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We demonstrated the efficient guided laser action in a diode-pumped YAG/Nd:YAG/YAG ceramic planar waveguide produced by tape casting and vacuum sintering technology for the first time to the best of our knowledge. In the regime of continuous wave operation, a maximum output power of 840 mW corresponding to the slope efficiency of 65% was achieved. During passively Q-switched operation, by replacing the dichroic mirror with graphene-oxide based output coupler, we obtained the stable pulse trains with the shortest pulse duration of 179 ns at a pulse repetition rate of 930 kHz which resulted in the single pulse energy of 221 nJ.
© 2015 Optical Society of America
1. Introduction
Lasers using planar waveguides as the gain media possess the advantages of high gain and high efficiency because of the architecture of waveguides permitting long high-intensity pump-intersection lengths [1–3]. The slab-like geometry of planar waveguide also provides a large cooling-surface-area-to-volume-ratio allowing effective heat dissipation. In contrast to the single crystals, high quality ceramic laser materials are highlighted due to its relatively low manufacturing costs, feasible fabrication process, including large grain size, high available doping concentrations and a much shorter period of fabrication [4–6]. The laser performance of ceramics had been proven to be equal or more superior to that of the traditionally used crystals. Accordingly, combining excellent properties of high-quality ceramics with the waveguide technology presents a promising prospective approach to construct high efficient lasers [7, 8]. Recently, laser emission from Nd:YAG ceramic waveguides using direct femtosecond-laser writing technique or ion implantation/irradiation had been realized [9, 10]. As an established ceramic fabrication technology, the combination of tape casting and vacuum sintering methodology has attracted constant interests in its potential to fabricate the multilayer composite ceramic waveguides in various substrates as a whole, not only permitting modification of the refractive index of a bulk material, also structuring materials with dissimilar refractive index [11, 12].
Compact pulsed lasers are beneficial for applications in several fields including nonlinear frequency conversion [13], telecommunications [14] and material processing [15]. The passively Q-switching operation with saturable absorber (SA) materials is one of the most recognized ways to achieve the compact pulsed laser source. In addition, the waveguide-based lasers also permit the integrated compact designs with stable output and efficient heat removal. In light of the above fact, waveguide configuration together with passively Q-switching operation would be alternative to meet the goal of miniature pulsed laser source. Indeed, passively Q-switched waveguide lasers based on semiconductor saturable absorber mirrors (SESAM), Cr4+:YAG and carbon nano-materials (carbon nanotubes or graphene) as SAs were successfully demonstrated [16–20]. Compared with other SAs, graphene-based SAs have more superior wide-range absorption, ultrafast recovery time, and moderate modulation depth, enabling it a promising candidate in effective passively Q-switched operation [21, 22]. Meanwhile, few-layered grephene-oxide based (GO-based) SAs has also been successfully demonstrated for passively Q-switched and mode-locking operation both in fiber lasers and bulk lasers [23–25] due to its similar properties to graphene in terms of ultrafast carrier dynamics and strong saturable absorption [26, 27]. To date, there had not been reports report on the GO-based passively Q-switched operation of planar waveguide to the best of our knowledge.
Here we presented the first experimental result on laser action in continuous-wave and GO-based Q-switched regimes realized by tape casting technique in a diode-pumped Nd:YAG ceramic planar waveguide. This work elucidated the suitability of tape casting together with vacuum sintering as the alternative fabrication technology for planar waveguide structure.
2. Experimental materials and configuration
Multilayer composite YAG/Nd:YAG/YAG ceramic planar waveguide utilized in the experiment was fabricated by the combination of tape casting process and vacuum sintering method, similar to the one described in [28]. High purity raw materials were processed to obtain slurries, which were then de-aired in a vacuum and cast to form thin tapes. Subsequently, the tapes with single layer thickness of 0.1 mm were cut into pieces then symmetrically stacked and laminated in slab-like geometry. The doping neodymium concentration in the single layer was 2 at.% on the stacking-site. The pores and interfaces between adjacent layers were eliminated from the sample of ceramic waveguide after annealing. All surfaces except the lateral were carefully mirror-polished, resulting in reduction of scattering losses and elimination of detrimental rounding effects. The composite YAG/Nd:YAG/YAG ceramic waveguide was fabricated theoretically with dimensions of 1.5 × 5 × 12.5 mm3, comprising two 0.7 mm thick undoped YAG outer layers and 0.1 mm thick Nd:YAG core with a concentration of 2 at.% Nd3+ by symmetrical arrangement. An index increase resulted from the neodymium doping leads to the guiding behavior.
Figure 1 showed the schematic plot of the laser experiment setup. The commercial multi-mode 808 nm laser diode (LIMO) with core diameter of 200 μm and numerical aperture of 0.22 was used as the pump source for the Nd:YAG ceramic planar waveguide. The pump beam was collimated by a spheric lens with 60 mm effective focal length and launched by an aspheric lens of f = 15 mm, providing a pump spot radius of 25 μm on the front facet of the active layer in ceramic planar waveguide. We constructed a simple plane-parallel quasi-monolithic Fabry-Perot type oscillator for evaluating the laser performance of Nd:YAG ceramic planar waveguide both in continuous-wave and passively Q-switched regimes. The input mirror had a dichroic coating with high transmission at pumping wavelength and high reflectivity (R = 99.99%) at lasing wavelength. In addition, both the input mirror and output coupler mirrors were adhered to the input and output facets of the waveguide without direct contact. We employed a spheric lens to collect laser emission from the waveguide, together with the filter to rule out the residual pump light.
3. Results and discussion
The absorbed pump power was estimated by the delivery pump power multiplied by the absorption coefficient, which is congruous with model described in Ref [29]. Specifically, the delivery pump power was derived from the incident pump power multiplied by the delivery efficiency. In our experimental configuration, the Nd:YAG waveguide was pumped by incident pump beam with a specified full-width half-maximum (FWHM) divergence . The delivery efficiency was calculated based on the following formula
Where applied the condition that the incident pump beam is less than angular acceptance of the waveguide, indicating the overlap of the diverging pump beam with numerical aperture of the YAG/air interface. Furthermore, the absorption coefficient was given with a simple assumption that the pump radiation delivered to the waveguide will be reduced by a factor, equal to the ratio of the doped to undoped area of the YAG layers, compared to uniformly doped material.Three separate output couplers were utilized for evaluating the laser performance of tape casting Nd:YAG ceramic planar waveguide with the transmittance between T = 5.7% and T = 14.1% at lasing wavelength respectively. Figure 2 illustrated the lasing action in continuous-wave regime. By using the output coupler with T = 5.7%, the maximum output power was obtained at the absorbed pump power of 940 mW, corresponding to the slope efficiency of 32% and the laser threshold of 85.8 mW. The best laser performance was achieved with the output coupler of T = 14.1%, giving a maximum output power of 840 mW at the absorbed pump power of 1350 mW. This represented a remarkable increase in laser performance with the slope efficiency of 65%. Its lasing threshold of 166 mW was slightly higher than that of output coupler with T = 5.7% and T = 8.5%. According to the method of Findlay and Clay [30], the roundtrip cavity loss at 1064 nm was estimated to be around 0.11 dB/cm by measuring the threshold versus the output coupling. This loss includes waveguide propagation loss and scattering loss at the air gap between cavity mirrors and the waveguide endface.
Figure 3 showed the obtained laser beam with near-filed intensity distribution (which was measured by Spiricon&Photon Laser Beam Profiler III.), illustrating mainly the multimode character. The spot diameters in the guided and un-guided directions were found to be 450 and 1500 μm, respectively. We attributed the relatively large near-field mode profile to the fact that Nd3+ ions diffused into the undoped substrate during the sintering process and could be evidence of the detection of ICP-MS [12].
In the Q-switching regime, a GO-based output coupler was utilized as SA to replace the dichroic mirror. The GO used in our experiment was synthesized though the oxidation and exfoliation of natural flake graphite with concentrated sulfuric acid and potassium permanganate. The red and green triangles in the right part of Fig. 4 presented the corresponding detected locations in the left part of Fig. 4, and according to the measurement result of atomic force microscopy (AFM), the thickness of the as-synthesized GO was around 1 nm which corresponding to the distribution of GO sheets in 1 to 3 layers.
The TEM image of monolayer GO sheets was shown in Fig. 5a, which was further confirmed by the selected electron diffraction (Fig. 5b) due to the fact that the intensity ratio of /﹥1 is a characteristic feature for monolayer graphene [31]. The transparent GO thin film was then fabricated by using a vacuum filtration method, similar to the previous report [32]. The GO-based SAs was fabricated by transferring these few-layered GO sheets onto the output coupler. With the GO against the surface of the output coupler, the resultant transmission of GO-based SAs was 12.5% at 1064 nm, as measured by the spectrophotometer. The modulation depth of our graphene oxide was ~2% and the nonsaturable loss was ~1% due to the weak scattering effects from large size graphene sheets.
Fig. 5 (a), (b) TEM images and the selected electron diffraction of monolayer GO sheets, respectively.
The linear section of the average output power versus absorber power plot depicted a slope efficiency of 41% for GO-based passively Q-switched operation as shown in Fig. 6. The lasing threshold was around 110 mW with the maximum output power of up to 265mW, corresponding to the absorbed power of 763mW. In comparison with the continuous-wave operation, a relatively lower average output power was achieved in this case, due to the nonsaturable loss of the GO-based SAs.
Fig. 6 Laser performance of Nd:YAG ceramic planar waveguide in GO-based passively Q-switched regime.
Figure 7 illustrated the typical Q-switched single-pulse under the absorbed pump power of 581 mW, with large-scale pulse train inserted. The temporal pulse profiles were studied with a fast photodiode and 1 GHz oscilloscope (Agilent DSO6102A).In this case, the laser operated at a repetition rate of 930 kHz with single-pulse energy of 221 nJ, and the pulse duration of 179 ns. It is worth noting that the stable Q-switched pulse trains were not achieved until the absorbed power up to 221 mW, below which the unstable Q-switching signal was observed. This is mainly due to the incomplete saturation of GO under low intra-cavity power.
The dependences of the repetition rate and the pulse duration on the absorbed pump power were shown in Fig. 8. The repetition rate was observed to fluctuate from 427 kHz to 930 kHz with the pulse duration decreasing from 540 ns to 179 ns upon increasing the absorbed pump power from 227 mW to 581 mW. Considering the low modulation depth associated with the quasi-monolayer of the GO, the relatively broad pulse was obtained. The high repetition rate may originate from the ultrafast recovery time of GO. Subsequently, the varying trend of pulse repetition rate is consistent with the passively Q-switched lasers analysis, as the repetition rate is proportional to intra-cavity intensity [33].Stable Q-switched pulses were observed over the whole experiment once the absorbed pump power above 221 mW, indicating the good quality of the GO and its resistance to damage.
We used a high resolution fiber-coupled optical spectrometer (Ocean Optics HR4000) to examine the lasing spectrum centered at 1064 nm both in continuous-wave and passively Q-switched regimes displayed in Fig. 9.No evident variation of output wavelength was observed.
The polarization property of output laser was analyzed by using a Glan-Laser prism in combination with a power meter. Experimental results turn out that it made no significant difference in individual polarization azimuth direction, which corresponding to the unpolarized or without well-determined polarization states.
4. Summary
In summary, the tape casting and vacuum sintering technique were applied in the fabrication of the Nd:YAG ceramic planar waveguide. Maximum continuous-wave output power of 840 mW was obtained with respect to the slope efficiency of 65%. Further performance optimization can be expected with the use of monolithic cavity configuration together with the index matching gel minimizing the Fresnel reflection. During the passively Q-switched operation with the GO as SA, a maximum output power of 265 mW with a slope efficiency of 41% was obtained. The shortest pulses duration of 179 ns with single-pulse energy of 221 nJ were achieved under the absorbed power of 581 mW. The compact, simplistic design and excellent thermal properties of this GO-based passively Q-switched tape casting Nd:YAG ceramic planar waveguide make it a promising pulse laser source of integrated devices with the potential for on-chip optical applications.
Acknowledgments
This work had been supported by the National Natural Science Foundation of China (Grant No.11404332 and No.61475067), the National High-Tech R&D Program of China (Grant No.2013AA014202). We would also like to thank for the financial aid of the natural science foundation of Fujian province (2013J05106), Fundamental Research Funds for the Central Universities and the Research Funds of Renmin University of China (No. 2014030192).
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