CN116147786B - Optical inter-pulse harmonic extraction method and device - Google Patents

Abstract

The invention discloses a method for extracting harmonic waves between optical pulses, and belongs to the technical field of microwave photons. Inputting the first optical pulse into an optical switch driven by a cosine microwave signal, changing the repetition frequency of the first optical pulse through the on-off of the optical switch to obtain a second optical pulse, wherein the cosine microwave signal is obtained by performing amplitude-phase regulation on a microwave signal sent by a voltage-controlled oscillator; using a phase discrimination module to discriminate the phase of the second optical pulse and the microwave signal sent by the voltage-controlled oscillator to obtain the phase difference between the second optical pulse and the microwave signal; and when the system is steady, the frequency of the microwave signal sent by the voltage-controlled oscillator is synchronized to the inter-harmonic wave of the repetition frequency of the first optical pulse, and the repetition frequency of the second optical pulse also realizes the frequency division of the repetition frequency of the first optical pulse. The invention also discloses a device for extracting the harmonic wave between the light pulses. The invention can break through the problem of frequency limitation of the microwave signals generated by the existing microwave photon phase discriminator.

Description

Optical inter-pulse harmonic extraction method and device

Technical field

The invention relates to a method and device for extracting harmonics between light pulses, and belongs to the technical field of microwave photons.

Background technique

The optical pulses generated by mode-locking technology have rich frequency components and extremely low jitter characteristics. The microwave signals with low phase noise characteristics extracted from them can improve the performance of communications, radar and other systems. Researchers have conducted a lot of research on how to extract low phase noise microwave signals. The most direct method is to use a photodetector to convert the optical pulse signal into an electrical pulse signal, and then select the appropriate frequency through a filter. This method requires a highly linear and high-power photodetector, but the microwave signal power obtained is often below 0dBm. In order to obtain microwave signals with lower residual noise, the mode-locked laser can be locked to an optically ultrastable cavity. Another approach is to build a microwave photonic phase detector and then build a phase-locked loop to lock the microwave signal to harmonics of the light pulse repetition frequency. This method effectively improves the power of the microwave signal obtained. Both of the above methods can greatly improve the phase noise performance of microwave signals at high frequency offsets through a Mach-Zehnder interferometer (MZI) frequency doubling structure. The direct frequency beating method produced a 12GHz microwave signal with a phase noise of -172dBc/Hz at a frequency offset of 10kHz (Xie , 11(1):44-47.), the microwave photon phase detector method obtained a microwave signal with a residual noise of -174dBc/Hz (Ahn C, Na Y, Hyun M, et al. Synchronization of an optical frequency comb and amicrowave oscillatorwith 53zs/Hz 1/2resolution and 10-20-level stability[J].Photonics Research, 2022,10(2):365-372.). It should be noted that although the MZI structure greatly improves the phase noise performance, doubling the pulse repetition frequency results in a reduction in the number of frequencies that can be extracted. How to extract more frequencies from stable light pulses has become an urgent problem to be solved. The traditional method is to frequency divide the generated microwave signal and then mix it to expand the obtainable frequency range. However, the active frequency divider will introduce additional phase noise, and the lack of mixing efficiency also greatly reduces the signal-to-noise ratio of the acquired microwave signal, thus worsening the phase noise. Using microwave photonics methods, the mode-locked laser can be synchronized with a signal generator, and the signal generator is used to generate a switching signal to drive the optical switch. When the light pulse passes through the switch, part of the pulse passes and part does not pass, thus reducing the pulse The repetition frequency expands the frequency range of the extracted signal. However, this method requires an external signal source. At the same time, when the pulse repetition frequency is relatively special (not 10MHz, 100MHz, but such as 133MHz, 204MHz), it increases the difficulty of synchronization. In order to avoid this switching structure, researchers have proposed using the nonlinear effect of electro-optic modulators to achieve fractional harmonic mode locking (Bahmanian M, Kress C, Scheytt J C. Locking ofmicrowave oscillators on the interharmonics of mode-locked laser signals [J].Optics Express,2022,30(5):7763-7771.). But this method requires high microwave power.

In summary, it can be seen that the current microwave signal generation based on microwave photon phase detectors mainly generates signals with the same frequency as the repeated harmonic frequency. How to use microwave photonic phase detectors to break through this repetition frequency limitation requires further research.

Contents of the invention

The technical problem to be solved by the present invention is to overcome the shortcomings of the existing technology and provide a method for extracting harmonics between light pulses, which can break through the frequency limitation problem of microwave signals generated by existing microwave photon phase detectors and simultaneously improve the repetition frequency of light pulses. Precise frequency division.

The present invention specifically adopts the following technical solutions to solve the above technical problems:

A method for extracting harmonics between optical pulses. The first optical pulse is input into an optical switch. The optical switch is driven by a cosine microwave signal. The repetition frequency of the first optical pulse is changed by turning the optical switch on and off to obtain the second optical switch. pulse, the cosine microwave signal is obtained by regulating the amplitude and phase of the microwave signal emitted by the voltage-controlled oscillator; using a phase identification module to identify the phase of the second optical pulse and the microwave signal emitted by the voltage-controlled oscillator, the difference between the two is obtained. The phase difference; using the phase difference as a feedback signal, feedback control is performed on the voltage controlled oscillator. When the entire system is in a steady state, the frequency of the microwave signal emitted by the voltage controlled oscillator is synchronized to the first light pulse repetition On the inter-harmonics of the frequency, the repetition frequency of the second optical pulse also achieves frequency division of the repetition frequency of the first optical pulse.

Further, before phase identification is performed between the second optical pulse and the microwave signal emitted by the voltage-controlled oscillator, the second optical pulse passes through a saturable absorber. The nature of the saturable absorber determines that high peak power pulses can pass through, while low peak power pulses are suppressed, thus further suppressing unnecessary time domain pulse components.

Preferably, the phase detection module includes a dual-polarization dual-drive Mach-Zehnder modulator, the radio frequency input of which is connected to a one-to-two microwave power splitter, the fiber output of which is connected to a polarizing beam splitter, and the two polarizing beam splitters The output port is connected to a low-speed balanced detector.

Preferably, the phase identification module can also be a Sagnac ring structure composed of a 2×2 optical coupler, a phase modulator, and a π/2 phase shift crystal connected end-to-end via optical fibers, and in the Sagnac ring structure An optical circulator is connected to the outside of the output end, and a low-speed balanced detector is connected to the two output ends of the loop.

Preferably, the optical switch is a Mach-Zehnder modulator.

Based on the same inventive concept, the following technical solutions can also be obtained:

An optical inter-pulse harmonic extraction device, including:

An optical switch is used to change the repetition frequency of the first optical pulse by turning the optical switch on and off to obtain a second optical pulse. The optical switch is driven by a cosine microwave signal, and the cosine microwave signal is passed through a voltage-controlled oscillator. The emitted microwave signal is obtained by adjusting the amplitude and phase;

The phase identification module is used to identify the phase of the second optical pulse and the microwave signal emitted by the voltage-controlled oscillator to obtain the phase difference between the two;

A feedback control module is used to perform feedback control on the voltage-controlled oscillator using the phase difference as a feedback signal. When the entire system is in a steady state, the frequency of the microwave signal emitted by the voltage-controlled oscillator is synchronized to the first optical signal. On the inter-harmonic of the pulse repetition frequency, the repetition frequency of the second optical pulse also achieves frequency division of the repetition frequency of the first optical pulse.

Further, the device also includes:

The saturable absorber is arranged between the output end of the optical switch and the input end of the phase detection module.

Preferably, the phase detection module includes a dual-polarization dual-drive Mach-Zehnder modulator, the radio frequency input of which is connected to a one-to-two microwave power splitter, the fiber output of which is connected to a polarizing beam splitter, and the two polarizing beam splitters The output port is connected to a low-speed balanced detector.

Preferably, the phase identification module can also be a Sagnac ring structure composed of a 2×2 optical coupler, a phase modulator, and a π/2 phase shift crystal connected end-to-end via optical fibers, and in the Sagnac ring structure An optical circulator is connected to the outside of the output end, and a low-speed balanced detector is connected to the two output ends of the loop.

Preferably, the optical switch is a Mach-Zehnder modulator.

Compared with the existing technology, the technical solution of the present invention has the following beneficial effects:

1. The present invention breaks through the frequency limitation of the microwave signal that can be extracted from the optical pulse repetition frequency, and realizes the extraction of harmonics between the pulse repetition frequencies;

2. While extracting the harmonics between the pulse repetition frequencies, the present invention realizes frequency division of the pulse repetition frequencies and obtains a new optical pulse sequence;

3. Compared with the traditional electrical frequency dividing and mixing method, this method retains the low phase noise characteristics of the mode-locked laser and causes less phase noise deterioration.

Description of the drawings

Figure 1 is a schematic structural diagram of a preferred embodiment of the optical inter-pulse harmonic extraction device of the present invention;

Figure 2 is a specific structural schematic diagram of the radio frequency amplitude and phase control module;

Figure 3 is a specific structural schematic diagram of the phase identification module;

Figure 4 is another specific structural schematic diagram of the phase identification module;

Figure 5 is a simulation diagram of the waveforms of the light pulse before and after passing through the optical switch and the response of the optical switch;

Figure 6 shows the verification results of the light pulse repetition frequency frequency division experiment;

Figure 7 shows the experimental verification results of harmonic extraction between optical pulses.

Detailed ways

In view of the shortcomings of the existing technology, the solution of the present invention is to use a voltage-controlled oscillator to generate a signal with a frequency that is the harmonic frequency between the repetition frequencies of the optical pulse, and use this signal to drive an optical switch. The optical pulse and microwave signal passing through the optical switch are The phase is compared in the phase detector, and the phase difference signal obtained is used to feedback control the voltage-controlled oscillator. When the system is in a steady state, the light pulse is divided after passing through the optical switch, and the output frequency of the voltage-controlled oscillator will be synchronized. to the interharmonics of the light pulse.

The optical inter-pulse harmonic extraction method proposed by the present invention is as follows:

The first optical pulse is input into an optical switch. The optical switch is driven by a cosine microwave signal. The repetition frequency of the first optical pulse is changed by turning the optical switch on and off to obtain a second optical pulse. The cosine microwave signal is generated by The microwave signal emitted by the voltage-controlled oscillator is obtained by adjusting the amplitude and phase; using a phase identification module to identify the phase of the second optical pulse and the microwave signal emitted by the voltage-controlled oscillator to obtain the phase difference between the two; using the phase difference as feedback signal to perform feedback control on the voltage-controlled oscillator. When the entire system is in a steady state, the frequency of the microwave signal emitted by the voltage-controlled oscillator is synchronized to the interharmonic of the repetition frequency of the first light pulse. The repetition frequency of the pulse also achieves frequency division of the repetition frequency of the first light pulse.

The optical inter-pulse harmonic extraction device proposed by the present invention includes:

An optical switch is used to change the repetition frequency of the first optical pulse by turning the optical switch on and off to obtain a second optical pulse. The optical switch is driven by a cosine microwave signal, and the cosine microwave signal is passed through a voltage-controlled oscillator. The emitted microwave signal is obtained by adjusting the amplitude and phase;

The phase identification module is used to identify the phase of the second optical pulse and the microwave signal emitted by the voltage-controlled oscillator to obtain the phase difference between the two;

A feedback control module is used to perform feedback control on the voltage-controlled oscillator using the phase difference as a feedback signal. When the entire system is in a steady state, the frequency of the microwave signal emitted by the voltage-controlled oscillator is synchronized to the first optical signal. On the inter-harmonic of the pulse repetition frequency, the repetition frequency of the second optical pulse also achieves frequency division of the repetition frequency of the first optical pulse.

In order to facilitate public understanding, the technical solution of the present invention is described in detail below through a preferred embodiment and in conjunction with the accompanying drawings:

As shown in Figure 1, the optical inter-pulse harmonic extraction device of this embodiment consists of a mode-locked laser, a radio frequency amplitude and phase control module, an optical switch module, a saturable absorber, a phase detection module, and a feedback controller , consisting of a voltage controlled oscillator and two directional couplers. As shown in Figure 1, the optical pulse signal generated by the mode-locked laser is first input into an optical switch module; the RF signal generated by the voltage-controlled oscillator is divided into two channels through the first directional coupler, and one channel is used as the RF output of the system. The other channel is divided into two channels through the second directional coupler. One channel drives the optical switch module after passing through the radio frequency amplitude and phase control module, and the other channel inputs the phase detection module; the light pulse passing through the optical switch module then passes through a saturable absorber, and then It is divided into two channels by the optical coupler, one is used as the optical signal output of the system, and the other is input to the phase detector module to obtain the phase difference information with the radio frequency signal; the output of the phase detector controls the voltage-controlled oscillator through the feedback controller. .

The radio frequency amplitude and phase control module in this embodiment is shown in Figure 2 and is a link including a radio frequency amplifier, a radio frequency attenuator and a radio frequency phase shifter.

The optical switch module in this embodiment uses a Mach-Zehnder modulator. The switching state of the optical switch is determined by the bias state of the Mach-Zehnder modulator and the amplitude and phase of the driving signal. Its large-bandwidth characteristics can help extract data within a wide bandwidth range. of radio frequency signals.

The phase identification module can adopt various existing phase identification solutions. One structure is shown in Figure 3, which includes a dual-polarization dual-drive Mach-Zehnder modulator, and its radio frequency input is connected to a one-to-two microwave power splitter. , its optical fiber output is connected to a polarizing beam splitter, and the two output ports of the polarizing beam splitter are connected to a low-speed balanced detector.

Another structure of the phase identification module is shown in Figure 4, which includes a Sagnac ring structure composed of a 2×2 optical coupler, a phase modulator, and a π/2 phase-shift crystal connected end-to-end via optical fibers, and in this An optical circulator is connected to the outside of one output end of the Sagnac ring structure, and a low-speed balanced detector is connected to the two output ends of the loop.

In order to facilitate public understanding, the basic principles of the technical solution of the present invention are further theoretically explained in detail below. In order to simplify the process, the relevant mathematical derivation is performed under the steady-state conditions of the system.

Since the pulse width of the optical pulse is extremely narrow, the output of the mode-locked laser can be written as:

Where P is the peak power of a single pulse, f _rep is the repetition frequency of the pulse, T _rep is the minimum positive period of the pulse, and δ is the impulse function.

The radio frequency signal generated by the voltage controlled oscillator is:

where A is the amplitude of the radio frequency signal, is the phase of the radio frequency signal and is close to 0, N is an integer. The frequency of the radio frequency signal is the (N+p) harmonic of the pulse repetition frequency. After passing through the RF amplitude and phase control module, it can be expressed as:

m(t)=acos(2π(N+p)f _rep t),(p=1/2,1/3,2/3)

where a is the adjusted amplitude. After passing through the phase shifter, the sinusoidal signal is converted into a cosine signal.

The optical switch module uses a Mach-Zehnder modulator and is driven by a radio frequency signal after amplitude and phase control. The optical pulse emitted by the mode-locked laser can be expressed as:

where V _π and θ are the half-wave voltage and bias phase of the Mach-Zehnder modulator, respectively. In order to achieve repeated frequency division, some pulse time domain components need to be suppressed, which can be achieved by adjusting the sizes of a and θ. Consider first the situation when p=1/2. At this time, the amplitude of the pulse has shown that the period is twice the original period, so only the pulse components when n=0 and 1 need to be considered. Here, it is assumed that when n=0, the pulse amplitude is not 0, and when n=1, the pulse amplitude is 0. In this way, the following conditions can be obtained:

When the above conditions are met, frequency division by 2 can be achieved theoretically. When n=0, the pulse amplitude remains unchanged. When n=1, the pulse amplitude is 0. The optimal conditions for frequency division can be obtained:

At this time, the output of the Mach-Zehnder modulator is:

It can be seen that the pulse repetition period has doubled, that is, the repetition frequency has been divided by two.

Consider again the situation when p=1/3. At this time, the amplitude of the pulse has shown that the period is three times the original period, so only the pulse components when n=0, 1 and 2 need to be considered. Here, it is assumed that when n=0, the pulse amplitude is not 0, and when n=1 and 2, the pulse amplitude is 0. In this way, the following conditions can be obtained:

When the above conditions are met, frequency division by 3 can be achieved theoretically. When n=0, the pulse amplitude remains unchanged. When n=1 and 2, the pulse amplitude is 0, and the optimal conditions for frequency division can be obtained:

At this time, the output of the Mach-Zehnder modulator is:

It can be seen that the pulse repetition period has become three times the original, that is, the repetition frequency has been divided by three. For the situation when p=2/3, it is basically the same as the situation when p=1/3, and the repetition frequency is also divided by 3. To sum up, the newly generated light pulse train can be recorded as:

Figure 5 shows the simulation results. During simulation, the pulse repetition frequency was set to 102MHz, and the radio frequency signals were selected to be 51MHz ((a) in Figure 5), 34MHz ((b) in Figure 5) and 68MHz ((c) in Figure 5). It can be seen that the three achieve frequency division by 2, frequency by 3 and frequency by 3 respectively.

Considering that in practical applications, the extinction ratio of the Mach-Zehnder modulator is limited, usually around 20dB, and the high extinction ratio model can reach 25dB, so its output pulse can further pass through a saturable absorber to further suppress unnecessary The optical pulse component is then input into the phase detection module.

Taking the phase identification module in Figure 3 as an example to illustrate the principle of phase identification. There is a Mach-Zehnder modulator on the upper and lower arms of the dual-polarization dual-drive Mach-Zehnder modulator; there is a 90° polarization rotator behind the lower Mach-Zehnder modulator, so that the light passing through the upper and lower arms has Mutually orthogonal polarization states; the RF signal of the voltage-controlled oscillator is divided into two paths through a power divider, which drives the Mach-Zehnder modulators on the upper and lower arms respectively. The two modulators are biased at complementary orthogonal biases. point. At this time, the output of the dual-polarization dual-drive Mach-Zehnder modulator can be written as:

where β is the modulation depth. The two orthogonal polarization states are split into two paths through a polarization beam splitter. A low-speed balanced detector is used to detect the average optical power difference between the two paths. Its output is:

From the above equation, we can see that the phase of the radio frequency signal is extracted. Then through a feedback controller, the phase difference is compensated to the voltage-controlled oscillator, thus maintaining synchronization and extracting the interharmonics of the original pulse repetition frequency.

The feasibility of the solution of the present invention is further verified through an experiment. As shown in Figure 6, (a) to (c) show the time domain waveforms of the pulse before and after frequency division, and (d) to (f) show the spectrum of the pulse before and after frequency division. It can be found that the time period of the pulse has been expanded by 2 times and 3 times, and the repetition frequency has been reduced by 1/2 and 1/3; Figure 7 (a) ~ (c) respectively show the phase noise of the extracted inter-pulse harmonics Figure, the dotted line is the phase noise curve of the voltage-controlled oscillator in the free oscillation state. After synchronization with the mode-locked laser is achieved, the phase noise has been greatly improved, as shown in the solid line in Figure 7, at 10kHz frequency deviation Phase noise reaches below -130dBc/Hz.

Claims (10)

1. The method is characterized in that a first optical pulse is input into an optical switch, the optical switch is driven by a cosine microwave signal, the repetition frequency of the first optical pulse is changed through the on-off of the optical switch, a second optical pulse is obtained, and the cosine microwave signal is obtained by amplitude-phase regulation and control of a microwave signal sent by a voltage-controlled oscillator; using a phase discrimination module to discriminate the phase of the second optical pulse and the microwave signal sent by the voltage-controlled oscillator to obtain the phase difference between the second optical pulse and the microwave signal; and when the whole system is in a steady state, the frequency of the microwave signal sent by the voltage-controlled oscillator is synchronized to the inter-harmonic wave of the repetition frequency of the first optical pulse, and the repetition frequency of the second optical pulse also realizes the frequency division of the repetition frequency of the first optical pulse.

2. The method of claim 1, wherein the second optical pulse is passed through a saturable absorber prior to phase discrimination of the second optical pulse with the microwave signal from the voltage controlled oscillator.

3. The method of claim 1 or 2, wherein the phase discrimination module comprises a dual-polarization dual-drive mach-zehnder modulator, the radio frequency input of which is connected with a one-to-two microwave power divider, the optical fiber output of which is connected with a polarization beam splitter, and two output ports of the polarization beam splitter are connected with a low-speed balance detector.

4. The method for extracting harmonic wave between optical pulses according to claim 1 or 2, wherein the phase discrimination module is a Sagnac loop structure composed of a 2 x 2 optical coupler, a phase modulator and a pi/2 phase shift crystal connected end to end in sequence via an optical fiber, an optical circulator is connected to the outer side of one output end of the Sagnac loop structure, and two output ends of the loop are connected to a low-speed balance detector.

5. A method of extracting an optical inter-pulse harmonic as claimed in claim 1 or 2 wherein the optical switch is a mach-zehnder modulator.

6. An optical inter-pulse harmonic extraction apparatus, comprising:

the optical switch is used for changing the repetition frequency of the first optical pulse through the on-off of the optical switch to obtain a second optical pulse, and the optical switch is driven by a cosine microwave signal, wherein the cosine microwave signal is obtained by performing amplitude-phase regulation and control on a microwave signal sent by the voltage-controlled oscillator;

the phase discrimination module is used for carrying out phase discrimination on the second optical pulse and the microwave signal sent by the voltage-controlled oscillator to obtain the phase difference between the second optical pulse and the microwave signal;

and the feedback control module is used for carrying out feedback control on the voltage-controlled oscillator by taking the phase difference as a feedback signal, when the whole system is in a steady state, the frequency of a microwave signal sent by the voltage-controlled oscillator is synchronized to the inter-harmonic wave of the repetition frequency of the first optical pulse, and the repetition frequency of the second optical pulse also realizes the frequency division of the repetition frequency of the first optical pulse.

7. The optical pulse-to-pulse harmonic extraction apparatus as in claim 6 further comprising:

the saturable absorber is arranged between the output end of the optical switch and the input end of the phase discrimination module.

8. The device for extracting harmonic between optical pulses as claimed in claim 6 or 7 wherein the phase discrimination module comprises a dual polarization dual drive mach-zehnder modulator having a radio frequency input coupled to a one-to-two microwave power divider and an optical fiber output coupled to a polarization beam splitter having two output ports coupled to a low speed balanced detector.

9. The device for extracting harmonic between optical pulses according to claim 6 or 7, wherein the phase discrimination module is a Sagnac loop structure formed by a 2 x 2 optical coupler, a phase modulator and a pi/2 phase shift crystal connected end to end in sequence via an optical fiber, an optical circulator is connected to the outer side of one output end of the Sagnac loop structure, and two output ends of the loop are connected to a low-speed balance detector.

10. An optical inter-pulse harmonic extraction apparatus as claimed in claim 6 or 7 wherein the optical switch is a mach-zehnder modulator.