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OPTICAL FREQUENCY MEASUREMENT AND CONTROL USING DUAL OPTICAL-FREQUENCY COMBS

This invention is a photonic-chip optical synthesizer, which produces a laser output in optical fiber with a user-defined optical frequency. The invention is based on microresonator optical frequency combs and other heterogeneously integrated photonic components such that revolutionary performance in terms of low power consumption, ultracompact size, and ultralow noise is possible.

The new element of this invention is the use of all photonic chip components for the creation of an optical frequency synthesizer, which enables low cost and highly scalable fabrication through semiconductor processing techniques as well as dramatic advances in size, weight, and power metrics. The invention works by leveraging two key concepts: (1) optical frequency combs can be created by purely nonlinear optical means through microresonator technology, hence the frequency comb needed for optical synthesis is dramatically smaller and simpler than in previous cases, and (2) heterogeneous photonic integration technology built on advanced semiconductor processing makes it possible to robustly connect the different pieces of an optical synthesizer in a small and highly scalable manufacturing process.

The optical frequency synthesizer invented solves the problem of generating an SI-calibrated laser source in a small, low-cost, manufacturable package. Whereas current optical synthesis technology is largely confined to the research laboratory, the technology of this invention makes it possible to create a synthesizer capable of being deployed to the field and adopted by non-specialists.

An example of an inventive dual-comb optical-frequency comb generator is illustrated schematically in FIG. 1 and comprises: (a) a tunable comb-generating laser 101; (b) a coarse-comb generator 300; (c) a fine-comb generator 500; (d) a second harmonic generator 405; (e) a coarse-comb offset photodetector 203; (f) a dual-comb offset photodetector 204; and (g) a fine-comb photodetector 205. The tunable comb-generating laser 101 generates a comb-generating laser signal 71 at an optical frequency νp. In the drawings, optical signals are indicated with solid lines with arrows, while electrical signals are indicated with dashed lines with arrows. The comb-generating laser signal 71 is split into first and second portions 73 and 75, respectively (if needed or desired, amplified by optical amplifiers 103 and 105, respectively). In the drawings, splitting or combining of optical signals is indicated at points where arrows converge or from which they diverge, respectively, and can be achieved in any suitable way, e.g., waveguide or fiber splitters, combiners, or couplers in implementations in which optical signals propagate as optical modes supported by waveguides or fibers, or free-space beamsplitters in implementations in which optical signals propagate as unconfined, free-space optical beams.

The coarse-comb generator 300 receives the first portion 73 of the comb-generating laser signal 71 (amplified by optical amplifier 103 if needed or desired; see below), and generates a coarse optical-frequency comb 83 having optical frequencies νN=νp+NΔ (where N is an integer (positive, negative, or zero) and Δ is a coarse-comb frequency spacing). The coarse optical-frequency comb 83 spans at least an octave of optical frequency. The fine-comb generator 500 receives a second portion 75 of the comb-generating laser signal 71 (amplified by optical amplifier 105 if needed or desired; see below) and generates a fine optical-frequency comb 85 having optical frequencies νN′=νp+N′Δ′ (where N′ is an integer (positive, negative, or zero) and Δ′ is a fine-comb frequency spacing). The second harmonic generator 405 receives a portion 83b of the coarse optical-frequency comb 83; the portion 83b includes a comb optical signals at optical frequency νN1 and can be amplified by optical amplifier 401 (if needed or desired). The second harmonic generator 405 generates from the comb optical signal at νN1 a second harmonic optical signal 83b′ at optical frequency 2νN1.

A coarse-comb offset photodetector 203 receives the second harmonic optical signal 83b′, and also receives another portion 83a of the coarse optical-frequency comb 83 that includes a comb optical signal at an optical frequency νN2≈2νN1. From the optical signals at νN2 and 2νN1, the coarse-comb offset photodetector 203 generates a coarse-comb offset electrical signal 93 at a coarse-comb offset frequency f0=|2νN1−νN2| (i.e., the beat note between 2νN1 and νN2; see FIG. 10). A dual-comb offset photodetector 204 receives portions 83c and 85c of the coarse and fine optical-frequency combs 83 and 85, respectively, including comb optical signals at optical frequencies νp+Δ and νp+MΔ′, or νp−Δ and νp−MΔ′ (where M is the positive integer closest to Δ/Δ′). From those comb optical signals, the dual-comb offset photodetector 204 generates a dual-comb offset electrical signal 94 at a dual-comb offset frequency f1=|Δ−MΔ′| (i.e., the beat note between νp+Δ and νp+MΔ′; see FIG. 10). The fine-comb photodetector 205 receives a portion 85a of the fine optical-frequency comb 85, and generates a fine-comb-spacing electrical signal 95 at the fine-comb frequency spacing Δ′.

Measurement of the frequencies of the electrical signals 93, 94, and 95 provides measured values for the frequencies f0, f1, and Δ′. In particular, because the coarse optical-frequency comb 83 spans an octave, the frequency f0 represents an absolute phase offset of the coarse-comb frequencies νN from corresponding pure harmonics (i.e., the coarse comb 83 is self-referencing), and enables a determination (along with measured values of Δ′ and f1) of the optical frequency νp or the coarse-comb spacing Δ. Knowledge of νp and Δ or Δ′ in turn enables determination of the comb frequencies νN=νp+NΔ or νN′=νp+N′Δ′ and their use as an absolute frequency scale for measurement of other optical frequencies. The dual-comb generator can include one or more electronic processors or circuit elements structured, connected or programmed so as to determine at least νp using measured values of Δ′, f0, and f1. The one or more electronic processors or circuit elements can be further structured, connected or programmed so as to determine Δ, νN, or νN′ using measured values of Δ′, f0, and f1.

If there is any drift or fluctuation of the optical frequency νp or the comb spacings Δ or Δ′ (i.e., if those quantities are dynamic, e.g., in response to environmental or mechanical perturbations), there will be corresponding drift or fluctuation of νN and νN′. In some examples that drift or fluctuation can be tolerated, and the values of νp, νN, or νN′ can be recalculated using differing measured values of Δ′, f0, and f1 as those values change with time. Alternatively, in some examples, one or more servo controllers can be employed to stabilize one or more of Δ, Δ′, or νp. Any suitable servo controller can be employed; a common implementation includes a phase-locked loop that phase-locks the controlled frequency to a frequency reference. Any suitable frequency reference can be employed including, e.g., frequency references based on one or more of a reference oscillator, an atomic transition, direct digital synthesis, or harmonic frequency multiplication. A common implementation includes a quartz reference oscillator, direct digital synthesis, and harmonic multiplication (if needed) to provide reference electrical signals at one or more selected reference frequencies.

Abstract: 

This invention is a compact, low power, and integrated frequency synthesizer capable of generating extremely broad bandwidth and highly coherent electromagnetic radiation, ranging from the optical domain (500e12 Hz) down through the terahertz (1e12 Hz) and microwave domains (1e9 Hz). This optical-to-microwave synthesizer utilizes microresonator technology that is robust, compact, and can be integrated on a silicon chip using fabrication techniques similar to those employed for making integrated computer chips. Thus, a fully functioning implementation of this invention would be a low-cost, centimeter-sized device that operates with approximately 1W of electrical power. Such optical synthesis capabilities would provide unprecedented precision as a critical component in advanced communications, remote sensing, RADAR, LIDAR, analog-to-digital conversion, timing and precision navigation.

Benefits: 

Commercial optical frequency synthesizers are based on technology with orders of magnitude larger size, weight, power consumption, and cost. This technology for optical frequency synthesis is fundamentally chip scale and is fully compatible with high volume semiconductor manufacturing techniques. Hence, the synthesizer technology not only allows an ultracompact form factor to be realized but the pathway for manufacturing many units is straightforward.

Inventors: 

Scott Papp, Kartik Srinivasan, Scott Diddams, Kerry Vahala, and John Bowers

Patent Number: 
10,067,031
Technology Type(s): 
Laser and Optics, Manufacturing, Electronics, Optical Technology, Photon Physics
Internal Laboratory Ref #: 
16-034US1
Patent Issue Date: 
September 4, 2018
Lab Representatives
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