Laser generating device

Abstract

Embodiments of the present application provide a laser generating device for improving the low repetition frequency of the laser device in the related art. The laser generating device comprises a pump source, a first laser cavity, a connecting device and a second laser cavity. The first laser cavity comprises a pump end, a first port and a second port. The pump end is connected with the pump source. The first laser cavity absorbs pump light and generates a first light beam and a second light beam. The first laser cavity further modulates the first light beam and the second light beam to generate a nonlinear phase difference. The polarization state of the second light beam is perpendicular to the polarization state of the first light beam. The connecting device comprises a first optical fiber connected with the first port and a second optical fiber connected with the second port. The second laser cavity modulates the first light beam and the second light beam to generate a fixed phase difference. The fixed phase difference and the nonlinear phase difference are superimposed on each other, so that the first light beam and the second light beam interfere, and then a laser beam is generated. It is also beneficial to reduce the length of the optical fiber and improve the repetition frequency of the laser generating device.

Description

Technical Field

[0001] This application relates to the field of laser technology, and more particularly to a laser generating device. Background Technology

[0002] A laser generator is a device that produces short-pulse lasers. Short-pulse lasers have very weak thermal effects and influences when interacting with materials, thus enabling cold processing. Laser generators can be applied to applications requiring high precision, such as drilling or cutting glass, dicing semiconductor wafers, and laser medical procedures.

[0003] Laser generators can achieve mode-locking based on non-linear amplifying loop mirrors (NALMs). The working principle of a NALM is that mode-locking is achieved by colliding two beams of light propagating in opposite directions within an optical fiber loop at their intersection. However, in related technologies, laser devices using NALMs for mode-locking have relatively low repetition rates. Summary of the Invention

[0004] The purpose of this application is to provide a laser generating device to improve the problem of low repetition rate of laser devices that achieve mode locking based on nonlinear amplification loop reflectors.

[0005] To achieve the above objectives, this application provides a laser generating device. The laser generating device includes a pump source, a first laser cavity, a connecting device, and a second laser cavity. The connecting device is located between the first and second laser cavities. The pump source emits pump light. The first laser cavity includes a pump end, a first port, and a second port. The pump end is connected to the pump source. The first laser cavity absorbs the pump light and generates signal light, which includes a first beam and a second beam. The first laser cavity also modulates the first and second beams to generate a nonlinear phase difference; wherein the polarization state of the second beam is perpendicular to the polarization state of the first beam. The connecting device includes a first optical fiber and a second optical fiber. The first optical fiber is connected to the first port and modulates the received signal light into the first beam. The second optical fiber is connected to the second port and modulates the received signal light into the second beam. The connecting device also includes a modulation device located on the optical paths of the first and second optical fibers away from the first laser cavity. The modulation device is used to combine or separate the first and second beams. The second laser cavity is used to modulate the first beam and the second beam to generate a fixed phase difference, forming and outputting a laser beam; the second laser cavity is also used to reflect the signal light from the first laser cavity back to the first optical fiber and the second optical fiber.

[0006] By configuring a first laser cavity, a connecting device, and a second laser cavity, the pump light emitted from the pump source is absorbed by the first laser cavity to generate signal light. This signal light is then transmitted sequentially to the connecting device and the second laser cavity, and reflected back into the first laser cavity by the second laser cavity. The first and second beams can propagate in opposite directions within the first laser cavity, facilitating modulation of the first and second beams to produce a nonlinear phase difference. When signal light A is transmitted again from the first laser cavity to the second laser cavity via the connecting device, the first optical fiber of the connecting device is connected to the first port, enabling the first optical fiber to modulate the received signal light into the first beam; the second optical fiber of the connecting device is connected to the second port, enabling the second optical fiber to modulate the received signal light into the second beam. After being combined and collimated by the connecting device, the first and second beams are transmitted into the second laser cavity, allowing the second laser cavity to modulate the first and second beams to produce a fixed phase difference. Since the second laser cavity can reflect the first beam and the second beam back into the first laser cavity, after multiple round trips, the nonlinear phase difference generated by the first beam and the second beam in the first laser cavity and the fixed phase difference generated in the second laser cavity are superimposed on each other, so that the first beam and the second beam can interfere in the cavity and thus generate a laser beam with ultrashort pulses.

[0007] Furthermore, since this application incorporates a first optical fiber and a second optical fiber in the connecting device, with the first optical fiber connected to the first port and the second optical fiber connected to the second port, no optical fiber connection is required between the connecting device and the second laser cavity. Therefore, in practical applications, the connecting device only needs two fusion splices, which helps reduce the length of the optical fiber in the laser generator, thereby improving the repetition rate of the laser generator.

[0008] In some embodiments, both the first and second optical fibers are polarization-maintaining fibers, with the slow axis of the first fiber perpendicular to the slow axis of the second fiber. Since polarization-maintaining fibers can maintain the polarization state of incident light, the slow axis of the first fiber can be perpendicular to the slow axis of the second fiber, ensuring that the polarization state of the beam transmitted within the first fiber is perpendicular to the polarization state of the beam transmitted within the second fiber.

[0009] In some embodiments, the modulation device includes a first polarizing crystal, and the connecting device further includes a collimating lens located on the optical path of the first polarizing crystal away from the first laser cavity; or, the collimating lens is located on the optical path between the first optical fiber, the second optical fiber, and the first polarizing crystal. The first polarizing crystal enables beam combining and separating of beams with different polarization states. The collimating lens is used to collimate the beam. Taking the collimating lens located on the optical path of the first polarizing crystal away from the first laser cavity as an example, when the first beam and the second beam are transmitted from the first laser cavity to the connecting device, the first polarizing crystal can combine the first beam and the second beam. The combined signal light is then collimated by the collimating lens and further transmitted to the second laser cavity. When the second laser cavity reflects the combined signal light back to the connecting device, the collimating lens expands the signal light, and the first polarizing crystal separates the expanded signal light into the first beam and the second beam.

[0010] In some embodiments, the first laser cavity includes a wavelength division multiplexer, a gain fiber, and a passive fiber. The wavelength division multiplexer includes a signal input terminal, a common output terminal, and a pump terminal. The common output terminal is connected to one end of the gain fiber, the other end of the gain fiber is connected to a first port, the signal input terminal is connected to one end of the passive fiber, and the other end of the passive fiber is connected to a second port.

[0011] With the above configuration, the wavelength division multiplexer, gain fiber, and passive fiber together form a first laser cavity, which is roughly annular in shape, with the gain fiber located at an asymmetrical position within the annular cavity. When the first beam enters the first laser cavity through the first port, it is first transmitted to the gain fiber and amplified. The amplified first beam is then transmitted from the gain fiber to the common output of the wavelength division multiplexer. The wavelength division multiplexer decomposes the amplified first beam into signals of different wavelengths. The first beam with the same wavelength as the pump light is transmitted back to the pump source from the pump end of the wavelength division multiplexer. The first signal light used to further form the laser beam is transmitted from the signal input end of the wavelength division multiplexer to the passive fiber, and then from the passive fiber to the second port. When the second beam enters the first laser cavity through the second port, it is transmitted from the signal input end of the wavelength division multiplexer (WDM) to the WDM itself. Simultaneously, since the pump light is transmitted from the pump end of the WDM to the WDM, the WDM couples the second beam with a different wavelength to the pump light, and transmits it to the gain fiber through the common output end of the WDM. The gain fiber absorbs the pump light and also amplifies the second beam. The amplified second beam is then transmitted to the first port through the gain fiber.

[0012] In some embodiments, the first laser cavity further includes a connecting optical fiber connected between the gain optical fiber and the first port. The connecting optical fiber can be an undoped polarization-maintaining fiber. With the above configuration, the cavity length of the laser generator can be further adjusted by adjusting the length of the connecting optical fiber, thereby adjusting the repetition frequency of the laser generator.

[0013] In some embodiments, the first laser cavity further includes a coupler, which includes at least a first input terminal, a first output terminal, and at least one second output terminal. The first input terminal is connected to a signal input terminal, the first output terminal is connected to a second port, and the second output terminal is used to output a laser beam. By providing a coupler, the output path of the laser beam can be further increased. In this application embodiment, the number of second output terminals is not limited and can be set according to actual conditions.

[0014] In some embodiments, the second laser cavity includes a phase biasing device, which comprises a Faraday rotator and a waveplate. The Faraday rotator is located in the optical path between the connecting device and the waveplate. The phase biasing device may include a Faraday rotator and a waveplate, with the Faraday rotator positioned in the optical path between the connecting device and the waveplate. The Faraday rotator is used to rotate the polarization direction of the incident beam by a certain angle, and the rotation direction is only related to the internal magnetic field and independent of the light transmission direction. The waveplate, also known as a phase delay plate, enables a fixed phase difference to be generated between a first beam and a second beam with mutually perpendicular polarization states.

[0015] In some embodiments, the second laser cavity includes a polarization-independent beam splitter located in the optical path between the connecting device and the Faraday rotator, or the polarization-independent beam splitter located in the optical path on the side of the phase biasing device away from the connecting device.

[0016] In some embodiments, the second laser cavity further includes a dispersion compensation device and a mirror. The dispersion compensation device includes at least one set of grating pairs, each grating pair including a first grating and a second grating disposed opposite to each other. When the polarization-independent beam splitter is located on the optical path between the connector and the Faraday rotator, the dispersion compensation device is located on the optical path between the phase bias device and the mirror. When the polarization-independent beam splitter is located on the optical path opposite to the connector on the phase bias device, the dispersion compensation device is located on the optical path between the polarization-independent beam splitter and the mirror. The dispersion compensation device may include at least one set of grating pairs, each grating pair including a first grating and a second grating disposed opposite to each other. The dispersion compensation device can compensate for dispersion generated within the optical fiber. The first grating and the second grating can be one of a diffraction grating, a transmission grating, or a reflection grating. The mirror is used to reflect the first and second beams from the connector back.

[0017] In some embodiments, the second laser cavity further includes a second polarizing crystal and a chirped fiber Bragg grating. When the polarization-independent beam splitter is located on the optical path between the connector and the Faraday rotator, the second polarizing crystal is located on the optical path between the phase-biasing device and the chirped fiber Bragg grating. When the polarization-independent beam splitter is located on the optical path of the phase-biasing device away from the connector, the second polarizing crystal is located on the optical path between the polarization-independent beam splitter and the chirped fiber Bragg grating. With the above configuration, the signal light from the phase-biasing device, after entering the second polarizing crystal, is filtered into signal light with only one polarization direction. After entering the chirped fiber Bragg grating, the signal light undergoes dispersion compensation, and is simultaneously reflected back by the chirped fiber Bragg grating, and is sequentially transmitted to the second polarizing crystal, the phase-biasing device, the connector, and the first laser cavity. The transmission process of the signal light in the phase-biasing device, the connector, and the first laser cavity can be as described in the above embodiments, and will not be repeated here.

[0018] In some embodiments, the laser generating device further includes an output optical fiber, which is located on the optical path of the polarization-independent beam splitter on the side opposite to the first laser cavity. With this configuration, a high-intensity laser beam transmitted from the first laser cavity to the second laser cavity can be coupled into the output optical fiber, so that it can be output to the desired scene through the output optical fiber.

[0019] In some embodiments, the laser generating device further includes a photodetector located on the optical path of the polarization-independent beam splitter on the side opposite to the second laser cavity. With this configuration, a weaker laser beam transmitted from the second laser cavity to the first laser cavity can be received by the photodetector. The photodetector can convert the optical signal into an electrical signal, which allows for observation of the timing of the laser pulses. Attached Figure Description

[0020] Figure 1 A structural block diagram of a mode-locked laser based on a nonlinear amplification loop mirror is provided for related technical embodiments;

[0021] Figure 2 A structural block diagram of a laser generating device provided in an embodiment of this application;

[0022] Figure 3 A structural block diagram of a connection device provided in an embodiment of this application;

[0023] Figure 4 for Figure 3 Cross-sectional views of the first and second optical fibers;

[0024] Figure 5 A structural block diagram of another laser generating device provided in the embodiments of this application;

[0025] Figure 6 A structural block diagram of another laser generating device provided in the embodiments of this application;

[0026] Figure 7 A structural block diagram of another laser generating device provided in the embodiments of this application;

[0027] Figure 8 A structural block diagram of another laser generating device provided in the embodiments of this application;

[0028] Figure 9 A structural block diagram of another laser generating device provided in the embodiments of this application;

[0029] Figure 10 This is a structural block diagram of another laser generating device provided in an embodiment of this application. Detailed Implementation

[0030] The technical solutions of the embodiments of this application will be described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0031] In the following description, the terms "first," "second," etc., are used for descriptive convenience only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0032] In the embodiments of this application, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or description. Any embodiment or design that is described as "exemplary" or "for example" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0033] In the embodiments of this application, the directional indications used to explain the structure and movement of different components, such as up, down, left, right, front, and back, are relative. These indications are appropriate when the components are in the positions shown in the figures. However, if the description of the component positions changes, these directional indications will also change accordingly.

[0034] This application provides a laser generating device. Light of a certain wavelength can exist stably within the resonant cavity of the laser device; these light waves satisfying the resonance condition are the longitudinal modes of the laser device. When the phase relationship between the longitudinal modes is locked, the laser generating device exhibits mode-locking. Through mode-locking, the laser generating device can obtain ultrashort coherent pulses in the time domain, possessing high peak power and a wide spectrum in the frequency domain.

[0035] Mode locking can be achieved using a nonlinear amplifying loop mirror (NALM). The working principle of the NALM is to achieve mode locking by colliding and adding two beams of light that are transmitted in opposite directions in the optical fiber loop at their intersection.

[0036] Figure 1 This diagram illustrates the structure of a mode-locked laser 90 based on a nonlinear amplifying loop mirror, providing insights into related technologies. Figure 1 As shown, the mode-locked laser 90 may include a loop portion 92 and a linear arm portion 91, wherein the loop portion 92 and the linear arm portion 91 are combined by an optical fiber coupler 93. The linear arm portion 91 includes a pump source, a fiber grating 911, and an optical fiber polarizer 912 connected in sequence, and the loop portion 92 includes a phase biasing unit 921 and an optical fiber amplifier 922.

[0037] When the aforementioned mode-locked laser 90 is operating, the pump light emitted from the pump source is coupled into the linear arm section 91, and then enters the loop section 92 via the coupler 93. Part of the beam propagates clockwise within the loop section 92, while the other part propagates counterclockwise. The optical field of the counterclockwise propagating beam is amplified by the fiber amplifier 922 and then enters the phase bias unit 921, while the optical field of the clockwise propagating beam is amplified again by the fiber amplifier 922 after passing through the phase bias unit 921. Therefore, the accumulated nonlinear phase of the counterclockwise propagating beam is greater than that of the clockwise propagating beam, resulting in a nonlinear phase difference related to light intensity. This loop section is the nonlinear amplification loop reflector.

[0038] Simultaneously, the phase biasing unit 921 also creates a fixed phase difference between the optical fields of the clockwise propagating beam and the counterclockwise propagating beam, thereby achieving mode-locked self-starting. Further, the clockwise and counterclockwise propagating beams, after being combined by the fiber coupler 93, enter the linear arm section 91. A portion of the beam is output as a laser beam from one end of the linear arm section 91 (e.g.,...). Figure 1 As shown, the end of the linear arm portion 91 away from the loop portion 92 may include two output paths, and the other part of the beam is reflected by the fiber grating 911 and retransmitted back to the loop portion 92 via the fiber polarizer 912 and the fiber coupler 93.

[0039] However, since the fiber polarizer 912, phase bias unit 921, and fiber amplifier 922 are all fiber optic devices, the fiber coupler 93 needs to be fused with each of these devices during use, resulting in a longer fiber length in the mode-locked laser 90. For example, the fiber length at both ends of a single fusion splice is greater than or equal to 5 cm, meaning the fiber length increases by at least 10 cm. In related technologies, the fiber coupler 93 can form three fusion splices with other fiber optic devices, thus increasing the fiber length by at least 30 cm.

[0040] Since the repetition rate of the mode-locked laser 90 is inversely proportional to the cavity length, and the fiber length in related technologies is relatively long, the cavity length of the mode-locked laser 90 is also relatively long, resulting in a lower repetition rate. Here, repetition rate refers to the pulse repetition rate (PRR), which is the number of pulses output per second by the mode-locked laser 90. Because high repetition rate (PRR) lasers are key technologies for scientific research and industrial applications such as high-speed optical sampling, precise distance measurement, and precision lidar, the application scenarios of the mode-locked laser 90 in related technologies are relatively limited.

[0041] Figure 2 This is a structural diagram of a laser generating device 100 provided in an embodiment of this application. Figure 2 As shown, the laser generating device 100 includes a pump source 11, a first laser cavity 10, a connecting device 30, and a second laser cavity 20. The pump source 11 emits pump light B and is connected to the first laser cavity 10. The connecting device 30 is located between the first laser cavity 10 and the second laser cavity 20. For example, it is located... Figure 2 The one on the left is the first laser cavity 10, located in... Figure 2 The one on the right is the second laser cavity 20.

[0042] The first laser cavity 10 includes a pump end 121, a first port 101, and a second port 102. The pump end 121 is connected to a pump source 11, and the first laser cavity 10 is used to absorb pump light B and generate signal light A. For example, the first laser cavity 10 can have a ring structure, wherein the pump end 121 is connected to the pump source 11 so that the first laser cavity 10 can couple the pump light B emitted by the pump source 11 into the cavity, thereby facilitating the absorption of pump light B and the generation of signal light A by the first laser cavity 10. The first port 101 and the second port 102 are used to output signal light A, so that signal light A from the first laser cavity 10 can be transmitted to the connecting device 30 via the first port 101 and the second port 102.

[0043] The signal light A includes a first beam A1 and a second beam A2. The polarization state of the first beam A1 is perpendicular to the polarization state of the second beam A2. For example, the first beam A1 can be an S-beam with its polarization vector in the vertical direction; the second beam A2 can be a P-beam with its polarization vector in the horizontal direction. The following explanation uses the example of the first beam A1 being an S-beam and the second beam A2 being a P-beam. Of course, in some other embodiments, the first beam A1 can also be a P-beam and the second beam A2 can also be an S-beam.

[0044] Figure 3 This is a structural block diagram of a connecting device 30 provided in an embodiment of this application. Figure 3 As shown, the connecting device 30 includes a first optical fiber 31 and a second optical fiber 32. Combined with... Figure 2 As shown, the first optical fiber 31 is connected to the first port 101, and the second optical fiber 32 is connected to the second port 102. As shown in the above embodiment, when the laser generator 100 starts generating signal light A (that is, when the laser generator 100 starts up), a portion of signal light A is transmitted from the first port 101 to the first optical fiber 31, and another portion of signal light A is transmitted from the second port 102 to the second optical fiber 32.

[0045] The first optical fiber 31 is used to receive signal light A and output a first beam A1; the second optical fiber 32 is used to receive signal light A and output a second beam A2. With the above configuration, after signal light A is transmitted to the first optical fiber 31, the first optical fiber 31 will output the first beam A1 to the second laser cavity 20; after signal light A is transmitted to the second optical fiber 32, the second optical fiber 32 will output the second beam A2 to the second laser cavity 20.

[0046] Combination Figure 2 and Figure 3 The second laser cavity 20 is used to reflect the signal light A from the first laser cavity 10 back to the first optical fiber 31 and the second optical fiber 32. When the signal light A is transmitted from the first laser cavity 10 to the connecting device 30, as described in the above embodiment, the first beam A1 and the second beam A2 are transmitted to the second laser cavity 20. When the second laser cavity 20 reflects the signal light A back to the connecting device 30, the signal light A can be modulated into a pre-polarized light that is polarized only along one direction, and its polarization direction is at a 45° angle to the initial polarization state of the first beam A1 and the second beam A2. The deflected signal light A is further transmitted to the first optical fiber 31 and the second optical fiber 32. The first optical fiber 31 receives the deflected signal light A from the second laser cavity 20 and outputs the first beam A1, and the second optical fiber 32 receives the deflected signal light A from the second laser cavity 20 and outputs the second beam A2.

[0047] The first port 101 is used to receive the first beam A1 and output the second beam A2, and the second port 102 is used to receive the second beam A2 and output the first beam A1. With this configuration, the first beam A1, reflected from the second laser cavity 20, is transmitted through the first port 101 into the first laser cavity 10, and then transmitted out of the first laser cavity 10 through the second port 102; similarly, the second beam A2, reflected from the second laser cavity 20, is transmitted through the second port 102 into the first laser cavity 10, and then transmitted out of the first laser cavity 10 through the first port 101. In other words, within the first laser cavity 10, the first beam A1 and the second beam A2 can be transmitted in opposite directions.

[0048] The first laser cavity 10 is used to modulate the first beam A1 and the second beam A2 to generate a nonlinear phase difference. By setting the first laser cavity 10, the first beam A1 and the second beam A2 are respectively given nonlinear phase shifts, and the nonlinear phase shifts of the first beam A1 and the second beam A2 are different, that is, there is a nonlinear phase difference between the first beam A1 and the second beam A2. This nonlinear phase difference is related to the light intensity. This light intensity-related nonlinear phase difference increases with the increase of light intensity, thereby forming a saturable absorption effect, which allows the high-intensity portion of the pulsed light to have high transmittance and the low-intensity portion to have low transmittance, which is beneficial for obtaining stable pulse output.

[0049] After being modulated by the first laser cavity 10, the first beam A1 and the second beam A2 are transmitted again to the connecting device 30. As described in the above embodiment, the first port 101 outputs the second beam A2, and the first optical fiber 31 is used to receive the signal light A and output the first beam A1, so that the first optical fiber 31 can receive the second beam A2 output by the first port 101 and then output the first beam A1. At the same time, the second port 102 outputs the second beam A2, and the second optical fiber 32 is used to receive the signal light A and output the second beam A2, so that the second optical fiber 32 can receive the first beam A1 output by the second port 102 and output the second beam A2.

[0050] Continue to refer to Figure 3 The connecting device 30 also includes a modulation device 33, which is located on the optical paths of the first optical fiber 31 and the second optical fiber 32 away from the first laser cavity 10. The modulation device 33 is used to combine or separate the first beam A1 and the second beam A2. The first beam A1 and the second beam A2 from the first optical fiber 31 and the second optical fiber 32 are transmitted to the modulation device 33. After the modulation device 33 combines the first beam A1 and the second beam A2, they are transmitted into the second laser cavity 20. When the second laser cavity 20 reflects the signal light A back to the first laser cavity 10, the modulation device 33 is used to separate the reflected signal light A.

[0051] After the modulator 33 combines the first beam A1 and the second beam A2, the second laser cavity 20 modulates the first beam A1 and the second beam A2 to generate a fixed phase difference, forming and outputting a laser beam. Through the above configuration, the nonlinear phase difference and the fixed phase difference generated by the first beam A1 and the second beam A2 are superimposed on each other, so that the first beam A1 and the second beam A2 can interfere within the cavity, thereby generating a laser beam with ultrashort pulses.

[0052] In summary, by configuring a first laser cavity 10, a connecting device 30, and a second laser cavity 20, the pump light B emitted by the pump source 11 is absorbed by the first laser cavity 10 to generate signal light A. Signal light A is transmitted through the connecting device 30 to the second laser cavity 20 and then reflected back into the first laser cavity 10. The first beam A1 and the second beam A2 can be transmitted in opposite directions within the first laser cavity 10, thus facilitating the modulation of the first beam A1 and the second beam A2 by the first laser cavity 10 to generate a nonlinear phase difference. When signal light A is transmitted again from the first laser cavity 10 to the second laser cavity 20 through the connecting device 30, the first optical fiber 31 of the connecting device 30 is connected to the first port 101, so that the first optical fiber 31 receives signal light A and outputs the first beam A1; the second optical fiber 32 of the connecting device 30 is connected to the second port 102, so that the second optical fiber 32 receives signal light A and outputs the second beam A2. After the first beam A1 and the second beam A2 are combined by the connecting device 30, they are transmitted into the second laser cavity 20, so that the second laser cavity 20 modulates the first beam A1 and the second beam A2 to produce a fixed phase difference. Since the second laser cavity 20 can reflect the first beam A1 and the second beam A2 back into the first laser cavity 10, after multiple round trips, the nonlinear phase difference generated by the first beam A1 and the second beam A2 in the first laser cavity 10 and the fixed phase difference generated in the second laser cavity 20 are superimposed, so that the first beam A1 and the second beam A2 can interfere within the cavity, thereby generating a laser beam with an ultrashort pulse.

[0053] Meanwhile, since this application incorporates a first optical fiber 31 and a second optical fiber 32 in the connecting device 30, with the first optical fiber 31 connected to the first port 101 and the second optical fiber 32 connected to the second port 102, no optical fiber connection is required between the connecting device 30 and the second laser cavity 20. Therefore, in practical applications, the connecting device 30 only needs two fusion splices, which helps reduce the length of the optical fiber in the laser generator 100, thereby improving the repetition frequency of the laser generator 100.

[0054] Figure 4 for Figure 3 Cross-sectional view of the first optical fiber 31 and the second optical fiber 32. (Refer to...) Figure 4Both the first optical fiber 31 and the second optical fiber 32 can be polarization-maintaining fibers, and the slow axis sa of the first optical fiber 31 can be perpendicular to the slow axis sa of the second optical fiber 32. This application does not limit the type of polarization-maintaining fiber; for example, the first optical fiber 31 and the second optical fiber 32 can be panda polarization-maintaining fibers, side-hole polarization-maintaining fibers, flat polarization-maintaining fibers, bow-tie polarization-maintaining fibers, etc. The polarization-maintaining fiber includes two main transmission axes, namely the fast axis fa and the slow axis sa. Because the polarization-maintaining fiber can maintain the polarization state of the incident light, the slow axis sa of the first optical fiber 31 can be perpendicular to the slow axis sa of the second optical fiber 32, ensuring that the polarization state of the beam transmitted in the first optical fiber 31 and the polarization state of the beam transmitted in the second optical fiber 32 are perpendicular to each other.

[0055] Continue to refer to Figure 2 The first laser cavity 10 may include a wavelength division multiplexer 12, a gain fiber 14, and a passive fiber 13.

[0056] The wavelength division multiplexer 12 includes a signal input terminal 122, a common output terminal 123, and a pump terminal 121. As described in the above embodiment, the pump terminal 121 is connected to the pump source 11 so that the pump light B emitted by the pump source 11 can enter the wavelength division multiplexer 12 through the pump terminal 121. The wavelength division multiplexer 12 is used to couple the pump light B into the cavity.

[0057] One end of the gain fiber 14 is connected to the common output terminal 123, and the other end of the gain fiber 14 is connected to the first port 101. With this configuration, the gain fiber 14 can absorb pump light B and generate signal light A. The wavelength of signal light A is greater than the wavelength of pump light B, so that the generated signal light A can further form a laser beam. In some embodiments, the gain fiber 14 may include rare earth ions. For example, rare earth ions may include ytterbium (Yb), erbium (Er), holmium (Ho), thulium (Tm), etc. Rare earth ions cause fiber modification and exhibit laser effects. The working principle is that pump light B is coupled into the rare earth ion-doped gain fiber 14, and then the rare earth ions in the gain fiber 14 absorb the energy of pump light B and undergo energy level transitions, thereby generating signal light A.

[0058] One end of the passive optical fiber 13 is connected to the signal input terminal 122, and the other end of the passive optical fiber 13 is connected to the second port 102. By setting the passive optical fiber 13, signal light A can be transmitted between the wavelength division multiplexer 12 and the second port 102.

[0059] With the above configuration, the wavelength division multiplexer 12, the gain fiber 14, and the passive fiber 13 together form a first laser cavity 10. The first laser cavity 10 is roughly a ring-shaped cavity, and the gain fiber 14 is located at an asymmetrical position within the ring-shaped cavity.

[0060] When the first beam A1 enters the first laser cavity 10 through the first port 101, the first beam A1 is first transmitted to the gain fiber 14 and amplified by the gain fiber 14. The amplified first beam A1 is transmitted from the gain fiber 14 to the common output terminal 123 of the wavelength division multiplexer 12. The wavelength division multiplexer 12 decomposes the amplified first beam A1 into different wavelength components. The wavelength components used to further form the laser beam are transmitted from the signal input terminal 122 of the wavelength division multiplexer 12 to the passive fiber 13, and then from the passive fiber 13 to the second port 102.

[0061] When the second beam A2 enters the first laser cavity 10 through the second port 102, the second beam A2 is transmitted to the wavelength division multiplexer 12 through the signal input terminal 122. At the same time, since the pump light B is transmitted to the wavelength division multiplexer 12 through the pump terminal 121, the wavelength division multiplexer 12 couples the second beam A2 and the pump light B together, and transmits them to the gain fiber 14 through the common output terminal 123. The gain fiber 14 generates signal light A from the pump light B, and at the same time, the gain fiber 14 amplifies the second beam A2. The amplified second beam A2 is transmitted to the first port 101 through the gain fiber 14.

[0062] In summary, when signal light A is reflected back to the first laser cavity 10 by the second laser cavity 20, the first beam A1 propagates counterclockwise within the first laser cavity 10 and is first amplified by the gain fiber 14 within the first laser cavity 10, resulting in a larger nonlinear phase shift for the first beam A1. The second beam A2 propagates clockwise within the first laser cavity 10 and is subsequently amplified by the gain fiber 14 within the second laser cavity 20, resulting in a smaller nonlinear phase shift for the second beam A2. After transmission through the connecting device 30, the first beam A1 and the second beam A2 generate a nonlinear phase difference related to light intensity.

[0063] Reference Figure 3 The modulation device 33 may include a first polarizing crystal 331, and the connecting device 30 may include a collimating lens 34. The first polarizing crystal 331 enables the combining and separating of beams with different polarization states. The collimating lens 34 is used to collimate the beam. For example, the collimating lens 34 may be located on the optical path of the first polarizing crystal 331 away from the first laser cavity 10. When the first beam A1 and the second beam A2 are transmitted from the first laser cavity 10 to the connecting device 30, the first polarizing crystal 331 can combine the first beam A1 and the second beam A2. The combined signal light A is then collimated by the collimating lens 34 and further transmitted to the second laser cavity 20. When the second laser cavity 20 reflects the combined signal light A back to the connecting device 30, the collimating lens 34 focuses the signal light A, and the first polarizing crystal 331 separates the focused signal light A into the first beam A1 and the second beam A2.

[0064] In some other embodiments, the collimating lens 34 may also be located in the optical path between the first optical fiber 31, the second optical fiber 32, and the first polarizing crystal 331. Similarly, when the first beam A1 and the second beam A2 are transmitted from the first laser cavity 10 to the connecting device 30, the first beam A1 and the second beam A2 can first be collimated by the collimating lens 34, and then combined by the first polarizing crystal 331 before being transmitted to the second laser cavity 20; when the second laser cavity 20 reflects the combined signal light A back to the connecting device 30.

[0065] Continue to refer to Figure 2 The second laser cavity 20 may include a phase biasing device 21 and a polarization-independent beam splitter 24. The phase biasing device 21 may include a Faraday rotator 211 and a waveplate 212. The Faraday rotator 211 is located in the optical path between the connecting device 30 and the waveplate 212. The Faraday rotator 211 is used to rotate the polarization direction of the incident beam by a certain angle, and the rotation direction is only related to the internal magnetic field and is independent of the light transmission direction. The waveplate 212, also known as a phase retarder, can generate a fixed phase difference between the first beam A1 and the second beam A2, whose polarization states are perpendicular to each other.

[0066] The second laser cavity 20 may further include a dispersion compensation device 22 and a reflector 23, with the dispersion compensation device 22 located in the optical path of the phase bias device 21 and the reflector 23. The dispersion compensation device 22 may include at least one set of grating pairs, each pair comprising a first grating 221 and a second grating 222 disposed opposite to each other. The dispersion compensation device can compensate for dispersion generated within the optical fiber. The first grating 221 and the second grating 222 may be one of a diffraction grating, a transmission grating, or a reflection grating. The reflector is used to reflect the first beam A1 and the second beam A2 from the connecting device 30 back.

[0067] The following description uses the example of a Faraday rotator 211 rotating the polarization direction of the incident beam 45° clockwise, and the first grating 221 and the second grating 222 being diffraction gratings capable of diffracting beams with a vertical polarization direction.

[0068] With the above configuration, the phase bias device 21, the polarization-independent beam splitter 24, the dispersion compensation device 22, and the reflector 23 together constitute the second laser cavity 20, which has a roughly linear structure.

[0069] When signal light A is transmitted from the first laser cavity 10 to the second laser cavity 20, the signal light A after being combined and collimated by the first beam A1 and the second beam A2 via the connecting device 30 is transmitted into the Faraday rotator 211. The Faraday rotator 211 can rotate the polarization direction of signal light A clockwise by 45°, that is, at this time, the polarization directions of the first beam A1 and the second beam A2 are both rotated clockwise by 45°. When signal light A enters the waveplate 212 from the Faraday rotator 211, the waveplate 212 performs phase bias on the rotated signal light A to produce a fixed phase difference between the first beam A1 and the second beam A2, but at this time, the polarization state of signal light A does not change. When signal light A is transmitted from waveplate 212 to dispersion compensation device 22, the polarization direction of the beams diffracted by the first grating 221 and the second grating 222 is vertical. This polarization direction forms a 45° angle with the polarization directions of the rotated first beam A1 and second beam A2. Only half the intensity of the light fields of the first beam A1 and second beam A2 can be diffracted by the first grating 221 and the second grating 222. The diffracted first beam A1 and second beam A2 have the same polarization direction, and both are vertical. The diffracted first beam A1 and second beam A2 are further transmitted to reflector 23.

[0070] When signal light A is reflected back to dispersion compensation device 22 by mirror 23, the first grating 221 and the second grating 222 can diffract the signal light A so that it can be transmitted to waveplate 212. Waveplate 212 performs phase bias on the reflected first beam A1 and second beam A2, as described in the above embodiment. At this time, the polarization directions of the first beam A1 and the second beam A2 are both vertical. After the first beam A1 and the second beam A2 are transmitted to Faraday rotator 211, Faraday rotator 211 can rotate the first beam A1 and the second beam A2 clockwise by 45°. After the first beam A1 and the second beam A2 are reflected back to connection device 30, part of them are received by the first optical fiber 31 so that the first optical fiber 31 outputs the first beam A1 to the first port 101; the other part is received by the second optical fiber 32 so that the second optical fiber 32 outputs the second beam A2 to the second port 102. The transmission process of the first beam A1 and the second beam A2 into the first laser cavity 10 can be as described in the above embodiment, and will not be repeated here.

[0071] The second laser cavity 20 may also include a polarization-independent beam splitter 24. The polarization-independent beam splitter 24 is used to separate the energy of the incident light, and the polarization state of the outgoing light is unchanged compared to the incident light.

[0072] like Figure 2As shown, the polarization-independent beam splitter 24 can be located in the optical path between the connecting device 30 and the Faraday rotator 211, and the dispersion compensation device 22 is located in the optical path between the phase bias device 21 and the reflector 23. When signal light A is transmitted from the first laser cavity 10 to the second laser cavity 20, a portion of signal light A is transmitted through the polarization-independent beam splitter 24 and then transmitted into the phase bias device 21; a portion of signal light A is reflected by the polarization-independent beam splitter 24 and serves as the output of the laser generator 100 (e.g., ...). Figure 2 (Output from the upper end). When signal light A is reflected back from the second laser cavity 20 to the first laser cavity 10, a portion of signal light A is transmitted through the polarization-independent beam splitter 24 and then transmitted into the connecting device 30; a portion of signal light A is reflected by the polarization-independent beam splitter 24 and serves as the output of the laser generator 100 (e.g., ...). Figure 2 (Output from the middle to the lower end).

[0073] In some other embodiments, the polarization-independent beam splitter 24 is located on the optical path of the phase bias device 21 on the side opposite to the connector 30. Figure 5 This is a structural block diagram of another laser generating device provided in an embodiment of this application. Figure 5 As shown, the polarization-independent beam splitter 24 can be located in the optical path between the phase biasing device 21 and the dispersion compensation device 22, and the dispersion compensation device 22 is located in the optical path between the polarization-independent beam splitter 24 and the reflector 23. When signal light A is transmitted from the first laser cavity 10 to the second laser cavity 20, a portion of signal light A is transmitted through the polarization-independent beam splitter 24 and then transmitted into the dispersion compensation device 22; a portion of signal light A is reflected by the polarization-independent beam splitter 24 and serves as the output of the laser generator 100 (e.g., ...). Figure 5 (Output from the upper end of the middle). When signal light A is reflected back from the second laser cavity 20 to the first laser cavity 10, a portion of signal light A is transmitted through the polarization-independent beam splitter 24 and then transmitted into the phase bias; a portion of signal light A is reflected by the polarization-independent beam splitter 24 and serves as the output of the laser generator 100 (e.g., ...). Figure 5 (Output from the middle to the lower end).

[0074] In summary, by setting up a polarization-independent beam splitter 24, two outputs of the signal light A that travels back and forth within the first laser cavity 10 and the second laser cavity 20 can be achieved.

[0075] During the transmission of signal light A from the first laser cavity 10 to the second laser cavity 20, signal light A is consumed. For example, when signal light A is transmitted to the polarization-independent beam splitter 24, a portion of signal light A forms a laser beam and is output. Furthermore, when signal light A is transmitted to the dispersion compensation device 22, only a portion of signal light A is diffracted by the first grating 221 and the second grating 222. Therefore, the intensity of the laser beam output when transmitted from the first laser cavity 10 to the second laser cavity 20 is stronger, while the intensity of the laser beam output when transmitted from the second laser cavity 20 to the first laser cavity 10 is weaker.

[0076] Figure 6 This is a structural block diagram of another laser generating device provided in an embodiment of this application. Figure 6 As shown, the laser generating device 100 may further include an output optical fiber 40, which is located on the optical path of the polarization-independent beam splitter 24 on the side opposite to the first laser cavity 10. With the above arrangement, a high-intensity laser beam transmitted from the first laser cavity 10 to the second laser cavity 20 can be coupled into the output optical fiber 40 so that it can be output to the desired scene through the output optical fiber 40.

[0077] For example, the output fiber 40 can be a fast-axis cutoff polarization-maintaining single-mode fiber, which can achieve uniaxial operation, thereby outputting a laser beam with the same polarization state. In other examples, the output fiber 40 can be a single-mode fiber, and an analyzer can be placed in the optical path between the output fiber 40 and the polarization-independent beam splitter 24 to achieve the output of a laser beam with the same polarization state.

[0078] Furthermore, continue to refer to Figure 6 The laser generating device 100 may further include a photodetector 50, which may be located on the optical path of the polarization-independent beam splitter 24 on the side opposite to the second laser cavity 20. With the above arrangement, a weak laser beam transmitted from the second laser cavity 20 to the first laser cavity 10 can be received by the photodetector 50. The photodetector 50 can convert the optical signal into an electrical signal, and the timing state of the laser pulse can be observed through the electrical signal.

[0079] Figure 7 A structural block diagram of another laser generating device provided in the embodiments of this application; Figure 8 This is a structural block diagram of another laser generating device provided in an embodiment of this application. Figure 7 and Figure 8As shown, the first laser cavity 10 in the above embodiments may further include a connecting optical fiber 15. In some embodiments, the connecting optical fiber 15 is connected between the gain optical fiber 14 and the first port 101. For example, one end of the connecting optical fiber 15 is connected to the end of the gain optical fiber 14 away from the wavelength division multiplexer 12, and the other end of the connecting optical fiber 15 is connected to the first port 101. With the above configuration, the cavity length of the laser generator 100 can be further adjusted by adjusting the length of the connecting optical fiber 15, thereby adjusting the repetition frequency of the laser generator 100.

[0080] In embodiments where the first laser cavity 10 further includes a connecting optical fiber 15, the structure of the second laser cavity 20 can be as shown in any of the above embodiments, and its working principle is as described in the above embodiments, and will not be repeated here. For example, as Figure 7 As shown, the polarization-independent beam splitter 24 is located on the optical path of the phase-biasing device 21 on the side opposite to the connecting device 30. Alternatively, as... Figure 8 As shown, the polarization-independent beam splitter 24 can be located in the optical path between the connecting device 30 and the Faraday rotator 211.

[0081] Figure 9 A structural block diagram of another laser generating device provided in the embodiments of this application; Figure 10 This is a structural block diagram of another laser generating device provided in an embodiment of this application. Figure 9 and Figure 10 As shown, the first laser cavity 10 may further include a coupler 16. The coupler 16 includes a first input terminal 161, a first output terminal 162 and at least one second output terminal 163. The first input terminal 161 is connected to the signal input terminal 122, the first output terminal 162 is connected to the second port 102, and the second output terminal 163 is used to output a laser beam.

[0082] Among them, such as Figure 9 As shown, coupler 16 can be, for example, a 1×2 coupler, meaning that the number of second output terminals 163 in coupler 16 can be one, and the second output terminal 163 can be located between coupler 16 and the second port 102. With the above configuration, when the first beam A1 enters through the first port 101 and propagates clockwise within the first laser cavity 10, the first beam A1 is first transmitted through the connecting fiber 15 to the gain fiber 14 and amplified by the gain fiber 14; the amplified first beam A1 is then transmitted to the common output terminal 123 of the wavelength division multiplexer 12, where the wavelength division multiplexer 12 decomposes the amplified first beam A1 into different wavelength components. A portion of the first beam A1 is transmitted from the signal input terminal 122 of the wavelength division multiplexer 12 to the first input terminal 161 of coupler 16, and then output from the second output terminal 163 of coupler 16.

[0083] Alternatively, coupler 16 can also be a 2×2 coupler, meaning that there can be two second output terminals 163 in coupler 16, with one second output terminal 163 located between coupler 16 and the second port 102, and the other coupler 16 located between coupler 16 and wavelength division multiplexer 12. As described in the above embodiment, when the first beam A1 propagates clockwise within the first laser cavity 10, a portion of the first beam A1 can be output from the second output terminal 163 located between coupler 16 and the second port 102. When the second beam A2 enters through the second port 102 and propagates counterclockwise within the first laser cavity 10, the second beam A2 enters through the first output terminal 162 of coupler 16. A portion of the second beam A2 is transmitted to wavelength division multiplexer 12 through the first input terminal 161 of coupler 16, and the other portion of the second beam A2 is output from the other second output terminal 163 of coupler 16, which is located between coupler 16 and wavelength division multiplexer 12.

[0084] In summary, by setting the coupler 16, the output path of the laser beam can be further increased. In this embodiment, the number of second output terminals 163 is not limited and can be set according to actual conditions.

[0085] In some embodiments, the second laser cavity 20 may further include a second polarizing crystal 25 and a chirped fiber Bragg grating 26, with the second polarizing crystal 25 located in the optical path between the phase bias device 21 and the chirped fiber Bragg grating 26. The chirped fiber Bragg grating 26 (CFBG) is a fiber Bragg grating (FBG) whose refractive index or grating period varies along the axial direction, used for fiber dispersion compensation. With the above configuration, signal light A from the phase bias device 21, after entering the second polarizing crystal 25, is filtered into signal light A with only one polarization direction. Signal light A then enters the chirped fiber Bragg grating 26 for dispersion compensation, and is simultaneously reflected back into the second polarizing crystal 25 by the chirped fiber Bragg grating 26, and sequentially transmitted to the phase bias device 21, the connecting device 30, and the first laser cavity 10. The transmission process of signal light A within the phase bias device 21, the connecting device 30, and the first laser cavity 10 can be as described in the above embodiments and will not be repeated here.

[0086] In embodiments where the first laser cavity 10 further includes a coupler 16, the structure of the second laser cavity 20 can be as shown in any of the above embodiments, and its working principle is as described in the above embodiments, and will not be repeated here. For example, as Figure 10 As shown, the polarization-independent beam splitter 24 is located on the optical path of the phase biasing device 21 on the side opposite to the connecting device 30. For example, the second polarizing crystal 25 is located on the optical path between the polarization-independent beam splitter 24 and the chirped fiber Bragg grating 26. Or, as... Figure 9 As shown, the polarization-independent beam splitter 24 can be located in the optical path between the connecting device 30 and the Faraday rotator 211, and the second polarization crystal 25 is located in the optical path between the phase biasing device 21 and the chirped fiber Bragg grating 26.

[0087] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A laser generating device, characterized in that, include: A pump source, a first laser cavity, a connecting device, and a second laser cavity, wherein the connecting device is located between the first laser cavity and the second laser cavity; The pump source is used to emit pump light; The first laser cavity includes a pump end, a first port, and a second port. The pump end is connected to the pump source. The first laser cavity is used to absorb the pump light and generate signal light. The signal light includes a first beam and a second beam. The first laser cavity is also used to modulate the first beam and the second beam to generate a nonlinear phase difference. The polarization state of the second beam is perpendicular to the polarization state of the first beam. The connecting device includes a first optical fiber and a second optical fiber. The first optical fiber is connected to the first port and is used to receive the signal light and output the first beam. The second optical fiber is connected to the second port and is used to receive the signal light and output the second beam. The connecting device also includes a modulation device located on the optical path of the first optical fiber and the second optical fiber away from the first laser cavity. The modulation device is used to combine or separate the first beam and the second beam. The second laser cavity is used to modulate the first beam and the second beam to generate a fixed phase difference, forming and outputting a laser beam; the second laser cavity is also used to reflect the signal light from the first laser cavity back to the first optical fiber and the second optical fiber.

2. The laser generating device according to claim 1, characterized in that, Both the first optical fiber and the second optical fiber are polarization-maintaining fibers, and the slow axis of the first optical fiber is perpendicular to the slow axis of the second optical fiber.

3. The laser generating apparatus according to any one of claims 1-2, characterized in that, The modulation device includes a first polarizing crystal, and the connection device further includes a collimating lens, which is located on the optical path of the first polarizing crystal away from the first laser cavity; or, the collimating lens is located on the optical path between the first optical fiber and the second optical fiber and the first polarizing crystal.

4. The laser generating apparatus according to any one of claims 1-3, characterized in that, The first laser cavity includes a wavelength division multiplexer, a gain fiber, and a passive fiber; The wavelength division multiplexer includes a signal input terminal, a common output terminal, and a pump terminal. The common output terminal is connected to one end of the gain fiber, and the other end of the gain fiber is connected to the first port. The signal input terminal is connected to one end of the passive fiber, and the other end of the passive fiber is connected to the second port.

5. The laser generating device according to claim 4, characterized in that, The first laser cavity further includes a connecting optical fiber, which is connected between the gain optical fiber and the first port.

6. The laser generating apparatus according to claim 4 or 5, characterized in that, The first laser cavity further includes a coupler, which includes at least a first input terminal, a first output terminal, and at least one second output terminal. The first input terminal is connected to the signal input terminal, the first output terminal is connected to the second port, and the second output terminal is used to output the laser beam.

7. The laser generating apparatus according to any one of claims 1-6, characterized in that, The second laser cavity includes a phase biasing device, which includes a Faraday rotator and a waveplate, with the Faraday rotator located in the optical path between the connecting device and the waveplate.

8. The laser generating device according to claim 7, characterized in that, The second laser cavity includes a polarization-independent beam splitter located in the optical path between the connecting device and the Faraday rotator, or located in the optical path on the side of the phase biasing device away from the connecting device.

9. The laser generating device according to claim 8, characterized in that, The second laser cavity further includes a dispersion compensation device and a mirror. The dispersion compensation device includes at least one set of grating pairs, and the grating pairs include a first grating and a second grating disposed opposite to each other. When the polarization-independent beam splitter is located in the optical path between the connecting device and the Faraday rotator, the dispersion compensation device is located in the optical path between the phase biasing device and the mirror. When the polarization-independent beam splitter is located on the optical path on the side of the phase bias device away from the connecting device, the dispersion compensation device is located on the optical path between the polarization-independent beam splitter and the mirror.

10. The laser generating device according to claim 8, characterized in that, The second laser cavity further includes a second polarizing crystal and a chirped fiber Bragg grating; when the polarization-independent beam splitter is located in the optical path between the connecting device and the Faraday rotator, the second polarizing crystal is located in the optical path between the phase biasing device and the chirped fiber Bragg grating; when the polarization-independent beam splitter is located in the optical path on the side of the phase biasing device away from the connecting device, the second polarizing crystal is located in the optical path between the polarization-independent beam splitter and the chirped fiber Bragg grating.

11. The laser generating apparatus according to any one of claims 8-10, characterized in that, It also includes an output optical fiber, which is located on the optical path of the polarization-independent beam splitter on the side away from the first laser cavity.

12. The laser generating apparatus according to any one of claims 8-11, characterized in that, It also includes a photodetector located on the optical path of the polarization-independent beam splitter on the side away from the second laser cavity.

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