Channel waveguides at 1540 nm in X- and Z-cut LiNbO3 are formed by one-step multienergy MeV O2+ implantation. Single perpendicular mode in both planar and channel waveguides is demonstrated. In the ne channel waveguide, double transverse modes are observed because of the large index difference and broad width of the channel. A single transverse mode is obtained in the no channel waveguide. The thickness of the waveguide is less than 2 μm, and the compactness of the waveguide is attributed to the large index difference between the guiding region and the low-index barrier. The mode patterns and the waveguide loss are measured and analyzed. The loss of 0.9 and 0.7 dB/cm are obtained from the no and ne channel waveguides, respectively.
©2005 Optical Society of America
Lithuium niobate (LN) is a versatile material used widely in optoelectronic, opto-acoustic, and piezoelectric applications [1–2]. The stripe-geometric lithium niobate waveguide is highly needed because it constitutes the basic structure for many optoelectronics devices, such as optical switch, modulator, coupler, multiplexer, etc. Stripe-geometric lithium niobate waveguides can be fabricated by etching, in-diffusion, or ion exchange. The ridge waveguide and Ti-diffused lithium niobate are the typical examples. As an alternative method, ion implantation is usually employed to form planar waveguides in lithium niobate [3–4]. Channel waveguides by He implantation was demonstrated in KNbO3 and Ga4GdO(BO3)3[5–7]. In both cases the implantation, sometimes with the aid of photolithography or the masking method, were carried out in two steps: a planar structure by relatively high energy ion implantation was formed in advance, followed by a low multienergy implantation to form the channel structure. Channel waveguide in KNbO3 was also demonstrated by the single homogeneous He+ ion irradiation, in which a structured photoresist with wedged edges serves as an implantation mask . In this article we report first the channel lithium niobate waveguide at 1.54 μm formed by one-step multienergy O2+ implantation. Because the refractive index barrier is caused by multienergy implantation, the waveguide with less leakage loss is obtained. For comparison, both planar and channel waveguides are formed simultaneously by employing the same implant parameters. The properties of waveguides are investigated using prism and end-face coupling methods. In the planar waveguide, only single guiding mode is demonstrated in the perpendicular direction to the surface of the sample. Two transverse modes and one transverse mode are observed in the ne and no channel waveguides, respectively. The losses of the channel waveguides are measured and analyzed. This is a continuative work of our former research ; the experimental results provide verification to our analysis in the former report.
2. Experiment and analysis
Three units of energy O2+ at 3.0 MeV; 3.6 MeV; and 4.5 MeV, respectively, are implanted into X-cut and Z-cut lithium niobate with moderate beam doses. In order to get a flat damage profile at the end of the ion track, the ratio of beam dose for each energy is chosen at 0.45:0.55:1.2. The summed dose for each sample is from 4.4×1014 ions/cm2 to 1.3×1015 ions/cm2, which corresponds to the O2+ peak concentration from 1 to 3×1020 ions/cm3. Implantation is carried out at room temperature, and the samples are titled 70 off the normal direction of the sample surface in to avoid channelling effect. The channel structure is formed with the help of the lithography technique. The 15 μm channels [Au: Does this mean you have fifteen separate micrometer channels?], which are arranged at intervals of 50 μm, are exposed to the implantation. The remaining area is covered by photoresist. The thickness of the photoresist is estimated to be about 2 μm. According to the TRIM simulation, the LN damage profile caused by implantation is given in Fig. 1. It can be seen that the intensive-damage region is concentrated at the end of the ions track, at about 1.7μm below the sample. For the photoresist-covered area, the LN intensive damage lies just beneath the surface of LN because of the stopping effect of the photoresist on the impinging O2+ ion. At the joint between the exposed and covered areas, the damaged region may slightly interpenetrate because of the transverse straggling of implanted O2+ ions. According to our previous experiments and analysis [9–10], the extraordinary refractive index (ne) of LN along the ion track is raised when the implanted O2+ ion dose is relatively low, which can cause an ne index-raised guiding region. Meanwhile, a slight reduction of ordinary index (no) is induced in this region. At the intensive-damage layer, an index-reduced barrier is formed because of the reduction of material density and the partly amorphous nature of this layer. In consequence, two kinds of waveguides can be formed in such implanted LiNbO3 in terms of refractive index: for ne, a waveguide may be formed with the structure of an extraordinary index-raised guiding region sandwiched between the surface (air) and the low index barrier ; for no, a typical barrier-confined waveguide is formed. Such an implanted ne channel waveguide has better optical confinement and a more-compact structure, because the index difference between the index-raised guiding region and the index-reduced barrier may be larger than or comparable to that of the in-diffusion and exchange waveguides.
By adjusting the thickness of the photoresist (corresponding to the location of index-reduced barrier), we formed the implanted ne channel that has the configuration of an extraordinary index-raised channel “encapsulated” by the low-index barrier and air. The schematic of the implanted-channel waveguide formation is given in Fig. 2(a). The dark areas represent the damaged region. The dashed curve describes the profile of damage in a sectional view. The microscopic cross section of the channel waveguide formed by O2+ implantation is shown in Fig. 2(b).
After implantation all the samples are annealed at 250°C for 20 min in air ambient. Waveguide measurements are carried out with prism coupling and end-face coupling. For the planar waveguide structure, a single mode at 1540 nm is demonstrated by dark-mode measurement; the mode profile is shown in Fig. 3. The location of a possible index barrier is situated at about 1.7 μm beneath the sample surface (see Fig. 1). This value is smaller than the typical thickness of the diffusion single-mode LN waveguide (usually 3–5 μm), which implies the possibility that a more-compacted single-mode waveguide can be formed by implantation. The effect of compressed waveguide size can be attributed to the big index difference between the guiding region and the index barrier.
The measurement for the channel waveguide is carried out by use of the end-face coupling method. For mode distribution measurement, a BT&D EFA 1000 erbium fiber amplifier is used. A tunable bandpass filter is connected to the EFA to select a wavelength of 1540 nm. The output light of the waveguide is coupled out by a 100× microscope objective. The mode pattern is detected by a computer-compatible video camera. For the ne and no channel waveguides, the typical output optical intensities exhibit Gaussian profiles, which are given in Fig. 4. As can be seen, two transverse waveguide modes can be supported in each ne channel waveguide because of the relatively large index difference and the broad width of channels (~15μm). When we slightly adjust the location of input focus on the input end face of each ne channel waveguide, the output intensity pattern experiences a process of deformation from an asymmetric single transverse mode to good symmetric double modes, as shown in Fig. 4(a). The intensity pattern of the modes shows good consistency with the waveguide channel structure, i.e., the intensity profile exhibits a nearly uniform ellipsoidal shape for the optical confinement at the down side of the pattern. This optical confinement comes from the index interface between the raised index ne in the guiding region and the reduced ne in the index barrier. Near the sample surface, the optical intensity is concentrated because of the intensive optical confinement from the air-LiNbO3 interface. It can be expected that if we shrink the channel width a single-transverse-mode waveguide at 1540 nm can be obtained. For the no barrier-confined waveguide, only a single transverse mode is observed, as shown in Fig. 4(b). This can be simply attributed to the small index difference between the guiding and barrier regions, since the ordinary index in the guiding region is reduced after implantation. Comparing the mode patterns from the two kinds of waveguides, it can be found that the ne waveguide provides a better optical confinement.
The channel waveguide loss at 1540 nm is measured before and after annealing. The light source used (1540 nm) is from Santec TSL-80 tunable semiconductor laser. Since the measurement setup we used is more suitable for the single-mode waveguide, the no single-mode waveguide is measured first. The measured waveguide loss is in the range of 0.9–1.2 dB/cm, depending on the width of each waveguide. This value is quite low in comparison with our previous LiNbO3 samples. As we know, high loss in implanted waveguides is caused mostly due to the high implant dose and the optical leakage from the index barrier. Because the higher the implant dose is, the more defects are produced in the guiding region, which may cause enhanced absorption and scattering loss. In the present experiment, the highest O2+ dose is only 1.3×1015 ions/cm2; the magnitude is at least one order lower than that of a typical He-implanted waveguide. With moderate annealing, those defects can be removed easily. Meanwhile, the index barrier become broad due to three units of energy O2+ implantation, this widened barrier plays an important role in reducing the leakage loss. The loss in the ne channel waveguide is also measured using the same setup. Being as similar as the no channel waveguide, a high optical attenuation is observed in the as-implanted samples. After annealing at 250°C for 20 min, the waveguide loss decreases dramatically, i.e., it reaches as low as 0.7 dB/cm. Because the ne channel waveguide supports double transverse modes, the measured result is only an estimate. However, it can be expected that the lower loss can be obtained in the single-mode ne channel waveguide. To investigate and improve the light propagation in the implanted channel waveguides, we are carrying out more experiments to optimize the implantation and annealing condition.
Implanted planar and channel waveguides at 1540 nm are formed by one-step MeV multienergy O2+ implantation. The single-mode planar waveguide, as well as the spatially distributed, single perpendicular mode and double- or single-transverse modes in channel waveguides are demonstrated through prism coupling and end-face coupling. According to our simulation and analysis, the thickness of the waveguide is approximately 1.7 μm—smaller than the size of typical diffusion LiNbO3 waveguide. The output intensity patterns show good symmetry in profile, indicating a quite uniform waveguide structure. Low loss of 0.9 and 0.7 dB/cm is obtained in the channel no and ne waveguides, respectively. Since the waveguide loss is governed by factors such as the implant parameters and annealing conditions, improved experiments are being carried out. As is known, implantation has many unique advantages for forming waveguides, especially in optical crystals . This method provides the possibility for forming a more-compact waveguide structure and may be generalized to the other optical crystals such as KTP, to form the nonlinear optical channel structure.
This work is supported by the National Natural Science Foundation of China under Grant No: 10375037.
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