Narrow-linewidth vertical external cavity surface emitting laser using single-frequency pumping

Abstract

The invention belongs to the technical field of laser equipment, and particularly discloses a narrow linewidth vertical external cavity surface emitting laser utilizing single-frequency pumping, which comprises a single-frequency pumping system and a narrow linewidth laser system, and is characterized in that the single-frequency pumping system comprises a pumping unit, a gain medium, a polarization and wavelength tuning element, a folding mirror, a frequency conversion crystal and a single-frequency pumping light output mirror; the narrow linewidth laser system comprises a semiconductor laser chip and a coupling output mirror. The single-frequency pumping system controls a laser longitudinal mode in a pumping resonant cavity by utilizing second harmonic self-suppression in a frequency conversion crystal, and generates high-power tunable single-frequency pumping light with controllable polarization in combination with polarization control and wavelength tuning characteristics of a polarization and wavelength tuning element. The single-frequency pump light is used as a pumping source of the vertical external cavity surface emitting laser, spontaneous radiation in a cavity in a laser system is inhibited, quantum noise of the laser is reduced, and narrow linewidth output can be obtained in a long laser resonant cavity without inserting a linewidth narrowing element.

Description

Technical Field

[0001] The invention belongs to the technical field of laser equipment, and in particular relates to a narrow-linewidth vertical external cavity surface emitting laser utilizing single-frequency pumping. Background Art

[0002] Narrow linewidth lasers have excellent optical properties such as narrow laser linewidth, long coherence length, and ultra-low phase noise. They have very important application value in coherent optical communications, lidar, atomic sensing, precision spectroscopy, and atomic cooling. In recent years, they have been at the forefront of research in the field of laser technology.

[0003] As a new type of semiconductor laser, the emission wavelength of the vertical external cavity surface emitting laser can be designed according to the energy band engineering. The wavelength range covers visible light to near infrared. By placing mode control elements and wavelength tuning elements in the high-precision external cavity structure, continuously tunable single-frequency output can be achieved. It has great application potential in the fields of atomic and molecular physics and high-precision spectroscopy. However, since the current pumping method of this laser mainly adopts multi-mode laser diode pumping, under normal circumstances, the laser usually outputs multiple longitudinal modes. In order to achieve single-frequency output, it is often necessary to shorten the length of the laser resonant cavity or place a mode control element in the resonant cavity. Since the line width of the laser is inversely proportional to the length of the laser resonant cavity, and the filtering accuracy of the mode control element is limited, the output line width of this type of laser is not narrow enough without active frequency stabilization. Summary of the invention

[0004] The purpose of the present invention is to provide a narrow linewidth vertical external cavity surface emitting laser using single-frequency pumping, so as to solve the problem that the vertical external cavity surface emitting laser is not narrow enough in output linewidth due to the influence of the laser resonant cavity length and the filtering accuracy of the intracavity mode control element in achieving single frequency under the existing multi-mode laser diode pumping.

[0005] In order to achieve the above-mentioned object, the technical solution of the present invention is: a narrow-linewidth vertical external cavity surface emitting laser using single-frequency pumping, comprising a single-frequency pumping system and a narrow-linewidth laser system, wherein the single-frequency pumping system is used to generate single-frequency pumping light and emit it to the narrow-linewidth laser system; the narrow-linewidth laser system is used to receive the single-frequency pumping light and form narrow-linewidth output light;

[0006] The single-frequency pumping system comprises a pumping unit, a gain medium, a polarization and wavelength tuning element, a folding mirror, a frequency conversion crystal and a single-frequency pumping light output mirror; the pumping unit is used to emit primary pumping light to the gain medium; the gain medium is used to absorb the energy of the primary pumping light and generate first stimulated radiation, and reflect the first stimulated radiation to the folding mirror; the folding mirror is used to reflect the first stimulated radiation to the single-frequency pumping light output mirror; a pumping resonant cavity is formed between the gain medium, the folding mirror and the single-frequency pumping light output mirror; the first stimulated radiation is The pump resonant cavity forms fundamental frequency light oscillation; the polarization and wavelength tuning element is located between the gain medium and the folding mirror, and the polarization and wavelength tuning element is used to control the polarization state of the fundamental frequency light and tune the wavelength of the fundamental frequency light; the frequency conversion crystal is located between the folding mirror and the single-frequency pump light output mirror, and the frequency conversion crystal can convert the fundamental frequency light into double frequency light; the frequency conversion crystal has the second harmonic self-suppression characteristic, and can suppress the laser longitudinal mode in the pump resonant cavity; the single-frequency pump light output mirror is used to output the single-frequency pump light to the narrow linewidth laser system;

[0007] The narrow linewidth laser system includes a semiconductor laser chip and a coupling output mirror. The semiconductor laser chip is used to absorb the energy of the single-frequency pump light and generate second stimulated radiation, and reflect the second stimulated radiation to the coupling output mirror. A laser resonant cavity is formed between the semiconductor laser chip and the coupling output mirror. The second stimulated radiation forms narrow linewidth output light under the action of the laser resonant cavity.

[0008] Furthermore, the semiconductor laser chip includes a reflective layer, an active layer and a protective layer arranged in sequence, the reflective layer is used to reflect the second stimulated radiation, the active layer can achieve light amplification of the second stimulated radiation, and the protective layer is used to prevent the semiconductor laser chip from being oxidized.

[0009] Furthermore, the pump unit includes a pump source, and the pump source is used to emit primary pump light.

[0010] Furthermore, the pump unit further comprises a first collimating and focusing lens, and the primary pump light is incident on the gain medium after being collimated and focused by the first collimating and focusing lens.

[0011] Furthermore, a second collimating and focusing lens is provided between the single-frequency pump light output mirror and the semiconductor laser chip, and the single-frequency pump light is incident on the semiconductor laser chip after being collimated and focused by the second collimating and focusing lens.

[0012] Furthermore, the gain medium is mounted on a first heat sink, and the semiconductor laser chip is mounted on a second heat sink.

[0013] Furthermore, the gain medium is made of solid gain material or semiconductor gain material.

[0014] Furthermore, the polarization and wavelength tuning element is placed at the Brewster angle.

[0015] Furthermore, the polarization and wavelength tuning element can be rotated with the normal line of the plane where the element is located as an axis, so as to tune the wavelength of the fundamental frequency light.

[0016] Furthermore, the folding mirror is coated with a first high-reflection film layer that reflects the wavelength of the fundamental frequency light and the wavelength of the doubled frequency light; the single-frequency pump light output mirror is coated with a second high-reflection film layer that reflects the wavelength of the fundamental frequency light and a first high-transmission film layer that transmits the wavelength of the doubled frequency light; and the frequency conversion crystal is coated with a second high-transmission film layer that transmits the wavelength of the fundamental frequency light and the wavelength of the doubled frequency light.

[0017] The working principle of this technical solution is that the gain medium absorbs the energy of the primary pump light and generates the first stimulated radiation. The first stimulated radiation forms a fundamental frequency light oscillation under the action of the pump resonant cavity formed by the gain medium, the folding mirror, and the single-frequency pump light output mirror. Then, a polarization and wavelength tuning element and a frequency conversion crystal are placed in the pump resonant cavity, which respectively control the polarization state of the fundamental frequency light, tune the output wavelength of the fundamental frequency light, realize the frequency conversion between the fundamental frequency light and the doubled frequency light, and suppress the longitudinal mode in the pump resonant cavity, thereby obtaining a high-power tunable single-frequency pump light with controllable polarization.

[0018] On this basis, the single-frequency pump light is collimated and focused by the second collimating and focusing lens and incident on the semiconductor laser chip. The semiconductor laser chip generates the second stimulated radiation by absorbing the energy of the single-frequency pump light, and the second stimulated radiation is optically amplified by the active layer in the semiconductor laser chip. The amplified second stimulated radiation forms laser oscillation between the reflective layer and the coupling output mirror of the semiconductor laser chip, thereby achieving narrow linewidth output light in a longer laser resonant cavity without inserting any linewidth narrowing element.

[0019] The beneficial effect of this technical solution is that the longitudinal mode of the laser in the pump resonant cavity is effectively controlled by using the second harmonic self-suppression in the frequency conversion crystal, and the polarization control and wavelength tuning characteristics of the polarization and wavelength tuning elements are combined to generate polarization-controllable high-power tunable single-frequency pump light. By using this single-frequency pump light as the pump source of the vertical external cavity surface emitting laser, the spontaneous radiation in the laser resonant cavity is suppressed, and the quantum noise of the laser is reduced from the source, so that in a longer laser resonant cavity, it is possible to obtain narrow linewidth output light without inserting any linewidth narrowing element. The present invention can solve the problem that the vertical external cavity surface emitting laser is affected by the length of the laser resonant cavity and the filtering accuracy of the intracavity mode control element in achieving single frequency, resulting in its output linewidth not being narrow enough, and promote the application of narrow linewidth vertical external cavity surface emitting lasers in precision spectroscopy and atomic cooling and other application fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic structural diagram of a narrow linewidth vertical external cavity surface emitting laser using single frequency pumping according to the present invention;

[0021] Figure 2 This is a structural diagram of a semiconductor laser chip;

[0022] Figure 3 is a structural diagram of a second collimating and focusing lens;

[0023] Figure 4 is the output power diagram of single-frequency pump light;

[0024] Figure 5 This is the output linewidth diagram of single-frequency pump light. DETAILED DESCRIPTION

[0025] The following is further described in detail through specific implementation methods:

[0026] The figure marks in the drawings of the specification include: pump source 1, first collimating and focusing lens 2, first heat sink 3, gain medium 4, polarization and wavelength tuning element 5, folding mirror 6, frequency conversion crystal 7, single-frequency pump light output mirror 8, single-frequency pump light 9, second collimating and focusing lens 10, semiconductor laser chip 11, second heat sink 12, coupling output mirror 13, narrow linewidth output light 14, reflection layer 15, active layer 16, protective layer 17, glass material 18, third high-transmittance film layer 19.

[0027] The following will be combined with the drawings in 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.

[0028] The embodiment is basically as shown in the attached Figure 1-5 As shown: A narrow linewidth vertical external cavity surface emitting laser using single frequency pumping, such as Figure 1 As shown, it includes a single-frequency pumping system and a narrow-linewidth laser system. The single-frequency pumping system is used to generate a single-frequency pumping light 9 and emit it to the narrow-linewidth laser system; the narrow-linewidth laser system is used to receive the single-frequency pumping light 9 and form a narrow-linewidth output light 14;

[0029] The single-frequency pumping system includes a pumping unit, a gain medium 4, a polarization and wavelength tuning element 5, a folding mirror 6, a frequency conversion crystal 7 and a single-frequency pumping light output mirror 8. The pumping unit is used to emit primary pumping light to the gain medium 4; the pumping unit includes a pumping source 1 and two sets of first collimating and focusing lenses 2, the pumping source 1 is used to emit primary pumping light, and the pumping source 1 is a semiconductor laser diode with a wavelength smaller than the wavelength of the laser of the single-frequency pumping system. The primary pumping light is collimated and focused by the two sets of first collimating and focusing lenses 2 and then incident on the gain medium 4.

[0030] The gain medium 4 is made of solid gain material or semiconductor gain material. The gain medium 4 is installed on the hollow first heat sink 3. The gain medium 4 is used to absorb the energy of the primary pump light and generate the first stimulated radiation, and reflect the first stimulated radiation to the folding mirror 6; the folding mirror 6 is used to reflect the first stimulated radiation to the single-frequency pump light output mirror 8. The folding mirror 6 is plated with a first high-reflection film layer that reflects the wavelength of the fundamental light and the wavelength of the doubled frequency light. A pump resonant cavity is formed between the gain medium 4, the folding mirror 6 and the single-frequency pump light output mirror 8; the first stimulated radiation forms a fundamental frequency light oscillation under the action of the pump resonant cavity.

[0031] The polarization and wavelength tuning element 5 is located between the gain medium 4 and the folding mirror 6. The polarization and wavelength tuning element 5 is used to control the polarization state of the fundamental frequency light and tune the wavelength of the fundamental frequency light. Specifically, the polarization and wavelength tuning element 5 is placed at the Brewster angle to control the polarization state of the fundamental frequency light. The polarization and wavelength tuning element 5 can rotate with the normal of the plane where it is located as the axis to tune the wavelength of the fundamental frequency light.

[0032] The frequency conversion crystal 7 is located between the folding mirror 6 and the single-frequency pump light output mirror 8. The frequency conversion crystal 7 can convert the fundamental frequency light into the double frequency light. The frequency conversion crystal 7 has the second harmonic self-suppression characteristic and can suppress the laser longitudinal mode in the pump resonant cavity. The two sides of the frequency conversion crystal 7 are respectively coated with a second high-transmittance film layer that transmits the wavelength of the fundamental frequency light and the wavelength of the double frequency light. The single-frequency pump light output mirror 8 is used to output the single-frequency pump light 9 to the narrow linewidth laser system. The single-frequency pump light output mirror 8 is coated with a second high-reflection film layer that reflects the wavelength of the fundamental frequency light and a first high-transmittance film layer that transmits the wavelength of the double frequency light.

[0033] The narrow linewidth laser system includes a semiconductor laser chip 11, a second collimating and focusing lens 10 and a coupling output mirror 13. The second collimating and focusing lens 10 is arranged between the single-frequency pump light output mirror 8 and the semiconductor laser chip 11. The single-frequency pump light 9 is collimated and focused by the second collimating and focusing lens 10 and then incident on the semiconductor laser chip 11. The structure of the second collimating and focusing lens 10 is as follows: Figure 3 As shown, it is made of glass material 18, and the two end surfaces are coated with a third high-transmittance film layer 19 that transmits the wavelength of the single-frequency pump light 9. The semiconductor laser chip 11 is mounted on the second heat sink 12. The semiconductor laser chip 11 is used to absorb the energy of the single-frequency pump light 9 and generate second stimulated radiation, and reflect the second stimulated radiation to the coupling output mirror 13. A laser resonant cavity is formed between the semiconductor laser chip 11 and the coupling output mirror 13; the second stimulated radiation forms a narrow line width output light 14 under the action of the laser resonant cavity.

[0034] like Figure 2 As shown, the semiconductor laser chip 11 includes a reflective layer 15, an active layer 16 and a protective layer 17 arranged in sequence, wherein the reflective layer 15 is used to reflect the second stimulated radiation. The active layer 16 can realize light amplification of the second stimulated radiation, and the active layer 16 is a semiconductor material and has a multi-quantum well structure. The protective layer 17 is used to prevent the semiconductor laser chip 11 from being oxidized.

[0035] The specific implementation process is as follows:

[0036] The gain medium 4 absorbs the energy of the primary pump light and generates the first stimulated radiation. The first stimulated radiation forms a fundamental frequency light oscillation under the action of the pump resonant cavity formed by the gain medium 4, the folding mirror 6, and the single-frequency pump light output mirror 8. Then, a polarization and wavelength tuning element 5 and a frequency conversion crystal 7 are placed in the pump resonant cavity, which respectively control the polarization state of the fundamental frequency light, tune the output wavelength of the fundamental frequency light, realize the frequency conversion between the fundamental frequency light and the doubled frequency light, and suppress the longitudinal mode in the pump resonant cavity, thereby obtaining a high-power tunable single-frequency pump light 9 with controllable polarization.

[0037] Subsequently, the single-frequency pump light 9 is collimated and focused by the second collimating and focusing lens 10 and incident on the semiconductor laser chip 11. The semiconductor laser chip 11 generates the second stimulated radiation by absorbing the energy of the single-frequency pump light 9, and the second stimulated radiation is optically amplified by the active layer 16 in the semiconductor laser chip 11. The amplified second stimulated radiation forms laser oscillation between the reflective layer 15 of the semiconductor laser chip 11 and the coupling output mirror 13, thereby achieving narrow linewidth output light 14 in a longer laser resonant cavity without inserting any linewidth narrowing element.

[0038] The following is a specific example to illustrate:

[0039] The pump source 1 is a semiconductor laser diode with an operating wavelength of 808 nm, and the gain medium 4 is an ytterbium-doped solid gain material or a semiconductor gain material with an output wavelength of 1020 nm. The folding mirror 6 is a plano-concave reflector with a curvature radius of 100 mm, and the concave surface is coated with a first high-reflection film layer with high reflectivity at wavelengths of 1020 nm and 510 nm. The single-frequency pump light output mirror 8 is a plano-concave reflector with a curvature radius of 50 mm, and its concave reflective surface is coated with a high-reflectivity film layer (second high-reflection film layer) and a high-transmittance film layer (first high-transmittance film layer) at wavelengths of 1020 nm and 510 nm, respectively, and the plane surface will also be coated with a film layer with high transmittance at a wavelength of 510 nm (first high-transmittance film layer), which is used for the output of 510 nm doubled frequency light (single-frequency pump light).

[0040] The polarization and wavelength tuning element 5 is a birefringent filter with a thickness of 1 mm, which is placed at the Brewster angle. At this time, the 1020nm fundamental frequency light is in a horizontal polarization state. The polarization and wavelength tuning element 5 is rotated around the normal of the plane in which it is located as the axis to tune the wavelength of the fundamental frequency light. The frequency conversion crystal 7 is a nonlinear frequency doubling crystal, which is located between the folding mirror 6 and the single-frequency pump light output mirror 8. When the phase matching is satisfied, the frequency conversion crystal 7 will realize the conversion of the laser wavelength from 1020nm to 510nm. At this time, the 510nm frequency doubling light is in a vertical polarization state. In addition, the second harmonic self-suppression characteristics of the frequency conversion crystal 7 can be used to suppress the longitudinal mode of the laser in the pump resonant cavity. Combined with the polarization control and wavelength tuning characteristics of the polarization and wavelength tuning element 5, a polarization-controllable high-power tunable 510nm single-frequency pump light 9 can be generated, and its output power and linewidth are respectively as follows: Figure 4 and Figure 5 As shown, in the single-frequency output state, the laser power and Lorentz linewidth can reach 5.37 W and 161.4 Hz respectively.

[0041] Subsequently, a single-chip focusing lens is used to focus the 510nm single-frequency pump light 9, and the light is incident on a semiconductor laser chip 11 with an output wavelength of 852nm. The semiconductor laser chip 11 is cut into a size of 4mm*4mm*10μm and welded on a second heat sink 12. The reflective layer 15 of the semiconductor laser chip 11 is composed of 30 pairs of GaAs / AlAs materials, the active layer 16 is composed of 10 pairs of InGaAs / GaAs materials, and the protective layer 17 is made of GaAs material. Due to the use of single-frequency laser pumping, spontaneous radiation in the laser resonant cavity can be suppressed, and the quantum noise of the laser can be reduced, thereby achieving narrow linewidth output in the 852nm band.

[0042] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0043] The above is only an embodiment of the present invention. The common sense such as the known specific structure and characteristics in the scheme is not described in detail here. The ordinary technicians in the relevant field know all the common technical knowledge in the technical field of the invention before the application date or priority date, can obtain all the existing technologies in the field, and have the ability to apply the conventional experimental means before that date. The ordinary technicians in the relevant field can improve and implement this scheme in combination with their own abilities under the enlightenment given by this application. Some typical known structures or known methods should not become obstacles for ordinary technicians in the relevant field to implement this application. It should be pointed out that for those skilled in the art, without departing from the structure of the present invention, several deformations and improvements can be made, which should also be regarded as the protection scope of the present invention, which will not affect the effect of the implementation of the present invention and the practicality of the patent. The protection scope required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A narrow linewidth vertical external cavity surface emitting laser using single frequency pumping, characterized in that: It comprises a single-frequency pumping system and a narrow-linewidth laser system, wherein the single-frequency pumping system is used to generate single-frequency pumping light (9) and emit it to the narrow-linewidth laser system; the narrow-linewidth laser system is used to receive the single-frequency pumping light (9) and form narrow-linewidth output light (14); The single-frequency pumping system comprises a pumping unit, a gain medium (4), a polarization and wavelength tuning element (5), a folding mirror (6), a frequency conversion crystal (7) and a single-frequency pumping light output mirror (8); the pumping unit is used to emit primary pumping light to the gain medium (4); the gain medium (4) is used to absorb the energy of the primary pumping light and generate first stimulated radiation, and reflect the first stimulated radiation to the folding mirror (6); the folding mirror (6) is used to reflect the first stimulated radiation to the single-frequency pumping light output mirror (8); a pumping resonant cavity is formed between the gain medium (4), the folding mirror (6) and the single-frequency pumping light output mirror (8); The first stimulated radiation forms fundamental frequency light oscillation under the action of the pump resonant cavity; the polarization and wavelength tuning element (5) is located between the gain medium (4) and the folding mirror (6), and the polarization and wavelength tuning element (5) is used to control the polarization state of the fundamental frequency light and tune the wavelength of the fundamental frequency light; the frequency conversion crystal (7) is located between the folding mirror (6) and the single-frequency pump light output mirror (8), and the frequency conversion crystal (7) can convert the fundamental frequency light into double frequency light; the frequency conversion crystal (7) has a second harmonic self-suppression characteristic and can suppress the laser longitudinal mode in the pump resonant cavity; the single-frequency pump light output mirror (8) is used to output the single-frequency pump light (9) to the narrow linewidth laser system; The narrow linewidth laser system comprises a semiconductor laser chip (11) and a coupling output mirror (13); the semiconductor laser chip (11) is used to absorb the energy of the single-frequency pump light (9) and generate second stimulated radiation, and reflect the second stimulated radiation to the coupling output mirror (13); a laser resonant cavity is formed between the semiconductor laser chip (11) and the coupling output mirror (13); and the second stimulated radiation forms narrow linewidth output light (14) under the action of the laser resonant cavity.

2. The narrow linewidth vertical external cavity surface emitting laser using single frequency pumping according to claim 1, characterized in that: The semiconductor laser chip (11) comprises a reflective layer (15), an active layer (16) and a protective layer (17) which are arranged in sequence; the reflective layer (15) is used to reflect the second stimulated radiation; the active layer (16) can achieve light amplification for the second stimulated radiation; and the protective layer (17) is used to prevent the semiconductor laser chip (11) from being oxidized.

3. The narrow linewidth vertical external cavity surface emitting laser using single frequency pumping according to claim 1, characterized in that: The pump unit comprises a pump source (1), and the pump source (1) is used to emit primary pump light.

4. The narrow linewidth vertical external cavity surface emitting laser using single frequency pumping according to claim 3, characterized in that: The pump unit further comprises a first collimating and focusing lens (2), and the primary pump light is incident on the gain medium (4) after being collimated and focused by the first collimating and focusing lens (2).

5. The narrow linewidth vertical external cavity surface emitting laser using single frequency pumping according to claim 1, characterized in that: A second collimating and focusing lens (10) is also provided between the single-frequency pump light output mirror (8) and the semiconductor laser chip (11), and the single-frequency pump light (9) is incident on the semiconductor laser chip (11) after being collimated and focused by the second collimating and focusing lens (10).

6. The narrow linewidth vertical external cavity surface emitting laser using single frequency pumping according to claim 1, characterized in that: The gain medium (4) is mounted on a first heat sink (3), and the semiconductor laser chip (11) is mounted on a second heat sink (12).

7. The narrow linewidth vertical external cavity surface emitting laser using single frequency pumping according to claim 1, characterized in that: The gain medium (4) is made of solid gain material or semiconductor gain material.

8. The narrow linewidth vertical external cavity surface emitting laser using single frequency pumping according to claim 1, characterized in that: The polarization and wavelength tuning element (5) is placed at the Brewster angle.

9. The narrow linewidth vertical external cavity surface emitting laser using single frequency pumping according to claim 1, characterized in that: The polarization and wavelength tuning element (5) can be rotated with the normal line of the plane where it is located as an axis, so as to tune the wavelength of the fundamental frequency light.

10. The narrow linewidth vertical external cavity surface emitting laser using single frequency pumping according to claim 1, characterized in that: The folding mirror (6) is coated with a first high-reflection film layer that reflects the wavelength of the fundamental frequency light and the wavelength of the doubled frequency light; the single-frequency pump light output mirror (8) is coated with a second high-reflection film layer that reflects the wavelength of the fundamental frequency light and a first high-transmission film layer that transmits the wavelength of the doubled frequency light; and the frequency conversion crystal (7) is coated with a second high-transmission film layer that transmits the wavelength of the fundamental frequency light and the wavelength of the doubled frequency light.

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