Pulsed Lasers and Auto-Trigger Mode
DataRay has long offered products with the capability of measuring pulsed lasers in addition to continuous wave (CW) lasers. This blog post will review how capturing pulsed lasers with a camera beam profiler differs from capturing CW lasers, and how the additional challenges with pulsed lasers are handled to provide useful measurements with the software’s Auto-Trigger mode. This is a part of a series of blog posts about measuring pulsed lasers; the next blog post will describe the External Trigger mode.
Pulsed Laser Overview
Pulsed lasers differ from CW lasers because instead of outputting light continuously, they output short pulses with an associated pulse width that occur at a specific pulse repetition rate (PRR). The pulse width can be very short, and can have a very high pulse energy. A CW laser and pulsed laser that both have the same average power over time will not have the same peak power because the pulsed laser has no output between pulses. So, if the average power remains constant, the pulse energy increases with shorter pulses. These pulsed laser parameters in addition to the wavelength, will determine how the laser will be measured.
Figure 1: Representation of a pulsed laser. The waveform is HIGH during the laser pulse; the waveform is LOW between pulses.
DataRay’s camera-based beam profilers use a sensor array to collect light from a laser beam which is ultimately read by the DataRay software to visualize the intensity distribution of the beam and calculate parameters such as beam width or ellipticity. Each pixel of the sensor array effectively has an electronic shutter to control how long it is exposed to the laser light. Every frame returned from the camera is a result of every pixel in the sensor array collecting light for the length of the exposure time displayed in the DataRay software. The exposure time is the amount of time the electronic shutter stays open to capture each image. Some sensor array’s electronic shutter is not capable of beginning the exposure for every pixel at the exact same time, but will instead employ a rolling shutter to expose the pixels in the array line by line. This poses a clear problem for measuring pulsed lasers with a rolling shutter, which is further detailed in the associated application note. A camera that begins and ends the exposure of every pixel at the exact same time is said to have a global shutter. A global shutter is always recommended for measuring pulsed lasers. DataRay’s flagship WinCamD-LCM with a large 1” CMOS sensor features a global shutter. Our WinCamD-UCD12, WinCamD-UCD15, and WinCamD-UCD23 cameras also have global shutters.
Figure 2: Representation of the electronic global shutter. The shutter is open and exposing when the waveform is HIGH; the shutter is closed when the waveform is LOW. The length of time between exposures when the waveform is LOW depends on how fast the frame is read out. Smaller capture blocks will be read out faster and result in higher frame rates.
A camera with a global shutter allows the DataRay software to be used to capture single pulses, partial pulses, an average of several pulses, or every pulse depending on the laser parameters. To capture single pulses, the electronic shutter must be open for the length of only a single, complete pulse width. To ensure the camera is capturing each pulse as intended, the DataRay software offers several trigger modes to synchronize the exposure time to the laser pulses. Capturing single pulses may not provide enough light for the sensor for adequate signal-to-noise ratio, so the Imager Gain can be increased as needed.
It is recommended to first find the beam using the default mode with auto-exposure to ensure the beam is centered on the sensor array. Once the beam is found, the Setup dialog should be used to choose an appropriate capture block size containing the beam. Pressing CTRL+T or right-clicking on the Trigger delay scroll control will open the Trigger Setup dialog box. Here the user may select auto trigger mode.
Auto Trigger Mode
Auto trigger is the simplest triggering mode for pulsed lasers, and doesn’t require any external trigger. There is no formal synchronization of the shutter to the laser pulse train, but the exposure time can be carefully chosen to best capture single pulses. The recommendation for capturing single pulses is to set the exposure time to 0.95*(1/PRR). This will expose the sensor array for slightly less than the amount of time between the start of each pulse, ensuring that capturing more than one full pulse is not possible.
Figure 3: The green waveform shows when the camera is exposing. The blue waveform shows when the laser is pulsing. The red waveform shows when the exposure and pulse overlap; this is the relevant data read by the software.
Since Auto-Trigger has no formal synchronization, it is possible to occasionally capture partial pulses. Figure 5 shows one way this can occur. If the exposure time is shortened, capturing two partial pulses in a single exposure can be avoided but the chances of capturing no pulse or one partial pulse increase as shown in Figure 6. Auto-Trigger mode handles this by offering the ability to reject frames if they do not meet a certain peak intensity criteria. With careful setting of the exposure time and Min/Max ADC %, the probability of capturing partial pulses can be minimized and partial/empty frames can be rejected.
Figure 4: Auto-Trigger Min/Max ADC %. A narrower range can be set to reject partial frames.
Figure 5: The fourth exposure shown here captures two partial pulses since Auto-Trigger mode is not formally synchronized with the pulse train.
Figure 6: The exposure time is shortened so that a single exposure cannot capture more than one pulse. However, the last two exposures here have each captured a partial pulse. If the Auto-Trigger Min/Max ADC % is set properly, these last two exposure will be rejected and ignored.
DataRay offers a selection of cameras that can be used to analyze pulsed lasers. Our software offers an Auto-Trigger mode for simple unsynchronized measurements of laser pulses, that can be sufficient for many applications. There is also an External Trigger mode that accepts an external input signal to synchronize the exposures with laser pulses for more controlled measurements. The external trigger will be explored in the next blog post. If you have any questions regarding beam measurement methods for your application, please contact us at email@example.com. We have years of experience in laser beam proﬁling and would love to discuss a solution for your system.