An optical powermeter (or laser power meter) is a device for the measurement of the power in a laser beam. Most powermeters are based on the principle of a thermal detector: optical power is converted to heating power in some absorber structure with a black coating, and the resulting temperature rise (or actually the temperature difference between the absorber and the mount) is measured, e.g. with a thermopile. Such thermal powermeters (see Figure 1) are useful for average powers between ≈ 0.01 W and several kilowatts; typically, some water cooling is required for powers above roughly 10 W. They are fairly robust (although too tight focusing onto the absorber should be avoided), moderately precise, usable in a wide wavelength range (with fairly wavelength-independent sensitivity), and relatively slow.
When a thermal powermeter is set for high sensitivity (responsivity), for example for a maximum power below 100 mW, its mount should not be touched with the hand during measurements. This is because any warming of the mount can induce temperature differences which modify the reading.
Faster and more sensitive powermeters can be made with photodiodes. These can also be used for measurement of high powers when a suitable attenuator is used. They are, however, less robust than thermal powermeters. Also, their sensitivity is rather wavelength-dependent, so that a photodiode-based powermeter usually requires the user to make a setting for the wavelength. The device then uses an internal calibration table to compensate for the wavelength-dependent responsivity. Obviously, this method does not work for beams with a very broadband or variable optical spectrum, whereas it may be perfectly convenient in situations where the laser wavelength is rarely changed.
A possible problem with all kinds of optical powermeters is the uniformity of response. For thermal powermeters, a lack of uniformity can result from a dependence of the resulting absorbance or temperature distribution for different beam positions. For photodiodes, a nonuniform response can easily result from damage with too high optical intensities.
Hints for Selecting a Device
For finding the most appropriate type and model of optical powermeter, a significant number of aspects needs to be considered. The most basic aspects are:
- the maximum allowed optical power (for a limited or unlimited time interval)
- the number and spacing of measurement ranges
- the lowest measurement range and the noise level caused e.g. by temperature fluctuations
- the accuracy and precision, uniformity of response, and possibly the option to have the device recalibrated
- the range of allowed wavelengths, and whether a setting has to be made concerning the momentarily used wavelength
Some additional aspects of possible practical relevance are:
- the speed (response time, measurement bandwidth, power sampling rate): e.g. laser alignment can be very much faster and easier with a sufficiently fast device
- the dimensions of the measurement head, and mechanical means to place it stably and adjust it to different beam heights
- options to use different measurements heads, e.g. for different power levels
- the possibility to use the display device with other measurement heads, for example for pyroelectric laser energy meters
- analog and/or digital display (depending on the circumstances, analog or digital display may be more convenient)
- internal data storage for recording the measurement history
- computer interface for automatic data recording and remote control
- protection against light coming from the side (e.g. ambient light in sensitive measurements)
- requirements for cooling
- powering with (rechargeable) batteries and/or AC adapter.
Optical power meters are mostly used for temporary purposes, e.g. when testing laboratory setups or doing maintenance operations. For permanent monitoring of powers, one often uses optical power monitors, which can be integrated into systems. Such power monitors are also available in fiber-coupled form.
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