Double-Clad Photonic Crystal Fiber Laser Is Side-Pumped
Probably the most common technique for pumping fiber lasers is to pump them through the ends of the fiber, focusing the light into the inner cladding. A drawback of this is that it does not tolerate high-power pumping with multiple sources. To overcome this, conventional lasers sometimes are side-pumped by coupling pump light into the inner cladding; for example, with small mirrors etched into the fiber along its length. But side-pumping of double-clad photonic crystal fibers (PCFs) had been unexplored.
Recently two scientists, at NKT Research and at its subsidiary, Crystal Fibre A/S, both in BirkerØd, Denmark, demonstrated a side-pumped PCF laser whose coupling efficiency exceeds 90 percent.
A multimode, angle-polished fiber coupled pump light into the inner core of the fiber laser through a 500-µm-long region in which the airholes of the outer cladding had been collapsed by heating. The coupling angle (γ) was 20°.
PCFs have been subjected to intense investigation in laboratories around the world because they can be designed to have optical characteristics that cannot be obtained in conventional fibers. The same PCF can be single-mode for virtually any wavelength, for example, or its zero-dispersion wavelength can be shifted into the visible region.
PCFs are fabricated by painstakingly stacking dozens of hollow glass tubes around a central glass rod to create a preform, and the fiber is subsequently drawn by conventional techniques. The fiber then consists of a solid core surrounded by a cladding perforated by longitudinal airholes from the hollow tubes. Because of these airholes, the average effective refractive index of the cladding is less than that of the core, so light is guided in the core by total internal reflection.
A number of laboratories have demonstrated double-clad fibers comprising a core, an inner cladding of lower-index glass and an outer cladding perforated by airholes that further lower the effective refractive index. Laser designers prefer double-clad fiber for fiber lasers because the pump light can be efficiently coupled into the high-numerical-aperture inner cladding, yet it excites only a single transverse mode in the single-mode core. A strong enticement for using double-clad PCF for fiber lasers is that the index of the outer cladding can be lower than that of a conventional double-clad fiber, so the numerical aperture of the inner cladding will be even larger.
The scientists in Denmark achieved side-pumping by heating the fiber with a fusion splicer so that the airholes collapsed. The pump radiation was coupled from a multimode, angle-polished fiber into the inner cladding (see figure). An index-matching gel or a UV-curable optical adhesive filled the space between the coupling fiber and the fiber laser.
The researchers' measurements showed that 90 percent of the power in the pump fiber was successfully coupled into the fiber laser's inner cladding, and that nearly all (99.5 percent) of the light that was coupled into the cladding propagated in the forward direction. The coupling efficiency was not critically dependent on positioning or alignment of the coupling fiber relative to the laser.
A 2.5-m length of double-clad PCF with an Yb-doped core formed a resonator for the researchers' laser experiments. The Fresnel reflections (4 percent) from the ends of the fiber served as laser mirrors. The 976-nm pump light was coupled into the fiber very close to one end to maximize the length it would propagate along the fiber.
Adding the output power from both ends of the laser, the scientists measured up to 43 mW of single-mode power, with a slope efficiency of 81 percent. They believe that this demonstration will pave the way for improved, high-power fiber lasers.
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