Dichroic beam splitters

Dichroic beam splitter is currently getting important for the state-of-the-art
spectrograph such as KCRM / Keck, GMACS / GMT, WFOS / TMT, or for
multi-channel CCD cameras such as HiPERCAM / GTC to take images in
different colours simultaneously.

Dichroic beam splitter splits the incident beam to two directions, eventually
gives an influence to the performance of at least two or more cameras
in terms of wavefront error, throughput, or available wavelength range.

Essential requirements for astronomical dichroic beam splitter are as follows.

a) High reflection in (often broad) band
b) High transmission in (often broad) band
c) Sharp transition from reflection to transmission or the opposite
d) Minimized flatness error after coating
e) Minimized transmitted wavefront error after coating

Both a) and b) contribute the high overall throughput of the instrument.

c) contributes to minimize unavailable wavelength range.
The key technique is to reduce the separation of p, s-polarization at
the specified AOI.

Both d) and e) contribute the image quality of the instrument.

There are few vendors in the world that can achieve all the requirements.
Asahi is one of potential vendors to provide such dichroic beam splitters.

We provided the KCRM 670mm x 265mm, the world's largest dichroic
beam splitter in 2020.
The dichroic met the challenging specifications requested by Caltech.
figure KCRM 670mm x 265mm dichroic figure KCRM flatness and wavefront

CaF2 based IR dichroic beam splitter

figure CaF2 based dichroic beam splitter for PWFS/FIU at Keck
Silicon, sapphire, or CaF2 is usually used as a substrate for IR optics.
However, regarding silicon, the wavelengths from optical region to Y band are not available due to the absorption.
Sapphire is a good material in terms of transmission from optical to IR region, but too solid to polish the surface precisely.
This is disadvantageous to make good surface flatness, which must be essential for dichroic beam splitters.

CaF2 is also good in terms of transmission, but difficult to be handled.
Because it is fragile and very sensitive against thermal shock.
Asahi succeeded in making CaF2 based and wedged 100mm dichroic and provided it to the instrument at Keck in Hawaii.