JP2009124342A - Optical transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To increase the transmission capacity by increasing the number of multi-values of a multi-level code, in a WDM multiplex transmission system. <P>SOLUTION: In this optical transmission system provided with a transmitter for generating a plurality of optical signals with different wavelenghs, a receiver for performing power composition of the plurality of optical signals transmitted from the transmitter via an optical fiber to be one multi-level code; the transmitter is configured so as to transmit the plurality of optical signals while making the power of one or more optical signals from among the plurality of optical signals which differ from the power of the other optical signals. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical transmission system in which a transmission capacity is increased when a plurality of optical signals having different wavelengths generated by a light source means are transmitted.

  A wavelength division multiplexing (WDM) transmission system has been put to practical use as an optical communication system. In a general WDM transmission system, as shown in FIG. 7, the transmitter 100 combines optical signals of a plurality of wavelengths generated by a plurality of light sources / modulators 110 by a wavelength multiplexer 120 and passes through an optical fiber 300. The receiver 200 demultiplexes the combined optical signal for each wavelength using the wavelength demultiplexer 210, and converts the signal into an electrical signal using the optical receiver 220 for each demultiplexed wavelength channel. To do.

  On the other hand, a WDM multi-level transmission scheme has been proposed as an economical transmission capacity expansion technique in the downstream transmission scheme of the one-to-many communication, particularly in the receiver apparatus (Non-Patent Document 1). As shown in FIG. 8, the transmitter 100 multiplexes optical signals of a plurality of wavelengths generated by a plurality of light sources / modulators 110 by a wavelength multiplexer 120 and transmits it via an optical fiber 300. In the receiver 200, the signal is received as a multi-level logic code by the single optical receiver 230.

In the WDM multilevel transmission system shown in FIG. 8, since optical signals of a plurality of wavelengths are collectively received by one optical receiver 230 without being demultiplexed and intensity modulation type multilevel logic encoding is performed, an expensive optical demultiplexing is performed. Economical expansion of transmission capacity can be realized without using the device 210. This method is particularly effective as a downlink transmission method in one-to-many communication such as PON as shown in FIG. 9 used in an access network. Reference numeral 400 denotes an optical distributor (power splitter). Since this WDM multi-level transmission method can reduce the cost of the receivers 200 of the user in-home devices existing on the network, a high economic effect can be obtained.
Four people from Miyoshi, “Proposal of multi-wavelength reception multilevel technology”, 2007 IEICE Communication Society, B-10-58, September 2007

However, in the WDM multi-level transmission method of Non-Patent Document 1, the power of the optical signals output from the plurality of light sources / modulators 110 is almost the same, so the multi-level number of the multi-level code is “number of light sources / modulators”. +1 ". For example, when three light sources / modulators 110 are used, there are four types of multilevel codes, “0”, “1”, “2”, and “3”. Therefore, the amount of information that can be transmitted at once is log ₂ (N + 1), where N is the number of light sources / modulators 110. Since the amount of information is N in the WDM transmission method described with reference to FIG. 7 described above, the WDM multilevel transmission method of Non-Patent Document 1 has a problem that the increase in the amount of information is small as the number of light sources / modulators 110 increases. There is. FIG. 10 shows a comparison result of the information amount between the WDM transmission system and the WDM multilevel transmission system.

  On the other hand, in the PON system, the distance to the user connected to one branch varies, and the attenuation of the signal in the optical fiber increases as the distance increases. Therefore, transmission power and transmission speed are limited so that a signal can be received at the longest distance of the specification. In other words, it is inefficient for receivers that are close to each other because high power and low speed transmission are performed in accordance with a long distance even though transmission with less power and higher speed transmission are possible. There is a problem.

  An object of the present invention is to provide an optical transmission system in which the transmission capacity is increased and the influence of attenuation in the transmission path is improved.

In order to achieve the above object, an invention according to claim 1 includes a transmitter that generates a plurality of optical signals having different wavelengths, and the plurality of optical signals transmitted from the transmitter via an optical transmission line. In an optical transmission system including a receiver that combines powers into one multilevel code, the transmitter uses the power of one or more optical signals of the plurality of optical signals as the power of other optical signals. It is characterized by transmitting differently.
According to a second aspect of the present invention, in the optical transmission system according to the first aspect, the transmitter includes a plurality of light source means for generating a plurality of optical signals having different wavelengths, and the transmitter includes: One or more includes a power control unit for controlling the power of the generated optical signal or a power attenuation unit for attenuating the power of the generated optical signal.
According to a third aspect of the present invention, in the optical transmission system according to the second aspect, the transmitter transmits all light as the power of each of the optical signals having different wavelengths transmitted from the plurality of light source units increases. It is characterized in that it is assigned to a wavelength close to the average value of the signal wavelengths.
According to a fourth aspect of the present invention, in the optical transmission system according to the first aspect, the receiver includes variable multilevel code conversion means for converting the received analog signal into a multilevel code having a desired multilevel number. It is characterized by.
According to a fifth aspect of the present invention, a time division multiplexing optical communication network is configured by connecting one transmitter according to the second or third aspect and a plurality of receivers according to the fourth aspect through an optical transmission line. In the range where the level difference of the received signal can be identified, the multi-level number is increased for a receiver having a small attenuation amount in the communication network with the transmitter among the plurality of receivers. The multi-value number is reduced for a receiver having a large attenuation in a communication network with the transmitter.
According to a sixth aspect of the present invention, in the optical transmission system according to the fifth aspect, among the plurality of receivers, a short communication time is allocated and set to a receiver having a large multi-level number of the set multi-level code. A long communication time is allocated to a receiver having a small multi-level number of the multi-level code.

  According to the present invention, since the power of one or more optical signals of a plurality of optical signals is transmitted differently from the power of other optical signals, the number of light source means in the WDM multilevel transmission system It is possible to generate a multi-level code with a multi-level number higher than that of the conventional one, and to increase the transmission capacity. Also, the reception characteristics can be improved by assigning the wavelength to the average wavelength side as the power of the transmission optical signal is larger. In addition, by enabling conversion to a multi-level code having a desired multi-level number on the receiving side, if this is applied to the PON system, the transmission capacity of the entire PON branch is increased, and a nearby user can be increased. On the other hand, high-speed transmission can be performed by increasing the multi-level number of the multi-level code, and transmission with low power consumption can be performed. Furthermore, for a receiver that cannot increase the number of multi-level codes, that is, with a large signal attenuation, it is possible to increase the minimum guaranteed bandwidth for that receiver by increasing the allocation time. Become.

<Example 1 (corresponding to claim 1)>
In the first embodiment, in the WDM multi-level transmission method, the transmitter transmits one or more optical signals of a plurality of optical signals having different wavelengths different from other optical signals.

  FIG. 1 is an explanatory diagram of the optical signal. In FIG. 1, an example using four light sources / modulators (light source means) will be described. Here, the wavelengths assigned to the optical signals generated from the four light sources and modulators are represented by λ1, λ2, λ3, and λ4, and the power is represented by the height. In the conventional method, as shown in FIG. 1A, since the optical signals of the output wavelengths λ1 to λ4 of the light sources and modulators have the same power, the multilevel number of multilevel codes that can be generated is 5 (“ 0 ”,“ 1 ”,“ 2 ”,“ 3 ”,“ 4 ”).

  On the other hand, in this embodiment, the power of the optical signal for each light source / modulator is changed. For example, as shown in FIG. 1B, the powers of the optical signals of the wavelengths λ1 to λ4 of the four light sources / modulators are “1”, “1/2”, “1/4”, “1/8”. Assuming that the number of multi-level codes is 16 by “0”, “1/8”,..., “15/8”. Note that the multi-value number increases even if the powers of the optical signals having wavelengths λ1 to λ4 are not all different values. For example, as shown in FIG. 1 (c), when the powers of the optical signals having wavelengths λ1 to λ4 are “1”, “1”, “1/2”, “1/4”, the multilevel code multilevel The number is 12.

  In this way, by making the power of at least one optical signal of a plurality of optical signals having different wavelengths different from those of other optical signals, the number of light sources / modulators is more than the conventional multi-value number. A multi-level code can be generated, and the transmission capacity can be increased.

<Example 2 (corresponding to claim 2)>
In the second embodiment, the transmitter includes a plurality of light sources / modulators (light source means) that generate a plurality of optical signals having different wavelengths, and one or more of the plurality of light sources / modulators generate light. Power control means for controlling the power of the signal or power attenuation means for attenuating the power of the generated optical signal is provided.

  FIG. 2 is an explanatory diagram of the transmitter 100. In FIG. 2A, the transmitter 100 includes a power control circuit 130 that performs power control of an optical signal for each of the LD driver circuits 111 of the three light sources / modulators 110. Reference numeral 112 denotes a laser diode. Here, the LD driver circuit 111 and the power control circuit 130 constitute power control means.

  Since an ordinary laser diode changes the power of an optical signal according to the amount of current input, the current may be controlled for each wavelength, that is, for each laser diode 112, so that the power of the optical signal of each wavelength is appropriate. Then, a part of the light output generated by the laser diode 112 is monitored, and the power control circuit 130 performs feedback control of the current amount so as to obtain a constant output (not shown).

  In FIG. 2B, an optical attenuator 113 for attenuating the power of the optical signal is added after the laser diode 112, and the attenuator 113 is controlled by the power control circuit 130. Here, the power control circuit 130 and the optical attenuator 113 constitute power attenuating means.

<Example 3 (corresponding to claim 3)>
In the third embodiment, in the transmitter, for each of the optical signals having different wavelengths transmitted from a plurality of light sources / modulators (light source means), the larger the power is, the average value of the wavelengths of all the optical signals in the optical signal. A wavelength close to is assigned.

  FIG. 3 is an explanatory diagram of the allocation of the optical signal. Here, the three light sources / modulators 110 of the second embodiment (FIG. 2) are used, and the power (intensity ratio) of the optical signal of each light source / modulator 110 is “1”, “1/2”, “1”. A case where / 4 "is set will be described. In general, in the WDM multiplex transmission system of Non-Patent Document 1, there is a shift in the time until the optical signal of each wavelength arrives at the receiver due to the influence of chromatic dispersion. For this reason, when the threshold determination is performed in the optical receiver, the deviation affects the preceding and following codes, so that the reception characteristics are deteriorated.

  Therefore, in this embodiment, an optimum combination of the power of the transmission optical signal and the wavelength arrangement that reduces the influence of intersymbol interference during reception is proposed. In the present embodiment, the wavelengths close to the arithmetic average value of the wavelengths of the three optical signals to be used are assigned in order from the power of the optical signal. Here, as shown in FIG. 3B, the largest optical signal with power “1” is assigned to the average wavelength, that is, an intermediate frequency, and the smallest optical signal with power “1/4” is low. An optical signal having a power of “½” is assigned to a high frequency. In FIG. 3A, wavelengths close to the average value of the wavelengths of the three optical signals to be used are assigned in order from the power having the smallest power.

  FIGS. 3A and 3B show eye patterns obtained by the optical transmission simulation when the present embodiment is not applied and when it is applied. However, the wavelength interval was 100 GHz, the wavelength dispersion was 20 ps / nm / km, the modulation frequency of each light source / modulator 110 was 2.5 GHz, and the transmission distance was 5 km.

  When using a transmitter in which the power of the optical signal of each wavelength is set as shown in FIGS. 3A and 3B, the eye opening is small in both the intensity direction and the time direction in FIG. Judgment is difficult. On the other hand, in the case of (b) in which the wavelength arrangement is controlled according to the present embodiment, the eye opening is greatly opened in both the intensity and the time direction as compared with the case of (a), and the threshold value determination is (a ).

<Example 4 (corresponding to claim 4)>
In the fourth embodiment, the receiver includes variable multi-level code conversion means for converting the received analog signal into a multi-level code having a desired multi-level number.

  FIG. 4 is a diagram showing the configuration of the optical receiver 230 of this embodiment. The optical receiver 230 includes a photoelectric conversion circuit 231, an A / D conversion circuit 232, a multi-value discrimination circuit 233, a multi-value number setting circuit 234, and a threshold value setting circuit 235. The A / D conversion circuit 232, the multilevel discrimination circuit 233, the multilevel number setting circuit 234, and the threshold setting circuit 235 constitute variable multilevel code conversion means.

  The input optical signal is converted into an analog electric signal by the photoelectric conversion circuit 231. When the obtained electrical signal is converted into a multi-value signal, the multi-value number is variable, so that the threshold value for making a discrete value also changes. Therefore, the threshold value setting circuit 235 sets the threshold value of the A / D conversion circuit 232 based on the set multi-value number information from the multi-value number setting circuit 234. The output of the A / D conversion circuit 232 normally has a fixed bit width, and various discrete signals can be output according to a set threshold value. Therefore, the output of the A / D conversion circuit 232 is discriminated by the multi-value discrimination circuit 233 based on the set multi-value number information from the multi-value number setting circuit 234, so that it is converted into an electric multi-value signal having a correct format.

<Example 5 (Claim 5)>
In the fifth embodiment, one transmitter and a plurality of receivers are connected by an optical fiber to form a time division multiplexing optical communication network, and a multi-level code is within a range in which a level difference of received signals can be identified. Controls the multivalued number of. And among a plurality of receivers, a multi-value number is increased for a receiver having a small amount of attenuation in the communication network with the transmitter, and the amount of attenuation in the communication network with the transmitter is For large receivers, the multi-level number is made small.

  In the PON system and the bus type system in which one optical fiber is shared by a plurality of users, the optical fiber length to each user is different, and thus the attenuation amount of the optical signal is different. In addition, the reception power of the optical signal for each user differs depending on the difference in the number of connectors used to attenuate the optical signal. When the power of the optical signal is attenuated, the level difference between the multi-level codes is reduced, and the signal is buried in noise, making it difficult to identify.

  Therefore, in the conventional system, it is necessary to output an optical signal having a power that can identify the level difference of the multilevel signal when the user with the largest attenuation receives.

  On the other hand, in this embodiment, the reception level difference of a user with a small amount of power attenuation such as a short distance is larger than that of a user with a large amount of attenuation. We propose increasing the number of values and increasing the bandwidth.

  FIG. 5 shows an example using a light source / modulator that generates three optical signals having wavelengths λ2, λ3, and λ4. Assuming that the powers of optical signals of wavelengths λ2, λ3, and λ4 at transmission are the same “a”, “a”, and “a” (dBm), a multi-level code that combines optical signals of wavelengths λ2 and λ3 at the time of transmission The power difference between P1 and the multilevel code P2 obtained by combining the optical signals of wavelengths λ2, λ3, and λ4 is “a”. However, as the attenuation amount r increases, the power difference decreases. The power difference between the multi-level codes P1 ′ and P2 ′ is “ar” (FIG. 5A).

  Similarly, in the case of multilevel codes P3 and P4 in which the powers of optical signals of wavelengths λ2, λ3, and λ4 at transmission are “a”, “a”, and “a-3” (dBm), the power difference is Although “a-3”, the power difference between the multilevel codes P3 ′ and P4 ′ on the receiving side is “a-3-r” (FIG. 5B). Further, in the case of multilevel codes P5 and P6 in which the powers of optical signals of wavelengths λ2, λ3, and λ4 at the time of transmission are “a”, “a-3”, and “a-6” (dBm), the power difference Is "a-6", but the power difference between the multilevel codes P5 'and P6' on the receiving side is "a-6-r" (FIG. 5 (c)).

  In order to receive without causing a reception error in practice, a power difference of a certain level or more is required between the received multilevel codes. Let this power difference be b. Since the multi-level code can be identified in the range where the power difference at the time of reception exceeds b, assuming that the attenuation in the transmission path is r, when the power difference at the time of transmission is “a” (dBm), “ Reception is possible within a range of r <a−b ″. When the power difference during transmission is “a-3” (dBm), “a <6” (dBm) has “r <a−b−3” and “r <a−b−6”, respectively. Can be received in the range of.

  In the PON or bus type system using time division multiplexing, since the communication time is divided for each user, by increasing the multi-level number of the multi-level code for the user having a small attenuation r as described above, The bandwidth can be increased.

<Example 6 (corresponding to claim 6)>
In the sixth embodiment, among a plurality of receivers, a short communication time is allocated to a receiver having a large multi-level number of a set multi-level code, and a long communication is performed to a receiver having a small multi-level number of a set multi-level code. Allocate time.

  The method shown in the fifth embodiment has a characteristic similar to ADSL, in which a user with a small amount of attenuation has a high transmission rate and a user with a large amount of attenuation has a low transmission rate. However, ADSL is an SS system in which each user's communication is independent, whereas a PON or bus type shares a fiber in a multiplex system.

  Therefore, in this embodiment, in the PON or bus type system using time division multiplexing, the communication time allocated to each user is controlled, and the transmission speed is high for users with a small amount of attenuation and a large number of multi-level codes. Therefore, a short communication time T3 is assigned, and a user with a large attenuation and a small multi-value number of the multi-value code is assigned a long communication time T1 because the transmission speed is slow (FIG. 6). In FIG. 6, P11 to P16 are multilevel codes obtained by multiplexing optical signals of three wavelengths. Thereby, it is possible to increase the transmission speed of a distant user while increasing the overall transmission capacity.

6 is an explanatory diagram of an optical signal according to the first embodiment. 6 is a block diagram illustrating a configuration of a transmitter according to Embodiment 2. FIG. It is explanatory drawing of the wavelength allocation of the optical signal of Example 3. FIG. FIG. 10 is a block diagram illustrating a configuration of an optical receiver according to a fourth embodiment. It is explanatory drawing of the multi-value code attenuation | damping of Example 5. FIG. It is explanatory drawing of the communication time allocation of the multi-level code of Example 6. It is a block diagram of the conventional WDM transmission system. It is a block diagram of the conventional WDM multiplex transmission system. It is a block diagram of the downlink transmission system of one-to-many communication. It is a comparison characteristic figure of the transmission information amount of a WDM transmission system and a WDM multiplex transmission system.

Explanation of symbols

100: transmitter 110: light source / modulator 111: LD driver circuit 112: laser diode 113 113 optical attenuator 120: wavelength multiplexer 200: receiver 210: wavelength demultiplexer 220: light Receiver: 230: Optical receiver, 231: Photoelectric conversion circuit, 232: A / D conversion circuit, 233: Multi-value discrimination circuit, 234: Multi-value number setting circuit, 235: Threshold setting circuit 300: Optical fiber 400: Light Distributor

Claims (6)

A transmitter that generates a plurality of optical signals having different wavelengths, and a receiver that combines the plurality of optical signals transmitted from the transmitter via an optical transmission path to form one multi-level code. In the optical transmission system,
The transmitter transmits the power of one or more optical signals out of the plurality of optical signals different from the power of other optical signals. The optical transmission system according to claim 1,
The transmitter comprises a plurality of light source means for generating a plurality of optical signals having different wavelengths, and a power control means for controlling the power of the optical signal generated by one or more of the plurality of light source means, or An optical transmission system comprising power attenuating means for attenuating the power of a generated optical signal. The optical transmission system according to claim 2,
The transmitter, for each of the optical signals having different wavelengths transmitted from the plurality of light source means, assigns a wavelength closer to the average value of the wavelengths of all the optical signals as the power is larger . The optical transmission system according to claim 1,
An optical transmission system characterized in that the receiver comprises variable multilevel code conversion means for converting a received analog signal into a multilevel code having a desired multilevel number. A transmitter according to claim 2 or 3 and a plurality of receivers according to claim 4 are connected by an optical transmission line to form a time division multiplexing optical communication network, and a difference in received signal level is identified. To the extent possible
Among the plurality of receivers, for a receiver having a small amount of attenuation in the communication network with the transmitter, the multilevel number is increased, and the amount of attenuation in the communication network with the transmitter is increased. An optical transmission system characterized in that the multi-value number is reduced for a receiver having a large value. In the optical transmission system according to claim 5,
Among the plurality of receivers, a short communication time is allocated to a receiver having a large multi-level number of a set multi-level code, and a long communication time is allocated to a receiver having a small multi-level number of a set multi-level code. An optical transmission system characterized by

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