Patent Application
20230361532
The present disclosure generally relates to optical devices and more particularly to optical sources.
A tunable laser is a laser in which the wavelength of operation can be altered in a controlled manner using filters to output the target wavelength. The tuning values vary over temperature and can require complex control systems to keep the tunable laser aligned during operation. A fixed laser is simpler to control; however, it is difficult to implement fixed lasers in photonic integrated circuits (PICs) due to calibration issues, power issues, and process control issues, such as process variation exhibited in modern PIC fabrication techniques.
The following description includes discussion of figures having illustrations given by way of example of implementations of embodiments of the disclosure. The drawings should be understood by way of example, and not by way of limitation. As used herein, references to one or more “embodiments” are to be understood as describing a particular feature, structure, or characteristic included in at least one implementation of the inventive subject matter. Thus, phrases such as “in one embodiment” or “in an alternate embodiment” appearing herein describe various embodiments and implementations of the inventive subject matter, and do not necessarily all refer to the same embodiment. However, they are also not necessarily mutually exclusive. To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number refer to the figure (“FIG.”) number in which that element or act is first introduced.
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the disclosure is provided below, followed by a more detailed description with reference to the drawings.
In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide an understanding of various embodiments of the inventive subject matter. It will be evident, however, to those skilled in the art, that embodiments of the inventive subject matter may be practiced without these specific details. In general, well-known instruction instances, structures, and techniques are not necessarily shown in detail.
As discussed, a PIC can implement a tunable laser, in which the laser can be tuned to output light of different wavelengths. In some example embodiments, the tunable laser can implement one or more optical filters to obtain a target wavelength of the optical system. The tuning values can vary over different temperatures, which can necessitate fast control loops that are integrated close to the PIC to ensure the tuners are aligned during operation. A fixed wavelength (e.g., single mode) silicon photonic laser with DFB can be configured in the PIC such that no wavelength calibration is required, which can reduce calibration cost and can further allow faster module bootup time, thereby reducing power consumption and simplifying laser control. A DFB laser can be implemented as an integrated PIC laser in which the laser resonator comprises of a periodic structure in the laser gain medium, which functions as a distributed Bragg reflector in the wavelength range of laser action. In some example embodiments, a distributed-feedback laser has multiple axial resonator modes, but there is typically one mode which is preferable in terms of losses; thus, single-frequency operation can be implemented.
While some non-silicon based DFB sources can implement facet coatings (e.g., anti-reflectivity (AR) coating, high-reflectivity (HR) coating) to obtain higher power, this approach is incompatible with silicon based photonic DFBs because coatings cannot be applied to facets of a silicon based photonic DFB. Additionally, these approaches waste power as the coatings have defects which can waste portions of the light (e.g., 20%). Additionally, these approaches suffer from reliability issues due the coatings. Additionally, these approaches exhibit poor feedback tolerance and are more sensitive to feedback and reflections. Additionally, applying the coating requires access to both the DFB output sides to apply the coatings, and coatings cannot be applied to silicon designs having integrated sources that are integrated in the middle the design, thereby making such access impossible.
To address the foregoing, a silicon photonic symmetric DFB can be implemented to provide light to the PIC in an approach that has a similar power efficiency as non-silicon photonic DFBs by forming a grating in the III-V layer, and by utilizing power from one output or both outputs of the silicon photonic symmetric DFB.
The bends in the routing of the silicon photonic symmetric DFB can be configured such that they are low loss and without reflection to the silicon photonic symmetric DFB, in contrast to bends of III-V based DFBs that cause high loss and high reflection and thus cannot be used to implement symmetric DFBs. In fiber based DFBs, large and expensive components are required to adjust and stabilize the phase of both outputs to combine them in a 2×1 coupler. The large size of fiber based DFB laser prevents their use in typical multiple lane transceivers such as ethernet applications (e.g., a multiple lane transceiver in which both laser outputs are utilized, such as combined for a single lane or each output running a separate lane).
In some example embodiments, a silicon photonic symmetric DFB is configured such that no wavelength adjustment is required in operation, which increases the power efficiency while achieving high optical mode stability. In some example embodiments, the silicon photonic symmetric DFB outputs to two waveguides and couples the light using a 2×1 optical combiner, in which the waveguides are fully symmetric waveguides to reduce phase errors, and thermal phase tuners provide optical phase matching at the input to the 2×1 optical combiner which outputs the optical beam. In some example embodiments, the silicon photonic symmetric DFB outputs to two different waveguides which drive separate optical lanes with the same operating wavelength, which achieves high power efficiency due to the optical power from both output ports being used.
One additional challenge is that while gratings can be fabricated in silicon (e.g., in a silicon waveguide), this type of processing requires specialized equipment and design processes that may be not be practical in some manufacturing environments. To this end, in some example embodiments, a grating is formed in the III-V structure and is then bonded to the silicon structure, as discussed in further detail below.
Continuing, the second symmetric DFB laser 205B (e.g., set to a second wavelength that is higher than the first wavelength) outputs to a coupler 210B, which combines the light, which is then modulated by the modulator 215B and output via an output port 220B. Further, the third symmetric DFB laser 205C (e.g., set to a third wavelength that is higher than the second wavelength) outputs to a coupler 210C, which combines the light, which is then modulated by the modulator 215C and output via an output port 220C. Further, the fourth symmetric DFB laser 205D (e.g., set to a fourth wavelength that is higher than the third wavelength) outputs to a coupler 210D, which combines the light, which is then modulated by the modulator 215D and output via an output port 220D.
Similarly, for the third and fourth lanes, one side of the silicon photonic integrated symmetric DFB laser 305B can provide light of a given wavelength (e.g., Ai) for a third lane in which the light is modulated by a modulator 310C and then output via output port 315C. Further, the other side of the silicon photonic integrated symmetric DFB laser 305B provides light of the given wavelength (e.g., Ai) for a fourth lane, where the light is modulated by a modulator 310D and then output via output port 315D, where the third and fourth lanes receive half of the light power provided by the silicon photonic integrated symmetric DFB laser 305B. In some example embodiments, the multi-lane silicon based DFB architecture 300 does not include heaters, and the light emanating from either side of the silicon photonic integrated symmetric DFB laser 305A may be out of phase, but the light from the different sides are not combined (e.g., in a 2×1 coupler as in
The architecture 450 illustrates a low loss approach in which the 1×2 coupler 410 is omitted and instead a symmetric DFB 455 provides light for both lanes, as discussed above with reference to
At operation 520, the light is combined. For example, with reference to
At operation 525, the light is modulated. For example, the modulator 215A modulates light of the first lane, which is light from both sides of the first symmetric DFB laser 205A which is combined via coupler 210A. As an additional example, the modulator 310A in
At operation 530, the light is output from the device. For example, the each lane of light is output from respective output ports (e.g., output ports 220A-220D of
In some example embodiments, heaters are added on both sides (e.g., heater 110A, heater 110B) but only one is used at a given time to compensate for small positive or negative phase imbalance, due to process variation in fabrication of the PIC having the symmetric DFBs. At operation 605, the electrical current for the symmetric DFB laser (e.g., silicon based DFB laser 105) is set to a nominal value (e.g., 100 milliamps). At operation 610, the maximum power for one of the heaters is recorded. For example, the power of the heater 110A is swept while the value of the monitor photodiode 120 is monitored, and the power value for the heater 110A is recorded when the monitor photodiode (MPD) reading is maximized.
At operation 615, the maximum power for another of the heaters is recorded. For example, the power of the heater 110B is swept while the value of the monitor photodiode 120 is monitored, and the power value for the heater 110B is recorded when the MPD reading is maximized.
At operation 620, it is determined whether heater 110A or heater 110B is more efficient (e.g., which has less power usage at maximum MPD reading) when the MPD reading is maximized, and heater power is applied to the most efficient of the heaters to phase balance the arms.
At operation 625, the electrical current of the symmetric DFB is adjusted until the target optical power is reached on MPD. At operation 630, the heater values and electrical current settings are saved to memory (e.g., flash memory) of the optical system (e.g., optical transceiver 700) to be implemented when the system is initialized for operation. In some example embodiments, the method 600 is performed multiple times for additional DFBs in the device (e.g., DFBs 205A-205D), and the respective values for each lane are stored at operation 630.
At operation 635, the optical system having the one or more symmetric DFBs is initialized for operation (e.g., in the field, in a product) and the stored values are applied to the one or more symmetric DFBs and one or more heaters for efficient operation of the optical device.
In some example embodiments, the PIC 820 includes silicon on insulator (SOI) or silicon based (e.g., silicon nitride (SiN)) devices, or may comprise devices formed from both silicon and a non-silicon material. Said non-silicon material (alternatively referred to as “heterogeneous material”) may comprise one of III-V material, magneto-optic (MO) material, or crystal substrate material. III-V semiconductors have elements that are found in group III and group V of the periodic table (e.g., Indium Gallium Arsenide Phosphide (InGaAsP), Gallium Indium Arsenide Nitride (GainAsN), Aluminum Indium Gallium Arsenide (AlInGaAs)). The carrier dispersion effects of III-V-based materials may be significantly higher than in silicon-based materials, as electron speed in III-V semiconductors is much faster than that in silicon. In addition, III-V materials have a direct bandgap, which enables efficient creation of light from electrical pumping. Thus, III-V semiconductor materials enable photonic operations with an increased efficiency over silicon for both generating light and modulating the refractive index of light. Thus, III-V semiconductor materials enable photonic operation with an increased efficiency at generating light from electricity and converting light back into electricity.
The low optical loss and high quality oxides of silicon are thus combined with the electro-optic efficiency of III-V semiconductors in the heterogeneous optical devices described below; in embodiments of the disclosure, said heterogeneous devices utilize low loss heterogeneous optical waveguide transitions between the devices' heterogeneous and silicon-only waveguides.
MO materials allow heterogeneous PICs to operate based on the MO effect. Such devices may utilize the Faraday Effect, in which the magnetic field associated with an electrical signal modulates an optical beam, offering high bandwidth modulation, and rotates the electric field of the optical mode, enabling optical isolators. Said MO materials may comprise, for example, materials such as iron, cobalt, or yttrium iron garnet (YIG). Further, in some example embodiments, crystal substrate materials provide heterogeneous PICs with a high electro-mechanical coupling, linear electro-optic coefficient, low transmission loss, and stable physical and chemical properties. Said crystal substrate materials may comprise, for example, lithium niobate (LiNbO3) or lithium tantalate (LiTaO3).
In the example illustrated, the PIC 820 exchanges light with an external light source 825 via an optical fiber 821, in a flip-chip configuration where a top-side of the PIC 820 is connected to the organic substrate 860 and light propagates out (or in) from a bottom-side of the PIC 820 facing away (e.g., towards a coupler), according to some example embodiments. The optical fiber 821 can couple with the PIC 820 using a prism, grating, or lens, according to some example embodiments. The optical components of PIC 820 (e.g., optical modulators, optical switches) are controlled, at least in part, by control circuitry included in ASIC 815. Both ASIC 815 and PIC 820 are shown to be disposed on copper pillars 814, which are used for communicatively coupling the PICs via organic substrate 860. PCB substrate 805 is coupled to organic substrate 860 via ball grid array (BGA) interconnect 816 and may be used to interconnect the organic substrate 860 (and thus, ASIC 815 and PIC 820) to other components of the optical-electrical device 800 not shown (e.g., interconnection modules, power supplies, etc.).
As discussed above, while DFB lasers can have gratings fabricated in the silicon waveguide, the processing uses specialized lithography equipment to generate a grating pattern with sufficiently small dimensions. Unfortunately, silicon foundries may not have lithography capabilities for grating fabrication and generally it requires heavy capital investment in further equipment (e.g., deep UV lithography equipment). Further, development time of the Si grating process can be significant, and the poor repeatability of the process is still problematic. Further, moving production wafers out of the Si foundry to do the grating step elsewhere increases cycle time and risk contamination.
In some example embodiments, the gratings are formed in the III-V structure using III-V epitaxy growth, and optionally regrowth. In some example embodiments, the III-V epitaxial structure is first half-way grown, then grating is patterned and etched, and the laser structure is finished by regrowth to embed the grating inside the materials. In some example embodiments, the III-V is grown to specification and a top-surface grating is etched and no regrowth occurs (e.g., the III-V epi die is flip-chip bonded to the SOI using the top-surface such that the mode is adiabatically coupled to the silicon waveguides in the SOI). One advantage of forming DFB gratings in the III-V structure is that it parallelizes manufacturing processes between the foundries: for example, between a III-V manufacturing facility that produces the III-V grating structure in parallel with a silicon wafer manufacturing facility to completes the silicon wafer front end processing. Further, a DFB with the grating in the III-V structure avoids extra process steps in the Silicon foundry beyond the existing SiPh process flow (e.g., used to design the silicon wafer). In this way, many Si foundries can more readily be used to manufacturing a DFB laser with wafer bonding processes. For instance, a given SiPh foundry may be configured for 500 nm Silicon thickness in the SOI wafer, while other SiPh foundries may be configured for a 220 nm Silicon thickness; however, it can be difficult or not possible to form gratings in when the Silicon is as thin as 220 nm. As such, forming the grating in III-V structure enables the design and fabrication processes to become insensitive to SOI thickness, which allows us to implement this concept to any SOI structure including 220 nm Si.
In some example embodiments, the etched III-V structure 925 is bonded to the silicon structure using plasma enhanced wafer bonding. For example, (1) a III-V epitaxial wafer is patterned with DFB gratings and alignment marks to align the III-V epitaxial structure on the silicon; (2) the III-V epitaxial wafer is mounted face down on UV release tape and the singulation process is performed on the backside of the III-V epitaxial wafer to protect the frontside surface (e.g., top-surface grating, bonding side) from damage and contamination; and (3) each III-V epitaxial die is accurately bonded to a target SOI using the alignment marks such that the grating and active region is disposed over the narrow width of the silicon waveguide and the tapers of the silicon waveguide are disposed under respect SOA regions of the III-V die.
In some example embodiments, the etched III-V structure 925 is bonded to the silicon structure using micro-Transfer Printing (uTP). For example, (1) a III-V epitaxial wafer is patterned with DFB gratings and alignment marks to align the III-V epitaxial structure on the silicon; (2) the III-V epitaxial wafer is singulated into III-V epitaxial dies using uTP process of etching and undercutting; and (3) each III-V epitaxial die is accurately bonded to a target SOI using the uTP stamp process.
In some example embodiments, the etched III-V structure 925 is then cleaved into small rectangles (e.g., epitaxial dies) using the alignment marks on the etched III-V structure 925 to align cleave locations to the gratings. The etched III-V structure 925 (e.g., an epitaxial die) is then bonded to the SOI structure to form the bonded structure 950. In some example embodiments, the bonded structure 950 is then further processed to form additional circuit components, and vias and metallic pads are integrated into the bonded structure 950 to provide current and drive the symmetric DFB laser.
In some example embodiments, the light propagates from the active region 1033 to a first SOA region 1030 and a second SOA region 1035, which couple the light from the III-V structure 1010 to the silicon waveguide 1025 of the silicon structure 1005 via tapers in the silicon waveguide 1025 that are formed under the respective first SOA region 1030 and a second SOA region 1035.
The tapered portions of the silicon waveguide 1025 taper to a narrow width section (e.g., taper from 2 um to ˜0.5 um) of the silicon waveguide that extend along the active region 1033 to minimize coupling from the III-V structure 1010 to the silicon structure 1005 along that section. That is, to keep the light in the III-V material so that the mode is completely distributed inside the gain section of the III-V structure 1010 in order to maximize modal gain and power efficiency.
In some example embodiments, the grating 1020 is formed along a longitudinal direction of the active region 1033, and terminates at the first SOA region 1030 and a second SOA region 1035, such that the mode selection of the output light from the active region is completed within the active region 1033 via the grating 1020 (e.g., the grating 1020 provides optical feedback such that multimode light that would otherwise be generated by the gain material is instead generated as two-mode or single mode light). In some example embodiments, the grating 1020 extends outside the active region 1033, e.g., partially into the SOA regions of the III-V layer, to add reflectivity to the cavity or other modify the coupling of the light.
In some example embodiments, a quarter wave shift (QWS) feature 1015 is formed in a middle portion of the grating 1020 (e.g., changing the grating teeth spacing to add a peak) to generate a symmetric cavity to refine the mode selection (e.g., from two-mode light to single mode light, light at a fixed wavelength) to provide light symmetrically from each end of the active region 1033. In some example embodiments, an asymmetric DFB structure can be formed by positioning a QWS feature toward one end of the cavity of the active region 1033. In some example embodiments, the grating is configured as an adiabatic chirped grating or non-uniform grating, which can be configured per a given design to further tailor the mode and power fraction toward one end of the active region 1033. In some example embodiments, the DFB having the grating in the III-V structure is a distributed phase delay DFB. In some example embodiments that implement the symmetric DFB structure (e.g., with a middle QWS feature), a reflector can be integrated in the silicon waveguide 1025 to reflect half of the light from one end port of the silicon waveguide 1025 to the other port in order to maximize output from the other port.
In view of the disclosure above, various examples are set forth below. It should be noted that one or more features of an example, taken in isolation or combination, should be considered within the disclosure of this application.
The following are example embodiments: Example 1. A photonic integrated circuit distributed feedback laser comprising: a III-V semiconductor structure comprising an active region and a grating etched on a bonding surface of the III-V semiconductor structure to provide optical feedback to the active region to generate output light that is output from a first side of the active region and that is further output from a second side of the active region; and a silicon structure comprising a silicon waveguide to receive the output light from the first side and the second side of the active region of the III-V semiconductor structure, the III-V semiconductor structure bonded to the silicon structure such that the bonding surface having the grating is bonded to a surface of the silicon structure to optically couple the active region to the silicon waveguide.
Example 2. The photonic integrated circuit distributed feedback laser of example 1, wherein the first side and the second side of the active region are separated by the grating that is etched on the bonding surface.
Example 3. The photonic integrated circuit distributed feedback laser of any of examples 1 or 2, wherein the output light is single mode light.
Example 4. The photonic integrated circuit distributed feedback laser of any of examples 1-3, wherein the grating provides optical feedback to generate the single mode light.
Example 5. The photonic integrated circuit distributed feedback laser of any of examples 1-4, wherein the grating is configured to apply a quarter wave shift to the active region to form the output light.
Example 6. The photonic integrated circuit distributed feedback laser of any of examples 1-5, wherein the quarter wave shift of the grating generates single mode light as the output light.
Example 7. The photonic integrated circuit distributed feedback laser of any of examples 1-6, wherein the grating is configured to apply the quarter wave shift in a middle portion of the grating.
Example 8. The photonic integrated circuit distributed feedback laser of any of examples 1-7, wherein the grating is a non-uniform grating that shifts an optical distribution towards one of: the first side of the active region, or the second side of the active region.
Example 9. The photonic integrated circuit distributed feedback laser of any of examples 1-8, wherein the III-V semiconductor structure comprises a first semiconductor optical amplifier to couple light from the first side of the active region to the silicon waveguide.
Example 10. The photonic integrated circuit distributed feedback laser of any of examples 1-9, wherein the III-V semiconductor structure comprises a second semiconductor optical amplifier to couple light from the second side of the active region to the silicon waveguide of the silicon structure.
Example 11. The photonic integrated circuit distributed feedback laser of any of examples 1-10, wherein the silicon waveguide comprises a narrow width section that is proximate to the active region of the III-V semiconductor structure that is bonded to the silicon structure, the narrow width section minimizing coupling from the active region to the narrow width section of the silicon waveguide.
Example 12. The photonic integrated circuit distributed feedback laser of any of examples 1-11, wherein the silicon waveguide comprises one or more widened sections that are wider than the narrow width section to couple the output light from the III-V semiconductor structure to the silicon waveguide.
Example 13. The photonic integrated circuit distributed feedback laser of any of examples 1-12, wherein the output light is coupled from the III-V semiconductor structure to the silicon structure without facet coating the III-V semiconductor structure.
Example 14. The photonic integrated circuit distributed feedback laser of any of examples 1-13, wherein the III-V semiconductor structure is bonded to the silicon structure using plasma based wafer bonding.
Example 15. The photonic integrated circuit distributed feedback laser of any of examples 1-14, wherein the III-V semiconductor structure is bonded to the silicon structure using transfer printing based bonding.
Example 16. The photonic integrated circuit distributed feedback laser of any of examples 1-15, wherein the grating is a top-surface grating and no regrowth of III-V material is applied to the top-surface grating.
Example 17. The photonic integrated circuit distributed feedback laser of any of examples 1-16, wherein grating teeth of the grating are filled with a dielectric material to reduce coupling efficiency.
Example 18. A method for manufacturing a photonic integrated circuit distributed feedback laser comprising: etching a grating on a III-V semiconductor structure, the III-V semiconductor structure comprising an active region to generate light, the grating being etched on a bonding surface of the III-V semiconductor structure to provide optical feedback to the active region to generate output light that is output from a first side of the active region and that is further output from a second side of the active region; and bonding the III-V semiconductor structure to a silicon structure, the silicon structure comprising a silicon waveguide to receive the output light from the III-V semiconductor structure, the III-V semiconductor structure bonded to the silicon structure such that the bonding surface having the grating is bonded to a surface of the silicon structure to optically couple the active region to the silicon waveguide.
Example 19. The method of example 18, wherein the first side and the second side of the active region are separated by the grating that is etched on the bonding surface.
Example 20. The method of any of examples 18 or 19, wherein the grating is etched such that a quarter wave shift is applied to the active region to form the output light.
In the foregoing detailed description, the method and apparatus of the present inventive subject matter have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present inventive subject matter. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.
