Semiconductor tunable ring laser, photonic integrated circuit, and optoelectronic system including the same

Abstract

To provide a semiconductor tunable ring laser (1) including a laser cavity (2) having a closed loop optical path, and an optical filter (3) disposed within the laser cavity and configured as a transmission type optical filter when the semiconductor tunable ring laser is in use.SOLUTION: An optical filter includes a first MZI-based tunable frequency filter section (3a) including a first 2×2 MMI output splitter (9) having an x / (100-x) splitting ratio, and x is not equal to 50. The first 2×2 MMI output splitter provides the laser cavity with an optical monitoring port at which a first non-zero fraction T1 of optical radiation sufficient for wavelength locking and / or power monitoring outside the laser cavity is presented. The present invention also relates to a PIC (100) including a semiconductor tunable ring laser according to the present invention, and to an optoelectronic system (200) including such a PIC.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a semiconductor tunable ring laser. The present invention also relates to a photonic integrated circuit (PIC) comprising a semiconductor tunable ring laser according to the present invention. The present invention further relates to an optoelectronic system comprising a PIC according to the present invention. The optoelectronic system according to the present invention can be used, by way of example but not exclusively, for telecommunications applications, light detection and ranging (LIDAR) or sensor applications. [Background technology]

[0002] In many optoelectronic systems, which may be used for, by way of example but not exclusively, telecommunications applications, light detection and ranging (LIDAR), or sensor applications, semiconductor lasers are a key element for generating a stable beam of optical radiation with a narrow spectrum. Many different types of semiconductor lasers are known, such as semiconductor tunable ring lasers, which comprise a laser cavity with a closed-loop optical path. An advantage of semiconductor tunable ring lasers, compared to, for example, distributed Bragg reflector lasers or Fabry-Perot lasers, is that the laser cavity of a semiconductor tunable ring laser does not require an on-chip reflector or facet reflector to achieve stimulated emission of photons. This advantage of semiconductor tunable ring lasers is beneficial for design and fabrication-related issues related to advanced chip-in-cell and optoelectronic systems that require reduced complexity.

[0003] To keep the lasing wavelength of a known semiconductor tunable ring laser stable, a control loop is typically required that includes a sensor capable of generating a sensor signal on which the control loop can act. Typically, the sensor is an optical sensor that is capable of generating a sensor signal based on a small fraction of the optical radiation tapped off from the amount of optical radiation that constitutes the main optical output of the semiconductor tunable ring laser and directed toward the sensor. Typically, the small fraction of the optical radiation is a few percent of the main optical output of the semiconductor tunable ring laser. Tapping off a small fraction of the optical radiation from the main optical output of the semiconductor tunable ring laser and directing only a small amount of optical power toward the sensor of the control loop for lasing wavelength stabilization purposes not only makes the main optical output totally unavailable for the application in which the semiconductor tunable ring laser is used, but also causes optical losses that reduce the overall efficiency of the semiconductor tunable ring laser.

[0004] Based on the above, there is a need to provide a semiconductor tunable ring laser that has improved overall efficiency despite removing a small fraction of the optical radiation and some optical losses associated with lasing wavelength stabilization purposes. Summary of the Invention [Problem to be solved by the invention]

[0005] It is an object of the present invention to provide a semiconductor tunable ring laser with improved overall efficiency, such that the semiconductor tunable ring laser according to the present invention is able to avoid or at least reduce at least one of the above-mentioned and / or other disadvantages associated with known semiconductor tunable ring lasers.

[0006] It is another object of the present invention to provide a PIC that includes a semiconductor tunable ring laser according to the present invention.

[0007] It is yet another object of the present invention to provide an optoelectronic system comprising a PIC according to the present invention, which can be used for, by way of example but not exclusively, telecommunications applications, LIDAR or sensor applications. [Means for solving the problem]

[0008] Aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features from the independent claims, as appropriate, and not only as explicitly set out in the claims. Furthermore, all features may be substituted by other technically equivalent features.

[0009] At least one of the above objects is achieved by providing a semiconductor tunable ring laser, comprising: a laser cavity having a closed-loop optical path; an optical filter disposed within the laser cavity and configured as a transmission-type optical filter when the semiconductor tunable ring laser is in use, a first MZI-based tunable frequency filter section, a first 1×2 MMI input splitter, a first optical input port disposed in optical communication with the closed-loop optical path of the laser cavity; a first optical output port, and ○ Second optical output port a first 1×2 MMI input splitter comprising a first multimode waveguide section provided with: a first 2×2 MMI output distributor, ○ a second optical input port, ○ A third optical input port, a third optical output port disposed in optical communication with the closed-loop optical path of the laser cavity; and a fourth optical output port configured and arranged as an optical monitoring port for the laser cavity; a first 2×2 MMI output divider comprising a second multimode waveguide section provided with a first light-guiding structure comprising a first optical waveguide arranged to optically interconnect a first optical output port of the 1x2 MMI input splitter and a second optical input port of the first 2x2 MMI output splitter, the first light-guiding structure being configured to provide a first optical path length between the first optical output port of the first 1x2 MMI input splitter and the second optical input port of the first 2x2 MMI output splitter; and a second light-guiding structure comprising a second optical waveguide arranged to optically interconnect a second optical output port of the first 1x2 MMI input splitter and a third optical input port of the first 2x2 MMI output splitter, the second light-guiding structure being configured to provide a second optical path length between the second optical output port of the first 1x2 MMI input splitter and the third optical input port of the first 2x2 MMI output splitter, the second optical path length being different from the first optical path length. wherein the second multimode waveguide section of the first 2×2 MMI output splitter is configured to achieve an x / (100−x) splitting ratio, where x has a value not equal to 50, and the x / (100−x) splitting ratio is such that, when the semiconductor tunable ring laser is in use, the x / (100−x) splitting ratio is independent of the phase relationship between the optical radiation at the second optical input port of the first 2×2 MMI output splitter and the optical radiation at the third optical input port of the first 2×2 MMI output splitter; a first non-zero fraction T1 of optical radiation of the amount T0 of optical radiation incident on the second optical input port of the first 2×2 MMI output splitter and on the third optical input port of the first 2×2 MMI output splitter is present at the fourth optical output port of the first 2×2 MMI output splitter, the first non-zero fraction T1 of optical radiation being sufficient to enable wavelength locking and / or power monitoring outside the laser cavity; and A second fraction T2=1-T1 of the optical radiation of the optical radiation amount T0 is present at the third optical output port of the first 2×2 MMI output splitter, thereby remaining in the closed-loop optical path of the laser cavity. a first MZI-based tunable frequency filter section selected to enable an optical filter comprising: This is achieved by a semiconductor tunable ring laser comprising:

[0010] Thus, the semiconductor tunable ring laser according to the present invention allows for the first non-zero fraction T1 of optical radiation to be directly removed from within the laser cavity, i.e., from the amount of optical radiation that constitutes the main optical output of the semiconductor tunable ring laser, rather than after or outside the laser cavity, as is done in semiconductor tunable ring lasers known in the art. Surprisingly, it has been found that directly removing the first non-zero fraction T1 of optical radiation from within the laser cavity results in a reduction in the overall optical losses of the semiconductor tunable ring laser according to the present invention, compared to the overall optical losses of semiconductor tunable ring lasers known in the art in which the first non-zero fraction T1 of optical radiation is removed after or outside the laser cavity. Thus, the semiconductor tunable ring laser according to the present invention has improved overall efficiency compared to semiconductor tunable ring lasers known in the art, despite the removal of the first non-zero fraction T1 of optical radiation and any optical losses associated with lasing wavelength stabilization purposes.

[0011] The insight of the present invention is that the x / (100-x) distribution ratio of the first 2x2 MMI output distributor, where x has a value not equal to 50, enables a first non-zero fraction T1 of the optical radiation to always be present at the fourth optical output port of the first 2x2 MMI output distributor, regardless of the phase relationship between the optical radiation at the second optical input port of the first 2x2 MMI output distributor and the optical radiation at the third optical input port of the first 2x2 MMI output distributor.

[0012] An advantage of the above insight is that it does not require the inclusion of a dedicated optical emission tapping structure inside the laser cavity that is constructed and arranged to allow for the removal of optical emission from within the cavity for wavelength locking and / or power monitoring purposes outside the laser cavity. Those skilled in the art will appreciate that the inclusion of a dedicated optical emission tapping structure would introduce additional losses and potentially detrimental reflections that would reduce the overall efficiency of the semiconductor tunable ring laser.

[0013] Another advantage of the above insight is that no control loop is required to ensure that a minimum amount of optical radiation is available for wavelength locking and / or power monitoring outside the laser cavity. As a result, fewer components are required to achieve lasing wavelength stabilization of the semiconductor tunable ring laser according to the present invention compared to semiconductor tunable ring lasers known in the art. To enable wavelength locking and / or power monitoring outside the laser cavity, the first non-zero fraction T1 of optical radiation is sufficient if, for example, the first non-zero fraction T1 of optical radiation is in the range of 1% to 15% of the amount of optical radiation T0 incident on the second optical input port of the first 2×2 MMI output splitter and on the third optical input port of the first 2×2 MMI output splitter.

[0014] In one embodiment of the semiconductor tunable ring laser according to the present invention, the value of x ranges from 64 to 80. Thus, the x / (100-x) splitting ratio of the first 2x2 MMI output splitter can range from 64 / 36 to 80 / 20. As a result, the first MZI-based tunable frequency filter section has a maximum transmission that can range from 85% to 99% and a minimum transmission that can range from 1% to 15%.

[0015] In one exemplary embodiment of a semiconductor tunable ring laser according to the present invention, the value of x is 72. In this case, the splitting ratio of the first 2x2 MMI output splitter is 72 / 28, and the first MZI-based tunable frequency filter section has a maximum transmission of 95% and a minimum transmission of 5%.

[0016] In one embodiment of the semiconductor tunable ring laser according to the present invention, the optical filter comprises: a second 1x2 MMI input splitter, ● a fourth optical input port disposed in optical communication with the third optical output port of the first 2×2 MMI output splitter of the first MZI-based tunable frequency filter section, and thereby in optical communication with the closed-loop optical path of the laser cavity; ● A fifth optical output port, and ● 6th optical output port a second 1×2 MMI input splitter comprising a third multimode waveguide section provided with: a first 2×1 MMI output combiner, ● 5th optical input port, ● A sixth optical input port, and a seventh optical output port disposed in optical communication with the closed-loop optical path of the laser cavity; a first 2×1 MMI output coupler comprising a fourth multimode waveguide section provided with - a third light-guiding structure comprising a third optical waveguide arranged to optically interconnect a fifth optical output port of the second 1x2 MMI input splitter and a fifth optical input port of the first 2x1 MMI output coupler, the third light-guiding structure being configured to provide a third optical path length between the fifth optical output port of the second 1x2 MMI input splitter and the fifth optical input port of the first 2x1 MMI output coupler; a fourth light-guiding structure comprising a fourth optical waveguide arranged to optically interconnect a sixth optical output port of the second 1x2 MMI input splitter and a sixth optical input port of the first 2x1 MMI output coupler, the fourth light-guiding structure being configured to provide a fourth optical path length between the sixth optical output port of the second 1x2 MMI input splitter and the sixth optical input port of the first 2x1 MMI output coupler, the fourth optical path length being different from the third optical path length; and a second MZI-based tunable frequency filter section comprising:

[0017] As a result of the first MZI-based tunable frequency filter section and the second MZI-based tunable frequency filter section, the above-described embodiments of the semiconductor tunable ring laser according to the present invention have improved frequency tunability, which is advantageous for PICs and optoelectronic systems that include the semiconductor tunable ring laser according to the present invention. Such PICs and optoelectronic systems can be used, by way of example but not exclusively, in telecommunications applications, light detection and ranging (LIDAR) or sensor applications.

[0018] In one embodiment of the semiconductor tunable ring laser according to the present invention, the optical filter comprises: a third 1x2 MMI input splitter, ● a seventh optical input port disposed in optical communication with the seventh optical output port of the first 2×1 MMI output coupler of the second MZI-based tunable frequency filter section, and thereby in optical communication with the closed-loop optical path of the laser cavity; ● An eighth optical output port, and ● 9th optical output port a third 1×2 MMI input splitter comprising a fifth multimode waveguide section provided with: a second 2×1 MMI output combiner, ● 8th optical input port, ● A ninth optical input port, and a tenth optical output port disposed in optical communication with the closed-loop optical path of the laser cavity; a second 2×1 MMI output coupler comprising a sixth multimode waveguide section provided with a fifth light-guiding structure comprising a fifth optical waveguide arranged to optically interconnect an eighth optical output port of the third 1x2 MMI input splitter and an eighth optical input port of the second 2x1 MMI output coupler, the fifth light-guiding structure being configured to provide a fifth optical path length between the eighth optical output port of the third 1x2 MMI input splitter and the eighth optical input port of the second 2x1 MMI output coupler; a sixth light-guiding structure comprising a sixth optical waveguide arranged to optically interconnect a ninth optical output port of the third 1×2 MMI input splitter and a ninth optical input port of the second 2×1 MMI output coupler, the sixth light-guiding structure being configured to provide a sixth optical path length between the ninth optical output port of the third 1×2 MMI input splitter and the ninth optical input port of the second 2×1 MMI output coupler, the sixth optical path length being different from the fifth optical path length; a third MZI-based tunable frequency filter section comprising:

[0019] As a result of the first MZI-based tunable frequency filter section, the second MZI-based tunable frequency filter section, and the third MZI-based tunable frequency filter section, the above-described embodiments of the semiconductor tunable ring laser according to the present invention have even further improved frequency tunability. As a result, PICs and optoelectronic systems comprising semiconductor tunable ring lasers according to the above-described embodiments of the present invention can be used for more advanced applications in, for example but not exclusively, the fields of telecommunications, light detection and ranging (LIDAR), or sensors.

[0020] In one embodiment of a semiconductor tunable ring laser according to the present invention, the closed loop optical path of the laser cavity includes: a gain section, a tenth optical input port disposed in optical communication with the tenth optical output port of the second 2×1 MMI output coupler of the third MZI-based tunable frequency filter section, and thereby in optical communication with the closed-loop optical path of the laser cavity; and 11th optical output port a gain section comprising: a second 2x2 MMI output divider, an eleventh optical input port disposed in optical communication with the eleventh optical output port of the gain section, and thereby in optical communication with the closed-loop optical path of the laser cavity; a twelfth optical input port disposed in optical communication with an optical reflector disposed outside the closed-loop optical path of the laser cavity; a twelfth optical output port disposed in optical communication with the first optical input port of the first 1×2 MMI input splitter of the first MZI-based tunable frequency filter section, and thereby in optical communication with the closed-loop optical path of the laser cavity; and a thirteenth optical output port constructed and arranged to allow optical power to be coupled out of the laser cavity for use in applications other than wavelength locking and / or power monitoring purposes; a second 2×2 MMI output divider comprising a seventh multimode waveguide section provided with is provided.

[0021] The second 2x2 MMI output splitter can be configured to have any suitable split ratio depending on the requirements of the application in which the semiconductor tunable ring laser according to the present invention is used.

[0022] In one embodiment of the semiconductor tunable ring laser according to the present invention, the first MZI-based tunable frequency filter section is configured to have a first free spectral range, the second MZI-based tunable frequency filter section is configured to have a second free spectral range, and the third MZI-based tunable frequency filter section is configured to have a third free spectral range, and the first free spectral range, the second free spectral range, and the third free spectral range are different from each other. In this way, the stability of the semiconductor tunable ring laser according to the present invention can be improved.

[0023] In one embodiment of the semiconductor tunable ring laser according to the present invention, the semiconductor tunable ring laser is an InP-based tunable ring laser. Those skilled in the art will appreciate that InP-based semiconductor materials are the semiconductor materials of choice for fabricating semiconductor tunable ring lasers that can be used, by way of example but not exclusively, in telecommunications, light detection and ranging (LIDAR), or sensor applications. InP-based technology enables the monolithic integration of both active components, such as light-emitting and / or light-absorbing optical devices, and passive components, such as light-guiding and / or light-switching optical devices, in one PIC on a single die.

[0024] According to another aspect of the present invention, there is provided a PIC comprising a semiconductor tunable ring laser according to the present invention, the PIC being a hybrid integrated PIC or a monolithically integrated PIC. Based on the above, those skilled in the art will understand that a PIC according to the present invention can benefit from the advantages provided by the semiconductor tunable ring laser according to the present invention.

[0025] The advantage of the hybrid integrated PIC is that the semiconductor tunable ring laser can be, for example, an InP-based tunable ring laser combined with a Si-based optoelectronic device, and therefore the PIC according to the present invention can be used in any semiconductor technology domain, such as the domain of silicon photonics.

[0026] Another advantage of the hybrid integrated PIC according to the present invention is that the semiconductor tunable ring laser can be replaced, which may be required, for example, in the event of laser malfunction or after the laser stops functioning.

[0027] An advantage of monolithically integrated PICs is that both active and passive optoelectronic devices can be integrated on the same semiconductor substrate, such as on an InP-based substrate. Moreover, monolithic integration of active and passive optoelectronic devices can be less cumbersome and, in some cases, requires less die area than hybrid integration of active and passive optoelectronic devices.

[0028] In one embodiment of a PIC in accordance with the present invention, the PIC includes an optical emission monitoring assembly optically coupled to the fourth optical output port of the first 2x2 MMI output splitter of the first MZI-based tunable frequency filter section of the optical filter of the semiconductor tunable ring laser. The optical emission monitoring assembly can be part of a control loop constructed and arranged to control the optical performance of the semiconductor tunable ring laser in accordance with the present invention.

[0029] In one exemplary embodiment of a PIC according to the present invention, the optical radiation monitoring assembly comprises a wavelength locker that can be constructed and arranged to stabilize the lasing wavelength of the semiconductor tunable ring laser according to the present invention when the semiconductor tunable ring laser is in use using a first non-zero fraction T1 of the optical radiation present at the fourth optical output port of the first 2×2 MMI output splitter of the first MZI-based tunable frequency filter section of the intracavity optical filter of the semiconductor tunable ring laser.

[0030] According to yet another aspect of the present invention, there is provided an optoelectronic system comprising a PIC according to the present invention, the optoelectronic system being one of a transmitter, a receiver, a transceiver, a coherent transmitter, a coherent receiver, and a coherent transceiver. The optoelectronic system may be used for telecommunications applications, LIDAR, or sensor applications, by way of example and not by way of limitation. Based on the above, those skilled in the art will understand that any one of the above-mentioned transmitters, receivers, and transceivers can benefit from the advantages provided by a PIC according to the present invention, which comprises a semiconductor tunable ring laser according to the present invention.

[0031] Further features and advantages of the present invention will become apparent from the description of exemplary, non-limiting embodiments of a semiconductor tunable ring laser, a PIC, and an optoelectronic system according to this invention.

[0032] Those skilled in the art will understand that the described embodiments of the semiconductor tunable ring laser, the PIC, and the optoelectronic system are merely exemplary in nature and should not be construed as limiting the scope of protection in any way. Those skilled in the art will recognize that alternatives and equivalent embodiments of the semiconductor tunable ring laser, the PIC, and the optoelectronic system can be devised and reduced to practice without departing from the scope of protection of the present invention.

[0033] Reference will be made to the figures on the accompanying drawings, which are schematic in nature and, therefore, are not necessarily drawn to scale. Furthermore, equal reference numerals represent equal or similar parts. In the accompanying drawings: [Brief explanation of the drawings]

[0034] [Figure 1] 1 is a schematic top view of a first exemplary non-limiting embodiment of a semiconductor tunable ring laser according to the present invention; [Figure 2] 2 is a schematic top view of a second exemplary non-limiting embodiment of a semiconductor tunable ring laser according to the present invention. FIG. [Figure 3] FIG. 2 is a schematic top view of a third exemplary non-limiting embodiment of a semiconductor tunable ring laser according to the present invention. [Figure 4] FIG. 10 is a schematic diagram of the output transmission as a function of the phase difference of an MZI-based tunable frequency filter section comprising a 1×2 MMI input splitter configured to achieve a 50 / 50 splitting ratio and a 2×1 MMI output combiner configured to achieve a 50 / 50 combining ratio. [Figure 5] FIG. 10 is a schematic diagram of the output transmission as a function of the phase difference of an MZI-based tunable frequency filter section comprising a 1×2 MMI input splitter configured to achieve a 50 / 50 split ratio and a 2×2 MMI output splitter configured to achieve a 72 / 28 split ratio. [Figure 6A]FIG. 1 is a comparison of the total optical output power of three semiconductor tunable ring lasers A, B, and C known in the art, each of which includes a conventional 85 / 15 power splitter located after or outside the laser cavity to provide optical radiation that can be used for lasing wavelength stabilization purposes, and a semiconductor tunable ring laser D according to the present invention, in which the laser cavity is directly tapped off using a 2×2 MMI power splitter configured to achieve a 72 / 28 split ratio to provide optical radiation that can be used for lasing wavelength stabilization purposes. [Figure 6B] FIG. 1 is a comparison of the total photoinduced current of three known semiconductor tunable ring lasers A, B, and C with the semiconductor tunable ring laser D according to the present invention, where the total photoinduced current is the sum of the photoinduced current resulting from the main optical output and the photoinduced current resulting from a small portion of the optical radiation directed to a photosensor in the control circuit for stabilizing the laser oscillation wavelength. [Figure 7] 1 is a schematic top view of a first exemplary non-limiting embodiment of a PIC according to the present invention, comprising a semiconductor tunable ring laser according to the present invention; [Figure 8] 1 is a schematic top view of a first exemplary non-limiting embodiment of an optoelectronic system according to the present invention, comprising a PIC according to the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0035] FIG. 1 is a schematic top view of a first exemplary, non-limiting embodiment of a semiconductor tunable ring laser 1 according to the present invention, comprising a laser cavity 2 having a closed-loop optical path and an optical filter 3 disposed within the laser cavity 2. The optical filter 3 is configured as a transmission-type optical filter when the semiconductor tunable ring laser 1 is in use. The optical filter 3 comprises a first MZI-based tunable frequency filter section 3a, a second MZI-based tunable frequency filter section 3b, and a third MZI-based tunable frequency filter section 3c arranged in a serial configuration inside the laser cavity 2. Those skilled in the art will appreciate that the number of MZI-based tunable frequency filter sections is exemplary and non-limiting. Any suitable number, such as 1, 2, 3, 4, 5, 6, etc., can be envisioned depending on the specific requirements that the semiconductor tunable ring laser must meet.

[0036] 1 includes a first 1×2 MMI input splitter 4 including a first multimode waveguide section 5 provided with a first optical input port 6, a first optical output port 7, and a second optical output port 8 arranged in optical communication with the closed-loop optical path of the laser cavity 2. The first multimode waveguide section 5 is configured to achieve a 50 / 50 split ratio. The first MZI-based tunable frequency filter section 3 a further includes a first 2×2 MMI output splitter 9 including a second multimode waveguide section 10 provided with a second optical input port 11, a third optical input port 12, a third optical output port 13 arranged in optical communication with the closed-loop optical path of the laser cavity 2, and a fourth optical output port 14 configured and arranged as an optical monitoring port for the laser cavity 2. According to the first exemplary non-limiting embodiment of the semiconductor tunable ring laser 1 shown in FIG. 1, the fourth optical output port 14 is positioned in optical communication with the wavelength locker 120 of the optical emission monitoring assembly 110.

[0037] The first optical output port 7 of the 1x2 MMI input splitter 4 and the second optical input port 11 of the first 2x2 MMI output splitter 9 are optically interconnected via a first light guiding structure 15 comprising a first optical waveguide 16. The first optical waveguide 16 is configured to provide a first optical path length between the first optical output port 7 and the second optical input port 11. The second optical output port 8 of the first 1x2 MMI input splitter 4 and the third optical input port 12 of the first 2x2 MMI output splitter 9 are optically interconnected via a second light guiding structure 17 comprising a second optical waveguide 18. The second optical waveguide 18 is configured to provide a second optical path length between the second optical output port 8 and the third optical input port 12. According to the first exemplary, non-limiting embodiment of the semiconductor tunable ring laser 1 shown in FIG. 1 , the second optical path length is shorter than the first optical path length because the second optical waveguide 18 is shorter than the first optical waveguide 16. Due to the difference between the first and second optical path lengths, the first MZI-based tunable frequency filter section 3 a can be interpreted as an asymmetric MZI-based tunable frequency filter section. Those skilled in the art will understand that the above-described implementation of the difference between the first and second optical path lengths, i.e., by applying the first and second light guiding structures 15 and 17 comprising the first and second optical waveguides 16 and 18, respectively, having different lengths, is merely a non-limiting example. Another non-limiting example (not shown) of how to implement the difference between the first and second optical path lengths is to apply a first light guiding structure 15 and a second light guiding structure 17 comprising a first optical waveguide 16 and a second optical waveguide 18, respectively, having equal lengths, wherein the first light guiding structure 15 further comprises a ring-shaped structure optically associated with the first optical waveguide 16 to establish a ring-loaded structure, which results in the first optical path length provided by the first light guiding structure 15 being different from the second optical path length provided by the second light guiding structure 17.

[0038] The second multimode waveguide section 10 of the first 2×2 MMI output splitter 9 is configured to achieve an x / (100−x) splitting ratio, where x has a value not equal to 50. On the one hand, the x / (100−x) splitting ratio is selected so as to enable a first non-zero fraction T1 of the amount of optical radiation T0 incident on the second optical input port 11 and on the third optical input port 12 of the first 2×2 MMI output splitter 9 to be present at the fourth optical output port 14 of the first 2×2 MMI output splitter 9 when the semiconductor tunable ring laser 1 is in use, regardless of the phase relationship between the optical radiation at the second optical input port 11 of the first 2×2 MMI output splitter 9 and the optical radiation at the third optical input port 12 of the first 2×2 MMI output splitter 9, and the first non-zero fraction T1 of the optical radiation is sufficient to enable wavelength locking and / or power monitoring outside the laser cavity 2. On the other hand, the x / (100-x) splitting ratio is chosen to enable, when the semiconductor tunable ring laser 1 is in use, a second fraction T2=1-T1 of the optical radiation of the amount T0 to be present at the third optical output port 13 of the first 2x2 MMI output splitter 9, regardless of the phase relationship between the optical radiation at the second optical input port 11 and the optical radiation at the third optical input port 12. In this way, the second fraction T2=1-T1 of the optical radiation remains in the closed-loop optical path of the laser cavity.

[0039] As mentioned above, the value of x can range from 64 to 80. Thus, the x / (100-x) splitting ratio of the first 2x2 MMI output splitter can range from 64 / 36 to 80 / 20. As a result, the first MZI-based tunable frequency filter section has a maximum transmission that can range from 85% to 99% and a minimum transmission that can range from 1% to 15%. In one exemplary embodiment of a semiconductor tunable ring laser according to the present invention, the value of x is 72. In this case, the splitting ratio of the first 2x2 MMI output splitter is 72 / 28, and the first MZI-based tunable frequency filter section has a maximum transmission of 95% and a minimum transmission of 5%.

[0040] The second MZI-based tunable frequency filter section 3b of the optical filter 3 shown in Figure 1 comprises a second 1x2 MMI input splitter 19 comprising a third multimode waveguide section 20 configured to achieve a 50 / 50 splitting ratio. The third multimode waveguide section 20 is provided with a fourth optical input port 21, a fifth optical output port 22, and a sixth optical output port 23. The fourth optical input port 21 is disposed in optical communication with the third optical output port 13 of the first 2x2 MMI output splitter 9 of the first MZI-based tunable frequency filter section 3a. Thus, the fourth optical input port 21 is disposed in optical communication with the closed-loop optical path of the laser cavity 2.

[0041] The second MZI-based tunable frequency filter section 3b further comprises a first 2×1 MMI output coupler 24 comprising a fourth multimode waveguide section 25 configured to achieve a 50 / 50 coupling ratio. The fourth multimode waveguide section 25 is provided with a fifth optical input port 26, a sixth optical input port 27, and a seventh optical output port 28 disposed in optical communication with the closed-loop optical path of the laser cavity 2. The fifth optical output port 22 of the second 1×2 MMI input splitter 19 and the fifth optical input port 26 of the first 2×1 MMI output coupler 24 are optically interconnected via a third light guiding structure 29 comprising a third optical waveguide 30. The third optical waveguide 30 is configured to provide a third optical path length between the fifth optical output port 22 and the fifth optical input port 26. The sixth optical output port 23 of the second 1×2 MMI input splitter 19 and the sixth optical input port 27 of the first 2×1 MMI output combiner 24 are optically interconnected via a fourth light-guiding structure 31 comprising a fourth optical waveguide 32. The fourth optical waveguide 32 is configured to provide a fourth optical path length between the sixth optical output port 23 and the sixth optical input port 27. According to the first exemplary, non-limiting embodiment of the semiconductor tunable ring laser 1 shown in FIG. 1 , the fourth optical path length is shorter than the third optical path length because the fourth optical waveguide 32 is shorter than the third optical waveguide 30. Due to the difference between the fourth optical path length and the third optical path length, the second MZI-based tunable frequency filter section 3b can be interpreted as an asymmetric MZI-based tunable frequency filter section. Those skilled in the art will understand that the above considerations regarding how the difference between the optical path lengths can be established apply mutatis mutandis.

[0042] The third MZI-based tunable frequency filter section 3c of the optical filter 3 shown in Figure 1 comprises a third 1x2 MMI input splitter 33 comprising a fifth multimode waveguide section 34 configured to achieve a 50 / 50 split ratio.

[0043] The third multimode waveguide section 34 is provided with a seventh optical input port 35, an eighth optical output port 36, and a ninth optical output port 37. The seventh optical input port 35 is placed in optical communication with the seventh optical output port 28 of the first 2×1 MMI output coupler 24 of the second MZI-based tunable frequency filter section 3 b. As such, the seventh optical input port 35 is placed in optical communication with the closed-loop optical path of the laser cavity 2.

[0044] The third MZI-based tunable frequency filter section 3c further comprises a second 2×1 MMI output coupler 38 comprising a sixth multimode waveguide section 39 configured to achieve a 50 / 50 coupling ratio. The sixth multimode waveguide section 39 is provided with an eighth optical input port 40, a ninth optical input port 41, and a tenth optical output port 42 disposed in optical communication with the closed-loop optical path of the laser cavity 2. The eighth optical output port 36 of the third 1×2 MMI input splitter 33 and the eighth optical input port 40 of the second 2×1 MMI output coupler 38 are optically interconnected via a fifth light guiding structure 43 comprising a fifth optical waveguide 44. The fifth optical waveguide 44 is configured to provide a fifth optical path length between the eighth optical output port 36 and the eighth optical input port 40. The ninth optical output port 37 of the third 1×2 MMI input splitter 33 and the ninth optical input port 41 of the second 2×1 MMI output combiner 38 are optically interconnected via a sixth light-guiding structure 45 comprising a sixth optical waveguide 46. The sixth optical waveguide 46 is configured to provide a sixth optical path length between the ninth optical output port 37 and the ninth optical input port 41. According to the first exemplary, non-limiting embodiment of the semiconductor tunable ring laser 1 shown in FIG. 1 , the sixth optical path length is shorter than the fifth optical path length because the sixth optical waveguide 46 is shorter than the fifth optical waveguide 44. Due to the difference between the sixth and fifth optical path lengths, the third MZI-based tunable frequency filter section 3c can be interpreted as an asymmetric MZI-based tunable frequency filter section. Those skilled in the art will understand that the above considerations regarding how the difference between the optical path lengths can be established apply mutatis mutandis.

[0045] 1 , the closed-loop optical path of the laser cavity 2 is provided with a gain section 47 comprising a tenth optical input port 48 and an eleventh optical output port 49. The tenth optical input port 48 is disposed in optical communication with the tenth optical output port 42 of the second 2×1 MMI output coupler 38 of the third MZI-based tunable frequency filter section 3 c. In this manner, the tenth optical input port 48 is disposed in optical communication with the closed-loop optical path of the laser cavity 2.

[0046] The closed-loop optical path of the laser cavity 2 is further provided with a second 2×2 MMI output splitter 50 comprising a seventh multimode waveguide section 51 provided with an eleventh optical input port 52 disposed in optical communication with the eleventh optical output port 49 of the gain section 47 and thereby in optical communication with the closed-loop optical path of the laser cavity 2. The seventh multimode waveguide section 51 is further provided with a twelfth optical input port 53, a twelfth optical output port 55, and a thirteenth optical output port 56. The twelfth optical input port 53 is disposed in optical communication with an optical reflector 54 disposed outside the closed-loop optical path of the laser cavity 2. The twelfth optical output port 55 is disposed in optical communication with the first optical input port 6 of the first 1×2 MMI input splitter 4 of the first MZI-based tunable frequency filter section 3 a and thereby in optical communication with the closed-loop optical path of the laser cavity 2. The thirteenth optical output port 56 is constructed and arranged to allow optical power to be coupled out of the laser cavity 2 for use in applications other than wavelength locking and / or power monitoring purposes. The second 2x2 MMI output splitter 50 can be configured to have any suitable split ratio depending on the requirements of the application in which the semiconductor tunable ring laser 1 according to the present invention is used.

[0047] 2 is a schematic top view of a second exemplary, non-limiting embodiment of a semiconductor tunable ring laser 1 according to the present invention, comprising a laser cavity 2 having a closed-loop optical path and an optical filter 3 disposed within the laser cavity 2. The optical filter 3 is configured as a transmission-type optical filter when the semiconductor tunable ring laser 1 is in use. The optical filter 3 comprises a first MZI-based tunable frequency filter section 3a, a second MZI-based tunable frequency filter section 3b, and a third MZI-based tunable frequency filter section 3c, as shown in FIG. 1. However, the order in which the three MZI-based tunable frequency filter sections are arranged in a consecutive configuration inside the laser cavity 2 is different. According to the second exemplary, non-limiting embodiment of the semiconductor tunable ring laser 1 shown in FIG. 2, the first MZI-based tunable frequency filter section 3a is disposed between the second MZI-based tunable frequency filter section 3b and the third MZI-based tunable frequency filter section 3c. Those skilled in the art will understand that the number of MZI-based tunable frequency filter sections is exemplary and non-limiting, and any suitable number, such as 1, 2, 3, 4, 5, 6, etc., can be envisioned depending on the specific requirements that the semiconductor tunable ring laser needs to meet.

[0048] 2, the fourth optical input port 21 of the second 1×2 MMI input splitter 19 of the second MZI-based tunable frequency filter section 3 b is disposed in optical communication with the closed-loop optical path of the laser cavity 2. The seventh optical output port 28 of the first 2×1 MMI output coupler 24 of the second MZI-based tunable frequency filter section 3 b is disposed in optical communication with the first optical input port 6 of the first 1×2 MMI input splitter 4 of the first MZI-based tunable frequency filter section 3 a.

[0049] The third optical output port 13 of the first 2×2 MMI output splitter 9 of the first MZI-based tunable frequency filter section 3 a is disposed in optical communication with the seventh optical input port 35 of the third 1×2 MMI input splitter 33 of the third MZI-based tunable frequency filter section 3 c. The fourth optical output port 14 of the first 2×2 MMI output splitter 9 of the first MZI-based tunable frequency filter section 3 a is configured and arranged as an optical monitoring port of the laser cavity 2. According to a second exemplary, non-limiting embodiment of the semiconductor tunable ring laser 1 shown in FIG. 2 , the fourth optical output port 14 is disposed in optical communication with a wavelength locker 120 of the optical emission monitoring assembly 110.

[0050] The tenth optical output port 42 of the second 2×1 MMI output combiner 38 of the third MZI-based tunable frequency filter section 3 c is placed in optical communication with the tenth optical input port 48 of the gain section 47. The twelfth optical output port 55 of the second 2×2 MMI output splitter 50 is placed in optical communication with the fourth optical input port 21 of the second 1×2 MMI input splitter 19 of the second MZI-based tunable frequency filter section 3 b.

[0051] 3 is a schematic top view of a third exemplary, non-limiting embodiment of a semiconductor tunable ring laser 1 according to the present invention, comprising a laser cavity 2 having a closed-loop optical path and an optical filter 3 disposed within the laser cavity 2. The optical filter 3 is configured as a transmission-type optical filter when the semiconductor tunable ring laser 1 is in use. The optical filter 3 comprises a first MZI-based tunable frequency filter section 3a, a second MZI-based tunable frequency filter section 3b, and a third MZI-based tunable frequency filter section 3c, as shown in FIGS. 1 and 2. However, the order in which the three MZI-based tunable frequency filter sections are arranged in a consecutive configuration inside the laser cavity 2 is different. According to a third exemplary, non-limiting embodiment of the semiconductor tunable ring laser 1 shown in FIG. 3, the order of the MZI-based tunable frequency filter sections is reversed compared to the order of the MZI-based tunable frequency filter sections shown in FIG. 1, i.e., the order as viewed from left to right: the third MZI-based tunable frequency filter section 3c is followed by the second MZI-based tunable frequency filter section 3b, which is followed by the first MZI-based tunable frequency filter section 3a. Those skilled in the art will understand that the number of MZI-based tunable frequency filter sections is exemplary and non-limiting. Any suitable number, such as 1, 2, 3, 4, 5, 6, etc., can be envisioned depending on the specific requirements that the semiconductor tunable ring laser needs to meet.

[0052] 3 , the seventh optical input port 35 of the third 1×2 MMI input splitter 33 of the third MZI-based tunable frequency filter section 3 c is disposed in optical communication with the closed-loop optical path of the laser cavity 2. The tenth optical output port 42 of the second 2×1 MMI output coupler 38 of the third MZI-based tunable frequency filter section 3 c is disposed in optical communication with the fourth optical input port 21 of the second 1×2 MMI input splitter 19 of the second MZI-based tunable frequency filter section 3 b. The seventh optical output port 28 of the first 2×1 MMI output coupler 24 of the second MZI-based tunable frequency filter section 3 b is disposed in optical communication with the first optical input port 6 of the first 1×2 MMI input splitter 4 of the first MZI-based tunable frequency filter section 3 a.

[0053] The third optical output port 13 of the first 2×2 MMI output splitter 9 of the first MZI-based tunable frequency filter section 3 a is disposed in optical communication with the tenth optical input port 48 of the gain section 47. The fourth optical output port 14 of the first 2×2 MMI output splitter 9 of the first MZI-based tunable frequency filter section 3 a is configured and arranged as an optical monitoring port of the laser cavity 2. According to the third exemplary, non-limiting embodiment of the semiconductor tunable ring laser 1 shown in FIG. 3 , the fourth optical output port 14 is disposed in optical communication with the wavelength locker 120 of the optical emission monitoring assembly 110. The twelfth optical output port 55 of the second 2×2 MMI output splitter 50 is disposed in optical communication with the seventh optical input port 35 of the third 1×2 MMI input splitter 33 of the third MZI-based tunable frequency filter section 3 c.

[0054] Those skilled in the art will understand that each of the three above-described exemplary, non-limiting embodiments of the semiconductor tunable ring laser 1 according to the present invention allows for the first non-zero fraction T1 of optical radiation to be removed directly from within the laser cavity 2, i.e., from the amount of optical radiation that constitutes the main optical output of the semiconductor tunable ring laser, rather than after or outside the laser cavity, as is done in semiconductor tunable ring lasers known in the art. As described above, the semiconductor tunable ring laser 1 according to the present invention has improved overall efficiency compared to semiconductor tunable ring lasers known in the art, despite the removal of the first non-zero fraction T1 of optical radiation and any optical losses associated with lasing wavelength stabilization purposes.

[0055] For each of the three exemplary non-limiting embodiments of the semiconductor tunable ring laser 1 according to the present invention, the first MZI-based tunable frequency filter section 3a is configured to have a first free spectral range, the second MZI-based tunable frequency filter section 3b is configured to have a second free spectral range, and the third MZI-based tunable frequency filter section 3c is configured to have a third free spectral range. The first free spectral range, the second free spectral range, and the third free spectral range are different from each other. In this way, the stability of the semiconductor tunable ring laser 1 according to the present invention can be improved.

[0056] Furthermore, each of the three above-described exemplary, non-limiting embodiments of the semiconductor tunable ring laser 1 according to the present invention can be implemented as an InP-based tunable ring laser. As noted above, InP-based semiconductor materials are the semiconductor materials of choice for fabricating semiconductor tunable ring lasers, which can be used, by way of example and not exclusively, in telecommunications, light detection and ranging (LIDAR), or sensor applications. InP-based technology enables the monolithic integration of both active components, such as light-emitting and / or light-absorbing optical devices, and passive components, such as light-guiding and / or light-switching optical devices, in one PIC on a single die.

[0057] FIG. 4 is a schematic diagram of the output transmission as a function of the phase difference of an MZI-based tunable frequency filter section including a 1×2 MMI input splitter configured to achieve a 50 / 50 split ratio and a 2×1 MMI output combiner configured to achieve a 50 / 50 combination ratio. Those skilled in the art will understand that the diagram of output transmission versus phase difference shown in FIG. 4 applies to the second MZI-based tunable frequency filter section 3b and the third MZI-based tunable frequency filter section 3c shown in FIGS. 1, 2, and 3. For clarity, the second MZI-based tunable frequency filter section 3b will be referred to in the following description. FIG. 4 shows that for a 180-degree phase difference between the third light-guiding structure 29 and the fourth light-guiding structure 31, full transmission to the fourth light-guiding structure 31 is achieved, and no transmission to the third light-guiding structure 29 is achieved. When using the second MZI-based tunable frequency filter section 3b to extract radiation from the laser cavity 2, the second MZI-based tunable frequency filter section 3b must be biased a little away from this point, i.e., a 180-degree phase difference between the third light-guiding structure 29 and the fourth light-guiding structure 31. This requires a control loop that must not interfere with any other control loops already running, such as, for example, a control loop for stabilizing the lasing wavelength of a semiconductor tunable ring laser. As mentioned above, the insight of the present invention is that the coupling ratio of the MMI output coupler can be chosen to ensure that at least a minimum amount of light will always reach the tap port or the monitoring port.

[0058] According to the above insight, the x / (100-x) splitting ratio, where x is not equal to 50, of the first 2×2 MMI output splitter 9 of the first MZI-based tunable frequency filter section 3a shown in FIGS. 1-3 enables a first non-zero fraction T1 of the optical radiation to always be present at the fourth optical output port 14 of the first 2×2 MMI output splitter 9, regardless of the phase relationship between the optical radiation at the second optical input port 11 of the first 2×2 MMI output splitter 9 and the optical radiation at the third optical input port 12 of the first 2×2 MMI output splitter 9. An advantage of the above insight is that a control loop is no longer required to ensure that a minimum amount of optical radiation is available for wavelength locking and / or power monitoring outside the laser cavity. As a result, fewer components are required to achieve lasing wavelength stabilization of the semiconductor tunable ring laser 1 according to the present invention compared to semiconductor tunable ring lasers known in the art. To enable wavelength locking and / or power monitoring outside the laser cavity, the first non-zero fraction T1 of the optical radiation is sufficient if it is, for example, in the range of 1% to 15% of the amount of optical radiation T0 incident on the second optical input port 11 of the first 2x2 MMI output splitter 9 and on the third optical input port 12 of the first 2x2 MMI output splitter 9.

[0059] 5 is a schematic diagram of the output transmission as a function of the phase difference of an MZI-based tunable frequency filter section comprising a 1×2 MMI input splitter configured to achieve a 50 / 50 split ratio and a 2×2 MMI output splitter configured to have a 72 / 28 split ratio. Those skilled in the art will appreciate that the above-described MZI-based tunable frequency filter section can be implemented as an exemplary, non-limiting embodiment of the first MZI-based tunable frequency filter section 3a of the semiconductor tunable ring laser 1 according to the present invention. Considering any one of FIGS. 1-3, it becomes clear that the 1×2 MMI input splitter with a 50 / 50 split ratio corresponds to the first 1×2 MMI input splitter 4 of the first MZI-based tunable frequency filter section 3a, and that the 2×2 MMI output splitter with a 72 / 28 split ratio corresponds to the first 2×2 MMI output splitter 9 of the first MZI-based tunable frequency filter section 3a.

[0060] 5 shows that in the case of a 180-degree phase difference between the first light-guiding structure 15 and the second light-guiding structure 17 of the first MZI-based tunable frequency filter section 3a, a maximum transmission of 95% to the second light-guiding structure 17 is achieved, and a minimum transmission of 5% to the first light-guiding structure 15 is always achieved. In this way, a control loop to ensure that a minimum amount of light reaches the wavelength locker 120 is no longer required, since it can be guaranteed by design that at least 5% of the optical radiation incident on the second optical input port 11 of the first 2×2 MMI output splitter 9 and on the third optical input port 12 of the first 2×2 MMI output splitter 9 reaches the wavelength locker 120, which is arranged in optical communication with the fourth optical output port 14 of the first 2×2 MMI output splitter 9, which serves as an optical monitoring port for the laser cavity 2.

[0061] As described above, by directly removing the first non-zero fraction T1 of the optical radiation from within the laser cavity 2 at the fourth optical output port 14 of the first 2×2 MMI power splitter 9, it is possible to avoid the use of a lossy coupler placed after or outside the laser cavity 2, as is done in semiconductor tunable ring lasers known in the art. Wafer probe measurements have shown that the omission of the lossy coupler results in a reduction in the overall optical loss of the semiconductor tunable ring laser 1 according to the present invention. 6A is a diagram comparing the total optical output power of three semiconductor tunable ring lasers A, B, and C known in the art, each of which includes a conventional 85 / 15 power splitter placed after or outside the laser cavity to provide optical radiation that can be used for lasing wavelength stabilization purposes, and a semiconductor tunable ring laser D according to the present invention, in which the laser cavity is directly tapped using a 2×2 MMI power splitter configured to achieve a 72 / 28 split ratio to provide optical radiation that can be used for lasing wavelength stabilization purposes. FIG. 6A shows that the optical output power of the semiconductor tunable ring laser D according to the present invention is higher than that of each of the known semiconductor tunable ring lasers A, B, and C. This is not simply because the total photo-induced current of semiconductor tunable ring laser D, which is the sum of the photo-induced current resulting from the primary optical output of semiconductor tunable ring laser D and the photo-induced current resulting from the first non-zero fraction T1 of optical radiation directed to the optical sensor of the control circuit for stabilizing the lasing wavelength, is higher than the total photo-induced current of each of the known semiconductor tunable ring lasers A, B, and C, and therefore a smaller non-zero fraction T1 of optical radiation is removed in the case of semiconductor tunable ring laser D in accordance with the present invention compared to the respective non-zero fraction T1 of optical radiation removed in the case of known semiconductor tunable ring lasers A, B, and C. This is shown in FIG.

[0062] 6A and 6B, one skilled in the art will appreciate that directly removing the first non-zero fraction T1 of the optical radiation from within the laser cavity results in a reduced overall optical loss for the semiconductor tunable ring laser D in accordance with the present invention compared to the overall optical losses for the three known semiconductor tunable ring lasers A, B, and C, in which the first non-zero fraction T1 of the optical radiation is removed after or outside the laser cavity. Thus, the semiconductor tunable ring laser D in accordance with the present invention has improved overall efficiency compared to the three known semiconductor tunable ring lasers A, B, and C.

[0063] 7 is a schematic top view of a first exemplary, non-limiting embodiment of a PIC 100 according to the present invention, comprising a semiconductor tunable ring laser 1 according to the present invention. The semiconductor tunable ring laser 1 can be construed as being monolithically integrated with other optoelectronic devices (not shown) of the PIC 100.

[0064] According to an exemplary, non-limiting embodiment of the PIC (not shown), the semiconductor tunable ring laser can be hybrid-integrated with other optoelectronic devices in the PIC. An advantage of enabling hybrid integration of the semiconductor tunable ring laser according to the present invention is that the semiconductor tunable ring laser can be used in any semiconductor technology domain, such as the domain of silicon photonics. Another advantage of enabling hybrid integration of the semiconductor tunable ring laser according to the present invention is that the semiconductor tunable ring laser can be replaced. Replacement of the semiconductor tunable ring laser may be required, for example, in the event of laser malfunction or after a laser outage.

[0065] An advantage of monolithically integrating a semiconductor tunable ring laser with other optoelectronic devices (not shown) on the same semiconductor substrate as that illustrated schematically in FIG. 7 is that the monolithic integration of the semiconductor tunable ring laser 1 and other optoelectronic components may be less cumbersome and, in some cases, require less die area than the hybrid integration. Therefore, the costs associated with the monolithic integration of active and passive optoelectronic devices may be less than the costs associated with the hybrid integration. Additionally, monolithic integration may enable the PIC 100 to have a smaller footprint, which is beneficial for reducing the cost of the PIC.

[0066] The PIC 100 can be an InP-based PIC. Those skilled in the art will appreciate that the most versatile technology platform for PICs, particularly for PICs that can be applied in telecommunications, LIDAR, or sensor applications, uses wafers comprising InP-based semiconductor materials. InP-based technology allows for the monolithic integration of both active components, such as light-emitting and / or light-absorbing optical devices, and passive components, such as light-guiding and / or light-switching optical devices, in one PIC on a single die.

[0067] Based on the above, those skilled in the art will understand that the PIC 100 according to the present invention can benefit from the advantages offered by the semiconductor tunable ring laser 1 according to the present invention.

[0068] 8 is a schematic diagram of a first exemplary, non-limiting embodiment of an optoelectronic system 200 according to the present invention, comprising a PIC 100 according to the present invention. The optoelectronic system 200 can be used, by way of example and not exclusively, in telecommunications applications, LIDAR, or sensor applications. The optoelectronic system 200 can be, for example, one of a transmitter, a receiver, a transceiver, a coherent transmitter, a coherent receiver, and a coherent transceiver. Based on the above, one skilled in the art will understand that the optoelectronic system 200 according to the present invention can benefit from advantages provided by the PIC 100 according to the present invention.

[0069] The present invention can be generally described as relating to a semiconductor tunable ring laser 1 comprising a laser cavity 2 having a closed-loop optical path and an optical filter 3 disposed within the laser cavity 2 and configured as a transmission-type optical filter when the semiconductor tunable ring laser 1 is in use. The optical filter 3 comprises a first MZI-based tunable frequency filter section 3a comprising a first 2×2 MMI output splitter 9 having an x / (100−x) splitting ratio, where x is not equal to 50. The first 2×2 MMI output splitter 9 provides an optical monitoring port to the laser cavity 2, at which a first non-zero fraction T1 of optical radiation is presented, sufficient for wavelength locking and / or power monitoring outside the laser cavity 2. The present invention also relates to a PIC 100 comprising the semiconductor tunable ring laser 1 according to the present invention, and to an optoelectronic system 200 comprising such a PIC 100.

[0070] It will be apparent to those skilled in the art that the scope of the present invention is not limited to the examples discussed above, and that several amendments and modifications thereof are possible without departing from the scope of the present invention as defined by the appended claims. In particular, combinations of the inherent features of various aspects of the present invention may be made. An aspect of the present invention may be further advantageously enhanced by adding features described with respect to another aspect of the present invention. While the present invention has been illustrated and described in the drawings and description, such illustration and description should be considered illustrative or exemplary only and not restrictive.

[0071] The present invention is not limited to the disclosed embodiments. Variations of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the figures, the description, and the appended claims. In the claims, the word "comprising" does not exclude other steps or elements, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be advantageously used. Any reference signs in the claims should not be interpreted as limiting the scope of the present invention.

Claims

1. A semiconductor wavelength tunable ring laser (1), a laser cavity (2) with a closed-loop optical path; an optical filter (3) arranged in the laser cavity (2) and configured as a transmission type optical filter when the semiconductor tunable ring laser (1) is in use, a first MZI-based tunable frequency filter section (3a), a first 1x2 MMI input splitter (4), a first optical input port (6) arranged in optical communication with the closed loop optical path of the laser cavity (2); a first optical output port (7), and ○ Second optical output port (8) a first 1x2 MMI input splitter (4) comprising a first multimode waveguide section (5) provided with: a first 2x2 MMI output divider (9), a second optical input port (11), a third optical input port (12), a third optical output port (13) disposed in optical communication with the closed-loop optical path of the laser cavity (2); and a fourth optical output port (14) configured and arranged as an optical monitoring port for said laser cavity (2); a first 2x2 MMI output splitter (9) comprising a second multimode waveguide section (10) provided with a first light guiding structure (15) comprising a first optical waveguide (16) arranged to optically interconnect the first optical output port (7) of the 1x2 MMI input splitter (4) and the second optical input port (11) of the first 2x2 MMI output splitter (9), the first light guiding structure (15) being configured to provide a first optical path length between the first optical output port (7) of the first 1x2 MMI input splitter (4) and the second optical input port (11) of the first 2x2 MMI output splitter (9); a second light-guiding structure (17) comprising a second optical waveguide (18) arranged to optically interconnect the second optical output port (8) of the first 1x2 MMI input splitter (4) and the third optical input port (12) of the first 2x2 MMI output splitter (9), the second light-guiding structure (17) being configured to provide a second optical path length between the second optical output port (8) of the first 1x2 MMI input splitter (4) and the third optical input port (12) of the first 2x2 MMI output splitter (9), the second optical path length being different from the first optical path length; and wherein the second multimode waveguide section (10) of the first 2x2 MMI output splitter (9) is configured to achieve an x / (100-x) splitting ratio, where x has a value not equal to 50, and the x / (100-x) splitting ratio is such that, when the semiconductor tunable ring laser (1) is in use, the x / (100-x) splitting ratio is equal to or greater than the phase relationship between the optical radiation at the second optical input port (11) of the first 2x2 MMI output splitter (9) and the optical radiation at the third optical input port (12) of the first 2x2 MMI output splitter (9). the amount of optical radiation T incident on the second optical input port (11) of the first 2x2 MMI output splitter (9) and on the third optical input port (12) of the first 2x2 MMI output splitter (9); ０ The first non-zero fraction T of the optical radiation １ is present at the fourth optical output port (14) of the first 2x2 MMI output splitter (9), and the first non-zero fraction T of optical radiation １ is sufficient to allow wavelength locking and / or power monitoring outside the laser cavity (2); and The amount of light radiation T ０ A second fraction T of the optical radiation ２ = 1 - T １ is present at the third optical output port (13) of the first 2x2 MMI output splitter (9), thereby remaining in the closed loop optical path of the laser cavity (2). a first MZI-based tunable frequency filter section (3a) selected to enable an optical filter (3) comprising: A semiconductor tunable ring laser (1) comprising:

2. 2. The semiconductor tunable ring laser (1) of claim 1, wherein the value of x is in the range of 64 to 80.

3. The optical filter (3) a second 1x2 MMI input splitter (19), a fourth optical input port (21) disposed in optical communication with the third optical output port (13) of the first 2x2 MMI output splitter (9) of the first MZI-based tunable frequency filter section (3 a), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); a fifth optical output port (22), and Sixth optical output port (23) a second 1x2 MMI input splitter (19) comprising a third multimode waveguide section (20) provided with: a first 2x1 MMI output combiner (24), a fifth optical input port (26); a sixth optical input port (27), and a seventh optical output port (28) disposed in optical communication with the closed-loop optical path of the laser cavity (2); a first 2x1 MMI output coupler (24) comprising a fourth multimode waveguide section (25) provided with a third light guiding structure (29) comprising a third optical waveguide (30) arranged to optically interconnect the fifth optical output port (22) of the second 1x2 MMI input splitter (19) and the fifth optical input port (26) of the first 2x1 MMI output coupler (24), the third light guiding structure (29) being configured to provide a third optical path length between the fifth optical output port (22) of the second 1x2 MMI input splitter (19) and the fifth optical input port (26) of the first 2x1 MMI output coupler (24); a fourth light guiding structure (31) comprising a fourth optical waveguide (32) arranged to optically interconnect the sixth optical output port (23) of the second 1x2 MMI input splitter (19) and the sixth optical input port (27) of the first 2x1 MMI output coupler (24), the fourth light guiding structure (31) being configured to provide a fourth optical path length between the sixth optical output port (23) of the second 1x2 MMI input splitter (19) and the sixth optical input port (27) of the first 2x1 MMI output coupler (24), the fourth optical path length being different from the third optical path length; 2. The semiconductor tunable ring laser (1) of claim 1, further comprising a second MZI-based tunable frequency filter section (3b) comprising:

4. The optical filter (3) a second 1x2 MMI input splitter (19), a fourth optical input port (21) disposed in optical communication with the third optical output port (13) of the first 2x2 MMI output splitter (9) of the first MZI-based tunable frequency filter section (3 a), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); a fifth optical output port (22), and Sixth optical output port (23) a second 1x2 MMI input splitter (19) comprising a third multimode waveguide section (20) provided with: a first 2x1 MMI output combiner (24), a fifth optical input port (26); a sixth optical input port (27), and a seventh optical output port (28) disposed in optical communication with the closed-loop optical path of the laser cavity (2); a first 2x1 MMI output coupler (24) comprising a fourth multimode waveguide section (25) provided with a third light guiding structure (29) comprising a third optical waveguide (30) arranged to optically interconnect the fifth optical output port (22) of the second 1x2 MMI input splitter (19) and the fifth optical input port (26) of the first 2x1 MMI output coupler (24), the third light guiding structure (29) being configured to provide a third optical path length between the fifth optical output port (22) of the second 1x2 MMI input splitter (19) and the fifth optical input port (26) of the first 2x1 MMI output coupler (24); a fourth light guiding structure (31) comprising a fourth optical waveguide (32) arranged to optically interconnect the sixth optical output port (23) of the second 1x2 MMI input splitter (19) and the sixth optical input port (27) of the first 2x1 MMI output coupler (24), the fourth light guiding structure (31) being configured to provide a fourth optical path length between the sixth optical output port (23) of the second 1x2 MMI input splitter (19) and the sixth optical input port (27) of the first 2x1 MMI output coupler (24), the fourth optical path length being different from the third optical path length; 3. The semiconductor tunable ring laser (1) of claim 2, further comprising a second MZI-based tunable frequency filter section (3b) comprising:

5. The optical filter (3) a third 1x2 MMI input splitter (33), a seventh optical input port (35) disposed in optical communication with the seventh optical output port (28) of the first 2×1 MMI output coupler (24) of the second MZI-based tunable frequency filter section (3 b), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); an eighth optical output port (36); and 9th optical output port (37) a third 1x2 MMI input splitter (33) comprising a fifth multimode waveguide section (34) provided with a third 1x2 MMI input splitter (33), the fifth multimode waveguide section (34) being configured to achieve a 50 / 50 split ratio; a second 2x1 MMI output combiner (38), an eighth optical input port (40); a ninth optical input port (41), and a tenth optical output port (42) disposed in optical communication with the closed-loop optical path of the laser cavity (2); a second 2x1 MMI output coupler (38) comprising a sixth multimode waveguide section (39) provided with a fifth light guiding structure (43) comprising a fifth optical waveguide (44) arranged to optically interconnect the eighth optical output port (36) of the third 1x2 MMI input splitter (33) and the eighth optical input port (40) of the second 2x1 MMI output coupler (38), the fifth light guiding structure (43) being configured to provide a fifth optical path length between the eighth optical output port (36) of the third 1x2 MMI input splitter (33) and the eighth optical input port (40) of the second 2x1 MMI output coupler (38); a sixth light guiding structure (45) comprising a sixth optical waveguide (46) arranged to optically interconnect the ninth optical output port (37) of the third 1x2 MMI input splitter (33) and the ninth optical input port (41) of the second 2x1 MMI output coupler (38), the sixth light guiding structure (45) being configured to provide a sixth optical path length between the ninth optical output port (37) of the third 1x2 MMI input splitter (33) and the ninth optical input port (41) of the second 2x1 MMI output coupler (38), the sixth light guiding structure (45) being different from the fifth optical path length; 4. The semiconductor tunable ring laser (1) of claim 3, further comprising a third MZI-based tunable frequency filter section (3c) comprising:

6. The optical filter (3) a third 1x2 MMI input splitter (33), a seventh optical input port (35) disposed in optical communication with the seventh optical output port (28) of the first 2×1 MMI output coupler (24) of the second MZI-based tunable frequency filter section (3 b), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); an eighth optical output port (36); and 9th optical output port (37) a third 1x2 MMI input splitter (33) comprising a fifth multimode waveguide section (34) provided with a third 1x2 MMI input splitter (33), the fifth multimode waveguide section (34) being configured to achieve a 50 / 50 split ratio; a second 2x1 MMI output combiner (38), an eighth optical input port (40); a ninth optical input port (41), and a tenth optical output port (42) disposed in optical communication with the closed-loop optical path of the laser cavity (2); a second 2x1 MMI output coupler (38) comprising a sixth multimode waveguide section (39) provided with a fifth light guiding structure (43) comprising a fifth optical waveguide (44) arranged to optically interconnect the eighth optical output port (36) of the third 1x2 MMI input splitter (33) and the eighth optical input port (40) of the second 2x1 MMI output coupler (38), the fifth light guiding structure (43) being configured to provide a fifth optical path length between the eighth optical output port (36) of the third 1x2 MMI input splitter (33) and the eighth optical input port (40) of the second 2x1 MMI output coupler (38); a sixth light guiding structure (45) comprising a sixth optical waveguide (46) arranged to optically interconnect the ninth optical output port (37) of the third 1x2 MMI input splitter (33) and the ninth optical input port (41) of the second 2x1 MMI output coupler (38), the sixth light guiding structure (45) being configured to provide a sixth optical path length between the ninth optical output port (37) of the third 1x2 MMI input splitter (33) and the ninth optical input port (41) of the second 2x1 MMI output coupler (38), the sixth light guiding structure (45) being different from the fifth optical path length; 5. The semiconductor tunable ring laser (1) of claim 4, further comprising a third MZI-based tunable frequency filter section (3c) comprising:

7. The closed loop optical path of the laser cavity (2) includes: a gain section (47) including: a tenth optical input port (48) disposed in optical communication with the tenth optical output port (42) of the second 2×1 MMI output coupler (38) of the third MZI-based tunable frequency filter section (3 c), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); and 11th optical output port (49) a gain section (47) comprising: a second 2x2 MMI output divider (50), an eleventh optical input port (52) disposed in optical communication with the eleventh optical output port (49) of the gain section (47), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); a twelfth optical input port (53) disposed in optical communication with an optical reflector (54) disposed outside the closed-loop optical path of the laser cavity (2); a twelfth optical output port (55) arranged in optical communication with the first optical input port (6) of the first 1x2 MMI input splitter (4) of the first MZI-based tunable frequency filter section (3 a), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); and a thirteenth optical output port (56) constructed and arranged to allow optical power to be coupled out of said laser cavity (2) for use in applications other than wavelength locking and / or power monitoring purposes; a second 2x2 MMI output splitter (50) comprising a seventh multimode waveguide section (51) provided with The semiconductor tunable ring laser (1) according to claim 5, wherein

8. The closed loop optical path of the laser cavity (2) includes: a gain section (47) including: a tenth optical input port (48) disposed in optical communication with the tenth optical output port (42) of the second 2×1 MMI output coupler (38) of the third MZI-based tunable frequency filter section (3 c), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); and 11th optical output port (49) a gain section (47) comprising: a second 2x2 MMI output divider (50), an eleventh optical input port (52) disposed in optical communication with an eleventh optical output port (49) of said gain section (47), and thereby in optical communication with said closed loop optical path of said laser cavity (2); a twelfth optical input port (53) disposed in optical communication with an optical reflector (54) disposed outside the closed-loop optical path of the laser cavity (2); a twelfth optical output port (55) arranged in optical communication with the first optical input port (6) of the first 1x2 MMI input splitter (4) of the first MZI-based tunable frequency filter section (3 a), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); and a thirteenth optical output port (56) constructed and arranged to allow optical power to be coupled out of said laser cavity (2) for use in applications other than wavelength locking and / or power monitoring purposes; a second 2x2 MMI output splitter (50) comprising a seventh multimode waveguide section (51) provided with The semiconductor tunable ring laser (1) according to claim 6, wherein

9. The optical filter (3) a second 1x2 MMI input splitter (19), a fourth optical input port (21) disposed in optical communication with the closed loop optical path of the laser cavity (2); a fifth optical output port (22), and a sixth optical output port (23); a second 1x2 MMI input splitter (19) comprising a third multimode waveguide section (20) provided with: a first 2x1 MMI output combiner (24), a fifth optical input port (26); a sixth optical input port (27), and a seventh optical output port (28) disposed in optical communication with the first optical input port (6) of the first 1×2 MMI input splitter (4) of the first MZI-based tunable frequency filter section (3 a), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); a first 2x1 MMI output coupler (24) comprising a fourth multimode waveguide section (25) provided with a third light guiding structure (29) comprising a third optical waveguide (30) arranged to optically interconnect the fifth optical output port (22) of the second 1x2 MMI input splitter (19) and the fifth optical input port (26) of the first 2x1 MMI output coupler (24), the third light guiding structure (29) being configured to provide a third optical path length between the fifth optical output port (22) of the second 1x2 MMI input splitter (19) and the fifth optical input port (26) of the first 2x1 MMI output coupler (24); a fourth light guiding structure (31) comprising a fourth optical waveguide (32) arranged to optically interconnect the sixth optical output port (23) of the second 1x2 MMI input splitter (19) and the sixth optical input port (27) of the first 2x1 MMI output coupler (24), the fourth light guiding structure (31) being configured to provide a fourth optical path length between the sixth optical output port (23) of the second 1x2 MMI input splitter (19) and the sixth optical input port (27) of the first 2x1 MMI output coupler (24), the fourth optical path length being different from the third optical path length; 2. The semiconductor tunable ring laser (1) of claim 1, further comprising a second MZI-based tunable frequency filter section (3b) comprising:

10. The optical filter (3) a second 1x2 MMI input splitter (19), a fourth optical input port (21) disposed in optical communication with the closed loop optical path of the laser cavity (2); a fifth optical output port (22), and Sixth optical output port (23) a second 1x2 MMI input splitter (19) comprising a third multimode waveguide section (20) provided with: a first 2x1 MMI output combiner (24), a fifth optical input port (26); a sixth optical input port (27), and a seventh optical output port (28) disposed in optical communication with the first optical input port (6) of the first 1×2 MMI input splitter (4) of the first MZI-based tunable frequency filter section (3 a), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); a first 2x1 MMI output coupler (24) comprising a fourth multimode waveguide section (25) provided with a third light guiding structure (29) comprising a third optical waveguide (30) arranged to optically interconnect the fifth optical output port (22) of the second 1x2 MMI input splitter (19) and the fifth optical input port (26) of the first 2x1 MMI output coupler (24), the third light guiding structure (29) being configured to provide a third optical path length between the fifth optical output port (22) of the second 1x2 MMI input splitter (19) and the fifth optical input port (26) of the first 2x1 MMI output coupler (24); a fourth light guiding structure (31) comprising a fourth optical waveguide (32) arranged to optically interconnect the sixth optical output port (23) of the second 1x2 MMI input splitter (19) and the sixth optical input port (27) of the first 2x1 MMI output coupler (24), the fourth light guiding structure (31) being configured to provide a fourth optical path length between the sixth optical output port (23) of the second 1x2 MMI input splitter (19) and the sixth optical input port (27) of the first 2x1 MMI output coupler (24), the fourth optical path length being different from the third optical path length; 3. The semiconductor tunable ring laser (1) of claim 2, further comprising a second MZI-based tunable frequency filter section (3b) comprising:

11. The optical filter (3) a third 1x2 MMI input splitter (33), a seventh optical input port (35) disposed in optical communication with the third optical output port (13) of the first 2x2 MMI output splitter (9) of the first MZI-based tunable frequency filter section (3 a), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); an eighth optical output port (36); and 9th optical output port (37) a third 1x2 MMI input splitter (33) comprising a fifth multimode waveguide section (34) provided with a third 1x2 MMI input splitter (33), the fifth multimode waveguide section (34) being configured to achieve a 50 / 50 split ratio; a second 2x1 MMI output combiner (38), an eighth optical input port (40); a ninth optical input port (41), and a tenth optical output port (42) disposed in optical communication with the closed-loop optical path of the laser cavity (2); a second 2x1 MMI output coupler (38) comprising a sixth multimode waveguide section (39) provided with a fifth light guiding structure (43) comprising a fifth optical waveguide (44) arranged to optically interconnect the eighth optical output port (36) of the third 1x2 MMI input splitter (33) and the eighth optical input port (40) of the second 2x1 MMI output coupler (38), the fifth light guiding structure (43) being configured to provide a fifth optical path length between the eighth optical output port (36) of the third 1x2 MMI input splitter (33) and the eighth optical input port (40) of the second 2x1 MMI output coupler (38); a sixth light guiding structure (45) comprising a sixth optical waveguide (46) arranged to optically interconnect the ninth optical output port (37) of the third 1x2 MMI input splitter (33) and the ninth optical input port (41) of the second 2x1 MMI output coupler (38), the sixth light guiding structure (45) being configured to provide a sixth optical path length between the ninth optical output port (37) of the third 1x2 MMI input splitter (33) and the ninth optical input port (41) of the second 2x1 MMI output coupler (38), the sixth light guiding structure (45) being different from the fifth optical path length; 10. The semiconductor tunable ring laser (1) of claim 9, further comprising a third MZI-based tunable frequency filter section (3c) comprising:

12. The optical filter (3) a third 1x2 MMI input splitter (33), a seventh optical input port (35) disposed in optical communication with the third optical output port (13) of the first 2x2 MMI output splitter (9) of the first MZI-based tunable frequency filter section (3 a), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); an eighth optical output port (36); and 9th optical output port (37) a third 1x2 MMI input splitter (33) comprising a fifth multimode waveguide section (34) provided with a third 1x2 MMI input splitter (33), the fifth multimode waveguide section (34) being configured to achieve a 50 / 50 split ratio; a second 2x1 MMI output combiner (38), an eighth optical input port (40); a ninth optical input port (41), and a tenth optical output port (42) disposed in optical communication with the closed-loop optical path of the laser cavity (2); a second 2x1 MMI output coupler (38) comprising a sixth multimode waveguide section (39) provided with a fifth light guiding structure (43) comprising a fifth optical waveguide (44) arranged to optically interconnect the eighth optical output port (36) of the third 1x2 MMI input splitter (33) and the eighth optical input port (40) of the second 2x1 MMI output coupler (38), the fifth light guiding structure (43) being configured to provide a fifth optical path length between the eighth optical output port (36) of the third 1x2 MMI input splitter (33) and the eighth optical input port (40) of the second 2x1 MMI output coupler (38); a sixth light guiding structure (45) comprising a sixth optical waveguide (46) arranged to optically interconnect the ninth optical output port (37) of the third 1x2 MMI input splitter (33) and the ninth optical input port (41) of the second 2x1 MMI output coupler (38), the sixth light guiding structure (45) being configured to provide a sixth optical path length between the ninth optical output port (37) of the third 1x2 MMI input splitter (33) and the ninth optical input port (41) of the second 2x1 MMI output coupler (38), the sixth light guiding structure (45) being different from the fifth optical path length; 11. The semiconductor tunable ring laser (1) of claim 10, further comprising a third MZI-based tunable frequency filter section (3c) comprising:

13. The closed loop optical path of the laser cavity (2) includes: a gain section (47) including: a tenth optical input port (48) disposed in optical communication with the tenth optical output port (42) of the second 2×1 MMI output coupler (38) of the third MZI-based tunable frequency filter section (3 c), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); and 11th optical output port (49) a gain section (47) comprising: a second 2x2 MMI output divider (50), an eleventh optical input port (52) disposed in optical communication with the eleventh optical output port (49) of the gain section (47), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); a twelfth optical input port (53) disposed in optical communication with an optical reflector (54) disposed outside the closed-loop optical path of the laser cavity (2); a twelfth optical output port (55) arranged in optical communication with the fourth optical input port (21) of the second 1x2 MMI input splitter (19) of the second MZI-based tunable frequency filter section (3b), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); a thirteenth optical output port (56) constructed and arranged to allow optical power to be coupled out of said laser cavity (2) for use in applications other than wavelength locking and / or power monitoring purposes; a second 2x2 MMI output splitter (50) comprising a seventh multimode waveguide section (51) provided with The semiconductor tunable ring laser (1) according to claim 11, wherein

14. The optical filter (3) a third 1x2 MMI input splitter (33), a seventh optical input port (35) disposed in optical communication with the closed loop optical path of the laser cavity (2); an eighth optical output port (36); and 9th optical output port (37) a third 1x2 MMI input splitter (33) comprising a fifth multimode waveguide section (34) provided with a third 1x2 MMI input splitter (33), the fifth multimode waveguide section (34) being configured to achieve a 50 / 50 split ratio; a second 2x1 MMI output combiner (38), an eighth optical input port (40); a ninth optical input port (41), and a tenth optical output port (42) disposed in optical communication with the fourth optical input port (21) of the second 1×2 MMI input splitter (19) of the second MZI-based tunable frequency filter section (3 b), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); a second 2x1 MMI output coupler (38) comprising a sixth multimode waveguide section (39) provided with a fifth light guiding structure (43) comprising a fifth optical waveguide (44) arranged to optically interconnect the eighth optical output port (36) of the third 1x2 MMI input splitter (33) and the eighth optical input port (40) of the second 2x1 MMI output coupler (38), the fifth light guiding structure (43) being configured to provide a fifth optical path length between the eighth optical output port (36) of the third 1x2 MMI input splitter (33) and the eighth optical input port (40) of the second 2x1 MMI output coupler (38); a sixth light guiding structure (45) comprising a sixth optical waveguide (46) arranged to optically interconnect the ninth optical output port (37) of the third 1x2 MMI input splitter (33) and the ninth optical input port (41) of the second 2x1 MMI output coupler (38), the sixth light guiding structure (45) being configured to provide a sixth optical path length between the ninth optical output port (37) of the third 1x2 MMI input splitter (33) and the ninth optical input port (41) of the second 2x1 MMI output coupler (38), the sixth light guiding structure (45) being different from the fifth optical path length; 10. The semiconductor tunable ring laser (1) of claim 9, further comprising a third MZI-based tunable frequency filter section (3c) comprising:

15. The closed loop optical path of the laser cavity (2) includes: a gain section (47) including: a tenth optical input port (48) arranged in optical communication with the third optical output port (13) of the first 2x2 MMI output splitter (9) of the first MZI-based tunable frequency filter section (3 a), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); and 11th optical output port (49) a gain section (47) comprising: a second 2x2 MMI output divider (50), an eleventh optical input port (52) disposed in optical communication with the eleventh optical output port (49) of the gain section (47), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); a twelfth optical input port (53) disposed in optical communication with an optical reflector (54) disposed outside the closed-loop optical path of the laser cavity (2); a twelfth optical output port (55) arranged in optical communication with the seventh optical input port (35) of the third 1x2 MMI input splitter (33) of the third MZI-based tunable frequency filter section (3c), and thereby in optical communication with the closed-loop optical path of the laser cavity (2); a thirteenth optical output port (56) constructed and arranged to allow optical power to be coupled out of said laser cavity (2) for use in applications other than wavelength locking and / or power monitoring purposes; a second 2x2 MMI output splitter (50) comprising a seventh multimode waveguide section (51) provided with The semiconductor tunable ring laser (1) according to claim 14, wherein

16. 6. The semiconductor tunable ring laser of claim 5, wherein the first MZI-based tunable frequency filter section (3 a) is configured to have a first free spectral range, the second MZI-based tunable frequency filter section (3 b) is configured to have a second free spectral range, and the third MZI-based tunable frequency filter section (3 c) is configured to have a third free spectral range, and the first free spectral range, the second free spectral range, and the third free spectral range are different from each other.

17. 12. The semiconductor tunable ring laser (1) of claim 11, wherein the first MZI-based tunable frequency filter section (3 a) is configured to have a first free spectral range, the second MZI-based tunable frequency filter section (3 b) is configured to have a second free spectral range, and the third MZI-based tunable frequency filter section (3 c) is configured to have a third free spectral range, and the first free spectral range, the second free spectral range, and the third free spectral range are different from each other.

18. 15. The semiconductor tunable ring laser (1) of claim 14, wherein the first MZI-based tunable frequency filter section (3 a) is configured to have a first free spectral range, the second MZI-based tunable frequency filter section (3 b) is configured to have a second free spectral range, and the third MZI-based tunable frequency filter section (3 c) is configured to have a third free spectral range, and the first free spectral range, the second free spectral range, and the third free spectral range are different from each other.

19. 2. The semiconductor tunable ring laser (1) of claim 1, wherein the semiconductor tunable ring laser (1) is an InP-based tunable ring laser.

20. A photonic integrated circuit PIC (100) comprising a semiconductor tunable ring laser (1) according to claim 1, the PIC (100) being a hybrid integrated PIC or a monolithically integrated PIC.

21. 21. The PIC (100) of claim 20, comprising an optical radiation monitoring assembly (110) optically connected to the fourth optical output port (14) of the first 2x2 MMI output splitter (9) of the first MZI-based wavelength-tunable frequency filter section (3a) of the optical filter (3) of the semiconductor wavelength-tunable ring laser (1).

22. 21. An optoelectronic system (200) comprising the PIC (100) of claim 20, wherein the optoelectronic system (200) is one of a transmitter, a receiver, a transceiver, a coherent transmitter, a coherent receiver, and a coherent transceiver.

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