Method and apparatus for resonantly enhancing pulsed laser light for practical applications and sensitive measurements

Abstract

A method for passively enhancing pulsed laser light by coherently adding laser pulses in an enhancement cavity 20, the method comprising: rep , and the frequency comb line spacing is the repetition frequency f rep generating a sequence of seed laser pulses 1 having a frequency comb spectrum 3 including a frequency comb line 4 equal to , and coupling the seed laser pulses 1 via a first plate-like coupling element 25 into an enhancement cavity (20) including at least two cavity mirrors 21, 22, 23, 24 having a metallic surface and spanning a cavity beam path 26 of a resonator length L, the enhancement cavity 20 emitting a fundamental transverse mode TEM 00 and higher order transverse cavity mode TEM nm , each having a series of cavity resonance frequencies 5 and cavity offset frequencies 6; and generating at least one enhanced circulating cavity pulse 2 per cavity length by coherent superposition of seed laser pulses 1 in an enhancement cavity 20, the frequency comb spectrum 3 being a harmonic comb spectrum 3 with the seed comb offset frequency being missing, and the enhancement cavity 20 being configured such that the round-trip carrier envelope phase shift of the circulating cavity pulses 2 is in the fundamental transverse mode TEM. 00 , where N is an integer equal to or greater than 1, to provide frequency overlap for multiple cavity resonant frequencies 5 with multiple frequency comb lines 4 along a frequency comb spectrum 3. Further described is a laser pulse enhancement device and its applications, for example in field resolved spectroscopy.

Description

[Technical field]

[0001] The present invention relates to a method for passively enhancing pulsed laser light by coherently adding laser pulses in an enhancement cavity. Furthermore, the present invention relates to a laser pulse enhancement device comprising an enhancement cavity for passively enhancing pulsed laser light by coherently adding laser pulses. The main application of the present invention is in the field of laser spectroscopy. Further applications can be used in the technical fields of laser pulse modification and / or high intensity harmonic pulses, soft x-rays, hard x-rays and terahertz wave generation. [Background technology]

[0002] The following prior art is referenced herein, which describes the technical background and related art of the present invention: [1] RJ Jones et al., Optics Letters 27, 1848 (2002) [2]KR Lykke, Optics Commun.157,88(1998) [3] J. Torgerson et al., Optics Commun. 161, 264 (1999) [4]E.Fill et al. “A Pellicle Coupled Optical Resonator” in 2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference,OSA Technical Digest(Optical Society of America,2019),paper cf_p_60 [5] S. Holzberger et al., Optics Letters 40, 2165 (2015) [6] A. Foltynowicz et al.,Phys.Rev.Lett.107,233002(2011) [7]A. Foltynowicz et al., Appl. Phys. B 110, 163 (2012) [8]O. Axner et al., “NICE-OHMS - Frequency Modulation Cavity Enhanced Spectroscopy - Principles and Performance” in “Cavity Enhanced Spectroscopy and Sensing”, G. Gagliardi and H. P. Look (eds), Springer Series in Optical Sciences 179, Springer Berlin, Heidelberg 2014 [9]A. Khodabakhsh et al., Optics Letters 39, 5034 (2014)

[10] R. Gotti et al., Scientific Reports 10, 2523 (2020)

[11] G. Zhao et al., Optics Letters 43, 715 (2018)

[12] H. J. Zeng, et al. Introductory lecture: Advances in ion spectroscopy: From astrophysics to biology. Faraday Discuss. 217, 8 - 33 (2019)

[13] D. Romanini et al., “Introduction to Cavity Enhanced Absorption Spectroscopy” in “Cavity Enhanced Spectroscopy and Sensing”, G. Gagliardi and H. P. Look (eds), Springer Series in Optical Sciences 179, Springer Berlin, Heidelberg 2014

[14] International Publication No. WO 2016 / 102056

[15] I. Pupeza et al., Nature 577, 52 (2020)

[0003] Coherent addition of laser pulses in a passive resonator (enhancement cavity) has been described by RJ Jones et al. in [1]. A sequence of phase-coherent equidistant pulses (seed laser pulses) is coupled into a resonator device, which may contain two mirrors (linear cavity) or three, four or more mirrors arranged in a ring resonator shape. The resonator device forms an optical path that is adjusted such that the input optical pulses are linearly superimposed in succession to form at least one circulating cavity optical pulse. The circulating cavity optical pulse is usually obtained by coupling a seed laser pulse into the cavity using a slightly transparent mirror, but coupling via a plate or wedge has been described by KR Lykke [2], J. Torgerson et al. [3] and E. Fill et al. [4].

[0004] Such conventional resonant enhancement of pulsed laser light using passive enhancement cavities has severe limitations in terms of the spectral bandwidth of the circulating pulses. In particular, the limited bandwidth and dispersion of the cavity mirrors prevent the enhancement of a broad frequency comb within the resonator. However, a broad spectral bandwidth is particularly important for the generation of ultrashort pulses and for sensitive absorption spectroscopy (e.g. field-resolved spectroscopy, trace gas detection using FRS).

[0005] In the frequency domain, the input optical pulses give a frequency comb consisting of frequency comb lines spanning the frequency comb spectrum. The frequency comb lines have a comb line spacing equal to the repetition rate of the input optical pulse sequence and the input comb offset frequency. An additional requirement for broadband enhancement is that the optimal offset frequency (OOF) of the resonator, determined by the optics and geometric phase [5], and the input comb offset frequency are matched to each other. Especially in FRS, a harmonic comb (a comb with the input comb offset frequency equal to zero) is used, so the input offset frequency cannot be adjusted and broadband methods are required to adjust the OOF of the resonator cavity.

[0006] There are several possibilities to tune the resonator cavity to a specific OOF. The most effective approach in the NIR is to use CE phase-shift tuned cavity mirrors, as proposed in [5]. However, this approach has not been demonstrated in the MIR spectral range and leads to bandwidth limitations.

[0007] The use of resonant cavities for trace gas detection has been described in the past, as follows: In 2011, it was shown that the use of cavities allows shot-noise limited absorption spectroscopy. However, the spectral bandwidth in this experiment was only 30 nm, centered at 1530 nm [6]. The first mid-infrared cavity experiment was performed by the same group, which showed a sensitivity of 8 ppb in the wavelength range 3500-4100 nm [7]. The most sensitive method of trace gas detection is NICE-OHMS (Noise-resistant Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy) [8]. However, this technique is inherently narrowband and therefore not suitable for the analysis of complex gas mixtures. Furthermore, attempts were made to implement the NICE-OHMS technique together with a frequency comb, as described by A. Khodabakhsh et al. [9]. Nevertheless, the spectral range of this experiment was extremely narrow, ranging from 1585-1590 cm, which corresponds to 20 nm. -1 It only extends between

[0008] Although there have been advances in sensitivity in recent years, reported spectral bandwidths are still very limited, with measurements of less than 5 minutes in a bandwidth of 18 nm at 1.54 μm and a sensitivity of 10 -10 cm -1 CO 2 and C 2 H 2 It has been reported by R. Gotti et al.

[10] . As reported by G. Zao et al., applying a cavity finesse of 5000, acetylene can be detected at 6518 cm in a bandwidth of 15.6 nm. -1

[11]

[0009] However, no existing technology achieves the sensitivity required to perform direct broadband spectroscopy of molecular ions stored in an ion trap, and the total number of molecules is typically less than 10 6

[12] This would broaden the range of possible applications, from astrophysics to analytical chemistry, biochemistry, and quantum science simulations. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] International Publication No. 2016 / 102056 [Non-patent literature]

[0011] [Non-Patent Document 1] RJ Jones et al., Optics Letters 27(20), 1848-1850(2002) [Non-Patent Document 2] KR Lykke, Optics Commun. 157, 88-92 (1998) [Non-Patent Document 3] J. Torgerson et al., Optics Commun. 161, 264-266 (1999) [Non-Patent Document 4] E.Fill et al. “A Pellicle Coupled Optical Resonator” in 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference,OSA Technical Digest(Optical Society of America,2019),paper cf_p_60 [Non-Patent Document 5] S. Holzberger et al., Optics Letters 40(10), 2165‐2168(2015) [Non-Patent Document 6] A.Foltynowicz et al.,Phys.Rev.Lett.107(23),233002(2011)

Non-licensed Document 7

Non-licensed literature 9

Non-licensed literature 10

Non-licensed Document 11

Non-licensed Document 12

Non-licensed Document 13

[0012] The object of the present invention is to provide an improved method of enhancing pulsed laser light using a passive enhancement cavity and an improved laser pulse enhancement device, avoiding the limitations of the prior art, especially with regard to bandwidth and dispersion. In particular, it is an object to provide enhancement of harmonic combs (i.e. zero offset frequency combs), as used, for example, in field-resolved spectroscopy (FRS). In particular, enhanced pulsed laser light is provided by coherently adding pulses to improve spectroscopic performance, especially with large spectral bandwidth, high intensity and / or minimal dispersion effects. Furthermore, a laser pulse enhancement device is provided, which has a compact structure that allows stable operation in practical applications. Furthermore, novel applications of pulsed laser light are provided.

[0013] The above mentioned object is solved by a method for passively enhancing pulsed laser light and a laser pulse enhancement device with the features of the independent claims. Preferred embodiments and applications of the invention are defined in the dependent claims. [Means for solving the problem]

[0014] According to a first general aspect of the present invention, the above mentioned object is solved by a method for passively enhancing pulsed laser light by coherently adding laser pulses in an enhancement cavity, the method comprising the steps of: rep , and the frequency comb line spacing is the repetition frequency f rep and coupling the seed laser pulses via a first plate-like coupling element into an enhancement cavity comprising at least two cavity mirrors having metallic surfaces (hereinafter also denoted metallic cavity mirrors or metallized cavity mirrors) and spanning a cavity beam path of a resonator length L, the enhancement cavity being configured to generate a fundamental mode (TEM 00 ) and higher-order transverse cavity modes (TEM nm ), each having a series of cavity resonant frequencies and cavity offset frequencies; and generating at least one enhanced circulating cavity pulse per cavity length by coherent superposition of seed laser pulses in the enhancement cavity.

[0015] According to the present invention, the frequency comb spectrum is a harmonic comb spectrum with a missing seed comb offset frequency. Further, according to the present invention, the enhancement cavity is configured such that the round-trip carrier envelope phase (CEP) shift of the circulating cavity pulses is proportional to the fundamental transverse mode TEM. 00, where N is an integer equal to or greater than 1, to provide frequency overlap for multiple cavity resonant frequencies with multiple frequency comb lines along the frequency comb spectrum. The CEP shift is the amount of phase change between the electric carrier light field and the pulse envelope of the circulating pulse during one round trip in the enhancement cavity. Providing frequency overlap means aligning the seed comb and / or enhancement cavity such that the cavity resonant frequencies are located at the frequencies of the frequency comb lines.

[0016] According to a second general aspect of the present invention, the above mentioned object is solved by a laser pulse enhancement device, which is adapted to passively enhance pulsed laser light by coherent addition of laser pulses, the device having a repetition rate f rep , and the frequency comb line spacing is the repetition frequency f rep and an enhancement cavity including at least two cavity mirrors having metallic surfaces and spanning a cavity beam path of a resonator length L, wherein the enhancement cavity is configured to generate a fundamental mode (TEM 00 ) and higher-order transverse cavity modes (TEM nm ), each having a series of cavity resonant frequencies and a cavity offset frequency, and a first plate-like coupling element positioned to couple a seed laser pulse into the enhancement cavity, wherein the enhancement cavity is adapted for coherent superposition of the seed laser pulse coupled into the enhancement cavity to generate at least one enhanced circulating cavity pulse.

[0017] In accordance with the present invention, a laser source is configured to generate a seed laser pulse having a harmonic comb spectrum with a missing seed comb offset frequency, and an enhancement cavity is configured such that the round-trip carrier envelope dephasing of the circulating cavity pulses results in a fundamental transverse mode TEM. 00 and wherein the laser source device and the enhancement cavity are adapted to provide frequency overlap for a plurality of cavity resonant frequencies with a plurality of frequency comb lines along the frequency comb spectrum. Preferably, the laser pulse enhancement device of the present invention or an embodiment thereof is adapted to perform a method according to the first general aspect of the present invention or an embodiment thereof.

[0018] The enhancement cavity is a linear cavity or has a ring resonator shape. The cavity mirrors may be flat or curved mirrors, preferably at least one curved mirror provides a focused cavity beam path (cavity beam path with at least one focus). The enhancement cavity preferably does not have an optical field amplification medium adapted to amplify the optical field of the circulating cavity pulse(s). An important advantage of the present invention is that, unlike [5], by providing metallic cavity mirrors (i.e. mirrors with metallic surfaces), the dispersion of the pulses circulating in the cavity can be made very small, which in principle allows operation with a very wide bandwidth. Preferably, all mirrors of the enhancement cavity are metallic mirrors.

[0019] Metallic cavity mirrors are used despite their general phase shifting effect. Specifically, metallic mirrors have a CEP shift of 180°. The total CEP shift (round trip phase shift) of the cavity consists of the total CEP shift of the mirrors plus the geometric phase shift (Gouy phase). The geometric phase shift can be 0° or 180° only at the stable edges of the cavity. Furthermore, the total CEP shift and the optimal offset frequency (OOF) of the resonator are related to each other. An OOF equal to 0° is the same as a total CEP shift equal to 0°. Before the present invention, it was thought that it was practically impossible to obtain a total CEP shift equal to 0° and to set the OOF to 0°, i.e. to enhance a harmonic comb with a cavity having metallic mirrors and a geometric phase shift not equal to 180° or 0°.

[0020] However, the inventors of the present application have found a new way to set the optimal offset frequency of the resonator, so that the invention allows to provide a wide bandwidth characteristic of the pulse enhancement using metallic mirrors, so that, contrary to [4], the cavity resonance can be aligned with the seed comb line. First, the cavity can be adjusted to the fundamental transverse mode TEM, preferably by changing the focal shape of the enhancement cavity, in particular by setting the distance between at least two mirrors and / or the deflection angle of at least one mirror with respect to the cavity beam path. 00 The round trip carrier envelope phase shift relative to is adjusted to 360° / N. Furthermore, as outlined below, the cavity resonant frequency is overlapped, specifically matched, with the frequency comb lines of the seed laser pulses, specifically by modifying the cavity length by a corresponding factor or by exciting higher order transverse cavity modes. Advantageously, frequency matching can be obtained over an extended portion of the comb spectrum, preferably over the entire comb spectrum, to enhance the broadband harmonic comb.

[0021] Broadband operation of the enhancement cavity refers to enhancing laser pulses, preferably with a central wavelength of 1000 nm, in particular 5 μm-20 μm, and a spectral bandwidth of at least 100 nm, in particular at least 5000 nm, and / or at most 20 μm, in particular at most 12 μm. Preferably, the enhanced frequency comb of the cavity pulses covers at least one octave of optical frequencies. As an example, a central wavelength in the range of 5 μm-20 μm is particularly important since it covers the characteristic spectral response range of biological samples (the so-called fingerprint region). The enhancement cavity is capable of enhancing laser pulses with the above-mentioned parameters and is different from conventional enhancement cavities for harmonic generation, which use pulses with a central wavelength such as 800 nm and a spectral bandwidth such as 200 nm. Such conventional enhancement cavities are not limited to harmonic combs and may use dielectric mirrors.

[0022] According to a further important advantage of the present invention, the spectral transfer function between the cavity input and output has such low dispersion that the pulse width of the broadband pulse is not affected or is negligibly affected. This property is important for the application of the present invention in FRS, since maintaining the broadband property means providing an enhanced pulse for a short time, which minimizes the temporal overlap of the pulse exciting the sample and the sample response, thus reducing the intensity noise in FRS.

[0023] According to a preferred embodiment (hereinafter referred to as the first embodiment of the invention, see FIG. 3), the frequency overlap is L * =(c / f rep ) / N, and adjusting the fundamental transverse mode (TEM 00 ) is given by exciting the fundamental cavity resonance frequency, c / L *For each cavity resonant frequency, every Nth frequency comb line has a frequency overlap, which is equal to: rep ) as 1 / N of cavity length L * Frequency matching of every Nth frequency comb line is obtained by choosing the cavity length L * The setting of is obtained, for example, by setting the distance between at least two mirrors of the cavity. The first embodiment has a particular advantage with regard to the ease of adjusting the enhancement cavity by shortening the resonator length.

[0024] According to an alternative preferred embodiment (hereinafter referred to as the second embodiment of the invention, see FIG. 4), the frequency overlap is achieved by splitting the n+m+1=N higher order transverse cavity modes (TEM modes) of the enhancement cavity. nm The fundamental cavity length L of the enhancement cavity is given by L = (c / f rep ) is maintained according to the TEM. The cavity resonant frequency spacing is equal to c / L, but nm The Gouy phase-induced contribution to the offset frequency of the cavity resonance is 00 The second embodiment has a particular advantage in terms of efficiency improvement, since all frequency comb lines of the comb spectrum contribute to the enhancement, with N times the Gouy phase induced contribution to the offset frequency of the resonance, and each of the cavity resonances with one of the frequency comb lines is given a frequency overlap.

[0025] Preferably, the second embodiment is combined with shaping the optical field of the seed laser pulse to generate higher order modes of the enhancement cavity (TEM) excited by the circulating cavity pulse. nm) is improved spatial overlap with the enhancement cavity. In a preferred example, the laser source device is operated such that its output includes a seed laser pulse having a geometric mode structure that is matched to the higher order mode. Alternatively, the laser source device is operated such that its output includes a seed laser pulse having a fundamental mode structure, and a mode matching device is provided between the laser source device and the enhancement cavity to match the mode structure with the geometry of the higher order cavity mode.

[0026] Preferably, the seed laser pulses are generated by difference frequency generation (DFG). Therefore, the laser source device is preferably adapted to generate seed laser pulses by difference frequency generation. Particularly preferably, the laser source device comprises a pulsed laser source emitting laser pulses for driving the DFG, particularly preferably with a duration below 20 fs, and the DFG device is arranged for intra-pulse DFG of the pulses output by the pulsed laser source. The output of the DFG device provides the seed laser pulses, which have essentially zero offset frequency and high phase stability.

[0027] The first plate-shaped coupling element is provided for coupling the seed laser pulse into the enhancement cavity by reflecting it towards the cavity beam path. The first plate-shaped coupling element is arranged in the cavity beam path. To improve the broadband enhancement, the first plate-shaped coupling element preferably includes at least one of the following features, which provide advantages in terms of reducing the losses experienced by the cavity pulse circulating in the cavity and adjusting the power coupling ratio to match the optimum value determined by the reflectivity of the cavity mirrors:

[0028] Preferably, the first plate-like coupling element comprises a pellicle, a plate, or a wedge element. Advantageously, such exemplary coupling elements minimize the dispersion introduced into the enhancement cavity.

[0029] Advantageously, the first plate-shaped coupling element may have a one-sided anti-reflection coating, which avoids etalon effects (leading to bandwidth limitations) and additional losses due to power being reflected out of the cavity.

[0030] Furthermore, the first plate-like coupling element has a surface that can be oriented at an angle equal to or close to the Brewster angle with respect to the cavity beam path. When using a thin wedge, one wedge surface is oriented at or close to the Brewster angle. Again, etalon effects are avoided.

[0031] According to another variant, if the first plate-shaped coupling element has low transmission losses in the wavelength range from 7 μm to 12 μm, special advantages are obtained for enhancing frequency combs in this wavelength range and for using cavity pulses for spectroscopic measurements.

[0032] According to a preferred material choice, the first plate-shaped coupling element may be made of polyethylene or diamond, which, unlike commercially available pellicles made of nitrocellulose, etc., exhibits strong absorption in the mid-infrared range.

[0033] Preferably, the first plate-shaped coupling element has a thickness below the central wavelength of the seed laser pulse, in order to avoid interference effects in an advantageous manner. Particularly preferably, the first plate-shaped coupling element has a thickness in the range of 100 nm to 500 μm.

[0034] According to a further preferred variant of the invention, one of the cavity mirrors is arranged outside the resonator plane, in which the cavity beam path into and out of the first plate-like coupling element extends. In this non-planar embodiment (see FIG. 6), losses per cavity round trip can be reduced in an advantageous manner by providing an enhanced circulating cavity pulse at the out-of-plane mirror and at the plate-like coupling element with a selected polarization. Preferably, the enhanced circulating cavity pulse is reflected at the cavity mirror arranged outside the resonator plane with s-polarized light at small angular incidence, while the enhanced circulating cavity pulse passes through the first plate-like coupling element with p-polarized light. In particular, the mirror reflection of s-polarized light allows a better reflectivity than that of p-polarized light. Furthermore, the p-polarized light at the plate-like coupling element allows the Brewster's angle of the plate-like coupling element to be exploited.

[0035] According to another embodiment of the invention, a part of the circulating enhanced cavity pulse(s) is coupled out of the enhancement cavity after interacting with the sample, for example for spectroscopic measurements. The pulse coupling out of the cavity is preferably performed via the first plate-shaped coupling element or via another second plate-shaped coupling element. Using a single first plate-shaped coupling element is advantageous in terms of reducing the complexity of the setup. The out-coupling of the part of the circulating cavity pulse with the first plate-shaped coupling element may include deflecting a part of the circulating optical field out of the cavity, this part of the circulating optical field not interfering with the part of the seed laser pulse transmitted by the first plate-shaped coupling element. Alternatively, the first plate-shaped coupling element may be arranged such that the cavity beam path is folded back such that the coupling element crosses the cavity beam path at one or two points. Alternatively, using a second plate-shaped coupling element is advantageous in terms of individually optimizing the coupling of the seed laser pulse into the cavity and the coupling of the optical pulse out of the cavity. The second plate-shaped bonding element preferably comprises at least one of the preferred features of the first plate-shaped bonding element described above.

[0036] According to a preferred application of the invention in spectroscopy, a portion of the seed laser pulse transmitted through the first plate-shaped coupling element is subjected to spectroscopic measurement. Advantageously, in this transmitted beam, the transmitted light field of the seed laser pulse interferes with a portion of the circulating light field deflected out of the cavity. If the cavity does not contain a sample, destructive interference preferably occurs, and the transmitted beam contains almost no power. If the cavity contains a sample, the transmitted beam mainly contains the spectroscopic response of the sample, i.e. the molecular signal.

[0037] Thus, according to a particularly advantageous embodiment of the invention, the sample to be investigated, preferably a gaseous sample such as a gas, vapor or aerosol, is fed into the enhancement cavity such that the circulating cavity pulses interact with the sample, and sample-specific information is extracted from the circulating cavity pulses that have interacted with the sample. For example, an absorption measurement is performed and a sample-specific change in the frequency comb spectrum of the circulating cavity pulses compared to the frequency comb spectrum of the seed laser pulse is detected. Regarding preferred device features of the laser pulse enhancement device, the enhancement cavity is adapted to accommodate the sample to be investigated and the circulating cavity pulses interact with the sample, and a detection device is arranged for extracting sample-specific information from the circulating cavity pulses, such as for absorption measurements by detection of sample-specific changes in the frequency comb spectrum of the circulating cavity pulses compared to the frequency comb spectrum of the seed laser pulse. The detection device is preferably arranged outside the enhancement cavity for performing measurements such as field-resolved spectroscopy measurements according to the available settings.

[0038] With the technique of the present invention, a preferably wide harmonic comb can be effectively coupled into the cavity and enhanced. This is important for generating short pulses that circulate in the cavity. The main application of a broadband cavity is therefore in highly sensitive absorption spectroscopy. The effective absorption length of a sample contained in a standing wave resonant cavity can be enhanced by a factor of 2 / πF, where F is the finesse of the cavity

[12] . The most important for detecting trace gas molecules etc. in the ambient gas is the wavelength of about 500-1500 cm. -1 The so-called optical spectral fingerprint region, which spans the entire spectrum, and the so-called functional group region of the spectrum (1500–4000 cm -1 ) In order to measure many gases or gas mixtures with high sensitivity, the widest possible spectral range is investigated. According to the preferred application of the invention, optical frequency ranges of more than one octave are easily achievable.

[0039] The sample may be placed in an absorption tube that accommodates a portion of the cavity beam path of the enhancement cavity for interaction inside the cavity. Advantageously, the absorption tube limits the volume that the sample can be accommodated, so that the number of samples investigated may be reduced. Alternatively, the sample may be delivered by a delivery device to a limited area of ​​the cavity beam path using delivery lines and / or nozzles. Thus, another important feature of the invention is the enhanced sensitivity combined with a small irradiation volume. This is advantageous when applying the invention to direct absorption spectroscopy of thin samples, such as electrospray and molecular jet. Also, in other applications of the invention, the concentration of power in a small volume allows driving optical nonlinearities.

[0040] Alternatively, the sample may be placed in a container that houses the enhancement cavity, this embodiment having advantages resulting from easier delivery of the sample to the cavity.

[0041] Another possible application takes advantage of the high finesse exhibited at wavelengths as small as 1 μm, which can be used in dual wavelength experiments such as molecular orientation with lasers simultaneously resonating in the near infrared.

[0042] In the main application in spectroscopy, the method for passively enhancing pulsed laser light is a measurement method, such as a spectroscopy method, and the laser pulse enhancement device is a measurement device, such as a spectroscopy device, both of which represent independent subjects of the present invention.

[0043] The features disclosed in the context of a method for passively enhancing pulsed laser light and its embodiments or a spectroscopic measurement method and its embodiments also represent preferred features of the inventive laser pulse enhancement device and spectroscopic measurement device and their embodiments. The aforementioned aspects and inventive features and preferred features also apply to the present method, in particular with regard to the configuration of the laser pulse enhancement device and spectroscopic measurement device, as well as the dimensions and configuration of the individual components described in connection with the device. The preferred embodiments, variants and features of the invention described above can be combined with each other as necessary.

[0044] Further details and advantages of the invention are explained below with reference to the accompanying schematic drawings. [Brief description of the drawings]

[0045] [Figure 1] 1 illustrates a feature of a preferred embodiment of the present invention using a linear cavity. [Diagram 2] 1 illustrates a feature of a preferred embodiment of the present invention using a bowtie cavity. [Diagram 3] An example of a first embodiment is shown using a shortened cavity by a factor of N to enable broadband enhancement of the harmonic comb using metallic mirror coatings. [Figure 4] A second example embodiment is shown using a cavity operated in a higher order transverse mode to enable broadband enhancement of the harmonic comb using metallic mirror coatings. [Diagram 5] 13 shows an embodiment in which pulses are coupled into and out of a bowtie cavity by a single plate-like coupling element. [Figure 6] A three-mirror cavity for traveling wave manipulation is shown. [Figure 7] 1 shows a two-mirror cavity for traveling wave manipulation using a wedge as a coupling element. [Figure 8] 1 shows a linear cavity that can be used for sensitive absorption measurements. [Figure 9] An embodiment using a wedge-coupled cavity for sensitive absorption measurements is shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] An embodiment of the invention is described below with reference to the matching of the seed comb line with the resonant frequency of the enhancement cavity and with an exemplary configuration of the enhancement cavity. It is emphasized that the practical implementation of the invention is not limited to the described examples of laser sources and cavities, and correspondingly allows for other usable types of laser sources and cavities, shapes and configurations. Details of the generation of seed laser pulses or of the performance of absorption measurements by a sample are not shown or described, as they are known per se.

[0047] FIG. 1 shows an embodiment of a laser pulse intensifier 100 having a laser source device 10 and an enhancement cavity 20. The enhancement cavity 20 in this embodiment is a linear cavity and comprises two mirrors 21, 22, which have gold surfaces and span the cavity beam path 26 with a resonator length L. At least one of the mirrors 21, 22 is curved to ensure the stability of the resonator. As a first plate-like coupling element 25, a pellicle, plate or wedge is placed in the enhancement cavity 20. The coupling element 25 crosses the cavity beam path 26. As an example, the coupling element 25 is a pellicle made of polyethylene with a thickness of 20 μm. The advantage of a linear cavity is its simplicity and the need for only a single coupling element 25 for input / output coupling.

[0048] The laser source device 10 includes, for example, an amplified mode-locked laser and a DFG unit (details not shown). The seed laser pulse 1 has a repetition frequency f rep and the frequency comb line spacing is the repetition frequency f rep and a frequency comb spectrum 3 containing frequency comb lines 4 equal to 1000 Hz (schematically shown in Figures 3B and 4B). For example, the seed laser pulses 1 have a central wavelength of 8-10 µm, a duration of 50-100 fs, and a repetition frequency of 10 to several hundreds of MHz.

[0049] The seed laser pulse 1 is coupled into the enhancement cavity 20 via an optional semi-transparent mirror 11 and a first plate-like coupling element 25 by illuminating the coupling element 25 at an angle close to the Brewster angle. The first plate-like coupling element 25 is, for example, a polyethylene pellicle with a thickness of 23 μm. Although the reflectivity of the input coupling element 25 is low, destructive interference between the part of the seed pulse transmitted through the input coupling element and the intracavity beam reflected from the input coupling element leads to efficient power coupling into the cavity. Although the amplitude ratio of the input coupled light is relatively small, the power enhanced in the cavity is large due to the low losses in the cavity. Furthermore, the low losses induced by the pellicle or plate and the low losses of the cavity mirrors 21, 22 lead to a high cavity finesse. As a result, the effective absorption length of the absorption cell containing a sample of the trace gas is increased by a significant factor (see FIG. 8).

[0050] The laser pulses coupled into the enhancement cavity 20 are coherently superimposed, so that one or more enhanced cavity pulses 2 are generated per cavity length, which circulate between the mirrors 21, 22 (see double-headed arrows). After each pass through the coupling element 25, a portion 7 of the cavity pulse 2 is deflected out of the enhancement cavity 20 and reaches the application site 30A via the semi-transparent mirror 11. For spectroscopic applications, if a sample is included in the cavity beam path 26, the portion of the seed laser pulse transmitted through the coupling element 25 is coherently superimposed with the portion of the circulating pulse interacting with the sample and is collected by the detection device 30, e.g. for FRS.

[0051] According to a first embodiment of the invention, in order to align the frequency comb lines 4 of the frequency comb spectrum 3 with the cavity resonance frequency 5, the cavity length L is set as described below with reference to Fig. 3. Alternatively, according to a second embodiment of the invention, the seed laser pulse 1 is spatially shaped and coupled to higher order transverse modes of the cavity as described below with reference to Fig. 4.

[0052] FIG. 2 shows features of an alternative embodiment of a laser pulse enhancement device 100 having a laser source device 10 and an enhancement cavity 20, which is a so-called bowtie cavity folded by four gold-coated mirrors 21, 22, 23, 24. Two mirrors (23, 24) are curved mirrors that define a cavity focus 26A of a cavity optical path 26 between them. The advantage of the bowtie cavity of FIG. 2 is that it allows more flexibility in adjusting parameters such as finesse and CE phase. Also, due to the traveling wave operation of the cavity, the first plate-like coupling element 25 is traversed only once during a circulation period. Only one reflection at the plate-like coupling element per cavity round trip allows impedance matching, loss reduction and optimal enhancement. This traveling wave advantage is also obtained in the embodiments of FIGS. 3-7 and 9. Furthermore, a cavity focal point 26A is created between the curved mirrors, which is essential for some applications such as harmonic generation and x-ray generation.

[0053] A seed laser pulse 1 is generated in a laser source device 10, e.g. as in Fig. 1, and coupled into an enhancement cavity 20 where it is coherently superimposed, thus generating e.g. one circulating enhanced cavity pulse 2, which circulates along the cavity beam path 26 (see arrows). To align the frequency comb lines 4 of the frequency comb spectrum 3 of the seed laser pulse 1 with the cavity resonance frequency 5, means of the first or second embodiment of the present invention are provided, as shown in Figs. 3 and 4.

[0054] According to a first variant of this embodiment, two coupling elements 25, 25A, such as pellicles or plates, are provided. The first plate-shaped coupling element 25 is arranged to couple a seed laser pulse 1 originating from a laser source device 10 into an enhancement cavity 20. According to the direction of the arrow, the laser pulse circulates in the cavity and the cavity pulse 2 is enhanced each time it passes through the first plate-shaped coupling element 25 by being coherently superimposed with another seed laser pulse 1.

[0055] The optional second plate-shaped coupling element 25A is arranged at a distance from the first plate-shaped coupling element 25, for example, in the part of the cavity beam path between the two plane mirrors 21 and 22. The second plate-shaped coupling element 25A may have a lower reflectivity compared to the first plate-shaped coupling element 25. Each time the cavity pulse 2 passes, a part 7 of the cavity pulse 2 is reflected out of the cavity, and the reflected part contributes less than the seed laser pulse 1 coupled into the cavity. In an embodiment, the first coupling element 25 and the second coupling element 25A are a polyethylene film with a thickness of 20 μm, or a diamond wedge with an average thickness of 350 μm, or a parallel plane diamond plate with a thickness of 350 μm and a one-sided anti-reflection coating.

[0056] If the cavity pulse 2 at the cavity focus 26A is used, for example, for spectroscopic measurements or to drive optical processes within the enhancement cavity 20, a second coupling element (shown in dashed lines) may be provided for diagnostic purposes only, or it may be arranged to couple light to the application site 30A.

[0057] According to a second variant of the embodiment of Fig. 2 (preferred configuration for spectroscopy applications), the measurement device 30 is arranged to collect the beam transmitted by the first coupling element 25 and coherently superimposed with the part of the circulating pulse interacting with the sample (right dashed arrow), and the second plate-like coupling element 25A is omitted. This configuration is advantageous for time-resolved background-free spectroscopy such as FRS, because the enhancement is not reduced by the second plate and it results in less superimposition of molecular signals, since the excitation pulse (circulating enhancement pulse) is at least partially suppressed by destructive interference.

[0058] A first embodiment of the invention (frequency matching by cavity length adjustment) is illustrated in Figure 3, where Figure 3A shows a variation of a bowtie enhancement cavity 20 (as shown in Figure 2) having four mirrors 21, 22, 23, 24, where a first plate-like coupling element 25 is a wedge element located at the cavity focus between mirrors 23 and 24. Mirrors 21, 22, 23, 24 span a cavity beam path 26 with a cavity length L. As shown in Figure 3B, the enhancement cavity 20 has a fundamental mode TEM frequency band including a series of fundamental cavity resonance frequencies 5 and a cavity offset frequency 6. 00 In the first embodiment of the invention, the higher order modes of the enhancement cavity 20 are not used.

[0059] In the time domain (Figure 3A), the seed laser pulse 1 is rep The seed laser pulse is emitted by the laser source device 10 and enters the enhancement cavity 20 in the fundamental mode TEM of the enhancement cavity 20.00 The fundamental Gaussian mode TEM is consistent with 00 In the frequency domain (Figure 3B), the frequency comb spectrum 3 of the seed laser pulse 1 has an optical field of f rep The seed laser pulse 1 provides a harmonic comb, so that the frequency comb line 4A is equal to f rep (the input comb offset frequency is equal to zero).

[0060] To match the frequency comb line 4 and the cavity resonant frequency 5 to each other, at least one of the mirrors 21, 22, 23, and 24 of the enhancement cavity 20 is adjusted to obtain a round-trip carrier envelope phase shift ΔΦ=360° / N, and the resonator length L is set according to equation 1, e.g., N=3.

number

[0061] A second embodiment of the invention (frequency matching by coupling to higher order cavity modes) is shown in Fig. 4, where Fig. 4A exemplarily shows a bowtie enhancement cavity 20 with four mirrors 21, 22, 23, 24 and a first plate-like coupling element 25 (wedge) located at the cavity focus between mirrors 23 and 24. The mirrors 21, 22, 23, 24 span the cavity beam path 26 with a resonator length L. In the second embodiment, as shown in Fig. 4B, a higher order transverse mode of the enhancement cavity 20 is used, which has a range of higher mode cavity resonant frequencies 5, including a cavity resonant offset frequency 6 that is a multiple of the cavity resonant frequency of the fundamental transverse mode.

[0062] In the time domain (Figure 4A), the seed laser pulse 1 is rep The seed laser pulse 1 enters the enhancement cavity 20 at 1000 Hz. The seed laser pulse 1 is emitted by the laser source device 10 in the fundamental Gaussian mode TEM. 00 is generated, and then transformed by the mode matching device 40 so that the optical field is tuned to the higher order modes TEM of the enhancement cavity 20. nm , where n+m+1=N, where N is a natural number equal to or greater than 2. The mode matching device 40 comprises at least one absorbing mask and / or phase mask, and optionally a cylindrical optical element, to geometrically shape the optical field to match the higher order modes TEM of the enhancement cavity 20. nm Shape of the TEM 11 or TEM 20 The cascade connection is configured to match the mode.

[0063] In the frequency domain (Figure 4B), the frequency comb spectrum 3 of the seed laser pulse 1 has a difference of f rep 3, the seed laser pulse 1 provides a harmonic comb, so frequency comb line 4A is similarly positioned at 0 (the input comb offset frequency is equal to zero).

[0064] In deviation from the first embodiment, the resonator length L is set according to equation 2, ie according to the resonant full length cavity of a conventional cavity.

number

[0065] Adjusting the resonator length according to the first embodiment or coupling to higher order modes according to the second embodiment is not limited to the bowtie shape of the enhancement cavity 20, but is also possible with any other configuration of the enhancement cavity, such as the embodiments of Figures 1 and 2 described below, or the embodiments of Figures 5 to 9.

[0066] According to Fig. 5, the enhancement cavity 20 of the laser pulse enhancement device 100 has a bowtie cavity shape as in Fig. 2, with four cavity mirrors 21, 22, 23 and 24 spanning the cavity beam path 26. In this embodiment, a single plate-shaped coupling element 25 is provided for input / output coupling. The coupling element 25 is arranged to cross the cavity beam path 26 in two different beam path parts, resulting in slightly different angles of the surface of the coupling element 25 with respect to the cavity beam path 26. Compared to the part 7 of the enhanced cavity pulse(s) coupled outside the enhancement cavity 20 to the application site 30, the seed laser pulse 1 originating from the laser source device 10 may be coupled into the enhancement cavity 20 from another direction. This embodiment has the advantage that the angle between the beams reflected from the plane mirrors is very small.

[0067] FIG. 6 shows a non-planar embodiment of the laser pulse intensifier 100, in which the traveling wave enhancement cavity 20 uses only three mirrors 21, 22, and 23. The plane mirror 21 is operated at high angles of incidence. The advantage of this enhancement cavity 20 is that it has a lower loss rate than a cavity with four mirrors. In a preferred realization of this design, the plane in which the incident and reflected beams spread at the plane mirror 21 is perpendicular to the plane in which the incident and transmitted beams spread at the first plate-like coupling element 25. For example, the latter plane is the horizontal xy plane (perpendicular to the plane of the drawing) of the platform supporting the laser pulse intensifier 100, and the plane mirror 21 extends parallel to the horizontal xy plane. In this embodiment, the radiation may be p-polarized with respect to the first plate-like coupling element 25 and s-polarized with respect to the plane mirror 21, resulting in a very high reflectivity at high angles of incidence.

[0068] The laser pulse intensifier 100 of Fig. 7 has a traveling-wave enhancement cavity 20 with only two mirrors 21, 22. Unlike the embodiment of Fig. 1, the cavity beam path 26 follows a ring shape instead of folding back on itself. This is made possible by coupling the seed laser pulse 1 through a coupling element 25 that includes a tilted wedge. This design combines the advantages of a traveling-wave cavity (only one pass through the coupling element 25 per circulation) with the low losses of a linear cavity.

[0069] According to FIG. 8, the laser pulse intensifier of the invention is configured as a spectroscopic measurement device 200, comprising a laser source device 10, an enhancement cavity 20 and a detector device 32. The spectroscopic measurement device 200 is adapted for the detection and measurement of highly sensitive spectroscopic trace gases. The enhancement cavity 20 has a linear cavity configuration as shown in FIG. 1 and comprises two mirrors 21, 22 and a pellicle-like coupling element 25. In addition to this, an absorber tube 31 is arranged in the enhancement cavity 20. The absorber tube 31 is a straight, longitudinally extending hollow tube, containing the cavity beam path 26. The axial end of the absorber tube 31 may be open towards the enhancement cavity 20 or may be closed by a thin foil window, or alternatively by an AR coating or Brewster window. The absorber tube 31 is filled with a gas sample 8 to be analyzed, such as a molecular gas or a gas mixture.

[0070] A seed laser pulse 1, for example with a central wavelength of 8-10 μm, a duration of 50-100 fs and a repetition rate of tens to hundreds of MHz, is coupled into the enhancement cavity 20 via a coupling element 25 and coherently added to the enhanced cavity pulse 2 circulating in the enhancement cavity 20. The cavity pulse 2 repeatedly passes through the absorption tube 31 and is absorbed by the sample 8. After absorption, a molecular reaction with the shape of the tail of the optical field following the cavity pulse 2 is coherently re-emitted. The tail of the optical field is specifically determined by the spectroscopic characteristics of the molecules of the sample 8.

[0071] Repeated absorption and resonant reactions by the sample 8 result in a linearly enhanced optical field tail that is coupled out of the enhancement cavity 20 and reaches the detector 32. A large part of the original circulating cavity pulse 2 is rejected by destructive interference (see dashed arrows), while a part of the cavity pulse 2 that is modified by absorption is deflected to the detector 32. In the frequency domain, the generation of the optical field tail leads to a sample-specific change in the (complex) frequency comb spectrum of the circulating cavity pulse 2 compared to the (complex) frequency comb spectrum of the seed laser pulse 1. The detector 32 is configured to sense the sample-specific change, for example by field-resolved spectroscopy. Details of the generation of the optical field tail and its detection are described, for example, in

[14] and

[15] .

[0072] FIG. 9 shows another embodiment of a spectroscopic measurement device 200 with an enhancement cavity 20 for trace gas detection, taking the design of FIG. 7 as an example. The enhancement cavity 20 with two mirrors 21, 22 and a wedge-shaped coupling element 25 is housed in an airtight container 33 with an input window 34 and an output window 35. The container 33 is filled with a gaseous sample 8 to be analyzed, such as a gas or a gas mixture. In this way, the entire cavity beam path 26 of the enhancement cavity 20 is also enhanced by a factor of 2 / πF and applied to the measurement of absorption. A seed laser pulse 1 is fed into the enhancement cavity 20 via the input window 34 and the coupling element 25. As explained with reference to FIG. 8, the molecular reactions of the sample 8 (tail of the optical field, part 7 of the cavity pulse 2) are coupled out of the cavity 20 to the detection device 32, while the main cavity pulse 6 is rejected by interference.

[0073] The features of the invention disclosed in the above description, the drawings and the claims may be of use both individually and in any combination or subcombination for implementing the invention in different embodiments thereof.

Claims

1. A method for passively enhancing pulsed laser light by coherently adding laser pulses in an enhancement cavity (20), comprising: Repetition frequency (f ｒｅｐ ), and the frequency comb line spacing is the repetition frequency (f ｒｅｐ generating a sequence of seed laser pulses (1) having a frequency comb spectrum (3) including frequency comb lines (4) equal to Coupling the seed laser pulse (1) via a first plate-like coupling element (25) into an enhancement cavity (20) having metal surfaces and including at least two cavity mirrors (21, 22, 23, 24) spanning a cavity beam path (26) of a resonator length L, the enhancement cavity (20) being configured to emit a fundamental transverse mode (TEM) ００ ) and higher-order transverse cavity modes (TEM ｎｍ ), each having a set of cavity resonant frequencies (5) and cavity offset frequencies (6); and generating at least one enhanced circulating cavity pulse (2) per cavity length by coherent superposition of the seed laser pulses (1) in the enhancement cavity (20), The frequency comb spectrum (3) is a harmonic comb spectrum (3) with a missing seed comb offset frequency, The enhancement cavity (20) is configured such that the round-trip carrier envelope phase shift of the circulating cavity pulse (2) is ００ 360° / N, where N is an integer equal to or greater than 2; A frequency overlap is provided for the plurality of cavity resonant frequencies (5) with the plurality of frequency comb lines (4) along the frequency comb spectrum (3). A method comprising:

2. The frequency overlap is L where c is the speed of light. ＊ = (c / f ｒｅｐ adjusting the resonator length L of the enhancement cavity (20) according to (T) / N; and adjusting the fundamental transverse mode (TEM ００ ), which is given by exciting The method of claim 1.

3. The frequency overlap is determined by the frequency of the higher order transverse cavity modes (TEMs) of the enhancement cavity (20), where n+m+1=N. ｎｍ ) is given by exciting one of The method of claim 1.

4. The seed laser pulse (1) is generated by difference frequency generation. The method according to any one of claims 1 to 3.

5. The first plate-shaped connecting element (25) comprises: the first plate-like coupling element (25) comprises a pellicle, a plate, or a wedge element; the first plate-shaped coupling element (25) having a one-sided anti-reflection coating; the first plate-like coupling element (25) having a surface oriented at an angle equal to or close to the Brewster's angle with respect to the cavity beam path (26); The first plate-shaped coupling element (25) is transparent in a wavelength range of 7 μm to 12 μm; characterised in that the first plate-shaped bonding element (25) is made of polyethylene or diamond; the first plate-shaped coupling element (25) having a thickness below the central wavelength of the seed laser pulse (1); The first plate-shaped bonding element (25) has a thickness in the range of 100 nm to 500 μm. The method according to any one of claims 1 to 4, comprising at least one of the following features:

6. one of the cavity mirrors (21) is arranged outside the resonator plane in which the cavity beam path (26) extending into and out of the first plate-shaped coupling element (25) extends, in particular the enhanced circulating cavity pulse (2) is reflected by the cavity mirror (21) arranged outside the resonator plane with a small angular incidence of s-polarized light, while the enhanced circulating cavity pulse (2) passes through the first plate-shaped coupling element (25) with p-polarized light. The method according to any one of claims 1 to 5.

7. - providing a sample (8) to be investigated in the enhancement cavity (20), where the circulating cavity pulse (2) interacts with the sample (8); and extracting sample-specific information from the circulating cavity pulse (2) that has interacted with the sample (8). The method according to any one of claims 1 to 6.

8. coupling an output portion (7) of the circulating cavity pulse (2) out of the enhancement cavity (20) via the first plate-like coupling element (25) or a further second plate-like coupling element (25A) after interaction with the sample (8) within the cavity, The method of claim 7.

9. The sample (8) is a gas sample; the sample (8) being arranged in an absorption tube (31) accommodating a part of the cavity beam path (26) of the enhancement cavity (20); the sample (8) being provided in a limited area of ​​the cavity beam path (26); The sample (8) is placed in a container (33) that contains the enhancement cavity (20). The method of claim 8 , comprising at least one of the following features:

10. 1. A laser pulse enhancement device (100) adapted to passively enhance pulsed laser light by coherent addition of laser pulses, comprising: Repetition frequency (f ｒｅｐ ), and the frequency comb line spacing is the repetition frequency (f ｒｅｐ a laser source device (10) configured to generate a sequence of seed laser pulses (1) having a frequency comb spectrum (3) including frequency comb lines (4) equal to An enhancement cavity (20) including at least two cavity mirrors (21, 22, 23, 24) having metallic surfaces and spanning a cavity beam path (26) of a resonator length L, said enhancement cavity (20) being configured to emit a fundamental transverse mode (TEM ００ ) and higher-order transverse cavity modes (TEM ｎｍ ), each having a series of cavity resonant frequencies (5) and cavity offset frequencies (6); a first plate-like coupling element (25) arranged to couple the seed laser pulse (1) into the enhancement cavity (20); The enhancement cavity (20) is adapted for coherent superposition of the seed laser pulse (1) coupled into the enhancement cavity (20) to generate at least one enhanced circulating cavity pulse (2); The laser source device (10) is configured to generate the seed laser pulse (1) having a harmonic comb spectrum (3) with a missing seed comb offset frequency; The enhancement cavity (20) is configured such that the round-trip carrier envelope phase shift of the circulating cavity pulse (2) is ００ is adjusted to 360° / N with respect to the laser source device (10) and the enhancement cavity (20) are adapted to provide a frequency overlap of a plurality of the cavity resonant frequencies (5) with a plurality of the frequency comb lines (4) along the frequency comb spectrum (3); A laser pulse intensifier (100).

11. The resonator length L of the enhancement cavity (20) is L, where c is the speed of light. ＊ = (c / f ｒｅｐ ) / N, The laser source device (10) and the enhancement cavity (20) are configured to convert the seed laser pulse (1) into the fundamental transverse mode (TEM) of the enhancement cavity (20). ００ ) The laser pulse boost device of claim 10.

12. A mode matching device (40) disposed between the laser source device (10) and the enhancement cavity (20) is configured to mode-shape the seed laser pulse (1) and to match the seed laser pulse (1) with the n+m+1=N higher-order transverse cavity modes (TEMs) of the enhancement cavity (20). ｎｍ ) The laser pulse boost device of claim 10.

13. The laser source device (10) is adapted to generate the seed laser pulse (1) by difference frequency generation. The laser pulse boosting device according to any one of claims 10 to 12.

14. The first plate-shaped connecting element (25) comprises: the first plate-like coupling element (25) comprises a pellicle, a plate, or a wedge element; the first plate-shaped coupling element (25) having a one-sided anti-reflection coating; the first plate-like coupling element (25) having a surface oriented at an angle equal to or close to the Brewster's angle with respect to the cavity beam path (26); The first plate-shaped coupling element (25) is transparent in a wavelength range of 7 μm to 12 μm; characterised in that the first plate-shaped bonding element (25) is made of polyethylene or diamond; the first plate-shaped coupling element (25) having a thickness below the central wavelength of the seed laser pulse (1); The first plate-shaped bonding element (25) has a thickness in the range of 100 nm to 500 μm. The laser pulse boost device according to any one of claims 10 to 13, comprising at least one of the following features:

15. one of the cavity mirrors (21) is arranged outside a resonator plane in which the cavity beam path (26) extending into and out of the first plate-shaped coupling element (25) extends, in particular the laser source device (10) and the enhancement cavity (20) are configured such that the enhanced circulating cavity pulse (2) passes through the first plate-shaped coupling element (25) with p-polarized light while being reflected by the cavity mirror (21) arranged outside the resonator plane with a small angular incidence of s-polarized light. The laser pulse boosting device according to any one of claims 10 to 14.

16. the first plate-shaped coupling element (25) or a further second plate-shaped coupling element (25A) is arranged to couple an output portion (7) of the circulating cavity pulse (2) out of the enhancement cavity (20); The laser pulse boosting device according to any one of claims 10 to 15.

17. the enhancement cavity (20) is adapted to accommodate a sample (8) to be investigated such that the circulating cavity pulse (2) interacts with the sample (8); a detection device (30) arranged to extract sample-specific information from the circulating cavity pulse (2) that has interacted with the sample (8); The laser pulse boosting device according to any one of claims 10 to 16.

18. The sample (8) is a gas sample; characterised in that the enhancement cavity (20) comprises an absorption tube (31) accommodating a part of the cavity beam path (26) of the enhancement cavity (20) and further accommodating the sample (8); the enhancement cavity (20) comprises a feeding device configured to feed the sample (8) to a limited region of the cavity beam path (26); The enhancement cavity (20) is arranged in a container (33) filled with the sample (8). The present invention includes at least one of the following features:

20. The laser pulse boost device of claim 17.

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