The present teaching relates to thin film lithium niobate (TFLN) photonic circuits. These devices have demonstrated significant advantages for functions such as high frequency optical modulation.
The present teaching, in accordance with preferred and exemplary embodiments, together with further advantages thereof, is more particularly described in the following detailed description, taken in conjunction with the accompanying drawings. The skilled person in the art will understand that the drawings, described below, are for illustration purposes only. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating principles of the teaching. The drawings are not intended to limit the scope of the Applicant's teaching in any way.
The present teaching will now be described in more detail with reference to exemplary embodiments thereof as shown in the accompanying drawings. While the present teaching is described in conjunction with various embodiments and examples, it is not intended that the present teaching be limited to such embodiments. On the contrary, the present teaching encompasses various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. Those of ordinary skill in the art having access to the teaching herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present disclosure as described herein.
Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the teaching. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment.
It should be understood that the individual steps of the method of the present teaching can be performed in any order and/or simultaneously as long as the teaching remains operable. Furthermore, it should be understood that the apparatus and method of the present teaching can include any number or all of the described embodiments as long as the teaching remains operable.
Thin film lithium niobate photonic circuits are useful for high frequency optical modulation. However, the coupling losses of thin film lithium niobate waveguides to optical fiber are currently much higher than for competing technologies due to the large difference in mode sizes between the optical fiber and the thin film lithium niobate waveguide. For example, at a wavelength of 1550 nm, the mode size of Corning SMF-28 fiber is approximately 10 μm, while the mode size of a thin film lithium niobate waveguide is only 1.2 μm.
One approach to mode matching is to taper the thin film lithium niobate waveguide. See, for example, Low-Loss Fiber-to-Chip Interface for Lithium Niobate Photonic Integrated Circuits, L He, Mian Zhang, Marko Loncar, et al., Optics Letters, 44, pp 2314-2317, 2019, which is incorporated herein by reference. The tapering expands the thin film lithium niobate mode. However, the mode can only be expanded enough to match a lensed fiber spot size of 2 μm. This small spot size combined with the twelve-micron air gap between the fiber and chip results in high sensitivity to environmental conditions, such as temperature and vibration. Coupling losses as low as 1.7 dB have been demonstrated but are not repeatable.
Another approach to mode matching is to taper the optical fiber. See, for example, Efficient Light Coupling Between an Ultra-Low Loss Lithium Niobate Waveguide and an Adiabatically Tapered Single Mode Optical Fiber, Ni Yao, et al., Optics Express V. 28, Issue 8, pp 12416-12423 (2020), which is also incorporated herein by reference. The fiber taper is typically several millimeters long and fragile as the tip diameter is only 1.4 μm. The small mode size requires precise alignment accuracy. Furthermore, because the tapered fiber requires an air-SiO2 interface for mode confinement, it cannot be epoxied to the chip, resulting in high sensitivity to environmental conditions.
The present teaching relates to expanding the thin film lithium niobate mode by using a Ge doped waveguide on top of the thin film lithium niobate waveguide. This allows for the expansion of the mode on the thin film lithium niobate chip to match that of standard single mode fiber, allowing the fiber to be epoxied to the end of the chip. This provides a stable junction that is robust to vibration, temperature, etc. with low Fresnel reflection. In one particular embodiment of the thin film lithium niobate chip according to the present teaching, the extent of the taper on the chip is only 400 microns long. For this particular configuration, simulations predict a coupling loss of only 0.36 dB indicating that the present teaching provides a significant performance advantage over the prior art.
Between the fiber interface and the nominal thin film LiNbO3 waveguide 206, there is a relatively straight transitional waveguide 208 having a width W2 and a length L4. A LiNbO3 ridge 210 tapers to a width W3 at the LiNbO3 slab 212 over length L3. The LiNbO3 slab 212 tapers from a width of W4 to a width of W2 over length L2.
The transitional waveguide 202, LiNbO3 slab 212, and LiNbO3 ridge 210 can be configured so that there is a taper over a length that is relatively short compared to the length of the full thin film LiNbO3 device. This configuration provides mode expansion on the thin film LiNbO3 chip to match that of standard single mode fiber. The mode expansion feature allows the input optical fiber to be epoxied to the end of the chip. The mode expansion feature also provides a stable junction that is robust to vibration, temperature, etc. with low Fresnel reflection. Simulations predict coupling losses of only a fraction of a dB, which is a large performance advantage over prior art configurations.
The LiNbO3 slab then tapers from a width of W2 to a width of W1 over length L5. The transitional waveguide material thickness and refractive index are chosen to provide optimal mode matching to the fiber. The refractive index of the transitional waveguide material is typically only a few percent higher than that of the SiO2 sub-layer.
Devices according to one particular embodiment of the present teaching can be fabricated in the following way. According to one specific physical implementation, devices are fabricated from a 500 μm thick commercially available x-cut LiNbO3-on-Silicon substrate, which is formed by bonding a 600 nm thick LiNbO3 film to the top of a 4 μm thick SiO2 film. Waveguide patterns are photolithographically defined and etched to 300 nm to produce rib waveguides in the LiNbO3. The LiNbO3 slab etch pattern is then photolithographically defined and etched. The transition layer is then deposited with an appropriate thickness. The transition waveguide etch pattern is photolithographically defined and etched. A final SiO2 layer is deposited over all surfaces. The end facets of the waveguides are then etched to produce good coupling to the optical fibers. It should be understood that devices according to the present teaching can be fabricated in numerous other ways and that this is just one example.
Simulations were performed to characterize the mode converter for fiber-to-thin film LiNbO3 coupling according to the present teaching. The particular embodiment investigated uses a Ge doped waveguide on top of the thin film LiNbO3 waveguide. Referring to
While the Applicant's teaching is described in conjunction with various embodiments, it is not intended that the Applicant's teaching be limited to such embodiments. On the contrary, the Applicant's teaching encompasses various alternatives, modifications, and equivalents, as will be appreciated by those of skill in the art, which may be made therein without departing from the spirit and scope of the teaching.
The present application is a non-provisional application of U.S. Provisional Patent Application Ser. No. 63/144,629 entitled “Mode Converter for Optical Fiber to Thin Film LiNbO3 Coupling” filed on Feb. 2, 2021. The entire content of U.S. Provisional Patent Application Ser. No. 63/144,629 is herein incorporated by reference.
Number | Date | Country | |
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63144629 | Feb 2021 | US |