JP4810514B2 - Optical transmission system and optical transmission method - Google Patents

Description

  The present invention relates to optical multiple access transmission. In particular, it relates to an increase in capacity and simplification of the receiver configuration.

  Conventionally, time-division multiplexing (TDM) and wavelength-division multiplexing (WDM: Wavelength Division Multiplexing) have been widely used for multiplexing for expanding the communication capacity of an optical transmission system. .

  In the case of normal WDM, a light source having a plurality of wavelengths different from each other is prepared on the transmission side, a plurality of information signals are converted by optical signals of each wavelength, and they are combined by an AWG (Arrayed Waveguide) or the like to generate an optical signal. Propagate the transmission path.

  On the receiving side, following the reverse procedure, the optical signal is separated into a plurality of optical wavelengths by the AWG and then optically received, and each information signal is demodulated, thereby realizing multiplex transmission with extremely wide bandwidth.

  On the other hand, in the TDM system, a plurality of signals are replaced with higher-speed signals on the transmission side, rearranged and multiplexed on the time axis, propagated through the optical fiber, and a plurality of time-multiplexed signals are transmitted on the reception side. By separating the signal into low-speed signals, it is possible to realize multiplex transmission that is relatively simple and economical.

  FIG. 9 is a diagram for explaining a functional configuration of a conventional general WDM transmission system. The signal from the information signal generator 6 is encoded by the encoder 5 and enters N transmitters 4 (Tx1 to TxN). An optical fiber in which N transmitters 4 (Tx1 to TxN) are converted into optical signals having different wavelengths λ1 to λN, and each wavelength is poly-polymerized by an optical multiplexer / demultiplexer 3 (AWG or the like) through an optical waveguide. Incident on the transmission line 1.

  On the receiver side, according to the reverse procedure, the optical multiplexer / demultiplexer 2 separates each wavelength component, and the optical signals of the respective wavelengths λ1 to λN enter the receiver 7 (Rx1 to RxN) through the waveguide.

  The optical multiplexers / demultiplexers 2 and 3 do not necessarily have a wavelength combining / separating action like AWG, and may simply function as a power splitter. However, in that case, it is necessary to insert a wavelength filter that transmits only λj (any of j = 1 to N) before the receiver 7 (Rx1 to RxN), and a branching loss occurs.

  The output of the receiver 7 (Rx1 to RxN) is decoded by the decoder 8, and an information signal is extracted.

Robert D. Feldman, Senior Member, IEEE, E.I. E. Harstead, S .; Jiang, Thomas H. et al. Wood, Senior Member, IEEE, Fellow. OSA. and Martin Zirngibl, “An Evaluation of Architecture inducing Wavelength Division Multiplexing for Broad-Band Fiber Access,” JOURNAL OFAV LOVEL VOL. 16, NO. 9, SEPTEMBER 1998

  In the WDM system as described above, an optical wavelength combining / separating device, a plurality of light sources or photodetectors are indispensable in both the transmitting and receiving units in proportion to the capacity expansion, and there is a problem in economy due to the application of expensive optical components. is there.

In particular, P-MP (Point (Passive Optical Network))
In the case of a to multipoint) type network, the user side device is required to be inexpensive, so that it is difficult to apply.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide an optical transmission system and method capable of easily expanding the capacity at low cost.

  In the present invention, since the signal transmission speed can be suppressed to a relatively low speed by applying the WDM system to the transmission side, the capacity scalability can be easily increased. The optical signal is based on the chromatic dispersion characteristics of the optical fiber, and the optical signals of the respective wavelength components are separated from each other in the time domain with propagation.

  On the receiving side, an optical signal can be received and demodulated as a simple TDM by installing a receiver at a position where these optical signals are just converted from a parallel signal in WDM to a serial signal in TDM.

  As a result, a receiver configuration that does not require a plurality of wavelength multiplexing or separation wavelength devices and does not require a simple configuration is possible, and a user device of a P-MP network such as a PON can be easily configured.

  Of course, as usual, it is also possible to receive and demodulate after wavelength separation as a wavelength multiplexed signal, and the flexibility to mix WDM and TDM receivers in one P-MP is also realized. can do.

  The proposal of such a multiplex transmission system has not been seen in the past.

  Furthermore, the optical signal is based on the chromatic dispersion characteristics of the optical fiber, and the optical signals of each wavelength component are separated into different wavelengths on the transmission side by introducing on the transmission side the reverse characteristics of the characteristics that separate each other in the time domain. By receiving the same modulated signal synchronously on the receiving side, the received signal power can be increased, and as a result, the optical transmission can be extended and widened by improving the sensitivity of the receiver.

  In addition, since a plurality of wavelengths are used in the above-described long extension and wide area, reduction in wavelength use efficiency becomes a problem, but when a combination of wavelengths is used as information, when received as a single optical reception power, As a result, the received power is improved, and wavelength information is separated by a wavelength filter or the like and each wavelength information is identified, so that it is possible to increase the amount of information and improve the wavelength utilization efficiency.

  That is, the present invention includes means for synchronizing N information signals and aligning transmission timing as parallel transmission, and means for converting the parallel signals into optical signals of different wavelength carriers and optically multiplexing them. As a result of propagation through this optical fiber transmission line, a transmitter, an optical fiber transmission line having a finite dispersion value or a dispersion-controllable optical fiber transmission line in the wavelength range for propagation of the output optical signal of this transmitter, and Means for receiving an optical signal at a position where each wavelength component is separated in the time domain by the dispersion effect and the parallel signal sequence is converted into a serial signal sequence, and the optical signal received by the receiving unit is decoded into an information signal And an optical transmission system.

  Alternatively, the present invention controls the transmission timing on the transmission side so that M (M <N) information signals can be received synchronously on the reception side, and for (N−M) information signals. Is a transmitter having means for synchronizing transmission signals and aligning transmission timings as parallel transmission, means for converting the parallel signals into optical signals of different wavelength carriers, and optical multiplexing, and an output of the transmitter An optical fiber transmission line having a finite dispersion value in a use wavelength region in which an optical signal is propagated or an optical fiber transmission line capable of controlling dispersion, and the M transmission signals as a result of propagation through the optical fiber transmission line are time domain And (NM) transmission signals at the position where each wavelength component is separated in the time domain by the dispersion effect and the parallel signal string is converted into a serial signal string. It means for receiving a signal, an optical transmission system comprising a receiver and means for decoding an optical signal means for this reception is received into the information signal.

  In this case, for example, the M transmission signals have information as a combination of wavelengths and are signal-converted on the transmission side so that the power sum satisfies a power level that allows signal discrimination as optical power. On the receiving side, some receivers identify information as a sum of power, and after wavelength-separating the signal, the optical signal is received and information is decoded by converting it into the time domain.

  In addition, instead of the optical multiplexing means for converting an information signal into an optical signal, multiplexing and entering the optical fiber transmission line, the light sources are arranged two-dimensionally, and the light source elements emit light at different wavelengths. And means for coupling the emitted light to the optical fiber.

  The present invention can also be viewed from the viewpoint of an optical transmission method. That is, in the present invention, a transmitter synchronizes N information signals, aligns transmission timings as parallel transmission, converts the parallel signals into optical signals of different wavelength carriers, and optically multiplexes and transmits the signals. An optical fiber transmission line having a finite dispersion value in the use wavelength region or an optical fiber transmission line capable of controlling dispersion propagates an output optical signal of the transmitter, and a receiver propagates through the optical fiber transmission line. As a result, an optical transmission method that receives an optical signal at a position where each wavelength component is separated in the time domain by a dispersion effect and a parallel signal sequence is converted into a serial signal sequence, and decodes the received optical signal into an information signal. is there.

  Alternatively, the present invention controls the transmission timing on the transmission side so that the transmitter can receive M (M <N) information signals synchronously on the reception side, and (N−M) pieces of information. Synchronize transmission signals for signals, align transmission timings for parallel transmission, convert these parallel signals to optical signals of different wavelength carriers, transmit them by optical multiplexing, and have a finite dispersion value in the wavelength range of use An optical fiber transmission line having an optical fiber transmission line or a dispersion-controllable optical fiber transmission line propagates an output optical signal of the transmitter, and a receiver propagates the optical fiber transmission line to obtain M transmission signals. In addition to synchronous reception in the time domain, for the (N−M) transmission signals, each wavelength component is separated in the time domain due to the dispersion effect, and the optical signal is transmitted at a position where the parallel signal string is converted into a serial signal string. It receives an optical transmission method for decoding an optical signal the received and the information signal.

  In this case, for example, the M transmission signals have information as a combination of wavelengths and are signal-converted on the transmission side so that the power sum satisfies a power level that allows signal discrimination as optical power. On the receiving side, some receivers identify information as a sum of power, and after wavelength-separating the signal, the optical signal is received and information is decoded by converting it into the time domain.

  In addition, instead of the process in which the transmitter converts the information signal into an optical signal, optically multiplexes and enters the optical fiber transmission line, each light source element of the light source in which the light source elements are arranged in two dimensions is caused to emit light at different wavelengths. The emitted light can be coupled to an optical fiber.

  According to the present invention, the capacity can be increased easily at low cost. In particular, a user device of a P-MP network such as PON can be easily configured.

  Embodiments of the present invention will be described with reference to FIGS. FIG. 1 is a diagram for explaining a functional configuration of an embodiment of the present invention. When the signal from the information signal generator 6 is encoded by the encoder 5, the bit rate is increased by N times the multiplex number (detailed in FIG. 2). The output of the encoder 5 is converted from an electrical signal to an optical signal by the transmitter 4 (Tx1 to TxN). At that time, the optical signals in the transmitters 4 (Tx1 to TxN) are synchronized by the control circuit 9 that performs timing adjustment, for example, each wavelength is poly-polymerized by the optical multiplexer / demultiplexer 3 after being aligned at the same timing. The wave is incident on the optical fiber transmission line 1. The light emitted from the optical fiber transmission line 1 enters the receiver 7 (Rx), is converted from an optical signal to an electric signal, is decoded by the decoder 8, and the information signal is demodulated.

  FIG. 2 is a diagram for explaining optical signal patterns in the transmitter Tx and the receiver Rx. FIG. 2A is a diagram illustrating transmission frames and optical pulses in N transmitters Tx1 to TxN. (B) is an example of the optical pulse of the receiver Rx. In FIG. 2A, the period of the encoded signal is Tf, and is subdivided into slots (time Ts) by the number N of multiplexing.

  In the operation of the encoder 5 shown in FIG. 1, the information signal depends on the signal modulation method having the period Tf and the pulse width Ts. Here, as an example, a case where it is converted into an NRZ code is shown. The output signals S1 to SN of the transmitters Tx1 to TxN in FIG. 2A are adjusted by the operation of the control circuit 9 so that, for example, the signals of each λ1 are all synchronized in the S1 slot.

  The combined output of all of the transmitters Tx1 to TxN is incident on the optical fiber transmission line 1, but using a plurality of light sources can lower the load on the light source (extend the life) compared to a single light source. It becomes possible.

  Due to the chromatic dispersion effect of the optical fiber transmission line 1, the wavelength multiplexed signals are time-separated and an optical signal delay time between adjacent wavelengths is created approximately equal to Ts (detailed in FIGS. 3 and 4). Since the λ1 to λN components are dispersed in each slot as shown in FIG. 2B, the reception signal of the device Rx can be received as a continuous TDM signal having a period Ts.

  That is, a parallel optical pulse train is converted into a serial train as the optical fiber propagates. Decoding after receiving Rx is possible by performing processing in the Tf cycle.

  FIG. 3 is a schematic diagram of the spectrum of output light from each transmitter Tx. The N transmitters Tx1 to TxN have a center wavelength of Δλ arranged at equal intervals, and have a finite spectral width δλ due to primary modulation and encoding. Here, Δλ is set sufficiently larger than δλ.

  FIG. 4 shows a propagation time difference caused by the propagation of the signals of λ1 to λN through the optical fiber transmission line 1.

  The dotted line in the figure is a part of the delay curve of the optical fiber transmission line 1, and the chromatic dispersion value can be regarded as substantially constant in a relatively narrow region of normal dispersion (about several nm), and therefore the delay amount is the wavelength difference Δλ. Can be approximated proportionally. That is, Δτ∝Δλ.

  Here, an optical fiber which is a transmission line is used to obtain the effect of dispersion, but any optical fiber can be used as long as it provides dispersion, and the present invention is not necessarily limited to an optical fiber.

  From the above, parallel / serial conversion becomes possible by making Δτ equal to the slot width Ts. Conversely, if the chromatic dispersion value D and the length L of the optical fiber are fixed, the delay amount Δτ is Δτ / Δλ. = DL is given, and the condition can be satisfied by selecting the light source so as to have an appropriate Δλ.

  Further, by setting δλ to about 10% or less of Δλ, it is possible to easily suppress interference between adjacent channels. A practical numerical example is shown below.

  When the optical fiber length of the subscriber system is 10 km, the chromatic dispersion value is 10 ps / km / nm, the number of wavelengths is 10 waves (1.5 μm band), the transmitter (Tx) side speed is 1 Gbps, and the receiver (Rx) side speed is 10 Gbps. In this case, Δτ (= Ts) is 0.1 ns, and Δλ is about 1 nm. If the primary modulation rate is 1 Gbps, δλ is about 0.01 nm, and δλ / Δλ << 1 is sufficiently established.

  FIG. 5 shows a configuration example in the case where a device having a two-dimensional array shape (referred to as VICSEL) such as a VCSEL is used as an optical transmitter in the same configuration as FIG. The signal from the information signal generator 6 is encoded by the encoder 5 and then enters the VICSEL driver circuit 14, and each output thereof is sent to the VICSEL 15. Here, each light source element in the VICSEL 15 has an array of light source elements that emit light at a preset wavelength interval, similarly to the conventional example of FIG. 9 and the embodiment of FIG. 1, and an optical signal from each light source element is It is coupled to the optical fiber transmission line 1 through the optical coupling unit 16.

  The optical coupling unit 16 can also be realized by, for example, a method of condensing and coupling with an objective lens, or a method of directly contacting the VISSEL section with the optical fiber section when the size of the VICSEL 15 is equal to the core diameter. . For the optical signal propagated through the optical fiber transmission line 1, the information signal can be demodulated by the same processing as in FIG.

  FIG. 6 shows an example of a PON often used in a subscriber system as an example of an application form of the present invention. On the transmission side, a transmitter side function (described in FIG. 1) by WDM and a conventional TDM transmitter side function 19 are arranged in parallel. The output from the transmitter using the WDM described in FIG. 1 propagates through the optical fiber transmission line 1 via the optical multiplexer / demultiplexer 3 and the optical power multiplexer 17, is branched by the power splitter 18, and enters the receiver 7. To do.

  Here, as can be seen from FIG. 2B, when the bit string having the period Ts is received by the TDM receiver side function 20, no wavelength information of the optical signal is used, and therefore the receiver 7 side of the present invention is used. It is not always necessary to make a pair with the transmitter 4 configured by WDM, and it is easy to establish communication even when paired with a simple high-speed TDM transmitter (having the same bit rate). I understand. That is, if the TDM transmitter-side function 19 in FIG. 6 is a device that transmits at a bit rate of 1 / Ts, the receiver 7 can be used in principle for the TDM transmitter-side function 19 in principle. Become. In this case, the TDM receiver side function 20 of the prior art can be omitted. The present invention is also characterized by such versatility.

  FIG. 7 shows an example of realizing optical reception power increase using WDM as an example of an application form of the present invention. The transmitter 4 is a transmitter Tx1 to TxN that realizes the wavelength / time conversion type transmission method shown in the previous section, and the receiver 7 is a receiver Rx1 to RxN corresponding to each transmitter Tx1 to TxN.

  Here, the transmitter 4 and the receiver 7 perform operations different from the above method. The transmitter 4 outputs so that the reception timing of each wavelength is the same in the receiver 7. That is, the delay time for each wavelength between the transmitter 4 and the receiver 7 is measured, and a signal is transmitted to the receiver 7 side using the inverse characteristic (delaying the transmission time). At that time, the transmission signals 109 to 111 are controlled to be the same at each wavelength. On the receiver 7 side, the reception signals 112 to 114 synchronized with each wavelength can be received, so that the reception power can be improved, and as a result, the reception sensitivity can be improved. As an effect, it is possible to extend the length of the optical transmission path and widen the optical network.

  FIG. 8 shows an example of realizing an increase in optical reception power using WDM as an example of an application form of the present invention. FIG. 7 shows an example of a method for improving the wavelength utilization efficiency. The reception power improvement using the copy by the wavelength of the transmission signals 109 to 111 shown in FIG. 7 has a problem in terms of wavelength utilization efficiency because wavelength information is not used. A method for further improving the proposed method of FIG. 7 is proposed. In this system, there are a receiver 7 that collectively receives only optical power ignoring wavelength characteristics, and a receiver 122 that includes a conversion circuit 13 that identifies the wavelength and converts it to a time axis.

  For example, in FIG. 8, 0 and 1 signals are selected for the transmission signals 115 to 120 for the receiver 7 that ignores the wavelength and identifies only the optical power as the reception signal. In other words, as an example, consider a case where the selection circuit 12 selects four information signal wavelengths (115, 116, 117, 118, 119, 120) when the level is 1 and selects 2 when the level is 0 (1). There is no problem if 5 levels are selected and 1 level is selected as 0 level). Here, selection defines the state of light emission at that wavelength. In the case of the receiver 7 ignoring the wavelength characteristics and receiving light as power, it is received as power level 4 as 1 level and power level 2 as 0 level, and 1 as 1 level and 0 as 0 level. The reception power can be improved by 3 dB compared with the case of reception.

On the other hand, the receiver 122 having the conversion circuit 13 that recognizes the wavelength characteristics and converts the wavelength into the time axis recognizes the combination of wavelengths transmitted by the transmitter 4 as the received signals 123 and 124. That is, in the case of this example, the combinations of signal wavelengths (115, 116, 117, 118, 119, 120) are combinations of selecting four out of six, so there are 15 combinations. This is the same for both the 1st level and the 0th level. (In addition, in the case of the example shown in parentheses above as another example, there are 6 combinations because 5 combinations are selected from 6). In the case of 15 combinations, the wavelength The use efficiency of log ₂ 15 can be improved as compared with the case where characteristics are not taken into consideration.

Note that the transmitter TX _N in FIG. 8 uses a method using a combination of wavelength data for improving sensitivity and wavelength utilization efficiency mainly shown in FIG.
(1) A method of converting and receiving from the wavelength axis to the time axis using the chromatic dispersion shown in FIG. 1 (2) It is generally shown that the conventional TDM communication method shown in FIG. 6 can coexist.

Therefore, the transmitter TX _N may include the information signal generator 6, the encoder 5, the transmitter 4, the optical multiplexer / demultiplexer 3, and the control circuit 9 shown in FIG. 1, or the TDM transmission shown in FIG. The device side function 19 may be used. Further, the transmitter TX _N may include the selection circuit 12, the transmitter 4, and the optical multiplexer / demultiplexers 3 and 10 illustrated in FIG. In this case, the receiver R _N without a reference numeral in FIG. 8 left, a receiver opposed to the transmitter TX _N.

  By applying the present invention, the capacity can be increased relatively easily by combining the conventional TDM and WDM. In particular, when the cost reduction of the user side device such as PON is an important requirement, it is extremely effective that a WDM signal can be received without waste with a simple TDM receiver, and the conventional TDM system is not limited to WDM. However, it can be applied as a receiver as it is.

  Furthermore, the reception power can be increased by performing synchronous batch reception in the time domain using WDM. Accordingly, it is possible to extend the length of the optical transmission path and widen the area, and it is possible to improve the amount of signal information depending on the wavelength by considering the axis of wavelength combination and optical power.

The figure for demonstrating the function structure of this invention Example. The figure for demonstrating the pattern of the optical signal in the transmitter Tx and the receiver Rx. The schematic diagram of the spectrum of the output light from each transmitter Tx. The figure which shows the propagation time difference which arises when the signal of each (lambda) 1- (lambda) N propagates an optical fiber transmission line. The figure which shows the structural example at the time of using the device (it is described as VICSEL) of a two-dimensional array shape like VCSEL as an optical transmitter in the structure similar to FIG. The figure which shows the example in the case of PON often used in a subscriber system as an example of the application form of this invention. The figure which shows the example which implement | achieves the optical reception power increase using WDM as an example of the application form of this invention. The figure which shows the example which implement | achieves the optical reception power increase using WDM as an example of the application form of this invention. The figure for demonstrating the function structure of the conventional general WDM transmission system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber transmission line 2, 3, 10, 11 Optical multiplexer / demultiplexer 4 Transmitter (Tx1-TxN)
5 Encoder 6 Information signal generator 7, 122 Receiver (Rx1 to RxN)
8 Decoder 9 Control circuit 12 Selection circuit 13 Conversion circuit 14 Driver circuit 15 VICSEL
16 Optical coupling unit 17 Optical power multiplexer 18 Power splitter 19 TDM transmitter side function 20 TDM receiver side function 101, 103 to 105, 109 to 111, 115 to 120 Transmission signal 102, 106 to 108, 112 to 114, 121, 123, 124 Received signal

Claims (6)

The transmission side controls transmission timing so that M (M <N) information signals can be received synchronously on the reception side, and transmission signal synchronization is established for (NM) information signals. Means for aligning transmission timing as parallel transmission;
A transmitter comprising means for converting the parallel signals into optical signals of different wavelength carriers and optical multiplexing; and
An optical fiber transmission line having a finite dispersion value in the use wavelength region for propagating the output optical signal of the transmitter, or an optical fiber transmission line capable of controlling dispersion;
As a result of propagating through this optical fiber transmission line, M transmission signals are synchronously received in the time domain, and for (NM) transmission signals, each wavelength component is separated in the time domain due to the dispersion effect, Means for receiving an optical signal at a position where the parallel signal string is converted into a serial signal string;
An optical transmission system comprising: a receiver provided with means for decoding the received optical signal into an information signal. The M transmission signals are transmitted by a combined signal wavelength , and signal conversion is performed on the transmission side so that the combined power sum satisfies a power level that allows signal discrimination as optical power. the optical transmission system according to claim 1, wherein the receiver performs information identification as a power sum, information receiving and decoding optical signal by converting the signal to the time domain after the wavelength separation of the. Means for converting an information signal into an optical signal, optically multiplexing the optical signal, and entering the optical fiber transmission line instead of the optical multiplexing means, wherein the light sources are two-dimensionally arranged, and each light source element emits light at a different wavelength;
The optical transmission system according to claim 1, further comprising means for coupling the emitted light to an optical fiber. The transmitter controls transmission timing on the transmission side so that M (M <N) information signals can be received synchronously on the reception side, and transmits (NM) information signals. Take signal synchronization, align transmission timing as parallel transmission, convert these parallel signals to optical signals of different wavelength carriers, optically combine and transmit,
An optical fiber transmission line having a finite dispersion value in the use wavelength region or an optical fiber transmission line capable of controlling the dispersion propagates the output optical signal of the transmitter,
As a result of the propagation of the optical fiber transmission line by the receiver, the M transmission signals are synchronously received in the time domain, and the wavelength components of the (NM) transmission signals are changed in the time domain due to the dispersion effect. An optical transmission method in which an optical signal is received at a position where the parallel signal sequence is converted into a serial signal sequence, and the received optical signal is decoded into an information signal. The M transmission signals are transmitted by a combined signal wavelength , and signal conversion is performed on the transmission side so that the power sum of the combination satisfies a signal level that can be identified as optical power. The optical transmission method according to claim 4 , wherein the receiver of the unit performs information identification as a power sum, receives the optical signal by performing wavelength separation after the signal is separated, and decodes the information. Instead of the step of the transmitter converting the information signal into an optical signal, optically combining it, and entering the optical fiber transmission line, each light source element of the light source in which the light source elements are arranged in two dimensions is caused to emit light at different wavelengths, and The optical transmission method according to claim 4 or 5 , wherein the emitted light is coupled to an optical fiber.

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