KR100217427B1 - A cold-start wdm optical transmission system and standardized frequency generation method thereof - Google Patents

Abstract

end. TECHNICAL FIELD This invention relates to a wavelength division multiplexing (WDM) optical transmission system.

I. The technical problem to be solved by the present invention is to implement a cold start wavelength division multiplexing (WDM) optical transmission system and a method for generating the standard frequency that transmit lasers can automatically generate a stable light output at the standard frequency.

All. SUMMARY OF THE INVENTION The present invention proposes a wavelength division multiplex (WDM) optical transmission system using a distributed feedback (DFB) laser and a passive optical signal demultiplexer. The DFB lasers measure wavelengths under normal operating conditions, and select lasers operating within half of a channel spacing on both sides from the standard frequency for each channel. The WDM optical transmission system automatically operates at the standard frequency only when the system is switched on, even if the selected lasers do not input the operating conditions necessary for operating at the standard frequency in advance. Characterized in that it is adjusted.

la. Significant use of the invention: wavelength division multiplexed light transmission systems.

Description

Cold start type wavelength division multiplex optical transmission system and its standard frequency generation method {A COLD-START WDM OPTICAL TRANSMISSION SYSTEM AND STANDARDIZED FREQUENCY GENERATION METHOD THEREOF}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength division multiplexed optical transmission system, and more particularly to a cold start type wavelength division multiplexed optical transmission system that enables transmitting lasers to automatically generate stable light output at a standard frequency and a method for generating a standard frequency for use in the system. It is about.

In general, wavelength-division-multiplexed (WDM) optical transmission system is a system for multiplexing multiple transmission lasers having different wavelengths into one optical fiber and greatly increasing transmission capacity per optical fiber. A system with an advantage. Using this system, even if each laser operates at a relatively low transmission speed, the transmission capacity per fiber can be greatly increased, so that the transmission capacity can be greatly increased without replacing the already installed optical fiber with a dispersion transition fiber. However, in a WDM system each laser needs to be operated only at a specific wavelength specified for the life of the system. In response to this need, the International Telecommunication Union (ITU) (hereinafter referred to as ITU) is currently working to standardize the wavelengths. Therefore, all lasers to be used in future WDM optical transmission systems must be operated at the standard frequency recommended by the ITU.

Recently, various types of variable wavelength lasers have been proposed for use in WDM optical transmission systems. However, there are still great difficulties in mass production of these variable-wavelength lasers, and there is also a lack of reliability and stability. Thus, in the WDM optical transmission system, as in the conventional optical transmission system, the distributed-feedback (DFB) The laser is expected to be used. However, the tunable wavelength capability of DFB lasers is very limited and has the disadvantage that it is never easy to produce lasers that operate accurately at standard frequencies.

Therefore, the most realistic method is to select lasers operating near the standard frequency among the many DFB lasers produced for each channel and adjust them to operate at the standard frequency using temperature and the like. Such adjustments must be made precisely at the factory producing the optical transmission device or at the site using the optical transmission device. Each laser must also be managed to operate at the standard frequency for the life of the system. However, the frequency of the laser may be a frequency of several tens of gigahertz [GHz] or more due to an aging effect even if the operating conditions such as temperature and current are kept constant. This shift in laser frequency reduces the gain margin of the system or causes crosstalk of the signal, resulting in communication failure. These points can cause many difficulties in installing and operating the WDM optical transmission system. In particular, if there is a case that the skilled workers in the field need to adjust the wavelength of the laser in order to improve the degraded transmission quality, serious problems such as communication loss may occur.

Accordingly, an object of the present invention is to provide a WDM optical transmission system having a function of automatically adjusting the frequency and light output of a laser so that each laser operates stably at a standard frequency.

It is another object of the present invention to provide a WDM optical transmission system that manages lasers to operate stably at standard frequencies for the life of the system.

It is still another object of the present invention to provide a WDM optical transmission system that improves a gain margin of a system that decreases as the frequency of a laser transitions.

It is still another object of the present invention to provide a WDM optical transmission system which prevents crosstalk of a signal caused by a change in the frequency of a laser.

It is still another object of the present invention to provide a WDM optical transmission system for preventing a communication failure from occurring as the frequency of a laser transitions.

Still another object of the present invention is to provide a WDM optical transmission system that can be more conveniently installed and maintained / managed.

It is another object of the present invention to provide a method for implementing an apparatus for generating a standard frequency for use in a WDM optical transmission system.

The present invention for achieving these objects proposes a WDM optical transmission system using a DFB laser and a passive optical signal demultiplexer. The DFB lasers measure wavelengths under normal operating conditions (for example, when a current of 40 mA is applied at 25 ° C.) to select lasers operating within half of a channel interval from the standard frequency to each channel. The WDM optical transmission system according to the present invention automatically operates at the standard frequency only when the system is switched on, even if the selected lasers do not input the operating conditions necessary for operating at the standard frequency, and each laser outputs as much light as desired. Characterized in that to adjust the generation. Therefore, if the system proposed in the present invention is used, it is not necessary to precisely adjust each laser to operate at the corresponding standard frequency at the production plant or installation site of the system, and ensure that these lasers operate at the standard frequency for the lifetime of the system. It becomes possible.

WDM optical transmission system according to the present invention; A device providing information on standard frequencies at regular intervals, transmission lasers of different channels, each of which is used to select two or more distributed feedback lasers each operating near the standard frequency, and the selected DFB laser A passive multiplexer that multiplexes the output of the signals into a single optical fiber, a control device that controls the selected DFB lasers to operate automatically at the standard frequency and generate a constant output, and is controlled by the control device and then transmitted through the optical fiber And a passive optical demultiplexer for demultiplexing two or more wavelength division multiplexed optical signals from the transmission lasers.

In the above, the device for providing information on the standard frequency at regular intervals is implemented as a solid etalon filter. Then, the transmission lasers are multiplexed to the passive multiplexer, and some of the outputs of the passive multiplexer are provided to the etalon filter to be used for frequency stabilization, and most of the remaining outputs of the multiplexer are transmitted through the optical fiber. Communication is achieved by separation by the demultiplexer. When the output of the passive multiplexer is one, a 2 × 2 star coupler may be added to generate two outputs required for transmission and control. The control device; A photodetector for converting the light output passing through the etalon filter into an electrical signal, a phase detection detector for detecting a first differential signal of the etalon filter, and a first differential signal of the filter detected by the phase detection detector It consists of a control unit to stabilize to the correct frequency using.

Since the phase detector detects the first derivative signal of the etalon filter according to the laser frequency, if the signal of the phase detector is 0 and the slope is negative, the frequency of the laser is exactly equal to the resonance frequency of the filter. Can be. The controller is configured to find a point where the slope of the phase detection detector is negative and its value becomes zero. The optical transmission system moves the frequency of each laser to an area where the slope of the first derivative signal of the filter is negative and the value approaches zero by the controller, and then stabilizes the peak at the filter peak within a short time. And a current control portion for finely adjusting the applied current of the laser by proportionally amplifying and integrating the differential differential signal. The current control part is composed of a proportional amplifier and an integrator, and the switch is set to operate after the frequency is moved around the standard frequency by the controller.

1 is a view showing the transfer characteristics of the etalon filter implemented as a standard frequency generator according to the present invention.

2 is a view showing the configuration of a cold start wavelength division multiplex optical transmission system according to the present invention.

3 is a diagram showing the first order differential signal of a filter obtainable from a phase detection detector and a transfer characteristic of a synchronized etalon filter according to the present invention.

4 shows the light spectrum of transmission lasers measured prior to performing a cold start procedure in accordance with the present invention.

5 shows the light spectrum of transmission lasers measured after performing a cold start procedure in accordance with the present invention.

DETAILED DESCRIPTION A detailed description of preferred embodiments of the present invention will now be described with reference to the accompanying drawings. In the following description of the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted. Terms to be described later are defined in consideration of functions in the present invention, and may be changed according to the intention or custom of a user or a designer, and the definitions should be made based on the contents throughout the present specification.

In the following description, a standard frequency generator that provides a series of reference information occupied by a constant bandwidth from a standard frequency (wavelength) will be described. An etalon filter may be used as the standard frequency generator. A cold start WDM system will be described in which data communication can be performed even when the laser wavelength or optical power is not manually adjusted using the etalon filter. Such a WDM system has the advantage that the installation and maintenance of the system is simplified because there is no need to accurately adjust the frequency of the lasers in the factory producing the system or the place where the system is installed. In addition, the continuous adjustment operation of each laser to operate at the standard frequency will improve the reliability of the WDM system.

Several different methods can be used to generate the standard frequency, but the most practical of these is the use of synchronized etalon filters. To use an etalon filter as a standard frequency generator, all filters must be able to provide a resonant frequency that matches the standard frequency recommended by the ITU. The m-th resonant frequency v_m of the etalon filter is given by Equation 1 below.

In Equation 1, m is the mode number, L is the free spectral range (hereinafter referred to as FSR), C is the speed of light, n is the refractive index, l is the thickness of the filter is the angle of incidence of the optical signal.

As the etalon filter, it is preferable to use a solid etalon filter having excellent mechanical stability and a simple structure. However, it is virtually impossible to fabricate many etalon filters with exactly the same thickness and refractive index. This difference in length and refractive index can be corrected by adjusting the angle of incidence, but note that two different absolute frequencies are required for this. For example, adjusting the angle of incidence θ so that the resonant frequencies v_m and v_m-k of the etalon filter correspond to the absolute frequencies v_1 and v_2, respectively, and generate (k + l) resonant frequencies between the two absolute frequencies. The etalon filter will provide exactly the same resonant frequency. This is possible by adjusting the angle of incidence θ to change the mode number m and FSR L so that the above two conditions can be satisfied by adjusting two variables.

A synchronized etalon filter, a technique already proposed by the present inventors, can be used as the standard frequency generator of the present invention. Techniques for such synchronized etalon filters are described in Y. C. Chung and L. W. Stulz, Synchronized etalon filters for standardizing WDM transmitter laser wavelengths, IEEE Photonics Technol. Lett., Vol. 5, no. 2, pp. 186-189, Feb. 1993 and in Y. C. Chung and L. W. Stulz, Synchronized etalon filters, US Patent Number: 5,434,877, Jul. 18, 1995. The synchronized etalon filter can correct the difference in thickness or refractive index between the etalon filters by adjusting the angle of incidence using the absolute frequency of the laser stabilized on the transition lines of two different Krypton atoms. As a result, the resonant frequencies (eg, 195.575 ± n × 0.1034 [THz]) of the respective filters will be perfectly matched.

However, since the transition line of the atom is determined by the physical properties of the atom, the resonance frequency of the etalon filter fabricated in this way will also be determined according to the properties of the atom. Note that there is no practical use if the resonant frequency of such a filter does not match the standard frequency recommended by the ITU. Therefore, in the present invention, even if any standard frequency is recommended in the ITU, it is intended to propose a method for manufacturing an etalon filter having a resonant frequency corresponding thereto. That is, in the present invention, the absolute frequency used to correct the etalon filter has an arbitrary value rather than a specific value such as the transition line of the atom. An absolute frequency having an arbitrary value can be obtained by stabilizing the laser's frequency to an arbitrary value by using a commercially available wavemeter.

Since the ITU final recommendation on the standard frequency has not been finalized, an etalon filter corresponding to the case where the standard frequency of the WDM optical transmission system is 193.686 ± n × 0.100 [THz] is described. Where n is the channel number. A 1.04 mm thick glass (fused-silica glasses) was coated with a TiO ₂ / SiO ₂ multilayer thin film to produce a solid etalon, followed by compensating the incident angle using lasers stabilized at two absolute frequencies. The method used here uses a laser stabilized at Kr ls ₂ -2p ₈ transition lines with a wavelength of 1.54782 μm (193.686 THz) as the first absolute frequency, but the second absolute frequency uses a wavemeter. Laser was stabilized at 1.56072 μm (192.086 THz). By using this randomly chosen second absolute frequency, the channel spacing can be exactly 100 GHz. The angle of incidence of the etalon filter is adjusted by using a broadband light source and an optical spectrum analyzer to observe the propagation characteristics of the etalon filter and to form 17 channels between two absolute frequencies. Since the mth and (m + 16) th resonant frequencies of the adjusted etalon filter correspond to the absolute frequencies of 1.54782 μm and 1.56072 μm, respectively, the mode number m must be 1936 and the FSR must be 100 GHz. The angle of incidence of the etalon filter is adjusted after the filter is installed on a support made of metal such as duralumin, and after the adjustment is completed, the incident angle is oxidized by oxidizing a portion of the support mass which is adjusted using an oxidizing agent such as hydrochloric acid. Completely fixed.

Figure 1 shows the transmission characteristics (light spectrum) of the etalon filter measured after the adjustment of the angle of incidence is completed. In this figure, the two high peaks represent the light output of the lasers stabilized at the absolute frequency and can be fabricated to generate an arbitrary absolute frequency by using a wavelength meter so that the etalon filter generates a resonant frequency at 193.686 ± n × 0.100 THz. Shows that there is. Thus, if both absolute frequencies are chosen to have arbitrary values (using two lasers stabilized to arbitrary values as the absolute frequency using a wavelength meter), then whatever ITU recommends as the standard frequency, It will be possible to produce a matching etalon filter.

An example of implementing a cold start WDM optical transmission system using an etalon filter implemented through the above process is shown in FIG. 2. It is assumed here that the standard frequency of the WDM optical transmission system is 1547.82 ± n × 0.8 nm (193.686 ± n × 0.100 THz). n represents a channel number.

3 is a transfer characteristic curve of an etalon filter shown to explain a cold start method. In this figure, the dotted line is the first derivative of the transfer characteristic of the etalon filter, which can be easily obtained using the phase sensitive detector shown in Fig. 2, and is used to stabilize the laser frequency at the peak of the resonant frequency. It is. Since the resonant frequency of the etalon filter is the same as the standard frequency of the WDM optical transmission system, the transmission laser of each channel selects lasers operating near the resonant frequency corresponding to each channel when operated normally. For example, lasers operating within 1/2 of the channel interval (area A in FIG. 3) can be selected and used from each resonance frequency to both sides. Hereinafter, a four-channel WDM optical transmission system will be described. To implement this four-channel WDM optical transmission system, lasers operating at 1552.97 nm, 1553.61 nm, 1554.61 nm, and 1555.24 nm, respectively, were used when 40 mA was applied at 25 ° C. Therefore, these lasers can be used as transmission lasers for channel numbers 6, 7, 8, and 9, respectively.

FIG. 4 shows that the selected lasers were operated under normal conditions (40 mA, 25 ° C.) and measured and showed significant irregularities in channel spacing and light output. The present invention is treated as described below to remove such irregularities so that the characteristics as shown in FIG.

In a cold start WDM optical transmission system, each laser is modulated with different sinusoids of 5 to 9 kHz each using an applied current. This is to find the first derivative of the etalon filter and to distinguish each channel. By modulating the applied current of each laser as described above, the Brillouin scattering effect can be reduced. The output of each laser is input to a 4 × 4 star coupler 20, one of the outputs of the star coupler 20 to the transmission fiber, and the other to the etalon filter 50. The 4 × 4 molded coupler was used as a multiplexer and if the multiplexer had one output, such as an integrated optic waveguide grating, a 2 × 2 star coupler was added to control and transmit. You get two outputs. The signal passing through the etalon filter 50 is distributed by the PIN photodetector 60 to four phase detection detectors after the optical signal is converted into an electrical signal. The optical detector 60, the phase detector 70, the proportional amplifier 80, the integrator 90, the switch 110 and the controller 100 constitute a control device of the WDM optical transmission system.

When driving the cold start WDM optical transmission system, the controller 100 of the controller automatically sets the temperature so that each laser is operated at 20 ° C., which is 5 ° C. lower than the normal condition. However, the applied current of each laser is 40 mA as in normal conditions. Since the channel spacing of the system to be implemented is 100 GHz and the frequency variability of the DFB laser is 10 GHz / ° C., the selected lasers operate in the region B of FIG. 3. The controller 100 of the control device first increases the operating temperature of the laser to move the frequency to an area where the value of the phase detector is positive. This is to prevent the lasers near the area D of FIG. 3 from stabilizing at the frequencies of other channels. Then, the applied current of each laser is adjusted while measuring the light output using the light output detection signal Pwr from the light detector mounted in the package of the transmission laser 10 so that the light output of each laser becomes 0 dBm. Next, the operating temperature of the laser is adjusted until each laser operates in the area C of FIG. This process can be confirmed by observing the slope of the first order differential signal of the filter obtained from the phase detection detector 70. The controller 100 recognizes that the laser operates in the area C when the first differential signal decreases as the operating temperature of the laser increases. When it is confirmed that the laser operates in the area C of FIG. 3, the controller 100 automatically drives the switch 110 to operate a current controller consisting of a proportional amplifier 80 and an integrator. This current control part finely adjusts the applied current of the laser to precisely stabilize the frequency of each laser to the resonant frequency of the desired etalon filter for a short time. The change in the light output due to the current change occurring in this process is compensated again by using the temperature control.

5 is a light spectrum of DFB lasers obtained by performing the above process. This figure shows that the cold start system automatically adjusts the frequency and light output of each laser to stabilize the channel spacing and light output to 100 ± 1 GHz and 0 ± 0.1 dBm, respectively. After powering up the system, it takes about 20 seconds for the lasers in each channel to transmit a 2.5 Gb / s signal without error (BER <10 ^-9 ). The system continuously monitors and readjusts the frequency and light output of each laser to ensure long-term stability. Therefore, when the system becomes inoperative due to a power failure or the like, the system can be easily recovered by using the readjusted operating conditions of the lasers already stored in the memory device. Therefore, the cold start process needs to be performed again only when the transmission laser needs to be replaced.

As described above, the cold start type WDM optical transmission system according to the present invention automatically adjusts each laser to operate at the standard frequency even if it does not measure and inputs the operating conditions necessary for each laser to operate at the standard frequency. Control to generate light output. Such a system has the following advantages. First, the frequency of the lasers used in the WDM optical transmission system need not be precisely adjusted at the production plant of the system or the site where the system is installed, thereby simplifying the installation, operation, and management of the WDM optical transmission system. Secondly, the reliability of the WDM optical transmission system is improved by continuously correcting the operating conditions so that each laser operates at a standard frequency. Third, by using a cold start control device, the selection condition of the DFB lasers that can be used in the WDM optical transmission system can be alleviated, thereby making it possible to lower the price of the laser for the WDM optical transmission system. In conclusion, the cold start controller can simplify the installation, maintenance and management of the WDM optical transmission system and improve reliability.

Meanwhile, in the detailed description of the present invention, specific embodiments have been described, but various modifications are possible without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be determined not only by the scope of the following claims, but also by the equivalents of the claims.

Claims (9)

A method of implementing an apparatus for providing a standard frequency for use in a wavelength division multiplexing optical transmission system, The apparatus is implemented as an etalon filter, and the etalon filter is corrected by adjusting the incident angle of the etalon filter by using two lasers stabilized at an arbitrary frequency to correct the difference in thickness or refractive index of the etalon filter. And a resonant frequency that matches any standard frequency. 2. The method of claim 1, wherein the device is implemented with a solid etalon filter, wherein two lasers for adjusting the angle of incidence of the etalon filter using a wavelength meter are stabilized at an arbitrary frequency. 3. A method according to claim 2, wherein at least one of the two lasers for adjusting the angle of incidence of the etalon filter is stabilized at an arbitrary frequency using a wavelength meter. The part according to any one of claims 1 to 3, wherein the etalon filter is mounted on a support made of metal such as duralumin, and the incident angle is adjusted, followed by oxidation of a portion for adjusting the angle of the support using an oxidizing agent such as hydrochloric acid. Thereby fixing the angle of incidence. In a wavelength division multiple optical transmission system, An element providing information on a standard frequency at regular intervals, Transmission lasers of different channels, each of which is used to select two or more distributed feedback lasers each operating near the standard frequency, A passive multiplexer for multiplexing the output of the selected lasers; A control device for controlling the selected distributed feedback lasers to automatically operate at the standard frequency and generate a desired light output; And a passive optical demultiplexer for demultiplexing two or more wavelength division multiplexed optical signals from the transmitting lasers transmitted through an optical fiber after being controlled by the controller. 6. The optical transmission system according to claim 5, wherein the device for providing information on standard frequencies at regular intervals is a solid etalon filter. 6. The output of the passive multiplexer multiplexing the transmission lasers is provided to the etalon filter, and the other output of the multiplexer is transmitted to the demultiplexer through the optical fiber. Optical transmission system. The method of claim 5, wherein the control device A phase detection detector for detecting a first order differential signal from the etalon filter; And a control unit for changing an operating temperature of the transmission lasers to match the resonant frequency of the filter by using the signal detected by the phase detection detector. The method of claim 8, wherein the control device When the frequency of each laser is operated near the resonance frequency of the etalon filter that provides the standard frequency, the current control part composed of the proportional amplifier and the integrator is automatically operated to more precisely stabilize the laser frequency at the resonance frequency within a short time. System characterized in that the.

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