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5SRS-assisted pulse frequency conversion in mode-locked fiber lasers and its application in a deep tissues multiphoton microscopy
D.S. Kharenko12*, E.E. Evmenova1, V.M. Volosi12, V.D. Efremov1, A.A. Antropov1, S.A. Babin12
1-IA&E SBRAS, 1 ac. Koptyug ave., Novosibirsk 630090, Russia 2- Novosibirsk State University, 1 Pirogova str., Novosibirsk 630090, Russia
Fiber lasers differ from other types by a large number of nonlinear effects that affect the mode of generation. Such effects are often the limiting factor for achieving, for example, high peak power or short pulse duration. But they also open the door for large variability in output parameters such as repetition frequency or carrier wavelength. The last one has great importance for biological applications like multiphoton microscopy (MPM) [1]. To maximize the penetration depth into a sample and image contrast, it is preferable to use laser radiation with a wavelength near 1.3 or 1.7 microns. To date, many approaches to obtaining pulses in these spectral regions have been demonstrated, both by direct generation methods using novel doped active fibers [2] and by nonlinear effects, in particular stimulated Raman scattering (SRS) [3,4]. However, the best results can be obtained by combining both approaches.
I this work, we present the latest results in the generation of ultrashort pulses by the stimulated Raman scattering effect as well as its amplification in all fiber schemes for further application in MPM. Both wavelength ranges may be covered by changing the pump wavelength from 1.09 to 1.55 ^m. The latter has the advantage of being easy to control the net dispersion of the external cavity, so dispersion-managed solitons can be generated. Such solitons often have a shorter duration and do not require an external grating compressor, which can be important in some applications. We have also demonstrated that the highly chirped pulses can be effectively amplified by continuous wave radiation due to the SRS effect. But the best results in terms of pulse energy and efficiency were obtained in a hybrid amplification scheme based on a combination of Raman and Bi-doped fiber stages. In this case, the peak power of the pulse compressed down to 650 fs exceeds 12 kW which should be suitable for deep tissue multiphoton microscopy.
In conclusion, we demonstrated that the SRS effect provides an effective way for pulse carrier frequency conversion in an all-fiber scheme. As a result, a robust and relatively low cost laser system can be built to operate with a multiphoton scanning microscope.
The work was supported by the state budget of the Russian Federation (IA&E SB RAS project No FWNG-2024-0015.
[1] S. Fan, S. Wang, C. Yang, F. Wise, L. Kong, Advances of Mode-Locking Fiber Lasers in Neural Imaging, Advanced Optical Materials, vol. 11, p. 2202945 (2023).
[2] A. Khegai, et al, NALM-based bismuth-doped fiber laser at 1.7^m, Optics letters, vol. 43, p. 1127 (2018).
[3] E.A. Evmenova, A.A. Antropov, D.S. Kharenko, All-fiber amplification of highly chirped dissipative solitons around a 1.3 micron using stimulated Raman scattering, Optics letters 48, 6444 (2023).
[4] I. Zhdanov, et al, Raman dissipative soliton source of ultrashort pulses in NIR-III spectral window, Optics Express, vol. 31, p. 35156 (2023).
