Gentec-EO – Laser Power Meter & Laser Energy Meters

Well established in this field for over 30 years Gentec Electro Optics has been a leader in the field of laser power and energy measurement. The average power density damage threshold of 100 kW/cm2 that we introduced with the WB series in the mid 1990’s is still unsurpassed. Gentec-EO also offers you broadband spectrally flat power detectors for general use in the UP12E-H5 & UP19K-H5 series, high peak power pulse damage resistance for specific UV and IR bands with the UP19K-V series, and high average power detectors in the air and water cooled High Power UP25N-H9 & UP55N-H9 for the big jobs. All our detectors are available in OEM version and different size disks as well. Whatever your need Gentec Electro Optics has a solution.



How They Work
The basic laser power meter is essentially a thermopile. The more familiar application for thermopiles, in fact where the common name “thermo electric cooler” comes from, is when a voltage is applied to cool one side of the thermopile and whatever it is bonded to. Thermopiles for laser power measurement however are used in the opposite fashion. That is, a temperature difference is used to create a voltage. On one side is material heated by the laser and on the other is a heat sink. The laser energy absorbed by that material is converted to heat. With the hot absorber on one surface and the cold heat sink on the other, there is a temperature difference across the thermo electric device as the heat flows through it. This temperature difference causes the thermopile to generate a voltage. That voltage is proportional to the temperature difference which in turn is proportional to the laser power. The monitor measures this voltage to provide the laser power reading in watts. Figure 1 shows the fundamentals of the thermopile-based power detectors. The Absorber The optically absorbing material is one of the most important parts of the detector. That is because its properties define much of the performance of the detector, especially its resistance to pulse damage. This material absorbs most of the light energy from the laser and converts it to heat. A fraction is reflected that can vary from a few percent to 50 percent of the total optical power, depending on the material and intended application. How much is shown by the spectral absorptivity response curve for the material. With an absorber like our broadband H coating, around 90% of the power may be absorbed across a very wide range of wavelengths (190 nm to 11 microns) with small variations. This is called a spectrally flat absorber. It is efficient and because of its low thermal mass it transfers the heat quickly.

The Humble
Beginning A thermopile is simply an array of thermocouples connected in series and close together. The fundamental technology of all state-of-the-art thermal laser power meters actually goes back to 1821! That is when Thomas Seebeck joined two wires of dissimilar materials together at both ends and discovered electrical current flowing when he heated one end. Moreover, he found that the voltage between junctions was proportional to the temperature difference between them. That is called the Seebeck voltage and became the basis for the thermocouple. Years later Lord Kelvin (William Thomson) explained it. Essentially, the heat causes electrons to diffuse away from one end of a wire to the other. Since the effect is different for different metals, there is a net difference in voltage where the metals join, hence Seebeck’s voltage. Peltier made his contribution in 1834 by observing that heat could be made to flow into, or out of, the junction depending on which way you make the current flow. Modern thermocouples are made by the joining of specially formulated metal alloys and even specially doped semiconductor materials.

The Thermocouple
A practical view of a thermocouple is essentially 2 wires of different metals attached at both ends like in Figure 2. One junction goes to the “hot” side of the device and the other goes to the “reference” or cold side of the device. In laser power measurement, the hot junction is placed next to the absorber and the other next to the heat sink. Any temperature difference between the two junctions causes a voltage difference between them. That electrical voltage is proportional to the temperature difference, therefore to laser power. This is the voltage that is measured by the power monitor to provide the power reading.



The Thermopile
The amount of voltage that can be produced by one thermocouple is small, so an array of thermocouples is connected in series to increase sensitivity and multiply the output. In the array, instead of the two wires being joined twice to each other, each wire is joined to two wires of the other type, but a different one at each end. The junctions alternate back and forth so that each wire has a junction on the hot side, and another on the cold side. This is easier to visualize in Figure 1. The more numerous and closer together the junctions are, the more sensitive the thermopile will be. So it gives more voltage for the same laser power.

Disk Thermopile
Two kinds of thermopiles are used in laser power measurement. One is the disk thermopile shown in Figure 1 and the other is the wafer-type thermopile. The disk is made of one set of junctions laid out radially. One set of junctions is arrated under the aperture while the alternate set is near the edge of the disk which is attached to a massive heat sink. The laser power heats the absorber in the center and creates a temperature difference between the center and the edge. The thermocouples generate a voltage corresponding to this difference. The primary difference with the second type is that the heat flows radially through the disk which can handle more average power, especially with blown air or water cooling. The disk thermopile also has a much faster natural response time. Gentec-EO offers a complete line that combined a new technology disk with different cooling way like heat-sink, fan or water cooling module.

Wafer-Type Thermopile
The second one resembles a wafer, or sandwich, with thermocouples running between the two sides. One rectangular face of the wafer thermopile receives the heat. That creates a large temperature gradient across the small distance to the other face that is in contact with the massive heat sink. The array of solid state thermocouples in the thermopile generates a voltage proportional to this gradient just like in the disk thermopile. Because of the close spacing of the thermocouples to each other, the resulting large number of thermocouples in the wafer, and the large temperature gradient across the two surfaces, the output voltage of this thermopile is the most sensitive to laser power and the least sensitive to beam position and size.

Anticipation
The voltage response of a thermopile to the incoming power is predictable. It can be modeled. All Gentec-EO monitors have circuitry and software that model the incoming pulse and accurately predict its peak value before it actually occurs. This “anticipation” circuitry allows the wafer type thermopiles to have a much faster accelerated response time when used with a Gentec-EO monitor than the natural response time of the device.

DamageThreshold
Average Power An average power that is too high simply overheats the detector until it damages the thermocouple junctions. As a consequence, the thermopile itself and the cooling system determine the average power capacity of the detector. This is also what you risk if you exceed the manufacturer’s specification for too long. To avoid the overheats, we offer the UP series which can, with is newest design, take a lot more heat than the usual thermopile.

Average Power Density
Concentrating too much energy into too small an area can damage the absorber. Hence, the absorber determines how much energy and power density the detector can take. There are two fundamental types of damage. The first is from slow thermal effects and the second from short pulse impacts. The slow thermal damage is due to local heating when the average power density is too high. The result is melting, vaporizing and/or cracking of the absorber. CW, QUASI-CW and lasers with high repetition rates, such as used in micromachining can create high average power density, especially with small beam diameters. For these demanding laser beams we offer the W5 series which has, at 100 kW/cm2, the highest average power density threshold available today. For the most challenging cases, expanding the beam is often the easiest way to reduce the power density to something manageable.

Peak Power Density (Pulsed)
When the pulse energy is concentrated into too short a time, as well as space, it explosively vaporizes some of the absorber material at the surface. That ablates or knocks away some of the absorber. When the thermopile underneath is eventually exposed, the sensitivity may be affected too much for the application. The VM and VH series (volume absorbers) are designed to take the concentrated pulse energy by distributing it through a volume instead of just on the absorber surface. Unlike the broader band materials which absorb the energy right on the surface, the energy is absorbed throughout the thickness of the material. That spreads the energy throughout a cylindrical volume rather than just over an area of the beam diameter. Energy densities greater than 30 J/cm2 and peak power densities above 100 GW/cm2 can be handled this way depending on the wavelength. If damaged by excessive pulse energy density or peak pulse power density our absorbers can be easily replaced in the field.

Wavelength
The other important consideration is wavelength. Energy from the longer wavelengths, like Mid and Far IR tends to penetrate deeper into the absorber. Damage from exceeding the specification may occur first at the absorber-thermopile interface and work its way up to the surface. In the shorter wavelengths the energy is concentrated closer to the absorber surface. In the case of UV the photons are so energetic and concentrated on the surface that they cause electronic as well as optical-thermal damage. Essentially, they knock electrons out of atoms in the absorber material. Gentec-EO offers broadband absorbers for all kind of wavelength as well as absorber for specific wavelength. In practice, a combination of the two mechanisms is often at play and both may be visible. If your application is pushing the limits pay attention to the damage thresholds provided by the manufacturer and the spectral absorptivity curve for the material to adjust for wavelength where necessary.

The Bottom Line
Damage to the absorber surface, whatever the mechanism (even if you scratch it), is only an issue when it changes the ratio of power reflected versus absorbed at your laser wavelength. Visible discolorations may not mean much at our wavelength of your laser if it is outside of the visible light spectrum. Then again they might. If more power is reflected, less will be absorbed so the detector will be less sensitive than when it was calibrated. When this damage is severe enough, and covers enough of the area under the beam to affect the accuracy required by the application, you should send the detector for recalibration, and possibly service. For many applications an annual recalibration is good policy.

Quality
Besides our attention to accuracy Gentec-EO detectors are some of the sturdiest on the market. That makes them ideal for OEM applications that require robust instrumentation. You see our thoughtful quality even in the supporting features like the cables and stands.

Cables
All Gentec-EO power detectors come equipped with a top of line high quality audio cable. This pliant cable provides long flex life and outstanding EMI shielding. It is standard on our detectors and another example of the high quality mark we set in the market.

Solutions for Many Needs
Lasers come in many different varieties to serve a multitude of applications but most have one common requirement. That is to know how much laser power or energy there is somewhere in the optical train, from the laser to the target. The following tables will help you quickly locate the Gentec-EO products that best suit your need. As a former laser manufacturer, we are experts in solving laser power and energy measurement problems. Please consult your Gentec-EO representative for help selecting the right product for your application.