High repetition rate femtosecond fiber laser based on nonlinear amplifying ring mirror and reproducible mode locking

Abstract

The present invention provides a high repetition rate femtosecond fiber laser based on a nonlinear amplifying loop mirror and regenerative mode locking, comprising a fiber oscillator and an optoelectronic feedback loop; the fiber oscillator further comprises a main oscillation loop and a nonlinear amplifying loop mirror; the main oscillation loop comprises a first pump source, a first wavelength division multiplexer, a first erbium-doped fiber, an isolator, a first fiber coupler, a second fiber coupler and a phase modulator; the nonlinear amplifying loop mirror comprises a first fiber coupler, a second erbium-doped fiber, a second wavelength division multiplexer, a second pump source, an optical phase shifter and a dispersion compensation fiber; the optoelectronic feedback loop comprises a third fiber coupler connected in sequence by part of the output end of the second fiber coupler, a photodetector, a bandpass filter, a radio frequency amplifier, a radio frequency phase shifter, a radio frequency amplifier and a phase modulator. The laser provided by the present invention can generate light pulses with high repetition rate, high supermode suppression ratio and femtosecond level.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber lasers, and in particular to a high repetition rate femtosecond optical fiber laser based on a nonlinear amplifying ring mirror and reproducible mode locking. Background Art

[0002] Ultrafast fiber lasers have gained tremendous development due to their advantages such as alignment-free, maintenance-free, compact size, and low cost. Fiber lasers that emit femtosecond light pulses at high repetition rates have a variety of applications, including frequency comb generation, ultra-low noise microwave generation, and high-speed optical sampling.

[0003] For fiber mode-locked lasers operating at the fundamental frequency of the cavity, the pulse repetition frequency is inversely proportional to the cavity length. To generate a GHz repetition frequency optical pulse train, the corresponding physical cavity length is usually limited to a few centimeters, which poses a huge challenge to the size of the gain fiber and devices required for the implementation of fiber lasers. Therefore, fiber lasers with a GHz repetition frequency usually require harmonic mode locking, which operates at the harmonic frequency of the cavity fundamental frequency. Harmonic mode locking technology has been widely used in mode-locked lasers. Although pulse trains up to several GHz can be obtained by passive harmonic mode locking, low supermode suppression ratio and power efficiency are still issues of concern. Active mode locking can easily force the laser to operate at harmonic frequencies up to tens of GHz by periodically modulating the intracavity loss using a microwave-driven modulator. However, the pulse width generated by active mode locking alone is limited to a few picoseconds (ps) or more. At the same time, the supermode noise of the harmonic mode-locked laser is also a key factor affecting its application value.

[0004] Several fiber laser schemes are available that can generate GHz optical pulse trains approaching 1 ps or even lower.

[0005] The first is a soliton actively mode-locked fiber laser, which can compress the pulse width of the actively mode-locked laser by controlling the anomalous group velocity dispersion and nonlinearity of the fiber laser cavity. Experiments have shown that an actively mode-locked fiber laser using soliton pulse compression can generate a 10GHz, 1.3ps optical pulse sequence, and the laser operates in the 1550nm band. Similarly, in the 1.1μm band, a 10GHz, 1.1ps optical pulse sequence was generated using a soliton fiber laser. However, the pulse width of the optical pulses generated by the soliton fiber laser is still difficult to reach the femtosecond level. In addition, in an actively mode-locked fiber laser, the effective cavity length changes with temperature (i.e., the cavity length thermal drift effect), which will be converted into fluctuations in the repetition frequency of the laser, and further detuning with the modulation signal will lead to instability of the laser. Soliton fiber lasers rely on dispersion-shifted optical fibers of up to hundreds of meters to accumulate nonlinearity, which makes the laser cavity susceptible to environmental interference. Although experiments have shown that the laser cavity length can be shortened as much as possible by selecting optical fibers with large nonlinearity and dispersion, specially prepared specialty fibers are still inevitable.

[0006] Another fiber mode-locked laser scheme is the active-passive hybrid mode-locked laser, which mainly utilizes the instantaneous response characteristics of the passive saturable absorption effect to produce femtosecond pulses while maintaining the high pulse repetition rate that active harmonic mode-locking can provide. For example, by inserting single-walled carbon nanotubes as saturable absorbers into an active mode-locked fiber laser, a 730 femtosecond (fs) optical pulse sequence can be generated at a 4GHz pulse repetition rate. In addition, experiments have shown that a 9.2GHz, 440fs optical pulse sequence can be generated using active-passive hybrid mode-locking. The laser uses highly nonlinear optical fibers to excite high-order solitons. At the same time, a semiconductor saturable absorption reflector is installed in the cavity to remove the base of the high-order solitons, thereby achieving stable generation of short pulses. However, in the proven active-passive hybrid mode-locked fiber lasers, the pulse repetition rate is still limited. In addition, the real saturable absorber used in the current active-passive hybrid mode-locking scheme is easily degraded with time and increased laser intensity, resulting in photoinduced damage, which limits the service life of the laser.

[0007] In principle, for the average power that the laser gain medium can provide, a higher pulse repetition frequency will correspond to a lower single pulse energy, and also to a lower pulse peak power, which will make it difficult for the intracavity pulse to obtain sufficient nonlinear effects, and thus difficult to achieve a certain intracavity pulse shaping to produce femtosecond optical pulses. Therefore, generating a femtosecond optical pulse sequence at a high pulse repetition frequency is of great research significance and application value. This type of laser has potential application value in the fields of frequency comb generation, ultra-low noise microwave generation, high-speed optical sampling, and optical arbitrary waveform generation. Summary of the invention

[0008] In view of this, an embodiment of the present invention provides a high repetition rate femtosecond fiber laser based on a nonlinear amplifying ring mirror and regenerative mode locking to eliminate or improve one or more defects existing in the prior art and solve the pulse width limitation and supermode noise problems existing in traditional actively mode-locked lasers.

[0009] In one aspect, the present invention provides a high repetition rate femtosecond fiber laser based on a nonlinear amplifying ring mirror and reproducible mode locking, the laser comprising:

[0010] A fiber oscillator; the fiber oscillator comprises a main oscillation loop and a biased nonlinear amplifying loop mirror;

[0011] The main oscillation loop includes a first pump source, a first wavelength division multiplexer, a first erbium-doped optical fiber, an isolator, a first optical fiber coupler, a second optical fiber coupler and a phase modulator; the first pump source is connected to the pump end of the first wavelength division multiplexer, the common end of the first wavelength division multiplexer is connected to the input end of the first erbium-doped optical fiber, the output end of the first erbium-doped optical fiber is connected to the input end of the isolator, the output end of the isolator is connected to the input end of the first optical fiber coupler, the output end of the first optical fiber coupler is connected to the input end of the second optical fiber coupler, the second optical fiber coupler has a part of its output end connected to the input end of the phase modulator according to a preset coupling ratio, and the output end of the phase modulator is connected to the through end of the first wavelength division multiplexer;

[0012] The nonlinear amplifying ring mirror comprises the first fiber coupler, a second erbium-doped fiber, a second wavelength division multiplexer, a second pump source, an optical phase shifter and a dispersion compensating fiber; according to a preset coupling ratio, part of the output end of the first fiber coupler is connected to the second erbium-doped fiber, and part of the output end is connected to the dispersion compensating fiber; the other end of the second erbium-doped fiber is connected to the common end of the second wavelength division multiplexer, the pump end of the second wavelength division multiplexer is connected to the second pump source, the through end of the second wavelength division multiplexer is connected to the input end of the optical phase shifter, and the output end of the optical phase shifter is connected to the dispersion compensating fiber;

[0013] The optoelectronic feedback loop comprises a third optical fiber coupler, a photodetector, a bandpass filter, an RF amplifier, an RF phase shifter, an RF amplifier and the phase modulator which are sequentially connected to the output end of the remaining part of the second optical fiber coupler.

[0014] In some embodiments of the present invention, an isolator is provided between the first erbium-doped optical fiber and the first optical fiber coupler to ensure unidirectional operation of the main oscillation loop and block reflected light of the nonlinear amplifying loop mirror.

[0015] In some embodiments of the present invention, the coupling ratio of the first optical fiber coupler is 80:20, the coupling ratio of the second optical fiber coupler is 80:20, and the coupling ratio of the third optical fiber coupler is 70:30.

[0016] In some embodiments of the present invention, it further comprises:

[0017] The 80% output end of the first optical fiber coupler is connected to the second erbium-doped optical fiber, and the 20% output end is connected to the dispersion compensation optical fiber;

[0018] The 80% output end of the second optical fiber coupler is connected to the input end of the phase modulator, and the 20% output end is connected to the input end of the third optical fiber coupler;

[0019] The 70% output end of the third optical fiber coupler is laser output, and the 30% output end is connected to the photoelectric detector.

[0020] In some embodiments of the present invention, both the first pump source and the second pump source are semiconductor laser diodes.

[0021] In some embodiments of the present invention, the modulation bandwidth of the phase modulator is set to 0.1-40 GHz.

[0022] In some embodiments of the present invention, the phase bias of the optical phase shifter is set to 0-π to bias the transmission curve of the nonlinear amplifying loop mirror to suppress supermode noise.

[0023] In some embodiments of the present invention, the net cavity dispersion of the laser is regulated to -0.015±0.02ps by the dispersion compensation fiber. 2 , to form a soliton-like pulse shaping mechanism.

[0024] In some embodiments of the present invention, the operating wavelength of the laser is in the 1530-1580 nm band. In some embodiments of the present invention, the following further comprises:

[0025] The photodetector converts the optical signal output by the third optical fiber coupler into an RF signal, the bandpass filter filters out harmonic signals in the RF signal that are integer multiples of the cavity fundamental frequency and within the passband of the bandpass filter, the RF amplifier amplifies the RF signal output by the bandpass filter, the RF phase shifter adjusts the phase of the RF signal output by the RF amplifier to match the optical pulse sequence in the optical fiber oscillator, and the RF amplifier amplifies the RF signal output by the RF phase shifter and drives the phase modulator to periodically modulate the optical oscillator.

[0026] The beneficial effects of the present invention are at least:

[0027] The present invention provides a high repetition rate femtosecond fiber laser based on a nonlinear amplifying ring mirror and regenerative mode locking, which can generate an optical pulse sequence with a high pulse repetition frequency and a femtosecond pulse width by utilizing the intracavity pulse compression and soliton-like pulse shaping mechanism of a biased nonlinear amplifying ring mirror; the transmission characteristics of the biased nonlinear amplifying ring mirror are utilized to achieve high supermode noise suppression of active harmonic mode locking; the full polarization-maintaining structure is adopted to avoid polarization-related environmental instability problems caused by mechanical vibration, temperature fluctuation, etc.; the all-fiber format is adopted to free the laser from complex alignment and maintenance, and at the same time, the service life of the fiber laser is improved. The laser provided by the present invention has a compact structure, low cost and is easy to control, and has potential application value in the fields of frequency comb generation, ultra-low noise microwave generation, high-speed optical sampling, and optical arbitrary waveform generation.

[0028] Additional advantages, purposes, and features of the present invention will be described in part in the following description, and will become apparent to those skilled in the art after studying the following, or may be learned from the practice of the present invention. The purposes and other advantages of the present invention may be achieved and obtained by the structures specifically indicated in the specification and the accompanying drawings.

[0029] Those skilled in the art will appreciate that the objectives and advantages that can be achieved with the present invention are not limited to the above specific description, and the above and other objectives that can be achieved by the present invention will be more clearly understood from the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The drawings described herein are used to provide a further understanding of the present invention, constitute a part of the present application, and do not constitute a limitation of the present invention. In the drawings:

[0031] Figure 1 Schematic diagram of the structure of a high repetition rate femtosecond fiber laser based on a nonlinear amplifying ring mirror and reproducible mode locking in one embodiment of the present invention.

[0032] Figure 2 Schematic diagram of a transmission curve of a laser nonlinear amplifying ring mirror in one embodiment of the present invention.

[0033] Figure 3 This is a radio frequency spectrum diagram of a laser output pulse sequence in one embodiment of the present invention.

[0034] Figure 4 Schematic diagram of the autocorrelation trace and oscilloscope trace of a laser output pulse sequence in one embodiment of the present invention.

[0035] Figure 5 FIG. 4 is a spectrum diagram of an output pulse sequence of a laser in one embodiment of the present invention.

[0036] Description of reference numerals:

[0037] 1: first pump source; 2: first wavelength division multiplexer; 3: first erbium-doped optical fiber;

[0038] 4: isolator; 5: first optical fiber coupler; 6: second optical fiber coupler;

[0039] 7: phase modulator; 8: second erbium-doped optical fiber; 9: second wavelength division multiplexer;

[0040] 10: Second pump source; 11: Optical phase shifter; 12: Dispersion compensation fiber;

[0041] 13: third optical fiber coupler; 14: photodetector; 15: bandpass filter;

[0042] 16: RF amplifier; 17: RF phase shifter; 18: RF amplifier. DETAILED DESCRIPTION

[0043] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments and the accompanying drawings. Here, the illustrative embodiments of the present invention and their descriptions are used to explain the present invention, but are not intended to limit the present invention.

[0044] It should also be noted that, in order to avoid obscuring the present invention due to unnecessary details, only structures and / or processing steps closely related to the solutions according to the present invention are shown in the accompanying drawings, while other details that are not closely related to the present invention are omitted.

[0045] It should be emphasized that the term “include / comprises” when used herein refers to the presence of features, elements, steps or components, but does not exclude the presence or addition of one or more other features, elements, steps or components.

[0046] It should also be noted that, unless otherwise specified, the term “connection” herein may refer not only to a direct connection but also to an indirect connection involving an intermediate.

[0047] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the accompanying drawings, the same reference numerals represent the same or similar components, or the same or similar steps.

[0048] In order to solve the problems of pulse width limitation and supermode noise in traditional active mode-locked lasers, the present invention proposes a high repetition rate femtosecond fiber laser based on nonlinear amplifying ring mirror and reproducible mode locking, such as Figure 1 As shown, the laser includes two parts: a fiber oscillator and an optoelectronic feedback loop. The fiber oscillator includes two parts: a main oscillation loop and a biased nonlinear amplifying loop mirror. Specifically:

[0049] The main oscillation loop includes a first pump source 1 , a first wavelength division multiplexer 2 , a first erbium-doped optical fiber 3 , an isolator 4 , a first optical fiber coupler 5 , a second optical fiber coupler 6 and a phase modulator 7 .

[0050] The connection relationship is: the first pump source 1 is connected to the pump end of the first wavelength division multiplexer 2, the common end of the first wavelength division multiplexer 2 is connected to the input end of the first erbium-doped optical fiber 3, the output end of the first erbium-doped optical fiber 3 is connected to the input end of the isolator 4, the output end of the isolator 4 is connected to the input end of the first optical fiber coupler 5, the output end of the first optical fiber coupler 5 is connected to the input end of the second optical fiber coupler 6, the second optical fiber coupler 6 has a part of its output end connected to the input end of the phase modulator 7 according to a preset coupling ratio, and the output end of the phase modulator 7 is connected to the through end of the first wavelength division multiplexer 2.

[0051] The nonlinear amplifying loop mirror comprises a first fiber coupler 5 , a second erbium-doped fiber 8 , a second wavelength division multiplexer 9 , a second pump source 10 , an optical phase shifter 11 and a dispersion compensation fiber 12 .

[0052] The connection relationship is: according to the preset coupling ratio, part of the output end of the first fiber coupler 5 is connected to the second erbium-doped fiber 8, and part of the output end is connected to the dispersion compensation fiber 12; the other end of the second erbium-doped fiber 8 is connected to the common end of the second wavelength division multiplexer 9, the pump end of the second wavelength division multiplexer 9 is connected to the second pump source 10, the straight-through end of the second wavelength division multiplexer 9 is connected to the input end of the optical phase shifter 11, and the output end of the optical phase shifter 11 is connected to the dispersion compensation fiber 12.

[0053] The optoelectronic feedback loop includes a third fiber coupler 13, a photodetector 14, a bandpass filter 15, a radio frequency amplifier 16, a radio frequency phase shifter 17, a radio frequency amplifier 18 and a phase modulator 7 which are sequentially connected by the remaining output end of the second fiber coupler 6.

[0054] In the main oscillation loop:

[0055] The first pump source 1 is connected to the pump end of the first wavelength division multiplexer 2 for pump coupling. A pump source refers to a source that can provide energy to excite another system. In optics, pump coupling refers to effectively matching the pump beam with the excited optical medium, such as laser material or amplifier medium, to ensure that as much energy as possible is transferred to the excited medium to achieve efficient excitation or amplification.

[0056] In some embodiments, the first pump source 1 is a semiconductor laser diode, illustratively, whose central wavelength is at 976 nm.

[0057] The common end of the first wavelength division multiplexer 2 is connected to the input end of the first erbium-doped optical fiber 3, wherein the first erbium-doped optical fiber 3 is used to provide a main oscillation loop gain for the laser.

[0058] In some embodiments, the first erbium-doped optical fiber 3 is a polarization-maintaining erbium-doped optical fiber, which is intended to maintain the polarization state of the optical signal and ensure that the polarization state is not easily changed when the light is transmitted in the optical fiber.

[0059] In some embodiments, the dispersion value of the first erbium-doped optical fiber 3 at a wavelength of 1550 nm is -20.4 ps. 2 / km, and the absorption coefficient at a wavelength of 1530nm is 55dB / m.

[0060] In some embodiments, an isolator 4 is provided between the first erbium-doped optical fiber 3 and the first optical fiber coupler 5 to ensure unidirectional operation of the main oscillation loop and to block the reflected light of the nonlinear amplifying loop mirror.

[0061] The output end of the isolator 4 is connected to the input end of the first optical fiber coupler 5, the output end of the first optical fiber coupler 5 is connected to the input end of the second optical fiber coupler 6, the output end of the second optical fiber coupler 6 is connected to the input end of the phase modulator 7, and the phase modulator 7 is connected to the through end of the first wavelength division multiplexer 2, thereby forming a unidirectional main oscillation loop.

[0062] In some embodiments, the coupling ratio of the first fiber coupler 5 is 80:20, and the coupling ratio of the second fiber coupler 6 is 80:20. The 80% output end of the second fiber coupler 6 is connected to the input end of the phase modulator 7, and the 20% output end is connected to the input end of the third fiber coupler 13 for laser output and photoelectric feedback.

[0063] In a nonlinear amplifying loop mirror:

[0064] The second pump source 10 is connected to the pump end of the second wavelength division multiplexer 9 for pump coupling of the nonlinear amplifying loop mirror. The second pump source 10 has the same principle as the first pump source 1, so it will not be described in detail.

[0065] In some embodiments, the second pump source 10 is a semiconductor laser diode, illustratively, with a central wavelength at 976 nm.

[0066] The common end of the second wavelength division multiplexer 9 is connected to one end of the second erbium-doped optical fiber 8, wherein the second erbium-doped optical fiber 8 is used to provide amplification asymmetry for the nonlinear amplifying loop mirror and provide gain for the optical fiber oscillator.

[0067] In some embodiments, the second erbium-doped optical fiber 8 is a polarization-maintaining erbium-doped optical fiber, which is intended to maintain the polarization state of the optical signal and ensure that the polarization state is not easily changed when the light is transmitted in the optical fiber.

[0068] In some embodiments, the dispersion value of the second erbium-doped optical fiber 8 at a wavelength of 1550 nm is -20.4 ps. 2 / km, and the absorption coefficient at a wavelength of 1530nm is 55dB / m.

[0069] The through end of the second wavelength division multiplexer 9 is connected to the input end of the optical phase shifter 11, the output end of the optical phase shifter 11 is connected to the dispersion compensating fiber 12, and the first fiber coupler 5 has a preset coupling ratio, such as 80:20 in the above example, and part of the output end is connected to the second erbium-doped fiber 8, and part of the output end is connected to the dispersion compensating fiber 12 to form a biased nonlinear amplifying ring mirror.

[0070] In some embodiments, the phase bias of the optical phase shifter 11 is set to 0-π to bias the transmission curve of the nonlinear amplifying loop mirror, thereby suppressing the supermode noise of the laser. Preferably, the phase bias of the optical phase shifter 11 is set to

[0071] In some embodiments, the dispersion value of the dispersion compensating optical fiber 12 at a wavelength of 1550 nm is 25.5 ps. 2 / km, and the nonlinear coefficient is 5.6 / W / km, which is used to control the net cavity dispersion and increase the asymmetric base of the nonlinear amplifying loop mirror.

[0072] In some embodiments, the net cavity dispersion of the laser is adjusted to -0.015±0.02ps by using the dispersion compensation fiber 12. 2 , to form a soliton-like pulse shaping mechanism. Preferably, the net cavity dispersion of the laser is adjusted to -0.014 ps 2 .

[0073] In the optoelectronic feedback loop:

[0074] Taking the coupling ratio of 80:20 as an example, part of the output end of the second fiber coupler 6, that is, 20% of the output end is connected to the third fiber coupler 13. The third fiber coupler 13 then uses part of the output end as laser output and part of the output end as connected to the photodetector 14 for photoelectric conversion according to its coupling ratio.

[0075] In some embodiments, the coupling ratio of the third optical fiber coupler 13 is 70:30, 70% of the output end of the third optical fiber coupler 13 is laser output, and 30% of the output end is connected to the photodetector 14 .

[0076] The photodetector 14 converts the optical signal output by the third optical fiber coupler 13 into an RF signal. The bandpass filter 15 filters out the harmonic signal in the RF signal output by the photodetector 14 that is an integer multiple of the cavity fundamental frequency and is within the passband of the bandpass filter 15. The RF amplifier 16 preliminarily amplifies the RF signal output by the bandpass filter 15. The RF phase shifter 17 adjusts the phase of the RF signal output by the RF amplifier 16 to match the optical pulse sequence in the optical fiber oscillator. The RF amplifier 18 power-amplifies the RF signal output by the RF phase shifter 17 and drives the phase modulator 7 to perform periodic modulation on the optical oscillator.

[0077] In some embodiments, the modulation bandwidth of the phase modulator 7 is set to 0.1-40 GHz. Preferably, the modulation bandwidth of the phase modulator 7 is set to 40 GHz, the insertion loss is 2.8 dB, and the radio frequency half-wave voltage is 4.1 V@1 GHz.

[0078] In some embodiments, the operating wavelength of the laser provided by the present invention is 1530-1580 nm.

[0079] The high repetition rate femtosecond fiber laser based on nonlinear amplifying ring mirror and reproducible mode locking provided by the present invention is further described below in conjunction with specific embodiments and experimental data.

[0080] In this laser, in addition to the polarization-maintaining dispersion compensation fiber, the entire fiber oscillator is connected by a polarization-maintaining single-mode fiber. The length of the polarization-maintaining dispersion compensation fiber 12 is set to 12m, the length of the first erbium-doped fiber is set to 0.6m, and the length of the second erbium-doped fiber 8 is set to 0.6m. The overall cavity length is 27.54m, the corresponding cavity fundamental frequency is 7.4MHz, and the corresponding net cavity dispersion is -0.014ps 2 The net cavity dispersion supports soliton-like pulse shaping, which in turn supports narrow pulse width and high pulse peak strategy.

[0081] In this laser, the total pump power is 920mW and the average output power is 13.98mW. Based on the above definition, the laser is tested and the experimental data are as follows: Figure 2 , Figure 3 , Figure 4 , Figure 5 shown.

[0082] like Figure 2 The figure shows the transmittance curve of the nonlinear amplifying loop mirror biased in the laser. The figure shows the transmittance of the nonlinear amplifying loop mirror as a function of the nonlinear phase difference, where the blue curve is the case without phase bias and the red curve is the case when the phase bias of the optical phase shifter is set to Since the relaxation time of erbium-doped fiber is in the millisecond range, in high-repetition-rate fiber lasers that use erbium-doped fiber as a gain medium, the pulse period is in the picosecond range and is much smaller than the relaxation time of erbium ions. Independent pulses cannot obtain the same gain saturation and have amplitude fluctuations, i.e., supermode noise. Therefore, Figure 2 In the embodiment shown, a biased nonlinear amplifying loop mirror is used to simultaneously compress the intracavity pulses and suppress the amplitude fluctuations of the laser pulses, i.e., supermode noise. When the phase is biased, the two pulses propagating in opposite directions in the nonlinear amplifying ring mirror biased by the laser can obtain The nonlinear phase difference of about 100 μm is used. At this time, the transmittance of the front and rear edges of the pulse will be compressed due to the increase of the nonlinear phase difference, while the transmittance of the pulse center will be limited due to the decrease of the nonlinear phase difference. Therefore, in the biased nonlinear amplifying loop mirror, the pulse in the laser cavity is not only compressed, but also the amplitude fluctuation is suppressed, so as to obtain a pulse sequence with narrow pulse width and high supermode noise suppression ratio.

[0083] It should be emphasized that in high repetition rate fiber lasers, due to the low energy of a single pulse, it is impossible to obtain sufficient nonlinear phase difference, so phase bias is necessary.

[0084] like Figure 3 As shown, this is the RF spectrum of the laser output pulse sequence in the experiment. Figure 3 (a) is the RF spectrum with a width of 100 MHz and centered at 10 GHz. The measurement resolution bandwidth is 2 kHz and the measurement mode is maximum hold. Figure 3 (a) shows that the repetition frequency of the laser output pulse train is 10.001991 GHz and the supermode noise suppression ratio is 84 dB. Figure 3 (b) is the RF spectrum with a frequency range from 0 to 25 GHz and a measurement resolution bandwidth of 3 MHz. Figure 3 (b) shows the laser output pulse repetition frequency of 10 GHz and the harmonic frequency of 20 GHz, with no spurious frequencies in the entire spectrum range.

[0085] like Figure 4 Shown are the autocorrelation trace and oscilloscope trace of the laser output pulse sequence in the experiment. Figure 4 (a) is the autocorrelation trace of the pulse. Assuming the pulse is Gaussian and the deconvolution factor of the autocorrelation function and the pulse is 1.414, it shows that the pulse width is 495fs. Figure 4 (b) is the oscilloscope trace of the pulse train, showing a uniform pulse train with a period of 100 ps, ​​corresponding to a pulse repetition frequency of 10 GHz.

[0086] like Figure 5As shown, this is the spectrum of the laser output pulse sequence in the experiment, with a measurement resolution of 0.02nm. Figure 5 The central wavelength of the spectrum is shown to be 1558nm and the 3dB spectrum width is 7.2nm. Figure 5 A comb spectrum with an interval of 0.08 nm is shown, which also corresponds to a pulse repetition frequency of 10 GHz.

[0087] In summary, the present invention provides a high repetition rate femtosecond fiber laser based on nonlinear amplifying ring mirror and regenerative mode locking, which can generate optical pulse sequences with high pulse repetition frequency and femtosecond pulse width by utilizing the intracavity pulse compression and soliton-like pulse shaping mechanism of the biased nonlinear amplifying ring mirror; the transmission characteristics of the biased nonlinear amplifying ring mirror are utilized to achieve high supermode noise suppression of active harmonic mode locking; the full polarization-maintaining structure is adopted to avoid polarization-related environmental instability problems caused by mechanical vibration, temperature fluctuation, etc.; the all-fiber format is adopted to free the laser from complex alignment and maintenance, and at the same time, the service life of the fiber laser is improved. The laser provided by the present invention has a compact structure, low cost and is easy to control, and has potential application value in the fields of frequency comb generation, ultra-low noise microwave generation, high-speed optical sampling, and optical arbitrary waveform generation.

[0088] It should be clear that the present invention is not limited to the specific configuration and processing described above and shown in the figures. For the sake of simplicity, a detailed description of the known method is omitted here. In the above embodiments, several specific steps are described and shown as examples. However, the method process of the present invention is not limited to the specific steps described and shown, and those skilled in the art can make various changes, modifications and additions, or change the order between the steps after understanding the spirit of the present invention.

[0089] In the present invention, features described and / or illustrated for one embodiment may be used in the same or similar manner in one or more other embodiments, and / or combined with features of other embodiments or replace features of other embodiments.

[0090] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the embodiments of the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A high repetition rate femtosecond fiber laser based on a nonlinear amplifying loop mirror and reproducible mode locking, characterized in that: The laser comprises: A fiber oscillator; the fiber oscillator comprises a main oscillation loop and a biased nonlinear amplifying loop mirror; The main oscillation loop includes a first pump source, a first wavelength division multiplexer, a first erbium-doped optical fiber, an isolator, a first optical fiber coupler, a second optical fiber coupler and a phase modulator; the first pump source is connected to the pump end of the first wavelength division multiplexer, the common end of the first wavelength division multiplexer is connected to the input end of the first erbium-doped optical fiber, the output end of the first erbium-doped optical fiber is connected to the input end of the isolator, the output end of the isolator is connected to the input end of the first optical fiber coupler, the output end of the first optical fiber coupler is connected to the input end of the second optical fiber coupler, the second optical fiber coupler has a part of its output end connected to the input end of the phase modulator according to a preset coupling ratio, and the output end of the phase modulator is connected to the through end of the first wavelength division multiplexer; The nonlinear amplifying ring mirror comprises the first fiber coupler, a second erbium-doped fiber, a second wavelength division multiplexer, a second pump source, an optical phase shifter and a dispersion compensating fiber; according to a preset coupling ratio, part of the output end of the first fiber coupler is connected to the second erbium-doped fiber, and part of the output end is connected to the dispersion compensating fiber; the other end of the second erbium-doped fiber is connected to the common end of the second wavelength division multiplexer, the pump end of the second wavelength division multiplexer is connected to the second pump source, the through end of the second wavelength division multiplexer is connected to the input end of the optical phase shifter, and the output end of the optical phase shifter is connected to the dispersion compensating fiber; A photoelectric feedback loop; the photoelectric feedback loop comprises a third fiber coupler, a photodetector, a bandpass filter, a radio frequency amplifier, a radio frequency phase shifter, a radio frequency amplifier and the phase modulator connected in sequence by the remaining output end of the second fiber coupler, wherein the photodetector converts the optical signal output by the third fiber coupler into a radio frequency signal, the bandpass filter filters out harmonic signals in the radio frequency signal that are integer multiples of the cavity fundamental frequency and are within the passband of the bandpass filter, the radio frequency amplifier amplifies the radio frequency signal output by the bandpass filter, the radio frequency phase shifter regulates the phase of the radio frequency signal output by the radio frequency amplifier to match the optical pulse sequence in the fiber oscillator, and the radio frequency amplifier amplifies the radio frequency signal output by the radio frequency phase shifter and drives the phase modulator to perform periodic modulation on the fiber oscillator.

2. The high repetition rate femtosecond fiber laser based on nonlinear amplifying loop mirror and reproducible mode locking according to claim 1, characterized in that: The coupling ratio of the first optical fiber coupler is 80:20, the coupling ratio of the second optical fiber coupler is 80:20, and the coupling ratio of the third optical fiber coupler is 70:

30.

3. The high repetition rate femtosecond fiber laser based on nonlinear amplifying loop mirror and reproducible mode locking according to claim 2, characterized in that: Also includes: An 80% output end of the first optical fiber coupler is connected to the second erbium-doped optical fiber, and a 20% output end is connected to the dispersion compensation optical fiber; The 80% output end of the second optical fiber coupler is connected to the input end of the phase modulator, and the 20% output end is connected to the input end of the third optical fiber coupler; The 70% output end of the third optical fiber coupler is laser output, and the 30% output end is connected to the photoelectric detector.

4. The high repetition rate femtosecond fiber laser based on nonlinear amplifying loop mirror and reproducible mode locking according to claim 1, characterized in that: The first pump source and the second pump source both adopt semiconductor laser diodes.

5. The high repetition rate femtosecond fiber laser based on nonlinear amplifying loop mirror and reproducible mode locking according to claim 1, characterized in that: The modulation bandwidth of the phase modulator is set to 0.1~40GHz.

6. The high repetition rate femtosecond fiber laser based on nonlinear amplifying loop mirror and reproducible mode locking according to claim 1, characterized in that: The phase bias of the optical phase shifter is set to 0~ , to bias the transmission curve of the nonlinear amplifying loop mirror and suppress supermode noise.

7. The high repetition rate femtosecond fiber laser based on nonlinear amplifying loop mirror and reproducible mode locking according to claim 1, characterized in that: The net cavity dispersion of the laser is regulated to be -0.015±0.02ps by the dispersion compensation fiber. 2 , to form a soliton-like pulse shaping mechanism.

8. The high repetition rate femtosecond fiber laser based on nonlinear amplifying loop mirror and reproducible mode locking according to claim 1, characterized in that: The operating wavelength of the laser is in the 1530-1580 nm band.

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