We evaluated the optical properties required for the design of a solar-pumped laser for -doped (ZBLAN) glass. The quantum efficiency (QE) of near-infrared emission from the state of using sunlight as an excitation source was 70%. The product of the stimulated emission cross section and the radiative lifetime () of the was . The integrated absorption strength in the region was the largest among -doped fluoride glasses. The high QE, large product, and large integrated absorption strength indicate that -doped ZBLAN is one of the most promising materials for solar-pumped lasers.
© 2011 Optical Society of America
A solar-pumped laser is a device that converts incoherent sunlight into a coherent laser beam. In recent years, solar-pumped lasers have been attracting much attention as novel renewable energy sources in optical communications, energy transfer in space , and an energy cycle using magnesium . The development of solar-pumped lasers has a long history. Kiss et al. have achieved continuous wave laser action at in the system at liquid neon temperature () using the Sun as the pumping source in 1963 . That was soon after the first laser oscillation using ruby by Maiman in 1960 . Since then, solar-pumped lasers have been attained with [5, 6], garnet (YAG) single crystal [1, 6, 7], and , , and . Recently, a solar-pumped laser with an output of about and an optical-to-optical conversion efficiency of 4.3% was achieved using , transparent ceramic .
The reliability, cost performance, and optical-to-optical conversion efficiency of the solar-pumped laser system must be raised considerably, in order to expand the use of solar-pumped lasers worldwide. We believe these issues can be resolved by utilizing -doped fiber lasers made of glass materials because of the optical confinement provided by the waveguide structure combined with the excellent laser properties of . It is necessary for fiber lasers to make use of glass material, which can be drawn into an optical fiber easily. -doped glass laser media has been used in laser fusion and fiber laser applications. However, the possibility of using a low-energy pumping source, such as the Sun, has not been considered, because one of the main targets of these applications is to obtain high-irradiance light sources. It should be elucidated which properties of the host glass dominate the efficiency under sunlight pumping. As the first trial, we have chosen (borosilicate) glass, which has excellent rare-earth solubility and can hold up to without evidence of Nd clustering . However, the quantum efficiency (QE) of the near-infrared emission of the -doped borosilicate glass under sunlight excitation was as low as 21% due to the high phonon energy .
(ZBLAN) glass, found in 1975 , has the wide transparent window between about 0.22 and , minimum loss as low as , low effective phonon energy less than , and high allowable doping levels up to of rare-earth ions . These properties should make ZBLAN glass a promising candidate as a host medium for a solar-pumped laser. In this paper, we present the optical properties of -doped ZBLAN glass required for the design of a solar-pumped laser. In particular, we focus on the QE of near-infrared emission under sunlight excitation and discuss the dependence of the QE using the Sun and monochromatic light sources on concentration.
Glass compositions prepared in this study were (, 0.05, 0.1, 0.2, 0.5, 1.0, 3.0, 5.0), in mole percent. The raw materials were mixed thoroughly in an alumina mortar and then melted in an electric furnace at for with an atmosphere in platinum crucibles. Subsequently, the glass melts were quenched by pouring them onto stainless steel plates preheated to . Each glass was then annealed at for . The glasses were cut to a thickness of around and then polished to optical quality to optical measurements.
The density of the glass was measured by the Archimedes method.
Optical absorption spectra were recorded by a double-beam monochromator (PerkinElmer, Lambda 900) over a range of .
The refractive indices were measured by a prism coupler (Metricon, 2010) at the wavelengths of 632.8, 974, 1320 and .
Judd–Ofelt (JO) analysis [15, 16] was carried out to calculate the radiative lifetime and the stimulated emission cross section for ions. The parameters used in the JO analysis were taken from Refs. [17, 18].
The emission lifetime () of the state of was estimated from the e-folding time of a emission decay curve. Emission from the sample excited by a Ti:sapphire laser (Coherent, 890) was dispersed by a single monochromator with a diffraction grating blazed at and detected by a near-infrared photomultiplier tube (Hamamatsu Photonics, H9170-75). The decay curve of the emission was obtained by a digital oscilloscope (Yokogawa, DL-1620).
The procedure of QE measurement using an integrating sphere was the in same manner described elsewhere . A schematic figure of the QE measurement system is shown in Fig. 1. Sunlight was focused onto the tip of an optical fiber bundle by a lens with a focal length of and a diameter of . They were fixed on an altazimuth mount with an automatic Sun tracking system (Vixen, SKYPOD) on a tripod. Two different photonic multichannel analyzers (PMAs) were used sequentially as spectrometers and detectors for the QE measurements. One is a visible PMA (Hamamatsu, C9220-02) covering the wavelength range of with a resolution of , and the other is a near-infrared PMA (Ohtsuka Denshi, Photal MCPD-5000) covering the wavelength range of with a resolution of . The spectral sensitivities of these PMAs were calibrated with standard light sources by the manufacturers. The QE measurement for one sample was completed within . It was confirmed that the spectral intensity and the shape of sunlight was unchanged during a series of the QE measurements. All QE measurements using sunlight were performed between 11 o’clock in the morning and 2 o’clock in the afternoon on bright and clear days.
The QE measurements under monochromatic wavelength excitation was also done using a xenon lamp and a Ti:sapphire laser as excitation sources in the excitation wavelength ranges of and , respectively.
2B. Analysis of Concentration Quenching Parameters
The decreasing QE as concentration increases is found to be accurately described by the decay function20, 21]. According to the Förster model , γ is given by 1) describes cross-relaxation processes enhanced by energy migration. In order to explain energy migration between donors, Burshtein introduced the hopping model , in which the excited-state population redistributes randomly over the donors at a characteristic hopping rate given by 22] 1) as  21, 24]25] for -doped ZBLAN glass. Thus is treated as the only fitting parameter in the fitting of Eq. (5) to the experimental results.
3A. Absorption and Emission Cross Sections and Emission Lifetime
The density was for undoped glass and linearly increased with x to for . The refractive indices were 1.500, 1.494, 1.492, and 1.490 for 632.8, 974, 1330, and , respectively. The JO parameters obtained from the integrated absorption intensities, density, and refractive indices were , , and . The JO parameters of ZBLAN glass were smaller than those of high-index glasses, such as lead bismuth gallate and chalcogenide , but were comparable with those of the other fluoride glasses .
Figure 2 shows the absorption and stimulated emission cross-section spectra of doped in ZBLAN glass. The absorption cross section was calculated by
The integrated absorption in the region has been used a measure of the absorption strengths, which is related to absorption efficiency for broadband exci tation sources . The integration absorptions relative to of ED-2 (or LG670), a high-gain lithium calcium aluminosilicate glass , are shown in Table 1. The relative integrated absorption of ZBLAN in the present work was 0.91, which is slightly lower than those of silicate (ED-2) and phosphate (LG750) glasses but larger than those of the other fluoride glasses, such as (ZBN) and (ZBL). This means that -doped ZBLAN can absorb sunlight most effectively among these -doped fluoride glasses.
The stimulated emission cross section of the transition was at , which is about twice as large as that of silica glass and almost similar to those of the other fluoride glasses, such as ZBN and ZBL, as shown in Table 1. The stimulated emission cross sections of the and for ZBLAN were at and at .
Figure 3 shows the emission lifetime of the state of doped in ZBLAN glass. The emission lifetime was about at low x and decreased at more than , showing the occurrence of concentration quenching in the high x regime.
The product of the stimulated emission cross section and the radiative lifetime () of a laser transition is recognized as a figure of merit of the laser transition, because is proportional to the slope efficiency and inversely proportional to the threshold pump power of a laser. of the transition of for several glasses are tabulated in Table 1. for ZBLAN was , which is more than twice as large as that for silica glass and comparable to that for LG750. This indicates that ZBLAN glass is one of the most preferable glass materials for -doped lasers.
3B. Quantum Efficiencies
Figure 4 shows (a) the spectral intensities from the integrating sphere with a fluoride glass sample of and without the sample under sunlight irradiation and (b) the difference spectrum. The sharp absorption and emission lines due to can be seen in Fig. 4b. Though broad absorption due to the host (including defects and impurities) was found for the absorption spectrum of -doped borosilicate glass , such broad absorption seems negligibly small for ZBLAN glass.
Figure 5 shows the radiative QE () of the state of and the internal quantum efficiencies under excitation () and under sunlight ().
was about in the low x regime and decreased with x due to the concentration quenching for . The inconsistency that exceeds 100% (that is, ) for the low x regime is most probably due to the intrinsic inaccuracy of in JO analysis  and the low nonradiative transition rate due to the low phonon energy of ZBLAN glass (). Similar inconsistencies were found also for several glasses [21, 26, 27]. Thus is thought to be for the low x regime.
was as high as 88% in the low x regime, which is comparable to of the QE of Czochralski grown crystal for excitation  but is lower than 94%  obtained by the thermal lens methods for -doped ZBLAN glass. decreased with increase in x to 25% at due to concentration quenching.
reached a maximum of 70% at . This high shows that ZBLAN glass is very attractive for solar-pumped laser applications. For the higher x, decreased with increase in x due to concentration quenching as well as and . In contrast, decreased with the decrease in x in the low x regime, which was not found for and .
4A. Effects of Nonradiative Transition and Ion–Ion Interactions on Quantum Efficiency
The nonradiative relaxation processes of excited rare-earth ions are usually attributed to multiphonon emission relaxation, concentration quenching through energy migration between rare-earth ions and the energy transfer to hydroxyl () groups.
The nonradiative relaxation rate via multiphonon emission is given by 34], and . The glass was prepared in dry atmosphere, and any evidence of the presence of in the samples was not observed in the absorption spectrum in the infrared region. This indicates that the rate of nonradiative energy transfer to groups () is negligibly small. As shown in Table 1, was , which corresponds to . Thus, the QE defined by is expected to be in the low x regime in which the concentration quenching does not occur.
The concentration quenching in the high x regime was evaluated by fitting of Eq. (5) to the measured radiative QE. was fixed at 0 during the fitting because the nonradiative transitions due to the multiphonon emission and the energy transfer to groups were negligible as mentioned above. The best fitting was obtained as from a pa rameter set of , , (Table 2). validates the applications of Burshtein’s hopping model to doped in ZBLAN glass. and of ZBLAN glass in the present work were comparable to those of the previously reported ZBLAN glass  and phosphate  glasses within the experimental errors.
4B. Effects of Absorptions by Other Than on Quantum Efficiency under Sunlight Excitation
The internal QE () of photoluminescence is given by
The absorption efficiency () is given by
The relaxation efficiency () is the efficiency that the excited relaxes to the initial state of emission () and is given by35].
The escaping efficiency () is the efficiency that the photons emitted by excited escape out of the sample. decreases by reabsorption by ions and glass host and thus a function of emission wavelength.
We can discuss and using Eq. (9), because they are internal QEs for different excitation wavelengths. , , and do not exceed 100% by definition. Thus and are always lower than . can be assumed to 100% for near-infrared emission from as mentioned above. Both and terms in Eq. (9) are common to and . Thus, the difference between and arises from the dependence of the remaining term on wavelength. It is thought that is nearly 100% at the wavelengths of the absorption peak of and is considerably lower at other wavelengths. Consequently, is always lower than for the same host and for the same concentration (x).
The dependences of and on the concentration can be understood from the dependence of on the concentration and concentration quenching. As the concentration increases, in Eq. (10) increases although is constant. Thus, increases with increasing of the concentration. On the other hand, decreases with increasing of the concentration by concentration quenching. Because for excitation, the term in would be almost 100% even for lower concentrations, and decreases monotonically with increasing in the concentration. In contrast, as a consequence that is determined by the product of and , gives a maximum at a specific concentration ( in this case).
There is other evidence that the absorption of host affects QE. Figure 6 shows the excitation wavelength dependence of QEs of near-infrared emission from state of doped in ZBLAN glass of . The curve in Fig. 6 is the absorption spectrum of the glass. Apparently, the positive correlation with the QE and the absorption coefficient can be seen. That is, the QE at an absorption peak wavelength of is higher than that at around the peak. Thus, in principle, the QE obtained using a broadband excitation source, such as sunlight, becomes lower than that obtained using a monochromatic light source at the absorption peak wavelength of .
Optical properties required for the design of a solar-pumped laser were evaluated for -doped ZBLAN glass. The QE of the near-infrared emission from the state of -doped ZBLAN glass was directly measured by an integrating sphere method using sunlight as an excitation source was as high as 70%. The stimulated emission cross section of the emission was , and the radiative lifetime obtained by JO analysis was . Thus, their product is , which is the largest among those of doped in glass media. The integrated absorption strength in the region of -doped ZBLAN was the largest among -doped fluoride glasses. These optical properties show that -doped ZBLAN glass is one of the most promising gain media for solar-pumped lasers. It was revealed that the QE decreased more remarkable for sunlight excitation than for monochromatic excitations at the wavelength of a absorption peak. The reduction in QE under sunlight is mainly due to optical loss by the host glass (including defects and impurities) in the case of - doped ZBLAN glass. This fact would be a clue to design more efficient glass host for solar-pumped lasers.
This work was supported by the Japan Society for the Promotion of Science as a part of the Grant-in-Aid for Scientific Research for Young Scientists (B) (20760039) and by the Ministry of Education, Culture, Sports, Science, and Tech nology as part of the Private University High-Tech Research Center Program (2011–2015).
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