1. Introduction

Optical waveguides in amplifying media offer compressed active volumes and, as a result, much higher optical intra-cavity intensities than bulk materials. Consequently, laser oscillation with reduced lasing thresholds may be realized in waveguide configurations, having comparable efficiencies with respect to bulk lasers [1,2]. In practice, two-dimensionally (2D) confined waveguide structures (typically channel or ridge geometries) allow for more compact geometries and stronger spatial confinement of light fields, exhibiting superior guiding performance with respect to the one-dimensional or planar counterparts [3]. In addition, 2D waveguides can be efficiently connected to optical fibers for construction of integrated photonic systems, provided that the physical shapes and correlated modal profiles have suitable dimensions.

In the past years there has been larger progress in the development of compact, directly-injected near-infrared wavelength diode lasers based on InGaAs [4,5] and InGaAsN [6,7] quantum wells grown on GaAs substrates. However, neodymium-doped yttrium aluminum garnet (Nd:Y₃Al₅O₁₂ or Nd:YAG) crystal is still one of the favorite gain media for solid-state lasers owing to their outstanding fluorescence, thermal and mechanical properties. Planar and channel waveguides have been produced in Nd:YAG crystals by using ion beam irradiation [8–12] or femtosecond (fs) laser inscription [13,14], and laser operation in both configurations has been realized. Recently, ridge waveguides have been fabricated in Nd:YAG crystal by utilizing fs laser ablation of an ion-irradiated planar waveguide layer [15]. Although such fs-laser-ablated ridge waveguide lasers have shown better performance than the planar ones, the maximum laser output power was still limited, which was at least partly attributed to the additional losses from the considerable roughness of the side walls (~1 µm estimated by SEM images) of the ridges. On the other hand, precise diamond blade dicing is becoming increasingly a fascinating technique to construct ridge waveguide structures owing to the ability of combined cutting and surface polishing. This method allows for smooth surfaces and consequently low propagation losses. As of yet, precise diamond blade dicing has been successfully utilized to manufacture high-quality ridge waveguide structures in lithium niobate (LiNbO₃) crystals [16–18], a material having a moderate hardness of ~4.5.

In this work, ridge waveguides in Nd:YAG crystals have been fabricated by combining swift heavy ion irradiation and precise diamond blade dicing. Under optical pumping, waveguide lasers at 1064 nm wavelength with low pumping threshold and efficient cw output have been realized.

2. Experiments in details

The Nd:YAG (doped by 1 at.% Nd³⁺ ions) crystal wafer used in this work was cut to dimensions of 10 × 10 × 2 mm³ and optically polished. One 10 × 10 mm² surface was irradiated with carbon (C⁵⁺) ions at energy of 15 MeV and a fluence of 3 × 10¹⁴ ions/cm² utilizing the 3 MV tandem accelerator at Helmholtz Zentrum Dresden-Rossendorf, Germany. In this way a slightly buried planar waveguide layer was formed with a thickness of ~8.4 μm through the electronic damage of the irradiated region [8]. Afterwards, diamond blade dicing was used to construct two series of 10 ridges each with varying depth d (4.5 µm and 9 µm) and widths w (from 7 μm to 32 μm) on the planar waveguide surface, see Fig. 1 . Crystalline Nd:YAG has a Mohs hardness of 8.5, which places enormous demands on the dicing procedure. We used a 200 µm thick resin-bonded blade with a diameter of 52 mm (DISCO Corp., P1A853SD5000R10MB01). The blade was set to 20.000 rpm and the cutting speed used was 0.1 mm/s. With these conditions the chipping and cracking observed at the edges of the ridges was minimized.

 

Fig. 1 The schematic of (a) 15 MeV C⁵⁺ ions irradiation and (b) diamond blade dicing processes for the Nd:YAG ridge waveguides fabrication.

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In order to investigate the roughness of the diced sidewalls, we used scanning electron microscopy (SEM) to image the end-facet and ridges of the waveguide [see Fig. 2(a) ]. From the magnified inset in Fig. 2(b) for this ridge with width w = 22 μm and height d = 9 μm, it is obvious that the side-wall is considerably smooth. This is a clear advantage when compared with ridges fabricated by fs-laser ablation [15]. To characterize the guidance properties of the diced waveguide, we utilized a typical end-facet coupling method to measure the modal profile of guided modes using a Helium-Neon laser with a wavelength of 632.8 nm. The measured near-field intensity distribution of the TE₀₀ mode of the waveguide is illustrated in Fig. 2(c). As one can see from the figure, the boundary of the near-field image is in good agreement with the respective geometry of the ridge structure. A similar mode profile has been obtained for TM modes.

 

Fig. 2 (a) SEM image of the cross section of a diced Nd:YAG ridge waveguide; (b) magnification of region marked by dashed line from (a); (c) measured near-field profile of TE₀₀ mode of the ridge waveguide at wavelength 632.8 nm.

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The propagation losses were determined by the back-reflection method [19] using a HeNe laser at wavelength 632 nm (see Fig. 3 ). By assuming similar damping at 1064 nm wavelength, we have estimated the transmission loss of the ridge from Fig. 2 (w = 22 μm, d = 9 μm) to be ~1.7 dB/cm. For comparison, the propagation loss of the planar waveguide in the same sample was measured to be ~1.0 dB/cm. In a second experiment, the total insertion losses (coupling, Fresnel and propagation losses) of the Nd:YAG waveguides were determined by transmission measurements using a linearly polarized cw laser at wavelength 1064 nm. When accounting for Fresnel reflection losses of 0.4 dB per surface, the coupling losses into planar and ridge waveguides were estimated to be ~1.5 dB and ~2.0 dB. Overall, these results demonstrate the high quality of the fabricated side walls defining the ridge, which leads only to minor additional scattering losses of the fabricated 2D waveguide. For the other ridges, similar damping coefficients have been measured, being slightly larger for those ridges with only 4.5 μm dicing depth, which is probably due to increased leakage of light from the shallow ridges into the planar waveguide layer. It is pointed out that for both, ridge and planar waveguides, no noticeable difference of the propagation losses for TE and TM polarizations has been observed.

 

Fig. 3 The schematic of the experimental setup for propagation loss measurement.

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In this work, the waveguide laser experiment was performed by using an end-pumped system at room temperature. We utilized a polarized light beam at 808 nm generated from a tunable cw Ti:sapphire laser (Coherent MBR 110) as the pump source. A convex lens with focal length of 25 mm was used to couple the laser beam into the waveguides. The cavity contains two additional dielectric mirrors. The input mirror has a high transmission of 98% at 808 nm and high reflectivity >99% at ~1064 nm, and the output mirror has a reflectivity >99% at 808 nm and a transmittance of 60% at 1064 nm. Both mirrors were adhered to the two end-facets of the waveguide sample, constructing a Fabry-Perot cavity. The generated laser emission was collected by a 20 × microscope objective lens (numerical aperture N.A. = 0.4), imaged by an infrared-sensitive CCD and characterized by a spectrometer and a powermeter.

3. Results and discussion

We calculated the electronic and nuclear stopping power by utilizing the Stopping and Range of Ions in Matter (SRIM) 2010 code [20] to investigate the modification of the ion irradiated region of the sample, as shown in Fig. 4 . We find the nuclear stopping power S_n to be practically absent within the range (0 ~8) μm, climbing to ~0.2 keV/nm only at a depth of ~8.4 μm. In the case of electronic stopping power, the significantly higher value of S_e increases from the sample surface to a broad maximum at approximately ~6 μm beneath, peaking at about 2.1 keV/nm. Therefore, it is evident to suppose that the electronic damage is the major factor contributing to the formation of the waveguide layer, which is similar to the previously reported swift-heavy ion-irradiated Nd:YAG ridge and planar waveguides [15,21,22].

 

Fig. 4 The electronic stopping power (red dashed line), nuclear stopping power (green dashed line) curves as well as the refractive index profile (blue solid line) of the Nd:YAG waveguide as a function of the depth from the sample surface.

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By assumption of a step-like refractive index profile [12] and measurement of the N.A. of the waveguide, we can estimate the refractive index change of the waveguide using the equation $\Delta n={\sin }^2{\Theta }_m/(2n)$ [23]. Here n = 1.8297 is the bulk refractive index at 632.8 nm, and Θ_m is the maximum beam divergence of the light entering or leaving the waveguide. According to the measured Θ_m ≈7°, the effective refractive index increase can be estimated to Δn ≈ + 0.004.

Figure 5 illustrates the room temperature laser emission spectrum measured from the output end-facets of the Nd:YAG ridge waveguides, when the absorbed power is above the lasing threshold. The central wavelength of the laser emission is at 1064 nm with a Full-Width Half-Maximum (FWHM) of 0.4 nm, clearly denoting laser oscillation at the main transition ⁴F_3/2 → ⁴I_11/2 of Nd³⁺ ions [24]. The inset shows the TE₀₀ mode distribution of the waveguide laser at 1064 nm, which clearly exhibits a single mode profile, which suggests an interesting feature for future practical applications.

 

Fig. 5 Laser emission spectrum from the Nd:YAG ridge waveguides. The inset depicts the laser modal profile at the lasing wavelength of ~1064 nm.

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Figure 6 depicts the dependence of the cw laser output power on the absorbed pump power for both, ridge and planar waveguides. All the experimental results for ridge and planar waveguides were obtained under their respective optimized conditions by using the same pumping system. From the linear fit (solid lines) to the experimental data, we have determined the slope efficiencies of the ridge and planar waveguide lasers to be Φ_r = 43% and Φ_p = 20%, respectively. In addition, the corresponding lasing thresholds for the 1064 nm oscillations are P_r,th = 79 mW and P_p,th = 111 mW, respectively. As the absorbed pump power increases, the output power reaches maximum values of P_{r,max_out} = 84 mW and P_{p,max_out} = 41 mW at absorbed pump powers of 265 mW and 318 mW, respectively. This corresponds to optical conversion efficiencies of 32.8% for the ridge and 12.9% for the planar waveguide laser. With respect to these data, one can conclude that the ridge waveguide possesses a superior laser performance with reduced lasing threshold, higher slope efficiency as well as higher output laser power. Compared with ridge waveguides produced by fs-laser ablation [15], the diamond-diced waveguides also demonstrate superior quality of the side walls of the ridges, which results in a low-loss structure, higher slope efficiency and larger maximum output power. These results suggest that ridge waveguide structures fabricated by combining precise diamond blade dicing and swift carbon ion irradiation will be a promising platform for integrated laser generation in Nd:YAG.

 

Fig. 6 The cw waveguide laser output powers at 1064 nm as a function of the absorbed light power at 808 nm. The triangular and rectangular symbols stand for the data of ridge and channel waveguides, respectively. The solid lines represent the linear fit of the experimental data.

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4. Summary

We have successfully fabricated Nd:YAG ridge waveguides with smooth side walls by using diamond dicing of planar waveguides fabricated by swift-heavy ion implantation. Efficient waveguide lasers at 1064 nm have been realized through optical pumping at 808 nm. The maximum laser power is ~84 mW with a slope efficiency as high as ~43%. Future work will concentrate on further optimizing the waveguide parameters (e.g. adjusting the dicing depth, use of surface coatings to prevent chipping, or annealing treatments to reduce losses), as well as to improve coupling efficiencies with respect to the pump beam. However, our results indicate that ridge waveguides produced by precise diamond blade micromachining of planar waveguide layers are promising candidates for compact light sources, and the described universal method may be adapted to a quite large number of different active materials.