Grating-assisted co-directional vertical coupler optical filter having well-suppressed sidelobe characteristics and method of fabricating the same

[0002] 1. Field of the Invention

[0003] The present invention relates to a tunable optical filter(hereinafter, called TOF) having a wavelength selective property used in an optical communication of a wavelength division multiplexing(hereinafter, called WDM) and method fabricating the same and, more particularly, to an optical filter whose sidelobe disturbing characteristics of an optical filter by weighting coupling efficiency between waveguides is controlled upon applying a selective region growth method in a wavelength selective variable semiconductor optical filter and method of fabricating the same.

[0004] 2. Discription of the Prior Art

[0005] Generally, the structure for realizing the optical filter are several, however, there are often used a Grating-Assisted Co-directional Coupler(GACC) filter coupled in vertical using a semiconductor material whose an integration of an optical device is good. In the structure of a conventional grating assisted co-directional coupler optical filter, there is generated an optical couple upon inserting lattices between two optical waveguides as shown in FIG. 1, the light incident in one waveguide is used as a signal light upon being taken out in the other waveguide. At this time, the lattice 13 functions as coupling light between two waveguides, when two waveguides are asymmetric, only any special wavelength is coupled to other waveguide. The grating assisted co-directional coupler as above can change the wavelength of light generated to a second waveguide by changing the refractivity of the waveguide using a current injection or electric field effect. When an i-th wavelength (^λi) is outputted from the wavelength variable optical filter as above to the second waveguide, if a (i+1)-th or (i−1)-th wavelength are outputted to the second waveguide, there is generated an error for detecting an i-th wavelength. Accordingly, to generate only the i-th wavelength to the second waveguide, it is ideal that an optical wavelength spectrum coupled to the second waveguide from the first waveguide must be 0 on the (i−1)-th wavelength adjacent to the (i+l)-th wavelength. However, the optical filter structure shown in FIG. 1 has a form such as an output spectrum coupled to the second waveguide is shown in a slender line 1 of FIG. 3 relative to an optical wavelength change and a small peak value 1acalled a sidelobe from the right and left of the central wavelength. Since a channel adjacent according to the position of this sidelobe is influenced, the remove of the sidelobe is essential so that the optical filter can be practical. This sidelobe is a cause to keep an optical couple efficiency between two waveguides constant because the distance between two waveguides is constant as shown in FIG. 1.

[0006] In conventional, a first and second waveguide were fabricated on the same plane through a photo-mask work such as a semiconductor process in the form of plane. However, in the case of fabricating an optical filter in the plane form, the distance control between two waveguides is easy, however there is a shortcoming that the wide band is very enlarged because the shape of the waveguide is not even and it is difficult to insert the lattice, therefore, there was performed an optical filter fabrication using LiNbO3 single crystal without performing the crystal growth. However, in the case of an optical filter using the semiconductor, because the crystal growth can be possible by an epitaxial growth method, there was possible the optical filter fabrication in the form of a vertical couple, in the case of a crystal growth, it is possible to fabricate an optical filter of an excellent characteristic because the distance between two waveguides is accurately controlled and a parameter control giving an effect on the characteristic of a filter such as a thickness, width of the waveguide is accurately achieved.

[0007] It is an object of the present invention to provide an optical filter whose sidelobe is controlled upon changing the waveguide distance as well as accurately controlling a characteristic since a first waveguide and a second waveguide are fabricated in the form of vertical couple to restrain a sidelobe, namely, one of an important characteristic of the filter and improve the characteristic of the optical filter due to providing a spatial additional value with an optical couple efficiency by means of a selective area growth(SAG) between two waveguides and method of fabricating the same.

[0008] In order to accomplish the above object, the present invention provides a grating assisted vertical coupling optical filter whose sidelobe is controlled. Such an optical filter comprises: an n-type InP substrate; an InGaAsP first waveguide layer accumulated in the thickness of 0.3˜0.8 μm on said n-type InP substrate, limited in spatial to have the width of 1˜3 μm and the length of 3˜10 mm; an n-type InGaAsP grating layer formed in the separation of 0.1˜0.5 μm from the first waveguide layer, limited in spatial to have the thickness of 0.02˜0.2 μm and the period of 10˜30 μm; an n-type InP layer accumulated in the thickness of 1˜3 μm on the n-type InGaAsP lattice layer; a p-type InGaAsP second optical waveguide layer accumulated in the thickness of 0.2˜1 μm on said n-type InP, limited in spatial to have the width of 1˜5 μm of and the length of 3˜10 mm; and a p-type InP clad layer accumulated in the thickness of 1˜3 μm on the second optical waveguide layer wherein, the distance between the two waveguides nearest at the middle portion of the optical filter is 0.5˜1.5 μm, and farthest at both ends of the optical filter is 2˜4 μm.

[0009] In order to accomplish the above object, the present invention provides a method of fabricating a grating assisted vertical coupling optical filter. Such a method comprises steps of: growing an InGaAsP of material consisting a first waveguide on a InP substrate by an organic metal vapor deposition method(OMVPE), to be patterned to have the width of 1˜3 μm; forming an InP layer and a grating layer thereon, to be patterned to have the period of 10˜40 μm; patterning a dielectric thin film deposited thereon; forming an InP layer thereon by a selective area growth method, and controlling the thickness in spatial; growing an InGaAsP of material consisting a second waveguide on the InP substrate, forming a second waveguide upon being patterned to have the width of 1˜5 μm in the strip form; and controlling a sidelobe upon growing said InP thereon.

[0010] The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments, when taken in conjunction with the accompanying drawings, in which:

[0011] FIG. 1 is a sectional view of a conventional vertical coupling grating assisted wavelength variable optical filter.

[0012] FIG. 2 is a sectional view of a vertical coupling grating assisted wavelength variable optical filter of the present invention.

[0013] FIG. 3 is transmission characteristics comparative view of a vertical coupling grating assisted wavelength variable optical filter of the present invention and a conventional vertical coupling grating assisted wavelength variable optical filter.

[0014] FIG. 4ais a construction view of a substrate pattern before SAG growth to achieve functional form of growth thickness according to the present invention.

[0015] FIG. 4bis a change view of InP thickness according to the position in FIG. 4a.

[0016] FIG. 5 is an experimental result view of the embodiment of FIG. 4a.

[0017] FIGS. 6athrough 6fare fabrication processes view of a vertical coupling grating assisted wavelength variable optical filter of the present invention.

[0018] Hereinafter, the present invention will be explained in detail with reference to drawings attached.

[0019] FIG. 2 is a construction view of a vertical coupling grating assisted wavelength variable optical filter suggested in the present invention.

[0020] As shown in FIG. 2, an optical filter of the present invention comprises an n-type InP substrate 21, an n-type InP buffer layer accumulated in the thickness of 0.5˜1 μm thereon, an InGaAsP first waveguide layer 22 accumulated in the thickness of 0.3˜0.8 μm thereon, limited in spatial to have the width of 1˜3 μm, and the length of 3˜10 μm, an n-type InGaAsP grating layer 23 formed in the distance of 0.1˜0.5 μm from the first waveguide layer 22, limited in spatial to have the period of 10˜30 μm, the thickness of 0.02˜0.2 μm, an n-type InP layer 24 accumulated in the thickness 1˜3 μm, a p-type InGaAsP second optical waveguide layer 25 accumulated in the thickness of 0.2˜1 μm, limited in the space to have the width of 1˜5 μm, and the length of 3˜10 mm thereon, and a p-type InP clad layer 26 accumulated in the thickness of 1˜3 μm thereon.

[0021] In this structure, there is consisted an InP between InGaAsP first and second waveguide layers whose a band gap is high, a middle portion distance thereof is nearest as the distance of 0.5˜1.5 μm, both end terminals is farthest as the distance of 2˜4 μm. If a coupling coefficient between two waveguides satisfies the following formula known as Hamming function, the sidelobe is controlled as shown in the thick line in FIG. 3. That is, when the middle portion of an optical filter in FIG. 2 is z=0, and the coupling coefficient K(z) satisfies the following formula, the sidelobe is suppressed largely. $\kappa \left(z\right)={\kappa }_0\left[1+0.852\cos \left(\frac{2\pi z}{L}\right)\right]$

[0022] The bandwidth is increased in the optical filter constructed as above. However, even though the bandwidth is increased, the sidelobe may be reduced so that the optical filter can be used in the actual system. Therefore, the optical filter constructed in the vertical couple form as above has a special advantage.

[0023] The optical filter having the construction above is illustrated in the basic structure only, if the electrode is formed in order to inject current into the optical filter, it can be operated by a waveguide variable optical filter. That is, there are formed a p-type electrode to an upper surface of the optical filter, and a n-type electrode to a lower surface of the optical filter so that a wavelength change of the couple light by means of the current injection or electric field effect can be realized.

[0024] Further, if a double lattice is formed so that the n-type InGaAsP lattice layer can be suitable to the respective lights TE- and TM- polarized, it can be operated by the wavelength variable optical filter which is polarization-independent. That is, the n-type InGaAsP lattice layer period can be formed independent to the polarization upon forming the lattice periods corresponded to TE polarization and TM polarization in a double simultaneously.

[0025] The embodiments to realize the construction of the present invention are shown in FIGS. 4a, 4b, and FIGS. 6athrough 6f. In the semiconductor crystal growth, if a dielectric thin film pattern is formed on a SiNx dielectric thin film 42 portion on the semiconductor surface in FIG. 4a, since the lattice structure of the dielectric thin film is different from the semiconductor, the crystal is not grown on the dielectric thin film, but on the semiconductor. The crystal growth method as above is called a selective area growth (SAG). In the SAG, because growth materials(when growing InP, In-atom and P-atom) arrived on the dielectric thin film are not grown on the dielectric thin film, they move to the side surface, accordingly, to be grown in the thickness on the boundary area of the dielectric thin film and the semiconductor. Accordingly, the wider the pattern of the dielectric thin film is, the more the growth material is moved to the lateral. Therefore, the growth of the boundary area is formed in the thicker. In the case forming the dielectric thin film as FIG. 4a, the A-region of FIG. 4ais thicker than the B- region, therefore, it can be applied to embody the optical filter structure as FIG. 2. FIG. 4bshows the thickness of InP grown on the semiconductor 43 between the dielectric thin film patterns according to the position(x). There can be attained the proposed purpose to embody the structure of FIG. 2 by changing the width of the dielectric thin film. In FIG. 4, it is suitable that the maximum width (W1) of the dielectric thin film is 100˜200 μm, the semiconductor width (W2) between the dielectric thin film pattern is 2˜30 μm. FIG. 5 shows an experiment result grown. The grown thickness is changed according to the change of W1, W2 of FIG. 4a, accordingly, which can sufficiently embody the structure in FIG. 2.

[0026] A method of fabricating a grating assisted vertical coupling optical filter is illustrated through FIGS. 6a-6cas follows: first, after growing an InGaAsP material constructing a first waveguide 62 on the InP substrate by a vapor deposition method(VPE) or an organic metal vapor deposition method (OMVPE) or liquid deposition method(LPE), then to patternized to have a constant width (FIG. 6a). An InP layer 63 and grating layer 64 are grown thereon, to be patterned with a rectangular type whose only the grating layer is designed(FIG. 6b). After depositing an dielectric thin film thereon, the lattice pattern is formed as shown in FIG. 6c. When forming the lattice pattern, the width of the middle portion of the optical filter can be narrow, and the width of the end portion can be wide, to control the thickness between two waveguides by a selective area growth method.

[0027] If growing the InP crystal on the lattice pattern by a selective area growth method an InP epitaxial layer 66 whose thickness is controlled spatially as FIG. 6dis obtained. After growing the InGaAsP layer of a second waveguide a second waveguide 67 (FIG. 6e) is formed after being patterned in the form of strip to have a constant width. If the waveguide in the semiconductor is formed upon growing InP on the whole surface, the optical filter is accomplished (FIG. 6f) with the structure shown in FIG. 2.

[0028] Because the optical filter fabricated as described above does not have a constant value of the optical coupling efficiency between two waveguides but can provide the spatial change, to obtain a waveform whose the sidelobe providing the disturbance for real application is removed. Accordingly, the crosstalk characteristic between adjacent channels of WDM communication can be considerably improved.

[0029] As described above, although the present invention has been described in detail with reference to illustrative embodiments, the invention is not limited thereto and various modifications and changes may be effected by one skilled in the art within the scope of the invention.