C-Band Er/Yb Fiber Laser Generates 43 W
Tunable lasers in the eye-safe, 1.5- to 2.0-µm region are useful for remote optical sensing, range-finding and free-space communication. Erbium-doped fiber lasers have the necessary gain bandwidth to cover shorter wavelengths of this spectral region, but they have several drawbacks. The erbium ion doesn't absorb well at the wavelengths of readily available diode lasers. Moreover, it is difficult to perform routine tasks such as splicing and writing gratings in double-clad fibers that have nonstandard outer diameters, so, for tuning, most cladding-pumped, tunable lasers rely on external, lens-coupled external gratings -- an awkward add-on that diminishes the lasers' robustness.
Figure 1. The resonator of the tunable fiber laser included both double-clad, Er/Yb doped fiber with a 30-µm core, and single-mode fiber into which a fiber Bragg grating had been written. The loss of the splice between the fibers was estimated to be less than 0.2 dB. ©2004 IEEE.
Recently, researchers at Southampton University in the UK have gotten around both issues and demonstrated a fiber laser that can generate more than 40 W, tunable from 1532 to 1567 nm. To excite the erbium ions with the 975-nm light from diode lasers, they adopted the relatively well-known technique of codoping the fiber with ytterbium, which absorbs the pump energy and transfers it to the erbium.
Fiber Bragg grating
To tune the laser across its spectral bandwidth, they successfully spliced a standard single-mode fiber with a fiber Bragg grating (FBG) onto the codoped, double-clad large-core fiber with an outer diameter of 400 µm. The reflectivity of the FBG and, hence, the wavelength of the laser, was tuned by longitudinally compressing the grating.
Figure 2. The laser produced up to 43 W of near-diffraction-limited (M2 <1.7) output, with no roll-off from nonlinear scattering or deleterious thermal effects at the upper end. ©2004 IEEE.
The laser's resonator was formed between the Fresnel reflection of the facet at one end of the fiber and the single-mode FBG at the other (Figure 1). The end facet of the fiber behind the FBG was cut at an oblique angle to eliminate residual feedback. The 30-µm core of the 3.5-m-long, double-clad fiber normally would support multiple transverse modes, but because it was spliced to the single-mode fiber and because both fibers were within the resonator, the laser oscillated in a single mode. (The laser's M2 was <1.7.) The scientists milled the inner cladding of their double-clad fiber into a D-shape before drawing, to suppress circularly symmetric pump-light modes that would not couple to the core.
Multiple laser-diode stacks provided the pump power for the fiber laser. Their outputs were focused with collimating and focusing lenses onto the face of the double-clad fiber, and approximately 90 percent of the diode lasers' output was coupled into the fiber. The pumped end of the double-clad fiber was held in a temperature-controlled metal V-groove to prevent thermal damage to the fiber coating resulting from any nonguided pump power or by the heat generated in the laser cycle.
At the output, a dichroic mirror separated the ~1550-nm light from the incoming 975 pump. A second dichroic mirror dumped parasitic ~1.1-µm light radiated by excited ytterbium ions. The output power reached a maximum of 43 W with 138 W of pump power launched into the fiber, and this level was constant across the approximately 35-nm tuning range.
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