JP2024119566A - Local light generating device, receiving device, and local light generating method - Google Patents

Abstract

[Problem] It is possible to generate local light phase-locked to a data signal and realize coherent homodyne detection without using an optical modulator. [Solution] A local light generating device comprising: a direct modulation light source unit that generates a sideband having a frequency that is a predetermined distance away from the frequency of the pilot tone signal by injection locking output light output from a local laser to a pilot tone signal and modulating the output light; and a second filter that removes optical signals other than the frequency of the sideband. [Selected Figure] Figure 4

Description

The present invention relates to an optical injection locking method, a local light generating device, a receiving device, and a local light generating method for use in ultra-multilevel digital coherent optical transmission.

In recent optical communications, research is being conducted into digital coherent multilevel transmission, which simultaneously encodes information in the amplitude and phase of light. Among the various multilevel modulation methods, quadrature amplitude modulation (QAM) in particular allows for the highest degree of modulation, and can maximize frequency utilization efficiency. In this type of highly efficient QAM transmission, highly accurate phase synchronization between the data signal and the local laser is essential to accurately extract the optical phase information.

As optical phase synchronization methods used in digital coherent multilevel transmission, the carrier phase estimation method using a digital signal processor (DSP) (Non-Patent Document 1), the optical phase-locked loop (OPLL) method (Non-Patent Document 2), and the optical injection locking method (Non-Patent Document 3) have been researched and developed so far.

The digital carrier phase estimation method has the advantage that it does not require a hardware feedback control circuit, but as the modulation speed and multi-value level of the data signal increase, the amount of calculations required by the DSP for carrier phase estimation increases. Furthermore, the accuracy of the phase estimation becomes insufficient, which causes degradation of the demodulation characteristics.

On the other hand, the analog optical PLL method, which uses a hardware feedback control circuit, has the advantage of enabling highly accurate optical phase synchronization that is independent of the multilevel level of the modulation signal or the modulation format. The optical injection locking method can achieve extremely accurate optical phase synchronization without using a feedback circuit. In addition, since there is no need to use an expensive narrow-linewidth semiconductor laser as the local laser, it is possible to reduce the cost of the transmission system, and this technology has attracted attention in the field of digital coherent optical transmission in recent years.

According to Non-Patent Document 4, coherent QAM transmission with an extremely high degree of multi-level (1024 values) is reported, in which a local laser is phase-locked to the data signal using an unmodulated pilot tone transmitted through an optical fiber simultaneously with the QAM data signal. However, in this method, in order to achieve homodyne detection of the QAM data, it is necessary to frequency-convert the oscillation frequency of the local laser, which is injection-locked to the pilot tone, using an optical frequency shifter. An optical modulator and an optical filter are used as the optical frequency shifter, and it is difficult to say that the configuration of the optical phase-locked system is necessarily simple.

K. Kikuchi, “Digital coherent optical communication systems: fundamentals and future prospects,” IEICE Electron. Express, vol. 8, no. 20, pp. 1642-1662, 2011. K. Kasai, J. Hongo, M. Yoshida, M. Nakazawa, “Optical phase-locked loop for coherent transmission over 500 km using heterodyne detection with fiber lasers,” IEICE Electron. Express, vol. 4, no. 3, pp .77-81, 2007. Y. Wang, K. Kasai, M. Yoshida, M. Nakazawa, “120 Gbit/s injection-locked homodyne coherent transmission of polarization-multiplexed 64 QAM signals over 150 km,” Opt. Express, vol. 22, no. 25 , p.31310, 2014. Y. Wang, S. Okamoto, K. Kasai, M. Yoshida and M. Nakazawa, “Single-channel 200 Gbit/s, 10 Gsymbol/s-1024 QAM injection-locked coherent transmission over 160 km with a pilot-assisted adaptive equalizer,” Opt. Express, vol. 26, no. 13, pp. 17015-17024, (2018).

Conventional optical injection locking circuits use relatively expensive optical modulation, which increases the cost of the system.
An object of the present invention is to provide an optical injection locking system, a local light generating device, a receiving device, and a local light generating method that realize coherent homodyne detection without using an optical modulator.

One aspect of the present invention is a local light generating device that includes a direct modulation light source unit that generates a sideband having a frequency that is a predetermined amount away from the frequency of the pilot tone signal by injection locking the output light output from a local laser to a pilot tone signal and modulating the output light, and a second filter that removes optical signals other than the frequency of the sideband.

According to the present invention, it is possible to generate local light that is phase-locked to a data signal and realize coherent homodyne detection without using an optical modulator.

FIG. 1 is a diagram illustrating an example of the configuration of a coherent multilevel optical transmission system 1 according to an embodiment of the 4 is a diagram showing an example of frequency characteristics of an optical signal transmitted by a transmitting device 12. FIG. 1 is a diagram showing an example of frequency characteristics of a QAM data signal and a local oscillator light for homodyne detection; FIG. 2 is a diagram illustrating an example of the configuration of a local light generating device 32. 4 is a flowchart showing an operation of the local light generating device 32.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram showing an example of the configuration of a coherent multi-level optical transmission system 1 according to the present embodiment.
The coherent multi-level optical transmission system 1 includes an optical transmission line 11, a transmitting device 12, and a receiving device 13. In the coherent multi-level optical transmission system 1, an optical signal transmitted by the transmitting device 12 is transmitted to the receiving device 13 via the optical transmission line 11. The optical transmission line 11 is, for example, an optical fiber.

The transmitting device 12 includes a DSP (Digital Signal Processor) 21 , a D/A converter 22 , a laser 23 , and a polarization multiplexing IQ modulator 24 .
The DSP 21 outputs the digital IQ data signal and the unmodulated pilot tone signal to the D/A converter 22. The D/A converter 22 converts the digital IQ data signal and the unmodulated pilot tone signal into analog signals. The D/A converter 22 outputs an analog baseband signal of an in-phase component (I) and a quadrature component (Q) and a pilot tone signal.
The output light of the laser 23 is input to a polarization multiplexing IQ modulator 24 and is IQ modulated by the analog baseband signal and pilot tone signal output from the D/A converter 22 .

With the above configuration, the transmitting device 12 transmits the QAM signal and the pilot tone signal to the receiving device 13 via the optical transmission path 11. Fig. 2 is a diagram showing an example of the frequency characteristics of the optical signal transmitted by the transmitting device 12. The optical signal transmitted by the transmitting device 12 includes a pilot tone signal of a single frequency _fTone and a QAM data signal having a spread around the frequency _fc .

The receiving device 13 includes a distributor 31 , a local light generating device 32 , a detector 33 , an A/D converter 34 , and a DSP 35 .
The distributor 31 distributes the signal received from the transmitter 12, and outputs the distributed signal to the local light generating device 32 and the detector 33. The local light generating device 32 generates local light having the center frequency of the QAM data signal (f _c in the example shown in FIG. 2 ) as a single frequency, based on the optical signal input from the distributor 31. A detailed configuration of the local light generating device 32 will be described later.

The detector 33 performs homodyne detection on the QAM data signal input from the distributor 31 based on the local light generated by the local light generator 32. Fig. 3 is a diagram showing an example of frequency characteristics of the QAM data signal and the local light for homodyne detection. Depending on the frequency of the local light, the detector 33 may perform heterodyne detection or intradyne detection.
The detector 33 outputs analog signals corresponding to I and Q in QAM to the A/D converter 34. The detector 33 includes, for example, a polarization diversity 90 degree optical hybrid circuit 331 and a balanced photodetector 332. The detector 33 causes interference between the local light and the QAM data signal by, for example, the polarization diversity 90 degree optical hybrid circuit 331, and extracts the in-phase component and quadrature component of the QAM data signal.

The A/D converter 34 converts the analog signal input from the detection unit 33 into a digital signal. The A/D converter 34 outputs the digital signal to the DSP 35. The DSP 35 processes the digital signal input from the A/D converter 34.

Figure 4 is a diagram showing an example of the configuration of the local light generating device 32. The local light generating device 32 includes a first filter 321, a circulator 322, a direct modulation light source unit 323, and a second filter 324.

The first filter 321 filters the optical signal input from the distribution unit 31 and removes the light of the frequency of the QAM data signal. The first filter 321 outputs the filtered optical signal to the circulator 322. The optical signal output from the first filter 321 to the circulator 322 does not include the QAM data signal, but only the pilot tone signal.

The circulator 322 outputs the optical signal input from the first filter 321 to the direct modulation light source unit 323. The circulator 322 outputs the optical signal input from the direct modulation light source unit 323 to the second filter 324.

The direct modulation light source unit 323 includes, for example, a gain switching type semiconductor laser or an electroabsorption modulator integrated semiconductor laser. The direct modulation light source unit 323 performs injection locking by forcibly injecting a pilot tone signal input from the circulator 322 into the self-oscillation system. The direct modulation light source unit 323 performs injection locking of output light output from, for example, a local laser to the pilot tone signal. This synchronizes the self-oscillation system with the pilot tone signal. This generates local light at the frequency of the pilot tone signal, and modulates the output light in the direct modulation light source unit 323 to generate single-frequency light (sideband) even at a frequency that is a predetermined magnitude away from the frequency of the pilot tone signal. The direct modulation light source unit 323 is set so that the frequency of the sideband is the center frequency f _c of the QAM data signal. Depending on the setting of the frequency of the sideband by the direct modulation light source unit 323, the detection unit 33 may perform heterodyne detection or intradyne detection.
The direct modulation light source unit 323 outputs the directly modulated signal to the circulator 322 .

The optical signal directly modulated by the direct modulation light source unit 323 is input to the second filter 324 via the circulator 322. The second filter 324 filters the optical signal input from the circulator 322 to remove light other than the light having the frequency of the QAM data signal. The second filter 324 outputs the filtered optical signal to the detection unit 33.
The frequency of light removed by the second filter 324 may be changed depending on the frequency of the sideband generated by the directly modulated light source section 323 .

As a result, the local light generating device 32 generates local light having a specific frequency, such as the center frequency of the QAM data signal, as a single frequency.

Figure 5 is a flowchart showing the operation of the local light generating device 32. The first filter 321 filters the optical signal input from the distributor 31 and removes the light of the frequency of the QAM data signal (step S101). Then, the direct modulation light source unit 323 injection-locks the output light output from the direct modulation light source unit 323 to the pilot tone signal (step S102). Then, the direct modulation light source unit 323 modulates the injection-locked output light to generate a sideband (step S103). If the direct modulation light source unit 323 includes a gain-switching semiconductor laser, it modulates the bias current to generate a sideband. If the direct modulation light source unit 323 includes an electric field-absorption modulator integrated semiconductor laser, it applies an electric field to the modulator unit associated with the electric field absorption modulator integrated semiconductor laser to modulate the output light and generate a sideband. Then, the second filter 324 removes light other than the light of the frequency of the QAM data signal from the directly modulated optical signal (step S104).

As described above, the local light generating device 32 can generate sidebands having a frequency set by the direct modulation light source unit 323 by using the pilot tone signal to perform injection locking by the direct modulation light source unit 323. This can further simplify the configuration of the coherent multilevel optical transmission system 1.

Other Embodiments
Although one embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to the above, and various design changes, etc. are possible within the scope that does not deviate from the gist of the present invention.

1 Coherent multilevel optical transmission system, 11 Optical transmission path, 12 Transmitter, 13 Receiver, 21 DSP (Digital Signal Processor), 22 D/A converter, 23 Laser, 24 Polarization multiplexing IQ modulator, 31 Distribution section, 32 Local light generating device, 321 First filter, 322 Circulator, 323 Direct modulation light source section, 324 Second filter, 33 Detector, 331 Polarization diversity 90 degree optical hybrid circuit, 332 Balanced photodetector, 34 A/D converter, 35 DSP (Digital Signal Processor)

Claims (5)

a direct modulation light source unit that generates a sideband having a frequency that is separated by a predetermined amount from the frequency of the pilot tone signal by injection locking output light output from a local laser to a pilot tone signal and modulating the output light;
and a second filter for removing optical signals other than the sideband frequency. The local light generating device according to claim 1, further comprising a first filter that removes the QAM data signal from a signal including the pilot tone signal and the QAM data signal, and outputs the pilot tone signal to the direct modulation light source unit. the sideband frequency is the center frequency of the QAM data signal;
the sideband and the QAM data signal are subjected to homodyne detection;
The local light generating device according to claim 2 . The local light generating device according to claim 1 ,
A detection unit that detects a QAM data signal based on a local light generated by the local light generating device;
a distributor that distributes a signal including the pilot tone signal and the QAM data signal and outputs the signal to the local light generating device and the detector;
A receiving device comprising: The output light from a local laser, which is a directly modulated light source, is injection locked to a pilot tone signal,
modulating the output light with the direct modulation light source to generate a sideband having a frequency that is a predetermined amount different from the frequency of the pilot tone signal;
removing optical signals other than the sideband frequency;
A method for generating local light.

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