Liquid crystal cells – elements and systems – Liquid crystal optical element – Liquid crystal etalon
Reexamination Certificate
1998-09-16
2003-04-08
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Liquid crystal optical element
Liquid crystal etalon
C349S089000, C349S090000
Reexamination Certificate
active
06545739
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tunable wavelength-selective filter capable of tunably selecting any optical signal from among wavelength-division multiplexed optical signals in an optical fiber, and to a manufacturing method for fabricating a high performance, inexpensive tunable wavelength-selective filter.
2. Description of the Related Art
Optical communication systems employing optical fibers have been rapidly put into practice because they can transmit large capacity information at a high speed. At present, however, they can only transmit only optical pulses with a particular wavelength. If they can transmit optical pulses at multiple different frequencies, they will be able to transmit larger capacity of information. This technique is called wavelength-division multiplexing (WDM), and is intensively researched. To achieve the wavelength-division multiplexing, a tunable wavelength-selective filter is necessary which can select only an optical signal of a desired wavelength from among optical pulses with multiple wavelengths.
As conventional tunable wavelength-selective filters of such a kind, there are a grating filter which controls its angle with a motor, a tunable wavelength-selective filter which moves its dielectric filter with a motor, and an etalon filter which controls its cavity length with a piezoelectric element. However, since they are mechanically controlled, they have drawbacks of being slow in response, bulk in module size, and expensive in cost.
To solve the foregoing problems of the tunable wavelength-selective filter, the assignee of the present application proposed a filter capable of varying the optical gap of a Fabry-Perot etalon by applying a voltage across a nematic liquid crystal material filled in the etalon as disclosed in Japanese Patent Application Laying-open No. 220,618/1992. Since the tunable wavelength-selective filter is a planar type device, it has an advantage of being able to be arrayed easily.
The filter using the nematic liquid crystal material, however, has a problem in that it cannot achieve a tuning rate beyond the maximum response rate of the nematic liquid crystal material of about a few milliseconds. A high tuning rate is an essential characteristic for implementing the wavelength-division multiplexing (WDM) in a high speed LAN (local-area network). In addition, since the liquid crystal filters utilize the orientations of the liquid crystal materials varied by applied voltages, their variations in refractive index are not uniform depending on the polarization state or direction of a plane of polarization of incident light. This causes so-called polarization dependence in which the tuning characteristics vary in response to the plane of polarization, which presents a problem in practical use. It is reported that a tunable wavelength-selective filter employing a chiral smectic A has a high response rate of about 10 microseconds. However, it has only a narrow tunable range, and its polarization dependence has yet to be improved (A. Sneh and K. Johnson, “High-speed continuously tunable liquid crystal filter for WDM network”, J. Lightwave Technol. vol. 14, p. 1067, 1996). Accordingly, to implement the WDM in high speed optical LANS, it is desired to develop tunable wavelength-selective filters of being able to solve the foregoing two problems, that is, fast response, polarization-independent tunable wavelength-selective filters of highly practical utility.
On the other hand, low cost, highly reliable, fixed wavelength filters with optical fibers have been developed (T. Oguchi, J. Noda, H. Hanafusa and S. Nishi, “Dielectric multilayered interference filters deposited on polyimide films”, Electronics Letters, Vol. 27, pp. 706-707, 1991).
The fixed wavelength filters with optical fibers are each fabricated by forming a dielectric mirror film on a polyimide film, by dicing it into few millimeter squares, and by embedding them into slots formed in the optical fibers. The fixed wavelength filters with optical fibers employ the polyimide film instead of glass as a substrate, which makes it possible for the dielectric mirror to be diced into few millimeter squares, and to be thinned to about a few tens of micrometers of thickness, enabling them to be inserted into the slots formed in the optical fibers. This obviates optical components such as lenses, and tedious alignment, making it possible to implement a very cheap and highly reliable fixed wavelength filters. However, it is difficult for the fixed wavelength filters with optical fibers to tunably control the filter transmission wavelength.
SUMMARY OF THE INVENTION
The present invention is implemented to solve the foregoing problems involved in the conventional techniques. It is therefore an object of the present invention to provide a polarization-independent tunable wavelength-selective filter with a high response rate and high speed tuning capability by utilizing as a cavity a material in which liquid crystal droplets are dispersed in a light transmissive characteristic medium.
Another object of the present invention is to provide a tunable wavelength-selective filter capable of increasing its tunable range per voltage by adding plasticizer into a polymer material in which the liquid crystal droplets are dispersed and by thus reducing the driving voltage of the liquid crystal droplets, although the liquid crystal droplets have an inclination to increase the driving voltage as their sizes decrease.
Still another object of the present invention is to provide a tunable wavelength-selective filter which is lower in cost and higher in reliability than the conventional tunable wavelength-selective filters, and capable of implementing higher density and smaller size by combining with arrayed optical fibers.
Another object of the present invention is to provide a tunable wavelength-selective filter capable of improving its spectrum characteristics, and of extending its wavelength tunable range to the entire wavelength range by stacking multiple tunable wavelength-selective filters or by using a planar mirror or prism mirror.
A still another object of the present invention is to provide a tunable wavelength-selective filter capable of simply constituting an optical multiplexer or demultiplexer by using a planar mirror.
In order to solve the problems aforementioned, a tunable wavelength-selective filter of a Fabry-Perot etalon type according to the present invention is constructed such that it comprises: a first layer including a transparent electrode and an optical mirror layer; a third layer including a transparent electrode and an optical mirror layer; and a second layer which is composed of a material with a refractive index variable with electric field, and sandwiched between the first layer and the third layer, wherein the material having a refractive index variable with electric field is composed by dispersing liquid crystal droplets equal to or less than 150 nm in diameter in a light transmissive characteristic medium such as a polymer or silica glass.
In the above structure, the optical mirror layer consists of a dielectric multilayer mirror with an optical reflectance of equal to or more than 95% for 1.5 &mgr;m wavelength light, and the glass substrate has a finishing surface precision of equal to or less than &lgr;/10, where &lgr; is a wavelength of light that traverses the tunable wavelength-selective filter.
Further, in the above structure, the liquid crystal droplets consist of nematic liquid crystal droplets, and the polymer contains a plasticizer.
The wavelength-selective filter can be arranged in such a manner that it comprises on its second side one of a prism mirror and a plane mirror, wherein one of a light beam and a light beam array, which is incident on its first side, passes through it, reflects off one of the prism mirror and the plane mirror, and passes through it, and is emitted from its first side.
A fabrication method of a tunable wavelength-selective filter of a Fabry-Perot etalon type according to the presen
Hayashi Takayoshi
Hirabayashi Katsuhiko
Matsumoto Shiro
Sakata Seizou
Sugiyama Yasuyuki
Fitch Even Tabin & Flannery
Nippon Telegraph and Telephone Corporation
Parker Kenneth
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