# O-Band Multimode Interference Coupler Power Combiner Using Slot-Waveguide Structures
###### Abstract
Several combiners have been introduced in recent works such as diffractive, anisotropic photonic crystal-based (photonic bandgap) combiners, and holographic combiners; holographic combiners are thin optical combiners that use diffraction to transform an incident wavefront into another. They can be used as lenses or as non-flat mirrors and have several applications in photonic optical systems [25]. Another widely spread photonic combiner is the photonic crystal-based (photonic bandgap) combiner. Its structure and operating mechanism, as presented in [26], are determined by specific features such as photonic bands, photonic bandgap, resonant interaction of photons with the periodic lattice, and constructive and destructive interferences [26]; however, one of the best known drawbacks of photonic crystal devices is the extreme cost of fabrication. Among the recent and many other combiners, waveguide combiners are based on total internal reflection (TIR) propagation of the entire field in an optical waveguide, essentially acting as a transparent periscope with a single entrance taper and often many exit tapers. When the number of TIR bounces increases, one might refer to them as waveguide combiners. The main components of a waveguide combiner consist of the input and output couplers. These can be either simple prisms, microprism arrays, embedded mirror arrays, surface relief gratings (SRGs), thin or thick analog holographic gratings, meta-surfaces, or resonant waveguide gratings (RWGs) [3]. All of these have their specific advantages and limitations, which will be discussed here. Waveguide combiners have been used historically over a vast range of applications, ranging from AR combiners, such as planar optical interconnections, to LCD backlights [27]. The photonic power combiner is a key component in transmitter systems that require a high-power level. There are usually two types of photonic power combiners, which are power combiners and multiplexers also called wavelength combiners. MMI couplers can be designed as a power combiner for combining many optical signals with the same wavelength or different wavelengths. However, the power loss is relatively high in such components. For example, researchers demonstrated a 4 \(\times\) 1 MMI coupler that has a very low power of 6 dB for each operated wavelength [18]. Some examples of multiplexer or wavelength couplers based on waveguide technologies are ring-resonator filters [28], and photonic bandgap filters [29] have been demonstrated. Wavelength combiners typically have a short band, which can cause distortion between the operating wavelengths. In addition, wavelength combiners in a practical system can reach a lower power level because of the fabrication process errors, which can happen as a result of small deviations in the dimensions of the device [30]. With the amount of information generated by 5G applications, cloud computing is always fueling the need for increasing data bitrates and better energy-efficient photonic interconnections in data center systems [31]. Silicon photonic transmitters depend on high-speed modulator devices, such as micro-ring, Mach-Zehnder, and electro-absorption devices, which are used to satisfy evolving bandwidth and energy requirements as they can combine energy efficiency, compact footprint, and complementary metal-oxide semiconductor compatibility [32; 33]. The back reflection is the amount of light reflected from the output waveguide; such a phenomenon can occur due to the differences in the reflective indexes between the waveguide materials along the boundaries. Therefore, one of the main problems that can limit the performance of the transmitter system is the back reflection coming back to the laser source [13]. The silicon MMI coupler can gain reflections from the self-imaging mechanism, because of its high index difference between silicon and silica (SiO2) [13].