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Supercontinuum generation in a highly nonlinear fiber

Posted: 16 Dec 2003 ?? ?Print Version ?Bookmark and Share

Keywords:supercontinuum generation? hnlf? fwm? osa?

Supercontinuum generation in micro-structured, tapered optical fibers and highly nonlinear fibers (HNLFs) has attracted considerable interest over the past few years. Currently, most of the work on this topic has dealt with supercontinua seeded by pulsed sources. For instance, octave-spanning supercontinuum generation has been successfully demonstrated by launching femtosecond laser pulses into the cores of these fibers.

Applications for supercontinuum generation include spectral slicing for wavelength division multiplexing, optical coherence tomography, sensing and device characterization.

Supercontinuum generation over a bandwidth of 100nm centered at 1,483.4nm with good long-term power stability was demonstrated experimentally in 1km of HNLF with a cw Raman fiber laser pump at 1,596nm. The full potential of the source was then shown by generating a supercontinuum with a bandwidth greater than 247nm in 4.5km of HNLF.

The pump source used was a diode-pumped Raman fiber laser with a tunable bandpass filter in the cavity, allowing the output wavelength to be varied from 1,570nm to1,600nm. The pump output was launched into a length of 1km of HNLF via a coupler - the 2 percent output port of which was used to monitor the power fluctuation of the pump source using an optical spectrum analyzer (OSA). The light coming out of the HNLF was attenuated using a variable optical attenuator (VOA) before it was fed into a second OSA to study the continuum generation.

The HNLF used in the experiment had a zero dispersion wavelength close to 1,594nm. The spectra of the continuum were acquired as a function of the power launched into the HNLF and of the pump wavelength. The OSA resolution was set at 0.05nm.

A 1,485nm line appeared in the input spectra representing the residual light coming from the Raman fiber laser pump itself. Additional weaker lines were also present at longer wavelengths. In the anomalous dispersion regime with a pump centered at 1,596nm and a launch power of 904mW, the 20dB bandwidth (measured from the peak of the pump laser) of the continuum was 100nm, spanning a wavelength range from 1,568nm to 1,668nm. In comparison, the 20dB bandwidth of the corresponding input spectrum was only 2.8nm.

Continuum generation in the anomalous dispersion regime can be accounted for by a combination of stimulated Raman scattering (SRS) and parametric gain due to 4-wave mixing (FWM). The effect of SRS can be seen in a broad Stokes peak visible at 1,693nm - that grows as the input pump power is increased. When the pump is positioned in the anomalous dispersion regime, the growth of a similar Stokes wave also occurs, but it is now accompanied by FWM.

Unlike the normal dispersion regime, phase-matching (hence, efficient FWM) in the anomalous dispersion regime is ensured by the balance between the negative contribution of material dispersion and the positive contribution of fiber nonlinearity. Sidebands that appear on opposite sides of the pump wavelength can be interpreted as the result of FWM phase-matched by self-phase modulation--also referred to as modulation instability.

The locations of the sidebands were qualitatively predicted by using a value of 12.8? for the effective area of the single mode propagating in the HNLF and typical values for nonlinearity and dispersion. The dispersion was used as a fitting parameter, but its small value is comparable to experimentally measured values. The combined effect of SRS and FWM was responsible for the large gain that was observed in the anomalous dispersion regime as the launch power was increased.

Device characterization requires a reliable source with long-term stability. We investigated the stability of the continuum spectrum generated in the HNLF of length on kilometer by a pump positioned at 1,596nm and a launch power of 1050mW over a period of more than an hour.

Spectra of the continuum were acquired at a regular time interval of 5mins for 70mins. The cw continuum spectrum at time t = 0. The continuum spectrum showed good long-term stability with a maximum standard deviation in power of only 0.5dB over a period of 70mins.

The experiment was repeated with a longer length of HNLF of 4.5km. The pump was positioned in the anomalous dispersion regime at 1,596nm, as before. The increased redistribution of the pump power over the supercontinuum is seen by the decrease in power at 1,596nm relative to the background supercontinuum. The effect was more pronounced compared to the previous results, where only 1km of HNLF was used.

Data for wavelengths greater than 1,770nm could not be obtained due to spectral limitations of the OSA. Nevertheless, a bandwidth greater than 247nm can be achieved by pumping the HNLF with 898mW of power. This indicates that using HNLFs, a supercontinuum spanning the S, C and L bands, can be generated using a single pump source. To the best of our knowledge, this is the broadest supercontinuum generated using a cw pump.

- Akheelesh Abeeluck, K. Brar, J. Bouteiller

OFS Labs

- S. Radic

Bell Labs

Lucent Technologies

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