JP2001094534A - Optical transmitter - Google Patents

Abstract

(57) [Summary] To collectively adjust an optical power profile of a WDM signal light. SOLUTION: An optical transmitting terminal 10 has a wavelength λ1.
Signal light sources 20-1 to 20-n that output signal light of .about..lambda.n.
Optical amplifiers 22-1 to 22-n for optically amplifying output lights of the respective signal light sources 20-1 to 20-n, and optical amplifiers 22-1 to 22-2
A wavelength multiplexer 24 that wavelength-multiplexes the 2-n output light and a gain equalizer 26 that adjusts the optical power profile of the output light of the wavelength multiplexer 22 with respect to wavelength are provided. The gain equalizer 26 is composed of a wavelength filter capable of adjusting the transmittance distribution with respect to wavelength within a range including the wavelengths λ1 to λn. The output light of the optical transmitting terminal 10 propagates through the optical fiber transmission line 12 and enters the optical receiving terminal 14.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter, and more particularly, to an optical transmitter for outputting wavelength division multiplexed signal light.

[0002]

2. Description of the Related Art Wavelength division multiplexing transmission type optical transmitters include:
A plurality of signal lights having different wavelengths are wavelength division multiplexed and output to an optical fiber transmission line. It is desirable that the S / N ratio of each signal light be constant on the receiving side. In the case of an ideal optical transmission line in which the loss of each wavelength is the same, the intensity of each signal light may be made uniform on the transmission side. However, the loss characteristics of the optical fiber transmission line depend on the wavelength. Therefore, conventionally, in order to obtain preferable transmission characteristics, the output light power level of each signal light generation device is individually adjusted in the transmission device. WDM
Such a pre-adjustment of the optical power profile or gain shape of the signal light is called pre-emphasis. For example, as shown in FIG. 9A, when the optical fiber transmission line has a loss characteristic such that the loss is the lowest at the center of the signal wavelength band and the loss increases toward the periphery of the signal wavelength band,
In the transmission device, as shown in FIG. 9B, the intensity of the signal light located around the signal wavelength band is higher than the intensity of the signal light located at the center. The power level of each signal light at the receiving terminal station is relatively uniform as shown in FIG.

[0003]

Conventionally, since the power of the signal light of each wavelength is individually adjusted, time and labor are required. The burden increases as the number of wavelengths increases.

[0004] Further, since the power is usually adjusted in the direction of lowering the signal light power, depending on the wavelength, S at the transmitting side may be different.
The / N ratio decreases. Further, in the case of the optical amplification repeater transmission line, since the noise light generated in the optical amplifier in the fiber transmission line is superimposed, it is extremely difficult to make the S / N ratios of the respective signal lights at the receiving terminal stations approximately equal. It is.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical transmitter capable of adjusting the optical power profile of WDM signal light at a time.

The present invention also provides a method for reducing the S / N ratio without lowering the S / N ratio.
It is an object of the present invention to provide an optical transmission device capable of adjusting an optical power profile of a WDM signal light collectively.

[0007]

An optical transmitter according to the present invention comprises: a WDM signal light source for outputting a plurality of wavelength-multiplexed signal lights having different wavelengths; and a light transmission means capable of changing the transmittance with respect to the wavelength. And an optical power profile adjusting means for adjusting the distribution of optical power with respect to the wavelength of the output light of the WDM signal light source.

By providing the optical power profile adjusting means, the optical power profiles of the wavelength multiplexed signal light can be collectively adjusted to a shape suitable for the optical transmission line.

When the optical power profile adjusting means is constituted by one or more passive optical filters, the signal light S
The optical power profile of the WDM signal light can be adjusted collectively without deteriorating the / N ratio.

The passive optical filter constituting the optical power profile adjusting means comprises, for example, any one of a Mach-Zehnder interference optical filter, an etalon optical filter, a fiber grating, an acousto-optical variable optical filter, and a combination thereof.

The WDM signal light source includes, for example, a plurality of signal light sources for generating signal lights having different wavelengths, and wavelength multiplexing means for multiplexing the signal lights output from the plurality of signal light sources. Further, the WDM signal light source includes a plurality of signal light sources that generate signal lights of different wavelengths, a plurality of optical amplifiers that respectively amplify the signal lights output from the plurality of signal light sources, and an output light of the optical amplifier. Wavelength multiplexing means for wavelength multiplexing.

[0012]

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

FIG. 1 is a schematic block diagram of an embodiment of the present invention. The optical transmitting terminal 10 wavelength-multiplexes the signal light having the wavelengths λ1 to λn and outputs the multiplexed signal light to the optical fiber transmission line 12.
The signal light transmitted through the optical fiber transmission line 12 is attenuated according to the loss characteristic inherent in the optical fiber transmission line 12 and enters the optical receiving terminal 14. The optical fiber transmission line 12 generally includes an optical amplification relay transmission line including one or a plurality of optical amplification repeaters, but may be a non-repeater optical transmission line. The optical receiving terminal 14 separates the signal light of each wavelength from the received light,
The data carried by each signal light is demodulated.

The optical transmitting terminal 10 has wavelengths λ 1 to λ, respectively.
signal light sources 20-1 to 20-n that output n signal lights;
Optical amplifiers 22-1 to 22-n for optically amplifying output lights of the respective signal light sources 20-1 to 20-n, and optical amplifiers 22-1 to 22-n.
A wavelength multiplexer 24 that wavelength-multiplexes the n output lights and a gain equalizer 26 that adjusts the optical power profile of the output light of the wavelength multiplexer 22 with respect to the wavelength are provided. Gain equalizer 2
Reference numeral 6 denotes an optical filter whose transmittance distribution with respect to wavelength is adjustable within a range including the wavelengths λ1 to λn of the signal light.

FIG. 2 shows the signal light power spectrum distribution in each section and the transmission characteristics of the gain equalizer 26. That is, FIG. 2A shows the spectrum distribution of the output light of the multiplexer 24. FIG. 3B shows the transmission characteristic 30 of the gain equalizer 24.
(Solid line) and the transmission characteristic 32 (dashed line) or loss characteristic of the optical fiber transmission line 12 are shown. (C) is a gain equalizer 26
3 shows the spectral distribution of the output light of FIG. The horizontal axis indicates the wavelength, and the vertical axis indicates the optical power. 2D shows the spectrum distribution of the input light of the reception terminal station 14. FIG. 2 (a), (c),
In (d), the horizontal axis represents wavelength and the vertical axis represents optical power. In FIG. 2B, the horizontal axis represents wavelength, and the vertical axis represents transmittance (relative value).

The operation of this embodiment will be described with reference to FIG. The signal sources 20-1 to 20-n have different wavelengths λ1 to
λn, and outputs optical signals of optical amplifiers 22-1 to 22-n.
Amplifies the output signal light of each of the signal light sources 20-1 to 20-n to the same power level. Optical amplifiers 22-1 to 22
The power level of the −n output light may, of course, be slightly different. The wavelength multiplexer 24 includes optical amplifiers 22-1 and 22-2.
2-n output light is wavelength-multiplexed, and wavelength-multiplexed light having a spectrum distribution as shown in FIG.

The gain equalizer 26 converts the light intensity of each signal light included in the output light of the wavelength multiplexer 22 into the optical fiber transmission line 1.
2 is adjusted so as to compensate for the wavelength characteristic 32 (see FIG. 2B) of the transmittance, and as illustrated in FIG. 2C, wavelength multiplexed signal light having different power levels is output for each signal light. .
Specifically, the gain equalizer 26 is connected to the optical fiber transmission line 12.
The optical power of each signal light is adjusted in advance so that the optical power of the signal light having a large attenuation in the optical signal becomes larger than the signal light having a small attenuation.

The gain equalizer 26 preferably comprises a passive optical filter capable of changing the wavelength characteristic of transmittance. In this case, the S / N ratio of each signal light before and after the gain equalizer 26 does not deteriorate. That is, the transmission S /
The power level of each signal light can be adjusted without deteriorating the N ratio.

The purpose of gain equalization by the gain equalizer 26 is as follows.
In the receiving terminal station 14, the optical power difference between the signal lights is reduced so that a desired S / N ratio can be obtained for all the signal lights. In order to achieve this object, the wavelength characteristic of the transmittance of the optical fiber transmission line 12 is measured in advance so that the receiving terminal station 14 can obtain power levels and S / N ratios higher than desired values for all signal lights. The wavelength characteristic of the transmittance of the gain equalizer 24 is adjusted. Of course, at the receiving terminal 14,
The transmission characteristics of the gain equalizer 26 may be feedback-controlled such that the reception power levels of the respective signal lights are compared and the transmission power levels become flat.

The output light from the gain transmitter 26 propagates through the optical fiber transmission line 12, during which each signal light is attenuated by a transmittance characteristic 32 shown by a broken line in FIG. Each signal light that has propagated through the optical fiber transmission line 12 enters the reception terminal station 14.
In the light input to the receiving terminal station 14, as illustrated in FIG. 2D, the difference between the optical power and the S / N ratio of each signal light is relatively small, and a good WDM transmission characteristic can be obtained. When a passive optical filter is used, the S / N ratio is not deteriorated even by the gain equalizer 26, so that the WDM transmission characteristics are further improved.

As the gain equalizer 26, for example, a Mach-Zehnder type optical filter, an etalon filter, a fiber
Gratings and acousto-optic tunable filters, and combinations thereof, can be used.

As shown in FIG. 3, the transmission characteristics of a Mach-Zehnder type optical filter have a periodicity with respect to wavelength and its FSR (Free Spectral Ran).
Ge) is fixed. In FIG. 3, the horizontal axis represents wavelength and the vertical axis represents loss. The further down, the greater the loss. at present,
The FSR can be realized in a range of 4 nm to 100 nm by design. The depth is variable, for example, from 0 dB to 35
What can be changed in the range of dB has been realized. The wavelength is also variable. Since the wavelength can be changed beyond the value of FSR,
It is easy to move the wavelength having the lowest transmittance to the wavelength at which the loss of the optical fiber transmission line 12 becomes the maximum.

As shown in FIG. 4, the transmission characteristic of the etalon filter changes sinusoidally with respect to the wavelength. The wavelength is variable, but the depth and FSR are fixed. In FIG. 4, the horizontal axis represents wavelength, and the vertical axis represents loss. The further down, the greater the loss. As the gain equalizer 26, for example, a depth of 0.4 to
Those having 1.0 dB and an FSR of 15 to 30 nm can be used. Etalon filters with an FSR of 15 nm or less and etalon filters with an FSR of 30 nm or more can be manufactured. Although the depth cannot be changed, the depth can be doubled, for example, by stacking two etalon filters having the same characteristics.

The transmission characteristic of the fiber grating has no periodicity as shown in FIG. In FIG. 5, the horizontal axis represents wavelength and the vertical axis represents loss. The further down, the greater the loss.
The wavelength, depth and FSR are all fixed. Since there is no periodicity, the gain equalizer 26 having various gain equalization characteristics can be realized by cascade-connecting fiber gratings having different wavelengths, depths, and FSRs. For example, there is one having a center wavelength of 1550 nm and a depth of about 1.5 dB.

The transmission characteristics of the acousto-optic type variable optical filter are as follows.
As shown in FIG. 6, it has no periodicity. However, the wavelength is variable and is electrically controllable.

The signal wavelength band is 10 nm, and the center wavelength is 1
550 nm and the optical fiber transmission line 12
In the case where the transmittance characteristic has a peak with respect to the wavelength, a Mach-Zehnder optical filter having an FSR of about 25 nm may be used. For example, by using a Mach-Zehnder type optical filter having a maximum loss wavelength of 1550 nm and a depth of about 2.5 dB, after gain equalization, the signal light with the lowest optical power (signal light with a wavelength of 1550 nm) and the maximum light The optical power difference from the power signal light (the signal light of the peripheral wavelength) can be reduced to about 1 dB. Further, by using a Mach-Zehnder type optical filter having a wavelength of maximum loss of 1552 nm and a depth of about 1.2 dB, the signal light (wavelength of 15
The optical power difference between the signal light of 52 nm) and the signal light of the maximum optical power (signal light of the peripheral wavelength) can be about 0.5 dB.

FIG. 7 shows a configuration example of the gain equalizer 26 applicable when the transmittance characteristic of the optical fiber transmission line 12 has two or more peaks with respect to the wavelength. Two tunable optical filters 40 and 42 whose wavelength and depth can be freely changed are connected in cascade. The variable optical filter 40 includes an optical switch 44,
46 allows the selection of any one of the Mach-Zehnder optical filter 48 having a 25 nm FSR and the through path 50. The variable optical filter 42 has a configuration in which either the Mach-Zehnder optical filter 56 having an FSR of 15 nm or the through path 58 can be selected by interlocking optical switches 52 and 54.

In the variable optical filter 40, an optical filter 52
And the wavelength of the maximum loss is 1544 nm,
The depth is set to 4 dB, and the variable optical filter 42 selects the optical filter 56 and sets the wavelength of the maximum loss to 1
By setting 552 nm and the depth to 2 dB, a WDM signal light having an optical power profile as illustrated in FIG. 8 can be obtained.

[0029]

As can be easily understood from the above description, according to the present invention, the optical power profile of the WDM signal light can be realized in a desired shape by a very simple operation, and as a result, good transmission characteristics can be obtained. realizable. Further, by using a passive filter for shaping the optical power profile, the S / N ratio on the transmission side is not degraded, which again contributes to the improvement of transmission characteristics.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram of an embodiment of the present invention.

FIG. 2 is a schematic diagram of a signal light power spectrum distribution and transmission characteristics of a gain equalizer 26 in each unit of the present embodiment.

FIG. 3 is a schematic diagram of transmission characteristics of a Mach-Zehnder optical filter.

FIG. 4 is a schematic diagram of transmission characteristics of an etalon filter.

FIG. 5 is a schematic diagram of transmission characteristics of a fiber grating.

FIG. 6 is a schematic diagram of transmission characteristics of an acousto-optic variable optical filter.

FIG. 7 is a schematic block diagram of a configuration example of a gain equalizer 26.

8 is an example of an optical power profile obtained by using the gain equalizer 26 shown in FIG.

FIG. 9 is an explanatory diagram of pre-emphasis and its effect in a conventional example.

[Explanation of symbols]

 10: Optical transmitting terminal 12: Optical fiber transmission line 14: Optical receiving terminal 20-1 to 20-n: Signal light source 22-1 to 22-n: Optical amplifier 24: Wavelength multiplexer 26: Gain equalizer 30 : Transmission characteristic of gain equalizer 24 32: Transmission characteristic (loss characteristic) of optical fiber transmission line 12 40, 42: Variable optical filter 44, 46: Optical switch 48: Mach-Zehnder optical filter 50: Through path 52, 54: Optical Switch 56: Mach-Zehnder type optical filter 58: Through-pass

Claims (5)

[Claims] 1. A wavelength division multiplexed WDM signal light source for outputting a plurality of signal lights having different wavelengths, and a light transmission means capable of changing the transmittance with respect to the wavelength,
An optical power profile adjusting means for adjusting the distribution of optical power with respect to the wavelength of the output light of the WDM signal light source. 2. The optical power profile adjusting means,
2. The optical transmission device according to claim 1, comprising one or more passive optical filters. 3. The optical transmission device according to claim 2, wherein said passive optical filter is any one of a Mach-Zehnder interference optical filter, an etalon optical filter, a fiber grating, an acousto-optic variable optical filter, and a combination thereof. 4. The WDM signal light source according to claim 1, wherein the WDM signal light source comprises a plurality of signal light sources for generating signal lights of different wavelengths, and wavelength multiplexing means for wavelength multiplexing the signal lights output from the plurality of signal light sources. Optical transmitter. 5. A WDM signal light source comprising: a plurality of signal light sources for generating signal lights of different wavelengths; a plurality of optical amplifiers for optically amplifying signal lights output from the plurality of signal light sources; 2. The optical transmitter according to claim 1, further comprising wavelength multiplexing means for wavelength multiplexing the output light.