Scott C. Buchter, Helsinki University of Technology
Bo Andersen, Koheras AS
Supercontinuum sources are finding increasing use in applications that require a source with very broad spectral bandwidth and high spatial brightness. A common method of generating supercontinuum light has involved the coupling of the output from a high-power femtosecond laser into a photonic crystal, or holey, fiber, in which a variety of nonlinear optical effects contribute to extreme spectral broadening. Although effective, the use of this type of pump source results in a complicated and expensive system that heretofore has restricted it to the laboratory.
In the past year, advances in fiber design and the use of miniature Q-switched pump lasers have led to the emergence of a new generation of supercontinuum sources. These devices, although smaller than a shoebox, provide spectral coverage and output powers similar to their lab-bound predecessors. This technology may enable supercontinuum sources to find real-world applications in telecommunications, biotechnology and basic research, and in the devices being considered tools rather than experiments.
Figure 1. Advances in fiber design and the use of miniature Q-switched pump lasers have enabled the development of supercontinuum sources with promise for various applications. Here, the output of a dual-wavelength-pumped system, dispersed by a grating, displays broad spectral bandwidth and high spatial brightness. Courtesy of Scott C. Buchter.
The heart of these systems typically is a diode-pumped, passively Q-switched Nd:YAG laser that generates nanosecond or subnanosecond pulses with a peak power of a few kilowatts. The use of passive Q-switching significantly reduces complexity by eliminating additional control electronics. Although monolithic microchip lasers may be used, thermal effects that limit average output powers hamper their utility.
Researchers at Helsinki University of Technology in Finland and at Koheras AS in BirkerØd, Denmark, have chosen a short-cavity, discrete-element design for their pump lasers in supercontinuum sources. Average powers of greater than 400 mW at 1064 nm in a suitable pulse format are possible with less than 2 W of diode pump power. This radiation is coupled directly into a photonic crystal fiber or frequency-doubled to 532 nm before coupling.
Figure 2. The output spectrum of an IR-pumped system spans more than 1000 nm. The system displays an output power stability of better than 1 percent over 12 hours.
The decision to use photonic crystal fiber rather than standard single-mode fiber is a result of the greater control one has over the dispersion properties of the former. In the case of infrared-only pumping, the fiber typically is designed to have a zero dispersion wavelength slightly shorter than the pump, at around 1040 nm. Four-wave mixing then is the primary mechanism for spectral broadening. The device generates a spectrum spanning more than 1000 nm (Figure 2). The average output power is approximately 115 mW, with a stability of better than 1 percent over a period of 12 hours.
In the other device design, a portion of the IR pump beam is frequency-doubled to 532 nm, and both beams are coupled into a photonic crystal fiber. The fiber in this case has two zero-dispersion wavelengths: one near 750 nm, and the other near 1600 nm. Four-wave mixing again is responsible for the spectral broadening, but this time with two widely separated pump wavelengths. This approach generates smooth continua in the visible portion of the spectrum (Figure 3). The average output power is 25 mW. In both devices, the output is spatially single-mode.
Figure 3. The output spectrum of a dual-wavelength-pumped system reveals smooth continua in the visible region.
There are many applications for which supercontinuum sources are suited. Measuring dispersion and loss in optical networks requires a source (or sources) that covers the wavelength range of interest and that does so with high single-mode power. These new sources fulfill both requirements.
Similar characteristics also are desirable for use in optical coherence tomography, a noninvasive imaging technique that is analogous to ultrasound but that employs light. The axial resolution is inversely proportional to the spectral bandwidth of the source, and the transverse resolution is determined by the focused spot size. Thus, a supercontinuum laser source is required to achieve micron-scale resolution in all three dimensions.
Another application of interest is active hyperspectral imaging, in which a scene is captured in many -- i.e., more than 100 -- spectral bands. The resulting data set contains the reflectance spectrum for each pixel in the image. When a broadband, short-pulse illumination source is used in a scanning geometry, the time of flight also can be measured, yielding a 3-D hyperspectral picture. The use of active illumination combined with time-gated detection provides a significant increase in signal-to-noise ratios.
Besides these specific applications, compact and low-cost supercontinuum sources are proving to be excellent general-purpose laboratory sources. For example, they allow one to measure at once the absorption spectrum of a material over the range spanned by the continuum. In addition, their short pulse widths enable pump-probe studies of time-dependent phenomena.
Although these first sources are well-suited to a variety of applications, they will continue to evolve to meet future needs. Miniature supercontinuum sources with coverage of the UV and mid-IR portions of the spectrum, as well as devices with greatly enhanced average powers or pulse energies, are under development.