WO2003005499A2

WO2003005499A2 - Monolithic integrated microlaser with a circular resonator comprising just one mirror plane

Info

Publication number: WO2003005499A2
Application number: PCT/DE2002/002493
Authority: WO
Inventors: Michael Hamacher; Helmut Heidrich; Dominik Rabus; Martina Hentschel; Klaus Richter
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.; MAX-PLANCK-Gesellschaft zur Förderung der Wissenschaften e.V.
Priority date: 2001-07-03
Filing date: 2002-07-03
Publication date: 2003-01-16
Also published as: AU2002320912A1; DE10132479C1; WO2003005499A3

Abstract

Microlasers with a closed embodiment of the resonator in various geometrical forms are increasingly used. Up to now light decoupling has been achieved by means of a sub-micrometre gap with undirected light overflowing from the total internal reflection. The degree of decoupling which may be achieved as above is however poor and furthermore the production of gaps in the sub-micrometre range is difficult. The inventive microlaser (ML) thus comprises circular resonators (CR) formed such that characteristic reflection points (CPR1, CPR2, CPM) have stability islands which are as large as possible with high point density, as analysed by the Poincaré section calculation. The resonator modes (RM) occurring are guided in a directed manner into at least one monolithic integrated decoupling waveguide (WG), optically coupled directly to the circular resonator (CR) at a particularly favourable reflection point (CPM), with the front face (AF) thereof, by means of a butt coupling (IR).The degree of decoupling may thus be constructively adjusted by means of the parameters of the optical field overlap at the butt joint (IR). Said microlaser (ML) thus comprises at least one adjustable and switchable light valve (LV), by means of which the above is particularly suitable for various applications as monochromatic light source in continuous and pulsed operation.

Description

Monolithically integrated microlaser with a circular resonator that has only one mirror plane

description

The invention relates to a monolithically integrated microlaser with a circular resonator having only one mirror plane in a semiconductor layer structure for the planar formation of revolving resonator modes, which are reflected at reflection points in the circular resonator, and with at least one monolithically integrated coupling-out waveguide, which is assigned to a fixed reflection point of a stable resonator mode and has a refractive index comparable to the refractive index of the circular resonator, for forming an optical light valve for an adjustable light coupling into the end face of the coupling-out waveguide.

In addition to monolithically integrated injection lasers with a linear resonator arrangement (Fabry Perot laser) and monolithically integrated injection lasers with integrated Bragg filter function (distributed feedback laser and distributed Bragg reflector laser), lasers based on so-called "circular resonators" have also recently been published. These are micro-lasers with a closed design of the resonator in different geometric shapes, in particular triangular, circular and elliptical shapes. The shapes can be made entirely of semiconductor material (“disk resonators”) or only as a border (“ring resonator”) In all variants, only a mode formation parallel to the planar semiconductor layer of the micro laser is used Depending on the geometric design of the micro laser, types of stable, that is to say stationary, resonator modes are formed which reflect on the inner edge of the resonator be reflected. The stable resonator modes run clockwise and counterclockwise Resonator around, which also includes the case of reflection on own return paths in non-closed path types.

For practical applications, the optical decoupling of laser light from the circular resonator of the micro laser is of fundamental importance. From the article [I] "Spatial beam switching and bistability in a diode ring laser" by M.F. Booth et al. (Appl. Phys. Lett., Vol. 76, Feb. 2000, No. 9, pp. 1095-

1097) a corner of a triangular ring laser is designed so that the

The angle of total reflection (with a usual refractive index ratio of the material system used is approx. 17 °) is undershot and according to the Snellius refraction law see an unguided and undefined

Free-beam radiation takes place. The degree of decoupling must be covered by an additional coating of the optical facet

Process step can be set. The quality of the laser can be determined as the quotient of the captured energy to the uncouplable energy via the degree of decoupling.

A defined decoupling has not yet been achieved even in the case of circular disk resonators. In US Pat. No. 5,742,633, decoupling via a directional coupler structure as in the case of passive ring resonators is proposed. For this purpose, an optical fiber runs parallel to an asymmetrical disc resonator separated by a micro gap. Due to the near-field effect, light from the circular resonator passes into the waveguide in an undirected manner. Only the portion of light can be decoupled in a defined manner that emerges from the edge during the total internal reflection at the reflection point ("eventscent wave"). The decoupling is defined by the width of the microgap. The problem is that even for a decoupling of a maximum of 10% of the power is to form a submicron gap, which requires the costly use of a direct-writing electron beam exposure and which is associated with a relatively large error due to the process tolerances. In addition, the absolute situation is more difficult with the quadrupole-typical resonator modes which form in the disk resonator one high power density is only vaguely known. Due to the process tolerances in the manufacture of the individual epitaxially grown compound semiconductors and in the vertical design of the components, fluctuations in the optical properties of the panes are inevitable. Thus, an absolute determination of the radiation position and angle is extremely difficult. Therefore an analysis of the beam dynamics in the phase space of the disc resonator is important. The standard method for this is the creation of a so-called "Poincare section", in which the reflection angles χ occurring in the circular resonator or their values sin χ are plotted over the phase angle φ (circumferential angle) along the boundary line of the resonator. By connecting the reflection points directly one after the other in the chronological order of their occurrence one obtains a continuous and descriptive mode path for the ray course. In order to better illustrate the dynamics of the mode paths, the Poincare cut is created in which a sequence of points with the coordinates φ, sinχ is assigned to each mode path. For periodic mode paths, the reflection points in the resonator form closed curves or clusters and also appear in the Poincare section in the form of stability islands with a corresponding size and density assignment of reflection points adjacent. A Poincare section is therefore a "phase space portrait". The basis for this and for the formation of quadrupole-typical resonator modes in deformed circular resonators which can be described by trigonometric formation formulas can be found in the article [II] "High Power Directional Emission from Microlasers with Chaotic Resonators" by C. Gmachl et al. (Science, Vol. 280 , June 1998, pp. 1556-1564).

Further possibilities for the extraction from disk-shaped microresonators are described, for example, in the article [III] “FDTD Simulation of Photonic Devices and Circuits Based on Circular and Fan-Shaped Microdisks” by A. Sakai et al. (J. of Ligthwave Technol., Vol 17, No. 8, August 1999, pp. 1493-1499) in which the formation of dimensioned bar submicron columns is required. Fan-out disks are used as waveguide disks for decoupling. The decoupling in turn takes place via the near-field effect with reflected light.

The prior art on which the invention is based is described in the article [IV] "GalnAs / GaAs Micro-Are Ring Semiconductor Laser" by S. Mitsugi et al. (Jpn. J. Appl. Phys. Vol. 34, Febr 1995, pp. 1265-1269), which describes a monolithically integrated microlaser in a triangular “pie slice shape” with two straight and one curved reflection surface. In parallel in the planar semiconductor layer structure, a stable, periodic resonator mode is formed in a triangular shape, which is reflected at three fixed reflection points in the middle of the reflection surfaces. The known circular resonator has only one mirror plane, which runs from the tip between the two straight reflection surfaces to the center of the curved reflection surface. Via a micro-gap between a straight reflection surface and a coupling-out waveguide, light emerging from the circular resonator is coupled in via the near-field effect during the reflection. The coupling-out waveguide is assigned outside the mirror plane to the corresponding fixed reflection point at an acute angle to the reflection surface in such a way that the light reaches its end face. The amount of light to be decoupled can be set by choosing the refractive index of the waveguide below the refractive index of the resonator and by a gap width of less than 1 μm, so that an optical light valve with an adjustable decoupling is present. However, a maximum coupling efficiency of 1% can only be achieved. In addition, the critical submicron structuring of the intermediate space places great demands on production. Reproducible production is problematic.

The object of the present invention is therefore to be seen in connecting a monolithically integrated microlaser with a circular resonator of the type described at the beginning to a coupling-out waveguide in such a way that coupling-out powers can be achieved in a wide range, on the one hand, so that the quality of the microlaser can be dimensioned comprehensively. On the other hand, the constructive design should be as simple as possible, so that no production-technically complicated requirements arise and a reproducible and highly precise production is guaranteed.

In a generic microlaser with only one mirror plane and at least one coupling-out waveguide, which is assigned to a fixed reflection point of a periodic resonator mode, the solution for this is therefore that the fixed reflection point in a Poincare section computed for a given dimensioning of the circular resonator by comparison with other calculated stability islands, the largest possible and densely occupied stability islands of a occurring stable resonator mode are fixed locally and that in this characteristic reflection point the coupling waveguide is optically connected directly to the circular resonator via a butt coupling point for coupling out a directed light beam, the degree of coupling out being dimensioned by optical field overlap occurring at the butt coupling point between the stable resonator mode and the expressed mode in the coupling-out waveguide ter is adjustable.

In the microlaser according to the invention, directional light decoupling is achieved. It is not used, as in known micro-gaps and parallel profiles in the near-field effect, "spilling" light with a low power component in the region of any fixed reflection point, but at least one characteristic reflection point is directly "tapped" by a coupling waveguide directly abutting its end face, so that here the impinging stable resonator mode in the form of a directed light beam can emerge from the circular resonator in a targeted manner. Depending on the optical field overlap, the laser light partially passes into the coupling-out waveguide, the refractive index of which generally has only a slight positive or negative difference to the refractive index of the circular resonator. Thus, the decoupling point occurs despite it Pronouncedness for the laser light not as a "fault location", but rather as a "linear extension" of the resonator ring, as a result of which the light output losses are very low.

The known calculation and representation methods are used to determine the characteristic reflection point (see article [II]), but are interpreted with regard to the new aspect of the gap-free butt coupling. In the Poincare sections, the invention therefore specifically searches for the largest possible and densely populated stability islands and clustering of the reflection points as an indication of a particularly strong and stable, periodic resonator mode which has at least three, particularly excellent characteristic reflection points for the butt coupling of a coupling waveguide ( triangle mode). Vertically pronounced stability islands, which assign a large number of reflection angles to a phase angle in the circular resonator and thus to a single reflection point, form the optimal case. However, vertically and horizontally pronounced stability islands, which can still be assigned to a central phase angle and thus a circular resonator section that is at most as wide as the coupling-out waveguide, can be included, since the coupling-out waveguide not only covers an exit point but also an exit surface. The sought-after number of reflection points can be recognized in the Poincare cut by a massive expression of the stability islands in the form of closely adjacent black points. Depending on the calculation resolution selected, typical ring-shaped stability islands show the largest circle as a measure of the extent of the stability island, which is densely covered with reflection points.

The production of a butt coupling in the growth process of the semiconductor layers is uncomplicated and is technically mastered with the highest reproducibility. The alignment of the coupling waveguide is determined as usual according to the Snellius ^{' law of} refraction. Since the difference between the effective refractive indices of the circular resonator and in the conductor is below the 1% range, the decoupling waveguide can be oriented directly in the direction of the emerging resonator mode, an angle correction is not necessary. The measure of the outcoupled amount of light is determined by the field overlap between the emerging resonator mode and the mode that can be formed in the waveguide. Optical mode propagation fields can be easily represented with conventional computer programs. The dimensioning of, for example, field width transformers with single or double tapered waveguide ribs at cross-sectional transitions using this method is well known. The coupling portion can be adjusted by appropriate dimensioning of the waveguide height and width, that is to say the waveguide cross section. The quality of the microlaser according to the invention can thus be dimensioned and used in engineering terms. In this way, the efficiency of the microlaser with a low-dimensional circular resonator and an integrated, adjustable light valve can be made available as an optical output port, for example for implementing the smallest monochromatic light sources. The end face of the output port must always be designed at its end facing away from the circular resonator such that this end face cannot interfere with the laser characteristic as a competing resonator. A reflection can be sufficiently prevented, for example, by an anti-reflective coating to better than -40 dB or by an anti-reflective coating to better than -10 dB in connection with the formation of the chip interface at an angle of, for example, 7 °.

As with all valves, the switching function of a light valve is also important. This means that applications with intermittent operation with adjustable time windows can be implemented. Therefore, in a continuation of the micro laser according to the invention, it is provided that the coupling-out waveguide is designed as a switchable active waveguide with a separate contacting of the active layer. The circular resonator and waveguide thus have the same layer sequence and height and are particularly easy to grow up. Both contain active semiconductor material, so that the outcoupling waveguide can be supplied with an injection current via its own contact to the transparent point. This measure can be used to set when the light valve is opened (injection current flows, the output waveguide is optically transparent) and when it is closed (no injection current, the output waveguide is optically opaque).

In principle, all occurring fixed reflection points of periodic resonator modes with stable mode paths are suitable for the butt coupling with the coupling-out waveguide. The monolithic implementation using epitaxial growth of the layers and micro-structuring processes enables any radial positioning of the coupling-out waveguide on the circular resonator. As a rule, a characteristic reflection point for the impact coupling of a coupling-out waveguide is selected for the specific application. However, there are special applications in which the attachment of two or even three light valves to a circular resonator makes sense. These can either lead to an increase in the light output that can be coupled out, as a rule a proportion of 30% to maintain the resonator function should not be exceeded, or to an expansion of the possible uses, in particular for executing parallel functions. The light valves can be designed according to the same requirements as previously explained in connection with the one light valve.

According to a next development of the invention, a further design option is that the coupling-out waveguide is designed as an optical amplifier. An injection current is then applied to the output waveguide well above the transparent point. The light emerging from the microlaser is amplified as a traveling wave when it passes through the output waveguide once.

In addition to modulated operation, constant operation of the microlaser according to the invention with high constancy is also used. Therefore According to another embodiment of the microlaser according to the invention, there is the possibility that the coupling-out waveguide is designed as a transparent passive waveguide. In this case, the layer sequence up to and including the one or more active layers is removed. The optical field overlap determining the degree of decoupling can then be set, for example, by the lateral and vertical guidance of the optical field in a remaining waveguide rib made of quaternary material, in particular GalnAsP. For the design, a degree of freedom for setting the field overlap is also an etching of a rib into the underlying n-contact material, in particular InP. Furthermore, there is the technologically more complex possibility of producing the permanently transparent coupling-out waveguide using an area-selective epitaxial growth step.

The stable resonator modes in the circular resonator can have a clockwise and counterclockwise direction. At the characteristic reflection points, there are therefore two exit directions for a non-perpendicular passage. Both can be used for the light decoupling if, according to a next advantageous embodiment of the micro laser according to the invention, the decoupling waveguide behind the butt coupling point on the circular resonator has a fork for decoupling two directed light beams of a periodic resonator mode rotating in both directions of rotation. With only one butt coupling point, which can be less than 1 μm depending on the degree of coupling, two optical coupling paths can be created. Despite the simple manufacture, there is an increase in the number of possible uses. Both decoupling waveguides can be designed actively with and without a switching function or passively.

The characteristic reflection points are determined by determining the phase angle range for the selected mode path or its characteristic stability islands from the Poincare cut. This Determination of the characteristic reflection points is greatly simplified in the invention in that the symmetry of the circular resonators is reduced to a mirror level. In this way, for example, the case of the pie-like resonator, which has straight edges, is also recorded, but now taking into account the mirror plane for direct coupling of the waveguide. With resonators with only curved edges, however, a further simplification can occur. According to another development of the micro laser according to the invention, the characteristic reflection point is defined locally by the intersection of the mirror plane with the edge in the region of its smallest curvature if the circular resonator is deformed. Since the mirror plane has two points of intersection with the boundary, the second boundary condition required for the localization of the characteristic reflection point via the curvature of the boundary is given. The sought-after reflection point is now always in the single mirror plane where the slightest curvature occurs, that is, in the flatter, "blunt" area of the boundary. In the special case of the "piece of cake", the curvature of the reflection surface with the characteristic reflection point correspondingly goes to zero. The great advantage of choosing this fixed reflection point can be seen in the fact that all other reflection points outside the mirror plane can change their position due to inevitable manufacturing tolerances by an asymmetrical shift of the stable mode path so that they leave the area of a butt coupling. At the point of reflection on the mirror plane, the shifting of the two mode rail legs takes place symmetrically, so that it is always compensated for and the point of reflection on the mirror plane remains stationary.

The term “deformation” is used in the relevant literature for any deviation from the ideal circle towards the ideal ellipse, which has two semi-axes of different lengths

Sizing can be arbitrarily curved figures with a mirror plane be generated. Building on the ellipse, it can therefore be provided, according to a further development of the invention, that the circular resonator in egg-like form with the polar coordinates r, φ according to the standardized formation formula: r (φ) = R ₀ (1 + εi cos 2φ + ε ₂ cos 3φ )

is formed with a constant mean radius R ₀ and selectable deformation factors εi, ε ₂ . In the case of ει = ε ₂ = 0, there is again a circle with the constant radius Ro. If only ε ₂ = 0, a quadrupole curve is created. For εi ≠ O, ε ₂ ≠ O the cosine function produces a modulation of the mean radius R ₀ according to its sign and amount. In the present case, two deformation factors are chosen which enter into the educational function with different periods of the cosine in order to obtain a greater scope in the value assignment of the factors. With a simple deformation of the circular resonator, triangular paths are usually formed which deviate little from the shape of an equilateral triangle. A special implementation is a stable mode path in the form of a double triangle, in which the characteristic reflection point on the mirror plane is passed twice. All triangular paths that occur are characterized by a characteristic reflection point, which is immutable even with manufacturing tolerances, if, according to a next embodiment, the deformation factor εi has a value between 0.01 and 0.05 and the deformation factor ε _{2 has} a value of 0.01 and 0.1 is selected, depending on the combination of values, both factors can still fluctuate by up to ± 30%. The final, precise combinations of values, for example εi = 0.02 and ε ₂ = 0.03, should always be checked with the aim that large and densely populated stability islands appear in the Poincare cut, which correspond to distinctly stable modes. At the characteristic reflection point, two light beams emerge due to the two-way sense of rotation of the stable resonator mode Even when realizing the double triangle, the angle between the two rays arriving per direction of rotation is so small that both light rays can be detected by a common coupling-out waveguide, which then has a fork. The design of a mode line in the form of a double triangle is therefore not critical and can be assigned to the group of triangular mode lines.

A stable resonator mode in a simple triangular shape also results if, after another continuation of the microlaser according to the invention, the circular resonator in the form of a cut ellipse with a large semiaxis a and a small semiaxis b with the polar coordinates r, φ according to the standardized formation formula:

r (φ) = [(1 - εT ⁴ ] / [l + ε cos φ]

is formed with an average radius R ₀ = (1 -ε ² ) ^1/4 and a deformation factor ε = (a -b ² ) ^1/2 / a, which at a point predetermined by a selected section factor τ between a focal point and the The center of the ellipse is cut parallel to the small semiaxis b. Similar to the circular pie-shaped resonator, the cut-off ellipse has a straight reflection surface (with an infinitely small curvature). Due to the intersection with the mirror plane, there is a characteristic reflection point in the middle with absolute spatial stability even when manufacturing tolerances occur, and again with two The curvature of the ellipse provides additional stabilization of the relevant resonator modes. The distance from the cut parallel plane to the small semiaxis of the ellipse can be chosen as desired, in particular it can also be zero, so that the center of the ellipse is halved along the short semiaxis According to a next embodiment of the invention, particularly stable resonator mode in a triangular shape results if the deformation factor ε has a value in a range from 0.3 to 0.9, in particular between 0.75 and 0.85, and a value in the range between 0.25 and 0.75, in particular 0.5, is selected for the section factor τ

The eccentric combination of two disk resonators with different refractive indices and radii results in a combined resonator in the form of an "annular billiard" with an outer ring and an inner disk resonator. With a suitable choice of parameters, a large number of stable mode orbits can be found here, which are characteristic The resonators in the form of the annular billiard also have only one mirror plane and, accordingly, each have a reflection point which is also stationary under the influence of tolerances and which is particularly suitable for shock coupling of the coupling-out waveguide. Therefore, after a continuation of the circular resonator according to the invention, it is advantageous if in a training of the circular resonator in the form of an annular billiard from an outer ring resonator with a constant radius R ^ and an inner disc resonator with a smaller radius R ₂ , the center of which by an E xzentrizitätsfactor δ are shifted to each other and have different refractive indices n _{1 (} n ₂ , the characteristic reflection point is fixed locally by the intersection of the mirror plane with the boundary of the outer ring resonator at the location of the greater distance to the inner disc resonator. As a result of the eccentricity of the two resonators with respect to one another, they have a larger distance between the edges on the one side and a smaller distance along the mirror plane. The characteristic edge point lies on the side with the greater distance, but on the other side there are two further reflection points, which are particularly distinguished, as will be explained in more detail below.

However, an additional peculiarity occurs with the circular resonators in the form of an annular billiard. There are special stable mode orbits here, which begin or end with vertical impingement at the outer edge of the circular resonator. Then one is created here characteristic reflection point at which the condition of total internal reflection is violated and the resonator mode is transmitted vertically and for the most part leaves the resonator. The advantage of the vertical exit is the appearance of only one light steel, which can be completely detected with an output waveguide. A distinction can therefore be made between reflection points which are determined by the largest, most striking islands of stability in the Poincare cut outside or within a region of the violation of the total internal reflection. If the total internal reflection is violated, a large part of the laser light leaves the circular resonator (reflection and transmission). This is not normally desirable in order to achieve a high level of resonance, but can be used sensibly in combination with a light valve. A violation of the total internal reflection occurs for values of sinχ that lie between the positive and the negative reciprocal of the refractive index n of the respective material system: -1 / n <sinχ <1 / n. In the Poincare cut, the area of the violation of the total internal reflection is accordingly limited by an upper line with sinχ = 1 / n. Since each resonator mode can have two directions of rotation (clockwise and counterclockwise), there is a lower boundary line for sinχ = -1 / n.

According to a next embodiment of the microlaser according to the invention, it is therefore advantageously provided that when the circular resonator is designed in the form of an annular billiard from an outer ring resonator with a constant radius Ri and an inner disk resonator with a smaller radius R ₂ , the center points thereof by an eccentricity factor δ are shifted towards each other and have different refractive indices ni, n ₂ , two characteristic reflection points are fixed locally by the vertical impingement of the periodic resonator mode on the edge of the outer ring resonator. A large proportion of light transmits to the reflection points violating the condition of total internal reflection, a smaller proportion becomes dependent on the jump in refractive index reflected according to the Fresnel equation. In this case, the resonator modes in question advantageously strike the edge perpendicularly, so that only one emerging light beam results in one orientation. A maximum light output can therefore be dissipated via a single, non-forked output waveguide with a corresponding cross section. However, if the decoupled light output should be smaller than the light output determined by the transmission due to the selected field overlap, it must be accepted that the difference is emitted into the room unused. In the event that the majority of the mode is decoupled and the minor part is held in the resonator, a special laser is to be used, in which the stimulated emission with the remaining portion of the resonator mode (approx. 30%) remaining during transmission occurs.

Preferred, stable mode orbits with periodic resonator modes with and without reflection points with violation of the internal total reflection occur if, according to a further invention, for the smaller radius R ₂ values between 0.2 Ri and 0.8 Ri and for the eccentricity factor δ values between 0, 3 R ₂ and 1, 0 R ₂ can be selected, whereby the values can fluctuate in a range of up to ± 30%. Then there are mode tracks in closed and in open polygon shape. In principle, suitable mode trajectories result for all selected smaller radii R ₂ if the eccentricity factor δ is selected appropriately. With regard to technical applications, however, the choice of the smaller radii R ₂ in a central region is particularly favorable; the choice of the eccentricity factor δ then supports stable mode paths with particularly large stability island configurations. According to a next embodiment of the invention, the inner disk resonator can be formed by an air space with a refractive index ni = 1. The annular billiard then only consists of a ring resonator and is particularly simple and inexpensive to manufacture. For all the solutions mentioned and possible, locally narrow mode paths within the resonator are preferred. Here, fewer lines or even only one line are contained in the emission spectra, since fewer longitudinal vibrational states or even only one longitudinal vibrational state occur. The aim should always be a node of the standing longitudinal wave exactly in the characteristic reflection point. Since the preceding explanations deal with the radiation character of light, the aspect mentioned stands in the background with regard to the wave character of light. With regard to an embodiment of the circular resonator as an annular billiard, it is therefore particularly advantageous, according to a further embodiment of the invention, if the resonator modes capable of propagation are closely localized in the circular resonator, with in particular only one longitudinal • mode occurring.

Forms of embodiment of the invention and diagrams are explained below with reference to the schematic figures for further understanding of the invention. It shows:

1 shows a microlaser according to the invention with a deformed

Circular resonator and active output waveguide, Figure 2 shows a microlaser according to the invention with a deformed

Circular resonator and passive coupling-out waveguide, FIG. 3 shows a Poincare section and the associated characteristic mode path for a circular resonator in an egg-like manner

Form, Figure 4 is a Poincare section and the associated characteristic

Mode path for a circular resonator in the form of a cut-off ellipse, FIG. 5 shows a Poincare section and associated characteristic mode paths for a circular resonator in the form of an annular billiard with a low spatial resolution, FIG. 6 shows the Poincare cut and associated characteristic mode tracks according to FIG. 5 with a higher spatial resolution,

Figure 7 shows a Poincare section and the associated characteristic

Mode track for a circular resonator in the form of an annular billiard with a transmission,

FIG. 8 shows another Poincare section and the associated characteristic mode path for a circular resonator in the form of an annular billiard with a transmissive / refractive inner edge.

FIG. 1 shows a microlaser ML according to the invention with a deformed circular resonator CR in an egg-like form. The microlaser ML is grown in a monolithically integrated manner on a substrate SU in a known manner and, in the exemplary embodiment shown, consists of an n-contact layer CL (for example n-doped InP), an n-conducting light-guiding waveguide layer WL (for example n-doped quaternary GalnAsP) a refractive index n ü _»of an active layer AL which determines the emission wavelength (for example GalnAsP compound semiconductor with a correspondingly selected energy band gap) and a p-type cover layer SL including a p-contact layer (for example p-doped binary InP with a temporary p-contact layer) , for example GalnAs).

The boundary of the circular resonator CR is calculated in the illustrated embodiment according to the general formula:

r (φ) = Ro (1 + εi cos 2φ + ε ₂ cos 3φ).

This achieves the egg shape shown in FIG. 1 (for the parameter value selection see FIG. 3). The circular resonator CR has a single one Mirror level MP on, no other mirror levels exist. A stable, periodic resonator mode RM with a mode path MC is shown schematically, the properties of which are exploited in the microlaser ML according to the invention. The resonator mode RM has a triangular mode orbit MC and rotates clockwise and counterclockwise in the circular resonator CR (arrow directions). At characteristic reflection points CPRi, CPR ₂ , CPM, the resonator mode RM is totally reflected on the inner boundary IS. The characteristic reflection point CPM is distinguished in a special way since it coincides with the point of intersection of the mirror plane MP with the inner boundary IS of the circular resonator CR on its flatter side. Defined light components of the incident resonator mode RM can exit here in a directed manner. Due to the circulation of the resonator mode RM in both clockwise directions, the exit can take place in two directions in the extension of the respective leg of the triangular mode path MC.

In the characteristic reflection point CPM in the microlaser ML according to the invention, an output waveguide WG with an end face AF is optically connected directly by a shock coupling IR. The coupling-out waveguide WG, which forms an adjustable optical light valve LV, has an effective refractive index ΠW _G , which deviates slightly from the effective refractive index nwι_ of the circular resonator CR. A directional correction based on the effective refractive indices, which take into account the light propagation in limited space, according to Snellius' law of refraction can be omitted due to the small differences in refractive index when coupling out the light, but can also be carried out if necessary. Light from the excellent resonator mode RM can be coupled out in a defined manner via the directly connecting coupling waveguide WG. The measure of the laser light to be coupled out is determined by the measure of the overlap of the optical field of the emerging resonator mode RM at the characteristic reflection point CPM with the optical field of the developing one Fashion in the coupling-out waveguide WG (shown schematically in the figure by gray ellipses). It can be seen that the field can be adjusted in a simple manner via the height and width of the coupling-out waveguide WG. Furthermore, a fork BO of the coupling-out waveguide WG is shown in two waveguides WGi, WG ₂ . These are aligned so that both emerging light beams of the excellent resonator mode RM can be optimally received. In the case of a very small width in the butt coupling IR required due to the selected degree of coupling, the coupling waveguide WG or its bifurcated waveguide WGi, WG _{2 can then} , as shown in the selected example, then increase in width again in the course of the process, in order to provide a stronger waveguide for the light guidance to achieve in curvatures or an adaptation of the optical field for further signal routing.

Due to the possibility of the adjustability of the laser light to be coupled out, the quality of the microlaser ML according to the invention as a measure of the ratio between the light energy held in the resonator and the emitted light energy can be optimally matched to the respective intended use and dimensioned in terms of construction. The light energy to be decoupled is usually in a range between 10% and 30%.

The output waveguide WG shown in FIG. 1 has the same layer structure as the circular resonator CR, which is grown together with the microlaser ML. The decoupling waveguide WG thus also has light-active material and is designed as an active waveguide AWG. The waveguide layer WL can be switched optically opaque or optically transparent by a separate contact, which is not shown in the figure, so that a switchable light valve LV is present as a function of a threshold value current. Furthermore, the output waveguide WG can also be designed as an amplifier. In all cases, reflection at its output facing away from the microlaser ML must be prevented. This can by a good anti-reflective coating (better than -40dB) or by a lower anti-reflective coating (better than -10 dB) together with a formation of the chip interface at an angle of, for example, approximately 7 °.

Analogously to FIG. 1, FIG. 2 also shows an egg-shaped deformed microlaser ML with a butt-coupled coupling-out waveguide WG. In contrast to the coupling-out waveguide WG according to FIG. 1, the coupling-out waveguide WG shown here has no active layer AL and no cover layer SL and is therefore designed as a transparent passive waveguide PWG. In this case, the layer sequence up to and including the active layer (s) is removed again after the growth. There is also the possibility of an area-selective epitaxial growth step. The optical field overlap determining the degree of coupling is set by the lateral and vertical guidance of the optical field in the remaining waveguide rib RWG. The optical fields are again represented by corresponding gray ellipses. It can be seen in FIG. 2 that the surge coupling IR in the case of the passive coupling-out waveguide PWG can be made much wider than in the case of the active coupling-out waveguide AWG.

In FIGS. 3 and 4, Poincare sections that are particularly suitable for the microlaser ML according to the invention are shown with a plot of the calculated reflection angle sinχ over the phase angle φ. In this case, only the upper half of the Poincare section is shown in FIG. 3, which corresponds to the mode path in only one direction of rotation, in this case the resonator modes RM rotating in the clockwise direction. The reference to the aforementioned Poincare cuts is not to be regarded as conclusive for the invention. Overall, on the basis of the selection criteria mentioned, there are a large number of possible mode paths MC for differently designed circular resonators CR with only one mirror plane MP and at least one suitable for butt coupling IR of a coupling-out waveguide WG Reflection point, which is either calculated (CPR) or simplified by the intersection of the mirror plane MP with the resonator edge IS at a point of lower curvature (CPM) or in the annular billiard due to a violation of the condition of total internal reflection (CPT, see below) is. The current shape of the characteristic mode track MC is shown in FIGS. 3 and 4 in association with the Poincare cuts. Which points the mode line corresponds to in the Poincare cut can be determined by comparing the corresponding phase angles. As a rule, it corresponds to the periodic islands of stability in the Poincare sections, which correspond to clusters of reflection points as an indication of a particularly stable resonator mode.

FIG. 3 shows the Poincare section for a circular resonator CR with a mirror plane MP in an egg-like form with the standardized formation formula:

r (φ) = R ₀ (1 + εi cos 2φ + ε ₂ cos 3φ)

with ro as constant an assumed average radius (normalization size "1") and εi, ε ₂ as the selected deformation factors. εi is the value for the deformation factor 0.02 and the deformation factor ε _2, the value 0.03 is selected. The result A characteristic mode path MC in the form of a double triangle according to Figure 4. The assignment of the reflection points RP is indicated by arrows. The double triangle has an apex, which is the characteristic reflection point CPM (cf. arrows in the Poincare section) through the intersection of the mirror plane MP with the boundary of the circular resonator CR on its flatter side, the value for sinχ can be reduced with other value assignments down to a value of 0.3, which is about the reciprocal of a refractive index n ι_ = 3.3 of the light-guiding layer with conventional The material structure in the circular resonator CR corresponds to that for other typical parameter combinations two neighboring stability islands merge and a stable triangular path is created, which can be assigned to these pronounced stability islands.

FIG. 4 shows the Poincare section for a circular resonator CR in the form of a cut-off ellipse with the standardized formation formula:

r (φ) = [(1 - ε ² ) ^{l 4} ] / [H- ε cos φ]

with an average, variable radius R ₀ = (1-ε ² ) ^1/4 as a parameter with a constant area and a deformation factor ε = (a ² -b ² ) ^1/2 / a (numerical eccentricity). Due to the vertical section, the ellipse loses its double symmetry, so that only the large semi-axis a is at the same time mirror plane MP. The value 0.75 is chosen for the deformation factor ε. The ellipse shown is cut off half the distance c between a focal point FP and the center MT of the ellipse (a quarter of the distance between the two focal points) parallel to the small semi-axis b, so that the section factor τ has the value 0.5 in the selected exemplary embodiment. The ellipse is thus cut off on one half of the vertical semi-axis b. At least the ellipse can be exactly halved. In the case of the ellipse, the deformation factor ε can also be referred to as “numerical eccentricity” in the form of the quotient of the distance c to the major semi-axis a (ε = c / a).

The result is a stable, revolving resonator mode RM with a triangular mode path MC, which has a characteristic reflection point CPM at the point of intersection of the mirror plane MP with the section plane (see arrows in the Poincare section), in which two directed beams emerge and in a coupling waveguide, not shown, can be coupled. The curvature of the ellipse serves for the special stabilization of the resonator mode RM. The Poincare sections shown in FIGS. 5 to 8 relate to circular resonators CR in the form of an annular billiard with a mirror plane MP. This is an eccentric combination of a ring resonator RR with an inner disk resonator DR, which have different refractive indices n _{1 (} n ₂ and different radii Ri, R _2. The outer radius Ri is constant (standardization variable “1”), the inner radius R ₂ and the center distance between ring and disc resonator RR, DR (eccentricity factor δ) can be set in relation to the outer radius Ri. With a suitable choice of values for these two parameters (in particular R ₂ = 0.2..0.8 R _{1 (} δ = 0.3 .... 1.0 R ₂ , for example R ₂ = 0.6 Ri and δ = 0.35 or 0.85 or 0.65 R ₂ ), one finds stable mode paths MC, the characteristic reflection points with compliance (CPR, CPM, compare FIGS. 5, 6 and 8) and / or a violation of the total internal reflection (CPT, compare FIG. 7). In the first case, the light decoupling can be set between zero and maximum by the field overlap and the not decoupled The light portion remains in the circular resonator CR. In the second case, the greater part of the laser light generated leaves the circular resonator CR via the outer ring resonator RR. This share can then be used in full. In the event of partial use, the unused portion is released freely into the room, but the advantage in this case is the directed, vertical exit of the laser radiation from the ring resonator RR, so that the entire light output can be transmitted with a single, non-forked waveguide. In the first case, the total internal reflection is maintained, however, the exit angle between the two light beams is still small enough to be able to receive both beams with a common coupling-out waveguide, which can then bifurcate in the further course.

In FIGS. 5 and 6, for an annular circular resonator CR with the parameter value assignments R ₂ = 0.6 Ri and δ = 0.22 Rj, the Poincare cut in different resolutions and selected mode paths MCj shown. The refractive index n _{2 of} the inner disk resonator DR in the area of the refractive index nn (iA 3.3) of the outer ring resonator RR is selected such that total reflection always occurs on the inner disk resonator DR - just like on the outer edge of the outer ring resonator RR (" hard wall "). Characteristic reflection points CPRi, CPM are shown, in which shock coupling of a coupling-out waveguide is possible. The stable mode tracks MC ₄ and MC ₅ are particularly suitable for attaching a coupling-out waveguide at the point CPM. The mode tracks MC ₁ and MC ₃ are completely and mode path MCβ largely unsuitable for the microlaser according to the invention due to their poor stability and is shown here only for comparison of the stability islands resulting from the section calculation, which are shown in the Poincare section according to Figure 6 with a computational resolution of 250 mode paths even more pronounced with 250 points each to recognize and to judge accordingly.

In FIG. 7, a Poincare cut for a circular resonator CR in the form of an annular billiard with the parameters listed above is a stable resonator mode RM with a parameter value assignment of R ₂ = 0.5 Ri and δ = 0.25 Ri zigzag-shaped mode track MC shown. This begins and ends in two particularly excellent reflection points CPT-i, CPT ₂ , in which the condition of total internal reflection is violated. The resonator mode RM hits the outer boundary IS of the ring resonator RR perpendicularly at both reflection points CPT-i, CPT ₂ and transmits with a share of approximately 70%. With a decoupling waveguide (not shown further ₎ , a single, vertically emerging light beam with maximum light output can be received at these points CPT1, CPT ₂ .

The Poincare section according to FIG. 8 was created for the same parameter assignment as for FIGS. 5 and 6 with the difference that here the Refractive index n ₂ with 1 (air) was selected, which means that the annular billiard is formed only by a ring resonator RR with a refractive index ni = 3. Preferred mode paths MCi, MC ₂ with characteristic reflection points CRi, CPM also result here. The mode path MCi corresponds to the mode path MC according to FIG. 5. However, the mode path MC ₂ is particularly interesting, since here the resonator mode RM passes through the inner air space, so that the inner circle of the ring resonator RR can be referred to as a “soft wall” with transmission Such an annular billiard design is new and can be used particularly well for the microlaser according to the invention.

LIST OF REFERENCE NUMBERS

a big semi-axis

AF face

AL active layer

AWG active waveguide b small semi-axis

BO fork c route FP / MT

CL n contact layer

CPR characteristic reflection point for total reflection CPM characteristic reflection point at intersection MP / IR

CPT characteristic reflection point for transmission

CR circular resonator

DR disc resonator

FP focal point IR shock coupling i running index IS inner boundary

LV light valve

MC Modenbahn

ML microlaser

MP mirror plane

MT center n effective refractive index

P corner point

PWG passive waveguide r, R radius

RM resonator mode

RR ring resonator

RWG waveguide rib

SL p-type cover layer including p-type p-contact layer

SU substrate

WL waveguide layer

WG decoupling waveguide

ß central angle δ eccentricity factor ε deformation factor τ section factor φ phase angle

% Angle of reflection

Claims

claims

1. Monolithically integrated microlaser with a circular resonator having only one mirror plane in a semiconductor layer structure for the planar formation of revolving resonator modes, which are reflected at reflection points in the circular resonator, and with at least one monolithically integrated coupling-out waveguide, which is assigned to a fixed reflection point of a stable resonator mode and one with which of the circular resonator has a comparable refractive index, to form an optical light valve for an adjustable light coupling into the end face of the coupling waveguide, characterized in that the fixed reflection point (CPM) in a Poincare section computed for a given dimensioning of the circular resonator (CR) by comparison to other calculated stability islands, the largest possible and densely documented stability islands of a occurring stable resonator mode (RM) are fixed locally and that in this characteristic reflection point (CPM) the coupling-out waveguide (WG) is optically connected directly to the circular resonator (CR) via a coupling coupling point (IR) for coupling out a directed light beam, the degree of coupling out being dimensioned by dimensioning the coupling coupling point (IR) occurring optical field overlap between the stable resonator mode (RM) and the expressed mode in the output waveguide (WG) is adjustable.

2. Monolithically integrated semiconductor laser according to claim 1, characterized in that the output waveguide (WG) is designed as a switchable active waveguide (AWG) with a separate contacting of the active layer (WL) in the semiconductor layer structure.

3. Monolithically integrated semiconductor laser according to claim 1, characterized in that the Auskoppeiwellenleiter is designed as an optical amplifier.

4. Monolithically integrated semiconductor laser according to claim 1, characterized in that the output waveguide (WG) is designed as a transparent passive waveguide (PWG).

5. Monolithically integrated semiconductor laser according to one of claims 1 to 4, characterized in that the Auskoppeiwellenleiter (WG) behind the shock coupling point (IR) on the circular resonator (CR) a fork (BO) for coupling out two directed light beams of a periodic in both directions of rotation Has resonator mode (RM).

6. Monolithically integrated semiconductor laser according to one of claims 1 to 5, characterized in that in the case of a deformed design of the circular resonator (CR), the characteristic reflection point (CPM) through the intersection of the mirror plane (MP) with the boundary (IS) in the region of its smallest Curvature is fixed

7. Monolithically integrated semiconductor laser according to claim 6, characterized in that the circular resonator (CR) in egg-like form with the polar coordinates r, φ according to the standardized formation formula: r (φ) = Ro (1 + εi cos 2φ + ε ₂ cos 3φ) is formed with a constant mean radius R ₀ and selectable deformation factors εi, ε ₂ .

8. monolithically integrated semiconductor laser according to claim 7, characterized in that for the deformation factor εi a value between 0.01 and 0.05 and

Deformation factor ε ₂ a value of 0.01 and 0.1 is chosen, depending on

Value combination both factors can fluctuate by up to ± 30%.

9. Monolithically integrated semiconductor laser according to claim 6, characterized in that the circular resonator (CR) in the form of a cut ellipse with a large semiaxis a and a small semiaxis b with the polar coordinates r, φ according to the standardized formation formula: r (φ) = [(1 - ε ² ) ^1/4 ] / [l + ε cos φ] with an average radius Ro = (1-ε ² ) ^1/4 and a deformation factor ε = (a ² -b ² ) ^1/2 / a is formed, which is cut off at a point predetermined by a selected section factor τ between a focal point (FP) and the center point (MT) of the ellipse parallel to the small semi-axis b.

10. A monolithically integrated semiconductor laser according to claim 9, characterized in that a value in the range from 0.3 to 0.9, in particular between 0.75 and 0.85, for the deformation factor ε and a value for the section factor τ can be selected in a range between 0.25 and 0.75, in particular 0.5.

11. A monolithically integrated semiconductor laser according to one of claims 1 to 5, characterized in that when the circular resonator (CR) is designed in the form of an annular billiard from an outer ring resonator (RR) with a constant radius Ri and an inner disk resonator (DR) with a smaller radius R ₂ , the centers of which are shifted from one another by an eccentricity factor δ and which have different refractive indices ni, n ₂ , the characteristic reflection point (CPM) through the intersection of the mirror plane (MP) with the The boundary (IS) of the outer ring resonator (RR) is fixed at the location of the greater distance from the inner disc resonator.

12. A monolithically integrated semiconductor laser according to one of claims 1 to 5, characterized in that with an embodiment of the circular resonator (CR) in the form of an annular billiard consisting of an outer ring resonator (RR) with a constant radius Ri and an inner disk resonator (DR) a smaller radius R ₂ , the centers of which are shifted from one another by an eccentricity factor δ and which have different refractive indices, two characteristic reflection points (CPT) due to the periodic impact of the periodic resonator mode (RM) on the edge (IS) of the outer ring resonator (RR) locally are set.

13. Monolithically integrated semiconductor laser according to claim 11 or 12, characterized in that for the smaller radius R ₂ values between 0.2 Ri and 0.8 Ri and for the eccentricity factor δ values between 0.3 R ₂ and 1.0 R ₂ can be selected, whereby the values can fluctuate in a range of up to ± 30%.

14. Monolithically integrated semiconductor laser according to one of claims 1 1 to 13, characterized in that the inner disc resonator (DR) is formed by an air space with a refractive index n ₂ = 1.

15. Monolithically integrated semiconductor laser according to one of claims 7 to 14, characterized in that the propagatable resonator modes (RM) are closely localized in the circular resonator (CR), in particular only one longitudinal mode occurring.