1. Introduction

Wavelength division multiplexing is an integral part of modern optical communication systems. For greater transmission capacity, past studies have emphasized components for dense wavelength division multiplexing (DWDM). More recently, coarse wavelength division multiplexing (CWDM), in which the spacing between individual wavelengths is larger (20nm as specified in the International Telecommunications Standard G.694.2), has been examined because it can be implemented with less expensive components [1]. Broadband multiplexing components include: dielectric interference filters [2], concave diffraction gratings [3], polymer arrayed waveguide gratings (AWG’s) and Mach-Zehnder interferometers [4], and silica AWG’s [5]. Planar devices based on III–V semiconductors are also highly desirable because they can be monolithically integrated with active components such as lasers. While there has been a significant amount of work on III–V semiconductor AWG’s for DWDM [6], research on planar III–V semiconductor devices with broadband operation and larger channel spacing suitable for CWDM has been very limited. A two-channel InP-based AWG for the multiplexing of 1.31 µm and 1.55 µm wavelengths [7] and an InP-based AWG with 3.2 nm channel spacing have been reported [8]; however, the channel spacing for these devices is not suitable for broadband operation desired for CWDM standards.

In this letter, we describe a 4-channel AWG on InP with channel spacing as large as 20 nm. To our knowledge, this is the first report of a III–V semiconductor AWG which can have a broadband operation compatible with CWDM. This design also provided a polarization and thermal insensitive operation. The AWG presented in this study is not a trivial extension of a DWDM AWG - there were significant problems to overcome. It was conjectured that chromatic dispersion could significantly degrade the performance of the AWG due to the broadband operation. We found, however, that with proper design, the effects of chromatic dispersion could be minimized. Also, a standard horseshoe shape AWG, as depicted in Fig. 1, will not work for broadband. Instead we used an “S” shape design [5] which has never been implemented in semiconductor.

 

Fig. 1. A standard horseshoe design and an S-shape design AWG. Standard horseshoe design is not suitable for broadband operation, instead an S-shape AWG was designed to achieve broadband operation.

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2. Design and Fabrication

An illustration of the cross section of the ridge waveguides used for the AWG is shown in Fig. 2. In_0.85Ga_0.15As_0.33P_0.67 was chosen for the 0.3 µm thick, high-index waveguide core layer that was sandwiched between InP. The calculated refractive indices for the core and cladding material at 1.5 µm are 3.293 and 3.170 respectively [9]. Photoluminescence for the InGaAsP layer was at a wavelength of 1.1 µm, indicating that the semiconductor had an absorption edge near this wavelength and establishing a lower bound for the range of operating wavelengths for the device. All epi-layers were grown by metal-organic chemical vapor deposition on InP substrates. Ridges were formed by etching to 0.11 µm of the InGaAsP core layer. Stopping the etch before it reached the core minimized contact between the optical mode (also pictured in Fig. 2) and the surface of the structure, and minimized optical scattering loss at the surface. Etching to within 0.11 µm of the core ensured good lateral confinement of the optical mode and minimized bend loss. Simulated bend loss [10] is shown in Fig. 3 for a ridge waveguide of width 2.5 µm, which calculations indicate will support two guided modes. Simulations indicate that bend loss for the fundamental mode is negligible when the radius of curvature is greater than 900 µm. Based on this result, the AWG was designed so that no segments had radius of curvature less than 1000 µm. Actual waveguide width was 2.5 µm and it might be conjectured that the presence of a higher order mode would degrade device performance by causing ghost images. However, as indicated in Fig. 3, the higher order mode was expected to experience much higher loss. Furthermore, no adverse effects on device performance were observed that could be attributed to the presence of a higher order guided mode. A 300 nm SiO₂ layer was deposited on top of the semiconductor epi-layers by using e-beam evaporation for an etch mask. The SiO₂ mask layer was patterned using a 1.5 µm thick spin-coated layer of AZ 1813 photoresist. The SiO₂ surface was treated in an oxygen plasma for three minutes before spin coating, to promote adhesion. The photoresist was soft baked on a hot plate at 110 °C for two minutes immediately after spin coating. The photoresist was exposed through a quartz-titanium mask for 10 seconds to ultraviolet light with an irradiance of 276 mW/cm², developed in AZ 352 developer, and then post baked for five minutes on a hot plate at 110 °C. A buffered oxide etch was used to etch the SiO₂. An Oxford Instruments Plasma Lab 100 reactive ion etching system (RIE), equipped with an electron cyclotron resonance unit (ECR), was used to etch semiconductor epi-layers. Before etching, the RIE chamber was cleaned for two hours with an oxygen plasma and pre-treated for 20 minutes with an H₂/CH₄ plasma. Epi-layers were etched with a mixture of hydrogen (20 sccm) and methane (10 sccm). Chamber pressure was 60 mbar, RF power was 150 W, microwave forward power to the ECR was 500 W, and the current in ECR coil was 16.4 amps. The etch rate for InP was 0.7 nm/s. SiO2 was removed from patterned epi-layers with a buffered oxide etch. Ridge waveguides had lateral width about 1 µm less than the stripes on the mask that were used to define them, which is a result that we attribute to isotropic etching of SiO₂ during the patterning of the SiO₂ mask layer. The measured value for propagation loss for optical waveguides was 4.5 dB/cm. Figure 4 shows the layout of an arrayed waveguide grating for coarse wavelength division multiplexing of optical signals at 1470 nm, 1490 nm, 1510 nm, and 1530 nm. An “S-shaped” design was implemented in order to achieve broadband operation. An S-shape allows waveguides in the array to have arbitrarily small optical path difference, which gives the grating an arbitrarily large free spectral range. A physical path difference of 10.31 µm between adjacent waveguides was generated in the center arc portion of the AWG to give a free spectral range of approximately 70 nm. This value for the path difference meant that the AWG operated in its 22nd order. This relatively small path difference accounts for insensitivity to chromatic dispersion; changes in refractive indices due to broadband operation led to only small changes in optical path difference. There were nine waveguides in the array. A single waveguide at the input was coupled to the AWG array with a Rowland-type star coupler with radius of curvature of 136 µm at the input face of the coupler and 272 µm at the output face. The center points of the coupler faces were separated by 272 µm. The waveguide array was coupled to four output waveguides with an inverted, identical star coupler. Center-to-center spacing for all waveguides was 5 µm at the star couplers. Waveguides were separated by 100 µm at the output of the device. Calculations indicated that the difference in the spectral response of the AWG for TE and TM polarization would be a shift of just 1.1 nm, a value that was found to be small compared with the measured spectral width for an output channel. The overall size of the AWG was about 0.8 mm by 4 mm.

 

Fig. 2. Composition and dimernsions of ridge waveguides for the arrayed waveguide grating. Color contours indicate the simulated profile for the fundamental guided mode.

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Fig. 3. Simulated bend loss for the fundamental and second order mode of the ridge waveguide pictured in Fig. 2. Based on these results, AWG was designed so that no segments had a radius of curvature less than 1000 microns.

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Fig. 4. The layout of a four channel, S-shaped arrayed waveguide grating for broadband operation suitable for coarse wavelength division multiplexing. Optical path difference is generated in central arc of the waveguide array.

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Fig. 5. SEM pictures of the star coupler with some of the array waveguides and an individual waveguide in the arrayed waveguide grating, fabricated on InP.

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3. Characterization and results

Optical setup shown in Fig. 6 was used for device characterization. Light from a tunable laser was coupled into the AWG with an end tapered single-mode fiber. Light from individual output waveguides was focused onto a germanium detector using an infrared objective. The spectral response for the four channel AWG, with portions of three different grating orders, is displayed in Fig. 7. Within a diffraction order, each peak (wavelength channel) represents the output of an individual waveguide as the laser was scanned over its full range from 1460 nm to 1580 nm. The full width half maximum (FWHM) of the channels was approximately 11 nm. Crosstalk between wavelength channels was no larger than -16 dB. For a fixed input power, the variation in the output of waveguides at the center wavelength for each channel was no greater than approximately 2 dB. The total estimated on-chip loss was less than 8 dB, which includes the propagation, bend and waveguide to star coupler coupling losses. No extra peaks were observed; and we believe that high bend loss for higher order modes prevented ghost image effects from a higher order mode. Birefringence measurements, as shown in Fig. 8, revealed a TE-TM shift of approximately 1.2 nm in the output spectra which agreed with calculations mentioned above. This resulted in a polarization dependent loss value of approximately 0.6 dB. We have also tested thermal stability of the device by heating it over a wide temperature range, and the results are also shown in Fig. 8. It is observed that the output spectra shifts by approximately 3.5 nm over a temperature range of 60 °C, and the measured loss for this shift is less than 4 dB. These results show that the shifts due to birefringence and temperature fluctuations is small compared to the wide spectral peaks of the output spectra (~11nm), resulting in a device which is insensitive to polarization and temperature.

 

Fig. 6. Optical setup used for characterization of the broadband arrayed waveguide grating.

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Fig. 7. Spectral response for the arrayed waveguide grating with portions of three different grating orders displayed. Within a grating order, each peak is the output from a single waveguide as the laser is tuned.

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Fig. 8. Polarization and Temperature Insensitivity: Birefringence between TE and TM mode shifts the AWG spectrum by only 1nm. Increasing the AWG temperature from 25 °C to 85 °C shifted the spectrum by 3.5 nm. Only one peak is shown for clarity.

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4. Conclusions

We have described what we believe to be the first demonstration of an s-shaped arrayed waveguide grating fabricated on III–V semiconductors operating at wavelengths compatible with the coarse wavelength division multiplexing standards. The wide spectral peaks results in a device which is thermally stable over a wide temperature range and polarization insensitive. The four channel design can be easily modified for operation with 8 or 16 wavelength channels and operation over any portion of the coarse wavelength division multiplexing range, which extends from 1270 nm to 1610 nm. Simulations suggest several modifications to improve device performance: a doubling of the number of waveguides in the central array to reduce cross talk by 20 dB and reduce the insertion loss, an increase in the length of star couplers to reduce channel non-uniformity, and use of tapered waveguides at the star couplers to further reduce insertion loss. Waveguide spacing at the output star coupler could also be adjusted to fine tune the spacing between wavelength channels and minimize loss if the standardized CWDM channels are desired.