Application Cases

Experiment Name: Beam Purification Experiment of High-Power Nanosecond Solid-State Slab Laser

Test Equipment: Voltage amplifier, wavefront sensor, tip-tilt mirror, deformable mirror, laser, etc.

Experimental Process:

Figure 1: Schematic Diagram of the Hybrid Beam Purification System

The experimental setup of the hybrid adaptive beam purification system is illustrated in Figure 1. It mainly consists of four parts: The first part is a high-average-power, high-repetition-rate nanosecond slab laser. The second part is a low-order aberration automatic correction system, which employs a four-element optical structure optimized from a three-element correction system. The four lenses are independently installed in a spacing adjustment system. Based on the aberration information measured by the H-S sensor and under the control of the wavefront processor, the spacing adjustment system automatically adjusts the air gaps between the lenses to meet the requirements for large-amplitude low-order aberration correction and beam size matching. The third part is the AO correction system, which is an AO system with wavefront detection. It primarily consists of a tip-tilt mirror (TTM), a deformable mirror (DM), voltage amplifiers, a wavefront processor, and a wavefront sensor. The TTM is used to correct the overall tilt aberration of the beam, effectively releasing the stroke of the DM actuators and ensuring the pointing of the output beam. The TTM used in the experiment has a stroke of ±3 arcminutes. The DM is used to generate the conjugate wavefront of the incident wavefront, and after reflection, the wavefront aberration of the output beam can be well corrected. The DM used in the hybrid adaptive beam purification system is a 59-actuator continuous faceplate deformable mirror with an actuator stroke of ±3 µm.

Figure 2: Workflow Diagram of the Hybrid Beam Purification System

The workflow of the hybrid beam purification system is shown in Figure 2. First, H-S1 measures the initial aberration information of the beam emitted by the slab laser and feeds it back to the wavefront processor. The wavefront processor then drives the low-order aberration automatic correction system to operate using a direct correction strategy. It moves the four lenses to positions that satisfy both low-order aberration correction and size matching, performing preliminary correction on the large-amplitude low-order aberration components at the operating point. Subsequently, H-S2 measures the wavefront information after correction by the LOAC system and feeds the slope information of the residual aberration back to the wavefront processor for judgment. When the wavefront PV value of the residual aberration is ≥ 4 µm, the wavefront processor drives the LOAC system to operate using an adjustment correction strategy to address the issue of aberration variation at the operating point. When the wavefront PV value of the residual aberration is < 4 µm, the wavefront processor converts the slope information from H-S2 into voltage signals, sends them to the voltage amplifiers to generate the operating voltages for the TTM and DM, ultimately driving the DM to produce the corresponding surface shape, thereby achieving fine correction of the residual aberration. After the two-step correction by the purification system described above, the output beam can achieve near-diffraction-limited beam quality.

Experimental Results:

Figure 3: Parameters of the output beam after low-order aberration correction. (a) Beam wavefront information (µm), PV=1.91 µm, RMS=0.29 µm; (b) Far-field intensity distribution, β=2.86

The wavefront information and far-field intensity distribution after correction by the LOAC system are shown in Figure 3. Figure (a) shows the wavefront information of the output beam after correction only by the LOAC system. The wavefront PV value decreased from 26.47 µm to 1.91 µm, and the RMS value decreased from 6.12 µm to 0.29 µm. The corrected far-field intensity distribution is shown in Figure (b). After correction, the beam quality β factor improved from 18.42 times the diffraction limit to 2.86 times the diffraction limit, indicating a significant enhancement in beam quality.

Figure 4: Coefficients of various Legendre polynomials after correction by the LOAC system

Figure 4 shows the coefficients of various Legendre polynomials for the output beam after correction only by the LOAC system. The coefficients for the 4th and 6th terms, representing defocus and astigmatism, are significantly reduced. After LOAC system correction, the low-order aberration components in the output beam were effectively corrected, and the residual aberrations are mainly high-order. Among these, the coefficients for the 11th and 13th terms, which constitute the spherical aberration terms (11th, 13th, and 15th), are small, indicating that the optimized four-element structure effectively suppresses its own spherical aberration. The increase in the coefficients of the 15th, 21st, and 28th terms is mainly due to the "M"-shaped abrupt change in the wavefront. This "M"-shaped wavefront deformation is a common wavefront distortion in solid-state slab lasers, and the residual aberration profile is consistent with previous experiments.

Figure 5: Near-field spot intensity distribution before and after correction. (a) Before correction; (b) After correction

Based on the analysis of the wavefront information above, which indicated effective correction of low-order aberrations, the changes in the near-field spot morphology were also analyzed. The measurement results are shown in Figure 5.

The near-field intensity distribution of the incident beam before correction is shown in Figure (a), with a beam size of 7 mm × 35 mm and a spot morphology presenting an elongated shape with an aspect ratio of 1:5. The near-field intensity distribution after size transformation by the LOAC system is shown in Figure (b), with a beam size of 42 mm × 44 mm and the spot morphology transformed into a square with an aspect ratio approximately 1:1. After this correction, it meets the requirements for beam size matching in subsequent applications.

The experimental results demonstrate that after correction by the LOAC system, the issues of large-amplitude low-order aberrations in the initial incident beam and the small aspect ratio of the spot morphology were simultaneously resolved.

Recommended Voltage Amplifier: ATA-2081

Figure: ATA-2081 High-Voltage Amplifier Specifications

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