# Echelle grating

An echelle spectrometer has one low groove density grating (echelle) and one high groove density grating. https://en.wikipedia.org/wiki/Echelle_grating

After the echelle grating there will be many orders and not well dispersed spectrum. After the second grating the spectrum will be dispersed above each order.

The spectrogram bellow does not follow this description. On the spectrogram, In vertical directions are the different orders from the low groove density echelle grating. In horizontal is the dispersion from the high groove density grating.

What I would like to ask is how are the wavelengths arranged. It looks as on the 52 order there are many dispersed wavelengths of let say blue color(400nm, 398nm, 397nm..380nm). And the 53 order have completely different spectral content (379nm, 378nm,..360nm) but no 400nm or 380nm.

• – Farcher Aug 23 at 15:08
• Nice explanation but I am missing how one diffraction order have spectrum from 1.37um to 1.39um and another order has spectrum from 1.71um to 1.75um. – ole Aug 25 at 9:12
• I have added a diagram to my answer to your question. – Farcher Aug 25 at 11:24
• What do you mean by "spectral content" here? – Emilio Pisanty Aug 25 at 12:46
• if one order disperses spectrum 1um-2um, I would expect the second order to have the same 1um-2um spectrum. The orders may overlap they would disperse more or less the same wavelengths. I am missing why orders n=67 and n=20 order are with different spectrum one is IR the other UV. – ole Aug 29 at 13:19

One echelle grating will mix up all the wavelengths because at a position where there is a maximum for the $$25^{\rm th}$$ order “red” of wavelength $$800 \,\rm nm$$ there may be a maximum for the $$50^{\rm th}$$ order “blue” of wavelength $$400 \,\rm nm$$ as $$25\times 800 = 50 \times 400 =n \lambda$$.
There may also be other other wavelength $$\lambda$$ of order $$n$$ at, or very close, to that position.