A Laser Driver Circuit for High Speed Switching Applications
Original Publication Date: 2000-Mar-01
Included in the Prior Art Database: 2003-Jun-19
It is well known that a semiconductor diode laser needs to be properly biased at a given operating point before a high speed switching signal can be used to modulate the output of the laser. The switching signal should operate about this bias point such that the laser is continually emitting to ensure that its dynamic performance can be maintained throughout the switching range. This disclosure describes a control scheme which achieves his objective and allows the switching signal to be set with the bias control such that the laser can always be assured of operating within its correct range. A semiconductor diode laser emits light when driven by a current source which exceeds a certain threshold level (Ith), above this threshold the light output increases with drive current up to the operating limits of the device. When a switching signal is superimposed on the drive current, it can be seen that this modulates the output of the laser and the gain of the device can be expressed as: A Pmax Pmin (Imax Imin The gain and threshold current of a laser varies significantly between individual parts and also with parametric changes, such as temperature. Consequently the laser drive control circuitry needs to take account of these effects and typically it is necessary to provide some form of adjustment to set the operating point on the laser characteristic so that the laser is known to be operating around a known output power. This is clearly important to guarantee correct circuit operation and also to ensure compliance with laser safety power limits. However, it is also necessary to ensure that the extremes of the signal levels do not cause the laser to operate outside of its desired operating region. At the low end it is important not to fall below the threshold current of the laser which will cause the output of the laser to turn off and will seriously affect the switching speed of the device. At the high end, there is the problem of increased signal distortion as the laser output power starts to level off with increasing drive current and the laser reaches the limits of its operation. Consequently laser driver circuits have normally provided another adjustment for controlling the switching range or gain of the laser control circuitry, or have ensured that the switching range is well within the operating limits of the full range of laser devices. Both of these techniques have disadvantages, the additional adjustment introduces additional manufacturing complexity and cost whereas reducing the switching range lowers the efficiency of the driver system since the signal power is reduced as a proportion of the carrier power of the laser output. The system described uses a closed loop feedback control system to regulate Pav and then uses the same drive current to set the signal swing Pmax-Pmin ). In this way the signal power tracks the average power setting of the laser and the signal swing is set around the average power level for improved control.