The base of laser beam diagnostic is to know how much average power you got. Available off the shelves form different manufacturers are three main type of devices, based either on a photodiode, a thermopile or a pyroelectric detector.
Of course, many factors will influence the quality of a power meter, the most important being its calibration. One should go for power meters which calibration is traceable to a recognised standard (such as NIST).
Photodiodes: precision for low power lasers.
When a photon source, such as a laser, is directed at a photodiode detector, a current proportional to the light intensity and dependent on the wavelength is created. A photodiode sensor has a high degree of linearity over a large range of light power levels - from fractions of a nanowatt to about 2 mW (this higher limit depends a bit on the photodiode). Above that light level, corresponding to a current of about 1mA, the electron density in the photodiode becomes too great and its efficiency is reduced causing saturation and a lower reading. Most manufacturers offer a removable ND filter to allow extending somewhat the dynamic range of the power meter, generally up to about a watt maximum.
Photodiodes are generally made of silicon, thus their response is typically 350-1100 nm, and can be extended to 200-1100 nm. Occasionally one can find an off the shelf calibrated germanium or InGaAs photodiode which will allow precise measurement on the 800-1600 nm range. As you can see on the picture below, the typical response curve of a silicon photodiode is highly wavelength-dependent.
Silicon reponse curve
This importance of the wavelength dependence leads to two main drawbacks: you need to have a clear idea of the wavelength of your laser, since the power meter will ask you for it and the result will depend on the answer. Plus photodiode power meters are inappropriate for broadband light sources power measurements (for instance it is not the way forward when using femtosecond lasers).
On the positive side, photodiodes are relatively insensitive to temperature fluctuations, have a very small form factor, are fast (from a fraction of a second to some tens of microsecond response time, limited by the electronic) and are insensitive to vibrations. But their main and unique advantage lies in their ability to measure very small optical power.
Some manufacturers even offer a background light cancellation feature, which uses a second photodiode placed outside of the laser beam path but close enough to the measuring photodiode. The light measured by this second photodiode is considered as the background noise and subtracted to the reading of the first one.
Thermopiles: stability for medium and high powers
Using a thermopile sensor is a very robust and well established way to measure laser energy. The underlying principle is quite simple: it uses some thermocouples to measure the temperature gradient between the point where the laser beam hit the thermopile and the periphery where the heat is dissipated using a heatsink. It is then easy to calculate the incident laser power.
Thermopiles tend to be more accurate than photodiodes, but their sensitivity is lower. This means the error is lower in percentage, but they are unable to measure low power lasers. Typically their power range can go as low as a few hundreds of microwatt while some high power thermopile sensors can measure up to nearly 10 kW. Usable wavelength range commonly span 200-20,000 nm for a single broadband sensor.
On the down side, they are slow, at generally a couple of second response time despite software acceleration. Plus, since the measurement is based on heat exchange, a quick fluctuation of housing temperature will decrease the accuracy of the result. This is an issue for instance if the beam hits the housing or if you hold a low power thermopile by hand. Keep in mind that part of the beam energy is distributed outside the defined beam diameter, and this energy can hit the housing if your beam is too large.
Due to their slow response time, they are only really capable of measuring average power. They generally have an energy mode which allow them to measure the energy of a single pulse. Interestingly, the pulse width does not really matter: however short, the energy of the pulse will produce a heat increase and thus the meter will deliver a reading. However some thermopiles are better equipped to measure short pulses with high energy: in this situation the energy needs to be absorbed in the volume of the absorber and not only on its surface, otherwise there is a possibility to damage the sensor.
Because the measurement relies on thermal exchanges, thermopile technology is quite diverse. One can find sensor specialised on short pulses, some on long pulses, some give better results at specific wavelength, some have a spectrally flat response over hundreds of nanometer allowing broadband light measurement, and some have a slightly different technology, based on a Peltier device, which allows sub-second response time.
Pyroelectric: energy and power
Some applications absolutely need a pulse-to-pulse measurement of the energy. In those situation where an average reading of the power is not enough, a pyroelectric energy meter is the way forward.
Pyroelectricity is the ability of certain materials (generally a polar crystal or a ferroelectric) to generate an electrical potential when they are heated or cooled. When a pulse of light hits the detector, it heats it up and create that electric potential. The electrical voltage read by the measuring instrument is then proportional to the energy. Average power can be calculated by the electronic.
Pyroelectric energy meters are very fast (up to tens of kHz) and very broadband (typically 200-20,000 nm). These energy detectors will also make accurate measurements in spite of changing temperature in the environment or heating of the detector.
Unfortunately they are less durable and less accurate than thermopiles or photodiodes. They are also sensitive to vibrations and can’t measure continuous light (CW lasers) nor long pulses (it typically has to be less than 10 ms, but this varies a lot from detector to detector). It also has a maximum repetition rate. Therefore they should only be used when the measure of each pulse energy is necessary.
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![\overline{\left|i_{noise} \right|^{2}}=\left[2e\left(\overline{i_{s}}+\overline{i_{0}}\right)+\frac{4k_{B}T}{R} \right]\Delta\nu](http://optical-technologies.info/wp-content/cache/tex_6c5cf7809182158f7eb0f09b48d40344.png)
to avoid confusion. In this situation we have then:![\left(NEP_{\sqrt{\Delta\nu}}\right)^{2}=\frac{NEP^{2}}{\Delta\nu}=\frac{\overline{P_{opt\mid S/N=1}^{2}}}{\Delta\nu}=\left(\frac{h\nu}{e\eta}\right)^{2}\left[2e\left(\overline{i_{s}}+\overline{i_{0}}\right)+\frac{4k_{B}T}{R} \right]](http://optical-technologies.info/wp-content/cache/tex_eb587d620c3fe9d4a5dbc8f5fb6e171f.png)
. The smallest the NEP, the better is the detector.
. Since all of parameters we defined depend on the area of the detector, in some cases this introduces a bias in the detector comparison. Thus sometime is specified a specific detectivity D* (D star), defined as:
