2.4 W highly efficient simultaneous dual-wavelength laser operation of monoclinic Yb3+:Ca4LaO(BO3)3 crystals Yuexia Ji,1 Jiafeng Cao,1 Jinlong Xu,2 Zhenyu You,2 and Chaoyang Tu2,* 1
2
School of Mathematics and Physics, Anhui University of Technology, Maanshan, Anhui 243032, China
Key Laboratory of Photoelectric Materials Chemistry and Physics of CAS, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China *Corresponding author: [email protected] Received 30 April 2014; revised 8 July 2014; accepted 13 July 2014; posted 14 July 2014 (Doc. ID 210333); published 18 August 2014
We have studied the simultaneous dual-wavelength laser operation of the X-cut, Y-cut, and Z-cut Yb3 :Ca4 LaOBO3 3 crystals for the first time to the best of our knowledge. We analyzed the dualwavelength laser output power and emission spectra under several output coupler transmittances. The stable dual-wavelength lasers were generated by adjusting the laser elements. A dual-wavelength laser output power 2.46 W was obtained with a slope efficiency of 67.5% by employing the Y-cut crystal at 1029 and 1036 nm. The two wavelengths had nearly the same relative intensity. The two different emission wavelengths were found to change with crystal direction and output coupler transmittance. This laser has a possible application as the laser source in the generation of different terahertz waves. © 2014 Optical Society of America OCIS codes: (140.0140) Lasers and laser optics; (140.3380) Laser materials; (140.3615) Lasers, ytterbium. http://dx.doi.org/10.1364/AO.53.005517
1. Introduction
Yb3 -doped
crystals have attracted enormous attention in recent years because of their interesting and promising characteristics. The simple energy level scheme of only the excited state 2 F5∕2 and the ground state 2 F7∕2 avoids excited state absorption and upconversion losses, therefore a highly efficient laser can be obtained. The broad absorption band relaxes the temperature control of the InGaAs LD pump source. The wideband emission can support the tunable laser and ultrashort laser pulse generation. The long fluorescence lifetime is beneficial for highenergy Q-switching laser operation. 1559-128X/14/245517-05$15.00/0 © 2014 Optical Society of America
The Yb3 ion laser was first reported by Etzel et al. [1] with Yb3 -ion-doped silicate glass. Subsequently, much work has been done in the field of Yb3 lasers, such as the generation of ultrashort laser pulses using mode-locked technology [2–6], the output of high power based on Yb fiber lasers [7], stable microwave signals from a dual-wavelength Yb3 -doped Al2 O3 distributed feedback waveguide laser [8], continuous-wave laser output [9–11], and the popular and advanced research on dual-wavelength laser operation [12–14]. Known from recent reports, various techniques have been developed to achieve dualwavelength laser operation, concerning an externalcavity tapered amplifier [15], intracavity-coupled Raman conversions [16], a C-band semiconductor optical amplifier placed in the cavity [17], passive Q-switching with a two-crystal linear cavity [18], a 20 August 2014 / Vol. 53, No. 24 / APPLIED OPTICS
5517
dual-wavelength mode-locked laser with semiconductor saturable absorber mirror (SESAM) [19,20], a double-walled carbon nanotube saturable absorber [21], and so on. For the same polarization of dualwavelength lasers, researchers have turned the same polarization of dual-wavelength laser beams into orthogonally polarized dual-wavelength lasers by adopting extracavity polarization elements or an intracavity birefringent gain medium [22,23]. The different frequency mixing of the simultaneous dual-wavelength laser in appropriate nonlinear crystal allows the generation of terahertz (THz) waves, which have a huge potential application in nondestructive imaging of concealed objects, free-space communications, and the military [24–29]. However, the generation of THz waves requires that the lasers simultaneously emit at two wavelengths with a space from 1 to 10 nm, which is not easily obtained. From our previous work, we found the simultaneous dual-wavelength laser for X-cut 5 at.% Yb3 :Ca4 LaOBO3 3 (Yb3 :LaCOB) crystal with outpour coupler T 3%, which is of intense interest to us [30]. Then, we focused the research on a dualwavelength laser for X-cut, Y-cut, and Z-cut 5 at.% Yb3 :LaCOB crystals with different outpour couplers. In this paper, we demonstrate the dual-wavelength laser of the Yb3 :LaCOB crystals with X-cut, Y-cut, and Z-cut, respectively. For the X-cut crystal, the better output power was 1.76 W with an output coupler transmittance of 3%, emitting two wavelengths at 1037.5 and 1042.7 nm. For the Y-cut crystal, the better output power was 2.46 W with an output coupler transmittance of 5%, and the dualwavelength emissions were at 1029 and 1036 nm. And for the Z-cut crystal, the better output power was 0.80 W and the dual-wavelength lasers were emitted at 1029 and 1033.5 nm for output coupler transmittance of 2%. 2. Experiment Setup
The light propagation direction was along the refractive index principal axes X, Y, and Z, which are labeled as X-cut, Y-cut, and Z-cut, respectively. The 5 at.% Yb3 -doped LaCOB crystals were processed with the dimensions of 3 mm × 3 mm × 3 mm, 3 mm × 3 mm × 5 mm, and 3 mm × 3 mm × 3 mm for X-cut, Y-cut, and Z-cut, respectively. The configuration of the laser resonator is shown in Fig. 1. A fiber-coupled laser diode at 976 nm was used as a pump source. The pump beam with a core diameter of 200 μm and numerical aperture (NA) of
Fig. 1. Configuration of the laser experiment for the laser diodepumped Yb3 :LaCOB crystals. F, focus coupling lens; M1 , input mirror; M2 , output mirror. 5518
APPLIED OPTICS / Vol. 53, No. 24 / 20 August 2014
0.22 was collimated and focused into the crystal by a pair of antireflection-coated plano–convex lenses with f 1 ∶f 2 1∶1. In order to remove the thermal accumulation, all the crystals were wrapped with indium foil and mounted in a copper holder maintained at 9°C by a water cooler. The laser crystals were placed in a plano–concave cavity with ∼20 mm cavity length. The input plane mirror M1 was coated with high reflectivity (HR) in the wavelength range of 1040–1080 nm (R > 99.9%) and high transmittance (T > 92%) at the pumping wavelength of 976 nm. Three concave mirrors were employed as the output mirror M2. One of the concave mirrors had a 75 mm radius of curvature (ROC) and a transmittance of 3%, while two others had a 100 mm ROC and transmittances of 2% and 5%. 3. Experiment Results and Discussion
The continuous-wave dual-wavelength laser operations with X-cut, Y-cut, and Z-cut Yb3 :LaCOB crystals were all achieved as the pump power was increased above the threshold. The laser output power obtained below did not reach the saturation of output power. The absorption coefficient was measured to be about 0.09∕mm for X-cut, Y-cut, and Z-cut crystals. There were mixed wavelengths when the laser was output initially. However, by adjusting the position and inclination of the output couplers and the crystal, the mixed-wavelength laser was turned into a laser emitting at two wavelengths simultaneously with a space of
2
School of Mathematics and Physics, Anhui University of Technology, Maanshan, Anhui 243032, China
Key Laboratory of Photoelectric Materials Chemistry and Physics of CAS, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China *Corresponding author: [email protected] Received 30 April 2014; revised 8 July 2014; accepted 13 July 2014; posted 14 July 2014 (Doc. ID 210333); published 18 August 2014
We have studied the simultaneous dual-wavelength laser operation of the X-cut, Y-cut, and Z-cut Yb3 :Ca4 LaOBO3 3 crystals for the first time to the best of our knowledge. We analyzed the dualwavelength laser output power and emission spectra under several output coupler transmittances. The stable dual-wavelength lasers were generated by adjusting the laser elements. A dual-wavelength laser output power 2.46 W was obtained with a slope efficiency of 67.5% by employing the Y-cut crystal at 1029 and 1036 nm. The two wavelengths had nearly the same relative intensity. The two different emission wavelengths were found to change with crystal direction and output coupler transmittance. This laser has a possible application as the laser source in the generation of different terahertz waves. © 2014 Optical Society of America OCIS codes: (140.0140) Lasers and laser optics; (140.3380) Laser materials; (140.3615) Lasers, ytterbium. http://dx.doi.org/10.1364/AO.53.005517
1. Introduction
Yb3 -doped
crystals have attracted enormous attention in recent years because of their interesting and promising characteristics. The simple energy level scheme of only the excited state 2 F5∕2 and the ground state 2 F7∕2 avoids excited state absorption and upconversion losses, therefore a highly efficient laser can be obtained. The broad absorption band relaxes the temperature control of the InGaAs LD pump source. The wideband emission can support the tunable laser and ultrashort laser pulse generation. The long fluorescence lifetime is beneficial for highenergy Q-switching laser operation. 1559-128X/14/245517-05$15.00/0 © 2014 Optical Society of America
The Yb3 ion laser was first reported by Etzel et al. [1] with Yb3 -ion-doped silicate glass. Subsequently, much work has been done in the field of Yb3 lasers, such as the generation of ultrashort laser pulses using mode-locked technology [2–6], the output of high power based on Yb fiber lasers [7], stable microwave signals from a dual-wavelength Yb3 -doped Al2 O3 distributed feedback waveguide laser [8], continuous-wave laser output [9–11], and the popular and advanced research on dual-wavelength laser operation [12–14]. Known from recent reports, various techniques have been developed to achieve dualwavelength laser operation, concerning an externalcavity tapered amplifier [15], intracavity-coupled Raman conversions [16], a C-band semiconductor optical amplifier placed in the cavity [17], passive Q-switching with a two-crystal linear cavity [18], a 20 August 2014 / Vol. 53, No. 24 / APPLIED OPTICS
5517
dual-wavelength mode-locked laser with semiconductor saturable absorber mirror (SESAM) [19,20], a double-walled carbon nanotube saturable absorber [21], and so on. For the same polarization of dualwavelength lasers, researchers have turned the same polarization of dual-wavelength laser beams into orthogonally polarized dual-wavelength lasers by adopting extracavity polarization elements or an intracavity birefringent gain medium [22,23]. The different frequency mixing of the simultaneous dual-wavelength laser in appropriate nonlinear crystal allows the generation of terahertz (THz) waves, which have a huge potential application in nondestructive imaging of concealed objects, free-space communications, and the military [24–29]. However, the generation of THz waves requires that the lasers simultaneously emit at two wavelengths with a space from 1 to 10 nm, which is not easily obtained. From our previous work, we found the simultaneous dual-wavelength laser for X-cut 5 at.% Yb3 :Ca4 LaOBO3 3 (Yb3 :LaCOB) crystal with outpour coupler T 3%, which is of intense interest to us [30]. Then, we focused the research on a dualwavelength laser for X-cut, Y-cut, and Z-cut 5 at.% Yb3 :LaCOB crystals with different outpour couplers. In this paper, we demonstrate the dual-wavelength laser of the Yb3 :LaCOB crystals with X-cut, Y-cut, and Z-cut, respectively. For the X-cut crystal, the better output power was 1.76 W with an output coupler transmittance of 3%, emitting two wavelengths at 1037.5 and 1042.7 nm. For the Y-cut crystal, the better output power was 2.46 W with an output coupler transmittance of 5%, and the dualwavelength emissions were at 1029 and 1036 nm. And for the Z-cut crystal, the better output power was 0.80 W and the dual-wavelength lasers were emitted at 1029 and 1033.5 nm for output coupler transmittance of 2%. 2. Experiment Setup
The light propagation direction was along the refractive index principal axes X, Y, and Z, which are labeled as X-cut, Y-cut, and Z-cut, respectively. The 5 at.% Yb3 -doped LaCOB crystals were processed with the dimensions of 3 mm × 3 mm × 3 mm, 3 mm × 3 mm × 5 mm, and 3 mm × 3 mm × 3 mm for X-cut, Y-cut, and Z-cut, respectively. The configuration of the laser resonator is shown in Fig. 1. A fiber-coupled laser diode at 976 nm was used as a pump source. The pump beam with a core diameter of 200 μm and numerical aperture (NA) of
Fig. 1. Configuration of the laser experiment for the laser diodepumped Yb3 :LaCOB crystals. F, focus coupling lens; M1 , input mirror; M2 , output mirror. 5518
APPLIED OPTICS / Vol. 53, No. 24 / 20 August 2014
0.22 was collimated and focused into the crystal by a pair of antireflection-coated plano–convex lenses with f 1 ∶f 2 1∶1. In order to remove the thermal accumulation, all the crystals were wrapped with indium foil and mounted in a copper holder maintained at 9°C by a water cooler. The laser crystals were placed in a plano–concave cavity with ∼20 mm cavity length. The input plane mirror M1 was coated with high reflectivity (HR) in the wavelength range of 1040–1080 nm (R > 99.9%) and high transmittance (T > 92%) at the pumping wavelength of 976 nm. Three concave mirrors were employed as the output mirror M2. One of the concave mirrors had a 75 mm radius of curvature (ROC) and a transmittance of 3%, while two others had a 100 mm ROC and transmittances of 2% and 5%. 3. Experiment Results and Discussion
The continuous-wave dual-wavelength laser operations with X-cut, Y-cut, and Z-cut Yb3 :LaCOB crystals were all achieved as the pump power was increased above the threshold. The laser output power obtained below did not reach the saturation of output power. The absorption coefficient was measured to be about 0.09∕mm for X-cut, Y-cut, and Z-cut crystals. There were mixed wavelengths when the laser was output initially. However, by adjusting the position and inclination of the output couplers and the crystal, the mixed-wavelength laser was turned into a laser emitting at two wavelengths simultaneously with a space of
