Free-space optical communication system

Abstract

This invention provides a plurality of mutually incoherent fundamental light beams (R1~R N A modulation device (DM) that generates each fundamental light beam (R1~R N A modulation device modulates the same digital data (M) that is transmitted, and the fundamental light beam (R1~R N ) combines, thereby producing a light emission called "coupled" light emission (R c The present invention relates to a free-space optical communication system (1) comprising a processing device (DT) configured to generate coupled light emission (R). The system is also optically coupled downstream of the processing device (DT) and c The device includes a beam-expanding optical device (T) that receives light and propagates free-space transmitted radiation (I).

Description

[Technical Field]

[0001] This invention relates to an optical communication system designed to overcome transmission difficulties associated with wavefront distortion during the propagation of optical radiation. This distortion can arise from atmospheric disturbances during optical communication in free space. More generally, this distortion is caused by the propagation of the optical beam in its medium. This invention has particular applications in the field of free-space optical communication. [Background technology]

[0002] In free-space optical communication, the transmitter modulates optical radiation (usually generated by a laser) with the information to be transmitted. This optical radiation is emitted in the direction of the receiver, typically in the form of a narrow transmitting beam of a few centimeters to one meter in width. After propagating through its medium (air is used as an example for the remainder of this explanation, but the medium can be of any properties, such as water in the case of underwater communication), the optical radiation is collected by the receiver and demodulated to recover the transmitted information.

[0003] As radiation generated by a transmitter propagates, it is subject to atmospheric disturbances, particularly temperature and pressure fluctuations, which alter the optical refractive index and distortion of the radiation. These irregular disturbances, with fluctuating dynamics typically between 100 Hz and several kHz, result in deformations that affect the wavefront. More precisely, disturbances tend to spatially redistribute the energy within the radiation, generating random fluctuations in amplitude and phase. This deformation is embodied in the form of "speckle" patterns within spots formed by the projection of the beam onto a radiation collection device and by scintillation. This limits the data rate on the link between the transmitter and receiver, and therefore the quality of the link. This is especially true when the pupil of the radiation collection device is relatively small compared to the size of the beam when it reaches the device, and therefore little energy of the transmitted beam can be captured. This is particularly true for "uplink" communications to receivers on satellites, or for ground-based "point-to-point" communications where the transmitted beam propagates entirely through the atmosphere. [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] One object of the present invention is to propose an optical communication system that overcomes the aforementioned problems at least partially. More specifically, one object of the present invention is to provide a free-space optical communication system that makes communication more reliable and / or maintains a data rate despite disturbances caused by the propagation medium. [Means for solving the problem]

[0005] To achieve this objective, the subject matter of the present invention is: -A modulation device that generates multiple mutually incoherent fundamental light beams, each fundamental light beam modulating the same transmitted digital data with respect to the modulation device, - A processing device configured to be optically coupled downstream of a modulation device, to combine the fundamental light beam, and thereby generate light emission referred to as "coupled" light emission, We propose a free-space optical communication system comprising a beam-expanding optical device that is optically coupled downstream of a processing device, receives coupled light radiation, and propagates free-space transmitted radiation.

[0006] According to other advantageous non-limiting features of the present invention, either alone or in any technically feasible combination, - The processing device comprises at least one multiplane conversion device, - The multiplane conversion device comprises a plurality of optical components, each having a reflective surface for inducing the propagation of a fundamental light beam, and at least one of the reflective surfaces has a microstructure configured to combine the fundamental light beam and form coupled light emission in the process of multiple reflections. -The microstructure is also configured to shape the fundamental light beam that constitutes the coupled light emission. - The system is configured to adapt the coupled light emission to a flat-top shape. - The flat top shape has a circular cross section so as to form a ring or a disk, - The modulation device includes a plurality of light sources associated with a plurality of modulators to generate a plurality of basic light beams, - The light sources emit radiation of different wavelengths, - The modulation device includes a broadband source, - The optical communication system includes an offset optical fiber optically disposed between the output port of the processing device and the beam expansion optical device to guide the propagation of the recombined light beam, - Each of the basic light beams is adapted to a linear combination of modes having similar group delays, - The offset optical fiber (F) is a degenerate lifting fiber such as a step index fiber or an elliptical core fiber, - The optical communication system includes a shaping optical block disposed between the offset optical fiber and the beam expansion optical device, - The beam expansion optical device (T) is a telescope or a lens assembly.

Brief Description of the Drawings

[0007] Other features and advantages of the present invention will become apparent from the following detailed description of the present invention when reference is made to the accompanying drawings. [Figure 1] An optical communication system according to the present invention is shown. [Figure 2] An embodiment of a multi-plane conversion device is shown. [Figure 3] The ring shape of the recombined radiation is shown.

Mode for Carrying Out the Invention

[0008] For clarity, in the present disclosure, an optical radiation or an optical beam is defined as radiation formed from at least one mode of an electromagnetic field, and each mode forms a spatial-frequency distribution of the amplitude, phase, and polarization of the field.

[0009] The “shape” of radiation or beam means the transverse distribution of the amplitude and phase of the modes, or the combination of the transverse amplitude and phase distributions of the modes constituting this radiation.

[0010] Referring to [Figure 1], the optical communication system 1 according to the present invention is designed to generate transmitted radiation I that carries information to be transmitted by modulation. The transmitted light radiation is supplied to an optical receiver that can extract information from the received radiation. In the embodiment shown herein in [Figure 1], the optical receiver is located on a satellite SAT, but the communication system of the present invention is not limited to this particular application. Generally speaking, the transmitter and receiver can be located on land, at sea, or in space, and can propagate in any free space, the atmosphere in the case of land communications, or underwater in the case of ocean communications. The transmitter and receiver can be stationary or moving relative to each other.

[0011] The transmitted light radiation I takes the form of a narrow beam directed towards the receiver. As the emitted radiation propagates through free space, it is subjected to atmospheric disturbances, so the incident light radiation I reaching the base station housing the receiver has spatial and temporal variations in amplitude and phase. This phenomenon affects the shape of this radiation, which tends to take an unstable, irregular, and time-varying shape, spreading and distorting spatially, so that the energy actually received by the radiation collection device at the receiver is limited or even intermittent. The optical communication system of this disclosure 1 aims to overcome these difficulties, at least in part, by creating spatial diversity in the transmitted light radiation I, which allows the effects of the variation to be attenuated so that the optical receiver OR can use the radiation to decode the transmitted message. This attenuation of variation is based on the fact that each spatial mode of the transmitted light radiation I perceives a different atmospheric layer and therefore creates different speckle patterns on the target. If the light beams carried by these spatial modes are incoherent to each other, their intensities are added together and averaged to form a more uniform pattern.

[0012] For this purpose, the optical communication system 1 uses multiple fundamental light beams, mutually incoherent fundamental light beams R1~R N Includes a modulation device DM that generates each fundamental optical beam R1~R N It is modulated by the same digital data M that is transmitted. Generally speaking, increasing the number of fundamental light beams increases the spatial diversity of transmitted radiation I and the robustness of the link, but at the cost of making the system more expensive. In practice, fundamental light beams R1~R N The number may be 2 to 50, preferably 2 to 15.

[0013] "Mutually incoherent" is understood to mean that the light beams are unlikely to interfere with each other on a timescale comparable to or longer than the duration of a single symbol of digital data M. Typically, the target data rate is 10 Gbaud / s to 50 Gbaud / s, or even up to 100 Gbaud / s.

[0014] Digital data can be encoded and modulated according to any suitable protocol, such as amplitude modulation or intra-dynamic coherent modulation.

[0015] The modulation device DM comprises at least one light source associated with at least one modulator from which digital data is transmitted. Any modulation technique is preferred, such as acousto-optic modulation (AOM), electro-optic modulation (EOM), semiconductor optical amplifier (SOA), or direct modulation of a laser source.

[0016] The generated basic light beams R1~R N The incoherent nature of can be achieved in several ways available to those skilled in the art.

[0017] As an example, a light source can be selected to exhibit a broad spectrum. The width of the light source spectrum is determined by the target throughput and the number of fundamental light beams. For example, a 10 Gbaud transmission using 10 fundamental light beams would mean selecting a light source spectrum width greater than 100 GHz.

[0018] Each fundamental light beam can be formed from a portion of this spectrum, which is obtained by filtering.

[0019] Alternatively, the light emission generated by a broad-spectrum source and modulated by a modulator can be separated to create multiple fundamental light beams R1~R N This can be generated. The modulation device DM selects a delay that is large with respect to the coherence time of the light source but small with respect to the duration of the modulation symbol, thereby generating the fundamental light beam R1~R N The optical path may include multiple adjustable delays, each positioned within it. The delays can be achieved using optical fibers of different lengths, patch cords of different lengths, or free-space lines with fixed or adjustable delays.

[0020] For example, a broad-spectrum source could be an Amplified Spontaneous Emission (ASE) source, a Superluminal Emitting Diode (ESE), or an EDFA source (erbium ion-doped fiber optic amplifier). Alternatively, it could be a spectrally sliced ​​incoherent light source as described in D. Lee, VV Mai, and H. Kim, "Mitigation of Scintillation in FSOC Using RSOA-Based Spectrum-Sliced ​​Incoherent Light," IEEE Photonics Technology Letters, vol. 33, no. 5, pp. 227-230, March 1, 2021.

[0021] However, advantageously, the modulation device DM comprises a plurality of light sources associated with a plurality of modulators in order to independently generate a plurality of basic optical beams R1 to R N The light sources can be selected to emit radiation having wavelengths that are different from each other and that are sufficiently separated (e.g., different from 10 GHz or 100 GHz or several hundreds of GHz) to ensure their non-coherence. This approach is advantageous in that it can generate a high-power transmitted optical radiation I by combining a plurality of independent light sources.

[0022] Continuing with the description of [Figure 1], the optical communication system 1 also comprises a processing device DT optically coupled to the modulation device DM downstream of the modulation device DM. A plurality of optical connection fibers, e.g., a plurality of single-mode fibers, are optically arranged between the modulation device DM and the processing device DT to guide the basic optical beams R1 to R N to the input ports of the processing device DM.

[0023] The processing device DT is configured to combine the basic optical beams R1 to R N to generate a so-called "combined" optical radiation R c "Combined optical radiation" refers to the radiation formed by the co-propagation (i.e., propagation in a single direction) of the basic optical beams R1 to R N .

[0024] In addition to this combining function, the processing device can also be used to shape the basic optical beams R1 to R c constituting the combined optical radiation R N .

[0025] The processing device DT can be generated by bundling the ends of the connecting optical fibers parallel to each other. This fiber bundle can be collimated using a micro-lens if necessary. In either case, the optical radiation emerging from the fiber bundle forms the combined radiation R c . The fiber bundle can be arranged in a matrix, and more generally, the ends of the fibers within the bundle are the combined optical radiation R cTo optimally form the optical fiber bundle, it can be arranged linearly or planarly, for example, in a disk shape or inscribed within a disk. The optical fiber bundle can be arranged such that the ends of the fibers are arranged in a ring shape, where the central part of the ring is not provided with fiber and corresponds to the shielded central region of the telescope, as described in later sections of this disclosure.

[0026] The processing device DT also emits coupled light R c The basic light beams R1~R that make up the structure N The optical components may include, for example, at least one diffractive optical element (DOE), a spatial light modulator (SLM), an optical system that images or includes at least one lens, an axicon, and aspherical and non-planar optical elements such as at least one transmissive or reflective aspherical or free-form optical element.

[0027] However, preferably, the processing device comprises, instead of or in addition to the elements described above, at least one multiplane converter device, which is referred to as an "MPLC device" for the remainder of this specification. Such a device processes the coupled light emission R processed by the processing device. c The basic light beams R1~R that make up the structure N Spatial parameters, for example, the fundamental light beam R1~R N The propagation direction (this direction can be defined via the mean linear phase of the associated electromagnetic field), coupled light emission R c The basic light beams inside are R1~R N The position of the beams (defined as the position of the centroid of the beam intensity distribution in a plane perpendicular to the propagation direction of these beams), the fundamental light beams R1~R N The horizontal or vertical size (defined as the standard deviation of the horizontal or vertical limiting intensity distribution), coupled light emission R c The basic light beams inside are R1~R N This makes it possible to control the ellipticity and divergence of the curve effectively.

[0028] For completeness, it should be recalled that in an MPLC device, incident light radiation undergoes a series of reflections and / or transmissions, followed by free-space propagation of the radiation after each reflection and / or transmission. At least some of the optical components where reflections and / or transmissions occur and which guide the propagation of the incident radiation have microstructured zones that modify the incident light radiation.

[0029] The term "microstructured zone" means that the surface of an optical component has a relief on this zone, which can be broken down into "pixels," for example, which may have dimensions ranging from a few microns to several hundred microns. These may also be metasurfaces. Each raised portion or pixel of this raised portion has a variable height of at most a few microns or at most several hundred microns relative to the mean plane that defines the surface in question. Regardless of the microstructure nature of the zone, an optical component having such a zone forms a phase mask that introduces a local phase shift into the cross-section of radiation reflected therein or transmitted therein.

[0030] Therefore, the light radiation propagating within the MPLC device undergoes a series of local phase shifts separated by propagation. A sequence of these fundamental transformations (e.g., at least four consecutive transformations, e.g., 8, 10, 12, 14, or even at least 20) establishes an overall transformation of the spatial profile of the incident radiation. Thus, it becomes possible to construct a microstructure of finely structured reflection zones or transmission zones, particularly to transform a first light radiation having a specific shape into a second light radiation having a different shape.

[0031] The literature "Programmable unitary spatial mode manipulation," Morizur et al., J.Opt.Soc.Am.A / Vol.27, No.11 / November 2010, and N. Fontaine et al. (ECOC, 2017), "Design of High Order Mode-Multiplexers using Multiplane Light Conversion," U.S. Patent No. 9,250454 and U.S. Patent Application Publication No. 2017010463, include the theoretical basis and practical implementation examples of the MPLC device.

[0032] As described in detail in the aforementioned literature and referring to the example in [Figure 2], the optical components 2a and 2b that form the MPLC device, and the microstructured zones 3 supported by these optical components, are connected to the fundamental light beams R1-R N By combining them, and through multiple reflection processes, coupled light emission R c The microstructure is designed and configured to form a coupled light emission R c The basic light beams R1~R that make up the structure N It can be configured to form a shape.

[0033] The fundamental optical beams R1~R received at the input port N These are divided into families of "input" modes. Each input family mode has its own basic optical beams R1~R N The energy is transferred to the modes of the "output" mode family at the output port of the MPLC device, and each is shaped accordingly. The MPLC device is configured to match the input family modes and output family modes, respectively, particularly through the microstructure of the microstructured zone. It is a passive device, and its transfer function is particularly stable and robust.

[0034] For the purpose of this explanation, as an example, the family of input modes is the basic light beam R1~R NThis may include Gaussian modes that are spatially opposite to the input mode. The family of output modes may consist of N Hermitian-Gaussian modes or N Laguerre-Gaussian modes. The MPLC device is configured to associate the Gaussian modes of the input mode family with the modes of the output mode family. The optical beam R1~R received at the input port N The energy is delivered to the MPLC device, distributed, and adapted to the associated output mode.

[0035] Of course, the Hermit-Gaussian and Laguerre-Gaussian modes used as examples are for illustrative purposes only, and other modes can be selected to perform the transformation.

[0036] The output modes do not need to be spatially superimposed. This is especially true for coupled light emission R c This is when it is required that the beam be ring-shaped. This is shown in [Figure 3], and is a coupled "ring-shaped" light emission R formed by a combination of three fundamental light beams R1-R3. c This shows the shape of the beams. These beams have a ring-like sector shape and are evenly distributed across the entire ring without overlapping.

[0037] As shown in [Figure 3], the beam does not need to be perfectly continuous and completely cover the annular surface. In other embodiments, for example, the fundamental beam may be a Gaussian shape arranged spatially in a ring. Alternatively, the fundamental beam may be formed by groups of Hermitian Gaussian modes, each group arranged spatially within the ring.

[0038] Basic light beam R1~R NBy spatially extending the output modes of the MPLC device (more precisely, the output modes of the MPLC device) without overlapping, for example, in discontinuous annular sectors, the spatial energy density is limited in the coupled radiation and further in the downstream transmitted radiation I. Therefore, the energy density present in the transmitted radiation I can be controlled to be eye-safe, which is an essential safety standard that must be met in optical communication systems with free-space propagation.

[0039] The objective is generally to generate a transmitted beam with maximum far-field irradiance along its propagation axis. This configuration can be achieved by forming a planar transmitted beam (known in the industry as a "top-hat" beam) in the near field in a beam expanding device T. This beam can have a substantially circular cross-section, forming a disk or ring. Thus, the output mode of the MPLC device is the fundamental optical beam R1~R operated by this device. N The coupling can be advantageously selected to be as close as possible to such a flat beam.

[0040] Returning to the description of [Figure 1], the optical communication system 1 also includes a beam-expanding optical device T downstream of the processing device DT, which is optically coupled to the processing device DT. The beam-expanding optical device T emits coupled light R c It receives and propagates transmitted radiation I in free space.

[0041] The beam-expanding optical device T can take any preferred form, such as a telescope or a lens assembly.

[0042] As is well known, a telescope can be formed by a lens equipped with a concave mirror for the free-space propagation of light radiation. Here, the coupled radiation R received at the optical port P of the telescope cThis can be projected onto a concave mirror by a second planar or convex mirror of the objective lens O. When the second mirror is present, it results in the formation of a very low-intensity, blocked central zone within the transmitted light emission I. This prevents this energy from propagating in free space, thus resulting in coupled radiation R c In order to avoid unnecessarily distributing light energy in the central part, a ring-shaped coupled radiation R (as described in the two previous sections of this disclosure) is used. c The interesting reason is that it forms [this].

[0043] When the beam-expanding optical device T is a lens assembly, coupled radiation R c It can take the form of a disk.

[0044] The beam-expanding optical device T is oriented to point at and / or track a receiver installed on the satellite SAT. The optical communication system 1 may also include devices for guiding transmitted radiation I (such as transmitted radiation), and more generally for correcting any deviations in the pointing of the telescope T.

[0045] Modulation DM devices and processing DT devices consist of multiple precisely assembled optical or photonic elements and can be particularly sensitive to their operating environment. They are generally housed in cabinets, rooms, vehicles, or other shelters within the operation center. Therefore, it may be advantageous to mechanically isolate and offset the beam-expanding optical device T from other system components. This allows the beam-expanding optical device T, for example, when movable, to be positioned at a point favorable to the transmission of transmitted radiation I without inducing vibrations in the upstream optical chain.

[0046] To enable this distance, at least one offset optical fiber F is provided optically positioned between the output port of the processing device DT and the port P1 of the beam-expanding optical device T, thereby allowing the recombined optical beam R cThe propagation of the beam can be induced. This fiber is advantageously multimode. The fiber F can typically be 1m to 10m in length, or 1m to 50m in length. If the objective is simply to mechanically isolate the beam-expanding optical device T from the rest of the system, very short fibers of, for example, 10cm to 1m can also be provided.

[0047] When the output port of an MPLC device is coupled to such a multimode offset fiber, the modes of the output mode family are preferably selected to correspond to the intrinsic modes of this offset fiber. This configuration allows control over the mode content of the radiation emitted at the fiber output, and therefore its divergence, more generally the properties of the transmitted beam I, and especially its stability. For this purpose, it may be desirable to use the induction mode of the lowest refractive index offset fiber, which works favorably for low divergence. It is also possible to control the shape of the radiation emanating from the offset fiber F to match or maximize the energy density supplied to the pupil of the beam-expanding optical device T.

[0048] Therefore, advantageously, the offset fiber is a non-degenerate fiber and provides a single "clean" mode for at least some of the mode group. This could be, for example, a refractive index hopping fiber or an elliptic core fiber.

[0049] More generally, the modes of the output mode family can be selected such that each of these modes is formed by a linear combination of modes having similar group delays in the offset fiber. Similarity is understood to mean that these delays do not differ from one another by more than 10%, preferably within 5%, of the symbol delay (defined as the reciprocal of the transmission rate in GBaud units).

[0050] For example, each mode in the input mode family can be selected as a linear combination of modes from the same fiber mode group. It should be noted that modes within the same mode group in a fiber propagate at the same speed. Therefore, by configuring the MPLC device as proposed above, the temporal spread of the transmitted symbol is limited, enabling high-speed transmission exceeding, for example, 10GBd, 25GBd, or even 33GBd. By avoiding the propagation of the fundamental optical beam across several mode groups in the fiber, it is also ensured that the propagation time differences of the fundamental optical beam can be pre-compensated for on a beam-by-beam basis, for example, using a static delay element. This feature is particularly useful when the offset fiber is relatively long, for example, 10m or more.

[0051] Of course, this offset optical fiber is not essential, and coupled radiation R c It is also possible to optically couple these two elements by simple free-space propagation. This is particularly possible when the processing device DT is coupled to a ring shape, for example, as proposed in the previous section. c This is the case when molding.

[0052] The optical communication system described above enables the formation of transmitted radiation I with high mode diversity, and therefore the effects of radiation fluctuations caused by atmospheric disturbances are attenuated.

[0053] Naturally, the present invention is not limited to the embodiments described, and alternative embodiments may be added without departing from the scope of the invention as defined by the claims.

[0054] The communication system according to the present invention may include elements or devices other than those described in detail herein.

[0055] For example, when an offset fiber F is provided between a processing device DT and a beam-expanding optical device T, an optical block can be added to shape the radiation generated by this fiber F before it is incident on the beam-expanding optical device T. Thus, this optical shaping block can be used to adjust the shape of the recombined radiation to suit the properties of the beam-expanding optical device T, for example, as a disk in a lens assembly and as a ring in a telescope. This optical shaping block can be implemented using any suitable optical element, such as a diffractive optical component, a spatial phase modulator, an optical system that images or includes at least one lens, an axicon, an aspherical or free-form optical element, or an MPLC device.

[0056] It can be assumed that some of the components of the system can be used for both transmission and reception. This is especially true for beam-expanding optical devices T, such as telescopes or lens assemblies, offset fibers F, and even processing devices DT.

Claims

1. A free-space optical communication system (1), - Multiple mutually incoherent fundamental light beams (R １ ~R Ｎ A modulation device (DM) that generates each fundamental light beam (R １ ~R Ｎ ) modulates the same digital data (M) that is transmitted, with a modulation device (DM), - Optically coupled downstream of the modulation device (DM), the fundamental light beam (R １ ~R Ｎ ) combines, thereby producing "coupled" light emission (R ｃ A processing device (DT) configured to generate light radiation referred to as, - Optically coupled downstream of the processing device (DT), and the coupled light emission (R ｃ An optical communication system (1) comprising a beam-expanding optical device (T) that receives light and propagates free-space transmitted radiation (I).

2. The optical communication system (1) according to claim 1, wherein the processing device (DT) comprises at least one multiplane conversion device.

3. The multi-plane conversion device includes a plurality of optical components each having a reflecting surface for guiding the propagation of the basic optical beam (R １ to R Ｎ ). At least one of the reflecting surfaces combines the basic optical beam (R １ to R Ｎ ) and has a microstructure configured to form the combined light emission (R ｃ ) in the process of multiple reflections. The optical communication system (1) according to claim 2.

4. The aforementioned microstructure also emits coupled light (R ｃ The fundamental light beam (R １ ~R Ｎ The optical communication system (1) according to claim 3, configured to form ).

5. The coupled light emission (R ｃ An optical communication system (1) according to any one of claims 1 to 4, configured to conform to a flat-top shape.

6. The optical communication system (1) according to claim 5, wherein the flat top shape has a circular cross-section so as to form a ring or a disk.

7. The modulation device (DM) comprises a plurality of light sources associated with a plurality of modulators, and the plurality of fundamental light beams (R １ ~R Ｎ An optical communication system (1) according to any one of claims 1 to 6, which generates ).

8. The optical communication system (1) according to claim 7, wherein the light source emits radiation of different wavelengths.

9. The optical communication system (1) according to any one of claims 1 to 6, wherein the modulation device (DM) comprises a wide-spectrum source.

10. The optical communication system (1) according to any one of claims 1 to 9, comprising an offset optical fiber (F) optically arranged between the output port of the processing device and the beam expanding optical device (T) for guiding the propagation of the recombined light beam.

11. The aforementioned fundamental light beam (R １ ~R Ｎ The optical communication system (1) according to claim 10, when combined with claim 4, wherein each of the modes is adapted to a linear combination of modes having similar group delays.

12. The optical communication system (1) according to claim 10 or 11, wherein the offset optical fiber (F) is a degenerate lifting fiber such as a step-index fiber or an elliptic core fiber.

13. The optical communication system (1) according to any one of claims 10 to 12, comprising a shaped optical block disposed between the offset optical fiber (F) and the beam expanding optical device (T).

14. The optical communication system (1) according to any one of claims 1 to 13, wherein the beam expanding optical device (T) is a telescope or a lens assembly.

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