A subharmonic fourier domain mode locked optoelectronic oscillator and method

Abstract

The application discloses a kind of subharmonic Fourier domain mode-locked optoelectronic oscillators and methods, optoelectronic oscillator includes laser, electro-optic modulator, fiber delay loop, photoelectric detector, electric amplifier, tunable electric filter and electric power splitter, wherein, tunable electric filter is also connected with waveform generator;The light output by laser enters electro-optic modulator and is modulated in intensity, then is delayed by fiber delay loop and is photoelectrically converted in photoelectric detector, and the output signal enters tunable electric filter and is selected in frequency.By controlling the amplitude and period of the signal generated by waveform generator, the passband of the tunable electric filter is periodically scanned.If the integer multiple of the period is exactly equal to the sum of the integer multiple of the fiber delay loop delay and the delay of other parts of the oscillator, Fourier domain mode-locking can be achieved.The application can solve the problem of limited width when generating broadband signals by optoelectronic oscillator.

Description

Technical Field

[0001] This invention belongs to the field of optoelectronic technology, specifically relating to a subharmonic Fourier domain mode-locked optoelectronic oscillator and method. Background Technology

[0002] In recent years, optoelectronic oscillators have become the primary method for generating high-quality microwave signals in microwave photonics technology due to their ultra-high quality factor. However, although optoelectronic oscillators can generate frequency-tunable microwave signals with ultra-low phase noise, the generated signals are mostly single-frequency continuous microwave signals, which is difficult to meet the requirements of systems such as radar for broadband reconfigurable transmission signals. In recent years, domestic researchers have introduced the concept of Fourier domain mode-locking into optoelectronic oscillators, realizing optoelectronic oscillators capable of generating broadband signals. Based on this, researchers have successively reported various types of Fourier domain mode-locked optoelectronic oscillators, such as bandwidth-variable Fourier domain mode-locked optoelectronic oscillators and harmonic mode-locked Fourier domain optoelectronic oscillators, achieving broadband signal output with variable bandwidth and time width.

[0003] However, although the aforementioned optoelectronic oscillators can generate broadband frequency-modulated signals, the duration of the generated signal is always limited by the loop length of the optoelectronic oscillator, which cannot meet the urgent needs of practical systems such as radar for signals with long durations. To extend the duration of the generated signal, the only way for ordinary Fourier-domain mode-locked optoelectronic oscillators is to increase the length of the optical fiber in the loop. This, however, leads to increased system instability, and the dispersion introduced by the long fiber will greatly limit the signal quality generated by phase-modulated optoelectronic oscillators. Summary of the Invention

[0004] The main objective of this invention is to overcome the shortcomings and deficiencies of the prior art and to propose a subharmonic Fourier domain mode-locked optoelectronic oscillator and method.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A subharmonic Fourier domain mode-locked optoelectronic oscillator includes a connected laser, an electro-optic modulator, an optical fiber delay loop, a photodetector, an electrical amplifier, a tunable electrical filter, and an electrical power beam splitter, wherein the tunable electrical filter is also connected to a waveform generator.

[0007] Lasers, including fixed-wavelength or tunable lasers, are used to generate and output optical signals.

[0008] Electro-optic modulators are used to modulate microwave signals onto optical carriers;

[0009] Fiber optic delay loops are used to provide variable and sufficiently large delays in optoelectronic oscillators;

[0010] A photodetector is used to convert optical signals into electrical signals;

[0011] An electrical amplifier is used to amplify the input electrical signal to compensate for cavity loss.

[0012] A tunable electrical filter is used to filter electrical signals and tune the passband position.

[0013] A waveform generator is used to generate a drive signal that changes the center frequency of a tunable electric filter.

[0014] An electric power beam splitter is used to split the input electrical signal into two parts. One part is output as a microwave signal, and the other part is fed back to the radio frequency port of the electro-optic modulator to form an oscillation loop.

[0015] Furthermore, the fiber delay loop specifically includes a connected optical coupler, an optical amplifier, a fiber delay unit, and a tunable optical attenuator;

[0016] An optical coupler is used to couple part of the output signal of an electro-optic modulator into an optical fiber delay loop and input part of the signal into a photodetector;

[0017] An optical amplifier is used to process the input optical signal to compensate for the loss in the fiber optic delay loop.

[0018] Fiber optic delay unit, used to delay optical signals;

[0019] A tunable optical attenuator is used to provide the necessary loss to control the amount of optical power in the delay loop.

[0020] Furthermore, the optical coupler in the fiber delay loop is a standard 2×2 port coupler, used to couple a portion of the optical power into the fiber delay loop.

[0021] Furthermore, optical switches or other devices that achieve the same function can be used to replace the optical couplers in the fiber optic delay loop.

[0022] Furthermore, the fiber delay unit is a single-mode fiber or other delay device with the same effect.

[0023] Furthermore, the optical amplifier can be an optical fiber amplifier, a semiconductor optical amplifier, or an optical parametric amplifier.

[0024] Furthermore, the tunable electrical filter is a tunable filter with a narrow passband response.

[0025] The present invention also includes a method for generating a time-width-bandwidth variable signal based on the provided subharmonic Fourier domain mode-locked optoelectronic oscillator, which includes an optical signal generated by a laser being electro-optically modulated, then sent into an optical fiber delay loop, and finally undergoing photoelectric conversion in a photodetector.

[0026] The electrical signal after photoelectric conversion is filtered by a tunable electrical filter to obtain an oscillating frequency. When the cavity gain is greater than the loss, single-frequency oscillation output is achieved.

[0027] By adjusting the amplitude of the driving signal of the tunable electric filter, the passband response frequency of the filter is changed, causing it to scan according to a certain pattern, thereby realizing a time-varying filter. When the microwave signal is modulated by the electro-optic modulator and transmitted through the optical link back to the tunable electric filter, if the passband frequency of the filter is exactly the same as the signal frequency, Fourier mode locking is achieved. At this time, the opto-oscillator will output a broadband signal, and the signal type is determined by the waveform of the driving signal of the electric filter.

[0028] By changing the scanning period of the tunable filter to be greater than the loop delay, subharmonic Fourier domain mode-locking can be achieved.

[0029] Furthermore, to achieve Fourier mode-locking, the time it takes for the signal to travel one revolution within the photoelectric oscillator cavity is equal to an integer multiple of the signal generation period, i.e.:

[0030] T = nT s

[0031] Where T is the time it takes for the signal to travel one revolution within the annular cavity, T s The period of the signal itself is n, where n is a positive integer; the time required for the signal to propagate within the ring cavity is:

[0032] T = mT1 + T2

[0033] Where T1 is the delay of the fiber delay loop, T2 is the delay of the other parts of the oscillation loop excluding the fiber delay loop, and m is a positive integer;

[0034] have to:

[0035] nT s =mT1+T2

[0036] Wherein, the period T of the generated signal s Controlled by the scan period of the tunable electric filter;

[0037] Since the signal can undergo multiple delays in the fiber optic delay loop, i.e., m is variable, by controlling the scanning period of the tunable electrical filter, the output signal can be cyclically output multiple times in the fiber optic loop to satisfy the Fourier domain mode-locking condition, i.e., subharmonic Fourier domain mode-locking is achieved.

[0038] Furthermore, the gain of the optical amplifier and the attenuation of the tunable optical attenuator in the fiber delay loop should be controlled in combination to avoid self-oscillation in the delay loop while minimizing the loop loss as much as possible.

[0039] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0040] 1. This invention is based on a subharmonic Fourier domain mode-locked optoelectronic oscillator, which can generate frequency-modulated microwave signals with tunable time and bandwidth, and the signal generation time can break through the limitation of loop length, realizing the generation of large time-bandwidth product signals.

[0041] 2. This invention achieves the tuning of the output signal bandwidth and time width by controlling the frequency scanning range and period of the tunable electric filter, and has the characteristics of simple adjustment method and applicability to radar system. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the structure of the present invention;

[0043] Figure 2 This is a structural diagram of the optical fiber delay loop in this invention;

[0044] Figure 3 This is a schematic diagram illustrating the relationship between the frequency of the signal generated by a subharmonic Fourier domain mode-locked optoelectronic oscillator and time according to the present invention.

[0045] Explanation of reference numerals: 1-Laser; 2-Electro-optic modulator; 3-Fiber optic delay loop; 4-Photodetector; 5-Electrical amplifier; 6-Tunable electrical filter; 7-Electrical power splitter; 8-Waveform generator; 9-Optical coupler; 10-Tunable optical attenuator; 11-Fiber optic cable; 12-Optical amplifier. Detailed Implementation

[0046] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0047] Example

[0048] like Figure 1 As shown, the present invention provides a subharmonic Fourier domain mode-locked optoelectronic oscillator, comprising a laser 1, an electro-optic modulator 2, an optical fiber delay loop 3, a photodetector 4, an electrical amplifier 5, a tunable electrical filter 6, and an electrical power beam splitter 7, wherein the tunable electrical filter is further connected to a waveform generator 8.

[0049] like Figure 2 As shown, the fiber delay loop specifically includes a connected optical coupler 9, an optical amplifier 12, a fiber delay unit, and a tunable optical attenuator 10.

[0050] The laser's output is connected to the optical input of the electro-optic modulator, the electro-optic modulator's output is connected to the input of the fiber optic delay loop, and the fiber optic delay loop's output is connected to the optical input of the photodetector. All connections between these devices are made of optical fiber.

[0051] The electrical output port of the photodetector is connected to the input of the electrical amplifier. The output of the electrical amplifier is connected to the RF input of the tunable electrical filter. The output of the tunable electrical filter is connected to the input of the electrical power beam splitter. Part of the output of the electrical power beam splitter is connected to the electrical input of the phase modulator, and the other part is used for signal output. The output of the waveform generator is connected to the tunable electrical filter. All connections between the above devices are made using cables.

[0052] The working process in this embodiment is as follows:

[0053] The light output from the laser enters an electro-optic modulator for intensity modulation, then is delayed by an optical fiber delay loop before undergoing photoelectric conversion in a photodetector. The output signal then enters a tunable electrical filter for frequency selection. By controlling the amplitude and period of the signal generated by the waveform generator, the passband of the tunable electrical filter is periodically scanned. If an integer multiple of this period is exactly equal to an integer multiple of the delay of the optical fiber delay loop plus the sum of the delays of other parts of the oscillator, Fourier domain mode-locking can be achieved. At this point, the oscillator can output a broadband signal. The bandwidth and center frequency of the output signal are determined by the waveform amplitude, and the duration of the output signal is determined by the scanning period of the waveform.

[0054] In practical implementation, those skilled in the art can easily and readily replace the structure of this embodiment. For example, the positions of the optical amplifier, optical delay unit, and optical attenuator in the optical fiber delay loop can be interchanged; the positions of the tunable electrical filter and the electrical amplifier can be interchanged; an optical amplifier can be added to the optical path to amplify the signal; and an optical filter can be added to the optical path for filtering. Furthermore, the number, shape, and size of the components can be modified according to the actual situation.

[0055] In this embodiment, the optical coupler, optical amplifier, optical delay unit, and tunable optical attenuator in the optical fiber delay loop are all connected by optical fiber.

[0056] The laser is a distributed feedback (DFB) semiconductor laser, but it is not limited to using DFB semiconductor lasers; other types of lasers can be used to achieve the same effect.

[0057] The electro-optic modulator is a broadband intensity modulator with a bandwidth of 1-100 GHz, and can also replace other modulators that can achieve the intensity modulation effect.

[0058] The coupler in the fiber optic delay loop is a standard 2×2 port coupler, used to couple a portion of the optical power into the fiber optic delay loop. In addition to the coupler, other devices such as optical switches can also be used to achieve the same function.

[0059] The fiber delay unit can use fiber 11, such as single-mode fiber with a length of 10m-10000m. It is not limited to using single-mode fiber and can use other delay devices with the same effect.

[0060] The photodetector is a photodetector with a bandwidth of 1-100 GHz or other types of photodetectors that can achieve the same effect as a 1-100 GHz photodetector.

[0061] The amplifier has a bandwidth of 50kHz-100GHz and a gain greater than 10dB, or other types of amplifiers with a bandwidth of 50kHz-100GHz and a gain greater than 10dB.

[0062] The tunable electric filter is a bandpass filter with a center frequency bandwidth of 50kHz-100GHz and a bandwidth of 0.01MHz-1GHz.

[0063] The operating bandwidth of the electric power beam splitter is 1-100 GHz, but other electric power beam splitters with the same operating bandwidth of 1-100 GHz can also be used.

[0064] Optical amplifiers can be fiber optic amplifiers, semiconductor optical amplifiers, optical parametric amplifiers, etc.

[0065] A tunable electrical filter is a tunable filter with a narrow passband response. It can be a yttrium iron garnet (YIG) filter, a microwave photonic filter, or other types of optical filters with the same effect.

[0066] The passband position of the optoelectronic oscillator is determined by the center frequency of the tunable electrical filter, and can be tuned by changing the output of the waveform generator. By periodically adjusting the passband position of the tunable electrical filter, Fourier mode-locking can be achieved under the condition of satisfying Fourier mode-locking. At this time, a broadband signal can be generated, and the signal duration is determined by the scanning period of the tunable electrical filter. Furthermore, combined with the large delay characteristics of the fiber delay loop, the subharmonic Fourier mode-locking characteristics can be used to broaden the signal duration generated by the oscillator under the same fiber length.

[0067] Typically, to achieve Fourier mode-locking, the time it takes for the signal to travel one revolution within the photoelectric oscillator cavity should be strictly equal to an integer multiple of the signal generation period, i.e.:

[0068] T = nT s

[0069] Where T is the time it takes for the signal to travel one revolution within the annular cavity, T s Let n be the period of the signal itself, and n be a positive integer. The time required for the signal to propagate within the cavity is:

[0070] T = mT1 + T2

[0071] Where T1 is the delay of the fiber delay loop, T2 is the delay of the other parts of the oscillation loop excluding the fiber delay loop, and m is a positive integer; therefore, we can finally obtain:

[0072] nT s =mT1+T2

[0073] Wherein, the period T of the generated signal s The scanning period of the tunable electrical filter controls the signal's duration. Since the signal can undergo multiple delays (i.e., m is variable) within the fiber optic delay loop, controlling the scanning period of the tunable electrical filter allows the output signal to satisfy the Fourier domain mode-locking condition multiple times within the fiber optic delay loop, thus achieving subharmonic Fourier domain mode-locking. This method can alter the output signal's duration and address the limitation imposed by the oscillation loop delay, meeting practical application requirements.

[0074] like Figure 3 The figure shows a schematic diagram of the frequency of the signal generated by a subharmonic Fourier domain mode-locked photoelectric oscillator in this embodiment as a function of time. The solid line in the figure represents the frequency of the passband of the adjustable electrical filter as a function of time, and the dashed line represents the frequency of the electrical signal generated by the photoelectric conversion after the optical signal is delayed by different delay loops as a function of time.

[0075] To avoid excessive gain in the fiber delay loop leading to lasing, the gain of the optical amplifier and the attenuation of the adjustable optical attenuator in the loop should be reasonably controlled to ensure that the loop gain is less than 0dB but close to 0dB.

[0076] In another embodiment, a method for generating a variable-bandwidth signal based on the above-mentioned photoelectric oscillator is provided, which includes an optical signal generated by a laser being electro-optically modulated, then fed into an optical fiber delay loop, and finally undergoing photoelectric conversion in a photodetector.

[0077] The electrical signal after photoelectric conversion is filtered by a tunable electrical filter to obtain an oscillating frequency. When the cavity gain is greater than the loss, single-frequency oscillation output is achieved.

[0078] By adjusting the amplitude of the driving signal of the tunable electric filter, the passband response frequency of the filter is changed, causing it to scan according to a certain pattern, thereby realizing a time-varying filter. When the microwave signal is modulated by the electro-optic modulator and transmitted through the optical link back to the tunable electric filter, if the passband frequency of the filter is exactly the same as the signal frequency, Fourier domain mode locking can be achieved. At this time, the opto-oscillator will output a broadband signal, and the signal type is determined by the waveform of the driving signal of the electric filter.

[0079] By changing the scanning period of the tunable filter to be greater than the loop delay, subharmonic Fourier domain mode-locking can be achieved.

[0080] It should also be noted that, in this specification, terms such as "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0081] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A subharmonic Fourier domain mode-locked optoelectronic oscillator, characterized in that, It includes a connected laser, an electro-optic modulator, an optical fiber delay loop, a photodetector, an electrical amplifier, a tunable electrical filter, and an electrical power beam splitter, wherein the tunable electrical filter is also connected to a waveform generator; Lasers, including fixed-wavelength or tunable lasers, are used to generate and output optical signals. Electro-optic modulators are used to modulate microwave signals onto optical carriers; Fiber optic delay loops are used to provide variable and sufficiently large delays in optoelectronic oscillators; A photodetector is used to convert optical signals into electrical signals; An electrical amplifier is used to amplify the input electrical signal to compensate for cavity loss. A tunable electrical filter is used to filter electrical signals and tune the passband position. A waveform generator is used to generate a drive signal that changes the center frequency of a tunable electric filter. An electric power beam splitter is used to split the input electrical signal into two parts. One part is output as a microwave signal, and the other part is fed back to the radio frequency port of the electro-optic modulator to form an oscillation loop. The fiber optic delay loop specifically includes a connected optical coupler, an optical amplifier, a fiber optic delay unit, and a tunable optical attenuator; An optical coupler is used to couple part of the output signal of an electro-optic modulator into an optical fiber delay loop and input part of the signal into a photodetector; An optical amplifier is used to process the input optical signal to compensate for the loss in the fiber optic delay loop. Fiber optic delay unit, used to delay optical signals; A tunable optical attenuator is used to provide the necessary loss to control the amount of optical power in the delay loop.

2. The subharmonic Fourier domain mode-locked optoelectronic oscillator according to claim 1, characterized in that, The optical coupler in the fiber delay loop is a 2×2 port coupler, used to couple a portion of the optical power into the fiber delay loop.

3. The subharmonic Fourier domain mode-locked optoelectronic oscillator according to claim 1, characterized in that, The fiber delay unit is a single-mode fiber.

4. A subharmonic Fourier domain mode-locked optoelectronic oscillator according to claim 1, characterized in that, Optical amplifiers can be fiber optic amplifiers, semiconductor optical amplifiers, or optical parametric amplifiers.

5. A subharmonic Fourier domain mode-locked optoelectronic oscillator according to claim 1, characterized in that, A tunable electrical filter is a tunable filter with a narrow passband response.

6. A method for generating a time-bandwidth variable signal based on the subharmonic Fourier domain mode-locked optoelectronic oscillator according to any one of claims 1-5, characterized in that, The optical signal generated by the laser is electro-optically modulated and then sent into the fiber delay loop to complete the photoelectric conversion in the photodetector. The electrical signal after photoelectric conversion is filtered by a tunable electrical filter to obtain an oscillating frequency. When the cavity gain is greater than the loss, single-frequency oscillation output is achieved. By adjusting the amplitude of the driving signal of the tunable electric filter, the passband response frequency of the filter is changed, causing it to scan according to a certain pattern, thereby realizing a time-varying filter. When the microwave signal is modulated by the electro-optic modulator and transmitted through the optical link back to the tunable electric filter, if the passband frequency of the filter is exactly the same as the signal frequency, Fourier mode locking is achieved. At this time, the opto-oscillator will output a broadband signal, and the signal type is determined by the waveform of the driving signal of the electric filter. By changing the scanning period of the tunable filter to be greater than the loop delay, subharmonic Fourier domain mode-locking can be achieved.

7. The method for generating a variable time-bandwidth signal according to claim 6, characterized in that, To achieve Fourier mode-locking, the time it takes for the signal to travel one revolution within the photoelectric oscillator cavity must be an integer multiple of the signal period, i.e.: in, T This refers to the time it takes for the signal to travel one revolution within the annular cavity. T s The period of the signal itself. n It is a positive integer; The time required for the signal to propagate within the cavity is: in, T 1 represents the fiber optic delay loop delay. T 2 represents the delay of the parts of the oscillation loop other than the fiber delay loop. m It is a positive integer; have to: Among them, the period of signal generation T s Controlled by the scan period of the tunable electric filter; Because the signal can undergo multiple delays in the fiber optic delay loop, i.e. m Since the scanning period of the tunable electrical filter is variable, the output signal can be cyclically repeated multiple times in the fiber optic loop to satisfy the Fourier domain mode-locking condition, thus achieving subharmonic Fourier domain mode-locking.

8. The method for generating a variable time-bandwidth signal according to claim 6, characterized in that, In fiber optic delay loops, the gain of the optical amplifier and the attenuation of the tunable optical attenuator should be controlled in combination to avoid self-oscillation in the delay loop while minimizing loop loss.

Images