This document discusses the interpretation and basis for stated measurement accuracy of Ophir Laser Power/Energy meters.
1. General Discussion
2. Combination of Errors and Total Error
3. Analysis of Power and Energy Calibration Errors
4. Detailed Analysis of Power and Energy Calibration Errors
The Renowned German standards laboratory Physikalisch-‐Technische Bundesanstalt – PTB, has now developed a highly accurate calibration standard for calibrating Terahertz radiation based on a modified Ophir 3A-‐P meter.
The laser industry is advancing steadily with new wavelengths, higher powers and energies, and new applications all the time. As the power, energy and variety of new lasers advances, so measurement of these lasers has to advance.
Using the Ophir BC20 to Evaluate Scanned Effects in Light Show Applications
The assessment of laser beam exposure used for entertainment applications is a challenging undertaking; both the emission and the environment pose particular obstacles to persons with the responsibility of ensuring emissions are below the permissible exposure limits. This article discusses how use of Ophir’s BC20 detector is able to offer significant improvements in measurements quality over traditional laser power detectors intended for CW beams. In addition, the BC20 simplifies the measurement process and allows measurement of live effects, opening the way for assessments to be undertaken with a much greater degree of accuracy. This provides benefits to assessors, whether they are operators, venue safety staff, or regulators, who can now additionally monitor emissions and ensure they are not exceeded during performances.
Power and Single Shot Energy Sensors
Ophir provides two types of power sensors: Photodiode sensors and Thermal sensors. Photodiode sensors are used for low powers from picowatts up to hundreds of milliwatts and as high as 3W. Thermal sensors are for use from fractions of a milliwatt up to thousands of watts.
Thermal sensors can also measure single shot energy at pulse rates not exceeding one pulse every ~5s.
Repetitive Pulse Energy Sensors
For higher pulse rates, Ophir has pyroelectric energy sensors able to measure pulse rates up to tens of KHz. These are described in the energy sensor section, section 1.3.
A thermopile sensor has a series of bimetallic junctions. A temperature difference between any two junctions causes a voltage to be formed between the two junctions. Since the junctions are in series and the «hot» junctions are always on the inner, hotter side, and the «cold» junctions are on the outer, cooler side, radial heat flow on the disc causes a voltage proportional to the power input. Laser power impinges on the center of the thermopile sensor disk (on the reverse side of the thermopile), flows radially and is cooled on the periphery. The array of thermocouples measures the temperature gradient, which is proportional to the incident or absorbed power. In principle, the reading is not dependent on the ambient temperature since only the temperature difference affects the voltage generated and the voltage difference depends only on the heat flow, not on the ambient temperature.
Ophir has two types of energy sensors, pyroelectric and RP. Pyroelectric sensors are for measuring repetitive pulse energies and average powers at pulse rates up to 25000 pulses per second and pulse widths up to 20ms. RP sensors are specialty items mainly for very long pulse widths and very high average powers that cannot be measured by pyroelectric sensors. Note that single shot energy with pulse rates less than one pulse every 5s or so can be measured with thermal sensors described in the power sensor section
Most drivers get caught speeding at some time during their driving experience. A common scenario occurs when a policeman uses a LIDAR speed meter to indicate that a car is over the speed limit. When the car is caught and pulled over, the driver shows a surprised, innocent face, attempting to get out of a fine. But when the policeman shows the driver the reading on his LIDAR speed meter he knows he’s going to have to pay. Can the driver claim that he was within the speed limit, claiming that the LIDAR instrument is not calibrated recently?
Various LIDAR instruments may be used to measure speed, direction of motion of a motor vehicle, and the distance to another moving vehicle. LIDAR instruments are used by the police to enforce speed limits and to analyze car crashes or crime scenes in order to reconstruct the scenes.
A common thread running through many Frequently Asked Questions relates to damage of measuring sensors.
Many applications involve considerable powers and/or energies; since laser measurement has us deliberately putting a measuring instrument in harm's way, let's have a look at the various effects a laser beam can have on an instrument in its path.
The selection of a sensor to accurately measure the power of a laser or energy of a pulsed laser can seem like a simple and easy procedure. However, many times the selection process is limited to choosing a sensor that only meets the range of power or energy to be measured, leaving out several other essential criteria of the laser specifications; that without their consideration, can allow the wrong sensor to be selected, the laser to be measured inaccurately and likely to cause the sensor to fail prematurely.
Ophir photodiode sensors can be used for many years without any repairs when used with the proper laser optical setup. Many of our customers have sensors that are using their original absorber that are over ten years of age. We hope that this document will enable you to also enjoy the long life and reliable results that Ophir- Spiricon is known for.
We believe that Ophir pyroelectric sensors can be used for many years without repair when used with the proper laser optical setup. We hope that this document will enable you to also enjoy the long life and reliable results that Ophir-Spiricon is known for.
Careful measurements are considered when testing the optical power, current, voltage, wavelength, and temperature of high output laser diodes. The test system energizes and measures the laser parameters as it will be used in the application. In some critical constant wave (CW) applications, the required output power from the laser is pushing the laser’s maximum specifications. Therefore, an accurate, stable, low drift laser power meter is required.
- Challenge: ever increasing demand for more accurate measurement
- Solution: constant improvements in equipment and methods
- How do we calibrate laser Power / Energy?
- Basic method: stable laser and substitution
- What is expected accuracy in simple case?
(power cal and wavelength available at NIST)
This was demonstrated in a project undertaken recently, and is a typical example of what of multi-user systems see in the field.
One of the inconveniences in the measurement of laser power and energy is what to do with the cables connecting the display to the sensor. These cables are of a limited flexibility and they clutter the workspace where the measurement is to be done. Sometimes, due to their stiffness, a motion of the cable moves the sensors and misaligns the set up.
The main situations in which RP sensors are useful are
- For very long pulses >10ms and very large duty cycles such as are typical in many pulsed diode laser applications.
- For very high average powers greater than 50W with repetitive lasers.
- When you want to measure the temporal pulse shape as well as the power and energy.
- When you want to monitor for missing pulses
From the time of its invention, more than 30 years ago, the laser power meter was generally
comprised of two parts: a measurement head and a display box. It was always considered
better to have such an arrangement with a cable connecting the two because of the hazardous
nature of the laser beam. As the display of the results is separated from the measurement
head, so are the eyes of the operator separated from the laser beam.
Now that the PC is an indispensable part of the office and the laboratory, it is important to be
able to integrate measurement instruments to the PC, particularly instruments that can gather
large volumes of data. There is a need for a unified connectivity architecture wherein all
measurement heads are compatible with all display boxes and are then easily connected to the
From the time the first laser was built, physicists probably thought, “That’s great! Now how do we measure it?” Thus laser power and energy meters were born.
Since lasers are good sources of concentrated heat, it was probably assumed that heat sensing methods would best be employed for measurement. The simplest device to measure heat is a thermocouple. A simple device to measure light is a photodiode. So, some enterprising engineer designed and built such a device. Then they needed an instrument to display the results and give rapid feedback in order to tweak, align, or adjust the laser for maximum output. Early displays were basically analog meters that had a needle on a dial that went from left to right as the laser power went up.