Analysis of 400G optical module technology

Infiberone managerGVIP2018/3/7 17:19:37

Analysis of 400G optical module technology


With the discussion and formulation of IEEE 100 Gbit/s (hereinafter referred to as 100G) Ethernet standard, the global mainstream manufacturers are promoting the global deployment of 100G, and focus on 400G or even 1 Tbit/s system. Like 40G/100G, the deployment of 400G should be a gradual approach. In order to make better use of existing DWDM (dense wavelength division multiplexing) line resources and reduce investment costs, operators hope that 400G can deploy on existing networks instead of redesigning and constructing a new network to adapt to 400G transmission. This means that 400G must adapt to the network design planning of 100G/40G or 10G to achieve a mixed deployment of 400G and 100G/40G.


1, Technical analysis of 400G LAN optical module


The 400G LAN (LAN) interface optical module may continue to adopt the parallel transmission mode in 100G Ethernet. In February 2011, Finisar proposed a 400 GE module standard at the "Beyond 100GE" symposium, which mainly supported two applications of 400 GE-LR16 and 400 GE-SR16. The 400 GE-LR16 is using 16 * 25 G LAN WDM (1330, 1310, 1290 and 1270 nm) while 400 GE ~ SR16 is using a 16 x 25 G multimode optical fiber interface. In addition, the CAUI (additional unit interface) -16, CPPI (parallel physical interface) -16 electrical interface standard is defined in the physical layer. 16 * 25 G is only a linear extension of 100GE, and there are no other technical difficulties as long as the process meets the requirements. In contrast, fiber optic parallelism will have more space for development, but the need for higher density photonic integration technology can make it possible for the 400G business to be commercially available.


In addition, Finisar also presented other possible ways of 400G LAN application: First is the use of commercial 4OG EML (electro absorption modulated laser) technology composed of 10 * 40 G architecture; second is to rely on the improvement of EML technique, using 4 level amplitude modulation and DSP (digital signal processing) framework for dispersion compensation 8 x 50G; the third is 4 * 100G architecture, this architecture must use the amplitude phase modulation complex. For example, PM-QPSK (polarization multiplexed quadrature phase shift keying). But there is no commercialized demonstration of the technology.


NTT's research report also points out that in serial data transmission, 16 * 25G, 10 * 40G and 8X 50G are all possible. For 50 G, the modulation is also based on MZ (Maher - Ceng De) DQPSK (differential quadrature phase shift keying) modulation, or OOK (on-off keying) modulation. Each architecture has advantages and disadvantages in terms of volume, cost, power consumption and so on. From the present research results, the DML (direct modulation laser) fabrication process is simple, and the power consumption is low, but the ER (extinction ratio) is very small. EML fabrication process is complex, power consumption is relatively large, but the ER is larger, which can get a clear eye pattern. In addition, the quantum well EAM (electro absorption modulator) based on InGaA1As reduces the hole accumulation caused by the valence band offset in the modulation process, so it is suitable for high-speed modulation. Figure 1 to figure 3 are NTT spectra in OFC 2011 show for 400 GE 1300 nm, 50G EML, and the relationship between injection current and output power of the fiber and eye pattern after transmission of 10km and 40 km.

For the 400 GE system, the 50G OOK modulation is a better compromise choice due to the volume advantage of its entire transmitter. In this mode of modulation, DML is more difficult to realize than that. It can be predicted that the OOK modulation of EML and 8 * 50 G is better in the 400 GE system.


2, Technical analysis of 400G long distance transmission module


In the ITU-T/IEEE joint seminar, the Alcatel-Iucent report suggested that the interface line rate of OTU5 would reach 449.219 Gbit/s. With the increase of speed, the requirements of OSNR (optical signal to noise ratio), CD (chromatic dispersion), PMD (PMD) and nonlinearity are getting higher and higher. The dispersion tolerance of the 400G signal is only 0.5 ps/nm, which is 1/16 of 100G. 400G also has a challenge on OSNR, it is 6 dB higher than 100G. In particular, a far distance transmission can be achieved with a FEC ( forward error correction) 7% higher than the current . 400G is more challenging in PMD than 100G, and the 400G PMD capacity limit is only 0.25 PS, 1/4 for 100G.


2.1, Requirements for line width of laser


With the development of digital coherent reception technology, the high order multilevel modulation format is becoming more and more attractive in the DWDM system because of its high frequency spectrum efficiency. The phase noise characteristics of the local vibration light of the transmitter and receiver determine the bit error rate performance of the system. Table 1 lists the comparison between 400G and 100G for laser line width in different modulation formats. In the table, fTX said the transmitter laser linewidth, fLO said receiver local oscillator light linewidth.

400 Gbit/s光模块的技术浅析


2.2, Modulation formats and channel spacing feature


In order to meet the transmission requirements of 400G in the current DWDM system and improve the capacity of the system, the most important requirement for modulation code is that it can meet the requirements of SE (spectrum efficiency) and OSNR sensitivity, and has a very strong nonlinear tolerance.


The theoretical capacity of the single mode fiber is 8 bit/s/Hz, and the upper limit is 4 bit/s/Hz in the actual long distance transmission equipment and optical fiber. In the modern optical communication system, the carrier modulation format has great influence on the system performance. In order to achieve the 10G, 40 G hybrid deployment with the current network, the 80 wave 50 GHz interval must achieve high SE, which can be achieved by single carrier high-order modulation or multi carrier transmission. For a 448 G transmission system, considering the device frequency drift and the non-ideal characteristics of R0ADM (reconfigurable optical add drop multiplexer), it is required that 45 G 32QAM (quadrature amplitude modulation) modulation or 28 G PM (polarization multiplexing) -256QAM must be applied in practice. The electrical domain OFDM (orthogonal frequency division multiplexing) can also replace single carrier modulation. The complexity of DSP is the same as that of OFDM. However, because of the additional information such as cyclic prefix, preamble and training symbol overhead, OFDM is usually lower than that of the corresponding single carrier format.


In order to meet the 50 GHz DWDM channel interval, many theoretical research in 2010 are based on multilevel amplitude modulation. That is, PM-256QAM, there are 65536 constellations in all. Compared with 100G PM-QPSK, the density is increased by 8 times, and it is very sensitive to optical noise and XPM (cross phase modulation) /SPM (self-phase modulation), and the transmission distance is very short. From the presentation of the current single carrier high-speed QAM presentation, in the short term no matter the single carrier PM to 256QAM or the 32QAM of electric OFDM, both of them cannot be commercialized in the 448G transmission,


The first way to relax the requirement of SE is to discard the rigid requirements of 50 GHz WDM interval, such as 56 G PM, one 16QAM and 70~80 GHz WDM interval, SE is 6~5 bit/s/Hz, and there is enough support for ROADM system. Data center users prefer to adopt this flexible solution. Telecom operators with large and multi service mesh networks adhere to the 50 GHz interval standard. In order to be compatible with the boundary conditions of 50 GHz, an inverse multiplexed 448 G channel can be used to form two 224 G wavelengths. The modulation mode of 28 G PM-I6QAM can reach 4 bit/s/Hz net SE, which doubles the capacity of each fiber in WDM compared to 100G PM-QPSK. In addition, in order to achieve seamless upgrade deployment of 10G, 40G/100G to 400G, the requirement of adjustable bandwidth for WSS (wavelength selection switch) is put forward.


The second solution is to replace the single carrier 448 G signal with 32QAM or higher order modulated low speed photonic carrier. This method is called coherent WDM or coherent light OFDM. It is different from the DWDM inverse multiplexing, because it can obtain the same SE as the single carrier in the specific modulation format and the OSNR tolerance of coherent reception. A 448 G transmitter can be used with 10 individually modulated photon carriers. The receiver can detect and receive two groups (each of the 5 groups). From the above discussion, in order to obtain the highest possible subcarrier rate, can be processed in an electric field, the minimum number will keep parallel light is a more practical and more economical solution. Table 2 lists the performance comparison between the OFDM and the single carrier in the 400G system.


Compared with single carrier high sensitivity and excellent CD/PMD tolerance, CO (coherent light) -OFDM with multi polarization digital coherent reception is becoming more and more promising and has attracted widespread attention in the industry. In the future exploration of the structure of 400G optical module based on OFDM modulation, there are mainly 3 architectures: OOFDM based on FFT (fast Fourier transform), all-optical OFDM and electro-optic OFDM.


The traditional OOFDM using DSP/DAC based IFFT (inverse fast Fourier transform) demodulation signal synthesis and FFT, CD and PMD tolerance can be improved by inserting cyclic prefix or guard interval, training symbol overhead which will result in 10% or 20%, and will increase the line speed. Especially in the transmission line with periodic CD compensation, the transmission performance of multicarrier OFDM based on DSP will be well matched by the amplifier, and the skew between two PD must be small. In addition, the skew of multichannel PD and the unbalance of sensitivity will reduce the CMRR of the receiver. TIA also needs to maintain a good linearity of the signal to realize the pure digitization of the ADC.


For coherent detection, it is difficult to achieve commercialization by using discrete free space 90 degree mixer and optical balance detector to build coherent receiving system. From the ECOC in 2009, U2T and HHI demonstrated the monolithic integrated PLC 90 degrees. The mixer and the two receiver for high speed balanced PD, to the ECOC of 2010, U2T and HH1 again demonstrate the monolithic integrated two - way PLC 90 degrees. The mixer and 8 high speed balanced PD receivers. From the evolution and development trend of 100G transmission technology in recent years, it seems that the 400G line receiving technology has gradually become integrated.


Integrated receivers are mostly monolithic integrated and free space optical devices, and there is no satisfactory performance, reliability and low cost. In this regard, NTT uses silicon based PLC technology to integrate PBS (polarization beam splitter) and 90 degree optical mixer into a single DPOH (dual polarized optical mixer). A new multichannel collimator to lower the coupling loss between DPOH and PD, and inhibit the coupling deviation caused by the change of temperature. In addition, NTT has also developed a compact high speed optoelectronic converter with a chip level, and uses these technologies to produce an integrated coherent receiver. The micro optical collimation technology makes the sensitivity matching of PD based on PLC hybrid integrated devices and the temperature performance greatly improved.


Microelectronics is a silicon based plane process and optoelectronic devices are stereoscopic technology. Compared with the hybrid integrated technology, PIC (photonic integrated circuit) can significantly reduce the optical module volume and save the cost of the packaging. It is the mainstream technology in the future that the plate connection can make the match and balance of optical path easy and control the deviation effectively. At present, the Bell laboratory has already studied the integrated coherent detection technology of monolithic silicon, but there are many technical difficulties to break through. In the early days of 400G, the hybrid integration of silicon based PLC and free space optical devices would be a more mature commercial scheme.


3. Zoning detection technology


Digital coherent reception technology is widely considered as a promising technology in high-speed transmission field, because it can improve the OSNR sensitivity of the system and compensate for the transmission impairments of CD and PMD lines. Because of the "bottleneck" of the electric speed, the sampling rate of ADC will be limited to 100 GS/s for a long time. In order to effectively solve the problem in the transmission of 400G and even T bit/s channels, it is an effective method to adopt multi carrier modulation format.

Using wavelength independent detection, the receiver hardware complexity can be reduced by detecting multiple carrier points. It can not only be unconstrained by the bottleneck of ADC sampling rate, but also reduce the load of DSP.


 3.1 Processing capability and power consumption


At present, most of the optical devices in 100G system have been commercialized. But in coherent reception technology, the most important problem of mass production of ADC and DSP chips is the processing capability and power consumption. Though in 2010, Alcatel-Lucent has adopted 56 GS/s ADC/DSP of 70M+ gate in the long distance system of 112 G, but the prediction will face the same bottleneck in 400G system.


Nowadays, the technology of CMOS with smaller process size can achieve the combination of low power consumption and high speed and high density, but it is at the cost of higher noise and more mismatch. The most common solution is to increase the size of transistors (gate length or width), but it is not realistic here, which will introduce additional bandwidth and reduce power consumption. However, small size transistors mean that both S/H (sampling and hold) and ADC circuits are mismatched not only on the signal links but also on the clock link. The error caused by mismatch can be reduced by single chip calibration. In addition, the new sampling / demultiplexing architecture and real-time simple amplitude and temporal calibration can meet the demand of optical module for linearity, noise and bandwidth without requiring very short channel transistors, and the power consumption of ADC is less than 0.5W.


Because of its low cost, high integration and low power consumption, the Si CMOS process has now begun to replace the III-V element in some applications. In the commercially deployed optical transmission system, the existing 90 or 65 nm CMOS technology has been used in signal processing monolithic ASIC (ASIC) chip, and it can meet the performance and power consumption requirements of long distance optical network system. In the future use of 40 nm or smaller size process design, it will be able to be commercially available in short and high speed 400G systems.


4, Conclusion


400G long distance transmission makes optical communication enter into a new era. Optical communication is changing from single carrier modulation coherent detection to polarization multiplexing multi carrier multilevel phase modulation and array coherent detection. Photon integration, electronic integration and ADC/DSP technology will be the key to the commercial application of 400G optical communication module and system. With the urgent demand of Ethernet standardization, the requirement of optical parallelization will greatly promote the photon integration technology. In the next 2~3 years, 400G electro-optic OFDM related technology will gradually mature. Although the cost and power consumption of these devices are far from commercial use, but as the technology matured and related standards discussed and developed, the prelude to the business use of the 400G system is about to be opened.

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Member2018-03-13

文章不错!

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2018/3/13 16:13:50
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