Optics & optical coatings

You can obtain mirrors of high reflectance by coating polished glass substrates with metals such as aluminum (Al) and gold (Au). In addition, metals such as silver (Ag), platinum (Pt) and Chrome (Cr) are sometimes used for mirrors. Metal coatings reflect a very wide wavelength range, and have small dependency on incident angle.
Since metals absorb light that was not reflected, if you make the metal coating a little thicker, light does not transmit through to the glass substrate side.

Aluminum offers high reflectance throughout ultraviolet to infrared, but is prone to oxidation and cannot provide stable performance especially in the ultraviolet region. It is also easily scratched, which disenables wiping of dirt on the surface.
That is why mirrors fabricated with aluminum have a protective coating to protect the metal coating from oxidation or scratches. Protective coatings have the effect of maintaining reflectance in a certain wavelength range, but may lower reflectance in other wavelength ranges.
On the contrary, some special protective coatings increase the reflectance only in a certain wavelength range.

Chrome and other alloys (inconel) are used as partial reflection coatings.
Chrome has lower reflectance compared to aluminum or gold, and absorbs light strongly, making it unsuitable for mirrors, but its small fluctuation in reflectance and absorptance in a wide wavelength range makes it useful as reflective ND filters or beamsplitters.

When light enters a glass substrate, about 4% of reflection occurs and results in transmittance loss. However, you can change the reflectance of the glass substrate by coating the glass substrate with a dielectric material that has a lower refractive index than glass. When the thickness of dielectric coating is adjusted so that the optical path length (refractive index n × coating thickness d) is λ/4, the reflectance is minimized because reflections at the interface between the glass substrate and the dielectric coating, and between the dielectric coating and air cancel each other.
Note that it is impossible to achieve a reflectance of complete zero because the refractive index is limited by coating materials. Since the refractive index of glass substrate also has influence, the coating cannot provide an anti-reflection effect to all glass substrates.

Due to the small selection of coating materials for single-layer coating, reflection of a glass substrate remains to a certain extent. To deal with the limited coating materials, you can layer coatings to gain optimal anti-reflection effect.
In addition, it is possible to fabricate narrowband multi-layer anti-reflection coating (NMAR) that reduces reflectance in a specific wavelength, or multilayer (broadband) anti-reflection coating (MLAR) that reduces reflectance in a broad wavelength range.

You can fabricate a reflection coating with very high reflectance by layering a dielectric coating of high refractive index and that of low refractive index alternately on top of a glass substrate. Small reflection occurs at the interface between high refractive index and low refractive index.
The thickness of dielectric coating of all layers is adjusted to the optical path length of λ/4 (refractive index n × thickness of coating d), thus the light reflected by each layer has a matched phase causing the combined amplitude to increase. On the contrary, the multiple reflected lights that proceed in the direction of transmission cancel each other to zero.
If there is a sufficient number of layers of dielectric coating, incident light is attenuated and practically not transmitted. Attenuated light all shifts to reflected light. Since dielectric coatings do not absorb light, 100% of incident light becomes reflected light without any loss.

You can add a special effect to a multi-layer coating by changing its coating structure.
For example, you can design a coating according to various requirements such as broaden the wavelength band or on the contrary, narrow the transmission band extremely, prevent passage of different wavelength ranges or set the reflectance to an arbitrary value.

A dielectric multi-layer coating having a few dozen layers is sufficient to reflect the visible range, but if you need to handle a wavelength range including from ultraviolet to infrared range, then you need to combine three or more multi-layer coatings for the ultraviolet range, visible range, and infrared range.
Consequently, the number of coating layers increases extremely, and its fabrication requires very high technology.

If you irradiate a dielectric multi-layer coating with high power pulse laser, the laser power on the interface of the coating becomes extremely large so that it breaks the coating.
To prevent this, the coating structure and materials have been reviewed and special multi-layer coatings have been developed to avoid laser damage.

When you irradiate a broadband dielectric multi-layer coating with a femtosecond laser, wavelength dispersion occurs because the transmission path within the coating differs depending on the wavelength.
A low dispersion coating for femtosecond laser is designed so that the path length within the coating will not change depending on the wavelength to minimize the wavelength dispersion. In addition to that, its coating structure can withstand high power.

By inserting a metal coating inside a dielectric multi-layer coating, you can create an optic with properties that have been impossible to achieve before. This type of coating is called hybrid coating because it is a combination of dielectric multi-layer coating and metal coating.
In practice, it is used for broadband non-polarizing beamsplitters and bandpass filters. Unfortunately, this coating causes a little intensity loss due to absorption by metal.

When you change the incident angle of light against a multi-layer coating, the wavelength characteristics of transmittance and reflectance change.
If the incident angle of light is oblique against the thickness of each layer of a multi-layer coating, then the optical path length the light goes through within the coating becomes long. This is the reason why the transmittance and reflectance wavelength characteristics are of the longest wavelength at the incident angle of zero degree (perpendicular), and shifts to those of shorter wavelength.
This phenomenon is called a blueshift.

The thickness of dielectric coating or the refractive index of coating material change due to temperature, which also causes changes in the transmittance and reflectance wavelength characteristics of the multi-layer coating. Since this temperature dependency differs according to the manufacturing method and materials of coating, consideration needs to be given in the design stage of coating. Consult us about the use environment and temperature fluctuation range in advance.

Dielectric multi-layer coating may change with time depending on the use environment.
Although it differs depending on the manufacturing method and coating composition, a coating might swell and its wavelength characteristic may change a little when the coating is placed in a high temperature high humidity environment for a long time.
Special care is required for products of which transmittance and reflectance versus wavelength change significantly such as dichroic mirrors or band-pass filters.
Maintain laboratories and inside devices where optics are used at ordinary temperature and low humidity, and when optics are not in use, keep them in a dry storage such as Dry-Cabi®.

Optics have polarization properties when used other than at the incident angle of zero degree (perpendicular incidence).
There are two types of polarization properties, one is the property that transmittance and reflectance change between P polarization and S polarization, and the other is that phase difference changes between P polarization and S polarization.
The property of phase difference is difficult to control, and the coating products listed in this catalog do not guarantee the property. For example, if a linear polarized beam enters a coating product at a 45 degrees angle, the polarization of the output beam will not be a linear polarized beam at 45 degrees, but will become an elliptic polarized beam. However, the intensity of the output light becomes the average value of the P polarized component and the S polarized component, thus it will not affect the property of transmittance and reflectance when considering P polarization and S polarization separately.
If irradiating a glass substrate without coating with light at 45 degrees incidence, the reflectance of P polarization differs from that of S polarization.

Using a pulse laser with variable power in a system for irradiating a target substrate surface with appropriate density.
Place the target onto a XY translation stage for enabling the irradiation position to change after the end of each irradiation cycle. Oscillate repeatedly the laser onto the target substrate S times on a same position. Repeating 10 times the same irradiation with the same power onto the next position. Change the laser power for the next repeatedly 10 times irradiation operation, then repeat the operation within the limitation of the laser power.
Inspect the tested target with a 150× magnification microscope evaluate the presence of damaging spot.
Plot the damage occurrence ratio vs the power density data onto a graph. The result is the power density and the occurrence ratio curb to the maximum occurrence level. (The occurrence ratio is 0% and 0% up to maximum level).
This number is the laser damage threshold.

• The result may change depending on the wavelenght, the pulse width and the repetition frequency of the laser used for the test. The set up test condition must be consistent.
• Pulse laser exist in single mode laser (TEM00) and multi-mode laser. The TEM00 laser was used for the laser damage threshold test. The multi-mode laser has a particularity of having high local density energy, which is also called spike shape. It happens that even by applying a low laser damage threshold multi-mode laser with spike shape, the optics can be damaged. If a multi-mode laser with a frequent optics damage problem occurs, the multi-mode laser optics adjustment may be a problem. In this case, please check the mode pattern of the laser or contact the laser maker.