CN109510057B - Method for generating 1-micron-waveband high-peak-power nanosecond pulse laser - Google Patents

Abstract

The present invention relates to the output of high-energy short-pulse laser light. A method for generating 1 mu m wave band high peak power nanosecond pulse laser is characterized in that ytterbium element is doped in the core diameter of a common optical fiber, based on the characteristic absorption and emission spectrum of the ytterbium element, laser with the wavelength of 950nm is used as a pumping source, seed pulse light modulated by an optical fiber type intensity electro-optic modulator is amplified, wavelength selection and spontaneous radiation amplification inhibition are carried out by an optical fiber Bragg grating, and pulse selection is carried out by an acousto-optic modulator. Finally, the obtained wave length is 1010 +/-10 nm, the pulse width is 1-5 ns, and the repetition frequency is>1kHz, single pulse energy>A strong pulsed laser at 100 nJ. The technical method expands the lower limit of the wavelength of 1064 nm pulse laser commonly used at present to a position close to 1 μm, such as rubidium atom 6p →nCoherent excitation of specific atomic energy level structures such as the sridelberg state provides an effective tool.

Description

Method for generating high peak power nanosecond pulsed laser in 1μm band

technical field

The invention relates to the output of high-energy short-pulse laser, which uses ytterbium-doped fiber as the basic amplifying medium, fiber-type intensity electro-optic modulator and acousto-optic modulator as pulse-shaping device, and fiber Bragg grating as a filter fiber amplifier.

Background technique

Compared with common semiconductor lasers, fiber lasers have very good spot quality due to their fiber output characteristics, and the beam quality factor M ² value is close to 1, and due to the rich energy level structure of doped rare earth elements, the choice of There are many wavelengths, and for the same doping element, the wavelength tuning bandwidth is very wide. Since the internal resonator structure has no optical mirrors or gratings, its stability is very high. In the field of high-power laser amplifiers, due to the continuous development of fiber technology, such as the emergence of double-clad large numerical aperture fibers, fiber amplifiers using rare earth elements doped have the advantages of high power, wide pump wavelength range, and high pump utilization efficiency. As a result, fiber lasers/amplifiers have received more and more extensive attention and applications.

SUMMARY OF THE INVENTION

The invention provides a method for generating nanosecond pulse laser with high peak power in 1 μm waveband. Utilizing the broad absorption and emission spectrum of ytterbium element, the ytterbium-doped fiber absorbs the pump laser with a wavelength of 950 nm, and amplifies the pulsed seed laser shaped by iEOM in the 1010 nm band. A pulsed laser with a frequency of 1 kHz.

The technical scheme adopted in the present invention is: a method for generating a nanosecond pulse laser with a high peak power in a 1 μm band, which is characterized in that the installation is carried out in the following steps:

Step 1. The semiconductor laser emits a continuous seed laser with a wavelength range of 1000-1085 nm and a power of 100 mW, and the beam quality and polarization of the continuous seed laser are adjusted so that it exhibits a good fundamental mode Gaussian property and a stable linear polarization state;

Step 2, the continuous seed laser is coupled into the optical fiber intensity electro-optic modulator iEOM as incident light, and the continuous seed light source is shaped into pulse laser output through modulation;

Step 3: The pulsed laser output from the iEOM enters the first-stage pre-amplified ytterbium-doped fiber and the first-stage fiber Bragg grating in turn, and performs the first wavelength selection and the first spontaneous emission amplification suppression, so that the wavelength is 1010 ± 0.5nm , SNR>60dB;

Step 4: Input the pulsed laser that has undergone the first wavelength selection and the first spontaneous emission amplification and suppression into the first-stage acousto-optic modulator AOM1 for pulse selection, reduce the repetition frequency to 100kHz, and select the negative stage of the acousto-optic modulator Diffracted light, the pulse laser frequency is reduced by Δf=250MHz;

Step 5: Input the pulsed laser pulse selected by the first-stage acousto-optic modulator AOM1 into the second-stage pre-amplified ytterbium-doped fiber and the second-stage fiber Bragg grating in turn for the second wavelength selection and the second spontaneous emission amplification suppression, so that the The wavelength is 1010±0.5nm, and the signal-to-noise ratio is >60dB;

Step 6: Input the pulsed laser that has undergone the second wavelength selection and the second spontaneous emission amplification and suppression into the main amplifying ytterbium-doped fiber, and use the power-adjustable pump source to excite to obtain a pulse width of 1ns, a single pulse energy of 100nJ, and a repetition rate of 100nJ. 100kHz pulsed laser;

Step 7: Input the pulsed laser excited by the pump source into the second-stage acousto-optic modulator AOM2, perform pulse selection, reduce the repetition frequency to 1 kHz, select the positive first-order diffracted light of the acousto-optic modulator, and compensate the pulse laser frequency Δf=250MHz, the output wavelength is 1010nm, the pulse width is 1ns, the single pulse energy is 100nJ, and the repetition frequency is 1kHz.

As a preferred way: the optical fiber intensity electro-optic modulator iEOM chops the continuous seed light source into pulsed optical signals, and uses the signal source and the radio frequency amplifier to make the pulse rising edge < 100ps, pulse width 1ns, pulse width jitter < 10ps, repetition frequency The electrical pulse signal >1MHz and the pulse amplitude is the half-wave voltage of EOM is loaded into the RF input of the optical fiber intensity electro-optical modulator iEOM, and the same pulsed optical signal as the seed light source is shaped into a pulsed laser is obtained at its output port;

As a preferred method: when pulse modulation is performed by the optical fiber intensity electro-optic modulator iEOM, it is necessary to ensure that the amplitude and phase of the pulsed light output are stable. Using the bias DC controller, the additional frequency of the DC bias voltage is 1kHz, and the amplitude is ~ 100mVpp. The small modulated signal is then loaded into the bias DC input port of the fiber-optic intensity electro-optic modulator iEOM. The output end of the fiber-optic intensity electro-optic modulator iEOM separates 1% of the optical signal for detection by a photodiode, and feeds back the detection signal to the bias. Set the DC controller to demodulate and compensate the DC output voltage of the controller, so that the operating point of the optical fiber intensity electro-optic modulator iEOM is set in the MIN mode of the transfer function, and the DC bias voltage of the iEOM is stabilized to obtain the amplitude and phase. Stable light pulse output.

As a preferred way: the light pulse passes through the first-stage pre-amplified ytterbium-doped fiber and the first-stage fiber Bragg grating again after passing through the first-stage pre-amplification ytterbium-doped fiber and the first-stage fiber Bragg grating, so as to improve the utilization of the pump laser efficiency.

As a preferred way: the pump source in the amplification stage of the second-stage pre-amplification ytterbium-doped fiber adopts a fixed power output.

As a preferred way: the carrier frequencies of the two-stage acousto-optic modulators are the same as 250MHz. When the pulsed light passes through the acousto-optic modulator, the frequency shift of the seed light due to diffraction will be canceled.

As a preferred way: both levels of fiber Bragg gratings have a bandwidth of 0.5 nm, so by replacing Bragg gratings with different center wavelengths, the purpose of wavelength selection can be achieved.

As a preferred way: the change of the frequency of the seed laser caused by the dispersion effect of the fiber Bragg grating is the change in the pulse 100fs range, and the influence on the pulse width can be ignored.

As a preferred way: all fiber-based devices, including fiber-based intensity electro-optic modulators, two-stage pre-amplified ytterbium-doped fibers, fiber Bragg gratings, and main-amplified ytterbium-doped fibers, are connected by fiber splicing technology. The output of one stage device is connected to the input port of the next stage to reduce the attenuation caused by fiber coupling.

The working principle of the method of the present invention mainly includes two main processes of pulse generation and laser amplification:

Figure 1 shows the experimental principle of pulsed light generation and stabilization. The generation of pulsed light is realized using a fiber-optic intensity electro-optic modulator. Unlike the commonly used phase electro-optic modulators, the working principle of intensity-type electro-optic modulators (iEOMs) is based on the Mach-Zehnder interference effect. That is to say, the iEOM includes two optical waveguides a and b, one of which is connected to the electrode. When the laser is input, it is divided into beams, one beam is divided into the output end through the waveguide a, and the other beam is divided into the output end through the waveguide b. When a voltage is applied to the b-waveguide, its refractive index changes, and the phase of the laser light passing through the waveguide changes. When the two lasers overlap at the output port, interference will occur. If the applied voltage value makes the phase of the laser passing through the b-waveguide change by half a cycle, that is, its phase changes by π, destructive interference will occur when the two paths of light overlap, and the voltage value at this time is called the half-wave voltage. Therefore, if a pulse signal whose amplitude is half-wave voltage is input to the RF input port of the iEOM, the corresponding pulse laser will be output at the output end of the iEOM.

The energy amplification process of laser is based on the absorption and emission spectrum of rare earth elements. For the absorption and emission spectrum of ytterbium element, the wavelength range of the absorption spectrum is from 850nm to 1050nm, and there is a very narrow absorption pole around 970nm. The wavelength range of its emission spectrum is from 900 nm to 1150 nm, and the absorption maximum value is around 1030 nm. Since the absorption spectrum and the emission spectrum partially overlap, the seed laser generally less than 1030 nm will be amplified to a large extent at the same time. Therefore, the operating wavelength of most commercial fiber lasers in this band is around 1060 nm. The absorption consumption of the pump laser by the fiber and the amplification of the seed laser can be expressed by the rate equation:

表示泵浦光和种子光同向和反向传输时的功率，

和

分别表示发射和吸收截面，n1和n2表示 下能级和上能级的原子布居数，Γ_p/s表示泵浦光与种子光的截面重叠大小，即光 纤芯径与泵浦光传播区域的面积之比。Among them, + means that the pump light and seed light are transmitted in the same direction, and - means that the two are transmitted in the opposite direction

represents the power when the pump light and the seed light are transmitted in the same direction and in the opposite direction,

and

represent the emission and absorption cross-sections, respectively, n1 and n2 represent the atomic population of the lower and upper energy levels, Γ _p/s represents the cross-section overlap between the pump light and the seed light, that is, the fiber core diameter and the pump light propagation area area ratio.

Due to the wide emission spectrum of ytterbium element, in the process of fiber amplification, in addition to the signal light with a wavelength of 1010nm being amplified based on stimulated radiation, there are also other wavelengths greater than 1020nm. This part of the amplified spontaneous emission light (Amplified Spontaneous Emission, ASE). Here, the fiber Bragg grating is used to suppress the spontaneous emission light to achieve the purpose of wavelength selection. Figure 4 shows the transmission spectrum of the fiber Bragg grating used. It can be seen that in the wavelength range of 0.5nm, its reflectivity reaches 98.5%, and other wavelengths basically pass through, so the laser light of the selected wavelength is reflected when passing through the grating, and other wavelengths, especially the spontaneous emission amplification light with a wavelength greater than 1020nm Filtered out by grating transmission.

For the 1010nm laser amplification involved in the present invention, it is also possible to reduce the reabsorption of the wavelength laser by the fiber and suppress the stimulated Brillouin scattering effect and the amplification effect of spontaneous emission by limiting the length of the fiber.

Description of drawings

Figure 1 is a schematic diagram of the structure of using a fiber-optic intensity electro-optic modulator to achieve stable pulsed light output;

Fig. 2 is the design principle diagram of the method of the present invention, which includes the pulse shaping with the fiber intensity electro-optic modulator as the core and the pulse selection module with the acousto-optic modulator as the core; the two-stage fiber pre-amplification module; the two-stage fiber Bragg The grating wavelength selection module; the main fiber amplification module; the pump laser source for respectively exciting the pre-amplifying ytterbium-doped fiber and the main amplifying ytterbium-doped fiber;

Figure 3 is a computer simulation of fiber Bragg grating reflection and transmission. The simulation takes the lower wavelength limit of 1008 nm of the fiber amplifier as an example. In the range of the full width at half maximum of the reflected signal of 0.5 nm, the reflectivity of the grating reaches 98.5%, while the optical signals of other wavelengths have nearly 100% transmission;

Fig. 4 is the calculation and simulation result of fiber amplification based on the rate equation. The simulation takes the lower limit wavelength of the fiber amplifier of 1008 nm as an example, and shows the change of the peak power of the pulsed light when it passes through each device of the fiber amplifier in turn. The abscissa represents the ordinal number of the device. , where the shading represents the part of the two-stage pre-amplification Ytterbium-doped fiber and the main-amplification Ytterbium-doped fiber.

Detailed ways

The method for generating a nanosecond pulse laser with high peak power in 1 μm band includes the following steps: 1. The semiconductor laser emits a continuous type seed laser with a wavelength range of 1000-1085 nm and a power of about 100 mW. Adjust the beam quality and polarization to make it exhibit good fundamental mode Gaussian characteristics and stable linear polarization state; ②. Coupling the incident light into a fiber-based intensity electro-optic modulator (iEOM), and modulates the continuous seed light source in the time domain space Shaped into pulsed laser output; ③. The laser pulse output from the iEOM enters the first-stage pre-amplification fiber and the first-stage fiber Bragg grating in turn for wavelength selection and spontaneous emission amplification suppression; ④. The pulsed light passing through the first-stage pre-amplification fiber passes through The first-order acousto-optic modulator (AOM1) performs pulse selection, the repetition frequency is reduced to 100kHz, the negative first-order diffracted light of the acousto-optic modulator is selected, and the pulse laser frequency is reduced by Δf; ⑤. The pulsed laser enters the second-stage pre-amplified ytterbium-doped fiber and the second-stage fiber Bragg grating, which further suppresses spontaneous radiation amplification and improves the signal-to-noise ratio of pulsed light in the 1010nm band; , to obtain a pulsed laser with a pulse width of ~1ns, a single pulse energy of ~100nJ, and a repetition rate of ~100kHz; ⑦. The pulsed light passes through the secondary acousto-optic modulator (AOM2), and the pulse selection is performed, and the pulsed light repetition frequency is reduced to ~1kHz, And select the positive first-order diffracted light of the acousto-optic modulator, the pulse laser frequency compensates Δf, and returns to the same frequency as the seed laser. Finally, the output wavelength of the fiber is ~1010nm, the pulse width is ~1ns, the single pulse energy is ~100nJ, and the repetition frequency is ~1kHz. pulsed laser.

The main instrumentation of the present invention includes, a. a pulse shaping module. Focusing on fiber-optic intensity electro-optic modulator, including electro-optic modulator crystal with fiber as input/output port; high-bandwidth signal source and RF signal amplifier are used to provide electrical pulse signal with fast rising edge (<100ps); DC bias The voltage controller is used to provide the DC bias to set the working point of the electro-optic modulator and the feedback signal to stabilize the position of the working point. b. The single-clad Ytterbium-doped fibers with lengths of 3m, 2.5m and 2.2m are used as the primary and secondary pre-amplification ytterbium-doped fibers and the main-amplification ytterbium-doped fibers, which are respectively connected to the previous stage devices through fiber splicing technology. c. A two-stage fast acousto-optic modulator (carrier frequency 250MHz) is used as a pulse selection device to reduce the repetition frequency of the pulsed light from 3 MHz to 100 kHz and 1 kHz respectively. d. The two-stage fiber Bragg grating is used as a filter element to suppress the amplification of spontaneous radiation and improve the signal-to-noise ratio of the amplified signal light. e. Two-stage pump laser source, respectively, a pump source 1 with constant power is used to excite the two-stage pre-amplification ytterbium-doped fiber, and a power-adjustable pump source 2 is used to excite the main-amplifier ytterbium-doped fiber.

Claims (9)

1. The method for generating the 1.1 mu m wave band high peak power nanosecond pulse laser is characterized in that: the following steps are carried out

   Step one, a semiconductor laser emits continuous seed laser, the wavelength range of the continuous seed laser is 1000-1085 nm, the power of the continuous seed laser is 100mW, and the beam quality and the polarization of the continuous seed laser are adjusted to enable the continuous seed laser to present good fundamental mode Gaussian characteristics and a stable linear polarization state; coupling continuous seed laser serving as incident light into an optical fiber type intensity electro-optic modulator iEOM, and shaping a continuous seed light source into pulse laser output through modulation;

   step three, pulse laser output from iEOM sequentially enters a first-stage pre-amplification ytterbium-doped optical fiber and a first-stage optical fiber Bragg grating for first-time wavelength selection and first-time spontaneous radiation amplification inhibition, so that the wavelength is 1010 +/-0.5 nm, and the signal-to-noise ratio is greater than 60 dB;

   inputting the pulse laser subjected to the first wavelength selection and the first spontaneous radiation amplification inhibition into a first-stage acousto-optic modulator AOM1 for pulse selection, reducing the repetition frequency to 100kHz, selecting the negative first-stage diffraction light of the first acousto-optic modulator, and reducing the frequency of the pulse laser by delta f = 250 MHz;

   step five, sequentially inputting pulse laser selected by the primary acousto-optic modulator AOM1 pulse into a secondary preamplification ytterbium-doped fiber and a secondary fiber Bragg grating for secondary wavelength selection and secondary spontaneous radiation amplification inhibition, so that the wavelength is 1010 +/-0.5 nm, and the signal-to-noise ratio is greater than 60 dB;

   inputting the pulse laser subjected to the secondary wavelength selection and the secondary spontaneous radiation amplification inhibition into a main amplification ytterbium-doped fiber, and exciting by using a pump source with adjustable power to obtain pulse laser with the pulse width of 1ns, the single pulse energy of 100nJ and the repetition frequency of 100 kHz;

   and seventhly, inputting the pulse laser excited by the pumping source into a second-stage acousto-optic modulator AOM2 for pulse selection, reducing the repetition frequency to 1kHz, selecting the positive first-stage diffraction light of a second acousto-optic modulator, wherein the pulse laser frequency compensation delta f = 250MHz, and outputting the pulse laser with the wavelength of 1010nm, the pulse width of 1ns, the single pulse energy of 100nJ and the repetition frequency of 1 kHz.
2. 2. The method for generating a 1 μm-band high-peak-power nanosecond pulsed laser as claimed in claim 1, wherein: the optical fiber type intensity electro-optic modulator iEOM chops a continuous seed light source into pulse light signals, and utilizes a signal source and a radio frequency amplifier to load the pulse light signals with pulse rising edges of <100ps, pulse width of 1ns, pulse width jitter of <10ps, repetition frequency of >1MHz and pulse amplitude of EOM half-wave voltage to the radio frequency input of the optical fiber type intensity electro-optic modulator iEOM, and obtain the same pulse light signals as the pulse light signals shaped into pulse lasers by the seed light source at the output port of the optical fiber type intensity electro-optic modulator iEOM.
3. 3. The method for producing a 1 μm-band high-peak-power nanosecond pulsed laser according to claim 1 or claim 2, wherein: when the pulse modulation is carried out through the optical fiber type intensity electro-optic modulator iEOM, the amplitude and the phase of pulse light output need to be ensured to be stable, a bias direct current controller is utilized, a small modulation signal with the direct current bias voltage additional frequency of 1kHz and the amplitude of 100mVpp is loaded on a bias direct current input port of the optical fiber type intensity electro-optic modulator iEOM, 1% of optical signals are separated from the output end of the optical fiber type intensity electro-optic modulator iEOM and detected by a light diode, the detection signals are fed back to the bias direct current controller to be demodulated, the direct current output voltage of the controller is compensated, the working point of the optical fiber type intensity electro-optic modulator iEOM is set in the MIN mode of a transmission function, and the purpose of stabilizing the direct current bias voltage of the iEOM is achieved, so that the optical pulse output with stable amplitude and phase is obtained.
4. 4. The method for generating a 1 μm-band high-peak-power nanosecond pulsed laser as claimed in claim 1, wherein: the optical pulse passes through the first-stage pre-amplification ytterbium-doped optical fiber and the first-stage fiber Bragg grating again after passing through the first-stage pre-amplification ytterbium-doped optical fiber and the first-stage fiber Bragg grating, and therefore the utilization efficiency of the pump laser is improved.
5. 5. The method for generating a 1 μm-band high-peak-power nanosecond pulsed laser as claimed in claim 1, wherein: the pumping source of the second-stage pre-amplification ytterbium-doped fiber amplification stage adopts fixed power output.
6. 6. The method for generating a 1 μm-band high-peak-power nanosecond pulsed laser as claimed in claim 1, wherein: the carrier frequencies of the two stages of acousto-optic modulators are both 250MHz, and when pulse light passes through the acousto-optic modulators, the seed light frequency offset caused by diffraction can be counteracted.
7. 7. The method for generating a 1 μm-band high-peak-power nanosecond pulsed laser as claimed in claim 1, wherein: the two-stage fiber Bragg gratings have the bandwidth of 0.5nm, so the purpose of wavelength selection can be achieved by replacing Bragg gratings with different central wavelengths.
8. 8. The method for generating a 1 μm-band high-peak-power nanosecond pulsed laser as claimed in claim 1, wherein: the variation of the seed laser frequency caused by the dispersion effect of the fiber bragg grating varies in the range of 100fs of the pulse, which has a negligible effect on the pulse width.
9. 9. The method for generating a 1 μm-band high-peak-power nanosecond pulsed laser as claimed in claim 1, wherein: all devices based on optical fibers, including the optical fiber type intensity electro-optic modulator, the two-stage pre-amplification ytterbium-doped optical fiber, the optical fiber Bragg grating and the main amplification ytterbium-doped optical fiber, are connected with the output of the previous-stage device and the input port of the next stage by utilizing an optical fiber splicing technology so as to reduce attenuation caused by optical fiber coupling.

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