JP2014145898A - Method of controlling light intensity modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light intensity modulator using a LN crystal capable of operating within a predetermined range even when a dc drift occurs.SOLUTION: A light intensity modulator includes an optical waveguide 3, a modulation part, a drive part, and a bias supply part 6. When a bias voltage becomes a predetermined upper limit or greater, it is forcibly switched over to a voltage to be a stable working point at a negative polarity side. When the bias voltage becomes a predetermined lower limit or smaller, it is forcibly switched over to a voltage to be a stable working point at a positive polarity side.

Description

The present invention relates to a method for controlling a light intensity modulator, and more particularly to a method for controlling a light intensity modulator using a lithium niobate (LiNbO ₃ ) crystal.

In recent years, LiNbO ₃ crystals have been synthesized, and devices utilizing the ferroelectric properties have been developed. In particular, LiNbO ₃ (hereinafter also referred to as “LN”) crystal has attracted attention because of its large electro-optic constant, and development is proceeding with a focus on waveguide type electro-optic devices.

  A device using an LN crystal is a modulator. LN modulators are widely used in optical communications, and there are two types of light intensity modulators and phase modulators. The optical intensity modulator is an external modulator that converts an electrical signal into an optical signal, and the phase modulator has a function to change the phase of the propagating light with an external voltage, and is used to correct the chromatic dispersion of the optical signal and control the degree of polarization. Used.

A dc drift and a temperature drift are generated in the light intensity modulator.
Nagata and Minowa describe the drift phenomenon in LiNbO ₃ modulators for light speed optical communications.
Hereinafter, explanation will be made with reference to this document.
Nagata, Minowa: "Reliability of LiNbO3 modulators for light-speed optical communication", Journal of the Japan Reliability Society, Vol.21. No.2, pp.86-93, 1999

  The dc drift is a phenomenon in which a modulation waveform flows over time when a dc bias is applied in order to adjust the modulation state of the light output in the operation state of the light intensity modulator.

  On the other hand, the cause of temperature drift is very complicated, but the following mechanism is considered. That is, the electric charge induced on the chip surface due to the pyroelectric effect of the LN crystal, the temperature dependence of the internal stress due to the formation of the buffer layer and the surface electrode, and the like change the refractive index of the waveguide. As a result, the phenomenon that the modulation waveform drifts is called temperature drift.

  The dc drift is a phenomenon induced by applying a dc bias. Originally, the dc bias is applied in order to keep a light output that causes a temperature drift constant. Therefore, dc drift and temperature drift are intricately intertwined.

The physical mechanism that causes the dc drift is not yet clear. However, the behavior can be explained by an equivalent circuit relating to a dielectric constituting the device. FIG. 1 is a model of the most basic equivalent circuit shown by Kiyono et al.
Kiyono et al .: Study on DC drift improvement of LiNbO3 waveguide modulator, IEICE Technical Report, OCS95-66, 55/60 (1995)

As shown in FIG. 1, the LN modulator includes an LN single crystal substrate in which an optical waveguide is embedded, an SiO ₂ buffer layer covering the same, and a high-frequency surface electrode. When an equivalent circuit corresponding to this is examined, it can be seen that the initial drift is affected by the capacitance C of the constituent material, and the long-term drift is affected by the resistance R. In particular, since the resistance R _{LN of the} LN crystal is not sufficiently large, the applied dc bias slowly drops in voltage, which becomes a main factor of long-term drift. Further, the direction of the initial drift changes depending on the resistance R _B and the capacitance C _B of the buffer layer. In an actual modulator, the buffer layer is designed so as to induce a negative drift in the initial stage and cancel the positive drift by utilizing this fact.

On the other hand, to reduce drift,
・ Use a crystal plane that can ignore the pyroelectric effect.
・ Coating the semiconductor film to uniformly distribute the induced surface charge.
・ Use highly symmetric waveguide / electrode patterns to make the stress distribution uniform.
・ Select structure / process conditions that can reduce the internal stress itself.
It can be dealt with by such methods.

  FIG. 2 shows the temperature dependence of the peak position of the output modulation waveform measured for the LN light intensity modulator. During measurement, no dc bias is applied. Between the temperature of 0 ° C. and 70 ° C., the modulation waveform of about 2.5V drifts. That is, the dc bias voltage applied to keep the modulator output constant fluctuates by a maximum of 2.5 V in this sample, reflecting changes in the environmental temperature.

  In the commercial modulator, the influence of the pyroelectric effect is almost eliminated from the above-mentioned temperature drift factors, and the drift amount can be reduced to about 10% in FIG. 2 by changing the electrode design. However, aiming for higher speed devices requires a design that leads to an increase in internal stress, such as thickening the electrodes, and it is difficult to completely eliminate temperature drift.

There are two modulation methods of the modulator: direct modulation and external modulation. Among these, external modulation techniques that can cope with higher transmission rates are disclosed in, for example, Japanese Patent Laid-Open Nos. 03-251815 and 2008-134663.
Japanese Patent Laid-Open No. 03-251815 JP 2008-134663 A

  In the external modulator control system disclosed in Japanese Patent Laid-Open No. 03-251815, the drift direction of the operating point is detected from the phase of the low-frequency signal, and the operating point of the external modulator is the same as the drift direction according to the direction. The direction is controlled.

  In the method for controlling an optical modulator disclosed in Japanese Patent Application Laid-Open No. 2008-134663, the operating point deviation of the light modulating unit is determined based on the change in the signal light output from the optical modulating unit, and the operating point deviation is determined. The bias supply unit is controlled so as to be small.

  As described above, the modulator using the LN crystal has a problem of temperature drift, and thus the optical output fluctuates. In order to prevent this, a dc bias is applied. However, by applying this dc bias, dc drift is induced. Moreover, it is known that the amount of drift increases when dc drift occurs.

  Therefore, in Japanese Patent Application Laid-Open No. 2008-134663, control is performed so that the operating point deviation is reduced. However, no reference is made to the upper limit value and lower limit value of the operating point deviation.

  In the optical intensity modulator for optical combs, a problem has been pointed out that a large dc bias is often applied and a dc drift increases.

  The present invention provides a method of controlling a light intensity modulator that can be operated within a predetermined range even if a dc drift occurs in the light intensity modulator using, for example, an LN crystal.

In order to solve the above-described problems, a method for controlling a light intensity modulator according to the present invention includes:
A method for controlling a light intensity modulator including an optical waveguide, a modulation unit, a drive unit, and a bias supply unit,
The optical waveguide has a branching waveguide, a first branching optical waveguide and a second branching optical waveguide, and a synthetic waveguide,
The branching waveguide branches incident light into the first and second branching optical waveguides,
The synthetic waveguide synthesizes light emitted from the first and second branch optical waveguides,
The modulation unit uses the first electrode and the second electrode provided in the first and second branch optical waveguides respectively to control the refractive indexes in the first and second branch optical waveguides, It has a structure to obtain periodic light intensity characteristics according to the refractive index difference,
The drive unit applies a drive signal to at least one of the first and second electrodes so that the modulation unit performs a modulation operation corresponding to one cycle of the light intensity characteristic;
The bias supply unit adjusts an operating point by supplying a bias voltage for control to the modulation unit,
In the light intensity modulator, when the bias voltage exceeds a preset upper limit value, the bias voltage is forcibly switched to a bias voltage that is a stable operating point on the negative polarity side, or the bias voltage is preset. When the value is below the lower limit, the bias voltage is forcibly switched to a stable operating point on the positive polarity side.

  In the light intensity modulator control method according to the present invention, when the bias voltage exceeds a preset upper limit value or lower limit value, the bias voltage is operated so as to operate at a stable operating point on the negative electrode side and the positive electrode side, respectively. Therefore, the generated dc drift can be kept within a predetermined range.

It is a model of an equivalent circuit in the most basic LN modulator shown by Kiyono et al. It is a graph which shows the temperature dependence of the peak position of an output modulation waveform measured about the LN light intensity modulator. It is a figure which shows an example of the light intensity modulator which can apply the control method by this invention. It is a figure which shows an example of the bias control circuit which can apply the control method by this invention. It is a figure for demonstrating an operating point in the control method by this invention.

FIG. 3 shows an example of a light intensity modulator to which the control method according to the present invention can be applied.
The light intensity modulator 100 includes a light source 1, an input coupler 2, an optical waveguide 3, and an output coupler 4, and further includes an oscillator 5 and a bias control circuit 6.

  A light source 1 having a laser diode (LD) as a light emitting element emits light having a wavelength of 1550 nm, for example, and the emitted light is incident on an optical waveguide 3 through an input coupler 2.

  Further, the signal output from the oscillator 5 is branched into two after passing through the front attenuator 71 and the front RF amplifier 81, and one of them is passed through the first attenuator 72 and the first RF amplifier 82 to be the first branch. The other is supplied to the first electrode provided in the optical waveguide 31, and the other is supplied via the phase shifter 9 to the second electrode provided in the second branch optical waveguide 32.

  Further, the bias control circuit 6 is supplied with an optical input monitor signal and an optical output monitor signal. The bias control circuit 6 has a function of determining whether the bias voltage is equal to or higher than a preset upper limit value or lower than a preset lower limit value from the potential difference between the monitor signals.

  The bias control circuit 6 forcibly switches to a bias voltage that is a stable operating point on the negative polarity side when the bias voltage exceeds a preset upper limit value, or the bias voltage is preset. It has a function of forcibly switching to a bias voltage that is a stable operating point on the positive polarity side when the lower limit is reached.

  Specifically, as shown in FIG. 4, the potential difference signal output from the monitor signal input to the input 1 and input 2 of the bias control circuit by the differential amplifier 61 is set to a preset upper limit value or The lower limit value is compared with the OP amplifiers 62 and 63. When the potential difference signal exceeds the preset upper limit value or lower limit value for a predetermined time, the switches 64 and 65 are turned on to operate at the stable operating points on the negative electrode side and the positive electrode side, respectively. Therefore, the bias voltage is forcibly switched and output.

The operating point in the method of controlling the light intensity modulator according to the present invention will be described with reference to FIG. In FIG. 5, the horizontal axis represents the bias voltage, and the vertical axis represents the transmittance.
Here, it is assumed that the light intensity modulator is operating at the second operating point in the figure. In the optical intensity modulator, an optical signal input from the left side in the figure is modulated by the optical intensity modulator and emitted as a modulated optical signal.

  When the operating point drifts from the second operating point to a higher voltage due to the occurrence of dc drift, the operating point may become equal to or higher than the upper limit value of the operating point. In the present invention, when the operating point becomes equal to or higher than the upper limit value, the control is forcibly switched to the bias voltage at the first operating point which is a stable operating point on the negative polarity side in the figure. Furthermore, the first operating point is preferably a bias voltage closest to zero among the stable operating points on the negative polarity side.

  If the operating point drifts to a higher voltage, switching to the bias voltage to a stable operating point on the negative polarity side will cause the operating point to drift within a predetermined range even if the operating point drifts to a higher voltage. This is preferable because the light intensity modulator can be operated for a long time.

  Also, assuming that the light intensity modulator is operating at the first operating point in the figure, and the operating point drifts from the first operating point to a lower voltage due to the occurrence of dc drift, the operating point becomes the lower limit. It can happen below the value. In the present invention, when the operating point is equal to or lower than the lower limit value, the control is forcibly switched to the bias voltage at the second operating point, which is the stable operating point on the positive polarity side in the figure. Furthermore, the second operating point is preferably a bias voltage that is closest to zero among the stable operating points on the positive polarity side.

  If the operating point drifts to a lower voltage, switching to the bias voltage to a stable operating point on the positive polarity side will cause the operating point to drift within a predetermined range even if the operating point drifts to a lower voltage. This is preferable because the light intensity modulator can be operated for a long time.

The method for controlling an optical intensity modulator according to the present invention is preferably applied to an optical intensity modulator that is particularly suitable for generating an optical comb.
The “optical comb” is a laser beam in which light of various wavelengths is arranged at regular intervals like a comb tooth.

  In the method of controlling the light intensity modulator according to the present invention, the bias voltage is forcibly switched, so that the light intensity modulation is interrupted instantaneously. However, for optical comb applications, even if the light intensity modulation is momentarily interrupted, there is no problem, and the present invention can be preferably applied.

  According to the light intensity modulator control method of the present invention, even if the operating point drifts due to the occurrence of dc drift, the bias voltage is forcibly switched to a stable operating point. Can be operated stably. Therefore, the control method of the light intensity modulator according to the present invention can be preferably used particularly for an optical comb application in which dc drift is likely to occur.

100 light intensity modulator,
1 light source,
2 input coupler,
3 Optical waveguide,
31 1st branch optical waveguide,
32 second branched optical waveguide,
4 output coupler,
5 Oscillator,
6 Bias control circuit,
61 differential amplifier,
62,63 OP amplifier,
64, 65 switches,
71 Pre-stage attenuator,
72 first attenuator,
81 Pre-stage RF amplifier,
82 1st RF amplifier,
9 Phase shifter,

Claims (3)

A method for controlling a light intensity modulator including an optical waveguide, a modulation unit, a drive unit, and a bias supply unit,
The optical waveguide has a branching waveguide, a first branching optical waveguide and a second branching optical waveguide, and a synthetic waveguide,
The branching waveguide branches incident light into the first and second branching optical waveguides,
The synthetic waveguide synthesizes light emitted from the first and second branch optical waveguides,
The modulation unit uses the first electrode and the second electrode provided in the first and second branch optical waveguides respectively to control the refractive indexes in the first and second branch optical waveguides, It has a structure to obtain periodic light intensity characteristics according to the refractive index difference,
The drive unit applies a drive signal to at least one of the first and second electrodes so that the modulation unit performs a modulation operation corresponding to one cycle of the light intensity characteristic;
The bias supply unit adjusts an operating point by supplying a bias voltage for control to the modulation unit,
In the light intensity modulator, when the bias voltage exceeds a preset upper limit value, the bias voltage is forcibly switched to a bias voltage that is a stable operating point on the negative polarity side, or the bias voltage is preset. A control method for a light intensity modulator, wherein the bias voltage is forcibly switched to a bias voltage that is a stable operating point on the positive polarity side when the value is below the lower limit. In the control method of the light intensity modulator according to claim 1,
The method of controlling a light intensity modulator, wherein the operating point to be switched when the bias voltage is equal to or higher than a preset upper limit value is set to an operating point closest to zero on the negative polarity side. In the control method of the light intensity modulator according to claim 1,
The method of controlling a light intensity modulator, wherein the operating point to be switched when the bias voltage is equal to or lower than a preset lower limit value is set to an operating point closest to zero on the positive polarity side.

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