Optical waveguides – Polarization without modulation
Reexamination Certificate
2002-02-08
2004-11-02
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Polarization without modulation
C385S031000, C385S037000, C385S049000, C385S129000
Reexamination Certificate
active
06813399
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical devices, and more particularly, to optical devices such as optical interleavers and polarization-dependent optical isolators used for WDM (wavelength division multiplexing) optical communication.
2. Related Background Art
FIG. 21
shows an example of a conventional optical demultiplexer
900
made of a natural birefringent material, such as rutile. The demultiplexer
900
includes an ingoing optical fiber
901
, an ingoing lens
902
, a birefringent material
903
such as rutile, a first outgoing lens
904
, a first outgoing optical fiber
905
, a second outgoing lens
906
, and a second outgoing optical fiber
907
. Incident light that is coupled from the ingoing optical fiber
901
through the ingoing lens
902
into the birefringent material
903
is separated into ordinary light (TE wave)
909
and extraordinary light (TM wave)
908
. The ordinary light
909
and the extraordinary light
908
, which are separated by a distance proportional to the length of the birefringent material
903
, are then coupled by the outgoing side lenses into the first outgoing optical fiber
905
and the second outgoing fiber
907
.
Natural birefringent materials, such as rutile, have a polarization-dispersion surface (refractive index ellipsoid) as shown in FIG.
22
. For this reason, the light that is incident on the birefringent material
903
propagates in directions perpendicular to the dispersion surface of the ordinary light and to the dispersion surface of the extraordinary light, in accordance with the law of conservation of momentum.
However, the difference between the dispersion surface of ordinary light and the dispersion surface of extraordinary light for natural birefringent materials such as rutile is small, and consequently, the separation angle of ordinary and extraordinary light is small as well. Therefore, the length of the birefringent material has to be long, so that the demultiplexer
900
becomes large.
On the other hand, optical devices using photonic crystals in which the refractive index changes periodically have been proposed (see JP 2000-180789A, JP 2000-241762A, JP 2000-241763A and JP 2000-284225A). It should be noted that throughout this specification, “photonic crystal” means an artificial multi-dimensionally periodic structure having a periodicity of the same order as the wavelength of light.
Conventional optical isolators that use photonic crystals as polarizers use photonic crystals having a photonic band structure in which either TE waves or TM waves are reflected at the ingoing plane. Therefore, the photonic crystal is tilted from the optical axis such that the reflected light (returning light) from the photonic crystal is not coupled into the light-source side. Such an optical isolator reflects light that is incident light from the light-source side, so that a separate optical system or optical design is necessary to make sure that the reflected light is not coupled into the light-source side, which makes the structure more complicated.
In view of these problems, it is an object of the present invention to provide a small optical device, with which incident light can be demultiplexed.
SUMMARY OF THE INVENTION
In order to achieve the object of the present invention, an optical device (CL
1
) according to the present invention includes a first optical member separating an incident light of wavelength &lgr; into TE wave and TM wave; and an optical input portion, which inputs the incident light into the first optical member; wherein the first optical member has a periodically changing refractive index; wherein an angle defined by a first reciprocal lattice vector &agr;
1
and a second reciprocal lattice vector &agr;
2
of the first optical member at the wavelength &lgr; is not larger than 90°; wherein, in the direction of the first reciprocal lattice vector &agr;
1
, the wave number of the TE wave is larger than the wave number of the TM wave; wherein, in the direction of the second reciprocal lattice vector &agr;
2
, the wave number of the TE wave is smaller than the wave number of the TM wave; and wherein the optical input portion inputs the incident light in a direction that is parallel to a plane P
12
including the first reciprocal lattice vector &agr;
1
and the second reciprocal lattice vector &agr;
2
. With this optical device, the difference between the dispersion surfaces of the TE waves and the TM waves in the demultiplexing portion becomes large, and the separation angle of the TE waves and the TM waves can be set to be large. As a result, it is possible to separate TE waves and TM waves at a propagation distance at which the influence of diffraction can be ignored, and it is possible to reduce the number of optical parts, such as lenses, and to make the device smaller.
Furthermore, another optical device (CL
6
) of the present invention includes, in addition to this optical device (CL
1
), a phase retarder and an optical output portion; wherein the first optical member, the phase retarder and the optical output portion are arranged such that light that enters from the optical input portion is transmitted in that order; wherein the optical input portion inputs a plurality of p light beams (wherein p is an integer), whose wavelengths range from a wavelength &lgr;(1) equal to &lgr; and increase at constant wavelength intervals to a wavelength &lgr;(p), in a direction that is parallel to the plane P
12
; and wherein the phase retarder imparts a difference in polarization between light beams of odd-numbered wavelengths and light beams of even-numbered wavelengths. With this optical device, the first optical member and a phase retarder imparting a constant polarization difference between odd-numbered wavelengths and even-numbered wavelengths are integrated into one optical device. Therefore, combining two kinds of polarizations and two kinds of wavelengths (odd-numbered wavelengths and even-numbered wavelengths), this optical device can be applied for devices with a variety of functions. This optical device is particularly preferable for WDM optical communication with light of a plurality of wavelengths that are separated by a constant wavelength interval.
REFERENCES:
patent: 5998298 (1999-12-01), Fleming et al.
patent: 6188819 (2001-02-01), Kosaka et al.
patent: 6317554 (2001-11-01), Kosaka et al.
patent: 6621644 (2003-09-01), Tokushima
patent: 2001/0026659 (2001-10-01), Sekine et al.
patent: 2003/0169787 (2003-09-01), Aldaz et al.
patent: 2003/0174961 (2003-09-01), Hamada
patent: 11-271541 (1999-10-01), None
patent: 2000-56146 (2000-02-01), None
patent: 2000-180789 (2000-06-01), None
patent: 2000-224109 (2000-08-01), None
patent: 2000-232258 (2000-08-01), None
patent: 2000-241762 (2000-09-01), None
patent: 2000-241763 (2000-09-01), None
patent: 2000-284225 (2000-10-01), None
patent: 2001-13439 (2001-01-01), None
patent: 2001-51122 (2001-02-01), None
patent: 2001-74954 (2001-03-01), None
“Photonic-Crystal Slabs with a Small Variation in Refractive Index and Application to Optical Functional Devices” by Hidenobu Hamada (Technical Report of IEICE (The Institute of Electronics, Information and Communication Engineers) OPE 2001-107 (2001-107 (2001-12), pp. 19-24).
Pak Sung
Sanghavi Hemang
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