Four-Wave Mixing Trumps Homogeneous Gain in Erbium Laser
Although current wavelength-division multiplexing (WDM) systems require a different laser for each wavelength transmitted over an optical fiber, engineers envision more-efficient systems in which a single laser will emit all of the necessary wavelengths. Laboratories around the world are investigating approaches to building lasers capable of generating multiple wavelengths simultaneously.
The erbium-doped fiber laser is a favored candidate because the underlying technology has matured in telecom applications and could be adopted in WDM transmitters. But the lasers have a fundamental drawback as multiwavelength sources: They are homogeneously broadened, so different wavelengths trying to oscillate in the same laser compete. Inevitably, one emerges as the dominant wavelength and usurps the gain of the others, which are extinguished.
Recently, researchers at the Institute for Infocomm Research in Singapore demonstrated a novel technique to finesse the homogeneous broadening in erbium-doped fiber lasers. They capitalized on an effect that usually is deleterious in WDM — four-wave mixing — to force a laser to lase at up to three wavelengths simultaneously.
Four-wave mixing is a nonlinear effect that occurs in an optical fiber when waves interact to generate additional waves at new frequencies. It’s deleterious in a transmission fiber because it causes loss to each channel and introduces noise into adjacent ones. But the researchers in Singapore harnessed the effect to serve as a sort of photonic Robin Hood, coupling energy from the rich wavelengths into the poor ones so that no one wavelength would become dominant and all could coexist in the same laser.
Figure 1. The ring laser provided feedback at up to three wavelengths, depending on the settings of the variable optical attenuators (VOA1 and VOA2) between the fiber Bragg gratings (FBG2, FBG0 and FBG1). The reflectivities of the gratingsincreased from left to right as shown in the diagram; i.e., FBG2 <FBG0 <FBG1. Images ©2005 IEEE.
The researchers constructed a fiber-ring laser with a 51-m length of highly nonlinear photonic crystal fiber to maximize the four-wave mixing, an erbium-doped fiber amplifier (EDFA) for the gain medium, and three fiber Bragg gratings (FBGs). Variable optical attenuators (VOAs) between the gratings allowed the researchers to adjust the resonator feedback from each FBG. A circulator coupled light into and out of the gratings, and an isolator in the EDFA ensured unidirectional oscillation around the ring (Figure 1).
Starting with the simplest case, if the researchers maximized the loss in both VOAs, then only the grating labeled FBG2 in the figure provided feedback in the resonator, and the setup lased at the central wavelength of the grating: λ2.
Figure 2. By balancing the feedback provided by the fiber Bragg gratings, the researchers forced the homogeneously broadened laser to oscillate at two wavelengths simultaneously. Intracavity four-wave mixing coupled energy between the wavelengths, making balancing possible.
When the researchers opened VOA1 (that is, minimized its loss), both FBG2 and FBG0 provided feedback at their respective wavelengths: λ2 and λ0. But the reflectivity of FBG0 was greater than that of FBG2 (91 vs. 85 percent), so λ0 overwhelmed λ2 and usurped the gain of the homogeneous EDFA. The Robin Hood effect of the nonlinear fiber coupled some energy from λ0 into λ2, but not enough to overcome the greater feedback of λ0.
Likewise, when the scientists opened both attenuators, all three fiber Bragg gratings provided feedback. But the reflected signal from FBG1 was strongest, so the setup lased only at λ1.
By carefully adjusting the VOAs and the polarization controllers, the researchers balanced the feedback so that two wavelengths oscillated simultaneously (Figure 2). Such balancing would have been impossible without the photonic crystal fiber in the resonator; one wavelength or the other would have become dominant. But, acting like Robin Hood, the four-wave mixing coupled energy out of the stronger wavelength and into the weaker, ensuring that no wavelength became strong enough to dominate.
Four-wave mixing also coupled some energy into other wavelengths, but because there was too little resonator feedback, the wavelengths never achieved laser threshold. The smaller peaks in Figure 2 are only amplified spontaneous emission and are at least 30 dB below the laser peaks.
Figure 3. The researchers obtained up to three wavelengths from the erbium-doped fiber laser simultaneously.
Finally, by adjusting the VOAs so that the resonator feedback from all three FBGs was approximately equal, the researchers achieved lasing at three wavelengths simultaneously (Figure 3). The values of λ1, λ2 and λ3 were determined by the reflectivities of the respective FBGs. The investigators demonstrated tuning of the laser wavelengths by tuning the reflectivity of a grating.
- optical fiber
- A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.
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