Mid-infrared frequency-sweeping laser and method for implementing frequency-sweeping laser in mid-infrared band

Abstract

The present invention discloses a mid-infrared frequency sweeping laser and a method for realizing a frequency sweeping laser in a mid-infrared band. The mid-infrared frequency sweeping laser comprises: a continuous light laser, a wide-spectrum laser, a dichroic mirror assembly, a wavelength selection device, a periodic polarization crystal and a concave mirror. The continuous light laser is used as pump light; the wide-spectrum laser is used as signal light; the dichroic mirror assembly is used to synthesize the pump light and the signal light into one beam and inject it into the periodic polarization crystal, and maintain the intracavity oscillation of the idle light excited in the cavity; the periodic polarization crystal is used to convert the pump light and the signal light into idle light in the mid-infrared band; the concave mirror is used to reflect the idle light to the dichroic mirror assembly and then to the wavelength selection device; the wavelength selection device is used to select a certain wavelength of idle light in the idle light and reflect it to the periodic polarization crystal through the dichroic mirror for amplification, and output the amplified idle light. The present invention can increase the scanning frequency of the mid-infrared frequency sweeping laser.

Description

Technical Field

[0001] The invention relates to the technical field of lasers, and in particular to a mid-infrared frequency sweeping laser and a method for realizing a frequency sweeping laser in a mid-infrared band. Background Art

[0002] The existing mid-infrared (MIR) spectral region refers to the wavelength band from 2.5 to 25μm. A swept laser refers to a laser with tunable wavelength and very fast tuning speed. The output wavelength of this laser has a one-to-one correspondence with the time domain signal. At present, quantum cascade lasers (QCLs) and inter-band cascade lasers (ICLs) can already achieve swept laser output in the mid-infrared band. Such swept lasers can be used for rapid or even real-time detection of gases and swept source optical coherence tomography (SS-OCT) in the mid-infrared band.

[0003] A current implementation method of frequency-sweeping lasers is to place a wavelength-selective device in the laser cavity, use the wavelength-selective device to allow only one wavelength in the cavity to achieve laser oscillation output at a certain time, and then continuously switch the wavelength of the wavelength-selective device to achieve frequency-sweeping laser output. The frequency of the frequency-sweeping laser achieved in this way is limited by the frequency of the wavelength-selective device, the energy level lifetime of the doped ions, and the cavity length of the laser. However, since the energy level lifetime of doped ions in doped optical fibers or doped crystals is usually in the microsecond range, the frequency-sweeping lasers in the mid-infrared band are limited by the energy level lifetime of the doped ions, resulting in a relatively low scanning frequency of the mid-infrared frequency-sweeping lasers, which is only tens or hundreds of hertz. The too low scanning frequency limits the scanning speed of OCT and mid-infrared gas detection.

[0004] Therefore, the prior art still needs to be improved and developed. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a mid-infrared swept laser and a method for implementing a swept laser in the mid-infrared band, so as to solve the problem that the scanning frequency of the existing mid-infrared band swept laser is low, which limits the scanning speed of mid-infrared band OCT and mid-infrared gas detection technology.

[0006] The technical solution of the present invention is as follows:

[0007] A mid-infrared frequency sweeping laser comprises: a continuous light laser, a wide spectrum laser, a dichroic mirror assembly, a wavelength selection device, a periodic polarization crystal and a concave mirror; the continuous light laser is used as pump light; the wide spectrum laser is used as signal light; the dichroic mirror assembly is used to combine the pump light and the signal light into one beam and inject it into the periodic polarization crystal, and maintain the intracavity oscillation of the idle light excited in the cavity; the periodic polarization crystal is used to transform the pump light and the signal light into idle light in the mid-infrared band by using the difference frequency effect; the concave mirror is used to reflect the idle light to the dichroic mirror assembly and then to the wavelength selection device; the wavelength selection device is used to select an idle light of a certain wavelength in the idle light and reflect it to the periodic polarization crystal through the dichroic mirror for amplification and oscillation, wherein when the wavelength selection device switches to the idle light of the next wavelength, the wavelength selection device outputs the previous amplified idle light output.

[0008] The present invention further provides that the dichroic mirror assembly comprises: a first dichroic mirror and a second dichroic mirror; wherein,

[0009] The first dichroic mirror and the second dichroic mirror are arranged horizontally, and the first dichroic mirror is used to combine the pump light and the signal light into a beam and inject the beam into the second dichroic mirror;

[0010] The second dichroic mirror is used to inject the pump light and the signal light into the periodically polarized crystal.

[0011] The present invention further arranges that the first dichroic mirror has high transmittance to the pump light and high reflectivity to the signal light;

[0012] The continuous light laser is arranged horizontally with the first dichroic mirror;

[0013] The wide spectrum laser is arranged perpendicular to the second dichroic mirror.

[0014] The present invention further provides that the first dichroic mirror has high reflectivity to the pump light and high transmittance to the signal light;

[0015] The continuous light laser is arranged perpendicularly to the first dichroic mirror;

[0016] The wide spectrum laser and the first dichroic mirror are arranged horizontally.

[0017] The present invention further provides that the wavelength selection device comprises: an acousto-optic tunable filter, a first arbitrary waveform generator and a total reflective mirror; wherein,

[0018] The acousto-optic tunable filter is used to select an idler light of a certain wavelength according to the idler light, and maintain the selected idler light to oscillate and amplify in the cavity;

[0019] The first arbitrary waveform generator is connected to the acousto-optic tunable filter, and the arbitrary waveform generator is used to output a radio frequency signal to control the acousto-optic tunable filter to select an idler light of a certain wavelength;

[0020] The total reflective mirror is used to reflect the idler light selected by the acousto-optic tunable filter to the dichroic mirror assembly.

[0021] The present invention further provides that the wavelength selection device comprises: a polygonal reflector, a grating and a second arbitrary waveform generator; wherein,

[0022] The grating is used to disperse the idler light reflected by the concave mirror according to the wavelength and hit the polygonal reflector;

[0023] The polygonal reflector is used to select idle light of a certain wavelength, so as to re-reflect the idle light of a certain wavelength that meets the angle into the cavity;

[0024] The second arbitrary waveform generator is connected to the polygonal reflector, and the arbitrary waveform generator is used to control the rotation speed of the polygonal reflector to control the polygonal reflector to reflect idler light of a specific angle to the dichroic mirror assembly.

[0025] The present invention further provides that the periodically poled crystal is one of a periodically poled lithium niobate crystal, a periodically poled magnesium oxide-doped lithium niobate crystal or a periodically poled potassium titanyl phosphate crystal.

[0026] The present invention further provides that the dichroic mirror assembly, the periodically polarized crystal and the concave mirror are located on the same straight line.

[0027] Based on the same inventive concept, the present invention also provides a method for implementing a swept-frequency laser in the mid-infrared band applied to the above-mentioned mid-infrared swept-frequency laser, which comprises:

[0028] The pump light generated by the continuous light laser and the signal light generated by the wide spectrum laser are received by the dichroic mirror assembly, and a beam of light is synthesized and then injected into the periodically polarized crystal;

[0029] The periodically polarized crystal transforms the pump light and the signal light into idler light in the mid-infrared band by using the difference frequency effect;

[0030] Reflecting the idle light to the dichroic mirror assembly and then to the wavelength selection device through a concave mirror;

[0031] The wavelength selection device selects idle light of a certain wavelength from the idle light and reflects the idle light to the periodically polarized crystal through the dichroic mirror for amplification and oscillation, and outputs the amplified idle light.

[0032] The present invention is further configured that the idle light of a certain wavelength selected by the wavelength selection device is amplified multiple times by the periodically polarized crystal and then output by the wavelength selection device; wherein, when the wavelength selection device switches to the idle light of the next wavelength, the wavelength selection device outputs the previously amplified idle light.

[0033] The present invention provides a mid-infrared frequency sweeping laser and a method for realizing frequency sweeping laser in the mid-infrared band. The mid-infrared frequency sweeping laser includes: a continuous light laser, a wide-spectrum laser, a dichroic mirror assembly, a wavelength selection device, a periodic polarization crystal and a concave mirror. The present invention receives the pump light generated by the continuous light laser and the signal light generated by the wide-spectrum laser through the dichroic mirror assembly and synthesizes them into a beam of light and then injects them into the periodic polarization crystal. Thereafter, the periodic polarization crystal uses the difference frequency effect to convert the pump light and the signal light into idle light in the mid-infrared band, and reflects the idle light to the dichroic mirror assembly through the concave mirror and then to the wavelength selection device. The wavelength selection device can select a certain wavelength of idle light in the idle light and reflect it to the periodic polarization crystal through the dichroic mirror for amplification and oscillation. When the wavelength selection device switches to the idle light of the next wavelength, the previous amplified idle light is output to realize laser frequency sweeping output. In this way, the present invention obtains idler light after nonlinear conversion of pump light through a periodically poled crystal, selects idler light of a certain wavelength through a wavelength selector and re-reflects it to the periodically poled crystal for amplification and then outputs it to achieve mid-infrared band swept laser output, wherein the time for the periodically poled crystal to perform nonlinear conversion is very short, thereby being able to increase the scanning frequency of the mid-infrared band swept laser, and further increasing the scanning speed of mid-infrared band OCT and mid-infrared gas detection technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary personnel in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0035] Figure 1 It is a structural schematic diagram of the mid-infrared frequency-sweeping laser in the present invention.

[0036] Figure 2 This is the conversion efficiency bandwidth diagram of PPLN crystals in different bands.

[0037] Figure 3 It is a schematic structural diagram of an infrared frequency sweeping laser in one embodiment of the present invention.

[0038] Figure 4 The figure is a schematic structural diagram of an infrared frequency sweeping laser in another embodiment of the present invention.

[0039] Figure 5 It is a flow chart of a method for realizing a frequency sweeping laser in a mid-infrared band of the present invention.

[0040] The marks in the accompanying drawings are: 1. continuous light laser; 2. wide-spectrum laser; 3. dichroic mirror assembly; 31. first dichroic mirror; 32. second dichroic mirror; 4. periodically polarized crystal; 5. concave mirror; 6. wavelength selection device; 61. acousto-optic tunable filter; 62. first arbitrary waveform generator; 63. total reflection mirror; 64. polygonal reflection mirror; 65. grating; 66. second arbitrary waveform generator. DETAILED DESCRIPTION

[0041] The present invention provides a mid-infrared frequency sweeping laser and a method for implementing a frequency sweeping laser in the mid-infrared band. In order to make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0042] In the embodiments and the scope of the patent application, unless the text specifically defines the article, "a", "an", "the" and "the" may also include plural forms. If there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features.

[0043] It should be further understood that the term "comprising" used in the specification of the present invention refers to the presence of the features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof. It should be understood that when we refer to an element as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or there can be intermediate elements. In addition, the "connection" or "coupling" used herein can include wireless connection or wireless coupling. The term "and / or" used herein includes all or any unit and all combinations of one or more associated listed items.

[0044] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless specifically defined as herein.

[0045] In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the fact that ordinary technicians in the field can implement it. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0046] The inventors have found that swept lasers are mature spectral characterization tools in the near-infrared band, usually based on semiconductor or fiber lasers. Although this concept can be extended to the mid-infrared band, the lack of suitable gain media makes the implementation of mid-infrared swept sources very complicated. Although it is possible to down-convert, sweep sources or frequency combs through nonlinear parameters of near-infrared tunable lasers, solid gain materials such as Cr:ZnS can also be explored.

[0047] However, compared with solid / fiber swept sources, the performance of quantum cascade lasers and interband cascade lasers in the mid-infrared band is currently mediocre and has low power scalability. Such swept lasers can be used for rapid or even real-time detection of gases and mid-infrared OCT.

[0048] One application area of ​​mid-infrared swept lasers is gas sensing. Since there are strong rotation-vibration molecular absorption peaks in the mid-infrared band, gas molecules have strong absorption peaks in the mid-infrared spectral region. Sensitive detection of gas can be achieved by measuring the changes before and after these absorption peaks. The key to realizing this application is broadband mid-infrared light sources and detection technology. However, in these fields, mid-infrared band devices usually lag behind the performance of their near-infrared devices. Although many sensing systems based on molecular vibration peaks have been demonstrated at near-infrared wavelengths (0.7-2.5μm), these absorption features are several orders of magnitude weaker than the basic absorption of mid-infrared, which ultimately limits the detection sensitivity. However, mid-infrared band spectral detection equipment, such as spectrometers, are currently less used and developed later, so the price is relatively expensive, usually around one million, and the volume is relatively large. This greatly limits the practical application of mid-infrared sensitive gas measurement. Since the swept laser has the characteristics of one-to-one correspondence between spectral signals and time domain signals, the intensity of the spectral signal can be obtained by measuring the intensity of the time domain signal. The devices for time domain signal detection, such as photodetectors, are very cheap due to their maturity, and are basically about one-fiftieth of the price of a mid-infrared spectrometer. Therefore, by using this feature of swept frequency lasers, low-cost and fast mid-infrared gas spectrum detection can be achieved.

[0049] The second area is mid-infrared optical coherence tomography (OCT) technology. Traditional OCT technology generally uses near-infrared or visible light sources. Near-infrared light sources have low scattering and good penetration in biological samples, but cannot penetrate emerging ceramic or polymer material samples, while mid-infrared light sources have good penetration in these samples. Therefore, mid-infrared OCT has a good prospect in the field of industrial detection. At the same time, the use of OCT for non-invasive investigation of paintings to provide information required for effective restoration and help conservation, art history and archaeology is also an emerging application. Operating in the long-wave band of about 2μm, it reduces the scattering of typical artistic pigments and brings the advantage of increased transmission depth. At present, there is an SD-OCT system that uses a mid-infrared supercontinuum laser as a light source and uses a grating and CCD for detection. However, the current mid-infrared supercontinuum laser needs to be built using a femtosecond laser as a pump source, so it is very expensive. At the same time, if SD-OCT is to perform high-speed detection, it requires a high-speed CCD for detection. At the same time, because the current CCD is in the visible to near-infrared band and cannot detect the mid-infrared band, a down-conversion module is required, which makes the entire system complicated. At the same time, the price of high-speed CCD is also very expensive. Therefore, the price of mid-infrared SD-OCT is expensive and the structure is complex, which is not conducive to the promotion and use of mid-infrared OCT. Due to its unique characteristics, the swept light source and photodetector of SS-OCT are only one-fifth of the price of SD-OCT.

[0050] Currently, Dy:ZBLAN optical fiber and wavelength selection devices have been used to realize swept fiber lasers in the mid-infrared band. However, due to the energy level lifetime of the doped ions in the Dy:ZBLAN optical fiber, the energy level lifetime of the doped ions is usually in the microsecond range (usually hundreds of microseconds). Therefore, the swept lasers in the mid-infrared band are limited by the energy level lifetime of the doped ions, resulting in a relatively low scanning frequency of only tens or hundreds of Hz. The too low scanning frequency limits the scanning speed of OCT and mid-infrared gas detection.

[0051] In view of the above problems, the present invention provides a mid-infrared frequency sweeping laser and a method for realizing frequency sweeping laser in the mid-infrared band. The mid-infrared frequency sweeping laser includes: a continuous light laser, a wide-spectrum laser, a dichroic mirror assembly, a wavelength selection device, a periodic polarization crystal and a concave mirror. The present invention receives the pump light generated by the continuous light laser and the signal light generated by the wide-spectrum laser through the dichroic mirror assembly and synthesizes them into a beam of light and then injects them into the periodic polarization crystal. Thereafter, the periodic polarization crystal uses the difference frequency effect to convert the pump light and the signal light into idle light in the mid-infrared band, and reflects the idle light to the dichroic mirror assembly through the concave mirror and then to the wavelength selection device. The wavelength selection device can select a certain wavelength of idle light in the idle light and reflect it to the periodic polarization crystal through the dichroic mirror for amplification and oscillation. When the wavelength selection device switches to the idle light of the next wavelength, the previous amplified idle light is output to realize laser frequency sweeping output. In this way, the present invention obtains idler light after nonlinear conversion of pump light through periodic polarization crystal, selects idler light of a certain wavelength through wavelength selector and re-reflects to periodic polarization crystal for amplification and then outputs to realize mid-infrared band swept laser output, wherein the nonlinear conversion time of periodic polarization crystal is very short, thus being able to increase the scanning frequency of mid-infrared band swept laser, thereby being able to improve

[0052] High scanning speed for OCT and mid-infrared gas detection. In addition, because the idle light can be oscillated and amplified multiple times in the periodic 5-polarized crystal, a higher output power can be output by using an intracavity OPO (Optical Parament Oscillator). The higher power output can make the application of mid-infrared lasers more extensive without being limited by lower output power.

[0053] Please also see Figures 1 to 4 The present invention provides a preferred embodiment of a mid-infrared swept-frequency laser.

[0054] 0 Figure 1 As shown, the present invention provides a mid-infrared frequency sweeping laser, which includes: a continuous light

[0055] Laser 1, wide spectrum laser 2, dichroic mirror assembly 3, wavelength selection device 6, periodic polarization crystal 4 and concave mirror 5; the continuous light laser 1 is used as pump light; the wide spectrum laser 2 is used as signal light; the dichroic mirror assembly 3 is used to combine the pump light and the signal light into one beam

[0056] and injected into the periodically polarized crystal 4, and maintain the intracavity oscillation of the idler light excited in the cavity; the periodically polarized crystal 4 is used to utilize the difference frequency effect to convert the pump light and the signal light into the intermediate

[0057] The concave mirror 5 is used to reflect the idle light to the dichroic mirror assembly 3 and then to the wavelength selection device 6; the wavelength selection device 6 is used to select a certain wavelength of the idle light and reflect it to the periodically polarized crystal through the dichroic mirror.

[0058] The body 4 performs amplified oscillation, wherein when the wavelength selection device 6 switches to the idle frequency light of the next wavelength, the wavelength selection device 6 outputs the previous amplified idle frequency light output.

[0059] Specifically, the continuous light laser 1 is used as a pump source to output continuous pump light, and the wide spectrum laser 2 outputs signal light. The signal light can be continuous light or pulsed light. If it is pulsed light, the pulse period needs to be consistent with the external cavity (the dichroic mirror).

[0060] The periodic polarization crystal 4 and the concave mirror 5 form a resonant cavity, and the cavity length of the resonant cavity is matched. The periodic polarization crystal 4 and the dichroic mirror component 3 are arranged opposite to each other, and the concave mirror 5 and the periodic polarization crystal 4 are arranged opposite to each other.

[0061] After receiving the pump light generated by the continuous light laser 1 and the signal light generated by the wide spectrum laser 2, the dichroic mirror component 3 combines the pump light and the signal light into a beam of light and injects it into the cavity, so as to inject the pump light and the signal light into the periodically polarized crystal 4. Thereafter, the periodically polarized crystal 4 converts the pump light and the signal light into idler light in the mid-infrared band by using the difference frequency effect, and reflects the idler light to the dichroic mirror component 3 and then to the wavelength selection device 6 through the concave mirror 5, wherein the concave mirror 5 can maintain the collimation of the idler light in the resonant cavity, and the concave mirror 5 can be highly reflective to the idler light and highly transparent to the pump light and the signal light, thereby preventing the pump light from being reflected to the continuous light laser 1 and causing damage to the laser. After being reflected by the concave mirror 5, the idle light is further reflected by the dichroic mirror assembly 3 to the wavelength selection device 6. The wavelength selection device 6 can select idle light of a certain wavelength in the idle light and reflect it through the dichroic mirror assembly 3 to the periodically polarized crystal 4 for amplification and oscillation, and output the amplified idle light.

[0062] The principle that the periodically polarized crystal 4 can realize a wider mid-infrared swept-frequency laser is as follows: a wider difference frequency operation can be realized by performing quasi-phase matching through the periodically polarized crystal 4, and the near-infrared swept-frequency laser can be converted into a mid-infrared swept-frequency laser through the difference frequency. Among them, the quasi-phase matching technology refers to compensating for the phase mismatch between the pump light and the parametric light caused by the dispersion of light in the optical parametric process by periodically changing the spontaneous polarization direction of the crystal material, so as to obtain an enhancement of the nonlinear effect. For example, the wavelength of the pump light is 914nm, and the selected signal lights are 1500-1600nm and 1280-1380nm respectively. A PPLN crystal with a thickness of 5mm is used for simulation, and the wavelength bandwidth of the idler light obtained is as follows Figure 2 As shown by Figure 2 It can be seen that the 1500-1600nm band can be converted to the 2100-2300nm band by pumping with a 914nm laser, and the 1280-1330nm can be converted to the 2700-3200nm band. At the same time, the 3dB gain bandwidth of the two bands is hundreds of nm. Therefore, a wider mid-infrared swept laser can be achieved through the PPLN crystal.

[0063] In this way, the present invention obtains idler light after nonlinear conversion of pump light through periodic polarization crystal 4, selects idler light of a certain wavelength through wavelength selection device 6, and re-reflects to periodic polarization crystal 4 for amplification and then outputs to realize mid-infrared band swept laser output, wherein the periodic polarization crystal 4 performs nonlinear conversion for a very short time, and the required time is about 50fs, so the scanning frequency of the swept laser is limited to the wavelength selection device and the cavity length, and is not limited to the nonlinear conversion process. Compared with the traditional use of gain fiber or gain crystal, a faster sweep frequency can be achieved. Compared with the existing scanning frequency of tens or hundreds of Hz, the scanning frequency of the present invention can reach hundreds of kHz or even MHz, so the present invention can increase the scanning frequency of the mid-infrared band swept laser, and then can increase the scanning speed of OCT and mid-infrared gas detection. In addition, the present invention reflects the idler light into the resonant cavity through the wavelength selection device 6 for multiple oscillation amplification, and adopts the intra-cavity OPO method to output a larger power output. The higher power output can make the application of mid-infrared laser more extensive without being limited to lower output power. Moreover, since the idler light continuously oscillates in the cavity, the spectral linewidth of the output swept-frequency laser will continue to narrow, thereby making the swept-frequency laser have a longer coherence length.

[0064] It should be noted that an optical parametric oscillator (OPO) is a device for nonlinear optical frequency conversion, and an optical parametric oscillator generally includes a pump source, a resonant cavity, and a gain medium. The specific implementation method of an optical parametric oscillator is to convert a short-wavelength laser (pump light) into a long-wavelength laser (parametric light) via a nonlinear optical crystal in the resonant cavity. Parametric light has two different wavelengths, of which the shorter wavelength laser is usually called signal light, and the longer wavelength laser is called idler light, and the sum of the frequencies of the parametric light is strictly equal to the pump light frequency, satisfying the law of conservation of energy.

[0065] Please continue reading Figure 1 In a further implementation of an embodiment, the dichroic mirror assembly 3, the periodically polarized crystal 4 and the concave mirror 5 are located on the same straight line.

[0066] Specifically, after the dichroic mirror assembly 3 injects the pump light and the signal light into the periodically polarized crystal 4, the idle light in the mid-infrared band generated by the periodically polarized crystal 4 is reflected to the dichroic mirror assembly 3 via the concave mirror 5, and then the dichroic mirror assembly 3 is reflected to the wavelength selection device 6, and the wavelength selection device 6 reflects the idle light after wavelength selection and transmits it to the dichroic mirror assembly 3. After repeating this process several times, the selected idle light can continuously oscillate and amplify in the resonant cavity.

[0067] See also Figure 1 In a further implementation of an embodiment, the dichroic mirror assembly 3 includes: a first dichroic mirror 31 and a second dichroic mirror 32. The first dichroic mirror 31 and the second dichroic mirror 32 are arranged horizontally, the first dichroic mirror 31 is used to combine the pump light and the signal light into a beam and inject it into the second dichroic mirror 32; the second dichroic mirror 32 is used to inject the pump light and the signal light into the periodically polarized crystal 4.

[0068] Specifically, the first dichroic mirror 31 and the second dichroic mirror 32 are arranged side by side in parallel, wherein the first dichroic mirror 31 can be highly transparent or highly reflective to the pump light and the signal light, so as to combine the pump light and the signal light into one beam of light. The second dichroic mirror 32 can be highly transparent to the pump light and the signal light, so as to inject the pump light and the signal light into the periodically polarized crystal 4.

[0069] In some embodiments, the first dichroic mirror 31 is highly transparent to the pump light and highly reflective to the signal light. The continuous light laser 1 is arranged horizontally with the first dichroic mirror 31 ; the wide spectrum laser 2 is arranged vertically with the second dichroic mirror 32 .

[0070] Specifically, the first dichroic mirror 31, the continuous light laser 1 and the second dichroic mirror 32 are located on the same straight line, so that the pump light output by the continuous light laser 1 can be transmitted to the second dichroic mirror 32. The broadband laser 2 is vertically arranged to the first dichroic mirror 31, and can be located directly above or directly below the second dichroic mirror 32, so that the first dichroic mirror 31 can reflect the signal light output by the broadband laser 2 to the second dichroic mirror 32.

[0071] In some other embodiments, the first dichroic mirror 31 is highly reflective to the pump light and highly transparent to the signal light. The continuous light laser 1 is vertically arranged to the first dichroic mirror 31; the wide spectrum laser 2 is horizontally arranged to the first dichroic mirror 31.

[0072] Specifically, the broadband laser 2, the first dichroic mirror 31 and the second dichroic mirror 32 are located on the same straight line, so that the optical signal generated by the broadband laser 2 can be transmitted to the second dichroic mirror 32. The continuous light laser 1 is arranged vertically to the first dichroic mirror 31, for example, it can be located directly above or directly below the first dichroic mirror 31, so that the pump light generated by the continuous light laser 1 can be reflected from the first dichroic mirror 31 to the second dichroic mirror 32.

[0073] In a further implementation of an embodiment, the periodically poled crystal 4 is one of a periodically poled lithium niobate crystal (PPLN), a periodically poled magnesium oxide-doped lithium niobate crystal (PPKTP), or a periodically poled potassium titanyl phosphate crystal (MgO:PPLN), for example, PPLN. It should be noted that the present invention is not limited to the selection of the periodically poled crystal 4, as long as a wider conversion bandwidth can be achieved.

[0074] See also Figure 1 and Figure 3In some embodiments, the wavelength selection device 6 includes: an acousto-optic tunable filter 61, a first arbitrary waveform generator 62 and a total reflective mirror 63. The acousto-optic tunable filter 61 is used to select an idler light of a certain wavelength according to the idler light, and maintain the selected idler light to oscillate and amplify in the cavity; the first arbitrary waveform generator 62 is connected to the acousto-optic tunable filter 61, and the first arbitrary waveform generator 62 is used to output a radio frequency signal to control the acousto-optic tunable filter 61 to select an idler light of a certain wavelength; the total reflective mirror 63 is used to reflect the idler light selected by the acousto-optic tunable filter 61 to the dichroic mirror assembly 3.

[0075] Specifically, in this embodiment, the acousto-optic tunable filter 61 and the external cavity OPO technology are used to realize a mid-infrared swept-frequency laser. In this embodiment, a continuous light laser 1 with a continuous light of 914nm is used as a pump source, and a wide-spectrum laser 2 (1500-1600nm) outputs signal light. The signal light and the pump light are combined into one beam through the first dichroic mirror 31, wherein the first dichroic mirror 31 has high reflectivity for the signal light and high transmittance for the pump light. Then the signal light and the pump light are injected into the external cavity through the second dichroic mirror 32, and the second dichroic mirror 32 has high transmittance for the pump light and the signal light and high reflectivity for the idler light generated thereafter. Then the pump light and the signal light are injected into the periodically polarized crystal 4, for example, into a PPLN crystal, and the PPLN crystal generates idler light in the mid-infrared band (2.2μm) through difference frequency operation. After the idler light in the mid-infrared band is generated, the idler light in the mid-infrared band will be reflected into the acousto-optic tunable filter 61 through the concave mirror 5, wherein the acousto-optic tunable filter 61 is controlled by the first arbitrary waveform generator 62. The function of the concave mirror 5 is to maintain the collimation of the idler light cavity in the generated mid-infrared band, and the concave mirror 5 is highly transparent to the pump light and the signal light, so as to prevent the pump light laser from being reflected back into the pump light laser and causing damage to the laser.

[0076] The working principle of the acousto-optic tunable filter 61 is to select the corresponding wavelength of idler light by adding different radio frequency signals, and different radio frequency signals correspond to different wavelengths. The selected wavelength will be deflected at a certain angle, while other wavelengths will be directly output. In addition, by placing a total reflection mirror 63 in the deflection direction, the total reflection mirror 63 reflects the idler light of the selected wavelength to the second dichroic mirror 32 and then injects it into the periodically polarized crystal 4, so that the idler light of the selected wavelength is re-injected into the resonant cavity.

[0077] It can be seen that after the generated mid-infrared idle light enters the acousto-optic tunable filter 61, light of a certain wavelength will be selected, and the other light will be output. The idle light of this wavelength will be reflected back into the periodically polarized crystal 4 and amplified again by the pump light. After repeating the above process many times, the acousto-optic tunable filter 61 will switch to the next wavelength, and the amplified idle light will be output by the acousto-optic tunable filter 61. Continuously repeating the above process will achieve the swept frequency output of the mid-infrared laser. The sweeping speed of the swept frequency laser using the acousto-optic tunable filter 61 is generally several hundred kHz, and can even reach 1 MHz, which is nearly four orders of magnitude higher than the sweeping frequency of the previous mid-infrared swept frequency laser.

[0078] See also Figure 1 and Figure 4 In some other embodiments, the wavelength selection device 6 includes: a polygonal reflector 64, a grating 65 and a second arbitrary waveform generator 66. The grating 65 is used to disperse the idle light reflected by the concave mirror 5 according to the wavelength and hit the polygonal reflector 64; the polygonal reflector 64 is used to select the idle light of a certain wavelength to re-reflect the idle light of a certain wavelength that meets the angle into the cavity; the second arbitrary waveform generator 66 is connected to the polygonal reflector 64, and the second arbitrary waveform generator 66 is used to control the rotation speed of the polygonal reflector 64 to control the polygonal reflector 64 to reflect the idle light of a specific angle to the dichroic mirror assembly 3.

[0079] Specifically, in this embodiment, the polygonal reflector 64, the grating 65 and the external cavity OPO technology are used to realize a mid-infrared swept-frequency laser. A continuous light laser with a continuous light of 914nm is used as a pump source, and the wide-spectrum laser 2 (1500-1600nm) outputs signal light. The signal light and the pump light are combined into a beam through the first dichroic mirror 31. The first dichroic mirror 31 has high reflection for the signal light and high transmittance for the pump light. Then the signal light and the pump light are injected into the external cavity through the second dichroic mirror 32. The second dichroic mirror 32 has high transmittance for the pump light and the signal light and high reflection for the idle light generated afterwards. Then the pump light and the signal light are injected into the periodically polarized crystal 4, for example, injected into the MgO:PPLN crystal. The MgO:PPLN crystal generates idle light in the mid-infrared band (2.2μm) through difference frequency operation. After the idle light in the mid-infrared band is generated, the idle light in the mid-infrared band will be reflected onto the grating 65 through the concave mirror 5. The grating 65 disperses the idle light and hits the polygonal reflector 64, wherein the second arbitrary waveform generator 66 is used to control the rotation speed of the polygonal reflector 64. At this time, only light that meets a specific angle will be reflected back into the cavity by the reflector. The polygonal reflector 64 is continuously rotated by the control of the motor, so that the angle of the reflector also changes continuously with the control of the motor. At this time, light of different wavelengths will be injected into the oscillation cavity with the rotation of the reflector. The function of the concave mirror 5 is to maintain the collimation of the idle light cavity in the generated mid-infrared band. At the same time, the concave mirror 5 is highly transparent to the pump light and the signal light, preventing the laser from being damaged by the reflection into the pump light laser. At the same time, the concave mirror 5 is semi-transparent and semi-reflective to the idle light band, so it can be used as an output mirror for output. After the generated mid-infrared idle light enters the polygonal reflector 64 and grating 65, a certain wavelength of light will be selected, and the other light will be output. The idle light of this wavelength will be reflected back into the periodically polarized crystal 4 and amplified again by the pump light. After repeating the above process many times, the polygonal reflector 64 and grating 65 will switch to the next wavelength, and the amplified idle light will be output by the polygonal reflector 64. Repeating the above process continuously will achieve the swept frequency output of the mid-infrared laser.

[0080] It should be noted that the polygonal reflector 64 is limited by the rotation speed of the rotary motor, and the scanning frequency is generally several kHz to more than ten kHz, which is lower than the scanning speed using the acousto-optic tunable filter, but also dozens of times higher than the existing mid-infrared scanning laser.

[0081] It should be noted that the wavelength selection device usually controls the wavelength output by using an external timing signal. The wavelength selection device can be an acousto-optic tunable filter 61 or a combination of a multi-faceted reflector and a grating 65 pair, or an optical fiber Fabry-Perot tunable filter. Any device that can select the wavelength through an external signal pair can be considered as the wavelength selection device described in the present invention. At the same time, the wavelength selection device can be a structure with optical fiber coupled input or a structure with all-solid input. Similarly, the output form is not limited to solid or optical fiber.

[0082] See also Figure 5 In some embodiments, the present invention further provides a method for implementing a swept-frequency laser in a mid-infrared band applied to the mid-infrared swept-frequency laser described above, which comprises the steps of:

[0083] S100, receiving the pump light generated by the continuous light laser and the signal light generated by the wide-spectrum laser through a dichroic mirror assembly, synthesizing them into a beam of light and then injecting them into a periodically polarized crystal; the details are as described in an embodiment of a mid-infrared swept-frequency laser, which will not be repeated here.

[0084] S200, the periodically polarized crystal utilizes the difference frequency effect to convert the pump light and the signal light into idler light in the mid-infrared band; the details are as described in an embodiment of a mid-infrared swept-frequency laser, which will not be repeated here.

[0085] S300, reflecting the idle light to the dichroic mirror assembly and then to the wavelength selection device through a concave mirror; the details are as described in an embodiment of a mid-infrared swept-frequency laser, which will not be repeated here.

[0086] S400, the wavelength selection device selects a certain wavelength of idler light from the idler light and reflects it to the periodically polarized crystal through the dichroic mirror for amplification and oscillation, and outputs the amplified idler light. The details are as described in an embodiment of a mid-infrared swept-frequency laser, which will not be described in detail here.

[0087] In summary, the mid-infrared frequency-sweeping laser and the method for implementing the mid-infrared frequency-sweeping laser provided by the present invention have the following beneficial effects:

[0088] The pump light is nonlinearly converted by a periodically polarized crystal to obtain idler light, and a wavelength selector selects the idler light of a certain wavelength and re-reflects it to the periodically polarized crystal for amplification and then outputs it to achieve mid-infrared band swept laser output. The nonlinear conversion time of the periodically polarized crystal is very short, so the scanning frequency of the mid-infrared band swept laser can be increased, thereby increasing the scanning speed of OCT and mid-infrared gas detection.

[0089] The idle light is reflected into the resonant cavity through the wavelength selection device for multiple oscillation amplification, and the intracavity OPO is used to output a higher power output. The higher power output can make the application of mid-infrared lasers more extensive without being limited to lower output power.

[0090] As the idler light continuously oscillates in the cavity, the spectral linewidth of the output swept-frequency laser will continue to narrow, thereby making the swept-frequency laser have a longer coherence length.

[0091] It should be understood that the application of the present invention is not limited to the above examples. For ordinary technicians in this field, improvements or changes can be made based on the above description. All these improvements and changes should fall within the scope of protection of the claims attached to the present invention.

Claims

1. A mid-infrared swept-frequency laser, characterized in that: include: Continuous light lasers, broadband lasers, dichroic mirror assemblies, periodically polarized crystals, concave mirrors and wavelength selection devices; The continuous light laser is used as pump light; The wide spectrum laser is used as signal light; The dichroic mirror assembly is used to combine the pump light and the signal light into one beam and inject the beam into the periodically polarized crystal, and maintain the intracavity oscillation of the idler light excited in the cavity; The periodically polarized crystal is used to convert the pump light and the signal light into idler light in the mid-infrared band by utilizing the difference frequency effect; The concave mirror is used to reflect the idler light to the dichroic mirror assembly and then to the wavelength selection device; The wavelength selection device is used to select an idler light of a certain wavelength from the idler light and reflect it through the dichroic mirror to the periodically polarized crystal for amplification and oscillation; wherein, when the wavelength selection device switches to the idler light of the next wavelength, the previous amplified idler light is output.

2. The mid-infrared swept-frequency laser according to claim 1, characterized in that: The dichroic mirror assembly comprises: a first dichroic mirror and a second dichroic mirror; wherein, The first dichroic mirror and the second dichroic mirror are arranged horizontally, and the first dichroic mirror is used to combine the pump light and the signal light into a beam and inject the beam into the second dichroic mirror; The second dichroic mirror is used to inject the pump light and the signal light into the periodically polarized crystal.

3. The mid-infrared swept laser according to claim 2, characterized in that: The first dichroic mirror has high transmittance to the pump light and high reflectivity to the signal light; The continuous light laser is arranged horizontally with the first dichroic mirror; The wide spectrum laser is arranged perpendicular to the second dichroic mirror.

4. The mid-infrared swept laser according to claim 2, characterized in that: The first dichroic mirror is highly reflective to the pump light and highly transparent to the signal light; The continuous light laser is arranged perpendicularly to the first dichroic mirror; The wide spectrum laser and the first dichroic mirror are arranged horizontally.

5. The mid-infrared swept-frequency laser according to claim 1, characterized in that: The wavelength selection device comprises: an acousto-optic tunable filter, a first arbitrary waveform generator and a total reflective mirror; wherein, The acousto-optic tunable filter is used to select an idler light of a certain wavelength according to the idler light, and maintain the selected idler light to oscillate and amplify in the cavity; The first arbitrary waveform generator is connected to the acousto-optic tunable filter, and the arbitrary waveform generator is used to output a radio frequency signal to control the acousto-optic tunable filter to select an idler light of a certain wavelength; The total reflective mirror is used to reflect the idler light selected by the acousto-optic tunable filter to the dichroic mirror assembly.

6. The mid-infrared swept laser according to claim 1, characterized in that: The wavelength selection device comprises: a polygonal reflector, a grating and a second arbitrary waveform generator; wherein, The grating is used to disperse the idler light reflected by the concave mirror according to the wavelength and hit the polygonal reflector; The polygonal reflector is used to select idle light of a certain wavelength to reflect the idle light of a certain wavelength at a matching angle back into the cavity; The second arbitrary waveform generator is connected to the polygonal reflector, and the arbitrary waveform generator is used to control the rotation speed of the polygonal reflector to control the polygonal reflector to reflect idler light of a specific angle to the dichroic mirror assembly.

7. The mid-infrared swept-frequency laser according to claim 1, characterized in that: The periodically poled crystal is one of a periodically poled lithium niobate crystal, a periodically poled magnesium oxide-doped lithium niobate crystal or a periodically poled potassium titanyl phosphate crystal.

8. The mid-infrared swept-frequency laser according to claim 1, characterized in that: The dichroic mirror assembly, the periodically polarized crystal and the concave mirror are located on the same straight line.

9. A method for implementing a swept-frequency laser in a mid-infrared band applied to the mid-infrared swept-frequency laser according to any one of claims 1 to 8, characterized in that: include: The pump light generated by the continuous light laser and the signal light generated by the wide spectrum laser are received by the dichroic mirror assembly, and a beam of light is synthesized and then injected into the periodically polarized crystal; The periodically polarized crystal transforms the pump light and the signal light into idler light in the mid-infrared band by using the difference frequency effect; Reflecting the idle light to the dichroic mirror assembly and then to the wavelength selection device through a concave mirror; The wavelength selection device selects idle light of a certain wavelength from the idle light and reflects the idle light to the periodically polarized crystal through the dichroic mirror for amplification and oscillation, and outputs the amplified idle light.

10. The method for implementing a swept frequency laser in the mid-infrared band according to claim 9, characterized in that: The idle light of a certain wavelength selected by the wavelength selection device is amplified multiple times by the periodically polarized crystal and then output by the wavelength selection device; wherein, when the wavelength selection device switches to the idle light of the next wavelength, the wavelength selection device outputs the last amplified idle light.

Images