Post-fabrication tuning of silicon waveguide components using multi-shot ultrashort pulse laser irradiation

Abstract

Disclosed examples generally relate to post-fabrication tuning of silicon waveguide components using multi-shot ultrashort pulse laser irradiation. In at least one example, the assembly includes: (i) a laser irradiation system, comprising: a ultrashort pulse laser configured to emit a plurality of ultrashort laser pulses; an optical train arranged to direct the plurality of laser pulses to a target location on or proximate to the silicon waveguide component; and (ii) an optical characterization system configured to measure an optical response of the silicon waveguide component to determine a resonance wavelength of the component.

Description

POST-FABRICATION TUNING OF SILICON WAVEGUIDE COMPONENTS USING MULTI-SHOT ULTRASHORT PULSE LASER IRRADIATION CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims priority to, and the benefit of, United States Provisional Patent Application No. 63 / 765,154, filed on February 28, 2025.FIELD

[0002] The present invention generally relates to silicon photonics, and more particularly, to post-fabrication tuning of silicon waveguide components using multi-shot ultrashort pulse laser irradiation.BACKGROUND

[0003] Silicon photonics have emerged as a promising platform for advanced on chip integrated optical devices and advanced microelectronic circuits during the last decades. Owing to its favorable optical properties such as a high index of refraction and transparency to telecommunication wavelengths, the technology has attracted a large amount of interest from both the academic community and industry.

[0004] For example, silicon photonic integrated circuits (SiPICs) are micrometer-sized structures integrated on a silicon chip which are used to perform various optical information processing functions such as switching, multiplexing / demultiplexing and filtering. SiPICs typically include waveguides components such as couplers, Mach-Zehnder interferometers, microring resonators, gratings, etc., whose optical responses need to be accurately controlled for the proper operation of the whole circuit.

[0005] Over the past decade, the technology has seen tremendous developments from both academia and industry for various applications including fiber optic communication, neuromorphic computing for machine learning, sensors, LiDARs, and quantum computing. For example, one of the1WSLEGAL\055326\0051 l\43474905v3most promising applications of SiPICs is in replacing the electrical interconnects for data transmission between servers with optical interconnects.

[0006] To that end, a large refractive index refraction contrast of the waveguide in Silicon on Insulator (SOI) chips enables high-density device integration due to the high modal confinement. However, silicon photonics devices suffer from extreme sensitivity to fabrication precision and tolerance. Even highly advanced fabrication processes still introduce uncertainty in device shape and dimension, leading to a significant deviation from a device’s designed spectral response. For example, a variation of 1 nm width in a Si-based microring resonator waveguide can shift the operating resonance wavelength by 2 nm or more.

[0007] The fabrication errors mentioned above have prompted the development of methods of post-fabrication correction for permanently tuning the phase of devices to ensure their performance. Approaches include localized recrystallization via ion implantation, deposition of the photosensitive layer, e-beam trimming, UV laser trimming of hydrogenated amorphous silicon waveguide, visible laser annealing of hydrogen silsesquioxane cladding, and ultrasfast laser-induced crystalline modifications and ablation.SUMMARY

[0008] Examples herein provide for a multi-shot ultrashort pulse laser surface modification method and system for effecting permanent post-fabrication tuning of waveguide components, e.g., used in a silicon photonic circuits.

[0009] Disclosed examples can be employed for correcting the optical response properties of waveguide components to ensure proper operation of silicon photonic circuits. The physical mechanism for tuning is the change of refractive index of a thin layer of silicon. The increase of refractive index results from modifications to the crystalline structures. In contrast, the decrease of refractive index results from removal of a thin layer of silicon owing to ablation. These modifications can be achieved with a broad ranges of laser wavelengths, pulse durations, pulse numbers and laser fluences. The multi-shots thresholds are significantly lower than the single-shot thresholds because of the incubation effect.2WSLEGAL\055326\0051 l\43474905v3

[0010] In view of the foregoing, and in accordance with a broad aspect, there is provided a method for post-fabrication tuning of a waveguide component, comprising applying a multi-shot femtosecond laser to the waveguide component.

[0011] In another broad aspect, there is provided a system for post-fabrication tuning of a waveguide component, wherein the system comprises an ultrashort pulse laser, which is operated to apply multiple laser shots to the waveguide component.

[0012] In some examples, the waveguides component comprises a coupler, Mach-Zehnder interferometer, microring resonator, grating, etc.

[0013] In some examples, the method involves using low pulse energy pulses in a range a few nano-joules, which allows low-cost and compact laser systems to be used.

[0014] In some examples, the waveguide component is irradiated with 1 to 1000 laser pulses in the same location of the waveguide with selected laser fluence.

[0015] In some examples, the laser beam has 800 nm wavelength, 50 or 130 fs pulse duration, a Gaussian spatial profile with a beam waist radius of 13 pm, and pulse energies of 10 nJ to 0.5 pj to generate the applied range of laser fluences.

[0016] In some examples, the laser beam has 1030 nm wavelength, 350 fs pulse duration, a Gaussian spatial profile with a beam waist radius of 4.6 pm or 13 pm and pulse energies of 5 nJ to 0.2 pj to generate the applied range of laser fluences.

[0017] In some examples, the wavelength is 400 nm, 50 fs pulse duration, a Gaussian spatial profile with a beam waist radius of 6 pm, and pulse energies of 10 nJ to 0.2 pj to generate the applied range of laser fluences.

[0018] In some examples, the single or multi-shot irradiation allows bi-directi on tuning with positive or negative refractive index changes with acceptable induced losses.

[0019] In some examples, the multi-shot irradiation allows fine tuning with high resolution. For example, this includes control of the resonant wavelength of microring circuit to a few picometers.

[0020] In some examples, the method allows for post-fabrication trimming of silicon photonic integrated circuits.3WSLEGAL\055326\0051 l\43474905v3

[0021] In at least one broad aspect, there is provided an assembly for post-fabrication tuning of a silicon waveguide component, comprising: a multi-shot laser irradiation system, comprising: an ultrashort pulse laser configured to emit a plurality of ultrashort laser pulses; an optical train arranged to direct the plurality of laser pulses to a target location on or proximate to a waveguide portion of the component; and an optical characterization system configured to measure an optical response of the silicon waveguide component to determine a resonance wavelength thereof.

[0022] In some examples, the ultrashort pulse laser source is configured to emit the plurality of laser pulses at a visible to near-infrared wavelength, in a range from about 400 nm to about 2500 nm.

[0023] In some examples, the wavelength range is about 400 nm to 2500 nm.

[0024] In some examples, the plurality of ultrashort laser pulses have a pulse duration in a range from about 1 fs to about 10 ps.

[0025] In some examples, the plurality of ultrashort laser pulses have a pulse energy in a range from about 1 nJ to less than about 1 mJ per pulse.

[0026] In some examples, the optical characterization system comprises: one or more of (i) a broadband optical source; and (ii) a swept frequency source; a transmitter lensed fiber configured to couple light from broadband source and / or swept frequency source, into the silicon waveguide component; a receiver lensed fiber configured to receive light from the silicon waveguide component; and one or more of (i) an optical spectrum analyzer, and (ii) a photodetector, coupled to the receiver lensed fiber to measure light.

[0027] In some examples, the silicon waveguide-based photonic component comprises one of a microring resonator, a Mach-Zehnder interferometer, a directional coupler, a grating coupler, a Bragg grating, or a racetrack resonator.

[0028] In some examples, the laser system is configured to selectively produce either: (i) a positive resonance wavelength shift by inducing a permanent increase in effective refractive index at the target location, or (ii) a negative resonance wavelength shift by inducing material removal at the target location.4WSLEGAL\055326\0051 l\43474905v3

[0029] In some examples, at a selected laser fluence, the positive resonance wavelength shift is produced, by delivering to the target location, a first number of the pulses that is at or below a pulsecount threshold, and the negative resonance wavelength shift is produced, by delivering to the target location, a second number of the pulses that is greater than the pulse-count threshold.

[0030] In some examples, the plurality of ultrashort laser pulses are controlled to provide trimming resolution of resonance wavelength shift of about 2 to 3 picometers per pulse and a full trimming range equal to the Free Spectral Range.

[0031] In another broad aspect, there is provided a post-fabrication tuning of a silicon waveguide component using the above-noted assembly, comprising: positioning the silicon waveguide component relative to the laser irradiation system such that the optical train directs the plurality of ultrashort laser pulses to a target location on or proximate the silicon waveguide component; measuring, with the optical characterization system, an initial resonance wavelength of the silicon waveguide component; operating the femtosecond laser to apply a selected number of the plurality of ultrashort laser pulses to the target location; and measuring, with the optical characterization system, a resonance wavelength of the silicon waveguide component to determine a resonance wavelength shift relative to the initial resonance wavelength and a target resonance wavelength.

[0032] In some examples, the measuring and operating are automatically performed by a controller.

[0033] Other features and advantages of the present application will become apparent from the following detailed description taken together with the accompanying drawings. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the application, are given by way of illustration only, since various changes and modifications within the spirit and scope of the application will become apparent to those skilled in the art from this detailed description.BRIEF DESCRIPTION OF THE DRAWINGS

[0034] For a better understanding of the various embodiments described herein, and to show more clearly how these various embodiments may be carried into effect, reference will be made, by way of example, to the accompanying drawings which show at least one example embodiment, and5WSLEGAL\055326\0051 l\43474905v3which are now described. The drawings are not intended to limit the scope of the teachings described herein.

[0035] FIG. 1A is a setup for a laser irradiation and optical characterization assembly.

[0036] FIG. IB is a setup for a laser irradiation system.

[0037] FIG. 2 plots examples of measured original resonance wavelength (solid blue) and measured data (solid red) of a microring resonator after laser irradiation using 800 nm wavelength and 130 fs pulse duration with a laser fluence of 0.055 J / cm2for N = 10 pulses. The corresponding curve fittings of the measured data are shown in dashed lines, respectively.

[0038] FIG. 3 is a plot showing resonance wavelength shifts of the resonators after irradiation using 800 nm wavelength and 130 fs pulse duration with different fluences at a pulse number of N = 1, 10, and 100.

[0039] FIG. 4 shows examples of positive and negative resonance wavelength shifts with N =10 pulses of 800 nm wavelength and 130 fs pulse duration. The spectral response is shown before irradiation (blue curve) and after irradiation (red curve): (a) a positive wavelength shift of 1.36 nm was achieved when irradiated at 0.021 J / cm2; (b) and (c) are corresponding optical microscopy and SEM images respectively for case (a); (d) a negative wavelength shift of -3.26 nm was achieved at a fluence of 0.063 J / cm2; and (e) and (f) are corresponding optical microscopy and SEM images of the irradiated area respectively for case (d).

[0040] FIG. 5 is a plot of induced roundtrip losses for the microring resonators as a function of resonance wavelength shifts for the N = 10 (solid triangle symbol) and N = 100 (hollow circle symbol) pulses of 800 nm wavelength and 130 fs pulse duration.

[0041] FIG. 6 are SEM images and enlarged details of the irradiated area on microring resonators at different laser fluences for N = 10 pulses of 800 nm wavelength and 130 fs pulse duration, (a) 0.012 J / cm2. (b) 0.03 J / cm2. (c) 0.06 J / cm2. (d) 0.063 J / cm2.

[0042] FIG. 7 is a plot showing a Raman spectrum of an irradiated area of a microring after irradiation by 10 pulses of 800 nm wavelength and 130 fs pulse duration at different laser fluences from 0.021 J / cm2to 0.063 J / cm2, respectively. The figure insert presents a clear view of the amorphous silicon peak on a log scale.6WSLEGAL\055326\0051 l\43474905v3

[0043] FIG. 8 is an example of the Gaussian beam profile applied in an experiment: (a) the greyscale image of the Airy Ring beam was obtained from a CCD camera; and (b) corresponding intensity distribution profile of the beam and the fitting to Gaussian profile.

[0044] FIG. 9 is an example of SLED spectrum extracted on OSA with wavelength range from 1200 nm to 1700 nm. The figure insert presents the narrower bandwidth of the spectrum from 1540 nm-1560 nm.

[0045] FIG. 10 is an example of the resonance wavelength for a racetrack microring resonator by applying a real-time characterization system (blue curve) using SLED as the light source and output coupled to OSA for spectrum measurement, compared to a resonator measured by an alternative optical characterization system.

[0046] FIG. 11 is a spectrum of the accumulated resonance wavelength shifts corresponding to different pulse numbers of irradiation using 800 nm wavelength and 50 fs pulse duration at a fluence of 0.0232 J / cm2: (a) pulse numbers range from N= 0 to N= 390, and (b) pulse numbers range from N= 390 toN= 790.

[0047] FIG. 12 shows plots of the accumulated resonance wavelength shifts corresponding to different pulse numbers of irradiation using 800 nm wavelength and 50 fs pulse duration at a fluence of 0.0232 J / cm2: (a) the accumulated shift for N=1 to N= 390, and (b) the accumulated shift for N= 390 toN= 790.

[0048] FIG. 13 is a plot of showing induced roundtrip loss of microring resonators as a function of laser pulse number irradiated at a fluence of 0.0232 J / cm2from irradiation using 800 nm wavelength and 50 fs pulse duration.

[0049] FIG. 14 is a plot of showing resonance wavelength shifts of the resonators after each laser pulse irradiation at a fluence from 0.0232 J / cm2to 0.0502 J / cm2from irradiation using 800 nm wavelength and 50 fs pulse duration.

[0050] FIG. 15 shows a schematic configuration of a Mach-Zehnder interferometer (MZI) structure.

[0051] FIG. 16 shows normalized transmission power measured from the output of a 3dB MZI structure after each 800 nm wavelength and 50 fs pulse duration laser pulse irradiation, at a fluence of 0.0426 J / cm2, 0.0505 J / cm2and 0.0568 J / cm2, respectively.7WSLEGAL\055326\0051 l\43474905v3

[0052] FIG. 17A shows accumulated resonance wavelength shifts corresponding to different pulse numbers at a fluence of 0.02778 J / cm2from irradiation using 800 nm wavelength and 50 fs pulse duration.

[0053] FIG. 17B shows induced round-trip loss of microring resonators as a function of laser pulse number irradiated at a fluence of 0.02788 J / cm2from irradiation using 800 nm wavelength and 50 fs pulse duration.

[0054] FIG. 18 shows an example method for post-fabrication tuning of silicon waveguides, e.g., using the assembly of FIG. 1 A.

[0055] FIG. 19 shows an example hardware configuration for a controller.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0056] Disclosed examples provide for a system and method for post-fabrication tuning of silicon waveguide components, using multi-shot ultrashort pulse laser irradiationI. DEFINITIONS

[0057] Any term or expression not expressly defined herein shall have its commonly accepted definition understood by a person skilled in the art. As used herein, the following terms have the following meanings.

[0058] “Ultrashort pulse laser” means a laser system configured to emit ultrashort optical pulses, typically having a pulse duration in a range from about 1 femtosecond (fs) to about 10 picoseconds (ps) (i.e., an "ultrashort pulse").

[0059] “Fluence” refers to the energy per unit area delivered by a laser pulse and is a critical parameter in laser-matter interactions, typically reported in units of (J / cm2).

[0060] "Memory" refers to a non-transitory tangible computer-readable medium for storing information in a format readable by a processor, and / or instructions readable by a processor to implement an algorithm. The term "memory" includes a plurality of physically discrete, operatively connected devices despite use of the term in the singular. Non-limiting types of memory include solid-state, optical, and magnetic computer readable media. Memory may be non-volatile or volatile.8WSLEGAL\055326\0051 l\43474905v3Instructions stored by a memory may be based on a plurality of programming languages known in the art, with non-limiting examples including the C, C++, Python ™, MATLAB ™, and Java ™ programming languages.

[0061] “Multi-shot laser” refers to a laser system that emits (or is capable of emitting) a series or plurality of discrete pulses over time, rather than a single pulse. These pulses can be delivered at a controlled repetition rate and with a defined energy per pulse, enabling precise and repeatable interactions with a target material or optical system. The number of pulses, denoted as N (e.g. N= 2 to 10), may be varied to achieve cumulative effects.

[0062] “Multi-shot tuning curve” refers to a curve expressing resonance wavelength shifts as a function of laser fluences.

[0063] "Processor" refers to one or more electronic devices that is / are capable of reading and executing instructions stored on a memory to perform operations on data, which may be stored on a memory or provided in a data signal. The term "processor" includes a plurality of physically discrete, operatively connected devices despite use of the term in the singular. Non-limiting examples of processors include devices referred to as microprocessors, microcontrollers, central processing units (CPU), and digital signal processors.

[0064] “Silicon waveguide-based photonic component” (or "silicon waveguide component") means a photonic device formed on a silicon-based photonic platform, including a silicon-on-insulator (SOI) platform, and comprising at least one silicon waveguide portion configured to guide optical radiation and to provide a defined optical function.

[0065] "Silicon waveguide" means a structure formed in and / or on a silicon-based photonic platform (e.g., a Silicon on Insulator (SOI)) and configured to confine and guide optical radiation along a defined path by refractive index contrast relative to surrounding material.

[0066] “Silicon photonic integrated circuit” (SiPIC) means an integrated photonic circuit formed on a silicon-based substrate and comprising one or more silicon waveguides and associated photonic components which are configured, for example, to generate, route, filter, modulate, and / or detect optical signals on-chip.

[0067] "Resonant wavelength shift" means a change in a resonance wavelength of a silicon waveguide component relative to a baseline resonance wavelength, the change being induced by a9WSLEGAL\055326\0051 l\43474905v3tuning operation. A positive resonant wavelength shift corresponds to an increase in the resonance wavelength, and a negative resonant wavelength shift corresponds to a decrease in the resonance wavelength. Many devices such as Mach-Zehnder interferometers do not have “resonance wavelengths”. The “resonance wavelength” is used here as a proxy term to encompass phase shifts in other devices.II. GENERAL OVERVIEW

[0068] SiPICs are often fabricated using standard complementary metal-oxide-semiconductor (CMOS) processes, as known in the art, from the semiconductor industry, such as deposition, lithography (e.g., UV or electron beam), and dry etching.

[0069] Unfortunately, process variations across a wafer (or even a die) cause device dimensions and material parameters to deviate from specifications, resulting in sub-optimal operation or even failure of the device. These process variations are currently unavoidable (e.g. better than 1 nm control of all waveguide dimensions is required for proper operation) and therefore it is necessary to tune the device characteristics after fabrication in order to restore the proper operation of the device.

[0070] In view of the foregoing, disclosed examples provide for a new system and method for post-fabrication correction of errors in silicon PICs using multi-shot ultrashort pulse laser irradiation.

[0071] Particularly, by irradiating silicon photonic waveguides with low-energy femtosecond laser pulses, the surface of the silicon waveguide is modified, causing a change in its effective refractive index. By changing the effective index of the silicon waveguide, the optical responses of phase-sensitive devices such as microring resonators, Mach-Zehnder interferometers, and gratings can be tuned.

[0072] As used herein, “tow energy" with respect to a laser pulse, means a pulse energy that is sufficiently low to be deliverable by a laser source while still producing a detectable and controllable modification of a silicon waveguide under multi-shot irradiation. In some examples, "low energy" is in a range from about 1 nJ to less than about 1 mJ per pulse.

[0073] In at least one example, the multi-shot ultrashort pulse laser irradiation is broadly applied to silicon waveguide components, including silicon waveguide components integrated into SiPICs. By way of non-limiting example, this can include microring resonators, Mach-Zehnder10WSLEGAL\055326\0051 l\43474905v3interferometer arms, directional coupler regions, grating couplers or Bragg grating regions, racetrack resonators, or other phase-sensitive waveguide segments, e.g., in SiPICs.

[0074] Using the disclosed techniques, both positive and negative resonance wavelength shifts can be achieved with reasonable induced waveguide losses.

[0075] To that end, and without limitation to theory, the physical mechanism responsible for the positive resonance wavelength shift is the change of refractive index of a thin layer of silicon as a result of modifications to the crystalline structures (e.g. defect generation or amorphization) due to irradiation of single or multiple ultrashort laser pulses. The laser-irradiated layer has a slightly larger refractive index than the crystalline silicon resulting in a slight increase in the overall refractive index of the waveguide. In contrast, the physical mechanism responsible for the negative resonance wavelength shift is the removal of a thin layer of silicon as a result of ablation due to application of sufficient large laser fluences.

[0076] At least one advantage of the disclosed system is that, owing to the incubation effect, the laser fluence requirement for the multi-shot approach is significantly lower than the single-shot approach. This, in turn, allows for the use of low-cost compact laser systems.

[0077] Another advantage of multishot method is that it allows better tuning resolution than the single shot method since it is easier to accurately control the number of shots than the exact fluence required in the single-shot method.

[0078] As used herein, the “incubation effect” means a cumulative multi-pulse laser-material interaction in which successive laser pulses incident on a same location progressively increase defect density and / or absorption such that a threshold fluence for inducing a permanent modification and / or ablation is reduced relative to a single-pulse threshold.

[0079] When a same location of a component is irradiated with multiple ultrashort laser pulses, the ablation threshold depends on the number of incident pulses. In particular, a multi-shot ablation threshold can be significantly lower than a single-shot ablation threshold due to the incubation effect. Without being bound by theory, even at fluences well below a single-shot ablation threshold, successive pulses can progressively generate defects and increase absorption at the irradiated location, thereby reducing the fluence required for subsequent pulses to induce ablation.11WSLEGAL\055326\0051 l\43474905v3

[0080] As further used herein, the “ ablation threshold means a minimum laser fluence, for a specified pulse duration and number of pulses incident on a location, at which the laser irradiation causes material removal from silicon at the location.

[0081] While other methods for post-fabrication tuning of silicon waveguide components in SiPICs have been used, the disclosed method may offer one or more the following appreciated unique advantages: (i) the method can be used to perform post-fabrication tuning directly on SiPIC devices. Importantly, it can tune SiPIC devices with SiC>2 cladding; (ii) the method does not require additional fabrication steps of SiPIC or additional materials; (iii) the method is simpler and faster to implement than other methods; and (iv) the method is amenable to error correction at the wafer level (for high-throughput manufacturing). More broadly, the multi-shot irradiation method can potentially provide a low-cost and versatile method for post-fabrication tuning of silicon photonic devices.III. LASER IRRADIATION AND OPTICAL CHARACTERIZATION ASSEMBLY

[0082] FIG. 1 A shows a laser irradiation and optical characterization assembly 100, according to some examples.

[0083] As shown, assembly 100 includes: (i) a multi-shot ultrashort pulse laser irradiation system 102, and (ii) an optical characterization system 104. Systems 102, 104 may be arranged on a common support frame (e.g., holder) or separate frames. In some cases, the assembly 100 can include only one of the systems 102, 104, i.e., the systems may be provided separately.

[0084] Laser system 102 includes an ultrashort pulse laser source for applying a laser beam 106 to a silicon waveguide component 108 (e.g., a silicon ring resonator). The component 108 can be an isolated waveguide component, or a component integrated into an SiPIC. In at least one example, the system 102 applies visible or multi-near-infrared (NIR) ultrashort laser pulses to the component 108. This enables the resonance wavelength of the silicon component to be tuned to the desired value.

[0085] As used herein, “visible” means electromagnetic radiation having a wavelength in a range from about 400 nm to 700 nm; “near-infrared' or “NIR” means electromagnetic radiation having a wavelength longer than visible light and shorter than mid-infrared light, and having a wavelength in a range from about 700 nm to about 2500 nm.12WSLEGAL\055326\0051 l\43474905v3

[0086] As exemplified, laser system 102 is arranged to deliver the laser beam 106 towards a surface of the silicon waveguide component 108, e.g., along an optical axis that is substantially normal to the surface of component 108. The component 108 may be, for example, integrated into an SiPIC, and having a device layer side defining a top (front) surface on which one or more silicon photonic components (e.g., silicon waveguides) are formed, and an opposing bottom (back) surface. In use, the laser beam 106 is incident on the top surface and is focused at a selected target location on or proximate to a waveguide portion of the SiPIC (or on or proximate the silicon waveguide component, more generally).

[0087] The laser beam 106 may be applied through a microscope objective lens 110. In some cases, the objective lens 110 itself forms part of the laser system 102. A configuration for the laser system 102 is described in greater detail with respect to FIG. IB.

[0088] Optical characterization system 104 is used to measure the wavelength resonance shift, resulting from applying the ultrashort laser pulses 106 (e.g., resonant wavelength measurement). For example, the system is used to measure the degree of wavelength resonance shift after each laser beam pulse, or after a series or plurality of pulses. This may assist in determining the additional number of pulses (and the energy / duration / fluence of these pulses) that need to be applied to effect a target cumulative resonant wavelength shift.

[0089] It is appreciated that combining the laser irradiation system 102 and the optical characterization system 104 into a single assembly 100 provides practical advantages. For example, the integrated assembly enables closed-loop or near-real-time measurement of resonance wavelength shift after a given laser exposure. This, in turn, allows the operator or automated controller to determine whether additional pulses are needed, and to stop irradiation once a target shift is reached. This thereby improves tuning precision and reduces over-processing and induced loss. Significantly, the shared mechanical and optical reference frame (e.g., common stages, alignment features, and beam delivery optics) also reduces component handling, alignment time, and positional uncertainty between processing and measurement, which improves repeatability across devices and across a wafer. The assembly 100 has also been custom designed to ensure that the lighting / laser used from the laser system 102 and characterization system 104 are in orthogonal or non-overlapping planes to avoid interference when used concurrently.13WSLEGAL\055326\0051 l\43474905v3

[0090] In some examples, assembly 100 includes one or more stages 122, which can be configured as motion stages. For instance, a primary motion stage 122a may position the component 108 relative to the laser irradiation system 102 and the optical characterization system 104. This positioning enables laser exposure and optical measurement at different locations on the component 108. It is also possible that one or both lensed fibers 112a, 112b of the characterization system 104 (as described below) are mounted on respective secondary motion stages 122b, 122c. This arrangement can permit characterization at different locations without moving the component 108.

[0091] Each motion stage 122 may provide one or more degrees of freedom, such as ID, 2D, or 3D translation. In at least one example, the motion stages 122 may have a resolution of about 200 nm.

[0092] Additionally or alternatively, the laser subsystem 102 and / or objective lens 110 may be movable via a respective stage.

[0093] Assembly 100 can also include a controller 150, which may be coupled to various components of the assembly 100 (not shown). As discussed below, controller 150 may be used to automatically control various functions of the assembly 100. For example, controller 150 may coupled to the stages 122 to control movement thereof.IV. EXAMPLE MULTI-SHOT IRRADIATION LASER SYSTEM

[0094] FIG. IB illustrates an example multi-shot irradiation laser system 102, that may be used in assembly 100 (FIG. 1A).

[0095] Broadly, system 102 comprises: (i) a laser input source 152; and (ii) an optical train subassembly 154.

[0096] Laser Input Source 152

[0097] Laser input source 152 generates a laser beam 106 which is used to tune the component 108. In disclosed examples, the laser source 152 is an ultrashort pulse laser source which is used to achieve the required tuning using ultrashort duration laser pulses. It may be further configured to use a visible (e.g., about 400 nm) or near-infrared (NIR) wavelength (e.g., about 800 nm), allowing the resonance wavelength of the silicon photonic device to be tuned to the desired value. In some examples, the laser source 152 is a commercial Ti: sapphire laser system (Amplitude TW™ system).14WSLEGAL\055326\0051 l\43474905v3

[0098] Without limitation to theory, ultrashort pulses are used because their ultrashort duration deposits energy faster than significant thermal diffusion can occur. This, in turn, enables highly localized, repeatable modification of a thin silicon region (defects / amorphization or controlled onset of ablation), and with reduced collateral heating and lower induced loss compared to longer pulses at comparable effect. Ultrashort pulses also allow the generation of high laser intensity with low energies to access the multi-photon absorption process.

[0099] In some examples, an NIR wavelength is further used because it is absorbed by silicon through a linear and multi-photon processes. The multi-photon absorption process is important for a strong incubation effect. In contrast, for short wavelength visible light below about 600 nm, the linear absorption dominates leading to a weak incubation effect.

[0100] In some cases, the laser input source 152 is coupled to, and controlled, by controller 150.

[0101] In some examples, the laser beam has 800 nm wavelength, 50 or 130 fs pulse duration, a Gaussian spatial profile with a beam waist radius of 13 pm, and pulse energies of 10 nJ to 0.5 pj to generate the applied range of laser fluences.

[0102] In some examples, the laser beam has 1030 nm wavelength, 350 fs pulse duration, a Gaussian spatial profile with a beam waist radius of 4.6 pm or 13 pm and pulse energies of 5 nJ to 0.2 pj to generate the applied range of laser fluences.

[0103] (ii.) Optical Train Subassembly 154

[0104] The optical train 154 includes a series of mirrors, lenses, and other beam-conditioning optical components configured to direct, shape, attenuate, and focus the laser beam 106 onto a selected location of the component 108 (and with a desired spot size, pulse energy, and spatial profile).

[0105] As shown, the laser beam 106 follows an optical path 130 defined by train 154 and extending between: (i) an input end, at the input laser source 152, and (ii) an output end, at a target location along the component 108 received on the primary stage 122a.

[0106] The total number of pulses for irradiation may be controlled using a combination of a fast electric shutter (ES) 162 and a delay generator 168, to generate the specified number of shots required without otherwise shifting the beam profile and location. In some examples, the electric shutter 162 and delay generator are coupled to, and controlled by, the controller 150 (FIG. 1A).15WSLEGAL\055326\0051 l\43474905v3

[0107] The energy of the laser may be further modulated using a combination of a glan-polarizer 156 and a half-wave plate 158, near the input of the laser source 152. It may also be further calibrated using silicon photodiodes (PD1) 160 placed before the electric shutter. By tuning the angle of the half-wave plate 158, the energy deposited on the component 108 surface can be tuned by combining with the insertion of ND filters 170 ranging from OD0.5-OD3. In some cases, the shutter may be an electro-optic shutter controlled by the delay generator and control computer.

[0108] In some cases, an LED 172 is provided as a light source that is used for illuminating the component 108 for inspecting its surface morphology. An aperture (Al) is further used for beam alignment.

[0109] The laser beam may be further focused using a microscope objective 110. In some examples, microscope objective 110 is a 10 * long working distance objective for irradiating the component at 0° angle of incidence. This gives a Gaussian beam profile with 1 / e2spot radius, which may be approximately 13.1 pm (FIG. 8) by moving the target away from the focus position by a distance of 400 pm.

[0110] System 102 also includes various mirrors (M), including (Ml) and (M2). In some examples, mirrors (Ml) and (M2) are high-reflection dielectric mirrors, and further, may be optimized to reflect light centered around the laser beam wavelength (e.g., around 800 nanometers).

[0111] As exemplified, the laser beam 106 shares part of the optical paths after mirror (M5) and then focuses on the component 108 surface.

[0112] In at least one example, a CCD camera 164 with a focal length 166 achromatic lens are provided to monitor surface morphology and the beam.

[0113] The 800 nm wavelength beam can be converted by using a Second Harmonic Crystal (SHG) 174. Ml mirror is removed during the 400 nm wavelength operation.V. EXAMPLE OPTICAL CHARACTERIZATION SYSTEM

[0114] As further exemplified in FIG. 1 A, the optical characterization system 104 may use at least a pair of lensed fibers 112a, 112b. The lensed fibers 112 may be installed in a parallel plane to16WSLEGAL\055326\0051 l\43474905v3the component 108 surface, and diametrically opposing sides thereof. One lensed fiber 112a may act as a light transmitter, while the other lensed fiber 112b may act as a light reci ever.

[0115] As used herein, a "lensed fiber" refers to an optical fiber whose end is shaped or fitted with a miniature lens (for example, a tapered, spherical, or GRIN lens) to focus or collimate light at the fiber tip. The lensed fibers are used to efficiently couple light between the fiber and a waveguide component by producing a small, well-defined spot size and numerical aperture matched to the waveguide facet.

[0116] As further shown, transmitter fiber 112a is coupled to an optical light source 114 (via an optic fiber cable) to emit light 118 (e.g., broadband light). In some cases, a polarizer 116 is also interposed along the fiber length. This may be configured to set the polarization to a TE mode, and its mode may be variable operated by controller 150.

[0117] Light source 114 can be a super luminescent LED (sLED). As known in the art, SLEDs are high-power semiconductor light sources with a broadband output optical spectrum, similar to LEDs, but at high power levels, like laser diodes. These SLEDs are based on a PN junction embedded in an optical waveguide. When electrically biased in the forward direction, they show optical gain and generate amplified spontaneous emission over a wide range of wavelengths.

[0118] SLEDs are designed to have high single-pass amplification for the spontaneous emission generated along the waveguide, but they cannot achieve lasing action due to insufficient feedback. However, they possess high optical output powers and a smooth spectrum achieved by tilting the waveguide with respect to the end facets and applying an antireflection coating to the facets.

[0119] This combination advantageously translates into a small beam divergence, which enables the coupling of the output into a single-mode fiber with an efficiency similar to that of laser diodes. In disclosed examples, SLEDs can provide a light source with a broadband spectrum covering at least tens of nm range and an almost flat-band energy output. This significantly allows characterizing the component behavior along a range of wavelengths to identify a resonance shift.

[0120] In at least one example, the SLED provides a constant and similar output power level within the 150 nm range of wavelengths from 1450 - 1600 nm.

[0121] In other examples, the light source 114 can comprise any other broadband light source (e.g., covering at least tens of nm range and an almost flat-band energy output), including sources that17WSLEGAL\055326\0051 l\43474905v3are couplable to single- or multi- mode fibers. It is understood that a broadband source allows for measuring the spectrum of wavelengths passing through the silicon component 108 in order to quantify the resonant wavelength shift.

[0122] On the other end, the receiving lensed fiber 112b is positioned to receive light 118 travelling through the component 108. The receiving fiber 112 is coupled to an optical spectrum analyzer (OSA), which can be used for signal processing. The OSA can be used to display the full resonance wavelength spectrum instantaneously concurrently, while the light 118 is applied to the component.

[0123] Particularly, as also known in the art, OSAs are precise instruments used to measure optical spectra, which can be further analyzed. Some typical applications include characterizing light sources like lasers and LEDs, testing optical systems (such as wavelength division multiplexing systems in optical fiber communications networks, where one may need to test the optical powers of different wavelength channels and measure signal-to-noise ratios), measuring the wavelengthdependent transmissivity or reflectivity of optical systems or devices by comparing spectra with and without the device, and characterizing fiber amplifiers (e.g., telecom erbium-doped fiber amplifiers) in terms of wavelength-dependent gain and noise figure.

[0124] An example of the full spectrum of the SLED from the wavelength range of 1200 nm to 1700 nm measured by the OSA is shown in FIG. 9. The relatively constant output power from 1450-1600nm is used in the characterization experiments of the unirradiated and irradiated silicon microring resonator (SiMRRs). Most measurements used the flat spectra range of 1540 nm to 1560 nm, which is shown in the insert of FIG. 9.

[0125] To that end, the resonant wavelength can be determined based on the data output from OSA using techniques well known in the art.

[0126] In some examples, in addition or in the alternative to using an OSA, the system can use a swept frequency source (e.g. tunable laser) and photodetector (e.g. power meter).VI. EXAMPLE METHOD

[0127] FIG. 18 shows an example method 1800 for post-fabrication tuning, e.g., using the assembly 100.18WSLEGAL\055326\0051 l\43474905v3

[0128] At 1802, a silicon waveguide-based photonic component is initially fabricated (e.g., a microring resonator, a Mach-Zehnder interferometer, a directional coupler, or a grating coupler). The component may be fabricated using semiconductor processing techniques known in the art, including deposition, lithography, and etching processes compatible with CMOS fabrication.

[0129] In some cases, what is fabricated is a silicon photonic integrated circuit (SiPIC) formed on a silicon-based substrate (e.g., an SOI substrate) and including one or more one or more waveguide components.

[0130] In other cases, act 1802 is performed beforehand and is not explicitly part of the method.

[0131] At 1804, the fabricated silicon waveguide-based photonic component 108 is placed on the assembly 100, such as over the stage 122a. For example, the component 108 is positioned such that a SiPIC device layer side defines a top surface on which the silicon photonic components are formed. The motion stage 122a may then be operated to align a selected target location of a waveguide component of the component 108, with an optical axis of the laser irradiation system 102. In some examples, one or both lensed fibers 112a, 112b are also positioned using respective motion stages 122b, 122c to enable optical coupling to and from the component 108 at a selected location.

[0132] At 1806, the optical characterization system 104 is initially operated to determine a current resonant wavelength of the silicon component. For example, this can involve activating the optical light source 114 (FIG. 1 A) and observing the results on the OSA 120.

[0133] At 1808, the laser irradiation system 102 is operated to deliver a laser beam 106 to a selected target location on or proximate to a silicon waveguide forming part of the photonic component. The laser irradiation is applied as a plurality of pulses directed to a same target location along the waveguide portion to induce a permanent change in an effective refractive index and / or a geometry of the waveguide portion, thereby shifting an optical response of the waveguide component.

[0134] A pulse count of the lasers may be controlled by gating the laser beam 106 using the electric shutter 162 (FIG. IB) operated in synchronization with a pulse train of a laser source 152. Controller 150 may control the electric shutter 162 and one or more operating parameters of the laser irradiation system 102, e.g., pulse energy, pulse number, repetition rate, fluence, and / or a target19WSLEGAL\055326\0051 l\43474905v3position on the component 108. In some examples, the laser irradiation system 102 is configured to apply from 1 to 1000 pulses at the selected target location, although larger pulse numbers may be used.

[0135] The method can then return to 1806, such that the optical characterization system 104 is again operated to measure an optical response of the waveguide component and to determine a tuning result produced by the applied laser irradiation. The measured resonance wavelength is compared to a target resonance wavelength to determine a resonance wavelength shift caused by the applied laser irradiation.

[0136] To that end, the tuning performed at acts 1806-1808 may be performed as a closed-loop process in which characterization is performed after each pulse, and subsequent pulses are applied based on the measured resonance wavelength shift. In some cases, this is automated by the controller 150.

[0137] For example, after each pulse of the laser beam 106, the optical characterization system 104 measures a resonance spectrum of the waveguide component, and controller 150 (or an operator) determines whether additional pulse(s) are required to move the resonance wavelength toward the target resonance wavelength. If the process is automated, then a memory of the controller 150 can store the target shift, which is compared against the measured existing shift. Controller 150 can then operate the laser system and characterization system in a closed loop as needed until the target shift is achieved.

[0138] The process is repeated until a stopping condition is satisfied, such as a resonance wavelength being within a specified tolerance of the target resonance wavelength and / or an estimated induced loss being within an acceptable range.

[0139] In other examples, characterization is performed after a set of pulses rather than after each pulse. For example, the laser irradiation system 102 applies a predetermined burst of pulses (e.g., 5 pulses, 10 pulses, 50 pulses, or 100 pulses) to the selected target location. The optical characterization system 104 then measures the resonance spectrum to determine an updated resonance wavelength shift. Based on the measured shift, an additional burst of pulses may be applied, and the burst-and-measure sequence may be repeated until a target tuning result is achieved.20WSLEGAL\055326\0051 l\43474905v3

[0140] It is possible that the pulse energy / fluence / duration of the pulses may be adjusted (collectively, "pulse parameters"), as between different pulses to achieve greater or lower increments in the resonant phase shift.

[0141] In at least one example, the method provides bi-directional tuning. At a first range of irradiation conditions (e.g., lower fluence and / or lower accumulated dose), the tuning is dominated by an induced refractive index increase caused by laser-induced crystalline modifications. Further, at a second range of irradiation conditions (e.g., higher fluence and / or higher accumulated dose), the tuning is dominated by a reduction in effective index due to material removal by ablation. The irradiation conditions may be selected to produce a positive resonance wavelength shift, a negative resonance wavelength shift, or a combination thereof.

[0142] By way of further example, at a selected laser fluence, the positive resonance wavelength shift is produced by delivering to a target location on the waveguide, a first number of the pulses that is at or below a pulse-count threshold (or within a first pulse count range). Further, negative resonance wavelength shift is produced by delivering to the target location a second number of the pulses that is greater than the pulse-count threshold (or in a second pulse count range, that is greater than the first pulse count range).

[0143] Method 1800 may be repeated at multiple locations on the component 108 to tune multiple waveguide portions and / or to tune multiple segments of a same waveguide portion. This can involve operating the motion stage 122a and / or stages 122b, 122c to reposition the component 108 and / or the lensed fibers 112a, 112b. This allows the laser irradiation and characterization operations to be repeated at the different locations.

[0144] In some examples, prior to tuning a given silicon waveguide component, the assembly 100 is used to characterize a tuning response of silicon waveguide portions of a device with the same design under multi-shot irradiation to establish one or more tuning curves. For example, the laser irradiation system 102 is operated to irradiate test locations (or a different component altogether) with varying laser fluence and / or varying pulse number. Further, the optical characterization system 104 is operated to measure corresponding resonance wavelength shifts and, in some cases, induced loss.

[0145] The resulting measurements may then be used to identify operating regions including: (i) an increasing region in which resonance wavelength shift increases with fluence and / or pulse number, (ii) a plateau region in which the resonance wavelength shift saturates over a range of21WSLEGAL\055326\0051 l\43474905v3irradiation conditions, and (iii) a decreasing region in which the resonance wavelength shift decreases and may become negative due to material removal (see discussion further below).

[0146] After such characterization, the identified operating regions and associated irradiation parameters may be applied to tune another similar silicon waveguide component. For example, the controller 150 and / or an operator may select: (i) an irradiation condition in the increasing region to obtain a controllable positive shift, (ii) an irradiation condition in the plateau region to reduce sensitivity to laser-pointing uncertainty, or (iii) an irradiation condition in the decreasing region to obtain a negative shift. The irradiation condition may be further refined using a closed-loop procedure in which the optical characterization system 104 measures the resonance response after each pulse or after a predetermined burst of pulses and the irradiation is continued until a target tuning result is achieved.

[0147] In some examples, the tuning procedure provides a fine-tuning resolution on the order of a few picometers. For example, experimental results demonstrate that, under selected irradiation conditions, an incremental resonance wavelength shift of about 2.0 pm - 3.0 pm per pulse may be achieved, which is enabled by using ultrashort pulses. This picometer-scale resolution enables trimming of phase-sensitive silicon photonic devices, such as microring resonators and Mach-Zehnder interferometers. The trimming can compensate for fabrication-induced deviations, and can be equal to a free spectral range. The trimming can also align a device resonance to a target wavelength with a tolerance that may be smaller than a channel spacing or linewidth in wavelength-selective applications.VII. EXAMPLE HARDWARE CONFIGURATION FOR CONTROLLER

[0148] FIG. 19 shows an example hardware configuration for a controller 150. As shown, it may include a processor 1902 coupled to a memory 1904, and one or more a communication interface 1906 and an input / output (I / O) interface 1908.

[0149] Communication interface 1906 may comprise a cellular modem and antenna for wireless transmission of data to the communications network.

[0150] I / O interface 1910 can be any interface for coupling the controller to other external elements.22WSLEGAL\055326\0051 l\43474905v3

[0151] To that end, it will be understood by those of skill in the art that references herein to controller 150 as carrying out a function or acting in a particular way imply that processor 1902 is executing instructions (e.g., a software program) stored in memory 1904 and possibly transmitting or receiving inputs and outputs via one or more interfaces.VIII. EXPERIMENTAL RESULTS

[0152] The following section discusses experimental results, which use the laser irradiation system (FIG. 1 A) for finetuning of silicon waveguide components.

[0153] In these examples, the laser source 152 used was a commercial Tksapphire laser system (coherent legend). This laser generate pluses with pulse energy up to 0.5 mJ, a repetition rate of 1 kHz, and a pulse duration of ~ 130 fs at 800 nm wavelength. The pulse-to-pulse energy fluctuation from the laser output was within 15%.

[0154] The experimental results confirm that, using these laser parameters, in permanent tuning of silicon photonic waveguide devices, it is advantageous to use N = 10 for tuning when compared with using N = 100. This conclusion was determined for a number of reasons: (i) there are fewer fluctuations in the tuning curve in the plateau region for the N = 10 case, and (ii) the induced roundtrip losses in the waveguide is smaller for the N = 10 case and are kept below 0.5 dB in the microring waveguide. The loss increase may not be tolerable for very high Q factor devices, but it is still sufficient for many applications.

[0155] The experimental results also confirm that, as compared to the single-shot laser tuning, the multi-shot approach also offers several advantages. Due to the incubation effect, the thresholds for both negative and positive resonance wavelength shifts are significantly lower for the multi-shot approach. This helps reduce the range of laser fluences required for tuning, which allows the use of low-cost compact laser systems that deliver microjoule or sub-microjoule pulse energies. For example, a fluence of 0.1 J / cm2, which is above the ablation threshold, can be achieved with a Gaussian beam radius of 13.1 pm at 270 nJ.

[0156] Another advantage of multi-shot tuning over the single-shot approach is that the induced waveguide losses are significantly lower. For example, for a resonance wavelength shift of 3.4 nm the induced roundtrip loss for a single shot was 1.3 dB, while for the N = 10 multi-shot case,23WSLEGAL\055326\0051 l\43474905v3the induced losses for all positive wavelength shifts are below 0.5 dB. For negative wavelength shifts, single-shot irradiation with fluences near the ablation threshold induced significant roundtrip microring losses, causing the microring resonances to completely disappear in the measured spectra.

[0157] For the N = 10 multi-shot case, the induced roundtrip loss is also kept below 0.5 dB for most negative wavelength shifts. The result confirms that the more gentle “multi-shot laser nanomilling” process has an advantage compared to the single-shot ablation process resulting in reduced waveguide losses.

[0158] The alignment and pointing uncertainty for a Gaussian beam can lead to significant variations of the effective laser fluence on the waveguide surface. This can be mitigated by using a laser fluence value in the plateau region on the multi-shot tuning curve. In the N = 10 case, a constant resonance wavelength shift of 4.5 nm was observed in the plateau region from 0.028 J / cm2to 0.048 J / cm2. This wavelength shift is directly proportional to the beam waist radius of the Gaussian beam and can be easily changed by varying the microscope to the target distance while keeping the fluence within the plateau region. For example, it was confirmed that when reducing the beam waist radius 4 times to 3.3 pm, the resonance wavelength shift reduced correspondingly 4 times to ~1 nm. Another way to reduce the effect of laser-pointing uncertainty is to employ a top-hat beam profile, similar to the one used in our previous single-shot studies.

[0159] (i.) Example Microring Used in Experiments

[0160] Experiments were performed using microrings, in silicon photonic devices, having a diameter of 15 pm, and which were coupled to a waveguide of 500 nm in width with a gap of 260 nm. The waveguide was a rib waveguide with a rib height of 130 nm and a slab height of 90 nm fabricated on the Silicon-on-Insulator (SOI) platform. The silicon photonic device was air-cladded. The free spectral range of the microring was 5.82 nm.

[0161] During the multi-shot irradiation processing, the laser position was located on one side of the microring. Before and after irradiation, the chip was moved to the optical characterization system to measure the response of the microring resonator. The characterization system consisted of a wavelength tunable laser, a polarization controller to set the polarization to TE, and lensed fibers to couple light into and out of the silicon waveguide (e.g., FIG. 1 A).24WSLEGAL\055326\0051 l\43474905v3

[0162] FIG. 2 shows examples of the measured microring resonance spectra before and after irradiation. The roundtrip loss can be derived from the transmission at the through port using the measured power transmission versus frequency which is given by Equation (1).where Pinis the input optical power, Pthruis the transmitted light power at the through port, T is the field transmission coefficient of the coupling junctions between the waveguides and microring, (prt= r|eyt(2TZ / / 1)27TR is the roundtrip phase and artthe roundtrip field attenuation factor of the microring.

[0163] By curve-fitting the measured transmission spectrum at the through port with the expression in Equation (1), the roundtrip attenuation is obtained artalong with the transmission coefficient T. Examples of the curve fit are shown by the dashed lines in FIG. 2.

[0164] (ii.) Effect on Average Fluence of Laser Shot on Resonance Wavelength Shifts for Microring Resonators Using Multi-Shot Laser

[0165] In conducted experiments, silicon microring resonators were irradiated with 130 fs laser pulses, and the peak fluence of each pulse ranged from 0.0045 J / cm2to 0.069 J / cm2(pulse energy from 12 nJ to 186 nJ). The number of shots (N) at each irradiated location was either 10 or 100 pulses.

[0166] FIG. 3 shows a semilog plot of the tuning curves, i.e. resonance wavelength shifts versus the average fluence of each laser shot, for N = 1, 10, and 100 (N=l being a single shot).

[0167] The data points and error bars for the N = 10 and N = 100 tuning curves are the average and standard deviation, respectively, of 3 to 9 irradiated components. The errors in the resonance wavelength shifts are due to the laser pointing uncertainty, which can be larger than a micron. The laser pointing uncertainty can include the initial alignment uncertainty and the subsequent shot-to-shot variations due to laser beam pointing fluctuations.

[0168] It is also confirmed that the laser pointing uncertainty can lead to significant variation of the effective laser fluence across the 500 nm width of the waveguide. If the center of the Gaussian beam is positioned exactly at the middle of the 500 nm wide waveguide, the fluence across the 500 nm width is almost constant with the fluence at the two boundaries only 1% lower than that in the middle. However, if the center of the Gaussian beam is positioned 1 pm off the middle of the 500 nm width of the waveguide, the fluences at the two boundaries would be 92% and 79% of the peak value.25WSLEGAL\055326\0051 l\43474905v3

[0169] FIG. 3 exemplifies that the laser thresholds for causing a permanent wavelength shift are lower for the multi-shot cases (N= 10 and 100) compared to the single-shot case (N=l), being as low as 0.02 J / cm2for the 10-shot cases and 0.05 J / cm2for the single-shot case.

[0170] In addition, while the wavelength shift increase linearly with the fluence for the singleshot case (until reaching the ablation threshold of 0.19 J / cm2), the multi-shot tuning curves consist of 3 regions: (i) a region of the positive resonance wavelength shifts increase with increasing laser fluences (region A - "increasing region"), (ii) a region where the positive resonance wavelength shifts are approximately constant (region B - "plateau region") and (iii) a region where the positive resonance wavelength shifts decrease with increasing fluences and eventually the resonance wavelength shifts became negative (region C - "decreasing region").• Region A (“increasing region”): For N = 10 cases, for the fluence range of 0.0045 J / cm2to 0.014 J / cm2, the resonance wavelength shifts were below the noise level of 0.07 nm. Within region A from 0.014 J / cm2to 0.028 J / cm2, the resonance wavelength shifts increase with increasing fluences and rise to a value of around 4.5 nm at a fluence value of around 0.028 J / cm2.• Region B (“plateau region”): For the N = 10 case, in the fluence range from 0.028 J / cm2to 0.048 J / cm2, the resonance wavelength shifts are approximately constant at around 4.5 nm. For the N = 100 case, a plateau region was also observed with a slightly lower value at around 4 nm but the values had larger fluctuations than the N= 10 case. It is believed that the origins of the plateau region and the fluctuations in the N = 100 case, are a result of the fact that each laser shot may modify a small region in the light absorption layer until the layer is fully modified and reaches the observed saturation. The fluctuations in the N = 100 case may be the result of the laser- pointing uncertainty, energy fluctuations, and focal position variations where small variations in fluence from shot to shot have a larger effect on the incubation behavior. In prior evaluations of multi-shot laser nanomilling of copper, stochastic behavior of ablation events was observed. Such stochastic behavior in multi-shot laser nanomilling can also contribute to the large fluctuations in the N = 100 case.• Region C ("decreasing region): For the N = 10 case, for the laser fluence larger than 0.048 J / cm2, the resonance wavelength shifts decrease with increasing laser fluence likely because of the amount of material removal increases with increasing laser fluence. For the N = 100 case,26WSLEGAL\055326\0051 l\43474905v3the behavior is similar with the onset of the decrease of resonance wavelength shifts starting at a lower laser fluence value of 0.034 J / cm2. The decrease in the wavelength shift with increasing laser fluence is likely caused by a thin layer of material being removed by the ablation process, resulting in a decrease in the effective index of the waveguide. The resonance wavelength shift becomes negative when the negative effective index change caused by ablation is larger than the positive index change caused by amorphization of the silicon material. The fluence where the resonance wavelength shift becomes negative is lower for N = 100 than for N = 10, the values being 0.052 J / cm2for N = 100 and 0.063 J / cm2for N = 10. For the single-shot case, the ablation threshold where the wavelength shift became negative was much higher at 0.19 J / cm2.

[0171] FIGs. 4(a) and (d) show examples of positive and negative wavelength shifts, respectively, of the microring resonance spectra for the N = 10 case. The positive wavelength shift was obtained at a laser fluence of 0.055 J / cm2while the negative wavelength shift was obtained at a larger laser fluence of 0.065 J / cm2.

[0172] For the positive shift case, the SEM image of the device after irradiation in FIG. 4(c) shows no surface damage on the microring waveguide. However, for the negative shift case, a change in surface morphology on the waveguide can be observed in the SEM image in FIG. 4(f).

[0173] (iii.) Average Roundtrip Losses in Microring Resonator

[0174] Measurement of the averaged roundtrip losses in the waveguide before and afterirradiated microring resonator can provide information about the change in the quality of the microring resonators after laser processing.

[0175] The roundtrip losses a2is obtained by performing curve fitting of the measured resonance spectrum using Equation (1) (see FIG. 2).

[0176] FIG. 5 shows the change of roundtrip loss after irradiation versus the resonance wavelength shift for N = 10 and N = 100, respectively. For N= 10, the induced roundtrip losses are below 0.5 dB in both the negative and positive shift regimes. For N = 100, the induced roundtrip losses are slightly larger than the N = 10 case, with the largest loss value reaching 0.8 dB.

[0177] The error bars of the roundtrip loss in the plots are caused by uncertainty in achieving the best curve fit of the measured resonance spectra. One source of this error is due to the coherent27WSLEGAL\055326\0051 l\43474905v3back- scattering inside the resonator, which causes splitting of the resonance spectrum resulting in errors in the extracted roundtrip loss. The uncertainty in defining the background baseline of the resonance spectrum also contributed to the fitting error.

[0178] (iv.) Scanning Electron Microscope and Raman Study of Surface Morphology

[0179] Various laser-induced defects and phase transformations were evaluated by single and multi-shot irradiation of single crystal silicon with fluences above the melting threshold using Nearinfrared femtosecond laser pulses. Various numbers of pulses at 0.15 J / cm2, about 15% above the single shot melting threshold, with a wavelength of 1030 nm and a pulse width of 300 fs were used.

[0180] For irradiation with a single shot, it was observed that a top layer of amorphous silicon where the laser fluences are sufficient for melting and a dislocation layer underneath where the laser fluences are below the melting threshold but sufficient to induce dislocations and lattice distortions.

[0181] As the number of shots increased, the dislocation layer was enlarged and a small portion of the amorphous silicon layer was removed. Further increase in the number of shots significantly increased the dislocation region, leading to the formation of a high-density defect layer and the appearance of macroscopic damage to the surface layer.

[0182] The visible surface damage was in the form of Laser-Induced Periodic Surface Structures (LIPSS) and microholes which would be accompanied by relatively larger material removal.

[0183] Although the laser fluences used were significantly below the melting threshold, the mechanisms observed provided useful insights. Based on the results from Scanning Electron Microscopy (SEM) and Raman Spectroscopy with N = 10, the evolution of multi- shot femtosecond irradiation on crystal silicon with increasing fluences (these fluences are significantly lower than the single-shot melting threshold) are described as follows:• For laser fluences below 0.014 J / cm2, no modifications of the surface can be detected by SEM (FIG. 6(a)) or Raman Spectroscopy. Even if there are some laser-induced dislocations, the amount is sufficiently small so no change in the induced index refraction could be detected.• When laser fluences are increased to higher than 0.014 J / cm2("increasing" region A of FIG.3), the laser irradiation generated dislocations and lattice distortions due to plastic deformation and higher temperature. The amount of dislocations and lattice distortions grows with28WSLEGAL\055326\0051 l\43474905v3increasing fluence, leading to an increase in induced index refraction and wavelength shift. However, no modifications of the surface can be detected by SEM or Raman Spectroscopy (FIG. 7 yellow and gray traces).• As laser fluences are further increased to 0.028 J / cm2(beginning of the plateau region B), the increase of induced index refraction begins to saturate, as the generated dislocations and defects completely fill the light absorption layer. SEM and Raman Spectroscopy still show no evidence of surface modification (FIG. 6(b)) or generation of amorphous silicon (FIG. 7 gray trace).• When the fluences further increase to 0.05 J / cm2(end of the "plateau" region B and beginning of the "decreasing" region of FIG. 3), the decrease of induced index refraction started and eventually led to negative resonance wavelength shifts. The decrease in induced index refraction is the result of material removal due to surface ablation. Ablation occurs typically when the surface temperature rises beyond the melting point and this is expected to be signified by the presence of amorphous silicon and surface and sub-surface damages. At fluences of 0.06 J / cm2slight surface modifications can be observed from the SEM images (FIG. 6(c)) while no significant amorphous silicon band is present in the Raman spectroscopy measurement (FIG. 7 orange trace). At a higher fluence of 0.063 J / cm2, nanodroplets, nanostructures and nanocracks can be clearly observed from SEM images (FIG. 6(d)) and the amorphous silicon band can be clearly observed from the spectroscopy measurement (FIG 7 blue trace).

[0184] (v.) Experiments Using Mach-Zehnder Interferometer (MZI)

[0185] In addition to microring resonators (MRRs), Mach-Zehnder interferometers (MZI) are also key components of silicon photonic integrated circuit.

[0186] Experiments were conducted to examine the tuning behavior of an MZI-MRR structure using the multi-shot approach. A schematic diagram of the device is illustrated in FIG. 15. It includes two 3 dB splitters and two identical add / drop MRRs. The output of the MZI-MRR structure depends on the phase difference of the two arms and can be changed by modifying the refractive index of one arm using our multi-shot tuning approach.

[0187] Multiple 50 fs duration and 800 nm wavelength laser pulses of 13.4 pm radius Gaussian laser beam with a fluence of 0.0426 J / cm2, 0.0505 J / cm2and 0.0568 J / cm2, respectively, are used to29WSLEGAL\055326\0051 l\43474905v3irradiate the upper arm of the MZI structure. The normalized percentage of transmission power of the MZI-MRR structure after each laser pulse irradiation is shown in FIG. 15.

[0188] Here, the maximum output power achieved during the tuning process is defined as 100%, and the noise level on OSA 120 (-100 dBm) as 0%. As shown in FIG. 16, at a fluence of 0.0568 J / cm2, the T increased from 35% to 90% after the first shot (N = 1), and kept increasing to around 100% with the second shot (N = 2). A plateau region of around 100% T was observed for N = 3 and N = 4. For N = 5 to N = 7, T is linearly reduced to 32%. The next two shots from N = 8 to N = 10 kept reducing the T to 12%.

[0189] A similar trend can be achieved by applying the fluence of 0.0505 J / cm2and 0.0426 J / cm2. The output power of the MZI structure can be tuned from 72.9% to 100% using 31 pulses, and from 78.21% to 100% by applying 170 pulses, respectively. The experiment illustrated that a desirable output power transmission can be obtained by using the multi-shot tuning approach.

[0190] (vi.) Tuning result for Silicon Microring Resonators Using 1030 nm Wavelength from a Compact Industrial Fiber Laser

[0191] A compact industrial fiber-based femtosecond laser system, with a wavelength of 1030 nm and a pulse duration of 350 fs, was utilized in the experiment, generating a focused Gaussian beam profile on the surface of the component with I t'2spot radius of 13.4 um and of 4.6 um, respectively.

[0192] Silicon microring resonators were irradiated with sub -microjoule laser pulses of 350 fs duration, 2 different I t'2spot radius of 13.4 um or 4.6 um, and varying peak fluences, ranging from 0.02778 J / cm2to 0.04122 J / cm2. The Gaussian beam was positioned on the left side of the racetrack microring resonator structure.

[0193] (a) Results for using laser spot with 1 / e2radius o f 13.4 um

[0194] The shot-to-shot resonance wavelength shift for a silicon microring resonator (SiMRR) irradiated by fs pulses with a fluence of 0.02778 J / cm2are shown in FIG. 17A. For shot numbers N from 1 to 340, no significant shifts in the resonance peaks were observed.

[0195] From N = 340 to 400, approximately constant positive shifts in the resonance peaks are noted, with a maximum accumulated shift of 0.18 nm. In the range of N = 400 to 440, only very small positive shifts in the resonance peaks are observed. However, for shot numbers N = 440 to 940, negative shifts in the resonance wavelength spectra are detected. In this regime of negative shifts, the30WSLEGAL\055326\0051 l\43474905v3resonance curves gradually broaden, and the height of the resonance peaks slowly diminishes, suggesting an increase in propagation loss within the waveguide of the microring resonator.

[0196] According to calculations derived from a semi-log plot of the shifting curves, in the positive shift region, using 1030 nm femtosecond laser pulses can achieve approximately 2.75 pm / pulse. This indicates that utilizing a relative low-cost industrial fiber-based femtosecond laser in the 1030 nm range can provide a tuning resolution comparable to that of the 800 nm more expensive scientific Ti: Sapphire lasers.

[0197] The measurement of the averaged induced roundtrip losses in the waveguide of the SiMRR after each shot is recorded at 0.02778 J / cm2, as shown in FIG. 17B.

[0198] Although there are notable fluctuations due to the fitting and measurement uncertainties of the round-trip loss, the total induced round-trip losses remain below 0.12 dB in both the negative and positive shift regimes. This value is significantly lower than the results obtained at 800 nm, suggesting that using a femtosecond laser with a wavelength of 1030 nm may offer better control over the induced round-trip loss.

[0199] (b) Results for using laser spot with 1 / e2radius o f 4.6 um

[0200] Same laser fluences can be generated with lower laser energy by using a smaller laser spot. The cost of a laser system can be significantly reduced if the laser energy requirement is reduced.

[0201] In disclosed examples, the beam radius was reduced to 4.6 pm, which is approximately three times smaller than previous configurations. As a result, to achieve the same laser fluence, the required input energy from the laser source was now nine times lower, placing it in the nanojoules range.

[0202] The impact of using a 4.6 pm beam radius was also investigated on achieving both positive and negative resonant wavelength shifts with reasonable resolution. For instance, when applying a fluence of 0,03291. / c / ??2, a maximum positive shift of 0.15 nm can be achieved at N = 15. Thus, the tuning resolution of 15 pm is larger for the smaller 4.6 um radius laser spot as compared to the resolution of2.75 pm when a larger 13.4 um radius laser spot was used. However, a 15 pm resolution is sufficient for most tuning applications and it is also better than all the reported tuning resolutions found in the literatures for other permanent tuning techniques. It is expected the 15 pm resolution result can be further improved by optimizing the experimental conditions. Thus, these results demonstrate that a31WSLEGAL\055326\0051 l\43474905v3reduced beam radius with lower laser energy requirement still allows for precise tuning of the resonator.IX. FURTHER EXPERIMENTAL RESULTS

[0203] In additional experiments, silicon microring resonators were irradiated with 50 fs laser pulses, and the peak fluence of each pulse ranged from 0.0232 J / cm2to 0.0502 J / cm2. The quasiGaussian beam of 13.4 pm in radius at 800 nm wavelength was located at the right side of the racetrack microring resonator structure, with a laser pointing uncertainty of 0.8 pm for the initial alignment uncertainty and 1.8 um shot-to-shot variations due to laser beam pointing fluctuations.

[0204] The number of shots (N) at each irradiated location usually accumulated from 1 to a few hundred, and the resonance wavelength spectrum of the resonator was measured right after each shot.

[0205] In the examples, the laser generated laser pulses with pulse energy up to 20 mJ, a repetition rate of 10 Hz, and a pulse duration of 50 fs. During the experiment, a wavelength of 800 nm or 400 nm was applied. The 800 nm wavelength is the fundamental wavelength from the output of the laser system.

[0206] The shot-to-shot resonance wavelength spectra for a silicon microring resonator irradiated by fs pulses with a fluence of 0.0232 J / cm2are shown in FIG. 11.

[0207] As shown, for the shot number N = 1 to 390, positive shifts for the resonance wavelength spectra are observed. For N = 1 to 40, the spectra are on top of each other, indicating no significant shift in the resonance peaks (blue curves in FIG. 11(a)) is observed. For N= 40 to 150, approximately constant positive shifts of the resonance peaks (purple curves in FIG. 11(a)) are observed. For N = 150 to 390, very small positive shifts of the resonance peaks (green curves in FIG.11(a)) are observed.

[0208] Negative shifts for the resonance wavelength spectra are observed for the shot number N = 390 to 790. For N = 390 to 600, approximately constant negative shifts of the resonance peaks (orange curves in FIG. 11(b)) are observed. For N = 600 to 790, larger approximately constant negative shifts of the resonance peaks (red curves in FIG. 11(b)) are observed. In this negative shift32WSLEGAL\055326\0051 l\43474905v3regime, the resonance curves are slowly broadening, and the height of the resonance peaks are slowly diminishing, indicating increased propagation loss in the waveguide of the microring resonator.

[0209] The tuning curves, i.e. the total resonance wavelength shifts versus the pulse number and the corresponding resonance wavelength shifts at 0.0232 J / cm2is shown in FIG. 12.

[0210] As further shown, for N = 1 to 40 (“Region A”), no significant resonance wavelength shifts are observed. A value of approximately zero can be used to represent the shifts in region A. For N = 40 to 150 (“Region B”), constant positive shifts are observed, and a linear relation with a positive slope can be used to represent the shifts in this region. For N = 150 to 390 (“Region C”), very small constant positive shifts are observed, and a linear relation with a very small positive slope can be used to represent the shifts in this region. For N = 390 to 600 (“Region D”), constant negative shifts are observed, and a linear relation with a negative slope can be used to represent the shifts in this region. For N = 600 to 790 (“Region E”), larger constant negative shifts are observed, and a linear relation with a larger negative slope can be used to represent the shifts in this region.

[0211] Equations representing the data for regions A to E are summarized in Table 1. The linear regression fit of the data gives the equations for regions B to E. The tuning precision is given by the slope of the linear relation, and the uncertainty is given by the standard error of the linear regression fit of the data.Table 1 - Equations representing the data for regions A to E in FIG. 12

[0212] Measurement of the average roundtrip losses in the waveguide of the SiMRR after each shot is shown in FIG. 13.WSLEGAL\055326\0051 l\43474905v3

[0213] As shown, the total roundtrip losses are below 0.45 dB in positive and negative shift regimes. The roundtrip losses increase slowly from 0.08 dB to approximately 0.15 dB from N = 0 (unirradiated case) to N = 390, and a linear relation with a positive slope can represent the data. Beyond N = 390, the roundtrip losses increased rapidly to approximately 0.45 dB atN = 790. A linear relation with a larger positive slope can represent the data for N = 390 - 790. Equations representing the data for FIG. 13 are summarized in Table 2. The linear regression fits the data and gives the equations. The slope of the linear relation gives the induced loss per shot, and the standard error of the linear regression fit of the data gives the uncertainty.

[0214] Similar trends can be achieved at different energy fluences.

[0215] As shown in FIG. 14, the chosen energy fluence ranges from 0.0232 J / cm2to 0.0555 J / cm2, respectively. It clearly indicated that it is easier to achieve the desired positive shift value with smaller applied laser pulse numbers as the laser fluence increases.

[0216] The maximum total accumulated resonance wavelength shift increases with increasing laser fluences. For instance, the maximum accumulated resonance wavelength shift is 1.2 nm at 0.0502 J / cm2, which is 4 x larger than that of 0.0232 J / cm2(FIG. 14). Higher fluences can obtain the desired value of the resonance wavelength shift by reducing the number of shots but sacrificing tuning resolution.

[0217] The experimental data indicates that the resonance wavelength of a SiMRR can be tuned precisely with relatively low loss. A different strategy can be used depending on the desired resonance wavelength shift (AX) belong to one of the following cases: (i) 0 < AX < 2.5 nm or (ii) 2.5 nm < AX < 1. 2 nm.WSLEGAL\055326\0051 l\43474905v3

[0218] As an example for case (i), if the desired resonance wavelength shift is 0.2 nm, as can be seen from Table 1, we can use a laser fluence of 0.0232 / cm2and take 118 shots to achieve a 0.2 nm shift. At this fluence, the tuning resolution is 2.2 pm / shot and a round-trip loss of 0.09 dB for 118 shots (Table 2).

[0219] As an example for case (ii), if the desired resonance wavelength shift is 1.0 nm, we can start with a rough tuning by using a laser fluence of 0.0502 J / cm2. After 5 shots, a shift of 0.94 nm is achieved and a round-trip loss of 0.148 dB for the 5 shots.

[0220] This can then be followed with a fine-tuning by using a laser fluence of 0.0232 J / cm2, after taking 55 shots on another spot on the waveguide to get an additional shift of 0.06 nm to give a total shift of 1.0 nm.

[0221] For this second example, the fine-tuning resolution is 2.2 pm / shot and the total roundtrip loss is 0.21 dB. If the desired resonance wavelength shift is negative, the multi-shot approach can also be applied, but it will lead to larger round-trip losses.

[0222] F or example, if the desired wavelength shift i s -0.5 nm, if a laser fluence of 0.0232 J / cm2is used, the resonance wavelength can be achieved after 714 shots (Table 1) with a round-trip loss of 0.36 dB and a tuning resolution of 5 pm / step. If larger tuning wavelength range is desired (e.g. up to the Free Spectral Range value), it can be achieved by applying multishots tuning at more than one location on the same waveguide.

[0223] Silicon microring resonator tuning experiment using 400 nm wavelength, 50 fs pulse duration, and Gaussian spatial profile with a beam waist radius of 6 pm was also conducted. It was observed 400 nm wavelength irradiation has a weak incubation effect. This led to a higher tuning laser fluence requirements comparing to the 800 nm case.X. INTERPRETATION

[0224] Various systems or methods have been described to provide an example of an embodiment of the claimed subject matter. No embodiment described limits any claimed subject matter and any claimed subject matter may cover methods or systems that differ from those described below. The claimed subject matter is not limited to systems or methods having all of the features of any one system or method described below or to features common to multiple or all of the apparatuses35WSLEGAL\055326\0051 l\43474905v3or methods described below. It is possible that a system or method described is not an embodiment that is recited in any claimed subject matter. Any subject matter disclosed in a system or method described that is not claimed in this document may be the subject matter of another protective instrument, for example, a continuing patent application, and the applicants, inventors or owners do not intend to abandon, disclaim or dedicate to the public any such subject matter by its disclosure in this document.

[0225] Furthermore, it will be appreciated that for simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein may be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the embodiments described herein. Also, the description is not to be considered as limiting the scope of the embodiments described herein.

[0226] It should also be noted that the terms “coupled” or “coupling” as used herein can have several different meanings depending in the context in which these terms are used. For example, the terms coupled or coupling may be used to indicate that an element or device can electrically, optically, or wirelessly send data to another element or device as well as receive data from another element or device. As used herein, two or more components are said to be “coupled”, or “connected” where the parts are joined or operate together either directly or indirectly (i.e., through one or more intermediate components), so long as a link occurs. As used herein and in the claims, two or more parts are said to be “directly coupled”, or “directly connected”, where the parts are joined or operate together without intervening intermediate components.

[0227] It should be noted that terms of degree such as "substantially", "about" and "approximately" as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree may also be construed as including a deviation of the modified term if this deviation would not negate the meaning of the term it modifies.

[0228] Furthermore, any recitation of numerical ranges by endpoints herein includes all numbers and fractions subsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.90, 4, and 5). It is also to be understood that all numbers and fractions thereof are presumed to be modified by36WSLEGAL\055326\0051 l\43474905v3the term "about" which means a variation of up to a certain amount of the number to which reference is being made if the end result is not significantly changed.

[0229] The present invention has been described here by way of example only, while numerous specific details are set forth herein in order to provide a thorough understanding of the exemplary embodiments described herein. However, it will be understood by those of ordinary skill in the art that these embodiments may, in some cases, be practiced without these specific details. In other instances, well-known methods, procedures and components have not been described in detail so as not to obscure the description of the embodiments. Various modification and variations may be made to these exemplary embodiments without departing from the spirit and scope of the invention, which is limited only by the appended claims.37WSLEGAL\055326\0051 l\43474905v3

Claims

CLAIMS:

1. An assembly for post-fabrication tuning of a silicon waveguide component, comprising:a multi-shot laser irradiation system, comprising:an ultrashort pulse laser configured to emit a plurality of ultrashort laser pulses; an optical train arranged to direct the plurality of laser pulses to a target location on or proximate to a waveguide portion of the component; andan optical characterization system configured to measure an optical response of the silicon waveguide component to determine a resonance wavelength thereof.

2. The assembly of claim 1, wherein the ultrashort pulse laser source is configured to emit the plurality of laser pulses at a visible to near-infrared wavelength, in a range from about 400 nm to about 2500 nm.

3. The assembly of claim 2, wherein the wavelength range is about 400 nm to 2500 nm.

4. The assembly of any one of claims 1 to 3, wherein the plurality of ultrashort laser pulses have a pulse duration in a range from about 1 fs to about 10 ps.

5. The assembly of any one of claims 1 to 4, wherein the plurality of ultrashort laser pulses have a pulse energy in a range from about 1 nJ to less than about 1 mJ per pulse.

6. The assembly of any one of claims 1 to 5, wherein the optical characterization system comprises:one or more of (i) a broadband optical source; and (ii) a swept frequency source; a transmitter lensed fiber configured to couple light from broadband source and / or swept frequency source, into the silicon waveguide component;a receiver lensed fiber configured to receive light from the silicon waveguide component; andone or more of (i) an optical spectrum analyzer, and (ii) a photodetector, coupled to the receiver lensed fiber to measure light.

7. The assembly of any one of claims 1 to 6, wherein the silicon waveguide-based photonic component comprises one of a microring resonator, a Mach-Zehnder interferometer, a directional coupler, a grating coupler, a Bragg grating, or a racetrack resonator.38WSLEGAL\055326\0051 l\43474905v38. The assembly of any one of claims 1 to 7, wherein the laser system is configured to selectively produce either: (i) a positive resonance wavelength shift by inducing a permanent increase in effective refractive index at the target location, or (ii) a negative resonance wavelength shift by inducing material removal at the target location.

9. The assembly of claim 8, wherein, at a selected laser fluence,the positive resonance wavelength shift is produced, by delivering to the target location, a first number of the pulses that is at or below a pulse-count threshold, andthe negative resonance wavelength shift is produced, by delivering to the target location, a second number of the pulses that is greater than the pulse-count threshold.

10. The assembly of any one of claims 1 to 9, wherein the plurality of ultrashort laser pulses are controlled to provide trimming resolution of resonance wavelength shift of about 2 to 3 picometers per pulse and a full trimming range equal to the free spectral range.

11. A method for post-fabrication tuning of a silicon waveguide component using the assembly of any one of claims 1 to 10, comprising:positioning the silicon waveguide component relative to the laser irradiation system such that the optical train directs the plurality of ultrashort laser pulses to a target location on or proximate the silicon waveguide component;measuring, with the optical characterization system, an initial resonance wavelength of the silicon waveguide component;operating the femtosecond laser to apply a selected number of the plurality of ultrashort laser pulses to the target location; andmeasuring, with the optical characterization system, a resonance wavelength of the silicon waveguide component to determine a resonance wavelength shift relative to the initial resonance wavelength and a target resonance wavelength.

12. The method of claim 11, wherein the measuring and operating are automatically performed by a controller.39WSLEGAL\055326\0051 l\43474905v3

Images