CWDM4 Transmission Technology for Data Center

Evolution of Optical Transmission Technology in Data Center
With the popularization and application of mobile Internet, data center has developed rapidly and become an important infrastructure in the information society. The data center consists of a large number of servers. High speed and large capacity data transmission and exchange are needed between servers. The traditional cable transmission cannot meet the speed requirements. Optical fiber transmission technology has entered the data center since 2010, and has become the mainstream transmission technology.

In the early days, the scale of data center was small and the transmission distance needed was tens to hundreds of meters. In order to meet the requirements of high-speed and long-distance transmission, multi-mode fiber parallel transmission technology was usually used, and the dispersion performance of multi-mode fiber was continuously optimized. The OM4 standard multimode fiber can support 10G signal transmission for 550m.

However, the mainstream transmission rate of data center has entered the era of 100G, usually using 4×25G transmission scheme. The transmission rate of single channel is up to 25G. Multimode fiber cannot support such a high transmission rate. Single mode fiber is introduced into 100G transmission system. In fact, the cost of single-mode fiber is higher than that of multimode fiber. Because the transmission wavelength of single-mode fiber is 1310nm, while that of multimode fiber is 850nm, and the optoelectronic devices in 1310nm are much more expensive than those in 850nm.

The PSM4 scheme is used to transmit 4×25G optical signal through single-mode optical fiber. The bidirectional data transmission between a pair of transceiver modules is realized through 8 optical fibers. Each fiber transceiver of PSM4 only needs one laser, four channels of light splitting, and four modulators for output, so the cost of light source is saved. However, with the increase of transmission distance, the cost of optical fiber increases rapidly, so PSM4 is usually used in the scene of transmission distance below 500 meters.

For the application scenarios with transmission distance greater than 500 meters, in order to save the cost of optical fiber, the CWDM Technology in the telecommunication network is introduced into the data center, which is the CWDM4 transmission scheme. Through wavelength division multiplexing/demultiplexer, four wavelengths of 1271nm, 1291nm, 1311nm and 1331nm are transmitted in one optical fiber with an interval of 20nm. In this way, only two optical fibers are needed to realize bidirectional transmission between two optical fiber transceiver modules. CWDM4 can support 4×25G signal transmission, and has cost advantage over PSM4 in 500~2000m transmission distance.

CWDM4 Technical Scheme
The application of CWDM Technology in telecommunication network is very mature. International Telecommunication Union(ITU) defines 18 CWDM channels with 20nm interval in 1271-1611nm band. CWDM4 standard in data communication adopts four wavelengths, namely 1271-1331nm, which are close to the zero dispersion point of G652 single-mode fiber.

At present, QSFP28 is the mainstream packaging form of 100G optical fiber transceiver module in data center, which integrates four semiconductor lasers, photodetector array and driving circuit, as well as passive CWDM4 module. In order to integrate CWDM4 module into QSFP28 module, it needs to be miniaturized as much as possible, and the size requirement is more stringent than CCDWM module (a compact CWDM module) in telecom applications.

1)Z-block Technology
The first CWDM4 module is based on Z-block technology of thin film filter(TFF). As shown in Figure 1, eight TFF filters are pasted on an oblique prism in two groups. One group is used for multiplexing and the other group is used for demultiplexing. The transmission wavelengths of each filter are 1271nm, 1291nm, 1311nm and 1331nm respectively.

Figure 1. Z-block structure with CWDM4 filter

The wavelength division multiplexing emission optical path of the Z-block module is shown in Figure 2. Note that a high reflection film is coated on the back of the prism. The optical signals emitted from the four collimators on the right pass through the corresponding filters respectively, and reach the collimator on the left common end through different reflection times, and then couple to the output fiber. Because the light path in the prism is longer, reaching the order of 10 mm, a total of five collimators must be used. The coupling of reflected light path and collimating beam is very sensitive to the angle, so the integrated collimator array cannot be used. The alignment of each input collimator must be adjusted independently, so the assembly process is complex.

Figure 2. Multiplexing emission optical path of Z-block

The wavelength division demultiplexing receiving optical path of the Z-block module is shown in Figure 3. The optical signal at the common end is input from the left collimator. The optical signal of each channel passes through the corresponding filter plate after different reflection times, and then focuses on the corresponding unit on the photodetector array through the microlens. The photodetector array is mounted on the PCB board, as shown in Figure 3(b). In the horizontal plane, the beam which is decomposed and multiplexed needs to pass through a right angle prism to achieve 90 degree turn, and then incident on the photodetector along the vertical direction. The size of active region of photodetector is usually only Φ50μm, and the diameter of collimated beam transmitted in Z-block is much larger than that. Therefore, microlens is needed to focus, and the microlens needs to be adjusted up, down, left and right in the cross section of the vertical optical path, so as to aim the focused spot at the source region of photodetector. This adjusting focusing process also increases the complexity of Z-block assembly process.

Figure 3. Demultiplexing receiving optical path of Z-block

2) AWG Technology
In order to simplify the packaging process and reduce the size and cost, CWDM4 AWG chip based on integrated optical technology has been developed. AWG is the abbreviation of arrayed waveguide grating, which has been used in telecommunication network for a long time. AWG in telecommunication network is used to multiplex/demultiplex DWDM optical signals. The channel spacing is usually 200G or 100G (corresponding to wavelength spacing of 1.6nm or 0.8nm). Because the application scenario is mainly the backbone of the telecommunication network, it is not sensitive to the cost.

The AWG technology is introduced into CWDM4 transmission system of data center, and the wavelength spacing is increased to 20nm, which reduces the technical difficulties. However, in order to integrate into QSFP28 module and apply in scale, the AWG chip size and cost constraints are much more stringent. The mainstream CWDM4 AWG chip size can be controlled within 2mm×10mm. The earliest CWDM4 AWG chip has input/output ports at both ends, as shown in Figure 4. In order to wind the fiber easily and integrate it into the fiber transceiver module, CWDM4 AWG chip with one side input/output has been developed. The input port is wound to the output port by bending the waveguide, as shown in Figure 5. This design further simplifies the coupling process between waveguide and fiber array. Of course, due to the limited width of the chip, the bending radius of the waveguide is less than 1 mm, which will lead to a certain bending loss.

Figure 4. CWDM4 AWG chip structure – Bilateral I/O
Figure 5. CWDM4 AWG chip structure – Unilateral I/O

In a CWDM4 fiber transceiver module, two CWDM4 AWG chips are needed. One is used for multiplexing transmission of optical signals, the other is used for demultiplexing reception of optical signals. At present, the CWDM4 AWG chip at the transmitting end mainly adopts the unilateral input / output structure shown in Figure 5, while at the receiving end, each wavelength of demultiplexing will eventually be detected by the photodetector, and there is no need to couple to the single-mode fiber to continue transmission. For this reason, the CWDM4 AWG chip at the receiving end usually adopts the input/output structure on both sides as shown in Fig. 4. The output port adopts multimode optical waveguide, and the output end face is polished into a 45° inclined plane to realize the 90° turning of the light beam, which is incident on the photodetector array, and the latter is directly mounted on the PCB board.

This design has two advantages: one is to use multimode waveguide output, which can realize the flattening design of AWG passband spectral line and optimize the channel quality; the other is to directly incident the output light into the photodetector array after 90° turning, which saves the butt coupling between the waveguide array and the fiber array, and simplifies the assembly process.

3) Comb Filter Technology
Compared with Z-block technology, CWDM4 AWG chip with integrated optical technology has smaller size, simpler assembly process and lower cost. However, the pass band flatness of AWG devices is poor, the channel quality is degraded, and the loss is much larger than that of Z-block.

Some manufacturers have introduced ITL technology in telecommunication network into data communication. Figure 6 shows an optical comb filter based on integrated optical technology, which is composed of several cascaded MZI interference arms. In fact, optical comb filters in telecommunication networks are mainly used for DWDM applications. Considering the temperature stability, GTI resonator or birefringent crystal scheme is usually adopted, so integrated optical comb filters cannot meet the practical conditions.

Figure 6. Optical comb filter based on integrated optical technology

However, the channel spacing of CWDM4 transmission system is 20nm, and the tolerance of temperature drift is large, so the integrated optical comb filter can be used. Note the MZI interference arm in Figure 6. The optical path difference is achieved by bending the waveguide, which will cause loss. The radius of the optical waveguide should be larger than the minimum bending radius, which depends on the refractive index difference of the waveguide. In order to reduce the bending radius to reduce the chip size, the existing suppliers use silicon nitride waveguide. However, the higher the refractive index difference, the higher the yield and the higher the coupling loss.
Optical comb filter is a 1×2 port device. In order to realize 1×4 wavelength division multiplexing/demultiplexing, three comb filters need to be connected in series and parallel, as shown in Figure 7. The wavelength interval of ITL#1 is 20nm, and that of ITL#2 and ITL#3 is 40nm.

Figure 7. CWDM4 chip composed of three optical comb filters in series and parallel

The optical comb filter and AWG in CWDM4 system adopt integrated optical technology. The former has lower loss and better channel quality, but lower yield.

3. CWDM4 Technical Comparison
The advantages and disadvantages of Z-block, AWG and ITL are shown in Table 1.

Table 1. Advantages and disadvantages of Z-block, AWG and ITL

The comparison shows that Z-block technology has the advantages of low loss and good channel quality. The CWDM4 module based on Z-block technology can even support 100G signal transmission for 10km. However, the process of this technology is difficult, resulting in high cost. AWG technology has the biggest loss and the worst channel quality, but it has the lowest process difficulty and cost. It meets the demand of cost reduction in data center market and is gradually replacing Z-block technology in the market. ITL technology has the channel quality comparable to Z-block technology, and the loss is much smaller than AWG. The difficulty of assembly process is equivalent to AWG. The current problem is that the yield of chip is low. If this problem is solved, it will be the best CWDM4 solution.

Written by Zhujun Wan, Jianwei Feng HYC Co., Ltd

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