Mid-IR supercontinuum laser covers key spectral bands for spectroscopy

The commercialization of supercontinuum lasers is pursued in a variety of wavelength regions, primarily motivated by the high brightness offered by these sources. Supercontinuum sources covering the mid-infrared (mid-IR) region (2.5–20 µm) are particularly important for identifying and characterizing materials through spectroscopy, enabling key applications such as remote sensing, industrial inspection, and hyperspectral imaging. Supercontinuum sources are promising due to their ability to produce orders-of-magnitude higher brightness levels compared to traditionally used thermal mid-IR sources such as lamps and globars. Their brightness levels can also exceed those achieved using a synchrotron beamline, while offering a portable and more cost-effective solution. This article outlines the principle of operation of a supercontinuum laser system, discusses the architectural choices that govern its noise performance, and describes the design of a mid-IR supercontinuum laser product developed by Thorlabs (Newton, NJ).

Supercontinuum systems typically consist of a pulsed laser source (pump laser) and a nonlinear medium that broadens the relatively narrow spectrum of the pump laser to one or more octaves of bandwidth. A number of pump-laser architectures and nonlinear materials have been used to generate supercontinuum light. Fiber-based pulsed laser sources are especially attractive as pump lasers due to their ability to reach high average powers and the possibility of integrating such lasers into compact and environmentally stable platforms.

The most common nonlinear medium used for spectral broadening is a single-mode waveguide with third-order (Kerr) nonlinearity. Single-mode waveguides (for example, optical fibers) have the added benefit of being compatible with fiber-laser pump sources in particular due to a simple coupling between the pump source and the waveguide by way of direct splicing.

Supercontinuum noise and the femtosecond architecture

The nonlinear processes that spectrally broaden a pump laser to generate the supercontinuum can also introduce significant levels of shot-to-shot instability in the output spectrum. This instability can cause the output spectrum to fluctuate from pulse to pulse, which can in turn degrade the signal-to-noise ratio (SNR) of the source. In some applications, the gains achieved by the higher brightness of the supercontinuum source can be easily erased by the degraded SNR. For instance, when fast measurements are required for demanding applications like in-line process monitoring, a high SNR is critical. High SNR is also required to reliably detect low-level signals, making SNR optimization a primary consideration for supercontinuum source development.

The shot-to-shot stability (coherence) of the supercontinuum-generation process has been widely studied, and it is well established that a femtosecond (less than 100 fs pulse width) pump source is a fundamental requirement for achieving a shot-to-shot stable supercontinuum. The first commercialized supercontinuum sources, and the majority of products on the market, use picosecond and nanosecond pump architectures that are simpler to implement, but are prone to shot-to-shot noise challenges. While using a femtosecond architecture typically adds to system complexity, the emergence and commercialization of femtosecond mode-locked fiber lasers over the last few years has made such sources more readily available, more reliable, and less expensive. Femtosecond-pumped supercontinuum systems provide a path to low-noise, compact, and robust sources of high-brightness broadband light.

Femtosecond mid-IR supercontinuum laser

A mid-IR supercontinuum laser based on femtosecond pumping architecture could play a key role in a number of spectroscopy applications due to its inherently low noise performance. The realization of such a source requires two key building blocks: a femtosecond pump laser operating within or close to the mid-IR spectral region, and a nonlinear waveguide that provides good transmission in the mid-IR region as well as favorable dispersion and nonlinearity for spectral broadening in the femtosecond regime.

To address the first of these requirements, Thorlabs has developed a thulium-doped femtosecond fiber laser (Item # FSL1950) that operates close to 2.2 µm and generates pulses at 50 MHz with <100 fs pulse width and average power >500 mW. This laser operates in the longest-wavelength band that is accessible by silica-based fiber laser systems to enable efficient generation of a supercontinuum source in the mid-IR. The limited transmission window of silica (which cuts off around 2.3 µm) requires the use of a nonlinear waveguide based on a different material system to broaden the pump spectrum into the mid-IR region.

Indium fluoride glass, which can be drawn into optical fiber, provides a transmission window up to 5.5 µm, covering a significant portion of the mid-IR wavelength region (see Fig. 1). The material system chosen is especially attractive due to its environmental stability, mechanical strength, and long-term robustness. The nonlinear indium fluoride fiber is drawn with a step-index geometry in a special draw process that allows its core diameter to be accurately controlled. Through this tight control on the fiber geometry, a fiber with optimal dispersion profile for supercontinuum generation in the femtosecond regime and into the mid-IR region is designed and developed using an in-house draw process.

Figure 2 shows a diagram of the femtosecond mid-IR supercontinuum system consisting of the two building blocks described above. Numerical simulations of the pump pulse propagation in the fiber are shown in the spectral-evolution plot in Figure 2, where the horizontal axis is the wavelength and the vertical axis is the propagation distance along the fiber. As shown in the simulations, a pulse generated from the femtosecond pump laser at around 2.2 µm is broadened to more than one octave of bandwidth within the first few centimeters of propagation in the fiber.

FIGURE 2. For a fluoride fiber pumped with light at 2 µm from a femtosecond-fiber laser source (depicted at left), the supercontinuum spectrum evolves as the light propagates down the fiber, as shown in the spectral evolution plot (right).

The femtosecond thulium-doped fiber laser and the mid-IR nonlinear fiber described above are integrated into an all-fiber platform to produce a compact benchtop mid-IR supercontinuum laser, the SC4500. Integration is facilitated by an in-house splice recipe that has been developed to join the silica fiber that delivers pulses from the pump laser to the nonlinear fluoride fiber. The splice is achieved using low-power filament splicing with a Thorlabs GPX series glass-processing system.

Single-mode beam quality

A measured supercontinuum spectrum from this system is shown in Figure 3, covering almost two octaves of bandwidth from 1.3 µm to beyond 4.5 µm. The results show good agreement with the numerical simulations shown in Figure 2. Generating this broad band requires the use of a modest length of nonlinear fiber (shorter than 20 cm), enabling the supercontinuum generation in the shot-to-shot coherent regime to minimize the spectral noise of the source.

FIGURE 3. Supercontinuum spectrum (a) from a compact benchtop mid-IR supercontinuum laser (b). The spectrum was obtained without purging the laser cavity. The fine structure seen around 2.7 μm is due to water and CO2 absorption in the beam path of the measurement setup. The sharp drop-off at 4.2 μm is also due to CO2 absorption.

A key advantage of supercontinuum sources stems from their single-mode beam quality, leading to improved brightness levels. A single-mode circular beam profile from the SC4500 is compared to the collimated beam generated from a thermal source (see Fig. 4). This contrast in beam quality can manifest itself in major performance improvements in certain key applications. One example is the use of broadband mid-IR sources in open-path gas analyzers, where long-distance beam propagation over hundreds of meters would be required. Another example is in imaging applications, where a diffraction-limited focused beam from a laser can provide much better spatial resolution compared to what can be achieved using traditional incoherent sources.

FIGURE 4. A single-mode circular beam profile from the SC4500 (top) is contrasted with the collimated beam generated from a thermal source, which is spatially incoherent (bottom).

By carefully considering the tradeoffs of various methodologies and architectures, the key desirable features of high brightness, good beam quality, broad spectral coverage, and shot-to-shot stability can all be achieved. In the specific efforts described above, all of these key performance metrics are delivered in a robust and compact all-fiber benchtop solution, opening up new application spaces.