A calgo crystal regenerative cavity and its alignment method

Abstract

The application discloses a CALGO crystal regenerative cavity and an assembling and adjusting method thereof, and relates to the technical field of lasers.The regenerative cavity comprises a seed source, an isolator, a polarization beam splitter, a polarization control assembly, a first cavity mirror, a self-imaging assembly, a gain medium, a double dichroic mirror, a second cavity mirror, a pump source, an equivalent short cavity mirror, a low-power consumption mirror and a power meter.The equivalent short cavity is established through the equivalent short cavity mirror and the second cavity mirror, the cavity length is prolonged through the self-imaging assembly, and the assembling and adjusting are dynamically fed back through the low-power consumption mirror and the power meter, so that the femtosecond laser regenerative amplification output with the advantages of easy assembling and adjusting, high stability, wide spectrum and high power is realized.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and specifically to a CALGO crystal regeneration cavity and its assembly method. Background Technology

[0002] Femtosecond regenerative amplification technology achieves energy enhancement of ultrashort pulses through a regenerative amplification cavity. Its core is to inject a seed pulse into the cavity and use an optical switch (such as a Pockels cell) to control the pulse to pass through the gain medium multiple times. Through stimulated emission, the energy is amplified step by step, and finally a high-energy femtosecond pulse is output.

[0003] Currently, due to the long rise and fall delay times of electro-optic driving, femtosecond regenerating cavities require very long cavity lengths, typically 1.5 meters or more, to avoid the unstable regions of electro-optic rise and fall delays. Excessive cavity length makes cavity assembly, tuning, and oscillation startup difficult, and also reduces stability. Consequently, seed injection for effective regeneration and amplification becomes challenging, and power stability deteriorates. Furthermore, the narrowing of the femtosecond laser amplification spectrum, leading to a larger minimum pulse width compression after amplification, is also a major problem for femtosecond lasers. Summary of the Invention

[0004] The purpose of this invention is to solve the problem mentioned in the background art that excessively long cavity length makes it difficult to assemble, tune, and start oscillate the resonant cavity, and also leads to poor stability. Therefore, this invention proposes a CALGO crystal regeneration cavity and its assembly and tuning method.

[0005] A first aspect of this invention provides a CALGO crystal regeneration cavity, the regeneration cavity comprising a seed source, an isolator, a polarizing beam splitter, a polarization control assembly, a first cavity mirror, a self-imaging assembly, a gain medium, a dichroic mirror, a second cavity mirror, and a pump source; wherein:

[0006] The seed source is connected sequentially to the optical paths of the isolator and the polarizing beam splitter;

[0007] A polarization control component is disposed between the polarization beam splitter and the first cavity mirror, and is used to adjust the polarization state of the beam;

[0008] The gain medium and the dichroic mirror are positioned between the second cavity mirror and the self-imaging assembly;

[0009] The pump source is used to pump the gain medium;

[0010] The self-imaging component is disposed between the polarizing beam splitter and the gain medium to form a folded-back optical path to extend the physical cavity length of the regeneration cavity, and to image the beam state at the corresponding position of the gain medium to the position of the first cavity mirror.

[0011] The first and second laparoscopic mirrors form the boundary of the long cavity regeneration chamber.

[0012] Specifically, the polarization control component includes a quarter-wave plate and a Pockel cell.

[0013] Specifically, the gain medium is a ytterbium-doped crystal, Yb:CALGO.

[0014] Specifically, the gain medium is soldered onto a gold-plated copper heat sink and connected to an external circulating water cooling system.

[0015] Specifically, the pump source is a high-power semiconductor laser.

[0016] Specifically, the self-imaging assembly includes at least two imaging mirrors and multiple planar mirrors.

[0017] Specifically, the isolator is equipped with an escape port, and the polarization control component switches the beam polarization state under preset conditions, so that the amplified beam is output through the escape port of the isolator.

[0018] A second aspect of this invention provides a method for assembling and adjusting a CALGO crystal regeneration cavity, the method comprising:

[0019] Step 1: Use the light from the seed source as the reference light to perform optical path reference calibration;

[0020] Step 2: Insert an equivalent short cavity mirror at the corresponding position of the gain medium. A short resonant cavity is formed between the equivalent short cavity mirror and the second cavity mirror.

[0021] Step 3: Turn off the seed source and insert a low-loss mirror into the short resonant cavity;

[0022] Step 4: Pump the gain medium and adjust the equivalent short cavity mirror to make the short resonant cavity oscillate.

[0023] Step 5: Remove the equivalent short cavity mirror, adjust the second cavity mirror, and detect the power of the light reflected by the low-loss mirror. When the highest long cavity oscillation power is measured, the adjustment of the long cavity regeneration cavity is completed, and the low-loss mirror is removed.

[0024] Step 6: Turn on the seed source to inject seed light, adjust the driving parameters of the polarization control component, and optimize the regeneration amplification output.

[0025] The beneficial effects of this invention are:

[0026] 1. The self-imaging component significantly increases the optical cavity length within a limited space by using a folded optical path, so that the optical cavity length of the regeneration cavity meets the femtosecond pulse round-trip timing requirements.

[0027] 2. During assembly and adjustment, a short resonant cavity is constructed, leveraging its low loss and ease of oscillation initiation to quickly establish resonance conditions. This is followed by a transition to a long cavity, with optimal parameter adjustment achieved through power monitoring. This method effectively overcomes the complexity of directly initiating oscillation in a long cavity, significantly improving the assembly and adjustment success rate and system stability. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structure of a CALGO crystal regeneration cavity provided in an embodiment of the present invention;

[0029] Figure 2 This is a schematic diagram of the assembly and adjustment structure of a CALGO crystal regeneration cavity provided in an embodiment of the present invention;

[0030] The labels in the diagram represent the following: 1-Seed source, 2-Isolator, 3-Polarization beam splitter, 4-Polarization control component, 401-Quarter-wave plate, 402-Pockell cell, 5-First cavity mirror, 6-Self-imaging component, 601-First imaging mirror, 605-Second imaging mirror, 602, 603, 604, 606, and 607 are plane mirrors; 7-Gain medium, 8-Dichroic mirror, 9-Second cavity mirror, 10-Pump source, 11-Equivalent short cavity mirror, 12-Low-loss mirror, 13-Mirror, 14-First power meter, 15-Second power meter, and 16-Third power meter. Detailed Implementation

[0031] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features and effects of the present invention is provided in conjunction with the accompanying drawings and preferred embodiments.

[0032] This invention provides a CALGO crystal regeneration cavity. See also... Figure 1 , Figure 1 This is a schematic diagram of a CALGO crystal regeneration cavity provided in an embodiment of the present invention. The regeneration cavity includes a seed source 1, an isolator 2, a polarizing beam splitter 3, a polarization control component 4, a first cavity mirror 5, a self-imaging component 6, a gain medium 7, a dichroic mirror 8, a second cavity mirror 9, and a pump source 10; wherein:

[0033] Seed source 1 is connected in sequence to isolator 2 and polarizing beam splitter 3. Isolator 2 prevents interference from reflected light to the seed source.

[0034] The polarization control component 4 is located between the polarization beam splitter 3 and the first cavity mirror 5 and is used to adjust the polarization state of the beam.

[0035] Pump source 10 is used to pump the gain medium 7.

[0036] Gain medium 7 and dichroic mirror 8 are disposed between second cavity mirror 9 and self-imaging component 6.

[0037] The self-imaging component 6 is disposed between the polarizing beam splitter 3 and the gain medium 7.

[0038] The first cavity mirror 5 and the second cavity mirror 9 form the boundary of the long cavity regeneration cavity. The light beam travels back and forth between the first cavity mirror 5 and the second cavity mirror 9 multiple times and is amplified by the gain medium 7.

[0039] In one implementation, seed source 1 can be provided with an ultrashort pulse seed using a mode-locked fiber optic oscillator.

[0040] In one implementation, the isolator 2 has an escape port, which serves as the output channel for the amplified pulse.

[0041] In one implementation, the polarization control component 4 includes a quarter-wave plate 401 and a Pockel cell 402.

[0042] The quarter-wave plate 401 works in conjunction with the Pockel cell 402 to achieve precise control of the beam polarization state through voltage adjustment, thereby completing the injection of the seed pulse, multiple round-trip amplifications within the cavity, and the directional extraction of the final amplified pulse. This structure achieves fully electronically controlled pulse gating, with fast response speed and high control precision, which is beneficial to improving the stability and repeatability of the regeneration amplification process.

[0043] In one implementation, the pump source 10 can be a high-power semiconductor laser (such as a 978nm or 980nm fiber-coupled pump module), and the energy conversion efficiency can be improved through a multi-pass pump structure. The dichroic mirror 8 is coated with a dichroic film to separate the pump light from the signal light.

[0044] In one implementation, the self-imaging component 6 includes at least two imaging mirrors (concave mirrors) and multiple mirrors (plane mirrors).

[0045] At least two imaging mirrors constitute an imaging system, which accurately images the beam state at the corresponding position of the gain medium 7 to the first cavity mirror 5, ensuring the mode quality of the beam after long-distance back propagation; multiple planar mirrors are used to fold the optical path, which greatly increases the optical cavity length in a limited space, so that the optical cavity length of the regeneration cavity meets the femtosecond pulse round-trip timing requirements, while maintaining the overall structure compactness.

[0046] In one embodiment, the gain medium 7 is a ytterbium-doped crystal Yb:CALGO.

[0047] Yb:CALGO crystals have a wide emission spectrum of approximately 80 nm, which can effectively suppress the spectral narrowing caused by gain narrowing during the amplification of femtosecond pulses. This helps maintain a wider spectral bandwidth, thereby supporting a shorter pulse compression width and improving the peak power of the output pulse.

[0048] In one implementation, the gain medium 7 is soldered onto a gold-plated copper heat sink and connected to an external circulating water cooling system to control the operating temperature at around 25°C. The gold-plated copper heat sink has excellent thermal conductivity, and combined with the external circulating water cooling system, it enables efficient and uniform heat dissipation from the gain medium. This effectively reduces the thermal lensing effect, avoids beam quality degradation and efficiency reduction caused by temperature rise, and improves the long-term stability and reliability of the system.

[0049] In one implementation, the light regeneration and amplification process of the CALGO crystal regeneration cavity is as follows:

[0050] The seed light emitted from seed source 1 passes through isolator 2 and is directed towards polarizing beam splitter 3. The seed light is horizontally polarized, satisfying the transmission polarization requirement of polarizing beam splitter 3. After transmission, the seed light passes through quarter-wave plate 401 and an unpowered Pockel cell 402 before reaching the first cavity mirror 5. It is then reflected back to polarizing beam splitter 3. Since there is no voltage on Pockel cell 402, after the seed pulse passes through quarter-wave plate 401 twice, it changes from horizontally polarized to vertically polarized, satisfying the reflection polarization requirement of polarizing beam splitter 3. The light is then reflected by polarizing beam splitter 3 and directed towards the first imaging mirror 601, thus injecting light into the resonant cavity.

[0051] The first imaging mirror 601 and the second imaging mirror 605 constitute a self-imaging system, which positions the gain medium 7 at the corresponding locations ( Figure 2 The image (beam state) at the intermediate short cavity mirror 11 is imaged onto the second cavity mirror 5, achieving self-imaging and regeneration cavity extension. Therefore, the seed light reflected by the first imaging mirror 601 passes sequentially through mirrors 602, 603, 604, the second imaging mirror 605, 606, and 607 and is directed towards the gain medium 7 for the first amplification, and then reflected by the dichroic mirror 8 and directed towards the second cavity mirror 9.

[0052] The reflected light passes through the gain medium 7 again, forming a secondary amplification. The amplified light passes through the various mirrors between the reflector 607 and the polarizing beam splitter 3, and then passes through the quarter-wave plate 401 and the Pockel cell 402 before being directed towards the first cavity mirror 5. At this time, the voltage on the Pockel cell has been applied to the quarter-wave voltage, and its combination with the quarter-wave plate 401 achieves the effect of a half-wave plate. After the amplified pulse passes through the quarter-wave plate 401 and the Pockel cell 402 twice, its vertical polarization state remains unchanged, and it continues to be reflected by the polarizing beam splitter 3 towards the gain medium 7, forming a fourth amplification in one round trip.

[0053] This amplification process continues until the number of amplifications of the optical pulse meets the preset requirements. Then, the voltage on the Pockel cell is turned off. At this time, the light reflected by the polarizing beam splitter 3 and directed towards the first cavity mirror 5 passes through the quarter-wave plate 401 and the Pockel cell 402 twice. The amplified optical pulse changes from vertically polarized light to horizontally polarized light and is directed towards the polarizing beam splitter 3. At this time, the amplified optical pulse is in a horizontally polarized state, which meets the transmission polarization state requirement of 3. It passes through the polarizing beam splitter 3 and is directed towards the isolator 2.

[0054] The light pulses output after regeneration and amplification are reversed and passed through isolator 2, exited through the escape port, and then passed through reflector 13 to power meter 14, where the power of regeneration and amplification is measured.

[0055] In one embodiment, the small emitter cross-sectional area of ​​the Yb:CALGO gain dielectric makes gain extraction difficult, and the relatively long cavity length of the regenerating cavity further complicates the debugging of the regenerating cavity's oscillation startup. Therefore, this invention provides a dedicated debugging method for CALGO crystal regenerating cavities. See also... Figure 2 , Figure 2 This is a schematic diagram of the assembly and adjustment structure of a CALGO crystal regeneration cavity provided in an embodiment of the present invention. The components indicated by dashed lines, the equivalent short-cavity reflector 11 and the low-loss reflector 12, are insertable and removable components, which can be flexibly inserted or removed during assembly and adjustment. The specific assembly and adjustment process includes:

[0056] Step 1, Optical path reference calibration: Using the light from seed source 1 as the reference light, align the pitch and yaw of each device between isolator 2 and second cavity mirror 9.

[0057] Step two, constructing the short cavity: Insert the equivalent short cavity reflector 11 at the corresponding position of the gain medium 7, and adjust the pitch and yaw of the equivalent reflector 11 to be consistent. A short resonant cavity is formed between the equivalent short cavity reflector 11 and the second cavity mirror 9. Due to the relatively short cavity length and low cavity loss, it is easy to adjust the cavity to start oscillation.

[0058] Step 3, Short Cavity Oscillation Preparation: Turn off seed source 1 and insert a low-loss mirror 12 into the short resonant cavity. Power meters 15 and 16 are respectively installed in the two reflected optical paths of the low-loss mirror 12. The low-loss mirror 12 is a partial reflector, which can interact with the intracavity oscillating laser without significantly disrupting the original resonant state, coupling a portion of its energy outside the original optical path for detection. This signal output method introduces minimal additional loss to the resonant cavity, allowing the laser oscillation in the short cavity to be maintained.

[0059] Step 4, short cavity oscillation: turn on the pump source 10 to pump the gain medium 7, adjust the equivalent short cavity reflector 11 to make the short cavity between the equivalent short cavity reflector 11 and the second cavity mirror 9 oscillate, and measure the light spot at the power meter 15 and the power meter 16.

[0060] Step 5, Adjusting to form a long cavity: Remove the equivalent short cavity reflector 11, fine-tune the second cavity mirror 9 so that the highest long cavity oscillation power is measured at the power meter 15 or power meter 16. At this point, the long cavity regeneration cavity is adjusted. Then remove the low-loss reflector 12.

[0061] Step 6: Turn on seed source 1, inject seed light into the regeneration cavity, and set the driving parameters of Pockel cell 402, including pulse width and delay. By optimizing and adjusting the electro-optic pulse width and delay, the maximum regeneration amplified laser power is measured at power meter 14, and the regeneration laser amplification adjustment is completed.

[0062] The CALGO crystal regeneration cavity and its assembly and adjustment method provided in this invention achieve easy assembly and adjustment, high stability, wide spectrum and high power femtosecond laser regeneration and amplification output through equivalent short cavity guidance, self-imaging extended cavity and dynamic feedback assembly and adjustment, providing a good foundation and guarantee for subsequent traveling wave amplification and pulse width compression.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention are within the scope of the claims of the present invention.

Claims

1. A CALGO crystal regeneration cavity, characterized in that, The regeneration cavity includes a seed source (1), an isolator (2), a polarization beam splitter (3), a polarization control assembly (4), a first cavity mirror (5), a self-imaging assembly (6), a gain medium (7), a dichroic mirror (8), a second cavity mirror (9), and a pump source (10); wherein: The seed source (1) is connected in sequence to the optical paths of the isolator (2) and the polarizing beam splitter (3); A polarization control component (4) is disposed between a polarization beam splitter (3) and a first cavity mirror (5) for adjusting the polarization state of the beam; The gain medium (7) and the dichroic mirror (8) are disposed between the second cavity mirror (9) and the self-imaging assembly (6); The pump source (10) is used to pump the gain medium (7); The self-imaging component (6) is disposed between the polarizing beam splitter (3) and the gain medium (7) to form a folded optical path to extend the cavity length of the regeneration cavity and to image the beam state at the corresponding position of the gain medium (7) to the position of the first cavity mirror (5). The first laparoscope (5) and the second laparoscope (9) form the boundary of the long cavity regeneration cavity.

2. The CALGO crystal regeneration cavity according to claim 1, characterized in that, The polarization control component (4) includes a quarter-wave plate (401) and a Pockel cell (402).

3. The CALGO crystal regeneration cavity according to claim 1, characterized in that, The gain medium (7) is a ytterbium-doped crystal Yb:CALGO.

4. The CALGO crystal regeneration cavity according to claim 3, characterized in that, The gain medium (7) is welded onto a copper-plated gold heat sink and connected to an external circulating water cooling system.

5. A CALGO crystal regeneration cavity according to claim 1, characterized in that, The pump source (10) is a high-power semiconductor laser.

6. The CALGO crystal regeneration cavity according to claim 1, characterized in that, The self-imaging component (6) includes at least two imaging mirrors and multiple planar mirrors.

7. A CALGO crystal regeneration cavity according to claim 2, characterized in that, The isolator (2) is equipped with an escape port. The polarization control component (4) switches the polarization state of the beam under preset conditions, so that the amplified beam is output through the escape port of the isolator (2).

8. A method for assembling and adjusting a CALGO crystal regeneration cavity as described in claim 7, characterized in that, Includes the following steps: Step 1: Use the light from seed source (1) as the reference light to perform optical path reference calibration; Step 2: Insert an equivalent short cavity mirror (11) at the corresponding position of the gain medium (7), and a short resonant cavity is formed between the equivalent short cavity mirror (11) and the second cavity mirror (9); Step 3: Turn off the seed source (1) and insert a low-loss mirror (12) into the short resonant cavity. Step 4: Pump the gain medium (7) and adjust the equivalent short cavity mirror (11) to make the short resonant cavity start oscillating; Step 5: Remove the equivalent short cavity mirror (11), adjust the second cavity mirror (9), detect the power of the light reflected by the low-loss mirror (12), and when the highest long cavity oscillation power is measured, the long cavity regeneration cavity adjustment is completed, and the low-loss mirror (12) is removed. Step 6: Turn on the seed source (1) to inject seed light, adjust the driving parameters of the polarization control component (4), and optimize the regeneration amplification output.

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