Micromachined Force Scale for Optical Power Measurement by Radiation Pressure Sensing

Abstract

We introduce a micromachined force scale for laser power measurement by means of radiation pressure sensing. With this technique, the measured laser light is not absorbed and can be utilized while being measured. We employ silicon micromachining technology to construct a miniature force scale, opening the potential to its use for fast in-line laser process monitoring. Here we describe the mechanical sensing principle and conversion to an electrical signal. We further outline an electrostatic force substitution process for nulling of the radiation pressure force on the sensor mirror. Finally, we look at the performance of a proof-of-concept device in open-loop operation (without the nulling electrostatic force) subjected to a modulated laser at 250 W and find its response time is less than 20 ms with noise floor dominated by electronics at 2.5 W/√Hz.

Non-Absorbing, Point-of-Use, High-Power Laser Power Meter

“…A detailed review of the electronics and sensitivity optimization for this system is outlined in Ref. [6]. In prototype tests tracing the open-loop output of a single spring sensor, we measure noise levels below 1 W/√Hz, which agree with our expected value of 0.44 W/√Hz.…”

Radiation pressure measurement using a macroscopic oscillator in an ambient environment

“…The main trend in light pressure studies in recent years has been to miniaturize a mechanical oscillator to the nano-micro scale for a higher sensitivity to the radiation pressure 4,6,7,25 . However, optical forces in those nanomicromechanical systems have been directly accompanied by photothermal effects due to short thermal time constants of the miniaturized resonators 6,7,[26][27][28][29] , which has required further sophisticated techniques to discern them from the radiation pressure effects.…”

MEMS Non-Absorbing Electromagnetic Power Sensor Employing the Effect of Radiation Pressure

“…To obtain a calibration between a measured electrical signal and the applied optical power, an accurate knowledge of electro-mechanical parameters (spring constant, capacitor plate spacing and capacitance gradient with respect to plate spacing) of our transducer is needed. For this purpose, we developed an opto-electronic characterization technique, where we apply a time-varying electrostatic bias voltage to the capacitor's electrodes and measure the axial displacement of the mirrors [9]. Simultaneously we record the geometrical capacitance measured by a LCR meter as a function of capacitor plate spacing (Figure 2a), yielding the knowledge of the capacitance gradient.…”

“…The signal generator was modulated by a square wave of 1 Hz frequency and 50% duty cycle. To decrease the contribution of the un-correlated noise we triggered the oscilloscope from the same modulation source and used waveform averaging [9]. The experiment was performed in a Faraday cage padded with foam microwave absorbers.…”