Zinc-indiffused MgO:PPLN waveguides for blue / UV generation via VECSEL pumping

We present the design and characterisation of a zinc-indiffused periodically poled lithium niobate ridge waveguide for second harmonic generation of ~390 nm light from 780 nm. We use a newly developed, broadband near-infrared vertical-external-cavity surface-emitting laser (VECSEL) to investigate the potential for lower footprint nonlinear optical pump sources as an alternative to larger commercial laser systems. We demonstrate a VECSEL with an output power of 500 mW, containing an intracavity birefringent ﬁlter for spectral narrowing and wavelength selection. In this ﬁrst demonstration of using a VECSEL to pump a nonlinear waveguide, we present the ability to generate 1 mW of ~390 nm light with further potential for increased efﬁciency and size reduction.


INTRODUCTION
The generation of efficient and small-footprint UV sources for ion trapping is a topic of recent interest driven by a desire to create portable quantum systems for timing and navigation. Routes towards achieving low-footprint 369.5 nm sources are of particular interest for the Doppler cooling of Yb + ions [1]. It is preferable to use frequency doubling in the process of generating this specific wavelength as near-IR lasers are commercially available and may be frequency locked to molecular iodine at 739 nm [2]. It has been suggested in recent work by Mulholland et al. [3], during the development of a complete device for all Yb + ion trapping sources, that a 100 µW CW laser is the minimum requirement for this cooling application. Other lasers required for Yb + ion trapping include the photoionisation, repumping and state clearout sources but these wavelengths are typically longer with more readily available commercial sources.
The visible/near-IR pump source typically used for singlepass frequency doubling to the blue/UV regime is a titanium sapphire laser [4][5][6], although the target wavelength can also be achieved via intracavity doubling with a Nd:YAG crystal and a suitable nonlinear media [7] or a ring cavity containing the nonlinear crystal where an external pump laser provides the infrared source [8,9]. While these lasers are ideal for laboratoryuse and initial characterisation, their large footprint prohibits their use in volume critical applications.
For low size, weight and power (SWAP) applications, periodically-poled lithium niobate (PPLN) remains a popular choice for frequency doubling due to its tailorable quasi-phasematched (QPM) structure and high d 33 nonlinear coefficient [10]. PPLN waveguides are often favourable to bulk crystals due to the higher conversion efficiency attained from tighter confinement and overlap of the pump and generated signal mode compared to focused Gaussian beams [11]. Theoretically, conversion efficiency in waveguides scales quadratically with length in contrast to the linear scaling observed in bulk nonlinear crystals; this is the case for the focused condition shown to be optimal for efficient second harmonic generation (SHG) in the low-conversion regime [12]. Short focal length optics also reduce the potential size of the nonlinear optical system. In this paper, we present zinc-indiffused PPLN diced ridge waveguides phasematched for~780 to 390 nm SHG as a route to achieve compact, low power blue/UV wavelengths. Our large mode area approach [13] provides advantages in terms of coupling efficiency and power handling compared to recent lithium niobate on insulator research which has demonstrated high conversion efficiencies for low input powers [14]. We combine this with the first demonstration of a vertical external-cavity surfaceemitting laser (VECSEL) as the pump source for a χ (2) nonlinear waveguide, investigating the goal of a potential small-footprint frequency doubling solution. We have chosen to investigate VECSELs as a single epitaxial growth yields hundreds of laser gain chips, making the technology extremely cost effective. Tailored active region design opens pathways to narrow linewidth operation [15][16][17][18], broadband sources for mode-locking [19][20][21] and wide tunability [22], while tailored external cavity design allows for multiple wavelength generation [23,24] and variable pulse repetition rates [25]. Previous work incorporating nonlinear crystals into VECSELs has accomplished deep UV generation using second harmonic generation with a single intra-cavity crystal [26], and third harmonic generation with two birefringently phasematched, intracavity crystals [27]. Recently, high-power fundamental emission in the 780 nm region has been demonstrated at the watt level [28]. Vitally, VECSELs offer the ability to design shorter external cavities, such as that presented by Kuznetsov et al. [29]. This combination of wavelength tunability, output power, potential for size reduction and beam quality in comparison to laser diodes provide an appropriate source for an investigation of blue/UV sources in combination with our nonlinear waveguides.

PPLN WAVEGUIDE FABRICATION
Our PPLN substrates are fabricated by applying a patterned photoresist to the -z face of a 0.5 mm z-cut, 5 % magnesium doped LiNbO 3 (MgO:LN). An electric field greater than the material's coercive field is applied to the crystal via liquid electrodes to achieve domain reversal with a pre-defined period, Λ, to satisfy phasematching conditions where ∆β = β 2ω − 2β ω , representing the propagation constants of the associated SHG and fundamental modes. We investigated poling periods ranging from 2.2 µm to 2.4 µm to account for the discrepancy of the effective index of the zinc-indiffused waveguides from the Sellmeier equation and coefficients for bulk MgO:LN [30]. The resulting periodically poled domains traverse the entire 0.5 mm thickness of the crystal; due to the short period the PPLN grating width was restricted to 200 µm to minimise local merging. The waveguide fabrication process flow is illustrated schematically in Fig. 1. These fabrication steps are optimised via several iterative stages, starting with the deposition of a metallic zinc layer on unpoled LN followed by diffusion to form a planar waveguide. A study of the planar modal size is presented in Fig. 2(a) in which the modal size of a range of zinc-film thicknesses and indiffusion temperatures are displayed. The mode field diameter (MFD) was measured by imaging the waveguide output onto a calibrated InGaAs CCD camera. The measurement technique is further detailed in Ref. [31]. We proceeded with diffusion of a 150 nm-thick metallic zinc layer at 900 • C for 1 hour in an oxygen environment as this gives the smallest MFD and the highest probable overlap of pump and SHG guided modes. The waveguide was also single-mode in the y-axis at~780 nm. Zinc deposition is carried out on a 50 mm × 10 mm PPLN chip. The ridge waveguide and end facets are formed via ultra-precision dicing. Waveguides widths were diced ranging from 5.7 µm to 8 µm, in each of the grating periods. The~780 nm mode was evidently multi-modal in the x-axis for widths above 7 µm. A scanning electron microscope image of one of these waveguides can be seen in Fig. 2(b). The blue/UV wavelength was multimodal in all fabricated waveguides. Further discussions on the waveguide platform, characterisation methodologies and its application in 1560 to 780 nm frequency doubling can be found in our recent work [31].
Initial characterisation of the second harmonic generation of the zinc-indiffused PPLN waveguides was carried out using a low-power, tuneable external cavity diode laser (ECDL) 780 nm laser (Sacher Lasertechnik). This source is fibre-coupled with a polarisation-maintaining fibre (PMF). The input coupling to the PPLN waveguide (PPLN:WG) was optimised via adjustable collimation and a focussing aspheric lens. The output was approximately collimated prior to prism-separation of the pump and signal beams. The 390 nm beam was directed to a silicon photodiode and detected using lock-in amplification. The waveguides phasematched at a pump wavelength of~782.1 nm at room temperature with a 2.2 µm period grating, with the highest efficiency in a 5.7 µm-wide ridge waveguide. The numerical aperture of the waveguide at the pump wavelength is approximately 0.11 and an MFD of 4.58 µm and 5.9 µm in the x-and y-axes, respectively. The resulting phasematching spectrum for this waveguide is displayed in Fig. 3. The measurements with the ECDL 780 nm source provide a detailed phasematching spectrum of the waveguide prior to the temperature-tuned phasematching required for SHG in further experiments utilising a VECSEL as the pump source; they also provide a coupling efficiency for comparison. While this phasematching spectrum is close the theoretical curve, any deviations from the ideal sinc 2 (∆βL/2) curve are known to result from fabrication non-uniformity [32,33]. The theoretical curve is calculated for a 14 mm waveguide, which is the length of the waveguide used in this work -indicating that the effective length must be fairly close to the actual physical length, which provides a general indicator of the poling quality and waveguide uniformity.

VECSEL CAVITY DESIGN AND CHARACTERISATION
The VECSEL used in this work is a top emitter consisting of an active region similar to that which was described in Ref. [28]. The epitaxial surface of a single 4 × 4 mm gain chip sample was capillary bonded to a 350 µm × 5 × 5 mm silicon carbide (SiC) heat spreader using isopropyl alcohol. The two components were clamped to a copper heat sink with a 100 µm thick layer of indium foil between the copper and the SiC as a thermal compound. The temperature of the copper heat sink was controlled with a PID servo and Peltier such that heat is exchanged into a water cooled copper block. The entire assembly is mounted in a high resolution micro-positioning mount.
The external cavity, shown schematically in Fig. 4, is formed from a 100 mm spherical concave high reflecting fold mirror and a plane output coupler mirror with reflectivity of 97.5 % at 780 nm. The gain-sample to fold-mirror distance is approximately 55 mm, and the fold mirror to output coupler distance is approximately 200 mm. In this external cavity configuration, the fundamental laser spatial mode has a radius of~48 µm (1/e 2 ) at the gain sample's surface and, with a cavity fold full angle of~24 • , the fundamental mode radii at the plane OC mirror are 244 µm and 295 µm in the tangential and sagittal directions, respectively.
We use a Coherent Milennia EV laser as the VECSEL pump source and utilise up to 7 W of pump light at 532 nm. The pump light is incident on the gain sample at approximately 25 • to the gain surface normal, and approximately 22 % of the pump light is spectrally reflected from the gain sample. The pump light is focused with a 75 mm achromatic lens which is mounted on a linear translation stage to allow for pump spot size adjustment at the gain surface. The pump source emits a fundamental Gaussian spatial mode and, using the 75 mm lens, the pump beam waist was significantly smaller than the fundamental cavity spatial mode waist at the gain sample. Therefore instead of using a longer focal length lens, the pump focusing lens is located specifically such that the pump light is brought to a waist in free space just before the gain sample, yielding a pump spot on the gain sample of radius~55 µm. Inside the laser cavity, a 4 mm thick quartz birefringent filter plate (BRF) is used to tune the emission wavelength. Spectral tuning of the VECSEL is achieved in steps of approximately 0.3 nm, due to the free spectral range of the etalon formed by the SiC heat spreader.
During operation, the gain structure's copper heat sink is maintained at 13.5 • C. The VECSEL output power as a function of absorbed pump power, both with and without the intra-cavity BRF, is shown in Fig. 5(a). On insertion of the BRF into the laser cavity, the additional losses cause an increase in the threshold pump power. The spectral filtering effect of the BRF prevents the laser from operating on more than one SiC etalon fringe, and the resulting linewidth of the VECSEL remains narrow over the entire pump power and spectral tuning range. The VEC-SEL emission was monitored on an Optical Spectrum Analyser (Yokogawa AQ6370D) with 0.02 nm resolution, and an example laser spectrum is inset in Fig. 5(a). Efficient coupling to a single mode waveguide forward of the laser cavity requires a good Gaussian beam profile. A beam profile of the VECSEL emission is given in Fig. 5(b), which represents the collimated beam approximately 75 cm from the output coupler.

BLUE/UV GENERATION
The PPLN is housed in a PV10 oven (Covesion Ltd.) capable of 0.01 • C temperature stability and mounted to a 5-axis translation stage. Insertion loss of the launched VECSEL beam is typically 4.7 dB using a 8 mm focal length aspherical lens, F1 in Fig. 4. This is in comparison to an insertion loss of 3 dB when using an  optimised beam from a fibre-launched zoom collimator configuration, as shown schematically in our previous work [13]. This difference is due to a higher degree of alignment control and the ability to adjust the beam diameter when using an adjustable launch. Further investigation with input coupling focal length will be carried out in future work. As shown in Fig. 4, the output is collimated, and the spatially overlapped SHG signal and pump beams are separated using a dichroic mirror (DM) and monitored using silicon photodiodes. A further band-pass filter is implemented prior to the SHG photodiode, PD2, to ensure isolation of the correct signal. The loss of SHG power associated with the DM and BPF is 0.37 dB and is accounted for in the presented data.
At the operational wavelength of the VECSEL in use (782.86 nm), the PPLN was temperature tuned to 33 • C to achieve phasematching, as demonstrated in Fig. 6. A maximum signal power of~1 mW was achieved with a pump throughput power of 120 mW, corresponding to an efficiency of 6.9 %/W, where efficiency is defined as η = P 2ω /P 2 ω . To further study the conversion efficiency, the power was controlled via a variable attenuator in the~780 nm beam path prior to the PPLN waveguide. The throughput power corresponds to the measured VECSEL power at PD1, as shown in Fig. 4. Analysing the resulting data presented in Fig. 7 by linear regression fitting to the efficiency for pump powers up to 50 mW (low power / linear regime), we find an efficiency of 5.45 ± 0.08 %/W.
The quadratic curve associated to the SHG power, P 2ω , as presented in Fig. 7, is calculated as per the efficiency of this device. Normalised to the nonlinear waveguide's relation to length, we obtain an efficiency of 2.78 %/W·cm 2 . While we note this is comparatively low to other blue-generating nonlinear waveguides [6], our frequency conversion utilised the first-order mode of the SH mode, reducing our nonlinear overlap integral [34]. Using the commercial waveguide modelling software FIMMWAVE (Photon Design Ltd.) as a numerical means for evaluating the overlap integral of the fundamental 780 nm mode with either the fundamental or first-order 390 nm modes, we would expect a two-fold increase in the nonlinear overlap integral. Hence, a theoretical quadrupling of the conversion efficiency will be expected [11], however, unfortunately in this experiment and with the available poling periods, we were unable to access this regime.
In future work, we expect to be able to target this phasematching condition and improve our conversion efficiency. Further, investigating improvements into the grating structure for this fine-period grating, 2.2 µm, will also benefit this overall UV generation scheme. This is additional to utilising MgO:PPLN which will allow wavelength generation further into the UV region than other second-order nonlinear waveguiding technologies and our zinc-indiffused platform will not be damage threshold limited for the optical powers required for the relevant applications [13].

CONCLUSIONS
We present the first demonstration of SHG from a VECSEL gain structure in a χ (2) nonlinear waveguide. Using a zinc-indiffused PPLN ridge waveguide with 2.2 µm poling period, we have achieved over 1 mW of UV/Blue SHG output power at 391.5 nm from the 783 nm VECSEL. While this initial result yielded a conversion efficiency of less than 6.9 %/W, optimisation of the VECSEL gain structure and further research into short-period poling of the lithium niobate both provide routes for improved efficiency of this UV laser source. Recent research presented by Kahle et al. [22] will be of use in further work in frequency doubling a~739 nm laser source post-optimisation of the gain structure towards the 369.5 nm required for Doppler cooling of Yb + ions.

ACKNOWLEDGEMENT
We would like to thank Patrik Rajala and Sanna Ranta for growth of the VECSEL structure.