JP2020150316A - Optical transmitter receiver and optical transmission and reception method - Google Patents

Abstract

To obtain an optical transmitter receiver capable of obtaining an uplink optical signal, from which downlink high frequency modulation signal components are completely removed, without complicating the structure on the optical receiver side, in optical transmission.SOLUTION: In an optical transmitter receiver where broadband optical signal, generated by an OLT 10 by using a WDM-PON using multiple wavelengths, is transmitted by means of an optical fiber 20 and received by multiple ONUs 30, the ONU 30 includes a signal branch part 311 for branching the optical signal into a carrier component and multiple signal components by dividing the optical signal into a path imparted with a differential delay τ and a path imparted with a phase rotation φ, a signal multiplexer 312 performing signal multiplexing again of the carrier component and multiple signal components, and a return branch part 313 using only the carrier component of the optical signal as the optical signal for uplink.SELECTED DRAWING: Figure 1

Description

The present invention relates to a transmission system using wavelength division multiplexing using an optical fiber, and particularly in a public network using an optical fiber, an optical signal is branched and merged by a passive element, and one optical fiber line is connected by a plurality of subscribers. The present invention relates to an optical transmission / reception device and an optical transmission / reception method suitable for wideband signalization (multiplexing) of a shared Passive optical network (hereinafter referred to as “PON”).

Currently, PONs used in FTTH lines that lay optical fibers from base stations of telecommunications carriers to each household are time-division multiplexed using a single wavelength with a branching device interposed in the middle of the optical fiber network. It is shared by multiple users. In order to expand the usable band in the future, the use of WDM-PON using a plurality of wavelengths is being studied.

A generally known problem with WDM-PON is communication from ONU (Optical Network Unit: optical line terminal on the user side) to OLT (Optical Line Terminal: optical line terminal on the accommodation station side). The stability of the wavelength light source for uplink is mentioned. Since it is necessary for the OLT side in WDM-PON to accurately receive signals from each ONU side arriving at various powers, it is necessary that the wavelength is accurately controlled in the vertical communication. However, since the ONUs are physically separated from the OLT, it is difficult to accurately control the wavelengths of each other.

In order to solve this problem, a method of reusing a downlink optical signal as an uplink optical signal by using a reflective semiconductor optical amplifier (RSOA) is known. By this method, since the upstream wavelength becomes the same as the downstream wavelength, the upstream wavelength can be grasped and the above-mentioned problem can be solved. However, when this method is used, since the downlink signal contains not only the carrier component of the light reused for the uplink signal but also the modulation component, the modulation component must be suppressed in order to perform accurate control. There is a problem that it does not become.

Non-Patent Document 1 discloses that when the downlink optical signal is reused for the uplink signal, the downlink modulation component is suppressed by intentionally using the saturation region of RSOA as a method for suppressing the modulation component. Has been done.

Further, Non-Patent Document 2 discloses that bandwidth expansion can be realized by making the structure of RSOA a two-section type.

S. O Duill et al, "Efficient modulation cancellation using reflective SOAs," Opt. Express 20, B587-B594 (2012). P. Zhou et al, "Two-section RSOA with enhanced modulation-cancelling effect for self-seeded colorless WDM transmitter," in Proceedings of European Conference on Optical Communication (ECOC), paper M.2.E (2016).

However, according to the method described in Non-Patent Document 1, there is a problem that the modulation band is sacrificed and it is difficult to apply it to a wide band signal. For example, on the receiving side of an FTTH line, when a base station is accommodated by WDM-PON in addition to users in each home, Radio-over Fiber (RoF) technology and WDM-PON may be used together. In the situation where RoF and WDM-PON are used in combination, it is difficult to apply this method because the RF signal of RoF usually reaches several tens of GHz.

When the bandwidth expansion is realized by making the structure of RSOA proposed in Non-Patent Document 2 into a two-section type, there arises a problem that device creation becomes complicated and costs increase.

The present invention has been proposed in view of the above circumstances, and an object of the present invention is to provide an optical transmission / reception device and an optical transmission / reception method suitable for use of a wideband signalized (multiplexed) WDM-PON.

In order to achieve the above object, the optical transmitter / receiver according to claim 1 of the present invention uses an optical fiber (20) to transmit a wideband optical signal generated by an optical transmitter (OLT10) by using WDM-PON using a plurality of wavelengths. It is an optical transmitter / receiver that transmits and receives light from a plurality of optical receivers (ONU30), and is characterized in that the optical receiver (ONU30) includes the following parts.
A signal branching portion (311) that branches the optical signal into a carrier component and a plurality of signal components by dividing the optical signal into a path to which a delay difference τ is given and a path to which a phase rotation φ is given. ..
A feedback branching portion (313) that uses only the carrier component of the optical signal as an uplink optical signal.

2. The optical transmitter / receiver according to claim 1
The delay difference τ and the phase rotation φ are obtained when the angular frequency of the carrier component is ωC and the modulation frequency of the signal component is ωRF.
τ = π / ωRF
φ ＝ −ωCτ
It is characterized by satisfying.

A third aspect of the present invention is the optical transmission / reception device according to the first aspect.
The optical receiver (ONU30) includes a reflective semiconductor optical amplifier (33).
The reflection type semiconductor optical amplifier (33) is characterized in that the carrier component of the optical signal is fed back as an uplink optical signal.

4. The optical transmitter / receiver according to claim 1 to 3.
The optical receiver (ONU30) is a base station.

A fifth aspect of the present invention is an optical transmission / reception method in which a wide band optical signal generated by an optical transmitter by using WDM-PON using a plurality of wavelengths is transmitted by an optical fiber and received by a plurality of optical receivers.
The optical signal transmitted by performing only intensity modulation is branched into a carrier component and a signal component, and then
The signal component and the carrier component are combined again to form a receiving side signal, and at the same time,
It is characterized in that only the carrier component of the optical signal is fed back and used as an upstream optical signal.

According to the present invention, by branching only the carrier component of the downlink optical signal and using it as the uplink optical signal, the downlink high-frequency modulated signal component is used without complicating the structure on the optical receiver side. Can be completely removed, so that an optical transmission / reception system suitable for WDM-PON using a plurality of wavelengths can be obtained.

It is a block diagram which shows the system structure of the optical transmission / reception device of this invention.

An optical transmission system (optical transmission method) using the optical transmission / reception device of the present invention will be described with reference to FIG.
The optical transmission system includes an OLT 10 on the accommodating side that transmits an optical signal of multiple wavelengths, an optical fiber 20 that transmits an optical signal of multiple wavelengths, and a plurality of optical signals that receive each optical signal branched according to the wavelength by a WDM splitter 21. It is configured to include an ONU 30 on the user side of the above. From the OLT10 side, RoF transmission using WDM-PON is assumed, and when a signal of a certain wavelength is transmitted from the OLT 10 to the ONU30 functioning as a base station, the signal is extracted by the WDM splitter 21 and then each ONU30. To reach.

The OLT 10 includes a transmitter 11 that transmits a two-sided band signal that has been subjected to intensity modulation only, and a photodiode (PD) 13 as a receiver that receives an uplink signal via a circulator 12. The multi-wavelength double-sided band signal transmitted from the transmitter 11 is transmitted to the optical fiber 20. The transmitted two-sided band signal Ein (t) can be represented by a function of Equation 1.

However, Ac represents the carrier component of light, and ARF represents the amplitude of the modulated signal component. Further, in Equation 1, a sine wave signal is assumed as the modulation signal, but even if the signal has a band component, if the modulation frequency is sufficiently large with respect to the frequency width, Equation 1 is approximately used. Can be represented.

The ONU 30 includes an asymmetric Mach-Zehnder interferometer 31 that branches an optical signal, a photodiode (PD) 32 as a photoelectric converter, and a reflective semiconductor optical amplifier 33 that feeds back an optical signal for uplink.

The asymmetric Mach-Zehnder interferometer 31 divides the optical signal into a path to which a delay difference τ is given and a path to which a phase rotation φ is given, thereby dividing the optical signal into a carrier component and a modulation component (signal component). It is provided with a signal branching section 311 that branches into a signal branching section 311, a signal joining section 312 that re-merges a carrier component and a modulation component, and a feedback branching section 313 that uses only the carrier component of the optical signal as an uplink optical signal. There is.
Further, it is provided with a control system that adjusts the values of the angular frequency ωC and the modulation frequency ωRF in the asymmetric Mach-Zehnder interferometer 31 to appropriate values described later by detecting the wavelength of the branched carrier component. The optical signal synthesized by the signal combiner 312 is input to the photodiode (PD) 32 as a downlink optical signal and converted into an electric signal.

An electrode or a temperature changing portion for adjusting the delay difference τ and the phase rotation φ is installed in the optical fiber of each path. The phase rotation φ is adjusted by changing the refractive index of the optical fiber by adjusting the voltage applied to the electrodes or the temperature by a heater or the like to change the optical path length. The delay difference τ is realized by giving a delay difference according to the modulation frequency ωRF on the optical circuit in advance, but in order to exactly match the delay difference according to ωRF, φ is displayed on the optical circuit. Similarly, an electrode for fine adjustment or a temperature changing part is provided.

The signal component E1 (t) and the carrier component E2 (t) branched by the signal branching portion 311 can be represented by the functions of the equation 2 and the equation 3, respectively.

The optical signal passes through the optical fiber (transmission line) 20 from the OLT 10 side and is guided to the ONU 30 side. A two-sided band signal is one in which sidebands are generated on both sides of a carrier wave on the spectrum when the carrier wave is amplitude-modulated with a signal wave current, and this is transmitted together with the carrier wave. In multicarrier transmission, such a plurality of double-sided band signals are simultaneously conveyed.

In the present invention, if only the carrier component of the branched optical signal is fed back as an uplink optical signal, an uplink optical signal in which the high-frequency modulation signal component is completely removed can be obtained without using a complicated circuit. It focuses on that.
The optical signal received by the ONU 30 is guided to the asymmetric Mach-Zehnder interferometer 31, and in the signal branching portion 311, a delay difference τ is given to the lower pass in the upper pass, and a phase rotation φ is given in the lower pass. It is configured as follows.
After that, the upper and lower paths are led to the 2x2 coupler. At this time, if "τ" and "φ" satisfy the following conditions (1) and (2), the output of one side of the 2x2 coupler Only the carrier component of the optical signal is output from, and only the signal component is output from the output on the other side.

τ = π / ωRF equation (1)
φ ＝ −ωCτ equation (2)

In the equation, ωC means the angular frequency of the carrier of the optical signal, and ωRF means the modulation frequency of the signal component of the optical signal.

In order to realize this configuration, it is necessary to adjust the asymmetric Mach-Zehnder interferometer 31 so that the delay difference τ and the phase rotation φ satisfy the equations (1) and (2), and therefore the angle. Detect frequency ωC and modulation frequency ωRF. The angular frequency ωC is detected from the wavelength of the carrier component obtained at the signal branching unit 311, and the modulation frequency ωRF is detected from the electric signal (RF power) output from the photodiode (PD) 32.

The angular frequency ωC of the carrier is detected from the wavelength from the carrier component. For example, a spectrum analyzer detects the frequency at which the voltage peaks and sets it as the angular frequency ωC.

The modulation frequency ωRF detects the frequency at which the output from the output RF power, which is an electric signal after being received by the PD 32, is maximized, and is set as the modulation frequency ωRF.

Since "τ" and "φ" are determined by the equations (1) and (2) from the detected angular frequency ωC and modulation frequency ωRF, control by the control system so that the delay difference τ and the phase rotation φ become predetermined values. Is done. This control is performed by adjusting the voltage applied to the electrodes installed at positions sandwiching each path after the signal branching portion 311 and adjusting the refractive index of the optical fiber of each path by changing the ambient temperature. Will be done.

In the ONU 30, a reflective semiconductor optical amplifier (RSOA) 33 is connected to the tip of the feedback branching portion 313. The reflection type semiconductor optical amplifier includes a reflection unit (mirror), a modulation unit, and an amplification unit. The carrier component branched via the feedback branching unit 313 is reflected by the reflection unit, modulated into a folded signal, and amplified. The signal is an optical signal for uplink (ONU to OLT).
The separated carrier components are branched into two, one is passed through the optical circulator 34 and then led to the reflective semiconductor optical amplifier 33, and the other is recombined with the modulation component at the signal combiner 312. After that, it is detected by the photodiode (PD) 32. The optical signal detected here is a signal including a normal carrier (carrier component) and a bilateral band (modulation component).

On the other hand, since the light guided to the reflective semiconductor optical amplifier 33 has only the carrier component, it is modulated by the upstream data stream using the carrier, and then passed through the circulator 35 and transmitted to the OLT 10 side. Will be done.
In this example, a reflective semiconductor optical amplifier (RSOA) 33 is used, but even if the branched carrier component is directly fed back to the OLT 10 side as an optical signal for uplink (not reflected and amplified). Good.

According to the above-mentioned optical transmitter / receiver, the signal branching portion 311 of the asymmetric Mach-Zehnder interferometer 31 branches into an upper path to which a delay difference τ is given and a lower path to which a phase rotation φ is given, and τ = π /. By satisfying ωRF and φ = −ωCτ, the optical signal can be divided into a carrier component and a signal component.

Then, the signal component and the carrier component are recombined at the signal combine section 312 to obtain an optical signal on the light receiving side, and the carrier component is used as an uplink optical signal via the feedback branch section 313. It is possible to obtain an optical signal for uplink in which high frequency components are completely removed without using a complicated circuit.

In the system of the optical transmitter / receiver described above, a signal merging section 312 for re-merging the carrier component and the signal component is provided, and the combined wave signal is input to the photoelectric converter (photodiode 32). However, for example, after branching into a carrier component and a signal component with the asymmetric Mach-Zehnder interferometer 31, the carrier component may be discarded and a new wave may be combined with the local light.

10 ... OLT (optical transmitter), 11 ... transmitter, 20 ... optical fiber, 30 ... ONU (optical receiver), 31 ... asymmetric Mach-Zehnder interferometer, 32 ... photodiode (photoelectric converter), 33 ... reflective type Semiconductor optical amplifier (RSOA), 311 ... signal branching section, 312 ... signal combining section, 313 ... feedback branching section.

Claims (5)

An optical transmitter / receiver that transmits a wideband optical signal generated by an optical transmitter by using WDM-PON using multiple wavelengths via an optical fiber and receives it by multiple optical receivers.
The optical receiver
A signal branching portion that branches the optical signal into a carrier component and a plurality of signal components by dividing the optical signal into a path that gives a delay difference τ and a path that gives a phase rotation φ.
A feedback branch that uses only the carrier component of the optical signal as an uplink optical signal,
An optical transmitter / receiver characterized by being equipped with. The delay difference τ and the phase rotation φ are obtained when the angular frequency of the carrier component is ωC and the modulation frequency of the signal component is ωRF.
τ = π / ωRF
φ ＝ −ωCτ
The optical transmitter / receiver according to claim 1. The optical receiver includes a reflective semiconductor optical amplifier.
The optical transmission / reception device according to claim 1, wherein the carrier component of the optical signal is fed back as an uplink optical signal by the reflective semiconductor optical amplifier. The optical transmitter / receiver according to claim 1 to 3, wherein the optical receiver is a base station. This is an optical transmission / reception method in which a wideband optical signal generated by an optical transmitter by using WDM-PON using multiple wavelengths is transmitted by an optical fiber and received by a plurality of optical receivers.
The optical signal transmitted by performing only intensity modulation is branched into a carrier component and a signal component, and then
The signal component and the carrier component are combined again to form a receiving side signal, and at the same time,
An optical transmission / reception method characterized in that only the carrier component of the optical signal is fed back and used as an uplink optical signal.