Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
2002-08-26
2004-08-10
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C385S003000, C385S008000, C385S027000, C385S039000, C385S050000, C385S014000
Reexamination Certificate
active
06775455
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a silicon mesa structure integrated in a glass-on-silicon waveguide for transmission and modulation of electromagnetic radiation.
The invention also relates to a method of manufacturing a silicon mesa structure integrated in a glass-on-silicon waveguide for transmission and modulation of electromagnetic radiation.
2. Description of the Related Art
Glass on silicon is the most promising system of materials which has been developed till now with a view to manufacturing integrated optics. The manufacturing process is inexpensive and compatible with silicon based microelectronics. A large number of low loss passive components may be manufactured on large substrates, good low loss coupling to optical fibres is possible, and low rate (less than 1 MHz) modulation has been demonstrated by the use of the photo-elastic effect. The first active components (amplifiers and lasers) in glass-on-silicon waveguides have been demonstrated, but to provide optical sources and detectors for the communication wavelengths (1.3-1.6 &mgr;m), it is necessary to use hybrid integration. Also, modulation at frequencies above 1 GHz has been demonstrated by the use of hybrid integration with semiconductor materials. This is complicated as well as expensive. This has led to experiments with direct integration of polymer material in glass-on-silicon waveguides for use as optical modulators. The use of polymer materials is associated with a reduced service life of the component and thereby a reduced reliability of the modulator.
For the transfer of information through an optical communications system it is required that a property of the light is changed in accordance with the information. Information may thus be transferred to a light wave by changing its intensity, phase, frequency, polarization or direction in analog or digital form. In particular, modulation of the intensity of light at high frequencies (greater than 1 GHz) is of great importance in connection with the input of information into communications systems. Likewise, modulation of the direction is of potentially great importance in optical time division multiplexing and demultiplexing.
Planar optical waveguides for modulation of light are well-known. The modulation is generated e.g. by varying the refractive index or the propagation loss in the waveguide. This may be achieved e.g. by changing the concentration of free charge carriers in a semiconductor material. Thus, silicon may be used as a modulator, the concentration of free charge carriers being controlled through a pn-junction. The development of such semiconductor based optical components has taken place in relative isolation with respect to glass-on-silicon components.
Silicon based modulators are typically manufactured by depositing low-doped epitaxial silicon directly on a high-doped silicon substrate or an oxidized silicon substrate (in a Silicon-on-insulator structure (SOI)). A one-dimensional waveguide (film waveguide) is provided, as low-doped silicon exhibits a higher refractive index than high-doped silicon owing to the difference in concentration of free charge carriers in the materials. A two-dimensional waveguide may be provided by forming a ridge in an epitaxial material and through doping adjacent areas of the core. An electro-optical modulator is provided in this type of waveguide by introducing p-type and n-type doping in the silicon material in the sheath layers of the three-dimensional waveguide. These layers may be introduced in a lateral direction as well as in a vertical direction, so that n-type and p-type materials are present on their respective sides of the core of the waveguide. The doping may be carried out so that a single pn-diode or a number of these (through which unipolar or bipolar transistor structures may be formed) are created. Another method of controlling the number of free charge carriers in the waveguide is by making a metal oxide semiconductor transistor (MOSFET) structure. This structure, too, may be provided longitudinally of or transversely to the waveguide.
It is typical of silicon based waveguides that they exhibit relatively high propagation losses because of the high-doped substrates. The use of SOI substrates reduces this by increasing the distance between the waveguide and the high-doped substrate. The reduce the propagation loss to below 0.1 dB/cm it is necessary to use insulated layers which are at least 2 &mgr;m thicker than what is available as standard SOI wafers. For silicon to be of real interest as a medium in an integrated optical modulator, it is necessary that the rapid electrically controllable properties of silicon must be combined with passive low loss integrated waveguides (preferably less than 0.1 dB/cm). It has not been possible to satisfy this requirement with the previously proposed semiconductor based structures.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a rapid and reliable electro-optical modulator for glass-on-silicon waveguides. The object of the invention is achieved by introducing a silicon mesa structure into the core or the sheath of a glass-on-silicon waveguide. Hereby, the reliable and rapid electrically controllable modulation of the silicon mesa structure are combined with the low propagation loss of the glass-on-silicon waveguide.
A waveguide may be provided in the silicon mesa structure by making the core of the waveguide of low-doped monocrystalline silicon, while the sheath may be provided by doping parts of the mesa structure and/or making parts such that these preferably consist of silicon dioxide. Here, the structure may advantageously be made of the silicon substrate material. This gives the advantage that the use of expensive epitaxial silicon or SOI substrates can be avoided. To achieve optical wave guidance in a waveguide composed of a silicon mesa structure made of the silicon substrate material, it is necessary to remove excess silicon substrate material below the mesa structure. A further advantage is that the propagation loss in the silicon mesa structure is reduced by removing excess silicon substrate material below it.
By changing the concentration of free charge carriers in the silicon waveguide, it is possible to change the effective refractive index in it. This results in a change in the optical path, which leads to a change in the phase of the light in the waveguide over the given extent. Optical path is here a measure of the time it takes the light of a given wavelength and type to pass through the silicon waveguide.
For the change of the refractive index through variation in the concentration of the free charge carriers in the waveguide, it is expedient if the silicon waveguide is constructed as a diffused pin diode. By applying an electrical signal in the forward direction or reverse direction of the diode, charge carriers may be injected into or be depleted from the diode. To achieve fast modulation, the modulator, in a preferred embodiment, will be based on the depletion of charge carriers. This is achieved by biasing the diode in the reverse direction and modulating this voltage.
It is an advantage if the silicon modulator is constructed such that the glass-on-silicon waveguide is recessed in the silicon substrate. This ensures that the optical axes of the glass-on-silicon waveguide and the silicon waveguide coincide. This enables coupling of light from the glass-on-silicon waveguide to the silicon waveguide (and vice versa). It is a further advantage if the glass-on-silicon waveguide is constructed such that the glass-on-silicon waveguide is recessed in the silicon substrate and the upper sheath glass is limited to cover just the recessed part of the waveguide. This structure reduces the double refraction known in glass on silicon, which occurs because of the difference in thermal expansion of the silicon substrate and the glass structure. The advantage is not limited to a glass-on-silicon waveguide with a silicon mesa structure inserted into the core and/or the sheath
Jacobson & Holman PLLC
Knauss Scott Alan
NKT Research A/S
Sanghavi Hemang
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