FIG. 8 9_ 97 __L__ 81 as ___i; 95
INTEGRATED OPTICAL FILTER APPARATUS
BACKGROUND OF THE INVENTION
This application claims the priority of Korean Patent Application No. 10-2005-0007241, filed on Jan. 26, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an optical filter apparatus, which separates or combines incident light according to wavelengths, and, more particularly, to an integrated optical filter apparatus, which can be used for a miniaturized waveguide and can reduce crosstalk between adjacent signals.
2. Description of the Related Art
In general, optical filters are employed in optical signal transceivers using multiple wavelengths, which separate and receive an optical signal or combine and transmit optical signals via an optical waveguide according to wavelengths. In the meantime, to integrate optical devices with electronic devices and manufacture cheap miniaturized optical devices, studies of silicon-based optical waveguides have recently been conducted. Accordingly, optical filters used together with the silicon-based optical waveguides are required to more efliciently separate a multiple-wavelength signal transmitted via the miniaturized optical waveguides.
FIG. 1 is a schematic diagram of a conventional optical filter apparatus. Referring to FIG. 1, a conventional optical filter apparatus 5 separates an optical signal beam LO transmitted via an optical waveguide 1 into first and second light beams L 1 and L2 having first and second wavelengths A1 and A2 to be respectively read by a first photo-detector 2 and a second photo-detector 3. That is, the optical filter 5 reflects a component of the first wavelength A1 from the optical signal beam LO to produce the first light beam L1, and transmits a component of the second wavelength A2 from the optical signal beam LO to produce the second light beam L2, such that the optical signal beam LO is separated into the first light beam L1 and the second light beam L2. Accordingly, the first and second light beams L 1 and L2 transmitted via the optical waveguide 1 and separated by the optical filter 5 are respectively received by the first and second photo-detectors 2 and 3 to detect optical signals.
Here, to separate an optical signal, the optical filter apparatus 5 is formed by alternately stacking material layers 6 and 7 with different refractive indices (e.g., layers made of SiO2 and Si) in a multiple layer structure. Here, the thickness of each of the material layers 6 and 7 is detennined to be about one fourth of the first wavelength A1. Accordingly, the optical filter 5 can reflect the first light beam L 1 of the first wavelength A1 and transmit the second light beam L2 of the second wavelength A2.
FIG. 2 is a graph illustrating a relationship between reflectance and wavelength of the optical filter 5 constructed as above.
Referring to FIG. 2, the first wavelength A1 is 1490 mn and the second wavelength A2 is 1550 mn. In the graph, since the reflectance of the light of the first wavelength A1 is 0 dB, the light of the first wavelength A1 is reflected 100% and is not directed to the second photo-detector 3. However, since the reflectance of the light of the second wavelength A2 is greater than approximately -13 dB, about 5% of the light is not transmitted through an incident surface 5a of the optical filter 5 but is reflected to be directed together with the light of the first wavelength A1 to the first photo-detector 2. Accordingly,
the light beam of the first wavelength A1 does not affect signal detection by the second photo-detector 3, but the light beam of the second wavelength A2 appears as crosstalk when the first photo-detector 2 detects a signal from the first light beam L1, thereby failing to maintain crosstalk at less than -30 dB required for high quality signal detection.
The present invention provides an integrated optical filter apparatus, which can be integrated into a miniaturized waveguide and can reduce crosstalk between adjacent signals.
According to an aspect of the present invention, there is provided an integrated optical filter apparatus comprising: a first optical filter reflecting a light beam of a first wavelength and transmitting a light beam of a second wavelength from an extemally incident light beam; and a second optical filter facing the first optical filter and reflecting the light beam of the first wavelength reflected by the first optical filter back to the first optical filter, wherein the light beam of the first wavelength is reflected by the first optical filter at least twice to remove noise within the light beam of the first wavelength.
According to another aspect of the present invention, there is provided an integrated optical filter apparatus comprising: an optical filter reflecting a light beam of a first wavelength and transmitting a light beam of a second wavelength from an extemally incident light beam; and a reflecting member facing the optical filter and reflecting the light beam of the first wavelength reflected by the optical filter, wherein the light beam of the first wavelength is reflected by the optical filter at least twice to remove noise within the light beam of the first wavelength.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:
FIG. 1 is a schematic diagram of a conventional optical filter apparatus;
FIG. 2 is a graph illustrating a relationship between reflectance (dB) and wavelength of the optical filter apparatus of FIG. 1;
FIG. 3 is a schematic diagram of an integrated optical filter apparatus according to an exemplary embodiment of the present invention;
FIG. 4 is a graph illustrating a relationship between reflectance (dB) and wavelength of the integrated optical filter apparatus of FIG. 3;
FIG. 5 is a schematic diagram of an integrated optical filter apparatus according to another exemplary embodiment of the present invention;
FIG. 6 is a schematic diagram of an integrated optical filter apparatus according to still another exemplary embodiment of the present invention;
FIG. 7 is a schematic diagram of an integrated optical filter apparatus according to yet another exemplary embodiment of the present invention;
FIG. 8 is a schematic diagram of an integrated optical filter apparatus according to a further exemplary embodiment of the present invention; and
FIG. 9 is a schematic diagram of an integrated optical filter apparatus according to another exemplary embodiment of the present invention.
DETAILED DESCRIPTION OF ILLUSTRATIVE, NON-LIMITING EMBODIMENTS OF THE INVENTION
The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The described exemplary embodiments are intended to assist the understanding of the invention, and are not intended to limit the scope of the invention in any way.
Referring to FIG. 3, an integrated optical filter apparatus according to an exemplary embodiment of the present invention includes first and second optical filters 20 and 25 facing each other with a space therebetween. The integrated optical filter apparatus of the present exemplary embodiment separates an optical signal beam transmitted via a waveguide 11 into first and second light beams L21, and L22 having central wavelengths 701' and 702' to be respectively received by the a first photo-detector 13 and a second photo-detector 15. Particularly, 701' denotes the wavelength reflected by the first and second optical filters 20 and 25 and 702' denotes the wavelength transmitted through the first optical filter 20.
Here, each of the first and second optical filters 20 and 25 is formed by stacking one or more pairs of layers 21 and 23. The paired layers 21 and 23 have different refractive indices, and each of the paired layers 21 and 23 has a thickness of
where 70 denotes a central wavelength of each of the light beams reflected by the optical filter and “n” denotes a positive integer.
It is preferable, but not necessary, that the first layer 21 be made of SiO2 and the second layer 23 be made of Si, SiNX or SiON. Here, which one of the first and second layers 21 and 23 is placed at the top of the other may be changed. If the first layer 21 is made of SiO2 and the second layer 23 is made of Si, SiNX, or SiON, a difference between the refractive indices of the two materials can be increased. Accordingly, even though only a few pairs of first and second layers 21 and 23 are stacked, a reflectance near to 1 for a desired wavelength can be easily obtained. For example, when the first and second layers 21 and 23 are respectively made of SiO2 and Si, a refractive index difference therebetween can be higher than 2.
The first optical filter 20 constructed as above reflects the first light beam L21 of the first wavelength 701' and transmits most of the second light beam L22 of the second wavelength 702' of the light beam emitted from the waveguide 11 and obliquely incident on a first incident surface 20a such that the light beam emitted from the waveguide 11 is separated into the first and second light beams L21 and L22. Here, the most of the second light beam L2 2 transmitted through the first optical filter 20 is received by the second photo-detector 15. Part of the second light beam L22 is reflected by the incident surface 20a of the first optical filter 20 and is directed to the second optical filter 25 along the same path as the reflected first light beam L21.
The second optical filter 25 is an optical filter having the same structure as the first optical filter 21, and performs the same function as the first optical filter 21 for the incident first and second light beams L21 and L22. That is, the first light beam L21 reflected by the first optical filter 20 and incident on the second optical filter 25 is totally reflected by a second incident surface 25a of the second optical filter 25 to the first
optical filter 20. Meanwhile, most of the second light beam L22 incident on the second optical filter 25 is transmitted through the second optical filter 25 and part of the second light beam L22 is reflected by the second incident surface 25a of the second optical filter 25 and is directed to the first optical filter 20 along the same path as the first light beam L21, reflected by the second incident surface 25a of the second optical filter 25. Since the second light beam L22 directed together with the first light beam L21 to the second optical filter 25 is removed in this way, the influence of the second light beam L22 on the light reception of the first photo-detector 13 can be minimized.
The light beams reincident on the first optical filter 20 are transmitted or reflected according to wavelengths, and the reflected beams are directed to the first photo-detector 13 while the light beam of the second wavelength 702' is filtered in the space between the first optical filter 20 and the second optical filter 25.
FIG. 4 is a graph illustrating a relationship between reflectance and wavelength of the integrated optical filter apparatus constructed as above. Referring to FIG. 4, the first wavelength 701' is 1490 mn, the second wavelength 702 is 1550 mn, segment A represents characteristics of a conventional filter apparatus with no second optical filter 25, segment B represents characteristics of the filter apparatus with the second optical filter 25 when the first light beam L21 is reflected five times by the first and second optical filters 20 and 25, and segment C represents characteristics of the filter apparatus with the second optical filter 25 when the first light beam L21 is reflected ten times by the first and second optical filters 20 and 25.
Referring to FIG. 4, since the reflectance of the light beam of the first wavelength 701' is 0 dB and thus the light beam of the first wavelength 701' is reflected 100%, the light beam is not directed to the second photo-detector 15. Since the conventional optical filter apparatus does not employ the second optical filter, the reflectance of the light beam of the second wavelength 702' is approximately -13 dB as shown by segment A. However, when the optical filter apparatus employs the second optical filter 25, the reflectance of the light beam of the second wavelength 702' is much less than -50 dB as shown by segment B and segment C. The light beam of the second wavelength 702' is transmitted through the first or second optical filter 20 or 25 and is rarely directed to the first photodetector 13.
Further, when a reflectance standard rarely affected by crosstalk is set to be less than -30 dB required for high quality signal detection, the bandwidth WB including the second wavelength 702' is approximately 17 mn in case of segment B, and the bandwidth WC including the second wavelength 702' is approximately 33 mn in case of segment C. Accordingly, noise can be reduced even with a wavelength fluctuation error of the second light beam L22 in that bandwidth.
Referring to FIG. 5, an integrated optical filter apparatus according to another exemplary embodiment of the present invention includes first and second optical filters 41 and 43 facing each other with a space therebetween, and a plurality of additional optical filters 45, 46, 47, and 48. The integrated optical filter apparatus separates an optical signal beam transmitted via a waveguide 31 into light beams respectively having central wavelengths 701", 702", . . . , 70,,_1", 70,," to be read by a plurality of photo-detectors 35, 36, 37, and 38.
Since the first and second optical filters 41 and 43 have the same structure and are substantially the same as the first and second optical filters 20 and 25 of the exemplary embodiment illustrated in FIG. 3, a detailed explanation thereof will not be given. Meanwhile, each of the first and second optical filters 41 and 43 transmits most of the light beam of the wavelength
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