Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
1999-04-23
2001-08-21
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Particular coupling structure
C385S001000, C385S002000, C385S003000, C385S040000, C385S132000, C250S227110, C257S022000, C257S021000, C257S184000
Reexamination Certificate
active
06278820
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a traveling-wave photodetector and more particularly to a traveling-wave photodetector having high power operation and large bandwidth.
BACKGROUND ART
Photodetectors convert modulated light into electrical signals. Photodetectors are used in many areas including, but not limited to, antenna remoting, spacecraft RF interconnects, analog and digital links, and other applications using a fiber optic link having a high spur-free dynamic range.
Solid state photodetectors typically operate by incident light or photons being absorbed to create electrons and holes, known as carriers. A device such as a p-i-n (PIN) diode sweeps away the carriers to electrical contacts to produce a photocurrent. A PIN diode is formed when p-type and n-type semiconductor regions are separated by a layer of intrinsic semiconductor material.
An example of a prior art surface coupled PIN photodetector
100
is shown in FIG.
1
. The goal in a surface coupled PIN is to absorb as much of the light in as short a distance as possible. The entire absorbing region is made from a narrow-bandgap material, which is covered with highly doped material, forming ohmic junctions that are transparent to both electrons and holes.
Conventional surface-coupled PIN photodetectors have a small surface area and a large bandwidth, but are limited in the amount of optical power they can absorb. Excessive power may damage the photodetector, and produces saturation effects that reduce performance. It is possible, by making the surface area of the device larger, so that the optical power can be spread out over a large area, to reduce the damage and increase the saturation threshold. However, this reduces the RF bandwidth of the device by the corresponding increase in capacitance. A simple PIN photodetector is not capable of unlimited high-power operation and large bandwidth.
Presently, a traveling-wave photodetector
200
having a larger surface area is known in the prior art and is shown in FIG.
2
. The photodetector
200
has a thin absorbing layer
202
to achieve low absorption per unit length. The layer
202
is positioned between two non-absorbing layers
204
. However, the thin absorbing layer has a significant drawback. The electron-hole pairs that are generated within the layers become trapped and can only leave the layers by way of a thermal process. The thermal process is inherently slow, which means that any photodetector made using thin layers will have an upper frequency response that cannot exceed a few Gigahertz. It has been suggested to use compositional grading. While this process alleviates the trapping problem, it does not eliminate it.
SUMMARY OF THE INVENTION
The traveling-wave photodetector of the present invention has high-power operation and large bandwidth. The present invention is a traveling-wave photodetector having an epitaxial layer structure that spreads out the absorbed power, minimizing the thermal heating of the device. The present invention operates such that light travels through an optical waveguide that is weakly absorbing, and in which electrical current generated by the absorbed light is collected by electrodes that are attached to the waveguide. The power absorption is spread out over a large area, while the intrinsic bandwidth of the absorbing structure is maintained. The problem of slow thermal processes is avoided and the frequency response is limited only by the transit time of the detector. Devices with responses well in excess of 100 GHz may be possible.
The present invention allows for a high-speed PIN photodetector to be fabricated by proven techniques, and combines the PIN structure with a separate non-absorbing waveguide structure in a manner that allows light from the waveguide to slowly leak into the absorbing PIN structure as light propagates along the waveguide. This greatly reduces the effective absorption per unit length of the photodetector, without compromising the performance of the detector. Spreading out the absorbed light reduces thermal heating and saturation effects. The traveling wave electrode structure for the PIN photodiode eliminates frequency degradation effects due to detector capacitance.
It is an object of the present invention to overcome the drawbacks associated with photodetectors having thin layers. It is another object of the present invention to provide high-power, large bandwidth operation of a traveling-wave photodetector.
It is a further object of the present invention to provide a traveling-wave photodetector that has an epitaxial layer structure that spreads out absorbed power and minimizes thermal heating. It is yet a further object of the present invention to have light travel through an optical waveguide that is weakly absorbing. It is still a further object of the present invention to collect the electrical current absorbed by the light at electrodes attached to the waveguide.
Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.
REFERENCES:
patent: 5270532 (1993-12-01), Hietala et al.
patent: 6177686 (2001-01-01), Gutierrez-Aitken
Font Frank G.
Gudmestad Terje
Punnoose Roy M.
The Boeing Company
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