Integration of different-period distributed Bragg reflectors (DBRs) is required in constructing an intra-board optical interconnection device with wavelength division multiplexing (WDM). Interference exposure method with cylindrical Lloyd mirror optics and mask aligner was discussed for integrating those DBRs simultaneously. DBRs were designed to give coupling efficiency higher than 80 % with coupling length of 0.6 mm and crosstalk noise less than -10 dB with 3nm wavelength separation for optical interconnection using eight wavelengths around 850nm. Interference exposure system was developed and integration of eight DBRs by two times exposure was demonstrated.
© 2006 Optical Society of America
Intra-board chip-to-chip optical interconnection is expected to be a key technology for constructing a future ultrahigh-performance information-processing unit . Two-dimensional (2-D) parallel transmission of optical signals will be a solution for bandwidth higher than terabits per second, since single-channel bandwidth is limited by direct modulation speed of light source, that is vertical-cavity surface-emitting laser (VCSEL), to ~10 Gbits/s. A topic is how to connect the optical signals from a 2-D array of VCSELs connected to a transmitter chip to a 2-D array of photodiodes (PDs) connected to a receiver chip. Most of configurations proposed and demonstrated so far utilize imaging systems with micro-optic or diffractive optic components [2–8]. As another approach, an integrated optic configuration has been proposed and investigated [9–14]. The integrated optic configuration uses a thin-film waveguide and has advantages of alignment stability, reduction of weight and size, fabrication by planar processes, and compatibility with a current printed circuit board.
An issue is coupling of free-space waves, which are radiated from VCSELs or focused to PDs, to or from guided waves in the thin-film waveguide. Another issue is 2-D optical transmission in the waveguide. To settle both issues, we have investigated utilization of wavelength-division-multiplexing (WDM) technique with free-space-wave add-drop multiplexers. Schematic view of the intra-board chip-to-chip WDM optical interconnection is depicted in Fig. 1. Each electronic chip with an optoelectronic interposer integrating a 2-D array of VCSELs and a 2-D array of PDs is surface-mounted on a thin-film optical waveguide. VCSELs of different wavelengths around 850 nm are integrated in line along the guided-wave propagation direction to transmit WDM optical signals. The add-drop multiplexer consists of a focusing grating coupler (FGC) and a distributed Bragg reflector (DBR). Prototype integrated optic interconnection device was designed and fabricated, and signal transmission of sub-Gbits/s per channel has already been demonstrated .
One of the next issues is the development of high-throughput fabrication process applicable for future mass production. Both FGCs and DBRs in the prototype device were fabricated by electron-beam (EB) direct writing technique. EB direct writing is time consuming and not suitable for high-throughput fabrication. FGC has curved and chirped grating line pattern but can be fabricated by photolithography process provided the lithography mask is made by EB direct writing technique, indicating no problem in future mass production. On the other hand, the photolithography is not suitable for the fabrication of DBRs, since the grating periods are to be precisely adjusted for compensating wavelength error of VCSELs to be integrated and guided-mode-index error introduced in previous fabrication processes. Interference exposure is an alternative technique capable of easy period adjustment. An interference exposure system was developed, and the required adjustability of the interference fringe periods has been demonstrated . However, integration of different-period DBRs by using uniform period interference fringes requires a series of exposures with precisely controlled period difference. Simultaneous interference exposure of different-period DBRs would provide significant improvement in the integration throughput.
DBRs are integrated in the SiO2 based glass waveguide and couples TE0 and TE1 guided waves contradirectionally. Grating periods are around 290 nm with the period pitch of 1 nm when WDM wavelengths are around 850 nm with the wavelength pitch of 3 nm . The different-period DBRs are located along guided-wave propagation direction (z direction). We can expose the different-period gratings at the same time by using two-beam interference exposure in which the incidence angle of one beam is properly varied along z.
In this report, we discuss the application of a cylindrical mirror and a metal mask to a prototype interference exposure system. The cylindrical mirror is used to generate the interference fringes of a linear chirp. The metal mask having slit windows is used to limit the exposure area to segmented DBRs of chirp gratings.
2. Design consideration
Developed interference exposure system is illustrated in Fig. 2. The system consists of a UV laser of wavelength 244 nm, a beam expander, a deflection mirror attached to a rotation stage, and a cylindrical mirror attached to a mask aligner. UV laser beam is expanded and collimated, and then deflected downward by the deflection mirror of which angle is precisely controlled. A half of the deflected beam directly illuminates a photoresist-coated waveguide sample through a metal mask having several slit windows, while the other half illuminates the sample after being reflected by the cylindrical mirror. Period of the interference fringes is not constant but chirped. Several chirped DBRs of different periods are integrated at the same time by the exposure through the slit windows.
A chirp rate α of the interference fringes should be discussed with the resultant DBR characteristics. The rate α in this report is defined with the fringe period ʌ(z) by
Coupled mode equations by DBR having a can be expressed by
where A(z) and B(z) represent the fields of the incident and the reflected guided modes, respectively, and κ is coupling coefficient. Phase mismatching factor Δ for wavelength λ was given by
λc denotes the center of coupling wavelength and N 0 and N 1 are mode indices of TE0 and TE1 guided modes, respectively.
Calculated coupling efficiencies of three DBRs are depicted in Fig. 3 for a case that α = 21.0 mm-2 and κ= 5.0 mm-1. Coupling length L is 0.6 mm, and three λc are 845 nm, 848 nm, and 851 nm. The maximum efficiency is expected to be higher than 90 %. Coupling efficiency higher than 80% is predicted for 1 nm wavelength range. The cross talk noise depends on a difference in λc between neighboring wavelength channels. The noise is predicted to be lower than -10 dB in the same wavelength range for 3 nm difference in λc. For giving such difference in Ac, difference in grating periods of DBRs is to be 1 nm when N 0 + N 1 = 2.934.
Two examples of interference fringe periods with α= 21 mm-2 are plotted by two lines against z position in Fig. 4. Both lines are partially solid and partially broken. By masking broken parts by the metal mask, we can expose only solid parts on each line. The mask having four slits of 0.6 mm width with 1.2 mm separation serves for exposing four chirped DBRs of L = 0.6 mm with the period pitch of 1 nm simultaneously. Two times exposure provides eight chirped DBRs within 7 mm length when the sample is displaced against the mask by 0.9 mm and ʌ(0) is properly adjusted by the deflection mirror rotation. In the case of Fig. 4, ʌ(0) for the two lines are 290.17 nm and 286.66 nm.
Longer L or smaller α results in higher efficiency but the number of DBRs becomes smaller for the limited integration area. Higher κ gives not only higher coupling efficiency but also higher cross talk.
The period of the interference fringes ʌex(z) can be written as
where λ ex is the wavelength of the UV laser and 244 nm in this case, and the incidence angles ϕ and γare illustrated in Fig. 2. γvaries along z. Tangent of cylindrical mirror surface is gradually tilted. The surface tilt angle δ at height h measured from the sample surface for the cylindrical mirror of curvature radius R is given by
R was determined to be 2.3 m for obtaining α = 21.0 mm-2 with ϕ= 24.9°.
3. Experimental results and discussions
Eight chirped gratings were integrated by two times exposure on a Ge:SiO2/SiO2 glass waveguide on a high reflection layer which was designed and used for the intra-board optical interconnection under investigation [10–14]. A photoresist KRFM151Y provided by JSR Corporation was coated on top of the waveguide. Thickness of the photoresist was 0.14 μm. Interference exposure was done by the developed system. The used metal mask had four slit windows of 0.6 mm width with 1.2 mm spacing. Thickness of the metal mask was 50 μm. Window-edge walls were taper-etched to avoid their shadows on the waveguide. The waveguide sample was aligned and contacted to the metal mask with alignment precision of a few microns. Precision of the deflection mirror rotation was 2.5×10-4 rad/div, meaning that precision in the control of the fringe period was 0.015 nm/div . Dose was 240 mJ/cm2 at the brightest part in the interference fringe.
Photograph of the diffracted lights from the fabricated DBRs is shown in Fig. 5. Since the period was shorter than 290 nm, the diffraction angle was very large even for blue light. When the photograph was taken, the fabricated sample surface did not face toward camera but largely tilted. As a result, only a part was just in focus. Size of each DBR was 0.6 mm × 2 mm.
Average periods of the fabricated DBRs were measured from the diffraction angle in the Littrow mounting. Measured example was summarized in Table 1. A set from no. 1 to no. 4 was simultaneously exposed. Another exposure was done for the other set from no. 5 to no. 8. We confirmed integration of different-period DBRs with acceptable period accuracy by the developed interference exposure system. However, the period difference was not the same as predicted value of 1.0 nm but slightly wider by ~10 %. We think the main cause of this discrepancy is distortion of the cylindrical mirror. The mirror is made of dielectric glass with high reflection coating, and its rear face was contacted and fixed to a metal block by an adhesive at high temperature. Difference in coefficient of thermal expansion between the mirror and the metal block may bend the mirror surface. The bend of a-few-micron displacement at both edges of the cylindrical mirror reduces the curvature radius by 10 %, resulting in the increase of α by 10 %.
Coupling wavelengths of four DBRs of the longer period set were also measured. Measurement setup is the same as described in Ref. . A laser beam from a wavelength tunable laser diode was coupled into the waveguide. A guided wave reflected by DBRs was coupled out and detected when the wavelength was varied. Obtained coupling wavelengths were 851.2 nm, 848.0 nm, 844.6 nm and 841.2 nm. They show good agreement to the predicted values when the above-mentioned increase of α is taken into account.
Integration of different-period DBRs for intra-board optical interconnection was discussed with an interference exposure method using a cylindrical mirror. The coupling efficiency higher than 80 % and the cross talk noise lower than -10 dB are expected for DBRs of 0.6 mm coupling length and chirp rate of 21 mm-2. Simultaneous exposure of four DBRs was done with a metal mask, and an integration of eight DBRs by two times exposure was demonstrated. Experimental results on the grating periods and the coupling wavelengths showed good agreement with the designed values, indicating usefulness and application possibility of this technique for high throughput production of WDM optical interconnection board.
This research work was financially supported in part by International Communications Foundation in Japan, and in part by a Grant-in-Aid for Scientific Research (A) No. 15206008 of Japan Society for the Promotion of Science.
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