Non-linear photonic devices are utilized to fabricate devices such as frequency comb (microcomb) generators, which are optical devices capable of generating very sharp and equidistant frequency lines in response to an input frequency. Frequency combs are useful in a number of applications, including optical communications, conversion from optical frequency ranges to RF/Microwave frequency ranges, light detection and ranging (LIDAR), spectroscopy, and timekeeping.
Typically, a laser fabricated on a first chip is connected via fiber or chip-to-chip coupling to the non-linear photonic device (i.e., frequency comb generator) to generate the desired output frequencies in response to the input provided by the laser. This increases the size, cost and power consumption of the non-linear photonic device.
It would be beneficial to integrate lasers and non-linear photonic devices on a simple integrated circuit. However, non-linear materials most commonly utilized are dielectrics, which provide low nonlinear optical coefficients and therefore require strict requirements on the quality factors of the cavities in order to operate efficiently. These requirements increase the cost of fabrication of the non-linear photonic devices. In addition, the dielectric material utilized for non-linear photonic devices are not easily integrated with active components (e.g., lasers) due to incompatibilities between design and fabrication of semiconductor materials and dielectric materials.
Utilization of photonic integration to assemble laser and nonlinear device on a same chip would therefore be beneficial, which can make the whole system low cost and scalable.
According to one aspect, an integrated laser/non-linear device includes a semiconductor/dielectric substrate, a nonlinear device fabricated on the semiconductor/dielectric substrate, and a pump laser fabricated on the same semiconductor/dielectric substrate.
According to another aspect, an integrated non-linear laser includes a waveguide and a resonator laser coupled to the waveguide via a directional coupler, wherein at least a portion of the resonator laser includes a gain section and at least a portion of the resonator laser comprises a non-linear waveguide.
According to another aspect, a frequency comb generator includes a resonator laser, a nonlinear resonator ring, and a waveguide. In some embodiments, the nonlinear resonator ring is coupled to the resonator laser to receive an input optical signal at a first frequency or first plurality of frequencies and to generate in response a frequency comb output. The waveguide is coupled via a coupler to the nonlinear resonator to receive the frequency comb optical output in response to the first frequency of first plurality of frequencies of the input optical signal.
According to another aspect, A frequency comb generator includes a gain waveguide, a no-linear resonator, and a waveguide. The non-linear resonator is coupled to the gain waveguide to receive an input optical signal at a first frequency or first plurality of frequencies and to generate in response a frequency comb output. The waveguide coupled via a coupler to the non-linear resonator to receive the frequency comb optical output in response to the first frequency of first plurality of frequencies of the input optical signal.
According to some embodiments, the present disclosure describes utilization of a semiconductor/dielectric substrate to fabricate non-linear devices having an intrinsic quality factor Q equal to or greater than 1.5×. For example, in some embodiments, Aluminum-Gallium-Arsenide on insulator (AlGaAsOI) may be utilized as the nonlinear material. The high-quality factor Q, high Kerr nonlinear coefficients, and compact mode volume allow for ultra-efficient frequency comb generation. For example, in one embodiment frequency comb generation was initiated at approximately 36 micro-watts (μW). In addition, fabrication of nonlinear devices based on semiconductor material is much simpler as compared with typical high Q non-linear platforms using dielectric material. For example, based on the utilization of AlGaAsOI as the non-linear material, ultra-efficient monolithically integrated laser and frequency comb generators can be fabricated, as well as hybrid nonlinear waveguides with integrated gain sections. In other embodiments, dielectric materials (included silicon nitride, silica, Ta2O5, LiNbO3, diamond, Hydex, MgF2) may be utilized to fabricate nonlinear devices, wherein the heterogeneous bonding of the dielectric substrate with the semiconductor substrate utilized to fabricate the active device (e.g., laser) to provide a monolithically integrated laser-nonlinear photonic device, substrates may be utilized.
As discussed in more detail below, the nonlinear waveguide is fabricated as part of a nonlinear device (e.g., microresonator, supercontinuum waveguide) that receives an optical input at a first frequency or first plurality of frequencies and generates in response a plurality of comb frequencies related to the input frequency. It is desirable that the nonlinear waveguide 16 be highly nonlinear while also being optically efficient. In some embodiments, this is achieved by utilizing a material having a high nonlinear coefficient, high index contrast, and high quality factors. In particular, in some embodiments, the nonlinear waveguide is fabricated from a semiconductor material, such as GaAs, GaN, InSb, InAs, InxGa1-xN, AlxGa1-xAs, InxGa1-xAs1-yP1-y, InxGa1-xAs1-ySb1-y, InxGa1-xSb1-yP1-y, InxGayAl1-x-yAs, InxGayAl1-x-yN where 0<x<1, 0<y<1 formed within an insulating SiO2 layer on a semiconductor substrate (e.g., silicon substrate). For example, in some embodiments the waveguide is an AlGaAsOI waveguides allows the generation of devices such as ultra-efficient frequency comb generators. In other embodiments, the nonlinear waveguide is fabricated from a dielectric material, such as silicon nitride, silica, Ta2O5, LiNbO3, diamond, Hydex, and MgF2. In some embodiments, the dielectric waveguide is fabricated on a dielectric substrate and heterogeneously integrated via wafer bonding to the semiconductor substrate utilized to fabricate the active device (e.g., laser).
For example, AlGaAs provides a very high nonlinear optical coefficient, which makes it a very attractive nonlinear optical material for use in nonlinear devices such as frequency comb generators. Table 1 provides a list of linear and nonlinear optical properties of various materials suitable for chip-based nonlinear photonics, wherein linear optical properties are described by the refractive index and nonlinear optical properties are described by the Kerr nonlinear coefficient (η2) (m2 W−1).
3 × 10−20
5 × 10−18
6 × 10−18
8 × 10−20
As shown in Table 1, (Al)GasAs provides a Kerr nonlinear coefficient of approximately 2.6×10−17, which is approximately two orders of magnitude higher than that of Si3N4 (η2=2.5×10−19) one hundred times greater than Si3N4 (η2=2.3×10−19).
Another benefit of AlGaAs is the relatively large bandgap of the material as compared to other commonly used semiconductor materials, such as Silicon (Si) (1.1 eV) or Indium-Phosphide (InP) (1.34 eV). In some embodiments, the nonlinear material is comprised of AlxGa1-xAs, wherein the ratio of Aluminum to Gallium can be modified to vary the bandgap of waveguide from 1.42 eV (872 nm) to 2.16 eV (574 nm). The bandgap associated with AlGaAs avoids two photon absorption (TPA) at the two most important telecom band (1310 nm and 1550 nm). In one embodiment, the value of x AlxGa1-xAs is selected to be 0.2 (Al0.2Ga0.8As) in order to generate a frequency comb at C-band wavelengths. In some embodiment, higher Al levels (e.g., greater than 0.2) may be utilized when targeting shorter pump wavelengths.
In addition, AlGaAs provides for intrinsic quality factor Q of approximately 1.5×106. In general, the quality factor of a given cavity/waveguide describes the loss resulting from interaction of the optical signal with the walls, the loss associated with the dielectric material filling the cavity, and losses associated with undisclosed holes in the cavity geometry. In some embodiments, the nonlinear waveguide 16 is fully etched with sub-micron dimensions, which provides anomalous group velocity dispersion (GVD) at the wavelength of the input frequency (e.g., pump wavelength). For example, telecommunications systems typically operate in the 0 band and the C band. In some embodiments, the AlGaAsOI nonlinear waveguide 16 provides strong material dispersion compensated by the waveguide geometry. In some embodiments, the thickness of the AlGaAs nonlinear waveguide 16 is set to be approximately 400 nm, at which the calculated GVD is anomalous at C band wavelength for waveguides with several different widths (e.g., 600 nm illustrated by line 22, 700 nm illustrated by line 24, and 800 nm illustrated by line 26) as shown in
In addition to the dimensions of the nonlinear waveguide 16, reducing sidewall roughness reduces the propagation loss associated with the nonlinear waveguide 16. In particular, because the mode size of the waveguide is small there is significant interaction of the mode with the waveguide sidewall. In addition, the strong index contrast between the nonlinear AlGaAs and the SiO2 cases an increase in the scattering loss. Reducing the sidewall roughness during fabrication will therefore reduce the propagation losses within the waveguide. In some embodiments, the patterned photoresist is reflowed after the lithography process. For example,
In addition to utilizing a reflow of the photoresist after the lithography process, in some embodiments scattering loss (sidewall roughness) is further reduced utilizing an optimized dry etch process. In some embodiments, an inductively coupled plasma (ICP) etch is utilized for both the hard mask and the AlGaAs. In particular, in some embodiments CHF3/CF4/N2 gases are utilized for etching the SiO2 and Cl2/N2 is utilized for etching the AlGaAs. For example, as shown in
In some embodiments, a surface passivation treatment is applied to the waveguide surface to reduce absorption caused by defect states at the surface of the materials. For example, in some embodiments a 5 nm thick Al2O3 layer is deposited utilized an atomic layer deposition (ALD) technique, which surrounds the nonlinear waveguide 16 and passivates the AlGaAs surface. In other embodiments, other methods may be utilized to reduce surface dissipation such as wet nitridation.
As discussed above, the dispersion of the waveguide plays a critical role. To characterize the GVD of the waveguides a ring resonator was fabricated having a 100 μm radius and a free spectral range (FSR) of 118 GHz. The resonance frequency of a mode family was measured as a function of relative mode number μ, relative to a reference resonance at wo, which is around 1550 nm. The resonance frequency ωμ of the modes can be expended in Taylor series as:
ωμ=ω0+μD1+½μ2D2+⅙μ3D3+. . . (1)
where D1/2π refers to the FSR around ω0 and D2 is related to the GVD β2 by
Considering the compact mode size (˜0.28 μm2) and small radius of the ring, it is reasonable to believe that this platform has waveguide loses that are comparable to the state of the art fully etched silicon on insulator (SOI) or even many commonly used dielectric waveguides.
In particular,
Referring now to
In the embodiment shown in
With respect to
With respect to the embodiment shown in
Referring now to
With respect to the nonlinear waveguide 60, in some embodiments the waveguide is fabricated utilizing a nonlinear semiconductor material and/or nonlinear dielectric material. As described above, nonlinear semiconductor material includes materials such as GaAs, GaN, InSb, InAs, InxGa1-xN, AlxGa1-xAs, InxGa1-xAs1-yP1-y, InxGa1-xAs1-ySb1-y, InxGa1-xSb1-yP1-y, InxGayAl1-x-yAs, InxGayAl1-x-yN where 0<x<1, 0<y<1. Nonlinear dielectric materials may include silicon nitride, silica, Ta2O5, LiNbO3, diamond Hydex, and MgF2.
In some embodiments, nonlinear materials such as AlGaAs provides relatively high gain. For example, the gain region 58 provides gain (i.e., lasing) while the nonlinear waveguide 60 provides the nonlinearity and dispersion required to generate the frequency comb. In some embodiments, the integrated comb laser 50 is capable of generating a frequency comb in the O frequency band, the C frequency band, and/or both the O and C frequency bands. The frequency comb output is coupled to the waveguide 54 via coupler 56.
In contrast,
Referring to
One of the benefits of the arrangements illustrated in
Referring now to
The chip-scale laser frequency comb 120 shown in
The DFB laser 122 generates a continuous-wave laser output that is provided to thermo-optic phase tuner 124. In some embodiments, the thermo-optic phase tuner 124 is comprised of a thermo-optic resistive heater that provides optical phase control. The output of the thermo-topic phase tuner 124 is coupled into a high-Q microring resonator 126, wherein Kerr non-linear four-wave mixing generates soliton microcombs. In some embodiments, the high-Q microring resonator 126 is comprised of Si3N4 that exhibits anomalous group velocity dispersion (GVD) in the telecommunication C band and has a free spectral range (FSR) of 100 GHz. In some embodiments, DFB laser 122 directly pumps the microring resonator 126 without an intermediate optical isolator, and the entire device is electronically operated via laser current control and phase control.
In some embodiments, the continuous wave laser output (shown by solid line 130) is coupled into the microresonator 126 and partially backscattered, as illustrated by dashed line 132. The backscattered signal triggers self-injection locking that assists soliton formation inside the microresonator 126. In some embodiments, the locking is optimized by controlling the laser current Ilaser. In some embodiments, laser self-injection locking (11, 12, 13, 14) leverages the narrow-band optical feedback at desired phase relations from a high-Q microresonator 126 to stabilize the pump laser and pulls the laser frequency toward the microring resonance. In this scenario, soliton microcombs can form when optimum laser-microresonator frequency detuning is reached. The DFB laser wavelength increases with increasing laser current, as the grating index increases as a result of injected electrical power heating. Consequently, certain gain currents trigger comb generation when the laser wavelength coincides with a micro-resonator resonance. The comb generation region resides where the laser is red-detuned to the resonance (as shown in
In some embodiments, fabrication begins with fabrication of the Si3N4 PIC on a Si substrate using a Damascene process with 4-mm-thick thermal wet silicon dioxide (SiO2). The PIC pattern is exposed with deep ultra-violet (DUV) stepper lithography and dry-etched into the SiO2 substrate to form the waveguide preform. Stoichiometric Si3N4 is deposited on the patterned SiO2 preform by using low-pressure chemical vapor deposition (LPCVD), filling the trenches and forming waveguide cores. Chemical-mechanical polishing (CMP) is used to remove excess Si3N4, planarize the wafer front surface, and control the Si3N4 waveguide height (e.g., 780 nm). Afterward, spacer SiO2 of 300-nm thickness is deposited on the Si3N4 substrate. The entire substrate is further annealed (e.g., at 1200° C.) to drive out the residual hydrogen content in Si3N4 and SiO2 and to densify the spacer SiO2. A second CMP is performed to create a flat and smooth wafer surface. In some embodiments, the measured root mean square (RMS) roughness of the wafer surface measured by atomic force microscopy (AFM) is 0.27 nm, enabling direct substrate bonding with an SOI wafer.
In some embodiments, to achieve high bonding yield, vertical channels for outgassing are etched before wafer bonding. Coarse alignment is required to bond blank films on the target areas of the patterned substrate. Fine alignment of patterns on different layers with an accuracy within 100 nm is enabled by DUV stepper lithography. After removing the Si substrate and buried SiO2 layer of the bonded SOI wafer, the Si device layer is processed to create waveguide structures with different etch depths, including shallow-etched Si rib waveguides for the lasers and phase tuners, fully etched hole structures for gratings, and thin Si tapers for mode conversion between the Si waveguide and underlying Si3N4 waveguide. In some embodiments, InP-based MQW gain material is then bonded to the patterned Si device at the active regions. In some embodiments, the InP process starts with InP substrate removal, which may include InP mesa etches. In some embodiments, P-type InP, InAlGaAs MQW, and N-type InP etching are performed by selective dry etching and wet etching. P- and N-type contact metals are deposited on the P—InGaAs layer and N—InP layer, respectively. In some embodiments, the excess Si on top of Si3N4 microresonators is removed before laser passivation by hydrogen-free deuterated SiO2 deposition (27). Vias are then etched for laser electrical contact, followed by proton implantation on the laser mesa structure to reduce electrical current leakage. In some embodiments, heater and probe metals are deposited at the end of the full process and the entire wafer is then diced into dozens of dies or chips to facilitate testing. In some embodiments, the InP/Si-to-Si rib waveguide transition loss is below 1 dB, and the Si-to-Si3N4 mode conversion efficiency is simulated to be above 90%. Assuming all devices share the same design, the overall device yield is determined primarily by the SOI bonding and InP bonding yields. In the current wafer, we have achieved bonding yields that enable thousands of complete laser-microresonator devices, each of which has a footprint as small as 1.6 mm2.
In some embodiments, an integrated laser/resonator includes a semiconductor/dielectric substrate, a pump laser fabricated on the substrate, and a microresonator fabricated on the same substrate, wherein the microresonator is coupled to the pump laser. In some embodiments, The integrated laser/resonator of claim 17, wherein the non-linear layer is comprised of GaAs, GaN, InSb, InAs, InxGa1-xN, AlxGa1-xAs, InxGa1-xAs1-yP1-y, InxGa1-xAs1-ySb1-y, InxGa1-xSb1-yP1-y, InxGayAl1-x-yAs, InxGayAl1-x-yN where 0<x<1, 0<y<1, dielectric materials such as silicon nitride, silica, Ta2O5, LiNbO3, diamond Hydex, and MgF2.
Although the embodiments described above were provided with respect to frequency comb generation, the nonlinear effects described may be utilized in other devices such as Second Harmonic Generation (SHG) devices, Optical Parametric Oscillation (OPO) devices, Stimulated Brillouin Scattering (SBS) devices, and Raman Scattering devices.
This invention was made with Government support under Grant (or Contract) No. HR0011-15-C-0055, awarded by the Department of Defense Advanced Research Projects Agency (DARPA). The Government has certain rights in this invention
Number | Date | Country | |
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63086156 | Oct 2020 | US |