May 11, 2023
Diffractive optical elements (DOEs) have supported innovations in industries such as healthcare for years. From intraocular lens implants to dermatological treatments, the applications of this technology are growing by leaps and bounds.
Diffractive components consist of micro-structures at the wavelength scale and utilize the wave characteristics of light to achieve their function. This means that light is deflected based on the index of refraction and the physical geometry of the lens, where the periodic structures that make up the geometry is at the same scale as the wavelength of the light itself. Therefore, different wavelengths (i.e., colors) scatter at different angles, depending on the properties of the diffraction grating, which can be tailored to specific applications.
In this way, diffractive optics are particularly powerful for steering light. Besides, DOEs are flat, thin, and lightweight, which make them the preferred solutions in compact high-performance systems.
Here, we’ll walk through several existing and emerging applications where diffractive optics improve quality of care and share some considerations for implementing this technology.
Confocal laser scanning microscopy (CLSM) is an optical imaging technique that increases resolution and contrast. In this application, DOEs can serve as wavelength-dependent focusing mechanisms for modulating the depth of an image, enabling noninvasive cross-sectioning of tissues. CLSM’s specificity and sensitivity enable medical professionals to identify certain diseases and cancers that are traditionally difficult to detect.
Diffractive optics enable faster image acquisition and better image quality by leveraging line scanning of light rather than single point scanning, the latter of which requires more integration time. These elements can also reduce the impact of optical aberrations on performance, as aberrations can cause reduced image sharpness. Further, with a small objective size, DOEs can be placed in an endoscope for in situ detection.
When designing a CLSM system with DOEs, in addition to the complex design and manufacturing requirements, it’s important to consider that DOEs can decrease the signal-to-noise ratio, which increases the need for sensitive detectors. Further, nonuniform illumination is more likely necessary, depending on the quality of the light source selected. All these factors could impact the quality of the overall system.
Optical coherence tomography (OTC) offers similar benefits as CLSM, specifically for the eye. It uses light waves to take cross-sectional images of the retina so an ophthalmologist can measure the thickness of each retinal layer. These measurements guide diagnoses and provide guidance for individualized treatment for conditions like glaucoma, diabetic retinopathy, and macular diseases.
As with CLSM, diffractive optics supply faster image acquisition speed, which supports better image quality. In OCT, diffractive optics are used in the spectrometer for light analysis or in the super luminescent diode (SLD) for illumination purposes. The spectrometer serves as an intensity modulation detector, as the information it collects can be processed to generate OCT images. In the latter case, diffractive optics are used to scan over the SLD spectrum, eliminating the need for a spectrometer to generate an OCT image. Both techniques facilitate and enhance OCT speed and image quality.
Diffractive optics have allowed spectral domain and swept-source OCT systems to replace time-domain OCT through increased image resolution, imaging depth, and reduced noise. Diffractive optics simplify OCT system design, which reduces cost. However, there needs to be a plan for managing potential decreases in light intensity or optical alignment issues caused by the use of DOEs in OCT.
In addition to diagnostic applications, diffractive optics are a foundational technology for medical implants too. Cataract surgery, one of the most common surgical procedures today, involves replacing a patient’s natural lens with an artificial intraocular lens (IOL), like a multifocal IOL. As the disease progresses, the crystalline lens of the eye can become opaque due to increased light scattering, which impairs vision and may cause blindness in extreme cases.
With these minimally invasive IOL implants, patients experience immediate vision improvements, reduced halo effects, and an increase in their depth of focus. Multifocal IOLs, mentioned above, can be classified as refractive, diffractive, or combined. Refractive multifocal IOLs implement different refractive power annular zones, so focusing relies on the pupil dynamics. This arrangement makes refractive multifocal IOLs sensitive to centering. On the other hand, diffractive IOLs implement diffractive microstructures to enable multiple viewing distances at the same time, which makes these lenses more tolerant to their positioning. Combined multifocal IOLs make use of both refractive and diffractive techniques and can generate an array of different solutions that can be customized to the patient’s unique needs. Recent studies have demonstrated that making IOL selections based on an individual’s pre-existing conditions has improved patient satisfaction.
That said, these combined diffractive multifocal lenses attempt to imitate the natural accommodation capabilities of the human eye with simultaneous imaging. They’re not perfect. Patients may experience longer visual adaptation time and reduced night vision due to reduced light transmission. As cataracts become more prevalent in active adults, there’s a growing demand for premium multifocal IOLs to improve achievable image quality at different focus points. However, because of their advanced design, multifocal IOLs come at a higher cost to patients.
There’s an increasing amount of significant research and development happening to better understand the role of diffractive optics in emerging and existing healthcare technologies. We are seeing metalenses opening the door for DOEs in endoscopy and innovation in noninvasive health monitoring, among others. Engineers are exploring these solutions on an application-by-application basis because it’s not always a good fit. For example, since high-power laser surgery equipment needs a high energy threshold, it’s not an ideal fit for DOEs.
Even with a great idea, the cost to manufacture DOEs at scale is still a barrier — and that won’t change until we can leverage fabrication techniques from small-scale component manufacturing to better compete with other optics solutions.
For more information on optics in healthcare, watch our webinar, Modeling the Human Eye and Designing Presbyopia Correction in Zemax and Speos.