Spectroscopic photoacoustic microscopy using a photonic crystal fiber supercontinuum source

Abstract

Photoacoustic microscopy (PAM) provides high resolution images with excellent image contrast based on optical absorption. The compact size and high repetition rate of pulsed microchip lasers make them attractive sources for PAM. However, their fixed wavelength output precludes their use in spectroscopic PAM. We are developing a tunable optical source based on a microchip laser that is suitable for spectroscopic PAM. Pulses from a 6.6 kHz repetition rate Q-switched Nd:YAG microchip laser are sent through a photonic crystal fiber with a zero dispersion wavelength at 1040 nm. The highly nonlinear optical propagation produces a supercontinuum spectrum spanning 500-1300 nm. A tunable band pass filter selects the desired wavelength band from the supercontinuum. Our PAM system employs optical focusing and a 25 MHz spherically focused detection transducer. En-face imaging experiments were performed at seven different wavelengths from 575 to 875 nm. A simple discriminant analysis of the multiwavelength photoacoustic data produces images that clearly distinguish the different absorbing regions of ink phantoms. These results suggest the potential of this compact tunable source for spectroscopic photoacoustic microscopy.

Optical coherence tomography today: speed, contrast, and multimodality

“…Photoacoustic imaging uses the photoacoustic effect-a certain chromophore absorbs a short laser pulse's energy; this absorbed energy is partially converted to heat; then under thermoelastic expansion, the heat is further converted to a local pressure rise and, hence, an acoustic wave-the photoacoustic wave; by detecting the laser pulse-induced photoacoustic waves, one can reconstruct in 3-D the chromophore distribution in the illuminated volume. 217 Different from OCT that detects backscattered light, photoacoustic imaging detects acoustic waves. Since the photoacoustic waves are inherently broadband, detection of different bandwidths of the photoacoustic wave can generate a variety of different imaging depths ranging from several hundred micrometers by submicron-resolution PAM (Ref.…”

Spectrally Encoded Photoacoustic Microscopy Using a Digital Mirror Device

“…In low-energy wavelength scanning mode, the laser pulse energy was reduced to ∼3 nJ at 586 nm to mimic the achievable spectral density of a broadband photonics crystal supercontinuum fiber laser. 7 The low-energy wavelength scanning mode yielded a PA SNR of 3:1 at 586 nm. When the transient PA peak signal was mingled with the excessive noise, the amplitude of the Hilbert-transformed PA A-line signal did not accurately reflect the optical energy deposition distribution, yielding errors greater than 30%.…”

“…6,7 By rapidly scanning across the individual wavelengths, optical absorption spectra are measured. However, optical filtering by excluding all wavelengths but one from the broadband laser beam is not energy-efficient.…”

“…However, optical filtering by excluding all wavelengths but one from the broadband laser beam is not energy-efficient. Although the PA signal-to-noise ratio (SNR) for tunable-laserbased PA microscopy is usually limited by the ANSI safety standard, 8 broadband sources, such as a photonic crystal fiber laser 7 or a laser diode array, 9 have such low pulse spectral densities that their pulse energy levels become the limiting factor for the PA SNR. For example, Takashi et al built a wavelengthagile PA microscope using a photonic crystal fiber supercontinuum source.…”

“…For example, Takashi et al built a wavelengthagile PA microscope using a photonic crystal fiber supercontinuum source. 7 Because of the inadequacy in pulse energy and spectral density, the bandwidth of the tunable band pass filter had to be widened to 40 nm around each tuned wavelength, which significantly sacrificed spectral resolution and made separation of different optical absorbers challenging.…”