Q: Is metallic or dielectric mirror better for use with polarized light?
A: It depends on the characteristics of the light (wavelength, type of polarization, etc.), the specific properties of the reflective coating, and the application (angle of incidence, polarization preservation requirement, etc.). In many cases, standard metallic mirrors more or less preserve polarization after reflection, and standard dielectric mirrors can also roughly preserve S or P linearly polarized light after reflection. However, standard dielectric mirrors are not typically recommended for circularly or elliptically polarized light. But as these general guidelines do not always apply for every application, Newport suggests trying a mirror with your application before sourcing a larger quantity of mirrors if preserving polarization is critical to your application.
Q: Newport offers several mirror substrates. What is best for my application?
A: Borofloat® 33 is a good substrate for most general purpose applications. It is a high quality borosilicate glass that offers low thermal expansion and high thermal shock resistance at a moderate cost. For applications requiring high thermal stability, Zerodur substrates are ideal. It is a glass ceramic material with a coefficient of thermal expansion approaching zero and excellent homogeneity throughout an entire piece of material. When high-energy damage thresholds are the primary concern, Fused Silica substrates should be considered. It is a synthetic, non-crystalline, colorless, amorphous silicon dioxide of extremely high purity. And for the lowest cost solution (with lower performance requirements), float glass substrates may be used. Please see Optical Materials for more information.
Q: I see visible scratches and pits in my mirror or lens, how will these imperfections affect light reflection or transmission?
A: These imperfections are specified by a scratch-dig designation, with the first number indicating the maximum width allowance for a scratch and the second number stating the maximum diameter for a dig in hundredths of a millimeter. The value indicating the scratch is an arbitrary number from 10 to 80, determined by visual comparison to standards defined in U.S. Military specification MIL-PRF-13830B - the lower the number, the less visible the scratches are, and vice versa. Scratches and digs will result in light being scattered, with lower scratch-dig specs causing less scatter.
For the most demanding laser systems, such as intra-cavity and moderate to high power lasers, 10-5 and 20-10 scratch-dig is recommended. For many general purpose and research applications which can tolerate little scattered light, 40-20 scratch-dig is suitable. And for less critical applications where cost is a priority over scattered light, or if a substantial amount of light is available, 60-40 scratch-dig can be used.
Q: For low light applications, what are the best optics specifications?
A: A successful low light application must preserve every photon possible. The first way to assist with this is to choose mirrors and lenses with low scratch-dig specifications - such as 20-10 and 10-5 - to reduce scattered (i.e., wasted) light. Next, select mirrors with high reflective coatings - Newport offers many standard dielectric mirrors with average reflectivity greater than 99%. Then, utilize lenses with high performing anti-reflection coatings to improve transmission efficiency - Newport offers several standard coatings with average reflectivity per surface of less than 0.5%, compared to typical reflectivity per surface of 4% for uncoated lenses.