Photonics Technology Overview

Document sections:

  1. Lightspan LS-1 Series Transparent Thixotropic Gels
  2. Lightspan LS-2 Series Optical Coatings
  3. Lightspan LS-3 Series Transparent Curing Encapsulation Gels
  4. Lightspan LS-5 Series High Stability Optical Fluids
  5. Lightspan LS-6 Series Optical Thermosets

Lightspan LS-1 Series Transparent Thixotropic Gels

Lightspan's transparent thixotropic gels are comprised of an optical grade polymer fluid immobilized in a nanoparticle powder. Unlike the two-part curing encapsulation gels described below, the thixotropic gels have no curing characteristics and no intrinsic shelf life limitation prior to dispensing. In appearance they are like a clear mayonnaise (Some of the gels appear hazy in bulk quantities, but are clear in thin film). By their nature they are thixotropic, meaning that they do not, like normal Newtonian fluids, have a well-defined viscosity. Rather, their viscosity varies inversely with flow rate or shear rate of the gel. At rest or zero shear rate, thixotropic gels are mechanically stable and will not migrate. When shear forces are applied to a gel, as for example when a gel is injected from a syringe into an optical assembly, the gel viscosity decreases dramatically and allows the gel to flow with the characteristics of a viscous fluid. A familiar material with similar thixotropy is toothpaste; when pressure is applied to a toothpaste tube, the paste flows easily out the neck of the tube. But when shear forces are removed the paste assumes a stable configuration on top of the toothbrush bristles. Lightspan transparent thixotropic gels are designed to encapsulate and protect sensitive components from dust, moisture, solvents, and mechanical vibration while being transparent enough to permit efficient transmission of optical signals. Originally designed for improving the return loss in single mode mechanical fiber splices, these products now find use in a number of photonics and optics applications.

These products are available as single-component ready-to-use materials. They are packaged in syringes or cartridges for convenient dispensing. Packaged gel is ultrafiltered, fully deaerated, and free of bubbles and voids.

As with its other products, Lightspan prepares the constituents of its thixotropic gels to minimize presence of trace microscopic dust contamination and dissolved ionic species which are known to degrade optical semiconductors.

Lightspan thixotropic gels are available with a range of refractive index from 1.46 (index matching to fused silica) to 1.59 (index matching to crown glass or polycarbonate). For standard gel products, the gel consistency is designed to be in the range of 220 to 250 on the cone penetration scale (ASTM D-1403). This is equivalent to an “apparent” viscosity in the range of 1,000,000 cP (ASTM D-1084) and is reminiscent of a mayonnaise-like material. Lightspan can customize these gels for different refractive index, and different apparent viscosities. A rigid gel is sometimes useful in mobile applications where sudden mechanical shock could jar the gel free from its intended location. A softer gel is sometimes useful in applications where the gel must flow rapidly as it is dispensed into or spread through an optical assembly.

Standard packages for Lightspan thixotropic gels are 0.5cc and 30cc syringes.

Typical applications for Lightspan transparent thixotropic gels:

  • Fiber & light pipe splicing

    Transparent thixotropic gels were originally developed for use in the fiber-to-fiber butt joint in single-mode mechanical telecommunication splices. The presence of the gel displaces the air gap between fibers and forms a bridge of material with refractive index similar to that of the fiber, allowing a dramatic improvement in return loss for the splice. Additionally, the gel performs a protective encapsulation function, protecting the joint from dust, rain, and solvents. In this application, the gel is selected to provide an index match to the fiber waveguide material. For fused silica splices, use Lightspan LS-1246 (1.46 index).

  • Small flat panel displays

    Placement of a gel between the cover glass and the light emitting elements can improve image brightness and image contrast in the presence of ambient sunlight. Because thixotropic gels exhibit a relatively high viscosity compared to the curable encapsulation gels in their uncured state, thixotropic gels are usually only suitable for injection into smaller display assemblies such as are used on hand-held devices or instrument readouts. For more information on index matching, review our application notes.

  • Low-stress pigtailing

    When a fiber is butt-joined to a planar element such as a lens or an optical semiconductor, a gel can provide suitable index matching without the risk of excessive strain on the adjoining components. In this application, since the gel does not rigidly restrain the parts as would be done by an optical epoxy, the physical alignment of the parts must be controlled externally by other means such as V-grooves, or other alignment jigging.

  • Test & troubleshooting

    A transparent thixotropic gel is a convenient index-matching medium for making quick optical test connections. Unlike a curing gel, a ready-to-use thixotropic gel requires no cure time to set up and is easier to remove subsequent to testing.

  • Optically clear matrix media

    Photonically active powders, nematic crystals, dyes, fibers, bubbles, or particles can be homogenized into and suspended in Lightspan gels to form filters, optical test standards, switch elements, photonic crystals, gratings, etc. Lightspan can assist you in proper homogenization techniques or prepare your mixture to your specifications.

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Lightspan LS-2 Series Optical Coatings

Lightspan's transparent optical coating products are synthetic polymer solids dissolved in a high purity volatile carrier solvent.These products are applied to a substrate by dip-coating, spin-coating, or casting. Once the carrier solvent has evaporated off, the coating fraction remains behind, forming a transparent optical quality film. Coating process parameters may be varied in order to control the film thickness; alternatively, Lightspan offers suitable solvent diluents for decreasing the coating concentration in order to reduce film thickness. Information regarding deposition processing is included on each product datasheet.

Lightspan coatings are designed for use with sensitive photonics and electro-optics. Once the coating solvent has been evaporated off, the coating itself is chemically inert, and will withstand high temperatures such as are encountered in subsequent soldering or heat staking operations. Low refractive index coatings such as LS-2233 (refractive index = 1.33) are used to treat fibers, lenses, and other components in order to improve total internal reflection (TIR) at grazing angle of incidence, or to protect optical surfaces from exposure to dust or ambient atmosphere.

Coating constituents and their carrier solvents are manufactured so as to minimize the presence of trace ionic species which are known to degrade optical components, semiconductors, and electronics. All Lightspan coatings are provided with test data for physical properties and for refractive index vs. wavelength, thermo-optic coefficients, and optical absorption vs. wavelength over the UV-Visible-Near IR spectrum. Most of Lightspan's optical gel products can also be prepared in a coating format; if this is of interest, contact Lightspan to discuss a customized coating for your application.

Standard packages for Lightspan coatings are 30cc and 480cc bottles. The 30cc bottle comes with a micropipette for laboratory prototyping work.

Typical applications for Lightspan coatings:

  • Fiber & light pipe cladding

    A low index coating on the outside of silica fiber or on the inside of fluid light pipe improves the total internal reflection (TIR) at grazing incidence, thereby reducing the light leakage at bends. In multichannel telecom assemblies this helps reduce channel crosstalk. In UV liquid light pipes, it reduces transmission loss over the length of the pipe. In flexible plastic optical fiber (POF) lightguides, it reduces the db/cm losses especially at minimum radius bends.

  • Transparent film

    Thin transparent films can be cast by spin-coating or molding. Examples are: UV transparent films for semiconductor UV-lithography, or transfer AR films to improve image brightness on small flat panel displays.

  • Semiconductor AR coating

    Photonic semiconductors may benefit from use of a quarter wave thick anti-reflection coating for improvement in optical efficiency by reducing Fresnel reflections.

  • Lens coatings

    The low index coating LS-2233 has a very low surface energy, making it ideal as a moisture and dust repellent and protective coating for lenses and other solid optics components. It also serves as an effective AR coating when applied in a quarter wave thickness.

  • Optical quality release agent

    For casting or molding of optical quality components from thermoset plastics, it is usually advisable to coat the mold with a coating which allows release of the molded part without contaminating, discoloring, or “frosting” of the optical surfaces of the molded part. Lightspan LS-2233 is recommended for this application.

  • Optical quality barrier film

    In assembly operations which call for application of organic coatings or adhesives in a well defined surface pattern, it is often desirable to lay down a barrier coating which will impede lateral migration of the organic coating under surface tension forces. Lightspan LS-2233, with its very low surface energy and its compatibility with other optical materials, is usually recommended for this application.

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Lightspan LS-3 Series Transparent Curing Encapsulation Gels

Lightspan transparent curing encapsulation gels are crosslinkable synthetic polymer fluids designed for use with sensitive photonics and electro-optics. Lightspan gels are designed to encapsulate and protect sensitive components from dust, moisture, solvents, and mechanical vibration while being transparent enough to permit inspection of the potted assembly or serve as a highly efficient optical coupling medium. These products are available as two-component materials, packaged in double-barrelled side-by-side cartridges for convenient dispensing. Pot life for these materials is designed to be at least 8 hours at room temperature. This allows ample time for careful dispensing of the material into an assembly and for any trapped air bubbles to rise to the surface. Pot life can be extended by cooling the mixed material; conversely, the cure rate can be accelerated by heating the material. Information on the specific time/temperature cure parameters is included in each product data sheet.

Once the gel is cured, it has the consistency of a very soft elastomer, like a somewhat sticky gelatin. It is firm enough that it is stationary and will not migrate. This mechanical consistency allows the gel to support large dimensional strain without building up sufficient forces to cause substrate delamination or fracture of sensitive optics. The yield strength of the gel is low enough to permit wire bonds to slice through the gel during thermally induced micromotion without risking wire bond failure. Lightspan uses gel cure chemistries which permit crosslinking of the encapsulation gel without outgassing of noxious reaction products. Gel constituents are manufactured and processed so as to minimize the presence of trace ionic species which are known to degrade optical semiconductors.

Lightspan encapsulation gels are available with a range of refractive index from 1.38 to 1.57. For standard gel products, the cured hardness is designed to be low on the Shore 00 durometer scale (“gelatin-like”, ASTM D-2240) but the degree of crosslinking in these products can be customized for firmer elastomeric consistencies up to the range of Shore A durometer (“soft rubber”). Lightspan can also customize these products for different cure profiles, cure temperatures, and packaging options. All Lightspan gels are provided with test data for physical properties and for refractive index vs. wavelength, thermo-optic coefficients, and optical absorptions vs. wavelength over the UV-Visible-Near IR spectrum.

Standard packages for Lighstpan gels are 50cc, 200cc, and 400cc cartridges. Cartridges are designed for use with Lightspan CAPgun dispensers.

Typical applications for Lightspan curing encapsulation gels:

  • Flat panel displays

    Placement of a gel between the cover glass and the light emitting elements can improve brightness and image contrast in the presence of ambient sunlight. Lightspan LS-3252 is usually recommended for this application.

  • HBLEDs and Photodetectors

    Use of a high index soft encapsulation material between the photonic semiconductor and the cover lens can improve light efficiency by reducing Fresnel reflections or increasing the size of the light extraction cone. Lightspan LS-3252 or LS-3357 is usually recommended for this application. For more information on light extraction, review our application notes.

  • Encapsulation of hybrid electro-optic circuit boards

    Applications similar to conventional printed circuit board encapsulation where there is concern about the rigidity or chemical incompatibility of conventional encapsulants with sensitive optical components.

  • Potting of packaged modules

    Assemblies such as transponders, transceivers, detector arrays, fiber Bragg gratings, fiber delay lines, etc., can be partially or fully encapsulated within their outer containers, providing gentle mechanical support, and strain relief. Lightspan LS-3238 is usually recommended for this application.

  • Tacking/staking

    Sensitive joints between neighboring photonic components can be staked or supported in place for subsequent assembly, soldering, or test operations without risk of high stresses which would be caused when using epoxies for this purpose. Lightspan LS-3238 is usually recommended for this application.

  • Optical index matching

    Lightspan encapsulation gels have excellent transparency properties and will resist yellowing during exposure to heat or high radiation flux. They are ideal for use as a mechanically pliable refractive index matching medium between adjoining optical elements. For more information on index matching, review our application notes.

  • Optically clear matrix media

    Photonically active powders, nematic crystals, dyes, fibers, bubbles, or particles can be suspended in Lightspan gels and “frozen” in place using a snap thermal cure to form filters, optical test standards, switch elements, photonic crystals, gratings, etc. Lightspan can assist you in proper homogenization techniques or prepare your mixture to your specifications.

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Lightspan LS-5 Series High Stability Optical Fluids

Lightspan transparent optical fluids are chemically inert, synthetic polymers designed for use with sensitive photonics and electro-optics. These materials are designed to have excellent purity, thermooxidative stability, ultra-low volatility, and radiation stability. Lightspan offers standard fluids with refractive index ranging from 1.29 to 1.67 to index match with AR coatings such as MgF2 (1.38), silica (1.46) up to dense flint glass (1.7). As a rule, Lightspan's fluids cover a viscosity range from about 100 cP (“a light oil”) to about 20,000 cP (“like honey”). While lower viscosity values can be provided as custom products, these often come with the trade-off of higher volatility. Similarly, very high viscosity fluids (> 20,000 cP) are sometimes provided but these materials are more difficult to dispense and so are not as commonly used. Unlike Lightspan's other gel or coating products, Lightspan's fluids are true fluids and will flow and migrate if not contained by a physical structure or by a surface energy barrier film (Lightspan LS-2233 Optical Coating is suitable as a barrier film).

Lightspan can provide custom fluids with arbitrary refractive index and viscosity in the range of 1.3 to 1.7. All fluids are provided with test data for physical properties and for refractive index vs. wavelength, thermo-optic coefficients, and optical absorption vs. wavelength over the UV-Visible-Near IR spectrum. For more information on index matching, review our application notes.

Standard packages for Lightspan fluids are 30cc and 480cc bottles. The 30cc bottle comes with a micropipette for laboratory prototyping work.

Typical applications for Lightspan Fluids:

  • Projection display components

    High refractive index fluids can be used to fill air gaps in high temperature lens assemblies in order to displace dust, permit cooling action, or reduce interfacial reflections.

  • Metrology fluids

    When used as a bath, or a temporary index matching medium, an optical fluid of suitable refractive index can provide an optically transparent connection between an optical test signal and an optical device under test. The classic example of this application is that of “immersion oil&rdquol; used to improve the light transmission between a microscope objective and a microscope slide. A present day analog of this application is as a fiber test probe contact fluid for direct optical measurements of wafer scale photonic circuits. Similarly, fluids such as Lightspan LS-5257, and SantoLight SL-5267 serve as excellent “contact fluids&rdquol; for use in refractometry.

  • Fluid filled lenses or microchannel waveguides

    The thermo-optic coefficient (dnD/dT) of Lightspan fluids is in the range of negative 2 to 5 x 10-4/ºC, much larger than most optical plastics and glasses. It is large enough to permit use in self compensating lenses in laser beam optics, for example. In high power density optics, the use of an optical fluid as a lens or prism medium permits the optical component to be cooled by using the optical fluid as a heat transfer coolant.

  • Test & troubleshooting

    An optical fluid is a convenient index-matching medium for making quick optical test connections. A fluid is often easier to inject into and to clean out of an optical assembly.

  • Optical signal processing medium

    Optical fluids can be used as the working medium in bubble-action optical switches, thermocapillary action switches or attenuators.

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Lightspan LS-6 Series Optical Thermosets

Lightspan's transparent optical thermosets are heat-curable soft polymer plastics designed for use with sensitive photonics and electro-optics. These materials are designed for casting or injection molding of high performance optical components such as lenses, blob-tops and underfills which must maintain high optical transmission, even after exposure to the high temperatures used in next generation lead-free solder processes. They exhibit sufficient strength and hardness to be molded or cast as discrete optical parts or films, or to be overmolded onto other assemblies with well-defined optical dimensions and finishes. Lightspan thermosets are supplied in a ready-to-use cartridge which allows off-line mixing in a preset 1:1 ratio, prior to casting or loading into an injection molder reservoir. Once mixed, the uncured thermoset fluid has a pot life of 3 days, allowing ample time for completion of molding runs. A variety of mold heat cure schedules may be selected ranging from a low temperature cure for several hours at 100ºC, to under a minute at 200ºC. Post-curing can also be employed to maximize material hardness. Details on cure schedule options are provided in individual product data sheets.

Once the thermoset is fully cured, it has the consistency of a soft plastic with a hardness on the high end of the Shore A durometer scale. While it is hard enough to hold its molded dimensions and to permit handling, it is still somewhat elastomeric in nature and will therefore take up strain relief during environmental heat cycling which cannot be accommodated by hard plastics or epoxies. Further, the chemical compositions of Lightspan's thermosets are designed to be far more resistant to sustained high temperatures and radiation flux than conventional materials. Under these conditions, they exhibit superior resistance to yellowing, hazing, and cracking. This makes them ideal for applications such as high brightness LED (HBLED) encapsulants or blob tops, flat panel display backlight, arrays, or transparent underfills for VCSEL flip chip mounts. For more information on LED applications, review our application notes.

All Lightspan optical thermosets are provided with test data for physical properties and for refractive index vs. wavelength, thermo-optic coefficients, and optical absorption vs. wavelength over the UV-Visible-Near IR spectrum.

Standard packages for Lightspan optical thermosets are 50cc, 200cc, and 400cc cartridges. Cartridges are designed for use with Lightspan CAPgun dispensers.

Typical applications for Lightspan optical thermosets:

  • HBLEDs, photodetectors, lasers

    Use of a thermoset to encapsulate the emitter/detector die protects the die from mechanical stresses and exposure to dust, dirt, and moisture. If the thermoset exhibits a high refractive index, it can also improve light output efficiency by reducing Fresnel reflections or increasing the size of the light extraction cone. Lightspan LS-3357 or LS-6257 is usually recommended for this application. For more information on light extraction, review our application notes.

  • Discrete optics

    Using a casting, or injection molding process, discrete optical elements such as lenses, prisms, and cover plates can be formed which withstand lead-free solder reflow temperatures, and high radiation flux environments. If the durometer values of the thermoset are sufficiently rigid to ensure tight dimensional fits with adjacent mounting features, the thermoset can be overmolded onto a mechanical exoskeletal support such as a part made with a high temperature engineering structural plastic or suitable metal or ceramic.

  • Polymer waveguide cores

    Planar waveguide cores and microchannel capillary waveguide cores may be fabricated using Lightspan thermosets. Besides low transmission loss, good refractive index contrast with the core index significantly higher than the waveguide cladding index is usually required in order to minimize signal leakage, channel crosstalk, and bending losses. Lightspan LS-6257 is usually recommended for these applications.

  • Flat panel displays

    Encapsulation of backlight assemblies allows improved light output efficiency and mechanical protection of the backlight LED arrays.

  • Optically clear underfills

    As with conventional flip chip underfills, which are employed to spread stresses between a flip chip and its substrate, an optically clear underfill can provide both the stress relief function and also efficient light transmission. Lightspan LS-6257 is an ideal material for use as an underfill in VCSEL flip chip assemblies.

  • Optically clear matrix media

    Photonically active powders, nematic crystals, dyes, fibers, bubbles, or particles can be suspended in Lightspan optical thermosets and “frozen” in place to form filters, optical test standards, switch elements, photonic crystals, gratings, etc. Lightspan can assist you in proper homogenization techniques or prepare your mixture to your specifications.

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