Random fiber laser based on fiber Bragg grating

Abstract

The invention provides a random fiber laser based on a fiber bragg grating. The random fiber laser comprises a light input end, a gain fiber and a random feedback mechanism, the light input end receives pump light from a pump source; the gain optical fiber is configured to receive the pump light and output a first optical signal in a first wavelength range; the random feedback mechanism comprises a feedback optical fiber and a fiber bragg grating string formed on the feedback optical fiber, and the fiber bragg grating string receives the first optical signal and provides random feedback for the first optical signal to output a second optical signal in a second wavelength range smaller than the first wavelength range; the fiber bragg grating string comprises a plurality of grating arrays, a plurality of grating periods of the plurality of grating arrays are gradually increased in sequence in the direction of incidence of the first optical signal to the fiber bragg grating string, and the distance between adjacent fiber bragg gratings in the plurality of fiber bragg gratings is random. Therefore, the length of the optical fiber can be shortened, and wide tuning can be realized.

Description

Technical Field

[0001] The present disclosure relates to a fiber laser, and in particular to a random fiber laser based on a fiber Bragg grating. Background Art

[0002] Random fiber lasers utilize the random distributed feedback mechanism of optical fiber, combined with the nonlinear gain or doping active gain of optical fiber, to make photons undergo multiple scattering and amplification in the optical fiber, thereby generating random lasers.

[0003] Early random fiber lasers mainly rely on the randomness of Rayleigh scattering feedback in the optical fiber. Since Rayleigh scattering feedback has wavelength-independent scattering characteristics, this non-selective scattering mechanism can provide feedback at any wavelength, enabling random fiber lasers to achieve a wide tunable range.

[0004] However, due to the weak Rayleigh scattering coefficient in optical fiber, in order to achieve laser output with the required light intensity, very long optical fiber is required, which usually requires a length of several kilometers to tens of kilometers. The use of long optical fiber significantly increases the volume and complexity of random fiber lasers, which poses a challenge to practical applications and integration. Summary of the invention

[0005] The present disclosure is proposed in view of the above situation, and its object is to provide a random fiber laser based on a fiber Bragg grating that can achieve wide tunability while shortening the fiber length.

[0006] To this end, a first aspect of the present disclosure provides a random fiber laser based on a fiber Bragg grating, comprising an optical input end, a gain fiber and a random feedback mechanism; the optical input end is configured to receive pump light from a pump source; the gain fiber is configured to receive the pump light and output a first optical signal in a first wavelength range, wherein the gain fiber has a gain medium excited by the pump light; the random feedback mechanism comprises a feedback fiber and a fiber Bragg grating string formed on the feedback fiber, the fiber Bragg grating string is configured to receive the first optical signal and provide random feedback for the first optical signal to output a second optical signal in a second wavelength range, wherein the second wavelength range is smaller than the first wavelength range, the fiber Bragg grating string comprises a plurality of grating arrays corresponding to different grating periods, in the direction along which the first optical signal is incident to the fiber Bragg grating string, the plurality of grating periods of the plurality of grating arrays are sequentially increased, each of the plurality of grating arrays comprises a plurality of fiber Bragg gratings, and the distance between adjacent fiber Bragg gratings in the plurality of fiber Bragg gratings is random.

[0007] In the present disclosure, multiple grating periods of multiple grating arrays are increased in sequence, so as to obtain continuous wavelengths to achieve continuous wide tunability. In addition, the distances between adjacent fiber Bragg gratings in multiple fiber Bragg gratings are random, which can provide random feedback. In addition, the first optical signal can be continuously reflected by multiple fiber Bragg gratings. The more reflections and the longer the time, the higher the light intensity of the formed second optical signal, so that the Q value of the second optical signal can be improved in a short optical fiber. Thus, wide tunability can be achieved while shortening the length of the optical fiber, and the Q value can be improved.

[0008] In addition, in the random fiber laser involved in the present disclosure, optionally, the optical input end is a wavelength division multiplexer, and the optical input end includes two input ports and one output port, and the two input ports receive the pump light and the second optical signal respectively and transmit them to the gain optical fiber through the output port. Thus, the second optical signal can be amplified while reducing the volume of the laser.

[0009] In addition, in the random fiber laser involved in the present disclosure, optionally, the gain fiber is an erbium-doped fiber, and the first wavelength range is 1530nm to 1570nm. In this case, the first wavelength range can be in the C band, thereby facilitating the generation of a target laser in the C band.

[0010] In addition, in the random fiber laser involved in the present disclosure, optionally, the second wavelength range is 1540nm to 1560nm. In this case, a wavelength range with a wavelength span as large as possible can be obtained when the feedback fiber length is short, which is convenient for achieving wide tunability while covering the C band as much as possible, and a flatter reflection spectrum can be obtained.

[0011] In addition, in the random fiber laser involved in the present disclosure, optionally, the second wavelength range is not greater than 20 nm. Thus, limiting the wavelength span to the above range facilitates tuning within a range of tens of nanometers and can obtain a relatively flat reflection spectrum.

[0012] In addition, in the random fiber laser involved in the present disclosure, optionally, the gain fiber is an erbium-doped fiber, the length of the gain fiber is in the order of meters, and the length of the fiber Bragg grating string is in the order of meters.

[0013] In addition, in the random fiber laser involved in the present disclosure, optionally, when the gain of the first optical signal is greater than the loss, the random feedback mechanism is further configured to receive the first optical signal and transmit it to output a third optical signal.

[0014] In addition, in the random fiber laser involved in the present disclosure, optionally, a circulator is further included, the circulator includes a first port, a second port and a third port, the first port receives the first optical signal and emits it to the random feedback mechanism through the second port, and the second optical signal enters the optical input end through the third port to amplify the second optical signal through the gain optical fiber. In this case, a ring-shaped laser cavity can be built to facilitate repeated amplification of the second optical signal, which can improve the efficiency of the laser.

[0015] In addition, in the random fiber laser involved in the present disclosure, optionally, it also includes a first isolator, which is arranged between the optical input end and the circulator, so that the circulator can only pass unidirectional optical signals.

[0016] In addition, in the random fiber laser involved in the present disclosure, optionally, a filter is further included, the filter is configured to limit the available output wavelength range, the second optical signal is input to the optical input end, and the filter is arranged between the optical input end and the random feedback mechanism. Thus, the second optical signal of a specific wavelength range can be selected to be input to the optical input end.

[0017] According to the present disclosure, a random fiber laser based on a fiber Bragg grating is provided, which is capable of achieving wide tunability while shortening the fiber length. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present disclosure will now be explained in further detail, by way of example only, with reference to the accompanying drawings.

[0019] Figure 1A 1 is a schematic diagram showing a first embodiment of a laser according to an example of the present disclosure.

[0020] Figure 1B Schematic diagram showing a second embodiment of a laser according to the present disclosure example.

[0021] Figure 2 is a schematic diagram showing a random feedback mechanism involved in examples of the present disclosure.

[0022] Figure 3 is a schematic diagram showing a reflection spectrum of a random feedback mechanism involved in an example of the present disclosure.

[0023] Figure 4 Schematic diagram showing a grating array involved in the examples of the present disclosure.

[0024] Figure 5A Schematic diagram showing a third embodiment of the laser according to the present disclosure example.

[0025] Figure 5BSchematic diagram showing a fourth embodiment of the laser according to the present disclosure example.

[0026] Figure 6 is a schematic diagram showing the spectrum of target laser light involved in the example of the present disclosure. DETAILED DESCRIPTION

[0027] Hereinafter, with reference to the accompanying drawings, preferred embodiments of the present disclosure are described in detail. In the following description, the same symbols are given to the same components, and repeated descriptions are omitted. In addition, the accompanying drawings are only schematic diagrams, and the ratio of the dimensions of the components to each other or the shapes of the components, etc. may be different from the actual ones. It should be noted that the terms "including" and "having" in the present disclosure and any variations thereof, such as a process, method, system, product or device including or having a series of steps or units, are not necessarily limited to those steps or units clearly listed, but may include or have other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0028] First, relevant terms involved in the present disclosure are introduced.

[0029] "Meter level" can mean that the length of an object is between 1 meter and 10 meters.

[0030] "Random feedback" can refer to an optical feedback mechanism inside the laser due to the random distribution of scattering centers.

[0031] "Flatness" may refer to the consistency of light intensity at different wavelengths in a spectrum (eg, a gain spectrum or a reflectance spectrum). The higher the flatness of a spectrum, the more consistent the light intensity at different wavelengths in the spectrum.

[0032] The "gain spectrum" may be the optical spectrum output by a gain fiber.

[0033] "Reflection spectrum" may refer to the spectrum formed by reflection from a random feedback mechanism.

[0034] As mentioned above, the existing scheme will increase the volume and complexity of the random fiber laser. In addition, the above-mentioned long fiber not only increases the cost and physical size of the random fiber laser, but also greatly increases the mode and causes nonlinear effects in the fiber, increasing the noise and instability of the laser. The inventors have found through research that random period or random distance refractive index modulation can be introduced into the optical fiber to form a random fiber grating or a random fiber Bragg grating, thereby providing random feedback with higher light intensity over a shorter fiber length.

[0035] The inventors further discovered that the preparation length and refractive index modulation depth of random fiber gratings are limited, and the reflection spectrum of random fiber gratings has a low Q value (quality factor) characteristic. In addition, the light intensity between the wavelengths of the reflection spectrum of random fiber gratings has a large difference (i.e., the consistency of light intensity is low), so that random lasers are only generated at some specific wavelengths, and continuous tunability of the laser cannot be achieved.

[0036] Therefore, the inventors provide some solutions, which provide random feedback through the fiber Bragg grating string involved in the examples of the present disclosure, so as to solve at least part of the above problems.

[0037] The random fiber laser (hereinafter referred to as laser) based on fiber Bragg grating involved in the examples of the present disclosure can achieve wide tunability while shortening the fiber length. In addition, the laser involved in the examples of the present disclosure can also be called a random laser or a random feedback laser. In addition, for the sake of distinction, the laser output by the laser is referred to as a target laser.

[0038] Examples of the present disclosure will be described in detail below with reference to the accompanying drawings. Figure 1A 1 is a schematic diagram showing a first embodiment of a laser 1 according to an example of the present disclosure. Figure 1B 1 is a schematic diagram showing a second embodiment of the laser 1 according to the example of the present disclosure. It should be noted that, for ease of understanding, the schematic diagram of the laser 1 according to the example of the present disclosure schematically shows the relevant components in a simplified manner.

[0039] In some examples, reference Figure 1A , the laser 1 may include an optical input end 11. The optical input end 11 may be configured to receive an optical signal. In addition, the optical signal received by the optical input end 11 may be transmitted to a component in the laser 1 (e.g., a gain fiber 12 or a random feedback mechanism 13 described later).

[0040] In some examples, reference Figure 1A , the optical input end 11 can be configured to receive an optical signal from a pump source 2, and the received optical signal can be a pump light. Thus, it is convenient to excite the gain medium in the laser 1. In some examples, the wavelength of the pump source 2 can be 980nm (nanometer). In some examples, the laser 1 can also include a pump source 2.

[0041] In some examples, reference Figure 1B The optical input end 11 can be configured to receive an optical signal (ie, a second optical signal described later) from a random feedback mechanism 13 (described later). Thus, it is convenient to allow the optical signal of the random feedback mechanism 13 to enter the gain optical fiber 12.

[0042] In some examples, the optical input end 11 may be a wavelength division multiplexer. The wavelength division multiplexer may allow multiple optical signals of different wavelengths to be transmitted through the same optical fiber. Thus, the volume of the laser 1 may be reduced. In some examples, for the optical input end 11 of the wavelength division multiplexer, refer to Figure 1B The optical input end 11 may include two input ports (ie, a first input port P11 and a second input port P12 ) and one output port P13 .

[0043] In addition, the two input ports of the optical input end 11 can respectively receive the pump light and the second optical signal and output them through the output port P13. Thus, the second optical signal can be amplified while reducing the volume of the laser 1. For example, the two input ports of the optical input end 11 can respectively receive the pump light with a wavelength of 980nm and the second optical signal with a wavelength of 1540nm. In some examples, the optical signals received by the two input ports can be transmitted to the gain fiber 12 through the output port P13.

[0044] In some examples, reference Figure 1A and Figure 1B , the laser 1 may include a gain fiber 12. In some examples, the gain fiber 12 may be configured to receive the pump light and output a first optical signal in a first wavelength range. In addition, the first wavelength range may be associated with the gain fiber 12. That is, different gain fibers 12 may correspond to different first wavelength ranges, and when the gain fiber 12 is determined, the first wavelength range may also be determined.

[0045] In some examples, the gain fiber 12 may also be configured to amplify the gain of the incident optical signal in the first wavelength range, and accordingly, the first optical signal may include the amplified incident optical signal. That is, the gain fiber 12 may provide gain. In other words, within the first wavelength range, the gain fiber 12 may play an amplifying role. In some examples, the incident optical signal may be the second optical signal received by the optical input end 11.

[0046] In some examples, the gain fiber 12 may have a gain medium. The gain medium may be excited by pump light. Under the excitation of the pump light, the gain medium may achieve stimulated radiation and amplify the gain of the incident light signal. Specifically, after the gain medium in the gain fiber 12 absorbs the energy of the pump light, the ions in the gain medium transition from the ground state to the high energy level. Since the high energy level is unstable, the ions quickly transition to the metastable energy level, i.e., the upper laser energy level, in a non-radiative manner. The ions have a long lifetime in the upper energy level, so they can gradually accumulate as the pump light continues to achieve a population inversion, thereby achieving stimulated radiation and amplifying the gain of the incident light signal.

[0047] In some examples, the gain fiber 12 may be an erbium-doped fiber. In this case, the first wavelength range can be in the C band, thereby facilitating the generation of a target laser in the C band. In some examples, for the gain fiber 12 being an erbium-doped fiber, the first wavelength range may be 1530 nm (nanometers) to 1570 nm. In other examples, the gain fiber 12 may also be replaced with other optical fibers except the erbium-doped fiber according to the wavelength band of the target laser.

[0048] In addition, the flatness of the gain spectrum generated by gain fibers 12 of different lengths may be different. That is, the flatness of the first optical signal generated by gain fibers 12 of different lengths may be different. In some examples, the length of the gain fiber 12 may be determined by the flatness of the target laser. Thus, it is convenient to improve the flatness of the target laser. In some examples, the length of the gain fiber 12 may be determined based on the flatness of the target laser and the flatness of the reflection spectrum of the random feedback mechanism 13. Specifically, the gain fiber 12 and the random feedback mechanism 13 are two major factors that affect the flatness of the target laser. The random feedback mechanism 13 cannot maintain the same flatness as the theoretical simulation due to the influence of parameters such as the writing power, so it is more effective to improve the flatness of the target laser from the perspective of controlling the length of the gain fiber 12.

[0049] In some examples, for erbium-doped fiber, the length of the gain fiber 12 can be in the order of meters. In this case, it is convenient to improve the flatness of the first optical signal so as to improve the flatness of the target laser, thereby reducing the jump of the target laser. In some examples, for erbium-doped fiber, the length of the gain fiber 12 can be 1 meter. In this case, the flatness of the first optical signal can be improved.

[0050] In some examples, reference Figure 1A and Figure 1B , the gain fiber 12 can be connected to the optical input end 11. Thus, the optical signal output by the optical input end 11 can be received.

[0051] In some examples, reference Figure 1A and Figure 1B , the laser 1 may include a random feedback mechanism 13. The random feedback mechanism 13 may be configured to receive an optical signal and provide random feedback for the optical signal to output a feedback optical signal. That is, in the laser 1, the random feedback mechanism 13 may provide random feedback.

[0052] In some examples, the optical signal received by the random feedback mechanism 13 may be a first optical signal, and the corresponding feedback optical signal may be referred to as a second optical signal. The random feedback mechanism 13 is described below by taking the optical signal received by the random feedback mechanism 13 as the first optical signal as an example, which does not limit the present disclosure. In some examples, when the gain of the first optical signal is greater than the loss, the random feedback mechanism 13 may also be configured to receive the first optical signal and transmit it to output a third optical signal (i.e., the target laser).

[0053] In some examples, reference Figure 1A and Figure 1B , the random feedback mechanism 13 can be connected to the gain fiber 12. Thus, the first optical signal can be received.

[0054] Figure 2 is a schematic diagram showing a random feedback mechanism 13 involved in the example of the present disclosure. Figure 3 is a schematic diagram showing a reflection spectrum of the random feedback mechanism 13 involved in the example of the present disclosure.

[0055] In some examples, reference Figure 2 The random feedback mechanism 13 may include a feedback optical fiber 131 and a fiber Bragg grating string 132 formed on the feedback optical fiber 131 .

[0056] In addition, the feedback optical fiber 131 may be an optical fiber. In some examples, the feedback optical fiber 131 may be a standard single-mode optical fiber. Thus, it is easy to obtain a target laser corresponding to the communication band (eg, 1550 nm) of the standard single-mode optical fiber.

[0057] In addition, the fiber Bragg grating string 132 may include a fiber Bragg grating and be configured to provide random feedback. In some examples, the fiber Bragg grating may be a second-order Bragg grating. In some examples, the fiber Bragg grating may be strongly scattered.

[0058] In some examples, when the optical signal input to the random feedback mechanism 13 is a first optical signal in a first wavelength range, the fiber Bragg grating string 132 can be configured to receive the first optical signal and provide random feedback for the first optical signal to output a second optical signal in a second wavelength range. Specifically, the fiber Bragg grating string 132 can reflect the first optical signal to output a second optical signal.

[0059] In some examples, the second wavelength range may be smaller than the first wavelength range. That is, the second wavelength range may be within the first wavelength range and not equal to the first wavelength range. Thus, it is convenient to obtain a relatively flat second optical signal, and it is convenient to cooperate with the gain fiber 12 to improve the flatness of the target laser. In addition, when the second optical signal in the second wavelength range passes through the gain fiber 12, it is convenient to be amplified by the gain fiber 12. As an example of the reflection spectrum of the fiber Bragg grating string 132 (i.e., the reflection spectrum of the random feedback mechanism 13), refer to Figure 3 When the reflection spectrum is in the range of 1540nm to 1560nm, for example, although the light intensity decreases slightly as the wavelength increases, it is relatively flat.

[0060] In some examples, the gain fiber 12 can be combined to suppress the situation where the wavelength increases and the light intensity decreases. In some examples, for the situation where the wavelength increases and the light intensity decreases in the reflection spectrum, the light intensity of the gain fiber 12 can be increased when the wavelength in the corresponding range increases. Therefore, the cooperation between the gain fiber 12 and the random feedback mechanism 13 can improve the flatness of the target laser. For example, by adjusting the length of the erbium-doped fiber, the high point of the light intensity of the gain spectrum of the erbium-doped fiber can correspond to the low point of the light intensity of the reflection spectrum of the fiber Bragg grating string 132.

[0061] In some examples, the wavelength span of the second optical signal may be no greater than 20 nm. As the wavelength span is larger, the fiber Bragg grating that needs to be written is longer, and after a certain length, the flatness of the reflection spectrum decreases. Therefore, limiting the wavelength span within the above range facilitates tuning within a range of tens of nanometers when the length of the feedback optical fiber 131 is at the meter level, and a relatively flat reflection spectrum can be obtained. Preferably, the wavelength span of the second optical signal may be 20 nm. That is, it can be tuned within a range of 20 nm. Thus, a tuning range suitable for most scenarios can be obtained, and a flatter reflection spectrum can be obtained.

[0062] In some examples, when the wavelength range of the first optical signal is 1530 nm to 1570 nm, the second wavelength range may be 1540 nm to 1560 nm (refer to Figure 3 In this case, a wavelength range with a wavelength span as large as possible can be obtained when the length of the feedback optical fiber 131 is short, so that wide tunability can be achieved while covering the C band as much as possible, and a flatter reflection spectrum can be obtained.

[0063] In some examples, the fiber Bragg grating string 132 can have high reflectivity. In some examples, the reflectivity of the fiber Bragg grating string 132 can be 20 db.

[0064] In some examples, when the gain of the first optical signal is greater than the loss, the fiber Bragg grating string 132 can also receive the first optical signal and transmit it to output a third optical signal.

[0065] In some examples, the fiber Bragg grating string 132 can be fabricated using femtosecond lasers.

[0066] In some examples, referring to Figure 2 , the fiber Bragg grating string 132 can include multiple grating arrays 1321, and the multiple grating arrays 1321 can correspond to different grating periods. Different grating periods can correspond to different wavelengths in the second optical signal.

[0067] In addition, the grating array 1321 can include fiber Bragg gratings, and the grating period of the grating array 1321 is the grating period of the fiber Bragg gratings in the grating array 1321. Since the grating period of the fiber Bragg grating is fixed, the range of the reflection spectrum is narrow, and the wide tunability of the laser wavelength cannot be achieved. In this case, by changing the grating periods of the multiple grating arrays 1321, the second optical signal can have multiple wavelengths, thus facilitating wide tunability. In addition, there can be a spacing between adjacent grating arrays 1321 among the multiple grating arrays 1321. As an example, Figure 2 shows a schematic of two grating arrays 1321, and the two grating arrays 1321 can include a first array A1 and a second array A2.

[0068] In some examples, the multiple grating periods of the multiple grating arrays 1321 can increase or decrease sequentially. In this case, it is convenient to obtain continuous wavelengths to achieve continuous wide tunability.

[0069] In some examples, along the direction in which the first optical signal is incident on the fiber Bragg grating string 132 (hereinafter simply referred to as the incident direction), the multiple grating periods of the multiple grating arrays 1321 can increase sequentially. That is, in the incident direction, the grating period of the grating array 1321 can gradually increase.

[0070] In some examples, the wavelength increment corresponding to the increment of the grating period can be negatively correlated with the continuity of the wavelengths in the second optical signal. That is, the larger the wavelength increment, the less continuous the wavelengths. In some examples, the wavelength increment corresponding to the increment of the grating period can be determined by simulation and debugging on the writing platform.

[0071] In some examples, the increment of the grating period can be the same. In some examples, the wavelength increment corresponding to the increment of the grating period can be 0.2 nm. In this case, the continuity of the wavelength can be improved, thereby achieving continuous wide tunability, and the length of the feedback fiber 131 is more appropriate. For example, for a second-order Bragg grating with a refractive index of 1.4401, an increment of the grating period of 0.1388 nm can correspond to a wavelength increment of 0.2 nm. If the number of grating arrays 1321 is 100, the length of the fiber Bragg grating string 132 can be approximately 4 meters.

[0072] In some examples, the number of grating arrays 1321 can be 50 to 150. Thus, it is convenient to obtain a tuning range of tens of nanometers and the length of the feedback fiber 131 is in the meter level. For example, the number of grating arrays 1321 can be 50, 70, 90, 100, 120 or 150. In some examples, the number of grating arrays 1321 can be 100. In this case, when the wavelength increment is 0.2 nm, a tuning range of 20 nm can be obtained, and the length of the fiber Bragg grating string 132 does not exceed 5 meters.

[0073] Figure 4 is a schematic diagram showing a grating array 1321 involved in the example of the present disclosure.

[0074] In some examples, reference Figure 4 , each grating array 1321 may include a plurality of fiber Bragg gratings. In some examples, the length of the grating array 1321 may be determined based on the number and length of the fiber Bragg gratings in the grating array 1321 .

[0075] In some examples, the number of fiber Bragg gratings in grating array 1321 can be adjusted according to the laser intensity requirement. In some examples, the number of fiber Bragg gratings in grating array 1321 can be positively correlated with the light intensity of random feedback (ie, the light intensity of the reflection spectrum).

[0076] In some examples, the number of fiber Bragg gratings in the grating array 1321 may be 6. Thus, a reflection spectrum with a good random effect can be obtained, and the random feedback light intensity is convenient for forming a laser intensity suitable for most scenes. As an example, Figure 4 Six fiber Bragg gratings are shown, and the six fiber Bragg gratings may include a first grating M1, a second grating M2, a third grating M3, a fourth grating M4, a fifth grating M5, and a sixth grating M6.

[0077] In some examples, the length of the fiber Bragg grating in the grating array 1321 may be negatively correlated with the saturation of the reflection spectrum. That is, the longer the length of the fiber Bragg grating in the grating array 1321 is, the easier it is for the light intensity of the reflection spectrum to be saturated.

[0078] In some examples, the lengths of the fiber Bragg gratings in the grating array 1321 may be the same. In some examples, the lengths of the fiber Bragg gratings in the grating array 1321 may be 0.1 mm (millimeter). Thus, the possibility of light intensity saturation of the reflection spectrum can be reduced.

[0079] In some examples, reference Figure 4 , the grating period of each fiber Bragg grating in the grating array 1321 can be fixed, and multiple fiber Bragg gratings can be the same. In addition, the grating period of the fiber Bragg grating is fixed, which means that the distances between the modulation points (also called refractive index change points) in the fiber Bragg grating are equal. As an example of the grating period of the fiber Bragg grating, Figure 4 FIG. 2 shows a schematic diagram of the grating period T of the first grating M1.

[0080] In some examples, the distances between adjacent fiber Bragg gratings in a plurality of fiber Bragg gratings may be random. Thus, random feedback can be provided. In addition, the first optical signal can be continuously reflected by a plurality of fiber Bragg gratings. The more reflections there are and the longer the time is, the higher the light intensity of the formed second optical signal is. The Q value of the second optical signal can be increased in a short optical fiber (e.g., an optical fiber with a length of meters), and the line width of the second optical signal can be reduced. Thus, wide tunability can be achieved while shortening the length of the optical fiber.

[0081] In addition, since the narrower the line width, the fewer modes there are (i.e., more mode competition is prevented), the more stable the laser 1 is, and the weaker the noise is. In addition, the narrower the line width, the better the filtering effect of ASE (Amplified Spontaneous Emission) noise. Thus, the stability of the laser 1 can be improved and the noise of the laser 1 can be reduced.

[0082] In some examples, the distances between adjacent fiber Bragg gratings can be randomly distributed in the range of 1 mm to 10 mm. In this case, more fiber Bragg gratings can be arranged on the shorter feedback fiber 131, which can increase the light intensity of the second optical signal. For example, taking 6 fiber Bragg gratings as an example, the distances between adjacent fiber Bragg gratings can be 5 mm (millimeter), 8 mm, 2 mm, 7 mm, and 3 mm, respectively. As an example of the distance of the fiber Bragg grating, Figure 4 The distance D between the third grating M3 and the fourth grating M4 is shown.

[0083] In some examples, the length of the fiber Bragg grating string 132 may be in the order of meters. In this case, it is convenient to obtain a more suitable light intensity and wavelength span of the second optical signal with a shorter fiber Bragg grating string 132 .

[0084] In some examples, the length of the fiber Bragg grating string 132 may be determined by the light intensity and wavelength span of the second optical signal. In some examples, the length of the fiber Bragg grating string 132 may be positively correlated with the light intensity and wavelength span of the second optical signal.

[0085] In some examples, when the laser 1 generates a target laser of the C-band, the length of the fiber Bragg grating string 132 can be 3m (meter) to 5m. For example, the length of the fiber Bragg grating string 132 can be 3m, 4m, or 5m. Preferably, the length of the fiber Bragg grating string 132 can be 4m. In this case, a more suitable light intensity (e.g., -10db) and a larger wavelength span (e.g., 20nm) of the second optical signal can be obtained under a shorter fiber Bragg grating string 132, and the influence of noise can be reduced relative to kilometer-level optical fibers (e.g., Rayleigh scattering optical fibers).

[0086] In addition, in order to facilitate the understanding of the fiber Bragg grating string 132, the example of the present disclosure also provides a method for preparing the fiber Bragg grating string 132. It should be noted that this does not limit the present disclosure. The basic idea of ​​the random feedback mechanism 13 has been disclosed in the present disclosure. Those skilled in the art can make corresponding adjustments as needed to prepare a random feedback mechanism 13 that can output a target laser of a corresponding wavelength range and light intensity. In this embodiment, the fiber Bragg grating can be written by changing the grating period and random distance to obtain the fiber Bragg grating string 132. In this way, a fiber Bragg grating string 132 with a wide bandwidth and a high Q value can be obtained.

[0087] Specifically, in the femtosecond laser etching system, a femtosecond laser with a pulse energy of 62nJ (nanojoule), a wavelength of 515nm, and a pulse repetition rate of 200kHz (kilohertz) is focused on a standard single-mode optical fiber (i.e., a type of feedback optical fiber 131) through a 63x oil-immersion objective lens, and a fiber Bragg grating (i.e., a grating array 1321) of random distance is written in the optical fiber by point-by-point technology to form a strongly scattered fiber Bragg grating that can provide random feedback. In addition, by changing the grating period, multiple grating arrays 1321 corresponding to multiple wavelengths are written.

[0088] In this case, a fiber Bragg grating string 132 with wide bandwidth and high Q value can be formed, breaking through the technical bottleneck of using short-length optical fiber feedback to form a widely tunable and narrow-linewidth laser 1. It should be noted that the present disclosure is not limited to the above parameters for preparing the fiber Bragg grating string 132, and those skilled in the art can adjust or select other preparation methods as needed.

[0089] The structure of the fiber Bragg grating string 132 is described below by taking a 4-meter fiber Bragg grating string 132 as an example, wherein the second wavelength range is 1540 nm to 1560 nm, and the fiber Bragg grating is a second-order Bragg grating. It should be noted that this does not limit the present disclosure.

[0090] The fiber Bragg grating string 132 can be composed of a grating array 1321 with an increment of 100 grating periods of 0.1388 nm, and the grating period ranges from 1.0693 μm (micrometers) to 1.0832 μm, which can correspond to a second wavelength range of 1540 nm to 1560 nm, wherein the wavelength increment corresponding to the increment of 0.1388 nm can be 0.2 nm.

[0091] Each grating array 1321 may be composed of 6 identical fiber Bragg gratings. The length of each fiber Bragg grating may be 0.1 mm, and it may be composed of a plurality of modulation points with the same grating period. The distances between adjacent fiber Bragg gratings are randomly distributed in the range of 1 mm to 10 mm, for example, 5 mm, 8 mm, 2 mm, 7 mm, and 3 mm, respectively. In this case, the total number of modulation points may be approximately 55,800.

[0092] In some examples, when the laser 1 includes both the gain fiber 12 and the random feedback mechanism 13, the light intensity of the target laser (hereinafter referred to as laser intensity) can be determined by the gain fiber 12 and the random feedback mechanism 13. That is, the laser intensity can be determined by the light intensity of the gain spectrum of the gain fiber 12 and the light intensity of the reflection spectrum of the random feedback mechanism 13. For example, the laser intensity can be determined by combining the gain spectrum with a light intensity of -10 db (decibel) and the reflection spectrum with a light intensity of -20 db.

[0093] In some examples, the second optical signal output by the random feedback mechanism 13 can be input to the optical input end 11. In this case, when the gain fiber 12 is connected to the optical input end 11, the second optical signal can be repeatedly amplified until the gain is greater than the loss and then the target laser is output.

[0094] Figure 5A 1 is a schematic diagram showing a third embodiment of the laser 1 according to the present disclosure example. Figure 5B 1 is a schematic diagram showing a fourth embodiment of the laser 1 according to the present disclosure example. Figure 6 is a schematic diagram showing the spectrum of the target laser light involved in the example of the present disclosure.

[0095] In some examples, reference Figure 5A , the second optical signal can enter the optical input end 11 through the circulator 14. That is, a ring cavity can be formed. In this case, a ring-shaped laser cavity can be built to facilitate repeated amplification of the second optical signal, which can improve the efficiency of the laser 1. Specifically, refer to Figure 5A , the laser 1 may include a circulator 14. The circulator 14 may be configured to receive the first optical signal and transmit it to the random feedback mechanism 13, and receive the second optical signal and transmit it to the optical input end 11.

[0096] In some examples, reference Figure 5A , the circulator 14 may include a first port P21, a second port P22, and a third port P23. The first port P21 may receive a first optical signal and emit it to the random feedback mechanism 13 through the second port P22, and a second optical signal of the random feedback mechanism 13 may enter the optical input end 11 through the third port P23. In some examples, the second optical signal of the random feedback mechanism 13 may be incident to the second port P22 and enter the optical input end 11 through the third port P23.

[0097] In some examples, the second optical signal entering the optical input end 11 can be amplified by the gain fiber 12. That is, the gain fiber 12 can also be configured to amplify the second optical signal. Thus, it is convenient to make the gain of the optical signal in the laser 1 greater than the loss to output the target laser.

[0098] In some examples, the circulator 14 may be unidirectionally isolating, thereby limiting the optical signal from propagating in the direction opposite to the direction in which the second optical signal enters the optical input end 11 .

[0099] In some examples, reference Figure 5A , the laser 1 may include a filter 15. The filter 15 may be configured to limit the available output wavelength range (i.e., the wavelength range of the target laser). In some examples, the filter 15 may be tunable. Thus, optical signals of different wavelengths can be selected to output target lasers of corresponding wavelengths.

[0100] In addition, the filter 15 can be set at any position that can limit the output wavelength range. Figure 5A When the second optical signal is input to the optical input end 11, the filter 15 can be arranged between the optical input end 11 and the random feedback mechanism 13. Thus, the second optical signal of a specific wavelength range can be selected to be input to the optical input end 11. That is, the filter 15 can be used to adjust the wavelength of the second optical signal input to the optical input end 11.

[0101] In some examples, the parameters of the filter 15 can be adjusted according to the wavelength range of the target laser. For example, for a target laser with a wavelength range of 1540nm to 1560nm, the bandwidth of the filter 15 can be tuned from 32pm (picometer) to 650pm, and the wavelength range can be 1480nm to 1620nm.

[0102] In some examples, reference Figure 5B , the laser 1 may include a first isolator 16. The first isolator 16 may be configured to restrict the optical signal from entering the random feedback mechanism 13 in the opposite direction of the direction in which the second optical signal enters the optical input end 11. In some examples, the first isolator 16 may be disposed between the optical input end 11 and the circulator 14. Thus, the circulator 14 can pass only unidirectional optical signals. In some examples, the first isolator 16 may be disposed between the optical input end 11 and the circulator 14, and directly connected to the optical input end 11.

[0103] In some examples, reference Figure 5B , the laser 1 may include a second isolator 17. The second isolator 17 may be configured to limit the external optical signal from entering the random feedback mechanism 13. In this case, the risk of forming multi-mode light due to the influence of the external optical signal can be reduced, and the formation of single-mode light can be facilitated. In some examples, the third optical signal can be output after passing through the second isolator 17.

[0104] In addition, in order to facilitate understanding of the laser 1 involved in the example of the present disclosure, the example of the present disclosure also provides a specific example of the laser 1.

[0105] Specifically, refer to Figure 5B , the laser 1 may include an optical input end 11 of a wavelength division multiplexer, a gain fiber 12 of an erbium-doped fiber, a circulator 14, a random feedback mechanism 13, a second isolator 17, a filter 15 and a first isolator 16. The wavelength division multiplexer may include a first input port P11, a second input port P12 and an output port P13, the first input port P11 may be connected to the pump source 2, the output port P13 may be connected to one end of the erbium-doped fiber, the circulator 14 may include a first port P21, a second port P22 and a third port P23, the other end of the erbium-doped fiber may be connected to the first port P21, the second port P22 may be connected to the random feedback mechanism 13 and then connected to the second isolator 17. The third port P23 may be connected to the filter 15, the first isolator 16 and the second input port P12 of the wavelength division multiplexer in sequence. The target laser is output from one end of the second isolator 17.

[0106] Based on the above-mentioned laser 1, the pump light emitted by the pump source 2 enters the gain fiber 12 which is an erbium-doped fiber after passing through the optical input end 11 of the wavelength division multiplexer. After the erbium-doped ions absorb the pump light energy, the population inversion occurs, thereby generating ASE light (i.e., a type of first optical signal). For example, the pump light with a central wavelength of 980nm can generate a first optical signal with a first wavelength range of 1530nm to 1570nm through the erbium-doped fiber. In addition, when stimulated radiation occurs, the gain fiber 12 can also amplify the incident optical signal in the first wavelength range.

[0107] ASE light can enter the random feedback mechanism 13 through the circulator 14. The optical signal entering the random feedback mechanism 13, in the second wavelength range, when the gain is less than the loss, can be reflected so as to enter the wavelength division multiplexer through the third port P23 of the circulator 14 to continue to be amplified by the erbium-doped fiber and enter the random feedback mechanism 13. For example, an optical signal with a wavelength of 1550nm entering the erbium-doped fiber can output an optical signal with a wavelength of 1550nm and enhanced light intensity; in the second wavelength range, when the gain is greater than the loss, it can be transmitted from the random feedback mechanism 13 to output the target laser.

[0108] In addition, bidirectional laser oscillation can be prevented by isolating the circulator 14 unidirectionally through the first isolator 16. In addition, in order to block the strong Fresnel reflection of the fiber end face, a second isolator 17 can be placed after the random feedback mechanism 13, and the target laser is output from the second isolator 17.

[0109] As described above, the optical signal entering the random feedback mechanism 13 can enter the wavelength division multiplexer after being reflected by the random feedback mechanism 13, and a filter 15 can be set on the optical path of the reflected optical signal entering the wavelength division multiplexer, so as to obtain a target laser with wide tunability, narrow line width and low noise by changing the wavelength of the filter 15. As an example, Figure 6 A schematic diagram showing the spectrum of the target laser is shown.

[0110] In some examples of the present disclosure, random feedback with wide bandwidth and high Q value is realized by using fiber Bragg gratings with random distances in the random feedback mechanism 13, and the optical gain is amplified in combination with the gain fiber 12. In addition, a circulator 14 is used to construct a ring cavity in a shorter laser cavity length to make the structure of the laser 1 compact, thereby providing a compact, widely tunable, narrow linewidth and low noise laser 1.

[0111] In some examples, the output end of the laser 1 can be connected to a spectrometer and an interferometer (such as a Mach-Zehnder interferometer) in sequence, and by changing the wavelength of the filter 15, the spectrum and line width of the target laser at different wavelengths can be observed.

[0112] The laser 1 involved in the example of the present disclosure can improve the light intensity of random feedback within a wide wavelength range of tens of nanometers and has a high Q value. At the same time, combined with the wide bandwidth gain of erbium-doped optical fiber, it can realize a widely tunable and narrow linewidth laser 1 in a short-distance feedback system.

[0113] The laser 1 involved in the example of the present disclosure greatly shortens the laser cavity length compared with the previous technology, achieves at least one of the purposes of wide tunability, continuous tunability, narrow linewidth and single-mode laser output, and has the advantages of simple structure, easy production, low cost, and excellent performance.

[0114] Although the present disclosure is specifically described above in conjunction with the accompanying drawings and examples, it is to be understood that the above description does not limit the present disclosure in any form. Those skilled in the art may modify and change the present disclosure as needed without departing from the essential spirit and scope of the present disclosure, and these modifications and changes all fall within the scope of the present disclosure.

Claims

1. A random fiber laser based on a fiber Bragg grating, characterized in that: It includes an optical input end, a gain optical fiber and a random feedback mechanism; the optical input end is configured to receive pump light from a pump source; The gain fiber is configured to receive the pump light and output a first optical signal in a first wavelength range, wherein the gain fiber has a gain medium excited by the pump light; the random feedback mechanism comprises a feedback fiber and a fiber Bragg grating string formed on the feedback fiber, the fiber Bragg grating string is configured to receive the first optical signal and provide random feedback for the first optical signal to output a second optical signal in a second wavelength range, wherein the second wavelength range is smaller than the first wavelength range, the fiber Bragg grating string comprises a plurality of grating arrays corresponding to different grating periods, in a direction along which the first optical signal is incident on the fiber Bragg grating string, a plurality of grating periods of the plurality of grating arrays are increased in sequence, each of the plurality of grating arrays comprises a plurality of fiber Bragg gratings, and the distance between adjacent fiber Bragg gratings in the plurality of fiber Bragg gratings is random.

2. The random fiber laser according to claim 1, characterized in that: The optical input end is a wavelength division multiplexer, and the optical input end includes two input ports and one output port. The two input ports receive the pump light and the second optical signal respectively and transmit them to the gain optical fiber through the output port.

3. The random fiber laser according to claim 1, characterized in that: The gain optical fiber is an erbium-doped optical fiber, and the first wavelength range is 1530 nm to 1570 nm.

4. The random fiber laser according to claim 3, characterized in that: The second wavelength range is from 1540 nm to 1560 nm.

5. The random fiber laser according to any one of claims 1 to 4, characterized in that: The second wavelength range is no greater than 20 nm.

6. The random fiber laser according to claim 1, characterized in that: The gain optical fiber is an erbium-doped optical fiber, the length of the gain optical fiber is in the order of meters, and the length of the fiber Bragg grating string is in the order of meters.

7. The random fiber laser according to claim 1, characterized in that: When the gain of the first optical signal is greater than the loss, the random feedback mechanism is further configured to receive the first optical signal and transmit it to output a third optical signal.

8. The random fiber laser according to claim 1, characterized in that: It also includes a circulator, which includes a first port, a second port and a third port. The first port receives the first optical signal and emits it to the random feedback mechanism through the second port. The second optical signal enters the optical input end through the third port to amplify the second optical signal through the gain optical fiber.

9. The random fiber laser according to claim 8, characterized in that: The optical circuit further includes a first isolator disposed between the optical input end and the circulator.

10. The random fiber laser according to claim 1, characterized in that: The optical feedback mechanism further comprises a filter configured to limit an available output wavelength range, wherein the second optical signal is input to the optical input end, and the filter is disposed between the optical input end and the random feedback mechanism.

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