GHz pulse string generation device and generation method

Abstract

The invention discloses a GHz pulse train generation device and method, and relates to the technical field of laser, and the device comprises a seed light source, a first acousto-optic modulator, a 50: 50 coupler, an optical fiber loop, and a third acousto-optic modulator. Wherein the fiber loop comprises a second acousto-optic modulator, a first chirp long-period fiber grating, a polarization-maintaining double-clad fiber and a second chirp long-period fiber grating. By using the first chirp long-period fiber grating, the polarization-maintaining double-clad fiber and the second chirp long-period fiber grating, the dispersion effect in a fiber loop can be compensated, and the conversion rate of the chirp long-period fiber grating is relatively high, so that the transmission loss in the fiber loop can be reduced; group delay ripples of the chirp long-period fiber bragg grating and the optical fiber high-order LP0m mode are very low, the shape and the width of pulses in a GHz pulse string on a time domain can be controlled, and finally, the optical fiber of the chirp long-period fiber bragg grating is a polarization-maintaining double-clad optical fiber, so that the nonlinear effect can be effectively inhibited.

Description

Technical Field

[0001] This application relates to the field of laser technology, and particularly to a GHz pulse train generating device and a generating method. Background Art

[0002] High-repetition-rate ultrashort pulse light sources, especially GHz light sources, have significant potential advantages in the field of material processing. GHz ultrashort pulse light sources can improve processing efficiency, reduce the required pulse energy threshold, and weaken the thermal effect in materials. Therefore, the generation of GHz repetition rate is extremely important in the field of material processing.

[0003] However, the chirped Bragg fiber grating in the current mainstream GHz pulse train generating device will cause high transmission loss, introduce unnecessary higher-order dispersion, increase the difficulty of controlling the shape and width of the pulses in the GHz pulse train in the time domain, and all the fibers in the mainstream GHz pulse train generating device are polarization-maintaining single-mode fibers, which are prone to generate nonlinear effects, thus affecting the shape and width of the pulses in the GHz pulse train in the time domain and frequency domain. Summary of the Invention

[0004] This application provides a GHz pulse train generating device and a generating method. By using a chirped long-period fiber grating and fiber LP 0m higher-order modes, the device and method can compensate for the dispersion effect in the fiber loop, reduce the transmission loss in the fiber loop, and reduce the group delay ripple, which is beneficial to controlling the shape and width of the pulses in the GHz pulse train in the time domain. At the same time, the device and method replace part of the fibers in the fiber loop with polarization-maintaining double-clad fibers, which can effectively suppress nonlinear effects.

[0005] In a first aspect, this application provides a GHz pulse train generating device, including a seed light source, a first acousto-optic modulator, a 50:50 coupler, a fiber loop, and a third acousto-optic modulator. In the GHz pulse train generating device, the seed light source is connected to the first acousto-optic modulator through a fiber, the first acousto-optic modulator is connected to the 50:50 coupler through a fiber, the 50:50 coupler and the third acousto-optic modulator are connected through a fiber, and the 50:50 coupler and the fiber loop are connected through a fiber. The device includes:

[0006] The fiber loop includes a second acousto-optic modulator, a first chirped long-period fiber grating, a polarization-maintaining double-clad fiber, a second chirped long-period fiber grating, and a polarization-maintaining single-mode gain fiber;

[0007] The 50:50 coupler includes 4 ports, where the first port is connected to the first acousto-optic modulator, the second port is connected to the second acousto-optic modulator in the fiber loop, the third port is connected to the third acousto-optic modulator, and the fourth port is connected to the polarization-maintaining single-mode gain fiber in the fiber loop;

[0008] A seed light source for outputting an initial signal optical pulse sequence;

[0009] A first acousto-optic modulator for intensity-modulating the initial signal optical pulse sequence and transmitting the initial signal optical pulses required for the GHz pulse train in the initial signal optical pulse sequence to a 50:50 coupler one by one through a first port;

[0010] A 50:50 coupler for splitting the initial signal optical pulse into two signal optical pulse trains, transmitting the first signal optical pulse train after splitting to an optical fiber loop through a second port, and transmitting the second signal optical pulse train after splitting to a third acousto-optic modulator through a third port, wherein the number of signal optical pulses in the first signal optical pulse train is the same as that in the second signal optical pulse train;

[0011] The 50:50 coupler and the optical fiber loop act together to synthesize the initial signal optical pulse and the first signal optical pulse train received through a fourth port, so as to increase the number of signal optical pulses in the first signal optical pulse train and the second signal optical pulse train after splitting;

[0012] A second acousto-optic modulator for transmitting the first signal optical pulse train to a first chirped long-period fiber grating;

[0013] The first chirped long-period fiber grating for converting the energy of the first signal optical pulse train from the fiber fundamental mode to the fiber LP 0m higher-order mode;

[0014] A polarization-maintaining double-clad fiber for conducting the first signal optical pulse train in the fiber LP 0m higher-order mode to a second chirped long-period fiber grating;

[0015] The second chirped long-period fiber grating for converting the energy of the first signal optical pulse train from the fiber LP 0m higher-order mode to the fiber fundamental mode and transmitting the converted first signal optical pulse train to a polarization-maintaining single-mode gain fiber;

[0016] The polarization-maintaining single-mode gain fiber for transmitting the converted first signal optical pulse train to the 50:50 coupler through a fourth port;

[0017] A third acousto-optic modulator for outputting the second signal optical pulse train to obtain a GHz pulse train when the number of signal optical pulses in the second signal optical pulse train is the same as the number of signal optical pulses required in the GHz pulse train;

[0018] Wherein, the first chirped long-period fiber grating and the second chirped long-period fiber grating are connected through the fiber LP 0mDispersion effects in a high-order mode compensation optical fiber loop are compensated; and the group delay ripples of the first chirped long-period fiber grating and the second chirped long-period fiber grating are less than a first threshold value, which is beneficial to controlling the shape and width of the pulses within the first optical signal pulse train and the second optical signal pulse train in the time domain.

[0019] Optionally, the GHz pulse train generating device further includes: a wavelength division multiplexer and a semiconductor pump laser,

[0020] The semiconductor pump laser is used to transmit the energy of the pump light to the wavelength division multiplexer, so as to provide the energy of the pump light to the polarization-maintaining single-mode gain fiber;

[0021] The wavelength division multiplexer is used to collectively transmit the pump light and the converted first optical signal pulse train transmitted by the second chirped long-period fiber grating to the polarization-maintaining single-mode gain fiber;

[0022] The polarization-maintaining single-mode gain fiber is further used to convert the energy of the pump light into the energy of the converted first optical signal pulse train, so as to compensate for the transmission loss of the first optical signal pulse train in the optical fiber loop.

[0023] Optionally, the GHz pulse train generating device further includes: an isolator,

[0024] The isolator is used to block the reflected pump light.

[0025] Optionally, the GHz pulse train generating device further includes: a fusion splice point,

[0026] The fusion splice point is arranged at the connection between the second acousto-optic modulator and the first chirped long-period fiber grating, and at the connection between the second chirped long-period fiber grating and the wavelength division multiplexer, and is used to enable the first optical signal pulse train to be transmitted in the fiber fundamental mode between the polarization-maintaining single-mode fiber and the polarization-maintaining double-clad fiber.

[0027] Optionally, the pigtails of the seed light source, the first acousto-optic modulator, the 50:50 coupler, the second acousto-optic modulator, the wavelength division multiplexer, and the third acousto-optic modulator are all polarization-maintaining single-mode fibers.

[0028] Optionally, the pulse period within the first optical signal pulse train is the transmission time of the first optical signal pulse train in the optical fiber loop minus the period of the initial optical signal pulse sequence, and the period within the first optical signal pulse train is the same as the period within the second optical signal pulse train.

[0029] Optionally, when the first acousto-optic modulator satisfies the first acousto-optic modulator off condition, it is used to block the transmission of the initial optical signal pulse to the 50:50 coupler until the next GHz pulse train needs to be generated, and the first acousto-optic modulator off condition is that the initial optical signal pulses required for the GHz pulse train have been transmitted to the 50:50 coupler one by one;

[0030] When the second acousto-optic modulator meets the second acousto-optic modulator off condition, it is used to block the transmission of the first signal optical pulse train to the first chirped long-period fiber grating until the next GHz pulse train needs to be generated. The second acousto-optic modulator off condition is that the number of signal optical pulses in the first signal optical pulse train is the same as the number of signal optical pulses required in the GHz pulse train;

[0031] When the third acousto-optic modulator meets the third acousto-optic modulator off condition, it is used to block the output of the second signal optical pulse train. The third acousto-optic modulator off condition is that the number of signal optical pulses in the second signal optical pulse train is different from the number of signal optical pulses required in the GHz pulse train.

[0032] In a second aspect, the present application provides a method for generating a GHz pulse train, which is applied to the GHz pulse train generating device described in the first aspect. The method is characterized in that the method includes:

[0033] Output an initial signal optical pulse sequence through a seed light source, and transmit the initial signal optical pulse in the initial signal optical pulse sequence to a 50:50 coupler through a first port;

[0034] Divide the initial signal optical pulse into two signal optical pulse trains through a 50:50 coupler, transmit the divided first signal optical pulse train to the optical fiber loop through a second port, and transmit the divided second signal optical pulse train to the third acousto-optic modulator through a third port. The number of signal optical pulses in the first signal optical pulse train is the same as that in the second signal optical pulse train;

[0035] Synthesize the initial signal optical pulse and the first signal optical pulse train received through the fourth port through the 50:50 coupler and the optical fiber loop to increase the number of signal optical pulses in the divided first signal optical pulse train and the divided second signal optical pulse train;

[0036] Convert the energy of the first signal optical pulse train through the optical fiber loop, and transmit the converted first signal optical pulse train to the 50:50 coupler through the fourth port. The energy conversion is realized through the first chirped long-period fiber grating, polarization-maintaining double-clad fiber, and the second chirped long-period fiber grating. The signal optical pulses between the first chirped long-period fiber grating and the second chirped long-period fiber grating are transmitted by the polarization-maintaining double-clad fiber;

[0037] When the number of signal optical pulses in the second signal optical pulse train is the same as the number of signal optical pulses required in the GHz pulse train, output the second signal optical pulse train through the third acousto-optic modulator to obtain a GHz pulse train.

[0038] Optionally, converting the energy of the first signal optical pulse train through the optical fiber loop and transmitting the converted first signal optical pulse train to the 50:50 coupler through the fourth port includes:

[0039] Transmit the first signal optical pulse train to the first chirped long-period fiber grating through a second acousto-optic modulator;

[0040] Convert the energy of the first signal optical pulse train from the fiber fundamental mode to the fiber LP 0m higher-order mode through the first chirped long-period fiber grating;

[0041] Conduct the first signal optical pulse train in the fiber LP 0m higher-order mode to the second chirped long-period fiber grating through a polarization-maintaining double-clad fiber;

[0042] Convert the energy of the first signal optical pulse train from the fiber LP 0m higher-order mode to the fiber fundamental mode through the second chirped long-period fiber grating, and transmit the converted first signal optical pulse train to a 50:50 coupler through a fourth port.

[0043] Optionally, output an initial signal optical pulse sequence through a seed light source, and transmit an initial signal optical pulse in the initial signal optical pulse sequence to a 50:50 coupler through a first port, including:

[0044] Output an initial signal optical pulse sequence through a seed light source, and transmit the initial signal optical pulse sequence to a first acousto-optic modulator;

[0045] Perform intensity modulation on the initial signal optical pulse sequence through the first acousto-optic modulator, and transmit the initial signal optical pulses required for the GHz pulse train to the 50:50 coupler one by one through the first port.

[0046] It can be seen that the present application has the following beneficial effects:

[0047] The present application provides a GHz pulse train generating device, including a seed light source, a first acousto-optic modulator, a 50:50 coupler, an optical fiber loop and a third acousto-optic modulator, wherein the optical fiber loop includes a second acousto-optic modulator, a first chirped long-period fiber grating, a polarization-maintaining double-clad fiber and a second chirped long-period fiber grating. By using the first chirped long-period fiber grating, the polarization-maintaining double-clad fiber and the second chirped long-period fiber grating, the device can compensate for the dispersion effect in the optical fiber loop, and the conversion rate of the chirped long-period fiber grating is relatively high, which can reduce the transmission loss in the optical fiber loop; the group delay ripple of the chirped long-period fiber grating is very low or even non-existent, and there is no group delay ripple in the optical fiber higher-order mode at all. Therefore, it is beneficial to control the shape and width of the pulses in the GHz pulse train in the time domain; and the optical fiber of the chirped long-period fiber grating is a polarization-maintaining double-clad fiber, and the internally transmitted light beam is the fiber LP 0m higher-order mode, and its mode field area is large, which can effectively suppress the nonlinear effect. Description of the Drawings

[0048] To more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments recorded in the present application. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings.

[0049] Figure 1 It is a schematic structural diagram of a mainstream traditional GHz pulse train generating device in the embodiments of the present application;

[0050] Figure 2 It is a schematic structural diagram of an embodiment of a GHz pulse train generating device in the embodiments of the present application;

[0051] Figure 3 It is a schematic flowchart of an embodiment of a method for generating a GHz pulse train in the embodiments of the present application;

[0052] Figure 4 It is a schematic flowchart of another embodiment of a method for generating a GHz pulse train in the embodiments of the present application. Detailed implementation manners

[0053] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art belong to the scope of protection of the present invention.

[0054] The "first" in the names such as "first acousto-optic modulator" and "first chirped long-period fiber grating" mentioned in the embodiments of the present application is only used as a name identifier and does not represent the first in order. This rule also applies to "second", "third", etc.

[0055] It should be noted that the user information (including but not limited to user device information, user personal information, etc.) and data (including but not limited to data for analysis, stored data, displayed data, etc.) involved in the present application are all information and data authorized by the user or fully authorized by all parties, and the collection, use, and processing of relevant data need to comply with the relevant laws, regulations, and standards of relevant countries and regions.

[0056] To facilitate the understanding of the GHz pulse train generating device, first, Figure 1 A common structure of a traditional GHz pulse train generating device is briefly described.

[0057] Figure 1 The solid arrows represent the signal light transmission direction, and the dashed arrows represent the pump light transmission direction.

[0058] (1) Seed light source: Used to output an initial signal optical pulse sequence with a repetition frequency on the order of 10 MHz to 100 MHz. The output pigtail of the seed light source is a polarization-maintaining single-mode fiber.

[0059] (2) First acousto-optic modulator: Serves as an optical switch. It should be noted that an optical switch is an optical path conversion device mainly used in fiber optic transmission systems, fiber optic test systems, and fiber optic sensing multi-point detection systems. It can perform physical switching or logical operations on optical signals in an optical transmission line or an integrated optical circuit. That is, when the first acousto-optic modulator generates a GHz pulse train, the transmittance of the first acousto-optic modulator is set to the highest value. According to the number of pulses in the required pulse train, an appropriate number of initial signal optical pulses are gated. Then, the transmittance of the first acousto-optic modulator is set to 0 to block the signal optical pulses until the next pulse train needs to be generated. And the pigtails at both ends of the first acousto-optic modulator are polarization-maintaining single-mode fibers.

[0060] (3) 50:50 coupler: First, it divides an appropriate number of initial signal optical pulses transmitted from the first acousto-optic modulator into two parts with equal power. The signal optical pulses in one part are transmitted one by one into an optical fiber loop containing devices such as a circulator, a chirped Bragg grating, and a second acousto-optic modulator, and the signal optical pulses in the other part are transmitted one by one to the third acousto-optic modulator. The pigtails of the four ports of the 50:50 coupler are all polarization-maintaining single-mode fibers.

[0061] (4) Circulator: First, it transmits a series of GHz pulse trains with an increasing number of pulses (1, 2, 3,...) transmitted from the 50:50 coupler to a polarization-maintaining single-mode gain fiber. Then, it transmits the reflected pulse train transmitted from the polarization-maintaining single-mode gain fiber to the second acousto-optic modulator. The pigtails of the three ports of the circulator are all polarization-maintaining single-mode fibers.

[0062] (5) Second acousto-optic modulator: Also serves as an optical switch. That is, when generating a GHz pulse train, the transmittance of the second acousto-optic modulator is set to the highest value, and a series of GHz pulse trains with an increasing number of pulses (1, 2, 3,...) in the optical fiber loop are gated. The pulse trains in the optical fiber loop can be further combined with the initial signal optical pulses transmitted from the first acousto-optic modulator to form a pulse train with a larger number of pulses. When the number of pulses in the pulse train gated by the second acousto-optic modulator meets the requirements for synthesizing the required GHz pulse train, the transmittance of the second acousto-optic modulator is set to 0 to block the signal optical pulses until the next pulse train needs to be generated. The pigtails at both ends of the second acousto-optic modulator are polarization-maintaining single-mode fibers.

[0063] (6) Polarization-maintaining single-mode gain fiber: First, it absorbs the pump light and stores the energy of the pump light in the polarization-maintaining single-mode gain fiber. Second, it converts the energy stored in the polarization-maintaining single-mode gain fiber (the energy of the pump light) into the energy of the signal light pulse, thereby compensating for the transmission loss of the signal light pulse in the fiber loop and being able to control the envelope shape of the GHz pulse train, achieving three shapes: flat, increasing, and decreasing.

[0064] (7) Chirped Bragg fiber grating: First, it is used to reflect the signal light pulse and keep the signal light pulse continuing to transmit in the fiber loop. Second, it applies chirp to the signal light pulse to compensate for the dispersion effects of other devices and fibers in the loop, thereby ensuring that each pulse in the GHz pulse train has the same pulse width and chirp. The fiber used in the chirped Bragg fiber grating is polarization-maintaining single-mode fiber. Figure 1 It can be seen that it is used to reflect the signal light pulse and keep the signal light pulse continuing to transmit in the fiber loop. Second, it applies chirp to the signal light pulse to compensate for the dispersion effects of other devices and fibers in the loop, thereby ensuring that each pulse in the GHz pulse train has the same pulse width and chirp. The fiber used in the chirped Bragg fiber grating is polarization-maintaining single-mode fiber.

[0065] (8) Semiconductor pump laser: Acts as a pump source and provides pump energy to the polarization-maintaining single-mode gain fiber.

[0066] (9) Isolator: Blocks the reflected pump light, thereby preventing damage to the semiconductor pump laser.

[0067] (10) Pump laser protector: Blocks the transmission of the signal light pulse to the semiconductor pump laser, thereby preventing damage to the semiconductor pump laser.

[0068] (11) Third acousto-optic modulator: Also acts as an optical switch. It receives a series of GHz pulse trains with the number of pulses increasing sequentially (1, 2, 3,...) from a 50:50 coupler. When the number of pulses in the pulse train is the same as the required pulse train, the transmittance of the third acousto-optic modulator is set to be non-zero and can vary with time. Therefore, this pulse train can be gated and the envelope shape can be edited. At other times, the transmittance of the third acousto-optic modulator is set to 0 to block the signal light pulse until the next pulse train that meets the requirements appears. Among them, the pigtail fibers at both ends of the third acousto-optic modulator are polarization-maintaining single-mode fibers.

[0069] Currently, there are several technical drawbacks in the mainstream traditional GHz pulse train generation devices:

[0070] (1) Due to the low reflectivity of the chirped Bragg fiber grating, usually less than 40%, it causes a high transmission loss in the fiber loop.

[0071] (2) The chirped Bragg fiber grating has group delay ripples, which will introduce unnecessary high-order dispersion and is not conducive to controlling the shape and width of the pulses in the GHz pulse train in the time domain.

[0072] (3) All the optical fibers in the mainstream traditional GHz pulse train generating device are polarization-maintaining single-mode optical fibers, which transmit the fundamental mode of the optical fiber internally, and have a small mode field area (on the order of 10μm 2 ), and are prone to generating nonlinear effects, thereby affecting the shape and width of the pulses in the GHz pulse train in the time domain and frequency domain.

[0073] It can be seen from this that the disadvantages of the existing technology appear in two parts: the chirped Bragg fiber grating and all the optical fibers in the device are polarization-maintaining single-mode optical fibers. In response to this, the present application uses a low-loss chirped long-period fiber grating and an optical fiber LP 0m higher-order mode to replace the chirped Bragg fiber grating, and replaces some of the optical fibers in the optical fiber loop with polarization-maintaining double-clad optical fibers, that is, the optical fibers of the chirped long-period fiber grating, to overcome the above-mentioned technical disadvantages.

[0074] The specific explanation of how to overcome the technical disadvantages is as follows:

[0075] (1) Both the chirped long-period fiber grating and the optical fiber LP 0m higher-order mode adopt a transmission working mode, which can apply chirp to the signal light pulse to compensate for the dispersion effect in the optical fiber loop;

[0076] (2) The chirped long-period fiber grating has a high conversion rate, which can reduce the transmission loss in the optical fiber loop;

[0077] (3) The group delay ripple of the chirped long-period fiber grating is very low or even non-existent, and the optical fiber higher-order mode has no group delay ripple at all. Therefore, it is beneficial to control the shape and width of the pulses in the GHz pulse train in the time domain;

[0078] (4) Some of the optical fibers in the optical fiber loop (the optical fibers of the chirped long-period fiber grating) are polarization-maintaining double-clad optical fibers, and the internally transmitted light beam is the optical fiber LP 0m higher-order mode, and its mode field area is large (on the order of 10 3 μm 2 ), which can effectively suppress nonlinear effects.

[0079] To facilitate understanding of the specific structure of the GHz pulse train generating device provided by the embodiments of the present application, the following will be combined with the attached Figure 2 figures for description. Among them, Figure 2 the solid arrows in the figures represent the signal light transmission direction, while the dashed arrows represent the pump light transmission direction.

[0080] In this application, a seed light source, a first acousto-optic modulator, a 50:50 coupler, an optical fiber loop, and a third acousto-optic modulator are provided. The optical fiber loop includes a second acousto-optic modulator, a first chirped long-period fiber grating, polarization-maintaining double-clad fiber, and a second chirped long-period fiber grating. In the GHz pulse train generating device, the seed light source is connected to the first acousto-optic modulator through an optical fiber, the first acousto-optic modulator is connected to the 50:50 coupler through an optical fiber, the 50:50 coupler and the third acousto-optic modulator are connected through an optical fiber, and the 50:50 coupler and the optical fiber loop are connected through an optical fiber;

[0081] The optical fiber loop includes a second acousto-optic modulator, a first chirped long-period fiber grating, polarization-maintaining double-clad fiber, a second chirped long-period fiber grating, and polarization-maintaining single-mode gain fiber;

[0082] The 50:50 coupler includes 4 ports. The first port is connected to the first acousto-optic modulator, the second port is connected to the second acousto-optic modulator in the optical fiber loop, the third port is connected to the third acousto-optic modulator, and the fourth port is connected to the polarization-maintaining single-mode gain fiber in the optical fiber loop;

[0083] The seed light source is used to output an initial signal optical pulse train;

[0084] The first acousto-optic modulator is used to perform intensity modulation on the initial signal optical pulse train and transmit the initial signal optical pulses required for the GHz pulse train in the initial signal optical pulse train to the 50:50 coupler one by one through the first port;

[0085] The 50:50 coupler is used to divide the initial signal optical pulse into two signal optical pulse trains, transmit the divided first signal optical pulse train to the optical fiber loop through the second port, and transmit the divided second signal optical pulse train to the third acousto-optic modulator through the third port. The number of signal optical pulses in the first signal optical pulse train is the same as that in the second signal optical pulse train;

[0086] The 50:50 coupler and the optical fiber loop act together to synthesize the initial signal optical pulse and the first signal optical pulse train received through the fourth port to increase the number of signal optical pulses in the divided first signal optical pulse train and the divided second signal optical pulse train;

[0087] The second acousto-optic modulator is used to transmit the first signal optical pulse train to the first chirped long-period fiber grating;

[0088] The first chirped long-period fiber grating is used to convert the energy of the first signal optical pulse train from the fiber fundamental mode to the fiber LP 0m higher-order mode;

[0089] The polarization-maintaining double-clad fiber is used to transmit the first signal optical pulse train in the fiber LP 0mThe high-order mode is conducted to the second chirped long-period fiber grating;

[0090] The second chirped long-period fiber grating is used to transfer the energy of the first signal optical pulse train from the fiber LP 0m high-order mode to the fiber fundamental mode and transmit the converted first signal optical pulse train to the polarization-maintaining single-mode gain fiber;

[0091] The polarization-maintaining single-mode gain fiber is used to transmit the converted first signal optical pulse train to the 50:50 coupler through the fourth port;

[0092] The third acousto-optic modulator is used to output the second signal optical pulse train when the number of signal optical pulses in the second signal optical pulse train is the same as the number of required signal optical pulses in the GHz pulse train, so as to obtain the GHz pulse train.

[0093] The above briefly introduces the main components of the GHz pulse train generating device. Among them, in this application, the first chirped long-period fiber grating and the second chirped long-period fiber grating are used to replace the chirped Bragg fiber grating in the mainstream traditional GHz pulse train generating device through the fiber LP 0m high-order mode to achieve dispersion compensation; and the fibers used for the first chirped long-period fiber grating and the second chirped long-period fiber grating are polarization-maintaining double-clad fibers.

[0094] Among them, the first chirped long-period fiber grating and the second chirped long-period fiber grating can compensate for the dispersion effect in the fiber loop through the fiber LP 0m high-order mode; the group delay ripple of the first chirped long-period fiber grating and the second chirped long-period fiber grating is less than the first threshold, which can effectively control the shape and width of the pulses in the first signal optical pulse train and the second signal optical pulse train in the time domain; at the same time, the energy conversion efficiency of the first chirped long-period fiber grating and the second chirped long-period fiber grating can be as high as 99%, thereby reducing the transmission loss in the fiber loop. Among them, the group delay ripple of the chirped long-period fiber grating in the embodiment of this application is in the sub-picosecond to several picosecond order of magnitude, and the specific value depends on the high-precision measurement technology. For example, the first threshold in the embodiment of this application can be 0.5 ps.

[0095] Moreover, the fibers used for the first chirped long-period fiber grating and the second chirped long-period fiber grating are polarization-maintaining double-clad fibers, and the light beam between the two is conducted in the fiber LP 0m high-order mode, and its mode field area is large (the order of magnitude reaches 10 3 μm 2 ), which can effectively suppress the nonlinear effect.

[0096] That is, by changing some components in the optical fiber loop as described above, the disadvantages of the prior art can be overcome, achieving the effects of reducing the transmission loss in the optical fiber loop, effectively controlling the shape and width of the pulses in the GHz pulse train in the time domain, and effectively suppressing the nonlinear effect.

[0097] It should be noted that since the transmission time T2 of the signal light pulse train in the optical fiber loop is slightly longer than the period T1 of the initial signal light pulse sequence, each time the first signal light pulse train in the optical fiber loop passes through the fourth port and is transmitted to the 50:50 coupler, the 50:50 coupler will further synthesize the first signal light pulse train with the initial signal light pulse, thereby continuously increasing the number of signal light pulses in the divided first signal light pulse train and the divided second signal light pulse train. During the entire pulse train synthesis process, the number of signal light pulses in the first signal light pulse train and the second signal light pulse train is always equal and continuously increasing (1, 2, 3,...). When the number of signal light pulses in the second signal light pulse train is consistent with the number of signal light pulses required in the GHz pulse train, the third acousto-optic modulator outputs the second signal light pulse train to obtain the GHz pulse train. That is, the pulse period in the first signal light pulse train is the transmission time of the first signal light pulse train in the optical fiber loop minus the period T of the initial signal light pulse sequence, i.e., T = T2 - T1, and the repetition frequency is f = 1 / T. And it is consistent with the period in the first signal light pulse train and the period in the pulses in the second signal light pulse train. Therefore, by controlling the length of the optical fiber loop, the transmission time T2 can be controlled, thereby adjusting the repetition frequency f in the pulse train.

[0098] Of course, the GHz pulse train generating device in the embodiments of the present application is not only the above components. As Figure 2 can be seen, the GHz pulse train generating device further includes: a semiconductor pump laser and a wavelength division multiplexer.

[0099] The semiconductor pump laser is used to transmit the energy of the pump light to the wavelength division multiplexer to provide the energy of the pump light to the polarization-maintaining single-mode gain fiber.

[0100] The wavelength division multiplexer is used to collect and transmit the pump light and the converted first signal light pulse train transmitted by the second chirped long-period fiber grating into the polarization-maintaining single-mode gain fiber.

[0101] The polarization-maintaining single-mode gain fiber is further used to convert the energy of the pump light into the energy of the converted first signal light pulse train to compensate for the transmission loss of the first signal light pulse train in the optical fiber loop.

[0102] In addition, the GHz pulse train generating device further includes: an isolator.

[0103] The isolator is used to block the reflected pump light.

[0104] It can be seen thatFigure 1 the mainstream traditional GHz pulse train generating device in Figure 2 differs from the GHz pulse train generating device in the embodiment of the present application in terms of the components for receiving pump light. Specifically, the mainstream traditional GHz pulse train generating device is provided with a pump laser protector because the reflectivity of the chirped Bragg fiber grating is low and a part of the signal light will pass through the grating. Therefore, in order to prevent the signal light from entering the pump laser, the pump laser protector is arranged between the grating and the semiconductor pump laser. The chirped Bragg fiber grating is ineffective for the pump light, so the pump light can be directly injected into the polarization-maintaining single-mode gain fiber through the chirped Bragg fiber grating. In the present application, the chirped Bragg fiber grating is replaced by a chirped long-period fiber grating. On the one hand, the signal light will not transmit towards the pump laser direction, so there is no need to use a pump laser protector. On the other hand, a wavelength division multiplexer is used to inject the pump light into the polarization-maintaining single-mode gain fiber.

[0105] In addition, in Figure 2 the GHz pulse train generating device in the embodiment of the present application in , the pigtails of some components (such as the pigtails of the seed light source, the first acousto-optic modulator, the 50:50 coupler, the second acousto-optic modulator, the wavelength division multiplexer, and the third acousto-optic modulator) are polarization-maintaining single-mode fibers. In order to enable the signal light pulse to be transmitted between the polarization-maintaining single-mode fiber and the polarization-maintaining double-clad fiber, the GHz pulse train generating device in the embodiment of the present application is also provided with fusion splices.

[0106] The fusion splices are arranged at the connection between the second acousto-optic modulator and the first chirped long-period fiber grating, and at the connection between the second chirped long-period fiber grating and the wavelength division multiplexer, for enabling the GHz pulse train to be transmitted between the polarization-maintaining single-mode fiber and the polarization-maintaining double-clad fiber in the fiber fundamental mode.

[0107] When the above device generates a GHz pulse train, the first acousto-optic modulator and the second acousto-optic modulator are both in the on state, and the third acousto-optic modulator is in the on state when outputting the GHz pulse train. The three perform the operations described above. When the first acousto-optic modulator, the second acousto-optic modulator, and the third acousto-optic modulator meet the closing conditions, the following corresponding operations will be performed:

[0108] When the first acousto-optic modulator meets the closing condition of the first acousto-optic modulator, it is used to block the transmission of the initial signal light pulse to the 50:50 coupler until the next GHz pulse train needs to be generated. The closing condition of the first acousto-optic modulator is that the initial signal light pulses required for the GHz pulse train have been transmitted to the 50:50 coupler one by one;

[0109] When the second acousto-optic modulator meets the second acousto-optic modulator off condition, it is used to block the transmission of the first signal optical pulse train to the first chirped long-period fiber grating until the next GHz pulse train needs to be generated. The second acousto-optic modulator off condition is that the number of signal optical pulses in the first signal optical pulse train is the same as the number of signal optical pulses required in the GHz pulse train;

[0110] When the third acousto-optic modulator meets the third acousto-optic modulator off condition, it is used to block the output of the second signal optical pulse train. The third acousto-optic modulator off condition is that the number of signal optical pulses in the second signal optical pulse train is different from the number of signal optical pulses required in the GHz pulse train.

[0111] To facilitate understanding of the specific implementation of the GHz pulse train generation method provided in the embodiments of the present application, the following will be combined with the attached Figure 3 for description.

[0112] It should be noted that the main body implementing the GHz pulse train generation method can be the GHz pulse train generation device provided in the embodiments of the present application. That is Figure 3 is a schematic flowchart of a GHz pulse train generation method provided in the embodiments of the present application. This method can also be applied to a GHz pulse train generation device, and the GHz pulse train generation device can be, for example, as Figure 2 shown in the GHz pulse train generation device.

[0113] As Figure 3 shown, the method includes the following steps S301 to S305:

[0114] S301: Output an initial signal optical pulse sequence through a seed light source, and transmit the initial signal optical pulse in the initial signal optical pulse sequence to a 50:50 coupler through a first port.

[0115] In order to generate GHz pulse trains, this application first outputs an initial signal optical pulse sequence through a seed light source, and transmits the initial signal optical pulse in the initial signal optical pulse sequence to a 50:50 coupler through a first port; divides the initial signal optical pulse into two signal optical pulse trains through the 50:50 coupler, transmits the first divided signal optical pulse train to an optical fiber loop through a second port, and transmits the second divided signal optical pulse train to a third acousto-optic modulator through a third port, where the number of signal optical pulses in the first signal optical pulse train is the same as that in the second signal optical pulse train; synthesizes the initial signal optical pulse and the first signal optical pulse train received through a fourth port through the 50:50 coupler and the optical fiber loop to increase the number of signal optical pulses in the first divided signal optical pulse train and the second divided signal optical pulse train; converts the energy of the first signal optical pulse train through the optical fiber loop and transmits the converted first signal optical pulse train to the 50:50 coupler, where the energy conversion is achieved through a first chirped long-period fiber grating, a polarization-maintaining double-clad fiber, and the second chirped long-period fiber grating, and the signal optical pulses between the first chirped long-period fiber grating and the second chirped long-period fiber grating are transmitted by the polarization-maintaining double-clad fiber; finally, when the number of signal optical pulses in the second signal optical pulse train is consistent with the number of signal optical pulses required in the GHz pulse train, output the second signal optical pulse train through the third acousto-optic modulator to obtain the GHz pulse train.

[0116] As an example, S301 may include: outputting an initial signal optical pulse sequence through a seed light source and transmitting the initial signal optical pulse sequence to a first acousto-optic modulator; and intensity-modulating the initial signal optical pulse sequence through the first acousto-optic modulator and transmitting the initial signal optical pulses required for the GHz pulse train one by one to the 50:50 coupler through the first port.

[0117] For the initial signal optical pulse sequence output by the above-mentioned seed light source, the order of magnitude of the repetition frequency can range from 10 MHz to 100 MHz, and the output pigtail fiber of the seed light source is a polarization-maintaining single-mode fiber.

[0118] When generating the GHz pulse train, the first acousto-optic modulator acts as an optical switch and is in the on state. It is necessary to set the transmittance of the first acousto-optic modulator to the maximum value. When an appropriate number of initial signal optical pulses are gated according to the number of pulses in the required pulse train in the GHz pulse train and meet the closing condition of the first acousto-optic modulator, that is, the required number of initial signal optical pulses have been transmitted to the 50:50 coupler through the first port, then set the transmittance of the first acousto-optic modulator to 0 to block the signal optical pulses until the next pulse train needs to be generated. For example, if the number of pulses in the required pulse train is 4, then after gating 4 initial signal optical pulses, block the signal optical pulses.

[0119] In this process, first, using a seed light source and a first acousto-optic modulator, the required number of initial signal light pulses can be transmitted to a 50:50 coupler through a first port, so that subsequent GHz pulse trains can be generated by the 50:50 coupler and the optical fiber loop.

[0120] S302: Divide the initial signal light pulses into two parts of signal light pulse trains through a 50:50 coupler, transmit the first signal light pulse train after division to the optical fiber loop through a second port, and transmit the second signal light pulse train after division to a third acousto-optic modulator through a third port. The number of signal light pulses in the first signal light pulse train is the same as that in the second signal light pulse train.

[0121] As an example, S302 may include: dividing an appropriate number of initial signal light pulses transmitted by the first acousto-optic modulator into two parts with equal power. The signal light pulse train (the first signal light pulse train) in one part is transmitted into the optical fiber loop containing devices such as a second acousto-optic modulator, a first chirped long-period fiber grating, a second chirped long-period fiber grating, and a wavelength division multiplexer one by one through the second port, and the signal light pulse train (the second signal light pulse train) in the other part is transmitted to the third acousto-optic modulator one by one through the third port.

[0122] In this process, the 50:50 coupler divides the initial signal light pulses into two outputs with equal amplitude, and the pigtails of the four ports of the 50:50 coupler are all polarization-maintaining single-mode optical fibers.

[0123] S303: Synthesize the initial signal light pulses and the first signal light pulse train received through a fourth port through the 50:50 coupler and the optical fiber loop to increase the number of signal light pulses in the first signal light pulse train and the second signal light pulse train after division.

[0124] Since the transmission time of the signal light pulse train in the optical fiber loop is slightly longer than the period of the initial signal light pulse sequence, each time the first signal light pulse train in the optical fiber loop is transmitted to the 50:50 coupler through the fourth port, the 50:50 coupler will further synthesize the first signal light pulse train and the initial signal light pulses, thereby continuously increasing the number of signal light pulses in the first signal light pulse train and the second signal light pulse train after division. So that the number of signal light pulses in the first signal light pulse train and the second signal light pulse train after division can be increased to the number of signal light pulses required for the GHz pulse train, thereby generating the GHz pulse train.

[0125] S304: Convert the energy of the first signal optical pulse train through an optical fiber loop, and transmit the converted first signal optical pulse train to a 50:50 coupler through the fourth port. The energy conversion is achieved through a first chirped long-period fiber grating, a polarization-maintaining double-clad fiber, and a second chirped long-period fiber grating. The signal optical pulses between the first chirped long-period fiber grating and the second chirped long-period fiber grating are transmitted by the polarization-maintaining double-clad fiber.

[0126] As an example, S304 may include: S3041, transmitting the first signal optical pulse train to the first chirped long-period fiber grating through a second acousto-optic modulator; S3042, converting the energy of the first signal optical pulse train from the fiber fundamental mode to the fiber LP 0m higher-order mode through the first chirped long-period fiber grating; S3043, conducting the first signal optical pulse train in the fiber LP 0m higher-order mode to the second chirped long-period fiber grating through the polarization-maintaining double-clad fiber; S3044, converting the energy of the first signal optical pulse train from the fiber LP 0m higher-order mode to the fiber fundamental mode through the second chirped long-period fiber grating, and transmitting the converted first signal optical pulse train to the 50:50 coupler through the fourth port.

[0127] When the second acousto-optic modulator in S3041 generates a GHz pulse train, the second acousto-optic modulator acts as an optical switch and is in the on state. It is necessary to set the transmittance of the second acousto-optic modulator to the maximum value to gate a series of GHz pulse trains with an increasing number of pulses (1, 2, 3,...) in the optical fiber loop. Subsequently, when the number of signal optical pulses in the first signal optical pulse train is consistent with the number of required signal optical pulses in the GHz pulse train, the second acousto-optic modulator closing condition is satisfied, and it is necessary to set the transmittance of the second acousto-optic modulator to 0 to block the transmission of the first signal optical pulse train to the first chirped long-period fiber grating until the next pulse train needs to be generated.

[0128] The first chirped long-period fiber grating in S3042 converts the energy of the first signal optical pulse train from the fiber fundamental mode to the fiber LP 0m higher-order mode, and the conversion efficiency is greater than 99%, that is, the first chirped long-period fiber grating can reduce the transmission loss in the optical fiber loop. At the same time, the first chirped long-period fiber grating can produce a dispersion effect. The dispersion sign (normal dispersion or anomalous dispersion) and magnitude are closely related to the chirp function, and the group delay ripple of the first chirped long-period fiber grating is very low or even non-existent, which can effectively control the shape and width of the pulses in the first signal optical pulse train in the time domain. The fiber used for the first chirped long-period fiber grating is a polarization-maintaining double-clad fiber.

[0129] The polarization-maintaining double-clad fiber in S3043 includes a core, an inner cladding, and an outer cladding. The core is a single-mode waveguide that can only conduct the fundamental mode, while the multimode waveguide formed by the core and the inner cladding can stably conduct LP 0m high-order modes, and the order of magnitude of the mode field area reaches 10 3 μm 2 , which can effectively suppress the nonlinear effect, and the LP 0m high-order modes of the optical fiber have flexibly designable waveguide dispersion, so that different magnitudes of normal dispersion or anomalous dispersion can be formed.

[0130] It should be noted that since the optical fiber used in the first chirped long-period fiber grating and the second chirped long-period fiber grating is a polarization-maintaining double-clad fiber, while the remaining components in the GHz pulse train generating device use polarization-maintaining single-mode fibers. For this reason, it is necessary to set the chirped long-period fiber grating to be connected to other components through a fusion point, so that the signal light pulses in the first signal light pulse train are transmitted between the polarization-maintaining single-mode fiber and the polarization-maintaining double-clad fiber with almost no loss in the fundamental mode.

[0131] In S3044, the second chirped long-period fiber grating converts the energy of the first signal light pulse train from the LP 0m high-order mode to the fundamental mode of the optical fiber, and the conversion efficiency is greater than 99%, that is, the second chirped long-period fiber grating can reduce the transmission loss in the optical fiber loop. At the same time, the second chirped long-period fiber grating can generate a dispersion effect, and the dispersion sign (normal dispersion or anomalous dispersion) and magnitude are closely related to the chirp function, and the group delay ripple of the second chirped long-period fiber grating is very low or even non-existent, which can effectively control the shape and width of the pulses in the first signal light pulse train in the time domain. The optical fiber used in the second chirped long-period fiber grating is a polarization-maintaining double-clad fiber.

[0132] The embodiment of the present application further includes converting the energy of the pump light into the energy of the signal light pulses to compensate for the transmission loss of the signal light pulses in the optical fiber loop. Therefore, a semiconductor pump laser is required to provide pump energy to the polarization-maintaining single-mode gain fiber. Specifically, first, the semiconductor pump laser needs to send pump light to the wavelength division multiplexer, and the wavelength division multiplexer transmits the pump light and the first signal light pulse train into the polarization-maintaining single-mode gain fiber, so that after the polarization-maintaining single-mode gain fiber absorbs the pump light, it stores the energy of the pump light in the polarization-maintaining single-mode gain fiber, and converts the stored energy of the pump light into the energy of the signal light pulses, thereby compensating for the transmission loss of the signal light pulses in the optical fiber loop. By adjusting the pump light power, the envelope shape of the GHz pulse train can also be controlled to achieve three shapes: flat, increasing, and decreasing. An isolator is also included between the semiconductor pump laser and the polarization-maintaining single-mode gain fiber to block the reflected pump light and prevent damage to the semiconductor pump laser.

[0133] In this process, by using the first chirped long-period fiber grating, polarization-maintaining double-clad fiber, and the second chirped long-period fiber grating, the transmission loss in the fiber loop can be reduced, the shape and width of the pulses within the GHz pulse train in the time domain can be effectively controlled, and the nonlinear effects can be effectively suppressed. The specific reasons are as explained above and will not be elaborated here.

[0134] In this application, in order to generate a GHz pulse train containing multiple signal optical pulses, it is necessary to increase the number of signal optical pulses in the signal optical pulse train. Since the transmission time of the first signal optical pulse train in the fiber loop is greater than the period of the initial signal optical pulse sequence, the first signal optical pulse train after being transformed in the fiber loop will still be combined with the initial signal optical pulses subsequently transmitted to the 50:50 coupler. For example, if there is only 1 signal optical pulse in the initial first signal optical pulse train, after passing through the fiber loop and being transmitted to the 50:50 coupler, it can be combined into 1 initial signal optical pulse and 1 signal optical pulse. Subsequently, after splitting into two paths, the first signal optical pulse train and the second signal optical pulse train obtained respectively contain 2 signal optical pulses. Repeating this process, finally, the first signal optical pulse train and the second signal optical pulse train are respectively GHz pulse trains containing multiple signal optical pulses.

[0135] S305: When the number of signal optical pulses in the second signal optical pulse train is the same as the number of signal optical pulses required in the GHz pulse train, the second signal optical pulse train is output through the third acousto-optic modulator to obtain the GHz pulse train.

[0136] When the above-mentioned third acousto-optic modulator is used as an optical switch to generate a GHz pulse train, it is in the on state. That is, when the number of signal optical pulses in the second signal optical pulse train is the same as the number of signal optical pulses required in the GHz pulse train, it means that the second signal optical pulse train can be output. Therefore, the transmittance of the third acousto-optic modulator can be set to be non-zero to output the second signal optical pulse train to obtain the GHz pulse train and the envelope shape can be edited. When the number of signal optical pulses in the second signal optical pulse train is not the same as the number of signal optical pulses required in the GHz pulse train, meeting the closing condition of the third acousto-optic modulator, the transmittance of the third acousto-optic modulator needs to be 0 to block the signal optical pulses until the next pulse train that meets the requirements appears.

[0137] In this process, the output of the GHz pulse train is controlled by using the third acousto-optic modulator. That is, when the number of signal optical pulses in the GHz pulse train meets the required number, the third acousto-optic modulator can be controlled to be in the on state to output the GHz pulse train.

[0138] It can be seen that in the embodiments of this application, by using chirped long-period fiber gratings and fiber LP 0mThe high-order mode replaces the chirped Bragg fiber grating, which can apply chirp to the signal optical pulse to compensate for the dispersion effect in the fiber loop; at the same time, the chirped long-period fiber grating has a high conversion rate, which can reduce the transmission loss in the fiber loop; and the group delay ripple of the chirped long-period fiber grating is very low or even non-existent, while the fiber high-order mode has no group delay ripple at all. Therefore, it is beneficial to control the shape and width of the pulses in the GHz pulse train in the time domain; finally, part of the fiber in the fiber loop (the fiber of the chirped long-period fiber grating) is polarization-maintaining double-clad fiber, and the internally transmitted light beam is the fiber LP 0m high-order mode, which has a large mode field area (the order of magnitude reaches 10 3 μm 2 ), and can effectively suppress the nonlinear effect.

[0139] To make the method provided in the embodiments of the present application clearer and easier to understand, the following combines Figure 4 to illustrate a specific example of applying this method to Figure 2 the GHz pulse train generating device shown.

[0140] As Figure 4 shown, this embodiment may include the following steps S401 - S411:

[0141] S401: Use a polarization-maintaining single-mode fiber through a seed light source to output an initial signal optical pulse sequence to the acousto-optic modulator 1.

[0142] In the embodiments of the present application, the repetition frequency of the initial signal optical pulse sequence is on the order of 10 MHz to 100 MHz, and the output pigtail of the seed light source is a polarization-maintaining single-mode fiber. The acousto-optic modulator 1 may be, for example, the first acousto-optic modulator in the above embodiments.

[0143] S402: Use the acousto-optic modulator 1 to transmit the initial signal optical pulses required for the GHz pulse train in the initial signal optical pulse sequence one by one to the 50:50 coupler through a polarization-maintaining single-mode fiber via the first port.

[0144] During the generation of the GHz pulse train, the acousto-optic modulator 1 will set the transmittance to the maximum value, and select the initial signal optical pulses of the required number of pulses according to the number of pulses required for the GHz pulse train to the 50:50 coupler. Then the transmittance will be set to 0 to block the signal optical pulses until the next pulse train needs to be generated. The pigtails at both ends of the acousto-optic modulator 1 are polarization-maintaining single-mode fibers.

[0145] S403: Divide the initial signal optical pulse into two parts with equal power through the 50:50 coupler to obtain the first signal optical pulse train after division and the second signal optical pulse train after division.

[0146] S404: Use a polarization-maintaining single-mode fiber through a 50:50 coupler to transmit the first signal optical pulse train to the acousto-optic modulator 2 in the optical fiber loop through the second port.

[0147] The acousto-optic modulator 2 can be, for example, the second acousto-optic modulator in the above embodiment.

[0148] S405: Use the acousto-optic modulator 2 to transmit the first signal optical pulse train to the chirped long-period fiber grating 1 through a polarization-maintaining single-mode fiber.

[0149] During the generation of the GHz pulse train, the acousto-optic modulator 2 will set the transmittance to the maximum value and select a series of first signal optical pulse trains with an increasing number of pulses in the optical fiber loop in sequence. That is, the pulse train in the optical fiber loop can be further combined with the initial signal optical pulse transmitted by the acousto-optic modulator 1 to form a pulse train with a larger number of pulses. The chirped long-period fiber grating 1 can be, for example, the first chirped long-period fiber grating in the above embodiment.

[0150] When the number of signal optical pulses in the first signal optical pulse train is the same as the number of signal optical pulses required in the GHz pulse train, the transmittance of the acousto-optic modulator 2 will be set to 0 to block the first signal optical pulse train until the next pulse train needs to be generated. The pigtail fibers at both ends of the acousto-optic modulator 2 are both polarization-maintaining single-mode fibers.

[0151] S406: Use the chirped long-period fiber grating 1 to convert the energy of the first signal optical pulse train from the fundamental mode to the fiber LP 0m higher-order mode.

[0152] In this application, the conversion rate of the chirped long-period fiber grating 1 is greater than 99%, and chirp is applied to the signal optical pulse. The pigtail fibers at both ends of the chirped long-period fiber grating 1 are both polarization-maintaining double-clad fibers. Therefore, a fusion point is required between the chirped long-period fiber grating 1 and the acousto-optic modulator 2 to enable the signal optical pulse to be transmitted in the fundamental mode between the polarization-maintaining single-mode fiber and the polarization-maintaining double-clad fiber with almost no loss.

[0153] S407: Use the polarization-maintaining double-clad fiber to conduct the first signal optical pulse train in the fiber LP 0m higher-order mode to the chirped long-period fiber grating 2.

[0154] In the polarization-maintaining double-clad fiber used in this application, the mode field area of the LP 0m higher-order mode is large, which can effectively suppress the nonlinear effect. The chirped long-period fiber grating 2 can be, for example, the second chirped long-period fiber grating in the above embodiment.

[0155] S408: Use the chirped long-period fiber grating 2 to convert the energy of the first signal optical pulse train from the fiber LP 0mThe high-order mode is converted to the fundamental mode of the optical fiber, and the converted first signal optical pulse train is transmitted to the wavelength division multiplexer.

[0156] In this application, the conversion rate of the chirped long-period fiber grating 2 is greater than 99%, and chirping is applied to the signal optical pulse. Moreover, the pigtails at both ends of the chirped long-period fiber grating 2 are polarization-maintaining double-clad optical fibers. Therefore, a fusion joint is required between the chirped long-period fiber grating 2 and the wavelength division multiplexer, so that the signal optical pulse is transmitted in the fundamental mode between the polarization-maintaining single-mode optical fiber and the polarization-maintaining double-clad optical fiber with almost no loss.

[0157] S409: Receive the pump light transmitted by the semiconductor pump laser through the wavelength division multiplexer, and transmit the pump light and the converted first signal optical pulse train into the polarization-maintaining single-mode gain fiber.

[0158] In this application, the semiconductor pump laser acts as a pump source to provide pump energy for the polarization-maintaining single-mode gain fiber, and isolators are provided in the semiconductor pump laser and the wavelength division multiplexer to block the reflected pump light, thereby preventing damage to the semiconductor pump laser.

[0159] S410: Convert the energy of the pump light into the energy of the converted first signal optical pulse train through the polarization-maintaining single-mode gain fiber, and transmit the converted first signal optical pulse train to the 50:50 coupler through the fourth port.

[0160] S411: Synthesize the initial signal optical pulse and the first signal optical pulse train received through the fourth port through the 50:50 coupler and the optical fiber loop to increase the number of signal optical pulses in the divided first signal optical pulse train and the divided second signal optical pulse train.

[0161] In order to obtain the first signal optical pulse train and the second signal optical pulse train with an increasing number of signal optical pulses, in this application, the initial signal optical pulse and the first signal optical pulse train in the optical fiber loop are synthesized through the 50:50 coupler and the optical fiber loop, thereby increasing the number of signal optical pulses in the divided first signal optical pulse train and the divided second signal optical pulse train.

[0162] S412: Transmit the second signal optical pulse train to the acousto-optic modulator 3 through the third port by using the polarization-maintaining single-mode optical fiber through the 50:50 coupler.

[0163] The acousto-optic modulator 3 can be, for example, the third acousto-optic modulator in the above embodiment.

[0164] S413: When the number of signal optical pulses in the second signal optical pulse train is consistent with the number of signal optical pulses required in the GHz pulse train, output the second signal optical pulse train through the acousto-optic modulator 3 to obtain the GHz pulse train.

[0165] During the process of generating a GHz pulse train, the acousto-optic modulator 3 receives a second signal optical pulse train with an increasing number of pulses transmitted from a 50:50 coupler. When the number of signal optical pulses in the second signal optical pulse train is the same as the number of signal optical pulses required in the GHz pulse train, the transmittance of the acousto-optic modulator 3 is not 0 and can vary with time. Therefore, the pulse train can be gated and the envelope shape can be edited. At other times, the transmittance of the acousto-optic modulator 3 is set to 0 to block the signal optical pulses until the next pulse train that meets the requirements appears. The acousto-optic modulator 3 can be, for example, the third acousto-optic modulator in the above embodiment.

[0166] This embodiment provides a method for generating a GHz pulse train. By using a chirped long-period fiber grating and a fiber LP 0m high-order mode to replace the chirped Bragg fiber grating, chirp can be applied to the signal optical pulses to compensate for the dispersion effect in the fiber loop. At the same time, the chirped long-period fiber grating has a high conversion rate, which can reduce the transmission loss in the fiber loop. And the group delay ripple of the chirped long-period fiber grating is very low or even non-existent, while the fiber high-order mode has no group delay ripple at all. Therefore, it is beneficial to control the shape and width of the pulses in the GHz pulse train in the time domain. Finally, the fiber of the chirped long-period fiber grating is a polarization-maintaining double-clad fiber, and the internally transmitted beam is a fiber LP 0m high-order mode, and its mode field area is large, which can effectively suppress the nonlinear effect.

[0167] From the description of the above embodiments, those skilled in the art can clearly understand that all or part of the steps in the above embodiment methods can be implemented by means of software plus a general hardware platform. Based on such an understanding, the technical solution of the present application can be embodied in the form of a software product, which can be stored in a storage medium, such as read-only memory (ROM) / RAM, magnetic disk, optical disk, etc., including several instructions for causing a computer device (which can be a personal computer, a server, or a network communication device such as a router) to execute the methods described in each embodiment or some parts of the embodiments of the present application.

[0168] Each embodiment in this specification is described in a progressive manner. For the same or similar parts among the embodiments, reference can be made to each other. Each embodiment focuses on the differences from other embodiments. In particular, for the apparatus embodiments, since they are basically similar to the method embodiments, the description is relatively simple. For the relevant parts, reference can be made to the description in the method embodiments. The apparatus embodiments described above are merely illustrative. The modules described as separate components may or may not be physically separated, and the components shown as modules may or may not be physical modules, that is, they may be located in one place or distributed to multiple network units. Some or all of the modules can be selected according to actual needs to achieve the objectives of the solution in this embodiment. A person of ordinary skill in the art can understand and implement it without creative work.

[0169] The above description is only an exemplary embodiment of the present application and is not intended to limit the protection scope of the present application.

Claims

1. A GHz pulse train generating device, characterized in that: The device comprises a seed light source, a first acousto-optic modulator, a 50:50 coupler, an optical fiber loop and a third acousto-optic modulator, wherein the seed light source and the first acousto-optic modulator are connected via an optical fiber, the first acousto-optic modulator and the 50:50 coupler are connected via an optical fiber, the 50:50 coupler and the third acousto-optic modulator are connected via an optical fiber, and the 50:50 coupler and the optical fiber loop are connected via an optical fiber; The optical fiber loop includes a second acousto-optic modulator, a first chirped long-period fiber grating, a polarization-maintaining double-clad optical fiber, a second chirped long-period fiber grating and a polarization-maintaining single-mode gain optical fiber; The 50:50 coupler comprises four ports, wherein the first port is connected to the first acousto-optic modulator, the second port is connected to the second acousto-optic modulator in the optical fiber loop, the third port is connected to the third acousto-optic modulator, and the fourth port is connected to the polarization-maintaining single-mode gain optical fiber in the optical fiber loop; The seed light source is used to output an initial signal light pulse sequence; The first acousto-optic modulator is used to modulate the intensity of the initial signal light pulse sequence, and transmit the initial signal light pulses required for the GHz pulse train in the initial signal light pulse sequence to the 50:50 coupler through the first port one by one; The 50:50 coupler is used to split the initial signal light pulse into two signal light pulse trains, and transmit the first signal light pulse train after splitting into the optical fiber loop through the second port, and transmit the second signal light pulse train after splitting into the third acousto-optic modulator through the third port, wherein the number of signal light pulses in the first signal light pulse train and the second signal light pulse train is the same; The 50:50 coupler works together with the optical fiber loop to synthesize the initial signal light pulse with the first signal light pulse string received through the fourth port to increase the number of signal light pulses in the first signal light pulse string and the second signal light pulse string after being divided; The second acousto-optic modulator is used to transmit the first signal light pulse train to the first chirped long-period fiber grating; The first chirped long-period fiber grating is used to convert the energy of the first signal light pulse train from the fiber fundamental mode to the fiber LP 0m Advanced mode; The polarization-maintaining double-clad optical fiber is used to transmit the first signal light pulse train to an optical fiber LP 0m The high-order mode is guided to the second chirped long-period fiber grating; The second chirped long-period fiber grating is used to transfer the energy of the first signal light pulse train from the optical fiber LP 0m The high-order mode is converted into the optical fiber fundamental mode, and the converted first signal light pulse train is transmitted to the polarization-maintaining single-mode gain optical fiber; The polarization-maintaining single-mode gain optical fiber is used to transmit the converted first signal optical pulse train to the 50:50 coupler through the fourth port; The third acousto-optic modulator is used to output the second signal light pulse train when the number of signal light pulses in the second signal light pulse train is consistent with the number of signal light pulses required in the GHz pulse train, so as to obtain the GHz pulse train; The first chirped long-period fiber grating and the second chirped long-period fiber grating are connected via an optical fiber LP 0m The high-order mode compensates for the dispersion effect in the optical fiber loop; and the group delay ripple of the first chirped long-period fiber grating and the second chirped long-period fiber grating is less than a first threshold, which is conducive to controlling the shape and width of the pulses in the first signal light pulse train and the second signal light pulse train in the time domain.

2. The GHz pulse train generating device according to claim 1, characterized in that: The GHz pulse train generating device further comprises: a wavelength division multiplexer and a semiconductor pump laser, The semiconductor pump laser is used to transmit the energy of the pump light to the wavelength division multiplexer to provide the energy of the pump light to the polarization-maintaining single-mode gain optical fiber; The wavelength division multiplexer is used to collect and transmit the converted first signal light pulse train transmitted by the pump light and the second chirped long-period fiber grating into the polarization-maintaining single-mode gain optical fiber; The polarization-maintaining single-mode gain optical fiber is also used to convert the energy of the pump light into the energy of the converted first signal light pulse string to compensate for the transmission loss of the first signal light pulse string in the optical fiber loop.

3. The GHz pulse train generating device according to claim 2, characterized in that: The GHz pulse train generating device further comprises: an isolator, The isolator is used to block the reflected pump light.

4. The GHz pulse train generating device according to claim 2, characterized in that: The GHz pulse train generating device further comprises: a fusion point, The fusion point is arranged at the connection between the second acousto-optic modulator and the first chirped long-period fiber grating, and at the connection between the second chirped long-period fiber grating and the wavelength division multiplexer, so as to enable the first signal light pulse train to be transmitted in the fiber fundamental mode between the polarization-maintaining single-mode fiber and the polarization-maintaining double-clad fiber.

5. The GHz pulse train generating device according to claim 2, characterized in that: The pigtails of the seed light source, the first acousto-optic modulator, the 50:50 coupler, the second acousto-optic modulator, the wavelength division multiplexer and the third acousto-optic modulator are all polarization-maintaining single-mode optical fibers.

6. The GHz pulse train generating device according to claim 1, characterized in that: The pulse period in the first signal light pulse train is the transmission time of the first signal light pulse train in the optical fiber loop minus the period of the initial signal light pulse sequence, and the period in the first signal light pulse train is consistent with the period in the second signal light pulse train.

7. The GHz pulse train generating device according to claim 1, characterized in that: When the first acousto-optic modulator meets the first acousto-optic modulator closing condition, the first acousto-optic modulator is used to block the transmission of the initial signal light pulse to the 50:50 coupler until the next GHz pulse train is required to be generated, and the first acousto-optic modulator closing condition is that the initial signal light pulses required for the GHz pulse train have been transmitted to the 50:50 coupler one by one; When the second acousto-optic modulator meets the second acousto-optic modulator closing condition, the second acousto-optic modulator is used to block the transmission of the first signal light pulse train to the first chirped long-period fiber grating until the next GHz pulse train is required to be generated, and the second acousto-optic modulator closing condition is that the number of signal light pulses in the first signal light pulse train is consistent with the number of signal light pulses required in the GHz pulse train; The third acousto-optic modulator is used to block the output of the second signal light pulse train when the third acousto-optic modulator closing condition is met, and the third acousto-optic modulator closing condition is that the number of signal light pulses in the second signal light pulse train is inconsistent with the number of signal light pulses required in the GHz pulse train.

8. A method for generating a GHz pulse train, characterized in that: The GHz pulse train generating device according to any one of claims 1 to 7, wherein the method comprises: Outputting an initial signal light pulse sequence through the seed light source, and transmitting an initial signal light pulse in the initial signal light pulse sequence to the 50:50 coupler through the first port; Splitting the initial signal light pulse into two signal light pulse trains through the 50:50 coupler, transmitting the first signal light pulse train after splitting into two signal light pulse trains through the second port to the optical fiber loop, and transmitting the second signal light pulse train after splitting into two signal light pulse trains through the third port to the third acousto-optic modulator, wherein the first signal light pulse train and the second signal light pulse train have the same number of signal light pulses; synthesizing the initial signal light pulse with the first signal light pulse string received through the fourth port through the 50:50 coupler and the optical fiber loop to increase the number of signal light pulses in the first signal light pulse string and the second signal light pulse string; The first signal light pulse train is subjected to energy conversion through the optical fiber loop, and the converted first signal light pulse train is transmitted to the 50:50 coupler through the fourth port, wherein the energy conversion is realized by the first chirped long period fiber grating, the polarization-maintaining double-clad optical fiber and the second chirped long period fiber grating, and the signal light pulse between the first chirped long period fiber grating and the second chirped long period fiber grating is transmitted by the polarization-maintaining double-clad optical fiber; When the number of signal light pulses in the second signal light pulse train is consistent with the number of signal light pulses required in the GHz pulse train, the second signal light pulse train is output through the third acousto-optic modulator to obtain the GHz pulse train.

9. The method according to claim 8, characterized in that The method of converting the energy of the first signal optical pulse train through the optical fiber loop and transmitting the converted first signal optical pulse train to the 50:50 coupler through the fourth port includes: transmitting the first signal light pulse train to the first chirped long-period fiber grating through the second acousto-optic modulator; The energy of the first signal light pulse train is converted from the fiber fundamental mode to the fiber LP mode by the first chirped long-period fiber grating. 0m Advanced mode; The first signal light pulse train is transmitted to the optical fiber LP through the polarization-maintaining double-clad optical fiber 0m The high-order mode is guided to the second chirped long-period fiber grating; The energy of the first signal light pulse train is transferred from the optical fiber LP to the optical fiber LP by the second chirped long period fiber grating. 0m The high-order mode is converted into the optical fiber fundamental mode, and the converted first signal optical pulse train is transmitted to the 50:50 coupler through the fourth port.

10. The method according to claim 8, characterized in that The step of outputting an initial signal light pulse sequence through the seed light source and transmitting an initial signal light pulse in the initial signal light pulse sequence to the 50:50 coupler through the first port comprises: Outputting the initial signal light pulse sequence through the seed light source, and transmitting the initial signal light pulse sequence to the first acousto-optic modulator; The intensity of the initial signal light pulse sequence is modulated by the first acousto-optic modulator, and the initial signal light pulses of the number required for the GHz pulse train are transmitted one by one to the 50:50 coupler through the first port.

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