The planar waveguides have been fabricated in fused quartz by 3.0 MeV oxygen ion implantation at dose of 1×1015ions/cm2. The guiding modes were observed at wavelengths of both 633 nm and 1539 nm before and after annealing at 320°C, 450°C and 500°C in 60 min characterized by the prism-coupling method. The width of the waveguide structure induced by oxygen ion implantation is about 3 microns. After suitable annealing, the minimum propagation loss of the waveguide in fused quartz can be reduced to -0.14 dB/cm.
©2004 Optical Society of America
Fused quartz is ultra pure, single component glasses (SiO2). Because of the refractive index which is closed to the value of fiber for coupling to the fiber easily, and the dielectric properties and very high electrical receptivity of fused quartz over a wide range of temperatures; as well as low thermal conductivity, therefore it can be used to fabricate integrated devices [1, 2]. The waveguide is structure of the fundamentals of the high-technology field which related to aspects of integrated optics such as optical communication, optical signal processing, and optical computing . In recent years ion implantation has gradually become a effective method for fabricating waveguide structures in most optical materials due to it has controllability and reproducibility that are superior to those of other techniques such as epitaxial growth, metal diffusion, and ion exchange [4, 5]. More recently the implantation of heavy ions has attracted much attention for use in waveguide fabrication because it usually requires a much lower dose compared with light ions implantation, such as He and H [6–8]. The wavelength at 1540 nm is preferred in optical fiber communication systems due to a minimum of fiber loss. Therefore, the property studies of the waveguides at telecommunication wavelength (~1540 nm) are desired [9, 10].
Thermal annealing is a means of altering the residual effects of densification, viscous flow, and anisotropic deformation in the implanted layer . In present letter, we have reported the fabrication of the waveguides by MeV oxygen ion implantation, and observed the guiding mode at the wavelengths of both 633 nm and 1539 nm before and after annealing. Concurrently, the influence of the annealing conditions on refractive index profile and propagation loss of the waveguide has also been investigated. An understanding of such dependences is necessary for the effective utilization of this technology in waveguide fabrication.
2. Experimental details
The fused quartz samples with sizes of 30×5×2mm3 were optically polished and cleaned before ion implantation, the refractive index of the fused quartz was 1.4572(as the measured at the wavelength of 633nm). The MeV oxygen ion implantations were performed at the 1.7 MV tandem accelerator of Peking University. The samples were implanted by 3.0MeV oxygen ions with dose of 1×1015ions/cm2 at room temperature. The ion beam was scanned to ensure a uniform implantation over the samples with typical intensity of about 80nA/cm2. The samples were annealed using furnace in the temperature from 200°C to 700°C in air.
Prism-coupling method was used to observe the waveguide dark modes. The dark modes of the optical waveguide were measured with a model 2010 prism coupler made in Metricon, USA. The laser beams with wavelengths of 633 nm and 1539 nm were struck on the base of the prism, which was brought into contact with the waveguide. The reflected beam from the base of the prism was detected by a photodetector. The angle of incident laser beam could be varied by means of a rotary table on which the prism, waveguide and photodetector were mounted. The intensity of light striking the photodetector was plotted as a function of the incident angle, a sharp drop in the intensity profile would correspond to a propagation mode. A computer was controlled the measurement system. The refractive index of the prism used in the measurement was 1.9649, the index resolution was ±0.0005, and the index accuracy was less than 0.001.
The moving fibre method was used to measure the propagation loss of the waveguide . When a 633nm He-Ne laser beam excited a waveguide mode, a fibre probe scanned down the length of the propagation streak to measure the scattered light from the waveguide. The light intensity detected was plotted as a function of the propagation distance. The equation which describes the decay of light in waveguide is I(x)=I0 exp(-αx), where α is the decay constant, I0 is the initial power, x is the distance, I(x) is the transmitted power through a waveguide mode. It was assumed that the intensity of the light scattered out of the waveguide was proportional to the transmitted power in the waveguide. The whole measurement was performed with the Metricon 2010 prism coupler.
3. Results and discussion
After the samples of fused quartz were ion implanted, the samples were annealed at different temperatures, the temperatures chosen as follows: A, as implanted; B, annealing at 320°C, 60min; C, annealing at 450°C, 60min and D, annealing at 500°C, 60min. Table 1 lists measured propagation modes and their effective refractive indices at different temperatures A, B, C and D obtained at the wavelength of 633 nm. Fig. 1 shows the measured relative intensity of the light (TE polarized) reflected from the prism of the incident light for the fused quartz waveguide formed by 3.0 MeV O+ ion implantation at the dose of 1×1015 ions/cm2 before and after annealing with the wavelengths 633 nm and 1539 nm, respectively. A1 and D1 represent the results before and after annealing at 500°C in 60min measured with 633nm wavelength. The reflection drop corresponds to the dark mode, each dark mode corresponds to the effective refractive index Nm . As indicated in A1 of Fig. 1, five obvious modes were observed, and the values of effective index corresponding to the mode were listed in Table 1, their effective indices Nm of the first two modes (TE0 and TE1) were relative sharp, and the corresponding Nm values are higher than that of virgin fused quartz (n=1.4572), which usually means a good confinement of light propagation. The last four dips (TE2~TE4) seemed to be broader to some extent, and the corresponding mode may be called “leaky” mode, which could be the result of optical interference between the multiple reflections occurring at the interfaces in the waveguides. After annealing, as indicated in D1 of Fig. 1 and Table 1, there are still excited two relative sharp dips (TE0 and TE1), and the effective index are higher than that of virgin fused quartz. But the value of effective index was slightly smaller than that corresponding to A1. The similar results were excited in the annealed process B and D as detail in the Table 1.
A waveguide is frequently characterized by its normalized width V, which can determine the total number of waveguide modes. V is found from the following equation :
V=2πd( - )1/2 / λ, where λ is the wavelength of light, d is the waveguide width, and n1 and n2 are the refractive index in the waveguide and in the substrate, respectively. The larger the value of V is, the larger the mode number becomes. As the A2 and D2 in Fig. 1, only a mode was excited before and after annealing at 500°C in 60min at the wavelength of 1539nm, the values of effective index corresponding to the mode were 1.4505 and 1.4468 before and after annealing, each of the values higher than the refractive index of virgin fused quartz (n= 1.4440 with the wavelength 1539 nm). This is attractive in the present optical fiber communication.
Many methods have been developed to reconstruct the refractive index profile, including the inverse Wentzel-Kramers-Brillouin and the parameterized index profile reconstruction. In the present work, we have used the reflectivity calculation method (RCM) to reconstruct the refractive index profile . RCM was used to successfully to characterize the non-stationary waveguide, particularly ion-implanted waveguide, a least-squared fitting program based on RCM was available to calculate the refractive index profile by adjusting certain parameters until the theoretical mode indices match the experimental ones within a satisfactory error. Figure 2 shows the refractive index profiles (at the wavelength of 633 nm) of the fused quartz waveguide formed by 3.0 MeV O+ ion implantation under four different treatments A, B, C and D. E represents the refractive index in virgin fused quartz (n= 1.4572). Table 1 gives the comparison of the measured mode indices with the fitted values of the indices based on RCM. It is found that the measured effective refractive indices were in agreement with the calculated values with less than 10-3. The refractive index was increased in the guiding region with a width of about 3 microns beneath the sample surface, therefore, the region between the surface and substrate barrier becomes a waveguide layer. As we can see, when the annealing condition changes from A to D, the refractive index in the guiding region decreases gradually, but the waveguide structure still excite the guiding modes at both 633 nm and 1539 nm, respectively.
The propagation loss is a significant parameter in the evaluation of waveguides usefulness for many applications. Figure 3 shows the natural logarithm of the scattered intensity of light in the zero-order mode vs. propagation distance along the fused quartz waveguides formed by ion implantation at the different annealing treatments A, B, C and D in air with the wavelengths of 633 nm, respectively. The propagation loss was -6.81dB/cm before annealing, and -1.09 and -0.37dB/cm after annealing at 320°C and 450°C in 60 min in air, respectively. The minimum value of the loss was only about -0.14dB/cm after annealing at 500°C in 60 min in air; this is a very low loss value in the waveguide. The measurements taken at various points on the sample are usually repeatable to ±10%. Figure 4 shows the photos of the 633 nm light propagating from right to left through the fused quartz waveguide formed by ion implantation with prism coupling: A as implanted and B after annealing treatment at 500°C in air, respectively. This means the fused quartz waveguide has a good performance. The low loss of the waveguides suggests that the thermal treatment can be used to decrease waveguide propagation loss, and hence, form a well-confined waveguide by oxygen ion implantation in fused quartz.
In summary, planar fused quartz waveguides formed by 3.0 MeV oxygen ion implantation at doses of 1×1015ions/cm2 were reported. We observed the guiding modes both at the wavelengths 633 nm and 1539 nm before and after annealing at 320°C, 450°C and 500°C. After suitable annealing, the minimum propagation loss of the fused quartz waveguide can be reduced to -0.14 dB/cm. The results also provide a possibility for channel fused quartz waveguide fabrication by standard photolithographic procedures and ion implantation.
This work is supported by the National Natural Science Foundation of China (Grant No. 10035010) and the Key Laboratory of Heavy Ion Physics of Education Ministry (Peking University).
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