System for supercontinuum generation

Abstract

To provide a system for generating a supercontinuum with a waveguide structure that facilitates efficient light coupling, enables the generation of a flat and broad supercontinuum spectrum, and is robust against damage from high-repetition-rate pump sources.SOLUTION: A system (16) comprises a frequency comb generator (15) and a waveguide structure (9) coupled to the frequency comb generator (15). The waveguide structure (9) comprises different sections: an untapered section (1) which has predetermined first cross-sectional outer diameters (8, 8a, 8b), a down tapered transition section (2) leading to a tapered waist section (3) having second cross-sectional outer diameters (7, 7a, 7b) smaller than the first cross-sectional outer diameters (8, 8a, 8b), Followed by an up-taper transition section (4) extending to an untapered output section (5) which reverts to third cross-sectional outer diameters (6, 6a, 6b) larger than the second cross-sectional outer diameters (7, 7a, 7b).SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to the technical fields of photonics and waveguide technology, with a particular focus on systems that generate supercontinuum light. This technology is relevant to a variety of applications, including optical communications, spectroscopy, signal processing, and metrology. The present invention addresses the challenges associated with generating broad and consistent spectral ranges in waveguide systems, which is crucial for increasing the efficiency and effectiveness of optical systems in scientific, industrial, and telecommunications applications. [Background technology]

[0002] Supercontinuum generation, the process of generating broadband optical spectra, has seen significant advances, with a wide range of applications in fields such as metrology, spectroscopy, biology, spectrometer calibration, and telecommunications. Traditional approaches to achieving supercontinuum generation often use waveguide structures, including microstructured optical fibers or planar waveguides on substrates such as wafers. These structures exploit nonlinear optical phenomena to broaden the spectrum of an input light source (usually a laser). This process is highly sensitive to properties such as the geometry and material composition of the waveguide structure.

[0003] Microstructured optical fibers, essential for supercontinuum generation, are distinguished by a distinctive configuration of air holes surrounding a solid or hollow core. This design allows for precise manipulation of the fiber's dispersion characteristics, which is crucial for efficient supercontinuum generation. The dispersion profile of these waveguide structures is crucial because it determines how light of different wavelengths travels through the fiber and affects the phase-matching conditions essential for nonlinear optical interactions.

[0004] Tapered waveguide structures, in which the cross-sectional outer diameter of the waveguide (such as the outer diameter of a cylindrical shape) varies along its length, have been utilized to enhance supercontinuum generation. These structures facilitate engineering the dispersion characteristics of the waveguide over its entire length, thereby enabling more effective broadening of the input spectrum. This modification of dispersion characteristics contributes to broader and more uniform supercontinuum generation.

[0005] One of the key challenges in supercontinuum generation is achieving a flat spectral output while maintaining coherence. Fluctuations in the intensities of different spectral components can limit the usefulness of supercontinuums in precision applications. Achieving a flat, broad, and coherent supercontinuum is particularly challenging, as most attempts result in a loss of coherence. Coherence is essential for applications such as spectrometer calibration, dual-comb spectroscopy, or beat signal generation with other multiple laser sources, such as continuous-wave lasers.

[0006] The prior art has explored various configurations of microstructured fibers and waveguides to address these challenges. For example, U.S. Patent No. 6,275,999 details a method for fabricating microstructured fibers and highlights the ability to customize the zero dispersion wavelength (ZDW) of these fibers. This document emphasizes the importance of the ZDW in the context of supercontinuum generation, as it directly impacts the phase-matching conditions essential for nonlinear processes.

[0007] Patent document 2 details the generation of ultrashort pulses using a laser cavity incorporating a nonlinear optical loop mirror. These pulses serve as a pump source for supercontinuum generation, highlighting the influence of the pump laser's properties on the efficiency and quality of the supercontinuum.

[0008] Patent document 3 provides insight into supercontinuum generation using tapered microstructured optical fibers. It describes a configuration in which the core diameter of the original, untapered fiber is greater than 7 μm and the pump wavelength is within the normal dispersion range. This document also addresses the issue of fiber damage at high optical powers and proposes the use of an end cap at the fiber input end to increase the damage threshold.

[0009] Recent academic studies have made significant contributions to our understanding of supercontinuum generation in tapered waveguides. Zhang et al. [1] and Jiang et al. [2] explored the effect of tapering on the supercontinuum spectrum. These studies revealed that the shape of the tapered transition and the length of the tapered section significantly change the spectral characteristics of the supercontinuum. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] US 10 698 155 B2 [Patent Document 2] EP 2 637 265 A1 [Patent Document 3] US 11 221 445 B2 [Non-patent literature]

[0011] [Non-Patent Document 1] Supercontinuum generation of 314.7 W ranging from 390 to 2400 nm by tapered photonic crystal fiber (Optics Letters, 46(6), 1429-1432, 2021) [Non-patent document 2] Transition profile control for broadband visible supercontinuum generation in tapered PCF (CLEO: Science and Innovations, 2015, paper JW2A.96) Summary of the Invention [Problem to be solved by the invention]

[0012] Prior art in the field of supercontinuum generation using tapered microstructured fibers faces several challenges and limitations, which the present invention aims to address.

[0013] First, efficient coupling of light into the fiber is crucial, especially when the available pump power is limited. Small core sizes (mode field diameter < 3 μm) should be avoided due to their high numerical aperture (NA), which complicates coupling. For planar waveguides, providing an inverse or double inverse taper at the coupling end improves efficiency. These short tapers are specifically designed for input / output coupling and should not be confused with tapered transitions used for dispersion compensation. In general, larger fiber diameters simplify coupling due to lower NA, improve alignment tolerances, and reduce the sensitivity of the coupling optics to angular deviations.

[0014] Second, prior art has shown that without careful tuning of the taper shape, the resulting supercontinuum tends to be structured and inhomogeneous across the spectrum. Achieving a flat, wide, and uniform supercontinuum requires precise control of the taper transition shape. This involves tuning a variety of parameters, including the length of the downtaper and uptaper transitions, the cross-sectional outer diameter of the taper waist, and other geometric features of the fiber.

[0015] Another important issue arises when using high-repetition-rate pump sources, such as those at GHz or multi-GHz frequencies. Because nonlinear processes scale with pulse energy, maintaining a constant pump pulse energy is essential to achieve consistent supercontinuum spectral coverage across a range of repetition rates. Increasing the repetition rate by a factor of 10 or even 100 results in a proportional increase in average power. This scaling poses a challenge: tapered microstructured fibers are susceptible to damage at high input power levels, especially at the input taper transition. This damage, which often leads to fiber burnout or melting, cannot be resolved by simply adding an end cap to the fiber input, as proposed in the prior art. To avoid coherence loss, nonlinear amplification of amplified spontaneous emission, and soliton splitting, it is desirable to use short fiber lengths of just a few centimeters, rather than the several meters seen in conventional implementations.

[0016] Therefore, the problem underlying the present invention is to overcome these limitations by providing a supercontinuum generation system having a waveguide structure that facilitates efficient optical coupling, enables the generation of a flat and broad supercontinuum spectrum, and is robust against damage from high repetition rate pump sources. [Means for solving the problem]

[0017] According to the present invention, the problems identified in the field of supercontinuum generation are solved by a system for generating a supercontinuum with a novel waveguide structure, which is defined in independent claim 1, further advantageous developments of which are outlined in the dependent claims.

[0018] Specifically, the present invention encompasses a system for generating a supercontinuum, the system comprising: a frequency comb generator, which may be a femtosecond (fs) mode-locked laser, a microcavity-based frequency comb, or an electro-optic frequency comb generated by modulating a continuous wave (cw) laser with, for example, an electro-optic modulator; A waveguide structure coupled to a frequency comb generator, comprising: an untapered input section having a predetermined first outer cross-sectional diameter; a down-tapered transition section leading to a tapered waist section having a second outer cross-sectional diameter dimension smaller than the first outer cross-sectional diameter dimension; and an up-tapered transition section extending to an untapered output section returning to a third outer cross-sectional diameter dimension larger than the second outer cross-sectional diameter dimension and preferably the same as or similar to the first outer cross-sectional diameter dimension (i.e., within a ±10% tolerance of the first outer cross-sectional diameter dimension), wherein the input section is configured to have a region of anomalous dispersion and at least one region of normal dispersion, and the tapered waist section is configured to have only a region of normal dispersion over a wavelength range essential for supercontinuum generation.

[0019] Frequency comb generators may incorporate additional components to enhance their functionality, such as fiber amplifiers and input pulse dispersion management tools, including pulse compression and pulse shaping. These components prepare the light before it enters the waveguide structure. In specific applications, such as the AstroComb, the system utilizes a fiber-based femtosecond (fs) laser, a filter cavity, a power amplifier, and a pulse compressor.

[0020] The waveguide structure of our system provides efficient dispersion management. The configuration of this waveguide structure, including precise cross-sectional outer diameter dimensions, allows optimal nonlinear interactions with precise dispersion control, which is crucial for achieving flat, broad supercontinuum spectra. In this context, "flat" refers to a spectrum with minimal structural variations, with intensity fluctuations of only 3 dB to 5 dB over a 100 nm range, excluding the pump region. A "broad" supercontinuum has a spectral width of at least 500 nm and can span an octave or more. For example, it can range from 500 nm to 1600 nm using a 1 μm pump wavelength, or from 900 nm to 2400 nm using a 1.5 μm pump wavelength. This fine-tuned dispersion control highlights our invention's ability to generate supercontinuum spectra that meet stringent application requirements in precision photonics.

[0021] In the context of the present invention, the wavelength range of interest for supercontinuum generation typically extends from 500 nm to 1600 nm when the wavelength of the pump light source is approximately 1 μm. Such a configuration ensures optimal phase-matching conditions and promotes the nonlinear interactions necessary to generate a coherent, broad supercontinuum. Alternatively, the wavelength range of interest for supercontinuum generation may extend from 400 nm to 1200 nm, preferably from 500 nm to 1000 nm, for example.

[0022] For a pump wavelength of 1.5 μm, the spectrum can range from 900 nm to 2400 nm, or from 1200 nm to 2200 nm, or other combinations within that range.

[0023] The present invention significantly advances supercontinuum generation by using a femtosecond (fs) or picosecond (ps) mode-locked laser as the pump source, strategically positioning the pump wavelength within the anomalous dispersion region of the original, untapered waveguide structure. Supercontinuum generation occurs primarily at the tapered transition, and by carefully shifting the dispersion profile through controlled transition shaping, a flat and broad supercontinuum is obtained, extending to the point where no group velocity dispersion (GVD) zero crossings occur. This method effectively eliminates the spectral gap and maintains spectral integrity throughout the entire tapered waist.

[0024] This waveguide structure is configured to be compatible with either fs or ps laser pumps, demonstrating its resiliency and flexibility for use with various pump sources (frequency comb sources), accommodating different comb mode spacings and enhancing its suitability for high-repetition-rate applications. This robust and adaptable design is crucial for generating stable, coherent supercontinuum light, making it highly valuable for precision metrology and optical communications applications.

[0025] According to one embodiment of the present invention, the down-taper transition and the up-taper transition each have a length between 5 cm and 15 cm, preferably between 8 cm and 12 cm.

[0026] Precise control of the length allows for optimal shaping of the supercontinuum spectrum to accommodate different wavelength requirements, while maintaining the structural integrity of the waveguide.

[0027] The waveguide structure can be implemented as a waveguide on a planar substrate structure, particularly a photonic integrated circuit (PIC). Furthermore, in applicable configurations, regardless of substrate implementation, the down-taper transition and the up-taper transition can each have a length between 3 mm and 30 mm.

[0028] In this embodiment, we introduce the adaptation of waveguide structures to a photonic integrated circuit (PIC) format. This offers the advantages of miniaturization and integrated optics, as well as improved waveguide fabrication precision, potentially enabling more compact and efficient supercontinuum sources. Another advantage of PIC implementation is that it allows for shorter structures and lower required power levels, since the nonlinearities of common PIC waveguide materials such as silicon nitride and lithium niobate are often higher than those of fused silica, a common material in optical fibers.

[0029] The waveguide structure can be configured as a tapered microstructure fiber, with the untapered input section having a core diameter in the range of approximately 3 μm to 5 μm. Further, in the same or a different configuration, the untapered input section can have two zero-dispersion wavelengths ZDW1 and ZDW2, with ZDW1 located within a wavelength range of approximately 900 nm ± 40 nm and ZDW2 located at wavelengths greater than 2000 nm, thereby establishing an anomalous dispersion region between the zero-dispersion wavelengths ZDW1 and ZDW2.

[0030] These specifications establish an effective anomalous dispersion region between ZDW1 and ZDW2, which is essential for broadband and flat supercontinuum generation, improving the fiber's performance for a variety of optical applications.

[0031] A further development includes configuring the waveguide structure for use with a pump light source, the pump light source being an ultrashort pulse laser, the wavelength of the ultrashort pulse laser being located within an anomalous dispersion range established between zero dispersion wavelengths ZDW1 and ZDW2.

[0032] As the taper progresses, the maximum of group velocity dispersion (GVD) shifts toward shorter wavelengths until it reaches a point where GVD at the pump wavelength becomes negative, after which the entire GVD curve shifts below zero, marking the transition into the normal dispersion region.

[0033] This configuration optimizes the interaction between the pump laser and the dispersion characteristics of the waveguide. By designing the waveguide to match the pump laser wavelength within the specified anomalous dispersion range, the supercontinuum generation efficiency is significantly improved. This strategic placement of the pump wavelength promotes spectral broadening and ensures effective phase-matching conditions, which are essential for generating high-quality, flat supercontinuum. This development is a careful integration of the waveguide's physical properties and the operating parameters of the pump light source, resulting in a synergistic enhancement of the overall supercontinuum generation process.

[0034] The downtaper transition can be configured to gradually blue-shift until both zero-dispersion wavelengths ZDW1 and ZDW2 disappear, facilitating full supercontinuum generation within this section and resulting in a spectrum free of strong modulation.

[0035] Such a configuration addresses and overcomes the challenge of spectral gaps that can degrade the quality and usefulness of the supercontinuum. By ensuring that the generated spectrum is free of strong modulation, this configuration significantly improves the applicability and performance of waveguides in a variety of applications requiring a broad, uninterrupted supercontinuum, including spectroscopy, metrology, and telecommunications. This development demonstrates a keen understanding of the complex interplay between waveguide geometry, dispersion properties, and nonlinear optics, resulting in a waveguide capable of generating superior supercontinuum spectra.

[0036] The length of the downtaper transition can be configured to produce a flat supercontinuum spectral envelope.

[0037] The length of the downtaper transition can be configured to generate a flat supercontinuum spectral envelope, balancing the trade-off that longer transitions increase conversion efficiency but also increase the risk of coherence loss. Spectral flatness is primarily determined by tapering to the point where the zero-dispersion wavelength (ZDW) vanishes, as previously discussed. Careful control of the taper length and profile optimizes the spectral characteristics and performance of supercontinuum generation. To elucidate the physical concept behind coherent supercontinuum generation, refer to the group delay curves plotted in Figure 5. The phase-matching condition required to generate a new wavelength is that the pump wavelength and the newly generated light travel at the same speed. Observing the transition from scale 1.00 to scale 0.30 clearly shows the shift of the isovelocity points toward shorter wavelengths, pinpointing the generation path of specific wavelengths. Below scale 0.40, the spectral width expansion stops because the isovelocity points no longer exist. Tapering to this critical point is therefore advantageous in that it prevents the formation of multiple conversion paths that can lead to spectral modulation, interference, and degradation of coherence.

[0038] Considering typical pulse energies in the picojoule (pJ) to nanojoule (nJ) range and the inherent nonlinearity of the fused silica in the photonic crystal fiber used for the taper, the taper length must be 5 cm to 15 cm to achieve effective spectral broadening. In contrast, for silicon nitride waveguides, which have much higher nonlinearities and smaller waveguide dimensions, the waveguide length required for effective broadening is significantly reduced, from 1 mm to 30 mm.

[0039] The concept of a "flat" supercontinuum is essential for applications where uniform intensity over a wide spectral range is desirable. In this context, "flat" means minimizing intensity variations across the generated supercontinuum spectrum. This uniformity ensures that all regions of the spectrum are equally represented, enhancing the usefulness of supercontinuums in a variety of applications. By using a computational model that simulates the optical properties of the waveguide, the optimal length of the downtaper transition can be calculated before the waveguide is fabricated. This length is used in the fabrication process.

[0040] The tapered waist does not have a zero dispersion wavelength, and optical signals can be transmitted without significantly changing their spectral characteristics.

[0041] The location of zero group velocity dispersion (GVD) influences the dispersion regime. At the end of the taper transition, all relevant wavelengths shift into the normal dispersion regime. This shift causes the input pulse to broaden in time and lose peak energy, thereby reducing its capacity for further nonlinear interactions.

[0042] The absence of a zero-dispersion wavelength at the tapered waist avoids the complexities of dispersion-related effects and halts nonlinear effects, enabling a supercontinuum that is not only broad but also highly uniform and coherent.

[0043] As explained, this configuration does not introduce multiple paths for generating a particular wavelength, thereby avoiding interference that could otherwise lead to significant spectral structuring.

[0044] This property is particularly useful in precision spectroscopy, frequency metrology, spectrometer calibration, and other scientific endeavors where spectral consistency is paramount.

[0045] Preferably, the tapered waist is of variable length and is specially adapted to optimize supercontinuum generation for different spectral requirements.

[0046] By varying the length of the tapered waist, the waveguide structure can be optimized for different applications, such as telecommunications, medical imaging, spectrometer calibration, scientific research, etc. This flexibility ensures that the supercontinuum generated is optimal for the intended use, whether that be a broader spectrum, more intense light at a specific wavelength, or other specific properties.

[0047] More preferably, the uptaper transition is configured to maintain spectral integrity regardless of the presence or absence of a zero-dispersion wavelength within the uptaper transition, and also avoid strong back reflections by providing an adiabatic impedance match, ensuring consistent transmission of the supercontinuum.

[0048] This uptaper transition configuration preserves the spectral integrity of the supercontinuum as light passes through this section of the waveguide. This is crucial for applications requiring a stable and reliable supercontinuum spectrum, as significant changes in that spectrum can affect system performance and accuracy. Furthermore, this configuration enables effective transmission of the supercontinuum regardless of whether or not a zero-dispersion wavelength (ZDW) is present in the uptaper.

[0049] Preferably, the length of the up-taper transition is selected to be the same as or different from the length of the down-taper transition, among other things, thereby allowing for customized taper configurations.

[0050] The uptaper is configured to avoid further changes in the spectral and coherence properties, and therefore may be configured to be significantly shorter than the downtaper, possibly by a factor of 2 to 5, to minimize its impact on the optical properties of the system.

[0051] The ability to adjust the length of the uptaper transition independently from the downtaper transition allows for a high degree of customization of the waveguide structure. This flexibility is beneficial for tailoring the waveguide properties to specific applications and experimental requirements. This allows for influencing the dispersion profile and nonlinear interactions within the waveguide structure, leading to optimized supercontinuum generation for different wavelength ranges and spectral characteristics.

[0052] Preferably, the untapered output section restores the two zero-dispersion wavelengths ZDW1 and ZDW2 as in the untapered input section, ensuring consistent transmission characteristics of the supercontinuum generated in the waveguide structure.

[0053] When configured in this way, tapered microstructured fibers ensure consistent and predictable transmission characteristics throughout the entire fiber, which is crucial for applications where the integrity and stability of the generated supercontinuum are paramount.

[0054] Preferably, the waveguide structure is covered with an index-matching material to mitigate damage when operated with high repetition rate GHz pump sources at high average powers above 1 W.

[0055] The use of index-matching materials provides additional protection against thermal and mechanical stresses that can occur at high power levels. This increased durability allows the waveguide structure to withstand extended use without performance degradation, improving reliability in demanding applications. The presence of the index-matching material also helps dissipate heat more efficiently and prevent localized heating. This allows the waveguide to handle higher power levels from GHz repetition rates of pump sources without damage, extending the operating range of the device.

[0056] To further refine the taper transition, it is beneficial to explore various shapes other than a linear taper, such as parabolic, exponential, or even step-like configurations. These alternative shapes are particularly effective in more precisely controlling the dispersion characteristics. Furthermore, grading the slope of the taper (starting more gradually and becoming steeper toward the end) can effectively mitigate the abrupt cutoff of the dispersion curve at short wavelengths and improve the flatness of the supercontinuum spectrum. Computational simulations support these modifications and optimize the taper transition. Preferred embodiments incorporate a taper in which the first half of the taper is less steep than the second half, optimizing supercontinuum generation across the entire spectrum.

[0057] In summary, this invention represents a significant advance in the field of photonic waveguide structures for supercontinuum generation. It introduces a novel configuration featuring different cross-sectional dimensions along different sections of the waveguide, optimizing dispersion characteristics to enhance supercontinuum generation. Specifically, it incorporates an untapered input section with anomalous and normal dispersion regions, a tapered waist section with exclusively normal dispersion, and an untapered output section. This configuration enables efficient optical coupling, improved spectral coherence, and broader applicability, addressing key limitations of existing supercontinuum generation technologies.

[0058] Further advantages and features of the present invention will become apparent from the following detailed description of the preferred embodiments, which proceeds with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0059] [Figure 1] 1 is a schematic diagram of a supercontinuum generation system according to the present invention, featuring a frequency comb generator coupled to a waveguide structure. [Figure 2]1 is a cross-sectional view of a microstructured optical fiber having a central core and concentric air-hole rings, illustrating dispersion control in a waveguide structure utilized in a supercontinuum generation system according to the present invention. [Figure 3] FIG. 1 is a detailed schematic diagram of a tapered microstructured fiber as an example of a waveguide structure utilized in a supercontinuum generation system according to the present invention, showing various sections including untapered input and output sections, a downtaper transition section, a tapered waist section, and an uptaper transition section. [Figure 3a] 1A and 1B show alternative embodiments of waveguide structures with rectangular cross sections implemented on a photonic integrated circuit PIC, illustrating the adaptability of the present invention to substrate-based configurations. [Figure 4] Dispersion curves at various scales of the waveguide structure (tapered microstructured fiber) utilized in the supercontinuum generation system according to the present invention, showing the dispersion engineering essential for generating flat and coherent supercontinuum over a wide wavelength range. [Figure 5] Group velocity curves of a waveguide structure (tapered microstructured fiber) at different downtaper scales, showing the wavelength-dependent group velocity, which is important for dispersion management in supercontinuum generation. [Figure 6a] Simulation results showing the spectral change of the waveguide structure (bottom plot) and the generated supercontinuum (top plot), demonstrating that a supercontinuum is generated when tapered to a diameter that eliminates anomalous dispersion. [Figure 6b] Simulation results for a waveguide structure showing a contrasting scenario to Figure 6a, demonstrating spectral changes (bottom plot) and supercontinuum generation (top plot) while the tapered waist maintains anomalous dispersion. [Figure 7a] Simulation results for a waveguide structure with an excessively short downtaper transition, showing the change in the spectrum (bottom graph) and the resulting supercontinuum (top graph), highlighting the importance of taper length accuracy. [Figure 7b] Simulation results for a waveguide structure with an excessively long downtaper transition, showing the change in the spectrum (bottom graph) and the resulting supercontinuum (top graph), highlighting the importance of taper length accuracy. [Figure 8] Graphical comparison of simulated and experimental supercontinuum spectra for waveguide structures, highlighting the impact of different air hole configurations on the spectral output. DETAILED DESCRIPTION OF THE INVENTION

[0060] The present invention advances the field by introducing a waveguide structure tailored for efficient and robust supercontinuum generation. This detailed description refers to the accompanying drawings, which show in detail various embodiments and aspects of the invention.

[0061] The overall system 16 according to the present invention is shown in FIG. 1 and is configured to generate a supercontinuum and comprises two main components: a frequency comb generator 15 and a waveguide structure 9 .

[0062] The frequency comb generator 15, located on the right side of Figure 1 and incorporating a pump source 18, serves as the origin of the broadband frequency comb. It generates a spectrum of frequencies using methods such as mode-locking or modulation. This component is crucial because it provides the initial optical input necessary for shaping into a supercontinuum. The frequency comb generator 15 can be realized as a femtosecond mode-locked laser, a microcavity-based frequency comb, or an electro-optic frequency comb, which is generated by modulating a continuous-wave (cw) laser.

[0063] The waveguide structure 9 is divided into multiple sections: an input section 1, a tapered-waist section 3, and an output section 5. The input section 1, located at the far right of the waveguide structure 9 in Figure 1, receives light (frequency comb output) from a frequency comb generator 15. It is configured to have an anomalous dispersion regime in at least one wavelength range that is important for initiating the supercontinuum generation process. The tapered-waist section 3 is the region where the actual modulation of the optical frequency occurs, dynamically changing the dispersion characteristics. This is achieved by physically tapering the dimensions of the waveguide. The output section 5, located at the far left of the waveguide structure 9 in Figure 1, transmits the modulated light and exhibits supercontinuum properties. This section is configured to return to a larger cross-sectional outer diameter to maintain the integrity of the transmitted light.

[0064] In terms of overall system interaction, light flows from frequency comb generator 15 into input 1 of waveguide structure 9, where it undergoes complex nonlinear interactions and dispersion modifications throughout the structure, and finally emerges as a supercontinuum at output 5. Frequency comb generator 15 and waveguide structure 9 are coupled in a way that minimizes light and coherence losses, which are essential for an efficient supercontinuum.

[0065] Figure 2 shows a cross-section of a microstructured optical fiber (MOF) that functions as a waveguide structure 9, which has a tapered configuration essential for effective supercontinuum generation. The MOF 9 comprises a solid core 10 defined by the innermost circumference of concentrically arranged air holes 11 and 12. The microstructured cladding arrangement depicted in Figure 2 surrounds the core 10, forming a specific geometric configuration that is crucial for managing the dispersion characteristics of the fiber 9.

[0066] The core 10 is made of fused silica, which can be doped with materials such as germanium to enhance its optical properties. It is designed with dimensions that optimize its interaction with light, particularly in terms of coupling efficiency and dispersion management. The air holes 11 and 12 are strategically sized and positioned to tune the dispersion properties of the MOF 9, thereby enabling manipulation of the phase-matching conditions essential for nonlinear optical processes, including supercontinuum generation.

[0067] This depiction shows the subtle engineering behind MOF 9, where the diameter of air holes 11 and 12 can be varied to create an elliptical core shape, thereby imparting polarization-maintaining capabilities to fiber 9. The major axis of elliptical core 10, determined by the placement of the air holes, will be referred to as the core diameter of fiber 9 for purposes of this invention.

[0068] In the context of the present invention, the core diameter is carefully selected to fall within the range of approximately 3 μm to 5 μm, matching the numerical aperture (NA) specifications of commercially available aspheric lenses. This alignment is crucial to ensure efficient coupling of pump light into the MOF, a factor that determines the effectiveness of supercontinuum generation, especially when operating under pump-power-limited conditions.

[0069] Furthermore, the cladding configuration shown demonstrates the ability of MOF 9 to maintain structural integrity while promoting desired optical properties. Cladding designs featuring multiple ring-shaped air holes 11, 12, and 13, including but not limited to those shown in Figure 2, expand the versatility of fiber 9. This enables a variety of fiber taper strategies that can affect the shape and coherence of the generated supercontinuum, thereby addressing issues identified in the prior art regarding structured supercontinuums lacking the flatness and coherence desired for certain applications, such as spectrometer calibration and dual-comb spectroscopy.

[0070] In the embodiment shown in Figure 2, the core diameter of MOF9 is precisely 4.05 μm, meticulously crafted to match the numerical aperture (NA) of a specific commercially available aspheric lens (such as the Thorlabs C230TMD-B). This precise calibration between the core size of MOF9 and the aspheric lens is crucial for achieving efficient optical coupling, a key factor in generating supercontinuum when utilizing limited pump power.

[0071] As shown in FIG. 2, the core and cladding structure demonstrates the ingenious application of various air hole diameters, which not only allows for customized elliptical cores 10 beneficial to maintaining polarization properties, but also serves as evidence of the optimized configuration of the waveguide structure 9 according to the present invention.

[0072] This configuration facilitates effective phase-matching conditions, which are crucial for the nonlinear optical processes that drive supercontinuum generation in MOF9.

[0073] 3 is a schematic diagram of a tapered microstructured optical fiber (MOF), an example of a waveguide structure 9 essential to a supercontinuum generation system 16 in accordance with the present invention. This diagram visually illustrates the key components of the fiber taper essential to the waveguide structure 9 in accordance with the present invention. It begins with an untapered input section 1, establishing the point of entry for the pump light. This initial segment seamlessly narrows to a downtapered transition section 2, which then proceeds to a defined tapered waist section 3. Following this intermediate section is an uptapered transition section 4, which ultimately extends to an untapered output section 5, completing the fiber taper structure.

[0074] 3 depicts the tapering process of the waveguide structure 9, which is crucial to the present invention. This figure highlights the initial uniform outer cross-sectional diameter dimensions corresponding to the outer diameters 8, 6 of the non-tapered sections 1, 5, gradually decreasing to a narrower outer waist diameter 7. The constant outer diameters 6, 8 characteristic of the non-tapered sections 1, 5 are typically maintained at standard fiber dimensions (e.g., 125 μm) to facilitate seamless integration with existing fiber connectors.

[0075] The downtaper transition 2 and uptaper transition 4 are depicted in visual detail and may each have lengths that are either consistent for symmetry or vary for asymmetric configurations. The central tapered waist 3 is depicted as an integral part of the waveguide structure 9, and its length can be adjusted. The inclusion of this section 3 is essential to the structural integrity of the taper; omission of this section 3 may lead to increased vulnerability to physical stresses, including damage from bending.

[0076] Figure 3 illustrates the structure of the waveguide structure 9 as claimed in claims 1 to 15, showing its versatility and flexibility for practical applications. This versatility is manifested in the ability to tailor the taper to a symmetric or asymmetric configuration and to adjust the length of the tapered waist 3, thereby meeting different requirements for supercontinuum generation.

[0077] Thus, Figure 3 illustrates our approach to creating a taper that meets the functional requirements for efficient supercontinuum generation while also incorporating structural features that improve the durability and adaptability of MOF9 across a variety of optical configurations.

[0078] FIG. 3a shows an alternative embodiment within the scope of the present invention, depicting a waveguide structure 9 having a rectangular cross-section located on a substrate 14, specifically designed for photonic integrated circuits (PICs). This embodiment demonstrates the flexibility of the present invention to accommodate waveguide formats other than fiber-based. The illustrated waveguide structure 9 is primarily rectangular, may be fabricated from silicon nitride, and has an untapered thickness 6b, 8b of approximately 800 nm and an untapered width 6a, 8a of approximately 1200 nm. Alternatively, the waveguide structure 9 can be tapered in only one dimension, i.e., the waveguide thickness can remain constant while the width decreases, or vice versa.

[0079] This waveguide configuration is configured to incorporate one or two zero-dispersion wavelengths (ZDWs). Specifically, a short-wavelength ZDW, designated ZDW1, is likely within the range of approximately 900 nm ± 40 nm (for pump wavelengths of approximately 1 μm). In contrast, a long-wavelength ZDW, designated ZDW2, exists at wavelengths greater than 1500 nm (for pump wavelengths of approximately 1 μm). This dispersion profile indicates that pump wavelengths, such as those emitted by Yb-doped fiber lasers typically operating near 1040 nm, fall within the anomalous dispersion region of the initial untapered waveguide structure. This characteristic is crucial for efficient supercontinuum generation. For a pump wavelength of 1.5 μm, the entire dispersion profile shifts to longer wavelengths, with ZDW1 located just below 1.5 μm and ZDW2 located well above 1.5 μm.

[0080] Furthermore, it is possible to position the zero dispersion wavelength (ZDW) slightly below the pump wavelength. This position depends on variables such as the size of the air holes in the waveguide structure and the contrast between the core and the surrounding material. Mastering the generation of a flat, broad, and coherent supercontinuum spectrum is a major challenge in the field, but is essential for advanced applications such as spectrometer calibration and dual-comb spectroscopy.

[0081] 3a illustrates the adaptability of the present invention to a variety of waveguide structures 9, extending its usefulness beyond traditional microstructured fiber designs. This versatility is evident from the specification that for substrate-based waveguide structures 9, such as photonic integrated circuits (PICs), the lengths of the downtaper transition 1 and uptaper transition 5 can range from 3 mm to 30 mm. This adaptability enhances the applicability of the present invention in a variety of optical and photonic systems.

[0082] FIG. 4 shows a graphical representation of the dispersion curves of tapered microstructured fibers (MOFs) (see FIGS. 1 and 2) at various downtaper scales, revealing the wavelength vs. dispersion characteristics essential for the waveguide structure 9 outlined in this invention.

[0083] These curves, taken from MOF9 shown in Figures 1 and 2, demonstrate the effect of tapering on the dispersion profile, a key factor in supercontinuum generation. The solid curve labeled "Scale_1.00" represents the dispersion of the original, untapered fiber (corresponding to outer diameters 6 and 8 in Figure 3). This solid curve contains two zero-dispersion wavelengths (ZDWs): ZDW1 at approximately 901 nm and ZDW2 above 2000 nm. Between these ZDWs lies the anomalous dispersion region, which corresponds to the wavelengths emitted by typical ultrashort-pulse lasers, such as Yb-doped fiber lasers operating around 1 μm.

[0084] The transformative effect of the tapering process on the dispersion profile of a waveguide structure 9, specifically a microstructured optical fiber (MOF), can be observed in Figure 4. The figure shows how the dispersion characteristics change as the diameter 7 of the fiber 9 at the taper waist 3 is systematically reduced through the tapering process.

[0085] The dashed curve labeled "Scale_0.60" represents the dispersion curve of MOF 9 when diameter 7 in Figure 3 is tapered to 60% of the original diameters 6 and 8. This adjustment results in a significant shift in the zero dispersion wavelengths (ZDWs): ZDW1 moves to 792 nm and ZDW2 moves to 1814 nm, highlighting the dynamic nature of the dispersion regime as the taper scale changes.

[0086] Continuing this trend, the dotted curve "Scale_0.40" and the dash-dot curve "Scale_0.30" in Figure 4 correspond to MOF9 when the diameter of the tapered waist region 3 is further reduced to 40% and 30%, respectively, from the original diameter. These curves show a gradual shift in the ZDW, and the "Scale_0.30" curve is particularly notable for its complete absence. This absence marks a complete transition to a region free of anomalous dispersion, a key element for the flat, coherent supercontinuum generation targeted in this invention.

[0087] Thus, Figure 4 serves as an important graphical representation of how specific variations in the taper scale of MOFs, an important example of a waveguide structure, directly affect their dispersion properties, providing a roadmap for optimizing supercontinuum generation in various application scenarios.

[0088] The tapering configuration of the waveguide structure 9, as depicted in Figures 2 and 2a, plays a crucial role in shaping the dispersion curve as depicted in Figure 4, which is a fundamental element for realizing a flat, broadband, and coherent supercontinuum. Realizing such a supercontinuum is essential for applications such as spectrometer calibration, dual-comb spectroscopy, and simultaneous detection of beat signals from continuous-wave (cw) lasers of different wavelengths.

[0089] Maintaining the coherence of the supercontinuum is crucial, especially when a non-optimal tapered waveguide structure can compromise the broadband frequency comb structure. Therefore, careful tuning of the tapering parameters, as shown in Figure 4, is essential to maintaining the integrity and functional capabilities of the supercontinuum in accordance with the objectives of the present invention.

[0090] The tapering rate, a key factor in shaping the dispersion characteristics of a waveguide structure, is defined as the reduction in the cross-sectional outer diameter per unit length. For example, uniformly tapering a waveguide structure from a core diameter of 5 μm to 1 μm over an 8 cm length results in a tapering rate of 0.5 μm / cm. Adjusting this rate allows for precise control of supercontinuum generation, addressing a variety of wavelength targets from the visible (approximately 400 nm to 900 nm) to the mid-infrared region. By selecting the pump wavelength and waveguide material (e.g., silicon nitride on silicon wafer (SiN), silicon on insulator (SiOI), lithium niobate on insulator (LNOI), tantalate, or aluminum nitride (AlN)), the supercontinuum can be further tailored to the needs of specific applications.

[0091] Figure 4 effectively illustrates the effect of varying the taper dimensions in MOF 9 on dispersion characteristics. This figure highlights the relationship between modifying the taper geometry and dispersion characteristics, which is crucial for optimizing supercontinuum generation in waveguide structure 9, as described in embodiments of the present invention. These strategic modifications facilitate adaptation of waveguide structure 9 to achieve desired supercontinuum properties, particularly in terms of coherence and spectral width.

[0092] Figure 5 shows group velocity curves for a tapered microstructured fiber (MOF), one embodiment of the waveguide structure 9 defined in this invention. These curves are mapped at various downtaper scales to illustrate the interplay between wavelength and group velocity, which is crucial for managing dispersion in supercontinuum generation. The plotted curves are based on the MOF 9 shown in Figure 2. The downtaper scales applied to the taper waist diameter 7 (see Figure 3) of the MOF 9 are varied to illustrate how changes in the cross-sectional dimensions of the MOF 9 at different points along its length affect the group velocity and, therefore, the dispersion characteristics that are essential for effective supercontinuum generation.

[0093] In Figure 5, paralleling the observations in Figure 4, each curve shows the interplay between wavelength and group velocity at different scales of fiber diameter reduction. The solid curve labeled "Scale_1.00" shows the group velocity profile of MOF 9 in its original, untapered state, correlated with its total diameter (shown as outer diameters 6 and 8 in Figure 3). This solid curve shows two distinct turning points, one at 901 nm and the other above 2000 nm, that indicate the zero-dispersion wavelength (ZDW). These turning points define the transition between normal and anomalous dispersion and are important for understanding dispersion dynamics within the waveguide structure 9 according to the present invention.

[0094] Figure 5 illustrates the effect of tapering on group velocity at various scales. The dashed "Scale_0.60" curve, the dotted "Scale_0.40" curve, and the dash-dot "Scale_0.30" curve represent different fiber diameter reductions. In the dashed "Scale_0.60" curve, group velocity transition points are located at 792 nm and 1814 nm, indicating a shift in the zero-dispersion wavelength (ZDW) caused by tapering. This shift is further illustrated in the dotted "Scale_0.40" curve, where transition points appear at 790 nm and 983 nm. Notably, in the dash-dot "Scale_0.30" curve, a continuous rise is observed without a clear turning point. This indicates the absence of a ZDW and a complete shift into the normal dispersion regime, which is essential for achieving the intended supercontinuum generation in the waveguide structure 9 of the present system 16.

[0095] 5 is crucial in elucidating the effect of tapering rate on the dispersion characteristics of the waveguide structure 9. The tapering rate is quantified as the reduction in diameter per unit length (e.g., tapering from 5 μm to 1 μm core diameter over an 8 cm length is 0.5 μm per cm), which directly affects the dispersion characteristics of the waveguide structure 9. These characteristics are essential for the efficiency and quality of supercontinuum generation according to embodiments of the present invention.

[0096] Thus, Figure 5 also effectively demonstrates the adaptability of the waveguide structure 9 to various materials and target wavelength ranges, which is an important aspect of the present invention. By using materials such as SiN on Si wafers, Si on insulator, LNOI, tantalate, or AlN, the waveguide can facilitate supercontinuum generation over a broad spectrum. This spectrum can range from visible light (400 nm to 900 nm) to mid-infrared wavelengths, depending on the pump wavelength and specific tapering parameters employed in the waveguide structure 9.

[0097] Figure 5 is crucial in showing how the degree of tapering of MOF9 affects the group velocity, a fundamental aspect of the present invention. This relationship is essential for generating flat, coherent, and broadband supercontinuum spectra. Such spectra are particularly useful for applications such as spectrometer calibration, dual-comb spectroscopy, or simultaneous detection of beat signals from continuous-wave (cw) lasers of different wavelengths, demonstrating the practicality of the present invention.

[0098] Figures 5a and 5b are useful in illustrating the effect of tapering on supercontinuum generation in microstructured fiber 9, consistent with the inventive concepts and embodiments discussed herein. These figures elucidate how different approaches to tapering affect both the spectral changes and supercontinuum generation associated with the present invention.

[0099] The lower graph in Figure 6a shows a simulation of the spectral changes in a tapered microstructure fiber 9 (an example of a waveguide structure) when tapering reduces the fiber diameter to the point where anomalous dispersion is eliminated. The simulation is based on parameters such as a pump laser with a central wavelength of 1040 nm, a pulse width of 130 fs, and a pulse energy of 0.18 nJ.

[0100] The taper configuration shown in Figure 3 is composed of several distinct sections: a 20 mm long untapered input section 1, a 69.1 mm long downtapered transition section 2, a 55 mm long central tapered waist section 3, followed by a 69.1 mm long uptapered transition section 4, and terminating in a 20 mm long untapered output section 5. This symmetrical configuration exhibits a taper waist outer diameter 7 of 38 μm, which corresponds to approximately 30% (0.3 times) of the original untapered diameter 6, 8 (125 μm) of the fiber 9.

[0101] The tapering process effectively eliminates the zero dispersion wavelengths (ZDWs) in the tapered waist section 3, thereby achieving perfect supercontinuum generation within the down-taper transition section 2. After this section 2, the spectrum of the supercontinuum remains almost unchanged in the subsequent sections 3, 4, and 5.

[0102] The upper graph in Figure 6a shows the supercontinuum generated at the output of tapered fiber 9. This supercontinuum features a spectral profile that is flat within 10 dB in the visible spectrum, demonstrating the effectiveness of the specific tapering technique used to achieve flat, broadband supercontinuum generation.

[0103] The lower graph of Figure 6b illustrates a different scenario from Figure 6a, showing the spectral changes of a tapered microstructured fiber 9 in which the tapering process does not lead to the elimination of anomalous dispersion in the tapered waist 3. This illustrates a modified tapering result that is not encompassed by the present invention, in which the tapering technique employed maintains anomalous dispersion in the tapered waist 3, thereby affecting the properties of the generated supercontinuum.

[0104] In the simulation shown in Figure 6b, the basic taper configuration is similar to that of Figure 6a, with a notable difference in the taper waist 3. Here, the outer diameter 7 of the taper waist is set to 50 μm, corresponding to 0.4 times the original diameters 6 and 8 of the fiber 9. This proportional reduction in diameter preserves anomalous dispersion in the taper waist, as evidenced by the presence of zero-dispersion wavelengths (ZDWs) at 790 nm and 983 nm. This variation in the taper waist diameter 7 represents an alternative embodiment outside the scope of this invention and highlights the impact of taper dimensions on dispersion characteristics and the resulting supercontinuum properties.

[0105] In the scenario depicted in Figure 6b, the tapered configuration allows the supercontinuum generation to extend beyond the downtaper transition section 2, through the tapered waist section 3, and on to the subsequent uptaper transition section 4 and untapered output section 5. However, as shown in the lower graph of Figure 6b, there is a discernible spectral gap in the range of 680 nm to 780 nm, highlighting the impact that the cross-sectional outer diameter dimensions of the tapered waist section have on the continuity and uniformity of the generated supercontinuum spectrum.

[0106] The top graph in Figure 6b shows the output supercontinuum spectrum for a configuration in which anomalous dispersion is not completely eliminated at the tapered waist 3. In contrast to Figure 6a, this spectrum shows a significant spectral gap, highlighting the importance of a complete transition to the normal dispersion region at the tapered waist 3 in order to achieve a continuous, gap-free supercontinuum spectrum consistent with the objectives of this invention.

[0107] Figures 5a and 5b effectively demonstrate the important role of precise tapering in the microstructured fiber 9 of the present invention in affecting the characteristics of the supercontinuum generation. Figure 6a illustrates a scenario in which the tapering is tailored to completely eliminate anomalous dispersion at the taper waist, resulting in a flat and broad (broadband) supercontinuum. This contrasts with Figure 6b, in which preserving anomalous dispersion at the taper waist results in a supercontinuum with a spectral gap. As shown in Figure 6a, the effectiveness of generating a coherent and flat supercontinuum spectrum is crucial for applications requiring a coherent light source, such as spectrometer calibration and dual-comb spectroscopy.

[0108] Figure 7a provides simulation insights into supercontinuum generation in a tapered microstructured fiber (waveguide structure 9), particularly under conditions of insufficient length of the downtaper transition 2. The lower plot of Figure 7a captures the spectral evolution within the microstructured fiber 9 where the length of the downtaper transition 2 is limited to 20 mm. This constrained length is insufficient to promote full supercontinuum development, a key characteristic for achieving optimal broadband spectral coverage.

[0109] As shown in Figure 3, the tapering specifications include a taper waist diameter 7 of 38 μm, which is 0.3 times smaller than the original diameters 6, 8 of the untapered (input and output) fiber sections 1, 5. However, as is evident in the upper plot of Figure 7a, this reduction in the length of the downtaper transition section 2 results in the supercontinuum spectrum no longer exhibiting the desired broad bandwidth. This simulation result highlights the critical importance of ensuring an adequate length of the downtaper transition section 2 in order to achieve the full potential of supercontinuum generation within the waveguide structure 9.

[0110] Figure 7b investigates the effect of an overextended downtaper transition 2 on supercontinuum generation in a tapered microstructure fiber, representative of waveguide structure 9. The lower plot in Figure 7b shows the spectral change when the length of the downtaper transition 2 is extended to 200 mm. This excessive length prevents the desired supercontinuum characteristics from being effectively expressed, resulting in an irregular and incomplete spectrum, as visualized in the upper plot in Figure 7b. This result deviates from the ideal flat and broad supercontinuum spectrum and emphasizes the importance of properly balancing the length of the downtaper transition for optimal broadband supercontinuum generation.

[0111] The taper dimensions used in the simulation in Figure 7b are consistent with the parameters outlined in Figure 3, demonstrating that an inappropriate length for the downtaper transition 2 can have a significant impact on the quality of the supercontinuum output. A downtaper transition that is too short or too long can result in suboptimal supercontinuum performance. Therefore, accurately calibrating the length of the downtaper transition 2 is crucial for achieving a well-defined, flat supercontinuum envelope. The optimal taper length for achieving a balanced supercontinuum spectrum, both in width and flatness, typically ranges from approximately 5 cm to 15 cm, with a more preferred range of 5 cm to 10 cm. This highlights the importance of accuracy in determining the taper length for effective and efficient supercontinuum generation within the waveguide structure 9.

[0112] The findings from Figures 6a and 6b collectively highlight the crucial role that precise taper length plays in designing waveguide structures that efficiently generate high-quality supercontinuum. Accurate taper length calibration is essential not only for optimizing the supercontinuum spectrum but also for maintaining the structural resilience of the microstructured fiber. This aspect becomes particularly important in high-power applications where the integrity of the fiber is paramount.

[0113] Furthermore, as shown in Figure 3, the employment of index-matching material 17 plays an important role in protecting fiber 9 from potential damage caused by leakage of high-power light. This protective measure is essential to maintain the performance and extend the operational life of fiber 9 in a variety of demanding optical devices.

[0114] Figure 8 shows a critical comparison of simulated and experimental supercontinuum spectra, demonstrating the practical effectiveness of the waveguide structure 9 outlined in this invention. Included in this graph is the simulated supercontinuum curve derived from the parameters established in Figure 6a. This curve is essential for assessing the fidelity of the experimental results to the theoretical model.

[0115] Figure 8 shows experimental results obtained from two types of tapered microstructured fibers 9, each with a unique air-ring configuration, as shown in Figure 2. The first set of experimental data was obtained from fiber 9 with a four-ring taper configuration, and the second set was obtained from fiber 9 with a seven-ring taper configuration. These experimental setups were consistent with the taper geometry and pump parameters used in the simulations, allowing a direct comparison of theoretical predictions with actual results.

[0116] A key finding in Figure 8 is the flat supercontinuum spectra produced in both experimental scenarios, distinguished by their gap-free nature. This demonstrates the success of the tapering strategy described in this work to achieve a coherent and scalable supercontinuum. The experimental data are in good agreement with the simulation results, primarily in the short wavelength region, highlighting the reliability of the simulations in predicting real fiber behavior.

[0117] In the experimental setup, tapered microstructured fiber 9, both in four-ring and seven-ring configurations, was tested at a pump repetition rate of 25 GHz. The setup required an average input power of 10 W to achieve the simulated pulse energy of 0.18 nJ. To counter potential damage from such high-power operation, the entire taper, including the downtaper transition 2, the taper waist 3, and the uptaper transition 4, was enveloped in index-matching gel (index-matching material 17). This precaution was essential to prevent strong light leakage and overheating, which could damage the fiber. The index-matching gel (index-matching material 17) served a dual role: it facilitated the removal of infrared light from the cladding of fiber 9 and also served as a thermal conductor, directing excess heat to a metal plate coupled to fiber 9.

[0118] Figure 8 highlights the flexibility in configuring the taper of the waveguide structure 9. Experimental data shows that effective results can be obtained by varying the lengths of the down-taper transition 2 and the up-taper transition 4, opening the door to asymmetric taper configurations. This configuration flexibility allows, in accordance with embodiments of the present invention, the creation of custom-made waveguide structures 9 optimally tuned to meet specific requirements for supercontinuum generation.

[0119] In summary, Figure 8 bridges the gap between theoretical modeling and experimental verification and demonstrates the practical effectiveness of the waveguide structure 9 for achieving the desired supercontinuum properties. This effectiveness is crucial for applications requiring high repetition rates and high power inputs, where the durability and adaptability of the tapered fiber 9 are key factors.

[0120] The above description has meticulously detailed an innovative approach to developing waveguide structures 9 for supercontinuum generation. It encompasses a thorough examination of the processes involved in tapering microstructured fibers 9 and elucidates their resulting impact on supercontinuum generation. The above description demonstrates the adaptability of these waveguide structures 9 across various embodiments and highlights their compatibility with a variety of configurations and material types. This adaptability greatly expands practical applications.

[0121] The precision demonstrated in the fabrication of these waveguide structures9, supported by the consistency of experimental results with theoretical predictions, demonstrates the effectiveness of this invention in high-power environments. This innovation successfully achieves a coherent, flat, and broad supercontinuum spectrum. Therefore, this invention represents a significant breakthrough in photonics, providing advanced solutions in areas such as spectrometer calibration, dual-comb spectroscopy, simultaneous detection of beat signals from continuous-wave (cw) lasers of different wavelengths, and various high-precision optical applications.

[0122] 1. Non-tapered input section 2 Down-taper transition 3 Tapered waist 4 Up-tapered transition 5 Non-tapered section 6, 8 outer diameter 6b, 8b thickness 7 Tapered waist diameter 9 Waveguide structure 10 cores 11, 12, 13 Air vents 14 PCB 15 Frequency Comb Generator 16 Supercontinuum Generation System 17 Refractive index matching material 18 Pump Source PIC Photonic Integrated Circuit ZDW1, ZDW2 Zero Dispersion Wavelength

Claims

1. A system (16) for generating a supercontinuum, comprising: a frequency comb generator (15); a waveguide structure (9) coupled to the frequency comb generator (15); The waveguide structure (9) an input portion (1) having a predetermined first cross-sectional outer diameter dimension (8, 8a, 8b); a down-taper transition section (2) leading to a tapered waist section (3) having a second cross-sectional outer diameter dimension (7, 7a, 7b) smaller than the first cross-sectional outer diameter dimension (8, 8a, 8b); an up-tapered transition section (4) following the tapered waist section (3) and extending to an untapered output section (5) returning to a third cross-sectional outer diameter dimension (6, 6a, 6b) greater than the second cross-sectional outer diameter dimension (7, 7a, 7b); The system, wherein the input section (1) is configured to have an anomalous dispersion region and at least one normal dispersion region, and the tapered waist section (3) is configured to have only normal dispersion regions over a wavelength range essential for supercontinuum generation.

2. 2. The system (16) of claim 1, wherein the third outer cross-sectional diameter dimension (6, 6a, 6b) is substantially the same as the first outer cross-sectional diameter dimension (8, 8a, 8b) within a tolerance of ±10%.

3. 3. The system (16) according to claim 1 or 2, wherein the down-taper transition (2) and the up-taper transition (4) each have a length between 5 cm and 15 cm when the waveguide structure (9) is implemented as a microstructured optical fiber (MOF), or between 3 mm and 30 mm when the waveguide structure (9) is implemented as a waveguide on a planar substrate structure (14), in particular a photonic integrated circuit (PIC).

4. 4. The system (16) according to any one of claims 1 to 3, wherein the waveguide structure (9) is configured as a tapered microstructured fiber, and the untapered input section (1) preferably has a core diameter in the range of about 3 μm to 5 μm.

5. 5. The system of claim 1, wherein the non-tapered input section has two zero-dispersion wavelengths ZDW1 and ZDW2, ZDW1 being located in a wavelength range of approximately 900 nm ± 40 nm and ZDW2 being located at wavelengths greater than 2000 nm, whereby an anomalous dispersion region is established between the zero-dispersion wavelengths ZDW1 and ZDW2.

6. 6. The system of claim 5, wherein the waveguide structure is configured for use with a pump source, the pump source being an ultrashort pulse laser, the wavelength of the ultrashort pulse laser being located within the anomalous dispersion range established between the zero dispersion wavelengths ZDW1 and ZDW2.

7. 7. The system (16) of claim 5 or 6, wherein the down-tapered transition section (2) is configured such that the zero-dispersion wavelengths ZDW1 and ZDW2 are gradually blue-shifted and eventually both disappear, facilitating complete supercontinuum generation within the section (2) and producing a spectrum without strong modulation.

8. The system (16) of any one of claims 1 to 7, wherein the length of the down-tapered transition (2) is configured to produce a flat supercontinuum spectral envelope.

9. 9. The system (16) according to any one of claims 1 to 8, wherein the tapered waist portion (3) does not have a zero dispersion wavelength, and optical signals can be transmitted without substantially changing their spectral characteristics.

10. 10. The system (16) of any one of claims 1 to 9, wherein the tapered waist (3) has a variable length and is specifically adapted to optimize supercontinuum generation for different spectral requirements.

11. 11. The system (16) of claim 1, wherein the uptaper transition (4) is configured to ensure consistent transmission of a supercontinuum while maintaining spectral integrity regardless of the presence or absence of a zero-dispersion wavelength within the uptaper transition (4).

12. 12. The system (16) of any one of claims 1 to 11, wherein the length of the up-taper transition (4) is specifically selected to be the same as or different from the length of the down-taper transition (2), thereby allowing for customized taper configurations.

13. 13. The system (16) of claim 5, wherein the untapered output section (5), like the untapered input section (1), restores the two zero-dispersion wavelengths ZDW1 and ZDW2, ensuring consistent transmission characteristics for the supercontinuum generated in the waveguide structure (9).

14. 14. The system (16) of any one of claims 1 to 13, wherein the waveguide structure (9) is covered with an index-matching material (17) to mitigate damage when operated with a high repetition rate GHz pump source (18) at high average powers above 1 W.

15. A method for generating a supercontinuum using the system (16) of any one of claims 1 to 14, comprising the steps of: coupling light from the frequency comb generator (15) into the non-tapered input section (1) of the waveguide structure (9); and collecting the resulting supercontinuum output from said untapered output (5); The generated supercontinuum exhibits a spectral width of at least 500 nm and spans an octave, for example, ranging from 500 nm to 1600 nm, when the waveguide structure (9) is excited with a laser having a central wavelength around about 1 μm, or ranging from 900 nm to 2400 nm when the waveguide structure (9) is excited with a laser (15) having a central wavelength around about 1.5 μm.

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