Laser system for generating single-sideband modulated laser radiation

Abstract

There is a need for laser systems which generate single-sideband modulated laser radiation in a predictable manner, namely with a frequency spacing between the carrier and sideband in the (high) MHz to GHz range. For this purpose, the invention proposes a laser system which has a laser source (1) that emits laser radiation (2). A first modulation device is provided for modulating the frequency of the laser radiation (2), said frequency modulation being carried out using a frequency modulation shift at a modulation frequency. A second modulation device which is independent of the first modulation device is provided for modulating the amplitude of the laser radiation, said amplitude modulation being carried out using an amplitude modulation shift at the same modulation frequency. The frequency modulation and the amplitude modulation are coupled together in a phase-locked manner, wherein the frequency modulation shift, the amplitude modulation shift, and the relative phase of the frequency modulation and the amplitude modulation are set in such manner that the spectrum of the frequency- and amplitude-modulated laser radiation has a carrier (4', 4'') and a single sideband (5', 5'') which is spaced from the carrier (4', 4'') by the value of the modulation frequency.

Description

[0001] Laser system for generating single-sideband modulated laser beams

[0002] The invention relates to a laser system having a laser light source and a modulation device for modulating the laser radiation so that the spectrum of the laser radiation comprises a carrier and a sideband.

[0003] Furthermore, the invention relates to the use of such a laser system for generating an artificial guide star or for exciting optical transitions in a quantum information system.

[0004] Finally, the invention also relates to a method for generating single-sideband modulated laser radiation.

[0005] Known laser systems use sideband-modulated laser radiation that is amplified and, for example, frequency doubled. This can be used, for example, to resonantly excite fluorescence in sodium atoms in the mesosphere. This creates a point-like artificial star ("guide star") that can be used as a reference for astronomical telescopes with adaptive optics. The requirements for suitable laser systems to generate such artificial guide stars are high. Ideally, a high power of 20 W or more is required, with a linewidth of a few GHz down to less than 5 MHz at the sodium resonance of 589 nm (sodium D line). To achieve sufficient intensity for the artificial guide star, the interaction between the laser and the few sodium atoms in the mesosphere must be as intensive as possible.It should be noted that the electronic excitation scheme of the sodium atom is not a pure two-level system. Of particular importance is the hyperfine splitting of the excited fluorescence line. Hyperfine splitting causes the fluorescence electron to be optically pumped into a non-resonant state after several excitation cycles, i.e., a state from which there is no resonant optical transition at the laser frequency (fluorescence frequency). After this time, the electron is no longer available for fluorescence, resulting in an immediate loss of the corresponding atom for fluorescence backscattering. This can be counteracted by adding a second spectral component to the laser light, namely one at a so-called pump-back frequency in addition to the one at the fluorescence frequency. The fluorescence frequency excites the fluorescence of the corresponding sodium resonance.The pump-back frequency is detuned from the fluorescence frequency by the amount corresponding to the hyperfine splitting of the respective sodium line. This causes the excitation electron to be "pumped back" from the non-resonant state and thus becomes available for the fluorescence process again. This significantly increases the intensity of the fluorescent light. The pump-back frequency is typically generated by frequency modulation (sideband modulation) of the laser. In this case, the laser spectrum consists of a carrier at the fluorescence frequency and at least one sideband at the pump-back frequency.

[0006] In known laser systems, a semiconductor laser is used as the laser light source, with the pump-back sideband being generated by sinusoidal modulation of the laser diode's injection current. Alternatively, the laser radiation can be modulated using an electro-optical modulator arranged in the beam path behind the laser light source. Sinusoidal modulation always creates sidebands arranged symmetrically around the carrier, i.e., with lower and higher frequencies than the carrier. However, usually only one of the sidebands is required for the application; in the best case, the second sideband at least does not interfere with the application.

[0007] In such laser systems, it can be disadvantageous that the portion of the optical power in the unused sideband does not contribute to fluorescence enhancement, thus reducing the laser's efficiency in generating fluorescent light. Accordingly, less power is available in the actually used sideband. Furthermore, in many applications, the unused second sideband may generate additional stray light (Rayleigh scattering), thus reducing the signal-to-noise ratio.

[0008] In the context of a laser-generated artificial guide star, a laser radiation spectrum without a superfluous second sideband can offer further significant advantages: To increase the coupling of the laser radiation to the atomic transition, frequency modulation of the laser ("chirping") is often used, in which the interaction of the laser with different velocity classes of the atomic ensemble is controlled. Any unnecessary frequency component in the laser spectrum can reduce the efficiency of this method. Furthermore, the narrowband laser can generate Raman signals at both the carrier frequency and the sideband frequencies (see, for example, Vogt et al., "Detection and Implications of Laser-Induced Raman Scattering at Astronomical Observatories", Phys. Rev. X, 2017), which interfere with the instruments used.With optical single-sideband modulated (OSSB) laser radiation, the number of Raman lines can be reduced by up to one third.

[0009] The frequency separation between carrier and sideband in the applications of interest lies in the high MHz to GHz range (e.g., 1.7 GHz for sodium hyperfine splitting). Direct generation of single-sideband modulated laser radiation, e.g., through serrodyne (sawtooth) modulation, is practically impossible due to the high-frequency Fourier components involved.

[0010] Sideband-modulated, particularly single-sideband-modulated, laser radiation is of interest for other applications besides the creation of artificial guide stars in astronomy. In many applications in atomic physics, quantum technology, including quantum information technology, and spectroscopy, it is essential to maximize the interaction time of a laser beam with an atom or an ensemble of atoms. These applications include, among others, laser cooling of atoms and, more generally, the generation of fluorescence signals, e.g., in spectroscopy. To maximize the interaction time, two-level systems that allow cyclic excitation are often used. Examples of these are generally alkali atoms or alkaline earth ions, where hyperfine splitting in the ground state must be taken into account.In order to increase the efficiency of laser cooling or the generation of resonance fluorescence of these alkali atoms or alkaline earth ions, it is therefore helpful and common to provide a second laser frequency (typically by means of a complete second laser system) in addition to the carrier that drives the quasi-cyclic transition in the atom, whereby the frequency separation of both components corresponds to the hyperfine splitting of the atomic ground state in order to bring the atoms back to the ground state of the quasi-cyclic transition by optical pumping so that they can contribute to the fluorescence.

[0011] The second sideband, which is always present in conventionally modulated laser radiation, always requires optical power, which is particularly disadvantageous when the laser is operated at its power limit.

[0012] Against this background, there is a need for a laser system and a method that generates single-sideband modulated laser radiation in a practical manner, with a frequency spacing between carrier and sideband in the (high) MHz to GHz range.

[0013] To this end, the invention proposes a laser system comprising the following: a laser light source configured to emit laser radiation during operation of the laser system, a first modulation device configured for frequency modulation of the laser radiation, wherein the frequency modulation is effected with a frequency modulation swing at a modulation frequency, a second modulation device independent of the first modulation device configured for amplitude modulation of the laser radiation, wherein the amplitude modulation is effected with an amplitude modulation swing at the modulation frequency, wherein the frequency modulation and the amplitude modulation are coupled to one another in a phase-locked manner, and wherein the frequency modulation swing, the amplitude modulation swing and the relative phase of the frequency modulation and the amplitude modulation are set in such a way thatthat the spectrum of the frequency- and amplitude-modulated laser radiation has a carrier and a single sideband spaced from the carrier by the amount of the modulation frequency.

[0014] The invention is based on the finding that it is possible to generate a single-sideband spectrum, ie a spectrum which has a carrier and a single sideband, by concatenating frequency and amplitude modulation of an optical carrier signal (i.e. the laser radiation generated by the laser light source) with suitable parameter selection with regard to modulation deviation and relative phase of frequency and amplitude modulation, wherein the sideband in the spectrum is spaced from the carrier, ie from the frequency of the carrier signal, by the amount of the modulation frequency.

[0015] This approach can also be implemented with real modulation techniques, which rarely generate pure frequency modulation or pure amplitude modulation, but usually comprise a mixture of both modulation types. For the purposes of the invention, frequency modulation is thus understood to mean a modulation that predominantly effects a frequency modulation of the laser radiation. Accordingly, amplitude modulation within the meaning of the present disclosure is a modulation that predominantly effects an amplitude modulation of the laser radiation. In other words, the first and second modulation devices differ from one another with regard to their respective modulation ranges for the frequency and amplitude modulation.In the first modulation device, the modulation deviation of the frequency modulation is larger than in the second modulation device; accordingly, the modulation deviation of the amplitude modulation is smaller in the first modulation device than in the second modulation device. It can be seen that by appropriately adjusting the modulation parameters, a single-sideband spectrum can always be generated when the modulations imposed by the individual modulators have different components of frequency modulation and amplitude modulation.

[0016] It should be noted that the designations "first" and "second" modulation devices do not imply a specific order of arrangement in the beam path of the laser radiation. Amplitude modulation following frequency modulation of the laser radiation is possible, as is the reverse order.

[0017] The approach of the invention has a number of advantages:

[0018] • By saving optical power for the additional, unnecessary spectral sideband, it is possible to increase the power in the remaining two optical signal components (carrier and single sideband) while maintaining the same overall power of an optical amplifier system. This is particularly relevant if the laser system as a whole (without the inventive solution) is already operating at its power limit, but additional output power is required.

[0019] • Laser systems in which the frequency of the carrier or sideband is to be changed quickly and / or with a large frequency shift ("laser frequency chirping") can be efficiently implemented using the invention. Conventional implementations, e.g., with suppression of the unwanted sideband by a spectral filter (which can only be adjusted comparatively slowly with regard to its passband), do not allow this, or only to a limited extent.

[0020] • The invention enables the realization of single-sideband laser systems at high modulation frequencies for which serrodyne modulation (sawtooth waveform) is impractical or at least extremely difficult. • The invention enables the generation of single-sideband modulated laser radiation at wavelengths or frequencies for which no (usually fiber-coupled) complex I / Q modulators (e.g. dual-drive or dual-parallel Mach-Zehnder modulators) are available. In contrast to the approach of the invention, the conventional generation of OSSB spectra with such modulators is based on the interaction of several electro-optical modulators and phase shifters in an interferometric arrangement. Several control loops are necessary to maintain the necessary interference conditions. The approach of the invention is significantly less complex and can in principle be implemented without any restrictions with regard to the wavelength.

[0021] • The invention enables the realization of extremely robust single-sideband laser systems in a compact design (e.g. compared to a narrow-band spectral filter whose filter frequency must be precisely controlled and stabilized).

[0022] • The invention enables the realization of single-sideband laser systems in combination with (nonlinear) optical amplifiers or frequency conversion systems that influence the spectrum of the laser radiation. Frequency and amplitude modulation can be used to "precondition" the laser radiation, so that the laser radiation exhibits the desired single-sideband modulated spectrum only at the output. The effects of amplification or frequency conversion on the spectrum are compensated for in advance (more details below).

[0023] • The invention enables the realization of single-sideband laser systems with standard components. In particular, in conventional MOPA systems ("master oscillator / power amplifier"), OSSB modulation can be implemented entirely without additional optical components. In one possible embodiment, the laser light source is a diode laser with at least one laser diode, wherein the first modulation device is configured to modulate the injection current of the laser diode. Modulating the injection current is a suitable method for generating spectral sidebands. This type of "electronic" modulation is particularly easy to implement. Thus, the laser system can be provided significantly more cost-effectively than laser systems that operate with alternative modulators, such as electro-optical modulators.

[0024] Alternatively, at least one of the first and second modulation devices may comprise an electro-optical modulator, an acousto-optical modulator, an electro-absorption modulator, and / or an interferometric modulator. Such modulators are common and commercially available components that can be used for frequency or amplitude modulation in the laser system according to the invention in a manner known per se.

[0025] In a further possible embodiment, the laser system comprises at least one optical amplifier configured to amplify the laser radiation. The optical amplifier serves to amplify the generated laser radiation up to the desired power in one or more stages. The optical amplifier can simultaneously be designed, for example, as a second modulation device. The gain of the optical amplifier can be easily modulated in order to implement amplitude modulation of the laser radiation. A Raman fiber amplifier, such as that disclosed in EP 2 081 264 A1, is suitable as an optical amplifier. In addition, reference is made to the publication by Luke R. Taylor et al. (“50 W CW visible laser source at 589 nm obtained via frequency doubling of three coherently combined narrow-band Raman fiber amplifiers”, Optics Express, Vol. 18, No. 8, 8540). However, other types of amplifier are also possible.

[0026] In another possible embodiment, the laser system comprises a nonlinear frequency converter configured for frequency conversion, in particular for frequency doubling, of the frequency- and amplitude-modulated laser radiation. The frequency converter transforms the frequency of the laser radiation into the desired frequency range, e.g., by frequency doubling and / or sum frequency generation.

[0027] In one possible embodiment of the laser system, the frequency modulation sweep, the amplitude modulation sweep, and the relative phase of the frequency modulation and the amplitude modulation are adjusted such that the spectrum of the frequency- and amplitude-modulated laser radiation only exhibits a carrier and a single sideband at the output of the optical amplifier or the nonlinear frequency converter. A further finding of the invention is that, as already mentioned above, by adjusting the frequency and amplitude modulation sweep and the relative phase of the frequency and amplitude modulation, a type of preconditioning can be applied to the spectrum of the modulated laser radiation, which specifically ensures that only the ultimately usable laser radiation, i.e., after amplification and, if applicable, frequency conversion, exhibits the desired single-sideband spectrum.After frequency and amplitude modulation, the laser radiation spectrum will generally not initially be a single-sideband spectrum. This only becomes apparent after amplification and, if necessary, frequency conversion. This preconditioning option is superior to conventional techniques, which can only generate a single-sideband spectrum before amplification and frequency conversion. With such techniques, the optical amplification and subsequent frequency conversion will in many cases result in the spectrum ultimately exhibiting unwanted sidebands. This can be avoided by the invention.

[0028] With regard to the applications mentioned at the beginning, the frequency of the carrier in the spectrum of the laser radiation in a possible design of the laser system corresponds to a fluorescence frequency and the frequency of the remaining sideband to a pump-back frequency, whereby the fluorescence frequency is resonant with the transition frequency of an (approximately cyclic) optical transition, e.g. in an atom, i.e. a line in the electronic excitation spectrum of the atom, in an atom ensemble or in an atom-like solid-state system (e.g. quantum dot) as a building block of a quantum information system, and the frequency separation of the pump-back frequency from the fluorescence frequency corresponds to the hyperfine splitting of the optical transition.To create an artificial guide star by fluorescence excitation of sodium atoms in the mesosphere, the fluorescence frequency of the laser radiation should correspond to a wavelength of 589 nm, while the frequency separation of the pump-back frequency from the carrier frequency should be 1.7 GHz. This frequency separation corresponds to the hyperfine splitting of the sodium line in question.

[0029] The invention also proposes a method for generating single-sideband modulated laser radiation, comprising the steps

[0030] Generation of laser radiation, frequency modulation of the laser radiation with a frequency modulation swing at a modulation frequency,

[0031] Amplitude modulation of the laser radiation with an amplitude modulation swing at the modulation frequency, wherein the frequency modulation and the amplitude modulation are coupled to one another in a phase-locked manner and wherein the frequency modulation swing, the amplitude modulation swing and the relative phase of the frequency modulation and the amplitude modulation are set in such a way that the spectrum of the frequency- and amplitude-modulated laser radiation has a carrier and a single sideband which is spaced from the carrier by the amount of the modulation frequency.

[0032] In this case, optical amplification and / or frequency conversion of the frequency- and amplitude-modulated laser radiation can additionally be performed. The frequency modulation sweep, the amplitude modulation sweep, and the relative phase of the frequency modulation and the amplitude modulation are expediently adjusted such that the spectrum of the frequency- and amplitude-modulated laser radiation only exhibits a carrier and a single sideband after the optical amplification and / or the nonlinear frequency conversion, which sideband is spaced from the carrier by the amount of the modulation frequency.

[0033] In one possible design, the frequency of the laser radiation is periodically varied between an initial value and a final value in addition to the frequency modulation (called "chirping" or "frequency chirping"). This allows, in the Laser Guide Star application, the interaction of the laser radiation with different velocity classes of the ensemble of sodium atoms in the mesosphere to be controlled. The frequency swing of the chirping will be significantly larger than that of the frequency modulation, while the speed, i.e., the frequency of the chirping, will be significantly smaller than the modulation frequency.

[0034] Exemplary embodiments of the invention are explained in more detail below with reference to the figures. They show:

[0035] Fig. 1 : schematic representation of a first

[0036] Embodiment of a laser system;

[0037] Fig. 2: Spectra of the laser radiation after

[0038] Frequency modulation (a) and amplitude modulation (b);

[0039] Fig. 3: schematic representation of a second

[0040] Embodiment of a laser system;

[0041] Fig. 4: Spectra of the laser radiation after

[0042] Frequency and amplitude modulation (a), after frequency conversion (b) and after frequency conversion with adjustment of the modulation parameters (c);

[0043] Fig. 5 schematic representation of a third

[0044] Example of a laser system.

[0045] In the following description of the figures, the same reference symbols and terms are used for the same and corresponding elements.

[0046] The laser system of Fig. 1 comprises a diode laser 1 as a laser light source, which emits laser radiation 2. A first output of a high-frequency signal generator 3 is connected to an electronic signal input of the diode laser 1, so that the injection current of a laser diode (not shown) of the diode laser 1 is modulated with a modulation frequency according to the supplied signal. This causes a frequency modulation of the laser radiation 2. The high-frequency signal generator 3, together with the laser diode, thus forms a first modulation device within the meaning of the invention.

[0047] Fig. 2a) shows the typical spectrum (intensity I as a function of frequency f) of the thus frequency-modulated laser radiation 2 with a carrier 4 and two sidebands 5 and 6 arranged symmetrically to the carrier 4. The distance between the carrier 4 and each of the sidebands 5, 6 is equal to the modulation frequency, i.e., the frequency of the output signal of the high-frequency signal generator 3.

[0048] In Fig. 1, an optical amplifier 7 is connected downstream of the diode laser 1 in the beam path. This amplifier emits amplified laser radiation 2' at its output. The amplifier 7 has an electronic signal input coupled to a second output of the high-frequency signal generator 3 via a phase shifter 8. The signal supplied to the amplifier 7 via the signal input modulates the optical amplification, i.e., amplitude modulates the laser radiation 2'. Thus, the high-frequency signal generator 3, together with the amplifier 7, forms a second modulation device within the meaning of the invention.

[0049] The signal at the second output of the high-frequency signal generator 3 has a fixed phase relationship to the signal present at the first output. The signals present at the two outputs are therefore phase-locked. Accordingly, the frequency and amplitude modulation of the laser radiation are phase-locked. The signal amplitudes at the two outputs of the high-frequency signal generator 3 (and thus the frequency modulation swing and the amplitude modulation swing) are, in contrast, freely adjustable. Furthermore, the phase shifter 8 can be used to adjust the relative phase between the modulation signals supplied to the diode laser 1 and the amplifier 7, i.e., the relative phase between frequency and amplitude modulation. Fig. 2b) shows the spectrum of the laser radiation 2' at the output of the amplifier 7. It can be seen that by suitable selection of the modulation parameters, i.e.,The frequency modulation sweep, the amplitude modulation sweep, and the relative phase, achieve single-sideband modulation. The spectrum shows the carrier 4' and, in contrast to Fig. 2a), only a single sideband 5'. Minor spectral components that may occur in practice at other sideband frequencies, as faintly visible in Fig. 2b), can be ignored. The intensity of the single sideband 5' is at least an order of magnitude stronger.

[0050] In the embodiment of Fig. 3, a non-linear frequency converter 9 is connected downstream of the optical amplifier 7, which emits at its output a laser radiation 2" which is frequency-converted with respect to the laser radiation 2'.

[0051] Fig. 4a) shows the spectrum of the single-sideband modulated laser radiation 2' at the output of the amplifier 7, analogous to Fig. 2b). Fig. 4b) shows the corresponding spectrum of the laser radiation 2" at the output of the frequency converter 9. It can be seen that the spectrum now again has a (frequency-converted) carrier 4" and two sidebands 5", 6". Due to the non-linear frequency conversion, two sidebands 5", 6" have again been created from the spectrum of Fig. 4a). By suitable adaptation of the modulation parameters, i.e. the frequency modulation deviation, the amplitude modulation deviation and the relative phase in the arrangement of Fig. 3, single-sideband modulation can again be achieved in this exemplary embodiment, as Fig. 4c) shows. With the modulation parameters corresponding to Fig. 4c), however, the laser radiation 2' at the output of the amplifier 7 is no longer single-sideband modulated.However, these modulation parameters cause a preconditioning of the laser radiation 2' in such a way that at the output of the frequency converter 9, ie in the laser radiation 2", the desired single-sideband modulation is again obtained only with (frequency-converted) carrier 4" and sideband 5".

[0052] Fig. 5 shows an embodiment which is suitable, for example, for the Laser Guide Star application mentioned. The laser system shown in Figure 1 again comprises a laser light source 1, which is a diode laser. The laser light source 1 is connected to the high-frequency signal generator 3, so that the injection current of the laser diode (not shown) of the laser light source 1 is high-frequency modulated. The modulation frequency is 1.7 GHz. This modulation frequency gives the spectrum of the radiation emitted by the laser light source 1 a sideband which forms the basis for generating radiation at the pump-back frequency corresponding to the hyperfine splitting of the sodium D line. The spectrum of the radiation emitted by the laser light source 1 has a component at a carrier frequency of 1178 nm. The frequency spacing between the carrier frequency and the two sidebands is + / - 1.7 GHz, corresponding to the modulation frequency.The frequency-modulated laser radiation is fed via an optical fiber to a non-modulated semiconductor amplifier 10 as the first amplification stage. Downstream of this amplifier, again connected via an optical fiber, is an amplitude modulator 11 based on an electro-optical modulator, controlled by the second output of the high-frequency signal generator 3 via the phase shifter 8. The modulation properties (with regard to the frequency and amplitude modulation components) of the amplitude modulator 11 differ significantly from those of the diode laser 1, which is advantageous for the principle of the invention. From the amplitude modulator 11, the laser radiation is in turn fed via an optical fiber to a Raman fiber amplifier 12. The amplifier fiber (not shown) of the fiber amplifier 12 is optically pumped by a pump laser (not shown).The Raman fiber amplifier 12 amplifies the laser radiation at the carrier frequency and at the sideband frequencies. The amplification bandwidth of the Raman fiber amplifier 12 is correspondingly large. At the output of the Raman fiber amplifier 12, the total power of the laser radiation is approximately 30 to 40 W; even more than 100 W is practically feasible. The thus amplified laser radiation is finally fed to a resonant frequency converter 13. This is a nonlinear crystal located within an optical resonator (neither shown). The (doubly resonant) frequency converter 13 converts the amplified radiation by frequency doubling and sum frequency generation.The spectrum of the single-sideband modulated laser radiation at the output of the frequency converter 13 then comprises, with appropriate setting of the modulation parameters, intensities at a fluorescence frequency (carrier) and a pump-back frequency (sideband), whereby the fluorescence frequency corresponds to the sodium D-line and the frequency separation of the pump-back frequency from the fluorescence frequency corresponds to the hyperfine splitting of the corresponding sodium D-line. The radiation power at the output of the frequency converter 13 can be well above 20 W, which is advantageous, for example, for the generation of an artificial guide star for astronomical telescopes with adaptive optics. The single-sideband modulated spectrum of the amplified laser radiation results from frequency doubling or sum frequency generation by means of the frequency converter 13. The carrier is generated at the fluorescence frequency of 589 nm.This central spectral line is created by frequency doubling the original carrier frequency. Furthermore, the amplified spectrum exhibits a single sideband spaced 1.7 GHz from the fluorescence frequency. This sideband is created by sum frequency generation from the carrier and the sideband of the original spectrum (the laser radiation at the output of the Raman fiber amplifier 12). The sideband at 1.7 GHz obtained by sum frequency generation is present at the pump-back frequency. For high fluorescence yield, the intensity at the pump-back frequency should be at least 10% of the intensity at the fluorescence frequency. To achieve this, sum frequency generation is used. This takes advantage of the fact that the resulting intensity from sum frequency generation behaves like the product of the intensities of the fundamental light fields.

[0053] - Patent claims -

Claims

Patent claims 1. A laser system comprising a laser light source (1) configured to emit laser radiation (2) during operation of the laser system, a first modulation device configured for frequency modulation of the laser radiation, wherein the frequency modulation occurs with a frequency modulation swing at a modulation frequency, a second modulation device independent of the first modulation device, configured for amplitude modulation of the laser radiation, wherein the amplitude modulation occurs with an amplitude modulation swing at the modulation frequency, wherein the frequency modulation and the amplitude modulation are coupled to one another in a phase-locked manner, and wherein the frequency modulation swing, the amplitude modulation swing, and the relative phase of the frequency modulation and the amplitude modulation are adjusted such that the spectrum of the frequency- and amplitude-modulated laser radiation (2', 2") has a carrier (4', 4") and a single sideband (5', 5"),which is spaced from the carrier (4', 4") by the amount of the modulation frequency., 2. Laser system according to claim 1, wherein the laser light source (1) is a diode laser with at least one laser diode, wherein the first modulation device is arranged to modulate the injection current of the laser diode.

3. Laser system according to claim 1 or 2, wherein at least one of the first and second modulation means comprises an electro-optical modulator, an acousto-optical modulator, an electro-absorption modulator and / or an interferometric modulator.

4. Laser system according to one of claims 1 to 3, further comprising at least one optical amplifier (7, 10, 12) arranged to amplify the laser radiation (2, 2') in one or more stages.

5. Laser system according to one of claims 1 to 4, further comprising a non-linear frequency converter (9) configured for frequency conversion, in particular for frequency doubling of the frequency- and amplitude-modulated laser radiation (2').

6. Laser system according to claim 4 or 5, wherein the frequency modulation sweep, the amplitude modulation sweep and the relative phase of the frequency modulation and the amplitude modulation are adjusted in such a way that the spectrum of the frequency- and amplitude-modulated laser radiation at the output of the optical amplifier (7, 10, 12) or the non-linear frequency converter (9) has a carrier (4', 4") and a single sideband (5', 5").

7. Laser system according to claim 6, wherein the frequency of the carrier (4', 4") in the spectrum of the frequency- and amplitude-modulated laser radiation at the output of the optical amplifier (7, 10, 12) or of the non-linear frequency converter (9) corresponds to a fluorescence frequency and the frequency of the single sideband (5', 5") corresponds to a pump-back frequency, wherein the fluorescence frequency is resonant with a transition frequency of an optical transition and the frequency separation of the pump-back frequency from the fluorescence frequency is resonant with the hyperfine splitting of the optical transition.

8. Laser system according to claim 7, wherein the fluorescence frequency of the transition frequency of the sodium line at a wavelength of 589 nm and the frequency separation of the pump-back frequency from the fluorescence frequency is 1.7 GHz.

9. Use of a laser system according to one of claims 1 to 8 for generating an artificial guide star (“laser guide star”) for astronomical telescopes with adaptive optics.

10. Use of a laser system according to one of claims 1 to 8 for exciting optical transitions in a quantum information system.

11. Method for generating single-sideband modulated laser radiation, comprising the steps Generation of laser radiation (2), frequency modulation of the laser radiation with a frequency modulation swing at a modulation frequency, Amplitude modulation of the laser radiation with an amplitude modulation swing at the modulation frequency, wherein the frequency modulation and the amplitude modulation are coupled to one another in a phase-locked manner and wherein the frequency modulation swing, the amplitude modulation swing and the relative phase of the frequency modulation and the amplitude modulation are set in such a way that the spectrum of the frequency- and amplitude-modulated laser radiation has a carrier (4', 4") and a single sideband (5', 5") which is spaced from the carrier (4', 4") by the amount of the modulation frequency.

12. The method of claim 11, further comprising optically amplifying the frequency and amplitude modulated laser radiation.

13. The method according to claim 11 or 12, further comprising a nonlinear frequency conversion of the frequency and amplitude modulated laser radiation.

14. The method according to claim 12 or 13, wherein the frequency modulation sweep, the amplitude modulation sweep and the relative phase of the frequency modulation and the amplitude modulation are adjusted in such a way that the spectrum of the frequency- and amplitude-modulated laser radiation after the optical amplification or the non-linear frequency conversion has a carrier (4', 4") and a single sideband (5', 5") which is spaced from the carrier by the amount of the modulation frequency.

15. The method according to any one of claims 11 to 14, wherein the frequency of the laser radiation is periodically varied between an initial value and a final value in addition to the frequency modulation. - Summary -

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