Fiber lasers with phosphate photonic crystal and multicore fibers

Heavily doped optical fibers of very short length are attractive solutions for laser sources in compact integrated devices and high power single frequency lasers. ¹ Utilizing heavily Er-Yb-codoped phosphate step-index fibers 7-cm in length, a record-high output power density of 1.33 W/cm of slightly multimode (M ²<4.0) at 1.5 μm was achieved while the single-mode output power density was only ∼0.5 W/cm because of its reduced core size [1, 2]. To obtain large-mode-area single-mode output, optical fibers with photonic crystal (PC) cladding have been investigated for the interesting endless single-mode behavior. Novel phosphate PC fibers with doped cores have been manufactured and large fundamental mode (>400 μm ²) operations have been achieved with properly designed PC fibers [3]. Therefore, phosphate PC fibers provide the potential to build high power fiber laser of very short active length. We demonstrated around 5 W from a PC fiber laser with only 3.5 cm of active fiber length (see Fig. 1) [4]. In this device the single-mode output power density has been significantly improved to 1.34 W/cm with a slope efficiency of ∼20% with respect to the launched pump. The output spectrum is centered at 1535 nm with a linewidth of ∼2 nm. Furthermore, utilizing a phosphate PC fiber of 3.8 cm length in combination with a narrow linewidth fiber Bragg grating, single frequency output at 1534 nm has been obtained and confirmed with heterodyne measurements. A maximum single frequency output power of 2.3 W is demonstrated and the slope efficiency is ∼12% up to pump power of 20 W. This is the first single-frequency fiber laser that is fabricated with PC fiber, instead of the standard step-index fiber, in the active laser cavity. We also demonstrated the application of multicore fibers to build compact fiber lasers with even higher output powers. Fig. 2 shows that more than 15 W of 1.5 μm signal power can be generated from a 10 cm long fiber that contains 19 active cores. Through fiber design the emission from these cores is coherently combined through evanescent field coupling, as will be described in the presentation. In summary, active phosphate PC fibers with large fundamental mode areas have been demonstrated for miniature high-power high-brightness and single frequency fiber lasers at 1.5 μm. This work is supported by the Air Force Office of Scientific Research through a MRI program No. F49620-02-1-0380, National Sciences Foundation grant No. 0335101, and the state of Arizona TRIF Photonics Initiative.