CN111129932A - Device for generating broadband supercontinuum laser and manufacturing method of crystal in device - Google Patents
Device for generating broadband supercontinuum laser and manufacturing method of crystal in device Download PDFInfo
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- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/09—Processes or apparatus for excitation, e.g. pumping
- H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
- H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
Abstract
The invention provides a device for generating broadband supercontinuum laser by utilizing the synergistic effect of second-order and third-order nonlinear optical effects, which comprises a pumping light source and a nonlinear laser crystal, and is characterized in that the pumping light source is a near-infrared femtosecond pulse laser source, the frequency of the femtosecond pulse laser provided by the pumping light source has the bandwidth of nearly 100nm, and the femtosecond pulse laser has the characteristic of high peak power; the nonlinear laser crystal is made of a ferroelectric crystal material and comprises a series of unit cells, and the length of the unit cells in the light propagation direction changes along the light propagation direction according to a continuous chirp change rule. In the nonlinear laser crystal, the spectral width of a pumping spectrum can be effectively extended from 100nm to nearly 300-500nm by utilizing the obvious third-order nonlinear optical effect, and the supercontinuum and completely coherent laser which can cover the bandwidth from visible to near-infrared wave band to nearly 500nm is generated by utilizing the second-order nonlinear frequency conversion; the structure is controllable, the preparation is easy, and the design is flexible; high energy conversion efficiency, super-continuous laser spectrum broadband and small device size.
Description
Technical Field
The invention relates to the technical field of laser, in particular to a device for generating broadband supercontinuum laser by utilizing the synergistic effect of second-order and third-order nonlinear optical effects and a corresponding method for manufacturing a nonlinear laser crystal.
Background
Since the birth of laser, the laser technology has great application value in the fields of national defense, medical treatment, industry, communication and the like. Laser technology has been under tremendous development in recent decades and has long been widely used in various areas of modern science. However, the frequency of the laser output is limited by the energy level of the gain medium, and an output of an arbitrary frequency cannot be generated. In addition to searching for new laser crystal materials, people often need to utilize second-order nonlinear frequency conversion technology (frequency doubling, sum frequency, difference frequency, parametric amplification, etc.) to obtain laser with required frequency.
In order to generate laser output with required frequency by using a second-order nonlinear frequency conversion technology, the most central problem is to solve the phase matching problem in the nonlinear process, however, the phase matching cannot be automatically satisfied due to chromatic dispersion in the nonlinear material. In practical applications, people usually use the birefringence of the crystal to compensate the dispersion of the crystal, so as to achieve the purpose of phase matching. However, birefringence matching has limitations such as walk-off effects and small nonlinear coefficients. Meanwhile, due to the limitation of the physical characteristics of the crystal, the crystal dispersion compensation in the second-order nonlinear frequency conversion process can only be carried out in a very small frequency range, so that the laser can only be output in a limited bandwidth, and the practical application of the laser is limited.
In order to solve this problem, the concept of nonlinear photonic crystals (i.e. artificial microstructure crystal materials in which the sign of the nonlinear coefficient has a periodic variation) has been proposed to achieve quasi-phase matching. The quasi-phase matching technology rearranges the phase of the light wave by periodically changing the spontaneous polarization direction of the crystal, thereby compensating the phase mismatch in the nonlinear process, greatly increasing the flexibility and controllability of the second-order nonlinear process, opening up a revolutionary new stage of the nonlinear frequency conversion technology by a brand-new thought method, and greatly promoting the development of the laser technology.
The preparation of the nonlinear photonic crystal needs to use ferroelectric crystal materials, and common materials include lithium niobate, lithium tantalate, potassium titanyl phosphate crystal and the like. The ferroelectric polarization direction represents the sign of the nonlinear coefficient (corresponding to positive and negative domains, respectively), the spontaneous polarization direction of the ferroelectric crystal is easily reversed by overcoming the coercive field inside the ferroelectric crystal with an applied electric field that will cause the nonlinear coefficient of the ferroelectric crystal to change sign (e.g., from positive to negative). When the applied electric field is periodically distributed in space, the positive and negative distribution of the nonlinear coefficient in the ferroelectric nonlinear crystal can be periodically regulated and controlled, so that the nonlinear photonic crystal is constructed, and the quasi-phase matching is realized.
Since the concept of quasi-phase matching technology is proposed, various periodic, quasi-periodic or non-periodic nonlinear photonic crystals are designed and prepared to meet the application requirements of diversified nonlinear optical frequency conversion, for example, quasi-phase matching of two or more wavelengths is simultaneously realized by using a single nonlinear photonic crystal. Because the existing nonlinear photonic crystal structure cannot simultaneously regulate and control phase matching conditions in various nonlinear processes of different wavelength components, the energy utilization efficiency of a nonlinear conversion process of broadband laser is greatly limited, and the application expansion of nonlinear optical technology to femtosecond pulse laser and other ultra-wideband laser technologies is also limited. For the birefringent matching technology, the bandwidth of phase matching which can be provided by the birefringent crystal is more limited, and one birefringent crystal can only apply efficient nonlinear frequency conversion to continuous laser or narrow-band pulse laser with a certain specific frequency, so that broadband super-continuous laser output cannot be obtained.
On the other hand, when the ultrashort ultrastrong broadband laser pulse is transmitted in a nonlinear optical medium, universal third-order nonlinear optical interaction (without the limitation of symmetry of a medium crystal atomic structure) can be generated, such as self-phase modulation, four-wave mixing, stimulated raman effect and the like, and the spectrum range and the wavelength range of the ultrashort laser pulse can be effectively widened. It is expected that if the second-order nonlinear optical effect and the third-order nonlinear optical effect can be effectively combined, the supercontinuum laser with wider bandwidth can be more effectively generated, and the synergistic effect is rarely reported in the literature.
The super-continuous laser light source has the advantages of high brightness, strong power, wide frequency coverage range and the like, and has wide application value in the fields of basic science, information, medical treatment, environmental detection and the like. At present, the method for generating the supercontinuum laser utilizes the third-order nonlinear optical effect to widen the frequency range of the pump laser. The method for internationally generating the supercontinuum laser source mainly utilizes the interaction of the photonic crystal fiber and the high-power ultrashort pulse laser and utilizes the optical field local effect to improve the pumping energy density so as to more effectively excite the third-order nonlinear optical effect to widen the spectrum range of the pumping laser. The scheme still has the defects of low conversion efficiency, and insufficient broadening of the spectrum broadening range only in a visible spectrum band or an infrared spectrum band. In addition, the supercontinuum laser light source in this solution is not a completely coherent laser light source, that is, the radiation light within a specific narrow band is coherent laser light, but there is no good coherence between the radiation light rays of different colors, which limits the application range of the light source. For example, the ultra-continuous laser light source cannot be used to generate ultra-short pulse laser light.
Disclosure of Invention
According to one aspect of the application, a device for generating broadband ultra-continuous laser is provided, which comprises a pumping light source and a nonlinear laser crystal, wherein the pumping light source is a near-infrared femtosecond pulse laser light source (the spectral bandwidth of the near-infrared femtosecond pulse laser is about hundred nanometers, and the femtosecond pulse laser has the characteristic of high peak power, the peak power can far exceed the optical damage power threshold of a crystal material under continuous laser pumping, and can reach GW/cm2Of the order of magnitude) of the pump light source, the pump light source producing a broadband supercontinuum laser output upon illumination through the nonlinear laser crystal, the nonlinear laser crystal comprising a series of cells whose lengths in a light propagation direction vary along the light propagation direction according to a continuous chirp variation.
Wherein, the nonlinear laser crystalThe length of each of the unit cells in the z direction
According to the formula
Calculating and determining, wherein z represents a position coordinate in a z direction corresponding to a certain cell, the position coordinate is a coordinate at the beginning of the cell, the z direction is the light propagation direction, and the z direction is the light propagation direction
The polarization period required by the frequency doubling process corresponding to the central wavelength of the near-infrared femtosecond pulse laser source is chirp degree.
Wherein the length L of the nonlinear laser crystal is the length of N unit cells
Sum of said polarization period and said chirp degree
And the numerical value of the cell cycle number N of the nonlinear laser crystal is combined to enable the reciprocal lattice vector of the nonlinear laser crystal to be presented as a plurality of reciprocal lattice vector bands distributed at different positions.
The plurality of reciprocal lattice vector bands in the nonlinear laser crystal respectively correspond to nonlinear frequency conversion processes participated by the broadband super-continuous laser with different wave bands, and each reciprocal lattice vector band effectively compensates the nonlinear frequency conversion process participated by the broadband super-continuous laser with continuous spectrum distribution.
The central wavelength of the pump light source is 1300nm, the nonlinear laser crystal can effectively broaden the spectrum of the pump light source, and the bandwidth of the pump light source is expanded from the initial 100nm to the range of nearly 300-500nm, so that more frequency components can participate in the nonlinear frequency conversion process.
The inverted lattice vector bands respectively correspond to the nonlinear frequency conversion processes of second and third harmonics of the broadened wave band of the pumping light source, effective phase compensation is provided for the nonlinear frequency conversion processes, and the wave bands of the second and third harmonics can be combined to form the broadband super-continuous laser output covering the whole visible-near infrared wave band with the bandwidth of nearly 500 nm. Due to the wide designed reciprocal lattice vector band, the output laser can be tuned in a wide frequency spectrum. Due to the interaction of the high-strength femtosecond pump light and the nonlinear crystal, the strong third-order nonlinear effect can be excited, the bandwidth of the pump light can be effectively widened, and richer frequency components can be provided for nonlinear frequency conversion. And then, second and third harmonic waves are simultaneously excited under the action of second-order nonlinear optics, so that the output laser can cover visible to near-infrared wave bands and has a bandwidth of nearly 500nm, and completely coherent broadband supercontinuum laser can be obtained.
The nonlinear laser crystal is made of a ferroelectric crystal material; the ferroelectric crystal material is a lithium niobate crystal material, a magnesium-doped lithium niobate crystal material or a lithium tantalate crystal material.
According to another aspect of the present application, there is provided a method for manufacturing a nonlinear laser crystal in the above apparatus for generating broadband supercontinuum laser, comprising the following steps:
step 1) preparing a mask having a specific pattern, the specific pattern on the mask corresponding to a series of cells of the nonlinear laser crystal, the length of the series of cells in a light propagation direction changing along the light propagation direction according to a continuous chirp variation;
step 2) spin-coating photoresist on the upper and lower surfaces of the nonlinear laser crystal, and transferring and curing the specific pattern on the mask plate on the photoresist by utilizing a photoetching technology;
and 3) respectively contacting the upper surface and the lower surface of the nonlinear laser crystal with a conductive medium, and adding an electric field which is larger than the coercive field of the crystal on the upper surface and the lower surface of the nonlinear laser crystal through the conductive medium, so that the polarization direction in the region which is not coated with the photoresist on the nonlinear laser crystal is reversed, and the region which is reserved with the photoresist still keeps the original polarization direction.
Wherein, step 1) still includes: trying different polarization periods, chirp levels
The number N of the cellular cycles and the length L of the nonlinear laser crystal, and determining the polarization period sequence of the cellular corresponding to various parameter value combinations according to a formula, wherein
The length of the unit cells in the z direction is expressed, the z direction is a light propagation direction, then inverse lattice vector band combinations corresponding to polarization period sequences of the unit cells are simulated by carrying out Fourier transformation on positive and negative domain structures of the unit cells, wave bands of finally output laser are simulated according to a phase mismatch condition, an optimal parameter value combination is preferably selected according to the required wave band of the output laser, and the specific pattern on the mask is determined according to the polarization period sequences of the unit cells corresponding to the optimal parameter value combination.
Wherein, step 1) still includes: if the simulated wave band of the finally output laser is continuous and can cover the wave band from visible to near infrared spectrum, the corresponding parameter value combination is judged to be the optimal parameter value combination, and the specific pattern on the mask is determined according to the cellular polarization period sequence corresponding to the optimal parameter value combination.
A final aspect of the invention utilizes high peak power (GW/cm)2Magnitude) near-infrared band femtosecond pulse laser and lithium niobate crystal material, and the frequency spectrum of the pump laser is remarkably broadened from 100nm magnitude to 300-500nm magnitude, thereby being greatly beneficial to obtaining broadband super-continuous laser generation covering visible and near-infrared bands through second-order nonlinear optical effect (second and third harmonic generation).
Compared with the prior art, the invention has the following technical effects:
1. according to the scheme of the invention, the pumping spectrum is effectively broadened by using the third-order nonlinear effect of the nonlinear laser crystal, and the efficient nonlinear frequency conversion is realized by using the second-order nonlinear optical effect, so that the broadband supercontinuum laser covering the bandwidth of near 500nm from visible to near-infrared wave bands can be generated.
2. The crystal has the advantages of controllable structure, easy preparation and flexible design.
3. The laser generating device has high energy conversion efficiency and ultra-wideband and super-continuous laser spectrum.
4. The device size of the super-continuous laser generating device is small.
Drawings
Embodiments of the invention are described in detail below with reference to the attached drawing figures, wherein:
FIG. 1 shows a spectral profile of a pumped near-infrared femtosecond pulsed laser in one embodiment of the invention;
FIG. 2 shows a schematic design of a chirped nonlinear photonic crystal in an embodiment of the present invention;
FIG. 3 shows a reciprocal lattice-vector band distribution plot for a nonlinear photonic crystal in one embodiment of the present invention; wherein the ordinate is an effective nonlinear coefficient, and the abscissa is a numerical value of a reciprocal lattice vector;
FIG. 4 is a graph showing a spectrum distribution after broadening of the third-order nonlinear optical effect of a lithium niobate crystal in an embodiment of the present invention;
fig. 5 shows the supercontinuum distribution of the chirped lithium niobate nonlinear photonic crystal under the synergistic effect of the second-order and third-order nonlinear optical effects according to one embodiment of the invention.
Detailed Description
In one embodiment of the invention, the device for generating the broadband ultra-continuous laser comprises a pump light source and a nonlinear laser crystal, wherein the pump light source is a near-infrared femtosecond pulse laser light source; the nonlinear laser crystal includes a series of cells whose lengths in a light propagation direction vary along the light propagation direction in accordance with a continuous chirp variation. The central wavelength range of the pump light is 1200-1500nm, the continuously adjustable single pulse energy is about 50 uJ, the Fourier limit of the pulse width is about 50 fs, the pulse repetition frequency is 1 kHz, the corresponding average power is about 60 mW, and the pulse laser is a completely coherent broadband pump light. Fig. 1 shows the spectral distribution of the near-infrared pumped femtosecond pulse laser in the present embodiment at different center wavelengths, and the spectral width of the pumping light source is about 100 nm. In the present invention, parameters such as the laser wavelength, the single pulse energy, the pulse length, the pulse repetition frequency, the average power, and the like of the near-infrared femtosecond pulse laser as the pump light can be adjusted according to the actual situation.
In this embodiment, the chirped nonlinear photonic crystal is used as a nonlinear photonic crystal that receives the pump light and outputs broadband supercontinuum laser covering a bandwidth of approximately 500nm from the visible to near-infrared band. In this embodiment, the nonlinear laser crystal refers to a nonlinear superlattice crystal having a laser frequency conversion function. The chirped nonlinear photonic crystal is a photonic crystal formed by applying a proper change rule to a polarization period on the basis of a periodic nonlinear photonic crystal, namely applying continuous chirp change to the length of the polarization period along the light propagation direction. For ease of understanding, a periodic nonlinear photonic crystal is first introduced.
The traditional periodic nonlinear photonic crystal is formed by a series of unit cells which are arranged periodically. Each cell contains one positive domain and one negative domain. The lengths of the respective unit cells in the light propagation direction are equal, for example, the lengths of the respective unit cells in the light propagation direction are equal to the lattice period of the nonlinear photonic crystal. The reciprocal lattice vector of the periodic nonlinear photonic crystal is a series of discrete lines with periodic intervals, and the reciprocal lattice vector can only provide effective phase compensation for nonlinear frequency conversion of laser with a certain wavelength. Therefore, the periodic nonlinear photonic crystal cannot be applied to a nonlinear frequency conversion process of a broadband laser (e.g., a femtosecond pulse laser).
The chirped nonlinear photonic crystal of the embodiment is based on a periodic nonlinear photonic crystal, and a proper change rule is applied to a polarization period, that is, a continuous chirp change is applied to the length of the polarization period along the light propagation direction. In this embodiment, the chirped nonlinear photonic crystal is composed of a series of cells, wherein the lengths of the negative domain and the positive domain of each cell are simultaneously changed so that the lengths of the series of cells satisfy the rule of chirp change. Fig. 2 shows a schematic design diagram of the chirped nonlinear photonic crystal in the present embodiment. Referring to fig. 2, a coordinate system is established with the transmission direction of the laser light at the leftmost end (i.e., the pump light input end) of the photonic crystal as the z-direction shown in fig. 2. The length of the polarization period is the length (also referred to as width) of the unit cell of the photonic crystal in the z direction, the length of the series of unit cells in the z direction changes along the z direction according to a continuous chirp change rule, and the "continuous chirp change" is a value of a function curve of the length of the series of unit cells in the photonic crystal in the z direction changing from small to large or from large to small along the chirp.
Starting from actual requirements, a polarization period, a chirp degree and a sample length L (namely a photonic crystal length) required by a frequency doubling process corresponding to the central wavelength of a proper pump light are selected, and then the length of each unit cell in the z direction is determined according to the polarization period, the chirp degree and the sample length L
Wherein z represents a position coordinate in the z direction corresponding to a certain cell. In this embodiment, the position coordinate of one cell in the z direction is the coordinate of the start of the cell, for example, the position coordinate of the first cell is 0, and the length of the first cell in the z direction is 0, so that the position coordinate of the second cell is the coordinate of the boundary between the first cell and the second cell.
In this example, the chirp rate was 14.2 μm-2The wavelength of the pump light was set to 1.45 μm and the crystal length was about 1.1 cm. Polarization period required by frequency doubling process corresponding to center wavelength
The refractive index of fundamental light having a wavelength is shown, and the wavelength is shown
The refractive index of light of double frequency of the fundamental frequency light.
As shown in fig. 3, the reciprocal lattice vector distribution of the chirped nonlinear photonic crystal having the above structure and parameters is a plurality of reciprocal lattice vector bands, each of which can provide reciprocal lattice vectors for the original frequency bandwidth range of the supercontinuum pulsed laser and the second-order nonlinear frequency conversion process (such as frequency doubling and sum frequency) within the extended frequency bandwidth range to compensate for the phase mismatch.
The energy density of the pumping near-infrared femtosecond pulse laser is very high, a remarkable third-order nonlinear optical effect is generated in the laser crystal, and the bandwidth of the pumping laser is remarkably expanded. Fig. 4 shows the original spectral distribution of the pump laser with a center wavelength of 1300nm in this embodiment, and the pump spectral distribution after passing through the nonlinear crystal used in this embodiment. The high-intensity pump laser generates remarkable third-order nonlinear optical effect broadening in the nonlinear crystal, and the spectrum of the pump laser is expanded from the initial spectral width of near 100nm to near 300-500 nm. Therefore, the second-order nonlinear frequency conversion can not only occur in the original spectrum range of the pump laser, and the spectrum broadening effect of the third-order nonlinear optical process provides richer frequency components for the second-order nonlinear effect (such as frequency doubling and sum frequency), so that the generation of the broadband supercontinuum laser becomes possible.
Fig. 5 shows the spectrum of the super-continuum laser output by the nonlinear laser crystal of this embodiment under the excitation of the pulse laser with different pump wavelengths, and the wavelength of the output laser can completely cover the broadband super-continuum laser output with the bandwidth of near 500nm in the visible to near-infrared band.
In this embodiment, the chirped nonlinear photonic crystal is made of a lithium niobate crystal material. The lithium niobate is a ferroelectric nonlinear crystal material with excellent comprehensive performance, has large nonlinear coefficient, has the transparent range of 310-5000nm and covers the wavelength range from ultraviolet to middle infrared. In other embodiments, the chirped nonlinear photonic crystal can also be made of magnesium-doped lithium niobate crystal or lithium tantalate crystal material.
It should be noted that, in the currently popular super-continuous light source of photonic crystal fiber, since the length of the optical fiber is about 1 meter, and the optical fiber includes a complex porous micro-nano structure, the geometric structure along the direction of the optical fiber has large non-uniformity (much larger than that of the lithium niobate crystal material), for example, the side wall of the hole has a plurality of concave-convex micro-nano structures, which has a strong diffuse scattering effect on the optical wave. In such long distance process, various third-order nonlinear interactions (self-phase modulation, four-wave mixing, raman scattering, etc.) occur, there is no way to maintain the coherence of the pump pulse laser, so that the coherence of the generated super-continuum light source is far inferior to that of the pump pulse laser, and also far inferior to that of the super-continuum laser generated by the interaction of the pump pulse laser of this embodiment and the lithium niobate nonlinear crystal higher harmonic, and in addition, the method has limited laser spectrum broadening capability, and cannot generate the broad spectrum super-continuum laser in this embodiment. Therefore, the super-continuum light source is a partially coherent laser light source in nature, and is not a truly fully coherent laser light source. In the embodiment, only a single nonlinear crystal is utilized to successfully realize the high-efficiency fully coherent broadband supercontinuum laser output which can cover the bandwidth from visible light to near infrared light and is near 500 nm.
As described in the foregoing, in the foregoing embodiments, the chirped nonlinear photonic crystal is constituted by a series of cells in which the lengths of the negative and positive domains of each cell are varied such that the lengths of the series of cells satisfy the rule of chirp variation. Such chirped nonlinear photonic crystal structures are not unique.
In addition, as mentioned above, the reciprocal lattice vector distribution of the chirped nonlinear photonic crystal is a plurality of reciprocal lattice vector bands, and the design scheme of these reciprocal lattice vector bands is not unique, wherein the width (i.e. bandwidth) of the band and the center position of the band, and the band-to-band spacing can be flexibly adjusted as required, as long as the series of reciprocal lattice vector bands can respectively correspond to the nonlinear frequency conversions (e.g. frequency doubling, sum frequency, etc.) of the second and third harmonics of the pump light (i.e. fundamental light) band, so as to provide effective phase compensation for these nonlinear frequency conversions. The pump spectrum is effectively broadened by matching with the third-order nonlinear optical effect, so that more pump frequency components can participate in nonlinear frequency conversion, and wider spectral output is generated. The wave band combination of the multiple order subharmonics can completely cover the whole visible to near infrared wave band near 500nm bandwidth broadband super-continuous laser output, and the completely coherent super-continuous laser can be obtained.
In another embodiment of the present invention, a method for manufacturing a nonlinear laser crystal in the laser generating apparatus is provided, which includes the following steps:
step 1) structural design of a nonlinear laser crystal. According to the spectral distribution of the pumping laser, firstly, the frequency components of the pumping spectrum participating in the nonlinear process are determined, and the phase mismatch distribution in the generation process of the corresponding second harmonic and third harmonic is analyzed by combining the refractive index dispersion of the crystal material, so that the distribution range of the cell period of the required nonlinear laser crystal can be roughly determined. According to the formula
And adjusting the initial polarization period, the chirp degree and the number N of the cellular periods to obtain a cellular period sequence of the nonlinear laser crystal, so that the cellular period sequence falls within the distribution range of the cellular periods. Meanwhile, due to the consideration of the laser crystal preparation process, the total length L of the nonlinear laser crystal cannot be too long, so that the chirp degree and the number N of the cell cycles need to be adjusted, so that the total length L of the designed nonlinear laser crystal meets the process requirements.
And 2) analyzing the nonlinear laser crystal. Each cell in the nonlinear laser crystal comprises a positive domain and a negative domain, the Fourier transform is carried out on the designed crystal positive and negative domain structure crystal, the distribution of inverted lattice vector bands corresponding to a polarization period sequence of the cell can be simulated, the peak value in the inverted lattice band represents the structural inverted lattice vector which can be provided by the designed crystal structure, and effective phase compensation can be provided for phase mismatch in the nonlinear process. And the final output laser wave band can be simulated by matching the phase mismatch condition of the nonlinear process.
And 3) optimizing the structure of the nonlinear laser crystal. If the simulated output laser wave band can not completely meet the design requirement, the initial polarization period, the chirp degree, the cell period number N and the proportion of positive and negative domains in the cells are finely adjusted, the analysis of the step 2) is repeated, and the optimal parameter value combination is preferably selected until the simulated final laser output wave band is continuous and can cover visible to near infrared spectrum wave bands. And determining the pattern on the mask according to the cell polarization periodic sequence corresponding to the parameter value combination.
Step 4) preparing a mask with a specific pattern, wherein the specific pattern on the mask corresponds to a series of unit cells of the nonlinear laser crystal; the length of the series of cells in the light propagation direction varies along the light propagation direction according to a continuous chirp variation;
step 5) spin-coating photoresist on the upper and lower surfaces of the nonlinear laser crystal, and transferring and curing the specific pattern on the mask plate on the photoresist by utilizing a photoetching technology;
and 6) respectively contacting the upper surface and the lower surface of the nonlinear laser crystal with a conductive medium, and adding an electric field which is larger than the crystal coercive field on the upper surface and the lower surface of the nonlinear laser crystal through the conductive medium, so that the polarization direction in the region which is not coated with the photoresist on the nonlinear laser crystal is reversed, and the region which is reserved with the photoresist still keeps the original polarization direction.
Finally, it should be noted that the above examples are only intended to describe the technical solutions of the present invention and not to limit the technical methods, the present invention can be extended in application to other modifications, variations, applications and embodiments, and therefore all such modifications, variations, applications, embodiments are considered to be within the spirit and teaching scope of the present invention.
Claims (10)
1. An apparatus for generating broadband supercontinuum laser, comprising a pump light source and a nonlinear laser crystal, wherein the pump light source is a near infrared femtosecond pulse laser light source, the pump light source generates broadband supercontinuum laser output after irradiating through the nonlinear laser crystal, and the nonlinear laser crystal comprises a series of unit cells, and the length of the unit cells in the light propagation direction changes along the light propagation direction according to continuous chirp change.
2. The apparatus of claim 1, wherein the length of each of the unit cells in the nonlinear laser crystal in the z direction is larger than the length of each of the unit cells in the nonlinear laser crystal in the z direction
According to the formula
Calculating and determining, wherein z represents a position coordinate in a z direction corresponding to a certain cell, the position coordinate is a coordinate at the beginning of the cell, the z direction is the light propagation direction, and the z direction is the light propagation direction
A polarization period required by a frequency doubling process corresponding to the central wavelength of the near-infrared femtosecond pulse laser source,
is the chirp degree.
3. The apparatus according to claim 2, wherein the length L of the nonlinear laser crystal is the length of all N of the unit cells
And the numerical combination of the polarization period, the chirp degree and the number N of the cellular periods of the nonlinear laser crystal enables the reciprocal lattice vector of the nonlinear laser crystal to be presented as a plurality of reciprocal lattice vector bands distributed at different positions.
4. The apparatus according to claim 3, wherein the plurality of reciprocal lattice-vector bands in the nonlinear laser crystal correspond to nonlinear frequency conversion processes involving the broadband supercontinuum laser in different wavelength bands, respectively, and each reciprocal lattice-vector band effectively compensates the nonlinear frequency conversion process involving the broadband supercontinuum laser with a continuous spectrum distribution.
5. The apparatus as claimed in claim 3, wherein the central wavelength of the pump light source is 1300nm, the nonlinear laser crystal can effectively broaden the spectrum of the pump light source, and the bandwidth of the pump light source is expanded from 100nm to approximately 300-500nm, so that more frequency components can participate in the nonlinear frequency conversion process.
6. The apparatus of claim 5, wherein the plurality of reciprocal lattice vector bands correspond to the second and third harmonic nonlinear frequency conversion processes of the broadened band of the pump light source, respectively, so as to provide effective phase compensation for the nonlinear frequency conversion processes, and enable the second and third harmonic bands to combine to form the broadband supercontinuum output covering the entire visible-near infrared band with a bandwidth of approximately 500 nm.
7. The device for generating broadband supercontinuum laser according to claim 1, wherein the nonlinear laser crystal is made of ferroelectric crystal material; the ferroelectric crystal material is a lithium niobate crystal material, a magnesium-doped lithium niobate crystal material or a lithium tantalate crystal material.
8. A method for making the nonlinear laser crystal in claim 1, comprising the steps of:
step 1) preparing a mask having a specific pattern, the specific pattern on the mask corresponding to a series of cells of the nonlinear laser crystal, the length of the series of cells in a light propagation direction changing along the light propagation direction according to a continuous chirp variation;
step 2) spin-coating photoresist on the upper and lower surfaces of the nonlinear laser crystal, transferring the specific pattern on the mask plate and curing the specific pattern on the photoresist;
and 3) respectively contacting the upper surface and the lower surface of the nonlinear laser crystal with a conductive medium, and adding an electric field which is larger than the coercive field of the crystal on the upper surface and the lower surface of the nonlinear laser crystal through the conductive medium, so that the polarization direction in the region which is not coated with the photoresist on the nonlinear laser crystal is reversed, and the region which is reserved with the photoresist still keeps the original polarization direction.
9. The method for manufacturing a nonlinear laser crystal according to claim 8, wherein the step 1) further comprises: trying different polarization periods, chirp levels
The number N of the cellular cycles and the length L of the nonlinear laser crystal, and determining the polarization period sequence of the cellular corresponding to various parameter value combinations according to a formula, wherein
The length of the unit cells in the z direction is expressed, the z direction is a light propagation direction, then inverse lattice vector band combinations corresponding to polarization period sequences of the unit cells are simulated by carrying out Fourier transformation on positive and negative domain structures of the unit cells, wave bands of finally output laser are simulated according to a phase mismatch condition, an optimal parameter value combination is preferably selected according to the required wave band of the output laser, and the specific pattern on the mask is determined according to the polarization period sequences of the unit cells corresponding to the optimal parameter value combination.
10. The method for manufacturing a nonlinear laser crystal according to claim 9, wherein the step 1) further comprises: if the simulated wave band of the finally output laser is continuous and can cover the wave band from visible to near infrared spectrum, the corresponding parameter value combination is judged to be the optimal parameter value combination, and the specific pattern on the mask is determined according to the cellular polarization period sequence corresponding to the optimal parameter value combination.
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112099286A (en) * | 2020-09-29 | 2020-12-18 | 清华大学 | Optical harmonic generator and method for producing the same |
| CN114637154A (en) * | 2022-02-28 | 2022-06-17 | 西安交通大学 | Cascade periodic polarization electro-optic crystal structure for optical phased array |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN106848809A (en) * | 2017-03-06 | 2017-06-13 | 李志远 | A kind of generation is visible to infrared band pole broadband, the device of super continuous laser |
| CN107516806A (en) * | 2016-06-16 | 2017-12-26 | 中国科学院物理研究所 | Produce the device and non-linear laser crystal preparation method of supercontinuum laser |
| CN107732642A (en) * | 2017-10-01 | 2018-02-23 | 李志远 | Produce the device and non-linear laser crystal preparation method of the visible super continuous laser near infrared band wideband adjustable |
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Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN107516806A (en) * | 2016-06-16 | 2017-12-26 | 中国科学院物理研究所 | Produce the device and non-linear laser crystal preparation method of supercontinuum laser |
| CN106848809A (en) * | 2017-03-06 | 2017-06-13 | 李志远 | A kind of generation is visible to infrared band pole broadband, the device of super continuous laser |
| CN107732642A (en) * | 2017-10-01 | 2018-02-23 | 李志远 | Produce the device and non-linear laser crystal preparation method of the visible super continuous laser near infrared band wideband adjustable |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112099286A (en) * | 2020-09-29 | 2020-12-18 | 清华大学 | Optical harmonic generator and method for producing the same |
| CN112099286B (en) * | 2020-09-29 | 2021-09-03 | 清华大学 | Optical harmonic generator and method for producing the same |
| CN114637154A (en) * | 2022-02-28 | 2022-06-17 | 西安交通大学 | Cascade periodic polarization electro-optic crystal structure for optical phased array |
| CN114637154B (en) * | 2022-02-28 | 2023-12-22 | 西安交通大学 | Cascaded periodically polarized electro-optic crystal structure for optical phased array |
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