An electro-optic Q-switch using an LN crystal as an alternative to Brewster angle cutting

Abstract

The present invention discloses an electro-optic Q-switch for replacing a LN crystal cut at the Brewster angle, which includes a prism, a first electro-optic crystal, and a second electro-optic crystal. The prism, the first electro-optic crystal, and the second electro-optic crystal are arranged in sequence along the light transmission direction. The working surface of the prism includes two light-transmitting surfaces and two reflecting surfaces. Anti-reflection films are coated on both light-transmitting surfaces, and high-reflection films are coated on the two reflecting surfaces according to whether total reflection occurs at the interface of the light. The first electro-optic crystal and the second electro-optic crystal adopt the transverse electro-optic effect, transmit light along the X-axis or Y-axis direction, and apply an electric field along the Z-axis. The main axes of the first electro-optic crystal and the second electro-optic crystal rotate 90° relative to each other around the light transmission direction, and the Z-axes of the first electro-optic crystal and the second electro-optic crystal form an angle of 45° with the normal direction of the incident surface. The present invention not only meets the requirement of generating a specific translation of the light in the laser cavity, but also effectively solves the problem of poor temperature stability of the traditional LN electro-optic Q-switch cut at the Brewster angle.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser devices, and more specifically, relates to an electro-optic Q-switch that replaces a LN crystal cut at the Brewster angle. Background Art

[0002] In the field of solid laser technology, the emergence and development of Q-switching technology is an important breakthrough in the history of laser development. Laser Q-switching technology is a technology that compresses laser energy into pulses with extremely narrow widths, thereby increasing the laser peak power by several orders of magnitude. On the one hand, high-peak-power Q-switched laser pulses interact with matter to produce a series of new phenomena and new technologies of great significance, directly promoting the development of nonlinear optics; on the other hand, Q-switched laser pulses with extremely narrow pulse widths have promoted the development of applications such as pulsed laser ranging, lidar, and high-speed holography. It can be said that laser Q-switching technology has become an important technology that cannot be replaced in many application fields.

[0003] An electro-optic Q-switch is the core component in electro-optic Q-switching technology, which is made by encapsulating certain crystals with electro-optic effects. Lithium niobate (LN) crystal is a traditional electro-optic crystal with excellent performance. For lasers with small single-pass gains in the near and mid-infrared regions, LN crystals cut at the Brewster angle are mostly used as electro-optic Q-switches. This kind of LN crystal cut at the Brewster angle transmits light along the optical axis direction and applies an electric field along the X axis; it simultaneously acts as a phase retarder and a polarizer in the laser, avoiding the loss caused by inserting redundant optical elements into the resonator; and the light-transmitting surface of the crystal does not need to be coated with an antireflection film because of the Brewster angle incidence, effectively reducing the processing cost, and was widely used in the initial stage of the development of electro-optic Q-switching technology. However, due to the inevitable stress and electro-optic inhomogeneity in the growth process of the LN crystal, there is a light leakage phenomenon in the switch under low-temperature conditions, which cannot meet the growing high and low temperature performance requirements of space lasers and military lasers.

[0004] Potassium titanyl phosphate (KTP) and rubidium titanyl phosphate (RTP) crystals are a new type of practical electro-optic crystal material developed in the 1980s. Experiments have verified that when a temperature compensation design using two KTP or RTP crystals of the same size is adopted, the influence of temperature on static birefringence can be offset, and it has high temperature stability in a wide temperature range. However, the optical path of the KTP and RTP electro-optic Q-switches in the laser cavity is different from that of the LN electro-optic Q-switch cut at the Brewster angle, and the incompatibility between the two results in the inability of the KTP and RTP electro-optic Q-switches to be applied to the mature laser resonator design scheme, which greatly increases the R & D cost.

[0005] Therefore, how to provide an electro-optic Q-switch that replaces a LN crystal cut at the Brewster angle is an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0006] Aiming at the deficiencies of the existing solutions, the purpose of the present invention is to propose a Q-switch for an LN electro-optic crystal that replaces Brewster angle cutting, which has a simple structure and is easy to adjust.

[0007] To achieve the above purpose, the present invention adopts the following technical solutions:

[0008] An electro-optic Q-switch for an LN crystal that replaces Brewster angle cutting, comprising: a prism, a first electro-optic crystal, and a second electro-optic crystal, wherein the prism, the first electro-optic crystal, and the second electro-optic crystal are arranged in sequence along the light passing direction; the working surfaces of the prism include two light passing surfaces and two reflecting surfaces, antireflection films are coated on both light passing surfaces, and high-reflection films are coated on the two reflecting surfaces according to whether total internal reflection will occur at the interface of the light;

[0009] The first electro-optic crystal and the second electro-optic crystal adopt the transverse electro-optic effect, pass light along the X-axis or Y-axis direction, and apply an electric field along the Z-axis;

[0010] The main axes of the first electro-optic crystal and the second electro-optic crystal rotate 90° relative to each other around the light passing direction, and the Z-axes of the first electro-optic crystal and the second electro-optic crystal form an angle of 45° with the normal direction of the incident surface.

[0011] Preferably, the prism is a rhombic prism, a right-angle prism, or an equilateral prism, and the number of prisms is one or more.

[0012] Preferably, the material of the prism is N-BK7 or fused silica crystal.

[0013] Preferably, the sizes of the first electro-optic crystal and the second electro-optic crystal are exactly the same.

[0014] Preferably, the first electro-optic crystal and the second electro-optic crystal are potassium titanyl phosphate crystals or rubidium titanyl phosphate crystals.

[0015] Preferably, the light passing aperture of the working surface of the prism is greater than or equal to the light passing aperture of the first electro-optic crystal or the second electro-optic crystal.

[0016] Preferably, the first electro-optic crystal or the second electro-optic crystal is processed into a square column shape according to the electro-optic application direction, the light passing surfaces are precisely optically polished, and the four side surfaces are finely ground.

[0017] Preferably, the incident light is P-polarized light.

[0018] The beneficial effects of the present invention are as follows:

[0019] (1) The present invention uses a combination of a prism, a first electro-optic crystal, and a second electro-optic crystal to form a new electro-optic Q-switch. After the incident light passes through this switch, it generates an equivalent translation to that of an LN electro-optic Q-switch cut by Brewster's angle.

[0020] (2) The electro-optic Q-switch proposed by the present invention adopts a double-crystal temperature compensation design, greatly improving the temperature adaptability of the device, and can meet the normal use of the laser within the range of -60°C to +80°C.

[0021] (3) By reasonably setting the sizes of the prism, the first electro-optic crystal, and the second electro-optic crystal, the electro-optic Q-switch proposed by the present invention can equivalently replace an LN electro-optic Q-switch cut by Brewster's angle of the same specification. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained according to the provided drawings.

[0023] Figure 1 The drawings are schematic structural diagrams of the present invention.

[0024] Figure 2 The drawings are structural diagrams of a laser using an LN electro-optic Q-switch cut by Brewster's angle.

[0025] Figure 3 The drawings are structural diagrams of a laser using the electro-optic Q-switch of the present invention.

[0026] Among them, in the drawings,

[0027] 11 - Prism; 12 - First electro-optic crystal; 13 - Second electro-optic crystal; 221 - Output mirror; 222 - Laser working substance; 223 - Polarizer; 214 - LN electro-optic Q-switch; 225 - Total reflection mirror; 224 - Electro-optic Q-switch. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0029] Please refer to the attached Figure 1, the present invention provides an electro-optic Q-switch that replaces the LN crystal cut at the Brewster angle, including: a prism 11, a first electro-optic crystal 12, and a second electro-optic crystal 13. Among them, the prism 11, the first electro-optic crystal 12, and the second electro-optic crystal 13 are arranged in sequence along the light transmission direction. To increase the operability, the prism 11, the first electro-optic crystal 12, and the second electro-optic crystal 13 are fixed in the overall structure; the working surfaces of the prism 11 include two light-transmitting surfaces and two reflecting surfaces. Anti-reflection films are coated on both light-transmitting surfaces, and high-reflection films are coated on both reflecting surfaces. Among them, whether high-reflection films are coated on the two reflecting surfaces is determined according to whether total internal reflection occurs when light rays reach the interface.

[0030] The first electro-optic crystal 12 and the second electro-optic crystal 13 adopt the transverse electro-optic effect, transmit light along the X-axis or Y-axis direction, and apply an electric field along the Z-axis;

[0031] The main axes of the first electro-optic crystal 12 and the second electro-optic crystal 13 rotate 90° with respect to each other around the light transmission direction, and the Z-axes of the first electro-optic crystal 12 and the second electro-optic crystal 13 form an angle of 45° with the normal direction of the incident surface.

[0032] In the present invention, the function of the prism is to make the incident light rays produce a specific translation without changing the polarization state of the light. Further explanation: the prism includes four working surfaces: a front light-transmitting surface, a front reflecting surface, a rear reflecting surface, and a rear light-transmitting surface. Among them, the two light-transmitting surfaces are parallel, the two reflecting surfaces are parallel, and the included angle between the light-transmitting surface and the reflecting surface is 45°. The translation amount of the light rays is uniquely determined by the relative distance between the two reflecting surfaces.

[0033] The working principle of the electro-optic Q-switch of the present invention is as follows: The electro-optic Q-switch of the present invention is divided into two working parts. Among them, the prism serves as the light ray translation part, equivalent to the light ray translation generated by the Brewster angle cutting surface of the LN crystal, and the electro-optic crystal part (including the first electro-optic crystal and the second electro-optic crystal) serves as the electro-optic modulation part, equivalent to the phase delay generated by the LN crystal.

[0034] In this embodiment, the prism adopts a rhombic prism, and the number of prisms is one.

[0035] In this embodiment, the material of the prism is N-BK7 or fused silica crystal.

[0036] In this embodiment, the sizes of the first electro-optic crystal and the second electro-optic crystal are exactly the same. Cuboids or cubes can be used. Using two crystals with the same size in combination can eliminate the influence of the change of static birefringence with temperature on the polarization state of the laser.

[0037] In this embodiment, the first electro-optic crystal and the second electro-optic crystal are potassium titanyl phosphate (KTP) crystals or rubidium titanyl phosphate (RTP) crystals.

[0038] In this embodiment, the light-transmitting aperture of the working surface of the prism is greater than or equal to the light-transmitting aperture of the first electro-optic crystal or the second electro-optic crystal.

[0039] In this embodiment, the first electro-optic crystal or the second electro-optic crystal is processed into a square column shape according to the electro-optic application direction. The light-transmitting surface is precisely optically polished, and the four side surfaces are finely ground. An antireflection film is coated on the light-transmitting end face of the first electro-optic crystal or the second electro-optic crystal, and an electrode layer is coated on the Z surface.

[0040] In addition, the electro-optic Q-switch formed by combining the prism, the first electro-optic crystal, and the second electro-optic crystal proposed by the present invention can use multiple prisms to translate the light.

[0041] In this embodiment, the incident light is P-polarized light. The reflection coefficients of the reflecting surface of the prism for P-polarized light and S-polarized light are not the same. When the incident polarized light contains both P-component and S-component, the polarization state of the incident light will change after passing through the prism, which will cause the failure of the electro-optic Q-switch.

[0042] The present invention uses a combination of a prism, a first electro-optic crystal, and a second electro-optic crystal to form a new type of electro-optic Q-switch. After the incident light passes through this switch, it generates an equivalent translation to that of the LN electro-optic Q-switch cut at the Brewster angle. The electro-optic Q-switch proposed by the present invention adopts a double-crystal temperature compensation design, which greatly improves the temperature adaptability of the device and can meet the normal use of the laser in the range of -60°C to +80°C. By reasonably setting the sizes of the prism, the first electro-optic crystal, and the second electro-optic crystal, the electro-optic Q-switch proposed by the present invention can equivalently replace the LN electro-optic Q-switch cut at the Brewster angle of the same specification.

[0043] The following Figure 1 The working principle of the electro-optic Q-switch is further described as follows: A beam of P-polarized light with a wavelength of 1064 nm and a polarization state along the horizontal direction is incident perpendicularly onto the prism of the electro-optic Q-switch along the light-transmitting direction. The front and rear light-transmitting surfaces of the prism are coated with an antireflection film with a wavelength of 1064 nm. After the light transmits through the front light-transmitting surface of the prism, total reflection occurs on the front reflecting surface and the light deflects by 90°. Then, total reflection occurs again on the rear reflecting surface and the light deflects by 90° and exits from the rear light-transmitting surface. From the above analysis, it can be understood that the incident light undergoes two total reflections after passing through the prism, resulting in a lateral displacement, but the propagation direction and polarization state of the outgoing light remain unchanged. The first electro-optic crystal and the second electro-optic crystal transmit light along the X-axis direction, and the Y and Z principal axes of the crystal rotate 90° relative to each other around the light-transmitting direction, and the Z-axis of the crystal forms an angle of 45° with the polarization direction of the P light. When no voltage is applied to the first electro-optic crystal and the second electro-optic crystal, the polarization state of the P-polarized light exiting from the prism remains unchanged after passing through the second electro-optic crystal; when a half-wave voltage is applied to the first electro-optic crystal and the second electro-optic crystal, the P-polarized light becomes S-polarized light; when a quarter-wave voltage is applied to the first electro-optic crystal and the second electro-optic crystal, the P-polarized light becomes circularly polarized light.

[0044] It should be noted that the reflection coefficients of the reflecting surface of the prism for P-polarized light and S-polarized light are not the same. When the incident polarized light contains both P-component and S-component, the polarization state of the incident light will change after passing through the prism, resulting in the failure of the electro-optic Q-switch.

[0045] To further illustrate the replaceability of the electro-optic Q-switch of the present invention and the LN electro-optic Q-switch cut at the Brewster angle, the following is combined with Figure 2 and Figure 3 , and the working principles of the two switches applied in the laser resonator are compared.

[0046] Figure 2 For a laser with an LN electro-optic Q-switch cut at the Brewster angle, an output mirror 221, a laser working medium 222, a polarizer 223, an LN electro-optic Q-switch 214, and a total reflection mirror 225 are sequentially arranged on the oscillation optical path of the laser. Its working principle is that under the action of an external excitation source, the laser working medium 222 emits fluorescence with a specific wavelength. After passing through the polarizer 223, a P-polarized light with a high degree of polarization is formed. The two end faces of the LN crystal are cut at the Brewster angle corresponding to the laser wavelength emitted by the working medium. After the P-light is incident on the un-voltage-applied LN electro-optic Q-switch 214, the optical path changes and the original polarization state is maintained. Then, through the total reflection mirror 225, the light returns along the original path. During the entire cycle, the polarization state of the light always remains unchanged, and the loss in the cavity is at a low value. The light reflects back and forth in the cavity to form an oscillation and generate laser output. When a quarter-wave voltage is applied to the LN electro-optic Q-switch 214, the P-polarized light becomes circularly polarized light after passing through the LN electro-optic Q-switch 214. The circularly polarized light is reflected by the total reflection mirror 225. When it is incident on the LN electro-optic Q-switch 214 again, the S-light is reflected, and the remaining P-light is still circularly polarized light after passing through the LN electro-optic Q-switch, but the intensity is attenuated by half. After multiple reflections, the loss of light in the cavity is too large to form an oscillation and generate laser output. The laser is turned off when a quarter-wave voltage is applied to the LN electro-optic switch and turned on when the quarter-wave voltage is removed.

[0047] Figure 3 For a laser applying the electro-optic Q-switch of the present invention, its structure is exactly the same as that of Figure 2 the laser with an LN electro-optic Q-switch cut at the Brewster angle. When the P-polarized light formed by the polarizer 223 passes through the prism part of the electro-optic Q-switch 224, a lateral translation occurs, and the translation effect is the same as that of Figure 2The translation of the LN crystal shown is the same in translation and does not change the polarization state of light. When no voltage is applied to the first electro-optic crystal and the second electro-optic crystal, the P-polarized light emerging from the prism remains unchanged in polarization state after passing through the second electro-optic crystal; the light oscillates with very little loss in the laser cavity to produce laser output. When a quarter-wave voltage is applied to the electro-optic Q-switch 224, the P-polarized light becomes circularly polarized after passing through the electro-optic Q-switch 224, is reflected by the total reflection mirror 225, and then becomes S-polarized light after passing through the electro-optic Q-switch 224 again. The S-polarized light is perpendicular to the polarization direction of the polarizer, and the light has too much loss in the cavity to form an oscillation to produce laser output.

[0048] Based on the above analysis, it can be understood that the electro-optic Q-switch provided by the present invention does not perturb the precise control of the laser polarization state in the laser cavity while changing the optical path in the laser cavity, and has the same effect as the LN electro-optic Q-switch cut at the Brewster angle in the laser resonator, and can be equivalently replaced.

[0049] In this specification, each embodiment is described in a progressive manner. The key point of each embodiment is to illustrate the differences from other embodiments. For the same or similar parts among the embodiments, reference can be made to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and reference can be made to the description of the method part for related parts.

[0050] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An electro-optic Q-switch for replacing a Brewster angle cut LN crystal, characterized in that, Including: A prism, a first electro-optical crystal, and a second electro-optical crystal, wherein the prism, the first electro-optical crystal, and the second electro-optical crystal are arranged in sequence along the optical path direction; the working surfaces of the prism include two optical surfaces and two reflection surfaces, antireflection films are coated on both optical surfaces, and high-reflection films are coated on the two reflection surfaces according to whether total internal reflection occurs at the interface of the light rays. The first electro-optical crystal and the second electro-optical crystal adopt the transverse electro-optical effect, have light passing along the X-axis or Y-axis direction, and an electric field is applied along the Z-axis. The main axes of the first electro-optical crystal and the second electro-optical crystal rotate 90° relative to each other around the optical path direction, and the Z-axes of the first electro-optical crystal and the second electro-optical crystal form an angle of 45° with the normal direction of the incident surface. The first electro-optical crystal and the second electro-optical crystal have exactly the same size. The incident light is P-polarized light, and the reflection coefficients of the reflection surface of the prism for P-polarized light and S-polarized light are different. The two optical surfaces of the prism are parallel, the two reflection surfaces are parallel, and the included angle between the optical surface and the reflection surface is 45°. The optical aperture of the working surface of the prism is greater than or equal to the optical aperture of the first electro-optical crystal or the second electro-optical crystal. The working principle of the electro-optical Q-switch using a LN crystal to replace the Brewster angle cutting is as follows: The electro-optical Q-switch is divided into two working parts. The prism serves as the light ray translation part, equivalent to the light ray translation generated by the Brewster angle cutting surface of the LN crystal. The first electro-optical crystal and the second electro-optical crystal serve as the electro-optical modulation part, equivalent to the phase delay generated by the LN crystal. The incident light undergoes two total internal reflections after passing through the prism to generate a lateral displacement, and the propagation direction and polarization state of the outgoing light remain unchanged. The first electro-optical crystal and the second electro-optical crystal have light passing along the X-axis direction, the Y and Z main axes of the crystal rotate 90° relative to each other around the optical path direction, and the Z-axis of the crystal forms an angle of 45° with the polarization direction of the P light.

2. An electro-optic Q-switch for replacing the Brewster angle cutting LN crystal according to claim 1, characterized in that, The prism is an orthorhombic prism, a right prism, or an equilateral prism, and the number of prisms is one or more.

3. An electro-optical Q-switch for replacing the Brewster angle cutting LN crystal according to claim 2, characterized in that, The material of the prism is N-BK7 or fused silica crystal.

4. An electro-optic Q-switch for replacing a Brewster angle cut LN crystal according to claim 1, characterized in that, The first electro-optical crystal and the second electro-optical crystal are potassium titanyl phosphate crystals or rubidium titanyl phosphate crystals.

5. An electro-optical Q-switch for replacing the Brewster angle cutting of an LN crystal according to claim 1, characterized in that, The first electro-optical crystal or the second electro-optical crystal is processed into a square column shape according to the electro-optical application direction, the optical surface is precisely optically polished, and the four side surfaces are finely ground.

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