Pulsed Laser Source

Pulsed fiber lasers with manageable low average power have found a wide range of applications in medicine, bio-medical imaging, and sensing. In particular, pulsed lasers with nanosecond pulse duration are widely used in Photoacoustic Microscopy (PAM), an in-vivo imaging technique based on the photoacoustic effect, which provides better penetration depth compared to other imaging techniques.

We have built a nanosecond pulsed laser source based on self-Q-switching. An SQS laser can be described as a half-open cavity laser. On one side of the cavity there is strong feedback, whereas on the other side cascaded Rayleigh scattering (RS) and stimulated Brillouin scattering (SBS) provides dynamic feedback in the cavity.At the beginning of every cycle the population inversion in the cavity builds up, and lasing occurs as a result of reflection from the highly reflective element. but due tohigh loss of the cavity, population inversion is not exhausted. On the onset of SBS in the ring mirror, a pulse is formed, and it undergoes double pass amplification in the cavity. An intense pulse is generated at the output. It exhausts all the population going back to the previous state. Then the process repeats itself. [1]-[3]

Video: Working principle of a self-Q-switched fiber laser.

Application of nanosecond pulsed laser source to Photoacoustic imaging

Previously, PAM has been extensively explored in the UV-visible and near infrared-I (700 to 1000nm) window, which covers the absorption bands of critical biological materials like haemoglobin, DNA/RNA and melanin [4] [5]. However, despite having advantages like high maximum permissible exposure (MPE) and low blood absorption for deep tissue imaging, the second NIR (NIR-II,1000 to 1700nm) window is still relatively less explored by PAM, primarily due to the unavailability of a readily available tunable nanosecond laser source with kilohertz range repetition rate and at least microjoule pulse energy. NIR-II window is used for in vivo liver, brain, skin, and tumor imaging, and it covers the absorption peaks of lipids for imaging lipid-rich tissue.

Our pulsed cascaded Raman fiber laser pumped with our nanosecond pulsed laser source provides ~µJ pulse energy with ~100ns pulse duration and ~kHz repetition rates for the entire range of NIR-II, reported for the first time [6].

Pulse energy of our pulsed cascaded Raman fiber laser for the entire range of NIR-II, best reported so far.

Resources

S. V. Chernikov, Y. Zhu, J. R. Taylor, and V. P. Gapontsev, “Supercontinuum self-Q-switched ytterbium fiber laser,” Opt. Lett., OL, vol. 22, no. 5, pp. 298–300, Mar. 1997, doi: 10.1364/OL.22.000298.
A. A. Fotiadi, P. Mégret, and M. Blondel, “Dynamics of a self-Q-switched fiber laser with a Rayleigh–stimulated Brillouin scattering ring mirror,” Opt. Lett., OL, vol. 29, no. 10, pp. 1078–1080, May 2004, doi: 10.1364/OL.29.001078.
A. Lobach, R. V. Drobyshev, A. A. Fotiadi, E. V. Podivilov, S. I. Kablukov, and S. A. Babin, “Open-cavity fiber laser with distributed feedback based on externally or self-induced dynamic gratings,” Opt. Lett., OL, vol. 42, no. 20, pp. 4207–4210, Oct. 2017, doi: 10.1364/OL.42.004207.
J. Yao and L. V. Wang, “Photoacoustic microscopy,” Laser & Photonics Reviews, vol. 7, no. 5, pp. 758–778, 2013, doi: 10.1002/lpor.201200060.
L. V. Wang, “Tutorial on Photoacoustic Microscopy and Computed Tomography,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 14, no. 1, pp. 171–179, Jan. 2008, doi: 10.1109/JSTQE.2007.913398.
A. Goswami, S. Dash, R. Deheri, S. Arun, and V. R. Supradeepa, “Pulsed Cascaded Raman Fiber Laser with Wide Wavelength Tunability,” in Proceedings of the 2022 Conference on Lasers and Electro-Optics Pacific Rim (2022), paper CTuP1D_02, Optica Publishing Group, Aug. 2022, p. CTuP1D_02. Accessed: Mar. 31, 2023. [Online]. Available: https://opg.optica.org/abstract.cfm?uri=CLEOPR-2022-CTuP1D_02

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