Shanghai North Optics has developed a double surface polishing method for the production of High Precision Thin Optical Windows or Substrates, where the surfaces of the windows or substrates are polished in one go, achieving accurate control of surface flatness while elevating the surface quality to 10/5 S/D. The high-precision thin windows manufactured in our factory are of outstanding functionalities that could meet the criteria for ultra-thin (0.1mm, 0.2mm) windows or substrates with high surface flatness (Lambda/10 or Lambda/4 @633nm), oriented for applications for precision optical systems such as Femtosecond lasers, whose market share is surging in recent years.

The double surface polishing method is a processing technique, where both of the two surfaces of optics complete their polishing procedure in one go. The optics are not put on pitch plates during the procedure, so that is no stress-releasing problem caused by bonding and detachment between the optics being polished and the carrier plate, which eliminates the variation of surface flatness of the window substrate to a significant extent.

High Precision Thin windows made from BK7, UV Fused Silica, and Quartz Crystals are available in North Optics, the windows feature superior surface quality of 10/5 Scratch/Dig, parallelism of 10 arcsec, with minimum thickness down to 0.03mm, diameter down to 0.5mm. N-BK7 is beneficial wavelengths from the Visible to IR spectrum (350-2000nm), and UV Fused Silica is recommendable for UV wavelengths due to low absorption in the UV range. Both of them are renowned optical glass materials with substantial mechanical strength and thermal stability and can endure operations under various conditions. Besides, windows made from Quartz Crystal are also provided.

The standard specifications of Thin BK7, UVFS, and Quartz Crystals Windows offered in North Optics include 12.7mmx0.2mm with λ/2 flatness, 12.7mmx1.0mm with λ/10 flatness, 25.4mmx0.2mm with λ/2 flatness and 25.4mmx1.0mm with λ/10 flatness, in addition to other custom dimensions. The windows could be configured with mounts for ease of handling, both uncoated window substrates and custom AR coatings are available.

Interference Test Result (Measured by Zygo)

Femto-line laser systems are developing rapidly in recent years, subsequently, one of its most crucial components, the demand for Ultra-thin (typically 0.1mm, 0.2mm) windows or substrates with high surface flatness (Lambda/10 or Lambda/4 @633nm) are surging.

The classical single surface polishing method, though utilized conventionally and commonly in the production of high precision laser grade optics products,  practical implementation has proved its difficulty and unreliability while processing high flatness optics with miniature thickness. The reason is that, during the actual operation of classical single surface polishing, the polished optics are often put on the pitch plate to accomplish the polishing procedure, the pitch plate and the optic are often bonded, inducing stress to the optics. You might manage to attain high surface quality(surface flatness and surface scratch/dig) while the optic is on the pitch plate, but once the optic is detached, the stress that comes from bonding is released, leading to a slight change of the form of the optics, and the surface flatness will be lowered. The thinner the optics, the more severe the impact of stress-releasing, since even the mildest deviation of the surface flatness could be significant as a proportion of the total thickness. To achieve high surface flatness in ultra-thin windows is always challenging for manufacturers.

To deal with the problem, Shanghai North Optics has successfully developed a double surface polishing method for processing high-precision optical windows or substrates, where both of the two surfaces of optics complete their polishing procedure in one go. The optics are not put on pitch plates during the procedure, so there is no stress-releasing problem, which effectively eliminates the variation of surface flatness after it is taken off the pitch plates. The double surface polishing method has not been incorporated in precise optics fabrication before because the finished optics produced by this method have the defect of low surface quality (that is, larger S/D). Shanghai North Optics has overcame this obstacle by improving the polishing machine, raising the surface quality to 10/5 S/D, while exploiting the benefits of the double surface polishing method of precise surface flatness. Furthermore, excellent parallelism(10 arcsec) is also realized on the UVFS Ultra-thin Windows or Substrates.