Photodetector For Optical Communication System.docx

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Photodetector for Optical Communication System Eljire Bagas Lewi1, Ir. Akhmad Hambali, M. T.2 Jurusan Teknik Telekomunikasi Telkom University, Bandung Jl. Telekomunikasi No. 1, Bandung 1 2

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Abstract-At the transmitting end of the optical fiber communication system, the light source is modulated with a low-frequency baseband electrical signal, and then the modulated light signal is transmitted via the optical fiber. Due to factors such as attenuation and dispersion of optical fibers, when a dimmed signal is transmitted to the receiving end, it becomes weak and has waveform distortion. The role of the optical receiver is to detect the weak light signal first, then convert it into an electrical signal, and then restore to the original baseband signal through amplification, shaping, regeneration, and decoding. Therefore, the core component of the optical receiver is a photodetector[3]. I. INTRODUCTION The abrupt increase of internet traffic has put tremendous pressure on communication capabilities. Three effective solutions, improving the transmission rate of a single channel, increasing the number of channels, and adopting an advanced modulation format technique, are usually employed to increase communication capacities. In the future optical fiber communication system in which the transmission rate continues to increase, whether adopting optical time division multiplexing, orthogonal frequency division multiplexing or wavelength division multiplexing (OTDM, OFDM or WDM) technology, high-speed photodetectors (PDs) are crucial devices for achieving the photoelectric conversion of the signal, and it

directly determines the performance of the communication system. A high-speed optical signal demodulation system requires specially designed PDs and detection systems, which need to meet the following factors: (1) broad bandwidth to accommodate the instantaneous variation of the incoming signal, (2) large response to the incident optical signal, (3) minimum of noise added by the detection systems. speed optical signal demodulation system r equires specially designed PDs and detectio n systems, which need to meet the following factors:(1) broad bandwidth to accommodate the instan taneous variation of the incoming signal, (2 ) large response to the incident optical sig nal,(3) minimum of noise added by the detec tion systems. In an optical communication link, there are two main ways to carry out light detection, named by direct detection and coherent detection. Direct detection means that the signal light is directly incident on the photosensitive surface of the PD, which only responds to the intensity of the incident light radiation, so direct detection is widely used due to its simple and practical advantages. Coherent detection refers to the process that the signal light and the intrinsic light, which satisfying the phase matching condition are mixed in the PD, and then the difference frequency signal is output. Photoelectric coherent detection has many advantages,

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such as strong detection capability, high signal to noise ratio, good filterability and high stability and reliability[1, 2]. II. THEORY In the entire optical fiber communication system, the photoelectric detection device mainly has two purposes: one is for terminal reception of the communication system, and the upper part of the figure is the schematic diagram of terminal receiving of the digital optical fiber communication system; the second is photoelectric conversion for the relay station. After the signal processing continues to transmit, the lower part of the figure below depicts the relay station’s photoelectric conversion process[3].

Figure 1. working process diagram of optical communication system[3] 2.1. Photoelectric Detection Principle The photodetector is an optoelectronic device made by utilizing the photoelectric effect of a semiconductor. It converts the change of the optical signal into the change of the photocurrent, and reflects the change rule of the information. According to different conversion parameters, the semiconductor photodetector has two basic types: photoconductive type and photodiode type. The conductance of the photoconductive type detector changes with the change of luminous flux. The photodiode is always operated in the reverse bias state. It belongs to the inner photoelectric effect device, and the incident photon does not directly bombard photoelectrons, but merely

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raises the internal electrons from the lower energy level to the higher energy level. The differential resistance of the photodetector does not change with the luminous flux, and the generated photocurrent is proportional to the luminous flux. Both types of semiconductor photodetection devices have very fast response speeds, but have their own characteristics and different uses. In optical fiber communication systems, the most widely used photodetector is a photodiode because of the small size and long life of such detectors[3]. 2.2. Photodetector Operating Characteristic Parameters The main function of the photodetector is to convert the optical power signal transmitted from the optical fiber into a current signal, which carries the information of the source. Photoelectric detector basic parameters including the following 6 main features[3]:  Photocurrent When the incident optical power of the photodetector changes, the photocurrent also changes linearly, thereby converting the optical signal into an electrical signal.  Quantum efficiency Quantum efficiency, ie, photoelectric conversion efficiency, represents the degree to which the total number of photons received by the photodetector can be converted into the total number of electrons of the photogenerated current.  Responsiveness TheResponsiveness, also called photoelectric conversion sensitivity, is represented by r, which reflects how much light power is converted into photo-generated current.

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 Cutoff wavelength Only when the incident photon energy is greater than the bandgap of the detection device material, photogenerated carriers can be generated, forming a photocurrent. Therefore, for any photoelectric detection device made of any material, there is a minimum frequency or maximum wavelength that can be detected, ie, the upper cutoff wavelength.  Dark current The dark current represents the reverse current that occurs in the absence of light. It affects the receiver’s signalto-noise ratio and is an important quality parameter.  Response time The response time (speed) indicates the ability of the photodetector to respond to the optical signal. 2.3. Fiber Optic Communication System Requirements for Photodetection Devices. The role of the photoelectric detection device is to use the photoelectric effect to convert the optical signal into an electrical signal. The main requirements for photodetection devices are: high sensitivity at the operating wavelength in order to improve the photoelectric conversion efficiency; fast response, good linearity, frequency bandwidth, and the speed of photoelectric conversion is higher than the operating speed of the system, reaching hundreds of Mbit/s From s to thousands of Gbit/s, the bit rate of the communication system can be increased; the additional noise caused by the detection process is small, various measures are taken to reduce the internal noise of the system, and the signal to noise ratio is improved; the cost is low, the reliability is high, and the volume

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is small Small, long life, the photo-sensitive surface of the detector is matched with the core diameter of the optical fiber to improve the coupling efficiency; the operating voltage is as low as possible and easy to use. In optical fiber communication systems, the photodetector devices that meet the above requirements are the most commonly used PIN photodiodes and avalanche photodiodes (APD)[3]. 2.4. Photoelectric detection device There are many materials that can be used to make photodiode detectors, such as Si, Ge, GaAs, InGaAs, GaAsP, InGaAsP, and the like. According to the photodiode detection PN junction, it can be divided into PN junction type, PIN junction type, Schottky barrier junction type, heterojunction and avalanche photodiode detectors. According to the wavelength response to light to distinguish, photodiode detector can be divided into infrared type, ultraviolet type, blue silicon type. Among them, the photodiode made of Si material, its typical peak response wavelength of 0.94 μm, its series is also more; PIN photodiode and avalanche photodiode APD’s response time is short, so suitable for high-speed transmission applications; Ge The photodiode of the material is also one of the widely used optoelectronic devices. Since its band gap is smaller than Si, it has higher sensitivity in the long wavelength band, but since the Ge material has a relatively large current, the noise is also high. InGaAs photodiodes are one to two orders of magnitude lower, so photodiodes of InGaAs compound materials are widely used. In order to meet the requirements of optoelectronic integrated circuits, integrated optical photodetectors can be fabricated using various waveguide effects[3].

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III. PACKAGING FOR PHOTODETECTORS

3.2. Effect of connecting ways on module performance

As we know, the bandwidth of the PD chip has exceeded 100 GHz; at such high operating frequency, the efficient design of packages and interconnects is quite challenging in terms of the loss due to the resonances in transmission line structures, launch transitions, impedance mismatch as well as electrical loss of electrodes. Moreover, to achieve the target of high capacity, multi-channel modules is a nice choice for the WDM system. Currently, hybrid integration packaging has been adopted for high-speed modules instead of the traditional packaging design, and it is a trend to integrate many functional components on one single substrate. In this section, some technologies of the packaging are introduced in the following[4].

Flip-chip, tape-bonding, and wirebonding are widely adopted for interconnecting the chips based on different substrates whose transmission performances up to 120 GHz. Many works have been conducted into the behavior of wire-bonding and flip-chip. Compared with wire-bonding technology, flip-chip technology generally has a superior performance in terms of small solder bumps and less inductive parasitics. However, the length of bonding wires can be fabricated to be fairly short with the development of wire-bonding technology. So wire-bonding is considered as another attractive connecting method in highspeed applications due to its advantages such as low cost, robustness, high thermal tolerance, and convenient fabrication process. The length of the wire bond used in the PD packaging between different components should be precisely controlled respectively. The inductance introduced by wire bonding causes peaking gain, which can be used to extend the bandwidth of the PD module, while the wire bond connecting the PD and TIA should be kept as short as possible. A method to reduce the inductance between the PD and TIA is making the TIA mounted very close to the PD chip and coplanar with it. What is more, the AC current through the PD to the ground must meet minimal inductance, because it causes a ground like an open circuit, which gives rise to a sharp decrease in gain, so the bonding wire connecting to the ground should also be short as possible. It worth noting that introducing additional capacitance that can increase module noise and reduce bandwidth should be avoided.

3.1. Transmission Line Design Single-ended PDs usually use coaxial output connectors whose cavity diameters are 1.8, 2.4, or 2.92 mm, depending on their speed. The interface of the connector pin and the internal transmission line are very crucial at such high speeds, because misalignment between them can cause degradation in the electrical return loss. In addition, it is necessary to cautiously control the shape and quantity of the solder used in the connection. The design of transmission lines has a significant impact on module performance. Generally, coplanar waveguides have better signal shielding and lower dispersion than microstrip lines, but a slightly higher loss[4].

3.3. Optical Coupling Figure 2. Structure of a PD module using a catadioptric system[5]

Highly efficient optical coupling is one of the most challenging aspects in the

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packaging process. This is primarily because the active areas of the high-speed PD are small which makes optical coupling extremely sensitive to any shifts in the optical fiber. Many types of compact ROSAs that were integrated with surface-illuminated PDs adopting various optical systems have been demonstrated. As shown in Figure 2, a PD module using a catadioptric system was demonstrated by the Kiyohide Sakai group in 2009; this optical system consists of a BK7 ball lens and a plastic-molded offset parabolic mirror[59]. In 2013, two different forms of demultiplexer

Figure 3. Schematic configuration of AWG[6]

Figure 4. Schematic of the optical DMUX block[7]

(DMUX) used for optical coupling for ROSA are presented by the NTT Corporation and Chungnam National University, respectively. As shown in Figure 3, a waveguide grating (AWG) DMUX is used in array module packaging, another optical DMUX block which is composed of local area networkwavelength division multiplexing (LAN-WDM) thin-film filters and an optically transparent quartz block is adopted shown in Figure 4. In 2017, Isaac et al. presented a coupling approach, which uses vertical grating couplers to illuminate the III–

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V modified (UTC-PD) in a silicon PIC, as shown in Figure 5[8].

Figure 5. A coupling approach using vertical grating[8] IV.

APPLICATION AND PROSPECT

PDs must meet various performances in terms of bandwidth, output power, sensitivity, power consumption and dynamic range for different applications such as optical communication systems, radio over fiber systems and millimeter terahertz signal generation systems. For optical communication systems, the capacity is constantly pursued by researchers, which is up to 100 GbE or even 400 GbE. Therefore, highspeed detectors and detector array receiver modules are still research hotspots for optical communication systems. The sensitivity is a significant parameter for freespace optical (FSO) communication systems, because the optical carrier after a long distance of transmission will become divergent and unstable, which makes it difficult for the detector to capture the optical signal. To overcome this problem, a big area of the photo-surface is strictly demanded, whereas this is often accompanied by a decrease in the speed of response. Therefore, a PD with new structure needs to be developed urgently. PD with high linearity and high power are vital for analog photonic links and they have been used in many various microwave photonic applications such as phased array radars, ROF systems, etc. As a high frequency mixing component, a PD which is able to deliver a very high photocurrent level will bring numerous advantages such as improving link gain, reducing the noise figure, and making the

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linearity of the PD better. The saturation current characteristic is a significant factor that determines the spurious-free dynamic range (SFDR) and signal to noise ratio (SNR) of the system. In particular, the non-linearity of the PD determines the upper bound of the spurious-free dynamic range of the system where a linear modulator has been used. In addition, for millimeter terahertz signal generation systems, a high-speed PD with big saturation currents can broaden the frequency range of the generated signal and the intensity of the signal produced by the PD is boosted by increasing the incident optical power. As a consequence, it is conducive to transmit signals and reduce the cost of the system. Therefore, the PD with high saturation currents got the enthusiasm of the researchers[4]. V.

REFERENCES

[1] Kaneda N, Pfau T, Zhang H, et al. Field demonstration of 100-Gb/s real-time coherent optical OFDM detection. The European Conference on Optical Communication, 2014: 1 [2] Zhou X, Zhong K, Huo J, et al. 112Gbit/s PDM-PAM4 transmission over 80-km SMF using digital coherent detection without optical amplifier. International Symposium on Communication Systems, Networks and Digital Signal Processing, 2016: 1 [3] [Online] : https://www.fiberoptictel.com/twomajor-uses-of-photodetectors-inoptical-communication-systems/ [4] Zhao, Zeping. High-Speed Photodeteectors in Optical Communication System. 2017. Chinese Academy Of Sciences, Beijing. [5] Caillaud C, Chanclou P, Blache F, et al. High sensitivity 40 Gbit/s preamplified SOA-PIN/TIA receiver module for high speed PON. European Conference on Optical Communication, 2014: 1

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[6] DoiY, Oguma M, Ito M, et al. Compact ROSA for 100-Gb/s (4 × 25 Gb/s) ethernet with a PLC-based AWG demultiplexer. National Fiber Optic Engineers Conference, 2013: NW1J.5 [7] Lee J K, Kang S K, Huh J Y, et al. Highly alignment tolerant 4 × 25 Gb/s ROSA module for 100G ethernet optical transceiver. 39th European Conference and Exhibition on Optical Communication, 2013: 1 [8] Isaac B, Song B, Xia X, et al. Hybrid integration of UTC-PDs on silicon photonics. CLEO: Science and Innovations, 2017: SM4O.1

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