Tunable soliton pulses emerge from microstructured fiber

Sunny Bains

At Lucent Technologies Bell Laboratories (Holmdel, NJ), researchers have found a way to exploit fiber nonlinearities that aren't normally attainable in a practical device to change the wavelength of soliton pulses. They achieved this by designing a microstructured tapering fiber that allows low-loss (adiabatic) coupling into the narrow fiber waist. The result is an unusually high intensity, which dramatically increases nonlinearity. Not only does the device allow for soliton self-frequency shifting in 15 cm, but it is easily coupled with ordinary single-mode fiber and can be coated (packaged) without causing its performance to suffer.

The basic structure of the air-silica microstructured fiber (ASMF) is a single-mode core surrounded by six air holes (see Fig. 1). The low refractive index of these channels serves to tightly confine light in the fundamental mode, thus making the outer cladding almost irrelevant to the waveguiding process. This fact makes such fibers robust to their external environment, including coatings. The structure initially has a diameter of 132 µm and is designed to couple easily with standard single-mode fiber. To produce the taper, the fibers are heated in a flame and stretched, with the temperature carefully controlled to produce sufficient plasticity without causing the air holes to collapse.¹ Narrowing takes place over 6 mm, a distance calculated to allow adiabatic (lossless) coupling into the fiber waist; in the real device, the loss from the fundamental mode was just 0.1 dB.

Conditions inside the fiber waist are designed to enhance soliton self-frequency shift, a result of intrapulse stimulated Raman scattering. This occurs when a soliton propagates along any fiber with anomalous group-velocity-dispersion characteristics, and often produces a downshift as the higher frequencies of the soliton spectrum are converted to lower frequencies. In conventional fibers, this process is marred by the fact that the dispersion is not uniform and includes a zero-dispersion wavelength; thus the shape of the pulse is distorted during propagation.

In microstructured fibers, however, there is more flexibility: the large change in refractive index between the core and cladding means that waveguide (as opposed to material) dispersion can be very high and is easy for the designer to manipulate. Thus, the zero-dispersion wavelength can be forced into a region where it will do no harm (the visible range), and the waveguide and material dispersion can be balanced to produce a flat dispersion/wavelength curve (see Fig. 2).²

In the tapered ASMF designed at Bell Labs, as the confinement tightens, not only does the dispersion of the fiber increase to become approximately uniform across communications wavelengths, but the intensity increases to 15 to 20 times its original value, thus dramatically increasing the rate of the soliton self-frequency shift. In addition, by manipulating the actual intensity (by changing the power of the incoming solitons), the amount of shift—and so the resultant wavelength—could be specified.

To test their device, researchers used a Ti:sapphire-pumped optical parametric oscillator to produce communications-wavelength pulses that were coupled (through air) into the tapered ASMF. Light propagated from the wide entrance to the device, through the taper to a 15-cm-long, 2-µm-core waist, and then back out through the wide exit channel. They found they were able to convert 60% of incident photons to the frequency-shifted soliton—a self-frequency shift of 20%. As expected, they were able to vary the wavelength simply by varying soliton power (see Fig. 3). In terms of length of device for performance, says the Lucent team, the tapered ASMF is two orders-of-magnitude shorter than the best previously reported result.

For more information, contact B. J. Eggleton at [email protected] about the fiber design or X. Liu at [email protected] about the soliton experiments.

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