Optical modulation system

Abstract

The present invention relates to an optical modulation system based on the interaction of two collinear light beams having different wavelengths passing through a cuvette containing a solution of Cs2SnI6, comprising a pumping laser emitter emitting a modulated pumping beam with a wavelength between 355 nm and 1550 nm and with a variable frequency; a test laser emitter emitting a continuous wave test beam at 633 nm and with a power limited to 10 mW; a beam splitter where the pumping and test beams are combined to strike on the cuvette; a pass filter that removes the signal from the pumping beam; a spatial analysis camera; a time / frequency analysis equipment with a photodetector, a commercial lock-in amplifier and an oscilloscope; and a signal generator.

Description

[0001] OPTICAL MODULATION SYSTEM

[0002] DESCRIPTION

[0003] Object of the invention

[0004] The present invention concerns a novel type of optical modulation system based on the interaction of two light beams having different wavelengths passing through a cuvette filled with a solution of perovskite nanocrystals.

[0005] The invention falls within the industrial sector related to optical switching with direct application in telecommunications and photonics systems and, more specifically, within the technology known as "all-optical" modulation.

[0006] Background of the invention

[0007] It is known that, in the context of telecommunications or electronics, modulation is a technique by which the original form of an information signal is converted into a form more suitable for transmission or processing. This technique is essential in many applications, including high-speed data transmission systems, satellite communication systems, radio and television broadcasting, mobile communications, fibre optic systems, data centres, or radars, among others.

[0008] The present invention relates to optical modulators, which are devices that manipulate light by modifying its properties, thus opening up a wide range of applications including, among others, telecommunications, imaging, optogenetics, optical manipulation with optical fibres, microscopy, optical tweezers, optical cryptography, quantum computing or 3D printing.

[0009] As a general rule, known devices are based on the modification of the refractive index of the material from which they are made by means of an external signal, which can be electrical, thermal or optical. In particular, the most common approach to developing said functionality has been the use of "electro-optical" type modulators whereby a light carrier is controlled by an electrical signal.

[0010] However, it is known that there is a more efficient way which consists of an interaction between two beams of light travelling at different wavelengths. The advantage of this alternative is that it avoids intermediate electrical conversions, which introduce losses and reduce the efficiency of the final device, which can represent a future disruptive technology to break the current barriers used by techniques limited by switching speeds and high-power dissipation. Full optical control of light can be achieved by non-linear processes, whereby the refractive index and the absorption of a given material changes with the power density of the light beam passing therethrough. However, the magnitude of the non-linear coefficients of the most common materials is usually very low, resulting in the use of too high powers and / or the need to use ultra-short pulses, which, among other limitations, increases the cost and power dissipation. In fact, the most common solutions used today use very complex optical architectures which require complicated and expensive technology.

[0011] Specifically, among others, solutions have been proposed based on the displacement of the resonance in photonic structures such as cavities or metasurfaces; controlling the transmission of light through a medium by means of a saturable absorption process; and using systems that increase light-matter interaction, such as polaritons or surface plasmons. These strategies, in addition to requiring high power and complicated manufacturing processes, are based on indirect modulation methods and in some cases require very specific conditions, such as characterisation at cryogenic temperatures, to be observed. In fact, it is well known that modulation between two optical signals is extremely complex to obtain due to the absence of a material with sufficiently high non-linear coefficients and appropriate modulation techniques, as simple as possible, to exploit said properties efficiently.

[0012] In this sense, the present invention is based on a simpler optical modulation technique based on the use of emerging materials with high non-linear properties within a specially designed system to obtain an optimised modulation / switching of light with light, which is known as "all-optical" modulation.

[0013] In this regard, document ‘Ecofriendly Perovskites with Giant Self-Defocusing Optical Response’ by Suarez Isaac, et al. is known, which discusses the nonlinear and self-defocusing properties of lead-free perovskite nanocrystals (Cs2Snle). This document discloses significant changes in the refractive index under the striking of relatively small intensities (< 1 GW cm-2), for which it discloses an experimental system that tests the aforementioned non-linear properties of Cs2Snle, wherein the system comprises a laser source with a beam aligned with a lens which is followed by a cuvette of Cs2Snle nanocrystal solution, and after the beam exit, a CCD is located behind a second lens which analyses the shape and intensity of the beam, and where the self-defocusing phenomenon is observed and analysed. However, this document neither suggests nor proposes the implementation of any modulation device, only the characterisation of the non-linear parameters.

[0014] Also known is the disclosure in document "Perovskite CsPbX3: A Promising Nonlinear Optical Material and Its Applications for Ambient All-Optical Switching with Enhanced Stability", by Leiming Wu, et al. in which the changes in the non-linear refractive index are presented, through several experimental set-ups, one experiment being of special interest in which the striking of two beams of different wavelengths in a cuvette of non-linear material (CsPbh QD and / or CsPbBrxh-x NS) is shown, and the possibilities of these non-linear materials in the context of all- optical modulation are indicated. For this purpose, a suitable modulation system is required that allows exploiting said properties efficiently. However, in this document the final ring formation is the result of a reorientation of the nanocrystals with the striking beam, which induces a birefringence of the medium and therefore requires another physical mechanism different from that of the present invention; the cuvette is illuminated with beams which are not collinear and which are adjusted with different lenses, i.e. forming a certain angle, or even illuminating different regions of the cuvette; and this proposed switching is caused by interferences produced between the rings formed by the two beams illuminating the cuvette, usually both of high power, and indirectly observing changes of the signal intensity by spatially filtering the light passing through the cuvette before being collected by a photodetector. Also, this publication does not use a modulated laser, which is essential when characterising the effect as a function of time and frequency. This implies an incomplete characterisation. Furthermore, it should be noted that, in contrast to what is disclosed in this background, the present invention describes a system based on the use of two collinear test and pumping beams that allows the waveguide formation indicated in that background to be exploited more efficiently as the beams travel through the same region of the cuvette and, in addition, defines a system whereby the effect can be characterised as a function of time and frequency.

[0015] Considering the techniques known in the state of the art, the present invention differs from any of them in the fact of using two collinear beams and defining a complete characterisation system based on an illumination with the aforementioned two collinear beams, which allows obtaining the advantages of: reducing the power of the test beam below the non-linear threshold so that it does not alter the effective waveguide formation and is coupled to the non-linear modes of propagation, thus undergoing a much more efficient phase shift; where it should be noted that the previously mentioned background requires using two beams, pumping and test, both of high power, and without significant effects on the switching result; that the use of two collinear beams facilitates subsequent integration with other types of optical structures, such as waveguides or fibre optics, which is not possible with the systems described in this background which are based on a non-collinear illumination; and, the system described in the present invention allows characterising the effect as a function of time and frequency, which is essential for a complete characterisation of the effect, something that is neither described nor mentioned in any of the systems known in the prior art.

[0016] The applicant is not aware of any optical modulation system that is either similar to or as advantageous as the one described and claimed below.

[0017] Description of the invention

[0018] The invention is a novel type of optical modulation system that solves, in its various embodiments, the problems and limitations present in the prior art, and where it should be noted that an "all-optical" modulation system is a system that modifies the parameters (amplitude, phase, polarisation) of an optical signal, called the test signal, by means of another optical signal, or pumping signal, travelling at a different wavelength.

[0019] In this sense, as previously mentioned, the all-optical modulator system of the present invention allows to efficiently exploit the non-linear properties of a solution of Cs2Snle nanoparticles; unlike any other known technology, it is based on striking or illuminating with two collinear beams of light into the cuvette, one for testing and the other for pumping, so that there is a non-linear interaction therebetween; it analyses the modulation or switching processes between the two beams, both spatially and temporally, allowing the results to be studied and figures of merit to be determined; and it allows the resulting modulated beam to be coupled to another system for subsequent application, such as study in an oscilloscope or an electrical amplifier. Going into detail on the configuration of the system, it comprises: a laser emitter emitting a beam modulated in a wavelength range from 355 nm to 1550 nm as a pumping signal and having a variable frequency between 1 Hz and 1 MHz, and controlled by a signal generator, and which may comprise neutral filters at its output to control the power; wherein preferably the range is between 455 nm and 1064 nm, and in one embodiment of the invention a 1064 nm Nd:Yag laser pulsed at 1 ns and modulated at 20 kHz is used; a laser emitter emitting a continuous wave HeNe beam with a wavelength of 633 nm as a test signal with a power limited to 10 mW with a neutral filter so that it does not generate non-linear effects by itself; a plurality of signal alignment mirrors; at least one focus lens, wherein there may be a lens for a lens in excitation to increase the power density and / or, there may be at least another lens in collection depending on the active area of the photodetector. a beam splitter lens, also known as a beam splitter, whereby the pumping and test beams are combined in a collinear fashion so that they collaterally strike on the cuvette; a cuvette filled with a solution of Cs2Snl6nanoparticles dissolved in toluene; in a preferred embodiment of the invention, the cuvette is 1 mm thick and is filled with a solution of a nonlinear material that exhibits a nonlinear refractive index change; and at a Cs2Snl6concentration of at least 50 mg / ml; wherein the two pumping and test beams pass through the cuvette and interact such that the pumping beam alters the refractive index of the non-linear material and creates an effective waveguide which affects the propagation properties of the test beam, throughout the visible and infrared spectrum; and, where, as a consequence of the formation of an effective waveguide within the solution, the spatial distribution of the test beam is changed in the form of concentrical rings by the intensity of the pumping power; a pass filter, which removes the pumping signal from the detection, which is a high pass filter when the wavelength of the pumping signal beam is shorter than the wavelength of the test beam, and which is a low pass filter in the case where the pumping signal beam is at a longer wavelength than the test signal beam; i.e., it is a high pass filter when the wavelength of the pumping beam is comprised between 355 nm and 633 nm; and it is a low pass filter when the wavelength of the pumping beam is between 633 nm and 1550 nm; and 633 mm being either of the two types of filters; a spectral analysis CCD camera, whereby the spatial distribution of the output signal is displayed as a function of pumping power; time / frequency analysis equipment, comprising a photodetector that collects the optical output signal and converts it into an electrical signal, covering the bandwidth required by the system of up to 1 MHz an oscilloscope that measures the signal as a function of time; and an electrical signal amplifier, preferably an electrical amplifier synchronised to a frequency of that which is commercially known as a lock-in amplifier, hereinafter referred to as a lock-in type amplifier, which treats the signal so that it can be easily distinguished from noise, so that it detects the signal at a given frequency given by the synchronising signal; thereby eliminating noise and improving the signal level; and allowing a frequency sweep to characterise the frequency response; and a signal generator that controls the modulated laser parameters and provides the synchronising signal to the lock-in amplifier and oscilloscope.

[0020] With this configuration, the system allows illuminating the cuvette with the two collinear beams, improving the interaction therebetween; and obtaining a total characterisation of the signal, since it studies the effects of spatial phase modulation with a CCD camera; measures the signal as a function of time with the oscilloscope; measures the signal as a function of frequency with the lock-in type amplifier; and performs a sweep of the pumping parameters (frequency or power density) and studies how they influence the spatial, temporal or frequency response; in addition to the fact that it would allow the system to be extended to other test wavelengths, by simply replacing the HeNe laser with a continuous wave laser at a different wavelength; and using a different pumping laser.

[0021] In this sense, the amplifier synchronised to the modulation frequency, or lock- in amplifier, has the advantage of allowing noise to be eliminated and the frequency at which it is synchronised to be easily changed, so that the analysis of the frequency response is straightforward. Other types of amplifiers, mainly those not synchronised to a frequency, have more noise and would require a second device to filter or allow observation of the full bandwidth. For example, using a network analyser or directly with the oscilloscope, but the former is much more expensive and the latter is less accurate. The use of a "lock-in" type amplifier allows for an optimal solution within the efficiency of the system.

[0022] With these aspects in mind, the present invention discloses a novel all-optical modulation technique based on the interaction of two light beams having different wavelengths, test and pumping beams, passing through a cuvette filled with a solution of a non-linear material. When the power of the pumping beam exceeds a certain threshold, there is a change in the refractive index of the material and consequently a change in the shape of the beam passing through the cuvette. In particular, the fact that the non-linear material is defocused causes the output beam to widen and form a series of concentrical rings. The demonstration of optical modulation lies in the fact that the refractive index variation generated by the pumping beam is followed by a low-power test beam, resulting in the shape of the test beam also being modified.

[0023] All in all, this system is considered to be a significant step forward in the implementation of light beam switches compared to that which has been known to date. In this sense, the present invention aims at the application of this system, in a direct way, in telecommunications and integrated optics technologies. Nevertheless, since it is based on beam shape change, it is also usable in other fields such as sensors or imaging, e.g. with the functionality of: on / off with optical control of a carrier at a given wavelength, resulting in "all-optical" switching of the signal; acting as a phase and shape modulator of the test beam, since the frequency of the pumping beam can be controlled; in spatial multiplexing of the modulated signal; because the effect is achieved only in the area where the beam is focused, it could be contemplated to focus the beam in specific areas of the cuvette, or even to use several pump beams, so that depending on the cuvette region, the light undergoes a different modulation process; definition of the light beam profile with optical control; detection of particles / gases by means of refractive index variations resulting in more / less variation of the beam shape; and imaging.

[0024] It should be noted that, throughout the description and claims, the term "comprises" and the variants thereof are not intended to exclude other technical features or additional elements. Brief description of the figures

[0025] In order to complete the description and to help a better understanding of the features of the invention, a set of figures and drawings is presented in which the following is represented by way of illustration and not limitation:

[0026] Figure 1 shows a schematic representation of the optical modulation system which is the subject of the present invention.

[0027] Figure 2 shows images of a beam passing through a cuvette (6) of Cs2Snle with a pulsed laser at 1064 nm (1 ns, 2 KHz) and which is detected by a CCD camera (8), where it is observed that when pumping power is increased, the beam broadens and diffracts into a set of concentrical rings. This would be an example of a system as disclosed in document "Ecofriendly Perovskites with Giant Self-Defocusing Optical Response".

[0028] Figure 3 shows images of the illumination of a cuvette (6) of Cs2Snle with two collinear wavelengths and having a pass filter (7), namely a low pass filter, and is detected by a CCD camera (8), and demonstrating that a non-linear interaction occurs therebetween; specifically, in addition to the pulsed beam at 1064 nm (1 ns, 20 kHz), a test beam at 633 nm is introduced the power of which is limited to less than 10 W / cm2so that it does not generate non-linear effects by itself; and as a result, it can be seen that the refractive index variations induced by the pumping beam affect the test beam such that the light at 633 nm passing through the cuvette undergoes a variation in shape with the power of the pumping beam, i.e. widening and generating concentrical rings depending on the power, and where the optical phase modulation of a beam is demonstrated.

[0029] Figure 4 shows images of a complete optical modulation with a system such as defined in the present invention, which illuminates the cuvette (6) with two collinear beams at different wavelengths oriented by a beam splitter lens (4), one of them of continuous wave and low power (633 nm), and another one at 450 nm power sufficient to generate non-linear effects and modulated with a square wave at different frequencies (1 Hz and 1 MHz); and wherein the "all-optical" modulation is demonstrated, after passing through the high-pass filter (7) and with a CCD camera (8), because the test beam can be observed to switch with the variations of a modulated beam, the frequency of which has been swept between 1 Hz and 1 MHz.

[0030] Figure 5 shows images of the shape of the test signal with the "all-optical" modulation at frequencies between 1 Hz and 50 MHz. Figure 6 shows images of the time response with the "all-optical" modulation of the pumping and test signal at different frequencies between 10 Hz and 10 kHz. In this figure, the blue colour signal (CA) corresponds to the pumping signal and the red colour signal (CR) to the test signal, where SD is the signal detected in an arbitrary unit and T is the time in ms; and image (A) is for a frequency of 10 Hz, image (B) is for 100 Hz, image (C) is for 1 kHz, and image (D) is for 10 kHz.

[0031] Detailed description of an embodiment of the invention

[0032] One embodiment of the optical modulation system of the present invention, which is based on the interaction of two collinear light beams having different wavelengths passing through a cuvette filled with a solution of perovskite nanocrystals, is such that it comprises: a pumping laser emitter (1 ) emitting a signal modulated at 450 nm as a pumping beam (a) and having a variable frequency between 1 Hz and 1 MHz, and controlled by a signal generator; which may comprise neutral filters at its exit to control the power; and wherein in a preferred embodiment of the invention a Nd:Yag laser pulsed at 1 ns and modulated at 20 kHz is used; a test laser emitter (2) emitting a 633 nm HeNe continuous wave signal as a test beam (b) with a power limited to 10 mW with a neutral filter so as not to generate non-linear effects of its own; a plurality of pumping and test beam alignment mirrors (3); a beam splitter lens (4), also known as a beam splitter, whereby the pumping (a) and test (b) beams are combined in a collinear fashion so that they collaterally strike on the cuvette; at least one excitation focus lens (5) for the purpose of increasing the power density and / or there may be at least one collection lens depending on the active area of the photodetector; a cuvette (6) filled with a solution of Cs2Snl6nanoparticles dissolved in toluene; in a preferred embodiment of the invention, the cuvette is 1 mm thick and is filled with a solution of a non-linear material exhibiting a non-linear refractive index change; and at a Cs2Snl6concentration of at least 50 mg / ml; wherein the two pumping (a) and test (b) beams pass through the cuvette (6) and interact such that the pumping beam alters the refractive index of the nanoparticles and creates an effective waveguide which affects the propagation properties of the test beam; and as a consequence of the formation of an effective waveguide within the solution, the spatial distribution of the test beam is modified in the form of concentrical rings by the intensity of the pumping power, a pass filter (7), which is a high pass filter, whereby the pumping signal is removed from the detection, and which can be replaced by a low pass filter in the case where the pumping signal is at a wavelength longer than the test signal; a spectral analysis camera (8) or CCD camera detector, which displays the spatial distribution of the output signal as a function of pumping power; time / frequency analysis equipment, comprising: a photodetector (9) that collects the optical output signal from the test beam (b) and converts same into an electrical signal, which covers the required system bandwidth of up to 1 MHz; an oscilloscope (10) that measures the signal as a function of time; and a lock-in type signal amplifier (11 ) that treats the signal so that it can be easily distinguished from noise by detecting the signal at a given frequency given by the synchronising signal; thereby eliminating noise and improving the signal level; and allowing a frequency sweep to characterise the frequency response; and a signal generator (12) that controls the modulated laser parameters and provides the synchronising signal to the lock-in amplifier and oscilloscope; so that the system allows obtaining a total characterisation of the signal, since it studies the effects of spatial phase modulation with a camera; measures the signal as a function of time with the oscilloscope; measures the signal as a function of frequency with the lock-in amplifier; and sweeps the pumping parameters (frequency or power density), and studies how they influence the spatial, temporal or frequency response.

[0033] In another embodiment of the invention, the system components are the same except that the pumping laser emitter (1 ) emits a modulated signal at 1064 nm as the pumping beam (a), and the pass filter (7) is a low pass filter.

[0034] The optical modulation of the embodiment of the present invention is demonstrated by analysing the variation of the test beam with different pumping frequencies, and allowing a complete characterisation of the effect. From Figures 2 to 6, it can be seen that the present invention describes a novel "all-optical" modulation technique based on the interaction of two light beams having different wavelengths, test and pumping beams, passing through a cuvette filled with a solution of a non-linear material, so that when the power of the pumping beam exceeds a certain threshold there is a change in the refractive index of the material, and consequently a change in the shape of the beam passing through the cuvette. In particular, the fact that the non-linear material is defocused causes the output beam to widen and form a series of concentrical rings. The demonstration of optical modulation lies in the fact that the refractive index variation generated by the pumping beam is followed by a low-power test beam, resulting in the shape of the test beam also being modified; and, from here, as seen in the last figure, the optical modulation process is demonstrated using a pumping beam modulated with a square wave.

Claims

CLAIMS1 . An optical modulation system comprising: a pumping laser emitter (1 ) emitting a modulated signal as a pumping beam (a); at least one focus lens (5); a cuvette (6) containing a solution of Cs2Snle nanoparticles where the modulated signal strikes; and a spatial analysis CCD camera (8) where the spatial distribution of the modulated output signal is displayed as a function of the pumping power; and which is characterised in that the pumping laser emitter (1 ) emits a beam at a wavelength comprised between 355 nm and 1550 nm and at a variable frequency between 1 Hz and 1 MHz; comprises a test laser emitter (2) emitting a continuous wave signal at 633 nm as a test beam (b) with a power limited to 10 mW with a neutral filter; comprises a plurality of pumping (a) and test (b) beam alignment mirrors (3); comprises a beam-splitter lens (4) where the pumping beam (a) and test beam (b) are combined in a collinear fashion to collaterally impact on the cuvette (6); and wherein both beams pass through the cuvette (6) and interact such that the pumping beam alters the refractive index of the nanoparticles and an effective waveguide is created which affects the propagation properties of the test beam (b); comprises a pass filter (7) which removes the signal from the pumping beam (a) from analysis in the spatial analysis camera (8) and from analysis in a time / frequency analysis equipment; comprising time / frequency analysis equipment comprising: a photodetector (9) that collects the optical output signal from the test beam (b) and converts same into an electrical signal; an oscilloscope (10) that measures the signal as a function of time; and an amplifier (11 ) synchronised to the modulation frequency that eliminates noise and measures the signal as a function of frequency; and comprising a signal generator (12) that controls the parameters of the pumping laser emitter (1 ) and provides a synchronising signal to the amplifier (11 ) and the oscilloscope (10).

2. The system according to claim 1 , wherein the pass filter (7) is a high pass filter when the wavelength of the pumping beam (a) is comprised between 355 nm and 633 nm.

3. The system according to claim 1 , wherein the pass filter (7) is a high pass filter when the wavelength of the pumping beam (a) is comprised between 633 nm and 1550 nm.

4. The system according to claim 1 , wherein the pumping laser emitter (1 ) is a Nd:Yag type laser pulsed at 1 ns and modulated at 20 kHz.

5. The system according to claim 1 , wherein the pumping laser emitter (1 ) comprises neutral filters at its power control output.

6. The system according to claim 1 , wherein the cuvette (6) is filled with the solution of Cs2Snle nanoparticles dissolved in toluene, at a Cs2Snle concentration of at least 50 mg / ml.

7. The system according to claim 1 , wherein the test laser emitter (2) emits a HeNe continuous wave signal.

8. The system according to claim 1 , wherein the signal amplifier (11 ) is a commercial lock-in amplifier.

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