Semiconductor laser device for the generation of a periodic refractive index distribution and/or periodic gain distribution.

0001The invention relates to optical semiconductor transmitters or semiconductor amplifiers with a periodic structure.

0002The transmission of messages via optical fibers enables very high data rates. Because of the dispersion behavior of optical fibers, lasers that emit a single longitudinal mode are required for long-distance message transmission. It is known from EP-A-0 149 462 that the number of modes emitted by the laser can be reduced by integrating an optical grating into a semiconductor laser, hereinafter referred to as HL laser. Such lasers are called DFB (Distributed Feedback) lasers. It can be shown that a DFB laser also emits in two longitudinal modes, which is not optimal with regard to the energy conversion from electrical to radiation energy.

0003It is also known from EP-A-0 213 965 to connect an HL laser to a tunable resonator in order to achieve a reduction in the number of longitudinal modes in this way. The resonator specified in this document also contains an optical grating, so that in principle two longitudinal modes are also emitted in this solution.

0004The object of the invention is to construct an optical HL transmitter or HL amplifier in such a way that it amplifies only a single mode and thus enables the emission of a single mode. This object is achieved by a device with the combination of features of the main claim. Further developments and refinements of the HL transmitter or HL amplifier are specified in the subclaims.

0005The arrangement according to the invention has the advantage that it can be used universally for a wide variety of areas in optical communication. This arrangement can be used as:<ul id="ul0001" list-style="none"><li>1. passive interference filter,</li><li>2nd active interference filter that can be tuned in its bandwidth,</li><li>3rd active, longitudinally single-mode transmission laser.</li></ul>

0006Several exemplary embodiments of the invention are described below and explained in more detail with reference to FIGS. 1 to 5. Show it:<ul id="ul0002" list-style="none"><li>1a shows the basic structure of the HL amplifier or HL laser,</li><li>1b shows the course of the refractive indices and the bandgap in such an arrangement,</li><li>2a shows a first embodiment,</li><li>2b shows a second embodiment,</li><li>2c shows a third embodiment,</li><li>3 shows the principle of the passive or active interference filter,</li><li>4 shows the ratio of incident intensity to reflected intensity in an interference filter,</li><li>5 shows the amplification of a single longitudinal mode in such a structure sequence.</li></ul>

0007Fig. 1a shows the basic structure of the optical HL laser or HL amplifier. A layer is designated by 10, which consists of a first HL material, which has the refractive index n1 and the Banbd distance E1. 11 denotes a second layer which has the refractive index n2 and the band gap E2. The layer sequence consisting of the two layers 10 and 11 is repeated a number of times, as can be seen in FIG. 1a. The thicknesses of the individual layers are designated d1 and d2. They are different depending on the application. A coordinate system is identified by 12, which is intended to identify the spatial position of the semiconductor arrangement. The direction of propagation of the light processed by the amplifier or the direction of propagation of the light during laser operation is the z-direction. In the upper diagram of FIG. 1b, the course of the refractive index in the z direction of the semiconductor arrangement according to FIG. 1a shown. In the diagram below, the course of the band gap in the z direction is plotted at 14. In the example chosen, the material with the smaller band gap E1 has the larger refractive index n1. The reverse case can also be realized.

0008In principle, a layer sequence is also conceivable that consists of more than two different semiconductor materials. In FIG. 1, materials with refractive indices n1 to nj and band gaps E1 to Ej would then have to be provided. The exemplary embodiments described below consist only of two different semiconductor materials. A structure according to Figure 1a can be created using Ga_xIn_1-x As_y P_1-y and InP are generated. In the first-mentioned material, the band gap can be varied depending on the composition; here a band gap E1 of 1.0 eV is selected with a refractive index n1 of 3.4. The second selected material InP has a band gap E2 of 1.35 eV and a refractive index n2 of 3.2. In principle, other material systems can also be used without losing the functional principle.

0009So far, only the basic structure of the layer sequence has been shown in FIG. 1. FIG. 2 shows three exemplary embodiments which relate to the embedding of the layer sequence according to FIG. 1 in an HL component. In Fig. 2a, 20 denotes a semi-insulating substrate, 21 an n-semiconductor, advantageously n-InP in the selected example and 22 a p-semiconductor (advantageously p-InP). 23 and 24 denote contacts which supply the two semiconductors 21 and 22 with current. The laser or the optical amplifier according to FIG. 1a is embedded between the semiconductors 21 and 22. As in FIG. 1a, the layer sequence is identified by 10 and 11. The coordinate system denoted by 12 also reflects the spatial position of the semiconductor arrangement in this case. Fig. 2b shows a further exemplary embodiment, in which the laser or optical amplifier is located on an n-semiconductor (eg n-InP), which is designated by 25. The resulting arrangement is embedded in a p-type semiconductor (eg p-InP). 23 and 24 designate the power supply as in FIG. 2a. The spatial position of the arrangement is represented by the coordinate system 12. 2c shows a third exemplary embodiment of the HL laser or HL amplifier. The end face of the layer sequence consisting of the two layers 10 and 11 is embedded here in an n-semiconductor 27 (for example n-InP) and a p-semiconductor 28 (for example p-InP). The contacts 23 and 24 serve for the power supply. As already shown in FIGS. 2a and 2b, the spatial position is represented by the coordinate system 12.

0010If an external voltage is applied to the structures shown in accordance with the polarity indicated, the injected charge carriers collect in the semiconductor region with the smaller band gap, here designated E1. In this way, both a spatially periodic gain distribution and a periodic refractive index distribution are realized along the z-axis. These different periodicities can be used advantageously for different types of components; these are, for example<ul id="ul0003" list-style="none"><li>1. passive interference filter,</li><li>2nd active, narrowband amplifying interference filters,</li><li>3rd optical, narrowband repeaters,</li><li>4th single-mode transmission laser.</li></ul>

0011The physical mode of operation of this semiconductor structure as a passive interference filter is shown in FIGS. 3 and 4 and the relevant mathematical equations are summarized in the appendix.

0012The physical mode of operation of this semiconductor structure as an active interference filter or narrow-band amplifier will be briefly explained below. If the semiconductor structure is pumped (current injection) and the energy of the incident light beam lies between that of the two band gaps E1 and E2, the incoming light is amplified in the areas of E1. The principle of operation is similar to that in the case of the unpumped structure or the passive interference filter.

00135 results in a further type of use of the semiconductor arrangement shown, namely for generating a single-mode emission. Essentially, the spatially periodic gain distribution is used. Due to the periodic structure and the periodic distribution of the amplification, a distribution is forced on the radiation in the semiconductor, which leads to a single-mode emission. For this purpose, the semiconductor structure is pumped over the laser threshold, standing waves being formed in the semiconductor. These are reinforced in the antinodes, but not in the vibration nodes. In this way, only a single vibration mode is formed over the entire length, which then leads to single-mode emission. The structure thus represents a single-mode laser that only oscillates in one mode and not, in principle, as in the DFB lasers previously used, in two modes. This eliminates the problem of one<img file="EP0333090A2_D0001.tif" />-Phase adjustment area, which is to be provided in the conventional DFB lasers to realize single-mode operation.

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0014From the right, a light beam labeled 30 falls on the arrangement according to FIG. 3. The energy of the photons of the light beam 30 is smaller than the band gap E 1 (only with the passive interference filter). A lambda / 4 antireflection coating is identified by 35. The known layers 10 and 11 follow this anti-reflective layer several times in succession. The rays 31 reflected on the lambda / 4 layer cancel each other out by interference. The reflected light is composed of the rays labeled 32, 33, 34 ... For reasons of clarity, no further reflected rays have been shown. For the light wave ψ reflected on the periodic structure_ref applies:<maths id="math0001" num=""><img file="EP0333090A2_D0002.tif" /></maths>

0015In a first approximation, the following applies to the reflected intensity:<maths id="math0002" num=""><img file="EP0333090A2_D0003.tif" /></maths>

0016In equation (1) r is the reflection coefficient of the electric field with<maths id="math0003" num=""><img file="EP0333090A2_D0004.tif" /></maths> δ is the phase shift between neighboring partial waves. Equation (1) reaches maxima for the following values ​​of δ / 2:<maths id="math0004" num=""><img file="EP0333090A2_D0005.tif" /></maths>

0017The layer thicknesses d1 and d2 are thus calculated<maths id="math0005" num=""><img file="EP0333090A2_D0006.tif" /></maths>

0018Constructive interference occurs for these values ​​of d1 and d2. λ is the emission wavelength of the incident light beam 30.

00194 shows the course of the reflected intensity from equation (1) as a function of the phase shift δ.

0020It is now possible to manufacture an interference filter from semiconductors, the focus wavelength (maximum reflection of the filter) can be set as required by the thicknesses d1 and d2, and the half width by choosing the reflection amplitude coefficient r, or by choosing the refractive indices n1 and n2 can also be freely set within wide limits. The considerations made up to now apply generally to any emission wavelength λ of the incident light beam and to the unpumped semiconductor structure.