Experimental photoluminescence and lifetimes at wavelengths including beyond 7 microns in Sm 3 + -doped selenide-chalcogenide glass fibers

: 1000 ppmw Sm 3 + -doped Ge 19.4 Sb 9.7 Se 67.9 Ga 3 atomic % chalcogenide bulk glass and unstructured ﬁber are prepared. Near- and mid-infrared absorption spectra of the bulk glass reveal Sm 3 + electronic absorption bands, and extrinsic vibrational absorption bands, due to host impurities. Fiber photoluminescence, centred at 3.75 µm and 7.25 µm, is measured when pumping at either 1300 or 1470 nm. Pumping at 1470 nm enables the photoluminescent lifetime at 7.3 µm to be measured for the ﬁrst time which was ∼ 100 µs. This is the longest to date, experimentally observed lifetime in the 6.5-9 µm wavelength-range of a lanthanide-doped chalcogenide glass ﬁber.


Introduction
The mid-infrared (MIR) region (3-50 µm wavelength range) [1] houses the fundamental vibrational absorption bands of molecules. MIR light sources of sufficient power output would enable the real-time detection of target molecules, an attractive prospect for a variety of industries and the environmental, food and drink, fossil fuel, defense and medical sectors [2]. Walsh et al. [3] have recently compared the attributes of both currently available and future potential MIR laser sources. They emphasized the advantages of active lanthanide-doped MIR solid-state lasers, of high power, or energy, output; narrow spectral linewidth; a wide variety of pulse widths and pulse repetition rates; broad tunability; efficient operation via direct diode laser pumping and excellent beam quality. Lanthanide-doped MIR solid-state fiber lasers have the added potential advantages of diffraction-limited output and compactness. With medical applications in mind, then lanthanide-doped MIR solid-state fiber lasers have the potential to operate at wavelengths between 6-9 µm and so be resonant with the so-called amide I, II and III tissue absorption bands at wavelengths: 5.90-6.15 µm (due to C = O of amide); ∼6.45 µm (N-H, C-H) and 7.63-8.33 µm (N-H, C-H), respectively, [4] with the capacity to provide a new generation of precision surgical lasers [5] to potentially minimise collateral damage to surrounding tissue. In addition, lanthanide-doped MIR solid-state fiber lasers operating as pulsed lasers in the MIR spectral region are needed in the wavelength range 4-7 µm for pumping chalcogenide glass fiber MIR supercontinuum lasers, to enable a compact, all-fiber solution, for use in MIR endoscopic probes to provide in vivo chemical mapping of tissue for disease diagnostics [6,7].
To achieve such longer wavelength lanthanide-doped MIR solid-state fiber lasers, a low phonon energy host is prerequisite [8,9]. Er 3+ -ZBLAN, and Ho 3+ -InF 3 , fibers have been demonstrated to lase at room temperature at 3.78, and 3.92 µm, respectively, [10,11]. This probably represents the long wavelength limit of laser operation beyond which the phonon energy of these host matrices favours non-radiative emission.
Selenide-based chalcogenide fibers, on the other hand, present lower phonon energies than the sulfides of 250-300 cm −1 and display a large low-loss transmission window (∼1-10 µm) with lowest intrinsic loss around 5.5-6.5 µm [8]. GeSbGaSe host glasses have been shown to support radiative transitions emitting out to 8 µm in a Tb 3+ doped chalcogenide composition [14]. Despite this, no MIR lasing has yet been recorded in chalcogenide fiber. Fabrication of such glass fibers requires careful purification to exhibit low concentrations of extrinsic impurities such as: oxygen, hydrogen, carbon, and silica, and be free of devitrification.
The work presented in this Paper details the development of a (GeSbSe) 97 Ga 3 at. % glass doped with 1000 ppmw Sm 3+ which was drawn to fiber. MIR bulk absorption spectra, fiber optical loss, photoluminescence (PL) and PL lifetimes are presented. Here, we report the longest to date, experimentally observed, photoluminescence lifetime of ∼ 0.1 ms for a lanthanide ion doped chalcogenide glass fiber, within the 6.5-9 µm range.

MIR optical loss of fiber
Optical loss of the 1000 ppmw Sm 3+ -doped (GeSbSe) 97 Ga 3 at. % glass fiber in the 3-9 µm wavelength range was measured using the cut-back method. The IFS 66/S, Bruker FTIR was used with purging air, as above, and a GloBar © blackbody source, KBr beamsplitter and liquid N 2 cooled indium antimonide (InSb, InfraRed Associates Inc.) and mercury-cadmium-telluride (MCT, Kolmar Technologies: V100-1B-7) detectors, to cover the NIR and MIR spectral regions, respectively. Further measurement details may be found in [19].

Fiber-end-collection of PL intensities
1000 ppmw Sm 3+ -doped (GeSbSe) 97 Ga 3 at. % glass fiber of 75 mm length and 415 ± 3 µm diameter was end-pumped (see Fig. 1) and the PL intensity detected at around 2.7, 3.7 and 7.3 µm wavelength, as detailed below. Sm 3+ ions doped in the selenide-chalcogenide glass fiber host were excited by either a 1470 nm laser diode (SemiNex: 4PN-127) or 1300 nm laser diode (SemiNex: 4PN-116). The pump laser beams were chopped in order to reduced thermal background noise from the sample, optical elements and other laboratory equipment. For example, the remaining pump power exiting the fiber-end, opposite to that pumped, heated a long-pass filter, down-circuit ( Fig. 1 and see below) and produced thermal background noise. Chopping the pump laser beam meant not only that the absolute heat load of the sample was reduced, but also that blackbody radiation from the sample, optical components and etc. were reduced as they were not modulated by the chopper. Thus, the pump beam was electronically chopped by means of a function generator (GW INSTEK GFG-3015) of low frequency, ∼20 Hz; the chopping period of 50 ms was chosen to be long compared to the Sm 3+ MIR PL life-times (∼100 µs, see Section 3).
The resulting PL emission, from the fiber end opposite to that pumped ( Fig. 1), was focused through a pair of Ge aspheric lenses (Edmund Optics: code 89-607, AR (anti-reflection coated) for 3-12 µm wavelength range), each of NA (numerical aperture): 1, and focal length: 12.7 mm. Lenses of short focal length/high NA were best suited for efficient collection of the fiber PL, and selected to match the high refractive index (n) of the unstructured chalcogenide glass fiber (e.g. n ∼2.55 at 3-10 µm, for undoped Ge 20 Sb 10 Se 70 at. % [20]) and high fiber NA (>>1). This approach was instigated at the Wrocław University of Technology and has been used in [21]. The Ge lens pair also acted as a 2 µm long-pass filter, thus any residual pump signal exiting the fiber-end was eliminated before reaching the next long-pass filter and monochromator further along the optical circuit (see Fig. 1, and below).
The PL exiting the fiber, resulting from excitation with the 1300 or 1470 nm laser diode, was first directed through the Ge lens pair, then passed through a long-pass filter of cut-on wavelength 3 µm (Spectrogon: 71M09339) into a monochromator (MSH-150, Quantum Lot (focal length: 150 mm), calibrated by Bentham in 2018) operating with a 4000 nm blazed, 150 line/ mm grating and onto the detector. Detection of the signal was achieved with a thermoelectric cooled (200 K) MCT (mercury cadmium telluride) detector (Vigo System: PVI-4TE-8), operating in the spectral range: 3 µm to 9 µm. Additionally, the thermoelectric cooled (200 K) MCT (Vigo System: PVI-4TE-5) was used operating in the spectral range: 2 µm to 6.0 µm. The PL exiting the fiber, resulting from excitation with the 1300 or 1470 nm laser diode, was directed through the Ge lens pair, then passed through a long-pass filter of cut-on wavelength 6.15 µm (Spectrogon: 71M09001) then into the same monochromator as above, but operating with a 9000 nm blazed, 100 line/ mm grating, and then onto the same MCT detectors as above. All measured spectra were corrected for system response
A function generator (GW INSTEK GFG-3015) coupled to laser diode drivers (Thorlabs: LBC240C) was used to produce a square-wave signal, which modulated the laser diode, switching it on and off. The resultant periodic PL excitation from the fiber under the side emission set-up (Fig. 2), and close to the pumped fiber end, was then directed through the pair of Ge lenses and through the long-pass filter (cut-on at 3 µm or 6.15 µm) into the MCT detector. The MCT detector was used to measure the intensity decay of the fiber PL side-emission. The signal from the MCT photodetector was directed to an oscilloscope. In order to suppress noise, averaging over several thousand excitation processes was performed several times.
3.4.2. PL decay centered at 7.25 µm, cut-on filter at 6.15 µm, fiber-side collection Figure 8 presents the PL decay of the of the 1000 ppmw Sm 3+ -doped (GeSbSe) 97 Ga 3 glass fiber using the MCT detector (PVI-4TE-8) and cut-on filter at 6.15 µm; the PL was centered at 7.25 µm (see Fig. 5(b)). As low pump power as possible was used in order to minimize heating of the sample. The decay was measured from the fiber-side. The measured lifetime was fitted using a single exponential function. The calculated PL lifetime was in the range of 0.1 ms; measurements were repeated 3 times: 0.0975 ms, 0.0971 ms and 0.104 ms. A single exponential was fitted, but was not a perfect fit. It is suggested that this is because there is more than one transition contributing: 6 H 13/2 → 6 H 9/2 at around 7.25 µm and 6 H 11/2 → 6 H 9/2 at around 7.8 µm; these two transition have a similar branching ratio [15]. Also, emission from ( 6 F 1/2 , 6 F 3/2 , 6 H 15/2 ) → 6 H 11/2 may contribute to the MIR emission but this has a low branching ratio [15].

Development of low optical loss host glass system for long wavelength
The much-investigated host glass system GeAsSeGa is a current benchmark for rare earth ion doped chalcogenide glass fibers. Fiber losses as low as 1.16 dB/m at 6.56 µm have been measured for Dy 3+ doped glass [26] with others reporting losses of 2-3 dB/m [27][28][29]. To the authors' knowledge, no emission beyond the 3.5-6 µm wavelength range exhibited by Pr 3+ has been recorded from a GeAsSeGa fiber [29,30]. Accessing longer wavelength rare earth ion emissions may therefore require the development of active chalcogenide fibers of comparatively lower phonon energy. Ga 5 Ge 20 Sb 10 Se 65 at. % fibers doped with 500 ppmw Tb 3+ has facilitated 8 µm emission, with a background loss of 2.5 dB/m [14]. 2000 ppmw Pr 3+ Ge 15 As 16 Se 63 In 3 I 3 at. % and 500 ppmw Pr 3+ GeAsInSe fibers have been fabricated with losses ca. 1 dB/m, and 4.3 dB/m, respectively [27,28]. The low loss of the former fiber can be attributed to an intricate distillation process. Despite being comprised of lower phonon energy constituents, no emission beyond 6 µm was measured from these fibers. The fiber reported herein exhibited a fiber loss to rival the GeAsSeGa system (2.15 dB/m at 6.04 µm) while also facilitating longer wavelength (7.3 µm) emission.

Fiber PL and PL lifetime
Pumping with 1300 nm into the 6 F 7/2 level resulted in a less intense emission despite the increase in pump power when compared to pumping at 1470 nm. Yet the 6 F 7/2 has a higher absorption coefficient of 0.72 cm −1 at 1300 nm, whilst that of the 6 F 5/2 manifold was 0.58 cm −1 at 1470 nm. The disparity in emission intensity may be explained by the lower quantum efficiency when pumped at 1300 nm.
To the authors' knowledge, this Paper reports the first decay lifetime measured for a Sm 3+ doped chalcogenide fiber at 7.25 µm. Judd-Ofelt analysis for a similar host glass in [15] predicted a ∼4.3 ms lifetime, however, the measured lifetime (∼ 0.1 ms) here was considerably faster, yet similar to that found for Dy 3+ at 7 µm (∼43 µs) [31]. The authors of [15] commented on the unlikelyhood of a Sm 3+ 6 H 13/2 → 6 H 11/2 radiative emission at 7.77 µm (Fig. 5) due to its small energy gap, yet it is suggested in the current work that this emission is indeed active.

Conclusions
1000 ppmw Sm 3+ -doped (GeSbSe) 97 Ga 3 glass was successfully drawn to an unstructured fiber with minimum loss of 2.15 dB/m at 6.04 µm wavelength. The fiber provided emission in the 3.25-4.5 µm and 6.23-9 µm wavelength range when pumped with 1300 nm or 1470 nm laser diodes. The first recorded Sm 3+ lifetime at 7.25 µm was measured to be 0.1 ms.