Energetic ultrafast fiber laser sources tunable in 1030-1215 nm for deep tissue multi-photon microscopy

Watt-level all polarization-maintaining femtosecond fiber laser source at 1100 nm

We demonstrate a compact watt-level all polarization-maintaining (PM) femtosecond fiber laser source at 1100 nm. The fiber laser source is seeded by an all PM fiber mode-locked laser employing a nonlinear amplifying loop mirror. The seed laser can generate stable pulses at a fundamental repetition rate of 40.71 MHz with a signal-to-noise rate of >100 dB and an integrated relative intensity noise of only ∼0.061%. After two-stage external amplification and pulse compression, an output power of ∼1.47 W (corresponding to a pulse energy of ∼36.1 nJ) and a pulse duration of ∼251 fs are obtained. The 1100 nm femtosecond fiber laser is then employed as the excitation light source for multicolor multi-photon fluorescence microscopy of Chinese hamster ovary (CHO) cells stably expressing red fluorescent proteins.

All-normal dispersion widely tunable dual-wavelength mode-locked fiber laser based on NALM

We experimentally and numerically demonstrate the all-normal dispersion (ANDi) ytterbium (Yb)-doped fiber laser based on nonlinear amplifying loop mirror (NALM) mode-locked, which allows tunable single-wavelength and dual-wavelength outputs. The pulses tuning ranges of the dual-wavelength are from 1032.24 nm to 1053.13 nm and from 1047.94 nm to 1069.05 nm, and the repetition frequency difference varies from 1766Hz to 1834Hz. To our knowledge, this is the widest dual-wavelength tuning range of Yb-doped fiber lasers based on NALM mode-locked. We test for 90 minutes and have high stability in both single-wavelength and dual-wavelength. In addition, the pulsed collision dynamics between two solitons at different wavelengths are numerically studied. Numerical results show that during the pulse collision, the two solitons pass through each other and maintain their properties, which also confirms the particle nature of the isolated wave. Our research contributes to the dynamics of dual-wavelength solitons collision in NALM mode-locked fiber laser and provides what we believe to be is a new idea for tunable Yb-doped dual-comb sources.

Three-photon excited fluorescence imaging in neuroscience: From principles to applications

The development of three-photon microscopy (3PM) has greatly expanded the capability of imaging deep within biological tissues, enabling neuroscientists to visualize the structure and activity of neuronal populations with greater depth than two-photon imaging. In this review, we outline the history and physical principles of 3PM technology. We cover the current techniques for improving the performance of 3PM. Furthermore, we summarize the imaging applications of 3PM for various brain regions and species. Finally, we discuss the future of 3PM applications for neuroscience.

High power ytterbium rod-type fiber laser delivering tunable picosecond pulses

We designed an all-normal dispersion ytterbium rod-type fiber laser oscillator delivering picosecond pulses which are continuously tunable both in central wavelength and pulse duration. This system delivers self-mode-locked pulses with an average power up to 25 W. At a repetition rate of 78 MHz, it yields picosecond laser pulses, of which the central wavelength and pulse duration can be tuned between 1010 nm - 1060 nm and 4.5 ps - 1.8 ps, respectively. The tunability is obtained by adjusting the position and the width of a slit which acts as a spectral bandwidth filter, placed close to the center of a 4f-folded zero dispersion line inserted in the laser cavity. This oscillator delivers nearly Fourier limited pulses with at most a 1.2 time-bandwidth product. A numerical model accounts well for the behavior of this all-normal dispersion fiber oscillator.

Compact multicolor two-photon fluorescence microscopy enabled by tailorable continuum generation from self-phase modulation and dispersive wave generation

By precisely managing fiber-optic nonlinearity with anomalous dispersion, we have demonstrated the control of generating plural few-optical-cycle pulses based on a 24-MHz Chromium:forsterite laser, allowing multicolor two-photon tissue imaging by wavelength mixing. The formation of high-order soliton and its efficient coupling to dispersive wave generation leads to phase-matched spectral broadening, and we have obtained a broadband continuum ranging from 830 nm to 1200 nm, delivering 5-nJ pulses with a pulse width of 10.5 fs using a piece of large-mode-area fiber. We locate the spectral enhancement at around 920 nm for the two-photon excitation of green fluorophores, and we can easily compress the resulting pulse close to its limited duration without the need for active pulse shaping. To optimize the wavelength mixing for sum-frequency excitation, we have realized the management of the power ratio and group delay between the soliton and dispersive wave by varying the initial pulse energy without additional delay control. We have thus demonstrated simultaneous three-color two-photon tissue imaging with contrast management between different signals. Our source optimization leads to efficient two-photon excitation reaching a 500-µm imaging depth under a low 14-mW illumination power. We believe our source development leads to an efficient and compact approach for driving multicolor two-photon fluorescence microscopy and other ultrafast investigations, such as strong-field-driven applications.

Sub-60-fs, compact 1100-nm fiber laser system based on double-pass pre-chirp managed amplification

An ytterbium-doped, single-stage, double-pass nonlinear fiber amplification system was fabricated for amplifying an 1100-nm mode-locking fiber laser. Pre-chirp managed amplification (PCMA) was applied in realizing the nonlinear amplification process with an all-polarization-maintaining (PM) fiber construction. The system can deliver 19.8-nJ, 58.7-fs, 24.4-MHz amplified signal pulses with a 10-dB spectral range spanning from 1049 nm to 1130 nm. Further experimental investigations were conducted in exploring the dynamics of the double-pass nonlinear amplification process. This compact 1100-nm ultrafast fiber laser can be implemented for multi-photon microscopy (MPM) with deep penetration depth.

Generation of 56.5 W femtosecond laser radiation by the combination of an Nd-doped picosecond amplifier and multi-pass-cell device

We demonstrate the generation of high average power femtosecond laser radiation by combination of an Nd-doped picosecond amplifier and a multi-pass cell device. With this efficient and robust scheme, the pulse duration of a picosecond amplifier is compressed from 9.13 ps to 477 fs, corresponding to a compression factor of 19.1. The average power before and after pulse compression is 77 W and 56.5 W respectively, so the overall transmission reaches 73.4%. The presented scheme offers a viable route toward low-cost and simple configuration high power femtosecond lasers driven by Nd-doped picosecond amplifiers.

High-speed wavelength-swept femtosecond source from 1055 to 1300 nm using a GHz femtosecond fiber laser

In this Letter, we demonstrate a high-speed broadband wavelength-swept femtosecond source (WFS) that leverages the soliton self-frequency shift (SSFS) and intensity-wavelength encoding technologies. The optical wavelength of the high-speed WFS can be continuously swept from 1055 nm to nearly 1300 nm at a sweeping rate of 100 kHz. This WFS is especially seeded by a femtosecond mode-locked all-fiber laser at 1055 nm that has a fundamental repetition rate of ∼1.0 GHz, a maximum output power of 7 W, and a compressed pulse width of 220 fs. It is anticipated that this high-speed broadband WFS can be a promising source for applications that require fast wavelength scanning and high-speed data processing.

Application of Multiphoton Microscopic Imaging in Study of Gastric Cancer

Multiphoton microscopy (MPM) imaging relies on the nonlinear interaction between ultrashort optical pulses and the samples to achieve image contrast. Featuring larger penetration depth, less phototoxicity, 3-dimensional sectioning capability, no need for labeling, MPM become a powerful medical imaging technique that can identify structural characteristics of tissues at the cellular and subcellular levels. In this review paper, we introduce the working principle of MPM imaging, present the current results of MPM imaging applied to the study of gastric tumors, and discuss the future prospects of this interdisciplinary research field.

Black Phosphorus/Polymers: Status and Challenges

As a newly emerged mono-elemental nanomaterial, black phosphorus (BP) has been widely investigated for its fascinating physical properties, including layer-dependent tunable band gap (0.3-1.5 eV), high ON/OFF ratio (10⁴ ), high carrier mobility (10³ cm² V^-1 s^-1 ), excellent mechanical resistance, as well as special in-plane anisotropic optical, thermal, and vibrational characteristics. However, the instability caused by chemical degradation of its surface has posed a severe challenge for its further applications. A focused BP/polymer strategy has more recently been developed and implemented to hurdle this issue, so at present BP/polymers have been developed that exhibit enhanced stability, as well as outstanding optical, thermal, mechanical, and electrical properties. This has promoted researchers to further explore the potential applications of black phosphorous. In this review, the preparation processes and the key properties of BP/polymers are reviewed, followed by a detailed account of their diversified applications, including areas like optoelectronics, bio-medicine, and energy storage. Finally, in accordance with the current progress, the prospective challenges and future directions are highlighted and discussed.

Low noise, self-phase-modulation-enabled femtosecond fiber sources tunable in 740-1236 nm for wide two-photon fluorescence microscopy applications

We have demonstrated widely tunable Yb:fiber-based laser sources, aiming to replace Ti:sapphire lasers for the nJ-level ultrafast applications, especially for the uses of nonlinear light microscopy. We investigated the influence of different input parameters to obtain an expansive spectral broadening, enabled by self-phase modulation and further reshaped by self-steepening, in the normal dispersion regime before the fiber damage. We also discussed the compressibility and intensity fluctuations of the demonstrated pulses, to reach the transform-limited duration with a very low intensity noise. Most importantly, we have demonstrated clear two-photon fluorescence images from UV-absorbing fluorophores to deep red dye stains.

Protein-crystal detection with a compact multimodal multiphoton microscope

There is an increasing demand for rapid, effective methods to identify and detect protein micro- and nano-crystal suspensions for serial diffraction data collection at X-ray free-electron lasers or high-intensity micro-focus synchrotron radiation sources. Here, we demonstrate a compact multimodal, multiphoton microscope, driven by a fiber-based ultrafast laser, enabling excitation wavelengths at 775 nm and 1300 nm for nonlinear optical imaging, which simultaneously records second-harmonic generation, third-harmonic generation and three-photon excited ultraviolet fluorescence to identify and detect protein crystals with high sensitivity. The instrument serves as a valuable and important tool supporting sample scoring and sample optimization in biomolecular crystallography, which we hope will increase the capabilities and productivity of serial diffraction data collection in the future.

Femtosecond two-color source synchronized at 100-as-precision based on SPM-enabled spectral selection

We demonstrate both numerically and experimentally that self-phase modulation-enabled spectral selection generates wavelength tunable energetic pulses that are tightly synchronized to the excitation pulses. The synchronization quantified by relative timing jitter is at the 100-as precision level, at least 10 times lower than can be achieved by Raman soliton pulses derived from the same source laser. This ultrafast two-color source is suitable for many important applications that require tight pulse synchronization.

Ultrafast Fiber Lasers: An Expanding Versatile Toolbox

Ultrafast fiber lasers have gained rapid advances in last decades for their intrinsic merits such as potential of all-fiber format, excellent beam quality, superior power scalability, and high single-pass gain, which opened widespread applications in high-field science, laser machining, precision metrology, optical communication, microscopy and spectroscopy, and modern ophthalmology, to name a few. Performance of an ultrafast fiber laser is well defined by the laser parameters including repetition rate, spectral bandwidth, pulse duration, pulse energy, wavelength tuning range, and average power. During past years, these parameters have been pushed to an unprecedented level. In this paper, we review these enabling technologies and explicitly show that the nonlinear interaction between ultrafast pulses and optical fibers plays the essential role. As a result of rapid development in both active and passive fibers, the toolbox of ultrafast fiber lasers will continue to expand and provide solutions to scientific and industrial problems.

Noise-like pulse generation around 1.3-µm based on cascaded Raman scattering

Based on cascaded Raman scattering, near-infrared (NIR) noise-like pulses (NLPs) were successfully demonstrated using a Yb-doped fiber amplifier system. Through a nonlinear fiber amplifier using a germanium-zirconia-silica Yb³⁺-doped single mode fiber as a gain fiber, the fourth-order Stokes wave (4^th-SW) can be excited to extend the emission peak of approximately 1.2-µm and a 3-dB bandwidth of approximately 130 nm. To further shift the wavelength more efficiently toward 1.3 µm, filtered NLPs with an emission peak at 1075 nm were adopted as seeded pulses to excite the fifth-order Stokes wave (5^th-SW) because of the better conversion efficiency of stimulated Raman scattering without gain competition with Yb-doped fiber. The generated NIR NLPs were shown to be an excellent light source for the photoluminescence emission from three photon absorption of perovskite to illustrate the red shift of the emission peak owing to the reabsorption effect.

Label-free multiphoton microscopic imaging as a novel real-time approach for discriminating colorectal lesions: A preliminary study

BACKGROUND AND AIM

Although various endoscopic technologies have been developed to increase the visual diagnostic accuracy of colorectal precancerosis and early carcinoma, the biopsy-dependent pathology still remains the golden standard. During routine endoscopy, real-time optical histological diagnosis is desired. Without fluorescent label, multiphoton microscopy (MPM) imaging directly reveals live cellular morphology and tissue microenvironment based on intrinsic two-photon excited fluorescence and second harmonic generation signals. Its high-imaging resolution and performance are comparable with the histopathology. We thus aimed to initially investigate the original features of colorectal diseases under MPM and evaluate its potential for real-time diagnosis.

METHODS

Experimental and diagnostic cohorts were designed. Multiphoton images of 40 ex vivo fresh tissues confirmed pathologically of colorectal normal tissues, hyperplastic polyps, adenomas, and adenocarcinomas were collected. Features were recorded to establish diagnostic standards with MPM. For the second cohort with 92 fresh tissues, we distinguished the various colorectal diseases with conclusive MPM features.

RESULTS

Through the investigation, the colorectal diseases were presented differences in the crypt opening, gland structure, epithelial cells, and collagen fibers. With the typical features, we preliminarily tested the diagnostic efficiency and found that its sensitivity for distinguishing normal, hyperplastic polyps, adenoma and adenocarcinoma was 88.89%, 76.47%, 83.33%, and 97.92%, while the specificity was 99.32%, 94.00%, 94.92%, and 94.12%, respectively.

CONCLUSION

The real-time multiphoton microscopic imaging can be effective to identify the colorectal lesions with high resolution. Via integrating with the endoscopes in the future, it could promote precise optical diagnosis in clinics.

High-performance tunable, self-similar fiber laser

We report a tunability study of a mode-locked Yb-doped self-similar fiber laser. The center wavelength is tunable from 1030 to 1100 nm without losing mode locking by adjusting a narrow intracavity spectral filter. Undesirable amplified spontaneous emission is suppressed by heating up the gain fiber. As a tunable mode-locked fiber laser, the performance (∼100  fs and ∼4  nJ) is exceptional owing to a unique self-similar pulse evolution.

Multimodal imaging platform for optical virtual skin biopsy enabled by a fiber-based two-color ultrafast laser source

We demonstrate multimodal label-free nonlinear optical microscopy in human skin enabled by a fiber-based two-color ultrafast source. Energetic femtosecond pulses at 775 nm and 1250 nm are simultaneously generated by an Er-fiber laser source employing frequency doubling and self-phase modulation enabled spectral selection. The integrated nonlinear optical microscope driven by such a two-color femtosecond source enables the excitation of endogenous two-photon excitation fluorescence, second-harmonic generation, and third-harmonic generation in human skin. Such a 3-channel imaging platform constitutes a powerful tool for clinical application and optical virtual skin biopsy.

Slide-free virtual histochemistry (Part I): development via nonlinear optics

Histochemistry is a microscopy-based technology widely used to visualize the molecular distribution in biological tissue. Recent developments in label-free optical imaging has demonstrated the potential to replace the conventional histochemical labels/markers (fluorescent antibodies, organic dyes, nucleic acid probes, and other contrast agents) with diverse optical interactions to generate histochemical contrasts, allowing "virtual" histochemistry in three spatial dimensions without preparing a microscope slide (i.e. labor-intensive sample preparation). However, the histochemical information in a label-free optical image has often been rather limited due to the difficulty in simultaneously generating multiple histochemical contrasts with strict spatial co-registration. Here, in the first part (Part I) of this two-part series study, we develop a technique of slide-free virtual histochemistry based on label-free multimodal multiphoton microscopy, and simultaneously generate up to four histochemical contrasts from in vivo animal and ex vivo human tissue. To enable this functionality, we construct and demonstrate a robust fiber-based laser source for clinical translation and phenotype a wide variety of vital cells in unperturbed mammary tissue.

All-fiber dissipative soliton Raman laser based on phosphosilicate fiber

We propose and demonstrate an all-fiber, synchronously pumped Raman laser based on phosphosilicate fiber (P-doped fiber) for deep tissue multiphoton imaging. The laser operates in a dissipative soliton regime and produces 2.2 ps chirped pulses (compressible to 317 fs) with energy up to 9.2 nJ, 0.3 W average power and at 1240 nm center wavelength. We have also found a new cross-polarization Raman lasing operation that offers access to an important wavelength near 930 nm for calcium imaging.

Investigation of the long wavelength limit of soliton self-frequency shift in a silica fiber

We explore the long wavelength limit of soliton self-frequency shift in silica-based fibers experimentally and using numerical simulation. We found that the longest wavelength soliton generated by soliton self-frequency shift is approximately 2500 nm because the soliton loses its energy rapidly at wavelength beyond 2400 nm due to material absorption by silica and water. We demonstrate 1580-2520 nm wavelength-tunable, high-pulse energy soliton generation using soliton self-frequency shift in a large-mode-area silica fiber pumped by a compact fiber source. Soliton pulses with pulse width of ~100 fs and pulse energy up to 73 nJ were obtained. Second harmonic generation of the solitons enables a wavelength-tunable femtosecond source from 950 nm to 1260 nm, with pulse energy up to 21 nJ. Using such energetic pulses, we demonstrate in vivo two-photon excited fluorescence imaging of vasculature and neurons in a mouse brain at wavelength beyond the tuning range of a mode-locked Ti:Sapphire lasers.

Energy scalable, offset-free ultrafast mid-infrared source harnessing self-phase-modulation-enabled spectral selection

We demonstrate a high-power offset-free ultrafast mid-infrared (IR) laser source based on difference-frequency generation (DFG). Powerful signal pulses are obtained by filtering the rightmost spectral lobe of the optical spectra broadened by fiber-optic nonlinearities dominated by self-phase modulation. The resulting mid-IR pulses are tunable from 7 to 18 μm with up to 5.4 mW average power. We experimentally and numerically investigate power scaling of this DFG source and demonstrate that increasing the signal power is an efficient approach for generating high-power mid-IR pulses.

Several new directions for ultrafast fiber lasers [Invited]

Ultrafast fiber lasers have the potential to make applications of ultrashort pulses widespread - techniques not only for scientists, but also for doctors, manufacturing engineers, and more. Today, this potential is only realized in refractive surgery and some femtosecond micromachining. The existing market for ultrafast lasers remains dominated by solid-state lasers, primarily Ti:sapphire, due to their superior performance. Recent advances show routes to ultrafast fiber sources that provide performance and capabilities equal to, and in some cases beyond, those of Ti:sapphire, in compact, versatile, low-cost devices. In this paper, we discuss the prospects for future ultrafast fiber lasers built on new kinds of pulse generation that capitalize on nonlinear dynamics. We focus primarily on three promising directions: mode-locked oscillators that use nonlinearity to enhance performance; systems that use nonlinear pulse propagation to achieve ultrashort pulses without a mode-locked oscillator; and multimode fiber lasers that exploit nonlinearities in space and time to obtain unparalleled control over an electric field.

Megawatt peak power tunable femtosecond source based on self-phase modulation enabled spectral selection

Wavelength widely tunable femtosecond sources can be implemented by optically filtering the leftmost/rightmost spectral lobes of a broadened spectrum due to self-phase modulation (SPM) dominated fiber-optic nonlinearities. We numerically and experimentally investigate the feasibility of implementing such a tunable source inside optical fibers with negative group-velocity dispersion (GVD). We show that the spectral broadening prior to soliton fission is dominated by SPM and generates well-isolated spectral lobes; filtering the leftmost/rightmost spectral lobes results in energetic femtosecond pulses with the wavelength tuning range more than 400 nm. Employing an ultrafast Er-fiber laser and a dispersion-shifted fiber with negative GVD, we implement an energetic tunable source that produces ~100-fs pulses tunable between 1.3 µm and 1.7 µm with up to ~16-nJ pulse energy. Further energy scaling is achieved by increasing the input pulse energy to ~1-μJ and reducing the fiber length to 1.3 cm. The resulting source can produce >100-nJ femtosecond pulses at 1.3 µm and 1.7 µm with MW level peak power, representing an order of magnitude improvement of our previous results. Such a powerful source covers the 2nd and the 3rd biological transmission window and can facilitate multiphoton deep-tissue imaging.

Compact fs ytterbium fiber laser at 1010 nm for biomedical applications

Ytterbium-doped fiber lasers (YDFLs) working in the near-infrared (NIR) spectral window and capable of high-power operation are popular in recent years. They have been broadly used in a variety of scientific and industrial research areas, including light bullet generation, optical frequency comb formation, materials fabrication, free-space laser communication, and biomedical diagnostics as well. The growing interest in YDFLs has also been cultivated for the generation of high-power femtosecond (fs) pulses. Unfortunately, the operating wavelengths of fs YDFLs have mostly been confined to two spectral bands, i.e., 970-980 nm through the three-level energy transition and 1030-1100 nm through the quasi three-level energy transition, leading to a spectral gap (990-1020 nm) in between, which is attributed to an intrinsically weak gain in this wavelength range. Here we demonstrate a high-power mode-locked fs YDFL operating at 1010 nm, which is accomplished in a compact and cost-effective package. It exhibits superior performance in terms of both short-term and long-term stability, i.e., <0.3% (peak intensity over 2.4 μs) and <4.0% (average power over 24 hours), respectively. To illustrate the practical applications, it is subsequently employed as a versatile fs laser for high-quality nonlinear imaging of biological samples, including two-photon excited fluorescence microscopy of mouse kidney and brain sections, as well as polarization-sensitive second-harmonic generation microscopy of potato starch granules and mouse tail muscle. It is anticipated that these efforts will largely extend the capability of fs YDFLs which is continuously tunable over 970-1100 nm wavelength range for wideband hyperspectral operations, serving as a promising complement to the gold-standard Ti:sapphire fs lasers.

Formation, element-migration and broadband luminescence in quantum dot-doped glass fibers

All solid-state PbS quantum dot (QD)-doped glass precursor fibers avoiding crystallization during fiber-drawing process are successfully fabricated by melt-in-tube technique. By subsequent heat treatment schedule, controllable crystallization of PbS QDs can be obtained in the glass precursor fibers, contributing to broad near-infrared emissions from PbS QD-doped glass fibers. Nevertheless, we find that element-migration and volatilization of sulfur simultaneously happen during the whole fiber-drawing process, because of the huge difference between the melting temperature of core glass and the fiber-drawing temperature. Element-migration pathways along the fiber length were revealed. Such PbS QD-doped glass fiber with broadband emissions will be a potential application as gain medium of broadband fiber amplifiers and fiber lasers.

Er-fiber laser enabled, energy scalable femtosecond source tunable from 1.3 to 1.7 µm

We demonstrate that energetic femtosecond pulses tunable from 1.3 to 1.7 µm can be achieved using self-phase modulation enabled spectral broadening followed by spectral lobe filtering. Based on a home-built 5-W Er-fiber laser system operating at 31-MHz repetition rate, we obtain femtosecond pulses that can be continuously tuned from 1.3 to 1.7 µm with >4.5 nJ pulse energy. We further optimize the spectral broadening process using a fiber with larger mode area and scale up the pulse energy to >10 nJ; the resulting pulse duration is as short as ~50 fs. Such a widely tunable, energetic femtosecond source is well suited for driving a laser scanning microscope to perform deep tissue multiphoton microscopy.