US-12619125-B2 - Low-threshold supercontinuum generation in bulk dielectrics and semiconductors

Abstract

Controlling a low-threshold femtosecond supercontinuum (fs SCG) in a bulk nonlinear material (BNLM) with a positive thermo-optic coefficient (dn/dT>0 K −1 ) is provided by coupling light at a first wavelength output by a fs oscillator at a full pulse repetition PRR into the BNLM. The coupling of light produces a nonlinear lens of the coupled beam in the BNLM which is insufficient to provide intensity of the light sufficient to reach ta threshold of the fs SCG. To raise the pulse energy and reach the SCG threshold, light at a second wavelength different from the first wavelength is absorbed in the BNLM to form a thermal lens in the BNLM which assist the nonlinear lens in creating the SCG.

Inventors

• Sergey Vasilyev
• Mike Mirov
• Igor Moskalev

Assignees

• IPG PHOTONIC CORPORATION

Dates

Publication Date

20260505

Application Date

20211229

Claims (20)

1. 1 . A method of controlling femtosecond supercontinuum (fs SCG) in a bulk nonlinear material (BNLM) with a positive thermo-optic coefficient (dn/dT>0 K −1 ), comprising: coupling light, which is emitted by a fs pulse oscillator at a first wavelength, into the BNLM, thereby producing nonlinear focusing of the coupled fs pulses in the BNLM, wherein the fs oscillator operates at a full repetition rate; providing interaction between the BNLM and light at a second wavelength different from the first wavelength and absorbable by the BNLM, thereby forming a thermal lens in the BNLM, wherein the nonlinear focusing of the fs pulses and thermal lens cumulatively generate the fs supercontinuum at the full repetition rate of the fs oscillator.
2. 2 . The method of claim 1 , wherein the BNLM is selected to have linear absorption, nonlinear absorption or linear and nonlinear absorption at the first and second wavelengths, the first wavelength length being selected from the near-IR to the MID-IR spectral range extending between 1 and 10 μm.
3. 3 . The method of claim 2 , wherein the interaction between the BNLM and first wavelength includes partially converting the first wavelength into at least one or more additional wavelengths upon multi-photon absorption or nonlinear three-wave or four-wave mixing or a combination thereof of the first wavelength.
4. 4 . The method of claim 1 , wherein the BNLM is selected from amorphous, single crystal and polycrystalline materials, with third order nonlinearity (χ (3) ≠0) and, optionally, second order nonlinearity (χ (2) ≠0), the single crystal materials being a YAG, BBQ, ZGP, CaF 2 , ZnS, ZnSe, or GaSe, and the amorphous materials including silicate or non-silicate glasses.
5. 5 . The method of claim 4 , wherein the BNLM is selected from a quasi-phase-matched or random quasi-phase-matched single crystal or polycrystalline materials with the second order nonlinearity (χ (2) ≠0) which enables one of sum frequency mixing, difference frequency mixing, optical parametric generation or optical rectification or a combination thereof, and is selected from PPLN, PPSLT, PPKTP, OP-GaAs, OP-GaP, polycrystalline ZnS, and polycrystalline ZnSe.
6. 6 . The method of claim 4 , wherein the BNLM is selected from TM:II-VI semiconductors including single crystal and polycrystalline Cr:ZnS, Cr:ZaSe, Fe:ZnS, Fe:ZnSe.
7. 7 . The method of claim 1 , wherein the absorption of the second wavelength induces radial temperature distribution along a cross-section of light acting as a thermal guide, thereby forming the thermal lens.
8. 8 . The method of claim 1 , wherein the interaction between the BNLM, first wavelength and second wavelength includes partially converting the first and second wavelengths into at least one or more additional wavelengths upon nonlinear three-wave and four-wave mixing of the first and second wavelengths, the first and second wavelengths co-propagating or counter propagating in the BNLM.
9. 9 . The method of claim 1 further comprising optimizing the SCG, thereby achieving a broadest spectrum of the fs pulses at an output of the BNLM at a possibly lowest energy and peak power of the fs pulses at an input of the BNLM, wherein the optimization of the SCO includes: (a) adjusting a beam size of the light at the first wavelength incident on the BNLM, (b) adjusting an average power and a beam size of the light at the second wavelength, (c) identifying an optimal temporal distribution of the fs pulses at a location of self-focusing upon inserting an optical element which is selected from bulk optical materials (YAG, ZnSe), or a combination of VBG and dispersive mirrors, thereby positively or negatively pre-chirping the fs pulses upstream from the BNLM, (d) selecting the BNLM to have material dispersion, thereby compressing the pre-chirped fs pulses, or (e) identifying a location of the self-focusing of the pre-chirped fs pulses inside the BNLM and compressing the pre-chirped pulses to a shortest possible pulse duration within the identified location, or (f) a selective combination of (a) through (e).
10. 10 . The method of claim 1 , wherein the BNLM is configured as a gain medium or non-gain medium at the first wavelength.
11. 11 . The method of claim 1 , wherein a threshold of the self-focusing in the presence of the thermal lens is lower than the threshold in the absence of the thermal lens by at least a factor of two (2).
12. 12 . An optical schematic for controlling femtosecond supercontinuum generation (fs SCG), comprising: a fs oscillator outputting light having a train of fs pulses at a full PRR, which ranges between 10 MHz to 10 GHz, at a first wavelength; and a bulk nonlinear material (BNLM) receiving the fs pulses with a pulse energy inducing local nonlinear focusing which is insufficient for reaching a threshold of the fs SCG, the BNLM having a positive thermo-optic coefficient (dn/dT>0 K −1 ) and configured to absorb light at a second wavelength, which is different from the first wavelength, the absorbed light inducing heat dissipation through a cross-section of the first wavelength which forms a thermal lens along a length of the BNLM, wherein the thermal lens increases the intensity of the nonlinear focused fs pulses at the first wavelength to the threshold of the fs SCG.
13. 13 . The optical schematic of claim 12 , wherein the BNLM is selected to have linear absorption, nonlinear absorption or linear and nonlinear absorption at the first and additional wavelengths, the first wavelength length being selected from the near-IR to the MID-IR spectral range extending between several hundred nanometers (nm) and 10 μm.
14. 14 . The optical schematic of claim 12 , wherein the BNLM is selected from amorphous, single crystal and polycrystalline materials, with the third order nonlinearity (χ (3) ≠0), the selected BNLMs with the third order nonlinearity include a subgroup of BNLM with the second order nonlinearity (χ (2) ≠0).
15. 15 . The optical schematic of claim 14 , wherein the BNLMs of the subgroup with the second order nonlinearity each are selected from single crystal or polycrystalline materials which include birefringent phase matched, quasi-phase-matched or random quasi-phase-matched materials, wherein the birefringent phase matched materials being one of LN, LBO, BBQ, KTP, ZGP, GaSe, the quasi-matched materials being one of PPLN, PPSLT, PPKTP, OP-GaAs, OP-GaP, and the random quasi-matched materials being one of polycrystalline ZnS or polycrystalline ZnSe.
16. 16 . The optical schematic of claim 15 , wherein the BNLM materials each are configured for three-wave mixing (TWM) including a nonlinear process which is selected from one or combination of second harmonic generation (SHG), sum- and difference-frequency generation, optical rectification and parametric generation.
17. 17 . The optical schematic of claim 14 , wherein the amorphous materials include silicate and non-silicate glasses, and single crystal materials include oxides (YAG, BBO) phosphides (ZGP), fluorides (CaF2) or sulfides and selenides (ZnS, ZnSe, GaSe), and the BNLM is selected from TM:II-VI semiconductors including single crystal and polycrystalline Cr:ZnS, Cr:ZnSe, Fe:ZnS, Fe:ZnSe which are configured for laser interactions, three-wave mixing, four-way mixing of the first and additional wavelengths to produce the second wavelength and multi-photon absorption.
18. 18 . The optical schematic of claim 17 further comprising an auxiliary laser source outputting an additional wavelength coupled into the BNLM so that the additional and first wavelengths co-propagate or counter-propagate, the additional wavelength being the second wavelength absorbed in the BNLM, or pumping the first wavelength or nonlinearly interacting with the BNLM and first wavelength to provide the three-wave mixing resulting in generation of the second wavelength, wherein the auxiliary laser source operates in a continuous wave or pulsed regimes.
19. 19 . The optical schematic of claim 18 further comprising a lens arrangement located between the auxiliary laser source and the BLNM and configured to controllably change a beam size of light at the additional wavelength, wherein the auxiliary laser source is configured to controllably adjust an average power of light at the additional wavelength or the fs oscillator and the BNLM and configured to change a beam size of the light at the first wavelength.
20. 20 . The optical schematic of claim 12 further comprising a dispersive element located between the fs oscillator and BNLM and including one or more of undoped YAG plates, ZnSe plates, mirrors with chromatic dispersion, volume Bragg gratings (VBG) to apply a chirp to the fs pulses, the BNLM being configured with material dispersive characteristics to compress the pre-chirped fs pulses, a beam size of the first and second wavelengths and the power of the second wavelength being controlled so that the nonlinear focusing and thermal lens have a common focal location within the BNLM.

Description

This application is a 35 USC 371 national stage entry of PCT/US2021/065532, filed on Dec. 29, 2021, which claims the benefit of priority under 35 U.S.C. § 119 (e) to U.S. Provisional Application Ser. No. 63/131,577, titled LOW-THRESHOLD SUPERCONTINUUM GENERATION IN BULK DIELECTRICS AND SEMICONDUCTORS, filed on Dec. 29, 2020, all of which are hereby incorporated by reference in their entirety. BACKGROUND OF DISCLOSURE Filed of the Disclosure This disclosure relates to methods of femtosecond supercontinuum generation (SCG) and supercontinuum laser sources. In particular, the disclosure relates to a method and system of generating low-threshold SCG in bulk dielectric and semiconductor nonlinear materials (NLM) by generating a thermal lens effect along with a nonlinear self-focusing effect. Background of the Disclosure The SCG is the formation of broad continuous spectra by propagation of high-power pulses through nonlinear media. The SCG with fs pulses (fs SCG) in particular attracts attention because it yields optical spectra that combine a broad, super-octave bandwidth with a high spatial and temporal coherence. Therefore, fs SCG is crucial for a number of important applications including, among others, the generation of optical frequency combs, arbitrary optical waveform synthesis and generation of attosecond pulses. The optical frequency combs, which is equivalent to fs pulse trains, with a Watt-level average power P between 0.1 and 10 W, relatively high pulse repetition rate (PRR) or frequency fR ranging between 107-1010 Hz and hence low pulse energy W=Pav/fR=0.1−100 nJ are indispensable to spectroscopy, sensing, microscopy and imaging. The techniques for fs SCG in specifically designed optical nonlinear fibers and waveguides, such as a silicon nitride (Si3N4), are well established. However, the use of the fibers and waveguides characterized by confined geometry comes at the expense of an increased complexity and reduced overall efficiency of the laser system. Further, the nonlinear fibers and waveguides have intrinsic limitations on power and coherence of SCG and require precise alignment. Bulk materials, including, for example, transparent amorphous solids (e.g. silicate and non-silicate optical glasses), crystals (e.g. oxides, fluorides, phosphides) and semiconductors (silicon, germanium, and other III-V and II-VI materials) also support fs SCG. The advantages of fs SCG in these materials include, among others, relative simplicity and thus low cost, flexibility, and possibility to scale peak and average power. In these materials laser propagation is not restricted by the material cross section profile which allows for relaxed alignment sensitivity. Moreover, SCG in some bulk materials features compression of femtosecond input pulse to even shorter output pulse comprising only few optical cycles. For example, USPP 2021/0124236 and U.S. Pat. Nos. 10,216,063 10,216,063 and 10,483,709, which are all co-owned with the subject matter application and incorporated herein by reference in their entirety, all teach SCG in a random quasi-phase-matched gain medium such as a polycrystalline zinc sulfide doped with Cr2+ ions. Femtosecond SCG in nonlinear media is governed by interplays between nonlinearities of the selected bulk material, nonlinear absorption and chromatic dispersion. The physical picture of fs-SCG could be understood in the framework of filamentation: the interplay between self-focusing, self-phase modulation, multiphoton absorption/ionization-induced free electron plasma. The interaction among these physical phenomena leads to the appearance of a filament “a dynamic structure with an intense core, that is able to propagate over extended distances much larger than the typical diffraction length while keeping a narrow beam size without the help of any external guiding mechanism” (A. Couairon and A. Mysyrowicz, Femtosecond filamentation in transparent media, Phys. Rep. 441, 47-190 (2007).) An important consequence of the filament formation is very strong nonlinear broadening of the pulse spectrum, i.e., bandwidth of the output spectrum is much larger than that of the input spectrum. FIGS. 1A and 1B illustrate a standard set-up for fs SCG in bulk materials. The presence of filamentation in FIG. 1A results in strong broadening, but if it is absent, as shown in FIG. 1B, the output spectrum is not broadened. The initial stage of filament formation is governed by self-focusing: χ(3) nonlinearity of the medium induces the intensity-dependent refractive index: n(I)=n0+n2I, where I is the intensity, n0 is the linear refractive index, n2 is the nonlinear refractive index. The local intensity is higher at the center of the beam and lower at its edges. Therefore bulk χ(3) medium with n2>0 acts like an intensity-dependent lens. The self-focusing threshold is defined by the critical power PCrit, which, in turn, is defined by the parameters of the gain medium including the refractive index's nonlinear and linear co