DE3317065A1 - Method and device for producing individual short laser pulses - Google Patents

Abstract

A method is described for producing individual short laser pulses, in which a laser medium which can be stimulated is excited by means of a pump pulse in order to produce a population inversion lying above a laser threshold value, and, after the emission of the short radiation pulse, the population inversion is held below the threshold value, which is characterised in that energy is initially extracted from the laser medium, which can be stimulated jointly by two resonators, by means of a first relaxation process at a predetermined threshold value with a relatively short time constant and a relatively low quality, and from the first resonator in the form of the desired individual short laser radiation pulse, and the inversion in the laser medium is subsequently held below the predetermined threshold value of the first resonance process, by means of a second relaxation process, of relatively long time constants and a relatively high quality in the second resonator which is coupled to the first. Furthermore, devices for carrying out this method are specified, which essentially contain two optical resonators of different length and different resonator life, whose optical axes both pass through one and the same active laser medium, which preferably consists of a dye solution.

Description

Method and device for generating individual

short laser pulses There are already a number of methods for Generation of short laser pulses in addition to the active medium and resonator of the laser other technical devices, such as Kerr cells, Pockels cells, Cuvettes with dye solutions, acousto-optical deflectors or rotating prisms or require mirrors and are therefore quite expensive. In other methods, e.g. in so-called synchronous pumping, to generate ultra-short pulses in the picosecond range there is no further effort to operate in the laser that generates the short pulses, but a considerable effort in pump laser, which in turn already has short impulses, even if only half-value of a few hundred picoseconds must generate.

From the publication by D. Roess: Giant Pulse Shortening by Resonator Transients; J. Appl. Phys. 37, 2004 - 2006 (1966) is a method of generation short laser pulses known in which a laser passes through another pulsed laser is pumped transiently and thereby generates relaxation oscillations, their shows Licher course from the temporal course of the pump pulse, the optical and geometric data of the active medium as well as the resonator data with the aid of the so-called rate equations are calculated karin.

These equations are two coupled nonlinear differential equations, the first of which is the change with time of the inverse ion of the active medium and the second the change in the photon density in the resonator over time (which is the output power proporti, onal is) describes.

A typical solution of these equations for an arbitrarily selected one An example and a specific pump pulse amplitude is shown in FIG. In this are from top to bottom the timing of the pump pulse (curve A), darlll der Ve r'1 äu f 'd (r inversion in the active medium (curve B) and finally the photon density in the resonator (curve C) is shown.

It can be seen that (as a result of the strong non-linearity of the process) The photon density takes the form of a series of separate pulses of decreasing Amplitude assumes. If you now decrease the pump light amplitude, it decreases also the amplitude of the generated relaxation impulses, until finally only the the first is left over, while the following no longer appear, as this is the case necessary swell inversion no longer achieved in the further course of the pump impulse will.

In order to achieve such a single pulse, the half-value is as short as possible to get 1 is (! 'i necessary to use a resonator life that is short opposite to is the pump pulse duration. Furthermore, the pump pulse amplitude must not be too high be used, since there is then the risk that a second relaxation oscillation pulse is already present can train. The optimal choice of the 'various parameters to achieve a as short a single pulse as possible with a given half-pulse width. the end of the publication .I.Q.

Yao: Optimum Operational Par ame-te r s Of 'rhe Ultrashort Cavity Laser Appl. Phys. Lett. 41, 136-138 (1982). One can in general With this method, pulses can be achieved that are up to a factor of one compared to the pump pulse 10 are shortened. Particular care must be taken to ensure that the Pump pulse amplitude does not exceed the threshold value for di (- 'Generation of the second relaxation pulse increases, see also Chinlon Lin "Studies of Relaxation Oscil lations in Organic Dye Lasers, IEEE Journ. Quant.

Electronics Vol. QE II, No. Aug. 8, 1975, 602-609.

Another method to reduce the duration of a laser pulse, consists in that one through the active medium of this laser during the pulse pass another laser beam in a different direction, its intensity is much stronger and thus quickly breaks down the inversion of the active medium, su that the original laser emission can no longer be sustained can and this original laser pulse aborts. The creation of the second, tlc first suppressive laser beam, can be done in various ways; e.g. by building a second resonator that connects to the first resonator has a common active medium. If the second resonator has a higher quality than the first so the number of photons in the second resonator is im generally grow in over the first resonator at a certain point in time and thus be in cier position, n, ie inversion in the common active medium snow I reduce it than the laser beam generated in the first resonator. This as well as various other methods with which laser radiation pulses up to a half-width of anything below 1 ns are described in the following publications: A. Andreoni, P. Benetti, and C.A. Sacchi: Subnanosecond Pulses brom A Single-Cdvity Dye Laser - Appl. Phys. 7: 61-64 (1975); A. Eranian, P. Dezauzier, and 0. De Witte: 2-nsec Pulses From Double Cavity Dye Laser. - Opt. Commun. 7: 150-154 (1973); H. Inomata and A. I. Carswell: Simultaneous Tunable Two-Wavelength Ultraviolet Dye Laser. - Opt. Commun.

22: 278-282 (1977); H. Lotem and R.T. Lynch, Jr .: Double-Wavelength Laser.

- Appl. Phys. Lett. 27, 344-346 (1975).

The aim of the invention is to provide a simple method with single pulses can be generated that are independent of the length of the pump pulse have a pulse duration that can be freely selected within wide limits.

This object is achieved by the method characterized in claim 1 solved. Further developments of this method and advantageous devices are part of it Implementation are the subject of subclaims.

In the invention, the two alternatives mentioned above are therefore used Procedures combined in a non-obvious way, namely, it takes place at the same time Exploitation of relaxation impulses through the settling of a short optical Resonator and the suppression of all further relaxation impulses after the first by a second, coupled resonator. Let through this combination Surprisingly, individual, very short laser radiation pulses are generated.

The present method, in which the two resonators common predefined stimulable laser medium by means of a first relaxation process Threshold value, relatively short time constant and relatively low quality energy the first resonator in the form of the desired single short laser radiation pulse is removed and then the inversion in the laser medium by a second Relaxation process with a relatively long time constant and a relatively high quality in the second, with the first coupled resonator under dz, ..

predetermined threshold value of the first resonator is maintained, and the present devices are generally applicable to those lasers which are in turn pumped by pulsed lasers. The procedure is characterized by surprising conceptual simplicity and the facilities for realization the process require a particularly low technical effort.

The following are exemplary embodiments of the invention with reference explained in more detail on the drawings.

1 shows a diagram to which reference has already been made above became; Fig. 2 is a schematic illustration to explain the Principle of the invention; Fig. 3 is a diagram of the time course of a stimulation or pump radiation pulse; 4 shows a diagram of the time course of an output radiation pulse of a laser according to an embodiment of the invention, and FIGS. 5, 6 and 7 are schematic Illustrations of preferred embodiments of the invention.

The present method of producing short individual coherent Radiation impulses are of such a general nature that it is in principle applicable to all types of laser can be used, but for the sake of simplicity, a very special example will be given first for the visible spectral range, which is particularly easy to see through and is of particular importance for the application. Figure 2 shows schematically a Laser device producing a dye cuvette with a square cross-section of 1 mm edge length. this cuvette is on two abutting surfaces with Mirror coatings M1 and M2 provided and on a third surface with an anti-reflective coating AR. Surface M1 is parallel to the mirror at a distance of a few centimeters Mirror M3. The three mirrors M1, M2, M3 should have the highest possible reflection coefficients have in the visible spectral range, for example from thick silver layers or even better because of the higher load capacity made of dielectric multilayers exist. The cuvette is in a Direction perpendicular to the plane of the paper traversed by a dye solution serving as a stimulable, active laser medium LM, E.g. from a Rhod (rnirl - (. (ß-solution of a suitable concentration, for example 10 3 mol / l. This solution is by the radiation of a pump laser P, for example pumped in such a way, for example by the pump laser beam PS so through a cylindrical lens C dividing line is focused, which lies in the plane of the paper and roughly along the diagonal extending from the puncture points of the distal edges of mirrors M1 and M2 is spanned by the plane of the paper. As a pump laser, for example, a Xenon chloride excimer lasers are used whose output radiation pulses the have the course shown in the oscillogram of FIG.

In the following only a qualitative consideration of the function is given but that can be done by setting up the corresponding rate equations and solving them the same was verified in quantitative detail with the help of digital computers. The active laser medium LM, in this case the rhodamine 6G solution, is from surrounded by two crossed laser resonators. The first, long resonator is used by the Mirror surfaces M1 and M3 formed, while the second resonator from the mirror M2 and the opposite, non-coated glass-air interface M4 of Cuvette is formed. The latter has a Fresnel's in the visible spectral range Reflection coefficient of approximately 0.04 if normal optical glass or quartz glass is used for the cuvette walls.

If now the cuvette with an intense, steeply rising pump pulse is irradiated, provided that the pump pulse amplitude is appropriate, it already arises in the rising part of the pump pulse a laser oscilloscope lation in short resonator, because of the short resonator length of only a little more than 1 mm very quickly settles into the steady state and thereby a first, very short relaxation pulse that emerges as a laser beam from the cuvette with a photodiode P or other suitable measuring instrument for examination of the temporal pulse course, for example with a streak camera, detected can be.

In the long resonator, on the other hand, the laser threshold becomes very large reached later, since a resonator length of a few centimeters is used here, thus a few times corresponding to ten times the resonator length of the short resonator. According to the invention, the length of this second resonator is now set so that The first relaxation pulse arising in this resonator occurs precisely when the first relaxation pulse from the short resonator has ended and the second The relaxation pulse of the short resonator has not yet started. With others In other words, the relaxation pulse begins in the long resonator in the time span between the end of the first relaxation pulse in the short resonator and the point in time at which the next relaxation pulse would begin in the short resonator, i.e. in Fig. 1 in the period between about 5 and 6 ns. The first relaxation pulse of the long resonator now builds the inversion of the medium below the laser threshold the corresponding critical inversion of the short resonator. She will after that too no longer rise significantly above this value and thus always significantly below remain the threshold value of the short resonator because of the high output losses 96% due to the uncoated surface of the cuvette a very high Sctlwellerl value In contrast to the long resonator, which has very low losses through the maximum having reflecting mirrors M1 and M2. It will be for the rest of the time Duration of the pump pulse a laser oscillation in the long resonator occur, but not in a short resonator, so that the goal is achieved at Generate pump pulses of any length only a single short laser pulse, which emerges from the non-reflective wall of the cuvette.

In Fig. 4 is in the upper part of the generated in this way Impulse reproduced as recorded with a streak camera. The scored Pulse half width is 120 ps. In the lower part there is a relaxation pulse train reproduced from the short resonator as it arises when the mirror M3 is removed or a piece of paper is placed between the cuvette and the mirror M3 suppress the laser oscillation in the long resonator. You can see very clearly the suppression of the second and generated by the action of the long resonator all subsequent relaxation impulses in the short resonator. An investigation into polarization of the laser beam from the short resonator showed that with unpolarized pump radiation the first relaxation pulse was completely linearly polarized, the electrical Vector lay parallel to the plane of the paper, while the later relaxation impulses only were partially polarized or from the superposition of two perpendicular to each other polarized pulses passed, which is the irregular sequence of the later relaxation pulses explained with the mirror M3 removed or blocked. This polarization of the first Momentum can be derived from the orthogonal position of the transition moments explain the emitting and absorbing dipole in the chosen dye molecule. In the case of other dye molecules, the two dipoles of which are parallel to one another, would result a perpendicular polarization of the first impulse. The later occurring relaxation impulses of different polarization when the Mirrors M3 can be explained by the rotation of the excited dye molecules in the solution.

A closer analysis of the rate equations shows that with higher Pump pulse amplitude, as already indicated in the lower half of FIG. 4, between the first and second relaxation pulse, the laser power is no longer quite goes down to zero. Nevertheless, if the length of the second resonator always completely suppress the further emission from the short resonator.

To the shortest possible half width of the generated in the short resonator To achieve a single pulse, the resonator life of the short resonator has to be achieved keep it as small as possible. The resonator life or time constant of a resonator mainly depends on the resonator length, i.e. the cycle time of the laser radiation and also depends on the reflectivity of the resonator mirror, see e.g. OPTICS COMMUNICATIONS 23, No. 3, Dec. 1977, 440-442. In the present case, the resonator life can be defined as the period of time in which the photon density in a passive Resonator (i.e. a resonator in which no photons are active in a laser medium generated) has dropped to 1 / e. This decline is due to the Decoupling of the laser radiation, e.g.

through a partially transparent mirror, and other losses, for example diffraction losses, reabsorption in the active medium. Absorption in the reflector layers, etc. All of this can be summarized in an effective reflectivity R and the resonator lifetime tc is then defined as follows for the case that R is not significantly different from 1 (ie for the case of high resonator quality): tc = L / ßc (1-R)) J, where L is the optical path length in the resonator (simple path), c the speed of light and R the effektí.ve reflectivity (in the case of high reflection factors R1, R2 of the two mirrors mean.

The shortest possible resonator life of the short resonator can once achieved by keeping the optical length of the resonator small in the present case only a few millimeters (the active medium is 1 mm long, the optical path in the short resonator is therefore 1 mm multiplied by the refractive index in the solution plus the optical path length in the two cuvette walls, the e.g. (IllS glass or quartz glass are made) and that the quality of the resonator can still be maintained keeps it low through a high level of coupling. It also shows a detailed analysis of the computational Solving the rate equations that it is particularly advantageous if one considers the proportion the spontaneous emission, which goes into the solid angle of the laser beam, as low as possible holds because this part of the spontaneous emission ignites and stops the laser oscillation initially slow fanning of the laser oscillation delayed the same until the pumping power has become very high and then a relaxation pulse with a steeper rising edge and accordingly steeper sloping edge can also be generated, so that overall the half width becomes very short. One can achieve this by looking at the transverse dimensions of the active medium is at most the same selects its longitudinal dimension, as has happened here, but advantageously even less. One could for example, instead of the reflection on the non-reflective surface of the cuvette use an external mirror that has small dimensions and to a certain extent The distance from the cuvette is set up parallel to the cuvette wall to be anti-reflective will. In this case one can also use spectral selection using filters, Prism or grating for the amplification of the laser oscillation in question Press the fraction of the spontaneous emission even further down to get the first relaxation pulse to shorten even further. Always is for the minimum achievable half width the rate of rise of the pump light pulse is significant. These can be e.g. increase by dividing the pump light pulse by first passing it through a saturable absorber can go, which in a known manner the low The initial intensity of the pump light pulse is largely suppressed and only the high one Lets the intensity of the pulse peak practically unattenuated.

It should not be difficult for the average person skilled in the art to find various indicate other embodiments of such coupled resonators in which- in a first relaxation pulse in the manner described in a short resonator is generated and in a longer resonator of higher quality then to a later one When a laser oscillation begins, all other relaxation oscillations suppressed in the short resonator. Some examples are in Figs. 5, 6 and 7 shown.

In the embodiment according to FIG. 5, the first resonator, in which the laser threshold value is first is enough, through two walls W1 and W2 of a cuvette K are formed, which are used as the active laser medium beis.j> ielswe-i sE 'contains a suitable dye solution. J) he one wall W 1 is with a die] .electric Multi-layer mirroring provided, which is designed so that it is for the Pump laser radiation P [i] has the smallest possible reflectivity for the wavelength However, the laser radiation LS has the highest possible reflectivity. The other Wall W2 has a relatively low reflectivity for the laser radiation wavelength of e.g. 10%, and a correspondingly high transmittance. The wall W2 can be designed so that the pump radiation as largely as possible into the cuvette reflected. The reflections are preferably located on the inner surfaces the walls of the cuvette, which are preferably made of gas or quartz.

The second, long resonator is limited by two reflectors M5, M6, which reflect the laser radiation as well as possible and are arranged so that the The radiation path between them goes through the wall W2 and is reflected on the wall W1 as can be seen from FIG.

The pump radiation is radiated into the cuvette through the wall W1, the laser radiation emerges in the opposite direction through the wall W2.

In the embodiment according to FIG. 6, the cuvette is designed similarly as in the embodiment according to FIG. 5. The second, long resonator is here, however designed as a ring resonator, for example, four reflectors M7 to M10 if possible high reflectivity for the laser radiation may contain. In the illustrated embodiment, reflectors are arranged so that the beam path in the ring resonator goes axially through the cuvette, i.e. essentially parallel to the two walls W1 and W2.

In the case of the laser devices according to FIGS. 5 and 6, the pump radiation also irradiated through one of the walls of the cuvette, not shown in these figures be, i.e. by one that runs parallel to the plane of the drawing. In this case then the reflectivity R of the wall W1 is not subject to any restrictions subjected to the pump laser wavelength.

In the embodiment according to FIG. 7, the first, short resonator is bounded by a first reflector M11 and a second reflector M12, which are perpendicular to a first optical axis ° 1, shown in dash-dotted lines, and in a predetermined one first optical distance are arranged along this axis. The reflector M11 has the highest possible reflectivity for the laser radiation, the second reflector M12 is designed to be partially reflective in the usual way, so that the laser radiation can emerge.

The second, long resonator is made up of two reflectors M13 and M14 limited, which have the highest possible eflexionsfator for the laser radiation should. The reflectors M13 and M14 are perpendicular to a second optical axis ° 2 and at a predetermined second optical distance which is substantially greater than the distance between the reflectors M11 and M12 is arranged.

The optical axes O and ° 2 intersect, i.e. they get lost as a whole essentially perpendicular to one another and intersect in a cuvette K, which contains the active laser medium. The walls penetrated by the radiation (J of the cuvette are preferably at Brewster's angle, at least with respect to the beam path 02 arranged.

This embodiment has the particular advantage that no wavelength-selective Mirrors are needed, so it is at least currently preferred.

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Claims (10)

Method and device for generating individual short laser pulses A method for generating individual short laser pulses, in which a stimulable laser medium, common to two resonators, by a pump pulse is excited to a population inversion lying above a laser threshold value to generate, and the population inversion after the emission of the short radiation pulse is kept below the threshold, characterized 9 that the stimulable Laser medium first predetermined threshold value by means of a first relaxation process, relatively short time constant and relatively low quality energy from the first resonator is taken in the form of the desired individual short laser radiation pulse and on- closing before the next short relaxation radiation pulse The inversion in the laser medium can develop through a second relaxation process relatively long time constant and relatively high quality in the second, coupled with the first Resonator is kept below the predetermined threshold of the first resonator. 2. Device for performing the method according to claim 1 with two optical resonators having different optical lengths, one in both resonators arranged, common stimulable laser medium; and one with the stimulable laser medium coupled excitation or pump radiation source and, characterized in that the optical resonators (M1, M3; M2, M4) and the pump radiation source (P) are dimensioned so that in the longer resonator (M1, M3) the laser emission threshold is reached after in the first resonator (M2, M4) a first laser radiation pulse has occurred and has ended and before the Threshold value for a new laser emission reached again in the first resonator will. 3. Device according to claim 2, characterized in that the first Resonator through two opposite, parallel walls one that can be stimulated Medium (LM) containing cuvette is formed, wherein the one wall is a highly reflective], ektierenden Has mirror (M2), while the other wall (M4) is designed to be partially reflective and that the second resonator is through a perpendicular to the first two walls, a highly reflective mirror (M1) having the wall of the cuvette and another highly reflective mirror (M3) is formed in the. Distance from the cuvette whose side is arranged opposite the third wall. 4. Device according to claim 3, characterized in that the fourth Wall of the cuvette is designed to be anti-reflective. 5. Device according to claim 3, characterized in that the first Resonator is formed by two parallel wound (W1, W2) of a cuvette, one of which one (Wl) has a high reflection factor for the laser wavelength and a low one Reflection factor for the pump radiation wavelength and the second a relative one low reflection factor and a relatively high transmission factor for the laser wavelength and that the second resonator is provided by two highly reflective (+ - rende mirrors (M5, M6) so arranged; that the beam path between these The reflection goes through the second wall of the cuvette and is reflected on the first wall becomes (Fig. 5). 6. Device according to claim X, characterized in that the first Resonator is formed by two parallel walls (W1, W2) of a cuvette and that the second resonator contains several highly reflective mirrors (M7 to M10) which form a ring resonator whose beam path is mentioned between the two Cuvette walls (W1, W2) goes through the cuvette. 7. Device according to claim 2, characterized in that the first Resonator is limited by two reflectors (M11, 12) which are transverse to an optical Axis and are arranged at a predetermined first distance along this, wherein the first reflector has the highest possible reflectivity for the laser radiation while the second reflector (M12) the laser radiation partially reflected, but mostly transmitted; that the second resonator by two Reflectors (M13, M14) are limited perpendicular to a second, the first crossing optical axis crosses the first at a predetermined second distance along this Axis, which is substantially greater than the first distance, are arranged; and that the active laser medium is arranged at the intersection of the two optical axes is. 8. Device according to 6, characterized in that the active laser medium is arranged in a cuvette, the walls of which at least with the second optical Axis are arranged at Brewster's angle. 9. Device according to one of claims 2 to 8, characterized in that that one resonator is at least five times longer than the other. 10. Device according to one of claims 2 to 9, characterized in that that the active medium is a dye solution.