Optical power detector with broad spectral coverage, high detectivity, and large dynamic range

Abstract

Optical power measurements are needed in practically all technologies based on light. Here, we report a general-purpose optical power detector based on the photoacoustic effect. Optical power incident on the detector’s black absorber produces an acoustic signal, which is further converted into an electrical signal using a silicon-cantilever pressure transducer. We demonstrate an exceptionally large spectral coverage from ultraviolet to far infrared, with the possibility for further extension to the terahertz region. The linear dynamic range of the detector reaches 80 dB, ranging from a noise-equivalent power of 6 n W / H z to 600 mW (independent of signal averaging time).

Cantilever-based photoacoustic sensor for terahertz range

“…Candle soot absorber shows higher detection responsivity than carbon nanotube from UV to infrared region, but the responsivity of soot material decreases as the incident radiation wavelength increases, while that of carbon nanotube absorber stays approximately constant. 11,13 In the THz range, the carbon nanotube absorber has a factor of two higher responsivity than the candle soot absorber. 8 For that reason the results in the following sections are mainly given for the carbon nanotube material.…”

“…These results at 214 µm wavelength agree with our earlier measurements at 0.633 µm wavelength. 13 Figure 2. Spatial responsivity of the PA sensor (a) and photograph of the carbon nanotube absorber (b).…”

“…The noise-equivalent power is 160 nW/√Hz with almost five orders of magnitude linear dynamic range of THz radiation detection. The lowest detectable THz power of the PA detector with carbon nanotube absorber material is expected not to be as low as in the case of candle soot absorber in the visible wavelength range 13 because of a factor three lower responsivity. Furthermore, high ambient vibration level in the laboratory room with the THz laser increases the noise-equivalent power.…”

“…In our previous studies, we have reported a robust silicon cantilever-based pressure sensor for photoacoustic (PA) detection from UV to infrared region [11][12][13] and demonstrated its performance also for THz detection 8 . The window transmittance is a key parameter for the detection process to extend the spectral sensitivity towards the THz range.…”

“…A linear dynamic range of nearly six orders of magnitude can be achieved with PA detection, extending the detectable power range from nanowatt level to several milliwatts, or even up to 600 mW. 12,13 The absorber material can be damaged at power levels above 1 W. Nevertheless, the silicon cantilever is robust and can tolerate vibration amplitudes corresponding to high radiation power levels, if there would be a suitable absorber material for that purpose.…”