Pulse Width Modulation (PWM) refers to controlling the optical output of a laser via manipulation of the source that drives the gain medium. For our RF-excited sealed CO2 lasers, this is accomplished by adjusting the Pulse Repetition Frequency (PRF) and Duty Cycle (DC). Our default operation for most products is a PRF of 2.5kHz, which has a signal period of 400µs. By gating this signal with a 0-100% DC, we can adjust the pulse length of the signal produced. An important note to keep in mind, when setting up your ideal pulsing characteristics, is that for standard lasers the rise and fall times, or the response from the time the electrical signal is given to when the optical signal reaches maximum output, is approximately 200µs. For lasers utilizing the Super Pulse technology, this rise/fall time is reduced to 100µs, but the pulse length is limited to 400µs with a DC of 25%.
For example, if you were operating an L5-FC laser and want to produce pulses of 200µs, the RF driver is by default operating at 2.5kHz. You would simply need to use a DC of 50% so that the laser sees an “on” signal for 200µs followed by an “off” signal for 200µs to complete one 400µs period. In theory, this would produce a 5W output for 200µs and 0W for 200µs, thus an average power of 2.5W with peak power of 5W. However, due to the rise/fall time associated with the RF energy coupling the gain medium to produce the optical output, you can expect the actual average power to be lower, with the peak power of 5W only achieved momentarily at the end of the 200µs rise time when the RF input signal switches from “on” to “off”.