JP2005266282A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator capable of keeping light output constant with simple constitution. <P>SOLUTION: The optical modulator is equipped with an optical modulator which inputs projection light from a laser module and varies a light absorption coefficient according to a voltage applied from an optical modulator driver circuit to project modulated light, a light absorption current detecting circuit which detects a light absorption current of the optical modulator, a comparing circuit which detects the difference signal between an average value of the light absorption current detected by the light absorption current detecting circuit and a light absorption current corresponding to the voltage applied to the set optical modulator, and a temperature control circuit which controls the temperature of a temperature controller heating or cooling the optical modulator by using the difference signal from the comparing circuit. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical modulator that emits modulated light by changing a light absorption coefficient according to an applied voltage with respect to incident light, and in particular, can maintain a light output constant even when there is a change in wavelength of incident light. The present invention relates to an optical modulator that can be used.

An optical modulator of conventional example 1 (see Patent Document 1) will be described.
In a distributed feedback (EA / DFB) semiconductor laser integrated with an electroabsorption optical modulator, the difference (ΔH) between the wavelength of oscillation light of the distributed feedback semiconductor laser element and the absorption edge wavelength of the optical modulator region is calculated. Keeping constant is important in terms of transmission characteristics. The wavelength of the oscillation light of the distributed feedback semiconductor laser element portion changes at a rate of about 0.1 nm per unit temperature. The band gap wavelength of the optical modulator region changes at a rate of about 1 nm per unit temperature. Therefore, when the temperature is raised to change the temperature of the entire distributed feedback semiconductor laser to perform precise wavelength control of the oscillation light, ΔH becomes smaller at a rate of 0.9 nm / ° C. and is kept constant. Can not.

Therefore, a function for changing the absorption edge wavelength of the electroabsorption optical modulator in accordance with the oscillation wavelength of the laser light is required. For example, the absorption edge wavelength can be changed by disposing a heating element such as a heater near the electroabsorption optical modulator and controlling the temperature of the optical modulator. Here, it is assumed that the temperature of the electroabsorption optical modulator and the laser element can be controlled independently. Therefore, in order to control the absorption edge wavelength of the electroabsorption optical modulator, a feedback mechanism, that is, means for measuring the absorption edge wavelength and feeding back to the temperature control system is required.
The distributed feedback semiconductor laser element and the optical modulator area are made independent by loading a heater or other heating element only on the laser element, or by loading a heater or other heating element only on the electro-absorption optical modulator. In the case of temperature control, the temperature of the distributed feedback semiconductor laser element portion can be controlled by a thin film heater or a Peltier element.
FIG. 3 shows current-voltage characteristics when a forward bias is applied to the electroabsorption optical modulator.
Since the relationship between the current and the voltage is determined by the band gap of the semiconductor, it varies depending on the temperature. When the current value is determined, the voltage value is uniquely determined. Therefore, when the drive current I is kept constant, the voltage value V is determined to be one value depending on the temperature. Further, when the driving voltage is kept constant, the current value is determined to be one value depending on the temperature.

The absorption edge wavelength control mechanism and the optical transmission module of the electroabsorption optical modulator according to the conventional example 1 will be described with reference to FIGS.
In the vicinity of the electroabsorption optical modulator 40-1, a monitoring stripe pattern 40-4 of a waveguide different from the optical modulator and an electrode associated therewith are provided. The stripe pattern is preferably as close as possible to the stripe pattern of the electroabsorption optical modulator, but it is necessary that the stripe pattern be separated so as not to affect the operation of the electroabsorption optical modulator and the optical waveguide. A sufficiently small constant current (for example, 5 mA) is supplied to the pattern from the constant current source 70-1. When the voltage value at this time is measured by the voltmeter 70-2, the voltage value is uniquely determined by the temperature, and the absorption edge wavelength λEA of the electroabsorption optical modulator can be calculated from the voltage value. Therefore, by feeding back the monitored voltage to the temperature adjusting means 70-4 for controlling the heater or the like and changing the temperature of the optical modulator, λEA can be set to an optimum value in accordance with the oscillation wavelength of the laser light.
As an example, a case where the temperature control of the electroabsorption optical modulator is performed by the thin film resistance heater 40-3 will be described. A constant current I is made to flow through the monitor stripe pattern. Consider a temperature control system that reads the voltage value V at this time and changes the current of the thin film resistance heater 40-3 according to this value. Suppose that the oscillation wavelength of the laser becomes λ1. At this time, in order to change the temperature of the optical modulator from T ₂ to T ₁ so that the absorption edge wavelength λEA of the optical modulator 40-1 becomes λ1-ΔH, a voltage is applied using an absorption edge wavelength monitor control circuit. it may be set the drive current of the thin film resistive heater so that the value becomes V _1. When the temperature adjusting means of the optical modulator is the Peltier element 40-2, the driving current of the Peltier element may be set in the same manner.

The optical modulator assembly of Conventional Example 2 will be described with reference to FIG. 6 (see Patent Document 2).
The electroabsorption modulator 102 is monolithically integrated with the semiconductor laser 101. The heater element 105 is very close to the modulator to control the modulator temperature essentially independently of the laser temperature.
The temperature sensors 103 and 104 are used to measure the temperature of the laser and the modulation medium. The temperature sensor may be a thin film sensor such as a respective platinum resistance thermometer, or a semiconductor sensor, and the semiconductor sensor is, for example, a pn junction or a thermistor.
In many situations, the temperature of the laser changes as a result of self-heating during use and changes in ambient temperature, resulting in a change in the wavelength of the light it generates. The temperature of the light modulator can be controlled to compensate the light from the laser for wavelength changes.
The absorption edge of the modulation element is adjusted to an optimum characteristic by monitoring the extinction ratio of the photocurrent induced by the optical modulator by the extinction ratio monitor unit 107. The extinction ratio of the photocurrent is induced by the photocurrent induced by the optical signal when the optical signal passes through the modulator (on state) and the optical signal when the optical signal is absorbed by the modulator (off state). The ratio between the photocurrent and the photocurrent.
The drive current for the heater element is set to a level that maximizes this ratio.
JP 2002-323865 (paragraphs 0007 to 0009, paragraphs 0013 to 0016, FIGS. 1 and 2) JP 2003-57613 (paragraphs 0018 to 0021, paragraph 0035)

In the absorption edge wavelength control mechanism of the electroabsorption modulator of Conventional Example 1, the current value or voltage value measured with the temperature monitor stripe pattern in the absence of light incidence is used as the driving system for the thin film heater (control circuit, temperature adjusting means). ) Cannot be adapted to dynamic wavelength changes.
In the modulator assembly of Conventional Example 2, a high-speed signal corresponding to the optical modulator driving signal for detecting the H level and the L level of the photocurrent in order to monitor the extinction ratio of the photocurrent induced by the modulator. A processing circuit is required.
An object of the present invention is to provide an optical modulator having a simple configuration that can cope with a dynamic change and can maintain a constant optical output.

  An optical modulator that inputs light emitted from the laser module and changes the light absorption coefficient according to the applied voltage from the optical modulator driver circuit to emit modulated light, and light absorption that detects the light absorption current of the light modulator Outputs a difference signal between the current detection circuit, the average value of the light absorption current detected by the light absorption current detection circuit, and the reference light absorption current corresponding to the voltage applied to the set light modulator. And a temperature control circuit for controlling the temperature of the temperature regulator that heats or cools the optical modulator using the difference signal from the comparison circuit.

  In the present invention, light absorption that flows when an optical modulator generates light intensity-modulated signal by light absorption with a constant amount of light incident and a high-speed electrical signal (modulation signal) applied. Since the current is monitored and the temperature of the optical modulator is controlled based on the difference between the average value of the light absorption current and the reference light absorption current, there is no need for a high-speed signal processing circuit and only a simple electric circuit. In addition, it can handle dynamic changes in light wavelength.

FIG. 1 shows the configuration of an optical modulator (Embodiment 1) according to the present invention.
An optical modulator is an optical modulator element that modulates the intensity of incident light by applying a voltage (optical modulator drive signal) from an optical modulator driver circuit to cause light absorption by the Franz Keldisch effect or the quantum Stark effect. It comprises a built-in light modulator module 4 and a laser module 1 containing a laser element that supplies light of a certain intensity to the built-in light modulator module 4. A general laser module 1 includes a temperature detector 1-3 for controlling the temperature of the laser element 1-1 and a temperature controller for heating or cooling, for example, a Peltier element 1-4 and a laser element 1-1. A monitoring photodiode element 1-2 that receives monitor light for performing control to keep the intensity of emitted light constant, performs O / E conversion, and outputs an electrical signal proportional to the intensity of the emitted light is incorporated. An optical fiber (optical waveguide) driven by a temperature control circuit 3 that receives this monitor temperature signal and controls it to a desired temperature and a laser driver circuit 2 that includes a function that receives the monitor light amount signal and controls the intensity of the emitted light to a constant level. A constant intensity of light is supplied inside.

The optical modulator module 4 includes an optical modulator driver circuit 5 including a function of adjusting an amplitude and a bias level in order to effectively apply an electric signal (optical modulator driving signal) whose voltage changes to the optical modulator element. Similarly to the laser module, it is driven by a temperature controller incorporated for temperature control of the optical modulator element, for example, a temperature control circuit (absorption edge wavelength control) 8 for a temperature variable device of the Peltier element 4-2.
The light controlled to a constant intensity that is incident on the optical modulator element 4-1 receives absorption corresponding to the voltage change of the optical modulator drive signal from the modulator driver circuit 5 in the optical modulator element, and the intensity is modulated. An optical signal is formed. At this time, carriers corresponding to the absorbed light intensity are generated in the optical modulator element 4-1, and this is taken out by the light absorption current detection circuit 6 as a light absorption current. This light absorption current value uniquely corresponds to the intensity of the outgoing modulated light.

  The comparison circuit 7 outputs a difference signal between the average value of the light absorption current and the reference light absorption current. The reference light absorption current is the specification of the output signal light required for the transmitter including the laser element and the light modulator element (in the case of the extinction ratio, in the case of the eye mask pattern, in the case of the Q value, or The light that flows as a result of adjusting the optical modulator drive signal for the wavelength of a laser element (any wavelength, which happens to be used when making adjustments) The value of absorption current is shown. Since the light modulator element changes the light intensity (both ON and OFF) of the emitted light by absorbing the light, if the wavelength of the incident light is changed to the long wavelength side, for example, the light absorption is The light absorption current also decreases. Therefore, in this case, the temperature of the light modulator element is increased so that the value of the light absorption current becomes the reference value determined as described above. As a result, the light absorption current becomes the reference value. If they match, the light absorption state is the same as when the reference value is determined, that is, the light intensity in each of the ON / OFF states of the outgoing signal light is the same as when the reference is determined and meets the required specifications. Become. The comparison circuit 7 uses a comparator (comparator) to apply a reference voltage corresponding to the reference light absorption current to one input terminal and a voltage corresponding to the light absorption current from the light absorption current detector to the other terminal. The difference is output.

The temperature control circuit 8 controls the temperature of the light modulator element so that the light absorption current from the light absorption current detector matches the value of the reference light absorption current.
Therefore, the optical modulator can be configured by using a widely used module, element, or circuit, and the manufacturing cost can be reduced.

A second embodiment will be described with reference to FIG.
For example, if the wavelength of the light incident on the modulator element changes due to changing the control temperature of the laser element, the intensity of the light emitted from the optical modulator is assumed to be unchanged if the electrical signal applied to the modulator element is unchanged. At the same time it appears as a level change, it changes by the amount corresponding to it.

As an example in which the present invention is actively used, the laser module 1 having a built-in wavelength tunable laser element 1-10 is used in combination with an optical modulator module having an automatic wavelength tuning function as described above. A tunable optical modulator can be configured.
The light emitted from the wavelength tunable laser element 1-10 is input to the laser light wavelength monitor unit 9 to detect the light wavelength, and the arithmetic circuit 10 outputs a correction signal to the temperature control circuit 3 so as to obtain a desired light emission wavelength. The temperature control circuit 3 controls the temperature of the laser element 1-10 to change to a desired emission wavelength. The light absorption current detection circuit 6 detects the light absorption current and inputs a difference signal between the average value of the light absorption current from the comparison circuit 7 and the reference light absorption current to the temperature control circuit 8.
The temperature control circuit 8 controls the Peltier element 4-2 so as to automatically change the light absorption edge wavelength of the light modulator element by the amount of change of the incident light before the change of the incident light wavelength.
Although the case where the laser module and the optical modulator module are serially connected by an optical fiber has been described here, the same applies to a module in which the above functions are integrated in a hybrid or monolithic manner.

The figure which shows the optical modulator (Example 1) of this invention. The figure which shows the optical modulator (Example 2) of this invention. The figure which shows the current-voltage characteristic when a forward bias is applied to an electroabsorption optical modulator. The figure which shows the absorption edge wavelength control mechanism (conventional example 1) of an electroabsorption type optical modulator. The figure which shows an optical transmission module (conventional example 1). The figure which shows an optical modulator assembly (conventional example 2).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Laser module, 2 ... Laser driver circuit, 3 ... Temperature control circuit, 4 ... Optical modulator module, 5 ... Optical modulator driver circuit, 6 ... Optical absorption current detection Circuit, 7 ... Comparison circuit, 8 ... Temperature control circuit (absorption edge wavelength control)

Claims (1)

A laser module that supplies light of constant intensity;
An optical modulator driver circuit;
An optical modulator that inputs the emitted light from the laser module, changes the light absorption coefficient according to the applied voltage from the optical modulator driver circuit, and emits the modulated light;
A temperature controller for heating or cooling the light modulator;
A light absorption current detection circuit for detecting the light absorption current of the light modulator;
A comparison circuit that detects a difference signal between an average value of the light absorption current detected by the light absorption current detection circuit and a reference light absorption current that is a light absorption current corresponding to a voltage applied to the set light modulator;
An optical modulator comprising: a temperature control circuit that controls the temperature of the temperature controller using a difference signal from the comparison circuit.

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