- Blue Fiber Laser Generates More Than 200 mW
Thulium-doped ZBLAN laser balances photobleaching and photodarkening.
Small blue lasers are difficult to come by but have many interesting applications, including laser displays, fluorescence microscopy, flow cytometry and spectroscopy. Recently, scientists at the Center for Optics, Photonics and Lasers (COPL) in Quebec City have demonstrated a blue fiber laser that may, after further refinements, be suitable for such applications.
ZBLAN glass is a mixture of zirconium, barium, lanthanum, aluminum and sodium fluorides that is frequently used in mid-infrared fiber lasers because its transmission extends to longer wavelengths than that of conventional silica glass. Rare-earth-doped fluoride glasses such as ZBLAN also are useful for generating visible light through a process called up-conversion pumping, in which multiple pump photons are absorbed sequentially by the laser medium. In this case, the scientists used a thulium-doped ZBLAN fiber pumped at 1108 nm to generate light in the blue spectral region at 482 nm.
The Tm3+:ZBLAN blue fiber is subject to photodarkening, for reasons not perfectly understood. In the currently proposed explanation, multiple pump photons can excite even higher-lying energy levels, which spontaneously emit ultraviolet photons. These photons are energetic enough to ionize the glass, creating holes and electrons that subsequently lead to absorption centers and photodarkening of the glass.
However, the blue laser light is absorbed at the absorption centers and tends to bleach them. Thus, a delicate balance exists between photodarkening (caused by pump light) and photobleaching (resulting from laser light). Until now, however, the equilibrium of that balance has always limited the power of such lasers to a few tens of milliwatts — or, at worst, has made the laser inoperable.
The COPL scientists configured an experiment to study the photodarkening while the laser was operating (Figure 1). To do so, they launched an argon-ion laser in the ZBLAN fiber so that its 514-nm photons could photobleach the absorption centers while the laser was operating.
Figure 1. An Yb:fiber laser provided 1108-nm infrared photons that populated high-lying energy levels in the Tm3+:ZBLAN fiber. A high-reflecting (HR) mirror and an output coupler (OC) defined the ZBLAN laser’s Fabry-Perot resonator. The two polarizing prisms and rotatable waveplate at the top of the figure provided a variable attenuator for the pump power. Detectors D1, D2 and D3 measured incoming pump power, pump power transmitted through the ZBLAN fiber and blue laser power, respectively. The argon laser on the right side of the figure was added in the second part of the experiment to launch photobleaching 514-nm photons into the ZBLAN fiber. Reprinted with permission of IEEE Photonics Technology Letters.
In their first experiment, where the argon laser was not used, they obtained what they believe is the highest stable blue (482 nm) output yet reported from such a laser: 204 mW. The fact that they used a shorter pump wavelength than previous investigators (1108 nm vs. 1120 to 1140 nm) could perhaps explain their results.
The investigators observed an unambiguous hysteresis in the ZBLAN laser’s transfer function (Figure 2). The output power increased linearly with the pump at low power, but saturated noticeably at higher power. They paused only 30 s before recording each of the blue circles in Figure 2. When the laser power leveled off (at ~230 mW), they paused for 8 min until the power gradually decreased to 204 mW and stabilized. When they reduced the pump power, the laser power decreased almost linearly.
Figure 2. The decrease in slope efficiency at higher increasing pump power was caused by photodarkening of the ZBLAN fiber. The linear decrease of laser power with pump power suggests a constant photoinduced fiber loss of approximately 2.7 dB/m.
To demonstrate the photobleaching in real time and the potential path to even higher blue laser power, the scientists launched several hundred milliwatts worth of 514-nm photons from an argon-ion laser into the operating ZBLAN laser. They started with a completely bleached ZBLAN fiber (i.e., no absorption centers) and blocked the argon laser beam at time t = 0, watching the laser power diminish afterward as the fiber photodarkened (Figure 3). Then they unblocked the argon laser and saw the ZBLAN laser power return close to its initial value.
Figure 3. After an interruption of 20 min, turning the argon laser back on restored the ZBLAN fiber laser output power to 98 percent of its initial value. All powers shown here are normalized to their values at t = 0.
The not-quite-complete recovery (∼98 percent) repeated itself in numerous experiments, but 100 percent transparency was subsequently obtained by launching the 514-nm power into the fiber when the pump laser was turned off.
IEEE Photonics Technology Letters, Jan. 15, 2007, pp. 112-114.
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