Abstract
Frequency-doubled femtosecond Er-doped fiber laser is a low-cost and portable excitation source suitable for multiphoton endoscopy. The frequency-doubled wavelength at 780nm is used to excite the intrinsic fluorescence signal. The frequency-doubling with a periodically poled MgO : LiNbO3 (PPLN) is integrated in the distal end of the imaging head to achieve fiber connection. The imaging speed is further improved by optimizing the excitation laser source. A 0.3-mm length of PPLN crystal is selected and the Er-doped fiber laser is manipulated to match its bandwidth with the acceptance bandwidth of the PPLN. Through this optimization, a reduced pulsewidth of 80fs of the frequency-doubled pulse is achieved. All-fiber dispersion compensation and pulse compression by single mode fiber is conducted, which makes the fiber laser directly fiber-coupled to the imaging head. An imaging speed of 4 frames / s is demonstrated on ex vivo imaging of unstained biological tissues, which is 10 times faster than our previous study using a 1-mm-long PPLN. The results show that miniature multiphoton endoscopy using frequency-doubled Er-doped fiber laser has great potential for clinical applications.
Highlights
Multiphoton microscopy (MPM) is an effective and noninvasive imaging tool that enables the visualization of living tissues and organs.[1,2] In MPM, near-infrared femtosecond pulsed lasers are used to excite nonlinear contrast signals such as two-photon excitation fluorescence (TPEF) and second harmonic generation (SHG).[3,4,5] In order to utilize MPM in clinical applications, miniature MPM endoscopy needs to be developed
In MPM endoscopy, the excitation laser pulse and the MPM signals can be guided through flexible optical fiber, and various tissue locations can be conveniently imaged via a miniature probe.[6,7,8,9]
Since the Er-doped fiber (EDF) laser is directly coupled into the imaging head through a standard single mode fiber (SMF) and the emitted MPM signal is directed to the photomultiplier tube (PMT) by an multimode fiber (MMF), the MPM endoscopy system is compact and fiber connected
Summary
Multiphoton microscopy (MPM) is an effective and noninvasive imaging tool that enables the visualization of living tissues and organs.[1,2] In MPM, near-infrared femtosecond pulsed lasers are used to excite nonlinear contrast signals such as two-photon excitation fluorescence (TPEF) and second harmonic generation (SHG).[3,4,5] In order to utilize MPM in clinical applications, miniature MPM endoscopy needs to be developed. Huland et al.[8] developed an MPM endoscopy system where the pulse from a Ti:Sapphire laser was delivered by a hollow core photonic bandgap fiber (PBF). The frequency-doubled EDF laser had a freespace output and was recoupled into a hollow core PBF for delivering 780-nm pulses This process required precise alignment for fiber recoupling, introduced coupling loss, and lost the convenience of direct fiber connection. A miniature fiber-optic MPM endoscopy system based on the frequency-doubled EDF laser was demonstrated in our previous work.[9] The laser pulse was directly coupled via a piece of SMF to the imaging probe where the pulse was frequencydoubled in the distal end of the probe. Using the optimized frequency-doubled pulse as the excitation source, MPM imaging with significantly increased speed is demonstrated by ex vivo imaging of unstained biological tissues. It is shown that the miniature MPM endoscopy with the frequency-doubled EDF laser has great potential in translating MPM imaging into clinical applications
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