Patent Application
20100238536
The invention is related to microphotonics, and in particular to designs of planar-integrated optical isolators using silicon material.
Integrated optical isolators, namely the devices that allow the transmission of light in only one direction, are necessary to prevent unwanted reflection back into the laser, and are thus an important component in integrated photonic systems. Optical isolation, for example, unidirectional transmission is typically realized by means of nonreciprocal magneto-optical effects of ferromagnetic materials such as Faraday rotation and nonreciprocal phase shift. However, most ferromagnetic materials have lattice constants different from that of silicon and therefore cannot be monolithically grown on a silicon substrate; in addition, ferromagnetic materials are often comprised of ferro-metallic alloys, exotic oxides or semiconductors doped with transition metal ions, none of which is compatible with the standard silicon CMOS fabrication process due to contamination issues.
Because of such incompatibilities, currently magneto-optic isolators are made in bulk garnet-based materials. Such bulk isolators are not amenable to planar integration and their cost is also high since optical-quality crystals are required to maximize the desired magneto-optic effect. Recently, magneto-optic isolator devices employing hybrid architecture have been demonstrated, in which magnetically active garnet materials are bonded onto a silicon substrate. Compared to such a hybrid design, a monolithic solution requires simpler equipment, is less demanding in terms of fabrication and more cost effective, and is thus a more attractive approach. In summary, optical isolators that can be monolithically integrated onto a silicon platform have been highly desirable but still remain to be developed.
According to one aspect of the invention, there is provided a magneto-optical isolator device. The isolator device includes a substrate and a bottom cladding layer that is formed on the substrate. An optical resonator structure is formed on the bottom cladding layer. The resonator structure includes crystalline or amorphous diamagnetic silicon or silicon-germanium so as to provide non-reciprocal optical isolation. A top cladding layer is formed on the resonator structure. One or more magnetic layers positioned on the top cladding layer or between the top cladding or bottom cladding layers and the optical resonator structure.
According to another aspect of the invention, there is provided a method of forming a magneto-optical isolator device. The method includes providing a substrate forming a bottom cladding layer on the substrate and positioning an optical resonator structure on the bottom cladding layer. The resonator structure includes crystalline or amorphous diamagnetic silicon or silicon-germanium so as to provide non-reciprocal optical isolation. Also, the method includes forming a top cladding layer on the resonator structure. Furthermore, the method includes positioning one or more magnetic layers on the top cladding layer or between the top cladding or bottom cladding layers and the optical resonator structure.
The invention provides optical isolator designs using silicon/silicon-germanium materials to achieve very low manufacturing cost, compatibility with planar microphotonic integration and possibly scalable performance improvement. In one example, the invention provides an optical waveguide coupled to a planar optical resonator made of silicon on insulator (SOI) material or silicon-germanium alloy grown on silicon and in the form of micro-ring, micro-disk or micro-racetrack.
An in-plane magnetic field is applied by patterned permanent ferromagnetic films on both sides of the waveguides comprising the resonator. The diamagnetic nature of silicon/silicon-germanium leads to nonreciprocal phase shift in the SOI waveguides, which lifts the degeneracy of counter-propagating resonant modes in the resonator. The operating wavelength of the device is specifically chosen to be resonant with backward propagating waves in the resonator; therefore, the back-reflected light is coupled into the resonator and dissipated while the forward-propagating light remains unaffected by the resonator and hence leads to an optical isolation effect. In another example, the isolator device includes a silicon/silicon-germanium resonator coupled to two optical waveguides, as well as magnetic films to provide magnetization in silicon. The operating wavelength is chosen so that the forward-propagating wave is resonant in the resonator: light from the laser can thus be coupled into the resonator and then coupled into the other waveguide as the output, whereas reflected light remains in one waveguide and cannot be fed back into the laser.
Silicon is a diamagnetic material and thus has traditionally been regarded as non-magnetically active. However, the fact that silicon has very low optical loss in the 1310 and 1550 nm telecommunication bands has been overlooked for optical isolator applications. The measurement has yielded Verdet constant of doped and undoped single crystalline silicon in the range of 12-17 deg/(T·cm) at 1550 nm wavelength. Further, the Verdet constant of silicon at 1310 and 1550 nm bands can be further improved by addition of germanium to form silicon-germanium alloys. Since silicon-germanium alloys can be monolithically grown on silicon substrate, such compatibility offers significant competitive edge for cost reduction and process improvement over current bulk magneto-optic device.
An inventive magneto-optic isolator device 2 incorporating a resonator structure 4 is schematically shown in
The top and bottom cladding layers described in
Magnetic field applied using patterned magnetic films leads to magnetization in the Si/SiGe waveguide core. If the isolator device is designed for transverse magnetic (TM) polarization, the top and bottom cladding layers can include patterned magnetic films being placed on both sides of the waveguide core, providing an in-plane magnetic field perpendicular to the light propagation direction. Alternatively, if the isolator device operates with transverse electric (TE) polarization, a continuous magnetic film layer can be deposited on top of the waveguide structure to yield a magnetic field perpendicular to the substrate. The inherent structural asymmetry in the rib/ridge structure can thus produce non-reciprocal phase shift for TM polarized light. In a microdisk resonator, similar rib/ridge structures can be used for TM polarization; the in-plane structural asymmetry of a micro-disk also allows micro-disk isolator operation with TE polarized light.
The resonant frequency degeneracy of light propagating in clockwise, corresponding to forward-propagating wave in
In another embodiment, the isolator device 80 includes of two optical waveguides, one for optical input 82 and one for output 84, both coupled to a Si/SiGe micro-resonator 86, as is illustrated in
In embodiments described herein, high isolation ratio in a Si/SiGe resonator isolator device can be achieved by: 1) minimized peak full-width-at-half-maximum (FWHM) of the resonant peak; and 2) large resonant peak separation between forward and backward propagating waves. The peak width is inversely proportional to optical propagation loss in the resonator. Both crystalline and hydrogenated amorphous silicon exhibit optical loss in the telecommunication wavebands as low as a few dB/cm (<1 dB/cm for single crystalline Si). The optical band gap of silicon-germanium alloys can be continuously tuned from 1.1 eV to 0.7 eV by adjusting the alloy composition, and low optical absorption loss at telecommunication bands can be achieve in Si-rich SiGe alloys, suitable for isolator application.
The peak separation is related to the propagation phase shift in the waveguiding structure, which is determined by the Verdet constant of the waveguide materials and the asymmetry of the optical guiding mode. The Verdet constant is correlated to the refractive index dispersion by the well-known Becquerel formula:
Alloying silicon with germanium increases the material dispersion at telecommunication wavelengths, and thereby the SiGe Verdet constant can be increased. Multilayer guiding structures with ferromagnetic or paramagnetic overlayers, such as oxide crystals including garnets, perovskites or spinels, transition metal ion doped semiconductors, oxide or chalcogenide glasses, with Faraday rotation or Verdet constant of sign opposite to that of Si/SiGe can be used to enhance the resonant modal asymmetry in the optical resonator 4, as is schematically shown in
Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.
