Laser drivers control and manage the operation of lasers used for switching in optical networks such transmission systems, optical transmitters and section regenerators. They provide adjustable laser bias and modulation currents, accept differential clock and data input signals, and may include an adjustable pulse-width control circuit to minimize pulse-width distortion. To interface with laser diodes, laser diode drivers require a series-damping resistor for impedance matching. Typically, these resistors minimize the optical output aberrations caused by signal reflections at the electrical interface. Features for laser drivers include high or low-side safety switches, laser failure monitoring, and temperature compensation.
Laser drivers carry product specifications for absolute maximum ratings such as supply voltage, storage temperature range, operating junction temperature, processing temperature, and current. Specifications may include a single +5 V or -5.2 V power supply, 108 mA supply current, and direct modulation to 10.7 Gbps. Laser drivers may also offer 50-ohm on-chip input termination resistors, programmable modulation current to 100 mA, programmable laser bias current to 100 mA, and a 25-ps rise time. Laser drivers with adjustable pulse-width control, a selectable data retiming latch, electrostatic discharge (ESD) protection, and an internal-series damping resistor are also available.
Selecting laser drivers requires an analysis of electrical characteristics. DC electrical characteristics include power supply voltage, power supply current, single-ended input resistant, bias current-setting range, bias current-setting error, bias current temperature stability, and bias off-current. Signal inputs are also an important specification to consider. AC electrical characteristics for laser drivers include input data rates, modulation current setting range, modulation current setting error, modulation current temperature stability, modulation off-current, output current rise time, output current fall time, and setup or hold time. Parameters also include pulse-width adjustment range, overshoot, driver random jitter, driver deterministic jitter, and input return loss.
Laser drivers are designed for use in SONET and SDH applications. Synchronous optical network (SONET) defines the rates and formats for synchronous data transmission over fiber-optic networks. SONET is specified in ANSI T1.105, ANSI T1.106, and ANSI T1.117 from the American National Standards Institute (ANSI). SONET is technically consistent with synchronous digital hierarchy (SDH), a European standard from UTI-T. Although SDH is not identical to SONET, the two standards are close enough that some hardware manufacturers provide equipment that conforms to both standards. For laser drivers, SONET OC-192 and SDH STM-64 are applicable standards.