Abstract
A universal principle to achieve different optical hybrids is summarized based on general nonoverlapping-image MMI couplers. An optical 72 $^{\circ }$ hybrid is proposed and demonstrated which is achieved by a 2 × 5 multimode interference (MMI) coupler with length of 89.5 $\mu$ m and width of 8.0 $\mu$ m. The device is fabricated in silicon-on-insulator (SOI) with 220 nm thick top silicon layer and the buried oxide layer is 3 $\mu$ m thick. The proposed device exhibits an extinction ratio larger than 20 dB from 1470.8 nm to 1581.2 nm. The excess loss of optical hybrid is about 1.4 dB and a phase deviation less than $\pm 6.0^{\circ }$ is obtained with the bandwidth of 51.5 nm.
Highlights
Multimode interference (MMI) coupler is a kind of multi-port optical structure which performs the basic function of beam splitting and combining
The optical hybrid is a kind of advanced device which is central to many applications such as coherent transmission systems [19] and phase sensitive detection [20], [21], which can be achieved by MMI couplers
Based on the presented principle of optical hybrids, we proposed three kinds of 72◦ optical hybrids based on 5 × 5 nonoverlapping-image MMI coupler
Summary
Multimode interference (MMI) coupler is a kind of multi-port optical structure which performs the basic function of beam splitting and combining. Light input from any port will excite several modes with different propagation constant. One or more images of the input light field appear at different lateral direction when multi-modes propagate through the region of coupler. The MMI couplers applied for the hybrid should provide certain relative phase difference at different output ports when light is incident from different input ports [22], [23]. The optical phase matrix of general nonoverlapping-image MMI coupler is analyzed with the deduction of important phase principles. The positions of input and output ports are determined by xp = (p − 0.5)W/N, xq = (q − 0.5)W/N with p, q = 1, 2 · · · N. relative phase information is interpreted, which is instructive to design hybrid with different degrees. The fabricated device is based on 220-nm silicon-on-insulator (SOI) platform and the testing results are well matching the simulated characteristic
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