Fiber Ring Laser Generates 50 Simultaneous Wavelengths
A scientist at Exfo Electro-Optical Engineering Inc. in Vanier, Quebec, Canada, has demonstrated a simple fiber ring laser capable of generating 50 simultaneous wavelengths uniformly spaced at 50 GHz between 1308 and 1322 nm. Such a laser could be useful in telecom test-and-measurement applications such as characterization of chromatic dispersion and polarization-mode dispersion. It also could find use in optical metrology and sensing.
Hongxin Chen based his laser on the flat, wide gain of a state-of-the-art semiconductor optical amplifier. Other multiwavelength lasers have employed an erbium-doped fiber amplifier as the gain medium. Although the fiber amplifiers provide a wide, flat gain, they also have an important drawback compared with semiconductor optical amplifiers: The gain of an erbium-doped fiber amplifier is homogeneous -- that is, stimulated emission at one wavelength reduces the entire population inversion, so gain at other wavelengths is diminished. In a homogeneously broadened laser, multiple wavelengths compete for the gain, and one or several wavelengths tend to dominate. (See "Multiwavelength Laser Utilizes Erbium-Doped Fiber Amplifier," page 99, for a solution to that problem.)
The gain in a semiconductor optical amplifier, on the other hand, is inhomogeneous. At a given wavelength, it is much less dependent on lasing at nearby wavelengths. Different wavelengths compete much less, so an inhomogeneously broadened multiwavelength laser is inherently more stable than a homogeneously broadened one.
Figure 1. The straightforward fiber-ring configuration generates 50 simultaneous wavelengths at the transmission peaks of the Fabry-Perot filter.
The configuration of the Exfo laser is straightforward (Figure 1). Besides the gain medium, the fiber resonator includes a Fabry-Perot filter with transmission peaks spaced at 50 GHz that forces the laser to oscillate only at those frequencies. An isolator forces unidirectional oscillation around the ring, and a polarization controller provides an intracavity mechanism to compensate for the slight polarization dependent gain. The total intracavity loss is about 10 dB (10 percent), most of which is due to the Fabry-Perot filter.
Figure 2. The gain of the semiconductor optical amplifier is flat across a wide spectral region. The four bottom curves show the amplified spontaneous emission at four drive currents, and the top curve shows the small-signal gain at one drive current. ©OSA.
The semiconductor optical amplifier's measured small-signal gain is flat to within 1.2 dB over the spectral range from 1280 to 1310 nm (Figure 2). Together with the gain's inhomogeneity, this results in an output spectrum flat to within 3 dB across the 50 output wavelengths (Figure 3). The signal-to-spontaneous-noise ratio is about 30 dB.
Figure 3. The laser's output spectrum consists of 50 wavelengths with a uniform 50-GHz separation and power variation of less than 3 dB. Total output at all 50 wavelengths is 23 dB (0.5 mW). ©OSA.
A noise peak at ~8 MHz is the result of beating of the longitudinal modes of the 25-m-long fiber resonator. Chen believes that this noise could be diminished by driving the amplifier harder and forcing it into a more saturated mode, thereby extinguishing some of the weaker longitudinal modes.
Other improvements he contemplates include enhancing the spectral flatness of the output by decreasing the semiconductor optical amplifier's gain ripple and by lessening the polarization mode dispersion of the intracavity components. The laser also could be extended to C- and L-band operation with a suitable semiconductor amplifier.
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