Supercontinuum generation in a highly nonlinear fiber uses a continuous wave pump

Supercontinuum generation in microstructured and tapered optical fibers as well as highly nonlinear fibers (HNLFs) has attracted considerable interest over the past few years. Most of the work to-date 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 100 nm centered at 1483.4 nm with good long-term power stability was demonstrated experimentally in 1 km of HNLF with a cw Raman fiber laser pump at 1596 nm. The full potential of the source was then shown by generating a supercontinuum with a bandwidth greater than 247 nm in 4.5 km 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 1570-1600 nm. The pump output was launched into a length of one km of HNLF via a coupler, the 2% 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 1594 nm. Spectra of the continuum were acquired as a function of the power launched into the HNLF and also as a function of the pump wavelength. The OSA resolution was set at 0.05 nm.

A 1485 nm 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 1596 nm and a launch power of 904 mW, the 20-dB bandwidth (measured from the peak of the pump laser) of the continuum was 100 nm, spanning a wavelength range from 1568 nm to 1668 nm. In comparison, the 20-dB bandwidth of the corresponding input spectrum was only 2.8 nm.

Continuum generation in the anomalous dispersion regime can be accounted for by a combination of stimulated Raman scattering (SRS) and parametric gain due to four-wave mixing (FWM). The effect of SRS solely can be seen in a broad Stokes peak visible at 1693 nm, which 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.

By using a value of 12.8 square microns for the effective area of the single mode propagating in the HNLF, and typical values for non-linearity and dispersion, the locations of the sidebands were qualitatively predicted. 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.

Applications such as device characterization require 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 1596 nm and a launch power of 1050 mW over a period of more than an hour.

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

The experiment was repeated with a longer length of HNLF of 4.5 km. The pump was positioned in the anomalous dispersion regime at 1596 nm, as before. The increased redistribution of the pump power over the supercontinuum is seen by the decrease in power at 1596 nm relative to the background supercontinuum. The effect was more pronounced compared to the previous results, where only 1 km of HNLF was used.

Data for wavelengths greater than 1770 nm could not be obtained due to spectral limitation of the OSA. Nevertheless, a bandwidth greater than 247 nm can be achieved by pumping the HNLF with 898 mW 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.

This article will be presented in full at the Optical Fiber Communications conference in a paper titled, "Supercontinuum generation in a highly nonlinear fiber using a continuous wave pump."