1. Introduction

Laser sources operating in the eyesafe wavelength regime around 1.5–1.6 µm have numerous applications including, remote sensing, ranging and free-space communications. Direct (in-band) pumping of Er:YAG with an Er, Yb fiber laser [1–6] or a diode laser [7–9] is rapidly emerging as one of the most promising routes to this wavelength regime owing to the prospect of high-average output power in both continuous-wave (cw) and Q-switched modes of operation. The use of a fiber-based pump laser is particularly attractive as this allows the use of Er:YAG crystals with low erbium ion concentrations to minimize the detrimental impact of energy-transfer-upconversion (ETU) on laser performance [10] and avoids the need for cryogenic cooling to achieve high lasing efficiencies [8]. One of the main attractions of the hybrid fiber-bulk laser approach is very low quantum defect heating in the bulk laser medium, which greatly simplifies power scaling in a laser geometry that also offers the potential for high pulse energy in Q-switched mode. This approach has been successfully applied to Er-doped and Ho-doped lasers operating in the ~1.6 and ~2.1 µm wavelength regimes. In recent work, we demonstrated hybrid lasers based on Er:YAG with >60 W of cw output [1] and with >15 mJ pulse energy in Q-switched mode on the ⁴I_13/2→⁴I_15/2 transition at 1645 nm [2,10]. However, for some remote sensing and ranging applications this operating wavelength is a little inconvenient, since there are some atmospheric absorption lines due to methane which are in very close proximity necessitating careful selection and control of the lasing wavelength. Er:YAG also has a transition between the same upper and lower manifolds at 1617 nm (see Fig. 1), which lies in a region of the spectrum where there are no atmospheric absorption lines. This transition benefits from a higher emission cross-section, but has much more pronounced three-level character requiring ~14 % of the Er³⁺ ions to be excited to the upper manifold to reach transparency compared with ~9 % for the 1645 nm transition. As a result, the threshold pump power for 1617 nm operation is generally much higher than for 1645 nm operation and hence standard resonator configurations normally lase at 1645 nm. Operation of Er:YAG at 1617 nm has been achieved either by employing additional wavelength discriminating components intracavity (e.g. etalons) [5,6] to suppress the line at 1645 nm or by operating at cryogenic temperatures where the re-absorption loss at 1617 nm is dramatically reduced [8]. In both cases, the highest average powers reported to date are below <6 W using wavelength discrimination [6] and <0.32 W for quasi-cw operation at 78 K [8].

Here, we report the results of an experimental study on 1617 nm operation of hybrid Er:YAG lasers at high pump powers and discuss how various factors (including Er³⁺ doping level and cavity design) influence laser performance. Based on the results of this study and using a simple strategy for power scaling we have demonstrated an Er:YAG laser, in-band pumped by a cladding-pumped Er, Yb fiber laser at 1532 nm, with 31 W of cw output at 1617 nm for 72 W of incident pump power at room temperature. To the best of our knowledge, this is the highest cw output power reported to date for an Er:YAG laser operating on the 1617 nm line.

 

Fig. 1. Er:YAG energy-level diagram showing the laser transitions at 1617 nm and 1645 nm.

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2. Experiment

The hybrid Er:YAG laser configuration used in our experiments is shown in Fig. 2. The Er, Yb fiber pump laser was constructed in-house [11] and comprised a ~2.5 m length of double-clad fiber with a 30 µm diameter (0.22 NA) Er, Yb-doped phospho-silicate core surrounded by a 400 µm diameter D-shaped pure silica inner cladding. The fiber was coated with a low refractive index (n=1.375) fluorinated polymer outer-cladding giving a calculated NA of 0.49 for the inner-cladding pump guide. Operation at the absorption peak in Er:YAG at 1532 nm was achieved with wavelength selective feedback provided by an external cavity containing a diffraction grating (600 lines/mm) in the Littrow configuration. A relatively long focal length collimating lens (120 mm) was used in the external cavity to ensure that the spectral selectivity of the grating was sufficient to achieve a narrower lasing bandwidth (~0.4 nm) than the Er:YAG absorption bandwidth (~4 nm). Feedback for lasing at the opposite end of the fiber was provided by the ~3.6 % Fresnel reflection from a perpendicularly-cleaved facet. Pump light was provided by two polarization-combined nine-bar diode pump modules at 976 nm. The output beam from the combined pump modules was split spatially into two beams of roughly equal power using a knife-edge mirror allowing pumping of the Er, Yb fiber from both ends. In this way, heat loading was more uniformly distributed along the fiber reducing the likelihood of thermally-induced damage to the polymer outer-coating. Using this arrangement the Er, Yb fiber yielded a maximum output power of 120 W at 1532 nm in a beam with M²<5 for ~440 W of launched pump power. At this power level, the fiber laser was prone to damage so, in order to ensure reliable operation, the laser operated at power levels below 75 W.

A simple four-mirror folded resonator was employed for the Er:YAG laser. This comprised a plane pump input-coupler with high reflectivity (> 99.8 %) at the lasing wavelength (1600–1650 nm) and high transmission (>95 %) at the pump wavelength (1532 nm), two concave mirrors (R1 and R2) of 100 mm radius-of-curvature with high reflectivity (>99.8 %) at both the lasing and the pump wavelengths and a plane output coupler. A range of output couplers with transmissions of 10 %, 20 %, 30 % and 50 % at the lasing wavelength were available for our study. To investigate the influence of Er³⁺ concentration on performance, three Er:YAG rods with doping levels of 0.25 at.%, 0.5 at.% and 1.0 at.% and with respective lengths of 58 mm, 29 mm and 15 mm were employed. The crystal lengths were selected so that all three crystals had approximately the same pump absorption efficiency at low pump powers (i.e. in the absence of ground-state bleaching). The latter was measured to be ~98% indicating that the absorption coefficient in Er:YAG for 1532 nm pumping is ~260 m^-1/at.%. Both end faces of the Er:YAG rods were antireflection coated for the 1.5 to 1.7 µm wavelength range covering both pump and lasing wavelengths. The Er:YAG rods were mounted in a water-cooled aluminum heat-sink maintained close to room temperature at 17°C and positioned at the mid-point of the resonator arm defined by the two curved mirrors (R1 and R2). The physical length of this arm of the resonator was ~125 mm and the total physical length of the resonator was ~365 mm resulting in calculated a TEM₀₀ waist radius of ~80 µm. The angle of incidence on the curved mirrors was made very small (<10°) to minimize astigmatism. The pump beam from the Er, Yb fiber laser was coupled into the resonator via the plane input-coupler and then focused to a waist radius of ~75 µm in the Er:YAG rod with the aid of curved mirror R1. An uncoated fused silica etalon of 100 µm thickness was used to provide the wavelength discrimination (when necessary) to ensure lasing on the 1617 nm line.

 

Fig. 2. Schematic diagram of Er:YAG resonator. IC : input coupler mirror (AR at 1532 nm and HR at 1600–1700 nm). OC : output coupler mirror (Transmission (T) of 10 %, 20 %, 30 % or 50 % at 1600–1700 nm).

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3. Results and discussion

At threshold for laser oscillation, the round-trip gain must equal the fractional loss of the laser cavity, hence

where σ_g is the gain cross-section, N is the active ion doping concentration, l is the length of the gain medium, T is the transmission of the output coupler and L is the round-trip cavity loss (excluding the output coupling loss). The gain cross-section depends on the effective emission and absorption cross-sections (σ_e and σ_a) for the transition and on the population densities, N₂ and N₁, in the upper manifold (⁴I_13/2) and lower manifold (⁴I_15/2) respectively via the relation [4]:

where the inversion parameter β=N₂/(N₁+N₂)≈N₂/N in the absence of energy-transfer-upconversion. To enforce lasing on the 1617 nm line requires the resonator to be configured so that the threshold for 1617 nm operation is lower than for any of the other laser transitions from ⁴I_13/2 to ⁴I_15/2. Usually, the 1645 nm line has the lowest threshold due to its weaker three-level character (i.e. lower effective absorption cross-section), even though the 1617 nm transition has a much higher effective emission cross-section. However, this does leave open two options for wavelength selection. The first, and most obvious, approach is to use loss discrimination (e.g. an intracavity etalon) to select the 1617 nm line. The second, and perhaps the most simple approach is to exploit the fact that the gain cross-section, σ_g increases more rapidly with inversion parameter, β for the 1617 nm line than for the 1645 nm line (see Fig. 3). The net result is that at high inversion densities the gain cross-section at 1617 nm is higher than at 1645 nm. At room temperature (300 K), this requires at least 35% of the Er³⁺ ions to be excited to the ⁴I_13/2 manifold. In practice, this can be achieved by simply increasing the threshold using a much higher transmission output coupler, without the need for additional wavelength selective intracavity components.

 

Fig. 3. Calculated gain cross-sections at 1617 nm and 1645 nm as a function of the population inversion parameter.

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Preliminary experiments were conducted using the Er:YAG rod with 0.5 at.% doping level and using the intracavity etalon to select 1617 nm operation. The results for laser output power as a function of incident pump power for three different output coupler transmissions (10, 20 and 30%) are shown in Fig. 4(a). Also, for comparison the output power for 1645 nm operation versus pump power (i.e. without the etalon present in the cavity) is also shown. It can be seen that the laser power increases with output coupler transmission at 1617 nm. However, the output powers at 1617 nm are somewhat lower than at 1645 nm. Moreover, there is a very pronounced roll-over in output power at 1617 nm as pump power is increased beyond ~60 W in contrast to the situation at 1645 nm. Figure 4(b) shows the 1617 nm performance with an output coupler transmission of 50%. In this case an etalon was not required. The threshold pump power was ~5.2 W and the slope efficiency with respect to incident pump power was ~42% up to a pump power of ~45 W. At higher pump power the output power rolls over very sharply reaching a maximum output power of only 16 W. This is considerably lower than for the same resonator with 20% and 30% output coupler transmissions. We attribute the roll-over in power at 1617 nm to more pronounced three-level character (i.e. increased re-absorption loss) due to a rise in temperature resulting from increased thermal loading at high pump powers. The situation is further exacerbated by energy-transfer-upconversion which acts to further increase heat loading when operating at high excitation densities. This is evident from the more dramatic roll-over in power for the laser with a 50% output coupler transmission.

 

Fig. 4. Output power versus incident pump power for Er:YAG laser with 0.5 at.% doping level (a) using output couplers with transmissions of 10%, 20% and 30%. (The solid symbols represent 1617 nm operation with an etalon and the open symbols represent 1645 nm operation). (b) Output power at 1617 nm with output coupler transmissions of 20%, 30% (with etalon) and 50% (without etalon).

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We repeated the experiment with Er:YAG rods with 0.25 at.% and 1.0 at.% doping levels using the 50% transmitting output coupler. Figure 5(a) shows the output power as a function of pump power for the three doping levels used in our study. The thermal loading density, and hence temperature rise in the 0.25 at.% doped rod is at least a factor-of-two lower than for the 0.5 at.% doped rod due to the lower doping concentration and reduced upconversion losses. As a consequence, we observed no roll-over in output power up to the maximum available pump power of 75 W. In contrast, the 1.0 at.% doped rod has a much higher thermal loading density and hence temperature rise and, as expected, the laser performed far worse reaching a maximum output power of only 3 W. These results support our assertion that the roll-over in power is due to increased three-level behaviour due to thermal loading and is exacerbated by energy-transfer-upconversion. Thus, the use of low Er³⁺ doping levels in conjunction with effective thermal management is crucial for power scaling on the 1617 nm transition in continuous-wave and Q-switched modes of operation.

 

Fig. 5. Er:YAG laser output power at 1617 nm versus pump power for (a) different Er³⁺ doping levels using an output coupler with 50% transmission and (b) an optimized cavity design using a 0.25 at. % crystal.

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Figure 5(b) shows the output power at 1617 nm versus pump power for an optimized resonator design using the 0.25 at.% Er:YAG rod. In this case, the two 100 mm radius-of-curvature mirrors were replaced by mirrors with 150 mm radius-of-curvature and the resonator length was adjusted to give a larger calculated TEM₀₀ beam waist radius of ~100 µm and hence a better spatial overlap with the pumped region. The threshold pump power was ~4.1 W and the slope efficiency with respect to incident pump power was ~47 %. There was no roll-over in output power up to the maximum available pump power and the laser yielded a maximum output power of 31 W at 1617 nm in a beam with M²≈2.2 for 72 W of incident pump power.

4. Summary

Operation of hybrid in-band pumped Er:YAG lasers at 1617 nm at high power levels in continuous-wave or in Q-switched modes of operation is much more challenging than for operation on the more familiar 1645 nm line. Our results suggest that thermal loading due to quantum defect heating and energy-transfer-upconversion, and the associated increase in temperature and lower level re-absorption loss is the main reason. We conclude that the use of a low Er³⁺ doping level and effective thermal management is vital for power scaling on this transition. Using this simple power scaling strategy we have demonstrated on Er:YAG laser, pumped by a high-power Er, Yb fiber laser at 1532 nm, with a continuous-wave output power of 31 W at 1617 nm for 72 W of incident pump power and with a corresponding slope efficiency of 47%. Further scaling of the output power and extension to Q-switched mode of operation may well benefit from the use of even lower erbium doping levels.