Fiber Ring Laser Is Wavelength-Switchable Around 1.5 µm
Multiwavelength lasers are useful for interrogating fiber optic sensors and for testing fiber telecommunications systems, and some day could even serve as telecom transmitters. It sometimes is useful for the laser to produce many wavelengths simultaneously, but a laser capable of switching among its wavelengths, producing only one at a time, can readily find many applications.
Erbium-doped fiber often serves as the gain medium for multiwavelength lasers because it provides the broad, relatively flat gain required. The wavelength-selecting mechanism in these lasers is usually somewhat complex, but investigators in Asia recently demonstrated a simple wavelength- switching scheme based on intracavity polarization.
The collaboration included scientists with Hong Kong Polytechnic University and with Asahi Glass Co. Ltd. in Yokohama, Japan. The laser, which lased at any one of about 35 wavelengths between 1545 and 1573 nm, was configured as a simple fiber ring (Figure 1). An optical circulator, as well as an isolator, ensured unidirectional, clockwise oscillation around the ring. The intracavity etalon had a free spectral range of 100 GHz, or 0.8 nm at 1.5 µm.
Figure 1. The laser was tuned in 100-GHz steps, from one resonance of the etalon filter to the next, by adjusting wavelength transmitted through the polarizer with the dispersive polarization controller. Images ©2005 IEEE.
The gain medium was lanthanum-codoped bismuth-oxide glass, in which erbium is much more soluble than in normal silicate glass. The researchers used an 84.6-cm-long fiber made of this material, with an erbium concentration of 6470 parts per million by weight. Such a high concentration enabled them to use a shorter length of fiber to produce the necessary gain and, as a result, they shortened the resonator, making it less susceptible to environmental perturbations than a longer one would be. They coupled 67 mW of pump power at 1480 nm into the erbium-doped fiber through the circulator.
Figure 2. Although the laser lased at only one wavelength at a time, these superimposed spectra show all 35 wavelengths that it produced. The inset shows the spectrum of a single wavelength.
The scientists tuned the laser from one resonance of the etalon to the next (i.e., in 100-GHz steps) by adjusting the electrically actuated, programmable polarization controller. Because the controller was dispersive, only a single wavelength was optimally aligned for transmission through the intracavity polarizer, and all other wavelengths experienced a greater polarization loss. Because erbium-doped fiber is homogeneously broadened -- that is, all wavelengths compete for the same gain -- the favored wavelength usurped all the gain and prevented the others from reaching threshold.
Figure 3. A two-hour test of the laser demonstrated good wavelength and power stability. The researchers attributed the wavelength variation during the first 10 minutes to the warm-up time of the optical spectrum analyzer.
The output power varied by less than 3 dB as the researchers tuned the laser across the 35 wavelengths (Figure 2). Adjacent modes were suppressed by better than 45 dB. The researchers applied a 500-Hz square wave to the polarization controller to switch it between wavelengths, and they observed a switching speed of approximately 10 kHz. In a demonstration of long-term stability, they measured a wavelength stability of better than 1 pm and a power stability of better than 0.15 dB during a two-hour test (Figure 3).
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