Chirped pulse amplification system and method based on hybrid gain medium

Abstract

The invention discloses a chirped pulse amplification system and a chirped pulse amplification method based on a mixed gain medium, which efficiently restrain the problem of gain narrowing in the chirped pulse amplification process under the condition of not depending on spectrum shaping by utilizing the characteristic that the central wavelengths of emission spectrums of different types of gain media are different. And optical elements are not additionally introduced, so that energy loss caused by spectrum shaping in the amplification process is avoided, and a reliable technical path is provided for realizing efficient broadband amplification and sub-picosecond output.

Description

Technical Field

[0001] The present invention relates to the field of laser technology, and in particular, to a chirped pulse amplification system and method based on a hybrid gain medium. Background Art

[0002] High-energy high-repetition-rate sub-picosecond lasers are an important research branch of advanced laser technology and can meet the application requirements in fields such as high-energy density physics research under ultrafast time scales and secondary radiation sources. Compared with femtosecond lasers such as titanium sapphire, a higher secondary radiation flux can be obtained on the sub-picosecond time scale. Chirped pulse amplification can fully extract the energy storage of the medium and is the main method for obtaining near-infrared high-energy sub-picosecond lasers.

[0003] In the broadband optical pulse amplification process, the problem of gain narrowing is inevitably faced. The essential reason for generating gain narrowing is the uneven emission spectrum intensity of the gain medium. Before pulse amplification, it is necessary to shape the spectrum of the injected pulse by means of modulation or filtering to achieve the purpose of suppressing gain narrowing.

[0004] Common spectral shaping means include spatial light modulators, programmable acousto-optic modulators, superstructured fiber gratings, and birefringence filtering, etc. However, spatial light modulators are bulky, programmable acousto-optic modulators and superstructured fiber gratings are difficult to apply to high-energy picosecond pulses, birefringence filtering is limited by the actual efficiency, and these methods will all cause significant losses. Existing means for suppressing gain narrowing are difficult to meet the application requirements of high-energy picosecond lasers. Summary of the Invention

[0005] In view of this, the present invention provides a chirped pulse amplification system and method based on a hybrid gain medium. By utilizing the different central wavelengths of the emission spectra of different types of gain media, the system effectively suppresses the gain narrowing problem in the chirped pulse amplification process and can efficiently achieve sub-picosecond-level pulse output without spectral shaping.

[0006] A chirped pulse amplification system based on a hybrid gain medium includes a fiber laser, a beam expander, a shaping device, an isolation device, a chirped pulse amplifier, and a compression device that are sequentially arranged along the light propagation direction on the seed light amplification optical path.

[0007] The fiber laser is used to output a chirped broadband nanosecond pulse as the seed light.

[0008] The beam expander is used to expand the seed light.

[0009] The shaping device is used to control the spot shape, intensity distribution, and spot size of the expanded seed light.

[0010] The isolation device is used to ensure that the seed light only enters the chirped pulse amplifier in the forward direction and the amplified optical pulse does not propagate backward.

[0011] The chirped pulse amplifier includes a power amplifier component and a beam transmission encoding device. There are two sets of power amplifier components, which are arranged oppositely so that the seed light reflects back and forth between them for single-pass or multi-pass amplification. The beam transmission encoding device is arranged between the two sets of power amplifier components to change the transmission direction of the beam refracted from one of the power amplifier components during the amplification process so that it enters the other opposite power amplifier component. The peak wavelengths of the emission spectra of the gain media of the two sets of amplification components are different.

[0012] Specifically, the structures of the two sets of power amplifier components are the same, and each includes a sheet-shaped gain medium, a cooling medium arranged outside the gain medium, a packaging board arranged outside the cooling medium for encapsulating the gain medium and the cooling medium into one body, and a laser pumping device arranged outside the packaging board. The surface of the gain medium in contact with the cooling medium is provided with a broadband high-reflection film for seed light and a high-transmission film for pump light, and the other opposite surface is provided with a broadband high-transmission film for seed light and a high-reflection film for pump light. Both surfaces of the packaging board are provided with high-transmission films for pump light.

[0013] Specifically, the pump intensity of the pump light emitted by the laser pumping device of each set of power amplifier components is determined according to the absorption spectrum of the corresponding gain medium.

[0014] Specifically, the beam transmission encoding device is a reflecting mirror or an encoding prism.

[0015] When the seed light is amplified in a single pass between the two sets of power amplifier components, a reflecting mirror is placed at the beam reflection position on this amplification optical path.

[0016] When the seed light is amplified in multiple passes between the two sets of power amplifier components, a reflecting mirror group or an encoding prism is placed at both the beam reflection position of the single-pass amplification optical path and the beam reflection position between adjacent two-pass amplification optical paths.

[0017] Specifically, the reflecting mirror or the encoding prism is provided with a high-reflection film for seed light.

[0018] Specifically, one or more beam expanding devices are provided.

[0019] Specifically, the compression device includes two pairs of dielectric film gratings with the same line density.

[0020] Specifically, the dielectric film grating is a reflective dielectric film grating or a transmissive dielectric film grating.

[0021] Specifically, the spectral range of the seed light output by the fiber laser matches the emission line shapes of the gain media of the two sets of amplification components.

[0022] A chirped pulse amplification method based on the above-mentioned system specifically includes the following steps:

[0023] The fiber laser emits broadband nanosecond pulses with chirp as the seed light;

[0024] The beam expander expands the seed light, and the expanded seed light enters the shaping device. After the shaping device controls the spot shape, intensity distribution, and spot size of the seed light, it enters the isolation device. The seed light output from the isolation device is incident on one group of power amplifier components of the chirped pulse amplifier at a specific angle. This power amplifier component uses its active mirror configuration to amplify the seed light injected into it. The amplified seed light refracts out of this power amplifier component and, under the action of the beam transmission coding device, is reflected back and incident on another opposite power amplifier component for amplification. This process of repeated reflection realizes single-pass or multi-pass amplification between the two power amplifier components;

[0025] The beam that has undergone single-pass or multi-pass amplification is input into the compression device, and the compression device compresses the beam.

[0026] The beneficial effects of the present invention are:

[0027] By utilizing the characteristic that the emission spectral center wavelengths of different types of gain media are different, the present invention efficiently suppresses the gain narrowing problem in the chirped pulse amplification process without relying on spectral shaping, and does not introduce additional optical elements, avoiding energy loss caused by spectral shaping during the amplification process. It efficiently realizes the output of high-energy sub-picosecond optical pulses, providing a reliable technical path for realizing efficient broadband amplification and sub-picosecond output. Description of the Drawings

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the following-described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0029] Figure 1 It is a schematic diagram of the system composition of the chirped pulse amplification system of the present invention.

[0030] Figure 2 It is a schematic diagram of the structure of the chirped pulse amplifier.

[0031] Figure 3 It is a diagram showing the relationship between the output spectrum and the number of passes of the chirped pulse amplification system in the embodiments of the present invention.

[0032] Figure 4 It is a diagram showing the relationship between the output energy and the number of passes of the chirped pulse amplification system in the embodiments of the present invention.

[0033] The meanings of the reference numerals in the figure are as follows:

[0034] 1 is a fiber laser,

[0035] 2 is a beam expander,

[0036] 3 is a beam shaper,

[0037] 4 is an isolator,

[0038] 5 is a chirped pulse amplifier, 5-1 is the seed light incident position, 5-2 is the seed light output position, 5-3 is the gain medium, 5-4 is the cooling medium, 5-5 is the encapsulation board, 5-6 is the laser pumping device, and 5-7 is a mirror or a coded prism.

[0039] 6 is a compressor. Detailed implementation manners

[0040] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be described below through specific embodiments shown in the accompanying drawings. However, it should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, the descriptions of well-known structures and technologies are omitted to avoid unnecessarily confusing the concepts of the present invention.

[0041] To better understand the technical solution of the present invention, the present invention will be described in detail below with reference to the accompanying drawings.

[0042] The present invention provides a chirped pulse amplification system based on a hybrid gain medium, including a fiber laser 1, a beam expander 2, a beam shaper 3, an isolator 4, a chirped pulse amplifier 5 and a compressor 6 arranged on the seed light amplification optical path. The fiber laser 1, the beam expander 2, the beam shaper 3, the isolator 4, the chirped pulse amplifier 5 and the compressor 6 are arranged in sequence along the light propagation direction.

[0043] The fiber laser 1 is used to output a chirped broadband nanosecond pulse as the seed light. The output spectral range thereof matches the emission line shape of the two gain media 5-3, and the single pulse energy, time waveform, spectral distribution and repetition frequency of the seed light output by the fiber laser 1 can be selected according to specific requirements.

[0044] The beam expander 2 is used to expand the seed light. On the entire seed light amplification optical path, one or more beam expanders 2 can be arranged according to the specific requirements of beam amplification. Each beam expander 2 can also be arranged in multiple stages to achieve multi-stage amplification of the beam. The beam expander 2 can use a Galilean configuration or a Kepler configuration according to requirements to achieve high-quality beam expansion.

[0045] The shaping device 3 is used to control the spot shape, intensity distribution, and spot size of the expanded seed light. The spot shape of the seed light adjusted by the shaping device 3 is preferably matched with the shape of the gain medium. For example, if the gain medium used is square, the shaping device 3 adjusts the spot shape to square; if the gain medium used is circular, the shaping device 3 adjusts the spot shape to circular. In this embodiment, the shaping device 3 adjusts the spot of the seed light to a square spot.

[0046] The isolation device 4 is used to ensure that the seed light only enters the chirped pulse amplifier 5 in the forward direction and the amplified optical pulse will not be transmitted in the reverse direction to ensure the safe use of the fiber laser 1. The isolation device 4 can use a Faraday isolator or other optical isolation elements or structures.

[0047] The chirped pulse amplifier 5 includes a power amplifier component and a beam transmission coding device. There are two sets of power amplifier components, and the two sets of power amplifier components are arranged opposite to each other so that the seed light makes a round trip or multiple round trips between them for amplification. The beam transmission coding device is arranged between the two sets of power amplifier components to change the transmission direction of the beam refracted from one of the power amplifier components during the amplification process so that it enters the other opposite power amplifier component. The peak wavelengths of the emission spectra of the gain media of the two sets of amplifier components are different.

[0048] Specifically, the structures of the two sets of power amplifier components are the same, and each includes a sheet-shaped gain medium 5-3, a cooling medium 5-4 arranged outside the gain medium 5-3, a packaging board 5-5 arranged outside the cooling medium 5-4 for encapsulating the gain medium and the cooling medium into one body, and a laser pumping device 5-6 arranged outside the packaging board 5-5. The surface of the gain medium 5-3 in contact with the cooling medium 5-4 is provided with a broadband high-reflection film for seed light and a high-transmission film for pump light, and the other opposite surface is provided with a broadband high-transmission film for seed light and a high-reflection film for pump light. That is, for each gain medium 5-3, its outer side is coated with a broadband high-reflection film for seed light and a high-transmission film for pump light, and its inner side is coated with a broadband high-transmission film for seed light and a high-reflection film for pump light to ensure that the seed light makes multiple round trips between the two gain media for amplification. Both surfaces of the packaging board 5-5 are provided with high-transmission films for pump light. The pump light can pass through the packaging board 5-5, the cooling medium 5-4, and the surface of the gain medium coated with the high-transmission film for pump light and enter the interior of the gain medium to be absorbed, and then is reflected by the surface of the gain medium coated with the high-reflection film for pump light and absorbed again. The double-pass absorption of the pump light in the gain medium can improve the energy storage.

[0049] The gain medium 5-3, the cooling medium 5-4 and the encapsulation plate 5-5 form a three-piece structure, and the cooling medium 5-4 is isolated from the outside through the gain medium 5-3 and the encapsulation plate 5-5. Pumping light highly transmissive films are provided on both surfaces of the encapsulation plate 5-5 to ensure that the pumping light can penetrate the encapsulation plate 5-5 and enter the gain medium. In this embodiment, the laser pumping device 5-6 uses a laser diode array, the area array size of the laser diode array matches the size of the gain medium 5-3, and the emission wavelength of the laser diode array is set to 940 nm.

[0050] The above two sets of power amplifier components can be placed completely symmetrically or staggeredly, that is, the chirped pulse amplifier 5 can be a symmetric structure with two gain media 5-3 as the symmetry center, or an asymmetric structure in which the two gain media 5-3 are placed opposite and staggered.

[0051] The amplification configuration of the above power amplifier components for the light beam is an active mirror configuration. In this configuration, after the seed light enters the interior of the gain medium 5-3 of the first set of power amplifier components at a certain angle from 5-1, since pumping light highly transmissive films are provided on both surfaces of the encapsulation plate 5-5 and a pumping light highly transmissive film is provided on the outer surface of the gain medium, the pumping light can also pass through the encapsulation plate 5-5 and the cooling medium 5-4 and enter the interior of the gain medium 5-3. The seed light is reflected by the outer surface of the gain medium and then refracted out of the gain medium through the inner surface of the gain medium. Only transmitting in this gain medium can make the output pulse have better beam quality; after the signal light is refracted out of the gain medium, it is projected onto the beam transmission coding device, and the beam transmission coding device changes the transmission direction of the gain medium so that it returns and enters the interior of the gain medium of the second set of power amplifier components placed opposite and realizes the same transmission in this gain medium and then projects out, and then under the action of the beam transmission coding device, it returns and enters the interior of the gain medium 5-3 of the first set of power amplifier components again. In this way, multiple passes of amplification are carried out between the two sets of power amplifier components, and it exits from 5-2 and is transmitted into the compression device 6 for compression.

[0052] In this application, single-pass amplification is defined as the seed light passing through the upper and lower gain media once each.

[0053] When the seed light only performs single-pass amplification between the two sets of power amplifier components, the beam transmission coding device is a mirror, and the mirror is placed on one side of the two gain media 5-3. The up, down, left, and right positions of the mirror can be specifically adjusted according to actual usage requirements. Preferably, a seed light highly reflective film is also coated on the mirror.

[0054] When the seed light is multi-pass amplified between two sets of amplifier components, mirror groups or encoding prisms are placed at the beam turning positions of the single-pass amplification optical path and between the beam turning positions of adjacent two-pass amplification optical paths to achieve multi-pass amplification of the seed light between two gain media, realizing efficient extraction of pump energy and high-energy pulse output after amplification. Preferably, a high-reflection film for the seed light is also coated on the mirror.

[0055] Since the peak wavelengths of the emission spectra of the gain media of the two sets of amplifier components are different, the pump intensity of the pump light emitted by the laser pump device 5-6 of each set of amplifier components is determined according to the absorption spectrum of its corresponding gain medium 5-3.

[0056] The compression device 6 includes two pairs of dielectric film gratings with the same line density. In actual use, gratings with appropriate scale densities are selected according to specific requirements, and the grating size and the distance between the two pairs of gratings are designed according to specific requirements. In this embodiment, the dielectric film grating is a reflective dielectric film grating or a transmissive dielectric film grating.

[0057] The specific implementation manners of the present invention will be described in detail below through specific examples.

[0058] In this embodiment, the upper and lower sheet-like gain media 5-3 are respectively selected as Yb:YAG and Yb:YGG. The peak wavelength of the emission spectrum of Yb:YAG is 1030 nm, and the full width at half maximum is 9 nm; the peak wavelength of the emission spectrum of Yb:YGG is 1024 nm, and the full width at half maximum is 22 nm. Using this hybrid gain medium amplification configuration can effectively reduce the spectral red shift and gain narrowing problems during the amplification process. The doping concentrations of the two gain media are both 1.75%, the aperture of the gain medium is 60 mm × 40 mm, and the thickness is 6 mm. To ensure that the two gain media 5-3 have similar amplification capabilities, therefore, Yb:YAG and Yb:YGG respectively adopt 15 kW / cm 2 and 12 kW / cm 2 pump intensities, the small-signal gain of Yb:YAG is 0.8358 / cm, and the small-signal gain of Yb:YGG is 0.8744 / cm.

[0059] End-face pumping is performed using the laser pump device 5-6. The laser pump device 5-6 is a laser diode array, with a maximum output capacity of 150 kW, a pulse width of 1 ms, and an output current of 400 A. Through the beam transmission encoding device, the seed light is amplified six times.

[0060] The optical fiber laser 1 outputs chirped optical pulses with an energy of approximately 200 μJ, a pulse width of approximately 5 ns, a spectral full width at half maximum (FWHM) of approximately 9 nm, and a repetition rate adjustable range of 1 Hz to 1 kHz, which is used as the seed light for the chirped pulse amplifier 5. After passing through the beam expander 2 and the shaping device 3, the seed light is modulated into a square spot with a size of 5 mm × 5 mm. After passing through the isolation device 4, approximately 100 μJ of seed light is injected into the chirped pulse amplifier 5.

[0061] The spectral evolution of the output pulse is as Figure 3 shown. As the number of amplification passes increases, the spectral bandwidth gradually decreases, which is mainly caused by gain narrowing. At the same time, it should be noted that due to the different emission spectral peaks of Yb:YGG and Yb:YAG, the speed of gain narrowing is effectively reduced. For the six-pass output, under the same pumping conditions, the output bandwidth (FWHM) of the hybrid gain medium is 2.7 nm, which is larger than that of the single-medium amplification configuration.

[0062] During the multi-pass amplification process, the variation of the output energy with the number of amplifier passes is as Figure 4 shown. The output of the sixth pass is 3.1 J, and the B integral is less than π. Considering the actual compression efficiency of each grating ~95% and the efficiency of the entire compressor ~80%, the pulse energy after compression is approximately 2.5 J, the Fourier transform limited pulse width of the output pulse is 0.6 ps, and the corresponding peak power is approximately 4.2 TW. In this embodiment, the final output fluence of the seed light is 12.4 J / cm 2 , which can be supported by current optical components and coating technologies without causing damage to the devices or media.

[0063] The present invention also provides a chirped pulse amplification method, which amplifies chirped pulses based on the above chirped pulse amplification system, and specifically includes the following steps:

[0064] The optical fiber laser 1 emits broadband nanosecond pulses with chirp as the seed light;

[0065] The beam expander 2 expands the seed light. The expanded seed light enters the shaping device 3. After the shaping device 3 controls the spot shape, intensity distribution, and spot size of the seed light, it enters the isolation device 4. The seed light output from the isolation device 4 is incident on one of the power amplifier components of the chirped pulse amplifier 5 at a specific angle. This power amplifier component uses its active mirror configuration to amplify the seed light injected into it. The amplified seed light refracts out of this power amplifier component and is reflected back into another opposite power amplifier component for amplification under the action of the beam transmission coding device. This process is repeated back and forth to achieve single-pass or multi-pass amplification between the two power amplifier components;

[0066] The beam that has undergone single-pass or multi-pass amplification is input into the compression device 6, and the compression device 6 compresses the beam.

[0067] It should be clear that the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

Claims

1. A chirped pulse amplification system based on a hybrid gain medium, characterized in that: The invention comprises a fiber laser (1), a beam expansion device (2), a shaping device (3), an isolation device (4), a chirped pulse amplifier (5) and a compression device (6) which are arranged in sequence along the light propagation direction on the seed light amplification optical path. The fiber laser (1) is used to output broadband nanosecond pulses with chirps as seed light; The beam expansion device (2) is used to expand the seed light; The shaping device (3) is used to control the spot shape, intensity distribution and spot size of the seed light after beam expansion; The isolation device (4) is used to ensure that the seed light is input into the chirped pulse amplifier (5) only in the forward direction and the amplified light pulse will not be transmitted in the reverse direction; The chirped pulse amplifier (5) comprises a power amplifier component and a light beam transmission encoding device. The power amplifier components are provided in two groups. The two groups of power amplifier components are arranged relative to each other so that the seed light is folded back between the two groups for single-pass or multi-pass amplification. The light beam transmission encoding device is arranged between the two groups of power amplifier components so as to change the transmission direction of the light beam refracted from one of the power amplifier components during the amplification process so that the light beam enters the other opposite power amplifier component. The emission spectrum peak wavelengths of the gain media of the two groups of amplifier components are different.

2. The chirped pulse amplification system based on hybrid gain medium according to claim 1, characterized in that: The two groups of power amplifier components have the same structure, both comprising a sheet-shaped gain medium (5-3), a cooling medium (5-4) arranged outside the gain medium (5-3), a packaging plate (5-5) arranged outside the cooling medium (5-4) for packaging the gain medium and the cooling medium into one, and a laser pumping device (5-6) arranged outside the packaging plate (5-5); a surface of the gain medium (5-3) in contact with the cooling medium (5-4) is provided with a seed light broadband high-reflection film and a pump light high-transmittance film, and the other opposite side surface is provided with a seed light broadband high-transmittance film and a pump light high-reflection film, and both surfaces of the packaging plate (5-5) are provided with pump light high-transmittance films.

3. The chirped pulse amplification system based on hybrid gain medium according to claim 2, characterized in that: The pumping intensity of the pumping light emitted by the laser pumping device (5-6) of each group of power amplifier components is determined according to the absorption spectrum of the corresponding gain medium (5-3).

4. The chirped pulse amplification system based on hybrid gain medium according to claim 1, characterized in that: The light beam transmission encoding device is a reflector or an encoding prism (5-7), When the seed light is amplified in a single pass between the two groups of power amplifier components, a reflector is placed at the beam return position on the amplified light path; When the seed light is multi-pass amplified between two groups of power amplifier components, a reflector group or a coding prism is placed at the beam return position of the single-pass amplification optical path and the beam return position between two adjacent pass amplification optical paths.

5. The chirped pulse amplification system based on hybrid gain medium according to claim 4, characterized in that: The reflector or the coding prism is provided with a seed light high reflection film.

6. The chirped pulse amplification system based on hybrid gain medium according to claim 1, characterized in that: The beam expansion device (2) is provided with one or more.

7. The chirped pulse amplification system based on hybrid gain medium according to claim 1, characterized in that: The compression device (6) comprises two pairs of dielectric film gratings with the same line density.

8. The chirped pulse amplification system based on hybrid gain medium according to claim 7, characterized in that: The dielectric film grating is a reflective dielectric film grating or a transmissive dielectric film grating.

9. The chirped pulse amplification system based on hybrid gain medium according to claim 1, characterized in that: The spectral range of the seed light output by the optical fiber laser (1) matches the emission line type of the gain medium of the two groups of amplification components.

10. A chirped pulse amplification method based on the system according to any one of claims 1 to 9, characterized in that: The specific steps include: The fiber laser (1) emits broadband nanosecond pulses with chirps as seed light; The beam expansion device (2) expands the seed light, and the expanded seed light enters the shaping device (3). The shaping device (3) controls the spot shape, intensity distribution and spot size of the seed light and then enters the isolation device (4). The seed light output from the isolation device (4) is incident at a specific angle into one of the power amplifier components of the chirped pulse amplifier (5). The power amplifier component amplifies the seed light injected therein by using its activated reflector configuration. The amplified seed light is refracted from the power amplifier component and, under the action of the beam transmission encoding device, is folded back and incident into another opposite power amplifier component for amplification. The folding back is repeated in this way to realize single-pass or multi-pass amplification between the two power amplifier components. The light beam amplified in a single pass or multiple passes is input into a compression device (6), and the compression device (6) compresses the light beam.

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