Mode-locked laser phase noise measurement system without dispersion management

Abstract

The invention relates to the technical field of photoelectric measurement, and discloses a mode-locked laser phase noise measuring system without dispersion management, which comprises a signal generating unit, a signal adjusting unit and a signal measuring unit, the device comprises a mode-locked laser for generating laser signals, a frequency multiplication module connected to the output end of the mode-locked laser, and a signal extraction and screening module connected to the output end of the frequency multiplication module. The signal adjusting unit is connected with the output end of the signal extraction module and comprises a signal frequency shift module and a signal extraction module which are connected in sequence; the signal measurement unit is connected with the output end of the signal extraction module and comprises a low-pass filter and a signal analyzer; by constructing a brand-new mode-locked laser phase noise measurement structure, the problem that an accurate dispersion management unit needs to be added when phase noise is measured through a traditional dual-wavelength delay self-heterodyne method can be solved, and the phase noise measurement device is simple in overall structure, accurate in measurement and high in efficiency.

Description

Technical Field

[0001] The invention relates to the technical field of photoelectric measurement, and in particular to a mode-locked laser phase noise measurement system without dispersion management. Background Art

[0002] In recent years, the excellent performance of mode-locked lasers (MLLs) and optical frequency combs has attracted widespread attention. More and more applications are being used in precision spectroscopy, optical atomic clocks, biomedical imaging, ultra-low phase noise microwave synthesis, photon analog-to-digital conversion, photon-based radar, etc. Therefore, in order to better apply them, it is crucial to characterize the repetition frequency phase noise of MLLs.

[0003] In the face of ultra-low noise lasers, traditional measurement methods can no longer meet the measurement requirements. Direct spectrum analysis is limited by the bandwidth of electronic equipment. In addition, the shot noise in the photodiode will also add additional phase noise, which will hinder the phase noise performance of the MLL. Balanced optical cross correlation (BOC) and optical heterodyne methods have excellent measurement sensitivity. However, they both require an additional reference laser, and the repetition rate phase noise should be lower than or close to the laser under test, which is difficult to achieve in the test. To overcome this problem, a dual-wavelength delayed self-heterodyne method can be used, which demonstrates the ability to accurately measure the phase noise of the MLL. In addition, this method does not require an additional reference source, which can greatly reduce the experimental cost. However, since the repetition frequency of the mode-locked laser is usually below GHz, which is much smaller than the bandwidth of traditional optical filters, this method cannot obtain the two comb teeth of different bands of the MLL. The group velocity dispersion of different comb teeth is different. In order to obtain a beat frequency with a high signal-to-noise ratio at the interference output, precise dispersion management is required in the fiber delay arm. The present invention improves the comb teeth extraction device of the traditional dual-wavelength time-delay self-heterodyne method, obtains two directly usable comb teeth from the mode-locked laser, and avoids the problem of different dispersion of multiple comb teeth after passing through the delay optical fiber. Summary of the invention

[0004] The purpose of this section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

[0005] In view of the above problems existing in the existing mode-locked laser phase noise measurement, the present invention is proposed.

[0006] Therefore, the purpose of the present invention is to provide a mode-locked laser phase noise measurement system that does not require dispersion management, which solves the problem that a precise dispersion management unit needs to be added when measuring phase noise using the traditional dual-wavelength time-delay self-heterodyne method.

[0007] Accurately measure the phase noise of a mode-locked laser without dispersion management.

[0008] To solve the above technical problems, the present invention provides the following technical solutions: a mode-locked laser phase noise measurement system that does not require dispersion management, the measurement device comprising a signal generating unit, a signal adjusting unit and a signal measuring unit, wherein the signal generating unit comprises a mode-locked laser that generates a laser signal and a frequency doubling module connected to the output end of the mode-locked laser, and a signal extraction and screening module connected to the output end of the frequency doubling module; the signal adjusting unit is connected to the output end of the signal extraction module, and comprises a signal frequency shifting module and a signal extraction module that are connected in sequence; and the signal measuring unit is connected to the output end of the signal extraction module, and comprises a low-pass filter and a signal analyzer.

[0009] As a preferred solution of the mode-locked laser phase noise measurement system without dispersion management described in the present invention, the frequency doubling module includes a plurality of Mach-Zehnder interferometers, and the Mach-Zehnder interferometers are connected in sequence.

[0010] As a preferred solution of the mode-locked laser phase noise measurement system without dispersion management described in the present invention, the length difference between the upper and lower arms of the Mach-Zehnder interferometer of the next stage is equal to half of the length difference of the Mach-Zehnder interferometer of the previous stage, and the length difference of the Mach-Zehnder interferometer of the first stage is half of the cavity length of the mode-locked laser.

[0011] As a preferred solution of the mode-locked laser phase noise measurement system without dispersion management described in the present invention, the signal extraction and screening module includes a first filter and two optical circulators connected to its output end, the second input ends of the two optical circulators are both connected to the laser, and the output ends of the two optical circulators are both connected to the first coupler.

[0012] As a preferred solution of the mode-locked laser phase noise measurement system without dispersion management of the present invention, the temperature and current of the laser are adjusted so that the laser corresponds to the comb wavelength filtered out by the first filter.

[0013] As a preferred solution of the mode-locked laser phase noise measurement system without dispersion management described in the present invention, the signal frequency shift module includes a second coupler and a third coupler, and a delay fiber and an acousto-optic modulator respectively connected between the second coupler and the third coupler.

[0014] As a preferred solution of the mode-locked laser phase noise measurement system without dispersion management described in the present invention, wherein: the acousto-optic modulator is driven by a voltage-controlled oscillator.

[0015] As a preferred solution of the mode-locked laser phase noise measurement system without dispersion management described in the present invention, the signal extraction module includes a second filter, a plurality of signal extraction branches connected to the output end of the second filter, and a mixer connected to the output end of each of the signal extraction branches.

[0016] As a preferred solution of the mode-locked laser phase noise measurement system without dispersion management described in the present invention, the signal extraction branch includes a photodetector, a bandpass filter and a radio frequency amplifier connected in sequence.

[0017] Beneficial effects of the present invention:

[0018] The present invention can solve the problem of adding an accurate dispersion management unit when measuring phase noise using the traditional dual-wavelength time-delay self-heterodyne method by constructing a new mode-locked laser phase noise measurement structure, and has a simple overall structure, accurate measurement and high efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them:

[0020] Figure 1 The figure is a schematic diagram of the overall structure of the mode-locked laser phase noise measurement system without dispersion management according to the present invention.

[0021] Figure 2 The figure is a schematic diagram of the specific connection direction of the mode-locked laser phase noise measurement system without dispersion management of the present invention.

[0022] Figure 3 Schematic diagram of the radio frequency spectrum structure of the mode-locked laser after three-time frequency doubling in Example 1 of the present invention.

[0023] Figure 4 Schematic diagram of the phase noise power spectrum density structure of the mode-locked laser measured by different delayed optical fibers in Example 1 of the present invention.

[0024] Figure 5 This is a schematic diagram of the radio frequency spectrum structure of the mode-locked laser after six times frequency doubling in Example 2 of the present invention.

[0025] Figure 6 Schematic diagram of the phase noise power spectrum density structure of the mode-locked laser measured by different delayed optical fibers in Example 2 of the present invention. DETAILED DESCRIPTION

[0026] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings.

[0027] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0028] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0029] Secondly, the present invention is described in detail with reference to the schematic diagram. When describing the embodiments of the present invention in detail, for the sake of convenience, the cross-sectional diagrams showing the device structure will not be partially enlarged according to the general scale, and the schematic diagrams are only examples, which should not limit the scope of protection of the present invention. In addition, in actual production, the three-dimensional dimensions of length, width and depth should be included.

[0030] Example

[0031] Reference Figure 1 , which is an embodiment of the present invention, provides a mode-locked laser phase noise measurement system that does not require dispersion management. The measurement device includes a signal generating unit 100, a signal adjusting unit 200 and a signal measuring unit 300, wherein the signal generating unit 100 is used to send out an initial laser signal and output it after processing; the signal adjusting unit 200 is used to adjust the signal output by the signal generating unit 100 after processing and extract the target signal, and finally output it, and the signal measuring unit 300 performs measurement.

[0032] Specifically, the signal generating unit 100 includes a mode-locked laser 101 for generating a laser signal, a frequency doubling module 102 connected to the output end of the mode-locked laser 101, and a signal extraction and screening module 103 connected to the output end of the frequency doubling module 102; wherein the mode-locked laser 101 outputs an initial laser signal, and the frequency doubling module 102 is used to frequency-double and amplify the initial laser signal, so that the greater the repetition frequency of the laser signal, the fewer the comb teeth; and the signal extraction and screening module 103 is used to injection-lock the laser signal after frequency doubling and amplification to facilitate the subsequent measurement process.

[0033] Furthermore, the frequency multiplication module 102 includes a plurality of Mach-Zehnder interferometers 102a, and each Mach-Zehnder interferometer 102a is connected in sequence.

[0034] The length difference between the upper and lower arms of the next-stage Mach-Zehnder interferometer 102a is equal to half of the length difference of the previous-stage Mach-Zehnder interferometer 102a, and the length difference of the first-stage Mach-Zehnder interferometer 102a is half of the cavity length of the mode-locked laser 101.

[0035] Specifically, the length difference between the upper and lower arms of the first-stage Mach-Zehnder interferometer 102a, i.e., the optical path difference, is half of the cavity length of the mode-locked laser 101. At this time, the pulse interval of the mode-locked laser passing through the Mach-Zehnder interferometer becomes half of the original, and the frequency interval becomes twice of the original; the length difference between the upper and lower arms of the Mach-Zehnder interferometer 102a of the next stage is equal to half of the Mach-Zehnder interferometer 102a of the previous stage.

[0036] Theoretically, the higher the pulse repetition frequency multiplier (i.e., frequency doubling module 102), the greater the repetition frequency of the mode-locked laser 101, the fewer the number of comb teeth entering the DFB laser 103c, and the more stable the injection locking result; however, considering that the side mode suppression ratio of the radio frequency spectrum will decrease as the number of levels increases, the injection locking effect will be unsatisfactory, and a three-level pulse repetition frequency multiplier is ultimately preferred. The ultimate goal of frequency doubling is to reduce the number of comb teeth within the locking bandwidth of the DFB laser 103c, so that the locking effect between the comb teeth and the DFB laser 103c is better.

[0037] The signal extraction and screening module 103 comprises a first filter 103a and two optical circulators 103b connected to the output end thereof, the second input ends of the two optical circulators 103b are both connected to a DFB laser 103c, and the output ends of the two optical circulators 103b are both connected to a first coupler 103d.

[0038] The first filter 103a is used to separate the laser signal after frequency doubling into two comb teeth of different wavelengths; the comb teeth in the laser signal are used to be injection-locked with the DFB laser 103c to effectively amplify the single comb teeth in the mode-locked laser 101, that is, by adjusting the temperature and current of the DFB laser 103c, the wavelength of the comb teeth filtered out by the DFB laser 103c and the first filter 103a is made to correspond, that is, the central wavelength of the DFB laser 103c needs to be as close to the center position of the first filter 103a as possible. After extracting a single comb tooth of the mode-locked laser 101, the two laser signals are coupled and output into the signal adjustment unit 200 through the first coupler 103d.

[0039] Furthermore, the signal adjustment unit 200 is connected to the output end of the signal extraction module 103, and includes a signal frequency shift module 201 and a signal extraction module 202 connected in sequence; wherein the signal frequency shift module 201 is used to move and adjust the beat frequency signal in the laser signal to reduce the interference of background noise, and the signal extraction module 202 is used to extract the target laser signal from the adjusted laser signal to output the measurement.

[0040] Specifically, the signal frequency shift module 201 includes a second coupler 201a and a third coupler 201b, and a delay optical fiber 201c and an acousto-optic modulator 201d respectively connected between the second coupler 201a and the third coupler 201b, forming a delayed self-heterodyne beat frequency system to generate a beat frequency signal for phase noise measurement. The acousto-optic modulator 201d is driven by a voltage-controlled oscillator 201e; wherein the second coupler 201a uses a coupler with a 50:50 splitting ratio, which is used to divide the laser signal into two beams, wherein one laser beam passes through the delay optical fiber 201c in the delay arm, and the other laser beam passes through the acousto-optic modulator 201d in the reference arm, so that the beat signal is at the driving frequency, which can prevent the interference of background noise; different lengths of the delay optical fiber 201c have different sensitivities for phase noise measurement, and increasing the length of the delay optical fiber can improve the sensitivity of the measurement system, but the increase in length reduces the measurement bandwidth of the system; the role of the acousto-optic modulator 201d is to move the beat signal to a certain frequency, which can reduce the interference of background noise. After the signal delay of the delay arm and the reference arm, the two laser signals are coupled again and output through the third coupler 201b into the signal extraction module 202.

[0041] Furthermore, the signal extraction module 202 includes a second filter 202a, a plurality of signal extraction branches 202b connected to the output end of the second filter 202a, and a mixer 202c connected to the output end of each signal extraction branch 202b.

[0042] The signal extraction branch 202b may be provided with multiple paths, and preferably two paths in this embodiment; the signal extraction branch 202b includes a photodetector 202b-1, a bandpass filter 202b-2 and a radio frequency amplifier 202b-3 connected in sequence.

[0043] Among them, the coupled laser signal is separated into two laser signals with at least two wavelengths through the second filter 202a, enters the signal extraction branch 202b, and passes through the photodetector 202b-1, the bandpass filter 202b-2 and the radio frequency amplifier 202b-3 in sequence; among them, the beat frequency signal in the laser signal is obtained through the photodetector 202b-1; when passing through the bandpass filter 202b-2, the remaining unnecessary interference signals will be filtered out; when passing through the radio frequency amplifier 202b-3, the required beat frequency signal will be amplified.

[0044] The beat signal outputted by the signal extraction branch 202b is mixed in the mixer 202c and outputted to the signal measurement unit 300 for measurement and analysis.

[0045] Specifically, the signal measuring unit 300 is connected to the output end of the signal extraction module 202 , and includes a low-pass filter 301 and a signal analyzer 302 .

[0046] When the mixing signal output by the mixer 202c passes through the low-pass filter 301, an error signal close to a straight line is filtered out, and this error signal contains the phase noise of the repetitive signal; finally, the filtered mixing signal is analyzed using the signal analyzer 302 to obtain the phase noise of the mode-locked laser 101.

[0047] Based on the measurement device in the above-mentioned embodiment 1, two mode-locked lasers based on the nonlinear deflection effect are used to measure the phase noise. The cavity lengths of the two are different, and the corresponding repetition frequencies are also different.

[0048] Example 1

[0049] Specifically, the repetition frequency of the first mode-locked laser is 192 MHz, and the cavity length is 0.96 m. After three-stage cascading, the repetition frequency output by the mode-locked laser 101 is doubled to 1.536 GHz.

[0050] In the signal extraction and screening module 103, the temperature of the first DFB laser is 15.7° C., and the temperature of the second DFB laser is 45.9° C., and the corresponding wavelengths are 1540 nm and 1560 nm, respectively.

[0051] In the signal frequency shift module 201, the signal of the voltage-controlled oscillator 201e driving the acousto-optic modulator 201d is a 40MHz sinusoidal signal, the corresponding bandpass filter 202b-2 is 40MHz, and the lengths of the delay optical fiber 201c used are 100m, 2000m and 5000m respectively.

[0052] The above is a description of the specific structural parameters of the complete experimental device, and the test device is built based on this.

[0053] For Example 1, the mode-locked laser 101 is turned on, and the optical path difference between the two arms of each level of Mach-Zehnder interferometer 102a is finely adjusted to suppress unnecessary comb teeth. The RF spectrum of the mode-locked laser 101 after passing through the MZI-PRRM (frequency doubling module 102) is as follows: Figure 3 As shown, the repetition frequency is successfully doubled from 192MHz to 1.536GHz, and the signal-to-noise ratio of the 1.536GHz signal is improved by 30dB.

[0054] The frequency-doubled mode-locked laser 101 is filtered out by the first filter 103a to obtain the comb teeth of the two wavelengths of 1540nm and 1560nm, and the interval between the two wavelengths is 20nm. By utilizing the characteristics of the optical circulator 103b, the optical circulator 103b is injected into the DFB laser 103c located in the optical circulator 2 port from the port 1. After the comb teeth are injection-locked with the DFB laser 103c, a single comb tooth in the mode-locked laser 101 is effectively amplified.

[0055] The measurement of phase noise is mainly based on the dual-wavelength self-heterodyne method. After passing through the asymmetric Mach-Zehnder interferometer, the two wavelengths of laser light are separated into 1540nm and 1560nm wavelength laser light by an optical filter (the second filter 202a). A 40MHz beat frequency signal is obtained on the photodetector 202b-1. The frequency shift of 40MHz can reduce the interference of background noise. The two beat frequency signals are passed into the two input ports of the mixer 202c for mixing. Finally, the error signal close to DC is filtered out by the low-pass filter 301. This error signal contains the phase noise of the repetitive signal. The phase noise of the mode-locked laser 101 can be analyzed using the signal analyzer 302. The phase noise power spectrum density of the mode-locked laser 101 under different delay optical fibers 201c is as follows: Figure 4 as shown in .

[0056] Example 2

[0057] The repetition frequency of the second mode-locked laser is 54 MHz, and the cavity length is 3.8052 m. After six-stage cascade connection, the repetition frequency output by the mode-locked laser 101 is doubled to 3.4 GHz.

[0058] In the signal extraction and screening module 103, the temperature of the first DFB laser is 53.939° C., and the temperature of the second DFB laser is 57° C., and the corresponding wavelengths are 1553 nm and 1564 nm, respectively.

[0059] In the signal frequency shift module 201, the signal of the voltage-controlled oscillator 201e driving the acousto-optic modulator 201d is a 40MHz sinusoidal signal, the corresponding bandpass filter 202b-2 is 40MHz, and the lengths of the delay optical fiber 201c used are 300m, 1000m and 5000m respectively.

[0060] The above is a description of the specific structural parameters of the complete experimental device, and the test device is built based on this.

[0061] For Example 2, the mode-locked laser 101 is turned on, and the optical path difference between the two arms of each level of Mach-Zehnder interferometer 102a is finely adjusted to suppress unnecessary comb teeth. The RF spectrum of the mode-locked laser 101 after passing through the MZI-PRRM (frequency doubling module 102) is as follows: Figure 5As shown, the repetition frequency is successfully doubled from 54MHz to 3.4GHz, and the signal-to-noise ratio of the 3.4GHz signal is improved by 30dB.

[0062] The frequency-doubled mode-locked laser 101 is filtered out by the first filter 103a to obtain the comb teeth of the two wavelengths of 1553nm and 1564nm, and the interval between the two wavelengths is 11nm. By utilizing the characteristics of the optical circulator 103b, the optical circulator 103b is injected into the DFB laser 103c located in the optical circulator 2 port from the port 1. After the comb teeth are injection-locked with the DFB laser 103c, a single comb tooth in the mode-locked laser 101 is effectively amplified.

[0063] The measurement of phase noise is mainly based on the dual-wavelength self-heterodyne method. After passing through the asymmetric Mach-Zehnder interferometer, the two wavelengths of laser are divided into 1553nm and 1564nm wavelength lasers by the second filter 202a, and a 40MHz beat frequency signal is obtained on the photodetector 202b-1. The frequency shift of 40MHz can reduce the interference of background noise. The two beat frequency signals are passed into the two input ports of the mixer 202c for mixing. Finally, the error signal close to DC is filtered out by the low-pass filter 301. This error signal contains the phase noise of the repetitive signal. The phase noise of the mode-locked laser 101 can be analyzed using the signal analyzer 302. The phase noise power spectrum density of the mode-locked laser 101 under different delay fibers 201c is as follows: Figure 6 as shown in .

[0064] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A mode-locked laser phase noise measurement system without dispersion management, characterized in that: include, A signal generating unit (100) comprising a mode-locked laser (101) for generating a laser signal, a frequency doubling module (102) connected to an output end of the mode-locked laser (101), and a signal extraction and screening module (103) connected to an output end of the frequency doubling module (102); A signal adjustment unit (200), connected to the output end of the signal extraction module (103), comprising a signal frequency shift module (201) and a signal extraction module (202) connected in sequence; and The signal measuring unit (300) is connected to the output end of the signal extraction module (202), and comprises a low-pass filter (301) and a signal analyzer (302).

2. The mode-locked laser phase noise measurement system without dispersion management according to claim 1, characterized in that: The frequency doubling module (102) comprises a plurality of Mach-Zehnder interferometers (102a), and each of the Mach-Zehnder interferometers (102a) is connected in sequence.

3. The mode-locked laser phase noise measurement system without dispersion management according to claim 2, characterized in that: The length difference between the upper and lower arms of the Mach-Zehnder interferometer (102a) of the next stage is equal to half of the length difference of the Mach-Zehnder interferometer (102a) of the previous stage, and the length difference of the Mach-Zehnder interferometer (102a) of the first stage is half of the cavity length of the mode-locked laser (101).

4. The mode-locked laser phase noise measurement system without dispersion management according to claim 3, characterized in that: The signal extraction and screening module (103) comprises a first filter (103a) and two optical circulators (103b) connected to the output ends thereof, the second input ends of the two optical circulators (103b) are both connected to a DFB laser (103c), and the output ends of the two optical circulators (103b) are both connected to a first coupler (103d).

5. The mode-locked laser phase noise measurement system without dispersion management according to claim 4, characterized in that: The temperature and current of the DFB laser (103c) are adjusted so that the DFB laser (103c) corresponds to the comb wavelength filtered out by the first filter (103a).

6. The mode-locked laser phase noise measurement system without dispersion management according to claim 5, characterized in that: The signal frequency shift module (201) comprises a second coupler (201a) and a third coupler (201b), and a delay optical fiber (201c) and an acousto-optic modulator (201d) respectively connected between the second coupler (201a) and the third coupler (201b).

7. The mode-locked laser phase noise measurement system without dispersion management according to claim 6, characterized in that: The acousto-optic modulator (201d) is driven by a voltage-controlled oscillator (201e).

8. The mode-locked laser phase noise measurement system without dispersion management according to claim 7, characterized in that: The signal extraction module (202) comprises a second filter (202a), a plurality of signal extraction branches (202b) connected to the output end of the second filter (202a), and a mixer (202c) connected to the output end of each of the signal extraction branches (202b).

9. The mode-locked laser phase noise measurement system without dispersion management according to claim 8, characterized in that: The signal extraction branch (202b) comprises a photoelectric detector (202b-1), a bandpass filter (202b-2) and a radio frequency amplifier (202b-3) which are connected in sequence.

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