1. Introduction

Lithium niobate (LiNbO₃) is an important material for a wide variety of photonic applications due to its excellent electro-optical, acousto-optical and nonlinear properties. The easy implementation of low-loss waveguides [1–4] in rare-earth doped LiNbO₃ allows for the development of waveguide amplifiers as well as waveguide lasers [5].

In particular, a great deal of attention was devoted to the development of Er³⁺ and Nd³⁺ doped LiNbO₃ waveguide lasers emitting around the 1.5 µm telecom wavelength range (Er³⁺: ⁴I_13/2 →⁴I_15/2) and 1.08 µm (Nd³⁺: ⁴F_3/2 →⁴I_11/2), respectively [6–11]. The absorption spectra of Er³⁺ and Nd³⁺ allow for effective pumping at 980 nm (Er³⁺: ⁴I_15/2 → ⁴I_11/2) and 814 nm (Nd³⁺: ⁴I_9/2 →⁴F_5/2) [12–15]. However, the use of the short wavelength pump can cause substantial photorefractive damage in LiNbO₃ waveguides, which puts constraints on the possible laser performance. Photorefractive damage in Er:LiNbO₃ waveguide lasers can be strongly suppressed by pumping Erbium at 1486 nm (Er³⁺: ⁴I_15/2 → ⁴I_13/2) [16]. Furthermore, the appearance of photorefractive damage can be suppressed by MgO co-doping. In the past, Nd:MgO:LiNbO₃ both channel and femtosecond laser-written waveguide lasers with slope efficiencies up to 34% have been reported [17,18]. Amin et al. reported continuous-wave lasing at room temperature with a slope efficiency of 40% in Nd:Ti:LiNbO₃ channel waveguides by using Z-propagating waveguides and σ-polarized pump light to suppress optical damage [19]. However, a further improvement of the laser performance can be achieved by using a ridge waveguide geometry due to the smaller mode fields and improved overlap of modes at different wavelengths, when compared to their channel counterparts. Furthermore, the occurrence of photorefractive damage can be minimized due to the ridge geometry [20].

Up to date, little attention was paid to the development of Nd:LiNbO₃ ridge waveguide lasers. In 2016 Mendívil et al. reported the demonstration of a Nd:LiNbO₃ ridge waveguide laser with a maximum output power of 1.1 mW at 1085 nm and a slope efficiency of 7% [21]. The ridge waveguides fabricated by fs-laser ablation technique in combination with Zn in-diffusion showed rather high propagation losses of 4 dB/cm [21]. Compared to fs-laser ablation, optical grade diamond blade dicing enables the fabrication of low-loss ridge waveguides (< 0.2 dB/cm) [3]. Recently, a novel fabrication method comprised of ridge definition by diamond blade dicing followed by three-side Er and Ti in-diffusion for the development of highly efficient Er:Ti:LiNbO₃ ridge waveguide amplifiers and lasers has been reported [16,22]. This method improves the amplifier and laser performance by allowing for an optimization of the overlap between the Er diffusion profile and the guided modes.

Besides the improvement of laser performance, the benefits of using ridge waveguide geometry also help to enhance the efficiency of nonlinear devices. The realization of efficient nonlinear processes in Ti:LiNbO₃ ridge waveguides requires a periodic domain inversion to achieve quasi-phase-matching (QPM). Recently, a method for local periodic poling of Ti-diffused ridge waveguides in LiNbO₃ has been demonstrated [23]. In contrast to previous approaches, this method allows an optimization of the ridge waveguide geometry for nonlinear optic applications such as QPM difference frequency generation.

Previously reported self-frequency-doubled Nd:LiNbO₃ waveguide lasers utilizing direct phase matching showed rather low conversion efficiencies due to the poor overlap of fundamental and harmonic [24], which can be improved by the above mentioned method. Thus, the combination of the ability to develop highly efficient lasers in LiNbO₃ and to achieve local QPM allows for the development of efficient hybrid optical devices on the same substrate emitting in the mid-infrared such as the example shown in Fig. 1. Here one section of a ridge waveguide is Nd-, Er- and Ti-doped to realize a Nd:Er:Ti:LiNbO₃ ridge waveguide laser operating simultaneously at λ_{laser, Er} = 1056/1053 nm and λ_{laser, Nd} = 1085 nm. By inserting an out-coupling mirror, which is highly reflecting (HR) at the pump wavelengths of λ_{pump, Er} = 1486 (or 980 nm) and λ_{pump, Nd} = 814 nm, only the laser signals will pass the periodically-poled part, thereby to be converted into a DFG signal around 3.6 µm wavelength suitable for applications such as gas sensing.

 

Fig. 1 Schematic of a ridge waveguide structure for implementation of a Nd:Er:Ti:LiNbO₃ laser and subsequent difference frequency generation on the same LiNbO₃ substrate.

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In this paper, we report on the development of efficient neodymium-doped titanium in-diffused ridge waveguide lasers in LiNbO₃. For sample fabrication we used the above mentioned technique of three-side evaporation and in-diffusion for Ti and Nd incorporation into pre-defined ridges. Stable laser operation at 1084.7 nm with a maximum output power of 108 mW and a slope efficiency of 34% has been obtained in a 12 mm long ridge waveguide for a coupled pump power of 402 mW at 814 nm.

2. Fabrication and characterization of Nd:Ti:LiNbO₃ ridge waveguides

2.1 Fabrication

For Nd:Ti:LiNbO₃ ridge waveguide fabrication, congruent x-cut LiNbO₃ wafers with 1 mm thickness and optical grade surfaces were used. First, ridges with widths ranging from 9 µm to 16 µm and heights of 16 µm and 19 µm were prepared by diamond blade dicing along the y-axis of the crystal. After that, two 10 nm-thick Nd layers were evaporated at symmetric angles ± 60° with respect to the substrate normal [22]. Afterwards the sample was annealed at 1125°C for 216 hours. For waveguiding in the depth direction, 85 nm-thick Ti layers were deposited on top of the ridges and the sidewalls at angles of ± 70 ° and in-diffused for 15 hours at 1120 °C. To improve the photorefractive damage resistance, two 7.5 nm-thick indium tin oxide (ITO) layers were deposited at ± 45 ° by reactive sputtering in an Ar-O₂ atmosphere and additionally annealed for 6 hours at 450 °C. Finally, smooth waveguide end facets for light in- and out-coupling were achieved by diamond blade dicing perpendicular to the y-axis.

2.2 Ridge waveguide characterization

The propagation losses of the prepared ridge waveguides were measured by using the Fabry-Pérot resonator technique [25]. To measure the propagation losses in the wavelength region of the pump and laser light we used a titanium-sapphire laser and a laser diode operating at 1064 nm, respectively. The losses of the ridge waveguides were found to be 1.0 dB/cm and 0.6 dB/cm at 860 nm and 1064 nm, respectively. The Fabry-Pérot spectra were recorded near 860 nm wavelength to separate the scattering losses from those caused by the Nd³⁺ absorption. By measuring the absorption of π-polarized light at 814 nm and using the π-polarized absorption cross section of [14] a neodymium doping concentration inside the ridges of 8.5·10¹⁹ cm⁻³ was estimated.

Near-field intensity profiles of the fundamental and higher order transverse modes at 800 nm and 1064 nm were measured by using the prism coupling technique to excite the different modes. For all ridge widths we observed guiding of several transverse modes. Figure 2 shows the two lowest order guided modes at 800 nm (σ-polarization) and 1064 nm (π-polarization) of a 9.5 µm wide ridge waveguide, which supports up to three transverse modes.

 

Fig. 2 Mode profiles of a 9.5 µm wide ridge waveguide: (a) and (b) show the recorded mode intensities at 800 nm (σ-polarization) of the lowest order guided modes. (c) and (d) show the two lowest order guided modes at 1064 nm (π-polarization).

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3. Laser characterization in Nd:Ti:LiNbO₃ ridge waveguides

3.1 Laser setup

For the measurement of the laser characteristics of the Nd:Ti:LiNbO₃ ridge waveguides, we have used the laser setup shown schematically in Fig. 3. The excitation source was a titanium-sapphire laser with an emission wavelength of 814 nm and a maximum output power of 2 W. To adjust the pump power, we used a combination of a half-wave plate and an optical diode. The pump polarization was adjusted by means of a second half-wave plate. To separate the out-coupled laser beam from the pump a dichroic mirror was used. The pump beam was coupled into the ridge waveguide by an anti-reflection coated lens with a focal length of 13 mm. At the output end facet, the laser signal and the residual pump light were collected by a lens with a focal length of 4.6 mm. A long-pass filter with a cut-on at 900 nm was used to separate the laser signal from the residual pump light. For spectral measurements and waveguide mode imaging, an additional beam splitter was used to reflect 30% of the out-coupled light into an optical spectrum analyzer (OSA) or onto an infrared camera.

 

Fig. 3 Schematic diagram of the experimental setup of the Nd:Ti:LiNbO₃ ridge waveguide laser operating at 1085 nm.

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We measured the laser output for two different output coupler reflectivities. In the first laser setup, we achieved laser operation with feedback provided solely by Fresnel reflection (~14%) at the waveguide end facets. In the other setup, in order to achieve a lower pump threshold, a mirror of reflectivity > 99% at 1085 nm (HR) and 99% transmission at 814 nm was inserted between the in-coupling lens and the sample. The air gap between the mirror and the sample was adjusted by means of a piezo drive to achieve the highest coupled pump power.

3.2 Results and discussion

In a first experimental investigation, Nd:Ti:LiNbO₃ samples without ITO layers were investigated. Because of photorefractive effects the laser operation around 1085 nm was observed to be unstable. As mentioned above, the sample was then covered with a 7.5 nm thick ITO layer to improve the optical damage resistance. In addition, we noticed that pumping with σ-polarized light benefits laser stability. The laser emission was π-polarized, independent of pump polarization and output coupling. In order to achieve stable and efficient laser operation σ-polarized pump light was chosen for the laser measurements. The best lasing results were obtained with a 9.5 µm wide and 12 mm long ridge waveguide [see Fig. 4(a)].

 

Fig. 4 Laser performance of a 9.5 µm wide Nd:Ti:LiNbO₃ ridge waveguide: (a) Slope efficiency vs crystal length for a configuration with feedback solely provided by Fresnel reflection. Inset: Spectrum of the laser output from the 12 mm long ridge waveguide. (b) Output power of the 12 mm long ridge waveguide laser vs coupled pump power for a configuration with feedback provided solely by Fresnel reflection (red line) and with inserted HR mirror (blue line).

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In Fig. 4(b), the laser output power is plotted against the coupled pump power for the above mentioned two laser configurations. Here, the output power is the sum of the measured laser output power at both waveguide end facets (see Fig. 2). The coupling efficiency into the ridge waveguide for the pump light, which was determined by the overlap integral between the measured mode profiles of the guided and unguided pump mode, was 70%. For the laser configuration with an inserted HR mirror, the pump threshold was 59 mW. A slope efficiency of 18% and a maximum laser power of 46 mW were achieved. The laser emission spectrum was measured with a 0.02 nm resolution spectrometer (OSA) and was centered at 1084.7 nm with a FWHM of ~0.4 nm, see the inset of Fig. 4(a). As can be seen in that inset, the laser emission has exhibited clear multi-longitudinal-mode behavior.

With feedback provided solely by Fresnel reflection (~14%) at the waveguide end facets, laser operation was achieved at a pump threshold of 83 mW. For the maximum coupled pump power of 402 mW we reached an output power of 108 mW with a slope efficiency of 34%. To the best of our knowledge, this represents the highest slope efficiency ever reported for a Nd:LiNbO₃ ridge waveguide laser and is comparable to the previously reported results in Nd:LiNbO₃ channel waveguides (see Table 1). It should be noted that in contrast to channel waveguides with equally high slope efficiencies, the laser threshold is significantly higher, which can be explained by the propagation losses, the non-optimal output coupling and the guidance of several transverse modes. In contrast to Nd:Ti:LiNbO₃ channel waveguide lasers [26], stable laser output was achieved without chopping the pump, which we assume is due to both, the improved photorefractive damage resistance by using a ridge waveguide geometry [20] and the additional ITO coating. We observed laser operation with an output power variation of less than 10%, measured over a time period of 30 minutes, at even the highest coupled pump power of 402 mW corresponding to an intensity of ~490 kW/cm².

Table 1. Performance of previously reported Nd:LiNbO₃ waveguide lasers. The abbreviations (c) and (a) stand for coupled and absorbed pump power, respectively.

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4. Summary

In conclusion, we presented Nd:Ti:LiNbO₃ ridge waveguide lasers with high slope efficiencies of 34% at a wavelength of 1084.7 nm pumped by a Ti:Sapphire laser emitting at 814 nm. For the waveguide fabrication we used a novel technique of three-side evaporation and in-diffusion for Nd and Ti incorporation into pre-defined ridges. An improvement of the photorefractive damage resistance by ITO coating has been observed, which enables stable laser operation even at pump intensities as high as ~490 kW/cm². Thus, this fabrication technique allows for the development of highly efficient hybrid optical devices emitting in the mid-infrared such as that depicted in Fig. 1. Future experiments will be focused on further improvement of the photorefractive damage resistance, e.g. through MgO or ZnO co-doping. Furthermore, we will optimize the output coupling and the fabrication parameters of the Nd:Ti:LiNbO₃ ridge waveguides to achieve single-transverse mode operation at the laser wavelength and to reduce propagation losses, which may allow for a further improvement of the laser performance.