RU2684929C1 - Apparatus for compressing optical impulse on diffraction plates with possibility of control of compressed impulse duration - Google Patents

Abstract

FIELD: optics.SUBSTANCE: invention relates to laser equipment and relates to an optical pulse compression device. Device includes a housing, a fixed platform on which tables with positioners of diffraction grids are installed and a mirror installed on guide rods movable platform with adjusting screw, which axis is parallel to guide rods, and tables installed on movable platform with positioners of diffraction grids. Diffraction gratings form two mirror-symmetric groups of one fixed and one movable grating with parallel working surfaces of the grids in each group. Mirror is mounted on fixed platform such that laser beam incident on mirror diffracted by one of fixed gratings and reflected from it beam propagate in one plane, perpendicular to propagation plane along optical system of laser pulse device.EFFECT: technical result consists in expansion of dispersion range of unit pulse speed of device.1 cl, 11 dwg

Description

The invention relates to the field of quantum electronics, namely, laser technology, and can be used to compress the optical laser pulse obtained in the technique of mode synchronization, in order to reduce its duration and increase the peak power. The device can also be used to study the compressibility of the pulse and search for the minimum pulse duration.

Currently, almost all laser systems that generate high power pulses use the CPA method (chirped-pulse amplification), amplify ultrashort pulses stretched to several nanoseconds, and then compress them to their original duration [Strickland D., Mourou G. Opt. Commun., 56, 219 (1985)]. In traditional CPA systems with direct amplification, as well as in systems based on parametric amplification of optical parametric CPA chirped pulses [Dubietis A., Butkus R., Piskarskas A.P. IEEE J. Sel. Top. Quantum Electron., 12, 163 (2006)], to compress a chirped laser pulse, laser compressors are used on one pair of parallel diffraction gratings [Treacy Edmond B. IEEE J. Quantum Electron., 5, 454 (1969)], providing a negative group dispersion speeds (long waves stay relatively short). The optical system of a laser compressor on one pair of parallel diffraction gratings is known [Treacy Edmond V., Optical Pulse Compression With Diffraction Gratings, IEEE Journal of Quantum Electronics, vol. 5, pp. 454, 1969]. This system uses a pair of parallel diffraction gratings at an angle to the laser beam to compress a pulse of light radiation. The angle of incidence on the first diffraction grating relative to the normal is chosen near (~ ± 10 °) the angle of the Lithuanian diffraction grating in order to ensure a high reflection coefficient of ≈92-95%. For laser radiation with a central wavelength of 1053 nm, the angle of incidence on the first diffraction grating is ≈55-75 °. The use of such an optical scheme to achieve the desired group dispersion leads to the need to increase the size of the diffraction gratings, which is technologically complicated.

From US Pat. No. 7,684,450 B2 (IPC: H01S 3/08, H01S 3/10; 03/23/2010) a variable compressor selected as a prototype on the diffraction gratings of a laser pulse source is known as a prototype. As follows from the description of this compressor, it contains the first, second diffraction gratings and a mirror. The second grating is located on a movable platform made with the possibility of moving the second grating in a direction perpendicular to the plane of the mirror. The optical path of the laser radiation passes between the first and second gratings and the mirror. The laser beam received through the input signal to the compressor, falls on the first grating, diffracts from it and then goes to the second grating, then diffracts from the second grating to the mirror. Then the beam is reflected from the mirror and returns back to the second grating and diffracts from it to the first grating. The beam then diffracts from the first grid back through the compressor inlet. The second grating is located on a movable platform made with the possibility of moving the second grating in a direction perpendicular to the plane of the mirror. The movement of the moving platform with the second grid is controlled by the controller. The controlled movement of the second grating provides a change in the optical path length of the laser beam in the compressor, and, thus, a controlled change (regulation) of the dispersion of the group velocity of the compressor. In this technical solution, a change in the dispersion can also be carried out by controlled movement of the first grating, the first and second gratings simultaneously, as well as by moving the mirror. As noted in the description of this compressor, the number of grids can be more than two.

The main disadvantage of this device is the limited range of adjustment of the dispersion of the device, since The use of such an optical scheme to achieve the greatest possible expansion of the range of variation of the dispersion, due to the increase in the size of the light beam and the large angle of incidence, necessitates an increase in the size of the diffraction gratings, which is technologically complicated. At present, the overall size of the available diffraction gratings is ≈500 × 400 mm.

The technical result achieved by the claimed invention consists in expanding the range of change (regulation) of the dispersion of the group velocity of the pulse device.

The technical result is achieved by the fact that the optical pulse compression device on diffraction gratings with the ability to adjust the duration of the compressed pulse, which contains fixed and movable diffraction gratings mounted on a movable platform, as well as a mirror, differs from the prototype in that it contains a body: with a fixed platform on which tables with diffraction grating positioners and a mirror are installed; with a movable platform mounted on guide rods in the housing; with an adjusting screw with a handle installed in a threaded hole of the movable platform, the axis of which is parallel to the guide rods, which together form the mechanism for moving the mobile platform along the guide rods along the axis of the screw; with tables mounted on a movable platform with diffraction grating positioners. The tables with the diffraction grating positioners are rotatable relative to their vertical axis, and the diffraction grating positioners are made with diffraction grating holders and the adjusting screws of the holders, thus ensuring the required spatial orientation and fixation of the diffraction gratings installed on the tables. Diffraction gratings form two mirror-symmetric with respect to a plane passing through the axis of the adjusting screw of the moving platform and perpendicular to the propagation plane through the optical system of the laser pulse device, groups of one fixed and one moving grating with parallel arranged and facing each other working planes of the gratings in each group. The mirror of the optical system of the device is mounted on a fixed platform so that the laser beam diffracted by one of the fixed gratings and reflected from the fixed gratings propagate in the same plane perpendicular to the propagation plane through the optical system of the laser pulse device. In this case, the input laser beam, directed perpendicularly to the direction of movement of the moving gratings and to the plane of symmetry of two groups of gratings, goes to another fixed grating, oriented to the input beam with an angle of incidence α, which is chosen from the relation

where: λ _с is the central wavelength of the spectrum of a laser pulse entering the device, d is the period of the diffraction grating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of the optical system of the device, where: 1 — the laser beam entering the device, incident on angle α, on a fixed diffraction grating 3; 4 and 5 - movable diffraction gratings placed on the movable platform 8 conventionally indicated in the diagram, having the ability to move in the directions indicated by the line with arrows; 6 - the second fixed grid; 7 - a mirror; 20 - boundary wavelength component of the laser pulse spectrum with a wavelength λ _r ; 21 - component of the spectrum of a laser pulse with a central wavelength λ _with ; 22 - boundary short-wave component of the laser pulse spectrum with a wavelength λ _b .

FIG. 2 is a diagram showing the fulfillment of the criterion for the conservation of the spectral composition of a laser beam diffracted from a moving grating, where: L is the width of the diffraction grating; M - the distance between the centers of the fixed and movable gratings.

FIG. 3 is a diagram showing the violation of the criterion for the conservation of the spectral composition of a laser beam diffracted from a moving grating, where: L is the width of the diffraction grating; > M is the distance between the centers of the fixed and movable grids, exceeding the maximum possible value.

FIG. 4 shows a general view of the device, where: 3 and 6 are fixed diffraction gratings mounted on a fixed platform 13; 4 and 5 - movable diffraction gratings mounted on the movable platform 8; 7 - a mirror (in the figure it is shown as one of the possible variants, an option with an angle mirror); 8 - mobile platform; 9 - device case; 10 - guide rods; 11 - adjusting screw with a handle 12; 13 - fixed platform; 14 - rotary tables; 15 - diffraction grating positioner.

FIG. 5 shows a device with a cross section in a vertical plane passing through the axis of the adjusting screw 11 with the handle 12, showing the mechanism for moving the movable platform 8 mounted on the guide rods 10 The adjusting screw is installed in the threaded hole 23 of the movable platform. Rotate the handle 12 to move the platform 8 along the guides 10.

FIG. 6 shows the diffraction grating positioner 15, where 16 is the grating holder 16 installed in the positioner; 17 - holder adjusting screws, with the help of which the grating is positioned and fixed.

FIG. 7 shows a view of the device, demonstrating the passage of a laser beam through the optical system of the device, where: 1 is the laser beam entering the device; 2 - - laser beam entering the device.

FIG. 8 - FIG. 11 with a specific example demonstrate the results obtained in the implementation of this invention:

in fig. 8 shows the pulse profile before compression (line 24) and after compression (line 25);

in fig. 9 shows the initial pulse (line 26) and its chirp (line 27);

in fig. 10 shows a compressed pulse (line 28) and a compressed pulse chirp (line 29);

in fig. 11 shows the spectrum of a compressed pulse.

The optical system of the device, as shown in FIG. 1, consists of four diffraction gratings and one mirror. Two lattices (3 and 6) are installed stationary, two others (4 and 5) are located on a mobile platform, the mirror is installed stationary. The operation of the device is based on the phenomenon of group velocity dispersion, which determines the relative time delay of the spectral components of a pulse in the device due to the passage of spectral components of a pulse of optical paths of various lengths between the points of entry into the device and the output.

The incoming pulse 1, as shown in FIG. 1 and FIG. 7, enters the grating 3 and diffracts from it into the grating 4. From the grating 4 spatially separated spectral components of the pulse diffract in parallel into the grating 5. From the grating 5 spectral components diffract into the grating 6. From the grating 6 the spectral components of the pulse diffract in a reconstructed original form spatially combined component of the pulse and the transverse beam profile. Then the beam hits the mirror 7. The mirror can be made both flat and angular. A corner mirror 7, as shown in FIG. 7, in the first action, reflects the beam in a direction perpendicular to the plane of propagation of the pulse, and in the second, it directs the beam in the opposite direction through the gratings from 6 to 3 in a plane parallel to the plane of direct passage of the beam. Beam 2 coming out of the device is spatially divided with incoming beam 1. When using optical paths instead of an angle mirror, the optical paths of the rays during forward and reverse travel will coincide.

Adjusting the magnitude of the dispersion of the group pulse velocity of the device is performed by moving the movable platform 8, on which the grids 4 and 5 are located.

The decision to implement a device with adjustable dispersion based on an optical system consisting of four diffraction gratings, the simultaneous movement of two of them, based on the choice of the orientation of the gratings with respect to the incoming beam, as well as the selection of the parameters of the gratings (diffraction grating period). The criterion for selecting the device parameters (the angle of grating orientation with respect to the incoming beam, the grating period) is the need to preserve the spectral composition of the pulse when the beam is diffracted from moving gratings 4 and 5, provided that the incoming beam is perpendicular and the moving direction of moving gratings 4 and 5. When moving a pair of moving of gratings, the illumination area of these gratings by the spectrum of the diffracted beam must not extend beyond the boundaries of the working surface of these gratings. The fulfillment of this condition is shown in FIG. 2 and FIG. 3. The position of the movable grid 4, as shown in FIG. 2, is the most distant from the fixed grating 3, in which the spectral composition of the beam diffracted from the grating 4 to the grating 5 is still conserved. If the distance between the gratings exceeds the maximum allowable value, this will lead to the loss of a part of the spectral components of the pulse, as shown in FIG. 3, and to malfunction of the device. For the device to work properly, it is necessary to determine the range of parameters for which the criterion of conservation of the spectral composition of the pulse passing through the device is fulfilled.

The diffraction grating equation is well known (Agraval G., Nonlinear fiber optics: Trans. From English - Moscow: Mir, 1996. - 323 p., P. 150.):

here α is the angle of incidence of the beam on the diffraction grating, β is the angle of diffraction of the beam from the diffraction grating, λ is the radiation wavelength, d is the period of the diffraction grating. Angles α and β are defined in the area

We write equation (1) for the shortwave and longwave boundaries of the spectrum:

Here βb, βr are the diffraction angles of the incoming beam from the grating 3 for the shortwave and longwave boundaries of the spectrum, respectively, λb, λr are the wavelengths of the shortwave and longwave borders of the spectrum (Fig. 2 and Fig. 3).

A necessary condition for the conservation of the spectral composition of the pulse upon reflection from the grating 4 is to limit the position of the boundaries of the illuminated region of the grating 4 by the spectrum diffracted from the grating 3 of the beam. It can be shown that this condition is satisfied for the short-wave part of the spectrum if:

and for the long-wave part of the spectrum, if:

- ширина подвижных решеток, М - расстояние между центрами неподвижной и подвижной пары решеток.Here

- the width of the movable gratings, M - the distance between the centers of the fixed and moving pair of gratings.

получаем максимальные величины углов βb, βr отклонения спектральных компонент границ спектра импульса. Подставляя эти величины в систему уравнений (2) можно рассчитать величины длин волн λb, λr границ спектрального диапазона работы устройства при перемещении подвижных решеток 4 и 5 на расстояние М.Setting the incoming angle a and solving the system (3) for a given distance between the pairs of gratings M and the width of the gratings

we obtain the maximum values of the angles βb, βr of the deviations of the spectral components of the boundaries of the spectrum of the pulse. Substituting these values into the system of equations (2), one can calculate the wavelengths λb, λr of the boundaries of the spectral range of the device when moving the moving gratings 4 and 5 by the distance M.

And back, specifying the wavelengths λb, λr determining the required spectral range of the device, you can calculate from (2) the deviation angles βb, βr of the data of the spectral components. Substituting these values into (3) for a given lattice width, one can obtain the maximum magnitude of the displacement of the lattices M at which the operation of the device remains correct.

Thus, the systems of equation (2), (3) determine the region of parameters for the correct operation of the device in which, with simultaneous movement of the gratings 4, 5, the incoming beam diffracted from the grating 3 remains within the working area of the gratings 4 and 5.

The dispersion of the proposed device is equal to (Agraval G., Nonlinear fiber optics: Trans. From English. - M: Mir, 1996. - 323 p., P. 151.):

Here ω _с is the carrier frequency of the pulse, β _с is the angle of diffraction of the beam of the carrier frequency of the pulse, d is the period of the diffraction grating, M is the distance between the centers of the gratings.

The angle of diffraction of the beam of the carrier frequency of the pulse β _with is determined by solving the equation:

The optimal angle of incidence of the incoming beam α corresponds to the case when the angle between the diffraction direction of the spectral component of the pulse with the central wavelength from the grating 3 and the direction of the beam entering the device is straight:

α-β _c = 90 °.

Then from (5) we obtain the equation for the optimal angle of the beam entrance to the device:

The design and operation of the device are explained in FIG. 1 - FIG. 7

The device comprises a housing 9: with a fixed platform 13, on which tables 14 are installed with positioners 15 diffraction gratings and a mirror 7; mounted on the guide rods 10 in the housing of the movable platform 8; with an adjusting screw 11 with a handle 12 installed in the threaded hole 23 of the movable platform, the axis of which is parallel to the guide rods, which together form the mechanism for moving the mobile platform along the guide rods 10 along the axis of the adjusting screw; The incoming beam 1, with a spectrum limited by the wavelengths λb, λr (Fig. 1), directed perpendicular to the direction of movement of the movable platform 8 with moving gratings 4 and 5 and to the plane of symmetry of two mirror-symmetric groups of gratings, falls at an angle α on the grating 3 The angle α is chosen from the ratio

where: λ _с is the central wavelength of the spectrum of a laser pulse entering the device, d is the period of the diffraction grating.

The gratings are mounted on tables 14 with positioners of 15 diffraction gratings. Diffraction gratings form two mirror-symmetrical with respect to the plane passing through the axis of the adjusting screw 11 of the movable platform 8 and perpendicular to the propagation plane through the optical system of the laser pulse device 1, groups of one fixed and one movable grating (grating 3 - grating 4 and grating 6 - grating 5 ) with parallel located and facing each other working planes of the gratings in each group. The tables 14 are rotatable about their vertical axis, and the positioners of the 15 diffraction gratings are made with holders of 16 diffraction gratings and adjusting screws of the holders 17, thus ensuring the necessary spatial orientation and fixation of the diffraction gratings mounted on the tables 3, 4, 5 and 6.

From the lattice 3, the beam diffracts in the angular range bounded by the angles βb and βr,

on the moving grating 4. From the grating 4, the beam diffracts into the grating 5. From the grating 5, the beam diffracts into the grating 6. From the grating 6, the spatially reconstructed beam diffracts into the mirror 7. The mirror 7 of the device’s optical system is mounted on a fixed platform 13 in such a way that A laser beam diffracted by a fixed grating 6 incident on a mirror and a beam reflected from it propagate in the same plane perpendicular to the propagation plane through the optical system of the laser pulse device. The mirror 7, at the same time, can be both flat and angular. Angular, as represented in this particular example in FIG. 7, mirror 7 converts the beam into a plane parallel to the plane formed by the incoming beam as it propagates from lattice 3 to lattice 6. From mirror 7, the beam is reflected and passes the opposite way to that entering in parallel plane passing through the lattice from 6 to 3.

Lattices 3 and 4, as well as 5 and 6 are parallel to each other. The angle between the gratings 4 and 5 is equal to twice the angle α of the incident beam on the grating 3.

When moving the movable platform 8 from the fixed platform 13 increases the length of the optical path, and, consequently, the dispersion of the device. In this case, due to the fact that in this technical solution there is a simultaneous movement of two lattices, then with the same lattice sizes, we are able to double the range of variation of the dispersion.

Thus, the essential features of this technical solution allow to achieve the stated technical result.

An example embodiment of the invention

FIG. 8 and FIG. 11 shows an example of a pulse compression device. The profile of the pulse before compression (Fig. 8, line 24) and after compression (Fig. 8, line 25) is shown. FIG. 9 shows the initial pulse (line 26) and its chirp (line 27), the corresponding squeezed pulse (line 28) and the squeezed pulse chirp (line 29) are shown in FIG. 10. The spectrum of this pulse is shown in FIG. eleven.

The initial pulse had a width of about 20 ps (half the height of the pulse). The rate of change of chirp in the middle part of the pulse is 0.1 nm / ps.

The maximum compression of this pulse by the device was obtained with a device dispersion equal to -2.21 ps ² .

The profile and chirp of a compressed pulse is shown in FIG. 10. The duration of the compressed pulse was about 1 ps. It can be seen that in the middle part the compressed pulse is almost cleared.

Calculate the parameters of the device for a given value of the dispersion.

For a diffraction grating of 600 lines / mm, using (6), we calculate the optimal angle α of the orientation of the grating 3 for a central wavelength of 1035 nm. We get that α = 1.24 rad. For a given α, using the system of equations (2), we obtain βb = -0.335 rad, βr = -0.328 rad. Using (3) taking into account the fact that the transverse size of the lattice is 2 cm, we obtain the maximum possible distance between pairs of gratings 2.4 m.

Thus, the range of parameters for the correct operation of the device for a given pulse spectrum and lattice parameters has been determined (the optimal lattice orientation angle and the displacement range are indicated).

The distance between the grids required for these device parameters, at which the considered pulse is maximally compressed (Fig. 8, line 25) and the dispersion is -2.21 ps ² , is calculated using (4). We get: M = 0.69 cm.

Claims (3)

A device for compressing an optical pulse on diffraction gratings with the ability to adjust the duration of a compressed pulse, comprising stationary and movable diffraction gratings mounted on a movable platform, as well as a mirror, characterized in that it includes a housing with a fixed platform on which tables with diffraction gratings and a mirror are mounted; with a movable platform mounted on guide rods in the housing; with an adjusting screw with a handle installed in a threaded hole of the movable platform, the axis of which is parallel to the guide rods, which together form the mechanism for moving the mobile platform along the guide rods along the axis of the screw; installed on a movable platform tables with diffraction grating positioners; the tables with the diffraction grating positioners are rotatable relative to their vertical axis, and the diffraction grating positioners are made with diffraction grating holders and the adjusting screws of the holders, thus ensuring the necessary spatial orientation and fixation of the diffraction gratings mounted on the tables; the diffraction gratings form two mirror-symmetrical with respect to a plane passing through the axis of the adjusting screw of the moving platform and perpendicular to the propagation plane through the optical system of the laser pulse device, groups of one fixed and one moving gratings with parallel arranged and facing each other working planes of the gratings in each group; the mirror of the optical system of the device is installed on a fixed platform so that the laser beam diffracted by one of the fixed gratings and the beam reflected from it propagate in the same plane perpendicular to the propagation plane through the optical system of the laser pulse device; while the input laser beam, directed perpendicular to the direction of movement of the moving gratings and to the plane of symmetry of the two groups of gratings, goes to another fixed grating, oriented to the input beam with the angle of incidence α, which is chosen from the relation

, where λ _с is the central wavelength of the spectrum of a laser pulse entering the device, d is the period of the diffraction grating.

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