Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1997-10-10
2001-07-31
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Plural
Reexamination Certificate
active
06269202
ABSTRACT:
BACKGROUND OF INVENTION
This invention relates to optical communication devices and in particular to a fiber optic filter useful in wavelength division multiplexing and demultiplexing. The invention is particularly useful for reducing the wavelength shift of filtering devices against the temperature change. This invention also has applications in altering the wavelength or other optical characteristics of filters and other optical devices.
In recent years, optical fiber technology for telecommunication has progressed rapidly. While the theoretical transmission capacity of the single mode optical fiber has been recognized in the industry as extremely high from the day such fiber was introduced, much of the capacity has not been utilized. For the increasing demand for bandwidth, such as in the transmission of video images and graphics, much attention has been directed lately toward the maximal utilization of the bandwidth of the single mode fiber. Wavelength division multiplexing (WDM) is one of the most viable schemes of maximizing bandwidth utilization of single mode fiber.
In a WDM system, multiple signal sources emitting at different wavelengths, &lgr;
1
, &lgr;
2
, . . . , &lgr;
n
, are coupled into the same single-mode fiber by means of a multiplexer. After the signals of different wavelengths are transmitted through the fiber to a desired destination, the multiple wavelength signals carried by the respective multiple wavelengths must then be separated by means of a demultiplexer into separate optical channels, each wavelength being carried by a different channel to a detector.
WDM based systems have evolved rapidly from early two channel systems to the current 16 channel system. International Telecommunication Union (ITU) has even proposed a 45 channel system utilizing wavelength range from 1533 to 1565 nm with channel spacing of 100 GHz (about 0.8 nm). Furthermore, a WDM system of channel spacing of 50 GHz is being fostered. It challenges optical component manufacturers to provide ultra-narrow bandwidth filters with highly stable pass wavelength against environmental temperature change. This invention is related to the enhancement of the temperature stability of the pass band of the filtering devices.
WDM multiplexers and demultiplexers can be made by employing thin film filters, diffraction grating, waveguides, Bragg in-fiber grating. The WDM employing thin film filters is widely used because of its excellent optical characteristics such as lower loss and higher channel isolation comparing with other technologies.
Filters are usually formed by a stack of thin films made by the deposition processes. The control of center wavelength accuracy of the filter during the deposition process and its stability against temperature change are extremely challenging, particularly for the dense WDM such as 100 GHz spacing system. The shift of the center wavelength under temperature change of 100 degree C. is required to be within 0.1 nm or smaller to prevent the shift from interfering with the adjacent channels.
The optical thickness of thin films responds to the temperature change and therefore the location of the center wavelength shifts with the temperature. The temperature coefficient of the center wavelength shift depends on the film structure, film materials, deposition process and others. Lower temperature coefficient is desired. The typical temperature coefficient of the filter made by the state of art deposition process ranges from 0.003 to 0.01 nm° C. A temperature change of 100° C. would cause a wavelength shift of about 0.3 nm to 1 nm, which is too large for dense WDM applications.
It is therefore desirable to introduce an improved filter system with a stable characteristic frequency over a large temperature range.
SUMMARY OF THE INVENTION
This invention is based on the observation that optical characteristics such as frequency or wavelength of a filter can be altered by changing the stress in the filter. Changing the stress in the filter alters the optical thicknesses of the thin film(s) in the filter, thereby also changing its frequency characteristic. If the change in stress is so as to reduce or cancel the wavelength shift caused by thermal expansion or contraction of the thin films, a filter with stable wavelength characteristic over a range of temperatures is achieved.
In this application, the characteristic frequency of an optical interference filter can include the center frequency of a band pass filter and an edge frequency of a low pass or high pass filter.
One aspect of the invention is directed towards an apparatus for filtering an optical signal, comprising an optical interference filter having a characteristic frequency and means for applying stress to the filter to compensate for effects of temperature on the characteristic frequency.
Another aspect of the invention is directed towards a method for filtering an optical signal, comprising providing an optical interference filter; causing change of stress in the filter and passing said optical signal through the filter.
One more aspect of the invention is directed towards a method for making an optical filter, comprising providing an optical assembly including an optical interference filter having two sides, said filter connected on one side to a first GRIN lens structure and on another side to a second GRIN lens structure; passing the assembly through a tube having a temperature expansion coefficient that is different from that of the assembly and attaching one side of the tube to the first lens structure and another side of the tube to the second lens structure.
Yet another aspect of the invention is directed towards a wavelength division multiplexer/demultiplexer, comprising an optical assembly that includes an optical interference filter having two sides and two GRIN lens structures, said filter connected on one side to a first GRIN lens structure and on another side to a second GRIN lens structure; means for applying stress to the filter to alter a characteristic frequency of the filter; one or more input optical channels carrying light of one or more wavelengths to the assembly and one or more output optical channels carrying light of one or more wavelengths from the assembly.
An additional aspect of the invention is directed towards an optical isolator assembly, comprising an optical isolator; an input optical channel supplying light to the isolator; an output optical channel carrying light from the isolator and means for applying stress to the isolator to alter the isolation characteristic of the isolator.
Yet one more aspect of the invention is directed towards an optical assembly, comprising an optical device having an optical characteristic that changes with stress in the device; an input optical channel supplying a collimated light beam to the device; an output optical channel carrying a collimated light beam from the device and means for applying stress to the device to alter the optical characteristic of the device.
Another aspect of the invention is directed to a method for optical processing, comprising providing an optical device having an optical characteristic that changes with stress in the device; supplying a collimated light beam to the device; delivering a collimated light beam from the device; and applying stress to the device to alter the optical characteristic of the device.
Still one more aspect of the invention is directed towards an optical assembly, comprising an optical device having an optical characteristic that changes with stress in the device; a piezoelectric member connected to the device and means for applying a voltage to the member to alter the optical characteristic of the device.
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patent: 3614188 (1971-10-01), Seeley
patent: 3949259 (1976-04-01), Kostlin et al.
patent: 4548478 (1985-10-01), Shirasaki
patent: 4949005 (1990-08-01), Parham et al.
patent: 5138219 (1992-08-01), Krisl et al.
patent: 5266238 (1993-11-01), Haacke et al.
patent: 5463494 (1995-10-01), Hobrock
patent: 5579420 (1996-11-01), Fukushima
patent: 5859717 (1999-01-01), Scobey et al.
Chiang Brian
Lee Ho-Shang
Lo Ming-Ching
Dicon Fiberoptics, Inc.
Skjerven Morrill & MacPherson LLP
Ullah Akm E.
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