Tunable Laser Diode

1. Field of the Invention

The present invention is directed generally to semiconductors and more specifically to a tunable semiconductor laser diode.

2. Description of the Related Art

Laser light sources that are tunable over a broad range are required for advanced optical communication systems. Simple operation and a broad tuning range are required of such laser diodes. A tunable laser diode that is divided into an amplifier region, a coupler region and an absorber region over its length and that is referred to as an ACA laser is described in the publication by S. Illek et al., "Co-Directionally Coupled Twin-Guide Laser Diode for Broadband Electronic Wavelength Tuning" in Electronics Letters 27, 2207-2208 (1991). Two waveguides are arranged vertically relative to one another and separated from one another by a cladding layer. Light that propagates in a first mode is generated and amplified in the upper waveguide. This wave is coupled into the lower waveguide in the coupler region. The absorber region prevents the emission of the mode in the upper waveguide. Laser mode becomes possible due to the exchange of the radiation capacity between the two modes guided in the two waveguides.

The publication by I. Kim et al., "Broadly Tunable InGaAsP/InP Vertical-Coupler Filtered Laser With Low Tuning Current" in Electronics Letters 29, 664-666 (1993) discloses a VCF laser diode wherein an amplifier region, a coupler region and what is referred to as a window region are arranged following one another. The amplifier region and the coupler region have separate power terminals. The amplifier region has a quantum well layer structure. The suppression of longitudinal side modes is relatively slight given the two structures disclosed in said publications since the wavelength-selective filtering and tuning occurs only in a sub-area of the overall laser diode. This can lead to an inadequate and unstable single-mode emission.

This problem can be essentially avoided when the filtering is distributed over the entire laser length, as, for example, in the DFC laser structure from the publication by M. C. Areann et al., "Widely Tunable Distributed Forward Coupled (DFC) Laser" in Electronics Letters 29, 793-794 (1993). Two waveguides are likewise arranged vertically relative to one another and coupled in this structure. Absorption regions that form a filtering grating in longitudinal direction are arranged vertically relative thereto in longitudinal direction.

Another proposed solution for suppressing side modes is disclosed in the publication by M. Oeberg et al., "74 nm Wavelength Tuning Range of an InGaAsP/InP Vertical Grating Assisted Codirectional Coupler Laser with Rear Sampled Grating Reflector" in IEEE Photonics Technology Letters 5, 735-738 (1993). The functions of filtering and tuning are thereby separated from one another and are separately optimized. As in the lasers set forth above, the codirectional mode coupling with its relatively slight spectral filtering of the (spectrally proximate) side modes is employed for broadband tuning. The far selection for modes at a greater spectral distance is good in the codirectional mode coupling (particularly when the coupler length corresponds to a coupling length). A great suppression of the side modes is therefore achieved by the additional employment Of a specific Bragg grating structure as laser end mirror. The good near selection of this specific Bragg grating, however, is acquired at the expense of a poor far selection (ambiguity). Due to the good far selection of the codirectional coupler, however, this does not mean a deterioration of the operating properties will result for the overall laser structure. The critical disadvantage of this combination--in addition to the relatively great length--is the critical synchronization of the filter function with the tuning function. This requires an extremely exact adjustment of two control currents and has a decisively negative effect on practical manipulation.

An object of the present invention is to specify an optimally short laser diode that is as simple as possible in technological terms and that can be tuned unambiguously, reproducibly and largely single-mode over an extremely broad wavelength range with only one tuning current.

This object is achieved in the tunable laser diode of the present invention. The tunable laser diode has a first and a second waveguide arranged in a layer plane as layers vertically above one another with respect to the layer plane and separated from one another by a cladding layer, the first and second waveguides being arranged so closely adjacent to one another so that a mode coupling occurs between the waveguides. The first waveguide is located only in a coupling region that has a length less than the entire length of a section of the laser diode provided as a laser resonator. The first and second waveguides have dimensions and material combinations such that a coupling of two different modes occurs in the coupling region. The coupling region has at least one interconnected section having a length equal to a natural multiple of the coupling length of the two different modes. The laser also has an active layer and a tuning layer arranged vertically relative to one another with respect to the layer plane in the first and second waveguide in the coupling region, and means for separately injecting currents into the active layer and into the tuning layer.

In the laser of the invention, the filter and tuning functions are not separated from one another; near and far selection can nonetheless be separately optimized. Only one control current is required because of the self-alignment within this laser diode. The combination of the filtering and tuning in one function unit also makes it possible to manufacture extremely short components. Typical lengths of the laser diode of the invention therefore are 200-600 .mu.m. In contrast, the overall structure must usually be a few mm long with previously known components having separate optimization of near and far selection. The functioning of the laser of the invention is based on the principle of the Mach-Zehnder interferometer that is vertically integrated into the laser diode. The abbreviation VMZ laser shall therefore be used below.

In the VMZ laser, a further waveguide coupled to the waveguide provided for the guidance of the laser light is arranged vertically relative to the guidance of the laser light. This further waveguide is present only in a sub-section of the overall length of the laser diode. The filter effect is produced by the coupling of the modes propagatable in the two waveguides. A good near selection as well as a good far selection can be achieved by a providing the second waveguides in a plurality of sections of different length.

A description of the VMZ laser diode of the present invention follows with reference to the figures.

FIGS. 1-4 show various exemplary embodiments of the VMZ laser of the invention in longitudinal section.

FIG. 5 shows filter curves for various embodiments of the present invention.

FIG. 6 shows the layer structure of an embodiment of the present invention in longitudinal section.

FIGS. 7 and 8 show the cross sections of the present invention referenced in FIG. 6.

In the simplest embodiment of the VMZ laser shown in a schematic longitudinal section in FIG. 1, the length of the laser resonator is divided into three sections. In succession, these sections are: a first single region 11 having the length S1, a double region 12 having the length F1 and a second single region 13 having the length S2. Respectively only one waveguide 4 is present in the single regions, only a transversal mode (mode T) being propagatable therein. A further waveguide 2 is arranged vertically parallel to the first waveguide 4 in the double region 12 provided for the filtering. Two guided modes R and S are propagatable in this double region 12. In the simple illustration of FIG. 1, the waveguide 4 present over the entire length of the component is applied on a lower cladding layer or on a substrate 5. This waveguide 4 is buried into a cladding layer 8 in which the second waveguide 2 is embedded. The mode T of the single regions 11, 13 is coupled with the two modes R and S of the double region 12 at the boundary planes H1, H2. Since the two modes R and S have different phase velocities (different effective refractive indices, wherein the difference of the effective refractive indices is approximately 0.1), a constructive interference of the modes R and S excited by mode T at the boundary surface H1 at the boundary surface H2 given renewed coupling into mode T is only possible at specific, discreet wavelengths. The coupling length is the determining factor for specifying these wavelengths. Constructive interference thus occurs when the length F1 of the double region 12 is a whole multiple of the coupling length. Since the coupling length is equal to the quotient of wavelength and difference between effective refractive indices, the wavelengths of the constructive interference occur as a whole fraction of the product of length F1 of the double region 12 and the difference between the effective refractive indices: .lambda.=F1.times.(n1-n2)/N, where n1 and n2 are effective refractive indices and N is a natural number.

When N is extremely large, these resonant wavelengths lie extremely close to one another and at approximately identical spacing (given negligible dispersion of the effective refractive indices). When the length F1 is extremely long, the near selection can therefore be extremely high in the immediate environment of a select, resonant wavelength, whereas, on the other hand, a deterioration of the far selection occurs clue to the ambiguity of the filter curve.

These properties are illustrated with reference to FIGS. 5a-5c, which show the fundamental form of the filter curves. FIG. 5a shows the curve of the transmission characteristic (filter curve) in the wavelength range around a specific resonant wavelength .lambda..sub.0 for an extremely large N (for example, N=20). The course of the filter curve is schematically shown in FIG. 5b for N=1. The flatter course of the resonance maximum in this case causes a slight selection of the closest wavelengths. The far selection, however, is substantially greater than in the example of the filter curve of FIG. 5a.

Given the VMZ laser diode, the tuning of the wavelength occurs by modifying the difference of the effective refractive indices with charge carrier injection into one of the waveguides 2, 4 in the double region 12. To this end, electrical contacts are applied to the upper sides of the diode and the regions of the cladding layer 8 are doped such that currents can be separately injected into the two waveguides 2, 4. As shown in FIG. 1, the overall laser resonator is limited by the laser end mirrors M1, M2.

A near selection that is greater by approximately 3 dB is possible with the structure modified according to FIG. 2, since the length F1 of the double region 12 here is effectively doubled because of the reflection of the radiation at the right-hand resonator end mirror M2. A simultaneous optimization of the near selection and far selection is possible by series connection of a plurality of filter regions of different length, as schematically shown in FIGS. 3 and 4. In FIG. 3, a first single region 11, a second single region 13, a third single region 15 and a fourth single region 17 having the respective lengths S1, S2, S3 and S4 are shown. Each of these regions are interrupted by a first double region 12, a second double region 14 and a third double region 16 having the respective lengths F1, F2 and F3. The second waveguide 2 provided for the coupling is correspondingly divided into sections having the lengths F1, F2 and F3 in longitudinal direction. Each of these regions are embedded into the cladding layer 8. Due to the different lengths of these different sections, a good selection of a single wavelength from the overall spectrum is achieved. A first double region 12 having an especially great length F1 and a second double region 14 having a substantially shorter length F2 are provided in the structure shown in FIG. 4. A combination of the good near selection, as shown by the filter curve of FIG. 5a, with the good far selection as shown by the filter curve of FIG. 5b thus derives. The filter curve of FIG. 5c is therefore achieved as transmission characteristic for this embodiment of FIG. 4. It is thereby particularly significant that a uniform and equally strong tuning occurs given the common contacting of the various sections, so that the two filter curves fundamentally shift synchronously in common on the wavelength scale without further measures when the wavelength is modified by modifying the tuning current. This represents a critical simplification in the manipulation of the VMZ laser diode (particularly compared to the structure disclosed in the initially cited publication by Oeberg et al. ) since the user need not be concerned with the mutual adjustment of near and far selection.

The layer structure of a practical embodiment of the VMZ laser shall be set forth with reference to FIG. 6-8 and the attached tables. FIG. 6 shows the longitudinal section of a VMZ laser diode according to the exemplary embodiment of FIG. 1. The waveguide 4 is applied as a stripe-shaped layer onto a substrate 5 or onto the uppermost layer of an epitaxially overgrown semiconductor wafer. The further waveguide 2 is embedded between cladding layers 1, 3 present on this waveguide 4 in the section (double region) provided for the coupling. The sections outside the coupling regions (single regions) are filled up with the cladding layers 6. In the embodiment of FIG. 6, the further waveguide 2 is 3-layer and has an active layer 2a, a parting layer 2s and a tuning layer 2t. A tuning layer can be alternatively arranged at that side of the parting layer facing away from the substrate. The active layer is then situated between the parting layer and the substrate. Excellent tuning properties occur when the active layer and the tuning layer are arranged together in one of the waveguides because the tuning range is enlarged--by approximately 30-50% in this case--due to the interaction (via the optical losses produced by the tuning) between the tuning layer and the active layer. It is also possible to fashion both waveguides single-layer, whereby the one waveguide then represents the active layer and the other waveguide represents the tuning layer. The waveguides 2, 4 can also be produced as quantum well layers or multi-quantum well layers.

The sections entered in FIG. 6 are shown in FIGS. 7 and 8. The lateral cladding layers 7 that limit the stripe-shape layer structure of the double waveguide are shown in FIGS. 7 and 8. Highly doped contact layers 9, 10a, 10b are respectively applied onto the various regions of the cladding layers. The remaining semiconductor material is doped such that the cladding layer 1, the cladding layer 3, the waveguide 4, the substrate 5 and the contact layer 9 on the cladding layer 1 are doped for electrical conduction of a first conductivity type. The parting layer 2s, the upper cladding layer 6 in the single regions, the lateral cladding layers 7 and the contact layers 10a and 10b on the lateral cladding layers 7 are doped for electrical conduction of the opposite conductivity type. For example, the first conductivity type is advantageously p-conduction and the second conductivity type is n-conduction. The respective contacts Ke, Kn, Ks for the electrical conduction toward the outside are applied onto the contact layers 9, 10 and on the substrate 5. The region between the contact layers on the upper side can be planarized with an oxide layer Ox.

Typical material combinations and dopings as well as the thicknesses of the individual layers are recited in attached Table 1 for an exemplary embodiment in the material system of InP. When the active layer and the tuning layer are each respectively established by one of the two waveguides 2, 4, the cladding layer 3 located therebetween for the substrate 5 and the upper cladding layer 1 must be correspondingly doped with the opposite conductivity type. The cladding layer 3 is then n-InP and the waveguides 2, 4 are undoper InGaAsP. Further parameters of the disclosed exemplary embodiment recited in Table 1 include:

a wavelength of 1.5 .mu.m

a strip width W of 1-2 .mu.m.

Effective index difference between mode R and S: approximately 0.1

Basic period .LAMBDA.: 15 .mu.m

Dispersion .beta.:2

Optical confinement factor in the tuning layer: approximately 30%

Table 2 recites typical length dimensions, the spacing of the resonant wavelengths and the tuning region for the embodiments of FIGS. 1, 2 and 4. A typical value is respectively recited in parenthesis in Table 1 for the recited regions.

The double regions wherein two waveguides are present vertically relative to one another are referred to in common as coupling region. Dependent on the embodiment of the VMZ laser diode, this coupling region can have one or more interconnected sections. In the simplest embodiment, the lower waveguide 4 is applied directly onto a substrate 5. Instead of a substrate, this semiconductor layer can also be a lower cladding layer 5. When the active layer or the tuning layer is fashioned in the lower waveguide 4 and a doped, lower cladding layer 5 is present for the power supply into this lower waveguide layer 4, the appertaining terminal contact can be applied on the upper side, so that a semi-insulating substrate can also be used. The measurers provided for the separate current injection into the active layer and into the tuning layer correspond to those of a known TTG laser diode, particularly to those of the initially cited ACA laser diode.

Although other modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.