DD145588A1 - OPTICAL-OPTICAL METHOD FOR THE CONCENTRATION AND SELECTION OF LASER IMPULSES - Google Patents

Abstract

Dia invention is intended by simple means a reduction of Laser pulses on pulse lengths in the range of 10 ~ ^ s to 10 ~ '' s and a selection of Eiazel pulses from the pulse trains ensure mode-locked laser. According to the invention this by the fast tuning of the optical path length of a formed as a Fabry-Perot interferometer semiconductor plate reached. The buckle tuning the optical path length and the associated tuning of the transmission of the semiconductor plate takes place by varying the refractive index n, which is determined by in produced the semiconductor plate generated free charge carriers becomes. These free charge carriers are either through the . shortening or to be selected laser pulse itself or generated by a cleaved from this laser pulse portion. This causes no synchronization problems, application area The invention is primarily the investigation faster kinetic Processes in solids, molecules and other atomic systems, -Fig.2 -

Description

Title of the invention

Optical-optical method for shortening and selecting laser pulses

Field of application of the invention

The invention relates to a method and an arrangement for shortening and selecting laser pulses.

Numerous applications of laser radiation require radiation pulses as short a temporal half- _{width ΠΓ} (10 "" ¹² ST _n £ -10 "^ s).

The field of application of such laser pulses is ultra-short-time spectroscopy. For example, the elucidation of many dynamic properties of solids, molecules, and other atomic systems. Thus one obtains among other things statements about relaxation and diffusion processes in solids and Fesikörperschichten, the z. B. are of great importance for the development of microelectronic devices, Bsslaxationsprozesse in molecules _s to problems of Beaktioifskinetik etc.

In the case of actively and passively Q-switched lasers, it is often necessary to produce an impulse shortening in the case of lasers which are active in lasers and which deliver pulse trains of several short individual pulses. A relatively complex pulse selection is necessary for this purpose. B, the shortening of the Strahlisigsimpulse of Q-switched low-pressure CO lasers and C3 hybrid lasers whose. Half-widths

*Q /CL >

in the range of 10 s £ t _k £ 10 s, as well as the selection of single pulses from the pulse trains of mode-synchronized CO ₂ ^ TS or Nd; YAG *. * Laser.

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Characteristic of the known technical solutions

For example, the current state of the art in shortening laser pulses of "X.sub.x, 40 ss can be characterized by the following examples: A known method (Appl. Phys. Letters 22, 660 (1975)) relies on the generation of high lacunar carrier layers in germanium by Hublstiaserbeschuß and the exploitation of the short-lived plas- sesareflexion to "excise" short pulses of CO2 (T _w some rs) from a much longer impulse (T ^>, 100 ns.) Ib in recent years (Appl.Phys. Letters 25, 580 (197 ² O intensively studied the method of ^iS Free Induction Decay ". Very short CO laser pulses (1ζ, ~ 100 ps) z. B. can be obtained by this process, as by means of a first optical Burefcfaruchs in nitrogen Pulses _with extremely steep trailing edge (JV-C 100 ps) and then sends them through a very narrow banded filter, which may be a Schiaalband iklisorber or a Fabry-Perot Inter ferometer (opt.Letters ly 161 (1977))

 Also used for shortening ns-CO laser pulses

Another method (Appl.Ptiys.Letters ^ 2, 598 (1978)) a schnei «lea switch on the basis of germanium, which makes use of the effect of the plasma reflection in two ways. Bsi could use a niodon-synchronized Nd: Slas laser as the pump laser for the switch

CO "-Raser" pulses of f ^ - Sps are generated. One possibility for shortening · ps pulses of mode-locked solid-state lasers is the combined use of single-ended and single-mode lasers. Multiphoton absorbers iAnn, d. Phys. " 21 (1976) p. 309). Another further known variant for the rapid switching of laser radiation is Appl. Phys «Letters ^ 0_, 280 (1977). , They use "an electro-optic crystal _t of in a Faljr ^ -.." Perot ~ interferometer (FPI) is by rapid change in the SpannuHg the crystal succeeds on the change in the optical path length of the FPI its transmission, ie "the Inten Parts of the first and third traversing ais are mainly the feofae apparatus expenditure, oils difficult synchronization of two

Pulse laser and the relatively low repetition frequency of the pulses, which is determined by the solid state laser. The disadvantages of the improved method result from the required optical oscillation, which on the one hand requires high laser powers (J-MW) or, on the other hand, brings about stability problems. The fourth ferfaaren allows only relatively low pulse shortening and ^"is:% rar 'of interest when applied to ps pulses is sufficient to ierfcämralichen method ers" It is a further disadvantage of this method is that organization with a certain Absorberkombi- only laser pulses. a relatively narrow wavelength

* rich can be shortened. Finally, the fifth method requires high quality crystals and a high voltage power supply with very short voltage rise times.

The current state of the art in the selection of individual pulses from a pulse train is characterized by the following examples: A pulse selection by means of a Pockels cell and a Glan-Thomson prism, which is located in the resonator of a mode-synchronized Nd; Glass lasers are located (Phys.Rev.Letters j21_, (1963)). By a fast thyratron switch, the Pockels cell was driven with an A / 2 voltage 1 so that with the help of the Slaa-Thomson prism a single pulse could be coupled out of the laser resonator.

The known method (Opt Comm · 2 , 215 (1970)) of pulse selection outside the resonator exploits the Kerr effect, generally Hiirobenzene. An optically ignited spark gap steers the KerrwZelie located between two crossed Glan-Thomson Prisraen so that a single pulse from the pulse train is selected by the second Glan-Thomson prism. ;.

In summary, it can be stated that many different variants of the single pulse selection by means of kerr and Pockels cells are already known. All methods require high voltages ("> 10 KV), fast pulse rise times (<Ins), and stable friggeriapulse to ensure satisfactory operation, and exact synchronization of the cooking pause and laser pulse is required.

Object of the invention

The object of the invention is, on the one hand, to provide a method for shortening laser pulses to half-value widths in the range of IG * s * c <ΐ * _Η z> 10 * s, which is characterized by the following features. The apparatus required for pulse shortening should be as much as possible In particular, the synchronization problems that have arisen in many methods used so far should be avoided.

The pulse shortening should already be effective for relatively low pulse power * {^ 1 (Ar) *

Bas method should ensure the highest possible repetition frequency of the laser pulses.

The shortened laser pulses should have the highest possible amplitude stability and low losses in the peak power. Me impulse length should be practicable * as far as possible to be tunable. .,.

The method is intended to ensure the shortening of laser pulses of the widest possible wavelength range and on the other hand to provide a method for the selection of individual pulses from the pulse trains mode-synchronated laser, which is characterized by the following features:

Applicability, even in the case of long-term succession of impulse trains, low losses in the peak power of the selected individual

Efficacy over a wide wavelength range.

Explanation of the essence of the invention

The invention is based on the object of providing a method for shortening and selecting laser pulses. If a Fabry * »PerQ ^ interferometer (FPI) is irradiated with light of wavelength A and power P, the optical path length nd (« - »refractive index of the medium between the reflecting surfaces, d - their distance) is shown Power Pr is a sequence of Airy's terrestrial maxisia with the period Δ- ^ / 3, the amplitude of which in the lossless case corresponds to the value ven P »

- 5 - ig ^

If it is now possible to change the refractive index η so quickly that, for example, rapidly reaching e.g. is swept over a full period of the function P (nd), the momentary "on-hook" of the FPI when sweeping the maximum transmittance can act as an "optical gate" useful for pulse shortening or pulse selection. In pulse shortening, the opening time of the optical gate T must be small against the half width ^ of the incident pulse; In addition, the power maximum of the incident impulse should coincide with a transmission maximum of the FPI. In the pulse selection, the optical gate must be just opened so that only one pulse of the incident pulse train (possibly the pulse of maximum power) is transmitted with the least possible loss.

The main component of the switch is a plane-parallel semiconductor plate, which acts as an FPI. To increase the FPI quality, the end surfaces of the plate can be mirrored to the desired extent. The transmittance T of such a plate is described by the following equation:

with d - thickness of the semiconductor plate η - refractive index

K - »reflectance of the two end faces <£ '·· absorption coefficient of the semiconductor

The fast tuning of T should be done by a variation of η. In this case, the required change in the refractive index ¹ Δη can be caused by the generation of sufficiently many free charge carriers N in the semiconductor. Prerequisite for the function of this method are on the one hand Rekombinations- and diffusion times that are as large as possible against the pulse duration of the incoming Laserirapulses or dia duration of the Irapulszuges oino sufficiently strong on summation u.ng the free charge carriers during, dor irradiation time in To allow focus volume. On the other hand, a sufficiently homogeneous charge carrier toleration over the beam cross-section of the beam to be switched must be present.

. IfSSi A

The relationship between & s and N is in the Drudenäherung by

Gl, 2 given:

with «a, m - effective masses of electrons or holes

tr - angular frequency of the laser radiation

e · Eleraentarladutig

As from Eq. 1, the absorption coefficient of the semiconductor plate also plays an important role in the course of the function E ₁ - (n - d). If the product oC-d is of the order of 1 », there is a considerable reduction in the maximum transmission and a deterioration in the quality of the FPI. The contribution of the free charge carriers to cL is given by Eq. 3 given:

with jüu up ~ mobility of the electrons or holes

c - vacuum speed of light

In unfavorable conditions of ta undu. the influence of vontn the TransKsission compared to the influence can be so great that no optical-optical switch with a- such a semiconductor material (OÖS) can be realized "In many Halbleiterniate" rials habes ta ursdyU- jedocfe such values "that the Switching process from Af £. is not affected. In some favorable cases, it is even so that the first {desired) shift is affected by absorption aterial not v /, while the white "· direct growth \ ^r on N * possibly still occurring (unwanted)

Transmissionm xiiaa by the then already essential δ <£ are greatly reduced *

Such a case must be realized in particular in the selection of a single pulse from a pulse train.

As processes which lead to the generation of the free charge carriers, increase in multiphoton absorptions of the set laser pulses come into question * The laser radiation depends on the length of the laser beam and the band gap Ig of the lead particle it hangs, which process occurs. The lasansiienhatig between the ira Zeitintervall dt he "enε'-free charge carriers dN and the irradiated laser intensity I: the y / lläläge λ .; is calculated for the four practically usable slopes by the following equations k. described:

i S # 4 7

d Tv ^- ^ ^ - AX ^ eii: one- photon absorption of Nq ts. RT ^ eLi: two-photon absorption. ., _ ' ¹ - _T. Two-photon absorption /

1 h / T) T 2 ι χ three-photon absorption

1, Bt C and D are the absorption rates of the corresponding processes, I ₁ the intensity of the green wave and I the intensity of their second harmonic. The Gl. 4. show that the type of absorption process with a given temporal pulse profile of the laser radiation has a significant influence on the rapidity of the generation of the free charge carriers and thus on the switching process. He is fastest for the three-band absorption.

The use of single-photon absorption is possible only when using very thin semiconductor plates (d 4. 10 m), while the use of Mehrphotonenabsorption allows the transmission of larger thicknesses. An essential criterion for the choice of the plate thickness is the desired speed of the switching process, this is in principle limited by the construction time t of the field in the 3PPI. It is determined by d, η and R of the following;

So z. eat. for a Si plate (n = 3.565) with the beading d s 100 / Um and a reflectivity B = 0 »*) a t. from c & 12 ps.

On. An essential feature of the invention is for all lusführungsvarianten, which may be very different in detail, in that the switching (of the free carrier generating) and the switched (the FPI effect exploiting) pulse are obtained from the same laser pulse. As a result, no synchronization problems occur, since the only synchronization required is the collapse of a transmission maxima of the FPI with a desired point in the temporal power profile of the incident pulse or pulse train simply by tempering of the parallelepiped semiconductor plate (hereinafter referred to as switching crystal).

embodiment

The described method can be realized in various embodiments, which can be adapted to the particular application. The variants described below can be used especially for pulse shortening or selection of CO laser * radiation. They are based on the simultaneous absorption of 3 laser photons (ThPA) and the simultaneous absorption of 2 photons of the 2nd harmonic laser radiation (TPA) ia tellurium single crystals

 Embodiment 1 according to Fig. Ii

Biese variant represents the simplest possible realization »The incoming laser pulse, which simultaneously represents the switching beam (for the production of the free charge carriers by IhPA) and the beam to be switched» is focused by means of a lens 1 on a switching crystal 2. Ba only in the center of the spatial intensity profile, the stranding of the generated free charge carriers is sufficiently homogeneous, the switched central parts of the beam are hidden by means of a diaphragm 3. This variant is characterized by its antecedent structure and is particularly suitable for experiments ₈ which require only low radiation power, so that the large losses in the 0OS play a minor role

 Ausführangsbeispiel 2 according to FIG. 2:

This variant is intended to ensure that practically over the entire cross section of the beam to be switched, the switching process has the same time characteristic. Two preconditions must be fulfilled. On the one hand, the beam to be switched on its own must not generate any charge carriers and, on the other hand, it must only pass through the central parts of the focus volume of the switching beam. For this, the incoming I, aserimpuls _s whose. Wavelength / Ξ- 11,2 / Ura must be to avoid the ThPA au; through the beam splitter k. divided into the switching and the beam to be switched. Via a deflecting mirror 5, the switching beam leached a lens 6 _S which focused ilia into a unit ^{7 for the} generation of the 2nd harmonic of the gyro wave (SIIG 1 unit),

4 | • 9 * *

The 2-harmonic is focused on the switching crystal 2 by passing through a filter 8 "which serves to block off the grand wave, by means of a lens 9 at a small angle to the incidence solder. The Te-switching crystal is antireflection-coated for the '2nd harmonic and mirrored to the desired extent for the fundamental wave'

The beam to be switched passes via a delay line 10 to a short focal length lens 11, which bundles it at a vertical incidence in the focus volume of the switching beam.

"This variant is characterized by the fact that the spatial beam profile of the switched beam is practically maintained and the power losses are relatively low.

Claims (5)

invention claim ! 1. Optical-optical "learn for shortening and selecting Lsserimpulsen _t characterized in that by He" - generating free Ladsngstrager by means of a switching light beam on a "or Mehrphötonenabsorption in a medium, the optical path length of an interference-capable assembly is dsrchgestiramt such a way that a to be switched beam., is of the same beam of light as the switching beam herrUiirt _9, a shortened pulse * excised ^, or a single pulse from a selected in the "pulse sequence. 2. Arrangement for carrying out the method according to item 1, characterized in that in the beam path of a focused laser beam of sufficient power that at the same time the * agile free charge carriers provided switching and the beam to be switched embodied, a vertically irradiated, plaaparallel and on both sides teilverspie- gemte semiconductor plate and behind it a means for blanking the switched central parts of the beam contains. 3 «arrangement for carrying out the method according to item I _8, characterized in that they for dividing the laser beam in the sclialtenden and the beam to be switched a Stra.hlteiler, a SHG-sincerity in the beam path of the switching beam, means for focusing the scarf. to a fundamental beam reflecting crystal reflective to the fundamental wave, which is arranged such that its incidence slot subtends a small angle with the incident switching beam and means for time delay and vertical focusing in the center of the focus volume of the switching beam. Λ. .- 11 - Arrangement according to item 3 »characterized in that means are provided for the spatial separation of the portion of the beam to be switched, which is reflected by the switching crystal at a small angle to the incidence solder, and its further use as a switching beam. 5. Arrangement for carrying out the method according to item 1, characterized in that in the beam path of the focused at a small angle to the incident slot on the switching crystal switching beam behind the switching crystal, with this a PPI high quality forming transparent plate is arranged and that means are provided, of the reflected at the front surface of the switching crystal portion, which embodies the beam to be switched to lead perpendicular to the back of the transparent plate and to focus in the focus volume of the switching beam. J,