OPTICAL CONTACT VS GLUE: BONDING EFFECTS IN PBS & NPBS CUBES

How optical cubes are constructed?

Alien Photonics optical cubes (PBS - standard, high energy, broadband and NPBS) are constructed by joining two right angle prisms. The hypotenuse of one prism is coated with polarizing or non-polarizing beamsplitting coating.

Gluing prisms to get PBS or NPBS cube

One way to join cubes is to glue two right angle prisms, with key production steps ranging from choosing and applying the right adhesive to curing. Alien Photonics offers both PBS cubes and NPBS cubes.

Bonding prisms using optical contact method

Another way to bond cubes is optical contact. While the procedure is not very complicated, but it requires high quality prisms and clean environment. This process allows to produce High Power PBS cubes and resistant broadband cubes.

How glue affects performance of optical cubes?

Several mechanisms contribute to performance of glued optical cubes (both PBS and NPBS). Alien Photonics cubes are produced with well controlled prism alignment and curing geometry. However, the fundamental limitations of glue remain.

Wavefront distortion (error) in glued cubes

Even when using Alien Photonics high flatness, e.g. λ/10 prisms, the adhesive layer can degrade the final transmitted wavefront to λ/6 or even λ/4, depending on glue behavior and curing conditions.

• Uneven curing. The adhesive shrinks as it hardens, pulling the prisms unevenly. This creates low-order phase errors.
• Layer thickness variation. Even a few micrometers of thickness difference across the interface changes the optical path and introduces additional wavefront distortion.

Glue-caused wedge between prisms

If done incorrectly, glued interface can introduce an unintended wedge between the prisms, resulting in beam deviation. This effect remains within acceptable deviation limits when the assembly is controlled, but two mechanisms can increase it:

• Uneven applying adhesive layer. Micrometer-level thickness differences across the glue layer form a slight tilt. This causes a slight change in the output beam angle, effect is more visible in larger cubes.
• Prism shift during curing. Adhesive shrinkage can pull or slightly slide one prism relative to the other, increasing the wedge (and beam deviation).

Chromatic aberrations in glued cubes

Optical adhesives typically have different dispersion characteristics than glass, meaning their refractive index changes differently with wavelength. As a result, a glue introduces wavelength-dependent phase and angle variations:

• Chromatic beam and path shifts across the spectrum,
• Reduced performance in broadband or multi-wavelength systems.

While single-band cubes are unaffected, for Alien Photonics broadband PBS and NPBS cubes only certain adhesives are used.

Light scattering in glue layer

The adhesive layer can introduce additional scattering due to defects formed during application or curing. Typical sources include:

• Nano- and micro-bubbles trapped during dispensing.
• Partially cured regions.
• Defects or inclusions within the glue volume.

These defects scatter light and degrade optical performance, leading to: power loss, worsened M², reduced beam quality, lower extinction ratio, overall contrast and transmission degradation in glued PBS cubes.

Even with careful adhesive handling, a glue layer can never be completely free of scattering centers — this is an inherent limitation of glued cubes compared to optical contact.

Wavefront distortion caused by optical contact bonding

Optical contacting is the cleanest way to join two prism surfaces and produce optically contacted PBS. However, this method is not perfect. If the contacting procedure is not done correctly, the interface can introduce its own set of aberrations and imperfections - something Alien Photonics prevents through optical inspection.

Partial Optical Contact

If the two surfaces differ slightly in figure, the contact region closes only in isolated patches rather than across the entire surface. This creates irregularities or low-order aberrations. At Alien Photonics, this scenario is avoided by ensuring that chosen prisms have surface flatness across aperture, so the interface forms uniformly rather than in islands.

Air Gaps (Sub-Micron Voids)

Extremely thin voids can produce small fringes in single band and broadband PBS cubes. These gaps appear only when the surface is too rough or matching is done incorrectly. Alien Photonics prevents this by ensuring that only prisms with precision polished surfaces are used and the bonded region is verified.

Thermal Stability in glued and optically contacted cubes

Thermal behavior is one of the strongest differentiators between glued cubes and optically contacted cubes. The adhesive layer in glued assemblies reacts to temperature changes, while an optically contacted interface behaves almost like a monolithic block of glass.

Thermal stability of glued cubes

The thermal behavior of glued cubes is dominated by the adhesive layer. Optical glues typically have a much higher CTE (thermal expansion coefficient) than glass. As temperature changes, the glue expands or contracts at a different rate than the prisms, generating:

• Mechanical stress at the interface.
• Slow beam drift as the prism orientation shifts by microns (or arcseconds).
• Changes in wavefront error.

Even with controlled curing and minimal glue thickness, the adhesive layer remains the dominant source of thermal instability, since its expansion behavior cannot be 100% matched to glass.

Thermal stability of optically contacted cubes

An optically contacted interface contains no glue layer and therefore exhibits thermal behavior almost identical to a monolithic block of glass. So, temperature changes produce:

• negligible stress.
• minimal shift or drift.
• stable wavefront across the entire operating range.

In practice, an optically contacted cube maintains alignment and optical performance even under significant temperature swings, making it the preferred choice for high-power, precision, and long-term stable systems.

LIDT glued vs optical contact

Laser-induced damage threshold (LIDT) is where glued and optically contacted cubes differ the most. The presence (or absence) of adhesive layer and the type of beamsplitting coating completely determines how the cube performs in high-power optical systems.

LIDT of glued PBS cubes

A glued PBS cube always contains an adhesive layer, which becomes the primary limiting element. Even when high-quality prisms and coatings are used, the glue introduces:

• Absorption far higher than glass or dielectric coatings.
• Local heating and thermal runaway.
• Index mismatch
• Photochemical degradation

This makes glued cubes unsuitable for high-power, Q-switched, or ultrafast systems. Typical LIDT of Alien Photonics standard glued PBS cubes (single band and broadband) is 0.3-0.5 J/cm² @ 1064, 10 ns, 10 Hz.

LIDT of optically contacted PBS cubes

An optically contacted interface contains no adhesive layer. The two prism surfaces behave almost like a monolithic optic, and the only limiting factor of PBS and Broadband PBS becomes the dielectric coating itself.

Advantages:

• no organic absorption.
• no thermal runaway.
• no photochemical breakdown.

In practice, optically contacted PBS cubes handle: high-power CW, nanosecond and picosecond pulses, high repetition rates, industrial micromachining and scientific laser systems that glued cubes cannot tolerate. They are also more favorable in quantum applications.

LIDT of glued and optically contacted NPBS

In Non-polarizing cubes LIDT is limited by both the glue and the metallic or metal-dielectric beamsplitter coating inside the cube. Thin metal films (e.g. Al, Ag or other metal) have intrinsically lower damage thresholds than dielectric PBS coatings. Even an optically contacted NPBS would still be limited by the metal-based coating. So NPBS cubes remain low-/mid-power components even if assembled perfectly.

Optical contact vs glue cubes - comparison

*NPBS cubes: LIDT is limited by coating itself.

Glued cubes are cost-effective but fundamentally limited by the adhesive layer, while optically contacted cubes deliver far superior stability, cleanliness, and power handling — making them the clear choice for any high-performance optical system.

Still not sure which one to choose ? Contact us for consultation!