We demonstrated continuous wave laser operation with a tunable range of over 300 nm in the visible and near-infrared regions (479–497, 515–548, 597–737, 849-960 nm) using a single 9-cm Pr3+-doped ZBLAN fiber pumped by a GaN laser. The total tunable range was 6469 cm−1, which is wider than that of conventional Ti:sapphire lasers.
©2009 Optical Society of America
Visible and near-infrared (NIR) light sources can be widely used for many applications, such as laser TVs, laser projectors, optical diagnostic systems, and bio-medical laser microscopes. An ideal light source for these applications is a compact wide-band tunable laser that covers the visible to NIR regions. Optical parametric oscillators and super-continuum lasers, for example, can cover the wavelength range; however, these systems are large, complicated, and expensive.
As it shows many emission bands in these wavelength ranges , we have focused on Pr3+-doped ZBLAN fiber (PDF) as a candidate for a wide-band tunable laser medium. The fluorescence spectrum, ground state absorption (GSA) spectrum, and excited state absorption (ESA) spectrum from 1G4 of Pr3+-doped ZBLAN glass are shown in Fig. 1 [2,3]. Although fewer absorption bands occur around the 500–550 and 600–750 nm regions, seamless tunable laser operation has not been obtained over these bands [4–6]. We assume that the primary reason for the restricted tunable range was an excess cavity loss due to high background loss caused by redundant long fluoride fibers, for example, 0.70 dB/m at 450 nm and 0.35 dB/m at 635 nm.
In addition, GaN blue laser diodes (LDs) have recently been considered as a next-generation pumping source for Pr3+-doped solidstate lasers [7–11]. Because of the high absorption cross section of Pr3+ ions around 442 nm, PDF can perfectly absorb a blue pump laser even at less than 10 cm. Thus, we can use a short fiber for the laser to suppress background loss.
In this study, we set up a low-loss tunable laser cavity to achieve continuous wave (CW) wide-band tunable laser operation and clarify the maximum tunable range of a GaN-LD-pumped PDF laser from the visible to the NIR.
2. Experimental setup
We prepared a PDF with Pr3+ concentration of 3000 wt-ppm having a core diameter of 3.8–3.9 µm, cladding diameter of 125 µm, and numerical aperture of 0.22. The molar-percentage composition of the core glass is: ZrF4(53.00)–BaF2(22.00)–LaF3(2.19)–YF3(2.00)–AlF3(3.50) –NaF(17.00)–PrF3(0.31), abbreviated to Pr:ZBLAN. We used a transverse multimode 448-nm GaN LD (Nichia NDB7112E) as a pump source. The absorption coefficient of the PDF was 0.20 cm−1 at 448 nm.
Figure 2 shows the experimental setup of the tunable fiber laser system. The pump coupling optics was constructed of two aspherical lenses and a cylindrical lens pair. We prepared three dichroic mirrors at the pump side for blue–green, orange–“deep-red,” and NIR laser operation: 448 nm AR/470–570 nm HR, 448 nm AR/520–735 nm HR, and 448 nm AR/740–1000 nm HR, respectively. The opposite end of the fiber was cleaved at 10° to suppress Fresnel reflection less than −50 dB. We also prepared two aspherical lenses (NA = 0.53) with different AR coatings (400–600 nm, 600–1000 nm) between the angle-cleaved fiber end and the prism. The emission from the PDF was dispersed by a prism with a MgF2-AR coating. The laser oscillation wavelength was selected by rotating a broadband HR mirror. The round-trip prism reflection loss, which was averaged by two polarizations, was 0.8 dB from 448 to 974 nm. Therefore the corresponding output coupling ratio was 17%. The laser wavelength was measured using an ANDO AQ6315A optical spectrum analyzer (OSA) by monitoring one of the surface reflection of the prism.
3. Results and discussion
First, we estimated the pump-coupling efficiency of an undoped ZBLAN fiber having the same fiber parameters as the PDF. The measured maximum coupled pump power into the un-doped fiber core was 252 mW, while the incident pump power to the fiber was 460 mW. The surface flatness and the angle of the cleaved fiber ends were confirmed for each cleaving to achieve a low-loss cavity, since the pump coupling efficiency and cavity loss depended heavily on the condition of the fiber ends. In addition, the quality of the surface and AR coating of the aspherical lens used in the laser cavity also greatly affected the width of the laser tunable range. We used an aspherical lens of 40-scratch/20-dig grade (MIL-O-13830A specification) in the laser cavity.
When we pumped the PDF at maximum pump power, tunable laser oscillation over a 300-nm range was achieved from the 9-cm PDF. Figure 3 shows the seamless tunability of the PDF laser (drawn with a 5-nm interval) within the emission band of Pr3+-doped ZBLAN fiber. The tunable ranges and the corresponding transitions are shown in Fig. 4 and Table 1 . The total tunable range of the PDF laser was wider than that of conventional Ti:sapphire lasers (650–1100 nm) in the wavenumber. This is the first report of seamless tunable laser operation for transitions from multiple upper levels (3P0,1) to multiple lower levels (3H4,5,6, 3F2,3,4, 1G4) of a PDF laser. For the sake of simplicity, 3P1 has been substituted for (3P1 + 1I6) in this letter .
The regions around 477, 588, and 1016 nm, in which laser oscillation were not observed, correspond to GSA: 3H4 → 3P0, 3H4 → 1D2, and 3H4 → 1G4, respectively (Fig. 1). The region around 830 nm corresponds to ESA, 1G4 → 3P0,1,2 (Fig. 1). Therefore, laser operation over these absorption bands is considered to be impossible. We could not obtain tunable laser operation from 497 nm to 515 nm, although neither GSA nor ESA exists there. We suppose that the gain was smaller than the cavity loss over the wavelength range. These tunable wavelength ranges will be widened if we achieve lower cavity loss, for example, if the cavity mirror on the pump side is directly coated on the fiber end or the aspherical lens is replaced with one of higher surface quality.
Although it seemed that the transitions from 1D2 might contribute to widening the tunable range in the NIR region (3P0 → 1D2 → 3H5, 6, 3F2), we disproved that possibility as follows. Figure 5 shows the fluorescence spectrum of Pr:ZBLAN under excitation at 580 nm (3H4 → 1D2), measured by fluorescence spectrometer FP-6500 (JASCO) and PMA-11 (Hamamatsu). The largest emission was observed at 595 nm, but other emission corresponding to the transitions from 1D2 (686, 795, 848 nm) were small enough to be negligible.
The maximum output powers and slope efficiencies are summarized in Fig. 6 and Table 2 . The horizontal axis in Fig. 6 indicates the absorbed pump power estimated from the coupled pump power and residual pump power. Slope efficiencies of more than 29% were obtained except at 907 nm.
The threshold pump powers for 488 nm and 521 nm were larger than that of other wavelengths. In the case of 488 nm, GSA (3H4 → 3P0) makes the threshold large. In the case of 521 nm, the population of 3P1 was reduced to below that of level 3P0 (Δ600 cm−1) by multi-phonon relaxation. Therefore, it is difficult to create a population inversion between 3P1 and 3H5.
The wide-band tunability of the PDF laser is also applicable to short pulse generation. Although the chromatic dispersion of ZBLAN fiber was −240 ps/km/nm at 635 nm, it is possible to compensate for the dispersion (−0.048 ps/nm) in the case of a very short fiber (10 cm).
We demonstrated CW tunable PDF laser operation over a 300-nm range (479–497, 515–548, 597–737, and 849–960 nm) pumped by a single GaN LD from a single 9-cm PDF. The total tunable range is 6469 cm−1, which was wider than that of conventional Ti:sapphire lasers. The revealed ultra-wide tunability of PDF lasers is useful for laser TVs, biomedical applications, and other uses. Lasers in the cyan–green region are significant for expanding the color reproduction range of laser TVs and laser projections. The wide tunablitiy of PDF lasers also has potential for short pulse generation at visible wavelengths.
References and links
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