Fiber lasers are used extensively in the field of manufacturing, health care and communications. Frequency doubling is a widely used method for producing laser radiation with twice the frequency (or half the wavelength) of the original laser source. This technique is employed to produce visible light from lasers that emit in the Infrared region of the electromagnetic spectrum.
It is well known that Silica glass is the preferred material for optical waveguide applications and enjoys widespread use in the existing fiber network . Silica is also widely used as a substrate for a variety of fiber lasers. While silica does have a number of attractive qualities it does not possess a significant second-order non-linearity (SON), meaning that is cannot be used as a frequency doubler . One material that possesses a large SON and is used in a variety of applications (including doubling) is lithium niobate, however, it (and other non-linear materials) is not particularly compatible with the existing silica based infrastructure as it does not come in the form of a fiber and would introduce loss and complicate the laser design. Ideally, a non-linear device would have all the desirable characteristics of silica as well as the SON of lithium niobate. Some breakthroughs in the late 80’s and early 90’s, such as thermal poling, allowed for the introduction of an artificial non-linearity in silica, however, only small SON’s compared to lithium niobate have been achievable through these means [3,4].
Thermal poling works by placing a large electric field across a piece of glass which is then heated up to an elevated temperature. Ions within the material are mobile at high temperatures and begin to migrate in the strong field. After the heat is turned off these ions are frozen in place.
All silica fiber frequency doubler
Fig. 1 Multi-layer silica structures
The charge separation in the material results in a strong permanent electric field which produces an artificial SON in the glass. As I mentioned before this is usually a weak effect that is only present very near anode. So all is lost, right? Maybe not!
Based on observations that charges tended to accumulate between regions of different dopants we performed a study (with Ksenia Yadav and Dr. Jacques Albert) on poling in multi-layer structures similar to the ones in Fig. 1.