ultrasensitive integrated photonic current sensor that incorporates a siliconbased single-mode-multimode-single-mode waveguide(SMSW) structure. This kind of SMSW structure is placed over a direct current carrying power resistor, which produces Joule's heat to change the temperature of the SMSW and further results in the change of the effective refractive index between different propagating modes. Interference occurs when the modes recombine at the second single mode waveguide. Finally, the current variation is measured by monitoring the shift in the output spectrum of the multimode interferometer. In low current, the wavelength shift has almost linear dependence: Δλ∝ Ic. This effect can be used as a current sensor with a slope efficiency of 4.24 nm/A in the range of 0–200 m A.