CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 of International Patent Application PCT/FR2018/053325, filed Dec. 17, 2018, designating the United States of America and published as International Patent Publication WO 2019/129949 A1 on Jul. 4, 2019, which claims the benefit under Article 8 of the Patent Cooperation Treaty to French Patent Application Serial No. 17/63315, filed Dec. 28, 2017.

TECHNICAL FIELD

The present disclosure relates to an optical device for manipulating light radiation. It relates more particularly to a device of this kind comprising means allowing for the precise assembly thereof or, in a more general manner, precise relative positioning of the optical parts of which it is made up. The means can be implemented during the assembly of the device, or in order to check the correct functioning thereof during use.

BACKGROUND

U.S. Pat. No. 9,250,454 and US2017010463 disclose optical devices, which are known by the acronym MPLC (Multi Plane Light Conversion) and make it possible to perform any kind of unitary spatial transformation of light radiation.

From a theoretical perspective, and as has been established in “Programmable unitary spatial mode manipulation,” Morizur et al, J. Opt. Soc. Am. A/Vol. 27, no. 11/November 2010, a unitary spatial transformation can essentially be broken down into a succession of primary transformations, each primary transformation affecting the profile of the transverse phase of the light radiation. In practice, and without this in any way restricting the technology, the MPLC components typically apply between 3 and 25 primary transformations.

The document “G. Labroille, P. Jian, N. Barré, B. Denolle, and J. Morizur, “Mode Selective 10-Mode Multiplexer based on Multi-Plane Light Conversion,” inOptical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2016), paper Th3E.5.” discloses a particular embodiment of an MPLC device. The device comprises a support on which an entry stage has been arranged, making it possible to guide the injection of incident light radiation into the device, an output stage for guiding the extraction of the transformed light radiation from the device, a mirror that is arranged opposite a reflective optical element in order to form a multi-passage cavity and makes it possible to reflect the incident light radiation on the optical element a plurality of times. The optical element comprises a microstructured main surface for applying a modification of the transverse phase signal to each reflection of the incident radiation.

The parts that make up a device of this kind must be positioned and oriented relative to one another, at a very high degree of accuracy. This accuracy is necessary in order to ensure that the incident light radiation intercepts the optical element exactly in the region of the microstructured zone, in order to provide it with a selected transformation. This need for precision in arranging the optical parts making up the device is all the more significant because one single part may intercept the optical path of the incident radiation several times (typically between 3 and 25 times), as has just been stated, with the result that a minimal deviation in the positioning or orientation with respect to the required positioning can have a significant effect on the correct functioning of the device.

Furthermore, the optical positioning tolerances (on the order of a micrometer and microradian, with regard to the angular precision) are much smaller than the geometric manufacturing tolerances of the parts (at least if parts that can be produced at a reasonable cost are sought), with the result that the position of these parts, in the assembly positions thereof, generally cannot be established in advance with the necessary precision.

In order to assemble the pieces onto the support, and, in particular, in order to form the multi-passage cavity, firstly the entry stage, the mirror, and the optical element are positioned roughly with respect to one another. Incident light radiation is injected into the cavity formed in this manner, and the intensity of the light signal at the outlet of the cavity is detected. The relative position and orientation of the entry stage, the mirror and the optical element are adjusted as finely as possible, according to all the available degrees of freedom, in order to optimize the detected intensity. The optimal relative positioning and orientation are fixed temporarily, for example, by means of adjustable retaining clamps.

Alignment cubes (or more generally alignment parts) having planar faces and having mutually perpendicular connecting faces are then used in order to fix the entry stage, the mirror and the optical element in the optimal support positions thereof, by means of an adhesive layer.

The quality of the optical alignment of the assembly, and, in particular, that of the cavity, is determined at least in part by the relative positioning and orientation of the parts with respect to one another, achieved on the basis of the detection of the intensity of the output light signal. Although this approach has demonstrated its effectiveness, this method results in relative positioning and orientation of the parts, which can be further improved.

It would also be desirable to be able to evaluate the correct mutual arrangement of the optical parts while the product is in use, in order to simply verify the correct functioning thereof or to make it possible to compensate for differences in positioning and orientation that would result.

BRIEF SUMMARY

With a view to achieving at least one of the goals, embodiments of the present disclosure include an optical device formed of a plurality of optical parts arranged on a support, at least one optical element of which includes a main surface provided with a first microstructured zone for intercepting incident light radiation, which propagates along a first determined optical path, the first microstructured zone spatially modifying the phase of the incident light radiation according to a determined spatial profile, and for forming transformed light radiation, by means of a plurality of reflections or transmissions on the optical element(s).

According to the present disclosure, the optical device comprises an entry stage for guiding the injection of positioning light radiation, according to a second optical path. The main surface of the optical element comprises a second textured zone that is designed to reflect the positioning light radiation and to back-propagate the positioning radiation along the second optical path.

Any deviation, even small, of the positioning or the orientation of one of the optical parts affects the propagation of the back-propagated radiation and the power of the radiation, which can be collected in the region of the entry stage. This very sensitive measure makes it possible to ensure very precise positioning of the parts to be positioned.

According to other advantageous and non-limiting features of the present disclosure, taken individually or in any technically possible combination:the optical device comprises an output stage for the transformed light radiation;the incident light radiation is injected via the entry stage;the transformed light radiation is extracted via the entry stage;the optical element is transparent with respect to the incident light radiation, and the transformed radiation is formed, at least in part, by means of transmission of the incident radiation through the transparent optical element;the optical element is reflective, and the transformed radiation is formed, at least in part, by means of reflection of the incident light radiation on the reflective optical element;the optical device comprises a mirror that is arranged opposite the reflective optical element in order to form a multi-passage cavity and to make it possible to project the incident light radiation and the positioning light signal along the first and the second optical path, respectively, a plurality of times.the second microstructured zone is designed so as to apply a transformation to the positioning radiation during the reflections or the transmissions preceding the backpropagation reflection;the optical element is a wave plate, a deformable mirror, or a spatial light modulator;the support is a planar support or a frame;the entry stage comprises at least one positioning optical fiber for injecting the positioning radiation.

According to another aspect, the present disclosure includes an assembly or testing system for the optical device, the system comprising a light source that is optically connected to the positioning optical fiber, and a device for measuring the power of the back-propagated light radiation.

According to other advantageous and non-limiting features of this aspect of the present disclosure, taken individually or in any technically possible combination:the assembly or testing system comprises a circulator and/or a separator between the light source and the positioning optical fiber, in order to extract the back-propagated light radiation toward the measuring device;

And according to another aspect, further embodiments of the present disclosure include an evaluation method for the optical device, comprising the following steps:

a. injecting positioning light radiation into the optical device via the entry stage;

b. measuring the power of the back-propagated signal.

According to other advantageous and non-limiting features of this method, taken individually or in any technically possible combination:the evaluation method comprises a step of determining the operational state of the optical device, in accordance with the measured power value;the evaluation method comprises a step that aims to temporarily place the optical parts in a relative manner, with respect to one another, before injecting the positioning light radiation;the evaluation method comprises:a step of adjusting the relative placement of the optical parts in order to achieve a target measurement of the power or to maximize the measurement of the power and establish a determined position and a determined orientation;a step for fixing the mutual relative placement of the optical parts in the determined position and orientation; the step of temporary placement comprises definitive fixing of the optical element on a planar support.

