Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-06-09
2001-06-26
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S031000
Reexamination Certificate
active
06253008
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical filter suitable for an optical transmission system or the like; and, in particular, to an optical filter in which a long-period grating in an optical waveguide such as an optical fiber causes, in light having a predetermined wavelength, a core mode to effect mode coupling with a cladding mode, thereby attenuating core mode light having the predetermined wavelength.
2. Related Background Art
Optical filters have transmission characteristics depending on the wavelength of light and can be used, for example, in an optical transmission system together with an optical amplifier, so as to equalize the wavelength dependence of gain of the optical amplifier. Namely, in an optical amplifier which optically amplifies incident light in a predetermined wavelength band and outputs thus amplified light, its gain upon optical amplification may vary depending on the wavelength. Also, the intensity of incident light may vary depending on its wavelength, thereby the intensity of the light outputted after being optically amplified may vary depending on the wavelength as well. An optical filter having such a transmission characteristic as to yield a loss peak wavelength at the same wavelength as the peak wavelength of the output light from the optical amplifier equalizes the wavelength dependence of the intensity of light outputted from the optical amplifier.
As such an optical filter, an optical fiber (optical fiber type optical filter) formed with a long-period grating in a predetermined area (filter area) has been known. This optical fiber type filter is preferably used due to its small insertion loss in optical fiber transmission systems or the like. In this type of optical filter, the long-period grating formed in the optical fiber causes a part of core mode light to effect mode coupling with cladding mode light, thereby attenuating a desirable light component. The transmission characteristic of the optical filter depends on the propagation constant of core mode light, the propagation constant of cladding mode light, the coupling constant between core mode light and cladding mode light, and the period of change in refractive index (grating period) in the long-period grating, and can be analytically obtained by solving mode coupling equations.
SUMMARY OF THE INVENTION
As a result of studies of the above-mentioned prior art, the inventors have found the following problems. Namely, in the above-mentioned prior art, when making or using an optical filter, the loss peak wavelength of the optical filter is generally caused to coincide with the peak wavelength of the core mode light outputted from the optical amplifier or the peak wavelength of gain of the optical amplifier. However, the transmission characteristic of an optical filter equipped with a long-period grating has a plurality of loss peak wavelengths, and each loss peak wavelength has a ripple (side peak) in the vicinity thereof.
FIGS. 1 and 2
show transmission characteristic charts of an optical filter having a long-period grating with a grating length of 40 mm and a grating period of 401.5 &mgr;m.
As shown in
FIG. 1
, when observed in a wide wavelength range, the optical filter has a plurality of loss peak wavelengths. For example, the transmittance of core mode light becomes about −3.4 dB, −2.0 dB, and −1.0 dB at its loss peak wavelengths of 1547.5 nm, 1420 nm, and 1360 nm, respectively. These wavelengths are those satisfying a phase matching condition between core mode and cladding mode.
On the other hand, as shown in
FIG. 2
, in a wavelength band of 1530 nm to 1565 nm near one loss peak wavelength of 1547.5 nm, there are wavelengths where the loss attains a maximum value, in addition to the loss peak wavelength. Namely, not only the loss of core mode light is maximized at a loss peak wavelength of 1547.5 nm (main peak), but also the loss is maximized at a wavelength of 1538 nm which is shorter than the loss peak wavelength, at a wavelength of 1557 nm which is longer than the loss peak wavelength, and the like (side peaks). When ripples are thus generated in the transmission characteristic of an optical filter, light components which should not have been attenuated would attenuate as well.
For example, T. Erdogan, J. Opt. Soc. Am. A/Vol. 14, No. 8, pp. 1760-1773 (1997) and Japanese Patent Application Laid-Open No. 9- 236720 disclose techniques for restraining ripples such as those mentioned above from occurring. However, the optical filter disclosed in the former literature is unclear in terms of a specific restraining method, and each of the optical filters disclosed in these literatures has a transmission characteristic asymmetrical with respect to the loss peak wavelength.
In order to overcome the above-mentioned problems, it is an object of the present invention to provide an optical filter in which a long-period grating formed in an optical waveguide attenuates core mode light having a predetermined wavelength by causing it to couple with cladding mode light, the optical filter having a symmetrical transmission characteristic with respect to the loss peak wavelength and suppressing the loss caused by ripples; and a method of making the same.
The optical fiber according to the present invention comprises an optical waveguide, such as an optical fiber or the like, including a core region having a predetermined refractive index and a cladding region, provided at an outer periphery of the core region, having a lower refractive index than the core region; and a long-period grating provided in at least the core region of the optical waveguide.
In particular, a filter area provided with the long-period grating is an area where change of the refractive index of the core region periodically occurs along an advancing direction of signal light. The periodical change of the refractive index along the advancing direction of the signal light can be defined by a refractive index modulation function. In the refractive index modulation function, the interval between individual points exhibiting an average refractive index in the filter area is constant along the advancing direction of the signal light, whereas, in the refractive index modulation function, values (indicating refractive index in the filter area) of maximum points and values (indicating refractive index in the filter area) of minimum points vary along the advancing direction of the signal light. Also, this refractive index modulation function has such a form that a first function, which passes through the maximum points of the refractive index modulation function, and a second function, which passes through the minimum points of the refractive index modulation function, project opposite each other with respect to an axis indicating the average refractive index of the filter area.
Here, as explicitly indicated in U.S. Pat. No. 5,703,978 as well, the above-mentioned long-period grating is a grating which induces coupling (mode coupling) between core mode light and cladding mode light which propagate through an optical waveguide such as optical fiber, and is clearly distinguishable from a short-period grating which reflects a light component having a predetermined wavelength. Also, for yielding a strong power conversion from the core mode light to the cladding mode light, the grating period (pitch) in the long-period grating is set such that the optical path difference between the core mode light and the cladding mode light becomes 2&pgr;. Thus, since the long-period grating acts to couple the core mode light to the cladding mode light, the core mode light attenuates over a narrow band centered at a predetermined wavelength (hereinafter referred to as loss peak wavelength).
The optical filter according to the present invention may comprise, at a predetermined position in the core region, a plurality of long-period gratings having periods of refractive index change, i.e., grating periods, different from each other with the same loss peak wavelength.
As a result of th
Harumoto Michiko
Shigehara Masakazu
Bovernick Rodney
Kim Ellen E.
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
LandOfFree
Optical filter and method of making the same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical filter and method of making the same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical filter and method of making the same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2483110