DEVICE FOR AMPLIFYING AND / OR TRANSPORTING ELECTROMAGNETIC RADIATION

Description

[0001] The invention relates to a device for amplifying electromagnetic radiation, comprising a radiation source for generating the electromagnetic radiation and an amplifier for amplifying the generated electromagnetic radiation.

[0002] Such devices are known and are used, for example, in the generation of high-energy laser radiation. The quality of the electromagnetic radiation is usually impaired during amplification and / or transport by the occurrence of dispersion and / or nonlinear effects. While the influence of dispersion can generally be easily compensated, nonlinear effects cause a significant deterioration in the quality of the laser radiation. The dominant nonlinear effect here is the Kerr effect. This manifests itself, for example, in the form of self-phase modulation. The Kerr effect is proportional, among other things, to the peak intensity and the interaction length in the amplifier or transport medium.To reduce the influence of Kerr nonlinearity, it is known to use temporally stretched laser pulses, also called chirped laser pulses, for example, to generate ultrashort laser pulses and then amplify these chirped laser pulses. This method is used in so-called CPA (Chirped Pulse Amplification) systems, in which the amplified laser pulses are usually finally compressed. Furthermore, it is known to control the incompressible phase caused by nonlinear effects using pulse shaping.

[0003] The effects of nonlinear effects occur in optical fibers even at very low power levels because the laser radiation is guided in the form of modes. On the one hand, the laser radiation is confined to small mode field areas, resulting in high intensities; on the other hand, the laser radiation propagates virtually diffraction-free, which in turn leads to large interaction lengths. Since self-phase modulation is inversely proportional to the mode field area and proportional to the fiber length, the use of novel microstructured fibers, such as LMA fibers (Large Mode-Area Fibers) or LMA-PCF fibers (Large Mode-Area Photonic Crystal Fibers), with large mode field areas and simultaneously short fiber lengths, can significantly reduce the influence of nonlinear effects.

[0004] A disadvantage of the known methods for reducing nonlinear effects is that relatively complex and costly structural modifications to existing devices are required to achieve the desired result. The associated adaptation of the individual components to each other is also relatively complex.

[0005] The document US 2001 / 017867 discloses a device for amplifying electromagnetic radiation, comprising a radiation source for generating the EM radiation and a fiber amplifier for amplifying the EM radiation, which is not linearly but circularly polarized in the amplifier, wherein the amplifier imposes a nonlinear phase on the EM radiation caused by nonlinear effects.

[0006] Based on this, the object of the invention is to provide a device for amplifying or transporting electromagnetic radiation, which offers a very simple and cost-effective way of reducing the influence of non-linear effects.

[0007] This object is achieved by a device as described in claim 1.

[0008] The invention is based on the finding that the coefficient of the nonlinear refractive index depends on the polarization of the electromagnetic radiation. This coefficient is approximately 1.5 times greater for linearly polarized electromagnetic radiation than for circularly polarized electromagnetic radiation. Thus, the inventive approach can reduce nonlinear effects such as self-phase modulation, cross-phase modulation, self-focusing, four-wave mixing, and the like. The inventive approach for reducing nonlinear effects is comparatively simple and can also be incorporated into existing systems without requiring complex structural modifications. Furthermore, the inventive approach can be combined almost seamlessly with the conventional approaches for reducing nonlinear effects described above.One of the advantages is that reducing nonlinear effects leads to an increase in the peak power of the electromagnetic radiation. Furthermore, the threshold for self-focusing can be shifted upwards.

[0009] The electromagnetic radiation propagating in the amplifier is elliptically or circularly polarized.

[0010] According to an advantageous embodiment of the invention, the radiation source generates linearly polarized electromagnetic radiation, and a polarization converter is arranged in the beam path between the radiation source and the amplifier. The polarization converter facilitates the conversion of the linearly polarized electromagnetic radiation such that the electromagnetic radiation propagating in the amplifier is largely non-linearly polarized.

[0011] The amplifier, for example, an optical fiber, imparts a nonlinear phase to the electromagnetic radiation caused by nonlinear effects. This phase can be significantly reduced in a simple manner using the inventive approach, thus increasing the quality or power of the amplified electromagnetic radiation.

[0012] The polarization converter is preferably designed as a quarter-wave plate. This represents a very simple and cost-effective implementation.

[0013] According to a further advantageous embodiment of the invention, the polarization state of the electromagnetic radiation propagating in the amplifier is determined by the position r > 0.9 on the one hand and | ε | > 30°, | ε | > 35° or | ε| > 40° on the other hand, on the Poincaré sphere. The choice of one of these polarization states has proven particularly suitable for reducing the nonlinear effects. However, other polarization states can also be selected, which are ideally tailored to the respective application. To clarify the parameter selection, reference is made to the Figure 5 shown Poincaré sphere representation, in which linearly polarized electromagnetic radiation is located at the equator, whereas circularly polarized electromagnetic radiation is found at the poles.

[0014] The invention provides that the amplifier comprises a largely optically isotropic medium. According to the invention, this medium is a waveguide or a gas. The inventive approach to reducing nonlinear effects has proven particularly effective and advantageous for such media.

[0015] According to a further advantageous embodiment of the invention, the electromagnetic radiation is formed from pulsed electromagnetic radiation pulses or from continuous electromagnetic radiation. The occurrence of nonlinear effects can be observed in both types of electromagnetic radiation. The reduction of nonlinear effects according to the invention applies equally to both types of radiation and leads to similarly good results.

[0016] According to a further advantageous embodiment of the invention, a polarization splitter is arranged in the beam path between the radiation source and the polarization converter, and a mirror is connected downstream of the amplifier, with the mirror reversing the radiation direction of the amplified electromagnetic radiation. In this embodiment of the invention, the use of a radiation source that generates linearly polarized electromagnetic radiation is advantageous. The generated electromagnetic radiation is first passed through the polarization splitter, for example, a polarization splitter cube, and then converted by the polarization converter into non-linearly polarized electromagnetic radiation. This radiation is then amplified by the amplifier, for example, a fiber amplifier.Nonlinear effects can occur in the amplifier, which can be significantly reduced by the inventive selection of the largely nonlinearly polarized electromagnetic radiation propagating in the amplifier. The amplified, largely nonlinearly polarized electromagnetic radiation is then reflected back by the mirror and amplified again by the amplifier, whereby very minimal nonlinear effects also occur in this second amplification stage due to the selection of largely nonlinearly polarized electromagnetic radiation. The twice amplified, largely nonlinearly polarized electromagnetic radiation then passes through the polarization converter again and becomes amplified, linearly polarized electromagnetic radiation whose polarization plane is perpendicular to that of the initial unamplified, linearly polarized electromagnetic radiation.The doubly amplified, linearly polarized electromagnetic radiation is finally coupled out of the device via the polarization splitter for further use. This design of the device is thus very effective despite its relatively compact design.

[0017] Alternatively, it is proposed that another polarization converter be connected downstream of the amplifier, which converts the largely non-linearly polarized electromagnetic radiation into linearly polarized electromagnetic radiation. In this configuration, the electromagnetic radiation passes through the amplifier only once.

[0018] It is further proposed that a stretcher be arranged in the beam path between the polarization converter and the amplifier to temporally stretch the electromagnetic radiation pulses, and that a compressor be connected downstream of the amplifier or the further polarization converter to compress the amplified electromagnetic radiation pulses. This embodiment of the invention relates to the implementation of the inventive concept in a CPA system with the associated advantages.

[0019] Furthermore, it is proposed that a stretcher for temporally stretching the electromagnetic radiation pulses be arranged in the beam path between the radiation source and the amplifier, and that a compressor for temporally compressing the amplified electromagnetic radiation pulses be connected downstream of the amplifier or further polarization converter.

[0020] It is further proposed that the device is configured for spectral shaping of the electromagnetic radiation pulses.

[0021] The invention thus provides a device which serves to reduce non-linear effects in the amplification of electromagnetic radiation in a very simple and cost-effective manner and which can simultaneously be easily combined with existing devices and with other methods for reducing non-linear effects.

[0022] Further advantages and features of the present invention are explained in more detail with reference to the exemplary embodiments shown in the figures. Figure 1: an embodiment of the device according to the invention, Figure 2: a further embodiment of the device according to the invention, Figure 3: a further embodiment of the device according to the invention, Figure 4: a further embodiment of the device according to the invention, and Figure 5: a representation of the Poincaré sphere.

[0023] Figure 1 shows a first embodiment of the device 1 according to the invention. This device has a radiation source 2, which generates largely non-linearly polarized electromagnetic radiation in the form of circularly or elliptically polarized electromagnetic radiation 3. The circularly or elliptically polarized electromagnetic radiation 3 is subsequently amplified by the amplifier 4. The non-linear effects occurring here are significantly reduced by the use of circularly or elliptically polarized electromagnetic radiation 3.

[0024] Figure 2 shows a further embodiment of the device 11 according to the invention. This has a radiation source 12 for generating linearly polarized electromagnetic radiation 13. The linearly polarized electromagnetic radiation 13 is subsequently converted into circularly or elliptically polarized electromagnetic radiation 17 by a polarization converter 16 arranged in the beam path between the radiation source 12 and the amplifier 14. The circularly or elliptically polarized electromagnetic radiation 17 is then amplified by the amplifier 14, whereby amplified, circularly or elliptically polarized electromagnetic radiation 15 is produced. This embodiment of the device 11 according to the invention can be used as an alternative to the Figure 1 shown can be used.

[0025] Figure 3shows a further exemplary embodiment of the device 21 according to the invention. In this embodiment, a radiation source 22 is used, which generates linearly polarized electromagnetic radiation 23. This linearly polarized electromagnetic radiation 23 is then passed through a polarization splitter 28, which is permeable to linearly polarized electromagnetic radiation 23 in the polarization state as generated by the radiation source 22. The linearly polarized electromagnetic radiation 23 passed through the polarization splitter 28 is then converted into circularly or elliptically polarized electromagnetic radiation 27 by means of the polarization converter 26. This circularly or elliptically polarized electromagnetic radiation 27 is then amplified by means of the amplifier 24.The amplified, circularly or elliptically polarized electromagnetic radiation 25 is then reflected by a mirror 30, which reverses the radiation direction of the amplified, circularly or elliptically polarized electromagnetic radiation 25. The reflected amplified, circularly or elliptically polarized electromagnetic radiation 25 is amplified again by the amplifier 24 and then converted by the polarization converter 26 into linearly polarized electromagnetic radiation 29, the polarization plane of which, however, is perpendicular to the original linearly polarized electromagnetic radiation 23 generated by the radiation source 22, as indicated by the dots and arrows. The doubly amplified, linearly polarized electromagnetic radiation 29 is then coupled out of the device 21 via the polarization splitter 28.

[0026] A further embodiment of the device 31 according to the invention is shown in Figure 4shown. In this embodiment, the radiation source 32 generates dispersively stretched, linearly polarized electromagnetic radiation 33. Instead, however, it can also be provided that the radiation source generates linearly polarized electromagnetic radiation, which is then dispersively stretched by means of a stretcher connected downstream of the radiation source in the beam path. The dispersively stretched, linearly polarized electromagnetic radiation 33 is then converted by means of the polarization converter 36 into dispersively stretched, circularly or elliptically polarized electromagnetic radiation 37. This is then amplified by means of the amplifier 34. The amplified, dispersively stretched, circularly or elliptically polarized electromagnetic radiation 35 is converted back into dispersively stretched, linearly polarized electromagnetic radiation 39 by means of the further polarization converter 38 connected downstream of the amplifier 34.This is finally compressed in time by means of a compressor 40, which enables the emission of high-energy, linearly polarized electromagnetic radiation 41 of high quality.

[0027] The embodiments described with reference to the figures serve to explain and are not restrictive.

Claims

1. A device (1) for amplifying electromagnetic radiation, comprising a radiation source (2) for generating the electromagnetic radiation and an amplifier (4) for amplifying the generated electromagnetic radiation, wherein the electromagnetic radiation propagating in the amplifier (4) is polarised largely non-linearly, specifically elliptically or circularly, characterised in that the amplifier (14, 24, 34) of the electromagnetic radiation impresses a non-linear phase, caused by non-linear effects, which is reduced compared to the non-linear phase with linear polarisation of the electromagnetic radiation, and wherein the amplifier (14, 24, 34) comprises an optically isotropic medium, wherein the optically isotropic medium is a waveguide or a gas.

2. The device (11, 21, 31) according to claim 1, characterised in that the radiation source (12, 22, 32) generates linearly polarised electromagnetic radiation (13, 23, 33) and a polarisation converter is arranged in the beam path between the radiation source (12, 22, 32) and the amplifier (14, 24, 34).

3. The device (11, 21, 31) according to claim 2, characterised in that the polarisation converter (16, 26, 36) is a quarter-wave plate.

4. The device (1, 11, 31) according to claim 2 or 3, characterised in that a further polarisation converter (38) is arranged downstream of the amplifier (4, 14, 34) and converts largely non-linearly polarised electromagnetic radiation (5, 15, 35) into linearly polarised electromagnetic radiation (39).

5. The device (11, 21, 31) according to any one of claims 2 to 4, characterised in that a stretcher for temporal stretching of the electromagnetic radiation pulses is arranged in the beam path between polarisation converter (16, 26, 36) and amplifier (14, 24, 34) and a compressor (40) for temporal compression of the amplified electromagnetic radiation pulses is arranged downstream of the amplifier 914, 24, 34) or the further polarisation converter (38).

6. The device (1) according to any one of claims 2 to 5, characterised in that a stretcher for temporal stretching of the electromagnetic radiation pulses is arranged in the beam path between radiation source (2, 12, 22, 32) and amplifier (4, 14, 24, 34) and a compressor for temporal compression of the amplified electromagnetic radiation pulses is arranged downstream of the amplifier (4, 14, 24, 34) or further polarisation converter (38).

7. The device (21) according to any one of claims 2 to 6, characterised in that a polarisation splitter (28) is arranged in the beam path between radiation source (22) and polarisation converter (26) and a mirror (30) is arranged downstream of the amplifier (24), wherein the mirror (30) reverses the radiation direction of the amplified electromagnetic radiation (25).

8. The device (11, 21, 31) according to any one of claims 1 to 7, characterised in that the electromagnetic radiation is formed from pulsed electromagnetic radiation pulses or from a continuous electromagnetic radiation.