A nonlinear loop mirror passively mode-locked laser based on pulse carving technique

Abstract

The application discloses a nonlinear loop mirror passive mode-locked laser based on pulse chopping technology, which comprises a feedback loop module, a fiber-coupled beam splitter 5 and a Sagnac loop module, wherein the Sagnac loop module comprises an electro-optic modulator 6, a passive optical fiber 7 and a phase biasing device 13; the electro-optic modulator 7 is asymmetrically arranged in the loop and is used for chopping continuous light input from the feedback loop into pulse light and providing a linear phase shift of π; and the phase biasing device generates a linear phase difference to compensate for the deficiency of the accumulated nonlinear phase difference in the fiber loop. The application uses the pulse chopping technology of the electro-optic phase modulator based on the Sagnac loop to chop the continuous light or long pulse light input into the Sagnac loop into picosecond-level pulses, completes the mode-locking process, solves the self-starting problem of the NOLM passive mode-locked laser, can realize the output of ultrashort pulse width laser, reduces the complexity of the system and prolongs the service life of the device.

Description

Technical Field

[0001] This invention belongs to the field of lasers, specifically a passive mode-locked laser based on a nonlinear loop mirror using pulse chopping technology. Background Technology

[0002] High-power, high-energy ultrafast fiber lasers possess enormous market application prospects due to their advantages such as high stability, high beam quality, and compact structure. They are crucial tools for cutting-edge research in fields like precision machining, advanced science, and astronomical exploration. Ultrafast fiber laser oscillators based on passive mode-locking technology are fundamental to achieving high-power, high-energy fiber laser output. They require both long-term operational stability and high environmental tolerance to meet the needs of applications ranging from laboratory research to industrial use. Fiber lasers based on nonlinear loop mirror (NLM) mode-locking can achieve a fully polarization-maintaining structure, providing a foundation for long-term operational stability. Furthermore, they are insensitive to external environmental factors (temperature, humidity, and fiber stress variations), making them ideal light sources for subsequent amplification to achieve high-power, high-energy pulsed lasers. Currently, fiber lasers based on nonlinear loop mirror mode-locking face the major challenge of self-starting. After the pump power reaches the mode-locking threshold, it is often necessary to apply artificial interference to achieve mode-locking again. Based on current optimizations of the pulse evolution dynamics within the fiber seed source, mode-locking can be repeatedly initiated under artificial interference. However, for system-integrated fiber lasers, current excitation or mechanical stress is required to achieve mode-locking, increasing the complexity of the entire system and potentially affecting the surface quality of fiber devices, thus reducing their lifespan. Therefore, achieving mode-locked self-starting without external interference is of great significance and value. Summary of the Invention

[0003] This invention provides a passive mode-locked laser with a nonlinear loop mirror based on pulse chopping technology to overcome the shortcomings of the prior art.

[0004] This invention is achieved through the following technical solution:

[0005] A passive mode-locked laser with a nonlinear loop mirror based on pulse chopping technology includes a feedback loop module, an optical fiber coupler beam splitter, and a Sagnac loop module.

[0006] As described above, a passive mode-locked laser based on pulse chopping technology using a nonlinear loop mirror includes a feed loop module comprising a first pump source, a first wavelength division multiplexer, a first gain fiber, a fiber isolator, an output coupler, and a fiber filter. The first pump source, wavelength division multiplexer, gain fiber, fiber isolator, output coupler, and fiber filter are connected in sequence to generate continuous light.

[0007] As described above, in a passive mode-locked laser with a nonlinear loop mirror based on pulse chopping technology, the fiber optic coupler is connected between the fiber optic isolator and the output coupler in the feedback loop, and is used to split a beam of light input from the feedback loop into two beams of different intensities for opposite transmission within the Sagnac loop.

[0008] As described above, a passive mode-locked laser based on pulse chopping technology using a nonlinear loop mirror includes a Sagnac loop module comprising an electro-optic modulator, a passive optical fiber, and a phase biaser. The electro-optic modulator is positioned asymmetrically within the loop to chop the continuous light input from the feedback loop into pulsed light and provide a linear phase shift of π. Two pulsed beams of different intensities propagate in opposite directions within the Sagnac loop, experiencing different nonlinear phase shifts, and ultimately interfering in the fiber-coupled beam splitter. The interfering pulses, under the influence of the Sagnac loop transmission curve, exhibit high transmittance in the central portion and low transmittance on the flanks, achieving pulse filtering. The filtered pulses continue to oscillate within the cavity, undergoing the above process multiple times until stable operation is achieved, completing the mode-locking process. The phase biaser generates a linear phase difference to compensate for the insufficient nonlinear phase difference accumulated in the fiber loop.

[0009] As described above, a passive mode-locked laser based on pulse chopping technology using a nonlinear loop mirror includes a Sagnac loop module comprising an electro-optic modulator, a passive optical fiber, a phase biaser, a second pump source, a second wavelength division multiplexer, and a second gain fiber. The second gain fiber is pumped by the second pump source and connected to the loop along with the second wavelength division multiplexer. Changing the pump intensity of the second pump source amplifies the two opposing light beams, increasing the light intensity within the nonlinear loop. This, combined with the electro-optic phase modulator, completes pulse chopping and nonlinear phase shift modulation, achieving stable mode-locked operation.

[0010] The advantages of this invention are: This invention utilizes the pulse chopping technology of an electro-optic phase modulator based on the Sagnac loop to chop the continuous light or long pulse light input to the Sagnac loop into picosecond-level pulses, thereby completing the mode-locking process. This solves the self-starting problem of NOLM passive mode-locked lasers, enabling ultra-short pulse width laser output, while reducing system complexity and improving device lifespan. Attached Figure Description

[0011] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0012] Figure 1This is one of the structural schematic diagrams of the passive mode-locked laser of the present invention;

[0013] Figure 2 This is the second schematic diagram of the passive mode-locked laser of the present invention;

[0014] Figure 3 The transmittance and reflectance curves in the fiber nonlinear loop are given when the splitting ratio is 45:55.

[0015] Figure 4 A simplified conceptual diagram of the optical chopper provided by this invention.

[0016] Reference numerals in the figures: 1. First pump source; 2. First wavelength division multiplexer; 3. First gain fiber; 4. Fiber isolator; 5. Fiber coupler splitter; 6. Electro-optic phase modulator; 7. Passive fiber; 8. Fiber coupler output; 9. Fiber filter; 10. Second pump source; 11. Second wavelength division multiplexer; 12. Second gain fiber; 13. Phase offset device. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] A passive mode-locked laser with a nonlinear loop mirror based on pulse chopping technology includes a feedback loop module, an optical fiber coupler beam splitter 5, and a Sagnac loop module.

[0019] Preferably, the feed loop module includes a first pump source 1, a first wavelength division multiplexer 2, a first gain fiber 3, a fiber isolator 4, an output coupler 8, and a fiber filter 9. The first pump source 1, wavelength division multiplexer 2, gain fiber 3, fiber isolator 4, output coupler 8, and fiber filter 9 are connected in sequence to generate continuous light.

[0020] Preferably, the fiber optic coupler splitter 5 is connected between the fiber optic isolator 4 and the output coupler 8 in the feedback loop, and is used to split a beam of light input from the feedback loop into two beams of light of different intensities for transmission in opposite directions within the Sagnac loop.

[0021] Preferably, the Sagnac loop module includes an electro-optic modulator 6, a passive optical fiber 7, and a phase biaser 13. The electro-optic modulator 7 is located asymmetrically in the loop and is used to cut the continuous light input from the feedback loop into pulsed light and provide a linear phase shift of π. Two pulsed light beams with different intensities propagate in opposite directions in the Sagnac loop and experience different nonlinear phase shifts. Finally, they interfere in the fiber optic coupler. Under the action of the transmission curve of the Sagnac loop, the central part of the pulse has high transmittance and the two wing parts have low transmittance, which realizes pulse screening. The pulse after screening continues to oscillate in the cavity. After going through the above process multiple times, stable operation is finally achieved, and the mode-locking process is completed. The phase biaser generates a linear phase difference to compensate for the lack of nonlinear phase difference accumulated in the fiber optic loop.

[0022] Preferably, the Sagnac loop module includes an electro-optic modulator 6, a passive optical fiber 7, a phase biaser 13, a second pump source 10, a second wavelength division multiplexer 11, and a second gain optical fiber 12. The second gain optical fiber 12 is pumped by the second pump source 10 and connected to the loop along with the second wavelength division multiplexer 11. Changing the pump intensity of the second pump source 10 amplifies the two opposing light beams, increases the light intensity in the nonlinear loop, and works with the electro-optic phase modulator 6 to complete pulse chopping and nonlinear phase shift modulation, thereby achieving stable mode-locked operation.

[0023] Preferably, the emission wavelength of the first pump source 1 corresponds to the absorption spectrum of the first gain fiber 3. The rare earth ions doped in the first gain fiber 3 can be ytterbium ions (Yb3+), erbium ions (Er3+), thulium ions (Tm3+), etc., but are not limited to the listed rare earth doped particles; the center wavelength of the fiber isolator 4 corresponds to the emission spectrum of the gain fiber; the fiber coupler 5 in the device has a 2×2 port type; the spectral range of the passive fiber 7 in the device is adapted to the wavelength of the established laser; the rise time τ of the electro-optic phase modulator 6 in the device affects the duration of the chopping pulse. Therefore, using an electro-optic phase modulator with a wider response bandwidth and a shorter rise time is more conducive to obtaining a short-duration chopping pulse. Further, taking an electro-optic phase modulator with a response bandwidth of 10 GHz as an example, if the response time is 100 ps and its rise time is 10% of the response time, then the rise time is 10 ps. The rise time limitation of the generation of chopper pulses is limited to only 10ps level chopper pulses; the center wavelength of the fiber filter in the device must be adapted to the center wavelength of the established laser.

[0024] Preferably, the dimensions of all device pigtails must be compatible, and all device connections must be fiber optic coupling connections.

[0025] Preferably, the emission wavelength of the first and second pump sources 10 corresponds to the absorption spectrum of the second gain fiber 12, and the rare earth ions doped in the second gain fiber 12 correspond to the first gain fiber 3.

[0026] Example 1

[0027] A passively mode-locked laser based on a nonlinear optical loop mirror (NOLM) using pulse chopping technology, such as Figure 1 As shown, it includes a first pump source 1, a first wavelength division multiplexer 2, a first gain fiber 3, an optical fiber isolator 4, an optical fiber coupler beam splitter 5, an electro-optic phase modulator 6, a passive optical fiber 7, an optical fiber coupler output 8, and an optical fiber filter 9 arranged sequentially along the optical path.

[0028] The first pump source 1 enters the resonant cavity from port a of the first wavelength division multiplexer 2, and pumps the first gain fiber 3 through port b to achieve population inversion. Combined with the resonant cavity, continuous light output is achieved. The continuous light is split into two beams by the fiber coupler beam splitter 5, and propagates along the Sagnac loop in clockwise and counterclockwise directions, respectively. When the electro-optic phase modulator 6 is turned on, the two beams are chopped and a linear phase shift of π is applied. At the same time, the electro-optic phase modulator 6 is asymmetrically positioned in the fiber loop, and the passive fibers at its left and right ends have a length difference ΔL = |L left -L right This length difference causes the spatial positions where the two opposing light beams begin to produce a phase change of π to be unequal, thus forming a time window with a relative phase difference of π. The size of this time window is determined by ΔL and the response time τ of the electro-optic phase modulator 6, where n is the refractive index of the passive fiber and c is the speed of light in vacuum. The pulse width of the initial chopping pulse can be adjusted by the length difference between the two ends of the fiber in the electro-optic phase modulator 6. The pulsed light propagating in opposite directions achieves interference mode-locking in the fiber coupler beam splitter 5 and is transmitted from port d. The light output from port c is lost by the fiber isolator 4. The transmitted pulsed light is output through port e of the fiber coupler outputter 8, and the remaining part continues to propagate in the cavity. It passes through the fiber filter 9 to filter out the two ends of the spectrum, retaining the light near the center wavelength. It is then amplified by the gain fiber and maintains stable oscillation in the two loops.

[0029] The basic components for generating the chopped pulses are the fiber-coupled beam splitter 5, the electro-optic phase modulator 6, and the passive fiber 7. The fiber-coupled beam splitter 5 employs a small splitting ratio to ensure that the relative phase difference between the two opposing beams originates primarily from the electro-optic phase modulator 6, thereby enabling effective control of the nonlinear phase in the loop by the electro-optic phase modulator. The electro-optic phase modulator 6 continuously chops the two beams input to the nonlinear loop, achieving pulsed light modulation output. The length of the passive fiber 7 in the Sagnac loop is designed so that the accumulated nonlinear phase shift difference between the two light pulse peaks meets the transmission trend requirements and interferes within the fiber-coupled beam splitter 5, achieving stable NOLM mode-locked operation.

[0030] Simultaneously, a phase biaser 13 is provided. In some applications, high repetition rate ultrashort laser pulses are required. However, simply increasing the loop fiber length to obtain a nonlinear phase difference is not conducive to obtaining high repetition rate lasers. Therefore, to meet special application requirements, if the accumulation of the nonlinear phase difference required to reach the mode-locking point is insufficient, a phase biaser 10 can be added to compensate using the linear phase difference generated by the phase biaser. The phase bias generated by the phase biaser can be... Any of the above, but not limited to these.

[0031] Example 2

[0032] A passively mode-locked laser based on a nonlinear amplifying loop mirror (NALM) using pulse chopping technology, such as... Figure 2 As shown:

[0033] In this example, NALM passive mode-locking technology is used to establish mode-locked laser. The splitting ratio of the fiber-coupled beam splitter 5 is not required. In the Sagnac loop described in Example 1, a second gain fiber 12 with asymmetric placement is added. The second gain fiber 12 is pumped by the first and second pump sources 10 and connected to the nonlinear loop along with the second wavelength division multiplexer 11. The pump intensity of the first and second pump sources 10 is changed to amplify the two beams propagating in opposite directions, increasing the light intensity in the nonlinear loop. In conjunction with the electro-optic phase modulator 6, pulse chopping and nonlinear phase shift modulation are completed to achieve stable mode-locked operation.

[0034] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A passively mode-locked laser with a nonlinear loop mirror based on pulse chopping technology, characterized in that: This includes a feedback loop module, an optical fiber coupler beam splitter, and a Sagnac loop module; The feed loop module includes a first pump source, a first wavelength division multiplexer, a first gain fiber, a fiber isolator, an output coupler, and a fiber filter. The first pump source, wavelength division multiplexer, gain fiber, fiber isolator, output coupler, and fiber filter are connected in sequence to generate continuous light. The fiber optic coupler splitter is connected between the fiber optic isolator and the output coupler in the feedback loop, and is used to split a beam of light input to the feedback loop into two beams of light of different intensities, which are then transmitted in opposite directions within the Sagnac loop. The Sagnac loop module includes an electro-optic modulator, a passive optical fiber, and a phase biaser. The electro-optic modulator is located in an asymmetrical position in the loop and is used to cut the continuous light input from the feedback loop into pulsed light and provide a linear phase shift of π. Two pulses of light with different intensities propagate toward each other in the Sagnac loop and undergo different nonlinear phase shifts. They eventually interfere in the fiber-coupled beam splitter. The pulses after interference have high transmittance in the center and low transmittance on the wings under the action of the transmission curve of the Sagnac loop, thus achieving pulse screening. The filtered pulses continue to oscillate in the cavity. After going through the above process multiple times, stable operation is finally achieved, and the mode-locking process is completed. The phase offset device generates a linear phase difference to compensate for the insufficient nonlinear phase difference accumulated in the fiber optic loop.

2. The passive mode-locked laser based on pulse chopping technology using a nonlinear loop mirror according to claim 1, characterized in that: The Sagnac loop module includes an electro-optic modulator, a passive optical fiber, a phase biaser, a second pump source, a second wavelength division multiplexer, and a second gain fiber. The second gain fiber is pumped by the second pump source and connected to the loop along with the second wavelength division multiplexer. Changing the pump intensity of the second pump source amplifies the two opposing light beams, increases the light intensity in the nonlinear loop, and works with the electro-optic phase modulator to complete pulse chopping and nonlinear phase shift modulation, achieving stable mode-locked operation.

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