Grating Enables Tunable Fiber Laser for Telecom

Breck Hitz

Today's fiber optic telecom systems use a different fixed-frequency semiconductor laser to transmit each channel into the optical fiber. If these individual lasers were tunable -- that is, capable of switching from one wavelength channel to another -- it would add great flexibility to the network. Even better, if a single laser were capable of generating all the channels simultaneously, it would not only add flexibility, but also simplify the network design.

For these reasons, researchers worldwide are investigating techniques to tune lasers and to build devices capable of simultaneous operation at multiple wavelengths. Now scientists from the National Taiwan University of Science and Technology in Taipei and from Tsing-Hua University in Beijing have achieved both goals using an arrayed waveguide grating and a fiber ring laser.

Figure 1. An arrayed waveguide grating is an interferometer fabricated in a planar waveguide.

An arrayed waveguide grating is an interferometer whose operating principle is much like that of an ordinary diffraction grating (Figure 1). The waves that enter travel through different channels just as they reflect from different lines of a diffraction grating, and as they emerge, they interfere with each other. As is the case for a diffraction grating, different wavelengths interfere constructively in different directions. Arrayed waveguide gratings have found widespread application in multiplexing and demultiplexing wavelength-division-multiplexed signals, and several research groups have investigated them as intracavity elements of tunable lasers.

Figure 2. The ring fiber laser could be tuned from one channel to another across 32 nm in the C-band.

The engineers placed an arrayed waveguide grating inside the resonator of a fiber ring laser (Figure 2). An erbium-doped fiber amplifier, which included an optical isolator, served as the laser's gain medium. A 1XN optical switch selected which of the arrayed waveguide grating's 40 channels oscillated in the laser. The channels were separated by 100 GHz (0.8 nm), and the laser could be tuned across 32 nm in the C-band by changing the position of the switch. The average power in each of the channels was 2 dBm.

Figure 3. A resonator configuration featuring variable optical attenuators in each channel and balancing the four oscillating channels against each other oscillated at four frequencies simultaneously.

With a slight modification, the laser produced four simultaneous wavelengths separated by 200 GHz (Figure 3). But because thermal gain broadening is the dominant broadening mechanism in an erbium-doped fiber amplifier, each erbium ion can contribute to the gain of any channel, and the channels can compete with one another for the population inversion. As in any competitive situation, the dominant channel will saturate the gain and minimize the power available to other channels. The scientists were able to overcome this problem by placing variable optical attenuators in each channel and balancing the four oscillating channels against each other so that the power variation from one channel to another was only 0.5 dB, but that the power in each channel was reduced to -10 dBm.

The glass fiber in an erbium-doped fiber amplifier is an amorphous -- not crystalline -- material, so erbium ions at different locations experience different electric fields from the nuclei surrounding them. At room temperature, thermal broadening overwhelms this effect, but at liquid-nitrogen temperature, the thermal bandwidth of each erbium ion becomes small enough that the ion can support only one of the channels permitted to oscillate by the arrayed waveguide grating.

The electric-field effect thus causes different ions to support different channels, eliminating competition among the channels. The scientists speculate that cooling the amplifier to liquid-nitrogen temperature could result in more stable and much more powerful multiwavelength oscillation of the laser.