FR2877444A1 - DEVICE FOR THE PROGRAMMABLE CONTROL OF THE AMPLITUDE AND THE SPECTRAL PHASE OF LIGHT PULSES FROM AN OPTICAL MIXER - Google Patents

Abstract

Programmable control device for the amplitude and the spectral phase of light pulses at the output of an optical mixer (M) effecting the collinear interaction of optical signals coming from two channels (V1, V2), comprising: - on the said channels (V1, V2), respectively two pulse stretchers (S1, S2) lengthening the short light pulses generated respectively by two laser sources (L1, L2), on at least one of the aforesaid channels (V1, V2 ), a programmable controller (P1, P2) for controlling the amplitude and the phase of the signals originating from the aforesaid pulse stretchers (S1, S2), and at the output of the aforesaid optical mixer (M) a compressor (C ) pulses from said optical mixture (M).

Description

The present invention relates to a device for programmable control

  the amplitude and spectral phase of light pulses from an optical mixer.

  It is more particularly intended to obtain pulses of high energy, wide spectral band, whose duration, amplitude and content in spectral phase or frequency are programmable.

  In general, it is known that pulsed laser sources make it possible to obtain a large variety of different wavelengths by nonlinear optical methods. For example, a wavelength source interacting in a non-linear crystal will produce a light beam of half-wavelength X / 2 by frequency doubling. More generally two sources of frequencies col and w2 will produce a resulting frequency beam: w3 = w1 + w2 and / or a resulting frequency beam: w3 = kwl-w2l.

  When a pulsed source is available, time pulse shaping may be performed which may be described as the introduction of frequency dependent amplitude and delay on the source spectrum. In the context of nonlinear optical interactions, it is conceivable to carry out this operation by an indirect method, in which the shaping is carried out on one of the two original beams and the desired shaping is produced on the beam resulting from the nonlinear interaction. The difficulty inherent in this approach is that the nonlinear optical interaction requires the simultaneity of the two pulses, ie their temporal overlap. It is clear, therefore, that to satisfy this time recovery condition, the shaping will be limited to delays of value less than the duration of the longer of the two pulses.

  It is possible to reduce this constraint by extending the duration of at least one of the pulses by a dispersive device that produces a delay that depends approximately linearly on the frequency (frequency drift). A resulting pulse is then produced at the output of the non-linear mixture which also has a frequency drift and which can then be compressed by a dispersive device having a delay as a function of the frequency opposite to that of the elongation. The devices for performing these elongation and compression functions are known elsewhere and generally use combinations of diffractive gratings or dispersive prisms.

  However, the elongation of the pulses will reduce the instantaneous optical power thereof and will therefore reduce the efficiency of the nonlinear interaction. Such an approach will therefore be effective only in a configuration allowing a high conversion efficiency despite the increase in the duration of the pulses. The conversion efficiency of nonlinear interactions is strongly determined by a phase matching condition between the two incident beams and the resulting beam.

  Indeed, the mixing of two optical signals E1 and E2 in a nonlinear optical crystal to obtain an output signal E3 whose frequency w3 is the sum or the difference of the frequencies w1 and 0) 2 of the signals E1 and E2, n ' is truly effective only when the signals E1 and E2 are very narrow spectral band signals, or even pure frequency signals.

  When the spectral band of one of the signals increases, the necessary phase tuning is no longer achieved for all frequencies in the crystal only over a short length of this crystal. This must then be chosen thin and the conversion efficiency of the input signals to the output signal decreases because the interaction length is not sufficient. The larger the spectral band of one of the signals, the thinner the crystal must be and the lower the conversion efficiency.

  To overcome these disadvantages, crystals consisting of a stack of thin crystals whose birefringence is reversed periodically were made to artificially increase the interaction length.

  Non-collinear interactions have also been proposed. In a non-collinear interaction, the phase tuning varies with the angle of incidence and the frequency of one of the signals to widen the spectral band in which the phase tuning is maintained.

  However, rather than considering a variation of the phase agreement with the frequency-dependent angle, it is possible to envisage a variation of the phase agreement with time provided that at each instant of the interaction the two signals E1 and E2 have in the crystal a pair of neck and neck frequencies that satisfy the phase matching condition. Under these conditions the two signals E1 and E2 must have approximately the same duration to interact constantly in the crystal and must have variations in their frequencies in time such that at each instant the phase agreement is achieved. When a frequency variation in time is given on one of the signals E1 for example by means of a pulse stretcher, the frequency variation in the time of the signal E2 is imposed by the orientation of the non-optical crystal. selected linear and by the frequency law of the signal E1.This was done by Boscheron et al. in 1996 in the context of the pulse frequency tripling of an Nd: glass laser to optimize the triplement conversion efficiency in the Nd: glass band.

  It turns out that all of these state-of-the-art methods rely on complex optical assemblies and that none of them can be programmed in amplitude and in phase with high efficiency and wide bandwidth. pulses from an optical mixer.

  The object of the invention is therefore more particularly to eliminate this drawback.

  The present invention involves associating an indirect temporal shaping method with an extension technique that maintains the phase agreement in a large spectral width.

  To this end, it proposes a programmable control device for the amplitude and the spectral phase of light pulses at the output of an optical mixer M effecting the collinear interaction of optical signals coming from two channels VI, V2, comprising: the aforesaid channels VI, V2, respectively two pulse stretchers S1, S2 elongating the short light pulses respectively generated by two laser sources L1, L2, on at least one of the aforementioned channels VI, V2, a programmable controller PI, P2 for controlling the amplitude and the phase of the signals originating from the aforementioned pulse stretchers S1, S2, and at the output of the aforesaid optical mixer M a compressor C pulses issuing from said optical mixture M. Advantageously a second programmable controller PI, P2 for controlling the amplitude and the phase of the signals originating from the aforesaid pulse stretchers S1, S2, is inserted on the second channel VI, V2.

  The aforementioned drawers S1, S2 of the short broadband pulses generate long pulses modulated in frequency of essentially identical duration and at least ten times greater than the minimum duration of the short pulses, to allow both the maintenance of the approximate agreement. phase in a wide bandwidth and the programming of the amplitude and the phase of the pulses from the mixer.

  Moreover, the ratio of the spectral bands of the signals generated by the above-mentioned laser sources LI, L2, and the sign of the slopes of the frequency modulated signals originating from the stretchers are determined by the mixing crystal and its orientation in order to optimize the conversion efficiency. of this mixing crystal in a very wide bandwidth.

  Advantageously, the above-mentioned programmable controllers P1, P2, for controlling the amplitude and the phase of the signals originating from the above-mentioned 20 pulse stretchers S1, S2 may be: programmable acousto-optical dispersive filters called AOPDF, or zero dispersion optical delay lines having an intermediate focal plane in which is placed a spatial control device of the phase and / or the amplitude.

  One embodiment of the invention will be described below, by way of non-limiting example, with reference to the accompanying drawings in which: FIG. 1 is a schematic representation of the device according to the invention, FIG. schematic representation of the device according to the invention for a first example of application, and Figure 3 is a schematic representation of the device according to the invention for a second application example.

  In the example shown in FIG. 1, two laser sources L1 and L2 generate short pulses E1 (t) and E2 (t) of central optical frequencies (Bi and col, two pulse stretchers Si and S2 transform the pulses short E1 (t) and E2 (t) of duration il and r2 and spectral widths B1 and B2, in long pulses E '1 (t) and E'2 (t) with the same spectral widths B 1 and B2 and durations Ti and T2 ten to one hundred times longer than the initial pulses E1 (t) and E2 (t), two programmable controllers P1 and P2 control the amplitude and the phase of the pulses E'1 (t) and E'2 ( t), an optical mixer M, consisting of a thick non-linear optical crystal, performs the collinear interaction of the signals E "1 (t) and E" 2 (t) issued from the programmable control devices P1 and P2, and a compressor C compresses the output pulses E3 (t) of the mixer M, the optical frequency co3 of these pulses E3 (t) being given by: (03 = c01 + (02 or: (03 = 01'w2l The above-mentioned blo cs L1, Si, P1, constitute the path V1; the aforesaid blocks L2, S2, P2 constitute the channel V2.

  The aforesaid laser sources LI and L2 generate short pulses which are written: E, (t) = A, (t). cos (w, t) E2 (t) = A2 (t). cos (w2t) AI (t) and A2 (t) being the time amplitudes of these pulses of duration -ci and T2E At the output of the aforesaid stretchers S1 and S2 the pulses E'I (t) and E'2 (t) 10 are written: E ', (t) = A', (t). cos (w, t B, t2 / 27i) E'2 (t) = A'2 (t). cos (w2t B2t2 / 2T2) A'1 (t) and A'2 (t) being the temporal amplitudes of the elongated pulses modulated in frequency of duration TI and T2.

  At the output of the aforesaid programmable controllers PI and P2, the pulses E "1 (t) and E" 2 (t) are written in the first order: E "1 (t) = E ', (t) H, (t / a,) E "2 (t) = E'2 (t) H2 (t / a2) where HI (t / al) and H2 (t / a2) are the impulse responses, in amplitude and in phase, of the controllers programmable PI and P2, generated by electrical signals H1 (t) and H2 (t) applied to the programmable controllers, where al and 112 are scaling factors, and 0 is the convolution product.

  When the pulses E "1 (t) and E" 2 (t) have the same duration T and are synchronized to arrive at the same time in the optical crystal mixer M, the conversion efficiency of this mixing crystal M is optimum for all mixing frequencies, if the pulse frequency modulations E "1 (t) and E" 2 (t) are adjusted by the above-mentioned programmable controllers P1 and P2 and by the choice of slopes: Pi = B1 / T and p2 = B2 / T drawers, respectively S1, S2, so that at each instant, the nonlinear crystal sees on its axis of interaction a pair of two frequencies: w1 (t) and w2 (t) for which the agreement phase is realized. These frequency laws: (D1 (t) and / or co2 (t) therefore depend on the choice of the mixing crystal M and the choice of the colinear interaction axis.

  The examples shown in Figures 2 and 3 are particular applications according to the invention.

  In the example represented in FIG. 2, short pulses E1 (t) of duration: tif = 50fs and spectral width B1 = 20nm, originating from a laser (Ti: Sa) L whose wavelength is centered around 1 = 800 nm (w1 = 2.35, 1015) are applied: on the one hand on the channel V1 to the stretcher S1 which delivers pulses of 3 to 10ps, - on the other hand on the channel V2 to a band filter F which reduces the band B2 to B2 = B1 / 4 = 5nm and accordingly extends the short pulses to i2 = 200fs; these 200 ff pulses are then sent to an optical parametric amplifier APO which generates an "idler" signal and a useful signal E2 (t) of duration t2 = 200fs centered around X2 = 1350 nm (co2 = 1.40, 1015); this useful signal E2 (t) is then applied to the stretcher S2 which delivers pulses of 3 to 10ps.

  Downstream of the stretchers S 1 and S 2 are the previously described structure; the pulses from the mixer M are centered around the frequency: w 3 = w 1 + w 2 = 3.75 × 10 15 is X 3 = 500 nm.

  In the example shown in FIG. 3, on the channel V2, a regenerative or multi-pass type optical amplifier ARO is placed after the programmable controller P2 and before a doubling crystal CR (of 2 harmonic generation).

  The channel V1 consists of a stretcher S1 and a programmable controller P1.

  On the channel V2, the signal E "2 (t) thus obtained before the optical mixer M, is at the frequency w2 = 2w1 and the signal from the optical mixer M is an amplification of the signal E" 1 (t) at the frequency w3 = w2 - wl = col.

Claims (8)

- 10 - Claims   1. Programmable control device for the amplitude and the spectral phase of light pulses at the output of an optical mixer (M) effecting the collinear interaction of optical signals coming from two channels (V1, V2), characterized in that it comprises: on the aforesaid channels (V1, V2), respectively two pulse stretchers (S1, S2) elongating the short light pulses generated respectively by two laser sources (L1, L2), on at least one of the aforesaid channels (V1, V2), a programmable controller (P1, P2) for controlling the amplitude and the phase of the signals originating from the aforesaid pulse stretchers (S1, S2), and at the output of the aforesaid optical mixer ( M) a compressor (C) pulses from said optical mixture (M).   2. Device according to claim 1, characterized in that a second programmable controller (P1, P2) for controlling the amplitude and phase of the signals from said pulse stretchers (S1, S2) is inserted on the second way (VI., V2).   3. Device according to claim 1, characterized in that the aforesaid stretchers (Si, S2) short broadband pulses generate long pulses modulated in frequency of substantially the same duration and at least ten times greater than the minimum duration of the pulses brief.   4. Device according to claims 1 and 3, characterized in that the ratio of the spectral bands of the signals generated by the aforesaid laser sources (L1, L2) and the sign of the slopes of the frequency-modulated signals from the drawers (SI, S2) are determined by the mixing crystal (M) and its orientation to optimize the conversion efficiency of this mixing crystal in a very wide bandwidth.   5. Device according to claim 1 or 2, characterized in that the aforesaid programmable controllers (PI, P2) for controlling the amplitude and the phase of the signals originating from the aforesaid pulse stretchers (S1, S2) are programmable acousto-optical dispersive filters called AOPDF.   6. Device according to claim 1 or 2, characterized in that the aforesaid programmable controllers (PI, P2) for controlling the amplitude and phase of the signals originating from the aforesaid pulse stretchers (S1, S2) are zero dispersion optical delay lines having an intermediate focal plane in which is placed a spatial control device of the phase and / or the amplitude.   7. Device according to claim 1, characterized in that the signal at the output of the aforesaid compressor (C) has for instantaneous frequency the sum of the instantaneous frequencies of the signals generated by the aforesaid laser sources (LI, L2).   8. Device according to claim 1, characterized in that the output signal of the aforesaid compressor (C) has instantaneous frequency difference instantaneous frequencies of the signals generated by the aforesaid laser sources (LI, L2).