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Advantages of Polymer and Hybrid Glass-Polymer Optics

Valentina Doushkina, Qioptiq Polymer, Inc.

Polymer materials with optical properties are being thrust into the design forefront for new, sophisticated electro-optical applications, enabling commercial markets and applications including medical disposables, bar-code scan/recognition, security and fingerprint scanners, motion and presence sensors, CCD cameras and laser collimation. Current advancements in polymer technology and injection molding capabilities have increased the manufacturability and precision of polymer optics.

Designing and manufacturing optics and optical systems using polymers alone or hybrid polymer-glass applications provide complete custom capability and offer new optical and optomechanical solutions for a variety of applications. Integrated mounting provides configuration flexibility, design freedom, and simplified optical alignment, mechanical design, assembly and packaging. A system can be assembled and aligned in a single manufacturing step.1

Polymer optics: Why and where

In many optical products and applications, traditional optical glass has been successfully replaced with optical-grade polymers. The main reasons that polymer optics are chosen for a variety of devices, applications and markets are the low cost of materials and the fabrication techniques, which offer high production volume with high precision and fast repeatability.

Among other advantages is high impact resistance: Polymers do not split like glass, making this type of optics highly useful in terms of durability and cost efficiency in applications such as head-up displays, goggles, and medical and biomedical disposable optics. The modern days of nanotechnology are known for our fascination with compact customer products; e.g., mobile phones, cameras and microprojectors that can be about the size of a credit card, offering low weight, affordability, multifunctionality and long-lasting battery charge.

Thanks to their low density or low weight by volume, polymers are well adapted for making cutting-edge-technology products lighter and smaller. Polymers are between two and a half and five times lighter than comparable glass products2 and are suitable for difficult and sophisticated refractive, reflective and diffractive substrates with spherical, aspherical and cylindrical prescriptions, thus reducing the number of optical components needed in a given optical system. Molding is the most repeatable, consistent and economical way to produce complex-shaped optics in large volume or to integrate them onto a common substrate. Optical-grade polymers exhibit high light transmittance and are comparable to high-grade crown glasses. The optical-grade polymer market is growing rapidly; new polymers with low birefringence as well as higher and more stable refractive indices are available, offering design flexibility not possible with glass optics on their own.

Grinding and polishing of glass optics is costly and time-consuming, whereas injection or compression molding of precision polymer optics is inexpensive and offers design capability and flexibility. Mounting features can be integrated with optical components, simplifying the assembly and resulting in a compact product and reduced price. A comparison of the design requirements, manufacturing lead time, tolerances of finished products and production cost between glass and equivalent polymers in many applications shows the superiority of polymers.

To utilize polymers, the optical designer must have a solid understanding of the injection molding process and know how to design optics with polymers. Having the molding manufacturing company involved in product design at an early stage is beneficial for cost-effective manufacturing of high-performance optics.

Several diamond-turned and/or injection-molded precision polymer optical components manufactured at Qioptiq Polymer Inc. are shown in Figure 1.


Figure 1.
Qioptiq Polymer Inc. offers a variety of precision polymer optical components. Images courtesy of Qioptiq Polymer Inc.


Advantages of hybrid optics

In glass-polymer hybrids, a fairly thin layer of aspherized optical polymer is added to a conventional spherical glass lens. Most optical-grade polymers have great transmission – comparable to crown glass – so the overall transmission of the hybrid is comparable to that of glass optics. The advantage of hybrids is the freedom of glass selection because the glass component is manufactured conventionally.

Glass-polymer hybrids are widely used in advanced optical systems, making some applications feasible by reducing component weight, lowering production cost and enhancing the safety and appeal of products. Applications range from consumer photographic zoom lenses to professional fast-zoom lenses.

The properties of glass and polymers complement one other; for example, a nonhydroscopic glass component facing the outer environment protects the inner aspherized polymer component of a hybrid, while the latter offers a large numerical aperture, wide-field angles, a diffraction-limited highly resolved image and a compact design with a minimal number of optical components.

Another advantage is that they have the thermal stability of glass and the low manufacturing cost of polymer-molded optics. When the glass component carries the optical power and the thin layer of the aspherized polymer component corrects for aberrations, the thermal behavior of the hybrids is as stable as that of glass optics – or even better: Optical glass has a positive dn/dt, while the optical polymers have a negative dn/dt, enhancing the thermal stability of a glass-polymer hybrid. The refractive index of optical glass or polymer changes with the temperature; dn/dt is the temperature coefficient of the refractive index defined from the curve showing the relationship between glass temperature and refractive index. The temperature coefficient of refractive index (for light of a given wavelength) changes with wavelength and temperature.3

An off-the-shelf catalog lens was modeled as an aspherized glass-polymer hybrid and analyzed for image quality and thermal stability. The sharpness of an imaging system or of a component of the system is characterized by a parameter called modulation transfer function (MTF), or spatial frequency response. The polychromatic diffraction MTF of the glass doublet alone is shown in Figure 2a and is 20 percent for 60 cycles per millimeter, while the aspherized hybrid shown exhibits 82 percent MTF for 60 cycles per millimeter (Figure 2B). This is an economical way to create a high-performance diffraction-limited lens.


Figure 2.
The off-the-shelf glass doublet was analyzed via modeling for thermal stability. Image (A) shows the polychromatic diffraction MTF of the glass doublet at 0 °C, and (B) shows the same glass doublet at 70 °C.


The off-the-shelf glass doublet was analyzed via modeling for thermal stability; the results are shown in Figures 3A and B. The polychromatic diffraction MTF variation within the temperature range from 0 to 70 °C is about 10 percent.

The modeled aspherized glass-polymer hybrid also was analyzed for thermal stability. The polychromatic diffraction MTF of the hybrid is shown in Figures 4A and B. The variation of the MTF within the temperature range from 0 to 70 °C is only 1 percent, compared with that of the glass doublet, which was 10 percent.

Typical optical polymers

Acrylic (polymethylmethacrylate, or PMMA) is the most commonly used polymer. It has good clarity and high transmittance in the visible, whereas polystyrene is a good achromatic pair with acrylic. Polycarbonate is similar to polystyrene and has high impact strength and good performance over broad temperatures – up to 120 °C. Polymers have a tendency to absorb water, which can significantly alter some of their key properties. Those containing only hydrogen and carbon (polyethylene and polystyrene) are nonhydroscopic (highly water resistant), whereas polymers having oxygen or oxyhydrogen groups are hydroscopic.


Figure 3.
The polychromatic diffraction MTF of the hybrid was analyzed at 0 °C and (b) at 70 °C.


Zeonex/Zeonor – cyclo olefin polymer – has much lower water absorption, high transparency and purity, good moldability, low birefringence (one-third of polycarbonate), high heat resistance – 100 to 160 °C – and good chemical resistance to acids, bases, alcohols and fragrances.

Ultem PolyEtherImide, an amber transparent high-performance polymer, combines high strength and rigidity at elevated temperatures with long-term heat resistance. It excels in reusable medical applications requiring repeated sterilization and is available in FDA-compliant colors. OKP4 has a high refractive index of 1.6 or more and has extremely low birefringence and high fluidity.2

Innovative hybrid glass-polymer optical solutions on a component or module level offer the thermal stability of glass with the low manufacturing cost of polymers as well as enhanced optical performance and a reduced number of components. The latter characteristics are not achievable when polymer or glass optics are used on their own.

Meet the author


Valentina Doushkina, MSc, is director of engineering at Qioptiq Polymer Inc. in La Verne, Calif.; Email: valentina.doushkina@ca.qioptiq.com.

References

1. V. Doushkina and E. Fleming (2009). Optical and mechanical design advantages using polymer optics, Proc SPIE, Vol. 7424, 74240Q.

2. IDES Prospector. www.ides.com.

3. www.oharacorp.com


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