A system for measuring phase noise of a mode-locked laser without dispersion management

Abstract

The application relates to the technical field of photoelectric measurement, and discloses a mode-locked laser phase noise measurement system without dispersion management, which comprises a signal generation unit, a signal adjustment unit and a signal measurement unit. The signal generation unit comprises a mode-locked laser for generating a laser signal, 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 adjustment unit is connected to the output end of the signal extraction module and comprises a signal frequency shift module and a signal extraction module connected in sequence. The signal measurement unit is connected to the output end of the signal extraction module and comprises a low-pass filter and a signal analyzer. The mode-locked laser phase noise measurement structure is newly constructed, the problem that an accurate dispersion management unit needs to be added when a traditional dual-wavelength delay self-heterodyne method is used to measure phase noise is solved, and the overall structure is simple, the measurement is accurate, and the efficiency is high.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic measurement technology, and in particular to a mode-locked laser phase noise measurement system that does not require dispersion management. Background Technology

[0002] In recent years, the superior performance of mode-locked lasers (MLLs) and optical frequency combs has attracted widespread attention. They are increasingly being used in precision spectroscopy, optical atomic clocks, biomedical imaging, ultra-low phase noise microwave synthesis, photonic analog-to-digital conversion, photonics-based radar, and other applications. Therefore, characterizing the repetition frequency phase noise of MLLs is crucial for their effective application.

[0003] Traditional measurement methods are insufficient for ultra-low noise lasers (MLLs). Direct spectral analysis is limited by the bandwidth of electronic devices. Furthermore, shot noise in photodiodes adds additional phase noise, hindering the phase noise performance of MLLs. Balanced optical cross-correlation (BOC) and optical heterodyne methods offer excellent measurement sensitivity. However, both require an additional reference laser, and the repetition rate phase noise should be lower than or close to that of the laser under test, which is difficult to achieve in practice. To overcome this problem, a dual-wavelength delayed self-heterodyne method can be used, demonstrating the ability to accurately measure the phase noise of MLLs. Moreover, this method does not require an additional reference source, significantly reducing experimental costs. However, since the repetition frequency of mode-locked lasers is typically below GHz, much smaller than the bandwidth of traditional optical filters, this method cannot obtain the two combs of the MLL at different wavelengths. Since the group velocity dispersion differs between the combs, precise dispersion management is required in the fiber delay arm to obtain a high signal-to-noise ratio beat frequency at the interference output. This invention improves the traditional comb extraction device of dual-wavelength delay self-heterodyne method, obtaining two usable comb teeth from the mode-locked laser, thus avoiding the problem of different dispersion of multiple comb teeth after passing through the delay fiber. Summary of the Invention

[0004] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

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

[0006] Therefore, the purpose of this 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 is required when measuring phase noise using the traditional dual-wavelength delay self-heterodyne method.

[0007] The phase noise of the mode-locked laser was accurately measured without dispersion management.

[0008] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a mode-locked laser phase noise measurement system without dispersion management. This measurement device includes a signal generation unit, a signal adjustment unit, and a signal measurement unit. The signal generation unit includes a mode-locked laser that generates a laser signal and a frequency multiplier module connected to the output of the mode-locked laser, as well as a signal extraction and filtering module connected to the output of the frequency multiplier module. The signal adjustment unit is connected to the output of the signal extraction and filtering module and includes a signal frequency shifting module and a signal extraction module connected in sequence. The signal measurement unit is connected to the output of the signal extraction module and includes a low-pass filter and a signal analyzer.

[0009] As a preferred embodiment of the dispersion-free mode-locked laser phase noise measurement system of the present invention, the frequency doubling module includes a plurality of Mach-Zehnder interferometers, and each of the Mach-Zehnder interferometers is connected in sequence.

[0010] As a preferred embodiment of the dispersion-free mode-locked laser phase noise measurement system of the present invention, wherein: the length difference between the upper and lower arms of the next-level Mach-Zehnder interferometer is equal to half the length difference of the previous-level Mach-Zehnder interferometer, and the length difference of the first-level Mach-Zehnder interferometer is half the cavity length of the mode-locked laser.

[0011] As a preferred embodiment of the dispersion-free mode-locked laser phase noise measurement system of the present invention, the signal extraction and filtering module includes a first filter and two optical circulators connected to its output. The second input of each of the two optical circulators is connected to a laser, and the output of each of the two optical circulators is connected to a first coupler.

[0012] As a preferred embodiment of the dispersion-free mode-locked laser phase noise measurement system of the present invention, wherein: 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 embodiment of the dispersion-free mode-locked laser phase noise measurement system of the present invention, the signal frequency shifting module includes a second coupler and a third coupler, as well as a delay fiber and an acousto-optic modulator respectively connected between the second coupler and the third coupler.

[0014] As a preferred embodiment of the dispersion-free mode-locked laser phase noise measurement system of the present invention, the acousto-optic modulator is driven by a voltage-controlled oscillator.

[0015] As a preferred embodiment of the dispersion-free mode-locked laser phase noise measurement system of the present invention, the signal extraction module includes a second filter, a plurality of signal extraction branches connected to the output of the second filter, and a mixer connected to the output of each of the signal extraction branches.

[0016] As a preferred embodiment of the dispersion-free mode-locked laser phase noise measurement system of the present invention, the signal extraction branch includes a photodetector, a bandpass filter and an RF amplifier connected in sequence.

[0017] The beneficial effects of this invention are as follows: By constructing a novel phase noise measurement structure for mode-locked lasers, this invention can solve the problem of needing to add a precise dispersion management unit when measuring phase noise using the traditional dual-wavelength delay self-heterodyne method. Furthermore, the overall structure is simple, the measurement is accurate, and the efficiency is high. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0019] Figure 1 This is a schematic diagram of the overall structure of the mode-locked laser phase noise measurement system that does not require dispersion management according to the present invention.

[0020] Figure 2 This is a schematic diagram showing the specific connection path of the mode-locked laser phase noise measurement system that does not require dispersion management according to the present invention.

[0021] Figure 3 This is a schematic diagram of the spectral structure of the mode-locked laser after three frequency doublings in Example 1 of the present invention.

[0022] Figure 4 This is a schematic diagram of the phase noise power spectral density structure of the mode-locked laser measured with different delay fibers in Example 1 of the present invention.

[0023] Figure 5 This is a schematic diagram of the spectral structure of the mode-locked laser after six frequency doublings in Example 2 of the present invention.

[0024] Figure 6 This is a schematic diagram of the phase noise power spectral density structure of the mode-locked laser measured with different delay fibers in Example 2 of the present invention. Detailed Implementation

[0025] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0026] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0027] 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 phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0028] Secondly, the present invention is described in detail with reference to the schematic diagrams. When detailing the embodiments of the present invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not according to the usual scale. Furthermore, the schematic diagrams are merely examples and should not limit the scope of protection of the present invention. In addition, actual fabrication should include three-dimensional spatial dimensions of length, width, and depth. Example

[0029] Reference Figure 1 As an embodiment of the present invention, a mode-locked laser phase noise measurement system without dispersion management is provided. This measurement device includes a signal generation unit 100, a signal adjustment unit 200, and a signal measurement unit 300. The signal generation unit 100 is used to emit an initial laser signal and output it after processing. The signal adjustment unit 200 is used to adjust the signal output by the signal generation unit 100 after processing and extract the target signal, and finally output it, which is then measured by the signal measurement unit 300.

[0030] Specifically, the signal generation unit 100 includes a mode-locked laser 101 that generates a laser signal, a frequency multiplier module 102 connected to the output of the mode-locked laser 101, and a signal extraction and filtering module 103 connected to the output of the frequency multiplier module 102. The mode-locked laser 101 outputs an initial laser signal, the frequency multiplier module 102 is used to amplify the initial laser signal by frequency multiplication, so that the repetition frequency of the laser signal is higher and the number of comb teeth is lower. The signal extraction and filtering module 103 is used to lock the frequency-amplified laser signal to facilitate the subsequent measurement process.

[0031] Furthermore, the frequency multiplication module 102 includes several Mach-Zehnder interferometers 102a, and each Mach-Zehnder interferometer 102a is connected in sequence.

[0032] The length difference between the upper and lower arms of the next-level Mach-Zehnder interferometer 102a is equal to half the length difference of the previous-level Mach-Zehnder interferometer 102a, and the length difference of the first-level Mach-Zehnder interferometer 102a is half the cavity length of the mode-locked laser 101.

[0033] 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 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 the original. The length difference between the upper and lower arms of the next-stage Mach-Zehnder interferometer 102a is equal to half of the length of the previous-stage Mach-Zehnder interferometer 102a.

[0034] Theoretically, the higher the number of stages in the pulse repetition frequency multiplier (i.e., frequency multiplier module 102), the higher the repetition frequency of the mode-locked laser 101, and the fewer comb teeth enter the DFB laser 103c, resulting in a more stable injection locking effect. However, considering that the side-mode suppression ratio of the emission spectrum decreases with increasing stages, leading to an unsatisfactory injection locking effect, a three-stage pulse repetition frequency multiplier is ultimately preferred. The ultimate goal of frequency multiplication is to reduce the number of comb teeth within the locking bandwidth of the DFB laser 103c, thereby improving the locking effect between the comb teeth and the DFB laser 103c.

[0035] The signal extraction and filtering module 103 includes a first filter 103a and two optical circulators 103b connected to its output. The second input of each optical circulator 103b is connected to a DFB laser 103c, and the output of each optical circulator 103b is connected to a first coupler 103d.

[0036] The first filter 103a separates the frequency-doubled laser signal into two comb teeth with different wavelengths. By locking the comb teeth in the laser signal with the DFB laser 103c, a single comb tooth in the mode-locked laser 101 is effectively amplified. This is achieved by adjusting the temperature and current of the DFB laser 103c so that its wavelength corresponds to the comb tooth wavelength filtered by the first filter 103a; specifically, the center wavelength of the DFB laser 103c needs to be as close as possible to the center of the first filter 103a. After extracting a single comb tooth from the mode-locked laser 101, the two laser signals are coupled and output to the signal adjustment unit 200 via the first coupler 103d.

[0037] Furthermore, the signal adjustment unit 200 is connected to the output terminal of the signal extraction and screening module 103, and includes a signal frequency shifting module 201 and a signal extraction module 202 connected in sequence; wherein, the signal frequency shifting module 201 is used to shift and adjust the beat frequency signal in the laser signal to reduce the interference of background noise, while the signal extraction module 202 is used to extract the target laser signal from the adjusted laser signal to output the measurement.

[0038] Specifically, the signal frequency shifting module 201 includes a second coupler 201a and a third coupler 201b, as well as a delay fiber 201c and an acousto-optic modulator 201d respectively connected between the second coupler 201a and the third coupler 201b, forming a delay self-heterodyne beat frequency system to generate a beat frequency signal for phase noise measurement. The acousto-optic modulator 201d is driven by the voltage-controlled oscillator 201e. The second coupler 201a, employing a 50:50 splitting ratio, splits the laser signal into two beams. One beam passes through the delay fiber 201c in the delay arm, while the other passes through the acousto-optic modulator 201d in the reference arm, ensuring the beat frequency signal is at the driving frequency, thus preventing background noise interference. Different lengths of the delay fiber 201c result in varying sensitivities for phase noise measurement; increasing the fiber length improves the system's sensitivity but reduces its bandwidth. The acousto-optic modulator 201d shifts the beat frequency signal by a specific frequency, further reducing background noise interference. After signal delay by the delay arm and reference arm, the two laser beams are coupled again by the third coupler 201b and output into the signal extraction module 202.

[0039] Furthermore, the signal extraction module 202 includes a second filter 202a, a plurality of signal extraction branches 202b connected to the output of the second filter 202a, and a mixer 202c connected to the output of each signal extraction branch 202b.

[0040] The signal extraction branch 202b can be configured with multiple paths, and in this embodiment, two paths are preferred. The signal extraction branch 202b includes a photodetector 202b-1, a bandpass filter 202b-2, and an RF amplifier 202b-3 connected in sequence.

[0041] The coupled laser signal is separated into two laser signals with at least two wavelengths by the second filter 202a and enters the signal extraction branch 202b. The signals then pass through the photodetector 202b-1, the bandpass filter 202b-2, and the radio frequency amplifier 202b-3 in sequence. The photodetector 202b-1 obtains the beat frequency signal from the laser signal. The bandpass filter 202b-2 filters out unwanted interference signals. The radio frequency amplifier 202b-3 amplifies the desired beat frequency signal.

[0042] The beat frequency signal output from signal extraction branch 202b is mixed in mixer 202c and then output to signal measurement unit 300 for measurement and analysis.

[0043] Specifically, the signal measurement unit 300 is connected to the output of the signal extraction module 202, and includes a low-pass filter 301 and a signal analyzer 302.

[0044] The mixed signal output by mixer 202c is filtered out as a near-straight line error signal when it passes through low-pass filter 301. This error signal contains phase noise of the repetitive signal. Finally, the phase noise of mode-locked laser 101 can be obtained by analyzing the filtered mixed signal using signal analyzer 302.

[0045] Based on the measurement device in Embodiment 1 above, two mode-locked lasers built on nonlinear deflection effects are used to measure phase noise. The two lasers have different cavity lengths and corresponding repetition frequencies.

[0046] Example 1

[0047] Specifically, the repetition frequency of the first mode-locked laser is 192MHz and the cavity length is 0.96m. After three cascades, the repetition frequency of the mode-locked laser 101 is doubled to 1.536GHz.

[0048] In the signal extraction and filtering module 103, the temperature of the first DFB laser is 15.7℃, and the temperature of the second DFB laser is 45.9℃, with corresponding wavelengths of 1540nm and 1560nm, respectively.

[0049] In the signal frequency shifting module 201, the signal driving the voltage-controlled oscillator 201e of the acousto-optic modulator 201d is a 40MHz sine wave signal, the corresponding bandpass filter 202b-2 is 40MHz, and the lengths of the delay optical fibers 201c used are 100m, 2000m and 5000m respectively.

[0050] The above describes the specific structural parameters of the complete experimental setup, which forms the basis for the constructed test device.

[0051] For Example 1, the mode-locked laser 101 is turned on, and the optical path difference between the two arms of each stage of the Mach-Zehnder interferometer 102a is finely adjusted to suppress unwanted comb teeth. The emission 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 was successfully multiplied from 192MHz to 1.536GHz, and the signal-to-noise ratio of the 1.536GHz signal was improved by 30dB.

[0052] The frequency-doubled mode-locked laser 101 is filtered by a first filter 103a to produce comb teeth with wavelengths of 1540 nm and 1560 nm, with a wavelength interval of 20 nm. Utilizing the characteristics of the optical circulator 103b, the comb teeth are injected from port 1 of the optical circulator 103b into the DFB laser 103c located at port 2 of the optical circulator. After the comb teeth are locked with the DFB laser 103c, the individual comb teeth in the mode-locked laser 101 are successfully amplified.

[0053] For the phase noise measurement, the method is mainly based on dual-wavelength self-heterodyne. Two wavelengths of laser light are separated into 1540nm and 1560nm wavelengths by an optical filter (second filter 202a). A 40MHz beat frequency signal is obtained on photodetector 202b-1. A 40MHz frequency shift reduces background noise interference. The two beat frequency signals are then mixed at the two input ports of mixer 202c. Finally, a near-DC error signal, containing phase noise from the repetitive signal, is filtered out by low-pass filter 301. The phase noise of mode-locked laser 101 can be analyzed using signal analyzer 302. The phase noise power spectral density of mode-locked laser 101 under different delay fibers 201c is shown below. Figure 4 As shown in the image.

[0054] Example 2

[0055] The second mode-locked laser has a repetition frequency of 54MHz and a cavity length of 3.8052m. After six stages of cascading, the repetition frequency of the mode-locked laser 101 is doubled to 3.4GHz.

[0056] In the signal extraction and filtering module 103, the temperature of the first DFB laser is 53.939℃, and the temperature of the second DFB laser is 57℃, with corresponding wavelengths of 1553nm and 1564nm, respectively.

[0057] In the signal frequency shifting module 201, the signal driving the voltage-controlled oscillator 201e of the acousto-optic modulator 201d is a 40MHz sine wave signal, the corresponding bandpass filter 202b-2 is 40MHz, and the lengths of the delay optical fibers 201c used are 300m, 1000m and 5000m respectively.

[0058] The above describes the specific structural parameters of the complete experimental setup, which forms the basis for the constructed test device.

[0059] For Example 2, the mode-locked laser 101 is turned on, and the optical path difference between the two arms of each stage of the Mach-Zehnder interferometer 102a is finely adjusted to suppress unwanted comb teeth. The emission 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 was successfully multiplied from 54MHz to 3.4GHz, and the signal-to-noise ratio of the 3.4GHz signal was improved by 30dB.

[0060] The frequency-doubled mode-locked laser 101 is filtered by a first filter 103a to produce comb teeth with wavelengths of 1553 nm and 1564 nm, with a wavelength interval of 11 nm. Utilizing the characteristics of the optical circulator 103b, the comb teeth are injected from port 1 of the optical circulator 103b into the DFB laser 103c located at port 2 of the optical circulator. After the comb teeth are locked with the DFB laser 103c, the individual comb teeth in the mode-locked laser 101 are successfully amplified.

[0061] For the phase noise measurement, the method is mainly based on dual-wavelength self-heterodyne. Two wavelengths of laser light are separated into 1553nm and 1564nm wavelengths by a second filter 202a after passing through an asymmetric Mach-Zehnder interferometer. A 40MHz beat frequency signal is obtained on photodetector 202b-1. A 40MHz frequency shift reduces background noise interference. The two beat frequency signals are then mixed at the two input ports of mixer 202c. Finally, a near-DC error signal, containing phase noise from the repetitive signal, is filtered out by low-pass filter 301. The phase noise of the mode-locked laser 101 can be analyzed using signal analyzer 302. The phase noise power spectral density of the mode-locked laser 101 under different delay fibers 201c is shown below. Figure 6 As shown in the image.

[0062] It should be noted that 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 preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A phase noise measurement system for a mode-locked laser without dispersion management, characterized in that: include, The signal generation unit (100) includes a mode-locked laser (101) that generates laser signals, a frequency multiplier module (102) connected to the output of the mode-locked laser (101), and a signal extraction and filtering module (103) connected to the output of the frequency multiplier module (102). A signal adjustment unit (200), connected to the output of the signal extraction and filtering module (103), includes a signal frequency shifting module (201) and a signal extraction module (202) connected in sequence; and, The signal measurement unit (300) is connected to the output of the signal extraction module (202) and includes a low-pass filter (301) and a signal analyzer (302). The frequency doubling module (102) includes a plurality of Mach-Zehnder interferometers (102a), and each of the Mach-Zehnder interferometers (102a) is connected in sequence; The signal extraction and filtering module (103) includes a first filter (103a) and two optical circulators (103b) connected to its output. The second input of each of the two optical circulators (103b) is connected to a DFB laser (103c), and the output of each of the two optical circulators (103b) is connected to a first coupler (103d).

2. The mode-locked laser phase noise measurement system without dispersion management according to claim 1, characterized in that: The length difference between the upper and lower arms of the next-level Mach-Zehnder interferometer (102a) is equal to half the length difference of the previous-level Mach-Zehnder interferometer (102a), and the length difference of the first-level Mach-Zehnder interferometer (102a) is half the cavity length of the mode-locked laser (101).

3. The mode-locked laser phase noise measurement system without dispersion management according to claim 1, characterized in that: The temperature and current of the DFB laser (103c) are adjusted so that the wavelength of the DFB laser (103c) corresponds to the wavelength of the comb teeth filtered out by the first filter (103a).

4. The mode-locked laser phase noise measurement system without dispersion management according to claim 3, characterized in that: The signal frequency shifting module (201) includes a second coupler (201a) and a third coupler (201b), as well as a delay fiber (201c) and an acousto-optic modulator (201d) respectively connected between the second coupler (201a) and the third coupler (201b).

5. The mode-locked laser phase noise measurement system without dispersion management according to claim 4, characterized in that: The acousto-optic modulator (201d) is driven by a voltage-controlled oscillator (201e).

6. The mode-locked laser phase noise measurement system without dispersion management according to claim 5, characterized in that: The signal extraction module (202) includes a second filter (202a), a plurality of signal extraction branches (202b) connected to the output of the second filter (202a), and a mixer (202c) connected to the output of each of the signal extraction branches (202b).

7. The mode-locked laser phase noise measurement system without dispersion management according to claim 6, characterized in that: The signal extraction branch (202b) includes a photodetector (202b-1), a bandpass filter (202b-2), and an RF amplifier (202b-3) connected in sequence.

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