JP2003139653A - Method and apparatus for measurement of characteristic of light modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and an apparatus wherein the characteristic such as the half-wave voltage value, the chirp parameter value or the like of a light modulator in a high-frequency modulation can be measured precisely. SOLUTION: With reference to the light modulator in which incident light is branched into two or more beams of light, in which a phase modulation is performed by applying an electrical signal to at least one from among the branched beams of light and in which an intensity-modulated light signal is generated by compositing the branched beams of light, the spectral distribution of the light signal is measured, and a characteristic value regarding the intensity modulation of the light modulator is calculated on the basis of a measured value regarding the measured spectral distribution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Mach-Zehnder interferometer type optical modulator (hereinafter referred to as an optical modulator) used for optical communication and optical measurement.
(For example, MZ type optical modulator), the incident light is branched into two or more, and an electric signal is applied to at least one of the branched lights to perform phase modulation, and then the branched light is The present invention relates to a method and apparatus for measuring characteristics of an optical modulator that generates an intensity-modulated optical signal by combining, and particularly measures characteristics such as half-wave voltage value and chirp parameter value of the optical modulator during high frequency modulation. Method and device.

[0002]

2. Description of the Related Art Due to the recent increase in communication demand for voice, data, etc., there is an increasing demand for an optical communication system capable of high-speed communication of a large amount of data. In such a large capacity and high speed communication system, an MZ type optical modulator capable of high speed modulation operation for data generation is used.

The basic operation of the MZ type optical modulator will be described. As shown in FIG. 1, the MZ type optical modulator includes LiNbO.
_{On the} substrate 10 having an electro-optical effect such as _3, an optical waveguide 20 for guiding a light wave, an electrode (not shown) for applying a high speed modulation signal in the microwave band to the light wave, and the like are configured. To be done. The operation principle of the MZ type optical modulator is that the light input from one end of the optical waveguide 20 is branched on the way and the refractive index changes depending on the magnitude of the voltage of the electric signal applied from the signal source 30. As a result of passing through the substrate, a difference in speed occurs between the light beams, and when the branched light beams merge, a phase shift occurs between them, and the combined optical output changes in intensity according to the electrical signal. Indicates.

FIG. 2 is a graph showing changes in the optical output intensity with respect to the input voltage of the signal source 30 applied to the MZ type optical modulator. The point a in FIG. 2 is the “On operation” state in which the light passing through the MZ optical modulator is maximized, and the point b is the “Off operation” state in which the light passing through is minimized. The voltage required for the switching operation between "operation" and "Off operation" (the magnitude of the input voltage value between points a and b) is called a half-wave voltage (V _π ), and the characteristics of the optical modulator are It is one of the important parameters to evaluate.

A method for measuring the half-wave voltage is shown in FIG.
, The voltage of the waveform shown as "electrical input waveform",
It is applied to the MZ type optical modulator from the signal source 30 of FIG. 1, and the light intensity waveform of the output light from the MZ type optical modulator is directly observed by a sampling oscilloscope or the like. The light intensity waveform of the output light observed by the sampling oscilloscope is shown as "observation waveform of the optical modulator output" in FIG. Then, the voltage applied to the optical modulator, that is, the input amplitude value of the “electrical input waveform” in FIG. 3 is measured in the state where the amplitude value of the observed waveform is maximum as shown in FIG. The half-wave voltage of the modulator was measured.

[0006]

The characteristics of the optical modulator, such as the half-wavelength voltage value, change depending on the frequency of the electric signal applied to the optical modulator even if the optical modulator is the same. Moreover,
With the increase in speed and capacity of optical communication in recent years, the driving frequency of the optical modulator has been increased, and it has been required to measure characteristics such as an accurate half-wave voltage value even at a frequency of 10 GHz or higher. However, when a measuring instrument such as a sampling oscilloscope that directly observes the waveform is used, if the frequency of the applied voltage becomes high, it becomes difficult to observe the accurate waveform due to the frequency characteristic problem of the light receiving system. Moreover, it is more difficult to accurately measure the characteristics of the optical modulator such as the half-wave voltage value due to the effects of insufficient output of the signal generator and amplifier that drive the optical modulator, and distortion of the waveform due to harmonics. Was becoming.

An object of the present invention is to solve the above problems and provide a method and apparatus for accurately measuring characteristics such as half-wavelength voltage value and chirp parameter value of an optical modulator during high frequency modulation. Is.

[0008]

In order to solve the above problems, in the characteristic measuring method for an optical modulator according to claim 1, the incident light is branched into two or more, and an electric signal is output to at least one of the branched lights. Is applied to perform phase modulation, and then the spectral distribution of the optical signal is measured with respect to an optical modulator that generates an intensity-modulated optical signal by combining the branched lights. A characteristic value relating to intensity modulation of the optical modulator is calculated from a measured value relating to a spectral distribution.

The optical modulator characteristic measuring method according to a second aspect is the optical modulator characteristic measuring method according to the first aspect, wherein the electric signal comprises a bias voltage and a modulation signal.
The measured value is the light intensity value P _0b of the emitted light of the spectrum component when the light intensity of the emitted light in the same spectrum component as the incident light is maximum due to the variable adjustment of the bias voltage, and the same spectrum component as the incident light. Is the light intensity value P _1c of the emitted light of the component related to the modulation frequency when the light intensity of the emitted light is minimum, and the light intensity value P _0a of the emitted light when the modulation signal is not applied. When calculating the characteristic value related to the intensity modulation of the container, a value obtained by normalizing the light intensity value P _0b and the light intensity value P _1c by the light intensity value P _0a is used.

According to a third aspect of the present invention, there is provided an optical modulator characteristic measuring method according to the first or second aspect, wherein the characteristic value relating to the intensity modulation of the optical modulator is the optical modulation characteristic. It is characterized by a half-wave voltage value of the container.

According to a fourth aspect of the present invention, there is provided an optical modulator characteristic measuring method according to the first or second aspect, wherein the characteristic value relating to intensity modulation of the optical modulator is the optical modulation characteristic. It is the chirp parameter value of the vessel.

An optical modulator characteristic measuring device according to a fifth aspect is characterized by using the optical modulator characteristic measuring method according to any one of the first to fourth aspects.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to preferred examples, but the scope of the present invention is not limited to the preferred examples. The principle of measurement of the present invention will be described. FIG. 4 is a schematic diagram of a measurement system used in the present invention. The light wave emitted from the laser is adjusted to a polarization suitable for modulation by the optical modulator by the polarization controller, and is incident on the optical modulator to be measured. A bias voltage from a DC power supply and an electric signal from an oscillator are superimposed and applied to this optical modulator. The optical modulator modulates the incident light in accordance with the superimposed applied voltage. The outgoing light modulated by the optical modulator enters the optical spectrum analyzer, and the light intensity distribution according to the frequency component of the outgoing light is measured.

The structure of the optical waveguide of the MZ type optical modulator to be measured is shown in FIG. When the incident light is introduced into the waveguide 1, the Y-branch 1 splits the light into two, and the branched light propagates through the respective waveguides 2 and 3 and is combined by the Y-branch 2 and finally the waveguide. It is emitted via 4.

Now, the electric field E _i exp (jω ₀
Consider a case in which light having t) is incident. When the MZ type optical modulator is not modulated, it can be assumed that the emitted light is the same as the incident light. Therefore, the light intensity value P _{0a of the} emitted light is
Can be expressed by the following equation 1.

[Formula 1]

Next, consider the case where modulation is performed in the waveguides 2 and 3. As in the unmodulated case, the electric field E _i e
The light having the wavelength xp (jω ₀ t) is made incident, and the waveguides 2, 3
Modulated by, and further combined by Y branch 2 electric field component E of the light
(T) can be expressed by the following Expression 2.

[Formula 2]

Here, A ₁ and A ₂ are parameters (modulation index) representing the depth of modulation, ω _m is the angular frequency of the modulation signal,
φ ₁ and φ ₂ are the phase of the modulation signal which is different depending on each waveguide, φ
_B1 and φ _B2 represent the phase depending on the initial state of the waveguide. Equation 2 shows that the combined light waves have various frequency components. Here, among such frequency components, the same frequency component as the incident light (when n = 0 in Equation 2), +1
Attention is paid to the next higher-order term (when n = 1 in Expression 2). The same frequency component (ω ₀ ) as the incident light and the + 1st-order higher-order term (ω ₀ +
The light intensity of ω _m ) is measured. Fig. 6 shows the measured waveform of the optical spectrum analyzer. The light intensity of each of these components is
It can be expressed by the following equation 3 (light intensity P ₀ of the same frequency component as the incident light) and equation 4 (light intensity P _{1 of the} + _1st- order higher order term).

[Formula 3]

[Formula 4]

Further, when the bias voltage applied to the optical modulator is adjusted by the DC power source so that the light intensity of the emitted light having the same frequency component (ω ₀ ) as the incident light is set to the maximum (a in FIG. 2). (A bias voltage is set at the point), and the light intensity value P of the emitted light having the same frequency component (ω ₀ ) as the incident light
_0b can be expressed by the following expression 5 by the above expression 3.

[Formula 5]

Further, when the bias voltage is adjusted so that the light intensity of the emitted light having the same frequency component (ω ₀ ) as the incident light is minimized (the state where the bias voltage is set at point b in FIG. 2). ), The light intensity value P _1c of the emitted light of the + 1st-order high-order term (ω ₀ + ω _m, which is a component related to the modulation frequency) can be expressed by the following Expression 6 from Expression 4 above.

[Formula 6]

Here, the light intensity of the outgoing light that has not been modulated
Degree value P_0aThen, P mentioned above_0b, P _1cStandardize
Then, the following equations 7 and 8 are obtained.

[Formula 7]

[Formula 8]

The parameters A ₁ and A ₂ representing the modulation depth are calculated using the equations 7 and 8. Note that Equation 7 and Equation 8
Becomes a transcendental equation, but since A ₁ and A ₂ occupy small values, the solution can be specified. Then, after calculating A ₁ and A ₂ , the half-wave voltage V _π of the optical modulator can be calculated by the following Expression 9. However, V _m represents the voltage amplitude from the oscillator is applied as a modulation signal.

[Formula 9]

As described above, according to the characteristic measuring method of the optical modulator of the present invention, the above-mentioned light intensity values P _0a , P are obtained from the spectral distribution of the light emitted from the optical modulator obtained by using the optical spectrum analyzer. _0b and P _1c are measured, and the half wavelength voltage value of the optical modulator is calculated based on the light intensity value. Therefore, it is not necessary to directly observe the waveform of the light intensity of the emitted light as in the conventional case, and the characteristics of the optical modulator can be measured even at high frequencies. Further, when measuring the half-wave voltage of the optical modulator, conventionally, the voltage amplitude of the modulation signal applied to the optical modulator was variably adjusted to a range equal to or more than the half-wave voltage value. Since it is possible to calculate the half-wave voltage using the voltage amplitude value Vm of the modulation signal, it is not necessary to adjust the modulation signal to the half-wave voltage value as in the conventional case, and the voltage amplitude of the modulation signal is It is not necessary to change the value. Further, since only the high-order first-order term is measured by the optical spectrum analyzer, the measurement is not affected by high-frequency components such as an amplifier, so that the half-wave voltage can be measured more accurately.

In the MZ type optical modulator, the waveguide 2 shown in FIG.
Due to the difference in modulation due to the difference in the modulation voltage applied to 3 and 3, not only the intensity of light but also the phase is modulated at the time of multiplexing.
This phenomenon is called chirping, and the magnitude of chirping is given by the α parameter expressed by the following Expression 10.

[Formula 10]

In equation 10, dI is the amount of change in light intensity, and dI
Since φ represents the amount of phase change of light, and A1 and A2 are calculated by the specific light intensity values measured from the spectral distribution of the emitted light as described above, the α parameter can be calculated.

The present invention is not limited to the above-mentioned characteristic measuring method of the optical modulator. For example, the + 1st-order higher-order term (spectral component of frequency ω ₀ + ω _m ) used in the characteristic measuring method is Also included are those that can be used in place of the first-order higher-order terms (spectral components of the frequency ω ₀ −ω _m ) to obtain similar effects.

Next, a measuring device using the characteristic measuring method of the optical modulator of the present invention will be described. FIG. 7 is a block diagram of an apparatus for automatically measuring the characteristics of the optical modulator of the present invention. The laser light having the frequency ω ₀ passes through the polarization controller, is polarized in a certain direction, and enters the MZ type optical modulator. The optical modulator is formed by an oscillator of DC power and the frequency omega _m, the modulation signal oscillating at the frequency omega _m is applied around the specific bias voltage.

The modulated signal is input to the optical modulator, and at the same time, the voltage amplitude value Vm, the bias voltage value, etc. of the modulated signal are detected and monitored by the RF power meter through the branch circuit. The numerical value measured by the RF power meter is input to the control computer and used as one of the detection signals for automating the characteristic measurement of the optical modulator described later.

The incident light is modulated in accordance with the modulation signal applied to the optical modulator and is emitted from the optical modulator as outgoing light. A spectrum distribution, which is a light intensity distribution with respect to frequency, of the emitted light is measured by an optical spectrum analyzer. The measurement value of the optical spectrum analyzer, especially the frequency ω _{0 of the} laser light, the + 1st-order high-order term (frequency ω ₀ +
ω _m . Alternatively, it may be a -1st-order high-order term. ), The measured value of the light intensity is input to the control computer.

The process of automatically measuring the characteristic measurement of the optical modulator will be described. Laser, polarization controller,
In a state in which various equipment required for measurement such as an optical spectrum analyzer is operating, first, according to an instruction from the control computer, the application of the modulation signal to the optical modulator by the DC power source and the oscillator is stopped, and the frequency in the non-modulation state is stopped. The light intensity P _0a at ω ₀ is measured, and the measured value is loaded into the control computer. Next, according to an instruction from the control computer, a modulation signal is applied to the optical modulator, the light intensity at the frequency ω ₀ from the optical spectrum analyzer is measured, and the result is input to the control computer. In the control computer, the output voltage of the DC power supply that regulates the bias voltage of the modulation signal is changed in accordance with the monitoring of the light intensity at the frequency ω _{0, and} the light intensity monitored in accordance with the change is the maximum light intensity. Determine P _0b . Further, in the control computer, the DC voltage that regulates the bias voltage of the modulation signal is adjusted in accordance with the monitoring of the light intensity at the frequency ω ₀ .
The output voltage of the power supply is changed, and the state in which the light intensity to be monitored is minimized is determined in accordance with the change, and the light intensity P _1c of the + 1st-order higher-order term (frequency ω ₀ + ω _m ) in that state is detected. Measure with a spectrum analyzer and capture the measured values.

Next, based on the respective light intensity values P _0a , P _0b and P _{1 c loaded} into the control computer, A1 and A2 are determined by solving the above equations 7 and 8, and further, Using Equations 9 and 10 above, the half-wave voltage Vπ,
The chirp parameter (α parameter) is calculated and determined. The frequency ω ₀ , the frequency ω _m, and the voltage amplitude value V
For each value such as m, a preset value may be input to the control computer depending on the laser or oscillator used when measuring the characteristics of the optical modulator, or an optical spectrum analyzer or RF power meter may be used. You may use the value measured by. Further, in the above-described measuring device, the control computer also executes the acquisition of measured values and various calculations.
It may be executed by using a storage device or an arithmetic device provided separately from the control computer.

The result of measurement using the measuring device of FIG.
This is shown in FIGS. FIG. 8 is a characteristic measurement result of the optical modulator when a modulation signal having a measurement frequency of 10 GHz is applied.
Here, the voltage amplitude value Vm of the modulation signal was varied in the range of 2 to 9 V, and the half-wavelength voltage value and the chirp parameter at each voltage amplitude value Vm were calculated. As shown in FIG. 8, both the half-wave voltage value and the chirp parameter show almost constant values regardless of the voltage amplitude, and the measuring method and apparatus of the present invention properly measure the characteristics of the optical modulator. Can understand. Regarding the chirp parameter, when the voltage amplitude value of the modulation signal is larger than the half-wave voltage value,
When the voltage amplitude value of the modulation signal is small, the measurement result of the characteristics of the optical modulator fluctuates slightly. This is because when the voltage amplitude value of the modulation signal is increased to a value equal to or greater than the half-wave voltage, the waveform of the modulation signal is distorted, and the light intensity of the light emitted from the optical modulator deviates from the theoretical value or causes noise. This is considered to be because a large amount is included, and accurate measurement becomes difficult.
Also, if the voltage amplitude value of the modulation signal is too small, the optical spectrum distribution reflecting the modulation such as the + 1st-order higher-order term is not clearly formed, and the influence of noise is strongly exerted, which makes accurate measurement difficult. Conceivable.

FIG. 9 shows a characteristic measurement result of the optical modulator when a modulation signal having a measurement frequency of 10 to 40 GHz is applied. As shown in FIG. 9, by using the measuring method and apparatus of the present invention, it is possible to effectively measure even high frequency characteristics without directly observing the fluctuation of the light emitted from the optical modulator as in the conventional case. It will be possible.

[0033]

As described above, according to the present invention,
It is possible to measure the characteristics of the optical modulator by measuring the spectral distribution of the optical signal emitted from the optical modulator. Without needing
Inexpensive and highly accurate measurement is possible. Moreover, when the light intensity of the emitted light in the same spectrum component as the incident light is maximum, the light intensity value P of the emitted light of the spectrum component is obtained.
_0b and the light intensity value P _1c of the emitted light of the component related to the modulation frequency when the light intensity of the emitted light in the same spectrum component as the incident light is the minimum, the light intensity of the emitted light when the modulation voltage is not applied By using the value standardized by the value P _0a, it is possible to easily calculate the half-wavelength voltage value and the chirp parameter related to the optical modulator.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an MZ type optical modulator.

FIG. 2 is a graph showing characteristics of output light intensity with respect to an applied voltage of the MZ type optical modulator.

FIG. 3 is a graph showing the relationship between the electrical input waveform and the optical output waveform of the MZ type optical modulator.

FIG. 4 is a block diagram showing an example of a characteristic measuring method of an optical modulator of the present invention.

FIG. 5 is a configuration diagram of an optical waveguide of an MZ type optical modulator.

FIG. 6 is a graph showing a spectral distribution of an optical signal emitted from an optical modulator.

FIG. 7 is a schematic diagram showing an example of a characteristic measuring apparatus for an optical modulator of the present invention.

FIG. 8 is a graph showing the influence of changes in the input voltage amplitude of the modulation signal on various characteristic values of the optical modulator measured using the present invention.

FIG. 9 is a graph showing the relationship between various characteristics of the optical modulator and the frequency of the modulation signal, which is measured using the present invention.

[Explanation of symbols]

10 MZ type optical modulator 20 optical waveguide 30 signal sources

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Satoshi Oikawa             2-31-19 Shiba, Minato-ku, Tokyo Communication and broadcasting             Within the mechanism (72) Inventor Masayuki Izutsu             4-2-1 Kanaikitamachi, Koganei City, Tokyo Independent             Communications Research Institute (72) Inventor Kaoru Nikuma             2-31-19 Shiba, Minato-ku, Tokyo Communication and broadcasting             Within the mechanism (72) Inventor Tetsuya Kawanishi             4-2-1 Kanaikitamachi, Koganei City, Tokyo Independent             Communications Research Institute F term (reference) 2G086 EE12                 2H079 AA02 AA13 DA03 HA11 KA18                       KA19 KA20

Claims (5)

[Claims] [Claim 1] An incident light is split into two or more beams, phase modulation is performed by applying an electric signal to at least one of the split lights, and then the intensity-modulated light is synthesized by combining the split lights. The optical modulator that generates a signal is characterized in that the spectral distribution of the optical signal is measured, and the characteristic value related to the intensity modulation of the optical modulator is calculated from the measured value related to the measured spectral distribution. Optical modulator characteristics measurement method. 2. The method of measuring the characteristics of an optical modulator according to claim 1, wherein the electric signal is composed of a bias voltage and a modulation signal, and the measured value is the same as that of the incident light by variably adjusting the bias voltage. The light intensity value P _0b of the emitted light of the spectrum component when the light intensity of the emitted light in the spectrum component is maximum, and the component related to the modulation frequency when the light intensity of the emitted light in the same spectrum component as the incident light is the minimum Is the light intensity value P _1c of the emitted light and the light intensity value P _0a of the emitted light when the modulation signal is not applied, and when calculating the characteristic value related to the intensity modulation of the optical modulator, the light intensity value P _{1c 0a} , the light intensity value P _0b and the light intensity value P 0
_1c and a standardized value are used to measure the characteristic of the optical modulator. 3. The characteristic measuring method for an optical modulator according to claim 1, wherein the characteristic value related to intensity modulation of the optical modulator is a half wavelength voltage value of the optical modulator. Measuring method for characteristics of optical modulator. 4. The method for measuring the characteristic of an optical modulator according to claim 1, wherein the characteristic value relating to the intensity modulation of the optical modulator is a chirp parameter value of the optical modulator. Optical modulator characteristics measurement method. 5. A characteristic measuring apparatus for an optical modulator, which uses the characteristic measuring method for an optical modulator according to any one of claims 1 to 4.