Self-injection feedback amplification output single-frequency laser modulation system and method

Abstract

The application provides a self-injection feedback amplification output single-frequency laser modulation system and method, and relates to the technical field of lasers. An energy source is used for outputting excitation light; a coupling lens system is arranged on an output path of the energy source; an input-output mirror is arranged between the coupling lens system and a laser oscillation unit; the laser oscillation unit is arranged at the rear end of the input-output mirror; a feedback unit is arranged on an emergent light path of the laser oscillation unit; and the input-output mirror, the laser oscillation unit and the feedback unit constitute an external cavity self-injection feedback structure. The application creatively proposes an external input-output mirror, a laser oscillation unit and a feedback unit outside the cavity, which constitute an external cavity self-injection feedback structure, wavelength screening and external reflection of the feedback unit are used to realize specific wavelength locking, and the noise with a wavelength close to the specific wavelength is removed, so that laser with a specific wavelength and high purity can be generated, the cavity loss is reduced and the optical path is simplified in structure, and the input-output mirror is integrated to reduce the number of system optical components.

Description

Technical Field

[0001] This application relates to the field of laser technology, and in particular to a self-injection feedback amplification output single-frequency laser modulation system and a self-injection feedback amplification output single-frequency laser modulation method. Background Technology

[0002] The existing lasers mainly fall into three categories: DFB semiconductor lasers, DFB fiber lasers, and traditional intracavity etalon frequency-selective NPRO lasers.

[0003] However, existing mainstream solutions suffer from problems such as cavity loss, complex optical paths, and a large number of required optical components. In particular, when the goal is to generate lasers of a specific wavelength, but the excitation light output from the energy source contains clutter with wavelengths close to the desired wavelength, the lasers generated by existing mainstream solutions cannot adequately meet the requirements. Summary of the Invention

[0004] In view of the above problems, this application proposes a self-injection feedback amplification output single-frequency laser modulation system and a self-injection feedback amplification output single-frequency laser modulation method to overcome the shortcomings of the prior art.

[0005] In a first aspect, embodiments of this application provide a self-injection feedback amplification output single-frequency laser modulation system, comprising: Energy source, used to output excitation light; A coupling lens system, positioned in the output path of the energy source, is used to focus the excitation light onto the laser oscillation unit; The input / output mirror is positioned between the coupling lens system and the laser oscillation unit. It is used to transmit the focused excitation light to the laser oscillation unit, reflect part of the feedback laser from the feedback unit back to the laser oscillation unit, and transmit the remaining part of the feedback laser to form a single-frequency output laser and output it. The laser oscillation unit is located at the rear end of the input and output mirrors. It is used to absorb the focused excitation light and feedback laser to generate population inversion, form a ring oscillating laser and emit it from the output end. The feedback unit is set in the output optical path of the laser oscillation unit. It is used to reflect the ring oscillation laser of a specific wavelength with high efficiency to form a feedback laser and suppress the ring oscillation laser of other wavelengths. The input / output mirror, the laser oscillation unit, and the feedback unit constitute an external cavity self-injection feedback structure.

[0006] Optionally, the coupling lens system includes: a combination of two achromatic spherical lenses; Each achromatic spherical lens has a focal length range of 5mm to 50mm and is coated with an anti-reflective coating. Two achromatic spherical lenses are combined to focus the excitation light into a spot with a diameter in the range of 100μm to 500μm, so as to match the incident end face size of the laser oscillation unit.

[0007] Optionally, the input / output mirror is a composite coated mirror, with an anti-reflection coating on its light-incident surface facing the energy source to make its transmittance greater than 98%, and a laser partial reflection coating on its light-outceasing surface facing the laser oscillation unit, so that the transmitted portion of the feedback laser is used as a single-frequency output laser, and the reflected portion of the feedback laser is reflected back to the laser oscillation unit. The surface accuracy of the input / output mirror is less than [a certain value]. / 10, the parallelism of the lens is less than 10″, among which λ is the wavelength.

[0008] Optionally, the laser oscillation unit includes: a single non-planar ring cavity, a discrete ring cavity, or a semiconductor gain chip; The single non-planar annular cavity is a laser crystal with multiple non-planar optical reflective surfaces combined to form a closed annular optical path inside the laser crystal. Laser crystals used in a single non-planar ring cavity include any one of Nd:YAG crystal, Nd:YVO4 crystal, or Yb:YAG crystal. The laser crystal used in the single non-planar ring cavity has an external size ranging from 7×7×12mm to 12×12×25mm and contains 5 to 7 non-planar optical reflective surfaces.

[0009] Optionally, the feedback unit includes: a reflective volume Bragg grating, a transmissive total reflection mirror combination, or a standard etalon high reflection mirror combination; The reflective volume Bragg grating forms a volume phase grating by writing a periodic refractive index modulation structure into a photosensitive glass material. The grating period and refractive index modulation of the volume phase grating are configured such that ring oscillating lasers that satisfy the Bragg condition are highly reflective, while other ring oscillating lasers that do not satisfy the Bragg condition are transmitted or lost. The reflective volume Bragg grating is a replaceable grating. By replacing the reflective volume Bragg grating with different wavelengths, the wavelength of the feedback laser can be switched accordingly. The grating thickness and refractive index modulation of the reflective volume Bragg grating are configured to make the diffraction efficiency of the ring oscillating laser at the corresponding wavelength greater than 90%, so that the wavelength threshold for the laser oscillation unit to start oscillation is more than 10 times lower than the wavelength threshold for the laser oscillation unit to start oscillation; thus achieving single-frequency laser output. The transmissive total reflection mirror assembly includes: a combination of a transmissive volume Bragg grating and a rear total reflection mirror, wherein the transmissive volume Bragg grating transmits a ring oscillating laser, and the rear total reflection mirror reflects the feedback laser. The standard etalon and high-reflection mirror combination includes a combination of a narrowband FP standard etalon and a high-reflection mirror. The narrowband FP standard etalon is used for wavelength selection of the ring oscillating laser, and the high-reflection mirror reflects the feedback laser.

[0010] Optionally, it further includes: a transmissive volume Bragg grating; the feedback unit includes: a reflective volume Bragg grating. The transmissive volume Bragg grating is disposed on the optical path from the laser oscillation unit to the feedback unit; The transmissive volume Bragg grating forms a volume phase grating that satisfies the Bragg diffraction condition by writing a periodic refractive index modulation structure into the photosensitive glass material. The grating period and refractive index modulation of the volume phase grating are configured to enable high transmission of the ring oscillating laser that satisfies the Bragg condition, while the remaining ring oscillating lasers that do not satisfy the Bragg condition are reflected or lost. Only when the wavelength and incident angle of the incident light simultaneously satisfy the Bragg condition will the transmissive volume Bragg grating produce high transmission of light that satisfies the Bragg condition, while other light that does not satisfy the Bragg condition will be reflected or lost. The reflective volume Bragg grating forms a volume phase grating by writing a periodic refractive index modulation structure into a photosensitive glass material. The grating period and refractive index modulation of the volume phase grating are configured such that ring oscillating lasers that satisfy the Bragg condition receive high reflection, while other ring oscillating lasers that do not satisfy the Bragg condition are transmitted or lost. The reflective volume Bragg grating is a replaceable grating. By replacing the reflective volume Bragg grating with different wavelengths, the wavelength of the feedback laser can be switched accordingly. The grating thickness and refractive index modulation of the reflective volume Bragg grating are configured to make the diffraction efficiency of the ring oscillating laser at the corresponding wavelength greater than 90%, so that the wavelength threshold for the laser oscillation unit to start oscillation is more than 10 times lower than the wavelength threshold for the laser oscillation unit to start oscillation; thereby achieving single-frequency laser output.

[0011] Optionally, it also includes a temperature control base; The temperature control base is installed at the target position of the reflective volume Bragg grating to adjust the temperature inside the reflective volume Bragg grating, thereby changing the wavelength to achieve wavelength fine-tuning; the target position refers to the position of the reflective volume Bragg grating other than the position where it receives the ring oscillating laser. Within the wavelength fine-tuning range, the angle of the reflective volume Bragg grating can be finely adjusted accordingly, and the incident angle of the ring oscillating laser can be changed without replacing the reflective volume Bragg grating.

[0012] Optionally, the linewidth of the feedback laser generated after the ring oscillating laser passes through the transmissive volume Bragg grating and the reflective volume Bragg grating is smaller than the linewidth of the feedback laser generated only by the reflective volume Bragg grating.

[0013] Optionally, the energy source includes: an LD pump source or an optical fiber output solid-state laser.

[0014] In a second aspect, embodiments of this application provide a self-injection feedback amplification output single-frequency laser modulation method, applied to the self-injection feedback amplification output single-frequency laser modulation system described in any one of the first aspects, comprising: Excitation light is generated by an energy source, focused by a coupling lens system, and then incident on the laser oscillation unit through the input and output mirrors. The laser oscillation unit absorbs and focuses the excitation light to generate population inversion, forming a ring oscillating laser and outputting it from the output end to the feedback unit. The feedback unit only reflects the ring oscillating laser of a specific wavelength with high efficiency, forming a feedback laser that is incident on the input and output mirrors, while the ring oscillating laser of other non-specific wavelengths is filtered out by transmission. The input and output mirrors reflect part of the feedback laser back to the laser oscillation unit to form self-injection feedback. Lasers of specific wavelengths preferentially start oscillation and quickly reach a steady state, while lasers of non-specific wavelengths are suppressed. The input and output mirrors also transmit the remaining portion of the laser back to form a single-frequency output laser and output it.

[0015] The self-injection feedback amplification output single-frequency laser modulation system proposed in this application creatively proposes an external input / output mirror, a laser oscillation unit, and a feedback unit outside the cavity. The three constitute an external cavity self-injection feedback structure. By utilizing the wavelength filtering and external reflection of the feedback unit, a specific wavelength locking is achieved, and clutter with wavelengths close to the specific wavelength is removed. This can generate lasers with a specific wavelength and high purity. At the same time, the structure reduces cavity loss and simplifies the optical path.

[0016] Furthermore, by relying on a customizable feedback unit, the crystal of the same laser oscillation unit can achieve multi-wavelength tunable output, breaking through the limitation of fixed wavelength in existing lasers. The integrated input and output mirrors with composite coating realize three functions: excitation light transmission, feedback laser coupling, and single-frequency laser output, simplifying the number of optical components in the system and possessing broad application prospects and high practicality. Attached Figure Description

[0017] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 This is a structural diagram of a distributed feedback semiconductor laser; Figure 2 This is a structural diagram of a distributed feedback fiber laser; Figure 3 This is a structural diagram of a conventional intracavity etalon frequency-selective NPRO laser; Figure 4 This is a modular diagram of a self-injection feedback amplification output single-frequency laser modulation system according to an embodiment of this application; Figure 5 This is a structural diagram of a self-injection feedback amplification output single-frequency laser modulation system formed in a preferred manner, as illustrated in the embodiments of this application. Figure 6 This is a schematic diagram of normal incidence reflective volume Bragg grating diffraction exemplified in the embodiments of this application; Figure 7 The simulated grating thickness and wavelength illustrated in the embodiments of this application are examples. The influence of reflective volume Bragg grating diffraction efficiency spectrum; Figure 8 The simulated refractive index modulation exemplified in the embodiments of this application affects the wavelength. The influence of reflective volume Bragg grating diffraction efficiency spectrum; Figure 9 This is a photon number density curve exemplified in the embodiments of this application, which satisfies the self-injection feedback wavelength; Figure 10 This is an example of a threshold curve representing the self-injection feedback wavelength, as illustrated in the embodiments of this application. Figure 11 This is a structural diagram of a preferred self-injection feedback amplification output single-frequency laser modulation system exemplified in the embodiments of this application. Detailed Implementation

[0018] The embodiments of this application will now be described in detail. Examples of these embodiments are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0019] The inventors discovered that current mainstream traditional lasers mainly fall into three categories: DFB semiconductor lasers, DFB fiber lasers, and traditional intracavity etalon-selective NPRO lasers. Distributed feedback semiconductor lasers (DFB lasers) are high-performance single-longitudinal-mode semiconductor lasers. Their core innovation lies in integrating periodic Bragg gratings into the laser resonant cavity, thereby achieving distributed optical feedback. DFB lasers primarily rely on the distributed reflection of the gratings to provide positive feedback, achieving stable single-longitudinal-mode output.

[0020] Reference Figure 1 The diagram shows the structure of a DFB laser, typically using an InP or GaAs substrate, consisting of an n-type layer, an active region, a grating layer, a p-type layer, and an electrode layer. The grating is formed through etching to periodically modulate the refractive index or gain. Antireflection coatings are deposited on both end faces to minimize end-face reflections and ensure that feedback originates almost entirely from the grating. When a forward current is injected into the pn junction, charge carriers enter the active region, achieving population inversion. Photons generated by spontaneous emission propagate in the waveguide and are amplified through stimulated emission. When the light wave encounters the periodic grating, Bragg scattering occurs, resulting in strong coupling between the forward and backward waves. According to coupled-mode theory, only specific wavelengths satisfying the Bragg condition can achieve maximum feedback, forming laser oscillation. By selecting the grating period and effective refractive index, the Bragg wavelength is made to the desired wavelength, while other wavelengths are suppressed due to phase mismatch, thus achieving extremely high side-mode rejection ratio and narrow linewidth output.

[0021] Distributed feedback fiber lasers (DFB-FLs) are single-longitudinal-mode fiber lasers based on fiber Bragg gratings (FBGs). They directly inscribe periodic Bragg gratings into rare-earth-doped active fibers to achieve distributed optical feedback and wavelength selectivity.

[0022] Reference Figure 2 The diagram shows the structure of a DFB-FL, where a grating runs through the entire resonant cavity, providing continuous distributed feedback to achieve extremely narrow linewidth, stable single-mode output, and high coherence. This scheme combines fiber laser gain with distributed Bragg reflection. Pump light is injected into the active fiber, and rare-earth ions absorb the pump light energy, transitioning from the ground state to a higher energy level, achieving population inversion. When the pump power exceeds a threshold, metastable ions generate signal light through stimulated emission. As the light wave propagates in the fiber, it encounters a periodically refractive index-modulated grating, causing strong coupling between the forward and backward propagating waves. Only specific wavelengths satisfying the Bragg condition receive the strongest feedback, forming a standing wave mode that is continuously amplified. By selecting the grating period and effective refractive index, the Bragg wavelength is made to the desired wavelength; other wavelengths are suppressed due to phase mismatch and extremely weak feedback, thus achieving narrow linewidth output and extremely high side-mode rejection ratio.

[0023] A traditional intracavity etalon-selective NPRO laser, taking a single-frequency laser output from a crystal based on a continuous diode end-pumped laser as an example, refers to... Figure 3 As shown, a diode-pumped laser crystal is used. Utilizing the energy level transitions of the laser crystal, and through cavity mirror coating and the insertion of a frequency-selective element such as an etalon within the cavity, the target laser oscillates within the resonant cavity, thus achieving continuous-wave laser output. This method, by employing cavity mirror coating and etalon frequency selection, effectively suppresses strongly competing wavelengths, resulting in a pure wavelength. Further research by the inventors revealed that these mainstream solutions mainly suffer from the following problems: 1) DFB semiconductor lasers suffer from stringent grating etching processes, high production costs, severe spatial hole burning and thermal distortion under high-power conditions, and an output power limit of only tens of milliwatts.

[0024] 2) DFB fiber lasers suffer from high environmental temperature sensitivity, large wavelength temperature drift, stringent phase shift grating writing accuracy, and output power limitations due to the concentration of rare earth doping in the fiber, preventing high-power output.

[0025] 3) Traditional intracavity inserted etalon frequency-selective lasers suffer from problems such as high cavity loss, high pump threshold, low optical-to-optical conversion efficiency, and too many intracavity components, making optical path debugging cumbersome.

[0026] 4) Existing mature single-frequency lasers have the disadvantages of fixed output wavelength, inability to flexibly switch bands, and poor equipment versatility.

[0027] Further in-depth research revealed that multi-gain spectrum NPRO crystals face challenges such as similar stimulated emission cross-sections, easy synchronous lasing of adjacent wavelengths, and the inability of crystal coating processes to achieve single-wavelength cutoff and naturally multi-wavelength output. For example, traditional multi-gain spectrum NPRO crystals can generate wavelength A lasers through crystal coating processes because the excitation light output from the energy source does not contain clutter with wavelengths close to A, or if it does contain clutter but with wavelengths significantly different from A, the clutter can be removed through crystal coating. However, if wavelength B lasers are required, but the excitation light output from the energy source contains clutter with wavelengths close to B, the multi-gain spectrum NPRO crystal coating process cannot remove the clutter. This clutter is then also lased, resulting in a laser that does not meet the requirements.

[0028] To address the aforementioned problems, the inventors have creatively proposed a self-injection feedback amplification output single-frequency laser modulation system and a self-injection feedback amplification output single-frequency laser modulation method, as described in this application. The technical solution of this application is explained in detail below.

[0029] This application discloses a self-injection feedback amplification output single-frequency laser modulation system, referring to... Figure 4The modular diagram shown includes: an energy source, a coupling lens system, input / output mirrors, a laser oscillation unit, and a feedback unit. The energy source is used to output excitation light; the coupling lens system is positioned along the output path of the energy source and is used to focus the excitation light onto the laser oscillation unit.

[0030] The input / output mirror, positioned between the coupling lens system and the laser oscillation unit, transmits the focused excitation light to the laser oscillation unit, reflects a portion of the feedback laser from the feedback unit back to the laser oscillation unit, and transmits the remaining feedback laser to form a single-frequency output laser. In other words, the input / output mirror is a key component for achieving efficient coupling between the excitation light, the ring oscillating laser, and the feedback laser. While transmitting the focused excitation light to the laser oscillation unit, it also reflects a portion of the feedback laser emitted by the feedback unit back to the laser oscillation unit, allowing for better oscillation and the formation of a superior ring oscillating laser. Furthermore, it transmits the remaining feedback laser to form a high-performance, narrow-linewidth single-frequency output laser.

[0031] The laser oscillation unit is located at the rear end of the input and output mirrors. The laser oscillation unit is used to absorb the focused excitation light and feedback laser to generate population inversion, forming a ring oscillating laser and emitting it from the output end. The feedback unit is located in the output light path of the laser oscillation unit. The feedback unit is used to efficiently reflect the ring oscillating laser of a specific wavelength to form a feedback laser and suppress the ring oscillating laser of other wavelengths.

[0032] In this structure, the input / output mirrors, laser oscillation unit, and feedback unit constitute an external cavity self-injection feedback structure. This structure utilizes the wavelength filtering and external reflection of the feedback unit to achieve specific wavelength locking, eliminating clutter with wavelengths close to the specific wavelength. This allows for the generation of high-purity laser light of a specific wavelength while structurally reducing cavity loss and simplifying the optical path. Customizable feedback units enable multi-wavelength tunable output from the same laser oscillation unit, overcoming the limitations of fixed-wavelength lasers. The integrated composite-coated input / output mirrors achieve excitation light transmission, feedback laser coupling, and single-frequency laser output, reducing the number of optical components in the system.

[0033] In one embodiment of this application, the energy source preferably includes either an LD pump source or a fiber-optic output solid-state laser. The preferred choice is an LD pump source, which provides the excitation energy required for population inversion in the laser oscillation unit, a prerequisite for laser oscillation. Using a semiconductor laser pump source like an LD pump source ensures that its emission wavelength is precisely matched to the absorption peak of the crystal used to excite the oscillation unit. The pump light emitted by the LD pump source is a continuous wave output with a wide linewidth. After being coupled out via a pigtail, it passes through a coupling lens system before being incident on the input / output mirror. The LD pump source has an output wavelength precisely matched to the absorption peak of the crystal used to excite the oscillation unit (e.g., 808nm for Nd:YVO4 / Nd:YAG, 976nm for Yb crystal, etc.), an output power of 500mW to 10W, and is coupled out via a pigtail. It is positioned at the very front of the optical path to provide the pump energy required for population inversion in the crystal.

[0034] In one embodiment of this application, preferably, the coupling lens system includes: a combination of two achromatic spherical lenses; each achromatic spherical lens has a focal length range of 5mm to 50mm and is coated with an anti-reflection film; the combination of the two achromatic spherical lenses is used to focus the excitation light into a spot with a diameter in the range of 100μm to 500μm to match the incident end face size of the laser oscillation unit. This design can better shape and focus the excitation light.

[0035] In one embodiment of this application, preferably, the input / output mirror is a composite coated lens. The light-incident surface of the input / output mirror facing the energy source is coated with an anti-reflection film, resulting in a transmittance greater than 98%. The light-outgoing surface of the input / output mirror facing the laser oscillation unit is coated with a partial laser reflection film, allowing the transmitted portion of the feedback laser to be used as a single-frequency output laser, and the reflected portion of the feedback laser to be reflected back to the laser oscillation unit. The surface accuracy of the input / output mirror is less than [a certain value]. / 10, the parallelism of the lens is less than 10″, among which λ is the wavelength.

[0036] Input / output mirrors are key components for achieving efficient coupling of excitation light, ring oscillating laser, and feedback laser. Ideally, the mirrors should be made of optical glass substrates with multilayer dielectric films deposited on the surface to achieve high transmittance and high reflectance across different wavelength bands. For example, for the pump wavelength of the pump source, the mirror has extremely high transmittance (greater than 98%), allowing the pump light to pass through the mirror without loss and efficiently couple into the excitation / oscillation unit crystal. For the feedback laser, the mirror has partial reflectivity, ensuring sufficient feedback laser light self-injected back into the excitation / oscillation unit crystal to participate in oscillation, while also allowing some feedback laser light to be transmitted to form the final single-frequency laser output.

[0037] Furthermore, the surface accuracy and parallelism of the input and output mirrors must be strictly controlled to avoid introducing additional optical loss or wavefront distortion, which would affect the coupling efficiency and linewidth characteristics of the feedback laser. The input and output mirrors are generally made of quartz optical glass substrate with double-sided dielectric films: pump band transmittance >98%, mirror parallelism <10″, and surface accuracy λ / 10; they are placed between the coupling lens system and the laser oscillation unit; allowing for high transmission of excitation light, partial back transmission of the feedback laser back to the laser oscillation unit using the crystal, and partial transmission of the feedback laser for output.

[0038] In one embodiment of this application, preferably, the laser oscillation unit includes: a monolithic non-planar ring cavity, a discrete ring cavity, or a semiconductor gain chip. Among these three, a monolithic non-planar ring cavity is preferred. The monolithic non-planar ring cavity is a laser crystal with multiple non-planar optical reflective surfaces combined to form a closed ring optical path within the laser crystal. The laser crystal used in the monolithic non-planar ring cavity includes any one of Nd:YAG crystal, Nd:YVO4 crystal, or Yb:YAG crystal. The external dimensions of the laser crystal used in the monolithic non-planar ring cavity range from 7×7×12mm to 12×12×25mm, and it contains 5 to 7 non-planar optical reflective surfaces. The non-planar structure of the monolithic non-planar ring cavity forms a closed ring optical path within the laser crystal through the combination of multiple reflective surfaces. The non-planar reflection design of the laser crystal achieves the effect of an optical one-way device. Utilizing the magneto-optical effect, the light can only oscillate counterclockwise or clockwise in one direction within the cavity, effectively suppressing the spatial hole-burning effect and achieving single-mode operation.

[0039] In one embodiment of this application, preferably, the feedback unit includes: a reflective volume Bragg grating, a transmissive total reflection mirror combination, or an etalon high-reflection mirror combination. The transmissive total reflection mirror combination includes: a combination of a transmissive volume Bragg grating and a rear total reflection mirror, wherein the transmissive volume Bragg grating transmits the ring oscillating laser, and the rear total reflection mirror reflects it to form the feedback laser; the etalon high-reflection mirror combination includes: a combination of a narrowband FP etalon and a high-reflection mirror, wherein the narrowband FP etalon is used for wavelength selection of the ring oscillating laser, and the high-reflection mirror reflects it to form the feedback laser.

[0040] Of the three options mentioned above, the preferred feedback unit is a reflective volume Bragg grating. This grating is formed by writing a periodic refractive index modulation structure into the photosensitive glass material. The grating period and refractive index modulation of this volume phase grating are configured such that ring-shaped oscillating lasers satisfying the Bragg condition receive high reflectivity, while other ring-shaped oscillating lasers that do not satisfy the Bragg condition are transmitted or lost. The Bragg condition is satisfied only if both the wavelength and the incident angle of the incident laser simultaneously meet it; otherwise, it is not considered satisfied.

[0041] The reflective volume Bragg grating is a replaceable grating. By replacing the reflective volume Bragg grating with different wavelengths, the wavelength of the feedback laser is switched accordingly. The grating thickness and refractive index modulation of the reflective volume Bragg grating are configured to ensure that the diffraction efficiency of the ring oscillating laser at the corresponding wavelength is greater than 90%, so that the wavelength threshold for the laser oscillation unit to start oscillation is more than 10 times lower than the wavelength threshold for the laser oscillation unit not to start oscillation. This achieves single-frequency laser output.

[0042] With the structure of the self-injection feedback amplification output single-frequency laser modulation system described above, if it is now necessary to generate B-wavelength laser, although the excitation light output by the energy source contains clutter with wavelengths close to B-wavelength, the narrowband filtering of the reflective volume Bragg grating + self-injection feedback structure can effectively remove the clutter with wavelengths close to B-wavelength, so that the B-wavelength laser oscillates preferentially, locking a single B-wavelength, avoiding the ultra-difficult narrowband crystal coating, and generating a B-wavelength single-frequency laser that meets the requirements.

[0043] The structure of the self-injection feedback amplification output single-frequency laser modulation system formed by the above-mentioned preferred combination is referenced. Figure 5 As shown, Figure 5 The example shown uses a pump source as the energy source, a single non-planar annular cavity as the laser oscillation unit, and a reflective volume Bragg grating as the feedback unit. Figure 5 Simplified diagrams of the various structures are shown as examples.

[0044] by Figure 5 The structure shown was used to fabricate a self-injection feedback amplification output single-frequency laser modulation system, which was then tested. (Refer to...) Figure 6 The diagram shows a normal incidence reflection volume Bragg grating diffraction pattern, where R and S represent the amplitudes of the incident and diffracted light, respectively. and , Let K be the spatial position vector, and K be the grating vector, with a magnitude of... , For the grating period, Let be the incident light propagation vector. Let be the propagation vector of the diffracted light, and satisfy . . Let be the angle between the incident light and the z-axis. Let be the angle between the incident light and the z-axis within the medium. The angle between the grating vector and the z-axis is denoted as .

[0045] According to coupled-wave equation theory, non-tilting... The diffraction efficiency of the reflective grating η The formula is:

[0046] In the above formula, , Refractive index modulation, The mismatch parameter is expressed as follows: . This indicates the coupling strength between the incident wave and the diffracted wave; the thicker the grating, the greater the refractive index modulation. The larger the value, the stronger the coupling. n It represents the refractive index. It is the thickness of the grating; Wavelength; This represents the total phase mismatch after normalization; This indicates that the Prague condition is strictly satisfied, and the mismatch is 0. The larger the value, the more severe the deviation from the Bragg condition, and the faster the diffraction efficiency decreases. Represents a hyperbolic sine function, in the formula This represents the oscillatory term in coupled-wave theory that describes the periodic energy exchange of a wave within a grating.

[0047] Reference Figure 7 The simulated grating thickness shown is related to wavelength. The influence of the diffraction efficiency spectrum of the reflective volume Bragg grating, referring to Figure 8 The simulated refractive index modulation shown affects the wavelength. The effect of reflective volume Bragg grating diffraction efficiency on the spectrum. The horizontal axis represents the change in wavelength, and the vertical axis represents the diffraction efficiency.

[0048] from Figure 7 It can be seen that as the thickness increases (d3>d2>d1, d3 has the largest thickness and d1 has the smallest thickness), the diffraction efficiency at the Bragg peak wavelength is significantly improved, approaching 1 at d3. At the same time, the main lobe bandwidth narrows and the side lobe amplitude increases, which means that the spectral selectivity is enhanced but stronger side lobe interference is introduced.

[0049] from Figure 8 It can be seen that as the refractive index modulation increases (n13>n12>n11, n13 has the largest refractive index modulation, and n11 has the smallest refractive index modulation), the Bragg peak diffraction efficiency also approaches 1. The parameters of the reflective volume Bragg grating can be comprehensively considered according to the target wavelength.

[0050] To verify the effectiveness of the self-injection feedback amplification output single-frequency laser modulation system proposed in this application, simulation tests were conducted, and the results were obtained. Figure 9 The photon number density curves shown satisfying the self-injection feedback wavelength and Figure 10 The graph shown represents the threshold curve that satisfies the self-injection feedback wavelength.

[0051] Figure 9 The horizontal axis represents time (μs), and the vertical axis represents photon number density (m). -3The solid line represents the photon number density curve for self-injection feedback, i.e., the photon number density that meets the self-injection feedback wavelength; the dashed line represents the photon number density curve for non-self-injection feedback, i.e., the photon number density that does not meet the self-injection feedback wavelength. It can be seen that the photon number density rises rapidly within tens of microseconds, then quickly reaches a steady state. The required wavelength is reflected back into the cavity by the reflective bulk Bragg grating (i.e., the cavity of the laser crystal used in the laser oscillation unit). Photons are continuously amplified and accumulated within the cavity, eventually reaching a very high steady-state value. The dashed line represents the photon number density that does not meet the self-injection feedback wavelength. The photon number density rises slowly, and the final steady-state value differs from the case that meets the self-injection feedback wavelength by nearly 10 times. Photons are transmitted through the reflective bulk Bragg grating, unable to form resonance, and the photons have an extremely short residence time within the cavity, resulting in significant losses and preventing accumulation.

[0052] Figure 10 The horizontal axis represents the diffraction efficiency of the reflective volume Bragg grating, where 1 indicates a diffraction efficiency of 100%. The vertical axis represents the wavelength threshold P. th (W), the solid line represents the wavelength threshold that satisfies the self-injection feedback, which decreases as the diffraction efficiency of the reflective volume Bragg grating increases. When the diffraction efficiency of the reflective volume Bragg grating is greater than 90%, the wavelength threshold that satisfies the self-injection feedback is more than 10 times lower than the wavelength threshold that does not satisfy the self-injection feedback, represented by the dashed line. This means that more lasers of specific wavelengths are reflected back to the laser crystal of the laser oscillation unit, thus the wavelengths that satisfy the self-injection feedback will oscillate preferentially. Uploaded tests verified the effectiveness of the self-injection feedback amplified output single-frequency laser modulation system proposed in this application.

[0053] In one embodiment of this application, to obtain a better linewidth, the self-injection feedback amplification output single-frequency laser modulation system may further include: a transmissive volume Bragg grating; (Refer to...) Figure 11 The diagram shows a preferred self-injection feedback amplification output single-frequency laser modulation system. Figure 11 The example shown uses a pump source as the energy source, a single non-planar annular cavity as the laser oscillation unit, and a reflective volume Bragg grating as the feedback unit.

[0054] A transmissive volume Bragg grating is placed in the optical path from a single non-planar annular cavity to a reflective volume Bragg grating. The transmissive volume Bragg grating forms a volume phase grating that satisfies the Bragg diffraction condition by writing a periodic refractive index modulation structure into the photosensitive glass material. The grating period and refractive index modulation of this volume phase grating are configured to ensure high transmission of the ring oscillating laser light that satisfies the Bragg condition, while reflecting or losing light that does not. Only when the wavelength and incident angle of the incident light simultaneously satisfy the Bragg condition will the transmissive volume Bragg grating produce high transmission of light satisfying the Bragg condition; other light that does not satisfy the Bragg condition will be reflected or lost. The linewidth of the feedback laser generated after the ring oscillating laser emitted from the single non-planar annular cavity passes through the transmissive and reflective volume Bragg gratings is smaller than the linewidth of the feedback laser generated only by the reflective volume Bragg grating (i.e., without the transmissive volume Bragg grating). This single-frequency laser with a better linewidth is output by the input / output mirrors.

[0055] Furthermore, a preferred self-injection feedback amplification output single-frequency laser modulation system may also include a temperature control base; the temperature control base is installed at the target position of the reflective volume Bragg grating to adjust the temperature inside the reflective volume Bragg grating, thereby changing the wavelength to achieve wavelength fine-tuning; the target position refers to the position of the reflective volume Bragg grating other than the position where it receives the ring oscillating laser; within the wavelength fine-tuning range, the corresponding arrangement angle of the reflective volume Bragg grating is finely adjusted, and the incident angle of the ring oscillating laser can be changed without replacing the reflective volume Bragg grating.

[0056] In other words, if a temperature-controlled base is installed at the target location of the reflective volume Bragg grating, the wavelength can be fine-tuned. Since the wavelength can be fine-tuned, the corresponding incident angle can also be fine-tuned. With this design, different wavelengths of laser light can be generated without replacing the reflective volume Bragg grating. Of course, it is understandable that for wavelengths outside the fine-tuning range, or for reflective volume Bragg gratings without a temperature-controlled base, if the required wavelength changes, then a reflective volume Bragg grating with the corresponding required wavelength needs to be replaced to generate the corresponding laser light; if the incident angle changes and the Bragg condition is no longer met, the reflective volume Bragg grating also needs to be replaced. Those skilled in the art have not yet solved the problem of generating single-frequency laser light of different wavelengths at any incident angle without replacing the reflective volume Bragg grating. This is the problem the inventors need to solve next.

[0057] Based on the aforementioned self-injection feedback amplification output single-frequency laser modulation system, this application also proposes a self-injection feedback amplification output single-frequency laser modulation method, which is applied to the self-injection feedback amplification output single-frequency laser modulation system described in any of the above claims, and includes: Step S1: Excitation light is generated by an energy source, focused by a coupling lens system, and then incident on the laser oscillation unit through the input and output mirrors.

[0058] Step S2: The laser oscillation unit absorbs the focused excitation light to generate population inversion, forming a ring oscillating laser and outputting it to the feedback unit.

[0059] Step S3: The feedback unit only reflects the ring oscillating laser of a specific wavelength with high efficiency, forming a feedback laser that is incident on the input and output mirrors, while the ring oscillating laser of other non-specific wavelengths is filtered out by transmission.

[0060] Step S4: The input-output mirror reflects part of the feedback laser back to the laser oscillation unit to form self-injection feedback. Lasers of specific wavelengths oscillate preferentially and quickly reach a steady state, while lasers of non-specific wavelengths are suppressed due to lack of feedback and high threshold. At the same time, the input-output mirror also transmits the remaining feedback laser to form a single-frequency output laser and outputs it.

[0061] Those skilled in the art can understand the above method by combining it with the structure of the aforementioned self-injection feedback amplification output single-frequency laser modulation system through simple reasoning, and will not be elaborated further.

[0062] In summary, the self-injection feedback amplification output single-frequency laser modulation system proposed in this application creatively proposes an external input / output mirror, a laser oscillation unit, and a feedback unit outside the cavity. These three components constitute an external cavity self-injection feedback structure. By utilizing the wavelength filtering and external reflection of the feedback unit, a specific wavelength locking is achieved, eliminating clutter with wavelengths close to the specific wavelength. This can generate lasers with a specific wavelength and high purity. At the same time, it reduces cavity loss and simplifies the optical path from a structural perspective.

[0063] Frequency selection is achieved by using an external reflective volume Bragg grating, eliminating the need for any frequency selection components inside the resonant cavity. This reduces fabrication difficulty, cavity loss, pump threshold, and assembly / adjustment processes, while increasing single-frequency laser output power to the watt level. By replacing the reflective volume Bragg gratings with different wavelengths or replacing the single non-planar ring cavity, the same optical path enables rapid switching of output laser wavelengths. Relying on the narrowband filtering and self-injection feedback structure of the reflective volume Bragg grating, it prioritizes oscillation and locks onto a single wavelength, avoiding the need for ultra-difficult narrowband crystal coating. The laser crystal of a solid single non-planar ring cavity is used as the gain source, and the temperature drift of the solid structure is much lower than that of optical fiber. The external reflective volume Bragg grating can be freely replaced and is not constrained by fiber doping.

[0064] By leveraging a customizable feedback unit, the crystal of the same laser oscillation unit can achieve multi-wavelength tunable output, breaking through the limitation of fixed wavelength in existing lasers. The integrated input and output mirrors with composite coating realize three functions: excitation light transmission, feedback laser coupling, and single-frequency laser output, simplifying the number of optical components in the system and possessing broad application prospects and high practicality.

[0065] Although preferred embodiments of the present application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of the present application.

[0066] Finally, it should be noted that in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or terminal device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or terminal device. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or terminal device that includes said element.

[0067] The embodiments of this application have been described above with reference to the accompanying drawings. However, this application is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of this application without departing from the spirit and scope of the claims. All of these forms are within the protection scope of this application.

Claims

1. A self-injection feedback amplification output single-frequency laser modulation system, characterized in that, include: Energy source, used to output excitation light; A coupling lens system, positioned in the output path of the energy source, is used to focus the excitation light onto the laser oscillation unit; The input / output mirror is positioned between the coupling lens system and the laser oscillation unit. It is used to transmit the focused excitation light to the laser oscillation unit, reflect part of the feedback laser from the feedback unit back to the laser oscillation unit, and transmit the remaining part of the feedback laser to form a single-frequency output laser and output it. The laser oscillation unit is located at the rear end of the input and output mirrors. It is used to absorb the focused excitation light and feedback laser to generate population inversion, form a ring oscillating laser and emit it from the output end. The feedback unit is set in the output optical path of the laser oscillation unit. It is used to reflect the ring oscillation laser of a specific wavelength with high efficiency to form a feedback laser and suppress the ring oscillation laser of other wavelengths. The input / output mirror, the laser oscillation unit, and the feedback unit constitute an external cavity self-injection feedback structure.

2. The self-injection feedback amplification output single-frequency laser modulation system according to claim 1, characterized in that, The coupling lens system includes: a combination of two achromatic spherical lenses; Each achromatic spherical lens has a focal length range of 5mm to 50mm and is coated with an anti-reflective coating. Two achromatic spherical lenses are combined to focus the excitation light into a spot with a diameter in the range of 100μm to 500μm, so as to match the incident end face size of the laser oscillation unit.

3. The self-injection feedback amplification output single-frequency laser modulation system according to claim 1, characterized in that, The input / output mirror is a composite coated mirror. Its light-incident surface facing the energy source is coated with an anti-reflection film, making its transmittance greater than 98%. Its light-outceasing surface facing the laser oscillation unit is coated with a laser partial reflection film, so that the transmitted part of the laser is fed back as a single-frequency output laser, and the reflected part of the feedback laser is reflected back to the laser oscillation unit. The surface accuracy of the input / output mirror is less than [a certain value]. / 10, the parallelism of the lens is less than 10″, among which λ is the wavelength.

4. The self-injection feedback amplification output single-frequency laser modulation system according to claim 1, characterized in that, The laser oscillation unit includes: a single non-planar ring cavity, a discrete ring cavity, or a semiconductor gain chip; The single non-planar annular cavity is a laser crystal with multiple non-planar optical reflective surfaces combined to form a closed annular optical path inside the laser crystal. Laser crystals used in a single non-planar ring cavity include any one of Nd:YAG crystal, Nd:YVO4 crystal, or Yb:YAG crystal. The laser crystal used in the single non-planar ring cavity has an external size ranging from 7×7×12mm to 12×12×25mm and contains 5 to 7 non-planar optical reflective surfaces.

5. The self-injection feedback amplification output single-frequency laser modulation system according to claim 1, characterized in that, The feedback unit includes: a reflective volume Bragg grating, a transmissive total reflection mirror combination, or a standard etalon high reflection mirror combination; The reflective volume Bragg grating forms a volume phase grating by writing a periodic refractive index modulation structure into a photosensitive glass material. The grating period and refractive index modulation of the volume phase grating are configured such that ring oscillating lasers that satisfy the Bragg condition are highly reflective, while other ring oscillating lasers that do not satisfy the Bragg condition are transmitted or lost. The reflective volume Bragg grating is a replaceable grating. By replacing the reflective volume Bragg grating with different wavelengths, the wavelength of the feedback laser can be switched accordingly. The grating thickness and refractive index modulation of the reflective volume Bragg grating are configured to make the diffraction efficiency of the ring oscillating laser at the corresponding wavelength greater than 90%, so that the wavelength threshold for the laser oscillation unit to start oscillation is more than 10 times lower than the wavelength threshold for the laser oscillation unit to start oscillation; thus achieving single-frequency laser output. The transmissive total reflection mirror assembly includes: a combination of a transmissive volume Bragg grating and a rear total reflection mirror, wherein the transmissive volume Bragg grating transmits a ring oscillating laser, and the rear total reflection mirror reflects the feedback laser. The standard etalon and high-reflection mirror combination includes a combination of a narrowband FP standard etalon and a high-reflection mirror. The narrowband FP standard etalon is used for wavelength selection of the ring oscillating laser, and the high-reflection mirror reflects the feedback laser.

6. The self-injection feedback amplification output single-frequency laser modulation system according to claim 1, characterized in that, Also includes: Transmissive volume Bragg grating; the feedback unit includes: reflective volume Bragg grating; The transmissive volume Bragg grating is disposed on the optical path from the laser oscillation unit to the feedback unit; The transmissive volume Bragg grating forms a volume phase grating that satisfies the Bragg diffraction condition by writing a periodic refractive index modulation structure into the photosensitive glass material. The grating period and refractive index modulation of the volume phase grating are configured to enable high transmission of the ring oscillating laser that satisfies the Bragg condition, while the remaining ring oscillating lasers that do not satisfy the Bragg condition are reflected or lost. Only when the wavelength and incident angle of the incident light simultaneously satisfy the Bragg condition will the transmissive volume Bragg grating produce high transmission of light that satisfies the Bragg condition, while other light that does not satisfy the Bragg condition will be reflected or lost. The reflective volume Bragg grating forms a volume phase grating by writing a periodic refractive index modulation structure into a photosensitive glass material. The grating period and refractive index modulation of the volume phase grating are configured such that ring oscillating lasers that satisfy the Bragg condition receive high reflection, while other ring oscillating lasers that do not satisfy the Bragg condition are transmitted or lost. The reflective volume Bragg grating is a replaceable grating. By replacing the reflective volume Bragg grating with different wavelengths, the wavelength of the feedback laser can be switched accordingly. The grating thickness and refractive index modulation of the reflective volume Bragg grating are configured to make the diffraction efficiency of the ring oscillating laser at the corresponding wavelength greater than 90%, so that the wavelength threshold for the laser oscillation unit to start oscillation is more than 10 times lower than the wavelength threshold for the laser oscillation unit to start oscillation; thereby achieving single-frequency laser output.

7. The self-injection feedback amplification output single-frequency laser modulation system according to claim 5, characterized in that, It also includes a temperature control base; The temperature control base is installed at the target position of the reflective volume Bragg grating to adjust the temperature inside the reflective volume Bragg grating, thereby changing the wavelength to achieve wavelength fine-tuning; the target position refers to the position of the reflective volume Bragg grating other than the position where it receives the ring oscillating laser. Within the wavelength fine-tuning range, the angle of the reflective volume Bragg grating can be finely adjusted accordingly, and the incident angle of the ring oscillating laser can be changed without replacing the reflective volume Bragg grating.

8. The self-injection feedback amplification output single-frequency laser modulation system according to claim 6, characterized in that, The linewidth of the feedback laser generated after the ring oscillating laser passes through the transmissive volume Bragg grating and the reflective volume Bragg grating is smaller than the linewidth of the feedback laser generated only by the reflective volume Bragg grating.

9. The self-injection feedback amplification output single-frequency laser modulation system according to claim 1, characterized in that, The energy source includes: an LD pump source or a fiber-optic output solid-state laser.

10. A method for modulating a single-frequency laser output with self-injection feedback amplification, characterized in that, The self-injection feedback amplification output single-frequency laser modulation system according to any one of claims 1-9 comprises: Excitation light is generated by an energy source, focused by a coupling lens system, and then incident on the laser oscillation unit through the input and output mirrors. The laser oscillation unit absorbs and focuses the excitation light to generate population inversion, forming a ring oscillating laser and outputting it from the output end to the feedback unit. The feedback unit only reflects the ring oscillating laser of a specific wavelength with high efficiency, forming a feedback laser that is incident on the input and output mirrors, while the ring oscillating laser of other non-specific wavelengths is filtered out by transmission. The input and output mirrors reflect part of the feedback laser back to the laser oscillation unit to form self-injection feedback. Lasers of specific wavelengths preferentially start oscillation and quickly reach a steady state, while lasers of non-specific wavelengths are suppressed. The input and output mirrors also transmit the remaining portion of the laser back to form a single-frequency output laser and output it.

Images