Cordless power meters offer convenient answer

28 Apr 2003

Having the right optical power meter is vital, but the range available can make purchasing a complicated business. Our independent guide aims to help you make your decision.

From Opto & Laser Europe May 2003

Look around any laser laboratory and it's almost certain that you'll find an optical power meter. However, with a wide range of technologies, sizes and specifications on offer, purchasing this invaluable piece of equipment can be a tough task. Here are a few tips that Opto & Laser Europe has gathered.

The first point to note is that power meters can be divided into two types, each of which detects power by a different method. The first type uses thermal (thermopile) detectors, while the second type relies on semiconductor photodiodes. A special kind of pyroelectric thermal detector is also available for measuring pulse energies, rather than power (see "Detectors explained" below).

Today, most power meters are modular. They typically consist of two parts: a main control unit that features a read-out display, signal-processing electronics and an interface for transferring data to a computer, and a detector head that is placed in the path of the light beam to be measured. Many manufacturers sell a wide variety of detector heads designed for different power levels. These simply plug into the main unit and can greatly enhance the flexibility of a power meter.

If you're looking for a general-purpose power meter for measuring continuous-wave (CW) laser powers of more than a few milliwatts, a thermopile-based solution is probably most appropriate. Thermopile detectors offer a very wide, flat spectral response with a high damage threshold. They are a popular choice for those working with more powerful lasers such as Nd:YAG, Ti:sapphire, carbon dioxide, excimer and argon ion. Detector heads for making measurements from around 0.5 mW up to 10 kW are commercially available with a spectral range from 200 nm to 11 µm.

If, on the other hand, you need to make very sensitive power measurements at a wavelength of less than 2 µm, a semiconductor photodiode may be better. These can measure powers as small as a fraction of a picowatt and are often available with heads that are specially designed for accepting optical fibre connectors for those working in the telecommunications field. Semiconductor-based meters are also available for making pulse measurements such as pulse energy and peak power.

If you need to measure light from a strongly diverging source such as a laser diode or the end of a bare optical fibre, consider an integrating sphere.

If you are looking for a low-cost, convenient solution for quick but less accurate measurements, battery-powered handheld semiconductor and thermopile probes are now available. However, these can't transfer data to a computer and aren't as precise as more sophisticated models.

Before you buy a power meter, ask yourself the following questions.

Can it measure the power of CW beams or the peak power/energy of individual pulses?

Most meters are designed for one task, although some are compatible with heads that allow both. Thermopile-based systems are often useful for measuring the power of CW lasers or the average power of pulsed lasers, above a few milliwatts. Photodiode-based systems can also make both pulsed-power and CW measurements but are often limited to low powers. Both photodiodes and thermopiles can calculate or infer the energy per pulse, while pyroelectric detectors can directly measure pulse energy.

What is the maximum and minimum detectable power?

Is a probe with the appropriate performance available? If you need to measure very low power levels (below 1 mW) then a photodiode-based solution is often best. Sensitive, low-noise photodiode heads that can measure sub-picowatt power levels are now available. By contrast, if you are checking power levels in the watt or kilowatt regime a thermopile solution is best.

What response time do I need?

Thermopile probes often need to be left in the beam for a few seconds. Semiconductor detectors, on the other hand, are designed to have a fast response.

Can the meter connect to a wide range of probes in case my requirements change?

Probes can usually be disconnected from the meter and exchanged with a different model to suit different power ranges or wavelengths. See what's on offer and how much the individual probes cost.

Is the meter's calibration traceable to internationally recognized standards such as NIST?

If the results from the meter are to be trusted it is vital that it has been properly calibrated. For complete confidence check to see if the meter is traceable to a standards body such as the US National Institute of Science and Technology (NIST). Also find out at what wavelength the calibration was performed - ideally you want it to be as close to your operating conditions as possible. A properly calibrated meter will be able to make a measurement with an uncertainty of less than 2-3%. One last point: make sure that you recalibrate the meter regularly, otherwise it may start to lose its accuracy. Most firms recommend annual recalibration.

How large is the meter's detector entrance aperture?

Make sure that the diameter of your light beam is not larger than the detector. If you need to measure a strongly diverging beam then consider using your detector with an integrating sphere, which is guaranteed to collect all the light.

Can it be connected to a computer for automated data collection or the download of results?

Meters can often connect to a computer through a USB, GPIB or RS-232 interface. If you're taking lots of measurements that you want to turn into graphs or are working in a production-line environment, this can be a very useful feature.

Does the meter have an analogue or a digital display?

Today, many meters come with a display that only gives a digital reading of the power being measured. While this is fine for recording a single measurement, it's not so convenient when you are trying to align an optical system and maximize its output power. In the latter case, some form of analogue display is much more useful. Even if the digital display on a meter shows a reading to several decimal places, that level of accuracy is not guaranteed. If a measurement has an uncertainty of no better than 2-3% then it follows that any digit after the third on the display is meaningless.

Detectors explained Thermopile detectors Thermopiles - also known as thermal detectors - measure optical power by sensing the heat that is released into an absorber when it is irradiated by a light beam. The detector head is made of two parts: an absorbing front surface and a cooled heat sink. A thermocouple built into the head is used to generate an electrical signal that relates to the difference in temperature between the absorber and the heat sink. This signal is passed to the control unit for conversion into an optical power reading.

Pyroelectric detectors This type of thermal detector uss a ferroelectric crystal instead of a thermocouple to sense the temperature difference. Incident photons heat the crystal, causing an electric current to flow. This kind of detector is used for directly measuring the energy of an optical pulse. It cannot be used for measurements of optical power.

Semiconductor photodiode detectors Photodiodes are usually made from silicon, germanium and indium arsenide, and directly convert incoming photons into an electrical signal. This kind of detector offers a very fast and sensitive response, but they are highly wavelength-dependent.

Integrating sphere detectors This is a spherical dome that is used to collect and homogenize the light before it enters a detector. It ensures that all the power is collected from a strongly divergent source and is a good way to obtain very accurate, reproducible results.