1. Introduction

High power diode-pumped mid-IR laser sources operating in the 2.5 – 3.0 μm spectral range are of great importance for medical applications, hazardous chemical detection, remote atmospheric sensing and pollution monitoring. Today the most developed sources in the mentioned spectral range are ~2.7-3-μm Er³⁺-doped lasers diode pumped at ~0.96-0.98 μm, which can operate in a CW [1], Q-switched [2] and even cascaded laser regimes (such as ~1.6 μm/3 μm fiber laser [3]). These lasers have penetrated commercial markets recently. E.g., the world's first commercially available, hermetic 2.94 μm laser module [4] based on end-pumped monolithic Er:YAG laser design, described in [5], is available for variety of applications. The concept of Mid-IR laser sources based on Er³⁺-doped gain materials directly diode pumped into the ⁴I_11/2 upper laser level at 0.97-0.98 μm and operating on the ~2.7-3-μm ⁴I_11/2 ⇒ ⁴I_13/2 transition was essentially developed in [6], where slope efficiency of 36% which exceeds the quantum defect limit was also demonstrated. This concept was further developed in [7] and after latest commercial offerings [4] stands out as one of the most productive approaches to obtaining significant Mid-IR power in the ~2.7-3-μm domain. It is very promising due to its potential for laser’s quantum defect (QD)-limited, and beyond [6], optical-to-optical efficiency operation combined with the highest electrical-to-optical efficiency of the 0.97-0.98 μm GaAs laser diodes, design simplicity and, hence, reliability and compactness. Laser power scaling in Mid-IR well over 10 W of average power with nearly diffraction limited beam quality, and yet simple design, is increasingly important for direct infrared countermeasures (DIRCM) applications. This goal is not an easy one to achieve with otherwise very promising ⁴I_15/2 ⇒ ⁴I_11/2 ⇒ ⁴I_13/2 pump-lase scheme due to its high quantum defect, around 70%, and, hence, significant heat deposition associated with laser power scaling. Fiber lasers, which are known to be less susceptible to beam quality deterioration due to heat deposition, are easier to scale to moderate average powers in the 3-μm spectral range, and continuous wave (CW) power of ~20 W has been recently obtained from Er-doped single-mode ZBLAN fiber laser [8]. It is well known that majority of stand-off detection and numerous other applications require pulsed laser regimes with substantial per-pulse energy. Meantime, obtaining scalable amounts of pulsed energy (beyond very few millijoules) out of fiber lasers with the same average power level is simply not feasible due to severe nonlinear limitations and damage issues in fibers. Thus, development of high power bulk solid state lasers in the ~3-μm spectral range and their power scaling with QD-limited performance is of high practical importance. Naturally, due to a QD of ~70%, laser materials with thermal conductivity higher than that of conventional YAG, such as cubic sesquioxides [9] would be a preferable design option for 0.97-0.98-μm GaAs diode-pumped high power ~3-μm laser.

Among known sesquioxides, we have recently identified [10] the Er³⁺:Y₂O₃ crystal as the most suitable gain medium for ~3-μm laser operation. It is due to the fact that its maximum phonon energy is Ω_max = 591 cm⁻¹ [11] (~6 phonons per ⁴I_11/2 − ⁴I_13/2 energy gap of ΔE ≈3500 cm⁻¹), which warrants the ⁴I_11/2 ⇒ ⁴I_13/2 transition in Er³⁺:Y₂O₃ to be predominantly radiative. The low-phonon host requirement is critical for Er-doped mid-IR lasers at ~3-μm for minimizing the upper laser level multi-phonon (non-radiative) decay rate, and, hence, maximizing the ⁴I_11/2 upper laser level lifetime. The upper laser level lifetime for the Er³⁺(2at.%):Y₂O₃ was measured to be 2.4 and 4.2 ms for room and liquid nitrogen temperature (LNT), respectively [10]. Among other known means to facilitate significant power scaling with good beam quality is cryogenic cooling of laser gain medium, benefits of which are presented in detail in [12, 13]. Using this approach we recently demonstrated diode-pumped cryogenically cooled ~2.7-μm Er³⁺:Y₂O₃ ceramic laser with ~1.6 W of CW output power and nearly diffraction-limited output, and this output was strictly pump power-limited [14]. In addition to providing significant thermal management benefits, cryogenic cooling is known to cause significant narrowing of gain material’s emission and absorption features [12,13], which is also the case with Er³⁺:Y₂O₃ [14]. So, nearly QD-limited 27.5% optical-to-optical slope efficiency in [14] was largely achieved due to implementation of sufficiently narrowband high power pump source, a surface-emitting distributed feedback (SE-DFB) laser diode (single emitter). The output spectral width of this SE-DFB pump, ~0.3 nm full width at half maximum (FWHM) at 974 nm described in [15], was almost a perfect match to the major absorption line of the Er³⁺:Y₂O₃ gain material at 77K [14]. Significant power scaling of this nearly QD-limited efficiency Er³⁺:Y₂O₃ laser would require much more powerful and yet sufficiently narrowband pump source. Reported here are the results of further significant power scaling of diode-pumped Er³⁺:Y₂O₃ laser taking advantage of the development and implementation of advanced, power-scaled, SE-DFB pump source. Nearly 14 W of true CW output at ~2.7 μm was achieved with optical-to-optical slope efficiency of ~26%. We believe that this is the highest power ever reported from Er³⁺-doped bulk crystalline laser operating in a ~3-μm spectral range.

2. Experimental details and results

In order to further power scale the Er³⁺:Y₂O₃ laser we designed and implemented the narrowband ~200-W SE-DFB pump laser. Power scaling from the original narrow-band 974 nm single-emitter pump source described in [15] was achieved by using a two-dimensional (2-D) array of SE-DFB single emitters. Surface emission from these diode lasers was achieved by using 2nd order surface grating which diffracts light normal to waveguide propagation direction. Their high spatial brightness was realized by using curved grating with the curvature that matched the phase of the propagating waves. An array of these single emitters was spatially combined and fiber-coupled. ZEMAX model of the combining architecture is shown in Fig. 1 . Each SE-DFB laser has an output beam which is naturally collimated in the longitudinal direction right out of the chip due to the second-order diffraction from the grating. In the lateral direction, it has an 8° full-angle divergence [15]. Four columns and six rows of SE-DFB single emitters were arranged in a quasi-circular geometry to contain within an area encircled by the back-projected numerical aperture (NA) cone of the fiber as shown in Fig. 1(a). In each column, the devices were aligned along their longitudinal direction; hence, a single cylindrical lens was used per column to collimate all the devices as shown in Fig. 1(c). The output beams from the array were collimated in both directions after the four cylindrical lenses as shown in Fig. 1(b). Lateral cross-section of the beams is shown in Fig. 1(c). An aspheric lens was then used for fiber coupling the array emission into a 1000 μm 0.22 NA fiber. The total power out of fiber at 25°C on the laser heatsink was measured to be ~200 watts with the operating current of ~18 A.

 

Fig. 1 ZEMAX layout of two-dimensional array of SE-DFB lasers (a), cross-sectional view of collimated beams after the cylindrical lenses shown in (c) and a lateral cross-section of the optical layout (c).

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Each single emitter device in the 2-D array had a narrow spectral width of ~0.3 nm FWHM. The pitch of the grating was controlled to within ~0.1 nm by a holographic exposure system used to form grating pattern. Nevertheless, variation of the center wavelength of up to 0.5 nm is observed between individual single emitters across the entire 2-D SE-DFB array, presumably due to variation in effective index of the lasers across the wafer resulting from compositional and thickness inhomogeneity. We used devices coming from a single wafer and mounted them on a water-cooled heatsink. Due to the specific coolant flow pattern in our current design, one can also see thermal gradient across the heatsink resulting in another ~0.1 nm inhomogeneous variation in center wavelength of single emitters. As a result, the measured total broadening of the spectral bandwidth goes from 0.3 nm (single emitter) to almost 1 nm for the array at this development stage. Figure 2 indicates the measured spectral output of the 2-D array of twenty SE-DFB single emitters used in this effort for pumping cryogenically cooled ~2.7-μm Er³⁺:Y₂O₃ laser. Spectral distribution is well centered in order to match the strongest ⁴I_15/2 ⇒ ⁴I_11/2 absorption line, 974 nm, of the Er³⁺:Y₂O₃ gain material at 77K.

 

Fig. 2 Spectral distribution of the 200-W output of a 2D SE-DFB laser array (20 single emitters). Measured spectral width is ~1 nm FWHM centered at 974 nm.

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Experimental setup of the 2-D SE-DFB diode-pumped Er³⁺:Y₂O₃ laser is shown in Fig. 3 along with a few experimental components used for measurement of laser parameters. A 2 at.% Er³⁺:Y₂O₃ ceramic sample from Konoshima Chemical Co. was used for our laser experiments, essentially the same ‘spectroscopic grade’ sample which was used in [14]. The spectroscopic properties of the sample used in this experiment were studied earlier and are detailed in [14]. It is important to mention that cooling the sample from room to LNT results in more than the fivefold increase of emission cross section, essentially from ~1.0 × 10⁻¹⁹ cm² to 5.4 × 10⁻¹⁹ cm², respectively. The absorption cross-section at the pump wavelength of 974 nm increases by factor of ~2.6. The ceramic ‘slab’ was sized at 2.5 mm x 5 mm x 17 mm with the 2.5 mm x 5 mm faces AR-coated for the wavelength range of 2.5 – 3.0 μm. The slab was mounted in a standard liquid nitrogen dewar with two high optical quality AR-coated CaF₂ windows and end-pumped along the 17 mm direction by the fiber coupled 2-D SE-DFB diode laser module described above. A 100 – 120 mm long plano-plano laser cavity with the ZnSe dichroic pump mirror and ZnSe output couplers with reflectivities in a range of 80-98% was used in our experiments. The pump beam was collimated and focused through a dichroic cavity mirror into a ~1 mm diameter (1/e²) pump spot inside the laser sample.

 

Fig. 3 Experimental laser setup of cryogenically cooled Er³⁺(2%):Y₂O₃ laser pumped by a fiber-coupled 2D array of SE-DFB emitters at 974 nm.

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In order to analyze spectral and temporal outputs of the ~2.7-μm Er³⁺:Y₂O₃ laser the ACTON 2500i monochromator equipped with PbS detector for spectral measurements and fast response InSb photodetector for temporal measurements were used. All data were processed using a LabVIEW-based acquisition system.

As was already mentioned, cryogenic cooling is extremely beneficial for improving all thermal and thermo-optical properties of the gain material as well as for boosting up the peak emission cross section - which is extremely critical to achieving nearly QD-limited operation with the available laser sample of only ‘spectroscopic grade’ quality [14]. However, spectral line narrowing due to cryogenic cooling makes efficient absorption of diode laser pump power in a limited length laser sample more challenging. Thus the spectral width of our 2-D array of SE-DFB emitters, for a few technological reasons mentioned above, is already wider than the major 974-nm absorption feature of Er³⁺:Y₂O₃ gain material at 77K. This reduces the absorbed fraction of the pump power in the 17-mm long Er³⁺(2%):Y₂O₃ laser sample. Due to pump saturation effects (similar to recently described in [16]) absorbed fraction of the pump also varies with the pump power ranging between ~40% and 50%. Thus to properly infer optical-to-optical efficiency laser output power should be plotted versus absorbed pump power. The well-known up conversion from lower laser level ⁴I_13/2 allowed us to achieve and sustain true CW high efficiency lasing at 2.7 μm from the Er³⁺(2%):Y₂O₃ laser sample despite the nominally self-terminating nature of the ⁴I_11/2 ⇒ ⁴I_13/2 laser transition [14]. At the first sight, the 2 at.% Er³⁺ doping level may seem relatively low in order to sustain the mentioned upconversion process. Meanwhile, due to the more compact lattice structure of Y₂O₃ host the actual number density of RE cations (ions/cm³) is much higher than that in many other hosts. E.g., it is ~2.5 times higher than that of YAG host material [9], which renders the Er³⁺ number density in Er³⁺(2%):Y₂O₃ to be equivalent of the Er³⁺ number density in YAG of 5at%. The efficient laser operation in pure CW mode in our experiments indicates sufficient depletion of lower laser level population at this concentration. The highest output power and the best laser efficiency results achieved so far were obtained with the 85%-reflecting output coupler and are presented in Fig. 4 . The laser was operating mostly at 2.71 μm, at times exhibiting two unstable and relatively weak spectral output components at 2.70 and 2.74 μm. The emission linewidth of the strongest 2.71 µm line was measured as ~0.3 nm FWHM, which is at the resolution limit of the spectrometer used in our experiments.

 

Fig. 4 True CW output versus absorbed pump power dependence obtained for the cryogenically-cooled Er³⁺(2%):Y₂O₃ ceramic laser at 2.7 μm, diode pumped at 0.974 μm.

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As can be seen from Fig. 4 the maximum achieved output power is ~14W and it was achieved with the slope efficiency of ~26%. While this high power CW result clearly indicates significant power scaling potential of the approach, relatively high threshold and somewhat lower optical efficiency vs. our previous results (at much lower power levels) [14] indicate that there is significant room for improvement via better pump beam re-shaping, and optimization of cavity configuration and outcoupling.

3. Conclusions

Reported here are the results of significant power scaling of ~2.7-μm Er³⁺:Y₂O₃ laser resonantly pumped into upper laser level ⁴I_11/2 by a fiber-coupled SE-DFB laser diode module at 974 nm. Some SE-DFB source development details presented along with the ~2.7-μm Er³⁺:Y₂O₃ laser power scaling results. Cryogenically cooled Er³⁺(2%):Y₂O₃ ceramic laser delivered ~14 W of true CW output at 2.7 μm which was achieved with optical-to-optical slope efficiency of ~26%. We believe that this is the highest power ever reported from Er³⁺-doped bulk crystalline laser in this spectral range. Experiments aimed at significant laser threshold reduction and low-loss Q-switching of the above nearly quantum defect-limited efficiency laser are in progress.