Widely tunable, narrow bandwidth polycrystalline ceramic Er:YAG laser with a volume Bragg grating Xiaoqi Zhang,1 Deyuan Shen,2,* Haitao Huang,3 Jun Liu,1 Xuan Liu,2 Jianing Zhang,2 Jian Zhang,3 Dingyuan Tang,3 and Dianyuan Fan1 2
1 Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China 3 School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China * [email protected]
Abstract: We report on a tunable polycrystalline ceramic Er:YAG laser with a total tuning range of 23.1 nm by using a volume Bragg grating. The whole tuning range was composed of three parts, 1614.2 nm - 1621.5 nm, 1629.3 nm - 1635.1 nm and 1639.2 nm - 1649.2 nm. The bandwidth of the output spectrum (FWHM) of the tunable Er:YAG laser was narrowed down to 99% at a center wavelength of 1570 nm, the fiber laser was locked at 1532 nm with a linewidth of 0.2 nm (FWHM) to match the narrow absorption spectrum of the Er:YAG ceramic. A polycrystalline Er:YAG ceramic of 0.5 at% Er3+-doping (developed at Jiangsu Normal University, China) was used as the gain medium in the experiment. It was cut and polished into dimensions of 5 × 5 × 28.4 mm3 and its end faces were both antireflection coated at 1500 nm - 1700 nm. The Er:YAG ceramic was covered with indium foil and mounted in a water-cooled copper heat sink maintained at a temperature of 15 °C. The plane input coupler (IC) was coated for high transmission (T > 95%) at the pump wavelength of 1532 nm and high-reflectivity (R > 99%) at 1600 nm - 1700 nm. The folding mirror with a 200 mm radius-of-curvature was highly reflective at 1500 nm - 1700 nm (R > 99.5%). Three output couplers with transmissions of 5%, 10% and 20% at the lasing wavelength were employed in the experiment. The wavelength tuning of the ceramic laser was achieved by using a VBG reflector (Optigrate Inc.), which was covered with indium foil and then mounted in a copper heat sink for heat removal. The VBG had dimensions of 6 mm × 8 mm × 2.75 mm and lased at 1650 nm at normal incident with a bandwidth of 1.8 nm (FWHM). To avoid the instabilities in oscillator performance caused by remaining Fresnel reflections, the grating was titled by a slant angle of 0.2° in the glass and the facets were broadband antireflection coated. In the laser cavity design, we mainly considered good mode matching between the pump beam and the laser mode. Another important issue is to maintain the high reflectivity of the VBG, which can be obtained if the angle distribution of the laser beam is wider than the grating angular selectivity [18]. The total laser cavity length was 427 mm and a small incident angle (< 10°) on the curve mirror was taken to minimize the astigmatism. The radius of the laser mode in the Er:YAG ceramic medium and the VBG was 125 μm and 300 μm, respectively. The spectrum of the laser output was measured by an optical spectrum analyzer (AQ6370C, Yokogawa) with a resolution of 0.02 nm.
#203640 - $15.00 USD (C) 2014 OSA
Received 3 Jan 2014; revised 28 Feb 2014; accepted 28 Feb 2014; published 19 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007154 | OPTICS EXPRESS 7156
Fig. 1. Schematic diagram of the tunable Er:YAG ceramic laser setup in the experiment. IC: Input coupler. OC: Output coupler.
3. Results and discussion The output power as a function of the wavelength is shown in Fig. 2. The widest total tuning range of 23.1 nm was obtained with the output coupler of 5% transmission at the maximum available pump power of 11.1 W due to the minimum cavity loss. The tuning range was punctuated into three parts and every part was symmetric with respect to its emission peak. The tuning range of the Er:YAG laser was mainly determined by three factors, i.e. the Er:YAG spectral gain, the output coupler transmission and the reflectivity of the VBG. Thus, the maximum continuous tuning range of 10 nm from 1639.2 nm to 1649.2 nm was obtained around 1645 nm emission peak. Due to the narrower bandwidth of the emission spectral of the 1617 nm transition compared with the 1645 nm transition, a tuning range of 7.3 nm from 1614.2 nm to 1621.5 nm was obtained at around the 1617 nm emission peak. Because of the low effective emission cross-section, the 1633 nm transition can meet the transparency condition with 11% population inversion, while the transparent condition for the 1645 nm and the 1617 nm transition is met 9% and 14%, respectively [1]. However, due to the weaker oscillating strength compared with those of the 1617 nm and 1645 nm, the gain cross-section of the 1633 nm transition cannot be higher than that of the 1617 nm and the 1645 nm transition simultaneously, even all population in the lower manifold is pumped up to the upper laser state. In other words, the 1634 nm transition cannot be obtained without a wavelength selector. In this experiment, we demonstrated the laser oscillation at around the 1630 nm for the first time. The total tuning range was 5.8 nm from 1629.3 nm to 1635.1 nm. A continuously tunable Er:YAG laser was not achieved due to the weak interaction between the shielded 4f electronic system and the vibration modes of the surrounding. A more convenient designation of the laser cavity should be implemented to make the tunable Er:YAG ceramic laser more competitive in the gas sensing systems, such as only one degree of freedom needed for the laser tuning operation [13]. As a comparative study, a free running Er:YAG ceramic laser was studied with the VBG replaced by a high-reflectivity (R > 99.5%) mirror at 1500 nm - 1700 nm. The output powers of the tunable and the free running Er:YAG ceramic lasers as a function of the incident pump power are given in Fig. 3. In the tuning range of 1639.2 nm-1649.2 nm, the tunable laser yielded a maximum output power of 3.7 W at 1644.7 nm with the output coupler transmission of 10%, corresponding to a slope efficiency of 42.3% with respect to the absorbed power. The M2 parameter for the output beam was determined to be 1.2 at the incident pump power of 11.1 W (NanoScan, Photon Inc.). In the tuning range from 1629.3 nm to 1635.1 nm, the maximum output power of 1.4 W was obtained at 1633.0 nm, corresponding to a slope efficiency of 20.9% with respect to the incident power. With the same output coupler the
#203640 - $15.00 USD (C) 2014 OSA
Received 3 Jan 2014; revised 28 Feb 2014; accepted 28 Feb 2014; published 19 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007154 | OPTICS EXPRESS 7157
performance of the tunable Er:YAG laser is not as good as that of the free running laser, which is mainly due to the single-pass pumping scheme caused by the VBG. It is worth noting that with the decrease of the wavelength, the folding angle of the laser cavity increased, which results in not only greater astigmatisms but also the reduction of the effective reflectivity of the VBG. According to the Bragg condition λ = λ0 cosθ, where λ0 is the wavelength at normal incidence, the internal incident angle is 11° and the external angle is 35° when the reflected wavelength is 1617 nm. Therefore the slope efficiency of the tunable laser operating at around 1617 nm is much smaller than that of the laser operating at 1645 nm.
Fig. 2. The output power of the tunable Er:YAG ceramic laser as a function of the wavelength.
The output spectra of the tunable and free running Er:YAG ceramic lasers are shown in Fig. 4. The bandwidth of the output spectrum (FWHM) of the tunable Er:YAG laser was smaller than 0.05 nm over the whole tuning range, which demonstrates the spectral discrimination property of the volume Bragg grating. The shift of the laser center wavelength did not exceed 0.13 nm when the pump power increased from the threshold to the maximum. We attributed this to the slight thermal laser wavelength shift of the VBG [19, 20]. The 1617 nm transition of the free running laser was obtained by using an output coupler with a transmission of 30%. The spectrum bandwidth (FWHM, 0.06 nm) of the 1617 nm free running laser is narrower than that of the 1645 nm laser (FWHM, 0.5 nm) due to the much higher laser threshold.
Fig. 3. The output powers of the tunable and free running Er:YAG ceramic lasers as a function of the incident pump power (FR: free running). (a) Operation at 1645 nm emission peak, (b) operation at round at 1617 nm and 1633 nm emission peaks.
#203640 - $15.00 USD (C) 2014 OSA
Received 3 Jan 2014; revised 28 Feb 2014; accepted 28 Feb 2014; published 19 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007154 | OPTICS EXPRESS 7158
Fig. 4. Comparison between the output spectra of the free running laser (1617 nm with an output coupler of 30% transmission, 1645 nm with an output coupler of 10% transmission) and the tunable laser (with an output coupler of 10% transmission) at the maximum incident pump power of 11.1 W.
4. Conclusion We have demonstrated a tunable, narrow-bandwidth Er:YAG ceramic laser by using a volume Bragg grating. The total tuning range of 23.1 nm was composed of three parts, i.e. 1614.2 nm −1621.5 nm, 1629.3 nm - 1635.1 nm and 1639.2 nm −1649.2 nm. At the maximum incident pump power of 11.1 W, a maximum output power of 3.7 W at 1644.7 nm was obtained with a slope efficiency of 37.3% with respect to the incident power. In the tuning range of 1629.3 nm - 1635.1 nm, we obtained a maximum output power of 1.4 W at 1633.0 nm, corresponding to a slope efficiency of 20.9% with respect to the incident pump power. The bandwidth of the output spectrum of the tunable Er:YAG ceramic laser did not exceed 0.05 nm (FWHM) over the whole tuning range. Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC 61078035, 61177045), the Research Fund for the Doctoral Program of Higher Education of China (RFDP: 20110071110016) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
#203640 - $15.00 USD (C) 2014 OSA
Received 3 Jan 2014; revised 28 Feb 2014; accepted 28 Feb 2014; published 19 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007154 | OPTICS EXPRESS 7159
1 Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China 3 School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China * [email protected]
Abstract: We report on a tunable polycrystalline ceramic Er:YAG laser with a total tuning range of 23.1 nm by using a volume Bragg grating. The whole tuning range was composed of three parts, 1614.2 nm - 1621.5 nm, 1629.3 nm - 1635.1 nm and 1639.2 nm - 1649.2 nm. The bandwidth of the output spectrum (FWHM) of the tunable Er:YAG laser was narrowed down to 99% at a center wavelength of 1570 nm, the fiber laser was locked at 1532 nm with a linewidth of 0.2 nm (FWHM) to match the narrow absorption spectrum of the Er:YAG ceramic. A polycrystalline Er:YAG ceramic of 0.5 at% Er3+-doping (developed at Jiangsu Normal University, China) was used as the gain medium in the experiment. It was cut and polished into dimensions of 5 × 5 × 28.4 mm3 and its end faces were both antireflection coated at 1500 nm - 1700 nm. The Er:YAG ceramic was covered with indium foil and mounted in a water-cooled copper heat sink maintained at a temperature of 15 °C. The plane input coupler (IC) was coated for high transmission (T > 95%) at the pump wavelength of 1532 nm and high-reflectivity (R > 99%) at 1600 nm - 1700 nm. The folding mirror with a 200 mm radius-of-curvature was highly reflective at 1500 nm - 1700 nm (R > 99.5%). Three output couplers with transmissions of 5%, 10% and 20% at the lasing wavelength were employed in the experiment. The wavelength tuning of the ceramic laser was achieved by using a VBG reflector (Optigrate Inc.), which was covered with indium foil and then mounted in a copper heat sink for heat removal. The VBG had dimensions of 6 mm × 8 mm × 2.75 mm and lased at 1650 nm at normal incident with a bandwidth of 1.8 nm (FWHM). To avoid the instabilities in oscillator performance caused by remaining Fresnel reflections, the grating was titled by a slant angle of 0.2° in the glass and the facets were broadband antireflection coated. In the laser cavity design, we mainly considered good mode matching between the pump beam and the laser mode. Another important issue is to maintain the high reflectivity of the VBG, which can be obtained if the angle distribution of the laser beam is wider than the grating angular selectivity [18]. The total laser cavity length was 427 mm and a small incident angle (< 10°) on the curve mirror was taken to minimize the astigmatism. The radius of the laser mode in the Er:YAG ceramic medium and the VBG was 125 μm and 300 μm, respectively. The spectrum of the laser output was measured by an optical spectrum analyzer (AQ6370C, Yokogawa) with a resolution of 0.02 nm.
#203640 - $15.00 USD (C) 2014 OSA
Received 3 Jan 2014; revised 28 Feb 2014; accepted 28 Feb 2014; published 19 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007154 | OPTICS EXPRESS 7156
Fig. 1. Schematic diagram of the tunable Er:YAG ceramic laser setup in the experiment. IC: Input coupler. OC: Output coupler.
3. Results and discussion The output power as a function of the wavelength is shown in Fig. 2. The widest total tuning range of 23.1 nm was obtained with the output coupler of 5% transmission at the maximum available pump power of 11.1 W due to the minimum cavity loss. The tuning range was punctuated into three parts and every part was symmetric with respect to its emission peak. The tuning range of the Er:YAG laser was mainly determined by three factors, i.e. the Er:YAG spectral gain, the output coupler transmission and the reflectivity of the VBG. Thus, the maximum continuous tuning range of 10 nm from 1639.2 nm to 1649.2 nm was obtained around 1645 nm emission peak. Due to the narrower bandwidth of the emission spectral of the 1617 nm transition compared with the 1645 nm transition, a tuning range of 7.3 nm from 1614.2 nm to 1621.5 nm was obtained at around the 1617 nm emission peak. Because of the low effective emission cross-section, the 1633 nm transition can meet the transparency condition with 11% population inversion, while the transparent condition for the 1645 nm and the 1617 nm transition is met 9% and 14%, respectively [1]. However, due to the weaker oscillating strength compared with those of the 1617 nm and 1645 nm, the gain cross-section of the 1633 nm transition cannot be higher than that of the 1617 nm and the 1645 nm transition simultaneously, even all population in the lower manifold is pumped up to the upper laser state. In other words, the 1634 nm transition cannot be obtained without a wavelength selector. In this experiment, we demonstrated the laser oscillation at around the 1630 nm for the first time. The total tuning range was 5.8 nm from 1629.3 nm to 1635.1 nm. A continuously tunable Er:YAG laser was not achieved due to the weak interaction between the shielded 4f electronic system and the vibration modes of the surrounding. A more convenient designation of the laser cavity should be implemented to make the tunable Er:YAG ceramic laser more competitive in the gas sensing systems, such as only one degree of freedom needed for the laser tuning operation [13]. As a comparative study, a free running Er:YAG ceramic laser was studied with the VBG replaced by a high-reflectivity (R > 99.5%) mirror at 1500 nm - 1700 nm. The output powers of the tunable and the free running Er:YAG ceramic lasers as a function of the incident pump power are given in Fig. 3. In the tuning range of 1639.2 nm-1649.2 nm, the tunable laser yielded a maximum output power of 3.7 W at 1644.7 nm with the output coupler transmission of 10%, corresponding to a slope efficiency of 42.3% with respect to the absorbed power. The M2 parameter for the output beam was determined to be 1.2 at the incident pump power of 11.1 W (NanoScan, Photon Inc.). In the tuning range from 1629.3 nm to 1635.1 nm, the maximum output power of 1.4 W was obtained at 1633.0 nm, corresponding to a slope efficiency of 20.9% with respect to the incident power. With the same output coupler the
#203640 - $15.00 USD (C) 2014 OSA
Received 3 Jan 2014; revised 28 Feb 2014; accepted 28 Feb 2014; published 19 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007154 | OPTICS EXPRESS 7157
performance of the tunable Er:YAG laser is not as good as that of the free running laser, which is mainly due to the single-pass pumping scheme caused by the VBG. It is worth noting that with the decrease of the wavelength, the folding angle of the laser cavity increased, which results in not only greater astigmatisms but also the reduction of the effective reflectivity of the VBG. According to the Bragg condition λ = λ0 cosθ, where λ0 is the wavelength at normal incidence, the internal incident angle is 11° and the external angle is 35° when the reflected wavelength is 1617 nm. Therefore the slope efficiency of the tunable laser operating at around 1617 nm is much smaller than that of the laser operating at 1645 nm.
Fig. 2. The output power of the tunable Er:YAG ceramic laser as a function of the wavelength.
The output spectra of the tunable and free running Er:YAG ceramic lasers are shown in Fig. 4. The bandwidth of the output spectrum (FWHM) of the tunable Er:YAG laser was smaller than 0.05 nm over the whole tuning range, which demonstrates the spectral discrimination property of the volume Bragg grating. The shift of the laser center wavelength did not exceed 0.13 nm when the pump power increased from the threshold to the maximum. We attributed this to the slight thermal laser wavelength shift of the VBG [19, 20]. The 1617 nm transition of the free running laser was obtained by using an output coupler with a transmission of 30%. The spectrum bandwidth (FWHM, 0.06 nm) of the 1617 nm free running laser is narrower than that of the 1645 nm laser (FWHM, 0.5 nm) due to the much higher laser threshold.
Fig. 3. The output powers of the tunable and free running Er:YAG ceramic lasers as a function of the incident pump power (FR: free running). (a) Operation at 1645 nm emission peak, (b) operation at round at 1617 nm and 1633 nm emission peaks.
#203640 - $15.00 USD (C) 2014 OSA
Received 3 Jan 2014; revised 28 Feb 2014; accepted 28 Feb 2014; published 19 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007154 | OPTICS EXPRESS 7158
Fig. 4. Comparison between the output spectra of the free running laser (1617 nm with an output coupler of 30% transmission, 1645 nm with an output coupler of 10% transmission) and the tunable laser (with an output coupler of 10% transmission) at the maximum incident pump power of 11.1 W.
4. Conclusion We have demonstrated a tunable, narrow-bandwidth Er:YAG ceramic laser by using a volume Bragg grating. The total tuning range of 23.1 nm was composed of three parts, i.e. 1614.2 nm −1621.5 nm, 1629.3 nm - 1635.1 nm and 1639.2 nm −1649.2 nm. At the maximum incident pump power of 11.1 W, a maximum output power of 3.7 W at 1644.7 nm was obtained with a slope efficiency of 37.3% with respect to the incident power. In the tuning range of 1629.3 nm - 1635.1 nm, we obtained a maximum output power of 1.4 W at 1633.0 nm, corresponding to a slope efficiency of 20.9% with respect to the incident pump power. The bandwidth of the output spectrum of the tunable Er:YAG ceramic laser did not exceed 0.05 nm (FWHM) over the whole tuning range. Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC 61078035, 61177045), the Research Fund for the Doctoral Program of Higher Education of China (RFDP: 20110071110016) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
#203640 - $15.00 USD (C) 2014 OSA
Received 3 Jan 2014; revised 28 Feb 2014; accepted 28 Feb 2014; published 19 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.007154 | OPTICS EXPRESS 7159
