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We report the realization of a solar-pumped fiber laser (SPFL) using a double-clad (a center core/ an inner clad working also as optical waveguide/ an outer clad) Nd-doped fluoride optical fiber as a laser medium. With a compact off-axis parabolic mirror of 5 cm in aperture diameter, the natural sunlight is concentrated by a factor 104, and introduced partly into the core of the fiber and partly into the inner clad in which the light is guided in some distance and transferred to the core after all. We have obtained clear laser spectrum characteristics with approximately 0.01 nm full-width-half-maximum of the laser line at the peak wavelength of 1053.7 nm, a low-lasing threshold of 49.1 mW, a slope efficiency of 6.6%, and a total efficiency of 1.76%. Further optimization of the medium properties, optical cavity, and concentration technique will yield higher efficiency and lower threshold.
©2012 Optical Society of America
1. Introduction
A solar-pumped laser (SPL) is an optical device that converts incoherent sunlight with a wide spread of spectrum and low areal energy density into a monochromatic coherent light beam. SPLs are expected to be promising core devices in such technologies as satellite communications, space debris management, sunlight energy transmission from space, and energy cycling by converting sunlight to chemical energy [1–3]. After the first reports on solar-pumped laser based on a Dy2+:CaF2 crystal system [4–10] in 1963, several improvements have been reported. More recently, for the lasing medium, transparent co-doped (Nd,Cr) YAG ceramics have been popularly investigated where Cr3+ sensitises Nd3+ ion emissions. A conversion efficiency from solar energy after corrector to laser energy was reported as 4.3% with a 9% slope efficiency, which are the best efficiencies obtained so far using (Cr3+,Nd3+):YAG ceramics [9]. Laser efficiency, cooling systems, and beam quality, however, still remain challenging issues. A solar-pumped fiber laser has been proposed [10] in 1997. After that one pumped by halogen lamp has been demonstrated [11], but not by natural sunlight. There are substantial differences between natural and simulated-sunlight in spectra, intensities, and concentration techniques. By overcoming these differences, we addressed the ultimate objective, that is, realization of fiber lasing under natural sunlight pumping. In this paper, we report the realization of a solar-pumped fiber laser (SPFL) operating under natural sunlight using a Nd-doped fluoride optical fiber.
2. Solar-pumped fiber laser system setup
Figure 1 shows a schematic diagram of the solar-pumped fiber laser system investigated. Intense sunlight is necessary because laser properties, such as slope efficiency and lasing threshold, are strongly dependent on excitation power density achieved in the laser medium rather than total amount of pump power. To increase power densities, the concentration ratio C of an optical concentrator must be increased. For sunlight, we have
where R and T are the respective reflectance and transmittance of the concentration optics, sinθf is the numerical aperture (NA) of the optical concentrator, θsun is the half view angle of the sun from earth, and sinθsun depends on the solar radius and the average distance between earth and sun. For solar light, C is determined by NA and R or T of the focusing optics. We selected an off-axis parabolic mirror because it is free from chromatic aberration and provides a relatively high NA with high reflectivity in the visible range. With NA of 0.5 and R of 0.92, C is 10626. A diameter of the parabolic mirror is 50 mm.
Fig. 1 Schematic set-up of the solar-pumped fiber laser experiment. L, lens; LPF, long-pass filter; MMF, multi-mode fiber; OAPM, off-axis parabolic mirror; PM, power meter; SA, spectrum analyzer; ZBLAN-DCF, ZBLAN double-clad fiber.
The accompanying temperature increase in the laser medium as C increases produces changes in lasing properties. Generally, beam quality and laser efficiency deteriorate; at worst thermal stress destroys the lasing medium. Heat has its origins in (1) the quantum defect, i.e., the energy difference between excitation photon and lasing photon, and (2) absorption losses in the laser medium. Whereas (2) can be reduced by improving the material, (1) cannot be reduced in principle. The lasing wavelength λL and energy conversion efficiency η are related as follows:
where λs and λe are the shortest and longest wavelengths representing the absorption range of the medium, respectively. For a Nd3+-doped Y3Al5O12 crystal, often used as the medium in solar-pumped rod lasers, the theoretical upper limit for solar energy conversion efficiency at a wavelength of 1.06 μm is calculated to be 48%. The remaining energy generates heat within the medium, which needs cooling so as not to cause a decrease in a figure of merit of the laser medium which is the product of the excited state lifetime and the stimulated emission cross section of active ion. To date, there are no reports of a continuous wave solar-pumped laser without an active cooling system. Efficient cooling under a constant volume is brought about by sufficient specific surface area. The efficient cooling can be realized by either disk-like or fiber-like shape of medium. Given the shape, we need to consider the absorbance of the medium over a wide range of wavelengths. A fiber-type is able to absorb almost all excitation light propagating along its axial direction because of the long optical path. In contrast, a disk-type medium cannot absorb same amount of excitation sunlight because of its extremely short interaction length. For this reason, a fiber-type medium is more suitable for our purposes.Directing all excitation light into the core is challenging because its size for a single-mode fiber is smaller than the smallest solar image capable of being provided by typical optical mirrors or lenses. A diameter of the sun image at the focal point using an off-axis parabolic mirror is about 0.5 mm. The double-clad structure of core, inner cladding, and outer cladding for a SPFL is considered adequate to absorb excitation sunlight efficiently, where the active ions in the core absorbs the light propagating in the inner cladding. Almost all sunlight captured ultimately contributes to the activation of core ions. Overall, the fiber-type shape with the double-clad structure gives a medium configuration satisfactory for use in the solar-pumped laser.
Glass materials subject to sunlight excitation have not been studied before. We reported that Nd-doped ZBLAN glass showed high quantum efficiencies for emission [12, 13]. Using the Judd-Ofelt analysis, the stimulated emission cross-section σ in the transition 4F3/2 → 4I11/2 for Nd3+ doped in ZBLAN glass was calculated to be σ = 2.95 × 10−20 cm2 for glass with a dopant concentration 0.5 mol. %. With a fluorescence lifetime τ = 496 μsec for glass, the quantity important to lasing and inversely proportional to the lasing threshold takes the value στ = 1.45 × 10−23cm2sec, which is much higher than στ = 0.75 x10−23cm2sec for Nd-doped silica glass co-doped with Al [14]. We constructed the SPFL resonator from Nd-doped ZBLAN double-clad fiber with a mode field diameter about 5 μm, an inner cladding diameter 125 μm and a cladding numerical aperture 0.5, and reflecting mirrors of 98.0% at 1050 nm.
3. Results and discussion
Lasing experiments by sunlight were performed at longitude 137°3′E and latitude 35°10′8”N under clear skies with the occasional cumuli. Figure 2(a) shows the spectra of sunlight before and after passing through the Nd-doped ZBLAN fiber. Direct solar radiation spectra are calculated by Bird’s model [15]. A calculation condition is as shown below. (1) elevation angle is 37°, (2) aerosol optical depth at the wavelength of 500 nm is 0.27, (3) total column ozone is 0.34 cm, (4) surface albedo is 0.20, (5) total perceptible vapor water is 1.42 cm, (6) 137° 3′ east in longitude, and (7) 35° 10′ north in latitude. (1)-(4) are followed standard condition values of ISO and JIS and (5)-(7) are local values.
Fig. 2 Solar-pumped fiber laser spectra (a) the spectra of sunlight before and after passing through the Nd-doped ZBLAN fiber (b) the lasing spectra obtained at different times
Absorption bands of the Nd3+ dopant are observed. Sunlight was completely absorbed around the bands at wavelengths of 520, 575, 740, 795 and 867 nm. In contrast, we observed strong intensity transmissions of blue light with a wavelength from 410 to 510 nm, red light from 590 nm to 725 nm, and near-infrared light from 890 nm up to the lasing wavelength. The absorbed sunlight power was 56%. Detailed in Fig. 2(b) are lasing spectra obtained at different times showing complex, congested features that vary considerably, partly because of inhomogeneous broadening from Nd ions in glass and longitudinal-mode hopping. All spectra obtained show a peak around the 1053 nm wavelength; many laser line peaks between 1052 and 1054 nm are observed. In one example, the full-width-half-maximum of the laser line at the peak wavelength of 1053.7 nm is approximately 0.01 nm.
Figure 3 plots SPFL output power versus input sunlight power captured in the inner cladding of the fiber. The input sunlight power is controlled by changing the opening ratio of a slit located in front of the parabolic mirror. A clear lasing threshold at 49.1 mW is confirmed by natural sunlight excitation. The total efficiency was 0.88%. The maximum laser output power was 0.57 mW. By fabricating a bundled structure of fibers, a higher output power will be achieved. The slope efficiency of a single side output above threshold was 3.3%. Because the reflectivity of both ends of the fiber is the same, the output power from the other side is estimated to be equivalent, thereby doubling the slope efficiency and total efficiency to 6.6% and to 1.76%, respectively. The ratio E of the sunlight power within the wavelength range of the Nd absorption bands to that over the range 350 nm to 4000 nm is expressed as
where λL is the lasing wavelength of 1.05 μm, I(λ) is the spectral intensity per unit wavelength, and ‘a’ equals 1 for wavelengths in the Nd absorption bands, otherwise 0. The air mass 1.5 (AM 1.5) spectrum and the absorption spectrum of Nd-doped ZBLAN glass yield E of about 44%. The theoretical limit of the slope efficiency, i.e., the theoretical energy conversion efficiency including quantum defects from the parts of AM 1.5 spectrum corresponding to Nd absorption bands to the lasing wavelength of 1.05 μm, was calculated as 14.0%. The reasons for the lower value of slope efficiency compared to the theoretical one are (1) high mirror reflectivity, i.e., a laser light confinement that is too high, (2) low absorption efficiency of excitation sunlight, and (3) losses in the laser cavity by the mirrors and fiber.In conclusion, we have demonstrated the operation of a solar-pumped fiber laser (SPFL) using highly concentrated natural sunlight and a Nd-doped fluoride optical fiber. With threshold of 49.1 mW, we estimated the total slope efficiency at 6.6% and the total efficiency at 1.76%. The realization of SPFL using double-clad single mode fiber overcame challenges concerning beam quality, cooling, and energy conversion efficiency. Moreover, this opens the way for compact solar-pumped lasers that contrasts with those large-scale and high-power systems previously reported. Further research for sensitisers besides Nd active over the absorption bands will lead to an improved use of sunlight. Optimization of the laser medium properties, optical cavity and concentration technique will lead to higher efficiencies and lower thresholds as well.
Acknowledgments
This work was supported in part by the Japanese Ministry of Culture, Sports, and Education (MEXT) Support Program for Forming Strategic Research Infrastructure (2011–2015).
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