When a small, diffraction-limited spot size is required, a reflective design is a natural choice.
STEFAAN VANDENDRIESSCHE, EDMUND OPTICS
Laser systems costs are falling, leading to their increased use in a host of demanding applications from machining to hyperspectral imaging. Often in these cases, a small, diffraction-limited spot size is required. When specifying a microscope objective for this purpose, system designers will typically first consider refractive or transmissive objectives — but there are many good reasons to consider a reflective objective design. The main advantages are achromaticity, high power handling and the availability of high numerical aperture (NA) designs. For these reasons, reflective objectives are often the ideal choice for applications such as ultrafast laser machining, Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy of semiconductor wafers.
When selecting a reflective microscope objective design and manufacturer, it is important to analyze actual performance and not just design performance. Courtesy of Edmund Optics.
Since light microscopy was invented in the 1620s, its importance as an optical technique has increased exponentially. Initially developed to magnify small objects to a size discernable by the human eye using ambient light, light microscopy is now often coupled with technological advancements that enhance its capabilities. The human eye is no longer the main detector, having been replaced by a wide variety of sensors. In confocal laser scanning microscopy, for example, the ambient light source has been replaced by a laser, bringing with it the advantages of high-intensity light and a single wavelength. Moreover, microscope objectives can collect light from a small spot over a wide angle, which has led to their use in reverse in many laser material processing applications; that is, the objectives are now also used to tightly focus a collimated laser beam into a small, very intense spot that allows for precise welding, engraving or cutting of target material.
Reflective microscope objectives were first designed as inverted reflective telescopes1. These initial designs were severely hampered by optical aberrations — in particular coma and spherical wavefront deformation2. Design refinement and the use of aspherical mirror surfaces led to a significant improvement in performance and a gradual increase in the use of reflective objectives3.
Reflective objectives have very low aberrations at high NAs, an example of which shows λ/10 wavefront performance. Courtesy of Edmund Optics.
Objective selection
The choice between a refractive or reflective microscope objective in a laser system is not an obvious one, but the key benefits and disadvantages of both designs are relatively straightforward.
Refractive microscope objectives consist of many glass elements, in some cases well over 10 elements. Different Abbe numbers and refractive indices of each individual element are used to control chromatic aberrations.
In contrast to refractive microscope objectives, which often require upward of 10 elements to achieve high performance over a broad wavelength range, reflective microscope objectives often consist of just two reflective surfaces. Courtesy of Edmund Optics.
This leaves refractive microscope objectives with an undesirable trade-off. Refractive designs must balance the need for a greater number of elements to correct for wider wavelength ranges against the desire to minimize the number of elements and keep price, weight and size under
control.
As is clear from the name, reflective microscope objectives use reflective surfaces to focus the laser beam. Reflections off mirror surfaces are inherently independent of wavelength and thus achromatic, so improving wavelength-independent performance does not require additional elements in the system. The chromatic performance of reflective objectives is limited by the reflectivity of the coatings of the mirror surfaces. With many, often metallic, options available that can span from the deep UV to the far-infrared, reflective objectives have much better chromatic dispersion than refractive objectives. While this is obviously a benefit for white light applications, it is surprisingly important in many laser applications as well.
Reflective microscope objectives, distinguishable from their refractive counterparts by their shorter length, larger diameter and central obscuration, use reflective surfaces to focus the laser beam. Reflections off mirror surfaces are inherently independent of wavelength and thus achromatic, giving reflective objectives an advantage over refractive objectives. Courtesy of Edmund Optics.
The relevance of chromatic aberrations is obvious for applications using tunable lasers or systems that use multiple lasers at different wavelengths. It is, however, also crucial for ultrafast lasers. There is a significant amount of chromatic aberration in most refractive microscope objectives. Any propagation through glass disperses an ultrafast pulse, distorting it and lengthening the laser pulse. In reflective objectives, the only component affecting the pulse dispersion is the reflective coating of the elements. These effects are orders-of-magnitude smaller than for refractive objectives, and can even be corrected for with specialized coating designs if necessary. As ultrafast lasers rapidly become more important for laser machining purposes, reflective objectives are an excellent choice for ultrafast laser materials processing.
Superior optical performance
Reflective objectives can be designed to exhibit impressive optical performance over a wide operating wavelength range. Aspherical mirror surfaces can provide very low aberrations at high-operating NAs — a feat difficult to reproduce with refractive designs. Moreover, for a given magnification, reflective designs have longer working distances than refractive microscope objectives. In addition, as the light passing through reflective designs does not need to propagate through glass, there is no change of refractive index of the medium with changes in temperature, so reflective designs are, by nature, significantly more athermal than refractive designs.
The main drawback to reflective designs is the central obscuration in the objective. For most designs, the smaller the central obscuration, the larger the wavefront aberration. While using aspherical mirror surfaces helps counteract these aberrations, this trade-off needs to be made for each reflective objective optical design. The central obscuration not only reduces the transmitted energy, it also reduces the size of the central spot while slightly intensifying the outer rings of the resulting Airy disc pattern.
Finally, there are often also smaller radial obscurations due to the mounting of the central mirror. These are usually less detrimental to performance and easier to minimize, although doing so can come at the cost of stability and price.
There is no laser propagation through glass in reflective designs, so there is no concern with material absorption. This leads to an inherently higher power-handling capability for reflective designs over refractive designs. This advantage is somewhat offset by the central obscuration of most designs. Nonetheless it is often possible to safely dispose of the obscured energy with negligible detrimental effects for the rest of the system. Thus, while some of the input energy is lost, the objective as a whole can still handle higher power than a refractive design.
Reflective and refractive microscope objectives are usually easy to distinguish simply by looking at their packaging. In order to achieve high NA, reflective objectives typically have much larger diameters than refractive designs. But they have fewer elements, so they are often also shorter than refractive objectives. Finally, the lack of multiple glass elements makes reflective microscope objectives lighter than refractive designs.
Selection process
Reflective objectives are different creatures than refractive objectives, and an experienced manufacturer will reduce design to production time and often be able to deliver higher-performing stock designs. Theoretical design performance, toleranced design performance and real-world performance are related, but not identical. It is critical to any project’s success to make sure decisions are made based on the performance of the actual product that will be assembled into the system and not just the theoretical design.
The increasing demands for high-NA, high-performance broadband microscope objectives for applications such as hyper-spectral imaging, FTIR spectroscopy and laser materials processing has led to a renewed focus on reflective microscope objectives. The strength of these reflective designs lies predominantly in their very high NA, relatively long working distances relative to their magnifications, and broadband achromatic performance. This makes them an excellent choice for many laser applications, particularly for ultrafast lasers, which are finding increased use in material processing applications.
Meet the author
Stefaan Vandendriessche is a product line manager for laser optics at Edmund Optics Inc.; email: [email protected].
References
1. C.R. Burch (1947). Reflecting microscopes. Proc Phys Soc, Vol. 59, Issue 1, p. 41.
2. D.S. Grey (1951). Computed aberrations of spherical schwarzschild reflecting microscope objectives. J Opt Soc Am, Vol. 41, Issue 3, pp. 183-192.
3. W. Walecki et al. (2011). Dispersion free all reflective confocal microscope objective. Proc SPIE, Vol. 8036, p. 803612.