Deep in vivo two-photon microscopy with a low cost custom built mode-locked 1060 nm fiber laser

All-fiber chirp tuning of ultrashort pulses via long chirped fiber Bragg grating pair

Chirp tuning of ultrashort pulses is crucial for nonlinear fiber amplification and nonlinear dynamics investigations. Here we demonstrate all-fiber chirp tuning via a chirped fiber Bragg grating (CFBG) pair. Two identically long CFBGs were placed reversely to cancel out most of their huge dispersion (∼40 ps/nm or ∼22.4 ps2 @1030 nm), while the controllable temperature gradient along them could be used for precise chirp tuning with a tuning range of ∼ps2, verified by dispersion measurement and ultrashort pulse broadening. This relatively large chirp tuning could be used in prechirp management in nonlinear fiber amplifiers, exemplified by the optical spectrum tailoring therein. In addition, we also show this precise chirp tuning capability could be very helpful for pulse temporal quality diagnosis, which is indispensable for seed pulse optimization. We believe this all-fiber chirp tuning technique would find wide applications in nonlinear amplification and ultrafast nonlinear dynamics investigations.

Microvascular plasticity in mouse stroke model recovery: Anatomy statistics, dynamics measured by longitudinal in vivo two-photon angiography, network vectorization

This manuscript quantitatively investigates remodeling dynamics of the cortical microvascular network (thousands of connected capillaries) following photothrombotic ischemia (cubic millimeter volume, imaged weekly) using a novel in vivo two-photon angiography and high throughput vascular vectorization method. The results suggest distinct temporal patterns of cerebrovascular plasticity, with acute remodeling peaking at one week post-stroke. The network architecture then gradually stabilizes, returning to a new steady state after four weeks. These findings align with previous literature on neuronal plasticity, highlighting the correlation between neuronal and neurovascular remodeling. Quantitative analysis of neurovascular networks using length- and strand-based statistical measures reveals intricate changes in network anatomy and topology. The distance and strand-length statistics show significant alterations, with a peak of plasticity observed at one week post-stroke, followed by a gradual return to baseline. The orientation statistic plasticity peaks at two weeks, gradually approaching the (conserved across subjects) stroke signature. The underlying mechanism of the vascular response (angiogenesis vs. tissue deformation), however, is yet unexplored. Overall, the combination of chronic two-photon angiography, vascular vectorization, reconstruction/visualization, and statistical analysis enables both qualitative and quantitative assessments of neurovascular remodeling dynamics, demonstrating a method for investigating cortical microvascular network disorders and the therapeutic modes of action thereof.

Pulse train gating to improve signal generation for in vivo two-photon fluorescence microscopy

Significance

Two-photon microscopy is used routinely for in vivo imaging of neural and vascular structures and functions in rodents with a high resolution. Image quality, however, often degrades in deeper portions of the cerebral cortex. Strategies to improve deep imaging are therefore needed. We introduce such a strategy using the gating of high repetition rate ultrafast pulse trains to increase the signal level.

Aim

We investigate how the signal generation, signal-to-noise ratio (SNR), and signal-to-background ratio (SBR) improve with pulse gating while imaging in vivo mouse cerebral vasculature.

Approach

An electro-optic modulator with a high-power (6 W) 80 MHz repetition rate ytterbium fiber amplifier is used to create gates of pulses at a 1 MHz repetition rate. We first measure signal generation from a Texas Red solution in a cuvette to characterize the system with no gating and at a 50%, 25%, and 12.5% duty cycle. We then compare the signal generation, SNR, and SBR when imaging Texas Red-labeled vasculature using these conditions.

Results

We find up to a 6.73-fold increase in fluorescent signal from a cuvette when using a 12.5% duty cycle pulse gating excitation pattern as opposed to a constant 80 MHz pulse train at the same average power. We verify similar increases for in vivo imaging to that observed in cuvette testing. For deep imaging, we find that pulse gating results in a 2.95-fold increase in the SNR and a 1.37-fold increase in the SBR on average when imaging mouse cortical vasculature at depths ranging from 950 to 1050    μ m .

Conclusions

We demonstrate that a pulse gating strategy can either be used to limit heating when imaging superficial brain regions or used to increase signal generation in deep regions. These findings should encourage others to adopt similar pulse gating excitation schemes for imaging neural structures through two-photon microscopy.

All-fiberized 1840-nm femtosecond thulium fiber laser for label-free nonlinear microscopy

We report an all-fiberized 1840-nm thulium-fiber-laser source, comprising a dissipative-soliton mode-locked seed laser and a chirped-pulse-amplification system for label-free biological imaging through nonlinear microscopy. The mode-locked thulium fiber laser generated dissipative-soliton pulses with a pre-chirped duration of 7 ps and pulse energy of 1 nJ. A chirped-pulse fiber-amplification system employing an in-house-fabricated, short-length, single-mode, high-absorption, thulium fiber delivered pulses with energies up to 105 nJ. The pulses were capable of being compressed to 416 fs by passing through a grating pair. Imaging of mouse tissue and human bone samples was demonstrated using this source via third-harmonic generation microscopy.

Evaluation of a gain-managed nonlinear fiber amplifier for multiphoton microscopy

Two-photon excited fluorescence microscopy is a widely-employed imaging technique that enables the noninvasive study of biological specimens in three dimensions with sub-micrometer resolution. Here, we report an assessment of a gain-managed nonlinear (GMN) fiber amplifier for multiphoton microscopy. This recently-developed source delivers 58-nJ and 33-fs pulses at 31-MHz repetition rate. We show that the GMN amplifier enables high-quality deep-tissue imaging, and furthermore that the broad spectral bandwidth of the GMN amplifier can be exploited for superior spectral resolution when imaging multiple distinct fluorophores.

Pulse train gating to improve signal generation for in vivo two-photon fluorescence microscopy

Significance

Two-photon microscopy is used routinely for in vivo imaging of neural and vascular structure and function in rodents with a high resolution. Image quality, however, often degrades in deeper portions of the cerebral cortex. Strategies to improve deep imaging are therefore needed. We introduce such a strategy using gates of high repetition rate ultrafast pulse trains to increase signal level.

Aim

We investigate how signal generation, signal-to-noise ratio (SNR), and signal-to-background ratio (SBR) improve with pulse gating while imaging in vivo mouse cerebral vasculature.

Approach

An electro-optic modulator is used with a high-power (6 W) 80 MHz repetition rate ytterbium fiber amplifier to create gates of pulses at a 1 MHz repetition rate. We first measure signal generation from a Texas Red solution in a cuvette to characterize the system with no gating and at a 50%, 25%, and 12.5% duty cycle. We then compare signal generation, SNR, and SBR when imaging Texas Red-labeled vasculature using these conditions.

Results

We find up to a 6.73-fold increase in fluorescent signal from a cuvette when using a 12.5% duty cycle pulse gating excitation pattern as opposed to a constant 80 MHz pulse train. We verify similar increases for in vivo imaging to that observed in cuvette testing. For deep imaging we find pulse gating to result in a 2.95-fold increase in SNR and a 1.37-fold increase in SBR on average when imaging mouse cortical vasculature at depths ranging from 950 μm to 1050 μm.

Conclusions

We demonstrate that a pulse gating strategy can either be used to limit heating when imaging superficial brain regions or used to increase signal generation in deep regions. These findings should encourage others to adopt similar pulse gating excitation schemes for imaging neural structure through two-photon microscopy.

Two-Photon Absorption: An Open Door to the NIR-II Biological Window?

The way in which photons travel through biological tissues and subsequently become scattered or absorbed is a key limitation for traditional optical medical imaging techniques using visible light. In contrast, near-infrared wavelengths, in particular those above 1000 nm, penetrate deeper in tissues and undergo less scattering and cause less photo-damage, which describes the so-called "second biological transparency window". Unfortunately, current dyes and imaging probes have severely limited absorption profiles at such long wavelengths, and molecular engineering of novel NIR-II dyes can be a tedious and unpredictable process, which limits access to this optical window and impedes further developments. Two-photon (2P) absorption not only provides convenient access to this window by doubling the absorption wavelength of dyes, but also increases the possible resolution. This review aims to provide an update on the available 2P instrumentation and 2P luminescent materials available for optical imaging in the NIR-II window.

Evaluation of resonant scanning as a high-speed imaging technique for two-photon imaging of cortical vasculature

We demonstrate a simple, low-cost two-photon microscope design with both galvo-galvo and resonant-galvo scanning capabilities. We quantify and compare the signal-to-noise ratios and imaging speeds of the galvo-galvo and resonant-galvo scanning modes when used for murine neurovascular imaging. The two scanning modes perform as expected under shot-noise limited detection and are found to achieve comparable signal-to-noise ratios. Resonant-galvo scanning is capable of reaching desired signal-to-noise ratios using less acquisition time when higher excitation power can be used. Given equal excitation power and total pixel dwell time between the two methods, galvo-galvo scanning outperforms resonant-galvo scanning in image quality when detection deviates from being shot-noise limited.

Aberration Correction to Optimize the Performance of Two-Photon Fluorescence Microscopy Using the Genetic Algorithm

Due to less light scattering and a better signal-to-noise ratio in deep imaging, two-photon fluorescence microscopy (TPFM) has been widely used in biomedical photonics since its advent. However, optical aberrations degrade the performance of TPFM in terms of the signal intensity and the imaging depth and therefore restrict its application. Here, we introduce adaptive optics based on the genetic algorithm to detect the distorted wavefront of the excitation laser beam and then perform aberration correction to optimize the performance of TPFM. By using a spatial light modulator as the wavefront controller, the correction phase is obtained through a signal feedback loop and a process of natural selection. The experimental results show that the signal intensity and imaging depth of TPFM are improved after aberration correction. Finally, the method was applied to two-photon fluorescence lifetime imaging, which helps to improve the signal-to-noise ratio and the accuracy of lifetime analysis. Furthermore, the method can also be implemented in other experiments, such as three-photon microscopy, light-sheet microscopy, and super-resolution microscopy.

Two-Photon Fluorescence Microscopy and Applications in Angiogenesis and Related Molecular Events

The role of angiogenesis in health and disease have gained considerable momentum in recent years. Visualizing angiogenic patterns and associated events of surrounding vascular beds in response to therapeutic and laboratory-grade biomolecules have become a commonplace in regenerative medicine and the biosciences. To aid imaging investigations in angiogenesis, the two-photon excitation fluorescence microscopy (2PEF), or multiphoton fluorescence microscopy is increasingly utilized in scientific investigations. The 2PEF microscope confers several distinct imaging advantages over other fluorescence excitation microscopy techniques - for the observation of in-depth, three-dimensional vascularity in a variety of tissue formats, including fixed tissue specimens and in vivo vasculature in live specimens. Understanding morphological and subcellular changes that occur in cells and tissues during angiogenesis will provide insights to behavioral responses in diseased states, advance the engineering of physiologically-relevant tissue models and provide biochemical clues for the design of therapeutic strategies. We review the applicability and limitations of the 2PEF microscope on the biophysical and molecular-level signatures of angiogenesis in various tissue models. Imaging techniques and strategies for best practices in 2PEF microscopy will be reviewed.

High-pulse-energy multiphoton imaging of neurons and oligodendrocytes in deep murine brain with a fiber laser

Here we demonstrate high-pulse-energy multiphoton microscopy (MPM) for intravital imaging of neurons and oligodendrocytes in the murine brain. Pulses with an order of magnitude higher energy (~ 10 nJ) were employed from a ytterbium doped fiber laser source at a 1-MHz repetition rate, as compared to the standard 80-MHz Ti:Sapphire laser. Intravital imaging was performed on mice expressing common fluorescent proteins, including green (GFP) and yellow fluorescent proteins (YFP), and TagRFPt. One fifth of the average power could be used for superior depths of MPM imaging, as compared to the Ti:Sapphire laser: A depth of ~ 860 µm was obtained by imaging the Thy1-YFP brain in vivo with 6.5 mW, and cortical myelin as deep as 400 µm ex vivo by intrinsic third-harmonic generation using 50 mW. The substantially higher pulse energy enables novel regimes of photophysics to be exploited for microscopic imaging. The limitation from higher order phototoxicity is also discussed.

jYCaMP: an optimized calcium indicator for two-photon imaging at fiber laser wavelengths

Femtosecond lasers at fixed wavelengths above 1,000 nm are powerful, stable and inexpensive, making them promising sources for two-photon microscopy. Biosensors optimized for these wavelengths are needed for both next-generation microscopes and affordable turn-key systems. Here we report jYCaMP1, a yellow variant of the calcium indicator jGCaMP7 that outperforms its parent in mice and flies at excitation wavelengths above 1,000 nm and enables improved two-color calcium imaging with red fluorescent protein-based indicators.

Si3N4 photonic integration platform at 1 µm for optical interconnects

Vertical-cavity surface-emitting lasers (VCSELs) are the predominant technology for high-speed short-range interconnects in data centers. Most short-range interconnects rely on GaAs-based multi-mode VCSELs and multi-mode fiber links operating at 850 nm. Recently, GaAs-based high-speed single-mode VCSELs at wavelengths > 1 µm have been demonstrated, which increases the interconnect reach using a single-mode fiber while maintaining low energy dissipation. If a suitable platform for passive wavelength- and space-multiplexing were developed in this wavelength range, this single-mode technology could deliver the multi-Tb/s interconnect capacity that will be required in future data centers. In this work, we show the first passive Si₃N₄ platform in the 1-µm band (1030-1075 nm) with an equivalent loss < 0.3 dB/cm, which is compatible with the system requirements of high-capacity interconnects. The waveguide structure is optimized to achieve simultaneously single-mode operation and low bending radius, and we demonstrate a wide range of high-performance building blocks, including arrayed waveguide gratings, Mach-Zehnder interferometers, splitters and low-loss fiber interfaces. This technology could be instrumental in scaling up the capacity and reducing the footprint of VCSEL-based optical interconnects and, thanks to the broad transparency in the near-infrared and compatibility with the Yb fiber amplifier window, enabling new applications in other domains as optical microscopy and nonlinear optics.

Two-photon microscope using a fiber-based approach for supercontinuum generation and light delivery to a small-footprint optical head

In this Letter, we report a low-cost, portable, two-photon excitation fluorescence microscopy imager that uses a fiber-based approach for both femtosecond supercontinuum (SC) generation and light delivery to the optical head. The SC generation is based on a tapered polarization-maintaining photonic crystal fiber that uses pre-chirped femtosecond narrowband pulses to generate a coherent SC spectrum with a bandwidth of approximately 300 nm. Using this approach, high-power, near-transform-limited, wavelength-selectable SC pulses are generated and directly delivered to the imaging optical head. Preliminary testing of this imager on brain slices is presented, demonstrating a high signal-to-noise ratio and sub-cellular imaging capabilities to a depth of approximately 200 µm. These results demonstrate the suitability of the technology for ex vivo and potentially in vivo cellular-level biomedical imaging applications.

Comparing the performance of a femto fiber-based laser and a Ti:sapphire used for multiphoton microscopy applications

Ti:sapphire laser systems are the more extended excitation sources in multiphoton (MP) microscopy. Although tunable, the cost, size, and lack of portability often limit their use in some research fields. Femtosecond fiber-based lasers represent an attractive alternative since they are portable, compact, and affordable. Most MP applications using these devices employ wavelengths beyond 1000 nm. This work evaluates the performance of a mode-locked fiber-based laser emitting at 780 nm (within the spectral region often used with Ti:sapphire devices) for use in MP imaging microscopy. MP images acquired with this laser system have been compared with those obtained with a "regular" solid-state source. Results herein show that the images recorded with both laser sources were similar, independently of the depth location of the imaged plane. The structural information contained in the images hardly differed. Moreover, the images of deeper layers improved by means of adaptive optics were also similar. These ultrafast laser sources are expected to enhance the impact of MP microscopy in basic research, as well as in biomedical environments.

Custom fabrication and mode-locked operation of a femtosecond fiber laser for multiphoton microscopy

Solid-state femtosecond lasers have stimulated the broad adoption of multiphoton microscopy in the modern laboratory. However, these devices remain costly. Fiber lasers offer promise as a means to inexpensively produce ultrashort pulses of light suitable for nonlinear microscopy in compact, robust and portable devices. Although encouraging, the initial methods reported in the biomedical engineering community to construct home-built femtosecond fiber laser systems overlooked fundamental aspects that compromised performance and misrepresented the significant financial and intellectual investments required to build these devices. Here, we present a practical protocol to fabricate an all-normal-dispersion ytterbium (Yb)-doped femtosecond fiber laser oscillator using commercially-available parts (plus standard optical components and extra-cavity accessories) as well as basic fiber splicing and laser pulse characterization equipment. We also provide a synthesis of established protocols in the laser physics community, but often overlooked in other fields, to verify true versus seemingly (partial or noise-like) mode-locked performance. The approaches described here make custom fabrication of femtosecond fiber lasers more accessible to a wide range of investigators and better represent the investments required for the proper laser design, fabrication and operation.

Toolbox for In Vivo Imaging of Host-Parasite Interactions at Multiple Scales

Animal models have for long been pivotal for parasitology research. Over the last few years, techniques such as intravital, optoacoustic and magnetic resonance imaging, optical projection tomography, and selective plane illumination microscopy developed promising potential for gaining insights into host-pathogen interactions by allowing different visualization forms in vivo and ex vivo. Advances including increased resolution, penetration depth, and acquisition speed, together with more complex image analysis methods, facilitate tackling biological problems previously impossible to study and/or quantify. Here we discuss advances and challenges in the in vivo imaging toolbox, which hold promising potential for the field of parasitology.

Polymer dots enable deep in vivo multiphoton fluorescence imaging of microvasculature

Deep in vivo imaging of vasculature requires small, bright, and photostable fluorophores suitable for multiphoton microscopy (MPM). Although semiconducting polymer dots (pdots) are an emerging class of highly fluorescent contrast agents with favorable advantages for the next generation of in vivo imaging, their use for deep MPM has never before been demonstrated. Herein, we characterize the multiphoton properties of three pdot variants and perform deep in vivo MPM imaging of cortical rodent microvasculature. We find pdot brightness exceeds conventional fluorophores, including quantum dots, and their broad multiphoton absorption spectrum permits imaging at wavelengths better-suited for biological imaging and confers compatibility with a range of longer excitation wavelengths. This results in substantial improvements in signal-to-background ratio (>3.5-fold) and greater cortical imaging depths (z = 1,300 µm). Ultimately, pdots are a versatile tool for MPM due to their extraordinary brightness and broad absorption, enabling interrogation of deep structures in vivo.

Labeling Microglia with Genetically Encoded Calcium Indicators

Genetically encoded calcium indicators (GECIs) have become widely used for Ca²⁺ imaging in cultured cells as well as in living organisms. Transduction of microglia with viral vectors encoding GECIs provides a convenient means to label microglia for in vivo Ca²⁺ imaging. We describe a method using microglia-specific microRNA-9-regulated viral vector, to label microglial cells with a ratiometric GECI (Twitch-2B). This method enables longitudinal recording of both transient and sustained elevations of Ca²⁺ in microglia in live animals.

Designing a large field-of-view two-photon microscope using optical invariant analysis

Conventional two-photon microscopy (TPM) is capable of imaging neural dynamics with subcellular resolution, but it is limited to a field-of-view (FOV) diameter [Formula: see text]. Although there has been recent progress in extending the FOV in TPM, a principled design approach for developing large FOV TPM (LF-TPM) with off-the-shelf components has yet to be established. Therefore, we present a design strategy that depends on analyzing the optical invariant of commercially available objectives, relay lenses, mirror scanners, and emission collection systems in isolation. Components are then selected to maximize the space-bandwidth product of the integrated microscope. In comparison with other LF-TPM systems, our strategy simplifies the sequence of design decisions and is applicable to extending the FOV in any microscope with an optical relay. The microscope we constructed with this design approach can image [Formula: see text] lateral and [Formula: see text] axial resolution over a 7-mm diameter FOV, which is a 100-fold increase in FOV compared with conventional TPM. As a demonstration of the potential that LF-TPM has on understanding the microarchitecture of the mouse brain across interhemispheric regions, we performed in vivo imaging of both the cerebral vasculature and microglia cell bodies over the mouse cortex.

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.

In situ high-resolution thermal microscopy on integrated circuits

The miniaturization of metal tracks in integrated circuits (ICs) can cause abnormal heat dissipation, resulting in electrostatic discharge, overvoltage breakdown, and other unwanted issues. Unfortunately, locating areas of abnormal heat dissipation is limited either by the spatial resolution or imaging acquisition speed of current thermal analytical techniques. A rapid, non-contact approach to the thermal imaging of ICs with sub-μm resolution could help to alleviate this issue. In this work, based on the intensity of the temperature-dependent two-photon fluorescence (TPF) of Rhodamine 6G (R6G) material, we developed a novel fast and non-invasive thermal microscopy with a sub-μm resolution. Its application to the location of hotspots that may evolve into thermally induced defects in ICs was also demonstrated. To the best of our knowledge, this is the first study to present high-resolution 2D thermal microscopic images of ICs, showing the generation, propagation, and distribution of heat during its operation. According to the demonstrated results, this scheme has considerable potential for future in situ hotspot analysis during the optimization stage of IC development.

Optimizing pulse compressibility in completely all-fibered Ytterbium chirped pulse amplifiers for in vivo two photon laser scanning microscopy

A simple and completely all-fiber Yb chirped pulse amplifier that uses a dispersion matched fiber stretcher and a spliced-on hollow core photonic bandgap fiber compressor is applied in nonlinear optical microscopy. This stretching-compression approach improves compressibility and helps to maximize the fluorescence signal in two-photon laser scanning microscopy as compared with approaches that use standard single mode fibers as stretcher. We also show that in femtosecond all-fiber systems, compensation of higher order dispersion terms is relevant even for pulses with relatively narrow bandwidths for applications relying on nonlinear optical effects. The completely all-fiber system was applied to image green fluorescent beads, a stained lily-of-the-valley root and rat-tail tendon. We also demonstrated in vivo imaging in zebrafish larvae, where we simultaneously measure second harmonic and fluorescence from two-photon excited red-fluorescent protein. Since the pulses are compressed in a fiber, this source is especially suited for upgrading existing laser scanning (confocal) microscopes with multiphoton imaging capabilities in space restricted settings or for incorporation in endoscope-based microscopy.

Multiphoton in vivo imaging with a femtosecond semiconductor disk laser

We use an ultrafast diode-pumped semiconductor disk laser (SDL) to demonstrate several applications in multiphoton microscopy. The ultrafast SDL is based on an optically pumped Vertical External Cavity Surface Emitting Laser (VECSEL) passively mode-locked with a semiconductor saturable absorber mirror (SESAM) and generates 170-fs pulses at a center wavelength of 1027 nm with a repetition rate of 1.63 GHz. We demonstrate the suitability of this laser for structural and functional multiphoton in vivo imaging in both Drosophila larvae and mice for a variety of fluorophores (including mKate2, tdTomato, Texas Red, OGB-1, and R-CaMP1.07) and for endogenous second-harmonic generation in muscle cell sarcomeres. We can demonstrate equivalent signal levels compared to a standard 80-MHz Ti:Sapphire laser when we increase the average power by a factor of 4.5 as predicted by theory. In addition, we compare the bleaching properties of both laser systems in fixed Drosophila larvae and find similar bleaching kinetics despite the large difference in pulse repetition rates. Our results highlight the great potential of ultrafast diode-pumped SDLs for creating a cost-efficient and compact alternative light source compared to standard Ti:Sapphire lasers for multiphoton imaging.

In vivo multiphoton imaging of a diverse array of fluorophores to investigate deep neurovascular structure

We perform high-resolution, non-invasive, in vivo deep-tissue imaging of the mouse neocortex using multiphoton microscopy with a high repetition rate optical parametric amplifier laser source tunable between λ=1,100 and 1,400 nm. By combining the high repetition rate (511 kHz) and high pulse energy (400 nJ) of our amplifier laser system, we demonstrate imaging of vasculature labeled with Texas Red and Indocyanine Green, and neurons expressing tdTomato and yellow fluorescent protein. We measure the blood flow speed of a single capillary at a depth of 1.2 mm, and image vasculature to a depth of 1.53 mm with fine axial steps (5 μm) and reasonable acquisition times. The high image quality enabled analysis of vascular morphology at depths to 1.45 mm.

In vivo deep two-photon imaging of neural circuits with the fluorescent Ca2+ indicator Cal-590

In vivo two-photon Ca2+ imaging has become an effective approach for the functional analysis of neuronal populations, individual neurons and subcellular neuronal compartments in the intact brain. When imaging individually labelled neurons, depth penetration can often reach up to 1 mm below the cortical surface. However, for densely labelled neuronal populations, imaging with single-cell resolution is largely restricted to the upper cortical layers in the mouse brain. Here, we review recent advances of deep two-photon Ca2+ imaging and the use of red-shifted fluorescent Ca2+ indicators as a promising strategy to increase the imaging depth, which takes advantage of reduced photon scattering at their long excitation and emission wavelengths. We describe results showing that the newly introduced fluorescent Ca2+ -sensitive dye Cal-590 can be used to record in vivo neuronal activity in isolated cortical neurons and in neurons within populations in depths of up to 900 μm below the pial surface. Thus, the new approach allows the comprehensive functional mapping of all six cortical layers of the mouse brain. Specific features of Cal-590-based in vivo Ca2+ two-photon imaging include a good signal-to-noise ratio, fast kinetics and a linear dependence of the Ca2+ transients on the number of action potentials. Another area of application is dual-colour imaging by combining Cal-590 with other, shorter wavelength Ca2+ indicators such as OGB-1. Overall, Cal-590-based two-photon microscopy emerges as a promising tool for the recording of neuronal activity at depths that were previously inaccessible to functional imaging of neuronal circuits.

Two-color multiphoton in vivo imaging with a femtosecond diamond Raman laser

Two-color multiphoton microscopy through wavelength mixing of synchronized lasers has been shown to increase the spectral window of excitable fluorophores without the need for wavelength tuning. However, most currently available dual output laser sources rely on the costly and complicated optical parametric generation approach. In this report, we detail a relatively simple and low cost diamond Raman laser pumped by a ytterbium fiber amplifier emitting at 1055 nm, which generates a first Stokes emission centered at 1240 nm with a pulse width of 100 fs. The two excitation wavelengths of 1055 and 1240 nm, along with the effective two-color excitation wavelength of 1140 nm, provide an almost complete coverage of fluorophores excitable within the range of 1000-1300 nm. When compared with 1055 nm excitation, two-color excitation at 1140 nm offers a 90% increase in signal for many far-red emitting fluorescent proteins (for example, tdKatushka2). We demonstrate multicolor imaging of tdKa-tushka2 and Hoechst 33342 via simultaneous two-color two-photon, and two-color three-photon microscopy in engineered 3D multicellular spheroids. We further discuss potential benefits and applications for two-color three-photon excitation. In addition, we show that this laser system is capable of in vivo imaging in mouse cortex to nearly 1 mm in depth with two-color excitation.

Compact diode laser source for multiphoton biological imaging

We demonstrate a compact, pulsed diode laser source suitable for multiphoton microscopy of biological samples. The center wavelength is 976 nm, near the peak of the two-photon cross section of common fluorescent markers such as genetically encoded green and yellow fluorescent proteins. The laser repetition rate is electrically tunable between 66.67 kHz and 10 MHz, with 2.3 ps pulse duration and peak powers >1 kW. The laser components are fiber-coupled and scalable to a compact package. We demonstrate >600 μm depth penetration in brain tissue, limited by laser power.

Improved deep two-photon calcium imaging in vivo

Two-photon laser scanning calcium imaging has emerged as a useful method for the exploration of neural function and structure at the cellular and subcellular level in vivo. The applications range from imaging of subcellular compartments such as dendrites, spines and axonal boutons up to the functional analysis of large neuronal or glial populations. However, the depth penetration is often limited to a few hundred micrometers, corresponding, for example, to the upper cortical layers of the mouse brain. Light scattering and aberrations originating from refractive index inhomogeneties of the tissue are the reasons for these limitations. The depth penetration of two-photon imaging can be enhanced through various approaches, such as the implementation of adaptive optics, the use of three-photon excitation and/or labeling cells with red-shifted genetically encoded fluorescent sensors. However, most of the approaches used so far require the implementation of new instrumentation and/or time consuming staining protocols. Here we present a simple approach that can be readily implemented in combination with standard two-photon microscopes. The method involves an optimized protocol for depth-restricted labeling with the red-shifted fluorescent calcium indicator Cal-590 and benefits from the use of ultra-short laser pulses. The approach allows in vivo functional imaging of neuronal populations with single cell resolution in all six layers of the mouse cortex. We demonstrate that stable recordings in deep cortical layers are not restricted to anesthetized animals but are well feasible in awake, behaving mice. We anticipate that the improved depth penetration will be beneficial for two-photon functional imaging in larger species, such as non-human primates.