TECHNOLOGICAL FIELD

The present invention is in the field of laser systems and is relevant to laser systems including arrays of gain elements. The invention relates in particular to laser system utilizing photonic topological states for selected lasing modes.

BACKGROUND

Diode lasers (also known as semiconductor lasers) are the most commonly used lasers system, with applications ranging from optical telecommunications to compact disks and much more. The gain in semiconductor lasers is high (in terms of gain per unit length), but the power emitted from a single semiconductor laser is limited, and generally does not exceed a few Watts. Extensive research is and has been carried out to develop arrays of semiconductor lasers, where neighboring emitters are coupled to one another, with the goal to overcome the power limitation of a single element and the critical damage limitations, while maintaining a narrow spectrum and a stable emission field that is robust against variations in pumping power, fabrication defects and disorder. In principle, laser array structures are capable of providing high laser power. However, such laser array structures tend to provide emission with many spatial and spectral modes simultaneously, where the modal structure of the laser array system (near field and spectrum) vary with the pumping strength, thereby greatly reducing the coherence properties of the laser and the stability to variations in the pumping and environmental conditions. Likewise, when the number of laser elements is increased, the coherence between emitters far apart from one another is greatly reduced. Despite many methods suggested to control the emission pattern of laser diode arrays, current technology is still not able to produce laser array systems operating as a single high power coherent laser source. Consequently, the most common application of laser array systems is as pump modules (instead of flashtubes) for solid state lasers.

Topological insulators are phases of matter characterized by topological states (typically edge states) having spatial configuration/path that is stable and robust to local variations. In the past decade, the principles of topological insulators have been demonstrated in various physical systems, ranging from photonics and microwaves to acoustics, mechanics and cold atoms. Specifically, with electromagnetic waves, topological insulators have been demonstrated in in systems of coupled elements, such waveguides or resonators. In general, the scientific understanding considers that to support topological modes within a system, the system need to be such that can be represented as Hermitian system, i.e. a passive system that contain no gain. Adding gain to topological insulators (in all systems) makes the system non-Hermitian, which was believed to break the topological properties, specifically to break the topological protection of transport in the system, rendering the system susceptible to defects and imperfections.

U.S. Pat. No. 9,052,448 describes two-dimensional coupled resonator optical waveguide arrangements and systems, devices, and methods thereof. Networks of coupled resonator optical waveguides are arranged so as to exploit topological properties of these optical networks. Such arrangement affords topological protection against disorders or perturbations in the network that may hinder or block photon flow. As a result of a disorder, photons traversing along edge states of the array are rerouted based on the disorder or perturbation. Photon routing in the network is accordingly protected against disorder or defects.

GENERAL DESCRIPTION

There is a need in the art for novel laser array configuration enabling emission of coherent optical radiation while maintaining stable single-mode emission at high pumping powers. Further, there is need for laser array configuration having robust emission with respect to variations in environment, defects, fabrication imperfections, and changes in pumping power.

The present invention provides a topological insulator laser system configured with an array of optical elements arranged to be coupled between them. The optical elements of the array include optical elements carrying selected gain material and configured to provide emission in one or more wavelength ranges in response to pumping energy. The arrangement of the optical elements in the array and the coupling levels between the optical elements are selected such that the array supports one or more topological modes associated with spatial configuration of the array. More specifically, the spatial arrangement and coupling parameters provide one or more topologically stable modes having certain spatial path. The laser system further includes a pumping unit configured to provide pumping energy (e.g. optical and/or electrical pumping) directed onto a group of optical elements associated with at least a portion of the spatial region of the topological mode, and an output port optically coupled to one or more optical elements of the array to provide extraction of a portion of the power from said laser system. Generally, to provide efficient out coupling of light, the output port is preferably coupled to one or more optical elements located along trajectory of the topological modes. Typically, such one or more optical elements are located at edges of the array.

Topological insulators are well known in electronic solid-state systems. In such systems, the bulk of solid-state element is an insulator having an energy gap between electronic valance states and conducting states. Topologically conducting surface states are special states that fall within the bulk energy gap and allow electrical conduction along surface of the bulk, by allowing electrons/holes to shift between states having small energetic difference. A substantially similar effect may occur in photonic devices formed by coupled optical elements such as resonators, waveguides etc. that are arranged in one- two-or three-dimensional arrays. Optical coupling between elements of the array gives rise to crystal-like behavior supporting optical modes and defining certain band structure (as in photonic crystals). Given the spatial arrangement and properties of the optical elements of the array, the band structure of the system and the wavefunctions of the various photonic states are determined, and can be calculated using various known techniques. Further, various configurations of arrays of optical elements supporting topological edge modes are described herein in more details below.

In some configuration, the array of optical elements may be a two-dimensional array including a plurality of optical elements (e.g. optical resonators or ring/oval resonators) arranged at selected arrangement. The optical elements of the array are optically coupled between them allowing light components to be transferred from one element to its neighboring elements. Arrangement of the array provides that coupling between optical elements of different rows (or columns) of the array applies certain phase variation 2πα going in a given direction in the array (for example, clockwise) and opposite phase variation (−2πα) in the opposite direction, where a is non-integer (e.g. 0.1<α<0.8, and generally α≠0.5). This variation in coupling phase shift affects band structure of the array creating one or more topological modes, characterized as band-gap crossing modes. These topological modes typically have non-zero group velocity and propagate along edges of the array, thereby providing corresponding one or more topological paths for light propagation along edges of the array.

Further, according to the present invention, the optical elements of the array include selected gain material enabling emission of light of one or more selected wavelength ranges in response to pumping energy. Light intensity occupying the one or more topological modes of the array cause stimulated emission into the topological modes. Such optical intensity may be out-coupled at one or more selected locations along path of the topological modes to provide stable and coherent laser emission. The stability of the topological modes results in limited to no interaction with other modes of the array maintaining single-mode emission for a wide range of pumping power. In some configurations where two or more topological modes are supported by the array, the laser emission may contain two or more lasing modes corresponding to the topological modes of the array. Generally, such two or more lasing modes may have substantially similar spatial mode structure allowing the modes to be combined to a single beam.

In this connection, the optical elements may be configured as waveguide resonators. At least some of the resonators, including resonators located in spatial region of the topological modes, are doped with rare earth ions, or embedded with quantum dot arrangement, enabling stimulated emission/absorption of light of selected wavelength range. For example, the optical elements of the array may be resonators formed as resonating waveguide including core formed of semiconductor diode gain material. Such diode gain material may include layers of quantum Inx=0.74Ga1-xAsy=0.57P1-ywells (e.g. having thickness of 10 nm each) sandwiched between cladding layers of Inx=0.56Ga1-xAsy=0.93P1-y(e.g. having thickness of 20 nm), all grown on a p-type substrate such as Indium Phosphide (InP) and covered by a protective layer. According to some other configuration, the gain material may comprise one or more Nitride compounds configured to provide optical emission in response to pumping energy. In some configurations, the Nitride compound may comprise one or more layers of comprising Gallium related compound.

Generally, the gain material of the optical elements generates matter-light interaction in which photons are absorbed and/or emitted by the gain material. To provide optical emission, a group of one or more optical elements are pumped by optical and/or electrical pumping provided by the pumping unit. The group of optical elements that are pumped is selected to include at least some of the optical elements along the spatial region associated with the topological modes. Accordingly, at least some of the optical elements of the array, that do not take part in light propagation along the one or more topological modes, act as loss to the system, as less pumping energy is applied thereto (or no pumping at all).

It should be noted that generally, gain/loss light-matter interaction causes the system to be non-Hermitian in nature. As indicating above, it is assumed that such non-Hermitian system does not support topological modes. However, the inventors of the present invention have identified that introduction of gain material within optical elements associated with one or more topological modes of the array, combined with proper pumping condition, provides efficient and stable optical emission associated with the topological modes with limited to no mixing with other optical modes. This enable the use of a plurality of optical elements participating in lasing to provide high power, while maintaining single-mode and stable optical emission of the laser system.

Further, the topological insulator laser system as described herein displays stable and robust lasing in a single spatial and temporal mode, that remains the same mode for a wide range of pumping power, and is robust to defects, disorder and fabrication imperfections in the system, enabling stable, single-mode lasing where a plurality of gain elements shares a combined, coherent, lasing mode. Moreover, due to the topological nature of the lasing mode, the laser system can maintain operation while supporting high out-coupling of power. Simply put, the topological design of the laser array system makes very many laser emitters operate together as a single highly coherent high power laser source.

Generally, to support topological modes without backscattering between modes, time reciprocity within the system is typically broken. More specifically, a system is reciprocal if it is symmetric with respect to time, i.e. lunching the output of the system in the opposite direction results in recovery of the corresponding input. Reciprocity of the system may be broken utilizing various configurations and techniques. Conventionally, introduction of external magnetic field onto the system is used for breaking of reciprocity as light path along field lines or against field lines accumulated different phase shifts. However, as mentioned above, the inventors of the present invention have identified that providing suitable array structure enabling symmetry of optical coupling within the array (e.g. provide phase shift difference between coupling directions) in addition to the nonlinearity provided by gain saturation associated with lasing action of the optical elements, maintains a system state in which reciprocity is broken and supports topological modes of the system. This causes the array to provide lasing power primarily in one direction, without scattering backwards even in the presence of defects, disorder, and fabrication imperfections.

As indicated above, the use of topological lasing mode of the system provides an active lasing mode that is shared by a plurality of optical elements (e.g. resonators carrying gain material) while maintaining a stable and coherent mode. Moreover, the topological nature of the mode provides lasing that is robust to local imperfections such as defects in the array, or certain level of disorder, e.g. variation on the characteristic of the optical resonators of the array due to fabrication or operational conditions. Further, the topological lasing mode has well defined path within the array, enabling to maintain strong coupling to an output port by providing the output port along path of the topological mode. This allows extraction of high radiation intensity as compared to conventional laser systems, wherein over extraction causes loss in the extracted mode and may stop emission of the laser. According to the present technique, optical emission within the topological mode is effectively insulated from other modes of the array, thereby enabling high out-coupling loss without directing optical emission to other non-topological modes. Combined with the use of a plurality of gain elements, this enables high power laser system providing high efficiency and stable mode of coherent emission robust to disorder and imperfections.

Further, the laser system described herein may include a plurality of gain carrying optical elements that are not located in spatial region of the topological mode, and accordingly, such optical elements can be passive (no gain nor loss), partially pumped, or generating high loss. This reduces participation of array regions that are not associated with the topological modes in optical emission, thus assisting to make sure that only the topological modes lase. The inventors of the present invention have understood, that such optical elements are typically required for maintenance of the topological lasing mode and provide regions of evanescence coupling decay of light intensity from the topological lasing mode to the bulk of the array (e.g. exponential decay in light intensity coupled from the edges and into bulk of the array), thereby maintaining stability of the laser system. Accordingly, as described in more details further below, the bulk regions may be used by the topological mode to maintain the path of the topological mode in case of edge defects that limit light propagation within the topological modes. This provide additional path options that may be generated by the topological mode when the edge related path is obstructed.

To this end, the topological insulator laser system, according to some embodiments of the invention may be formed by a two-dimensional array of optical elements, e.g. ring or helical resonators, arranged such that light components coupled in one direction between different rows (or columns) of the array accumulates certain phase shift opposite to light components coupled the opposite (backwards) direction. The resonators of the array are impregnated with selected gain material, by addition of quantum wells or dots, rare earth ions or any other gain material within the resonators, thus enabling each of the resonators to support at least one lasing mode. Further, the topological insulator laser system comprises a pumping unit configured to provide pumping power to at least a group of the optical elements located along at least a portion of the array to provide lasing of one or multiple topological modes of the system. The pumping energy is preferably selected to provide saturated pumping of the gain material and enable at least local (within the relevant optical elements) lasing activity. Typically, the topological insulator laser system also comprises an output port, optically coupled to one or more of the optical elements along edge regions of the array. The output port is configured to provide out-coupling of a portion of light inside the laser array to thereby provide output radiation from the laser system.

Generally, the gain material and local configuration of each of the optical elements are selected to provide certain overlap between at least a portion of the emission spectrum of the gain material and supported spectrum of the optical elements. For example, in the case of resonators (such as ring, helical of any other resonators), the gain material is selected to emit radiation in wavelength range corresponding with resonance range of the resonators. Such resonance range of the resonators is selected in accordance with material composition (e.g. refractive index) and physical dimensions thereof. The pumping power is preferably selected to be above the pumping level that provides gain equal to the threshold gain level (determined by the losses in the array). Under such conditions, the array lases, which means that at every lasing mode the gain is clamped to the threshold gain (by the nonlinear action of gain saturation) and the pumping power above threshold is transformed into the optical power of the lasing mode. In the topological array system, the topological lasing mode moves around the array (with a certain group velocity determined by the dispersion curve), and in doing that it forces the gain elements to laser at the same frequency. It is important that the pumping region is preferably selected to overlap with a region associated with the topological mode. It should however be noted that stability of the topological lasing mode enables active lasing of the system when only a portion of the region associated with the topological mode undergo pumping.

The topological mode of the topological insulating laser as described herein, is typically an edge mode propagating in clockwise and/or counterclockwise directions along peripheral edges of the array. Generally, such topological mode has certain width corresponding to radiation intensity within optical elements in one or more lines (rows and columns) of the array that are internal with respect to the edge. Accordingly, the region associated with the topological mode may have certain width (herein referred to as width of the topological mode), corresponding to evanescence decay of light intensity of the topological modes into the bulk of the array. Such width may be within the range of 1-4 unit cells of the array. In some configuration, the selected region for pumping corresponds with width of the topological mode.

Thus, according to one broad aspect, the present invention provides a laser system comprising:an array of optical elements comprising optical elements comprising gain material, the optical elements of the array are optically coupled between them and are spatially arranged supporting at least one topological mode associated with certain spatial region of the array;a pumping unit configured to provide pumping of a group of the optical elements of the array within at least a portion of the spatial region corresponding with said topological mode; andat least one output port optically coupled to one or more of the optical elements associated with said topological mode, said at least one output ports being configured for extracting a portion of light intensity from said laser system.

According to some embodiments the array of optical elements may be configured as a photonic crystal having a band structure associated with photonic states within said photonic crystal, said band structure comprising one or more photonic states crossing a gap between bands of the photonic states; said one or more photonic states crossing said gap correspond with the at least one topological mode within the array.

The array may be a two-dimensional array, said at least one topological mode being associated with at least one edge states travelling along periphery of said array.

According to some embodiments the optical elements may be arranged with spatial variation between them providing phase shift to light components coupled between rows of the array in one direction different with respect to opposite direction.

According to some embodiments the coupling between said optical elements of the array may be configured such that coupling between optical elements of different rows introduce selected phase shift to light coupling in one direction and different phase shift to light coupling in the opposite direction.

According to some embodiments the optical elements are optical resonators. For example, the optical elements may be ring resonators, S-shape resonators, and may comprise one or more helical portion.

According to some embodiments the array may comprise at least two optical links coupling between said optical resonators of the array, said optical links are accommodated with selected spatial shifts with respect to an axis between corresponding resonators thereby providing phase difference associated with two optical paths connecting said resonators through the links.

The optical links may be arranged with shifts with respect to an axis between corresponding resonators, said shifts vary between different rows of the array. The shifts in location of the optical links may vary by predetermined step level between different rows of the array.

According to some embodiments the array may be configured as a two-dimensional array, said array comprises a plurality of optical links coupling between adjacent optical resonators, optical links of different rows of the array are shifted with respect to optical links of neighboring rows by a predetermined shift, thereby providing predetermined shift in phase accumulation of light passing through said links in clockwise or counterclockwise directions.

According to some embodiments the optical resonators may be directly coupled between them, and wherein at least one of internal resonator geometry and coupling between neighboring resonators is varied along the array providing said optical topological mode.

According to some embodiments the optical resonators may be arranged in a hexagonal array forming first hexagonal arrangements characterized first coupling constant within the hexagonal arrangement and a second coupling constant, lower with respect to the first coupling constant between neighboring hexagonal arrangements.

According to some embodiments the optical resonators may be characterized by first resonant frequency, said array comprises a plurality of link couplers located between neighboring resonators of the array to provide coupling between them, said link couplers having a second resonant frequency different than said first resonant frequency. The link couplers are positioned to be strongly coupled to neighboring resonators at said first resonant frequency.

According to some embodiments the array may have lattice geometry selected from: rectangular, hexagonal, honeycomb or having a triangular lattice of sub-array units where each unit is formed by honeycomb lattice.

The at least one topological mode may have non-zero state velocity.

According to some embodiments the gain material may comprise at least one Nitride compound. The Nitride compound may comprise at least one Gallium containing layer.

According to some embodiments the at least one output port may comprise at least one optical fiber arranged along a selected and wherein a region of said optical fiber is positioned in close proximity to at least one optical element located at edge of the array to thereby enable out coupling of optical emission from one or more topological modes of the array.

The laser system may generally be configured to operate under zero magnetic field conditions. More specifically, the topological modes are supported by the arrangement of the array, while not requiring external magnetic field being constant or partially varying.

According to yet another broad aspect, the present invention provides a topological laser system comprising an array of optical resonators arranged in predetermined arrangement, said optical resonators being configured with gain material selected for emission of optical radiation in certain wavelength range, a plurality of optical link couplers located between adjacent optical resonators providing coupling of optical intensity between the optical resonators, output coupling unit located in proximity to one or more of optical resonators along edge of the array and configured for coupling selected portion of light out of the laser system, and a pumping unit configured for pumping a selected region along edges of the array; wherein said plurality of optical link couplers being arranged with selected shifts with respect to axis defining rows of the array, said selected shifts varying between rows of the array.

According to yet another broad aspect, the present invention provides a topological laser system comprising:

an array of optical resonators arranged in hexagonal lattice and defining a plurality of hexagonal arrangements characterized by first coupling rate within said hexagonal arrangements and second coupling rate lower than the first coupling rate between neighboring hexagonal arrangement, said optical resonators comprising optical resonators carrying gain material configured for emitting optical emission at wavelength range associated with resonant frequency of said resonators;

a pumping unit configured for providing pumping energy to at least a portion along edges of said array; and

at least one output port comprising optical waveguide coupled to one or more resonators along edges of said array.

According to yet another broad aspect, the present invention provides a topological laser system comprising:

an array of optical resonators characterized by first resonant frequency, and a plurality of link couplers having second resonant frequency different than the first resonant frequency, said link resonators arranged between neighboring resonators along lattice vectors defined by the array, said link couplers being strongly coupled to the neighboring resonators at said first resonant frequency, said optical resonators comprising optical resonators carrying gain material configured for emitting light at wavelength corresponding with said first resonant frequency;

a pumping unit configured for providing pumping energy to at least a portion along edges of said array; and

at least one output port comprising optical waveguide coupled to one or more resonators along edges of said array.

According to yet another broad aspect, the present invention provides a laser system comprising:an array of optical elements comprise optical elements carrying gain material, the optical elements of the array are optically coupled between them and are spatially accommodated such that the array supports at least one topological mode associated with certain spatial region of the array;a pumping unit configured to provide pumping of a group of the optical elements of the array within at least a portion of the spatial region corresponding with said topological mode; andat least one output port optically coupled to one or more of the optical elements associated with said topological mode, said at least one output ports being configured for extracting a portion of light intensity from said laser system.

The optical elements may be configured as semiconductor laser cavities such as resonators, ring resonators, helical resonators, and S-ring resonators.

According to some embodiments, the array of optical elements may be configured to provide a photonic crystal having a band structure associated with photonic states within said photonic crystal, said band structure comprising one or more photonic states crossing a gap between bands of the photonic states; said one or more photonic states crossing said gap correspond with the at least one topological mode within the array.

According to some embodiments, the array is a two-dimensional array, said at least one topological mode being associated with at least one edge states travelling along periphery of said array.

According to some embodiments, coupling between said optical elements of the array is configured such that coupling between optical elements of different rows introduce selected phase shift to light coupling in one direction and different phase shift to light coupling in the opposite direction.

According to some embodiments, the optical elements are optical resonators.

According to some embodiments, the optical resonators are ring resonators.

According to some embodiments, the optical resonators comprise at least one helical portion.

According to some embodiments, the array comprises at least two optical links coupling between said optical resonators of the array, said optical links are accommodated with selected shifts with respect to an axis between corresponding resonators thereby providing phase difference associated with two optical paths connecting said resonators through the links.

According to some embodiments, the array is configured as a two-dimensional array and comprises a plurality of optical links coupling between adjacent optical resonators, optical links of different rows of the array are shifted with respect to optical links of neighboring rows by a predetermined shift, thereby providing predetermined shift in phase accumulation of light passing through said links in clockwise or counterclockwise directions.

According to some embodiments, the optical resonators are directly coupled between them, and wherein at least one of internal resonator geometry and coupling between neighboring resonators is varied along the array providing said optical topological mode.

According to some embodiments, the array has lattice geometry selected from: rectangular, hexagonal, honeycomb or having a triangular lattice of sub-array units where each unit is formed by honeycomb lattice.

According to some embodiments, at least one topological mode has non-zero state velocity.

According to some embodiments, the gain material comprises at least one Nitride compound.

According to some embodiments, the Nitride compound comprises at least one Gallium containing layer.

