Fine adjustment screws work like most other screws, with consumers and businesses using them to secure objects. However, fine adjustment screws are characterized by the presence of more threads per inch.

Fine adjustment screwin vernier caliper

Characterized by the presence of external threading, screws are one of the world’s most versatile and commonly used fasteners. They are designed to secure objects by removing materials while simultaneously creating helical grooves. Fine adjustment screws serve this same purpose, but they feature a unique design that distinguishes them from other types of screws. So, what is a fine adjustment screw exactly?

Screw adjustmentmechanisms

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A fine adjustment screw is a type of screw that’s designed with 40 to 100 threads per inch. Of course, this is significantly more threads than traditional screws. As a result, fine adjustment screws can be “adjusted” for increased precision.

Needless to say, that's way too complicated. So the two-prism compressor was invented to simplify it.  But it's still complex, and maintaining its alignment when the laser beam into it wanders even slightly is a pain.

Coarseadjustment screw

Even better, you can place two or three prisms in it (the achievable negative GDD will be 4 or 9 times that of a single-prism device). Or you can place a grating and a prism (Chauhan, et al.) in it for even more GDD. This also provides the ability to compensate for higher-order dispersion.  All without the need for careful alignment.

Linearadjustment screw

Ultrafine AdjustmentScrews

It's based on the fact that angular dispersion introduces negative chirp. But angular dispersion also introduces other spatio-temporal distortions, so three additional prisms are required to compensate for them.

So we invented a single-prism pulse compressor (Akturk, et al.), which takes advantage of the precision of corner cubes, which retro-reflect a pulse parallel to its incident direction to within a few arcseconds. This device is much easier to work with:  a single knob  changes the chirp and another changes the wavelength. Also, it cannot be misaligned, and it cannot introduce any spatio-temporal distortions!

Also, modeling the dispersion of this device wouldn't have been possible without a very nice, elegant equation we derived (Chauhan and Trebino) for the total dispersion of an arbitrary number of dispersive elements of any kind.

Fine adjustment screws have 40 to 100 threads per inch. In comparison, ultra-fine adjustment screws have 100 to 500 threads per inch. Therefore, ultra-fine adjustment screws can contain over five times as many threads per inch than conventional fine adjustment screws. According to Wikipedia, ultra-fine adjustment screws are incredibly difficult to produce, especially if they contain more than 250 threads per inch.

Most ultrashort laser pulses pass through glass or other materials on their way to their final destination, which lengthens the pulse and introduces positive chirp into the pulse (the red precedes the blue).  The well-known prism (or grating) pulse compressor compensates for this effect, reducing the pulse length to its original and eliminating the chirp.

Fine adjustment screwin microscope

If an application requires the use of a screw in a specific area of the object or objects, a fine adjustment screw may be used. Optical mirrors, for example, are often mounted using fine adjustment screws. The screw can be driven deeper into the optical mirror, or it can be driven farther out of the optical mirror, to achieve the correct position. Other types of screws offer less-precise adjustments, making them a poor choice for applications such as this.

In addition to fine adjustment screws, there are also ultra-fine adjustment screws. As you may have guessed, ultra-fine adjustment screws have even more threads per inch than their fine adjustment counterparts.

Fine adjustment screws are typically made of stainless steel. Stainless steel has become the preferred material for adjustment screws because of its ability to withstand rust and degradation. With a chromium content of about 10.5%, it’s naturally protected against rust and corrosion. The chromium is placed on the exterior of steel, resulting in a barrier that inhibits oxidation. Fine adjustment screws made of stainless steel typically last longer and offer a higher level of security and support than those made of other materials.

How do you operate a pulse compressor?  You vary the chirp (GDD) by moving any prism into or out of the beam, introducing more or less glass (positive chirp). Or you can change the separation between the 1st and 2nd prisms (and also the 3rd and 4th prisms, maintaining these two distances equal). Changing your pulse's wavelength requires tilting all four prisms by the same amount.