CPA-ready femtosecond pulses at 1 MHz from a custom recycled output Mamyshev oscillator.

A cost-effective fiber laser architecture is introduced in which the output seed pulse is stretched and then returned in the oscillator for an additional single-pass amplification without spectral broadening. It is implemented in an all-PM-fiber configuration based on a Mamyshev oscillator with a low repetition rate of 1 MHz. It features a linear oscillator bounded by two offset chirped fiber Bragg gratings accompanied by a third one acting as a pulse recycling filter. The latter tailors the pulse profile in amplitude and phase to seed femtosecond chirped-pulse amplification systems without additional pre-amplification nor pulse stretching. A single-pump prototype generating 200-nJ, 100-ps pulses compressible to 290 fs at 1030 nm and at 960 kHz is demonstrated. Furthermore, simulations show how this new oscillator architecture can provide tailored seed pulses with high enough spectral energy density and low enough nonlinear phase to generate sub-200 fs, 40 µJ, > 180 MW pulses from an all-fiber setup involving a single tapered-fiber power amplifier, without pulse picking.

Femtosecond Mamyshev oscillator at 920 nm.

A femtosecond all-PM-fiber Mamyshev oscillator (MO) at 920 nm is presented. It is based on a neodymium-doped fiber with a W-type index profile that effectively suppresses the emission around 1064 nm. The linear cavity is bounded by two near-zero dispersion fiber Bragg gratings with Gaussian reflectivity profiles. The laser is self-starting and generates up to 10-nJ pulses at a repetition rate of 41 MHz. The pulses can be compressed to 53 fs with a grating-pair compressor. To our knowledge, this is the first Mamyshev oscillator and also the highest energy femtosecond fiber oscillator demonstrated in this spectral region.

Bringing metasurfaces to analytical lens design: stigmatism and specific ray mapping.

We propose a method to design the exact phase profile of at least one metasurface in a stigmatic singlet that can be made to implement a desired ray mapping. Following the generalized vector law of refraction and Fermat's principle, we can obtain exact solutions for the required lens shape and phase profile of a phase gradient metasurface to respect particular ray conditions (e.g., Abbe sine) as if it were a freeform refractive element. To do so, the method requires solving an implicit ordinary differential equation. We present comparisons with Zemax simulations of illustrative designed lenses to confirm the anticipated optical behaviour.

Modeling the short wavelength infrared laser radiance reflection on the sea surface.

The sea surface is a complex dynamic structure dependent on atmospheric conditions, and for which physical and chemical properties change from water to foam. Its roughness determines how the surface reflects, absorbs, and emits radiance, and depends on multiple parameters such as wind speed and direction, and foam and turbulence induced from natural waves or from object displacement. In this paper, a model description is given for laser reflection on the sea surface in open water driven by the wind. The model allows calculation of the reflected laser radiance from the sea surface toward a receiver as a function of the incoming laser radiance with a known beam intensity profile. Each subarea of the sea surface seen by one pixel of the receiver is considered as an ensemble of facets, where each facet is defined by its x and y directional slopes. The wind speed and orientation determine the probability density function of the sea surface facet slope occurrence. In this paper, we have analytically expressed the reflected radiance on the sea surface as a function of the wind speed, receiver range, receiver heading, laser position, laser output aperture, and laser incoming radiance. Using the tolerance ellipse, the reflected radiance expression was approximated, and both direct and approximated results were compared. The richness in behavior of the reflected radiance and its dependence on the geometry of the problem were studied showing the impact of the receiver position, the laser position, heading, and beam divergence.

Millimetric scale two-photon Bessel-Gauss beam light sheet microscopy with three-axis isotropic resolution using an axicon lens.

Significance: Typical light sheet microscopes suffer from artifacts related to the geometry of the light sheet. One main inconvenience is the non-uniform thickness of the light sheet obtained with a Gaussian laser beam. Aim: We developed a two-photon light sheet microscope that takes advantage of a thin and long Bessel-Gauss beam illumination to increase the sheet extent without compromising the resolution. Approach: We use an axicon lens placed directly at the output of an amplified femtosecond laser to produce a long Bessel-Gauss beam on the sample. We studied the dopaminergic system and its projections in a whole cleared mouse brain. Results: Our light sheet microscope allows an isotropic resolution of 2.4 µm in all three axes of the scanned volume while keeping a millimetric-sized field of view, and a fast acquisition rate of up to 34 mm2/s. With slight modifications to the optical setup, the sheet extent can be increased to 6 mm. Conclusion: The proposed system's sheet extent and resolution surpass currently available systems, enabling the fast imaging of large specimens.

Multi-megawatt pulses at 50 MHz from a single-pump Mamyshev oscillator gain-managed amplifier laser.

We have developed a compact all-PM-fiber ytterbium-doped Mamyshev oscillator-amplifier laser system generating compressed pulses of 102 nJ and 37 fs, thus having over 2 MW of peak power, at a repetition rate of 52 MHz. The pump power from a single diode is shared between a linear cavity oscillator and a gain-managed nonlinear amplifier. The oscillator is self-started by pump-modulation and a linearly polarized single-pulse operation is achieved without filter tuning. The cavity filters are near-zero dispersion fiber Bragg gratings with a Gaussian spectral response. To our knowledge, this simple and efficient source has the highest repetition rate and average power among all-fiber multi-megawatt femtosecond pulsed laser sources and its architecture holds potential for generating higher pulse energies.

Study of the impact of wavefront aberrations on the characterization of ultrashort laser pulses with GRENOUILLE.

In recent years, GRENOUILLE has emerged as a relatively simple technique to fully characterize the electric field of an ultrashort laser pulse in a single shot. It does so by spatially mapping the delays on the transverse spatial coordinate and by mapping the frequencies on the angular coordinate of the orthogonal direction. Because of this spatial mapping, an aberrated wavefront could distort and affect the measurement of the pulse. It is shown here experimentally how these aberrations can affect the measurement using a deformable mirror to induce various aberrations in the wavefront. This can result in distortions of the spectral or temporal profile of the retrieved pulse, and a decrease of the intensity of the second-harmonic signal generated by the nonlinear crystal. Additionally, the signatures of some of the distortions of the trace resemble those previously identified as being caused by pulse-front tilt or spatial chirp and could be interpreted as such while being in fact caused by aberrations. This can complicate the identification of the real source of the distortions, since a purely spatial effect can cause distortions similar to those created by dispersion-based phenomena or other types of spatiotemporal couplings.

Impact of wavefront aberrations on the duration of few-cycle laser pulses.

It is generally difficult to define the duration of few-cycle laser pulses in the presence of spatiotemporal coupling. The pulse temporal width can indeed vary locally across the pulse front and spatially varying delays can complicate the definition of the temporal pulse length over the whole pulse front. However, the simple formalism of the global pulse length can be used to define the duration of such pulses. The variation of the rms temporal pulse width and the maximum instantaneous intensity of this global pulse is used here to investigate the impact of various aberrations. This is done for a collimated Gaussian few-cycle pulse propagating in a vacuum with no dispersion as a perfect plane wave of uniform, Gaussian, and super-Gaussian spatial profiles and for various local temporal pulse widths. It is shown that the temporal global profile of an aberrated pulse front can lose its Gaussian profile even for low amplitudes of aberration. This results in an increase of the rms temporal width and a decrease of the maximum instantaneous intensity of the global pulse, depending on the type of aberration. This is generally associated with a decrease in the performance for optical systems using few-cycle pulses.

Clarifications on the Gouy phase of radially polarized laser beams.

The Gouy phase, which describes how a field accumulates an additional phase shift around the focus compared to a spherical wave, is investigated for the strongly focused TM01 beam in free space. It is shown, using both analytical and numerical solutions, that when the effects of the edges of focusing optics are small, the complete variation of the on-axis Gouy phase of the longitudinal electric field is 2π. It is true even in the strongly focused limit. Moreover, we show, using an appropriate effective wavevector, how to retrieve the Gouy phase when the diffraction caused by the edges is dominant at the focus. We further investigate the off-axis Gouy phase of the TM01 beam and show that any slight variation from the optical axis causes a variation of the Gouy phase of the longitudinal electric field to reduce to π.

All-fiber Mamyshev oscillator enabled by chirped fiber Bragg gratings.

We present a linear cavity self-polarizing Mamyshev oscillator at 1550 nm made fully of polarization-maintaining fibers. The cavity filters are chirped fiber Bragg gratings with a Gaussian reflectivity profile that allow for greater reflectivity, larger bandwidth, and dispersion control. This mode-locked fiber laser architecture shows an unprecedented simplicity while delivering 21.3-nJ pulses compressed to 108 fs with a competitive 22.3% pump conversion efficiency. Mode locking is initiated with an external saturable absorber mirror. Numerical simulations show how nonlinearity can be managed with highly dispersive filters inside a Mamyshev oscillator.

Generation of optical Y-junction Bessel beams.

A method is proposed to split the central spot of zero-order Bessel beams into two parallel spots along the propagation axis of the beam. A magnetic-liquid deformable mirror is used to provide the required phase profile combining an axicon and a phase step. The obtained Y-junction Bessel beam has been characterized; the 80 µm central spot of the Bessel beam is split into two spots of the same size that have been propagated over a length exceeding 15 cm. The observations are consistent with the predictions of a numerical model. Potential applications of Y-junction Bessel beams are discussed.

Analytical inversion of the focusing of high-numerical-aperture aplanatic systems.

We propose a method for an analytical inversion of the electric and magnetic fields at the focus of a high-NA aplanatic system to obtain incident light beam distribution. Our approach is based on an inverse Fourier transform of the Richards-Wolf formalism for targeted longitudinal fields along the radial or axial directions at the non-paraxial focus. Analytical solutions are discussed for both axial and radial focal fields for a radially polarized incident light beam, and a criterion is defined to access a physically valid solution. We also validate the method according to results found in the literature. Finally, we show how the method can be generalized to other incident field distributions.

Femtosecond fiber Mamyshev oscillator at 1550 nm.

We investigated the possibility of reaching nanojoule-level pulse energies in a femtosecond erbium-doped fiber Mamyshev oscillator. In experiments, lasers generate stable pulse trains with energy up to 31.3 nJ, which is comparable to the highest achieved by prior ultrafast erbium fiber lasers. The pulse duration after a grating compressor is around 100 fs. However, as the pulse energy increases, the pulse quality degrades significantly, with a substantial fraction of the energy going into a picosecond pedestal. Numerical simulations agree with the experimental observations, and allow us to identify the gain spectrum and the nonlinearity of the erbium-doped fibers as challenges to the operation of such oscillators at high pulse energy.

Resolution enhancement in laser scanning microscopy with deconvolution switching laser modes (D-SLAM).

Laser scanning microscopy is limited in lateral resolution by the diffraction of light. Superresolution methods have been developed since the 90s to overcome this limitation. However superresolution is generally achieved at the expense of a greater complexity (high power lasers, very long acquisition times, specific fluorophores) and limitations on the observable samples. In this paper we propose a method to improve the resolution of confocal microscopy by combining different laser modes and deconvolution. Two images of the same field are acquired with the confocal microscope using different laser modes and used as inputs to a deconvolution algorithm. The two laser modes have different Point Spread Functions and thus provide complementary information leading to an image with enhanced resolution compared to using a single confocal image as input to the same deconvolution algorithm. By changing the laser modes to Bessel-Gauss beams we were able to further improve the efficiency of the deconvolution algorithm and obtain images with a residual Point Spread Function having a width of 0.14 λ (72 nm at a wavelength of 532 nm). This method only requires a laser scanning microscope and is not dependent on certain specific properties of fluorescent proteins. The proposed method requires only a few add-ons to classical confocal or two-photon microscopes and can easily be retrofitted into an existing commercial laser scanning microscope.

Generation of optical Bessel beams with arbitrarily curved trajectories using a magnetic-liquid deformable mirror.

We propose a new strategy to curve the trajectory of the central lobe of a zero-order Bessel beam and a first-order Bessel beam along their propagation axis. Our method involves modifying the phase of a beam that is incident on an adaptive mirror. As examples, we show that the most intense lobe of the beam can follow a parabolic trajectory, a cubic trajectory, or a trajectory made by a combination of these orders. By using a phase correction emulating the effect of cylindrical mirrors, the central lobe always preserves its symmetry. Theoretical simulations were reproduced in the laboratory using a magnetic-liquid deformable mirror. The parabolic trajectory of the 60-µm central spot of a zero-order Bessel beam exhibits a 0.6-mm off-axis shift after 30-cm-length propagation.

Resolution enhancement in confocal microscopy using Bessel-Gauss beams.

Laser scanning microscopy is limited in lateral resolution by the diffraction of light. We show that we can obtain twenty percent improvement in the resolution of confocal microscopy using Bessel-Gauss beams with the right pinhole size compared to conventional Gaussian beam based confocal microscopy. Advantages of this strategy include simplicity of installation and use, linear polarization compatibility, possibility to combine it with other resolution enhancement and superresolution strategies. We demonstrate the resolution enhancement capabilities of Bessel-Gauss beams both theoretically and experimentally on nano-spheres and biological tissue samples without any residual artifacts coming from the Bessel-Gauss beam side lobes with a resolution of 0.39λ. We also show that the resolution enhancement of Bessel-Gauss beams yields a better statistical colocalization analysis with fewer false positive results than when using Gaussian beams. We have also used Bessel-Gauss beams of different orders to further improve the resolution by combining them in SLAM microscopy (Switching LAser Modes : Dehez, Optics Express, 2013) achieving a resolution of 0.2λ.

Watt-level fiber-based femtosecond laser source tunable from 2.8 to 3.6 µm.

The development of compact and reliable ultrafast sources operating in the mid-infrared region could lead to major advances in both fundamental and applied sciences. In this Letter, we report on a simple and efficient laser system based entirely on erbium-doped fluoride glass fibers that generates high-energy Raman soliton pulses tunable from 2.8 to 3.6 µm at a high average output power. Stable 160 fs pulses at 3.4 µm with a maximum energy of 37 nJ, a corresponding average output power above 2 W, and an estimated peak power above 200 kW are demonstrated. This tunable source promises direct applications in laser processing of polymers and biological materials.

Exact vectorial model for nonparaxial focusing by arbitrary axisymmetric surfaces.

We present a new approach, based on Richards-Wolf formalism, to rigorously model nonparaxial focusing of radially and azimuthally polarized electromagnetic beams by axisymmetric systems without a single-point focus. Our approach is based on a combined method that uses ray tracing and diffraction integrals. Our method is validated by comparing known results obtained with a parabolic mirror. Our integral representation of the focused beams, compliant with diffraction theory, is thoroughly discussed and solved for various conics that, so far, have not been treated analytically. The extension of the method to other polarization states is straightforward.

Vector similariton erbium-doped all-fiber laser generating sub-100-fs nJ pulses at 100 MHz.

Erbium-doped mode-locked fiber lasers with repetition rates comparable to those of solid-state lasers and generating nJ pulses are required for many applications. Our goal was to design a fiber laser that would meet such requirements, that could be built at relatively low cost and that would be reliable and robust. We thus developed a high-fundamental-repetition-rate erbium-doped all-fiber laser operating in the amplifier similariton regime. Experimental characterization shows that this laser, which is mode-locked by nonlinear polarization evolution, emits 76-fs pulses with an energy of 1.17 nJ at a repetition rate of 100 MHz. Numerical simulations support the interpretation of self-similar evolution of the pulse in the gain fiber. More specifically we introduce the concept of vector similariton in fiber lasers. The coupled x- and y- polarization components of such a pulse have a pulse profile with a linear chirp and their combined power profile evolves self-similarly when the nonlinear asymptotic regime is reached in the gain fiber.

All-fiber amplifier similariton laser based on a fiber Bragg grating filter.

This article presents, for the first time to our knowledge, an all-fiber amplifier similariton laser based on a fiber Bragg grating filter. The laser emits 2.9 nJ pulses at a wavelength of 1554 nm with a repetition rate of 31 MHz. The dechirped pulses have a duration of 89 fs. The characteristic features of the pulse profile and spectrum along with the dynamics of the laser are highlighted in representative simulations. These simulations also address the effect of the filter shape and detuning with respect to the gain spectral peak.