CHIP-BASED FREQUENCY COMB GENERATOR WITH MICROWAVE REPETITION RATE

• US 20120320448A1
• Filed: 06/15/2012
• Published: 12/20/2012
• Est. Priority Date: 06/17/2011
• Status: Active Grant

• Current Assignee: California Institute of Technology
• Original Assignee: California Institute of Technology
• Inventors: LI, Jiang, LEE, Hansuek, CHEN, Tong, VAHALA, Kerry
• Granted Patent: US 8,917,444 B2
• Time in Patent Office: Days
• US Class Current: 359/332
• CPC Class Codes: G02F 1/2257 the optical waveguides being made of semiconducting material
• CPC Class Codes: G02F 1/353 Frequency conversion, i.e. wherein a light beam is generated with frequency components different from those of the incident light beams
• CPC Class Codes: G02F 2201/063 ridge; rib; strip loaded
• CPC Class Codes: G02F 2201/17 Multi-pass arrangements, i.e. arrangements to pass light a plurality of times through the same element, e.g. by using an enhancement cavity
• CPC Class Codes: G02F 2201/58 Arrangements comprising a monitoring photodetector
• CPC Class Codes: G02F 2202/09 inorganic glass
• CPC Class Codes: G02F 2202/105 single crystal Si
• CPC Class Codes: G02F 2203/15 involving resonance effects, e.g. resonantly enhanced interaction
• CPC Class Codes: G02F 2203/54 Optical pulse train (comb) synthesizer
• CPC Class Codes: G02F 2203/56 Frequency comb synthesizer
• CPC Class Codes: G02F 2203/69 Arrangements or methods for testing or calibrating a device

Abstract

A frequency comb generator fabricated on a chip with elimination of a disadvantageous reflow process, includes an ultra-high Q disk resonator having a waveguide that is a part of a wedge structure fabricated from a silicon dioxide layer of the chip. The disk resonator allows generation of a frequency comb with a mode spacing as low as 2.6 GHz and up to 220 GHz. A surface-loss-limited behavior of the disk resonator decouples a strong dependence of pumping threshold on repetition rate.

Claims

1. A chip-based frequency comb generator comprising:

a chip-based disk resonator comprising a waveguide that is a part of a wedge structure fabricated from a silicon dioxide layer, the disk resonator having a low-threshold microwave to millimeter wave repetition rate control, with repetition rates ranging from 2.6 GHz when the disk resonator has a diameter of 25 mm to 220 GHz when the disk resonator has a diameter of 3 mm.

2. The frequency comb generator of claim 1, wherein the repetition rates include 4.4 GHz, 6.8 GHz, 8.8 GHz, 16.4 GHz, 22 GHz, 33 GHz, 51 GHz and 66 GHz when the disk resonator has a respective diameter in correspondence to the respective repetition rate.

3. The frequency comb generator of claim 1, wherein the disk resonator is configured to generate a frequency comb comprising about 1900 comb lines.

4. The frequency comb generator of claim 3, wherein the frequency comb is generated when the disk resonator is provided a pump power at about 200 mW.

5. The frequency comb generator of claim 3, wherein the disk resonator has a diameter of about 2 nun and generates the frequency comb with a repetition rate of about 33 GHz and a span ranging from about 132.0 nm to about 1820 nm.

6. The frequency comb generator of claim 1, wherein the disk resonator has a diameter of about 1 mm and generates a frequency comb with a span exceeding ¾

octave and a continuous spectrum ranging from about 1180 nm to about 2020 nm.

7. The frequency comb generator of claim 1, wherein the wedge structure is fabricated in part by eliminating a foot region in a silicon dioxide layer of an integrated circuit, and having a linear sloping surface with a linear slope angle ranging from >

7 degrees to ≦

90 degrees, the linear sloping surface further characterized by an absence of the foot region.

8. A chip-based frequency comb generator comprising:

a chip-based disk resonator comprising a waveguide that is a part of a wedge structure fabricated from a silicon dioxide layer, the disk resonator having a microwave to millimeter wave repetition rate output when provided with a pump power threshold as low as 1 mW.

9. The frequency comb generator of claim 8, wherein the chip-based disk resonator has an optical Q factor that varies substantially linearly with diameter for free spectral range values in the microwave to millimeter wave range.

10. The frequency comb generator of claim 9, wherein the disk resonator is characterized in part by a Q factor of at least 875 million.

11. The frequency comb generator of claim 10, wherein the disk resonator is configured to operate exclusively in a scattering-loss-limited mode when generating the frequency comb, and wherein the frequency comb is generated with a free-running beat line-width of less than 200 Hz.

12. The frequency comb generator of claim 10, wherein operating exclusively in the scattering-loss-limited mode comprises decoupling of a dependency between the pump power threshold and the repetition rate.

13. The frequency comb generator of claim 12, wherein the pump power threshold is less than 5 mW for repetition rates between 4.4 air and 220 GHz.

14. The frequency comb generator of claim 8, wherein the circular disk resonator comprises a waveguide component fabricated by executing steps comprising:

placing a silicon substrate in a furnace;

introducing steam into the furnace;

raising a temperature inside the furnace to a first temperature level wherein a silicon dioxide layer is formed on a major surface of the silicon substrate;

eliminating a moisture content in the silicon substrate by heating the silicon substrate at a second temperature level in an oxygen-rich environment;

forming a first assembly by applying a photo-resist layer upon a portion of the major surface of the silicon dioxide layer;

immersing the first assembly into a bath containing an etching solution selected for etching silicon dioxide;

forming a second assembly by allowing the etching solution to act upon the silicon dioxide layer of the first assembly for a first period of time that is selected in order to;

a) expose a portion of the silicon substrate, and b) form a wedge structure in the silicon dioxide layer;

forming a third assembly by extending the first period of time by a second period of time in order to eliminate a foot region formed upon a sloping surface of the wedge structure;

after eliminating the foot region, forming a fourth assembly by removing the photo-resist layer from the third assembly; and

forming a waveguide component from the fourth assembly by exposing the fourth assembly to a xenon difluoride (XeF₂) environment that eliminates a portion of the silicon substrate and forms a support pillar below the wedge structure.

15. The frequency comb generator of claim 14, wherein the first period of time is further selected to allow the etching solution to act upon the silicon dioxide layer to form a slope angle ranging from about 7 degrees to about 90 degrees in the sloping surface of the wedge structure.