All RF-excited CO2 lasers use pulse-width modulation (PWM) to control their output power.
The laser gas discharge is a dynamic and non-linear load on its RF driver. While the load impedance changes with the input RF power, the matching network between the RF driver and the laser (load) can only match to a fixed impedance. A mismatch between the RF driver and the laser would over-stress the RF driver, causing it to fail prematurely, or result in inconsistent laser performance. Therefore, it is essential to maintain the input RF power constant to avoid a mismatch. With PWM, this is achieved when the RF power is either on or off without intermediate levels.
When the frequency is high enough, the practical result of PWM is the same as changing power level.
Two factors contribute to this fact:
- Laser output response is slower than the PWM electric signal. As can be seen in the following two scope traces. The upper trace is laser power, the faint straight line below the upper trace designates zero power, the second trace is the envelope of RF power, and the bottom trace (half shown) is the TTL (0/5 volt) modulation signal.
It is clear from these traces that at low frequency the laser waveform more or less follows the modulation signal (A), but at high frequencies it does not (B). At high frequencies, the adjustment of the duty cycle in the control PWM signal translates to the adjustment of the level of laser power. This behavior is similar to the output of an RC circuit driven by a square wave with much longer (A) or shorter (B) periods than the RC time constant.
- Material response is slower than the rate at which the laser power changes. If the material has a low thermal conductivity and the laser beam changes relatively slowly, the laser power is averaged by the material. Here the effect of the PWM is the same as a continuous wave with the same average power. Materials with low thermal conductivity include paper, plastic, wood, glass, fabric, etc.
PWM is achieved with a TTL signal (0 or 5 Volts)
All the Access Laser products have a BNC port on the RF driver where the customer can supply a pulse signal at a wide range of frequencies and duty cycles. Zero volts turns off the laser, and 5 volts turns on the laser. Please consult with the factory if you plan to use a PWM frequency of 100 kHz or higher. Laser controllers from Access Laser provide a knob for PWM control at a fixed frequency: 15 kHz for lasers using model RF4 RF driver, and 2.5 kHz for lasers using other RF drivers. The customer can adjust the average power using this knob. Typically, a CW-capable laser can be adjusted down to a 10% duty cycle and still maintain good power stability.