Robust hollow-fiber-pigtailed 930 nm femtosecond Nd:fiber laser for volumetric two-photon imaging

Sub-50 fs, 0.5 W average power Nd-doped fiber amplifier at 920 nm

We develop an all polarization-maintaining (PM) 920 nm Nd-doped fiber amplifier delivering a train of pulses with ∼0.53 W average power and sub-50 fs duration. The sub-50 fs pulse benefits from the pre-chirping management method that allows for over 60 nm broadening spectrum without pulse breaking in the amplification stage. By virtue of the short pulse duration, the pulse peak power can reach to ∼0.31 MW in spite of the moderate average power. These results represent a key step in developing high-peak-power pulse Nd-doped fiber laser systems at 920 nm, which will find important applications in fields such as biomedical imaging, ultrafast optical spectroscopy, and excitation of quantum-dot single photon sources.

Efficient three-level continuous-wave and GHz passively mode-locked laser by a Nd3+-doped silicate glass single mode fiber

Nd³⁺-doped three-level (⁴F_3/2-⁴I_9/2) fiber lasers with wavelengths in the range of 850-950 nm are of considerable interest in applications such as bio-medical imaging and blue and ultraviolet laser generation. Although the design of a suitable fiber geometry has enhanced the laser performance by suppressing the competitive four-level (⁴F_3/2-⁴I_11/2) transition at ∼1 µm, efficient operation of Nd³⁺-doped three-level fiber lasers still remains a challenge. In this study, taking a developed Nd³⁺-doped silicate glass single-mode fiber as gain medium, we demonstrate efficient three-level continuous-wave lasers and passively mode-locked lasers with a gigahertz (GHz) fundamental repetition rate. The fiber is designed using the rod-in-tube method and has a core diameter of 4 µm with a numerical aperture of 0.14. In a short 4.5-cm-long Nd³⁺-doped silicate fiber, all-fiber CW lasing in the range of 890 to 915 nm with a signal-to-noise ratio (SNR) greater than 49 dB is achieved. Especially, the laser slope efficiency reaches 31.7% at 910 nm. Furthermore, a centimeter-scale ultrashort passively mode-locked laser cavity is constructed and ultrashort pulse at 920 nm with a highest GHz fundamental repetition is successfully demonstrated. Our results confirm that Nd³⁺-doped silicate fiber could be an alternative gain medium for efficient three-level laser operation.

Watt-level gigahertz femtosecond fiber laser system at 920 nm

We demonstrate a watt-level femtosecond fiber laser system at 0.9 µm with a repetition rate of >1 GHz, which is the highest value reported so far for a fundamental mode-locked fiber laser. The fiber laser system is seeded by a fundamental mode-locked fiber laser constructed with a home-made highly Nd³⁺-doped fiber. After external amplification and pulse compression, an output power of 1.75 W and a pulse duration of 309 fs are obtained. This compact fiber laser system is expected to be a promising laser source for biological applications, particularly two-photon excitation microscopy.

Ultrafast fiber laser at 0.9 µm with a gigahertz fundamental repetition rate by a high gain Nd3+-doped phosphate glass fiber

Fiber lasers, owing to the advantages of excellent beam quality and unique robustness, play a crucial role in lots of fields in modern society. Developing optical glass fibers with superior performance is of fundamental importance for wide applications of fiber lasers. Here, a new Nd³⁺-doped phosphate single-mode fiber that enables a high gain at 0.9 µm is designed and fabricated. Compared to previous Nd³⁺-doped silica fibers, the developed phosphate fiber exhibits a significant gain promotion, up to 2.7 dB cm^-1 at 915 nm. Configuring in a continuous-wave fiber laser, this phosphate fiber can provide a slope efficiency of 11.2% in a length of only 4.5 cm, about 6 times higher than that of Nd³⁺-doped silica fiber. To showcase its uniqueness, an ultrafast fiber laser with ultrashort cavity is constructed, such that an ultrashort pulse train with a fundamental repetition rate of up to 1.2 GHz is successfully generated. To the best of our knowledge, this is the highest fundamental repetition rate for mode-locked fiber lasers at this wavelength range - two orders of magnitude higher than that of prior works. These results indicate that this Nd³⁺-doped phosphate fiber is an effective gain medium for fiber amplifiers and lasers at 0.9 µm, and it is promising for two-photon biophotonics that requires long-term operation with low phototoxicity.

High energy (>40 nJ), sub-100 fs, 950 nm laser for two-photon microscopy

Compact and high-energy femtosecond fiber lasers operating around 900-950 nm are desirable for multiphoton microscopy. Here, we demonstrate a >40 nJ, sub-100 fs, wavelength-tunable ultrafast laser system based on chirped pulse amplification (CPA) in thulium-doped fiber and second-harmonic generation (SHG) technology. Through effective control of the nonlinear effect in the CPA process, we have obtained 92-fs pulses at 1903 nm with an average power of 0.89 W and a pulse energy of 81 nJ. By frequency doubling, 95-fs pulses at 954 nm with an average power of 0.46 W and a pulse energy of 42 nJ have been generated. In addition, our system can also achieve tunable wavelength from 932 nm to 962 nm (frequency doubled from 1863 nm to 1919 nm). A pulse width of ∼100 fs and sufficient pulse energy are ensured over the entire tuning range. Finally, we applied the laser in a two-photon microscope and obtained superior imaging results. Due to a relatively low repetition rate (∼ 10 MHz), similar imaging quality can be achieved at significantly reduced average power compared with a commercial 80 MHz laser system. At the same time, the lower average power is helpful in limiting the thermal load to the samples. It is believed that such a setup, with its well-balanced optical characteristics and compact footprint, provides an ideal source for two-photon microscopy.

Development of a deep-ultraviolet pulse laser source operating at 234 nm for direct cooling of Al+ ion clocks

We report on the development of a 250-MHz 234 nm deep-ultraviolet pulse source based on a flexible wavelength-conversion scheme. The scheme is based on a frequency-doubled optical parametric oscillator (FD-OPO) together with a cascaded frequency conversion process. We use a χ⁽²⁾ nonlinear envelope equation to guide the design of an intra-cavity OPO crystal, demonstrating a flexible broadband tunable feature and providing as high as watt-level of a frequency-doubled signal output centered at 850 nm, which is served as an input wave for the cascaded frequency conversion process. As much as 3.0 mW of an average power at 234 nm is obtained, with an rms power stability of better than 1% over 20 minutes. This deep-ultraviolet pulse laser source can be used for many applications in quantum optics and for direct laser cooling of Al⁺ ion clocks.

Two-photon microscopy with a frequency-doubled fully fusion-spliced fiber laser at 1840 nm

We introduce a fiber-based laser system providing 130 fs pulses with 3.5 nJ energy at 920 nm at a 43 MHz repetition rate and illustrate the potential of the source for two-photon excited fluorescence microscopy of living mouse brain. The laser source is based on frequency-doubling high-energy solitons generated and frequency-shifted to 1840 nm in large mode area fibers. This simple laser system could unleash the potential of two-photon microscopy techniques in the biology laboratory where green fluorescent proteins with two-photon absorption spectrum peaking around 920 nm are routinely used.

Rapid volumetric imaging with Bessel-Beam three-photon microscopy

Owing to its tissue-penetration ability, multi-photon fluorescence microscopy allows for the high-resolution, non-invasive imaging of deep tissue in vivo; the recently developed three-photon microscopy (3PM) has extended the depth of high-resolution, non-invasive functional imaging of mouse brains to beyond 1.0 mm. However, the low repetition rate of femtosecond lasers that are normally used in 3PM limits the temporal resolution of point-scanning three-photon microscopy. To increase the volumetric imaging speed of 3PM, we propose a combination of an axially elongated needle-like Bessel-beam with three-photon excitation (3PE) to image biological samples with an extended depth of focus. We demonstrate the higher signal-to-background ratio (SBR) of the Bessel-beam 3PM compared to the two-photon version both theoretically and experimentally. Finally, we perform simultaneous calcium imaging of brain regions at different axial locations in live fruit flies and rapid volumetric imaging of neuronal structures in live mouse brains. These results highlight the unique advantage of conducting rapid volumetric imaging with a high SBR in the deep brain in vivo using scanning Bessel-3PM.