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Abstract
We demonstrated continuous wave operation of an in-band pumped Er:YAG planar waveguide laser with the output of 75 W at 1645 nm and a slope efficiency of 64% with respect to the absorbed pump power at 1532 nm.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The waveguide lasers occupy a ‘space’ between fiber lasers and solid state lasers borrowing an advantageous pump and laser beam confining feature of the former and better spectroscopic properties and thermal conductivity of the latter, associated with its crystalline nature. This unique combination makes waveguide-based laser architectures very promising for power scaling.
The scaling potential of a cladding pumped planar waveguide (PWG) laser [1] is based on (i) its ‘thermal efficiency’ due to a much higher surface area to laser volume ratio with respect to conventional bulk solid-state laser gain media, and (ii) its capability to accommodate substantial amounts of pump from a relatively inexpensive, powerful laser diode bar stack (LDBS). Such pumping is possible due to the (i) high numerical aperture (NA) of the PWG cladding, and (ii) a large aspect ratio of the PWG cladding which makes it possible to simply couple a LDBS output into PWG [2, 3].
Cladding pumped PWG lasers based on Nd- and Yb-doped single-crystalline and ceramic host materials have been studied quite extensively, and powers ranging from a few Watts to a few kilowatts have been achieved in the 1 µm wavelength domain (e.g., [1–7]). Meanwhile, very little research has been presented on the eye-safe Er-doped PWGs operating at around 1.6 µm so far. In [8], Shepherd, et al the authors took advantage of Yb co-doping in order to achieve sufficient absorption at ~980 nm. No research data was found on resonantly-clad-pumped crystalline Yb-free, Er-doped PWG lasers. This paper presents the first results obtained with a clad-pumped laser based on a Yb-free, Er:YAG-core PWG utilizing in-band excitation. Continuous wave (CW) operation of a resonantly pumped, 200 mm long Er:YAG PWG laser was demonstrated with a 75 W output at 1645 nm and a slope efficiency of 64% with respect to the absorbed pump power at 1532 nm.
2. Experimental setup
A 200 mm long planar waveguide with a 50 μm × 4 mm cross-section, single-crystalline Er3+(1%):YAG core and a 1 mm × 4 mm single-crystalline Yb3+(1%):YAG inner cladding was fabricated by Onyx Optics, Inc. (see Fig. 1) with an AFB® (Adhesive-Free Bond) process, commonly known as ‘diffusion bonding’. In order to reduce the Numerical Aperture (NA) of the waveguide’s core, an inner cladding was made of Yb3+-doped YAG instead of un-doped YAG, which was previously used in PWG structures [2,3]. Note that the Yb:YAG is fully transparent for the 1532 nm pump.
Ytterbium co-doping of YAG increases the index of refraction of the cladding with respect to the Er-doped core by only ~4.8 × 10−5, which reduces the NA of the waveguide to an ultra-low value of 0.0132.
Indeed, if the refractive index of YAG, nYAG = 1.81744 [9], then co-doping YAG with 1% Ytterbium (cladding) or 1% Erbium (core) increases its index of refraction by ΔnYb ~1.6 × 10−4 and by ΔnEr ~2.08 × 10−4 correspondingly [10]. As a result, the NA of the core for these particular doping concentrations becomes:
Due to the thin, 50 µm of the waveguide core and its small NA, the diffraction limited divergence of the laser beam should be expected in this guiding direction. The criterion of a diffraction limited output of a square waveguide with an a × a core is B < 1.37. Where the B-number is the “square” analog of the V-number for round fibers define [11,12]:
In our waveguide, with a = 50 µm and λ = 1.64 µm, in the vertical, or guiding, direction B is equal to 0.86.To complete the PWG design, a 1 mm thick sapphire plate was bonded to the bottom of the Yb:YAG cladding. It served as a second cladding to Yb:YAG, and the large difference in refraction indices resulted in an inner cladding NA of ~0.46. This sapphire plate also ensured more efficient conductive cooling of the waveguide from the bottom. The top surface of the inner cladding was coated by a thin (~1.5 - 2 µm) SiO2 protective layer, forming the second, upper cladding.
Both ends of the Er:YAG PWG were anti-reflection (AR) coated for the 1500-1680 nm wavelength band. The PWD, wrapped in Indium foil, was clamped between two water-cooled copper blocks for efficient cooling, maintained at 16 C. The optical-to-optical efficiency of the resonantly, cladding-pumped Er:YAG-core PWG laser was assessed in a simple power oscillator configuration. Its simplified optical layout is presented in Fig. 2.
Fig. 2 A simplified optical layout of the resonantly cladding-pumped Er:YAG planar waveguide laser. Also pictured is a 10-bar LDBS (right) and a copper waveguide housing
We used a microchannel-cooled, fast and slow axis collimated (FAC/SAC), commercial 10-bar LDBS (QPC Lasers, BrightLock® Stacked Array with Distributed Bragg Gratings, 1532.5 nm at 20 CO) as a linearly polarized pump source with ~2.5 nm bandwidth.
Its wavelength could be temperature-tuned by varying the coolant temperature to achieve maximum laser efficiency. The LDBS operated at a constant current of 75 A to maintain its peak wavelength. The combination of a polarizer and a half-wave plate was used as a variable pump power attenuator.
The LDBS pump was focused into the inner cladding of the PWG by an aspheric lens with a focal length of 45 mm through a flat dichroic mirror (T > 98% at 1520 - 1540 nm, and R > 99.5% at 1650 nm). The latter served as a high reflector in the laser cavity. An angular divergence of the pump in the horizontal direction (~0.07 rad) was approximately three times larger than that in the vertical direction. As a result we were able to use only a single aspheric lens in lieu of two cylindrical ones in order to focus the astigmatic pump beam into the PWG inner cladding. The pump spot size, measured on the front surface of the PWG, was about 3.2 mm × 1 mm. This lens also provided the best NA matching of the focused pump beam and the PWG cladding. The pump beam completely fills the 1 × 4 mm cross section of the cladding in about 12 mm away from the entrance of the waveguide.
A laser cavity was formed by a flat dichroic mirror as mentioned above and a concave output coupler with the 2 m radius of curvature. Both cavity mirrors were butt-coupled to the waveguide ends. A dichroic mirror placed at 45° angle (see Fig. 2) allowed for the simultaneous measurement of both the PWG laser output at 1645 nm and the unabsorbed fraction of the pump power at 1532 nm. The PWG laser beam divergence was measured using a CCD camera (Spiricon, model LW230) placed in the focal plane of the concave mirror with a radius of curvature of 25 cm.
We determined the absorbed power by simultaneously measuring the incident pump power, the laser output power and the residual pump power transmitted through the output coupler. We also took into account that portion of the pump, which reflected back into the PWG by the output coupler after the first pass. The reflections of the output couplers at the pump wavelength were measured in advance.
3. Results
Figure 3(a) shows how the CW output power of the Er:YAG-core PWG laser depends on the incident pump power. The maximum output of 75 W and the best optical-to-optical efficiency of 37% were achieved with the ROC = 70%.
Fig. 3 Output power of the Er:YAG-core PWG laser versus incident pump power at 1532 nm (a), and versus absorbed pump power (b), during CW operation. Dashed lines represent linear data fit. Solid lines represent simulation results.
In Fig. 3(b) the output power of the Er:YAG-core PWG laser is plotted versus the absorbed pump power, revealing the power scaling potential of this waveguide design with a slope efficiency of 64%. As can be seen, the dependence is perfectly linear, so power scaling at this point is pump power limited.
The optical-to-optical efficiency versus the incident pump power was relatively modest, because only ~60-70% of the incident pump was absorbed in the clad-pumped PWG. There are a few reasons for this could be the case. The core to clad area ratio was 1:20, which significantly weakened the effective absorption of the Er(1%):YAG core. In addition, the spectral width of the pump source was noticeably wider than the main absorption band of Er3+ in YAG centered at 1532.1 nm.
The absorption spectrum, associated with resonant 4I15/2 → 4I13/2 transitions of the Er3+, consists of several narrow peaks between 1524 and 1540 nm, see Fig. 4(a). The blue line in the graph shows the emission spectrum of the LDBS pump. This visible pump-absorption spectral mismatch weakens the PWG effective absorption even further. As a result, overall pump absorption in the waveguide varied from ~71% at threshold; to ~61% of the incident power at the maximum available pump. The observed weakening of the pump absorption is most likely caused by the pump saturation effect [13]. We also estimated the localized core heating as a possible reason for deteriorating of the pump absorption efficiency and concluded that the temperature of the Er:YAG core increases only by 3-4 degrees at the maximum available pump.
Fig. 4 (a) Absorption cross-section of the 4I15/2 → 4I13/2 transitions of Er3+ in YAG at 300 K, red line. Spectrum of the laser diode module, blue line. (b) Pump and laser spectral lines.
The output beam divergence was measured at the maximum CW output power of 75 W, see Fig. 5. As expected, in the guiding Y-direction it was diffraction limited ~3.2 × 10−2 rad. In the horizontal X-direction the laser output was highly multimode (~0. 165 rad).
4. Simulations
We developed a numerical model of a cladding pumped waveguide laser for analyzing and optimizing its performance. It was based on general quasi-four-level rate equations for an end-pumped laser. We took into account both the energy transfer up-conversion process and the pump saturation effect. The latter is the most important issue for resonantly pumped Er:YAG due to the strong peak absorption cross-section (~3 × 10−20 cm2) and a long fluorescence lifetime (~7.7 ms) of the 4I13/2 manifold. The influence of energy transfer up-conversion in Er:YAG lasers becomes noticeable if Er-ions doping reaches 0.25% [14] and becomes extremely detrimental with 1%-doping and higher. In simulations we used the up-conversion parameter CUP = 3.5 × 10−18 cm3/sec [15].
We divided the waveguide into several sections (m = 80…100) along its optical axis. For pump fluence, travelling through the m-th section, we made two assumptions: (i) the pump is homogenously distributed across the cladding, and (ii) the reduction of pump power in transition between two sections m and m + 1, caused by absorption in the core, is directly proportional to the ratio (χ) of core-to-cladding cross sections. Note that in this model, the absorption coefficient of the core is modified only by pump saturation and up-conversion and is not considered “diluted” proportionally to χ. The basic details of a similar model can be found in [16], Ter-Gabrielyan et al. A full model description will be reported elsewhere.
Calculated dependences of the laser output power on the incident pump power are plotted in Fig. 3(a) with solid lines. The graph shows that the experimental and simulated data agree well if the passive loss in the waveguide core is taken as ~0.004 cm−1, that is not far from a typical value of passive loss in the laser crystals.
5. Conclusion
We demonstrated continuous wave operation of an in-band pumped Er:YAG planar waveguide laser with the output of 75 W at 1645 nm and slope efficiency of 64% with respect to the absorbed pump power at 1532 nm. This is the first resonantly pumped, eye-safe laser based on an Er:YAG-core planar waveguide with NA-control provided by co-doping cladding with Ytterbium ions. To the best of our knowledge, this is the highest output power and the highest efficiency reported for an eye-safe, cladding-pumped PWG laser.
References and links
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