Method and system for providing pulsed light

Abstract

The present disclosure relates to a method of providing light. The method comprises: providing a chamber filled with a gas; injecting a pump pulse train into the chamber with pulses being spaced by a pump pulse interval; generating a frequency-converted pulse train by frequency conversion via stimulated Raman scattering with the gas, wherein the pulses of the pump pulse train induce coherent oscillations of the gas which facilitates said stimulated Raman scattering, the coherent oscillations having a dephasing time; and outputting the frequency-converted pulse train. A ratio of the pump pulse interval to the dephasing time is at most 10.0. Some pulses of the pump pulse train each serves both to: generate a pulse of the frequency-converted pulse train via coherent oscillations induced by previous pulse(s) of the pump pulse train; and enhance the coherent oscillations for the conversion of a subsequent pulse of the pump pulse train. [Fig. 2b]

Description

[0001] METHOD AND SYSTEM FOR PROVIDING PULSED LIGHT

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method for providing light. The present invention further relates to a system for providing light.

[0004] BACKGROUND OF THE INVENTION

[0005] Pulsed light sources find a wide range of applications within science and technology. However, conventional technologies have certain limitations with respect to either repetition rate, pulse energy, wavelength ranges, or conversion efficiency.

[0006] Rare-earth-ion based mode-locking and optical amplification constitute a basic way of generating GHz high-repetition-rate ultrashort pulsed lasers with relatively high energy in the (sub-)microjoule level. Some are well-developed to a compact all-silica-fibre structure for real-world applications. However, due to the limited bandwidth of rare-earth-ion based gain media, the wavelengths of such lasers are typically limited to few narrow spectral regions in the near-infrared region. Rare-earth-ion doped soft-glass fibre mode-locking and optical amplification have been widely demonstrated for mid-infrared high energy ultrafast generation, but the repetition rate is limited to a level of only tens of MHz. The emerging mode-locked quantum-cascade laser technology can generate GHz-repetition-rate femtosecond pulses in the mid- and far-infrared regions. However, the pulse energy is only at the femtojoule level.

[0007] Under this context, nonlinear frequency conversion has become the focus of attention for high repetition rate ultrafast laser generation at wavelengths out the gain range of rare- earth-ion doped gain media. A significant scientific challenge is that no known physical mechanism allows nonlinear frequency conversion of ultrafast lasers with high repetition rate (GHz) and high pulse energy (e.g., (sub-)microjoule) at unconventional wavelengths like ultraviolet and mid-infrared regions. The root of this scientific challenge is how to address contradiction between nonlinear frequency conversion efficiency and extreme thermal effects. This is because nonlinear frequency conversion is the basic approach for generating ultrafast lasers at unconventional wavelengths. Specifically, due to the conservation of energy, a high pulse energy (e.g. 1 pJ) and high repetition rate (e.g. 1 GHz) ultrafast laser is naturally associated with a high average laser power (e.g. 1 kW), which induces extreme thermal effects in the tight / focused laser-medium interaction region. On the other hand, a high pulse energy (typically at least tens of microjoule) is required to enable efficient frequency conversion, because the efficiency of nonlinear frequency conversion increases with laser's instantaneous intensity in a nonlinear manner.

[0008] Within the field of non-linear frequency conversion, various nonlinear mechanisms have been employed, but pump pulses with (sub-)microjoule energy are typically needed to enable a relatively high conversion efficiency, limiting the repetition rate to below tens of MHz to maintain a moderate average power and thus to avoid thermal damage effect. A popular mechanism is sum / difference frequency conversion (e.g., optical parametric oscillator) using solid materials (e.g., lithium triborate or beta barium borate) as nonlinear frequency conversion medium. In the case of GHz high repetition rate, these kinds of methods suffer from a poor conversion efficiency and low pulse energy in the ultraviolet and mid-infrared region due to the thermal damage caused by the high average power. Further, some schemes involve cascaded crystals which require a large footprint and has poor stability against environmental vibrations.

[0009] Gas can potentially be used as medium to withstand the extreme thermal effect because it has a higher ionization(damage) threshold than solid materials. Different approaches have been proposed to enable and exploit gas nonlinear effects, aiming to convert the frequency of ultrafast lasers. A typical approach is "cavity-enhanced harmonic generation", which is based on the harmonic generation in a gas medium for high repetition rate pulsed laser generation at ultraviolet region. To reach the high threshold of the harmonic generation of gas, the laser intensity is significantly enhanced through bounces back and forth within the cavity. However, higher repetition rates lead to excessive plasma accumulation within the focus region of the laser beam in the optical cavity which limits the maximum reported repetition rate to 250 MHz.

[0010] Gas-filled hollow-core fibre laser technology is another candidate for non-linear frequency conversion, with the gas acting as nonlinear frequency conversion medium, while the hollowcore fibre plays the role of gas chamber where the gas-light interaction occurs within the hollow-core region of the fibre. Based on this configuration, different nonlinear frequency conversion mechanisms of gas such as stimulated Raman scattering, harmonic generation, dispersive wave generation have been exploited. Nevertheless, in similarity with other nonlinear frequency conversion methods, a high pump pulse energy at tens microjoule is conventionally needed to enable a relatively high frequency conversion efficiency. Such high pulse energy limits the repetition rate to kHz or MHz level, because at GHz repetition rate the average pump power will be scaled up to hundreds of kilowatt level which is too high.

[0011] Thus, there is a need for pulsed light sources which are simultaneously capable of providing a high repetition rate, preferably in the GHz range, and a high pulse energy, preferably in the nJ range, with reduced limitations regarding wavelength of the output, in particular with reduced limitations of wavelengths with respect to the ultraviolet region and the mid-infrared region.

[0012] SUMMARY OF THE INVENTION

[0013] On the above background, it is an object of preferred embodiments of the present disclosure to provide pulsed light which has a high repetition rate, a high pulse energy, and reduced limitations regarding wavelength. Furthermore, it is an object of some preferred embodiments of the present disclosure to enable efficient Raman frequency conversion with significantly reduced pump pulse energy and average power. It is moreover an object of some preferred embodiments of the present disclosure to reduce the footprint and the use of free-space optics of light sources.

[0014] A first aspect of the present disclosure relates to a method of providing pulsed light, the method comprising the steps of: providing a chamber filled with a gas; injecting a pump light pulse train into the chamber, wherein power peaks of pulses of the pump light pulse train are temporally spaced at most by a pump pulse interval; generating a frequency-converted pulse train by non-linear frequency conversion of the pump light pulse train via stimulated Raman scattering of the pump light pulse train with the gas within the chamber, wherein the pulses of the pump light pulse train induce and / or enhance coherent oscillations of the gas within the chamber which facilitates said stimulated Raman scattering, the coherent oscillations having a dephasing time and being vibrational oscillations and / or rotational oscillations; and outputting the frequency-converted pulse train from the chamber; wherein a ratio of the pump pulse interval to the dephasing time is at most 10.0 such that at least some pulses of the pump light pulse train each serves both to: generate a pulse of the frequency-converted pulse train via said stimulated Raman scattering facilitated by the coherent oscillations of the gas induced and / or enhanced by at least one previous pulse of the pump light pulse train; and induce and / or enhance coherent oscillations of the gas for said stimulated Raman scattering for conversion of at least one subsequent pulse of the pump light pulse train.

[0015] As pulses of a pump light pulse train enters the chamber and interacts with the gas, coherent oscillations are induced in the gas molecules through stimulated Raman scattering effect. These oscillations can be vibrational oscillations and / or rotational oscillations and can facilitate stimulated Raman scattering for Raman frequency conversion of the pump light pulse train into a frequency-converted pulse train. When a pump pulse has passed the molecule(s), the excited coherent oscillations will dephase or decohere until a new pump pulse enters the chamber to interact with the gas molecule(s).

[0016] Aspects of the present disclosure utilize a particular upper ratio of the pump pulse interval to the dephasing time. In particular, a ratio of the pump pulse interval to the dephasing time, which is at most, e.g., 10.0 may ensure that a coherent oscillation induced by one pulse is still present when the next pulse enters the chamber, which improves non-linear frequency conversion of the latter pulse and enhances the coherent oscillations already present. By injecting a train of pulses into the chamber to pump the gas, a single pulse from the injected train of pulses can both be (at least partially) converted into a frequency-converted pulse via coherent oscillations induced and / or enhanced by previous pulse(s), and induce and / or enhance coherent oscillations for subsequent pulse(s). This scheme can ensure that significant coherent oscillations are present when gas molecules interact with a new pulse from the pump light pulse train, thereby improving conversion efficiency.

[0017] Hence, aspects of the present disclosure can, in turn, significantly reduce the threshold of stimulated Raman scattering, while also providing avoiding damaging, e.g., the nonlinear medium.

[0018] Within the scope of the present disclosure, a particular or specific vibrational mode and / or rotational mode of molecular coherent oscillations is used or selected.

[0019] To achieve a particular ratio of the pump pulse interval to the dephasing time, it is generally desirable to provide a short pump pulse interval, and / or to provide an appropriate gas at appropriate pressure and temperature which in combination provides a relatively long dephasing time. As an example, a pump laser providing a pump light pulse train at a repetition rate at 1.0 GHz, corresponding to a pump pulse interval of 1.0 ns, can be paired with a chamber filled with a molecular hydrogen gas at room temperature and a pressure of a 10 bar, corresponding to a dephasing time of ~0.57 ns for the Q(l) vibrational mode. Therefore, the ratio between the pump pulse interval and dephasing time is 1.75.

[0020] Within the scope of the present disclosure, the gas can have a constant temperature and pressure, or a gradient in pressure and / or temperature within the chamber. If at least a portion of the gas has a dephasing time such that the ratio of the pump pulse interval to this (local) dephasing time is at most 10.0, then the advantages with respect to light generation presented herein can potentially be obtained, and such examples thus lie within the scope of the disclosure.

[0021] Thereby, within aspects of the present disclosure, it is possible to efficiently generate a frequency-converted pulse train having a high repetition rate, for example a repetition rate on the order of 1 GHz, and a high pulse energy, for example a pulse energy on the order of 100 nJ. Further, such solutions impose less restrictions to the wavelength of frequency- converted pulse train. In particular, it is possible to realize a frequency-converted pulse train having a centre wavelength in the ultraviolet region or in the mid-infrared region. Moreover, examples of the present disclosure can be provided with a small footprint with minimal reliance on free-space optics, for example by using a hollow-core fibre as chamber for the gas. Furthermore, the power of pulses of the pump light pulse train required to perform frequency-conversion can be reduced to a low level of hundreds or few thousand watts.

[0022] Stimulated Raman scattering is a non-linear optical process that occurs when high-intensity laser light interacts with a material, such as a gas. During the gas-light interaction, each pump pulse drives gas molecules into coherent oscillation due to simulated Raman scattering effect, meaning that excited molecules coherently oscillate at the frequency corresponding to the energy difference between the ground state and the state of the selected vibrational / rotational mode of Raman (anti-)Stokes. This coherent oscillation in turn couples with the pump pulse, to generate new photons at shifted frequencies, at the Stokes and antiStokes wavelengths relative to the original laser frequency. Consequently, a constructive feedback loop involving the pump, Raman, and gas molecule is triggered, resulting in a cascaded amplification of the Raman (anti-)Stokes signal. Thereby, stimulated Raman scattering can be employed to generate new frequencies of light by exploiting the vibrational transitions of molecules within the medium. In the perspective of quantum mechanics, coherent oscillation means that the gas molecules are excited from the ground state to the vibrational / rotational state, and the phases of these exited gas molecules are synchronized / aligned in a certain manner, to make the stimulated Raman scattering process efficient. A key principle in this stimulated Raman scattering process is that the stronger the oscillation coherence of gas molecule(s), the higher the efficiency of the frequency-converted Raman (anti-)Stokes pulses. Generally, non-linear frequency conversion according to the present disclosure covers any type of Raman frequency conversion. Thereby, non-linear frequency conversion according to the present disclosure may also or alternatively be referred to Raman frequency conversion.

[0023] After inducing coherent oscillations in a medium, these oscillations will gradually dephase towards a random oscillation phase (i.e., thermal equilibrium state). The dephasing rate can be quantified in terms of a dephasing time. This dephasing time, often denoted as T2, is thus a measure of how quickly the phase coherence of molecular vibration / rotation is lost. It varies over gas pressure, gas temperature, and gas species. The dephasing time is determined by both the population relaxation time from the vibrational / rotational state of the gas molecules and the pure dephasing time.

[0024] In the context of the present disclosure, a practically measured dephasing time can be used.

[0025] Theoretically, the dephasing time T2 can be expressed as l / T2=l / (2Ti) + l / TPd, where Ti is the longitudinal relaxation time also known as the population relaxation time, and TPd is the pure dephasing time, which accounts for dephasing of the phase of molecular coherence that do not involve population relaxation.

[0026] Practically, the dephasing time T2 can be measured using stimulated Raman gain spectroscopy. A specific procedure for performing Raman gain spectroscopy to obtain the dephasing time in a step-by-step process is outlined below, with reference to the following two scientific publications:

[0027] [1] W. K. Bischel and M. J. Dyer, "Temperature dependence of the Raman linewidth and line shift for the Q(l) and Q(0) transitions in normal and para-H2", Phys. Rev. A (Coll Park) 33(5), 3113-3123 (1986).

[0028] [2] G. C. Herring, M. J. Dyer, and W. K. Bischel, "Temperature and density dependence of the linewidths and line shifts of the rotational Raman lines in N2 and H2", Phys. Rev. A (Coll Park) 34(3), 1944-1951 (1986).

[0029] The step-by-step process for measuring the dephasing time of a gas is as follows:

[0030] • A gas chamber, filled with the gas at certain pressure and temperature conditions, is provided. To reduce the needed laser energy, and thus lower the experimental complexity, a hollow-core fibre can be used. • Inject a pulsed pump laser and a continuous wave probe laser into the gas chamber. There are eight requirements: i) The linewidth of both lasers needs to be at least 10 times less than the Raman gain width of the relevant vibrational / rotational mode of selected gas. This is because using a sufficiently narrow linewidth is necessary to ensure measurement accuracy. As an example, for a typical gas, such as hydrogen at a 5 bar pressure and a room temperature, the pump laser needs to have a time-averaged linewidth of < 100 MHz, and the probe laser can have a time-averaged linewidth of < 10 MHz. However, for other gases with other temperatures and pressures, other linewidths may be preferable. Ultimately, the linewidths should preferably be narrower than the spectral feature to be resolved, i.e., the Raman gain profile, ii) The frequency difference between the pump and probe lasers needs to match the frequency of the relevant vibrational / rotational mode of selected gas, so that the pump energy can be converted to probe through stimulated Raman scattering for the selected mode, iii) The frequency(wavelength) of either the pump or the probe needs to be precisely tuneable (for example, step size < 50 MHz), to scan the Raman gain profile, iv) The fluctuation of the centre frequency and linewidth of the pump and probe lasers should preferably be less than 10 MHz and 50 MHz, respectively, v) The relative peak power noise (defined as the ratio of the standard deviation of laser pulse peak power to the mean value of laser pulse peak power) of the pump laser should be less than 1%; The relative power noise of the probe (defined as the ratio of the standard deviation of laser power to the mean value of laser power) should be less than 1%. vi) The stimulated Raman scattering should obey the small signal Raman-gain rule, i.e., the pump pulse energy should not be depleted over 1 % of the output pump energy from the gas chamber without having the probe laser, vii) The pump and probe should have a Gaussian-like spatial laser beam profile, and the laser beam of the pump and probe should be overlapped in the gas-laser interaction region, to enable the stimulated Raman scattering, viii) Stimulated Raman scattering of the selected gas should not occur with the presence of only the probe or pump laser. This means that, with either of the pump or probe, any generated Raman (anti-)Stokes signal from the pump and probe should have a power / energy less than 0.01% of the pump / probe output energy from the gas chamber.

[0031] • Scan the frequency of either the pump laser or the probe laser, and use a photodetector to record the variation of the probe laser signal at the output end of the gas chamber. Note that the pump laser and environmental lights should be completely blocked during measuring the probe. The photodetector should detect the probe laser in its quasi-linear response region, meaning that the probe laser power might need to be properly attenuated before coupling into the photodetector. • Use a lock-in amplifier to process the voltage signal captured by the photodetector. The reference frequency of the lock-in amplifier needs to be locked to the repetition rate of the pump laser. Then a time-averaged voltage data is given by the lock-in amplifier.

[0032] • At each relative frequency between the pump laser and the probe laser, calculate the Raman gain Gkaman as: where Vprobe wlth pumpis the averaged voltage value of probe laser detected by lock-in amplifier when the pump laser is on, and Vprobe wlthout pumpis the averaged voltage value of probe laser detected by lock-in amplifier when the pump laser is off. Then, the Raman gain coefficient g Raman C3 n be calculated as: where Ipis the pump intensity, and L is the gas-laser interaction length.

[0033] • Tuning the frequency(wavelength) of the pump or probe laser, and measure the Raman gain coefficient g aman at different frequency values of the pump or probe laser. Then plot the measured Raman gain coefficient g Raman 3S a function of the corresponding frequency of the pump laser or probe laser. As a result, measured data comprising a peak as a function of frequency is obtained.

[0034] • Fit the measured data with Voigt profile. Then, calculate the FWHM Av of the Voigt fitted curve.

[0035] • Calculate dephasing time with T2=l / (nAv), where Av is the spectral width (FWHM) of the Raman gain.

[0036] A reference gas chamber can be used to improve the accuracy of the measured dephasing time [1,2]. As a calibration, the dephasing time of the Q(l) vibrational transition (4155 cm1Raman shift coefficient) in hydrogen can be measured with the experimental setup as a reference, to validate that the experimental setup is qualified to measure the dephasing time of the selected gas. In such a case, hydrogen should preferably have a purity of 99.99% or higher. The hydrogen used in the gas cell should be at room temperature (i.e., at a temperature in a range from 20 °C to 25 °C) and in at a pressure in a range of 1-30 bar). The dephasing time of the Q(l) vibrational transition in hydrogen is calculated with the equation (2) in Ref. [1] : 0.152 x (T - 298) + 4.85 x 10“4X (T - 298)2) x p where v is the FWHM Raman linewidth in MHz, p is the density in units of amagat (amg), and T is the temperature in K. The gas density p in amagat and pressure P can be converted using the ideal gas law: P(bar) 273.15 o(amagat) = - x - - -

[0037] 5 J101.325 (273.15 + 79

[0038] The error between the calculated and measured Av is calculated as error

[0039] The experimental setup for measuring the gas dephasing time is considered to be qualified if the error is less than 10%.

[0040] For a pump light pulse train provided with a particular repetition rate, the pump pulse interval can be calculated as the inverse of the repetition rate. For example, a pump light pulse train provided with a repetition rate of 1.5 GHz, the pump pulse interval is 0.67 ns. In case the injected pump light pulse train does not have a well-defined repetition rate, the temporal spacing of power peaks of the pulses of the pump light pulse train can be used to determine the pump pulse interval. In this case, the pulse interval is perhaps inconsistent, and the method applies with the ratio between the minimum pulse interval and molecular dephasing time, e.g., at most 10.0. Generally, and particularly for a pump light pulse train having an irregular temporal power profile, the pump pulse interval may be calculated via analysis of the power envelope of the pump light pulse train. Let P(t) denote the temporal envelope of the optical power associated with the optical field of the pump light pulse train. The function P(t) represents the slowly varying power envelope obtained by filtering out the high-frequency carrier oscillations of the optical field. Two temporally adjacent power peaks occurring at times tl and t2 are considered to belong to two separate pulses if the following conditions are satisfied : 1) Temporal separation condition: |t2 - tl|> 5 ps; and 2) Power extinction ratio condition: ER =m'n rp!t'>'‘!t>>> 10~rhetemporal distance between two such rninte[ti,t2]r>(7) separate pulses can then define the pump pulse interval.

[0041] A (pump light) pulse train according to the present disclosure may be defined as a laser(- generated) field comprising a sequence of at least three pulses in substantially similar or the substantially the same wavelength, where neighbouring pulses of the pulse train has at most a pulse interval of 10 ns.

[0042] Preferably, the pump light pulse train comprises at least three pulses, for example at least four pulses, for example at least five pulses, for example at least 10 pulses. If the pump pulse interval is at most 10 ns, the pump light pulse train should preferably comprise at least five pulses. A certain minimum number of pulses of the pump light pulse train can ensure that the process in which a single pulse from the injected train of pulses can both be (at least partially) converted into a frequency-converted pulse via coherent oscillations induced and / or enhanced by a previous pulse, and induce and / or enhance coherent oscillations for a subsequent pump pulse occurs efficiently.

[0043] Further, depending on saturation and dephasing effects of the coherent oscillations relative to the rate at which coherent oscillations are excited, a certain number of pulses can be required before efficient, steady-state operation is achieved. A certain number of minimum pulses may ensure that such efficient, steady-state operation is achieved.

[0044] According to examples of the present disclosure, the chamber is a core of a hollow-core fibre.

[0045] Preferably, a diameter of the hollow centre of the hollow-core fibre is at most 200 pm.

[0046] Using the core of a hollow-core fibre as a chamber for the gas advantageously permits a low footprint, since the fibre can be coiled, while maintaining a long distance in which the pump light pulse train can interact with the gas.

[0047] According to examples of the present disclosure, the hollow-core fibre is an anti-resonant hollow-core fibre.

[0048] A limit of the gas-filled hollow-core fibre technology is the transmission range of the hollowcore fibre, since a broad and low-loss transmission range is the prerequisite of nonlinear frequency conversion toward ultraviolet and mid-infrared regions. The provision of an anti- resonant hollow-core fibre enables low loss over a broad spectral range from, e.g., 200 nm to 5 pm. An anti-resonant hollow-core fibre has silica capillaries as the fibre's cladding, thereby forming a hollow-core region for the propagation of the most laser beam energy.

[0049] According to examples of the present disclosure, the pulse interval is at most 10.0 nanoseconds, for example at most 5.0 nanoseconds, for example at most 2.0 nanoseconds, for example at most 1.0 nanoseconds, for example at most 0.3 nanoseconds, such as at most 0.1 nanosecond.

[0050] According to examples of the present disclosure, the gas is any of: molecular hydrogen; carbon dioxide; molecular deuterium; and any combinations thereof.

[0051] Typically, a single species of gas is primarily used, but combinations of different gas species can also be employed.

[0052] According to examples of the present disclosure, a pressure of the gas is in a range from 0.01 bar to 100.0 bar, for example in a range from 0.1 bar to 30.0 bar, for example in a range from 1.0 bar to 12.0 bar, for example in a range from 2.0 bar to 8.0 bar, such as in a range from 3.0 bar to 6.0 bar.

[0053] According to examples of the present disclosure, a temperature of the gas is in a range from -50°C to 100°C, for example in a range from 0°C to 50°C, for example in a range from 5°C to 40°C, such as in a range from 10°C to 30°C.

[0054] Although examples according to the present disclosure can be realized using a broad range of pressures and or temperatures, typical examples are at room temperature and at moderate pressures of a few or tens of bars. This also serves as an advantage of examples of the present disclosure: the frequency-converted pulse train can be realized without requiring control of temperature or special means for realizing very low or very high pressures.

[0055] According to examples of the present disclosure, a centre wavelength of pulses of the pump light pulse train is in a range from 1.0 pm to 2.0 pm, for example in a range from 1.06 pm to 1.07 pm, in a range from 1.5 pm to 1.6 pm, or in a range from 1.9 pm to 2.0 pm.

[0056] As examples, the pump laser providing pump light pulse train can be a Yb-doped fibre laser having a centre wavelength in a range from 1.06 pm to 1.07 pm, an Er-doped fibre laser having a centre wavelength in a range from 1.5 pm to 1.6 pm, or a Tm-doped fibre laser having a centre wavelength in a range from 1.9 pm to 2.0 pm.

[0057] According to examples of the present disclosure, a centre wavelength of pulses of the frequency-converted pulse train is in a range from 200 nm to 5 pm, for example in a range from 200 nm to 700 nm, such as in a range from 200 nm to 500 nm; or for example in a range from 2.5 pm to 5.0 pm, such as in a range from 3.0 pm to 5.0 pm. The centre wavelength of pulses of the frequency-converted pulse train can, e.g., be a spectral peak wavelength of the pulses, or a spectrally weighted average of the pulses.

[0058] Generally, a particular wavelength of the pulses of the frequency-converted pulse train can be achieved by adequately selecting the gas, the pump laser, the length of the chamber, etc. Thereby, anti-Stokes Raman scattering or Stokes Raman scattering may be induced to decrease or increase wavelength accordingly.

[0059] For example, by using the Q(l) vibrational stimulated Raman scattering of hydrogen gas (4155 cm1Raman shift coefficient), diverse Raman wavelength can be generated :

[0060] (1) By using the mature Er-doped high repetition rate ultrafast fibre laser at 1550 nm as a pump laser, and employing Stokes Raman scattering to generate a frequency- converted pulse train, then a 1storder Q(l) vibrational Raman Stokes at 4354 nm can be generated.

[0061] (2) By using the mature Yb-doped high repetition rate ultrafast fibre laser at 1060 nm as a pump laser, and employing Stokes Raman or anti-Stokes Raman scattering to generate a frequency-converted pulse train, then a 1storder vibrational Raman Stokes doped high repetition rate ultrafast laser at 1894 nm can be generated, and 1st- 10thorder vibrational Raman anti-Stokes high repetition rate ultrafast lasers can be generated at 736 nm, 564 nm, 457 nm, 384 nm, 331 nm, 291 nm, 259 nm, 234 nm, 213 nm, and 196 nm, respectively.

[0062] In contrast to certain conventional solutions, examples of the present disclosure in particular permit generation of pulsed light having a high repetition rate and a high pulse energy in the ultraviolet region, such as from 200 nm to 700 nm or from 200 nm to 400 nm, and in the mid-infrared region, such as from 2.5 pm to 5 pm.

[0063] According to examples of the present disclosure, the stimulated Raman scattering is any of anti-Stokes Raman scattering and Stokes Raman scattering.

[0064] According to examples of the present disclosure, the energy of at least some pulses of the frequency-converted pulse train is at least 10 pJ, such as at least 10.0 nJ, preferably at least 10 pJ when the stimulated Raman scattering is anti-Stokes Raman scatting, and preferably at least 10.0 nJ when the stimulated Raman scattering is Stokes Raman scattering.

[0065] Generally, Stokes Raman scattering is more efficient than anti-Stokes Raman scattering. Thereby, the pulse energies and conversion efficiencies which are achievable may vary depending on which of these two physical processes which is employed. According to examples of the present disclosure, the ratio of the pump pulse interval to the dephasing time is at most 6.0, for example at most 4.0, for example at most 2.0, for example at most 1.5, such as at most 1.0.

[0066] Generally, by decreasing the ratio of the pump pulse interval to the dephasing time, less dephasing occurs in the duration between the arrival of pulses, and thereby the efficiency of the conversion of the pulses from the pump light pulse train into the frequency-converted pulse train can be increased.

[0067] According to examples of the present disclosure, the frequency-converted pulse train is applied to perform any of: irradiation of a photocathode to provide a free electron source for light generation; micromachining of a material; non-linear microscopy; and satellite communication.

[0068] Generally, the methods and systems disclosed herein has several promising applications.

[0069] One application is as a short-wavelength laser generation via a high-brightness electron source. The development of high-brightness electron sources stands as a critical challenge for current and next-generation short wavelength (extreme UV and X-Ray) light source facilities such as free electron laser and synchrotron radiation, which play crucial roles in precision frequency metrology and photoemission spectroscopy. Realization of high-brightness electron sources can be possible by irradiation of a semiconductor photocathode pulsed light. Nevertheless, semiconductor photocathodes, while efficient, have a short lifetime and require a high vacuum environment. To circumvent these challenges, metal photocathodes can be employed, but these require a deep UV GHz high-repetition-rate ultrafast laser, which examples of the present disclosure may offer.

[0070] One other application is micromachining of a material. Conventional micromachining through laser ablation via near-infrared ultrafast lasers in burst mode is limited by precision offered by these types of lasers. The frequency-converted pulse train offered by examples according to the present disclosure may potentially be capable of improving this precision, particularly when employing shorted wavelengths of frequency-converted light.

[0071] One other application is Earth-satellite free-space optical communication. Conventional technologies typically rely on transmitters at the near-infrared atmosphere window (approximately 800 nm - 1700 nm), leading to a poor performance against adverse weather condition (e.g., fog, dust, turbulence). By instead utilizing frequency-converted light according to the present disclosure, particularly frequency-converted light at greater wavelengths, such as from 2.5 pm to 5.0 pm, these poor performance against, e.g., adverse weather conditions may be mitigated.

[0072] One other application is non-linear microscopy. Here, second / third harmonic imaging and two / three-photon excitation fluorescence imaging can be utilized. Examples according to the present disclosure may be capable of improving speed and contrast of images acquired, while reducing non-linear photodamaging or photobleaching of a sample, preferably a biological sample. Further, examples according to the present disclosure may be capable of doing so at wavelengths where light attenuation in a biological specimen reaches a minimum, for example at wavelengths in a range from 1150 nm to 1400 nm, such as from 1200 nm to 1350 nm, thereby maximizing the penetration depth and mitigating heating-induced damage.

[0073] One other application is preparation of a permanently-lived molecular coherence system, which can be used for frequency conversion of a weak auxiliary laser through Raman (anti- )Stokes generation. In this context, 'permanently' means that the molecular coherence keeps existing while pump light pulse train is injected. The auxiliary light can be pulsed or continuous wave in the temporal domain.

[0074] According to examples of the present disclosure, the pump pulse interval is at least 5.0 ps, for example at least 15.0 ps, for example at least 40.0 ps, such as at least 100 ps.

[0075] According to examples of the present disclosure, an energy of each pulse of the pump light pulse train is at least 10 nJ, for example at least 30 nJ, such as at least 50 nJ; and / or is at most 3.0 pJ, for example at most 2.0 pJ, such as at most 1.0 pJ.

[0076] In some examples, the energy of each pulse of the pump light pulse train is however greater than 10 pJ, preferably in combination with the pump pulse interval being at least 10 ns.

[0077] According to examples of the present disclosure, a pulse width of each pulse of the pump light pulse train is greater than the period of the coherent oscillations of the gas; and / or a pulse width of each pulse of the pump light pulse train is greater than 100 fs, for example greater than 400 fs, such as greater than 1.0 ps.

[0078] As an example, the Q(l) vibrational Raman frequency shift coefficient of H2 is 4155 cm-1, corresponding to an oscillation period of 8.03 fs. Therefore, the pump pulse width should preferably be longer than 8.03 fs, to avoid the impulsive Raman scattering.

[0079] A pulse width is typically defined as a full-width-at-half-maximum pulse width. By providing a pulse width which is greater than a period of the coherent oscillations of the gas, impulsive Raman scattering may be avoided. Using gas as Raman medium, impulsive Raman scattering may typically require a pulse width which is less than 100 fs.

[0080] According to examples of the present disclosure, the pump light pulse train is generated by a single light source or pump laser, and / or wherein the pulses of the pump light pulse train have substantially the same wavelength.

[0081] According to examples of the present disclosure, the method comprises the steps of: injecting auxiliary light into the chamber while the gas within the chamber undergoes coherent oscillations induced and / or enhanced by the pump light pulse train, the auxiliary light being separate from the pump light pulse train; generating frequency converted auxiliary light by non-linear frequency conversion of the auxiliary light via stimulated Raman scattering of the auxiliary light within the chamber; and outputting the frequency-converted auxiliary light from the chamber.

[0082] Thereby, the pump light pulse train can be used to provide coherent oscillations which are then employed to perform frequency-conversion of light separate from the pump light pulse train, i.e. perform frequency-conversion of auxiliary light. Hence, in such a scenario, the step of outputting the frequency-converted pulse train from the chamber may also optionally be omitted, for example in case only the frequency-converted auxiliary light is of interest. Correspondingly, an optical outlet may also be suitable for outputting the frequency- converted auxiliary light from the chamber, and not necessarily for outputting the frequency- converted pulse train from the chamber. Utilization of auxiliary light further allows versatile light generation. The auxiliary light is separate in the sense that it may be provided from a separate laser source which is independent on the pump laser. However, it is typically not spatially and / or temporally separated from the pump light pulse train, at least not within the chamber.

[0083] A second aspect of the present disclosure relates to a system for providing pulsed light, the system comprising : a pump laser for emitting a pump light pulse train, wherein power peaks of pulses of the pump light pulse train are temporally spaced at most by a pump pulse interval; a chamber, wherein the pump laser is optically coupled to the chamber for injecting the pump light pulse train into the chamber; a gas within the chamber for generating a frequency-converted pulse train by non-linear frequency conversion of the pump light pulse train via stimulated Raman scattering of the pump light pulse train with the gas within the chamber, wherein the pulses of the pump light pulse train induce and / or enhance coherent oscillations of the gas within the chamber which facilitates said stimulated Raman scattering, the coherent oscillations having a dephasing time and being vibrational oscillations and / or rotational oscillations; and an optical outlet for outputting the frequency-converted pulse train from the chamber, wherein a ratio of the pump pulse interval to the dephasing time is at most 10.0 such that at least some pulses of the pump light pulse train each serves both to: generate a pulse of the frequency-converted pulse train via said stimulated Raman scattering facilitated by the coherent oscillations of the gas induced and / or enhanced by at least one previous pulse of the pump light pulse train; and induce and / or enhance coherent oscillations of the gas for said stimulated Raman scattering for conversion of at least one subsequent pulse of the pump light pulse train. A system according to the second aspect of the present disclosure may provide any of the same or similar advantages and effects as the method according to the first aspect of the present disclosure.

[0084] According to examples of the present disclosure, said chamber is a first chamber, said gas is a first gas, and said optical outlet is a first optical outlet, wherein the system further comprises: a second chamber, wherein the frequency-converted pulse train from the first chamber is optically coupled to the second chamber for injecting the frequency- converted pulse train into the second chamber; a second gas within the second chamber for generating a further frequency- converted pulse train by non-linear frequency conversion of the frequency- converted pulse train from the first chamber via stimulated Raman scattering of the frequency-converted pulse train from the first chamber with the second gas within the second chamber; a second optical outlet for outputting the further frequency-converted pulse train from the second chamber.

[0085] By arranging two chambers in series as outlined above, light may be efficiently converted to additional frequency ranges, which can otherwise be difficult to reach. The second gas may optionally be different from the first gas.

[0086] The first chamber and the second chamber may be separate hollow core fibres, such as separate anti-resonant hollow-core fibres.

[0087] According to examples of the present disclosure, said dephasing time is a first dephasing time, wherein the pulses of the frequency-converted pulse train from the first chamber induce and / or enhance coherent oscillations of the second gas within the second chamber which facilitates said stimulated Raman scattering, the coherent oscillations having a second dephasing time and being vibrational oscillations and / or rotational oscillations, and wherein a ratio of the pump pulse interval to the second dephasing time is at most 10.0 such that at least some pulses of the frequency-converted pulse train from the first chamber each serves both to: generate a pulse of the further frequency-converted pulse train via said stimulated Raman scattering facilitated by the coherent oscillations of the second gas induced and / or enhanced by at least one previous pulse of the frequency-converted pulse train from the first chamber; and induce and / or enhance coherent oscillations of the second gas for said stimulated Raman scattering for conversion of at least one subsequent pulse of the frequency-converted pulse train from the first chamber.

[0088] According to examples of the present disclosure, the ratio of the pump pulse interval to the second dephasing time is at most 6.0, for example at most 4.0, for example at most 2.0, for example at most 1.5, such as at most 1.0.

[0089] According to examples of the present disclosure, the system according to the second aspect is arranged to perform the method according to any examples under the first aspect.

[0090] BRIEF DESCRIPTION OF THE DRAWINGS

[0091] Embodiments of the invention will now be further described by reference to the accompanying drawings, in which:

[0092] Fig. 1 illustrates a system according to the present disclosure,

[0093] Fig. 2a-b illustrate coherent oscillations in a gas and generation of a frequency-converted pulse train at two different ratios of pump pulse interval to dephasing time,

[0094] Fig. 3 illustrates method steps according to the present disclosure,

[0095] Figs. 4(a)-(d) illustrate a simulation which demonstrates principles of the present disclosure,

[0096] Figs. 5(a)-(c) illustrate experimental evidence of a frequency-converted pulse train according to an example of the present disclosure, Figs. 6a-d illustrate another simulation which demonstrates principles of the present disclosure,

[0097] Fig. 7 illustrates another system according to the present disclosure,

[0098] Figs. 8a-d illustrate an additional experimental demonstration of principles of the present disclosure,

[0099] Fig. 9 illustrate exemplary spectra of frequency-converted pulse trains according to the present disclosure, and

[0100] Figs. lOa-c illustrate further exemplary spectra of frequency-converted pulse trains according to the present disclosure.

[0101] DETAILED DESCRIPTION

[0102] Fig. 1 illustrates a system 1 according to the present disclosure. The system 1 comprises a pump laser 2 for emitting a pump light pulse train 3. The pulses of the pump light pulse train 3 are temporally spaced by a pump pulse interval 4 indicated in the illustration and corresponding to the inverse repetition rate of the pump light pulse train 3.

[0103] The system 1 further comprises a chamber 5. The pump laser 2 is optically coupled to this chamber 5 for injecting the pump light pulse train 3 into the chamber. The optical coupling may, for example, be implemented via an optical fibre, or free-space optics.

[0104] Moreover, the system 1 comprises a gas 6 within the chamber 5. This gas 6 is suitable for generation of a frequency-converted pulse train 8 by non-linear frequency conversion of the pump light pulse train 3 via stimulated Raman scattering of the pump light pulse train 3. That is, the light of the pump light pulse train 3 undergoes stimulated Raman scattering with the gas 6 within the chamber 5 to thereby convert as least some of the light of the pump light pulse train 3 into light of the frequency-converted pulse train 8, thereby generating the frequency-converted pulse train 8.

[0105] The pulses of the pump light pulse train 3 induce and enhance coherent oscillations of the gas 6, these coherent oscillations being vibrational oscillations and / or rotational oscillations. These enhanced coherent oscillations reduce the threshold of the stimulated Raman scattering. After a pulse of the pump light pulse train 3 has induced or enhanced these coherent oscillations of the gas 6, these oscillations remain in the gas 6, but the coherence of the oscillations gradually decreases due to dephasing processes. Thereby, the coherent oscillations have a dephasing time which characterise the rate at which the coherence of the oscillations decrease.

[0106] In practice, the dephasing time depends on the species of gas, the temperature, and the pressure of the gas.

[0107] In the present example, the pump pulse interval 4 and the dephasing time is engineered such that the ratio of the pump pulse interval to the dephasing time is below 10.0 or preferably below 2.0.

[0108] As a result, when a new pulse of the pump light pulse train 3 enters the chamber 5, some of the amplitude of the coherent oscillation induced by previous pulse(s) of the pump light pulse train 3 remains. Said new pulse can then enhance and utilize the coherent oscillations, thereby increasing efficiency of the non-linear frequency conversion of that new pulse in comparison with the efficiency of the Raman frequency conversion of the previous pulse. In a similar manner, the efficiency of Raman frequency conversion of subsequent pulses is also increased.

[0109] When injecting the pump light pulse train 3 with a short pump pulse interval 4 relative to the dephasing time, at least some pulses of the pump light pulse train 3 can thereby each serve a dual purpose. Firstly, such a pulse can contribute to the generation of a pulse of the frequency-converted pulse train 8 via stimulated Raman scattering facilitated by the coherent oscillations of the gas 6 induced and / or enhanced by at least one previous pulse of the pump light pulse train 3. Secondly, such a pulse can induce and / or enhance coherent oscillations of the gas 6 for stimulated Raman scattering for conversion of a subsequent pulse of the pump light pulse train 3 into light for a pulse of the frequency-converted pulse train 8.

[0110] Finally, the system 1 comprises an optical outlet 7 for outputting the frequency-converted pulse train 8 from the chamber 5. At the outlet 7, the pump light pulse train 3 and the frequency-converted pulse train 8 may spatially and temporally overlap. If desired, these two pulse trains can be separated, for example by using a dichroic mirror / beamsplitter.

[0111] Fig. 2a-b illustrate coherent oscillations 9 in a gas and generation of a frequency-converted pulse train 8 at two different ratios of pump pulse interval 4 to dephasing time 10. In particular, Fig. 2a illustrates an example in which the pump pulse interval 4 is long relative to the dephasing time 10, whereas Fig. 2b illustrates an example in which the pump pulse interval is shorter than two times of the dephasing time 10. Each of the subfigures considers a portion of gas within a chamber as exemplified in relation to Fig. 1, for example a portion of gas near the optical outlet. The horizontal axis in each of the subfigures corresponds to time. Further, each of the subfigures illustrates, from top to bottom, the amplitude of coherent oscillations 9, the amplitude of pulses of the pump light pulse train 3, and the amplitude of pulses of the frequency-converted pulse train 8. Finally, both the pump pulse interval 4 and the dephasing time 10 is indicated by horizontal arrows.

[0112] In Fig. 2a, the left-most pulse of the pump light pulse train 3 induces coherent oscillations 9 in the gas. As illustrated, the amplitude of these coherent oscillations gradually decreases, characterised by a decoherence time 10, until the next pulse of the pump light pulse train 3 induce coherent oscillations anew. Since the oscillations coherence 9 dephases to a negligible level when this next pulse enters, the coherent oscillations already induced by the previous pulse does not necessarily contribute significantly to the frequency conversion of said next pulse into a pulse of the frequency-converted pulse train 8. Accordingly, the conversion efficiency of pulses from the pump light pulse train 3 into pulses of the frequency-converted pulse train is relatively low in this example.

[0113] In Fig. 2b, the pump pulse interval 4 between pulses of the pump light pulse train 3 is smaller than in the example illustrated in Fig. 2a. As the first pulse (left-most pulse in the illustration) of the pump light pulse train 3 interacts with the gas, coherence oscillations 9 are induced in the gas and the amplitude of these coherent oscillations gradually decreases characterised by a decoherence time 10. However, in contrast to the example illustrated in Fig. 2a, the amplitude of the coherent oscillations 9 is not negligible when the second pulse interacts with the gas. Thereby, as this second pulse interacts with the gas, the amplitude of the coherent oscillations 9 is further enhanced. As a result, the amplitude of the coherent oscillations 9 is greater just after said second pulse has interacted with the gas than just after the first pulse has interacted with the gas. Proceeding to the third pulse of pump light pulse train 3, this third pulse enhances the amplitude of the coherent oscillations 9 even further. Each consecutive pulse of the pump light pulse train 3 will continue to enhance the amplitude of coherent oscillations 9, for example until saturation effects limit further enhancement of the coherent oscillations 9, or until the pump light pulse train 3 terminates.

[0114] Since the amplitude of coherent oscillations 9 is enhanced, so is the conversion efficiency of light of the pump light pulse train 3 into light of the frequency-converted pulse train 8, even if the pump pulse energy is as low. This is also indicated in Fig. 2b. Moving from left to right in Fig. 2b, corresponding to forward in time, amplitude of the pulses of the frequency- converted pulse train 8 increase in amplitude. Correspondingly, the amplitude of the pulses of the pump light pulse train 3 decrease, since an increasing fraction of this light is converted into light of the frequency-converted pulse train 8. Note that Figs. 2a-b are merely schematic illustrations. In Fig. 2a, the ratio of the pump pulse interval to the dephasing time 10 is illustrated so as to correspond to approximately three. However, such a ratio may actually provide the desired enhancement of coherent oscillations, even though the illustration indicates that this enhancement does not occur within the duration of a few pulses.

[0115] Fig. 3 illustrates method steps S1-S4 according to the present disclosure. The method according to this example is a method of providing pulsed light.

[0116] In a step SI of the method, a chamber filled with gas is provided. The chamber can be any chamber capable of hosting a gas. Preferably, however, the chamber is the core of a hollowcore fibre, preferably an anti-resonant hollow-core fibre.

[0117] In another step S2 of the method, a pump light pulse train is injected into the chamber. Power peaks of pulses of the pump light pulse train are temporally spaced at most by a pump light pulse interval. The pump light pulse train can be provided by a pump laser.

[0118] In another step S3 of the method, a frequency-converted pulse train is generated. Said generation of the frequency-converted pulse train occurs by non-linear frequency conversion of the pump light pulse train via stimulated Raman scattering of the pump light pulse train. Pulses of the pump light pulse train induce and / or enhance coherent oscillations of the gas within the chamber. These coherent oscillations facilitate the stimulated Raman scattering. The coherent oscillations are vibrational oscillations and / or rotational oscillations of (molecules of) the gas. Further, the coherent oscillations have a dephasing time indicative of a rate at which the oscillation coherence decreases.

[0119] The ratio of the pump pulse interval to the dephasing time is at most 10.0. Thereby, at least some pulses of the pump light pulse train each serves both to: generate a pulse of the frequency-converted pulse train via the stimulated Raman scattering facilitated by coherent oscillations of the gas induced and / or enhanced by at least one previous pulse of the pump light pulse train; and induce and / or enhance coherent oscillations of the gas for said stimulated Raman scattering for conversion of at least one subsequent pulse of the pump light pulse train.

[0120] In another step S4 of the method, the frequency-converted pulse train is outputted from the chamber. This frequency-converted pulse train is thus provided as the pulsed light which is provided according to the method.

[0121] Figs. 4(a)-(d) illustrate a simulation which demonstrates principles of the present disclosure. The theoretical framework for the simulation is the classical one-dimension coupled vector wave equation described in the following scientific publication:

[0122] [3] A. Picozzi, C. Montes, J. Botineau, and E. Picholle, "Inertial model for stimulated Raman scattering inducing chaotic dynamics", Journal of the Optical Society of America B 15(4), 1309-1314 (1998).

[0123] This model couples the pump wave, Raman Stokes wave, and the oscillation coherence of gas molecules. The slowly-varying pulse envelope approximation is adopted, which is valid for pulse widths at tens of femtosecond or longer in the wavelength range from ~200 nm to 5 pm. An H2-filled anti-resonant hollow-core fibre is used as the chamber for the gas, and the dispersion of this fibre is accounted for in this equation by including a dispersion walk-off effect between each pump pulse and corresponding Raman pulse.

[0124] For the pump pulse train, a train of 35 pulses with a 0.200 ns pump pulse interval is used (corresponding to a repetition rate of 5.00 GHz). The pulse width (FWHM) is 300 fs, and the centre wavelength is 1.53 pm. The pump pulse train is also illustrated in Fig. 4(a) having a horizontal axis providing time in units of ns, and a vertical axis providing power in units of MW. For simplicity, only the 1storder vibrational Raman Stokes generation of H2 with a frequency shift coefficient of 4155 cm1is taken into account, which provides a wavelength of the frequency-converted pulse train in the mid-infrared region at 4.20 pm. Since Raman (anti-)Stokes pulses arise from quantum noise, such quantum noise is also included into the simulation model based on equation (9) in the following scientific publication:

[0125] [4] R. G. Smith, "Optical Power Handling Capacity of Low Loss Optical Fibers as Determined by Stimulated Raman and Brillouin Scattering," AppL Opt. 11, 2489-2494 (1972).

[0126] Fibre loss values of 0.008 dB / m at 1.53 pm, and 0.1 dB / m at 4.3 pm are included, corresponding to those of an actual anti-resonant hollow-core fibre. The core diameter of the fibre is 81 pm, the simulated mode-field diameter is 51 pm, and the length of the fibre is 4.5 m. To accurately represent this pump pulse train in the simulation, a relatively wide time window of 8 ns with a fine step size of 6.1 fs was simulated, see Fig. 4(a). The pump pulse energy is set to only 600 nJ, corresponding to 2 MW peak power and 3 kW average power. The simulated pressure of H2 is set to 30 bar, and the temperature is set to 298 K, corresponding to a dephasing time of 0.204 ns calculated via equation (2) of the scientific publication [1] referred to above. Thereby, the dephasing time which is slightly longer than the pulse-to-pulse interval of 0.200 ns, i.e., the ratio of the pump pulse interval to the dephasing time is below 10.0, more specifically below 2.0. The results of the simulation are illustrated in Figs. 4(b)-(d), each horizontal axis representing time in units of ns. In Fig. 4(b), the left vertical axis is the power of light in units of MW, and the right vertical axis provides the molecular coherence. The presented results correspond to the simulated values at 4.5 m fibre length, i.e., at the optical outlet.

[0127] In Fig. 4(b), both the frequency-converted pulse train 8 and the residual pump light pulse train 3 are shown. The residual pump light pulse train is the remainder of the pump light which has not been converted or lost through other processes, such as absorption, at the optical outlet. A dashed line is inserted to indicate the approximate upper boundary of the simulated power of the frequency-converted pulse train.

[0128] Evidently, the coherent oscillation, is gradually and periodically enhanced with the number of successive pulses of the pump light pulse train which have interacted with the gas. Further, the power of successive pulses of the frequency-converted pulse train 8 increases as well. The maximum conversion efficiency provided in the simulation is approximately 27 % corresponding to a pulse energy of 59 nJ. This is calculated as the quantum efficiency, i.e., frequency-converted pulse energy x Raman wavelength / (pump pulse energy x pump wavelength). The pulse energy of pulses of the residual pump light pulse train is 50 nJ.

[0129] Further simulations (not shown) provide that a similar conversion efficiency when considering a single pulse, and not a pulse train having a ratio of the pump pulse interval to the dephasing time being at most 10.0, requires a pulse energy of the pump light pulse train of 7.2 pJ, which more than an order of magnitude greater than the simulated pulse energy of 600 nJ used in Fig. 4(a)-(d).

[0130] Fig. 4(c) presents the evolution process of the simulated pulses along the Fh-filled anti- resonant hollow-core fibre, and Fig. 4(d) shows the dispersion walk-off between the 35thpump pulse and the corresponding frequency-converted pulse along the propagation of the fibre. A dashed line is inserted in Fig. 4(c) to indicate the approximate boundary where frequency conversion is essentially completed. As evident, the conversion of pump pulses into frequency-converted pulses occurs gradually at an earlier position of the fibre with increasing pulse number. This occurs due to the increasing molecular coherence illustrated in Fig. 4(b).

[0131] Fig. 4(d) illustrates that the Raman pulse is slowly separated from the pump pulse in the time domain since each pump pulse and the corresponding Raman pulse propagate with slightly different velocities along the fibre due to the fibre's dispersion. Note that this dispersion walk-off effect does not necessarily significantly affect the frequency conversion efficiency, as frequency conversion is typically mostly completed before the dispersion walk-off occurs. Figs. 5(a)-(c) illustrate experimental evidence of a frequency-converted pulse 8 train according to an example of the present disclosure.

[0132] The experiments were performed using an anti-resonant hollow-core fibre as chamber for the gas. A cross-sectional view of the fibre is illustrated in Fig. 5(c). The fibre used had a length of 5 m, a core diameter of 32.8 pm, and a < 0.1 dB / m loss in the approximate range of 1-2 pm. A Yb-doped fibre laser followed by Yb-doped fibre amplifiers was used as a pump laser to provide a pump light pulse train 3. The pump light pulse train 3 comprises approximately 25 pulses having a central wavelength of 1045 nm, a linewidth of approximately 0.2 nm, a pulse width of approximately 8 ps. The energy of the entire burst of pulses was measured to be 4.3 pj, with a maximum energy of a pulse estimated to be 350 nJ with a peak power of 44 kW. The pump pulse interval was 0.52 ns, corresponding to a repetition rate of 1.92 GHz.

[0133] In a first set of measurements, the core of the fibre was filled with CO2 as gas, provided at a pressure of 2 bar and at room temperature, which provides a dephasing time greater than 1 ns. Further, these conditions provide a relatively high gain and a relatively long Raman shift coefficient of 1385 cm . Injecting the pump light pulse train provides a 1storder vibrational Raman Stokes line at 1242 nm at the outlet, corresponding to the frequency-converted pulse train 8 illustrated in Fig. 5(a).

[0134] The horizontal axis of Fig. 5(a) corresponds to time in units of nanoseconds, and the vertical axis is the voltage of the separate photodiodes which measure the intensities of the pump light pulse train 3 and the frequency-converted pulse train in units of mV.

[0135] The figure illustrates the pump light pulse train 3 and the frequency-converted pulse train 8. The temporal separation of the two pulse trains is an artefact of the measurement performed using different photodiodes. At the optical outlet of the anti-resonant hollow-core fibre, the two pulse trains overlap spatially and temporally. In practice, they can be separated using a dichroic beamsplitter.

[0136] The frequency-converted pulse train 8 has the same internal repetition rate of 1.92 GHz as the pump light pulse train 3. The energy of the entire frequency-converted pulse train was measured to be 1.3 pJ, corresponding to a quantum efficiency of 34%. The maximum pulse energy was estimated to be 130 nJ.

[0137] Fig. 5(b) illustrates a measured optical spectrum showing the wavelength conversion. Evidently, both the pump light pulse train 3 at 1042 nm, and a frequency-converted pulse train 8 at 1242 nm are present in the measured spectrum. In a second set of measurements (not shown), the core of the fibre was filled with CFU as gas, which, at few bar pressures, typically has a dephasing time on the order of tens of picoseconds. Yet, CFk has previously often been used for Raman Stokes generation due to its high vibrational Raman gain coefficient. Using a pump light pulse train having a centre wavelength of 1045 nm, the 1storder vibrational Raman Stokes is calculated to be 1503 nm. However, when injecting the pump light pulse train, no Raman signal was observed. Further attempts at varying and optimizing relevant parameters did not yield observations of a Raman signal. This indicates that the ratio of the pump pulse interval to the dephasing time influence the efficiency of generating a frequency-converted pulse train.

[0138] The following provides an example of how a suitable pump laser and a suitable pump light pulse train is prepared.

[0139] As a first step, active mode-locking technology is used to provide an Yb-doped fibre laser source having a centre wavelength in the 1 pm wavelength region, with a repetition rate at 1 GHz, corresponding to a pump pulse interval of 1 ns.

[0140] As a next step, this light from this laser source is optically amplified such that the pulse energy reaches 0.5 pJ, corresponding to an average power of each pulse of approximately 0.5 kW. As an example, the linewidth of the amplified laser is less than 0.5 nm.

[0141] As an example, the pump light pulse train of this light source is injected into an anti-resonant hollow-core fibre filled with H2 at a pressure of 5 bar and a temperature in a range from 20°C to 25°C, and a frequency-converted pulse train is generated at 1.9 pm and output from the outlet of the hollow-core fibre, with the frequency-conversion being first-order vibrational Stokes Raman scattering.

[0142] Generally, gas can be supplied to a hollow-core fibre by means of one or more gas cells fluidly coupled to one or both ends of the hollow-core fibre.

[0143] Figs. 6a-d illustrate another simulation which demonstrates principles of the present disclosure.

[0144] The simulation models 1storder Q(l) vibrational Raman Stokes generation at 1.9 pm wavelength in a chamber in the form of an anti-resonant hollow-core fibre having a length of 5 m and filled with gas in the form of H2 at a pressure of 16 bar, which provides a dephasing time of approximately 0.375 ns. The numerical model used for the simulation are onedimensional coupled unidirectional Maxwell-Bloch equations. The pump light pulse train comprises a 19 ns burst of identical Gaussian pulses injected at with different pump pulse intervals, corresponding to different repetition rates of 3 GHz, 2 GHz, and 1 GHz, respectively. The pulse width of the pulses is 23 ps and the energy of each pulse is 100 nJ.

[0145] Fig. 6a illustrates the quantum efficiency for the conversion of a pulse of the pump light pulse train into a pulse of the frequency-converted pulse train. The figure illustrates the efficiency along the length of the anti-resonant hollow-core fibre for various pulses when simulating a repetition rate at 3 GHz, where the different lines from bottom to top correspond to the first pulse, the third pulse, the fifth pulse, the seventh pulse, the ninth pulse, the eleventh pulse, the thirteenth pulse, and the sixtieth pulse. Evidently, the quantum efficiency initially increases with each subsequent pulse, supporting the notion that the coherent oscillations increase for a low ratio of the pump pulse interval to the dephasing time. For the sixtieth pulse, the final quantum efficiency is 75.2%. In the case of conventional stimulated Raman scattering relying on a single pulse, a further simulation shows that a quantum efficiency of 75.2% requires a pump pulse energy of up to around 5 pJ at 23 ps pulse width, i.e., a pulse energy which is 50 times greater than the one used in the simulations illustrated in Figs. 6a- d.

[0146] Figure 6b illustrates the evolution of molecular coherence in logarithm scale along the hollowcore fibre for the first 22 pulses, where the horizontal axis represents time and the vertical axis represents length along the fibre. After a few pulses, the molecular coherence is increased along the length of the fibre. Further, after each pulse, the molecular coherence gradually decreases until the next pump pulse arrives.

[0147] Fig. 6c illustrates intensities of various light components 3, 8, 19 as well as the amplitude of the molecular coherence 9 at the outlet of the fibre. For the initial pulses, the molecular coherence 9 in the bottom sub-figure is very low. After a certain number of pulses, from -3 ns and onwards in the figure, the molecular coherence 9 transits into a steady periodical enhancement-dephasing process. This steady enhancement-dephasing process endlessly repeats until the last pump pulse arrives. In this steady regime, the pulses of the pump light pulse train 3 are steadily converted to pulses of the frequency-converted pulse train 8 with a uniform peak power of ~1.95 kW and pulse energy of ~44 nJ. Note that the enhancement- dephasing process shows a quick decline from approximately -7 ns into the subsequent steady periodical region. This decline trend is attributed to the depletion of the pump pulse energy toward frequency-converted (Raman) pulse. When the residual pump light pulse train 19 is significantly depleted and dispersive separated from its corresponding Raman pulse, that Raman pulse could be slightly back-converted to the pump wavelength through first order Raman anti-Stokes conversion process. Fig. 6d illustrates quantum efficiency of the 20thpulse for the three repetition rates of (from top to bottom) 3 GHz, 2 GHz, and 1 GHz, as well as for a single pulse (bottom-most trace). Decreasing the repetition rate from 3 GHz to 1 GHz results in a quantum efficiency at the end of the fibre going from 74% to 1.6 x 10-4%.

[0148] Fig. 7 illustrates another system 1 according to the present disclosure.

[0149] The system 1 comprises a pump laser 2 for emitting a 1 pm pump light pulse train 3 from an endcap. After the endcap, a lens 11 collimates the beam. The beam then passes through a bandpass filter 12, a wedged optical window 13, a neutral density filter 14, a half-wave plate 15, and a lens 11 which is used to inject the pump light pulse train 3 into a chamber 5 in the form of an anti-resonant hollow-core fibre 17 with gas cells 16 at both ends. The wedged optical window 13 redirects a small portion of light through a neutral density filter 14 and into a first photodiode 18a, which resultingly provides a measurement indicative of the temporal profile of the pump light pulse train 3. After the hollow-core fibre 17, the light is recollimated by a lens 11. A bandpass mirror splits up the frequency-converted pulse train 8 from the residual pump light pulse train 19. The residual pump light pulse train 19 passes through a neutral density filter 14, a bandpass filter 12, and a lens 11 prior to arrival at a second photodiode 18b which then provides a measurement indicative of the temporal profile of the residual pump light pulse train 19. The frequency-converted pulse train 8 passes through a neutral density filter 14 and a lens 11 and arrives at a third photodiode 18c, which then provides a measurement indicative of the temporal profile of the frequency-converted pulse train 8.

[0150] Figs. 8a-d illustrate an additional experimental demonstration of principles of the present disclosure. The setup illustrated in Fig. 7 was used. Optical bursts having repetition rates ranging from 500 MHz to 3 GHz was used as exemplified pump in Fig. 8a where the top subfigure illustrates a burst having a repetition rate of 500 MHz and the bottom subfigure illustrates a burst having a repetition rate of 3 GHz. The number of pulses within each burst pump are ~ 11 at 500 MHz, ~ 22 at 1 GHz, ~ 41 at 2 GHz, and ~ 71 at 3 GHz, respectively, corresponding to a calculated average pulse energy of 982 nJ, 786 nJ, 678 nJ, and 478 nJ, respectively. The wavelength of the pump light pulse train is 1063 nm, the linewidth is approximately 0.13 nm, and the average pulse width is 23 ps, independent of repetition rate and wavelength. The gas chamber is a H2-filled anti-resonant hollow-core fibre with a gas pressure of 16 bar. The polarization orientation of the pump laser is configured with a halfwave plate to optimize the vibrational Raman efficiency.

[0151] Under these conditions, the pump light pulse train can be used to generate a frequency- converted pulse train at 1.9 pm. Fig. 8b illustrates measured temporal profiles at a repetition rate of 2 GHz, with the top subfigure showing the pump light pulse train 3, the middle subfigure showing the frequency-converted pulse train 8, and the bottom subfigure showing the residual pump light pulse train 19. Similarly to the simulations in Fig. 6c, the residual pump light pulse train 19 initially has pulses which are not substantially converted into frequency-converted (Raman) pulses.

[0152] For the average pump energy of 786 nJ, the Raman burst average power is measured to be 3.17 mW, corresponding to an average quantum efficiency of 23.0 %. The number of Raman pulses within the burst is ~37, leading to an estimated average pulse energy of 85 nJ. Unlike the simulation of Fig. 6a-d, the quantum efficiency for an individual Raman pulse cannot be measured experimentally. Instead, the average quantum efficiency is measured. Yet, the quantum efficiency of an individual Raman pulse is expected to exceed the average quantum efficiency, since the pump pulses at the leading edge of the burst undergo little Raman conversion but are still included in the calculation of the average efficiency.

[0153] Fig. 8c illustrates a comparison of quantum efficiencies versus average pump energy for different repetition rates. From left to right, the repetition rate used for the measurement traces are 3 GHz, 2 GHz, 1 GHz, and 500 MHz. The H2 pressure is 16 bar. At 3 GHz repetition rate, the threshold of 1.9 pm Raman pulses was reached with an average pump energy as low as ~120 nJ. The maximum average quantum efficiency of 23 % is obtained at the pump energy of only ~200 nJ, which corresponds to 600 W average power at continuous repetition rate. Decreasing the repetition rates results in a greater threshold for efficient generation of a frequency-converted pulse train up to 900 nJ at 500 MHz repetition rate.

[0154] Fig. 8d illustrates a comparison of quantum efficiencies versus pressure for different repetition rates. From top to bottom, the repetition rates for the measurement traces are 3 GHz, 2 GHz, 1 GHz, and 500 MHz. The average pump pulse energy is set to the maximum at each repetition rate.

[0155] Changing the gas pressure has a non-trivial influence onto the efficiency of the disclosed approach. Increasing gas pressure brings the advantage of higher Raman gain coefficient but shortens the molecular dephasing time which has both advantageous and disadvantageous impacts on the Raman conversion efficiency. The disadvantage is that the dephasing time of molecular coherence becomes short, which imposes a negative impact on the multi-pulse enhancement process of molecular coherence. On the other hand, reducing the dephasing time means further suppression of the transient Raman scattering regime, which is advantageous to boost the Raman efficiency. Therefore, in determining the optimal pressure for maximizing Raman frequency conversion efficiency, the Raman gain coefficient, the suppression of transient Raman scattering, and the contribution of the memory effect of stimulated Raman scattering must be balanced. This optimal pressure also relies on the parameters of pump light pulse train and the gas chamber.

[0156] In Fig. 8d, it can be seen that the pressure for the maximum quantum efficiency varies at different pump repetition rates. Above 10 bar pressure, the dephasing time is approximately 0.2 ns. However, for the pump light pulse train at 500 MHz repetition rate, a quantum efficiency of approximately 5% is nevertheless achieved, even though this corresponds to a ratio of the pump pulse interval to the dephasing time of 10.

[0157] Fig. 9 illustrate exemplary spectra of frequency-converted pulse trains according to the present disclosure. The spectrum in the top subfigure is recorded using 16 bar H2 and exhibits light at 1.9 pm. In comparison, the spectrum in the middle subfigure is provided using 5 bar CO2 and exhibits light at 1.25 pm. The spectrum in the bottom subfigure is recorded using 3 bar CH4, which provides a short dephasing time of approximately 30 ps. Consequently, no spectral features, beyond the pump light at approximately 1063 nm, are visible in the spectrum.

[0158] Figs. lOa-c illustrate further exemplary spectra of frequency-converted pulse trains according to the present disclosure.

[0159] A first measurement scheme illustrated in Fig. 10a is recoded using pump light pulse train at a 3 GHz repetition rate and 478 nJ average pulse energy, to pump the H2-filled anti-resonant hollow-core fibre at 30 bar pressure. Due to the increased pressure (in comparison with conditions for top subfigure of Fig. 9), the phase matching condition of Raman anti-Stokes generation is better met which yields spectral features at 534 nm and 737 nm, corresponding to first and second order Q(l) Raman anti-Stokes lines, respectively. Although the measured average power of these two Raman anti-Stoke lines is less than 100 pW due to phase mismatching, it implies the feasibility of the disclosure on Raman anti-Stokes generation.

[0160] A second measurement scheme uses a second chamber, in the form of a second anti- resonant hollow-core fibre, arranged serially after the first fibre. Further, the wavelength of the pump light pulse train is varied. The first fibre is filled with 2 bar CO2 to convert the 1034-1085 nm pump light pulse train into a frequency-converted pulse train at 1207-1277 nm. The second fibre is filled with H2 to convert the frequency-converted pulse train from the first fibre into a further frequency-converted pulse train at 2.42-2.54 pm when the pump light is tuned from 1034 nm to 1055 nm.

[0161] The results are illustrated in Figs. 10b and 10c, where Fig. 10b shows spectra recorded between the two fibres, and Fig. 10c shows spectra recorded after the second fibre. Several measurements arising from different wavelengths of the pump light pulse train are superimposed in the figures. The measurements in Fig. 10c also exhibits S(l) rotational Raman (anti-)Stokes generation of H2.

[0162] List of figure references:

[0163] 1 system

[0164] 2 pump laser

[0165] 3 pump light pulse train

[0166] 4 pump pulse interval

[0167] 5 chamber

[0168] 6 gas

[0169] 7 optical outlet

[0170] 8 frequency-converted pulse train

[0171] 9 coherent oscillations

[0172] 10 dephasing time

[0173] 11 lens

[0174] 12 bandpass filter

[0175] 13 wedged optical window

[0176] 14 neutral density filter

[0177] 15 half-wave plate

[0178] 16 gas cell

[0179] 17 anti-resonant hollow core fibre

[0180] 18 photodiode

[0181] 19 residual pump light pulse train

[0182] S1-S4 method steps

Claims

1. 32CLAIMS1. A method of providing pulsed light, the method comprising the steps of: providing a chamber filled with a gas; injecting a pump light pulse train into the chamber, wherein power peaks of pulses of the pump light pulse train are temporally spaced at most by a pump pulse interval; generating a frequency-converted pulse train by non-linear frequency conversion of the pump light pulse train via stimulated Raman scattering of the pump light pulse train with the gas within the chamber, wherein the pulses of the pump light pulse train induce and / or enhance coherent oscillations of the gas within the chamber which facilitates said stimulated Raman scattering, the coherent oscillations having a dephasing time and being vibrational oscillations and / or rotational oscillations; and outputting the frequency-converted pulse train from the chamber; wherein a ratio of the pump pulse interval to the dephasing time is at most 10.0 such that at least some pulses of the pump light pulse train each serves both to: generate a pulse of the frequency-converted pulse train via said stimulated Raman scattering facilitated by the coherent oscillations of the gas induced and / or enhanced by at least one previous pulse of the pump light pulse train; and induce and / or enhance coherent oscillations of the gas for said stimulated Raman scattering for conversion of at least one subsequent pulse of the pump light pulse train.

2. A method according to claim 1, wherein the chamber is a core of a hollow-core fibre, preferably wherein a diameter of the hollow centre of the hollow-core fibre is at most 200 pm.

3. A method according to claim 2, wherein the hollow-core fibre is an anti-resonant hollowcore fibre.

334. A method according to any of the preceding claims, wherein the pulse interval is at most 10.0 nanoseconds, for example at most 5.0 nanoseconds, for example at most 2.0 nanoseconds, for example at most 1.0 nanoseconds, for example at most 0.3 nanoseconds, such as at most 0.1 nanosecond.

5. A method according to any of the preceding claims, wherein the gas is any of: molecular hydrogen; carbon dioxide; molecular deuterium; and any combinations thereof.

6. A method according to any of the preceding claims, wherein a pressure of the gas is in a range from 0.01 bar to 100.0 bar, for example in a range from 0.1 bar to 30.0 bar, for example in a range from 1.0 bar to 12.0 bar, for example in a range from 2.0 bar to 8.0 bar, such as in a range from 3.0 bar to 6.0 bar.

7. A method according to any of the preceding claims, wherein a temperature of the gas is in a range from -50°C to 100°C, for example in a range from 0°C to 50°C, for example in a range from 5°C to 40°C, such as in a range from 10°C to 30°C.

8. A method according to any of the preceding claims, wherein a centre wavelength of pulses of the pump light pulse train is in a range from 1.0 pm to 2.0 pm, for example in a range from 1.06 pm to 1.07 pm, in a range from 1.5 pm to 1.6 pm, or in a range from 1.9 pm to 2.0 pm.

9. A method according to any of the preceding claims, wherein a centre wavelength of pulses of the frequency-converted pulse train is in a range from 200 nm to 5 pm, for example in a range from 200 nm to 700 nm, such as in a range from 200 nm to 500 nm; or for example in a range from 2.5 pm to 5.0 pm, such as in a range from 3.0 pm to 5.0 pm.

10. A method according to any of the preceding claims, wherein the stimulated Raman scattering is any of anti-Stokes Raman scattering and Stokes Raman scattering.

11. A method according to any of the preceding claims, wherein the energy of at least some pulses of the frequency-converted pulse train is at least 10 pJ, such as at least 10.0 nJ, preferably at least 10 pJ when the stimulated Raman scattering is anti-Stokes Raman scatting, and preferably at least 10.0 nJ when the stimulated Raman scattering is Stokes Raman scattering.

12. A method according to any of the preceding claims, wherein the ratio of the pump pulse interval to the dephasing time is at most 6.0, for example at most 4.0, for example at most 2.0, for example at most 1.5, such as at most 1.0.

13. A method according to any of the preceding claims, wherein the frequency-converted pulse train is applied to perform any of: irradiation of a photocathode to provide a free electron source for light generation; micromachining of a material; non-linear microscopy; and satellite communication.

14. A method according to any of the preceding claims, wherein the pump pulse interval is at least 5.0 ps, for example at least 15.0 ps, for example at least 40.0 ps, such as at least 100 ps.

15. A method according to any of the preceding claims, wherein the pump light pulse train comprises at least three pulses, for example at least four pulses, for example at least five pulses, for example at least 10 pulses.

16. A method according to any of the preceding claims, wherein an energy of each pulse of the pump light pulse train is at least 10 nJ, for example at least 30 nJ, such as at least 50 nJ; and / or is at most 3.0 pJ, for example at most 2.0 pJ, such as at most 1.0 pJ.

17. A method according to any of the preceding claims, wherein a pulse width of each pulse of the pump light pulse train is greater than a period of the coherent oscillations of the gas; and / or a pulse width of each pulse of the pump light pulse train is greater than 100 fs, for example greater than 400 fs, such as greater than 1.0 ps.

18. A method according to any of the preceding claims, wherein the pump light pulse train is generated by a single light source or pump laser, and / or wherein the pulses of the pump light pulse train have substantially the same wavelength.

19. A method according to any of the preceding claims, wherein the method comprises the steps of: injecting auxiliary light into the chamber while the gas within the chamber undergoes coherent oscillations induced and / or enhanced by the pump light pulse train, the auxiliary light being separate from the pump light pulse train; generating frequency converted auxiliary light by non-linear frequency conversion of the auxiliary light via stimulated Raman scattering of the auxiliary light within the chamber; and outputting the frequency-converted auxiliary light from the chamber.

20. A system for providing pulsed light, the system comprising : a pump laser for emitting a pump light pulse train, wherein power peaks of pulses of the pump light pulse train are temporally spaced at most by a pump pulse interval; a chamber, wherein the pump laser is optically coupled to the chamber for injecting the pump light pulse train into the chamber; a gas within the chamber for generating a frequency-converted pulse train by non-linear frequency conversion of the pump light pulse train via stimulated Raman scattering of the pump light pulse train with the gas within the chamber, wherein the pulses of the pump light pulse train induce and / or enhance coherent oscillations of the gas within the chamber which facilitates said stimulated Raman scattering, the coherent oscillations having a dephasing time and being vibrational oscillations and / or rotational oscillations; and an optical outlet for outputting the frequency-converted pulse train from the chamber,36 wherein a ratio of the pump pulse interval to the dephasing time is at most 10.0 such that at least some pulses of the pump light pulse train each serves both to: generate a pulse of the frequency-converted pulse train via said stimulated Raman scattering facilitated by the coherent oscillations of the gas induced and / or enhanced by at least one previous pulse of the pump light pulse train; and induce and / or enhance coherent oscillations of the gas for said stimulated Raman scattering for conversion of at least one subsequent pulse of the pump light pulse train.

21. A system according to claim 20, wherein said chamber is a first chamber, said gas is a first gas, and said optical outlet is a first optical outlet, wherein the system further comprises: a second chamber, wherein the frequency-converted pulse train from the first chamber is optically coupled to the second chamber for injecting the frequency- converted pulse train into the second chamber; a second gas within the second chamber for generating a further frequency- converted pulse train by non-linear frequency conversion of the frequency- converted pulse train from the first chamber via stimulated Raman scattering of the frequency-converted pulse train from the first chamber with the second gas within the second chamber; a second optical outlet for outputting the further frequency-converted pulse train from the second chamber.

22. A system according to claim 21, wherein said dephasing time is a first dephasing time, wherein the pulses of the frequency-converted pulse train from the first chamber induce and / or enhance coherent oscillations of the second gas within the second chamber which facilitates said stimulated Raman scattering, the coherent oscillations having a second dephasing time and being vibrational oscillations and / or rotational oscillations, and37 wherein a ratio of the pump pulse interval to the second dephasing time is at most 10.0 such that at least some pulses of the frequency-converted pulse train from the first chamber each serves both to: generate a pulse of the further frequency-converted pulse train via said stimulated Raman scattering facilitated by the coherent oscillations of the second gas induced and / or enhanced by at least one previous pulse of the frequency-converted pulse train from the first chamber; and induce and / or enhance coherent oscillations of the second gas for said stimulated Raman scattering for conversion of at least one subsequent pulse of the frequency-converted pulse train from the first chamber.

23. A laser system according to any of claims 20-22, wherein the system is arranged to perform the method according to any of claims 1-19.

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