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Mode-Locked Laser Has Potential as OC-768 Transmitter

Photonics Spectra
Oct 2004
Breck Hitz

A research group at Massachusetts Institute of Technology's Lincoln Laboratory in Lexington, Mass., has designed, built and operated a 40-GHz mode-locked hybrid fiber/semiconductor laser that has potential as the transmitter in next-generation (OC-768) fiber optic telecom systems. The scientists demonstrated its capabilities in a 40-Gb/s transmission experiment over a 45-km link.

Most of today's fiber optic telecom systems rely on directly or externally modulated semiconductor lasers as the source of the optical pulses that carry information over the fiber, but it is unclear how well those techniques will work in next-generation systems with 40-Gb/s data rates that are four times faster than today's fastest rates. An alternative that is being studied in many laboratories is the mode-locked laser, operated at a harmonic of its fundamental frequency to generate a 40-GHz pulse train. In a typical mode-locked laser, a single pulse circulates in the laser resonator, creating an output pulse each time it hits the output coupler. In a harmonically mode-locked laser, multiple pulses circulate simultaneously, creating an output pulse train at a multiple of the fundamental frequency.

Both fiber lasers and semiconductor lasers have been considered as mode-locked sources for telecom systems, but each type has its disadvantages. A semiconductor laser's resonator is difficult to adjust and stabilize without adding an external mirror, an undesirable complexity that limits the tunability of the mode-locking repetition rate and the emission wavelength. Moreover, there is inherent coupling loss between a semiconductor laser and optical fiber. Coupling loss is much less problematic between a fiber laser and a fiber, but fiber lasers are relatively complex devices, requiring another laser as an optical pump and a separate, bulk device as the mode-locking modulator.

Research groups at other laboratories recently investigated means of combining the best characteristics of both types of lasers. These hybrid lasers use a device that is like a semiconductor laser, called a semiconductor optical amplifier (SOA), as the source of gain and a fiber as the laser resonator. This design eliminates the pump laser required by conventional fiber lasers, but it retains the desirable fiber-based resonator.

Mode-Locked Laser Has Potential as OC-768 Transmitter
Figure 1. The gain medium of the hybrid laser source is a semiconductor optical amplifier (SOA), but the resonator is optical fiber. Combining the electroabsorption modulator (EAM) and the SOA into one monolithic device significantly reduces the laser's complexity.

The integration of the SOA and an electroabsorption mode-locking modulator in the Lincoln Lab team's laser into a single, monolithic device significantly simplifies the design (Figure 1). An SOA essentially is a mirrorless semiconductor laser, a semiconductor diode that produces photons when electron-hole pairs combine. An electroabsorption modulator is also a semiconductor diode, but it is reverse-biased to absorb photons and create electron-hole pairs. Because the two devices have similar semiconductor structures, they can be combined in a monolithic instrument. (One of the electroabsorption modulators depicted in Figure 1 provided the loss modulation to mode-lock the laser. The second served as a data encoder and was not used in the experiment described here.)

The laser's fundamental mode-locking frequency was 17 MHz, so the researchers operated it at a very high harmonic to produce a 40-GHz pulse train. They generated pulses shorter than 4 ps that could be tuned with an intracavity bandpass filter between 1540 and 1567 nm. They observed the shortest pulses -- 2.2 ps in duration -- at ~1560 nm. Mode-locking was very stable, requiring only one or two manual adjustments of the resonator length per day to maintain the spectral width, output power and central wavelength to within 7.5 percent of the nominal values over a 100-hour period.

Mode-Locked Laser Has Potential as OC-768 Transmitter
Figure 2. The openness of the eye pattern obtained when the laser transmitted random data over a 45-km link indicates the source's potential for application as a transmitter in a fiber optic telecom system.

To demonstrate the laser's potential in a telecom system, the researchers amplified the output and applied 40-Gb/s random data onto the pulse train with a separate lithium-niobate modulator. This modulated signal was injected into a 45-km fiber link, and amplified and detected at the other end. The open eye pattern (Figure 2) confirms that the pulses are suitable for fiber optic telecom transmission.

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