Hybrid semiconductor saturable absorber mirror (SESAM) output coupler

Abstract

A hybrid semiconductor saturable absorber mirror (SESAM) output coupler for mode-locking and coupling an output of a laser system. The hybrid SESAM output coupler includes a curved base substrate. The hybrid SESAM output coupler further includes a dielectric mirror deposited onto the curved base substrate such that a curvature of the dielectric mirror matches a curvature of the curved base substrate. The hybrid SESAM output coupler further includes a semiconductor saturable absorber bonded to the dielectric mirror such that a curvature of the semiconductor saturable absorber matches the curvature of the curved base substrate. The hybrid SESAM output coupler may be configured to mode-lock one or more laser beams generated by the laser system and simultaneously optically couple each of the mode-locked laser beams into a mode-locked laser output.

Description

HYBRID SEMICONDUCTOR SATURABLE ABSORBER MIRROR (SESAM)OUTPUT COUPLERCROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 519,498 filed August 14, 2023, the specification(s) of which is / are incorporated herein in their entirety by reference.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Grant No. FA9550-17-1-0246 awarded by Air Force Office of Scientific Research. The government has certain rights in the invention.FIELD OF THE INVENTION

[0003] The present invention is directed to a hybrid semiconductor absorber mirror output coupler for use in mode-locked laser devices.BACKGROUND OF THE INVENTION

[0004] Semiconductor saturable absorber mirrors (SESAMs) find numerous applications in terms of improving the efficiency and function of laser devices. SESAMs are optical devices that allow for improvements to passive mode locking of ultrafast solid-state lasers. Implementations of SESAMs have resulted in significant improvements in terms of pulse widths, average power, repetition rates, and overall performance in laser devices.

[0005] The improvements brought about by SESAMs have allowed solid-state laser devices to expand their usage to medical, industrial, meteorological, and communications applications. The design and components of SESAMs also allow for optical devices with increased precision with the ability to efficiently control the magnitude and phase of the optical absorption and reflection, as well as the saturation fluence and temporal response.

[0006] Vertical-external-cavity surface-emitting lasers (VECSELs) are semiconductor lasers that are typically optically pumped. Due to the solid-state qualities of VECSELs, SESAMs may be able to provide improvements to the performance of these laser devices. Furthermore, a specialized design for a SESAM may allow for components of a VECSEL to be combined to provide for a more resource-efficient laser system with high performance. This specialized design may allow for simple two-element linear optical cavities capable of high repetition rates andgreater cavity stability.BRIEF SUMMARY OF THE INVENTION

[0007] It is an objective of the present invention to provide devices and systems that allow for a hybrid semiconductor saturable absorber mirror output coupler for use in a laser system, as specified in the independent claims. Embodiments of the invention are given in the dependent claims. Embodiments of the present invention can be freely combined with each other if they are not mutually exclusive.

[0008] The hybrid SESAM enables a greatly simplified optical layout for developing short-pulse laser sources. Because the hybrid SESAM acts as both the saturable absorber needed to mode lock the laser and optical cavity output coupler, one can make simple linear optical cavities that support high repetition rates and greater cavity stability. This could be an enabling technology for many laser systems, and in particular for VECSEL-based frequency combs.

[0009] The present invention features a hybrid semiconductor saturable absorber mirror (SESAM) output coupler for mode-locking and coupling an output of a laser system. The hybrid SESAM output coupler may comprise a curved base substrate. The hybrid SESAM output coupler may further comprise a dielectric mirror deposited onto the curved base substrate such that a curvature of the dielectric mirror matches a curvature of the curved base substrate. The hybrid SESAM output coupler may further comprise a semiconductor saturable absorber bonded to the dielectric mirror such that a curvature of the semiconductor saturable absorber matches the curvature of the curved base substrate. The hybrid SESAM output coupler may be configured to mode-lock one or more laser beams generated by the laser system and simultaneously optically couple each of the mode-locked laser beams into a mode-locked laser output.

[0010] In some embodiments, the hybrid SESAM device may comprise a single-crystal semiconductor saturable absorber component bonded to a dielectric mirror acting as an output coupler. In some embodiments, the mirror may comprise a distributed Bragg reflector (DBR). The device may further comprise a quantum well, a spacer layer disposed between the mirror and the quantum well, and a cap disposed on an end of the quantum well.

[0011] A unique and inventive technical feature of the present invention is the implementation of a dielectric mirror in a SESAM component. Without wishing to limit the invention to any theory or mechanism, it is believed that said technical feature advantageously provides for a SESAMcomponent that is configured to act as both a mode-locking saturable absorber and an output coupler for a laser system, providing said laser system with simple linear optical cavities capable of high repetition rates and greater cavity stability. None of the presently known prior references or work has the technical feature of the present invention.

[0012] Furthermore, this inventive technical feature of the present invention is counterintuitive. The reason that it is counterintuitive is because it contributed to a surprising result. One of ordinary skill in the art would observe that dielectric mirrors struggle with thermal conductivity, and would likely limit the present invention’s compatibility with high average power operation in thin-disk laser systems. Surprisingly, the present invention is able to achieve consistent mode-locked pulsing operation over a long period of time scalable to a plurality of wavelengths despite the limit on power operation of the laser system. Thus, the inventive technical feature of the present invention contributed to a surprising result and is counterintuitive.

[0013] Another unique and inventive technical feature of the present invention is the implementation of an active region of the gain chip of the laser system matching a repetition rate of the active region of the saturable absorber. Without wishing to limit the invention to any theory or mechanism, it is believed that said technical feature advantageously provides for adjustable high-repetition-rate mode-locked laser systems scalable to various wavelengths. None of the presently known prior references or work has the technical feature of the present invention.

[0014] Another unique and inventive technical feature of the present invention is an optimized dielectric mirror bonded to a separately optimized semiconductor saturable absorber. Without wishing to limit the invention to any theory or mechanism, it is believed that said technical feature advantageously provides for a hybrid SESAM output coupler with no flat optics or substrates, allowing for compact focusing cavities with optimized optics. None of the presently known prior references or work has the technical feature of the present invention.

[0015] Furthermore, this inventive technical feature of the present invention is counterintuitive. The reason that it is counterintuitive is because it contributed to a surprising result. One of ordinary skills in the art would expect that these optimized components would be incompatible with each other due to the different materials making them up. Furthermore, one of ordinary skills in the art would find it difficult to bond these curved components due to their tight radii of curvature potentially causing fractures. Surprisingly, the present invention is able to effectively bond the curved dielectric mirror to the saturable absorber and achieve linear two-element lasercavities in the 1-15 GHz range despite the tight curvature and the incompatible materials due to the use of a curved substrate upon which the dielectric mirror is deposited on.

[0016] Furthermore, with the curved components of the SESAM of the present invention with tight curvatures, one of ordinary skill in the art would expect that the spot size of the laser at the output coupler would be too large to be functional with the use of curved materials. The use of a single linear two-element cavity would increase the spot size even more. Surprisingly, the laser systems of the present invention are still able to achieve efficient mode-locking despite the increased spot size. Thus, the inventive technical feature of the present invention contributed to a surprising result and is counterintuitive.

[0017] Any feature or combination of features described herein are included within the scope of the present invention provided that the features included in any such combination are not mutually inconsistent as will be apparent from the context, this specification, and the knowledge of one of ordinary skills in the art. Additional advantages and aspects of the present invention are apparent in the following detailed description and claims.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0018] The features and advantages of the present invention will become apparent from a consideration of the following detailed description presented in connection with the accompanying drawings in which:

[0019] FIG. 1 shows a schematic of a laser system, showing the output coupling and mode-locking capabilities of the SESAM of the present invention.

[0020] FIG. 2 shows an alternate embodiment of a laser system having just one cavity arm. The hybrid SESAM of the present invention is still capable of output coupling but only executes mode-locking in the embodiment depicted.

[0021] FIG. 3 shows an alternate embodiment of a laser system having just one cavity arm but multiple lasers generated by the gain chip that are subsequently mode-locked and coupled by the hybrid SESAM of the present invention.

[0022] FIG. 4 shows a schematic of the SESAM device of the present invention.

[0023] FIG. 5 shows a plot of the increase in spectral bandwidth from the amorphous dielectric DBR / mirror compared to the semiconductor DBR / mirror.

[0024] FIG. 6 shows a plot of the flatness that can be achieved in GDD of an amorphous DBR mirror compared to that of the semiconductor DBR.DETAILED DESCRIPTION OF THE INVENTION

[0025] Following is a list of elements corresponding to a particular element referred to herein:

[0026] 100 hybrid SES AM output coupler

[0027] 110 dielectric mirror

[0028] 120 semiconductor saturable absorber component

[0029] 121 spacer layer

[0030] 122 quantum well

[0031] 123 cap

[0032] 130 curved base substrate

[0033] 200 laser system

[0034] 210 gain chip

[0035] 220 pumping source

[0036] 230 cavity

[0037] 240 optical element

[0038] The term “mode lock” is defined herein as a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds or femtoseconds. This not only produces pulses but also produces an amplitude-stable, periodic, and equally spaced train of pulses.

[0039] The term “diffraction grating” is defined herein as an optical component with a periodic structure that diffracts light into several beams traveling in different directions.

[0040] The term “distributed Bragg reflector (DBR)” is defined herein as a structure formed from multiple layers of alternating materials with different refractive index, or by periodic variation of some characteristic (such as height) of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide.

[0041] The term “frequency comb” is defined herein as a laser source whose spectrum consists of a series of discrete, equally spaced frequency lines.

[0042] The term “transverse mode profile” is defined herein as a particular electromagnetic field pattern of the radiation in the plane perpendicular (i.e., transverse) to the laser's propagation direction.

[0043] The term “saturable” is defined herein as capable of being saturated.

[0044] The term “output coupler” is defined herein as a component in a laser cavity system that allows for the extraction of a portion of the light from the laser's intracavity beam.

[0045] Referring now to FIGs. 1-3, the present invention features a hybrid semiconductor saturable absorber mirror (SESAM) output coupler (100) for mode-locking and coupling an output of a laser system. The hybrid SESAM output coupler (100) may comprise a curved base substrate (130). The hybrid SESAM output coupler (100) may further comprise a dielectric mirror (110) deposited onto the curved base substrate (130) such that a curvature of the dielectric mirror (110) matches a curvature of the curved base substrate (130). The hybrid SESAM output coupler (100) may further comprise a semiconductor-saturable absorber (120) bonded to the dielectric mirror (110) such that a curvature of the semiconductor-saturable absorber (120) matches the curvature of the curved base substrate (130). The hybrid SESAM output coupler (100) may be configured to mode-lock one or more laser beams generated by the laser system (200) and simultaneously optically couple each of the mode-locked laser beams into a mode-locked laser output.

[0046] In some embodiments, a radius of the curvature (RoC) of the curved base substrate (130), an RoC of the dielectric mirror (110), and / or an RoC of the semiconductor saturable absorber (120) may be 5 to 50 cm. In some embodiments, the RoC of the curved base substrate (130), the RoC of the dielectric mirror (110), and / or the RoC of the semiconductor saturable absorber (120) may be 5 to 10 cm. In some embodiments, the RoC of the curved base substrate (130), the RoC of the dielectric mirror (110), and / or the RoC of the semiconductor saturable absorber (120) may be 10-50 cm. In some embodiments, the radii of curvature for the base substrate, the dielectric mirror, and the semiconductor saturable absorber may all be identical. In other embodiments, two or more of the radii of curvature for the base substrate, the dielectric mirror, and the semiconductor saturable absorber may differ by 1 to 5 cm.

[0047] In some embodiments, the semiconductor saturable absorber (120) may comprise an epitaxially-grown compound semiconductor. In some embodiments, the dielectric mirror (110) comprises oxides, fluorides, or a combination thereof deposited onto the curved base substrate (130) by a physical vapor deposition process. In some embodiments, the oxides may comprise hafnium oxide, niobium oxide, aluminum oxide, silicon oxide, tantalum pentoxide, titanium oxide, or a combination thereof. In some embodiments, the fluorides may comprise lithium fluoride, calcium fluoride, barium fluoride, zirconium fluoride, or a combination thereof.

[0048] In some embodiments, the laser system (200) may comprise one or more gain media (210) configured to generate the one or more laser beams. The laser system (200) may further comprise a single linear two-element cavity (230) optically coupled to the one or more gain media (210) such that the one or more gain media (210) direct the one or more laser beams through a length of the cavity (230). The hybrid SESAM output coupler (100) may be optically coupled to the cavity (230) such that the one or more laser beams are directed into the hybrid SESAM output coupler (100). In some embodiments, each laser beam of the one or more laser beams generated by the laser system (200) may comprise a frequency of 1 to 50 GHz. In some embodiments, each laser beam of the one or more laser beams generated by the laser system (200) may comprise a frequency of 1 to 10 GHz. In some embodiments, each laser beam of the one or more laser beams generated by the laser system (200) may comprise a frequency of 10 to 50 GHz. In some embodiments, each laser beam of the one or more laser beams generated by the laser system (200) may comprise a frequency of 10 to 15 gHz. In some embodiments, the laser system (200) may comprise a cavity of any length.

[0049] In some embodiments, the one or more laser beams generated by the laser system (200) intersect with the dielectric mirror (110) and the semiconductor saturable absorber (120) such that a laser spot is generated. In some embodiments, a diameter of the laser spot may be equal to a diameter of the one or more laser beams when generated by the one or more gain media (210) such that a ratio of the size of the laser when it is initially generated by a gain medium and the size of the laser spot on the mirror / absorber is about 1 to 1. In some embodiments, the size of the laser spot may be greater than the size of the laser when it is initially generated by the gain medium. In some embodiments, the size of the laser spot may be smaller than the size of the laser when it is initially generated by the gain medium. In some embodiments, the laser system (200) may further comprise one or more optical elements (240) disposed in the cavity (230), configured to alter one or more properties of the one or more laser beams. In some embodiments, the one or more optical elements (240) may comprise lenses, filters, mirrors, prisms, diffraction gratings, beamsplitters, or a combination thereof. In some embodiments, the one or more properties may comprise wavelength, linewidth, transverse mode profile, beam size, beam position, or a combination thereof of the one or more laser beams.

[0050] In some embodiments, the one or more gain media (210) may comprise one or more first active regions configured to generate the one or more laser beams at a first repetition rate. The semiconductor saturable absorber (120) may comprise a second active region independent of theone or more first active regions configured to mode-lock the one or more laser beams generated by the laser system at a second repetition rate. The first repetition rate may be equal to the second repetition rate. In some embodiments, the one or more gain media (210) of the laser system (200) may comprise a semiconductor gain chip or a solid-state emitter. In some embodiments, the laser system (200) may further comprise a pumping source (220) operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams. In some embodiments, the laser system (200) may comprise a vertical-extemal-cavity surface-emitting laser (VECSEL) system. In some embodiments, the laser system (200) may comprise a membrane-extemal-cavity surface-emitting laser (MECSEL) system.

[0051] In some embodiments, the dielectric mirror (110) may comprise a distributed Bragg reflector (DBR). In some embodiments, the semiconductor saturable absorber (120) may comprise a spacer layer (121) bonded to the DBR, a quantum well (122) coupled to the spacer layer (121), and a cap (123) coupled to the quantum well (122). In some embodiments, the curved base substrate (130) may have a thickness of 6 mm. In some embodiments, the dielectric mirror (110) may have a thickness of 2 to 5 pm. In some embodiments, the spacer layer (121) may have a thickness of 3 to 10 nm. In some embodiments, the quantum well (122) may have a thickness of 1 to 9 nm. In some embodiments, the cap (123) may have a thickness of 3 to 30 nm.

[0052] The present invention features a laser system (200). In some embodiments, the laser system (200) may comprise one or more gain media (210) configured to generate one or more laser beams. The laser system (200) may further comprise a laser pumping source operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams. The laser system (200) may further comprise a single linear two-element cavity (230) optically coupled to the one or more gain media (210) such that the one or more gain media (210) direct the one or more laser beams through a length of the cavity (230). The laser system (200) may further comprise a hybrid semiconductor saturable absorber mirror (SESAM) output coupler (100) comprising a curved base substrate (130), a dielectric mirror (110) deposited onto the curved base substrate (130) such that a curvature of the dielectric mirror (110) matches a curvature of the curved base substrate (130), and a semiconductor saturable absorber (120) bonded to the dielectric mirror (110) such that acurvature of the semiconductor saturable absorber (120) matches the curvature of the curved base substrate (130), configured to mode-lock the laser beam and simultaneously optically couple the one or more laser beams into a mode-locked laser output.

[0053] In some embodiments, the one or more gain media (210) of the laser system (200) comprise a semiconductor gain chip or a solid-state emitter. In some embodiments, the laser system (200) may further comprise a pumping source (220) operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams. In some embodiments, the dielectric mirror (110) may comprise a distributed Bragg reflector (DBR). The semiconductor saturable absorber (120) may comprise a spacer layer (121) bonded to the DBR, a quantum well (122) coupled to the spacer layer (121), and a cap (123) coupled to the quantum well (122). In some embodiments, the one or more gain media (210) may comprise one or more first active regions configured to generate the one or more laser beams at a first repetition rate. The semiconductor saturable absorber (120) may comprise a second active region independent of the one or more first active regions configured to mode-lock the one or more laser beams generated by the laser system at a second repetition rate. The first repetition rate may be equal to the second repetition rate.

[0054] The present invention features a semiconductor laser system. The laser system may comprise one or more gain media (210) comprising one or more active regions configured to generate one or more laser beams at a first repetition rate. The laser system may further comprise a pumping source (220) operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams. The laser system may further comprise a single linear two-element cavity (230) optically coupled to the one or more gain media (210) such that the one or more gain media (210) direct the one or more laser beams through a length of the cavity (230). The laser system may further comprise one or more optical elements (240) disposed in the cavity (230), configured to alter one or more properties of the one or more laser beams. The laser system may further comprise a hybrid semiconductor saturable absorber mirror (SESAM) output coupler (100) comprising a curved base substrate (130), a distributed Bragg reflector (DBR) deposited onto the curved base substrate (130) such that a curvature of the DBR matches a curvature of the curved base substrate (130), and a semiconductor saturable absorber (120) bonded to the DBR such that a curvature of thesemiconductor saturable absorber (120) matches the curvature of the curved base substrate (130). The semiconductor saturable absorber (120) may comprise a spacer layer (121) bonded to the DBR, a quantum well (122) coupled to the spacer layer (121), a cap (123) coupled to the quantum well (122), optically coupled to the cavity (230) such that the one or more laser beams are directed into the hybrid SESAM output coupler (100), and an active region independent of the one or more active regions of the one or more gain media (210). The hybrid SESAM output coupler (100) may be configured to mode-lock the laser beam at a second repetition rate and simultaneously optically couple the one or more laser beams into a mode-locked laser output. The first repetition rate may be equal to the second repetition rate.

[0055] The present invention features a hybrid semiconductor saturable absorber mirror (SESAM) output coupler (100) for mode-locking and coupling an output of a laser system. In some embodiments, the hybrid SESAM output coupler (100) may comprise a curved base substrate (130). The hybrid SESAM output coupler (100) may further comprise a dielectric mirror (110) deposited onto the curved base substrate (130). The hybrid SESAM output coupler (100) may further comprise a semiconductor saturable absorber (120) bonded to the dielectric mirror (110), configured to mode-lock one or more laser beams generated by the laser system (200) and simultaneously optically couple each of the mode-locked laser beams into a mode-locked laser output.

[0056] In some embodiments, the laser system (200) may comprise one or more gain media (210) configured to generate the one or more laser beams. The laser system (200) may further comprise a single linear two-element cavity (230) optically coupled to the one or more gain media (210) such that the one or more gain media (210) direct the one or more laser beams through a length of the cavity (230). The hybrid SESAM output coupler (100) may be optically coupled to the cavity (230) such that the one or more laser beams are directed into the hybrid SESAM output coupler (100).

[0057] In some embodiments, the laser system (200) may further comprise one or more optical elements (240) disposed in the cavity (230), configured to alter one or more properties of the one or more laser beams. In some embodiments, the one or more optical elements (240) may comprise lenses, filters, mirrors, prisms, diffraction gratings, beamsplitters, or a combination thereof. In some embodiments, the one or more properties may comprise wavelength, linewidth, transverse mode profile, beam size, beam position, or a combination thereof of the one or morelaser beams. In some embodiments, the one or more gain media (210) may comprise one or more first active regions configured to generate the one or more laser beams at a first repetition rate. The semiconductor saturable absorber (120) may comprise a second active region configured to mode-lock the one or more laser beams generated by the laser system at a second repetition rate. In some embodiments, the first repetition rate may be equal to the second repetition rate.

[0058] In some embodiments, the one or more gain media (210) of the laser system (200) may comprise a vertical-external-cavity surface-emitting laser (VECSEL) gain chip or a solid-state emitter. In some embodiments, the laser system (200) may further comprise a pumping source (220) operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams.

[0059] In some embodiments, the dielectric mirror (110) comprises a distributed Bragg reflector (DBR). The semiconductor saturable absorber (120) may comprise a spacer layer (121) bonded to the DBR, a quantum well (122) coupled to the spacer layer (121), and a cap (123) coupled to the quantum well (122). In some embodiments, the dielectric mirror (110) may be configured to act as a broadband mirror and provide customized group delay dispersion profiles for ultrafast laser systems.

[0060] In some embodiments, the present invention features a laser system (200). In some embodiments, the laser system (200) may comprise one or more gain media (210) configured to generate one or more laser beams. The laser system (200) may further comprise a laser pumping source operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams. The laser system (200) may further comprise a single linear two-element cavity (230) optically coupled to the one or more gain media (210) such that the one or more gain media (210) direct the one or more laser beams through a length of the cavity (230). The laser system (200) may further comprise a hybrid semiconductor saturable absorber mirror (SESAM) output coupler (100) comprising a dielectric mirror (110) bonded to a semiconductor saturable absorber (120), optically coupled to the cavity (230) such that the one or more laser beams are directed into the hybrid SESAM output coupler (100), configured to mode-lock the laser beam and simultaneously optically couple the one or more laser beams into a mode-locked laser output.

[0061] In some embodiments, the laser device may further comprise one or more optical elements (240) disposed in the cavity (230), configured to alter one or more properties of the one or more laser beams. In some embodiments, the one or more optical elements (240) may comprise lenses, filters, mirrors, prisms, diffraction gratings, beamsplitters, or a combination thereof. In some embodiments, the one or more properties may comprise wavelength, linewidth, transverse mode profile, beam size, beam position, or a combination thereof of the one or more laser beams. In some embodiments, the laser device may comprise a vertical-external-cavity surface-emitting laser (VECSEL). In some embodiments, the laser system (200) may further comprise a pumping source (220) operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams.

[0062] In some embodiments, the dielectric mirror (110) may comprise a distributed Bragg reflector (DBR), wherein the semiconductor saturable absorber (120) comprises a spacer layer (121) bonded to the DBR, a quantum well (122) coupled to the spacer layer (121), and a cap (123) coupled to the quantum well (122). In some embodiments, the dielectric mirror (110) may be configured to act as a broadband mirror and provide customized group delay dispersion profiles for ultrafast laser systems. In some embodiments, the one or more gain media (210) may comprise one or more first active regions configured to generate the one or more laser beams at a first repetition rate. The semiconductor saturable absorber (120) may comprise a second active region configured to mode-lock the one or more laser beams generated by the laser system at a second repetition rate. In some embodiments, the first repetition rate may be equal to the second repetition rate.

[0063] In some embodiments, the present invention features a vertical-external-cavity surface-emitting laser (VECSEL). In some embodiments, the system may comprise one or more gain media (210) comprising one or more active regions configured to generate one or more laser beams at a first repetition rate. The system may further comprise a pumping source (220) operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams. The system may further comprise a single linear two-element cavity (230) optically coupled to the one or more gain media (210) such that the one or more gain media (210) direct the one or more laser beams through a length of the cavity (230). The system may further comprise one or more optical elements (240) disposed in the cavity(230), configured to alter one or more properties of the one or more laser beams. The system may further comprise a hybrid semiconductor saturable absorber mirror (SESAM) output coupler (100) comprising a distributed Bragg reflector (DBR), a spacer layer (121) bonded to the DBR, a quantum well (122) coupled to the spacer layer (121), a cap (123) coupled to the quantum well (122), optically coupled to the cavity (230) such that the one or more laser beams are directed into the hybrid SESAM output coupler (100), and an active region configured to mode-lock the laser beam at a second repetition rate and simultaneously optically couple the one or more laser beams into a mode-locked laser output. In some embodiments, the first repetition rate may be equal to the second repetition rate.

[0064] In some embodiments, the present invention features a hybrid semiconductor saturable absorber mirror (SESAM) output coupler (100) comprising a curved base substrate (130), an amorphous oxide multilayer mirror (110) deposited onto the curved base substrate (130), and a single-crystal semiconductor saturable absorber (120) bonded to the amorphous oxide multilayer mirror (110), configured to mode lock one or more laser beams of a laser system (200) and simultaneously optically couple each of the mode-locked laser beams into a mode-locked laser output.

[0065] In some embodiments, the laser system (200) may comprise one or more gain media (210) configured to generate the one or more laser beams, and a single linear two-element cavity (230) optically coupled to the one or more gain media (210) such that the one or more gain media (210) direct the one or more laser beams through a length of the cavity (230). The hybrid SESAM output coupler (100) may be optically coupled to the cavity (230) such that the one or more laser beams are directed into the hybrid SESAM output coupler (100). In some embodiments, the laser system (200) may further comprise one or more optical elements (240) disposed in the cavity (230), configured to alter one or more properties of the one or more laser beams. In some embodiments, the one or more optical elements (240) may comprise lenses, filters, mirrors, prisms, diffraction gratings, beamsplitters, or a combination thereof. In some embodiments, the one or more properties may comprise wavelength, linewidth, transverse mode profile, beam size, beam position, or a combination thereof of the one or more laser beams.

[0066] In some embodiments, the one or more gain media (210) of the laser system (200) may comprise a vertical-external-cavity surface-emitting laser (VECSEL) gain chip or a solid-state emitter. In some embodiments, the laser system (200) may further comprise a pumping source(220) operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams. In some embodiments, the amorphous oxide multilayer mirror (110) may comprise a distributed Bragg reflector (DBR). The single-crystal semiconductor saturable absorber (120) may comprise a spacer layer (121) bonded to the DBR, a quantum well (122) coupled to the spacer layer (121), and a cap (123) coupled to the quantum well (122). In some embodiments, the amorphous oxide multilayer mirror (110) may be configured to act as a broadband mirror and provide customized group delay dispersion profiles for ultrafast laser systems.

[0067] Referring now to FIG. 4, in some embodiments, the hybrid SESAM output coupler (100) may comprise a semiconductor saturable absorber component bonded to a dielectric mirror acting as an output coupler. In some embodiments, the hybrid SESAM output coupler (100) may comprise a distributed Bragg reflector (DBR) acting as the mirror. The DBR may comprise a plurality of pairs of SiO2wafers in combination with one or more thin film layers of TiO2, Ta2O5, hafnia, zirconia, any other high-refractive-index metal oxide material, or a combination thereof. In some embodiments, the semiconductor saturable absorber component of the hybrid SESAM output coupler (100) may comprise one or more quantum wells, a spacer layer disposed between the mirror and the quantum well, and a cap disposed on an end of the quantum well. In some embodiments, the quantum well may comprise InGaAs. In some embodiments, the spacer layer may comprise GaAs. In some embodiments, the cap may comprise GaAs.

[0068] In some embodiments, as shown in FIG. 1, the laser system (200) may comprise a plurality of cavities each having its own gain chip, configured to direct a laser beam into the single hybrid SESAM output coupler for mode-locking and coupling. In some embodiments, as shown in FIG. 2, the laser system (200) may comprise a single cavity having a single gain chip, configured to direct a laser beam into the single hybrid SESAM output coupler for mode-locking. In some embodiments, as shown in FIG. 3, the laser system (200) may comprise a single cavity having a single gain chip configured to generate a plurality of laser beams and direct them into the single hybrid SESAM output coupler for mode-locking and coupling. However, any combination of numbers of cavities, gain media and laser beams may be implemented in a laser system compatible with the hybrid SESAM output coupler of the present invention. In some embodiments, the laser system (200) comprises a single, linear, two-element laser cavity. In some embodiments, the dielectric mirror may have a 5-10 cm radius of curvature (RoC).

[0069] In some embodiments, the laser system implementing the hybrid SESAM output coupler of the present invention may act as a VECSEL-based frequency comb, capable of high repetition rates and greater cavity stability.

[0070] EXAMPLE 1 - The following is a non-limiting example of the present invention. It is to be understood that said example is not intended to limit the present invention in any way. Equivalents or substitutes are within the scope of the present invention.

[0071] The hybrid semiconductor saturable absorber mirror (SESAM) output coupler of the present invention was produced by bonding the saturable absorber to a dielectric mirror deposited onto a curved base substrate, shifting the complexity from the epitaxial growth to the fabrication process. This technique also removed a damage mechanism observed in GaAs / Al(Ga)As-based SESAMs, whereby two-photon absorption (TPA) degrades device performance by damaging the top distributed Bragg reflector (DBR) layers. This fabrication technique previously led to the VECSEL variant membrane-external-cavity surface-emitting-laser (MECSEL) allowing for continuous-wave performance at wavelengths typically difficult to achieve in VECSELs due to thermal performance or growth complexity. It has also been used to improve the thermal performance of SESAMs. In these previous examples, the semiconductor active structure was bonded to planar heat spreaders, while in this case the active structure was bonded to strongly-curved output couplers.

[0072] The semiconductor portion of the hybrid SESAM was grown by solid source molecular beam epitaxy (MBE). The quantum well (QW) and cap were grown in reverse order on a 300 nm Alo.7Gao.3As etch stop layer. The resulting absorber (after bonding) has a 5 nm GaAs cap, an 8.35 nm Ino.19Gao.8iAs QW, and a GaAs spacer layer used to optimize the field overlap with the QW. The output coupler was a commercially available dielectric mirror with 99.4% reflectivity and GDD of < 20 fs2at A = 1030 nm.

[0073] The devices were realized by transferring the GaAs-based epi onto the dielectric-coated glass substrate with low-temperature direct bonding. First, an 8 mm diameter disc was lithographically defined and chemically etched, followed by a wet chemical coring process to generate individual die. Post singulation, the epi and substrate surfaces were cleaned and plasma-activated to prepare for bonding. The parts were then pressed into contact using a convex press for the curved surface, forcing the epi disc to conform to the fused silica substrate. After initial contact, the parts were annealed to set the bond. Selective etching removed the remnantGaAs substrate, terminating on the AlGaAs etch stop layer. This layer was then removed, leaving a hybrid SESAM with < 200 nm thick semiconductor saturable absorber on the dielectric coating.

[0074] A linear cavity consisting of a resonant periodic gain (RPG) chip and the 10 cm RoC hybrid SESAM was set up to operate at a repetition rate of 4.2 GHz. This led to a cavity mode size of 127 pm (159 pm) on the gain chip (hybrid SESAM) leading to a ratio of the modal areas of 0.64. The gain chip was pumped at 808 nm. The VECSEL achieved stable single-pulse mode locking with 160 mW average output power at ~ 1030 nm and 410 fs pulse duration. The laser maintained consistent mode locked operation over a 14-hour test, with negligible change in pulse duration and spectrum. In these initial results, the repetition rate of the laser could be easily adjusted from 2.8 GHz to 4.2 GHz. The hybrid SESAM output coupler demonstrated a versatile approach for adjustable high-repetition-rate mode-locked VECSELs scalable to various wavelengths owing to the ability to maintain the same active region of the gain chip and the saturable absorber.

[0075] EXAMPLE 2 - The following is another non-limiting example of the present invention. It is to be understood that said example is not intended to limit the present invention in any way. Equivalents or substitutes are within the scope of the present invention.

[0076] A distributed Bragg reflector (DBR) centered at a wavelength of 1030 nm consisting of 12 pairs of SiO2and Ta2O5was used to design the mirror (DBR) portion of the hybrid SESAM. The resulting DBR showed comparable performance in terms of reflectivity, bandwidth, and group delay dispersion (GDD) to commercially available output couplers. The hybrid SESAM was completed with a GaAs spacer layer between the quantum well and the dielectric DBR and a thin GaAs cap layer after the quantum well (QW). The top cap layer, h^Ga^As quantum well thickness, and indium concentration of the quantum well was kept consistent with prior work at Lap=5 nm, tQW= 8.35 nm, and x = 0.19. The resulting spacer layer between the quantum well and the dielectric DBR was used to maximize the field overlap within the QW. The semiconductor saturable absorber region minimally perturbed the reflectivity of the dielectric DBR. Notably, when compared to a traditional SESAM, the DBR bandwidth was nearly double due to the large refractive index contrast available in sputtered metal-oxide-based mirrors.

[0077] The semiconductor region of the hybrid SESAM was grown by solid source molecular beam epitaxy (MBE). To enable bonding this structure was grown in reverse on a 300 nm Alo.7Gao.3As sacrificial layer grown at a temperature of 600 C. Subsequently the saturableabsorber structure comprised of the 5 nm GaAs capping layer, 8.35 nm Ino.19Gao.8iAs QW, and lastly the GaAs spacer layer was grown. The QW and 1.5 nm of the GaAs spacer layer were grown at a temperature of 485 C while the rest of the GaAs was grown at 580 C. A beam equivalent pressure (BEP) ratio of V:III was maintained between 10-12 during the growth. The resulting surface roughness was less than 1 nm RMS.

[0078] A commercially available dielectric mirror with a 10 cm RoC and a diameter of 12.7 mm was chosen for the bonding of the hybrid SESAM. The DBR was centered at 1030 nm with a reflectivity of 99.4% ± 0.3% and a GDD of < |20fs2|, similar to the ’surrogate’ mirror used in the design discussed above. The DBR was terminated with a SiO2layer and also shows an RMS roughness < 1 nm. The backside of the substrate has an anti -refl ection coating with R < 0.25%.

[0079] Fabrication entailed lithographically patterning and etching the epitaxial structure to define 8 mm diameter discs. The GaAs wafer was then thinned to ~ 100 pm and individual die were separated via a wet-etch-based coring process. The thinned epitaxial discs and dielectric-coated silica substrate were thoroughly cleaned to remove organic contamination and particulates. To prepare the surface for bonding, a plasma activation process was employed and the parts were pressed into contact using a convex press, forcing the epitaxial disc to conform to the curvature of the fused silica substrate. Next, the parts were annealed to set the bond. Selective etching removed the remnant GaAs substrate, terminating on the embedded high aluminum content Al GaAs etch stop layer. This layer was then removed, leaving the ~ 150 nm thick semiconductor saturable absorber on the dielectric coating and completing the hybrid SESAM. Owing to the high surface quality of all materials (with the epi and sputtered SiO2surfaces exhibiting microroughness values less than 1.0 nm as probed by AFM), the present invention achieved repeatable high-quality bonding, even with the relatively strong curvature.

[0080] With a GaAs-based RPG gain chip designed for 1030 nm operation and the fabricated 10 cm RoC hybrid SESAM, a linear cavity was constructed. The gain chip was pumped at 808 nm. Stable mode-locking was achieved between cavity lengths of 5.35 cm and 3.57 cm corresponding to repetition rates of 2.8 GHz and 4.2 GHz. The mode field diameter decreased from 376 pm to 302 pm over that range leading to an increase in the ratio of the modal areas of the gain chip relative to the SESAM from 0.46 to 0.64. At the highest repetition rate mode locked pulses of 410 fs were achieved with an average power of 160 mW.

[0081] As the ‘active’ region for both the VECSEL chip and the hybrid SESAM were essentiallythe same structure, this enabled straightforward translation of this cavity configuration and resulting mode-locked VECSELs to a variety of other wavelengths. The SiO2mirror substrate limited high-power performance due to the lower thermal conductivity as well as added complexity to thermal tuning of the saturable absorption relative to the gain.

[0082] Lastly, at the current operation wavelength of 1030 nm, one common mechanism of SESAM degradation was alleviated. Two-photon absorption has been shown to be a common damage mechanism in the aluminum containing layers of traditional SESAMs. This led to droop over time in average output power from the mode-locked laser and often degradation of the SESAM to a state where mode-locking could no longer be achieved at the damaged spot on the SESAM. The use of a wide-bandgap metal-oxide-based dielectric mirror obviated two-photon absorption in the Al(Ga)As DBR layers. The laser cavity operating at a 4.2 GHz repetition rate was run continuously for fourteen hours. The power, pulse-width, and spectral performance of the mode-locked VECSEL showed no appreciable changes from initial measurements to the end of the fourteen hour period.

[0083] Although there has been shown and described the preferred embodiment of the present invention, it will be readily apparent to those skilled in the art that modifications may be made thereto which do not exceed the scope of the appended claims. Therefore, the scope of the invention is only to be limited by the following claims. In some embodiments, the figures presented in this patent application are drawn to scale, including the angles, ratios of dimensions, etc. In some embodiments, the figures are representative only and the claims are not limited by the dimensions of the figures. Reference numbers recited in the below claims are solely for ease of examination of this patent application, and are exemplary, and are not intended in any way to limit the scope of the claims to the particular features having the corresponding reference numbers in the drawings. In some embodiments, descriptions of the inventions described herein using the phrase “comprising” includes embodiments that could be described as “consisting essentially of’ or “consisting of’, and as such the written description requirement for claiming one or more embodiments of the present invention using the phrase “consisting essentially of’ or “consisting of’ is met.

Claims

WHAT IS CLAIMED IS:

1. A hybrid semiconductor saturable absorber mirror (SESAM) output coupler (100) for mode-locking and coupling an output of a laser system, the hybrid SESAM output coupler (100) comprising a curved base substrate (130), a dielectric mirror (110) deposited onto the curved base substrate (130) such that a curvature of the dielectric mirror (110) matches a curvature of the curved base substrate (130), and a semiconductor saturable absorber (120) bonded to the dielectric mirror (110) such that a curvature of the semiconductor saturable absorber (120) matches the curvature of the curved base substrate (130), configured to mode-lock one or more laser beams generated by the laser system (200) and simultaneously optically couple each of the mode-locked laser beams into a mode-locked laser output.

2. The hybrid SESAM output coupler (100) of claim 1, wherein a radius of the curvature of the curved base substrate (130), a radius of the curvature of the dielectric mirror (110), and a radius of the curvature of the semiconductor saturable absorber (120) is 5 to 50 cm.

3. The hybrid SESAM output coupler (100) of claim 1, the semiconductor saturable absorber (120) comprises an epitaxially-grown compound semiconductor.

4. The hybrid SESAM output coupler (100) of claim 1, the dielectric mirror (110) comprises oxides, fluorides, or a combination thereof sputtered onto the curved base substrate (130) by a physical vapor deposition process.

5. The hybrid SESAM output coupler (100) of claim 1, wherein the laser system (200) comprises: a. one or more gain media (210) configured to generate the one or more laser beams; and b. a single linear two-element cavity (230) optically coupled to the one or more gain media (210) such that the one or more gain media (210) are further configured to direct the one or more laser beams through a length of the cavity (230); wherein the hybrid SESAM output coupler (100) is optically coupled to the cavity (230) such that the one or more laser beams are directed into the hybrid SESAM output coupler (100).

6. The hybrid SESAM output coupler (100) of claim 5, wherein each laser beam of the oneor more laser beams generated by the laser system (200) comprise a frequency of 1 to 50 GHz.

7. The hybrid SESAM output coupler (100) of claim 5, wherein the one or more laser beams generated by the laser system (200) intersect with the dielectric mirror (110) and the semiconductor saturable absorber (120) such that a laser spot is generated, wherein a diameter of the laser spot is equal to a diameter of the one or more laser beams when generated by the one or more gain media (210).

8. The hybrid SESAM output coupler (100) of claim 5, wherein the laser system (200) further comprises one or more optical elements (240) disposed in the cavity (230), configured to alter one or more properties of the one or more laser beams.

9. The hybrid SESAM output coupler (100) of claim 8, wherein the one or more optical elements (240) comprise lenses, filters, mirrors, prisms, diffraction gratings, beamsplitters, or a combination thereof.

10. The hybrid SESAM output coupler (100) of claim 8, wherein the one or more properties comprise wavelength, linewidth, transverse mode profile, beam size, beam position, or a combination thereof of the one or more laser beams.

11. The hybrid SESAM output coupler (100) of claim 5, wherein the one or more gain media (210) comprise one or more first active regions configured to generate the one or more laser beams at a first repetition rate; wherein the semiconductor saturable absorber (120) comprises a second active region independent of the one or more first active regions configured to mode-lock the one or more laser beams generated by the laser system at a second repetition rate; wherein the first repetition rate is equal to the second repetition rate.

12. The hybrid SESAM output coupler (100) of claim 5, wherein the one or more gain media (210) of the laser system (200) comprise a semiconductor gain chip or a solid-state emitter.

13. The hybrid SESAM output coupler (100) of claim 12, wherein the laser system (200) further comprises a pumping source (220) operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one ormore gain media (210) such that the one or more gain media (210) generate the one or more laser beams.

14. The hybrid SESAM output coupler (100) of claim 1, wherein the dielectric mirror (110) comprises a distributed Bragg reflector (DBR), wherein the semiconductor saturable absorber (120) comprises a spacer layer (121) bonded to the DBR, a quantum well (122) coupled to the spacer layer (121), and a cap (123) coupled to the quantum well (122).

15. A laser system (200) comprising a. one or more gain media (210) configured to generate one or more laser beams; b. a laser pumping source operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams; c. a single linear two-element cavity (230) optically coupled to the one or more gain media (210) such that the one or more gain media (210) are further configured to direct the one or more laser beams through a length of the cavity (230); and d. a hybrid semiconductor saturable absorber mirror (SESAM) output coupler (100) comprising a curved base substrate (130), a dielectric mirror (110) deposited onto the curved base substrate (130) such that a curvature of the dielectric mirror (110) matches a curvature of the curved base substrate (130), and a semiconductor saturable absorber (120) bonded to the dielectric mirror (110) such that a curvature of the semiconductor saturable absorber (120) matches the curvature of the curved base substrate (130), configured to mode-lock the laser beam and simultaneously optically couple the one or more laser beams into a mode-locked laser output.

16. The system (200) of claim 15, wherein the one or more gain media (210) of the laser system (200) comprise a semiconductor gain chip or a solid-state emitter.

17. The system (200) of claim 16, wherein the laser system (200) further comprises a pumping source (220) operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams.

18. The system (200) of claim 15, wherein the dielectric mirror (110) comprises a distributed Bragg reflector (DBR), wherein the semiconductor saturable absorber (120) comprises a spacer layer (121) bonded to the DBR, a quantum well (122) coupled to the spacer layer (121), and a cap (123) coupled to the quantum well (122).

19. The system (200) of claim 15, wherein the one or more gain media (210) comprise one or more first active regions configured to generate the one or more laser beams at a first repetition rate; wherein the semiconductor saturable absorber (120) comprises a second active region independent of the one or more first active regions configured to mode-lock the one or more laser beams generated by the laser system at a second repetition rate; wherein the first repetition rate is equal to the second repetition rate.

20. A semiconductor laser system comprising: a. one or more gain media (210) comprising one or more active regions configured to generate one or more laser beams at a first repetition rate; b. a pumping source (220) operatively coupled to the one or more gain media (210), configured to generate light energy and deliver the light energy to the one or more gain media (210) such that the one or more gain media (210) generate the one or more laser beams; c. a single linear two-element cavity (230) optically coupled to the one or more gain media (210) such that the one or more gain media (210) are further configured to direct the one or more laser beams through a length of the cavity (230); d. one or more optical elements (240) disposed in the cavity (230), configured to alter one or more properties of the one or more laser beams; and e. a hybrid semiconductor saturable absorber mirror (SESAM) output coupler (100) comprising a curved base substrate (130), a distributed Bragg reflector (DBR) deposited onto the curved base substrate (130) such that a curvature of the DBR matches a curvature of the curved base substrate (130), and a semiconductor saturable absorber (120) bonded to the DBR such that a curvature of the semiconductor saturable absorber (120) matches the curvature of the curved base substrate (130), the semiconductor saturable absorber (120) comprising a spacer layer (121) bonded to the DBR, a quantum well (122) coupled to the spacer layer(121), a cap (123) coupled to the quantum well (122), optically coupled to the cavity (230) such that the one or more laser beams are directed into the hybrid SESAM output coupler (100), and an active region independent of the one or more active regions of the one or more gain media (210), configured to mode-lock the laser beam at a second repetition rate and simultaneously optically couple the one or more laser beams into a mode-locked laser output; wherein the first repetition rate is equal to the second repetition rate.

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