Fiber optics will eventually dominate the networking and communications market due to its higher bandwidth, better signal quality, immunity from electromagnetic (EM) radiation and lighter weight. However, domination can only be realized if the optical transceivers become smaller, cheaper and lower power. Most data communications systems use laser modules in high- performance optical network transceivers. Currently, fiber-optic transceiver designs have no easy means for adjusting the modulation current of the laser based upon the temperature of the laser itself and maintaining a constant light output or extinction ratio. This has led to widespread use of thermal electric coolers (TECs) to hold the laser at a constant temperature, an approach that increases the footprint, cost and power consumption of the transceiver. Eliminating the (TEC) in the transceivers while maintaining optimized performance over a wide range of ambient temperatures is key. Automatically adjusting modulation and bias current to fit individual laser diode characteristics as well as aging effects also obsoletes costly manual calibration procedures. Methods to achieve optimal laser power over temperature and time are shown in this paper with a proven reference design. The reference design includes a 2.5Gbps OC48 1310nm transmitter laser module, a differential laser driver, and an adaptive power controller with non-linear control and performance test results.

Existing Laser Solutions

Available laser solutions for long-reach applications such as telecom and/or enterprise switches use the TEC and associated control electronics to keep the laser diode at a constant temperature. This extra cooling is necessary for high power lasers designed to drive long distances. The TEC reduces the diode temperature to increase operating life at the higher power and to keep the needed modulation and bias current constant. Unfortunately, the TEC also increases the size and cost of the laser module as well as the solution complexity due to the power devices needed to control the TEC. An example of a transceiver is shown in Figure 1 and an internal schematic of a laser module is shown in Figure 2.

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