WO2019188192A1

WO2019188192A1 - Optical transmitter

Info

Publication number: WO2019188192A1
Application number: PCT/JP2019/009706
Authority: WO
Inventors: 山崎　裕史; 橋本　俊和
Original assignee: 日本電信電話株式会社
Priority date: 2018-03-29
Filing date: 2019-03-11
Publication date: 2019-10-03
Also published as: JP2019174747A

Abstract

The present invention provides an optical SSB transmitter which sufficiently suppresses fading caused by wavelength dispersion by a simple configuration. This optical transmitter is provided with: an electric signal generation means which generates a driving electric signal comprising an RF carrier wave with a frequency f_s and a baseband modulation signal component with a frequency f_s or less; a laser light source which oscillates at a frequency f₀; a Mach-Zehnder modulator which is biased at a Null point and push-pull driven by the driving electric signal, and modulates light outputted from the laser light source; and an optical filter which is connected to a stage subsequent to the Mach-Zehnder modulator, has a transmission bandwidth of f_s or more, and has a transition region between a transmission band and a stop band, the transition region being present within a frequency range of frequencies from f₀-f_s to f₀ or frequencies from f₀ to f₀+f_s.

Description

Optical transmitter

The present invention relates to an optical transmitter used for optical fiber communication. More specifically, the present invention relates to an optical SSB transmitter that performs optical SSB modulation.

In recent years, due to the increasing demand for communication between data centers, etc., there is an interest in technology for realizing large-capacity and low-cost optical transmission with a transmission distance of several tens to several hundreds km. In particular, in recent years, a technique for realizing the above by an intensity modulation / direct detection (IMDD) system having a simple configuration of an optical transceiver has been actively studied.

In this transmission distance region, it is desirable to use the C band (wavelength 1.5 μm band) which is the lowest loss band of the optical fiber, but the influence of the wavelength dispersion of the optical fiber becomes a problem in the C band. In particular, in normal IMDD, so-called fading, in which the intensity of a specific frequency component of a signal spectrum is significantly attenuated in a baseband signal after intensity detection due to wavelength dispersion, is a serious problem.

This is a phenomenon that occurs because a normal light intensity modulation signal is a double sideband (DSB) modulation signal. For this reason, it is known to perform one sideband (Single Sideband, SSB) modulation that removes one of the sidebands of the optical signal spectrum as one countermeasure against fading.

In order to generate an optical SSB signal, a method of using an optical IQ modulator (sometimes referred to as an SSB modulator) as in Non-Patent Document 1 and removing one sideband by interference between an I component and a Q component It has been known. Optical SSB modulation based on the same principle can be performed by driving the arms of a Mach-Zehnder modulator (MZM) with drive signals whose phases are shifted from each other as in Non-Patent Document 2.

Further, as in Non-Patent Document 3 or Non-Patent Document 4, an optical IQ modulator is used to suppress the light source frequency component simultaneously with the modulation, and instead, an optical carrier is set up at one of the band edges of the modulated signal spectrum and transmitted. A method for generating an optical SSB signal by effectively shifting the optical carrier frequency is also known.

Furthermore, as in Non-Patent Document 5, a method of attenuating one sideband with an optical filter after generating a normal optical DSB signal is called a VSB (Vestinal Sideband) method, which is also a kind of SSB, Signal degradation due to fading can be reduced.

However, the method for generating an optical SSB or VSB signal in a conventional optical SSB transmitter has the following problems.

First, FIG. 1 shows a configuration (a) of an optical SSB transmitter using an IQ modulator as an optical SSB transmitter of Conventional Example 1, and spectrums (b) and (c) of signals of each part. The conventional optical SSB transmitter 10 shown in FIG. 1A uses an IQ modulator 12 to convert a laser light carrier of, for example, f ₀ = 193 THz from a laser light source 11 and a modulated electrical signal (b) from the left side by 90 °. The optical SSB signal (c) whose spectrum is shown at the right end is generated by optical modulation with the two systems of electrical signals shifted from each other.

In the conventional optical SSB transmitter 10 in FIG. 1A, it is necessary to generate two systems of modulated electrical signals whose phases are shifted by 90 degrees in order to drive the IQ modulator 12, There is a problem that the configuration of the drive system becomes complicated, such as the need for two driver amplifiers. This problem also applies to a conventional optical SSB transmitter using a Mach-Zehnder modulator (MZM) driven by two systems.

FIG. 2 shows an optical VSB transmitter (a) using an optical filter as an optical SSB transmitter of Conventional Example 2, and spectrums (b) to (d) of signals of each part. In the optical VSB transmitter 20 of FIG. 2 (a), a modulated electric signal having a spectrum S (f) from the left side is applied to a laser light carrier of, for example, about f ₀ = 193 THz from the laser light source 21 in the light intensity modulator 22. The optical intensity is modulated in (b) to generate a DSB optical signal (c) having a spectrum in the center.

DSB optical signal shown in FIG. 2 (c), has a light spectrum with sidebands symmetrical spectral shape across the optical carrier of the central frequency f _0, the light of the lower sideband (LSB) It can be said that the signal S ^* (− f + f ₀ ) and the optical signal S (f−f ₀ ) in the upper sideband (USB) are in a complex conjugate pair relationship, and one is the other complex conjugate signal. When this DSB optical signal is passed through an optical filter 23 having a transmission spectrum indicated by a dotted line in FIG. 2D, the lower sideband is partially suppressed to generate a VSB optical signal (d).

The conventional optical VSB transmitter 20 shown in FIG. 2 is simple because only one optical modulator drive system is required, but generally the transmission characteristics of the optical filter are not sufficiently sharp with respect to the signal bandwidth. The sideband suppression becomes incomplete, and part of the LSB remains, fading suppression becomes incomplete, and there is a problem that fading cannot be suppressed.

The present invention has been made in view of such problems, and an object of the present invention is to provide an optical SSB transmitter that sufficiently suppresses fading due to chromatic dispersion while having a simple configuration. is there.

In order to achieve such an object, the present invention is characterized by having the following configuration.

(Structure 1 of the invention)
And an electric signal generating means for generating a driving electrical signal containing a component of the RF carrier and frequency f _s following baseband modulation signal of frequency f _s,
A laser light source that oscillates at a frequency f ₀ ;
A Mach-Zehnder modulator that is biased to a null point, is push-pull driven by the drive electrical signal, and modulates light output from the laser light source;
Which is connected downstream of the Mach-Zehnder modulator, a transmission bandwidth f _s or more, and either a transition region of the transmission band and blocking band from the frequency f ₀ -f _s from f ₀ or frequency f ₀ of f ₀ + f _s And an optical filter existing in the frequency range of the optical transmitter.

(Configuration 2)
The electrical signal generating means includes
A DSP that synthesizes and generates the RF carrier and the baseband modulated signal as a digital signal;
The optical transmitter according to Configuration 1, further comprising: a DAC that converts a digital signal generated by the DSP into an analog signal.

(Structure 3 of the invention)
The electrical signal generating means includes
RF carrier wave generating means for generating the RF carrier wave as an analog signal;
A DSP for generating the baseband modulation signal as a digital signal;
A DAC that converts the baseband modulation signal generated by the DSP into an analog signal;
The optical transmitter according to Configuration 1, further comprising an adder that adds the output of the RF carrier generation means and the output of the DAC as an analog signal.

(Configuration 4)
The electrical signal generating means includes
RF carrier wave generating means for generating an RF carrier wave of the frequency fs as an analog signal;
The optical transmitter according to Configuration 1, further comprising analog modulation signal generation means for generating the baseband modulation signal as an analog signal.

As described above, according to the present invention, it is possible to realize an optical SSB transmitter capable of sufficiently suppressing fading due to chromatic dispersion while having a simple configuration in which an MZM is driven by a single electric signal. Can do.

It is the figure (a) which shows the structure of the conventional optical SSB transmitter, and the figure which shows the spectrum of the signal of each part (b), (c). FIG. 2A is a diagram showing a configuration of a conventional optical VSB transmitter, and FIGS. 2B to 2D are diagrams showing signal spectra of respective units. FIG. 2A is a diagram illustrating a configuration of an optical SSB transmitter according to a first embodiment of the present invention, and FIGS. 2B to 2D are diagrams illustrating signal spectra of respective units. FIG. 4A is a diagram showing a configuration of an optical SSB transmitter according to a second embodiment of the present invention, and FIG. 5B is a diagram showing a spectrum of a signal at each section.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[First embodiment]
FIG. 3 is a diagram schematically showing the configuration (a) of the optical SSB transmitter according to the first embodiment of the present invention and the spectrums (b) to (d) of the signals of each part. The optical SSB transmitter 300 in FIG. 3A includes a drive signal generation unit 310, a laser light source 320, an MZM (Mach-Zehnder modulator) 330, and an optical filter 340.

The drive signal generation unit 310 is an electric signal generation unit that generates a drive electric signal for driving the MZM 330, and includes a DSP (digital signal processing circuit) 311 and a DAC (digital-analog converter) 312. An amplifier may be used after the DAC 312 as necessary. As shown in the spectrum of FIG. 3B, the drive signal output from the DAC 312 includes a baseband modulation signal 381 having a bandwidth B and an RF carrier 382 having a frequency f _s ≧ B.

The MZM 330 is a push-pull type (zero chirp type), and the DC bias is adjusted to a null point (a point at which the intensity of the light source frequency component is minimum). As a result, in the DSB optical signal output from the MZM 330, the component of the light source frequency f ₀ is suppressed as shown in FIG. 3C, and instead, the RF carrier 382 is added to the side band edge f ₀ ± f _s. The derived optical carrier is shifted and stands.

The transmission spectrum of the optical filter 340 is indicated by a dotted line in FIG. 3D, and the transmission spectrum is adjusted so as to suppress one of the shifted optical carriers. Hereinafter, in this example the descriptions which suppresses the optical carrier of the USB side of f ₀ + f _s. At this time, the transmission spectrum of the optical filter 340 transmits the optical carrier of f ₀ -f _s on the LSB side and the optical signal component 384 in the range of f ₀ -f _s to f ₀ , and f ₀ + f _s on the USB side. As long as the optical carrier is suppressed, it is sufficient. That is, as the optical filter 340, an optical filter having a transmission bandwidth of f _s or more and a transition region between the transmission band and the stop band in the range of frequencies f ₀ to f ₀ + f _s may be used. Here, the difference in light transmittance between the transmission band and the blocking band is typically 20 dB or more. Hereinafter, the frequency of the optical carrier wave is assumed to be f _c = f ₀ −f _s .

The output light of such an optical filter 340, as shown in the spectrum of FIG. 3 (d), the results in the presence of positive only signal components viewed from f _c on the frequency axis, this by f _c An optical SSB signal 384 serving as an optical carrier wave is generated. Here, the optical filter 340, an optical carrier of f ₀ + f _s may be sufficient even be suppressed.

Of the optical signal output from the optical filter 340, the optical SSB signal component 384 in the range of the frequency f ₀ -f _s to f ₀ is used for data transmission, and the image component 385 having the frequency f ₀ or higher is essentially unnecessary. is there. Since this image component 385 exists on the same side as the optical signal component 384 on the frequency axis when viewed from the optical carrier wave with f _c = f ₀ -B, it does not cause fading, and is analog or It can be easily removed using a low-pass filter or the like in the digital domain. Furthermore, the image component 385 can be used in principle for improving signal quality by using diversity combining as shown in Non-Patent Document 6.

As in general optical SSB transmission, the intensity of the optical carrier wave with f _c = f ₀ -B is sufficiently larger than the intensity of other signal components, that is, the sum of the intensity of the optical signal component 384 and the image component 385. adjust. In general, in optical SSB transmission, the penalty due to signal-signal beat interference (SSBI) increases as the optical carrier strength decreases with respect to the strength of other signal components. The ratio with the intensity is +10 dB or more, but the specific setting of the intensity ratio depends on the required specifications of the system and the processing method on the receiving side. This is the same as the conventional optical SSB transmission. However, it should be noted that in this example, the intensity of the optical carrier must be increased by an amount corresponding to the intensity of the image signal 385 compared to the conventional example.

(Relationship between baseband modulation signal and optical signal component)
The relationship between the baseband modulation signal 381 and the output optical signal component 384 in the first embodiment will be described. In the following description, for simplicity, the frequency dependence and nonlinearity of the MZM response are ignored, and the scaling factor is omitted.

In conventional optical SSB transmission, when the positive frequency component of the spectrum of the input signal to the optical transmitter is represented by S (f) (see FIG. 1B), the output optical SSB signal is simply expressed as + f. _c only those upconverted, i.e. positive frequency component of the spectrum is S (f-f _c) + signal as expressed in the optical carrier frequency f _c] (see Fig. 1 (c)).

On the other hand, the optical signal component 384 (see FIG. 3D) output in the first embodiment is obtained by shifting the negative frequency component of the baseband modulated signal 381 (see FIG. 3B) by + f _0. Equivalent to. Therefore, if the signal input to the DSP 311 is a signal whose positive frequency component of the spectrum is represented by S (f), simply converting this to DA and adding an RF carrier as it is will result in an output optical SSB signal. A signal equivalent to the conventional method cannot be obtained.

When it is desired to obtain an optical signal whose positive frequency component is represented by S (f−f _c ) as the optical signal component 384 in FIG. A signal whose frequency component is S ^* (− f + f _s ), that is, a signal represented by a waveform obtained by inverting S (f) around f _s / 2 on the frequency axis and taking a complex conjugate may be used. This is because the negative frequency component of the spectrum of the baseband modulation signal 381 at this time is S (f + f _s ), and if this is frequency shifted by + f _0, S (f + f _s −f ₀ ) = S (f−f _c ). . That is, the driving electric signal generated by the electric signal generating means (310) is an RF carrier having a frequency f _s and a complex conjugate signal S ^* (−f) of the modulated electric signal S (f) only at the frequency f _{s of the} RF carrier. The shifted baseband modulated signal S ^* (− f + f _s ) is included. Here, the bandwidth B of the drive electrical signal is equal to or less than the frequency f _{s of the} RF carrier.

Note that the process of inverting the spectrum as described above to obtain the complex conjugate may be performed not on the transmission side but on the reception side. That is, if a signal whose spectrum positive frequency component is represented by S (f) is used as it is as the baseband modulation signal 381, the spectrum positive frequency component is represented by S ^* (− f + f ₀ ) as the optical signal component 384. As a result, the signal having the positive frequency component of the spectrum represented by S ^* (− f + f _s ) is obtained as an electrical signal on the receiving side. At this time, if the electric signal is inverted on the frequency axis around f _s / 2 and the complex conjugate is taken, the positive frequency component of the spectrum is S (f) as in the original baseband modulation signal. A signal as represented can be obtained. Such reception-side processing can be realized, for example, by using a receiver including an analog-digital converter (ADC) and a DSP.

The driving electric signal obtained by adding the RF carrier 382 to the baseband modulation signal 381 as described above is generated as a digital signal by the DSP 311 in FIG. 3A and converted into an electric waveform of an analog signal by the DAC 312 to be converted into an MZM 330. Used for driving.

As described above, in the optical transmitter according to the first embodiment, unlike the conventional optical SSB signal generation using the IQ modulator and the dual-system drive MZM, the drive system is achieved by using the single-system drive push-pull MZM330. To keep it simple. In addition, although the optical filter 340 is used, unlike the conventional VSB method, since the optical carrier wave is set at the band edge in the MZM 330, the residual component due to the “round” of the optical filter 340 does not cause fading. That is, according to the first embodiment of the present invention, the above-described problems of the conventional technology can be solved.

In the above description, the description has been made on the assumption that the optical carrier of f ₀ + f _s is suppressed among the two optical carriers included in the output of the MZM 330 and f _c = f ₀ −f _s is used as the optical carrier. It is also possible to suppress the optical carrier of f ₀ −f _s and use f _c = f ₀ + f _s as the optical carrier. As the time optical filter 340, the optical signal component of the range of the optical carrier and f ₀ of f ₀ + f _s of the f ₀ + f _s is transmitted, and the optical carrier of f ₀ -f _s is used as such as to suppress That's fine. The baseband modulation signal 381 does not need to be changed.

[Second Embodiment]
FIG. 4 is a diagram schematically showing the configuration (a) of the optical SSB transmitter according to the second embodiment of the present invention and the spectrums (b) to (e) of the signals of the respective parts. The configuration of the optical SSB transmitter 400 in FIG. 4A is the same as that of the first embodiment shown in FIG. 3A except for the drive signal generator 410, and therefore only the drive signal generator 410 will be described. To do.

In the second embodiment, the drive signal generation unit 410 in FIG. 4A is an electric signal generation unit that generates a drive electric signal for driving the MZM 430, and includes a DSP 411, a DAC 412, an RF carrier wave generation unit 413, and an adder. 414. In the second embodiment, the DSP 411 and the DAC 412 are used only for generating the baseband modulation signal 481 in FIG. 4B, and the RF carrier 482 in FIG. 4E is generated separately by the RF carrier generation unit 413. The adder 414 adds the baseband modulation signal 481 (b).

As described above, since the intensity of the optical carrier needs to be sufficiently larger than the other signal intensities, the intensity of the RF carrier 482 needs to be sufficiently greater than the intensity of the baseband modulation signal 481.

For this reason, the DSP 411 and the DAC 412 are used only for the generation of the baseband modulation signal 481, and the RF carrier 482 is separately generated as an analog signal by the RF carrier generation unit 413, and is added by the adder 414 as an analog signal in the analog domain. This reduces the burden (circuit scale and power consumption) on the DSP 411 and DAC 412 in terms of the bit width and dynamic range of the digital signal, and the quality (signal-to-noise ratio) of the baseband modulation signal 481 compared to the first embodiment. Etc.) can be improved.

As the RF carrier wave generation unit 413, for example, a clock signal source synchronized with the DAC 412 can be used. Further, in order to adjust the intensity and phase of each of the baseband modulation signal 481 and the RF carrier wave 482, an amplifier, a phase shifter, and the like may be disposed in front of the adder 414.

In the second embodiment and the first embodiment, the DSP 411 (311) and the DAC 412 (312) positively correct the positive frequency component S (f−f _c ) of the spectrum of the signal to be transmitted as the optical signal component 484 (384). The method of generating a signal whose frequency component is represented by S ^* (− f + f _s ) and using it as the baseband modulation signal 481 (381) has been described. The processing corresponding to this is performed on the receiving side, such as ADC and DSP. You may implement | achieve by using.

That is, if an arbitrary signal whose positive frequency component is represented by X (f) is used as the baseband modulation signal 481 in the configuration of the second embodiment, the positive frequency component is X ^* (− in the conventional SSB transmitter. Since a signal corresponding to a signal represented by f + f _s ) is transmitted, the negative frequency component of the baseband received signal after intensity detection is shifted by + f _s on the receiving side. After performing the processing to make the positive frequency component, necessary processing such as channel equalization may be performed.

Further, in the case of the configuration in which the RF carrier wave generation unit 413 is separated as in the second embodiment, a modulation method that can be realized without performing a spectrum operation on the receiving side and using a DAC such as NRZ as a baseband modulation method. If used, the DSP 411 and the DAC 412 can be replaced with an analog modulation signal generating unit that operates in an analog manner and generates a baseband modulation signal as an analog signal.

According to the present invention described above, it is possible to realize an optical SSB transmitter capable of sufficiently suppressing fading due to chromatic dispersion while having a simple configuration in which an MZM is driven by one electric signal.

10, 20, 300, 400

Optical SSB transmitter

11, 21, 320, 420 Laser light source 12 IQ modulator 22

Optical intensity modulator

23, 340, 440

Optical filter

310, 410

Drive signal generator

330, 430 MZM (Mach-Zehnder modulation) vessel)
311 and 411 DSP (digital signal processing circuit)
312 and 412 DAC (digital-analog converter)
381, 481

Baseband modulation signal

382, 482

RF carrier wave

384, 484

Optical signal component

385, 485 Image component 413 RF carrier wave generator 414 Adder

Claims

And an electric signal generating means for generating a driving electrical signal containing a component of the RF carrier and frequency f s following baseband modulation signal of frequency f s,
A laser light source that oscillates at a frequency f 0 ;
A Mach-Zehnder modulator that is biased to a null point, is push-pull driven by the drive electrical signal, and modulates light output from the laser light source;
Which is connected downstream of the Mach-Zehnder modulator, a transmission bandwidth f s or more, and either a transition region of the transmission band and blocking band from the frequency f 0 -f s from f 0 or frequency f 0 of f 0 + f s And an optical filter existing in the frequency range of the optical transmitter.
The electrical signal generating means includes
A DSP that synthesizes and generates the RF carrier and the baseband modulated signal as a digital signal;
The optical transmitter according to claim 1, further comprising a DAC that converts a digital signal generated by the DSP into an analog signal.
The electrical signal generating means includes
RF carrier wave generating means for generating the RF carrier wave as an analog signal;
A DSP for generating the baseband modulation signal as a digital signal;
A DAC that converts the baseband modulation signal generated by the DSP into an analog signal;
The optical transmitter according to claim 1, further comprising an adder that adds the output of the RF carrier wave generation unit and the output of the DAC as an analog signal.
The electrical signal generating means includes
RF carrier wave generating means for generating an RF carrier wave of the frequency f s as an analog signal;
The optical transmitter according to claim 1, further comprising: an analog modulation signal generation unit configured to generate the baseband modulation signal as an analog signal.