Fiber Laser Pumps Fiber Lasers for Telecommunications Applications

Breck Hitz

A coalition of British and Russian telecom engineers has developed a fiber laser system that may boost performance and cut costs when substituted for the traditional diode laser transmitters in wavelength division multiplexing (WDM) networks. The group, which comprises researchers from the University of Southampton and Southampton Photonics, both in the UK, and from Milon Laser of St. Petersburg, Russia, believes that this is the first fiber-laser-pumped fiber laser developed as a telecom transmitter.

The output of the 977-nm Yb-fiber laser is delivered to eight individual Er-Yb distributed feedback fiber lasers, which generate a 50-GHz comb of wavelength division multiplexing frequencies.

The transmitter in a WDM system is one of the most critical components, both in terms of system performance and system cost. Semiconductor lasers employing distributed feedback usually fill this role, but distributed feedback fiber lasers are strong competitors for the job. The latter are inherently high-power devices with low insertion loss, capable of producing single-frequency, polarized output with high signal-to-noise ratios. A drawback of fiber laser transmitters, however, has been that they require an expensive, grating-stabilized semiconductor laser to pump them. But now the coalition reports pumping the fiber laser transmitters with an inexpensive fiber laser, removing a major barrier to commercial deployment.

In the configuration of the new system, the 977-nm output of an ytterbium-doped fiber laser pumps eight erbium-ytterbium co-doped fiber laser WDM transmitters (see figure, facing page). Each distributed feedback fiber laser transmitter is separated from its neighbor by 50 GHz, so the system produces a standard WDM 50-GHz comb.

The 977-nm pump laser itself is cladding pumped with an inexpensive, broad-stripe laser diode at 915 nm. Approximately 1 W of 977-nm radiation emerges from the laser, but losses in the 3-dB couplers and fibers of the splitting tree reduce the power delivered to each distributed feedback fiber laser to 90 mW. Under these conditions, each of the eight lasers produces about 21 mW.

Reduced costs

In a series of tests, the engineers compared the performance of the distributed feedback fiber lasers pumped with the fiber laser with that under laser diode pumping. They found that the slope efficiencies of the distributed feedback lasers were the same whether the pump source was the fiber laser or a diode laser and that the relative intensity noise was independent of the pump source.

The bandwidths of the eight distributed feedback lasers were considerably greater when they were pumped with the fiber laser, however: 260 kHz vs. 16.7 kHz with diode laser pumping. The broadening, the engineers believe, results from power variations of the fiber pump laser, but they believe that it is not detrimental to the system's performance as a WDM transmitter.

Using only one inexpensive fiber laser instead of eight expensive diode lasers to pump eight distributed feedback fiber laser transmitters can reduce the total cost of the transmitter package. This development makes distributed feedback fiber lasers more competitive with the traditional semiconductor lasers for use as WDM transmitters.