Highly efficient in-band pumped Er:YAG laser with 60 W of output at 1645 nm

Abstract

Laser sources operating in the eyesafe wavelength regime around 1.5-1.6 µm have applications in a number of areas including, remote sensing, ranging and free-space communications. For many of these applications, the requirement for high output power is often supplemented by the need for high efficiency and good beam quality. This combination of operating characteristics is very difficult to achieve in conventional diode-pumped solid-state lasers based on erbium doped crystals (sensitised with Yb) owing to the relatively high fractional heat loading which results from the large quantum defect (~ 40%) and energy-transfer-upconversion. To alleviate this problem, attention has recently turned to singly-doped crystals (e.g. Er:YAG) and in-band pumping using an Er,Yb fibre laser. This approach has the advantage that most of the waste heat is generated in the fibre, which is largely immune to thermal effects, and quantum defect heating in the Er-doped crystal is very small (~7%). Using this hybrid laser scheme, we have demonstrated ~60 W of continuous-wave output from an Er:YAG laser at 1645 nm with a slope efficiency of 80 % 1. However, for some remote sensing applications this operating wavelength is a little inconvenient, since there are 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 from the same upper level manifold (4 I 13/2) at 1617 nm, which lies in a region of the spectrum where there are no atmospheric absorption lies. However, this transition has a much more pronounced three-level character and hence a much higher threshold pump power, so it has received little attention in spite of its obvious advantages for certain applications. Here, we report preliminary results for power scaling of an Er:YAG laser at 1617 nm in-band pumped by a high-power cladding-pumped Er,Yb fibre laser. The Er,Yb fibre pump laser was constructed in-house with wavelength selection provided by an external cavity containing a diffraction grating in the Littrow configuration. The Er,Yb fibre laser was pumped by two 9-bar pump modules at 975 nm and produced up to 120 W of output in a beam with M 2 ≈ 5. For efficient pumping of Er:YAG, the Er,Yb fibre laser operating wavelength was tuned to the absorption peak at 1532 nm. In this preliminary study, a simple four-mirror folded cavity was employed for the Er:YAG laser comprising a 29 mm long Er:YAG rod with 0.5 at.% Er 3+ concentration mounted in a water-cooled aluminium heat-sink, a plane pump in-coupling mirror with high reflectivity at 1617 nm and high transmission at 1532 nm, two concave mirrors of 100 mm radius of curvature and high reflectivity at 1617nm, a plane output coupler of transmission, 30% at 1617 nm, and a 100 µm thick fused silica etalon. The latter was used to select the 1617 nm line and suppress oscillation on the higher gain 1645 nm line. Figure 1 shows the Er:YAG output power at 1617 nm as a function of pump power. The Er:YAG laser yielded a m...

Resonantly diode-pumped continuous-wave and Q-switched Er:YAG laser at 1645 nm

“…This minimizes the waste heat from the pumping process, reducing thermally induced lensing and birefringence to yield a laser that can operate efficiently at a variety of power levels. Er:YAG lasers can be resonantly pumped at either 1470 nm or 1532 nm, using either laser diodes [2][3][4][5] or erbium-doped fiber lasers (EDFL) [6][7][8][9][10][11].…”

Passively Q-switched diode-pumped Er:YAG solid-state laser

“…The pump sources are either Er:Yb fiber lasers operating at 1532 nm [1] or laser diodes emitting at 1470 nm [2] and 1532 nm [3,4]. For several configurations, the laser emission naturally occurred at 1645 nm.…”

Efficient 1645 nm continuous-wave and Q‑switched Er:YAG laser pumped by 1532 nm narrow-band laser diode

“…One of the most promising eye-safe lasers, 1645 nm hybrid fiber-bulk erbium lasers, has been proven to be effective in achieving high output power and good beam quality [3][4][5][6][7]. The best results were reported to be a 60 W output with a slope efficiency of 80% for a continuous-wave (CW) operation [5] and 30 mJ pulses with a pulse width less than 20 ns at 20 Hz PRF for a Qswitched operation [7]. This two-step method, however, adds complexity, weight, and volume.…”

“…In addition, the optical-optical efficiency of the two-step system is limited by the efficiency of the first step: 1532 nm fiber lasers pumped by 976 nm laser diodes (LDs). Consequently, only 28% optical-optical efficiency is achieved for the two-step system [5,7].…”

“…The optical-optical and slope efficiencies are much lower than the quantum efficiency (~90%). The strong thermal effect in Er:YAG crystals is explained as a result of ETU [3,5,6,15], which is quadratic in the upper laser level population. Therefore, Er:YAG crystals with low Er 3+ doping concentration are required.…”