Optical waveguides – Optical fiber waveguide with cladding – Utilizing multiple core or cladding
Reexamination Certificate
2001-05-21
2003-12-30
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
Utilizing multiple core or cladding
C385S122000, C385S123000, C385S146000
Reexamination Certificate
active
06671444
ABSTRACT:
FIELD OF THE ART
This invention concerns an optical fiber that is used as a light source, for example, for wavelength division multiplexed optical transmission and with which a pumping optical pulse of a high peak output is made incident into the optical fiber to generate white light (super continuum light), which has a wide wavelength band at both sides of the pumping optical pulse.
BACKGROUND ART
With the development of optical communication technologies in recent years, wavelength division multiplexed optical transmission (WDM), with which a plurality of signal light, which mutually differ in wavelength, are multiplexed and transmitted through a single optical fiber, has come to be put to practical use. Wavelength division multiplexed optical transmission enables light of a plurality of wavelengths to be transmitted through a single optical fiber and is thus an optical transmission method that is suited for high-capacity, high-speed communication. Wavelength division multiplexed transmission is presently carried out by the application of an erbium-doped optical fiber type optical amplifier. Also, wavelength division multiplexed optical transmission is carried out in a wavelength band of 1.5 &mgr;m, which is the gain band of the abovementioned optical amplifier.
In recent years, higher transmission speeds are being desired in optical communication that apply the above-described wavelength division multiplexed optical transmission. Light sources, using super continuum (SC) light, which is pulsed light of a wide wavelength width and substantially rectangular shape, are being noted as means for satisfying this demand. For example, Japanese Unexamined Patent Publication No. 90737 of 1998 proposes an optical fiber that generates the abovementioned SC light and a light source that uses this optical fiber.
SC light is generated when a pumping optical pulse of high peak power is made incident into a nonlinear medium having a dispersion decreasing zone, with which the wavelength dispersion changes, for example, from positive dispersion (anomalous dispersion) to negative dispersion (normal dispersion), along the length direction, from the side of incidence of light towards the side of exit of the light. The generation of SC light is a phenomenon in which an optical Kerr effect occurs as the abovementioned pumping optical pulse propagates through the abovementioned dispersion decreasing zone, causing wavelength broadening and pulse compression of the pumping optical pulse, and four-wave mixing and other nonlinear optical effects occur additionally to lead to the generation of a short pulse that is broadened to a wide wavelength band. When light of new wavelengths are generated by nonlinear phenomena at both sides of the wavelength of the input pumping light and these light of new wavelengths propagate through the nonlinear medium, new light are generated again by nonlinear phenomena at both sides of the wavelengths of the former new light. It is considered that the wavelength width of the spectrum is broadened by the repeated generation of such new light, leading to the generation of a substantially rectangular optical pulse with optical intensity over a wide bandwidth.
As has been mentioned above, SC light is white light that is generated in the wavelength bands at both sides of a pumping optical pulse of high peak power that is made to enter an optical fiber or other nonlinear medium, and this SC light may be obtained with a single pumping light source and a single SC light generating optical fiber. Thus by dividing the SC light by means of a wavelength division device, a plurality of light that mutually differ in wavelength may be obtained in a far more economical manner in comparison to methods that require the preparation of the same number of light sources as the number of signals to be transmitted.
Presently, a 1.5 &mgr;m wavelength band erbium-doped optical fiber type optical amplifier is used for wavelength division multiplexed transmission. In the case where this optical fiber type optical amplifier is used to input pumping optical pulses into an optical fiber for SC light generation, SC light will be generated at the wavelength bands at both sides of the 1.5 &mgr;m wavelength band. However, with prior-art optical fibers for wavelength division multiplexed transmission, absorption and an accompanying trailing edge occur near a wavelength of 1.4 &mgr;m at the short wavelength side of the 1.5 &mgr;m wavelength band due to OH groups that are incorporated in the process of manufacture of the optical fiber. When such a portion in which the optical fiber transmission loss is large exists in the wavelength range in which SC light is generated, the intensity of the generated light becomes attenuated during propagation, thereby obstructing the broadening of the wavelength width of the spectrum and the making of the spectrum rectangular in shape. Thus the wavelength width of the spectrum of SC light was limited at the short wavelength side of the 1.5 &mgr;m wavelength band due to the abovementioned absorption, etc. by the OH groups.
Also, in generating SC light using an optical fiber, it is preferable that the polarized condition within the optical fiber not vary so that effective use can be made of the nonlinear phenomena within the optical fiber. The SC light generating optical fiber may thus be formed for example from a PANDA type polarization-maintaining optical fiber and thereby made to maintain the polarization. A PANDA type polarization-maintaining optical fiber is formed by providing a pair of stress-applying parts, which apply stress to the core, so as to sandwich the core from both sides. However, due to the effects of forming the stress-applying parts, SC light of adequate wavelength width could not be obtained from this type of PANDA type polarization-maintaining optical fiber even in the proposed example described above.
This invention has been made to solve the above problems. An object of this invention is to provide a white light generating optical fiber for wavelength division multiplexed transmission that has the following first to third characteristics. That is, the first characteristic of the optical fiber of this invention is that it can generate white light (SC light) that is adequately broadened in wavelength, the second characteristic is that the polarization is maintained so that nonlinear phenomena will occur efficiently, and the third characteristic is that the ill effects that accompany the maintaining of polarization can be avoided or lessened.
DISCLOSURE OF THE INVENTION
In order to achieve the above object, this invention provides the following arrangements as means for solving the problems. That is, the first arrangement of this invention is an optical fiber, which generates white light by means of nonlinear phenomenon in the wavelength bands at both sides of a pumping optical pulse that is input, and this optical fiber is characterized in being formed by covering the surroundings of the core by a first cladding, which is lower in refractive index than the core and covering the surroundings of the first cladding by a second cladding, which is higher in refractive index than the first cladding but lower in refractive index than the abovementioned core, by having, in at least part of the length direction of the optical fiber, a portion in which the wavelength dispersion gradient for the wavelength band at the wavelength side shorter than the wavelength of the pumping optical path is positive and the wavelength dispersion gradient for the wavelength band at the wavelength side longer than the wavelength of the pumping optical path is negative, by having, in at least part of the length direction of the optical fiber, a zone in which the wavelength dispersion for the wavelength of the pumping optical pulse varies from anomalous dispersion to normal dispersion from the side at which the pumping optical pulse is made incident towards the side at which light exits, and by having a transmission loss of the optical fiber that is 10 dB/km or less for light at a wa
Arai Shinichi
Kawanishi Satoki
Mori Kunihiko
Takara Hidehiko
Yagi Takeshi
Knobbe Martens & Olson Bear LLP.
Rojas Omar
Sanghavi Hemang
The Furukawa Electric Co. Ltd.
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