# O-Band Multimode Interference Coupler Power Combiner Using Slot-Waveguide Structures
###### Abstract  
Photonic transmitters that operate with a high data transfer rate (over 150 Gb/s) at the O-band range (1260-1360 nm) require at least 100 milliwatts of power to overcome the power losses that are caused by using high-speed modulators. A laser with higher power can probably handle this requirement; however, for the transmitter system, this solution can be problematic due to the nonlinear effects that can happen, which may lead to high noise in the transmitter system. Thus, to solve this issue, we propose a new design of a 2 \(\times\) 1 multimode interference (MMI) power combiner using silicon nitride (SiN) slot waveguide structures. The MMI power combiner and the SiN slot waveguide structures were optimized using the full-vectorial beam propagation method (FV-BPM) and the finite difference time domain (FDTD) method. After combining two sources, high efficiency was obtained of 94.8-97.6% from the total power after a short coupling length of 109.81 \(\upmu\)m over the O-band range with a low back reflection of 44.94 dB. Thus, the proposed device can be very useful for combining two O-band sources to gain a higher power level, which can be utilized to improve performances in transmitter systems. index terms; FV-BPM; MMI; coupler; slot-waveguide; O-band; combiner +
Footnote †: journal: Applied  
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Footnote 1: institutetext: Faculty of Engineering, Holon Institute of Technology (HIT), Holon 5810201, Israel; [EMAIL_ADDRESS] (S.K. ); [EMAIL_ADDRESS] (N.K.) 2  
the slot area in case of transverse magnetic or slab areas in case of transverse electric (TE) mode [9]. This structure is based on three layers, two high index layers (slabs) and a low index layer (slot), which ensures total internal reflection (TIR) and is an important light confinement mechanism of the slot waveguide [10]. One more important phenomenon of the slot waveguide operation is the discontinuity of the electric field at the boundaries between the different materials. The discontinuity is present in the perpendicular electric field component of the electromagnetic wave in TE mode at the slot boundaries. This analysis, which is based on [PERSON]'s equations, shows that the electric field is much more intense in the low refractive index slot region than in the high refractive index regions, resulting in a relatively higher power density in the slot than that in the high refractive index regions [8]. Slot waveguide structure is widely used in sensitive biochemical optical integrated sensors, such as ring resonator-based refractometric label-based optical sensing, and nano-opto-mechanical sensors. The conventional strip or rib waveguides are often used in biochemical sensors based on integrated photonics. In these waveguides, the light is guided based on total TIR in a high refractive index material, surrounded by a low refractive index material; the TIR confines light in the high refractive index material. However, the mentioned waveguides are more wavelength-dependent than slot waveguides because of the relatively high optical interference [11]. These great abilities make the slot waveguide a good candidate for designing new waveguide devices such as splitters [12], demultiplexers [13; 14], combiners [15], and optical amplification [16]. Multimode interference (MMI) based photonic devices is a key building block for designing photonic integrated circuits because of their unique structure, low polarization dependence, large bandwidth, and low excess loss [17; 18]. The operation principle of MMI devices is based on the self-imaging mechanism, which is the ability to reproduce the input field pattern in single or multiple images at periodic intervals over the propagation axis of the device over the waveguide [19]. MMI devices are used for many applications such as filters [20], temperature sensors [21], splitters [22; 23], and couplers [24].