The increasing demand in cloud has increased the need for high power efficiency, and to keep the power budget low, optical interconnects are a solution to meet current requirements. A brief analysis of different optical technologies is discussed in this paper. Further, this paper has discussed different approaches to enhance the existing data center interconnects. The application of rolled-up semiconductor tube optical cavity has been entailed to support the emerging demands.
Data center Interconnect is the internetworking of 2 or more different data centers.
This interconnectivity between enables them to work together and share the resources among them. The interconnectivity between different data centers are connected by using leased lines or internet or by using VPN. The market of Data center interconnect grows at exponential rate every year, which is much faster than the overall data center rate. Server to Server traffic is growing increasingly due to the application design and usage. In addition, to increase the performance and to keep the cost low, large cloud providers are needing to be relocated which is closer to the end users.
As a result, the data centers within the metro networks requires a high degree of connectivity.
The increase in the use of the internet services such as cloud computing, e-commerce, social networking has created the need for the powerful data centers. These services create a huge amount of traffic both inside and outside the data centers. The network bandwidth consumed by the data centers has outpaced the traditional telecommunications.
According to CISCO Global Cloud Index, by the year 2019 around 99% of the network traffic will be one or the other way related to data centers. The global data center traffic will reach around 10.4 Zettabytes annually.
Intra data center allows the Connections within the data centers. These connections can be from a few meters to 10 km.
They allow the connections between different data centers or the data centers within the building. Here, the connections between the data centers can be from 10 km to 80 kms.
Traffic between the different data centers are called as East-West Traffic and the traffic between the different users of the data centers are called as North-South Traffic.
Fig 2: DCI Market segmentation ( Jim Theodoras, 2016)
The above graph shows the DCI market segmentation by different customer type. Most of the DCI traffic are consumed by the ICPs which has made it economical for the users as well as to build their own networks. Another sub segment of this DCI market is the smaller ICPs and CNPs. They are of high importance since they provide the direct connections to the ISPs and the end users. Communication service providers (CSPs) is the next large segment in the DCI market. It has number of different providers types and even different data center operators. These data center operators (DCOs) do not have large volume of inter data center traffic common but have common traffic among the data centers. Another segment of DCI market consist of government, research and educational organizations. They usually grow among a single metro area.
Optical Interconnect is the means of communication by the optical fibre instead of traditional cables. Optical fibres are much preferred since they are capable of a much higher bandwidth from 10 Gbit/s to 100 Gbit/s. Electrical interconnect performance degrades when operating at the frequencies above 1 GHz due to signal latency, crosstalk and frequency dependent attenuation. In this case, Optical interconnects provide a solution since it has low crosstalk, negligible frequency dependent loss and much higher bandwidth. The important key factors about the fiber and the interconnects are
The speed and the complexity decide the high-performance data processing systems. Despite the advantages, optical interconnects are not commercially used. In order to make the optical interconnects economical and viable to the end users, a platform should be developed which integrates the optical interconnects into Si CMOS at low cost. As a result, low cost and high-performance CMOS compatible optical components are obtained. Integration of both the optical and the electrical components shows the development of low cost, high performance and ultra compact components that are compatible with the current technology. Large scale manufacturing of optical devices has been developed and implemented with the components that have different technology.
The above diagram shows the basic block diagram of an optical interconnect. The modulator has 2 inputs- an electrical input from CMOS circuit and an optical input from a laser source. The modulator converts the electrical signal into the conventional optical signal. Coupler is used to inject the light into the system.
Fig 3: Block diagram (Sumita Mishra, Naresh K Chaudhary, Kalyan Singh,2016)
Optical interconnects use the waveguide for transmission. The waveguide consists of a dielectric material with high refractive index surrounded by the materials of low refractive index. Optical router is used to route the light travelling in the different directions. The receiver section is responsible for the reconstruction of the electrical signal. The receiver consists of a photodetector followed by an amplifier. The photodetector is used to detect the photocurrent and convert them back to optical current. The amplifier is used to amplify the photocurrent and convert them back to conventional electrical signal.
Vertical cavity surface emitting laser (VCSEL) is the most commonly used as optical source for the short distance communications because of its high performance. VCSEL is a semiconductor laser diode which emits light in the direction perpendicular to the upper surface of the semiconductor. They can be manufactured at different wavelengths. The devices that are operating at 850 nm are used for the most high-speed interconnect technologies. The devices that are operating at 980 nm are based on GaAs/ AlGaAs based process technology. These wavelengths are most choice for ultra dense chip to chip level interconnects that are operating in the range of Tb/s. The devices that are operating in the range of 1200-1600 nm are commonly based on Indium Phosphide (InP) based process technology. The devices that are operating at a wavelength 1550 nm are not used in the current technology since they have many disadvantage due to its high thermal conductivity which occurs due to the unnecessary DBRs. Many researches are going into new materials such as InGaNAs(Sb) which has proven to provide high data rates with low power consumption.
Modulators use either off-chip laser or the On-chip laser. These sources are modulated at a frequency much higher than the threshold value in both 1-bit and 0-bit states. As a result, they consume more power and leads to performance degradation. In today’s scenario the critical issue is to keep the power consumption low and this can be achieved by parallelism. For this reason, Wavelength division multiplexing (WDM) are used which offers a massive parallelism. WDM are not part of Optical interconnect systems but they are considered as an important in order to build a high-performance Optical Interconnect system.
Heat that are generated from the laser source causes the wavelength shifts so it is always preferable to use a on chip modulator and modulate the light that is coming from the off-chip laser. There are different modulation techniques used which are based on thermo optic effect, electro optic effect, electro absorption effect and plasma dispersion effect. Thermo optic effect is the changes in the absorption coefficient and its refractive index which changes due to the changes in the temperature. Electro optic effect is the changes in the refractive index due to the changes in applied electric field. These techniques are effective in III-IV semiconductors, but Si refractive index produced is small. As a solution, plasma dispersion effect is used where the concentration of free carriers changes both the refractive index and the absorption coefficient.
Refractive modulators use single interference structure or multiple interference structure. The resonators can be used which enables modulation using compact devices. Absorptive modulators are mostly based on the changes in the optical absorption by applying electric field. Mostly Quantum well modulators are used for their high-speed applications.
The basic receiver section consists of a semiconductor photodetector followed by an electronic amplifier. The photo detector can convert the incoming optical signal into the electrical signal. The photodetector used here should provide high bandwidth and high sensitivity. There are many photodetectors such as
Of all these photodetectors, PIN are most commonly used for short distance applications. MSM photo detectors are used for the high-speed operation because of its low responsivity. APD provides the highest responsibility but they use a very expensive fabrication process. There are different materials used in photodetectors of which GaAs is suitable for wavelength up to 850 nm and they are most commonly used for high speed application, but they are very expensive to fabricate. The material Si can be used for the wavelength up to 1 ?m and for the wavelength of 1.7 ?m, InGaAs on InP is used. The photocurrent that are generated by the photodiode must be amplified with a minimum amount of added noise, so preamplifiers are used as first stage. Different amplifiers used are,
Waveguides are commonly used to guide the light that is travelling in the different direction. They have a very low attenuation and good optical properties. By reducing the optical field, the attenuation in the waveguide can be reduced. By increasing the width and decreasing the edge depth can reduce the overlap. Other components such as bends,and splitters are also essential for signal routing.
One or the other way, all the connections inside the data center are optical. Predominant interconnect between the datacenters is Multimode fibre which are used up to 40G. short reach use multiple parallel planes to achieve high speed application. Most short reach is of 100 G consist of 4 parallel lanes with each running at 25 Gbit/s. while designing any optical interconnects both the cost and speed are to be considered. In future, most of the optical interconnects are unplugged modules. Optical transceivers provide an optimal solution to maintain the high performance.
Requirements for metro networks differ from the requirements from those used in telecommunications networks. Due to the requirements of the signal latency a maximum length of about 100 km are used. Here direct detection techniques having low cost and line system must be carefully designed, whereas for the coherent technologies both the line system and the capacity must be carefully designed. However, it is not easy to design a coherent technology just same as direct detection method.
WAN:
Spectral efficiency and the reach are the two
important parameters considered while designing any system. Here Polarization division multiplexed Quadrature phase shift keying (PDM-QPSK) is used. Foe the next generation system Quadrature amplitude modulation can be used or even a higher order modulation formats are used. In order to achieve the high reach of the line system, coded modulation and high-level amplifications and digital processing systems are used. New parameters are introduced in order to increase the efficiency and the to reduce the cost of the network such as SDM and SDN.
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