We have implemented an extended depth of field optical system by wavefront coding with a micromachined membrane deformable mirror. This approach provides a versatile extension to standard wavefront coding based on fixed phase mask. First experimental results validate the feasibility of the use of adaptive optics for variable depth wavefront coding in imaging optical systems.
Optimization of the point spread function by means of sensor-less adaptive optics, based on direct imaging of the focal spot, suffers from errors due to enormous dynamic range of the focal intensity. Also, optimization algorithms based on the focal spot metrics only, are insensitive to other system parameters and can converge towrong" solutions. To improve the beam quality and the robustness of the global extremum, we have introduced dynamic feedback control of the camera sensitivity. To further increase the robustness of optimization, we introduced a regularization parameter in the form of some function of the system state, achieving its minimum together with the desired solution. Significant gain in achievable beam quality is shown in comparison with the implementation lacking those improvements. Proposed techniques are implemented in Beam Tuner software forne-tuning of laser and imaging systems with adaptive optics.
Micromachined membrane deformable mirror (MMDM) can serve as an ad hoc" free-form optical element. To test the repeatability and stability of the standard MMDM, we have conducted the test of surface figure during multiple thermal cycling, test of figure drift at elevated temperatures, and a long-term 16-day stability test of actively formed mirror figure. The average rms error did not exceed λ =25 at λ = 633 nm, after repeated cycling from -14 to +70 C, with return to the room temperature. The existing design provides ~10° stability in the temperature range of ~10°. Optimization of the design, eliminating astigmatism, would allow one to extend the temperature range to about 30. The long-term mirror figure instability at a constant temperature reaches λ/20 rms in 16 days. The P-V error with respect to the nearest sphere changes from λ/20 in the first day, to about λ/10 in the 16-th day. The tests show that MMDM is stable enough to make a reasonable alternative to free-form optics in applications that require various optical shapes to be formed with a single element.