SINGLE_HETEROSTRUCTURE_LASER_DRIVER

Published:2009/7/14 11:44:00 Author:May | From:SeekIC

One way to reliably achieve the fast rise-time pulse of short duration to power SH lasers is to use the phenomenon called avalanching of a transistor. Transistors have a breakdown voltage, abbreviated as Vceo; if this voltage is exceeded, the transistor will spontaneously conduct with its collectorto-emitter circuit open and the voltage applied to its collector. This produces a pulse whose rise time is extremely fast-on the order of 1 or 2 ns-and whose duration is related to the capacitance at the collector. The operation is as follows: Current rises at the base of transistor Q1 when the voltage on Cy has risen enough to jump over and break down the collector-to-emitter junction. For the 2N2222, this is typically 75 to 80 V. When the transistor breaks down, it sends a high-current pulse of energy through a low-value carbon film resistor R(s). With the 0.022-μF capacitor shown, the duration of the pulse to the laser is only about 75 ns. This capacitor must be specifically designed for low inductance. Note the high-speed switching diode D1. Its purpose is to damp out any reverse voltage that might occur in the circuit due to inductance after the laser turns off because a reverse voltage of only 3 V can destroy the SH laser. The rise time of this diode is comparable to that of the avalanche circuit, being only 6 ns before it begins to switch. The values of the capacitor and the current-limiting resistor R(s) depend upon the type of SH laser diode used.



Reprinted Url Of This Article:
http://www.seekic.com/circuit_diagram/Basic_Circuit/SINGLE_HETEROSTRUCTURE_LASER_DRIVER.html

Print this Page | Comments | Reading(3)