A device and method for measuring the half-wave voltage of a phase modulator.

Abstract

A device and method for measuring the half-wave voltage of a phase modulator, belonging to the field of optical fiber communication technology, solves the problems of complex structure, low accuracy, high cost, and complicated measurement methods in existing technologies for measuring the half-wave voltage of phase modulators. The device of this invention is very simple to set up, does not require judgment of extreme values, the system is stable and not easily affected by external factors. It is only necessary to adjust the voltage output of the second channel of the waveform generator and observe the change of polarization state in the polarization analyzer. The half-wave voltage can be measured by observing the polarization state measured by the polarization analyzer rotating half a circle on the Poincaré sphere. There is no need to manually adjust the polarization controller to maximize the optical power, nor is it necessary to judge the maximum and minimum values ​​of the optical power meter. The measurement device can measure the value of the half-wave voltage of the phase modulator, and the measurement is more stable and accurate.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber communication and relates to a device and method for measuring the half-wave voltage of a phase modulator. BACKGROUND

[0002] The half-wave voltage is a very important physical quantity representing the influence of voltage on phase difference during electro-optical modulation. There are three methods for measuring the half-wave voltage: 1) optical communication simulation method, in which the modulation signal is converted into an audio signal, a speaker is connected to the external modulation input, the sine signal is cut off when the modulation loading switch is turned on, the output signal is played through the power output port of the speaker, the volume can be controlled by the demodulation amplitude, and in the process of gradually increasing the direct current voltage, the sound will appear twice with the smallest volume and distortion, and the difference between the two voltages is the half-wave voltage; 2) extreme value measurement method, in which only a direct current voltage is applied to the crystal, and in the process of increasing the direct current voltage from small to large, the output light intensity will appear a minimum value and a maximum value, and the difference between the direct current voltages corresponding to the adjacent minimum value and maximum value is the half-wave voltage; 3) frequency multiplication modulation method, in which a direct current voltage and an alternating current signal are applied to the crystal at the same time, and when the direct current voltage is adjusted to the voltage value corresponding to the minimum value or maximum value of the output light intensity, the output alternating current signal appears frequency multiplication distortion, and the modulation waveform and signal waveform can be observed by an oscilloscope, and the difference between the direct current voltages corresponding to the adjacent frequency multiplication distortion is the half-wave voltage.

[0003] The above three methods have the following disadvantages: 1) in the optical communication simulation method, it is difficult to determine the minimum value of the sound volume appearing twice, and the accuracy of the obtained result is not high; 2) in the extreme value measurement method, the measurement accuracy is limited due to the instability of the light source and other factors, and in addition, the extreme value method for measuring the half-wave voltage also has the disadvantages of instability of the light path difference sensitivity, complex system, high cost, and susceptibility to external influences; 3) the frequency multiplication modulation method has high requirements for adjustment and is difficult to adjust to the best state.

[0004] In the prior art, the Chinese invention patent application No. CN107121585A and published on September 1, 2017 discloses a system and method for measuring the half-wave voltage of an electro-optical phase modulator, which comprises a port of a beam splitter, a first polarization controller, a polarization selection device, a phase modulator, a second polarization controller and another port of the beam splitter, which are connected in sequence to form a Sagnac loop structure, the light power meter output is adjusted to the maximum by adjusting the first polarization controller and the second polarization controller, then a pulse voltage is loaded to the phase modulator by a voltage source, the modulation voltage of the phase modulator is adjusted so that the reading of the light power meter output reaches the maximum and the minimum, and then the half-wave voltage is the difference between the maximum and the minimum, finally, the half-wave voltage is measured by curve fitting to improve the measurement accuracy of the system. However, the measurement system is complex, and the polarization controller needs to be adjusted and curve fitting is required, so the measurement method is complicated and prone to errors. SUMMARY

[0005] The technical problem solved by the present application is how to solve the problems of complex device structure, low precision, high cost and complex measurement method in the prior art for measuring the half-wave voltage of a phase modulator.

[0006] The present application solves the above technical problems by the following technical scheme:

[0007] A device for measuring the half-wave voltage of a phase modulator, comprising a waveform generator (1), a laser (2), a polarization analyzer (3), an optical power meter, a delay line (5) and a polarization maintaining fiber device; a first channel of the waveform generator (1) is connected with the input end of the laser (2), and the output end of the laser (2) is connected with the polarization maintaining fiber device through a first tail fiber (P1); a second channel of the waveform generator (1) is connected with the input end of a phase modulator (4), one output port of the phase modulator (4) is connected with the polarization maintaining fiber device through a second tail fiber (P2), the other output port is connected with one end of the delay line (5), and the other end of the delay line (5) is connected with the polarization maintaining fiber device through a fourth tail fiber (P4); the polarization analyzer (3) or the optical power meter is connected with the polarization maintaining fiber device through a third tail fiber (P3).

[0008] The device of the present application is very simple to build, does not need to make a judgment on the extreme value, is stable and not easily affected by the outside world, only needs to adjust the voltage output by the second channel of the waveform generator, and the change of the polarization state in the polarization analyzer can be observed, so that the half-wave voltage can be measured when the polarization state measured by the polarization analyzer rotates a full circle on the Poincare sphere; it is not necessary to manually adjust the polarization controller to make the optical power reach the maximum, and it is not necessary to judge the maximum and minimum values of the optical power meter, and the value of the half-wave voltage of the phase modulator can be measured by the built measurement device, which is more stable and accurate.

[0009] As a further improvement of the technical scheme of the present application, the light pulse emitted by the laser (2) enters from the first tail fiber (P1) of the polarization maintaining fiber device, and is divided into two polarized lights after passing through the polarization maintaining fiber device, one of the paths of the two polarized lights is: the second tail fiber (P2)→the phase modulator (4)→the delay line (5)→the fourth tail fiber (P4); the other path of the two polarized lights is: the fourth tail fiber (P4)→the delay line (5)→the phase modulator (4)→the second tail fiber (P2); the paths of the two polarized lights constitute a loop with equal distance and opposite direction, and the two polarized lights superimpose at the polarization maintaining fiber device to form a Sagnac loop; the position of the delay line (5) is adjusted so that the phase modulator (4) is placed at an asymmetric position of the Sagnac loop, the phase modulator (4) divides the Sagnac loop into two sections of optical fiber, and the time difference of the light pulse passing through the two sections of optical fiber is greater than the rising and falling time of the voltage signal loaded by the waveform generator (1).

[0010] As a further improvement of the technical scheme of the present application, the polarization maintaining fiber device is a polarization beam splitter (PBS) or a polarization circulation ring (PCIR).

[0011] As a further improvement of the technical scheme of the present application, if the laser (2) is connected at the first pigtail (P1), then the polarization analyzer (3) or the optical power meter is connected at the third pigtail (P3); if the polarization analyzer (3) or the optical power meter is connected at the first pigtail (P1), then the laser (2) is connected at the third pigtail (P3).

[0012] As a further improvement of the technical scheme of the present application, the first pigtail (P1), the second pigtail (P2), the third pigtail (P3) and the fourth pigtail (P4) all adopt polarization maintaining fibers.

[0013] As a further improvement of the technical scheme of the present application, the phase modulator (4) is a polarization maintaining phase modulator.

[0014] A measurement method using the measurement device, comprising the following steps:

[0015] Step one, the waveform generator (1) sends a signal to the phase modulator (4), changes the voltage value of the phase modulator (4) to change the polarization state of the light in the Sagnac ring, and measures the delay when the optical power is maximum;

[0016] Step two, set the delay of the waveform generator (1) to the delay when the optical power is maximum, adjust the voltage value of the second channel output of the waveform generator (1) by rotating the knob of the waveform generator (1), and observe the change of the polarization state in the polarization analyzer (3); when it is observed that the polarization state measured by the polarization analyzer (3) rotates a full circle on the Poincare sphere, the voltage value of the phase modulator (4) at this time is the half-wave voltage value to be measured.

[0017] As a further improvement of the technical scheme of the present application, the measurement method of the delay when the optical power is maximum in step one is:

[0018] Step 1), build a measurement device, replace the polarization analyzer (3) in the measurement device of the phase modulator half-wave voltage with an optical power meter;

[0019] Step 2), turn on the waveform generator (1), set the parameters of the first channel and the second channel of the waveform generator (1) to output respectively, and confirm that the pulse waveform output by the first channel and the sine waveform output by the second channel are normal;

[0020] Step 3) Adjust the phase value of the voltage output by the second channel by rotating the knob of the waveform generator (1), thereby adjusting the polarization state of the light from 0° to 360°. At this time, the optical power will change, and the reading on the optical power meter will change accordingly. Observe the reading of the optical power meter and record the delay when the reading reaches its maximum value.

[0021] As a further improvement to the technical solution of the present invention, the parameters in step 2) include: waveform, frequency, delay, high and low levels, and duty cycle.

[0022] As a further improvement to the technical solution of the present invention, the adjustment step value from 0° to 360° in step 3) is set to 1°.

[0023] The advantages of this invention are:

[0024] (1) The device of the present invention is very simple to set up. It does not require judgment of extreme values. The system is stable and not easily affected by external factors. It is only necessary to adjust the voltage output of the second channel of the waveform generator and observe the change of polarization state in the polarization analyzer. It can be observed that the polarization state measured by the polarization analyzer rotates half a circle on the Bonga sphere to measure the half-wave voltage.

[0025] (2) The present invention can measure the value of half-wave voltage of phase modulator by constructing a measuring device, and the measurement is more stable and accurate.

[0026] (3) The present invention can measure optical devices with polarization-maintaining fiber pigtails without manually adjusting the polarization controller to maximize the optical power, and without judging the maximum and minimum values ​​of the optical power meter. Attached Figure Description

[0027] Figure 1 This is a structural diagram of the device according to Embodiment 1 of the present invention;

[0028] Figure 2 This is a structural diagram of the device according to Embodiment 2 of the present invention;

[0029] Figure 3 This is a flowchart of the measurement method in Embodiments 1 and 2 of the present invention. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0032] Example 1

[0033] like Figure 1 As shown, a phase modulator half-wave voltage measurement device according to this embodiment includes a waveform generator 1, a laser 2, a polarization analyzer 3, an optical power meter (not shown in the figure), a delay line 5, and a four-port polarization beam splitter PBS.

[0034] The four-port polarization beam splitter has four pigtails P1, P2, P3, and P4, all of which are polarization-maintaining fibers. Pigtail P1 is connected to laser 2, pigtail P2 is connected to the in port of phase modulator 4, pigtail P3 is connected to polarization analyzer 3 or optical power meter, and pigtail P4 is connected to one end of delay line 5. Laser 2 is connected to the CH1 port of waveform generator 1. Phase modulator 4 is connected to the CH2 port of waveform generator 1, and the out port of phase modulator 4 is connected to the other end of delay line 5. The phase modulator 4 is a polarization-maintaining phase modulator.

[0035] The light pulse emitted by laser 2 enters through the pigtail P1 of the PBS and is split into two polarized beams after passing through the PBS. The path of one polarized beam is: PBS → pigtail P2 → phase modulator 4 → delay line 5 → pigtail P4 → PBS; the path of the other polarized beam is: PBS → pigtail P4 → delay line 5 → phase modulator 4 → pigtail P2 → PBS. The paths of the two polarized beams form a loop with equal path length and opposite direction, which superimposes at the PBS to form a Sagnac loop.

[0036] The position of delay line 5 is adjusted so that phase modulator 4 is placed in an asymmetrical position of Sagnac ring. Its time asymmetry must be greater than the rise and fall time of the voltage signal applied by waveform generator 1. That is, phase modulator 4 splits Sagnac ring into two optical fibers. The time difference of the optical pulse through these two optical fibers must be greater than the rise and fall time of the voltage signal applied by waveform generator 1 to ensure the accuracy of the measurement results.

[0037] like Figure 3 As shown, a method for measuring the half-wave voltage of a phase modulator is as follows:

[0038] Step 1: Waveform generator 1 sends a signal to phase modulator 4 to change the voltage value of polarization-maintaining phase modulator 4, thereby changing the polarization state of light in the Sagnac ring, and measuring the delay when the optical power is at its maximum.

[0039] Step 2: Set the delay of waveform generator 1 to the delay when the optical power is at its maximum. Adjust the voltage value output by the CH2 port of waveform generator 1 by rotating the knob of waveform generator 1. Observe the change of polarization state in polarization analyzer 3. When the polarization state measured by polarization analyzer 3 rotates half a circle on the Bonga sphere, the voltage value of phase modulator 4 at this time is the half-wave voltage value to be measured.

[0040] The method for measuring the delay when the optical power is at its maximum, as described in step one, is as follows:

[0041] 1. Figure 1 The polarization analyzer 3 in the middle was replaced with an optical power meter, and according to... Figure 1 Set up the measuring device;

[0042] 2. Turn on waveform generator 1, and set the waveform, frequency, delay, high and low levels, duty cycle and other parameters of the output of CH1 port and CH2 port of waveform generator 1 respectively, and confirm that the pulse waveform output by CH1 port and the sine waveform output by CH2 port are normal.

[0043] 3. By rotating the knob of waveform generator 1, adjust the phase (delay) value of the voltage output from port CH2, thereby adjusting the polarization state of the light from 0° to 360° (step set to 1°). At this time, the optical power will change, and the reading on the optical power meter will change accordingly. Observe the reading of the optical power meter and record the delay when the reading reaches its maximum value.

[0044] In this embodiment, the positions of laser 2 and polarization analyzer 3 or optical power meter can be interchanged. That is, if laser 2 is connected to pigtail P1, then polarization analyzer 3 or optical power meter is connected to pigtail P3. If polarization analyzer 3 or optical power meter is connected to pigtail P1, then laser 2 is connected to pigtail P3.

[0045] Example 2

[0046] like Figure 2 As shown, unlike Example 1, this example replaces the PBS in Example 1 with a circular polarization beam splitter (PCIR). The light emitted by the laser 2 enters from the pigtail P1 of the circular polarization beam splitter, and is split into two polarized beams by the PBS in the PCIR. The beams return to the PCIR through pigtails P2 and P4 respectively, with equal paths but opposite directions, and are superimposed to form a Sagnac ring.

[0047] like Figure 3 As shown, the measurement method in this embodiment is the same as that in Embodiment 1.

[0048] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A device for measuring the half-wave voltage of a phase modulator, characterized in that It comprises a waveform generator (1), a laser (2), a polarization analyzer (3), an optical power meter, a delay line (5) and a polarization maintaining fiber device; the first channel of the waveform generator (1) is connected with the input end of the laser (2), the output end of the laser (2) is connected with the polarization maintaining fiber device through a first pigtail (P1); the second channel of the waveform generator (1) is connected with the input end of the phase modulator (4), one output port of the phase modulator (4) is connected with the polarization maintaining fiber device through a second pigtail (P2), the other output port is connected with one end of the delay line (5), the other end of the delay line (5) is connected with the polarization maintaining fiber device through a fourth pigtail (P4); the polarization analyzer (3) or the optical power meter is connected with the polarization maintaining fiber device through a third pigtail (P3); The light pulse emitted by the laser (2) enters from the first pigtail (P1) of the polarization maintaining fiber device, and is divided into two polarized lights after passing through the polarization maintaining fiber device, the paths of the two polarized lights constitute a loop with equal distance and opposite direction, and the two polarized lights are superimposed at the polarization maintaining fiber device to form a Sagnac ring; the position of the delay line (5) is adjusted so that the phase modulator (4) is arranged at an asymmetric position of the Sagnac ring, and the phase modulator (4) divides the Sagnac ring into two optical fibers, and the time difference of the light pulse passing through the two optical fibers is greater than the rising and falling time of the voltage signal loaded by the waveform generator (1). The path of one of the two polarized lights is: the second pigtail (P2)→the phase modulator (4)→the delay line (5)→the fourth pigtail (P4); the path of the other polarized light is: the fourth pigtail (P4)→the delay line (5)→the phase modulator (4)→the second pigtail (P2).

2. A device for measuring the half-wave voltage of a phase modulator according to claim 1, characterized in that The polarization maintaining fiber device is a polarization beam splitter (PBS) or a polarization circle beam splitter (PCIR).

3. A device for measuring the half-wave voltage of a phase modulator according to claim 1 or 2, characterized in that If the laser (2) is connected at the first pigtail (P1), then the polarization analyzer (3) or the optical power meter is connected at the third pigtail (P3); if the polarization analyzer (3) or the optical power meter is connected at the first pigtail (P1), then the laser (2) is connected at the third pigtail (P3).

4. A device for measuring the half-wave voltage of a phase modulator according to claim 1, characterized in that The first pigtail (P1), the second pigtail (P2), the third pigtail (P3) and the fourth pigtail (P4) all adopt polarization maintaining fibers.

5. The apparatus of claim 1, wherein the phase modulator half-wave voltage is measured by, The phase modulator (4) is a polarization maintaining phase modulator.

6. A device for measuring the half-wave voltage of a phase modulator according to claim 1, characterized in that It comprises the following steps:

7. A measuring method employing the measuring device according to any one of claims 1 to 6, characterized in that, Step one: the waveform generator (1) sends a signal to the phase modulator (4), changes the voltage value of the phase modulator (4) to change the polarization state of the light in the Sagnac ring, and measures the delay time when the optical power is maximum; Step two: the delay time of the waveform generator (1) is set to the delay time when the optical power is maximum, the voltage value output by the second channel of the waveform generator (1) is adjusted by rotating the knob of the waveform generator (1), and the change of the polarization state in the polarization analyzer (3) is observed; when it is observed that the polarization state measured by the polarization analyzer (3) rotates a half circle on the Poincare sphere, the voltage value of the phase modulator (4) at this time is the half-wave voltage value to be measured. The measurement method of the delay time when the optical power is maximum in step one is:

8. The measurement method according to claim 7, characterized in that, ​ Step 1), build the measuring device, change the polarization analyzer (3) in the measuring device of the phase modulator half-wave voltage to an optical power meter; Step 2), turn on the waveform generator (1), set the parameters of the first channel and the second channel of the waveform generator (1) respectively, and confirm that the pulse waveform output by the first channel and the sine waveform output by the second channel are normal; Step 3), adjust the phase value of the voltage output by the second channel by rotating the knob of the waveform generator (1), so as to adjust the polarization state of the light from 0° to 360°, at this time the optical power will change, the reading on the optical power meter will change accordingly, observe the reading of the optical power meter, and record the delay when the reading is maximum.

9. The measurement method according to claim 8, characterized in that, The parameters in step 2) include: waveform, frequency, delay, high and low level, duty cycle.

10. The measurement method according to claim 8, characterized by, In step 3), the adjustment step value from 0° to 360° is set to 1°.

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