TECHNICAL FIELD

The field of the invention is that of on-chip photonic circuits, and more precisely that of optical coupling of a flared laser source to a waveguide of a photonic chip. The invention is notably applicable to the LIDAR field, LIDAR being the acronym of Light Detection and Ranging.

PRIOR ART

Various applications, such as LIDARs, require a semiconductor laser source that emits a high-power optical signal. Such a laser source may be a so-called flared laser source (also called tapered laser source), as notably described in the article by Wenzel et al. titledHigh-brightness diode lasers, C.R. Physique 4 (2003), 649-661.

A flared laser source conventionally comprises (located in the optical cavity) an active waveguide formed from a straight single-mode section followed by a flared section that ends in an emission surface of the laser source. The straight single-mode section may be a ridge section of small transverse dimensions, so as to form a spatial filter and to force the optical signal to be transverse single-mode, and the flared section may ensure the amplification of the optical signal. The emission surface of the laser source may then have a width of the order of one to several hundred microns. Such flared laser sources may thus emit an optical signal of high power, of the order of several watts for example.

However, it is important to be able to couple such a flared laser source to an output waveguide. Optical devices that ensure free-space optical coupling between the flared laser source and a single-mode optical fibre, such as the optical device described in the article by Delepine et al. titledHow to Launch1W Into Single-Mode Fiber from a Single1.48 μm Flared Resonator, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 7, No. 2, (2001), 111-123, already exist. This type of optical device comprises a set of collimating and focusing lenses that are particularly difficult to position relative to one another, and its coupling performance is highly sensitive to errors in relative position.

It would therefore be desirable to be able to couple the flared laser source to an output waveguide on a photonic chip. However, the intensity of the optical signal emitted by the flared laser source has a very elliptical spatial distribution, its width being of the order of several hundred microns along a horizontal axis, whereas the transverse dimensions of the optical mode supported by the output waveguide may be of the order of only one micron, this making optical coupling particularly difficult to obtain.

It will be noted that planar gradient-index lenses that are able to collimate the incident optical signal have already been produced on a photonic chip. An example of such planar lenses, here a so-called Luneburg lens, is described in the article by Kim et al. titledLuneburg Lens for Wide-Angle Chip-Scale Optical Beam Steering, Conference on Lasers and Electro-Optics (CLEO), 2019. However, planar gradient-index lenses are generally photonic crystals and hence they are formed from a plurality of holes of various sub-wavelength diameters produced in a thin silicon layer of an SOI substrate. The holes are arranged so that the effective index of the optical mode exhibits a predefined variation allowing the collimating function to be obtained. However, it turns out that these holes forming the photonic crystal of the lens are particularly complex to produce, and that imperfections in production greatly impact the performance of such an optical device.

DISCLOSURE OF THE INVENTION

The objective of the invention is to at least partially remedy the drawbacks of the prior art, and more particularly to provide a device for optically coupling, on a photonic chip, a flared laser source and an output waveguide that has a high performance, in terms of collection of the emitted optical signal, transmission of the optical signal thus collected, and/or focus onto the input of the output waveguide.

To this end, one subject of the invention is a coupling device suitable for ensuring optical coupling between a flared laser source and an output waveguide on a photonic chip.

The flared laser source comprises, along an optical axis Δ, a straight single-mode section, and a section that is flared (ie. tapered), in a main plane, and that ends in a surface of emission of the optical signal. It is configured to emit an optical signal a wavefront of which, in the main plane, is circular and centred on a position zhlocated in the flared section on the optical axis Δ.

According to the invention, the coupling device comprises, placed on the photonic chip:a coupler, which is intended to be coupled to the flared laser source so as to collect and transmit at least one portion of the emitted optical signal, and which comprises a curved and circularly arcuate output the centre of which is intended to be located at the position zh;a combiner, which is coupled to intermediate waveguides so as to receive the optical signals transmitted by the latter, and to focus them on to an input of the output waveguide; and which is a planar focusing lens comprising an input and an output bounding a free-propagation region of uniform refractive index, the input being curved and circularly arcuate and having a centre at a position zgson the output face where the input of the output waveguide is located;a network of intermediate waveguides, which are coupled to the curved output of the coupler so as to receive the collected optical signal and to transmit it to the curved input of the combiner; and which comprise: a connecting upstream section in which they are orthogonally connected to the curved coupler output; a connecting downstream section in which they are orthogonally connected to the curved combiner input; a correcting central section located between the connecting upstream and downstream sections, in which section an effective index associated with the guided modes is adjusted so that the optical paths of the intermediate waveguides between the curved output of the coupler and the curved input of the combiner are identical to one another.

The following are certain preferred but non-limiting aspects of this coupling device.

The intermediate waveguides may exhibit, in the correcting central section, a longitudinal variation in at least one transverse dimension, said longitudinal variation being predefined for each intermediate waveguide, so that the optical paths of the intermediate waveguides are identical to one another.

Each intermediate waveguide may exhibit, in the correcting central section, an adiabatic variation in width, passing from a minimum first value that is identical for each intermediate waveguide to a maximum value that is different from one intermediate waveguide to the next, before then decreasing to a second minimum value that is identical for each intermediate waveguide.

The coupling device may comprise waveguides that are referred to as additional waveguides, these waveguides having a refractive index different from that of the intermediate waveguides, lying solely in the correcting central section, and each being arranged facing and parallel to one intermediate waveguide, so as to permit modal coupling to the intermediate waveguide in question, the additional waveguides having a predefined length that differs from one additional waveguide to the next, so that the optical paths of the intermediate waveguides are identical to one another.

The coupling device may comprise thermo-optic phase-shifters arranged solely in the correcting central section so as to generate, via the application of a temperature to the intermediate waveguides, a predefined phase shift between the optical modes in the intermediate waveguides so that the optical paths of the intermediate waveguides are identical to one another.

The coupler may be formed from a network of tapers each taper being rectilinear and oriented in the direction of a given position intended to coincide with the position zh, the tapers being arranged laterally so that their downstream ends are placed on a circular arc the centre of which is intended to be located at the position zh, thus forming the curved output of the coupler.

The coupler may be a star coupler comprising an input face intended to be oriented orthogonally to the optical axis Δ, a curved output face, and a free-propagation region of uniform refractive index bounded by the input and output faces.

The invention also relates to a photonic chip comprising a coupling device according to any one of the preceding features, and the output waveguide coupled to the combiner in the position zgsof its curved output.

The photonic chip may comprise a silicon-on-insulator substrate.

The invention also relates to an optoelectronic system, comprising: an emissive chip comprising a flared laser source which comprises, along an optical axis Δ, a straight single-mode section, and a section that is flared in a main plane and that ends in a surface of emission of the optical signal, and which is configured to emit an optical signal a wavefront of which, in the parallel plane, is circular and centred on a position zhlocated in the flared section on the optical axis Δ; and a photonic chip according to any one of the preceding features.

The emissive and photonic chips may be joined to each other by an adhesive layer.

