DD297039A5 - ARRANGEMENT FOR FIELD-LIGHT GLAUTION AND SELECTIVE BACK-COUPLING OF LASERARRAYS - Google Patents

Abstract

Arrangement for far-field glazing and selective feedback of laser arrays. The arrangement is suitable, even with closely adjacent channels with strong structuring, as z. As is the case with gas laser arrays, to achieve a usable far-field smoothness. This can be achieved by using a 4f design by locating a phase structure in the Fourier plane of the array to compensate for the phase field of the array field, adapted to the structure of the coupled array field. Far field; Smoothing; Phasenflaechenkompensation; Fourier plane; Phase structure}

Description

Characteristic of the known state of the art

For near field compression of semiconductor laser arrays, a method has become known. In this case, an optical image of the laser array field takes place on the periodic phase structure of a binary grating.

It is known that by applying a binary grid a single fold or even localized field can be spatially amplified. In reverse, when specifying a type of duplication, d. H. Assuming that the array field is a spatial replication of the field of a single array channel, using the binary grid, a coherent superposition of the localized channel fields into a single localized field, using a 4f construction.

The described method has a number of disadvantages. The limitation to binary grating structures in semiconductor laser arrays has the disadvantage that an interpretation of the array field is carried out as a grating spectrum. In a first approximation, this condition only applies if the widths of the individual channels of the respective array can be considered large in relation to their distances from each other and if, moreover, it is an equidistant arrangement of the individual channels. In addition, there are the limitations that, on the one hand, the field profile within each channel is fixed to the shape of the diffraction peaks of the binary grating and, on the other hand, diffraction peaks of mostly low intensity at locations which are not characteristic of the presence of channels, ie have to be neglected outside the array.

In addition, a certain arbitrariness in the adaptation of the lattice fine structure, so the lattice structure within a period, can not be excluded.

Object of the invention

The aim of the invention is, compared to the known Vcrfahron simple, universal and relatively easy to adjust method for far field smoothing, which ensures maximum phase smoothing and thus compression.

Explanation of the essence of the invention

The invention has for its object to find a method that make the necessary in the application of semiconductor laser arrays preconditions and restrictions, in particular the need to interpret the array as a diffraction spectrum of a binary grid, superfluous and, for example, for gas laser scrarrays with tight adjacent, highly structured channels is applicable.

The solution of this object is achieved with an arrangement for far-field smoothing and selective feedback of laser arrays using a 4 f structure according to the invention characterized in that in the Fourior-level of the arrangement is provided for compensating the phase surface of the array field, the structure of the decoupled array field adapted phase structure located.

The concrete spatial structure of the decoupled array fields depends i.A. from the array geometry and the experimental conditions of the laser process and can be measured experimentally or theoretically calculated. Consequently, if one considers the manifold of possible array fields as given and selects one of them, a Fourier transformation of this field first takes place. If there is an (anti) symmetric field, the Fourier transform is pure (imaginary) real, and the smoothing to be achieved with the phase structure can be such that the zeroes of the Fourier transforms are determined and there the jumps of the ones in that Trap binary phase structure can be localized. This corresponds to an adaptation of the phase structure to this fixed far field of the array and is connected after further Fourier transformation of the smoothed far field with maximum possible compression.

exemplary

The essence of the invention will be explained in more detail in an embodiment shown in the drawing. Show it

Fig. 1: 4 f structure with array 1, phase structure 2, Solektionsblende 3 and feedback mirror 4 Fig. 2: Graphical determination of the jumps of the phase structure Fig.3: power loss coefficient of the feedback into the array.

An already existing weak phase coupling should be stabilized. The field distribution s (x, t) decoupled from an array 1 is thus given as a (temporally) incoherent mixture

N e (x, t) = Σ a _m s _m | x) exp (lw _m t)

m - 1

of a maximum of N supermodels of the form

s _m (.x) - Σ u (x-In (N + 1) / 2 | xc) 8ln (nmn / | N + 1 »n = 1

In this model, phase coupling, verbundon with dom occurrence of supermodes, is effected by weak evanescent field overlap of adjacent, uniformly (for all N array channels) uniformly assumed channel fields u (x). x _c is the channel spacing.

The goal pursued in the following should now be to aim for coherence, ie to suppress all coexisting supermodes, except for one which should be weakened as little as possible. Thus, if one fixes a particular supermode m for far-field smoothing and selective feedback from this manifold N, the phase structure g (k) 2 is adjusted in the first step to compensate for its far-field phase surface. The far field of S _n , (x) is obtained after Fourier transformation (through the first lens)

s _m (k) - (IF * 'u (k) f _m (k); f _m (k) - h _m (k) + (- 1F "hj- k)

h _m (k) a sin (n N [k xc-m / 2 (N + 1)]) / sin h [kx _c -m / 2 (N + 1)])

The phase structure is in this case binary (with the phases π and 0), the phase jumps are the zeros of s _m (k). Due to the weighting of the channels α sin (nmn / (N + 1)), even a lattice-like periodicity is present, apart from the slowly changing envelope u (k), which becomes limiting only at a greater distance from the axis. The Fourier variable k is related to the real length extension x _k = λ f _L k, where λ is the laser wavelength and f _{L is} the lens focal length. FIG. 2 illustrates the determination of the phase jump points f _m (k) - 0 (crosses) and the associated binary phase structure within a period k Xc c [-1], for the first (m = 1) and tenth (m = 10) Supermode of a 10-element array (N «= 10). From this, the minimum distance between adjacent phase jump points can be estimated in general terms as Ak "» 1 / (N Xc), which results in the spatial distance of these to Ax _k "· λ f _L / (N Xc) in the real phase structure λ »10μιη and x _c « · 3mm means at a focal length f _L 1 = 3cm, for example: Ax _k · · 100 / Νμιη This estimation must be taken into account when selecting a suitable manufacturing method for the phase structure to be adapted.

The feedback array fields S _n , (x) are obtained in the second step. This includes, inter alia, the selection by means of a diaphragm 3 and the feedback into the array after reflection at a plane or concave mirror 4. The power loss coefficient Tp shown in Fig. 3 is a quantitative criterion of the feedback and selection quality. Again related to an array of evanescent coupled channels, the phase structure was adapted to the first and tenth supermode, respectively, as in FIG. 2.

In Tp a 1 -P [S | / P (s), the ratio of the powers P of the feedback and decoupled fold distributions, in each case based on a single supermode m, is included. Depending on the aperture size, which in Fig. 3 assumes the array size related values of 0.5 (+), 1 (»), 1.5 (x), and 2 (o), then, after the feedback, the supermode In a first approximation, multiply a _m in s (x, t) by multiplying it by the corresponding values 1 - Tp, if we omit field overlap effects. Clearly, a preference for the (first) supermode to which the phase structure has been adapted corresponds to a seioction or to a (further) splitting of the laser supermysms of the individual supermodes, in the case of decoupling at a (nonselective) feedback mirror, which offers itself for the far-field-smoothed and therefore mirror-focused supermode, the a _{m should be provided} with a further (uniform) reduction factor.

Claims (1)

Arrangement for far-field smoothing and selective feedback of laser arrays using a 4f structure, characterized in that in the Fourler plane of the arrangement is provided for compensating the phase surface of the array field, adapted to the structure of the decoupled array array phase structure. Field of application of the invention The invention relates to an array for flat field smoothing and selective feedback of laser arrays using a 4f construction. Boi laser arrays with a coherent coupling of the individual Kanalfeldor of these arrays, a so-called phase coupling necessary, urr. It is not possible to achieve the coherent array array performance and to be able to focus the fields appropriately.