A TE-TM mode converter is proposed in a single trench GaInAsP/InP waveguide, which is fabricated by a single masking and etching process. Use of single-trench structure makes the design and the fabrication much simpler. The design of single-trench mode converter is described together with its fabrication in this article. We investigated the dependence of conversion efficiency on the waveguide width, trench depth, and trench position. Also, the wavelength dependence of mode conversion efficiency was calculated in a wavelength range between 1.5 µm to 1.58 µm. 95% TE-TM mode conversion was measured at a wavelength of 1.55 µm in a fabricated device with a 210-µm half-beat length.
©2009 Optical Society of America
Photonic integrated circuits (PICs) are composed of waveguide components that are, in general, polarization sensitive. Thus, optical mode converters are important components in PICs to manipulate the polarization state of lightwave. Also, the mode converter is a building block for a polarization diversity configuration, which enables one to build a polarization independent circuit . We proposed a polarization-independent waveguide optical isolator employing the nonreciprocal phase shift that is given only for TM mode in a Mach-Zehnder interferometer . To achieve polarization-independent operation, TE-TM mode converters are to be installed in constituent waveguides.
There have been proposed several types of waveguide mode converters such as a multi-sectional load converter , angled-facet waveguide mode converters [4–11], and a sloped slots converter . The asymmetrical angled-facet waveguide mode converters have been demonstrated to provide high conversion efficiency with a single-section of 250–720-µm length. Though the conversion efficiency up to 90% has been demonstrated, this type of device requires two different etching steps (wet and dry etching) to form the asymmetric structure of waveguide, i.e., an angled and a vertical sidewall. The angled sidewall can be fabricated by utilizing the anisotropic etching behaviour of semiconductors. However, its applicability depends strongly on the waveguide material.
The design of a passive mode converter is demonstrated in a GaAs/AlGaAs waveguide that has multiple trenches on the top of rib waveguide . It is shown to provide high conversion efficiency with an asymmetric structure fabricated in a single-step dry etching process. The asymmetry of waveguide is provided by multiple trenches with different depth and width, which are fabricated by utilizing the phenomenon called “Reactive Ion Etching (RIE) LAG”. This device has the advantages of high efficiency and low insertion loss. Moreover, it is possible to realize the monolithic integration with other optical waveguide devices. The fabrication technique of RIE LAG can be applied to a variety of materials.
In this paper, a single-trench type mode converter based on a GaInAsP/InP waveguide is designed and fabricated with a single masking and etching process. Use of a single-trench structure makes the design and the fabrication much simpler.
2. Operation principle and design
Figure 1 illustrates schematically the operation principle of a mode converter. By choosing the geometry and refractive index of an asymmetric waveguide, two eigen modes, orthogonally polarized to each other, exist whose optical axes are rotated by 45° with respect to the x- and the y-axis as is shown in Fig. 1(a). Let us consider a lightwave whose polarization is directed along the x-axis, i.e., TE mode, is launched into the asymmetric waveguide section through a symmetric waveguide (Fig. 1(b)). In the asymmetric waveguide section, two eigen modes are excited, and are propagated with different propagation constants, β1 and β2. After a propagation of a half beat-length, Lπ=π/(β1-β2), 90°-polarization rotation is achieved, and, thus, output becomes TM mode.
As shown in Fig. 2, a single trench in a waveguide core provides the asymmetry of waveguide. The degree of asymmetry can be controlled by adjusting the position, width and depth of the trench. The waveguide consists of a InGaAsP core (λg=1.42 µm, nc=3.45) grown on a (100) InP substrate (ns=3.17). We choose this composition of GaInAsP by considering the application to the polarization-independent interferometric optical isolator, in which the larger refractive index, hence longer λg, of GaInAsP waveguide core contributes to reduce the required length of nonreciprocal phase shifter. Since the GaInAsP bandgap wavelength of 1.42 µm is shorter by 0.13 µm than the operational wavelength of 1.55 µm, the measured loss of GaInAsP rib waveguide was <0.5 dB/mm at a wavelength of 1.55 µm.
where n(x, y) is a cross-sectional refractive-index distribution. The electric field components Ex and Ey are those of specified eigen mode in the x and the y direction, respectively. If R<1, the mode is mainly polarized along a vertical axis. If R>1, it is horizontally polarized. R=1 indicates that the optical axis of specified mode is rotated by 45° with respect to the x- and the y-axis. Therefore, it is required to make R=1 for achieving 100% mode conversion. We used a simulator based on the Finite-Element Method (FEM) to calculate the field of eigen mode in a single-trench waveguide for evaluating the parameter R.
Figure 3 plots the variation of R parameter as a function of the waveguide width in a 0.6µm-thick waveguide with a single trench of t=0.1 µm and D=0.4 µm. The wavelength is 1.55 µm. It can be concluded that a width W=1.02 µm yields a 45° rotation of the optical axis. Consequently, the 0.6-µm-thick and 1.02-µm-wide waveguide with a 0.1-µm-wide and 0.4-µm-deep trench located at t=0.1 µm is expected to provide a 90° polarization rotation in a half beat length (Lπ).
Variations in device parameters, such as the width of waveguide and the depth of trench, resulted from fabrication errors affect the conversion efficiency of mode converter. The conversion efficiency (PC) is given by
, where φ is the rotated angle of optical axis, L is the length of mode converter section, and Lπ is the half-beat length . Using the Eq. (2), we calculated the conversion efficiency for various trench positions t as a function of waveguide width as shown in Fig. 4(a), where h=0.6 µm, D=0.4 µm, and the trench section is 210-µm long. The operating wavelength is 1.55 µm. The calculated results show that the conversion efficiency decreases by 20% for a deviation of ± 0.01 µm from the optimal waveguide widths Wopt. The dependence of conversion efficiency on the trench position is only slight. The tolerance against width variation can be improved by using a wider and shallower trench. In order to fabricate a fabrication-tolerant structure, we have to use other layered structure than a single GaInAsP guiding layer on an InP substrate. The detail of this will be reported elsewhere.
Figure 4(b) shows the variation of conversion efficiency as a function of trench depth. In this calculation, the other parameters are kept constant at W=1.02 µm, h=0.6 µm, t=0.1 µm, and the trench section is 210-µm long. The operating wavelength is also 1.55 µm. It can be observed that the conversion efficiency decreases by 20% for a deviation of ±0.02 µm from the optimal trench depth (Dopt=0.4 µm). The wavelength dependence of conversion efficiency is shown in Fig. 5. In a wavelength range of 1.5–1.58 µm, the conversion efficiency decreases by 6% at 1.5 µm compared with the maximum value at 1.55 µm. Also, the Eq. (2) indicates that the conversion efficiency shows a sinusoidal dependence on the length of trench section. The efficiency decreases by 0.6% for a ± 10 µm deviation of length around the optimum condition.
In order to apply this mode converter to the polarization independent optical isolator , the reflection at the interface between the symmetric waveguide and the trench section should be minimized. We calculated the reflection at the interface (Fig. 1(b)) by using the Finite Difference Time Domain (FDTD) method. The input, transmitted and reflected power are defined as the real pointing vector flux including both TE and TM modes. The calculated transmittance and reflection are -0.09 dB and -23 dB, respectively, for the converter with W=1.02 µm, h=0.6 µm, D=0.4 µm, and t=0.1 µm.
3. Fabrication and measurement
The single trench mode converter consisting of a 0.6-µm-thick InGaAsP (λg=1.42 µm) waveguide core was fabricated on a (100) InP substrate using a single masking and etching process to produce a 0.4-µm-deep and 0.1-µm-wide trench. The RIE LAG enables one to realize such a structure in a single masking and etching process, because an etching rate is reduced in the trench fabricated with a narrow mask opening due to the reduction of etching reaction caused by the reduced supply of etching species. The waveguide pattern with a single trench was drawn by an e-beam lithography. The etching was done in CH4/H2 RIE. We found the appropriate etching condition so that a fully open area and a 0.1-µm-wide mask opening can provide a 0.6-µm-thick waveguide core with a 0.4-µm-deep trench in a single etching process.
The measurement is performed in a polarization maintaining transmission set up as shown in Fig. 6. The launched mode is propagated into the sample waveguide from laser diode (LD) operating at 1.55 µm followed by a fiber polarizer (POL) and a polarization maintaining fiber (PMF). The output light transmitted through the sample is imaged through a lens onto an infrared (IR) camera or a detector. The IR camera is used for observing an output near field pattern. The power of specified output polarization, i.e., TE- or TM-polarized light, was measured by an optical power meter through a polarizer placed between the lens and the detector.
We launched TE mode into waveguides. First, we checked that the conversion did not occur in a waveguide in which no trench was fabricated. The output light was TE-polarized. The variation of conversion efficiency in the single trench mode converter was measured as a function of waveguide width. The results are shown in Fig. 7. The maximum TE to TM mode conversion efficiency was measured to be 95% for a designed waveguide width Wopt=1.02 µm at a wavelength of 1.55µm, where h=0.6 µm, D=0.4 µm, t=0.1 µm, and the trench section was 210-µm long. As shown in Fig. 2, the cross section of fabricated waveguide is trapezoidal, where the upper part of waveguide is narrower by 60 nm than the lower part. The effective waveguide width, which is defined by the mean value of upper and lower waveguide width, is slightly smaller than the designed optimal width of Wopt=1.02 µm. Although there is a discrepancy between the theoretically optimal width and the effective width of fabricated device, the conversion efficiency decreases by 20% for a deviation of ±10 nm from the optimal waveguide width that gives the maximum conversion.
The conversion efficiency of mode converter is measured, when the wavelength varies from 1.5 µm to 1.58 µm. Figure 8 shows the measured and calculated conversion efficiency as a function of wavelength in a 210-µm-long mode converter. It can be observed that the conversion efficiency changes from 85% to 96% in the measured wavelength range. Although the measured efficiency is slightly less than the calculated one, the converter exhibits rather low wavelength dependence. The excess losses of single-trench type mode converter are measured to be 1.38 dB and 0.98 dB for TE and TM mode, respectively, at a wavelength of 1.5 µm compared with a 1-µm-wide and 0.6-µm-thick waveguide equipped with no trench.
Since a semiconductor waveguide optical isolator  can be designed with the fabricated waveguide geometry, the single-trench waveguide mode converter investigated in this paper can be applied to a polarization-independent semiconductor waveguide isolator with a single masking and etching process .
The design of single-trench waveguide TE-TM mode converter was discussed in a GaInAsP/InP waveguide. The waveguide mode converter was designed to obtain 90°-rotation of polarization. We also investigated the dependence of conversion efficiency on the waveguide width, trench depth, and trench position. Also, its wavelength dependence was calculated in a wavelength range between 1.5 µm to 1.58 µm. The calculated results showed that the designed converter exhibited slight dependence of the conversion efficiency on the variation of wavelength and trench position. We obtained a TE-TM mode conversion efficiency of 95% at a wavelength of 1.55 µm in a fabricated 210-µm-long single-trench waveguide mode converter. The wavelength dependence of conversion efficiency was measured in a fabricated device. The device exhibited high conversion efficiency in a wide operating bandwidth (1.5–1.58 µm). This type of mode converter can be applied, in principle, to a variety of waveguides composed of other material than GaInAsP.
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