Phase modulator half-wave voltage test system and method suitable for wide frequency band

Abstract

The invention discloses a phase modulator half-wave voltage testing system and method suitable for a wide frequency band. The phase modulator half-wave voltage testing system comprises a continuous light source, a phase modulator to be tested, a signal generator, an optical fiber adjustable delay line, a 3dB optical coupler and an optical power meter. The 3dB optical couplers comprise a first 3dB optical coupler and a second 3dB optical coupler, an optical signal output by the continuous light source is divided into an upper arm optical signal and a lower arm optical signal by the first 3dB optical coupler, the upper arm optical signal enters the phase modulator to be measured, the lower arm optical signal is connected with the optical fiber adjustable delay line, the second 3dB optical coupler couples the upper arm optical signal and the lower arm optical signal, and the optical fiber adjustable delay line is connected with the optical fiber adjustable delay line. And inputting into an optical power meter. According to the invention, the half-wave voltage of all frequencies in the bandwidth range of the phase modulator can be measured, the precision can reach 0.01 V, and the application range is wide; instruments and equipment used in the method are common equipment, the cost is low, the requirement for the power of the signal generator is only that the generated sinusoidal signal peak-to-peak value reaches 1.53 times of the half-wave voltage Vpi of the to-be-tested phase modulator, and the method is suitable for scenes such as manufacturer production testing and the like.

Description

Technical Field

[0001] The present invention relates to a phase modulator half-wave voltage testing system and method, and in particular to a phase modulator half-wave voltage testing system and method applicable to a wide frequency band. Background Art

[0002] An electro-optic phase modulator (EOPM) is an optoelectronic device that modulates the phase of an optical carrier wave based on the electro-optic effect. It has important applications in fiber-optic communications, optical sensing, quantum optics, and other fields. Half-wave voltage is a key performance metric for modulators, determining their modulation efficiency. The smaller the half-wave voltage, the higher the modulation efficiency. Determining the magnitude of the half-wave voltage is crucial. Performance evaluation is essential for both manufacturers and customers. Furthermore, when designing a system, knowing the specific half-wave voltage is crucial for correctly configuring the driver circuit, ensuring optimal modulator operation, and avoiding nonlinear distortion.

[0003] Existing methods for testing the half-wave voltage of a phase modulator mainly include the triangle wave method and the spectral method. A phase modulator modulates only the phase of the optical carrier without changing the optical intensity during modulation. At a low frequency, a triangle wave is injected into the RF modulation port of the phase modulator for modulation. Phase modulation is converted to intensity modulation using a Sagnac loop. The output optical signal undergoes photoelectric conversion and is received by an oscilloscope. The half-wave voltage of the phase modulator under test can be derived from the time difference between adjacent peaks and valleys of the signal on the oscilloscope, the frequency of the triangle wave signal, and the peak-to-peak value of the triangle wave. This method has the advantage of high accuracy, but is generally only used to test the half-wave voltage at lower frequencies. Testing high-frequency half-wave voltage requires a high-speed oscilloscope and AWG, and the test frequency must precisely match the Sagnac loop length, making it difficult to implement in practice. The spectral method modulates the phase modulator with a sinusoidal RF signal and directly observes the modulated optical signal output by the phase modulator using a spectrometer. The RF signal power is continuously increased until the optical carrier is completely suppressed, at which point the peak-to-peak value of the RF signal is 2.44 times the half-wave voltage. This method can be used to test the high-frequency half-wave voltage of a phase modulator, but it requires very high power from the RF signal; the peak-to-peak value needs to be 2.44 times the half-wave voltage, and it also requires a high resolution from the spectrometer. Furthermore, for low-frequency signals, it is difficult to clearly observe the carrier and sidebands on a spectrometer, so only the high-frequency half-wave voltage can be tested. Summary of the Invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a phase modulator half-wave voltage testing system and method suitable for a wide frequency band, which can measure the half-wave voltage of all frequencies within the bandwidth range of the phase modulator and has a wide range of applications.

[0005] Technical solution: The present invention includes a continuous light source, a phase modulator to be measured, a signal generator, an optical fiber adjustable delay line, a 3dB optical coupler, and an optical power meter. The 3dB optical coupler includes a first 3dB optical coupler and a second 3dB optical coupler. The optical signal output by the continuous light source is divided into an upper arm optical signal and a lower arm optical signal by the first 3dB optical coupler. The upper arm optical signal enters the phase modulator to be measured, and the lower arm optical signal is connected to the optical fiber adjustable delay line. The second 3dB optical coupler couples the upper and lower arm optical signals and then inputs them into the optical power meter.

[0006] The optical fiber adjustable delay line is used to change the phase difference between the upper and lower arms of the optical signal.

[0007] The sinusoidal signal generated by the signal generator is injected into the RF modulation port of the phase modulator to be tested to modulate the upper arm optical signal.

[0008] The light signal generated by the continuous light source is:

[0009] E in (t) = E0cos(ω0t) (1)

[0010] Where E0 represents the optical amplitude and ω0 represents the optical carrier angular frequency.

[0011] The upper arm optical signal and the lower arm optical signal are expressed as:

[0012]

[0013] Assume that the RF signal injected into the phase modulator to be tested is:

[0014] v m (t) = V m cos(ω m t) (3)

[0015] Among them, V m 、ω m Respectively represent the amplitude and angular frequency of the RF signal;

[0016] The output light field of the upper arm optical signal after being modulated by the phase modulator to be measured is expressed as:

[0017]

[0018] Where α1 represents the optical loss introduced by the upper arm, V π Represents the half-wave voltage of the phase modulator under test.

[0019] The output optical field of the lower arm optical signal after passing through the optical fiber adjustable delay line is expressed as:

[0020]

[0021] Where α2 represents the optical loss introduced by the lower arm, n represents the refractive index of the optical waveguide, ΔL represents the optical path difference between the upper and lower arms, and λ represents the wavelength of the optical carrier. Since ω0 = 2πc / λ, formula (5) can be simplified to:

[0022]

[0023] in,

[0024] The output light field after the upper and lower optical signals are coupled by the second 3dB optical coupler is expressed as:

[0025]

[0026] The output optical power is the time average of the optical field intensity and is expressed as:

[0027]

[0028] Introduced input optical power P in , substitute into formula (7) and simplify, the output optical power is expressed as:

[0029]

[0030] The cosine function of the AC part of formula (9) is expressed as:

[0031]

[0032] Expanding them separately, we can get:

[0033]

[0034] Among them, J n (x) represents the nth-order Bessel function of the first kind.

[0035] The optical power meter receives the average optical power. Substituting formulas (10), (11), and (12) into formula (9) and filtering out high-order components, we obtain:

[0036]

[0037] The sampling frequency of the optical power meter is less than the frequency of the sinusoidal signal.

[0038] A method for testing half-wave voltage of a phase modulator applicable to a wide frequency band comprises the following steps:

[0039] Step S1, building a phase modulator half-wave voltage test system suitable for a wide frequency band;

[0040] Step S2: Adjust the optical fiber adjustable delay line to change the phase difference of the optical signals in the upper and lower arms. At the same time, use an optical power meter to measure the output optical power and record it. Obtain a curve of the output optical power changing with the effective length of the optical fiber adjustable delay line. The effective length of the optical fiber adjustable delay line is L, and the effective length corresponding to the point with the minimum output optical power is L. B =L0;

[0041] Step S3: Turn on the signal generator to generate a frequency of f and a peak-to-peak value of V PP-1 A sinusoidal signal is injected into the RF modulation port of the phase modulator to be tested;

[0042] Step S4, adjust the optical fiber adjustable delay line again to change the phase difference of the upper and lower arms of the optical signal, and record the output average optical power to determine L B = L0, the output average optical power is the minimum or maximum point: If L B = L0 when the output average optical power is the minimum point, then increase the peak-to-peak value of the RF signal. If L B =L0, the output average optical power reaches its maximum value, which reduces the peak-to-peak value of the RF signal;

[0043] Step S5, repeat step S4 until the L B = The peak-to-peak value of the sinusoidal signal when the average optical power output changes from the minimum value to the maximum value at L0 PP-T ;

[0044] Step S6: According to the peak-to-peak value of the sinusoidal signal V PP-T Calculate the half-wave voltage of the phase modulator to be tested at frequency f, the calculation formula is V π =V PP-T / 1.53.

[0045] Beneficial effects: The present invention can measure the half-wave voltage of all frequencies within the bandwidth of the phase modulator with an accuracy of 0.01V and a wide range of applications. The instruments and equipment used in the present invention are all common equipment, and do not require expensive instruments and equipment such as high-speed oscilloscopes, high-frequency AWGs and high-resolution spectrometers. The cost is low, and the power requirement for the signal generator is only that the peak-to-peak value of the generated sinusoidal signal reaches the half-wave voltage V of the phase modulator to be measured. π It is 1.53 times that of the original data, which is suitable for scenarios such as manufacturer production testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 is a system block diagram of the present invention;

[0047] Figure 2 Schematic diagram of the first type of 0th-order Bessel function and its first two positive zeros disclosed in an embodiment of the present invention;

[0048] Figure 3Different sinusoidal signals V disclosed in the embodiments of the present invention PP Below, the average output optical power changes with the phase difference between the two arms;

[0049] Figure 4 is a flow chart of the present invention;

[0050] Figure 5 This is a comparison chart of the phase modulator half-wave voltage obtained by the half-wave voltage testing method disclosed in an embodiment of the present invention and the data provided by the manufacturer. DETAILED DESCRIPTION

[0051] The present invention will be further described below with reference to the accompanying drawings.

[0052] Example 1

[0053] like Figure 4 As shown, the half-wave voltage test method of a phase modulator applicable to a wide frequency band of this embodiment can realize half-wave voltage test of all frequencies within the bandwidth range of the phase modulator, and specifically includes the following steps:

[0054] Step S1, press Figure 1 As shown in the figure, a test link is constructed, where the first port of the first 3dB optical coupler 5a is connected to the continuous light source 1, the second port is connected to the optical input port of the phase modulator 2 to be tested, and the third port is connected to the optical input port of the optical fiber adjustable delay line 4; the first port of the second 3dB optical coupler 5b is connected to the optical power meter 6, the second port is connected to the optical output port of the phase modulator 2 to be tested, and the third port is connected to the optical output port of the optical fiber adjustable delay line 4; and the signal generator 3 is connected to the RF input port of the phase modulator 2 to be tested.

[0055] Turn on the continuous light source 1. Its output optical signal, acting as an optical carrier, is split into two beams by the first 3dB optical coupler 5a, designated as the upper arm and the lower arm. The upper arm optical signal enters the phase modulator 2 under test, while the lower arm optical signal is connected to the optical fiber adjustable delay line 4. The second 3dB optical coupler 5b couples the upper and lower arm signals as the output optical signal. The output optical signal is received by an optical power meter 6, whose sampling frequency should be less than the frequency of the sinusoidal signal. The optical fiber adjustable delay line 4 is used to vary the phase difference between the upper and lower arm optical signals. The sinusoidal signal generated by the signal generator 3 is injected into the RF modulation port of the phase modulator 2 under test, modulating the upper arm optical signal.

[0056] Step S2: Adjust the optical fiber adjustable delay line 4 to change the phase difference between the upper and lower arms of the optical signal. At the same time, use the optical power meter 6 to measure the output optical power and record it, and obtain the curve of the output optical power changing with the effective length of the optical fiber adjustable delay line 4. The effective length of the optical fiber adjustable delay line 4 is L, and the effective length corresponding to the point with the minimum output optical power is L. B =L0;

[0057] Step S3, turn on the signal generator 3, generate a frequency of f and a peak-to-peak value of V PP-1 A sinusoidal signal is injected into the RF modulation port of the phase modulator 2 to be tested;

[0058] Step S4, adjust the optical fiber adjustable delay line 4 again to change the phase difference of the upper and lower arms of the optical signal, and record the output average optical power to determine L B = L0, the output average optical power is the minimum or maximum point. B = L0 when the output average optical power is the minimum point, then increase the peak-to-peak value of the RF signal. If L B =L0, the output average optical power reaches its maximum value, which reduces the peak-to-peak value of the RF signal.

[0059] Step S5, repeat step S4 until the L B = L0 when the output average optical power changes from the minimum value to the maximum value of the sinusoidal signal peak-to-peak value V PP-T ;

[0060] Step S6: According to the peak-to-peak value of the sinusoidal signal V PP-T Calculate the half-wave voltage of the phase modulator 2 under test at frequency f, the calculation formula is V π =V PP-T / 1.53.

[0061] The light signal generated by the continuous light source 1 can be expressed as:

[0062] E in (t) = E0cos(ω0t) (1)

[0063] Where E0 represents the optical amplitude and ω0 represents the optical carrier angular frequency.

[0064] After the input optical signal is split into two beams by the first 3dB optical coupler 5a, the input optical signals of the upper and lower arms can be expressed as:

[0065]

[0066] Assume that the RF signal injected into the phase modulator to be tested is:

[0067] v m (t) = V m cos(ω m t) (3)

[0068] Among them, V m 、ω m Represent the amplitude and angular frequency of the RF signal respectively.

[0069] The output light field of the upper arm optical signal after being modulated by the phase modulator 2 to be measured can be expressed as:

[0070]

[0071] Where α1 represents the optical loss introduced by the upper arm, V π Represents the half-wave voltage of the phase modulator under test.

[0072] The output optical field of the lower arm optical signal after passing through the optical fiber adjustable delay line 4 can be expressed as:

[0073]

[0074] Where α2 represents the optical loss introduced by the lower arm, n represents the refractive index of the optical waveguide, ΔL represents the optical path difference between the upper and lower arms, and λ represents the wavelength of the optical carrier. Since ω0 = 2πc / λ, formula (5) can be simplified to:

[0075]

[0076] in,

[0077] The optical signals of the upper and lower arms are coupled through the second 3dB optical coupler 5b. The output optical field after coupling can be expressed as:

[0078]

[0079] The output optical power is the time average of the optical field intensity, which can be expressed as

[0080]

[0081] Introduced input optical power P in , substituting into formula (7) and simplifying, the output optical power can be expressed as:

[0082]

[0083] From formula (9), it can be seen that the output optical power consists of two parts: DC and AC. The AC part is related to the RF modulation signal and θ.

[0084] The cosine function of the AC part of formula (9) can be expressed as:

[0085]

[0086] Expanding them separately, we can get:

[0087]

[0088] Among them, J n(x) represents the first-order Bessel function of the first kind. From formulas (11) and (12), it can be seen that the AC part of the output optical power includes the DC component, odd harmonics, and even harmonics.

[0089] The optical power meter 6 used in this embodiment is a low-bandwidth instrument and cannot detect high-frequency modulated signals. Therefore, the optical power meter 6 receives the average optical power, which does not contain high-frequency components. Substituting formulas (10), (11), and (12) into formula (9) and filtering out high-order components, we can obtain:

[0090]

[0091] It can be seen from formula (13) that the curve of the average output optical power of the second 3dB optical coupler 5b changing with θ is related to the amplitude of the injected sinusoidal signal.

[0092] The curve of the first kind 0th order Bessel function J0(x) is as follows Figure 2 As shown, the first two zero points greater than 0 are 2.405 and 5.520 respectively. Therefore, the amplitude of the sinusoidal signal V m The size of determines the change trend of the output optical power. When 0<πV m / V π <2.405, that is, 0<V m <0.7655V π hour, The average optical power output has a minimum value when θ=(2n+1)π, n=0,1,2...; when 2.405<πV RF / V π <5.520, i.e. 0.7655V π <V RF <1.757V π hour, The average output optical power reaches a maximum value when θ=(2n+1)π. Figure 3 Shows V PP =2V RF =V π / 2,V π , 3V π / 2, 2V π Schematic diagram of the average optical power changing with the phase difference between the two arms when .

[0093] From the above we know Wherein ω0, n, and c are all constants. Therefore, θ is only related to the optical path difference ΔL between the upper and lower arms. Therefore, the value of θ can be controlled by changing the effective length of the optical fiber adjustable delay line 4.

[0094] When no sinusoidal signal is applied, the effective length of the optical fiber adjustable delay line 4 corresponding to the point where the output optical power is the minimum, that is, θ = (2n + 1)π, is L. B =L0, with peak-to-peak value V PP To express the amplitude of the sinusoidal signal, the above conclusion can be rewritten as: when 0<V PP <1.53V π When the average optical power output is L B = L0 has a minimum value; when 1.53V π <V PP <3.51V π When the average optical power output is L B = L0 has a maximum value. Therefore, the output optical power can be B =L0 when the sinusoidal signal V changes from the minimum value to the maximum value PP To determine the half-wave voltage of the phase modulator.

[0095] Assume that the half-wave voltage of the phase modulator is 3V to 3.5V and the required accuracy is 0.01V. Let V PP-1 is 4V, at this time, L B =L0 when the output optical power has the minimum value; V PP-2 is 6V, at this time, L B = L0 when the output optical power reaches its maximum value. PP-T In the range of 4V to 6V, the peak-to-peak value of the sinusoidal RF signal is determined by the dichotomy method. PP-T The possible range is:

[0096] L=2*0.5 n-2 =0.5 n-3 ,n>2 (14)

[0097] To achieve an accuracy of 0.01V, it is necessary to satisfy L<0.01, that is,

[0098] 0.5 n-3 <0.01 (15)

[0099] It can be solved that n>9.64, that is, a maximum of 10 repeated operations are required to obtain the half-wave voltage of the phase modulator, with an error of less than 0.01 V. In the automated testing of actual engineering production, this is a very fast process.

[0100] Through the above steps, the test of the half-wave voltage of the phase modulator is realized. This method does not require expensive instruments such as high-speed oscilloscopes. The required equipment are all conventional instruments. The power requirement for the signal generator is only that the peak-to-peak value of the sinusoidal signal reaches 1.53 times the half-wave voltage of the phase modulator to be tested. The half-wave voltage of all frequencies within the bandwidth range of the phase modulator can be measured, which is suitable for scenarios such as industrial production testing. Figure 5 The figure shows the half-wave voltage data of the phase modulator tested by the method described in this embodiment. As can be seen from the figure, the half-wave voltage tested by the method described in the present invention is consistent with the half-wave voltage data provided by the manufacturer.

[0101] Example 2

[0102] like Figure 4 As shown, the phase modulator half-wave voltage test system suitable for a wide frequency band of this embodiment includes a continuous light source 1, a phase modulator to be tested 2, a signal generator 3, an optical fiber adjustable delay line 4, a 3dB optical coupler and an optical power meter 6; the optical signal output by the continuous light source 1 is divided into two beams by a first 3dB optical coupler 5a, one of which is connected to the optical input port of the phase modulator to be tested 2, and the other is connected to the input port of the optical fiber adjustable delay line 4; the two optical signals from the optical output port of the phase modulator to be tested 2 and the output port of the optical fiber adjustable delay line 4 are coupled by a second 3dB optical coupler 5b, and the coupled optical signals are connected to the optical power meter 6; the RF modulation port of the phase modulator 2 to be tested is connected to the signal generator 3.

[0103] Signal generator 3 is a sinusoidal signal generator. The required peak-to-peak value of the sinusoidal signal should reach 1.53 times the half-wave voltage of the phase modulator under test. Continuous-wave light source 1, optical fiber variable delay line 4, 3dB optical coupler, and optical power meter 6 are all commonly used devices. Fiber optic variable delay line 4 is a continuously adjustable optical fiber variable delay line. Optical power meter 6 has a low bandwidth and cannot detect high-frequency RF signals. It measures the average optical power of the output signal.

[0104] The testing method of the phase modulator half-wave voltage testing system applicable to a wide frequency band in this embodiment is similar to that in the first embodiment and will not be described in detail.

Claims

1. A phase modulator half-wave voltage test system suitable for a wide frequency band, characterized in that: The device includes a continuous light source, a phase modulator to be measured, a signal generator, an optical fiber adjustable delay line, a 3dB optical coupler and an optical power meter; the 3dB optical coupler includes a first 3dB optical coupler and a second 3dB optical coupler. The optical signal output by the continuous light source is divided into an upper arm optical signal and a lower arm optical signal by the first 3dB optical coupler. The upper arm optical signal enters the phase modulator to be measured, and the lower arm optical signal is connected to the optical fiber adjustable delay line. The second 3dB optical coupler couples the upper and lower arm optical signals and then inputs them into the optical power meter.

2. The phase modulator half-wave voltage test system applicable to a wide frequency band according to claim 1, characterized in that: The optical fiber adjustable delay line is used to change the phase difference between the upper and lower arms of the optical signal.

3. The phase modulator half-wave voltage test system applicable to a wide frequency band according to claim 1, characterized in that: The sinusoidal signal generated by the signal generator is injected into the RF modulation port of the phase modulator to be tested to modulate the upper arm optical signal.

4. The phase modulator half-wave voltage test system applicable to a wide frequency band according to claim 1, characterized in that: The light signal generated by the continuous light source is: E in (t)=E0cos(ω0t)(1) Where E0 represents the optical amplitude and ω0 represents the optical carrier angular frequency.

5. The phase modulator half-wave voltage test system applicable to a wide frequency band according to claim 4, characterized in that: The upper arm optical signal and the lower arm optical signal are expressed as: Assume that the RF signal injected into the phase modulator to be tested is: v m (t)=V m cos(ω m t)(3) Among them, V m 、ω m Respectively represent the amplitude and angular frequency of the RF signal; The output light field of the upper arm optical signal after being modulated by the phase modulator to be measured is expressed as: Where α1 represents the optical loss introduced by the upper arm, V π Represents the half-wave voltage of the phase modulator under test.

6. The phase modulator half-wave voltage test system applicable to a wide frequency band according to claim 5, characterized in that: The output optical field of the lower arm optical signal after passing through the optical fiber adjustable delay line is expressed as: Where α2 represents the optical loss introduced by the lower arm, n represents the refractive index of the optical waveguide, ΔL represents the optical path difference between the upper and lower arms, and λ represents the wavelength of the optical carrier. Since ω0 = 2πc / λ, formula (5) is simplified to: in, 7. The phase modulator half-wave voltage test system applicable to a wide frequency band according to claim 6, characterized in that: The output light field after the upper and lower optical signals are coupled by the second 3dB optical coupler is expressed as: The output optical power is the time average of the optical field intensity and is expressed as: Introduced input optical power P in , substitute into formula (7) and simplify, the output optical power is expressed as: The cosine function of the AC part of formula (9) is expressed as: Expanding them separately, we can get: Among them, J n (x) represents the nth-order Bessel function of the first kind.

8. The phase modulator half-wave voltage test system applicable to a wide frequency band according to claim 7, characterized in that: The optical power meter receives the average optical power. Substituting formulas (10), (11), and (12) into formula (9) and filtering out high-order components, we obtain:

9. The phase modulator half-wave voltage test system applicable to a wide frequency band according to claim 1, characterized in that: The sampling frequency of the optical power meter is less than the frequency of the sinusoidal signal.

10. A method for testing half-wave voltage of a wide-band phase modulator according to any one of claims 1 to 9, characterized in that: The following steps are involved: Step S1, building a phase modulator half-wave voltage test system suitable for a wide frequency band; Step S2: Adjust the optical fiber adjustable delay line to change the phase difference of the optical signals in the upper and lower arms. At the same time, use an optical power meter to measure the output optical power and record it. Obtain a curve of the output optical power changing with the effective length of the optical fiber adjustable delay line. The effective length of the optical fiber adjustable delay line is L, and the effective length corresponding to the point with the minimum output optical power is L. B =L0; Step S3: Turn on the signal generator to generate a frequency of f and a peak-to-peak value of V PP-1 A sinusoidal signal is injected into the RF modulation port of the phase modulator to be tested; Step S4, adjust the optical fiber adjustable delay line again to change the phase difference of the upper and lower arms of the optical signal, and record the output average optical power to determine L B = L0, the output average optical power is the minimum or maximum point: If L B = L0 when the output average optical power is the minimum point, then increase the peak-to-peak value of the RF signal. If L B =L0, the output average optical power reaches its maximum value, which reduces the peak-to-peak value of the RF signal; Step S5, repeat step S4 until the L B = The peak-to-peak value of the sinusoidal signal when the average optical power output changes from the minimum value to the maximum value at L0 PP-T ; Step S6: According to the peak-to-peak value of the sinusoidal signal V PP-T Calculate the half-wave voltage of the phase modulator to be tested at frequency f, the calculation formula is V π =V PP-T / 1.53.

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