We demonstrated a laser-diode (LD) end-pumped continuous-wave (CW) tri-wavelength Nd:YAG ceramic laser operating at 1064, 1319 and 1338 nm. For the 1064 nm laser, one of the Nd:YAG polished end faces was used as the output coupler. As references, Nd:YVO4 and Nd:YAG crystal lasers were also investigated under the same structure. We found that the maximum output power came from Nd:YAG ceramic which was 1.74 W, corresponding an optical conversion efficiency of 16.3%. Using a three mirror cavity, we realized efficient multi-wavelength operation at 4F3/2—4F11/2 and 4F3/2—4F13/2 transitions for Nd:YAG ceramic, simultaneously. The maximum output power was 3.2 W, which included 1064, 1319, and 1338 nm three wavelength, and the optical conversion efficiency was 30%.
©2010 Optical Society of America
Solid state lasers emitting multiple wavelengths simultaneously have many applications in nonlinear optical frequency conversion, precision spectral analysis, medical instrumentation, and THz frequency generation, etc. Nd3+-ion doped host materials are popularly used in such lasers because of their high gain, good thermal and mechanical properties. For Nd3+ ions there are three main transitions, 4F3/2-4F9/2, 4F3/2-4F11/2, and 4F3/2-4F13/2, corresponding laser emitting at around 0.9, 1.1, and 1.3 μm, respectively. By proper choosing cavity parameters, simultaneous multi-wavelength operation can be achieved. In recent years, continuous-wave dual-wavelength lasers for different transitions have been reported for some crystals such as Nd:YAG, Nd:YVO4, Nd:GdVO4, Nd:YAP, and Nd:YAB [1–12]. As a new developed laser material, ceramic Nd:YAG has similar thermal, mechanical, and spectral characteristics with crystallized Nd:YAG, and additionally possesses several advantages including large scale production with low cost, large size, high concentration of Nd3+ ions, multiple structures, etc . By the time now, the best known potential application of ceramic Nd:YAG is to make high power lasers as a replacement for the conventional single-crystal Nd:YAG [13–15]. As hopeful THz generation sources, we have demonstrated two types of efficient dual-wavelength operations by ceramic Nd:YAG, one was 1052, 1064 nm output at 4F3/2-4F11/2 transition , and the other was 1319, 1338 nm output at 4F3/2-4F13/2 transition . Beside these, RGB (red, green, and blue) light source for projector and laser television is also one of the possible future applications for ceramic Nd:YAG . To realize this object, multi-wavelength laser operated simultaneously at 1.1 and 1.3 μm might be a good choice, with which the RGB three fundamental colors could be produced by further frequency conversion, as demonstrated by Liao et al . In this paper, we reported, for the first time to our knowledge, simultaneous multi-wavelength operation at two different transitions (4F3/2-4F11/2, 4F3/2-4F13/2) for ceramic Nd:YAG. The maximum output power at 1064, 1319, 1338 nm tri-wavelength was 3.2 W, and the optical conversion efficiency was 30%. Our research exhibited a bright future for ceramic Nd:YAG in producing compact, efficient color display instruments.
2. 1064 nm operation
Comparing with other laser mediums such as Nd:YVO4, Nd:GdVO4, and Nd:YAP crystals, the ratio of the stimulated emission cross-sections between 4F3/2-4F11/2 and 4F3/2-4F13/2 transitions for Nd:YAG crystal is larger (~5). Since Nd:YAG ceramic has similar spectral properties with Nd:YAG crystal, to realize simultaneously emitting at both transitions in Nd:YAG ceramic, it could be anticipated that the 4F3/2-4F11/2 transition should be restrained seriously either, just as the requirement for Nd:YAG crystal. For this purpose, we employed a rare cavity structure for 4F3/2-4F11/2 transition, i.e. 1064 nm emitting. We used one polished end face of Nd:YAG ceramic as the output coupler directly, and no partial reflection film was coated as output mirror. For multi-wavelength operation simultaneously, this structure could restrained 4F3/2-4F11/2 transition effectively by the low reflectivity (R ≈8%) of the output face. For comparison, a Nd:YVO4 crystal and a Nd:YAG crystal were also used under the same conditions.. In 1989, this structure was once attempted for fiber laser : a 7-m-long Er3+-doped fiber yields an output power of 1 mW for an absorbed pump power of 60 mW and a slope efficiency of 6.25%. To the best of our knowledge, the work demonstrated in this paper was the first time that such structure was used on ceramic laser or crystal laser. The compactness, output power, as well as conversion efficiency have been improved greatly because of the larger gain for these laser media, as shown in the following paragraphs.
The laser set-up was shown in Fig. 1 . The pump source was a fiber-coupled laser diode with central wavelength of 808 nm. Through a focusing system, the pump light was delivered into the laser medium with spot radius of 0.1 mm. The input mirror M1 is a plano one with anti-reflection (AR) coated at 808 nm on the pump face, high-reflection (HR) coated at 1064 nm and high-transmission (HT) coated at 808 nm on the other face. We used three laser materials in this experiment: a Nd:YAG ceramic (3 mm × 3 mm × 5 mm) with Nd3+ concentration of 2 at.%, a Nd:YAG crystal (3 mm × 3 mm × 8 mm) with Nd3+ concentration of 1.1 at.%, and a Nd:YVO4 crystal (3 mm × 3 mm × 8.7 mm) with Nd3+ concentration of 1 at.%. Their end faces were polished but not coated. To remove the residual heat, they were wrapped with indium foil and mounted in a water-cooled copper block. The temperature of the cooling water was controlled at 15 °C. The right end face of laser material was served as the output coupler. Considering the refractive index of 1064 nm was 1.8 for Nd:YAG ceramic and Nd:YAG crystal, 2.2 (//c) for Nd:YVO4 crystal, the reflectivity of output coupling face was 8% for Nd:YAG media, and 14% for Nd:YVO4 crystal, respectively. The output power was measured by a power meter (EPM 2000, Molectron Inc.).
The dependence of the output power on the absorbed pump power was shown in Fig. 2 . The best results were come from Nd:YAG ceramic: the output power reached 1.74 W when the pump power was 10.7 W corresponding an optical conversion efficiency of 16.3% and slope efficiency of 25.3%. For the Nd:YAG crystal, the pump threshold (Pth) was 6.4 W, the largest output was 0.51 W at a pump power of 10.6 W with an optical conversion efficiency of 4.8% and a slope efficiency of 12.1%. Benefited from larger stimulated emission cross-section and higher reflectivity of output coupling face, the pump threshold of Nd:YVO4 crystal was the lowest among three laser materials, which was 1.7 W. For Nd:YVO4, when the pump power was 4.8 W, its 1064 nm output reached 0.57 W. The optical conversion efficiency was 11.9%, and slope efficiency was 19.7%. The output became saturate and then the Nd:YVO4 crystal fractured when the pump power was increased further. For the cavity structure that one end face of laser medium was used as the output coupler directly, above experiments have proved that Nd:YAG ceramic was more appropriate than the other two laser materials. Usually, the Nd3+ concentration of Nd:YAG crystal was limited to 1 at.% ~1.5 at.%, to avoid optical quality degradation caused by fluorescence quenching, line broadening, straining, and other harmful effects. For high power laser operation, the Nd3+-ion doping level of Nd:YVO4 crystal was controlled to be less than 1 at.%, because higher concentration would lead the inferiority of thermal loading capacity and increased the fracturing possibility. From comparison, we have seen that Nd:YAG ceramic could preserved perfect optical and thermal properties at relatively higher Nd3+ concentration (2 at.%), and its laser output exhibited large power and high efficiency at the same time.
3. Multi-wavelength operation
Simultaneous CW multi-wavelength operation at 4F3/2—4F11/2 and 4F3/2—4F13/2 transitions was performed for Nd:YAG ceramic. The experimental set-up was shown in Fig. 3 , which was developed from the 1064 nm cavity of Fig. 1. The laser ceramic and the pump source were as the same as those used in 1064 nm single wavelength operation, which had been introduced above. A flat mirror M2 was inserted between M1 and Nd:YAG ceramic, served as the total reflector for 1.3 μm. It was HT coated at 808 nm, 1.06 μm, and HR coated at 1.3 μm. A flat Mirror M3 was placed at the right side of Nd:YAG ceramic, served as the output coupler for 1.3 μm. It was HT coated at 1.06 μm, and partial-reflection (PR) coated at 1.3 μm (T = firstname.lastname@example.org μm). In this way, a hybrid cavity was formed by three mirrors and one end face of Nd:YAG ceramic: 1.06 μm laser at 4F3/2—4F11/2 transition oscillated between M1 and right end face of Nd:YAG ceramic, and 1.3 μm laser at 4F3/2—4F13/2 transition oscillated between M2 and M3. The total length of this cavity was about 5 cm. It was a pity that we haven’t a mirror which was HT coated at 808nm and HR coated at 1.06 μm, 1.3 μm dual-wavebands. In that condition, it would replace M1, M2 and the laser cavity could be simplified to a more compact style, i.e. a two-mirror structure. To evaluate the composition of laser output, we deposed a filter between M3 and the power meter, which was HR coated at 1.3 μm and PR coated at 1.06 μm (T = 88%). At each pump level, we inserted the filter in the optical path and then removed it away, by analyzing the reading difference on the power meter we could determine the ratio between 1.06 μm and 1.3 μm. When the filter was inserted into the optical path, it was slightly tilting to the optical axis to avoid additionally disturbing to the hybrid laser cavity.
The total and respective output powers versus the absorbed pump power were shown in Fig. 4 . When the pump power increased to 0.6 W, 1.3 μm laser reached the threshold and oscillated firstly. The pump threshold of 1.06 μm was 4.6 W. Beyond this pump level, 1.3 μm laser became saturate and then decreased slowly, at the same time 1.06 μm component grew gradually. At a pump power of 6.4 W, the output power of 1.3 μm reached the maximum value, 1.54 W, corresponding to an optical conversion efficiency of 24%. At a pump power of 9.7 W, the output powers of 1.3 μm and 1.06 μm became similar, which were 1.25 W and 1.1 W, respectively. When the pump power was 10.7 W, we obtained a maximum multi-wavelength output power of 3.2 W, which contained 1 W 1338 & 1319 nm components and 2.2 W 1064 nm component. The total optical conversion efficiency was 30%. The spectra of output laser were detected by a spectrum analyzer (MS9710C, Anritsu Inc.). Figure 5 exhibited a typical spectrum at a pump power of 8.6 W, which contained three wavelengths, 1064 nm, 1319 nm, and 1338 nm. In order to show the relevant transitions clearly, its energy diagram was presented in the inset of Fig. 3.
Based on the analysis of Chen , the condition that the dual-wavelength oscillated at the same threshold can be given by:
Here R 1.06 and R 1.3 are the reflectivity values at the lasing wavelength of 1.06 μm and 1.3 μm, σ1.06 = 4.6 × 10−19cm2 and σ1.3 = 0.9 × 10−19cm2 are the emission cross-sections at 1.06 μm and 1.3 μm, and L 1.06 and L 1.3 are the roundtrip losses except output coupling for 1.06 μm and 1.3 μm, which can be assumed to be 0.02. Therefore, for the cavity described above where R 1.06 = 8%, R 1.3 is calculated to be 62%, i.e. the transmittance for the output coupler at 1.3 μm is 38%. In our available 1.3 μm output couplers, the largest transmittance is 20% which has been used in above experiment. It is far below the calculated value of 38%, so the thresholds of 1.06 μm and 1.3 μm are quite different, as shown in Fig. 4. For comparison, other output couplers with different transmission at 1.3 μm were also used in this experiment. When the transmittance of M3 became smaller, the threshold for the emission at 1.06 μm turned to be higher. For T = 13.1%, Pth (1.06) = 5.08 W, the largest multi-wavelength output power was 3.1 W. For T = 3.8%, Pth (1.06) = 5.73 W, the largest multi-wavelength output power was 2.85 W. These results indicated that for the special cavity structure of this paper decreasing the transmittance of 1.3 μm could not increase efficiency. In previous works lasing simultaneously at 1.06 μm and 1.3 μm [4, 7,9], output couplers with high reflectivity were preferred to be used on condition that the Eq. (1) was meet. With T = email@example.comμm and T = firstname.lastname@example.orgμm couplers, the total output power and the conversion efficiency were 5.6 W and 28% for Nd:YAG crystal , 3.8 W and 30% for Nd:GdVO4 crystal . With T = email@example.comμm and T = firstname.lastname@example.orgμm couplers, the total output power was 3.6 W and the conversion efficiency was 29% . For 1.06 μm and 1.3 μm components, the thresholds were similar, and the ratio was unchanged generally under their arrangements. By improving our experimental setup to approach their parameters, it is possible to obtain better output performance for 1.06 μm and 1.3 μm operation simultaneously in Nd:YAG ceramic.
For Nd:YAG ceramic, it was a challenge to realize efficient CW multi-wavelength operation at 1.06 and 1.3 μm, because for this material the ratio of the stimulated emission cross-sections between 4F3/2-4I11/2 and 4F3/2-4I13/2 transitions was relatively large (~5), just as for Nd:YAG crystal. To our knowledge, by time now there is no report on this subject. In this paper we attempted a rarely cavity structure for 1064 nm oscillating. We used one polished end face of laser material as the output coupler directly, and no partial reflection coating film was used as the output mirror. In this way, the coating procedure of laser material was saved, at the same time the 4F3/2-4F11/2 transition was restrained naturally and effectively by the low reflectivity (R ≈8%) of the output face. Under this condition, with a fiber coupled LD as the pump source, three laser materials (Nd:YAG ceramic, Nd:YAG crystal, and Nd:YVO4 crystal) were used for 1064 nm single wavelength operation. The best results came from Nd:YAG ceramic: the largest output power was 1.74 W, the optical conversion efficiency was 16.3%, and the slope efficiency was 25.3%. Based on this structure, we designed a three-mirror hybrid cavity for Nd:YAG ceramic, and realized 1064, 1319, 1338 nm tri-wavelength efficient CW operation. The total output power was 3.2 W, including 2.2 W 1064 nm laser, and 1 W 1319 & 1338 nm laser, corresponding an optical conversion efficiency of 30%. Since Nd:YAG ceramic permits higher Nd3+ doping level than Nd:YAG crystal does, and has higher thermal conductivity than Nd:YVO4 crystal, it can make up disadvantages of these two traditional laser materials, and possess high efficiency and high power characteristics simultaneously, which has been shown by our 1064 nm single wavelength operation experiment. From the experimental results, we can conclude that Nd:YAG ceramic is an excellent gain host to make high-efficiency, high-quality multi-wavelength all-solid-state lasers operated at different transitions simultaneously, as well as RGB three-fundamental-color lasers by further nonlinear frequency conversion.
This work is supported by the National Natural Science Foundation of China (60978027, 50925205 and 50990303), Natural Science Foundation of Shandong Province, China (ZR2009FM015), Innovation Fund for the Post-Doctoral Program of Shandong Province (200802029), China Postdoctoral Science Foundation funded project (200904501184), and Independent Innovation Foundation of Shandong University (2009TS129).
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