Principles and application of reduced beat length in MMI couplers

Abstract

A unified method was proposed to reduce the beat length of a multimode interference (MMI) coupler. By properly adjusting the phase difference of the N-fold images, the mode evolution is changed to generate self-images at a much shorter distance. The effect of adjusting the phase difference can be regarded as dividing the original MMI coupler into multiple sub-MMI couplers. Such an effect can be applied for both symmetric- and paired-interference cases. We applied the principle to design compact optical splitters operating at dual wavelength bands. The simulation shows that excellent performance with reduced coupler length can be obtained for splitters operating at both 1.3 and 1.55 mum bands.

Liquid crystal tunable claddings for polymer integrated optical waveguides

“…Each mode in the MMI travels according to its specific effective refractive index and thus an optical power distribution pattern is generated from the interference of the modes present in the MMI. The pattern repeats at a certain distance [11] called the “characteristic length”, L π ,…”

Optimized ARROW-Based MMI Waveguides for High Fidelity Excitation Patterns for Optofluidic Multiplexing

“…Unfortunately, shorter MMI waveguide designs are not feasible given the sensor’s application. In order to optimally excite the fluorescent labels at their excitation wavelengths the waveguide is lengthened to achieve a common multiple of the beat lengths for each color [15]. The width cannot be adjusted to shorten the length (Equation 1) as the width must be kept wide to provide adequate spacing between excitation beams, which determines Δt.…”

“…The width cannot be adjusted to shorten the length (Equation 1) as the width must be kept wide to provide adequate spacing between excitation beams, which determines Δt. In summary, as seen in telecommunications [15], shorter MMI waveguides can attain higher fidelity spot patterns and in order to obtain optimal spot patterns the etch depth and excitation waveguide width must be deep and narrow.…”

“…When used for multiplexing, the MMI waveguide needs to be designed such that the device length achieves high fidelity spots across multiple wavelengths. This requires a specific length L to get an integer numbers of spot patterns for different wavelengths at the liquid core (LC) intersection [15]. For our design which is meant to give N = 6 spots at 738 nm, N = 7 spots at 633 nm, N = 8 spots at 554 nm and N = 9 at 492 nm, the MMI waveguide for optofluidic multiplexing is 75 μ m wide by 1,932 μ m long or 100 μ m wide by 3,442 μ m long.…”

Buried Rib SiO2 Multimode Interference Waveguides for Optofluidic Multiplexing

“…In order to fit within the required wavelength range, MMI widths must be accurate to +/ − 0.8% of target widths [14]. Further, the MMI waveguide must be made long to attain a common multiple of the beat lengths across these wavelengths [16]. Typically this increase can be minimized by reducing the width.…”