Measuring laser power is a challenge. We can concentrate the lasaer pointer energy on a sensor and measure the resulting temperature rise, or measure the power of a given portion of the beam. For the first option, the laser must be switched from its normal propagation path, so it cannot be used in production. Between the two options, extremely powerful lasers used for metal cutting and welding can easily burn down sensors because of “improper handling.”
Apparently aware of these challenges, researchers at the national institute of standards and technology (NIST) recently designed a system to measure laser power in a high-kilowatt range, up to 500 kW, McCombs reports. They used a mirror to intercept and reflect 99.9% of the beam, allowing the beam to continue to perform its original function. The device, about the size of a shoebox, precisely offsets the error to measure the force of the photons hitting it (radiation pressure, a 100-kw beam, about 330 mg).
But the method was not compatible with lasers of much lower intensity, in the hundreds of watts, so the NIST researchers devised a completely different sensing scheme. Instead of measuring the tiny laser force hitting the mirror, they designed a conceptually simple “smart mirror” (figure 1). The new design is compact and fits into the optical path, so it does not interfere with the use of lasers, a big advantage in practical applications.
The core of this sensor is a MEMS based capacitive assembly consisting of two identical plates, each about 20 mm wide and 42.5 um apart (figure 2). The sensor element on the upper silicon plate is attached to the outer silicon ring by three narrow spiral brackets (265 um in width, 380 um in thickness, and 45 mm in length). It is made into a distributed Bragg reflector, a high-reflectivity mirror made of alternating layers of silicon and silica. By tuning the interval and distribution of alternate layers, it can achieve maximum reflectivity at required wavelengths, unlike conventional reflectors.