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We report on the fabrication and second harmonic generation from a periodically-poled MgO-doped lithium niobate ridge waveguide within the telecommunication L-band. The ridge waveguide is fabricated by carbon ion implantation and the following diamond blade dicing method. A normalized second harmonic conversion efficiency of 20.3%W−1cm−2 was obtained with a total insertion loss of 4.3dB at wavelength of 1612.7nm. Our analysis shows that at least ~70% of the second-order nonlinearity was preserved after the implantation and thermal annealing process.
© 2015 Optical Society of America
1. Introduction
Lithium niobate (LiNbO3) is an excellent material for nonlinear integrated optical devices due to its remarkable χ(2) nonlinearity and wide transparent wavelength range [1]. The long interaction length and robust structure of quasi-phase-matched (QPM) waveguide devices based on periodically poled LiNbO3 (PPLN) crystals allows for highly efficient nonlinear frequency conversion [2,3]. These waveguide devices have been investigated in many fascinating photonic applications, such as wavelength conversion based on sum or difference frequency generation [4–6], on-chip generation and manipulation of entangled photons and single-photon detection [7,8]. To realize waveguide structures in LiNbO3 substrates either proton exchange (PE) or metal in-diffusion (e.g. Ti) were mostly investigated and developed so far [9,10]. Furthermore, wafer bonding/lapping techniques were also reported as well as femtosecond laser writing [5,11].
As a flexible method to induce refractive index modulations inside optical materials, ion implantation has been adopted to produce low-loss waveguides in LiNbO3 [12]. During the implantation energetic ions are injected into the surface region of the crystal and stopped around a certain depth. Energy loss in this process can cause a considerable change of refractive indices (both ordinary and extraordinary) in the implanted region. By means of implantations with different ions and energies, planar and channel waveguide structures can be formed inside the crystal [12–14]. Second order nonlinear optical processes, based on the Cherenkov angular phase matching, modal phase matching and QPM have been demonstrated in LiNbO3 planar waveguides [15–17]. Second harmonic generation (SHG) via QPM has also been demonstrated in oxygen ion implanted LiNbO3 channel waveguides [18]. The reported normalized conversion efficiency was about 34.5%W−1cm−2 (SHG from 992nm to 496nm). However, nonlinear interactions in the telecommunication bands were not yet reported in such waveguides. Recent investigations show that low-loss planar waveguides with surface index increase can be formed in LiNbO3 by MeV carbon ion irradiation with ultra-low fluence [19,20]. Such implantation conditions should in principle allow for well-preserved second order nonlinearities.
Ridge-type waveguides are of considerable interest because of their strong light confinement in the lateral direction [6]. Ridge structures have been realized by several approaches, including wet etching [21,22], reactive ion etching [23] and ion beam assisted etching [24]. Recently “optical grade dicing” was emerging as a quite effective technique to fabricate ridge waveguides and structures with high aspect ratio and ultra-smooth sidewalls have been demonstrated [5,6,25]. Up to now, nonlinear interactions in diced ridges based on ion implanted waveguide have not been reported.
In this Letter, we describe the formation of low-loss MgO:PPLN ridge waveguides by means of low-fluence carbon ion implantation followed by ultra-precise dicing. Successful combination of these two methods allowed for efficient SHG of 1.6μm pump wavelength. Our work shows the potential of ion-implanted diced ridge waveguides for nonlinear frequency mixing in the telecommunication band.
2. Experiments
The z-cut PPLN (5% Mg doped) chip with grating period of Λ = 19.3μm (40% duty-cycle) was cut to dimensions of 18 × 2 × 0.5mm3. After an RCA cleaning process the sample was implanted by 7.5 MeV C3+ with a fluence of 3 × 1014ions/cm2. The ion beam was tilted 7°off the sample normal to eliminate any channeling effects. After the implantation the sample was annealed at 260°C for 30 minutes to improve the guiding properties of the waveguide. The dark mode spectra of the planar waveguide were investigated by a prism coupler (Metricon2010) before and after annealing. The implanted surface was then structured by the diamond dicing blade with high precision. During this process, the sidewalls along the cutting track are polished by the rotating resin-based blade simultaneously. Ridge waveguides with different widths (from 4μm to 16μm) and a constant depth of 10μm were fabricated. The whole procedure is depicted in Fig. 1.
The two end facets of the ridge waveguides were finally polished to allow for fiber-waveguide coupling. The final length of the waveguides was 17.2mm. During SHG measurements the waveguide was pumped by a laser (Santec210VF) which can be tuned from 1260nm to 1630nm. The pump light was launched into the ridge waveguide by direct endfacet-coupling using a polarization maintaining fiber. The output light from the ridge waveguide was collected by an objective lens and collimated into a power meter.
3. Results and discussion
Using the prism coupler method we found optical modes with effective refractive indices neff of 2.1919 (2.1904) for the as-implanted (annealed) sample and a wavelength of 632.8nm, and 2.1313 (2.1303) for as-implanted (annealed) conditions at 1539nm wavelength. In both cases extraordinary polarization was used. These effective indices were close to the substrate index nsub (2.1911 for 632.8nm, 2.1305 for 1539nm) and only a small decrease due to annealing was observed. Accordingly, we attribute these values to excitation of higher (or leaky) modes of the formed waveguide. On the other hand, fundamental modes of the waveguide can hardly be excited because of a buried refractive index profile. Recent investigations have indicated that such buried waveguides can be formed by ion implantation where real modes were not excitable using a prism coupler [26,27].
As a consequence, usual methods of index profile reconstruction like RCM [28] or WKB [29] cannot be applied here. Alternatively, Fig. 2 shows the energy deposition profile simulated by SRIM2008 software [30] and the corresponding index profile Δn632.8(z) at a wavelength of 632.8nm that was calculated according to the method described in [20]. The refractive index profile shows a small increase at the surface that starts to grow up to a maximum at a depth of 4μm. In this deeper region the index profile exhibits a typical dip that is attributed to the peak region of nuclear energy deposition, where partial amorphization of the crystalline material leads to a lowering of the refractive index.
Fig. 2 Simulated damage profile ED(z) (blue) and extraordinary index profile Δn632.8(z) (red) of the as-implanted planar waveguide.
The refractive index profile of the planar waveguides at different wavelengths and after annealing can be estimated by the following equation:
The dispersion of the refractive index profile is assumed to be the same as that of the substrate. Here the factor f<1 accounts for the (unknown) decrease of the refractive index profile due to annealing, which will be further discussed using simulations of the phase matching condition.
The fundamental modes of the formed ridge waveguides with index profile n(λ,z), constant height of 10μm and a width w were calculated for pump wavelength λ1 and SHG wavelength λ2 = λ1/2. The values obtained for the effective refractive indices of these modes, neff,i , i = 1,2, are then used to calculate the period Λ = (neff,2/λ2-2neff,1/λ1)-1 needed for phase-matching. In Fig. 3(a) we simulated a ridge with a width of w = 16µm at a temperature of 23°C and using λ1 = 1612nm and varying factor f. As can be seen, for f = 0.91 the corresponding grating period would be Λ = 19.3μm. When keeping f = 0.91 fixed but varying the waveguide width w the data in Fig. 3(b) is obtained. A change of 200 nm in the waveguide width results in a change of 0.005μm of the required grating period to get phase matching for λ1 = 1612nm. Furthermore, a waveguide with width w = 15.8μm could be phase matched at a wavelength of 1611.75nm using a grating with Λ = 19.3μm, i.e. a change in the width of Δw = 200nm shifts the phase matching wavelength by Δλ = 0.25nm.
Fig. 3 Calculated phase-matching conditions for ridge waveguides with a height of 10μm, width w and a refractive index profile according to Eq. (1). (a) Grating period Λ for a waveguide with w = 16μm as a function of factor f. (b) The same data for f = 0.91 but with varying width w.
We carried out the fiber-to-waveguide coupling in all the ridge waveguides having different widths. It was observed that ridges with widths below 12μm exhibited severe propagation losses for longer wavelengths due to a possible modal cut-off at around 1550 nm. Waveguide loss coefficients have been measured using the Fabry-Perot method and by tuning the wavelength using the following formula [31]:
where Imax and Imin are the maximum and minimum transmitted power, and R is the Fresnel reflectivity at the endfacet. Figure 4 shows the interference signal of the transmitted light of a ridge with width of 16μm (WG16) around the wavelength of 1612nm. The propagation loss deduced from this measurement is rather low and about 0.76dB/cm. The corresponding insertion loss is 4.3dB including Fresnel reflection losses, fiber-waveguide coupling efficiency and the propagation loss inside the waveguide at the wavelength of 1612nm.Second harmonic generation was demonstrated in the ridge waveguides by tuning the wavelength of the pump light at room temperature (23°C). Figure 5(a) presents the power of the SHG signal versus pump wavelength in ridge WG16. The corresponding calculated phase matching curve (sinc function) is also depicted in Fig. 5(a) as a reference. It is found that the QPM wavelength of WG16 is about 1612.7nm. While the here used grating period of 19.3μm was designed to allow for SHG of a pump wavelength of 1550nm in the MgO:LiNbO3 bulk material, the observed shift to longer wavelength can be convincingly understood by the waveguide dispersion properties discussed in the former section: If we assume a reasonable reduction of the calculated refractive index profile by a factor f = 0.91 that is due to the thermal annealing step a grating period of 19.3μm allows for phase matching in the diced ridge waveguide with width w = 16μm.
Fig. 5 (a) Experimental SHG tuning curve (red dots) compared with theoretical QPM sinc curve (blue dots); (b) measured second harmonic power (blue circles) vs. pump power of wavelength 1612.7 nm. The inset shows the output mode profile of the SHG wave.
While SHG was also demonstrated in WG12 and WG14 having widths of 12.5μm and 14.4μm, respectively, the best experimental result was obtained in WG16. In this sample the measured SHG power was 415nW at a pump power of 0.83mW (both values were measured after the rear facet), which results in a normalized conversion efficiency of 20.3%W−1cm−2. It should also be noted that the measured acceptance wavelength bandwidth (FWHM) of WG16 is approximately 0.84nm which is nearly 1.6 times larger than the calculated curve [see Fig. 5(a)]. This broadening may be attributed to the fluctuation of neff along the ridge that is due to small variations of the width of the diced ridge (but potentially also tiny inhomogeneities that result from a non-uniform implantation). During dicing small changes in the width of the formed ridge of typically ± (100-200)nm on a length scale of (1-2)mm at both ends of the ridge occur when the blade enters or leaves the substrate surface. This leads to small changes of the phase matching condition during propagation and is in quantitative agreement [see discussion of data from Fig. 3(b)] with the measured curve in Fig. 5(a).
Figure 5(b) shows the generated SHG power against the fundamental power for WG16 and the square fit curve. The inset shows the mode pattern of SHG signal measured at wavelength of ~806nm, which clearly indicates a single-mode generation and propagation of SHG signal in WG16. The coupled-modes equations in the waveguide with continuous wave pump can be demonstrated by the following equations [32]:
where Δk = 2k1− k2, A1,2 are the time-averaged amplitudes of pump and SHG mode, α1,2 are propagation losses, and deff is the effective second-order nonlinear coefficient in the implanted waveguide region. The effective area Seff of the ridge waveguide is calculated (see [4]):where the E1Norand E2Nor are the normalized mode profiles of pump and SHG signal. The calculated Seff is about 85μm2. When substituting the averaged amplitudes and the propagation losses into Eqs. (3,4), the intensities of harmonic and pump light versus the propagation length can be calculated by solving the coupled-modes equations. The possible degradation of deff due to ion implantation can be deduced by comparing the experimental and simulated values. We obtain a deff of 8.9pm/V (or d33 of 14pm/V) in the waveguide region taking into account the imperfect duty-cycle of the domain grating and the broadened QPM curve. Data from [1] shows that the d33 of 5mol%MgO:LiNbO3 is about 20.1pm/V at a wavelength of 1313nm. Comparing the measured deff with this value shows that ~70% of the second-order nonlinearity was preserved. We want to note that the present SHG measurements were performed for a wavelength of 1612.7nm, significantly larger than 1313nm. With the reduction of the d33 coefficient of MgO:LiNbO3 under longer wavelengths [1] it is thus reasonable to claim that this value of ~70% is a lower limit of preservation ratio of the d33 coefficient.4. Conclusion
Low-loss ridge waveguides were fabricated in 5% MgO:PPLN by carbon ion implantation followed by precision diamond-blade dicing. Second harmonic generation experiments in the telecommunication L-band at a wavelength of 1612.7nm have been realized in these ion-implanted waveguide. Waveguide dispersion was successfully modelled using reasonable assumptions for the generated refractive index profile. Our results demonstrate the nonlinear properties are mostly preserved after ion implantation and thus show the principal application potential of the fabricated PPLN ridge waveguides in the domain of integrated nonlinear optics.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (grants 11511130017 and 11375105), the State Key Laboratory of Nuclear Physics and Technology of China, and the Deutsche Forschungsgemeinschaft (DFG grant Ki482/15-1).
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