FR2530018A1 - DEVICE FOR DETERMINING THE BEHAVIOR IN THE TIME OF ULTRA-SHORT OPTICAL PULSES - Google Patents

Abstract

THIS IS A DEVICE USED TO DETERMINE THE BEHAVIOR OVER TIME OF ULTRA-SHORT OPTICAL PULSES. THE PULSE SHAPE AND THE PULSE DURATION IN TIME ARE DEDUCTED FROM A LINEAR AUTO-CORRELATION FUNCTION OBTAINED ACCORDING TO THE INTERFERENCE METHOD. TO THIS EFFECT, A PULSE BEAM IS DIVIDED BY A BEAM DIVIDER INTO TWO PARTIAL BEAMS, THESE PARTIAL BEAMS ARE REFLECTED BY APPROPRIATE REFLECTORS AND JOINED BY THE BEAM DIVIDER AND THE SHAPED INTERFERENCE CONFIGURATION IS EVALUATED, BEING A TRIBUTE TO THE DISPLACEMENT OF AT LEAST ONE OF THE REFLECTORS.

Description

25300 1 8

  -1 - "Device for determining-the time behavior of ultra-short optical pulses"

  The invention relates to a device for deter-

  the behavior over time of ultra-short optical pulses

  your. There are already several devices of this kind

  as well as in a first known device, a direct measurement

  made using fast optical detectors (usually

  avalanche diodes or PES field effect transistors) in cooperation with sampling oscilloscopes

  the optical pulse is measured directly.

  solution in time is delimited by the duration of rise and

  detection of the detector and the sampling system or,

  very terms, in general, by the electrical activation behavior of the detection system Currently, the width FWHM (Full Width Half Maximum) of such systems is about 50 to 100

  ps The determination of true optical pulse widths

  100 ps is possible, it is true, by deconvolution.

  quadratic behavior of the system, but presents possible systematic faults according to the pulse width and the shape

  In accordance with the spectral sensitivity curves,

  in question of the detector, it is generally possible to demonstrate the optical pulse for small pulse energies (> 310 14

  J) and in a wide range of wavelengths (some 100 nm).

  In another known device, the measurement is carried out using a fast "Streak camera" Here also the pulse shape with the limitation of a system of finite behavior, which is located in this system in the region picoseconds, it is true is measured in a direct way The power of resolution in the time of

  the Streak camera compared to the above method is superior to

  Viron 2 orders of magnitude, the spectral region is equivalent, the

  pulse energies needed too.

  Finally, there is a device for measuring

  This device, which serves the pulse width range, takes advantage of the physical principle of training.

25300 1 8

  -2- of the second harmonic (in English "second-harmonic generation"

  (SHG)) in a non-linear crystal (eg KDP), which is also

  used for frequency doubling of lasers for

  production of combined or very short wavelengths:

  of two D 1 and O 2 frequencies in the crystal

  to produce combination frequencies D 1 + u O 2

  The use for pulse width measurement is that the light of the optical transmitter consists of a series of optical pulses.

  is divided into two partial beams in a beam splitter.

  These partial beams are each reflected by a reflector with high reflection and joined in the nonlinear crystal Starting from equal distances between the reflectors and the beam splitter,

  it is possible to obtain a difference in the optimum path length

  only for the two partial beams by relative displacement of one of the reflectors with respect to the other and, as a result, a displacement in time of the pulses, with respect to one another. In accordance with the duration of piopagation of the light in the air, the relation T = between the difference in pulse propagation time and the difference in optical path length 4 z of the two partial beams is obtained. A displacement of 1 mm from a reflector from the position zero (symmetrical position) corresponds to a delay in the time of the partial pulses in

  the combined bundle of about 6.7 ps.

  The SHG signal in the eristal KDP occurs at a

  signal frequency t O and is proportional to the product of the in-

  1/2 overlapping partial beam voltages entering at the frequency

  t 1 (laser or transmitter frequency) In the case of an

  Partially pulsed, which means (AZ = 0), the SHG signal is maximal When the pulses do not overlap, which means that t Z> C 0/2 t TM, where TM is the maximum width optical pulses, the SHG signal disappears or then returns according to the exact theory to 1/3 of the

  maximum signal; for other explanations, it is necessary to

  to "Spectra Physics, Laser Technical Bulletin", February 1978,

  N 3 Given its linitations, the width of the SHG signal is therefore pro-

  proportional to the pulse duration of the optical pulse.

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  -3- ception of certain forms of pulse, for example the Gaussche form, it is possible to determine the pulse width at

  SHG Moreover, it is necessary to refer to the article

  "Autocorrelations function analysis"; in IEEE Journal, QE-16,

1980, No. 9, page 990).

  The time resolution of the SHG measurement is very high (greater than 0.1 ps), but because it is a

  non-linear effect in the case of SHG, the pulse energies op-

  In addition, the spectral region in

  which SHG occurs is strongly limited by the choice of

  It is necessary to satisfy the so-called "matt

  ching condition "And all, this process is very expensive, it is complicated and the adjustment is difficult and the process is limited to

  quite high pulse energies.

  The present invention aims at providing a device of the gen-

  mentioned in the preamble with a simple structure and

  particularly suitable for measuring the behavior over time of im-

  optical impulses of low impulse energies and small

absolute impulse givers.

  The device according to the invention takes advantage of the above-mentioned

  the possibilities described for the SHG process and the division

  However, we also take advantage of the interference attitude in the coherence length of the coherent light, coherence length which, in the case of pulsed laser beams, is also used in conjunction with interferometric devices. , is dependent on the duration of the program and, therefore,

  does, necessarily of the duration of the optical pulses.

  The method according to the invention is characterized by a beam splitter, two reflectors provided in the paths of

  two partial beams formed by the beam splitter, reflecting

  at least one of which can be moved in the direction of the axis of the corresponding partial beam, a photosensitive detector, inserted in the path of the interfering partial beams and which are united in a single beam by the beam splitter and a

  system for the processing of the detection signal.

  The first embodiment of the device is further characterized in that the reflectors are flat.

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  -4- after reflection by the reflectors and meeting, the two beams

  may evenly interfere with the entire trans-

  of the beam of radiation formed from the beams of

  This interference occurs only when the

  pathways of the partial beams do not differ more than the

  coherence length of pulsed laser light

  interference, a uniform light distribution

  on the cross-section of the combined beam, distribution of which

  the intensity is dependent on the relative phase of the beams

  The intensity varies periodically in the case of a variation

  continuous path length of one of the two beams

  by moving one of the reflectors A corresponding period

  to the displacement of a reflector on half the length

  of the light used The amplitude of the variation of inten-

  sity is dependent on the difference in absolute length of

  of the two partial beams, which means the difference in

  in the case of optimal positions of the two reflections

  and the duration of the optical emission.

  maximum tensile strengths for absolute optical path lengths

  equal readings of the partial beams and decrease in the case of a

  displacement of one of the beams as the pulses par-

  are less overlapping in the beams of radiation composed of

  For path length differences of a magnitude such that the partial pulses no longer overlap, i.e. the partial pulses are displaced relative to each other.

  other than the pulse width, it does not occur

terférence.

  The use of perpendicular reflectors is advantageous

  when the available intensities of the pulsed optical beam

  are small because of the unified light distribution

  form on the total cross-section of the compound beam, modulation of

  tensity is measured integrally on the total section of the

  beam in the case of displacement of one of the reflectors.

  According to another advantageous embodiment of the

  according to the invention, one of the reflectors is placed in an oblique position so that the normal to the plane of this reflector

  is inclined relative to the axis of the corresponding partial beam.

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  As interference configuration, a configuration of alternately light and dark bands occurs in concordance with the locally relative relative phase of the two partial beams. The distance of parallel strips juxtaposed is dependent on the angle of inclination of the reflector and of the wavelength of the pulsed light The maximum local intensity fluctuations between two juxtaposed bands are dependent on the difference in length

  of the partial beams and the duration of the optical pulses.

  In the case of displacement of one of the reflectors, the

  Interference management moves perpendicular to the direction

  Longitudinal displacement of the bands A displacement on half of the wavelength of the light used corresponds to the displacement of the interference configuration over a single band distance La

  modulation of local intensity between juxtaposed bands (contras-

  band) is maximum for optical path lengths equal to

  the partial beams, it decreases as in the case of displacement of one of the reflectors, the pulses of the partial beams in the compound beam overlap less in time and

  disappears in the case of relative length differences corresponding to

  the temporal displacement of the partial beams relative to

  more than the pulse width.

  This device offers the advantage that the displacement of the interference pattern on the cross section of the beam

  radiation provides direct data on the direction of

  In addition, the presence of interference, especially in the case of small modulation amplitudes (low overlap of partial pulses), is easier to achieve and the speed of movement of the two partial pulses relative to each other. determine that in the case of using perpendicular reflectors The interference pattern can be recorded after

point.

  According to another advantageous embodiment of the arrangement

  of the invention, prisms, including retroreflective prisms are used as reflectors At least one of

  prisms can be moved perpendicular to the ray axis-

  Thanks to this dislocation; In this case, the joined partial beams are displaced parallel to each other without being

25300 1 8

  -6- produce an additional difference in path length between

  partial beams Thus, the region in which can be

  the partial beams can be modified by moving one of the prisms perpendicular to the corresponding partial beam axis However, because in this case, several regions of the section of the two partial beams can interfere with each other, the configuration of interference thus formed contains information on the local behavior over time of the optical pulses. This information is then necessary to determine the pulse duration and the impuence form when the emission does not occur simultaneously in all the regions of the section of

  beam or when pulsed light passes through non-homogeneous

  genes, the various beam parts being subject to delays

  In both cases, the total impulse width of the

  is greater than that corresponding to the length of coherence

of the light used.

  The advantage inherent in the device according to the invention is therefore, among other things, the fact that in the case of using a retroreflective prism that can be moved perpendicularly to the axis of the beam, it is also possible to determine the duration

  total impulse in these cases from the variation of the con-

  figuration of interference during the movement of one of the

  reflecting reflections and from the variation in

  dicular. According to another characteristic of the device according to the invention, a diaphragm is provided between the beam portions

  and the detector, diaphragm whose opening diameter is adjusted

  ble and / or whose opening can be moved in one direction

  perpendicular to the axis of the compound beam A diagonal opening

  variable diameter membrane is then advantageously used in

  the case of using a reflector and / or a prism whose standard

  the base plane is inclined relative to the corresponding partial beam axis The mobile band-shaped interference pattern can be explored, point by point, using the diaphragm aperture The diameter of the diaphragm opening

  is small compared to the band spacing.

  An aperture of diaphragm, whose position can be

  regulated perpendicularly to the axis of the beam of radiation com-

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-7-

  posed is advantageously used in a device, whose

  Partial beams can be moved parallel to one another by moving a prism perpendicular to the axis of

  partial beam In this case, a variation of the position of the

  diaphragm opening perpendicular to the axis of the beam of

  compound and a variation in the diameter of the di-

  phremma allows to explore the interference pattern at various locations in the beam section respectively for multiple

  regions of overlapping partial beams and to record it.

  The resolution power is then dependent on the chosen diameter

for the diaphragm.

  The following description, with reference to the drawings

  nexés, all given as a non-limitative example, will make a good

  take how the invention can be realized.

  Figure 1 shows a device with two reflections

  perpendicular to the partial beams.

  FIG. 2 illustrates a device presenting a reflection

tipped.

  FIG. 3 shows a device provided with two prisms

of reflective components and

  Figures 4 and 5 show the measurement results obtained

  bare with the device according to the invention.

  FIG. 1 shows a device having two reflectors perpendicular to the partial beam. In this device, a laser beam LS, which has passed through a beam expander SA,

  is divided into two partial beams a, b in a divider desfais-

  An impulse P is divided by the beam splitter ST into two partial pulses P1 and P2. These partial beams a and b are reflected by two reflectors 51 and 52 perpendicular to the axes of the beams and are joined during the next pass by the beam. ST beam splitter In the embodiment shown in the drawing, the reflector 52 can be moved by a motor M

  in the direction of the partial beam axis A damping filter

  Variable DF ensures that in the area of Section B of the

  beam of radiation composed from the partial beams

  the partial beam intensities are approximately equal

  the partial beams a and b interfere with each other and in this

25300 1 8

  In this case, the intensity is the same throughout the cross-section B. The intensity is determined by the distance z 2 between the reflector 52 and the beam splitter ST.

  radiation of the compound radiation beam is inserted a di-

  diaphragm L whose opening has a diameter OL In the case in question, the diaphragm L is positioned centrally with respect to the compound radiation beam and the opening corresponds to the section B of the radiation beam (e L = B ); the diaphragm serves only to reduce the unwanted ambient light Behind the diaphragm L is a photosensitive detector D for converting the intensity of the combined radiation beam into a signal

  In the exemplary embodiment shown, is this signal

  recorded on the vertical axis (Y) of a recording device R, the horizontal axis (X) being controlled via the motor M and therefore in accordance with the distance z 2 of the reflector 52. The intensity is therefore recorded directly according to the distance Z 2 In the case of a symmetrical reflector arrangement

  (z = z 2 z 1 = 0), the overlap in time of

  partial ions P1 and P2 in the compound radiation beam is

  maximum (AT = 0) When moving the reflector 52, the intensi-

  té periodically varies on the detector, the period being given by: z => / 2, 'being the wavelength of the laser light p

  pulsed A displacement of the reflector 52 over a distance a z corres-

  poses at a difference in optical path length of 2 JE z between two partial beams and, thus, a displacement in time of the partial pulse P 2 with respect to the partial pulse P 1 Thus the displacement of the reflector 52 in a period e zp

  corresponds to a displacemant in time of the two impulses par-

  tials AT = X / c O 6 67 "10-3 ps (>) In the case of upward / downward movement of the reflector, the partial pulses

  Pl and P 2 are always more displaced, in time, one by

  port to another, the parts of pulses that can interfere with each other, that is to say which can always overlap in time, decreasing more and more As a result, the modulation amplitude of the interference signal for A z croissant becomes smaller For

253001 8

  -9- displacements a z such as for the resulting delays LN T he

  applies 8 T = 2, t z / c O J T (where To is the pulse width

  maximum pulse of the optical pulse P), the modulation disappears

  and there is a constant signal over time.

  The light intensity recorded with the recording device R as a function of the displacement A 2 z of the reflector 52 from the symmetrical position thus has the characteristic variation shown in FIG. 1, from which the pulse shape and the pulse duration of the laser pulse P using

  methods of autocorrelation analysis As the duration of impulse

  sion and the pulse form are uniquely correlated with the coherence length of light, it is possible to deduce

quantitative impulse.

  FIG. 2 represents a device presenting a reflection

  52, which is tilted at an angle V with respect to the axis of

partial skin.

  The elements of this device having the same function

  than the corresponding elements in Figure 1.

  The interfering partial beams a, b constitute a band-shaped interference configuration, the contrast of which

  interference, band spacing and tape position are tri-

  butaries of displacement ii z of reflector 52.

  In the path of the beams of the radiation beam

  Here is also inserted a diaphragm L whose opening diameter

  L is variable The diaphragm L is positioned centrally with respect to the axis of the compound radiation beam, and the section of the aperture is significantly smaller than half the distance d of interposed interference bands. X 1 Z <4 d / 2) The

  interference configuration which, when moving the reflec-

  52, moves in the direction perpendicular to the direction

  longitudinal band, can be explored by the light detector.

  In the exemplary embodiment shown, the signal of this

  The detector is recorded on the vertical axis (Y) of the recording device.

  R, the horizontal axis (X) being excited via

  motor M and therefore in accordance with the provision of the

  The luminous intensity at the point of opening of the di-

  The phrase is therefore recorded directly according to the posi-

  reflector z 2 -10- In the case of a symmetrical arrangement of the reflectors

  z z = 2 z 1 = 0), the overlap in time of the impulses

  Partial P1 and P2 in the compound radiation beam is maximal (L T = 0) Modulation amplitude of the configuration

  band-shaped interference is then also maximal.

  band pitch d is equal for all the bands and, given the interference condition, it is given in this case by d \ = / 2 tg Y In the case of a displacement of the reflector 52 from the

  symmetrical position, the interference pattern moves

  pendicular to the longitudinal direction of the bands towards the left

  to the right according to the direction of movement of the reflec-

  52 A displacement of the reflector 52 surd z 2 P => / 2 corres-

  the interference pattern, or a

  This displacement also corresponds to a delay in time of the pulse P 2 of Tp = / c O 6 67 1073 ps (\), / um

  where c is the speed of the laser light.

  o In the case of an increasing displacement d C z of the reflector 52, the partial pulses P 1 and P 2 are always more displaced in time with respect to each other and the impulse parts that can interfere with each other become As a result, the modulation amplitude of the interference signal for a larger and smaller z becomes smaller. For displacements z such as the resulting displacements in time i T = 2 a

  z / c O> AT 0, (i S T O being the pulse width of the impulse

  P), the interference contrast respectively the configuration

  band disappears completely and only a constant signal is de-

  The luminous intensity recorded with the aid of the

  Thus, registremnent R presents the variation represented qualitatively in FIG. 2 as a function of displacement z of the reflector 52, a variation from which the pulse shape and the duration can be deduced.

  of the impulse P according to the methods of the analysis of

  to-correlation When the two magnitudes are uniquely correlated

  form with the coherence length of the emission for the pulse,

* Quantitative values of the pulse can be deduced.

  FIG. 3 represents an embodiment showing

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-11

  two retroreflection prisms perpendicular to the axes of the

partial cels.

  The elements of this embodiment have, here also, the same function as the corresponding elements of FIGS. 1 and 2. The prism R P1 can be moved perpendicular to the axis of the partial beam a over a distance ar 1, whereas the prism RP 2 can be

  moved by the motor M in the direction of the axis of the beam

  In accordance with the known properties of a refractive prism,

  troreflexion, a displacement of the prism R Pl over a distance A r 1

  perpendicular to the axis of the partial beam has resulted in a displacement

  parallel of these partial beams on 2 t Ar 1 in the region of the section B of the compound radiation beam.

  optical path length of the partial beam a does not undergo

  The partial beams a and b can be integrated only if they overlap after displacement on 2 b r 1 The interference configuration in this overlap region is not only dependent on the distance z 2 from the prism RP 2, but also from the

  when the behavior in time of the impulse

  P laser beam in different regions of the beam is different In general, this is the case when the emission is not homogeneous

  on the beam section or if various beam

  have different delays, for example when

non-homogeneous optical media.

  In the path of the beams of the radiation beam

  A diaphragm is inserted, whose opening can be moved in the radial direction and whose opening diameter WL can be modified. Behind the diaphragm L is provided a photosensitive detector D for converting the light intensity into the aperture. light intensity aperture which is modulated in the case of a displacement of the prism RP 2 into an electrical signal This signal is recorded on the vertical axis (Y) of the recording device R. The horizontal axis (X) is simultaneously excited by means of the motor M, in accordance with the distance z 2 between the prism RP 2 and the beam splitter ST The intensity at the location of the aperture of the diaphragm can be measured directly according to the distance z 2 for various displacement values

radial r 1 of the prism RP 1.

-12-

  In the case of a symmetrical position of the prisms of retro-

  reflection (S z = z 1 -z 2 = 0, a r 1 = 0), the overlap, both over time and over time, of the partial pulses

  and P 2 is maximal In the case of displacement of the PR 2 prism, the inten-

  sity is subject to periodic variations, which are the same in the

  the total beam B of the compound radiation beam, the period A z = p

  / 2 This period a z corresponds to a delay of the impulse

  p tial P 2 with respect to the partial pulse P i of Tp = \ / c 6.67 10-3 ps (>), where O o is the velocity of the laser light In the case of increasing displacement z, the Partial pulses are more and more displaced in time with respect to each other, so that the impulse portions that can interfere with one another become smaller and smaller. increasing

  becomes smaller For trips to z such as for

  time placements resulting it applies to T = 2 iz / c> A To, where To is the pulse width of the laser pulse) we only observe a constant intensity Similar qualitative behavior is determined when the prism R Pl is displaced in the direction perpendicular to the axis of the partial beam a and when, starting from the symmetrical position of the prism RP 2 (z 2 z 1 = z = 0), the amplitude of modulation is recorded, here also, depending on the displacement A z of the prism RP 2 However, in this case,

  the non-corresponding parts of the partial beams a and b overlap

  so that, if the time behavior of the laser pulse varies in the section of the beam, the maximum modulation regions move according to the displacement A r of the prism R P1 and

  of the displacement & r 1 of the diaphragm L The intensity curves

  registered with the recording device R therefore have a different variation for different values of S r 1, as shown in Figure 3 For the curv C, A z 1 is equal to zero and for the curve d, az n In this case, a complete analysis of these modulation curves with respect to the shape and position of maxima versus A r 1 and dea r L provides data on the coherence length of the impulse (duration of emission) and time and location structure of impulse

  (variation on the section of the beam).

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  FIGS. 4 and 5 show examples of interference configurations measured using the device according to the invention

  and the resulting variations of the auto-correlation function.

  FIG. 4 shows the intensity variations, as a function of z, in the compound radiation beam, as measured with a

  device according to Figure 1 As a pulsed light source was used

  a synchronously pumped synchronous pump color laser

  In this case, one period corresponds to a wavelength variation of 0.25 u / M respectively to a delay in time of the partial pulse P 2. of 5.7 10 3 ps (c Tp = 2 A zp / O /) From the variations of

  the amplitude of modulation of i z can be determined the variations

  The time-dependent characteristics of the Imo (A T) T mx (jAtT = 0) Imod () max correlation function (Figure 5 shows such a variation for the impulse of a synchronously pumped synchronous pump color laser. mode (curve K) and for a pulse of a krypton ion laser locked in mode (curve R) The upper horizontal scale m corresond with the curve K and the lower horizontal scale N with the curve L.

  Compared to state of the art devices

  that (fast detector + sampling, Streak camera, SHG), the dis-

  positive in accordance with the invention offers the following advantages: In all embodiments, the structure is compact, simple and inexpensive Adaptation to various light sources is possible without expensive adjustment Depending on the quality of the optical elements, measurement can perform without adaptation of these

  elements in a large spectral region of wavelengths of la-

  Because interference is a linear effect, it is possible

  to measure modulations for extremely high pulse powers.

  The sensitivity is correspondingly high and can be increased further, for example by means of the locking technique. Thus, the minimum pulse energies that can always be detected without a locking technique are, for example, about 10-9 Ws, while using the techniques of

  locking, they are about 10 i 12 Ws of the self-function

  correlation obtained according to the interference method, can be reconstructed the explicit shape of the optical pulse of a

25300 1 8

  A simpler way than in the case of non-linear SHG measurement; since the measured signal is directly proportional to the overlapping regions of the pulses, variations in the time of the pulse can be obtained from the simple reconstruction

  in the time of the self-correlation curve The resolving power

  in time is very high, whereas at the same time

  Calibration marks for the time axis can be obtained (1

  6 diode 6 67 10-3 ps (\), see Figure 4), simultaneously from the periodicity of the modulation variations on A z The resolution power over time is determined by the accuracy of the mechanical shift elements and exceeds 10 -2 ps The measure

  Interference modulation curves do not require detection of

  The motor speed can be chosen very slowly and corresponds to the sensitivity of the detector Pulse series

  fast repeats can be integrated as a "single light"

  "without the information on the pulse structure over time is lost A device according to the invention allows to

  to measure time delays in an enlarged optical impulse

  tially with great power of resolution over time, for example

greater than 10-1 ps.

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-15-

Claims (7)

  1 Device for determining the behavior in the   ultra-short optical pulse time, characterized by a division   beam detector, two reflectors provided in the paths of two partial beams formed by the beam splitter, reflectors of which at least one can be moved in the direction of the axis of the corresponding partial beam, a photosensitive detector, inserted into the beam, path of the partial beams which interfere and which are united in a single beam by the beam splitter and a system   for the processing of the detection signal.   2 Device according to claim 1, characterized in that an element for adjusting the intensity of this partial beam   is inserted into the path-of at least one of the partial beams.   Device according to claim 1 or 2, characterized in that   that a diaphragm is provided between the beam splitter and the   and its aperture diameter is adjustable and / or the aperture can be moved in a direction perpendicular to the axis of the compound beam. 4 Device according to claim 2 or 3, characterized in that   that the reflectors are planar reflectors.   Device according to claim 4, characterized in that   one of the reflectors is inclined relative to the axis of the beam corresponding tiel.   Device according to claim 1, 2 or 3, characterized in   that the reflectors are formed by prisms.   7 Device according to claim 6, characterized in that   prisms are retroreflective prisms.   8 Device according to claim 6 or 7, characterized in that at least one of the prisms can be moved in one direction   perpendicular to the axis of the corresponding partial beam.   9 Device according to claim 6, characterized in that the base plane of one of the prisms is inclined with respect to the axis the corresponding partial beam.