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Fresnel Rhomb Retarder

Fresnel rhombs are very good achromatic retarders as change of retardance with wavelength arises only from the change in refractive index. Total internal reflections produce a phase shift and single reflection can be arranged to produce a retardance of λ/8. Two reflections therefore produce a λ/4 retardation and a half-wave retardation can achieved with 4 internal reflections.

δ_p = 2 arctan [ n √(n² sin²θ_i - 1) / cosθ_i ]

δ_s = 2 arctan [ √(n² sin²θ_i - 1) / ( n cosθ_i ) ]

Where angle of incidence θ_i > θ_c critical angle.

The over all advance phase shift for total internal reflection is:

ΔΦ = δ_p - δ_s

The total internal reflections of BK7 n=1.5066 @1064nm produce 45° phase difference at two angles of incidence: about 49.0° and 54.25°.

For fused silica n=1.45 is not enough to produce 45° phase shift.

ZnSe n=2.472 total internal reflection produce 90° phase difference at angles of incidence: about 29.2° and 35.9°.

TIR of BK7 phase shift table

BK7 n=1.5066 at 1064nm
θ_i	48	49	50	51	52	53	54	54.25	55	55
ΔΦ	44.095	44.956	45.462	45.684	45.675	45.477	45.118	45.005	44.623	44.013

TIR of fused sililca phase shift table

Fused Silica n=1.4585 at 588nm
θ_i	50	51	52	53	54	55	56
ΔΦ	41.03	41.74	42.12	42.25	42.18	41.93	41.53

Features of Fresnel Rhomb Retarder

- Low Temperature Sensitivity
- Broader Wavelength Range

Specifications of Fresnel Rhomb Retarder

Material: N-BK7, ZnSe
Dimension Tolerance: +0.0/-0.1mm
Parallelism <5 arc second
Surface Quality: 10-5
Clear Aperture: >90% (80% with mount)
Wavefront Distortion: λ/10@633nm
Retardance Variation: 2% 600-1550nm 5%@400nm
AR Coating: Available upon request
Mount: Optional
Wavelegth Range: 400-1550nm, 8-12μm