## External Cavity Laser

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.
 Recognitions: Science Advisor 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...de_lasers.html The external diffractive element provides the mechanism for narrowing the linewidth.
 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.