Integrated tunable optical filter

An integrated tunable optical filter comprising a substrate made of a semiconducting material. The substrate includes first and second sections. The first section forms a tunable transmission filter, based on a codirectional coupler having a low selectivity. The second section forms a reflection filter with a reflection spectrum containing a number of peaks. A first injector, designed for current injection into said first section, is provided. Thus, the filter response of the first section is shifted in wavelength over a large wavelength range. There is also a second injector, designed for current injection into the second section. As a result, the reflective spectrum of the second section is slightly shifted in wavelength, in such a way that one reflection peak of the reflection spectrum corresponds to the coupling wavelength of the first section. Consequently, the total filter response has a very narrow bandwidth and wide tunability.

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.

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 .mu.m, inclusive. Typical values for the period of 
the sawtooth-shaped structure are below 1 .mu.m. 
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 
.lambda., 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.sub.codir of said section 2, the 
period of the rib-shaped structure 41 and the effective refractive indices 
n.sub.2, and n.sub.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.sub.a and n.sub.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 .DELTA..lambda..sub.3db is the spectral bandwidth of the 
codirectional coupler: 
##EQU3## 
where: .lambda. is the wavelength, 
.DELTA. is the period of the rib-shaped structure in the first section 
L.sub.codir is the length of the first section 
n.sub.a, n.sub.b are the effective refractive indices of the two waveguides 
in the first section, 
L.sub.1 is the length of the sawtooth-shaped structure grating in one 
period of the second section, 
L.sub.2 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 .DELTA..lambda..sub.1, the tuning range of the 
codirectional structure, and .DELTA..lambda..sub.2, 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: 
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Codirectional coupler section 
Length L.sub.codir 900 .mu.m 
n.sub.a 3.210 
n.sub.b 3.307 
.LAMBDA. 16 .mu.m 
Coupling coefficient 17.5 cm.sup.-1 
Broken sawtooth-shaped structure 
Length 900 .mu.m 
N.sub.eff 3.24 
L.sub.1 75 .mu.m 
L.sub.2 7.5 .mu.m 
.LAMBDA.samgrat 0.2413 .mu.m 
Coupling coefficient 10 cm.sup.-1 
Filter response 
Central wavelength .lambda. = 1.550 .mu.m 
Tuning range 50 nm 
Change of refractive index 
0.3 nm 
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