Super-continuum spectrum generation system

Abstract

The invention relates to a system (16) for generating a super-continuum spectrum. The 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 portions: a non-conical input portion (1) having a specified first cross-sectional outer dimension (8, 8a, 8b), a lower conical transition portion (2) leading to a conical waist portion (3) having a second cross-sectional outer dimension (7, 7a, 7b) smaller than the first cross-sectional outer dimension (8, 8a, 8b), followed by an upper conical transition portion (4) extending to a non-conical output portion (5), the non-tapered output portion (5) restores to a third cross-sectional outer dimension (6, 6a, 6b) greater than the second cross-sectional outer dimension (7, 7a, 7b). The parts (1, 2, 3, 4, 5) are meticulously configured to exhibit different dispersion regions, which is crucial to efficient super-continuum spectrum generation. One key aspect of the invention is precise calibration of the tapering process. This calibration ensures the elimination of abnormal dispersion zones in the cone waist portion (3), which is crucial for achieving flat and coherent supercontinuum spectra.

Description

TECHNICAL FIELD

[0001] The present invention relates to the technical field of photonics and waveguide technology, and in particular to a system for generating supercontinuum. The technology is applicable to a variety of applications such as optical communications, spectroscopy, signal processing, and metrology. The present invention addresses challenges associated with producing a wide and consistent spectral range in waveguide systems, which is critical for improving the efficiency and effectiveness of optical systems in scientific, industrial, and telecommunication applications. BACKGROUND

[0002] Supercontinuum generation is a process of producing a wideband optical spectrum that has seen significant progress due to its wide-ranging applications in metrology, spectroscopy, biology, spectrometer calibration, and telecommunications, among others. Traditional methods of achieving supercontinuum typically involve the use of waveguide structures, including microstructured optical fibers and planar waveguides on wafer-like substrates. These structures utilize nonlinear optical phenomena to broaden the spectrum of an input light source, usually a laser. The process is highly sensitive to the characteristics of the waveguide structure, including its geometry and material composition.

[0003] Microstructured optical fibers are an integral part of supercontinuum generation, with their unique air-hole structure surrounding a solid or hollow core. This design enables precise manipulation of the fiber's dispersion properties, which are critical for efficient supercontinuum generation. The dispersion profile of these waveguide structures is crucial as it determines how various wavelengths of light travel through the fiber, affecting the phase-matching conditions necessary for nonlinear optical interactions.

[0004] Tapered waveguide structures, where the cross-sectional outer dimensions (such as the outer diameter in a cylindrical configuration) vary along the waveguide length, have been used to enhance supercontinuum generation. These structures help manipulate the dispersion properties of the waveguide throughout its length, enabling more efficient broadening of the input spectrum. This alteration in the dispersion properties facilitates the generation of a wider, more uniform supercontinuum.

[0005] A significant challenge in supercontinuum generation is achieving a flat spectral output while maintaining coherence. Fluctuations in the intensity of different spectral components can limit the usefulness of a supercontinuum in precise applications. Achieving a flat, wide, and coherent supercontinuum is particularly challenging, as most attempts result in the loss of coherence, which is critical for applications such as spectrometer calibration, dual-comb spectroscopy, or generating beatnotes with multiple other laser sources like continuous-wave lasers.

[0006] The state of the art has investigated different configurations of microstructured optical fibers and waveguides to tackle these challenges. For example, US 10698155 B2 details the fabrication of microstructured optical fibers and highlights the ability to tailor the zero-dispersion wavelength (ZDW) of these fibers. This application emphasizes the importance of the ZDW in the context of supercontinuum generation, as it directly influences the phase matching condition necessary for nonlinear processes.

[0007] EP 2637265 A1 details the generation of ultrashort pulses using a laser resonator incorporating a nonlinear optical loop mirror. These pulses can act as a pump source for supercontinuum generation, highlighting the influence of the pump laser characteristics on the supercontinuum efficiency and quality.

[0008] US 11221445 B2 provides insights into the generation of supercontinuum using tapered microstructured optical fibers. It describes a configuration where the core diameter of the untapered original fiber is greater than 7 pm and the pump wavelength is in the normal dispersion regime. This application also addresses the issue of fiber damage at high optical powers, suggesting the use of end caps at the input end of the fiber to increase the damage threshold.

[0009] Recent academic research has made significant contributions to understanding supercontinuum generation in tapered waveguides. Zhang et al. in "Supercontinuum generation of 314.7 W ranging from 390 to 2400 nm by tapered photonic crystal fiber" (Optics Letters, 46(6), 1429-1432, 2021) and Jiang et al. in "Transition profile control for broadband visible supercontinuum generation in tapered PCF" (CLEO: Science and Innovations, 2015, Paper JW2A.96) have explored the influence of tapering on the supercontinuum spectrum. These studies highlight how the shape of the taper transition and the length of the tapered section significantly alter the spectral properties of the supercontinuum.

[0010] The state of the art in the field of supercontinuum generation using tapered microstructured optical fibers faces several challenges and limitations, which this application aims to address.

[0011] Firstly, it is critical to efficiently couple light into the fiber, especially in cases where the available pump power is limited. Small core sizes (mode field diameter < 3 pm) should be avoided as they have a high numerical aperture (NA) which complicates the coupling. For planar waveguides, a reverse taper or double reverse taper at the coupling end can improve the efficiency. These short tapers are specifically designed for input and output coupling and should not be confused with the taper transitions used for dispersion modification. Generally, larger fiber diameters simplify the coupling due to lower NA, improve the alignment tolerance, and reduce the sensitivity to angular misalignments of the coupling optics.

[0012] Secondly, the prior art shows that the generated supercontinuum spectrum tends to be structured or non-uniform across the spectrum if the taper shape is not finely tailored. To achieve a flat, wide and uniform supercontinuum spectrum, the taper transition shape must be precisely controlled. This involves adjusting various parameters such as the length of the down-taper and up-taper transitions, the cross-sectional outer dimension (outer diameter) at the taper waist, and other geometric features of the fiber.

[0013] Another important issue arises when using a pump source with a high repetition rate, such as GHz or multi-GHz frequency. Since the nonlinear processes are proportional to the pulse energy, it is crucial to keep the pump pulse energy constant for achieving consistent supercontinuum spectral coverage across various repetition rates. Therefore, increasing the repetition rate by a factor of 10 or even 100, the average power is proportionally increased. This scaling poses a challenge as the taper microstructured fiber is prone to damage at high levels of incident power, especially within the down-taper transition at the input side. This damage often leads to fiber burning and melting, which cannot be addressed simply by adding an end cap at the fiber input as suggested in some prior art. To avoid the loss of coherence, nonlinear amplification of amplified spontaneous emission, and soliton fission, it is suggested to use a shorter fiber length - only a few centimeters instead of several meters as in some prior implementations.

[0014] Therefore, the underlying problem of the present invention is to overcome these limitations by providing a system for generating a supercontinuum with a waveguide structure that facilitates efficient light coupling, enables the generation of a flat and wide supercontinuum spectrum, and is robust against damage from high repetition rate pump sources. SUMMARY

[0015] According to the invention, the problems found in the field of supercontinuum generation are solved by a system for generating a supercontinuum with a novel waveguide structure. The system is defined in independent claim 1, and further advantageous developments are outlined in the dependent claims.

[0016] In particular, the invention comprises a system for generating a supercontinuum, the system comprising: a frequency comb generator, which can be a femtosecond (fs) mode-locked laser, a microresonator-based frequency comb, or an electro-optic frequency comb generated by modulating a continuous wave (cw) laser with, for example, an electro-optic modulator; and

[0017] a waveguide structure coupled to the frequency comb generator and comprising: a non-tapered input section having a specified first cross-sectional outer dimension, a lower tapered transition section leading to a tapered waist section having a second cross-sectional outer dimension smaller than the first cross-sectional outer dimension, followed by an upper tapered transition section extending to a non-tapered output section that restores to a third cross-sectional outer dimension larger than the second cross-sectional outer dimension, and preferably identical or similar to the first cross-sectional outer dimension, i.e. within a tolerance range of + / - 10%, wherein the input section is configured to have an anomalous dispersion region and at least one normal dispersion region, and the tapered waist section is configured to have only normal dispersion regions in a wavelength range critical for supercontinuum generation.

[0018] The frequency comb generator can contain additional components with enhanced functionalities, such as a fiber amplifier and input pulse dispersion management tools, including pulse compression and pulse shaping. These components prepare the light before it enters the waveguide structure. For certain applications such as AstroComb, the system utilizes a fiber-based fs laser, a filtering cavity, a power amplifier, and a pulse compressor.

[0019] The waveguide structure in the system according to the invention provides efficient dispersion management. The configuration of this waveguide structure involves precise cross-sectional outer dimensions, enabling optimal nonlinear interactions through precise dispersion control, which is critical for a flat and wide supercontinuum spectrum. In this context, “flat” refers to a spectrum with minimal structure variation - the intensity variation is kept between 3dB to 5dB over a 100nm range, excluding the pump region. A “wide” supercontinuum includes a spectral width of at least 500nm, possibly extending over an octave, for example from 500nm to 1600nm when using a 1pm pump wavelength, or from 900nm to 2400nm when using a 1.5pm pump wavelength. This fine-tuned dispersion control emphasizes the ability of the invention to produce a supercontinuum spectrum that meets the stringent application requirements in high-precision photonics.

[0020] In the context of the invention, the wavelength range critical for supercontinuum generation can typically span 500nm to 1600nm when using a pump source with a wavelength of about 1pm. This configuration ensures that the optimal phase matching conditions are met and facilitates the necessary nonlinear interactions to produce a coherent and wide supercontinuum. Alternatively, the wavelength range critical for supercontinuum generation can span, for example, 400nm to 1200nm, preferably 500nm to 1000nm.

[0021] For a pump wavelength of 1.5 pm, the spectrum can span 900 nm to 2400 nm, or 1200 nm to 2200 nm, or other combinations within this range.

[0022] The present invention can significantly advance supercontinuum generation by employing a femtosecond (fs) or picosecond (ps) mode-locked laser as the pump source. It strategically places the pump wavelength within the anomalous dispersion region of the original non-tapered waveguide structure. Supercontinuum generation mainly occurs in the tapered transition section, where the dispersion profile is carefully shifted by controlled transition shaping, resulting in a flat and wide supercontinuum spectrum extending to a point where the group velocity dispersion (GVD) zero-crossing does not occur. This method effectively eliminates spectral gaps and maintains spectral integrity on the tapered waist.

[0023] The waveguide structure is configured to be compatible with fs or ps laser pumping, and it shows flexibility and flexibility in using various pump sources (frequency comb sources), adapting to different comb spacing, and enhancing its applicability to high repetition rate applications. This robust and adaptable design is crucial for generating stable and coherent supercontinuum light, which makes it highly valuable in precision metrology and optical communication.

[0024] According to one embodiment of the invention, the length of the lower tapered transition section and the upper tapered transition section is between 5 cm and 15 cm, preferably between 8 cm and 12 cm.

[0025] The precise control of the length allows the optimal shaping of the supercontinuum spectrum to meet different wavelength requirements while maintaining the structural integrity of the waveguide.

[0026] The waveguide structure can be implemented as a waveguide on a planar substrate structure, in particular a photonic integrated circuit (PIC). Furthermore, in applicable configurations, the length of the lower tapered transition section and the upper tapered transition section can be between 3 mm and 30 mm, regardless of the substrate implementation.

[0027] This embodiment introduces the adaptability of the waveguide structure to the photonic integrated circuit (PIC) format. This enables the advantages of miniaturization and integrated optics, which, combined with the improved precision of waveguide manufacturing, can lead to more compact and efficient supercontinuum sources. Another advantage of the PIC implementation is that the nonlinearity of typical PIC waveguide materials such as silicon nitride or lithium niobate is generally higher than that of fused silica, the typical material of optical fibers, resulting in a shorter structure and a lower required power level.

[0028] The waveguide structure can be configured as a tapered microstructured fiber with a non-tapered input section having a core diameter in the range of about 3-5 pm. Further, in the same or different configurations, the non-tapered input section can have two zero-dispersion wavelengths ZDW1 and ZDW2, where ZDW1 is located in the wavelength range of about 900 nm ± 40 nm and ZDW2 is located in the wavelength range of greater than 2000 nm, thereby establishing an anomalous dispersion region between the zero-dispersion wavelengths ZDW1 and ZDW2.

[0029] These specifications establish an effective anomalous dispersion region between ZDW1 and ZDW2, which is critical for generating a broad and flat supercontinuum spectrum, thereby enhancing the performance of the fiber in various optical applications.

[0030] Further developments involve configuring the waveguide structure for use with a pump source that is an ultra-short pulse laser, and the wavelength of the ultra-short pulse laser is located in the anomalous dispersion range established between the zero-dispersion wavelengths ZDW1 and ZDW2.

[0031] As the taper evolves, the maximum of the group velocity dispersion (GVD) shifts towards shorter wavelengths, resulting in the GVD at the pump wavelength becoming negative. Subsequently, the entire GVD curve shifts below zero, indicating a transition to the normal dispersion region.

[0032] This configuration optimizes the interaction between the pump laser and the waveguide dispersion characteristics. By designing the waveguide to align with the wavelength of the pump laser in a specified anomalous dispersion range, the efficiency of supercontinuum generation is significantly improved. This strategic placement of the pump wavelength ensures effective phase matching conditions, which are critical for promoting the broadening of the optical spectrum and generating a high-quality, flat supercontinuum spectrum. This development represents a thoughtful integration of the physical characteristics of the waveguide with the operational parameters of the pump source, thereby synergistically enhancing the overall supercontinuum generation process.

[0033] The lower taper transition section can be configured such that the zero-dispersion wavelengths ZDW1 and ZDW2 gradually blue-shift until they both disappear, thereby facilitating the generation of a fully supercontinuum spectrum within this section and producing a spectrum without strong modulation.

[0034] This configuration addresses and overcomes the challenge of spectral gaps, which can reduce the quality and utility of the supercontinuum spectrum. By ensuring that the generated spectrum is free of strong modulation, this configuration aspect significantly improves the applicability and performance of the waveguide in various applications that require a broad and uninterrupted supercontinuum spectrum, such as in spectroscopy, metrology, and telecommunications. This development demonstrates a keen understanding of the complex interplay between the waveguide geometry, dispersion characteristics, and nonlinear optics, ultimately resulting in a waveguide capable of producing an exceptional supercontinuum spectrum.

[0035] The length of the lower taper transition can be configured to generate a flat supercontinuum spectral envelope.

[0036] The length of the lower taper transition can be configured to generate a flat supercontinuum spectral envelope, balancing the trade-off of longer sections improving conversion efficiency but also increasing the risk of coherence loss. As previously mentioned, the flatness of the spectrum is primarily determined by the point at which the gradual decrease to zero dispersion wavelength (ZDW) disappears. Careful control of the taper length and profile can optimize the spectral characteristics and performance of the supercontinuum generation. For clarification of the physical concepts behind coherent supercontinuum generation, please refer to the group delay curve depicted in Figure 5 The phase matching condition required to generate new wavelengths demands that the light of the pump wavelength and the newly generated light propagate at the same speed. When observing the transition from scale 1.00 to scale 0.30, it becomes clear that the isovelocity point moves towards shorter wavelengths, precisely tracing the generation path of a specific wavelength. Below scale 0.40, no isovelocity point leads to the stop of the spectral broadening. Therefore, the gradual decrease to this critical moment is advantageous as it prevents the formation of multiple conversion paths that can lead to significant spectral modulation, interference, and reduced coherence.

[0037] Considering the typical pulse energies in the range of picojoules (pJ) to nanojoules (nJ) and the inherent nonlinearities of fused silica in photonic crystal fibers used for the taper, the taper length should be between 5 cm and 15 cm to achieve efficient spectral broadening. In contrast, in silicon nitride waveguides with higher nonlinearities and smaller waveguide dimensions, the waveguide length required for efficient broadening is significantly reduced, ranging from 1 mm to 30 mm.

[0038] The concept of a "flat" supercontinuum is crucial for applications requiring a broad spectrum of uniform intensity. In this context, "flat" refers to minimizing the variation in intensity across the generated supercontinuum spectrum. This uniformity ensures that all parts of the spectrum are equally represented, enhancing the utility of the supercontinuum in various applications. Using a computational model of the waveguide's optical properties, the optimal length of the lower taper transition can be calculated before the waveguide is manufactured. This length is then used during the manufacturing process.

[0039] The taper waist section can not have a zero dispersion wavelength, enabling the transmission of optical signals without significantly altering their spectral characteristics.

[0040] The location of the zero group velocity dispersion (GVD) affects the dispersion regime. At the end of the taper transition, all relevant wavelengths are moved into the normal dispersion regime. This movement leads to the temporal spreading of the input pulse, resulting in a loss of peak energy and thus reducing its ability for further nonlinear interactions.

[0041] The absence of a zero-dispersion wavelength in the tapered waist avoids the complexity of dispersion-related effects, preventing nonlinear effects and enabling a supercontinuum that is not only broad but also very uniform and coherent.

[0042] As mentioned above, this configuration does not introduce multiple paths to generate a specific wavelength, thus avoiding interference that can lead to significant spectral structure.

[0043] This feature is particularly beneficial in precision spectroscopy, frequency metrology, spectrometer calibration, and other scientific work where spectral consistency is crucial.

[0044] Preferably, the tapered waist section has a variable length, particularly adapted to optimize supercontinuum generation for different spectral requirements.

[0045] By varying the length of the tapered waist section, the waveguide structure can be optimized for different applications, such as telecommunications, medical imaging, spectrometer calibration, and scientific research. This flexibility ensures that the generated supercontinuum is best suited for the intended use, whether it requires a broader spectrum, stronger light at certain wavelengths, or other specific characteristics.

[0046] More preferably, the upper tapered transition section is configured to ensure consistent transmission of the supercontinuum, maintaining spectral integrity regardless of the presence of a zero-dispersion wavelength within the upper tapered transition section, also avoiding strong back reflections by providing an adiabatic impedance match.

[0047] This configuration of the upper tapered transition section ensures that the spectral integrity of the supercontinuum is maintained when the light passes through this section of the waveguide. This is crucial for applications that require a stable and reliable supercontinuum spectrum, as any significant changes to the spectrum can affect the performance and accuracy of the system. Additionally, this configuration allows for efficient transmission of the supercontinuum regardless of the presence of a zero-dispersion wavelength (ZDW) in the upper tapered section.

[0048] Preferably, the length of the upper tapered transition section is specifically chosen to be the same or different from the length of the lower tapered transition section, enabling a customized tapered configuration.

[0049] The upper taper is configured to avoid further changes to the spectral and coherence properties. Therefore, it can be configured to be much shorter than the lower taper, possibly 2 times or up to 5 times shorter, to ensure minimal impact on the optical properties of the system.

[0050] The ability to adjust the length of the upper tapered transition section independently of the lower tapered transition section allows for a high degree of customization of the waveguide structure. This flexibility is beneficial for tailoring the properties of the waveguide according to specific application or experimental requirements. This affects the dispersion distribution and nonlinear interactions within the waveguide structure, optimizing supercontinuum generation for different wavelength ranges or spectral characteristics.

[0051] Preferably, the non-tapered output section restores the two zero-dispersion wavelengths ZDW1 and ZDW2 as in the non-tapered input section, ensuring consistent transmission characteristics of the generated supercontinuum spectrum in the waveguide structure.

[0052] When configured in this manner, the tapered microstructured optical fiber can ensure consistent and predictable transmission characteristics throughout the fiber. This consistency is crucial for applications where the integrity and stability of the generated supercontinuum spectrum is of utmost importance.

[0053] Preferably, the waveguide structure is surrounded by an index-matching material to mitigate damage when operated at high average powers exceeding 1 W with high repetition rate GHz pump sources.

[0054] The use of an index-matching material can provide additional protection against thermal and mechanical stresses that can arise at high power levels. This enhancement in robustness ensures that the waveguide structure can withstand prolonged usage without degradation in performance, resulting in higher reliability in demanding applications. The presence of the index-matching material aids in more efficient heat dissipation and prevents localized heating. This enables the waveguide to handle higher power levels from GHz repetition rate pump sources without causing damage, thereby expanding the operating range of the device.

[0055] To further refine the taper transition, it is beneficial to explore various geometrical shapes beyond the linear taper, such as parabolic, exponential, or even stepped configurations. These alternative shapes are particularly effective in more precisely managing the dispersion characteristics. Additionally, using a gradient in the taper slope—starting with a more gradual angle and increasing the steepness towards the end—can effectively mitigate the abrupt cutoff of the dispersion curve at shorter wavelengths, thereby enhancing the flatness of the supercontinuum spectrum. Computational simulations support these modifications, optimizing the taper transition. The preferred embodiment incorporates a taper where the slope of the first half is smaller than the slope of the second half, thereby optimizing the supercontinuum generation across the entire spectrum.

[0056] In summary, the present invention represents a significant advancement in the field of photonic waveguide structures for supercontinuum generation. It introduces a novel configuration with different cross-sectional dimensions along different sections of the waveguide, optimizing the dispersion characteristics for enhanced supercontinuum generation. Specifically, it comprises a non-tapered input section with anomalous and normal dispersion regions, a tapered waist section with only normal dispersion, and a non-tapered output section. This configuration allows for efficient light coupling, improved spectral coherence, and broader application potential, thereby addressing key limitations of existing supercontinuum generation techniques. BRIEF DESCRIPTION OF DRAWINGS

[0057] Other advantages and features of the present invention will become apparent by reference to the following detailed description of preferred embodiments, when considered in conjunction with the accompanying drawings. Said drawings depict:

[0058] Figure 1 : Schematic of a system for generating supercontinua according to the present invention, with a frequency comb generator coupled to a waveguide structure.

[0059] Figure 2 : Cross-section of a microstructured optical fiber with a central core and concentric air hole rings, illustrating dispersion control in a waveguide structure used in a supercontinuum generation system according to the present invention.

[0060] Figure 3 : Detailed schematic of a tapered microstructured optical fiber, as an example of a waveguide structure used in a supercontinuum generation system according to the present invention, showing various sections, including non-tapered input and output sections, lower taper transition, taper waist, and upper taper transition sections.

[0061] Figure 3a : Illustration of an alternative embodiment of a waveguide structure with a rectangular cross-section implemented on a photonic integrated circuit (PIC), showing the adaptability of the present invention to substrate-based configurations.

[0062] Figure 4 : Waveguide structure (tapered microstructured optical fiber) used in a supercontinuum generation system according to the present invention

[0063] Dispersion curves at different scales, illustrating the dispersion management necessary to generate a flat coherent supercontinuum over a wide wavelength range.

[0064] Figure 5 : Group velocity curves of the waveguide structure (tapered microstructured optical fiber) at different lower taper scales, illustrating the wavelength-dependent group velocity, which is crucial for dispersion management in supercontinuum generation.

[0065] Figure 6a : Simulation results showing the spectral evolution (lower plot) and the generated supercontinuum (upper plot) of a waveguide structure, illustrating the generated supercontinuum when the diameter is gradually reduced to eliminate anomalous dispersion. Figure 6b : Simulation results of a waveguide structure, in contrast to Figure 6a , where the spectral evolution (lower plot) and supercontinuum generation (upper plot) are demonstrated with anomalous dispersion maintained at the taper waist section. Figure 7a : Simulation results of a waveguide structure with an excessively short lower taper transition, showing the spectral evolution (lower plot)

[0066] and the resulting supercontinuum (upper plot), highlighting the criticality of taper length precision. Figure 7b : Simulation results of a waveguide structure with an excessively long lower taper transition, showing the spectral evolution (lower plot)

[0067] and the resulting supercontinuum (upper plot), highlighting the criticality of taper length precision. Figure 8: graphical comparison of simulated and experimental supercontinuum spectra of waveguide structures, highlighting the impact of different air-hole configurations on the spectral output. DETAILED DESCRIPTION

[0068] The present invention advances the field by introducing a waveguide structure tailored for efficient and robust supercontinuum generation. The detailed description refers to the accompanying drawings, which show in detail various embodiments and aspects of the invention.

[0069] The overall system 16 according to the present invention, as shown in Figure 1 , is configured to generate a supercontinuum and consists of two main components: a frequency comb generator 15 and a waveguide structure 9.

[0070] The frequency comb generator 15, located Figure 1 on the right and containing a pump source 18, acts as the origin of a wideband frequency comb. It employs methods such as mode-locking or modulation to produce a frequency spectrum. This component is crucial as it provides the initial light input necessary to carve into a supercontinuum. The frequency comb generator 15 can manifest as a femtosecond mode-locked laser, a microresonator-based frequency comb, or an electro-optic frequency comb, which is generated by modulating a continuous wave (cw) laser.

[0071] The waveguide structure 9 is divided into several sections: an input section 1, a tapered waist section 3, and an output section 5. The input section 1 is located Figure 1 at the far right of the waveguide structure 9, receiving light from the frequency comb generator 15 (frequency comb output). It is configured to have an anomalous dispersion region in at least one wavelength range, which is crucial for the initiation of the supercontinuum generation process. The tapered waist section 3 is the region where the actual modulation of light frequencies occurs, dynamically changing the dispersion properties by physically tapering down the waveguide dimensions. The output section 5 is located Figure 1 at the far left of the waveguide structure 9. It transmits the modified light, exhibiting supercontinuum characteristics. This section is configured to revert to a larger external dimension of the cross-section, preserving the integrity of the transmitted light.

[0072] In terms of overall system interaction, light flows from the frequency comb generator 15 into the input section 1 of the waveguide structure 9, where it undergoes complex nonlinear interactions and dispersion modifications across the structure, ultimately emerging as a supercontinuum through the output section 5. The frequency comb generator 15 and the waveguide structure 9 are coupled in such a way that ensures minimal light loss and coherence, which is crucial for efficient supercontinuum generation.

[0073] Figure 2 A cross-sectional view of a microstructured optical fiber (MOF) used as the waveguide structure 9 is shown, which has undergone a tapering process crucial for efficient supercontinuum generation. The MOF 9 includes a solid core 10 defined by the innermost circumference of air holes 11 and 12 arranged in a concentric pattern.Figure 2 The microstructured cladding arrangement shown surrounds the core 10, forming a specific geometric configuration that is critical to managing the dispersion properties of the optical fiber 9.

[0074] The core 10 is made of fused silica, which can be doped with materials such as germanium to enhance its optical properties. Its design dimensions are optimized for interaction with light, particularly in terms of coupling efficiency and dispersion management. The air holes 11 and 12 are strategically sized and positioned to tailor the dispersion properties of the MOF 9, enabling manipulation of the phase matching conditions that are critical to nonlinear optical processes, including supercontinuum generation.

[0075] This description demonstrates the fine engineering behind the MOF 9, where the diameters of the air holes 11 and 12 can be varied to produce an elliptical core shape, endowing the optical fiber 9 with polarization maintaining capabilities. For the purposes of this invention, the major axis of the elliptical core 10, as determined by the air hole configuration, is denoted as the core diameter of the optical fiber 9.

[0076] In the context of this invention, the core diameter is carefully chosen to be in the range of about 3 pm to 5 pm, aligning with the numerical aperture (NA) specifications of commercially available aspherical lenses. This alignment is critical to ensuring efficient coupling of the pump light into the MOF 9, which is a determinant of supercontinuum generation efficiency, particularly when operating under limited pump power conditions.

[0077] Furthermore, as shown, the cladding configuration demonstrates the ability of the MOF 9 to facilitate the necessary optical properties while maintaining structural integrity. The design of the cladding has multiple annular air holes 11, 12, 13, including but not limited to Figure 2 The air holes shown extend the versatility of the optical fiber 9. It allows for different fiber tapering strategies that will impact the shape and coherence of the generated supercontinuum, addressing the issues found in the prior art related to structured supercontinuum that lack the flatness and coherence required for certain applications such as spectrometer calibration or dual comb spectroscopy.

[0078] In the context of this invention, the core diameter of the MOF 9 is carefully chosen to be in the range of about 3 pm to 5 pm, aligning with the numerical aperture (NA) specifications of commercially available aspherical lenses. This alignment is critical to ensuring efficient coupling of the pump light into the MOF 9, which is a determinant of supercontinuum generation efficiency, particularly when operating under limited pump power conditions. Figure 2 In the embodiment shown, the core diameter of the MOF 9 is precisely 4.05 pm, carefully made to align with the numerical aperture (NA) of a specific commercially available aspherical lens, such as Thorlabs C230TMD-B. The precise calibration between the core dimensions of the MOF 9 and the aspherical lens is critical to achieving efficient light coupling, which is an important factor in generating supercontinuum with limited pump power.

[0079] As shown in the embodiment, the MOF 9 is designed to be compatible with a specific commercially available aspherical lens, such as Thorlabs C230TMD-B. The precise calibration between the core dimensions of the MOF 9 and the aspherical lens is critical to achieving efficient light coupling, which is an important factor in generating supercontinuum with limited pump power. Figure 2As shown, the core and cladding architecture exhibits the creative application of different air hole diameters, which not only allows for the customization of the elliptical core 10, favoring the preservation of the polarization properties, but also demonstrates the optimized configuration of the waveguide structure 9 according to the present invention. This configuration facilitates the efficient phase matching conditions, which are critical for the nonlinear optical processes driving the supercontinuum generation within the MOF 9.

[0080] Figure 3 A schematic diagram of a tapered microstructured optical fiber (MOF) is shown, which is an example of the waveguide structure 9 that is a component of the supercontinuum generation system 16 according to the present invention. The diagram visually illustrates the basic components of the fiber taper, which are components of the waveguide structure 9 according to the present invention. It starts with a non-tapered input section 1, which establishes the entry point for the pump light. This initial segment seamlessly narrows into a lower taper transition section 2, which continues to a defined taper waist section 3. Following this intermediate section is an upper taper transition section 4, which eventually extends to a non-tapered output section 5, completing the fiber taper structure.

[0081] Figure 3 The tapering process of the waveguide structure 9, which is critical to the present invention, is depicted. The diagram explicitly indicates that the initial uniform cross-sectional outer dimension corresponding to the outer diameters 8, 6 of the non-tapered sections 1, 5 gradually decreases to the narrower waist outer diameter 7. The consistent outer diameters 6, 8 of the non-tapered sections 1, 5 are typically maintained at standard fiber dimensions, such as 125 pm, to facilitate seamless integration with established fiber connectors.

[0082] The lower taper transition section 2 and the upper taper transition section 4 are visually detailed, with the length of each transition section either matching the symmetry or varying according to the asymmetric configuration. The central taper waist section 3 is depicted as a component of the waveguide structure 9, the length of which can be adjusted. The inclusion of this section 3 is critical to the robustness of the taper structure, as omitting it can lead to increased vulnerability to physical stresses, including damage from bending.

[0083] Figure 3 The construction of the waveguide structure 9 as claimed in claims 1 to 15 is exemplified, demonstrating the versatility and flexibility of practical applications. This versatility is manifested in the ability to tailor the taper into symmetric or asymmetric configurations, as well as adjusting the length of the taper waist section 3, thereby meeting different requirements for supercontinuum generation.

[0084] Therefore, Figure 3 A creative method of creating a taper is shown, which meets the functional requirements of efficient supercontinuum generation while also incorporating structural features to enhance the durability and adaptability of the MOF 9 under different optical configurations.

[0085] Figure 3aAn alternative embodiment within the scope of the present invention is shown, depicting a waveguide structure 9 with a rectangular cross-section on a substrate 14, tailored specifically for a photonic integrated circuit (PIC). This embodiment demonstrates the flexibility of the present invention in accommodating non-fiber-based waveguide formats. The waveguide structure 9 shown is primarily rectangular, can be made of silicon nitride, and has a non-tapered thickness 6b, 8b of about 800 nm and a non-tapered width 6a, 8a of about 1200 nm. Alternatively, it can be tapered in only one dimension, i.e. the thickness of the waveguide remains constant while the width decreases, or vice versa.

[0086] This waveguide configuration is configured to contain one or two zero-dispersion wavelengths (ZDWs). Specifically, the shorter-wavelength ZDW, ZDW1, can be located in the range of about 900 nm ± 40 nm (pump wavelength of about 1 pm). In contrast, the longer-wavelength ZDW, ZDW2, is found at wavelengths beyond 1500 nm (pump wavelength of about 1 pm). This dispersion profile indicates that a pump wavelength, such as one emitted from a Yb-doped fiber laser that typically operates around 1040 nm, is aligned within the anomalous dispersion region of the initial non-tapered waveguide structure. This characteristic is crucial for efficient supercontinuum generation. For a pump wavelength of 1.5 pm, the entire dispersion profile shifts to longer wavelengths, with ZDW1 slightly below 1.5 pm and ZDW2 well above 1.5 pm.

[0087] Furthermore, it is possible to achieve positioning of the zero-dispersion wavelength (ZDW) at a location slightly below the pump wavelength. This placement depends on variables such as air hole size and contrast between the core and surrounding material in the waveguide structure 9. Mastering the generation of flat, broad, and coherent supercontinuum spectra is a significant challenge in the field, but it is an important component of advanced applications such as spectrometer calibration and dual-comb spectroscopy.

[0088] Figure 3a The adaptability of the present invention to different waveguide structures 9 is demonstrated, extending its use beyond traditional microstructured fiber designs. This versatility is evident throughout the specification, where for a substrate-based waveguide structure 9, such as a photonic integrated circuit (PIC), the length of the lower and upper taper transition sections 1, 5 can be between 3 mm and 30 mm. This adaptability enhances the applicability of the present invention in various optical and photonic systems.

[0089] Figure 4 Graphical representations of dispersion curves for tapered microstructured optical fibers (MOFs) (see Figure 1 and Figure 2 ) are shown at various lower taper scales, elucidating the relationship between wavelength and dispersion characteristics of the waveguide structures 9 outlined in the present invention.

[0090] These curves are derived from Figure 1 andFigure 2 depicted MOF 9, demonstrating the impact of tapering on the dispersion profile, which is a key factor in supercontinuum generation. The solid line labeled "scale_1.00" represents the dispersion of the original untapered fiber (corresponding to Figure 3 in the outer diameter 6, 8). This solid line includes two zero-dispersion wavelengths (ZDWs): ZDW1 is approximately 901 nm, and ZDW2 is beyond 2000 nm. Between these ZDWs, there exists an anomalous dispersion region, which is suitable for wavelengths emitted by typical ultra-short pulse lasers, such as Yb-doped fiber lasers operating around 1 μm.

[0091] In Figure 4 , we observe a transformative effect of tapering on the dispersion profile of waveguide structures 9, particularly microstructured optical fibers (MOFs). This graph demonstrates how the dispersion characteristics evolve as the diameter 7 of the fiber 9 in the tapered waist section 3 is systematically reduced through the tapering process.

[0092] The dashed line labeled "scale_0.60" represents the dispersion curve of the MOF 9 when the diameter 7 is gradually reduced to 60% of its original diameter 6, 8 in Figure 3 . This adjustment results in a significant shift of the zero-dispersion wavelengths (ZDWs): ZDW1 moves to 792 nm, and ZDW2 moves to 1814 nm, highlighting the dynamic nature of the dispersion region as the tapering scale changes.

[0093] Continuing this trend, Figure 4 in the point curve "scale_0.40" and the dash-dot curve "scale_0.30" correspond to the MOF 9 when the diameter 7 in the tapered waist section 3 is further reduced to 40% and 30% of the original, respectively. These curves illustrate the progressive shift of the ZDWs, with the "scale_0.30" curve being particularly noteworthy as it lacks any ZDWs. This absence implies a complete transition to a region without anomalous dispersion, which is a key factor in generating the flat coherent supercontinuum targeted by the present invention.

[0094] Therefore, Figure 4 is an important graphical representation of how specific changes in the tapering scale of a MOF, which is an important example of waveguide structures 9, directly affect its dispersion characteristics, providing a roadmap for optimizing supercontinuum generation in various application scenarios.

[0095] As Figure 3 and Figure 3a illustrate, the tapering configuration in waveguide structures 9 plays a crucial role in shaping the dispersion curve, as Figure 4As shown, this is the basis for achieving a flat, wide and coherent supercontinuum spectrum. Achieving this type of supercontinuum spectrum is crucial for applications such as spectrometer calibration or dual comb spectroscopy or simultaneous detection of beat signals with different wavelength cw lasers.

[0096] It is essential to preserve the coherence of the supercontinuum spectrum, especially when the wideband frequency comb structure is at risk of being disrupted by waveguide structures that have undergone suboptimal tapering processes. Thus, as shown in Figure 4 , careful adjustment of the tapering parameters is essential to preserve the integrity and functional capabilities of the supercontinuum spectrum in accordance with the purposes of the present invention.

[0097] The tapering rate, which is a key factor in shaping the dispersion characteristics of the waveguide structure 9, is defined as the reduction of the cross-sectional outer dimension per unit length. For example, a uniform gradual reduction of the waveguide structure from a core diameter of 5 pm to 1 pm over a length of 8 cm results in a tapering rate of 0.5 pm per cm. Adjusting this rate allows for precise control of the supercontinuum spectrum generation, adapting to different wavelength targets ranging from visible light (approximately 400 nm to 900 nm) to the mid-infrared range. The choice of pump wavelength and waveguide material, such as silicon nitride (SiN) on a silicon chip, silicon-on-insulator (SiOI), lithium niobate on insulator (LNOI), tantalate, or aluminum nitride (AIN), further enables the supercontinuum spectrum to adapt to specific application requirements.

[0098] Figure 4 Effectively demonstrates the impact of different tapering dimensions in the MOF 9 on its dispersion characteristics. The graph highlights the relationship between taper modification and dispersion characteristics, which is crucial for optimizing supercontinuum spectrum generation in the waveguide structure 9, as described in embodiments of the present invention. These strategic changes contribute to the adaptation of the waveguide structure 9 to achieve the desired supercontinuum spectrum characteristics, particularly in terms of coherence and spectral width.

[0099] Figure 5 The group velocity curves of a tapered microstructured optical fiber (MOF) are shown, which is one embodiment of the waveguide structure 9 described in the present invention. These curves are plotted at different tapering scales, illustrating the interaction between wavelength and group velocity, which is crucial for managing dispersion in supercontinuum spectrum generation. The depicted curves are based on Figure 2 the MOF 9 shown in Figure 3 The tapering scales applied to the taper waist diameter 7 (see ) of the MOF 9 vary, indicating how the variation in cross-sectional dimensions of the MOF 9 at different points along its length affects the group velocity, thereby affecting the dispersion characteristics necessary for effective supercontinuum spectrum generation.

[0100] In Figure 5 , with Figure 4parallel, each curve depicts the interaction between wavelength and group velocity for different scales of fiber diameter reduction. The solid curve labeled "scale_1.00" represents the group velocity distribution of MOF 9 in its original, non-tapered form, in relation to its full diameter (as indicated by the outer diameter 6, 8 in Figure 3 The outer diameter 6, 8 in

[0101] In Figure 5 The effect of tapering on group velocity is depicted in different scales in

[0102] Figure 5 is key to elucidate the impact of tapering rate on the dispersion properties of waveguide structure 9. The tapering rate, quantified as the reduction of diameter per unit length— for example, 0.5 pm per centimeter when tapering the core diameter from 5 pm to 1 pm over a length of 8 cm— directly influences the dispersion properties of waveguide structure 9. These properties are indispensable for the efficiency and quality of supercontinuum generation, according to embodiments of the present application.

[0103] Thus, Figure 5 Also effectively demonstrates the adaptability of waveguide structure 9 to various materials and target wavelength ranges, which is a key aspect of the present application. By using materials such as SiN on Si wafers, Si on Insulator, LNOI, Tantalates, or AIN, among others, the waveguide can facilitate supercontinuum generation over a broad spectrum. The range of this spectrum can vary from visible light (400 nm to 900 nm) to mid-infrared wavelengths, depending on the pump wavelength and the specific tapering parameters employed in waveguide structure 9.

[0104] Figure 5is key to describing how tapering in MOF 9 influences the group velocity, which is a fundamental aspect of the present invention. This relationship is critical to generating flat, coherent and broad supercontinuum spectra. Such spectra are particularly beneficial for applications such as spectrometer calibration and dual comb spectroscopy or simultaneous detection of beat signals with different wavelength cw lasers, demonstrating the utility of the present invention.

[0105] Figure 6a and Figure 6b are helpful in demonstrating the influence of tapering on supercontinuum generation within microstructured optical fiber 9, in line with the inventive concepts and embodiments explored in this specification. These figures illustrate how different tapering methods related to the present invention influence the spectral evolution and generation of supercontinuum.

[0106] Figure 6a The lower plot in Figure 1 1 depicts the simulation of spectral evolution within a tapered microstructured optical fiber 9 (one example of a waveguide structure), demonstrating the effect of tapering when the fiber diameter is reduced to the point where anomalous dispersion is eliminated. The simulation is based on parameters including a pump laser with a central wavelength of 1040 nm, a pulse duration of 130 fs, and a pulse energy of 0.18 nJ.

[0107] Figure 3 The tapering configuration shown includes several distinct sections: a non-tapered input section 1 of length 20 mm, a lower taper transition section 2 extending for 69.1 mm, a central tapered waist section 3 of length 55 mm, followed by an upper taper transition section 4 of also 69.1 mm in length, and finally a non-tapered output section 5 of length 20 mm. The outer diameter 7 of this symmetrically configured taper waist is 38 pm, which is approximately 30% (0.3 times) of the original non-tapered diameter 6, 8 of the fiber 9 (i.e. 125 pm).

[0108] The tapering process effectively removes the zero-dispersion wavelength (ZDW) at the waist section 3. This results in the complete generation of a supercontinuum spectrum within the lower taper transition section 2. After this section 2, the supercontinuum spectrum remains essentially unchanged in the subsequent sections 3, 4 and 5.

[0109] Figure 6a The upper plot in Figure 1 1 shows the supercontinuum generated at the output end of the tapered fiber 9. This supercontinuum is characterized by a flat spectral profile over a 10 dB range in the visible spectrum. This demonstrates the effectiveness of the specific tapering method used to achieve flat and broad supercontinuum generation.

[0110] Figure 6b The lower plot in Figure 1 1 shows the supercontinuum generated at the output end of the tapered fiber 9. This supercontinuum is characterized by a flat spectral profile over a 10 dB range in the visible spectrum. This demonstrates the effectiveness of the specific tapering method used to achieve flat and broad supercontinuum generation. Figure 6aDifferent cases illustrate the spectral evolution in the tapered microstructured optical fiber 9, where the tapering process does not eliminate the anomalous dispersion at the taper waist section 3. This depicts a variant tapered outcome not included in the present invention, where the tapering method employed maintains the anomalous dispersion in the taper waist section 3, thereby affecting the properties of the supercontinuum generated.

[0111] In Figure 6b the simulation shown, the base tapering configuration is similar to the tapering configuration in Figure 6a , but with a significant difference in the taper waist section 3. Here, the outer diameter 7 at the taper waist is set to 50 pm, which is 0.4 times the original diameter 6, 8 of the optical fiber 9. This proportional reduction in diameter maintains the anomalous dispersion at 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 illustrates an alternative embodiment outside the scope of the present invention, highlighting the influence of tapering dimensions on the dispersion properties and the resulting supercontinuum spectrum properties.

[0112] In Figure 6b the scenario shown, the tapering configuration results in the generation of a supercontinuum spectrum that extends beyond the lower taper transition section 2, continuing through the taper waist section 3, into the subsequent upper taper transition section 4, and the non-tapered 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 influence of the outer dimensions of the taper waist cross-section on the continuity and uniformity of the generated supercontinuum spectrum.

[0113] Figure 6b The upper graph of Figure 6a shows the output supercontinuum spectrum under a configuration where the anomalous dispersion in the taper waist section 3 is not fully eliminated. In contrast to , this spectrum exhibits a clear spectral gap, emphasizing the importance of a full transition to the normal dispersion regime in the taper waist section 3 to achieve a continuous, gap-free supercontinuum spectrum spectrum that aligns with the objectives of the present invention.

[0114] Figure 6a Figure 6b Efficiently illustrate the critical role of precise tapering in the microstructured optical fiber 9 in influencing the supercontinuum generation properties, in accordance with the present invention. Figure 6a Illustrate the case where the tapering is adjusted to fully eliminate the anomalous dispersion at the taper waist, resulting in a flat and wide supercontinuum spectrum. This is in stark contrast to Figure 6b , where the maintenance of anomalous dispersion at the taper waist leads to a supercontinuum spectrum with spectral gaps. As shown in Figure 6b , the efficiency of generating a coherent and flat supercontinuum spectrum spectrum is crucial for applications that require a coherent light source, such as spectrometer calibration and dual-comb spectroscopy. Figure 6a

[0115] Figure 7a ​Simulation insights into supercontinuum generation in tapered microstructured optical fibers (waveguide structure 9) are provided, particularly under conditions where the lower tapered transition section 2 is not long enough. Figure 7a The lower graph in FIG. 1 1 captures the spectral evolution within the microstructured optical fiber 9, with the length of the lower tapered transition section 2 limited to 20 mm. This constrained length is insufficient to facilitate full development of the supercontinuum spectrum, which is a key characteristic to achieve optimal broadband spectral coverage.

[0116] As shown in FIG. 1 1, Figure 3 the taper specifications include a tapered waist diameter 7 of 38 pm, which is 0.3 times smaller than the original diameters 6, 8 of the non-tapered (input and output) fiber sections 1, 5. However, as shown in the upper graph of FIG. 1 1, Figure 7a the shortening of the length of the lower tapered transition section 2 results in a supercontinuum spectrum that does not exhibit the desired broadband width. This simulation result underscores the criticality of ensuring a lower tapered transition section 2 of appropriate length to realize the full potential of supercontinuum generation within the waveguide structure 9.

[0117] Figure 7b The effect of an excessively long lower tapered transition section 2 in a tapered microstructured optical fiber representative of the waveguide structure 9 on supercontinuum generation is investigated. Figure 7b The lower graph in FIG. 1 1 shows the spectral evolution when the length of the lower tapered transition section 2 is lengthened to 200 mm. This excessively long length hinders the effective development of the desired supercontinuum characteristics, ultimately resulting in an irregular and incomplete spectrum, as shown in the upper graph of FIG. 1 1. Figure 7b This result deviates from the ideal flat and wide supercontinuum spectrum, underscoring the importance of appropriately balancing the lower tapered transition length to achieve optimal broadband supercontinuum generation.

[0118] Figure 7b The taper dimensions used in the simulation are consistent with the parameters outlined in FIG. 1 1, Figure 3 indicating that an inappropriate length of the lower tapered transition section 2 can significantly impact the quality of the supercontinuum output. Either too short or too long a lower tapered transition length can result in suboptimal supercontinuum characteristics. Therefore, precise calibration of the length of the lower tapered transition section 2 is critical to obtaining a well-defined and flat supercontinuum envelope. In terms of width and flatness, the optimal taper length to achieve a balanced supercontinuum spectrum is typically in the range of about 5 cm to 15 cm, with a more preferred range of 5 cm to 10 cm. This underscores the importance of precision in determining the taper length to effectively and efficiently generate a supercontinuum spectrum within the waveguide structure 9.

[0119] Figure 6a and Figure 6bThe findings collectively underscore the critical role of precise taper length in designing waveguide structures 9 that efficiently generate high-quality supercontinua. Precise taper length calibration is not only vital for optimizing supercontinuum spectra but also for preserving the structural resilience of the microstructured fiber 9. This aspect becomes particularly important in high-power applications where the integrity of the fiber 9 is of utmost importance.

[0120] Furthermore, as Figure 3 indicated, the use of index-matching material 17 plays a critical role in protecting the fiber 9 from potential damage due to high-power leakage. This protective measure is crucial for maintaining the performance of the fiber 9 and extending its lifespan in various demanding optical settings.

[0121] Figure 8 A key comparison between simulated and experimental supercontinuum spectra is shown, demonstrating the real effectiveness of the waveguide structures 9 outlined in the present invention. The figure includes simulated supercontinuum curves derived from the parameters established in Figure 6a This curve is vital for assessing the fidelity of experimental results to the theoretical model.

[0122] Figure 8 Experimental results from two different tapered microstructured fibers 9 are shown, each with a unique configuration of air-hole rings, as Figure 2 indicated. The first set of experimental data comes from a fiber 9 with a 4-ring taper configuration, and the second set comes from a fiber 9 with a 7-ring taper. These experimental setups are consistent with the taper shapes and pumping parameters used in the simulations, allowing for a direct comparison between theoretical predictions and actual results.

[0123] Figure 8 A key takeaway from the present invention is the flat supercontinuum spectra generated in both experimental scenarios, characterized by their gapless nature. This demonstrates the success of the tapering strategy described in the present invention in achieving coherent and stretched supercontinua. The experimental data closely align with the simulated results, primarily in the shorter wavelength region, emphasizing the reliability of the simulations in predicting real fiber behavior.

[0124] In the experimental setup, the tapered microstructured fiber 9 with 4-ring and 7-ring configurations was tested at a pump repetition rate of 25 GHz. This setup required 10 W of average input power to achieve the same 0.18 nJ pulse energy as in the simulation. To counteract the potential damage of this high-power operation, the entire taper, including the lower taper transition 2, the taper waist 3, and the upper taper transition 4, was wrapped in an index-matching gel (index-matching material 17). This precaution was crucial to prevent the fiber from being damaged by strong light leakage and overheating. The index-matching gel (index-matching material 17) served a dual purpose: it helped to remove the infrared light from the cladding of the fiber 9 and acted as a heat conductor, directing the excess heat to a metal plate linked to the fiber 9.

[0125] Figure 8 The adaptability of configuring the taper for the waveguide structure 9 is highlighted. Experimental data show that changing the length of the lower taper transition 2 and the upper taper transition 4 can yield effective results, opening the door to asymmetric taper configurations. According to the inventive aspects of the present invention, this flexibility of configuration allows the creation of a customized waveguide structure 9 that is optimally adjusted to meet the specific requirements of supercontinuum generation.

[0126] In summary, Figure 8 The gap between theoretical modeling and experimental verification is bridged, demonstrating the practical effectiveness of the waveguide structures 9 in achieving the desired supercontinuum spectral properties. This effectiveness is crucial for applications that require high repetition rates and high-power input, where the flexibility and adaptability of the tapered fiber 9 are key factors.

[0127] The above description elaborates on the innovative approach to developing waveguide structures 9 for generating supercontinuum. It includes a thorough examination of the processes involved with tapered microstructured fibers 9 and elucidates their impact on supercontinuum generation. The above description showcases the adaptability of these waveguide structures 9 in various embodiments, highlighting their compatibility with different configurations and material types. This adaptability greatly expands their practical applications.

[0128] The agreement between experimental results and theoretical predictions highlights the precision exhibited in fabricating these waveguide structures 9, thereby validating the effectiveness of the present invention in high-power environments. This innovation cleverly achieves coherent, flat, and broad supercontinuum spectra. Therefore, the present invention represents a significant breakthrough in photonics, introducing advanced solutions in the fields of spectrometer calibration, dual-comb spectroscopy, simultaneous detection of beatnotes with different wavelengths of cw lasers, and various high-precision optical applications.

Claims

1. A system (16) for generating a supercontinuum comprising: a frequency comb generator (15); and a waveguide structure (9) coupled to the frequency comb generator (15) and comprising: a non-tapered input section (1) having a specified first cross-sectional outer dimension (8, 8a, 8b), a lower tapered transition section (2) leading to a tapered waist section (3) having a second cross-sectional outer dimension (7, 7a, 7b) smaller than the first cross-sectional outer dimension (8, 8a, 8b), followed by an upper tapered transition section (4) extending to a non-tapered output section (5), the non-tapered output section (5) reverting to a third cross-sectional outer dimension (6, 6a, 6b) larger than the second cross-sectional outer dimension (7, 7a, 7b), 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 in a wavelength range critical to supercontinuum generation. the third cross-sectional outer dimension (6, 6a, 6b) is substantially the same as the first cross-sectional outer dimension (8, 8a, 8b) within a tolerance range of + / - 10%.

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

3. The system (16) according to claim 1 or 2, wherein, the waveguide structure (9) is configured as a tapered microstructured optical fiber, the non-tapered input section (1) preferably has a core diameter in the range of about 3 pm to 5 pm.

4. The system (16) according to any one of claims 1 to 3, wherein, the non-tapered input section (1) has two zero dispersion wavelengths, ZDW1 and ZDW2, with ZDW1 being located in a wavelength range of about 900 nm ± 40 nm, and ZDW2 being located at a wavelength larger than 2000 nm, thereby establishing an anomalous dispersion region between the zero dispersion wavelengths ZDW1 and ZDW2.

5. The system (16) according to any one of claims 1 to 4, wherein, the waveguide structure (9) is configured for use with a pump source (18), the pump source (18) being an ultra-short pulse laser, and the wavelength of the ultra-short pulse laser is located in the anomalous dispersion range established between the zero dispersion wavelengths ZDW1 and ZDW2.

6. The system (16) of claim 5, wherein, the lower tapered transition section (2) is configured such that the zero dispersion wavelengths ZDW1 and ZDW2 are gradually blue-shifted until they both disappear, which facilitates a complete generation of a supercontinuum within this section (2) and produces a spectrum without strong modulation.

7. The system (16) according to claim 5 or 6, wherein, the length of the lower tapered transition section (2) is configured to generate a flat supercontinuum spectral envelope.

8. The system (16) according to any one of claims 1 to 7, wherein, the tapered waist section (3) has no zero dispersion wavelengths, thereby enabling transmission of an optical signal without significantly altering its spectral characteristics.

9. The system (16) according to any one of claims 1 to 8, wherein, the tapered waist section (3) has a variable length, in particular adapted to optimize supercontinuum generation for different spectral requirements.

10. The system (16) according to any one of claims 1 to 9, wherein, ​ 11. The system (16) according to any one of claims 1 to 10, wherein, The upper tapered transition section (4) is configured to ensure consistent transmission of the supercontinuum, preserving spectral integrity, regardless of the presence or absence of a zero dispersion wavelength within the upper tapered transition section (4).

12. The system (16) according to any one of claims 1 to 11, wherein, The length of the upper tapered transition section (4) is specifically selected to be the same or different from the length of the lower tapered transition section (2), enabling a customized tapered configuration.

13. The system (16) according to any one of claims 5 to 12, wherein, The non-tapered output section (5) restores the two zero dispersion wavelengths ZDW1 and ZDW2, as in the non-tapered input section (1), ensuring consistent transmission properties of the supercontinuum generated in the waveguide structure (9).

14. The system (16) according to any one of claims 1 to 13, wherein, The waveguide structure (9) is surrounded by an index matching material (17) to mitigate damage when operated at high repetition rates GHz pump sources (18) at high average powers exceeding 1 W.

15. A method of generating a supercontinuum using the system (16) of any one of claims 1 to 14, the method comprising the steps of: Light from a frequency comb generator (15) is coupled into the non-tapered input section (1) of the waveguide structure (9) and the generated supercontinuum output from the non-tapered output section (5) is collected, wherein the generated supercontinuum exhibits a spectral width of at least 500 nm, extending over an octave, such as from 500 nm to 1600 nm when the waveguide structure (9) is pumped with a laser having a center wavelength of about 1 pm or so, or from 900 nm to 2400 nm when the waveguide structure (9) is pumped with a laser (15) having a center wavelength of about 1.5 pm or so.

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