Ultra-short pulse width multi-pass stretching system and method

Abstract

The application discloses a kind of ultra-short pulse width multi-path broadening system and method, it is related to high-power ultra-short pulse amplification field.The multi-path broadening system includes at least one body bragg grating and polarization reflection unit;The multi-path broadening system utilizes optical path design to make pulse laser multiple times through body bragg grating, realizes the multi-path body bragg grating pulse broadening of pulse laser in system by adjusting the polarization state of light beam.Compared with prior art, the application utilizes optical path design to make pulse laser multiple times vertically through body bragg grating, realizes the multi-path body bragg grating pulse broadening of pulse laser in system by adjusting the polarization state of light beam, so as to realize the great broadening of pulse width to nanosecond above, greatly reduce the complexity of system without the help of additional devices such as optical fiber, increase the stability of system, and realize high-power laser output.

Description

Technical Field

[0001] This invention relates to the field of high-power ultrashort pulse amplification, and in particular discloses an ultrashort pulse width multi-pass stretching system and method based on a volume Bragg grating. Background Technology

[0002] In existing technologies, ultrashort pulse laser systems generally employ chirped pulse amplification (CPA) technology. The seed pulse is a femtosecond pulse with a pulse width typically within 1 ps (picosecond). Therefore, before amplification, the pulse width needs to be stretched to several hundred ps or even nanoseconds (ns). The wider the stretched pulse, the greater the system's output capability. Stretchers typically employ adjustable grating pairs or prisms, and the stretching amount of these devices is proportional to the optical path length within the device. To stretch to 1 ns, the device length generally needs to be close to 2 meters, occupying a huge amount of space and exhibiting low stability. This limits their use primarily in laboratory stabilization platforms, restricting the development of commercial ultrashort pulse lasers.

[0003] Currently, there exists a type of stretcher that is very small in size and easy to adjust in terms of optical path, but whose pulse stretching amount is not adjustable—the volume Bragg grating (VBG). However, due to limitations in materials and fabrication processes, the maximum stretching amount of this VBG is only 500 ps, ​​and it cannot provide a larger pulse stretching amount. The main reasons for this are as follows:

[0004] (1) Different axes result in poor dispersion accuracy

[0005] like Figure 1 As shown, since the optical path of the volume Bragg grating (VBG) is obliquely incident, the oblique incident angle is difficult to control, resulting in the inability to control the optical path difference. Since the dispersion is related to the optical path difference, and thus the dispersion is related to the oblique incident angle, the system dispersion cannot be precisely controlled, which brings an irreparable dispersion difference to the subsequent system compression and limits the system dispersion control accuracy.

[0006] (2) In the case of different axes, the aperture limits the light spot.

[0007] like Figure 2 As shown, the aperture of a volume Bragg grating is relatively small, typically 5*5mm. Due to the oblique incidence, both the incident and outgoing light spots need to be within the aperture. Therefore, the light spot can generally only be set to 2*2mm, or even smaller, 1*1mm, thus limiting the aperture size of the light spot.

[0008] (3) The tolerance power limits the damage threshold.

[0009] Since the damage threshold of the end face of a volume Bragg grating is generally 5 J / cm2, while for a 5*5mm aperture grating, the flux threshold can reach 125 J, due to oblique incidence, the spot size of the volume Bragg grating is limited to 1*1mm, thus limiting the flux threshold to only 5 J, which greatly restricts the damage threshold of the device and makes it extremely easy to be damaged.

[0010] (4) Dispersion limits the broadening.

[0011] The dispersion of a typical volume Bragg grating is 500 ps, ​​and the system's spanning is also 500 ps. Currently, it is mainly used in single-pass oblique incidence, so the spanning is only 500 ps, ​​which limits the system's spanning and thus the system output. Therefore, a larger dispersion is needed. Summary of the Invention

[0012] The purpose of this invention is to address the aforementioned problems by providing a multi-path broadening system based on a volume Bragg grating. This system achieves coaxial input and output of the optical path, ensures that the dispersion is precisely consistent with the design value, achieves a maximum incident light spot size of 5*5mm, has a damage flux threshold exceeding 100J, and doubles the dispersion broadening. It also boasts advantages such as small size and stable reliability.

[0013] The technical solution adopted in this invention is as follows:

[0014] An ultrashort pulse width multi-pass stretching system includes at least one volume Bragg grating and a polarization reflection unit for controlling the polarization state and angle of the incident light. The multi-pass stretching system utilizes optical path design to make the pulsed laser pass through the volume Bragg grating multiple times. By adjusting the polarization state of the beam, the pulsed laser achieves multi-pass volume Bragg grating pulse stretching in the system.

[0015] On the other hand, the present invention also provides a corresponding ultrashort pulse width multi-pass widening method, which is mainly based on the widening system to perform multi-pass widening. By adjusting the angle of the quarter-wave plate, the beam passing through the quarter-wave plate is transformed between P-polarized light / circularly polarized light / S-polarized light, so that the beam can complete the multi-pass widening of the target number of passes in the widening system.

[0016] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0017] (1) The multi-path stretching system and method provided by the present invention innovatively adopts the combination of polarization reflection unit and stretching branch to realize polarization state selection and multi-path stretching of incident pulse laser. By using optical path design, the pulse laser passes through the volume Bragg grating multiple times. By adjusting the polarization state of the beam, the pulse laser is realized to achieve multi-path volume Bragg grating pulse stretching in the system.

[0018] (2) The multi-path stretching system and method provided by the present invention make the incident beam and the outgoing beam perpendicular to and coaxial with respect to the volume Bragg grating. The optical path difference of the system is controllable, the dispersion control accuracy is high, the spot diameter is large, and the damage threshold of the device is also improved, making the device less susceptible to damage.

[0019] (3) The multi-path stretching system and method provided by the present invention achieves a maximum pulse width stretching to nanoseconds or more, greatly reducing the complexity of the system without the need for additional devices such as optical fibers, increasing the stability of the system, and achieving high-power laser output. Attached Figure Description

[0020] The present invention will be described by way of example and with reference to the accompanying drawings, wherein:

[0021] Figure 1 This is a schematic diagram of the optical path of a solid Bragg grating;

[0022] Figure 2 This is a schematic diagram of the light spot of a volume Bragg grating;

[0023] Figure 3 This is a schematic diagram of the structure of an ultrashort pulse width two-way stretching system according to the present invention;

[0024] Figure 4 This is a schematic diagram of the structure of an ultrashort pulse width four-pass widening system according to the present invention.

[0025] In the diagram: 1. Volume Bragg grating; 2. Quarter-wave plate; 3. Polarizing beam splitter; 4. Half-wave plate; 5. 0° high-reflectivity mirror; 6. Incident light; 7. First P-polarized light; 8. First circularly polarized light; 9. First S-polarized light; 10. Second circularly polarized light; 11. Second P-polarized light; 12. Outgoing light; 13. Third circularly polarized light; 14. Fourth circularly polarized light; 20. Laser seed source. Detailed Implementation

[0026] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Based on the embodiments in this application, other similar embodiments obtained by those skilled in the art without creative effort should all fall within the scope of protection of this application.

[0027] In this invention, unless otherwise explicitly specified and limited, the terms "connection," "fixed," etc., should be interpreted broadly. For example, "fixed" can mean a fixed connection, a detachable connection, or an integral part; it can mean a mechanical connection, an electrical connection, a physical connection, or a wireless communication connection; it can mean a direct connection or an indirect connection through an intermediate medium; it can mean the internal communication of two elements or the interaction between two elements, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0028] Example 1

[0029] like Figure 3 As shown, Figure 3 This invention relates to a two-way pulse width stretching system based on a volume Bragg grating. The two-way stretching system includes a laser seed source 20, a volume Bragg grating 1, a half-wave plate 4 for adjusting the polarization component of the incident light, and a polarization reflection unit for polarization state selection. The half-wave plate 4 and the polarization reflection unit work together to control the polarization state and angle of the highly polarized incident light 6 provided by the laser seed source 20, thereby realizing two pulse stretchings of the incident light 6 in the volume Bragg grating 1, and simultaneously realizing the input and output of the incident light 6 and the output light 12 perpendicular and coaxial with respect to the volume Bragg grating 1.

[0030] The polarization reflection unit includes a polarization beam splitter 3 (PBS), a quarter-wave plate 2, and a 0° high reflectivity mirror 5. The quarter-wave plate 2 is located between the volume Bragg grating 1 and the polarization beam splitter 3 (PBS).

[0031] The half-wave plate 4 achieves component control of the polarization state of the incident light 6 by adjusting the rotation angle. The accuracy of the rotation angle of the half-wave plate 4 determines the loss of the entire system.

[0032] The quarter-wave plate 2 is used to control the polarization state of the input and output light of the volume Bragg grating 1. By rotating the angle of the quarter-wave plate 2, the crystal axis is made to form a certain angle with the incident polarized light, thereby converting the polarization state of the outgoing beam between S-polarization state, circular polarization state, and P-polarization state.

[0033] The 0° high reflectivity mirror 5, as a reflection module, is mainly used to reflect the incident light beam. Its reflectivity is as high as 99.99%, and it is independent of the polarization state of the incident light. In a preferred embodiment, the 0° high reflectivity mirror 5 can also be replaced with other optical devices with high reflectivity.

[0034] The polarizing beam splitter 3 is composed of a pair of high-precision right-angle prisms connected together, with a polarizing beam splitting medium film coated on the hypotenuse of one of the prisms. This polarizing beam splitter 3 can split incident unpolarized light into two perpendicular linearly polarized beams, where the P-polarized light passes through completely, while the S-polarized light is reflected at a 45-degree angle, and the final exit direction of the S-polarized light forms a 90-degree angle with the P-polarized light.

[0035] In the two-way beam stretching system, the polarization beam splitter 3 reflects the incident light, which has been deflected by 90 degrees, in a direction 90 degrees from the incident light. The output light is then perpendicularly incident on the 0° high reflectivity mirror 5 and reflected perpendicularly and coaxially with its original polarization state. The reflected output light re-enters the polarization beam splitter 3. Since the polarization state has not changed, the reflected output light is also reflected in a direction 90 degrees from the incident light, thereby achieving the original path return control of the laser after the first pulse stretching.

[0036] In a preferred embodiment, the polarizing beam splitter 3 can also be replaced by a Nicol prism, a Gran-Thompson prism, or other beam splitting prisms, in combination with the 0° high reflectivity mirror 5 to return the laser beam broadened by the volume Bragg grating 1 in this embodiment of the invention along its original path.

[0037] Example 2

[0038] The optical path of the double-pass widening module is as follows: Figure 3 As shown, the incident light 6 provided by the laser seed source 20 first passes through the half-wave plate 4 and then reaches the polarization beam splitter 3 for the first time. By rotating the angle of the half-wave plate 4, the polarization component of the incident light is adjusted so that the light energy after being transmitted through the polarization beam splitter 3 is the strongest. At this time, the transmitted light is P-polarized light, which can also be called the first P-polarized light 7.

[0039] After the first P-polarized light 7 passes through the quarter-wave plate 2 for the first time, the first P-polarized light 7 is converted into the first circularly polarized light 8. Then, the first circularly polarized light 8 is incident perpendicularly on the body Bragg grating 1, completing the first pulse broadening. Then, the broadened first circularly polarized light 8 is reflected and output along the original path.

[0040] The first circularly polarized light 8, reflected by the volume Bragg grating 1, passes through the quarter-wave plate 2 for the second time. At this point, the polarization state of the first circularly polarized light 8 undergoes a transformation. Before adjusting the quarter-wave plate 2, the relationship between the crystal axis of the quarter-wave plate 2 and the polarization direction of the light is not clear, so the specific polarization state after passing through the quarter-wave plate 2 cannot be obtained. Considering that when the polarized light, after passing through the quarter-wave plate 2, reaches the polarization beam splitter 3 for the second time, the output light will undergo reflection and transmission, with the S-polarized light being reflected and the P-polarized light being transmitted. Therefore, it is necessary to rotate the angle of the quarter-wave plate 2 to convert the first circularly polarized light 8 into the first S-polarized light 9, which is then reflected and output from the polarization beam splitter 3.

[0041] After the first S-polarized light 9 is reflected at 90 degrees in the polarization beam splitter and then incident perpendicularly on the 0° high reflectivity mirror, all the light is reflected along the original path. The reflected light is still in the S-polarized state. After the first S-polarized light 9 enters the polarization beam splitter 3 for the third time, it will also be reflected at 90 degrees from the polarization beam splitter 3 again.

[0042] Then, the first S-polarized light 9 enters the quarter-wave plate vertically for the third time, at which point it transforms into the second circularly polarized light 10. The second circularly polarized light 10 is then incident vertically on the volume Bragg grating 1 for a second broadening, and the broadened second circularly polarized light 10 is reflected back from the volume Bragg grating 1 and output. After being reflected by the volume Bragg grating 1, the second circularly polarized light 10 passes through the quarter-wave plate 2 for the fourth time, and simultaneously, the circular polarization state of the beam changes to the P-polarization state. Then, the second P-polarized light 11 is incident vertically on the polarizing beam splitter 3 and is transmitted through the polarizing beam splitter 3 before being output vertically, resulting in the final output light 12 after two pulse broadenings.

[0043] It should be noted that the above descriptions of the number of times light passes through optical path devices are all relative to the number of times the device itself is passed through, in order to more clearly describe the overall trajectory of the optical path.

[0044] Example 3

[0045] Figure 4 This is a schematic diagram of a four-pass beam stretching system based on a volume Bragg grating provided in an embodiment of the present invention. In addition to a laser seed source 20 and a half-wave plate 4, the four-pass beam stretching system includes two volume Bragg gratings 1 and a polarization reflection unit for polarization state selection. In this embodiment, the polarization reflection unit includes a polarizing beam splitter prism 3 (PBS), two quarter-wave plates 2, and a 0° high-reflectivity mirror 5. Similar to the previous embodiment, the two quarter-wave plates 2 are respectively located between the two volume Bragg gratings 1 and the polarizing beam splitter prism 3, forming a first stretching branch and a second stretching branch, respectively, with the first stretching branch perpendicular to the second stretching branch.

[0046] As shown in the figure, when the incident light 6 provided by the laser seed source 20 passes sequentially through the half-wave plate 4, the polarizing beam splitter 3, the quarter-wave plate 2 of the first broadening branch, and the volume Bragg grating 1, the first broadening is completed. This first broadening process is the same as the first broadening process of the aforementioned two-way broadening system.

[0047] After the beam is first broadened, it is reflected and re-passed through the quarter-wave plate 2 to be converted into the first S-polarized light 9 and reaches the polarizing beam splitter 3. At this time, the first S-polarized light 9 is completely reflected by the polarizing beam splitter 3 at 90 degrees and enters the second broadened branch perpendicularly.

[0048] Then, the first S-polarized light 9 passes through the quarter-wave plate 2 and the volume Bragg grating 1 in the second broadening branch to complete the second broadening. After the second broadening, the second circularly polarized light 10 is reflected by the volume Bragg grating 1 and continues to pass through the polarization state conversion of the quarter-wave plate 2, the transmission of the polarization beam splitter 3, the reflection of the 0° high reflectivity mirror 5, and the transmission of the polarization beam splitter 3 in the second broadening branch before returning to the second broadening branch. After the polarization state conversion of the quarter-wave plate 2, it enters the volume Bragg grating 1 as the third circularly polarized light 13 to complete the third broadening.

[0049] After the third broadening, the beam is reflected along the second broadening branch and is reflected again by the polarizing beam splitter 3 at 90 degrees. It then re-enters the first broadening branch and, after polarization conversion, completes the fourth broadening and reflection output as the fourth circularly polarized light 14 in the volume Bragg grating 1 of the first broadening branch. Finally, the vertical output light 12 after four pulse broadenings is obtained.

[0050] In this embodiment, polarization transformation is achieved directly using polarization units, and four-way broadening of the input pulse laser is achieved by using two broadening branches in combination. At the same time, coaxial and perpendicular incident and outgoing light of the input and output light are also achieved.

[0051] In another preferred embodiment, the 0° high reflectivity mirror 5 can be replaced with a volume Bragg grating 1 to form a three-way broadening branch. In this case, the volume Bragg grating 1 can simultaneously reflect and broaden the beam, thus achieving a wider beam path through beam polarization state adjustment and direction adjustment.

[0052] Preferably, multi-path pulsed laser broadening can be achieved by using 1-3 broadening branches combined with polarization reflection without the need for additional devices such as optical fibers. The number of paths is preferably 2-6. Furthermore, the beam is incident and emitted perpendicularly to the volume Bragg grating. In addition, the relative positions of the devices in this broadening system do not strictly follow the description in the above embodiments. As long as the aforementioned devices can be used to achieve multi-path broadening of the beam, the relative positions of individual devices can be adjusted and expanded accordingly.

[0053] It should be noted that the embodiments of the present invention can also realize single-way widening, but the focus of this application is that, for composite applications, a single widening branch is needed to realize double-way or more-way widening, and multiple-way widening is realized by extending multiple widening branches. Therefore, single-way widening will not be described in detail.

[0054] Example 4

[0055] This embodiment provides a corresponding method for multi-pass widening of ultrashort pulse width, which is mainly based on any of the aforementioned widening systems. The key is to adjust the angle of the quarter-wave plate 2 so that the beam passing through the quarter-wave plate 2 can be transformed between P-polarized light / circularly polarized light / S-polarized light, so that the beam can complete the multi-pass widening of the target number of passes in the widening system.

[0056] Compared with the prior art, the multi-path broadening system provided in this embodiment of the invention innovatively adopts the combination of polarization reflection unit and broadening branch to achieve polarization state selection and multi-path broadening of incident pulsed laser. By using optical path design, the pulsed laser passes through the volume Bragg grating multiple times. By adjusting the polarization state of the beam, the pulsed laser achieves multi-path volume Bragg grating pulse broadening in the system, thereby achieving a maximum pulse width broadening to nanoseconds or more. This greatly reduces the complexity of the system, increases the stability of the system, and achieves high-power laser output.

[0057] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention's specification and drawings under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

[0058] Any feature disclosed in this specification (including any appended claims and abstract) may be replaced by other equivalent or similar features, unless specifically stated otherwise. That is, unless specifically stated otherwise, each feature is merely one example of a series of equivalent or similar features.

Claims

1. A multi-pass stretching system for ultrashort pulse width, characterized in that, The multi-path stretching system includes: a laser seed source (20), at least one bulk Bragg grating (1), a half-wave plate (4) for adjusting the polarization component of the incident light, and a polarization reflection unit for controlling the polarization state and angle of the incident light (6) provided by the laser seed source (20); The polarization reflection unit includes a polarization beam splitter (3) and at least one quarter-wave plate (2). The quarter-wave plate (2) is located between the volume Bragg grating (1) and the polarization beam splitter (3), and a quarter-wave plate (2) and a volume Bragg grating (1) form a broadening branch. The polarization reflection unit and the broadening branch work together to broaden the incident light (6) in the volume Bragg grating (1) through multiple passes, while realizing the input and output of the incident light (6) and the outgoing light (12) perpendicular and coaxial with respect to the volume Bragg grating (1). The polarization reflection unit also includes a reflection module. The reflection module works together with the polarization beam splitter (3) to reflect the beam to be broadened along the original path and broaden it for the next pass. The reflection module is a 0° high reflectivity mirror (5). The polarization beam splitter (3) reflects the incident light that has been deflected by 90 degrees in a direction that is 90 degrees from the incident light. The output light is perpendicularly incident on the 0° high reflectivity mirror (5) and then reflected perpendicularly and coaxially with the original polarization state. The reflected output light re-enters the polarization beam splitter (3). Since the polarization state has not changed, the reflected output light is also reflected in a direction that is 90 degrees from the incident light, thereby realizing the original path return control of the laser after the first pulse broadening. The number of stretching stages in the multi-stage stretching system is an integer greater than 2.

2. The ultrashort pulse width multi-pass stretching system as described in claim 1, characterized in that, The half-wave plate (4) adjusts the polarization state of the incident light (6) by rotating the angle, so that the adjusted polarized light is transmitted by the polarization beam splitter (3) and enters the broadening branch to complete the first broadening.

3. The ultrashort pulse width multi-pass stretching system as described in claim 1, characterized in that, The beam to be broadened enters the volume Bragg grating (1) vertically, is reflected, and exits vertically.

4. A method for multi-pass widening of ultrashort pulse width, characterized in that, The multi-pass stretching method is implemented based on any one of the multi-pass stretching systems in claims 1-3, and includes: Adjust the angle of the quarter-wave plate (2) so that the polarization state of the beam passing through the quarter-wave plate (2) can be changed between P polarization / circular polarization / S polarization; By rotating the angle of the half-wave plate (4), the polarization component of the incident light (6) is adjusted so that the light energy transmitted through the polarizing beam splitter (3) is the strongest.

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