Wavelength converter exploits DFB laser for cost reduction

John Wallace

The ability to transfer data between the 1310-nm and 1550-nm spectral windows via all-optical conversion makes networks more flexible. Wavelength conversion between the two windows can be achieved using semiconductor optical amplifiers (SOAs). If such devices are to be used in local-access systems, they must be inexpensive to make, while maintaining wavelength conversion over a large range of input powers.

Researchers at the University of Rochester (Rochester, NY) and the University of Tokyo (Tokyo, Japan) have developed a single-contact SOA useful for wavelength conversion between 1310 and 1550 nm. Aiming for the ultimate in cost reduction, the researchers have used a commercial distributed-feedback (DFB) laser driven below its lasing threshold as the SOA. Absorption of the modulated shorter-wavelength signal in the laser generates charge carriers that decrease the refractive index of the active material, actively controlling the transmission of a 1550-nm continuous-wave probe beam.

The data and probe beams were combined with a polarization-maintaining 3-dB coupler designed for use at 1550 nm (see figure). Approximately 80% of the 1310-nm light was passed by the coupler to the DFB SOA. Light was coupled into and out of the device using tapered fibers; the 1550-nm output signal was boosted by an erbium-doped fiber amplifier.

The polarity of the output signal (whether its highs match the highs or the lows of the input signal) depends on the precise wavelength of the probe beam-which, in the setup, was tunable. For example, when the probe beam is set at 1547.707 nm, the polarity of the out- put matches that of the input; for a wavelength of 1547.749 nm, the polarity is reversed. In fact, polarity reversal occurs when the probe wavelength is at or near the long-wavelength side of the device`s Bragg resonance, so that decreased probe power leads to lower refractive index and thus a Bragg resonance shifted further away from the probe wavelength.

The researchers found that, while polarity-preserved signals showed an almost linear transfer function, the polarity-reversed signals exhibited a highly nonlinear transfer function having an on-off effect-in other words, acting like a digital switch. At 155.5 Mbit/s, the device`s on-off ratio of four is constant over a 2-mW range of input signal peak powers. The digital nature of the transfer function has a natural advantage for wavelength conversion of digital data. The device is expected to be capable of converting all WDM wavelengths in the 1310-nm window, although the output wavelength will be a single 1550-nm-region channel, say the researchers. Additional 1550-nm-region WDM channels will be produced by designing a separate wavelength converter for each output channel. For more information, contact Drew Maywar at maywar@optics.rochester.edu.

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