Jocelyn Parent

A magnetic liquid deformable mirror for high stroke and low order axially symmetrical aberrations.

We present a new class of magnetically shaped deformable liquid mirrors made of a magnetic liquid (ferrofluid). Deformable liquid mirrors offer advantages with respect to deformable solid mirrors: large deformations, low costs and the possibility of very large mirrors with added aberration control. They have some disadvantages (e.g. slower response time). We made and tested a deformable mirror, producing axially symmetrical wavefront aberrations by applying electric currents to 5 concentric coils made of copper wire wound on aluminum cylinders. Each of these coils generates a magnetic field which combines to deform the surface of a ferrofluid to the desired shape. We have carried out laboratory tests on a 5 cm diameter prototype mirror and demonstrated defocus as well as Seidel and Zernike spherical aberrations having amplitudes up to 20 microm, which was the limiting measurable amplitude of our equipment.

Dynamic response of ferrofluidic deformable mirrors.

Ferrofluids can be used to make deformable mirrors having highly interesting characteristics (e.g., extremely large strokes and low costs). Until recently, such mirrors were thought to be restricted to corrections of frequencies lower than 10 Hz, thus limiting their usefulness. We present counterintuitive results that demonstrate that the limiting operational frequency can be increased by increasing the viscosity of the ferrofluid. We tested the response of ferrofluids having viscosities as high as 494 cP, finding that they could allow an adaptive optics correction frequency as high as 900 Hz. We also demonstrate that we can counter the amplitude loss due to the high viscosity by overdriving the actuators. The overdriving technique combines high current, short duration pulses with ordinary driving step functions to deform the mirror. The integration of a FDM in a complete closed-loop adaptive optics system running at about 500 Hz thus appears to be a realistic goal in the near future.

Spatial dependence of surface error slopes on tolerancing panoramic lenses

Surface irregularity errors are conventionally used to specify fabrication accuracy of spherical, aspheric, or plane surfaces. However, in some cases, the amplitude of the irregularities fails to fully describe the surface accuracy requirement when the pupil size is small compared to the surface diameter. In such cases, the irregularity slope will induce distortion. A spatially dependent representation of the irregularity slope is proposed and implemented to specify the surface accuracy. As an optical design example, we study in detail the case of the front surfaces of a fish-eye lens and a panomorph lens. Panoramic lenses are characterized by a small entrance pupil and by important distortion. For both lenses, we found that the novel field-dependent mathematical descriptor provided a nearly perfect agreement with Monte Carlo analyses and can be used to specify the spatially dependent irregularity requirement. The approach is not limited to wide-angle lenses.

Tolerancing panoramic lenses

Tolerancing a lens is a basic procedure in lens design. It consists in first defining an appropriate set of tolerances for the lens, then in adding compensators with their allowable ranges and finally in selecting an appropriate quality criterion (MTF, RMS spot size, wavefront error, boresight error...) for the given application. The procedure is straightforward for standard optical systems. However, it becomes more complex when tolerancing very wide angle lenses (larger than 150 degrees). With a large field of view, issues such as severe off-axis pupil shift, considerable distortion and low relative illumination must be addressed. The pupil shift affects the raytrace as some rays can no longer be traced properly. For high resolution imagers, particularly for robotic and security applications, the image footprint is most critical in order to limit or avoid complex calibration procedures. We studied various wide angle lenses and concluded that most of the distortion comes from the front surface of the lens. Consequently, any variation of the front surface will greatly affect the image footprint. In this paper, we study the effects on the image footprint of slightly modifying the front surface of four different lenses: a simple double-gauss for comparison, a fisheye lens, a catadioptric system (omnidirectional lens) and a Panomorph lens. We also present a method to analyze variations of the image footprint. Our analysis shows that for wide angle lenses, on which the entrance pupil is much smaller than the front surface, irregularities (amplitude, slope and location) are critical on both aspherical and spherical front surfaces to predict the image footprint variation for high resolution cameras. Finally, we present how the entrance pupil varies (location, size) with the field of view for these optical systems.

Wavefront correction with a ferrofluid deformable mirror: experimental results and recent developments

We present the research status of a deformable mirror made of a magnetic liquid whose surface is actuated by a triangular array of small current carrying coils. We demonstrate that the mirror can correct a 11 μm low order aberrated wavefront to a residual RMS wavefront error 0.05 μm. Recent developments show that these deformable mirrors can reach a frequency response of several hundred hertz. A new method for linearizing the response of these mirrors is also presented.

Design, fabrication and test of miniature plastic panomorph lenses with 180° field of view

We present two miniature all plastic megapixel panomorph lenses for consumer electronics (total track length (TTL) of 6.56 mm) and mobile devices (TTL of 3.80 mm) showing the unique challenges from specification, design, manufacturing and testing phases of these new generation of miniature 180° FoV wide-angle lenses.

Locally magnifying imager

We present a new optical system capable of changing in real-time, anywhere in the field of view, the magnification of the image, while potentially keeping the total field of view constant. This is achieved by using an active optic element to change the direction of some selected rays, thus creating controlled distortion. A mathematical description of such a system is presented, along with the fundamental limits on the amplitude of the active surface and on the F/# to keep the image quality. Experimental results obtained with a simple prototype using a ferrofluidic deformable mirror as the active surface are also presented. The local magnifications obtained are in agreement with the developed mathematical model.

Dedicated testing setup for panoramic lenses

Panoramic imaging is of growing importance in many applications around the world spurred by the development of digital imaging. Panoramic lens characteristics are unique and their careful characterization can be a challenge. For example, the price to pay for a large field of view in this type of lens is high distortion in the image. For vision applications like security or inspection, a precise knowledge of the distortion introduced by panoramic lenses is essential to produce natural unwrapped views to the operator. Of special concern is the image quality which must be uniformed over the entire field of view because all directions are equally important. In addition, two hemispheric images can also be stitched together to create a complete spherical image. For these reasons, we have developed a dedicated setup to study the distortion and the image quality produced by panoramic lenses. The test setup is made of a 75-cm radius cylindrical structure with targets placed on it. Using referenced equally-spaced targets, we obtained the radial image mapping curves for various azymuthal angles, allowing us to calculate the full-field resolution map. Also, transition targets were used to find field-dependent spatial frequency where the MTF is 50%. We tested four different panoramic lenses, two panomorph lenses and two fisheyes. For each lens, we discussed the experimental resolution and MTF curves and compared some of those results to theoretical design data.

Active imaging lens with real-time variable resolution and constant field of view

We present a lens with a constant total field of view and real-time variable resolution in certain zones of interest. This smart imaging lens uses an active optical element to modify as desired the local distortion. This way, while keeping the total field of view constant, the resolution can be increased in a zone of interest, at the expense of decreasing it somewhere in the remaining part of the field of view. We first present the concept of this lens, using a deformable mirror as the active surface. Computer simulations are done with Zemax in which a magnifying power of 2 in a zone of interest representing 10% of the full field of view is achieved, using a f=12.5 mm lens and a F/# of 18. Different combinations of theses parameters would allow different performances and results. We then present experimental results of this lens with a prototype built using a ferrofluidic deformable mirror as the active element. Experimental results of a zone of increased resolution with a magnification of 1.32 and a zone of decreased resolution with a magnification of 0.80 are obtained.

Consumer electronic optics: how small can a lens be: the case of panomorph lenses

In 2014, miniature camera modules are applied to a variety of applications such as webcam, mobile phone, automotive, endoscope, tablets, portable computers and many other products. Mobile phone cameras are probably one of the most challenging parts due to the need for smaller and smaller total track length (TTL) and optimized embedded image processing algorithms. As the technology is developing, higher resolution and higher image quality, new capabilities are required to fulfil the market needs. Consequently, the lens system becomes more complex and requires more optical elements and/or new optical elements. What is the limit? How small an injection molded lens can be? We will discuss those questions by comparing two wide angle lenses for consumer electronic market. The first lens is a 6.56 mm (TTL) panoramic (180° FOV) lens built in 2012. The second is a more recent (2014) panoramic lens (180° FOV) with a TTL of 3.80 mm for mobile phone camera. Both optics are panomorph lenses used with megapixel sensors. Between 2012 and 2014, the development in design and plastic injection molding allowed a reduction of the TTL by more than 40%. This TTL reduction has been achieved by pushing the lens design to the extreme (edge/central air and material thicknesses as well as lens shape). This was also possible due to a better control of the injection molding process and material (low birefringence, haze and thermal stability). These aspects will be presented and discussed. During the next few years, we don’t know if new material will come or new process but we will still need innovative people and industries to push again the limits.