Dispersion differs slightly from the other optical properties above in that it focuses on wavelength separation rather than overall transmission or intensity. Dispersion is best understood through the example of a rainbow: different wavelengths move through water droplets at different speeds. Thus, what enters a humid atmosphere as white light separates into distinct bands of its constituent wavelengths, which we perceive as different colors.

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There are three main types of dispersion that influence glass applications:

Material Dispersion
Different wavelengths travel at different speeds, with longer wavelengths moving faster than shorter ones. As a result, the light that moves through a glass transmitter will be separated by the different travel rates of light. This separation is called material dispersion.

Modal Dispersion
Modal dispersion is a form of wavelength separation based on the physical properties of the transmitter rather than the wavelength itself. This method of dispersion is often discussed in the context of fiber optics since the core of the fiber can transmit light faster than the cladding.

Chromatic Dispersion
Chromatic dispersion arises from a combination of material dispersion and modal dispersion. It specifically refers to the separation of wavelengths as they move through a transmitter. Manufacturers of optical equipment can use lenses with different materials and refraction indices to equalize differences in velocity and minimize chromatic dispersion.

Understanding the factors that influence dispersion helps to ensure clearer image quality and more pure light transmission.

Optical window : a complete guide

Before the discovery of glass and even until the development of its mass production with the invention of float glass, windows where almost non existant. This changed a lot in the last 3 centuries, and now windows are everywhere from our homes to our cars and mobile phones. Below we will focus more precisely on windows used in Photonics : the optical windows.

What is an optical window ?

An optical window if an optical component made of transparent material in a certain wavelength range that is usually used for protection of equipment while having little effect on the beam signal.

In a nutshell they are used to avoid external elements (water, air, objects..) to enter the equipment while keeping the optical parameters of a system.

Types of optical windows and applications

Optical windows can be of many different types, round, rectangular, oval or free form shapes. Note that round optics are usually easier to manufacture and to reach high optical specifications, due to their rotational symmetry.

Usually windows width are bigger than their thickness but very thick windows may be used for high pressure applications.

Outside of their shape optical windows can be characterized by their material and their spectrum of application (UV, visible, IR).

Optical windows may also be used as a substrate for further processing. Optical coatings can make them filters or mirrors and metallic coating to offer electromagnetic shielding properties.  Finally marking can be added on surface to make optical masks.

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 Technical details

Which optical materials ?

All optical materials can be used for optical manufacturing, below you&#;ll find the most common ones :

Usually the harder the material the longer will be the processing time and the higher the price.

What are the typical specifications of an optical window ?

Below table shows some typical specifications that can be reached when manufacturing optical windows:

SPECIFICATION USUAL SPECIFIC SIZE Diameter up to 500mm D from 0.5mm up to 1.5m TRANSMISSION Visible range UV to far infrared SCRATCH 

&

 DIG 20-10 10 -5 SURFACE IRREGULARITY λ/10 λ/20; λ/100 may be achieved with IBF (Ion Beam Figuring) SURFACE Roughness 5 to 15 Å RMS Down to 1 Å RMS (superpolishing) DIMENSIONAL TOLERANCE +/- 0,1mm +/- 0,05mm Thickness TOLERANCE +/- 0,05mm +/- 0,01mm PARALLELISM &#; 10 arcsec &#; 1 arcsec CLEAR APPERTURE 85-90% 95% DAMAGE THRESHOLD 5J/cm2 20J/cm2 COATING AR Custom filtering, mirror, masks, ITO coatings

Optical window manufacturing process explained.

The key quality point for an optical windows are the material and the polishing quality. Below is the most commun manufacturing steps followed by factories:

1. Raw material cutting.

2. Lapping/ Grinding and Polishing.  There are two main equipment to process the laping and polishing :
   • • Pitch polishing machine.  Made of a support in metal or granite and moving polishing pitches.
     • Double-Sided Lapping & Polishing machine. Made of a upper and lower turning lapping plates and parts are hold in geared carriers with thickness a little lower that the window.

   Both are working with polishing pads and polishing slurry which includes abrasive particules, which evolve along the process from gross to fine.  Polishing is the longer processing step for windows taking from some hours to half a day per batch.

3. Chamfering is used to break the sharp angles of the windows and so to protect people having to manipulate then, and also reduce the risk of edge breaks on the windows. This process is also called beveling, but can be replaced by rounding of the window edge.
4. Strengthening or annealing, are heat treatments that release the stress inside the raw material after manufacturing and therefore minimise the risk of breaking the window by either mechanical or thermal shock.
5. Cleaning the parts in ultrasound bath.

6. Coating, is done according to coating specifications.

Where to buy an optical window ?

Optical windows can be found in optical distributors of flat optics manufacturer.  According to the complexity of your specification and material you may need to look for specialist (UV, IR, VISIBLE, SAPPHIRE).

Information to provide to have a quick and accurate quote :

• Material
• Drawing or dimensions (the higher the width/thickness ratio the higher the price)
• Quantities. For polishing flat optics 3 parts are usual manufacturing MOQ and 5 parts or multiple of 5 optimum quantities.
• Optical specifications : surface aspect, parallelism  and optionally transmitted wavefront.
• Coating request if any

If you are looking for a supply solution or need support for your request for quotation, don&#;t hesitate to click on below  link to contact SINOPTIX for your optical window project.

Are you interested in learning more about Optical Glass Window? Contact us today to secure an expert consultation!