DE3401102A1 - Semiconductor diode laser - Google Patents

Abstract

A single-frequency diode laser having an active stripe (4a) which produces laser radiation and having at least one further stripe (11a, 12a, 13a, 21, 31, 231), which is or are arranged laterally with respect to said active stripe (4a) and is or are supplied with current below the value of the threshold current. <IMAGE>

Description

Semiconductor diode laser.

The present invention relates to a semiconductor diode laser, as indicated in the preamble of claim 1.

Single-frequency semiconductor diode lasers are known from the prior art (single-frequency semiconductor laser) known, namely those diode lasers that Generate radiation of only one frequency and thus only one oscillation mode and radiate. Details of such a diode laser can be found in general Review in IEEE Spectrum, December 1983, pages 38-45. There are in particular in the figure on page 43 there are numerous possibilities for realizing such single-frequency laser diode shown. Individual examples are more detailed Such laser diodes are described in the publications listed below: 4. Int. Conf. on Integrated Optics and Optical Fiber Comm., 27.-30. June 1983, Tokyo, Strings 24 and 25; Appl. Phys. Letters, 43: 530-532 (1983); Electronics Letters, Volume 19 (1983), No.22, pages 926/927.

The diode lasers described in these publications have to achieve single-frequency laser radiation such a structure that the laser radiation along its direction of propagation passes through more than one resonator. Such embodiments is based on a physical principle that already existed at the beginning of the sixties became known in particular for gas lasers. These several in the direction of radiation optical resonators (Fabry-Perot resonators) arranged one behind the other achieve through their cooperation, that the generated laser radiation at least practically only has a single frequency. Usually generated namely a laser such laser radiation that has a multimode spectrum and numerous, even if adjacent, includes frequencies of the radiation. The through a serial Multiple resonator structure with resonators with different resonance frequency spectrum passing laser radiation experiences its frequency selection for a matching one Natural or resonance frequency of the two resonators.

The mentioned publication t'4. Int. Conf. ... "shows in FIG as above-mentioned semiconductor diode laser, which consists essentially of two, with little Distance between adjacent semiconductor bodies exists, which are located on a heat sink. In the two semiconductor bodies there are active strips for generating laser radiation formed, which face each other and together an optical Have axis. Radiation from one active strip reaches the other strip and vice versa. The semiconductor bodies have at least one in the direction of this optical axis As a rule, the mutually different length dimensions L1, L2, which are also the length dimensions of the optical resonators formed by the active strips.

By modulating the electrical power to be supplied to the semiconductor bodies The modulated laser radiation can be generated by the current. By appropriate dimensioning the direct current component of the injected modulated current can actually be effective optical length of the respective containing the active strip in question determine optical resonator, so that overall a shift in the frequency position the generated single-frequency laser radiation is possible (see Fig. 2).

As can be seen, it is vital that the active stripes in the two more or less separated semiconductor bodies (diode 1 and diode 2 in Fig. 1 of the cited document) extremely precisely to one another are aligned and also in the course of in particular due to thermal stress So remain exactly aligned with the resulting changed conditions. Another, The disadvantage recognized only in connection with the invention is explained below.

The object of the present invention is to provide such a semiconductor diode laser with single-frequency laser radiation, which is not only technologically simple Is to be produced in a manner that preferably does not require any special adjustment effort, and yet the mechanical and optical stability required for operation owns. In addition, instabilities in the generation of radiation, and in particular undesired attenuation of the laser radiation can be avoided. In particular, the electronic tuning of the laser frequency does not lead to additional attenuation.

This object is achieved by a semiconductor diode laser that the Features of claim 1 has. Further developments and further training of the invention emerge from the subclaims.

Based on the knowledge that when optical resonators are connected in series in diode lasers the need for a structure that is as stable as possible Diode laser exists, the present invention is based on the knowledge that essential Advantages can be achieved by using the laser radiation generation provided., located in the semiconductor body with active strips the Formation of an optical resonator (Fabry-Perot resonator) adjacent in parallel at least one further active strip with an optical resonator in more detail the way to be executed. This associated active strip is the same as for Prior art discussed above is fed with current in such a way that its excitation remains below the threshold for its own generation of laser radiation.

In itself, this is the active strip that generates the laser radiation further active strips to be assigned, taken individually from the status discussed above known in the art.

In the invention, however, is the presence of a lateral optical Coupling between these two adjacent optical resonators or active ones Stripe, namely that of the active stripe generating the laser radiation and the active strip associated therewith. The coupling is based on the transverse to the optical axis adjacent arrangement of the two mentioned optical Resonators and the degree of coupling can be determined by their distance from each other and the Length dimension can be determined over which this coupling is effective. It it should be mentioned that it is sufficient and possibly even advantageous if only part of the resonator length of the associated optical resonator under these conditions subject to optical coupling. But it can also cover the entire length of the resonator be provided.

The aforementioned advantage over the prior art discussed above is that in the direction of propagation of the laser radiation in the invention In the diode laser there are no elements that cause additional attenuation. This also has the effect of improving the stability of the laser radiation.

Above, the basic principle of the invention was based on the Presence two (optically coupled to one another) optical resonators described, namely that of the active strip generating the laser radiation and that of the associated active strip that does not exceed the self-excitation threshold (by the fed-in current) is excited. It is particularly advantageous to the an optical resonator of the active strip generating the laser radiation two or more optical resonators of corresponding further active strips to be assigned, e.g.

on both sides of the first-mentioned optical resonator (in which the Laser radiation is generated). This first-mentioned optical resonator can also be on one or both sides of (each) in the longitudinal direction in a row one behind the other arranged assigned optical resonators (without own radiation generation) be flanked. Appropriately, the plurality of the optical resonator of the optical resonators to be assigned to the active strip generating the radiation (of the other assigned active strips) to adjust their optical length so that that the distances between the natural resonance frequencies in their respective spectrum are different differ from each other and also differ from the spacing of the natural resonance frequencies in the spectrum of the optical resonator of the active one that generates the laser radiation Differentiate stripes.

For the sake of completeness, reference should be made to the state of the art, which is based on another problem and its embodiment only apparently comes close to the invention. It is an integrated Semiconductor diode laser array, as it is practically identical in content in Appl. Phys. Letters, Vol. 43 (1983), pp. 421-423, Appl. Physics Letters, Vol. 43 (1983, pp. 521-523 and 4th Int. Conf. on Integrated Optics and Optical Fiber Comm. (1983), pp. 30/31 is specified. This diode laser -Array has a substrate body, in which a number of parallel diode lasers, which all generate radiation, are integrated. Their optical resonators range from one edge to the other opposite edge of the substrate body. This number is essentially parallel mutually aligned strip-shaped resonators of the known diode laser array are at such a distance from each other that optical coupling occurs between them, in order to still obtain phase-coupled radiation from all these diode lasers. With this state of the art, no single-frequency laser radiation can be generated, as this is a basic condition for the invention. For each diode laser, i. for each existing optical resonator, laser radiation generation is provided, what is particularly indispensable with regard to the provisions provided there Phase coupling within the array. This only apparently comes close to the invention State of the art is missing apart from the problem on which the invention is based, To generate laser radiation that is single-frequency even with modulation in the GHz range and whose frequency selection is optionally controllable, thus also the characteristic of an assignment at least one further optical resonator of a diode laser that does not generate radiation is to be fed with electricity, to an in itself usually generating laser radiation Diode laser, which is then due to at least a respective share the optical coupling present along the length of the resonator, single-frequency laser radiation would produce - as this is achieved according to the object of the invention.

Further explanations of the invention can be found in the following description to embodiments of the invention. Show it Fig.1 a representation that includes several variants of an embodiment of the invention, FIG. 2 shows a further variant and FIG. 3 shows a further variant with an associated optical resonator with particularly high quality.

In Fig. 1, 2 is a semiconductor body, e.g. made of gallium arsenide, designated. As is known from hetero-stripe diode lasers, is located on one main surface of a substrate 2 'a number of superposed, in particular epitaxially deposited layers 3, which together with the substrate 2 'form the semiconductor body 2 of the diode laser. Within one of these layers 3 is below one strip-shaped conductive covering 4 of the surface in a manner known per se the zone of a laser-active strip 4a in which when current flows, and when there the current density exceeds a certain threshold value due to self-excitation Laser radiation 6 is generated. Characterized in that on the semiconductor body 2 opposite one another End faces 5 and not visible 5 'are present, which are adjacent to air for the generated laser radiation is known to have a reflective effect, lies - how also well known - a (respective) optical resonator in the semiconductor body before. Due to the limited reflective effect of the surfaces 5 (and 5 ') occurs with 6 indicated laser beam from the active strip or from the optical Resonator of the one (already described) active strip 4a.

As is known, occurs within the associated optical resonator - Multiple back and forth movement of the generated laser radiation between the mentioned ones reflective surfaces 5, 5 '.

On the upper surface of the uppermost one of the layers 3, i.

on the surface of the entire substrate body consisting of parts 2 and 3, if the electrode assignment is present, it is shown with hatching for better identification is highlighted. It consists of a strip-shaped occupancy 4 and one itself adjoining it via a connecting strip with electrical connection Connection 8. The local position of the strip-shaped conductive occupancy 4 is determined the local position of the active strip 4a in which the laser radiation is generated and from which the laser radiation 6 emerges. In which of the layers 3 the laser radiation generated results from the choice of layer. Counter electrode to the strip-shaped The electrode covering 4 is the substrate body 2 with its electrical connection 9.

The details of the exemplary embodiment described above are known state of the art and do not require any for a person skilled in the art further detailed explanation.

As can be seen from the illustration in FIG mentioned surface of the layer system 3 a further strip-shaped conductive Occupancy 11 planned.

There are other corresponding strip-shaped conductive coverings denoted by 12 and 13. Each of these strip-shaped assignments preferably has its own connection 18, 18 ', 18 ", which is electrically conductive with the associated strip-shaped occupancy 11 or 12 or 13 is connected.

By the respective current flow between the connections 18 and the counter electrode 9, as has already been described in more detail for the active strip 4a is, below these conductive coatings 11 or 12 or 13 laser radiation generated if one considers the flowing current density of the respective threshold value for self-excited Can exceed radiation generation. There is one optical resonator in each case effective, the - as known - between opposing reflective surfaces is also present, as in connection with the strip-shaped conductive assignment 4 belonging optical resonator of the active strip 4a is described. So far the strip-shaped conductive coverings 11 to 13 at one end not up reach to the front surface of the substrate body - e.g. at both ends of the occupancy 13 and at each end of the assignments 11 and 12 - are from above into the Layer system 3 and optionally further into the semiconductor body 2 extensive pits, e.g. created by locally limited ion etching, are provided, each one end face labeled 14 within the substrate body 2 and his layer system 3 have. These inner end faces 14 are so flat and smoothly executed so that they have at least essentially the same reflective effect have, as is the case for the end faces 5, 5 '.

In the invention it is provided that the current flow or its current density smaller for the active strips 11a, 12a, 13a below the occupancies 11, 12 and 13 than the threshold value necessary for laser radiation generation. It lies there is thus an active strip with an optical resonator in front of, in which due of the current value lying below the laser threshold, laser radiation is still generated he follows. As can be seen in particular from FIG. 1, the assignments extend 11, 12, 13 on the one hand and the occupancy 4 on the other hand in at least substantially parallel alignment laterally adjacent to one another. To the lateral optical Coupling between the laser radiation generating active Stripes 4a and the one or more adjacently arranged active strips 11a, 12a, 13a to achieve with an optical resonator, the distances D1 and D1 are a maximum of about 30, preferably not more than 20, and in particular up to about 10 wavelengths of the laser radiation sized large.

This wavelength specification is that in the semiconductor material each occurring wavelength.

As can also be seen from FIG. 1, the respective active strips 11a, 12a, 13a or the conductive coatings that supply the current 11, 12, 13 not along the entire length of the active strip 4a or the strip-shaped Occupancy 4 extend, as Fig.1 shows.

In individual cases it may be sufficient that, in addition to the one corresponding to occupancy 4 active strip 4a (which generates the laser radiation 6) only a single one adjacent arranged, corresponding to occupancy 11 (not up to self-excitation with electricity fed) active strip 11a or 12a or 13a is present. To ensure, that even in unfavorable resp.

critical operating cases the laser radiation 6 is single-frequency or the active strip assigned to occupancy 4 supplies single-frequency radiation, it is advisable to use 4 strips on both sides of the strip-shaped, conductive coating conductive assignments 11, 12 and / or 13 to be provided.

Another variant would be to provide such an assignment, such as they are made up of the strip-shaped conductive strips shown one behind the other Allocations 12 and 13 taken together result.

As already mentioned at the beginning, the illustration of FIG. 1 contains a Multiple variants of specific embodiments of the invention. One more Another variant is shown in FIG. The ones contained in Fig.2 and for Fig.l already The reference numerals described have the same in each case in FIG meaning as in Fig.1. The strip-shaped conductive coating 21 in Figure 2 has a like the figure visible configuration, namely one end of it runs to a 90 "curvature as a portion 121 to a lateral edge surface of the substrate 2. This The shape or routing of the occupancy 21 (including the portion 121) forms one underneath running on the same path active strip with an optical Resonator, the reflective end faces of which with the edge faces of the semiconductor body 2 coincide. The dashed representation of the end of the portion near the edge 121 of the assignment 21 is intended to indicate that, in principle, the optical resonator of the Occupancy 21 associated active strip or its optical resonator already at the transition to the part shown in dashed lines already end in the semiconductor body can, as has already been described with reference to reference numeral 14. One embodiment 2, for example, it can be advantageous in that the length of the optical Resonator of the active strip assigned to occupancy 21 (which does not go up to Self-excitation is operated) a relatively large resonator length and thus greater Can have resonator quality. This can have a positive effect on the stability of the generation single-frequency laser radiation 6 affect.

Another variant of an embodiment according to the invention and in particular a further development of the embodiment according to FIG. 2 is shown in FIG which has a strip-like occupancy as shown, preferably elongated, Ring 131, the portion 231 of which meets the condition, is substantially adjacent parallel to the strip-shaped occupancy 4 of the radiation 6 generating active To stretch the strip. For this portion 231, the (essentially on the Lateral coupling based on distance D1.

As shown, the portion 231 of the ring 131 the straight-line strip-shaped portion 31 of this assignment connect, with also for this share has the coupling effect as for assignment 11. With the ring 131 a ring laser is implemented that has a particularly high resonator quality.

A variant of the embodiment according to FIG FIG. 3 is the one in which the portion 31 shown in FIG. 3 has been omitted and the coupling between the active one corresponding to the annular occupancy 131 Stripes and the active stripe 4a only correspond to the portion 231 via the portion 231 Path length extends.

By choosing the current strengths or current densities in the respective active Area, the resonators belonging to these areas (to each other) be tuned so that the result is the single-frequency laser radiation 6. This vote has a certain scope.

Within this tuning range, the In addition, the frequency value of this single-frequency radiation can be controlled with amperage to be changed.

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Claims (8)

Claims: semiconductor diode laser for single-frequency laser radiation, whose frequency is adjustable, with a plurality of laser-active strips with each optical resonator, these strips being in semiconductor material of which a strip is fed with electricity until at least self-excitation and of which another strip is not supplied with current to the point of self-excitation and wherein the optical resonators are matched to one another so that they are with respect to the spacing of the resonance frequencies of their respective resonance frequency spectrum from one another differ by the fact that these active strips (4a, league, 12a i3a) are located in a single semiconductor body (2); and that this several active strips, each with at least a portion of their total strip length at least substantially parallel to one another at a respective distance from one another are arranged laterally adjacent, so that there is a lateral optical coupling, their size by the respective selected distance (D1, D2) and the respectively selected Share of the total strip length can be determined as an effective coupling length. 2. Diode laser according to claim 1, g e k e n n z e i c h n e t thereby, that in addition to the active strip to be fed with current up to self-excitation (4, 4a) more than one additional additional active strip (11, 11a; 12, 12a; 131tal; 21; 31; 231) are provided. 3. Diode laser according to claim 2, g e k e n n z e i c h n e t thereby, that further active strips (11, ila; 12, 12a; 13 13a} on both sides of the Active laser radiation (6) generating Strip (4, 4a) arranged are. 4. Diode laser according to one of claims 1 to 3, g e -k e n nz e i c h n e t in that on at least one side of the laser radiation (6) generating active strip (4a) parallel to this strip one behind the other active strips (12a; 1 3a) are arranged. 5. Diode laser according to one of claims 1 to 4, g e -z e i c h n e t in that at least a portion of a further active strip (131) as Ring strip is formed (Fig.3). 6. Diode laser according to one of claims 1 to 5, g e -k e n n z e i c h n e t in that at least one of the further active strips (21) has a of the active strip (4 a) generating the laser radiation (6) in terms of direction has a different proportion (121). 7. Diode laser according to one of claims 1 to 7, g e -k e n nz e i c h n e t in that at least a plurality of the individual active strips associated conductive assignments (4, 11, 12, 13, 21, 31) have their own electrical Connection (8, 18, 18 ', 18 "...) has. 8. Diode laser according to one of claims 1 to 8, g e -k e n n z e i n e t in that at least one of the mirror surfaces of the optical resonator at least one of the active strips (13) by means of ion etching in the semiconductor body (2) generated pit is formed.