CN114814331B - Method for testing half-wave voltage of Mach-Zehnder modulator - Google Patents
Method for testing half-wave voltage of Mach-Zehnder modulator Download PDFInfo
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Abstract
The invention discloses a method for testing half-wave voltage of a Mach-Zehnder modulator. According to the invention, an optical carrier output by a laser enters a Mach-Zehnder modulator to be measured, a sinusoidal electrical signal is loaded to a modulation end of the Mach-Zehnder modulator to be measured, another sinusoidal electrical signal is loaded to a bias end of the Mach-Zehnder modulator to be measured, the modulated optical signal enters an optical detector for beat frequency, the amplitude average value of the electrical signals loaded by the bias end of the Mach-Zehnder modulator to be measured is changed, two groups of frequency spectrograms are obtained, data in the two groups of frequency spectrograms are analyzed to obtain the modulation coefficient of the Mach-Zehnder modulator to be measured, and then the half-wave voltage of the Mach-Zehnder modulator is obtained. The method is suitable for Mach-Zehnder modulators with any splitting ratio, can be used for respectively testing the half-wave voltage of the modulation end and the offset end of the Mach-Zehnder modulator, and has the advantages of simple device, convenience in operation and high resolution.
Description
Technical Field
The invention belongs to the field of testing of optoelectronic devices, and particularly relates to a method for testing half-wave voltage of a Mach-Zehnder modulator.
Background
The Mach-Zehnder modulator is a key device in an optical communication system and a microwave optical sub-link, half-wave voltage, namely two voltage difference values corresponding to the modulation of an electric signal to an optical carrier to enable the optical power of the optical carrier to change from maximum to minimum, are related to the frequency of a driving signal and serve as one of the most important parameters of the Mach-Zehnder modulator, and the realization of accurate measurement of the half-wave voltage difference values is very important for performance representation of the Mach-Zehnder modulator.
One of the more common methods for measuring the half-wave voltage of a mach-zehnder modulator is the extreme method. The method only uses one direct current source, directly measures the change condition of the light intensity of the output light signal along with the direct current voltage to obtain the transmission curve of the Mach-Zehnder modulator, obtains the direct current voltage value corresponding to the maximum value and the minimum value of the light intensity of the output light signal from the curve, and the difference between the two values is the half-wave voltage of the Mach-Zehnder modulator. The method has the disadvantages of long testing time and low testing precision. Because if the half-wave voltage value with higher precision is obtained, the step value of the direct current voltage needs to be reduced as much as possible, thereby increasing the test time, and the increase of the test time causes larger errors caused by the fluctuation of the output light power of the laser. In addition, the method can only obtain the static half-wave voltage of the bias end of the Mach-Zehnder modulator, and cannot obtain the dynamic half-wave voltage when the modulation end or the bias end of the Mach-Zehnder modulator loads an electric signal.
The other method is a frequency multiplication method, wherein direct current voltage and alternating current signals are loaded to a bias end and a modulation end of a Mach-Zehnder modulator respectively, and the phenomenon of frequency multiplication distortion of modulation signals can be observed on an oscilloscope when the output light intensity has a maximum value or a minimum value by adjusting direct current bias voltage. The difference of the direct current voltages corresponding to the two adjacent frequency multiplication distortions is the half-wave voltage of the modulation end of the Mach-Zehnder modulator. The disadvantage of this method is that when the dc bias is applied to a certain extent, the phenomenon of distortion is not easily observed, and the result obtained by observing the waveform is also relatively rough. In addition, the method can only test the half-wave voltage of the modulation end of the Mach-Zehnder modulator, but cannot test the half-wave voltage of the bias end of the Mach-Zehnder modulator.
Both the two methods can only test the half-wave voltage of a single port of the modulator, and the test precision is low, so that a quick, convenient and accurate test method is urgently needed to simultaneously test the half-wave voltage of the modulation end and the offset end of the Mach-Zehnder modulator.
Disclosure of Invention
The invention aims to provide a measuring method for measuring the half-wave voltage of a Mach-Zehnder modulator. The method is suitable for Mach-Zehnder modulators with any splitting ratio, can respectively realize the test of half-wave voltages of the modulation end and the offset end, and has the advantages of simple device, convenient operation and high resolution.
The technical scheme adopted by the invention is as follows:
step a: a testing device is set up, the optical carrier output by the laser (1) is directly sent to the Mach-Zehnder modulator (2) to be tested, and a signal generation module(3-1) output frequency of f d Amplitude of V d The sine electrical signal is loaded to the modulation end of the Mach-Zehnder modulator to be measured, and the output frequency of the signal generation module (3-2) is f 1 Amplitude average value is V 1 The sinusoidal electrical signal is loaded to the offset end of the Mach-Zehnder modulator to be measured, the modulated optical signal is sent to a photoelectric detector (4) for beat frequency, and a spectrum analysis module (5) is used for analyzing and measuring the obtained beat frequency signal;
step b: regulating V 1 The frequency of the output of the photodetector (4) is set to f 1 ±f d Has the maximum spectral line amplitude and is marked as A (f) 1 ±f d ) (ii) a Step c: regulating V 1 The frequency of the output of the photodetector (4) is set to f 1 Has the maximum spectral line amplitude and is marked as A (f) 1 )；
Step d: measuring A (f) obtained in step b 1 ±f d ) And step c measuring the resulting A (f) 1 ) According to the following formula
Determining the modulation factor m (f) of the Mach-Zehnder modulator to be measured d ) Wherein J 1 (·)，J 0 (. H) are 1-order and 0-order Bessel functions of the first kind respectively;
step e: according to V set in step a d And the modulation factor m (f) obtained in step d d ) According to the following formula
The frequency f of a driving signal corresponding to a modulation end of the Mach-Zehnder modulator to be measured is obtained d Half wave voltage V of time π (f d )；
Step f: change f d And repeating the steps a, b, c, d and e in sequence to obtain the modulation end of the Mach-Zehnder modulator to be measured at different frequencies f d Half wave voltage V of time π (f d )。
Drawings
Fig. 1 is a schematic structural diagram of a half-wave voltage testing method of a mach-zehnder modulator according to the present invention.
Fig. 2 and 3 are schematic diagrams of two groups of beat frequency lines measured in the embodiment of the invention.
FIG. 4 shows a half-wave voltage V measured at the modulation end of the Mach-Zehnder modulator to be measured in the embodiment of the present invention π (f d ) And frequency f d Graph of the relationship of (c).
FIG. 5 shows the half-wave voltage V of the bias terminal of the Mach-Zehnder modulator to be measured in the embodiment of the invention π (f 1 ) And frequency f 1 Graph of the relationship of (c).
Wherein, in fig. 1: the (1) is a laser; (2) a Mach-Zehnder modulator to be measured; (3-1) and (3-2) are two signal generating modules; (4) is a photoelectric detector; and (5) a spectrum analysis module.
Detailed Description
The present invention is further described with reference to the following drawings and examples, which are intended to illustrate only some of the embodiments of the present invention, and the scope of the invention is not limited by the examples, and should not be limited thereby.
As shown in fig. 1, the principle of the testing method of the half-wave voltage of the mach-zehnder modulator of the present invention is as follows: an optical carrier output by the laser (1) enters the Mach-Zehnder electro-optic modulator (2) to be measured, a sinusoidal electrical signal is output by the signal generation module (3-1) and loaded to the modulation end of the Mach-Zehnder modulator (2) to be measured, another sinusoidal electrical signal is output by the signal generation module (3-2) and loaded to the offset end of the Mach-Zehnder modulator (2) to be measured, the modulated optical signal is sent to the photoelectric detector (4) for beat frequency, and the obtained beat frequency spectral line is analyzed and measured by the spectrum analysis module (5). The modulation coefficient of the Mach-Zehnder modulator (2) to be measured can be obtained according to the amplitude information of the beat frequency spectral line, and then the half-wave voltage of the Mach-Zehnder modulator (2) to be measured is obtained.
In order to better understand the technical scheme of the invention, the following detailed description is made on the measurement principle of the invention:
the optical carrier signal output by the laser can be expressed as:
wherein E 0 And f 0 Respectively the amplitude and frequency of the optical carrier.
Two electrical signals v 1 (t)＝V 1 sin(2πf 1 t+θ 1 )+V DC And v d (t)＝V d sin(2πf d t+θ d ) Respectively loading the modulated optical carrier signals to a bias end and a modulation end of a Mach-Zehnder modulator to be measured, wherein the modulated optical carrier signals are as follows:
wherein gamma is the splitting ratio of the Mach-Zehnder modulator to be measured; m is 1 ＝πV 1 /V π (f 1 ) Is the modulation coefficient, m, caused by the electrical signal loaded to the bias terminal of the Mach-Zehnder modulator to be measured d ＝πV d /V π (f d ) Is the modulation coefficient, V, caused by the electrical signal loaded to the modulation end of the Mach-Zehnder modulator to be measured π (f 1 ) And V π (f d ) The half-wave voltages of the bias end and the modulation end of the Mach-Zehnder modulator to be measured are related to the frequency respectively;
shows a direct current component V loaded in an electric signal at a bias end of a Mach-Zehnder modulator to be measured DC The resulting phase change. The photocurrent formed by the modulated optical signal after the beat frequency of the photodetector is as follows:
where R is the responsivity of the photodetector, which is frequency dependent. The formula is developed to obtain:
the expression is obtained by expanding the expression into a Bessel function form:
wherein J p (·)、J q (. Cndot.) represents the first class Bessel functions of order p and order q, respectively. Obtaining the amplitude expressions of two beat frequency spectrum lines from the alternating current term as follows:
by adjusting the amplitude average value of the loaded electrical signal applied to the bias end of the Mach-Zehnder modulator to be measured, the amplitude average value can be correspondingly caused
A change in size. Firstly, make
The beat spectrum line amplitude expression becomes:
then make
The two beat spectral line amplitude expressions now become:
comparing the formula (9) with the formula (10) to obtain:
setting a frequency relationship f d Is much less than f 1 Then R (f) in the formula (11) 1 ±f d ) Approximately equal to R (f) 1 ) Then the equation becomes:
calculating the modulation coefficient m of the Mach-Zehnder modulator to be measured according to the equation d Then, according to the formula:
the frequency of the drive signal of the Mach-Zehnder modulator to be measured can be calculated to be f d Half wave voltage of time, here V d Is at a frequency f d The amplitude of the sinusoidal electrical signal.
The above process introduces the test of the half-wave voltage V at the modulation end of the Mach-Zehnder modulator π (f d ) If let the frequency be f 1 Takes the sine electric signal as the signal to be measured, so that f d Is much greater than f 1 The same principle can also test the half-wave voltage V of the bias end of the Mach-Zehnder modulator π (f 1 )。
Examples
In this example, the frequency of the optical carrier signal output by the laser used is 193.14THz, the optical power is 10dBm, the device under test is a Mach-Zehnder modulator, and the frequency is f d =1kHz, voltage peak-to-peak value V pp A sinusoidal electric signal of =200mV is loaded to the modulation end of the Mach-Zehnder modulator to be measured, and the other frequency is f 1 The sinusoidal electrical signal with 1MHz and 200mV of voltage peak-to-peak value is loaded to the Mach-Zehnder to be measuredA bias terminal of the modulator. By adjusting the magnitude of the average value of the amplitude of the electrical signal applied to the offset end of the Mach-Zehnder modulator to be measured, the beat frequency spectrum line with the frequency of 1MHz +/-1 kHz is obtained and shown in figure 2, and the beat frequency spectrum line with the frequency of 1MHz is shown in figure 3. The electric power value of the beat frequency line with the frequency of 1MHz-1kHz is-84.73 dBm, the electric power value of the beat frequency line with the frequency of 1MHz +1kHz is-84.77 dBm, the average value of the electric power value and the average value of the electric power value is-84.75 dBm, the electric power value of the beat frequency line with the frequency of 1MHz is-55.28 dBm, and a modulation coefficient m is calculated d =0.067, from formula V π ＝πV d /m d Calculating the half-wave voltage V of the modulation end of the Mach-Zehnder modulator to be measured when the frequency of the driving signal is 1kHz π (f d )＝4.689V。
Changing the frequency f of the electrical signal loaded to the modulation end of the Mach-Zehnder modulator to be measured d And obtaining the half-wave voltage of the modulation end of the Mach-Zehnder modulator to be measured under different modulation frequencies. The half-wave voltage of the offset end of the Mach-Zehnder modulator to be measured under different modulation frequencies can be obtained by the same method.
FIG. 4 shows a half-wave voltage V of the modulation end of the Mach-Zehnder modulator to be tested obtained according to the method for testing the half-wave voltage of the Mach-Zehnder modulator in the embodiment π (f d ) And a modulation frequency f d The relationship of (1).
FIG. 5 shows a half-wave voltage V at the bias terminal of the Mach-Zehnder modulator to be tested, obtained according to the method for testing the half-wave voltage of the Mach-Zehnder modulator in the embodiment of the present invention π (f 1 ) And a modulation frequency f 1 The relationship of (1).
Claims (2)
1. A method for testing half-wave voltage of a Mach-Zehnder modulator is characterized by comprising the following steps:
step a: a testing device is set up, the optical carrier output by the laser (1) is directly sent to the Mach-Zehnder modulator (2) to be tested, and the output frequency of the first signal generation module (3-1) is f d Amplitude of V d The sine electrical signal is loaded to the modulation end of the Mach-Zehnder modulator to be measured, and the output frequency of the second signal generation module (3-2) is f 1 Is of equal amplitudeMean value of V 1 The sinusoidal electrical signal is loaded to the offset end of the Mach-Zehnder modulator to be measured, the modulated optical signal is sent to a photoelectric detector (4) for beat frequency, and a spectrum analysis module (5) is used for analyzing and measuring the obtained beat frequency signal;
step b: regulating V 1 The frequency of the output of the photodetector (4) is set to f 1 ±f d Has the maximum spectral line amplitude and is marked as A (f) 1 ±f d ) (ii) a Step c: regulating V 1 The frequency of the output of the photodetector (4) is set to f 1 Has the maximum spectral line amplitude and is marked as A (f) 1 )；
Step d: measuring A (f) obtained in step b 1 ±f d ) And step c measuring the resulting A (f) 1 ) According to the following formula
Determining the modulation factor m (f) of the Mach-Zehnder modulator to be measured d ) Wherein J 1 (·)，J 0 (. Cndot.) is a first class Bessel function of order 1 and 0 respectively;
step e: according to V set in step a d And the modulation factor m (f) obtained in step d d ) According to the following formula
Calculating the frequency f of the drive signal of the Mach-Zehnder modulator to be measured corresponding to the modulation end d Half wave voltage V of time π (f d )；
Step f: change f d And repeating the steps a, b, c, d and e in sequence to obtain the modulation end of the Mach-Zehnder modulator to be measured at different frequencies f d Half wave voltage V of time π (f d )。
2. The method for testing the half-wave voltage of the Mach-Zehnder modulator as defined in claim 1, wherein the method is also suitable for measuring the half-wave voltage of the bias terminal of the Mach-Zehnder modulator.
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