JP3177361B2 - Optical transmitter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission device used in an optical CATV system, and more particularly to an optical transmission device for reducing deterioration of transmission characteristics due to chromatic dispersion.

[0002]

2. Description of the Related Art A multi-channel analog video signal light transmission system using a 1.3 μm wavelength semiconductor laser as a light source is currently widely used in domestic and overseas CATV systems. In this case, a transmission line has a zero-dispersion single mode fiber (hereinafter, Single Mode Fiber: S) in a 1.3 μm band.
Abbreviated as MF. ) Is the mainstream. However, in recent years, a light source having a wavelength band of 1.5 μm is being adopted in consideration of the introduction of an optical fiber amplifier which has been put into practical use. In this case, it is known that when an SMF already laid is used for a transmission line, waveform deterioration occurs due to chromatic dispersion characteristics of the fiber. In the case where a large number of analog video signals are frequency-multiplexed and the laser light is directly luminance-modulated and transmitted, there is a possibility that the second-order distortion is deteriorated and the distortion performance required for CATV cannot be satisfied depending on the transmission distance.

Therefore, conventionally, a single mode fiber (hereinafter, referred to as a dispersion mode) having a zero-dispersion wavelength in the 1.5 μm band.
Fiber: Abbreviated as DSF. ) Or a method of compensating for chromatic dispersion according to the transmission distance by using a special high-dispersion fiber having a chromatic dispersion characteristic opposite to that of SMF.

This special high dispersion fiber is a chromatic dispersion compensating fiber (hereinafter referred to as “Dispersion Compensation Fiber: D”).
Abbreviated as CF. ). The principle of chromatic dispersion compensation by DCF will be briefly described. SMF has a chromatic dispersion coefficient of about 17 ps / km / nm in the 1.5 μm wavelength band. 170 ps / nm for 10 km SMF
The delay difference of the optical signal depending on the wavelength is generated.
Therefore, the DCF having a negative chromatic dispersion coefficient of -170 ps / nm
Is connected in series with the SMF, the chromatic dispersion equivalent to 10 km of the SMF can be canceled.

[0005]

However, the above D
The SF is more expensive than the SMF, and the existing SMF cannot be used at all. For this reason, transmission line laying costs are enormous. When the DCF is used, it is necessary to determine the length of the DCF according to the length of the SMF used as a transmission path. Therefore, if the transmission distance changes, the length of the DCF needs to be changed. Therefore, in the CATV system, when the service area is expanded or transmitted to new subscribers' homes scattered, the optical transmission equipment needs to be changed.

The present invention has been made in view of such problems as DSF, DCF, etc., and by connecting a plurality of DCFs in series to an optical transmission device of a CATV center, the equipment is changed even when the CATV transmission system is expanded or changed. It is an object of the present invention to provide an optical transmission device which is economical and can respond without any problem.

[0007]

According to the present invention, there is provided a high-dispersion fiber having a negative chromatic dispersion coefficient for transmitting laser light, and a 1.times.n optical splitter connected to an output end of the high-dispersion fiber. K dispersion compensating means for outputting n branched lights are provided;
At least one of the branched lights from the dispersion compensating means is input to another dispersion compensating means, and k dispersion compensating means are connected in series to transmit laser light.

[0008]

According to the above means, if the laser signal light passes through the first dispersion compensating means, it will have a negative wavelength dispersion. By sequentially passing the signal light through the dispersion compensating means connected in series, a similar negative chromatic dispersion characteristic is added. These signal lights are split by each optical splitter provided in the dispersion compensating means. As a result, if the light passes through the k pieces of dispersion compensating means connected in series, k kinds of signal lights having negative chromatic dispersion characteristics can be obtained.

Therefore, according to the length of the SMF as the transmission path, the signal light capable of canceling the positive chromatic dispersion of the SMF can be selected from the k kinds of signal lights. For this reason,
Even if the transmission distance changes, it can respond flexibly.

[0010]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an optical transmission device according to one embodiment of the present invention. In FIG. 1, 1 is a wavelength of 1.5 μm.
Band semiconductor lasers, 21, 22, and 23 are optical fiber amplifiers for the 1.5-um band, 31, 32, and 33 are 1 × 2 optical splitters, and 41, 42, and 43 are high-dispersion optical elements having negative chromatic dispersion coefficients. The fibers 51, 52, 53, 54 are light output ends.
The high dispersion fibers 41 to 43 are respectively -70 ps / n
m has a negative chromatic dispersion coefficient.

Next, the operation of this embodiment will be described.

A multiplexed signal obtained by frequency-multiplexing a multi-channel analog video signal signal such as a broadcast wave has a wavelength of 1.
The luminance of the 5 μm band semiconductor laser light 1 is directly modulated. The modulated signal light is supplied to the optical fiber amplifier 2 for the 1.5-um wavelength band.
The light is amplified by 1 and split into two by a 1 × 2 optical splitter 31. One of the split lights is input to the high dispersion fiber 41, and the other split light is sent to the optical output end 54. The signal light input to the high-dispersion fiber 41 is again amplified by the optical fiber amplifier 22 for the wavelength of 1.5 μm and is split into two by the 1 × 2 optical splitter 32. To the other, the light output end 53
Sent to Similarly, the third high dispersion fiber 4
3 is output to an optical output terminal 51, and then 1 ×
The split light from the two-light splitter 33 is sent to the optical output end 52.

As described above, the optical transmission apparatus of the present embodiment
It has light output ends 51 to 54 having different wavelength dispersion characteristics. The chromatic dispersion coefficient of the SMF of the transmission line is 1 as described above.
It is 17 ps / nm in the 1.5 μm band per km. Therefore, the signal light transmitted from the optical output end 53 that has passed through the high dispersion fiber 41 of -70 ps / nm is about 4 k
m SMF length chromatic dispersion can be compensated. Similarly, the signal light transmitted from the optical output terminal 52 is approximately 8 km, and the optical output terminal 5
The signal light transmitted from 1 can compensate for the chromatic dispersion of about 12 km. Therefore, for transmission to a point having a different transmission distance, an optimum signal light may be selected from the optical output terminals 51 to 54, and it is possible to flexibly cope with a change in the system. Further, by connecting the high dispersion fibers 41 to 43 in series, there is an advantage that it is possible to cope with shortest dispersion fibers from a short distance to a long distance.

Next, an embodiment of the second invention will be described.
FIG. 2 is a block diagram showing the configuration of one embodiment of the present invention. In FIG. 2, components having the same functions as those in FIG. 1 are denoted by the same reference numerals, and detailed description is omitted. In FIG. 2, 61, 62 and 63 are 1 × 2 optical switches, and 7 is a 4 × 1 optical switch.

In this embodiment, 1 × 2 optical switches 61 to 61 are provided in the optical transmission apparatus according to the length of the SMF as the transmission path.
The combination of the dispersion compensating fibers can be selected by switching the 63 and 4 × 1 optical switches 7. Therefore, the chromatic dispersion can be immediately compensated for the change in the transmission distance only by switching the optical switch.

[0017]

As is apparent from the above description,
According to the present invention, an optical transmitter capable of supporting various transmission distances can be realized with a small number of dispersion compensating fibers. Because optical transmission is possible,
Capital investment of the entire system can be minimized.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a second exemplary embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1.5-micrometer wavelength semiconductor laser 21-23 Optical fiber amplifier for 1.5-micrometer wavelength band 31-33 1x2 optical splitter 41-43 High dispersion fiber with negative chromatic dispersion coefficient 51-54 Optical output terminal 61-63 1 × 2 optical switch 74 × 1 optical switch

Continuation of front page (72) Inventor Yoshiharu Kudo 1006 Kazuma Kadoma, Kazuma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) References JP-A-6-112907 (JP, A) JP-A 5-152645 (JP) , A) JP-A-5-3453 (JP, A) JP-A-62-265529 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 G02B 6/10

Claims (5)

(57) [Claims] 1. An optical transmitter for use in an optical CATV, comprising: a 1.5 μm wavelength semiconductor laser, a high dispersion fiber having a negative chromatic dispersion coefficient for transmitting the laser light, and the high dispersion fiber. And a 1 × n optical splitter that inputs n outputs and outputs n split lights. The dispersion compensating means is provided with k split lights, and the split light from one of them is provided. Wherein at least one of the dispersion compensation means is input to another dispersion compensation means, and the k dispersion compensation means are connected in series. 2. The output of a high dispersion fiber is 1.5 μm wavelength.
2. The optical transmission device according to claim 1, wherein the optical transmission device is amplified and branched by a band optical amplifier. 3. The optical transmission device according to claim 1, wherein the chromatic dispersion coefficients of the high dispersion fibers of the dispersion compensating means are the same. 4. The optical transmitter according to claim 1, wherein the optical splitter is a 1 × 2 optical splitter. 5. An optical transmitter for use in an optical CATV, comprising: a 1.5 μm-band semiconductor laser; and k serially connected high-dispersion coefficients having a negative chromatic dispersion coefficient for transmitting the laser light. A fiber, and an optical switch connecting the high-dispersion fiber, wherein the selection of the optical switch outputs the laser light to a next-stage high-dispersion fiber or an optical output end. Optical transmitter.