CN109510057B

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

Info

Publication number: CN109510057B
Application number: CN201811530663.9A
Authority: CN
Inventors: 张好; 张临杰; 肖连团; 贾锁堂
Original assignee: Shanxi University
Current assignee: Shanxi University
Priority date: 2018-12-14
Filing date: 2018-12-14
Publication date: 2020-07-10
Anticipated expiration: 2038-12-14
Also published as: CN109510057A

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 1-micron-waveband high-peak-power nanosecond pulse laser

Technical Field

The invention relates to the output of high-energy short pulse laser, which is an optical fiber amplifier taking ytterbium-doped optical fiber as a basic amplification medium, taking an optical fiber type intensity electro-optic modulator and an acousto-optic modulator as pulse shaping devices and taking an optical fiber Bragg grating as a filter.

Background

Compared with a common semiconductor laser, the optical fiber laser has the advantages that due to the characteristic of optical fiber output, the quality of light spots of output laser is very good, and the quality factor M of light beams is²The value is close to 1, the selectable wavelengths are very many due to the abundant energy level structure of the doped rare earth elements, and the wavelength tuning bandwidth is very wide for the same doped element. Because the internal resonant cavity structure has no optical lens or optical grating, the stability is very high. In the field of high-power laser amplifiers, due to the continuous development of optical fiber technology, for example, the appearance of double-clad large-numerical-aperture optical fibers, the rare earth element-doped optical fiber amplifier has the advantages of high power, wide pumping wavelength range, high pumping utilization efficiency and the like, so that the optical fiber laser/amplifier is more and more widely concerned and applied.

Disclosure of Invention

The invention provides a method for generating nanosecond pulse laser with high peak power in a 1-micron wave band. By utilizing wider absorption and emission spectrum of ytterbium element, the ytterbium-doped fiber absorbs the pump laser with 950nm wavelength, amplifies the pulse seed laser shaped by iEOM in 1010nm waveband, and finally obtains the pulse laser with 1ns pulse width, 100nJ single pulse energy and 1kHz repetition frequency.

The technical scheme adopted by the invention is as follows: the method for generating the 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 high-order property 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 negative first-stage diffraction light of the acousto-optic modulator, and reducing the frequency of the pulse laser by delta f to 250 MHz;

step five, 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 in sequence to perform 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, performing pulse selection, reducing the repetition frequency to 1kHz, selecting the positive first-stage diffraction light of the acousto-optic modulator, wherein the pulse laser frequency compensation delta f is 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.

As a preferred mode: the method comprises the following steps that an optical fiber type intensity electro-optic modulator iEOM chops a continuous seed light source into pulse light signals, the signal source and a radio frequency amplifier are utilized to load the pulse light signals with pulse rising edges of 100ps, pulse widths 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 the same pulse light signals as the pulse lasers shaped by the seed light source are obtained at the output port of the optical fiber type intensity electro-optic modulator iEOM;

as a preferred mode: 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 frequency of 1kHz and the amplitude of 100mVpp is added to direct current bias voltage, then the small modulation signal 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 detected signals are fed back to the bias direct current controller for demodulation, 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 direct current bias voltage of the iEOM is stabilized, so that the optical pulse output with stable amplitude and phase is obtained.

As a preferred mode: 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.

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

As a preferred mode: 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.

As a preferred mode: 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.

As a preferred mode: 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.

As a preferred mode: 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.

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

fig. 1 shows experimental principles of pulsed light generation and stabilization. The generation of the pulse light is realized by using a fiber-optic intensity electro-optic modulator. Unlike commonly used phase electro-optic modulators, the working principle of intensity electro-optic modulators (iEOM) is based on the Mach-Zehnder interference effect. The iEOM internally comprises two paths of optical waveguides a and b, wherein one path b is connected with an electrode, when laser is input, one beam of light is split to reach an output end through the waveguide a, the other beam of light is split to reach the output end through the waveguide b, when the waveguide b is loaded with voltage, the refraction rate of the waveguide b is changed, the phase of the laser passing through the waveguide is changed, and when the two paths of laser coincide at the output end, interference occurs. If the applied voltage value makes the laser phase change through the b waveguide by half a period, i.e. the phase changes by pi, destructive interference will occur when the two paths of light coincide, and the voltage value at this moment is called half-wave voltage. Therefore, if a pulse signal with half-wave voltage amplitude is input into the radio frequency input port of the iEOM, corresponding pulse laser is output at the output end of the iEOM.

The energy amplification process of the laser is based on the absorption and emission spectrum of the rare earth element, and for the absorption and emission spectrum of the ytterbium element, the wavelength range of the absorption spectrum is from 850nm to 1050nm, and the absorption spectrum has an absorption maximum value with a narrow width around 970 nm; the wavelength range of the emission spectrum of the fiber laser has an absorption maximum value from 900nm to 1150 nm near 1030nm, and the absorption spectrum and the emission spectrum are partially overlapped, so that the seed laser light which is generally smaller than 1030nm can be greatly reabsorbed while being amplified, and the working wavelength of most commercial fiber lasers in the band is about 1060 nm. The absorption consumption of the pump laser by the fiber and the amplification of the seed laser can be expressed by a rate equation:

wherein + represents the co-transmission of the pump light and the seed light, and-represents the reverse transmission of the two

Indicating the power of the pump light and the seed light when they are transmitted in the same direction and in the opposite direction,

and

representing emission and absorption cross sections, respectively, n1 and n2 representing atomic population numbers of lower and upper energy levels,_p/sthe cross-sectional overlap of the pump light and the seed light, i.e. the ratio of the core diameter of the optical fiber to the area of the propagation region of the pump light, is shown.

Since ytterbium element has a wide Emission spectrum, in the process of fiber amplification, a signal having a wavelength of 1010nm is Amplified based on a stimulated Emission form, and a Spontaneous Emission Amplified optical signal having a wavelength of other bands than 1020nm is also present, and thus it is necessary to suppress Amplified Spontaneous Emission (ASE) of this portion. The purpose of wavelength selection is achieved by suppressing spontaneous emission light by using the fiber Bragg grating. The transmission spectrum of the fiber bragg grating used is shown in fig. 4. It can be seen that within the wavelength range of 0.5nm, the reflectivity reaches 98.5%, and the wavelengths of other bands substantially all pass through, so that the laser light of the selected wavelength is reflected while passing through the grating, and the amplified spontaneous emission light of other bands, particularly wavelengths greater than 1020nm, is filtered out by the grating.

For the amplification of 1010nm laser light according to the present invention, it is also possible to reduce the reabsorption of laser light of that wavelength by the optical fiber by limiting the optical fiber length and to suppress the stimulated brillouin scattering effect and the amplification effect of spontaneous emission.

Drawings

FIG. 1 is a schematic diagram of a structure for realizing stable pulse light output by using an optical fiber type intensity electro-optic modulator;

FIG. 2 is a schematic diagram of the design of the method of the present invention including pulse shaping with a fiber-optic intensity electro-optic modulator as the core and a pulse selection module with an acousto-optic modulator as the core; a two-stage optical fiber pre-amplification module; a two-stage fiber Bragg grating wavelength selection module; a main optical fiber amplifying module; pump laser sources for respectively exciting the pre-amplification ytterbium-doped fiber and the main amplification ytterbium-doped fiber;

figure 3 is a computer simulation of the reflection and transmission of a fiber bragg grating. The simulation takes the lower limit wavelength 1008nm of the optical fiber amplifier as an example, the reflectivity of the grating reaches 98.5 percent in the range of 0.5nm of the full width at half maximum of the reflected signal, and the optical signals with other wavelengths have transmission close to 100 percent;

fig. 4 is a simulation result of fiber amplification calculation based on a rate equation, which takes the lower limit wavelength 1008nm of the fiber amplifier as an example, and shows the change of the peak power of the pulsed light when passing through each device of the fiber amplifier in sequence, the abscissa shows the ordinal number of the device, and the hatching shows the parts of the two-stage pre-amplification ytterbium-doped fiber and the main amplification ytterbium-doped fiber.

Detailed Description

The method for generating the 1 mu m waveband high-peak power nanosecond pulse laser comprises the following steps of ① emitting continuous seed laser with the wavelength range of 1000-1085 nm and the power of about 100 mW. to adjust the quality and polarization of a light beam so that the continuous seed laser presents good fundamental mode Gaussian characteristics and a stable linear polarization state, ② coupling incident light into an optical fiber type intensity electro-optic modulator (iEOM) and shaping the continuous seed light source into pulse laser output in a time domain space through modulation, ③. laser pulses output from the iM sequentially enter a primary pre-amplification optical fiber and a primary fiber Bragg pulse optical grating for wavelength selection and spontaneous radiation amplification inhibition, ④. pulse light of the primary pre-amplification optical fiber passes through the primary acousto-optic modulator (AOM1) for pulse selection, the repetition frequency is reduced to 100kHz, negative primary diffraction light of the acousto-optic modulator is selected, the pulse laser frequency is reduced by delta f, 86 ⑤. the pulse laser enters a secondary pre-amplification ytterbium fiber and the primary Bragg pulse optical fiber Bragg grating for pulse amplification, the pulse frequency is further inhibited from the kHz, the signal-to-noise ratio of the ytterbium doped optical modulator is increased, the pulse laser is increased by the pulse frequency of the primary pre-amplified ytterbium doped optical fiber, the primary pre-amplified optical fiber, the pulse optical fiber is adjusted pulse laser, the pulse frequency is adjusted by the primary pulse frequency of the secondary pre-amplified by the primary pulse laser, the secondary pulse frequency of the primary amplified ytterbium doped optical fiber is adjusted pulse laser, the primary amplified by the secondary amplified by the primary amplified optical fiber, the secondary amplified optical fiber is adjusted pulse frequency of the primary amplified optical fiber, the primary amplified.

The main instrument device of the invention comprises a pulse shaping module. The optical fiber type intensity electro-optic modulator is taken as a core, and comprises an electro-optic modulator crystal taking an optical fiber as an input/output port; a high bandwidth signal source and a radio frequency signal amplifier for providing an electrical pulse signal with a fast rising edge (<100 ps); the DC bias voltage controller is used for providing DC bias to set the working point of the electro-optic modulator and the stable working point position of the feedback signal. And b, taking the single-clad ytterbium-doped optical fibers with the lengths of 3m, 2.5m and 2.2m as the primary preamplification ytterbium-doped optical fiber, the secondary preamplification ytterbium-doped optical fiber and the main amplification ytterbium-doped optical fiber, and respectively connecting the primary preamplification ytterbium-doped optical fiber and the main amplification ytterbium-doped optical fiber with the preceding stage device through an optical fiber splicing technology. c. Two-stage fast acousto-optic modulator (carrier frequency 250MHz) is used as pulse selecting device to reduce the repetition frequency of pulse light from 3 MHz to 100kHz and 1kHz respectively. d. The two-stage fiber Bragg grating is used as a filter device to inhibit spontaneous radiation amplification and improve the signal-to-noise ratio of amplified signal light. e. The two-stage pump laser source is respectively a pump source 1 with constant power for exciting the two-stage pre-amplification ytterbium-doped fiber, and a pump source 2 with adjustable power for exciting the main amplification ytterbium-doped fiber.

Claims

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. 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. 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. 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. 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. 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. 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. 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. 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.