External Cavity Laser: Ultra Narrow Line Width

[narra]

Hello,

How does an external cavity laser produce ultra narrow line width when it's larger cavity should reduce the free spectral range and thus introduce more longitudinal modes?

Thanks.

 

[Andy Resnick]

The external cavity lasers I am familiar with are diode lasers that have an external diffraction grating and vertical-cavity surface emitting lasers that use an external distributed Bragg reflector:

http://www.rp-photonics.com/external_cavity_diode_lasers.html

The external diffractive element provides the mechanism for narrowing the linewidth.

 

[narra]

Hi,

Thanks for your reply. My understanding was that the diffraction grating just provided a means of tuning the central wavelength. I thought that perhaps the fact that the cavity was external was the key to offering ultra narrow linewidth ... although seems counter intuitive since a shorter cavity would promote fewer longitudinal modes.