EP1871565A1 - Femtosecond laser micromachining device with dynamic beam shaping - Google Patents

Abstract

The inventive device comprises a laser-type coherent light radiation source (1) transmitting ultrashort pulses, a dynamic beam shaping arrangement (2, 3, 4, 5, 6) and an arrangement (7, 8, 9, 10) for using and processing a laser beam. Said dynamic beam shaping system comprises an optical system consisting of a part (2) for actively modifying the laser source wavefront and detecting means (5), excepting a phase meter, used for three-dimensionally shaping the beam, wherein said parts are connected by a feed-back circuit (6) and the active modifying part comprises a first fixed or active component and a second active component.

Description

 FEMTOSECOND LASER MICRO-MACHINING DEVICE WITH DYNAMIC CONFORMATION

BEAM

The invention relates to the technical sector of micro-machining of various materials, including femtosecond laser.

It has already been proposed to perform micromachining operations using laser sources producing ultrashort pulses. There may be mentioned, for example, the teaching of US Pat. No. 6,285,002 and the teaching of patent EP 1,011,911, which concerns a machining of ultra-short pulse laser metals and alloys.

Reference should be made to the publications [Momma 1996]: C. Momma, B. N. Chichkov, S. Nolte, F. Von Alvensleben, A. Tunnermann,

H. Welling and B. Wellegehausen, Opt. Comm. 129, 134 (1996) and [The

Harzic 2002]: R. Harzic, N. Huot, Audouard E., Jonin C., Laporte P., S.

Valette, A. Fraczkievicz and R. Fortunier, Appl. Phys. Lett. 80, 3886

(2002), which show that laser processes with ultrashort pulses find a particularly advantageous application to perform micro-machining with extremely limited collateral thermal effect for sufficiently thin materials, in particular of thickness less than one millimeter.

Moreover, it appears from the publication [Cordingley 1993]

J. Cordingley, Appl. Opt. 32, 2538 (1993) that laser processes using fixed diffracting optical elements, allow a shaping of the beam before process. Ultra-short laser processes are also known using dynamic wavefront correction with phase measurement feedback [Sanner 2004] N. Sanner, N. Huot, Audouard E., Larat C., Laporte P. and JP Huignard. The technical results obtained can be considered satisfactory, while observing however that the use of a wavefront sensor (phase measurement) is a particularly high cost.

The object of the invention is to remedy these disadvantages in a simple, safe, effective and rational manner.

The problem that the invention proposes to solve is to be able to suppress the wavefront sensor which makes it possible to obtain a linear correction with a quasi-direct correspondence between address pixels and detected pixels and to use, according to invention, an optimization method for making the correction.

It has therefore been designed and developed a femtosecond laser micro-machining device with dynamic beam conformation of the type comprising: a source of coherent light radiation of laser type emitting ultrashort pulses;

a set of dynamic conformation of the beam;

a set of implementation and treatment of the laser beam,

According to the invention, in view of the problem posed, in order to ensure the dynamic compression of the beam without phase measurement, the dynamic conformation assembly of the beam comprises an optical system composed of an active part for modifying the wavefront of the beam. source laser and a detection means, excluding a phase meter, adapted to ensure the spatial shaping of the beam, said parts being connected by a feedback loop, the active part of modification containing a first fixed or active component and a second active component.

Given the problem of not using a wavefront sensor, the means capable of ensuring the spatial shaping of the beam is a simple detection system of the CDD camera type or photodetector.

To solve the problem of achieving computer convergence, the feedback loop implements an algorithm selected to adapt the phase functions performed by the first and second components, in order to optimize the result in the form of an assigned score. by criteria dependent on the selected simple detection system.

According to a feature underlying the invention, the action of a high-speed ultrashort pulse laser, with programmable shaping of the beam, without phase measurement, is of real industrial interest. In particular, it appears that the high rate decreases the rate of the process, while the programmable formatting makes it possible to structure the shape of the beam and to vary it by computer. In a particularly important way, the absence of phase measurement saves the use of a wavefront sensor or an interferometric device whose prices are prohibitive. The invention is described below in more detail with reference to the figures of the accompanying drawings in which: FIG. 1 is a purely schematic view of the main assemblies of the device according to the invention for implementing the laser method in the context of micromachining operations, in the case of direct transmission of the laser source; Figure 2 is a view similar to Figure 1, in the case of a transmission by reflection of the laser source.

The femtosecond laser micro-machining device comprises in combination a coherent light source of laser type emitting ultrashort pulses (1), a dynamic beamforming assembly (2), (3), (4), (5), ) and (6) and a set of implementation and processing of the laser beam (7), (8), (9) and (10) (micromachining operation for example).

The laser source (1) operates in mode-locked pulse mode and delivers ultra-short pulses of less than 100 ps duration and repetition rates greater than or equal to 1 kHz. The energies delivered for each pulse are generally greater than or equal to 1 nJ. The laser source emits at a wavelength compatible with the dynamic beam conformation assembly. By way of non-limiting example, a source may consist of an amplified femtosecond chain based on titanium doped Sapphire crystal emitting pulses of 4 μJ for a duration of 200 fs at a variable rate of 10 to 250 kHz. Without departing from the scope of the invention, other solutions can be envisaged. For example, a diode pumped femtosecond source based on Ytterbium ion doping emitting pulses of 100 μJ for a duration of 400 fs at a rate of 1 to 10 kHz can be used. It is also possible to use an amplified femtosecond source based on titanium doped sapphire crystal emitting pulses of about 1-1.5 mJ for a duration of 150 fs at a rate of 1-5 kHz.

The dynamic conformation assembly of the beam comprises an optical device composed of an active wavefront modification system (2) of the laser source (1) and a detection system without phase measurement (5) . The wavefront modification system (2) and the phase-less detection system (5) are connected by a feedback loop (6).

In a significant way and according to a characteristic underlying the invention and as will be indicated in the following description, the detection system (5) is not a wavefront sensor whatever it is or an interferometric device for phase measurement.

The active wavefront modification system (5) contains a first fixed or active component and a second active component.

The first component has a low spatial resolution in terms of wavefront sculpture.

When this component is fixed, it may be constituted by an afocal optical system composed of lenses and / or mirrors and which is capable of producing a curvature of the wavefront. This first component, when it is fixed, may also consist of a diffractive optical element performing a "pre-shaping" function modulated by the second component.

When this first component is active, it may be constituted by a deformable mirror (for example of the type marketed by CILAS France) or by a deformable membrane (for example of the type marketed at OKO Technologies, Japan) or by a valve optically addressed optically and more generally by any means for performing a modulation of the spatial phase with a fairly high dynamic (typically greater than or equal to 2π) with a low spatial resolution, especially with pixels less than or equal to 100 microns.

These different components are addressed electrically or optically and controlled by computer. They can work by reflection or transmission.

When the first component is active, it allows to realize the phase function necessary to obtain the desired formatting, without solving the details of this basic function. In this case, such details are made by the second active component.

This second component has a high spatial resolution with a pixel dimension less than or equal to 100 μm and a number of pixels of at least 100. This second active component is based on a liquid crystal layer and can, for example, be constituted by a spatial light modulator set in phase modulator and addressed electrically or be constituted by an optical valve optically addressed and more generally by any means performing this function with sufficient spatial resolution. This active component can operate by reflection or transmission having the function of refining the spatial shape of the wavefront.

According to an important characteristic of the invention, the detection system (5) combining with the feedback loop (6) makes it possible to assign a note to the formatting performed. In a fundamental way, the detection system is not a phase measuring device making it possible to compare the phase front produced with an expected phase front. It is a simple detection system of the CDD camera type or photodetector, after nonlinear crystal.

More generally, according to the invention, the system without a wavefront sensor may consist of any detection system whose function is to spatially shape the beam and to obtain a better result. By result, we mean a score assigned according to the criteria depending on the selected means, for example the image quality if it is with a CDD camera or frequency doubled intensity detected on a photodiode.

With this technical solution, the rating assigned depends on the pixel rate of the address at a time. As a result, there is no direct correspondence as is the case with a wavefront sensor. It is therefore necessary to optimize the entire dressing matrix at the same time. The note is integrated into the feedback loop (6).

Given these characteristics, to achieve the computer convergence, the feedback loop implements an algorithm to adapt the phase functions performed by the first and second components to optimize the note delivered by the detection part. For example, the so-called "genetic" or "revolutionary" algorithms can be used.

After dynamic conformation of the beam, the device comprises an objective (Fourier lenses (3), (4), for example) which focuses the beam thus structured and produced in its focal point or in its vicinity, a spot having the desired spatial distribution . If the minimum dimension of this spot does not correspond to the desired dimensions, a lens and / or mirror imaging device (7) can be added downstream.

At the image spot is arranged a sample (9) on which the laser process is performed. For example, the sample (9) is mechanically connected to a mobile assembly (10) controlled by computer. For example, this mobile assembly may be a set of motorized translations coupled or not to motorized rotation devices.

A set of scanner type (8) (galvanometric mirror system) can also be inserted on the optical path before forming the image spot so as to deflect the beam by computer control.

The advantages are apparent from the description, in particular it is emphasized and recalled the significant decrease in costs by the removal of the wavefront sensor using a simple detector capable of providing a feedback signal in connection with an algorithm selected to establish a test-error convergence procedure with improvement during each error test.

Claims

R E V E N D I C A T IO N S Femtosecond laser micro-machining device with dynamic beam conformation comprising: a source (1) of coherent light radiation of laser type emitting ultrashort pulses; a set of dynamic conformation of the beam (2), (3), (4), (5) and (6); an assembly for implementing and processing the laser beam (7), (8), (9) and (10), characterized in that the dynamic conformation assembly of the beam comprises an optical system composed of a part active modification of (2) the wavefront of the laser source and a detection means (5), excluding a phase meter, adapted to ensure the spatial shaping of the beam, said portions being connected by a feedback loop (6), the active modification part containing a first fixed or active component and a second active component. -2- Device according to claim 1, characterized in that the means adapted to ensure the spatial shaping of the beam is a simple detection system of the CDD camera type or photodetector. -3- Device according to claim 1, characterized in that the feedback loop (6) implements an algorithm selected to adapt the phase functions performed by the first and second components, in order to optimize the result in form a score assigned by criteria dependent on the selected simple detection system. -4- Device according to claim 1, characterized in that the first component has a low spatial resolution in terms of wavefront. -5- Device according to claim 4, characterized in that when the first component is fixed, it is constituted by an afocal optical system capable of being disordered so as to produce a curvature of the wavefront. -6- Device according to claim 4, characterized in that when the first component is fixed, it is constituted by a diffractive optical element adapted to perform a premise function shaped modulated by the second component. -7- Device according to claim 4, characterized in that when the first component is active, it is constituted by a system adapted to perform a modulation of the spatial phase with high dynamics and low spatial resolution. -8- Device according to claim 1, characterized in that the second component has a high spatial resolution. -9- Device according to claim 8, characterized in that it is based on a liquid crystal layer and operates by reflection or transmission being able to refine the spatial wavefront shaping.