Supercontinuum generation from an all-normal dispersion fiber and a frequency-conversion-element

Abstract

Disclosed is an all-normal-dispersion (ANDi)-based supercontinuum (SC) system, comprising a pulsed light-source configured to generate a train of light-pulses; an all-normal- dispersion (ANDi)-fiber optically coupled to the pulsed light-source and configured to generate a first supercontinuum-spectrum from the train of light-pulses, wherein the supercontinuum-spectrum is defined in a first frequency-domain; and a frequency- conversion-element configured to convert the supercontinuum-spectrum into a second frequency-domain, whereby the first supercontinuum-spectrum is converted to a second supercontinuum-spectrum.

Description

[0001] SUPERCONTINUUM GENERATION FROM AN ALL-NORMAL DISPERSION FIBER AND A FREQUENCY-CONVERSION-ELEMENT

[0002] Field of the invention

[0003] The present disclosure relates generally to supercontinuum generation in an all-normal dispersion fiber. Furthermore, the present disclosure relates to supercontinuum generation in the visible domain. The present disclosure further relates to generating light in the infrared region and / or ultraviolet region from an all-normal dispersion fiber.

[0004] Background

[0005] Supercontinuum light generation typically involves broadening the spectral content of a laser pulse through one or more non-linear processes to generate light spanning a wide range of wavelengths. These supercontinuum light sources are integral to numerous applications, including spectroscopy, optical imaging, and frequency metrology. Traditional approaches to supercontinuum generation often rely on non-linear processes such as self-phase modulation (SPM) and soliton dynamics in fibers with anomalous dispersion. However, these methods can introduce significant noise, limiting their utility for applications demanding high coherence and spectral stability.

[0006] All-normal dispersion (ANDi) fibers have emerged as an alternative non-linear medium for supercontinuum generation. Unlike anomalous dispersion fibers, ANDi fibers exhibit a purely normal group velocity dispersion (GVD) profile across the operating wavelength range, thereby suppressing soliton-related instabilities. In ANDi fibers, spectral broadening may occur through non-linear processes such as SPM and optical wave breaking, often resulting in smoother, and more stable spectra. This makes ANDi fibers particularly attractive for applications requiring consistent and reliable spectral performance.

[0007] Despite the advantages of ANDi fibers, challenges remain in extending supercontinuum generation to shorter wavelengths, particularly in the ultraviolet (UV) range. UV generation is hindered by limitations in fiber transparency, the fragility of solid-core fibers at UV wavelengths, and the complexities of efficient frequency-conversion to the UV domain.

[0008] Thus, there is a need for a new and improved supercontinuum system that addresses the issues and challenges mentioned herein. Summary

[0009] It is an objective of this disclosure to provide an all-normal dispersion (ANDi)- supercontinuum (SC) source. Further, it is an objective of this disclosure to provide an ANDi- SC source to provide an ANDi SC, particularly an ANDi-SC that generates UV light or extends into the ultraviolet and / or the infrared domain. Even further, it is an objective of this disclosure to provide an ANDi-SC source that is more optimal than current solutions to ANDi- SC sources.

[0010] These and other objectives have been solved by the all-normal dispersion- based supercontinuum system as defined in the claims and as described below in the present disclosure.

[0011] Accordingly, the above-mentioned challenges are addressed and solved by providing an all- normal-dispersion based supercontinuum system, comprising: a pulsed light-source configured to generate a train of light-pulses; an all-normal-dispersion fiber optically coupled to the pulsed light-source and configured to generate a first supercontinuum-spectrum from the train of light-pulses, wherein the supercontinuum-spectrum is defined in a first frequencydomain; and a frequency-conversion-element configured to convert the supercontinuum- spectrum into a second frequency-domain, whereby the first supercontinuum-spectrum is converted to a second supercontinuum-spectrum.

[0012] The general idea of converting one or more of the light pulses into a second train of lightpulses defined by a second frequency-domain and then use this second train of light pulses in an ANDi-fiber to generate an ANDi-SC in the UV-domain has previously been proposed, but never experimentally verified.

[0013] To generate ANDi-SC in the UV-domain, it is likely preferred to use a solid core photonic crystal fiber (PCF), since the dispersion of such a fiber in the UV-domain is flat and has an absolute low value. However, if a solid core ANDi-fiber is used together with UV-light, then such a solid core ANDi-fiber will rapidly (within days of operation) be damaged by the exposure to the UV-light, and thus not work in an industrial setting.

[0014] There is thus a need for an alternative setup to generate ANDi-SC in the UV-domain. In some embodiments, this is addressed by arranging the frequency-conversion-element downstream of the all-normal-dispersion fiber. Additionally, or alternatively, the train of light pulses generated by the pulsed light-source may each have a first center wavelength, which lies in a range from about 850 nm to about 1250 nm. This has the advantage that such wavelengths can be transmitted through solid core ANDi-fibers without any damage caused by UV-light.

[0015] In accordance with some embodiments, the present disclosure provides an ANDi-SC source that extends the SC spectrum into the ultraviolet and / or the infrared domain.

[0016] In one aspect of the disclosure, there is disclosed an all-normal-dispersion (ANDi)-based supercontinuum (SC) system, comprising:

[0017] - a pulsed light-source configured to generate a train of light-pulses;

[0018] - an all-normal-dispersion (ANDi)-fiber optically coupled to the pulsed light-source and configured to generate a first supercontinuum-spectrum from the train of light-pulses, wherein the supercontinuum-spectrum is defined in a first frequency-domain; and

[0019] - a frequency-conversion-element configured to convert the supercontinuum-spectrum into a second frequency-domain, whereby the first supercontinuum-spectrum is converted to a second supercontinuum-spectrum.

[0020] Advantageously, the frequency-conversion-element is arranged downstream of the all- normal-dispersion fiber. This is in contrast to some existing SC systems, which has the frequency-conversion-element arranged before the all-normal-dispersion fiber.

[0021] Using a frequency-conversion-element to for example convert an input wavelength of A to half of that, A / 2, sometimes referred to as frequency doubling or second harmonic generation (SHG), using a non-linear crystal, is generally known.

[0022] Generally, the conversion efficiency in a non-linear crystal is related to the optical power density of the light-pulses, the wavelength, and the polarization stability. Further, a single narrow wavelength band is preferred, since linewidth broadening and chirp reduce doubling efficiency.

[0023] The efficiency of SHG may be reduced by linewidth broadening in a crystal that is periodically poled. This may be because the poling period is linked directly to the phase velocity and / or the absolutely frequency of the light. Accordingly, for a low peak power input, a non-linear crystal may be poled to get “quasi phase matching”. In such a crystal, the phase may be matched over a relatively long length of the crystal to get enough light converted. For this reason, a frequency-conversion-element is typically placed before any kind of linewidth broadening element, in this case, the ANDi-fiber. However, the presently disclosed ANDi-based SC system comprises a frequency- conversion-element that is preferably arranged downstream of the ANDi-fiber, and therefore differs from SC systems where a frequency-conversion-element is placed upstream of the ANDi-fiber.

[0024] According to the presently disclosed ANDi-based SC system, this implies that not only a single narrow wavelength band is converted to another single narrow wavelength band, but instead the first SC spectrum is converted to a second SC spectrum. In view of the above, it would be expected that the conversion efficiency for such a system would be sub-optimal and not a preferred solution.

[0025] However, the inventors of the presently disclosed ANDi-based SC system have found that by arranging the frequency-conversion-element downstream of the ANDi-fiber, it is still possible to efficiently convert a SC spectrum rather than just a narrow wavelength band.

[0026] Additionally, the inventors of the presently disclosed ANDi-based SC system have realized that for embodiments utilizing a high peak power input, a relatively long length of a frequency-conversion-element and poling may not be needed. In other words, in embodiments with a high peak power input, the inventors have realized that a relatively short frequency-conversion-element with no poling can be used.

[0027] When using a relatively short frequency-conversion-element with no poling, the inventors have found that the frequency-conversion-element can be so short that the phase velocity would have limited walk-off over a very wide range of frequencies. With this discovery, the inventors have realized that it is possible to efficiently convert over a broad bandwidth.

[0028] As stated before, instead of converting a single wavelength to another single wavelength, the presently disclosed SC system converts multiple wavelengths (the first SC spectrum) to other multiple wavelengths (the second SC spectrum). This also has several advantages.

[0029] Firstly, if the second frequency-domain is in the UV-domain, then the first frequency-domain may not need to be in the UV-domain. In such embodiments, light in the UV-domain, i.e. UV-light, may not need to be emitted into the ANDi-fiber. Accordingly, the ANDi-fiber, for example in the form of a solid core fiber, may not need to be exposed to UV-light, and therefore not be damaged. Secondly, if the second frequency-domain is in the IR-domain, then I R-light may be generated without the use of tapered or all-solid PCF designs. The all-normal dispersion (ANDi)-fiber is a fiber that has only normal dispersion in a specific wavelength-region, for example from 0.5 pm to 2.0 pm. This wavelength-region may be referred to as the ANDi-wavelength range. The ANDi-wavelength range may extend from about 200 nm to 2100 nm, such as from about 400 nm to 1800 nm, such as from about 600 nm to 1400 nm. Furthermore, for an ANDi-fiber, the dispersion is low and has flat and only normal dispersion over the entire bandwidth of interest. One definition of low and flat normal dispersion is for example when the dispersion is with an absolute value between 0 and 1500 ps / nm / km. Another definition of low and flat normal dispersion is for example when the dispersion is with an absolute value between 0 and 500 ps / nm / km. Yet another definition of low and flat normal dispersion may be when the dispersion is with an absolute value between 0 and 100 ps / nm / km, such as between 0 and 50 ps / nm / km.

[0030] When the first SC is generated in the ANDi-fiber, i.e. in the ANDi-wavelength range, the generated SC spectrum may be referred to as an ANDi-SC. To generate the ANDi-SC, the ANDi-fiber may have a length configured to initiate both self-phase modulation (SPM) and optical wave-breaking (OWB) in the ANDi-wavelength range. Furthermore, to generate the ANDi-SC, the length of the ANDi-fiber may be further configured to develop the first SC spectrum, i.e. the ANDi SC, by the combined effect of SPM and OWB. In other words, the all-normal-dispersion fiber may have a sufficient length to initiate both self-phase modulation (SPM) and optical wave-breaking (OWB) in the fiber. Specifically, the length of the all- normal-dispersion fiber may be longer than what is known as the optical wave-breaking distance. In some embodiments, the length of the all-normal-dispersion fiber is between 1 cm and 300 cm, such as between 10 cm and 200 cm, such as between 10 cm and 100 cm.

[0031] Generally, the first supercontinuum-spectrum may be generated in the all-normal-dispersion fiber by one or more non-linear processes in the fiber. In some embodiments, the one or more non-linear processes are selected from the group of self-phase modulation (SPM) and optical wave-breaking (OWB), and / or combinations thereof. In preferred embodiments, the first supercontinuum-spectrum is generated by the combined effects of SPM and OWB. Since the first supercontinuum-spectrum is generated in an all-normal-dispersion fiber, modulation instability and the formation of solitons can ideally be entirely avoided. The length of the all-normal-dispersion fiber is preferably selected such that the non-linear processes that contribute to the generation of the first supercontinuum-spectrum are selfphase modulation (SPM) and / or optical wave-breaking (OWB). Another aspect is the compression of chirped pulses generated through combined nonlinear processes, such as self-phase modulation (SPM) and optical wave breaking, which presents several significant challenges. These processes typically produce pulses with highly complex spectral and temporal profiles, characterized by a mix of linear and nonlinear chirp across a broad bandwidth. The non-linear contributions, particularly those from optical wave breaking, often introduce higher-order dispersion components that are difficult to compensate using conventional techniques.

[0032] Second-order dispersion compensation is often insufficient for such pulses, as the higher- order terms, such as third-order dispersion, become increasingly prominent in shaping the temporal structure of the pulse. This higher-order dispersion may cause a nonuniform temporal broadening and complex phase distortions, making it challenging to achieve full recompression to the transform-limited pulse duration.

[0033] Another complication arises from the broad spectral bandwidth produced by optical wave breaking. As the bandwidth increases, the chirp becomes more non-linear, necessitating dispersion compensation techniques capable of handling a wide range of phase variations. Conventional compressors, such as diffraction gratings, often struggle to manage the intricate phase relationships introduced by these non-linear effects, leading to incomplete recompression or pulse distortion.

[0034] It is therefore a further objective of the present disclosure to address challenges regarding the compression of chirped pulses. In the present context, for an all-normal-dispersion based supercontinuum system, as the pulse travels through the all-normal-dispersion fiber, it is affected by non-linear processes and the pulse becomes chirped as a result of the dispersion properties of the fiber, specifically the normal dispersion in case of an ANDi-fiber. In accordance with some aspects of the present disclosure, it is an object to counteract, or at least partially compensate, such normal dispersion added to the pulse as it propagated through the ANDi-fiber.

[0035] In some embodiments, these challenges are addressed by providing an all-normal- dispersion based supercontinuum system as disclosed herein, wherein the supercontinuum system further comprises a compression stage configured to temporally compress the train of pulses.

[0036] In a further aspect of the present disclosure, an all-normal-dispersion based supercontinuum system is provided, which is configured for providing light in the infrared region, such as in the short- or mid-infrared region. Accordingly, the supercontinuum system may be configured for providing a supercontinuum spectrum that extends into the infrared region, or alternatively a spectrum that resides entirely in the infrared region. Thus, the second supercontinuum-spectrum, i.e., the frequency-converted supercontinuum spectrum, may include wavelengths from about 2 pm to about 8 pm, such as wavelengths from about 2 pm to about 5 pm, such as wavelengths from about 3.5 pm to about 4.5 pm.

[0037] In accordance with some embodiments of the present disclosure, this is achieved by providing an all-normal-dispersion based supercontinuum system, comprising: a pulsed light-source configured to generate a train of light-pulses; an all-normal-dispersion fiber optically coupled to the pulsed light-source and configured to generate a first supercontinuum-spectrum from the train of light-pulses, wherein the supercontinuum- spectrum is defined in a first frequency-domain; and a frequency-conversion-element configured to convert the supercontinuum-spectrum into a second frequency-domain, whereby the first supercontinuum-spectrum is converted to a second supercontinuum- spectrum, wherein at least a part of the second frequency-domain comprises frequencies that are lower than that of the first frequency-domain, wherein the lower frequencies reside in the infrared region, such as in the short- or mid-infrared region.

[0038] In accordance with some embodiments, the all-normal-dispersion based supercontinuum system comprises a pulsed light-source configured to generate a train of light-pulses; an all- normal-dispersion fiber optically coupled to the pulsed light-source and configured to generate a first supercontinuum-spectrum from the train of light-pulses; and a frequency- conversion-element, such as a non-linear crystal, configured to frequency convert the first supercontinuum-spectrum into a second supercontinuum-spectrum through difference frequency generation (DFG), wherein the second supercontinuum-spectrum covers wavelengths from about 2 pm to about 5 pm. In such embodiments, the supercontinuum system may also be referred to as a mid-infrared supercontinuum system.

[0039] Further details are described in the following.

[0040] Brief description of the drawings

[0041] The above and / or additional objects, features and advantages of the present disclosure, will be further described by the following illustrative and non-limiting detailed description of embodiments of the present disclosure, with reference to the appended drawing(s), wherein: Fig. 1 shows an example of an ANDi SC system according to the present disclosure.

[0042] Fig. 2 shows the dispersion (in ps / nm / km) for an all-normal-dispersion fiber according to the present disclosure.

[0043] Fig. 3 shows three different supercontinuum spectra from the all-normal-dispersion fiber according to the present disclosure.

[0044] Figs. 4-10 collectively show an example of the filtration and compression of an output pulse from an all-normal-dispersion fiber according to the present disclosure.

[0045] Fig. 4 shows an example of an output light pulse from the all-normal-dispersion fiber according to the present disclosure.

[0046] Fig. 5 shows an example of a pulse spectrogram of an output light pulse from the all- normal-dispersion fiber according to the present disclosure.

[0047] Fig. 6 shows an example of the spectral density versus wavelength of an output light pulse from the all-normal-dispersion fiber according to the present disclosure.

[0048] Fig. 7 shows an example of a pulse spectrogram of a filtered output light pulse from the all- normal-dispersion fiber according to the present disclosure.

[0049] Fig. 8 shows an example of the power versus time of a filtered output light pulse from the all-normal-dispersion fiber according to the present disclosure.

[0050] Fig. 9 shows an example of a pulse spectrogram of a filtered, and compressed, output light pulse from an all-normal-dispersion fiber according to the present disclosure.

[0051] Fig. 10 shows an example of a compressed pulse from a compression stage forming part of the supercontinuum system according to the present disclosure.

[0052] Detailed description

[0053] A better understanding of the features and advantages of the present disclosure will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of embodiments of the present disclosure are utilized, and the accompanying drawings. Although the detailed description contains many specifics, these should not be construed as limiting the scope of the disclosure but merely as illustrating different examples and aspects of the present disclosure. Frequency-conversion-element

[0054] In some embodiments, at least a part of the second frequency-domain comprises frequencies that are higher than that of the first frequency-domain. This means that the wavelengths in the first SC have been converted to lower wavelengths in the second SC. For example, a first SC that has a bandwidth from 600 nm to 1000 nm may be down- converted in wavelengths to a second SC having a bandwidth from 300 to 500 nm, which is therefore defined in the UV-domain, starting from 400 nm. Accordingly, using such an approach as described here provides that the bandwidth may be halved, for example in this case from a bandwidth of 400 nm to 200 nm. In some embodiments, dependent on the efficiency of the frequency-conversion-element, i.e. dependent on the efficiency of the downconversion, the bandwidth may be less than halved.

[0055] In embodiments as described above, the frequency-conversion-element may be a second harmonic generator, for example a non-linear crystal. Thus, the frequency-conversion- element may comprise, or constitute, a non-linear crystal configured for second harmonic generation (SHG). More generally, the non-linear crystal may be configured for sumfrequency generation (SFG). Examples of non-linear crystal include Beta Barium Borate (BBO), Lithium Triborate (LBO), Potassium Dihydrogen Phosphate (KDP), Deuterated KDP (DKDP), Bismuth Triborate (BiBO), Cesium Lithium Borate (CLBO), Zinc Oxide (ZnO), Barium Magnesium Fluoride (BaMgF4), Potassium Titanyl Phosphate (KTP), or a poled LBGO. In some embodiments, the non-linear crystal is a non-poled crystal. In some embodiments, quasi phase matching using periodic poling of the non-linear crystal can be used to increase the phase-matching bandwidth, or alternatively by using thin crystals and short pulses for pumping.

[0056] In relation to using the frequency-conversion-element for second harmonic generation (SHG), using a non-linear crystal, the non-linear crystal may have a length (in which the interaction is defined) that is less than 5 mm, such as around or less than 3 mm. In some embodiments, the non-linear crystal has a length that is less than 1 mm, preferably less than 500 microns, more preferably less than 300 microns, most preferably such as around 100 microns. Having an interaction length of less than 5 mm may provide that the phase velocity has limited walk-off over a very wide range of frequencies. In this embodiment, it is therefore possible to efficiently convert over a broad bandwidth, and therefore effectively provide an ANDi-SC, such as in the UV-domain. Having a short interaction length may in some embodiments imply that a relatively high peak power is also to be used. For example, peak powers of more than 50 kW may be used in combination with a non-linear crystal having a length of less than 5 mm. In such an example, the conversion efficiency may for example be around 20% or less. Although this is a relatively low conversion efficiency, it is noted that this is the average efficiency for all wavelengths in the first supercontinuum, and therefore still very efficient to generate an ANDi-SC, such as in the UV-domain.

[0057] In other embodiments, at least a part of the second frequency-domain comprises frequencies that are lower than that of the first frequency-domain. This means that the wavelengths in the first SC have been converted to higher wavelengths in the second SC. For example, a first SC that has a bandwidth from 600 nm to 1000 nm may be up-converted in wavelengths to a second SC having a bandwidth from 1200 to 2000 nm, which is therefore defined in the IR-domain, starting from 760 nm. Accordingly, using such an approach as described here provides that the bandwidth may be doubled, for example in this case from a bandwidth of 400 nm to 800 nm.

[0058] In other embodiments, at least a part of the second frequency-domain comprises frequencies that are lower than that of the first frequency-domain, wherein the lower frequencies reside in the infrared region, such as in the short- or mid-infrared region. As an example, the second supercontinuum-spectrum may comprise wavelengths that are higher than that of the first supercontinuum-spectrum, such as wavelengths from about 2 pm to about 5 pm, or from about 3 pm to about 4.5 pm. This may be achieved by using a frequency- conversion-element, such as a non-linear crystal, configured for difference frequency generation (DFG). In particular, the first supercontinuum-spectrum may be provided as the input to the frequency-conversion-element along with a mixing wavelength, e.g., provided by a mixing laser, such that a second, frequency-converted, supercontinuum-spectrum is generated through difference frequency generation (DFG), such as intra-pulse difference frequency generation. An example of a suitable non-linear crystal for such a purpose is lithium niobate (LiNbO3). In such embodiments, the supercontinuum system may constitute a mid-infrared broadband system.

[0059] In some embodiments, to extend the supercontinuum-spectrum into the mid-IR, the frequency-conversion-element may constitute a non-linear crystal configured for differencefrequency generation (DFG). Examples of suitable non-linear crystals include lithium niobate (LiNbO3), AgGaS2 (AGS), zinc germanium phosphide (ZGP), or gallium selenide (GaSe). As a specific example, the non-linear crystal may be an AgGaS2 (AGS) crystal for DFG, which may enable frequency-conversion up to about 8 pm, such that the second supercontinuum-spectrum may cover wavelengths up to about 8 pm. Preferably, the nonlinear crystal is positioned downstream of the all-normal-dispersion fiber (AN Di fiber). The supercontinuum system may further comprise a mixing laser configured to operate at a mixing wavelength suitable for phase-matching with the first supercontinuum spectrum. The mixing laser may be a single-wavelength laser. The mixing wavelength may be input into the non-linear crystal along with at least a part of the first supercontinuum spectrum, or the entire first supercontinuum spectrum. The mixing wavelength and the first supercontinuum spectrum may interact within the non-linear crystal, thereby generating a frequency- converted spectrum in the mid-IR range, such as through a DFG process. Such a design allows the frequency-converted supercontinuum spectrum to cover mid-IR wavelengths, such as between 2 pm and 5 pm, enabling applications in spectroscopy, gas sensing, optical coherence tomography, and biomedical imaging.

[0060] In some embodiments, the first supercontinuum spectrum has a bandwidth that is more than 200 nm, preferably more than 400 nm, more preferably more than 500 nm, most preferably more than 600 nm. In some cases, the bandwidth is more than 800 nm.

[0061] Pulsed light source

[0062] The pulsed light-source-device, according to the present disclosure, may be configured to generate a train of light-pulses. Such pulses may be defined to have a first center wavelength, which may then broadened in the ANDi-fiber to generate the first supercontinuum spectrum. Further, each of the pulses may be defined in terms of at least a peak-power, Po, and a pulse width, To. The pulse width is also called pulse duration.

[0063] In some embodiments, the pulsed light source generates the train of light-pulses with pulses that are defined to have a pulse duration of less than 10 ps, such as less than 1 ps, such as less than 500 fs, such as around or less than 250 fs. In other embodiments, the pulse duration is more than 20 fs, such as more than 50 fs. In further embodiments, the pulse duration is between 20 fs and 500 fs, such as between 50 fs and 250 fs. The pulsed light- source-device may be a femto-second pulse source. The here described pulse durations have been found to be optimal for ANDi-SC generation in an ANDI-SC system according to the present disclosure.

[0064] In some embodiments, the train of light pulses is defined by a first center wavelength, wherein the first center wavelength is shorter than 1100 nm, such as around 1064 nm, such as around 1050 nm, such as around 800 nm. In some embodiments, the train of light-pulses comprises pulses having a center wavelength selected in a range from about 850 nm to about 1250 nm, such as from about 950 nm to about 1150 nm, such as from about 1000 nm to about 1100 nm.

[0065] In some embodiments, the pulsed light-source-device is a mode-locked laser, such as a mode-locked femto-second laser, configured to provide the train of light-pulses. In some embodiments, the train of light-pulses comprises pulses, each having a pulse energy of less than 500 nJ, such as less than 250 nJ, such as less than 100 nJ, such as less than 50 nJ. The train of light-pulses may be generated at a predefined pulse repetition rate, such as at a pulse repetition rate between 100 kHz and 500 MHz, such as between 1 MHz and 250 MHz, such as between 1 MHz and 100 MHz. The pulsed light-source-device may be a fiber laser or a solid-state laser.

[0066] In some embodiments, the pulsed light-source-device is a femto-second pulse source configured to generate the train of light-pulses being transmitted into the ANDi-fiber, wherein the ANDi-fiber is a solid core fiber, and wherein the train of light pulses comprises lightpulses that are defined to have a peak power of more than 50 kW, such as around or more than 100 kW, such as around or more than 1000 kW. Such peak powers in combination with a solid core fiber have been found to be optimal for ANDi-SC generation in an ANDI-SC system according to the present disclosure.

[0067] In other embodiments, the pulsed light-source-device is a femto-second pulse source configured to generate the train of light-pulses being transmitted into the ANDi-fiber, wherein the ANDi-fiber is a hollow core fiber, wherein the train of light pulses comprises light-pulses that are defined to have a peak power of more than 1 MW, such as around or more than 100 MW, such as around or more than 1000 MW. Such peak powers in combination with a hollow core fiber have been found to be optimal for ANDi-SC generation in an ANDI-SC system according to the present disclosure.

[0068] An ANDi-fiber, or a non-linear fiber in general, may be defined by at least a non-linear coefficient, y, and a group-velocity dispersion, 2. With such a definition of the ANDi-fiber, a soliton number, N, can be defined for the pulsed-light source in combination with the ANDi- fiber. The soliton number for each of the pulses from the pulsed light-source-device in combination with the ANDi-fiber, can be defined by a soliton number, N, where N2= To2y P0 / IP2I . In some embodiments, each pulse in the train of light pulses is defined by a soliton number being between N=10 and N=1000. In some embodiments, the first supercontinuum- spectrum is generated without the formation of solitons in the all-normal-dispersion fiber and / or without significant modulation instability.

[0069] In some embodiments, the train of pulses is defined by a shot-to-shot noise that is less than 1 %, preferably less than 0.5%, more preferably less than 0.3%, most preferably less than 0.15%. Accordingly, the first and / or the second supercontinuum-spectrum may constitute a low-noise and coherent supercontinuum. As an example, a low-noise supercontinuum is useful in optical coherence tomography (OCT) systems.

[0070] All-normal-dispersion fiber

[0071] In some embodiments, the all-normal-dispersion fiber is a polarization maintaining fiber. This may avoid quantum noise amplification via incoherent polarization-dependent non-linear processes. In some embodiments, the all-normal-dispersion fiber is a microstructured fiber, such as a photonic crystal fiber, such as a solid-core photonic crystal fiber.

[0072] In some embodiments, the all-normal-dispersion fiber is birefringent, defining a fast axis and a slow axis, and wherein the train of light pulses is guided primarily along the fast axis. This may exclude coupling between the fundamental polarization modes, and thus also prevent polarization-dependent non-linear processes. The exclusion of coupling between the fundamental polarization modes may in some embodiments be fulfilled when the ANDi-fiber is a highly birefringent fiber, such as having a birefringence higher than 1.0-1 O’4. Furthermore, the polarization noise may be reduced (but not fully prevented) when the high energy component propagates along the fast axis (low index axis) of the fiber.

[0073] In some embodiments, the all-normal-dispersion fiber has an absolute dispersion between 0 ps / nm / km and 500 ps / nm / km from the wavelength ranging from 300 nm to 2000 nm. In the present disclosure, when referred to dispersion, this means the group velocity dispersion parameter, called D. Having such a dispersion, may generate high-quality SC spectra having a large spectral bandwidth. In other embodiments, the all-normal-dispersion fiber has an absolute dispersion between 0 ps / nm / km and 750 ps / nm / km in a predefined wavelength range, such as a wavelength range extending from 300 nm to 2000 nm. In other embodiments, the all-normal-dispersion fiber has an absolute dispersion between 0 ps / nm / km and 250 ps / nm / km in a wavelength range from about 600 nm to about 1400 nm. An exemplary and non-limiting dispersion curve is shown in figure 2. In accordance with some embodiments, the all-normal-dispersion fiber is a solid core fiber, wherein the core diameter of the solid core fiber is less than 10 microns, preferably less than 5 microns, such as less than 3 microns.

[0074] In accordance with other embodiments, the all-normal-dispersion fiber is a hollow core fiber, wherein the core diameter of the hollow core fiber is less than 100 microns, such as less than 80 microns, such as less than 60 microns. In some embodiments, the core diameter of the hollow core fiber ranges from 10 microns to 70 microns, more preferably from 10 microns to 50 microns, most preferably from 10 microns to 30 microns.

[0075] It may be advantageous to use a solid core fiber over a hollow core fiber because a solid core fiber may only need pulses laser light having peak powers in the kW regime, such as more than 50 kW, whereas a hollow core fiber may need much higher peak powers, such as more than 1 MW. In addition, to provide an ANDi-fiber in the form of a hollow core fiber, the hollow core fiber may need to be filled with a gas or liquid, even a pressurized gas. However, if these features are not seen as a difficult implementation, both solid core and a hollow core fiber may be suitable for generating ANDi-SC in the SC system as described in the present disclosure.

[0076] ANDi-SC system

[0077] In accordance with some embodiments, the pulsed light source, the all-normal-dispersion fiber, and the frequency-conversion element are configured together to frequency convert the first supercontinuum-spectrum such that at least a part of the second supercontinuumspectrum resides in an ultraviolet wavelength domain. In some embodiments, the ultraviolet wavelength domain is covers wavelengths of less than 400 nm, such as less than 380 nm, such as less than 320 nm. Providing an ANDi-SC in the UV-domain has been a desire in the community for a long time, and the inventors have now found an efficient method to generate ANDi-SC in the UV-domain using the presently disclosed ANDi-SC system. Additionally, the inventors have found an efficient method to generate ANDi-SC in the IR- domain, such as through difference frequency generation (DFG). In some embodiments, the infrared wavelength domain covers wavelengths from about 2 pm to about 8 pm, such as from about 2 pm to about 5 pm, such as from about 3 pm to about 4.5 pm.

[0078] Compression stage In some embodiments, the supercontinuum system further comprises a compression stage configured to compress the train of pulses to pulses with a pulse duration of less than 100 fs, preferably of less than 50 fs, such as less than 30 fs. It has been found that for high conversion efficiency, such pulses in the femtosecond regime may be optimal. In some embodiments, the compression stage is located between the all-normal-dispersion fiber and the frequency-conversion element. Specifically, the compression stage may be arranged downstream of the all-normal-dispersion fiber and upstream of the frequency-conversion element. The output from the compression stage may be a train of coherent, high peak power, light-pulses.

[0079] The compression stage may be selected from the group of prism compressors, grism compressors, grating compressors, chirped mirrors, fiber-Bragg gratings, adaptive optics, and / or combinations thereof. In some embodiments, the compression stage is a prism compressor comprising one or more prisms, such as two or more prisms, such as four or more prisms. The one or more prisms may comprise a prism material selected from the group of heavy glass, fused silica, BK7 glass, N-F2, N-SF11 , Calcium Fluoride (CaF2), Sapphire, Zinc Selenide (ZnSe), or other suitable materials. As an example, the prisms may be equilateral dispersive prisms.

[0080] In some embodiments, the compression stage is configured to at least partially compensate for second-order dispersion added in the all-normal-dispersion fiber. As an example, the compression stage may be configured to provide negative second-order dispersion having an absolute value of more than 400 fs2, such as more than 1000 fs2, such as more than 1600 fs2, such as more than 2000 fs2. As another example, the compression stage may be configured to provide negative second-order dispersion having a value from about -400 fs2to about -4000 fs2.

[0081] In some embodiments, the compression stage is configured to at least partially compensate for higher-order dispersion, such as third-order dispersion (TOD), fourth-order dispersion, or fifth-order dispersion. Advantageously, the compression stage is configured to at least partially compensate TOD having an absolute value of more than 900 fs3, such as more than 1100 fs3, such as more than 1300 fs3, such as more than 1500 fs3. In particular, the compression stage may be configured to at least partially compensate higher-order dispersion, such as third-order dispersion, added to the train of pulses in the all-normal- dispersion fiber. As an example, the compression stage may be configured to at least partially compensate higher-order dispersion, such as third-order dispersion, by providing negative values of higher-order dispersion. In some embodiments, the absolute value of the provided negative third-order dispersion by the compression stage is more than 900 fs3, such as more than 1100 fs3, such as more than 1300 fs3, such as more than 1500 fs3.

[0082] In some cases, dispersion compensation can be done efficiently with only group delay dispersion (GDD) compensation, such as in the central part of the supercontinuum where the broadening is mainly due to self-phase modulation (SPM). However, with the onset of optical wave breaking, the bandwidth of the supercontinuum typically increases even further. In the all-normal-dispersion fiber as disclosed herein, the OWB broadened light may experience large amounts of higher-order dispersion, primarily third order dispersion (TOD) as the non-linear effects takes place and the pulse propagates. In such cases, third-order dispersion (TOD) is preferably compensated in the compression stage.

[0083] Compensation for higher-order dispersion may include use of: Grating compressors, Prism compressors, Grism compressors, tunable fiber-Bragg gratings, Chirped dispersion compensating mirrors or compression schemes using adaptive optics such as spatial light modulators. Furthermore, combinations of the above schemes in sequence may be employed to produce the desired higher-order dispersion compensation.

[0084] An advantage of an all-normal supercontinuum based on the combined effect of SPM and OWB is that the bandwidth of such a supercontinuum can be approximately twice that of a supercontinuum originating from SPM alone.

[0085] Advantageously, the compression stage is configured to temporally compress each lightpulse at least in a region of the pulse that was generated from optical wave-breaking (OWB). Pulses generated from OWB are typically chirped, or highly chirped, from higher-order dispersion. Consequently, such pulses, or regions of such pulses, are often considered more challenging to compress.

[0086] Example 1 - An example of an ANDi-SC system according to the present disclosure:

[0087] Fig. 1 shows an example of an ANDi SC system 1 according to the present disclosure. The ANDi SC system 1 comprises a pulsed light-source 2 configured to generate a train of lightpulses 3. Further, the ANDi-SC system comprises an all-normal-dispersion fiber 4 optically coupled to the pulsed light-source 2 and configured to generate a first supercontinuum 5. The first supercontinuum is light composed of multiple wavelengths, thus forming a first supercontinuum spectrum from the train of light-pulses 3. The supercontinuum-spectrum is defined in a first frequency-domain. The ANDi-SC system further comprises a frequency- conversion-element 6 configured to convert the supercontinuum-spectrum into a second frequency-domain, whereby the first supercontinuum-spectrum is converted to a second supercontinuum-spectrum, i.e. a second supercontinuum 7.

[0088] The ANDi-fiber 4 is a solid core fiber having only normal dispersion between an absolute value of 0 ps / nm / km and 1500 ps / nm / km in an ANDi-wavelength range from at least 400 nm to 1000 nm. In this manner, the solid core fiber 4 is configured to generate the first SC 5 based on the only normal dispersion of the solid core fiber 4.

[0089] The solid core fiber 4 has a length configured to initiate optical wave-breaking, whereby the SC spectrum 5 defines an ANDi SC spectrum.

[0090] The frequency-conversion-element 6 comprises a non-linear crystal 8 configured for frequency-doubling the first frequency-domain to the second frequency-domain by second harmonic generation.

[0091] As can further be seen from Fig. 1 , there is an optical coupling between the pulsed lightsource 2 and the non-linear crystal 8. Further, there is an optical coupling between the non- linear-crystal 8 and the solid core fiber 4. In this example, all the optical coupling is via a plurality of optical elements, in this case a plurality of optical lenses 9. Finally, the non-linear crystal 8 is optically coupled to an optical lens 8 to transmit the ANDi-SC light 7, i.e. the second SC, here in the form of collimated light.

[0092] Further, the ANDi-SC system 1 comprises a compression stage 10 configured to temporally compress the first ANDi SC spectrum 5 to another first ANDi-SC spectrum 12 having shorter pulses than the first ANDi-SC spectrum. In this example, the compression stage comprises a plurality of prisms 11 , forming a so-called prism-compressor 10. The prism-compressor 10 is here used to shorten the pulses of the ANDi SC 5, since short pulses may be used / required in specific applications and / or to efficiently convert the pulses in the non-linear crystal 8.

[0093] In this example, the non-linear crystal 8 may for example be a non-poled BBO crystal with a length of less than 3 mm. Some advantages of using a BBO crystal include an excellent UV transparency and a high damage threshold. A BBO crystal is therefore suitable for high- intensity pulses, such as femtosecond or picosecond pulses. As another example, the nonlinear crystal 8 may be a Lithium Triborate (LBO) crystal. Some advantages of LBO include a broad transparency range extending into the UV, and a high thermal and mechanical stability. Thus, LBO is similarly suitable for use in demanding, high-power applications. Other non-linear crystals can be envisaged without departing from the scope of the disclosure.

[0094] The train of light pulses 3 is defined by a first center wavelength, wherein the first center wavelength is around 800 nm. When this wavelength is used, the non-linear crystal 8 is cut at 29 degrees, whereby normal incidence of the second ANDi-SC 12 will convert 800 nm light efficiently to 400 nm. The 800 nm wavelength is the center of the wavelength range of the generated ANDi-SC 12.

[0095] To provide the most efficient conversion of the ANDi-SC 12, the crystal has a length of less 1 mm, such as around 100 microns, and the pulsed light source 2 and / or the compression stage 10 generates the train of light-pulses with pulses that are defined to have a pulse duration of less than 250 fs, such as less than 200 fs. Such short pulses are typically accompanied by high peak powers, such as more than 50 kW. Using such pulse durations and peak powers maximize the bandwidth of the second ANDi-SC. This is because the short crystal length avoids walk-off due to high peak powers.

[0096] Fig. 2 shows the dispersion (in ps / nm / km) for an all-normal-dispersion fiber according to the present disclosure. The dispersion is shown versus the wavelength (in nm). As shown in the plot, the all-normal-dispersion fiber has only normal dispersion in a predefined wavelength range. Normal dispersion implies that the dispersion is negative, at least in the predefined wavelength range, but it may also have normal dispersion for all wavelengths. In this example, the all-normal-dispersion fiber has an absolute dispersion between 0 ps / nm / km and about 250 ps / nm / km in a wavelength range from about 600 nm to about 1400 nm. However, other values can be envisaged without departing from the scope of the disclosure. As an example, the dispersion may reach higher negative values for lower wavelengths, i.e. , wavelength less than 600 nm.

[0097] Fig. 3 shows three different supercontinuum spectra from the all-normal-dispersion fiber according to the present disclosure. Thus, each of the supercontinuum spectra constitute a first supercontinuum-spectrum in a first frequency-domain. The figure shows the spectral density (in dB / nm) versus the wavelength (in nm) for three different pulse energies (in nJ). The all-normal-dispersion fiber was optically pumped with a pulsed light-source configured to generate a train of light-pulses. In this example, the pulses in the train of light-pulses each have a pulse duration of about 120 fs. Other pulse durations and pulse energies can be envisaged without departing from the scope of the disclosure. In this particular example, the train of light pulses was provided at a pulse repetition rate of about 80 MHz. The pulse energy was varied, and the solid graph corresponds to a pulse energy of about 9.4 nJ, the dashed graph corresponds to a pulse energy of about 4.4 nJ, and the dotted graph corresponds to a pulse energy of about 0.8 nJ. The corresponding average pump powers are 755 mW, 351 mW, and 63 mW, respectively, each at a pulse repetition rate of 80 MHz. The center broadening is achieved by self-phase modulation. In this example it is especially prominent in the spectrum at low pulse energy. The broad wings of the spectrum, especially prominent at high pulse energy, consist of non-linear broadening from optical wave breaking.

[0098] Figures 4-10 collectively show an example of the filtration and compression of an output pulse from an all-normal-dispersion fiber according to the present disclosure. Specifically, in this example the output pulse is spectrally (and temporally) filtered downstream of the fiber, and subsequently temporally compressed to a pulse duration of about 16 fs (-10 dB bandwidth, 7 fs FWHM) in a compression stage forming part of the all-normal-dispersion based supercontinuum system according to the present disclosure. The compression of the pulse is shown in figures 9-10.

[0099] Fig. 4 shows an example of an output light pulse from the all-normal-dispersion fiber according to the present disclosure. The figure shows the power (in W) of the light pulse versus the time (in ps). In this example, the power of the pulse from the fiber exceeds 4 kW. The solid graph represents the output pulse from the fiber before any filtration, temporally and / or spectrally. The dashed graph represents a filtered output pulse, i.e. , a pulse that has been temporarily and / or spectrally filtered, downstream of the all-normal-dispersion fiber. Thus, the supercontinuum system may further comprise an optical filter configured to filter a predefined range of wavelengths of the first supercontinuum-spectrum. The optical filter may form part of any of the embodiments described herein. As evident from the solid graph, some dispersion, such as second-order and third-order dispersion, has been added to the pulse during propagation through the all-normal-dispersion fiber. In some aspects of the present disclosure, it is an object to counteract, or at least partially compensate, such dispersion added to the pulse in the fiber. Notice the center peak of the pulse contains primarily wavelengths generated from self-phase modulation, while the broad pedestal wings are generated from optical wave breaking.

[0100] Fig. 5 shows an example of a pulse spectrogram of an output light pulse from the all-normal- dispersion fiber according to the present disclosure. This spectrogram corresponds to the light pulse shown in figure 4. The spectrogram shows the time (in ps) versus the wavelength (in nm) of the light pulse after propagation in the all-normal-dispersion fiber. As evident from the figure, the light pulse comprises a range of wavelengths from about 450 nm to about 1900 nm. The wavelengths in the light pulse arrive at different times from about -4 ps to about 4 ps. The light pulse of this example is before being spectrally filtered and before any temporal compression in a compression stage.

[0101] Fig. 6 shows an example of the spectral density versus wavelength of an output light pulse from the all-normal-dispersion fiber according to the present disclosure. The solid graph represents the output pulse from the fiber before any filtration, temporally and / or spectrally. The dashed graph represents a filtered output pulse, i.e. , a pulse that has been temporarily and / or spectrally filtered, downstream of the all-normal-dispersion fiber. In this example, the output light pulse from the fiber comprises wavelengths from about 450 nm to about 1900 nm, whereas the filtered output pulse comprises wavelengths from about 500 nm to about 900 nm. The spectral filtration may also result in a temporal filtration as evident from fig. 4. The filtered part of the spectrum corresponds primarily to wavelengths generated by optical wave breaking which have accumulated significant phase in the high dispersion regime of the all-normal-dispersion fiber.

[0102] Fig. 7 shows an example of a pulse spectrogram of a filtered output light pulse from the all- normal-dispersion fiber according to the present disclosure. The spectrogram shows the time (in ps) versus the wavelength (in nm) of the light pulse. In this example, the filtered output pulse comprises wavelengths from about 500 nm to about 900 nm. The filtered pulse is similar to the filtered pulse shown in figures 4 and 6. The pulse spectrogram shows the pulse before compression, i.e., it shows a filtered, uncompressed pulse. As evident from the spectrogram, the pulse has been affected by both second-order dispersion and higher-order dispersion, such as third-order dispersion, in the all-normal-dispersion fiber. In some embodiments of the presently disclosed supercontinuum system, the system is configured to counteract the normal dispersion applied to the pulse in the all-normal-dispersion fiber. This may be achieved by a compression stage configured to at least partially compensate for second-order dispersion, and / or third-order dispersion, added in the all-normal- dispersion fiber, such as by providing predefined amounts of anomalous dispersion.

[0103] Fig. 8 shows an example of the power versus time of a filtered output light pulse from the all-normal-dispersion fiber according to the present disclosure. The graph shows the pulse before compression, i.e., it shows the optical power of a filtered, uncompressed pulse. As evident from the graph, the uncompressed pulse comprises optical powers from about 0 kW to about 3.5 kW. In this example, the pulse duration of the filtered, uncompressed pulse is about 3.9 ps in a 10 dB bandwidth. Other values of pulse durations and peak powers can be envisaged without departing from the scope of the disclosure. The filtered and uncompressed light pulse may be provided as the input to a compression stage forming part of the presently disclosed supercontinuum system.

[0104] Fig. 9 shows an example of a pulse spectrogram of a filtered, and compressed, output light pulse from an all-normal-dispersion fiber according to the present disclosure. The spectrogram shows the time (in ps) versus the wavelength (in nm) of the compressed light pulse. As evident, the spectrum is substantially flat in time, i.e., it forms a substantially horizontal line in the pulse spectrogram. This may be achieved by propagating a filtered pulse through a compression stage, such as a prism compressor, configured to temporally compress the filtered pulse. In this specific example, the filtered pulse corresponds to the one shown in figures 7-8, and was provided as the input to the compression stage. The pulse spectrogram shows the output from the compression stage. The compression stage may be configured to provide predefined negative amounts of second-order and third-order dispersion in order to at least partially compensate, or counteract, positive amounts of dispersion (normal dispersion) received during propagation of the all-normal-dispersion fiber. In this particular example, a group delay dispersion (GDD) of -2556 fs2was applied along with a third-order dispersion (TOD) of about -2311 fs3; both applied in the compression stage by a prism compressor. Other values of GDD and TOD can be envisaged without departing from the scope of the disclosure.

[0105] Fig. 10 shows an example of a compressed pulse from a compression stage forming part of an all-normal-dispersion based supercontinuum system according to the present disclosure. In this example, the supercontinuum system further comprises a compression stage configured to temporally compress the train of pulses to provide compressed pulses having a pulse duration of less than 30 fs, specifically in this example less than 20 fs, here about 16 fs in a -10 dB bandwidth. This corresponds to a compression factor of more than 200, here about 250. It is observed that the power of the compressed pulse is significantly higher than the uncompressed pulse shown in fig. 8. Thus, the output from the compression stage is a train of coherent, high peak power, light-pulses. Specifically, the peak power of the compressed pulses is in the kW range. In this example, the peak power exceeds 600 kW. Other values of pulse durations and peak powers can be envisaged without departing from the scope of the disclosure. In this example, the compression stage is configured to receive a filtered light pulse, such as the one shown in figures 7-8, and temporally compress the filtered light pulse to very short pulse durations, such as to pulse durations in the femtosecond range, such as pulse durations below 50 fs. The compressed light pulses may be provided to a frequency-conversion-element forming part of the all-normal-dispersion based supercontinuum system in accordance with some embodiments.

[0106] Further details of the disclosure

[0107] 1. An all-normal-dispersion based supercontinuum system, comprising:

[0108] - a pulsed light-source configured to generate a train of light-pulses; and

[0109] - an all-normal-dispersion fiber optically coupled to the pulsed light-source and configured to generate a first supercontinuum-spectrum from the train of light-pulses, wherein the supercontinuum-spectrum is defined in a first frequency-domain.

[0110] 2. The supercontinuum system according to item 1 , wherein the supercontinuum system further comprises a frequency-conversion-element configured to convert the supercontinuum-spectrum into a second frequency-domain, whereby the first supercontinuum-spectrum is converted to a second supercontinuum-spectrum.

[0111] 3. The supercontinuum system according to item 2, wherein at least a part of the second frequency-domain comprises frequencies that are higher than that of the first frequency-domain.

[0112] 4. The supercontinuum system according to any of the items 2-3, wherein at least a part of the second supercontinuum-spectrum comprises wavelengths that are lower than that of the first supercontinuum-spectrum.

[0113] 5. The supercontinuum system according to any of the items 2-4, wherein the second supercontinuum-spectrum extends to wavelengths less than 450 nm, such as less than 400 nm, such as less than 350 nm.

[0114] 6. The supercontinuum system according to any of the items 2-5, wherein the second supercontinuum-spectrum covers wavelengths from about 100 nm to about 800 nm, such as from about 200 nm to about 750 nm, such as from about 300 nm to about 700 nm.

[0115] 7. The supercontinuum system according to any of the items 2-6, wherein at least a part of the second frequency-domain comprises frequencies that are lower than that of the first frequency-domain.

[0116] 8. The supercontinuum system according to any of the items 2-7, wherein at least a part of the second supercontinuum-spectrum comprises wavelengths that are higher than that of the first supercontinuum-spectrum.

[0117] 9. The supercontinuum system according to any of the items 2-8, wherein the second supercontinuum-spectrum extends to wavelengths above 2 pm, such as above 3 pm, such as above 3.5 pm.

[0118] 10. The supercontinuum system according to any of the items 2-9, wherein the second supercontinuum-spectrum covers wavelengths from about 2 pm to about 8 pm, such as from about 2 pm to about 5 pm, such as from about 3 pm to about 4.5 pm.

[0119] 11. The supercontinuum system according to any of the preceding items, wherein the train of light-pulses comprises pulses having a pulse duration of less than 10 ps, such as less than 1 ps, such as less than 500 fs, such as around or less than 250 fs.

[0120] 12. The supercontinuum system according to any of the preceding items, wherein the train of light-pulses comprises pulses having a pulse duration selected in a range from about 10 fs to about 100 ps, such as from about 100 fs to about 20 ps, such as from about 250 fs to about 10 ps.

[0121] 13. The supercontinuum system according to any of the items 2-12, wherein the frequency-conversion-element is arranged downstream of the all-normal-dispersion fiber. 14. The supercontinuum system according to any of the preceding items, wherein the train of light-pulses comprises pulses having a pulse energy of less than 200 nJ, such as less than 50 nJ, such as less than 10 nJ.

[0122] 15. The supercontinuum system according to any of the preceding items, wherein the train of light-pulses comprises pulses having a pulse energy of between 0.1 nJ and 1 pj, such as between 0.5 nJ and 200 nJ, such as between 1 nJ and 100 nJ.

[0123] 16. The supercontinuum system according to any of the preceding items, wherein the train of light-pulses comprises pulses having a center wavelength selected in a range from about 850 nm to about 1250 nm, such as from about 950 nm to about 1150 nm, such as from about 1000 nm to about 1100 nm.

[0124] 17. The supercontinuum system according to any of the preceding items, wherein the train of light-pulses are generated at a predefined pulse repetition rate.

[0125] 18. The supercontinuum system according to item 17, wherein the predefined pulse repetition rate is between 100 kHz and 500 MHz, such as between 1 MHz and 250 MHz, such as between 1 MHz and 100 MHz.

[0126] 19. The supercontinuum system according to any of the preceding items, wherein the train of light pulses is defined by a first center wavelength.

[0127] 20. The supercontinuum system according to item 19, wherein the first center wavelength is shorter than 1100 nm, such as around 1064 nm, such as around 1050 nm, such as around 800 nm.

[0128] 21. The supercontinuum system according to any of the preceding items, wherein the group velocity dispersion (GVD) of the all-normal-dispersion fiber is entirely positive (normal) across a predefined operating wavelength range.

[0129] 22. The supercontinuum system according to any of the preceding items, wherein the all- normal-dispersion fiber has an absolute dispersion between 0 ps / nm / km and 750 ps / nm / km in a predefined wavelength range. 23. The supercontinuum system according to any of the preceding items, wherein the all- normal-dispersion fiber has an absolute dispersion between 0 ps / nm / km and 500 ps / nm / km in a predefined wavelength range.

[0130] 24. The supercontinuum system according to any of the items 21 -23, wherein the predefined wavelength range extends from about 200 nm to 2100 nm, such as from about 400 nm to 1800 nm, such as from about 600 nm to 1400 nm.

[0131] 25. The supercontinuum system according to any of the preceding items, wherein the all- normal-dispersion fiber is a polarization maintaining (PM) fiber.

[0132] 26. The supercontinuum system according to any of the preceding items, wherein the all- normal-dispersion fiber is a microstructured fiber, such as a photonic crystal fiber.

[0133] 27. The supercontinuum system according to any of the preceding items, wherein the all- normal-dispersion fiber is a solid core fiber.

[0134] 28. The supercontinuum system according to item 27, wherein the core diameter of the solid core fiber is less than 10 microns, such as less than 5 microns, such as less than 3 microns.

[0135] 29. The supercontinuum system according to any of the preceding items, wherein the all- normal-dispersion fiber is a hollow core fiber.

[0136] 30. The supercontinuum system according to item 29, wherein the core diameter of the hollow core fiber is less than 100 microns, such as less than 80 microns, such as less than 60 microns.

[0137] 31. The supercontinuum system according to any of the preceding items, wherein the first supercontinuum spectrum has a bandwidth of more than 200 nm.

[0138] 32. The supercontinuum system according to item 31 , wherein the bandwidth is more than 400 nm, such as more than 600 nm, such as more than 800 nm. 33. The supercontinuum system according to any of the items 2-32, wherein the frequency-conversion-element is a second harmonic generator.

[0139] 34. The supercontinuum system according to any of the items 2-33, where the frequency- conversion-element comprises, or constitutes, a non-linear crystal, such as a nonlinear crystal for second harmonic generation (SHG).

[0140] 35. The supercontinuum system according to item 34, wherein the non-linear crystal has a length that is less than 10 mm, such as less than 5 mm, such as around or less than 3 mm.

[0141] 36. The supercontinuum system according to any of the preceding items, wherein the pulsed light-source-device is a fiber laser or a solid-state laser.

[0142] 37. The supercontinuum system according to any of the preceding items, wherein the pulsed light-source-device is a femto-second pulse source.

[0143] 38. The supercontinuum system according to any of the preceding items, wherein the pulsed light-source-device is a mode-locked laser, such as a mode-locked femtosecond laser.

[0144] 39. The supercontinuum system according to any of the preceding items, wherein the pulsed light-source-device is a femto-second pulse source configured to generate the train of light-pulses being transmitted into the all-normal-dispersion fiber, wherein said fiber is a solid core fiber.

[0145] 40. The supercontinuum system according to item 39, wherein the train of light pulses comprises light-pulses that are defined to have a peak power of more than 50 kW, such as around or more than 100 kW, such as around or more than 1000 kW.

[0146] 41. The supercontinuum system according to any of the preceding items, wherein the pulsed light-source-device is a femto-second pulse source configured to generate the train of light-pulses being transmitted into the all-normal-dispersion fiber, wherein said fiber is a hollow core fiber.

[0147] 42. The supercontinuum system according to item 41 , wherein the train of light pulses comprises light-pulses that are defined to have a peak power of more than 1 MW, such as around or more than 100 MW, such as around or more than 1000 MW.

[0148] 43. The supercontinuum system according to any of the items 2-42, wherein the pulsed light source, the all-normal-dispersion fiber, and the frequency-conversion element are configured together to convert at least a part of the first supercontinuum-spectrum into an ultraviolet wavelength domain.

[0149] 44. The supercontinuum system according to item 43, wherein the ultraviolet wavelength domain comprises wavelengths of less than 400 nm, such as less than 380 nm, such as less than 320 nm.

[0150] 45. The supercontinuum system according to any of the items 2-42, wherein the pulsed light source, the all-normal-dispersion fiber, and the frequency-conversion element are configured together to convert at least a part of the first supercontinuum-spectrum into an infrared wavelength domain.

[0151] 46. The supercontinuum system according to item 45, wherein the infrared wavelength domain comprises wavelengths of more than 1 .4 pm, such as more than 2 pm, such as more than 3 pm.

[0152] 47. The supercontinuum system according to any of the preceding items, wherein the supercontinuum system further comprises a compression stage configured to temporally compress the train of pulses.

[0153] 48. The supercontinuum system according to item 47, wherein the compression stage is configured to temporally compress the train of pulses to compressed pulses having a pulse duration of less than 100 fs, such as less than 50 fs, such as around, or less than, 30 fs. 49. The supercontinuum system according to any of the items 47-48, wherein the compression stage is located between the all-normal-dispersion fiber and the frequency-conversion element.

[0154] 50. The supercontinuum system according to any of the items 47-49, wherein the compression stage is arranged downstream of the all-normal-dispersion fiber and upstream of the frequency-conversion element.

[0155] 51 . The supercontinuum system according to any of the items 47-50, wherein the output from the compression stage is a train of coherent, high peak power, light-pulses.

[0156] 52. The supercontinuum system according to any of the items 47-51 , wherein the compression stage is selected from the group of prism compressors, grating compressors, chirped mirrors, fiber-Bragg gratings, adaptive optics, and / or combinations thereof.

[0157] 53. The supercontinuum system according to any of the items 47-51 , wherein the compression stage is a prism compressor comprising one or more prisms, such as two or more prisms, such as four or more prisms.

[0158] 54. The supercontinuum system according to item 53, wherein the one or more prisms comprise a prism material selected from the group of heavy glass, fused silica, BK7 glass, Calcium Fluoride (CaF2), Sapphire, Zinc Selenide (ZnSe)

[0159] 55. The supercontinuum system according to any of the items 47-54, wherein the compression stage is configured to at least partially compensate for second-order dispersion added in the all-normal-dispersion fiber.

[0160] 56. The supercontinuum system according to any of the items 47-55, wherein the compression stage is configured to provide negative second-order dispersion having an absolute value of more than 400 fs2, such as more than 1000 fs2, such as more than 1600 fs2, such as more than 2000 fs2. 57. The supercontinuum system according to any of the items 47-56, wherein the compression stage is configured to provide negative second-order dispersion having a value from about -400 fs2to about -4000 fs2.

[0161] 58. The supercontinuum system according to any of the items 47-57, wherein the compression stage is configured to at least partially compensate for higher-order dispersion, such as third-order dispersion (TOD), fourth-order dispersion, or fifthorder dispersion.

[0162] 59. The supercontinuum system according to any of the items 47-58, wherein the compression stage is configured to at least partially compensate third-order dispersion having an absolute value of more than 900 fs3, such as more than 1100 fs3, such as more than 1300 fs3, such as more than 1500 fs3.

[0163] 60. The supercontinuum system according to any of the items 47-59, wherein the compression stage is configured to at least partially compensate higher-order dispersion, such as third-order dispersion, added to the train of pulses in the all- normal-dispersion fiber.

[0164] 61. The supercontinuum system according to any of the items 47-60, wherein the compression stage is configured to at least partially compensate higher-order dispersion, such as third-order dispersion, by providing negative values of higher- order dispersion.

[0165] 62. The supercontinuum system according to any of the items 47-61 , wherein the absolute value of the provided negative third-order dispersion by the compression stage is more than 900 fs3, such as more than 1100 fs3, such as more than 1300 fs3, such as more than 1500 fs3.

[0166] 63. The supercontinuum system according to any of the preceding items, wherein the all- normal-dispersion fiber has a sufficient length to initiate both self-phase modulation (SPM) and optical wave-breaking (OWB) in the fiber. 64. The supercontinuum system according to any of the preceding items, wherein the length of the all-normal-dispersion fiber is longer than the optical wave-breaking distance.

[0167] 65. The supercontinuum system according to any of the preceding items, wherein the length of the all-normal-dispersion fiber is between 1 cm and 300 cm, such as between 10 cm and 200 cm, such as between 10 cm and 100 cm.

[0168] 66. The supercontinuum system according to any of the preceding items, wherein the first supercontinuum-spectrum is generated in the all-normal-dispersion fiber by one or more non-linear processes in the fiber.

[0169] 67. The supercontinuum system according to item 66, wherein the one or more non-linear processes are selected from the group of self-phase modulation (SPM) and optical wave-breaking (OWB), and / or combinations thereof.

[0170] 68. The supercontinuum system according to items 66-67, wherein the first supercontinuum-spectrum is generated without the formation of solitons in the all- normal-dispersion fiber and / or without significant modulation instability.

[0171] 69. The supercontinuum system according to items 66-68, wherein the length of the all- normal-dispersion fiber is selected such that the non-linear processes that contribute to the generation of the first supercontinuum-spectrum is self-phase modulation (SPM) and / or optical wave-breaking (OWB).

[0172] 70. The supercontinuum system according to any of the preceding items, wherein the first supercontinuum-spectrum is generated from one or more non-linear processes, such as self-phase modulation (SPM) and optical wave-breaking (OWB), without significant modulation instability and / or without the formation of solitons.

[0173] 71. The supercontinuum system according to any of the preceding items, wherein the compression stage is configured to temporally compress each light-pulse at least in a region of the pulse that was generated from optical wave-breaking (OWB). 72. The supercontinuum system according to any of the preceding items, wherein the spectral density in the outer regions of the first supercontinuum-spectrum is higher than in a central region of the first supercontinuum-spectrum.

[0174] 73. The supercontinuum system according to any of the preceding items, wherein the supercontinuum system further comprises an optical filter configured to filter a predefined range of wavelengths of the first supercontinuum-spectrum.

[0175] 74. The supercontinuum system according to item 73, wherein the spectral density in the predefined range of wavelengths is higher than the spectral density in a central region of the first supercontinuum-spectrum.

[0176] 75. The supercontinuum system according to any of the items 73-74, wherein the optical filter is arranged downstream of the all-normal-dispersion fiber.

[0177] 76. The supercontinuum system according to any of the items 73-75, wherein the optical filter is arranged between the all-normal-dispersion fiber and the compression stage.

[0178] 77. The supercontinuum system according to any of the preceding items, wherein the first and / or the second supercontinuum-spectrum constitute a low-noise and coherent supercontinuum.

[0179] 78. The supercontinuum system according to any of the items 47-77, wherein the frequency-conversion-element is arranged downstream of the compression stage.

[0180] 79. The supercontinuum system according to any of the preceding items, wherein the supercontinuum system is configured to frequency convert at least a part of the first supercontinuum-spectrum into an infrared wavelength domain.

[0181] 80. The supercontinuum system according to any of the preceding items, wherein the frequency-conversion-element is configured to frequency convert at least a part of the first supercontinuum-spectrum into a second frequency-domain through a frequency mixing process, such as difference frequency generation (DFG). 81. The supercontinuum system according to any of the preceding items, wherein the supercontinuum system further comprises a mixing laser for providing a mixing wavelength in a difference frequency generation (DFG) process, such that the frequency-conversion-element is configured to frequency convert the first supercontinuum spectrum to lower frequencies through a DFG process.

[0182] 82. The supercontinuum system according to item 81 , wherein the mixing laser is a single-wavelength laser.

[0183] 83. The supercontinuum system according to any of the items 81 -82, wherein the mixing laser is configured to operate at a wavelength suitable for phase-matching with the first supercontinuum spectrum.

[0184] 84. The supercontinuum system according to any of the items 81 -83, wherein the lower frequencies reside in the infrared region, such as in the short- or mid-infrared region.

[0185] 85. The supercontinuum system according to any of the items 81 -84, wherein the lower frequencies of the frequency-converted supercontinuum spectrum correspond to wavelengths from about 2 pm to about 5 pm, such as from about 3.5 pm to about 4.5 pm.

[0186] 86. The supercontinuum system according to any of the preceding items, wherein the frequency-conversion-element comprises, or constitutes, a non-linear crystal for difference frequency generation (DFG).

[0187] 87. The supercontinuum system according to item 86, wherein the non-linear crystal is selected from the group of: lithium niobate (LiNbO3), AgGaS2 (AGS), zinc germanium phosphide (ZGP), or gallium selenide (GaSe).

[0188] Although some embodiments have been described and shown in detail, the disclosure is not restricted to such details, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilized, and structural and functional modifications may be made without departing from the scope of the present disclosure. Furthermore, the skilled person would find it apparent that unless an embodiment is specifically presented only as an alternative, different disclosed embodiments may be combined to achieve a specific implementation and such specific implementation is within the scope of the disclosure.

Claims

Claims1. An all-normal-dispersion based supercontinuum system, comprising:- a pulsed light-source configured to generate a train of light-pulses;- an all-normal-dispersion fiber optically coupled to the pulsed light-source and configured to generate a first supercontinuum-spectrum from the train of light-pulses, wherein the supercontinuum-spectrum is defined in a first frequency-domain; and- a frequency-conversion-element configured to convert the supercontinuum- spectrum into a second frequency-domain, whereby the first supercontinuum-spectrum is converted to a second supercontinuum- spectrum.

2. The supercontinuum system according to any of the preceding claims, wherein the first supercontinuum-spectrum is generated in the all-normal-dispersion fiber by self-phase modulation (SPM) and optical wave-breaking (OWB).

3. The supercontinuum system according to any of the preceding claims, wherein the length of the all-normal-dispersion fiber is selected such that the non-linear processes that contribute to the generation of the first supercontinuum-spectrum is self-phase modulation (SPM) and optical wave-breaking (OWB).

4. The supercontinuum system according to any of the preceding claims, wherein the length of the all-normal-dispersion fiber is between 1 cm and 300 cm, such as between 10 cm and 200 cm, such as between 10 cm and 100 cm.

5. The supercontinuum system according to any of the preceding claims, wherein the frequency-conversion-element is arranged downstream of the all-normal-dispersion fiber.

6. The supercontinuum system according to any of the preceding claims, wherein the frequency-conversion-element is a non-linear crystal configured for second harmonic generation (SHG) of the first supercontinuum-spectrum.

7. The supercontinuum system according to any of the preceding claims, wherein the second supercontinuum-spectrum extends to wavelengths less than 450 nm, such as less than 400 nm, such as less than 350 nm.

8. The supercontinuum system according to any of the claims 1 -5, wherein the frequency-conversion-element is configured to frequency convert at least a part of the first supercontinuum-spectrum into the second frequency-domain through difference frequency generation (DFG).

9. The supercontinuum system according to any of the claims 1 -5, or 8, wherein the frequency-conversion-element is a non-linear crystal configured for difference frequency generation (DFG), wherein the non-linear crystal is selected from the group of lithium niobate (LiNbO3) and AgGaS2 (AGS).

10. The supercontinuum system according to any of the preceding claims, wherein the second supercontinuum-spectrum covers wavelengths from about 2 pm to about 8 pm, such as from about 2 pm to about 5 pm.

11. The supercontinuum system according to any of the preceding claims, wherein the supercontinuum system further comprises a compression stage configured to temporally compress the train of pulses.

12. The supercontinuum system according to claim 11 , wherein the compression stage is arranged between the all-normal-dispersion fiber and the frequency-conversion element.

13. The supercontinuum system according to any of the claims 11 -12, wherein the compression stage is configured to temporally compress the train of pulses to compressed pulses having a pulse duration of less than 100 fs, such as less than 50 fs.

14. The supercontinuum system according to any of the claims 11 -13, wherein the compression stage is configured to temporally compress light-pulses of the train oflight-pulses from the fiber, at least in a region of the light-pulses that was generated from optical wave-breaking (OWB).

15. The supercontinuum system according to any of the claims 11 -14, wherein the compression stage is configured to at least partially compensate for second-order dispersion added to the train of pulses in the all-normal-dispersion fiber.

16. The supercontinuum system according to any of the claims 11 -15, wherein the compression stage is configured to apply a group delay dispersion (GDD) having a value of between -4000 fs2to -1000 fs2, such as between -3000 fs2to -2000 fs2.

17. The supercontinuum system according to any of the claims 11 -16, wherein the compression stage is configured to apply a group delay dispersion (GDD) having an absolute value of more than 2000 fs2.

18. The supercontinuum system according to any of the claims 11 -17, wherein the compression stage is configured to at least partially compensate for higher-order dispersion, such as third-order dispersion (TOD), fourth-order dispersion, or fifthorder dispersion.

19. The supercontinuum system according to any of the claims 11 -18, wherein the compression stage is configured to at least partially compensate third-order dispersion having an absolute value of more than 1000 fs3, such as more than 1500 fs3, such as more than 2000 fs3.

20. The supercontinuum system according to any of the claims 11 -19, wherein the compression stage is configured to apply a third-order dispersion (TOD) having an absolute value of more than 2000 fs3.21 . The supercontinuum system according to any of the claims 11 -20, wherein the compression stage is a prism compressor comprising two or more prisms, such as four or more prisms.

22. The supercontinuum system according to any of the preceding claims, wherein the first and / or the second supercontinuum-spectrum constitutes a low-noise and coherent supercontinuum.

23. The supercontinuum system according to any of the preceding claims, wherein the spectral density in the outer regions of the first supercontinuum-spectrum is higher than in a central region of the first supercontinuum-spectrum.

24. The supercontinuum system according to any of the preceding claims, wherein the supercontinuum system further comprises an optical filter configured to filter a predefined range of wavelengths of the first supercontinuum-spectrum.

25. An optical coherence tomography (OCT) system comprising the supercontinuum system according to any of the preceding claims.

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