The continued convergence of electronics and photonics on the chip scale can benefit from the voltage control of optical polarization for applications in communications, signal processing and sensing. It is challenging, however, to electrically manipulate the polarization state of light in planar optical waveguides. Here we introduce out-of-plane optical waveguides, allowing access to Berry's phase, a quantum-mechanical phenomenon of purely topological origin. As a result, electrically tunable optical polarization rotation on the chip scale is achieved. Devices fabricated in the silicon-on-insulator material platform are not limited to a single static polarization state. Rather, they can exhibit dynamic tuning of polarization from the fundamental transverse electric mode to the fundamental transverse magnetic mode. Electrical tuning of optical polarization over a 19?dB range of polarization extinction ratio is demonstrated with less than 1?dB of conversion loss at infrared wavelengths. Compact system architectures involving dynamic control of optical polarization in integrated circuits are envisioned.
We present a silicon microring resonator with a lithium niobate top cladding and integrated tuning electrodes. Submicrometer thin films of z-cut lithium niobate are bonded to silicon microring resonators via benzocyclobutene. Integrated electrodes are incorporated to confine voltage controlled electric fields within the lithium niobate thin film. The electrode design utilizes thin film metal electrodes and an optically transparent electrode wherein the silicon waveguide core serves as both an optical waveguide medium and as a conductive electrode medium. The hybrid material system combines the electro-optic functionality of lithium niobate with the high index contrast of silicon waveguides, enabling compact low tuning voltage microring resonators. Optical characterization of fabricated devices results in a measured loaded quality factor of 11,500 and a free spectral range of 7.15 nm in the infrared. The demonstrated tunability is 12.5 pm/V, which is over an order of magnitude greater than electrode-free designs.
Using full three-dimensional analysis we show that coupled periodic optical waveguides can exhibit a giant slow light resonance associated with a degenerate photonic band edge. We consider the silicon-on-insulator material system for implementation in silicon photonics at optical telecommunications wavelengths. The coupling of the resonance mode with the input light can be controlled continuously by varying the input power ratio and the phase difference between the two input arms. Near unity transmission efficiency through the degenerate band edge structure can be achieved, enabling exploitation of the advantages of the giant slow wave resonance.
We demonstrate vertical chip-to-chip light coupling using silicon strip waveguide cantilever couplers. The guided-wave couplers consist of silicon strip waveguides embedded within silicon dioxide cantilevers. The cantilevers deflect 90° out-of-plane via residual stress, allowing vertical light coupling between separate chips. A chip-to-chip coupling loss of 2.5 dB per connection is measured for TE polarization and 1.1 dB for TM polarization at 1550 nm wavelength. The coupling loss varies by less than±0.8 dB within the wavelength range from 1500 nm to 1565 nm for both polarizations. The couplers enable broadband and compact system architectures involving high speed vertical data transport between photonic integrated circuits.
A low power Mach-Zehnder interferometer thermo-optic switch using free-standing silicon-on-insulator strip waveguides is demonstrated. The air gap provides thermal isolation between the waveguide interferometer arms and the underlying silicon substrate. The highly confined optical modes of the strip waveguides enable miniature heated cross-sections. The heating efficiency from on-chip resistive heaters is enhanced. Measurements of fabricated devices using 100 microm arm lengths at 1550 nm wavelength result in a switching power of 540 microW, a 10% - 90% switching rise time of 141 micros, and an extinction ratio of 25 dB.
We demonstrate low-power thermo-optic-based optical bistability in a free-standing silicon ring resonator. A bistable optical response is achieved at reduced pump powers by thermally isolating the ring resonator from its supporting substrate with an air gap. The conversion efficiency from optical power to temperature change in the silicon core is enhanced. The optical transfer function of the resulting free-standing resonator exhibits a hysteresis loop for 80 microW input optical power. Similar nonthermally isolated resonators at the same detuning do not exhibit a bistable mode for input powers less than 2 mW.
An intra-chip coupling scheme from optical fibers to silicon strip waveguides is demonstrated. The couplers consist of silicon inverse width tapers embedded within silicon dioxide cantilevers that are deflected out-of-plane by residual stress. Deflection angles from 5 to 30 degrees are obtained and controlled by thermal annealing. Butt-coupling from tapered fibers or collimation-coupling from lensed fibers may be employed. The coupling scheme enables direct access to devices on the entire chip surface without dicing or cleaving the chip. Coupling efficiencies of 1.6 dB per connection for TE polarization and 2 dB per connection for TM polarization are achieved. The coupling efficiency shows little wavelength-dependence, with less than 1.6 dB fluctuation over the wavelength range of 1500 nm to 1560 nm.
Fringe visibility detection of the interaction of two bus spatial eigenmodes with a resonant cavity is investigated for the purpose of achieving a sensor platform with high sensitivity. The power distribution between the bus waveguide eigenmodes is modulated by the interaction with the cavity and is detected via fringe visibility lineshapes produced by twin-fiber interferometry. A test device is fabricated in a polymer-silica material system by a photolithographic process and is characterized by measuring the fringe visibility change as a function of analyte refractive index. Fringe visibility modulation from a straight two-mode waveguide coupled to a single mode ring resonator exposed to an aqueous glucose solution demonstrates a visibility change of 1.57 per weight percent, compared to a transmission change of 0.19 per weight percent for a single mode waveguide critically coupled to a ring with similar intrinsic quality factor. The demonstrated change in fringe visibility is 8.2 times larger.
An electric field sensor based on the indirect bonding of submicrometer thin films of lithium niobate to silicon microring resonators is presented using benzocyclobutene as an intermediate bonding layer. The hybrid material system combines the electro-optic functionality of lithium niobate with the high-index contrast of silicon waveguides, enabling compact and metal-free electric field sensors. A sensor is designed and fabricated using ion-sliced z-cut lithium niobate as the top cladding of a 20 ?m radius silicon microring resonator. The optical quasi transverse magnetic mode is used to access the largest electro-optic coefficient in the lithium niobate. Optical characterization of the hybrid device results in a measured loaded quality factor of 13,000 in the infrared. Operation of the device as an electric field sensor is demonstrated by detecting the fringing fields from a microstrip electrical circuit operating at 1.86 GHz. The demonstrated sensitivity to electric fields is 4.5 V m-1 Hz-1/2.
We demonstrate coupling from tapered optical fibers to 450 nm by 250 nm silicon strip waveguides using compact cantilever couplers. The couplers consist of silicon inverse width tapers embedded within silicon dioxide cantilevers. Finite difference time domain simulations are used to design the length of the silicon inverse width taper to as short as 6.5 ?m for a cantilever width of 2 ?m. Modeling of various strip waveguide taper profiles shows reduced coupling losses for a quadratic taper profile. Infrared measurements of fabricated devices demonstrate average coupling losses of 0.62 dB per connection for the quasi-TE mode and 0.50 dB per connection for the quasi-TM mode across the optical telecommunications C band. In the wavelength range from 1477 nm to 1580 nm, coupling losses for both polarizations are less than 1 dB per connection. The compact, broadband, and low-loss coupling scheme enables direct access to photonic integrated circuits on an entire chip surface without the need for dicing or cleaving the chip.
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