A pulsed solid-state laser

Abstract

A pulsed solid-state laser with the ability to stabilise the repetition rate (frep.) and carrier- envelope offset frequency (fceo), which can be used, among other things, in molecular spectroscopy, for the construction of optical atomic clocks, for the generation of attosecond pulses, in optical communications, for the construction of laser rangefinders, or in medical diagnostics, comprising a pumping laser (1), a set of lenses shaping the pumping beam (2) located downstream of the pumping laser (1), a laser resonator comprising an input coupler mirror (3a), an gain medium (3b) in the form of a crystal and an output coupler mirror (3c), as well as an electro-optical modulator located between the laser resonator components, characterised in that gain medium (3b) has the form of a crystal that demonstrates an electro-optical effect and which at the same time constitutes an electro-optical modulator for the purposes of stabilising the carrier-envelope offset frequency fceo or the repetition rate frep.

Description

[0001] A pulsed solid-state laser

[0002] The invention refers to a pulsed solid-state laser with the ability to stabilise the repetition rate (frep) and carrier-envelope offset frequency (fceo), wherein such a laser could be useful, among other things, in molecular spectroscopy, as well as for purposes such as the construction of optical atomic clocks, the generation of attosecond pulses, optical communications, the construction of laser rangefinders, or in medical diagnostics.

[0003] A known and commonly used solid-state laser comprises a pumping laser, a lens or a set of lenses that forms the pumping beam, and a laser resonator that comprises an input coupler mirror, a gain medium in the form of a crystal, an output coupler mirror and possibly some additional resonator mirrors. A mode-locked solid-state laser can be used to generate ultra-short laser pulses of the duration ranging from a few picoseconds to even less than 100 fs. One of the characteristics of a mode-locked laser is its carrier-envelope offset frequency (fceo), which describes the changes of the carrier-envelope phase (CEP) between successive pulses that are generated with a repetition rate of frep. The stabilisation of frequencies fceo and frepis necessary for the laser to become a frequency comb.

[0004] A frequency comb based on a solid-state laser is known, among others, from publication [D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, S. T. Cundiff. Carrier-Envelope Phase Control of Femtosecond Mode-Locked Lasers and Direct Optical Frequency Synthesis. Science 288, 635-639, 2000]. It is also known from US patent application US2002171835A1 as a method and device for samples analysis and specification and for optical communication improvement.

[0005] Frequency combs have a wide range of applications in basic science, as well as in non-laboratory applications. The first group of applications includes, for example, the use of frequency combs in precision spectroscopy [N. Picque, T. W. Hansch. Frequency comb spectroscopy. Nature Photonics 13, 146-157, 2019], for the construction of optical atomic clocks [T.L. Nicholson, S.L. Campbell, R.B. Hutson, G.E. Marti, B.J. Bloom, R.L. McNally, W. Zhang, M.D. Barrett, M.S. Safronova, G.F. Strouse, W.L. Tew, J. Ye. Systematic evaluation of an atomic clock at 2xl0-18total uncertainty. Nature Communications 6, 6896, 2015], or the generation of attosecond pulses and using them to study matter [A. Baltuska, T. Udem, M. Uiberacker, M. Hentschel, E. Goulielmakis,Ch. Gohle, R. Holzwarth, V. S. Yakovlev, A. Scrinzi, T. W. Hansch, F. Krausz. Attosecond control of electronic processes by intense light fields. Nature 421, 611-615, 2003],

[0006] The practical or industrial applications of frequency combs may include optical communications [Quantum-limited optical time transfer for future geosynchronous links. Nature 618, 721-726, 2023], ultra high precision rangefinders [E. D. Caldwell, L. C. Sinclair, N. R. Newbury & J.-D. Deschenes The time-programmable frequency comb and its use in quantum-limited ranging. Nature 610, 667-673, 2022], environmental monitoring [G. B. Rieker, F. R. Giorgetta, W. C. Swann, J. Kofler, A. M. Zolot, L. C. Sinclair, E. Baumann, C. Cromer, G. Petron, C. Sweeney, P. P. Tans, I. Coddington, N. R. Newbury. Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths. Optica 1, 290-298, 2014], or, finally, medical diagnostics related to diseases such as Covid- 19 [Q. Liang, Y.-C. Chan, J. Toscano, K. K. Bjorkman, L. A. Leinwan, R. Parker, E. S. Nozik, D. J. Nesbitt, J. Ye. Breath analysis by ultra-sensitive broadband laser spectroscopy detects SARS-CoV-2 infection. Journal of Breath Research 17, 036001].

[0007] To obtain a frequency comb, it is necessary to stabilise the laser’s fceo and frepfrequencies. A known and commonly used method to stabilise repetition rate frepinvolves the detection of the frepvalue by means of measurement of the laser light using a highspeed photodetector and subsequently the active stabilisation of frepin a phase-locked loop by the modification of the length of the laser resonator using a mirror installed on a piezoelectric shifter. This method is known, among others, from the above-mentioned paper [D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, S. T. Cundiff. Carrier-Envelope Phase Control of Femtosecond Mode-Locked Lasers and Direct Optical Frequency Synthesis. Science 288, 635-639, 2000].

[0008] Alternatively, frequency frepcan be stabilised by installing an electro-optical modulator in the resonator of a solid-state laser. This solution has been described, among others, in the paper [N. Torcheboeuf, G. Buchs, S. Kundermann, E. Portuondo-Campa, J. Bennes, S. Lecomte. Repetition rate stabilization of an optical frequency comb based on solid-state laser technology with an intra-cavity electro-optic modulator. Optics Express 25, 2215-2220, 2017]. An electro-optical modulator is a crystal that demonstrates an electro-optical effect, for example lithium niobate, LiNbOs, or lithium tantalate, LiTaOs, or barium borate, BaB2O4, or another. The crystal’s walls are connected to electrodes, which supply a voltage signal to the crystal’s walls, which changes the crystal’s refractiveindex and consequently changes the effective length of the optical path in the resonator. The utilisation of this effect allows one to control and stabilise the repetition rate frep.

[0009] Frequency fceo can be detected, for example, in an f-2f interferometer, 0-f interferometer, or a similar arrangement, which is known from the paper [H. R. Telle, G. Steinmeyer, A. E. Dunlop, J. Stenger, D. H. Sutter, U. Keller. Carrier-envelope offset phase control: a novel concept for absolute optical frequency measurement and ultrashort pulse generation. Applied Physics B 69, 327-332, 1999]. Especially in an f-2f interferometer arrangement, frequency fceo is detected using beating between the laser signal of the optical frequency of 2f and frequency -doubled frequency f (i.e. 2xf = 2f). This means that the laser signal must cover at least one optical octave, i.e. it must contain the frequency f and the twice-higher frequency 2f.

[0010] Once the carrier-envelope offset frequency fceo has been detected, its active stabilisation can be implemented in various ways, on the basis of the control of the laser parameters that affect the fceo frequency. The modulation of the power of the laser that pumps the laser used for the stabilisation the carrier-envelope offset frequency fceo is known, among others, from paper [R. Holzwarth, T. Udem, T. W. Hansch, J. C. Knight, W. J. Wadsworth, P. St. J. Russell. Optical Frequency Synthesizer for Precision Spectroscopy. Physical Review Letters 85, 2264-2267, 2000]. This method has been implemented for example in European patent application EP2866311A1. The described invention discloses a method of controlling frequency fceo in a thin-disk laser, using at least one modulated pumping laser diode that is powered by a current source with modulating capability.

[0011] The stabilisation of the carrier-envelope offset frequency fceo through the modulation of losses inside a solid-state laser is known from European patent application EP3053226A2. The described invention discloses a method of the optical-optical modulation of fceo via the illumination of a SESAM-type saturable absorber using an additional laser radiation source. The illumination of the saturable absorber affects the losses inside the laser resonator and causes a frequency shift in the fceo frequency. By way of controlling the optical source signal using the error signal, fceo stabilisation can be achieved.

[0012] The stabilisation of the carrier-envelope offset frequency fceo using an electro-optical modulator implemented inside an optical fibre laser resonator (intra-cavity) is known from the conference paper [W. Hansel, M. Giunta, M. Lezius, M. Fischer, R.Holzwarth. Electro-optic modulator for rapid control of the carrier-envelope offset frequency. 2017 Conference on Lasers and Electro-Optics (CLEO), San Jose, CA, USA, 2017]. As in the case of the technique of modulating the repetition rate frepusing an electro-optical modulator described above, this modulator is installed inside an optical fibre laser’s resonator as an additional component. The modification of voltage applied to this modulator changes its refractive index, which consequently changes frequency fceo of the laser signal and enables its stabilisation. This technique is also disclosed in US patent application US10720750B2, which describes the modification of frequency fceoin a laser via the modification of the phase (Acp) on the carrier wave of the laser pulse per each resonator round-trip. The technique of stabilising fceo with the use of an additional electro-optical modulator has so far only been implemented in optical fibre lasers, but not in solid-state lasers.

[0013] The use of an electro-optical modulator outside of the laser resonator (extracavity) in a solid-state laser amplifier arrangement, for the purposes of additional carrierenvelope phase stabilisation, is known from European patent application EP2656454B1. The described invention requires the laser to be stabilised using a different method, as the electro-optical modulator does not affect the laser itself, but only its output beam, which is amplified by an additional amplifier arrangement. This type of solution differs therefore from the method of active stabilisation of fceo in lasers. This technique has also been described in paper [O. Gobert, P.M Paul, J.F Hergott, O. Tcherbakoff, F. Lepetit, P. D 'Oliveira, F. Viala, and M. Comte. Carrier-envelope phase control using linear electrooptic effect. Opt. Express 19, 5410-5418, 2011].

[0014] The aim of the present invention is to resolve the problem of simplifying the design of the laser known from the state of the art, while simultaneously enabling the stabilisation of the repetition rate frepor the stabilisation of the carrier-envelope offset frequency (fce0).

[0015] A pulsed solid-state laser comprising a pumping laser, a set of lenses shaping the pumping beam, located downstream of the pumping laser, a laser resonator comprising an input coupler mirror, a gain medium in the form of a crystal and an output coupler mirror, as well as an electro-optical modulator located between the laser resonator components, according to the present invention, is characterised in that the gain medium has the form of a crystal that demonstrates an electro-optical effect, and whichat the same time constitutes an electro-optical modulator for the purposes of stabilising the carrier-envelope offset frequency fceo, or the repetition rate frep.

[0016] Preferably, the laser resonator is also equipped with additional mirrors, which are intended to extend the resonator’s length and thus to change the repetition rate frep.

[0017] Preferably, the additional mirrors can be dielectric dispersive mirrors that enable the control of the laser resonator’s dispersion.

[0018] The gain medium crystal that demonstrates an electro-optical effect can have the form, for example, of zinc sulphide doped with chromium ions, Cr:ZnS, or zinc selenide doped with chromium ions, Cr:ZnSe.

[0019] The performed tests demonstrated the possibility of electro-optical modulation in the gain medium, which enabled the removal of the above electro-optical modulator from the system as an additional unit. The solution according to the invention utilises a crystal with an electro-optical effect as the gain medium. The crystal walls of the gain medium are connected with electrodes that supply a voltage signal to the crystal walls, which changes the refractive index of the crystal and consequently enables the control and active stabilisation of the carrier-envelope offset frequency fce0, or the repetition rate frep.

[0020] The object of the invention has been shown in the form of drawings, in which fig. 1 presents a diagram of the solution according to the first embodiment of the invention, fig. 2 presents a diagram of the solution according to the second embodiment of the invention, fig. 3 presents an operating diagram of the solution for the stabilisation of carrier-envelope offset frequency fceo, fig. 4 presents an alternative operating diagram of the solution for the stabilisation of carrier-envelope offset frequency fceo to the diagram in fig. 3, and fig. 5 presents an operating diagram of the solution for the stabilisation of repetition rate frep.

[0021] A pulsed solid-state laser according to the first embodiment of the invention comprises a pumping laser 1, a set of lenses for the forming of the pumping beam 2 located downstream of the pumping laser 1, a laser resonator comprising an input coupler mirror 3a, a gain medium 3b in the form of a crystal and an output coupler mirror 3c. Gain medium 3b has the form of a crystal which demonstrates an electro -optical effect and which also constitutes an electro-optical modulator for the purposes of stabilising the carrier-envelope offset frequency fceo or the repetition rate frep. The crystal walls of gain medium 3b are connected with electrodes 3d that supply a voltage signal to the crystal walls, which changes the refractive index of the crystal. The crystal of gain medium 3b,which demonstrates an electro-optical effect, can have the form, for example, of zinc sulphide doped with chromium ions, Cr:ZnS, or zinc selenide doped with chromium ions, Cr:ZnSe.

[0022] A pulsed solid-state laser according to the second embodiment of the invention differs from the first embodiment in that the laser resonator, apart from an input coupler mirror 3a, a gain medium 3b in the form of a crystal and an output coupler mirror 3c, also comprises additional mirrors 3e, located between the input coupler mirror 3a and the output coupler mirror 3c, which extend the optical path length in the resonator in order to modify repetition rate frep. Additional mirrors 3e have the form of dielectric dispersive mirrors, which enable the control of the laser resonator’s dispersion.

[0023] According to other, further embodiments of the invention, the laser resonator can be provided with more additional mirrors 3e.

[0024] In the course of stabilisation of the carrier-envelope offset frequency fceo using the system according to the invention, the first step is its detection, which can be done, for example, using an f-2f interferometer, or 0-f interferometer system, or using a similar fceo frequency detector 5. The obtained electrical signal 6 is compared with electrical reference signal fref generated by the fref reference signal generator 7, using a phase detector 8 that generates an error signal for the proportional-integral-derivative PID controller 9. Electrical signal 10 from PID controller 9 is sent via electrodes 3d as a modulating (controlling) signal to the laser’s gain medium 3b, which acts as an electro-optical modulator. In this way, it is possible to stabilise the laser’s frequency fceo in accordance with reference signal fref, by electrically modulating gain medium 3b. Alternatively, frequency fceo can be stabilised to zero by dividing electrical signal fceo in a signal divider 11 and by sending two signals to phase detector 8 (without an additional reference signal). Alternatively, electrical signal fceo can be sent directly to PID controller 9, in order to stabilise frequency fceo to zero.

[0025] In the case of the stabilisation of repetition rate frep, the first step is its detection, which can be done using a high-speed photodetector - rate frepdetector 12. The obtained electrical signal 6 is compared with electrical reference signal fref using phase detector 8, which generates an error signal for PID controller 9. Electrical signal 10 from PID controller 9 is sent as a modulating (control) signal via electrodes 3d to the laser’s gain medium 3b, which acts as an electro-optical modulator. In this way, it is possible tostabilise the laser’s freprate in accordance with reference signal fref, by modulating gain medium 3b.

[0026] List of designations:

[0027] 1 - pumping laser

[0028] 2 - set of lenses for the forming of the pumping beam

[0029] 3a - input coupler mirror

[0030] 3b - gain medium in the form of a crystal

[0031] 3c - output coupler mirror

[0032] 3d - electrodes

[0033] 3e - additional mirrors

[0034] 4 - laser beam

[0035] 5 - fceo frequency detector

[0036] 6 - electrical signal

[0037] 7 - fref reference signal generator

[0038] 8 - phase detector

[0039] 9 - PID controller

[0040] 10 - electrical modulating signal for EOM

[0041] 11 - signal divider

[0042] 12 " frep frequency detector

Claims

Claims1. A pulsed solid-state laser comprising a pumping laser, a set of lenses shaping the pumping beam, located downstream of the pumping laser, a laser resonator comprising an input coupler mirror, a gain medium in the form of a crystal and an output coupler mirror, as well as an electro-optical modulator located between the laser resonator components, characterised in that the gain medium (3b) has the form of a crystal that demonstrates an electro-optical effect and which at the same time constitutes an electro-optical modulator for the purposes of stabilising the carrier-envelope offset frequency fceo, or the repetition rate frep.

2. The laser according to claim 1, characterised in that the laser resonator is provided with additional mirrors (3e) which, in order to change repetition rate frep, extend the optical path length inside the resonator.

3. The laser according to claim 2, characterised in that additional mirrors (3e) have the form of dielectric dispersive mirrors, which enable the control of the laser resonator’s dispersion.

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