Photoacoustic-photothermal probe

Sensor head | Performance | Applications | References | Contact

Pulsed photoacoustic and photothermal techniques are investigative methods in which short sub-ablation threshold excitation laser pulses are absorbed in a target absorber producing both acoustic (thermoelastic) and thermal waves. These waves act as carriers of information relating to the optical, acoustic and thermal properties of the target absorber and can be used to describe its constituents and structure. Applications include the characterisation of biological tissue and non-destructive testing of materials and structures. Whilst photoacoustic and photothermal techniques provide an inherently powerful means of characterising a target, their practical implementation can be problematic using conventional acoustic and thermal detection methods. This is particularly so when it is required that the generation and detection of the photoacoustic or photothermal signals take place on the same side of the target as is generally required for the in situ characterisation of biological tissues. In such cases the acoustic/thermal detector should be transparent so that it can be aligned coaxially with the excitation laser beam thus excluding most conventional piezoelectric and pyroelectric contact transducer configurations. Biomedical implementations also often require a miniature, flexible probe type format for minimally invasive use such as insertion via a biopsy needle or endoscope and this too represents a substantial challenge using existing methods. A miniature all-optical probe [1,6] that employs a transparent acoustically and thermally sensitive Fabry Perot sensor for making photoacoustic and photothermal measurements simultaneously has been developed that offers a potential solution to these limitations.

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Sensor head

Figure 1 Schematic of the sensor head probe.

Principles of operation: A schematic of the sensor head probe is shown in figure 1. A 380µm core diameter multimode optical fibre with a transparent Fabry Perot polymer film sensor mounted at its distal end is placed in contact with the target absorber (figure 1). Nanosecond, sub-millijoule optical pulses from a Q switched Nd:YAG laser are launched into the fibre, transmitted through the sensor and absorbed in the target producing thermal waves with a typical duration of the order of a few hundred milliseconds. In addition, rapid thermal expansion occurs generating ultrasonic thermoelastic waves with a typical duration of several hundred nanoseconds. Both thermal and thermoelastic waves are detected by the sensor at the tip of the fibre. The sensor itself comprises a transparent 50μm thick polymer film. This can either be deposited under vacuum directly on to the cleaved end of the fibre using the Parylene process or a discrete polymer film such as PET mounted at the fibre can be used. When illuminated by light launched into the fibre from a CW low power tuneable laser source, the polymer film acts as a low finesse Fabry Perot interferometer with the refractive index mismatches on either side of the film providing the mirrors of the interferometer. An incident thermal or thermoelastic wave changes the optical thickness of the film and hence the optical phase difference between the Fresnel reflections from either side of the film. This produces a corresponding intensity modulation in the light reflected from the sensing film which is then detected by a photodiode. Linear operation is achieved for small measurand-induced phase shifts by tuning the wavelength of the laser source so that the interferometer phase bias is set to the optimum quadrature point.

Figure 2 Sensor output (50mm water-backed PET film) in response to photoacoustic and photothermal signals generated in an ink-Tris absorber (m_a=70cm^-1). Inset shows complete thermal signal over expanded timescale. Fluence: 0.27mJ/mm², pulse duration: 5ns, repetition rate: 16 Hz. [1]

Experimental set-up and procedure: Figure 2 demonstrates the dual sensing ability of the system showing the photoacoustic and photothermal signals generated in an ink/Tris solution of absorption coefficient µ_a=70cm^-1 over different timescales. Figure 2 shows the short duration (400ns) photoacoustic signals. The step decrease, immediately following the initial thermoelastic wave is due to the initial heating of the target by the laser pulse and can be regarded as the onset of the rising edge of the thermal wave. This very slow increase in the thermal signal can be seen more clearly in the inset which shows an expanded timescale graph.

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Performance

The acoustic system sensitivity was obtained by comparing the sensor output to a calibrated 25MHz PVDF membrane hydrophone and was found to be 140 mV/MPa with an acoustic noise floor of 2 kPa over a 25 MHz measurement bandwidth and 30 averages. The dc thermal system sensitivity was established by placing the sensor head in a water bath and recording the sensor output as the temperature was varied. A calibrated thermocouple placed immediately adjacent to the sensor head was used as a reference. The dc thermal system sensitivity was found to be 32 mV/ºC with a thermal noise floor of 6.3 x 10^-3°C, also over a 25 MHz measurement bandwidth and 30 averages. By varying the temperature over 25°C, it was possible to observe a maximum and minimum of the interferometer transfer function (indicating a phase shift of π rad) giving a temperature (phase) sensitivity of 0.13rad/°C.

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Applications