Integrated tunable optical filter

This application is continuation application of Ser. No. 08/244,110, as PCT/BE93/00061, Sep. 17, 1993 published as WO94/07178, Mar. 31, 1994, now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to an integrated broadly tunable optical filter designed, in particular, for advanced WDM (Wavelength Division Multiplexing) applications as well as for use in spectroscopic testing of various optical components.

Broadly tunable optical filters are key components in optical communication systems. Two objectives dominate with such filters. The aims, on the one hand, are a broad tunable range and, on the other hand, high spectral selectivity. The planar integrated structures known hitherto, however, permit only one of these objectives at a time to be met.

A conventional filter structure, known as co-directional coupler structure, is described in "Broadly tunable InGaAsP/InP buried rib waveguide vertical coupler filter" (R. C. Alferness, L. L. Buhl, U. Koren, B. I. Miller, M. G. Young, T. L. Koch, G. A. Burrus, G. Raybon), Appl. Phys. Lett. 60(8), 24 Feb. 1992. This conventional structure consists of two asymmetrical waveguides having different effective refractive indices. The optical signal launched into the uppermost waveguide is launched into the lowermost waveguide and selectively reflected towards the uppermost waveguide. If the parameters of the waveguides are chosen carefully, a structure of this type can act as a selective filter.

The advantage of this conventional structure is the fact that a broad tuning range is provided, but it is difficult to obtain high spectral selectivity without constructing a long appliance.

On the other hand, a semiconductor optical structure is also known which can provide high selectivity. This conventional structure makes use of a waveguide having a grating grown thereon, that grating of which is periodically omitted. A structure of this type has already been used in a tunable laser (V. Jayaraman, D. A. Cohen, L. A. Coldten, "Demonstration of broadband tunability in a semiconductor laser using sampled gratings", Appl. Phys. Lett 60(19), 11 May 1992). Nevertheless, a structure of this type has hitherto not been used in a filter structure.

This conventional structure provides a comb-shaped reflection spectrum. The parameters thereof are the spacing between the peaks, the spectral bandwidth of the envelope of the comb spectrum, the peak maximums and the bandwidth of a reflection peak. The latter is probably the most important parameter, because it affects the selectivity of the structure. Most unfortunately, this parameter too limits the tuning range of the structure.

SUMMARY OF THE INVENTION

The object of the present invention is to present an optical filter structure which provides, at the same time, a large tuning range and high spectral selectivity in a compact integrated appliance.

In order to accomplish the abovementioned objective, the present invention makes provision for an integrated optical filter which comprises a semiconductor substrate having a common electrode on a first side thereof, the substrate comprising a first waveguide on which a grating is grown, said grating comprising a first part having a large period and a second part having a small period, from which said grating is periodically omitted. At the top of said first grating section and at a distance therefrom, a second waveguide extends in such a way that there is formed, in a first section of the substrate, a codirectional coupler section. Said second waveguide is covered by a semiconductor part whose topside carries a second electrode. Provided on the topside of the substrate at the top of the second grating section there is a third electrode. The refractive indices of both waveguides in the first section are controlled by current injection along said second electrode. The refractive index of the waveguide in the second section is controlled by current injection along said third electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail with reference to the drawing, in which

FIG. 1 diagrammatically represents a cross section of the structure according to the invention;

FIGS. 2 to 6 inclusive represent curves, by way of example, which illustrate the operation of the filter structure of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows an optical filter structure according to the invention. The integrated filter structure mainly comprises substrate 1 made of semiconductor material, for example InP, in which two sections 2 and 3 are formed. On the substrate a number of layers are grown from semiconductor material, for example InGaAsP, which form the lowermost waveguide 4 having a grating 40 grown thereon. Said grating 40 is grown so as to have two different geometrical structures, namely a first rib-shaped structure 41 having a long period in said first section 2, and a second periodically broken sawtooth-shaped or stripe-shaped structure 42 in said second section 3. The last mentioned sawtooth-shaped structure 42 is formed so as to have short-period striped regions 43 and alternating non-striped regions 44. Typical values for the period of the rib-shaped structure range between 10 and 50 μm, inclusive. Typical values for the period of the sawtooth-shaped structure are below 1 μm.

At the top of the first rib-shaped structure 41, and at a distance thereof, an uppermost waveguide 5 extends in such a way that there is formed, in the first second 2 of the substrate, a codirectional coupler section. On the topside of the first section 2 there is provided a first electrode 6, by means of which current injection can be coupled in into said first section. On the topside of the second section 3, too, a second electrode 7 is provided by means of which current injection can be coupled in into said second section. A common back electrode 8 is provided on the underside of the substrate.

An appliance according to the invention works as follows. An optical signal is passed into the uppermost waveguide 5, and at a certain wavelength λ, 100% of the optical power is coupled across to the lowermost waveguide 4. The spectral bandwidth of the codirectional coupler in section 2 is determined by the length L_codir of said section 2, the period of the rib-shaped structure 41 and the effective refractive indices n₂, and n_b of the two waveguides 5 and 4, respectively, according to the following relationship: ##EQU1##

Said coupled wavelength can be altered by varying the refractive indices n_a and n_b based on current injection by applying a voltage between electrodes 6 and 8. FIG. 2 represents an example of a filter response A of a codirectional coupler, and FIG. 3 represents the same filter response A', but shifted in wavelength by current injection. It should be noted that the filter response in section 2 has a low selectivity.

The optical signal which was coupled to the lowermost waveguide 4 ends up in the broken sawtooth-shaped structure 42. The reflection spectrum of said structure is comb-shaped as shown by B in FIG. 4, together with the filter response of the first section. The reflection peaks of the comb spectrum have a high selectivity. The selectivity is mainly determined by the number of periods in the broken structure 42 and the coupling strength of that structure.

The reflection peaks of the comb spectrum B can be changed slightly by current injection in said section when a voltage is applied between electrodes 7 and 8. Thus the Comb spectrum is slightly shifted in wavelength as shown by B' in FIG. 5. Thus it is possible to ensure that the wavelength of one of the reflection peaks corresponds to the central coupling wavelength of the first section 2.

This results in a filter response C having a very narrow bandwidth as shown in FIG. 6, which response is launched into the uppermost waveguide 5 and supplied to the output thereof.

The parameters of the two sections 2 and 3 cannot be chosen totally independently of one another, but must satisfy at least the following condition. This condition establishes a relationship between the spectral bandwidth of the codirectional coupler and the spacing between the peaks in the comb spectrum of section 3. This bandwidth must be smaller than the spacing between the peaks: ##EQU2## where Δλ_3db is the spectral bandwidth of the codirectional coupler: ##EQU3## where: λ is the wavelength,

Δ is the period of the rib-shaped structure in the first section

L_codir is the length of the first section

n_a, n_b are the effective refractive indices of the two waveguides in the first section,

L₁ is the length of the sawtooth-shaped structure grating in one period of the second section,

L₂ is the length of the non-striped portion of the grating structure in one period of the second section,

n is the effective refractive index of the waveguide in the second section.

Furthermore, the choice of the parameters is also restricted by the properties required of the overall structure. Two important properties are the magnitude of the tuning range and the selectivity. The tuning range is determined by both sections and is no greater than the maximum of two variables, namely Δλ₁, the tuning range of the codirectional structure, and Δλ₂, the spectral bandwidth of the envelope of the comb spectrum at the second section 3. ##EQU4##

Typical values in one illustrative embodiment in an InGaAsP substrate:

<pre xml:space="preserve" listing-type="tabular"> <!--Greenbook tabular data-->______________________________________Codirectional coupler sectionLength L_coupler 900 μm 3.210 3.307Å 16 μMCoupling coefficient 17.5 cm^-1Broken sawtooth-shaped structureLength 900 μmN_eff 3.24L₁ 75 μmL₂ 7.5 μm 0.2413 μmCoupling coefficient 10 cm^-1Filter responseCentral wavelength = 1.550 μmTuning range 50 nmChange of refractive index 0.3 nm______________________________________</pre>