ULTRAFAST LASER SOURCES AND METHOD

Abstract

There is provided an ultrafast laser source and a method for fabrication thereof, the system comprising a waveguide module and a compression module, wherein the waveguide module generates pulses of multidimensional solitary states from ultra-short-laser pulses and the compression module compresses the pulses of multidimensional solitary states at the output of the waveguide module, the method comprising generating pulses of multidimensional solitary states from ultrashort laser pulses; and compressing the pulses of multidimensional solitary states.

Claims

1. An ultrafast laser source, comprising: a waveguide module; and a compression module. wherein said waveguide module generates pulses of multidimensional solitary states from ultrashort-laser pulses and said compression module compresses the pulses of multidimensional solitary states at the output of the waveguide module.

2. The ultrafast laser source of claim 1, wherein said waveguide module comprises a hollow core waveguide of a core diameter of at least the wavelength of the ultrashort laser pulses.

3. The ultrafast laser source of claim 1, wherein said waveguide module comprises a hollow core waveguide of a core diameter comprised in a range between 50 microns and 10 mm.

4. The ultrafast laser source of claim 1, wherein said waveguide module comprises a hollow core waveguide of a core diameter of at least the wavelength of the ultrashort laser pulses, said waveguide being one of: rod hollow core fibers, stretched hollow core fibers, hollow core photonics crystal fibers and planar hollow waveguides.

5. The ultrafast laser source of claim 1, wherein said compression module comprises one of: a gas and a glass.

6. The ultrafast laser source of claim 1, comprising a power scaling module, said power scaling module controlling the chirp of the ultrashort laser pulses.

7. The ultrafast laser source of claim 1, comprising a power scaling module, said power scaling module controlling the chirp of the ultrashort laser pulses using at least one of: gratings, prisms, pulse shapers, deformable mirrors, wave shapers and chirped mirrors.

8. The ultrafast laser source of claim 1, comprising a spatial coupling module, said spatial coupling module controlling spatial coupling of ultrashort laser pulses to said waveguide module.

9. The ultrafast laser source of claim 1, comprising a spatial coupling module, said spatial coupling module controlling spatial coupling of the ultrashort laser pulses to said waveguide module using one of: focusing elements, spatial light modulators and deformable mirrors.

10. The ultrafast laser source of claim 1, wherein the ultrashort laser pulses have a pulse duration of at most 100 picoseconds and a pulse energy of at least 1 microjoule.

11. A method for generating high intensity ultrafast pulses, comprising generating pulses of multidimensional solitary states from ultrashort laser pulses; and compressing the pulses of multidimensional solitary states.

12. The method of claim 11, comprising generating the pulses of multidimensional solitary states in a hollow core waveguide of a core diameter of at least the wavelength of the ultrashort laser pulses.

13. The method of claim 11, comprising generating the pulses of multidimensional solitary states in a hollow core waveguide of a core diameter comprised in a range between 50 microns and 10 mm.

14. The method of claim 11, comprising generating the pulses of multidimensional solitary states in a hollow core waveguide of a core diameter comprised in a range between 50 microns and 1 mm.

15. The method of claim 11, comprising generating the pulses of multidimensional solitary states in a hollow core waveguide of a core diameter of at least the wavelength of the ultrashort laser pulses, the waveguide being one of: rod hollow core fibers, stretched hollow core fibers, hollow core photonics crystal fibers and planar hollow waveguides.

16. The method of claim 11, comprising compressing the pulses of multidimensional solitary states using one of: a gas and a glass.

17. The method of claim 11, comprising controlling the chirp of the ultrashort laser pulses.

18. The method of claim 11, comprising controlling the chirp of ultrashort laser pulses using at least one of: gratings, prisms, pulse shapers, deformable mirrors, wave shapers and chirped mirrors.

19. The method of claim 11, comprising generating the pulses of multidimensional solitary states in a hollow core waveguide of a core diameter of at least the wavelength of the ultrashort laser pulses, the method comprising controlling spatial coupling of the ultrashort lasers to the hollow core waveguide.

20. The method of claim 11, wherein the ultrashort laser pulses have a pulse duration of at most 100 picoseconds and a pulse energy of at least 1 microjoule.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In the appended drawings:

[0016] FIG. 1A is a schematic view of a system according to an embodiment of an aspect of the present disclosure;

[0017] FIG. 1B is a schematic view of power scaling according to an embodiment of an aspect of the present disclosure;

[0018] FIG. 1C is a schematic view of a detail of the system of FIG. 1A according to an embodiment of an aspect of the present disclosure;

[0019] FIG. 1D is a schematic view of compression according to an embodiment of an aspect of the present disclosure;

[0020] FIG. 2 shows the evolution of self-trapped solitary states with enhanced nonlinear spatiotemporal interactions in nitrogen-filled HCF;

[0021] FIG. 3A shows measured spectra for 700 fs input pulses after an N.sub.2-filled HCF (3 bar) with enhanced red-shifted spectra through stimulated Raman scattering (SRS): the output spectra when the HCF is filled with Ar (dark curve extending up to 900 nm), 1D simulation (dashed curve extending from 725 nm to 825 nm); the input spectra (lighter full curve), and the experimental spectra (dark curve extending up to 1100 nm);

[0022] FIG. 3B shows multidimensional solitary states (MDSS) spatial profiles obtained using an 830 nm long-pass filter after the fiber, for 8 single-shot consecutive pulses;

[0023] FIG. 3C shows the spatial profile of the laser beam at the output of the HCF after a band-pass filter; 760-790 nm;

[0024] FIG. 4A shows the Wigner function of the retrieved second-harmonic generation-frequency-resolved optical gating (SHG-FROG) trace, directly measured at the output of the HCF;

[0025] FIG. 4B shows the Wigner function of the retrieved second-harmonic generation-frequency-resolved optical gating (SHG-FROG) trace, directly measured after pulse compression with glass;

[0026] FIG. 4C shows the measured spectra (full line), the spectra predicted from a multidimensional simulation (dot line), and the retrieved spectra from the second-harmonic generation-frequency-resolved optical gating (SHG-FROG) measurements of the compressed pulses (stipple line); and

[0027] FIG. 4D shows the temporal profile of the sub-picosecond pulses before the hollow core fiber (in shade); the measured temporal profile of the compressed pulses after the HCF using second-harmonic generation-frequency-resolved optical gating (SHG-FROG) demonstrating 10.8 fs pulse duration (stipple line), the Fourier transform (FT) of the output spectra (dash-dot line), and the predicted pulse temporal profile from a multidimensional simulation (MD) (dot line).

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0028] The present invention is illustrated in further detail by the following non-limiting examples.

[0029] FIG. 1 illustrate spatiotemporal nonlinear enhancement according to an embodiment of an aspect of the present disclosure with a high-energy spatiotemporal laser source system using multidimensional interaction in a gas filled-hollow core fiber for spatiotemporal nonlinear enhancement.

[0030] As illustrated in FIG. 1A, the system comprises a spatial a waveguide module 50 and a compression module 60.

[0031] The ultra-short pulse laser source 10 to be compressed typically emits pulses with pulse duration below 100 picoseconds and pulse energy above 1 microjoule.

[0032] The waveguide module 50 comprises a hollow core waveguide selected with a hollow core diameter at least the wavelength of the ultra-short pulse laser source 10 to be compressed. The waveguide module 50 allows multidimensional interactions independently of specific waveguides. A range of hollow core waveguides may be used, such as rod HCFs, stretched HCFs, hollow core photonics crystal fibers (HC-PCF) and planar hollow waveguides, for example.

[0033] At the output of the waveguide module 50, the compression module 60 compresses the negatively chirped broadband output pulses of multidimensional solitary states (MDSS) of quadratic spectral phase to the Fourier transform-limited pulse duration. In the case of ultrafast laser sources 10 such as Ti:Sa and Yb lasers for example, a window of solid material such as fused silica or calcium fluoride for example may be used to directly compress the output pulses to few-cycle duration for example (FIG. 1D). Alternatively, the pulses may be compressed by propagation through a gas, such as dry air, dry nitrogen, or through a noble gas such as helium, for example, to avoid absorption in the mid-IR, regardless of their wavelength since gas provide positive chirp.

[0034] A power scaling module 30 is used to pre-chirp the input laser pulses thereby increasing the energy per pulse and reducing their input peak power. The power scaling module thus controls the input chirp of the pulses emitted by the laser source 10, using gratings, prisms, pulse shapers such as Dazzlers, deformable mirrors, wave shapers or chirped mirrors for example, without countering fundamental limitations such as gas ionization and beam collapse. The energy per pulse is thus increased at the output of the waveguide module by spatiotemporal nonlinear enhancement, leading to higher peak power, as shown in FIG. 1B by a longer arrow in the right handside compared to conventional Kerr nonlinear enhancement (shown with smaller arrow).

[0035] The hollow core waveguide is positioned using a multi-axis stage for movement and tilt relative to the incident beam to adjust to the initial spatial condition in a spatial coupling module 40. A focusing element such as a lens, a spatial light modulator, or deformable mirrors may be used for wavefront shaping to control spatiotemporal nonlinear interactions at the input coupling fiber and spectral information as feedback. The spatial coupling module 40 thus controls nonlinearity to balance diffraction and dispersion and achieves beam confinement at laser powers matching critical power for self-focusing. Thus, the spatial coupling module 40 controls spatial coupling to target spatiotemporal nonlinear enhancement and beam confinement.

[0036] Spatiotemporal nonlinear enhancement is achieved using shorter interaction length of fiber L.sub.2 than in conventional devices (L.sub.2<L.sub.1 in FIG. 1C); for example, a rod HCF of a length of about 1 meter may be used as compared to typically between about 3 and 6 meters of stretched HCF for compression of Yb laser with a duration of few hundreds of fs, which increases compactness of the system. The compression module 60 achieves multidimensional solitary states (MDSS) compression, which may be used in any spectral region, as a low-cost route for few-cycle pulse generation. In comparison, conventional spectral broadening devices using self-phase modulation (SPM) and multiple chirped-mirror sets with negative chirp to post-compress pulses (FIG. 1D) are complex and costly; besides, chirped mirrors are typically designed and manufactured only for a specific wavelength range.

[0037] The resulting system is compact and robust. The system delivers high-energy few-cycle pulses in a broad range of spectral regions, from below 200 nm to above 20000 nm, for example between about 200 nm and about 20000 nm, and maybe pumped with different lasers of pulse duration below 100 picoseconds with pulse energy above 1 microjoule, for example in the range between about 1 microjoule and about 1 Joule, accordingly. It may be pumped with different laser drivers at a number of wavelength such as titanium sapphire (Ti:Sa), ytterbium (Yb), fiber, slab, or disk, thulium, chromium, holmium and carbon dioxide (CO.sub.2) lasers or parametric devices such as optical parametric amplification (OPA), optical parametric chirped pulse amplification (OPCPA), and difference-frequency generation (DFG) (see FIG. 1A). Spatiotemporal effects are controlled by the spatial coupling module. The system may selectively enhance the nonlinearity by selecting combinations of hollow-core waveguides and gas medium for multidimensional nonlinear interactions. The beam at the output of the hollow-core waveguide may be controlled to have pure negative chirp for efficient direct compression to few-cycle pulse duration.

[0038] FIG. 2 schematically illustrates optimizing the focal spot size at the input of a large core HCF for obtaining high coupling efficiency into the fundamental mode. Under the weak guiding condition in the HCF, modes fall into groups of linearly polarized modes (LP.sub.mn). Self-focusing in the HCF corresponds to a transfer of energy from the fundamental mode (LP.sub.01) to higher LP.sub.mn modes, which have rotational symmetry with maximum intensity in the central part. In the high-intensity regime, typically in the range between about 10.sup.11 and about 10.sup.13 W/cm.sup.2, the effects of self-focusing and diffraction become significant and multiple LP.sub.On modes are created upstream of the fiber. These extended multidimensional pathways lead to Raman enhancement, and a resulting nonlinearity balances the potential diffraction. This nonlinear self-trapping mechanism, together with the ultra-low modal dispersion inherent to large core HCFs, causes the generated multimode beam to undergo efficient red-shifted spectral broadening through cascaded stimulated Raman scattering (SRS).

[0039] In an experiment, 700 fs high-energy pulses 5 mJ/pulse with a central wavelength of 780 nm were launched into a 3 m long, 500 μm hollow-core fiber filled with nitrogen. The 700 fs pulses were obtained by positively chirping 40 fs pulses from a Titanium-Sapphire amplifier. At the HCF output, coherent solitary states located at the leading edge of the initial driver pulse with a negative quadratic spectral phase were separated from the pump part by using a long-pass filter (above 830 nm). The pump part was selected using a band-pass filter (760 nm-790 nm). The broadband output pulses (above 830 nm) were subsequently compressed using a window of fused silica (FS).

[0040] Evolution of self-trapped solitary states with enhanced nonlinear spatiotemporal interactions in the nitrogen-filled HCF is shown in FIG. 2, schematically illustrating generation and propagation of the multimode beam (above the fiber), these multimodes composed of LP.sub.On modes being created upstream of the fiber as schematically shown on the left handside. These states are self-trapped in the LP.sub.On modes family with periodic energy exchange while propagating through the HCF. The evolution of the spectra is schematically depicted (below the fiber), starting with the input spectra on the left handside, to the broadened red-shifted spectra on the right handside. The broadband red-shifted spectra originate from enhanced cascaded stimulated Raman scattering due to multidimensional interactions over the interaction length L of the large core HCF. The output broadband pulses schematically shown on the far right handside have a negative chirp and are compressed by linear propagation in a window of fused silica, with normal dispersion below 1.3 microns. High-order harmonic generation (HHG) is used to confirm the spatiotemporal quality and peak power of the red-shifted output pulses.

[0041] FIG. 3A shows measured spectra for 700 fs input pulses after an N.sub.2-filled HCF (3 bar) with enhanced red-shifted spectra through stimulated Raman scattering (SRS), 1D simulation (dashed curve extending from 725 nm to 825 nm); the input spectra (lighter full curve), the output spectra when the HCF is filled with Ar (dark curve extending up to 900 nm), experimental spectra (dark curve extending up to 1100 nm). A self-trapped, highly stable, and clean beam is obtained. The red-shifted spectra in Raman active gas such as N.sub.2, N.sub.2O, or CO.sub.2 for example have stable spatial properties at peak powers comparable to the critical power for self-focusing. The stability of the self-trapped output beam is demonstrated in the multidimensional solitary states (MDSS) spatial profiles obtained using a 830 nm long-pass filter after the fiber, with 8 consecutive single-shot far-field images shown in FIG. 3B. FIG. 3C indicates strong spatial instability and coupling to higher-order modes (HOMs) near the input pump wavelength.

[0042] The Wigner function was used for time-frequency analysis of the output beam to investigate temporal properties. FIG. 4 show the temporal evolution of the self-trapped output beam driven by 5 mJ 700 fs pulses at 2500 mbar. FIG. 4A shows the Wigner function of the output pulses from the reconstructed second-harmonic generation-frequency-resolved optical gating (SHG-FROG) trace, where the dominant negative quadratic spectral phase at the leading edge of the pulse contains the majority of the energy, indicating the localization of the output beam at the leading edge of the driver pulse with negative chirp. Consequently, the beam was post-compressed down to 10.8 fs by propagating through a window of fused silica (FS) glass, with positive group velocity dispersion (GVD) below 1.3 microns (FIG. 4B). The corresponding spectra are presented FIG. 4C. FIG. 4D shows the temporal characterization of the compressed pulses. The ratio between the pump pulse duration and the compressed red-shifted pulse duration, of a value of −65, is much higher than the corresponding ratio achieved with pulse compression based on Kerr nonlinearities under the same HCF geometry. This demonstrates a compression ratio, defined as the value of the input pulse duration divided by the output pulse duration, of 65.

[0043] Thus, based on spatiotemporal Raman enhancement, a highly stable multimode beam is generated by inducing a phase-mismatch for intermodal four wave mixing using hollow core fiber (HCF) of a core diameter of at least the wavelength of the laser source, for example in a range between 50 microns and 10 mm, for example between 50 microns and 1 mm as presently commercially available.

[0044] In FIG. 3A, the measured spectra for the 700 fs input driver after the N.sub.2-filled HCF (3 bar) was obtained with different input pulse energies in a range between about 1 millijoule and about 15 millijoules by varying the pressure of N.sub.2 between about 5 bars and 500 mbars, showing versatility and adaptability of the system. For lower pulse energy, down to about 100 microjoules, similar spectra and pulse compression ratio were obtained with N.sub.2O using a 1 m long 250 microns core diameter rod HCF. The expected spectrum from a 1D simulation, and the experimental spectra for argon was determined at the same pressure. Although HCF is intrinsically considered as step-index fibers, the modal dispersion and losses scale inversely with the second and third power of the core size, respectively, and hollow core fibers (HCFs) provide ultra-low modal dispersion and small losses. In experiments, 700 fs, high-energy pulses (5 mJ/pulse) with a central wavelength of 780 nm were used as a pump, and the pump was focused on a spot size of −330 μm and coupled into a 3-meter-long HCF with 500 μm core diameter stretched between two holders. A translation stage was used at the fiber entrance for alignment at both ends and for optimizing the mode quality at output. Optimization of the coupling conditions was achieved by feedback adjustment of input coupling conditions while monitoring the output spectrum. The HCF was filled by nitrogen (N.sub.2) at the pressure of 2500 mbar.

[0045] There is thus provided a multimodal tunable laser source using spatiotemporal nonlinear enhancement via strong nonlinear mode-coupling and a platform such as molecular gas-filled HC-PCF, hollow core fiber (HCF), and planar hollow waveguides, operating for a wide wavelength range extending from below 200 nm to higher than 20000 nm, for example between about 200 nm and about 20000 nm, and high energy regime, in the range between about 1 microjoule and about 1 Joule depending on the diameter of the HCF and on the laser wavelength. For example, high-energy CO.sub.2 laser may be compressed using a large core HCF or planar waveguide. Besides, nonlinearity and dispersion may be tuned by varying the pressure and gas mixture. A wide window of transparency extending from the vacuum ultraviolet to the mid-infrared spectra range is obtained.

[0046] There is provided a method for controlling efficient, tunable, and power scalable multidimensional interactions with the ability to create stable high-energy multidimensional states.

[0047] There is provided a system operated for pulse energy of at least 1 microjoule, in a range of energy between about 1 microjoule and about 1 Joule of pulse energy for example, depending on the diameter of the waveguide and the laser wavelength, by selecting the waveguide geometry, generating a highly stable and localized output beam.

[0048] There is provided a compact, tunable, and power scalable ultrafast laser source based on spatiotemporal nonlinear enhancement. The present system and laser source provide broadband red-shifted spectra with the capability to continuously tune the central wavelength, independently of the driver wavelength, as alternatives to OPA for intense ultra-broadband long-wavelength infrared/mid-infrared driver sources. Furthermore, the s present system and laser source generate few-cycle pulses with a high degree of temporal coherency in a single HCF, as opposed to cascaded hollow core fibers (HCF) arrangement or multiple meters long hollow core fibers (HCF) as typically needed in the current state of art for high energy pulse compression. The present system and laser source may be pumped directly using commercial Yb, Tm, and Ho lasers with picosecond (ps) pulse durations for example. The system may efficiently operate in ps regime, and is scalable to power beyond the fundamental limitations of the current systems by pre-chirping the input pulses.

[0049] The present method. system and laser source allow a high degree of controllability based on spatiotemporal nonlinear effects, continuous tunability of the central wavelength of the output beam, high spatial quality, and confinement for output beam; clean negative chirp for output pulses resulting in efficient and direct post-compression to few-cycle pulse duration.

[0050] The scope of the claims should not be limited by the embodiments outlined in the examples but should be given the broadest interpretation consistent with the description.