Hybrid device for laser device

Abstract

The invention provides a hybrid device for a laser device, and relates to the technical field of laser, the hybrid device comprises a signal end collimator, a pumping end collimator, a spectroscope, an isolator and a beam expanding unit, the pumping end collimator and the signal end collimator are arranged on one side of the spectroscope, and the beam expanding unit is arranged on the other side of the spectroscope. Pump light output by the pump end collimator is reflected to the signal end collimator through the spectroscope; the isolator and the beam expanding unit are arranged in the emitting direction of the signal light, the pump light is focused at the second lens and passes through the first lens to form an annular light spot, and the annular light spot is coupled to an inner cladding of the first optical fiber and is totally reflected at the joint of the inner cladding and an outer cladding of the first optical fiber. The annular light spot penetrates through the fiber core of the first optical fiber for multiple times and is absorbed by the metal rare earth element doped in the fiber core, the signal light transmitted by the fiber core is amplified, and the amplified signal light is transmitted out of the spectroscope. According to the invention, the stability and reliability of the laser can be improved.

Description

Technical Field

[0001] The invention relates to the field of laser technology, and in particular to a hybrid device for a laser device. Background Art

[0002] Fiber lasers are widely used in laser engraving, laser cutting, laser marking, medical treatment, spectroscopy, optical communication, material processing, aerospace, etc. due to their high efficiency, high beam quality, compact structure, and high reliability. The main pumping method of existing high-power fiber lasers is cladding pumping of double-clad optical fiber. The pump light is transmitted in the inner cladding of the double-clad optical fiber and passes through the core many times. It is absorbed by the doped elements in the core, thereby efficiently converting the pump light into single-mode laser.

[0003] The Chinese patent with publication number CN114759423A discloses a hybrid device and a laser, wherein the hybrid device includes an input collimator, an output collimator, a pump light collimator, an isolator, a filter and a beam combining module. The input collimator is used to receive signal light, the isolator is used to transmit the signal light from the direction of the input collimator to the filter, the filter is used to filter the signal light to obtain a first light beam, the pump light collimator is used to receive pump light, the beam combining module is used to combine the first light beam and the pump light output by the pump light collimator to obtain a first combined light beam, and the output collimator is used to output the first combined light beam. However, when the pump light is coupled into the double-clad optical fiber, the pump light will enter the fiber core, and a photon darkening effect will occur under high power conditions, which will eventually reduce the threshold and output power of the laser and affect the overall performance of the laser. Therefore, it is very necessary to provide a hybrid device for a laser device to help improve the stability and reliability of the laser. Summary of the invention

[0004] In view of this, the present invention proposes a hybrid device for a laser device. By shaping the pump light into a ring beam, the direct coupling of the pump light into the fiber core is reduced, the photon darkening effect is effectively suppressed, and the stability and reliability of the laser are improved.

[0005] The present invention provides a hybrid device for a laser device, comprising a signal end collimator, a pump end collimator, a beam splitter, an isolator and a beam expansion unit, wherein:

[0006] The pump end collimator and the signal end collimator are arranged on one side of the beam splitter, and the pump light output by the pump end collimator is reflected by the beam splitter to the signal end collimator;

[0007] The signal end collimator comprises a signal source, a first optical fiber, a first lens and a second lens, wherein the first optical fiber, the first lens and the second lens are all arranged in the light emitting direction of the signal source, the first optical fiber is used to transmit the signal light emitted by the signal source, the pump light is focused at the second lens and passes through the first lens to form an annular light spot, the annular light spot is coupled to the inner cladding of the first optical fiber and is totally reflected at the junction of the inner cladding and the outer cladding of the first optical fiber, the annular light spot passes through the core of the first optical fiber for multiple times and is absorbed by the metal rare earth element doped in the core, the signal light transmitted by the core is amplified, and the amplified signal light is transmitted from the beam splitter;

[0008] The isolator and the beam expansion unit are arranged in the emission direction of the signal light;

[0009] The distance between the first lens and the second lens along the optical axis satisfies: 0.67 mm to 0.79 mm;

[0010] The physical base angle of the first lens is 21.30° to 23.27°, and the deflection angle between the refracted light of the first lens and the optical axis is 10.51° to 11.70°.

[0011] On the basis of the above technical solution, preferably, the pump end collimator includes a pump source, a second optical fiber and a third lens, the second optical fiber and the third lens are sequentially arranged along the light emitting direction of the pump source, and the wavelength of the pump light emitted by the pump source is 976nm.

[0012] On the basis of the above technical solution, preferably, the first optical fiber is a polarization-maintaining double-clad optical fiber doped with rare earth elements, the core diameter of the first optical fiber is 25 μm, the inner cladding diameter of the first optical fiber is 250 μm, the numerical aperture of the first optical fiber is 0.065, and the transmission wavelength of the first optical fiber is 1064 nm.

[0013] More preferably, the second optical fiber is a multimode optical fiber, the core diameter of the second optical fiber is 135 μm, the numerical aperture of the second optical fiber is 0.22, and the transmission wavelength of the second optical fiber is 976 nm.

[0014] More preferably, the beam expansion unit includes a fourth lens and a fifth lens, and the fourth lens and the fifth lens are sequentially arranged in the emitting direction of the signal light.

[0015] More preferably, the focal length ratio of the fifth lens (52) to the fourth lens (51) satisfies:

[0016] -5.5≤f5 / f4≤-3.5

[0017] Wherein, f4 represents the focal length of the fourth lens (51), and f5 represents the focal length of the fifth lens (52).

[0018] More preferably, the focal length of the first lens is 5.9 mm to 6.3 mm, the center thickness of the first lens is 2.5 mm to 3.5 mm, the focal length of the second lens is 4.2 mm to 4.3 mm, and the center thickness of the second lens is 2.5 mm to 3.5 mm.

[0019] More preferably, both sides of the first lens are coated with a broadband anti-reflection film including 976nm and 1064nm, both sides of the second lens are coated with a broadband anti-reflection film including 976nm and 1064nm, both sides of the third lens are coated with an anti-reflection film of the 976nm band, and both sides of the fourth lens and the fifth lens are coated with an anti-reflection film of 1064nm.

[0020] More preferably, the first lens is a plano-convex lens or an aspheric lens, the second lens is an aspheric lens, the third lens is a cone lens, the fourth lens is a plano-concave lens or a meniscus lens, and the fifth lens is a plano-convex lens or a biconvex lens.

[0021] More preferably, the distance between the second lens and the fourth lens along the optical axis is 45 mm to 55 mm.

[0022] The hybrid device for a laser device provided by the present invention has the following beneficial effects compared with the prior art:

[0023] (1) By shaping the pump light into an annular beam, the direct coupling of the pump light into the fiber core is reduced, and the photon darkening effect is effectively suppressed. The precise structural parameter design ensures the formation quality of the annular spot and realizes the efficient conversion of the pump light into the annular spot, so that the pump light is mainly coupled to the inner cladding of the double-clad fiber rather than the core. This not only improves the coupling efficiency of the pump light and the beam quality of the system, but also improves the beam shaping effect. At the same time, since the direct coupling of the pump light into the fiber core is reduced, the stability and reliability of the laser are improved. The overall design optimizes the coupling mode of the pump light, which helps to improve the overall performance and service life of the laser.

[0024] (2) Through precise fiber parameter configuration, efficient transmission and conversion of pump light and signal light are achieved. The larger core diameter and numerical aperture of the second fiber are conducive to the collection and transmission of 976nm pump light, while the first fiber adopts a polarization-maintaining double-cladding structure doped with rare earth elements. With the appropriate core diameter and small numerical aperture, it not only ensures the good transmission characteristics of the 1064nm signal light, but also provides a transmission channel for the pump light through the double-cladding structure. This parameter matching not only improves the coupling efficiency of the pump light, but also effectively suppresses the photon darkening effect, ensuring the stable operation and output performance of the laser. At the same time, the design of the polarization-maintaining characteristics also ensures the stability of the polarization state of the light beam, improving the overall beam quality output by the optical system. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0026] Figure 1 A schematic diagram of the structure of a hybrid device for a laser device provided by the present invention;

[0027] Figure 2 Schematic diagram of energy distribution of the annular light spot provided by the present invention.

[0028] Explanation of the reference numerals: 1. signal end collimator; 11. signal source; 12. first optical fiber; 13. first lens; 14. second lens; 2. pump end collimator; 21. pump source; 22. second optical fiber; 23. third lens; 3. spectroscope; 4. isolator; 5. beam expansion unit; 51. fourth lens; 52. fifth lens. DETAILED DESCRIPTION

[0029] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0030] See also Figure 1 The embodiment of the present application discloses a hybrid device for a laser device, comprising a signal end collimator 1, a pump end collimator 2, a beam splitter 3, an isolator 4 and a beam expansion unit 5, wherein:

[0031] The pump-end collimator 2 and the signal-end collimator 1 are arranged on one side of the beam splitter 3 , and the pump light output by the pump-end collimator 2 is reflected to the signal-end collimator 1 through the beam splitter 3 .

[0032] The pump end collimator 2 comprises a pump source 21, a second optical fiber 22 and a third lens 23. The second optical fiber 22 and the third lens 23 are sequentially arranged along the light emitting direction of the pump source 21. The wavelength of the pump light emitted by the pump source 21 is 976 nm.

[0033] The signal end collimator 1 includes a signal source 11, a first optical fiber 12, a first lens 13 and a second lens 14. The first optical fiber 12, the first lens 13 and the second lens 14 are all arranged in the light emitting direction of the signal source 11. The first optical fiber 12 is used to transmit the signal light emitted by the signal source 11. The pump light is focused at the second lens 14 and passes through the first lens 13 to form an annular light spot. The annular light spot is coupled to the inner cladding of the first optical fiber 12 and is totally reflected at the junction of the inner cladding and the outer cladding of the first optical fiber 12. The annular light spot passes through the core of the first optical fiber 12 many times and is absorbed by the metal rare earth elements doped in the core, amplifying the signal light transmitted by the core, and transmitting the amplified signal light from the beam splitter 3. And the annular light spot formed requires that the inner ring diameter of the first optical fiber 12 is greater than the core diameter of 25μm, and the outer ring diameter of the first optical fiber 12 is less than the inner cladding diameter of 250μm.

[0034] In this embodiment, efficient conversion of pump light to annular light spot is achieved. The design of the inner ring diameter of the annular light spot is larger than the core diameter and the outer ring diameter is smaller than the inner cladding diameter, which ensures that the pump light is mainly coupled to the inner cladding rather than the core. This selective coupling mechanism effectively avoids the photon darkening effect caused by the pump light directly entering the core. At the same time, the multiple reflections of the annular light spot in the inner cladding improve the absorption efficiency of the pump light and achieve efficient conversion of pump light to signal light. This structural design not only improves the power stability and reliability of the laser.

[0035] Furthermore, the physical base angle of the first lens 13 is 21.30° to 23.27°, and the deflection angle between the refracted light of the first lens 13 and the optical axis is 10.51° to 11.70°. The distance between the first lens 13 and the second lens 14 along the optical axis satisfies: 0.67mm to 0.79mm; the focal length of the first lens 13 is 5.9mm to 6.3mm, the center thickness of the first lens 13 is 2.5mm to 3.5mm, the focal length of the second lens 14 is 4.2mm to 4.3mm, and the center thickness of the second lens 14 is 2.5mm to 3.5mm.

[0036] In this embodiment, by accurately matching the physical bottom angle of the first lens 13 and the deflection angle of the refracted light, effective control of converting the incident Gaussian beam into a ring beam is achieved. At the same time, the focal length of the first lens 13 and the focal length of the second lens 14 form a reasonable optical structure, ensuring excellent imaging quality and beam control effect. At the same time, the compact spacing design adopted by the hybrid device significantly reduces the overall volume of the optical system and improves the space utilization efficiency. In addition, a reasonable tolerance range is set for each parameter, which not only ensures the stability of the optical performance, but also improves the manufacturability and yield rate of the product, so that the optical system has excellent engineering practicality and mass production capabilities while ensuring excellent optical performance.

[0037] Further, the first optical fiber 12 is a polarization-maintaining double-clad optical fiber doped with rare earth elements, the core diameter of the first optical fiber 12 is 25 μm, the inner cladding diameter of the first optical fiber 12 is 250 μm, the numerical aperture of the first optical fiber 12 is 0.065, and the transmission wavelength of the first optical fiber 12 is 1064 nm. The second optical fiber 22 is a multimode optical fiber, the core diameter of the second optical fiber 22 is 135 μm, the numerical aperture of the second optical fiber 22 is 0.22, and the transmission wavelength of the second optical fiber 22 is 976 nm.

[0038] In this embodiment, through precise fiber parameter configuration, efficient transmission and conversion of pump light and signal light are achieved. The larger core diameter and numerical aperture of the second optical fiber 22 are conducive to the collection and transmission of 976nm pump light, and the first optical fiber 12 adopts a polarization-maintaining double-cladding structure doped with rare earth elements, with a moderate core diameter and a small numerical aperture, which not only ensures the good transmission characteristics of the 1064nm signal light, but also provides a transmission channel for the pump light through the double-cladding structure. This parameter matching not only improves the coupling efficiency of the pump light, but also effectively suppresses the photon darkening effect, ensuring the stable operation and output performance of the laser. At the same time, the design of the polarization-maintaining characteristic also ensures the stability of the polarization state of the light beam, and improves the overall light beam quality output by the optical system.

[0039] The isolator 4 and the beam expander 5 are arranged in the emission direction of the signal light. The isolator 4 is used to block the return light, and the beam expander 5 includes a fourth lens 51 and a fifth lens 52, which are arranged in sequence in the emission direction of the signal light.

[0040] And the focal length ratio of the fifth lens 52 to the fourth lens 51 satisfies:

[0041] -5.5≤f5 / f4≤-3.5

[0042] Wherein, f4 represents the focal length of the fourth lens 51 , and f5 represents the focal length of the fifth lens 52 .

[0043] In this embodiment, excellent beam processing and control effects are achieved by rationally arranging the isolator 4 and the beam expansion unit 5 in the direction of signal light emission. Among them, the setting of the isolator 4 effectively blocks the return light, significantly improves the anti-interference ability and signal purity of the system, and avoids the damage and interference of the return light to the signal source. The beam expansion unit 5 adopts a combination design of the fourth lens 51 and the fifth lens 52. By precisely controlling the focal length ratio of the two lenses in the range of -5.5 to -3.5, it not only ensures the ideal beam expansion effect, but also maintains the high quality and uniformity of the light beam. This focal ratio design also provides a moderate adjustment space, which is conducive to fine adjustment in practical applications, while ensuring the assembly tolerance of the optical system. Overall, the optical system achieves efficient, stable and reliable beam processing functions through the protective effect of the isolator 4 and the precise control of the beam expansion unit 5.

[0044] In one example, the beam expansion unit 5 includes but is not limited to a fourth lens 51 and a fifth lens 52 , and may also be a combination of three lenses.

[0045] Furthermore, the distance between the second lens 14 and the fourth lens 51 along the optical axis is 45 mm to 55 mm. This distance design provides a suitable optical path for the transmission of the signal light, so that the signal light can be stably transmitted to the beam expansion unit 5 after being collimated by the second lens 14. This specific distance range not only provides sufficient installation space for optical components such as the isolator 4 in the middle, but also ensures that the signal light can enter the beam expansion unit 5 with appropriate beam parameters, thereby obtaining an ideal beam expansion effect.

[0046] Both sides of the first lens 13 are coated with a broadband anti-reflection film including 976nm and 1064nm, both sides of the second lens 14 are coated with a broadband anti-reflection film including 976nm and 1064nm, both sides of the third lens 23 are coated with an anti-reflection film of the 976nm band, and both sides of the fourth lens 51 and the fifth lens 52 are coated with an anti-reflection film of 1064nm.

[0047] In this embodiment, efficient transmission of light of different wavelengths is achieved through targeted coating design. Since the first lens 13 and the second lens 14 need to process 976nm pump light and 1064nm signal light at the same time, a broadband anti-reflection film design is adopted to ensure that light of both wavelengths can be efficiently transmitted. The third lens 23 is specially used for collimation of pump light and therefore only requires a 976nm band anti-reflection film. The fourth lens 51 and the fifth lens 52 are located in the beam expansion unit 5 and only transmit signal light, so a 1064nm single-wavelength anti-reflection film is adopted. This differentiated coating design not only significantly reduces the reflection loss of each optical interface and improves the transmittance of light, but also avoids unnecessary coating costs, optimizes the transmission efficiency of the entire optical system, and improves the overall performance and reliability of the laser.

[0048] The first lens 13 is a plano-convex lens or an aspheric lens, the second lens 14 is an aspheric lens, the third lens 23 is a cone lens, the fourth lens 51 is a plano-concave lens or a meniscus lens, and the fifth lens 52 is a plano-convex lens or a biconvex lens.

[0049] In this embodiment, the first lens 13 adopts a plano-convex or aspheric design, and the second lens 14 adopts an aspheric design. This combination can effectively reduce aberrations and improve beam quality. The third lens 23 adopts a conical lens design, which can accurately shape the pump light into an annular spot, ensuring that the pump light is mainly coupled to the inner cladding rather than the core; the fourth lens 51 adopts a plano-concave or meniscus design, and the fifth lens 52 adopts a plano-convex or bi-convex design. This combination constitutes an efficient beam expansion unit 5, which can achieve an ideal beam expansion effect. The design of the overall lens system not only ensures the optical performance, but also has good engineering feasibility, effectively improving the overall performance and reliability of the laser.

[0050] Implementation principle: The 976nm pump light output from the second optical fiber 22 in the pump-end collimator 2 passes through the third lens 23 to collimate the light beam and enter the beam splitter 3. The pump collimated light is reflected by the beam splitter 3 to the second lens 14 of the signal-end collimator 1 and then focused. It then passes through the first lens 13 to form an annular light spot, which is coupled into the inner cladding of the first optical fiber 12 and is totally reflected therein. At the same time, it continuously passes through the fiber core and is absorbed by the doped ions in the fiber core, thereby amplifying the core signal light. The signal light output from the core of the signal-end collimator 1 passes through the beam splitter 3, is transmitted through the isolator 4, and then passes through the beam expansion unit 5 to amplify the light spot.

[0051] In one example, the first optical fiber 12 , the first lens 13 , and the second lens 14 are assembled together through a glass tube and a metal tube to form a signal end collimator 1 , and the second optical fiber 22 and the third lens 23 are assembled together through a glass tube and a metal tube to form a pump end collimator 2 .

[0052] By shaping the pump light into an annular beam, the direct coupling of the pump light into the fiber core is reduced, and the photon darkening effect is effectively suppressed. The precise structural parameter design ensures the formation quality of the annular spot and realizes the efficient conversion of the pump light to the annular spot, so that the pump light is mainly coupled to the inner cladding of the double-clad optical fiber rather than the core. This not only improves the coupling efficiency of the pump light and the beam quality of the system, but also improves the effect of beam shaping. At the same time, since the direct coupling of the pump light into the fiber core is reduced, the stability and reliability of the laser are improved. The overall design optimizes the coupling mode of the pump light, which helps to improve the overall performance and service life of the laser.

[0053] Unless otherwise defined, the technical terms or scientific terms used in the present invention shall have the usual meanings understood by persons with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, words such as "one" or "one" do not indicate quantity restrictions, but indicate the existence of at least one. Words such as "connect" or "connected" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the object being described changes, the relative positional relationship also changes accordingly.

[0054] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A hybrid device for a laser device, characterized in that: It comprises a signal end collimator (1), a pump end collimator (2), a beam splitter (3), an isolator (4) and a beam expansion unit (5), wherein: The pump end collimator (2) and the signal end collimator (1) are arranged on one side of the beam splitter (3), and the pump light output by the pump end collimator (2) is reflected by the beam splitter (3) to the signal end collimator (1); The signal end collimator (1) comprises a signal source (11), a first optical fiber (12), a first lens (13) and a second lens (14); the first optical fiber (12), the first lens (13) and the second lens (14) are all arranged in the light emitting direction of the signal source (11); the first optical fiber (12) is used to transmit the signal light emitted by the signal source (11); the pump light is focused at the second lens (14) and passes through the first lens (13) to form an annular light spot; the annular light spot is coupled to the inner cladding of the first optical fiber (12) and is totally reflected at the junction of the inner cladding and the outer cladding of the first optical fiber (12); the annular light spot passes through the core of the first optical fiber (12) for multiple times and is absorbed by the metal rare earth element doped in the core, thereby amplifying the signal light transmitted by the core and transmitting the amplified signal light from the beam splitter (3); The isolator (4) and the beam expansion unit (5) are arranged in the emission direction of the signal light; The distance between the first lens (13) and the second lens (14) along the optical axis satisfies: 0.67 mm to 0.79 mm; The physical base angle of the first lens (13) is 21.30° to 23.27°, and the deflection angle between the refracted light of the first lens (13) and the optical axis is 10.51° to 11.70°.

2. The hybrid device for a laser device according to claim 1, characterized in that: The pump end collimator (2) comprises a pump source (21), a second optical fiber (22) and a third lens (23), wherein the second optical fiber (22) and the third lens (23) are sequentially arranged along the light emitting direction of the pump source (21), and the wavelength of the pump light emitted by the pump source (21) is 976 nm.

3. The hybrid device for a laser device according to claim 1, characterized in that: The first optical fiber (12) is a polarization-maintaining double-clad optical fiber doped with rare earth elements, the core diameter of the first optical fiber (12) is 25 μm, the inner cladding diameter of the first optical fiber (12) is 250 μm, the numerical aperture of the first optical fiber (12) is 0.065, and the transmission wavelength of the first optical fiber (12) is 1064 nm.

4. The hybrid device for a laser device according to claim 2, characterized in that: The second optical fiber (22) is a multimode optical fiber, the core diameter of the second optical fiber (22) is 135 μm, the numerical aperture of the second optical fiber (22) is 0.22, and the transmission wavelength of the second optical fiber (22) is 976 nm.

5. The hybrid device for a laser device according to claim 2, characterized in that: The beam expansion unit (5) comprises a fourth lens (51) and a fifth lens (52), and the fourth lens (51) and the fifth lens (52) are arranged in sequence in the emission direction of the signal light.

6. The hybrid device for a laser device according to claim 5, characterized in that: The focal length ratio of the fifth lens (52) and the fourth lens (51) satisfies: -5.5≤f5 / f4≤-3.5 Wherein, f4 represents the focal length of the fourth lens (51), and f5 represents the focal length of the fifth lens (52).

7. The hybrid device for a laser device according to claim 2, characterized in that: The focal length of the first lens (13) is 5.9 mm to 6.3 mm, the center thickness of the first lens (13) is 2.5 mm to 3.5 mm, the focal length of the second lens (14) is 4.2 mm to 4.3 mm, and the center thickness of the second lens (14) is 2.5 mm to 3.5 mm.

8. The hybrid device for a laser device according to claim 5, characterized in that: Both sides of the first lens (13) are coated with a broadband anti-reflection film including 976nm and 1064nm, both sides of the second lens (14) are coated with a broadband anti-reflection film including 976nm and 1064nm, both sides of the third lens (23) are coated with an anti-reflection film of the 976nm band, and both sides of the fourth lens (51) and the fifth lens (52) are coated with an anti-reflection film of 1064nm.

9. The hybrid device for a laser device according to claim 5, characterized in that: The first lens (13) is a plano-convex lens or an aspheric lens, the second lens (14) is an aspheric lens, the third lens (23) is a cone lens, the fourth lens (51) is a plano-concave lens or a meniscus lens, and the fifth lens (52) is a plano-convex lens or a biconvex lens.

10. The hybrid device for a laser device according to claim 5, characterized in that: The distance between the second lens (14) and the fourth lens (51) along the optical axis direction is 45 mm to 55 mm.

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