Optical waveguides – With optical coupler
Reexamination Certificate
2001-10-25
2004-08-24
Sangkavi, Hemang (Department: 2874)
Optical waveguides
With optical coupler
C385S027000, C385S039000, C385S042000, C385S050000
Reexamination Certificate
active
06782152
ABSTRACT:
FIELD OF INVENTION
This invention relates to method and apparatus for tuning the channels of an unbalanced Mach-Zehnder filter to the grids outlined by the International Telecommunication Union (ITU) standards and for the filter channel spacing adjustments by means of an ultraviolet (UV) laser.
BACKGROUND OF THE INVENTION
Wavelength division multiplexing (WDM) devices are used in optical communication systems to carry a number of channels in parallel to its maximum capacity. A number of different technologies exist for WDM components, including thin-film dielectric (interference) filters, planar arrayed waveguides, fiber Bragg gratings, diffraction gratings, and fused-cascaded Mach-Zehnder interferometers (MZI). To meet with the large bandwidth demand, the number of channels are increasing, thus decreasing the spacing between two adjacent channels. Using the Mach-Zehnder technology, channel spacing as narrow as 0.04 nm (2.5 GHz) can easily be fabricated. This spacing theoretically allows for 1600 channels in the C-band region. Furthermore, insertion loss and uniformity is very these components as they are fabricated from standard single-mode silica fiber. The MZI is an odd and even-numbered filter or interleaver. For example a 100 GHz filter results in two periodic sets of WDM channels, one carrying the odd-numbered channels and the other carrying the even-numbered channels. The channels of each of the two sets of channels thus operate with 200 GHz channel spacing.
An asymmetric Mach-Zehnder interferometer (MZI) device comprises two 3 dB couplers with two fibers, F
1
and F
2
, with different optical path lengths in between. The couplers are arranged to transfer half of the optical power centered at 1550 nm along each fiber. Thus, the first coupler splits the input signal into two equal parts sending it to the two optical fibers interconnecting the two couplers. The different optical length of fibers, F
1
and F
2
, leads to a phase delay between the two signals observed from the input ports of the second coupler. It is the difference between the two optical path lengths that determines the channel spacing of the sinusoidal wavelength response of the MZI. The resulting filter has two input and two output ports. These devices are used as wavelength selective devices in optical communication systems and can be used as optical interleavers.
In practical communication systems, the actual channel frequencies of the interleaver should match those of the International Telecommunication Union (ITU) grids that are standards that must be met by all optical interfaces for multi-channel systems. Usually after building MZI the wavelengths do not perfectly match that of the ITU grid and there is a need for further adjustment of the optical phase delays to move the wavelength peaks towards the predetermined locations or even to adjust the wavelength spacing of the filter by altering the optical path length. One way of adjusting the phase delay is to adjust the optical path lengths by exposing the fiber to a UV light so that the refractive index of the optical path is altered to achieve the desired phase delay. In the literature, UV tuning have been introduced for optical phase adjustments by exposing the fiber to a UV light by directing and focusing the UV on the desired location of the fiber surface. This conventional method needs careful operators and a complex optical setup to ensure that the desired fiber length is exposed to the UV source for achieving the desired wavelength shift (see FIG.
1
). In order to achieve the required phase adjustments, the UV light has to be fairly intense as well to reach to the core of the fiber through the polymer coating and cladding and to introduce a large enough refractive index change in a limited exposed area of the fiber. That might cause a permanent damage to that location and increase the optical loss in the MZI. In addition, in the previous arts the particulars of the UV exposure for UV tuning of optical devices are not discussed in detail.
Another major challenge in manufacturing of these filters is to make them environmentally robust, so that the wavelength response becomes stable against temperature variations and mechanical vibrations. As a result, tuning of the filter before its packaging is not very helpful because the optical phase delay can be arbitrarily changed due to local stresses or curves or the like on the fiber arms during the packaging process. These changes will in turn lead to possible unwanted changes in the location of the adjusted wavelengths. Therefore it is best to align the channels after the device has been stabilized for vibration and environmental conditions by an initial packaging. However the initial packaging has to insure that at least one section of the fiber in the MZI is exposed to the outside in order to shine the UV light on that section of the fiber. Alternatively, one can use a UV-transparent material on that section of the fiber. However, the UV-transparent material with the required characteristics may not be easily available or suitable for packaging. Furthermore both methods increase the complexity of packaging.
In this patent, we introduce a novel and simple method for adjusting the MZI interleaver channels to those of the ITU grid using a UV light source to create a change in the index of refraction (IOR) of the fiber arms of MZI filter after the device is packaged. Using this method of UV light irradiation of a silica fiber, it is possible to create or adjust a phase delay in a MZ filter by changing its optical path length difference through changing the fiber's index of refraction. In this invention, we launch the UV light into the MZI through one of its input ports in order to achieve the phase adjustment. Note that our irradiation method works from within the fiber, along its whole length, rather than on a localized section of the fiber's exterior surface. In this way, since the exposure length is the entire length of one of the arms of the MZI, the required refractive index change is much smaller than that of the localized UV tuning method and, therefore, the UV source power and duration of irradiation can be lower than the previous methods. Using a weaker UV source and shorter exposure time reduces the chances of damaging the fiber and the MZI. The reason for the success of this method is that the UV light stays in only one of the arms of the MZI. The coupler has a 50:50 coupling ratio at 1550 nm wavelength light source, however it shows negligible coupling ratio at the wavelength of the UV source. Therefore this method allows for selective UV tuning and changing the optical path length of one of the optical arms of the MZ device. Note that an angled output end or an index matching oil at the output end prevents the formation of unwanted grating in the treated fiber arm.
One major consideration for a MZ periodic filter is to minimize the descrepancy between the actual central frequency of the filter channels and the nominal central frequency of the ITU grid. It is known that the UV radiation of a doped-silica fiber causes a change in its refractive index. This property has been widely used for making index gratings on the fiber. For example in the U.S. Pat. No. 4,807,950, the UV light has been used to create a change in the IOR of Germanium (Ge)-doped silica fiber by aligning two UV beams to the surface of the fiber at a particular angle to imprint a grating. Yet in another invention (U.S. Pat. No. 4,474,427), index grating has been written on optical fiber by launching the visible light into the fiber core. A refractive index grating is formed when the reflected light from the other fiber end interferes with the forward propagation forming a standing wave having a period corresponding to half of the wavelength of the visible light. The UV light has also been used before for UV tuning a MZI device and a coupler. However, in both inventions the surface of the optical fiber or the coupler is subjected to the UV light at a specified angle. In the U.S. Pat. No. 6,031,948, part of the surf
Ahmadvand Nima
Hu Hanwu
Mohtat Nadereh
Katten Muchin Zavis & Rosenman
Knauss Scott Alan
Peleton Photonic Systems Inc.
Sangkavi Hemang
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