In optical communications networks, optical communications module (i.e., optical transceiver, transmitter and receiver modules) are used to transmit and receive optical signals over optical waveguides, which are typically optical fibers. An optical transceiver module includes a transmitter side and a receiver side. On the transmitter side, a laser light source generates a laser light beam and an optical coupling system receives the laser light beam and optically couples the laser light beam onto an end face of an optical fiber. The laser light source typically comprises one or more laser diodes that generate light beams of a particular wavelength or wavelength range. A laser diode driver circuit of the transmitter side outputs electrical drive signals that drive the laser diode. The optical coupling system typically includes one or more reflective, refractive and/or diffractive elements that couple the modulated light beam onto the end face of the optical fiber. On the receiver side, optical signals passing out of the end face of the optical fiber are optically coupled by an optical coupling system onto a photodiode, such as a P-intrinsic-N(P-I-N) diode, for example, by an optical coupling system of the transceiver module. The photodiode converts the optical signal into an electrical signal. Receiver circuitry of the receiver side processes the electrical signal to recover the data. The transmitter and receiver sides may use the same optical coupling system or they may use separate optical coupling systems.
In high-speed data communications networks (e.g., 10 Gigabits per second (Gb/s) and higher), certain link performance characteristics, such as relative intensity noise (RIN), for example, are dependent on properties of the laser light source and on the design of the optical coupling system. In most optical fiber applications, a trade-off exists between forward optical coupling efficiency of laser light from the laser light source into the end face of the optical fiber and back reflection of laser light from the end face of the optical fiber onto the laser light source. Back reflection increases RIN and degrades the performance of the laser light source. As optical communications links utilize increasingly higher data rates, reducing RIN becomes increasingly important. In optical links that use multimode laser light sources and multimode optical fibers (MMFs), it is desired to improve the mode matching between the laser modes and the fiber modes in the forward coupling to increase the link distance and reduce the sensitivity of the link to mode partition noise.
The traditional approaches for managing back reflection include using an edge-emitting laser diode with a fixed-polarization output beam in conjunction with an optical isolator, or using an angular offset launch in which either an angled fiber in a pigtailed transceiver package or a fiber stub is used to direct the light from the light source onto the end face of the link fiber at a non-zero degree angle to the optical axis of the link fiber. These approaches have advantages and disadvantages. The optical isolator may not have the desired effect if used with a laser light source that has a variable-polarization output beam, such as a vertical cavity surface emitting laser diode (VCSEL). Using an angled fiber pigtail or fiber stub can increase the complexity and cost of the transceiver packaging. Also, such approaches may not be suitable for applications where a standard optical fiber needs to be used.
A need exists for an optical coupling system for coupling a laser light beam onto an end face of an optical fiber that enables efficient coupling to many fiber modes while also reducing back reflections and, therefore, reducing RIN.