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We report on a 808 nm diode-pumped Nd:YAG narrow linewidth laser at 1834.25 nm using a compact two-mirror linear cavity. In free-running mode, a maximum output power of 1.10 W is obtained with a slope efficiency of about 11.1% at a cooling temperature of the laser crystal of 18°C. Decreasing this temperature down to 8°C increases the output power linearly up to 1.31 W. Shortening the laser cavity reduces the output power down to about 0.70 W but on a single longitudinal mode. These laser results represent the best laser performance ever achieved with any Nd-doped bulk material in the 1.8 µm mid-infrared spectral range.
© 2016 Optical Society of America
1. Introduction
Compact and efficient eye-safe laser sources in the 1.9 μm (1.8-2.1 μm) spectral region are very attractive for many applications including laser radar, remote sensing, spectroscopy, medical treatment, optical communications and metrology [1–3 ]. It is well known that trivalent Tm3+- and Ho3+-doped lasers, as well as Tm3+ and Ho3+-codoped lasers are the dominant laser sources operating in this emission domain. Many host materials, mainly fluorides and oxides such as YLF [4,5 ], LLF [6], KYF4 [7], CaF2 [8], BaY2F8 [9,10 ], KY3F10 [11], YAG [12,13 ], YVO4 [14,15 ], GdVO4 [16,17 ], LuVO4 [18,19 ], CaYAlO4 [20,21 ], and Lu2O3 [22,23 ], have been investigated and developed in the past for that purpose. However, the energy level structures of Tm3+ and Ho3+ ions are such that 1.9 μm lasers, because of unavoidable reabsorption losses, always operate on quasi-three level schemes requiring rather high excitation densities for efficient population inversion. Moreover, in most of the cases, the shortest achievable lasing wavelengths in the 1.8-2.1 µm wavelength domain are always limited to about 1.9 μm. To reach lower laser wavelengths, as in the present work, it is necessary to operate with Tm3+ lasers, and more specifically on broad-band Tm-doped fluorides such as Tm:CaF2 or Tm:KY3F10, the latest being operated, without any intracavity wavelength tuner, at a laser wavelength of 1845 nm and with a laser slope efficiency exceeding 42% [11]. Shorter laser wavelengths down to 1835nm could be obtained with Tm:CaF2 but with the aid of a wavelength selector and much lower laser slope efficiencies [11]. Indeed, intracavity wavelength tuners such as a etalon [24], a birefringent filter [7,12 ] or a volume Bragg grating [25], always add some additional losses which degrade the laser performance.
Therefore, in this work, we revisited Nd:YAG for laser generation around 1834 nm. Indeed, first, such Nd:YAG 1834 nm laser line operates on a true four-level scheme without any reabsorption problem. Secondly, the emission cross section of this Nd:YAG 1834 nm emission line would be about 3 × 10−21 cm2 [26], which is quite competitive with the Tm3+ or Ho3+ doped lasers [27]. Thirdly, as most of the Nd-doped materials, Nd:YAG can be pumped efficiently at 808 nm which is a more standard laser diode wavelength than that used around 790 nm for Tm-based laser systems.
The 1834 nm laser line of Nd:YAG has been already operated in the past with bulk samples, but in very unfavorable conditions giving rise to very poor laser performance [28,29 ]. For instance, in 1992, Kubo and Kane [29] operated a diode-pumped Nd:YAG laser system at about 1833 nm but only with a maximum output power of about 3 mW with a crystal cooled down −28°C. Much better laser performance was obtained later, but by operating in a waveguide configuration [30]. With such a Nd:YAG double-clad waveguide laser, lasing was indeed demonstrated at about 1833 nm with an absorbed pump power threshold of 300 mW, an output power of 400 mW and a laser slope efficiency of about 7.5%.
We show here in the present work that it is possible to operate a Nd:YAG bulk crystal at 1834 nm at the watt level and with a laser slope efficiency exceeding 11%, thus offering a real alternative to Tm- and Ho-based lasers for various types of applications.
2. Experimental setup
A schematic of the laser experimental setup is shown in Fig. 1 . The pump source is a commercially available AlGaAs laser diode emitting at 808 nm with a maximum output power of about 21 W. The output pump beam is collected by using a coupling fiber with a numerical aperture of 0.22 and a core diameter of 400 μm. The coupling and focusing lenses are both achromatic doublets with focal lengths of 50 mm, thus giving a minimum pump beam spot of about 400 μm diameter inside the laser crystal. The laser gain medium was a 5-mm-long Nd:YAG with a doping concentration of 0.5at.% and a cross section of 3 × 3 mm2. In order to alleviate thermal lensing effects, the laser crystal was wrapped inside an indium foil and mounted in a copper holder maintained at a temperature of 18°C.
The laser cavity is a simple two-mirror resonator with a physical length of about 48 mm. The flat input mirror (M1) was coated with a high transmission coating of 90% at 808 nm and high reflection of more than 99.9% at 1834 nm. To suppress the high-gain emission lines of Nd:YAG around 946, 1064, 1320 and 1440 nm, the input mirror coating was also highly transmittive (more than 85%) for all these potential lasing lines. Four concave mirrors, all with a radius of curvature of 50 mm, were used one after the other as output mirrors (M2). These four output mirrors all had transmissions of more than 80% for the above-mentioned high-gain emission lines and transmissions of about 0.51%, 1.85%, 2.20% and 4.06% at 1834 nm. They were also highly transmittive, as much as 90%, at the pump wavelength, thus not allowing for recycling the transmitted pump power back into the laser crystal.
3. Results and analysis
The emission spectrum of the Nd:YAG crystal was first measured at room temperature and shown in Fig. 2 by using an optical spectrum analyzer (Ocean Optics, NIRQuest) with a precision of about 3.2 nm . This spectrum is less well spectrally resolved around 1800 nm than the one reported in [26], but it is more complete on the long wavelength side and in better agreement with the Stark level structure of the considered 4F3/2→4I15/2 inter-manifold emission transition reported in [31]. Indeed, at room temperature and according to [31], there should be not only emission lines around 1740, 1758, 1785, 1796, 1806, 1824 and 1834, as reported in [26], but also around 2029, 2065, 2090 and 2129 nm.
Fig. 2 Emission spectrum of Nd:YAG crystal around 1.8 μm. Inset: full emission spectrum from 0.9 to 2.3 μm.
According to Fig. 2, the most intense line occurs around 1834 nm and is associated with a R1→W4 inter-Stark emission transition. According to [26], its stimulated emission cross section would be about 3 × 10−21 cm2, which is about 1/90 of the typical four-level 1064nm laser line or 1/14 of the quasi-three-level one at 946 nm. Therefore, it does not only mean that much attention must be paid to avoid lasing at these high-gain laser lines by using specific mirrors (see above). It also means that laser thresholds can be relatively high and that it can be difficult to reach high laser efficiencies”.
As indicated above, the laser measurements were carried out by using four output couplers with different transmissions. The resulting laser output versus absorbed pump power curves obtained at 1834 nm is reported in Fig. 3 . The maximum output power when the crystal temperature was maintained at 18°C reached 1.1 W and was obtained with the 2.2% transmittive output mirror. The threshold absorbed pump power was 3 W and the associated laser slope efficiency reached a value of about 11.1%. Increasing (up to 4.06%) or decreasing (down to 1.85%) the transmission of the output mirror both resulted in a degradation of the laser performance, with maximum output powers of about 0.69 W and 0.99 W and laser slope efficiencies of 7.8% and 10.6%, respectively. Using a 0.51% transmittive output mirror, we obtained a laser threshold of about 2.07 W along with a maximum output power of 0.76 W and a slope efficiency of about 6.8%.
It is worth noting that, in all cases, no temperature roll-off was observed thus indicating laser output powers could be easily increased further by increasing the input pump power. In particular, we consider that the output power could still be increased by optimizing the transmission of the output mirror between 2.20% and 4.06%. Performing a Findlay-Clay analysis with the four sets of data we could deduce intracavity round-trip losses of about 2.3%.
If there seems to be no temperature roll-off at a given sample temperature, it was noticed however that the output power could be almost linearly increased (see in Fig. 4 ), up to about 1.31 W only by lowering the sample temperature down to 8°C while keeping the pump power unchanged. By linearly fitting these data, the relation between the output power (Pout) and the sample temperature (T) can be expressed as Pout = −0.02T + 1.46. Thus, after a simple calculation, we can deduce that for our 5 mm long Nd:YAG crystal the power variation rate was about 0.36%/mm·K. A similar result was also found in the case of Nd:GGG [32]. The output beam quality of the 1834 nm laser was determined at the maximum output power of 1.31 W, thus at the maximum available pump power and for a sample temperature of 8°C. By using an infrared camera (Electrophysics, MicronViewer 7290A) and a 90/10 knife-edge technique (see in Fig. 5(a) ), M2 factors of about 2.51 and 2.53 could be measured in the x and y directions, respectively, thus attesting for a nearly perfect circular symmetry. It was also found that the beam quality obtained at the sample temperature of 8°C was better than that measured at 18°C with M2 factors of 3.23 and 3.39 in x and y directions, respectively.
Fig. 5 (a) Beam propagation factor measurements for the1834 nm laser and (b) 1834 nm laser spectrum.
In the end, a more accurate laser wavelength measurement was performed by using a Bristol Model 721B-IR laser wavelength meter with a wavelength resolution of 6 GHz (about 0.06 nm). The peak wavelength was found at 1834.25 nm with a FWHM of about 0.13 nm (see in Fig. 5(b)). Since the laser cavity was only about 48 mm long, it is easy to estimate that the measured narrow laser line could correspond to about 4 longitudinal modes (intracavity longitudinal mode spacing of about 0.03 nm). Such a narrow laser linewidth encouraged us to operate the laser in single-mode.
Two simple methods could be readily employed to reach this goal, either inserting a thin Fabry-Pérot etalon inside the laser cavity or shortening the laser cavity, both aiming at increasing the intracavity longitudinal mode spacing. Choice was made to increase this longitudinal mode spacing by shortening the cavity length down to about 10 mm (expected mode spacing of about 0.115 nm). Under these conditions (maintaining the cooling temperature at 8°C), a flat output mirror with the same coating as the 2.2% transmittive concave output mirror was used. The corresponding laser output versus absorbed pump powercurve is reported in Fig. 6(a) . The maximum output power and slope efficiency reduced to 0.70 W and 8.0%, respectively, and the laser threshold increased up to about 5.04W. On the other hand, it resulted (see in Fig. 6(b)) in a slightly shifted laser emission line now peaking at 1834.29 nm with a FWHM of about 0.12 nm. Such a FWHM is only slightly larger than the expected longitudinal mode spacing (0.115 nm), which proves that the laser was nearly operating on a single-mode. However, since the measured value has not changed significantly from that measured for a 48 mm long cavity, it is likely that both measured linewidths were overestimated due to the limited spectral resolution of the optical spectrum analyser.
Fig. 6 (a) Laser output power versus absorbed power curve using a 10-mm-long flat-flat laser cavity and (b) Laser emission spectrum of the 1834 nm laser using the 10-mm-long flat-flat laser cavity.
4. Conclusion
Using a simple two-mirror linear cavity, a diode-pumped Nd:YAG laser system was operated at 1834.25 nm with a maximum output power of about 1.10 W and a laser slope efficiency of about 11.1% while maintaining the temperature of the laser crystal at 18°C. By decreasing the temperature of the laser crystal down to 8°C, the maximum output power was increased up to 1.31 W. This represents, to the best of our knowledge, the best laser performance ever obtained at the considered laser wavelength with a Nd-doped bulk crystal. The temperature-dependent behavior of the laser output power showed a power variation rate of about 0.36%/mm·K, which implies that lowering the temperature of the laser crystal could be a simple method for power scaling. The beam quality of the 1834 nm laser was characterized by x and y direction beam propagation factors of about 2.51 and 2.53, respectively. It was finally demonstrated that the system could be operated on a single mode with an output power of 0.70 W at the laser wavelength of 1834.29 nm, by simply shortening the length of the laser cavity down to about 10 mm.
Acknowledgments
The authors wish to thank the financial support from National Natural Science Foundation of China (NSFC) (61575164), the Specialized Research Fund for the Doctoral Program of Higher Education (20130121120043), the Fundamental Research Funds for the Central Universities (2013121022), and Natural Science Foundation of Fujian Province of China (2014J01251).
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