We report on a simultaneous 2-photon and 3-photon signal acquisition method for label-free multiphoton microscopy. With dual excitation wavelengths of 1580 nm and 790 nm and a set of filter designs, multiple contrasts including second harmonic generation, third harmonic generation, and two-photon-excitation-fluorescence images are detected and separated. The spectrum of the nonlinear signals is measured and verified by a spectrometer. Depth-resolved multimodal images are demonstrated on a silicon photonic chip and leaf samples. The simultaneous 2- and 3-photon microscopy is shown to have great potential for label-free in-vivo imaging.
One major advantage of multiphoton microscopy (MPM) is that it can image below tissue surface and produce a stack of images showing sample structure at various depths. A miniature objective with depth scanning capability is needed for MPM endoscopy. Spherical aberration may be induced when changing the focusing depth during multiphoton microscope depth scanning, thus limiting the range over which images may be acquired. A specially designed miniature objective that minimizes spherical aberration across large range of focusing depths is presented. Simulations show that the 0.53 numerical aperture design can achieve on-axis diffraction limited focusing in water for depths from 0 μm to just over 1400 μm and a diffraction limited field of view of up to 290 μm for a 790 nm laser. In experiment, our multiphoton microscope demonstrates a field of view of 64 μm by 100 μm and a depth scanning range of 440 μm, limited by the scanning hardware. Depth scanning capability is confirmed by imaging 0.1 μm diameter fluorescent beads across the 440 μm range. Biological samples to a depth of 150 μm are imaged using the custom objective; the imaging depth is mainly limited by the absorption and scattering of the sample.
We report on a multimodal multiphoton microscopy (MPM) system with depth scanning. An Er-doped fiber laser with 1580 nm and 790 nm output provides the dual-wavelength multimodal capability and a shape-memory-alloy (SMA) based depth-scanning objective enables the depth scanning. Image stacks combining two-photon-excitation-fluorescence (TPEF), second-harmonic-generation (SHG), and third-harmonic-generation (THG) signals have been acquired on animal samples from the surface to over 200 μm underneath.
A laser diode end-pumped passively Q-switched YAG/Nd:YAG/Cr4+:YAG burst mode laser at 1064 nm is reported. The maximum burst energy of 13.2 mJ at a repetition rate of 100 Hz is got in a duration of 1 ms. The pulse repetition frequency increases linearly and peaks at 31.1 kHz in the pulse burst, while the pulse width keeps nearly constant around ~2 ns. The highest peak power of 373.7 kW is achieved at 10.2 kHz. The beam quality of passively Qswitched 1064 nm laser is also investigated.
An all-diode-pumped Nd:YAG burst-mode laser was demonstrated. A diode-laser-side-pumped Nd:YAG laser with Qswitcher, as laser oscillator, was employed to produce burst pulse directly. When the diode-laser worked at 10Hz and Qswitcher was operated at 10kHz, a maximum burst energy of 456mJ was obtained in the master oscillator with the highest optical efficiency of ~30%. Each burst included 19 pulses in the pumping duration of 2ms. A master oscillator and power amplifier architecture was adapted to scale the burst energy to meet PLIF system applications. The burst energy of 456mJ obtained from laser oscillator was amplified to 2.2J by use of three diode-laser-side-pumped Nd:YAG modules as laser amplifiers. The energy extraction efficiency of each stage was achieved to ~13%, ~22% and ~24%, respectively. The single pulse energy of 1064nm laser at 10 kHz reached to ~116 mJ with pulse width of 9.8 ns and a peak power of ~11.8 MW. The performances of pulse-burst laser we constructed can be better and improved greatly by use of more amplifiers.
Planar laser induced fluorescence (PLIF) diagnosis technology has been widely applied in flow field study and combustion diagnosis. Due to the complexity of the experimental environment of practical PLIF applications, The PLIF diagnostic system needs a good environment adaptation. In this paper, we reported a high energy Nd:YAG MOPA laser with repetition rate of 500Hz, which was applied in PLIF diagnostic system. A diode-laser side-pumped Nd:YAG module, which was pumped from five directions and optimized for better gain distribution, was employed to build a EO Q-switched Nd:YAG oscillator. A stable structure design of oscillator resonant cavity was used to improve the environmental adaptability of Nd:YAG laser. In our environmental adaptation experimentation, the laser oscillator has an energy fluctuation of <5% in the temperature range from 5°C to 45°C. In order to scale the pulse energy to meet the PLIF system requirements, we employed three 2500W diode-laser five-direction-side-pumped Nd:YAG modules as laser amplifiers to build MOPA system. Finally, the laser pulse energy of ~18mJ was amplified to 68mJ at 1064nm. Using KTP crystal as a frequency doubling crystal, we obtained a pulse energy of 35.6mJ at 532nm. The developed Nd:YAG laser has been used in our 500Hz-PLIF diagnostic system successfully.
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