1. Introduction

Lasers emitting at the wavelengths around 2 μm are attractive for soft material processing, bio- and environmental sensing, spectroscopy and frequency conversion further into the mid-infrared spectral range. Nowadays, there is a trend of laser development towards highly efficient, power scalable and compact (integrated) devices. Waveguide (WG) lasers featuring small mode areas leading to low laser thresholds and high laser intensities, as well as good overlap between the pump and laser modes leading to high slope efficiencies [1], meet these criteria. In particular, continuous-wave and mode-locked 2 μm WG lasers are promising as active element of distributed sensors, as well as compact seed sources for further amplification. Due to their inherently short cavity lengths, in the mode-locked regime of operation, such lasers could deliver ultrashort pulses at high repetition rates (in the GHz-range) [2] which is attractive for the so-called cold ablation of biological tissues [3].

Laser emission slightly above 2 μm is typically achieved using the ⁵I₇ → ⁵I₈ electronic transition of Holmium (Ho³⁺) ions. A common approach for excitation of Ho³⁺ is a codoping of the host matrix by both Tm³⁺ and Ho³⁺ ions. The former ions act as sensitizers and efficiently transfer the energy of electronic excitation to the latter ions, Tm³⁺(³F₄) → Ho³⁺(⁵I₇), Fig. 1(a). Tm³⁺ ions, in their turn, can be excited at 0.8 μm (into the ³H₄ pump level), e.g., by AlGaAs diode lasers, representing a conventional pumping scheme. Efficient continuous-wave bulk Tm,Ho lasers are known [4,5]. The main disadvantage of this pump scheme is a significant heat release due to (i) the non-radiative path from the ³H₄ and ³H₅ Tm³⁺ excited-states, (ii) parasitic energy-transfer upconversion (ETU) from the metastable states of both Tm³⁺ and Ho³⁺ ions, and (iii) energy loss due to the back energy transfer. The heating leads to severe thermo-optic effects even at moderate pump levels causing a thermal roll-over in the output dependence, laser ceasing or even crystal fracture. It is probably because of this reasons that the development of Tm,Ho WG lasers was not very intense so far [6–8]. Lancaster et al. reported on a Tm,Ho:ZBLAN glass WG fabricated by femtosecond direct laser writing (DLW) and delivering only 76 mW at 2052nm with a moderate slope efficiency of 20% [6].

 

Fig. 1. (a) The scheme of energy levels of Tm³⁺ and Ho³⁺ ions in LiYF₄ showing pump (blue) and laser (red) transition, ET – energy-transfer, CR – cross-relaxation, ETU – energy-transfer upconversion, R and NR – radiative and non-radiative relaxation, respectively; (b) absorption cross-sections, σ_abs, for the ³H₆ → ³F₄ transition of Tm³⁺ ions in LiYF₄ for π and σ polarizations, the spectrum of the Raman fiber laser (blue) is shown for comparison.

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An alternative pumping scheme of Tm³⁺,Ho³⁺-codoped materials is the direct excitation of Tm³⁺ ions to the ³F₄ state (the so-called Tm in-band pumping), Fig. 1(a). It may greatly reduce the energy losses originating from the non-radiative path of Tm³⁺ ions. Such approach has been studied recently for both bulk [9] and WG [10] lasers based on singly Tm³⁺-doped materials showing good prospects for both achieving high slope efficiency and power scaling. Loiko et al. reported on an in-band pumped Tm:LiYF₄ WG fabricated by Liquid Phase Epitaxy (LPE) generating a record-high output power of 2.05 W at 1881nm with a high slope efficiency of 78.3% [10]. Note that the in-band pumping of Tm³⁺ ions around 1.6–1.7 μm is feasible based on the well-developed Er fiber laser technology. It should be noted that singly Ho³⁺-doped materials can also be in-band pumped (directly to the ⁵I₇ state), e.g., by Tm bulk or fiber lasers emitting at ∼1.9 μm. In-band pumped Ho WG lasers have been reported [11,12]. The advantage of Tm³⁺,Ho³⁺ codoped materials as compared to singly Ho³⁺-doped ones (even if both are in-band pumped) is the combined gain bandwidth of both ions around 2 μm supporting broader range of wavelength tuning and the generation of shorter pulses in the mode-locked regime [13].

First in-band pumped bulk and WG Tm,Ho lasers were reported recently. Kalachev et al. used a Raman-shifted Er fiber laser to pump a Tm,Ho:LiYF₄ crystal yielding an output power of 400 mW at 2052nm with a high slope efficiency of 44% [14]. Kifle et al. applied a similar pump source for a Tm,Ho:KLu(WO₄)₂ WG laser fabricated by DLW; however, the authors detected parasitic Tm³⁺ colasing limiting the power scaling capabilities for Ho³⁺ radiation [15].

In the present work, we report on a power-scalable Tm,Ho waveguide laser operating solely on the ⁵I₇ → ⁵I₈ Ho³⁺ transition and based on the Tm in-band pumping scheme which mitigates the thermal effects and provides a relatively high slope efficiency. We have used a Tm,Ho:LiYF₄ planar WG fabricated by LPE to benefit from good thermal properties of this fluoride host material.

2. Experimental

2.1 Planar waveguide

Single-crystalline layers of RE:LiYF₄ (where RE = Gd, Tm and Ho) were grown by Liquid Phase Epitaxy. Bulk undoped LiYF₄ substrates with their surfaces parallel to the (001) crystallographic plane were used. The substrates were cut from a boule grown by the Czochralski method. They were double side polished with small surface roughness (few nm). For LPE, the batch composition was 73 mol% LiF – 27 mol% REF₃ (RE = Y, Gd, Tm, Ho). LiF was used as a solvent. The growth was performed in pure Ar atmosphere. The growth temperature was ∼734 °C and its duration was 45 min.

The optically passive Gd³⁺ ions (3.5 at.% in the layer) were intentionally added to enhance the refractive index contrast between the layer and the substrate (Δn = n_layer – n_substrate = 2×10⁻³, n_substrate = 1.4445 at the wavelength of ∼2 μm for π-polarization). The actual doping levels of laser-active ions (in the layer) were 4.5 at.% Tm and 0.5 at.% Ho, and the corresponding ion densities amounted to N_Tm = 6.2×10²⁰ at/cm³ and N_Ho = 0.70×10²⁰ at/cm³, respectively.

The top surface of the epitaxy was polished resulting in a layer thickness of 25 ± 1 μm, see Fig. 2. The input and output faces were polished as well and left uncoated. The planar WG sample was oriented for light propagation along the a-axis (a-cut) and its length t was 8.0 mm.

 

Fig. 2. Observation of a polished side facet of the Tm,Ho:LiYF₄ / LiYF₄ epitaxy: (a) optical microscope, crossed polarizers, ×10 objective; (b) confocal laser microscope, UV illumination, ×100 objective.

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3.2 Laser set-up

The scheme of the laser set-up is shown in Fig. 3(a). The bottom (substrate) part of the epitaxy was mounted on a passively cooled Cu-holder using a silver paint for better heat removal. The laser cavity was formed by two plane mirrors: a pump mirror (PM) coated for high transmission (T = 93%) at 1.68 μm and for high reflectance (HR) at 1.86–2.32 μm, and a set of plane output couplers (OCs) with a transmission T_OC at the laser wavelength in the range of 1.5% - 30%. Both cavity mirrors were placed as close as possible to the end-facets of the epitaxy. No index-matching liquid was used.

 

Fig. 3. (a) Scheme of the Tm,Ho waveguide laser: RFL: Raman fiber laser, CL and FL – collimating and focusing lenses, respectively, M: folding mirrors, PM: pump mirror, OC: output coupler, F: filter (a dichroic mirror); (b) Photograph of the pumped WG; (c) Measured single-pass pump absorption as a function of the incident pump power (circles), calculated small-signal absorption (line).

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As a pump source, we employed a home-made Raman fiber laser (RFL) based on an Er fiber master oscillator power amplifier and a single-mode polarization maintaining germanosilicate fiber [16]. The RFL delivered up to 3.5 W at 1679 nm (emission bandwidth: <1 nm) with linear polarization and a beam quality factor M² ≈ 1. The selected pump wavelength well matched the maximum of the ³H₆ → ³F₄ absorption band of Tm³⁺ ions in LiYF₄, cf. Figure 1(b). The output of the RFL was first collimated using an AR-coated spherical lens (f = 7.5 mm) and then focused at the input facet of the WG using an uncoated CaF₂ lens (f = 40 mm) resulting in a measured pump spot diameter 2w_P of 30 ± 5 μm. A photograph of the pumped WG is shown in Fig. 3(b). It shows green-yellow upconversion luminescence of Ho³⁺ ions.

The pump polarization corresponded to σ (TE) in the layer. Although it led to lower absorption cross-section as compared to π-polarization, Fig. 1(b), the use of σ-polarized pump ensured more uniform pump distribution through the whole length of the WG. Previously, this was found to be critical for optimization of Tm,Ho waveguide lasers as for relatively long WGs, unpumped regions act as sources of strong reabsorption losses. In our previous work [8], we intentionally detuned the pump wavelength from the absorption peak. In the present paper, the pump wavelength was fixed, and the polarization state of the pump radiation was changed to fit the σ polarization in the layer.

The pump coupling efficiency η_coupl = 91.1% was estimated from the geometrical overlap of the pump beam and the layer, as well as the Fresnel loss. The single-pass pump absorption was determined in a pump-transmission experiment, Fig. 3(c). A certain absorption saturation due to the ground-state bleaching was detected, as η_abs(1-pass) slowly decreased from the small-signal absorption, η_abs,0 = 1 – exp(–σ_abs^PN_Tmt) = 67.4%, where σ_abs^P = 0.23×10⁻²⁰ cm². All the OCs provided a reflectivity at ∼1.68 μm, so that the pumping was in double-pass. The total (2-pass) pump absorption taken at the threshold pump power slightly decreased with T_OC, in the range 76.8% - 68.9%.

The laser output was separated from the residual (non-absorbed) pump using a long-pass filter (a dichroic mirror). The spectra of laser emission were measured by an optical spectrum analyzer (Yokogawa, model AQ6375B). The mode profile at the output facet of the WG was captured using a short focal length CaF₂ lens (f = 15 mm) and a FIND-R-SCOPE near-IR camera. A scale calibration for the camera was provided by illuminating the epitaxy with near-IR light revealing the layer with a known thickness.

3. Waveguide laser performance

The Tm,Ho:LiYF₄ waveguide laser operating in the CW regime generated a maximum output power of 540 mW at 2051 nm with a slope efficiency η of 32.6% (vs. the absorbed pump power), a laser threshold of 337 mW and a maximum optical-to-optical conversion efficiency η_opt of 26.3% (vs. the pump power incident on the WG) by using an output coupler with T_OC = 20%, see Fig. 4(a). For higher output coupling, the laser performance deteriorated probably due to higher losses related by enhanced ETU. With increasing T_OC, the laser threshold gradually increased from 143 mW (0.5% OC) to 473 mW (30% OC). The input-output dependences were linear, and no signs of thermal roll-over was observed. Further power scaling was limited by the available pump power. No damage of the waveguide nor cavity mirrors was observed.

 

Fig. 4. Output performance of the in-band pumped Tm,Ho:LiYF₄ waveguide laser: (a) input-output dependences, η – slope efficiency; (b) typical spectra of laser emission (π-polarization); (c) evaluation of the propagation losses δ_loss using the Caird plot; (d) output performance of the same WG under conventional pumping (λ_P = 797 nm), T_OC = 7%.

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The typical spectra of laser emission are shown in Fig. 4(b). The waveguide laser operated solely on the ⁵I₇ → ⁵I₈ Ho³⁺ transition and no Tm³⁺ colasing was observed. The laser polarization was linear (π); it was naturally selected by the gain anisotropy, Fig. 5(a). For small output coupling (T_OC = 1.5% - 3%), the laser emission was observed around two wavelengths, 2052 and 2065 nm. For higher T_OC ≥ 5%, the laser operated only around 2051 nm. This behavior is due to the quasi-three-level nature of the Ho³⁺ laser scheme with reabsorption and it agrees with the gain spectra of Ho³⁺ ions in LiYF₄ (see below).

 

Fig. 5. (a) Stimulated-emission (SE) cross-section, σ_SE, spectra for the ⁵I₇ → ⁵I₈ transition of Ho³⁺ ions in LiYF₄ for π and σ polarizations; (b) effective gain cross-section, σ_g,eff, profiles for a 4.5 at.% Tm, 0.5 at.% Ho codoped LiYF₄ crystal for π-polarization, β_Tm = N(³F₄)/N_Tm is the inversion ratio for Tm³⁺ ions, grey curve – the luminescence spectrum from the epitaxy.

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The propagation losses in the Tm,Ho:LiYF₄ WG were evaluated by the Caird method [17], Fig. 4(c), i.e., by fitting the dependence of the inverse of the slope efficiency, 1/η, on the inverse of the output-coupling loss, 1/γ_OC, using the formula (1/η) = (1/η₀)[1 + 2(γ/γ_OC)], where γ = – ln(1 – L), L is the passive loss per pass, γ_OC = – ln(1 – T_OC), and η₀ is the intrinsic slope efficiency. The best-fit parameters are η₀ = 34.5 ± 5% and L corresponding to the WG propagation loss δ_loss = 0.19 ± 0.1 dB/cm. This value is close to ones reported previously for Yb:LiYF₄ (0.2 dB/cm at ∼1 μm) [18] and Tm:LiYF₄ (0.11 dB/cm at ∼1.9 μm) [19] planar waveguides produced by LPE.

For comparison, we studied the output performance of the same WG under conventional pumping employing a Ti:Sapphire laser tuned to 797 nm, Fig. 4(d). CW operation was achieved with the highest output coupling of 7% and the laser generated only 72 mW at 2051 nm with η = 24.0% and a laser threshold of 203 mW. A thermal roll-over was observed for an absorbed pump power >0.5 W and the laser ceased at > 0.55 W. This clearly proved better power scaling capabilities with the in-band pumping scheme.

To explain the observed spectral behavior of the Tm,Ho waveguide laser, the so-called effective gain cross-section, σ_g,eff, spectra [20] were calculated, Fig. 5(b). They account for both the absorption and stimulated emission of Tm³⁺ and Ho³⁺ ions around 2 μm, and a bidirectional energy transfer between them, Tm³⁺(³F₄) ↔ Ho³⁺(⁵I₇), leading to equilibrium populations of the excited multiplets [21]. For small inversion ratios for Tm³⁺ ions β_Tm < 0.05, a peak at 2065 nm is observed in the spectra. For intermediate 0.05 < β_Tm < 0.08, almost equal gain is observed at 2051 nm and 2065 nm. For large β_Tm > 0.08, the former peak dominates. This agrees with the spectral behavior of the Tm,Ho WG laser considering that higher output coupling loss implies higher inversion in the gain medium. Figure 5(b) also confirms that there is no possibility for Tm³⁺ colasing at ∼1.9 μm due to the reabsorption loss induced by Ho³⁺ ions in this spectral range.

The laser mode profile at the output waveguide facet is shown in Fig. 6(a). It has a horizontal stripe shape which is typical for planar waveguide lasers. The 1D intensity profiles in the horizontal (x) and vertical (y, || c-axis) directions are shown in Fig. 3(b). The mode size (at the 1/e² level) is 310 (x) × 24 (y) μm². The intensity profile along the y-axis is close to the Gaussian one, while a strongly multimode behavior is observed along the x-axis.

 

Fig. 6. Mode profile at the output facet of the Tm,Ho:LiYF₄ waveguide laser: (a) 2D profile, while lines mark the layer / substrate and the layer / air interfaces; (b) 1D intensity profiles in the horizontal (x) and vertical (y) directions. Dark-red curve – Gaussian fit of the intensity profile (vertical direction). The laser polarization (π) is vertical.

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In Table 1, we compare the output characteristics of Tm,Ho waveguide lasers reported so far. In this work, we report on the highest output power and slope efficiency achieved from any Tm,Ho waveguide laser. Moreover, no parasitic Tm³⁺ colasing was observed even for high output coupling, an effect which limited the laser efficiency in the previous studies [6,15].

Table 1. An Overview of Tm,Ho Waveguide Lasers Reported So Far^a

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4. Conclusion

To conclude, in-band pumping of Tm³⁺,Ho³⁺-codoped laser materials (in particular, optical waveguides) is a promising approach for efficient and power-scalable operation above 2 μm according to the ⁵I₇ → ⁵I₈ Ho³⁺ transition. In the present work, we report on the record-high output power from any Tm,Ho WG laser without any parasitic Tm³⁺ colasing. Further improvement of the laser efficiency is expected in the channel waveguide geometry owing to the better overlap of the pump and laser modes. Considering the combined gain profiles from both Tm³⁺ and Ho³⁺ ions and the location of the gain maximum >2 μm thus avoiding the structured water vapor absorption in the atmosphere, in-band pumped Tm,Ho WGs are promising for generation of ultrashort pulses at high repetition rates from mode-locked oscillators.