Optical waveguides – Temporal optical modulation within an optical waveguide – Electro-optic
Reexamination Certificate
2000-03-23
2001-10-02
Ullah, Akm E. (Department: 2874)
Optical waveguides
Temporal optical modulation within an optical waveguide
Electro-optic
Reexamination Certificate
active
06298177
ABSTRACT:
The invention relates to devices and methods of phase modulation of light transmitted in a semiconductor waveguide, and to optical switches including such phase modulation.
BACKGROUND OF THE INVENTION
Optical phase modulators may be used in various devices including optical switches and optical modulators including for example interferometers such as a Mach-Zehnder interferometer. Phase shifts may be caused by varying the refractive index of the light transmitting medium. In the case of a semiconductor material, such as silicon, the refractive index for transmitted light may be varied by thermo-optic effect or by varying the number of charge carriers in the optical path. Silicon is used for providing optical waveguides transmitting infra red light in the spectral region above 1.1 micron wavelength. Phase modulators are known for silicon waveguides using either thermal or current injection based PIN diode arrangements. In the case of thermal arrangements, means are provided to vary the temperature of the silicon waveguides and the known thermo-optic effect is used to vary the refractive index. Such devices are too slow in operation for many applications. In the case of current injection based PIN type diodes, a semiconductor diode is formed by locating intrinsic silicon between regions of doped silicon, one region being P type and the other N type. When an electrical potential is applied to the P and N regions to forward bias the diode, the doped regions inject charge carriers into the intrinsic silicon to cause the known free carrier dispersion effect. The intrinsic silicon into which the free carriers are injected is located in the optical path of the silicon waveguide and the change in concentration of free charge carriers alters the refractive index. The time taken for the injection of free carriers as well as their recombination time is not short enough for very high speed optical switches. The recombination time may be of the order of 0.01-10&mgr; seconds, and this can result in an optical switch speed no faster than 100 MHz. The requirement for switch speeds up to 1 GHz (
10
9
Hz) or faster is envisaged.
It is an object of the present invention to provide an improved device and method of effecting phase modulation in a silicon optical waveguide by use of carrier depletion.
SUMMARY OF THE INVENTION
The invention provides an optical phase modulator comprising a semiconductor waveguide formed from a semiconductor layer with an upstanding rib defining an optical transmission path, the semiconductor of the waveguide having both P-type and N-type doped regions forming at least one PN junction extending along the path of the rib for the length of a phase modulation region, said P-type semiconductor having a highly doped ohmic contact region on the surface of the semiconductor extending along the phase modulation region, said N-type semiconductor having a highly doped ohmic contact region on the surface of the semiconductor extending along the phase modulation region, anode and cathode terminals in contact respectively with the said ohmic contact regions and separated from each other by an insulating layer extending between the terminals, and selectively operable electric supply circuitry connected to said anode and cathode terminals to selectively reverse bias said PN junction or junctions and thereby extend the width of a carrier depletion zone at the or each junction to alter the refractive index along the waveguide.
In one embodiment one PN junction is formed and extends upwardly through the semiconductor from the substrate to the top of the rib and is located between the side walls of the rib.
Preferably the PN junction extends longitudinally along the length of the rib in a position offset from the midpoint of the rib width so that on reverse biasing of the junction the optical axis of the waveguide lies in the extended region of the depletion zone.
Preferably the PN junction is offset towards the side of the rib formed by N-type semiconductor so that on increase of the width of the depletion zone the midpoint of the rib width lies in an extended region of the depletion zone within P-type semiconductor.
Preferably the width of the rib and the position of the PN junction is such that when no potential bias is applied to the junction, the carrier depletion zone at the junction is offset laterally from the centre of the optical profile of light transmitted by the waveguide, but on application of reverse bias potential, the extended carrier depletion zone extends over the centre of the optical profile of light transmitted through the waveguide.
Preferably the width of the rib is such that on reverse bias being applied to the junction, the extended depletion zone occupies at least one half the width of the rib.
Preferably the anode and cathode terminals each comprise a metal layer such as aluminium or aluminium alloy.
The invention includes an optical interferometer having two parallel light transmitting arms, at least one of said arms including an optical phase modulator as aforesaid.
The invention includes an optical switch including an optical interferometer as aforesaid.
The invention includes a method of effecting an optical phase shift in a semiconductor waveguide formed from a semiconductor layer with an upstanding rib on the semiconductor layer defining an optical transmission path, the semiconductor of the waveguide comprising both P-type and N-type doped semiconductors forming at least one PN junction extending along the phase modulation region, said P-type semiconductor having a highly doped ohmic contact region on the surface of the semiconductor layer extending along the region at one side of the rib, said N-type semiconductor having a highly doped ohmic contact region on the surface of the semiconductor layer extending along the region at the opposite side of the rib, anode and cathode terminals in contact respectively with said ohmic contact regions and separated from each other by an insulating layer extending over the rib between said terminals, which method comprises applying a voltage to said anode and cathode terminals to reverse bias the or each PN junction and thereby increase the width of carrier depletion zone within the rib and change the refractive index along the waveguide.
In one embodiment a PN junction is formed upwardly through the rib and light is transmitted through the waveguide with the centre of the optical profile offset to one side of the PN junction, the offset being on the side formed by P-type semiconductor.
Preferably the centre of the optical profile is offset to one side of the depletion zone at the PN junction when no bias voltage is applied to the junction, but lies within the extended depletion zone when a reverse bias is applied by a bias voltage less than the breakdown voltage.
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Lionel Friedman, “Silicon Double-Iinjection Electro-Optic Modulator With Junction Gate Control”, Journal of Applied Physics (1988), Mar. 15, No. 6, pp 1831-1839.
Bookham Technology plc
Sughrue Mion Zinn Macpeak & Seas, PLLC
Ullah Akm E.
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