Oxide glasses for mid-infrared lasers

Lasers that emit light in the mid-IR wavelength range from 2.5 to 5μm are of great interest for numerous applications in a wide range of fields, including military countermeasure applications, medical diagnostics, and light detection and ranging (lidar) for atmospheric and chemical sensing and monitoring. Many organic molecules exhibit signature optical absorption in this range, which can be used for their detection and analysis. There are also bands—or windows—within this wavelength region characterized by very little absorption of light in the atmosphere (see Figure 1).

Figure 1. Atmospheric transmission spectra at 3m and 3km altitude between 3 and 5μm.

Options for compact, high-power laser sources in this spectral regime are currently rather limited because of the difficulty of finding glass and crystal host materials that are both sufficiently transparent at these mid-IR wavelengths and sufficiently robust and easy to fabricate. One such option is offered by fluoride glass, which—in the form of optical fiber—has previously been demonstrated to support laser oscillation at wavelengths as long as 3.9μm,¹ and up to 4.5μm using supercontinuum generation.² However, fluoride glass has not been widely accepted by industry because of its relatively poor stability, so alternatives are continually sought. Oxide glasses such as silica are significantly more robust than fluorides but have a higher phonon energy (1100cm⁻¹) than fluoride glasses (550cm⁻¹), thus limiting their useful IR transmission range to around 2.3μm. Other families of oxide glasses with lower phonon energies than silica and better stability than fluoride glasses include tellurite and germanate compounds based on the glass-network formers tellurium and germanium dioxide (TeO₂ and GeO₂), respectively. With phonon energies in the range of 650–900cm⁻¹, certain compositions of these glasses can transmit light up to and beyond 5μm (see Figure 2). They are of interest as host materials for mid-IR laser sources. To date, we have demonstrated lasing at 2.1μm in tellurite and germanate fibers and bulk glass.³ Lasing up to 4.9μm has also been achieved using a tellurite photonic-crystal-fiber-based supercontinuum source.⁴ An ongoing challenge with these glasses is removal of impurities such as hydroxyl ions (OH⁻), which are characterized by absorption in the wavelength region of interest. We (and others) have employed several techniques for glass purification that have resulted in significantly reduced OH⁻ absorption.

Figure 2. Near-IR absorption-coefficient spectrum of tellurite glass showing transmission to >5μm and broad absorption at 3.3μm caused by hydroxyl-ion impurities.

Rare-earth-doped solid-state glasses and glass fibers are good options for high-power lasers. There are several rare-earth-ion dopants that have suitable energy-level transitions for lasing in the mid-IR spectral region. Trivalent dysprosium (Dy³⁺), erbium (Er³⁺), and holmium (Ho³⁺) ions have transitions with energies corresponding to wavelengths of 3.0; 2.7; and 2.9 and 3.9μm, respectively, which can potentially be made to lase in oxide glasses such as tellurites and germanates. A solid-state laser must be ‘pumped’ by another light source, typically a high-power near-IR diode laser. The most common choices have wavelengths of either ∼800 or ∼940nm, which are readily available with high powers and at relatively low cost. Dy³⁺ and Er³⁺ions have absorption bands that can be pumped using these common wavelengths, while Ho³⁺ is usually pumped using an intermediate rare-earth ion (sensitizer ion) such as thulium (Tm³⁺) or ytterbium (Yb³⁺). Figure 3 shows the visible to near-IR absorption spectrum of a Dy³⁺-doped tellurite glass sample. We are developing rare-earth-doped novel oxide glasses for mid-IR laser applications and plan to demonstrate lasing from bulk glasses and optical fibers at 2.7–3.5μm in the near future.

Figure 3. Visible to near-IR absorption-coefficient spectrum of a trivalent-dysprosium (Dy³⁺)-doped tellurite-glass sample. The labels describe the Dy³⁺energy-level transitions responsible for the associated absorption peaks.