Polishing Precision Optics - Photonics and Optical Components Surface Finishing

What are Precision Optics

Precision Optics is the use of optical components and elements in electro-optic devices. These devices are used for the detection, sense, transmittance, and effects of light energy. It works with light energy of specific wavelengths and the changes in its spectrum. Multiple geometries and surfaces are being created to get optical properties according to requirements. These properties are dependent not only on the geometry of materials but also on the type of material used. It helps the manufacturers to acquire a required set of properties according to the need of the application. Specific designs are made to cater to the modern world's requirements. There are different types of precision optics applications including laser optics, infrared optics, achromatic optics, and optical prisms. It is the combination of glass and plastic optics to obtain a targeted set of properties. Precision optics has introduced a whole new concept of very small lead times and delivery times as well. Quick light signal processing has been achieved by using modern precision optics concepts. Miniature devices and machines are continuously developed using precision optics materials. Moreover, precision optics are useful for applications where you have to use light for precise measurement, structuring, analysis, and processing to a higher level of accuracy. Photonics Photonics is a technology that uses photons. Therefore, it deals with the study of photons generation, transmission, energy loss, modulation, signal generation and processing, amplification, switching, and sensing. Photonics is a modern field that is becoming popular for advanced optical devices. It considers the generation, transmission, control, and detection of photons. It works for the development of smart technology applications that are implemented in our daily lives.

What is Photonics Engineering

It is the branch of engineering that deals with the study of light energy and information to create advanced technology in multiple fields such as instruments for healthcare, construction, information technology, communication, etc. Almost every industry is requiring photonics engineers to make advances in the field. Replacement of conventional light transmittance procedures needs efficient photonics devices that are possible to be made by using expert engineering knowledge. It has helped the communication industry to move towards a better future with higher efficiency and low delivery times. Photonics engineering makes you capable of generating and studying nanostructured materials that can exceptionally perform well and be used in optical devices of the modern era. It also identifies the techniques, tests, and procedures with which optical structures are visualised, monitored, amended, and used. Multiple components are being developed for optoelectronic devices that add to the efficiency of electrical circuits.

Photonics and Optical Components Surface Finishing

Importance of Photonics

Photonics plays a major role in the development of modern technology including smart electronic devices, industrial-level machines, faster communication through fibre optics and the internet, reducing the size of devices, and making intricate structures with high efficiency. Principles of photonics are extensively used in different fields to create wonderful instruments. It is one of the major sources of innovation in industries that make use of optics. It generates new directions for material choice, processing, device fabrication, data storage and transmission, and many more. It offers a new way to overcome the limitations of storage, accuracy, and speed in conventional devices. It contributes to light-saving procedures that make processes sustainable in energy development. So, it is not only helping in cost-effectiveness but also saving the people and the environment. There are photonic sensors that are contactless and are used efficiently to detect any hazardous thing for humans. Photonics makes an efficient implementation of particulate properties of light for developing modern optoelectronic and photonic devices for telecommunication, laser, LEDs, and many more.

Difference between Optoelectronics and Photonics

Optoelectronics deal with light emitting devices or light detecting devices while photonics deal with the physical science of light and its applications. Optoelectronics is an electronics branch that is just specified for optical applications. Photonics is more towards optics study i.e., relating to physical properties and phenomena. Source of light, detection and control is studied under optoelectronics for diodes, transistors, and other optical components of electronic devices. Therefore, photonics mostly deals with the study of physics and relating it to the optical phenomenon of light. Optoelectronics is more towards the electrical implementation of a phenomenon that is studied in photonics. Both branches are interlinked for the development of optical devices for precise applications ranging from household applications to space technologies.

Surface Finishing of Optical Components

Surface finishing techniques are very crucial and specified depending on the type of material and its application. Rastering or rotating the optical components in a fluid is commonly used in surface finishing procedures. Surface properties are controlled by surface finishing processes. Roughness, atomic concentration, and related properties of optical components are optimised for optimal reflective, refractive, transmittance, and related properties. Polishing pads, ultra-polishing pads, contact polishing, and quasi-polishing pads are used for this purpose. The better the surface finishing technique used, the better will be the optical properties of the prepared material. Bowl feed polishing is a technique that is employed for the ultrasmooth surface of up to 0.3 nm RMS value. Bonnet polishing is also employed where the surface roughness of up to 80 nm is required in the optical components. If a further improved surface is required, slurry jet polishing is used. It can be seen that the surface finishing operations of optical components are more difficult and expensive as well due to the very precise needs of optical specifications. To view the structure of any optoelectronic components, it must be first cleaned efficiently for better visuals and image creation. Kemet offers excellent surface finishing processes for most optical components and materials, some of them are described below concerning their uses and surface finishing properties.

Beta-barium oxide

Beryllium Copper

Cadmium sulfide

Calcium fluoride

Fused Silica

Gallium arsenide

Germanium

Lithium niobate

Lithium triborate

Magnesium fluoride

Quartz

Sapphire

Silicon

Silicon carbide

YAG

Zinc Selenide

Beta Barium Borate

It is a compound that shows non-linear optical behaviour and is transparent in the range of 190 to 1300 nm approximately when a crystal layer of few mm is used. It can be used to produce quantum-linked photons. It generates harmonic generations that can be used in optical oscillators and optoelectronic devices. Moreover, it is resistant to ultraviolet radiation. Its properties can be altered to obtain a wide range of optical profiles that are extensively used to develop optical processing devices.

Beryllium Copper

It is an alloy of copper that is used extensively for non-magnetic properties and non-sparking qualities. Its properties are being tailored to use in oilfields, the landing gear of aeroplanes, mould making, and robotic welding. High strength and high electrical conductivity have made it important to be used in the electronics industry and hence, it is linked with optoelectronic devices for providing better contact properties. Optical communication devices have made their use very advantageous. It is used in repeaters' housing that strengthens the signals travelling over a long distance. It provides photonic characteristics that can be used in photoelectric devices for multiple applications.

Lapping & Polishing Beryllium Copper - Case Study

Test Requirements: Optical finish, no specific flatness required
Component/Material: Various Beryllium Copper samples

Stage 1

Machine Type: Kemet 15” diamond lapping/polishing machine
Lap Plate: Kemet XP
Abrasive Type/Grade: 3µ Type K STD Diamond slurry
Additional Pressure: 4 Kg

Stage 2

Machine Type: Kemet 15” diamond lapping/polishing machine
Lap Plate: ASFL-AW silk cloth
Abrasive Type/Grade: 3µ Type K STD Diamond slurry
Additional Pressure: 4 Kg

Stage 3

Machine Type: KemCol 15 - Chemical Mechanical Polishing Machine
Lap Plate: Chem Cloth
Abrasive Type/Grade: Col-K
Additional Pressure: 4 Kg

Process Breakdown
Stage	Plate/cloth type	Abrasive type/grade	Dispenser settings	Process time
1	Kemet XP	3µ Type K STD Diamond slurries	2 seconds spray every 30 seconds	15 minutes
2	ASFL-AW silk cloth	3µ Type K STD Diamond slurries	2 seconds spray every 30 seconds	5 minutes
3	Chem Cloth	Col-K CMP Slurry	Constant drip feed from peristaltic pump	5 minutes

Stage 1:
This is a grading stage to produce a good flat surface and finish. Initially we ran the component on a Kemet XP plate using a 3µ slurry with 4kgs weight for 15 minutes. This produced a good reflective finish with some scratches visible.

Stage 2:
This is a pre-polishing stage, which gives a good reflective finish and prepares the component for final polishing. The component was then run on an ASFL-AW silk cloth using a 3µ type K STD slurry for 5 minutes with 4kgs of weight to produce a 90% blemish free finish.

Stage 3:
Polishing stage, gives a blemish free mirror polish. Finally the component was run on a CHEM cloth using COL-K colloidal silica with 4kgs of weight for 5 minutes. This produced a blemish free mirror polish.

Recommended minimum equipment required to run 1 component

1 x Kemet 15” Diamond lapping machine or larger
1 x Kemet 15” Col-k Colloidal silicate lapping machine or larger
1 x Kemet XP plate
1 x Kemet Stainless Steel plate for Col-K
1 x ASFL-AW silk cloth
1 x Plastic faced control ring
3µ Type K STD diamond slurry
Col-K Colloidal Silicate

Before Polishing Beryllium Copper

After Polishing Beryllium Copper

Cadmium Sulfide

It is an inorganic yellow solid that is made by the combination of two crystal structures namely greenockite and hawleyite. It is widely employed as the material for light sensors that use photo resistors or light-dependent resistors. Optical switches are also using it to detect light signals when electrical signals are received. Biomedical lasers are also constructed using CdS in conjunction with polymers. It aids the photoconductivity behaviour of the laser. With semiconductor behaviour and non-magnetic, the compound is used in solar cells to capture light and convert it to electrical energy efficiently. Thus, optoelectronic cells can make use of this compound for higher efficiency and energy harvesting.

Calcium Fluoride

An inorganic compound that is white, mostly used in the manufacturing of optical components like lenses and windows. It finds several applications in thermal imaging systems that have multiple uses. Moreover, it is used in making lenses for spectroscopic techniques, telescopes, and lasers. One of its major applications is photolithography which is used in fused lenses. It provides a wide range of transparency from ultraviolet to infrared region. It provides a high refractive index with a very homogeneous behaviour. It makes the optical devices stable and hence, its use is advantageous.

Fused Silica

Fused silica is the amorphous form of silicon dioxide that is used mainly in optical instruments like lenses, mirrors, metrology components, and parts of optoelectronic devices. As we know, there is a very high demand for the surface finish to aid the reflection, refraction, or transmittance of light. To fulfil this demand, abrasive powders have been used for surface finishing. Cerium oxide particles are used as abrasive powders. Moreover, Kemet has developed a polishing procedure that is very effective in obtaining a highly flat and pure fused silica surface.

Gallium Arsenide

It is an important semiconductor material that is used for making integrated circuits and field effect transistors. It has found major applications in optical communication systems and control systems. It is used to manufacture linear devices such as oscillators and amplifiers. The faster mobility of electrons through its crystalline structure has made its properties superior to that of silicon. Rear mirrors and lenses are made by using this compound due to its cost-effectiveness and better properties. Optoelectronic and microelectronics have also made use of this compound for better electrical and optical properties.

Germanium

It is in elemental form and a semiconductor with a lustrous appearance similar to that of silicon. It is mostly used in optical applications such as cameras, microscopes, fibre optics, and infrared optics. Wide-angle lenses have been made using this material. Windows, prisms, and other optical components for optoelectronic devices are made through germanium due to their excellent light and laser qualities. It is a natural beam splitter (50%) without the use of coatings. It is used as a substrate material to build optical filters. Photodiode and transistors are also made from germanium which forms the basis of microelectronics.

Lapping & Polishing Germanium Parts - Case Study

Test Requirements: To lap germanium discs with surface finish Ra<0.01 µm, Rz<0.05 µm;flatness < 1µm; parallel < 3 µm.
Component/Material: 6 x 25mm diameter Germanium

Stage 1

Machine Type: Kemet 15” diamond lapping/polishing machine
Lap Plate: Cast Iron
Abrasive Type/Grade: Kemox 0800S
Additional Pressure: 4 Kg

Stage 2

Machine Type: Kemet 15” diamond lapping/polishing machine
Lap Plate: ASFL polishing cloth
Abrasive Type/Grade: Kemet Liquid diamond Type K 1 Micron standard
Additional Pressure: 4 Kg

Process Breakdown
Stage	Plate/cloth type	Abrasive type/grade	Process time
1 Lap	Cast iron	Kemox 080S	2 mins per side
2 Pol	ASFL Polishing pad	Kemet Liquid diamond Type K 1 Micron	5 Mins per side

Comments:
Flatness required: < 3 µm . Preferably 1 µm
Flatness achieved: 2 light band = 0.67 µm
Ra required Ra: 0.01 µm
Ra achieved: 0.059 µm
Rz required: 0.05 µm
Rz achieved: 0.0253 µm

Before Lapping & Polishing Germanium

Before Lapping germanium

After Lapping & Polishing Germanium

After Lapping germanium

Lithium Niobate

This compound exists in the trigonal crystal structure. It is used for optoelectronic properties when in the form of a single crystal. Major applications of this compound enlist mobile phones, optical waveguides, piezoelectric sensors, linear and non-linear optics, and optical modulators. Electromechanical coupling is the phenomenon that occurs in this compound to provide excellent optoelectronic properties. It is a piezoelectric material that is used in electro-optic devices providing a wide range of required properties. It stands as a signal control element in these devices to perform at very high efficiency. It works as an excellent modulator of electrical and optical signals.

Lithium Triborate

It is a non-linear optical crystal that has a wide transparency range, small walk-off angle, and non-linear coupling. A combination of desirable mechanical, optical, and chemical properties has made its use proprietary in optoelectronic devices. Moreover, it has a high damage threshold making the microelectronic and optoelectronic devices durable for a longer duration. It has also been used in lasers and low-refractive index lenses. Frequency doubling and tripling of high-power peaks of sapphire and dye lasers also make use of this compound. It makes the conversion of CW to quasi-CW radiation in optoelectronic devices.

Magnesium Fluoride

It is an inorganic white crystalline salt that is transparent over a wide range of wavelengths, making it applicable in optical development. Its use is not limited to simple optics but it also finds uses in space telescopes. Excimer laser application is the major use that has made it popular. It is mostly used for UV optics. It is cut in a special plane to provide the minimum amount of birefringence. Anti-reflection and multilayer coatings are being made from this compound due to its mechanical properties like durability, strength, and thermal shock resistance along with optical properties. Polarisers and optical protective coatings are also built from this compound.

Quartz

Quartz also known as silica is made by a framework of SiO4 tetrahedra that shares oxygen atoms and gives an overall formula of SiO2. It has a high optical transmission value due to its low thermal expansion coefficient. Also, it can bear very high operating temperatures owing to the thermal expansion coefficient. It gives a purity like a water drop and a combination of optical and mechanical properties makes it feasible for making optoelectronic device components. It is widely employed for making windows, lenses, and external bodies of optical components. Moreover, due to its chemical compatibility feature, it remains unreacted and increases the lifetime of optical devices. It is stable in a wide spectrum range of ultraviolet to infrared radiation.

Polishing Quartz - Case Study

Test Requirements: To lap and polish 2 samples of Quartz to flatness better than 1 um and Ra better than 1nm
Component/Material: 1 quartz glass tube (OD ~62mm) sample and 1 quartz glass rod (OD ~41mm)
Machine Type: Kemet 15” diamond lapping/polishing machine

Process Breakdown
Stage	Plate/cloth type	Abrasive type/grade	Process time
1 Lap	Cast iron	Kemox 0-800s	2o minutes
2 Pol	PSU-M Polishing pad	Kemox WC	50 minutes

The glass was held on the weights using a non-slip film.

Quartz sections were initially cut on Micracut 201 precision saw using diamond cut off wheel and a slow feed rate. This produced a clean cut with minimal chipping

After lapping & polishing, the middle of the part is flat to 1-2 light bands (< 1 micron).

Before Polishing Quartz

After Polishing Quartz

PR3 Plate Quartz Polishing Test

Work Size: φ100mm×6mmT
Polishing Condition: In-Situ method. The benefit of using the dresser is that it keeps the cutting rate and maintains the PR3 plate flatness.
Plate: 15” PR3 Plate Conditioned by facing unit
rpm: 40rpm　
Slurry: Cerium oxidie (Size 1~2µm 5%/wt)
Flow amount : 5ml /min
Process 1: Pressure 80g/cm2 30 min
Process 2: Pressure 30g/cm2 20 min
Flatness Result: 0.130µm（λ/4.88）

Sapphire

Sapphire is the form of aluminium oxide. It has very high durability that is second to that of a diamond. It has a high transmittance range of 150 to 6000nm. Due to the wide transmission spectrum, it has found a variety of applications in optoelectronics. Optical grade sapphire is used in optical electronics and devices with the optical axis. Although it is a costly material it is used for highly sensitive applications of control systems and satellite communication. With a refractive index of 1.76 to 1.77, sapphire is used for making optical components having precise measurements. Moreover, thinned sapphire presents better optical properties close to the transmission limits. Kemet also offers high-quality surface finishing processes for Sapphire.

Silicon

Silicon is a hard and brittle solid that is a semiconductor. Its crystal structure is similar to that of a diamond. Due to its semiconductor behaviour, it is used as a transistor in electrical devices. It is used in optoelectronics for making diodes and chips. Silicon chips are made from high-quality silicon wafers that are used in computers, mobile phone sims, and integrated circuits. Modern electronics also make use of silicon to provide excellent electrical conductivity and control. Sensors are also made from silicon and fibre optics are made due to their high-temperature performance. It offers high bandwidth and is computable.

Silicon Carbide (SiC)

SiC is a semiconductor that is synthetically made through crystalline silicon and carbon. It is mostly used in grinding wheels, cutting tools, and sandpaper. Due to its lightweight, it is used in scan mirrors, reflective imaging systems, semiconductor wafer handling, and mounts. SiC-mounted optic structures are used in optical devices. Its applications are extended to space-based telescopes. Precision micro-optical devices and nano-photonic devices are made from this compound. Optical resonators, junction field transistors, and resistors are made using the excellent mechanical properties of silicon carbide.

Polishing Silicon Carbide - Case Study

Test Requirements: To produce best surface finish on 65mmø Silicon Carbide blanks prior to pitch polishing.
Component/Material: Silicon Carbide blanks

Stage 1

Machine Type: Kemet 15 diamond lapping/polishing machine
Lap Plate: Kemet Iron with spiral grooves
Abrasive Type/Grade: Kemet Liquid Diamond Type K 14 Micron standard
Additional Pressure: 3.5Kgs for 1 part

Stage 2

Machine Type: Kemet 15 diamond lapping machine with facing unit
Lap Plate: Kemet Copper with spiral grooves
Abrasive Type/Grade: 6-KDS1488 (water-based)
Additional Pressure: 4 Kg

Stage 3

Machine Type: Kemet 15 diamond lapping machine with facing unit
Lap Plate: Pure Tin
Abrasive Type/Grade: 0.75-KDS1438 (water based)
Additional Pressure: 4 Kg

Process Breakdown
Stage	Plate/cloth type	Abrasive type/grade	Dispenser settings	Process time
1	Kemet Iron with spiral grooves	Kemet Liquid Diamond Type K 14 Micron standard	2 second spray every 40 seconds	5 - 10 minutes (Front Surface)
2	Kemet Copper with spiral grooves	6-KDS1488 (water-based) diamond slurries	2 second spray every 40 seconds	10 - 15 minutes
3	Pure Tin	0.75-KDS1438 (water based) diamond slurries	2 second spray every 40 seconds	1.5 - 2 hours

Results:

Stage 1: - This process removed 10 - 20 µm in 5 - 10 minutes. The surface finish achieved ranged from 0.08 ~ 0.084µm.

Stage 2: - This pre-polish step gives a semi-reflective surface with surface finish of Ra 0.056 ~ 0.060µm.

Stage 3: - The polishing process results in a mirror surface finish of Ra 0.007µm.

After Stage 1 Polishing Silicon Carbide

Before Polishing Silicon Carbide

Conclusion
In order to improve surface finish and flatness, pitch polishing should be used

Flatness was measured on a Zygo and was 0.341 µm (Peak to Valley)

Final Flatness After Polishing Silicon Carbide

After Polishing Silicon Carbide

Yttrium Aluminum Garnet (YAG)

It is an important compound that is a crystal specially used in lasers for medical purposes. Solid state lasers are mostly made from synthetically developed compounds of YAG. It has a reflection loss of 16.7% with a transmission range of 0.21 to 5.5 μm. It can control the pulse ranges in microseconds making it the most efficient material for making lasers. It provides excellent optical control properties that are widely used in optoelectronic and semiconductor devices.

Zinc Selenide

ZnSe is a light-yellow crystal that is an intrinsic semiconductor (bandgap of 2.70 eV at 25 °C). Its remarkably wide transmission range makes it an excellent material for optical use is 0.6 to 21.0 μm. Common applications of this compound are light emitting diodes and lasers. It emits blue light when doped with chromium. Lenses, output couplers, lasers, windows, and beam expanders are the most popular applications of ZnSe because of their low absorption percentage and visible transmission.

PR3 Polishing Plate For Optics

The Kemet PR3 Polishing Plate is a new alternative to using polyurethane pads for polishing precision optics. The plate consists of a thermally stable resin and can be used with cerium oxide, aluminium oxide and diamond slurries. Kemet PR3 can be supplied as complete plates in most diameters or as discs for fixing to your machine base plate.

Excellent Flatness produced – below 0.08 micron
Improved Edge Exclusion - No fall off at the edges.
Produces excellent surface finishes down to 1nm
Easy to machine.
Ideal for polishing optical glass

Polishing Precision Optics - Photonics and Optical Components Surface Finishing

What are Precision Optics

What is Photonics Engineering

Importance of Photonics

Difference between Optoelectronics and Photonics

Surface Finishing of Optical Components

Beta Barium Borate

Beryllium Copper

Lapping & Polishing Beryllium Copper - Case Study

Stage 1

Stage 2

Stage 3

Recommended minimum equipment required to run 1 component

Before Polishing Beryllium Copper

After Polishing Beryllium Copper

Cadmium Sulfide

Calcium Fluoride

Fused Silica

Gallium Arsenide

Germanium

Lapping & Polishing Germanium Parts - Case Study

Stage 1

Stage 2

Before Lapping & Polishing Germanium

After Lapping & Polishing Germanium

Lithium Niobate

Lithium Triborate

Magnesium Fluoride

Quartz

Polishing Quartz - Case Study

Before Polishing Quartz

After Polishing Quartz

PR3 Plate Quartz Polishing Test

Sapphire

Silicon

Silicon Carbide (SiC)

Polishing Silicon Carbide - Case Study

Stage 1

Stage 2

Stage 3

After Stage 1 Polishing Silicon Carbide

Final Flatness After Polishing Silicon Carbide

Yttrium Aluminum Garnet (YAG)

Zinc Selenide

PR3 Polishing Plate For Optics

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