Cube beamsplitter

A cube beamsplitter is formed by cementing two right-angle prisms (typically 5 – 50 mm side length) along their hypotenuse faces, with a thin-film beam-splitting coating deposited on one prism's hypotenuse before cementing. Light enters normal to one face, reflects partially off the embedded coating, and exits through two perpendicular output faces.

Operating principle. Incident light enters the cube normal to one outer face. Inside the cube, the light hits the cemented interface at 45° angle of incidence. The thin-film coating at the interface partially reflects (90° from the input direction) and partially transmits (continuing through the second prism). Both output beams exit normal to outer faces, so neither undergoes refraction or beam offset on exit.

Why cube geometry. Compared to a plate beam splitter:

1. No ghost reflections. The back surface of the splitting interface is immersed in glass (the second prism); the back-surface Fresnel reflection ($\sim$ 4% for air-glass) is replaced with a glass-glass interface reflection ($<$ 0.5% for typical index match). Ghost reflections are essentially eliminated.
2. Symmetric transmission paths. Both transmitted and reflected beams pass through the same total glass thickness, so chromatic dispersion is equal in both arms. Important for interferometers.
3. No lateral beam offset. Both input and output faces are normal to their respective beams; no refractive offset is introduced.
4. Mechanical robustness. Solid glass block; not fragile like a pellicle.
5. Compatible with high-power lasers. No thin membrane; damage threshold typically 10 – 1000 W/cm² CW.

Standard splitting ratios and types.

Type	Coating	Splitting ratio	Wavelength dependence
Non-polarizing (NPBS)	Hybrid metallic/dielectric	50 : 50 (also 30 : 70, 70 : 30, etc.)	Broadband (200 nm – 1 μm)
Polarizing (PBS)	Dielectric	TE/TM split typically 99 : 1	Narrow band (50 – 200 nm)
Dielectric NPBS	All-dielectric	50 : 50	Narrow band (50 – 200 nm)
Hot/cold mirror cube	Spectrally-selective dielectric	Wavelength-selective split	Used as compact dichroic
Plate-coupled (PBS with absorber)	Polarizing + waveplate combination	Polarization separation w/ unwanted state absorbed	Specialty

Polarizing cube (PBS) operation. A standard polarizing beam splitter cube uses a multilayer dielectric coating designed to operate at Brewster's angle inside the glass. At the embedded coating, p-polarized light (TM) experiences no reflection while s-polarized light (TE) experiences strong reflection. The cube splits unpolarized input into:

• p-polarized output transmitted straight through
• s-polarized output reflected at 90°

Typical PBS extinction ratio: $> 1000:1$ for transmitted polarization, $> 100:1$ for reflected. Useful only over a $\sim 50$ nm wavelength range centered on the design wavelength.

Specifications for production-grade cube BSes.

Parameter	Standard glass cube
Substrate	BK7 or fused silica
Side length	5 mm to 50 mm
Cement	UV-cure epoxy (most common); optical contact (high-power)
Surface flatness	$\lambda / 4$ at 633 nm
Wavefront error	$\lambda / 4$
Surface quality	40-20 to 20-10 scratch-dig
AR coating on outer faces	$R < 0.25$% over design band
Damage threshold (CW)	1 – 10 kW/cm²
Damage threshold (pulsed)	0.5 – 5 J/cm² (10 ns)
Useful temperature range	$-30$ to $+70$°C (epoxy cement limit)

Limitations.

1. Limited high-power performance. The epoxy cement at the splitting interface can absorb a fraction of light, heat up, and degrade. High-power cube BSes use optical contact (no cement; molecular bonding of the two prism halves) but cost is significantly higher.

2. Polarization sensitivity of "non-polarizing" cubes. Even nominally non-polarizing cube splitters have $\sim 5 - 15$% polarization-dependence in their split ratio, due to the inherent polarization dependence of Fresnel coefficients at 45° AOI.

3. Wavelength dependence of split ratio. Cube BS coatings are most easily designed for narrow bandwidth. A "broadband" 50:50 cube might be 45:55 at the wavelength extremes.

4. Wedge-induced beam deviation. Cementing imperfections can introduce a small (1 – 10 arcsec) wedge between the two cube halves, causing the transmitted beam to deviate from input direction by a few arcseconds. Not usually a problem but matters for high-precision interferometry.

Selection criteria.

Choose cube BS for:	Choose pellicle BS for:	Choose plate BS for:
General-purpose; robustness	Critical alignment, no ghosts	Low cost; high power
Inteferometers (symmetric paths)	Ultrafast (no GDD added)	Wide wavelength range
Wide range of split ratios	Critical wavelength precision	Compact optical setups

References: Saleh & Teich, Fundamentals of Photonics, Ch. 6 for the wave-optics-based treatment of beam splitters; Bass (ed.), Handbook of Optics (3rd ed., Vol. I, 2010), Ch. 16 for the comprehensive practical engineering treatment.