WO1998012582A1

WO1998012582A1 - High-efficiency focusing diffraction network and method for manufacturing same

Info

Publication number: WO1998012582A1
Application number: PCT/FR1997/001624
Authority: WO
Inventors: Eric Journot; Jérôme PAYE
Original assignee: Commissariat A L'energie Atomique
Priority date: 1996-09-18
Filing date: 1997-09-15
Publication date: 1998-03-26
Also published as: FR2753539B1; FR2753539A1

Abstract

The invention concerns a high-efficiency focusing diffraction network and method for manufacturing this network. This network comprises projections (12) forming successive arcs the step (Μ) of which decreases gradually from one side to the other of the element (11) bearing these projections and is obtained by forming a hologram capable of focusing the radiation to be focused with the network and by inscribing the interference figure contained in the hologram on one surface of the element. The invention is useful for focusing and collimating laser beams with intense flux or for modifying the wave front of laser beams.

Description

HIGHLY EFFICIENT FOCUSING DIFFRACTION NETWORK AND MANUFACTURING METHOD THEREOF

DESCRIPTION

TECHNICAL AREA

The present invention relates to a focusing diffraction grating of very high efficiency as well as a method of manufacturing this focusing diffraction grating.

It applies in particular to the focusing or to the collimation of laser beams with intense flux.

PRIOR STATE OF THE ART

To focus a laser beam, we have:

- standard refractive components, in which the phase is given by the curvature of these components, and

- diffractive components.

These are divided into several groups: • holograms, in which the phase of an incident light wave is modified by modulating the refractive index of the material where holograms are formed, and • kmoform lenses, in which the phase of an incident light wave is modified by etching the surface of these lenses. Also known are refractive-diffractive hybrid components which are standard refractive components at the surface of which a diffractive etching is formed. The simple spherical refractive components do not make it possible to achieve a focusing quality close to diffraction because of geometric aberrations.

To get rid of these aberrations, you must make these components aspherical, which is not very simple, or use several components (such as doublets, triplets for example).

In addition, the refractive components are bulky and heavy as soon as their diameter increases. On the contrary, the diffractive components are, for their part, compact and light and make it possible in theory to reach the diffraction limit.

However, the optical efficiency of the holographic focusing components is clearly less good than that of the refractive components because the light is distributed there in several orders of diffraction.

The kinoform components are, in turn, capable of focusing light with good efficiency but at the cost of a more complex realization.

In these kinoform components, the phase profile which focuses an incident light wave is in fact sampled on 2 levels and therefore etched in m successive stages.

For an efficiency greater than 901, 2 ^m > 8, i.e. m> 3, is required. STATEMENT OF THE INVENTION

The object of the present invention is to remedy the above drawbacks.

It relates to a diffraction grating focusing by transmission, intended to be used with radiation of determined wavelength, this grating being characterized in that it comprises a solid element which is transparent at this wavelength and on one face of which is formed a set of reliefs, this set being delimited by two planar levels, the reliefs forming successive arcs whose pitch decreases from one side to the other of the element, so that by lighting the element by radiation, under an oblique incidence depending on the average pitch of the arcs, this radiation is focused with great efficiency, that is to say that more than half of the incident light energy is focused.

By illuminating the element with radiation having the determined wavelength and diverging from the focal point, the diffraction grating is also capable of collimating this radiation with great efficiency.

Asymmetry of the reliefs risks reducing the diffraction efficiency. This is why, according to a preferred embodiment of the diffraction grating object of the invention, each relief, seen in cross section, has a plane of symmetry.

In this case, the reliefs may have, seen in cross section, a shape chosen from the group comprising slots, saw teeth and sinusoids.

Any other form having a plane of symmetry is of course usable. Preferably, the depth of the reliefs is determined so as to obtain maximum diffraction efficiency for the grating.

According to a particular embodiment of the diffraction grating object of the invention, the reliefs have, seen in cross section, the shape of slots and the filling rate of these slots as well as the depth of these are determined so as to obtain maximum diffraction efficiency for the grating.

According to a preferred embodiment of the invention, the average pitch of the successive arcs is equal to or close to the wavelength of the radiation with which the network is intended to be used, so that this radiation is focused with a very high efficiency when the element is illuminated by radiation, at an oblique incidence close to 30 °. The present invention also relates to a method of manufacturing a focusing diffraction grating intended to be used with radiation of determined wavelength, this diffraction grating comprising an element which is transparent at this wavelength, proceeds being characterized in that it comprises the following stages ^• - a hologram is formed capable of focusing the radiation, and - the interference figure contained in the hologram is written on one face of the element.

According to a preferred embodiment of the process which is the subject of the invention, this face is covered with a layer of photosensitive resin ("photoresist"), the hologram is formed in this layer, the layer thus isolated and the 'the face of the element is etched through the developed layer. Preferably, the hologram is formed at the wavelength of the radiation.

Also preferably, the hologram is formed under an oblique incidence close to 30 °. The diffraction grating object of the present invention is located at the border of holographic components and kinoform components: it achieves the focusing of an incident light beam as a hologram would but with the efficiency of a kinoform component.

In addition, it can be carried out in just two steps: recording the hologram then engraving a phase profile at 2 levels (m = 1).

Etching consists in fact of writing, on the surface of the transparent element, the interference figure which is recorded in the hologram.

This diffraction grating can therefore be considered as a surface hologram because its operating principle is identical to that of a standard hologram.

However, the index modulation is more important there because the two mediums involved are air (whose refractive index is equal to 1) and a material such as for example glass (whose refractive index is approximately 1 , 5), which makes it possible to have a modulated zone of much lower depth than that of the volume holograms while obtaining the same phase shifts.

The main advantages of the totally diffractive component constituted by the network ob _j and of the invention are the following:

- with this component, the focusing quality has no other limits than diffraction, and

- this component has a theoretical efficiency greater than 90% for an opening less than f / 3.5, f being the focal distance of the component, regardless of the incident polarization direction for which the network is optimized. It is specified that the profile of the focusing network can be optimized for any direction of polarization (linear polarization) during its production, but the network then operates with high efficiency only for this polarization. In addition, used in the conditions of

Bragg at first order diffraction, this component allows a deflection of 60 ° of the focused beam.

In addition, the component operates with monochromatic incident radiation. It is important to note that this component can in particular operate in the ultraviolet range with very good efficiency, unlike known diffractive optical components whose use is limited to the infrared and visible domains, and that this component can withstand fluxes. important incidents because its resistance to such flows is that of the material on which the diffraction grating is formed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments given below, by way of purely indicative and in no way limiting, with reference to the appended drawings in which: * FIG. 1 is a schematic cross-sectional view of a known diffraction grating, * Figures 2A, 3A and 4A schematically illustrate various possible forms for such a network, namely slots

(Figure 2A), saw teeth (Figure 3A) and sinusoids (Figure 4A), and Figures ^~ -2B, 3B and 4B schematically illustrate the existence of a plane of symmetry for these slots (Figure 2B), these saw teeth (FIG. 3B) and these sinusoids (FIG. 4B),

* Figure 5 is a schematic cross-sectional view of a particular embodiment of the diffraction grating object of the invention, * Figure 6A is a schematic and partial perspective view of the diffraction grating of Figure 5, and " ^• FIG. 6fî schematically illustrates a wave front usable for recording a network conforming to

The invention, and

* Figures 7 and 8 schematically illustrate steps of a method of manufacturing the diffraction grating of Figures 5 and 6A.-

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

Let's start by making a few reminders on the operation of a conventional diffraction grating 2, operating by transmission, with reference to Figure 1.

The operation of such a diffraction grating 2 is simple: an incident plane light wave 4, whose angle of incidence with the normal N at the diffraction grating 2 is noted θ _x , is diffracted and the angle of the diffracted wave 6 with the normal N is noted θ _d .

These two angles Θ-. and θ _d are linked by the following equation (1):

(1) smθ. + smθ _d = pλ / Λ.

In this equation: p represents the diffraction order considered, λ represents the wavelength of the light wave 4, and Λ represents the pitch of the diffraction grating 2.

For a diffraction grating operating by transmission, the sign conventions relating to the measurement of the angles θi and θd are opposite and the deviation D between the incident direction and a diffracted direction is such that:

D = θ, ι 0 _d .

For an incident wave, a diffraction direction therefore corresponds to a diffraction order.

It appears that, for an incident wave, several diffracted waves verify the previous equation (1).

To obtain a very high diffraction efficiency for a given order of diffraction p, using a lattice, one must place oneself in a configuration verifying the Bragg condition which is written:

(2) sιnθ _λ = pλ / 2Λ. In this case, the emergence angle θ _d of the diffracted beam is equal to θi and there is a total deviation D equal to 2θ _x .

We can show that the maximum diffraction efficiency is obtained for order 1, when Λ is equal to λ.

The deviation D is then equal to 2 x 30 °, that is to say 60 °.

When the angles θ _x and θ _d are equal but less than 30 ° (Λ> λ), the diffraction efficiency decreases slightly but remains greater than 90% for Θ- _. greater than 20 °, adapting the engraving depth of the network.

The diffraction orders available are only order 0 and order 1 since equation (1) is written in this particular case:

sinθi + sinθ _d = p.

For such a grating 2, the depth h of the etching of the material 8 (which is transparent at the wavelength λ) leading to the diffraction grating 2 can be optimized in order to produce the maximum diffraction efficiency in order +1 at the expense of order 0.

A simplistic interpretation of the action of network 2 in this configuration is that the relative phase shift introduced by the two parts of the etching on the diffracted wave is π for order 0. The conservation of energy during diffraction implies that the energy is then found in order 1.

All the calculations necessary for this network optimization can be carried out by software taking into account the vector nature of the electromagnetic field.

Indeed, the patterns of the diffraction grating (of the slots in the example shown) having dimensions close to the wavelength of the incident light wave, the scalar approximation of the electromagnetic field is not valid.

Now consider the optimization of the etching profile of the diffraction grating 2. This point is very important because the high efficiency of the grating depends on this profile.

The network 2, seen in cross section in Figure 1, consists of a succession of elementary patterns whose main characteristic is that they all have a plane of symmetry.

For the various achievable patterns, there is an optimal etching depth giving a maximum diffraction efficiency close to 100%.

Figures 2A, 3A and 4A show achievable patterns, namely slots (Figure 2A), saw teeth (Figure 3A) and sinusoids (Figure 4A).

FIGS. 2B, 3B and 4B show the existence of a plane of symmetry P for each of the patterns in FIGS. 2A, 3A and 4A.

We consider more particularly the slots of figure 2A whose spacing is noted a_ and the step b.

The ratio (b-a) / b is less than 1 by "filling rate".

The optimum depth to be given to the slots is also a function of this filling rate.

In general, for a given pattern, the etching depth of the network is minimum for a filling ratio equal to 0.5. We now consider a diffraction grating focusing by transmission, in accordance with the present invention, which is schematically represented in cross section in FIG. 5 and in partial perspective in FIG. 6 A -

This network ^" is intended to be used with a light beam 10 with parallel rays, of determined wavelength λ.

This network comprises a substrate 11 which is transparent at this wavelength λ and on one face of which a set of reliefs 12 is formed.

This set of reliefs is delimited by a lower plane level 13 and an upper plane level 14 which is parallel to the lower level 13. The reliefs 12 form successive arcs whose pitch Λ decreases from one side to the other of the substrate 11 as seen in FIG. 6 A.

In this way, when the face carrying the reliefs 12 is illuminated, by the beam 10, through the substrate 11, under an oblique incidence (the angle of incidence of the beam 10 relative to the normal N to the grating is still noted θj. in FIG. 5), this incidence being a function of the average pitch Λ of the arcs, this beam 10 is focused with great efficiency at a point noted F in FIG. 5.

The network can also function by lighting it on the side of the face carrying the reliefs.

The performances obtained would be identical except for slight aberrations brought by the crossing of the face without reliefs by an oblique and convergent beam.

What has been said previously with respect to the conventional network of FIG. 1 can be applied to the network according to the invention of FIGS. 5 and 6A in replacing the parameter Λ by Λ in formulas (1) and (2).

Thus each relief, seen in cross section, has a plane of symmetry P. In the example of Figures 5 and 6A reliefs have, seen in co ^transverse UPE, the shape of slots.

However, it is also possible to use reliefs forming for example saw teeth or sinusoids or having any other shape having a plane of symmetry.

The depth h of the reliefs 12 is determined so as to obtain maximum diffraction efficiency for the grating. As we saw above, we place ourselves to do this in the Bragg conditions at the first order of diffraction.

In this case, as we have seen, the emergence angle θ _d of the diffracted beam 16 is equal to θ, and there is a total deflection D equal to 2θ.

In the case of FIGS. 5 and 6A, where the reliefs have, seen in cross section, the form of slots, one can choose the filling rate (average) of these slots as well as the depth h of them so as to obtain high diffraction efficiency for the network.

Preferably, the average pitch of the successive arcs is equal to the wavelength λ of the light beam 10. Thus this light beam 10 is focused with great efficiency when the face on which the reliefs 12 are formed is illuminated, through the substrate 11, by the light beam 10 with an angle of incidence Θ-. 30 °. The substrate 11 is for example made of silica. One could also use such a substrate on one face of which would be deposited a layer of material transparent to the light beam of wavelength λ, the reliefs then being formed in this layer of transparent material.

With a silica substrate 11, the network according to the invention is capable of operating in the ultraviolet range.

With a material such as silica, the diffraction efficiency under Bragg conditions for an optimized etching of the reliefs 12 is greater than 90%, for an opening less than f / 3, whatever the direction of polarization of the light beam. incident 10 for which the profile is optimized.

It is specified that the optimizations are obtained using software of the kind of that which was mentioned above.

Thus we have seen how to obtain a diffraction grating by focusing transmission according to the invention, the network having a very high efficiency ^diffraction in the order 1 and performing a D 60 ° deflection.

For the wavelength λ considered, the function of the grating according to the invention of FIGS. 5 and 6A is to modify the incident light beam 10.

This network therefore transforms a wavefront 18 (plane or spherical) into a spherical wavefront 20.

In general, the network transforms an incident wavefront into an emerging wavefront.

When recording the network, the wave fronts used are the conjugates of the actual incident and emerging wave fronts (that is to say, those which intervene during the use of the component). There is no particular constraint on these FR wave fronts except that their radius of curvature R and their diameter D (see figure βjβ) verify R> 3.5D in order to maintain good efficiency of diffraction for the network produced.

The immediate applications for this diffraction grating are:

- standard spherical lenses which transform a plane wavefront into a spherical wavefront (hence a focusing at a point) or which transform a spherical wavefront into a plane wavefront (hence collimation),

- cylindrical lenses which transform a plane wavefront into a cylindrical wavefront (hence focusing on a line) or which transform a cylindrical wavefront into a plane wavefront (hence a collimation),

- conjugation, an incident spherical wavefront then being transformed into an emerging spherical wavefront.

The transformation of a wavefront 18

(plane or spherical) in a spherical wavefront 20 can be produced by a hologram recorded by interference between a spherical wave coming from a source point and a reference wave (plane or spherical).

The difference between the network according to the invention and such a hologram is the origin of the phase difference that is given to the incident light beam. In a hologram, the phase difference is given by a local difference in the refractive index of the material in which the hologram is formed, this difference being proportional to the amount of light present locally during the recording of this hologram. On the surface of the network according to the invention, it is the profile of the etching which gives the phase difference.

However, in both cases, the interference fringes are positioned spatially in the same places (to achieve ^ the same function).

It should be noted that the phase profile of the diffraction grating according to the invention is etched on the surface of the substrate 11 and not recorded in the volume of this substrate 11.

The degrees of freedom available for the profile of the etching are the shape, the depth and also the filling rate which, as we have seen, make it possible to optimize the diffraction efficiency for a single order.

In addition, the resistance of the focusing diffraction grating according to the invention to an incident light flux is identical to that of the material constituting the substrate 11 on which this grating is formed.

This resistance to light flux is much higher than that of bichroated gelatins used for recording holograms.

Explaining with reference to FIGS. 7 and 8, a method of manufacturing a diffraction grating focusing by transmission, of the kind which is shown in FIGS. 5 and 6A and which allows the focusing of the light beam 10 of wavelength, is explained. λ. The silica substrate 11 is used, which is transparent at this wavelength λ, and a hologram capable of focusing the light beam 10 is formed, then the interference figure contained in this hologram is written on one face of the substrate 11. To do this, this face is covered with a layer 22 of photosensitive resin whose thickness is equal to the depth h desired for the reliefs of the networks. The hologram is then formed in this layer of photosensitive resin 22.

To do this, a light source 24 capable of providing a light beam with parallel rays 26 of wavelength λ is used. The hologram interference figure is therefore preferably recorded at this wavelength of use λ because, otherwise, complex wave fronts which are difficult to obtain should be used. To form the hologram in the layer of photosensitive resin 22, the light beam 26 is sent at the angle of incidence Q _lt , preferably equal to 30 °, through the substrate 4, in the direction of the face carrying the layer of photosensitive resin 22. By means of a semi-transparent mirror 28 placed on the path of the beam 26, a part 30 of the latter is removed.

The part 30 thus removed is deflected by means of a mirror 32 so that the light beam reflected by this mirror 32 also has an angle of incidence θ _λ relative to the layer 22.

There is an opaque screen 34 pierced with a hole 36 in the path of this reflected beam so that this hole 36 constitutes a point light source. It is specified that the beam reflected by the mirror 32 is focused on the hole 36 by means of appropriate optics 38.

The diameter of the hole 36 is equal to a few times the wavelength λ. The light from the point source interferes, in the photosensitive resin layer 22, with the non-sampled part 40 of the beam 26, hence the formation of the hologram sought 42 in this layer (Figure 8).

The photosensitive resin layer 22 thus exposed is then developed and the substrate 11 is etched through the developed photosensitive resin layer. To do this, an ion etching process is used for example.

One thus obtains a focusing diffraction grating according to the invention, of the kind of that of FIGS. 5 and 6A "It is specified that the interference fringes of the hologram are curves of the same phase shift between a spherical wave surface and an inclined plane wave surface.

On one side of the substrate 11, the pitch of all the interference fringes is small, which will allow a large deflection of the light rays while on the other side of the substrate, the pitch is larger and the deviation given to light rays will be weaker. On the edges of the component, the interference fringes curve, resulting in a deflection of the light rays towards a point (focus).

Locally, the fringes can be considered as rectilinear and we are then in a case of conical diffraction: the incident wave vector is no longer in the plane defined by the normal to the substrate and the local normal to the fringes.

The variation of the pitch and the non-zero angle between this plane and the plane defined by the normal to the substrate and the incident wave vector cause the component no longer works exactly under Bragg conditions.

The diffraction efficiency therefore drops slightly at the edges. It was calculated that for an open component at f / 7, the overall diffraction efficiency decreased by 3% compared to the diffraction efficiency obtained under Bragg conditions.

It should however be noted that the diffraction efficiency of this component remains greater than 94% whatever the incident polarization.

For an opening equal to f / 3.5 and therefore greater than the previous one, the diffraction efficiency of the component nevertheless remains greater than 90% whatever the incident polarization direction for which the profile has been optimized.

The diffraction grating object of the invention therefore constitutes a focusing diffractive component of very high efficiency at a given wavelength, without polarization restriction.

This wavelength can be in the visible range or the ultraviolet range.

In the ultraviolet field, this component is the only focusing diffractive component of high efficiency.

The focusing quality is theoretically limited by diffraction.

The component can be produced in two stages: holographic recording of the optical function at the operating wavelength, then transfer of the interference pattern to the surface of the substrate.

The invention allows the incident beam and the focused beam to be inclined by 60 ° relative to each other. The incident beam can be collimated or not, the recording of the interference figure taking place in a holographic manner.

However, the aperture of the optics produced must not exceed f / 3.5 to maintain a high diffraction efficiency (greater than or equal to 90%).

The additional characteristic of this component is its resistance to a luminous flux, this resistance corresponding to that of the material in which the diffraction grating is formed.

The advantage of this component compared to standard diffractive kmoform optics is that its etching is binary and therefore simpler to carry out since it is not necessary to align with very high precision successive etching masks.

Of course, a diffraction grating according to the invention, of the kind which is represented in FIG. 5, also makes it possible to colli ate a diverging light beam sent towards the face of the substrate 11 where the reliefs 12 are 'other side of this substrate).

Claims

1. Diffraction grating focusing by transmission, intended for use with radiation of determined wavelength, this grating being characterized in that it comprises a solid element (11) which is transparent at this wavelength and on one face of which is formed a set of reliefs (12), this set being delimited by two flat levels, the reliefs forming successive arcs whose pitch decreases from one side to the other of the element, so that 'by illuminating the element with radiation, at an oblique incidence depending on the average pitch of the arcs, this radiation is focused with great efficiency.

2. Diffraction grating according to claim 1, characterized in that each relief (12), seen in cross section, has a plane of symmetry (P).

3. Diffraction grating according to claim 2, characterized in that the reliefs (12) have, seen in cross section, a shape chosen from the group comprising slots, saw teeth and sinusoids.

4. diffraction grating according to any one of claims 1 to 3, characterized in that the depth (h) of the reliefs is determined so as to obtain maximum diffraction efficiency for the grating.

5. Diffraction grating according to claim 3, characterized in that the reliefs (12) have, seen in cross section, the shape of slots and in that the filling rate of these slots as well as the depth of these are determined so as to obtain maximum diffraction efficiency for the grating.

6. diffraction grating according to any one of claims 1 to 5, characterized in that the mean pitch of the successive arcs is equal to or close to the wavelength of the radiation with which the grating is intended to be used, of so that this radiation is focused with great efficiency when the element (11) is illuminated by the radiation, at an oblique incidence close to 30 °.

7. A method of manufacturing a focusing diffraction grating intended to be used with radiation of determined wavelength, this diffraction grating comprising an element (11) which is transparent at this wavelength, this method being characterized in that it comprises the following steps: - a hologram (42) is formed capable of focusing the radiation, and - the interference figure contained in the hologram is written on one face of the element (11).

8. Method according to claim 7, characterized in that this face is covered with a layer of photosensitive resin (22), the hologram (42) is formed in this layer, the layer thus exposed is developed and the the face of the element is etched through the developed layer.

9. Method according to any one of claims 7 and 8, characterized in that the hologram (42) is formed at the wavelength of the radiation.

10. Method according to claim 9, characterized in that the hologram (42) is formed under an oblique incidence close to 30 °.