Driving method for electroabsorption modulators operating in burst mode

Abstract

The present invention relates to a method for driving an electroabsorption modulator operating in burst mode. The method for driving an electroabsorption modulator according to the present invention is a method for driving an electroabsorption modulator operating in burst mode, in which the magnitudes of the DC voltage and AC voltage for driving the electroabsorption modulator are reduced functionally over time from the point at which laser light is injected into the electroabsorption modulator and light absorption begins to proceed, thereby offsetting light absorption characteristics due to temperature changes in the electroabsorption modulator region.

Description

[Technical Field]

[0001] The present invention relates to a method for driving a modulator, and more particularly to a method for driving an electroabsorption modulator operating in burst mode. [Background technology]

[0002] Currently, light emitting devices used in high-speed optical communications are in the form of modulator-integrated semiconductor lasers. In a modulator-integrated semiconductor laser (Electro-Absorption Modulator Integrated DFB-LD: EML), the DFB-LD (Distributed Feed Back-Laser Diode) is operated continuously, and an optical modulator is attached to the optical output section of the DFB-LD to generate a modulated signal.

[0003] The operating characteristics of such an optical modulator are shown in Figure 1.

[0004] When no reverse voltage is applied, the modulator's absorption spectrum curve shifts to a longer wavelength as the absorption spectrum curve decreases when a reverse voltage is applied. In Figure 1, when the electric field E=0V / cm is applied to the B wavelength light source being pulled into the modulator, the modulator has a low absorption coefficient of B1. When a voltage of 5.4E4V / cm is applied to the modulator, the modulator's absorption spectrum changes to a high absorption coefficient of B2. The difference between the absorption coefficients of B1 and B2 changes the intensity of the light passing through the modulator. An element in which the intensity of light passing through such a modulator is determined by the magnitude of the reverse voltage applied to the modulator is called an electroabsorption modulator.

[0005] Depending on the bandgap of the modulator itself and the wavelength of the light being pulled in, the same modulator can operate at wavelengths C and D in Figure 1. Therefore, a well-defined relationship is required between the bandgap of the modulator itself and the wavelength being pulled in. Figure 2 shows the absorption spectrum of the modulator itself measured at a temperature of 300K, and the change in the absorption spectrum when a voltage is applied to a modulator maintained at 300K. However, all materials change temperature when they absorb energy, and this changes depending on the amount of energy absorbed and the heat dissipation of the material. The bandgap of semiconductor materials used as absorbing materials in modulators changes with temperature, and the absorption spectrum shifts from this changed bandgap wavelength due to the electric field. This is shown in Figure 2.

[0006] In Figure 3, the blue absorption spectrum curve represents the absorption spectrum curve as a function of temperature when no voltage is applied, while the ochre curve represents the absorption spectrum curve shifted with voltage at each temperature. Therefore, to obtain the correct extinction ratio (the ratio of the signal strength of a "1" signal to a "0" signal) and the light intensity passing through the modulator, the modulator temperature and applied voltage must be precisely adjusted. In other words, in Figure 2, the extinction ratio when zero voltage (E = 0 V / cm) and E = 5.4E4 V / cm are applied to an input light wavelength "B" at a temperature of 300 K will be different from the extinction ratio when zero voltage (E = 0 V / cm) and E = 5.4E4 V / cm are applied to the same input light wavelength at a modulator temperature of 310 K, and the average optical output will also change.

[0007] The optical signal generated by the modulator with "1" and "0" signals is transmitted through an optical fiber, and the "1" and "0" signals are extracted at the optical receiving end using the statistical power and signal extinction ratio of the "1" and "0" signals. Therefore, if the modulator is driven using the same voltage from the time of its operation, the temperature of the modulator changes over time, which causes changes in the average power and extinction ratio of the signal generated by the modulator, which acts as an error at the receiving end.

[0008] Conventional electroabsorption modulators operate in CW (continuous wave) mode, and after an initial period of time, the temperature of the modulator stabilizes and no additional temperature changes occur in the modulator region. However, in a modulator operating in burst mode (Figure 4), the drive changes between periods when no light is applied to the modulator and periods when light is applied to the modulator to form a modulated signal, so the temperature of the modulator's optical absorption region experiences continuous changes in response to burst operation, making it difficult to transmit signals using a burst mode modulator. [Prior art documents] [Patent documents]

[0009] (Patent Document 1) Republic of Korea Patent Registration No. 10-0362060 (Registered November 11, 2002) Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention has been proposed to solve the problems that occur in the conventional electroabsorption modulator operating in burst mode. The object of the present invention is to provide a method for driving an electroabsorption modulator that can solve the problem that, in an electroabsorption modulator operating in burst mode, the intensity of the light source drawn into the modulator changes depending on the operation of the light source, and as a result, the temperature of the modulator changes depending on the intensity of the light absorbed by the modulator. Under modulator driving conditions that do not take the temperature change of the modulator into consideration, the average intensity and extinction ratio of the light passing through the modulator change over time, making it difficult to receive a signal at the receiving end. [Means for solving the problem]

[0011] To achieve the above object, the method for driving an electroabsorption modulator according to the present invention is a method for driving an electroabsorption modulator that operates in burst mode, in which the magnitudes of the DC voltage and AC voltage that drive the electroabsorption modulator are functionally reduced over time from the point at which laser light is injected into the electroabsorption modulator and light absorption begins to progress.

[0012] At this time, the function by which the magnitude of the DC voltage and AC voltage decreases over time is determined in advance and recorded in memory.

[0013] Meanwhile, it is preferable that a laser diode, which is a light source, is integrally fabricated and operated at the optical input portion of the electroabsorption modulator.

[0014] Also, an optical amplifier can be integrally fabricated and operated at the optical output portion of the electroabsorption modulator.

[0015] The electroabsorption modulator may be a ridge type modulator having a deep ridge structure.

[0016] Meanwhile, the electroabsorption modulator generates a "1" signal and a "0" signal by applying a DC voltage +1 / 2 modulation voltage and a "1" signal by applying a DC voltage -1 / 2 modulation voltage, or by applying a modulation voltage with an equivalent process, and it is preferable that the decrease functions of the DC voltage and modulation voltage after the electroabsorption modulator is operated decrease exponentially with time.

[0017] In addition, the electroabsorption modulator generates a "1" signal and a "0" signal by applying a DC voltage +1 / 2 modulation voltage and a "1" signal by applying a DC voltage -1 / 2 modulation voltage, or a modulation voltage with an equivalent process, and the decreasing function of the DC voltage and modulation voltage after the electroabsorption modulator is activated can be a function that has the characteristic of offsetting the temperature rise of the electroabsorption modulator over time.

[0018] The laser diode may be configured with a reverse mesa structure with a heater mounted on top.

[0019] Preferably, the electroabsorption modulator has a reverse mesa structure with a heater mounted on top.

[0020] Meanwhile, a heater mounted on the top of the electroabsorption modulator is driven to offset the temperature change caused by light absorption of the modulator operating in burst mode.

[0021] Also, the heater mounted on the upper portion of the electroabsorption modulator may be mounted on the upper electrode of the modulator, separated by an insulating film.

[0022] The heater mounted on the top of the electroabsorption modulator may be connected to an external electrode in the form of an air bridge. [Effects of the Invention]

[0023] The method of driving an electroabsorption modulator operating in burst mode of the present invention has the effect of suppressing temperature changes due to light absorption in a modulator operating in burst mode, as well as changes in optical output and extinction ratio due to temperature changes, thereby facilitating signal discrimination at the optical receiving end. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a graph showing the change in absorption spectrum of an electroabsorption modulator depending on the applied voltage. [Figure 2] The operating principle of the modulator in response to the drive voltage is shown. [Figure 3] This is an example showing that when a modulator is driven with a constant driving voltage regardless of the temperature of the modulator, the extinction ratio of the modulated signal changes due to the change in the temperature of the modulator. [Figure 4] 1 shows an example of a burst mode operation of a modulator. [Figure 5]1 shows an example of the optical output characteristics of a modulator modulated by varying the modulator drive voltage to offset temperature changes in the absorption region of the modulator according to the present invention. [Figure 6] 1 shows an example of temperature change with light absorption time in a modulator region operating in burst mode according to the present invention. [Figure 7] 10 shows another example of the temperature change in the modulator region due to light absorption from the start of operation of the modulator according to the present invention. [Figure 8] 1 shows an example of an inverted mesa ridge structure light source to which a heater according to the present invention is attached. [Figure 9] 1 shows an example of a modulator having an inverted mesa ridge structure with a deep ridge structure to which a heater according to the present invention is attached. [Figure 10] An example of a heater driving method in a modulator equipped with a heater according to the present invention will be described. [Figure 11] An example of the temperature change of the modulator when burst on is started in the modulator according to the present invention, light absorption occurs, and the heater is turned off at the same time, and heat generation by the heater ceases is shown below. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, preferred, non-limiting embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0026] FIG. 5 shows an example of the optical output characteristics of a modulator according to the present invention modulated by varying the modulator drive voltage to compensate for temperature variations in the absorption region of the modulator.

[0027] Figure 5 shows the extinction ratio at a temperature of 300K under drive voltage conditions of E = 0V / cm and E = 5.4V / cm, shown in blue. The solid blue line is the absorption spectrum for 300K and E = 0V / cm, where there is almost no absorption of the "B" wavelength. The dotted blue line shows the absorption when an electric field of 5.4E4V / cm is applied, from which the initial extinction ratio is determined. However, once this modulator begins to absorb light and the temperature of the modulator increases to 310K, the extinction ratio becomes significantly larger when the same electric fields of 0V / cm and 5.4E4V / cm are applied. Therefore, if the electric field is maintained constant, the temperature of the modulator region will change over time, and the resulting change in the extinction ratio will be unavoidable.

[0028] In Figure 5, when the temperature of the modulator rises to 310K, it can be seen that when the electric field of the modulator is changed between 0V / cm and 2.1E4V / cm, an extinction ratio similar to that obtained at 300K, 0V / cm and 5.4E4V / cm is obtained.

[0029] As shown in Figure 5, the change in extinction ratio due to the temperature change caused by the start of burst mode and the change in the absorption curve in response to the temperature change of the modulator can be minimized by changing the drive voltage in a way that minimizes this change in extinction ratio at the start and end of the burst.

[0030] Although the present invention has been described using an electric field, since the length of the modulator in the electric field direction is fixed, changes in the electric field can be achieved by changing the voltage applied to the modulator. Typically, the modulator voltage is given as the sum of a DC voltage and an AC modulation voltage corresponding to the "1" and "0" signals, so changes in the extinction ratio due to changes in the modulator temperature can be offset by changes in the DC voltage and modulation voltage caused by changes in the modulator temperature. "1" signal voltage = DC voltage - 1 / 2 AC modulation voltage "0" signal voltage = DC voltage + 1 / 2 AC modulation voltage

[0031] However, the modulation voltage does not necessarily have to be realized in this manner, and various methods for applying voltages corresponding to "1" and "0" signals are possible.

[0032] Basically, the temperature change of the modulator induces a change in the absorption spectrum, and in order to determine the change in the modulation voltage to offset the change in the modulation characteristics of the modulator, it is first necessary to measure the temperature change of the modulator depending on the driving time of the modulator. The temperature change from the start of the modulator operation varies depending on the structure of the modulator and the intensity of the light drawn into the modulator, and Figures 6 and 7 show an example of the temperature change from the start of the modulator operation.

[0033] In Figures 6 and 7, the temperature change of the modulator can be modeled using exponential and higher-order fourth-power terms. Because the wavelength shift of the modulator varies almost linearly with temperature, as in the examples shown in Figures 6 and 7, the temperature change of the modulator is first modeled, the corresponding change in the modulator spectrum is inferred, and then the DC voltage and AC modulation voltage required to offset the corresponding changes in the modulator's characteristics, such as its extinction ratio, can be calculated as a function of time. While Figures 6 and 7 can vary in many ways, the key point is the characteristic of applying varying DC and AC modulation voltages to the modulator to offset the temperature change over time and the changes in the modulator characteristics at each time.

[0034] One possible method is to convert the changes in the DC voltage and AC modulation voltage of the modulator into a function from the start of the burst signal, and then adjust the actual modulation voltage of the modulator from this function. Alternatively, a time look-up table can be stored in memory, and the DC voltage and AC modulation voltage for driving the modulator can be read from this memory to drive the modulator.

[0035] To fabricate a modulator that operates in burst mode, a light source that operates in burst mode is required, and the modulator and light source are usually fabricated as a single unit. Such a light source is composed of a DFB-LD or DBR-LD (Distributed Bragg Reflector Laser Diode), and the light source wavelength itself changes depending on the burst mode operation of the light source. Therefore, it is necessary to suppress the change in the light source wavelength itself, and this can be achieved by incorporating an electric heater on one side or on top of the DFB-LD. To make it easy to incorporate such a heater, it is appropriate for the light source structure of the DFB-LD to be a reverse mesa-ridge structure.

[0036] 8 shows an example of an inverted mesa ridge light source equipped with a heater according to an embodiment of the present invention. The current for operating the light source is injected into the electrode (p-type electrode) for the DFB-LD shown in FIG. 8 . The heater is disposed on top of the electrode for the DFB-LD and driven complementarily with the current for operating the light source to offset temperature changes in the laser diode region due to burst mode operation of the laser diode. That is, the heater is mounted on top of the DFB-LD light source, and the DBB-LD light source has an inverted mesa ridge structure to facilitate mounting the heater on the top. The inverted mesa ridge refers to a structure in which the top of a clad layer made of p-InP / p-InGaAs layers is wider than the bottom. In FIG. 8 , since the electrode for driving the light source is disposed on top of the mesa, the heater is preferably fabricated overlapping the top electrode for driving the light source, separated by an insulating film.

[0037] The characteristics to be improved in the present invention are that the period during which the modulator absorbs light and modulates and transmits occurs in the form of a burst, and during periods other than the burst-on time, there is no light absorption in the modulator, causing the modulator to cool, and during the burst-on period, light absorption begins and the temperature of the modulator rises, causing the light absorption characteristics to change over time. Therefore, the above-mentioned method maintains the light absorption characteristics of the modulator constant by using a change in the modulator driving voltage to offset the temperature change of the modulator, and a method of attaching a heater to the modulator can be used as a method of maintaining the light absorption characteristics of the optical modulator constant after the burst-on.

[0038] FIG. 9 shows an example of a modulator having an inverted mesa ridge structure with a deep ridge structure to which a heater is attached according to an embodiment of the present invention.

[0039] As shown in Figure 9, two grooves are formed on both sides of the modulator with an inverted mesa ridge structure in the substrate. These grooves form a deep ridge structure, which improves the modulator's characteristics by concentrating the modulator's electric field in the active region. Figure 9 shows a method of maintaining a constant temperature of the modulator regardless of whether the modulator is operating, thereby preventing temperature changes due to optical absorption by the modulator and changes in optical modulation characteristics due to temperature changes, by attaching a heater to the modulator region and keeping the heater off when the modulator is performing its optical absorption / optical modulation / optical transmission functions, and operating the heater to prevent cooling when the modulator is not performing its optical absorption / optical modulation / optical transmission functions and cooling down.

[0040] In Figure 9, the heater is mounted on the top of the modulator, and the modulator preferably has an inverted mesa structure to facilitate mounting the heater on the top. The inverted mesa ridge refers to a structure in which the top of a cladding layer made up of p-InP / p-InGaAs layers is wider than the bottom. The bottom width of the ridge is 1-2 um so that light has a single transverse mode, while the top width of the ridge can be at least 0.5 um wider. It is more preferable for the cross section of the ridge to have a (111) plane, in which case the top width of the ridge is 2-3 um wider than the bottom width, making it easier to mount an additional heater.

[0041] In FIG. 9, it is preferable to minimize capacitance in the high-speed modulator, and therefore the heater electrode is preferably connected to the external electrode in an air bridge configuration to minimize parasitic capacitance. This air bridge-type electrode is fabricated so that the portion where the electrode metal is connected to the pad is suspended in the air, which minimizes parasitic capacitance that occurs at the heater pad connection portion and facilitates high-speed operation of the device. Furthermore, since the electrode for driving the modulator is disposed on top of the mesa, it is preferable that the heater be separated by an insulating film and fabricated overlapping the upper electrode for driving the modulator. For the sake of simplicity, an insulating film for protecting the InP is not shown in FIG. 9, but a surface protection layer such as an insulating film as in a typical modulator structure may be added.

[0042] Figure 10 shows a heater driving method for a modulator equipped with a heater. The heater is turned off during burst-on periods when optical absorption / optical modulation / optical transmission occurs in the modulator, and is activated during burst-off periods when no light enters the modulator, substituting for the optical absorption power occurring in the modulator, thereby maintaining a constant temperature of the modulator regardless of burst-on / off.

[0043] Figure 11 shows the temperature change of the modulator when burst on begins in the modulator, light absorption occurs, and at the same time the heater stops generating heat, eliminating the heater's heat generation. Curve (a) of Figure 11, the heater temperature, shows the temperature change of the modulator over time when there is no light absorption and the heater stops generating heat, while curve (b) of Figure 11, the light absorption temperature, shows the temperature change of the modulator when light absorption begins in the modulator with the heater not operating. Therefore, when both of these effects occur simultaneously, curve (c) of Figure 11 shows the average effect of both effects (a) and (b). As shown by curve (c) of Figure 11, the modulator temperature remains constant regardless of burst on / off, resulting in stable modulator characteristics.

[0044] In this way, in the present invention, the DC voltage and modulation voltage that drive the modulator are changed as a function of time, and the modulator is driven so as to offset the temperature change of the modulator over the modulator drive time, thereby making it possible to maintain constant the average intensity and extinction ratio of the modulator output optical signal.

[0045] On the other hand, in the aforementioned FIG. 9, the heater power modulation does not need to alternate between a predetermined power and zero power. The object of the present invention can be achieved as long as the heater modulation power offsets the change in heat quantity due to light absorption by the modulator.

[0046] Also, in Figures 8 and 9, it is explained that the light source such as a DFB-LD has an inverted mesa ridge structure, and that the modulator also has an inverted mesa ridge structure, but it is also possible to combine a light source such as a DBR-LD with a mesa structure and an inverted mesa modulator structure equipped with a heater.

[0047] As such, the present invention is not limited to the above-described embodiments, and it goes without saying that various modifications and variations can be made by a person having ordinary skill in the art to which the present invention pertains within the technical spirit of the present invention and the equivalent scope of the claims set forth below.

Claims

1. A method for driving an electroabsorption modulator operating in burst mode, comprising: A method for driving an electroabsorption modulator, characterized in that the magnitudes of the DC voltage and AC voltage for driving the electroabsorption modulator decrease functionally over time from the point at which laser light is injected into the electroabsorption modulator and light absorption begins to progress.

2. 2. The method of claim 1, wherein the function of decreasing the magnitude of the DC voltage and the AC voltage with the passage of time is predetermined and stored in a memory.

3. 2. The method of claim 1, wherein a laser diode as a light source is integrally fabricated and operated at an optical input portion of the electroabsorption modulator.

4. 2. The method for driving an electroabsorption modulator according to claim 1, wherein an optical amplifier is integrally fabricated and operated at an optical output portion of the electroabsorption modulator.

5. 2. The method of claim 1, wherein the electroabsorption modulator is a ridge-type modulator having a deep ridge structure.

6. The electroabsorption modulator generates a "1" signal and a "0" signal by applying a DC voltage + 1 / 2 modulation voltage, and a "1" signal by applying a DC voltage - 1 / 2 modulation voltage, or by applying a modulation voltage of an equivalent process; 2. The method of claim 1, wherein the DC voltage and the modulation voltage decrease exponentially with time after the electroabsorption modulator is activated.

7. The electroabsorption modulator generates a "1" signal and a "0" signal by applying a DC voltage + 1 / 2 modulation voltage, and a "1" signal by applying a DC voltage - 1 / 2 modulation voltage, or by applying a modulation voltage of an equivalent process; 2. The method for driving an electroabsorption modulator according to claim 1, wherein the decreasing function of the DC voltage and the modulation voltage after the electroabsorption modulator is activated is a function having a characteristic of offsetting the temperature rise of the electroabsorption modulator over time.

8. 4. The method of claim 3, wherein the laser diode has a reverse mesa structure with a heater mounted on top.

9. 2. The method of claim 1, wherein the electroabsorption modulator has a reverse mesa structure with a heater mounted on the top.

10. 10. The method of claim 9, wherein the heater mounted on the upper part of the electroabsorption modulator is driven to offset a temperature change caused by light absorption of the modulator operating in a burst mode.

11. 10. The method of claim 9, wherein the heater mounted on the upper portion of the electroabsorption modulator is mounted on the upper electrode of the modulator and separated by an insulating film.

12. 10. The method of claim 9, wherein the heater mounted on the electroabsorption modulator is connected to an external electrode in the form of an air bridge.

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