A narrow linewidth single longitudinal mode seed laser system and generation method

Abstract

The present invention discloses a narrow linewidth single longitudinal mode seed laser system and a generation method, which includes a VCSEL laser, a polarization-maintaining beam splitter, a mirror, and a half-wave plate. The beam emitted by the VCSEL laser is first split into a TM-mode beam and a TE-mode beam by the polarization-maintaining beam splitter. On the propagation paths of the TM-mode beam and the TE-mode beam, a number of mirrors are arranged, and the design of these mirrors enables the two-mode beams to form a ring circuit. A half-wave plate is provided on the said ring circuit to achieve the regulation of the polarization state of the beam. Through the setting of this system, a narrow linewidth (about 1MHZ) single longitudinal mode seed laser can be generated, providing a high-quality light source for fields such as optical communication and spectral analysis; at the same time, the structure is simple, easy to implement, and reduces the implementation complexity and cost of the narrow linewidth single longitudinal mode seed laser.

Description

Technical Field

[0001] The invention belongs to the technical field of lasers and relates to a narrow-linewidth single longitudinal mode seed laser system and a generation method thereof. Background Art

[0002] As a high-efficiency light source device, the core function of laser is to convert electrical energy into monochromatic, coherent and high-energy light beams. This feature makes lasers have broad application prospects in many fields such as optical communications, medical treatment, and material processing. With the continuous advancement of science and technology, the performance requirements for lasers are also increasing, especially in terms of line width and longitudinal mode characteristics.

[0003] A narrow linewidth laser means that the light it outputs has a very narrow spectrum width, that is, the frequency of the light wave is very concentrated. This feature is particularly important in many application scenarios. For example, in the field of laser communications, narrow linewidth lasers can achieve higher information transmission rates and longer transmission distances, thereby significantly improving the performance of communication systems. In addition, in scientific research, narrow linewidth lasers are also widely used in fields such as high-precision measurement and frequency stability experiments, providing more accurate tools for scientific research.

[0004] A single longitudinal mode laser refers to a laser that contains only one frequency and spatial mode in its beam. The beam generated by this laser is very spatially concentrated and can form a very fine light spot. This feature is particularly important in fields such as optical sensing and laser medicine. For example, in optical sensing, a single longitudinal mode laser can provide a more stable and accurate sensing signal; in laser medicine, a single longitudinal mode laser can achieve more precise and safe medical operations.

[0005] However, it is not easy to achieve a narrow-linewidth single-longitudinal-mode seed laser. Since the inherent gain linewidth of the laser working material is usually wide, traditional oscillators can hardly directly output narrow-linewidth lasers. In order to overcome this technical difficulty, researchers usually need to adopt a series of complex technical means. Among them, using filters, gratings and other devices to limit or select the number of longitudinal modes in the gain spectrum is a common method. However, these methods often also need to be combined with technical means such as temperature stabilization and optical feedback control to achieve more stable and precise laser output.

[0006] Although these methods can achieve the output of narrow-linewidth single-longitudinal-mode lasers to a certain extent, their complexity and cost are still high. Therefore, how to further simplify the technical means, reduce costs and improve the performance of narrow-linewidth single-longitudinal-mode lasers is still an important issue that needs to be solved in the current field of laser technology. Summary of the invention

[0007] The object of the present invention is to solve the technical problems of complex technology and high cost in realizing the output of a narrow linewidth single longitudinal mode laser in the prior art, and to provide a narrow linewidth single longitudinal mode seed laser system and a generation method.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] The first aspect of the present invention provides a narrow linewidth single longitudinal mode seed laser system, including a VCSEL laser, a polarization maintaining beam splitter, a mirror and a half-wave plate;

[0010] The beam emitted by the VCSEL laser is split by the polarization maintaining beam splitter into a TM mode beam and a TE mode beam; several mirrors are arranged on the propagation paths of the TM mode beam and the TE mode beam, so that the TM mode beam and the TE mode beam form a circular loop on the optical path; a half-wave plate is arranged on the circular loop.

[0011] Further, the circular loop is a square loop with four corners perpendicular.

[0012] Further, a beam splitter is arranged between the VCSEL laser and the polarization maintaining beam splitter.

[0013] Further, a collimating lens is arranged between the beam splitter and the VCSEL laser.

[0014] Further, after the beam emitted by the VCSEL laser is split by the beam splitter, one beam enters the photodetector, and the other beam passes through the polarization maintaining beam splitter.

[0015] Further, an optical isolator and a collimating lens are arranged between the photodetector and the beam splitter; after the beam is split by the beam splitter, it passes through the optical isolator and the collimating lens in sequence and then enters the photodetector.

[0016] The second aspect of the present invention provides a method for generating a narrow linewidth single longitudinal mode seed laser, including the following steps:

[0017] S1, splitting the beam emitted by the VCSEL laser to obtain a first beam and a second beam;

[0018] S2, performing polarization splitting on the second beam to obtain a TM mode beam and a TE mode beam;

[0019] S3, propagating the TM mode beam and the TE mode beam in opposite directions along the external ring cavity, and passing the TM mode beam and the TE mode beam through the same half-wave plate respectively during the propagation;

[0020] S4, after the TM mode beam and the TE mode beam pass through the circular loop respectively, they return along the original path and propagate to the VCSEL laser to complete one polarization rotation;

[0021] In S5, after the TM-mode beam and the TE-mode beam are reflected by the VCSEL laser, a secondary polarization rotation is performed; the secondary polarization rotation is the same as the primary polarization rotation method;

[0022] In S6, the VCSEL laser couples the TM-mode beam and the TE-mode beam after the secondary polarization rotation into the resonant cavity to achieve the output of a narrow-linewidth single-longitudinal-mode seed laser.

[0023] Furthermore, the angle of the half-wave plate can be adjusted as needed.

[0024] Furthermore, the VCSEL laser couples the TM-mode beam and the TE-mode beam after the secondary polarization rotation into the resonant cavity to achieve the output of a narrow-linewidth single-longitudinal-mode seed laser, specifically:

[0025] Under the relaxation oscillation frequency of the TE-mode beam and the resonance condition of the external ring cavity eigenmode, by adjusting the angle of the half-wave plate, the component ratio and phase of the TE-mode beam and the TM-mode beam are adjusted until the condition for beat frequency is achieved, forming the output of a narrow-linewidth single-longitudinal-mode seed laser.

[0026] Furthermore, by adjusting the pump current of the VCSEL laser, the relaxation oscillation frequency of the TE-mode beam and the eigenmode of the external ring cavity are made to resonate.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The present invention discloses a narrow-linewidth single-longitudinal-mode seed laser system. Utilizing the characteristics of the polarization-maintaining beam splitter, the present invention successfully separates the beam emitted by the VCSEL laser into two independent beams of TM mode and TE mode. This design not only ensures the purity of the two-mode beams but also provides a basis for subsequent regulation; by setting a number of reflectors on the propagation paths of the TM-mode beam and the TE-mode beam, a stable ring circuit is formed. This structure helps the beam to be reflected multiple times in the cavity, increasing the interaction time between the beam and the gain medium, thereby improving the stability and output efficiency of the laser; the half-wave plate set on the ring circuit can precisely regulate the phase relationship between the TM-mode beam and the TE-mode beam by rotating the half-wave plate, achieving the output of a narrow-linewidth (about 1 MHz) and single-longitudinal-mode laser; the laser system of the present invention has a compact structure, and the connection between components is simple and clear, making it easy to integrate with other optical elements or systems. This design not only improves the flexibility of the system but also provides convenience for subsequent expansion and upgrade; compared with traditional narrow-linewidth single-longitudinal-mode lasers, the components adopted in the present invention are relatively simple and have a lower cost, thus achieving higher cost-effectiveness.

[0029] The present invention provides a method for generating a narrow linewidth single longitudinal mode seed laser. Through the steps of beam splitting and polarization beam splitting, the beam emitted by the VCSEL laser is efficiently separated into two independent beams of TM mode and TE mode. This polarization separation not only ensures the purity of the two mode beams, but also provides a basis for subsequent polarization control; By propagating the TM mode beam and the TE mode beam in opposite directions along the external ring cavity respectively, the beam is reflected multiple times in the cavity, increasing the interaction time between the beam and the gain medium, thereby improving the stability and output efficiency of the laser. At the same time, the design of the external ring cavity also helps to suppress the noise and fluctuations of the laser; During the propagation of the TM mode beam and the TE mode beam, they respectively pass through the same half-wave plate. By rotating the half-wave plate, the polarization states of the two mode beams can be dynamically adjusted, thereby achieving fine control of the laser output; After the TM mode beam and the TE mode beam complete one polarization rotation, they are reflected by the VCSEL laser for secondary polarization rotation. This design of multiple polarization rotations helps to enhance the selectivity for specific modes (such as single longitudinal mode), thereby suppressing other unnecessary modes; The components adopted by this method are relatively simple and easy to integrate, providing guarantee for the flexibility and scalability of the system. By adjusting the parameters of the external ring cavity, the rotation angle of the half-wave plate, etc., further control and optimization of the laser output can be achieved. Description of the Drawings

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0031] Figure 1 Structural diagram of the narrow linewidth single longitudinal mode seed laser system for the embodiment of the present invention;

[0032] Figure 2 Input-output curves of two linear polarization modes of the laser for the embodiment of the present invention;

[0033] Figure 3 Time-domain optical output of the TE mode under beat frequency conditions for the embodiment of the present invention;

[0034] Figure 4 Spectrum measured from the time-domain optical signal for the embodiment of the present invention;

[0035] Figure 5 Block diagram of the method for generating a narrow linewidth single longitudinal mode seed laser for the embodiment of the present invention. Detailed Embodiments

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Components of the embodiments of the present invention described and marked in the accompanying drawings here can be arranged and designed in various different configurations.

[0037] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0038] It should be noted that like reference numerals and letters denote like items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0039] In the description of the embodiments of the present invention, it should be noted that if terms such as "upper", "lower", "horizontal", "inner", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the inventive product is customarily placed during use, it is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation of the present invention. In addition, terms such as "first", "second", etc. are only used for distinguishing descriptions and cannot be construed as indicating or implying relative importance.

[0040] In addition, if the term "horizontal" appears, it does not mean that the component is required to be absolutely horizontal, but it can be slightly inclined. For example, "horizontal" only means that its direction is more horizontal relative to "vertical", and does not mean that the structure must be completely horizontal, but it can be slightly inclined.

[0041] In the description of the embodiments of the present invention, it should also be noted that unless otherwise clearly specified and limited, if terms such as "set", "installed", "connected", "connected" are understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0042] The following further describes the present invention in detail with reference to the accompanying drawings:

[0043] SeeFigure 1 , the present invention provides a narrow linewidth single longitudinal mode seed laser system, which includes a VCSEL laser. After the light source emitted by the VCSEL laser passes through a collimating lens L, it is split by a beam splitter BS. In this embodiment, the beam splitter adopts a 50:50 ratio. One of the beams passes through an optical isolator ISO and enters a fast photodetector PD, and then signal acquisition is performed through an oscilloscope Scope. The other beam enters an external ring cavity after passing through a polarization-maintaining beam splitter prism PBS. The length of the external ring cavity is 1.35 m. The polarization-maintaining beam splitter prism PBS divides the beam into a TE-mode beam and a TM-mode beam. Then, the TE-mode beam and the TM-mode beam propagate in the clockwise and counterclockwise directions respectively. During the propagation process, they pass through a half-wave plate. At this time, the TE-mode beam and the TM-mode beam are respectively rotated by 90 degrees, that is, the TE mode becomes the TM mode and the TM mode becomes the TE mode. When the TE-mode beam and the TM-mode beam first reach the VCSEL laser after passing through the external ring cavity, they are reflected back, and then after experiencing another polarization rotation, they propagate to the VCSEL laser, and the VCSEL laser couples it into the resonant cavity.

[0044] The external ring cavity is realized by arranging a plurality of mirrors on the propagation paths of the TM-mode beam and the TE-mode beam, so that the TM-mode beam and the TE-mode beam form a ring circuit; a half-wave plate is arranged on the ring circuit; the ring circuit can be a square circuit with four corners perpendicular. A collimating lens L is arranged between the beam splitter BS and the VCSEL laser. An optical isolator ISO and a collimating lens L are arranged between the photodetector PD and the beam splitter BS; after the beam is split by the beam splitter BS, it passes through the optical isolator ISO and the collimating lens L in sequence and then enters the photodetector PD.

[0045] As Figure 5 shown, the present invention provides a method for generating a narrow linewidth single longitudinal mode seed laser, which includes the following steps:

[0046] S1, splitting the beam emitted by the VCSEL laser to obtain a first beam and a second beam;

[0047] S2, performing polarization splitting on the second beam to obtain a TM-mode beam and a TE-mode beam;

[0048] S3, propagating the TM-mode beam and the TE-mode beam in opposite directions along the external ring cavity respectively, and the TM-mode beam and the TE-mode beam respectively pass through the same half-wave plate during the propagation process;

[0049] S4, after the TM-mode beam and the TE-mode beam respectively pass through the ring circuit, they propagate back along the original path to the VCSEL laser to complete a polarization rotation;

[0050] S5. After the TM-mode beam and the TE-mode beam are reflected by the VCSEL laser, a secondary polarization rotation is performed; the secondary polarization rotation is the same as the primary polarization rotation method.

[0051] S6. The VCSEL laser couples the TM-mode beam and the TE-mode beam after the secondary polarization rotation into the resonant cavity to achieve the output of a narrow-linewidth single-longitudinal-mode seed laser.

[0052] An embodiment of the present invention provides a method for generating a narrow-linewidth single-longitudinal-mode seed laser, including:

[0053] S1. A light source is provided by the VCSEL laser. The VCSEL laser has a good temperature control and power supply control system, so the laser output has good stability; the beam emitted by the VCSEL laser passes through the collimating lens L to obtain collimated laser light.

[0054] S2. The collimated laser light is first split by a beam splitter. In this embodiment, a beam splitting prism is used, and the beam ratio of the beam splitting prism is set to 50:50; one path of light passes through an optical isolator and enters a fast photodetector. The fast photodetector has a bandwidth of 10 GHz, and then signal acquisition is performed through an oscilloscope. Since the optical isolator selects the TE mode, the measured time-domain signal represents the time-domain dynamic behavior of the TE mode.

[0055] S3. The other path of light passes through a polarization-maintaining beam splitting prism and enters an external ring cavity. The length of the external ring cavity is 1.35 m. After passing through the polarization-maintaining beam splitting prism, the TE and TM polarization modes of the laser are separated to obtain a TE-mode beam and a TM-mode beam; then the two polarization modes propagate in the external ring cavity in the clockwise and counterclockwise directions respectively. The external ring cavity is provided with a half-wave plate.

[0056] S4. When the TE-mode beam and the TM-mode beam pass through the half-wave plate for the first time, polarization rotation is achieved, then they are propagated to the VCSEL laser and reflected back, and then pass through the half-wave plate again to achieve the second polarization rotation, and then are propagated to the VCSEL laser. The VCSEL laser couples the TE-mode beam and the TM-mode beam into the resonant cavity of the VCSEL laser.

[0057] S5. By adjusting the pump current of the VCSEL laser, the relaxation oscillation frequency of the TE-mode beam is made to resonate with the eigenmode of the external ring cavity.

[0058] S6. Under the resonance condition, by rotating the half-wave plate, the component ratio and phase of the TE-mode beam and the TM-mode beam in the optical path are adjusted until the condition for beat frequency is achieved, thereby achieving the output of a narrow-linewidth single-longitudinal-mode seed laser.

[0059] The beat frequency signal achieved in this embodiment is at 7.6 GHz with a linewidth of only 10 MHz and can be continuously adjusted within a certain range. This greatly enhances the application range of the seed laser and provides a reliable, efficient, and practical implementation method for the practical application of narrow linewidth seed lasers. In addition, this method is expected to be extended to lasers in other frequency bands and has a certain universality.

[0060] A method for generating a narrow linewidth single longitudinal mode seed laser according to the present invention utilizes the characteristics of two linear polarization modes (TE and TM modes) of a semiconductor vertical cavity surface emitting laser (VCSEL), especially the advantage that the TE mode has a lower threshold. Through an external delay feedback mechanism, effective modulation of the laser gain is achieved. This design not only improves the efficiency of the laser but also realizes the selective enhancement of a specific mode (such as the TE mode), thereby suppressing other unnecessary modes. Through an external ring cavity structure and the adjustment of a half-wave plate, the ratio and phase relationship between the TE and TM modes can be precisely controlled. Under resonance conditions, the TE and TM modes produce a beat frequency effect, and this interaction causes the laser output to exhibit the characteristics of narrow linewidth and single longitudinal mode, which can significantly improve the resolution and accuracy of the system. By adjusting the laser pump current, the tunability of the relaxation oscillation frequency of the TE mode is achieved. This means that users can flexibly adjust the output frequency of the laser according to actual needs to meet the requirements of different application scenarios. The external delay feedback and ring cavity design are adopted, which have good stability and reliability. At the same time, the external delay feedback can suppress the noise and fluctuations of the laser, while the ring cavity structure helps to maintain the continuity and stability of the laser output. Further, the laser system structure of the present invention is compact and easy to integrate, and can be conveniently connected and expanded with other optical elements or systems. This provides convenience for its application in complex optical systems.

[0061] To prove the superiority of the present invention, the input-output characteristics, time-domain dynamic behavior, and frequency-domain characteristics of different polarization modes (TE and TM modes) of the laser are analyzed in detail to comprehensively evaluate its performance.

[0062] In the embodiment of the present invention, an oscilloscope is used to monitor and collect time-domain signals, and the data monitoring window is set to 20 μs. Through Fourier transform of the time-domain signals, frequency-domain characterization is performed.

[0063] See Figure 2 , Figure 2 is the input-output curve characterization of the two linear polarization modes of the laser. It can be seen from the figure that the threshold of the TE mode is much smaller than that of the TM mode; indicating that under the same input power, the TE mode is more likely to reach and maintain laser output, which shows that the present invention has higher efficiency and lower energy consumption in the TE mode.

[0064] SeeFigure 3 It is the time-domain optical output of the TE mode under the beat frequency condition. It can be seen that under the beat frequency condition, the time-domain optical output of the TE mode exhibits a rapid relaxation oscillation phenomenon. Relaxation oscillation is a dynamic process before the laser operates stably, and its rapidity indicates that the present invention has a faster response speed and better stability.

[0065] See Figure 4 is the spectrum measured corresponding to the time-domain optical signal. It can be seen from the figure that the main peak position is 7.6 GHz, and there is a small peak with a relatively weak intensity at the 15.2 GHz position. The inset is an enlarged view of the main peak mode, and it can be seen from the figure that the linewidth of this mode is only 1 MHz. The stability of the main peak position and the narrow linewidth (1 MHz) indicate that the laser system of the present invention has excellent monochromaticity and coherence in the frequency domain, and at the same time has a very high frequency stability.

[0066] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A narrow linewidth single longitudinal mode seed laser system, characterized in that: Includes VCSEL laser, polarization-maintaining beam splitter, reflector and half-wave plate; The light beam emitted by the VCSEL laser is divided into a TM mode light beam and a TE mode light beam by a polarization-maintaining beam splitter; a plurality of reflectors are arranged on the propagation paths of the TM mode light beam and the TE mode light beam, so that the TM mode light beam and the TE mode light beam form a ring loop on the optical path; a half-wave plate is arranged on the ring loop; the TM mode light beam and the TE mode light beam propagate in opposite directions along the ring loop respectively and then return along the original path to the VCSEL laser for polarization rotation; the TM mode light beam and the TE mode light beam are coupled into the resonant cavity after two polarization rotations, so as to complete the output of a narrow-linewidth single longitudinal mode seed laser.

2. The narrow linewidth single longitudinal mode seed laser system according to claim 1, characterized in that: The annular loop is a square loop with four vertical corners.

3. The narrow linewidth single longitudinal mode seed laser system according to claim 1, characterized in that: A beam splitter is arranged between the VCSEL laser and the polarization-maintaining beam splitter.

4. The narrow linewidth single longitudinal mode seed laser system according to claim 3, characterized in that: A collimating lens is arranged between the beam splitter and the VCSEL laser.

5. The narrow linewidth single longitudinal mode seed laser system according to claim 3, characterized in that: After the light beam emitted by the VCSEL laser is split by the beam splitter, one beam enters the photodetector and the other beam passes through the polarization-maintaining beam splitter.

6. The narrow linewidth single longitudinal mode seed laser system according to claim 5, characterized in that: An optical isolator and a collimating lens are arranged between the photoelectric detector and the beam splitter; after the light beam is split by the beam splitter, it passes through the optical isolator and the collimating lens in sequence and then enters the photoelectric detector.

7. A method for generating a narrow linewidth single longitudinal mode seed laser, characterized in that: The following steps are involved: S1, splitting the light beam emitted by the VCSEL laser to obtain a first light beam and a second light beam; S2, performing polarization splitting on the second light beam to obtain a TM mode light beam and a TE mode light beam; S3, propagating the TM mode beam and the TE mode beam in opposite directions along the external ring cavity respectively, and during the propagation process, the TM mode beam and the TE mode beam pass through the same half-wave plate respectively; S4, the TM mode beam and the TE mode beam pass through the circular loop and then propagate back along the original path to the VCSEL laser, completing a polarization rotation; S5, after the TM mode light beam and the TE mode light beam are reflected by the VCSEL laser, a secondary polarization rotation is performed; the secondary polarization rotation is performed in the same manner as the primary polarization rotation; S6, the VCSEL laser couples the TM mode beam and TE mode beam after secondary polarization rotation into the resonant cavity to achieve narrow linewidth single longitudinal mode seed laser output.

8. The method for generating a narrow linewidth single longitudinal mode seed laser according to claim 7, characterized in that: The angle of the half-wave plate can be adjusted as required.

9. The method for generating a narrow linewidth single longitudinal mode seed laser according to claim 7, characterized in that: The VCSEL laser couples the TM mode beam and TE mode beam after secondary polarization rotation into the resonant cavity to achieve narrow linewidth single longitudinal mode seed laser output, specifically: Under the relaxation oscillation frequency of the TE mode beam and the eigenmode resonance conditions of the external ring cavity, the component ratio and phase of the TE mode beam and the TM mode beam are adjusted by adjusting the angle of the half-wave plate until the beat frequency condition is achieved, forming a narrow linewidth single longitudinal mode seed laser output.

10. The method for generating a narrow linewidth single longitudinal mode seed laser according to claim 9, characterized in that: By adjusting the pump current of the VCSEL laser, the relaxation oscillation frequency of the TE mode beam and the eigenmode of the external ring cavity are resonated.

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