Laser Drivers

Laser drivers, also known as a laser diodes driver and sometimes as a laser diode controller, is a device used in converting very high-speed digital data to analog signals used to drive laser diodes. Laser diodes are used many applications including CD, DVD drives and fiber optic interfaces. A typical application is driving laser diodes in multirate LAN, MAN fiber channel and SONET/SDH interfaces. Data rates for these kinds of applications can reach tens of GHz for extremely high-speed systems.
They can provide closed loop control of the average power of a continuous wave laser diode. In this case, the control loop adjusts the bias current to the laser diode to maintain a constant current output of a monitoring photodiode (MPD). They can incorporate failure alarms for the end of life of a laser or degradation of a laser diode (impending fail).
In order for data to be transmitted by the laser, the digital data stream is converted into a modulated analog signal that modulates the laser current. The digital data stream arrives as a differential pair into a 50 Ohm terminated differential amplifier. This differential digital stream is converted into a differential current mode output designed to drive a 50 Ohm load. The laser diode is usually in series with resistors to provide matching. The devices accept PECL or CML style high-speed serial interfaces on their inputs. The also have enable inputs to enable the driver, and the polarity of the data may able to be selected and swapped.
The extremely high-speed devices supporting greater than 10GHz include equalizers for compensating for cable and connector losses. Such standards include interfaces like 10Gbit Ethernet (10GBASE-SR). Jitter noise is extremely important for high-speed communications. Jitter is a phase noise relative to the data clock that is embedded in the data stream itself. Jitter specifications of only a few picoseconds are required to ensure data integrity. Data is sent in a differential waveform that is analyzed by storing successive data bit transitions as they occur in the analog signal. As they overlap they create a pattern referred to as an eye. The jitter determines how open the eye is. It is desirable to have an open eye to ensure the data is correctly received with acceptable bit error rate (BER) by the remote receiver. Read more Read less