Laser and laser system

Abstract

The present disclosure relates to a laser and a laser system, the laser comprising: a first optical fiber circulator, an optical amplifier, an optical coupler, a second optical fiber circulator, and a Mach-Zehnder modulator connected in sequence, the first optical fiber circulator, a polarization rotation element, a first polarization element, a dispersive medium, a second polarization element, and the second optical fiber circulator connected in sequence, the Mach-Zehnder modulator further connected to the first optical fiber circulator and a pulse generation unit, and the dispersive medium used for dispersion control and reflection of light of different wavelengths input from both ends. The present disclosure blocks the light passing through the dispersive medium by the polarization element, avoids the light passing through the dispersive medium from being amplified by the optical amplifier again to form laser oscillation, affects the output of the swept light, and does not need to perform gain modulation of the optical amplifier and does not need to use two dispersive media, so that the overall structure is more simple and the cost is lower.

Description

Technical Field

[0001] This disclosure relates to the field of laser technology, and more particularly to a laser and a laser system. Background Technology

[0002] A swept laser is a light source that can output laser wavelengths that change rapidly over time. On average, a swept laser outputs broadband light, but at a certain point in time, it outputs monochromatic light. Therefore, the interference spectrum required for OCT imaging can be obtained by rapid acquisition using a single-point photodetector.

[0003] However, current sweep lasers are prone to laser oscillations, which can disrupt the original sweep output. Summary of the Invention

[0004] According to one aspect of this disclosure, a laser is provided, the laser comprising a first fiber circulator, an optical amplifier, an optical coupler, a second fiber circulator, a Mach-Zehnder modulator, a polarization rotation element, a first polarization element, a dispersive medium, a second polarization element, and a pulse generation unit, wherein,

[0005] The first fiber optic circulator, optical amplifier, optical coupler, second fiber optic circulator, and Mach-Zehnder modulator are connected in sequence. The first fiber optic circulator, polarization rotation element, first polarization element, dispersive medium, second polarization element, and second fiber optic circulator are also connected in sequence. The Mach-Zehnder modulator is further connected to the first fiber optic circulator and the pulse generation unit.

[0006] The dispersive medium is used to control the dispersion of light of different wavelengths input at both ends and reflect it, wherein some of the light input at one end of the dispersive medium is transmitted to the other end.

[0007] The first polarizing element and the second polarizing element have different polarization directions;

[0008] The polarization rotation element is used to change the polarization direction of light to align with the polarization direction of the first polarization element.

[0009] The pulse generation unit is used to emit a preset pulse signal so that the Mach-Zehnder modulator can adjust the intensity of light.

[0010] In one possible implementation, the dispersive medium comprises a chirped fiber Bragg grating.

[0011] In one possible implementation, the dispersion medium is made of polarization-maintaining fiber.

[0012] In one possible implementation, the polarization rotation element includes at least one of a polarization beam combiner / splitter, a waveplate, and a rotating polarization-maintaining fiber.

[0013] In one possible implementation, both the first polarization element and the second polarization element include fiber polarizers, and the optical amplifier includes a semiconductor optical amplifier.

[0014] In one possible implementation, the pulse generating unit includes:

[0015] A waveform generator is used to generate the preset pulse signal;

[0016] A radio frequency amplifier, connected to the waveform generator, is used to amplify the preset pulse signal.

[0017] In one possible implementation, the time it takes for light to circulate once in the laser is 1.5 to 2.5 times the stretching time, wherein the stretching time is the time it takes for light to travel from the first fiber circulator through the polarization rotation element, the first polarization element, the dispersive medium, and reflect back to the first fiber circulator.

[0018] In one possible implementation, the product of the laser's optical path length and the fiber's refractive index is equal to the product of the stretching time and the speed of light, such that the time it takes for light to circulate once in the laser is twice the stretching time.

[0019] Wherein, the optical path length is the sum of the length of the loop formed by the first fiber circulator, the optical amplifier, the optical coupler, the second fiber circulator, and the Mach-Zehnder modulator connected in sequence, and the length of the first fiber circulator, the polarization rotation element, the first polarization element, the dispersive medium, the second polarization element, and the second fiber circulator.

[0020] In one possible implementation, the first fiber optic circulator is used to receive light of a first wavelength, the second fiber optic circulator is used to receive light of a second wavelength, and the optical coupler is used to output laser light.

[0021] In one possible implementation, the laser includes a swept-frequency laser.

[0022] According to one aspect of this disclosure, a laser system is provided, the system comprising:

[0023] The laser mentioned above;

[0024] A first optical transmitter is used to emit light of a first wavelength into a first fiber optic circulator of the laser;

[0025] A second optical transmitter is used to emit light of a second wavelength into a second fiber optic circulator of the laser.

[0026] This disclosure proposes a laser comprising a first fiber circulator, an optical amplifier, an optical coupler, a second fiber circulator, a Mach-Zehnder modulator, a polarization rotation element, a first polarization element, a dispersion medium, a second polarization element, and a pulse generation unit. The first fiber circulator, optical amplifier, optical coupler, second fiber circulator, and Mach-Zehnder modulator are connected sequentially. The first fiber circulator, polarization rotation element, first polarization element, dispersion medium, second polarization element, and second fiber circulator are also connected sequentially. The Mach-Zehnder modulator is further connected to the first fiber circulator and the pulse generation unit. The polarization rotation element, first polarization element, and second polarization element ensure that light passing through the dispersion medium twice has different (e.g., mutually perpendicular) linear polarization states. The polarization element blocks the light passing through the dispersion medium, preventing it from being amplified again by the optical amplifier to form laser oscillations and affect the output of the swept light. Furthermore, this disclosure does not require optical amplifier gain modulation or two dispersion media, resulting in a simpler overall structure and lower cost.

[0027] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Other features and aspects of this disclosure will become clear from the following detailed description of exemplary embodiments with reference to the accompanying drawings. Attached Figure Description

[0028] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this disclosure and, together with the specification, serve to illustrate the technical solutions of this disclosure.

[0029] Figure 1a A schematic diagram of a conventional wavelength scanning fiber laser is shown. Figure 1b A schematic diagram of a Fourier mode-locked fiber laser is shown.

[0030] Figure 2a A schematic diagram of a stretch pulse mode-locked sweep laser is shown. Figure 2b A schematic diagram of the output waveform of a stretched pulse mode-locked swept laser is shown.

[0031] Figure 3a A schematic diagram of another stretch pulse mode-locked sweep laser is shown. Figure 3b A schematic diagram of the output waveform of a stretched pulse mode-locked swept laser is shown.

[0032] Figure 4 A schematic diagram of a laser according to an embodiment of the present disclosure is shown.

[0033] Figure 5 A schematic diagram of a laser according to an embodiment of the present disclosure is shown.

[0034] Figure 6 A schematic diagram of a laser system according to an embodiment of the present disclosure is shown. Detailed Implementation

[0035] Various exemplary embodiments, features, and aspects of this disclosure will now be described in detail with reference to the accompanying drawings. The same reference numerals in the drawings denote elements that have the same or similar functions. Although various aspects of the embodiments are shown in the drawings, they are not necessarily drawn to scale unless specifically indicated otherwise.

[0036] In the description of this disclosure, it should be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this disclosure and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this disclosure.

[0037] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this disclosure, "a plurality of" means two or more, unless otherwise expressly specified.

[0038] In this disclosure, unless otherwise expressly specified and limited, the terms "installation," "connection," "linking," "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this disclosure according to the specific circumstances.

[0039] The term “exemplary” as used herein means “serving as an example, embodiment, or illustration.” Any embodiment illustrated herein as “exemplary” is not necessarily to be construed as superior to or better than other embodiments.

[0040] In this document, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent three cases: A alone, A and B simultaneously, and B alone. Furthermore, the term "at least one" in this document means any combination of at least two of any one or more elements. For example, including at least one of A, B, and C can mean including any one or more elements selected from the set consisting of A, B, and C.

[0041] Furthermore, to better illustrate this disclosure, numerous specific details are set forth in the following detailed description. Those skilled in the art will understand that this disclosure can be practiced without certain specific details. In some instances, methods, means, components, and circuits well known to those skilled in the art have not been described in detail in order to highlight the main points of this disclosure.

[0042] Currently, there are various implementation schemes for swept-frequency lasers, such as traditional swept-frequency lasers, Fourier mode-locked fiber lasers, and stretched pulse mode-locked swept-frequency lasers.

[0043] Please see Figure 1a and Figure 1b , Figure 1a A schematic diagram of a conventional wavelength scanning fiber laser is shown. Figure 1b A schematic diagram of a Fourier mode-locked fiber laser is shown.

[0044] like Figure 1a As shown, traditional wavelength-scanning fiber lasers achieve wavelength-scanning laser output by rapidly adjusting a tunable filter within the cavity using a filter drive signal. The tuning frequency of the tunable filter is not matched to the cavity fundamental frequency (because the cavity fundamental frequency of a typical fiber laser is tens of MHz, and the scanning frequency of the FP filter cannot reach this level). When light of a certain wavelength reaches the filter again, it will be blocked due to the change in the wavelength transmitted through the filter. Therefore, only one wavelength is "activated" within the laser cavity (e.g., ...). Figure 1a During wavelength scanning, the laser oscillation for the scanning wavelength needs to be re-established by spontaneous emission (ASE). Therefore, the scanning speed of the laser wavelength is limited by the excitation time of the cavity gain medium. If the scanning speed exceeds a certain value, the output light intensity of the laser will decrease rapidly with the increase of scanning speed. The maximum scanning frequency that the laser can achieve is only 25kHz.

[0045] In 2006, R. Huber et al. proposed a novel sweeping laser oscillation mechanism—Fourier domain mode-locking. The structure of a Fourier domain mode-locked fiber laser is as follows: Figure 1b As shown, by adding several kilometers of optical fiber as a delay line inside the laser resonant cavity, the cavity fundamental frequency f is reduced. cavity The tuning frequency f of the tunable filter driveJust matches the laser cavity fundamental frequency f cavity Equal. Thus, when each wavelength returns to the filter after one cycle within the cavity, the filter is precisely tuned to that wavelength for passage. Therefore, each wavelength does not need to re-establish laser oscillation from spontaneous emission (ASE) in a new scan cycle; instead, it uses the wavelength from the previous cycle as a "seed" to form the laser output. All wavelengths are sequentially "stored" within the cavity (e.g., ...). Figure 1b In this method, different wavelengths pass through the filter in a specific order within one tuning cycle. Thus, the sweep speed is no longer limited by the laser oscillation settling time, as was the case with traditional short-cavity lasers, but only by the filter's tuning speed. Due to the limitations of the filter's tuning speed, several kilometers of fiber delay lines are typically added inside the cavity to meet the Fourier domain mode-locking conditions. Although this increases the laser cavity length, the scanning speed is significantly increased.

[0046] Please see Figure 2a and Figure 2b , Figure 2a A schematic diagram of a stretch pulse mode-locked sweep laser is shown. Figure 2b A schematic diagram of the output waveform of a stretched pulse mode-locked swept laser is shown.

[0047] The structure of a stretched pulse mode-locked swept laser is as follows: Figure 2aThe laser cavity shown, shaped like "θ", works as follows: An electro-optic intensity modulator (EOM) modulates the light in the optical fiber into a 200 ps pulse. This pulse is then spread according to wavelength by a chirped fiber Bragg grating (CFBG), meaning different wavelengths return at different positions, thus "stretching" and separating them (the pulse width is approximately 100 ns). Part of the light passes through a semiconductor optical amplifier (SOA) and is output, while the remaining light enters the CFBG from the opposite direction. Since its dispersion direction is reversed, it returns through the CFBG and is recompressed into a pulse. This cycle repeats, continuously stretching and compressing the laser within the cavity, while active mode-locking is achieved through the EOM. This scheme enables the output of a swept-frequency laser at a speed of approximately 10 MHz. After further amplification by the SOA at the end, the average output power reaches 95 mW. Since the CFBG cannot return all the light, some light will pass through the CFBG (as shown by the dashed arrow in the figure), pass through the circulator, and re-enter the SOA, thus forming a complete laser cavity. To avoid such oscillations, the SOA needs to be modulated. That is, when the light passing through the CFBG passes through the SOA, the SOA is closed, and when the light returning from the CFBG passes through the SOA, the SOA is opened to amplify the gain of this part of the light. The downside is that the duty cycle of the laser cavity output cannot exceed 50%, and more than half of the SOA gain is wasted. Therefore, this scheme requires the swept light to pass through a buffer after being output from the "θ" cavity. The swept light is split into two parts, and these two parts are merged after passing through different optical paths. This can double the duty cycle. At the same time, due to the Faraday mirror (FRM), circulator, and other devices, its output power will have a large loss. Therefore, before the final output, it needs to pass through another SOA to amplify the swept light.

[0048] Please see Figure 3a and Figure 3b , Figure 3a A schematic diagram of another stretch pulse mode-locked sweep laser is shown. Figure 3b A schematic diagram of the output waveform of a stretched pulse mode-locked swept laser is shown.

[0049] For example, Figure 2b , Figure 3b In this context, arb represents a relative unit, quantized to 0-1.

[0050] Another configuration of stretched pulse mode-locked swept laser is as follows: Figure 3aAs shown, two independent CFBGs are used, thus overcoming the problem of unnecessary oscillations caused by light passing through the CFBG. The duty cycle of the swept laser can be increased to over 90% without a buffer pool. However, due to limitations in the manufacturing process, the structures of the two CFBGs cannot be completely identical. Therefore, the light stretched by CFBG1 cannot be completely compressed by CFBG2 (dispersion mismatch), resulting in strong noise in the output of the swept light.

[0051] As can be seen from the above introduction, the main problem in stretched pulse mode-locked sweep lasers lies in how to handle the light penetrating the CFBG and the additional laser oscillations caused by this light, because these additional laser oscillations will disrupt the original sweep output. Existing solution 1 ( Figure 2a The SOA is modulated and its gain is rapidly switched. When light passing through the CFBG passes through the SOA, the SOA is turned off; when light returning from the CFBG passes through the SOA, the SOA is turned on, amplifying the gain of this portion of light. The downside is that the duty cycle of the laser cavity output cannot exceed 50%, and more than half of the SOA's gain is wasted. Therefore, this approach requires the swept light to pass through a buffer after exiting the "θ" cavity, splitting it into two parts. These two parts then undergo different optical paths before being combined, thus doubling the duty cycle. However, due to the passage of Faraday mirrors (FRMs), circulators, and other devices, there is significant power loss in the output. Therefore, before the final output, the swept light needs to pass through another SOA for amplification. This method is therefore more complex and increases costs.

[0052] Another option 2 ( Figure 3a This is a stretched pulsed mode-locked sweep laser employing dual CFBGs, which can achieve a duty cycle of over 90%. However, due to manufacturing limitations, the structures of the two CFBGs cannot be completely identical. Therefore, the light stretched by CFBG1 cannot be completely compressed by CFBG2 (dispersion mismatch), resulting in strong noise in the swept light output. Furthermore, the use of dual CFBGs significantly increases the cost of this laser.

[0053] Therefore, neither of these two approaches solved the problem of additional laser oscillations caused by the penetration of CFBG light. Moreover, solving this problem significantly increased the cost and complexity of the laser, and introduced noise, which further interfered with the laser.

[0054] In view of this, the present disclosure proposes a laser comprising a first fiber circulator, an optical amplifier, an optical coupler, a second fiber circulator, a Mach-Zehnder modulator, a polarization rotation element, a first polarization element, a dispersion medium, a second polarization element, and a pulse generation unit. The first fiber circulator, optical amplifier, optical coupler, second fiber circulator, and Mach-Zehnder modulator are connected in sequence. The first fiber circulator, polarization rotation element, first polarization element, dispersion medium, second polarization element, and second fiber circulator are also connected in sequence. The Mach-Zehnder modulator is also connected to the first fiber circulator and the pulse generation unit. The polarization rotation element, first polarization element, and second polarization element enable light passing through the dispersion medium twice to have different (e.g., mutually perpendicular) linear polarization states. The polarization element blocks the light passing through the dispersion medium, preventing the light passing through the dispersion medium from being amplified again by the optical amplifier to form laser oscillations and affect the output of the swept light. Furthermore, the present disclosure does not require optical amplifier gain modulation or the use of two dispersion media, thus the overall structure is simpler and the cost is lower.

[0055] Please see Figure 4 , Figure 4 A schematic diagram of a laser according to an embodiment of the present disclosure is shown.

[0056] like Figure 4 As shown, the laser includes a first fiber circulator 10, an optical amplifier 11, an optical coupler 12, a second fiber circulator 13, a Mach-Zehnder modulator 14, a polarization rotation element 15, a first polarization element 16, a dispersive medium 17, a second polarization element 18, and a pulse generation unit 20, wherein...

[0057] The first fiber optic circulator 10, optical amplifier 11, optical coupler 12, second fiber optic circulator 13, and Mach-Zehnder modulator 14 are connected in sequence. The first fiber optic circulator 10, polarization rotation element 15, first polarization element 16, dispersion medium 17, second polarization element 18, and second fiber optic circulator 13 are also connected in sequence. The Mach-Zehnder modulator 14 is also connected to the first fiber optic circulator 10 and the pulse generation unit 20.

[0058] The dispersive medium 17 is used to control the dispersion of light of different wavelengths input at both ends and reflect it, wherein some of the light input at one end of the dispersive medium 17 will be transmitted to the other end.

[0059] The first polarizing element 16 and the second polarizing element 18 have different polarization directions;

[0060] The polarization rotation element 15 is used to change the polarization direction of light to align with the polarization direction of the first polarization element 16.

[0061] The pulse generation unit 20 is used to emit a preset pulse signal so that the Mach-Zehnder modulator 14 adjusts the intensity of the light.

[0062] The specific implementation of the first fiber optic circulator 10, optical amplifier 11, optical coupler 12, second fiber optic circulator 13, Mach-Zehnder modulator 14, polarization rotation element 15, first polarization element 16, dispersive medium 17, second polarization element 18, and pulse generation unit 20 in this embodiment is not limited. Those skilled in the art can set it according to actual conditions and needs. The following is an exemplary description.

[0063] Please see Figure 5 , Figure 5 A schematic diagram of a laser according to an embodiment of the present disclosure is shown.

[0064] In one possible implementation, the dispersive medium 17 includes a chirped fiber Bragg grating or other types of media. For example, the chirped fiber Bragg grating can be used for dispersion control, thereby achieving pulse broadening and compression, and providing dispersion capability. Light entering from both ends of the chirped fiber Bragg grating will have opposite dispersion directions. For example, the chirped fiber Bragg grating can be a grating with a spatially periodically distributed phase formed within the fiber core. Its function is essentially to form a narrow-band (transmission or reflection) filter or mirror within the fiber core. This disclosure does not limit the specific implementation of the chirped fiber Bragg grating; those skilled in the art can implement it using related technologies.

[0065] The embodiments disclosed herein do not limit the specific parameters of the chirped fiber Bragg grating. Those skilled in the art can set the reflection center wavelength, bandwidth, reflectivity, and total dispersion provided by the chirped fiber Bragg grating to both ends according to actual conditions and needs.

[0066] In one example, the function of a CFBG is to allow different wavelengths to return at different locations, but some light will "leak through" or "pass through" the CFBG. For example, such as... Figure 5 As shown, the pulse stretching stage occurs when light enters the CFBG from the left CIR (first fiber circulator 10) and is reflected back to the left CIR, during which the pulse width of the reflected light is stretched; the pulse compression stage occurs when light enters the CFBG from the right CIR (second fiber circulator 13) and is reflected back to the right CIR.

[0067] In one example, such as Figure 5As shown, during the pulse compression phase, light propagates from right to left through the dispersive medium 17 (such as a chirped fiber Bragg grating, CFBG) (i.e., light from the second fiber circulator 13 propagates through the dispersive medium 17 to the first fiber circulator 10), and is polarized along the polarization direction of the second polarization element 18. Therefore, the light passing through the CFBG maintains its polarization direction (the effect is even better if the CFBG is made of polarization-maintaining fiber, PMF). The transmitted light is blocked by the first polarization element 16 (for example, a fast-axis fiber polarizer with a high extinction ratio is even better).

[0068] In one example, such as Figure 5 As shown, during the pulse stretching phase, if the light pulse propagates from right to left through the dispersive medium 17 (such as a CFBG) (i.e., light from the second fiber circulator 13 propagates through the dispersive medium 17 to the first fiber circulator 10), this embodiment of the present disclosure uses at least one of the following as a polarization rotation element 15 (other polarization rotation elements 15 can also be used) placed between the first circulator and the first polarization element 16 to change the polarization of the light to align with the polarization direction of the first polarization element 16. Thus, most of the light penetrating the dispersive medium 17 (such as a CFBG) retains its fast-axis polarization state (the polarization direction of the first polarization element 16) and is therefore blocked by the slow-axis polarizer (second polarization element 18) placed on the right. Therefore, this embodiment of the present disclosure solves the problem of additional laser oscillation caused by light penetrating the dispersive medium 17 (such as a CFBG).

[0069] In one possible implementation, the dispersive medium 17 can be made of polarization-maintaining optical fiber (PMF). Of course, other devices requiring optical fiber can also be manufactured using PMF. Thus, embodiments of this disclosure can reduce optical losses caused by optical depolarization. PMF overcomes the influence of environmental factors on the polarization state in the fiber during transmission by increasing the fiber's inherent birefringence properties, maintaining the polarization state of the transmitted light wave unchanged. This improves the stability and communication capacity of the optical transmission system and significantly reduces errors caused by polarization coupling in the optical path. For example... Figure 5 A solid double-headed arrow indicates the polarization direction, while a dashed arrow indicates the direction of light propagation.

[0070] For example, the first fiber optic circulator 10 and the second fiber optic circulator 13 can enable signal transmission along a specific port and can achieve signal isolation between each port. For example, the fiber optic circulator can be a multi-port non-reciprocal optical device in which light can propagate in one direction.

[0071] In one possible implementation, both the first polarization element 16 and the second polarization element 18 include an optical fiber polarizer, which enables light passing through the device to have only one linear polarization direction.

[0072] In one possible implementation, the optical amplifier 11 includes a semiconductor optical amplifier (SOA). This disclosure does not limit the specific implementation of the semiconductor optical amplifier 11. Those skilled in the art can implement it using relevant technologies according to actual conditions and needs.

[0073] In one example, the Mach-Zehnder modulator 14 (IM) can rapidly modulate the intensity (transmittance) of light, enabling active mode-locking of the laser. This disclosure does not limit the specific implementation of the Mach-Zehnder modulator 14, and those skilled in the art can implement it using relevant technologies according to actual conditions and needs.

[0074] The specific parameters of the Mach-Zehnder modulator 14 are not limited in the embodiments disclosed herein. Those skilled in the art can set the operating wavelength, modulation bandwidth and other parameters of the Mach-Zehnder modulator 14 according to the actual situation and needs.

[0075] In one possible implementation, such as Figure 5 As shown, the pulse generating unit 20 may include:

[0076] A waveform generator 210 is used to generate the preset pulse signal. The waveform generator 210 may integrate a function generator or receive a waveform function output by an external function generator 200, thereby generating the preset pulse signal according to the waveform function.

[0077] The radio frequency amplifier RF Amp is connected to the waveform generator 210 and is used to amplify the preset pulse signal.

[0078] This disclosure does not limit the type of waveform generator 210. For example, the waveform generator 210 can be any waveform generator 210, or a tunable picosecond pulse generator, or other types of waveform generator 210.

[0079] The embodiments disclosed herein do not limit the implementation method or amplification parameters of the radio frequency amplifier (RF Amp). Those skilled in the art can set them according to actual conditions and needs.

[0080] In this embodiment, the preset pulse signal is generated by the pulse generation unit 20, and the preset pulse signal is amplified and input to the Mach-Zehnder modulator 14 to achieve intensity modulation of the optical pulse signal in the laser.

[0081] In one possible implementation, embodiments of this disclosure may set the time for light to circulate once in the laser to be 1.5 to 2.5 times the stretching time, in order to improve the effective duty cycle of the laser output, wherein the stretching time is the time for light to travel from the first fiber circulator 10 through the polarization rotation element 15, the first polarization element 16, the dispersion medium 17 and be reflected back to the first fiber circulator 10.

[0082] Of course, the above description of the ratio of the time for light to circulate once in the laser to the stretching time is exemplary and should not be regarded as a limitation on the embodiments of this disclosure. In other embodiments, the time for light to circulate once in the laser and the stretching time may also be other multiples (such as greater than 2.5 times). Those skilled in the art can set this according to actual conditions and needs.

[0083] In one possible implementation, the product of the laser's optical path length and the fiber's refractive index is equal to the product of the stretching time and the speed of light, such that the time it takes for light to circulate once in the laser is twice the stretching time.

[0084] Wherein, the optical path length (cavity length of the laser) is the sum of the length of the loop formed by the sequential connection of the first fiber circulator 10, the optical amplifier 11, the optical coupler 12, the second fiber circulator 13, and the Mach-Zehnder modulator 14, and the length of the first fiber circulator 10, the polarization rotation element 15, the first polarization element 16, the dispersive medium 17, the second polarization element 18, and the second fiber circulator 13.

[0085] In this embodiment of the disclosure, by setting the product of the optical path length of the laser and the refractive index of the optical fiber to be equal to the product of the stretching time and the speed of light, the time for light to circulate once in the laser is twice the stretching time, which enables the laser to obtain a laser sweep frequency output with an effective duty cycle close to 100%.

[0086] In one possible implementation, the laser can be a frequency-swept laser.

[0087] In one possible implementation, such as Figure 5 As shown, the first fiber optic circulator 10 is used to receive light of a first wavelength, the second fiber optic circulator 13 is used to receive light of a second wavelength, and the optical coupler 12 is used to output laser light.

[0088] In one example, such as Figure 5 As shown, the first fiber circulator 10 receives light of the first wavelength, which, after passing through the polarization rotation element 15 and the first polarization element 16, reaches the dispersive medium 17 (such as a chirped fiber Bragg grating CFBG). The chirped fiber Bragg grating CFBG reflects most of the incident light, and the reflected light pulse is subjected to sufficient dispersion response, which stretches the pulse width of the light pulse. Since the polarization direction of the second polarization element 18 is different from that of the first polarization element 16 (such as perpendicular), although some light passes through the chirped fiber Bragg grating CFBG, the chirped fiber Bragg grating CFBG will be blocked by the second polarization element 18, thereby preventing laser oscillation.

[0089] Similarly, in one example, such as Figure 5 As shown, the second fiber circulator 13 receives light of the second wavelength, which, after passing through the second polarization element 18, reaches the dispersive medium 17 (such as a chirped fiber Bragg grating CFBG). The chirped fiber Bragg grating CFBG reflects most of the incident light, and the reflected light pulse is subjected to sufficient dispersion response, which compresses the pulse width of the light pulse. Since the polarization directions of the second polarization element 18 and the first polarization element 16 are different (such as perpendicular), although some light passes through the chirped fiber Bragg grating CFBG, the chirped fiber Bragg grating CFBG will be blocked by the first polarization element 16, thereby preventing laser oscillation.

[0090] For example, the first wavelength and the second wavelength can be different, so that two different wavelengths of light pulses can exist in the laser of this embodiment.

[0091] In one example, such as Figure 5As shown in the diagram, the propagation sequence of the light pulse in the laser can be as shown in the diagram ①->②->③->④->⑤->⑥->⑦->⑧. The preset pulse signal is generated by the pulse generation unit 20, and after being amplified, it is input to the Mach-Zehnder modulator 14. The Mach-Zehnder modulator 14 outputs a modulation wave to modulate the intensity of the light output from the second fiber circulator 13 (①). The light received by the first fiber circulator 10 or the light output from the second fiber circulator 13 is modulated and then passes through the polarization rotation element 15 (②) and the first polarization element 16 to reach the dispersion medium 17 (③). The chirped fiber Bragg grating (CFBG) reflects most of the incident light, and the reflected light pulse is subjected to sufficient dispersion, which stretches the pulse width. Since the polarization directions of the second polarization element 18 and the first polarization element 16 are different (e.g., perpendicular), although some light passes through the chirped fiber Bragg grating (CFBG), this chirped fiber Bragg grating (CFBG) will be affected by the second polarization element 18. To prevent laser oscillation, the light reflected from the chirped fiber Bragg grating (CFBG) output by the first fiber circulator 10 is sent to the optical amplifier 11 (④). The optical coupler 12 outputs laser light (e.g., 80% output). The second fiber circulator 13 receives the second wavelength light and / or a portion of the light output by the optical coupler 12 (⑤). After passing through the second polarization element 18 (⑥), the light reaches the dispersive medium 17 (e.g., a chirped fiber Bragg grating (CFBG)). The chirped fiber Bragg grating (CFBG) reflects most of the incident light (⑦). Furthermore, the reflected light pulse is subjected to sufficient dispersion, which compresses the pulse width. Since the polarization directions of the second polarization element 18 and the first polarization element 16 are different (e.g., perpendicular), although some light passes through the chirped fiber Bragg grating (CFBG), the CFBG is blocked by the first polarization element 16, thus preventing laser oscillation. The second fiber circulator 13 outputs the light reflected from the CFBG to the Mach-Zehnder modulator 14 (⑧). Of course, the above propagation sequence of the light pulse is exemplary and should not be considered as a limitation of this disclosure.

[0092] As can be seen, the embodiments of this disclosure do not require modulation of the gain medium (such as SOA) or the use of two independent dispersion media 17 (such as CFBG). In the laser, polarization elements and polarization rotation elements 15 are used so that the light passing through the dispersion medium 17 (such as chirped fiber Bragg grating) twice has different (such as mutually perpendicular) linear polarization states. The light passing through the dispersion medium 17 is blocked by polarization elements (such as fiber polarizers) to prevent the light passing through the dispersion medium 17 from being amplified again by the optical amplifier 11 to form laser oscillation and affect the output of the swept light. Compared with related technologies, the construction complexity and cost are reduced, and no noise is generated, which improves the accuracy of the laser generated by the laser. In addition, the embodiments of this disclosure reasonably adjust the cavity length of the laser and set the time for the light to circulate once in the laser to twice the duration of the stretching pulse, thereby obtaining a laser swept output with an effective duty cycle of almost 100%.

[0093] Furthermore, the optical amplifier 11 of this embodiment can achieve a continuous gain and double the duty cycle without the need for a buffer pool. Therefore, the output power of the laser does not need to be amplified by SOA again to reach more than 80mW, which is sufficient to meet the needs of OCT imaging. Compared with the dual CFBG scheme in related technologies, the scheme of this embodiment has better dispersion matching, so the noise of the output swept light is smaller.

[0094] Please see Figure 6 , Figure 6 A schematic diagram of a laser system according to an embodiment of the present disclosure is shown.

[0095] like Figure 6 As shown, the system includes:

[0096] The laser mentioned above;

[0097] A first optical transmitter 30 is used to emit light of a first wavelength to a first fiber optic circulator 10 of the laser;

[0098] The second optical transmitter 40 is used to emit light of a second wavelength to the second fiber optic circulator 13 of the laser.

[0099] This embodiment of the invention utilizes a first optical transmitter 30 to emit light of a first wavelength to the first fiber optic circulator 10 of the laser, and a second optical transmitter 40 to emit light of a second wavelength to the second fiber optic circulator 13 of the laser, resulting in two wavelengths of light pulses in the laser, thereby improving the laser's efficiency. Furthermore, since the laser employs polarization elements and polarization rotation elements 15, the light passing through the dispersion medium 17 (such as a chirped fiber Bragg grating) twice has different (e.g., mutually perpendicular) linear polarization states. The polarization elements (such as fiber polarizers) block the light passing through the dispersion medium 17, preventing the light passing through the dispersion medium 17 from being amplified again by the optical amplifier 11, thus avoiding laser oscillation and affecting the output of the swept light. Compared with related technologies, this reduces the construction complexity and cost, does not generate noise, and improves the accuracy of the laser generated by the laser.

[0100] Of course, please refer to the previous description for a detailed introduction to the laser, which will not be repeated here.

[0101] The various embodiments of this disclosure have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A laser, characterized in that, The laser includes a first fiber circulator, an optical amplifier, an optical coupler, a second fiber circulator, a Mach-Zehnder modulator, a polarization rotation element, a first polarization element, a dispersive medium, a second polarization element, and a pulse generation unit, wherein... The first fiber optic circulator, optical amplifier, optical coupler, second fiber optic circulator, and Mach-Zehnder modulator are connected in sequence. The first fiber optic circulator, polarization rotation element, first polarization element, dispersive medium, second polarization element, and second fiber optic circulator are also connected in sequence. The Mach-Zehnder modulator is further connected to the first fiber optic circulator and the pulse generation unit. The dispersive medium is used to control the dispersion of light of different wavelengths input at both ends and reflect it, wherein some of the light input at one end of the dispersive medium is transmitted to the other end. The first polarizing element and the second polarizing element have different polarization directions to block the light passing through the dispersive medium and prevent the light passing through the dispersive medium from being amplified again by the optical amplifier. The polarization rotation element is used to change the polarization direction of light to align with the polarization direction of the first polarization element. The pulse generation unit is used to emit a preset pulse signal so that the Mach-Zehnder modulator can adjust the intensity of light.

2. The laser according to claim 1, characterized in that, The dispersive medium includes a chirped fiber Bragg grating.

3. The laser according to claim 1 or 2, characterized in that, The dispersive medium is made of polarization-maintaining fiber.

4. The laser according to claim 1, characterized in that, The polarization rotation element includes at least one of a polarization beam combiner / splitter, a waveplate, and a rotating polarization-maintaining fiber.

5. The laser according to claim 1, characterized in that, Both the first polarization element and the second polarization element include fiber polarizers, and the optical amplifier includes a semiconductor optical amplifier.

6. The laser according to claim 1, characterized in that, The pulse generating unit includes: A waveform generator is used to generate the preset pulse signal; A radio frequency amplifier, connected to the waveform generator, is used to amplify the preset pulse signal.

7. The laser according to claim 1, characterized in that, The time it takes for light to circulate once in the laser is 1.5 to 2.5 times the stretching time, wherein the stretching time is the time it takes for light to travel from the first fiber optic circulator through the polarization rotation element, the first polarization element, the dispersive medium, and back to the first fiber optic circulator.

8. The laser according to claim 7, characterized in that, The product of the laser's optical path length and the fiber's refractive index is equal to the product of the stretching time and the speed of light, such that the time it takes for light to circulate once in the laser is twice the stretching time. Wherein, the optical path length is the sum of the length of the loop formed by the first fiber circulator, the optical amplifier, the optical coupler, the second fiber circulator, and the Mach-Zehnder modulator connected in sequence, and the length of the first fiber circulator, the polarization rotation element, the first polarization element, the dispersive medium, the second polarization element, and the second fiber circulator.

9. The laser according to claim 1, characterized in that, The first fiber optic circulator is used to receive light of a first wavelength, the second fiber optic circulator is used to receive light of a second wavelength, and the optical coupler is used to output laser light.

10. The laser according to claim 1, characterized in that, The laser includes a frequency-swept laser.

11. A laser system, characterized in that, The system includes: The laser as described in any one of claims 1 to 10; A first optical transmitter is used to emit light of a first wavelength into a first fiber optic circulator of the laser; A second optical transmitter is used to emit light of a second wavelength into a second fiber optic circulator of the laser.

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