While we're here, is there a difference between dielectric coated mirrors, dielectric mirrors, dichroic mirrors, and dichroic filters?:thinking:
At first, I was tempted just to post, "What is the sound of one photon bouncing off a tree in the woods, if not one was there to see it."
Or, "In the beginning there was man...."
And then run like hell.
OK, can of worms time, this could be a whole masters dissertation:
A bunch of terms, all meaning in reality the same thing. There is NO difference between a dielectric mirror and a dichroic filter, in concept , for example.
A conventional mirror is a film of a bright metal, on a polished substrate.
The outer electrons in the metal do a pretty good job, for the most part, of redirecting photons that hit them, and for bare silver or bare aluminum, hit between 70-80% reflection, over a broad range of wavelengths. Some do better at other wavelengths outside the visible, such as gold or silicon for the far IR.
Sometime in the 1920s or 1930s, as math and instruments got better, people noticed that some glass lenses exposed to chemicals or the elements, for many years, were much more efficient at transmitting light, and no longer showed the 4% loss from Fresnel Reflection on the exposed side. Snell's law and the index of refraction (How much a given material bends light by slowing its speed) Were already well known, but the technology of glassmaking was improving by leaps and bounds. Investigation of the weathering showed that minimal reflection loss happened when the coating was 1/4 wavelength thick, at the targeted color. It also showed that the coating had a lower index of refraction then the lens.
At first done chemically, (Zeiss GMBH, Germany) and then by vacuum deposition, the simple antireflection "V" coat was a revolution in optics, allowing more then 2 or 3 lenses to be used in a telescope with low loss and a dramatic reduction in ghost images.
The next step was to try depositing more then one layer. A modern reflective coating, in its simplest form, consists of alternating one fourth wave thick stacks of high and low index materials. Every time the light passes through the interface of a 90' Low to High stack, a good percentage of it has its phase shifted 180' and is thus reflected. More stacks results in more reflection, thus a 15 layer stack of coatings can approach 99% reflection for one wavelength.
Now this is where the fun starts. If you put down, just for example, 10 red stacks, and 10 green stacks, and 5 blue stacks, you can start to approach a white mirror. By changing the thickness and type of stacks, you can go the other way and make a bandpass filter. Ie put down a reflect red and a reflect green stack, blue can still pass through. By moving away from just 1/4 wave thicknesses, the math gets much more complex, but you can then really start to tune things and make it efficient.
Ok so far we're dealing with 0 degree incidence, where the light hits the mirror at exactly 90 degrees (A "normal", in geometry) At normal incidence, its easy to make a 1/4 wave thickness. Now here is the rub, if the simple dielectric coating, made for normal incidence, are tilted, the reflectance falls off as the cosine of the angle. This is why light show folks end up "angle tuning" their combining dichros. So if you know what your target angle is, you can make the coatings a little thicker or thinner, as needed to correct for the thickness error of the light hitting at a angle. Hence the ability to make a dichro for combining laser lines.
So all dielectric coatings use the same physics, its just the designers change the thickness and number of layers to tune for a given set of properties. It gets even more fun if you add thin metal layers , between the layers of high and low index materials. metal films less then 1/4 wavelength thick are just spacers, they don't do much, allowing for some even crazier interactions.
So a dichro, is a dielectric coating, which means "insulator" and can be from one to say 100 layers, tuned to do a specific function, either bandpass, band stop, lowpass or highpass, or antireflect, or a combination of those functions. Dichroic simply means, two colors. A dichroic mirror is a subset of dielectric optics, if you want to draw one of those Gawd Aweful "Venn Diagrams" they make you learn in Algebra.
Thus something like a Newport BD-1 coating, which is reflect 99% of S (vertical) polarization from 0 to 45 degree incidence, from 400 to 700 nanometers wavelength, is about a 80 layer stack.
Since dichroic/dielectric coatings are not that expensive, and are getting cheaper all the time, professionals wonder why laser hobbyists still buy aluminum (75%) or enhanced aluminum (90-93%) coatings and loose large amounts of expensive laser power. As the laser power goes up, in the visible, that loss turns to heat, and its not uncommon for a 1-2 watt visible laser to start to burn a cheap metal film coating if a speck of dust lands on it or it gets focused. Dielectric coatings can handle huge amounts of power.
Modern "hard" dielectric coatings are also scratch resistant, meaning they will last much, much , longer then a aluminum or silver metal mirror.
For a low gain laser amplifier, such as helium neon, where the per pass gain is perhaps one tenth of a per cent, it becomes essential to use dielectric mirrors for the laser cavity. The losses from even a overcoated aluminum mirror, would prevent lasing.
Sometimes you see the terms "enhanced aluminum" or "Overcoated Aluminum", this is a compromise, it is a metal film mirror with two or three dielectric layers coated over the aluminum to bring the reflectivity up while keeping costs down. These are used in things like photocopiers or video projectors where 95% reflection is good enough.
Let me give one example. A simple AL mirror, 70% reflection at 532 nm. Launch power is 1 watt. At after the first mirror, you have 700 mW, after the second mirror, you have .70 x 700 = 490 mW. So if you used simple AL mirrors in a laser show projector, your down to 1/2 power just after the galvos alone. (OUCH!, and I've seen projectors with simple AL mirrors!)
A good source for low cost Dielectric mirror is One Stop Laser Shop.
Steve
