High-Speed Digital Fiber-Optic Transmitter

Wavelength Division Multiplexed (WDM) solutions are available from the commercial sector in both single mode and multimode formats. However, these solutions are not readily adaptable to use in the harsh environmental conditions required by the Military and Aerospace segment, nor are they capable of the high passive loss requirements. For example, the commercially available solutions in the 850 to 940nm band (so called SWDM band) are typically rated from 0 to 70C and are manufactured using complex chip on board die placement and utilize molded plastic optics. Operation over extended temperature ranges and other environmental factors such as mechanical shock and vibration are not possible. Additionally, the receiver sensitivity is generally at -11dBm (OMA) and fiber coupled powers are in the -3dBm range. This provides a loss budget of 8dBm, of which only 2dB is allocated to passive (connector) loss. Transceivers in the traditional telecom wavelength range of 1310 to 1550nm operate on single mode optical fiber, which has shown to not be robust in the Navy operating environment. Here we propose a Wavelength Configurable Transmitter (WCT) in both the SWDM and CWDM band capable of operating in the extreme military environment; that offers an addition 7dB of passive loss budget; and that is built using traditional TO packaged lasers in a hermetic optical path. The design uses a strain relieved OM4 or OM5 fiber optic pigtail with an FC-PC connector and the optical power from the lasers is collimated through the hermetic assembly. A series of filters is used to combine the individual wavelengths and direct them to the multimode fiber output. The TO can packaged lasers are actively aligned to the lenses to maximize fiber coupled power. The TO can components will be laser welded in place and back filled with epoxy (or laser seam sealed) to complete the hermetic assembly. A flex circuit is used to connect the TO laser components to a printed circuit board containing the laser drivers and associated circuitry. VCSELs in the SWDM band with output power capable of meeting the loss budget are not commercially available, and one objective of this project is to design and fabricate these lasers. DFB lasers with sufficient coupling to multimode fiber are more commercially available and these will be utilized in the program. The target is minimum of +3dBm fiber launch power per wavelength. When coupled with the Wavelength Configurable Receiver (WCR) receiver being developed by PSI for the Navy, a 15dB total link budget is achievable. The proposed architecture offers a high degree of configurability. For example, SWDM and CWDM bands could even be mixed within the WCT. Furthermore, the WCT could be made bidirectional by mixing components from the WCR. The signaling speed of each channel can be readily customized to any rate up to 28Gbps, possibly even 28GBd, making the total data throughput 200Gbps.