1. Introduction

Solid-state laser sources based on the Er³⁺ ion operating in the eye-safe wavelength regime around 1.6 μm have seen rapid developed over the past few years owing to their applications in free-space optical communications and remote monitoring [1–3]. Due to its well-established noise reduction techniques, the tunable Er:YAG solid-state laser is suitable for atmospheric trace gas measurements of the critical greenhouse gases CO₂ and methane [4, 5].

With the development of the resonant pumping technology, the 1532 nm erbium-doped fiber laser and ultrahigh-brightness diode laser with a narrow linewidth (~0.17 nm) are available and well match the narrow absorption bandwidth of Er:YAG (~1 nm around 1532 nm) [6, 7]. The resonant pumping technology reduces the thermal lensing and thermal-stress-induced birefringence and is characterized by a high Stokes factor of 6.9%, which enables the laser operating efficiently at high power levels. All of these advantages of the resonant pumping technology make the tunable Er:YAG laser more efficient and competitive in the gas detecting application. Moreover, the tunable Er:YAG laser with a narrow bandwidth operating at around 1645 nm can accurately avoid the absorption of methane in the atmosphere and thus is more competitive than the free-running 1645 nm Er:YAG laser in the applications requiring free-space propagation over long distances.

In general, the tunable operation of solid-state lasers can be obtained by inserting spectrum discrimination filter into the cavity, such as etalon [8], dispersive prism [9] and birefringent filter [10]. All of these techniques, however, introduce additional losses and complicate the laser cavity. In addition, the selected spectrum is usually not sharp and the free spectral range of the etalon is limited by its structure. Volume Bragg gratings (VBGs) can serve as perfect spectrum selectors, because they typically have a narrow bandwidth on the sub-nanometer scale. Meanwhile, VBGs have excellent optical, thermal and mechanical stabilities in high-power operations. VBG is fabricated by periodically modulating the reflective index of a photo-thermo-refractive glass throughout the volume. The spectral selectivity and center wavelength depend on the thickness and period of the refraction index modulation. Recently, VBGs have been used as spectrum selectors in fiber, semiconductor and solid-state lasers [7, 11, 12]. In solid-state laser systems, tunable lasers based on Yb³⁺ (Yb:KYW laser with a tunable range of 53 nm [13]) and Tm³⁺ (Tm:LuYAG laser with a tunable range of 60 nm [14]) have been demonstrated. Due to the lower multiplicity of the Stark levels, the fluorescence spectrum of Er:YAG is not as smooth or broad as that of Tm:YAG and Yb:KYW. Earlier, single-crystal Er:YAG laser with a tuning range of 1.8 nm by an intra-cavity etalon has been reported [8].

Recently, transparent ceramics laser technology has been greatly developed due to lots of advantages over conventional single crystals, such as rapid and large volume fabrication, flexibility in doping concentration and profile, and low cost. It has been reported that transparent Er:YAG ceramics laser have nearly the same lasing performance as single crystal laser [6, 15–17]. In this paper, we demonstrated a tunable Er:YAG ceramic laser with a total tuning range of 23.1 nm by using a VBG as the wavelength selective element. The pump source was a cladding pumped Er, Yb fiber laser at 1532 nm. The bandwidth of the output spectrum was narrower than 0.05 nm over the whole tuning range. In a single-pass pump scheme, the tunable laser yielded a maximum output power of 3.7 W at 1644.7 nm with an absorbed power slope efficiency of 42.3% and an M² value of 1.2. At around the 1634 nm emission peak, the maximum output power of 1.4 W was obtained at 1633.0 nm, and the corresponding slope efficiency with respect to the incident power was 20.9%.

2. Experiments setup

A z-shaped laser cavity was designed for the tunable Er:YAG ceramic laser, as shown in Fig. 1.The pump source was an in-house constructed double-clad Er, Yb fiber laser [16]. The 4 m double-clad fiber was comprised by an Er, Yb-doped phosphor-silicate core with a diameter of 30 μm (0.22 NA) and a D-shaped pure silica inner cladding with a diameter of 350 μm (0.49 NA). A 967 nm diode laser source was used as the pump light, which was split spatially two beams of roughly equal power and pumped the Er,Yb-doped fiber from two ends. By employing an external cavity consisting of an antireflection coated collimating lens and a VBG with a high reflectivity of > 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.

 

Fig. 1 Schematic diagram of the tunable Er:YAG ceramic laser setup in the experiment. IC: Input coupler. OC: Output coupler.

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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].

 

Fig. 2 The output power of the tunable Er:YAG ceramic laser as a function of the wavelength.

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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 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. 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.

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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. 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.

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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.