CN113131321B - Low-threshold self-starting full-polarization-maintaining femtosecond fiber laser - Google Patents

Abstract

A low-threshold self-starting full-polarization-maintaining femtosecond fiber laser belongs to the technical field of lasers and aims to solve the problems that an existing full-polarization-maintaining femtosecond fiber laser is high in threshold and difficult to self-start, a first pumping source and a second pumping source are respectively connected with an a end of a first wavelength division multiplexer and a d end of a second wavelength division multiplexer and are injected into polarization-maintaining erbium-doped fibers; the c end of a signal end of the first wavelength division multiplexer is connected with a first polarization-preserving single-mode fiber, the other end of the first polarization-preserving single-mode fiber is connected with the input end of a polarization-preserving phase shifter, the output end of the polarization-preserving phase shifter is connected with the g end of a 2 x 2 coupler, the h end of the 2 x 2 coupler is connected with a polarization-related isolator, and the output end of the polarization-related isolator is connected with the k end of the coupler; the coupler is provided with two output ports, wherein the m end serves as a laser output end, the l end is connected with a second polarization-maintaining single-mode fiber, the other end of the second polarization-maintaining single-mode fiber is connected with the j end of the 2 x 2 coupler, and the second input end of the 2 x 2 coupler is connected with the f end of the second wavelength division multiplexer.

Description

Low-threshold self-starting fully polarization-maintaining femtosecond fiber laser

technical field

The invention relates to a low-threshold self-starting full polarization-maintaining femtosecond fiber laser, which belongs to the technical field of lasers.

Background technique

Femtosecond fiber lasers can generate ultrashort laser pulses with a pulse time domain width of the order of femtoseconds (10 ^-15 ) in a fiber resonator, which is of great importance in the fields of medical treatment, communication, material processing, ranging, and optical frequency combs. application. The generation of femtosecond laser is realized by the method of mode locking. The traditional mode locking fiber laser uses conventional quartz fiber as the resonant cavity, and its working state is easily affected by the external environment such as temperature and vibration. The mode-locked laser based on the all-polarization-maintaining fiber structure has little effect on the femtosecond pulse due to changes in the external environment, so the femtosecond laser produced has good stability and is suitable for practical applications under complex conditions, and has a compact structure, high efficiency and low cost. , suitable for mass production.

At present, there are two main methods for generating femtosecond laser in an all polarization-maintaining fiber resonator structure. Second pulse; the second is to use a nine-subcavity structure based on a linear phase shifter to generate conventional soliton pulses. Among them, the first method requires a longer cavity length to increase the nonlinear phase shift, and the larger transmission loss results in the structure generally having a higher mode-locking threshold. The second method reduces the mode-locking threshold by introducing a linear phase shift in the amplifier loop, but the output mode-locking pulse is of a single type, usually only a conventional soliton pulse.

The Chinese Patent Publication No. "CN103825177B" discloses "a pulsed fully polarization-maintaining fiber laser based on multiple nonlinear amplifying ring mirrors". or multiple nonlinear amplifying annular mirror cavities; the light field in the laser propagates clockwise, and sequentially passes through the main oscillator cavity, the front-stage nonlinear amplifying annular mirror cavity, one or more nonlinear amplifying annular mirror cavities in the subsequent stage, and the main oscillator The lower end of the beam splitter in the main oscillator cavity is used as the output end of the laser.

By adopting a cascade structure of multiple (two or more) nonlinear amplifying ring mirrors, the technology effectively suppresses amplitude noise, weakens pulse splitting, eliminates small pulses before and after pulses, and narrows pulses. However, when multiple nonlinear ring mirrors optimize the pulse output characteristics, additional transmission loss will be introduced, which will lead to an increase in the mode-locking threshold and a decrease in the conversion efficiency. In addition, the complex resonator structure is not conducive to the integration and miniaturization of the laser.

The Chinese Patent Publication No. "CN110896191A" discloses "a pulsed total polarization maintaining fiber laser based on multiple nonlinear amplifying ring mirrors", which is characterized in that the pigtail of the first semiconductor laser is complexed with the first polarization maintaining wavelength division complex. connected with the pump end of the device; the output end of the first polarization-maintaining wavelength division multiplexer is connected with the input end of the first polarization-maintaining single-mode fiber, and the output end of the first polarization-maintaining single-mode fiber is isolated from the first polarization-maintaining fiber The output end of the first polarization maintaining fiber isolator 4 is connected with the first input end of the second polarization maintaining fiber coupler. The input end of the first polarization-maintaining dispersion compensation fiber is connected with the first output end of the second polarization-maintaining fiber coupler, and the output end is connected with the input end of the first polarization-maintaining fiber coupler; The output end is connected with the second input end of the first polarization-maintaining wavelength division multiplexer to form a closed ring cavity.

This technology uses traditional nonlinear fiber ring mirrors to achieve mode locking, enabling the laser to generate high pulse energy lasers under low repetition frequency operation, and to achieve stable self-starting mode locking. However, the traditional nonlinear fiber ring mirror structure requires sufficient cavity length accumulation to achieve self-starting, which greatly increases the mode-locking threshold and reduces the repetition frequency of the mode-locking pulse.

SUMMARY OF THE INVENTION

In order to solve the problems of high threshold and difficult self-starting of the existing full polarization-maintaining femtosecond fiber laser, the invention proposes a low-threshold self-starting full polarization-maintaining femtosecond fiber laser. It can realize the self-starting of the mode-locked femtosecond laser under the working conditions of short cavity length and low threshold. The structure is simple and the femtosecond pulse is stable, which can further promote the mass production and industrialization of all polarization-maintaining femtosecond fiber lasers.

The present invention adopts the following technical solutions:

A low-threshold self-starting fully polarization-maintaining femtosecond fiber laser, characterized in that the laser includes a first pump source, a second pump source, a first polarization-maintaining wavelength division multiplexer, and a second polarization-maintaining wavelength division multiplexer , polarization-maintaining erbium-doped fiber, first polarization-maintaining single-mode fiber, polarization-maintaining phase shifter, 2×2 coupler, polarization-dependent isolator, second polarization-maintaining single-mode fiber and coupler;

The first pump source and the second pump source are respectively connected to the a-end of the first wavelength division multiplexer and the d end of the second wavelength-division multiplexer to form a bidirectional pumping structure, which is injected into the polarization-maintaining erbium-doped fiber; The signal end c of the first wavelength division multiplexer is connected to the first polarization-maintaining single-mode fiber, the other end of the first polarization-maintaining single-mode fiber is connected to the input end of the polarization-maintaining phase shifter, and the output end of the polarization-maintaining phase shifter It is connected to the g terminal of the 2×2 coupler, the h terminal of the 2×2 coupler is connected to the polarization dependent isolator, and the output terminal of the polarization dependent isolator is connected to the k terminal of the coupler; the coupler has two output ports, among which the m terminal As the laser output end, the l end is connected to the second polarization-maintaining single-mode fiber, and the other end of the second polarization-maintaining single-mode fiber is connected to the j end of the 2×2 coupler to form a one-way ring, and the second input of the 2×2 coupler The terminal is connected to the f terminal of the second wavelength division multiplexer to form an amplification ring.

In the laser structure, all the fibers and the pigtails of the devices are polarization-maintaining fibers, realizing a fully polarization-maintaining resonant cavity structure.

The polarization-maintaining erbium-doped fiber is a gain fiber, and its output laser is in the 1.55 μm band.

The polarization-maintaining phase shifter is composed of a fiber circulator, a fiber collimator, a Faraday optical crystal and a plane mirror, and is a reflective structure; the light transmitted clockwise is incident from the I port of the fiber circulator, and is output to the fiber collimator through the II port. The collimator is incident on the Faraday optical crystal through the fiber collimator, and the outgoing light is reflected by the plane mirror to the Faraday optical crystal, so that the polarization state is rotated by 90°, and the return light of the Faraday optical crystal returns to the optical fiber collimator and the optical fiber circulator in turn. Port II is finally output from port III; after the light transmitted counterclockwise is incident on port III of the fiber circulator, it is directly output from port I, and the polarization state remains unchanged.

The structure of the polarization-maintaining phase shifter is an integrated optical fiber device with a polarization-maintaining fiber as a pigtail.

The linear phase shift of the polarization-maintaining phase shifter is π/2, so as to realize the self-starting of the mode-locked laser.

Beneficial effects of the present invention:

The invention adopts the femtosecond fiber laser of the linear phase shifter, which has low threshold value, self-starting and good femtosecond pulse stability. By properly controlling the parameters such as the total dispersion value and nonlinear effect of the fiber resonator, several working states of traditional soliton pulse, dispersion-managed soliton pulse, self-similar pulse and noise-like pulse can be realized respectively.

The invention adopts a full polarization maintaining resonant cavity structure, has strong anti-interference ability, narrow output laser pulse width and good stability. The laser uses a polarization-maintaining phase shifter, which can realize the self-starting of femtosecond pulses without external excitation, has strong adaptability to complex environments, is suitable for mass production, and can be applied in large-scale industrial processing, scientific research and other fields .

Description of drawings

FIG. 1 is a schematic structural diagram of a low-threshold self-starting full polarization-maintaining femtosecond fiber laser of the present invention.

FIG. 2 is a schematic diagram of the structure of the nonlinear amplification loop.

FIG. 3 is a schematic diagram showing the relationship between the nonlinear amplification loop structure device and the phase shift amount.

Figure 4(a) is the soliton pulse spectrogram, (b) is the autocorrelation curve of the soliton pulse, (c) is the time domain sequence diagram of the soliton pulse, and (d) is the soliton pulse spectrogram.

Figure 5(a) is the noise-like pulse spectrogram, (b) is the noise-like pulse time-domain sequence diagram, (c) is the noise-like pulse autocorrelation curve, and (d) is the noise-like pulse spectrogram.

FIG. 6 is a schematic diagram showing the relationship between the average output power and the total pump power.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings.

As shown in Fig. 1, the low threshold self-starting full polarization-maintaining femtosecond fiber laser includes a first pump source 1, a second pump source 2, a first polarization-maintaining wavelength division multiplexer 3, and a second polarization-maintaining wave Demultiplexer 4, polarization-maintaining erbium-doped fiber 5, first polarization-maintaining single-mode fiber 6, polarization-maintaining phase shifter 7, 2×2 coupler 8, polarization-dependent isolator 9, second polarization-maintaining single-mode fiber 10 and coupler 11.

The first pump source 1 and the second pump source 2 are respectively connected to the a terminal of the first wavelength division multiplexer 3 and the d terminal of the second wavelength division multiplexer 4 to form a bidirectional pump structure, which is injected into the polarization maintaining Erbium-doped fiber 5, the signal end c of the first wavelength division multiplexer 3 is connected to the first polarization-maintaining single-mode fiber 6, and the other end of the first polarization-maintaining single-mode fiber 6 is connected to the input end of the polarization-maintaining phase shifter 7 connection, the output end of the polarization maintaining phase shifter 7 is connected to the first input end g end of the 2×2 coupler 8, the first output end h end of the 2×2 coupler 8 is connected to the polarization dependent isolator 9, and the polarization dependent isolation The output end of the coupler 9 is connected to the k end of the coupler 11, the m end of the coupler 11 is used as the laser output end, the l end is connected to the second polarization-maintaining single-mode fiber 10, and the other end of the second polarization-maintaining single-mode fiber 10 is connected to the 2× The second output end j of the 2-coupler 8 is connected to form a unidirectional loop, and the second input end i of the 2×2 coupler 8 is connected to the signal f end of the second wavelength division multiplexer 4 to form an amplification ring.

In the structure, the first polarization-maintaining single-mode fiber and the second polarization-maintaining single-mode fiber increase the nonlinear phase shift in the cavity, and the polarization-maintaining erbium-doped fiber, the polarization-maintaining phase shifter and the polarization-maintaining coupler together form a nonlinear amplification loop structure. It is used to realize the phase locking of multi-longitudinal modes in the resonant cavity, and the polarization maintaining phase shifter can increase the transmittance when the nonlinear phase shift of the nonlinear amplification loop is 0, thereby reducing the mode locking threshold. When the first pump source and the second pump source are turned on at the same time, when the gain in the resonator is sufficient, the self-starting of the mode-locked pulse can be realized without any additional mechanical or electrical control.

In the laser structure, all the fibers and the pigtails of the devices are polarization-maintaining fibers, realizing a fully polarization-maintaining resonant cavity structure.

The polarization-maintaining erbium-doped fiber 5 is used as a gain medium to generate a gain in the 1.55 μm band. In order to achieve laser output in other bands, the gain fiber can also select polarization-maintaining fibers with other doped elements, such as ytterbium, thulium, and holmium.

The first polarization-maintaining single-mode fiber 6 and the second polarization-maintaining single-mode fiber 10 are respectively connected to the amplification ring and the unidirectional ring to provide nonlinear phase shift, and at the same time, it can avoid excessive threshold value caused by excessive single loop loss. high.

The polarization-maintaining phase shifter 7 is connected to the amplifying ring, and a linear phase shift is introduced to promote the self-starting of the mode-locked laser and reduce the mode-locking threshold, and its position in the unidirectional ring is flexible and variable.

Turn on the first pump source 1 and increase the power to 40mW, the multi-longitudinal mode phase locking is realized by the nonlinear amplification loop, and the gain light is incident into the loop from the first input end of the 2×2 coupler 8. Due to the polarization-maintaining erbium doping The asymmetry of the placement of the fiber 5 in the loop, the light traveling in the clockwise and counterclockwise directions experiences different nonlinear phase shifts, when the two beams return to the 2×2 coupler 8 again and are separated by the second When the input end exits, the loop forms an intensity-dependent transmittance change, forms a saturable absorption effect, and realizes mode locking. Gradually increase the power of the first pump source 1 and the second pump source 2. When the total power of the two reaches 390mW, the conventional soliton pulse is automatically converted into a noise-like pulse, and the noise-like pulse state can be maintained until the pump power reaches the maximum. 700mW. During the operation, the first pump source 1 and the second pump source 2 are turned off and set to the pump power range required by the mode-locking pulse again, and the laser can automatically restore the corresponding mode-locking state.

The a, b, and c terminals of the first wavelength division multiplexer 3 and the d, e, and f terminals of the second wavelength division multiplexer 4 are the pump terminal, the common terminal, and the signal terminal, respectively. The g, i, h and j terminals of the 2×2 coupler 8 are the first input terminal, the second input terminal, the first output terminal and the second output terminal, respectively. The k, l and m terminals of the coupler 11 are the input terminal, the first output terminal and the second output terminal, respectively.

As shown in FIG. 2 , the polarization-maintaining phase shifter 7 is composed of a fiber circulator 7-1, a fiber collimator 7-2, a Faraday rotatory crystal 7-3 and a plane mirror 7-4, and is a reflective structure. The light transmitted clockwise is incident from the I port of the fiber circulator 7-1, and is output to the fiber collimator 7-2 through the II port, and is incident to the Faraday optical crystal 7-3 through the fiber collimator 7-2. The plane mirror 7-4 is reflected to the Faraday optical crystal 7-3, so that the polarization state is rotated by 90°, and the returning light of the Faraday optical crystal 7-3 returns to the fiber collimator 7-2 and the fiber circulator 7-1. Port II in turn , and finally output from port III; after the light transmitted counterclockwise is incident on port III of fiber circulator 7-1, it is directly output from port I, and the polarization state remains unchanged. Therefore, the phase shifter introduces an additional π/2 linear phase shift to the light between the clockwise and counterclockwise directions, thereby greatly improving the self-start probability of mode locking.

As shown in FIG. 3 , after the introduction of the polarization maintaining phase shifter 7, when the nonlinear phase shift amount of the nonlinear amplification loop is 0, its transmittance can be increased to be higher than 0.5. This process can be equivalent to that when the first pump source 1 and the second pump source 2 are just turned on, the nonlinear amplification loop has a large instantaneous transmittance, so that the mode-locked pulse is automatically activated.

Figure 4(a) shows the spectrum of the soliton pulse, the center wavelength is 1560nm, and the 3dB bandwidth is 3.7nm. Figure 4(b) is the autocorrelation curve of the soliton pulse, the pulse width is 863.8fs, Figure 4(c) is the soliton pulse sequence, the pulse time interval is 645.1ns, and Figure 4(d) is the soliton pulse spectrum, the repetition frequency is 1.6 MHz, the signal-to-noise ratio of the fundamental frequency is 52dB, which proves that the soliton pulse has good noise characteristics.

Figure 5(a) is the noise-like pulse spectrum, the 3dB bandwidth is 13.4nm, Figure 5(b) is the noise-like pulse envelope waveform, the single envelope width is 712ps, and Figure 5(c) is the noise-like pulse autocorrelation Curve, the peak width at the center of the base is 417.9fs, which corresponds to the average width of a single femtosecond pulse in the noise-like envelope. Figure 5(d) is the noise-like pulse spectrum, and the fundamental frequency SNR is 54dB.

The invention can generate two kinds of femtosecond pulses of traditional soliton and noise-like pulse in the same resonant cavity structure. As shown in Figure 6, when the total power of the first pump source 1 and the second pump source 2 is lower than 390mW, the laser works in the soliton mode-locked state, and when the total pump power is higher than 390mW, the laser works in a class Noise mode-locked state, so noise-like pulses have higher output power. In practical applications, the mutual conversion between soliton pulses and noise-like pulses can be achieved only by setting the pump power appropriately, and an appropriate working state can be selected according to actual needs. In the process of self-starting and state switching, the laser does not need a polarization state adjustment mechanism, and the operation is simple. If there is a specific requirement for the output power, the laser can be directly connected to the power amplifier module, such as the main oscillation power amplifier and the chirped pulse amplifier.

Claims (6)

1. a low-threshold self-starting fully polarization-maintaining femtosecond fiber laser, characterized in that the laser comprises a first pump source (1), a second pump source (2), a first polarization-maintaining wavelength division multiplexer (3) ), a second polarization-maintaining wavelength division multiplexer (4), a polarization-maintaining erbium-doped fiber (5), a first polarization-maintaining single-mode fiber (6), a polarization-maintaining phase shifter (7), and a 2×2 coupler ( 8), a polarization-dependent isolator (9), a second polarization-maintaining single-mode fiber (10), and a coupler (11); The first pump source (1) and the second pump source (2) are respectively connected to the a-end of the first polarization-maintaining wavelength division multiplexer (3) and the d-end of the second polarization-maintaining wavelength division multiplexer (4). connected to form a bidirectional pumping structure, and injected into the polarization-maintaining erbium-doped fiber (5); the signal end c of the first polarization-maintaining wavelength division multiplexer (3) is connected to the first polarization-maintaining single-mode fiber (6), and the first polarization-maintaining wavelength division multiplexer (3) The other end of a polarization-maintaining single-mode fiber (6) is connected to the input end of the polarization-maintaining phase shifter (7), and the output end of the polarization-maintaining phase shifter (7) is connected to the g end of the 2×2 coupler (8), The h end of the 2×2 coupler (8) is connected to the polarization dependent isolator (9), and the output end of the polarization dependent isolator (9) is connected to the k end of the coupler (11); the coupler (11) has two output ports , where the m end is used as the laser output end, the l end is connected to the second polarization-maintaining single-mode fiber (10), and the other end of the second polarization-maintaining single-mode fiber (10) is connected to the j end of the 2×2 coupler (8), A unidirectional ring is formed, and the second input end of the 2×2 coupler (8) is connected to the f end of the second polarization-maintaining wavelength division multiplexer (4) to form an amplification ring. 2 . The low-threshold self-starting fully polarization-maintaining femtosecond fiber laser according to claim 1 , wherein all the fibers and the pigtails of the devices in the laser structure are polarization-maintaining fibers, so as to realize a fully polarization-maintaining resonant cavity structure. 3 . 3 . The low-threshold self-starting fully polarization-maintaining femtosecond fiber laser according to claim 1 , wherein the gain fiber is selected as a polarization-maintaining erbium-doped fiber (5), and the output laser is in a 1.55 μm waveband. 4 . 4. The low-threshold self-starting full polarization-maintaining femtosecond fiber laser according to claim 1, wherein the polarization-maintaining phase shifter (7) is composed of a fiber circulator (7-1), a fiber collimator ( 7-2), a Faraday optical crystal (7-3) and a plane mirror (7-4), which is a reflective structure; the light transmitted clockwise is incident from the I port of the fiber circulator (7-1), and is output through the II port to the optical fiber collimator (7-2), incident on the Faraday optical crystal (7-3) through the optical fiber collimator (7-2), and the outgoing light is reflected by the plane mirror (7-4) to the Faraday optical crystal (7-3) ), the polarization state is rotated by 90°, and the return light of the Faraday optical crystal (7-3) returns to the fiber collimator (7-2) and the fiber circulator (7-1) port II in turn, and finally the port III Output; after the light transmitted counterclockwise is incident from the III port of the fiber circulator (7-1), it is directly output from the I port, and the polarization state remains unchanged. 5. The low-threshold self-starting full polarization-maintaining femtosecond fiber laser according to claim 4, wherein the structure of the polarization-maintaining phase shifter (7) is an integrated optical fiber device with a polarization-maintaining fiber as a pigtail . 6. The low-threshold self-starting full polarization-maintaining femtosecond fiber laser according to claim 4, wherein the linear phase shift of the polarization-maintaining phase shifter (7) is π/2, so as to realize the mode-locked laser Self-starting.

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