We report on fabrication of ridge waveguides formed in congruent periodically poled lithium niobate substrates using annealed and reverse proton exchange followed by diamond blade dicing. 1 W of second-harmonic generation at 775 nm has been obtained in a single-pass in 50 mm long ridge waveguides with internal conversion efficiency of 70%. At this power level, 97% pump depletion has been reached. Although elevated temperature operation and ridge geometry help to mitigate photorefractive damage (PRD) effects, nevertheless, at even higher second harmonic outputs significant power drop with blue shift and distortion of the SHG tuning curve have been observed indicating an onset of PRD.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The ferroelectric crystal lithium niobate (LiNbO3) with its wide transparency window, excellent electro-optical, acousto-optical and nonlinear properties is a material of choice for a wide variety of photonics applications. Furthermore, low-loss waveguides, the basic building blocks of integrated optics, can be easily implemented in this material leading to the development of efficient photonic devices, such as electro-optical modulators, nonlinear frequency converters, optical amplifiers and lasers . In particular, due to large χ(2) nonlinear coefficients and the possibility of robust domain inversion on a micrometer scale, nonlinear frequency conversion devices based on periodically poled lithium niobate (PPLN) waveguides have been used for a range of applications including optical telecommunications [2,3], generation of squeezed states  and entangled photon-pairs for quantum communication and information processing [5,6], laser cooling of atoms [7,8], realization of efficient narrow-linewidth laser sources  and supercontinuum generation . In the last decades, PPLN waveguides with channel and ridge geometries and with a quite different frequency conversion performance have been fabricated using a variety of methods. Regarding the nonlinear frequency doubling of the pump sources at around 1550 nm, impressive peak conversion efficiencies of up to 4600%/W (length-normalized efficiency up to 184%/W·cm2) have been reported in direct-bonded, thinned and subsequently etched PPLN ridge waveguides [11,12], as well as up to 3500%/W (150%/W·cm2) in annealed/reverse proton exchanged channel waveguides [2,13]. Even higher length-normalized efficiency of 2600%/W·cm2 was achieved by Wang et al.  in PPLN nanowaveguides with sub-µm2 cross-section. On the other hand, the watt-level SHG outputs, necessary e.g. for atom cooling applications, have been demonstrated in PPLN waveguides with relatively low conversion efficiencies [8,15–17]. In particular, 1.7 W of SHG at 5 W of coupled pump power was generated in a direct-bonded etched ridge waveguide with 54%/W (22%/W·cm2) conversion efficiency , and 1 W of SHG at 1.8 W of coupled pump was reported in a Zn-indiffused diced ridge waveguide with 101%/W (6.3%/W·cm2) . Furthermore, in the latter waveguide, an improved 2.5 W stable operation at 4.3 W of coupled pump has been recently demonstrated . In this paper, we report on the first annealed/reverse proton exchanged, diamond blade diced PPLN ridge waveguides with 510%/W (25%/W·cm2) conversion efficiency. In high power regime, ridge geometry  and operation at an elevated temperature of 120 °C  helped to mitigate detrimental PRD effects thus enabling us to generate up to 1 W of SHG at 775 nm at 70% pump power conversion.
2. Sample fabrication
The fabrication process of our PPLN ridge waveguides comprises the following four main steps: electric field periodic poling of a LiNbO3 substrate, proton exchange with subsequent annealing (annealed proton exchange, APE), reverse proton exchange (RPE) and ridge waveguide definition using precision diamond blade dicing. For the sample substrate, a rectangular 50 mm x 10 mm piece was cut from a standard optical-grade 0.5 mm thick z-cut LiNbO3 wafer. First, electric field poling with a quasi-phase-matching (QPM) period of 18.2 µm over 45 mm section of the 50 mm long substrate was conducted. To do this, a periodic grating chromium electrode was lithographically patterned on the substrate’s + z surface, and on the –z surface a standard medical conductive gel was directly applied to a 45 mm x 6 mm window positioned symmetrically within the sample area. While a voltage of 12 kV was applied across the sample thickness, we monitored charge that had flown through the substrate to ensure nearly perfect 50% duty cycle of the poling pattern. The sample was proton exchanged for 100 minutes at 200 °C in pure benzoic acid and then annealed in air for 18 hours at 350 °C. The in-depth refractive index profiles at 532 nm of the obtained planar waveguide after proton exchange and subsequently after annealing step were reconstructed based on the effective indices of the several guided TM modes measured using a prism coupler (Metricon 2010/M). The refractive index directly after proton exchange exhibited a typical step-like profile  with a 0.12 increase over the bulk extraordinary index and a depth of 0.9 µm. The half-bell-shaped refractive index profile after the annealing step is shown in Fig. 1(a) and had a maximum increase of 0.017 at the surface with 4.0 µm width at 50% of this maximum. A subsequent RPE step was conducted by immersing the sample for 7 hours at 330 °C in a Li-rich eutectic melt of LiNO3:KNO3:NaNO3 with a 37.5:44.5:18 mole percent ratio [20,21]. The refractive index profile after RPE, shown in Fig. 1(a), was calculated using the proton out-diffusion model of Ref. . Although prism coupling into guided modes of such a buried waveguide is hindered by the need of light tunneling through the proton out-diffused lower index surface layer, we were still able to measure the effective index of the fundamental mode at 1550 nm. The measured value of 2.1399 ± 0.0002 (≈ 2.3·10−3 above the bulk substrate index) was in good agreement with 2.1400 calculated using the RPE index profile of Fig. 1(a), which was adjusted to account for wavelength dispersion of proton exchanged LiNbO3 [22,23].
The simulated mode intensities of the RPE planar waveguide at 1550 nm and 775 nm, the two wavelengths involved in frequency doubling of a 1550 nm wave, are shown with dashed lines in Fig. 1(a). As can be seen, in the RPE waveguide the guided modes are pushed away from the surface layer with reduced (damaged) nonlinearity [24,25], and at the same time the more symmetric index profile leads to a significantly improved modes’ overlap integral [13,20,21]. Both these effects are advantageous for efficiency of nonlinear frequency conversion. In the next fabrication step, ridges with widths ranging from 7 µm to 15 µm and a height of 25 µm were cut using a precision wafer saw (Disco DAD322), see micrographs of Fig. 1(b). For cutting, a 100 µm wide resin-bonded blade with the finest available grit (6000, average synthetic diamonds’ diameter 2µm) at 20000 rev/min and a feed rate 6 mm/min were used. Smooth sample’s end facets were prepared under the same cutting conditions by dicing perpendicularly to the waveguide’s axis. Finally, after having measured the intrinsic propagation losses in the waveguides using Fabry-Pérot resonator technique , we cut the output facet of the PPLN chip at a 5.6° angle to avoid Fabry-Pérot effects in SHG measurements.
The microscope images of the cross-section of the fabricated ridge waveguides are presented in Fig. 1(b). The slight asymmetry of the ridge shape with a side-wall deviation from verticality of 7° (left side) and 3° (right side) was caused by the asymmetric profile of the cutting diamond blade. The width of the ridges at a 4 µm height below the ridge top was measured at different points along the ridges using a 3D optical profilometer (Keyence VK-X200). The average deviation of the measured widths from the target ones was below 0.3 µm while the width variation over the 50 mm sample length caused by blade consumption did not exceed 0.1 µm. The simulated two-dimensional intensities of the fundamental modes at 1550 nm and 775 nm of a 10 µm wide RPE ridge waveguide with a rectangular cross-section are shown in Fig. 1(c). Close to circular intensity profiles at 1550 nm are obtained for the ridge widths (8–10) µm which is favorable for efficient coupling using round beams, e.g. fiber butt-coupling. Our simulations show that in the vertical direction, besides the fundamental mode at both wavelengths, the ridge waveguides support two higher-order modes at 775 nm. In the lateral dimension within the ridge width range of (7–15) µm, 2 - 4 and 4 - 6 higher-order guided modes were found at 1550 nm and 775 nm, respectively. Based on the calculated effective refractive indices of the guided modes, for a poling period of 18.2 µm and a fundamental-to-fundamental mode SHG process, QPM wavelengths of 1595 nm, 1557 nm and 1540 nm were found for 7 µm, 10 µm and 15 µm wide PPLN ridges, respectively.
3. Experimental setup
A schematic of the experimental setup for SHG measurements in our RPE PPLN ridge waveguides is presented in Fig. 2. The low power sub-milliwatt output of a fiber-coupled tunable laser source (New Focus Venturi TLB 6600 H CL) was amplified by a polarization maintained Er-doped fiber amplifier (EDFA, Keopsys CEFA-C-PM-HP-37) with an output power of up to 4.5 W. A collimator and an aspheric lens combination was used to inject the pump light into the waveguides. To facilitate coupling optimization, the in-coupling optics was mounted on a 3-axis piezo positioning system (Thorlabs Nanomax MAX341) with an auto-align option for tracking the output SH maximum (or throughput pump power maximum, when needed). The waveguide chip was placed inside an oven (Covesion PV50) for operation at elevated temperature. The second harmonic and the residual pump exiting the sample were collected and collimated with another aspheric lens. A beam sampler (BS) on a flip mount was used to reflect a small part of the output onto an infrared CCD camera while an appropriate filter (F, Thorlabs FGL1000M or Thorlabs FGB25) was inserted to separately record intensities of the pump and SH waveguide modes. The SH and the pump beams were then split in a short-pass dichroic mirror with 950 nm cut-off wavelength (DM, Thorlabs DMSP950) and their powers measured with calibrated Si and Ge high-sensitivity power sensors, respectively. To prevent sensors’ saturation at higher power levels, neutral density filters as well as cross-check with a thermal power meter head were used. Furthermore, since the dichroic mirror has 2.5% reflectance at 775 nm, and Ge sensor has non-negligible sensitivity at this wavelength, a long-pass filter (F1, Thorlabs FGL1000M) was used in front of the Ge power sensor to block the reflected second harmonic light. By determining the SHG and the residual pump powers at the waveguide’s output, we accounted for the losses caused by the downstream optical components. To record the SHG and residual pump tuning curves, a swept-wavelength option of our laser and a digital oscilloscope were used.
4. Waveguide SHG characterization
Before cutting the sample’s output end facet at an angle, we evaluated propagation losses around 1550 nm by applying the Fabry-Pérot resonator formula  to the recorded low power scans of our tunable laser source. Given the excellent smoothness (RMS roughness <5 nm) and the measured deviation from verticality below 0.5°of the ridge facets within the area of the guided modes’ location, the bulk reflection at the air/LiNbO3 interface for the extraordinary wave (0.132 at 1550 nm) was used for the loss evaluation. In ridges with widths larger than 10 µm, very similar propagation losses of (0.15 ± 0.03) dB/cm were measured whereas those in the narrower (7–8) µm wide waveguides were found to be (0.27 ± 0.04) dB/cm. Also, for each waveguide, the pump coupling efficiency was estimated based on the overlap integral of the mode intensity recorded at the perpendicularly cut sample’s output facet with that of the in-coupling lens’s focal spot. Using this method, the background noise in the recorded images can lead to incorrect (usually overestimated) values for the coupling efficiency. Therefore, we have preliminary processed the recorded mode images by equating the background noise outside mode intensity contours to zero. Coupling efficiencies ranging from 83% to 94% were found, with the maximum values for ≈10 µm wide ridges with close to circular mode profiles. We carried out the SHG characterization of all available RPE PPLN ridge waveguides by scanning the laser wavelength around quasi-phase-matching point at low pump power of 5 mW. To ensure the same in-coupled pump power, laser source power was adjusted to account for different pump coupling efficiencies into the waveguides. The highest SHG output was measured in 10 µm wide PPLN ridges, the best of which was chosen for the comprehensive SHG characterization. To avoid PRD effects, all subsequent SHG measurement were conducted with the sample heated and maintained at 120 °C . The measured mode intensity profiles at the waveguide’s output for the pump and SH wavelengths, along with the corresponding averaged one-dimensional intensities, are shown in Fig. 3(a). The pump mode was recorded away from the QPM point, at 1552.6 nm, where SHG power was negligible. For the guided mode at 775.7 nm we used the generated second harmonic at QPM pump wavelength. In good agreement with the simulations of Fig. 1(c), mode sizes at 1/e2 peak intensity of 8.1 µm and 6.4 µm for the pump, and 8.0 µm and 4.8 µm for the second harmonic were obtained along the horizontal and vertical axes, respectively.
The measured low-power SHG tuning curve of the best efficiency ridge waveguide and the respective theoretically calculated curve, plotted in Fig. 2(b), show good agreement of the main peaks with a full width at half maximum (FWHM) of 0.22 nm. The small deviation in the wings of the measured curve from the typical sinc2 behavior results from the effective index variation along the waveguide [12,27,28] which is caused by imperfections in the fabrication process, in our case by in-homogenous proton in- and out-diffusion during PE and RPE steps as well as slight non-uniformity in the diced ridge width. We defined the peak SHG conversion efficiency as η = 100% PSH / (Pp-in)2, where PSH and Pp-in are the exiting SH power directly after the waveguide and the launched into the waveguide (coupled) pump power, respectively. During the measurements, Pp-in was calculated multiplying the pump power, measured after the in-coupling lens, by the product of the coupling efficiency (0.94 for this waveguide) with the transmittance (0.868 for the extraordinary wave at 1550 nm) at the air/LiNbO3 interface. From the parabola fit to the measured low-power dependence of PSH on Pp-in at QPM wavelength 1551.4 nm, shown in the inset of Fig. 3(b), η = 510%/W was extracted. For our waveguide with a 45 mm long periodically poled section this corresponds to a peak length-normalized efficiency of ηnorm ≈ 25%/W·cm2. The best reported to date SH efficiency values for proton-exchanged PPLN waveguides of 38%/W·cm2 and 150%/W·cm2 were measured in APE  and RPE  channel waveguides, respectively. Yet, in these works, SHG conversion efficiency formula with a square of the output pump power in the denominator was used. Using this definition, the conversion efficiency of 49%/W·cm2 in our waveguide can be found which is higher than that reported in APE channel waveguides  but still substantially lower than 150%/W·cm2 observed in buried RPE channel waveguides with comparable guided mode dimensions . The theoretical conversion efficiency for our waveguide calculated using the model, whose description can be found in Ref. , and the propagation losses of 0.15 dB/cm for both pump and SH waves, was estimated to be ≈125%/W·cm2. The reason for the large discrepancy between the measured and the theoretical values cannot be explained by fabrication imperfections and is under investigation.
The measured SHG characteristics versus Pp-in at a QPM wavelength of 1551.4 nm are shown in Fig. 4(a). Here PSH and the conversion efficiency of the pump into the SH wave, defined as ηSHG = 100% · PSH / Pp-in, are plotted as a function of Pp-in. Similar to Refs. [12,17,29], after the initial parabolic increase in the low pump depletion regime [inset of Fig. 3(b)], a quasi-linear increase of PSH for up to 1.2 W of coupled pump power was observed. At Pp-in = 1.2 W, the maximum ηSHG ≈ 70% was reached with 0.84 W of the second harmonic output while the residual pump power output was measured to be 0.025 W. At the same Pp-in at 1552.6 nm, slightly away from the phase-matching condition where PSH was negligible, we measured 0.87 W of pump light exiting the waveguide, indicating that at the QPM wavelength 1551.4 nm about 97% pump depletion, calculated based on the definition of Ref. , was reached. Furthermore, the fact, that the sum of PSH and the pump output at QPM wavelength (0.84 W + 0.025 W) is only slightly smaller than the pump output at 1552.6 nm (0.87 W), can serve as an indirect indication that the waveguide propagation losses at pump and SH wavelengths are quite similar. At higher SH powers, up to 10 µW of green light at 517 nm, resulting from non-phase-matched sum frequency generation of the pump and SH waves, was also measured at the waveguide’s output. With the further increase of Pp-in, sub-linear PSH grows and a decline of SHG efficiency was measured, while at Pp-in > 1.6 W a sharp drop and large fluctuations on a seconds’ time scale of both PSH and ηSHG were observed [not shown in Fig. 4(a)].
The explanation for this behavior can be found in Fig. 4(b) where the measured SHG wavelength tuning curves at different pump power levels are shown. The observed blue shift of the spectral peak at a pump power of 1.62 W is a clear indication of photorefractive damage . Interestingly, in the large conversion regime at coupled pump powers ≥1 W, we did not see any indication of the QPM spectrum narrowing as described in Ref.  and observed in different types of PPLN waveguides [17,30]. The possible explanation could be that even before the onset of strong PRD, the expected narrowing of the QPM peak is compensated by peak broadening caused by photorefractive effects. The slight QPM peak broadening with a blue shift is already noticeable at Pp-in = 1 W in Fig. 4(b). It is worth noting that up to Pp-in ≈ 3.5 W away from the QPM condition, our ridge waveguides showed no signs of PRD such as power fluctuation or mode profile degradation. However, at QPM wavelength, physical damage occurred with similar melting and reflowing effects as observed in Ref.  but always in a ridge section closer to the output waveguide’s facet, suggesting that the reason for this was PRD caused by the growing second harmonic wave.
To conclude, we have fabricated PPLN ridge waveguides using annealed and reverse proton exchange with subsequent diamond blade dicing. The highest reported to date SHG power of 1 W at 775.7 nm and internal conversion efficiency of 70% with 97% pump depletion is demonstrated for this type of PPLN waveguides. In the low power regime peak SHG conversion efficiency of 510%/W is measured in the ridge waveguides with a 4.5 cm long periodically poled section. The maximum attainable SHG power is limited by photorefractive damage effects necessitating PRD resistant, e.g. MgO doped, LiNbO3 substrates for even higher power operation. Enhancing the PRD resistance of RPE PPLN ridge waveguides and improving their peak SHG efficiency toward that of their channel counterparts  would allow for realization of stable multi-watt-level second-harmonic generation in ≈1 cm long PPLN ridge devices.
Deutsche Forschungsgemeinschaft (DFG Ki482/17-1).
The authors declare no conflicts of interest.
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