LT6107B - Method for forming bragg diffraction gratings by femtosecond bessel beams - Google Patents

Abstract

An object of the present invention is to provide a method for writing transparenthigh efficiency volume Bragg gratings or other modified refraction index patterns insidetransparent dielectric media by using ultrashort pulse and high intensity Gauss-Bessellaser beams.The method includes the following steps of: generating ultrashort light pulses,focusing the light pulses so that the light intensity inside the processed material exceedsthe optical damage threshold; temporally and spatially controlling the exposure in orderto limit the appearance of defects induced by thermal and other effects; forming andprewriting a desired pattern by using a focused Gaussian laser beam by scanning thedielectric medium; forming and focusing a Gauss-Bessel beam into the dielectricmedium; monitoring the near-field distribution of the Gauss-Bessel beam in relation withthe target medium; writing the desired pattern by scanning the dielectric medium inrelation to the incident beam and inspecting the performance of the inscribed opticaldevice.The diffraction gratings can be made of variable size by fabrication in multiple layers.

Description

The present invention relates to methods for producing a modified modified refractive index pattern or a Breg ultra-high efficiency bulk diffraction grating in a transparent dielectric medium using Gaussian-Bessel femtosecond laser beams.

TECHNICAL LEVEL

The diffractive grating is one of the key optical elements in photonics. Permeation gratings with the highest diffraction efficiency are required in high energy density and high peak power lasers, since even the slightest loss of absorption can cause significant damage and optical breakouts. Also, a small lattice optical nonlinearity is useful at high laser irradiations / intensities. To date, Bregian diffractive grays have generally been formed using photo-thermo-refractive glasses using interfering ultraviolet continuous-mode laser light (as described in Opt. Lett. 25 (23), pp. 1693-1695, 2000) or ultra-short laser pulses as described. U.S. Pat. US2006221449, published 05-10-2006). In order to be able to modify the photopolymer by optical radiation, photosensitivity of the recording medium (glass or photopolymer) is usually required, but this property can complicate the process of forming structures and limit possible applications.

When dielectric damage is used to record photonic derivatives in glasses and crystals, strong scattering occurs even when tens of nanometers in size are formed at the focal point (Opt. Mat. Express 4, pp. 783-795, 2011). An attractive solution would be to find a way to write free-form three-dimensional structures by light modifying the real refractive index n without causing absorption. Appropriate recording conditions have been realized in mechanically resistant solution-gel media and the formation of 3D derivatives can be achieved without the use of a photoinitiator at best resolutions less than wavelength. (Opt. Express 18 (10), pp. 10209-10221, 2010). Optical devices formed in solution-gel-resistant media are uncomfortable because they are difficult to integrate with photonic devices recorded in a glass volume.

Pure quartz with low non-linear absorption is a good material for forming micro-optical elements. Sharp focus and small focal point dimensions must be achieved when recording high-resolution diffraction gratings. However, when Gaussian spatial laser pulses are used, axial overlap and compaction of surface lattices is complicated. Also, with a sharp focus on the fiber, filamanetization is neither efficient nor well controlled (S. Juodkazis et al., "Studies on femtosecond pulse filamentation in glasses" in Int. Conf. Lasers, Applications, and Technologies: High-Power Lasers and Applications, ICONO, St. Petersburg, May 11-15, 2005). Therefore, in the present invention, we use Gaussian Bessel fibers to simultaneously obtain a small focal point size at axial lengths longer than tens of micrometers.

U.S. Pat. US2009274420, published 5/11/2009, describes a system and method for recording indelible diffractive grating in low phonon energy glasses, waveguides. The ultra-short pulses of light are generated and, in the form of two fibers, synchronously matched within the waveguide to form the desired lattice corresponding to the interfering image. The pulses of light are focused so that the intensity of light in the waveguide exceeds the filamentation threshold. The exposure of the waveguide with these light pulses is controlled in time and space to limit the deleterious effects of high intensity light pulses in the glass waveguide medium.

U.S. Pat. US2004184731, published September 23, 2004, describes a method and device for altering the refractive index of a material using an interfering image. The method and device include a laser source of ultrashort pulses. The electromagnetic radiation emitted by the laser propagates to a diffractive element located near the workpiece. Diffractive electromagnetic radiation forms an interfering image whose peaks are of sufficient intensity and can cause a refractive index change.

Previous inventions describe methods and systems for recording Breg cells in different dielectric media, but do not provide a method for producing high resolution and high efficiency (up to 90 percent) transparent Brego cells which are required in prior art applications. Patents describing techniques that use an interfering image generally have the disadvantage of using a phase mask to produce an interfering image.

THE SUBSTANCE OF THE INVENTION

In order to overcome the above drawbacks, the present invention provides a method for recording indelible high efficiency, volumetric, transparent Bregg grating in a transparent dielectric medium.

Sharp-focused Gaussian fibers allow good transverse and axial modification selectivity in bulk material, but axial selectivity can be understood more as a disadvantage than an advantage. On the other hand, Gaussian-Bessel fibers, even sharply focused to produce modifications of similar transverse dimensions, have the potential to affect a larger area in the axial direction. Thus, the present invention enables the use of this property of Gaussian-Bessel fibers in the production of Bregs of various thicknesses.

The method comprises the steps of: generating ultra-short pulses of light; focusing the light pulses so that the light intensity inside the workpiece exceeds the optical damage threshold; temporal and spatial control of exposure to limit the occurrence of defects caused by thermal or other phenomena; forming and recording a preliminary pattern using a focused laser Gaussian beam, scanning a dielectric medium; Forming and focusing Gaussian-Bessel fibers in a dielectric medium; Monitoring the near-field distribution of Gaussian-Bessel fibers with respect to the media being processed; recording the desired pattern by scanning the dielectric medium for the incident beam and verifying the performance of the recorded optical device.

Other features and advantages of the present invention will be better understood by reading the most preferred embodiments having regard to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

In order to better understand the invention and evaluate its practical applications, the following illustrative drawings are provided. The drawings are given by way of example only and in no way limit the scope of the invention.

FIG. 1 Shows an exemplary set of equipment for recording high performance Breg cells in transparent dielectric trays;

FIG. 2 Shows an exemplary set of equipment for forming and rendering Gaussian-Bessel fibers. 4

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS It is an object of the present invention to provide a method for

For recording Breg cells or other modified refractive index structures in transparent dielectric media using ultra-short pulses and high intensity

Gaussian-Bessel laser beams.

In the description given, the phrase "diffractive grating" refers to any periodic or aperiodic structure of a modulated refractive index indelibly recorded in a machining medium. It should be apparent to a person skilled in the art that the diffractive grating may be one or more channels, may be chirped, tilted, multi-modulated, or have more than one such characteristic.

In a preferred embodiment, the recording medium is a volumetric fused silica glass, and the recording substrate made of this material will be referred to as a "preform.

A laser source (2) is used which can act as a source of coherent electromagnetic wave beams having internal or external means to produce the required intensity (using means such as an optical power attenuator having a phase plate mounted on a rotary table (3) with a polarizer (4) )), pulse duration (from several femtoseconds to hundreds of picoseconds, but preferably from 160 fs to 2500 fs), repetition frequency, and wavelength ultrashort pulses. The wavelength must be chosen in such a way that linear absorption does not occur in the formation of the said refractive index derivatives. Most often, laser sources have an intensity distribution that can be approximated by a Gaussian function. It is assumed here that the spatial distribution of the fibers is exactly that, but one skilled in the art should have no difficulty in applying further aspects of the invention to a different type of laser beam. A fiber-forming optical element, such as a flat-convex axon (cone lens) (5) or other suitable element, is placed in an optical path where it converts a falling Gaussian beam to a Gaussian-Bessel beam. In addition, the focusing system (6) is designed to form a Gaussian-Bessel fiber joint having the desired parameters inside the sample, and is positioned within the fiber optical path between the optical elements and the sample. Such a focusing system may comprise several different lenses (7) and one or more objective lenses (8).

Proper placement of the specimen relative to the incident beam junction is controlled by the spatial distribution of the reflected or past radiation relative to the specimen.

In order to obtain the desired refractive index modification pattern, the specimen is moved (scanned) with respect to the incident beam bend using precision positioning tables (9), preferably motorized or controlled, using a computer (10) or a similar device. High-efficiency Breg lattices can be inscribed in the sample volume in layers, such that each successive layer is recorded by moving the sample parallel to the incident beam (axial shift) so that the modified areas overlap. This process will be referred to as stitching. The process must be monitored for optimal results.

The quality evaluation of the recorded device can be achieved by using an integrated optical microscope (6, 11) and / or illuminating the lattice with a white light continuum (BSC). BSC diffraction is used to evaluate the accuracy of axial stitching and the angular and spectral selectivity of the lattice.

In another embodiment, prototype lattices in a sample volume can be recorded in Gaussian mode (using the above method but without the use of a fiber-forming optical element) to find optimum recording parameters.

In another embodiment, Breg lattice and stitch defect inspection is performed by generating and focusing 1028 nm wavelengths, 280 fs pulse duration, Ep = 1.6 J energy pulses at 25 kHz repetition, NA = 0.03 lens, 2mm thick sapphire plate , f = 50 mm lens is used to collimate and refocus the CSF j Breg grid (not shown in the drawings). This embodiment is not limited to the specific values or devices described.

In another embodiment, the treatment medium can be replaced with almost any other dielectric material, such as fluoride, chalcogenide, chalcohalide, soda-lime, borosilicate, or any other type of optical glass having similar optical properties. It is important to mention that other materials may not achieve the high diffraction efficiency as in bulk fused quartz due to the material properties such as absorption, thermal conductivity and others. The workpiece can be any optical device that may have useful Bregg lattices or similar optical elements, such as an optical fiber or flat waveguide.

In another embodiment, said photomodified refractive index patterns are formed in a photopolymer volume.

At optimum formation parameters, resolution up to 1 micrometer and up to 90 percent diffraction efficiency were demonstrated.

DEFINITION OF INVENTION

Claims (12)

DEFINITION OF INVENTION 1. A method for recording high performance volumetric, transparent Breg cell grating or other similar periodic or aperiodic refractive index in a volume of a transparent dielectric medium, characterized in that a focused Gaussian-Bessel electromagnetic beam is used to modify the refractive index. 2. A method according to claim 1, characterized in that said Brego lattices are produced by stitching together several layers of modified refractive index patterns. 3. A method as claimed in any one of claims 1 to 2, wherein the efficiency of the stitching process of said Bregs is monitored by means of testing the quality and performance of said boxes. 4. The method of claim 3 wherein said scanning means is an optical microscope. 5. The method of claim 3, wherein said Brego lattice is screened by illuminating said lattice in a focused white light continuum. 6. A method according to claim 1, characterized in that the transparent dielectric medium is precisely aligned with respect to the focused Gaussian-Bessel beam by observing the distribution of the near-field Gaussian-Bessel beam with respect to said medium. 7. A process according to claim 1, wherein the transparent dielectric medium is made of volumetric fused quartz or other material having similar properties. 8. Method according to one of claims 1 to 7, characterized in that the wavelength of the electromagnetic radiation is such that no linear absorption occurs in the transparent dielectric medium. 9. An optical device having at least one Brego volumetric grating, wherein said volumetric Brego grating is manufactured using the method described in any one of claims 1-8. 10. An optical device having at least one planar waveguide, characterized in that said planar waveguide is manufactured using the method of any one of claims 1-8. 11. An optical device having at least one flicker mirror, wherein said flicker mirror is manufactured using the method of any one of claims 1-8. 12. An optical device having at least one two-dimensional photonic derivative, wherein said two-dimensional photonic derivative is manufactured using the method of any one of claims 1-8.