Optics & optical coatings
Guide | Technical Referance
Optical experiments use various optics with coatings.
In many cases, a coating is deposited on a polished glass surface, and this coating provides most of the properties of light required for optical experiments such as reflectance, wavelength characteristic, and polarization property. For this reason, mirrors and the like are used by directly irradiating their coating side with light to make full use of their performances. Coatings are mainly divided into two types, metal deposition coating and transparent film (dielectric coating) deposition coating.
Both of them are used as mirrors for optical experiments, but their usage, properties and various other points are different due to their completely different reflection mechanisms.
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.
Dielectric materials are transparent and colorless, having no large reflection or absorption like metals.
On the other hand, however, proper selection of materials and coating thickness can cause interference of light at the interface between a glass substrate, coating and air, creating special transmittance and reflectance wavelength characteristics.
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.
Dielectric multi-layer coatings can offer various wavelength characteristics, and are used for various optical devices. Meanwhile, leading edge research requires higher performance, and a dielectric multi-layer coating sometimes takes a very special structure consisting of 100 layers or more.
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.
Properties of Multi-layer Coating
While multi-layer coatings can achieve any reflectance and transmittance wavelength characteristics, they have various limitations.Incident Angle Dependence
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.
Similarly the reflectance of P polarization and that of S polarization change at the interface between each layer of coating, and in a multi-layer coating, fluctuation occurs in the wavelength characteristic in addition to significant fluctuation in the transmittance and reflectance between P polarization and S polarization.
For this reason, this catalog shows a P polarization and S polarization graph for transmittance and reflectance wavelength characteristics.
Coated optics have polarization properties even if a graph of P polarization and S polarization is not shown, and a graph of the average of P polarization and S polarization is listed for them.
Since most light sources other than laser light are unpolarized light, a graph of the average value of the P polarization and S polarization is sufficient, but in case of laser light which is a linear polarized beam, whether it takes the value of P polarization, S polarization or a value between them changes depending on the polarization direction.
Especially for coated optics that characteristically have the transmission band and reflection band switch in a narrow wavelength range such as a dichroic mirror, the wavelength where transmission switches to reflection differs depending on whether to use S polarization or P polarization.
Laser damage threshold
The optics used in laser oscillator or in laser processing system must have a high resistance to high power laser source incident over time without damaging the coated surface.
The resistance is determined by a numerical information called Laser damage threshold.
This term is frequently applied in optics selection for use in high power laser processing system and optical system design. The laser damage threshold evaluation method is regulated by the S-on-1 test method according to the international ISO 21254 standard.
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.
- The optics can be damaged easily even with a low power laser if the optics surface is dirty with dust or oil mark. Make sure that the optics are clean with no dust before applying to high power laser.
- The laser damage threshold value must be evalued under the similar test condition for a correct comparaison.
Every makers may test their optics with different test condition, it is not possible to judge the optics’s superiority by refering to the laser damage threshold value mentioned in the catalog. Sigma Koki provides to its customer a safety range of usage with strict test conditon.
But even using a below range laser damage threshold, the deterioration change with time is inevitable.
We support all types specifications of custom optics design and coating design which may not be found in our catalog. We are open to all kind of inquiries, ranging from project development to mass production optics.
Again, we accept to do customized coating on our standard no-coated optics, as well as a completed customized optics, also on supplied optics. Please do not hesistate to contact our Intenational sales division for all your inquiries.
Please fill out the inquiry form with your specifications required for us to support you smoothly.
For custom coating we may require a longer lead time, please contact our International Sales Division for details.