Gentec-EO – Laser Power Meter & Laser Energy Meters

Gentec Electro Optics provides a full range of products to meet your pulse energy measurement needs. They range from the tiny and sensitive QE4, the lean and portable QE12 and QE25 series, the large aperture QE50 series to our large world class custom calorimeters. Having introduced the first pyroelectric joulemeter over 30 years ago Gentec-EO is well established as an experienced source of energy measurement expertise. Be it in the laboratory or an OEM application Gentec-EO will have a solution.



How They Work
In the simplest terms, a pulse of light is absorbed by the surface of the detector and heats it up. That in turn, changes the temperature in a pyroelectric material underneath. This separates electrical charges in the pyroelectric which creates a voltage as the pulse of heat energy passes through it to a heat sink. The heat sink removes the heat energy to allow the pyroelectric to be ready for another pulse and to prevent it from over heating. The electrical voltage read by the measuring instrument is proportional to the energy. Figure 1 sketches out the basic structure of a pyroelectric joulemeter.



The Absorber
The business end of the detector is the absorber that coats the side of the pyroelectric that is exposed to the laser. That material absorbs most of the light energy from the laser and converts it to heat. A small fraction is reflected. How much is shown by the spectral response curve for the material. The thermal mass of the absorber and its thickness determine how quickly the heat can flow to the pyroelectric detector and hence its response time. Lowering the thermal impedance by using an absorber with a lower thermal mass or reducing the thickness of the absorber will increase its speed. The metallic MT coating is a good example. It allows for a measurement of each pulse up to 4000-6000 Hz.



The Pyroelectric
The heart of every Gentec-EO energy detector is a fast response pyroelectric material. It acts as a source of electrical current when subjected to changes in temperature provided by the absorber. Essentially it contains permanent electrical dipoles that are oriented in a specific direction. A rapid temperature change in the material will alter the orientation of these dipoles. That changes the internal electric field and causes an imbalance in electrical charge between the 2 large sides of the device. There are thin metal electrodes on these surfaces. They allow the charge to flow from one electrode into a circuit with a load resistor and then back to the crystal via the other electrode to eliminate the imbalance. The electrical current is converted into a voltage signal by the load resistor.

The Voltage Response
The result is a voltage pulse that rises quickly with the response time of the device to a level proportional to the laser energy (Figure 2). It then decays exponentially over a longer period of time that is a function of the pyroelectric device and load impedance. Figure 2 also shows that there is a longer recovery time to return to the initial state of the detector. This is a function of thermal phenomena and is not affected by the load impedance as are the rise and decay times. The integrated pulse energy over this period is proportional to the peak voltage.

The Measurement
The laser energy is given by the change in voltage divided by the sensitivity (in Volts/Joule) of the detector. The measured voltage is the change from the initial reference voltage to the maximum voltage of the pulse. The sensitivity is provided by Gentec-EO on our NIST-traceable calibration certificate. We measure this with extreme care with a well known laser energy provided by an NIST standard. This sensitivity is for the specific load impedance that is requested. The user can measure the voltage on an oscilloscope or computer data acquisition system and use the sensitivity value to make the energy measurement. An easier option is to read it directly in Joules from a Gentec-EO SOLO PE or DUO monitor.

Thermally Robust
The energy detector will make accurate measurements in spite of changing temperature in the environment or heating of the detector as long as the maximum voltage does not saturate. This is because it is the difference between the initial and peak voltages that measure the pulse energy. This relative measurement is good until the peak voltage is prevented from reaching its natural value by the maximum voltage available in the electronics.

Pulse Width Versus Rise Time
Usually the applied laser pulse must be shorter than the rise time of the detector for all of its energy to be represented by the peak voltage. Pulse energy received after the detector voltage has peaked will not be fully integrated into that value. For very long pulses, the peak voltage will actually represent peak power rather than pulse energy.

Damage Threshold
Excessive pulse energy that is concentrated into to a small area can damage energy detectors. For the most demanding laser beams we offer the broadband MB coating which has pulse energy density thresholds that are among the best in the world. Slight discoloration from short pulses is due to a modification of the organic material in the absorber that does not affect the detector calibration. If enough of the coating is removed by ablation to expose the metal electrode underneath, then the output voltage may be affected too much for the application. Too much average power, (that is above the manufacturer’s specification) can cause the detector to overheat. Contamination on the absorber surface can also interfere with the measurement or damage the detector by concentrating too much energy in one spot. Grease, dust, and fingerprints are some of the common contaminants to avoid.



Solutions for Many Needs
Gentec-EO provides a post, optical stand and cable with all energy detectors and calorimeters. Moreover they can all connect to both BNC and DB-15 terminations. All of the QE series detectors come with a DB-15 to interface with our SOLO PE and DUO monitors and with an optional DB-15 to BNC adapter for connecting to an oscilloscope. This connector contains an EEPROM with unique calibration information for each individual detector that allow the SOLO PE and DUO monitors to maximize its performance and accuracy.

QE Series Attenuators
The QEA and QEAS attenuators have been designed to extend the performance of the Quanta Energy detectors in the spectral range of 400 nm to 2.5 µm for the QEA and 190 nm to 400 nm for the QEAS. They transmit 15% to 20% of the incident energy, depending on wavelength, to the energy detector. The QEA has been tested to a peak pulse energy density of 7 J/cm2 for short pulses (7 ns), at 1064 nm. The QEAS can take up to 1 J/cm2 at 266 nm for short pulses (7 ns). They are easy to install. They simply slide onto the Quanta Energy Series detectors. The QEA and QEAS attenuators can be used with models QE12, QE25, and QE50.

QE-X Amplifier
Use this amplifier to measure even lower energy levels. The QE-X is a shielded electronic circuit with a gain of 10 designed to improve the signal to noise ratio to the monitor. The QE-X Amplifier is compatible with the SOLO and the DUO. The QE-X is connected directly between the head and the monitor, the latter providing the electrical power.