3μm high-repetition-rate high-power femtosecond fiber laser

Abstract

A 3μm high-repetition-rate high-power femtosecond fiber laser, comprising: a ring cavity structure (1) and an amplification unit (2). The ring cavity structure (1) comprises a first pumping source (11), a first erbium-doped gain fiber (12), a nonlinear polarization rotation unit (13), and a coupler (14). The first pumping source (11) emits 1.7μm pumped light to the first erbium-doped gain fiber (12); the first erbium-doped gain fiber (12) emits 3μm laser light to the nonlinear polarization rotation unit (13); the nonlinear polarization rotation unit (13) emits 3μm high-repetition-rate mode-locked laser light to the coupler (14); the coupler (14) inputs part of the 3μm high-repetition-rate mode-locked laser light into the amplification unit (2). The amplification unit (2) increases the average power of the 3μm high-repetition-rate mode-locked laser light. Using the 1.7μm fiber core pumping technology that enhances absorption in excited states outputs high-repetition-rate mode-locked pulses. In addition, using a 1.7μm high absorption rate and the short gain fiber can achieve the advantage of high gain, weaken the nonlinear effect in pulse amplification processes, and finally output high-average-power pulses.

Description

A 3μm high repetition rate, high power femtosecond fiber laser Technical Field

[0001] This invention relates to the field of laser technology, and more specifically, to a 3μm high repetition rate, high power femtosecond fiber laser. Background Technology

[0002] To achieve high repetition rate mode-locked pulse output, the length of the resonant cavity needs to be controlled, using the shortest possible active fiber as the gain medium. Compared to core-pumped methods, Er 3+ ZBLAN fiber lasers suffer from low cladding-pumped absorption, requiring longer gain fibers to achieve sufficient pump absorption to reach the lasing threshold, thus limiting the improvement of mode-locked pulse repetition frequency. Furthermore, the significant frequency difference between the 976 nm pump light and the 3 μm signal light results in a larger quantum defect and a lower Stokes limit, further restricting the efficiency of 3 μm laser generation. In other words, current 3 μm mid-infrared femtosecond fiber lasers have shortcomings in average power and repetition frequency, and face developmental bottlenecks. Summary of the Invention

[0003] To address the aforementioned problems, the purpose of this invention is to provide a 3μm high repetition rate, high power femtosecond fiber laser.

[0004] This invention provides a 3μm high repetition rate, high power femtosecond fiber laser, comprising: a ring cavity structure and an amplification unit; the ring cavity structure includes at least a first pump source, a first erbium-doped gain fiber, a nonlinear polarization rotation unit, and a coupler; the first pump source emits 1.7μm pump light to the first erbium-doped gain fiber, the first erbium-doped gain fiber receives the 1.7μm pump light and emits 3μm laser light to the nonlinear polarization rotation unit, the nonlinear polarization rotation unit receives the 3μm laser light and emits 3μm high repetition rate mode-locked laser light to the coupler, the coupler keeps a portion of the 3μm high repetition rate mode-locked laser light circulating in the ring cavity structure, and inputs the remaining portion of the 3μm high repetition rate mode-locked laser light into the amplification unit; the amplification unit is used to increase the average power of the 3μm high repetition rate mode-locked laser light.

[0005] Optionally, the annular cavity structure further includes: a first dichroic mirror disposed between the first pump source and the first erbium-doped gain fiber, the first dichroic mirror being used to transmit the 1.7μm pump light emitted by the first pump source to the first erbium-doped gain fiber, and to reflect the 3μm high repetition rate mode-locked laser transmitted by the coupler.

[0006] Optionally, the annular cavity structure further includes a gold mirror disposed between the first dichroic mirror and the coupler, the gold mirror being used to reflect the 3μm high repetition rate mode-locked laser transmitted by the coupler back to the first dichroic mirror.

[0007] Optionally, the annular cavity structure further includes a second dichroic mirror disposed between the first erbium-doped gain fiber and the nonlinear polarization rotation unit, the second dichroic mirror being used to reflect the 3μm laser emitted by the first erbium-doped gain fiber to the nonlinear polarization rotation unit.

[0008] Optionally, the annular cavity structure further includes a first wavelength division multiplexer disposed between the first pump source and the first erbium-doped gain fiber.

[0009] Optionally, the nonlinear polarization rotation unit includes: a first half-paddle, a first isolator, and a first quarter-paddle; the first half-paddle is used to change the polarization direction of the 3μm laser emitted by the erbium-doped gain fiber, the first isolator is used to ensure that the 3μm laser is transmitted in a single direction, and the first quarter-paddle is used to adjust the polarization state of the 3μm laser and output the 3μm high repetition rate mode-locked laser.

[0010] Optionally, the amplification unit includes at least: a second pump source, a second erbium-doped gain fiber, and a second isolator; the second pump source emits 1.7μm pump light to the second erbium-doped gain fiber, the second isolator receives the 3μm high repetition rate mode-locked laser shunted by the coupler and emits it to the second erbium-doped gain fiber, and the second erbium-doped gain fiber receives the 1.7μm pump light emitted by the second pump source and the 3μm high repetition rate mode-locked laser emitted by the second isolator, thereby increasing the average power of the 3μm high repetition rate mode-locked laser.

[0011] Optionally, if the annular cavity structure further includes the first wavelength division multiplexer, the amplification unit further includes a second wavelength division multiplexer disposed between the second pump source and the second erbium-doped gain fiber.

[0012] Optionally, if the annular cavity structure further includes the first dichroic mirror, the amplification unit further includes: a third dichroic mirror disposed between the second pump source and the second erbium-doped gain fiber; the third dichroic mirror is used to transmit the 1.7μm pump light emitted by the second pump source to the second erbium-doped gain fiber, and to reflect the 3μm high repetition rate mode-locked laser emitted by the second isolator to the second erbium-doped gain fiber.

[0013] Optionally, the amplification unit further includes: a second quarter lever and a second half lever sequentially disposed on the light-emitting side of the second isolator.

[0014] In the above-described solution of the present invention, Er is pumped using a 1.7μm fiber core. 3+ Doped ZBLAN fiber enables high repetition rate (PRR) mode-locked pulse output. Compared to cladding pumping, 1.7μm core pumping significantly improves the laser conversion efficiency in active fiber while reducing quantum loss. Higher gain can be achieved with shorter gain fiber, thus greatly increasing the repetition rate of the mode-locked fiber laser, reducing nonlinear effects, and suppressing pulse splitting. Subsequently, this high PPR mode-locked fiber laser is used as a seed source and combined with amplification unit 2. During the amplification of the high PPR signal light, the high repetition rate results in a lower peak pulse power at the same average power, avoiding pulse splitting caused by peak power clamping. Simultaneously, soliton self-compression balances the requirements of high average power and narrow pulse width, ultimately achieving stable output of a 3μm high PPR high-power femtosecond laser.

[0015] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0016] 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 only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 shows a schematic diagram of the general structure of the first 3μm high repetition rate high power femtosecond fiber laser provided in the embodiment of the present invention.

[0018] Figure 2 shows a detailed structural schematic diagram of the first 3μm high repetition rate high power femtosecond fiber laser provided in the embodiment of the present invention;

[0019] Figure 3 shows a schematic diagram of the general structure of the second type of 3μm high repetition rate high power femtosecond fiber laser provided in the embodiment of the present invention.

[0020] Figure 4 shows a detailed structural schematic diagram of the second type of 3μm high repetition rate high power femtosecond fiber laser provided in the embodiment of the present invention.

[0021] icon:

[0022] 1-Ring cavity structure, 2-Amplification unit, 11-First pump source, 12-First erbium-doped gain fiber, 13-Nonlinear polarization rotation unit, 14-Coupler, 15-First dichroic mirror, 16-Gold mirror, 17-Second dichroic mirror, 18-First wavelength division multiplexer, 21-Second pump source, 22-Second erbium-doped gain fiber, 23-Second isolator, 24-Second wavelength division multiplexer, 25-Third dichroic mirror, 26-Second quarter-paddle, 27-Second half-paddle, 131-First half-paddle, 132-First isolator, 133-First quarter-paddle, a-Collimating lens, b-Focusing lens. Embodiments of the present invention

[0023] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0024] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0025] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0026] This invention provides a 3μm high repetition rate, high power femtosecond fiber laser, as shown in Figures 1 to 4. Figures 1 and 2 show a first structural schematic diagram of the 3μm high repetition rate, high power femtosecond fiber laser, and Figures 3 and 4 show a second structural schematic diagram of the 3μm high repetition rate, high power femtosecond fiber laser. Referring to Figures 1 and 3, both types of 3μm high repetition rate, high power femtosecond fiber lasers include two main parts: a ring cavity structure 1 and an amplification unit 2. As shown in Figures 2 and 4 (for clarity and neatness, the reference numerals for the ring cavity structure 1 and the amplification unit 2 are not directly shown in Figures 2 and 4), the ring cavity structure 1 includes at least a first pump source 11, a first erbium-doped gain fiber 12, a nonlinear polarization rotation unit 13, and a coupler 14. The first pump source 11 can be a 1.7μm core pump for emitting 1.7μm pump light; the first erbium-doped gain fiber 12 uses commercially available Erbium-doped fiber. 3+ ZBLAN fiber; nonlinear polarization rotation unit 13 is a saturable absorber used to achieve passive mode-locking; the coupler 14 has a 50:50 ratio, which can split the light input to the coupler 14 evenly.

[0027] Specifically, as shown in Figures 2 and 4, the first pump source 11 emits 1.7 μm pump light to the first erbium-doped gain fiber 12. The first erbium-doped gain fiber 12 receives the 1.7 μm pump light and emits 3 μm laser light to the nonlinear polarization rotation unit 13. The nonlinear polarization rotation unit 13 receives the 3 μm laser light and emits 3 μm high repetition rate mode-locked laser light to the coupler 14. The coupler 14 keeps part of the 3 μm high repetition rate mode-locked laser light circulating in the ring cavity structure 1, and inputs the other part of the 3 μm high repetition rate mode-locked laser light into the amplification unit 2. As shown in Figure 2, the coupler 14 can transmit half of the input 3 μm high repetition rate mode-locked laser light into the subsequent optical path pulse cavity of the ring cavity structure 1, and reflect the remaining half of the laser light into the subsequently connected amplification unit 2. Since the average power of the 3μm high-repetition-rate mode-locked laser is relatively low when directly output from coupler 14, it needs to be amplified outside the ring cavity structure 1 to obtain a high average power. In this embodiment of the invention, the 3μm high-repetition-rate mode-locked laser in the input amplification unit 2 can be used as a seed light, and combined with the signal light amplification technology provided by the amplification unit 2, the average power of the 3μm high-repetition-rate mode-locked laser can be improved to achieve high average power pulse output.

[0028] The 3μm high repetition rate, high power femtosecond fiber laser provided in this embodiment of the invention utilizes a 1.7μm fiber core pump for Er 3+Doped ZBLAN fiber enables high repetition rate (PRR) mode-locked pulse output. Compared to cladding pumping, 1.7μm core pumping significantly improves the laser conversion efficiency in active fiber while reducing quantum loss. Higher gain can be achieved with shorter gain fiber, thus greatly increasing the repetition rate of the mode-locked fiber laser, reducing nonlinear effects, and suppressing pulse splitting. Subsequently, this high PPR mode-locked fiber laser is used as a seed source and combined with amplification unit 2. During the amplification of the high PPR signal light, the high repetition rate results in a lower peak pulse power at the same average power, avoiding pulse splitting caused by peak power clamping. Simultaneously, soliton self-compression balances the requirements of high average power and narrow pulse width, ultimately achieving stable output of a 3μm high PPR high-power femtosecond laser.

[0029] Optionally, as shown in Figure 2, which specifically illustrates a 3μm high repetition rate, high-power femtosecond fiber laser with a free-space structure, the ring cavity structure 1 further includes a first dichroic mirror 15 disposed between the first pump source 11 and the first erbium-doped gain fiber 12. This first dichroic mirror 15 can be obliquely positioned at 45 degrees to transmit the 1.7μm pump light emitted from the first pump source 11 to the first erbium-doped gain fiber 12, and the 3μm high repetition rate mode-locked laser transmitted through the reflection coupler 14. The first dichroic mirror 15 enables optical path adjustment and separation of the pump light and the high repetition rate mode-locked laser.

[0030] Optionally, as shown in Figure 2, the annular cavity structure 1 may further include a gold mirror 16 disposed between the first dichroic mirror 15 and the coupler 14. The gold mirror 16 may also be disposed at an angle to reflect the 3μm high repetition rate mode-locked laser transmitted by the coupler 14 to the first dichroic mirror 15, thereby adjusting the direction of the optical path so that the 3μm high repetition rate mode-locked laser can continuously circulate in the annular cavity structure 1.

[0031] Optionally, as shown in Figure 2, the annular cavity structure 1 may further include: a second dichroic mirror 17 disposed between the first erbium-doped gain fiber 12 and the nonlinear polarization rotation unit 13. The second dichroic mirror 17 may be obliquely positioned at 45 degrees to reflect the 3μm laser emitted by the first erbium-doped gain fiber 12 to the nonlinear polarization rotation unit 13.

[0032] It should be noted that in the 3μm high repetition rate, high power femtosecond fiber laser with the free-space structure shown in Figure 2, a collimating lens a can be placed between the first pump source 11 and the first dichroic mirror 15, and between the first erbium-doped gain fiber 12 and the second dichroic mirror 17, so that the 1.7μm pump light emitted by the first pump source 11 is collimated with the 3μm high repetition rate mode-locked laser emitted by the first erbium-doped gain fiber 12. Furthermore, a focusing lens b can also be placed between the first dichroic mirror 15 and the first erbium-doped gain fiber 12 to concentrate and focus the 1.7μm pump light into the first erbium-doped gain fiber 12.

[0033] Optionally, as shown in Figure 4, which illustrates the structure of a 3μm high repetition rate, high power femtosecond fiber laser with an all-fiber structure, the ring cavity structure 1 may further include a first wavelength division multiplexer 18 disposed between the first pump source 11 and the first erbium-doped gain fiber 12. This first wavelength division multiplexer 18 does not alter the beam; its function is simply to connect the first pump source 11 and the first erbium-doped gain fiber 12.

[0034] As can be seen, the 3μm high-repetition-rate, high-power femtosecond fiber laser provided in this embodiment of the invention can be applied in both space structures and all-fiber structures. Compared to free-space structures, the all-fiber structure increases mode-locking stability, and the laser structure is compact, easy to integrate, and can greatly expand its practical applications.

[0035] Optionally, as shown in Figures 2 and 4, the structure of the nonlinear polarization rotation unit 13 is consistent in both cases, including: a first half-paddle 131, a first isolator 132, and a first quarter-paddle 133. This structure, placed within the annular cavity structure 1, enables mode-locked pulse output. The first half-paddle 131 is used to change the polarization direction of the 3μm laser emitted by the first erbium-doped gain fiber 12, adjusting the 3μm laser from linearly polarized light to elliptically polarized light. The first isolator 132 ensures that the 3μm laser propagates in a single direction, and the first quarter-paddle 133 adjusts the polarization state of the 3μm laser, outputting a 3μm high-repetition-rate mode-locked laser.

[0036] Optionally, as shown in Figures 2 and 4, in both of the above cases, the amplification unit 2 includes at least: a second pump source 21, a second erbium-doped gain fiber 22, and a second isolator 23. The second pump source 21 emits 1.7μm pump light to the second erbium-doped gain fiber 22, and the second isolator 23 receives the 3μm high repetition rate mode-locked laser shunted by the coupler 14 and emits it to the second erbium-doped gain fiber 22. The second erbium-doped gain fiber 22 simultaneously receives the 1.7μm pump light emitted by the second pump source 21 and the 3μm high repetition rate mode-locked laser emitted by the second isolator 23. That is, in this embodiment of the invention, a high repetition rate mode-locked fiber laser composed of a ring cavity structure 1 is used as a seed source, and by combining it with 1.7μm core pumping technology to amplify the 2.8μm signal light, a high average power pulse output can be achieved. At the same time, by combining soliton self-compression, the requirements of high average power and narrow pulse width can be met, ultimately achieving an increase in the average power of the 3μm high repetition rate mode-locked laser.

[0037] Optionally, as shown in Figure 4, if the annular cavity structure 1 further includes a first wavelength division multiplexer 18, that is, in the case of a 3μm high repetition rate high power femtosecond fiber laser with an all-fiber structure, the amplification unit 2 may also include: a second wavelength division multiplexer 24 disposed between the second pump source 21 and the second erbium-doped gain fiber 22, for connecting the second pump source 21 and the second erbium-doped gain fiber 22.

[0038] Optionally, as shown in Figure 2, if the annular cavity structure 1 further includes a first dichroic mirror 15, that is, in the case of a 3μm high repetition rate high power femtosecond fiber laser in a free space structure, the amplification unit 2 may also include a third dichroic mirror 25 disposed between the second pump source 21 and the second erbium-doped gain fiber 22. The third dichroic mirror 25 is used to transmit the 1.7μm pump light emitted by the second pump source 21 to the second erbium-doped gain fiber 22, and to reflect the 3μm high repetition rate mode-locked laser emitted by the second isolator 23 to the second erbium-doped gain fiber 22.

[0039] Optionally, still as shown in Figure 2, in the case of a 3μm high repetition rate, high power femtosecond fiber laser with a free-space structure, the amplification unit 2 may further include: a second quarter-paddle 26 and a second half-paddle 27 sequentially disposed on the output side of the second isolator 23. The second quarter-paddle 26 changes the polarization state of the 3μm high repetition rate mode-locked laser, and the second half-paddle 27 changes the polarization direction of the 3μm high repetition rate mode-locked laser. Specifically, the combination of the second half-paddle 27 and the second quarter-paddle 26 can control the polarization state of the output laser (i.e., the 3μm high repetition rate mode-locked laser). By adjusting the angle of the second half-paddle 27 and the second quarter-paddle 26, the 3μm high repetition rate mode-locked laser can be converted from a linearly polarized state to an elliptical polarized state. Compared to a linearly polarized laser, an elliptical polarized laser can suppress the Raman effect during amplification. Therefore, the amplification efficiency and average power are improved.

[0040] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A 3μm high-repetition-rate, high-power femtosecond fiber laser, characterized in that, include: The ring cavity structure (1) and the amplification unit (2) include at least a first pump source (11), a first erbium-doped gain fiber (12), a nonlinear polarization rotation unit (13), and a coupler (14). The first pump source (11) emits 1.7 μm pump light to the first erbium-doped gain fiber (12), the first erbium-doped gain fiber (12) receives the 1.7 μm pump light and emits 3 μm laser light to the nonlinear polarization rotation unit (13), the nonlinear polarization rotation unit (13) receives the 3 μm laser light and emits 3 μm high repetition rate mode-locked laser light to the coupler (14), the coupler (14) keeps part of the 3 μm high repetition rate mode-locked laser light in the annular cavity structure (1) for circulation, and inputs the other part of the 3 μm high repetition rate mode-locked laser light into the amplification unit (2); the amplification unit (2) is used to increase the average power of the 3 μm high repetition rate mode-locked laser light.

2. The 3μm high repetition rate, high power femtosecond fiber laser according to claim 1, characterized in that, The annular cavity structure (1) further includes: a first dichroic mirror (15) disposed between the first pump source (11) and the first erbium-doped gain fiber (12), the first dichroic mirror (15) being used to transmit the 1.7μm pump light emitted by the first pump source (11) to the first erbium-doped gain fiber (12), and to reflect the 3μm high repetition rate mode-locked laser transmitted by the coupler (14).

3. The 3μm high repetition rate, high power femtosecond fiber laser according to claim 2, characterized in that, The annular cavity structure (1) further includes a gold mirror (16) disposed between the first dichroic mirror (15) and the coupler (14), the gold mirror (16) being used to reflect the 3μm high repetition rate mode-locked laser transmitted by the coupler (14) back to the first dichroic mirror (15).

4. The 3μm high repetition rate, high power femtosecond fiber laser according to claim 3, characterized in that, The annular cavity structure (1) further includes a second dichroic mirror (17) disposed between the first erbium-doped gain fiber (12) and the nonlinear polarization rotation unit (13), the second dichroic mirror (17) being used to reflect the 3μm laser emitted by the first erbium-doped gain fiber (12) to the nonlinear polarization rotation unit (13).

5. The 3μm high repetition rate, high power femtosecond fiber laser according to claim 1, characterized in that, The annular cavity structure (1) further includes a first wavelength division multiplexer (18) disposed between the first pump source (11) and the first erbium-doped gain fiber (12).

6. The 3μm high repetition rate, high power femtosecond fiber laser according to any one of claims 1 to 5, characterized in that, The nonlinear polarization rotation unit (13) includes: a first half-paddle (131), a first isolator (132), and a first quarter-paddle (133); the first half-paddle (131) is used to change the polarization direction of the 3μm laser emitted by the first erbium-doped gain fiber (12), the first isolator (132) is used to ensure that the 3μm laser is transmitted in a single direction, and the first quarter-paddle (133) is used to adjust the polarization state of the 3μm laser and output the 3μm high repetition rate mode-locked laser.

7. The 3μm high repetition rate, high power femtosecond fiber laser according to any one of claims 1 to 5, characterized in that, The amplification unit (2) includes at least: a second pump source (21), a second erbium-doped gain fiber (22), and a second isolator (23); The second pump source (21) emits 1.7μm pump light to the second erbium-doped gain fiber (22). The second isolator (23) receives the 3μm high repetition rate mode-locked laser shunted by the coupler (14) and emits it to the second erbium-doped gain fiber (22). The second erbium-doped gain fiber (22) receives the 1.7μm pump light emitted by the second pump source (21) and the 3μm high repetition rate mode-locked laser emitted by the second isolator (23), thereby increasing the average power of the 3μm high repetition rate mode-locked laser.

8. The 3μm high repetition rate, high power femtosecond fiber laser according to claim 7, characterized in that, In the case where the annular cavity structure (1) further includes the first wavelength division multiplexer (18), the amplification unit (2) further includes a second wavelength division multiplexer (24) disposed between the second pump source (21) and the second erbium-doped gain fiber (22).

9. The 3μm high repetition rate, high power femtosecond fiber laser according to claim 7, characterized in that, In the case where the annular cavity structure (1) further includes the first dichroic mirror (15), the amplification unit (2) further includes: a third dichroic mirror (25) disposed between the second pump source (21) and the second erbium-doped gain fiber (22). The third dichroic mirror (25) is used to transmit the 1.7μm pump light emitted by the second pump source (21) to the second erbium-doped gain fiber (22), and to reflect the 3μm high repetition rate mode-locked laser emitted by the second isolator (23) to the second erbium-doped gain fiber (22).

10. The 3μm high repetition rate, high power femtosecond fiber laser according to claim 9, characterized in that, The amplification unit (2) further includes: a second quarter-paddle (26) and a second half-paddle (27) sequentially disposed on the light-emitting side of the second isolator (23).

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