11 March 2010


Mitsubishi develops 25Gbps laser and photodiode for 100GbE

Tokyo-based Mitsubishi Electric Corp has developed transmission devices for high-speed, 100 Gigabit Ethernet (100GbE) applications that multiplex four wavelength channels of 25Gbps-speed optical signals in parallel.

The direct-modulation distributed-feedback (DFB) laser diode features modulated waveforms of 25Gbps, while the photodiode array for receiver modules has a responsivity of 0.88 amperes per watt (A/W). The products stem partly from the government project ‘R&D on High-speed Optical Transport System Technologies (High-Speed Low-power-consumption Optical Transport Technology for Ethernet)’ overseen by Japan’s Ministry of Internal Affairs and Communications.

Since transmission volumes over optical communication networks is rising rapidly, the IEEE is expected this June to set a standard for 100GbE, increasing the maximum transmission speed from the existing 10Gbps Ethernet standard. Specifically, within transmission distances of 100m to 10km (e.g. between local data centers and inside buildings), the 100GBASE-LR4 standard is being considered as a method for transmitting data at 100Gbps by wavelength multiplexing four channels of 1.3 micron-wavelength 25Gbps signals in parallel. To send and receive these optical signals, each end requires four optical semiconductor chips operating at 25Gbps: four DFB laser diodes to send optical signals and four photodiodes to receive the signals.

Mitsubishi Electric says that there is demand for 25Gbps DFB laser diodes that can be operated directly by modulation signals (and hence do not require external modulation devices), leading to lower energy consumption and lower transmission equipment costs. However, until now it has been difficult to raise the speed to 25Gbps because the mask margin — an index showing the quality of modulation waveforms (eye diagrams) — deteriorates at high speed.

The active layer of the new laser diode incorporates a buried active-layer structure that uses AlGaInAs. The active layer also has a short cavity and, by keeping the operating current density low as well as inhibiting the electrons in the active layer from dispersing at high temperature, Mitsubishi Electric has achieved low energy consumption and what is claimed to be a world-leading eye-diagram quality. Even at a very high operating temperature of 50ºC and low average current of 45mA, the new laser can achieve an optical output of 9dBm and a high mask margin of 26% or more. The device can hence enable up to 10km of transmission under the 100GBASE-LR4 standard with low energy consumption of 0.1W or less.

Regarding the receiver, Mitsubishi Electric adds that there is a tradeoff between gaining high speed by making optical absorption layers thinner and achieving high responsivity in the photodiodes, which has previously made it difficult to obtain sufficient responsivity at 25Gbps. The new photodiode has a highly reflective mirror at the bottom of its optical absorption layer, reflecting signal light so that it goes through the thin absorption layers twice to be converted into electrical signals. Mitsubishi Electric hence achieved operation at up to 25Gbps with a high responsivity of 0.88A/W. Integration of four photodiodes into one array chip helps to reduce the size of the receiver modules to less than 3cm3 (a third of the volume of four discrete photodiode modules in total).

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