Waveguide lasers in phosphate glasses made using UV-written gratings
Phosphate glass is an ideal host for rare-earth ions, which makes it an excellent choice for the design of compact fiber and integrated optic amplifiers and lasers. Indeed, compact Er-doped waveguide amplifiers1 and Er-doped single-frequency fiber lasers2 are already commercially available. Until recently, however, it has been difficult to fabricate volume gratings in these glasses with the standard UV-writing techniques used to produce Bragg gratings in silica fibers. Our team has performed an in-depth study of the photosensitivity of different phosphate glasses while developing UV-writing processes for the fabrication of high-quality Bragg gratings in both phosphate fibers and planar glass waveguides. These gratings significantly simplify the fabrication of single-frequency waveguides and fiber lasers.
The first experiments reported in planar slab waveguides using an Er-Yb-codoped Schott IOG-1 glass demonstrated the feasibility of UV-written gratings in phosphate glass.3 However, absorption from the silver diffused in the glass to form the waveguides limited the reflectivity to values that did not allow the formation of laser cavities. In our studies, we have used both undoped and active (i.e., Er-Yb-codoped) phosphate glasses. It was difficult to obtain high index changes in active IOG-1 glass with the maximum index modulation being only 1.5 x 10−5.4 In undoped IOG-1 glass however, we were able to optimize the process for highly stable planar waveguide gratings by fabricating the gratings prior to waveguide formation.
The reflectance and transmittance spectra of our first good quality waveguide grating in undoped IOG-1 phosphate glass are shown in Figure 1. A reflectance of 44% was achieved with a grating that was only 4mm-long with five minutes of exposure time. The measured reflectivity corresponds to an index modulation of 1 x 10−4.
To fabricate an integrated optic waveguide distributed Bragg reflector (DBR) laser, we used IOG glass, consisting of a hybrid substrate composed of both Er-Yb-codoped and undoped parts bonded to each other (see Figure 2).5 The Bragg grating was written to the undoped part of the substrate with an ArF excimer laser through a phase mask. The grating is the wavelength-selective output coupler of the laser while the Er-Yb-codoped part of the hybrid glass provides the gain required for laser operation. The measured laser output power as a function of pump power is presented in Figure 3. The inset shows the laser output spectrum at a resolution of 0.07nm, indicative of single-frequency operation.
Encouraged by our demonstration of successful planar waveguide gratings, we extended the work to studies of Bragg gratings in phosphate glass fibers. We fabricated the first Bragg gratings into passive fibers drawn from phosphate glass provided by NP Photonics.2 The UV-writing was again performed using an ArF excimer laser and a phase mask. We achieved stable gratings with reflectivities greater than 99%, and the estimated index change exceeded 10−4.6 More recently, we have also demonstrated efficient gratings in Er-Yb-codoped phosphate fibers. Our latest results include a tunable dual-wavelength fiber laser with up to 1W of output power based on cascaded distributed feedback (DFB).7
To summarize, we performed extensive studies of the photosensitivity properties of phosphate glass.8 UV-written planar waveguide gratings were demonstrated in undoped phosphate glass and utilized to fabricate a single-mode waveguide laser in a hybrid phosphate glass. We have also fabricated efficient gratings and UV-imprinted DFB lasers in phosphate glass fibers. Our high-power, dual-wavelength fiber laser sources are excellent candidates for applications in optical communications, sensing, ranging, and nonlinear optical wavelength conversion such as terahertz generation. Our future research will focus on fabricating compact multiple-wavelength DBR laser arrays on a single chip.
