A fiber laser outputting dual repetition rate femtosecond laser
By utilizing the refractive index difference between the fast and slow axes of the polarization-maintaining fiber within the resonant cavity of a fiber laser, a dual-repetition-frequency femtosecond laser with orthogonal polarization is generated, solving the problem of single-frequency output in traditional fiber lasers and achieving high coherence and low-cost dual-repetition-frequency femtosecond laser output.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- BEIJING CHANGCHENG INST OF METROLOGY & MEASUREMENT AVIATION IND CORP OF CHINA
- Filing Date
- 2026-01-21
- Publication Date
- 2026-06-02
AI Technical Summary
Traditional femtosecond fiber lasers can only output lasers with a single repetition frequency, making it difficult to realize dual femtosecond laser applications. This results in large system size, high cost, poor coherence, and low signal-to-noise ratio. Furthermore, existing single-cavity dual-comb solutions are difficult to precisely control optical path difference and are prone to interference.
By utilizing the refractive index difference between the fast and slow axes of the polarization-maintaining fiber within the resonant cavity of the fiber laser, two laser circulation paths with different optical paths are formed, generating a polarized orthogonal femtosecond laser with dual repetition frequencies. The laser is then separated and output through a polarization beam splitter, and polarization rotation mode-locking is achieved using a polarization control module and an optical isolation module.
It achieves dual-repetition-rate femtosecond laser output with simple device, compact structure, and low cost, improves coherence and suppresses common-mode noise, and can flexibly adjust the laser intensity ratio.
Smart Images
Figure CN121566258B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of ultrafast laser technology, and particularly relates to a fiber laser that outputs a femtosecond laser with dual repetition rates. Background Technology
[0002] Femtosecond lasers are laser systems characterized by periodic, ultrashort pulse sequences. Their time-domain pulse widths are extremely narrow (typically on the order of a few to hundreds of femtoseconds), while their frequency domain exhibits a highly uniform, equally spaced spectral line distribution. This unique time-frequency characteristic makes them advanced light sources with superior performance. Femtosecond lasers, and femtosecond optical frequency combs based on them, have brought revolutionary breakthroughs to modern precision measurement technology, achieving significant progress in many fields such as precision spectral analysis, absolute distance measurement, remote clock synchronization, and trace gas sensing.
[0003] In femtosecond laser measurement technology, two femtosecond lasers with slightly different repetition frequencies can form a dual-femtosecond laser / dual-comb system. Due to the minute difference in repetition frequencies between the two femtosecond lasers, during measurement, one femtosecond laser can scan and sample the other, acting like a "vernier," thereby improving measurement accuracy and reducing the sampling rate requirements of the detection device. Therefore, dual-femtosecond laser ranging and dual-femtosecond optical comb spectral analysis have become the main methods of femtosecond laser measurement.
[0004] However, traditional femtosecond fiber lasers can only output laser light with a single repetition frequency. To achieve dual femtosecond laser applications, two independent femtosecond lasers need to be built, which leads to a series of problems such as large size, high cost, poor coherence, and low signal-to-noise ratio. Simultaneously outputting dual-repetition-frequency femtosecond lasers through a single device, commonly known as single-cavity dual-comb, can reduce system size and cost, and help improve the coherence between the two femtosecond lasers. However, typical single-cavity dual-comb schemes require physically constructing two different laser circulation paths within the laser resonant cavity, such as forming two laser circulation paths through different diffraction orders of gratings within the laser cavity, or constructing two laser circulation paths through a polarization splitter and combiner module. For this scheme of outputting dual repetition frequencies through two separate laser circulation paths, accurately controlling the optical path difference between the two circulation paths is a challenge, and interference can easily occur during the splitting and combining of the two paths, leading to mode-locking failure of the laser, resulting in poor system feasibility and considerable complexity. Summary of the Invention
[0005] In view of the technical problems of existing dual-repetition-frequency femtosecond laser devices, such as system complexity, poor coherence, difficulty in implementation, and high cost, this invention proposes a fiber laser that outputs dual-repetition-frequency femtosecond lasers.
[0006] This invention discloses a fiber laser that outputs dual-repetition-rate femtosecond laser light. The fiber laser comprises: a pump source, a wavelength coupler, a gain medium, a first collimator, a second collimator, a first polarization control module, a second polarization control module, a first polarization beam splitter, a first polarization-maintaining collimator, a polarization-maintaining fiber, a second polarization-maintaining collimator, a second polarization beam splitter, and an optical isolation module; wherein,
[0007] The light emitted from the pump source is coupled into the gain medium through a wavelength coupler, amplifying the randomly generated spontaneous emission and thus generating amplified stimulated emission light; the amplified stimulated emission light passes through the wavelength coupler and the first collimator in sequence and is collimated into spatial light.
[0008] Spatial light is converted into periodically varying elliptically polarized light by the first polarization modulation module;
[0009] The slow axis of the first polarization-maintaining collimator forms an angle with the transmission axis of the first polarization beamsplitter. After the elliptically polarized light is transmitted through the first polarization beamsplitter, it is coupled into the polarization-maintaining fiber through the first polarization-maintaining collimator and decomposed into two beams with polarization directions along the fast axis and slow axis of the polarization-maintaining fiber. These beams form two cyclic paths with different optical paths in the resonant cavity of the fiber laser, thereby generating a dual-repetition-frequency femtosecond laser with orthogonal polarization. The dual-repetition-frequency femtosecond laser is then separated and output by the second polarization beamsplitter. The polarization directions of the dual-repetition-frequency femtosecond laser are respectively along the fast axis and slow axis of the polarization-maintaining fiber.
[0010] The second polarization-maintaining collimator includes a partial reflective surface. The two beams of light, after being split and transmitted by the polarization-maintaining fiber, arrive at the second polarization-maintaining collimator. After being reflected by the second polarization-maintaining collimator, they pass through the polarization-maintaining fiber and the first polarization-maintaining collimator, and after being reflected by the first polarization beam splitter, they pass through the optical isolation module and the second polarization control module in sequence to adjust the polarization state of the returning light before entering the second collimator. Then, they pass through the gain fiber and the wavelength coupler and arrive at the first collimator again, completing one cycle of two laser paths with different optical paths.
[0011] Optionally, the first polarization control module includes a first λ / 4 phase delay unit and a λ / 2 phase delay unit arranged sequentially; the second polarization control module is a second λ / 4 phase delay unit.
[0012] Optionally, each phase delay unit is a liquid crystal phase delay unit or an electronically controlled waveplate.
[0013] Alternatively, the wavelength coupler can be a wavelength division multiplexer or an optical fiber coupler.
[0014] Optionally, the gain medium is a rare-earth ion-doped optical fiber.
[0015] Alternatively, the rare earth ion-doped optical fiber may be erbium-doped, ytterbium-doped, thulium-doped, or holmium-doped optical fiber.
[0016] Optionally, the reflectivity of a portion of the reflective surface of the second polarization-maintaining coupler is 80% to 95%.
[0017] Optionally, the slow axis direction of the first polarization-maintaining coupler forms an angle of 10° to 80° with the transmission axis direction of the first polarization beam splitter.
[0018] Optionally, the first polarization control module, the second polarization control module, and the optical isolation module work together to achieve the polarization rotation mode-locking function.
[0019] In summary, the solution proposed in this invention has the following technical effects:
[0020] This invention utilizes the refractive index difference between the fast and slow axes of a polarization-maintaining fiber within the resonant cavity of a fiber laser to form two laser circulation paths with different optical paths, generating orthogonally polarized femtosecond laser outputs with dual repetition frequencies. It has the advantages of simple device, compact structure, and low cost.
[0021] The present invention generates dual-repetition-rate femtosecond lasers through the same resonant cavity, which can effectively suppress common-mode noise and has excellent coherence.
[0022] The intensity ratio between the two repetition frequency femtosecond lasers of the present invention can be adjusted by controlling the angle between the slow axis direction of the first polarization-maintaining collimator and the horizontal polarization direction, which provides high flexibility. Attached Figure Description
[0023] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific 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 from these drawings without creative effort.
[0024] Figure 1 This is a schematic diagram of the composition and structure of a fiber laser that outputs dual repetition frequency femtosecond lasers according to an embodiment of the present invention;
[0025] Figure 2 This is a schematic diagram showing the orientation relationship between the polarization axis of the polarization-maintaining fiber and the horizontal and vertical polarization directions in an embodiment of the present invention.
[0026] Among them, 1-pump source, 2-wavelength division multiplexer, 3-gain fiber, 4-first collimator, 5-second collimator, 6-first quarter wave plate, 7-half wave plate, 8-second quarter wave plate, 9-isolator, 10-first polarization beam splitter, 11-first polarization-maintaining collimator, 12-polarization-maintaining fiber, 13-second polarization-maintaining collimator, 14-second polarization beam splitter. Detailed Implementation
[0027] 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, and 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.
[0028] This invention discloses a fiber laser that outputs dual-repetition-rate femtosecond laser light. The fiber laser comprises: a pump source, a wavelength coupler, a gain medium, a first collimator, a second collimator, a first polarization control module, a second polarization control module, a first polarization beam splitter, a first polarization-maintaining collimator, a polarization-maintaining fiber, a second polarization-maintaining collimator, a second polarization beam splitter, and an optical isolation module; wherein,
[0029] The light emitted from the pump source is coupled into the gain medium through a wavelength coupler, amplifying the randomly generated spontaneous emission and thus generating amplified stimulated emission light; the amplified stimulated emission light passes through the wavelength coupler and the first collimator in sequence and is collimated into spatial light.
[0030] Spatial light is converted into periodically varying elliptically polarized light by the first polarization modulation module;
[0031] The slow axis of the first polarization-maintaining collimator forms an angle with the transmission axis of the first polarization beamsplitter. After the elliptically polarized light is transmitted through the first polarization beamsplitter, it is coupled into the polarization-maintaining fiber through the first polarization-maintaining collimator and decomposed into two beams with polarization directions along the fast axis and slow axis of the polarization-maintaining fiber. These beams form two cyclic paths with different optical paths in the resonant cavity of the fiber laser, thereby generating a dual-repetition-frequency femtosecond laser with orthogonal polarization. The dual-repetition-frequency femtosecond laser is then separated and output by the second polarization beamsplitter. The polarization directions of the dual-repetition-frequency femtosecond laser are respectively along the fast axis and slow axis of the polarization-maintaining fiber.
[0032] The second polarization-maintaining collimator includes a partial reflective surface. The two beams of light, after being split and transmitted by the polarization-maintaining fiber, arrive at the second polarization-maintaining collimator. After being reflected by the second polarization-maintaining collimator, they pass through the polarization-maintaining fiber and the first polarization-maintaining collimator, and after being reflected by the first polarization beam splitter, they pass through the optical isolation module and the second polarization control module in sequence to adjust the polarization state of the returning light before entering the second collimator. Then, they pass through the gain fiber and the wavelength coupler and arrive at the first collimator again, completing one cycle of two laser paths with different optical paths.
[0033] That is, forming two optically different loop paths in the resonant cavity of a fiber laser includes:
[0034] The two beams of light, after being split and transmitted by the polarization-maintaining fiber, reach the second polarization-maintaining collimator. After being reflected by the second polarization-maintaining collimator, they are transmitted again along the fast axis and the slow axis of the polarization-maintaining fiber, respectively, to the first polarization-maintaining collimator (i.e., the split and reflected light along the fast axis of the polarization-maintaining fiber is transmitted again along the fast axis of the polarization-maintaining fiber to the first polarization-maintaining collimator, and the split and reflected light along the slow axis of the polarization-maintaining fiber is transmitted again along the slow axis of the polarization-maintaining fiber to the first polarization-maintaining collimator). After being reflected by the first polarization beam splitter, the return light passes through the optical isolation module and the second polarization control module in sequence to adjust the polarization state before entering the second collimator. Then, it passes through the gain fiber and the wavelength coupler, and finally reaches the first collimator again, completing one cycle of two laser paths with different optical paths.
[0035] Optionally, the first polarization control module includes a first λ / 4 phase delay unit and a λ / 2 phase delay unit arranged sequentially; the second polarization control module is a second λ / 4 phase delay unit. Optionally, each phase delay unit is a liquid crystal phase delay unit or an electrically controlled waveplate.
[0036] Optionally, the wavelength coupler can be a wavelength division multiplexer or a fiber coupler. Alternatively, the wavelength coupler can also be a multiplexer based on a thin-film filter (TFF) or fiber grating (FBG).
[0037] Optionally, the gain medium is rare-earth ion-doped fiber. Optionally, the rare-earth ion-doped fiber is erbium-doped, ytterbium-doped, thulium-doped, or holmium-doped fiber.
[0038] Alternatively, the polarization beam splitter may be a prism-based (such as Glan-Taylor) or fiber-optic polarization beam splitter (F-PBS).
[0039] Optionally, the reflectivity of a portion of the reflective surface of the second polarization-maintaining coupler is 80% to 95%.
[0040] Optionally, the slow axis direction of the first polarization-maintaining coupler forms an angle of 10° to 80° with the transmission axis direction of the first polarization beam splitter.
[0041] Optionally, the first polarization control module, the second polarization control module, and the optical isolation module work together to achieve the polarization rotation mode-locking function.
[0042] Please see Figure 1This invention discloses a fiber laser that outputs dual-repetition-rate femtosecond laser light, comprising a pump source 1, a wavelength division multiplexer (WDM) 2, a gain fiber (gain medium) 3, a first collimator 4, a second collimator 5, a first quarter-wave plate (first λ / 4 phase delay unit) 6, a half-wave plate (λ / 2 phase delay unit) 7, a second quarter-wave plate (second λ / 4 phase delay unit) 8, an isolator (optical isolation module) 9, a first polarization beam splitter 10, a first polarization-maintaining collimator 11, a polarization-maintaining fiber 12, a second polarization-maintaining collimator 13, and a second polarization beam splitter 14. The first collimator 4, WDM 2, gain fiber 3, and second collimator 5 are sequentially connected via non-polarization-maintaining single-mode fibers. The first polarization-maintaining collimator 11 and the second polarization-maintaining collimator 13 are directly connected via the polarization-maintaining fiber 12. The first quarter-wave plate 6, the half-wave plate 7, the second quarter-wave plate 8, the isolator 9, the first polarization beam splitter 10, and the second polarization beam splitter 14 are located in the spatial optical path.
[0043] The present invention discloses a fiber laser that outputs a femtosecond laser with dual repetition frequency. The surface of the second polarization maintaining collimator 13 is coated with a partial reflective film, which is used as the output coupling mirror of the laser to output a certain proportion of light to the outside of the laser resonant cavity, while reflecting the remaining part of the light back to the resonant cavity.
[0044] This invention discloses a fiber laser that outputs a dual-repetition-frequency femtosecond laser. The laser cycle sequence within the laser resonant cavity is as follows: the collimated light from the first collimator 4 passes sequentially through the first quarter-wave plate 6 and the half-wave plate 7, is transmitted through the first polarization beam splitter 10, enters the first polarization-maintaining collimator 11, is transmitted through the polarization-maintaining fiber 12, and reaches the second polarization-maintaining collimator 13. The reflected light from the second polarization-maintaining collimator 13 passes again through the polarization-maintaining fiber 12 and the first polarization-maintaining collimator 11, is reflected by the first polarization beam splitter 10, passes sequentially through the isolator 9 and the second quarter-wave plate 8, enters the second collimator 5, and then is transmitted within the fiber through the gain fiber 3 and the wavelength division multiplexer 2, before reaching the first collimator 4 again, completing one laser cycle.
[0045] This invention discloses a fiber laser that outputs a dual-repetition-frequency femtosecond laser. The transmitted light from the first polarization beam splitter 10 can be decomposed into two beams with polarization directions along the fast axis and the slow axis of the polarization-maintaining fiber 12. Since the refractive indices of the fast and slow axes of the polarization-maintaining fiber are different, two cyclic paths with different optical paths are formed, thereby generating a dual-repetition-frequency femtosecond laser with orthogonal polarization. The polarization directions of the dual-repetition-frequency femtosecond laser are along the fast axis and the slow axis of the polarization-maintaining fiber 12, respectively.
[0046] In this invention, the term 'slow axis' refers to the polarization principal axis with a higher effective refractive index in the polarization-maintaining fiber; the term 'fast axis' refers to another orthogonal polarization principal axis with a lower effective refractive index in the polarization-maintaining fiber.
[0047] The present invention discloses a fiber laser that outputs dual repetition frequency femtosecond lasers, wherein the slow axis direction of the second polarization maintaining collimator 13 is aligned with the horizontal polarization direction or the vertical polarization direction of the second polarization beam splitter 14 to achieve the separate output of dual repetition frequency femtosecond lasers.
[0048] The present invention discloses a fiber laser that outputs a dual-repetition-frequency femtosecond laser. The slow axis direction of the first polarization-maintaining collimator 11 forms an angle with the horizontal polarization direction of the first polarization beam splitter 10. Adjusting the angle can control the intensity ratio of the two polarizations of the dual-repetition-frequency femtosecond laser.
[0049] The present invention discloses a fiber laser that outputs dual repetition frequency femtosecond lasers. The first quarter wave plate 6, the half wave plate 7, the second quarter wave plate 8, and the isolator 9 together realize polarization rotation mode-locking to ensure stable output of femtosecond pulses.
[0050] The present invention discloses a fiber laser that outputs a dual repetition frequency femtosecond laser, wherein the gain fiber 3 includes rare earth ion doped fibers such as erbium-doped, ytterbium-doped, thulium-doped, and holmium-doped fibers that have optical gain effects.
[0051] In this embodiment, pump source 1 is a 980nm single-mode pump source, gain fiber 3 is erbium-doped single-mode fiber (Er110-4 / 125, Liekki), and polarization-maintaining fiber 12 is a PM1550-XP with a length of 20cm. The operating wavelength of all other devices is 1550nm. A schematic diagram showing the orientation of the polarization axis of the polarization-maintaining fiber relative to the horizontal and vertical polarization directions is shown below. Figure 2 As shown. The second polarization-maintaining collimator 13 has a partially reflective coating on its surface, with a reflectivity of 88% and a transmittance of 12%, serving as the output coupling mirror of the laser. By adjusting the angles of the first quarter-wave plate 6, the half-wave plate 7, and the second quarter-wave plate 8, polarization rotation mode-locking of the laser is achieved, resulting in a stable femtosecond pulse output with a repetition frequency of 100MHz. The refractive index difference between the slow and fast axes of the polarization-maintaining fiber PM1550-XP is approximately 0.0003, and the optical path difference introduced by the 20cm polarization-maintaining fiber (passing twice) is 120μm. The repetition frequencies of the polarized orthogonal femtosecond lasers output by the second polarization beam splitter 14 are 100.428MHz and 100.424MHz, respectively.
[0052] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to 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 or all of the technical features therein, and such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.
Claims
1. A fiber laser outputting a dual repetition rate femtosecond laser, characterized by, The fiber laser includes: a pump source, a wavelength coupler, a gain medium, a first collimator, a second collimator, a first polarization control module, a second polarization control module, a first polarization beam splitter, a first polarization-maintaining collimator, a polarization-maintaining fiber, a second polarization-maintaining collimator, a second polarization beam splitter, and an optical isolation module; wherein, The light emitted from the pump source is coupled into the gain medium through a wavelength coupler, amplifying the randomly generated spontaneous emission and thus generating amplified stimulated emission light; the amplified stimulated emission light passes through the wavelength coupler and the first collimator in sequence and is collimated into spatial light. Spatial light is converted into periodically varying elliptically polarized light by the first polarization modulation module; The slow axis of the first polarization collimator forms an angle of 10° to 80° with the transmission axis of the first polarization beam splitter. The slow axis direction of the second polarization collimator is aligned with the horizontal or vertical polarization direction of the second polarization beam splitter. Elliptically polarized light is transmitted through the first polarization beam splitter and coupled into the polarization-maintaining fiber through the first polarization-maintaining collimator. It is decomposed into two beams with polarization directions along the fast axis and the slow axis of the polarization-maintaining fiber, forming two cyclic paths with different optical paths in the resonant cavity of the fiber laser. This generates a dual-repetition-rate femtosecond laser with orthogonal polarization. The dual-repetition-rate femtosecond laser is then separated and output by the second polarization beam splitter. The polarization directions of the dual-repetition-rate femtosecond laser are along the fast axis and the slow axis of the polarization-maintaining fiber, respectively. The second polarization-maintaining collimator includes a partial reflective surface; the reflectivity of the partial reflective surface of the second polarization-maintaining collimator is 80%~95%; the two beams of light, after being split and transmitted by the polarization-maintaining fiber, arrive at the second polarization-maintaining collimator, are reflected by the second polarization-maintaining collimator, pass through the polarization-maintaining fiber and the first polarization-maintaining collimator, are reflected by the first polarization beam splitter, and then pass through the optical isolation module and the second polarization adjustment module in sequence to adjust the polarization state of the return light before entering the second collimator. Subsequently, it passes through the gain fiber and the wavelength coupler, and then arrives at the first collimator again, completing one cycle of two laser paths with different optical paths.
2. The fiber laser of claim 1, wherein, The first polarization control module includes a first λ / 4 phase delay unit and a λ / 2 phase delay unit arranged sequentially; the second polarization control module is a second λ / 4 phase delay unit.
3. The fiber laser according to claim 2, characterized in that, Each phase delay unit is a liquid crystal phase delay unit or an electronically controlled waveplate.
4. The fiber laser according to claim 1, characterized in that, The wavelength coupler is either a wavelength division multiplexer or an optical fiber coupler.
5. The fiber laser according to claim 1, characterized in that, The gain medium is rare earth ion-doped optical fiber.
6. The fiber laser according to claim 5, characterized in that, Rare earth ion-doped optical fibers are erbium-doped, ytterbium-doped, thulium-doped, or holmium-doped optical fibers.
7. The fiber laser according to claim 1, characterized in that, By adjusting the angle between the slow axis direction of the first polarization-maintaining collimator and the transmission axis direction of the first polarization beam splitter, the relative intensity of the two femtosecond lasers with different repetition frequencies can be controlled.
8. The fiber laser according to claim 1, characterized in that, The first polarization control module, the second polarization control module, and the optical isolation module work together to achieve the polarization rotation mode-locking function.