The use of optical fibers as media for transmission of digital data is becoming more popular due to the high reliability and the large available bandwidth. For instance, short-reach interconnects over optical fiber on the order of a few hundred meters (m) are widely used in computer systems, data centers, and campus networks. Thus, high-capacity optical transceivers designed for short-distance transmission are in high demand, with the major tradeoffs among various solutions including performance, cost, and power consumption. Many short-reach optical transceivers operate at 1 gigabit per second (Gb/s) or 10 Gb/s and use intensity modulation and direct detection (IM-DD), one fiber pair, one wavelength, and one polarization state.
To increase the bit rate to 40 Gb/s and 100 Gb/s, both the signaling rate and the number of channels may be increased. One example is 4×25 Gb/s local area network wavelength-division multiplexing (LWDM) over a fiber pair. Another example is 4×25 Gb/s parallel single-mode 4-lane (PSM4) over parallel fibers for 100 Gigabit Ethernet (100 GbE). As Ethernet data rates scale from 100 GbE to 400 GbE, additional wavelengths and/or a higher symbol rate modulation are required to continue the IM-DD technology scheme. However, the additional wavelengths and the higher symbol rate modulation increase complexity and sensitivity penalties.
To improve system capacity and per-bit power dissipation, reduction in the number of wavelengths through the use of high-order modulation is required. Polarization multiplexing and coherent technology, which have high spectrum efficiency and sensitivity, are needed to further scale to next generation Ethernet data rates, for instance 4×250 G for Terabit Ethernet (TbE). Conventional coherent solutions require the use of frequency-locked and narrow-linewidth lasers for transmitters and local oscillators (LOs). However, the cost and complexity (e.g., frequency tracking) is too high for short-distance applications.