Optical transmission equipment and optical transmission system

Abstract

Reduce signal bandwidth tilt. [Solution] The optical receiver 100 receives and processes the received optical signal received via the optical transmission path. The optical receiver 100 includes a DSP 105 that outputs information regarding the deviation of the amplitude characteristics of the received signal (intraband tilt), a TOF 101 that filters the received signal, and a control unit 106 that controls the settings of the TOF 101 based on the intraband tilt. For example, the control unit 106 reduces the intraband tilt by variably controlling the center frequency of the TOF.

Description

Technical Field

[0001] The present invention relates to an optical transmission device and an optical transmission system.

Background Art

[0002] In recent years, optical amplifiers have been used in optical transmission systems. The optical amplifier is mounted, for example, inside a ROADM (Reconfigurable Optical Add / Drop Multiplexer) in a transmission line or inside an optical transceiver to amplify the transmission / reception optical power. By using an optical amplifier, the conventionally used electrical repeaters can be reduced, and cost reduction and power consumption reduction of the system can be achieved.

[0003] As one method for realizing a large capacity of an optical communication system, increasing the capacity per wavelength of an optical signal can be mentioned. The transmission capacity per wavelength is related to the baud rate of the optical signal. By increasing the baud rate, the transmission capacity can be increased without changing the reception sensitivity. On the other hand, problems such as a stricter requirement for the band characteristics of the transmission / reception device and an increase in the power consumption of the optical transmission device occur.

[0004] As a conventional technique, there is one that adjusts the optical channel spectrum by varying the frequency of an optical filter based on the signal error rate (BER: Bit Error Rate) to reduce the influence of non-target filtering. There is also one that corrects the characteristic variation of a balanced receiver by controlling the amplitude and delay of an optical signal. There is also one that reduces distortion with a compensation signal obtained by extracting a pilot tone included in a received signal by filtering operation. There is also one that reduces the influence of frequency-dependent imbalance between IQ sub-channels of a channel of a receiver and recovers data based on calibration data stored in advance. There is also one that compensates for the loss difference of optical signals for each wavelength generated in a wavelength division multiplexed signal with a loss compensator (see, for example, Patent Documents 1 to 5 below).

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Special Publication No. 2022-519282 [Patent Document 2] Japanese Patent Publication No. 2008-219765 [Patent Document 3] U.S. Patent Application Publication No. 2014 / 0286642 [Patent Document 4] U.S. Patent Application Publication No. 2012 / 0057863 [Patent Document 5] Japanese Patent Application Publication No. 11-275020 [Overview of the project] [Problems that the invention aims to solve]

[0006] As the baud rate of an optical signal increases, the bandwidth of the optical signal expands. Optical amplifiers generally have different amplification factors depending on the wavelength of light. When an optical signal with a wide bandwidth passes through an optical amplifier, the amplification factor changes within the signal bandwidth. When the baud rate increases, the deviation of the amplitude characteristic (amplification factor) within the signal bandwidth (called in-band tilt) increases, causing distortion in the signal spectrum and leading to deterioration of the characteristics of the received signal. In order to achieve high baud rates, it becomes necessary to reduce the in-band tilt and suppress the increase in distortion of the signal spectrum.

[0007] In one aspect, the present invention aims to reduce in-band tilt. [Means for solving the problem]

[0008] According to one aspect of the present invention, the optical transmission device is an optical transmission device that receives and processes a received signal of light received via an optical transmission path, and is required to include an output unit that outputs information regarding the deviation of the amplitude characteristics of the received signal, a TOF (Tunable Optical Filter) that filters the received signal, and a control unit that controls the setting of the TOF based on the information regarding the deviation of the amplitude characteristics. [Effects of the Invention]

[0009] According to one aspect of the present invention, the effect of reducing in-band tilt is achieved. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a diagram showing an optical transmission device according to Embodiment 1. [Figure 2] Figure 2 is a diagram showing an example of the amplification characteristics of an optical amplifier. [Figure 3] Figure 3 is an explanatory diagram of the deviation in amplitude characteristics for different baud rates. [Figure 4] Figure 4 shows an example of the internal functions of the DSP according to Embodiment 1. [Figure 5] Figure 5 is an explanatory diagram illustrating in-band tilt control by changing the center frequency of the TOF (Time-of-Flight). [Figure 6] Figure 6 is a diagram showing an example of the frequency characteristics of the compensation coefficients of an adaptive equalization circuit. [Figure 7] Figure 7 is an explanatory diagram illustrating an example of calculating in-band tilt from tap numbers. [Figure 8] Figure 8 is a flowchart showing an example of control of an optical transmission device according to Embodiment 1. [Figure 9] Figure 9 is a diagram showing an example of settings for whether or not control is implemented for each signal type. [Figure 10] Figure 10 shows an optical transmission system according to Embodiment 2. [Modes for carrying out the invention]

[0011] Embodiments of the disclosed optical transmission apparatus and optical transmission system will be described in detail below with reference to the drawings. The optical transmission apparatus of the embodiment is applied, for example, to an optical transceiver and an optical receiver. The optical receiver receives an optical digital coherent optical signal and reduces the in-band tilt by changing the setting of the optical filter based on the deviation of the amplitude level within the signal band (in-band tilt).

[0012] In the optical communication transmission of the embodiment, for example, an optical signal of quadrature amplitude modulation (QAM) is transmitted.

[0013] (Embodiment 1) FIG. 1 is a diagram showing an optical transmission device according to Embodiment 1. The optical transmission device shown in FIG. 1 is an optical receiver 100. The optical receiver 100 receives an optical signal transmitted by an optical transmitter via an optical transmission line. The optical receiver 100 includes a tunable optical filter (TOF) 101 as an optical filter, an optical amplifier 102, an integrated coherent receiver (ICR) 103, a laser diode (LD) 104, a digital signal processor (DSP) 105, and a control unit 106.

[0014] The TOF 101 filters the optical signal received via the optical transmission line and outputs the filtered optical signal to the optical amplifier 102. The TOF 101 is, for example, a band-pass filter and has a band-pass characteristic that allows only a predetermined signal band to pass through. The center frequency f0 of the band-pass characteristic of the TOF 101 is variable. For example, the TOF 101 changes the center frequency f0 by varying the control voltage (voltage value) Vf output by the control unit 106.

[0015] The AMP 102, which is an optical amplifier, optically amplifies the optical signal after passing through the TOF 101. For the AMP 102, for example, an erbium-doped fiber amplifier (EDFA), a fiber Raman amplifier (FRA), a semiconductor optical amplifier (SOA), etc. are used.

[0016] ICR103 performs reception processing such as optical detection of the received coherent light, optical-to-electrical signal conversion, optical attenuation, and polarization separation based on the laser light (local emission) of LD104. DSP105 performs processing such as analog-to-digital conversion and signal compensation of the received signal and outputs the received data. DSP105 also outputs information about the received signal (information about the deviation of the amplitude characteristics after optical amplification by AMP102) to the control unit 106. DSP105 has the function of an output unit that outputs information about the deviation of the amplitude characteristics.

[0017] The control unit 106 determines the in-band tilt of the received signal based on the output of the DSP 105 (information regarding the deviation of amplitude characteristics) and controls the TOF 101 to reduce the in-band tilt. In the configuration example shown in Figure 1, the control unit 106 changes the center frequency f0 by variably controlling the control voltage Vf of the TOF 101 based on the information regarding the deviation of amplitude characteristics.

[0018] In the first embodiment, the optical receiver 100 performs the following processing based on the control of the control unit 106 when receiving a received signal.

[0019] 1. The DSP105 transfers (outputs) information about the received signal (information about the deviation in amplitude characteristics) to the control unit 106. The information about the deviation in amplitude characteristics is, for example, the compensation coefficient of the adaptive equalization circuit (see Figure 4).

[0020] 2. The control unit 106 controls the setting of the TOF 101 based on the information of the received signal (information regarding the deviation of the amplitude characteristics).

[0021] 2-1. First, the control unit 106 calculates the in-band tilt of the received signal based on the information of the received signal (information regarding the deviation of the amplitude characteristics).

[0022] 2-2. The control unit 106 controls the TOF 101 to change its settings in a direction that reduces the in-band tilt. More specifically, for example, the control unit 106 determines whether the value (absolute value) of the in-band tilt is greater than a predetermined value (e.g., a threshold).

[0023] 2-3. If the absolute value of the tilt within the signal band is greater than a predetermined value (threshold), the control unit 106 changes the setting of the TOF 101 installed inside the optical receiver 100 in a direction that decreases the absolute value of the tilt within the signal band. The "setting" of the TOF 101 is, for example, the center frequency f0 of the TOF 101. Furthermore, the "predetermined value (threshold)" and the configurable range of the TOF 101 can be set to any value.

[0024] 3. The control unit 106 continues to perform the above process periodically while the device is operating.

[0025] (Explanation of technical background) Here, I will explain the technical background and its challenges.

[0026] Figure 2 is a graph showing an example of the amplification characteristics of an optical amplifier. The horizontal axis represents wavelength (Wavelength [nm]) and the vertical axis represents optical power (Power [dB]), showing the wavelength dependence of EDFA.

[0027] As shown in Figure 2, the optical power due to optical amplification differs with respect to wavelength. For example, the optical power with respect to wavelength is not flat but wavy, meaning that the optical power differs for each wavelength.

[0028] Figure 3 is an explanatory diagram of the amplitude characteristic deviations for different baud rates. Figure 3(a) shows the frequency spectrum of the received signal (main signal) for different baud rates. The horizontal axis represents frequency (Frequency [GHz]), and the vertical axis represents optical energy (Magnitude [dB]). As the baud rate increases to 64 Gbd, 96 Gbd, and 130 Gbd, the bandwidth of the signal (the flat characteristic portion) expands in the frequency direction.

[0029] Figure 3(b) shows the deviation of the amplitude characteristics after optical amplification at different baud rates. The horizontal axis represents wavelength, and the vertical axis represents the amplification factor of the optical amplifier. At low baud rates, the width of the signal bandwidth amplified by the optical amplifier is narrow, while at high baud rates, the width of the signal bandwidth amplified by the optical amplifier is wide. At low baud rates, the deviation of the amplitude characteristics after optical amplification is only Δ1, while at high baud rates, the deviation of the amplitude characteristics after optical amplification becomes Δ2, and the deviation Δ of the amplification factor within the signal bandwidth expands as the baud rate increases. As a result, when a wide bandwidth signal at a high baud rate passes through an optical amplifier, as shown in Figure 3(b), a deviation (intraband tilt) occurs in the optical amplification factor (amplitude characteristics) within the signal bandwidth.

[0030] Deviations in amplitude characteristics within the signal band (intraband tilt) lead to distortion of the signal spectrum, resulting in degradation of the characteristics of the received signal. To eliminate intraband tilt, for example, it is possible to reduce the wavelength dependence of the amplification factor by incorporating an optical equalizer, but this can be a factor in increasing costs.

[0031] In contrast, in Embodiment 1, the in-band tilt is calculated, and the in-band tilt is reduced using TOF based on the in-band tilt, thereby reducing the in-band tilt even when the width of the optical signal bandwidth is expanded due to the increased baud rate.

[0032] (Example of DSP internal functions) Figure 4 shows an example of the internal functions of a DSP according to Embodiment 1. The DSP 105 includes an ADC (Analog-to-Digital Converter) 401, a fixed equalization circuit 402, and an adaptive equalization circuit 403. The ADC 401 converts the input received signal (electrical signal) from digital to analog. The fixed equalization circuit 402 compensates for the wavelength dispersion of the optical transmission path. The adaptive equalization circuit 403 adaptively equalizes the output signal of the fixed equalization circuit 402. For example, the adaptive equalization circuit 403 performs frequency offset compensation, polarization mode dispersion compensation, carrier phase restoration, etc.

[0033] The information regarding the amplitude characteristic deviation described above is, for example, the compensation coefficient output by the adaptive equalization circuit 403. The DSP 105 may also include a waveform monitor 404 that monitors the waveform of the received signal output by the fixed equalization circuit 402. The spectral shape of the optical signal output by the waveform monitor 404 may be used as information regarding the amplitude characteristic deviation. The control unit 106 receives the information regarding the amplitude characteristic deviation output by the DSP 105.

[0034] (Example of in-band tilt control of a signal by changing the center frequency of the TOF) Figure 5 is an explanatory diagram of in-band tilt control by changing the center frequency of the TOF. In Figure 5, the horizontal axis represents frequency [GHz], and the vertical axis represents the amplitude [dB] of the received signal output by TOF101. The inventors discovered that in-band tilt can be controlled by changing the center frequency f0 of TOF101.

[0035] When the filter characteristic F1 (center frequency f0 = 0 GHz) is used, the optical spectrum output by TOF101 is S1. Similarly, when the TOF filter characteristic F2 (center frequency f0 = -60 GHz) is used, the optical spectrum output by TOF101 is S2. Thus, it was found that changing the center frequency f0 of TOF101 changes the spectral shape of the optical signal transmitted through TOF101.

[0036] When the filter characteristic F1 (center frequency f0 = 0 GHz), the optical spectrum S1 output by TOF101 has a tilt T1 within the signal band (before tilt correction). In contrast, by changing the center frequency f0 of TOF101 to Δf0 (-60 GHz), the spectral shape of the optical spectrum S2 output by TOF101 is shaped to have a tilt T2 within the signal band, which is gentler than the tilt T1 within the signal band (after tilt correction). In this way, by changing the center frequency f0 of TOF101, the spectral shape of the optical signal transmitted by TOF101 changes, and the tilt within the signal band can be reduced.

[0037] As shown in Figure 5 after tilt correction, the center frequency of the signal band indicated by the optical spectrum S2 and the center frequency f0 of the filter characteristic F2 of TOF101 differ in frequency by Δf0.

[0038] The control unit 106 calculates the in-band tilt of the received signal based on information regarding the amplitude characteristic deviation output by the DSP 105 (e.g., compensation signal). Then, the control unit 106 changes the center frequency f0 of the TOF 101 by variably controlling the control voltage Vf of the TOF 101 to reduce the calculated in-band tilt.

[0039] (Example of calculating tilt within the signal band) Figure 6 is a diagram showing an example of the frequency characteristics of the compensation coefficient of the adaptive equalization circuit. The horizontal axis represents frequency [GHz], and the vertical axis represents the amplitude of the received signal [dB]. The adaptive equalization circuit 403 dynamically generates the compensation coefficient based on the state of the received signal at the current time. The control unit 106 calculates the in-band tilt of the received signal based on the compensation coefficient.

[0040] Figure 6 shows the frequency characteristics a of the adaptive equalization circuit coefficients before tilt correction and b of the adaptive equalization circuit coefficients after tilt correction. By changing the center frequency of the optical filter, the spectral shape of the transmitted optical signal changes, and the in-band tilt is reduced from T1 to T2.

[0041] The procedure for calculating the in-band tilt from the compensation coefficient by the control unit 106 is shown below. 1. Convert the compensation coefficients expressed in the time domain to the frequency domain using the Fourier transform. 2a. The difference between any two points in the converted frequency domain characteristics (hereinafter referred to as frequency characteristics) is defined as the in-band tilt.

[0042] For example, suppose that the compensation coefficient is an FIR (Finite Impulse Response) filter with an arbitrary number of taps i, and that the compensation coefficient is expressed as [x1, x2, x3, ... xi]. For example, the FIR filter is provided in the adaptive equalization circuit 403. The control unit 106 assumes that the frequency characteristics obtained by transforming these compensation coefficients using a Fourier transform are [y1, y2, y3, ... yi]. In the above frequency characteristics, the difference between any two points, for example, y2 and y6, is defined as the in-band tilt.

[0043] 2b. In addition, the control unit 106 may define the in-band tilt as the difference between the average value of an arbitrary range in the frequency characteristics and the average value of an arbitrary range different from the above. For example, the in-band tilt can also be defined as the difference between the frequency characteristics of [average value of y2, y3, y4] and the average value of an arbitrary range different from the above, such as [average value of y6, y7, y8].

[0044] Figure 7 is an explanatory diagram illustrating an example of calculating the in-band tilt from the tap numbers. In Example 1 shown in Figure 7(a), the in-band tilt is defined as the difference between tap numbers 10 and 25 at any two points on the FIR. In Example 2 shown in Figure 7(b), the in-band tilt is defined as the difference between the average of tap numbers 8-10 and the average of tap numbers 23-25 ​​on the FIR.

[0045] (Control example of the optical transmission device in Embodiment 1) Figure 8 is a flowchart illustrating an example of control of an optical transmission device according to Embodiment 1. First, information about the received signal (information about the deviation of amplitude characteristics) is transferred from the DSP 105 to the control unit 106 (step S801).

[0046] Next, the control unit 106 calculates the in-band tilt (step S802). Then, the control unit 106 determines whether the in-band tilt is greater than a predetermined value (threshold) (step S803). If the in-band tilt is less than the predetermined value (step S803: No), the control unit 106 returns to the process in step S801. On the other hand, if the in-band tilt is greater than the predetermined value (threshold) (step S803: Yes), the control unit 106 proceeds to the process in step S804.

[0047] In step S804, the control unit 106 calculates the amount of deviation in the center frequency f0 of the TOF 101 that is necessary to keep the in-band tilt below a predetermined value (step S804). Then, the control unit 106 calculates the amount of change in the TOF 101 setting that corresponds to the calculated amount of deviation (step S805). For example, the amount of change in the setting is the amount of change in the center frequency f0 of the TOF 101 (see, for example, Figure 5).

[0048] Then, the control unit 106 applies the change in the setting to the TOF 101 (step S806) and returns to the process in step S801. In this way, the optical receiver 100 continues to periodically perform the process shown in Figure 8 while the device is operating.

[0049] The processing in steps S803 to S806 will be explained in more detail. The control unit 106 determines whether the absolute value of the signal band tilt is greater than a predetermined value. If the absolute value of the signal band tilt is greater than the predetermined value, the control unit 106 calculates the amount of shift in the optical filter's center frequency required to bring the calculated absolute value of the signal band tilt to the predetermined value. For example, the amount of shift in center frequency f0 by how many GHz is required for a 1 dB signal band tilt is set in advance as a set value for the shift amount.

[0050] The control unit 106 calculates the appropriate deviation amount in GHz for the in-band tilt calculated for the current received signal, based on the set value of the deviation amount. Then, the control unit 106 calculates the amount of change in the TOF 101 setting that corresponds to the calculated deviation amount. This "setting" is the control voltage (voltage value) Vf applied to the TOF 101, and the control unit 106 calculates (converts) the amount of change in the voltage value corresponding to the deviation amount from the correspondence between the center frequency f0 and the voltage value Vf, which are defined as product characteristics of the TOF 101. Then, the control unit 106 applies the calculated amount of change in the setting (calculated voltage value Vf) to the TOF 101.

[0051] (Example of handling multiple signal types) The optical receiver 100 described in Embodiment 1 can receive multiple signal types (categories) with different baud rates, for example, in response to an increase in baud rate. In this case, it is conceivable that there may be cases where the significance of implementing the present invention is diminished depending on the type of signal to be received. Therefore, the optical receiver 100 can be configured to set whether or not to implement the control of the embodiment described in Figure 8, etc., for each type of signal to be received. The types of received signals are classified according to the combination of baud rate and multi-level parameters. Therefore, the implementation of the above control differs for each signal type depending on the combination of baud rate and multi-level parameters.

[0052] Figure 9 is a diagram showing an example of setting whether or not to implement control for a given signal type. The control implementation setting value of 900 includes the sequential number of the signal type, the baud rate of the signal type, the multi-level intensity of the signal type, and whether or not to implement control. In Figure 9, the baud rate and multi-level intensity are set as parameters that constitute the signal type.

[0053] In the example of setting value 900 in Figure 9, if the received signal is signal type 1 (baud rate XXX Gbaud, multi-level degree XXX), the control unit 106 "implements" the control of Embodiment 1. If the received signal is signal type 2 (baud rate XXX Gbaud, multi-level degree XXX), the control unit 106 "does not implement" the control of Embodiment 1, i.e., does not implement it. In this way, the implementation / non-implementation of the control described in Embodiment 1 can be selected based on at least one parameter of the baud rate and multi-level degree of the received signal type, and the control described in Embodiment 1 can be applied only to the appropriate signal type.

[0054] (Examples of application to optical transmission systems) The optical transmission system includes a first optical transmission device (optical transmitter) that transmits a transmission signal (optical signal) onto a transmission path, and a second optical transmission device (optical receiver) that receives a reception signal (optical signal) via the transmission path. The optical receiver 100 of this optical transmission system can be the optical receiver described in Embodiment 1. This makes it possible to reduce in-band tilt and improve transmission performance even when the baud rate of the optical signal transmitted in the optical transmission system is increased. Furthermore, tilt can occur not only when there is an optical amplifier in the optical transmitter, but also when there is an optical amplifier in the middle of the optical transmission path. Even in that case, the in-band tilt can be corrected, so the in-band tilt can be reduced throughout the entire optical transmission system for both optical transmission and reception.

[0055] (Embodiment 2) Embodiment 1 described an example of controlling a TOF 101 mounted on an optical receiver 100. Embodiment 2 describes an example of controlling a TOF mounted on an optical transmitter, rather than controlling a TOF 101 mounted on an optical receiver 100.

[0056] Figure 10 shows an optical transmission system according to Embodiment 2. The optical transmission system consists of a first optical transmission device (optical transmitter) 1000 and a second optical transmission device (optical receiver) 1010 connected via an optical transmission path L.

[0057] The optical transmitter 1000 includes a DSP 1001, a CDM (Coherent Driver Modulator) 1002, an LD 1003, an optical amplifier (AMP) 1004, a TOF 1005, and a control unit 1006.

[0058] The DSP1001 performs digital-to-analog conversion, frame conversion, digital modulation processing at a predetermined baud rate and modulation scheme based on the settings of the control unit 106, symbol mapping, bandwidth characteristic compensation processing, and other operations on the transmitted data.

[0059] The CDM1002 converts the transmitted data into an optical signal based on the laser light (local emission) of the LD1003, and includes an optical modulator and a driver. The AMP1004 optically amplifies the optical signal after it has passed through the CDM1003. The AMP1004 filters the optical signal it outputs and outputs the filtered optical signal to the optical transmission path L. The TOF101 is a bandpass filter similar to the TOF101 described in Embodiment 1, and has a bandpass characteristic that transmits only a predetermined signal band. The center frequency f0 of the bandpass characteristic of the TOF101 is variable. For example, the center frequency f0 of the TOF1005 is changed by changing the control voltage (voltage value) Vf output by the control unit 1006.

[0060] The optical receiver 1010 has the same configuration as the optical receiver 100 described in Embodiment 1 and is denoted by the same reference numerals.

[0061] In the optical transmission system of Embodiment 2, the optical receiver 1010 and the optical transmitter 1000 perform the following processes.

[0062] (Processing on the optical receiver 1010 side) 1. The DSP105 transfers (outputs) information about the received signal (information about the deviation in amplitude characteristics) to the control unit 106. The information about the deviation in amplitude characteristics is, for example, the compensation coefficient of the adaptive equalization circuit (see Figure 4).

[0063] 2. The control unit 106 controls the setting of the TOF 101 based on the information of the received signal (information regarding the deviation of the amplitude characteristics).

[0064] 2-1. First, the control unit 106 calculates the in-band tilt of the received signal based on the information of the received signal (information regarding the deviation of the amplitude characteristics).

[0065] 2-2. The control unit 106 controls the TOF 101 to change its settings in a direction that reduces the tilt within the signal band. More specifically, for example, the control unit 106 determines whether the value (absolute value) of the tilt within the signal band is greater than a predetermined value (e.g., a threshold). If the control unit 106 determines that the value (absolute value) of the tilt within the signal band is greater than a predetermined value (e.g., a threshold), it sends setting change information D to the optical transmitter 1000.

[0066] (Processing on the optical transmitter 1000 side) 2-3. When setting change information D is received (when the absolute value of the tilt within the signal band is greater than a predetermined value (threshold)), the control unit 1006 changes the setting of the TOF 1005 installed inside the optical transmitter 1000 in a direction that reduces the absolute value of the tilt within the signal band. The "setting" of TOF 1005 is, for example, the center frequency f0 of TOF 101.

[0067] 3. The optical receiver 1010 (control unit 106) and the optical transmitter 1000 (control unit 1006) continue to perform the above processing periodically while the device is operating.

[0068] The configuration change information D can be transmitted from the optical receiver 1010 to the optical transmitter 1000 via a wired or wireless electrical or optical transmission system. For example, it can be transmitted by including it in the OSC (Optical Service Channel) on the optical transmission path from the optical receiver 1010 to the optical transmitter 1000.

[0069] The processing performed by the optical receiver 1010 and the processing performed by the optical transmitter 1000 can be changed as appropriate. In other words, the tilt within the signal band of the transmitted signal transmitted by the optical transmitter 1000 can be changed and controlled based on the information of the received signal received by the optical receiver 1010 (information regarding the deviation of amplitude characteristics).

[0070] For example, the control unit 106 on the optical receiver 1010 side may directly send the above "1. Information on the received signal from the DSP 105 (information on the deviation of amplitude characteristics)" to the optical transmitter 1000. In this case, the optical transmitter 1000 side only needs to perform the subsequent processing described in 2. (2-1. to 2-3.).

[0071] According to the embodiment 2 described above, by transmitting a transmission signal with reduced in-band tilt at the TOF 1005 of the optical transmitter 1000 based on the received signal on the optical receiver 1010 side, the in-band tilt of the entire optical transmission and reception system can be reduced. Furthermore, even if there are no optical amplifiers in the optical transmitter and optical receiver, the in-band tilt of the entire optical transmission system can be reduced if there is an optical amplifier in the optical transmission path.

[0072] Furthermore, another example of an optical transmission system configuration is that Embodiment 2 can be combined with Embodiment 1. Specifically, using Figure 10 as an example, optical filtering is performed by the TOF (first TOF) 1005 on the optical transmitter 1000 side and the TOF (second TOF) 101 on the optical receiver 1010 side. The control unit (second control unit) 106 on the optical receiver 1010 side controls the setting of the second TOF 101 based on information regarding the deviation of the received amplitude characteristics. Also, the control unit (first control unit) 1006 on the optical transmitter 1000 side controls the setting of the first TOF 1005 based on information regarding the deviation of the amplitude characteristics received from the optical receiver 1010 (second control unit 106). As a result, the TOF 1005 of the optical transmitter 1000 transmits a transmission signal with reduced in-band tilt, and the TOF 101 of the optical receiver 1010 reduces the in-band tilt of the received signal, thereby reducing the in-band tilt of the entire optical transmission system, including the optical transmission path. Furthermore, even if there are no optical amplifiers in the optical transmitter and optical receiver, the presence of optical amplifiers along the optical transmission path can reduce in-band tilt throughout the entire optical transmission system.

[0073] Incidentally, optical transmission devices require high-speed signal processing, and currently, dedicated DSPs are used. However, ASICs and FPGAs capable of high-speed processing can also be used. Furthermore, a high-speed CPU could be used as the control unit in the future. ASIC stands for Application Specific Integrated Circuit, and FPGA stands for Field Programmable Gate Array.

[0074] The optical receiver of the embodiment described above receives and processes the received optical signal received via an optical transmission path. The optical receiver includes an output unit (DSP) that outputs information regarding the deviation of the amplitude characteristics of the received signal (in-band tilt), a TOF that filters the received signal, and a control unit that controls the TOF setting based on the in-band tilt. The information regarding the deviation of the amplitude characteristics is, for example, information regarding the deviation of the amplitude characteristics after optical amplification by an optical amplifier. However, the optical receiver of the embodiment can control the in-band tilt based on the information regarding the deviation of the amplitude characteristics even in a configuration without an optical amplifier. This makes it possible to reduce the in-band tilt. Furthermore, since it is possible to reduce the in-band tilt at high baud rates and suppress the increase in signal spectrum distortion without providing an optical equalizer or the like, it is possible to improve transmission characteristics at low cost.

[0075] Furthermore, in the optical receiver of this embodiment, the control unit variably controls the center frequency of the TOF based on the in-band tilt. For example, the TOF has bandpass optical filter characteristics. By changing the center frequency of the TOF, the optical signal spectrum transmitted through the TOF can be changed. For example, the center frequency of the TOF can be set to a different value from the center frequency of the signal band of the received signal. This makes it possible to easily reduce the in-band tilt simply by changing the center frequency of the TOF.

[0076] Furthermore, in the optical receiver of this embodiment, the control unit calculates the in-band tilt of the signal using the output of the waveform monitor of the received signal or the compensation coefficient of the adaptive equalization circuit. In this way, the in-band tilt of the signal can be easily calculated using information output by the existing functions of the device.

[0077] Furthermore, in the optical receiver of this embodiment, the control unit compares the absolute value of the in-band tilt with a predetermined threshold, and if the absolute value of the in-band tilt is greater than the threshold, it controls the TOF setting in a direction that decreases the absolute value of the in-band tilt. This makes it possible to reduce the tilt (direction of tilt with respect to frequency) of the in-band tilt which has a large tilt.

[0078] Furthermore, in the optical receiver of this embodiment, the control unit selects whether or not to change the TOF settings for a predetermined signal type that includes at least one parameter of the multiple signal types of the received signal, namely the baud rate and the multi-level intensity. This makes it possible to apply processing to signal types that require reduction of in-band tilt.

[0079] Furthermore, the optical transmission system of this embodiment includes an optical transmitter that transmits an optical transmission signal to an optical transmission path, and an optical receiver that receives an optical reception signal via the optical transmission path. The optical transmitter transmits a transmission signal and, in accordance with the above configuration, controls the TOF setting in the optical receiver based on the in-band tilt of the signal. Thus, the optical transmission device can be applied to an optical transmission system that transmits and receives optical signals.

[0080] Furthermore, in the embodiment of the optical transmission system, the optical receiver may transmit information on in-band tilt, and the optical transmitter may control the TOF setting within the optical transmitter based on the in-band tilt.

[0081] Furthermore, the optical receiver may control the TOF setting within the optical receiver based on the in-band tilt, and the optical transmitter may control the TOF setting within the optical transmitter based on the in-band tilt transmitted from the optical transmitter. This allows the optical transmitter to transmit a signal with reduced in-band tilt, and the optical receiver to reduce the in-band tilt from the received signal.

[0082] With regard to the embodiments described above, the following additional information is disclosed.

[0083] (Note 1) An optical transmission device that receives and processes optical signals received via an optical transmission path, An output unit that outputs information regarding the deviation in the amplitude characteristics of the received signal, A TOF (Tunable Optical Filter) is used to filter the received signal, The system includes a control unit that controls the setting of the TOF based on information regarding the deviation of the amplitude characteristics. An optical transmission device characterized by the following features.

[0084] (Note 2) The optical transmission apparatus according to Note 1, characterized in that the control unit variably controls the center frequency of the TOF based on information regarding the deviation of the amplitude characteristics.

[0085] (Note 3) The optical transmission apparatus according to Note 1, characterized in that the control unit calculates the deviation of the amplitude characteristics using the output of the waveform monitor of the received signal or the compensation coefficient of the adaptive equalization circuit.

[0086] (Note 4) The optical transmission apparatus according to Note 1, characterized in that the control unit compares the absolute value of the deviation of the amplitude characteristics with a predetermined threshold, and if the absolute value of the deviation of the amplitude characteristics is greater than the threshold, controls the setting of the TOF in a direction that decreases the absolute value of the deviation of the amplitude characteristics.

[0087] (Note 5) The optical transmission apparatus according to Note 1, characterized in that the control unit selects whether or not to change the TOF setting for a predetermined signal type that includes at least one parameter of baud rate and multi-level, which are parameters of a plurality of signal types of the received signal.

[0088] (Note 6) The optical transmission apparatus according to Note 1, characterized in that the control unit sets the center frequency of the TOF to a different value from the center frequency of the signal band of the received signal.

[0089] (Note 7) The receiving signal is provided with an optical amplifier that optically amplifies the signal, The optical transmission apparatus according to Appendix 1, characterized in that the information relating to the deviation of the amplitude characteristics is information relating to the deviation of the amplitude characteristics after optical amplification.

[0090] (Note 8) The optical transmission device according to Note 1, characterized in that the TOF has bandpass optical filter characteristics.

[0091] (Note 9) In an optical transmission system including a first optical transmission device that transmits an optical transmission signal to an optical transmission path, and a second optical transmission device that receives an optical reception signal via the optical transmission path, The first optical transmission device transmits the transmission signal, The second optical transmission device, An output unit that outputs information regarding the deviation in the amplitude characteristics of the received signal, A first TOF (Tunable Optical Filter) performs filtering of the received signal, A first control unit that controls the setting of the first TOF based on information regarding the deviation of the amplitude characteristics, An optical transmission system characterized by the following:

[0092] (Note 10) The output unit transmits information regarding the deviation of the amplitude characteristics to the first optical transmission device. The first optical transmission device, A second TOF that filters the aforementioned transmission signal, The system includes a second control unit that controls the setting of the second TOF based on the received information regarding the deviation of the amplitude characteristics. The optical transmission system described in Appendix 9, characterized by the features described herein.

[0093] (Note 11) In an optical transmission system including a first optical transmission device that transmits an optical transmission signal to an optical transmission path, and a second optical transmission device that receives an optical reception signal via the optical transmission path, The first optical transmission device, A TOF (Tunable Optical Filter) is used to filter the aforementioned transmission signal, The system includes a control unit that controls the TOF setting based on information regarding the deviation of the amplitude characteristics, The second optical transmission device, The output unit has an output unit that outputs information regarding the deviation of the amplitude characteristics to the first optical transmission device. An optical transmission system characterized by the following: [Explanation of symbols]

[0094] 100 Optical receiver (optical transmission device) 101 TOF 102 AMP (Optical Amplifier) 103 ICR 104 LD 105 DSP 106 Control Unit 401 ADC 402 Fixed equalization circuit 403 Adaptive Equalization Circuit 404 Waveform Monitor 900 Control implementation setting value 1000 Optical Transmitter (First Optical Transmission Device) 1010 Optical receiver (second optical transmission device) D Settings change information L Optical transmission path T1 Intraband tilt (before tilt correction) T2 signal band tilt (after tilt correction) Vf Control voltage (voltage value)

Claims

1. An optical transmission device that receives and processes optical signals received via an optical transmission path, An output unit that outputs information regarding the deviation in the amplitude characteristics of the received signal, A TOF (Tunable Optical Filter) performs filtering of the received signal, The system includes a control unit that controls the setting of the TOF based on information regarding the deviation of the amplitude characteristics. An optical transmission device characterized by the following features.

2. The optical transmission apparatus according to claim 1, characterized in that the control unit variably controls the center frequency of the TOF based on information regarding the deviation of the amplitude characteristics.

3. The optical transmission apparatus according to claim 1, characterized in that the control unit calculates the deviation of the amplitude characteristics using the output of the waveform monitor of the received signal or the compensation coefficient of the adaptive equalization circuit.

4. The optical transmission apparatus according to claim 1, characterized in that the control unit compares the absolute value of the deviation of the amplitude characteristics with a predetermined threshold, and if the absolute value of the deviation of the amplitude characteristics is greater than the threshold, controls the setting of the TOF in a direction that decreases the absolute value of the deviation of the amplitude characteristics.

5. The optical transmission apparatus according to claim 1, characterized in that the control unit selects whether or not to change the TOF setting for a predetermined signal type that includes at least one parameter of baud rate and multi-level, which are parameters of a plurality of signal types of the received signal.

6. The optical transmission apparatus according to claim 1, characterized in that the control unit sets the center frequency of the TOF to a value different from the center frequency of the signal band of the received signal.

7. The system includes an optical amplifier that optically amplifies the received signal, The optical transmission device according to claim 1, characterized in that the information relating to the deviation of the amplitude characteristics is information relating to the deviation of the amplitude characteristics after optical amplification.

8. An optical transmission system including a first optical transmission device that transmits an optical transmission signal to an optical transmission path, and a second optical transmission device that receives an optical reception signal via the optical transmission path, The first optical transmission device transmits the transmission signal, The second optical transmission device is An output unit that outputs information regarding the deviation of the amplitude characteristics, A first TOF (Tunable Optical Filter) that filters the received signal, The system includes a first control unit that controls the setting of the first TOF based on information regarding the deviation of the amplitude characteristics. An optical transmission system characterized by the following:

9. The output unit transmits information regarding the deviation of the amplitude characteristics to the first optical transmission device. The first optical transmission device is A second TOF for filtering the aforementioned transmission signal, The system includes a second control unit that controls the setting of the second TOF based on the received information regarding the deviation of the amplitude characteristics. The optical transmission system according to feature 8.

10. An optical transmission system including a first optical transmission device that transmits an optical transmission signal to an optical transmission path, and a second optical transmission device that receives an optical reception signal via the optical transmission path, The first optical transmission device is A TOF (Tunable Optical Filter) is used to filter the aforementioned transmission signal, The system includes a control unit that controls the setting of the TOF based on information regarding the deviation of the amplitude characteristics, The second optical transmission device is The output unit has an output unit that outputs information regarding the deviation of the amplitude characteristics to the first optical transmission device. An optical transmission system characterized by the following:

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