集成 TOSA 和高速EML 芯片

Data communication services, such as social  networking services and video streaming, have been continuously growing in line with the spread of highly functional  mobile devices. As the communication traffic volume has  increased, efforts have been made to expand the capacity of  communication networks.

The capacity of optical transport systems used in  communication networks increases by reducing the size of  optical transceivers implemented in the converters and  achieving high-density implementation of multiple optical  transceivers.

The new transmitter that we have developed is  equipped with four electro-absorption modulator integrated  laser (EML) chips that are suitable for medium- and longdistance transmission. Integration has been achieved by an  optical multiplexing system of our proprietary design.

For optical multiplexing, the spatial multiplexing  method characterized by low optical loss is used. Two flexible printed circuits (FPCs) are used for electrical connection with the circuit board of the optical transceiver. The  high-frequency signals are separated from the lowfrequency signals. An LC receptacle incorporating a single  mode fiber (SMF) is used for the optical output port.

Figure 3 shows the schematic diagram of the optical  multiplexer. The optical beam from each laser is converted  into collimated beam by an optical lens. The P-polarization  parallel beams of Lane 0 and Lane 2 are multiplexed by  WDM filter #2. The parallel beams of Lane 1 and Lane 3  are converted from P-polarization to S-polarization by a  half-wave plate and multiplexed by WDM filter #1. The  two pairs of parallel beams are eventually multiplexed into  a single beam by a polarization beam combiner, and multiplexed to a fiber by a condenser.

There are two candidates for the high-speed light  source: direct modulation lasers (DMLs) and EMLs. It is  difficult for DMLs to achieve a high extinction ratio, and  they are characterized by significant distortion of the  optical waveform due to relaxation oscillation. Thus,  DMLs are unsuitable for medium- and long-distance transmission. Therefore, the new transmitter uses EML chips for  these targets.

The schematic diagram of the cross section of EML is  shown in Fig. 4. EML has an optical modulator integrated  in front of a distributed feedback (DFB) laser. To increase  the extinction ratio of EML, the modulator length needs to  be increased.

Fig1

For optical multiplexing, the spatial multiplexing  method characterized by low optical loss is used. Two flexible printed circuits (FPCs) are used for electrical connection with the circuit board of the optical transceiver. The  high-frequency signals are separated from the lowfrequency signals. An LC receptacle incorporating a single  mode fiber (SMF) is used for the optical output port.

The next-generation 200 and 400 Gbit/s systems  increase communication speed by using the PAM4 modulation instead of the non-return-to-zero (NRZ) modulation.  While the 200 Gbit/s system uses the four-wavelength  multiplexing method, the 400 Gbit/s system uses the eightwavelength multiplexing method (double that of the 200  Gbit/s system) for the signal transmission of 400 Gbit/s in  total.

The evaluation results using PAM4 modulation signals  are shown in Fig. 11. The signal source IC and linear laser  driver IC were used to input PAM4 modulation signals  (symbol rate: 25.6 Gbaud, signal pattern: PRBS 213-1) to  observe the optical waveform of the transmitter. The equalizer specified by the Transmitter and Dispersion Eye  Closure Quaternary (TDECQ) was used to achieve this  optical waveform.

We have developed 4-ch integrated transmitter with  an LD chip (incorporating an external modulator) that can  be mounted in the QSFP28 small 100 Gbit/s optical transceiver.

We verified favorable operating characteristics at the  transmission speed of 27.95 Gbit/s by using EMLs manufactured in-house. We also verified the characteristics of  the PAM4 modulation to be used in the 200 and 400 Gbit/s  systems. The device has been confirmed to be usable in  next-generation communication applications.