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

#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.

#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.