KR20200139608A - Method for controlling tunable light source - Google Patents

Abstract

The present invention may provide a method of constantly controlling a tunable light source with a change in a detuning state according to a change in the external temperature and the driving of a heater. According to an embodiment of the present invention, a tunable light source may comprise a gain medium and a phase controller. According to an embodiment of the present invention, a method of controlling the tunable light source may comprise the steps of: monitoring a first power of a first optical signal output from the gain medium at a first time and a second power of a second optical signal output from the gain medium at a second time; comparing the first power with the second power; and when the first power and the second power are different, adjusting a phase control current applied to the phase controller.

Description

How to control a variable wavelength light source{METHOD FOR CONTROLLING TUNABLE LIGHT SOURCE}

The present invention relates to a method of controlling a variable-wavelength light source, and more particularly, to a method for controlling a variable-wavelength light source to constantly change a detuning state due to a change in external temperature and driving a heater.

Research on a Wavelength Division Multiplexing (WDM)-based Passive Optical Network (PON) (hereinafter referred to as'WDM-PON') is actively being conducted. In WDM-PON, communication between the central base station and the subscriber is performed using each wavelength specified for each subscriber. And since WDM-PON uses a dedicated wavelength for each subscriber, it has excellent security, enables large-capacity communication services, and has different transmission technologies such as link rate and frame format for each subscriber or service. It has the advantage that it can be applied.

However, WDM-PON requires different light sources as many as the number of subscribers belonging to one remote node (RN), which places great economic burden on both users and operators in the production, installation, and management of light sources for each wavelength. In order to solve this problem, a method of applying a variable wavelength light source device capable of selectively changing the wavelength of a light source has been actively studied.

The present invention is to solve the above-described technical problem, the present invention can provide a method of constantly controlling a variable wavelength light source with a change in a detuning state due to a change in external temperature and driving a heater.

A variable wavelength light source according to an embodiment of the present invention may include a gain medium and a phase controller. A method of controlling a variable wavelength light source according to an embodiment of the present invention includes a first power of a first optical signal output from a gain medium at a first time and a second power of a second optical signal output from the gain medium at a second time. Monitoring the power, comparing the first power with the second power, and when the first power and the second power are different, adjusting the phase control current applied to the phase controller.

In one embodiment, the tunable light source may further include a monitor photodiode. In one embodiment, the method of controlling the tunable light source may further include detecting a first monitor current flowing through the monitor photodiode at a first time and a second monitor current flowing through the monitor photodiode at a second time. have.

The variable wavelength light source according to an embodiment of the present invention has a gain medium that generates and amplifies an optical signal according to an applied bias current, an external reflector that directly combines with the gain medium to form an external resonator, and reflectance of the external reflector It includes a phase adjuster that adjusts the phase to oscillate in a long wavelength region above the maximum wavelength, and a monitor photodiode that monitors a detuning state.

In one embodiment, the external reflector is characterized in that it comprises a Bragg grating.

In one embodiment, the external reflector may be a tunable reflector.

In one embodiment, the phase adjuster may be bonded to the gain medium.

In one embodiment, the phase adjuster may be bonded to an external reflector.

In an embodiment, the variable-wavelength light source according to the present invention may further include a high-frequency transmission medium for controlling an operation speed of an optical signal by applying a high-frequency signal to a gain medium.

In an embodiment, the monitor photodiode may be located in a portion where the output power of the highly reflective coating surface of the gain medium can be read.

The variable-wavelength light source according to an exemplary embodiment of the present invention can prevent a change in a detuning state due to environmental changes including external temperature by constantly maintaining an operating state under an optimal operating condition. Accordingly, by preventing a change in performance including mode hopping, the performance of the variable wavelength light source can be kept constant.

1 is a flowchart illustrating a method of controlling a variable wavelength light source according to an embodiment of the present invention.
FIG. 2 is a conceptual diagram illustrating an optical subscriber network system based on wavelength division multiplexing using a broadband light source.
3 is a diagram showing the configuration of an optical subscriber network system based on wavelength division multiplexing using a variable wavelength light source.
4A and 4B are conceptual diagrams showing operating characteristics of a variable wavelength light source and a position of a phase control electrode.
5 is a diagram showing device variables to be controlled in a variable wavelength light source.
6A and 6B are diagrams for explaining a detune loading effect in a variable wavelength light source.
7A, 7B, 7C, and 7D are views for explaining a detune loading effect that reduces chirps in a variable wavelength light source that directly modulates.
8A and 8B are diagrams for explaining a method of determining output power in a variable wavelength light source.
9A and 9B are graphs measuring power output to both ends of a variable wavelength light source.
10A, 10B, 10C, and 10D are graphs showing output powers according to a detune frequency in phases of each passive region in a variable wavelength light source.
11A and 11B are graphs showing a transmission penalty according to a change in output power when a detuning sweep is performed while changing a heater current of an external reflector at a constant temperature according to an embodiment of the present invention.
12A and 12B illustrate a change in rear output power and a corresponding change in transmission penalty when a detuning sweep is performed while simultaneously changing temperature and external reflector heater current to maintain a constant wavelength according to an embodiment of the present invention. This is the graph shown.
13 is a graph showing the relationship between the ambient temperature and the temperature of the gain medium and the graph showing the relationship between the ambient temperature and the oscillation wavelength of the laser.
14 are graphs showing relationships between ambient temperature and oscillation wavelength.
15 are graphs showing a relationship between an output of a phase control unit and a current of a monitor photodiode.

In the following, embodiments of the present invention will be described clearly and in detail to the extent that those skilled in the art can easily implement the present invention.

1 is a flowchart illustrating a method of controlling a variable wavelength light source according to an embodiment of the present invention. The tunable light source described in FIG. 1 may include a gain medium, a camouflage controller, and a monitor photodiode (the tunable light source and its respective components will be described later in detail in FIGS. 4A, 4B, and 5. ). In one example, the tunable light source may be a tunable laser. In another example, the tunable light source may be an external resonator type tunable laser.

In step S110, the variable wavelength light source detects, senses, senses, reads, reads monitor currents (or monitoring currents, monitor voltages, monitoring voltages) flowing through the monitor photodiode at different times. ), or can be monitored. In one example, the tunable light source may detect a first monitor current flowing through the monitor photodiode at a first time and a second monitor current flowing through the monitor photodiode at a second time. The tunable light source can detect the monitor current flowing through the monitor photodiode in real time.

In step S120, the tunable light source may monitor, read, read, or calculate powers (outputs, powers, output powers) of the optical signals output from the gain medium based on the detected monitor currents. In one example, the tunable light source may monitor a first power of the first optical signal output from the gain medium at a first time based on a first monitor current detected by the monitor photodiode at a first time and The second power of the second optical signal output from the gain medium at the second time may be monitored based on the second monitor current detected by the monitor photodiode at 2 hours. In another example, the tunable light source may monitor the powers of optical signals output from the gain medium in real time.

In step S130, the tunable light source may compare the monitored powers at different times. For example, the tunable light source may compare a first power monitored at a first time with a first power monitored at a second time. When the first power monitored at the first time is the same as the second power monitored at the second time, the power may be constant over time. When the first power monitored at the first time is different from the second power monitored at the second time, the power may not be constant over time. When the first power monitored at the first time is the same as the second power monitored at the second time (if the power over time is constant), the step of controlling the variable wavelength light source may be terminated. When the first power monitored at the first time is different from the second power monitored at the second time (if the power is not constant according to time), step S140 may proceed, and steps S110, S120, and S130 are repeated. Can be.

In step S140, the variable-wavelength light source may adjust the phase control current applied to the phase controller. The tunable light source may operate based on first and second detuning curves including optimal operating points obtained by a detuning sweep. Here, the first detuning curve may be obtained from a tunable light source operating at a first ambient temperature, and a second detuning curve may be obtained from a tunable light source operating at a second ambient temperature. The variable-wavelength light source can shift, change, change, or transition the operating state from the first point on the first or second detuning curve to the second point by adjusting the phase control current applied to the phase controller. have. The variable wavelength light source may acquire a plurality of detuning curves according to a change in ambient temperature (or external temperature). The variable-wavelength light source can shift the operating state to an optimal operating point on a plurality of detuning curves by adjusting the phase control current applied to the phase controller. Step S140 will be described in more detail in FIGS. 13 to 15.

In step S140 according to an embodiment, the variable-wavelength light source may adjust the phase control current by increasing (or decreasing) the level of the phase control current applied to the phase controller. In another example, the tunable light source may adjust the phase control current by increasing (or decreasing) a duty value of the phase control current applied to the phase controller. In another example, the tunable light source may adjust the phase control current by increasing (or decreasing) the waveform, frequency, wavelength, or direct current (DC) level of the phase control current applied to the phase controller. Examples of adjusting the phase control current described above will not limit the scope of the present invention. When the power output from the gain medium becomes constant, the method of controlling the tunable light source described in FIG. 1 may be terminated.

FIG. 2 is a conceptual diagram illustrating a Wavelength Division Multiplexing-Passive Optical Network (WDM-PON) based on a wavelength division multiplexing using a broadband light source. Referring to FIG. 2, the WDM-PON 100 may provide a convergence service of voice, data, and broadcast. The WDM-PON 100 uses a wavelength determined for each subscriber for communication between a center office (CO) and a subscriber. WDM-PON (100) is a base station transceiver (Optical Line Terminal; OLT) 110 placed on the side of the central base station (CO), a subscriber terminal device (Optical Network Unit; ONU or Optical Network Terminal; ONT) placed on the subscriber side ( 130) and an outdoor node (Remote Node; RN) 120. The base station transmitting/receiving device 110 and the outdoor node 120 are connected by a single-core feeder optical fiber 117, and the outdoor node 120 and the subscriber terminal device 130 are connected by a distribution optical fiber 125. I can.

Downlink light transmitted from the base station transceiver 110 to the subscriber terminal device 130 is from a broadband light source (BLS) 112 of the base station transceiver 110 to a first circulator (circulator or optical circulator) ( 114) and an Arrayed Waveguide Grating (AWG) 113 that performs wavelength division multiplexing and demultiplexing to the Reflective Semiconductor Optical Amplifier (RSOA) 111 of the base station transceiver 110 Can be.

The downlink light transmitted to the optical transmitter 111 of the base station transceiver 110 is again passed through the array waveguide grating 113, the first circulator 114, and the second circulator 115 through the feeder optical fiber 117. It is transmitted to the array waveguide grating 123 of the outdoor node (RN) 120, and is transmitted to an optical coupler 133 in the subscriber terminal device 130 through a distribution optical fiber. Here, the optical coupler 133 may be a circulator in various embodiments. Downlink light input to the optical coupler 133 may be transmitted to the optical transmitter 131 and the optical receiver 132 of the subscriber terminal device.

Uplink light transmitted from the subscriber station device 130 to the base station transceiver 110 may be transmitted in a direction opposite to the downlink light. The uplink light is transmitted from the optical transmitter 131 of the subscriber terminal device to the outdoor node 120 through the optical coupler 133 and the distribution optical fiber 125. The uplink light is transmitted to the base station transceiver 110 through the array waveguide grating (AWG) 123 of the outdoor node 120 and the feeder optical fiber 117. The transmitted uplink light may be transmitted to the optical receiver 116 of the base station transceiver 110 through the second circulator 115 and the grid 118 through the arrangement of the base station transceiver.

As in the embodiment of FIG. 2, the WDM-PON 100 using a broadband light source does not need to secure a separate light source at the subscriber end because the light source of the base station transmission and reception device 110 is also used in the subscriber terminal device 130. Thus, a color-independent system can be implemented. However, since the WDM-PON (100) using a broadband light source injects a seed light source using a separate broadband light source, and amplifies and modulates it by the RSOA (111), there is a speed limitation and thus a 10Gbps (Giga bit per second) class It is recognized in a way that is difficult to use in the system. To compensate for this, a device incorporating a reflective field absorption modulator has been proposed as an alternative.

3 is a diagram showing the configuration of an optical subscriber network system based on wavelength division multiplexing using a variable wavelength light source. 3, the WDM-PON 200 includes a base station transceiver (OLT) 210 placed on the central base station (CO) side, a subscriber terminal device (ONU or ONT) 230 placed on the subscriber side, and an outdoor node ( RN) 220 may be included. The base station transceiver 210 and the outdoor node 220 may be connected by a single-core feeder optical fiber 217, and the outdoor node 220 and the subscriber terminal device 230 may be connected by a distribution optical fiber.

The downlink light is transmitted from a Tunable Laser Diode (TLD) 211 of the base station transceiver 210 through a wavelength division multiplexing (WDM) filter 213 to the array waveguide grating (AWG) of the base station transceiver 210 ( 214). The transmitted downlink light is transmitted to the outdoor node 220 through the feeder optical fiber 217, and the downlink light passes through the array waveguide grating (AWG) 223 of the outdoor node 220 and the subscriber terminal through the distribution optical fiber 225. It is passed to a wavelength division multiplexing (WDM) filter 233 of the device 230. The transmitted downlink light may be transmitted to the light receiving unit (PD) 232 of the subscriber terminal device 230 through a wavelength division multiplexing (WDM) filter 233. The uplink light proceeds from the variable wavelength light source (TLD) 231 of the base station transceiver 210 of the subscriber terminal device 230 in the opposite direction to the downlink light and is transmitted to the light receiving unit (PD) 212 of the base station transceiver 210. Can be delivered.

In the embodiment of FIG. 3, unlike the embodiment of FIG. 2, in order to configure a system that does not depend on a specific wavelength, a tunable light source (TLD) 211 and 231 is provided with a base station transceiver 210 and a subscriber terminal device ( 230) included in each. The WDM-PON using the variable wavelength light source of FIG. 3 has a limitation that both the base station transmitting and receiving device 210 and the subscriber terminal device 230 must have a light source, but since it is a structure using a laser, speed compared to the embodiment of FIG. High performance can be achieved from the side. Therefore, it is important to make a reliable and high-performance wavelength tunable light source at low cost for WDM-PON using a tunable light source.

The external resonator-type tunable laser is advantageous in single mode lasing by filtering the oscillation light through the external resonator. However, even if the external resonator has a stable oscillation condition under a specific condition, the oscillation mode may shift when the applied current or external temperature changes. In this case, mode hopping to an adjacent mode may occur in the external resonator-type wavelength tunable laser, and in some cases, a phenomenon such as multi-mode lasing may be seen. When a phenomenon such as mode hopping or multi-mode lazing occurs, an error rate of transmission data may increase in an optical communication field using a constant wavelength in a single mode.

Therefore, in using an external resonator type tunable laser, it is important to determine a stable wavelength region under a given condition or a stable condition in a given wavelength region.

Hereinafter, in an embodiment of the present invention, for convenience of explanation, an external resonator type wavelength tunable laser mainly using a thermo-optic (TO) effect will be described. However, this is for convenience of description, and the method of controlling an external resonator-type laser according to an embodiment of the present invention is applied in the same manner to a general external resonator-type laser and can be applied according to an individual device method.

A tunable light source is a key device used in fields such as optical communication, spectroscopy, and sensors, and various technologies have been proposed to implement a tunable light source. The tunable light source is a single integrated laser using the Vernier effect using semiconductor devices including SG-DBR (sampled grating distributed Bragg Reflector), an external resonator laser using an external grating reflector, and an array of multiple single wavelength light sources. Typical examples are array lasers that are made into a shape and realize multiple wavelengths.

The tunable technology used in the external resonance type tunable laser is a technology that separates wavelengths by diffraction angles using MEMS (Micro Electro Mechanical Systems) technology, technology using thermo-optical effects, voltage or current such as liquid crystal. It can be classified as a technology that changes the wavelength by.

Although there have been many developments in the wavelength tunable method by single integration, it has a problem due to high price due to a yield problem. In the case of an array laser, its application is also limited due to its size and yield. In the case of the external resonator laser, stable laser driving is possible to some extent, but there are limitations due to the large size and high-speed operation that the external resonator laser has.

4A and 4B are conceptual diagrams showing operating characteristics of a variable wavelength light source and a position of a phase control electrode. 4A and 4B, the variable wavelength light sources 300a and 300b include a gain medium 301 (a gain medium or semiconductor gain medium), an external reflector 303 (or a polymer bragg reflector (PBR)), A phase controller 305, and a monitor photodiode 307 may be included. The gain medium 301 may be implemented to provide a gain required for oscillation of the light source. The gain medium 301 is made of a semiconductor material, crystal, or gas molecules, and can obtain a gain by pumping by external light or injection of current. In a semiconductor laser or the like, a mode converter or the like may be integrated to improve optical coupling with the external reflector 303. In FIGS. 4A and 4B, as an example, the length of the gain medium 301 (L _in1 , L _in2 ) or the distance between the high reflection coating surface HR and the low reflection coating surface AR in the gain medium 301 is It may be 500 micrometers (um).

The external reflector 303 may vary the wavelength of light output from the variable wavelength light sources 300a and 300b. The external reflector 303 is implemented to change the reflected center wavelength, and may change the wavelength by, for example, a thermo-optical effect or a plasma effect. The external reflector 303 may be implemented using a mirror having no wavelength selectivity or a Bragg Grating Reflector having wavelength selectivity outside the gain medium.

The variable wavelength light sources 300a and 300b may perform a wavelength variable function based on the external reflector 303. For example, the variable wavelength light sources 300a and 300b are wavelengths at which a change in the band through which the optical signal generated by the gain medium 301 passes or reflects is generated based on external current injection, temperature change, or angle control. It can be implemented using variable filters. The tunable light sources 300a and 300b may be implemented using a polymer material or a semiconductor material. The variable-wavelength light sources 300a and 300b may cause a change in refractive index due to a plasma effect or a thermo-optical effect, or may be implemented to enable a wavelength change by using a wavelength change due to a change in a diffraction angle.

When the tunable light sources 300a and 300b are implemented as a tunable laser based on the thermo-optical effect, the gain medium 301 implemented with a superluminescent diode (SLD) and the thermo-optical effect are large. It is possible to optically couple an external reflector formed with dispersion Bragg grating on a polymer or semiconductor material.

The tunable light sources 300a and 300b may adjust the wavelength using a thermo-optical effect. Specifically, the variable wavelength light sources 300a and 300b may form a heater electrode to adjust the refractive index of the device. That is, since the driving wavelength of the light source is varied by temperature change due to heat generation of the heater electrode, a control method for changing the wavelength may be easily performed. However, since the material of the tunable light sources 300a and 300b itself can change its wavelength sensitively to temperature, the oscillation characteristics may easily change due to changes in the external environment.

Therefore, the external resonator type variable wavelength light source having only the gain medium 301 and the external reflector 303 may have unstable oscillation mode due to changes in the external environment, and a phase controller 305 is required to solve this. Do.

The phase controller 305 may be implemented by being integrated into the external reflector 303 as shown in FIG. 4A (wavelength variable light source 300), and may be implemented by being integrated in the gain medium 301 as shown in FIG. 4B (wavelength Variable light source 300b). Both cases of FIGS. 4A and 4B are functionally the same, but differ in terms of characteristic control when an actual device is implemented.

Specifically, in order to implement the variable wavelength light sources 300a and 300b, parasitic reflection that impairs characteristics should not occur between the gain medium 301 and the external reflector 303, but the gain medium 301 is relatively low. Due to the imperfections of the reflective coating or the tilt structure, the low-reflective coating surface AR may have a residual reflectance r ₂ . The reflectance viewed by the gain medium 301 becomes the reflectance r _R of the cavity mirror generated by a combination of residual reflectance r ₂ and reflection by the external reflector 303. The highly reflective coating surface HR of the gain medium 301 may have a reflectance r ₁ .

In the case where the phase controller 305 is integrated and implemented in the external reflector 303 as shown in FIG. 4A, the phase of the oscillation mode is formed because the part where residual reflection occurs is located outside the phase controller 305 based on the external reflector 303. When the phase controller 305 is operated to change the value, the internal phase of the cavity mirror is also changed, so that the shape of the reflectance itself may be changed. As an example, the distance L _ext1 between the gain medium 301 and the phase controller 305 of FIG. 4A may be less than 3 mm (mm).

In the case of Fig. 4B, since the residual reflection is placed inside the phase controller 305, only the mode moves without changing the reflectance of the cavity mirror. Accordingly, in the case of FIG. 4B, although it is advantageous in terms of controlling the reflectance characteristic, there are problems of deteriorating the characteristics of the gain medium 301 due to heat generation, electrical isolation, and an increase in the length of the resonator. For example, the distance L _ext2 between the phase controller 305 and the external reflector 303 of FIG. 4B may be less than 3 mm (mm).

A monitor current determined according to the power output from the gain medium 301 may flow through the monitor photodiode 309. The monitor photodiode 309 may monitor the power output from the gain medium 301 based on the monitor current. The monitor photodiode 309 may be located on the rear surface of the tunable light sources 300a and 300b. The monitor photodiode 309 may be located at a portion where the output power of the highly reflective coating surface HR of the gain medium 301 can be read. The location of the monitor photodiode 309 shown in FIGS. 4A and 4B is exemplary and does not limit the scope of the present invention.

5 is a diagram showing device variables to be controlled in a variable wavelength light source. The tunable light source 400 includes a gain medium 401, an external reflector 403, a phase controller 405, a mode converter 407, a monitor photodiode 409, and a module (or referred to as a thermoelectric cooler; thermoelectric cooler; TEC). The gain medium 401, the external reflector 403, the phase controller 405, and the monitor photodiode 409 are the gain medium 301, the external reflector 303, the phase controller 305, and the monitor photodiode 409 of FIG. 4A. It may be substantially the same as the diode 309. A gain medium 401, an external reflector 403, a phase controller 405, a mode converter 407, and a monitor photodiode 409 may be located on the module TEC.

Referring to FIG. 5, a phase controller 405 is located on an external reflector 403, and a mode converter 407 may run on the gain medium 401. As an example, the distance L _ext3 between the mode converter 407 and the phase controller 405 may be less than 3 mm (mm). As an example, the length (L _in3 ) of the gain medium 401 or the distance between the high reflection coating surface HR and the low reflection coating surface AR in the gain medium 401 may be 500 micrometers (um). .

In order to match the wavelength of the wavelength variable light source 400, the entire module (TEC) temperature (T _module) phase controller 405 is applied to a current to the heating elements having a wavelength near the desired wavelength is formed on the fixed image while Bragg grating of Control. In this case, the desired wavelength can be produced, but chirp reduction due to the detuned loading effect can be obtained only for some wavelengths. That is, in order to obtain a desired degree of detuning at a desired wavelength, the phase (Ф1) of the passive region, the phase (Ф2) of the active region, and the oscillation wavelength (λ _PBR ) must be simultaneously adjusted. Accordingly, the phase controller 405, which is a controllable variable, the current applied to the external reflector 403, and the temperature of the entire module TEC must be simultaneously controlled.

6A and 6B are diagrams for explaining a detune loading effect in a variable wavelength light source. Each of the drawings will be described in more detail below.

6A is a graph showing a change in reflectivity (REFLECTIVITY) according to a wavelength of PBR of an external reflector implemented as a Bragg grating and a graph showing an oscillation mode in which long wavelength detuning is performed. Referring to FIG. 6A, under the condition that the oscillation mode of the external resonator type tunable laser does not move outside the stabilization region, the effective line width increase coefficient decreases due to an increase in the effective differential gain as the wavelength increases as the wavelength increases, thereby reducing the transmission penalty.

6B is a graph showing a bit-error rate (BER) according to detuning. Referring to FIG. 6B, the lines connected with a dotted line in a square shape are before transmitted bit error rate (BER) of a signal directly modulated at 10 Gbps at a wavelength of 1545 nm, and the lines connected with a solid line are 20 km. This is the bit error rate after transmitted. The same patterns 1 to 6 indicate the same conditions (eg, wavelength values to be detuned), and the arrows indicate the long wavelength detuning direction, and it can be seen that the penalty decreases as the detuning to the long wavelength increases. For example, a wavelength to be detuned may be longer as it goes from graph 1 to graph 6 in before transmitted and after transmitted.

7A, 7B, 7C, and 7D are views for explaining a detune loading effect that reduces chirps in a variable wavelength light source that directly modulates. 7A, 7B, 7C, and 7D, the effective line width enhancement coefficient α _{transient that} can be obtained from the device according to the change in the phase φ1 of the passive region described through the embodiment of FIG. 5 The change is shown. 6A, 6B, 6C, and 6D correspond to the phase of the passive region (Ф1), and the reflectivity of the external reflector (Reflectivity of PBR; r _PBR ) is the phase of the passive region (Ф1). Depending on the curve (Refectivity of r _R ) changes. A curve of the effective line width enhancement factor (α _transient ) that can be obtained according to this reflectance change is shown. That is, FIG. 7A shows a case where the phase (Ф1) of the passive area is 40 degrees, 7b shows a case where the phase (Ф1) of the passive area is 160 degrees, 7c is a case where the phase (Ф1) of the passive area is 280 degrees, and 7d is It shows the change of the effective line width enhancement factor (α _transient ) when the phase (Ф1) is 340 degrees.

7A, 7B, 7C, and 7D, as the point on the curve of the effective line width enhancement factor α _transient proceeds to the end of the long wavelength side stabilization region obtained by calculating the mode stabilization region, the effective line width enhancement factor of the material In a device with (α _transient ) of 5, the difference in the effective line width enhancement factor (α _transient ) that can be obtained according to the phase change is large. That is, in order to obtain the best chirp characteristics at a desired wavelength, it can be seen that the oscillation mode must be placed at the end of the long wavelength in a situation where the phase of the passive region is 160 degrees (Fig. 7b), and therefore additional element parameters need to be adjusted for this. .

The method that can be used to set long wavelength detuning suitable for long-distance transmission is a method of indirectly monitoring the wavelength change through OSA, and direct transmission characteristics through measurement of a transmission eye diagram or error rate penalty. There is a way to check. However, these methods are difficult to apply in an environment in which an actual device is used because a separate measuring device must be used. In addition, when the device has three control variables, there are too many combinations to find the condition, and this measurement method takes too long. On the other hand, optical power or voltage can be monitored simply without separate measurement or control equipment, and data can be quickly collected. Therefore, practical detuning control is possible using the correlation between the detuning state and changes in these values. The present invention discloses a method for performing detuning control through monitoring of optical power.

8A and 8B are diagrams for explaining a method of determining output power in a variable wavelength light source. 8A and 8B will be described with reference to FIG. 5. The tunable laser 400 may have an output that passes through the external reflector 403 and an output that comes out through the highly reflective coating surface (HR; reflectance r1) of the gain medium 401. Among them, the output through the external reflector 403 is POWER _PBR , the output through the high reflection coating surface (HR) of the gain medium 401 is POWER _HR , and the low reflection coating surface of the gain medium 401 ( AR) is named as POWER _AR .

In addition, POWER _AR of FIG. 8A represents the power output to the external reflector 403 on the surface having the residual reflectance r2 of the gain medium 401. Due to the influence of the external reflector 403 having the reflectance curve as shown in FIG. 6A, the POWER _AR is highest near the 0 detuned frequency where the reflectance is large as in FIG. 8A. Referring to FIG. 8B, it can be seen that the output power POWER _HR to the highly reflective coating surface HR to which the characteristics according to the wavelength are applied equally shows a similar tendency to that of the POWER _AR . The transmitted ratio of FIG. 8A represents the ratio of light passing through the external reflector 403 in the POWER _AR . It can be seen that this is the smallest near the 0 detune frequency due to the influence of the reflectance and increases as the reflectance goes up to both frequency domains where the reflectance decreases. As a result, since the POWER _PBR is determined by the power (POWER _AR ) exiting the gain medium 401 and the transmission ratio, it may have the form of a graph shown in FIG. 8B. When comparing the POWER _PBR and POWER _HR , it can be seen that the degree of power change according to the detuning differs greatly. In other words, it can be seen that the power _HR received only in one direction shows a much larger change in power than the POWER _PBR affected by the wavelength dependence of the reflectance in different directions, and such a large change is advantageous for monitoring characteristics.

9A and 9B are graphs measuring power output to both ends of a variable wavelength light source. In FIGS. 9A and 9B, graphs are shown for cases in which the phase control current PC current increases or decreases. Specifically, Figures 9a and 9b are measurements of power change due to detuning. When comparing two powers under the same operating conditions, as in the results of Figures 8a and 8b, the power _HR is received as the monitor photodiode 409 and measured. It can be seen that the ratio of the monitor current (m-PD current) to that of the POWER _PBR is also changed. In addition, it shows a much clearer power change at the end of the long wavelength detuning. From these two characteristics, it can be seen that in the present invention, it is preferable to use POWER _HR for detuning control through power monitoring of the variable wavelength light source 400.

10A, 10B, 10C, and 10D are graphs illustrating output powers (POWER _PBR and POWER _HR ) according to a detuned frequency in phases of each passive region in a variable wavelength light source. Specifically, FIG. 10A shows when the phase (Ф1) of the passive region is 40 degrees, 10b is when the phase (Ф1) of the passive region is 160 degrees, 10c is when the phase (Ф1) of the passive region is 280 degrees, and 10d is the passive region. It shows the change in the output power when the phase [phi]1 is 340 degrees. Referring to FIGS. 10A, 10B, 10C, and 10D, it can be seen that the maximum power that can appear due to detuning is the smallest in FIG. 10D having a phase condition with poor chirp characteristics. The detuning control method using power will be described in detail for the two device structures mentioned in FIGS. 3A and 3B, respectively.

The device having the structure of the phase controller 305 as shown in FIG. 3B has a phase (Ф1) of a constant passive region when detuning occurs by applying a current to the phase controller 305 (detuning sweep). Therefore, only the oscillation mode is detuned without changing the external reflectance curve. Accordingly, in this case, it is possible to obtain curves in the shape of FIGS. 10A, 10B, 10C, and 10D. If this is measured at different module temperatures and the heater current applied to the external reflector 303, one of the curves of Figs. 10A, 10B, 10C, and 10D is obtained, and the best chirp condition is applied. The device conditions can be set by selecting the conditions shown in Fig. 10B.

In the device structure as shown in FIG. 3A, when the phase controller 305 is operated, the phase (Ф1) of the passive passive region is continuously changed to fix this value and calculate the POWER _HR curves of FIGS. 10A, 10B, 10C, and 10D. You can't get the same result. However, in order to operate the mode at the end of the long-wavelength stabilization region, the minimum chipping condition may be set using the property of passing through a point where the detuned frequency of approximately 180 degrees is zero. That is, if the detuning sweep is performed while changing the current of the phase controller 305 for the module temperature under various conditions and the external reflector heater current, several curves are obtained. Among them, the curve with the smallest maximum value obtains the minimum overlapping characteristic. It corresponds to the possible module temperature and the condition of the heater current applied to the external reflector 303, and under this condition, long wavelength detuning is performed and sent to the end of the stabilization region to obtain the desired minimum overlapping condition.

11A and 11B are graphs showing a transmission penalty according to a change in output power when a detuning sweep is performed while changing a heater current of an external reflector at a constant temperature according to an embodiment of the present invention. That is, FIGS. 11A and 11B are monitor photos obtained when the detuning sweep is performed while changing the wavelength by changing only the heater current of the external reflector 303 while maintaining the temperature of the module (TEC). This is a graph showing the power penalty of transmitting a signal modulated directly at 10Gbps speed under the optimized chirp condition under the POWER _HR expressed as the output current (monitor current) of the diode 309 and each module temperature (T _module ) and heater current for 20km. .

Referring to FIGS. 11A and 11B, a condition in which the maximum value on the detuning sweep curve is the smallest has a transmission penalty value. As shown in the measurement results of FIGS. 11A and 11B, it can be seen that these conditions can be obtained only for a specific wavelength in a state where the temperature is kept constant. On the other hand, the point of long-wavelength detuning under the module temperature and heater current corresponding to the minimum chirping condition is set by giving a constant offset from the minimum power point during the detuning sweep. This is because hopping of the external resonator laser mode occurs due to the chirp around the minimum value.

12A and 12B illustrate changes in rear output power that appear when a detuning sweep is performed while simultaneously changing the temperature and the heater current of the external reflector in order to maintain a constant wavelength (λ=1545.32nm) according to an embodiment of the present invention. This is a graph showing the change in transmission penalty accordingly.

Specifically, FIGS. 12A and 12B show the detuning curve (or detuning sweep curve) measured by simultaneously changing the module temperature (T _module ) and the heater current of the external reflector to keep the wavelength constant, and the transmission penalty obtained under each condition. Is the result of measuring. Referring to FIGS. 12A and 12B, if the condition including the module temperature (T _module ) is set to an optimum value at any wavelength, the operation of the minimum chirp condition is possible.

13 is a graph showing the relationship between the ambient temperature and the temperature of the gain medium and the graph showing the relationship between the ambient temperature and the oscillation wavelength of the laser. Since many optical modules have a lot of change in characteristics according to temperature, a thermistor and a thermoelectric element are used to maintain a constant temperature. However, since the temperature control through this is not perfect, the temperature of the internal parts changes due to the external temperature change, as shown in FIG. 13, thereby changing the output light characteristics. That is, when the external temperature changes, the temperature distribution inside the package of the external resonator laser changes, and accordingly, the temperature of the other part of the laser increases even though the temperature of the part monitoring the temperature with the thermistor is kept constant. In this case, even when all conditions applied to the laser are maintained the same, the operating state (or detuning state) of the device is changed along the same path as the detuning curve of FIG. 11A or 12A.

13 shows a gain medium when the ambient temperature (or external environment temperature) changes while the temperature of the module is controlled in a form in which the temperature of the tunable filter of the tunable light source (eg, external resonator type laser) is kept constant. This is an example of measuring the change of the SLD Temperature of the submount of the SLD: 301 or 401 and the change of the PBR Peak Wavelength of the external reflector (303 or 403). As can be seen from the results, even if the temperature of the module is kept constant at 25 degrees (℃), it can be seen that the temperature near the gain medium (SLD; 301 or 401) changes, and the wavelength of the laser also changes. can see.

In this case, the characteristic change due to the temperature change in the module can appear in two ways. One is the change in the center wavelength of the wavelength variable filter according to the temperature change, and the other is the phase change of the oscillation mode according to the temperature change, that is, in the form of wavelength shift. Will appear. In the case of the measurement like Fig. 13rhk, since temperature control is performed in a form that keeps the temperature of the variable-wavelength filter constant, the change in the center wavelength will hardly occur, so the change in the oscillation wavelength is mostly the phase change caused by the temperature change. It will be the wavelength change according to.

As can be seen in FIG. 11A or 12A, such a phase change can be monitored by a current flowing through the rear monitor photodiode 309 or 409. Therefore, if the operating conditions of the module initially set to the optimum value change over time due to changes in the ambient temperature (or external temperature), the characteristics of the monitor current can be kept constant. Maintenance becomes possible. This method of maintaining the detuning state of the external resonator laser is a method that can be commonly applied to devices that operate devices in a continuous oscillation form or devices that operate through high-speed data modulation, and is essential for maintaining long-term characteristics under actual module usage conditions. It becomes the way.

14 are graphs showing relationships between ambient temperature and peak wavelength. Specifically, FIG. 14 is a case in which the temperatures of the tunable laser (eg, external cavity laser; ECL) and the external reflector (eg, polymer bragg reflector; PBR) are 25 degrees (℃). These are graphs showing the relationship between the ambient temperature and the peak wavelength of.

The same method can be applied to the case of controlling the temperature of the module by measuring the temperature at the submount where the gain medium (301 or 401) has risen. This makes the internal wavelength change due to the external temperature change relatively large, as shown in FIG. However, since most of the detuning state changes according to the phase change, the change in detuning can be prevented only by controlling the current flowing through the monitor photodiode 301 or 401 (eg, the monitor current).

15 are graphs showing a relationship between an output of a phase control unit and a current of a monitor photodiode. 15 will be described with reference to FIGS. 13 and 14. It is assumed that a point having an optimum operating state found through the detuning sweep shown in FIG. 11A or 12A is a point A on the first ambient temperature curve T _A1 .

When the device is operated under conditions that maintain the optimum operating state, the detuning curve may shift to the second ambient temperature curve (T _A2 ; Ambient Temperature 2) on the drawing due to external temperature changes. Therefore, the operating point can move to point B, which is not the optimal point. This is because the internal temperature change caused by the external temperature change is a change in the degree to which the phase is changed, and in reality, the shape of the curve is almost unchanged and the parallel movement occurs.

Since the optimal operating point on the second ambient temperature curve (T _A2 ; Ambient Temperature 2) is point C, the value (or level) of the monitor current flowing through the rear monitor photodiode (301 or 401) identical to the point A through feedback control By moving to the corresponding point C, the optimum operating point of the device can be maintained. At this time, since the detuning curve only moves in parallel, the value of the monitor current flowing through the monitor photodiode (301 or 401) corresponding to point A (that is, the value with constant power) is output by adjusting only the phase control current. You can do it.

The contents described above are specific examples for carrying out the present invention. The present invention will include not only the embodiments described above, but also embodiments that can be changed in design or easily changed. In addition, the present invention will also include techniques that can be easily modified and implemented in the future using the above-described embodiments.

100, 200: WDM-PON
300, 400: variable wavelength light source

Claims (1)

In a method for controlling a variable wavelength light source comprising a gain medium and a phase controller,
Monitoring a first power of a first optical signal output from the gain medium at a first time and a second power of a second optical signal output from the gain medium at a second time;
Comparing the first power with the second power; And
And when the first power and the second power are different, adjusting a phase control current applied to the phase controller.

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