JP5101455B2 - Optical communication system, optical transmitter, and optical transmission method - Google Patents

Description

  The present invention relates to an optical communication system that transmits modulated optical signals having a plurality of wavelengths, an optical transmission method, and an optical transmitter used in the optical communication system.

  A method of generating modulated optical signals having a plurality of wavelengths using conventional techniques will be described with reference to FIGS. 17 and 18. This method mainly multiplexes and transmits optical signals of a plurality of wavelengths on a single optical transmission line, and realizes so-called wavelength multiplexing (expansion of transmission capacity, service lineup, and increase in the number of subscribers). It is assumed to be applied to an optical communication system called a WDM (Wavelength Division Multiplexing) system.

  FIG. 17 shows a configuration example for generating and transmitting a modulated optical signal having a plurality of wavelengths by using the conventional technique. In the optical transmitter, first, a ramp-shaped signal having a predetermined repetition period (1 / B) output from the wavelength sweep signal generator is configured to have a predetermined signal amplitude with the configuration shown in the figure. The signal is adjusted by the sweep signal amplitude adjuster and input to the laser light source. Accordingly, the laser light source generates wavelength sweep light that outputs an optical signal that sweeps a predetermined wavelength range at a predetermined repetition period (1 / B). The wavelength sweep light is modulated by the modulation signal output from the modulation signal generator in the optical modulator and then transmitted to the optical transmission line.

As shown in FIG. 18, the modulation signal generator generates a signal obtained by multiplexing the data of a plurality of wavelength channels to be transmitted on the time axis, and inputs the signal to the optical modulator. Here, by synchronizing the output signal of the modulation signal generator and the wavelength sweep light in terms of time, as shown in FIG. 17, a plurality of wavelengths each having a transmission rate of B [b / s] (channels in FIG. 17). Several N) modulated optical signals can be generated at once. The multi-wavelength optical signal generated in this configuration is transmitted to the optical receiver via the optical transmission line, and the optical receiver extracts only the optical signal of the desired wavelength channel using an optical filter, etc. Can be selectively received.
According to this configuration, an optical signal having a plurality of wavelengths can be generated with a simple configuration of a single light source and an optical modulator. Is characterized in that a large-capacity optical communication system using wavelength division multiplexing can be constructed (see Non-Patent Document 1).
Tomohiro Taniguchi, Kiyoshi Narukawa, Naoya Sakurai, Hideaki Kimura, Shin Tsubokawa, “Proposal of Shared Optical Transmitter Using Wavelength Sweep Light in Optical FDM Access System”, Proceedings of 2007 Society Conference of IEICE, Japan IEICE August 29, 2007, B-10-64, page 258 Tomohiro Taniguchi, Naoya Sakurai, Hideaki Kimura, Shin Tsubokawa, “Transmission Optical Spectrum Shaping for Suppression of Adjacent Channel Interference in Wavelength Sweep Optical FDM System”, Proceedings of 2008 IEICE General Conference, The Institute of Electronics, Information and Communication Engineers Published March 5, 2008, B-10-57, p.340 Shinji Matsuo, Takaaki Sasuka, Toru Segawa, Naoki Fujiwara, Yasuo Shibata, Hiroshi Yasaka, Hiroyuki Suzuki, “Transmitter using a frequency-modulated SSG-DBR laser and wavelength filter”, Proceedings of the Society Conference of IEICE 2007 , The Institute of Electronics, Information and Communication Engineers August 29, 2007, C-4-14, p. 220

  As described above, in the conventional example, in the optical modulator, the wavelength sweep light having a repetition frequency equal to the symbol rate of each wavelength channel is modulated with a signal obtained by multiplexing the data of each wavelength channel on the time axis, Generates signal light for all channels at once. For this reason, if it is attempted to increase the symbol rate of each wavelength channel, it is necessary to set the repetition frequency of the wavelength sweep light to be high, which makes generation difficult. Further, the modulation signal obtained by multiplexing on the time axis described above becomes very fast, and the spectrum of the optical signal generated as a result of the optical modulation is greatly expanded as in the case of the prior art in FIG. End up. As a result, there is a concern that interference between adjacent channels increases and reception quality deteriorates.

  The present invention has been made in such a background, and without increasing the repetition frequency of the wavelength sweep light, and without degrading the reception quality due to interference of adjacent channels, the symbol rate of each wavelength channel. An object of the present invention is to provide an optical communication system, an optical transmitter, and an optical transmission method.

In order to achieve the above object, an optical communication system of the present invention includes an optical transmitter that outputs a modulated optical signal of a plurality of wavelength channels, and a predetermined signal among optical signals transmitted from the optical transmitter via an optical transmission line. An optical communication system comprising an optical receiver for extracting and receiving an optical signal of a wavelength channel, wherein the optical transmitter outputs an optical signal that sweeps a predetermined wavelength range at a predetermined repetition frequency. And an optical branching device for branching the output optical signal of the wavelength sweep light source into N (N is a natural number of 2 or more), and the data of each wavelength channel is separated into N in bit order, and each of these separated wavelengths A modulation signal generator for outputting N modulation signals obtained by multiplexing channel data on the time axis, and N output optical signals provided for the N output optical signals of the optical branching unit. wherein each of the optical signal And N optical modulators for modulating the respective modulated signals, with respect to the branched N optical signals, so that the time difference between the optical signal is equal to one-N-th of the reciprocal of the predetermined repetition frequency, respectively An optical delay unit that gives a time delay to the optical modulator, and an optical multiplexer that multiplexes the N optical signals that have passed through the optical modulator and the optical delay unit and sends them to an optical transmission line, the optical receiver comprising: An optical filter that extracts an optical signal of a desired wavelength channel from the optical signals transmitted from the optical transmitter, and a light receiving device that receives the output optical signal of the optical filter and reproduces the data of the desired wavelength channel It is characterized by providing a vessel.

  In the optical communication system according to the present invention, the optical transmitter and the 2kth optical signal passing through the (2k-1) th (k is a natural number) path among the paths through which the branched optical signal passes. It is preferable to further comprise a polarization adjuster that controls the polarization state so that the polarization directions of the optical signal passing through the path are orthogonal to each other, and the optical transmitter is provided at the subsequent stage of the optical multiplexer, It is preferable to further include an optical filter having a passing characteristic that suppresses a wavelength range in which wavelengths of optical signals that have passed through different paths approach.

The optical transmitter of the present invention includes a wavelength sweep light source that outputs an optical signal that sweeps a predetermined wavelength range at a predetermined repetition frequency, and an output optical signal of the wavelength sweep light source that is N (N is a natural number of 2 or more). ) and optical splitter for splitting the pieces, the data of each wavelength channels separated into N in bit order, these separate multiplexing N modulated signals obtained by the data of each wavelength channel in each time axis a modulation signal generator for outputting, provided corresponding to N output optical signal of the optical splitter, the N modulating the each of the N output optical signal at each of said N modulated signals An optical modulator, an optical delay device for giving a time delay to the N optical signals branched, and a time delay so that a time difference of the optical signal is 1 / N of a reciprocal of the predetermined repetition frequency; Pass through the modulator and the optical delay Multiplexes the N optical signals, characterized in that it comprises an optical multiplexer to be transmitted to the optical transmission path.

  The optical transmitter of the present invention includes an optical signal passing through the (2k-1) -th path (k is a natural number) and an optical signal passing through the second-k path among the branched optical signal paths. It is preferable to further include a polarization controller that controls the polarization state so that the polarization directions are orthogonal to each other, and suppresses a wavelength region in which the wavelength of the optical signal that has passed through a different path approaches the subsequent stage of the optical multiplexer. It is preferable to further include an optical filter having such pass characteristics.

The optical transmission method of the present invention generates an optical signal that sweeps a predetermined wavelength range at a predetermined repetition frequency, and branches the generated optical signal into N (N is a natural number of 2 or more). A step of splitting the data of each wavelength channel into N pieces in order of bits and multiplexing the data of each separated wavelength channel on the time axis to generate N modulation signals; A step of modulating each of the N optical signals with each of the N modulated signals , and a time difference of the optical signals with respect to the branched N optical signals, respectively, is N times the reciprocal of the predetermined repetition frequency. The method includes a step of providing a time delay so as to be 1, and a step of combining the N optical signals having the time difference and sending them to an optical transmission line.

  In the optical transmission method of the present invention, the optical signal passing through the (2k-1) -th path (k is a natural number) and the optical signal passing through the second-k path among the paths through which the branched optical signal passes. It is preferable that the method further includes a step of controlling the polarization state so that the polarization directions are orthogonal to each other, and after the optical signals are combined, the wavelength region in which the wavelengths of the optical signals that have passed through different paths approach each other is suppressed. Preferably, the method further includes a step.

  The optical communication system of the present invention is a wavelength division multiplexing optical communication system based on wavelength sweep light, without increasing the repetition frequency of the wavelength sweep light, and without degrading the reception quality due to interference of adjacent channels, It is possible to increase the symbol rate of each wavelength channel. Thereby, the capacity of the communication system can be increased.

Embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
A first embodiment of the optical communication system of the present invention will be described with reference to FIGS. An optical communication system according to the first embodiment includes an optical transmitter that outputs a modulated optical signal of a plurality of wavelength channels, and light of a predetermined wavelength channel among optical signals transmitted from the optical transmitter via an optical transmission line. The optical receiver is configured to extract and receive a signal. FIG. 1 shows a configuration example of an optical transmitter and an optical receiver according to the first embodiment of the present invention, and output signals in respective components. The optical transmitter includes a laser light source 11, optical modulators 12 ₁ and 12 ₂ , a wavelength sweep signal generator 13, a wavelength sweep signal amplitude adjuster 14, a modulation signal generator 15, and an optical branch as optical components. Device 16, optical delay device 17, and optical multiplexer 18.

  Wavelength sweep signal amplitude adjuster so that a ramp waveform signal having a predetermined repetition period (1 / B: equal to the symbol length of each wavelength channel) output from the wavelength sweep signal generator 13 has a predetermined signal amplitude. 14 is adjusted and input to the laser light source 11. Thereby, the laser light source 11 can generate the wavelength sweep light that outputs the optical signal that sweeps the predetermined wavelength range at the predetermined repetition period (1 / B).

  The wavelength sweep signal amplitude adjuster 14 is used to adjust the amplitude of the ramp-waveform wavelength sweep signal and generate an optical signal having a desired wavelength band and wavelength sweep amount. The AC component and the DC component are separated by a tee, etc., and the wavelength sweep amount is controlled by adjusting the AC component amplitude, and at the same time, the wavelength band of light emission is controlled by adjusting the DC component.

  As a method for generating the wavelength sweep light, for example, a distributed feedback (DFB) laser that generates a single spectrum optical signal is used as the laser light source 11, and the oscillation wavelength of the laser depends on the applied current. A method of generating wavelength sweep light using a changing chirp phenomenon is conceivable. Thereby, a wavelength sweep range of several tens GHz can be obtained with a repetition period of about several GHz. In this case, since the intensity fluctuation occurs simultaneously with the wavelength sweep, if necessary, a semiconductor optical amplifier (SOA) operated in a gain saturation region in the subsequent stage of the laser light source 11 is used. It is also useful to suppress the intensity fluctuation component (see Non-Patent Document 2).

  Further, in order to generate a wavelength sweep light having a higher speed and a wider bandwidth, a method of applying the above-described wavelength sweep signal to the phase adjustment region of a distributed reflection (DBR) laser (see Non-Patent Document 3) is also conceivable. ).

The wavelength sweep light generated in this way is branched into two outputs by the optical branching unit 16 and input to the optical modulators 12 ₁ and 12 ₂ , respectively. The output optical signals of the _two optical modulators 12 ₁ and 12 ₂ are multiplexed by the optical multiplexer 18 and then sent to the optical transmission line 20.

As shown in FIG. 2, the modulation signal generator 15 separates data of a plurality of wavelength channels to be transmitted every other bit, and generates signals (modulation signal 1 and modulation signal 2) obtained by multiplexing these on the time axis. And input to the optical modulators 12 ₁ and 12 ₂ , respectively. The optical modulators 12 ₁ and 12 ₂ modulate the wavelength sweep light with a signal obtained by multiplexing transmission data of each wavelength channel on the time axis. Here, the output signal of the modulation signal generator 15 and the wavelength sweep light are synchronized with each other in time, and further, the optical delayer 17 causes the optical signal passing through the path 2 to have different wavelengths with respect to the optical signal passing through the path 1. By giving a delay so as to be delayed by half the channel symbol length (1/2 B [s]), as shown in FIG. 1, a plurality of wavelengths each having a transmission rate of 2 B [b / s] (in FIG. 1, the number of channels) N) modulated optical signals can be generated collectively. In FIG. 1, the optical delay device 17 is installed on the path 2 side, but may be installed on the path 1 side. Further, although the optical delay device 17 is installed at the rear stage of the optical modulator, it may be installed at the front stage of the optical modulator.

  As shown in FIG. 1, the optical receiver includes an optical filter 21, a light receiver 23, and a low-pass filter 24. The optical filter 21 extracts only a desired wavelength channel from the multiple wavelength optical signals transmitted from the optical transmitter via the optical transmission path. Thereby, the data of the desired channel can be selectively received. The light receiver 23 receives the output optical signal from the optical filter 21 and reproduces data of a desired wavelength channel. At this time, the low-pass filter 24 is used to limit the noise band as necessary so that good reception characteristics can be obtained.

  Here, the adjacent channel interference in the first embodiment of the present invention will be described with reference to FIGS. 3 and 4. FIG. 3 shows an optical signal when the transmission speed is doubled in the above-described prior art. On the other hand, the optical signal in the first embodiment of the present invention is in a state as shown in FIG. As can be seen from FIG. 3 and FIG. 4, in the present invention, even if the transmission rate is doubled, the repetition frequency of the generated wavelength sweep light does not change, and it is only half that of the conventional technique. Therefore, the technical difficulty in realizing the wavelength sweep light source is small. Further, when attention is paid to the interference of the adjacent channel, it can be seen that the adjacent channel component included in the passband of the optical filter can be suppressed to be smaller than that of the conventional technique. As a result, it is possible to suppress degradation of reception quality due to adjacent channel interference when the transmission rate is increased.

  As an example of the configuration of an optical communication system to which such an embodiment is applied, for example, as shown in FIG. 5, an optical signal from an optical transmitter is respectively transmitted to a plurality of optical receivers via a single-core optical transmission line. Multiple optical receivers are connected while sharing a part of the optical transmission line via the connected single star (SS) system or an optical branching unit installed in the middle of the optical transmission line as shown in FIG. A passive optical network (PON) scheme is considered. In addition, as shown in FIG. 7, a WDM-PON system in which a WDM filter is installed in the middle of an optical transmission line and an optical signal is transmitted through a different path depending on the wavelength can be considered. In this case, an optical filter may be one that demultiplexes one wave for each output port, or one that demultiplexes in units of a plurality of wavelengths (wavelength groups).

(Second Embodiment)
A second embodiment of the optical communication system of the present invention will be described with reference to FIG. In the second embodiment, in addition to the configuration of the optical transmitter of the first embodiment, a polarization adjuster 19 that controls the polarization state of the optical signal passing through the path 1 is provided. With this polarization adjuster 19, at the output of the optical multiplexer 18, the optical signal passing through the path 1 is orthogonal so that the polarization directions of the optical signal passing through the path 1 and the optical signal passing through the path 2 are orthogonal to each other. Control the polarization state. In FIG. 8, the polarization adjuster 19 is installed on the path 1 side, but may be installed on the path 2 side. Further, although the polarization adjuster 19 is installed in the subsequent stage of the optical modulator, it may be installed in the previous stage of the optical modulator.

  In the first embodiment, since the polarization state is not particularly adjusted, when the polarization state of the optical signal that has passed through the path 1 and that of the optical signal that has passed through the path 2 coincide, There is a concern that this interference will occur and the reception quality at the optical receiver will deteriorate. On the other hand, in the second embodiment, as described above, in the optical transmitter, the polarization states of the optical signals passing through the two paths are adjusted in advance so as to be orthogonal to each other. As compared with the first embodiment, high quality reception characteristics can be expected. Further, when a polarization beam combiner using a birefringent material is used as the optical multiplexer 18, power loss at the time of multiplexing can be reduced.

In the above, the first and second embodiments have been described with respect to the configuration in which the wavelength sweep light is branched into two and modulated respectively. However, in these embodiments, a configuration in which three or more branches are also possible. That is, in the first embodiment, after the wavelength sweep light is branched into N (N is a natural number of 3 or more), the data of each wavelength channel is separated into N pieces in order of bits, and each of these separated wavelength channels is separated. Are modulated with signals obtained by multiplexing on the time axis, and the modulated optical signals are further delayed after being given a time difference of 1 / NB [s].
Furthermore, in the second embodiment, after the delay is given to the modulated optical signal so as to have a time difference of 1 / NB [s], the path (2k−1) among the paths through which the modulated optical signal passes. The polarization state is controlled so that the polarization directions of the optical signal passing through (k is a natural number) and the optical signal passing through the path 2k are orthogonal to each other.
In this case, the optical signal transmitted to the optical transmission line can be represented in FIG. As an example, FIG. 10 and FIG. 11 show the configuration of an optical modulator when N = 4 and the modulation signal input to each optical modulator.
As described above, according to the present invention, it is possible to increase the transmission rate of each wavelength channel without changing the repetition frequency of the wavelength sweep light and suppressing the influence of interference of adjacent channels.

(Third embodiment)
A third embodiment of the optical communication system of the present invention will be described with reference to FIGS. In the third embodiment, the configurations of the optical transmitter and the optical receiver are the same as those in the first and second embodiments. However, as shown in FIG. 12, the latter stage of the optical multiplexer in the optical transmitter. Is characterized in that the optical filter 21 is used.

  FIG. 13 shows the time-wavelength characteristics of the optical signal when the number of branches (N) of the wavelength sweep light is 2, and FIG. 14 shows the pass characteristics of the optical filter. As shown in FIG. 14, the optical filter 21 has a transmission characteristic that suppresses a wavelength region (indicated by a dotted line in FIG. 13) in which the wavelengths of optical signals that have passed through two paths at the same time approach each other. Yes. An optical filter having such pass characteristics can be realized by using, for example, FBG (Fiber Bragg Grating). In the above wavelength range, since the wavelengths of the two optical signals are close to each other, these interference components are detected by the photoreceiver, which may deteriorate the reception quality. In the present embodiment, in the optical transmitter, by suppressing such a wavelength range in advance using an optical filter, it is possible to suppress degradation in reception quality due to interference between two optical signals.

  When the number of wavelength sweep light branches (N) is 3 or more, as shown in FIG. 15, there are a plurality of wavelength regions in which the wavelengths of optical signals that have passed through different paths in the same time are close. In this case, as shown in FIG. 16, these wavelength regions can be suppressed collectively by using an optical filter having periodic pass characteristics. The optical filter having such pass characteristics can be realized by, for example, a Mach-Zehnder interference type filter.

It is a figure which shows the structural example of the optical communication system in the 1st Embodiment of this invention, and the output signal in each structure part. It is a figure which shows the modulation signal in 1st Embodiment. It is a conceptual diagram explaining the influence of the adjacent channel interference in a prior art. It is a conceptual diagram explaining the influence of the adjacent channel interference in this invention. It is a figure explaining a single star (SS) system. It is a figure explaining a passive optical network (PON) system. It is a figure explaining a WDM-PON system. It is a figure which shows the structural example of the optical transmitter in the 2nd Embodiment of this invention, and the output signal in each structure part. It is a figure which shows the optical signal in the form which expanded the number of branches in 1st and 2nd embodiment. It is a figure which shows the structural example of the optical transmitter in case the number of branches N = 4. It is a figure which shows the modulation signal in case the number of branches N = 4. It is a figure which shows the structural example of the optical transmitter in the 2nd Embodiment of this invention, and the output signal in each structure part. It is a figure which shows the optical signal and optical filter characteristic in case the number of branches N = 2 in 3rd Embodiment. It is a figure which shows the passage characteristic of an optical filter. It is a figure which shows the optical signal and optical filter characteristic in case the number of branches N is 3 or more in 3rd Embodiment. It is a figure which shows the passage characteristic of an optical filter. It is a figure which shows the structural example of the conventional optical communication system. It is a figure which shows the modulation signal in the conventional optical communication system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Laser light source 12 Optical modulator 13 Wavelength sweep signal generator 14 Wavelength sweep signal amplitude adjuster 15 Modulation signal generator 16 Optical branching device 17 Optical delay device 18 Optical multiplexer 19 Polarization adjuster 20 Optical transmission line 21 Optical filter 23 Receiver 24 Low-pass filter

Claims (9)

From an optical transmitter that outputs a modulated optical signal of a plurality of wavelength channels, and an optical receiver that extracts and receives an optical signal of a predetermined wavelength channel from optical signals transmitted from the optical transmitter via an optical transmission line An optical communication system
The optical transmitter is
A wavelength sweep light source that outputs an optical signal that sweeps a predetermined wavelength range at a predetermined repetition rate; and
An optical branching device for branching the output optical signal of the wavelength sweep light source into N (N is a natural number of 2 or more);
A modulation signal generator that outputs N modulated signals obtained by separating the data of each wavelength channel into N pieces in bit order and multiplexing the data of each separated wavelength channel on the time axis;
Provided corresponding to the N output optical signal of the optical splitter, and N optical modulator for modulating each of the N output optical signal at each of said N modulated signals,
An optical delay unit that gives a time delay to the branched N optical signals so that the time difference of the optical signals is 1 / N of the reciprocal of the predetermined repetition frequency;
An optical multiplexer that multiplexes N optical signals that have passed through the optical modulator and the optical delay device and sends them to an optical transmission line;
The optical receiver is:
An optical filter that extracts an optical signal of a desired wavelength channel from the optical signals transmitted from the optical transmitter;
An optical communication system, comprising: a light receiver that receives an output optical signal of the optical filter and reproduces data of the desired wavelength channel.   The optical transmitter includes: an optical signal that passes through a (2k-1) -th path (k is a natural number) of paths through which the branched optical signal passes; and an optical signal that passes through a second-k path. The optical communication system according to claim 1, further comprising a polarization controller that controls a polarization state so that polarization directions are orthogonal to each other.   The optical transmitter further includes an optical filter having a pass characteristic that suppresses a wavelength region in which a wavelength of an optical signal that has passed through a different path approaches, subsequent to the optical multiplexer. The optical communication system according to 1 or 2. A wavelength sweep light source that outputs an optical signal that sweeps a predetermined wavelength range at a predetermined repetition rate; and
An optical branching device for branching the output optical signal of the wavelength sweep light source into N (N is a natural number of 2 or more);
A modulation signal generator that outputs N modulated signals obtained by separating the data of each wavelength channel into N pieces in bit order and multiplexing the data of each separated wavelength channel on the time axis;
Provided corresponding to the N output optical signal of the optical splitter, and N optical modulator for modulating each of the N output optical signal at each of said N modulated signals,
An optical delay unit that gives a time delay to the branched N optical signals so that the time difference of the optical signals is 1 / N of the reciprocal of the predetermined repetition frequency;
An optical transmitter comprising: an optical multiplexer that multiplexes N optical signals that have passed through the optical modulator and the optical delay device, and sends them to an optical transmission line.   Of the branched optical signal paths, the polarization directions of the optical signal passing through the (2k-1) th (k is a natural number) path and the optical signal passing through the 2k path are orthogonal to each other. The optical transmitter according to claim 4, further comprising a polarization controller for controlling a polarization state.   6. The optical filter according to claim 4, further comprising an optical filter having a pass characteristic that suppresses a wavelength region in which a wavelength of an optical signal that has passed through a different path approaches, subsequent to the optical multiplexer. Optical transmitter. Generating an optical signal that sweeps a predetermined wavelength range at a predetermined repetition rate; and
Branching the generated optical signal into N (N is a natural number of 2 or more);
Separating the data of each wavelength channel into N in bit order, and multiplexing the separated data of each wavelength channel on the time axis to generate N modulated signals;
A step of modulating each of branched N optical signals at each of said N modulated signals,
Giving a time delay to the branched N optical signals such that the time difference of the optical signals is 1 / N of the reciprocal of the predetermined repetition frequency;
Combining N optical signals having the time difference and sending them to an optical transmission line;
An optical transmission method.   Of the paths through which the branched optical signal passes, the polarization directions of the optical signal passing through the (2k-1) (k is a natural number) path and the optical signal passing through the 2k path are orthogonal to each other. The optical transmission method according to claim 7, further comprising the step of controlling the polarization state to   The optical transmission method according to claim 7, further comprising a step of suppressing a wavelength region in which wavelengths of optical signals that have passed through different paths approach after the optical signals are multiplexed.

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