Optical filter and method of making the same

Details Optical filter and method of making the same Optical filter and method of making the same

1999-05-17

2002-01-08

Healy, Brian (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling function

C385S037000, C430S321000

Reexamination Certificate

active

06337937

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical component applicable to the field of optical communications or the like and, in particular, to an optical filter comprising a long-period grating for eliminating the wavelength dependence of the gain of a fiber amplifier doped with rare earth.
2. Related Background Art
A typical optical fiber communication system comprises an optical transmitter including a light source, an optical fiber line having one end connected to the optical transmitter, and an optical receiver connected to the other end of the optical fiber line. An optical amplifier for amplifying signal light in a predetermined wavelength band is installed in the optical fiber line. Such an optical fiber communication system often utilizes WDM signals in the band of 1.5 &mgr;m and employs, as its amplifier, a fiber amplifier doped with rare earth such as Erbium (Er) or the like. This erbium-doped fiber amplifier (EDFA) forms a population inversion of electron state within the EDFA in response to excitation light having a predetermined wavelength, and induces stimulated emission in response to light in the band of 1.5 &mgr;m incident thereon, thereby amplifying the incident light.
In such an optical fiber communication system, the amplified spontaneous emission (ASE) generated by the mutual action between the power of excitation light and Er ions within the EDFA becomes a noise component. The ASE lowers the gain and increases the noise figure. Also, since the ASE has a power distribution peaking at 1.53 &mgr;m, when optical amplification is repeated by a plurality of EDFAs, their gain may fluctuate among individual wavelength components of signal light (a wavelength dependence may occur in the amplification gain of the optical amplifier) under the influence of the power distribution of ASE. As a consequence, in a WDM (Wavelength Division Multiplexing) communication system for transmitting a plurality of signal light components having wavelengths different from each other, different gains are given to the respective channels (respective signal light components), whereby the bit error rate may become higher in some channels.
A technique using a long-period grating for overcoming these problems is disclosed in a paper titled “Broad-Band Erbium-Doped Fiber Amplifier Flattened Beyond 40 nm Using Long-Period Grating Filter” (IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 10,°OCT. 1997, pp. 1343-1345).
This long-period grating is a portion of the core region where the refractive index periodically changes along the axis of an optical waveguide, and is a grating which induces coupling between a core mode and a cladding mode in the signal light propagating through the optical waveguide. The period (pitch) of the grating is set such that the optical path difference between the core mode and cladding mode within one period becomes identical to a predetermined wavelength, and thereby yielding a strong power conversion from the core mode to the cladding mode. As a result, since the long-period grating acts to radiate the core mode to the cladding mode, the intensity of the core mode over a narrow band centered at a predetermined wavelength (loss wavelength) is attenuated.
The center wavelength of the wavelength spectrum of light radiated from the core to the cladding by the long-period grating, i.e., loss wavelength, is determined according to the following expression (1):
&bgr;
core
(lm)
−&bgr;
clad
(n)
=2&pgr;/&Lgr;  (1)
where l and m are the order of the core mode (1=0, m=1 in the fundamental mode LP01), &bgr;
core
(lm)
is the propagation constant defined by (lm), &bgr;
clad
(n)
is the propagation constant of the n-th order cladding mode, and &Lgr; is the grating period of the long-period grating.
Since the propagation constants &bgr;
core
(lm)
, &bgr;
clad
(n)
are parameters dependent on the wavelength, the loss wavelength of the long-period grating can be controlled when the long-period grating is formed with its grating period &Lgr; being adjusted in view of the above-mentioned expression (1).
On the other hand, &bgr;
core
(lm)
, &bgr;
clad
(n)
are dependent on the effective refractive indexes of the core and cladding, respectively. Consequently, when the grating period &Lgr; is set constant, the loss wavelength of the long-period grating mainly depends on the difference between the effective refractive index of the core and the effective refractive index of the cladding in the part formed with the long-period grating (grating forming region). Also, the effective refractive index of the core in the grating forming region can be considered in terms of the average value of modulated refractive index. Consequently, the difference between the effective refractive index of the core and the effective refractive index of the cladding in the grating forming region depends on the average refractive index of the core and the average refractive index of the cladding. Further, the amount of refractive index change (amplitude of refractive index modulation) in the region doped with GeO
2
changes in response to the irradiation amount of ultraviolet light for forming the grating. That is, the refractive index of the core itself also changes in response thereto. Eventually, the loss wavelength of the long-period grating can be controlled also by forming the long-period grating with the irradiation amount of ultraviolet light being adjusted, so as to regulate the difference between the effective refractive index of the core and the effective refractive index of the cladding.
SUMMARY OF THE INVENTION
As a result of studies of conventional optical filters, the inventors have found the following problems. Namely, since the gain of an EDFA such as that mentioned above depends on the wavelength, the optical power may fluctuate among the individual wavelength components of the WDM signal light amplified by the EDFA. Therefore, for wavelength components with higher and lower gains, long-period gratings exhibiting greater and smaller amounts of loss are prepared (and inserted in the transmission line), respectively, so as to homogenize the gain.
In general, however, in an optical filter formed by a combination of a plurality of long-period gratings, the overall transmission characteristic of the optical filter would not become the product (or sum in terms of dB) of the transmission characteristics of individual long-period gratings, thereby making it difficult to yield an optical filter having a desirable transmission characteristic as a whole.
In order to overcome the above-mentioned problems, it is an object of the present invention to provide an optical filter which, when applied to an optical transmission system having an optical amplifier, can eliminate the wavelength dependence of gain in the optical amplifier, while having a structure which can be made easily; and a method of making the same.
The optical filter according to the present invention comprises an optical waveguide having a core region with a predetermined refractive index and a cladding region, provided on an outer periphery of the core region, with a refractive index lower than that of the core region, wherein a plurality of long-period gratings are arranged, at least, in the core region. 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 to the cladding mode, the grating period (pitch) in the long-period grating is set such that the optical phase 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 to the cladding mode, the core mode atten

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