No single detector type is best for all applications. Choosing the optimal detector for a particular task thus requires matching detector characteristics to the type of measurement required.
The two most popular sensor types for measuring laser average power are quantum detectors and thermopile detectors. A quantum detector measures laser power by counting photons. Based on a semiconductor material such as indium gallium arsenide, the quantum detector converts incoming photons into charge carriers (electrons and holes), which are then summed as a voltage potential or a current through an amplified circuit. In contrast, a thermopile detector acts as a calorimeter. Incident light heats the device, and a circuit senses the heat differential between the detector and an attached heat sink. With appropriate calibration and conversion software in the power meter, both of these detector types can provide measurements in units of watts.
Thermopile detectors exhibit only small variations over a very broad operating range.
Quantum detectors count photons whose energy varies as a function of wavelength. At the same time, quantum detectors themselves suffer from a wavelength-dependent quantum efficiency. Their response is thus highly wave-length dependent. In addition, quantum detectors are limited in terms of spectral sensitivity, typically operating over a range of a few hundred nanometers. Because a thermopile responds to radiant energy, its response can vary by as little as 3 to 5% over the entire usable wavelength range. Thermopiles offer a very broad operating range; a single detector can operate from the UV to the IR spectral regions. Some thermopiles exhibit wavelength dependence, particularly at the end of the operating range, though a thorough discussion is outside the scope of this article.1
Both detectors will always give a reproducible relative response. With the appropriate meter, both types will also give high absolute accuracy, but the quantum detector is entirely dependent on the wavelength-response calibration curve stored in the detector. This can be problematic for broadband or multiwavelength lasers. Generally, it is best to avoid quantum detectors for such applications.
Both detector types can measure the average power of pulsed laser beams - within certain limits. At pulse repetition rates above 10 kHz, both detectors can be configured to characterize these lasers as though they were continuous wave, and therefore deliver accurate average-power measurements. The relatively slow response speed of thermopile sensors enables them to deliver high-accuracy average-power readings at repetition rates as low as a few hertz with most meters. Commercially available laser-power meters fitted with quantum detectors, however, will typically start to differentiate individual pulses below 10 kHz, introducing errors. Quantum detectors should thus be avoided for average-power measurement of pulsed lasers below the range of about 10 kHz, although custom solutions may be developed.
A quantum detector offers more sensitivity than a thermopile, so at power levels below the microwatt level, the quantum detector represents the only viable choice. The latest, most sensitive thermopiles now can work at powers as low as tens of microwatts by taking advantage of signal amplification in the power meter. At power levels above this point, it is therefore possible to use either detector, although some type of attenuation (neutral density filter) is required to protect a quantum detector from laser damage. Quantum detector response becomes nonlinear at higher powers due to saturation effects, so the thermopile typically represents a better choice at very high laser-power levels. Note that at sufficiently high powers, active cooling is required.
One final note: Many industrial users think that the power measurement must be made at the beam focus. This is not true and carries the highest risk of damaging the detector with the focused intensity. Instead, the beam can be measured well away from the focus, providing the beam does not overfill the detector head. oe
Chad Nelson is an applications engineer at Coherent Inc., Portland, OR.