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Pulsed Lasers: Pulse Energy & Imager Gain

Posted by Loren Jones on 1/10/2017 to General

Introduction

This is the third entry in a series of blog posts that explore DataRay’s pulsed laser measurement capabilities. The previous blog posts provided an overview of pulsed lasers and covered how individual pulses can be measured using Auto-Trigger mode or External Trigger mode to synchronize exposures to single pulses. This blog post will talk about the energy requirements involved with measuring pulsed lasers, and how the received signal can be controlled when the exposure time is fixed.

Pulse Energy

When capturing CW laser beams we are typically concerned with the power in Watts, which is a measurement of energy (Joules) per second. The amount of energy collected by the imaging sensor for a CW laser is therefore controlled by the length of exposure in seconds. But when using a pulsed laser with the goal of capturing single pulses, the exposure time cannot be freely adjusted to control the amount of energy collected by the sensor. The energy collected when measuring single pulses will be equal to the laser’s single pulse energy that reaches the sensor. The laser’s pulse energy is measured in Joules.

Attenuation

Both CW and pulsed lasers will usually need to be attenuated using one or more ND filters to reach an appropriate level for the measurement. A beam sampler may also be necessary for beams that could damage the ND filters. Typically, the attenuated beam is measured and the camera adjusts the exposure to maximize the dynamic range. Since adjusting the exposure time is not a solution when capturing single pulses with a fixed exposure time, the other way to fine-tune the exposure is to adjust the imager gain.

Chart 1: This plot displays the pulse energy limits for saturation of our camera sensors when using an ND-4 filter

Imager Gain

The imager gain adjusts how the analog signal from the imager is converted to digital units. DataRay’s beam profiling cameras use a CMOS or CCD sensor array to convert incoming light (photons) to electric charge (electrons), which is ultimately converted to digital units with an analog-to-digital converter. The imager gain defines how many digital units are generated per electron. Higher gain values result in a more sensitive sensor, since more digital units are generated per electron.


Figure 1: The Imager Gain slider in the DataRay software can be right-clicked to select Auto-Gain

By default, the gain is set so that the full well saturation of the sensor corresponds to the analog-to-digital converter’s full range; this is when the imager gain equals 1. A gain of 1 is selected to make use of the full range of the sensor since we usually have plenty of light when measuring lasers and can fine-tune the exposure time. But when measuring single pulses of pulsed lasers, the sensor may not be able to collect the full amount of light it is capable of collecting. For example, if the linear full well of the imaging sensor is 8000 electrons, a gain of 1 will set the ADC full range to correspond to 8000 electrons. If a single pulse only generates 4000 electrons, then much of the ADC range will be unused if a gain of 1 is selected. If the Imager Gain is increased to 2, then the sensor’s 4000 electrons will now correspond to the full ADC range therefore increasing the effective bit depth and reducing percentage of the signal that is noise.

Peak Signal (Electrons) Imager Gain Peak Signal ADC % Baseline Noise SNR
8000 1 90% 0.04% 2500:1
4000 1 45% 0.04% 1250:1
4000 2 90% 0.063% 1418:1
Table 1: The baseline noise contributes a higher percentage of the peak signal if a gain of 1 is used in photon-starved measurement scenarios such as lower energy single pulse measurements, so increasing the gain will improve this type of measurement

Auto Gain

The exposure control dialog shown in Figure 2 below is accessed by right-clicking the Imager Gain or Exposure Time box. When Enable auto gain adjustment is selected, Enable auto exposure adjustment is automatically deselected and the exposure time will now be fixed. The laser’s pulse repetition rate in kHz can be entered in the PRR field to automatically calculate a recommended exposure time for capturing individual pulses. The previous two blog posts talk about setting the correct exposure time for each triggering mode.


Figure 2: Exposure Control dialog in the DataRay software

Integrating Multiple Pulses

In cases where single pulses do not provide enough energy for a good measurement or if the pulse repetition rate is too high to measure only a single pulse within the camera’s minimum exposure time, then it will be necessary to increase the exposure time and capture multiple pulses per exposure. This type of measurement will provide the average pulse profile which is a useful measurement for pulsed lasers. Since this allows for an increased and adjustable exposure time, the imager gain can be set to 1 and the exposure time can be set to Auto.

Conclusion

DataRay’s camera beam profilers and included software provide the tools necessary for pulsed laser analysis. We offer several attenuation options, and can help choose the proper attenuation based on your laser specifications. The various triggering modes can be used to capture individual pulses, and the imager gain can be increased to improve lower pulse energy captures. Multiple pulses can also be measured in a single exposure, which can allow simpler measurements and provide an average pulse profile. Please feel free to contact us at support@dataray.com with your laser specifications and measurement requirements. We have years of experience in laser beam profiling and are happy to discuss a solution for your system.