Invited PapersAll-fiber chalcogenide-based mid-infrared supercontinuum source
Highlights
► An all-fiber mid-infrared supercontinuum source emitting over 1.9–4.8 μm. ► The laser includes commercial off-the-shelf parts and a chalcogenide fiber. ► Demonstrated 10 dB spectral flatness from 2.0 to 4.6 μm, and −20 dBm from 1.9 to 4.8 μm. ► The system output average power is 565 mW at 10 MHz. ► The long wavelength edge limit is caused by an extrinsic absorption in the fiber.
Introduction
Supercontinuum sources in the mid-infrared (2–5 μm) provide continuous wavelength coverage for a spectral band where only a few narrow line laser systems exists. Supercontinuum sources based on fiber laser pump sources benefit from developments by the optical fiber industry such as reduced size, weight and power requirements (when compared to bulk optical lasers and frequency converters), high wall plug efficiency and rugged packaging. In addition, fiber based supercontinuum sources have high brightness broadband outputs with well defined beam shapes and divergences. High brightness broadband sources in the mid-infrared with good beam quality and divergence are needed for spectroscopy and metrology as well as defense applications.
There have been several demonstrations of mid-infrared supercontinuum generation with optical fibers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Supercontinuum sources using fluoride have reached watt level powers but are restricted to wavelengths below 4.5 μm [6], [11]. The intrinsic fiber loss due to the multiphoton absorption edge in fluorides and tellurite fibers (∼4.5 μm) is the main limitation on the long-wavelength side for supercontinuum systems based on fluorides and tellurites [12]. Emission beyond the multiphoton absorption edge can be overcome using short lengths of fiber and high pump intensity and has been demonstrated in tellurite [7] and of fluoride fiber [8] but requires the use of high peak power pump lasers (Ti:Sapphire/optical parametric amplifier) and/or micron-sized suspended core fiber.
Chalcogenide fibers (fibers based on chalcogen elements S, Se and Te) are natural candidates for infrared supercontinuum sources due to their long wavelength multiphoton absorption edge. For example the multiphoton absorption edge for an As2S3 is 7.4 μm [12], and can be even further for Se-based and Te-based chalcogenide fibers. For applications involving nonlinear optical processes, chalcogenide fibers are even more suitable [13] given their high nonlinear refractive index n2 ( and ) [9] and high peak Raman gain gr ( [14], [15], [16], [17]. In addition, chalcogenides have high optical damage threshold (e.g. >12 GW/cm2 for picoseconds pulses at 2.4 μm in As2S3 [2]) and environmental stability (e.g. non-hydroscopic for most chalcogenide glasses including As2S3, As2Se3). It is the combination of wide transmission range, strong nonlinear properties, high optical damage threshold and environmental stability that make chalcogenide fibers an ideal system from fiber based supercontinuum laser sources.
The first mid-infrared fiber supercontinuum source was demonstrated using chalcogenide fibers in 2005 [1], [18] but was based on a optical parametric amplifier pumped by a Ti:Sapphire laser. The supercontinuum source was limited in spectral range (2–3.5 μm) and average power (<10 mW). In [2] we transitioned to a fiber based pump source which still emitted close to 2.5 μm, increasing the average power (∼140 mW) and extending the spectral range to cover from 1.5 to 4.8 μm. The supercontinuum laser system was based on a custom picosecond mode-locked laser seed system [19], amplified in a Er-doped fiber amplification stage, soliton-self frequency shifted to 2 μm and further amplified in a Tm-doped fiber amplification stage. In this paper, we transition to an all COTS based laser system still using a chalcogenide fiber in the last stage for high nonlinear mid-infrared supercontinuum source.
Section snippets
Experimental setup
Fig. 1 shows a schematic of the laser system developed for the supercontinuum source. A multistage master oscillator power amplifier geometry is used following a similar approach as the one described in [20]. An 40-ps, 10-MHz Er-fiber-based laser with 200 mW average output power was sourced from a commercial vendor and is used as a seed for the system. The use of a picosecond pulse duration seed minimizes the effect of Brillouin scattering as power is increased while being fairly insensitive to
Results
The laser system was characterized before and after the insertion of the chalcogenide fiber in the system. The optical beam’s wavelength prior to entering the chalcogenide fiber is centered around 2.45 μm and has approximately 100-nm bandwidth. At this stage, the output is approximately 1.4 W of average power. After propagating through the As2S3 chalcogenide fiber, 565 mW broadband supercontinuum is observed.
Fig. 2 shows the generated supercontinuum spectrum. The spectrum is measured with a
Discussion
Previous demonstrations of mid-IR supercontinuum generation in other materials systems used anomalous dispersion pumping for supercontinuum generation [9], [12], [21]. Here, normal dispersion pumping is utilized for supercontinuum generation. The process of supercontinuum generation in a normal dispersion fiber is known and is primarily an interplay between stimulated Raman scattering and self and cross-phase modulation. [20] We speculate that given the asymmetric power induced broadening
Conclusion
We present an all-fiber chalcogenide-based supercontinuum source with emission covering the wavelength range of 1.9–4.8 μm. The system architecture is based on a combination of silica commercial off-the shelf components and an As2S3 step index nonlinear optical fiber. The supercontinuum spectrum has 10 dB spectral flatness from 2.0 to 4.6 μm, −20 dBm points from 1.9 to 4.8 μm with a total output power of 0.565 W. We identify the current long-wavelength limit of the system to be due to an extrinsic
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