Cavity design for multi-wavelength laser

Abstract

A photonic element includes a bottom cladding, a top cladding, and a first waveguide between the bottom cladding and the top cladding. A first multi-wavelength grating is optically coupled with the first waveguide. The multi-wavelength grating is characterized by a grating strength that varies along a first axis based on a piecewise mathematical function. The first waveguide and the multi-wavelength grating collectively enable an output having a plurality of wavelengths.

Description

[0001] Cross-reference of related applications

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 556,069, filed February 21, 2024, the contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure generally relates to photonic elements, and more specifically, to photonic elements based on distributed feedback reflectors. Background Technology

[0004] A windowed sampling grating (WSG) is a modified Bragg grating suitable for generating multiple optical wavelengths. The grating intensity of a WSG (defined as the effective refractive index difference between the grating elements and the gap sections) varies along the longitudinal axis of the device according to a desired function to achieve multi-wavelength output. In other words, the reflectivity of the grating elements (i.e., the effective refractive index contrast at the grating elements) varies along the longitudinal axis of the grating according to a desired function. The refractive index distribution of a WSG can be specifically tailored to produce resonance and therefore high reflectivity at a particular set of wavelengths. For example, a WSG can be designed to generate 16 wavelengths spaced 1 nm apart, centered at 1310 nm. In contrast, a conventional Bragg grating produces reflection at a single wavelength, while a sampling grating produces a series of reflection peaks that are uniformly spaced across the wavelengths and have an amplitude distribution fixed by the sampling function.

[0005] Examples of WSG and illustrative methods for physically implementing WSG to produce a desired refractive index distribution can be found in co-pending U.S. Application No. 17 / 465,403 entitled “Windowed Sampled Grating and Method of Fabrication”.

[0006] WSGs can be used in several different applications. For example, they can be used in combination with gain media in passive waveguide systems to realize multi-wavelength lasers, as a source for providing a wavelength "comb," as a multi-wavelength filter, and the like. Multi-wavelength lasers emit laser light according to a set of wavelengths defined by the WSG, much like conventional single-wavelength lasers based on distributed Bragg reflectors. The grating can be completely passive and placed outside the amplifier section, thus forming a laser cavity between the grating and a back mirror, which can be another WSG or a broadband reflector, such as a facet, or it can be contained within the amplifier section, for example, in a distributed feedback (DFB) laser. In a DFB laser, the grating extends within the laser cavity to provide optical feedback that produces a narrow linewidth output. The DFB configuration is desirable because it eliminates the need to align the phase of the cavity mode with the grating passband, whereas this alignment is necessary when the grating is independent of the amplifier section.

[0007] As is well known, DFB lasers with gratings designed to reflect at a single wavelength emit laser light at two wavelengths on either side of the grating stopband unless the grating is engineered to contain a resonant cavity. Since it is generally desirable for lasers to emit light at a single wavelength, various methods have been developed to generate only that single wavelength. The most common method is to introduce a quarter-wave phase shift in the grating. For example, the grating may have a segment at its center that introduces an additional quarter-wave phase shift in combination with a low-reflectivity facet. Such DFB lasers are commonly referred to as quarter-wave-shifted DFB lasers. Alternatively, the grating may be cleaved in half and have a highly reflective coating on one side, producing an effect similar to that produced by a quarter-wave phase-shift segment of the grating laid across the highly reflective coated cleaved facet. These DFB lasers are commonly referred to as high-reflectivity / low-reflectivity facet lasers, often denoted as HR / AR lasers.

[0008] While HR / AR DFB lasers can be manufactured more efficiently and simply than quarter-wave shifted DFB lasers, their yield is lower because it is practically impossible to perfectly cleave the edges of the final grating teeth. Although HR / AR lasers incorporated into WSGs can theoretically be produced, the multiple laser emission wavelengths are unlikely to function correctly with random phase errors on the HR facets. This makes high-yield production difficult, if not impossible.

[0009] Unfortunately, quarter-wave shifted DFB lasers have well-known drawbacks. Two main difficulties in this design are spatial aperture burning, which limits optical linewidth and maximum output power; and reduced efficiency, attributed to wasted power emitted from the anti-reflective coating on the back side. Spatial aperture burning can be addressed by employing a "corrugated pitch modulation" (CPM) cavity design, where the λ / 4 segment is replaced by a longer grating segment with a pitch slightly different from the main DFB grating. Back-side power emission can be reduced by asymmetrically placing the cavity inside the grating.

[0010] Therefore, it is desirable to provide a multi-wavelength photonic element that incorporates quarter-wave shift and can be used, for example, to provide a quarter-wavelength shifted multi-wavelength DFB laser. Summary of the Invention

[0011] On one hand, this disclosure relates to multi-wavelength gratings suitable for providing multiple wavelength signals propagating in a waveguide, and multi-wavelength lasers incorporated into said multi-wavelength gratings. The apparatus according to this disclosure includes a multi-wavelength grating whose grating intensity and / or phase vary along the longitudinal axis of the apparatus according to a desired piecewise mathematical function to achieve a multi-wavelength output signal. In other words, the reflectivity of the grating elements (i.e., the effective refractive index contrast at the grating elements) and, in some cases, the grating pitch vary along the longitudinal axis of the grating according to a desired piecewise mathematical function. Therefore, when an optical signal propagating through the grating traverses the grating, the optical signal sees a change in reflection intensity at the transition between the grating elements and the gap material between the grating elements. Embodiments according to this disclosure are particularly suitable for use in conjunction with a gain medium in passive waveguide systems to realize multi-wavelength lasers, as sources for providing wavelength “combs,” as multi-wavelength filters, and the like.

[0012] An embodiment of the present disclosure is a photonic element comprising: a bottom cladding; a top cladding; a first waveguide located between the bottom cladding and the top cladding; and a first multi-wavelength grating optically coupled to the first waveguide, wherein the multi-wavelength grating is characterized by a grating intensity varying along a first axis based on a piecewise mathematical function; wherein the first waveguide and the multi-wavelength grating together achieve an output having multiple wavelengths.

[0013] In some embodiments, the first multi-wavelength grating is a windowed sampling grating (WSG). In other embodiments, the first multi-wavelength grating is a binary superimposed grating (BSG) or a phase grating (PG).

[0014] In some embodiments, a quarter-wave phase shift is introduced into the first multi-wavelength grating. In some embodiments, the quarter-wave phase shift is introduced by incorporating a corrugated pitch modulation (CPM) grating segment into the multi-wavelength grating.

[0015] In some embodiments, the first multi-wavelength grating has a plurality of grating segments, each characterized by a grating intensity varying along the first axis by a subfunction of the piecewise mathematical function based on each of the wavelengths. Each of the subfunctions is defined as a summation of an expression over the plurality of wavelengths, the summation representing the grating at a single wavelength defined in the expression.

[0016] On the other hand, this disclosure relates to a multi-wavelength DFB laser comprising an active layer and a grating element layer commonly located between an upper cladding layer and a lower cladding layer. A multi-wavelength grating is formed in the grating element layer, which is optically coupled to a gain layer such that the multi-wavelength grating functions as a mirror of the laser cavity. The grating element layer is located between the active layer structure and its upper or lower cladding structure. In a particular embodiment, the grating is located below the active layer. The grating elements of the multi-wavelength grating are openings in the grating element layer, wherein photolithographically defined features of the grating elements (e.g., their widths) vary along the longitudinal axis according to a piecewise mathematical function. The effective refractive index contrast and therefore effective reflectivity at each grating element are based on a value of its width or other photolithographically defined features.

[0017] The multi-wavelength DFB laser disclosed herein can be designed to emit laser light at a desired wavelength with high efficiency, high output power, and high directionality, wherein high directionality refers to the ability to guide the emitted output power along the desired direction. The cavity of the multi-wavelength DFB laser can also support arbitrary channel planes.

[0018] Furthermore, the multi-wavelength DFB laser according to this disclosure can be scaled with the number of emission wavelengths without necessarily affecting the size of the laser. Additionally, since the gratings in the WSG have relatively irregular pitch and effective refractive index, there is no unexpected feedback from one wavelength to another. Moreover, since the cavities of the lasers can be stacked on top of each other, the effects of spatial hole burning can be balanced.

[0019] This synopsis is provided to present, in a simplified form, the selection of concepts further described in the detailed description below. This synopsis is not intended to identify key or essential features of the claimed subject matter, nor is it intended to serve as an aid in determining the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to embodiments that address any or all of the shortcomings pointed out in any part of this disclosure. Attached Figure Description

[0020] Figure 1 A schematic diagram depicting a portion of an exemplary WSG according to this disclosure.

[0021] Figure 2 The length with phase shift at the center An example of a four-segment sinusoidal refractive index distribution for a multi-wavelength grating.

[0022] Figure 3 Showing source Figure 2 The WSG waveform of the summation of the sinusoidal refractive index distribution is shown in the figure.

[0023] Figure 4 The display is similar to Figure 2 The four single-wavelength piecewise sinusoidal refractive index distributions shown in the figure, but among them and The central section between them has a higher pitch than the other sections of the grating.

[0024] Figure 5 The display is by Figure 4 The complete analog waveform generated by summing the piecewise sine functions shown in the figure.

[0025] Figure 6 This describes the cold cavity reflection spectrum of an example of a real multi-wavelength CPM DFB laser.

[0026] Figure 7 exhibit Figure 6 Simulated refractive index distribution of the WSG of the laser.

[0027] Figure 8 The refractive index distribution of a four-channel multi-wavelength CPM DFB laser with a single-wavelength CPM segment having a forming cavity is shown.

[0028] Figure 9 exhibit Figure 8 The cold cavity reflection spectrum obtained from the CPM DFB laser

[0029] Figure 10 The simulated waveform of a WSG with a CPM cavity designed with non-uniform optical frequency spacing is shown.

[0030] Figure 11A A schematic plan view showing a portion of an illustrative embodiment of a laser including a WSG according to the present disclosure.

[0031] Figure 11B and 11C Representative examples depicting grating elements and gaps are shown respectively. Figure 11A A schematic diagram of the cross-section of the laser in the image.

[0032] Figure 12A This is a plan view of another example of a multi-wavelength CPM DFB laser.

[0033] Figure 12B and 12C They are along Figure 12AThe lines bb and cc in the middle are cut off Figure 12A A cross-sectional view of the laser. Detailed Implementation

[0034] A windowed sampling grating (WSG) is a specially modified Bragg grating in which the grating intensity (defined by the effective refractive index difference between the grating elements and the gap segments between the gratings) has a desired non-uniform distribution along the propagation direction within the grating. In some embodiments, the grating pitch varies along the propagation direction. This grating intensity and phase distribution are specifically tailored to produce resonance and therefore high reflectivity at a particular set of wavelengths.

[0035] WSG enables the creation of intracavity filters with emission spectra tailored for applications requiring multiple wavelengths (also known as “wavelength signals”), such as wavelength division multiplexing (WDM) systems. This emission spectrum then remains stable with respect to temperature variations, allowing for uncooled operation because the number of emitted laser modes will not change with temperature, and the pattern will drift with temperature due to thermo-optical effects (0.1 nm / C) rather than bandgap contraction (0.5 nm / C). Furthermore, WSG enables the creation of custom and arbitrary wavelength grids to meet any of the requirements of a wide range of WDM systems. It can produce wide-spaced grids for coarse wavelength division multiplexing (CWDM) systems, and tightly spaced grids for dense wavelength division multiplexing (DWDM), and anything in between. Gratings can be used in lasers to form distributed feedback (DFB) lasers.

[0036] As described in co-pending U.S. application Serial No. 17 / 465,403, the WSG can be described by a simulated refractive index distribution that varies along the length of the grating. Refractive index distribution This can be expressed by equations 1 to 6 as follows:

[0037]

[0038] Where N is the number of wavelengths reflected, such that in Equation 1, the refractive index distribution... Defined by the sum of N sinusoidal refractive index distributions. Each sine curve has... The refractive index range, The set defined by Equation 2 and the set defined by Equation 2 Defined pitch, The set defined by Equation 3 and a set of phases defined by Equation 4 By using the formula given in Equation 6, the desired emission wavelength can be determined. Export pitch. It is the set defined by Equation 5. These are window functions, such as Blackman or Tukey windows, which can be applied to waveforms to reduce the appearance of sidelobes. It can also be equal to 1. Then, the sum of the sinusoidal refractive index distributions is added as a refractive index perturbation to the effective refractive index of the waveguide. .

[0039] Figure 1 A schematic plan view depicting a portion of an exemplary WSG according to this disclosure. Grating 200 includes a series of grating elements 202-1 to 202-N, separated by gaps 204-1 to 204-N, respectively. Grating elements 202-1 to 202-N (collectively referred to as grating elements 202) are arranged along a longitudinal axis A1 and configured such that the grating intensity varies non-linearly along axis A1 in a generally smooth manner. Typically, grating elements 202 and gaps 204 are formed in a waveguide layer or grating element (GE) layer, which is operatively coupled to a waveguide layer and / or an active layer configured to provide optical gain (to collectively define the WSG laser).

[0040] The grating element 202 and the gap 204 can be formed in many well-known ways, such as by forming features in a suitable material layer, each of which is configured to define its desired reflectivity. As will be apparent to those skilled in the art, the reflectivity of the grating features can be controlled by controlling at least one of the feature width, feature depth, and gap distance, including structures located adjacent to each grating element and the like. Examples of structures that cause a change in the effective refractive index along the waveguide are described in co-pending U.S. Application Serial No. 17 / 465,403.

[0041] In the depicted example, each grating element 202-i (wherein) The grating intensity at point N is determined by the width of the grating element. The grating intensity variation is determined and defined by its neighboring gap 204i, which varies along the vertical axis A1 according to the singer function convolved with an infinite sampling function in the length domain. However, it should be noted that numerous grating intensity variations are within the scope of this disclosure, including but not limited to sine functions, sums of sine functions, convolved sine functions, and singer functions.

[0042] As previously mentioned, quarter-wave shifted DFB lasers operating at a single wavelength are well known. According to this disclosure, this can be achieved by […] along the longitudinal axis of the WSG. At a certain position, a phase shift is inserted into each of the individual sinusoidal refractive index distributions in Equation 1 to generate a quarter-wave shifted DFB laser operating at multiple wavelengths. At this position, the N sinusoidal refractive index distributions in Equation 1 now each represent a subfunction of a piecewise function, where each subfunction of the piecewise function is a WSG representing a portion of the entire grating. The subfunction is uniformly shifted in phase. .

[0043] Figure 2 The length of the phase shift at each center is shown in the figure. Examples of four sinusoidal refractive index distributions of the grating. The expression can then be summed to produce a quarter-wave shifted WSG function. This is mathematically expressed in Equation 7 as:

[0044]

[0045] Equation 7 is similar in principle to Equation 1, except that it is now a piecewise formula used to define different segments of the grating. Along a certain length of the grating... After that, additional A phase shift is added to the sine curve. Typically, for However, this value is not the only one that can be used; for example, some designs used for manufacturing constraints may require... yes It should be an integer multiple of the specified value. Note that if improved directionality is required, the phase shift can be placed asymmetrically. Figure 3 Showing source Figure 2 The WSG waveform showing the summation of the sinusoidal refractive index distribution is illustrated. Please note that... Figure 2 and 3 The waveform shown is much shorter and has a wider wavelength interval than that of an actual WSG DFB laser. This arrangement makes the phase shift visible compared to the rest of the grating.

[0046] As previously mentioned, quarter-wave shifted single-wavelength DFB lasers suffer from spatial aperture, caused by high power accumulation near the phase shift. Spatial aperture increases optical linewidth and the likelihood of longitudinal multimode under high current bias. The effects of spatial aperture can be mitigated by replacing the phase shift with a distributed phase shift in the form of grating segments with a pitch slightly different from that of most lasers. This essentially increases the laser cavity size from half the grating pitch to tens of micrometers, thereby significantly reducing the concentration of optical power and thus mitigating the effects of spatial aperture. Grating segments with this distributed phase shift are called corrugated pitch modulation (CPM) grating segments and are discussed in Makoto Okai et al., 1994, Jpn. J. Appl. Phys. 33 2563, the entire contents of which are hereby incorporated by reference.

[0047] The use of CPM grating sections can be extended to multi-wavelength DFB lasers to provide multi-wavelength CPM DFB lasers. Figure 4 The display is similar to Figure 2The four single-wavelength piecewise sinusoidal refractive index distributions shown in the figure, but among them and The central segment of the grating has a higher pitch than the other segments. These sinusoidal functions, individually used, represent instances of CPM DFB lasers. The summation of the waveforms represented by these functions results in the WSG of the central CPM grating segment with the higher pitch. Figure 5 The display is by Figure 4 The complete analog waveform produced by the sum of the piecewise sine functions shown is illustrated above. As with the quarter-wave shifted DFB above, the length and wavelength interval of this WSG have been chosen for clarity and do not necessarily represent an actual embodiment. These gratings have the mathematical form shown in Equation 8:

[0048]

[0049] Equation 8 is a finite sum of the piecewise formulas for individual grating segments of the WSG. The grating has M segments, and each , , Can be for each Different sets of values. Each grating segment Depend on Each subfunction is characterized, where each subfunction is defined as a summation of a sine expression over wavelength, the summation representing the grating at a single wavelength defined by Equation 6. Furthermore, segments do not need to have the same number of wavelength channels because... (It used to be the integers in equations 1 and 7) now contains A set of values. Figure 4 and 5 Instances in have , and collection , , For all three sections that are the same, except (The Bragg wavelength value in section 2) has been increased. Note that in this example, In other words, the CPM segment is asymmetrically positioned within the WSG. and The longer grating sections between the laser beams increase the reflection intensity of these sections, thereby directing more output power to the multi-wavelength CPM DFB laser. Side. In other embodiments, different grating segments may have different lengths, and CPM segments may be distributed symmetrically or asymmetrically in the WSG.

[0050] Figure 6This illustrates the cold cavity reflection spectrum of an example of a practical multi-wavelength CPM DFB laser. This device has four emission wavelengths spaced 1 nm apart, which can be seen through the four characteristic stopbands between 1300 and 1305 nm at the central notch. The simulated refractive index distribution of this WSG used for this laser is shown in [insert WSG value here]. Figure 7 As shown in the example, the refractive index distribution has been flattened to allow the production of a semiconductor multiwavelength CPM DFB laser using the techniques described in the previously mentioned U.S. Patent Application Serial No. 17 / 465,403. While this may be attributed to the cavity ratio compared to the pitch and the lower relative difference in pitch between the WPM and WSG segments... Figure 7 It is not easily seen, but the central group of the three peaks in the refractive index distribution has a grating pitch corresponding to a center wavelength of 1335 nm, while the rest of the grating has a grating pitch corresponding to a center wavelength of 1302.5 nm.

[0051] Another example of a multi-wavelength CPM DFB laser employs a WSG with a refractive index distribution, represented by a special case of Equation 8, in which the number of piecewise functions in the central cavity segment... It has only a length of 1. That is to say, there is a simple harmonic grating in the cavity segment. Figure 8 The refractive index distribution of this grating shown in the figure is similar in other ways to Figure 7 The grating in the comb, besides having a single wavelength grating segment as a cavity, needs to be detuned to the wavelength in the comb and needs to have a specific length for accumulation. or an integer multiple thereof. Figure 9 The image shows the cold cavity reflection spectrum of this laser. Again, the four stopband segments with a central defect are shown. (Compared to...) Figure 7 Compared to WSG, this grating has the advantage of being simpler to design and manufacture, but the length of the single-wavelength CPM segment before the cavity begins to decouple across the grid and stopband is shorter. Figure 7 The CPM segment is long. This can be seen as an advantage because multi-wavelength CPM DFB lasers will be more compact, or as a disadvantage because optical power will be more concentrated in a shorter CPM segment.

[0052] Figure 5 Another example demonstrating a complete analog waveform of WSG, the waveform having a set of ordered superperiods, two in and At the pitch of the reflector sections between, one in and At the pitch of the CPM segment between, and another at and The presence of these hyperperiods is related to the wavelength spacing in the comb of wavelengths reflected by the grating. If the optical frequency spacing in the comb is uniform, then the grating will exhibit a hyperperiodic appearance. While uniformly spaced combs are a potential application, it may be desirable to have non-uniformly spaced combs, for example, to suppress four-wave mixing or to have discrete wavelength groups. In this case, hyperperiods will not appear and the grating may exhibit a more disordered appearance, such as... Figure 10 As shown in the image.

[0053] Figure 11A This is a schematic plan view illustrating a portion of an illustrative embodiment of a laser including a WSG according to the present disclosure. Laser 600 includes a gain element 602 and a WSG 200, optically coupled to jointly define a multi-wavelength CPM DFB laser structure. The grating structure of the WSG 200 includes grating elements 202 and gaps 204, configured to define a waveguide-based grating whose refractive index varies along the vertical axis x according to a piecewise mathematical function.

[0054] WSG 200 corresponds to Figure 5 The simulated waveforms shown in the image. Figure 6 As shown in A, WSG 200 includes segments representing mirror function 620 and cavity function 630.

[0055] Figure 11B and 11C Schematic diagrams showing cross-sectional views of a laser structure 600 depicting representative examples of grating element 202 and gap 204. (The diagrams are shown along...) Figure 11A The lines aa and bb are used to cut off Figure 11B and 11C As shown, the layer structure of laser 600 includes a lower cladding layer 606, an active layer 608, a grating element (GE) layer 610, and a top cladding layer 612.

[0056] The lower cladding layer 606 is a conventional cladding layer suitable for inclusion in a DFB laser structure. In one example, the lower cladding layer 606 may comprise aluminum gallium arsenide (AlGaAs) formed on a substrate (not shown), such as a conventional gallium arsenide (GaAs) substrate. Gain elements 602 are defined by an active layer 608, which may be a GaAs layer including, for example, quantum elements (e.g., quantum dots, quantum scribing, quantum wells, etc.) to enable optical gain. The GE layer 610 formed on the active layer 608 may be a GaAs layer. The grating structure of the WSG 200 can be formed in the GE layer 610 by completely removing the layer in the gap region to expose the underlying active layer 608. In some embodiments, the grating structure of the WSG 200 is formed by partially etching the GE layer 610 in the gap region.

[0057] Each of the grating elements 202 comprises a portion of the GE layer 610 with a deposited thickness of t1. In other words, each of the grating elements is the unetched portion of the GE layer 610. In the depicted example, the refractive index of each grating element 202-i is based on its width. The refractive index is defined by the width of its corresponding adjacent gap 204-i. According to this disclosure, a generally smooth refractive index distribution along the longitudinal axis A1 can be achieved because the refractive index at each grating element is based on a photolithographically defined parameter of the grating element. Furthermore, although this parameter is the width of the grating element in the depicted example, any suitable parameter variation can be used without departing from the scope of this disclosure.

[0058] For example, in some embodiments, a smoothly varying refractive index function is achieved along the vertical axis A1 by including features on either side of each grating element. The spacing between the structure and the grating elements determines the refractive index at each grating element, and it varies along the length of the grating to change the refractive index distribution. Since this spacing is defined photolithographically, it can vary smoothly according to any desired piecewise mathematical function (e.g., a sine curve, a singer function, a combination of sine curves, a combination of sine curves and singer functions, etc.).

[0059] The upper cladding 612 is formed as a ridge structure on the GE layer 610 via a regeneration technique. The upper cladding 606 is similar to the lower cladding 604 and may include, for example, aluminum gallium arsenide (AlGaAs).

[0060] By varying the width (dimension along the y-axis, as shown) or depth (dimension along the x-axis, as shown) of this etched pattern, a WSG DBR or DFB is formed, where the grating elements of the grating are defined by the etched pattern. This type of waveguide can also be implemented in fiber Bragg reflectors by varying the refractive index of a segment along the waveguide.

[0061] It should be noted that although the depicted examples include WSGs formed in grating element layers included in the structure for this purpose, the inclusion of grating element layers is optional, and the WSGs may be implemented in other layers of the structure without departing from this disclosure. For example, in some embodiments, the WSG is at least partially formed in the following:

[0062] i. The top cladding of the structure; or

[0063] ii. A passive waveguide coupled to structured optics; or

[0064] iii. The active layer of the structure; or

[0065] iv. Any combination of i, ii and iii.

[0066] As will be apparent to those skilled in the art upon reading this specification, the completion of a laser according to this disclosure typically involves several additional conventional operations, such as device slicing, laser facet formation, and the like, which are not described herein. However, it should be noted that one or both facets of a laser according to this disclosure may, as needed, incorporate additional materials (e.g., a high-reflectivity layer, an anti-reflective coating, etc.) to ensure laser emission at the desired wavelength.

[0067] Figures 12A to 12C A schematic diagram showing another embodiment of a multi-wavelength CPM DFB laser 700 including a WSG 200 according to the present disclosure is shown. Figure 12A This is a plan view of the multi-wavelength CPM DFB laser 700. Figure 12B and 12C They are along Figure 12A The diagram shows a schematic cross-sectional view of the laser structure of laser 700, taken by lines aa and bb. This embodiment is fabricated using a hybrid integration technique that avoids the need for epitaxial regeneration. WSG 200 includes grating elements 202 and gaps 204. WSG 200 includes segments representing mirror functions 720 and cavity functions 730. As explained below, hybrid integration is used to bond contacts, cladding, and active layers to the semiconductor waveguide after forming WSG 200 in the semiconductor waveguide.

[0068] As shown in the cross-sectional view, laser 700 includes a lower cladding 704, a waveguide 706, a contact layer 708, an active layer 710, and an upper cladding 712 defined as a ridge. Waveguide 706 is a material layer suitable for transmitting light generated by the active layer 710. For example, in one embodiment, waveguide 700 comprises silicon, and the lower cladding comprises silicon dioxide. Waveguide 706 is optically coupled to the active layer 710 via contact layer 708, which facilitates bonding between the waveguide material and the material of the active layer 710.

[0069] The WSG 200 of the laser 700 is formed by etching wells 714 in waveguide 706, wherein the effective refractive index of the grating element is based on the values ​​of the width and depth of each well 714, at least one of which varies along the longitudinal axis of the laser 700 according to a piecewise mathematical function (e.g., a sine curve, a singer curve, etc.) as discussed above.

[0070] The contact layer 708, the functional layer 710, and the upper cladding layer 712 can be fabricated in a separate process by epitaxially growing layers on a sacrificial substrate, after which the layers can be flipped and bonded to the waveguide 706 using hybrid manufacturing techniques (e.g., direct bonding, plasma bonding, fused wire bonding, hot anodic bonding, and the like).

[0071] It should be noted that although many of the examples provided herein are based on III-V semiconductor-based DFB laser waveguides, embodiments according to this disclosure can be formed in any waveguide platform suitable for forming Bragg gratings, such as silicon nitride waveguides, doped silicon dioxide core waveguides, passive silicon-on-insulator waveguides, thin-film filters, and the like. Furthermore, the Bragg grating can be present on both sides of the laser gain material or only on one side (typically, different types of reflectors are on the other side) without departing from the scope of this disclosure.

[0072] In the examples presented above, the grating is described as a windowed sampling grating. More generally, any suitable multi-wavelength grating can be used. For example, instead of a windowed sampling grating, multi-wavelength gratings such as binary superimposed gratings (BSGs) and phase gratings, which use the same concepts and equations, can be used. Because the grating features of a BSG (e.g., grating elements and gaps) are typically formed by etching a single surface, a BSG has only two levels of refractive index. A phase grating is similar to a BSG, but is formed by etching a single-pitch, through... It consists of several uniform gratings of different lengths that are phase-shifted and separated.

[0073] Furthermore, as discussed above, Equations 7 and 8 describe analog waveforms that can be discretized using the method shown in U.S. Patent Application No. 17 / 465,403 [File No. 3218-008US1]. However, other discretization methods can be used instead. For example, the binary superimposed grating (BSG) discretization described in IA Avrutsky et al., “Design of Widely Tunable Semiconductor Lasers and the Concept of Binary Superimposed Gratings (BSGs)”, JSTQE 34 4 1998, can be applied to BSG-based laser cavities.

[0074] It should be understood that the embodiments taught in this disclosure are merely some examples, and many variations of this disclosure can be readily designed by those skilled in the art upon reading it, and the scope of the invention will be determined by the appended claims.

Claims

1. A photonic element comprising: Bottom cladding; Top cladding; The first waveguide is located between the bottom cladding and the top cladding; and A first multi-wavelength grating, optically coupled to the first waveguide, wherein the first multi-wavelength grating is characterized by grating intensity and / or phase varying along a first axis based on a piecewise mathematical function; and The first waveguide and the first multi-wavelength grating together achieve output signals at multiple wavelengths.

2. The photonic element according to claim 1, wherein the first multi-wavelength grating is a windowed sampling grating (WSG).

3. The photonic element according to claim 1, wherein the first multi-wavelength grating is a binary superimposed grating (BSG).

4. The photonic element according to claim 1, wherein the first multi-wavelength grating is a phase grating (PG).

5. The photonic element of claim 1, wherein the first multi-wavelength grating has a plurality of grating segments, each characterized by a grating intensity varying along the first axis by a subfunction based on the piecewise mathematical function, wherein each of the subfunctions is defined as a summation of an expression over the plurality of wavelengths, the summation representing a grating at a single wavelength defined in the expression.

6. The photonic element of claim 5, wherein the first multi-wavelength grating includes additional grating segments characterized by grating phase varying along the first axis by a subfunction based on the piecewise mathematical function, the subfunction generating a quarter-wavelength phase shift for each of the plurality of wavelengths.

7. The photonic element of claim 6, wherein the additional grating segment defines the summation of the corrugated pitch modulation grating, wherein the quarter-wave phase shift is distributed on the grating segment.

8. The photonic element of claim 6, wherein the additional grating segment is located off-center within the multi-wavelength grating.

9. The photonic element of claim 7, wherein the additional grating segment is located off-center within the multi-wavelength grating.

10. The photonic element of claim 1, wherein the wavelengths of the plurality of wavelengths of the output signal define a uniformly spaced frequency comb.

11. The photonic element of claim 1, wherein the wavelengths of the plurality of wavelengths of the output signal define a non-uniformly spaced frequency comb.

12. The photonic element of claim 1, wherein the first multi-wavelength grating is further characterized by a window function that limits the plurality of wavelength signals to a first wavelength range.

13. The photonic element of claim 1, further comprising a gain element layer optically coupled to the first waveguide, wherein the gain element layer comprises the first multi-wavelength grating.

14. The photonic element of claim 1, wherein the top cladding comprises the first multi-wavelength grating.

15. The photonic element of claim 1, wherein the first multi-wavelength grating is located below the first waveguide.

16. The photonic element of claim 1, further comprising a gain element optically coupled to the first multi-wavelength grating and the first waveguide, wherein the gain element, the first multi-wavelength grating, and the first waveguide together define a first laser that generates the plurality of wavelength signals.

17. The photonic element of claim 16, wherein the first waveguide includes the gain element.

18. The photonic element of claim 17, wherein the gain element comprises an active layer optically coupled to the first waveguide, wherein the active layer comprises at least one quantum element selected from the group consisting of quantum dots, quantum scribings, quantum wires and quantum wells.

19. The photonic element of claim 1, wherein the piecewise mathematical function comprises a sine curve or a singer function.

20. A method for providing a first output signal comprising a plurality of wavelength signals, the method comprising: To achieve the propagation of the first optical signal in the first waveguide located between the lower cladding and the upper cladding; and The first optical signal is optically coupled to a multi-wavelength grating, wherein the first multi-wavelength grating is characterized by grating intensity and / or phase varying along a first axis based on a piecewise mathematical function, wherein the multiple wavelength signals are based on the piecewise mathematical function.

21. The method of claim 20, wherein the first multi-wavelength grating is a windowed sampling grating (WSG).

22. The method of claim 20, wherein the first multi-wavelength grating is a binary superimposed grating (BSG).

23. The method of claim 20, wherein the first multi-wavelength grating is a phase grating (PG).

24. The method of claim 20, wherein the first multi-wavelength grating has a plurality of grating segments, each characterized by a grating intensity varying along the first axis by a sub-function based on the piecewise mathematical function, wherein each of the sub-functions is defined as a summation of an expression over the plurality of wavelengths, the summation representing a grating at a single wavelength defined in the expression.

25. The method of claim 24, wherein the first multi-wavelength grating includes additional grating segments characterized by grating phase varying along the first axis by a subfunction based on the piecewise mathematical function, the subfunction generating a quarter-wavelength phase shift for each of the plurality of wavelength signals.

26. The method of claim 25, wherein the additional grating segment defines the summation of the corrugated pitch modulation grating, wherein the quarter-wave phase shift is distributed on the grating segment.

27. The method of claim 25, wherein the additional grating segment is located off-center within the multi-wavelength grating.

28. The method of claim 26, wherein the additional grating segment is located off-center within the multi-wavelength grating.

29. The method of claim 20, wherein the wavelength signals of the plurality of wavelength signals of the output signal define a uniformly spaced frequency comb.

30. The method of claim 20, wherein the wavelength signals of the plurality of wavelength signals of the output signal define a non-uniformly spaced frequency comb.

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