Compact fs ytterbium fiber laser at 1010 nm for biomedical applications

Information-Distilled Generative Label-Free Morphological Profiling Encodes Cellular Heterogeneity

Image-based cytometry faces challenges due to technical variations arising from different experimental batches and conditions, such as differences in instrument configurations or image acquisition protocols, impeding genuine biological interpretation of cell morphology. Existing solutions, often necessitating extensive pre-existing data knowledge or control samples across batches, have proved limited, especially with complex cell image data. To overcome this, "Cyto-Morphology Adversarial Distillation" (CytoMAD), a self-supervised multi-task learning strategy that distills biologically relevant cellular morphological information from batch variations, is introduced to enable integrated analysis across multiple data batches without complex data assumptions or extensive manual annotation. Unique to CytoMAD is its "morphology distillation", symbiotically paired with deep-learning image-contrast translation-offering additional interpretable insights into label-free cell morphology. The versatile efficacy of CytoMAD is demonstrated in augmenting the power of biophysical imaging cytometry. It allows integrated label-free classification of human lung cancer cell types and accurately recapitulates their progressive drug responses, even when trained without the drug concentration information. CytoMAD  also allows joint analysis of tumor biophysical cellular heterogeneity, linked to epithelial-mesenchymal plasticity, that standard fluorescence markers overlook. CytoMAD can substantiate the wide adoption of biophysical cytometry for cost-effective diagnosis and screening.

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.

Multiphoton excitation imaging via an actively mode-locked tunable fiber-cavity SOA laser around 800 nm

In this study, an active mode-locked tunable pulsed laser (AML-TPL) is proposed to excite picosecond pulsed light with a rapid wavelength tunability of approximately 800 nm for multiphoton microscopy. The AML-TPL is schematically based on a fiber-cavity semiconductor optical amplifier (SOA) configuration to implement a robust and align-free pulsed light source with a duration of 1.6 ps, a repetition rate of 27.9271 MHz, and average output power of over 600 mW. A custom-built multiphoton imaging system was also built to demonstrate the imaging performance of the proposed AML-TPL by comparing with the commercial Ti:Sapphire femtosecond laser. Two-photon excited fluorescence images were successfully acquired using a human breast cancer cell line (MDA-MB-231) stained with acridine orange.

Passively mode locked thulium and thulium/holmium doped fiber lasers using MXene Nb2C coated microfiber

As a result of the emergence of two-dimensional (2D) materials for various opto-electronics applications, a new class of materials named MXenes have been attracting interests due to their outstanding nonlinear properties. In this work, an MXene niobium carbide (Nb2C) was proposed and demonstrated as a saturable absorber to induce mode-locking in thulium- and thulium/holmium-doped fiber lasers. The Nb2C solution was first prepared using the liquid exfoliation technique, and then deposited onto a microfiber for integration into the laser cavity. Stable mode-locking operation was observed in both laser cavities, where the center wavelengths of the laser were recorded at 1944 nm for the TDFL and 1950 nm for the THDFL. The generated pulses in the TDFL and THDFL had repetition rates of 9.35 and 11.76 MHz respectively, while their corresponding pulse widths were 1.67 and 1.34 ps. Both of the lasers were highly stable, having SNR values of more than 52 dB and showed no major fluctuations when tested for their long-term stabilities. The results demonstrate an excellent performance of the Nb2C as a saturable absorber, offering opportunities to further explore MXenes for future photonics devices.

Multiscale and Multimodal Optical Imaging of the Ultrastructure of Human Liver Biopsies

The liver as the largest organ in the human body is composed of a complex macroscopic and microscopic architecture that supports its indispensable function to maintain physiological homeostasis. Optical imaging of the human liver is particularly challenging because of the need to cover length scales across 7 orders of magnitude (from the centimeter scale to the nanometer scale) in order to fully assess the ultrastructure of the entire organ down to the subcellular scale and probe its physiological function. This task becomes even more challenging the deeper within the organ one hopes to image, because of the strong absorption and scattering of visible light by the liver. Here, we demonstrate how optical imaging methods utilizing highly specific fluorescent labels, as well as label-free optical methods can seamlessly cover this entire size range in excised, fixed human liver tissue and we exemplify this by reconstructing the biliary tree in three-dimensional space. Imaging of tissue beyond approximately 0.5 mm length requires optical clearing of the human liver. We present the successful use of optical projection tomography and light-sheet fluorescence microscopy to derive information about the liver architecture on the millimeter scale. The intermediate size range is covered using label-free structural and chemically sensitive methods, such as second harmonic generation and coherent anti-Stokes Raman scattering microscopy. Laser-scanning confocal microscopy extends the resolution to the nanoscale, allowing us to ultimately image individual liver sinusoidal endothelial cells and their fenestrations by super-resolution structured illumination microscopy. This allowed us to visualize the human hepatobiliary system in 3D down to the cellular level, which indicates that reticular biliary networks communicate with portal bile ducts via single or a few ductuli. Non-linear optical microscopy enabled us to identify fibrotic regions extending from the portal field to the parenchyma, along with microvesicular steatosis in liver biopsies from an older patient. Lastly, super-resolution microscopy allowed us to visualize and determine the size distribution of fenestrations in human liver sinusoidal endothelial cells for the first time under aqueous conditions. Thus, this proof-of-concept study allows us to demonstrate, how, in combination, these techniques open up a new chapter in liver biopsy analysis.

All-polarization-maintaining, all-normal-dispersion mode-locked fiber laser with spectral filtering in a nonlinear optical loop mirror

The spectral filtering effect is essential to dissipative dynamics in an all-normal-dispersion (ANDi) mode-locked fiber laser. In this study, we numerically and experimentally demonstrate the spectral filtering process of a nonlinear optical loop mirror (NOLM). Taking advantage of the 40/60 NOLM's spectral filtering ability, we designed a novel all-polarization-maintaining ANDi mode-locked fiber laser without using a separate spectral filter. The NOLM functions as an artificial saturable absorber and a spectral filter in an ANDi cavity. During mode locking, we observed that the NOLM decreased the spectral width of the pulse from 5.46 to 4.38 nm. The fiber laser generated 509-fs compressed pulses at the repetition rate of 13.4 MHz. Our work provides a promising novel and compact ANDi fiber laser for ultrafast photonic applications.

Depth-resolved volumetric two-photon microscopy based on dual Airy beam scanning

We demonstrate dual-Airy-beam-scanning-based volumetric two-photon microscopy (TPM) with depth-resolving capability. A pair of Airy beams with opposite acceleration is used as the excitation lights to sequentially illuminate the sample, and depth information can be resolved based on the deflection of the Airy beam. The depth-resolving range of the volumetric TPM is up to 32 μm. The advantages of the depth-resolved volumetric TPM are the depth-resolving capability over Bessel-beam-based TPM and less scanning times over traditional Gaussian-beam-based TPM. The depth-resolved volumetric TPM provides a promising fast imaging tool to study the dynamics in neural biology.

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.