CN101924319A - An all-fiber structured laser system for generating high-energy femtosecond pulses - Google Patents

Abstract

The invention provides an all-fiber structure laser system for generating high-energy femtosecond pulses, and aims to solve the technical problems of low pulse energy and poor stability of the conventional erbium-doped fiber laser. The all-fiber structure laser system comprises an annular laser used for generating high-energy pulses and a single-mode fiber used for pulse compression, wherein the annular laser comprises a wavelength division multiplexer, an erbium-doped fiber, a first polarization controller, a polarization-related isolator, a second polarization controller and an output coupler which are sequentially connected through the single-mode fiber in a cavity, and a pumping light source is arranged at the input end of the wavelength division multiplexer. The output end of the output coupler is connected with a single mode fiber positioned outside the cavity and used for compressing the high-energy pulse generated in the cavity. The invention has the advantages of simple structure, low price, good stability and the like, and particularly has the advantages of high pumping efficiency, convenient adjustment, easy optical fiber coupling and the like compared with a solid laser for generating high-energy femtosecond pulses.

Description

An all-fiber structured laser system for generating high-energy femtosecond pulses

technical field

The invention relates to an optical fiber laser system, in particular to an all-fiber structured laser system for generating high-energy femtosecond pulses.

Background technique

Due to its simple structure, low cost, high stability and no need for optical path alignment, all-fiber lasers have been deeply studied and widely used in the fields of optical communication, supercontinuum generation, and medical treatment. Fiber lasers can work in different states, including continuous light output and pulse output. According to the different output pulse characteristics after mode-locking, the optical pulses currently studied are mainly divided into four categories. The first is a conventional soliton operating in the negative dispersion region. The self-starting mode-locked pulse output is achieved through the interaction of fiber anomalous dispersion and Kerr nonlinear effect in the fiber laser cavity to reach a balance state. However, due to the energy quantization effect of solitons, the energy of this pulse is limited to within 0.1 nanojoule (nJ). The second is stretched-pulse. By adding a dispersion delay line in the cavity of the stretched pulse laser, the net dispersion in the cavity is close to 0. The pulse is periodically compressed and stretched during intracavity transmission. Because the average pulse width is increased and the nonlinear effect is weakened, the energy of a single pulse can reach about 1nJ, and the peak power can reach the order of kilowatts. The third type is a self-similar pulse with a slightly positive net dispersion in the cavity. The self-similar pulse laser realizes self-consistent pulse evolution by adding a negative dispersion device in the cavity. The shape of the pulse remains basically unchanged during the propagation process in the cavity, and it has a certain ability to resist light wave splitting. The fourth type is dissipative soliton. It is a new type of soliton pulse discovered in recent years, which is generated in a fiber laser with large positive dispersion. The formation of this kind of soliton is the comprehensive effect of gain dispersion, gain saturation and positive dispersion effect, nonlinear loss, etc., in which gain and loss play a leading role, so it is called dissipative soliton. Compared with traditional solitons, the pulse width is increased by two or three orders of magnitude, and the nonlinear effect is significantly suppressed. The optical pulse has a strong ability to resist light wave splitting, and the single pulse energy can reach the level of tens of nJ.

Although dissipative solitons have high pulse energy, the pulse width is generally relatively large, reaching tens of picoseconds (ps), and the pulse has a very large frequency chirp. This requires de-chirping the pulse to achieve pulse compression in the time domain, thereby obtaining high peak power. The general compression method includes using negative dispersion devices such as prism pairs, grating pairs, and Bragg gratings to perform chirp compensation on pulses. But these devices are non-fiber structures, not easy to adjust, and inconvenient to couple with fiber lasers. At the same time, these optical devices are difficult to manufacture and expensive, which is not conducive to the promotion and use of fiber lasers in engineering.

In order to obtain ultrashort pulses with higher energy, traditional femtosecond pulses can be amplified by fiber amplifiers, but this makes the laser system complex, which is not conducive to engineering use, and the amplified pulses may be distorted. Other ways to obtain high-energy pulses are to add special devices in the laser cavity, such as using a photonic crystal fiber (PCF) with a large mode field. By increasing the mode field area and reducing the energy density of the optical pulse, the energy of the optical pulse is increased. . However, the core diameters of PCF and single-mode fiber are not uniform, so they cannot be directly coupled, and space optics are required for connection. The fiber laser system is no longer an all-fiber structure, resulting in cumbersome adjustments, increased costs, and reduced stability. At present, there is no fiber laser with a simple structure, working at the communication wavelength, and capable of generating high-energy femtosecond pulses in China.

Contents of the invention

The invention provides an all-fiber structure laser system for generating high-energy femtosecond pulses, aiming to solve the technical problems of low pulse energy and poor stability of current erbium-doped (Er ³⁺ ) fiber lasers, making it suitable for use in optical communication, optical sensing, etc. , The field of optical detection has been widely used.

Technical scheme of the present invention is as follows:

The all-fiber structured laser system includes a ring laser and a single-mode fiber for generating high-energy pulses, the single-mode fiber is divided into two parts, one part is located inside the ring laser cavity, and the other part is located outside the ring laser cavity for pulse compression; The ring laser includes a wavelength division multiplexer, an erbium-doped fiber, a first polarization controller, a polarization-dependent isolator, a second polarization controller, and an output coupler sequentially connected by an intracavity single-mode fiber, wherein the wavelength division multiplexer The input end of the pump light source is provided with; The length of described erbium-doped optical fiber is 3 to 50m, and it has positive dispersion near 1550nm; The optical fiber has negative dispersion near 1550nm and the length is 50 to 500m.

The wavelength division range of the above wavelength division multiplexer is 980nm/1550nm.

The output ratio of the above-mentioned output coupler is 10%-80%, and the preferred output ratio is 70%.

The output power of the above pump light source is 100-200mW.

The above-mentioned pumping light source is a single-mode semiconductor laser with an operating wavelength near 980nm, and the working parameters of the single-mode semiconductor laser match the erbium-doped optical fiber. The suitable output power of the above-mentioned pump light source is 125mW, and the length of the erbium-doped optical fiber is about 18m.

The advantages of the present invention are as follows:

The components used in the fiber laser are common components used in common fiber lasers, and all of them have been commercialized, so the cost is very low.

The optical laser adopts an all-fiber structure and does not need a spatial light adjustment device, so its structure is simple, easy to adjust, and good in stability.

The fiber laser uses a single-mode fiber to compress light pulses, which greatly reduces the cost compared to other compression devices such as grating counterparts.

The fiber laser has high single pulse energy, narrow pulse width, and peak power of 100,000 watts, and can be used as a femtosecond source without amplification.

The erbium-doped fiber laser works in the 1550nm band, which corresponds to the optical communication band, so it has great application prospects in optical communication, optical sensing, and optical detection.

After optimizing parameters such as the length of the single-mode fiber, the length of the erbium-doped fiber, and the output ratio of the output coupler, the single pulse energy will be further increased, the compressed pulse width can be further reduced, and the peak power will be further increased. possible.

Description of drawings:

Fig. 1 is a structural representation of the present invention;

Fig. 2 is the spectrogram of the dissipation type pulse of the present invention at a pump power of 125mW;

Fig. 3 is the pulse train chart that the oscilloscope of the present invention records;

Fig. 4 is the spectrogram of the pulse of the present invention;

Fig. 5 is the autocorrelation graph of the pulse of the present invention;

Fig. 6 is an autocorrelation graph of the compressed pulse of the present invention.

The reference signs are as follows:

1-pump light source; 2-wavelength division multiplexer; 3-erbium-doped fiber; 4-first polarization controller; 5-polarization dependent isolator; 6-second polarization controller; 7-output coupler; 8 -Single-mode fiber for compression.

Detailed ways:

Referring to Fig. 1, the fiber laser laser system that the present invention provides to generate high-energy femtosecond pulses includes a pumping source 1 connected to each other through a single-mode optical fiber, a wavelength division multiplexer 2, an erbium-doped optical fiber 3, and a first polarization controller 4 , a polarization-dependent isolator 5 , a second polarization controller 6 , an output coupler 7 , and a single-mode fiber 8 . An erbium-doped optical fiber 3 is arranged between the wavelength division multiplexer 2 and the first polarization controller 4 . The length of the erbium-doped optical fiber 3 is 3 to 50m, and the best effect is 20m. The pump light source 1 is a single-mode semiconductor laser with an operating wavelength of 980nm. When the pump power is 125mW, the output pulse width is 23.1ps and the spectral width is 18nm. After the pulse passes through about 80m of single-mode fiber, the pulse is compressed to within 300fs in the time domain. The frequency division range of the wavelength division multiplexer 2 is 980nm/1550nm; the output ratio of the output coupler 4 is 70%. The erbium-doped fiber 3 has a model number Nufern EDFC-980-HP.

Working principle of the present invention and concrete device parameter are as follows:

Referring to Fig. 1, the present invention adopts the semiconductor laser that working wavelength is 980nm as the pump light source, and the maximum output power is 550mW, and it passes through a 980nm/1550nm WDM to carry out pumping and pumping to a section of 18m erbium fiber, the erbium-doped fiber The model is Nufern EDFC-980-HP, and the dispersion parameter D at 1550nm is about -42ps/nm/km. Backward feedback is suppressed by a polarization-dependent isolator to ensure unidirectional operation of the laser. The first polarization controller, the second polarization controller and the polarization-dependent isolator act together as a saturable absorber, thereby realizing self-starting mode-locking of the laser. The output rate of the output coupler is 70%, and 30% of the energy is left in the ring laser. This output rate can ensure that the output single pulse has a large energy without affecting the stability of the mode locking.

The pigtails of WDM, polarization-dependent isolator and output coupler are common single-mode fibers with a total length of 7.5m and a dispersion coefficient D of about 17ps/nm/km at 1550nm.

The total length of the laser is 25.5m, corresponding to the fundamental frequency of the cavity is 8.2MHz, the length of the erbium fiber is 18m, and the net dispersion β ₂ in the cavity is +0.8ps ² .

The device used for pulse compression is a single-mode optical fiber about 80m long, and the dispersion coefficient D is about 17ps/nm/km. The optimal length of the compressed optical fiber used is determined by the characteristics of the outgoing pulse.

In the experiment, a spectrum analyzer (YOKOGAWA-6370B) was used to measure the spectrum of the output pulse, and an autocorrelator was used to measure the width of the pulse before compression and after compression. After the photoelectric conversion of the output pulse, use an oscilloscope and a spectrum analyzer to observe the output pulse sequence and mode-locking state of the laser, respectively. The cavity shape design of this structure is adopted, and the size of the net dispersion in the cavity is controlled by rationally selecting the length of the erbium fiber and the length of the pigtail fiber of the device. Under the condition of large positive dispersion, high-energy pulses can be obtained by adjusting the polarization controller. At this time, the pulse width directly output is generally tens of ps, and the pulse is compressed by a single-mode fiber, and the pulse width can reach the femtosecond (fs) level.

The components of the ring fiber laser used in the experiment all use traditional fiber laser devices, which have the advantages of simple structure, low price, and good stability. It can solve the shortcoming of low pulse energy of traditional femtosecond fiber laser. After the output pulse is compressed, the peak power can reach the order of 100,000 watts. Compared with solid-state lasers that generate high-energy femtosecond pulses, it has the advantages of high pumping efficiency, convenient adjustment, and easy fiber coupling.

Concrete principle of the present invention and experimental result are as follows:

This type of laser can use nonlinear polarization rotation (NPR: Nonlinear PolarizationRotation) technology to achieve mode locking. When the pump power reaches the threshold of self-starting mode-locking, by adjusting the first polarization controller and the second polarization controller to change the polarization state of the fiber, the laser can achieve stable mode-locked pulse output. During the experiment, when the pump power reaches above 100mW, the laser can achieve stable mode-locked pulse output by adjusting the two polarization controllers. Continue to adjust the two polarization controllers to make the mode-locked pulse spectrum wider, and then fix the two polarization controllers. Increase the power of the pump light, and the laser is a single pulse output between 100 and 200mW. When the pump reaches more than 200mW, the laser tends to work in a multi-pulse state. Keeping the pump power at 125mW, the spectrum, pulse sequence, frequency spectrum and autocorrelation curve of the output single pulse are shown in Figure 2, Figure 3, Figure 4, and Figure 5, respectively. The experimental results shown in the figure can be explained by the following principles:

The initial white noise pulse in the fiber laser resonator is amplified by pumping after entering the erbium fiber, and at the same time, the pulse spectrum is broadened due to nonlinear effects such as strong self-phase modulation (SPM). However, due to the spectral filtering effect of the intracavity gain erbium fiber, the spectral width of the pulse cannot be increased infinitely, and the spectral broadening caused by SPM and the spectral filtering caused by the gain erbium fiber will reach a dynamic balance. In the time domain, because the erbium fiber has a large positive dispersion, the pulse is also broadened during the gain amplification process. After that, the pulse enters the equivalent saturable absorber formed by the polarization controller and the polarization-dependent isolator, and the two wings of the pulse are eliminated, resulting in a reduction in the pulse width in the time domain. This process is repeated until the initial input light wave pulse forms a self-consistent evolution in the cavity, and finally a stable mode-locked pulse output can be obtained, and the spectrum of the output pulse has a steep edge. It can be seen that the formed pulse is the result of the joint action of laser gain, nonlinear polarization rotation and nonlinear loss, so it is also called dissipative pulse. Since the chirp introduced by nonlinear effects such as SPM is positive, and the intracavity dispersion is also a large positive value, the combined effect of the two results in a relatively large pulse width of the dissipative pulse, which is increased compared with the traditional negative dispersion soliton. Two or three orders of magnitude, the peak power of the pulse in the cavity will be maintained at a low level, so that the effect of light wave splitting can be effectively avoided, and the output pulse energy is also increased by more than two orders of magnitude.

Referring to Figure 2, when the pump power is 125mW, the output spectrum is approximately rectangular, with a width of 18nm and a center wavelength of 1565nm. The autocorrelation curve of the pulse is shown in Figure 5. Its half-peak full width is 32.4 ps. If it is fitted with a Gaussian pulse, the pulse width can be obtained as 23.1 ps. Through calculation, it can be obtained that the time-bandwidth product is about 40, which is much larger than the time-bandwidth product of the non-chirped pulse. This shows that the dissipative pulse has a huge chirp, which is also consistent with the theoretical analysis. The measured output power is about 50mW, and the single pulse energy can be calculated to be close to 8nJ. Although the pulse at this time has relatively large energy, its pulse width is also relatively wide, and the peak power is still relatively low, which cannot meet the application requirements. For example, it does not meet the requirements for use as a femtosecond source.

Using single-mode fiber to dechirp the dissipative pulse realizes the pulse compression. The principle is:

The outgoing dissipative pulse is generated in the cavity with a large positive dispersion, and the pulse has a large positive linear chirp. When such pulses are transmitted in a single-mode fiber with negative dispersion, the single-mode fiber causes the pulse to be negatively chirped. In this way, when the initial pulse is transmitted in the single-mode fiber, the linear part of its chirp is compensated by the single-mode fiber, and the chirped pulse becomes an approximately chirp-free pulse, thereby realizing the compression of the optical pulse in the time domain. As shown in Figure 6. The original chirped pulse is compressed to less than 300fs after passing through about 80m of single-mode fiber. The sidelobes on both sides are mainly due to the incompressible nonlinear chirp. Other compressed pulse methods use grating pairs, chirped mirrors, etc. to perform dispersion compensation. The principle is similar to that of single-mode fiber compressed pulses, and both use the principle of dispersion compensation. However, these spatial optical devices are not in the structure of optical fibers, which is not conducive to adjustment. In addition, compared with single-mode optical fibers, they are expensive, difficult to fabricate, and have low environmental stability, which limits their use in engineering.

To sum up, this type of fiber laser solves the shortcomings of low pulse energy and large pulse width of traditional fiber lasers. Pulses with an energy up to 8nJ and a pulse width of 23.1ps were generated using a dissipative laser system. After a section of single-mode fiber compression, the pulse width becomes 290fs, and the peak power reaches 26kW. This femtosecond fiber laser has the advantages of high pulse energy, simple structure, easy adjustment, and good stability. It can be used directly as a femtosecond source without amplification, and has a wide range of applications in optical communications, optical detection, and medical treatment. prospect.

Claims (7)

1. all-fiber structure laser system that produces the high-energy femtosecond pulse, this system comprises ring laser and monomode fiber, it is characterized in that: described monomode fiber is divided into two parts, and a part is positioned at the ring laser chamber, and another part is positioned at outside the ring laser chamber; Described ring laser comprises wavelength division multiplexer, Er-doped fiber, first Polarization Controller, polarization relevant isolator, second Polarization Controller and the output coupler that connects by monomode fiber in the chamber successively, and wherein the input of wavelength division multiplexer is provided with pump light source; The length of described Er-doped fiber is 3 to 50m, and it has positive dispersion near 1550nm; The output of described output coupler is connected with the monomode fiber that is positioned at outside the chamber, and this monomode fiber has negative dispersion near 1550nm, and length is 50 to 500m.

2. the all-fiber structure laser system of generation high-energy femtosecond pulse according to claim 1 is characterized in that: the wavelength-division scope of described wavelength division multiplexer is 980nm/1550nm.

3. the fiber laser system of generation high-energy femtosecond pulse according to claim 2 is characterized in that: the output ratio of described output coupler is 10%-80%.

4. the novel optical fiber laser system of generation high energy pulse according to claim 3 is characterized in that: described pump light source power output is 100-200mW.

5. the fiber laser system of generation high-energy femtosecond pulse according to claim 4 is characterized in that: the output ratio of described output coupler is 70%.

6. according to the fiber laser system of the arbitrary described generation high-energy femtosecond pulse of claim 1 to 5, it is characterized in that: described pump light source is that operation wavelength is the single mode semiconductor laser of 980nm, this single mode semiconductor laser running parameter and Er-doped fiber coupling.

7. the fiber laser system of generation high-energy femtosecond pulse according to claim 6 is characterized in that: described pump light source power output is 125mW, and the length of described Er-doped fiber is 18m.

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