CN117996556B - Device for generating continuously tunable narrow-band laser in ultra-wide frequency spectrum - Google Patents

Abstract

The invention discloses a device for generating continuous tunable narrow-band laser in ultra-wide frequency spectrum, which comprises: narrow-band laser pump sources such as nanoseconds, picoseconds, even femtoseconds and the like can be tuned; the controllable nonlinear frequency conversion module is composed of two semi-cylindrical prisms and a single nonlinear polarized crystal. By utilizing the novel combined frequency conversion module and the PPLN sample internal angle tuning scheme, the nonlinear polarization crystal is obliquely incident on the pumping light source to carry out the frequency multiplication process, and the high-efficiency continuous tunable second harmonic which covers the fundamental frequency light from 750nm to 4900nm approximately and outputs an extremely wide spectrum can be generated through a 1-10-order multi-order quasi-phase matching mechanism. The invention has the advantages of small device size, easy preparation and processing of the semi-cylindrical prism, simple light path, strong mobility and wide application; the generated tunable frequency doubling laser has the advantages of wide covered frequency spectrum range and high energy conversion efficiency. The invention can be widely applied to the technical fields of laser technology and nonlinear frequency conversion.

Description

A device for generating continuously tunable narrow-band laser in an extremely wide spectrum

Technical Field

The invention relates to the fields of laser technology and nonlinear frequency conversion technology, and in particular to a device for generating continuously tunable narrow-band laser in an extremely wide frequency spectrum.

Background technique

Since the wavelength generated by the laser is affected by the energy level structure of the gain medium, many wavelengths cannot be generated directly. With the need for other wavelength light sources in practical applications, wavelength conversion technology has gradually attracted people's attention and become a key research project in nonlinear optics. The use of quasi-phase matching technology to achieve nonlinear frequency conversion has become an effective way to broaden the laser frequency. In recent years, narrow-linewidth lasers have been widely used in gravitational wave detection, lidar, distributed sensing, high-speed coherent optical communications and other fields. Therefore, it is of great significance to generate continuously tunable narrow-band lasers in an extremely wide spectrum.

A single periodically poled lithium niobate crystal can only provide a single reciprocal lattice vector to compensate for its phase mismatch under a specific order of quasi-phase matching, and output a single specific frequency wavelength. When no external method is used to expand the nonlinear frequency conversion, it is necessary to increase the number of crystal blocks to provide multiple reciprocal lattice vectors required for nonlinear frequency conversion. At present, there are many tuning methods, including temperature tuning, electric field tuning, angle tuning, etc. However, there are several problems with temperature tuning, including the requirement for high-precision temperature control equipment, the impact of temperature drift on wavelength stability, the long response time of high and low temperature switching, and the easy degradation of crystal life at high temperature. On the other hand, the combination of multiple tuning methods increases the complexity of experimental operation and the uncertainty of result analysis. Electric field tuning technology can solve the above problems to a certain extent. Nevertheless, the uniform electric field method does not provide tuning when applied to PPLN crystals. The electro-optical perturbations caused by the uniform electric field will produce phase matching changes of nonlinear parameter interactions, but in symmetrical crystals they will cancel each other out, and the periodic field only penetrates from the crystal surface to a certain depth. As an alternative tuning method in quasi-phase matching, the incident angle can indeed produce tunable narrow-band lasers by changing the angle at which the pump light is directly incident from the air to the end face of the crystal under the angle quasi-phase matching scheme. When it is assumed that the external incident angle can be ideally adjusted from 0-90°, it can be seen from Snell's refraction law that the actual angle at which the pump light enters the lithium niobate crystal is extremely small. When the nonlinear effect of frequency doubling occurs, the internal angle changes only approximately from 0-30°. In the experiment, the size of the crystal also limits the range of the incident angle. This means that when a tunable frequency doubling process is generated, the covered fundamental frequency spectrum range is very limited, which undoubtedly restricts the application value of the output wavelength of a single periodically polarized lithium niobate crystal under the angle quasi-phase matching scheme.

Summary of the invention

In order to solve at least one of the technical problems existing in the prior art to a certain extent, the present invention aims to provide a device for generating continuously tunable narrow-band laser in an extremely wide spectrum.

The technical solution adopted by the present invention is:

A device for generating continuously tunable narrow-band laser in an extremely wide spectrum, comprising:

Tunable nanosecond, picosecond, and even femtosecond narrow-band laser pump source modules;

The controllable nonlinear frequency conversion module is composed of two semi-cylindrical prisms and a single nonlinear polarization crystal, and the nonlinear polarization crystal is arranged between the two semi-cylindrical prisms;

Based on changing the internal angle of the pump light in the nonlinear polarization crystal, the nonlinear polarization crystal is rotated according to the quasi-phase matching condition, and the semi-cylindrical prism close to the nonlinear polarization crystal is moved back and forth to complete the three processes of light incidence, nonlinear effect generation, and light emission. Among them, the pump light is incident on the optical center of the first semi-cylindrical prism from different angles in the air, and the process does not change the angle deflection to achieve the first process. The light is incident on the nonlinear polarization crystal from the optical center, and the internal angle deflection occurs to produce a nonlinear effect. Finally, the light is emitted from the crystal exit port through the optical center of the second semi-cylindrical prism into the air for reception and collection.

The above scheme is further improved as follows: the pump light is incident on the nonlinear frequency conversion module at different angles in TE polarization mode, the vibration direction is parallel to the crystal axis z-axis, the light entering the nonlinear polarization crystal is extraordinary light, and the phase mismatch of the pump light at different internal angles of the nonlinear polarization crystal during the frequency doubling process is derived.

The above scheme is further improved as follows: the pump light is obliquely incident on the nonlinear polarization crystal to generate second harmonics. It is assumed that the phase of the crystal is matched in the x direction and mismatched in the y direction. Only the nonlinear frequency conversion in the y direction is considered, and quasi-phase matching is achieved by using the reciprocal lattice structure provided by the nonlinear crystal.

The above scheme is further improved as follows: the numerical combination of the polarization period, chirp, duty cycle, crystal specifications, etc. of the nonlinear polarization crystal is adjustable, so that the reciprocal lattice vector of the nonlinear polarization crystal appears as a number of narrow-band reciprocal lattice vector peaks or extremely narrow-band reciprocal lattice vector bands distributed at different positions, which can effectively compensate for the phase mismatch in the multi-order quasi-phase matching nonlinear frequency conversion process.

The above scheme is further improved as follows: the nonlinear polarization crystal has a polarization period of 24 μm, a chirp of 0, a light transmission length of 5 mm, a width of 12 mm, and provides a reciprocal lattice vector structure along the light transmission direction, which can effectively compensate for the phase mismatch in the 1-10 order quasi-phase matching nonlinear frequency conversion process.

The above scheme is further improved as follows: a plurality of narrowband reciprocal lattice vector peaks or reciprocal lattice vector bands of the nonlinear polarization crystal respectively correspond to tunable fundamental frequency lights of different wavelengths at different internal angles to participate in the nonlinear frequency conversion process, and each reciprocal lattice vector can effectively connect the phase mismatch amount of the nonlinear frequency conversion process compensated by the previous reciprocal lattice vector at a specific crystal internal angle, and a plurality of reciprocal lattice vectors of multi-order quasi-phase matching are used to generate extremely wide spectrum tunable second harmonics, and tunable frequency doublings are generated by 1-10 order quasi-phase matching, covering fundamental frequency light up to 750nm-2700nm, and tunable frequency doublings covering longer waves are continuously generated by 1 order quasi-phase matching alone, covering fundamental frequency light up to a wide spectrum range of 2700nm-4900nm, thereby generating efficient narrowband frequency doublings that are continuously tunable in the range of 325nm-2450nm. It should be noted that, based on the specific design parameters of the nonlinear polarization crystal given above, the continuously tunable high-efficiency narrowband frequency doubling range that can be produced by using the multi-order quasi-phase matching mechanism and the angle tuning scheme is only for better display and easy understanding, and should not be construed as limiting the scope of rights protected by the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

The above scheme is further improved as follows: the plane end of the first semi-cylindrical prism is close to the light-entering side of the nonlinear polarization crystal to complete the light incident process, and its refractive index is close to but still lower than that of the nonlinear polarization crystal; according to Snell's law, the incident angle of the semi-cylindrical prism is calculated from the internal angle of the nonlinear polarization crystal, and the pump light is controlled to be incident from the air vertically to the cylindrical end of the semi-cylindrical prism, and the direction passes through the optical center of the semi-cylindrical prism, and the incident angle is maintained during the propagation process.

The above scheme is further improved as follows: the plane end of the second semi-cylindrical prism is close to the light-emitting side of the nonlinear polarization crystal to complete the light emission process, and its refractive index is close to but still higher than that of the nonlinear polarization crystal; the manipulated light is incident from the exit port of the nonlinear polarization crystal through the optical center of the plane end of the second semi-cylindrical prism, and according to Snell's law, the light is deflected in the second semi-cylindrical prism and the refraction angle is maintained vertically to be emitted through the cylindrical end for reception and collection.

The above solution is further improved in that the material, radius and thickness of the two semi-cylindrical prisms can be changed, and the semi-cylindrical prisms can be moved to better combine with the nonlinear polarization crystal to form a controllable nonlinear frequency conversion module.

The above scheme is further improved as follows: the tunable narrowband laser pump source module includes a laser, a half-wave plate and a plano-convex lens; the laser is used to generate laser light, and the laser light passes through the half-wave plate and the plano-convex lens in sequence and then enters the nonlinear frequency conversion module.

The beneficial effects of the present invention include:

(1) The present invention discloses a device for generating a continuously tunable narrowband laser within an extremely wide spectrum. The nonlinear optical frequency conversion module composed of two semi-cylindrical prisms and a single nonlinear polarization crystal (PPLN) abandons the traditional method of directly focusing the incident light from the air into the sample. The traditional angle tuning method uses a novel combined frequency conversion module and a PPLN sample internal angle tuning scheme. Through a multi-order quasi-phase matching mechanism, the present invention is able to generate an extremely wideband monochromatic tunable laser of approximately 750nm-4900nm, effectively broadening the nonlinear frequency conversion spectrum.

(2) The narrowband nonlinear polarization crystal designed in the present invention has a flexible and controllable structure and is easy to prepare. It is a device that uses 1-10 order quasi-phase matching to generate a continuously tunable narrowband laser in an extremely wide spectrum, wherein the 1-10 order quasi-phase matching generates a high-efficiency frequency doubling covering the tunable fundamental frequency light from 750nm to 2700nm, and the 1st order quasi-phase matching alone continues to generate a high-efficiency frequency doubling covering the tunable fundamental frequency light from 2700nm to 4900nm. The energy conversion efficiency is high and it is suitable for narrowband lasers such as nanosecond, picosecond or even femtosecond pump sources.

(3) The device for generating continuously tunable narrow-band laser in an extremely wide spectrum of the present invention has the advantages of small device size, easy preparation and processing of the semi-cylindrical prism, simplified optical path, strong mobility and wide application.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the embodiments of the present invention or the drawings of related technical solutions in the prior art are introduced below. It should be understood that the drawings introduced below are only for the convenience of clearly describing some embodiments of the technical solutions of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative work.

FIG1 is a schematic diagram of an experimental device for generating a continuously tunable narrow-band laser in an extremely wide spectrum in this example;

FIG2 is a graph showing the Fourier coefficient spectrum of the periodically poled lithium niobate crystal and the phase mismatch curve of the second harmonic generation process at different internal angles in this embodiment, wherein the abscissa is the value of the reciprocal lattice vector, the left ordinate is the effective nonlinear coefficient, and the right ordinate is the matching fundamental wavelength;

FIG3 is a schematic diagram of the connection between the periodically poled lithium niobate crystal in this example using internal angle tuning and multi-order quasi-phase matching to achieve extremely wide spectrum efficient frequency doubling, wherein FIG3(a) is a schematic diagram of m=1 to m=5 order quasi-phase matching, and FIG3(b) is a schematic diagram of m=6 to m=10 order quasi-phase matching;

FIG4 is a schematic diagram of the incidence and emission of fundamental frequency light in the process of internal angle tuning produced by the periodically poled lithium niobate crystal and two semi-cylindrical prisms in this example, wherein FIG4(a) is a schematic diagram of the first-order quasi-phase matching demonstration, and FIG4(b) is a schematic diagram of the first-order and second-order quasi-phase matching demonstration.

Explanation of the reference numerals: laser 1; half-wave plate 2; plano-convex lens 3; rotating table 4; semi-cylindrical prisms 5 and 7; periodically poled lithium niobate crystal 6; filter 8; photoelectric probe 9; power meter 10; spectrometer 11; computer 12.

Detailed ways

The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and are not to be construed as limitations of the present invention. For the step numbers in the following embodiments, they are only provided for the convenience of explanation, and the order between the steps is not limited in any way, and the execution order of each step in the embodiment can be adaptively adjusted according to the understanding of those skilled in the art.

In the description of the present invention, it should be understood that descriptions involving orientations, such as up, down, front, back, left, right, etc., and orientations or positional relationships indicated are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

In the description of the present invention, "several" means one or more, "more" means more than two, "greater than", "less than", "exceed" etc. are understood to exclude the number itself, and "above", "below", "within" etc. are understood to include the number itself. If there is a description of "first" or "second", it is only used for the purpose of distinguishing the technical features, and cannot be understood as indicating or implying the relative importance or implicitly indicating the number of the indicated technical features or implicitly indicating the order of the indicated technical features.

In the description of the present invention, unless otherwise clearly defined, terms such as setting, installing, connecting, etc. should be understood in a broad sense, and technicians in the relevant technical field can reasonably determine the specific meanings of the above terms in the present invention in combination with the specific content of the technical solution.

Terminology explanation:

Extremely wide spectrum: An optical spectrum covering the ultraviolet-visible-infrared band. An optical spectrum involving only the visible-infrared band or the ultraviolet-visible band also falls within the scope of the understanding of extremely wide spectrum, that is, the tunable wavelength range involved exceeds the generally understood tens of nanometers.

As shown in FIG1 , this embodiment provides a device for generating continuously tunable narrow-band laser in an extremely wide spectrum, comprising:

Tunable nanosecond, picosecond, and even femtosecond narrow-band laser pump source modules;

The controllable nonlinear frequency conversion module is composed of two semi-cylindrical prisms and a single nonlinear crystal.

The tunable narrowband laser pump source module includes a laser 1, a half-wave plate 2 and a plano-convex lens 3. The laser 1 generates laser light, and the focused light spot is incident at different angles on a nonlinear frequency conversion module composed of two semi-cylindrical prisms 5 and 7 and a single 5% MgO-doped periodically polarized lithium niobate crystal 6 (i.e., a nonlinear polarized crystal). This process is achieved by rotating the frequency conversion module device placed on a rotating table. The internal angle β of the pump light in the periodically polarized lithium niobate crystal 6 is changed from 0° to 60.9°, and the pump light is adjusted to vertically incident on the cylindrical surface of the first semi-cylindrical prism at different angles from the air according to Snell's law, wherein it is ensured that the pump light always passes through the optical center direction of the semi-cylindrical prism. Furthermore, the pump light emitted from the plane optical center of the first semi-cylindrical prism can enter the periodically poled lithium niobate crystal to produce a frequency doubling nonlinear effect, and the light at the crystal output port is refracted again, propagates along the plane optical center of the second semi-cylindrical prism and is emitted into the air for collection and measurement. The filter 8 is used to filter the fundamental frequency light, and the filtered light reaches the photoelectric probe 9, which is connected to the power meter 10. The power meter 10 and the spectrometer 11 are used to record and measure the output light spectrum.

In this implementation case, as shown in FIG2, the Fourier coefficient spectrum of the periodically poled lithium niobate crystal and the phase mismatch curve of the second harmonic generation process at different internal angles, the horizontal axis is the value of the reciprocal lattice vector, the left vertical axis is the effective nonlinear coefficient, and the right vertical axis is the matching fundamental wavelength. The designed periodically poled lithium niobate crystal provides 10 reciprocal lattice vector peaks, corresponding to m = 1 to m = 10 order quasi-phase matching. When the pump light is incident normally, that is, β = 0°, according to the intersection of the phase mismatch curve and each reciprocal lattice vector peak, it can be seen that at this time, different reciprocal lattice vector peaks can provide effective phase compensation for the nonlinear frequency conversion process of different fundamental wavelengths and generate second harmonics of different frequencies. The internal angle β of the periodically poled lithium niobate crystal with the incident pump light is effectively utilized. Here, 0-80° is used as an example. The same reciprocal lattice vector peak provided by the crystal at different angles provides effective phase compensation for the nonlinear frequency conversion process of different fundamental wavelengths. At this point, the multi-order quasi-phase matching and internal angle tuning mechanism of periodically poled lithium niobate crystals can provide effective phase compensation for continuous nonlinear frequency conversion processes within an extremely wide spectrum, thereby producing efficient frequency doubling. In addition, compared with the effective Fourier spectrum of broadband chirped periodically poled lithium niobate crystals, periodically poled lithium niobate or narrowband chirped periodically poled lithium niobate crystals can provide larger Fourier coefficients, which will effectively improve the conversion efficiency of the output tunable laser wavelength.

In this implementation case, as shown in FIG3 , the internal angle tuning of the periodically poled lithium niobate crystal and the multi-order quasi-phase matching realize extremely wide spectrum efficient continuous frequency doubling. As shown in FIG3 (a), in the extremely wide spectrum efficient frequency doubling generation, the first inverse lattice vector peak covers the fundamental wavelength from about 1719nm to 4900nm in the angle range of β from 0-50°, which effectively utilizes the degenerate point of the phase mismatch curve; the second inverse lattice vector peak covers the fundamental wavelength from about 1278nm to 1719nm in the angle range of β from 0-60.9°; the third inverse lattice vector peak covers the fundamental wavelength from about 1112nm to 1278nm in the angle range of β from 0-48.85°; the fourth inverse lattice vector peak covers the fundamental wavelength from about 10 14-1112nm; the fifth inverted lattice vector peak covers the fundamental frequency wavelength from about 949-1014nm in the angle range of 0-37.2° in β; the sixth inverted lattice vector peak covers the fundamental frequency wavelength from about 897-949nm in the angle range of 0-34.4° in β; the seventh inverted lattice vector peak covers the fundamental frequency wavelength from about 857-897nm in the angle range of 0-31.9° in β; the eighth inverted lattice vector peak covers the fundamental frequency wavelength from about 799-857nm in the angle range of 0-40.3° in β; the ninth inverted lattice vector peak covers the fundamental frequency wavelength from about 777-799nm in the angle range of 0-28.4° in β; the tenth inverted lattice vector peak covers the fundamental frequency wavelength from about 750-777nm in the angle range of 0-26.3° in β. Each of the above-mentioned reciprocal lattice vectors with tunable fundamental wavelength compensation is narrow-band and can even be regarded as monochromatic light. At the same time, the design scheme of this nonlinear crystal reciprocal lattice vector is not unique. It only needs the distribution of these reciprocal lattice vectors to be narrow-band enough and to be able to effectively compensate for the frequency doubling process of the tunable pump laser light source, and to make the multi-order quasi-phase matching of the crystal compensate for the tunable fundamental light covering an extremely wide spectrum under the internal angle tuning scheme, so as to effectively produce extremely wide spectrum and efficient frequency doubling.

In this implementation case, as shown in FIG4 , the periodically poled lithium niobate crystal and two semi-cylindrical prisms cooperate to complete the incidence and emission of the fundamental frequency light in the process of internal angle tuning. FIG4 (a) is an example of m=1-order quasi-phase matching. When the internal angle β of the periodically poled lithium niobate crystal is 0°, 10°, 20°, 30°, 40° and 48.5°, respectively, according to the refractive index of the material selected for the first semi-cylindrical prism, it can be calculated by Snell's refraction law that the angle of the pump light incident on the semi-cylindrical prism and the horizontal direction is 0°, 12.37°, 24.96°, 38.05°, 52.32° and 67.32°, respectively, wherein the pump light is vertically incident on the cylindrical surface of the semi-cylindrical prism, and the direction passes through the optical center of the semi-cylindrical prism. In order to avoid the phenomenon of total reflection of the emitted light directly from the air interface after the nonlinear effect of the periodically poled lithium niobate crystal occurs, the light emission is better guided and further measured. The second semi-cylindrical prism can be used when the refraction angle is large. Here, a schematic diagram of the semi-cylindrical prism device moving up and down is constructed with β of 30°, 40°, and 48.5° to ensure that the light is incident and propagated along the optical center direction of the second semi-cylindrical prism and emitted into the air, which is convenient for subsequent filtering tests. Similarly, Figure 4 (b) is an example of m = 2nd order quasi-phase matching, and the internal angles β of the periodically poled lithium niobate crystal are selected as 0°, 20°, 40°, 60°, and 60.9° for specific demonstration, and the corresponding angles of the pump light incident on the first semi-cylindrical prism and the horizontal direction are 0°, 22.67°, 46.41°, 77.47°, and 80.04°, respectively. When the internal angle increases, the crystal can move up and down so that the pump light enters the crystal from the optical center of the first semi-cylindrical prism, and the interaction length in the y direction is kept at the maximum, and the light does not hit the side of the crystal. The semi-cylindrical prism device is constructed with β of 40°, 60°, and 60.9° to move up and down to receive the light emitted from the crystal, ensuring that the light is incident and propagated along the optical center direction of the second semi-cylindrical prism and emitted into the air. Finally, the nonlinear frequency conversion module based on the combination of two semi-cylindrical prisms and a single nonlinear crystal produces highly efficient frequency doubling with an extremely wide spectrum under the pumping of a tunable narrow-band laser.

In the above description of this specification, the description with reference to the terms "one embodiment/example", "another embodiment/example" or "certain embodiments/examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the claims and their equivalents.

The above is a specific description of the preferred implementation of the present invention, but the present invention is not limited to the above embodiments. Those skilled in the art may make various equivalent modifications or substitutions without violating the spirit of the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims (10)

1. A device for generating continuously tunable narrow-band frequency-doubled laser in an extremely wide spectrum, characterized by comprising: Tunable narrow-band laser pump source module; The controllable nonlinear frequency conversion module is composed of two semi-cylindrical prisms and a single nonlinear polarization crystal, and the nonlinear polarization crystal is arranged between the two semi-cylindrical prisms; The pump light is changed at an internal angle of the nonlinear polarization crystal, and the nonlinear polarization crystal is rotated according to the quasi-phase matching condition. At the same time, the semi-cylindrical prism close to the nonlinear polarization crystal is moved back and forth to complete the three processes of light incidence, nonlinear effect generation, and light emission. Among them, the pump light is incident from different angles in the air to the optical center of the first semi-cylindrical prism, and the angle deflection is not changed during the process to achieve the first process. The light is incident from the optical center to the nonlinear polarization crystal, and an internal angle deflection occurs to generate a nonlinear effect. Finally, the light is emitted from the crystal exit port through the optical center of the second semi-cylindrical prism into the air for reception and collection. The plane end of the first semi-cylindrical prism is in close contact with the light-incoming side of the nonlinear polarization crystal; a plurality of narrow-band reciprocal lattice vector peaks or reciprocal lattice vector bands of the nonlinear polarization crystal respectively correspond to tunable fundamental frequency lights of different wavelengths at different internal angles to participate in the nonlinear frequency conversion process, and each reciprocal lattice vector can effectively connect the phase mismatch amount of the nonlinear frequency conversion process compensated by the previous reciprocal lattice vector at a specific crystal internal angle, and generate extremely wide spectrum tunable second harmonics through a plurality of reciprocal lattice vectors of multi-order quasi-phase matching; the plane end of the second semi-cylindrical prism is in close contact with the light-emitting side of the nonlinear polarization crystal; The extremely wide spectrum is an optical spectrum covering the ultraviolet-visible-infrared band. 2. According to claim 1, a device for generating continuously tunable narrowband frequency-doubled laser in an extremely wide spectrum is characterized in that the pump light is incident on the nonlinear frequency conversion module at different angles in TE polarization mode, the vibration direction is parallel to the crystal axis z-axis, and the light entering the nonlinear polarization crystal is extraordinary light, and the phase mismatch of the pump light at different internal angles of the nonlinear polarization crystal during the frequency doubling process is derived. 3. According to claim 1, a device for generating continuously tunable narrowband frequency-doubled laser in an extremely wide spectrum is characterized in that the pump light is incident obliquely on a nonlinear polarization crystal to generate second harmonics, assuming that the phase of the crystal is matched in the x direction and mismatched in the y direction, and only considering the nonlinear frequency conversion in the y direction, quasi-phase matching is achieved by utilizing the reciprocal lattice structure provided by the nonlinear crystal. 4. According to claim 1, a device for generating continuously tunable narrowband frequency-doubled laser in an extremely wide spectrum is characterized in that the polarization period, chirp, duty cycle and crystal specifications of the nonlinear polarization crystal are adjustable, and it has a series of positive and negative domain structures of polarization directions along the y direction, so that the reciprocal lattice vector of the nonlinear polarization crystal appears as a number of narrowband reciprocal lattice vector peaks or extremely narrowband reciprocal lattice vector bands distributed at different positions, which can effectively compensate for the phase mismatch in the multi-order quasi-phase matching nonlinear frequency conversion process. 5. According to claim 4, a device for generating continuously tunable narrowband frequency-doubled laser in an extremely wide spectrum is characterized in that the nonlinear polarization crystal has a polarization period of 24 μm, a chirp of 0, a light-transmitting length of 5 mm, a width of 12 mm, and provides a reciprocal lattice vector structure along the light-transmitting direction, which can effectively compensate for the phase mismatch of the 1-10 order quasi-phase matching nonlinear frequency conversion process. 6. The device for generating a continuously tunable narrowband frequency-doubled laser in an extremely wide spectrum according to claim 5 is characterized in that the tunable frequency doubled is generated by 1-10 order quasi-phase matching, covering the fundamental frequency light up to 750nm-2700nm, and the tunable frequency doubled covering longer waves is continued to be generated by 1 order quasi-phase matching alone, covering the fundamental frequency light up to a wide spectrum range of 2700nm-4900nm, thereby generating a continuously tunable and efficient narrowband frequency doubled in the range of 325nm-2450nm. 7. According to claim 1, a device for generating continuously tunable narrowband frequency-doubled laser in an extremely wide spectrum is characterized in that the first semi-cylindrical prism is used to complete the light incident process, and its refractive index is close to but still lower than that of the nonlinear polarization crystal; according to Snell's law, the incident angle of the semi-cylindrical prism is calculated by the internal angle of the nonlinear polarization crystal, and the pump light is controlled to be incident from the air vertically to the cylindrical end of the semi-cylindrical prism, and the direction passes through the optical center of the semi-cylindrical prism, and the incident angle is maintained during the propagation process. 8. According to claim 1, a device for generating continuously tunable narrowband frequency-doubled laser in an extremely wide spectrum is characterized in that the second semi-cylindrical prism is used to complete the light emission process, and its refractive index is close to but still higher than that of the nonlinear polarization crystal; the manipulated light is incident from the nonlinear polarization crystal exit port through the optical center of the plane end of the second semi-cylindrical prism, and according to Snell's law, the light is deflected in the second semi-cylindrical prism and the refraction angle is maintained vertically to be emitted through the cylindrical end for reception and collection. 9. The device for generating continuously tunable narrow-band frequency-doubled laser in an extremely wide spectrum according to claim 1 is characterized in that the material, radius and thickness of the two semi-cylindrical prisms can be changed, and the semi-cylindrical prisms can be moved to better combine with the nonlinear polarization crystal to form a controllable nonlinear frequency conversion module. 10. The device for generating continuously tunable narrowband frequency-doubled laser in an extremely wide spectrum according to claim 1, characterized in that the tunable narrowband laser pump source module comprises a laser, a half-wave plate and a plano-convex lens; the laser is used to generate laser light, and the laser light passes through the half-wave plate and the plano-convex lens in sequence and then enters the nonlinear frequency conversion module.