Photoacoustic Microscopy using a Scanning Fourier Transform Spectrometer

Broadband fiber lasers have high pulse repetition rate and wide wavelength coverage, making them well suited for spectroscopic photoacoustic microscopy (PAM). These systems typically employ sequential wavelength scanning with an optical band-pass filter. This wastes the majority of spectral content and energy in each broadband laser pulse, and can lead to long data acquisitions times. Fourier transform infrared (FTIR) spectroscopy is a well-established technique in chemical analysis, where all wavelengths of a broadband infrared source are used simultaneously. An interferogram is produced by a Michelson interferometer with a variable optical delay line. A Fourier transform of the interferogram produces the desired spectral data with both fine spectral resolution and high signal-to-noise ratio. This paper reports our initial efforts to adapt the FTIR spectroscopy approach to PAM. Our laser uses nonlinear optical propagation in a photonic crystal fiber to produce broadband pulses (532 - 650 nm) with 7 uJ of energy at a 10 kHz repetition rate. These pulses are sent through a Michelson interferometer with a reference mirror mounted on a small audio speaker. The Michelson output is directed to an optically focused PAM system with a 25 MHz detection transducer. Test experiments on two silicone tubes containing different colored ink clearly produce different interferograms, and their resulting spectra are consistent with the expected absorption characteristics of each ink. Although the interferogram and photoacoustic spectra are degraded by pulse-to-pulse spectral fluctuations of the laser, we believe these preliminary results show the potential of a spectroscopic PAM system based on a scanning Fourier transform spectrometer.