Polarization independent waveguide structure

Details Polarization independent waveguide structure Polarization independent waveguide structure

2000-09-26

2003-11-25

Healy, Brian (Department: 2874)

Optical waveguides

Planar optical waveguide

Thin film optical waveguide

C438S031000

Reexamination Certificate

active

06654533

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to semiconductor optical waveguides and more particularly to a thick semiconductor optical waveguide that is substantially polarization independent, has increased coupling efficiency to optical fiber and superior uniformity and reproducibility.
BACKGROUND OF THE INVENTION
Fiber optic communication systems have gained widespread acceptance over the past few decades. With the advent of optical fiber, communication signals are transmitted as light propagating along a fiber supporting total internal reflection of the light propagating therein. Many communication systems rely on optical communications because they are less susceptible to noise induced by external sources and are capable of supporting very high speed carrier signals and increased bandwidth. Unfortunately, optical fiber components are bulky and often require hand assembly resulting in Sawer yield and higher costs. One modern approach to automating manufacture in the field of communications is integration. Integrated electronic circuits (ICs) are well known and their widespread use in every field is a clear indication of their cost effectiveness and robustness. A similar approach to optical communication components could prove very helpful.
In an attempt to integrate optical components, manufacturers try to miniaturise optical systems within a single chip. For example, an InP structure can be formed on a substrate and can act as a waveguide for conducting an optical signal. Typically, the waveguide structure is thin and acts as a two dimensional waveguide, thereby effecting polarization of a signal guided therein. In order to provide polarization independence, several approaches exist.
Integrated wavelength multi/demultiplexers are important components for wavelength division multiplexing (WDM) optical communication systems. Integration offers the advantages of compactness, reliability, and reduced packaging costs. Further, implementation in a semiconductor material, particularly the InGaAsP/InP system important for optical fiber communications systems, would permit monolithic integration of these passive devices with active ones, such as lasers, modulators, optical switches, and detectors, resulting in sophisticated wavelength sensitive photonic integrated circuits with complex functionalities.
As described above, one of the major drawbacks in an integrated wavelength multi/demultiplexers is the polarization sensitivity of the device. Since an optical signal propagating through an optical fiber has an indeterminate polarization state, the switching/routing devices must be substantially polarization insensitive. However, planar waveguides usually have different propagation constants for TE (transverse electric) and TM (transverse magnetic) waveguide modes. For wavelength multi/demultiplexers, this difference in propagation constants results in a wavelength shift in the spectral response peak or the passband of each wavelength channel. This wavelength shift is sensitive to the design of the planar waveguide, and can be as large as 3 nm or more. As WDM systems are being designed towards smaller and smaller channel spacing—currently from 1.6 nm to 0.8 nm and even less in the future, even a small polarization dependent wavelength shift (e.g. 0.3-0.4 nm) is of concern.
Two types of integrated wavelength multi/demultiplexers that have been widely investigated art; phased waveguide arrays and etched grating-on-a-chip spectrometers. Grating based devices require high quality, deeply etched grating facets. The optical loss of the device depends critically on the verticality and smoothness of the grating facets. However, the size of the grating device is much smaller than the phased array and the spectral finesse is much higher due to the fact that the number of teeth in the grating is much larger than the number of waveguides in the phased array. This allows the grating based device to have a larger number of channels available over its free spectral range (FSR) and consequently can he scaled-up easily to high density operation.
In waveguide array based devices, several approaches have been used to compensate for the polarization sensitivity; for example the insertion of a half wave plate in the middle of the waveguides array is described by H. Takahashi, Y. Hibino, and I. Nishi, in a paper entitled “Polarization-insensitive arrayed waveguide grating wavelength multiplexer on silicon”, Opt. Lett., vol. 17, no. 7, pp. 499-501, 1992. Alternatively, the use or non-birefringent waveguides with a square cross section has been described by J. B. D. Soole, M. R. Amersfoort, H. P. Leblanc, N. C. Andreadakis, A. Raijhel, C. Caneau, M. A. Koza, R. Bhat, C. Youtsey, and l. Adesida, in a paper entitled “Polarization-independent lnP arrayed waveguide filter using square cross-section waveguides”, Electron. Lett., vol. 32, pp. 323-324, 1996. Birefringence compensation using two different rib waveguides has been described by P. C. Chou, C. H. Joynerm M. Zimgibl, in U.S. Pat. No. 5,623,571 entitled “Polarization compensated waveguide orating router”. In the '571 patent the polarization compensation is not within the slab waveguiding region. This technique requires either two regrowth steps as described in the patent and in a paper by the same authors entitled “Polarization compensated waveguide grating router on InP”, Electron. Lett., vol. 31, pp. 1662-1664, 1995, or two etching steps as described by C. G, M. Vreeburg, C. G. P. Herben, X. J. M. Leijtens, M. K. Smit, F. H. Groen, J. J. G. M. van der Tol and P. Demeester, in a paper entitled “An improved technology for eliminating, polarization dispersion in integrated phasar demultiplexers”, in Proc. 23.sup.rd Conf. on Optical Comm. (FCOC'97), pp. 3.83-3.86, Edinburgh, UK, 1997. In addition to increases complexity in fabrication process, the reduced cladding layer thickness in the polarization compensating rib/ridge waveguides resulted in a reduced lateral index contrast, and consequently increased phase errors due to enhanced coupling between adjacent waveguides. In order to avoid radiation loss due to reduced index contrast, the polarization compensating waveguides need to he implemented in straight waveguide section, which leads to an additional straight section length of the arrayed waveguides and consequently a larger device size:. Yet another alternative in the attempt to overcome polarization sensitivity is dispersion matching with adjacent diffraction orders which has been described by M. Zirngibl, C. H. Joyner, I. W. Stulz, Th. Gaigge and C. Dragone, in a paper entitled “Polarization independent 8.times.8 waveguide grating multiplexer on InP”, Electron. Lett., vol. 29, pp. 201-201, 1993, and by L. H. Spiekman, M. R. Amersfoort, A. H. de Vreede, F. P. G. M. van Ham, A. Kuntze, J. W. Pedersen, P. Demeester, and M. K. Smit, in a paper entitled; “Design and realization of polarization independent phased array wavelength demultiplexers using different array order for TE and TM”, J. Lightwave Technol., vol. 14, pp. 991-995, 1996. Another approach is that of using layer structures with low birefringence by using thick guiding layer and low refractive index contrasts has been described by H. Bissessur, F. Gaborit, B. Martin, P. Pagnod-Rossiaux, J. L. Peyre and M. Renaud, in a paper entitled “16 channel phased array wavelength demultiplexer on InP with low polarization sensitivity”, Electron. Lett., vol, 30, pp. 336-337, 1994.
For diffraction grating based wavelength multi/demultiplexers, only the last two approaches are possible. In the polarization compensation method which attempts to match the TE and TM passband to two adjacent diffraction orders, the tree spectral range (FSR) of the grating needs to be chosen equal to the wavelength split between the to modes. In this case, the passband corresponding to the mth-order for TE will overlap with the (m-1)th order for TM. A severe drawback of this method is that the available FSR for WDA channels is limited by the polarization split, which is determined by the waveguide layer st

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