IOP PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY Meas. Sci. Technol. 19 (2008) 055301 (8pp) doi:10.1088/0957-0233/19/5/055301 Parallel detection of low modulation depth signals: application to picosecond ultrasonics R J Smith 1 , M G Somekh 1 , S D Sharples 1 , M C Pitter 1 , I Harrison 1 and C Rossignol 2 1 School of Electrical and Electronic Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, UK 2 Laboratoire de M´ ecanique Physique, UMR CNRS 5469, Universit´ e Bordeaux 1, 351 cours de la Lib´ eration, 33405 Talence, France Received 20 November 2007, in final form 15 February 2008 Published 19 March 2008 Online at stacks.iop.org/MST/19/055301 Abstract We demonstrate parallel detection of laser ultrasonic signals above 50 GHz, where the signals are encoded as small modulations on a large dc background signal. The measurement problem addressed here is generic for many situations, particularly pump/probe experiments, and this paper discusses the problems of moving from single point to parallel detection and practical solutions. This is achieved with a commercial detector array, custom interface electronics and a carefully selected phase stepping algorithm. The parallel detection of Brillouin oscillations illustrates the very low modulation depths that can be measured with this technique. Noise performance and projected improvements for future custom detectors are also considered. Keywords: arrays, demodulation, optical sensors, ultrasonic measurement (Some figures in this article are in colour only in the electronic version) 1. Introduction One recurring measurement problem in optical measurement is the need to recover weakly modulated signals in a large background. For instance, this occurs in nearly all pump/probe experiments where a small proportion of the modulation imposed on the pump beam is transferred to the probe beam. Examples include: photoreflectance [1], photothermal techniques [2] and laser ultrasonics [3]. These pump/probe techniques are usually performed as point measurements; an exciting and important challenge is to perform many such pump/probe measurements in parallel. This would greatly speed up measurements that are often time consuming, and will furthermore enable measurements to be made that are presently impractical. In this paper we show how modulated signals can be detected in parallel with comparable performance to point detection with a photodiode/lock-in amplifier combination. The method employed here uses a commercial array detector, some custom interface electronics and a suitable phase stepping algorithm to suppress the odd harmonics inherent to square wave modulation. In addition, we discuss future full custom designs that will surpass the performance of the array described in the present paper. The typical point detection approach is usually implemented with a modulator, a photodetector and a lock-in amplifier. Here, the signal of interest is mixed with a reference signal and integrated or band pass filtered to allow narrow- band detection. In the case of optical signals a mechanical chopper or acousto-optical modulator is often used to impose the reference frequency on the optical signal. The signal from the photodiode is used as the input to the lock-in amplifier and the signal driving the chopper or modulator is used as the frequency reference and this produces an extremely sensitive detection system [4]. Many experimental situations benefit from parallel detection schemes. This has been successfully achieved by integrating the signal over parts of the cycle and recovering the signal with a phase stepping algorithm. For instance, in thermography, where the temperature changes induced by a sinusoidally modulated heat source are recorded, parallel detection [5] is quite common. The method has also been applied to parallel heterodyne interferometry [6]. The 0957-0233/08/055301+08$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK