System for generating short optical pulses of a duration shorter than the period of the optical carrier using the principle of parametric amplification

Abstract

A generator of short optical pulses includes: a generator of optical pulses with a wavelength equal to /2 and a spectral width less than /3; a generator of optical pulses of a duration less than 10 picoseconds at a wavelength /3; a device for recombining the pulses from the generators; a parametric amplification device receiving the output of the recombination device as input; a filter extracting, from the output of the amplification device, a band centered about a wavelength ; a second harmonic generator receiving the output of the filter and generating a band centered over a wavelength /2; and a programmable device making it possible to temporally adjust the pulses corresponding to the bands, in order to allow for the generation of a pulse with a wavelength equal to /3 and of a duration less than the period of the optical cycle.

Claims

1. System for generating short optical pulses of a duration shorter than a period of an optical carrier using a principle of parametric amplification, which comprises: a first generator of optical pulses (P, B1) outputting pulses with a wavelength substantially equal to /2 and a spectral width less than /3; a second generator of optical pulses (B2) outputting pulses of a duration less than 10 picoseconds at a wavelength substantially equal to /3; a device (R1) for collinearly recombining the pulses from the first and second optical pulse generators; a first parametric amplification device (OPA1) receiving an output of the said recombination device (R1) as input; a filter (F2) extracting from the output of the first parametric amplification device (OPA1) a band centered about a wavelength substantially equal to ; a second harmonic generator (SH) receiving an output of said filter (F2) as input and generating a band centered about a wavelength substantially equal to /2; and a programmable device for controlling a delay and a spectral phase (DCDP2) making it possible to temporally adjust the pulses corresponding to said bands, in such a way as to allow for the generation, by constructive interference, of a pulse with a wavelength substantially equal to /3 and of a duration less than a period of an optical cycle.

2. Device according to claim 1, further comprising a dispersive device (C) carrying out a compression and bringing the pulses from the programmable device (DCDP2) to their minimum duration.

3. System according to claim 2, further comprising a second parametric amplification device (OPA2) receiving uncompressed pulses from the programmable device (DCDP2).

4. System according to claim 2, wherein said programmable device for controlling the delay and the spectral phase (DCDP2) is an acousto-optic programmable dispersive filter.

5. System according to claim 2, further comprising a pump source (P) for the generators, with said pump source comprising preferably means of amplification by materials doped with Erbium ions.

6. System according to claim 1, further comprising a second parametric amplification device (OPA2) receiving uncompressed pulses from the programmable device (DCDP2).

7. System according to claim 6, wherein said programmable device for controlling the delay and the spectral phase (DCDP2) is an acousto-optic programmable dispersive filter.

8. System according to claim 6, further comprising a pump source (P) for the generators, with said pump source comprising preferably means of amplification by materials doped with Erbium ions.

9. System according to claim 1, wherein said programmable device for controlling the delay and the spectral phase (DCDP2) is an acousto-optic programmable dispersive filter.

10. System according to claim 9, further comprising a pump source (P) for the generators, with said pump source comprising preferably means of amplification by materials doped with Erbium ions.

11. System according to claim 1, further comprising a pump source (P) for the generators, with said pump source comprising preferably means of amplification by materials doped with Erbium ions.

12. System according to claim 11, wherein the first generator is obtained using the pump source (P) by a non-linear optical process (B1).

13. System according to claim 11, wherein the second generator is obtained using the pump source (P) by a non-linear optical process.

14. System according to claim 1, further comprising means for synchronising the pulses from the first and second generators.

Description

(1) A method of implementing a device according to the invention is described hereinafter, by way of non-limiting examples, with reference to the annexed drawings wherein:

(2) FIG. 1 diagrammatically shows the structure of the optical system according to the invention; and,

(3) FIG. 2 shows the spectra obtained at various locations of the system of FIG. 1.

(4) The amplification of ultrafast laser pulses via parametric effect is described in literature. A signal pulse, with central optical frequency f1, interacts in a material having non-linear optical properties with a higher power pulse (pump pulse) and with optical frequency f. The frequency content of the signal pulse must be wide in order to allow for the production of ultrafast pulses, the pump pulse generally being of a narrower spectral band. During the interaction, a portion of the energy of the pump pulse is transferred to the signal pulse which is amplified. Simultaneously, a third pulse is generated and co-amplified. This third pulse is designated by the term idler and the relations for energy conservation impose on this pulse to have as a central frequency f2=ff1. The phase tuning constraint also imposes that the CEP of the idler pulse be equal, to within a constant, to the difference between the CEPs of the pump and signal pulses. If this difference is constant in a repeatable manner (i.e. from one pulse to another), then the CEP of the idler pulse is stable over time, even if neither the pump pulse or the signal pulse have this property.

(5) A remarkable choice for the optical frequency for the signal pulse is f1=2*f/3. Indeed, in this particular case, the optical frequency of the idler pulse is f2=ff1=f/3=f1/2. The optical frequency f1 of the signal pulse is then the exact harmonic of the idler pulse, of frequency f2. This choice of optical frequencies is hereinafter referred to as 3-2-1 in reference to the values of the of the optical wavelengths of the pump, signal and idler pulses (respectively /3, /2 and if is the wavelength of the idler).

(6) For the generating of high power pulses, the principle of the parametric amplification is combined with that of drift frequency amplification which consists in temporally extending the signal pulse and the pump pulse in order to reduce the instant power, with the amplified signal being recompressed later in order to obtain a short pulse. This is drift frequency parametric amplification, designated by the acronym OPCPA (Optical Parametric Chirped Pulse Amplification). At the output of the optical parametric amplifier, the idler pulse has a frequency drift that is, in the particular case of a quasi-monochromatic pump or with a spectral width less than that of the signal, a sign opposite that of the signal pulse. It is therefore not possible, in general, to use the single same device to recompress the idler pulses. In practice, it would be necessary to geometrically separate the signal and idler beams, to recompress them separately then to recombine them coherently. These operations are not trivial, have technical difficulties and major complexity and have not, as far as we know, been the subject of a publication. Moreover, the spatial separation of the beams would reintroduce possible sources of fluctuation in the relative phase between the signal and idler pulses, which complicates all the more so their recombining.

(7) This invention is placed in the context of an OPCPA architecture of the 3-2-1 type comprising at least two parametric amplifiers and for which the signal and pump pulses have a CEP difference that is over time. The invention introduces two additional devices into the OPCPA architecture: on the one hand, a stage of second harmonic generation inserted between two optical parametric amplifiers, on the other hand, a device for reversing the sign of the frequency drift and phase.

(8) The stage of second harmonic makes it possible to generate using the idler pulse from the first parametric amplifier (with optical frequency f1/3) a pulse with optical frequency 2*f1/3 which is identical to the optical frequency of the signal pulse of the first parametric amplifier. This identity is the direct result of the choice of 3-2-1 frequencies and makes it possible to continue the parametric amplification in the following amplification stages. As the idler pulse is CEP-stable, the idler pulse doubled in frequency is also and the CEP difference between the latter is also stable over time. A phase tuning of type 0 in addition allows the second harmonic to be polarised linearly like the idler pulse. The combination of the idler pulse and of its harmonic therefore has all of the properties sought: a spectral width greater than the octave, spatial and spectral coherency, a stable CEP. The harmonic pulses form a single pulse that can be described as a pulse with an optical carrier f/2 and spectral width >f/3. If the spectral width of the starting signal pulse is, for example, f/6 then the combined harmonic pulses cover a spectral width of f/2 which is a full octave with respect to the optical carrier.

(9) In order to proceed with the amplification of these pulses it is necessary to tune the frequency drifts, the delays and the absolute phases between these two bands with a precision in the delay of about one fraction of an optical period, typically much less than 1 fs. This ultra-precise operation can advantageously be carried out using a programmable dispersive device such as an acousto-optic programmable dispersive filter (AOPDF).

(10) Light sources that combine a parametric amplifier with external non-linear optical devices such as a second harmonic generator or a frequency difference generator with the purpose of increasing the range of frequencies that can be covered have been described in literature (Miller et al., PCT WO 2005/112207 A1).

(11) This invention uses such a combination, but it can be distinguished in particular from Miller et al in that: the invention allows for the co-amplification of the pulses with frequency f2 and 2*f2 in the same single parametric amplifier: same pump pulses, same polarisation, same direction of propagation, same type of non-linear crystal, the invention does not require a starting signal pulse that has a spectral width approaching or covering the octave; indeed, the invention makes it possible to generate the octave using a more reduced spectral width, the generation of the second harmonic is essential: it is not used to increase the spectral coverage of the invention.

(12) If the source of the signal at the frequency f1 were independent of the pump laser at the frequency f, the carrying out of the invention would require a complex synchronisation mechanism between these two laser sources, in order to ensure that they are simultaneously present in the parametric amplifier. A preferred embodiment of the invention consists therefore in generating, for example, the source of the signal using the pump pulse, according to a procedure described for example by Cerullo et al. (Ultrafast optical parametric amplifiers, Rev. Sci. Instrum. 74, 1 (2003)). Several mechanisms with a non-linear optical base are known for generating a wide band pulse, referred to as a supercontinuum, using an intense pulse of a narrower spectral band and with a different central frequency. This is for example the non-linear interaction with the glass in an optical fibre, the non-linear interaction with a solid crystal or the interaction with a gas in a hollow fibre. An interaction of this type will be used in order to generate using the pump of frequency fa wide spectrum pulse including the frequency 2*f/3. A spectral filter can extract from this wide spectrum the frequencies intended to form the signal pulse which is then amplified by the parametric amplifier.

(13) A second possibility is to generate the pump source using a signal pulse by an equivalent mechanism: a non-linear interaction is used to generate using the signal with frequency 2*f/3 a wide spectrum pulse including the frequency f. A spectral filter extracts from this wide spectrum the frequencies intended to form the pump pulse which is then amplified by laser power amplifiers.

(14) A third possibility, described, for example, by Adachi et al. (1.5 mJ, 6.4 fs parametric chirped-pulse amplification system at 1 kHz, Optics Letters, Vol. 32, Issue 17, pp. 2487-2489 (2007)) is to use a source that generates pulses that have a spectral band that covers both the frequency f and the frequency 2*f/3. A spectral filter extracts from this initial wide spectrum a frequency interval which is then amplified by laser power amplifiers and finally converted by second harmonic generation in order to form the pump pulse. A second spectral filter extracts from the initial wide spectrum the frequencies intended to form the signal pulse which is then amplified by the parametric amplifier.

(15) In sum the principle of the invention makes it possible, using a pump pulse with a narrow spectrum and frequency f and a synchronous signal pulse with a wide spectrum centred at the frequency f1=2*f/3, to generate and amplify a wide spectrum pulse at f/3 at the optical frequency f/2. This generation is provided by the following elements: a parametric amplification stage that creates, via frequency difference, a wave complementary to the frequency f2=ff1, the band filtering around f2=ff1 and the generating of a second harmonic 2*f2=f1 via a phase tuning of type 0, the turning over and the adjustment of the frequency drift of the pulse at the frequency 2*f2 so that this pulse interferes constructively with the pulse at the frequency f2 in the final compressor.

(16) In addition, the 3-2-1 choice, which is an essential character of the invention, guarantees that the pulses at frequencies f2 and 2*f2 can be co-amplified in the following parametric amplification stages using the same pump pulses as for the first parametric amplification stage.

(17) The invention shall be better understood by considering the following example embodiment shown in FIG. 1. The pump source P is constituted by a chain of generation and amplification of pulses with a base of amplifying materials containing Ytterbium ions. It emits intense pulses with a wavelength of /3, corresponding to the frequency f. In the example shown, /3=1030 nm. The optical output of this source is divided by a separator BS into beams injected respectively into three arms B1, B2 and B3. The beam of the first arm B1 is focussed in a YAG SC crystal making it possible to generate a supercontinuum that covers at least the range 1300-1900 nm. The output of this supercontinuum is used as a signal channel.

(18) The beam of the second arm B2 is centred on the wavelength /3 and the frequency f. The second arm B2 is used as a pump channel.

(19) The beams from the first arm B1 and from the second arm B2 are combined collinearly in a first recombination device R1 and injected into a first optical parametric amplifier OPA1 that carries out the frequency difference function. Advantageously, the signal pulse was modified, with recombining, by a first device for controlling the delay and the phase DCDP1 in order to obtain an optimum temporal overlapping of the pulses of the first and second arms B1, B2, in order to optimise the drift frequency parametric amplification mechanism. A first filter F1 is advantageously arranged between the output of the supercontinuum SC and the first device for controlling the delay and the phase DCDP1, in order to select a signal centred on the frequency /2=1545 nm corresponding to the frequency f1.

(20) Advantageously, the first optical parametric amplification device OPA1 has a MgO-doped Lithium Niobate crystal base, of which the polarisation is periodically inverted (Periodically Poled Lithium Niobate). This type of device provides a quasi phase tuning between the pulses with a wavelength /3 at 1030 nm and those comprised in the band 1300-1900 nm, allowing for the amplification over a wide spectrum band and in collinear configuration with a signal centred on the wavelength /3=1545 nm.

(21) An idler output centred on wavelength =3090 nm is selected by a second filter F2. The filter F2 is as such provided to extract from the output of the first amplifier OPA1 a band centred about a wavelength substantially equal to . In the example shown, this filter F2 is constituted of one or several dichroic mirrors.

(22) The output of the filter F2 is injected into a non-linear crystal SH, also of the MgO:PPLN type, operating as a second harmonic generator.

(23) A second device for controlling the delay and the phase DCDP2 controls the delay and the phase of the spectral components from the crystal SH. The idler and its harmonic interfere to form a pulse of which the average optical wavelength is centred on 2*/3=2060 nm, corresponding to a frequency equal to f/2. If the spectrum of the signal covers the range 1300-1900 nm then the idler and its second harmonic cover the entire spectral range 1125-4345 nm and this spectrum supports a pulse of sub-cycle duration.

(24) The beam of the third arm B3 is centred on the wavelength /3 and the frequency f. The third arm B3 is also used as a pump channel.

(25) A second recombination device R2 combines in a quasi-collinear manner the beam formed by the output of the second device for controlling the delay and the phase DCDP2 with the beam of the third pump arm B3 and directs the combined beams to a second optical parametric amplifier OPA2. This amplifier is of the same type as the first OPAL It increases the power of the pulse from the second device for controlling the delay and the phase DCDP2. Finally, a dispersive system C constituted, for example, of a thick blade of Silicon makes it possible to compress the pulses to their minimum duration and forms a pulse with sub-cycle duration.

(26) FIG. 2 diagrammatically shows the spectra obtained in the various steps of the system of FIG. 1: The spectrum 2a is the spectrum of the pump source P; The spectrum 2b is the spectrum of the supercontinuum at the output of the crystal Sc; The spectrum 2c is the spectrum obtained at the output of the first filter F1; The spectrum 2d is the spectrum obtained at the output of the first parametric amplifier OPA1; The spectrum 2e is the spectrum obtained at the output of the second filter F2; and The spectrum 2f is the spectrum obtained at the output of the second harmonic generator SH.

(27) It is obvious that the pulses from the arms B1 and B2 before the recombining R can be, without loss of generality, from two synchronous optical sources, wherein one of which is represented by the pump source P.