FR3061563A1 - OPTICAL COMPONENT WITH SPECTRAL SELECTIVITY FOR IMPROVING OPTICAL COMMUNICATION - Google Patents

Abstract

The invention relates to an optical component of spectral selectivity at depth of field for optical communication, comprising at least one diffractive annular grating (ALDA), defined by the following parameters: a minimum radius (Rmin); - a maximum radius (Rmax); - a period of the network (d); - said parameters being determined so as to obtain a spectral separation of two adjacent wavelengths λ + dλ and λ originating from a modulated light source external to said component, that is to say such as the maximum focusing distance (Zmax (λ + dλ)) of the wavelength λ + dλ is less than the minimum focusing distance (Zmin (λ)) of the wavelength λ, - said component being characterized in that said distances of maximum and minimum focalizations are calculated according to the following formulas: Zmax (λ + dλ) = (Rmax.d) / ((λ + dλ) .m) and Zmin (λ) = (Rmin.d) / (λ.m) , where m represents the diffraction order.

Description

Holder (s):

SUNPARTNER TECHNOLOGIES.

O Extension request (s):

® Agent (s): GLOBAL INVENTIONS.

(54) OPTICAL COMPONENT WITH SPECTRAL SELECTIVITY FOR IMPROVING OPTICAL COMMUNICATION.

FR 3,061,563 - A1 (57) The invention relates to an optical component of spectral selectivity at depth of field for optical communication, comprising at least one annular diffractive grating (ALDA), defined by the following parameters:

- a minimum radius (R _min );

- a maximum radius (R _max );

- a network period (d);

- Said parameters being determined so as to obtain a spectral separation of two adjacent wavelengths Z + dZ and Z coming from a modulated light source external to said component, that is to say such as the maximum focusing distance (Z _maX ( _{X + dA} p of the wavelength Z + dZ is less than the minimum focusing distance (Z _min ( _X p of the wavelength Z,

said component being characterized in that said maximum and minimum focusing distances are calculated according to the following formulas: Z _max (^ _{+ C} ix) = (R ^ ax-dV ((Z + dZ) .m) and Z _min ( _À ) = (R _min .d) / (Zm), where m represents the diffraction order.

Spectral selectivity optical component for improving optical communication

The present invention relates to communication devices by visible and invisible light (IR and UV) of optical laser communication type and / or of VLC (acronym for Visible Light Communication) or LiFi (acronym for Light-Fidelity) type.

STATE OF THE ART

Optical communication devices use light waves to transmit information between two distant points.

Free space optical communication devices generally use lasers or laser diodes as the transmission means, and photodiodes as the reception means. The light beam emitted by the laser or laser diode is a coherent beam. It can therefore be modulated in amplitude and in phase.

Communication devices using visible light (VLC or LiFi) generally include an LED (or a module of LEDs) forming a transmission means, and a photo detector forming a reception means.

The LED provides an inconsistent light signal (Visible / IR / UV), the intensity of which is modulated according to the information to be transmitted. In particular for the visible area, LED luminaires have the advantage of allowing the dual function of lighting and data transmission. Their physical characteristics make it possible to envisage bit rates of the order of Gbits / s for dedicated systems. This LED can consist of a single blue chip covered with a phosphor to allow white lighting, or consist of several chips (RGYB for example).

The first type, a blue chip associated with a luminophore, is predominant in LED bulbs, but it is also often the factor limiting the bandwidth due to the response time of the phosphors. A blue chip can reach a bandwidth of 20-100MHz, while this same blue chip associated with a phosphor will see its performance reduced to 2-3 MHz.

To overcome this problem, it is possible to filter the signal in blue before the photo detector in order to keep only the response of the chip. However, conventional filters drastically reduce the signal level, sometimes making communication impossible.

Indeed, conventionally, the use of a blue filter not associated with a converging lens, only allows the use of 10% of the signal available before the filter. In the case of a multi-chip LED, each LED can be addressed independently of its neighbor, which means that the light amplitude from each chip can be modulated independently. We then get rid of the problem of the filter set out above if we only use one communication channel. For example in an LED bulb composed of 4 chips with a distinct spectrum, only the intensity of a chip is modulated and the others are only complementary to generate the white color. But we actually lose the potential of this type of LED bulb which could emit within the 4 communication channels via the 4 chips with the distinct spectrum. Thus each chip generates a particular communication channel which can be processed independently.

Such a communication system is advantageous in that it can integrate as reception means most of the photo-detecting surfaces associated with an information processing system making it possible to analyze the variation in the amplitude of the received light signal and d '' deduce the transmitted signal. In particular, most photovoltaic surfaces restore the variations of the optical signal in variation of the electrical signal.

In a multi-user optical communication environment, several optical channels are necessary so that each user can benefit from internet-type communication. For this, a solution consists in allocating a spectral width of communication to each user while keeping a universal receiver (the means implemented from a protocol point of view are not the subject of this patent application and therefore will not be not detailed).

.

However, the reception system must be able to filter the signal in the right frequency range, thus enabling optical communication.

The use of several optical channels of defined spectral width also makes it possible to increase the speed of a communication by using, for example, technologies of type Multiple (Multiple-Input Multiple-Output). This type of system requires spectral selection means such as color filters on the receiver side. However, the filters conventionally used allow a spectral selection in wavelength but do not concentrate the light received filtered on the photo detector, drastically reducing the level of signal received.

Specific lenses can both perform this filter and concentrator function. Diffractive lenses also called Fresnel lenses achieve this double function but are extremely sensitive to de-focusing, that is to say to the variation of the distance between the emitter and the source. When the defocusing is too great, in the case of the use of so-called Fresnel lenses, the communication may be lost.

In order to overcome this problem, one solution consists in using lenses with depth of field. These optical elements are known as axicon. However, axicons are essentially achromatic optics, they do not make it possible to obtain both a large depth of field and a spectral selectivity.

To overcome this problem, a new class of chromatic diffractive optics at depth of field has been proposed recently. These diffractive optics are known under the terms of ALDA (acronym for Axicon Linear Diffractive Diaphragmed Annularly) and MALDA (acronym for Multiple Axicon Linear Diffractive Diaphragmed Annularly). They have been the subject of in-depth studies, the results of which have been the subject of scientific publications: [1] E. Bialic, J.-L. de Bougrenet de la Tocnaye, Multiple annular linear diffractive axicons, JOSA A, Vol . 28, Issue 4, pp. 523-533 (2011), [2] E. Bialic, J.-L. de Bougrenet de la Tocnaye, Multispectral imaging axicons, Applied Optics, Vol. 50, Issue 20, pp. 3638-3645 (2011), [3] E. Bialic, J.-L. de Bougrenet de la Tocnaye, Selective color.

imaging using weighted interleaved multiple annula linear diffractive axicons, Applied Optics, Vol. 51, Issue 20, pp. 4775-4782 (2012)).

However, the means proposed in the literature do not make it possible to optimize the spectral selectivity as a function of the depth of field. They do not describe how to make an optic of the ALDA or MALDA type making it possible to select different spectral bands without any overlap, the objective being for example to obtain an optic which performs several filtering functions, red, green, blue for example, all maintaining a depth of field for each color. This is why the present invention proposes an optimization of the design of MALDAs in order to solve this problem.

PURPOSE OF THE INVENTION

The main purpose of the invention is to propose an optical device associated with a photo detector capable of selecting a spectral width, concentrating the signal and ensuring a depth of field in order to increase the bandwidth of an optical communication channel, which increases the communication speed without impacting other communication channels that can be used simultaneously.

OBJECT OF THE INVENTION

The invention relates to a fixed, deformable or modular optical device making it possible to perform the function of spectral selectivity and of light concentrator in the selected spectral width while maintaining a large depth of field. This optical device makes it possible to select an optical channel for wired or wireless optical communication.

More specifically, the subject of the invention is an optical component of spectral selectivity at depth of field for optical communication, comprising at least one annular diffractive grating (ALDA), defined by the following parameters:

- a minimum radius (R _m in);

- a maximum radius (R _ma x);

- a network period (d);

- Said parameters being determined so as to obtain a spectral separation of two adjacent wavelengths λ + dA and λ coming from a modulated light source external to said component, that is to say such as the maximum focusing distance (Z _ma x (A + dA)) of the wavelength λ + dÀ is less than the minimum focusing distance (Z _min (A)) of the wavelength λ,

said component being characterized in that said maximum and minimum focusing distances are calculated according to the following formulas: Z _ma x (A + dA) = (Rmax .d) / ((À + dÀ) .m) and Zmin (A ) ^- (Rmin .d) / (À.m), where m represents the diffraction order.

According to one embodiment, the optical component according to comprises a plurality of annular diffractive gratings (ALDAs) arranged to form a MALDA

7? ^(B) mm respecting the MALDA recurrence formula

- Rmin ⁽ⁿ⁾ corresponds to the minimum radius of the ALDA (n),

- Rmax ⁽ⁿ⁾ corresponds to the maximum radius of the ALDA (n),

- Â _max corresponds to the maximum wavelength present in the modulated light source,

- Δλ corresponds to the spectral width of said source, and:

- the spectral width Δλ is equal dA-ε, where ε represents the separation precision;

- Àmax is equal to the maximum value of the wavelength that one wishes to separate from the others, which can be different from the maximum length present in said source.

Advantageously, the annular diffractive network (s) are generated by a spatial light modulator.

The invention also relates to a reception system for optical communication comprising a photoreceptor associated with an optical component according to one of the preceding claims, characterized in that it further comprises a mechanical system making it possible to modify the distance between the photoreceptor and said optical component, so as to modify the wavelengths coming from the modulated light source focusing on the photoreceptor.

Advantageously, the system further comprises an adjustment means making it possible to generate a homothety of ratio k of said optical component in order to increase or decrease the parameters (R _m in, Rmax and d) of the ALDA.

According to one embodiment, the system is such that the photoreceptor comprises a plurality of active photo elements arranged annularly so that each of the different spectral widths coming from the modulated light source is concentrated on one of said active photo elements.

Preferably, the photoactive elements are photovoltaic elements.

Advantageously, the dynamic optimized MALDA system allows the use of a single receiver, compared to known systems which use as many receivers as there are optical channels.

DETAILED DESCRIPTION OF THE INVENTION

The invention is now described in more detail using the description of Figures 1 to 5 indexed.

FIG. 1A is a block diagram which represents a portion of a binary annular circular network in top view. It describes the layout of an ALDA.

FIG. 1B is a block diagram which represents the propagation of light rays through an optic of the ALDA type.

Figure 2 is a block diagram showing the arrangement of a MALDA in top view.

,

FIGS. 3A, 3B and 3C are block diagrams which represent the propagation of light rays through an optic of the ALDA or MALDA type when a receiver is placed along the optical axis.

Figure 4 is a block diagram which shows a diffraction pattern through an optic of the ALDA or MALDA type which occurs at a determined distance along the optical axis.

FIG. 5 is a block diagram which represents a photovoltaic multi-channel photo detector adapted to the diffraction figure presented in FIG. 4.

The optics optimized according to the invention is composed of at least one ALDA (1) of amplitude or phase (binary or multi-level) defined by its opening (R _ma x), its minimum radius (Rmin) and its period (d), which will hereinafter be called ALDA design parameters. An example of amplitude ALDA is presented in figure 1. This example represents a top view of the binary ALDA which can be composed of a glass plate covered with chromium, whose chromium has been partially removed according to the ALDA design rules. The dark areas correspond to the chrome areas. The light areas represent the areas of transparency.

The principle of operation of a binary phase ALDA seen in section is recalled in Figure IB. A portion of circular binary phase grating 12 diffracts the incident light 11 from an external modulated light source (not shown here) according to a controlled geometric deviation, dependent on the incident wavelength (for example 21 for the length d red wave at 640 nm, 22 for the green wavelength at 520 nm, 23 for the blue wavelength at 460 nm) and ALDA design parameters. These different wavelengths focus along the optical axis 13, for the maximum radius of the ALDA at 31, 32 and 33 respectively.

The geometric relationship used to calculate the maximum focusing distances Z _ma x and minimum Z _m in for a wavelength λ is given by the formula:

λ.ιη /? il ^ πμχ where m is the order of diffraction considered (explained for example in the doctoral thesis entitled "Study of a new axiconic optical element: the multiple linear axicon diffractive annular" published on January 10, 2011 by Emilie Bialic). The focusing length for a wavelength λ is given by the difference ΔΖ = Z _ma x Zmin

MALDA (acronym for Multiple Axicon Linear Diffractive Annular) is an axonic diffractive optic with depth of field but whose chromaticity is controlled. MALDA is a combination of annularly joined diaphragm axicons, each of whose chromatic focal points coincides perfectly.

An example of a diagram in top view representing a known binary MALDA is presented in figure 2. This MALDA is composed of 3 ALDAs, the external ALDA is noted 41, the middle ALDA is noted 42, and the interior ALDA is noted 43. MALDA is therefore a combination of contiguous ALDAs, each of whose chromatic foci coincides perfectly with its neighbor in order to increase its energy efficiency. In order to generate ALDAs such that their respective foci coincide perfectly, it is necessary to respect the recurrence formula (described for example in patent FR2979439B1) as follows:

R ^w min

1Δλ d (") 'max

WHERE Rmin ⁽ⁿ⁾ corresponds to the minimum radius of the ALDA (n); Rmax ⁽ⁿ⁾ corresponds to the maximum radius of ALDA (n), Àmax corresponds to the maximum wavelength present in the source and Δλ corresponds to the spectral width of the source. Δλ therefore corresponds to the difference between the longest wavelength present in the source and the shortest wavelength present in the source. For example for a white LED type source, Δλ = 640-460 nm = 180 nm.

However, the description of MALDA in the state of the art does not ensure sufficient precision for the separation of wavelengths in the,

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AA = dA-e where ε represents the precision of the separation.

In order to calculate the MALDA optimized to achieve spectral selectivity without overlapping of the working wavelengths, it is necessary to proceed as follows in the calculations of the parameters of the optics:

1. Define the separation value of the two wavelengths that one wishes to separate spectrally dA.

2. Choose for A _ma x the longest wavelength that you want to separate from the other shorter wavelengths.

3. Choose for ΔΑ the value of dA-ε where ε represents the precision of the separation.

For example, consider a LED source composed of 3 separate chips centered on the wavelengths of 640 nm (red), 520 nm (green) and 460 nm (blue). With a width at half height of the emission spectrum of the three wavelengths of 10 nm.

1. The separation value dA = min (640 nm-520 nm, 520 nm - 460 nm) = 60 nm

2. The largest wavelength considered is the red wavelength Amax = 640 nm.

3. The spectral width to be considered is therefore AA = dA-e where ε is greater than the width at half height, for example 2 times the width at half height, or ε = 20nm. In this case, the half-height spectral width is Δλ = 60 nm - 20 nm = 40 nm.

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The opening of this MALDA R _ma x was fixed at 5 cm. The maximum focusing distance Z _ma x (640nm) of the red wavelength at 640 nm has been set at 30 cm.

This MALDA has the following characteristics:

ALDA number R _max in pm Rmin in pm d in pm 1 5000 4687 38 2 4687 4394 40 3 4394 4119 43 4 4119 3862 46 5 3862 3620 49 6 3620 3394 53

It is then easy to check thanks to the formula for calculating the focusing distances that there will be no overlap. For this, it suffices to calculate the maximum focusing distance of the red wavelength, the minimum focusing distance of the green wavelength to ensure that the red and green color is not overlapped.

Zmax (640nm) ⁼ (5 * 38) / 640 m = 30 cm Zmin (520nm) ⁼ (4,393 * 40) / 520 m = 33 cm

We have the relation Z _ma x (640nm) <Z _m in (520nm), so there is no overlap between the green and red wavelengths.

And proceed in the same way for the green and blue wavelength. Zm _a x (520nm) ⁼ (5 * 38) / 520 m = 36 cm

Zmin ^ ôOnnn ^ (4,393 * 40) / 460 m = 38 cm

We have the relation Z _ma x (520nm) <Z _min (46onm), so there is no overlap between the green and blue wavelengths.

The properties of MALDA type optics were used to produce an optimized optical component, capable of generating controlled spectral selectivity, at depth of field, and the radiometric efficiency of which is maximized in order to select the communication channels (in infrared light). red, visible or UV) in ambient. These components can be used in two modes.

According to a first mode, this element is used as an optical component of spectral selectivity with longitudinal depth of field. This first mode uses the focusing characteristic of MALDA to generate the properties of spectral selectivity.

Such an optical channel selectivity optical system is equipped at least with a photoreceptor (40) placed along the optical axis (13) of the MALDA at a distance determined according to the design parameters of the MALDA. For example, in order to change the optical communication channel, the receiving system may be composed of a broadband photoreceptor, capable of collecting all of the available wavelengths, a MALDA and a displacement system (not shown) mechanical, electrical or optical of the photoreceptor (40) along the optical axis (13) of the MALDA.

In particular, a photoreceptor 40 of dimension in adequacy with the mode of propagation of the wavelengths diffracted by the optimized MALDA is placed along the optical axis in the focusing zones (respectively 31, 32 and 33) of the lengths d wave to be selected (respectively 21, 22 and 23) for the optical channel as shown in the diagrams of the respective figures 3A, 3B and 3C.

The size of said photoreceptor (40) can be easily calculated by a person skilled in the art as a function of the data published in the literature (Reference: the above-mentioned thesis).

A similar system can be used to move the MALDA while keeping the photoreceptor in a fixed position. MALDA as a binary diffractive optic can be used both in transmission and in reflection. It can also be used in special cases as a multi-level diffractive optic. In the latter case, the calculations must be adjusted accordingly, taking into account the potential prismatic aspects induced by said optic.

Judiciously, it can also be envisaged to keep the optics of spectral selectivity and the photoreceptor fixed for problems of reduction of congestion. In this case, the spectral selectivity optic is an optic whose | ρς rarartprictini ipc cnnt mnrli ilahloc

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According to a first variant, the means for adjusting the spectral selectivity relates to the use of a MALDA on a flexible support whose stretching by mechanical, electrical, optical means or any combination of means generates a homothety of ratio k of l integrality of the optics, thus modifying its intrinsic parameters of focusing and therefore of spectral selectivity.

For example, consider a MALDA optimized for the maximum wavelength A _max = 640 nm which focuses at the distance Z _{max (} 640nm) = Rmax-d / (A _ma xm) Z _f fixed. When homothetic, the observation is always made at the distance Zf but Zf = k. R _max .kd / (A ₂ .m) by equalization, we obtain that λ ₂ = A _ma x / k ² . Either for example if we carry out a homothety with a ratio k = l, l, the focusing wavelength at the distance Z _f becomes λ ₂ = A _m ax / l, 21 = 530nm corresponding to the color green. The recurrence formula remains true.

According to a second variant, the means for adjusting the spectral selectivity relates to the use of a spatial light modulator (SLM) in place of a conventional diffractive optic. Spatial light modulators are optoelectronic matrix components based on liquid crystals or micromirrors. There are two main families of SLMs, those which are electrically addressed and those which are optically addressed which require the modulation of an addressing light beam. Addressed modulators are divided into two subcategories. The first are based on MEMS technology (acronym for "Micro Electro Mechanical Systems"). They group together for example the family of micro-mirror arrays (denoted DMD, acronym of “Digital Micromirror Device”) and the family of deformable mirrors (denoted DM, acronym of “Deformable Mirror”). The second are based on LCD technology.

SLM can generally be considered as a microscopic network of pixels composed either of micro-mirrors or of liquid crystals. Advantageously, this spatial light modulator can be modified in real time and makes it possible to modify its characteristics to reveal the equivalent of the diffractive MALDA making it possible to choose the right optical channel via spectral selectivity. This system is particularly advantageous for solving the problems of multi-user allocation with changes in optical carriers (an optical carrier corresponds to a communication channel and corresponds to a narrow spectral band of the order of twenty nanometers).

The second mode of use concerns the use of this element as an optical component with radial propagation properties, associated with a photoreceptor with a specific design. Said photoreceptor comprises a plurality of active photo elements arranged annularly so that each of the different spectral widths coming from the modulated light source is concentrated on one of said active photo elements. Advantageously, the photo active elements are photovoltaic elements.

The component is then used either upstream or downstream of the focusing zones of the different wavelengths.

FIG. 4 represents a cross-sectional view of the pattern of propagation of the incident light after passing through the MALDA and upstream of the first focusing zone of the longest wavelength. For example, it represents a cross section of Figure IB before the focusing zone 31. The first ring in the center, corresponds to the convergence ring of the longest wavelength 21 (for example the color red). The intermediate ring corresponds to the intermediate wavelength 22 (for example the color green). The outer ring corresponds to the convergence ring of the shortest wavelength 23 (for example the color blue).

Advantageously, the shape of the photoreceptor will be adapted to the shape of the diffraction pattern in the determined plane. It will then for example be made of a broadband or narrowband photosensitive material, distributed over all or part of the corresponding convergence ring. Thus, the distribution of the photosensitive material 51, 52, 53, a diagram of which is presented in FIG. 5, will be made according to the shape of the convergence ring, respectively 21, 22 and 23 shown diagrammatically in FIG. 4.

In order to recover the electrical signal resulting from the electro-optical conversion of the photosensitive material, interconnection buses 54 and 55 will be integrated.

¹⁴

Advantageously, it will, for example, be possible to use photosensitive materials which are easy to work with, such as photovoltaic cells in inorganic or organic thin layers, for example cells of amorphous silicon, CIGS or polymer type.

ADVANTAGES OF THE INVENTION

The invention achieves the set goals.

This type of optic can be used in inter-satellite visible light communication systems by making it possible to select the communication wavelengths in space corresponding to the wavelengths not present in the spectrum of the sun. It allows in addition to its filtering function to concentrate the received light and thus to increase the range of the communication. Advantageously, the weight of the diffractive optic is negligible compared to the association of a refractive optic and a filter.

According to another aspect, the invention differs from the literature by the use of dynamic optimized MALDA, that is to say allowing the modification in real time of the choice of the spectral width. Thus, the system will be able to switch from a blue optical channel to a green optical channel for example without changing the optic, which is not possible by filter systems where it is necessary to change the filter to change the spectral band of transmission.

Claims (7)

1 - Optical component of spectral selectivity at depth of field for optical communication, comprising at least one annular diffractive grating (ALDA), defined by the following parameters: - a minimum radius (Rmin); - a maximum radius (R _ma x); - a network period (d); - Said parameters being determined so as to obtain a spectral separation of two adjacent wavelengths λ + dA and λ coming from a modulated light source external to said component, that is to say such as the maximum focusing distance (Z _maX (A + dÀ)) of the wavelength λ + dÀ is less than the minimum focusing distance (Zm _in (À)) of the wavelength λ, said component being characterized in that said maximum and minimum focusing distances are calculated according to the following formulas: Z _{max (À + d} A) = (Rmax .d) / ((À + dÂ) .m) and Z _min ( A) = (Rmin .d) / (À.m), where m represents the order of diffraction. 2 - Optical component according to claim 1, comprising a plurality of annular diffractive gratings (ALDAs) arranged to form a MALDA respecting the formula for the recurrence of MALDAs, where - Rmin ⁽ⁿ⁾ corresponds to the minimum radius of the ALDA (n), - Rmax ⁽ⁿ⁾ corresponds to the maximum radius of the ALDA (n), - At _max corresponds to the maximum wavelength present in the modulated light source, - Δλ corresponds to the spectral width of said source, characterized in that: the spectral width ΔΑ is chosen to be equal to dA-ε, where ε represents the separation precision, and - Amax is equal to the maximum value of the wavelength that one wishes to separate from the others, which can be different from the maximum length present in said source. 3 - Optical component according to one of the preceding claims, characterized in that the annular diffractive network (s) are generated by a spatial light modulator. 4 - reception system for optical communication comprising a photoreceptor associated with an optical component according to one of the preceding claims, characterized in that it further comprises a mechanical system making it possible to modify the distance between the photoreceptor and said optical component, so as to modify the wavelengths coming from the modulated light source focusing on the photoreceptor. 5 - reception system for optical communication comprising an optical component according to one of claims 1 and 2, characterized in that it further comprises an adjustment means making it possible to generate a homothety of ratio k of said optical component to carry out increasing or decreasing the parameters (Rmin, Rmax and d) of ALDA. 6 - reception system according to one of claims 4 or 5, characterized in that it comprises a photoreceptor comprising a plurality of active photo elements arranged annularly so that each of the different spectral widths coming from the modulated light source is concentrated on one of said active photo elements. 7 - reception system according to claim 6, characterized in that the photoactive elements are photovoltaic elements. 1/4