Patent Description:
Stimulated Brillouin scattering (SBS) lasers may be used in a wide range of sensing and metrology applications. For example, SBS lasers may provide narrow linewidth optical emissions within small integrated photonics packages. Typically, to stimulate the Brillouin scattering, an external pump laser source (a laser source outside of an integrated photonics package) may provide a pump laser to a Brillouin cavity. To align the frequency of the pump laser with the resonance frequency of the Brillouin cavity, systems may include feedback loops, such as Pound-Drever-Hall (PDH) loops. The PDH loops typically include a phase modulator, detector, mixer, and proportional-integral-derivative (PID) controller. Frequently, at least some of the components of the PDH loop are fabricated outside of the integrated photonics package. "Single-mode Brillouin fiber laser passively stabilized at resonance frequency with self-injection locked pump laser", by Spirin V V ET AL discloses a single-mode Brillouin fiber laser passively stabilized at resonance frequency with self-injection locked pump laser. Document <CIT> discloses an assembly and a method, whereby the power of the laser light may be resonantly enhanced.

Systems and methods for a self-injection locked stimulated Brillouin scattering laser are provided herein. The system includes a pump laser source that provides a pump laser. The system further includes a
stimulated Brillouin scattering (SBS) resonator that receives the pump laser through a first port, wherein the SBS resonator scatters a portion of the pump laser to provide an SBS laser through the first port, and wherein a frequency shift of Brillouin scattering within the SBS resonator is an integer multiple of a free-spectral range for the SBS resonator. Also, the system includes a filter that receives the pump laser on a first filter port and the SBS laser on a second filter port, wherein the pump laser is output through the second filter port and the SBS laser is output through a drop port. Additionally, the system includes a pump laser path that couples the output pump laser from the SBS resonator into the pump laser source, wherein a frequency of the pump laser becomes locked to a frequency of the output pump laser at a resonance frequency of the SBS resonator. The system further comprises a pump laser filter coupled in the optical path between the SBS resonator and the pump laser source, wherein the pump laser filter is a bandpass filter configured to pass a single resonance frequency associated with the desired SBS laser frequency and attenuates the light at other resonance frequencies, and wherein the pump laser filter is arranged to ensure that the pump laser produced by the pump laser source locks to the single resonance frequency.

Understanding that the drawings depict only some embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail using the accompanying drawings, in which:.

In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the example embodiments.

In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments. However, it is to be understood that other embodiments may be utilized and that logical, mechanical, and electrical changes may be made.

Embodiments described herein provide for a self-injection locked stimulated Brillouin scattering (SBS) laser. In particular, an electrically pumped gain medium may be placed in the same external optical cavity as the associated Brillouin gain resonator. Using the self-injection locking effect, the resonance frequency of the Brillouin gain resonator may control the frequency of the emitted pump laser beam generated by the electrically pumped gain medium. Thus, the optical pump beam for stimulating the Brillouin laser may automatically align itself with the appropriate resonance frequency of the Brillouin resonator.

In embodiments using the self-injection locked SBS laser, the components of the SBS laser may be fabricated as a single integrated photonics device. For instance, as the SBS resonator automatically controls the frequency of the emitted pump laser to be at a resonance frequency of the SBS resonator, the self-injection locked SBS laser may be fabricated without control loops (such as the Pound-Drever-Hall (PDH) loop). By eliminating bulky and complex control loops, the SBS laser may be implemented on a single photonics chip. As the SBS laser can be made on a single chip, the SBS laser may be cheaper and less complex to manufacture.

<FIG> is a diagram illustrating a system <NUM> for injection locking a frequency of a laser (referred to interchangeably herein as a laser beam, optical beam, light beam, laser light, light, and the like) produced by a laser source <NUM> (such as a pump laser source) using a laser fed back from a resonator <NUM> (such as an SBS resonator, described in greater detail below). As shown, the system <NUM> includes a laser source <NUM>. The laser source <NUM> may be capable of producing a laser at a particular frequency. For example, the laser source <NUM> may be an electronic gain medium, a distributed feedback (DFB) semiconductor laser or other type of laser source. While the laser sources described herein are capable of being fabricated within an integrated photonics chip, the laser source <NUM> may also include other laser sources that may be coupled to a separately located resonator <NUM> using fiber optics, optical waveguides, or other transmissive media.

The laser source <NUM> is coupled through an optical path <NUM> to the resonator <NUM>. As used herein, the resonator <NUM> may any device through which light may resonate. For example, the resonator <NUM> may be a fiber optic coil, a series of mirrors, and the like. When the resonator <NUM> receives the laser from the laser source <NUM>, a portion of the received laser may resonate within the resonator <NUM> at a resonance frequency, where the resonance frequency is dependent upon the length of the fiber within the resonator <NUM>. Further, the resonator <NUM> provides an output laser at the resonance frequency from an output that is coupled through the optical path <NUM> to an input of the laser source <NUM>. Alternatively, the output laser from the resonator <NUM> may be reflected back into the resonator <NUM> and through the laser source <NUM>, where the output may again be reflected to the input of the laser source <NUM>.

The output laser provided by the resonator <NUM> to the laser source <NUM> through the optical path <NUM> injection locks the laser produced by the laser source <NUM> to the resonance frequency of the resonator <NUM>. As known to one having skill in the art, injection locking occurs when a first oscillator, producing a laser at a first frequency (a launch frequency), is disturbed by a second laser at a second frequency, where the second frequency is close to the first frequency. When the first and second frequencies are close enough to one another, the second laser may capture the first laser such that the frequencies of the first laser and the second laser become essentially identical. With regards to the system <NUM>, the output laser provided by the resonator <NUM> at the resonance frequency of the resonator <NUM> may capture the laser produced by the laser source <NUM> such that the laser produced by the laser source <NUM> is substantially equivalent to the resonance frequency of the resonator <NUM>.

<FIG> is a graph <NUM> illustrating the effects of injection locking within the system <NUM>. Without external feedback, the laser source <NUM> may produce a laser at frequency (ω<NUM>) because the longitudinal mode <NUM> may have a higher gain than other produced modes. As shown, the laser source <NUM> may produce laser light having various frequency components along the gain curve <NUM> when the overall gain is equal to the loss. As the resonance frequency <NUM> is within the frequencies having a substantive component within the gain curve <NUM>, a portion of the laser light emitted by the laser source <NUM> may resonate within the resonator <NUM>. Thus, the resonator <NUM> may provide an output laser at the resonance frequency <NUM> back to the laser source <NUM>. Accordingly, the laser produced by the laser source <NUM> may have higher overall gain near frequency <NUM> than the original laser frequency <NUM> due to the external feedback near frequency <NUM>. Thus, the laser produced by the laser source <NUM> may begin to lase at a frequency close to the resonance frequency instead of the frequency <NUM>. In certain embodiments, the output laser at the resonance frequency <NUM> may perturb the laser provided by the laser source <NUM> such that the launch frequency of the laser source <NUM> moves from the initial frequency <NUM> towards an injection locked frequency <NUM>. As shown, the injection locked frequency <NUM> is substantially equal to the resonance frequency <NUM>. Thus, the laser source <NUM> may provide a laser at a frequency that is locked to the resonance frequency <NUM> of the resonator <NUM>.

<FIG> is a block diagram of a system <NUM> for providing an SBS laser through the implementation of an injection-locked pump laser. The system <NUM> includes a pump laser source <NUM>. The pump laser source <NUM> may be any laser source capable of providing a laser at a desired frequency. For example, the pump laser source <NUM> may be an electrically pumped gain medium, a DFB laser, a DBR laser, a ring laser, or other type of laser-providing device. The pump laser source <NUM> may be coupled to an SBS filter <NUM>, wherein the SBS filter <NUM> merely passes the received pump laser through to a transmission port of the SBS filter <NUM>.

The system <NUM> includes an SBS resonator <NUM>. After receiving the pump laser from the pump laser source <NUM>, the SBS filter <NUM> couples the received pump laser into the SBS resonator <NUM>. A portion of the pump laser may propagate within the SBS resonator <NUM> at a particular resonance frequency of the SBS resonator <NUM>. For example, as discussed above with respect to <FIG>, the pump laser may have light at a range of frequencies along a gain curve, where one or more of the resonance frequencies of the SBS resonator <NUM> are within the range of frequencies. The portions of the light of the pump laser at one of the resonance frequencies may propagate within the SBS resonator <NUM>. Portions of the pump laser that are off resonance of the SBS resonator <NUM> may fail to propagate within the SBS resonator <NUM>.

The system <NUM> includes a pump laser path <NUM>, where the pump laser path <NUM> routes an output of the SBS resonator <NUM> to an input of the pump laser source <NUM>. The SBS resonator <NUM> provides the portion of the pump laser that is at the resonance frequency of the SBS resonator <NUM> to the pump laser path <NUM>. The pump laser path <NUM> then couples the resonant portion of the pump laser to an input of the pump laser source <NUM>. As the resonant frequency of the pump laser is near the resonant frequency of the SBS resonator <NUM> external feedback provided by the SBS resonator <NUM> causes the frequency of the pump laser to lock to the resonance frequency of the SBS resonator <NUM> due to effects of injection locking as described above with respect to <FIG>.

To couple the output of the SBS resonator <NUM> to the pump laser source <NUM>, the pump laser path <NUM> provides an optical path that couples an output from the SBS resonator <NUM> to the pump laser source <NUM>. The optical path may include an optical fiber, waveguide, mirrors that reflect the light through free space, or other optical transmission medium. Alternatively, the SBS resonator <NUM> or the pump laser path <NUM> may reflect the pump laser through the original input of the SBS resonator <NUM>. The back-reflected pump beam may then be provided back to the transmission port of the SBS filter <NUM>, where the SBS filter <NUM> passes the back-reflected pump beam through to the pump laser source <NUM> as an external feedback, where the back-reflected pump beam may then cause the frequency of the pump laser provided by the pump laser source <NUM> to lock to the resonance frequency of the SBS resonator <NUM> in accordance with the effects of injection locking described above with respect to <FIG>.

In certain embodiments, when the pump laser propagates in a particular direction within the SBS resonator <NUM>, a SBS laser may propagate in an opposite direction within the SBS resonator <NUM> due to stimulated Brillouin scattering. The generated laser is provided as an output from the SBS resonator <NUM> that is coupled into the SBS filter <NUM>. When the SBS filter <NUM> receives an SBS laser at the stimulated Brillouin scattering frequency, the SBS filter <NUM> couples the SBS laser to a drop port of the SBS filter <NUM>. The drop port of the SBS filter <NUM> may be coupled to an SBS output <NUM> of the system <NUM>. The SBS output <NUM> may provide the SBS laser to other devices coupled to the system <NUM> or to other components within a larger system that encompasses the system <NUM>.

As described above, by using injection locking of the pump laser source <NUM>, the different components within the system <NUM> may be fabricated within a single integrated photonics chip. In particular, the use of injection locking within the system <NUM> may allow the system <NUM> to lock the frequency of the pump laser produced by the pump laser source <NUM> to the resonance frequency of the SBS resonator <NUM> without implementing control loops having components that prevent the integration of the system <NUM> on a single integrated photonics chip. While the various components of the system <NUM> may be implemented within a single integrated photonics chip, the components of the system <NUM> may also be comprised of separate components or groups of components that are coupled to one another through combinations of optical transmission media that may include fiber optics, optical waveguides, reflectors, and/or free space optical transmissions.

<FIG> is a block diagram illustrating a system <NUM> for producing an SBS laser using an injection locked pump laser. In particular, the system <NUM> may be a detailed implementation of the system <NUM>. The system <NUM> includes a pump laser source <NUM>, an SBS filter <NUM>, an SBS resonator <NUM>, and a pump laser path <NUM> that function in a substantially similar manner to the pump laser source <NUM>, the SBS filter <NUM>, the SBS resonator <NUM>, and the pump laser path <NUM> described above in <FIG>.

In certain embodiments, the pump laser source <NUM> provides a pump laser substantially as described above in connection with the pump laser source <NUM> in <FIG>. The pump laser source <NUM> is coupled to an input transmission port <NUM>-a of an SBS filter <NUM>. As illustrated, the SBS filter <NUM> may be an optical add/drop filter. For example, the SBS filter <NUM> may include coupled optical waveguides having Bragg gratings formed therein. When light propagates through an input, the light may be coupled from one waveguide to the other based on the frequency of the light. With regards to the pump laser, when the SBS filter <NUM> receives a laser through the input port <NUM>-a at the frequency of the pump laser or near the resonance frequency of the SBS resonator <NUM>, the SBS filter <NUM> may keep the pump laser within the same waveguide and couple the pump laser to the output port <NUM>-b.

The output port <NUM>-b is coupled to an input port <NUM>-a of the SBS resonator <NUM>. As described above in <FIG>, components of the pump laser at one or more of the resonance frequencies of the SBS resonator <NUM> may propagate within the SBS resonator <NUM>. Accordingly, the SBS resonator <NUM> may provide components of the pump laser at one or more of the resonance frequencies of the SBS resonator <NUM> through the output port <NUM>-b.

The system <NUM> includes a pump laser filter <NUM> that is positioned between the SBS resonator <NUM> and the pump laser source <NUM>. In particular, the output port <NUM>-b of the SBS resonator <NUM> may be coupled to a pump laser filter <NUM> to attenuate undesired components of the pump laser that are at resonance frequencies of the SBS resonator <NUM> other than the resonance frequency of the SBS resonator <NUM> that incites the generation of the desired SBS laser. For example, the frequency range of the light produced by the pump laser source <NUM> may span multiple resonance frequencies of the SBS resonator <NUM>. Accordingly, the SBS resonator <NUM> may provide multiple optical beams associated with different resonance frequencies of the SBS resonator <NUM> through the output port <NUM>-b. To ensure that the pump laser produced by the pump laser source <NUM> locks to the correct resonance frequency, the pump laser filter <NUM> may be a bandpass filter that passes a single resonance frequency associated with the desired SBS laser frequency and attenuate the light at other resonance frequencies. Accordingly, the pump laser will lock to the desired resonance frequency.

As described above, when the pump laser propagates within the SBS resonator <NUM>, an SBS laser may be generated within the SBS resonator <NUM>. The SBS laser may propagate in an opposite direction from the direction of propagation of the pump laser within the SBS resonator <NUM>. For example, the pump laser enters the SBS resonator <NUM> at port <NUM>-a, may propagate in a clockwise direction within the SBS resonator <NUM>, and is coupled out of the SBS resonator <NUM> at port <NUM>-b. Conversely, the SBS laser may be generated to propagate in the counter-clockwise direction within the SBS resonator <NUM> and coupled out of the SBS resonator <NUM> at port <NUM>-a. While the terms "clockwise" and "counter-clockwise" are used herein to indicate a direction of propagation within the SBS resonator <NUM>, the terms are used to indicate that the SBS laser and the pump laser propagate in opposite directions within the SBS resonator <NUM>.

The SBS laser is coupled out of the port <NUM>-a from the SBS resonator <NUM>, and the system <NUM> couples the SBS laser into the SBS filter <NUM>. As described above, in relation to the pump laser, the SBS filter <NUM> may pass optical beams at the frequency of the pump laser through the SBS filter <NUM> without coupling the pump laser from one waveguide to another. However, the SBS filter <NUM> may couple optical beams at the frequency of the SBS laser from one waveguide to another. For example, the SBS filter <NUM> may receive the SBS laser on port <NUM>-b and couple the SBS laser into another waveguide such that the SBS laser is coupled out of the SBS filter <NUM> at port <NUM>-c. The port <NUM>-c may then be coupled to an SBS laser output port <NUM> for the system <NUM>.

<FIG> is a block diagram illustrating an additional system <NUM> for producing an SBS laser using an injection locked pump laser. The system <NUM> is largely similar to the system <NUM>. For example, the pump laser source <NUM>, the SBS filter <NUM>, the SBS resonator <NUM>, the SBS laser output port <NUM>, and the pump laser path <NUM> function substantially as described above with respect to the pump laser source <NUM>, SBS filter <NUM>, SBS resonator <NUM>, SBS laser output port <NUM>, and pump laser path <NUM> in <FIG>. However, the system <NUM> is different from the system <NUM> in that the system <NUM> may not include a pump laser filter, such as the pump laser filter <NUM> in <FIG>.

In some embodiments, the pump laser source <NUM> may be able to produce laser light at a narrow range of frequencies. In particular, the range of frequencies may be narrow enough to include a single resonance frequency of the SBS resonator <NUM>. When the range of frequencies includes a single resonance frequency, when the pump laser is returned along the pump laser path <NUM> from the SBS resonator <NUM>, the returned pump laser may be a narrow laser at a single resonance frequency. As the returned pump laser includes a single resonance frequency, the system <NUM> may operate without a pump laser filter used to eliminate unwanted signals at different resonance frequencies of the SBS resonator <NUM> that are not associated with the creation of the SBS laser.

<FIG> is a block diagram illustrating an additional system <NUM> for producing an SBS laser using an injection locked pump laser. The system <NUM> is similar to the system <NUM>. For example, the pump laser source <NUM>, the SBS filter <NUM>, the SBS resonator <NUM>, the SBS laser output port <NUM>, and the pump laser filter <NUM> function substantially as described above with respect to the pump laser source <NUM>, the SBS filter <NUM>, the SBS resonator <NUM>, the SBS laser output port <NUM>, and the pump laser filter <NUM> in <FIG>. However, the system <NUM> is different from the system <NUM> in that the mechanism for the pump laser path uses reflector <NUM>-a to direct the pump laser to an input of the pump laser source <NUM>.

In certain embodiments, when the pump laser beam enters the SBS resonator <NUM>, the pump laser may be coupled out of the SBS resonator <NUM> at the output port <NUM>-b. The output port <NUM>-b may be coupled to a reflector <NUM>-A. The reflector <NUM>-a may be a partial reflector or full reflector. Additionally, the reflector <NUM>-a may also be coupled to the output port <NUM>-a of the SBS resonator <NUM> such that a portion of the pump laser is reflected back into the SBS resonator <NUM>. The portion of the pump laser that is reflected back into the SBS resonator <NUM> may be coupled out of the SBS resonator <NUM> into the SBS filter <NUM>, coupled through the pump laser filter <NUM>, positioned between the SBS filter <NUM> and the pump laser source <NUM>, and into the pump laser source <NUM>, where the reflected pump laser locks the frequency of pump laser to a resonance frequency of the SBS resonator <NUM>.

<FIG> is a flowchart diagram of a method <NUM> for providing a self-injection locked SBS laser. As shown, the method <NUM> proceeds at <NUM>, where a pump laser is produced at a pump laser source. Additionally, the method <NUM> proceeds at <NUM>, where the pump laser is introduced into a first port of an SBS resonator. Further, the method <NUM> proceeds at <NUM>, where a resonator output is coupled back into the pump laser source, wherein the output laser locks the pump laser to a resonance frequency of the SBS resonator. Moreover, the method <NUM> proceeds at <NUM>, where an SBS laser is generated within the SBS resonator. For example, the Brillouin scattering of the pump laser within the SBS resonator may create an SBS laser that propagates in the opposite direction of the propagation direction of the pump laser.

Claim 1:
A system comprising:
a pump laser source (<NUM>) that provides a pump laser;
a stimulated Brillouin scattering, SBS, resonator (<NUM>) that receives the pump laser through a first port (<NUM>-a), wherein the SBS resonator (<NUM>) scatters a portion of the pump laser to provide an SBS laser through the first port (<NUM>-a), and wherein a frequency shift of Brillouin scattering within the SBS resonator (<NUM>) is an integer multiple of a free-spectral range for the SBS resonator (<NUM>);
a filter (<NUM>) that receives the pump laser on a first filter port (<NUM>-a) and the SBS laser on a second filter port (<NUM>-b), wherein the pump laser is output through the second filter port (<NUM>-b) and the SBS laser is output through a drop port (<NUM>-c); and
a pump laser path (<NUM>) that couples the output pump laser from the SBS resonator (<NUM>) into the pump laser source (<NUM>), wherein a frequency of the pump laser becomes locked to a frequency of the output pump laser at a resonance frequency of the SBS resonator (<NUM>), wherein the pump laser path (<NUM>) comprises an optical path that couples a transmission port (<NUM>-b) of the SBS resonator (<NUM>) to the pump laser source (<NUM>), further comprising:
a pump laser filter (<NUM>) coupled in the optical path between the SBS resonator (<NUM>) and the pump laser source (<NUM>), wherein the pump laser filter is a bandpass filter configured to pass a single resonance frequency associated with the desired SBS laser frequency and attenuates the light at other resonance frequencies, and wherein the pump laser filter is arranged to ensure that the pump laser produced by the pump laser source (<NUM>) locks to the single resonance frequency.