CN102025095A - A new fiber laser system that generates high-energy pulses - Google Patents

Abstract

The novel fiber laser system for generating high-energy pulses provided by the present invention includes a wavelength division multiplexer 2, an output coupler 4, a first polarization controller 5, a polarization-dependent isolator 6, and a second polarization controller 7 sequentially connected through an optical path The input end of the wavelength division multiplexer is provided with a pump light source 1, and an erbium-doped (Er) optical fiber 3 is arranged between the output end of the wavelength division multiplexer and the input end of the output coupler; the length of the erbium-doped optical fiber 5-30m. The invention can solve the technical problem that the output energy is limited due to the light wave splitting effect existing in the existing fiber laser. The device used in this fiber optic system is a common device commonly used in ordinary fiber lasers. Its structure is very simple and its cost is relatively low. Compared with traditional solid-state lasers, this laser adopts an all-fiber structure, which has high conversion efficiency, good beam quality, and convenient heat dissipation. , easy to couple with other optical fiber devices and so on.

Description

A new fiber laser system that generates high-energy pulses

Technical field:

The invention relates to a fiber laser system, in particular to a novel fiber laser system for generating high-energy pulses.

Background technique:

Due to its simple structure, low cost, easy adjustment and good stability, fiber lasers have been extensively researched and developed. According to the dispersion distribution characteristics of the fiber laser, the fiber mode-locked laser can be respectively excited to generate conventional negative dispersion soliton pulse (conventional soliton), stretched pulse (stretched pulse), self-similar pulse (self-similar pulse) and dissipative soliton pulse (dissipative soliton).

In the negative dispersion soliton fiber laser, the passive mode-locked laser working in the negative dispersion region, due to the balance of the anomalous dispersion in the cavity and the fiber Kerr nonlinearity, this laser can easily achieve stable soliton mode-locked output, but the traditional soliton Quantization effects in the pulses limit the maximum energy that can be output from such lasers. Due to the limitation of the soliton area theory, the energy of the soliton pulse is limited to about 0.1nJ. When the energy of the light pulse is high, it will cause the splitting of the light wave. Although the stretched pulse mode-locked laser inserts a dispersive delay line into the cavity, the mode-locked pulse undergoes periodical broadening and compression in the cavity, which reduces the peak power of the intracavity pulse and overcomes the nonlinear effect caused by the soliton mode-locked laser. Wave splitting, the output pulse energy can be an order of magnitude higher than that of conventional negative dispersion solitons, but its pulse energy still cannot reach a higher level.

Mathematically, the self-similar pulse is the self-similar solution of the nonlinear Schrödinger equation (Nonlinear Schrodinger Equation) with gain in the positive dispersion region of the fiber. When the self-similar pulse propagates at high power, the pulse shape does not change, and it has the ability to resist light wave splitting. Self-similar pulsed fiber lasers require negative dispersion elements to realize the self-consistent evolution of pulses in the cavity, and the total dispersion value in the cavity is maintained at a level slightly greater than zero. The intracavity dispersion distribution of stretched pulse and self-similar pulse laser can be simplified as shown in Fig.1. As shown in Figure 1a, the total dispersion β _net in the cavity of the stretched pulse laser is approximately 0; while in the self-similar pulse laser, as shown in Figure 1b, the β _net in the cavity is slightly greater than 0.

In order to obtain higher energy pulses, it is also possible to use special devices in the cavity, such as photonic crystal fibers (PCFs). density, preventing pulse splitting. In this method, since the PCF core diameter is much larger than that of ordinary single-mode optical fibers, it cannot be directly coupled with single-mode optical devices, and needs to be connected with spatial optical devices, resulting in complex cavity structure, cumbersome adjustment, and high cost. In order to overcome these shortcomings, it is necessary to propose a new pulse generation theory and method. With the deepening of research, optical solitons can also be formed in fiber lasers composed of all-normal-dispersion or net-normal-dispersion. Dissipative pulses generated in positive dispersion fiber lasers can be approximately described by the Ginzburg-Landau equation. Since the optical pulses formed in this type of laser are the result of the combined effects of laser gain dispersion, gain saturation, positive dispersion effects, and nonlinear losses, the formed optical pulses are also called dissipative optical pulses or dissipative optical pulses. soliton. The intracavity dispersion distribution of a dissipative pulsed laser is shown in Figure 1c. By further increasing the positive dispersion in the cavity and removing the negative dispersion device, a pulse output with wider pulse width and larger chirp can be obtained. Correspondingly, Pulse energy will also be able to be further increased. Recently, a research team from Cornell University in the United States has achieved many results in the research of generating dissipative pulses based on positive dispersion ytterbium (Yb) fiber lasers. Its output pulse spectrum has steep edges with a large positive chirp. Such pulses can accumulate more energy output, and after compression, the peak power can reach the kW or even MW level. At present, the results produced by this type of laser have not yet matured theoretical analysis in the world, and the experimental achievements are also increasing and improving day by day. However, the work done by this research group is mainly limited to ytterbium (Yb) fiber lasers, and there is less research on erbium (Er) fiber lasers that are used in communication bands and have broad development prospects. Domestic research on high-power fiber lasers is mostly limited to ytterbium-doped fiber lasers.

Invention content:

In order to solve the technical problem of limited output energy caused by the light wave splitting effect existing in existing fiber lasers, the present invention provides a novel fiber laser system for generating high-energy pulses.

Technical scheme of the present invention is as follows:

The novel fiber laser system for generating high-energy pulses provided by the present invention includes a wavelength division multiplexer, an output coupler, a first polarization controller, a polarization-dependent isolator, and a second polarization controller sequentially connected through an optical path; The input end of the wavelength division multiplexer is provided with a pump light source, and it is characterized in that: an erbium-doped (Er) optical fiber is arranged between the output end of the described wavelength division multiplexer and the input end of the output coupler; The length of the erbium fiber is 5-30m.

The above-mentioned erbium-doped optical fiber has a length of 20 m.

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

The output ratio of the above output coupler is 10%.

The output power of the above pump light source is 200-500mW.

The output power of the above pump light source is 500mW.

The above-mentioned pump light source is a single-mode semiconductor laser with an operating wavelength of 980nm.

The model used for the above erbium-doped fiber is Nufern EDFC-980-HP.

Description of drawings:

Figure 1 is the intracavity dispersion distribution diagram of stretched pulse, self-similar pulse and dissipative pulse laser;

Fig. 2 is a structural representation of the present invention;

Fig. 3 is the spectrogram when the pumping power of the dissipation type pulse of the present invention is 500mW;

Fig. 4 is that the oscilloscope of the present invention measures the dissipative pulse sequence diagram;

Fig. 5 is the pulse autocorrelation curve diagram of the present invention.

Explanation of attached marks:

1-pump light source; 2-wavelength division multiplexer; 3-erbium-doped (Er) fiber; 4-output coupler; 5-first polarization controller; 6-polarization dependent isolator; 7-second polarization control device.

The beneficial effects of the present invention are as follows:

1. The devices used in the fiber optic system are common devices commonly used in ordinary fiber lasers, and their structure is very simple and the cost is relatively low.

2. Compared with traditional solid-state lasers, this laser adopts an all-fiber structure, which has the advantages of high conversion efficiency, good beam quality, convenient heat dissipation, and easy coupling with other devices.

3. The laser is easy to adjust and easy to operate.

4. The erbium-doped fiber laser produced by this invention has broad application prospects in optical communication, optical ranging, photoelectric sensing, medical treatment, etc., because its wavelength is just in the communication window (1550nm) band. And the generated Gaussian pulse has a huge spectral width and high pulse energy. It can also be used as a broadband pulsed light source. After the pulse width is compressed by a negative dispersion device, it can also be used as a high-energy femtosecond pulse source.

5. High-energy pulses are directly output from the cavity without further amplification. And due to the limited experimental conditions, the pump source power level is low, the maximum power is only 500mW, resulting in no further increase in pulse energy. If the pump power is increased, higher energy pulses can be obtained.

Detailed ways:

Referring to Figure 2, the novel fiber laser system for generating high-energy pulses provided by the present invention includes a pump light source 1, a wavelength division multiplexer 2, an output coupler 4, a first polarization controller 5, and a polarization-dependent isolation An erbium (Er) doped fiber 3 is arranged between the wavelength division multiplexer 2 and the output coupler 4 . The length of the erbium-doped optical fiber 3 is 5-30m, 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 output power is 500mW, the output pulse spectrum envelope covers the range of 1530nm-1660nm. The frequency division range of the wavelength division multiplexer 2 is 980nm/1550nm; the output ratio of the output coupler 4 is 10%. The erbium-doped fiber 3 has a model number Nufern EDFC-980-HP.

The working principle and specific device parameters are as follows:

Referring to Fig. 2, the present invention adopts the single-mode semiconductor laser that working wavelength is 980nm as pumping light source, and its maximum output power is 500mW, then by the wavelength division multiplexer (WDM) of 980nm/1550nm to a section length is 20m doped erbium The fiber is used for pumping. The model of the erbium-doped fiber is Nufern EDFC-980-HP, and the dispersion parameter D at 1550nm is -42ps/nm/km. The backward feedback is suppressed by the polarization-dependent isolator to ensure the unidirectional operation of the ring cavity laser, and it cooperates with the first polarization controller and the second polarization controller to form an equivalent saturable absorber, thereby realizing self-starting mode locking. Among them, the output coupler is 10% output, and 90% stays in the cavity. This output ratio is reasonable, other ratios lead to higher pump power thresholds.

Other optical fibers in the cavity such as device pigtails are standard single-mode optical fibers with a total length of 3.8m, and their dispersion parameter D is 17ps/nm/km at 1550nm.

The total cavity length of the laser is 23.8m, so its mode-locked fundamental frequency is about 8.72MHz. Since the length of the erbium-doped fiber used is up to 20m, the total dispersion in the cavity reaches +1 ps ² ;

In the experiment, a spectrum analyzer (ANDO AQ-6315A) was used to measure the spectrum of the output laser, and after photoelectric conversion, the pulse sequence was observed with an oscilloscope (LeCroy SDA, 11GHz), and the pulse width was measured with an autocorrelator. With this cavity design structure, the dispersion value can be controlled and the nonlinear effect can be enhanced through reasonable selection of the length of the erbium-doped fiber. The longer the length of the erbium-doped fiber, the greater the dispersion value and the stronger the nonlinear effect. The novel high-energy pulses can be generated by carefully tuning the laser under conditions of extremely positive dispersion in the laser cavity.

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

The laser uses Nonlinear Polarization Rotation (NPR: Nonlinear Polarization Rotation) technology to achieve self-starting mode locking. When the pump power exceeds a certain threshold, a stable mode-locked pulse output can be obtained by adjusting the polarization states of the first polarization controller and the second polarization controller. In the experiment, the lowest pump power for self-starting mode-locking of this laser is only 45mW. However, when the pump power is increased in this state, multi-pulse output will be formed, so high-energy single-pulse output cannot be obtained. Keep the power of the pump source above 200mW, and continue to adjust the polarization controller. In a specific state, increase the pump power to the maximum and keep the number of pulses at one. At this time, high-energy wave-splitting dissipative pulse output can be obtained. . The typical output spectrum and pulse sequence of this type of pulse are shown in Figure 3 and Figure 4 below, respectively. According to the experimental results in Fig. 3-Fig. 4, the basic physical process of forming the mode-locked pulse is: the initial noise pulse in the laser cavity enters the erbium-doped fiber and is amplified by pumping, due to the strong self-phase modulation ( Nonlinear effects such as SPM (self-phase modulation) lead to the broadening of its spectrum and produce a positive frequency chirp. Because the erbium-doped fiber has a great positive dispersion, the width of the pulse is also widened. After that, it enters into 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 reduced pulse width. At the same time, because the pulse has a positive frequency chirp, the front part of the pulse is a red-shifted long-wave component, and the trailing edge is a blue-shifted short-wave component. The propagation speed of the short wave) is slow, and the wings of the pulse are eliminated in the saturable absorber, which leads to the filtering of the pulse in the frequency domain, that is, the spectral width of the pulse is reduced. This process is repeated until the initial input light wave pulse can be absorbed in the cavity A self-consistent evolution is formed, and finally a stable mode-locked pulse output can be obtained. 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 self-phase modulation is positive, and the intracavity dispersion is a large positive value, the combined effect of the two results in a large pulse width of the dissipative pulse. Compared with the general negative dispersion soliton increase It is more than two orders of magnitude larger, so the pulse peak power in the cavity will be maintained at a lower level, so that the light wave splitting effect can be effectively avoided, and the output pulse energy can also be increased by one or two orders of magnitude; and the greater the dispersion value of the laser, the The maximum pulse energy that can be obtained will also be higher.

Referring to Figure 3, when the pump power is 500mW, the output pulse spectral envelope covers the range of 1530nm-1660nm, including the C-band (1530-1565nm), L-band (1565-1625nm) and U-band (1625- 1675nm); its 3dB spectral width can reach 42nm.

Compared with other conventional dissipative solitons, this type of high-energy wave-free splitting pulse has distinctive features: First, its spectral width is extremely large, covering the C+L+U band, far exceeding the gain bandwidth of the erbium-doped fiber used in the experiment , the gain fiber used in the experiment is C-band erbium-doped fiber. This is because the length of the erbium-doped fiber used in the experiment is relatively long, and the pulse can be fully amplified in the gain fiber. From the gain characteristics of the erbium-doped fiber, it can be known that the spectrum of the output pulse will be strongly broadened in the case of high pump power, even The gain bandwidth of Erbium-doped fiber can be exceeded. Second, the spectral envelope is relatively smooth without any sidebands or steep edges, which is different from the typical spectral shape of ordinary dissipative soliton pulses (with limited gain bandwidth and steep edges).

It is worth noting that although the spectral width of this type of pulse is extremely large and the energy is high, it is similar to the noise-like pulses (noise-like solitons) generated in conventional negative dispersion soliton lasers, but due to the two working in positive/negative dispersion area, the mode-locking mechanism is completely different, so there is an essential difference between the two in the physical sense. This point can also be verified from Figure 5: the noise-like pulse autocorrelation line formed by the negative dispersion laser generally has a very narrow spike center and a very wide base; while the high-energy wave-splitting dissipative pulse The autocorrelation spectral line of is a smooth Gaussian type. Combined with the theoretical analysis of the physical characteristics of the pulse and the mode-locking process of the laser, it can be known that the pulse generated by this laser is a new type of pulse different from any other fiber laser in the past.

(P_avg表示脉冲的平均功率，F为脉冲重复频率8.72MHz，T表示标准时间单位)，经过计算得知，腔外输出的单脉冲能量约为3.44nJ。由于实验中采用的输出耦合器输出率为10％，因此可知，该类型高能量脉冲的单脉冲总能量约为34.4nJ。It can be seen from Fig. 5 that when the pump power is 500mw, the full width at half maximum of the pulse autocorrelation curve is 137ps; considering the pulse is Gaussian, the corresponding pulse width is 97ps respectively. With the increase of the pump power, the intensity of the pulse increases, and the pulse width also gradually increases. Since the output coupler of the laser is located just after the light wave passes through the erbium-doped fiber, it is the place where the pulse width in the cavity is the largest, and the output pulse also has a very large positive chirp. When the pump power is 500mW, the pulse 3dB spectral width The pulse width is 42nm, the center wavelength is 1590nm, and the pulse width is 97ps, so its time-bandwidth product (TBP) is about 483. At this time, the average power of the pulse outside the output cavity is about 30mW; from the single pulse energy calculation formula

(P _avg represents the average power of the pulse, F represents the pulse repetition frequency of 8.72MHz, and T represents the standard time unit). After calculation, the single pulse energy output outside the cavity is about 3.44nJ. Since the output rate of the output coupler used in the experiment is 10%, it can be seen that the total energy of a single pulse of this type of high energy pulse is about 34.4nJ.

After the above experiments and theoretical analysis, this type of laser can overcome the energy peak limitation of negative dispersion lasers, and exhibit completely different characteristics from traditional optical solitons, thereby realizing high-energy wave-splitting pulse output. The erbium-doped fiber laser produced by the invention has broad application prospects in optical communication, optical ranging, photoelectric sensing, medical treatment, etc., because its wavelength is just in the communication window (1550nm) band. And the generated Gaussian pulse has a huge spectral width and high pulse energy. After the pulse width is compressed by a negative dispersion device, it can also be used as a high-energy femtosecond pulse source.

Claims (8)

1. A novel fiber laser system generating high-energy pulses, the system comprising a wavelength division multiplexer, an output coupler, a first polarization controller, a polarization-dependent isolator and a second polarization controller sequentially connected by an optical path; The input end of the wavelength division multiplexer is provided with a pump light source, and it is characterized in that: an erbium-doped (Er) optical fiber is arranged between the output end of the described wavelength division multiplexer and the input end of the output coupler; The length of the erbium fiber is 5-30m. 2. The novel fiber laser system generating high-energy pulses according to claim 1, characterized in that: the length of the erbium-doped fiber is 20m. 3. The novel fiber laser system for generating high-energy pulses according to claim 1 or 2, characterized in that: the model of the wavelength division multiplexer is 980nm/1550nm. 4. The novel fiber laser system for generating high-energy pulses according to claim 3, characterized in that: the output ratio of the output coupler is 10%. 5. The novel fiber laser system for generating high-energy pulses according to claim 4, characterized in that: the output power of the pumping light source is 200-500mW. 6. The novel fiber laser system for generating high-energy pulses according to claim 5, wherein the output power of the pumping light source is 500mW. 7. The novel fiber laser system for generating high-energy pulses according to claim 6, wherein the pumping light source is a single-mode semiconductor laser with an operating wavelength of 980nm. 8. The novel fiber laser system for generating high-energy pulses according to claim 7, characterized in that: the model used for the erbium-doped fiber is Nufern EDFC-980-HP.

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