## 1 Introduction  
The In\({}_{0.53}\)Ga\({}_{0.47}\)As p-i-n photodetector is the most commonly utilized one for fiber optic communication networks in the 1.3-1.6 \(\upmu\)m wavelength range. GaAs is the most well developed compound semiconductor material for electronic devices due to its high drift mobility [1]. Most 10-Gbpsapplication specific integrated circuits (ASICs) have been implemented on GaAs and SiGe technologies with 0.5 \(\upmu\)m line width [2]. GaAs substrates have the advantages of low cost and large area in wafers. For some monolithic optoelectronic integrated circuit (OEIC) applications, it is useful to use GaAs substrates instead of InP because of the mature fabrication technology for electronic components on GaAs. Therefore, the In\({}_{0.53}\)Ga\({}_{0.47}\)As alloy lattice mismatched to the GaAs substrate has attracted considerable attention to the growth between these two layers for photodetector applications [3-5]. Among the technologies used to overcome the \(\sim\)4% lattice mismatch in In\({}_{0.53}\)Ga\({}_{0.47}\)As/GaAs hetero structure, a 1.5 \(\upmu\)m thick linearly graded In\({}_{x}\)Ga\({}_{1\textrm{-}}\)P buffered GaAs substrate was successfully demonstrated and reported to have a 10 Gb/s operation in an In\({}_{0.53}\)Ga\({}_{0.47}\)As p-i-n photodiode [5]. In this review paper, a simple, thin 1.7 \(\upmu\)m InP buffer layer was introduced and grown using solid source molecular beam epitaxy (MBE) to sufficiently suppress the dislocations between In\({}_{0.53}\)Ga\({}_{0.47}\)As/InP photodiode and AlGaAs/GaAs optical waveguide layers, as shown in Figure 1[6]. A hybrid packaging of discrete optical and optoelectronic components is inherently susceptible to strict alignment tolerances for high coupling efficiency and optical path-length control. These types of interconnection and assembly difficulties can be significantly reduced by monolithically integrating a large amount of devices onto a single chip. The integration of interconnected optical and electronic devices is an important area of investigation for applications within optical fiber systems [7]. OEICs focus primarily on the monolithic (single-substrate) integration of optically interconnected guide-wave optoelectronic devices, which combines various optical and electronic elements in a single-material chip to achieve optimized performance over systems using discrete components. The emphasis has been on the integration of the terminal optical transmit or receive device, or arrays of such devices without optical interconnections, with the associated amplification or signal-conditioning electronics. The mechanisms used to integrate waveguides and photodetectors mainly include butt coupling [8], grating assisted directional coupler [9] evanescent-wave [10], travelling-wave [11], and reflection approaches [12,13], as shown in Figure 2. The bonding and selective regrowth technologies are crucial to butt and grating coupling. An easier, simpler approach for non-regrowth technology can be applied  
Figure 1: Schematic diagram of cross-section of InP/In\({}_{0.53}\)Ga\({}_{0.47}\)As/InP double hetero-structure grown on five layer AlGaAs/GaAs waveguide [6]. directly to the evanescent/travelling and reflection types. Among the regrowth-free applications, the reflective integration between waveguide and photodetector is an efficient, compact, economical approach. Optical monitoring performance is typically controlled using a variable optical attenuator (VOA), erbium doped fiber amplifier (EDFA), multiplexer/demultiplexer (Mux/Demux), and optical switch to provide remote power adjustment capability for each optical transmission channel [14], as shown in Figure 3(a). Figure 3: **(a)** The waveguide based optical performance monitoring subsystem includes photo-detectors, the waveguide tap and main optical functions (M). **(b)** The directional coupler based waveguide tap photodetector monitor [15] **(c)** WDM receiver using reflection grating.