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
There are two main technologies commonly used today for measuring laser beam powers:
- Photodiode-based sensors, used for measuring low powers (from pW up to several hundred mW, typically); these are limited to spectral regions from the UV to the near IR, depending on the specific semiconductor used, and
- Thermal sensors, used for measuring higher powers; the most sensitive thermal sensors can measure from as low as tens of microwatts, and up to 100 KW and beyond.
In many industries LEDs are replacing traditional broadband light sources such as mercury, deuterium, Xenon, and quartz-halogen lamps. Systems and applications transitioning to LEDs are reengineered in terms of optics, electronics, heat management and more. Similarly, the equipment used by professionals to measure the output of these sources needs to be fitted for measuring LEDs.
Traditionally, broadband sources are measured using radiometers or photometers. These sensors have a relatively wide and uneven response over a certain spectral range such as UVA, UVB, UVC, etc. The sensor gives an integral of the measured spectral power density weighted by its own spectral response.
Modern laser applications demand ever-increasing accuracy in the measurement and control of the laser beam. Various types of sensors and instruments are in use today, the choice depending on the type of measurement needed.
However, even given the correct choice of equipment, there are measurement conditions that can seriously affect the accuracy of the readings obtained if not correctly taken into account. Examples include ambient temperature, incidence angle of the beam on the sensor, and others.
In this article we’ll examine to what degree incidence angle matters when you’re measuring laser beam power or energy. We will consider a number of common sensor types.
The beauty of integrating an OEM sensor into your laser system is that you get to call the shots. What matters to you the most? Accuracy? Size? Output type? Tell us what you need and we’ll make it happen. Julian Marsden, the head of our electrical engineering department, wrote up a white paper to show you some of the OEM energy sensor options that are available. Read the whole white paper here:
There seems to be a good deal of confusion when it comes to the terms “response time” and “integration time” of energy sensors. In this article we will clarify the meaning of these terms, as they apply to Ophir’s pyroelectric “Smart Sensors.”
Pyroelectric sensors use a pyroelectric crystal. When a laser pulse is absorbed, it is turned into a heat pulse in the crystal, and the crystal then generates an electric charge proportional to the heat absorbed. Since the two surfaces of the crystal are metal coated, the coated crystal in effect becomes a capacitor; the total charge generated is collected (and therefore the response is not dependent on beam size or position) and becomes a voltage pulse, which gets measured.
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.
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. 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.
We have included this document with your recent calibration order because we have noticed an out of tolerance condition obtained from your equipment when returned for calibration. This document was created to assist our valued customers in the proper care and maintenance of Ophir photodiode sensors. The following information is for reference only. If you have any reason to believe that the sensor is no longer performing within the original specifications, we always recommend that you send it in for repair and/or recalibration by our trained technicians to bring the unit back to the proper NIST traceable standards.
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.
This document was created to assist our valued customers in the proper care and maintenance of Ophir-Spiricon pyroelectric laser power sensors. The following information is for reference only. If you have any reason to believe that the sensor is no longer performing within the original specifications, we always recommend that you send it in for repair and/or recalibration by our trained technicians to bring the unit back to the proper NIST traceable standards.
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 document was created to assist our valued customers in the proper care and maintenance of Ophir thermal laser power sensors. The following information is for reference only. If you have any reason to believe that the sensor is no longer performing within the original specifications,we always recommend that you send it in for repair and/or recalibration by our trained technicians to bring the unit back to the proper NIST traceable standards. We believe that Ophir thermal sensors can be used for many years without any repairs 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 for which Ophir-Spiricon is known.
The Quasar wireless Bluetooth laser power and energy measurement interface allows quick and trouble free installation of complex measurement systems in an existing manufacturing environment, with a minimum of cable laying and disturbance to the facility’s operations.
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.
Ophir Photodiode sensors use silicon, germanium and InGaAs sensors together with built in and removable filters. The spectral response of these type of sensors vary widely with wavelength. When used with our smart displays or PC interfaces, the sensitivity factor for the relevant wavelength is automatically set when the user inputs the laser wavelength.
The need to accurately measure laser power and energy has increased as more of these systems are used in medical procedures and industrial processes. Although a fairly simple process, this measurement is not as straightforward as an electric power measurement. With lasers, more attention must be paid to the selection of the right sensor as since different sensors perform different measurements. Selecting the wrong sensor can destroy the laser.
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
Ophir standard power and energy measuring heads and displays all use smart head technology. This means that all the configuration and calibration information is stored in a small memory chip inside the smart head plug, so that when the head is plugged into the display the correct power and energy are read. Except for some OEM heads, this technology is used in all Ophir heads: pyroelectric, photodiode, scanned beams, medical heads, etc.
Calibration is perhaps the most important of our products. We have a complete line of calibration lasers so that we can always calibrate at or near the customer’s wavelength. These lasers include powers up to 400W and both CW and pulsed lasers. In addition, we have a number of heads calibrated at NIST used as calibration standards. Below is a list of the calibration wavelengths used at Ophir in calibrating our standard catalog heads. Usually the calibration is done at representative wavelengths within a band of wavelengths where the head is spectrally flat. The calibration then applies to any wavelength in this band. The specifications note the maximum additional error in each wavelength band due to variations incalibration between the wavelength of calibration and the wavelength of measurement.