Single frequency short pulse cavity dumping ring cavity laser and method

Abstract

The present invention belongs to the field of laser technology, and provides a single-frequency short-pulse cavity-dumped ring cavity laser and method, including a pump source and a unidirectional ring cavity, wherein the pump source is used to generate pump light, which is coupled into a low-gain crystal through a dichroic mirror; the unidirectional ring cavity comprises a dichroic mirror, a low-gain crystal, a first reflector, a second reflector, an optical rotator, a half-wave plate, a Pockels cell, a thin-film polarizer, a birefringent filter and a third reflector arranged in sequence, wherein the dichroic mirror is used to couple the pump light and the laser in the reflecting cavity, the optical rotator and the half-wave plate are used to realize the unidirectional operation of the laser in the ring cavity, an electro-optical crystal is arranged inside the Pockels cell, the thin-film polarizer is used to output light in a predetermined polarization state, and the birefringent filter is used to obtain laser light of a specified wavelength. The present invention solves the problem that it is difficult to realize short-pulse single-frequency laser output using a low-gain crystal.

Description

Technical Field

[0001] The present invention belongs to the field of laser technology, and specifically relates to a single-frequency short-pulse cavity-dumped ring cavity laser and a method, and in particular to a single-frequency short-pulse cavity-dumped ring cavity laser and a method based on a low-gain crystal. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Single-frequency, short-pulse lasers with pulse widths less than 10ns have the characteristics of high temporal and spatial resolution and have important applications in atmospheric lidar, gravitational wave detection, and high-resolution laser spectroscopy. Pulse width and spectral line width are two important parameters of such single-frequency pulse lasers. Due to the Fourier transform limit, the product of the two will not be less than a fixed value. Under the premise of ensuring the single-frequency output of the laser, further shortening the pulse width will improve the temporal resolution and peak power of the laser, thereby improving the detection accuracy.

[0004] The method to realize the above-mentioned single-frequency short-pulse laser is usually to introduce active Q-switching or passive Q-switching devices or apparatuses into the single-frequency laser structure (such as inserting an etalon or volume Bragg grating into the cavity, unidirectional ring cavity method, twisted cavity mode method and seed injection method, etc.), thereby realizing single-frequency pulse laser output. For crystals with different gain characteristics, the laser pulse width that can be obtained using the previous single-frequency pulse laser scheme is different. For high-gain crystals, such as Nd:YAG and Nd:YVO4, due to their large stimulated emission cross-section (the stimulated emission cross-section of Nd:YAG is 2.8×10 -19 cm 2 , the stimulated emission cross section of Nd:YVO4 is 25×10 -19 cm 2 ), the pulse width of the single-frequency pulse laser obtained by the above method can reach the order of 10ns. However, for low-gain crystals, such as alexandrite crystals, Tm:YAG crystals, Er:CaF2 crystals, etc., the laser pulse width that can be obtained by active or passive Q-switching of these crystals is generally in the order of hundreds of nanoseconds, which is difficult to be compared with high-gain crystals, and therefore does not meet the needs of practical applications; the reason is that the stimulated emission cross-section of these low-gain crystals is generally small (for example, the stimulated emission cross-section of alexandrite crystal is 7×10 -21 cm 2 The stimulated emission cross section of Tm:YAG is 2×10 -21 cm 2 , the stimulated emission cross section of Er:CaF2 is 2×10 -21 cm 2), the crystal gain is small. But it is worth noting that such low-gain crystals mostly work in special laser bands, such as alexandrite crystals working in the 700-800nm ​​band, Tm:YAG working in the 2-micron band, Er:CaF2 working in the 3-micron band, etc. These laser bands play an irreplaceable and key role in applications such as atmospheric lidar and high-resolution spectroscopy. Therefore, for such low-gain crystals working at special wavelengths, it is of great significance to solve the problem of achieving short pulse width and single-frequency laser output with a pulse width of less than 10ns. Summary of the invention

[0005] In order to solve the above problems, the present invention proposes a single-frequency short-pulse cavity-dumped ring cavity laser and method. The present invention utilizes a unidirectional ring cavity structure to eliminate the spatial hole burning effect, so that the laser can achieve single-frequency output; at the same time, combined with the cavity dumping scheme, short-pulse laser output with a smaller pulse width is achieved, effectively solving the problem that it is difficult to achieve short-pulse single-frequency laser output using low-gain crystals.

[0006] According to some embodiments, the present invention adopts the following technical solutions:

[0007] A single-frequency short-pulse cavity-dumped ring cavity laser comprises a pump source and a unidirectional ring cavity, wherein:

[0008] The pump source is used to generate pump light, which is coupled into the low-gain crystal through a dichroic mirror;

[0009] The unidirectional annular cavity comprises a dichroic mirror, a low-gain crystal, a first reflector, a second reflector, an optical rotator, a half-wave plate, a Pockels cell, a thin-film polarizer, a birefringent filter and a third reflector which are arranged in sequence; the dichroic mirror is used to couple pump light and reflect the laser in the cavity, the optical rotator and the half-wave plate are used to realize the unidirectional operation of the laser in the annular cavity, an electro-optical crystal is arranged inside the Pockels cell, the thin-film polarizer is used to output light of a predetermined polarization state, and the birefringent filter is used to obtain laser light of a specified wavelength; the first reflector, the second reflector and the third reflector are used to reflect the laser in the annular cavity; the laser operates unidirectionally in the annular cavity formed by the dichroic mirror, the first reflector, the second reflector and the third reflector, and output is realized at the thin-film polarizer.

[0010] As an optional implementation, the pump source is a laser light source, a light emitting diode or a laser diode.

[0011] As an optional implementation, the dichroic mirror is coated with an anti-reflection film for the pump light band and a high-reflection film for the laser band, and is used to couple the pump light and reflect the laser.

[0012] As a further feature, the dichroic mirror is a plano-concave lens with a certain curvature, and the incident angle is 0-10°.

[0013] As an optional implementation, the first reflector is coated with a high-reflection film for the laser band to reflect the laser, and the first reflector is a plano-concave lens with a certain curvature, and the incident angle is 0-10°;

[0014] The second reflector is arranged at the front end of the optical rotator, and the second reflector is a parallel plane mirror, and the incident angle is 0-10°.

[0015] As an optional implementation, the third reflector is coated with a high-reflection film for the laser wavelength band, and the third reflector is a parallel plane mirror with an incident angle of 0-10°.

[0016] As an optional implementation, the low-gain crystal is a crystal whose stimulated emission cross section is smaller than a set value and whose crystal gain is smaller than a set threshold.

[0017] As an optional implementation, the optical rotator includes a magneto-optical crystal and a permanent magnet. Permanent magnets are arranged on both sides of the magneto-optical crystal. The surface of the magneto-optical crystal is coated with an anti-reflection film for the laser band. When the linearly polarized laser passes through the optical rotator in the forward or reverse direction, its polarization state direction is rotated 45° in the same direction.

[0018] As an optional implementation, the half-wave plate is coated with an anti-reflection film for the laser wavelength band, and rotating the half-wave plate can change the polarization direction of the linearly polarized laser.

[0019] As an optional implementation, the inside of the Pockels cell is an electro-optical crystal, and the end faces of the electro-optical crystal are coated with an anti-reflection film for the laser band. When a certain high voltage is applied to both ends of the electro-optical crystal, the polarization state of the laser changes after passing through the Pockels cell during the duration of the high voltage.

[0020] As an optional implementation, the surface of the thin-film polarizer is coated with an anti-reflection film for the laser wavelength. When the laser is incident at the Brewster angle, the p-polarization state component is transmitted and the s-polarization state component is reflected.

[0021] As an optional implementation, the birefringent filter is a single piece or a combination of multiple pieces, and the wavelength tunable output is achieved by rotating the optical axis of the birefringent filter.

[0022] The working method of the cavity dumping ring cavity laser comprises the following steps:

[0023] In the first stage, no voltage is applied to the two ends of the Pockels cell, and the laser runs unidirectionally in the ring cavity and stores energy;

[0024] The second stage: a half-wave voltage is applied to both ends of the Pockels cell. After passing through the Pockels cell, the polarization state of the laser light in the forward unidirectional direction in the ring cavity changes from p-polarization to s-polarization. After passing through the thin-film polarizer, the s-polarized laser light is reflected to form cavity emptying output.

[0025] The laser in the cavity is incident on the birefringent filter at the Brewster angle, and laser components of different wavelengths produce birefringence in the birefringent filter. The ordinary light o and the extraordinary light e emitted from the birefringent filter produce a certain phase difference. When the phase difference is an integer multiple of 2π, the corresponding wavelength can completely pass through the birefringent filter. By rotating the birefringent filter, the completely transmitted wavelength can be changed, while other wavelengths have large losses in the cavity and cannot be output, that is, wavelength tunable output is achieved.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The present invention targets low-gain crystals and adopts a laser structure of a cavity-dumped ring cavity, wherein the unidirectional ring cavity eliminates the spatial hole burning effect and realizes single-frequency laser output, while utilizing cavity dumping to obtain pulsed laser output; since the laser pulse width obtained by the cavity dumping scheme is only related to the cavity length, the influence of the small stimulated emission cross-section of the low-gain crystal on the output laser pulse width is effectively avoided, thereby solving the problem that it is difficult for low-gain crystals to achieve short-pulse single-frequency laser output.

[0028] In addition, most low-gain crystals work in special laser bands. The laser provided by the present invention supplements the single-frequency short-pulse laser in the special band and can achieve a wide range of wavelength tunable output using a birefringent filter.

[0029] The laser provided by the present invention is of a fully solid-state structure, and the generated laser beam has good quality, good robustness, strong stability and is easy to maintain.

[0030] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0032] Figure 1 The overall structure diagram of a single-frequency short-pulse cavity-dumped ring cavity laser based on a low-gain crystal in one embodiment;

[0033] Among them, 1. pump source, 2. dichroic mirror, 3. low gain crystal, 4. first reflector, 5. second reflector; 6. optical rotator, 7. half-wave plate, 8. Pockels cell, 9. thin film polarizer, 10. output laser, 11. birefringent filter, 12. third reflector. DETAILED DESCRIPTION

[0034] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0035] It should be noted that the following detailed descriptions are all illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0036] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0037] In the absence of conflict, the embodiments in this application and the features in the embodiments may be combined with each other.

[0038] Embodiment 1

[0039] A single-frequency short-pulse cavity-dumped ring cavity laser comprises a pump source, a dichroic mirror, a low-gain laser crystal, a first reflector, a second reflector, an optical rotator, a half-wave plate, a Pockels cell, a thin-film polarizer, a birefringent filter and a third reflector.

[0040] The pump source is one of the light sources such as laser, light emitting diode, laser diode, etc. The pump source excites the laser crystal to cause the inversion of the number of particles inside the laser crystal to generate laser output;

[0041] The dichroic mirror is coated with an anti-reflection film for the pump light band and a high-reflection film for the laser band, and is used to couple the pump light into the crystal and reflect the laser;

[0042] In some preferred embodiments, the dichroic mirror is a plano-concave lens with a curvature of 100 mm and an incident angle of 0-10°.

[0043] The laser runs unidirectionally in a ring cavity composed of a dichroic mirror, a first reflecting mirror, a second reflecting mirror and a third reflecting mirror, and is output at a thin-film polarizer.

[0044] The low-gain crystal is a general term for a class of crystals (such as alexandrite crystal, Tm:YAG, Er:CaF2, etc.), and its common characteristics are small stimulated emission cross section and small laser single-pass gain.

[0045] The first reflector, the second reflector and the third reflector are coated with a high-reflection film for the laser band, which reflects the laser and makes it oscillate in the resonant cavity; in some preferred embodiments, the first reflector is a plano-concave lens with a curvature of 100 mm and an incident angle of 0-10°, and the second reflector and the third reflector are parallel plane mirrors with an incident angle of 0-10°.

[0046] The optical rotator is composed of a magneto-optical crystal and a permanent magnet. The permanent magnets are arranged at both ends of the magneto-optical crystal. The surface of the magneto-optical crystal is coated with an anti-reflection film for the laser band. When the linearly polarized laser passes through the optical rotator in the forward or reverse direction, its polarization state direction is rotated 45 degrees in the same direction. The optical rotator and the half-wave plate are combined to realize the unidirectional operation of the laser in the ring cavity.

[0047] The half-wave plate is coated with an anti-reflection film for the laser band. Rotating the half-wave plate can change the polarization direction of the linearly polarized laser. The combination of the half-wave plate and the optical rotator can realize the unidirectional operation of the laser in the resonant cavity.

[0048] The Pockels cell contains electro-optic crystals such as BBO, KD*P, RTP, and LGS. The end faces of the electro-optic crystals are plated with anti-reflection films for the laser band. A certain high voltage is applied to both ends of the electro-optic crystal. During the duration of the high voltage, the polarization state of the laser changes after passing through the Pockels cell. Preferably, a high voltage corresponding to a half-wave effect is selected and applied to both sides of the electro-optic crystal. During the duration of the high voltage, the Pockels cell is equivalent to a half-wave plate.

[0049] The surface of the thin film polarizer is coated with an anti-reflection film for the laser wavelength. When the laser is incident at the Brewster angle, the p polarization state component is transmitted, while the s polarization state component is reflected. The thin film polarizer also serves as an output port for the cavity dumping laser.

[0050] The birefringent filter can achieve wavelength tuning for a crystal with a stimulated emission line width and can output laser light of a specified wavelength.

[0051] The working process of the laser of this embodiment is that the pump light hits the low-gain crystal through the dichroic mirror to reverse the number of particles in its upper energy level.

[0052] In the first stage, no voltage is applied to the two ends of the Pockels cell, and the laser operates unidirectionally in the ring cavity. The specific physical process is that when the laser is running forward, the laser in the p-polarized state passes through the optical rotator and the half-wave plate in turn, and the polarization state does not change. Then it passes through the Pockels cell without voltage, and the polarization state does not change. Then it passes through the thin-film polarizer and operates unidirectionally in the forward direction in the ring cavity. When the laser is running reversely, the laser in the p-polarized state passes through the Pockels cell without voltage, and the polarization state does not change. After the last pass through the half-wave plate and the optical rotator, the laser polarization state becomes s-polarization. S-polarized light cannot pass through the polarizer, so the laser reverse cannot operate normally. Since the four cavity mirrors are highly reflective to the laser band, the laser cannot be output, and the laser stores energy in the ring cavity.

[0053] In the second stage, a half-wave voltage is applied to both ends of the Pockels cell. After passing through the Pockels cell, the polarization state of the laser light in the forward unidirectional direction in the ring cavity changes from p polarization to s polarization. After passing through the thin-film polarizer, all the s-polarized laser light is reflected to form cavity dumping output. Since the pulse width of cavity dumping is related to the cavity length of the resonant cavity, short-pulse single-frequency laser output with a pulse width of less than 10ns can be achieved.

[0054] Embodiment 2

[0055] like Figure 1 As shown, taking alexandrite crystal as an example, this embodiment provides a cavity-dumped ring cavity laser based on alexandrite crystal, including a pump source 1, a dichroic mirror 2, a low-gain crystal 3, a first reflector 4, a second reflector 5, an optical rotator 6, a half-wave plate 7, a Pockels cell 8, a thin-film polarizer 9, an output laser 10, a birefringent filter 11 and a third reflector 12 arranged in sequence along the optical path.

[0056] In this embodiment, the pump source 1 is a red light semiconductor laser with a central wavelength of 638 nm, fiber-coupled output, a core diameter of 400 μm, and a numerical aperture of 0.22.

[0057] In this embodiment, the dichroic mirror 2 is a plano-concave lens with a curvature of 100 mm, coated with an anti-reflection film for the 638 nm band and a high-reflection film for the 700-800 nm band, and the incident angle is 0-10°.

[0058] In this embodiment, the low gain crystal 3 is an alexandrite crystal with a size of 4×4×10 mm. 3 , c-axis cutting, doping concentration is 0.2at.%.

[0059] In this embodiment, the first reflector 4 is a plano-concave lens with a curvature of 100 mm, coated with a high-reflection film for the 700-800 nm band, and the incident angle is 0-10°; the second reflector 5 is a parallel plane mirror, coated with a high-reflection film for the 700-800 nm band, and the incident angle is 0-10°.

[0060] In this embodiment, the effective aperture of the optical rotator 6 is 5 mm, and is coated with an anti-reflection film for the 700-800 nm band; in this embodiment, the half-wave plate 7 is coated with an anti-reflection film for the 750 nm band.

[0061] In this embodiment, the electro-optical crystal inside the Pockels cell is a BBO crystal, and the half-wave voltage corresponding to 750nm is about 3.2kV.

[0062] In this embodiment, the insertion angle of the thin film polarizer forms a Brewster angle with the light path.

[0063] In this embodiment, the birefringent filter is a combination of three pieces of 0.5 mm, 1 mm, and 2 mm.

[0064] In this embodiment, the pump light 1 is hit on the alexandrite crystal through the dichroic mirror 2. At the beginning, no voltage is applied to the two ends of the Pockels cell 8. Since there are optical rotators 6 and half-wave plates 7 in the ring cavity, the laser can only run in one direction in the ring cavity. A half-wave voltage is applied to the two ends of the Pockels cell 8. At this time, after the forward-running laser passes through the Pockels cell 8, the polarization state direction rotates 90°, from p-polarized light to s-polarized light. When passing through the thin-film polarizer 9, the output laser 10 is reflected. Due to the limitations of the unidirectional ring cavity and the cavity emptying structure, the characteristics of the output laser are single-frequency lasers with short pulse widths. Since the emission spectrum of the alexandrite crystal is relatively wide, by rotating the birefringent filter 11, tunable laser output of different wavelengths can be achieved.

[0065] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made by those skilled in the art within the spirit and principle of the present invention without creative labor shall be included in the protection scope of the present invention.

Claims

1. A method for operating a single-frequency short-pulse cavity-dumped ring cavity laser, characterized in that: The laser comprises a pump source and a unidirectional ring cavity, wherein: The pump source is used to generate pump light, which is coupled into the low-gain crystal through a dichroic mirror; The unidirectional ring cavity comprises a dichroic mirror, a low-gain crystal, a first reflector, a second reflector, an optical rotator, a half-wave plate, a Pockels cell, a thin-film polarizer, a birefringent filter and a third reflector which are arranged in sequence; the dichroic mirror is used to couple pump light and reflect the laser in the cavity, the optical rotator and the half-wave plate are used to realize the unidirectional operation of the laser in the ring cavity, an electro-optical crystal is arranged inside the Pockels cell, the thin-film polarizer is used to output light of a predetermined polarization state, and the birefringent filter is used to obtain laser light of a specified wavelength; the first reflector, the second reflector and the third reflector are used to reflect the laser in the ring cavity; the laser operates unidirectionally in the ring cavity formed by the dichroic mirror, the first reflector, the second reflector and the third reflector, and output is realized at the thin-film polarizer; The working method comprises the following steps: In the first stage, no voltage is applied to the two ends of the Pockels cell, and the laser runs unidirectionally in the ring cavity and stores energy; The second stage: a half-wave voltage is applied to both ends of the Pockels cell. After passing through the Pockels cell, the polarization state of the laser light running in the forward unidirectional direction in the ring cavity changes from p-polarization to s-polarization. After passing through the thin-film polarizer, the s-polarized laser light is reflected to form a cavity-dumped output. Due to the limitations of the unidirectional ring cavity and the cavity-dumped structure, the output laser is characterized by a single-frequency laser with a short pulse width.

2. The operating method of a single-frequency short-pulse cavity dumped ring cavity laser as claimed in claim 1, characterized in that: The pump source is a laser light source, a light emitting diode or a laser diode.

3. The operating method of a single-frequency short-pulse cavity dumped ring cavity laser as claimed in claim 1, characterized in that: The dichroic mirror is coated with an anti-reflection film for the pump light band and a high-reflection film for the laser band, and is used to couple the pump light and reflect the laser. The dichroic mirror is a plano-concave lens with a certain curvature, and the incident angle is 0-10°. The first reflector is coated with a high-reflection film for the laser band to reflect the laser. The first reflector is a plano-concave lens with a certain curvature, and the incident angle is 0-10°; The second reflector is coated with a high-reflection film for the laser band to reflect the laser. The second reflector is a parallel plane mirror with an incident angle of 0-10°; The third reflector is plated with a high-reflection film for the laser wavelength band to reflect the laser. The third reflector is a parallel plane mirror with an incident angle of 0-10°.

4. The operating method of a single-frequency short-pulse cavity dumped ring cavity laser as claimed in claim 1, characterized in that: The low-gain crystal is a crystal whose stimulated emission cross section is smaller than a set value and whose crystal gain is smaller than a set threshold value.

5. The operating method of a single-frequency short-pulse cavity dumped ring cavity laser as claimed in claim 1, characterized in that: The optical rotator comprises a magneto-optical crystal and a permanent magnet. Permanent magnets are arranged on both sides of the magneto-optical crystal. The surface of the magneto-optical crystal is coated with an anti-reflection film for the laser band. When the linearly polarized laser passes through the optical rotator in the forward or reverse direction, the polarization state direction thereof is rotated 45 degrees in the same direction.

6. The operating method of a single-frequency short-pulse cavity dumped ring cavity laser as claimed in claim 1, characterized in that: The half-wave plate is coated with an anti-reflection film for the laser band, and rotating the half-wave plate can change the polarization state direction of the linearly polarized laser.

7. The operating method of a single-frequency short-pulse cavity dumped ring cavity laser as claimed in claim 1, characterized in that: The Pockels cell contains an electro-optical crystal, and both end faces of the electro-optical crystal are coated with an anti-reflection film for the laser band. When a certain high voltage is applied to both ends of the electro-optical crystal, the polarization state of the laser changes after passing through the Pockels cell during the duration of the high voltage.

8. The operating method of a single-frequency short-pulse cavity dumped ring cavity laser as claimed in claim 1, characterized in that: The surface of the thin-film polarizer is coated with an anti-reflection film for the laser wavelength. When the laser is incident at the Brewster angle, the p-polarization state component is transmitted and the s-polarization state component is reflected.

9. The operating method of a single-frequency short-pulse cavity dumped ring cavity laser as claimed in claim 1, characterized in that: The birefringent filter is a single piece or a combination of multiple pieces. Rotating the birefringent filter can achieve wavelength tunable output.

Images