Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2001-08-30
2003-08-26
Duverne, Jean (Department: 2839)
Optical waveguides
With optical coupler
Plural
Reexamination Certificate
active
06611638
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical wavelength multiplexing transmission of an optical fiber communication, and particularly to a constitution of an optical add/drop multiplexer used for an optical add/drop multiplexing section for carrying out communication between a plurality of nodes, a node constitution having an optical add/drop constitution, and an optical network.
In a long distance optical communication making use of an optical fiber, the transmission capacity by a single optical fiber was expanded rapidly by introduction of an optical wavelength multiplexing technique for placing independent signals on a plurality of different wavelengths in a single optical fiber. For example, by combining an optical fiber amplifier making use of the 1.5 &mgr;m band optical amplification characteristics of erbium doped fiber with the wavelength multiplexing technique, it becomes possible to place the rate of 10 Gbit/s on each wavelength to transmit 160 wavelengths for several hundreds km.
Further, recently, there has been introduced the optical add/drop multiplexing (separation and multiplexing) technique in which in the optical signal transmission between a plurality of spots, a common optical fiber transmission path is used to vary spots of traffic every wavelength of multiplexed signals, whereby optical signals are relayed and connected directly without converting them into electrical signals. The wavelength multiplexing technique and the signal optical add/drop multiplexing are combined whereby the communication between specific two spot nodes can be carried out every wavelength making use of the common optical fiber, thereby realizing the inexpensive optical transmission between many spots.
For demultiplexing from a multiplexed signal, roughly two methods have been employed. Generally, there are a method for separating all wavelengths, and a method for a single channel drop which takes out only a specific wavelength by a wavelength filter. For example, in the center node, electrical signals are taken out of all the optical signals and processed, and therefore, the former method which separates all the wavelengths is employed. However, in the local node which requires to separate only a few wavelength optical signals out of optical signals subjected to scores of or hundreds of wavelength multiplexing, it is not economical that all the wavelength signals are separated and multiplexed, and in a case of passing through a plurality of nodes, the shaping effect of a wavelength filter occurs. The shaping effect termed herein is a band narrowing effect in which even lowering of loss of 0.3 dB in the shape of a single filter, ten times of transmission result in loss of 0.3 dB, and the 3 dB band in total corresponds to a 0.3 dB band of a single filter. Since in the latter method, the constitution having the single drop combined is economical and the transmission wavelength signal is not taken out by a wavelength filter, there is no influence of the shaping effect of a wavelength filter; therefore the constitution was used at a spot for demultiplexing/multiplexing only a few wavelength optical signals.
For a single channel drop section, there is a method making use of a dielectric multilayer film filter comprising a construction having a plurality of dielectric layers having a great refractive index difference laminated and an optical fiber bragg grating filter comprising a construction in which an optical signal for transmitting an optical fiber delicately changes in refractive index. Light is reciprocal, and an optical channel of the single channel drop section is reversed to thereby constitute the single channel add section.
In the constitution of the wavelength add/drop making use of the dielectric multilayer film filter, a single channel drop filter is connected to an optical fiber subjected to wavelength multiplexing, only specific wavelength is separated and taken out, an optical signal of the other wavelength is let to pass through as it is to reach the single channel add filter of the same wavelength, and signal light of the same wavelength as the drop wavelength is multiplexed. The dielectric multilayer film filter performs demultiplexing and multiplexing with a thickness of a few wavelengths, and the filter constitution is a multilayer film so that excellent mass production is presented, but the rejection characteristics of demultiplexed/multiplexed light to the transmission path is not satisfactory in principle, which is about dozens of dB. The leakage light to the transmission path suffers several tens dB of loss in a subsequent filter, which is in total about 30 dB, producing a coherent cross of the demultiplexed/multiplexed signal and the optical signal. According to General Meeting SB-11-7 (p. 747) of The Institute of Electronics, Information and Communication Engineers in 1996, the coherent cross talk requires rejection of about 38 dB or more. Normally, in a system using the single wavelength add/drop multiplexing, there are many ring constitutions via a plurality of nodes or long distance transmissions via a plurality of nodes, accumulation of grain differences between wavelengths by repeating of relay becomes large. Referring to specific wavelength signals, as the leakage characteristics of the demultiplexing section, a light level is sometimes higher than an average determined value, and since the multiplexed optical level has an optical level independently thereof, the rejection characteristics as high as grain differences are further required to prevent the coherent cross talk. Conventionally, to improve the rejection characteristics, there is employed a method for inserting a further filter between the drop filter and the add filter to enhance the rejection characteristics, but an increase in pass-through loss of transmission light or an increase in cost are brought forth.
In the constitution of the wavelength add/drop multiplexing making use of a fiber bragg grating filter, the fiber bragg grating is a reflection reversing type filter in which light reversibly moves in the same optical fiber as the moving-in channel as described in General Meeting SB-11-7 (p. 747) of The Institute of Electronics, Information and Communication Engineers in 1996, and therefore, and it was necessary to make use of a circulator for taking out the demultiplexed light or multiplexing insert light. In the circulator, three input positions and output positions can be separated according to the light traveling direction, and thereby the reverse-traveling light can be output not to the incident fiber but the demultiplexed light taking out fiber. The fiber bragg grating is excellent in wavelength selectivity, and leakage of demultiplexed light to the transmission channel is rejected not less than 40 dB. However, the manufacturing process of the fiber bragg grating is easy and the cost is low, but a circulator which is complex in construction and has less effect of mass production is required, and so, the price is high to obtain the add/drop multiplexing function. Further, recently, higher modulation speed of an optical signal and higher density wavelength multiplexing are progressed, and the constitution of a system for multiplexing the rate of 10 Gbit/s at 100 GHz intervals or 50 GHz intervals has been studied.
We have obtained from our studies that in the fiber bragg grating, a great program as noted below occurs. When a spacing between wavelengths is narrow, a filter band of the fiber bragg grating becomes narrow, and it is necessary to make small a change in refractive index forming a grating, for example, it is necessary to lower it to about 5×10 for 100 GHz spacing. To obtaine high rejection characteristics in that state, it is necessary to have a longer reflection area because a reflection coefficient in an area lowers by a portion that the change in refractive index lowers, for example, it is necessary to have not less than 10 mm in the above-described example. However, in a case where a digital signal of 10 Gbit/s is input, a spatial dispersi
Duverne Jean
Sofer & Haroun LLP
LandOfFree
High performance optical add/drop multiplexer and optical... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High performance optical add/drop multiplexer and optical..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High performance optical add/drop multiplexer and optical... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3109869