Widely tunable brillouin laser based on vernier filter external cavity

Abstract

To provide a widely tunable Stimulated Brillouin Scattering (SBS) laser without active stabilization of a pump laser.SOLUTION: Systems and methods for an SBS laser based on a Vernier filter external cavity are provided. The SBS laser includes a gain chip and external cavity. An external cavity chip includes a first optical waveguide, a first optical resonator optically coupled to the gain chip via the first optical waveguide, a second optical waveguide, and a second optical resonator optically coupled to the first optical resonator via the second optical waveguide. The second optical resonator is configured to generate SBS light from pump light that has propagated through both the first and second optical resonators. The pump light is resonant to both the first and second optical resonators. The SBS light is only resonant to the second optical resonator. The SBS laser is configured to output the SBS light from an output port optically coupled to the second optical resonator.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Stimulated Brillouin scattering (SBS) lasers are a powerful technique that can provide extremely narrow linewidths, which can be extremely beneficial for use in optical sensing applications. However, one of the major drawbacks of SBS lasers is that they traditionally require a pump laser to operate. The pump laser must be actively stabilized to the resonator that generates the SBS laser, limiting the tuning range of the SBS laser to that of the pump laser. In current systems, the pump laser is stabilized to the optical cavity using either a PDH loop or self-injection locking. In the case of a PDH loop, a phase modulator between the pump laser and the SBS resonator is often also required due to the fairly high frequency of the modulation. In both cases, stabilization is achieved using an electrically implemented feedback loop, which inevitably increases the device's complexity and limits its flexibility.

[0002] For the reasons stated above, and others discussed herein, there is a need for a device that uses a spectrally broad gain medium to directly generate a widely tunable SBS laser without active stabilization of the pump laser. Summary of the Invention

[0003] In some aspects, a stimulated Brillouin scattering (SBS) laser is described herein. The SBS laser includes a gain chip and an external cavity chip. The external cavity chip includes a first optical waveguide, a first optical resonator optically coupled to the gain chip via the first optical waveguide, a second optical waveguide, and a second optical resonator optically coupled to the first optical resonator via the second optical waveguide. The second optical resonator is configured to generate SBS light from pump light propagating through both the first optical resonator and the second optical resonator. The pump light resonates with both the first optical resonator and the second optical resonator. The SBS light resonates only with the second optical resonator. The SBS laser is configured to output the SBS light from an output port optically coupled to the second optical resonator.

[0004] In some aspects, a system is described herein. The system includes a gain chip and a first optical resonator optically coupled to the gain chip via a first optical waveguide. The system further includes a second optical resonator optically coupled to the first optical resonator via a second optical waveguide. The second optical resonator is configured to generate SBS light from pump light propagating through both the first optical resonator and the second optical resonator. The pump light resonates with both the first optical resonator and the second optical resonator. The SBS light resonates only with the second optical resonator. The system further includes one or more circuits configured to set the wavelength of the SBS light.

[0005] In some aspects, methods are described herein. The method includes generating pump light using a gain chip. The method further includes providing the pump light from the gain chip to an external cavity chip, the external cavity chip comprising a first optical resonator and a second optical resonator. The method further includes using the second optical resonator to generate stimulated Brillouin scattering (SBS) light from the pump light propagating through both the first optical resonator and the second optical resonator. The pump light resonates with both the first optical resonator and the second optical resonator. The SBS light resonates only with the second optical resonator. The method further includes outputting the SBS light from an output port optically coupled to the second optical resonator. [Brief explanation of the drawings]

[0006] Example embodiments are described with additional specificity and detail using the accompanying drawings, with the understanding that the drawings depict example embodiments only and therefore should not be considered limiting in scope. [Figure 1] FIG. 1 is a diagram of an exemplary SBS laser. [Figure 2] FIG. 1 is a diagram of another exemplary SBS laser. [Figure 3] FIG. 1 is a block diagram of an exemplary system. [Figure 4] FIG. 1 is a flow diagram of an exemplary method of operating an SBS laser and system.

[0007] In accordance with common practice, the various illustrated features are not drawn to scale but rather to emphasize specific features relevant to the exemplary embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0008] In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific illustrative embodiments. It should be understood, however, that other embodiments may be utilized and that logical, mechanical, and electrical changes may be made. Furthermore, the methods presented in the drawings and specification should not be construed as limiting the order in which individual steps may be performed. Therefore, the following detailed description is not to be construed in a limiting sense.

[0009] The technique described herein utilizes a dual-cavity Vernier external cavity chip and a gain chip to generate a widely tunable pump laser while simultaneously automatically generating a stimulated Brillouin scattering (SBS) laser. The gain chip provides pump light to the external cavity chip, which resonates with both optical resonators that act as filters controlling the emission wavelength of the pump light. One optical resonator is configured to generate SBS light from the pump light propagating through both optical resonators, with the SBS light resonating only with the optical resonator that generates it. The SBS light is output from a port of the external cavity chip optically coupled to the optical resonator that generates the SBS light. Because the SBS light does not propagate back to the gain chip, a beneficial result of this design is that the SBS laser is essentially immune to back reflections and does not require the use of an external isolator.

[0010] FIG. 1 illustrates a diagram of an exemplary SBS laser 100. In the example shown in FIG. 1, the SBS laser 100 includes various components, including a gain chip 102 and an external cavity chip 104 optically coupled to the gain chip 102. In the example shown in FIG. 1, the external cavity chip 104 includes a first optical resonator 106 and a second optical resonator 108. In some examples, the first optical resonator 106 and the second optical resonator 108 are in a Vernier filtering configuration. In the example shown in FIG. 1, the second optical resonator 108 operates as an SBS resonator and is configured to generate SBS light 128.

[0011] The gain chip 102 is configured to generate and provide pump light 118 to the external cavity chip 104. The gain chip 102 is configured to establish injection locking, in which the transmission spectrum of the external cavity chip 104 provides optical feedback that propagates back into the gain chip 102, thereby naturally locking the emission wavelength of the pump light 118 from the gain chip 102 to one of the resonant frequencies of the first optical cavity 106 and the second optical cavity 108 of the external cavity chip 104. The gain chip 102 is configured to generate pump light 118 with a sufficiently high gain to exceed the Brillouin lasing threshold of the second optical cavity 108. The specific characteristics of the gain chip 102 are determined based on the desired operating wavelength and material platform for the SBS laser 100. In some examples, the characteristics of the gain chip 102 are also determined based on the desired linewidth of the SBS laser 100, since the Brillouin lasing threshold increases as the linewidth of the SBS laser 100 narrows. The drive current of the gain chip 102 can be adjusted (e.g., using a controller as discussed herein) to generate the pump light 118 at a desired power level such that the power level of the pump light 118 exceeds the Brillouin lasing threshold of the second optical cavity 108.

[0012] In some examples, a portion of the first edge 107 of the external cavity chip 104 is coupled to the gain chip 102 using edge coupling. In such examples, the edge of the gain chip 102 and the first edge 107 of the external cavity chip 104 are aligned and secured in place by epoxy, mounting, or other techniques. In some examples, techniques are used to improve or maximize the coupling efficiency between the gain chip 102 and the external cavity chip 104. For example, the mode profile of the pump light 118 exiting the gain chip 102 can be characterized, and an edge coupler (not shown) on the external cavity chip 104 can be designed for mode matching so that the mode support on the external cavity chip 104 more closely matches the mode profile of the mode exiting the gain chip 102. In other examples, the external cavity chip 104 is optically coupled to the gain chip 102 using a grating coupler or another technique.

[0013] 1 , the external cavity chip 104 includes a first optical resonator 106, a second optical resonator 108, a first optical waveguide 110, a second optical waveguide 112, a third optical waveguide 114, and a Bragg grating 116. The first optical waveguide 110 extends from a first edge 107 of the external cavity chip 104 coupled to the gain chip 102 to a second edge 109 of the external cavity chip 104, and the second optical waveguide 112 extends from a position offset from the first edge 107 to the second edge 109 of the external cavity chip 104. The first optical resonator 106 is positioned between the first optical waveguide 110 and the second optical waveguide 112. The third optical waveguide 114 extends from a location offset from the first edge 107 of the external cavity chip 104 to a Bragg grating 116 positioned at the other end of the third optical waveguide 114. The second optical resonator 108 is positioned between the second optical waveguide 112 and the third optical waveguide 114 and is offset from the Bragg grating 116.

[0014] In some examples, the first optical resonator 106, the second optical resonator 108, the cores of the optical waveguides 110, 112, and 114, and the Bragg grating 116 are formed of the same optical material. In some examples, the optical material is silicon nitride. In other examples, the optical material is silicon, silicon oxynitride, silicon carbide, diamond, or germanium. In some examples, a cladding material (e.g., silicon dioxide) having a lower refractive index than the optical material also forms part of the external cavity chip 104. It should be understood that the optical material and cladding material are examples, and other optical materials may also be used. The external cavity chip 104 can be fabricated using known integrated photonics fabrication techniques.

[0015] 1 and described herein, the first optical resonator 106 and the second optical resonator 108 are implemented as optical ring resonators. In other examples, the first optical resonator 106 and / or the second optical resonator 108 can be implemented as different types of resonators. For example, the first optical resonator 106 and / or the second optical resonator 108 can be implemented as a racetrack resonator or a Bragg resonator.

[0016] 1 , the Brillouin lasing threshold of the second optical cavity 108 is configured to be lower than the Brillouin lasing threshold of the first optical cavity 106. That is, the threshold power of the pump light 118 (and the threshold drive current of the gain chip 102) sufficient to induce SBS lasing in the second optical cavity 108 is lower than the threshold power of the pump light 118 (and the threshold drive current of the gain chip 102) that would be sufficient to induce SBS lasing in the first optical cavity 106. In some examples, the Brillouin lasing threshold of the second optical cavity 108 is configured to be substantially lower than the Brillouin lasing threshold of the first optical cavity 106. There are several ways to set the Brillouin lasing threshold of the second optical cavity 108 lower than the first optical cavity 106.

[0017] In some examples, the first optical resonator 106 and the second optical resonator 108 have different radii. The different radii are selected so that the resonances of the first optical resonator 106 and the second optical resonator 108 are aligned at a single wavelength over the spectral range in which gain exists, and the radius of the first optical resonator 106 is selected so as not to operate as an SBS laser. In such examples, the radii of the first optical resonator 106 and the second optical resonator 108 are selected so that the Brillouin lasing threshold of the second optical resonator 108 is lower than the Brillouin lasing threshold of the first optical resonator 106.

[0018] In other examples, the first optical resonator 106 and the second optical resonator 108 have the same radius, and the temperatures of the first optical resonator 106 and the second optical resonator 108 are controlled using one or more circuits (as discussed below) such that the resonances of the first optical resonator 106 and the second optical resonator 108 are matched at only one wavelength. In such examples, the temperatures of the first optical resonator 106 and the second optical resonator 108 are selected such that the Brillouin lasing threshold of the second optical resonator 108 is lower than the Brillouin lasing threshold of the first optical resonator 106.

[0019] In combination with configuring the Brillouin lasing thresholds of the first optical cavity 106 and the second optical cavity 108, the drive current of the gain chip 102 is selected such that only the second optical cavity 108 lases. For example, the drive current of the gain chip 102 is selected such that the power level of the pump light 118 exceeds the Brillouin lasing threshold of the second optical cavity 108 but does not exceed the Brillouin lasing threshold of the first optical cavity 106.

[0020] 1, the first optical resonator 106 of the external cavity chip 104 is optically coupled to the gain chip 102 via a first optical waveguide 110. In some examples, the external cavity chip 104 includes an output 132 at a second edge 109 of the external cavity chip 104 at the end of the first optical waveguide 110 that can be used to access the pump light 118. In other examples, a Bragg grating can be used to back-reflect the pump light 118, or a different feature can be a termination used to terminate the pump light 118 rather than including an output 132.

[0021] The first optical cavity 106 is configured to receive pump light 118 from the gain chip 102 via the first optical waveguide 110 at the first coupling region 120. In the example shown in Figure 1, the pump light 118 is coupled into the first optical cavity 106 at the first coupling region 120 and propagates through the first optical cavity 106 in a clockwise direction.

[0022] After propagating in the clockwise direction through the first optical resonator 106, at least a portion of the pump light 118 is coupled out of the first optical resonator 106 into the second optical waveguide 112 at the second coupling region 122. The pump light 118 is then coupled into the second optical resonator 108 via the second optical waveguide 112 at the third coupling region 124 and propagates in the counterclockwise direction through the second optical resonator 108.

[0023] After propagating in the counterclockwise direction through the first optical resonator 106, at least a portion of the pump light 118 is coupled out of the second optical resonator 108 into the third optical waveguide 114 at the fourth coupling region 126. The pump light 118 propagates through the third optical waveguide 114 to the Bragg grating 116, which reflects the pump light 118 back through the third optical waveguide 114 towards the second optical resonator 108. The pump light 118 is then coupled back into the second optical resonator 108 at the fourth coupling region 126 and propagates in the clockwise direction through the second optical resonator 108.

[0024] The first optical resonator 106 and the second optical resonator 108 are configured such that the resonances of the first optical resonator 106 and the second optical resonator 108 are rarely matched to one another. In some examples, the first optical resonator 106 and the second optical resonator 108 are configured such that the resonances of the first optical resonator 106 and the second optical resonator 108 are only matched at one wavelength and mismatched at all other wavelengths across the spectral range in which gain exists. This rare matching causes feedback to the gain chip 102 only at the dual-resonance wavelength. The pump light 118 resonates with both the first optical resonator 106 and the second optical resonator 108.

[0025] In operation, the first optical resonator 106 and the second optical resonator 108 act as filters to control the emission wavelength of the pump light 118. For the second optical resonator 108 to generate SBS light 128, the free-spectral range (FSR), set by the radius of the second optical resonator 108, must correspond to the frequency shift at which Brillouin scattering occurs. In some examples, the waveguides 110, 112, and 114 operate at 1550 nm. In such examples, if the pump light 118 is provided at one frequency, optical gain can be created at another frequency that is shifted down by approximately 11 GHz. To function as an SBS laser, the FSR is an integer fraction of the Brillouin gain shift (e.g., the FSR can be 5.5 GHz). When the pump light 118 is at one resonance and another resonance of the second optical resonator 108 matches the gain, the second optical resonator 108 generates SBS light 128 when the power level of the pump light 118 is sufficiently high. Thus, the second optical resonator 108 is configured to operate as an SBS resonator and generate SBS light 128 .

[0026] In some examples, the first optical cavity 106 is specifically designed to have a radius with a resonance that does not support SBS lasing, and the SBS light 128 resonates only with the second optical cavity 108. Because the SBS light 128 resonates only with the second optical cavity 108, the SBS light 128 is routed to the output 130 and does not return to the gain chip 102 as the pump light 118 does.

[0027] In some examples, additional Bragg gratings 134, 136 may be optionally included to reflect the pump light 118 and / or SBS light 128 back into the second optical cavity 108. In other examples, the ends of the second optical waveguide 112 and the third optical waveguide 114 at which the optional Bragg gratings 134, 136 are shown are instead optically coupled together to route the pump light 118 and / or SBS light 128 back into the second optical cavity 108. In either case, these techniques ensure that the SBS light 128 is routed only to the output 130.

[0028] It may be desirable to access the SBS light 128 at more than one output. In some examples, one or more of the additional Bragg gratings 134, 136 are omitted, and the waveguide extends to the first edge 107 of the external cavity chip 104 so that the SBS light 128 can be output at those locations in addition to the output 130. In some examples, the output 130 is replaced with a Bragg grating, and the SBS light 128 is instead accessed at one of different locations.

[0029] While a particular example includes the gain chip 102 coupled to a first edge 107 of the external cavity chip 104 and the pump light 118 passing through the first optical resonator 106 and the second optical resonator 108 in a particular direction, it should be understood that this is just one example and other configurations may be used. For example, the gain chip 102 may instead be coupled to a second edge 109 of the external cavity chip 104, and the pump light 118 may pass through the first optical resonator 106 and the second optical resonator 108 in the opposite direction.

[0030] FIG. 2 illustrates a diagram of another exemplary SBS laser 200. In the example shown in FIG. 2, the SBS laser includes various components, including a gain chip 202 and an external cavity chip 204 optically coupled to the gain chip 202. In the example shown in FIG. 1, the external cavity chip 204 includes a first optical resonator 206 and a second optical resonator 208. In some examples, the first optical resonator 206 and the second optical resonator 208 are in a Vernier filtering configuration. In the example shown in FIG. 2, the second optical resonator 208 operates as an SBS resonator and is configured to generate SBS light 232. However, in other examples, the first optical resonator 206 operates as an SBS resonator and is configured to generate SBS light 232. For instructional purposes, the following description of FIG. 2 focuses on an example in which the second optical resonator 208 is configured to generate SBS light 232.

[0031] The gain chip 202 is configured to generate and provide pump light 220 to the external cavity chip 204. The gain chip 202 is configured to establish injection locking, where the transmission spectrum of the external cavity chip 204 provides optical feedback that propagates back into the gain chip 202, thereby naturally locking the emission wavelength of the pump light 220 from the gain chip 202 to one of the resonant frequencies of the first optical resonator 206 and the second optical resonator 208 of the external cavity chip 204. The gain chip 202 is configured to generate pump light 220 with a sufficiently high gain to exceed the Brillouin lasing threshold of the second optical resonator 208. The specific characteristics of the gain chip 202 are determined based on the desired operating wavelength and material platform for the SBS laser 200. In some examples, the characteristics of the gain chip 202 are also determined based on the desired linewidth of the SBS laser 200, since the Brillouin lasing threshold increases as the linewidth of the SBS laser 200 narrows. The drive current of the gain chip 202 can be adjusted to generate the pump light 220 at a desired power level such that the power level of the pump light 220 exceeds the Brillouin lasing threshold of the second optical cavity 208 .

[0032] In some examples, a portion of the first edge 207 of the external cavity chip 204 is coupled to the gain chip 202 using edge coupling. In such examples, the edge of the gain chip 102 and the first edge 207 of the external cavity chip 204 are aligned and secured in place by epoxy, mounting, or other techniques. In some examples, techniques are used to improve or maximize the coupling efficiency between the gain chip 202 and the external cavity chip 204. For example, the mode profile of the pump light 220 exiting the gain chip 202 can be characterized, and an edge coupler (not shown) on the external cavity chip 104 can be designed for mode matching such that the mode support on the external cavity chip 204 more closely matches the mode profile of the mode exiting the gain chip 202. In other examples, the external cavity chip 204 is optically coupled to the gain chip 202 using a grating coupler or another technique.

[0033] 2, the external cavity chip 204 includes a first optical resonator 206, a second optical resonator 208, a first optical waveguide 210, a splitter 212, a second optical waveguide 214, a third optical waveguide 216, and a fourth optical waveguide 218. The first optical waveguide 210 extends from a first edge 207 of the external cavity chip 204, which is coupled to the gain chip 202, to the splitter 212. From the splitter 212, the second optical waveguide 214 and the third optical waveguide 216 extend to a second edge 209 of the external cavity chip 204. 2, the second optical waveguide 214 extends from the splitter 212 toward the top of the external cavity chip 204, and the third optical waveguide 216 extends from the splitter 212 toward the bottom of the external cavity chip 204. The fourth optical waveguide 218 extends from the second edge 209 of the external cavity chip 204 toward the first edge 207 and then loops back to the second edge 209. The first optical resonator 206 is positioned between the second optical waveguide 214 and the upper portion of the fourth optical waveguide 218, and the second optical resonator 208 is positioned between the third optical waveguide 216 and the lower portion of the fourth optical waveguide 218.

[0034] In some examples, the first optical resonator 206, the second optical resonator 208, the cores of the optical waveguides 210, 214, 216, and 218, and the splitter 212 are formed of the same optical material. In some examples, the optical material is silicon nitride. In other examples, the optical material is silicon, silicon oxynitride, silicon carbide, diamond, or germanium. In some examples, a cladding material (e.g., silicon dioxide) having a lower refractive index than the optical material also forms part of the external cavity chip 204. It should be understood that the optical material and cladding material are examples, and other optical materials may also be used. The external cavity chip 204 can be fabricated using known integrated photonics fabrication techniques.

[0035] 2 and described herein, first optical resonator 206 and second optical resonator 208 are implemented as optical ring resonators. In other examples, first optical resonator 206 and / or second optical resonator 208 can be implemented as different types of resonators. For example, first optical resonator 206 and / or second optical resonator 208 can be implemented as racetrack resonators or Bragg resonators.

[0036] 2, the Brillouin lasing threshold of the second optical cavity 208 is configured to be lower than the Brillouin lasing threshold of the first optical cavity 206. That is, the threshold power of the pump light 220 (and the threshold drive current of the gain chip 202) sufficient to induce SBS lasing in the second optical cavity 208 is lower than the threshold power of the pump light 220 (and the threshold drive current of the gain chip 202) that would be sufficient to induce SBS lasing in the first optical cavity 206. In some examples, the Brillouin lasing threshold of the second optical cavity 208 is configured to be substantially lower than the Brillouin lasing threshold of the first optical cavity 206. There are several ways to set the Brillouin lasing threshold of the second optical cavity 208 lower than the first optical cavity 206.

[0037] In some examples, the first optical resonator 206 and the second optical resonator 208 have different radii. The different radii are selected so that the resonances of the first optical resonator 206 and the second optical resonator 208 are aligned at a single wavelength over the spectral range in which gain exists, and the radius of the first optical resonator 206 is selected so as not to operate as an SBS laser. In such examples, the radii of the first optical resonator 206 and the second optical resonator 208 are selected so that the Brillouin lasing threshold of the second optical resonator 208 is lower than the Brillouin lasing threshold of the first optical resonator 206.

[0038] In other examples, first optical resonator 206 and second optical resonator 208 have the same radius, and the temperatures of first optical resonator 206 and second optical resonator 208 are controlled using one or more circuits (as discussed below) such that the resonances of first optical resonator 206 and second optical resonator 208 are matched at only one wavelength. In such examples, the temperatures of first optical resonator 206 and second optical resonator 208 are selected such that the Brillouin lasing threshold of second optical resonator 208 is lower than the Brillouin lasing threshold of first optical resonator 206.

[0039] In combination with configuring the Brillouin lasing thresholds of the first optical cavity 206 and the second optical cavity 208, the drive current of the gain chip 202 is selected such that only the second optical cavity 208 lases. For example, the drive current of the gain chip 202 is selected such that the power level of the pump light 220 exceeds the Brillouin lasing threshold of the second optical cavity 208 but does not exceed the Brillouin lasing threshold of the first optical cavity 206.

[0040] 2, the first optical resonator 206 is optically coupled to the gain chip 202 via a first optical waveguide 210, a splitter 212, and a second optical waveguide 214. In the example shown in FIG. 2, the external cavity chip 204 includes an output 236 at a second edge 209 of the external cavity chip 204 and at the end of the second optical waveguide 214 that can be used to access the pump light 220. In other examples, a Bragg grating can be used to back-reflect the pump light 220, or a different feature can be a termination used to terminate the pump light 220 rather than including an output 236.

[0041] Pump light 220 generated by the gain chip 202 is provided to the components of the external cavity chip 204 via a first optical waveguide 210 and a splitter 212. The first optical resonator 206 is configured to receive the pump light 220 from the gain chip 202 via a second optical waveguide 214 at a first coupling region 224. In the example shown in Figure 2, the pump light 220 is coupled into the first optical resonator 206 at the first coupling region 224 and propagates through the first optical resonator 206 in a clockwise direction.

[0042] After propagating in the clockwise direction through the first optical resonator 206, at least a portion of the pump light 220 decouples from the first optical resonator 206 at the second coupling region 226 and is provided to the fourth optical waveguide 218. The pump light 220 decouples from the first optical resonator 206 and is provided to the fourth optical waveguide 218, where it is then coupled to the second optical resonator 208 via the fourth optical waveguide 218 at the fourth coupling region 230. The pump light is coupled to the second optical resonator 208 at the fourth coupling region 230 and propagates in the clockwise direction through the second optical resonator 208. In some examples, after propagating in a clockwise direction through the second optical cavity 208, at least a portion of the pump light 220 decouples from the second optical cavity 208 in the third coupling region 228 and is provided as feedback to the gain chip 202 via the third optical waveguide 216, the splitter 212, and the first optical waveguide 210.

[0043] The second optical cavity 208 is also configured to receive pump light 220 from the gain chip 202 via the third optical waveguide 216 at a third coupling region 228. In the example shown in Figure 2, the pump light 220 is coupled into the second optical cavity 208 at the third coupling region 228 and propagates through the second optical cavity 208 in a counterclockwise direction.

[0044] After propagating in the counterclockwise direction through the second optical resonator 208, at least a portion of the pump light 220 decouples from the second optical resonator 208 at the fourth coupling region 230 and is provided to the fourth optical waveguide 218. This pump light 220 decouples from the second optical resonator 208 and is provided to the fourth optical waveguide 218, and is then coupled to the first optical resonator 206 via the fourth optical waveguide 218 at the second coupling region 226. The pump light 220 coupled into the first optical resonator 206 at the second coupling region 226 then propagates in the counterclockwise direction through the first optical resonator 206. In some examples, after propagating in the counterclockwise direction through the first optical resonator 206, at least a portion of the pump light 220 decouples from the first optical resonator 206 at the second coupling region 226 and is provided to an upper portion of the fourth optical waveguide 218 and to an output 238 that can be used to access the pump light 220 at this stage. In other examples, a Bragg grating can be used to back-reflect the pump light 220, or a different feature can be a termination used to terminate the pump light 220 rather than including the output 238. In some examples, after propagating in the counterclockwise direction through the first optical resonator 206, at least a portion of the pump light 220 decouples from the first optical resonator 206 at the first coupling region 224 and is provided to the gain chip 202 as feedback via the second optical waveguide 214, the splitter 212, and the first optical waveguide 210.

[0045] The first optical resonator 206 and the second optical resonator 208 are configured such that the resonances of the first optical resonator 206 and the second optical resonator 208 are rarely matched to one another. In some examples, the first optical resonator 206 and the second optical resonator 208 are configured such that the resonances of the first optical resonator 206 and the second optical resonator 208 are only matched at one wavelength and mismatched at all other wavelengths across the spectral range in which gain exists. This rare matching causes feedback to the gain chip 202 only at the dual-resonance wavelength. The pump light 220 resonates with both the first optical resonator 206 and the second optical resonator 208.

[0046] In operation, the first optical resonator 206 and the second optical resonator 208 act as filters to control the emission wavelength of the pump light 220. For the second optical resonator 208 to generate SBS light 232, the free spectral range (FSR), set by the radius of the second optical resonator 208, must correspond to the frequency shift at which Brillouin scattering occurs. In some examples, the waveguides 210, 214, 216, and 218 operate at 1550 nm. In such examples, if the pump light 220 is provided at one frequency, optical gain can be created at another frequency that is shifted down by approximately 11 GHz. To function as an SBS laser, the FSR is an integer fraction of the Brillouin gain shift (e.g., the FSR can be 5.5 GHz). When the pump light 220 is at one resonance and another resonance of the second optical resonator 208 matches the gain, the second optical resonator 208 generates SBS light 232 when the power level of the pump light 220 is sufficiently high. Thus, the second optical resonator 208 is configured to operate as an SBS resonator and generate SBS light 232 .

[0047] In some examples, the first optical cavity 206 is specifically designed to have a radius with a resonance that does not support SBS lasing, and the SBS light 232 resonates only with the second optical cavity 208. Because the SBS light 232 resonates only with the second optical cavity 208, the SBS light 232 is routed to the output 130 rather than returning to the gain chip 202 as the pump light 220 did.

[0048] 2, there is a little more flexibility in design compared to the SBS laser 100 described above with respect to FIG. 1 in that the Brillouin lasing threshold of either the first optical cavity 206 or the second optical cavity 208 can be configured to be lower than the Brillouin lasing threshold of the other optical cavity. In some examples, the Brillouin lasing threshold of the first optical cavity 206 is configured to be lower than the Brillouin lasing threshold of the second optical cavity 208. In other examples, the Brillouin lasing threshold of the second optical cavity 208 is configured to be lower than the Brillouin lasing threshold of the first optical cavity 206.

[0049] While a particular example includes the gain chip 202 coupled to a first edge 207 of the external cavity chip 204 and the pump light 220 passing through the first optical resonator 206 and the second optical resonator 108 in a particular direction, it should be understood that this is only an example and other configurations may be used. For example, the gain chip 202 may instead be coupled to a second edge 209 of the external cavity chip 204, and the pump light 220 may pass through the first optical resonator 206 and the second optical resonator 208 in the opposite direction.

[0050] As explained above, the SBS lasers 100, 200 operate as single frequency SBS lasers. However, it may be desirable to adjust the operating wavelength of the SBS lasers 100, 200 so that widely tunable SBS laser operation is achieved.

[0051] 3 is a block diagram of an example system 300 that can utilize the techniques for SBS laser generation described herein. In the example shown in FIG. 3, the system 300 includes an SBS laser 302, one or more wavelength tuning circuits 304, and one or more controller circuits 306 communicatively coupled to the one or more wavelength tuning circuits 304.

[0052] In some examples, SBS laser 302 is SBS laser 100, including gain chip 102 and external cavity chip 104, as described above with respect to Figure 1. In other examples, SBS laser 302 is SBS laser 200, including gain chip 202 and external cavity chip 204, as described above with respect to Figure 2. Reference numbers to particular features of SBS laser 302 include reference numbers for both SBS laser 100 and SBS laser 200.

[0053] The one or more wavelength tuning circuits 304 are configured to tune the emission wavelength of the SBS light 128, 232. In some examples, the one or more wavelength tuning circuits 304 are configured to tune the resonance of the first optical resonator 106, 206 and the second optical resonator 108, 208. In some examples, the one or more wavelength tuning circuits 304 are configured to tune the temperature of the first optical resonator 106, 206 and / or the temperature of the second optical resonator 108, 208. In such examples, the one or more wavelength tuning circuits 304 include heaters positioned proximate the first optical resonator 106, 206 and the second optical resonator 108, 208 of the external cavity chip 104, 204. In some examples, the one or more wavelength tuning circuits 304 include a first circuit configured to control the temperature of the first optical resonator 106, 206 and a second circuit configured to control the temperature of the second optical resonator 108, 208. In other examples, the one or more wavelength tuning circuits 304 include a single circuit configured to control the temperature of the first optical resonator 106, 206 and the second optical resonator 108, 208. In either case, the temperature of the first optical resonator 106, 206 and the temperature of the second optical resonator 108, 208 are controlled independently.

[0054] In some examples, the one or more controller circuits 306 provide control signals to the one or more wavelength tuning circuits 304 to control the adjustment of the emission wavelength of the SBS light 128, 232. In some examples, the one or more controller circuits 306 include respective controller circuits configured to provide control signals to the one or more wavelength tuning circuits 304 for adjustment of the temperature of the first optical resonator 106, 206 and the second optical resonator 108, 208. In other examples, the one or more controller circuits 306 include a single circuit configured to provide control signals to the one or more wavelength tuning circuits 304 for adjustment of the temperature of the first optical resonator 106, 206 and the second optical resonator 108, 208. In some examples, the one or more controller circuits 306 are configured to provide control signals based on feedback from the SBS laser 302. For example, the one or more controller circuits 306 are configured to receive an indication of the wavelength of the SBS light 128, 232 output by the SBS laser 302 and provide a control signal based on the indication of the wavelength of the SBS light 128, 232.

[0055] Independently controlling the temperatures of the first optical resonator 106, 206 and the second optical resonator 108, 208 allows for large relative shifts in the alignment of the resonances of the first optical resonator 106, 206 and the second optical resonator 108, 208 of the external cavity chip 104, 204. In some instances, the emission wavelength of the SBS light 128, 232 can be tuned by tens of nanometers, which would not be possible without the use of the first optical resonator 106, 206 and the second optical resonator 108, 208 as described herein.

[0056] In some examples, the one or more wavelength tuning circuits 304 are also configured to adjust the operation of the gain chips 102, 202 of the SBS laser 302 in addition to adjusting the temperature of the first optical cavity 106, 206 and / or the second optical cavity 108, 208. In some examples, the one or more wavelength tuning circuits 304 include a third circuit configured to adjust the drive current of the gain chips 102, 202 of the SBS laser 302. In other examples, the one or more controller circuits 306 are directly coupled to the gain chips 102, 202 of the SBS laser 302 and are configured to adjust the operation of the gain chips 102, 202 (e.g., by adjusting the drive current).

[0057] Although the one or more wavelength adjustment circuits 304 and the one or more controller circuits 306 are shown as separate components in FIG. 3, it should be understood that the one or more wavelength adjustment circuits 304 and the one or more controller circuits 306 can, in some examples, share at least some of the same circuitry.

[0058] 4 illustrates a flow diagram of an exemplary method 400 of operating an SBS laser. Common features discussed above with respect to the exemplary systems of FIGS. 1-3B can include similar features as those discussed with respect to method 400, and vice versa. In some examples, at least some blocks of method 400 are performed by an SBS laser (e.g., SBS lasers 100, 200, 302).

[0059] The method 400 includes generating pump light using a gain chip (block 402) and providing the pump light to an external cavity chip (block 404). In some examples, the external cavity chip is edge-coupled to the gain chip and includes two optical resonators optically coupled via optical waveguides, and injection locking is established to lock the emission wavelength of the pump light from the gain chip to the resonances of the two optical resonators. The pump light resonates with both of the optical resonators of the external cavity chip.

[0060] The method 400 further includes generating SBS light using two optical resonators of an external cavity chip coupled to the gain chip (block 406). In some examples, the two optical resonators are in a Vernier filtering configuration (e.g., as described above with respect to FIGS. 1-2). SBS is generated by only one of the two optical resonators from pump light that propagates through both of the two optical resonators of the external cavity chip. The SBS light resonates only with the optical resonator that generates the SBS light. In some examples, the radii of the optical resonators are selected so that the Brillouin lasing threshold of the optical resonator that generates the SBS light is lower than the Brillouin lasing threshold of the optical resonator that does not generate the SBS light.

[0061] The method 400 further includes outputting the SBS light from a port (block 408). In some examples, the port of the external cavity chip used to output the SBS light can be controlled by using a Bragg grating or by connecting a section of an optical waveguide to return the SBS light to the optical resonator that generates the SBS light.

[0062] Method 400 optionally includes adjusting the wavelength of the SBS light (block 410). In some examples, adjusting the wavelength of the SBS light includes adjusting the temperature of an optical resonator of an external cavity chip, which adjusts the resonance of the optical resonator. In some examples, adjusting the wavelength of the SBS light also includes adjusting the drive current of a gain chip to ensure that the power level of the pump light exceeds the Brillouin lasing threshold of only one of the optical resonators.

[0063] By using a Vernier filtering configuration for the optical resonator of the external cavity chip and configuring the optical resonator as described herein, the system and method enable the generation of SBS light with a very narrow linewidth (e.g., down to the mHz level, depending on the finesse of the optical resonator) without the need for complex control electronics or phase modulators to lock the pump light to the resonance of the optical resonator. The wavelength of the SBS light can be adjusted (e.g., using temperature control of the optical resonator) to generate a widely tunable SBS laser. Another advantage of the SBS laser designs described herein is that because the SBS light is resonant with only one of the optical resonators, these SBS lasers are not subject to back reflections and do not require the use of an external isolator.

[0064] In various aspects, the system elements, method steps, or examples described throughout this disclosure (such as, for example, system 300 or its components) may be implemented on one or more computer systems, including central processing units (CPUs), graphics processing units (GPUs), field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), and / or similar devices with hardware that execute code to implement those elements, processes, or examples, which code is stored in non-transitory data storage devices. These devices contain or function with software programs, firmware, or other computer-readable instructions to perform various methods, process tasks, computations, and control functions.

[0065] These instructions are typically stored on any suitable computer-readable medium used for storing computer-readable instructions or data structures. Computer-readable media can be implemented as any available medium that can be accessed by a general-purpose or specialized computer or processor, or any programmable logic device. Suitable processor-readable media can include storage or memory media such as magnetic or optical media. For example, storage or memory media can include volatile or non-volatile media such as conventional hard disks, Compact Disk-Read Only Memory (CD-ROM), Random Access Memory (RAM) (including, but not limited to, Synchronous Dynamic Random Access Memory (SDRAM), Double Data Rate (DDR) RAM, RAMBUS Dynamic RAM (RDRAM), Static RAM (SRAM), etc.), Read Only Memory (ROM), Electrically Erasable Programmable ROM (EEPROM), and flash memory. Suitable processor-readable media may also include transmission media such as electrical, electromagnetic, or digital signals conveyed over a communications medium such as a network and / or wireless link.

[0066] The methods and techniques described herein may be implemented in digital electronic circuitry, or with a programmable processor (e.g., a special-purpose processor or a general-purpose processor such as a computer), firmware, software, or combinations thereof. Apparatus embodying these techniques may include appropriate input and output devices, a programmable processor, and a storage medium tangibly embodying program instructions for execution by the programmable processor. Processes embodying these techniques may be performed by a programmable processor executing a program of instructions to perform a desired function by operating on input data and generating appropriate output. The techniques may advantageously be implemented in one or more programs executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Generally, the processor receives instructions and data from a read-only memory and / or a random-access memory. Suitable storage devices for tangibly embodying computer program instructions and data include, for example, all forms of non-volatile memory, including semiconductor memory devices (such as EPROM, EEPROM, and flash memory devices), magnetic disks (such as internal hard disks or removable disks), magneto-optical disks, and DVD disks. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs).

[0067] Illustrative Embodiments Example 1 includes a stimulated Brillouin scattering (SBS) laser, the SBS laser comprising a gain chip and an external cavity chip, the external cavity chip comprising a first optical waveguide and a first optical resonator optically coupled to the gain chip via the first optical waveguide, a second optical waveguide and a second optical resonator optically coupled to the first optical resonator via the second optical waveguide, the second optical resonator being configured to generate SBS light from pump light propagating through both the first optical resonator and the second optical resonator, the pump light resonating with both the first optical resonator and the second optical resonator and the SBS light resonating only with the second optical resonator, and the SBS laser being configured to output the SBS light from an output port optically coupled to the second optical resonator.

[0068] Example 2 includes the SBS laser of Example 1, where the radius of the first optical cavity is different from the radius of the second optical cavity.

[0069] Example 3 includes the SBS laser of example 1 or 2, wherein the temperature of the first optical resonator and / or the temperature of the second optical resonator are selected such that the resonances of the first optical resonator and the second optical resonator are rarely matched across a spectral range in which gain exists.

[0070] Example 4 includes the SBS laser of any of Examples 1-3, further comprising a third optical waveguide and a Bragg grating, wherein the first optical resonator is positioned between the first optical waveguide and the second optical waveguide, the second optical resonator is positioned between the second optical waveguide and the third optical waveguide, and the third optical waveguide is coupled to the Bragg grating.

[0071] Example 5 includes the SBS laser of example 4, wherein the Bragg grating is configured to reflect light back into the second optical cavity via the third optical waveguide.

[0072] Example 6 includes the SBS laser of example 5, further comprising one or more additional Bragg gratings coupled to the second optical cavity and configured to reflect the SBS light back toward the second optical cavity.

[0073] Example 7 includes the SBS laser of any of Examples 1-3, further comprising a splitter and a third optical waveguide, wherein the first optical waveguide and the third optical waveguide extend from the splitter, the first optical resonator is optically coupled to the gain chip via the splitter and the first optical waveguide, the second optical resonator is optically coupled to the gain chip via the third optical waveguide, the first optical resonator is positioned between the first optical waveguide and the second optical waveguide, and the second optical resonator is positioned between the second optical waveguide and the third optical waveguide.

[0074] Example 8 includes the SBS laser according to any one of Examples 1 to 7, wherein the first optical resonator and the second optical resonator are ring resonators.

[0075] Example 9 includes the SBS laser of any of Examples 1 to 8, wherein the first optical resonator and the second optical resonator are racetrack resonators or Bragg resonators.

[0076] Example 10 includes the SBS laser according to any one of Examples 1 to 9, wherein the Brillouin lasing threshold of the second optical cavity is lower than the Brillouin lasing threshold of the first optical cavity.

[0077] Example 11 includes the SBS laser of any of Examples 1-10, wherein the first optical cavity is configured to have a radius with a resonance not compatible with SBS lasing.

[0078] Example 12 includes a system, the system comprising: a gain chip; a first optical resonator optically coupled to the gain chip via a first optical waveguide; a second optical resonator optically coupled to the first optical resonator via a second optical waveguide, the second optical resonator configured to generate SBS light from pump light propagating through both the first optical resonator and the second optical resonator, wherein the pump light resonates with both the first optical resonator and the second optical resonator and the SBS light resonates only with the second optical resonator; and one or more circuits configured to set a wavelength of the SBS light.

[0079] Example 13 includes the system of example 12, wherein the one or more circuits include a first circuit and a second circuit, the first circuit configured to adjust a temperature of the first optical resonator, and the second circuit configured to adjust a temperature of the second optical resonator.

[0080] Example 14 includes the system of example 13, wherein the one or more circuits include a third circuit configured to adjust a drive current of the gain chip.

[0081] Example 15 includes the system of any of examples 12-14, wherein the radius of the first optical resonator is different from the radius of the second optical resonator.

[0082] Example 16 includes the system of any of examples 12-15, wherein the Brillouin lasing threshold of the second optical resonator is lower than the Brillouin lasing threshold of the first optical resonator.

[0083] Example 17 includes the system of any of Examples 12-16, wherein the one or more circuits are configured to receive an indication of a wavelength of the SBS light, and the one or more circuits are configured to adjust a temperature of the first optical cavity, a temperature of the second optical cavity, and / or a drive current of the gain chip based on the indication of the wavelength of the SBS light.

[0084] Example 18 includes a method, the method including: generating pump light using a gain chip; providing the pump light from the gain chip to an external cavity chip, the external cavity chip comprising a first optical resonator and a second optical resonator; generating stimulated Brillouin scattering (SBS) light from the pump light propagating through both the first optical resonator and the second optical resonator, using the second optical resonator, wherein the pump light resonates with both the first optical resonator and the second optical resonator and the SBS light resonates only with the second optical resonator; and outputting the SBS light from an output port optically coupled to the second optical resonator.

[0085] Example 19 includes the method of example 18, further including adjusting the wavelength of the SBS light.

[0086] Example 20 includes the method of example 18 or 19, further including adjusting a temperature of the first optical cavity, a temperature of the second optical cavity, and / or a drive current of the gain chip.

[0087] While specific embodiments have been illustrated and described herein, those skilled in the art will recognize that any arrangement which is expected to achieve the same purpose may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.

Claims

1. A stimulated Brillouin scattering (SBS) laser (100, 200), comprising: a gain chip (102, 202); an external cavity tip (104, 204), wherein the external cavity tip (104, 204) a first optical waveguide (110, 214); a first optical cavity (106, 206) optically coupled to the gain chip (102, 202) via the first optical waveguide (110, 214); a second optical waveguide (112, 218); a second optical resonator (108, 208) optically coupled to the first optical resonator (106, 206) via the second optical waveguide (112, 218); the second optical resonator (108, 208) is configured to generate SBS light (128, 232) from pump light (118, 220) propagated through both the first optical resonator (106, 206) and the second optical resonator (108, 208), the pump light (118, 220) resonating with both the first optical resonator (106, 206) and the second optical resonator (108, 208), and the SBS light (128, 232) resonating only with the second optical resonator (108, 208); 1. A stimulated Brillouin scattering (SBS) laser (100, 200), wherein the SBS laser (100, 200) is configured to output the SBS light (128, 232) from an output port optically coupled to the second optical cavity (108, 208).

2. 2. The SBS laser of claim 1, further comprising a third optical waveguide and a Bragg grating, wherein the first optical resonator is positioned between the first optical waveguide and the second optical waveguide, the second optical resonator is positioned between the second optical waveguide and the third optical waveguide, and the third optical waveguide is coupled to the Bragg grating.

3. 2. The SBS laser of claim 1, further comprising a splitter and a third optical waveguide, wherein the first optical waveguide and the third optical waveguide extend from the splitter, the first optical resonator is optically coupled to the gain chip via the splitter and the first optical waveguide, the second optical resonator is optically coupled to the gain chip via the third optical waveguide, the first optical resonator is positioned between the first optical waveguide and the second optical waveguide, and the second optical resonator is positioned between the second optical waveguide and the third optical waveguide.

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