Ultrafast optical pulse delivery with fibers for nonlinear microscopy

Three stage HCF fabrication technique for high yield, broadband UV-visible fibers

Hollow-core optical fibers can offer broadband, single mode guidance in the UV-visible-NIR wavelength range, with the potential for low-loss, solarization-free operation, making them desirable and potentially disruptive for a wide range of applications. To achieve this requires the fabrication of fibers with <300nm anti-resonant membranes, which is technically challenging. Here we investigate the underlying fluid dynamics of the fiber fabrication process and demonstrate a new three-stage fabrication approach, capable of delivering long (∼350m) lengths of fiber with the desired thin-membranes.

Biocompatible diameter-oscillating fiber with microlens endface

Optical fibers have been widely applied to life science, such as laser delivering, fluorescence collection, biosensing, bioimaging, etc. To resolve the challenges of advanced multiphoton biophotonic applications utilizing ultrashort laser pulses, here we report a flexible diameter-oscillating fiber (DOF) with microlens endface fabricated by using Polydimethylsiloxane (PDMS) elastomers. The diameter of the DOF is designed to longitudinally vary for providing accurate dispersion management, which is important for near-infrared multiphoton biophotonics that usually involves ultrashort laser pulses. The variation range and period of the DOF's diameter can be flexibly adjusted by controlling the parameters during the fabrication, such that dispersion curves with different oscillation landscapes can be obtained. The dispersion oscillating around the zero-dispersion baseline gives rise to a minimized net dispersion as the ultrashort laser pulse passes through the DOF - reducing the temporal broadening effect and resulting in transform-limited pulsewidth. In addition, the endface of the DOF is fabricated with a microlens, which is especially useful for laser scanning/focusing and fluorescence excitation. It is anticipated that this new biocompatible DOF is of great interest for biophotonic applications, particularly multiphoton microscopy deep inside biological tissues.

Original Solution of Coupled Nonlinear Schrödinger Equations for Simulation of Ultrashort Optical Pulse Propagation in a Birefringent Fiber

This paper discusses approaches to the numerical integration of the coupled nonlinear Schrödinger equations system, different from the generally accepted approach based on the method of splitting according to physical processes. A combined explicit/implicit finite-difference integration scheme based on the implicit Crank–Nicolson finite-difference scheme is proposed and substantiated. It allows the integration of a nonlinear system of equations with a choice of nonlinear terms from the previous integration step. The main advantages of the proposed method are: its absolute stability through the use of an implicit finite-difference integration scheme and an integrated mechanism for refining the numerical solution at each step; integration with automatic step selection; performance gains (or resolutions) up to three or more orders of magnitude due to the fact that there is no need to produce direct and inverse Fourier transforms at each integration step, as is required in the method of splitting according to physical processes. An additional advantage of the proposed method is the ability to calculate the interaction with an arbitrary number of propagation modes in the fiber.

A Dual Hollow Core Antiresonant Optical Fiber Coupler Based on a Highly Birefringent Structure-Numerical Design and Analysis

With the growing interest in hollow-core antiresonant fibers (HC-ARF), attributed to the development of their fabrication technology, the appearance of more sophisticated structures is understandable. One of the recently advancing concepts is that of dual hollow-core antiresonant fibers, which have the potential to be used as optical fiber couplers. In the following paper, a design of a dual hollow-core antiresonant fiber (DHC-ARF) acting as a polarization fiber coupler is presented. The structure is based on a highly birefringent hollow-core fiber design, which is proven to be a promising solution for the purpose of propagation of polarized signals. The design of an optimized DHC-ARF with asymmetrical cores is proposed, together with analysis of its essential coupling parameters, such as the extinction ratio, coupling length ratio, and coupling strength. The latter two for the x- and y-polarized signals were ~2 and 1, respectively, while the optical losses were below 0.3 dB/cm in the 1500–1700 nm transmission band.

Optical plasmon nanostrip probe as an effective ultrashort pulse delivery system

In this paper, we analyze the ultrafast temporal and spectral responses of optical fields in tapered and metalized optical fibers (MOFs) and optical plasmon nanostrip probes (NPs). Computational experiment shows that output pulses of the NPs are virtually unchanged in shape and duration for input pulses with a duration of >1 fs and are not sensitive to changes in the parameters of the probe (such as convergence angle and taper length), while local enhancement of the electric field intensity reaches 300 times at the NP apex. Compared with the NPs, MOFs lead to significant output pulse distortions, even for input pulses with a duration of 10 fs. In addition, the temporal response at the MOF apex is critically sensitive to changes in MOF parameters and cannot provide any significant local enhancement of the electric field. These findings reveal the high potential of optical plasmon nanostrip probes as an ultrashort pulse delivery system to nanometer-size areas and indicate that its usage can be promising for a wide variety of techniques studying ultrafast processes in nanoscopic volumes.

Three dimensional two-photon brain imaging in freely moving mice using a miniature fiber coupled microscope with active axial-scanning

We present a miniature head mounted two-photon fiber-coupled microscope (2P-FCM) for neuronal imaging with active axial focusing enabled using a miniature electrowetting lens. We show three-dimensional two-photon imaging of neuronal structure and record neuronal activity from GCaMP6s fluorescence from multiple focal planes in a freely-moving mouse. Two-color simultaneous imaging of GFP and tdTomato fluorescence is also demonstrated. Additionally, dynamic control of the axial scanning of the electrowetting lens allows tilting of the focal plane enabling neurons in multiple depths to be imaged in a single plane. Two-photon imaging allows increased penetration depth in tissue yielding a working distance of 450 μm with an additional 180 μm of active axial focusing. The objective NA is 0.45 with a lateral resolution of 1.8 μm, an axial resolution of 10 μm, and a field-of-view of 240 μm diameter. The 2P-FCM has a weight of only ~2.5 g and is capable of repeatable and stable head-attachment. The 2P-FCM with dynamic axial scanning provides a new capability to record from functionally distinct neuronal layers, opening new opportunities in neuroscience research.

In vivo multiphoton tomography and fluorescence lifetime imaging of human brain tumor tissue

High resolution multiphoton tomography and fluorescence lifetime imaging differentiates glioma from adjacent brain in native tissue samples ex vivo. Presently, multiphoton tomography is applied in clinical dermatology and experimentally. We here present the first application of multiphoton and fluorescence lifetime imaging for in vivo imaging on humans during a neurosurgical procedure. We used a MPTflex™ Multiphoton Laser Tomograph (JenLab, Germany). We examined cultured glioma cells in an orthotopic mouse tumor model and native human tissue samples. Finally the multiphoton tomograph was applied to provide optical biopsies during resection of a clinical case of glioblastoma. All tissues imaged by multiphoton tomography were sampled and processed for conventional histopathology. The multiphoton tomograph allowed fluorescence intensity- and fluorescence lifetime imaging with submicron spatial resolution and 200 picosecond temporal resolution. Morphological fluorescence intensity imaging and fluorescence lifetime imaging of tumor-bearing mouse brains and native human tissue samples clearly differentiated tumor and adjacent brain tissue. Intraoperative imaging was found to be technically feasible. Intraoperative image quality was comparable to ex vivo examinations. To our knowledge we here present the first intraoperative application of high resolution multiphoton tomography and fluorescence lifetime imaging of human brain tumors in situ. It allowed in vivo identification and determination of cell density of tumor tissue on a cellular and subcellular level within seconds. The technology shows the potential of rapid intraoperative identification of native glioma tissue without need for tissue processing or staining.

Two-photon excitation nanoparticles for photodynamic therapy

Integration of the two-photon excitation (TPE) technique and nanomaterials to construct TPE nanoparticle-based photosensitizers for PDT is summarized and reviewed.

Improving femtosecond laser pulse delivery through a hollow core photonic crystal fiber for temporally focused two-photon endomicroscopy

In this paper, we present a strategy to improve delivery of femtosecond laser pulses from a regenerative amplifier through a hollow core photonic crystal fiber for temporally focused wide-field two-photon endomicroscopy. For endomicroscope application, wide-field two-photon excitation has the advantage of requiring no scanning in the distal end. However, wide-field two-photon excitation requires peak power that is 10(4)-10(5) times higher than the point scanning approach corresponding to femtosecond pulses with energy on the order of 1-10 μJ at the specimen plane. The transmission of these high energy pulses through a single mode fiber into the microendoscope is a significant challenge. Two approaches were pursued to partially overcome this limitation. First, a single high energy pulse is split into a train of pulses with energy below the fiber damage threshold better utilizing the available laser energy. Second, stretching the pulse width in time by introducing negative dispersion was shown to have the dual benefit of reducing fiber damage probability and compensating for the positive group velocity dispersion induced by the fiber. With these strategy applied, 11 fold increase in the two photon excitation signal has been demonstrated.

Fluorescence lifetime imaging microscopy using near-infrared contrast agents

Although single-photon fluorescence lifetime imaging microscopy (FLIM) is widely used to image molecular processes using a wide range of excitation wavelengths, the captured emission of this technique is confined to the visible spectrum. Here, we explore the feasibility of utilizing near-infrared (NIR) fluorescent molecular probes with emission >700 nm for FLIM of live cells. The confocal microscope is equipped with a 785 nm laser diode, a red-enhanced photomultiplier tube, and a time-correlated single photon counting card. We demonstrate that our system reports the lifetime distributions of NIR fluorescent dyes, cypate and DTTCI, in cells. In cells labelled separately or jointly with these dyes, NIR FLIM successfully distinguishes their lifetimes, providing a method to sort different cell populations. In addition, lifetime distributions of cells co-incubated with these dyes allow estimate of the dyes' relative concentrations in complex cellular microenvironments. With the heightened interest in fluorescence lifetime-based small animal imaging using NIR fluorophores, this technique further serves as a bridge between in vitro spectroscopic characterization of new fluorophore lifetimes and in vivo tissue imaging.

Multiphoton microscopy with near infrared contrast agents

While multiphoton microscopy (MPM) has been performed with a wide range of excitation wavelengths, fluorescence emission has been limited to the visible spectrum. We introduce a paradigm for MPM of near-infrared (NIR) fluorescent molecular probes via nonlinear excitation at 1550 nm. This all-NIR system expands the range of available MPM fluorophores, virtually eliminates background autofluorescence, and allows for use of fiber-based, turnkey ultrafast lasers developed for telecommunications.

Fiber delivered probe for efficient CARS imaging of tissues

We demonstrate a fiber-based probe for maximum collection of the coherent anti-Stokes Raman scattering (CARS) signal in biological tissues. We discuss the design challenges including capturing the backscattered forward generated CARS signal in the sample and the effects of fiber nonlinearities on the propagating pulses. Three different single mode fibers (fused silica fiber, photonic crystal fiber and double-clad photonic crystal fiber) were tested for the probe design. We investigated self-phase modulation, stimulated Raman scattering (SRS) and four-wave-mixing (FWM) generation in the fiber: nonlinear processes expected to occur in a two-beam excitation based probe. While SPM and SRS induced spectral broadening was negligible, a strong non phase-matched FWM contribution was found to be present in all the tested fibers for excitation conditions relevant to CARS microscopy of tissues. To spectrally suppress this strong contribution, the pro design incorporates separate fibers for excitation light delivery and for signal detection, in combination with dichroic optics. CARS images of the samples were recorded by collecting the back-scattered forward generated CARS signal in the sample through a multi-mode fiber. Different biological tissues were imaged ex vivo in order to assess the performance of our fiber-delivered probe for CARS imaging, a tool which we consider an important advance towards label-free, in vivo probing of superficial tissues.