Integration of solid-state gain media with nonlinear photonics for narrow linewidth and mode-locked lasers on chip

Abstract

Integration of planar photonic circuit technology with ion-doped solid state gain media is provided by disposing the gain media in the cladding region of photonic circuit waveguides. This provides waveguide gain, but advantageously does not require any thin-film processing or patterning of photonic circuit features in the ion-doped solid state gain media. Numerous integrated photonic circuit applications are possible, such as : mode-locked lasers, low line width lasers, lasers with built-in wavelength conversion, on-chip entangled photon sources using spontaneous downconversion, lasers with built-in quadrature squeezing, large bandwidth tunable lasers, and ring laser gyroscopes.

Description

[0001] Integration of Solid-State Gain Media with Nonlinear Photonics for Narrow Linewidth and Mode- Locked Lasers on Chip by

[0002] Alexander White

[0003] Geun Ho Ahn

[0004] Kasper Van Gasse

[0005] Richard Luhtaru

[0006] Jelena Vuckovic

[0007] FIELD OF THE INVENTION

[0008] This invention relates to ion-doped solid state lasers .

[0009] BACKGROUND

[0010] Ion-doped solid-state gain media form the basis for some of the lowest-noise and most powerful lasers . They can also be used to create pulses with very high energy through mode-locking . As these lasers are currently extremely bulky and complex and require regular maintenance , it is thus highly desirable to use integrated photonics instead of tabletop optics to build the laser infrastructure . However, important photonic technology elements , such as low loss waveguides and thin films , tend to be very di f ficult to fabricate in ion-doped solid-state gain media . Accordingly, it would be an advance in the art to provide improved integration of photonic technology with ion-doped solid- state gain media . SUMMARY

[0011] Here we describe a method to create narrow linewidth and mode-locked lasers on-chip by integrating solid-state gain media with nonlinear photonic waveguides . As indicated above , solid-state gain media are di f ficult to manufacture into thin film photonics . However, in this work they are combined via hybrid integration with a photonic waveguide of a di f ferent material , forming hybrid guided modes with optical gain . I f the photonic waveguide material has a higher optical index than the gain medium, the gain medium can be arbitrarily thick and requires no processing ( otherwise , the gain medium must be made into a thin film) . With the presence of guided modes with gain, laser cavities can be formed . Due to the low optical nonlinearities present in most solid-state gain media, mode-locked lasers typically require an additional source of nonlinearity, which the photonic waveguides can provide . Finally, dispersion compensation can be achieved through the careful choice of waveguide material and cross-sectional geometry, or through lithographic geometry optimi zation with photonic inverse design .

[0012] In addition to creating short optical pulses through mode-locking, this technology has numerous applications enabled by the coincidence of optical gain and nonlinearity on chip . These include but are not limited to : Lasers with built-in wavelength conversion, On-chip entangled photon sources using spontaneous downconversion, Lasers with built- in quadrature squeezing, Large bandwidth tunable lasers , and Ring laser gyroscopes . Ultrafast lasers on chip that generate femtosecond and picosecond pulses have numerous applications in medicine ( such as ophthalmology, dermatology) , basic science ( including neuroscience , two photon microscopy) , and quantum technologies , where they would replace bulky traditional pulsed lasers. As the gain material can be selectively applied, the quality of some elements of the photonic circuit can be unaffected, enabling ultra-narrow linewidth lasing with high Q optical cavities. In addition to radically decreasing the size and cost of pulsed lasers, this technology allows for building mode- locked lasers into CMOS-compatible platforms.

[0013] BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIGs. 1A-C show a first embodiment of the invention.

[0015] FIGs. 2A-B show a second embodiment of the invention.

[0016] FIGs. 3A-B are optical microscope images of a fabricated device.

[0017] FIGs. 4A-C show modeling and characterization results for the device of FIGs. 3A-B.

[0018] FIGs. 5A-B show an embodiment of the invention including a periodically-poled nonlinear waveguide.

[0019] FIGs. 6A-C schematically show embodiments of the invention relating to additive pulse mode-locking.

[0020] FIGs. 7A-C schematically show embodiments of the invention that make use of nonlinear interferometers.

[0021] DETAILED DESCRIPTION

[0022] FIGs. 1A-C show a first embodiment of the invention.

[0023] Here FIG. 1A is a top view, FIG. IB is a corresponding side view, and FIG. 1C is an enlarged view of part of FIG. IB.

[0024] This exemplary embodiment is an apparatus including an optical waveguide including a waveguide core 104 surrounded by one or more cladding regions (102 and 106) . Waveguide core 104 is disposed on a planar substrate 102 , and waveguide core 104 has at least one optical nonlinearity . At least one of the cladding regions includes an ion-doped solid-state gain medium 106 configured to provide optical gain responsive to optical pumping of the ion-doped solid- state gain medium . Practice of the invention does not depend critically on how this pumping is done , so it is not shown . At least one mode of the optical waveguide has a mode shape 108 that extends into the ion-doped solid-state gain medium 106 . In this example evanescent part 110 of mode shape 108 extends into the ion-doped solid-state gain medium 106 , as shown on FIG . 1C . We have found that suf ficient gain can be obtained in this configuration, even though only a small part of mode shape 108 extends into gain medium 106 . The mode of the optical waveguide having mode shape 108 is thus ampli fied by the optical gain . The optical waveguide can be dispersion-engineered as needed for speci fic applications .

[0025] The apparatus can further include an optical resonator including the optical waveguide to provide a waveguide laser . FIG . 1A shows such a configuration, where resonator 104 ' can provides optical feedback to the optical waveguide suf ficient to establish lasing by having the round-trip gain exceed the round-trip loss . Any optical resonator suitable for photonic circuit integration can be employed, including but not limited to : ring resonators , ring resonators coupled to delay lines , coupled ring resonators , Fabry-Perot resonators , coupled Fabry-Perot resonators , whispering gallery resonators , and resonators including a nonlinear interferometer .

[0026] One important application of this work is mode-locked lasers . In this case , the optical nonlinearity can be saturable absorption to provide passive mode-locking . As indicated below, active modulation can also be used for mode-locking, leading to active mode-locking i f there is no saturable absorption, or leading to hybrid mode-locking i f active modulation and saturable absorption are both present .

[0027] Another application is low-linewidth lasers . For example the instantaneous line width of the waveguide laser can be 1 kHz or less (we ' ve seen 500 Hz in experiments , and this could be as low as 100 Hz ) . The low line width is because of two reasons .

[0028] 1 ) Optically pumped solid state lasers inherently have lower noise properties than semiconductor lasers .

[0029] 2 ) The other reason is that we also employ non-resonant pump configuration . We utili ze solid state laser as coherent gain with narrow line width resonator for emission, while the pump is non-resonantly absorbed in the delay path ( e . g . , as in examples below) with the laser gain . The non-resonant pump is helpful because even i f the pump laser has large frequency noise , that frequency noise does not get translated into the emission from the waveguide laser .

[0030] Additional applications include , but are not limited to : lasers with built-in wavelength conversion, on-chip entangled photon sources using spontaneous parametric downconversion, lasers with built-in quadrature squeezing, large bandwidth tunable lasers , and ring laser gyroscopes . Further comments follow with respect to some of these applications .

[0031] Squeezing : The squeezing fundamentally happens from application of second-order or third-order optical nonlinearity . The nonlinear interaction redistributes the quantum uncertainty between quadratures — producing a " squeezed state" where one quadrature has less noise than the standard quantum limit . When it happens with the gain simultaneously, the efficiency of the squeezing can be made higher, requiring less optical pumping due to the optical gain .

[0032] Large bandwidth tunable lasers: Here optical nonlinearity is used to broaden the lasing frequencies. By utilizing wide solid state gain (which already provides wide wavelength tunability) with optical nonlinearity, we can get even broader tunability.

[0033] Ring laser gyroscopes: These devices work by the measure of differential phase between CW and CCW modes of the rings. By combining nonlinearity with the solid state gain, we can improve laser properties to be more stable and the Kerr nonlinearity can cause the CW and CCW beams to shift their resonance differently depending on rotation- induced power imbalance. This means the rotation-induced phase shift gets amplified. Furthermore, the form factor is better because the nonlinearity is fully integrated with the laser .

[0034] Suitable ion-doped solid state gain media include, but are not limited to: sapphire, glass, YAG (yttrium aluminum garnet) , YALO (yttrium orthoaluminate) , YVO (yttrium orthovanadate) , YLF (yttrium lithium fluoride) , tungstates, borates, apatites, sesquioxides , fluorides, zinc chalcogenides, LiSAF (LiSrAlFe) , LiCAF (LiCaAlFe) , and forsterite .

[0035] Suitable ion dopants for the ion-doped solid state gain media include, but are not limited to: Ti, Nd, Yb, Er, Tm, Ho, Ce, and Cr.

[0036] Suitable materials for the waveguide core include, but are not limited to: silicon, silicon nitride, silicon carbide, lithium niobate, lithium tantalate, calcium fluoride, magnesium fluoride, AlGaAs, and tantala (Ta2Os) . Two exemplary embodiments are Nd:YAG bonded to silicon nitride on sapphire waveguides, and Ti: sapphire bonded to periodically poled lithium niobate waveguides on sapphire. This bonding can occur on a wafer scale or can be chipsetbased to provide sections of a larger photonic chip with gain without disrupting the rest of the chip. In addition, wafer scale commercial foundry based photonics can be employed to bring additional complexity and functionality to photonics. Further exemplary material stacks are Er-doped YVO4 on silicon dioxide, ruby on CaF on sapphire, Cr-doped ZnSe on AlGaAs on ZnSe, and Yb-doped YLF on tantala on YVO.

[0037] FIGs. 2A-B show a second embodiment of the invention. The main difference between this embodiment and the embodiment of FIGs. 1A-C is that here the ion-doped solid- state gain medium 202 is configured as an unpatterned slab disposed on part of the optical waveguide. By contrast, the example of FIGs. 1A-C had the ion-doped solid-state gain medium 106 configured as an unpatterned slab disposed on all of the optical waveguide. The ability to control which parts of the photonic circuit get gain is valuable for providing gain where it will be most useful and / or for reducing loss where loss is most harmful (e.g., near passive high-Q resonators) .

[0038] The gain medium and nonlinear waveguide (s) can be integrated in numerous ways. This includes but is not limited to:

[0039] • Direct bonding on chip or wafer scale

[0040] • Direct bonding of gain chiplets (e.g., with pick and place technology)

[0041] • Indirect bonding on chip or wafer scale (using adhesives or oxide deposition)

[0042] • Indirect bonding of gain chiplets (using adhesives or oxide deposition) • Growth of ion doped materials on waveguides

[0043] • Integration of thin film of solid state gain material on thin film photonic platform and subsequent lithography and etching of waveguide structures in the thin film photonic platform.

[0044] FIG. 2A also shows an optional feature of an actively driven modulator 204 disposed in the optical resonator 104' . As indicated above, such a configuration can be used for active or hybrid mode-locking of the waveguide laser.

[0045] FIGs. 3A-B are optical microscope images of a fabricated device. These are optical micrographs of Nd:YAG bonded on a delay line of silicon nitride on sapphire photonic circuit, allowing for controllable and localized optical gain. Here FIG. 3A has the bonded Nd:YAG gain medium in focus, and FIG. 3B has the rest of the photonic circuit in focus.

[0046] FIGs. 4A-C show modeling and characterization results for the device of FIGs. 3A-B. FIG. 4A is a mode simulation of this stack for the exemplary embodiment of Nd:YAG on silicon nitride on sapphire. High overlap in both the nonlinear nitride and the Nd:YAG gain allows the waveguide to provide both optical gain and nonlinearity. FIG. 4B shows measured net gain in this platform showing peak gain of 50dB when pumped with 808nm laser diode. FIG. 4G shows a measured laser curve in this platform.

[0047] FIGs. 5A-B show an embodiment of the invention including a periodically-poled nonlinear waveguide 502. Inclusion of a periodically-poled material having a second order optical nonlinearity in the waveguide core 104 is particularly useful for providing quasi-phasematching of a second order nonlinearity. Due to the variety of optical nonlinearities in waveguide material, there are multiple ways to induce modelocking. For example, nonlinear interferometry, additive pulse mode-locking, and Q-switching. Note with whispering gallery mode based schemes, the Kerr nonlinearity can ensure that the resonance frequency of the ring can be different in different directions, allowing for unidirectional lasing.

[0048] FIGs. 6A-C schematically show embodiments of the invention relating to additive pulse mode-locking. In all cases, gain is proved as described above, but not shown here. FIG. 6A shows additive pulse mode locking using a whispering gallery mode in resonator 604 and a delay line 602. Differential nonlinear phase accumulation makes pulsing 606 energetically favorable. FIG. 6B shows additive pulse mode locking using coupled Fabry-Perot cavities. Here the insertion of partial reflection 612 in a linear resonator formed by waveguide 608 and reflectors 610 leads to pulsing 606. This can also be done in a loop topology where one reflector is removed and the waveguide is connected back around in a circular path. FIG. 6C shows additive pulse mode locking in coupled whispering gallery mode resonators 614 and 616. Here coupling 618 between the two resonators is what leads to pulsing 606.

[0049] FIGs. 7A-C schematically show embodiments of the invention that make use of nonlinear interferometers. In all cases, gain is proved as described above, but not shown here. FIG. 7A shows nonlinear interferometer based mode locking in a ring configuration. An MZI 702 with unequal splitting in the arms provides a power dependent round trip loss in ring resonator 704 leading to mode locking. FIG. 7B shows nonlinear interferometer based mode locking in a Fabry-Perot configuration. Here 706 is a waveguide Fabry- Perot resonator. FIG. 7C shows nonlinear interferometry based Fabry Perot laser using directional coupling . With uneven spitting ratio in directional coupler 708 and uneven path lengths , the round trip loss can be made to decrease under higher powers , inducing mode locking . Here the directional coupler and / or waveguides can be nonlinear .

[0050] E . g . , a nonlinear phase shi ft in a waveguide could become a nonlinear loss because of the directional coupling, and this loss process could occur on faster time scales than saturable absorption . Additionally, all of the schemes illustrated in

[0051] FIGs . 6A-7C can be combined with electronic phase modulation to induce active mode locking or initiate mode locking . With nonlinear waveguides that have second-order nonlinearities , this can be done at very high speed with the electrooptic ef fect . Dispersion engineering via photonic inverse design can be employed to design resonators with desired broadband performance .

Claims

CLAIMS1 . Apparatus comprising : an optical waveguide including a waveguide core surrounded by one or more cladding regions ; wherein the waveguide core is disposed on a planar substrate ; wherein the waveguide core has at least one optical nonlinearity; wherein at least one of the cladding regions includes an ion-doped solid-state gain medium configured to provide optical gain responsive to optical pumping of the ion-doped solid-state gain medium; wherein at least one mode of the optical waveguide has a mode shape that extends into the ion-doped solid-state gain medium; wherein the at least one mode of the optical waveguide is ampli fied by the optical gain .2 . The apparatus of claim 1 , further comprising an optical resonator including the optical waveguide to provide a waveguide laser .3 . The apparatus of claim 2 , wherein the optical resonator is selected from the group consisting of : ring resonators , ring resonators coupled to delay lines , coupled ring resonators , Fabry-Perot resonators , coupled Fabry-Perot resonators , whispering gallery resonators , and resonators including a nonlinear interferometer .

4. The apparatus of claim 2, wherein the optical nonlinearity is saturable absorption, whereby the waveguide laser is passively mode-locked.

5. The apparatus of claim 2, further comprising an actively driven modulator disposed in the optical resonator, whereby the waveguide laser is actively mode-locked or hybrid mode- locked .

6. The apparatus of claim 2, wherein an instantaneous line width of the waveguide laser is 1 kHz or less.

7. The apparatus of claim 2, wherein the apparatus is configured as a system selected from the group consisting of: lasers with built-in wavelength conversion, on-chip entangled photon sources using spontaneous downconversion, lasers with built-in quadrature squeezing, large bandwidth tunable lasers, and ring laser gyroscopes.

8. The apparatus of claim 1, wherein the ion-doped solid- state gain medium is configured as an unpatterned slab disposed on top of part or all of the optical waveguide.

9. The apparatus of claim 1, wherein the ion-doped solid state gain medium is selected from the group consisting of: sapphire, glass, YAG (yttrium aluminum garnet) , YALO (yttrium orthoaluminate) , YVO (yttrium orthovanadate) , YLF (yttrium lithium fluoride) , tungstates, borates, apatites, sesquioxides , fluorides, zinc chalcogenides, LiSAF (LiSrAlFe) , LiCAF (LiCaAlFe) , and forsterite.

10. The apparatus of claim 1, wherein an ion dopant of the ion-doped solid state gain medium is selected from the group consisting of: Ti, Nd, Yb, Er, Tm, Ho, Ce, and Cr.

11. The apparatus of claim 1, wherein the waveguide core includes a material selected from the group consisting of: silicon, silicon nitride, silicon carbide, lithium niobate, lithium tantalate, calcium fluoride, magnesium fluoride, AlGaAs, and tantala (Ta2Os) .

12. The apparatus of claim 1, wherein the waveguide core includes a periodically-poled material having a second order optical nonlinearity.

13. The apparatus of claim 1, wherein the optical waveguide is dispersion-engineered.

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