Patent Application
20090202257
This application is based upon and claims the benefit of the priority of Japanese patent application No. 2008-028018, filed on Feb. 7, 2008, the disclosure of which is incorporated herein in its entirety by reference thereto.
This invention relates to an optical transmission line and optical transmission system. More particularly, the invention relates to a wavelength division multiplexing (WDM) optical fiber transmission line and wavelength division multiplexing optical fiber transmission system that compensates for wavelength dispersion.
DPSK (Differential Phase Shift Keying) modulation/demodulation is one effective method used in a WDM optical fiber transmission system owing to its high receiver sensitivity characteristic and low non-linearity characteristic. With DPSK, data “1” is assigned to a phase difference of π between adjacent bits and data “0” is assigned to a phase difference of zero between adjacent bits, by way of example.
In DPSK modulation/demodulation, use is made of the phase of light (and inversion of phase) as means for transmitting bits “0”, “1”. As a consequence, the system is readily susceptible to the effects of phase noise and it is difficult to exploit full performance under conditions where phase noise tends to increase.
In a submarine optical fiber cable transmission system that applies ON-OFF keying referred to as NRZ (Non-Return to Zero) or RZ (Return to Zero), NZ-DSF (Non-Zero Dispersion Shifted single-mode optical Fiber) generally is employed.
Patent Document 1 discloses dispersion management in which spans of sections LA1 to LA9 are equipped with respective NZ-DSFs (which have a zero dispersion wavelength at 1585 nm and the wavelength dispersion of which is about −0.2 ps/nm/km) and optical repeaters (EDF amplifiers) (one span=50 km), a span of a section LA10 is equipped with an SMF (Single Mode Fiber) (which has a zero dispersion wavelength at 1310 nm and the wavelength dispersion of which is about +18 ps/nm/km), one dispersion compensating interval LA is formed by nine spans and the SMF span, and two dispersion compensating intervals LA are connected between a transmit station and a receive station over a distance of 1000 km.
Patent Document 2 discloses an optical communication transmission system in which a repeater section has multiple stages of optical transmission fibers and optical amplifying repeaters. The dispersion value of the optical transmission fibers is different from zero in the signal optical wavelength, or regions in which dispersion value becomes zero are few. This patent document describes use of optical fiber having negative dispersion of −0.4 ps/nm/km, or values higher and lower than this value, or use of optical fiber having positive dispersion.
Patent Document 3 discloses an arrangement having a dispersion compensating fiber DCF every other several DSF sections. In blocks where the dispersion compensating fiber has been inserted, dispersion is not made zero with respect to the center wavelength of distribution in the signal optical wavelength. The final residual dispersion is made zero at the position where a receiver is installed.
[Patent Document 1] Japanese Patent Kokai Publication No. JP-P2004-228715A
[Patent Document 2] Japanese Patent Kokai Publication No. JP-A-6-85758
[Patent Document 3] Japanese Patent Kokai Publication No. JP-A-11-331074
The disclosures of Patent Documents 1 to 3 are incorporated by reference in this specification. An analysis of the related art in the present invention is given below.
With ON-OFF keying, as the amount of accumulation of wavelength dispersion in a transmission line increases, waveform distortion increases. The smaller the cumulative dispersion in a region, the better the transmission characteristic. In the vicinity of the center of signal wavelength band, average wavelength dispersion in NZ-DSF transmission line block 12 (see
In a DPSK modulated/demodulated signal, however, when the average wavelength dispersion of the NZ-DSF transmission line block is in the vicinity of zero, phase noise tends to increase and degradation of the transmission characteristic, such as in increase in signal waveform distortion after transmission, occurs, as illustrated in
Accordingly, an object of the present invention is to provide an optical transmission line and optical transmission system in which degradation of the transmission characteristic is avoided in DPSK modulation/demodulation.
The above and other objects may be attained by the present invention set forth below.
According to an aspect of the present invention, there is provided an optical transmission line for transmitting an optical signal by DPSK modulation, comprising an NZ-DSF transmission line block that includes N stages (where N is a prescribed positive integer) of NZ-DSFs serially connected, and at least one DCF placed at least as one stage among a preceding stage, succeeding stage and intermediate stage of the total N stages of NZ-DSFs; wherein an average wavelength dispersion value of the NZ-DSF transmission line block is made non-zero.
In an exemplary embodiment of the present invention, the NZ-DSF transmission line block has an optical repeater, which compensates for loss of the NZ-DSF, as a preceding stage or succeeding stage of one stage of the NZ-DSFs.
In an exemplary embodiment of the present invention, the NZ-DSF transmission line block has an optical repeater, which compensates for loss of the DCF, as a preceding stage or succeeding stage of the DCF.
In an exemplary embodiment of the present invention, the DCF makes the average wavelength dispersion value of the NZ-DSF transmission line block non-zero by allowing some dispersion, which is accumulated by the N stages of NZ-DSFs, to remain instead of applying 100% compensation, or by over-compensating for the accumulated dispersion.
In an exemplary embodiment of the present invention, the optical transmission line has a plurality of stages of the NZ-DSF transmission line blocks serially connected.
In an exemplary embodiment of the present invention, the optical transmission line has at least one block DCF that includes one or a plurality of DSFs, wherein a residual dispersion value at a receiving end at the terminus of the optical transmission line is made zero.
In an exemplary embodiment of the present invention, the block DCF is provided at least as any one stage among a preceding stage, intermediate stage and succeeding stage of the plurality of stages of the NZ-DSF transmission line blocks serially connected, wherein a residual dispersion value at the receiving end is made non-zero.
In an exemplary embodiment of the present invention, polarities of the average wavelength dispersion values of the plurality of stages of the NZ-DSF transmission line blocks are the same as one another.
In an exemplary embodiment of the present invention, the plurality of stages of the NZ-DSF transmission line blocks in which the average wavelength dispersion values are of positive polarity are serially connected, the plurality of stages of the NZ-DSF transmission line blocks in which the average wavelength dispersion values are of negative polarity are serially connected, and the residual dispersion value at the receiving end is made zero.
In an exemplary embodiment of the present invention, the plurality of stages of the NZ-DSF transmission line blocks include a first group of serially connected NZ-DSF transmission line blocks in which the average wavelength dispersion values are of positive polarity; and a second group of serially connected NZ-DSF blocks, as a preceding stage or succeeding stage of the first group of NZ-DSF transmission line blocks, in which the average wavelength dispersion values are of negative polarity; wherein the residual dispersion value at the receiving end of the optical transmission line is made zero.
In an exemplary embodiment of the present invention, the average wavelength dispersion value of the NZ-DSF transmission line block is equal to or less than −0.4 ps/nm/km. Alternatively, the average wavelength dispersion value of the NZ-DSF transmission line block is equal to or greater than +0.4 ps/nm/km.
In an exemplary embodiment of the present invention, the optical transmission line has a plurality of NZ-DSF transmission line blocks in which the average wavelength dispersion value is equal to or less than −0.4 ps/nm/km, and a plurality of NZ-DSF transmission line blocks in which the average wavelength dispersion value is equal to or greater than +0.4 ps/nm/km.
In an exemplary embodiment of the present invention, the optical signal is transmitted by WDM.
In accordance with another aspect of the present invention, there is provided an optical transmission system comprising the above-described optical transmission line; a transmitting unit for outputting signal light to the optical transmission line; and a receiving unit for inputting signal light from the optical transmission line.
In accordance with the present invention, it is possible to reduce signal waveform distortion after transmission and realize excellent transmission quality in a DPSK modulation system.
Still other features and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description in conjunction with the accompanying drawings wherein only exemplary embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out this invention. As will be realized, the invention is capable of other and different exemplary embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.
In an optical fiber transmission line according to the present invention, N spans of a span comprising an NZ-DSF 1 and an optical repeater 2 are serially connected. This is followed by a span comprising a DCF 3 and an optical repeater 4, whereby dispersion compensation is carried out. These components constitute a block referred to as an “NZ-DSF transmission line block”.
In a transmission system to which DPSK modulation is applied in the present invention, it is possible to reduce optical waveform distortion after transmission of a DPSK signal by setting the average wavelength dispersion of an NZ-DSF transmission line block (a value obtained by dividing the cumulative wavelength dispersion of the NZ-DSF transmission line block by the length of the NZ-DSF transmission line block) to a value equal to or less than −0.4 ps/nm/km or equal to or greater than +0.4 ps/nm/km over the WDM signal wavelength band. Exemplary embodiments of the present invention will now be described.
In the example of
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In this embodiment, as illustrated in
In order to describe the effectiveness of the configuration of the optical fiber transmission line of this embodiment,
When the average wavelength dispersion is in the vicinity of zero dispersion wavelength, degradation of the Q value increases. It will be understood that the Q value improves as the average wavelength dispersion departs from zero dispersion wavelength.
As a result, in order to fully exploit the transmission performance of a DPSK signal in an optical fiber transmission system that employs DPSK modulation and an NZ-DSF transmission line, an effective arrangement is one in which a value equal to or less than −0.4 ps/nm/km or equal to or greater than +0.4 ps/nm/km is set as the value of average wavelength dispersion of the NZ-DSF transmission line block, whereby transmission degradation in the vicinity of zero dispersion wavelength is avoided.
Next, optical fiber transmission lines constructed of NZ-DSF transmission line blocks will be described as other exemplary embodiments of the present invention. In the NZ-DSF transmission line block 5, the average wavelength dispersion is intentionally shifted from zero dispersion wavelength. As a result, the optical fiber transmission line comprising the NZ-DSF transmission line blocks leaves a large amount of negative dispersion.
When a DCF block for compensating for residual dispersion resulting from the NZ-DSF transmission line blocks 5 is placed in the transmission line, it is required that the number of times the DCF block is inserted be limited to the minimum number (one or two times) in order to effectively suppress waveform distortion of the DPSK modulated signal.
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Though the present invention has been described in accordance with the foregoing embodiments, the invention is not limited to these embodiments and it goes without saying that the invention covers various modifications and changes that would be obvious to those skilled in the art within the scope of the claims.
It should be noted that other objects, features and aspects of the present invention will become apparent in the entire disclosure and that modifications may be done without departing the gist and scope of the present invention as disclosed herein and claimed as appended herewith.
Also it should be noted that any combination of the disclosed and/or claimed elements, matters and/or items may fall under the modifications aforementioned.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-028018 | Feb 2008 | JP | national |
