Femtosecond lasers at 780 nm are vital sources for abundant applications, especially in two-photon fluorescence microscopy (TPFM)[
Chinese Optics Letters, Volume. 17, Issue 7, 071405(2019)
Femtosecond fiber laser at 780 nm for two-photon autofluorescence imaging
We present an Er-doped fiber (Er:fiber)-based femtosecond laser at 780 nm with 256 MHz repetition rate, 191 fs pulse duration, and over 1 W average power. Apart from the careful third-order dispersion management, we introduce moderate self-phase modulation to broaden the output spectrum of the Er:fiber amplifier and achieve 193 fs pulse duration and 2.43 W average power. Over 40% frequency doubling efficiency is obtained by a periodically poled lithium niobate crystal. Delivering through a hollow-core photonic bandgap fiber, this robust laser becomes an ideal and convenient light source for two-photon autofluorescence imaging.
Femtosecond lasers at 780 nm are vital sources for abundant applications, especially in two-photon fluorescence microscopy (TPFM)[
High repetition-rate laser pulses have attracted much attention for the low photobleaching rate[
Frequency doubling of femtosecond Er-doped fiber (Er:fiber) lasers[
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In this Letter, by combining fiber chirped pulse amplification (FCPA)[
The system consists of an Er:fiber femtosecond laser, a fiber stretcher, a two-stage amplifier, a transmission grating pair compressor, and a frequency doubling system, shown in Fig.
Figure 1.Schematic diagram of the laser system. PBS, polarization beam splitter; SM LD, single-mode laser diode; MM LD, multi-mode laser diode; WDM, wavelength division multiplexer; ISO, isolator.
It is important to match the dispersion at least to the third order to achieve pulses of about 100 fs. The dispersion at 1560 nm for all system components is listed in the Table
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The compressed pulses are frequency doubled by a 1-mm-long periodically poled lithium niobate (PPLN) crystal. The crystal is mounted on a copper holder for natural cooling. Two waveplates are utilized to rotate the polarization, ensuring maximum frequency doubling efficiency. A bandpass filter is used to remove the unwanted pulses from the frequency doubled pulses. Finally, the pulses at 780 nm are imported into a 1.5-m-long HC-PBF with negligible bending loss and nonlinearity[
The final output power after two-stage amplification is 3.6 W, and the optimized pulses are compressed to 193 fs. The average power of the compressed pulses is 2.43 W with the compression efficiency of 67.5%. To achieve this pulse duration, the DCF length was optimized.
When the DCF length was 13.4 m, the spectrum bandwidth of the pulses was reduced from 52.6 nm of the seed pulses to 12.9 nm after two-stage amplification, so the compressed pulses were over 200 fs accordingly. To cope with the spectrum narrowing, we introduced a certain amount of nonlinear phase shift by a shorter stretched pulse in the DCF. The nonlinear phase shift is defined by
The nonlinear phase shift and the residual TOD as functions of the DCF length are shown in Fig.
Figure 2.Results of the Er:fiber laser. (a) Calculated nonlinear phase shift and residual TOD in the FCPA system, (b) spectra and intensity autocorrelation traces under different DCF lengths.
By investigating the DCF length, we found that when the DCF length was 6.6 m, the pulse spectrum was about 20 nm, and the pulse duration was 193 fs with small side lobes, which is about 1.2 times the Fourier-transform-limited pulse duration.
Using the above optimized output pulses, we continued the frequency doubling and show the results in Fig.
Figure 3.Experimental results of the frequency doubling. (a) RF spectrum of the oscillator, (b) output average power, (c) optical spectrum, (d) intensity autocorrelation trace of the frequency doubled pulse.
Figure 4.Optical spectra of frequency doubling under different DCF lengths.
The output pulses from the HC-PBF were launched into a home-built two-photon microscope to image the rabbit intestine tissue and human skin. The average powers of the samples were 85 and 40 mW, respectively. Figure
Figure 5.(a) Experimental result of TPAM imaging of the rabbit intestine tissue
We have demonstrated an FCPA laser at 780 nm, which generates 256 MHz, 191 fs, and 1.01 W average power pulses. The pulse energy is 3.95 nJ with 20.68 kW peak power. Through careful TOD and nonlinearity management, high power and good quality pulses at 780 nm are obtained. Two-photon autofluorescence imaging of the rabbit intestine tissue and human skin with this laser was performed, and the result confirms the great potential of the laser to be used for diagnosis and therapeutic monitoring without the use of extrinsic labels. The integrated fiber laser source with a miniature TPFM system will be investigated in our future study.
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Wan Yang, Danlei Wu, Runlong Wu, Guanyu Liu, Bingying Chen, Lishuang Feng, Zhigang Zhang, Aimin Wang. Femtosecond fiber laser at 780 nm for two-photon autofluorescence imaging[J]. Chinese Optics Letters, 2019, 17(7): 071405
Category: Letters
Received: Nov. 30, 2018
Accepted: Apr. 12, 2019
Published Online: Jul. 9, 2019
The Author Email: Aimin Wang (wangaimin@pku.edu.cn)