Technique, Application and Noise Analysis of Purely Thermal-Wave Photopyroelectric Interferometry (PPEI) Andreas Mandelis and Chinhua Wang Photothermal and Optoelectronic Diagnostics Laboratories, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario M5S 3G8, Canada A novel purely thermal-wave interferometric technique based on photopyroelectric (PPE) detection using a thin film polyvinylidene fluoride (PVDF) has been developed. The interfering thermal waves within the PVDF thin film are generated by two intensity-modulated laser beams which are incident onto the two active surfaces of the PVDF thin film from the opposite directions. Applications of this technique to differential measurements of optical properties of solid laser crystals and the detection of trace hydrogen in nitrogen were carried out and were compared with the conventional methods. The major feature of the technique is the efficient suppression of the background PPE signal and the system noise, and therefore, the significant enhancement of the measurement sensitivity, precision, and signal dynamic range. A comprehensive theoretical and experimental analysis of the PPE interferometric (PPEI) system noise has been conducted to consolidate the basis of the technique. (Received June 29, 2000; Accepted August 19, 2000) The single-beam photopyroelectric (PPE) technique is a well-established photothermal method used for spectroscopic and thermal characterization of various materials as well as for studies of thermophysical properties of gases 1-6 . In all the conventional embodiments of the PPE technique 1,3,5,6 , a single excitation source is employed with the radiation impinging on either the front- (PPE), or the rear-surface (Inverse PPE, IPPE) of a PVDF transducer. The sensitivity and dynamic range, however, of the PPE measurement scheme can be compromised in the study of solid transparent materials, due to the substantial baseline signal from direct transmission of the incident light onto the detector. Moreover, the fluctuation of incident light also introduces a large optical noise to the measurement in the single-beam measurement scheme. In this work, we have developed a new purely-thermal-wave interferometric technique. The unique characteristic of the purely-thermal-wave interferometry is that the technique is based on the spatial interference of thermal-waves within the body of the pyroelectric transducer, independently of the sample. Unlike other prior (“conventional”) photothermal interferometric schemes 7, 8 , the new technique is not based on monitoring thermal waves resulting from direct optical interference patterns, such as those generated by two appropriately modulated laser beams (e.g. intensity, phase or polarization modulation). In the present coherence scheme, thermal waves interfere as they are induced by two intensity-modulated beams, split-off a single laser source and with a fixed phase-shift relationship between them. The usually large instrumental PPE baseline signal and a significant portion of the noise can be efficiently suppressed within the PVDF detector if the two laser beams are collinearly incident on opposite surfaces of the thin pyroelectric film, and with 180 o relative phase shift. In this fashion, much higher signal sensitivity and dynamic range PPE measurements than with the conventional single-beam PPE configurations are expected. This paper presents a generalized theory of a purely thermal-wave interferometry and several applications which have been implemented for different research aspects including measurement of optical properties of solid laser crystals and the design and development of a novel H 2 gas sensor. In addition, the noise suppression and the detectivity enhancement of the system will also be discussed. Theory of PPE Purely-Thermal-Wave Interferometry The most general configuration diagram for purely-thermal-wave PPE interferometry using a one-dimensional heat transfer model is shown in Fig.1. Two laser beams of intensities I 1 and I 2 , respectively, are split off of a laser source and are modulated at the same angular frequency ( ω ). They have a fixed, adjustable phase shift ( ϕ Δ ), and are incident onto the front- and rear-surfaces of a PVDF detector, passing through optically transparent sample and reference media, which, along with the PVDF sensor in the middle form the thermal-wave cavities g2 and g3 as shown in Fig.1. The sample and the reference have bulk Fig. 1 Schematic of a photopyroelectric interferometry for theoretical analysis. optical absorption coefficient r s β β , , surface absorptance r s A A , , and surface and bulk non-radiative energy conversion efficiency, ) ( ), ( λ η λ η b r b s , respectively. Light absorption by the sample-PVDF-reference system and nonradiative energy conversion to heat increases the temperature of the PVDF sensor, which results in a potential difference between the two surfaces of the transducer due to the photopyroelectric effect. The photopyroelectric signal from the PVDF detector is proportional to the average ac temperature of the PVDF film detector 5 , and can be written: ∫ + + + = d L l L l p dx x T S V ) , ( ) ( ) ( ω ω ω (1) Here ) (ω S is the instrumental transfer-function, which is Sample PVDF Reference (g1) (s) (g2) (p) (g3) (r) (g4) ) ( λ s A ) ( ) ( λ η λ β b s s ) ( λ s A ) ( λ r A ) ( ) ( λ η λ β b r r ) ( λ r A t j e I ω 1 ) ( 2 ϕ ω Δ + t j e I X m L d L l L d L l d L l L l l + + + + + + + + + + 1 1 0 2001 © The Japan Society for Analytical Chemistry s447 ANALYTICAL SCIENCES APRIL 2001, VOL.17 Special Issue