Addressing 100G Growing Pains

If you strip enough layers off of your optical networking system, you will get to the physical layer― the nuts and bolts of the backbone of communication. The receive optical sub assembly (ROSA) and a transmit optical sub assembly (TOSA) are the keys for making an optical transceiver. The anatomy of a ROSA and TOSA are fairly straightforward.  The TOSA contains the laser, which is used to transmit the optical signal down the multi-mode fiber (MMF) or single-mode Fiber (SMF). The ROSA typically contains a photodiode and a transimpedance amplifier. Each element must meet a set of optical and electrical requirements to satisfy the interface requirements of the transceiver.  First, it starts with light coming in from the MMF or SMF, which hits the photodiode and converts the light into a current.  The transimpedance amplifier (TIA) then takes that photodiode output current and converts it into voltage to be used in the system. The TIA is one of the most important ICs in the receive-side signal path and overwhelmingly determines the sensitivity and overload performance of the transceiver. 

Let’s focus a moment on the TIA.  TIAs mainly come in bare die form because they have to be wire-bonded to the photodiode and co-packaged to avoid the detrimental effects of parasitic inductance and capacitance.   Different TIAs are also needed to address various market segments. There is a lot of growth within the 10G market place, and there are multiple products available.  The burgeoning marketplace, though, is the 100G space. 

New products for this area are emerging as the demand for higher levels of density for the metro and data centers is increasing. Previous generations of 10G TIAs offered pin manipulation to address optimization needs.  As 100G market needs evolve, the demand for higher levels of granularity and resolution into TIA settings also increase. A two-wire serial interface is a unique solution that provides the flexibility for optimizing performance. For example, adjustment of the differential output voltage to minimize power and maintain the signal integrity, adjusting the bandwidth to optimize the response while minimizing the noise and adjusting the input threshold to set an optimal sampling point can all help add margin especially for CFP2, CFP4, and QSFP28 optical transceiver applications.

This increased bandwidth is just one of the innovative features incorporated into the ONET2804T, TI’s first 100G TIA.