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We demonstrate a tunable hollow waveguide distributed Bragg reflector consisting of a grating loaded slab hollow waveguide with a variable air-core. The modeling shows that a change in an air-core thickness enables a large shift of several tens of nanometers in Bragg wavelength due to a change of several percents in a propagation constant. We fabricated a slab hollow waveguide Bragg reflector with 620 µm long and, 190 nm deep 1st-order circular grating composed of SiO2, exhibiting strong Bragg reflection at 1558 nm with an air-core thickness of 10 µm for TM mode. The peak reflectivity is 65 % including fiber coupling losses, the 3 dB bandwidth is 2.8 nm and the grating-induced loss is less than 0.5 dB. We demonstrate a 3 nm wavelength tuning of the fabricated hollow waveguide Bragg reflector by changing an air-core thickness from 10 µm to 7.9 µm.
©2004 Optical Society of America
1. Introduction
Various tunable optical devices such as tunable filters, tunable lasers and tunable dispersion compensators are becoming important for future photonic networks with optical cross-connect and optical routing functions. In particular, Bragg grating structures have been widely used in tunable optical devices. So far, waveguide type Bragg reflectors based on dielectric or polymer waveguides have been reported. A thermo-optic effect has been used for tuning operations. Polymer materials show a large thermo-optic coefficient, which is about 20 times larger than that of SiO2 and thus wide wavelength tuning has been realized [1]. However, tunable devices utilizing a thermo-optic effect strongly depend on environment temperatures, thus those need a costly precise temperature controller. In addition, a tuning range is not large enough to meet demands for future DWDM networks. To solve these problems, we proposed a tunable hollow optical waveguide with a variable air-core [2]. Air-core hollow waveguides have some unique features such as temperature insensitive characteristics and large tunability in propagation constant using a variable air-core. We predicted a large tunability of a few percent in propagation constant [3].
We also proposed a hollow waveguide Bragg reflector with a wide tunability of Bragg wavelength [4]. We predict a large continuous change of several tens of nanometers in Bragg wavelength with a change, of a few microns, in an air-core thickness.
In this paper, we fabricate a tunable distributed Bragg reflector composed of a slab hollow waveguide with a 1st-order circular grating for the first time. We demonstrate a 3 nm wavelength tuning by changing an air-core thickness from 10 µm to 7.9 µm.
2. Structure of hollow waveguide Bragg reflector
Figure 1 shows the schematic structure of our proposed tunable hollow waveguide Bragg reflector. Two GaAs/AlAs DBRs are used to confine light in the air-core of a slab hollow waveguide. A 1st-order circular diffraction grating was formed on the DBR surface as shown in Fig. 1, which gives us reflection and focusing at an input port.
The dynamic change of an air-core thickness induces a large change in a propagation constant of guided modes. We carried out 2-dimensional full-vectorial simulation by using a simulator (FIMMWAVE, provided by Photon Design Company), which is based on a film-mode-matching method. The numerical simulation shows a large tunability of Bragg wavelength over several tens of nanometers in hollow waveguide Bragg reflectors as shown in Fig. 2.
Fig. 1. Schematic structure of tunable hollow waveguide distributed Bragg reflector with variable air-core.
Fig. 2. Calculated results of reflection spectra with different air-core for TE mode. The period, depth and length of grating are 780 nm, 250 nm and 7.8 mm, respectively.
In this calculation, we assume 19 pair GaAs/AlAs multilayer stack with refractive indices of GaAs and AlAs to be 3.4 and 2.9, respectively. Each layer is designed to be a quarter wavelength for oblique incident angles. A grating layer consists of SiO2 with a refractive index of 1.47. This structure gives us a high reflectivity even by using thin gratings because of the large contrast in refractive indices between the grating layer (SiO2) and an air-core.
3. Fabrication
We fabricated a 1st-order circular diffraction grating on the DBR surface. The 1st-order circular grating was formed on a GaAs/AlAs multilayer mirror by using sputtering of SiO2 followed by electron beam lithography and RIE dry etching. The SEM view and the AFM view of a fabricated 1st-order circular grating are shown in Fig. 3. The fabricated grating depth, pitch and length are 190 nm, 780 nm and 620 µm, respectively. The grating loaded DBR wafer and another same DBR wafer are faced each other. The air gap between the two DBR wafers is precisely controlled by a PZT actuator. The length of an input slab waveguide and the entire device are 100 µm and 3 mm, respectively.
Fig. 3. (a) SEM photographs and (b) AFM view of fabricated hollow waveguide Bragg reflector with 1st-order circular grating.
4. Characterization of hollow waveguide Bragg reflector
In order to measure Bragg reflection spectra of this device, we used the measurement setup composed of a broadband light source around 1550 nm, a circulator, a polarization controller and an optical spectrum analyzer. The input port of the device was directly coupled with a single mode fiber. The measured reflection spectra of the device with an air-core thickness of 10 µm are shown in Fig. 4. We observed strong Bragg reflection around 1558 nm for TM mode. The peak reflectivity is estimated to be 65 %, which includes a fiber coupling losses, and the 3 dB bandwidth is 2.8 nm. We confirmed that the grating-induced excess loss in transmission is less than 0.5 dB from the comparison of the insertion loss of a hollow waveguide with and without grating at a wavelength of 1600 nm. Noticeable sidelobes caused by the deep modulation of our grating can be seen, which would be suppressed using an apodization technique [1].
And then, reflection spectra show large polarization dependence because of the large polarization dependence of intensity distributions in an air-core as shown in Fig. 5. The grating depth of 190 nm is not sufficient to obtain Bragg reflection for TE mode because of a strong confinement of TE mode in the air-core. On the other hand, the intensity distribution for TM mode penetrates through the DBR region in comparison with the case of TE mode. Thus strong Bragg reflection generates for only TM mode because the overlap between the grating and the intensity distribution of a fundamental guided mode is larger. We will be able to reduce the large polarization dependence of the Bragg reflector shown in Fig. 4 by using hollow waveguides with a phase control layer [5].
Fig. 5. Calculated intensity distribution of the hollow waveguide with an air-core thickness of 10 µm.
We measured the Bragg reflection spectra for various air-core thicknesses as shown in Fig. 6. The Bragg wavelength shifts to a shorter wavelength side in proportion to an air-core thickness. We clearly observed the blue shift of 3 nm in Bragg wavelength while changing the air-core thickness from 10 µm to 7.9 µm for TM mode.
The measured reflection spectra show the split of the main reflection peak with decreasing an air-core thickness. This would be caused by the resonance between the Bragg reflection and the facet reflection of a SMF. The evaluation of wavelength tuning was carried out by taking this fact into account. The bandwidth of the reflection spectra increases as the air-core thickness decreases. This is because the overlap between a grating and a guided mode field increases as decreasing an air-core thickness, resulting in strong coupling of the grating.
Although the chromatic dispersion in an air-core waveguide is large, especially when the air-core thickness is comparable to wavelength, the dispersion in our hollow waveguide with a several micron air-core is estimated to be less than 0.01 psec/nm/cm [6]. Thus, the effect of the waveguide dispersion can be negligibly small in the proposed Bragg reflector.
Calculated tuning characteristics for TE and TM modes versus an air-core thickness are shown with experimental results in Fig. 7.
As shown in the figure, the tunability for TE mode is much larger than that of TM mode because TM mode is insensitive to the change of an air-core thickness due to the weak confinement in an air-core, which is shown in Fig. 5.
The measured Bragg wavelength is in an agreement with the value calculated from the following equation,
where Λ, neff and λBragg is the circular grating pitch, the effective refractive index of a fundamental guided mode calculated from the Maxwell’s equation solver and Bragg wavelength, respectively. The result indicates the potential tunable function of our hollow waveguide Bragg reflector. Large wavelength tuning can be expected in a narrow air-core. However, the strong coupling constant of the grating for a TM mode makes the observation difficult due to the distortion of reflection spectra. We expect much larger tuning for a TE mode as shown in Fig. 7.
Fig. 7. Calculation and experiment results on tunability of Bragg wavelength for TE and TM modes with variable air-core.
5. Conclusion
We successfully demonstrated a hollow waveguide distributed Bragg reflector with 1st-order circular grating for the first time, which shows a clear Bragg reflection at 1558 nm with an air-core thickness of 10 µm for TM mode. The peak reflectivity is 65 % including fiber coupling losses, the 3 dB bandwidth is 2.8 nm and the grating-induced loss is less than 0.5 dB. Using this hollow waveguide Bragg reflector, we achieved 3 nm wavelength tuning by changing the air-core thickness from 10 µm to 7.9 µm. We can expect much wider tunability of Bragg wavelength by optimizing grating parameters. The proposed concept may open up a new class of various tunable optical devices including tunable filters, lasers and dispersion compensators, which gives us unique features of wide tunability, compact size and temperature insensitivity.
Acknowledgments
The authors acknowledge Professor Emeritus Kenichi Iga of Tokyo Institute of Technology for his encouragement. This work was supported by the Grant-in-Aid for Creative Scientific Research from the Ministry of Education, Science, Sport and Culture (#14GS0212”).
References and links
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2. T. Miura, F. Koyama, and A. Matsutani, “Modeling and Fabrication of Hollow Optical Waveguide for Photonic Integrated Circuits,” Jpn. J. Appl. Phys. 41, 4785–4789 (2002). [CrossRef]
3. T. Miura and F. Koyama, “Novel Phase-Tunable Three-Dimensional Hollow Waveguides with Variable Air Core,” IEEE Photon. Technol. Lett. 15, 1240–1242 (2003). [CrossRef]
4. Y. Sakurai and F. Koyama, “Proposal of Tunable Hollow Waveguide Distributed Bragg Reflectors,” Jpn. J. Appl. Phys. 43, L631–L633 (2004). [CrossRef]
5. T. Miura and F. Koyama, “Low-Loss and Polarization-Insensitive Semiconductor Hollow Waveguide with GaAs/AlAs Multi-Layer Mirrors,” Jpn. J. Appl. Phys. 43, L21–L23 (2004). [CrossRef]
6. Y. Sakurai and F. Koyama, “Control of Group Delay and Chromatic Dispersion in Tunable Hollow Waveguide with Highly Reflective Mirrors,” submitted to Jpn. J. Appl. Phys.43 (2004).
