# How does spot size of a laser depend on wavelength?

Does the spot size of a laser depend on wavelength? We have a laser source that is fed into a spectrometer to set the wavelength we want. No other optics are used after the spectrometer. I was going to do the "knife-edge" experiment at a bunch of wavelengths (400-1100nm) to see if the spot size changes, but I figured I would ask here first.

• I'm not an expert in this field but if i recall correctly a laser beam is (in ideal case) a Gaussian beam and according to en.wikipedia.org/wiki/Laser_beam_quality the waist, therefore the spot, has a wavelength dependence. – Bonsay Jan 24 at 14:48

## 1 Answer

Yes it does, and that is mostly because a coherently illuminated slit/hole (diffraction limited case) has a main lobe whose angular extent is $$\approx k\frac {\lambda}{D}$$ where $$k\approx 1$$ and $$D$$ is the linear dimension of the slit. In the case of a laser source the output diaphragm determines the minimum extent of the beam.

• For laser diodes which have a tiny aperture the diffraction is huge but a collimating lens captures these beams asap (short focal length) and collimates the beam, the spectrometer as well as the knife edge will also diffract thereby increase the spot size. – PhysicsDave Jan 24 at 15:36
• @PhysicsDave sure but that just says it depends on where you define the effective coherent output. With your amplifier drive a dipole antenna having a near omni antenna pattern, a receiver thousands of meters away sees the effective "antenna" that is the parabolic dish sitting behind the dipole and then the dish's diameter controls the beamwidth. – hyportnex Jan 24 at 15:50