JP2001044934A - Wavelength multiplex optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the deterioration of transmission characteristic caused by the non-degeneration type 4-optical wave mixture and mutual phase modulation and to reduce the attenuation of short wavelength side signal beam caused by the induced Raman scattering by polarizing and synthesizing the signal beams of long and short wavelength bands of polarized waves orthogonal to each other which are controlled by a polarized wave control means. SOLUTION: A polarization maintaining type is secured for the optical modulators 12-1 to 12-n, optical multiplexers 13-1 and 13-2 and optical amplifiers 14-1 and 14-2 respectively and these parts are connected together via the polarization maintaining optical fibers 15. When it is supposed that all output polarized waves of light sources 11-1 to 11-n are equal to each other, one of both fibers 15 which connect the amplifiers 14-1 and 14-2 and a polarized beam splitter 16 is turned by 90 degrees so that the polarized waves of wavelength multiple signal beams of every wavelength band are set orthogonal to each other. Thus, the wavelength multiplex signal beams of every wavelength band can be polarized and synthesized by the splitter 16 and sent to an optical fiber transmission line 17.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a wavelength division multiplexing optical transmitter used in a broadband wavelength division multiplexing optical transmission system.

[0002]

2. Description of the Related Art On the transmitting side of a wavelength division multiplexing optical transmission system,
Light having different wavelengths is modulated by corresponding electric signals, and the signal light of each wavelength is wavelength-multiplexed by an optical passive component such as a wavelength filter and transmitted to an optical fiber transmission line. On the receiving side, the wavelength-division multiplexed signal light is demultiplexed for each wavelength by a wavelength filter or the like, and is demodulated into the original electric signal after each photoelectric conversion. In such a wavelength division multiplexing optical transmission system, the multiplexing and demultiplexing of a plurality of signal lights can be easily performed only by optical passive components, and large-capacity optical transmission is possible.

[0003] In recent years, in response to a rapid increase in communication traffic, research on a wideband WDM optical transmission system using a wide wavelength band has been advanced. In particular, the gain band of the erbium-doped fiber amplifier (wavelength 1530-1565 nm) and the gain band of the gain-shifted erbium-doped fiber amplifier (wavelength 1565 nm)
(Wavelength multiplexing optical transmission system) that simultaneously uses the wavelength band of the thulium-doped fiber amplifier (wavelength 1450-1500 nm).

FIG. 6 shows a configuration example of a conventional wavelength multiplexing optical transmitter used in a broadband wavelength multiplexing optical transmission system. Here, the wavelength bands λ1 to λk and the wavelengths λ (k + 1) to λn
Corresponds to the gain band of each of the rare earth-doped fiber amplifiers (hereinafter referred to as “optical amplifiers”). Note that n is an integer of 2 or more, and k is an integer of 1 to n-1.

In FIG. 1, light beams having wavelengths λ 1 to λ k output from light sources 61-1 to 61-k are applied to optical modulators 6 respectively.
2-1 to 62-k are modulated by corresponding transmission signals,
The light is multiplexed by the optical multiplexer 63-1 to become a wavelength multiplexed signal light,
The signal is amplified by the optical amplifier 64-1. On the other hand, the light source 61− (k +
1) The lights of wavelengths λ (k + 1) to λn output from to 61-n are modulated by corresponding transmission signals in optical modulators 62- (k + 1) to 62-n, respectively, The signal light is multiplexed by 63-2 to become a wavelength multiplexed signal light, and is amplified by an optical amplifier 64-2. The wavelength division multiplexed signal light of each wavelength band is
5, and are transmitted to the optical fiber transmission line 66.

[0006]

By the way, especially in a broadband wavelength division multiplexing optical transmission system using a plurality of wavelength bands at the same time on a dispersion-shifted fiber or a non-zero dispersion-shifted fiber, there occurs between signal lights in different wavelength bands. It has been pointed out that nonlinear characteristics called "non-degenerate four-wave mixing" and nonlinear effects called "cross-phase modulation" may cause deterioration in transmission characteristics (Reference: J.
Kani et al., "Bi-directional transmission for suppre
ssing inter-wavelength-band nonlinear interactions
in ultra-wide band WDM transmission systems ", IEE
E Photonics Technology Letters, vol.11, pp.376-37
8, 1999).

[0007] Further, in the above-mentioned wideband wavelength division multiplexing optical transmission system, there is a problem that signal light in a short wavelength band is attenuated by a nonlinear effect called "stimulated Raman scattering", and transmission characteristics are deteriorated. Stimulated Raman scattering is a phenomenon in which short-wavelength light becomes excitation light and amplifies long-wavelength light when two lights having different wavelengths propagate in a nonlinear medium. It is known that the generation efficiency increases as the wavelength interval between two lights increases, and reaches a maximum at about 100 nm.

Further, the WDM filter 65 for multiplexing signal lights of two or more different wavelength bands has a band where the transmittance crosses as shown in FIG. This band is a useless band to which a signal wavelength cannot be assigned (hereinafter, referred to as “dead band”), and hinders effective use of the band.

According to the present invention, in a broadband wavelength division multiplexing optical transmission system using a plurality of wavelength bands, transmission characteristics deterioration due to non-degenerate four-wave mixing and cross-phase modulation occurring between signal lights in different wavelength bands described above is reduced. In addition, a wavelength multiplexing optical transmitter that can reduce the attenuation of signal light on the short wavelength side due to stimulated Raman scattering and can effectively use the band without generating a dead band when combining signal lights of a plurality of different wavelength bands. The purpose is to provide.

[0010]

SUMMARY OF THE INVENTION A wavelength division multiplexing optical transmitter according to the present invention, when multiplexing a signal light of a short wavelength band and a signal light of a long wavelength band and transmitting the multiplexed signal light to an optical fiber transmission line, transmits each wavelength band. And the polarization combining is performed so that the polarizations of the signal lights are orthogonal to each other.

With this configuration, the efficiency of non-degenerate four-wave mixing between bands is reduced. This is because, out of three signal lights involved in the generation of non-degenerate four-wave mixing, the polarization of two signal lights in one wavelength band and the polarization of one signal light in the other wavelength band are different. This is because when orthogonal, the generation efficiency of non-degenerate four-wave mixing is reduced to 1/4 (Reference: K. Inoue, "Polar"
ization effect on four-wave mixing efficiency in a
single-mode fiber ", IEEE J. of Quantum Electronic
s, vol.28, pp.883-894, 1992).

Further, according to the configuration of the present invention, the efficiency at which the cross-phase modulation effect between the bands occurs is reduced. This is because the generation efficiency of the cross-phase modulation becomes 2/3 when the polarizations of the two signal lights related to the generation of the cross-phase modulation are orthogonal to each other in the optical fiber (reference: GPAgrawal, "Nonlinea").
r fiber optics second edition ", Academic press, p.2
44).

Further, according to the configuration of the present invention, the efficiency of attenuating the signal light in the short wavelength band due to stimulated Raman scattering generated in the signal light in the long wavelength band is reduced. This is because if the polarization of the pump light and the polarization of the signal light to be amplified are orthogonal in the optical fiber, the generation efficiency of stimulated Raman scattering becomes zero. The relationship between this polarization and the generation efficiency of stimulated Raman scattering is described in the reference (RHSt
olen, "Crosstalk due to stimulated Raman Scatterin
g in single-mode fibers for optical communication
in wavelength division multiplex systems ", IEEE J.
of Quantum Electronics, vol.15, pp.1157-1160, 197
Although this is described in 9), it relates to basic physical properties when two laser beams are incident on an optical fiber, and does not refer to the wavelength arrangement of a broadband WDM optical transmission system.

By the way, the change in the polarization state when the signal light propagates through the optical fiber transmission line has wavelength dependence. For this reason, the orthogonal polarization relationship between the signal light in the short wavelength band and the signal light in the long wavelength band may not be maintained up to the output end of the optical fiber transmission line. In this case, the effect of suppressing the non-degenerate four-wave mixing, cross-phase modulation, and stimulated Raman scattering is reduced.

On the other hand, the reference (Inoue, "Study of Four-Wave Mixing in Wavelength Division Multiplexing Optical Fiber Transmission System", Ph.D. It describes the result of how much the light is deviated from the orthogonal state as it propagates through the optical fiber. According to this, it is understood that the polarization between the signal lights separated by about 10 nm deviates from the orthogonal state by about 50% in the optical fiber transmission of about 25 km. A non-linear length (a distance where a non-linear effect is effective) in an optical fiber transmission of 80 km, which is a link distance specified by the ITU-T, is about 15 km. Therefore, it can be seen that the suppression of non-degenerate four-wave mixing, cross-phase modulation, and stimulated Raman scattering by the above method is effective at least when the interval between two wavelength bands is about 10 nm or less.

Furthermore, in the configuration of the present invention, a polarization combining means (polarization beam splitter) for performing polarization combining in a state where the polarization of the short wavelength band signal light and the polarization of the long wavelength band signal light are orthogonal to each other. Is used. This allows WD to be combined into two wavelength bands.
When the M filter is used, a dead band which is a problem does not occur, and the band can be effectively used.

[0017]

(First Embodiment) FIG. 1 shows a first embodiment of the wavelength division multiplexing optical transmitter according to the present invention.

In FIG. 1, wavelengths λ 1 to λ 1 generated by light sources 11-1 to 11-k and optical modulators 12-1 to 12-k are used.
The signal light of λk is multiplexed by the optical multiplexer 13-1 and amplified by the optical amplifier 14-1. Light source 11- (k + 1) to 11-n
The signal lights having wavelengths λ (k + 1) to λn generated by the optical modulators 12- (k + 1) to 12-n are multiplexed by the optical multiplexer 13-2, and are multiplexed by the optical amplifier 14-2. Amplified. Here, it is assumed that the band of wavelengths λ1 to λk and the band of wavelengths λ (k + 1) to λn correspond to the gain bands of the optical amplifiers 14-1 and 14-2.

In the present embodiment, the optical modulators 12-1 to 12-12
-N, optical multiplexers 13-1, 13-2, optical amplifier 14-
1 and 14-2 are of a polarization maintaining type, and each part is connected via a polarization maintaining optical fiber 15. Further, the optical amplifier 14-
The input ports 1 and 14-2 and the corresponding input ports of the polarization beam splitter 16 are connected by a polarization maintaining optical fiber 15. Here, assuming that the output polarizations of the light sources 11-1 to 11-n are all the same, one of the polarization maintaining optical fibers 15 connecting the optical amplifiers 14-1 and 14-2 and the polarization beam splitter 16 is connected. By rotating by 90 degrees, the polarization of the wavelength multiplexed signal light in each wavelength band becomes orthogonal. This allows the polarization multiplexing of the wavelength multiplexed signal light of each wavelength band by the polarization beam splitter 16 and transmission to the optical fiber transmission line 17. In addition,
Light sources 11-1 to 11-k and light sources 11- (k + 1) to 11-n
Output the polarizations orthogonal to each other, there is no need to rotate the polarization maintaining optical fiber 15 by 90 degrees.

FIG. 2 shows an example of the wavelength arrangement of signal lights having wavelengths λ1 to λn. Here, the wavelengths λ1 to λk are defined as short wavelength bands, and the wavelengths λ (k + 1) to λn are defined as long wavelength bands. The polarization of the signal light in each wavelength band is orthogonal, and the polarization is combined by the polarization beam splitter 16, so that no dead band occurs.

The short wavelength band is 1530-1565 nm, the long wavelength band is 15
When the wavelength is 65 to 1605 nm, the optical amplifier 14-1 uses an erbium-doped fiber amplifier, and the optical amplifier 14-2 uses a gain-shifted erbium-doped fiber amplifier. 1450 short wavelength band
If the long wavelength band is 1530-1605 nm, the optical amplifier 14-1 uses a thulium-doped fiber amplifier, and the optical amplifier 14-2 uses a broadband erbium-doped fiber amplifier. Here, the broadband erbium-doped fiber amplifier is
An erbium-doped fiber amplifier having a gain band at a wavelength of 1530 to 1565 nm and a gain shift erbium-doped fiber amplifier having a gain band at a wavelength of 1565 to 1605 nm can be connected in parallel.

(Second Embodiment) FIG. 3 shows a second embodiment of the wavelength division multiplexing optical transmitter according to the present invention. In the figure, light source 1
The signal lights of wavelengths λ1 to λk generated by 1-1 to 11-k and the optical modulators 22-1 to 22-k are combined with the optical multiplexer 23-.
1 and are amplified by the optical amplifier 24-1. Light source 1
1- (k + 1) -11-n and optical modulator 22- (k + 1) -2
The signal lights of wavelengths λ (k + 1) to λn generated by 2-n are multiplexed by the optical multiplexer 23-2 and amplified by the optical amplifier 24-2. Here, the wavelength band λ1 to λk and the wavelength λ (k + 1)
.Lambda.n correspond to the gain bands of the optical amplifiers 24-1 and 24-2.

In this embodiment, the optical modulators 22-1 to 22-2
Polarization controllers (PCs) 25-1 to 25-k are inserted between the optical modulators 22-1 and 25-k, respectively.
Polarization controllers (PC) 25- (k + 1) to 25-n are inserted between (k + 1) to 22-n and the optical multiplexer 23-2, respectively. Further, the optical amplifiers 24-1 and 24-2 are connected to corresponding input ports of the polarization beam splitter 16. The polarization controllers 25-1 to 25-n include the optical modulators 22-1 to 22-22.
-K of the signal light of wavelengths λ1 to λk output from
Wavelength λ output from optical modulators 22- (k + 1) to 22-n
Control is performed so that the polarizations of the signal lights of (k + 1) to λn are orthogonal at the input end of the polarization beam splitter 16. This allows
The wavelength multiplexed signal light of each wavelength band is converted into a polarization beam splitter 16.
, And can be transmitted to the optical fiber transmission line 17.

(Third Embodiment) FIG. 4 shows a wavelength multiplexing optical transmitter according to a third embodiment of the present invention. In the figure, light source 1
1-1 to 11-n, optical modulators 12-1 to 12-n, optical multiplexers 13-1 and 13-2, optical amplifiers 24-1 and 24-2
Is configured to multiplex and amplify the signal light of each wavelength band as in the first and second embodiments. Here, it is assumed that the band of the wavelengths λ1 to λk and the band of the wavelengths λ (k + 1) to λn correspond to the gain bands of the optical amplifiers 24-1 and 24-2.

In this embodiment, the optical modulators 12-1 to 12-12
−n and the optical multiplexers 13-1 and 13-2 are of a polarization maintaining type. Further, a polarization controller (PC) 25-1 is inserted between the optical multiplexer 13-1 and the optical amplifier 24-1, and a polarization controller (PC) 25-1 is inserted between the optical multiplexer 13-2 and the optical amplifier 24-2. Controller (P
C) Insert 25-2. Each of the light sources 11-1 to 11-1
1-n to the polarization controllers 25-1 and 25-2 via the polarization-maintaining optical fiber 15;
1, 24-2 and the corresponding input ports of the polarization beam splitter 16 are connected. Polarization controllers 25-1, 25-2
Is the polarization of the wavelength multiplexed signal light of wavelengths λ1 to λk multiplexed by the optical multiplexer 13-1 and the wavelength multiplexing of the wavelengths λ (k + 1) to λn multiplexed by the optical multiplexer 13-2. Control is performed so that the polarization of the signal light is orthogonal at the input end of the polarization beam splitter 16. As a result, the wavelength-division multiplexed signal light of each wavelength band is polarization-combined by the polarization beam splitter 16 and the optical fiber transmission path 17
Can be sent to

(Fourth Embodiment) FIG. 5 shows a wavelength multiplexing optical transmitter according to a fourth embodiment of the present invention. In the present embodiment, FIG.
In the configuration of the second embodiment shown in (1), a part of the output light of the polarization beam splitter 16 is received by the control circuit 32 via the optical coupler 31, and the light intensity of the wavelength multiplexed signal light in each wavelength band is maximized. Each of the polarization controllers 25-1 to 25-n is subjected to negative feedback control.

In the configuration of the third embodiment shown in FIG. 4, a part of the output light of the polarization beam splitter 16 is monitored so that the light intensity of the wavelength multiplexed signal light in each wavelength band is maximized. Each of the polarization controllers 25-1 and 25-2 may be subjected to negative feedback control.

The optical receiver corresponding to the wavelength division multiplexing optical transmitter in each of the embodiments described above demultiplexes the wavelength division multiplexed signal light for each wavelength using a wavelength filter or the like, and performs optical-electrical conversion to convert the transmission signal. What is necessary is just to demodulate. That is, the wavelength division multiplexing optical transmission system using the wavelength division multiplexing optical transmitter of the present invention does not need to perform polarization demultiplexing on the receiving side, unlike the so-called polarization multiplexing system.

[0029]

As described above, the wavelength division multiplexing optical transmitter of the present invention makes the polarizations of the short wavelength band signal light and the long wavelength band signal light orthogonal, combines the polarizations, and synthesizes the optical fiber transmission line. , It is possible to reduce signal deterioration due to non-degenerate four-wave mixing between bands and cross-phase modulation. Further, excess loss of signal light in a short wavelength band due to stimulated Raman scattering can be reduced.

Further, since the configuration is such that the signal light in the short wavelength band and the signal light in the long wavelength band are polarization-synthesized, a dead band which is a problem when a WDM filter is used does not occur, and the band is effectively used. be able to.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of a wavelength division multiplexing optical transmitter according to the present invention.

FIG. 2 is a diagram illustrating an example of a wavelength arrangement according to the first embodiment.

FIG. 3 is a block diagram showing a second embodiment of the wavelength division multiplexing optical transmitter according to the present invention.

FIG. 4 is a block diagram showing a third embodiment of the wavelength division multiplexing optical transmitter according to the present invention.

FIG. 5 is a block diagram showing a fourth embodiment of the wavelength division multiplexing optical transmitter according to the present invention.

FIG. 6 is a block diagram showing a configuration example of a conventional wavelength multiplexing optical transmitter used in a broadband wavelength multiplexing optical transmission system.

FIG. 7 is a diagram illustrating a relationship between a wavelength arrangement example of a conventional configuration and a transmission band of a WDM filter.

[Explanation of symbols]

 Reference Signs List 11 light source 12 optical modulator (polarization maintaining type) 13 optical multiplexer (polarization maintaining type) 14 optical amplifier (polarization maintaining type) 15 polarization maintaining optical fiber 16 polarization beam splitter 17 optical fiber transmission line 22 optical modulation Device 23 optical multiplexer 24 optical amplifier 25 polarization controller (PC) 31 optical coupler 32 control circuit

Claims (4)

[Claims] 1. A wavelength division multiplexing optical transmitter for multiplexing a signal light in a long wavelength band and a signal light in a short wavelength band and transmitting the multiplexed signal light to an optical fiber transmission line, wherein: Polarization control means for controlling the polarization of the signal light in the short wavelength band to be orthogonal, and the signal light in the long wavelength band and the signal light in the short wavelength band of the orthogonal polarization controlled by the polarization control means. And a polarization combining means for combining the polarizations of the wavelengths. 2. The signal light of the long wavelength band and the signal light of the short wavelength band are wavelength multiplexed signal lights obtained by wavelength multiplexing a plurality of signal lights having different wavelengths, respectively. 2. The wavelength division multiplexing optical transmitter according to claim 1, wherein the wavelength division multiplexing signal transmitter is configured to control the polarizations of the wavelength division multiplexing signal light to be orthogonal. 3. The polarization control means is configured to maintain or adjust each polarization state such that the polarization of the signal light (wavelength multiplexed signal light) in each wavelength band is orthogonal. The wavelength division multiplexing optical transmitter according to claim 1. 4. The polarization control means controls each polarization state such that the light intensity of the signal light (wavelength multiplexed signal light) in each wavelength band becomes maximum at the output terminal of the polarization combining means. 3. The wavelength division multiplexing optical transmitter according to claim 1, wherein the transmitter has a configuration.