A composite crystal consisting of a 1.5-mm-thick Er:Yb:YAl3(BO3)4 crystal between two 1.2-mm-thick sapphire crystals was fabricated by the thermal diffusion bonding technique. Compared with a lone Er:Yb:YAl3(BO3)4 crystal measured under the identical experimental conditions, higher laser performances were demonstrated in the sapphire/Er:Yb:YAl3(BO3)4/sapphire composite crystal due to the reduction of the thermal effects. End-pumped by a 976 nm laser diode in a hemispherical cavity, a 1.55 μm continuous-wave laser with a maximum output power of 1.75 W and a slope efficiency of 36% was obtained in the composite crystal when the incident pump power was 6.54 W. Passively Q-switched by a Co2+:MgAl2O4 crystal, a 1.52 μm pulse laser with energy of 10 μJ and repetition frequency of 105 kHz was also realized in the composite crystal. Pulse width was 315 ns. The results show that the sapphire/Er:Yb:YAl3(BO3)4/sapphire composite crystal is an excellent active element for 1.55 μm laser.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In solid-state laser, heat is inevitably generated in gain medium due to the existence of some intrinsic energy loss mechanisms, such as quantum defect and nonradiative transition of upper laser level . Thermal effects caused by the redistribution of the heat in the gain medium can reduce the output laser performances. Therefore, some techniques have been proposed to weaken the influence of the thermal effects [2–4]. Among them, using a composite crystal as a laser active element is a simple and effective method. Superior laser performances have been demonstrated in a composite crystal fabricated by the thermal diffusion bonding of a laser gain medium to one or two pure identical host crystals, in which the pure host crystal acts as a heat sink to effectively dissipate the heat generated in the pump region of the gain medium [4,5]. At present, high quality composite crystals, such as Nd3+:YAG/YAG, Yb3+:YAG/YAG and Nd3+:YVO4/YVO4, have been commercialized in some well-known companies, such as Onyx optics. Inc. In order to further enhance the heat-removal capability, a composite crystal consisting of a laser gain medium and a pure heterocrystal with a high thermal conductivity, such as diamond (~2000 Wm−1K−1), SiC (~490 Wm−1K−1), and sapphire (~40 Wm−1K−1), has also been fabricated by the diffusion bonding technique [6–8]. For example, 1064 nm continuous-wave (cw) lasers with maximum output powers of 11.4 and 6.7 W have been realized recently in the room-temperature bonding Nd3+:YAG/diamond and high-temperature bonding Nd3+:YAG/SiC composite crystals, respectively [6,7].
Laser around 1.55 μm is eye-safe and has important applications in lidar, satellite laser ranging, three-dimensional imaging, and environmental sensing [9,10]. Compared with those (350 mW cw output power and 20–30% slope efficiency) realized in the Er:Yb:phosphate glass, which is widely used as a commercialized 1.55 μm laser material [10–12], higher cw output power (~1.0 W) and slope efficiency (~35%) have been obtained in Er:Yb:RAl3(BO3)4 (Er:Yb:RAB, R = Y, Gd and Lu) crystals [13–15]. Therefore, the Er:Yb:RAB crystals have been considered as one kind of excellent gain media for 1.55 μm laser. However, due to the large quantum defect (about 37%) and low fluorescence quantum efficiency (7%) of the upper laser level [13–15], a large amount of absorbed pump power is converted to heat in the Er:Yb:RAB crystal and the 1.55 μm laser performances of the crystal are affected seriously [15,16]. When a sapphire crystal as a heat sink was closely contacted with an Er:Yb:RAB crystal through the use of screws, the thermal effects in the Er:Yb:RAB crystal can be obviously reduced and then the 1.55 μm laser performances were enhanced greatly [15,16]. However, a perfect optical contact between the faces of the two discrete crystals is difficult to be realized in this simple unbonded structure. Recently, an Er:Yb:YAB/YAB composite crystal was fabricated by the thermal diffusion bonding technique, and the maximum output power of 1.55 μm cw laser was increased from 350 mW in a lone Er:Yb:YAB crystal to 780 mW in the composite crystal under the identical experimental conditions .
Considering that sapphire crystal has a higher thermal conductivity (~40 Wm−1K−1 ) than that of pure YAB crystal (11–12 Wm−1K−1 ), a sapphire/Er:Yb:YAB/sapphire (sap/Er:Yb:YAB/sap) composite crystal was firstly fabricated to be used as a 1.55 μm laser active element in this work. Then, end-pumped by a 976 nm laser diode (LD), cw and passively Q-switched pulse laser performances of the composite crystal were investigated.
2. Material fabrication and characterization
The fabrication process of a sap/Er:Yb:YAB/sap composite crystal is briefly depicted in Fig. 1(a). A 1.5-mm-thick, c-cut Er(1.4 at.%):Yb(13 at.%):YAB crystal with a cross section of 3.0 mm × 3.0 mm were cut from a single crystal grown in our lab. The front and rear endfaces of the crystal were precisely polished to achieve a surface quality with scratch/dig specification of 20/10, flatness less than one-fourth wavelength at 633 nm, and parallelism better than 20 arcsec. Due to the similar refractive index (1.75 in 1.5–1.6 μm ) to the Er:Yb:YAB crystal and the high thermal conductivity, sapphire crystal was selected as a heat sink. Two 1.2-mm-thick, <0001> sapphire crystals with a cross section of 3.0 mm × 3.0 mm were brought from Shanghai Daheng Optics and Fine Mechanics Co., Ltd. The front and rear endfaces of both sapphire crystals were precisely polished to achieve a surface quality with scratch/dig specification of 40/20, flatness less than one-half wavelength at 633 nm, and parallelism better than 30 arcsec. Then, each polished endface of all the crystals was carefully cleaned to remove the dust particle. In order to overcome the lattice mismatch between the Er:Yb:YAB and sapphire crystals, SiO2 film was deposited onto each contact surface of all the crystals by the ion beam assisted electron beam evaporation. Finally, all the crystals were tightly pressed together and had been heated at an elevated temperature for several hours. At an elevated temperature, SiO2 films on the surfaces of different crystals can be bonded together by the atom thermal diffusion. Then, a sap/Er:Yb:YAB/sap composite crystal was successfully fabricated. Compared with the single-end bonded sap/Er:Yb:YAB composite crystal, in which the thermal effects can also be reduced, the heat generated in the gain medium can be more effectively dissipated in the dual-end bonded sap/Er:Yb:YAB/sap composited crystal, as reported in the Nd:YVO4 laser . A photograph of the composite crystal is shown in Fig. 1(b). Another 1.5-mm-thick, c-cut Er:Yb:YAB crystal with a cross section of 3.0 mm × 3.0 mm was also cut from the same as-grown crystal and polished for comparative studies.
Transmission spectra in 1400–1650 nm of both the sap/Er:Yb:YAB/sap composite and Er:Yb:YAB crystals were recorded by an UV-VIS-NIR spectrophotometer (Lambda-950, Perkin-Elmer) and are shown in Fig. 2. Transmissivity of the Er:Yb:YAB crystal within this range is about 85% when the absorption originating from the Er3+ is excluded. This value is close to the theoretical limit of 86% caused by the interface reflection between the air and crystal with the refractive index of 1.75 . However, the transmissivity of the composite crystal within this range is only 77–80%. The low transmissivity of the composite crystal may be caused by the deposited SiO2 film with a refractive index of about 1.46 , which is lower than those (1.75) of the Er:Yb:YAB and sapphire crystals. If another kind of film with a similar refractive index to the crystals is adopted in the future, Fresnel reflection loss of the composite crystal can be reduced. Absorption spectrum in 875–1050 nm of the composite crystal was also recorded and is shown in the inset of Fig. 2. The peak absorption coefficient at wavelength of 976 nm is 21.5 cm−1.
3. Arrangement of laser experiment
A simple end-pumped hemispherical resonator was adopted and the experimental setup is depicted in Fig. 3. Compared with the plano-plano microcavity, in which the crystal thickness determines the cavity length and affects the output laser performance, the hemispherical resonator with longer cavity length is more favorable for the comparison of the laser performances of the sap/Er:Yb:YAB/sap composite and lone Er:Yb:YAB crystals with different thicknesses. The active element was mounted inside a copper holder cooled by water at 20 °C. There is a hole with diameter of 1.5 mm in the center of the holder to permit the passing of laser beams. A cw 976 nm fiber-coupled LD with core diameter of 100 μm and numerical aperture of 0.15 from Dilas Inc. was used. After passing a telescopic lens system (TLS) consisting of two convex lenses, pumping beam with waist diameter of about 100 µm was focused in the active element. The position of the pumping beam waist was near the center of the active element, which is optimized by adjusting the active element to achieve the best laser performance during the experiment. An input mirror (IM) with 90% transmissivity at 976 nm and 99.8% reflectivity in 1.5–1.6 μm was directly deposited on the front surface of the active element. Three output mirrors (OMs) with identical curvature radius of 100 mm and different transmissivities (1.7%, 3.2% and 5.8%) in 1.5–1.6 μm were used. The resonator length was close to 100 mm. In order to investigate the passively Q-switched pulse performances of the composite crystal, a 0.95-mm-thick Co2+:MgAl2O4 crystal with a cross section of 3.0 mm × 3.0 mm was used as a saturable absorber and closely contacted with the composite crystal by screws. All the crystals were placed inside the same copper holder. The AR coated Co2+:MgAl2O4 crystal had an initial transmissivity of 95% in 1.5–1.6 μm. The waist diameter of the fundamental laser was about 130 μm inside the saturable absorber.
Laser spectrum was recorded by a monochromator (Triax550, Jobin-Yvon) with an integral time of 0.3 s and an accuracy of 0.02 nm. Using a convex lens with a 50-cm focal length to focus output laser beam, spatial profiles of the focused beam at various distances from the focusing lens were recorded with a Pyrocam III camera (Ophir Optronics). The beam radius was calculated by the 4-sigma method and the beam quality factor M2 can be estimated by fitting these data to the Gaussian beam propagation expression . Pulse profile was measured by a 2 GHz InGaAs photodiode connected to an oscilloscope (DSO6102A, Agilent).
4. Results and discussion
Figure 4(a) plots the cw output power realized in the sap/Er:Yb:YAB/sap composite crystal versus incident pump power for different OM transmissivities T. When the OM transmissivity was 5.8%, a maximum output power of 1.75 W was obtained at an incident pump power of 6.54 W, which is limited by the available output power of the used LD in our lab. The slope efficiency η was 36%. Above values are higher than those obtained in a 1.5-mm-thick, c-cut Er(1.5 at.%):Yb(11 at.%):YAB crystal reported previously (1.0 W maximum output power and 35% slope efficiency with respect to absorbed pump power) . The incident pump threshold was 1.53 W. The spectra of output lasers are shown in Fig. 4(b). The laser wavelength was blue-shifted from 1.6 to 1.55 μm when the OM transmissivity was increased from 1.7% to 5.8%. This phenomenon is attributed to the gain characteristic of the Er3+/Yb3+ codoped materials, in which the peak gain wavelength is blue-shifted with the increment of inversion density (or intracavity losses) [22,23]. A nearly circular output beam was observed and the beam quality factors M2 in the X and Y directions were fitted to be 2.02 and 2.16, respectively, at an incident pump power of 6.54 W and the OM transmissivity of 5.8%, as shown in Fig. 4(c). Figure 4(d) shows that the beam quality of output laser was improved with the decrement of pump power, and M2 was close to 1.1 when the pump power was 2.2 W.
Under the identical experimental conditions, laser performances of a lone 1.5-mm-thick, c-cut Er:Yb:YAB crystal with the same doping concentrations were also investigated. For the 5.8% OM transmissivity, output powers of both the sap/Er:Yb:YAB/sap composite and lone Er:Yb:YAB crystals as a function of incident pump power are compared in Fig. 5(a). A maximum output power of 1.45 W with a slope efficiency of 28% was obtained in the lone Er:Yb:YAB crystal at an incident pump power of 6.54 W. Therefore, when the sap/Er:Yb:YAB/sap composite crystal was used as laser active element, about 20% increment in maximum output power and 30% increment in slope efficiency were obtained. Furthermore, as shown in Fig. 5(b), the beam quality factors M2 in the X and Y directions of the Er:Yb:YAB laser were 3.76 and 4.06 at an incident pump power of 6.54 W, respectively, which are larger than those of the sap/Er:Yb:YAB/sap laser at the same pump power. Above results clearly demonstrate that output laser performances of the composite crystal are effectively improved due to the reduction of the thermal effects in the gain medium. However, the incident pump threshold of the composite crystal (1.53 W) was higher than that of the lone Er:Yb:YAB crystal (1.25 W), which may be caused by the larger Fresnel reflection loss of the composite crystal shown in Fig. 2. It can be expected that a further enhancement in laser performances of the composite crystal can be realized when its bonding quality is improved in the future.
Spectra of both the lasers are also shown in the inset of Fig. 5(a) at an incident pump power of 6.54 W. The oscillating wavelength of the sap/Er:Yb:YAB/sap laser was located around 1.55 μm, while that of the Er:Yb:YAB laser was blue-shifted to 1.52 μm. The difference of output wavelength may be caused by the variance of temperature in the gain media. As seen from the energy level diagram of Er3+ in YAB crystal shown in Fig. 5(c) , 1.55 μm laser is corresponding to the transition from the 2nd Stark level of the 4I13/2 multiplet to the 4th Stark level of the 4I15/2 multiplet, while 1.52 μm laser is corresponding to the transition from the 3rd Stark level of the 4I13/2 multiplet to the 2nd Stark level of the 4I15/2 multiplet. According to the Boltzmann statistics, a part of ions in a lower Stark level will populate in an upper Stark level in the same multiplet when temperature is increased. Therefore, the population-inversion density (i.e., gain coefficient) corresponding to 1.55 μm laser is reduced and the one corresponding to 1.52 μm laser is increased with the increment of temperature in the gain medium. Due to the lack of the sapphire heat sink, temperature in the gain medium of the Er:Yb:YAB laser is obviously higher than that of the sap/Er:Yb:YAB/sap laser. Therefore, it is more favorable to the realization of the 1.52 μm laser in the lone Er:Yb:YAB crystal at the same pump power. The spacing between the adjacent modes in the spectrum of the composite laser was close to 0.52 nm, and is in agreement with the theoretical mode spacing of 0.57 nm caused by the etalon effect associated to the 1.2-mm-thick sapphire crystal . However, due to the more serious thermal effects and the lack of the sapphire crystal as an etalon, the mode spacing in the lone Er:Yb:YAB laser was narrower and varied, which made the spectrum look like continuous.
For the passively Q-switched pulse laser of the composite crystal, the OM with a transmissivity of 5.8% and the Co2+:MgAl2O4 crystal with an initial transmissivity of 95%, which are available in our lab at present but unoptimized, were used in this work. A maximum average output power of 0.95 W was obtained at an incident pump power of 6.54 W, where the slope efficiency η was 20%, as shown in Fig. 6. Thus, the conversion efficiency from the cw to Q-switched regime of operation was close to 54%. It can be seen from the inset of Fig. 6 that the beam quality factors M2 in the X and Y directions of the pulse laser were fitted to be 1.48 and 1.61, respectively, at an incident pump power of 6.54 W. Compared to the 1.55 μm observed in the cw sap/Er:Yb:YAB/sap laser, wavelength of the pulse laser was blue-shifted to 1.52 μm, which was mainly caused by the increment of the intracavity losses originated from the insert of the saturable absorber with an initial transmissivity of 95%. Similar to the cw composite laser, several discrete longitudinal modes were observed in the spectrum of the pulse laser, which is also originated from the etalon effect caused by the sapphire crystal.
Repetition frequency and width of the sap/Er:Yb:YAB/sap pulse laser were recorded at various incident pump power. As shown in Figs. 7(a) and (b), the pulse repetition frequency changed from 5.9 to 105 kHz when the incident pump power was increased from 2.2 to 6.54 W. It can also be seen that the pulse-to-pulse amplitude fluctuation and interpulse time jittering at high pump power were far larger than those at low pump power due to the increment of the thermal effects in the gain medium. As shown in Fig. 7(c), there is an approximately linear increment between the pulse repetition frequency and the incident pump power. The pulse energy of the passively Q-switched sap/Er:Yb:YAB/sap laser was almost kept as a constant for various pump power and estimated to be about 10 ± 1 μJ. Above parameters are also higher than those obtained in the Er:Yb:YAB crystal with similar doping concentrations reported previously (60 kHz repetition frequency and 5.25 μJ energy) . Although the pulse energy obtained in the composite crystal is lower than those (20-30 μJ) realized in the Er:Yb:glass microchip lasers, the repetition frequency (105 kHz) is two orders of magnitude higher than those (about 1 kHz) of the glass lasers [25,26]. With the increment of the incident pump power, pulse width was almost kept at 315 ns, as shown in Fig. 7(d) recorded at an incident pump power of 6.54 W. The long decay time of the generated pulse may be caused by the unoptimized cavity parameters, such as cavity length, transmissivity of the OM, and initial transmissivity of the saturable absorber. When the cavity parameters are optimized in the future , the pulse laser performances, including pulse energy and width, can be enhanced.
A sap/Er:Yb:YAB/sap composite crystal was fabricated by the thermal diffusion bonding technique for the first time. In the composite crystal, the sapphire crystal with a high thermal conductivity can be used as an excellent heat sink to dissipate the heat generated in the Er:Yb:YAB crystal. Efficient cw and passively Q-switched pulse lasers around 1.55 μm were demonstrated in the composite crystal. When some technical parameters in the fabrication process, such as surface quality of the polished crystal, the kind of the bonded film as well as the heat treatment temperature and period, are optimized, the bonding quality of the composite crystal will be improved and then higher 1.55 μm laser performances can be expected.
National Key Research and Development Program of China (2016YFB0701002); Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).
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