Device for generating multi-wavelength and ultra-high-speed optical pulses

Abstract

A device for generating multiple wavelengths, comprising a pulse light source configured to emit a series of optical pulses, each of the optical pulses being defined by a central wavelength having a first pulse duration and a first bandwidth, and a first hollow-core optical fiber having a core diameter d, the first hollow-core optical fiber being configured to spectrally broaden the series of optical pulses, thereby generating a series of multi-wavelength pulses having a plurality of wavelengths, wherein the first hollow-core optical fiber has a dispersion selected such that the optical pulses are spectrally broadened mainly by self-phase modulation (SPM).

Description

Technical Field

[0001] The present disclosure generally relates to devices for generating multiple wavelengths. More particularly, the present disclosure relates to generating ultra-fast optical pulses.

Background Art

[0002] The generation of multiple wavelengths using ultra-short pulses is known in the field of spectral broadening of light from a pulsed light source having a single wavelength. The broadening of light from a pulsed light source can be diverse, but typically an optical fiber is used.

[0003] When using a fiber, the simplest way to broaden the optical pulse mainly relies on self-phase modulation. Here, the dominant non-linear optical effect in the optical fiber is due to the optical Kerr effect.

[0004] Self-phase modulation (SPM) has the effect of initially splitting a pulse spectrum having a Gaussian profile into two dominant spectral lobes, one of which is shifted to a shorter wavelength and the other is shifted to a higher wavelength. For the introduction of non-linear optical effects for broadening the spectrum of ultra-short optical pulses, see, for example, Non-Patent Document 1.

[0005] As an example of SPM, Non-Patent Document 2 shows a method that can expand an initial narrow pulse spectrum centered at 1030 nm so that the spectrum ranges from 825 to 1210 nm using spectral broadening from self-phase modulation. In the present disclosure, a solid-core fiber having low dispersion and a small mode field diameter (about 2.2 μm) enables fiber-optic broadening of the input optical spectrum dominated by self-phase modulation (SPM) plus self-steepening. Since the influence of dispersion is minimized, the resulting broadened spectrum features well-separated spectral lobes. Next, each of these spectral lobes can be selected by an optical bandpass filter to generate nearly transform-limited pulses. Without external compression, the filtered spectral lobes corresponded to ~100 fs (nearly transform-limited) pulses with >1 nJ pulse energy. One problem with this approach is that when using a fiber with a small mode field diameter (MFD) to expand the optical spectrum, the power coupled into the fiber is susceptible to environmental influences, making the resulting wavelength-tunable light source impractical for multiphoton microscopy (MPM) applications. For this reason, Non-Patent Document 2 proposed changing the solid-core optical fiber to a solid-core large mode area fiber.

[0006] The proposal by Non-Patent Document 2 is described in Non-Patent Document 3. Here, a large mode area (LMA) fiber having a large MFD up to 7.5 μm and appropriate dispersion (for SPM-dominated spectral broadening) enabled scaling up the pulse energy up to 20 nJ, but due to more power being coupled into the fiber, it was in the wavelength range of 1030 - 1215 nm at a repetition rate of 55 MHz. The wavelength range and pulse energy here were found to be practical for MPM applications. Using a grating-based compressor, a filtered pulse duration of about 190 fs was achieved from an initial 90 fs source. However, when the aim is to provide wavelengths below 950 nm as in the first example, a shift in the center wavelength of 200 nm becomes a problem.

[0007] To provide a laser that offers >1 nJ with ultrashort pulses, is wavelength tunable across the entire wavelength range of 800 - 1300, and is simultaneously not affected by environmental disturbances, a solution was proposed by Non-Patent Document 4. In this paper, extreme pulse compression was achieved by relying on self-phase modulation (SPM) that is moderately driven over an extended propagation distance. This avoided other harmful non-linear mechanisms from taking over and deteriorating the SPM process. The long propagation was obtained with a hollow-core fiber (HCF) up to 6 m in length. The HCF was filled with gas and pressurized to different levels to optimize the output stability and spectrum. This research is related to Patent Document 1.

[0008] One problem associated with the HCF as disclosed in both Non-Patent Document 4 and Patent Document 1 is that the pulses provided are not single-cycle. As indicated by the title of Non-Patent Document 4, the minimum pulse cycle is 1.5. Having a pulse cycle greater than 1 may not be optimal in various applications.

[0009] Therefore, it is desired to provide a wavelength-tunable and ultra-fast pulse device for multi-wavelength generation that is also single-cycle. Such a device is disclosed in Non-Patent Document 5. In this disclosure, a pulse light source operating with pulses having a wavelength of 1030 nm is first compressed in a first compression stage having an anti-resonant and krypton-filled single-ring photonic crystal fiber (SR-PCF). The first compression stage is designed to provide normal dispersion spectral broadening by self-phase modulation. This provides pulse compression of pulses from the pulse light source that are greater than 320 fs and down to 25 fs, together with phase compensation by a negatively chirped mirror. These compressed pulses are then further compressed to less than 4 fs using a second compression stage having another anti-resonant SR-PCF filled with neon at a pressure of 5 MPa (50 bar). The second compression stage is designed to provide soliton self-compression.

[0010] The effect of self-soliton compression clearly compresses the pulse into a desired ultra-fast pulse, but also washes out the two dominant spectral lobes such as those obtained from SPM broadening. Since these two lobes are desired in many applications, it is not desirable to use the setup as disclosed in Non-Patent Document 5.

[0011] Even when the second compression stage is not used, the system according to Non-Patent Document 5, particularly the first compression stage, has several problems. First, since a 3.8 m long SR-PCF is used and wound in one turn with a diameter of 0.8 m, the first compression stage is very long. Therefore, the system is very large. Second, since the SR-PCF contains krypton gas, the system is very expensive. Third, due to the long length of the SR-PCF, the polarization change is controlled by a pair of quarter-wave plates, complicating the system. Fourth, a combination of a broadband half-wave plate and a thin-film polarizer is used to control the power, further complicating the system.

[0012] The first compression stage as disclosed in Non-Patent Document 5 has been proposed because its purpose is to provide a scalable system for handling light sources with very high repetition rates. However, as described now, this incurs costs in that it is large, expensive, and very complex.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Non-Patent Documents

[0014]

Non-Patent Document 1

Problems to be Solved by the Invention

[0015] An object of the present disclosure is to provide a device for generating multiple wavelengths that overcomes the problems of the prior art. Furthermore, an object of the present disclosure is to provide a device for generating multiple wavelengths that is small, low-cost, and simple.

[0016] Furthermore, an object of the present disclosure is to provide a device for generating multiple wavelengths that provides an ultra-fast optical pulse that can be wavelength-variable.

Means for Solving the Problems

[0017] These and other objects are defined in the claims and are solved by a device for generating multiple wavelengths as described below in the present disclosure. In one aspect, a device for generating multiple wavelengths, a pulse light source configured to emit a series of optical pulses, each of the optical pulses being defined by a central wavelength having a first pulse duration and a first bandwidth, the pulse light source; a first hollow-core optical fiber having a core diameter D, configured to spectrally broaden the series of optical pulses, thereby generating a series of multi-wavelength pulses having a plurality of wavelengths, a first hollow-core optical fiber; and are provided. The first hollow-core optical fiber has a dispersion selected such that the optical pulses are spectrally broadened mainly by self-phase modulation (SPM), and a device is disclosed.

[0018] In this context, as described in the prior art in the background art section, when the spectral broadening is due to SPM, it is well understood that two spectral lobes are generated within the multi-wavelength pulse. Thus, the two spectral lobes characterize the spectral broadening due to SPM. Thus, for example, two spectral lobes, such as separated lobes, one having a shorter wavelength and the other having a higher wavelength and having more power than the others, can be identified in the spectrum after pulse broadening, the broadening is said to be dominated by SPM.

[0019] According to a first aspect, the dispersion is selected such that the first spectral lobe is centered at a first wavelength and the second spectral lobe is centered at a second wavelength. For example, the dispersion may be selected based on simulation or based on experiment.

[0020] Furthermore, according to a first aspect, the first hollow-core optical fiber is an antiresonant hollow-core optical fiber. Furthermore, according to a first aspect, the antiresonant hollow-core optical fiber has a core diameter D that is 80 micrometers or less.

[0021] Anti-resonant hollow-core optical fibers are terms and fibers well understood in the field of optical fibers. Different types of such fibers are presented, for example, in the paper "Design and properties of hollow anti-resonant fibers for the visible and near infrared spectral range" by Walter Belardi, Journal of Lightwave Technology, (USA), November 1, 2015, Vol. 33, No. 21, p. 4583-4590. Thus, in the case of anti-resonant hollow-core optical fibers, the diameter D is also a well-understood term. As understood from the paper by Belardi, anti-resonant hollow-core optical fibers include a plurality of anti-resonant structures (ARS), which can also be called cladding tubes or capillaries. Thus, the terms "cladding tube" and "capillary" are interchangeable. The cladding tube or capillary has a thickness t, which can also be called the capillary wall thickness. The capillary can be a single, double, triple, called a nest, and a free-core boundary where the capillaries do not touch each other, as described by Belardi. Typically, the capillary is circular and thus has an inner diameter d much smaller than the core diameter D, but the capillary can also have other forms such as droplets.

[0022] As described in the background section, the use of anti-resonant hollow-core optical fibers mainly for pulse broadening by SPM is well known, as demonstrated, for example, by Non-Patent Document 5.

[0023] However, achieving pulse broadening mainly by SPM using an anti-resonant hollow-core optical fiber with a core diameter D of 80 micrometers has not been demonstrated or proposed by Non-Patent Document 5. In contrast, to prevent the onset of modulation instability that can shift the pump pulse into the normal dispersion region and degrade the coherence of the compressed pulse, the use of a large core of 86 micrometers has been proposed by Non-Patent Document 5.

[0024] However, the inventors of the present disclosure have found that it is not necessary to use a large core to mainly provide pulse broadening by SPM, and thus the proposed solution is to use a smaller hollow core diameter D of 80 micrometers or less. By doing so, the inventors have found that the normal dispersion region can be achieved by other means, but a complete normal dispersion region is also not mainly required to provide pulse broadening by SPM. This feature and other features are described in the present disclosure.

[0025] Furthermore, using an antiresonant hollow core optical fiber having a core diameter D of 80 micrometers or less provides many advantages as also described in the present disclosure. Generally, the inventors of the present disclosure have found that by using an antiresonant hollow core optical fiber having a core diameter D of 80 micrometers or less, pulse broadening can be mainly provided by SPM so that a device for multi-wavelength generation is provided, whereby it is small, low-cost, and has low complexity.

[0026] The device according to the first aspect is small, low-cost, and has low complexity, and thus has several applications. Therefore, in a second aspect of the present disclosure, - multiphoton excitation, or - activating neurons in the brain, or - deep tissue multiphoton imaging, or - high-frequency voltage imaging in neurons, - ophthalmology, or - laser writing, or - spectroscopy, or - microscopy, or - a system configured for micromachining is provided, The system comprises a device according to the first aspect.

[0027] Further details of the present disclosure are provided in the following description. 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 accompanying drawings.

Brief Description of the Drawings

[0028]

Figure 1

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Mode for Carrying Out the Invention

[0029] The dispersion of the anti-resonant hollow-core optical fiber varies with the core diameter D. As can be seen in FIG. 1, the inventors of the present application performed numerical simulations on this dependency. In the simulation of FIG. 1, the anti-resonant capillary wall thickness was set to 300 nm, the hollow core was filled with argon, and at a pressure of 100 kPa (1 bar), the core diameter D varied from 20 nm to 80 nm. The dispersion (ps / (nm km)) is shown for wavelengths from 750 nm to 1250 nm. FIG. 1 clearly shows that the larger the diameter, the lower the dispersion. Furthermore, FIG. 1 also clearly shows that when the gas has a pressure of 100 kPa (1 bar), all dispersion values exceed zero, meaning that the dispersion is positive. Positive dispersion means that anomalous dispersion exists in the fiber. When dominant SPM is desired, high anomalous dispersion is not desired. This is because high anomalous dispersion leads to modulation instability dominating SPM. To provide pulse broadening when SPM is the dominant effect or when pulse broadening is mainly due to SPM, normal dispersion (i.e., negative dispersion) needs to occur in the hollow-core optical fiber. Anomalous dispersion can be shifted to normal dispersion by increasing the pressure of the gas.

[0030] This is shown in Fig. 2, where the pressure is increased to 1 MPa (10 bar). As can be seen from Fig. 2, in a hollow-core optical fiber with a core diameter D of 80 micrometers or more, normal dispersion can be obtained for wavelengths less than 1000 nm. For this reason, F. Kettig et al. proposed using an antiresonant hollow-core optical fiber with a core diameter of 86 micrometers and a capillary size of 730 nm and a gas pressure of 1 MPa (10 bar). Using a pressure of 1 MPa (10 bar) or less is advantageous because it may be easy to handle a pressure of this magnitude. This is also the reason why F. Kettig et al. used a pressure of 1 MPa (10 bar) in combination with a large core diameter such as 86 micrometers, and also the reason why Yon-Gyun Jon et al. used a pressure of 100 kPa (1 bar) in combination with an even larger core diameter of about 500 micrometers (however, in a non-antiresonant HCF). In other words, by using a large core diameter of the HCF (having a core diameter larger than 86 micrometers), using a long fiber (longer than 3.0 m), and a low gas pressure (lower than 1 MPa (10 bar)) is the most common method that mainly provides pulse broadening by SPM because the particularly low gas pressure provides simple gas handling and the large core diameter provides high power input stability. Therefore, in the field of pulse broadening, it has generally been considered that functions outside these limits do not function properly or at least do not function easily.

[0031] However, the inventors of the present disclosure challenged the common belief and experimented with using a much smaller core diameter, a much shorter fiber length, and a much higher pressure, and found that it was functioning.

[0032] In particular, as described in the summary of the present disclosure, the inventors found that they were able to demonstrate pulse broadening mainly by SPM by providing an antiresonant HCF having a core diameter of 80 micrometers or less.

[0033] This has not been described previously, and further insights into why this works will be described hereinafter. The first step for understanding the basic principle is by simulation. Figure 3 shows the results using an anti-resonant HCF with a diameter varying from 20 micrometers to 80 micrometers, where the capillary wall thickness is set to 300 nm and the hollow core is filled with argon gas at a pressure of 4 MPa (40 bar). It can be seen here that normal dispersion for wavelengths from 750 nm to 1100 nm is achieved for core diameters of 50 micrometers, 60 micrometers, 70 micrometers, and 80 micrometers. Further, it can be seen that using an anti-resonant hollow core fiber with a diameter from 20 micrometers to 40 micrometers has a very low anomalous dispersion at about 1100 nm, with the dispersion being less than 5 nm / (ps km). Further, using an anti-resonant hollow core fiber with a diameter from 20 micrometers to 40 micrometers has an even lower anomalous dispersion at about 900 nm, with the dispersion being less than 3 nm / (ps km) and also being -0.5 for a diameter of 40 micrometers and thus being normal here.

[0034] From these observations, it is concluded that in the range from 900 nm to 1100 nm, the anomalous dispersion is very low and soliton self - compression is limited. Furthermore, the inventors have found that the modulation instability gain band is shifted to a larger detuning and thus this may further limit the onset of MI. Additionally, the inventors have found that the design of the optical fiber (such as using an optimal capillary thickness) can position the MI band into the non - transmission resonance band of the fiber, further weakening the MI effect. Therefore, using an anti - resonant hollow - core optical fiber with a diameter of 80 micrometers or less results in a limited MI effect, whereby SPM can dominate as the main non - linear effect, particularly in the wavelength range from 900 nm to 1100 nm, although as can be seen from FIG. 3, this holds true all the way from 750 nm to 1250 nm. For this reason, the advantage of using an anti - resonant hollow - core optical fiber with a diameter of 80 micrometers or less is that it provides a low dispersion close to anomalous dispersion in the wavelength range from 750 nm to 1250 nm.

[0035] As described above, the capillary thickness affects the MI effect, and thus the pressure of 4 MPa (40 bar) used as an example to show the effect is not the only way to achieve dominant SPM pulse broadening. In fact, even with a lower pressure, a relatively low dispersion can be obtained, so SPM is the dominant effect and, of course, the hollow - core diameter is maintained below 80 micrometers. Other means of providing dominant SPM pulse broadening in an anti - resonant hollow - core fiber with a diameter of 80 micrometers or less, other than using a high pressure such as 4 MPa (40 bar), will be disclosed later in this application. Below, the gas pressure will be explained in a little more detail in order to understand the reasons why a high pressure is also advantageous compared to using a low gas pressure.

[0036] The gas pressure is, as described above, an important parameter in SPM pulse broadening. The reason is that the non - linearity (including SPM and others, so - called X) (3)This is because the (nonlinearity control) increases linearly with pressure. Therefore, by using a high gas pressure, SPM broadening occurs more rapidly, enabling the use of a short fiber length. Furthermore, a high gas pressure can cause the energy to shift to two spectral lobes. In this way, a high gas pressure can provide that the two spectral lobes contain most of the energy, such as more than 40% or more than 50% of the total energy.

[0037] Similar to the general idea that using a large hollow core diameter and a low pressure is the optimal way to obtain SPM broadening, Young-Gyun Jon et al. also reexamined and described the prior art, stating that regardless of the method used, the general strategy is to avoid other harmful nonlinear effects from distorting the spectral phase, and thus, to make the propagation "as minimally nonlinear as possible" in order to achieve a clearly defined output. Furthermore, Young-Gyun Jon et al. described that their method also depends on following the general strategy. This, in combination with using a hollow core fiber having a large core diameter, emphasizes again that using low nonlinearity, and thus low pressure, is the most common strategy for many reasons.

[0038] The inventors of the present disclosure understand other advantages and how it functions, so they do not rely on using the general strategy. One of the great advantages of using a hollow core optical fiber having a core diameter of 8 micrometers or less is that it enables the HCF to operate in or near a few-mode operation. In particular, the HCF enables operation with fewer modes compared to an HCF having a larger core diameter. In this way, by using a hollow core optical fiber having a core diameter of 80 micrometers or less, it is possible to operate without the need to curl the fiber to reduce parasitic modes.

[0039] Furthermore, another advantage of using a hollow-core optical fiber having a core diameter of 80 micrometers or less is that since the length of the fiber scales with its diameter, the length can be reduced to a very small size. In this way, the solution described in the claims provides a device for multi-wavelength generation that is small in size.

[0040] Furthermore, since the length of the fiber scales with its diameter, a third advantage of using a hollow-core optical fiber having a core diameter of 80 micrometers or less is that a high gas pressure is tolerated in a short fiber, which facilitates functionality.

[0041] Thus, a fourth advantage of using a hollow-core optical fiber having a core diameter of 80 micrometers or less is also that it provides low dispersion precisely at or near these wavelengths.

[0042] As an example showing the SPM broadened pulse, the inventors of the present disclosure performed an experimental setup, thereby showing an example of how the present disclosure can be implemented. In that setup, the inventors used an anti-resonant hollow-core optical fiber having an inner core diameter of 50 micrometers, where the length of the fiber was 0.3 m and the hollow core was filled with argon gas at a pressure of 3.6 MPa (36 bar). The pulse duration of the pulse light source was 300 fs and the central wavelength was 1030 nm. The SPM broadened pulse is shown in FIG. 4. Here, curve A clearly shows that the optical pulse is spectrally broadened mainly by self-phase modulation (SPM), thereby generating two spectral lobes within the multi-wavelength pulse. The first spectral lobe is centered around a first wavelength of approximately 1000 nm, and the second spectral lobe is centered around a second wavelength of approximately 1050 nm.

[0043] For comparison, the C and G curves do not show pulses that "are spectrally broadened mainly by self-phase modulation (SPM), thereby generating two spectral lobes within the multi-wavelength pulse". These curves are included here only to show spectra generated with suboptimal pulse energy and having no characteristic or isolated SPM side lobes at the edges.

[0044] As shown in this experimental setup, a high voltage of 40 nm was not used to obtain SPM broadening. It is disclosed below that, in addition to using a high pressure of about 4 MPa (40 bar), an antiresonant hollow-core optical fiber having an inner core diameter of 80 micrometers or less can still be used to obtain SPM broadening. Further features and examples are also disclosed below.

[0045] Hollow-core optical fiber and dispersion In one embodiment, the core diameter D ranges from 30 micrometers to 80 micrometers, preferably from 20 micrometers to 70 micrometers. These ranges have been found to be optimal. For example, as can be seen from FIG. 3, using 20 micrometers or more provides very low dispersion, but the difference between using 70 micrometers and using 80 micrometers is very limited. This shows both optimal conditions between 30 micrometers and 80 micrometers and between 20 micrometers and 70 micrometers.

[0046] In another embodiment, the core diameter D ranges from 30 micrometers to 60 micrometers. This range has also been found to be an optimal condition, if possible. For example, as can also be seen from FIG. 3, this is another optimal condition in that using 30 micrometers or more provides very low dispersion, but the difference between using 60 micrometers and using 70 micrometers is still very limited.

[0047] In yet another embodiment, the core diameter D ranges from 40 micrometers to 50 micrometers. This range may be optimal in terms of providing a good compromise between low dispersion and few mode operations.

[0048] In some embodiments, the core diameter D ranges from 50 micrometers to 80 micrometers. This range may be optimal in that it provides the lowest dispersion that can also be fully normal dispersion for a wide wavelength range, for example from 750 nm to 1100 nm. Referring to, for example, FIG. 3, anomalous dispersion is obtained using high pressure and thin capillary wall thickness.

[0049] As can be understood from the above embodiments, the choice of diameter may depend on various other parameters such as the application or the gas pressure, but may also depend on, for example, the choice of light source. In a preferred embodiment, the dispersion is selected to be less than an absolute value of 20 ps / (nm km) for wavelengths in the range from 900 nm to 1200 nm, such as in the range from 900 nm to 1100 nm. The wavelength range from 900 nm to 1200 nm is an interesting range as it provides many interesting applications as described in connection with the second aspect of the present disclosure. The inventors have found that selecting a dispersion less than an absolute value of 20 ps / (nm km) provides pulse broadening where SPM is the dominant effect.

[0050] In another preferred embodiment, the dispersion is selected to be less than an absolute value of 10 ps / (nm km) for wavelengths in the range from 900 nm to 1200 nm, such as in the range from 900 nm to 1100 nm. This also provides pulse broadening where SPM is the dominant effect and can actually be achieved by using a core diameter D between, for example, 20 micrometers and 80 micrometers even at low gas pressures, see for example FIGS. 1 to 3. Thus, in the most preferred embodiment, the dispersion is selected to be as close to zero as possible, such as less than 2 ps / (nm km) for wavelengths in the range from 900 nm to 1200 nm, such as in the range from 900 nm to 1100 nm.

[0051] In most embodiments, the first hollow-core optical fiber comprises a plurality of capillaries forming an antiresonant structure, and each capillary has a capillary wall thickness t of less than 800 nm, such as less than 500 nm, less than 400 nm, about 300 nm, etc. As described above, the fiber design can be optimized, for example, using the capillary wall thickness to adjust modulation instability. However, the wall thickness also causes resonance at certain wavelengths, so special care must be taken when determining the capillary wall thickness. For example, a capillary wall thickness of 500 nm provides resonances at 1000 nm, 500 nm, 333 nm, and 250 nm. This phenomenon is seen in FIG. 5, where the hollow-core optical fiber is simulated with a core diameter D of 50 micrometers and the hollow core is filled with argon gas at 4 MPa (40 bar). If it is desired to use wavelengths in the range from, for example, 900 nm to 1100 nm, 500 nm is not the optimal choice. For many applications of the devices of the present disclosure, the desired wavelength range is from 900 nm to 1100 nm, and thus, for this wavelength range, FIG. 6 shows how the capillary wall thickness varies the respective dispersion. From FIG. 6, if anomalous dispersion is desired in the described wavelength range, the optimal capillary wall thickness is 300 nm, or near this wall thickness. In certain cases, as simulated, this is of course for a device where the core diameter D is selected to be 50 micrometers and the argon gas is pressurized to 4 MPa (40 bar) inside the core. Thus, in a preferred embodiment, the capillary wall thickness is about 300 nm. Thus, this optimal capillary wall thickness is also shown in the simulations of FIGS. 1-3. Using a relatively thin capillary wall thickness of about 200 nm to 400 nm provides a wider wavelength region with a flat dispersion profile, as can also be seen from FIG. 6, which may be desired in some embodiments. However, using a wall thickness of about 300 nm may not always be optimal. For example, because manufacturing may be difficult, or simply because the wall thickness is relatively thin. In some embodiments, therefore, it may be desirable to use a thicker capillary wall thickness that is still less than 800 nm, such as about 750 nm, 700 nm, or 600 nm.As an example, the dispersion for a hollow core with various diameters and a capillary size of 750 nm, pressurized to 100 kPa (1 bar) with argon gas, over a wide wavelength range from 250 nm to 2250 nm, was calculated by simulation and is shown in FIG. 7. From this figure, in the range from 900 nm to about 1200 nm, the dispersion is positive but less than an absolute value of 20 ps / (nm km) for diameters in the range of 20 to 50 micrometers, and thus it can be seen that it is only slightly anomalous. Also, from this figure, it can be seen that in the range from 900 nm to about 1200 nm, the dispersion does not change much and is flat. Both the low flatness and the low absolute value of the dispersion can be very important parameters for achieving pulse broadening dominated by SPM. Therefore, both the low flatness and the low absolute value of the dispersion in the range from 900 nm to 1200 nm for hollow core fibers with diameters less than 80 nm are obtained not only by using a capillary wall thickness of about 750 nm as shown here, but also by using a capillary wall thickness less than 800 nm such as about 700 nm or 600 nm, so a relatively thick capillary wall thickness is a good solution for obtaining pulse broadening dominated by SPM.

[0052] In some setups, for example, it may be desirable to have a specific polarization in order to increase the sensitivity in the sample plane or the detector plane. Therefore, control of the polarization through the HCF and / or before and after the HCF may be desirable. Thus, in one embodiment, the first hollow core optical fiber is a polarization maintaining fiber.

[0053] In one embodiment, the first hollow-core optical fiber has a length of less than 3.0 m, more preferably less than 2.0 m, even more preferably less than 1.0 m, and most preferably less than 0.5 m, such as less than 0.3 m or less than 0.1 m. The length of the fiber is determined by the core diameter of the fiber, and thus, according to the present disclosure, by requiring that the hollow-core diameter needs to be less than 80 micrometers, in most embodiments, the length of the fiber can be scaled to a corresponding length. As described above, a short fiber length provides a compact device.

[0054] In some embodiments, the first hollow-core optical fiber is filled with air. The inventors of the present disclosure have found that in some embodiments, air can provide pulse broadening dominated by SPM. This embodiment provides a simple and low-cost device that does not require handling and acquisition of special types of gases.

[0055] In other embodiments, the first hollow core optical fiber is filled with a gas different from air, and the gas is preferably a Raman-inactive gas such as argon, helium, neon, krypton, and xenon, or a mixture thereof. These gases, together with air, provide different dispersions to the hollow core optical fiber, as shown in FIG. 8, which is provided by simulating a hollow core optical fiber having a hollow core diameter of 50 micrometers, a capillary wall thickness of 300 nm, and varying the gas type set to a pressure of 2 MPa (20 bar). Different gases have different advantages. From FIG. 8, it can be seen that both air and argon provide approximately the same dispersion profile, indicating that both air and argon are good candidates for providing low absolute dispersion. The use of helium and neon provides a flat dispersion curve over a long wavelength range. The use of krypton provides normal dispersion for the wavelength range from about 750 nm to 1100 nm, while xenon extends the normal dispersion region up to 1350 nm. Since krypton and xenon can have very high costs, in some embodiments, it may be desirable to use less expensive gases such as air, argon, helium, and neon to provide a low-cost device. Alternatively, the use of a Raman-active gas may be used. For example, since a Raman-active gas can shift Raman-active light away from a wavelength range where pulse broadening by SPM is dominant, such a gas under certain conditions can still provide pulse broadening dominated by SPM.

[0056] In other embodiments, the first hollow core optical fiber is filled with a gas different from air, and the gas is preferably a Raman-inactive gas such as argon, helium, neon, krypton, and xenon, or a mixture thereof. These gases, together with air, provide different dispersions to the hollow core optical fiber as shown in FIG. 8, which is provided by simulating a hollow core optical fiber having a hollow core diameter of 50 micrometers, a capillary wall thickness of 300 nm, and varying the gas type set at a pressure of 2 MPa (20 bar). Different gases have different advantages. From FIG. 8, it can be seen that both air and argon provide substantially the same dispersion profile, indicating that both air and argon are good candidates for providing low absolute dispersion. The use of helium and neon provides a flat dispersion curve over a long wavelength range. The use of krypton provides normal dispersion for a wavelength range from about 750 nm to 1100 nm, while xenon extends the normal dispersion region up to 1350 nm. Since krypton and xenon can have very high costs, in some embodiments, it may be desirable to use less expensive gases such as air, argon, helium, and neon to provide a low-cost device. Alternatively, the use of a Raman-active gas may be used. For example, since a Raman-active gas can shift Raman-active light away from a wavelength range where SPM-induced pulse broadening is dominant, such a gas under certain conditions can still provide pulse broadening dominated by SPM.

[0057] As described above, the general method for obtaining pulse broadening by SPM was by using a low pressure of less than 1 MPa (10 bar) because low non-linearity was desired in previous devices. Regarding embodiments related to using a higher pressure of 1 MPa (10 bar) or more, the inventors of the present disclosure propose here to obtain SPM-dominated pulse broadening by a method that does not rely on the general method for achieving it. One way to achieve a higher pressure in the hollow core is, as described in the present disclosure, by reducing the core diameter from the generally used diameter, thereby shortening the length of the fiber. Having a hollow core with gas under pressure, such as in a gas chamber, has several advantages. First, it provides great flexibility in that it can control the gas pressure, thereby controlling the dispersion, thereby creating a spectrum, and thereby making the SPM broadened pulse wavelength-variable. Second, it provides great flexibility in that it can control the non-linearity.

[0058] Accordingly, in a preferred embodiment, the device comprises a hollow core optical fiber and a gas chamber configured to hold a gas at a defined pressure, whereby the hollow core is filled with the gas at the defined pressure. For this reason, a short fiber is preferred for this embodiment. In a more preferred embodiment, the defined pressure is adjustable, whereby a wavelength-variable multi-wavelength device is obtained.

[0059] In the most preferred embodiment, the dispersion is further selected such that the optical pulse is spectrally broadened without significantly changing the temporal envelope of the optical pulse. The reason for this is clearly that if the temporal envelope of the optical pulse is allowed to change significantly, the pulse broadening is no longer dominated by SPM, and thus the generated spectrum no longer provides the characteristic SPM side lobes at the edges of the spectrum within the multi-wavelength range.

[0060] In another preferred embodiment, the dispersion is further selected such that it is selected to substantially prevent soliton self-compression. The reason for this is clearly that if soliton self-compression is allowed to occur significantly (either inside or after the first hollow-core optical fiber), the pulse broadening is no longer dominated by SPM, and thus the generated spectrum no longer provides the characteristic SPM side lobes at the edges of the spectrum within the multi-wavelength range.

[0061] Pulse light source According to a first aspect, the pulse light source is configured to emit a series of optical pulses, each of the optical pulses being defined by a central wavelength having a first pulse duration and a first bandwidth.

[0062] In one embodiment, the central wavelength is less than 1100 nm, such as about 1030 nm. This wavelength can be provided, for example, by a laser having Yb as the gain medium. In another embodiment, the central wavelength is less than 1600 nm, such as about 1550 nm.

[0063] Preferably, the wavelength may be selected to induce SPM in the hollow-core optical fiber. In yet another embodiment, the central wavelength is less than 550 nm, such as about 515 nm. In some embodiments, for example, when the wavelength is about 515 nm, the central wavelength is modified to provide a modified central wavelength that is longer than the central wavelength, mainly before the pulse is broadened by SPM. One way to provide the modified central wavelength is, for example, by using a pulse broadening and compression stage together with a filter for filtering out the modified central wavelength.

[0064] In another embodiment, the central wavelength is greater than 2100 nm, such as about 2460 nm. In some embodiments, for example, when the wavelength is about 2460 nm, the central wavelength is modified to provide a central wavelength that is shorter than the central wavelength before being mainly pulse broadened by SPM. One way to provide the modified central wavelength may be, for example, by using a harmonic conversion unit. The harmonic conversion may be configured to provide, for example, second harmonic generation or third harmonic generation.

[0065] In a preferred embodiment, the pulse duration is less than 1 ps, such as less than 500 fs. In a more preferred embodiment, the pulse duration is between 250 fs and 500 fs. Most preferably, the pulse duration is about 400 fs. By definition, an ultrashort pulse is a pulse with a duration less than 100 ps.

[0066] In one embodiment, the pulse light source further comprises a pulse compression stage, the pulse compression stage comprising a second hollow-core optical fiber configured to increase a first bandwidth to a second bandwidth while forming a linear chirp across each of the optical pulses, and a de-chirping unit configured to compensate for the linear chirp such that a first pulse duration is reduced to a second pulse duration. The technical effect of using the pulse compression stage in combination with a first hollow-core optical fiber having a dispersion selected such that the optical pulses are mainly spectrally broadened by self-phase modulation (SPM) is to achieve SPM broadening at a reduced fiber length. In other words, SPM broadening occurs with higher efficiency, especially as compared to onset effects such as MI, soliton self-compression, and optical wave breaking.

[0067] In related embodiments, the first pulse duration is decreased to the second pulse duration, whereby the shorter input pulse duration gives two broader spectral lobes. Thus, in related embodiments, the second pulse duration causes the first spectral lobe and the second spectral lobe to expand. This further provides a higher conversion efficiency and an even shorter pulse duration at the first wavelength and the second wavelength at which the two spectral lobes are located. Since a higher conversion efficiency is obtained, it is possible to achieve pulse broadening dominated by SPM using a low pressure such as about 100 kPa (1 bar) to 2 MPa (20 bar). Thus, this embodiment is very practical in many applications where short pulses with multiple wavelengths are required even at low gas pressures.

[0068] In most embodiments, the de-chirp unit is selected to comprise one or more of a chirp mirror, a grating, a prism, and a fiber. Using a grating provides tuning of the wavelength. Thus, the grating may be an alternative to adjusting the pressure of the gas to achieve wavelength variability or may be an add-on to wavelength variability.

[0069] In one embodiment, the first pulse duration is more than twice the second pulse duration, such as more than three times the second pulse duration, such as more than four times the second pulse duration. For example, in the latter case, the first pulse duration may be about 400 fs and the second pulse duration may be 100 fs.

[0070] SPM Broadening Spectrum and Filter Unit In one embodiment, the first wavelength is less than 980 nm, such as about 920 nm or 900 nm, and the second wavelength is greater than 1020 nm, such as about 1100 nm. The first wavelength of about 900 nm may be used for optical stimulation in some applications, for example, to avoid crosstalk with the imaging excitation path. The first wavelength of about 920 nm may be used for wide-field two-photon imaging of Ca or for voltage indicators in some applications. Since the two spectral lobes are spectrally separated from each other, the first wavelength of about 920 nm and the second wavelength of about 1100 nm may be used for neural activation and inhibition.

[0071] In another embodiment, the first wavelength may be located in the range of about 410 nm to 450 nm. The first wavelength in this vicinity can be used for ophthalmology in some applications. In most embodiments, the device further comprises a filter unit configured to filter out at least a first portion of the first spectral lobe and at least a second portion of the second spectral lobe. This filter unit may depend on the application and thus may be specified according to the application.

[0072] In a preferred embodiment, the first portion is a first band-filtered portion having a bandwidth of 10 nm to 30 nm, and the second portion is a second band-filtered portion having a bandwidth of 10 nm to 30 nm.

[0073] Applications According to a second aspect, - Multiphoton excitation, or - Activating neurons in the brain, or - Deep tissue multiphoton imaging, or - High-frequency voltage imaging in neurons, - Ophthalmology, or - Laser writing, or - Spectroscopy, or - Microscopy, or - A system for micromachining is disclosed, The system comprises a device according to the first aspect as described in detail above. Some of the applications with specific wavelengths for the two spectral lobes having specific purposes have already been described above by way of example. The system described above may be configured to transmit the two spectral lobes onto / into an object / sample.

[0074] In most embodiments of the second aspect, the system comprises a filter unit configured to filter out at least a first portion of the first spectral lobe and configured to filter out at least a second portion of the second spectral lobe.

[0075] In one embodiment of the second aspect, the light having the band-filtered portion is modulated independently after filtering. In another embodiment of the second aspect, the light having the band-filtered portion is separated into two hollow-core optical fibers. In this way, the two hollow-core optical fibers can be used as delivery fibers to a sample / object illuminated by the filtered light.

[0076] In yet another embodiment, the light having the band-filtered portion is coupled into a single hollow-core optical fiber. In this way, the single hollow-core optical fiber can be used as a delivery fiber to a sample / object irradiated by the filtered light. In one embodiment of the second aspect, the light having the band-filtered portion is modulated independently after filtering.

[0077] In a preferred embodiment of the second aspect, the system comprises a modulator configured to turn the light source on and off. The modulator may be in the form of, for example, an acousto-optic modulator (AOM).

[0078] Two examples of the system will now be described in more detail below. First Embodiment of the System According to the Second Aspect of Example 1 FIG. 10 shows an embodiment of the system according to the second aspect. The system includes a device according to the first aspect. The device includes a pulse light source 1 configured to emit a series of optical pulses, each of the optical pulses being defined by a central wavelength having a first pulse duration and a first bandwidth. The system further includes a first hollow-core optical fiber 2 having a core diameter D, the first hollow-core optical fiber being configured to spectrally broaden a series of optical pulses, thereby generating a series of multi-wavelength pulses having a plurality of wavelengths. The first hollow-core optical fiber 2 has a dispersion selected such that the optical pulses are spectrally broadened mainly by self-phase modulation (SPM), thereby generating two spectral lobes within the multi-wavelength pulse. The first spectral lobe is centered at a first wavelength, the second spectral lobe is centered at a second wavelength, the first hollow-core optical fiber 2 is an anti-resonant hollow-core optical fiber, and the core diameter d is 80 micrometers or less. The system includes a filter unit 3 configured to filter out at least a first portion of the first spectral lobe and configured to filter out at least a second portion of the second spectral lobe. The light having the band-filtered portion is split into two hollow-core optical fibers 4 and 5. In this way, the two hollow-core optical fibers 4 and 5 can be used as delivery fibers to a sample / object 6 illuminated by the filtered light.

[0079] In this embodiment, the system, more specifically, comprises a laser source 1 that provides 400 fs pulses at 1 MHz and 50 MW peak power. The light is launched into an anti-resonant hollow-core fiber (AR-HCF) 2 having a core diameter of 50 μm and a capillary wall thickness of 760 nm, thereby inducing an absolute value of dispersion of less than 10 ps / nm / km that is anomalous at a second anti-resonant band between 850 and 1500 nm and a pressure of 100 kPa (1 bar). The AR-HC fiber 2 is gas-filled with argon up to a pressure of 3 - 4 MPa (30 - 40 bar), shifting the dispersion in the 900 - 1100 nm band, in part or completely, to a smaller absolute value, either in the anomalous or normal dispersion regime. The length of the fiber 2 is 15 cm.

[0080] The light propagates through the AR-HC fiber 2 in a highly nonlinear and low-dispersion environment. For normal dispersion, this limits the onset of wave breaking, while for anomalous dispersion, soliton self-compression is limited and the MI gain band is shifted to a larger detuning that can also limit the MI onset. An optimal design of the optical fiber positions the MI band within the non-transmission resonance band of the fiber 2, further weakening its effect. All of the above non-linearities are limited, enabling self-phase modulation (SPM) to dominate as the main non-linear effect.

[0081] The pulse broadens from a bandwidth of <5 nm to 920 - 1100 nm through SPM and has a characteristic SPM structure with edge lobes containing high parts of the pulse energy. The light is emitted from the fiber 2 and heads towards the filter unit. In this embodiment, the filter unit comprises two band-pass filters, each band-pass filter having 10 - 20 nm centered on the edge lobes of the SPM broadened spectrum. The filtering element spatially separates the light at 920 nm from the light at 1100 nm, which can be both measured to have a duration of about 200 fs and contain 10 - 20% of the laser source power.

[0082] The two filtered optical bands can be focused into AR-HCFs (4 and 5) of any length for direct beam delivery to sample 6. Alternatively, the beam can be delivered in free space using mirrors to direct it towards the sample. If a delivery fiber is used, the fiber has a pressure of <100 kPa (1 bar) to prevent non-linear effects and dispersion broadening of the pulses. Specifically, the Kerr or cubic effect and the Raman effect are mitigated and the absolute value of the dispersion becomes <1 ps / (nm km).

[0083] Second Embodiment of the Device According to the First Aspect of Example 2 FIG. 11 shows an embodiment of a system according to a second aspect. The system comprises a device according to the first aspect. The device comprises a pulse light source 1 configured to emit a series of optical pulses, each of the optical pulses being defined by a central wavelength having a first pulse duration and a first bandwidth. The system further comprises a first hollow-core optical fiber 2 having a core diameter D, the first hollow-core optical fiber being configured to spectrally broaden a series of optical pulses, thereby generating a series of multi-wavelength pulses having a plurality of wavelengths, the first hollow-core optical fiber 2 having a dispersion selected such that the optical pulses are spectrally broadened mainly by self-phase modulation (SPM), thereby generating two spectral lobes within the multi-wavelength pulse, the first spectral lobe centered on a first wavelength and the second spectral lobe centered on a second wavelength, the first hollow-core optical fiber 2 being an anti-resonant hollow-core optical fiber and having a core diameter d of 80 micrometers or less. The system comprises a filter unit 3 configured to filter out at least a first portion of the first spectral lobe and configured to filter out at least a second portion of the second spectral lobe. The light having the band-filtered portion is split into two hollow-core optical fibers 4 and 5. In this way, the two hollow-core optical fibers 4 and 5 can be used as delivery fibers to a sample / object 6 illuminated by the filtered light. This is similar to the setup shown in FIG. 10, but FIG. 11 here has a light source 2 including a pulse compression stage, the pulse compression stage being configured to increase the first bandwidth to a second bandwidth while forming a linear chirp across each of the optical pulses, the second hollow-core optical fiber 7, and a de-chirp unit 8 configured to compensate for the linear chirp, thereby reducing the first pulse duration to a second pulse duration, the de-chirp unit 8 being selected to include two chirp mirrors.

[0084] In this example, the system, more specifically, comprises an AR-HCF7 in a pulse compression stage that has a core diameter of 50 μm, operates at a length of 10 cm and a pressure of 100 kPa (1 bar), but may have higher or lower pressures mainly to control non-linearity. The pulses in the AR-HCF7 of the compression stage are used to increase the bandwidth from ~3.9 nm to 16 nm. Next, the chirp mirror 8 compensates for the linear chirp accumulated over the central region of the pulse (with respect to the frequency domain) and reduces the pulse duration from 400 fs to 100 fs.

[0085] The pulses launched into the AR-HCF2 for SPM broadening (coming from the pulse compression stage) have a pulse duration of 100 fs and thereby have a peak power of 200 MW. SPM broadening occurs with higher efficiency compared to onset effects such as MI, soliton self-compression, and optical wave breaking. This is mainly due to the SPM effect that achieves complete expansion over a short and reduced fiber length.

[0086] Shorter input pulse durations also give broader lobes at 920 nm and 1100 nm, which provide higher conversion efficiency and shorter pulse durations at 920 nm and 1100 nm.

Claims

1. A device for generating multiple wavelengths, comprising: A pulse light source configured to emit a series of optical pulses, each of the optical pulses being defined by a central wavelength having a first pulse duration and a first bandwidth; A first hollow-core optical fiber having a core diameter D, configured to spectrally broaden the series of optical pulses, thereby generating a series of multi-wavelength pulses having multiple wavelengths; The first hollow-core optical fiber has a dispersion selected such that the optical pulses are spectrally broadened mainly by self-phase modulation (SPM); Thereby, two spectral lobes are generated within the multi-wavelength pulses; The first spectral lobe is centered on a first wavelength; The second spectral lobe is centered on a second wavelength; The first hollow-core optical fiber is an anti-resonant hollow-core optical fiber; The device, wherein the core diameter D is 80 micrometers or less.

2. The device according to claim 1, wherein the core diameter D is preferably in the range of 20 micrometers to 70 micrometers, more preferably in the range of 30 micrometers to 60 micrometers, still more preferably in the range of 40 micrometers to 50 micrometers, or in the range of 50 micrometers to 80 micrometers.

3. The dispersion is selected to be less than an absolute value of 20 ps / (nm km) for wavelengths in the range of 900 nm to 1200 nm, such as in the range of 900 nm to 1100 nm. More preferably, the dispersion is selected to be less than an absolute value of 10 ps / (nm km) for wavelengths in the range of 900 nm to 1200 nm, such as in the range of 900 nm to 1100 nm. The device according to claim 1 or 2.

4. The device according to any one of claims 1 to 3, wherein the first hollow-core optical fiber comprises a plurality of capillaries forming an anti-resonant structure, each of the capillaries having a capillary wall thickness t less than 800 nm, such as less than about 500 nm, such as less than about 400 nm, such as about 300 nm.

5. The first hollow-core optical fiber of claim 1 to 4 has a length of less than 3.0 m, more preferably less than 2.0 m, even more preferably less than 1.0 m, and most preferably less than 0.5 m, such as less than 0.3 m or less than 0.1 m, of the device according to any one of claims 1 to 4.

6. The first hollow-core optical fiber is filled with air, or the first hollow-core optical fiber is filled with a gas different from air, and the gas is preferably a Raman-inactive gas such as argon, helium, neon, krypton, and xenon, or a mixture thereof, of the device according to claim 1.

7. The gas or the air is pressurized to a pressure of 1 MPa or more, preferably about 2 MPa or more, more preferably 3 MPa or more, even more preferably 4 MPa or more, and most preferably 5 MPa or more, of the device according to claim 6.

8. The pulsed light source further comprises a pulse compression stage, The pulse compression stage is, A second hollow-core optical fiber configured to increase the first bandwidth to a second bandwidth while forming a linear chirp across each of the optical pulses, A de-chirp unit configured to compensate for the linear chirp, thereby reducing the first pulse duration to a second pulse duration, of the device according to any one of claims 1 to 7.

9. The de-chirp unit is, A chirp mirror, A grating, A prism, and One or more of the fibers, selected to be provided, of the device according to claim 8.

10. The first pulse duration is more than 3 times, such as more than 4 times, the second pulse duration, more than 2 times the second pulse duration, of the device according to claim 8.

11. The second pulse duration causes the first spectral lobe and the second spectral lobe to expand, of the device according to any one of claims 8 to 10.

12. The first wavelength is less than 980 nm, such as about 920 nm, and the second wavelength is greater than 1020 nm, such as about 1100 nm, of the device according to any one of claims 1 to 11.

13. A filter unit configured to filter out at least a first portion of the first spectral lobe and at least a second portion of the second spectral lobe, the device according to any one of claims 1 to 12, further comprising the filter unit.

14. The first portion is a first band-filtered portion having a bandwidth of 10 nm to 30 nm, and the second portion is a second band-filtered portion having a bandwidth of 10 nm to 30 nm, the device according to claim 13.

15. The dispersion is further selected such that the optical pulse is spectrally broadened without significantly changing the temporal envelope of the optical pulse, or is selected to significantly prevent soliton self-compression, or both, the device according to any one of claims 1 to 14.

16. A system comprising Multiphoton excitation, or Activating neurons in the brain, or Deep tissue multiphoton imaging, or High-frequency voltage imaging in neurons, or Laser writing, or Spectroscopy, or Microscopy, or Microfabrication, and is configured for A system comprising the device according to any one of claims 1 to 15.

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