Novel Broadband Light Sources—Guiding Light through Glass and Holes

F.G. Omenetto, M.R. Wehner, M.R. Ross, E.H. Gershgoren (P-23), A.V. Efimov (MST-10), A.J. Taylor (CINT)
Excerpted from LA-14202-PR

Introduction

Over the last seven years, photonic crystal fibers (PCFs) have become one of the success stories of modern photonics.¹ Starting as a highly speculative idea in 1991, it is now possible to obtain PCFs in different varieties with specifically tailored features.² A typical PCF consists of an array of microscopic holes (hollow capillaries with diameters precisely controllable in the range ~ 25 nm to ~ 50 nm) running along the fiber length (Figure 1). These holes act as optical barriers or scatterers which, suitably arranged, can trap light within a central core (either hollow or made of solid glass). The very large air-glass refractive-index difference opens up many new possibilities not available in standard fibers. For example, light can be guided in a hollow core by a photonic-bandgap effect. The revolutionary nature of the waveguides and their very high performance measured in terms of loss, nonlinearity, and dispersion control means that applications are emerging in many diverse areas of science and technology.

A Rainbow of Light

One of the immediate consequences of these new waveguides is the spawning of renewed interest in the study of nonlinear processes in optical fibers, specifically in regards to supercontinuum formation and various mixing and nonlinear frequency-conversion processes. A supercontinuum is formed when a short pulse of light (typically of subpicosecond duration) undergoes a nonlinear interaction with the material in which it propagates. Such interaction, in the appropriate conditions, leads to a dramatic broadening of the pulse’s bandwidth, and provides a “rainbow of colors” at the output. These conditions were traditionally obtained in the early 1980s by propagating high- intensity laser pulses through materials such as sapphire. PCFs provide the very desirable feature of confining and guiding the optical field in a glass core whose diameter does not exceed a few microns. Having a moderately high-intensity femtosecond laser pulse combined with the spatial confinement of the guided mode is enough to make the nonlinear interaction between light and glass the overwhelmingly dominant process governing pulse propagation in PCF.

Spectacular frequency-conversion phenomena occur in these conditions. We have described power-dependent generation of visible radiation by coupling femtosecond pulses at a wavelength of 1550 nm in a 95 cm segment of a “high-Δ” microstructured fiber (i.e., high-air filling in the cladding). Two bands of visible radiation were generated by a combination of temporal pulse splitting of the fundamental pulse followed by Raman self-frequency shifting of one of the split pulses and a subsequent third harmonic generation of both frequencies.³ The visible generated radiation is dependent on the polarization state of the input pulse coupled into the PCF.

One of the most dramatic manifestations of the nonlinear effects in PCFs is supercontinuum generation.⁴ When 100 fs pulses from titanium-sapphire lasers (λ ~ 800 nm) were coupled in a few meters of these fibers, the combination of nonlinear effects gave rise to a significant spectral broadening spanning nearly 1000 nm. This result provided a practical means to achieve an efficient pulsed white light source encompassing an “optical octave” (400–800 nm). This latter feature is of great value in realizing precision measurement techniques based on supercontinuum frequency combs (Figure 2).

The appeal of controlling supercontinuum as a practical source of light remains very high. The availability of such a broad-bandwidth source is extremely important for a variety of applications such as spectroscopic detection and interrogation, new laser source generation, broad-bandwitdh communication sources, and arbitrary signal generation. Typically made of silica, the PCFs used to generate supercontinuum radiation provide slightly in excess of one optical octave usually limited by the physical properties of the fiber itself (such as modes supported, wavelength-dependent absorption, dispersion etc.).

Summary of Some Experimental Results

We experimentally observed that the propagation of a pulse of fixed energy, though linearly polarized along different directions, yields very distinct visible components at the output.⁵ These results suggest a polarization-dependent selectivity for phase-matching (i.e., frequency conversion) according to the input polarization state. Such polarization-dependent selectivity provides a means to generate specific harmonics and, therefore, a means to control the output’s visible frequency through the input pulse polarization state. These approaches are promising for the research and development of all-fiber signal control and in-fiber ultrafast optical switching.⁶ Furthermore, through intramodal phase matching processes in PCFs, one can generate guided modes well into the ultraviolet region (λ < 250 nm)^7–9 (Figure 3).

In collaboration with the University of Bath, we have experimented with soft Schott glass (SF6) PCFs and studied supercontinuum formation in these new structures. This glass exhibits greater transparency in the infrared than silica and also has a higher nonlinear index of refraction, thereby enhancing the nonlinear interaction between the optical pulse and the glass. Initial results indicated a broader supercontinuum than what was conventionally achievable with silica-based PCFs, extending the spectral span to ~ 1400 nm after propagating pulses in a 75 cm piece of fiber.¹⁰

We have recently demonstrated the broadest supercontinuum formation ever recorded¹¹ by propagation λ = 1550 nm, ~ 100 fs pulses of 1 nJ energy in a short (Z = 4.7 mm) piece of the SF6 PCF. The measurement illustrated in Figure 4 compares a typical supercontinuum trace obtained with silica PCF. The present measurement is limited by the finite spectral sensitivity of the cooled mercury-cadmium-titanium detector, which does not go beyond 3 μm. The extensive span of this supercontinuum offers opportunities across multiple wavelength areas, covering seven optical octaves of 400 nm, for instance, and extending the wavelength ranges well into the infrared region. In addition to the spectral extent of this source, this result has given us further insight into the physical mechanism of supercontinuum.⁴

Conclusion

The study of these fibers and the nonlinear effects that take place in them is an area rich in opportunities in the basic and applied sciences. A glimpse of the breadth of the field is offered by the future possibilities that include new approaches to optical fiber sensors, high-power fiber lasers, sources for medical imaging, and hollow-core photonic crystal fibers (which open exciting avenues to realize enhanced, specialized Raman cells for high-sensitivity spectroscopy, or for the sensing of atmospheric contaminants such as molecules or viruses).

The future looks bright for guided light.

References

1. P. Russell, “Photonic crystal fibers,” Science 299, 358–362 (2003).
2. W.H. Reeves et al., “Supercontinuum generation in dispersion flattened photonic crystal fiber,” Nature 424, 511–515 (2003).
3. F.G. Omenetto et al., “Polarization dependent third harmonic generation in photonic crystal fibers,” Optics Express 11, 61–67 (2003).
4. J.K. Ranka, R.S. Windeler, and A.J. Stentz, “Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm,” Optics Letters 25, 25–27 (2000).
5. F.G. Omenetto et al., “Simultaneous generation of spectrally distinct third harmonic in photonic crystal fibers,” Optics Letters 26, 1558–1560 (2001).
6. F.G. Omenetto et al., “Polarization dependent third harmonic generation in photonic crystal fibers,” Optics and Photonics News 14, 36 (2003).
7. F.G. Omenetto, “Femtosecond pulses in optical fibers,” in Progress in Optics, E. Wolf (Ed.), (Elsevier, Amsterdam, 2002), 44, pp. 85–141.
8. A. Efimov et al., “Phase-matched third harmonic generation in microstructured fibers,” Optics Express 11, 2567–2576 (2003).
9. A. Efimov et al., “Nonlinear generation of very high order UV modes in microstructured fibers,” Optics Express 11, 910–918 (2003).
10. V.V. Ravi Kanth Kumar et al., “An extruded soft glass photonic crystal fiber for supercontinuum generation,” Optics Express 10, 1520–1525 (2002).
11. F.G. Omenetto et al., “An ultrabroad supercontinuum laser source,” (in preparation).

Acknowledgment

We gratefully acknowledge our collaborators at the University of Bath (Philip Russell and Jonathan Knight) for the continuous collaboration and for the endless supply of new PCF samples, and specifically Will Reeves and V.V.R.K. Kumar for the early work on SF6 fibers. This work was funded by the Laboratory-Directed Research and Development program.

For further information, contact Fio Omenetto, 505-665-5847, omenetto@lanl.gov.