WO2013130065A1

WO2013130065A1 - Unidirectional ring lasers

Info

Publication number: WO2013130065A1
Application number: PCT/US2012/027107
Authority: WO
Inventors: Di Liang; David Fattal
Original assignee: Hewlett Packard Development Co LP
Current assignee: Hewlett Packard Development Co LP
Priority date: 2012-02-29
Filing date: 2012-02-29
Publication date: 2013-09-06
Anticipated expiration: 2014-08-29
Also published as: EP2823539A1; US20150333479A1; US20150010035A1; CN104081597A; US9419405B2; EP2823539A4; KR20140130674A; US9130342B2

Description

UNIDIRECTIONAL RING LASERS

BACKGROUND

[OOOI3 A traveling-wave resonator laser, such as a ring laser, may be associated with bidirectional Iasing in two counter-propagating directions. The resonator may unpredictably las© in eiiher or both directions, regardless of input current biasing level, thereby reducing laser emission efficiency in a desired direction.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

IOOO23 FIG. 1 is a block diagram of a laser including a reflector according to an example.

IOOO33 FIGS. 2A-2E are block diagrams of reflectors according to examples. [OQQ43 FIG. 3 is a chart of a phase condition of a laser according to an example.

[0005] FIG. 4 is a block diagram of a laser array including a reflector according to an example.

[0006} FIG. 5 is a block diagram of a laser including a reflector according to an example.

[00073 FIG. 6 is a block diagram of a laser array including a plurality of reflectors according to an example.

[00083 FSG, 7 is a block diagram of a laser including a reflector according to an example.

[00091 FIG. 8 is a block diagram of a laser array including a plurality of reflectors according to an example. |0010J FIG. 9 is a flow chart based on generating light at an active ring according to an example.

DETAILED DESCRIPTION

[0011J Laser applications may include optica! interconnects, e.g., photonic data Sinks, where unidirectional lasing may be desirable for efficient and robust signal communication. Unidirectional lasing may be achieved based on the following examples, even when using a !aser cavity where lasing can take place at two counter-propagating directions, simultaneously or alternatively, due to the fundamental traveling-wave nature of the laser cavity. Example laser systems may trigger domination of unidirectional lasing in a traveling-wave resonator (e.g., a laser cavity) based on a ref Sector associated with a waveguide coupled to the resonator to emit iight. The reflector may encourage and/or promote unidirectional lasing domination in a preferred direction, by feeding/building lasing in the preferred direction to break symmetry/energy balance in the laser resonator cavity and lead to unidirectional domination.

[00123 Domination of unidirectional lasing may be associated with a gain/loss imbalance, or other asymmetry and/or manipulation, of the energy balance associated with the counter-propagating directions. Thus, the domination of unidirectional lasing is to favor a lasing direction by using available energy in the resonator for that lasing direction, while shutting off lasing in the other direction. Example lasers may be based on other traveling- wave resonators besides microrings. Thus, systems based on the example below may enable low power consumption, high modulation speeds, small footprints, and flexibility to form wavelength division multiplexing {WDM) light sources.

[00133 In an example, a lase system may include an active ring, a passive waveguide, and a reflector. The active ring is to generate light, e.g., based on a gain medium responsive to energy pumped into the active ring. The passive waveguide is associated with the active ring to capture generated light. The passive waveguide may be a bus waveguide, and the active ring may be coupled to the passive waveguide at a coupling point on the passive waveguide. The reflector associated with the passive waveguide is to cause captured Sight from the waveguide to be coupied into the active ring to trigger domination of unidirectional iasing in the active ring to generate light. In an example, the reflector may reflect light emissions in one direction from the waveguide back to the active ring to trigger domination of iasing in the active ring in another direction. Reflectors may be complementary metal-oxide-semiconductor (CMOS) compatible, and may be formed during fabricatio of ring lasers and/or other components without adding complexity or cost (e.g., based on lithography). The laser systems may have a very small footprint and enable dense integration.

[001 | FIG. 1 is a block diagram of a laser 100 including a reflector 130 according to an example. The reflector 130 is associated with waveguide 120. The waveguide 120 is coupled to active ring 110 at coupling point 122. The reflector 30 is positioned a distance d 123 from the coupling point 122.

[00153 ^he active ring 110 is to generate Sight in response to energy pumped into the active ring 110. For exampie_; the active ring 10 may be an electrically-driven hybrid silicon microring {e.g., a ring structure having ill-V material gain epitaxial layers bonded to a substrate) having electrodes to receive an injection current from an external energy pump such as a current source. In alternate examples, the active ring 110 may generate light based on energy received from an optica! energy pump. The active ring 110 may generate light bidirectionally, e.g., in counterclockwise 112 and/or clockwise 1 4 directions, and the directionality of light generation may be unpredictable due to characteristics of traveling-wave resonators such as a microing laser cavity.

[0016 in a laser resonator cavity, intensity of generated light is related to power level of energy pumped into the ring (e.g., a level of injection current or level of pump light intensity), such that intensity of generated light increases as energy pumping level increases. However, due to the bidirectional nature of the light propagation in a ring cavity, the directionality light generation in the active ring 110 could begin as simultaneously bidirectional, then switch to clockwise 114 (or counterclockwise 112), then switch to counterclockwise 112 (or counterclockwise 114), whether input power is varied or held constant. Dominant Iasing direction may be related to carrier injection level or other energy pumping into the laser cavity, but may be unpredictable when no steps are taken to break the physical symmetry (e.g., equal gain and loss) associated with the two iasing directions.

[0017J The laser 100 is to provide emitted light 128 to/from the waveguide 122. Thus, it is desirable to trigger domination of unidirectional Iasing 118 consistent with the desired direction of emitted light 126. Reflector 130 may be used to trigger domination of unidirectional Iasing 118. Clockwise 114 and counterclockwise 112 Iasing may occur at the same or similar wavelengths,

[0018] Light from clockwise 114 light generation may be coupled, including partially coupled, to the waveguide 120 as captured light 124. Reflector 130 may reflect, including partially reflected, the captured light 124, as reflected light 125. The reflected light 125 may be coupled, including partially coupled, into the active ring 110 as coupled light 1 16, traveling in the counterclockwise 112 direction. The coupled light 16 is to unbalance the counterclockwise 112 and clockwise 114 light generation in the active ring 110, and trigger domination of unidirectional iasing 18, Unidirectional iasing 1 18 enables the active ring 1 0 to efficiently convert received pump energy into emitted light 126, Reflected Sight 125, including a portion remaining that is not coupled into the active ring 1 0, may remain in the waveguide 120 and join with outcoupled counterclockwise 112 emissions (including emissions based on domination of unidirectional Iasing). The joined Sight may have identical or similar wavelengths, based on various factors including active ring 1 10, external energy pump, distance d, waveguide 120, coupling point 122, distance between the active ring 110 and waveguide 120, and other factors.

[0019] Although the illustrated examples show unidirectionai iasing and light emission based o the counterclockwise direction, other examples may be based on the clockwise direction with corresponding changes to the arrangement of components such as the reflector,

[00203 IGS. 2A-2E are block diagrams of reflectors 232A-232E according to examples. The reflectors may be added to an active ring and waveguide without having to modify the ring laser (e.g., without having to adjust internal iosses of the active ring). Reflectors may be passive to operate without consuming power, and may be tuned based on heating them or applying current/charge to adjust the reflection bandwidth.

IOO213 IG. 2A shows a facet reflector 232A coupled to the waveguide 220A. The facet reflector 232A may include a reflection coating, such as a pa rtiai/low reflection and full/high reflection coating, and may be a smooth vertical facet. The facet reflector 232A may operate over the entire wavelength range supported by the laser, ring, and/or waveguide, or a subset of wavelengths. The facet reflector 232A may provide a compact reflector structure that may be fabricated on-chi with other components of the laser system.

[ΟΟ223 FIG. 2B shows a teardrop reflector 232B coupled to the waveguide 220B via a y-branch 229B. Teardrop reflector 232B may equally split a Sight beam from the waveguide into two streams based on the y-branch 229B and teardrop 2348, and may reroute the split streams back to the same waveguide simultaneously. Although examples are shown with a y-branch, other combiners may be used in place of the y-branch, e.g., multimod interferometer ( Ml), directional coupler, or other connector. The teardrop reflector 232B may split the beam unevenly, and may support the entire wavelength range or a subset. The teardrop reflector 232B, including the teardrop 234B and y-branch 2298 may be fabricated on-chip, such as by patterning using photolithography, [0023] F!G. 2C shows a passive ring reflector 232C coupled to the waveguide 220C. Passive ring reflector 232C may include passive ring 236C, y-branch 229C, upper waveguide 235C, and lower waveguide 237C, The passive ring reflector 232C may include functionality similar to the teardrop reflector 232B. The passive ring reflector 232C also may function as a wavelength-selective add/drop component. The passive ring reflector 232C may include a resonance wavelength to be reflected, and may pass other wavelengths. For example, the passive ring reflector 232C may match its resonance wavelength with certain iasing wavelengths, such as the primary Iasing wavelength(s of the active ring. Thus, the passive ring reflector 232C may reflect only primary waveiength(s) λ₀, and allow other Iasing wavelengths {Kf , λ₂. As, . . ,) to be emitted from the open ports of the upper waveguide 235C and Iower waveguide 237C. The passive ring reflector 232C may trigger domination of unidirectional iasing at a particuiar wavelength (λο) associated with the passive ring reflector 232C, even when the active ring supports multiple iasing wavelengths. Accordingly, the light power reflected and coupled back into the active ring resonator cavity may trigger the domination of unidirectional iasing and light emission to the desired output port of the laser, providing single- wavelength u redirection ally dominated iasing. Thus, the passive ring reflector 232C may enable single-wavelength output for ring lasers that would otherwise !ase in multiple wavelengths, such as long-cavity ring lasers having a small free spectral range (FSR). The reflection bandwidth of the passive ring reflector 238C may be chosen to be much smaller than one FSR of the active ring laser.

[0024] FIG. 2D shows a passive ring reflector 232D coupied to the waveguide 220D, including a plurality of passive rings 236D, y-branch 220D (or other combiner), upper waveguide 235D, and iower waveguide 237D, The ring reflector 232 D may provide similar benefits as described above regarding passive ring reflector 232C, with the additional features of enabling muliipie specific wavelengths to be reflected by each of the plurality of passive rings 236D. The reflection bandwidth of the ring reflector 232D may be increased and/or flattened compared to a reflector based on a single passive ring, e.g., by vertically coupling the array of passive rings 236D between the upper waveguide 235D and lower waveguide 237D via the y-branch 229D. The large reflection bandwidth of the ring reflector 232D enables that reflector to trigger unidirectionally dominated Iasing for muitipie active rings coupled to the waveguide 220D.

[00253 FIG. 2E shows a Distributed Bragg Reflector (DBR) 232E coupled to the waveguide 220E. The DBR 232E may provide single-wavelength Iasing features similar to the passive ring reflector 238C, including designing the reflection bandwidth of the DBR 232E for long-cavity ring lasers having small FSR that usually lase in multiple wavelengths, such as designing the reflection bandwidth to be much smaller than one FSR of the ring laser. The reflection bandwidth of the DBR 232E may be increased and/or flattened by using a short and high-index-contrast grating structure for the DBR 232E, making the DB 232E perform similarly to the teardrop reflector 234B. Thus, similar to other reflector examples shown throughout, a singie DBR 232E may be used to trigger domination of unidirectional lasing in a bank of active ring !asers coupled to the waveguide 220E.

[0O26J FIG. 3 is a chart of a phase condition 340 of a laser according to an example. The phase condition 340 is shown in terms of wavelength 342 and intensity 344, for distance d 323 of 10 microns (solid black curve) and 100 microns (gray curve). Distance d 323 corresponds to the distance along the waveguide between the reflecto and coupling point, e.g., as shown in FS6. 1 regarding reflector 130 and coupling point 122.

[00273 The behavior shown in FIG. 3 may arise due to interference in a waveguide, where reflected counterclockwise light (e.g., 125 in Fig. 1 ) meets clockwise light (e.g., 112 in Fig, 1) such as at the coupiing point of the waveguide. Thus, the interference is affected by the optica! length between the reflector and the ring-waveguide coupiing point, i.e., d 323. Depending on the phase condition, which is related to d 323 and laser waveiength 342, constructive and/or destructive interference may occur, as shown in FIG. 3,

[0028] High vaiues for intensity 344, such as peaks where intensity 344 is approximately equal to 1 a,u., correspond with constructive interference. Low vaiues for intensity 344, such as valleys where intensity 344 is approximately 0 a.u., correspond to destructive interference. Thus, distance d 323 may be chosen in view of wavelength 342 to resuit in a peak at a desired waveiength 342. For example, a lasing waveiength 342 associated with the domination of unidirectional lasing of approximately 1540 nm may provide a peak intensity 344 when d 323 is chosen to be 00 μηι.

[0029] The value for d 323 also may he chosen to provide a large optica! bandwidth. In the example above, where d 323 was chosen to be 100 pm, the peak at approximateiy 1540 nm is sharp and fa!!s off rapidly as the waveiength 342 deviates from 1540 nm. Accordingly, the vaiue of distance d 323 of 100 pm may resuit in a shorter bandwidth wherein intensity 344 fails off as wavelength 342 fluctuates. |0030J in contrast to a large value of d 323 such as 100 prn, a shorter value of d 323 may provide larger optical bandwidth more tolerant of fluctuations in !asing wavelength 342. Thus, constructive interference may be maintained at a larger range of wavelengths 342, allowing for the lasing wavelength 342 to fluctuate while still providing high intensify 344. For example, consider d 323 of 100 pm and a wavelength 342 of 1533 nm where intensity 344 is approximately 1. The intensity 344 is maintained approximately above 0,8 a.u., even if the wavelength 342 fluctuates by approximately ± S nm. In examples described below, d may vary for muitipie rings sharing a reflector and/or waveguide where eac ring is coupled at a different distance from the shared reflector. In other examples described below, d may be chosen for each ring associated with its own reflector, even if muitipie rings are coupled to the same waveguide, by adjusting the reflector position relative to its associated ring and/or side waveguide.

[00313 ^ . 4 is a block diagram of a laser array 400 including a reflector 430 according to an example. The laser array 400 includes a plurality of n active rings 410 coupied to the waveguide 420, The rings 410 enable output of a plurality of corresponding wavelengths 442. The rings 410 may share one waveguide 420. and may be triggered into domination of unidirectional lasing based on one reflector 430.

[00323 The laser array 400 may be used to provide a wavelength division multiplexing (WDM) light source, including n active rings 410 to output n wavelengths. The laser array 400 enables avoidance of cross-talk between different wavelengths, because each of the n active rings 410 is excited by a different wavelength. The rings 410 may b arranged such that the distance d_n between the reflector 430 and the coupling point 422 for that ring provides a desired phase condition (e.g., constaictive interference) in view of the wavelength λ„ for that ring. In an example, an arrangement of the n active rings 410 ma provide some rings with constructive interference and some rings with destructive interference. In an example, an arbitrary number of active rings may be added to the waveguide 420_: e.g., to provide multi-channel WDM, and the one reflector may include a reflection bandwidth to reflect wavelengths to enable domination of unidirectional iasing in all the active rings.

[00333 PSG. 5 is a block diagram of a lase 500 including a reflector 530 according to an example. The waveguide includes a main waveguide 527 and a side waveguide 528 coupied to each other via a combiner 529. Combiner 529 may be a y-branch, muitimode interferometer (MM I), directional coupler, or other connector. The side waveguide 528 is shown coupled to the main waveguide 527 at an angle, and may be coupled at any angle including 90 degrees, acute, or obtuse angles. Reflector 530 is associated with side waveguide 528. The side waveguide 528 is coupied to active ring 510 at side coupling point 522, The reflector 530 is positioned a distance d 523 from the side coupling point 522. The active ring 510 also may be coupled to the main waveguide 527 via a main coupling point 521.

[0034J The laser 500 is to provide emitted Iight 528 to/from the main waveguide 527 based on triggered domination of unidirectional Iasing 518 consistent with the desired direction of emitted Iight 528. Reflector 530 and side waveguide 528 may be used to trigger domination of unidirectional iasing 518 in the active ring 510,

[OO353 Light from clockwise 514 iight generation may be coupled to the side waveguide 528 {and/or main waveguide 527} as captured light 524. Reflector 530 may reflect the captured light 524 as reflected light 525. The reflected light 525 may be coupled into the main waveguide 527, and may be coupled into the active ring 510 as coupled iight 516, Coupied Iight 516 traveling in the counterclockwise 512 direction is to trigger domination of unidirectional Iasing 518 in the active ring 510.

[0036] FIG. 8 is a block diagram of a laser array 600 including a plurality of reflectors 630 according to an example. The laser array 600 includes a plurality of n active rings 610 coupied to a plurality of n side waveguides 628. The n side waveguides 628 are each coupied to a corresponding one of the n reflectors 630 at a distance ά_η from a corresponding side coupling point 622, Each side waveguide 628 is coupied to main waveguide 627 via a combiner, shown as a y-branch 628 in the example of FIG. 8 (although other combiners may be used). The y-branch 829 can enable reflected light from the reflector, that is not coupled back into its corresponding ring in side waveguide 828, to enter the main waveguide 827 to become part of toe light output.

[0037] The rings 810 enable output of a plurality of corresponding wavelengths 642. Each ring 610 may be associated with its corresponding reflector 630 based on a corresponding distance d„, enabling each ring 610 to provide enhanced efficiency by tailoring a phase condition according to the wavelength for that particular ring 610 in view of the distance d 623 associated with that ring 610. Each reflector may be provided at a short distance d 623 from the coupling point, such that each ring may provide light wit constructive interference over a wide bandwidth of wavelength values/fluctuations.

[00383 FSG- 7 is a block diagram of a laser 700 including a reflector 730 according to an example. The waveguide Includes main waveguide 727 and side waveguide 728 spaced from each other. Reflector 730 is associated wit side waveguide 728, The side waveguide 728 is coupled to active ring 710 at side coupling point 722. The reflector 730 is positioned a distance d 723 from the sid coupling point 722, The active ring 710 also may be coupled to the main waveguide 727 via a main coupling point 721 ,

[0039] The laser 700 is to provide emitted light 728 to/from the main waveguide 727 based on triggered domination of unidirectional lasing 718 consistent with the desired direction of emitted light 726. Reflector 730 and side waveguide 728 may be used to trigger domination of unidirectional lasing 718 in the active ring 710.

[00403 Light from clockwise 714 light generation may be coupled to the side waveguide 728 (and/or main waveguide 727) as captured light 724, Reflector 730 may reflect the captured Sight 724 as reflected light 725. The reflected light 725 may be coupled into the active ring 710 as coupled light 716. Coupled Sight 716 traveling in the counterclockwise 712 direction is to trigger domination of unidirectional lasing 718 in the active ring 710.

[00413 ^G- 8 is a block diagram of a laser array 800 including a plurality of reflectors 830 according to an example. The laser array 800 includes a plurality of n active rings 810 each coupled to a plurality of n side waveguides 828, The n side waveguides 828 are each coupled to a corresponding reflector 830 at a distance ct_; from a corresponding side coupling point 822. Each side waveguide 828 is separated from main waveguide 827.

[0042] The rings 810 enable output of a plurality of corresponding wavelengths 842. Each ring 810 may be associated with its corresponding reflector 830 based on a corresponding distance d_!lt enabling each ring 810 to provide enhanced efficiency by tailoring a phase condition according to the wavelength for that particular ring 810 in view of the distance d 823 associated with that ring 8 0.

[00433 !G. 9 is a flow chart 900 based on generating light at an active ring according to an example. In block 9 0, light is generated at an active ring. For example, an active ring may include a gain medium that responds to optical pumping and/or electrical pumping (e.g., an injection current), !n block 920, generated Sight is captured at a passive waveguide associated with the active ring. The passive waveguide may be coupled to the active ring at a coupling point. The passive waveguide may include a side waveguide and a main waveguide, and the passive waveguide may be coupled to the active ring via the side waveguide and/or the main waveguide. The side waveguide may be separate from the main waveguide, and/or the side waveguide may be coupied to the main waveguide (e.g., via a y-branch).

[00443 in biock 930, dominatio of unidirectional lasing is triggered in the active ring to generate light based on a reflector associated with the passive waveguide to couple captured light from the waveguide into the active ring, in an example, the reflector is to reflect light in the waveguide from the non- dominant direction to the dominant direction, and the reflected light is coupled into the active ring to trigger unidirectional iasing in the active ring. Thus, light generated in the active ring due to unidirectional lasing i the dominant direction may be coupled to and output from the passive waveguide at high intensity, in block 940, a reflection bandwidth of the reflector is tuned to correspond to a iasing wavelength associated with the active ring, in an example, the reflector may include a tunable passive ring that reflects a wavelength to trigger domination of unidirectional lasing in the active ring, in step 950, domination of unidirectionai lasing is maintained over a range of current injection biasing ieveis. For example, the active ring may unidirectionally iase in response to a variety of ievels of external energy pumping.

Claims

WHAT IS CLAIMED IS:

1, A laser comprising:

an active ring to generate light;

a passive waveguide associated with the active ring to capture generated fight; and

a reflector associated with the passive waveguide to cause captured light from the waveguide to be coupled into the active ring to trigger domination of unidirectional iasing in the active ring to generate Sight.

2, The laser of claim 1, wherein the passive waveguide includes a coupling point, at a distance d from the reflector, associated with a phase condition based on d and laser wavelength, to provide constructive interference having high intensity over a large optical bandwidth.

3, The laser of claim 1, wherein the passive waveguide includes a main waveguide and a side waveguide, wherein the reflector is associated with the side waveguide and the side waveguide includes a side coupling point at a distance d from the reflector, associated with a phase condition based on d and laser wavelength, to provide constructive interference having high intensity over a large optical bandwidth.

4, The laser of claim 3, wherein the side waveguide is coupled to the main waveguide via a combiner.

5, The laser of claim 1 , wherein the active ring includes a gain medium responsive to an external energy pump to generate light.

8, The laser of claim 1. wherein the reflector includes a vertical smooth waveguide facet having a high-reflection coating.

7. The laser of claim 1 , wherein the reflector includes a teardrop shaped waveguide reflector.

8. The laser of claim 1 , wherein the reflector includes a passive ring waveguide reflector.

9. The laser of claim 1 , wherein the reflector includes a distributed bragg reflector (DBR).

10. A laser comprising;

an active ring to generate light in response to an external energy pump; a passive waveguide associated with the active ring to capture generated iight based on a coupling point; and

a reflector associated with the passive waveguide at a distance d from the coupling point to cause captured light from the waveguide to be coup!ed into the active ring to trigger domination of unidirectional iasing in the active ring to generate light.

11. The laser of claim 10, further comprising a plurality of active rings associated with the passive waveguide to generate light including a plurality of wavelengths,

12. The laser of claim 1 , further comprising n reflectors, wherein the passive waveguide includes a main waveguide and n side waveguides, wherein each reflector is associated with a corresponding side waveguide, and each side waveguide includes a side coupling point at a distance d_n from the corresponding reflector, associated with a phase condition based on d_n and laser wavelength λπ associated with each active ring.

13. A method of generating light, comprising:

generating light at an active ring; capturing generated light at a passive waveguide associated with the active ring; and

triggering domination of unidirectional !asing in the active ring to generate Sight based on a reflector associated with the passive waveguide to couple captured light from the waveguide into the active ring.

14. The method of claim 13_; further comprising tuning a reflection bandwidth of the reflector to correspond to a lasing wavelength associated with the active ring.

15. The method of ciaim 13, further comprising maintaining the domination of unidirectional lasing over a range of external energy pumping levels.