Method and device for one-shot measurement of the transient birefringence induced by a perturbation lying within the terahertz frequency range
View Patent ↗The present invention relates to a method and a device for direct, non-deformed one-shot measurement of the transient birefringence induced in an optical medium by a perturbation lying within the terahertz frequency range. The aim of the present invention is to alleviate the drawbacks of the prior art by providing a one-shot measurement method and a one-shot measurement device, these being based on the spectral encoding/decoding principle, which are compatible with all short pulse (UV-NIR) laser sources. In this regard, the invention provides a one-shot method for measuring the transient birefringence induced in an optical medium ( 12 ) by at least one terahertz perturbation ( 6 ), the method including a step of transmitting and spectrally encoding a pulsed optical signal ( 2 ). The encoding step, which includes the generation of a supercontinuum ( 3 ), is furthermore combined with a step of decoding the polarization ellipticity of the supercontinuum, induced by the perturbation ( 6 ) of the medium ( 12 ), by decomposing the electric field of the supercontinuum in two polarization directions and simultaneously measuring the intensities Is and Ip of the two components.
1. Method for the one-shot measurement of the transient birefringence induced in an optical medium ( 12 ) by at least one terahertz perturbation ( 6 ), the method comprising a step of emission and spectral coding of a pulsed optical signal ( 2 ), characterised in that the coding step, which comprises the generation of a supercontinuum ( 3 ), is combined with a step of decoding the polarisation ellipticity of the supercontinuum induced by the perturbation ( 6 ) of the medium ( 12 ), by decomposition of the electrical field of the supercontinuum in two polarisation directions and simultaneous measurement of the intensities I s and I p of the two components, the one-shot measurement consisting of encoding the temporal profile of the terahertz perturbation on the spectral profile of a single pulse of the supercontinuum.
2. One-shot measurement method according to claim 1 , also comprising a step of balancing the intensities I s and I p of the two components of the electrical field of the supercontinuum ( 3 ).
3. Measurement method according to claim 1 , in which the step of generating the supercontinuum ( 3 ) has a phase of propagating at least part of the pulsed optical signal ( 2 ) in a monofilament mode in order to be free of the spectral and temporal characteristics of the pulsed optical signal ( 2 ).
4. Measurement method according to claim 1 , comprising at least one step of optical compensation by correction of the chromatic and spatial aberrations of the device.
5. Measurement method according to claim 4 , comprising a step of shaping the spectral, spatial and temporal distribution of the supercontinuum ( 3 ).
6. Measurement method according to claim 1 , in which the coding step comprises a temporal shaping step comprising a phase of stretching the supercontinuum ( 3 ) consisting of propagating the supercontinuum ( 3 ) along an identical geometric path for all the wavelengths of the supercontinuum ( 3 ).
7. Measurement method according to claim 1 , the method comprising:
i. a step of spatial separation of the terahertz perturbation ( 6 ) into two analysis ( 6 a ) and reference ( 6 b ) analysis terahertz perturbations;
ii. a step of temporal offset of the two analysis ( 6 a ) and reference ( 6 b ) terahertz perturbations; and
iii. a step of encoding the two analysis ( 6 a ) and reference ( 6 b ) terahertz perturbations on a pulse of the supercontinuum ( 3 ).
8. Device for the one-shot measurement of the transient birefringence induced in an optical medium ( 12 ) by at least one terahertz perturbation ( 6 ), the device comprising a source ( 14 ) emitting a pulsed optical signal ( 2 ) and means ( 4 , 12 ) for the spectral coding of the pulsed optical signal ( 2 ), characterised in that the spectral coding means ( 4 , 12 ), which comprise means ( 4 ) of generating a supercontinuum ( 3 ), are combined, by means ( 30 ) of decomposing the electrical field of the supercontinuum in two polarisation directions S and P. with a unit ( 40 ) for the simultaneous measurement of the intensities I s and I p of the two components, the one-shot measurement consistin of encodin' the temporal profile of the terahertz perturbation on the spectral profile of a single pulse of the supercontinuum.
9. Device according to claim 8 , in which the means ( 4 ) of generating the supercontinuum ( 3 ) comprise means ( 17 , 18 ) of forming a monofilament in order to be free of the spatial and temporal characteristics of the pulsed optical source ( 14 ).
10. Measurement device according to claim 9 , the device comprising means ( 17 ) of focussing the optical pulses ( 2 ) in order to concentrate part of the energy until it attains the propagation threshold according to a monofilament mode in a non-linear medium ( 18 ).
11. Measurement device according to claim 9 , the device comprising means ( 17 ) of focussing the optical pulses ( 2 ) so as to inject some of these pulses into an optical fibre ( 18 ).
12. Measurement device according to claim 9 , the device comprising at least one focussing means ( 17 ) and a diaphragm placed on the optical path of the pulse ( 2 ) in order to attenuate the effects of spatial fluctuations ( 2 ) on the generation of the supercontinuum ( 3 ).
13. Device according to claim 9 , in which the decomposition means ( 30 ) comprise balancing elements ( 32 , 34 ).
14. Device according to claim 13 , in which the balancing elements consist of a quarter-wave plate ( 32 ) associated with a polariser ( 34 ).
15. Device according to claim 9 , in which the decomposition means ( 30 ) comprise means ( 35 , 36 , 38 ) of correcting the chromatic and spatial aberrations of the device.
16. Device according to claim 9 , in which the decomposition means ( 30 ) comprise two secondary polarising splitting plates ( 35 . 36 ) respectively disposed on the optical path of the two components of the supercontinuum ( 3 ) and each have a function opposite to that of the polariser ( 34 ).
17. Device according to claim 9 , in which the generation means ( 4 ) have means ( 19 ) of adapting the supercontinuum ( 3 ) consisting of dispersive optical elements for controlling the temporal stretching of the supercontinuum ( 3 ).
18. Device according to claim 9 , in which at least one of the two components of the electrical field of the supercontinuum ( 3 ) is propagated towards a spectrograph by means of at least one optical fibre.
19. Device according to claim 8 , the device also comprising means ( 50 ) of spatial separation of the terahertz perturbation ( 6 ) into two analysis ( 6 a ) and reference ( 6 b ) terahertz perturbations as well as means ( 52 ) for the temporal offsetting of one of the two analysis ( 6 a ) and reference ( 6 b ) terahertz perturbations.
20. A method for terahertz spectroscopy, said method comprising taking the one-shot measurement of the transient birefringence using the device of claim 8 and performing said terahertz spectroscopy from the one-shot measurement.
21. A method for diagnosis of beams of charged and accelerated particles, said method comprising taking the one-shot measurement of the transient birefringence using the device of claim 8 and performing said diagnosis from the one-shot measurement.