KR100944865B1 - Optical Wavelength Locking for the Optical Line Terminal of Wavelength Division Multiplexing Passive Optical Network - Google Patents

Abstract

The present invention relates to a method and apparatus for locking and stabilizing wavelengths of a plurality of optical transceivers required in a line termination device (OLT) of a wavelength division multiplexing passive optical access network (WDM-PON). A plurality of optical detectors capable of detecting a low speed optical signal in each channel of the splitter, a wavelength division multiplexer, an optical modulator for modulating a plurality of wavelength division multiplexed optical signals at a low speed, and a plurality of wavelength division multiplexed optical signals, respectively It may consist of a reflective wavelength reference or reflector that serves as a reference of the wavelength in locking to the position. The present invention can lock the wavelengths of a plurality of light sources at once using inexpensive optical components in a bidirectional optical communication environment and an environment with high optical noise.

Wavelength Division Multiplexing, Wavelength Stabilization, Passive Optical Subscriber Network, WDM-PON, Bidirectional Optical Communication

Description

Optical Wavelength Locking for the Optical Line Terminal of Wavelength Division Multiplexing Passive Optical Network

The present invention provides an optical transceiver (Optical Line Terminal, hereinafter referred to as 'OLT') of a Wavelength Division Multiplexing Passive Optical Network (WDM-PON). The present invention relates to a method of stabilizing and locking an optical transceiver (hereinafter referred to as TRX).

The present invention provides an optical transceiver (Optical Line Terminal, hereinafter referred to as 'OLT') of a Wavelength Division Multiplexing Passive Optical Network (WDM-PON). The present invention relates to a method of stabilizing and locking an optical transceiver (hereinafter referred to as TRX).

With the increase of Internet and multimedia communication traffic, it is necessary to greatly improve the transmission capacity of Subscriber Network. One way to improve it is to install WDM-PON.

WDM-PON is composed of an OLT 100, a remote node 200 (hereinafter referred to as "RN") and a network device 300 (Optical Network Unit, referred to as "ONU") as shown in FIG. . WDM-PON generally transmits down data and up data at different wavelengths. In addition, each ONU uses a different up / down wavelength.

As can be seen in Figure 1, in order for the WDM-PON to operate normally, the up and down wavelengths must be locked in the normal range. If a certain wavelength is shifted by an external factor such as a change in the outside air temperature, it causes interference with the wavelength of another channel, causing serious communication quality degradation.

Therefore, wavelength lock or stabilization of TRX in WDM-PON is a very important technique and much research has been done. The WDM-PON system having the wavelength alignment function described in Korean Patent Publication No. 10-0596406 selects the wavelength locking method of the optical multiplexer / wide multiplexer used in OLT or RN, not the wavelength lock of TRX. This method cannot be used in the bidirectional communication which is generally adopted in the passive optical subscriber network.

In addition, the "wavelength stabilization apparatus and method of the multi-channel light source and the optical comb generator and the injection lock device used therein" described in Patent Publication No. 10-0556209, it is possible to lock the wavelength of the TRX while bidirectional communication is possible. However, there are disadvantages in that expensive wavelength fixing, frequency shifter and optical comb generator are required. 5 is a configuration of a wavelength stabilization apparatus for a multi-channel light source using an optical comb generator.

In addition, in the "optical wavelength stabilization device for multi-channel" described in Patent Publication No. 10-0765001, the optical wavelength filter required to implement the invention is expensive with the current technology, it is difficult to use in bidirectional communication such as WDM-PON. .

All patents proposed to date have one or more disadvantages in application to wavelength stabilization in WDM-PON, which is required in recent industrial fields.

The present invention aims to lock and stabilize optical wavelengths of a plurality of TRXs used in OLT, in particular, in a WDM-PON which is a typical bidirectional optical communication device. In particular, in achieving the above object, it is easy to handle, low cost, and implements the wavelength lock function while using only optical components that can be easily purchased in the industrial field. In addition, there is an object of the invention to enable stable wavelength stabilization even in a bidirectional communication environment that is very vulnerable to reflection and noise.

In order to achieve the above object, it is easy to handle, low cost, and implements the wavelength lock function while using only optical components that can be easily purchased in the industrial field. The present invention is largely classified by function of one or more OLT card; Only one photomultiplexer / demultiplexer; One optocoupler; One optical modulator; It consists of one reflective multiwavelength reference. The optical multiplexer / demultiplexer is typically used as an arrayed waveguide grating (hereinafter referred to as an AWG), and the reflective multi-wavelength reference unit is made of a Fabry-Perot resonator and a mirror. It can consist of a combination.

The present invention is used in the OLT of a bidirectional WDM-PON, which enables to lock and stabilize the TRX wavelengths of the OLT in a simple and economical way. In particular, unlike the existing methods, all of the optical components required to implement the present invention are low-cost and readily available in the industrial field. In addition, in the bidirectional WDM-PON environment, which is vulnerable to reflection and noise, wavelength stabilization can be achieved without being influenced by uplink optical signals.

A first aspect of the present invention is directed to a line termination device (OLT) of a bidirectional wavelength division multiplexing passive optical subscriber network (WDM-PON) comprising a plurality of network devices (ONU); The line terminator includes a plurality of OLT cards including a variable wavelength optical transceiver (TRX); An optical multiplexer / demultiplexer connected to a plurality of OLT cards; A first tap coupler connected to a common port of the optical multiplexer / demultiplexer to branch an optical output; And a light modulator connected to a first output of the first tap coupler, a light output reflective multi-wavelength reference device or a light reflector connected to the optical modulator.

According to a second aspect of the present invention, the OLT card includes a second tap coupler connected to the optical multiplexer / demultiplexer; A connected photodetector of the second tap coupler; And a locking amplifier connected to the optical detector, wherein the locking amplifier receives the output of the optical detector and adjusts the wavelength of the variable wavelength optical transmitter and receiver of the OLT card.

According to a third aspect of the present invention, in the above aspect, the optical modulator modulates an input optical signal by using a clock, and the clock is used as a synchronization signal of the locking amplifier. Includes configuration of line termination devices.

Further, in a fourth aspect of the present invention, the optical multiplexer / demultiplexer is in the form of an arrayed waveguide grating (AWG), and the downlink light output from any OLT card of the line terminator. The wavelength of the signal and the wavelength of the uplink optical signal output from the network device (ONU) corresponding to the arbitrary OLT card are separated by a multiple of the free spectral range (FSR) of the array-waveguide-grafting. It includes a line terminator characterized in that.

In order to achieve the above object, the present invention is largely divided into one or more OLT card by function; Only one photomultiplexer / demultiplexer; One optocoupler; One optical modulator; It consists of one reflective multiwavelength reference.

The optical multiplexer / demultiplexer typically uses an arrayed waveguide grating (hereinafter, referred to as 'AWG').

The multi-wavelength reference unit may be configured by combining a Fabry-Perot resonator and an optical reflector, and such a configuration is obvious to those skilled in the art. For example, the conventional optical comb generator of FIG. 5 is configured by using a Fabry-Perot resonator, and the optical comb generator and the light reflector may be combined to easily implement the multi-wavelength reference device. Corresponds to the known configuration.

2 shows an embodiment of the invention. The OLT card 110a includes a variable wavelength TRX 111a capable of varying the wavelength by an external control signal I_cont; A tap coupler 114a for sending a portion of the received optical signal power to the photodetector (mPD) 113a; Photodetector 113a; And a locking amplifier 112a for amplifying the signal of the photodetector 113a. In order for the locking amplifier 112a to operate normally, a synchronization signal must be received from the outside, and the signal is received from the clock 141 located inside the optical modulator 140.

The AWG 120 multiplexes the outputs from the respective OLT cards 110a to 110n, and simultaneously demultiplexes the optical signals input from the respective ONUs 300 to deliver the OLT cards 110a to 110n. The transmission characteristic of the AWG 120 is as shown in FIG. As shown in (a) of FIG. 3, the downlink and uplink wavelength bands use bands separated by a free spectral range (FSR), respectively. Therefore, each port of the AWG 120 transmits two wavelengths bidirectionally for each port. For example, port 1 of the AWG 120 may pass the λ (1) down and λ (1) up signals. This characteristic of the AWG 120 is not further described as corresponds to techniques known in the art.

Optical signals of the respective OLT cards 110a to 110n multiplexed by the AWG 120 are transmitted downward to the RN 200. In this process, some optical power is branched by the tap coupler 130 of FIG. 2. The branched optical power is input to the reflective multi-wavelength reference unit 150 via the optical modulator 140. Reflection spectral characteristics of the reflective multi-wavelength reference unit 150 are shown in FIG. When comparing the reflection spectrum characteristics of the reflective multi-wavelength reference unit 150 and the transmission characteristics of the AWG, the reflection spectrum width of the reflective multi-wavelength reference unit 150 is narrower than the transmission characteristic spectrum of the AWG. Thus, the reflective multi-wavelength reference 150 is more precise as the wavelength reference. The reflective multi-wavelength reference unit 150 may be implemented using a Fabry-Perot optical filter and the like, and such a configuration corresponds to a technique known to those skilled in the art.

Once the wavelengths λ (1) down to λ (N) down of the optical signals input to the reflective multi-wavelength reference unit 150 coincide with or be greater than the center wavelength of the reflection spectrum of the reflective multi-wavelength reference unit 150 If not, the optical signals λ (1) down to λ (N) down input to the reflective multi-wavelength reference unit 150 are reflected. On the contrary, if the wavelengths of the optical signals input to the reflective multi-wavelength reference unit 150 deviate much from the spectral characteristics of the reflective multi-wavelength reference unit 150, the output of the reflective multi-wavelength reference unit 150 may be absent or very weak. will be.

Assuming that the wavelengths of the optical signals of the OLT cards 110a to 110n coincide with the optical spectra of the reflective multi-wavelength reference unit 150, as shown in FIG. Some will be input to the respective OLT cards 110a-110n via the AWG 120 again. If the signals fed back to the OLT are called λ (1) down_mod to λ (N) down_mod, the photodetectors located inside the respective OLT cards 110a to 110n have a signal size of λ (1) down_mod to λ (N) down_mod. The 113a to 113n monitors the wavelength of the optical signal output from the OLT cards 110a to 110n to determine the degree of deviation from the center wavelength of each port of the AWG 120, and the OLT card ( The variable wavelength TRXs 111a to 111n at 110a to 110n can be adjusted.

The optical modulator 140 is coupled to an optical path in which the downlink optical signals λ (1) down to λ (N) down are fed back to the OLT cards 110a to 110n by the reflective multi-wavelength reference unit 150. The reason for making the feedback optical signals to be lambda (1) down_mod to lambda (N) down_mod rather than lambda (1) down to lambda (N) down is as follows.

WDM-PON is a bidirectional communication device, and since each port of the AWG 120 passes not only the downlink signal wavelength but also the uplink signal wavelength, the signal input to the optical detectors 113a to 113n of the OLT cards 110a to 110n is λ. (1) As well as down_mod to λ (N) down_mod, the λ (1) up to λ (N) up signals are also input. Since only the signals required by the photodetectors 113a to 113n are λ (1) down_mod to λ (N) down_mod, the λ (1) up to λ (N) up signals may be regarded as noise. Due to the WDM-PON system design constraints, the λ (1) down_mod to λ (N) down_mod signals cannot be made large. Therefore, a special method is needed to detect λ (1) down_mod to λ (N) down_mod signals in the environment where not only λ (1) up to λ (N) up signals but also other noises exist.

An optical modulator 140 between the tap coupler 130 and the reflective multi-wavelength reference machine 150 exists for that purpose. The optical modulator amplitude modulates the optical signals input to the reflective multi-wavelength reference unit 150 at a low speed of several tens of kHz. Thus, λ (1) down to λ (N) down and λ (1) down_mod to λ (N) down_mod are the same wavelength but the former is not amplitude-modulated and the latter is amplitude-modulated at tens of kHz. The amplitude-modulated lambda (1) down_mod to lambda (N) down_mod and the upstream signals lambda (1) up to lambda (N) up are simultaneously detected by the respective photodetectors 113a to 113n. In this case, when the locking amplifiers 112a to 112n are used, only λ (1) down_mod to λ (N) down_mod signal components may be detected among the detected signals. Therefore, the influence of λ (1) up to λ (N) up can be eliminated. For normal operation of the locking amplifiers 112a to 112n, the clock 141 of the optical modulator 140 must also be input to the locking amplifiers 112a to 112n of the respective OLT cards 110a to 110n. The operating principle of the locking amplifier corresponds to a technique known to those skilled in the art.

Another embodiment of the invention is shown in FIG. 4. The difference between the embodiment of FIG. 2 and the embodiment of FIG. 4 is that in FIG. 4, a simple optical fiber reflector 160 is used instead of the reflective multi-wavelength reference machine 150. That is, in the configuration in which the optical wavelength of the OLT card is measured with respect to each wavelength reference by using the spectrum of the multi-wavelength reference unit 150 of FIG. It is a configuration to play the same role. In this case, as shown in the spectral characteristics of FIG. 3, since the AWG spectrum has a slower spectrum than the multi-wavelength reference unit 150, the accuracy of wavelength control will be reduced. In addition, the spectrum of the AWG can be changed according to the external temperature change depending on the configuration method such as whether temperature control is included, compared to the multi-wavelength reference machine in which the reference spectrum is correctly maintained. The disadvantage is that it can be a problem.

However, if the characteristics of the AWG 120 and the RN 200 exhibit the same characteristics regardless of the external temperature change, it is not necessary to use the reflective multi-wavelength reference unit 150. Alternatively, even when the transmission distance of the WDM-PON is sufficiently short, the power loss due to the difference in characteristics between the AWG 120 and the RN 200 is negligible, the simple optical fiber reflector 160 instead of the reflective multi-wavelength reference unit 150 is also used. Can be used to seek cost reduction.

The illustrated embodiments are intended to illustrate but not limit the invention. It will be apparent to those skilled in the art that various changes, modifications, or adjustments to the above embodiments can be made without departing from the spirit of the invention. Therefore, the protection scope of the present invention will be limited only by the appended claims, and should be construed as including all such changes, modifications or adjustments.

Fig. 1 Configuration of WDM-PON

2 structure of an OLT according to an embodiment of the present invention

Fig. 3 Spectral Characteristics of AWG and Reflective Multiwavelength Reference System

4 embodiment of the present invention using an optical fiber reflector

5 wavelength stabilization device of a multi-channel light source using the optical comb generator

Claims (4)

A line termination device (OLT) of a bidirectional wavelength division multiplexing passive optical subscriber network (WDM-PON) comprising a plurality of network devices (ONU); The line terminator includes a plurality of OLT cards including a variable wavelength optical transceiver (TRX); An optical multiplexer / demultiplexer connected to the plurality of OLT cards; A first tap coupler connected to a common port of the optical multiplexer / demultiplexer to branch an optical output; An optical modulator connected to a first output of the first tap coupler, an optical output reflective multi-wavelength reference device or a reflector connected to the optical modulator; The method of claim 1, The OLT card may include a second tap coupler connected to the optical multiplexer / demultiplexer; A connected photodetector of the second tap coupler; A locking amplifier connected to the photodetector, And the locking amplifier receives the output of the optical detector and adjusts the wavelength of the variable wavelength optical transmitter and receiver of the OLT card. The method of claim 2, The optical modulator modulates an input optical signal using a clock, and the clock is used as a synchronization signal of the locking amplifier. The method according to any one of claims 1 to 3, The optical multiplexer / demultiplexer is an array-waveguide grating (AWG) type and a network corresponding to the wavelength of the downlink optical signal output from any OLT card of the line terminator and the arbitrary OLT card. The wavelength of the uplink optical signal output from the device ONU is separated by a multiple of the free spectral range (FSR) of the array-waveguide-grating.

Images