JPH07202846A - Optical wavelength multiplex network system - Google Patents

Abstract

PURPOSE:To simultaneously transmit information signals of plural bit rates by using effectively a limited operating wavelength number with inexpensive configuration. CONSTITUTION:An external modulation system is adopted and mXn-sets of input information signals S11-Smn are divided into m-groups and a carrier light of n-kinds of wavelengths lambdal-lambdan obtained by light emitting devices 41-4n is distributed by using optical splitters 51-5n. By using the carrier light of n- kinds of wavelengths for each group, n-sets of information signals are modulated by an optical modulator 14 and synthesized in the unit of groups by star couplers SC1, SCm provided to each group and distributed to an i-system. Then an optional synthesis light is selected by the optical switch 36 provided to each system and a tunable optical filter 31 extracts a signal light of an optional wavelength and converted into an electric signal by a photoelectric converter 32 and a receiver 33 demodulates the information signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention modulates and outputs each information signal using carrier lights having different wavelengths at a transmitting node, multiplexes them by a star coupler for transmission, and separates wavelengths at a receiving node. The present invention relates to an optical wavelength division multiplexing (hereinafter, referred to as WDM) network system that selectively demodulates.

[0002]

2. Description of the Related Art A conventional WDM network system used in, for example, a broadcasting station is basically constructed as shown in FIG. Note that FIG. 9 shows the case where both the transmitting node and the receiving node have n channels.

In FIG. 9, the transmission nodes T1 to Tn have the same structure, and the light emission drive unit 11 and the light emission unit 1 are respectively provided.
2 is provided. The light emitting devices 12 of the respective transmission nodes T1 to Tn generate carrier lights of different wavelengths .lambda.1 to .lambda.n.

That is, the information signals S1 to Sn input to the respective transmission nodes T1 to Tn are respectively emitted from the light emission drive unit 1.
The signal is converted into a light emission drive signal by 1 and sent to the light emitting device 12 to directly drive the light emitting device 12. by this,
The light emitted from each light emitting device 12 is modulated according to the inputted light emission drive signal.

The modulated lights transmitted from the light emitting devices 12 of the respective transmitting nodes T1 to Tn are combined by the star coupler 2 and the variable wavelengths of the receiving nodes R1 to Rn (hereinafter referred to as tunable signals) are output through an n-channel output port. Refer to)
It is sent to the optical filter 31.

The tunable optical filter 31 separates the modulated light of any desired wavelength from the combined light from the star coupler 2, and the modulated light separated here is the light receiving device 3.
After being photoelectrically converted by 2, the signal is demodulated by the receiving device 33. That is, at each of the receiving nodes R1 to Rn, an information signal from an arbitrary transmitting node T1 to Tn can be selected and obtained.

By the way, in the WDM network system having the above configuration, since the modulated light of each wavelength is multiplexed in the star coupler and then distributed to a plurality of channels, it is necessary to compensate the distribution loss. For this distribution loss compensation, an erbium-doped fiber amplifier (hereinafter referred to as EDFA) which has been put into practical use in recent years can be used.

However, the EDFA has a finite gain band as shown in FIG. On the other hand, the wavelength interval for stable separation is uniquely determined from the wavelength stability of a practical light emitting device, the resolution of a tunable optical filter, and the like. Therefore, the number of wavelengths that can be used in the WDM network is limited, and the number of transmission signals is limited by the number of wavelengths that can be used.

Therefore, when transmitting a large number of types of information signals, it is common to employ a time division multiplexing system.
A system configuration according to this method is shown in FIG. In FIG. 11, the transmission node T1 is provided with an m: 1 time division multiplexer 13 in front of the light emission drive device 11. At this transmission node T1, a plurality of information signals S11, S12, ..., S1m
Are time-division multiplexed to increase the speed, and then the carrier light of wavelength λ1 is modulated and output.

Similarly, in the other transmission nodes T2 to Tn, a plurality of information signals S21 to S2m, ... Sn1 to Snm, respectively.
Are time-division-multiplexed to increase the speed, and then carrier lights having respective wavelengths λ2 to λn are modulated and output.

On the other hand, in each of the receiving nodes R1 to Rn, a 1: m demultiplexing device 34 and a selector switch 35 are installed in this order in the subsequent stage of the receiving device 33, and the high-speed signals time-division multiplexed by the 1: m demultiplexing device 34 are m. After separating into channels, the selector switch 35 selects a signal of an arbitrary channel.

As a result, the light of n kinds of wavelengths is used for n
All the m-channel information signals S11 to Snm can be transmitted to the receiving nodes R1 to Rn, and the receiving nodes R1 to Rn can be transmitted.
An information signal of an arbitrary channel can be selected and extracted by Rn.

In the WDM network system having the above configuration, a finite number of optical wavelengths can be effectively used and a large number of types of information signals can be transmitted. However, since the multiplexer and the demultiplexer generally shape the waveform of the transmission signal by the clock, the bit rate of the input signal must be unified for each multiplexer, and the mixture of multiple bit rate signals cannot be allowed. .

Further, since the speed of the signal is increased by the multiplexer, the number of multiplexed signals and the speedup are limited, and the speed of the input signal is also limited, which is a merit of the optical wavelength multiplexing technique. Will be damaged. Furthermore, because of the multiplexing for each transmitting node, the transmitting node itself is limited to the number of usable wavelengths.

From the above, the configuration shown in FIG. 12 has been conventionally considered. That is, in this system, m × n transmission nodes T11 to Tmn each composed of the light emission drive device 11 and the light emission device 12 are divided into m groups, and in each group, each transmission node has one of λ1 to λn. Assign different wavelengths. Then, the output of the transmission node of each group is input to the star couplers SC1 to SCm provided corresponding to each group, and after multiplexing, n
Receiving nodes R1 distributed to each system and provided for each system
Send to Rn.

Each of the receiving nodes R1 to Rn has an optical selector switch 36 at the input stage.
Can selectively derive any one of the transmitted lights from the star couplers SC1 to SCm. here,
The optical output of the star coupler SC1 is the combined light of S11 to S1n, S
The optical output of C2 is the combined light of S21-SC2n
The optical output of m is the combined light of Sm1 to Smn.

Therefore, in each of the receiving nodes R1 to Rn, the optical selector switch 36 causes the star coupler SC to operate.
A group on the transmitting side can be selected by selecting an optical output of 1 to SCm, and an arbitrary information signal in the group selected by the optical filter 31, the light receiving device 32, and the receiving device 33 described above can be taken out. be able to.

However, in the system having the grouping structure as described above, since the relatively expensive optical wavelength setting light emitting device 12 is required for each information signal, the cost becomes extremely high in a system having a large number of information signals. Will end up.
Further, the light wavelength setting light emitting device must be controlled accurately and stably in order to use the wavelength as effectively as possible, and a complicated light wavelength control device is required for each wavelength device. This also causes the cost and increase of the system.

By the way, the light emitting device in the above-mentioned conventional system employs a drive system in which the light emitting device is directly modulated by an input signal. In this direct modulation system, as shown in FIG. Due to the wavelength chirping due to the modulation, the emission spectrum width becomes wider than that when there is no modulation, and crosstalk with the adjacent channel signal occurs. Further, if the wavelength interval between adjacent channels is widened so that the above-mentioned crosstalk does not occur, the number of usable wavelengths decreases.

[0020]

As described above, in the conventional WDM network system, in order to effectively use the limited number of wavelengths to be used, if the time division multiplexing method is adopted, a multi-bit rate signal is transmitted. Mixing cannot be allowed, flexibility of the optical wavelength division multiplexing technique is impaired due to the limitation of the transmission signal speed, and the number of transmitting nodes is limited to the number of usable wavelengths.

A grouping structure is considered as an improvement measure, but it is difficult to realize the cost because a lot of expensive light emitting devices and complicated wavelength control devices are required. further,
Since the conventional system uses the direct modulation method,
There is also a problem that the emission spectrum width at the time of modulation widens, crosstalk with adjacent channel signals occurs, and the number of usable wavelengths decreases.

The present invention has been made to solve the above-mentioned problems, and it is possible to simultaneously transmit information signals of a plurality of bit rates with a low-cost structure and effectively use a limited number of wavelengths to be used. The purpose of the present invention is to provide a WDM network system capable of achieving a number of transmission nodes more than the number of wavelengths used, maintaining flexibility of WDM technology, and reducing the number of usable wavelengths to adjacent channel signals. An object is to provide a WDM network system capable of suppressing crosstalk.

[0023]

In order to solve the above problems, a WDM network system according to the present invention comprises n light emitting devices which emit carrier lights having different wavelengths,
Provided corresponding to each of these n light emitting devices,
An n number of optical splitters for respectively distributing the carrier light output of each light emitting device to m channels and one of the m × n channel carrier lights distributed by the n number of optical splitters are allocated, and the input carrier light is M × n optical modulators that modulate and output by individually input information signals,
Of the signal lights output from the m × n optical modulators, n-channel signal lights having different carrier light wavelengths are input, and after multiplexing, k channels (where k = n or k
≠ n) m optical star couplers with n inputs and k outputs and channel outputs of the m optical star couplers are respectively supplied, and the combined light of any one channel is selected according to the selection operation. I (where i ≤
k) optical switch circuits and i pieces provided corresponding to each of the i optical switch circuits and extracting signal light of an arbitrary wavelength from the combined light selected by the corresponding optical switch circuit by tuning operation Tunable optical filters, i photoelectric converters provided corresponding to the i optical filters, respectively, and receiving the signal light extracted by the corresponding optical filters, and the i photoelectric converters, respectively. And i demodulators for demodulating the information signal from the output of the corresponding photoelectric converter.

[0024]

In the WDM network system configured as described above, the external modulation method is adopted, m × n input information signals are divided into m groups, and carrier lights of n kinds of wavelengths are distributed and supplied to each group. Then, n pieces of information signals are externally modulated and output by using carrier lights of n kinds of wavelengths for each group, multiplexed by a group unit by a star coupler provided for each group, and distributed to the i system. Then, select any combined light with the optical switch provided for each system, extract the signal light of any wavelength with the tunable optical filter, convert it into an electrical signal with the photoelectric converter, and then receive the information signal with the receiving device. Are demodulated. This makes it possible to solve the above problems.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of a WDM network system according to the present invention. In FIG. 1, the same parts as those of the WDM network system shown in FIG. 6 are designated by the same reference numerals, and different parts will be described here.

In FIG. 1, a carrier light generator is provided separately from the transmitting node, and n light emitting devices 41 to 4n continuously generate carrier lights of n kinds of wavelengths λ1 to λn without modulation.
Each of the n optical splitters 51 to 5n for each wavelength distributes the light by m.

On the other hand, each of the transmission nodes T11 to Tmn is provided with a modulation driving device 13 and an optical modulator 14, and is divided into m groups of n each. Each group is supplied with a group of n kinds of carrier lights of wavelengths λ1 to λn, which are m-divided by the carrier light generator, and are allocated to n transmission nodes in the group.

At the transmission node T11, the carrier light having the assigned wavelength λ1 is input to the optical modulator 14. The optical modulator 14 modulates the carrier light based on the information signal input through the modulation driving device 13 and outputs it as transmission light. This modulation processing is the same for the other transmission nodes T12 to Tmn.

The transmission light output from each of the transmission nodes T11 to Tmn is input to the star couplers SC1 to SCm for each group and multiplexed for each group. Each star coupler SC
1 to SCm each receive the multiplexed light by n receiving nodes R1 to
Supply to Rn.

Each of the receiving nodes R1 to Rn has an optical selector switch 36 at the input stage. This optical selector switch 3
Reference numeral 6 selectively derives any one of the transmitted lights from the star couplers SC1 to SCm. That is, SC1 output S11
.About.S1n, SC2 outputs S21 to SC2n, and thereafter similarly, any of the multiplexed lights of SCm outputs Sm1 to Smn is selected.

Each of the receiving nodes R1 to Rn extracts the modulated light of an arbitrary wavelength from the combined light selected by the optical selector switch 36 by tuning the tunable optical filter 31, and photoelectrically converts it by the light receiving device 32. , By demodulating with the receiving device 33, an arbitrary information signal in the selected group is extracted.

That is, in the WDM network system having the above configuration, the external optical modulator 14 is arranged in each of the transmission nodes T11 to Tmn, and the transmission nodes T11 to Tmn are grouped into m according to the number n of usable wavelengths. . And
A carrier light generator is separately provided, and the light emitting devices 41 to 4 therein are provided.
Optical splitters 51 to
5n distributes and supplies to each group and allocates to each transmitting node in the group. Further, the output of the transmitting node is multiplexed by using the star couplers SC1 to SCm for each group and distributed and supplied to the receiving nodes R1 to Rk.

According to the above structure, since the light emitting device is shared by each group, even if the number of usable wavelengths is limited, it is possible to transmit more information signals by increasing the number of groups. Becomes Further, since the complicated wavelength setting control circuit is shared with the relatively expensive light wavelength setting light emitting device, the number of them can be significantly reduced, and the cost can be significantly reduced.

Since one information signal is modulated and output for each wavelength, high-speed signals of a plurality of bit rates can be transmitted and a flexible network can be constructed.

Furthermore, in each transmission node, the direct modulation method is eliminated and the stable carrier light supplied from the outside is modulated and output. Therefore, the modulation control is simple and the cost of the transmission node itself is reduced. Can be planned.

Further, according to this external modulation method, as shown in FIG. 2, the spread of the emission spectrum due to the chirping at the time of modulation can be suppressed. Therefore, crosstalk with the adjacent channel signal can be reduced,
There is no need to reduce the number of usable wavelengths. Moreover, as shown in comparison with the figure, since the spectrum width of the transmitted light is sufficiently narrow compared to the transmission characteristics of the optical filter, it is easy to set the tuning of the tunable optical filter on the receiving node side. You can

Furthermore, since time division multiplexing is not used,
By using the linear region of the optical modulator, it is possible to construct a flexible network that can be mixed with analog signals. By the way, in the WDM network system having the above configuration,
Optical splitters 51-5n and star couplers SC1-SCm
Each of the optical nodes R1 to
The optical power input to Rn will decrease.

In order to compensate for this distribution loss, a method of inserting an optical amplifier such as an erbium-doped fiber amplifier (EDFA) into each input section or output section of each star coupler SC1 to SCm can be considered. In such a method, since expensive optical amplifiers of m × n are required, the cost of the entire system increases.

Therefore, as shown in FIG. 3, the optical amplifiers 61 to 6n such as the above-mentioned EDFA are arranged immediately before the input to the optical splitters 51 to 5n so as to be commonly used as booster amplifiers for the respective light emitting devices 41 to 4n. . With this configuration, the number of expensive optical amplifiers can be reduced to n.

On the other hand, as the optical modulator 14 of each of the transmission nodes T11 to Tmn, it is realistic to use a Mach-Zehnder (MZ) type which is currently in practical use for high-speed modulation. FIG. 4 shows the basic configuration of the MZ type optical modulator.

In FIG. 4, reference numeral 141 denotes a polarizer having a polarization dependency that allows a light wave having only a polarization in a fixed direction as shown in FIG. 5 to pass therethrough and intercepts light waves having other polarization directions.
Reference numeral 2 is a substrate having an electro-optical effect such that when an electric field is applied to a light wave in a specific polarization direction, the refractive index changes and the propagation speed of the light wave can be controlled.

That is, this MZ type optical modulator optimizes the modulation efficiency by inputting only the light wave of the polarized wave in a fixed direction into the polarizer 141, and the input light wave passing through the polarizer 141 on the substrate 142 is designated as A. It is branched into two optical waveguides of B, an electric field is applied to one of the optical waveguides by an input signal S to shift the wavelength of the light wave by a half wavelength, and the light wave from the other optical waveguide is combined. is there. The combined optical outputs cancel each other only when an input signal is applied, and become an off level. Therefore, the input light wave can be digitally modulated by the input signal.

When such an MZ type optical modulator is used as the optical modulator 14 of the above-mentioned system, it is transmitted from the optical splitters 51 to 5n by using the polarization maintaining fiber which keeps the polarization direction of the input light wave constant. In general, the polarization direction of the carrier light to be transmitted is made to coincide with the passing polarization direction of the polarizer and is incident on the optical modulator 14. However, as shown in FIG.
The optical amplifiers 61 to 6n such as EDFA are connected to the optical splitter 5
When inserted in the input stage of 1 to 5n, the polarization state of the light wave changes in the optical amplifier due to environmental changes such as temperature, and the output optical power of the optical modulator 14 due to the polarization dependence of the polarizer 141. Fluctuates, and in some cases, the light output disappears.

As a method for solving this problem,
Instead of the above MZ type optical modulator having polarization dependence,
For example, a semiconductor electro-absorption (EA) type optical modulator having a small polarization dependence may be used.
The EA-type optical modulator is still under development and its cost is high, and the polarization dependency is not completely solved under the present circumstances.

As another method, all the optical transmission lines from the light emitting device to the optical modulator, including the optical amplifier and the optical splitter, are made of polarization-maintaining fiber, so that the above-mentioned polarization fluctuation in the optical amplifier can be prevented. Although it may be suppressed, the polarization-maintaining DEFA is still undeveloped and costly, which is not practical.

FIG. 6 shows a system configuration that solves the above problems. 6, the same parts as those in FIGS. 1 and 3 are designated by the same reference numerals and the description thereof will be omitted. This system is similar to the embodiment shown in FIG.
Optical amplifiers 61 to 6n based on EDFA are arranged at the input stage of n, the MZ type optical modulator having polarization dependence shown in FIG. 4 is used as the optical modulator 14, and the light emitting devices 41 to 4n and the optical amplifier 61 are used. To 6n between polarization scramblers 71 to 7
n is interposed.

The polarization scramblers 71 to 7n are shown in FIG.
As shown in (1), the constant polarization state of the input light wave is forcibly changed by the high-frequency drive voltage E to eliminate polarization deviation.

That is, as described above, the light emitting devices 41 to 4 are
When the carrier light having a constant polarization state sent from n is passed through the polarization scramblers 71 to 7n and the polarization scramble is applied by the high frequency drive voltage E, the low frequency generated in the optical amplifiers 61 to 6n due to environmental changes or the like. It is possible to ignore the influence of the polarization fluctuation of.

Here, when the input information signal is a digital signal as shown in FIG. 8A, the polarization scramblers 71 to 7 are provided.
The frequency of the driving voltage E of n is made sufficiently higher than the input information signal. At this time, the carrier light incident on the optical modulator 14 passes through the polarizer 141 and is sent to the substrate 142 only when the polarization state matches the polarization direction of the polarizer 141. Therefore, as shown in FIG. 8B, the output light of the optical modulator 14 becomes intensity-modulated light that is high-frequency modulated only when the input information signal is "1".

When the high-frequency intensity-modulated optical signals are input to the receiving nodes R1 to Rn, the receiving nodes R1 to Rn
Since the signal frequency band is limited on the Rn side, the high frequency component is smoothed by the band limitation as shown in FIG. Therefore, the original signal can be demodulated.

Therefore, according to the above configuration, since the polarization states of the carrier light are scrambled at high frequencies by the polarization scramblers 71 to 7n, polarization fluctuations of the carrier light and optical modulation in the optical amplifiers 61 to 6n are performed. The polarization dependence of the device 14 can be ignored, and the optical signals can be stably received at the receiving nodes R1 to Rn. Moreover, since it is only necessary to insert the existing polarization scramblers for the number of carrier lights, the cost is low and the realization thereof is sufficiently possible.

The present invention is not limited to the above embodiment, the number of distributions of the star coupler may be arbitrary, and the number of receiving nodes may be set arbitrarily. In addition, various modifications can be made without departing from the spirit of the invention.

[0053]

As described above, according to the present invention, it is possible to simultaneously transmit the information signals of a plurality of bit rates by using the limited number of wavelengths effectively with the low cost structure. It is possible to provide a system, in particular, it is possible to realize the number of transmitting nodes more than the number of wavelengths used,
(EN) Provided is a WDM network system capable of suppressing crosstalk with adjacent channel signals without impairing the flexibility of WDM technology, reducing the number of usable wavelengths, and facilitating tuning setting of an optical filter. be able to.

[Brief description of drawings]

FIG. 1 is a block optical circuit diagram showing a configuration of an embodiment of a WDM network system according to the present invention.

FIG. 2 is a spectrum characteristic diagram showing a relationship between a modulation light emission spectrum characteristic and an optical filter transmission characteristic in the above embodiment.

FIG. 3 is a block optical circuit diagram showing a configuration in the case where an optical amplifier is used for compensating an optical distribution loss in the above embodiment.

FIG. 4 is an optical circuit diagram showing a basic configuration of an MZ type optical modulator used for the external optical modulator of the above-described embodiment.

FIG. 5 is a waveform diagram showing a concept of polarization in order to explain polarization dependency of a polarizer of the MZ modulator.

FIG. 6 is a diagram showing an external modulator as an MZ type modulator, DE
It is a block optical circuit diagram which shows the system configuration for solving the problem of polarization fluctuation when FA is used.

7 is a block optical circuit diagram showing a specific configuration of a polarization scrambler used in the system of FIG.

FIG. 8 is a timing waveform chart for explaining a processing operation of the polarization scrambler.

FIG. 9 is a block circuit diagram showing a basic configuration of a conventional WDM network system.

FIG. 10 is a characteristic diagram showing a gain band characteristic of an EDFA used for star coupler distribution loss compensation of a WDM network system.

FIG. 11 is a block circuit diagram showing a configuration of a WDM network system adopting a conventional time division multiplexing system.

FIG. 12 is a block circuit diagram showing a configuration of a WDM network system having a conventional grouping configuration.

FIG. 13 is a spectrum characteristic diagram for pointing out a problem of a direct modulation method adopted in a conventional WDM network system.

[Explanation of symbols]

T1 to Tn, T11 to Tmn ... Transmitting node, R1 to Rk ... Receiving node, SC, SC1 to SCm ... Star coupler, 11
... Light emitting drive device, 12, 41 to 4n ... Light emitting device, 13 ...
m: 1 time division multiplexer, 14 ... Optical modulator, 141 ... Polarizer, 142 ... Substrate, 31 ... Tunable optical filter,
32 ... Light receiving device, 33 ... Receiving device, 34 ... 1: m time division separating device, 35 ... Selector switch, 36 ... Optical selector switch, 51-5n ... Optical splitter, 61-6n ... Optical amplifier (EDFA), 71 ... 7n ... Polarization scrambler.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location H04L 12/44 7831-5K H04L 11/00 340

Claims (7)

[Claims] 1. n that emits carrier lights having different wavelengths from each other
Light emitting devices, and the light emitting devices are provided corresponding to each of the n light emitting devices,
N optical splitters for distributing the carrier light output of each light emitting device to m channels respectively, and m × each for distribution by the n optical splitters.
One of the n-channel carrier lights is allocated, and the input carrier lights are respectively output from m × n optical modulators that modulate and output by the input information signals and m × n optical modulators. Of the signal lights, n-channel signal lights having mutually different carrier light wavelengths are input, and after multiplexing, k channels (where k = n or k
≠ n) m optical star couplers with n inputs and k outputs and the channel outputs of the m optical star couplers are respectively supplied, and the combined light of any one channel is selected according to the selection operation. I (where i ≦ k) optical switch circuits to be derived in a specific manner, and any of the combined lights selected by the corresponding optical switch circuits by tuning operation, provided corresponding to the i optical switch circuits. I tunable optical filters for extracting signal light of wavelengths, and i photoelectric converters provided corresponding to the respective i optical filters and receiving the signal light extracted by the corresponding optical filters. And an optical demodulator that is provided corresponding to each of the i photoelectric converters and demodulates an information signal from the output of the corresponding photoelectric converter. Tem. 2. The optical system according to claim 1, further comprising n optical amplifiers provided in the preceding stage of said n optical splitters and compensating for optical distribution loss of said optical splitters and said optical star couplers. Optical wavelength division multiplexing network system. 3. The optical wavelength multiplexing network system according to claim 2, wherein the optical amplifier is an erbium-doped fiber amplifier. 4. When all of the m × n optical modulators have polarization dependency, the optical modulators are further provided in front of the n optical amplifiers, and carry light from the n light emitting devices. 3. The optical wavelength division multiplexing network system according to claim 2, further comprising n polarization scramblers for performing polarization scrambling. 5. The polarization scrambler changes the polarization state of the carrier light faster than the frequency of the information signal input to the optical modulator. Optical wavelength division multiplexing network system. 6. The optical wavelength division multiplexing network system according to claim 4, wherein the optical modulator is a Mach-Zehnder type modulator. 7. An optical wavelength multiplexing network system for wavelength-multiplexing light waves of different wavelengths transmitted from a plurality of light sources by modulating the input signals of the optical modulators by optical modulators each having polarization dependence, An optical wavelength division multiplexer provided between the plurality of light sources and the optical modulator, comprising a polarization scrambler for varying the polarization state of each light wave faster than the frequency of the input signal. Network system.