Noncompressed and single-beam generation of carrier-envelope phase-stable optical pulses

Abstract

The present invention is particularly directed to an optical carrier frequency f p and pulse duration τ p from an input optical pulse with optical carrier frequency f i This invention relates to a method and system for generating carrier-envelope phase-stable (CEP-stable) optical pulses, which uses a birefringent medium, a nonlinear medium, a dispersive optical system, and a parametric device (DFG) in succession to achieve this generation.

Description

[Technical Field]

[0001] FIELD OF THE INVENTION Embodiments of the present invention generally relate to the field of optical systems and methods for generating carrier-envelope phase-stable (CEP-stable) optical pulses. [Background technology]

[0002] Pulsed laser sources are characterized by the temporal concentration of light they emit. The electric field of these pulses is generally described in terms of an envelope and a carrier. The electric field of these pulses is the product of an oscillating field (photocarrier) with a period T0 = λ0 / c and a slowly varying function (envelope) on the order of T0. For ultrafast laser sources, the duration of the envelope is typically several tens of femtoseconds (1 femtosecond = 10 -15 seconds), which is typically longer than the photoperiod in near- and mid-infrared light. For example, at 800 nm, the photoperiod is 2.7 femtoseconds.

[0003] For certain applications, optical pulses with a duration of a few cycles (hereafter referred to as few-cycle pulses) are advantageous. In this case, the electric field approaches a transient field, which is advantageous for applications such as harmonic generation in gases or solids. Such pulses allow for the generation of light in particular with a duration of a few tens of attoseconds (1 attosecond = 10 -18 This makes it possible to effectively generate attosecond pulses with durations reaching 67 attoseconds. An example of this is described in "K. Zhao et al. in 'Tailoring a 67 attosecond pulse through advantageous phase-mismatch', Optics Letters 37, 3891-3893 (2012)."

[0004] Carrier-envelope phase (CEP) refers to the relative phase between the envelope and the carrier. Shot-to-shot stability of the CEP is important for maintaining the characteristics of the electric field from pulse to pulse. Such stability is particularly important in the context of high-order harmonic generation, as explained by A. Baltuska et al. in “Attosecond control of electronic processes by intense light fields”, Nature 421, 611 (2003).

[0005] The CEP can be modulated by dispersive elements with unequal group and phase indices, such as bulk media (glass, gas, etc.) or pairs of angular dispersive elements (diffraction grating pairs, prism pairs, etc.). Variations in the optical beam path in such dispersive elements are the main cause of CEP variation or drift in ultrafast laser systems.

[0006] Filamentation is a nonlinear optical process that allows a beam of light to propagate through a medium without diffraction. This self-induced phenomenon has a λ 2 The laser peak power must be higher than a threshold power, called the critical power, approximately equal to / (8π*n0*n2), where n0 and n2 are the linear and nonlinear optical refractive indices of the propagation medium, respectively. Filamentation of subpicosecond or picosecond pulses can broaden the input pulse through self-phase modulation and other cascaded nonlinear processes, generating a broadband spectral continuum. Under some conditions (single filament regime), filamentation maintains shot-to-shot CEP stability. An example of a subpicosecond-generated continuum is described in "Femtosecond continuum generation in bulk laser host materials with sub-μJ pump pulses," Applied Physics B 97.3 (2009): 561.

[0007] Stabilization of the CEP often requires sophisticated and expensive equipment that is complex to implement. A distinction is usually made between so-called passive and active stabilization methods, depending on whether a feedback mechanism for stabilizing the CEP is present (active) or not (passive).

[0008] As an example of an active feedback mechanism for CEP stabilization, SW Huang et al. "High-energy pulse synthesis with sub-cycle waveform control for strong-field physics", Nature Photonics 5, 475-479 (2011) describes a source concept that includes an ultrafast pulse generator in which the CEP phase is actively stabilized, a device for extending the spectrum of these pulses into the infrared, two pulse shapers, two banks of optical parametric amplifiers non-collinearly pumped by two pump wavelengths, and a phase system controlled by a measurement device based on cross-correlation detection.

[0009] An example of a passive feedback mechanism for stabilizing CEP is presented at the 13th International Conference on Ultrafast Phenomena (OSA Technical Digest Series, Optical Society of America, 2002). th In the paper "A. Baltuska et al. "All-optical self-stabilization of carrier-envelope phase offset in few-cycle pulses by optical parametric amplifiers" at the International Conference on Ultrafast Phenomena, various optical concepts for passive stabilization by second-order nonlinear processes such as difference frequency generation (DFG) are described.

[0010] Ultrafast laser pulse generation via DFG is described in

[10] . In a material with second-order nonlinear optical properties, the carrier frequency fs The first optical pulse (hereafter referred to as the "signal" pulse) of p >f s During the interaction, part of the energy of the "pump" pulse is transferred to the "signal" pulse, amplifying it. At the same time, a third optical pulse is generated and co-amplified. This third pulse, called the "idler" pulse, has a carrier frequency of f p -f s The CEP of the "idler" pulse is equal to the difference, within a certain range, between the CEP of the "pump" signal and the CEP of the "signal" pulse. If this difference is repeatably constant (i.e., constant from pulse to pulse), the CEP of the "idler" pulse will be stable over time, even if the "pump" and "signal" pulses do not share this property. Because DFG is a localized and instantaneous process, difference frequency generation is limited to wavelengths where the optical group delay in the DFG medium is approximately equal.

[0011] The generation of passively stabilized few-cycle optical pulses via DFG can be divided into two categories.

[0012] The first category is intrapulse DFG, where the "signal" and "pump" waves are generated from a single broadband optical beam. An example of intrapulse DFG is described in C.P. Hauri et al., "Generation of intense, carrier-envelope phase-locked few-cycle laser pulses through filamentation," Appl. Phys. B 79(6), 673-677 (2004).

[0013] The second category, known as interpulse DFG, is two optical beams in which the "signal" and "pump" waves propagate along different optical paths, even if one beam is derived from the other. A common implementation of this structure is to generate the "signal" beam from the "pump" beam by filamentation within a bulk crystal. An example of interpulse DFG is described in the paper "G. Cirmi, C. Manzoni, D. Brida, S. De Silvestri, and G. Cerullo, "Carrier-envelope phase-stable, few-optical-cycle pulses tunable from visible to near IR," J. Opt. Soc. Am. B 25, B62-B69 (2008)" and is shown in Figure 1. Summary of the Invention [Problem to be solved by the invention]

[0014] In intrapulse DFG, the "pump" and "signal" beams share a common optical path, also known as a single beam. Therefore, the CEP stability of optical pulses generated by intrapulse DFG is, in principle, superior to that of interpulse DFG. However, intrapulse DFG requires ultrabroadband compressed pulses, which are difficult to generate and manage, especially at the μJ level. As an example, generating CEP-stable pulses of approximately 1 μm via DFG requires a bandwidth of over 300 THz. Compressing such bandwidth, i.e., equalizing the group delay of light at all frequency components, is a major technical challenge, requiring complex, meter-scale technical installations. An example of such an installation is described in "Synthesized Light Transients." Science 334.6053 (2011): 195-200.

[0015] Furthermore, known stabilized optical pulse generation systems via pulse-to-pulse DFG require the use of mirrors, which increase the overall system size. Furthermore, the presence of a beam splitter means that the light uses different paths, causing instability in the relative path lengths of the "pump" and "signal" beams and, therefore, the CEP of the "idler" beam.

[0016] Therefore, there is a need to improve the generation of CEP-stable optical pulses of a given optical carrier frequency from an input optical pulse by avoiding both beam splitting and pulse compression. [Means for solving the problem]

[0017] According to a first aspect, the present invention provides a method for generating a tunable optical signal at an optical carrier frequency f p and pulse duration τ p from an input optical pulse with optical carrier frequency f i A single-beam, non-compressive optical system for generating carrier-envelope phase-stable (CEP-stable) optical pulses of a birefringent medium (B) for receiving as input an input optical pulse and for providing as output a pair of linearly polarized pulses (PP), each of which has orthogonal polarizations along first and second directions (E1, E2) and a relative optical group delay that is approximately equal to the pulse duration (τ) of the input optical pulse; p ) or more, a nonlinear medium (NL) for receiving as input the output of the birefringent medium (B) and for providing as output pairs of linearly polarized pulses (PP) in which at least one of the two pulses in each pair is spectrally broadened; a transparent dispersive optical system (O) for receiving as input the output of a nonlinear medium (NL) and for providing as output pairs of linearly polarized pulses (PP) in which the two pulses of each pair overlap in time in whole or in part; - a parametric device (DFG) for receiving as input the output of a nonlinear medium (NL) and for providing as output the frequency difference between a frequency component polarized along a first direction (E1) and a frequency component polarized along a second direction (E2).

[0018] The optical system further comprises: a filter (F) for receiving as input the output of the parametric device (DFG) and extracting as output the frequency difference of the output of the parametric device (DFG), the filter may be a dichroic mirror; - a waveplate for controlling the polarization state of the input light pulses; an optical component arranged between the optical system (O) and the parametric device (DFG), adapted to receive as input the output of the dispersive optical system (O) and to focus the output of the dispersive optical system (O) into the parametric device (DFG), The birefringent medium (B) may have a thickness of 100 μm or more, and may contain at least one crystal selected from a calcite crystal, a quartz crystal, an α-BBO crystal, a YVO4 crystal, and a TeO2 crystal; the dispersive optical system (O) may comprise at least one material selected from a glass window, a YAG crystal, a thick lens, a calcite crystal, a quartz crystal, an α-BBO crystal, a YVO4 crystal, a TeO2 crystal; The nonlinear medium (NL) may comprise at least one material selected from a YAG crystal, a sapphire crystal, a calcium fluoride crystal, and a fused silica window; The parametric device (DFG) may be a second-order nonlinear crystal containing at least one material selected from β-BBO, LBO, LiNbO3, LiIO3, KTA, LGS, AGS; The parametric device (DFG) may be a nonlinear crystal characterized by a second-order susceptibility.

[0019] According to another aspect, the present invention provides a method for generating an optical pulse having an optical carrier frequency f p and pulse duration τ p from an input optical pulse with optical carrier frequency f i The present invention may be a method for generating carrier-envelope phase-stable (CEP-stable) optical pulses, the method comprising: -optical carrier frequency f p and pulse duration τ p providing an input optical pulse having - generating, from the input optical pulses provided thereto, a pair of linearly polarized pulses (PP) by a birefringent medium (B), each of the pair of linearly polarized pulses (PP) having orthogonal polarizations along first and second directions (E1, E2) and a relative optical group delay of approximately the pulse duration (τ) of the input optical pulses; p ) or more steps, - inducing filamentation of at least one of the two pulses of each pair by a nonlinear medium (NL), wherein at least one of the two pulses of each pair is spectrally broadened as a result of the filamentation; - overlapping (40) in time the two pulses of each pair output to the nonlinear medium (NL) by a dispersive optical system (O); and - generating a frequency difference between a frequency component polarized along a first direction (E1) and a frequency component polarized along a second direction (E2) by means of a parametric device (DFG).

[0020] The method may further include: - extracting (60) the frequency difference produced by the filter, - focusing (42) the output of the optical system (O) onto a parametric device (DFG) by means of an optical component, - adjusting (12) by means of a waveplate the energy split between the two pulses of the pulse pair produced by the birefringent medium.

[0021] According to another aspect, the invention can be a light source unit for generating carrier-envelope phase-stable (CEP-stable) optical pulses at an optical carrier frequency f from input optical pulses, the light source unit comprising: a pulse generator for delivering input optical pulses having an optical carrier frequency f and a pulse duration τ, the input optical pulses being linearly polarized; an optical system adapted to receive the input optical pulses; and an optical amplifier adapted to amplify the output of the optical system.

[0022] Systems, products and methods embodying the present invention will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0023] [Figure 1] 1 shows an example of a prior art inter-pulse DFG configuration. [Figure 2] 1 shows an example of the system principle of the present invention. [Figure 3] 2 shows another example of the system of the present invention. [Figure 4] 1 shows an example of a temporal profile of a light pulse in the system. [Figure 5] 1 shows a schematic block diagram of an example of the method of the present invention.

[0024] Similar or functionally similar elements among the figures will be assigned the same reference numbers unless otherwise noted. DETAILED DESCRIPTION OF THE INVENTION

[0025] Referring to Figure 2, the input optical pulse is converted to an optical carrier frequency f i An optical system has been proposed to generate carrier-envelope phase-stable optical pulses with an optical carrier frequency f p and pulse duration τ p The optical system is adapted to generate a CEP-stable light pulse from an input light pulse having a CEP-stable light pulse of 1 / 2 s. Thus, the optical system is suitable (or adapted) to receive an input light pulse and generate a CEP-stable light pulse, which is provided as an output of the optical system. The optical system is described below.

[0026] The system comprises a birefringent medium (B). Birefringence is the optical property of a material with a refractive index that depends on the polarization and propagation direction of light. The simplest birefringence is said to be uniaxial, meaning that there is a single direction that governs the optical anisotropy. Rotating the material around this axis, called the optic axis, does not change its optical behavior. For simplicity, only uniaxial crystals will be considered below. However, it will be understood that biaxial crystals can also be used to produce the desired optical properties.

[0027] Light with polarization perpendicular to the optical axis of the birefringent medium (B) experiences a refractive index n o ("normal"). Light with polarization in the direction of the optical axis of the birefringent medium (B) is governed by the optical index n e ("extraordinary"). The two polarizations (i.e., ordinary and extraordinary polarizations) are orthogonal along a first and second direction (E1, E2), respectively, and the first and second directions are orthogonal. The group delay of ordinary light and the group delay of extraordinary light are also different, i.e., the time it takes for a light pulse with polarization orthogonal to the optic axis to pass through a birefringent medium (B) is different from the time it takes for a light pulse with polarization along the optic axis to pass through a birefringent medium (B). The delay is denoted as τ below.

[0028] The retardation τ depends, inter alia, on the thickness of the birefringent medium (B). The retardation τ is given by the following equation (1): τ=|n g,e -n g,o |*L / c (1) In the formula, n g,e represents the group delay of the extraordinary light, n g,o represents the group delay of ordinary light, L represents the thickness of the medium, c represents the speed of light in a vacuum.

[0029] The birefringent medium (B) is adapted to split the input light pulse into two linearly polarized pulses. The two linearly polarized pulses at the output of the birefringent medium (B) are consecutive and separated by a time delay equal to τ. Thus, for each input light pulse, the birefringent medium (B) generates a pair of orthogonal linearly polarized pulses, one polarized pulse polarized in a direction perpendicular to the optical axis direction of the birefringent medium (B) (the first direction) (the ordinary polarized pulse) and the other polarized pulse polarized in the optical axis direction of the birefringent medium (B) (the second direction) (the extraordinary polarized pulse). The pulse pair may be consecutive pulses.

[0030] The birefringent medium (B) is therefore suitable for providing a linearly polarized pulse pair (PP) from an input optical pulse. The pulse pair comprises one ordinary and one extraordinary polarized pulse, which are orthogonally polarized along a first and second direction (E1, E2). Passage of the input optical pulse through the birefringent medium (B) introduces a relative optical group delay between the ordinary and extraordinary polarized pulses of the pair.

[0031] The delay between the pair of ordinary and extraordinary polarized pulses is approximately the pulse duration (τ p ) or more. This ensures that the pulse pairs do not overlap in time. In other words, the extraordinary and ordinary components of the electric field do not overlap in time.

[0032] The birefringent medium (B) avoids the use of a pulse splitter to split the input light pulse into two or more output pulses. Therefore, the system is more compact than the prior art depicted in Figure 1, as it does not require a delay line to introduce a delay between the at least two pulses. Furthermore, the system of the present invention is more compact compared to known systems, as no reflective optics are involved to generate the at least two pulses from the input light pulse.

[0033] For example, the birefringent medium (B) may be a calcite (CaCO3) crystal, a quartz plate (SiO2), an α-BBO (α-BaB2O4) crystal, a YVO4 crystal, or a TeO2 crystal.

[0034] By way of example, the thickness of the birefringent medium (B) may be 100 μm or more.

[0035] By way of example, the thickness of the birefringent medium (B) may be comprised between 1.5 and 6 mm.

[0036] As an example, the birefringent medium (B) may be cut at 90° from the optical axis.

[0037] By way of example, the birefringent medium (B) may be oriented such that the polarization direction of the input light pulse is at about 56° from the optical axis of the birefringent medium (B).

[0038] As a specific example, the birefringent medium (B) is a 1.5-6 mm thick calcite crystal, cut at 90° from the optic axis and oriented so that the polarization direction of the input light pulse is approximately 56° from the optic axis of the birefringent medium (B). Approximately 30% of the input light pulse energy is ordinary polarization, and approximately 70% of the input light pulse energy is extraordinary polarization. The birefringence delay τ introduced between the extraordinary and ordinary polarizations of the birefringent medium (B) is approximately 900 femtoseconds at 1030 nm. In this example, because the birefringence delay τ is larger than the pulse duration of the input light pulse (P), the extraordinary and ordinary components of the electric field barely overlap in time and can be considered independent pulses.

[0039] In Figure 4, we show examples of temporal profiles of optical pulses at different points in a system of the present invention. The temporal profiles represent the pulse intensity (W / m) over time (in femtoseconds). 2 ) represents a change in

[0040] A short optical pulse is supplied by a generator (P) not shown in Figure 4. The input optical pulse has an optical carrier frequency f p and pulse duration τ pThe short optical pulses form an input beam that is linearly polarized.

[0041] The time profile shown in 1a) is a wavelength λ along the first polarization direction (E1). p , pulse duration τ p 1a) represents an input light pulse (P) along the second polarization direction (E2). The first polarization direction (E1) is parallel to the ordinary polarization direction of the birefringent medium (B). The temporal profile shown in 1g) represents the temporal profile of an input light pulse (P) along the second polarization direction (E2). The second polarization direction (E2) is parallel to the extraordinary polarization direction of the birefringent medium (B). The temporal profile in 1g) is the same as the temporal profile in 1a), except that the input light pulse (P) along the extraordinary polarization direction (E2) carries more energy.

[0042] The temporal profile shown in 1b) represents the temporal profile at the output of the birefringent medium (B) of the input pulse of profile 1a). The profile of the polarized pulse is usually unchanged at the output of the birefringent medium (B). By "unchanged" we mean that profile 1b) is nearly identical to profile 1a), although slight changes in the pulse may occur due to optical dispersion, as is known in the art.

[0043] Temporal profile 1h) represents the temporal profile at the output of the birefringent medium (B) along the second direction of polarization (E2). The input to the birefringent medium (B) is a pulse of profile 1g). Temporal profile 1h) is shifted in time by the group delay τ with respect to temporal profile 1b. The extraordinary polarization pulse of temporal profile 1h) barely overlaps in time with the ordinary polarization pulse of temporal profile 1b), and the extraordinary polarization pulse and the ordinary polarization pulse can be considered as independent pulses. This is because the delay τ caused by the birefringent medium (B) is shifted in time by the pulse duration τ of the input light pulse. p This is possible because it is larger than

[0044] Referring back to Figure 2, the system further comprises a nonlinear medium (NL), which, as known in the art, is a medium whose polarization density responds nonlinearly to a pulsed electric field E.

[0045] The nonlinear medium (NL) is adapted to receive as input the output of the birefringent medium (B), i.e., to receive as input linearly polarized pulse pairs (PP). The nonlinear medium (NL) is suitable (or adapted) to broaden the spectrum of at least one of the two pulses of each pair received as input. The optical properties of the nonlinear medium (NL) make it possible to induce filamentation of the ordinarily polarized and / or extraordinary polarized pulses. Filamentation is performed as known in the art.

[0046] As an example, the extraordinary polarization pulse (a pulse polarized along the second direction (E2)) may be intense enough to induce filamentation and may be spectrally broadened. The normally polarization pulse is not intense enough to induce filamentation and remains essentially identical while propagating through the nonlinear medium (NL), i.e., the normally polarization pulse is essentially the same as the pulse at the output of the birefringent material (B). At the output of the nonlinear medium (NL), due to group delay dispersion, some wavelengths of the broadened extraordinary polarization pulse may experience the same optical group delay as the normally polarization pulse.

[0047] By way of example, the nonlinear medium (NL) may partially or entirely comprise one or more materials selected from a YAG (yttrium aluminum garnet) crystal, a sapphire crystal, a calcium fluoride crystal, and a fused silica window.

[0048] As an example, the nonlinear medium (NL) may include a YAG crystal and have a thickness of 4 to 15 mm.

[0049] As a specific example, the nonlinear medium (NL) may include a YAG crystal, with a thickness of 4 to 15 mm and an overlapping wavelength of about 655 nm.

[0050] Referring again to Figure 4, the temporal profile of 1c) represents the temporal profile of the pulse of profile 1b) that is fed as input to the nonlinear medium (NL). The profile of the normally polarized pulse remains unchanged at the output of the nonlinear medium (NL). Again, "unchanged" means that profile 1c) is nearly identical to profile 1b), although slight changes in the pulse may occur due to optical dispersion, as is known in the art.

[0051] The temporal profile in 1i) is the temporal profile at the output of the nonlinear medium (NL) along the second direction of polarization (E2). The temporal profile in 1i) is broadened due to a combination of spectral broadening and group delay dispersion. The wavelength λ s is produced by the spectral broadening produced by the nonlinear medium (NL).

[0052] As shown in FIG. 4, the output of the nonlinear material (NL) is received as input by a dispersive optical system (O). The dispersive optical system (O) is suitable (or adapted) for partially overlapping the ordinary and extraordinary polarization pulses in the time domain. One solution for this could be to temporally spread the pulses so that they partially overlap in the time domain. A second solution could be to use birefringent crystals similar to the birefringent medium (B) but with different orientations so that the pulses partially overlap in the time domain. It should be understood that both solutions can be combined. It should also be understood that the linearly polarized pulse pairs (PP) provided as the output of the nonlinear material (NL) may completely overlap in the time domain.

[0053] The dispersive optical system (O) may be a bulk dispersive optical system, for example, the dispersive optical system does not include mirrors and / or is an integrated assembly.

[0054] Referring again to Figure 4, the temporal profile 1d) represents the temporal profile of the pulse of profile 1c) at the output of the optical system (O), where the profile of the normally polarized pulse remains unchanged relative to the output of the nonlinear medium (NL).

[0055] The temporal profile 1j) shows the temporal profile at the output of the optical system (O) along the second direction (E2) of polarization. The temporal profile 1j) is stretched compared to the group delay dispersion of the temporal profile 1i) fed as input to the optical system (O). Interestingly, for wavelength λ along the second direction (E2), s and wavelength λ along the first direction (E1) p The group delays of the

[0056] By way of example, the dispersive optical system (O) may include at least one material selected from a glass window, a YAG crystal, a calcite (CaCO) crystal, a quartz crystal, an α-BBO (α-BaBO) crystal, an yttrium orthovanadate (YVO), a tellurium dioxide (TeO) crystal, etc. It should be understood that any material may be selected as long as it has optical properties that allow for temporally expanding or synchronizing the pulses.

[0057] As an example, the dispersive optical system may be a thick lens.

[0058] By way of example, the optical system (O) may be an SF11 (glass code 785258) glass window.

[0059] As a specific example, the optical system (O) may include an SF11 glass window having a thickness of 1 to 5 mm.

[0060] Returning to Figure 2, the system further comprises a (suitable) parametric device (DFG) adapted to receive the output provided by the dispersive optical system (O). The DFG is adapted to generate a frequency difference between frequency components polarized along a first direction (E1) and frequency components polarized along a second direction (E2). The DFG may be any suitable parametric device known in the art. Difference Frequency Generation where a pulse polarized along a first direction (E1) acts as a "pump" pulse and a pulse polarized along a second direction (E2) acts as a "signal" pulse.

[0061] Difference frequency generation is limited to wavelengths where the optical group delay in the DFG medium is equal. Therefore, the frequency difference generated is f p -f s and f p is the frequency of the normal polarized pulse, f s is the frequency of the extraordinary polarization pulse.

[0062] As an example, the parametric device (DFG) is a second-order nonlinear crystal comprising at least one material selected from β-BBO (low-temperature β phase of barium borate), LBO (lithium triborate, denoted LiB3O5), lithium niobate (LiNbO3), lithium iodate (LiIO3), LGS (lithium gallium sulfide, denoted LiGaS2), AGS (silver gallium sulfide, denoted AgGaS2), and KTA (potassium titanium arsenide).

[0063] As an example, a parametric device (DFG) is a nonlinear crystal characterized by a second-order nonlinear susceptibility.

[0064] As a specific example, the parametric device (DFG) is a 100 μm thick low-temperature beta-phase barium metaborate crystal cut at approximately 52.6° for Type I phase matching. Difference frequency generation between components at approximately 655 nm (the "pump" wave in the DFG crystal) and 1030 nm (the "signal" wave in the DFG crystal) produces an idler wave at approximately 1800 nm.

[0065] Returning to Figure 4, the temporal profile in 1e) shows the temporal profile at the output of the parametric device (DFG) along the first direction (E1), which is the temporal profile of the normally polarized pulse and the wavelength λ generated by difference frequency generation. i The temporal profile of a third light pulse (also called an "idler" pulse) having:

[0066] Because the difference between the CEP of the "pump" pulse and the CEP of the "signal" pulse is constant from pulse to pulse, the CEP of the "idler" pulse is passively stable, i.e., reproducible from pulse to pulse.

[0067] The temporal profile of 1k) is the temporal profile at the output of the parametric device (DFG) along the second direction (E2). The profile of the extraordinary polarization pulse is similar to the profile obtained at the output of the optical system (O).

[0068] An example optical system has been described above with reference to FIG. 2. Compared to interpulse DFG configurations, the present invention is single-beam (no beam splitting), inherently phase-stable, and insensitive to beam path variations. Indeed, prior art configurations such as those depicted in FIG. 1 require the use of a beam splitter to generate two beams from an input beam, a delay line to time-delay one of the two beams, and a mirror to regroup the two beams for DFG generation. The present invention enables the generation and transmission of two beams (one of which includes a delayed beam) in a single path, i.e., does not require mirrors and / or delay lines. Therefore, the system according to the present invention is more compact and more tolerant to beam disturbances. Furthermore, because a single optical path is shared by the "pump" and "signal" beams, the CEP is more stable over time. Compared to intrapulse DFG configurations, the present invention does not require an ultra-broadband input pulse or a compressed pulse for the DFG stage.

[0069] 2, the system may further comprise a filter (F) adapted to receive as input the output of the parametric device (DFG). The filter is adapted to detect when the optical system is at wavelength λ i The system is adapted to extract the frequency difference of the parametric device (DFG) output so as to output only the third optical pulse at the optical carrier frequency f i Generate only stable light pulses of CEP.

[0070] For example, filter may be a dichroic mirror.

[0071] As a specific example, the filter may be a dichroic mirror that removes wavelength components at approximately 655 nm and 1030 nm.

[0072] FIG. 3 illustrates an example of the optical system of FIG. 2 with additional optical elements. In this example, the system further includes a wave plate disposed between the input optical pulse generator and the birefringent medium (B). The wave plate is thus adapted to receive the input optical pulses generated by the generator (P). The wave plate is further adapted to adjust the energy ratio between the pulses of the pulse pair generated in the birefringent medium (B). The polarization state of the input optical pulses is controlled by the wave plate, e.g., the split energy is balanced between the two pulses of the pulse pair generated by the birefringent medium. This advantageously improves the filamentation performed in the nonlinear medium (NL).

[0073] Referring again to FIG. 3 , the optical system includes at least one optical component disposed between the optical system (O) and the parametric device (DFG). By "disposed between," we mean that the optical component is in the light (pulse) path between the optical system (O) and the parametric device (DFG). Thus, the optical component is adapted to receive the output of the dispersive optical system (O) as an input and to focus the output of the dispersive optical system (O) into the parametric device (DFG). Focusing the light improves the generation of frequency differences by the DFG. In this example, the optical component may be a lens adapted to focus an incident beam at its output.

[0074] At least one additional optical component (e.g., a lens) may be disposed between the birefringent medium (B) and the nonlinear medium (NL) to focus the output of the birefringent medium (B) onto the nonlinear medium (NL), thereby improving filamentation.

[0075] At least one further optical component (e.g., a lens) may be arranged between the parametric device (DFG) and the filter (F) to focus the output of the parametric device (DFG) onto the filter (F), thereby improving the filtering performed by the filter.

[0076] The elements of the system are arranged so that light generated by the pulse generator is transmitted sequentially from one element to another. For example, in FIG. 2, the input optical pulse is transmitted sequentially to a birefringent medium (B), a nonlinear medium (NL), a dispersive optical system (O), and a parametric device (DFG). If the system includes a filter, the pulse is finally transmitted to the filter. For example, in FIG. 3, the input optical pulse is transmitted sequentially to a wave plate, a birefringent medium (B), a first optional optical component (lens), a nonlinear medium (NL), a dispersive optical system (O), a second optional optical component (lens), a parametric device (DFG), a third optional optical component (lens), and a filter (F).

[0077] Another aspect of the present invention is the optical carrier frequency fp and pulse duration τ p from an input optical pulse with optical carrier frequency f i A method for generating carrier-envelope phase-stable (CEP-stable) optical pulses of is described below, which essentially performs the operations performed by the series of elements forming the present system or examples of the present system described above.

[0078] The above method can improve the generation of CEP-stable optical pulses, which are inherently phase-stable and unaffected by beam path fluctuations.

[0079] This method uses an optical carrier frequency f p and pulse duration τ p The method includes providing (10) an input optical pulse having a linear polarization. The input optical pulse is linearly polarized. The providing step may be performed by a pulse generator as described above.

[0080] The method further comprises generating (20) a pair of linearly polarized pulses (PP) from the provided input optical pulses, each of the pair of linearly polarized pulses (PP) having orthogonal polarizations along first and second directions (E1, E2) and a relative optical group delay that is approximately equal to the pulse duration (τ p ) and above. The generating step is carried out by the birefringent medium (B) described above.

[0081] The method then further comprises inducing (30) filamentation of at least one of the two pulses of each pair, wherein at least one of the two pulses of each pair is spectrally broadened as a result of the filamentation, the filamentation being performed by the nonlinear medium (NL) described above.

[0082] The method also includes a step (40) of temporally overlapping the two pulses of each pair from the output of the nonlinear medium (NL), which is performed by a dispersive optical system (O).

[0083] The method further comprises the step of generating (50) a frequency difference between a frequency component polarized along a first direction (E1) and a frequency component polarized along a second direction (E2), the generating step being performed by a parametric device (DFG).

[0084] The method may further comprise the step of extracting (60) the frequency difference produced, which is performed by a filter as described with reference to FIG.

[0085] The method may further comprise the step (42) of focusing the output of the birefringent medium (B) into a nonlinear medium (NL) and / or the output of the optical system (O) into a parametric device (DFG) and / or the output of the parametric device (DFG) into a filter.

[0086] The method may further comprise the step of adjusting (12) the energy split between the two pulses of the pulse pair produced by the birefringent medium, which may be performed by a wave plate.

[0087] Another aspect of the present invention is to convert an input optical pulse into an optical carrier frequency f i A light source unit for generating carrier-envelope phase-stable (CEP-stable) optical pulses of an optical carrier frequency f p and pulse duration τ p The optical system according to the present invention comprises a generator of optical pulses having a carrier-envelope phase-stable (CEP-stable) optical pulse, an optical system as described above, and an optical amplifier. The optical pulse is provided as an input to the optical system according to the present invention, which in turn provides a carrier-envelope phase-stable (CEP-stable) optical pulse as an output. The carrier-envelope phase-stable (CEP-stable) optical pulse is provided as an input to an optical amplifier which provides an amplified carrier-envelope phase-stable (CEP-stable) optical pulse as an output. The optical amplifier is therefore adapted to amplify the output of the optical system.

[0088] For example, the pulse generator of this light source unit generates an optical carrier frequency fp , pulse duration τ p The pulses produced by the generator are linearly polarized.

[0089] In one example, the pulse generator (P) may be an ytterbium laser pickup delivering short optical pulses at 1030 nm with pulse energy of approximately 10 μJ and 350 fs.

[0090] The optical amplifier directly amplifies the carrier-envelope phase-stable (CEP-stable) optical pulses generated by the optical system, i.e., there is no need to first convert the CEP-stable optical pulses into an electrical signal, as known in the art. The optical amplifier may be of any type or technology.

[0091] While the present invention has been described with reference to particular examples, more generally, those skilled in the art will recognize that various modifications can be made and equivalents substituted without departing from the scope of the invention. Additionally, modifications may be made to adapt a particular situation to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular examples disclosed, but rather include all examples falling within the scope of the appended claims.

Claims

1. Optical carrier frequency f ｐ and pulse duration τ ｐ from an input optical pulse having an optical carrier frequency f ｉ 1. A single-beam, non-compressive optical system for generating carrier-envelope phase-stable (CEP-stable) optical pulses of a birefringent medium (B), --receives as input said input light pulse; a birefringent medium (B) for providing as output pairs of linearly polarized pulses (PP), wherein each pair of linearly polarized pulses (PP) has orthogonal polarizations along first and second directions (E1, E2) and a relative optical group delay of approximately the pulse duration (τ ｐ ) or more, and a nonlinear medium (NL), --receives as input the output of said birefringent medium (B); said nonlinear medium (NL) for providing as output pairs of linearly polarized pulses (PP) in which at least one of the two pulses in each pair is spectrally broadened; a transparent dispersive optical system (O), --receives as input the output of said nonlinear medium (NL); - said transparent dispersive optical system (O) for providing as output pairs of linearly polarized pulses (PP) in which the two pulses of each pair overlap in time wholly or partially; - a parametric device (DFG), --receives as input the output of said transparent dispersive optical system (O), - the parametric device (DFG) for providing as output the frequency difference between frequency components polarized along the first direction (E1) and frequency components polarized along the second direction (E2).

2. the optical system further comprising: a filter (F), --receives as input the output of said parametric device (DFG); An optical system according to claim 1, further comprising the filter (F) for extracting as an output the frequency difference of the outputs of the parametric device (DFG).

3. The optical system of claim 2 , wherein the filter is a dichroic mirror.

4. 4. The optical system of claim 1, further comprising a waveplate for controlling the polarization state of the input light pulse.

5. The optical system (O) further comprises an optical component arranged between the optical system (O) and the parametric device (DFG), the optical component comprising: --receives as input the output of said dispersive optical system (O), An optical system according to any one of claims 1 to 4, comprising an optical component for focusing the output of said dispersive optical system (O) onto said parametric device (DFG).

6. The birefringent medium (B) has a thickness of 100 μm or more and is selected from the group consisting of calcite crystal, quartz crystal, α-BBO crystal, YVO ４ Crystal, TeO ２ 6. The optical system according to claim 1, comprising at least one crystal selected from the group consisting of:

7. The dispersive optical system (O) may be a glass window, a YAG crystal, a thick lens, a calcite crystal, a quartz crystal, an α-BBO crystal, a YVO ４ Crystal, TeO ２ 7. The optical system of claim 1, comprising at least one material selected from crystals.

8. The optical system according to any one of claims 1 to 7, wherein the nonlinear medium (NL) comprises at least one material selected from a YAG crystal, a sapphire crystal, a calcium fluoride crystal, and a fused silica window.

9. The parametric device (DFG) is made of β-BBO, LBO, LiNbO ３ , LiIO ３ 9. The optical system according to claim 1, wherein the second-order nonlinear crystal comprises at least one material selected from the group consisting of KTA, LGS, and AGS.

10. 9. The optical system according to claim 1, wherein the parametric device (DFG) is a nonlinear crystal characterized by a second-order nonlinear susceptibility.

11. Optical carrier frequency f ｐ and pulse duration τ ｐ from an input optical pulse having an optical carrier frequency f ｉ 1. A method for generating carrier-envelope phase-stable (CEP-stable) optical pulses of the optical carrier frequency f ｐ and pulse duration τ ｐ providing (10) an input light pulse having - generating (20) from said input light pulses by a birefringent medium (B) a pair of linearly polarized pulses (PP), each of which has orthogonal polarizations along first and second directions (E1, E2) and a relative optical group delay of approximately the pulse duration (τ ｐ ) or more of the generating step; - a step (30) of inducing filamentation of at least one of the two pulses of each pair by a nonlinear medium (NL), wherein at least one of the two pulses of each pair is spectrally broadened as a result of the filamentation; - overlapping (40) in time the two pulses of each pair at the output of said nonlinear medium (NL) by means of a dispersive optical system (O); - generating (50) by a parametric device (DFG) a frequency difference between frequency components polarized along said first direction (E1) and frequency components polarized along said second direction (E2).

12. The method further comprises: A method according to claim 11, comprising a step (60) of extracting the frequency difference produced by means of a filter.

13. The method further comprises: A method according to claim 11 or 12, comprising a step (42) of focusing, by means of an optical component, the output of said optical system (O) onto said parametric device (DFG).

14. The method further comprises:

14. The method according to any one of claims 11 to 13, further comprising the step (12) of adjusting the energy split between the two pulses of the pulse pair produced by the birefringent medium by means of a waveplate.

15. From the input optical pulse to the optical carrier frequency f ｉ a light source unit for generating a carrier-envelope phase-stable (CEP-stable) optical pulse of - optical carrier frequency f ｐ , pulse duration τ ｐ a pulse generator for delivering input optical pulses having a linear polarization; an optical system according to any one of claims 1 to 10, adapted to receive said input light pulses; - an optical amplifier adapted to amplify the output of the optical system.

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