Nathalie McCarthy

Two-photon excitation fluorescence microscopy with a high depth of field using an axicon

In conventional two-photon excitation fluorescence microscopy, the numerical aperture of the objective determines the lateral resolution and the depth of field. In some situations, as with functional imaging of dynamic events distributed in live biological tissue, an improved temporal resolution is needed; as a consequence, it is imperative to use optics with a high depth of field to simultaneously image objects at different axial positions. With a conventional microscope objective, increasing the depth of field is achieved at the expense of lateral resolution. To overcome this limitation, we have incorporated an axicon in a two-photon excitation fluorescence microscopy system; measurements have shown that an axicon provides a depth of field in excess of a millimeter, while the lateral resolution is maintained at the micrometer scale. Thus axicon-based two-photon microscopy has been shown to yield a high-resolution projection image of a sample with a single 2D scan of the laser beam while maintaining the improved tissue penetration typical of two-photon microscopy.

Single transverse mode oscillation from an unstable resonator Nd:YAG laser using a variable reflectivity mirror

Abstract Single transverse mode pulses were obtained from a Nd:YAG laser using an output mirror with a parabolic reflectivity profile in an unstable Cassegrain resonator. The experimental results show a smooth near-field beam intensity distribution and a narrow, single lobe far-field pattern. Output pulses of up to 150 mJ have been obtained with a 4 mm rod.

Effects of hard apertures on mode properties of resonators with gaussian reflectivity mirrors

The effects of hard apertures on the energy distribution, the far-field beam quality, the mode volume, and the discrimination against high-order modes in Cassegrain resonators with Gaussian reflectivity mirrors have been investigated both theoretically and experimentally. It has been shown that, in the far field, the fraction of energy in the secondary lobes remains small as long as the ratio of the design beam waist to the radius of the gain medium w_{b}/a is w_{b}/a > 0.7 , deterioration of the beam quality and diffraction losses counterbalanced a more efficient filling of the gain medium to limit the resonator efficiency. At low magnifications, resonators with Gaussian mirrors out perform resonators with Standard mirrors.

Numerical procedure for the lateral-mode analysis of broad-area semiconductor lasers with an external cavity

A numerical procedure for the investigation of the lateral modes of semiconductor lasers with an external cavity is described. The propagation of the optical field inside the semiconductor laser is carried out via a standard beam-propagation scheme, while the method of coordinate scaling with the generalized Huygens-Fresnel integral allows for a computationally efficient propagation of the field in the external cavity. Using an eigenfunction solver based on the Prony method, it is shown that a simple external-cavity configuration comprising a broad area laser, a collimation lens, and a uniform end reflector can exhibit a complex modal behavior. Compared to a solitary broad area laser, optimized external cavities can yield enhanced modal discriminations, resulting in a single-lateral-mode operation with output power in excess of 100 mW along with a clean, single-lobed far-field pattern. The spatial-filtering action of the broad-area laser is highlighted, and we also discuss of the role played by the wavefront curvature of the beam incident upon the broad-area laser for achieving an efficient suppression of the higher order modes along with minimum increase of the threshold current. >

Extended depth of field microscopy for rapid volumetric two-photon imaging

Two-photon fluorescence microscopy is an influential tool in biology, providing valuable information on the activity of cells deep inside the tissue. However, it is limited by its low speed for imaging volume samples. Here we present the design of a two-photon scanning microscope with an extended and adjustable depth of field, which improves the temporal resolution for sampling thick samples. Moreover, this method implies no loss of optical power and resolution, and can be easily integrated into most commercial laser-scanning microscopy systems. We demonstrate experimentally the gain in performance of the system by comparing volumetric scans of neuronal structures with a standard versus an extended depth of field system.

Extended two-photon microscopy in live samples with Bessel beams: steadier focus, faster volume scans, and simpler stereoscopic imaging

Two-photon microscopy has revolutionized functional cellular imaging in tissue, but although the highly confined depth of field (DOF) of standard set-ups yields great optical sectioning, it also limits imaging speed in volume samples and ease of use. For this reason, we recently presented a simple and retrofittable modification to the two-photon laser-scanning microscope which extends the DOF through the use of an axicon (conical lens). Here we demonstrate three significant benefits of this technique using biological samples commonly employed in the field of neuroscience. First, we use a sample of neurons grown in culture and move it along the z-axis, showing that a more stable focus is achieved without compromise on transverse resolution. Second, we monitor 3D population dynamics in an acute slice of live mouse cortex, demonstrating that faster volumetric scans can be conducted. Third, we acquire a stereoscopic image of neurons and their dendrites in a fixed sample of mouse cortex, using only two scans instead of the complete stack and calculations required by standard systems. Taken together, these advantages, combined with the ease of integration into pre-existing systems, make the extended depth-of-field imaging based on Bessel beams a strong asset for the field of microscopy and life sciences in general.

Apodizing holographic gratings for the modal control of semiconductor lasers

We first present the fabrication technique of apodizing holographic gratings. Gratings with a spatially variable reflectivity profile were obtained by the interference of two Gaussian beams on a glass plate covered with a photoresist. When the exposure time was short enough to avoid saturation of the photoresist, gratings with a quasi-Gaussian reflectivity profile for the beam reflected in the -1 order were produced; the reflectivity at the center could be as high as 71%, and the half-width of the reflectivity profile at the e-1 position could be as small as 180 µm. Apodizing gratings were used as the end mirror of the external cavity of a broad-area semiconductor laser. Single longitudinal- and lateral-mode operation was observed over the full range of allowed injection currents.

Influence of the axially varying quasi‐Fermi‐level separation of the active region on spatial hole burning in distributed‐feedback semiconductor lasers

The longitudinal‐mode characteristics of distributed‐feedback semiconductor lasers subjected to longitudinal spatial hole burning have been investigated using an improved numerical modeling scheme. The main new feature of the model is that it allows for the natural axial variations of the separation between the quasi‐Fermi levels (Fermi voltage) in the laser’s active region. This gives rise to a current density injected into the active region that varies along the laser axis, even for uniformly biased lasers. It is found that compared to the results obtained by assuming an uniform current density, the detrimental influence of the longitudinal spatial hole burning on important static characteristics of quarter‐wave‐shifted distributed‐feedback lasers, such as the gain margin and the lasing wavelength stability, is weakened. Therefore, the usual assumption of an uniform injected current density gives rise to an overestimated influence of the spatial hole burning, this overestimation being more important for...

Improvement of the lateral-mode discrimination of broad-area diode lasers with a profiled reflectivity output facet

The emission of a broad-area laser always contains several lateral modes (along the junction plane) even at low drive levels. To increase the discrimination against high-order lateral modes, we developed simple techniques for depositing a profiled thin layer of SiO on the output facet of a broad-area laser that has a 75-μm-wide injection current stripe. The profiled coating provided a nearly Gaussian reflectivity in the lateral direction (parallel to the junction plane). The resolved near-field spectra of the uncoated and coated lasers have been compared. The maximum output power in the single-lateral-mode regime was pushed to 25 mW with the profiled coating, compared with a corresponding power of less than 1 mW before deposition. Experimental results have confirmed the behavior predicted by our numerical simulations. This method is scalable to higher-power lasers.

Noise reduction in a synchronously-pumped picosecond dye laser with a phase-conjugate mirror

Abstract Noise reduction by more than 35 dB in the megahertz range has been observedin a synchronously-pumped mode-locked dye laser with a phase-conjugating external cavity. The noise reduction is accompanied by a lengthening of the laser pulse. Both effects occur for external cavities shorter than the laser cavity and for a wide range of cavity detuning. The phase-conjugate mirror offers the advantages of self-alignment of the external cavity and the ease to measure exactly the effective feedback level.