Polarization-dependent Wavelength Shift Research Articles

A compact all-fiber PDL-compensated acoustooptic wavelength monitor

A compact all-fiber acoustooptic wavelength monitor with passive polarization-dependent loss (PDL) compensation is demonstrated. A Sagnac loop interferometer is used to compensate the intrinsic PDL, and hence, the polarization-dependent wavelength shift of the fiber acoustooptic tunable filter (AOTF). This increases the extrinsic wavelength accuracy of the device to 0.05 nm, a three-fold improvement from a previously reported uncompensated device. The relaxation in the PDL specifications for the AOTF also results in a reduction of fiber length from 38 to 14 cm.

Elimination of PD-λ at all output ports of silica-based AWG

It is shown that the polarisation dependent wavelength shift (PD-λ) of a silica-based arrayed waveguide grating (AWG) is proportional to its output port location as a result of the birefringence in a slab waveguide. This PD-λ was eliminated to less than 0.01 nm by inserting a half waveplate across the slab waveguide.

Modal birefringence and power density distribution in strained buried-core square waveguides

The optical properties of engineered devices derive from the flow of energy within the structure, a flow that is governed by the interaction between device architecture and material properties. With evolving device and system sophistication, implementation of manufacturing processes becomes an exercise in global optimization. This task is addressed in the design phase by choosing metrics that correlate with critical operating parameters of the device. We illustrate two complementary metrics, arising from the same physical effect, that manifest at different length scales. By considering the thermoelastic properties of a doped-glass multilayer with an embedded waveguide, we reveal the impact of stress on modal birefringence and optical power distributions. The modal birefringence of a single-mode buried-core square waveguide irreducibly describes a discrete metric whose value derives from global strain contributions. It is an ideal metric for addressing polarization-dependent wavelength shifts in arrayed waveguide grating devices, which are particularly sensitive to material stresses over long length scales. By contrast, the power density distribution in the bound mode is a distributed metric dependent on local properties and is thus more suitable for devices whose operability is dependent upon shorter length scales. These complementary characterizations of stress-optical coupling are numerically assessed by finite element analysis. The material stresses are first found through solution of the structural problem, with subsequent solution of the the full-vector anisotropic Maxwell's equations as an eigenvalue problem. We found the modal birefringence to be primarily governed by the properties of the cladding material, the core properties having a negligible effect, while fine control may be achieved by varying the position of the interface between the lower and upper cladding. This demonstrates its suitability as a metric for devices primarily reliant on isolated waveguides. At shorter length scales, through contrast with the isotropic case, we showed that stress-optical coupling suppresses the in-plane symmetry axis of the logarithmic difference in spatial power densities, regardless of the degeneracy of the optical mode, with asymptotic behavior that effectively diverges. This revealed a metric of potential applicability for interferometric and evanescently coupled structures.

Etched-Diffraction-Grating-Based Planar Waveguide Demultiplexer on Silicon-on-Insulator

An optical wavelength demultiplexer with the etched diffraction grating (EDG) on the silicon-on-insulator (SOI) material is demonstrated. Fabricated by the wet-anisotropic-etching method, 90° turning mirrors are used to bend waveguides, and the size of the EDG-based demultiplexer is minimized to only 16×2.5 mm2. The crosstalk is below −16 dB. The on-chip loss is about 9.97 dB, which is composed of about 8.72 dB excess loss and 1.25 dB diffraction loss. The polarization-dependent central wavelength shift is below 0.13 nm, and the polarization-dependent loss is about 0.35 dB. The sources of the crosstalk and loss are discussed in details, and the related measures to improve the performance are also presented.

Planar waveguide echelle gratings in silica-on-silicon

Silica planar waveguide echelle grating demultiplexers with 48 channels and 256 channels are described and demonstrated. Polarization effects due to stress birefringence and polarization-dependent grating efficiency have been eliminated using a modified polarization compensator and grating design. The devices have a polarization-dependent wavelength shift of less than 10 pm, and a polarization-dependent loss below 0.2 dB. The 48-channel device has a measured crosstalk of -35 dB, an insertion loss better than 4 dB, and a uniformity of 1 dB across the C-band.

Birefringence as a function of upper-cladding layers in polymeric arrayed waveguide gratings

We investigated on the variation of polarization-dependent wavelength shifts in birefringent-polymeric arrayed waveguide gratings (AWG) along the each step of forming a core layer and a multi-layered upper cladding in fabrication processes. We found that the polarization-dependent wavelength shift is a function of the number of sublayers with a positive offset value depending on the thickness of the first sublayer and a negative slope depending on the thickness of the additional sublayers, resulting in a polarization-insensitive AWG with proper values of the thickness and the number.

Reflective arrayed waveguide grating multiplexer

A high-performance reflective arrayed waveguide grating multiplexer/demultiplexer designed for compatibility with silicon integrated circuit processing is described. The grating, folded by a flat reflecting surface, can be printed in a single-exposure field of a photolithographic stepper. Advanced plasma assisted deposition is used to prepare waveguides with very low loss and minimum birefringence. Multiplexers with 40 channels separated by 100 GHz show intrinsic insertion losses of -2.4 dB, channel uniformity less than 0.6 dB, and very low polarization dependent wavelength shift of 0.012 nm.

Simple method to adjust polarization dependence in polymeric arrayed waveguide gratings

Polarization-dependent wavelength shift of a birefringent-polymeric arrayed waveguide grating (AWG) is found to be adjusted simply by controlling the thickness of the first upper cladding layer while forming the upper cladding by several sublayers. Polarization-dependent wavelength shift is reduced to 0.08 from 4.24 nm in a birefringent-polymeric 16 /spl times/ 16 channel AWG on a silicon substrate by this method.

Silicon-dioxide waveguides with low birefringence

We describe the use of highly boron-doped silicon dioxide for the preparation of optical waveguides with very low birefringence. Plasma-enhanced chemical vapor deposition was used to vary the boron content from 5 wt% to 10 wt%, at a constant phosphorus content of 4.8%. A transition from compressive to tensile stress was observed at a boron concentration of 9.1%. Pedestal-type waveguides formed with the high-boron top cladding layer show low loss of 0.02 dB/cm. Arrayed waveguide grating devices with a polarization-dependent wavelength shift of 0.01 nm and excellent stability have been demonstrated.

Performance characteristics of a WDM channel monitor based on an all-fiber AOTF with an on-fiber photodetector

We demonstrated an all-fiber wavelength-division-multiplexing channel monitor based on a narrow-band acoustooptic tunable filter (AOTF) with a built-in on-fiber photodetector. By employing a frequency-shift keying (FSK) modulation scheme, intrinsic wavelength accuracy of 0.02 nm is obtained for single-channel detection and 0.05 nm for multiple-channel detection. The minimum resolvable channel spacing is 1.0 nm. The measured channel power accuracy is <1 dB. The demonstrated channel monitor showed a polarization-dependent wavelength shift of /spl sim/0.15 nm, which is limited by the polarization mode splitting of the AOTF itself. The influence of FSK frequency spacing and the effect of channel crosstalk are also discussed.

Compact polymeric wavelength division multiplexer

A compact polymeric wavelength division multiplexer based on a slab waveguide and a reflective transmission grating is presented for application within the low cost data-communications market. The device channel spacing is 800 GHz (6.4 nm), measures only 4/spl times/4 mm in size, displays a polarisation-dependent wavelength shift of 0.1 nm and crosstalk levels approaching -25 dB.

Fused III-V vertical coupler filter with reducedpolarisation sensitivity

A novel fused InGaAsP/InP-AlGaAs/GaAs vertical coupler filter is demonstrated. By using the large material dispersion between InGaAsP and AlGaAs and similar waveguide geometries, a 3.6 nm bandwidth and 5 nm polarisation dependent wavelength shift using a 1.1 mm long fused vertical coupler has been achieved.

Polarization-insensitive wavelength filters by birefringence compensation of vertical couplers

We demonstrate a novel birefringence compensation technique to reduce the polarization dependence of AlGaAs directional coupler wavelength filters operating in the 770 to 970 nm wavelength range. By using a multiple quantum well structure to compensate intrinsic waveguide birefringence, we fabricated vertical coupler filters with polarization-dependent wavelength shifts from 0.1 to 2 nm, significantly less than those achieved for conventional devices. Experimental results show that both the filter wavelength and insertion loss can be rendered polarization insensitive by this technique.