Laser system for Generating Single-Sideband Modulated Laser Radiation

Abstract

The invention relates to a laser system comprising a laser light source (1) that emits laser radiation during operation of the laser system, a modulation means (2) that brings about modulation of the laser radiation emitted by the laser light source (1) such that the spectrum of the laser radiation comprises a carrier (14) and two sidebands (13, 15) that are symmetrically distributed around the carrier, and at least one optical amplifier (5) that amplifies the radiation emitted by the laser light source (1). The invention proposes that an optical filter (4) be provided in the beam path of the laser radiation, upstream of the optical amplifier (5), which filter is intended for removing the spectral portion of the laser radiation at the frequency of one of the two sidebands (13). The laser system is suitable inter alia for generating an artificial guide star (laser guide star) for astronomical telescopes comprising adaptive optics. The invention furthermore relates to a method for generating single-sideband modulated laser radiation.

Claims

1. Laser system, comprising a laser light source that is configured to emit laser radiation during operation of the laser system, a modulation means that is configured for modulating the emitted laser radiation such that the frequency spectrum of the laser radiation comprises a carrier and two sidebands that are symmetrically distributed around the carrier, and at least one optical amplifier that is configured for amplifying the laser radiation, wherein an optical filter is arranged in the beam path of the laser radiation, upstream of the optical amplifier, which filter is configured to remove the spectral portion of the laser radiation at the frequency of one of the two sidebands.

2. Laser system according to claim 1, wherein the optical filter is an optical notch filter.

3. Laser system according to claim 1, wherein the optical filter is a fiber-Bragg grating, in particular a -phase-shifted fiber-Bragg grating, which grating transmits the spectral portion of the laser radiation at the frequency of one sideband, and reflects the spectral portions at the frequency of the carrier and the frequency of the other sideband.

4. Laser system according to claim 3, wherein a light-guiding fiber of the fiber-Bragg grating is thermally coupled to a temperature-control means, preferably a cooler, particularly preferably a thermoelectric cooler.

5. Laser system according to claim 3, wherein the light-guiding fiber of the fiber-Bragg grating is thermally coupled to a temperature-control means, preferably an electrical heating element, in particular a heating wire.

6. Laser system according to claim 1, further comprising a control loop comprising a sensor that is configured to derive a control variable from the laser radiation filtered by means of the optical filter, and a controller that is configured to stabilize the filter to the frequency of the sideband to be removed.

7. Laser system according to claim 4, wherein the controller is connected to the two temperature-control means.

8. Laser system according to claim 7, wherein the controller is configured to modulate the temperature of the fiber-Bragg grating, by means of actuating the further temperature-control means, so as to generate an error signal.

9. Laser system, comprising a laser light source that is configured to emit laser radiation during operation of the laser system, a modulation means that is configured for serrodyne modulation of the emitted laser radiation such that the frequency spectrum of the laser radiation comprises a carrier and at least one sideband, at least one optical amplifier that is configured for amplifying the laser radiation.

10. Laser system according to claim 9, wherein the modulation means comprises a sine wave generator and a non-linear transmission line connected downstream thereof, which are configured for generating a sawtooth modulation signal.

11. Laser system according to claim 1, further comprising a stabilization means that is assigned to the laser light source and that is configured to regulate the frequency of the carrier to a specifiable value.

12. Laser system according to claim 1, wherein, in the spectrum of the amplified laser radiation, the frequency of the carrier corresponds to a fluorescence frequency, and the frequency of the sideband corresponds to a back-pumping frequency, wherein the fluorescence frequency is resonant with a transition frequency of an optical transition, and the frequency spacing of the back-pumping frequency from the fluorescence frequency is resonant with the hyperfine splitting of the optical transition.

13. Laser system according to claim 12, wherein the fluorescence frequency of the transition frequency corresponds to the sodium line, at a wavelength of 589 nm, and the frequency spacing of the back-pumping frequency from the fluorescence frequency is 1.7 GHz.

14. Laser system according to claim 1, wherein the laser light source is a diode laser comprising at least one laser diode, wherein the modulation means is configured for modulating the injection current of the laser diode.

15. Use of a laser system according to claim 1 for generating an artificial guide star (laser guide star) for astronomical telescopes comprising adaptive optics.

16. Use of a laser system according to claim 1 for exciting optical transitions in a quantum information system.

17. Method for generating laser radiation, comprising the method steps of generating laser radiation by means of a laser light source; modulating the laser radiation such that the spectrum of the laser radiation comprises a carrier (14) and two sidebands that are symmetrically distributed around the carrier, and amplifying the laser radiation, wherein the modulated laser radiation passes through an optical filter prior to amplification, which filter removes the spectral portion of the laser radiation at the frequency of one of the two sidebands.

18. Method according to claim 17, wherein a start-up procedure comprising at least the following method steps is performed: activating the laser light source; activating the modulation of the laser radiation; detecting the characteristics of the optical notch filter; setting and stabilizing the notch filter to the frequency of the sideband to be removed.

19. Method according to claim 17, comprising the following further method steps: monitoring the power of the laser radiation fed to the optical amplifier; shutting down the optical amplifier as soon as the power of the laser radiation fed to the optical amplifier falls below a specifiable threshold value.

20. Method according to claim 17, wherein the frequency of the carrier is detuned from a first value to a second value, and specifically by a frequency spacing that is greater than the frequency spacing between the sideband and the carrier, wherein the frequency direction of the detuning is selected such that the spectral portion of the laser radiation at the frequency of the carrier is not removed by the notch filter during the detuning process.

21. Method for generating laser radiation, comprising the method steps of generating laser radiation by means of a laser light source; serrodyne modulation of the laser radiation such that the spectrum of the laser radiation comprises a carrier and two sidebands of different intensities that are symmetrically distributed around the carrier, and amplifying the laser radiation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of the laser system and the method will be explained in greater detail in the following, with reference to the drawings, in which:

[0045] FIG. 1: is a block diagram of the laser system;

[0046] FIG. 2: illustrates the spectral filter pattern;

[0047] FIG. 3: schematically shows the notch filter comprising temperature control;

[0048] FIG. 4: is a flow diagram illustrating the start-up process of the laser system, comprising sideband modulation;

[0049] FIG. 5: is a flow diagram illustrating the start-up process of the laser system, without sideband modulation;

[0050] FIG. 6: is a flow diagram illustrating the detuning process of the laser system;

[0051] FIG. 7: illustrates the stabilization of the notch filter and the detuning process;

[0052] FIG. 8: is a block diagram illustrating an alternative embodiment of the laser system, comprising serrodyne modulation.

DETAILED DESCRIPTION OF EMBODIMENTS

[0053] The laser system shown in FIG. 1 comprises a laser light source 1 which is a diode laser. The laser light source 1 is connected to a modulation means 2 that performs high-frequency modulation of the injection current of the laser diode (not shown) of the laser light source 1. The modulation frequency is 1.7 GHz. As a result of this modulation frequency, the spectrum of the radiation emitted by the laser light source 1 obtains a sideband that is the basis for the generation of radiation at the back-pumping frequency in accordance with the hyperfine structure of the sodium D-line. The spectrum of the radiation emitted by the laser light source 1 comprises a component at a carrier frequency of 1178 nm. The frequency spacing from the carrier frequency to the two sidebands is +1.7 GHz, depending on the modulation frequency. The spectrum comprising the carrier and the two sidebands arranged symmetrically thereto at the output of the laser light source 1 is indicated schematically in FIG. 1.

[0054] The modulated laser radiation is supplied to a first port of a circulator 3 by means of an optical fiber. The laser radiation then reaches an optical notch filter 4 via a second port. The frequency of the filter notch of the optical notch filter 4 is matched to the frequency of a sideband of the laser radiation. Accordingly, the notch filter 4 transmits only the sideband of the laser radiation, as indicated schematically at the output of the notch filter 4. The carrier and the other sideband are reflected by the notch filter 4, return to the second port of the circulator 3, and leave said circulator via the third port thereof.

[0055] From the third port of the circulator 3, the now single-sideband modulated laser radiation of the laser light source 1 is supplied to a Raman fiber amplifier 5. The amplifier fiber (not shown) of the fiber amplifier 5 is optically pumped by means of a pump laser (not shown). The Raman fiber amplifier 5 amplifies the laser radiation at the carrier frequency and at the frequency of the remaining sideband. The amplification bandwidth of the Raman fiber amplifier 5 is correspondingly large. At the output of the Raman fiber amplifier 5, the power of the laser radiation is approximately 30 to 40 W in total, and even more than 100 W can be achieved in practice.

[0056] The laser radiation thus amplified is supplied to a resonant frequency multiplier 6. In this case, this is a nonlinear crystal that is located inside an optical resonator. The frequency multiplier 6 converts the amplified radiation by means of frequency multiplication and sum-frequency generation. The spectrum of the radiation at the output of the frequency multiplier 6 comprises intensities at a fluorescence frequency and at a back-pumping frequency, wherein the fluorescence frequency corresponds to the sodium D-line, and the frequency spacing of the back-pumping frequency from the fluorescence frequency corresponds to the hyperfine splitting of the corresponding sodium D-line. In this case, the power of the radiation at the output of the frequency multiplier 6 can be significantly above 20 W, which is advantageous for example for generating an artificial guide star for astronomical telescopes comprising adaptive optics.

[0057] The spectrum of the amplified laser radiation results after the frequency multiplication or sum-frequency generation using the frequency multiplier 6. The carrier is generated at the fluorescence frequency of 589 nm. This central spectral line results from frequency doubling of the original carrier frequency. Furthermore, the amplified spectrum exhibits a sideband that is spaced apart from the fluorescence frequency by 1.7 GHz. This sideband results from the carrier and the sideband of the original spectrum by means of sum-frequency generation. Furthermore, after passing through the frequency multiplier 6 a further sideband is present that results from frequency doubling of the sideband from the original spectrum. However, said sideband is significantly attenuated and is therefore of no further relevance. The sideband obtained by sum-frequency generation, at 1.7 GHz, is provided at the back-pumping frequency. For high fluorescence, the intensity at the back-pumping frequency should be at least 10% of the intensity at the fluorescence frequency. In order to achieve this, sum-frequency generation is used. In this case, use is made of the fact that the resulting intensity in the case of sum-frequency generation behaves like the products of the intensities of the fundamental light fields.

[0058] In order to stabilize the notch filter 4, such that the notch frequency is matched to the undesired sideband of the laser radiation, a control loop is provided, comprising a photo diode 7 as the sensor, which photo diode detects the intensity of the sideband transmitted by the notch filter 4, as a control variable. The control loop further comprises a controller 8 that stabilizes the notch filter 4 at the frequency of the sideband to be removed. This is achieved for example in accordance with a lock-in scheme, for which purpose the controller 8 modulates the notch frequency and thus imposes an error signal on the control variable, which signal the controller 8 in turn derives, in narrow-band, from the signal of the photo diode 7. A regulating bandwidth of from a few Hz to a few 10 Hz is sufficient, in practice, for stabilizing the notch filter 4.

[0059] Advantageously, a -phase-shifted fiber-Bragg grating is used as the notch filter, which grating transmits the spectral portion of the laser radiation at the frequency of one sideband, and reflects the spectral portions at the frequency of the carrier and the frequency of the other sideband. The filter characteristics of a suitable FBG are shown in FIG. 2. The graph in FIG. 2 shows the reflectivity R as a function of the frequency F. It can be seen that the reflection spectrum 9 comprises a notch 10. This property results from the -phase discontinuity in the center of the fiber-Bragg grating. The spectral overall width 11 of the reflection spectrum 9 is in the range of over 10 GHz. The spectral width 12 of the notch 10 is less than 1 GHz (full width at half the height of the reflection curve 9). As can be seen, the optical notch filter formed by the FBG is sufficiently narrow-band and has a high degree of edge steepness, in order to separate the sideband 13, likewise shown in FIG. 2, from the carrier 14. Furthermore, the filter bandwidth is sufficiently large, specifically larger than the laser bandwidth (typically <10 MHz), in order that the undesired sideband 13 can be completely removed. At the same time, the filter bandwidth is sufficiently narrow compared with the spacing between the carrier and sideband, i.e. narrower than the modulation frequency (in this case 1.7 GHz). It can be seen that the filter notch 10 coincides with the sideband 13. The sideband 13 is thus transmitted by the filter 4 formed by the FBG. The carrier 14 and the desired sideband 15 are in the range of high reflectivity of the FBG (maximum reflectivity ca. 91%). These spectral components of the laser radiation are thus reflected. In this way, the sideband 13 is removed from the laser radiation by means of the transmitting notch filter 4, in order to thus maintain single-sideband modulated laser radiation.

[0060] FIG. 3 shows a light-guiding fiber 16 having inscribed refractive index modulation 17, which fiber forms the FBG. Introducing the -phase discontinuity 18 into the refractive index modulation 17 of the fiber-Bragg grating is achieved in that the spectral transmission exhibits a narrow band-pass resonance 10 which, as shown in FIG. 2, results in the center of the reflection spectrum 9. The spectral position of the filter notch 10 can be controlled by means of the thermal expansion of the light-conducting fiber 16. For this purpose, in FIG. 3 the light-conducting fiber 16 is attached to a printed circuit board (PCB) 19 which is in turn in thermal contact with a thermoelectric cooler (Peltier element) 20. Said cooler dissipates the heat to a heat sink 21 having a sufficiently large heat capacity. The fiber 16 is furthermore in thermal contact with an electrical heating element 22. Actuating the cooler 20 makes it possible to set a specified temperature of the light-conducting fiber 16 in thermal equilibrium, such that the filter notch 10 is at the desired frequency. The frequency position of the filter notch 10, and thus the intensity of the transmitted radiation of the sideband 13, is modulated accordingly by means of modulating a heater current flowing through the heating element 22. In this case, the modulation amplitude should be less than the spectral width 12 of the transmission. In this way, an error signal arises at the detector 7 (FIG. 1), which signal, as described above, can be used for active stabilization of the notch filter 4.

[0061] The method for actuating the notch filter 4 is non-trivial in practice, in order to provide an uninterrupted input signal for the optical amplifier 5, even in the case of different sideband modulation amplitudes, including complete shutdown of the modulation, as well as detuning of the carrier frequency, optionally by many times the sideband frequency. That is to say that it is necessary to ensure that the carrier frequency of the laser radiation never coincides with the frequency of the filter notch 10, i.e. the transmission frequency of the filter 4. Otherwise, there is a risk of damage to/destruction of the optical amplifier 5.

[0062] In this respect, FIG. 4 is a flow diagram illustrating the start-up process of the laser system, comprising sideband modulation. The procedure begins, in step 22, with starting up the semiconductor laser 1 by means of activating the injection current. In step 23, the stabilization of the laser emission is activated, e.g. in the conventional manner firstly by setting the temperature of the laser diode and of the injection current to a target value (known in advance), such that the semiconductor laser emits approximately at the desired frequency, and then by means of active stabilization at a suitable absolute reference (e.g. using a wavelength measuring device or by means of absorption spectroscopy). A rapid control loop, e.g. comprising the injection current as the manipulated variable, is used for this purpose. Thereafter, in step 24 the sideband modulation is activated, by means of direct modulation of the injection current of the laser diode or by means of modulation of the emitted laser radiation using an electro-optic modulator. In the next step, i.e. in step 25, detection of the filter characteristics of the notch filter 4 is performed, in order to thereby determine the spectral position of the filter notch 10. In this case, the notch filter 4 is detuned over a predefined spectral range (by actuating the cooler 20; see FIG. 3). At the same time, the course of the transmitted power is detected by means of the photo diode 7 (FIG. 1) and recorded. In step 26, an automatic analysis (e.g. peak detection) takes place on the basis of the recorded signals of the photo diode 7, as a result of which the spectral position of the undesired sideband 13, and thus the target value of the filter setting and the associated value of the manipulated variable of the notch filter 4, are determined. A storage oscilloscope or a suitably programmed embedded system, e.g. as a component of the controller 8, can be used for this purpose. In step 27, the corresponding initialization of the notch filter 4 then takes place, i.e. the notch filter 4 is set to the previously determined target value of the transmission frequency, e.g. by means of corresponding actuation of the cooler 20 (FIG. 3). A suitable (slow) control loop may be provided for stabilizing the temperature of the FBG that determines the transmission frequency. Finally, in step 28, the active stabilization of the notch filter 4 is activated, for which purpose, as described above, an error signal is generated by the heating element 22, and the manipulated variable is derived therefrom, by means of the controller 8, for setting the temperature of the FBG. In parallel thereto, in step 29 the continuous monitoring of the power of the power transmitted by the notch filter 4 is activated. An emergency shutdown of the optical amplifier 5 (e.g. by means of shutting down the pump radiation) is initiated as soon as a threshold value of the power on the photo diode 7 is exceeded. This is considered to be a sign that the carrier is transmitted from the notch filter 4, and accordingly too little power is reaching into the amplifier 5. The threshold value may be firmly specified, or may be dependent on the modulation amplitude.

[0063] The laser system should also be able to function without sideband modulation. A start-up procedure suitable for this purpose is illustrated in FIG. 5. The activation and stabilization of the laser light source again takes place, in steps 23 and 24. The activation of the modulation is omitted. In steps 25A and 26A, the notch filter 4 is tuned and the transmission system detected in the process is analyzed, wherein, in step 27A, the notch filter 4 is then initialized such that the filter notch 10 is located close to the (only available) carrier. In this case, the notch filter 4 is detuned until a low transmission signal is detected. The stabilization of the notch filter 4 is then activated in step 28A, using the low transmission signal as the target value of the control variable. As a result, the notch filter 4 is stabilized such that the filter notch 10 is located spectrally close beside the carrier, but the major part of the carrier is reflected. Alternatively, the notch filter 4 may be detuned by a specified frequency offset relative to the frequency of the carrier, such that it is reliably ensured, without actively stabilizing the notch filter 4, that a sufficient spacing exists between the carrier and the filter notch 10, and also that drifting cannot lead to the carrier being transmitted. In parallel thereto, the monitoring and emergency shutdown is activated in step 29, as described above with reference to FIG. 4.

[0064] In particular when generating an artificial guide star, it is necessary to regularly shift the carrier frequency from a first, resonant value to a second, non-resonant value, at which no resonance fluorescence occurs in the sodium layer. The detuning must take place at (significantly) more than the line width of the laser, and also more than the modulation frequency. In the detuned state, the Rayleigh scattering background which the laser generates on its path through the lower levels of the earth's atmosphere can be detected separately, in order to be used for correcting the astronomical image data. A procedure suitable for this purpose is shown in FIG. 6. In step 30, the active (lock-in) stabilization of the notch filter 4 is interrupted, and only the passive stabilization, at the previously determined target frequency of the filter notch 10, is maintained. Then, in step 31, the active stabilization of the laser light source 1 is deactivated and the frequency of the carrier is detuned from the value last set to the desired second value (e.g. by means of setting a previously specified diode temperature), wherein the frequency direction of the detuning is selected such that the spectral portion of the laser radiation at the frequency of the carrier does not pass through the notch filter 10 during the detuning process. In other words, the shift of the carrier 14 takes place in the direction away from the filter notch 10. Upon reaching the new target value of the carrier frequency, the stabilization of the laser light source 1 at the new carrier frequency is again activated. In step 32, the measurement of the background signal is then performed at the telescope. In step 33, back-detuning of the laser radiation to the resonant carrier frequency takes place. This step is critical because it is necessary to prevent, in this case, the carrier coming into superimposition with the filter notch 10. Even at a slight overshoot of the control variable (e.g. the diode temperature), the carrier would enter the region of the filter notch 10, as a result of which the input signal would break at the optical amplifier 5. Regulation that is carefully matched to the specific interaction between the notch filter 4 and the carrier frequency is necessary here. It is thus possible, for example, to compensate for overshoots of the slower temperature regulation of the laser diode by means of corresponding counter control using the injection current of the laser diode. Finally, in step 28, the stabilization of the notch filter 4 is activated again. The monitoring and emergency shutoff 29, as described above, takes place in parallel therewith.

[0065] FIG. 7 illustrates the above-described detuning process. In the left-hand graph of FIG. 7, the notch filter 4 is stabilized such that the sideband 13 to be removed corresponds exactly to the filter notch 10. The right-hand graph shows that the frequencies of the assembly of the three lines 13, 14, 15 (sidebands and carrier) are shifted together to the right, i.e. the frequency direction of the detuning is selected such that the spectral portion of the laser radiation at the frequency of the carrier 14 does not pass through the filter notch 10 during the detuning process. The filter characteristics of the notch filter 4 remain sufficiently stable during the detuning process, as a result of the passive stabilization.

[0066] In FIG. 8 and FIG. 1, mutually corresponding elements of the laser systems shown schematically in each case are denoted by the same reference signs. In the embodiment of FIG. 8, differently from FIG. 1 a serrodyne modulation is used. In this case, the laser radiation emitted by the laser light source 1 during operation is modulated by means of an electro-optic modulator 34. A sawtooth modulation signal is applied to said radiation. In this way, it is possible to purposely generate sidebands having different intensities, instead of the conventional two symmetrical sidebands. This makes it possible to generate the sideband that is involved subsequently, i.e. following the amplification, in the frequency multiplication or sum-frequency generation for generating the radiation at the back-pumping frequency, so as to be of a significantly higher intensity than the other, undesired sideband. At the output of the electro-optic modulator 34, FIG. 8 shows, in simplified form, the spectrum of the laser radiation having only the desired sideband. In this case, the sawtooth modulation waveform is generated by means of a sine wave generator (VCO) 35, in combination with a non-linear transmission line 36.