Optical: systems and elements – Optical amplifier – Particular active medium
Reexamination Certificate
2000-02-15
2001-11-13
Tarcza, Thomas H. (Department: 3662)
Optical: systems and elements
Optical amplifier
Particular active medium
C359S337100
Reexamination Certificate
active
06317256
ABSTRACT:
TECHNICAL FIELD
The present invention relates in general to optical signal splitting using a simultaneous optical splitter/amplifier (SOSA) and more particularly, to a SOSA whose gain and noise figure performance is equalized to obtain nearly uniform performance over a wider bandwidth compared to conventional splitter/amplifier devices. More particularly, the present invention provides a process for fabricating a SOSA having equalized gain and noise figures to extend the useful bandwidth of the device.
BACKGROUND OF THE INVENTION
There is a significant interest in the development of photonically implemented antennas for military and commercial communication and RADAR applications. Several candidate architectures under investigation are active phased arrays, direct radiating multiple beam antennas (MBA), and array-fed-reflector MBAs. Most demonstrations and experimental investigations to date have been constructed with relatively few elements and passive optical systems to and from the array elements. In the future, practical antenna systems achieving useful levels of performance will require active distribution of optical power to as many as hundreds or thousands of individual array elements.
Active optical distribution components, realized using the self-imaging properties of multi-mode interference (MMI) cavities, represent an integrated enabling technology for these types of systems. An active MMI based optical splitter also represents a potentially compact and reliable, electrically pumped unit for active distribution nodes in increasingly more prolific ground based digital or analog, single or multiple wavelength fiber-optic networks.
An example of such a splitting device is described in the related application by Zmudzinski. The related application discloses and claims an active multimode optical signal splitter with an optical multimode waveguide which allows for signal splitting and current pumping in order to provide optical power amplification and minimize losses due to signal splitting. The type of device described in the related application falls into the broader class of optical signal components known as the simultaneous optical splitter/amplifier or SOSA.
The SOSA has been established as a useful and potentially critical component in several optical systems such as phased array radar and optical signal processing. In essence, the SOSA is a MMI splitter, which has the added feature of optical gain. As the optical signal is split (attenuating the signal), the pumped semiconductor material provides gain to the light (amplifying the signal). The output of the SOSA consists of multiple outputs, each of which can have the same intensity as the original inputs.
While the SOSA may prove to be an integral component of optical signal distribution systems, it would be more useful if it could operate over an extended wavelength range or optical bandwidth, enabling its use in wavelength division multiplexing (WDM) systems. In conventional SOSAs, the optical bandwidth is a function of the splitter wavelength response and amplifier optical gain response, which are aligned in order to maximize performance for a single wavelength. This conventional configuration of a SOSA, however, limits their application to multi-wavelength signal distribution systems.
As such, an optical signal splitter/amplifier whose bandwidth is greater than the bandwidth of the individual splitter and gain components would provide numerous advantages.
SUMMARY OF THE INVENTION
The present invention provides a method and process of making an MMI splitter combined with optical gain material having equalized gain and noise figures that provides an optical splitter/amplifier with a wider optical bandwidth. The method of the present invention can be utilized to fabricate splitter/amplifier devices suitable for use in multiple wavelength optical signal distribution systems such as those employing wavelength division multiplexing.
In particular, the method of the present invention can be used for extending the useful bandwidth of a SOSA by equalizing gain and noise figure performance over an optical bandwidth spanning tens of nanometers. This equalization technique relies on a subtle yet important alteration of the physical configuration of the SOSA in order to obtain nearly uniform device performance over a wider optical bandwidth.
As such, disclosed is a method of designing the semiconductor optical amplifier material and the MMI splitter in order to achieve gain and noise figure equalization. The method includes the step of equalizing the device by designing the wavelength of peak optical gain for the semiconductor material to be different (typically by about 15 nm) from the wavelength for perfectly focused spots produced by the MMI splitter. In effect, the net gain, as measured by the light coupled to the output of the device, is the semiconductor gain multiplied by the insertion loss of MMI splitter.
Equalization is achieved such that when the gain of the optical semiconductor material is at a peak, the MMI splitter is slightly defocused. Likewise, when the MMI splitter is perfectly focused, the semiconductor gain is reduced from its peak value. In this manner, the net gain of the device is flattened over a wide range of wavelengths. The method also includes the step of equalizing the noise figure of the SOSA so that the noise figure and gain figure are equalized at the same time.
Also disclosed is a process for fabricating a simultaneous optical splitter/amplifier comprising the steps of epitaxially growing a substrate layer to a predetermined thickness associated with a desired peak wavelength response and lithographically patterning a set of optical waveguides on the substrate layer to form a multi-mode interference splitter with a desired perfect focus wavelength. Next, the necessary electrical contacts are formed and aligned with the multi-mode interference splitter. The geometry and quantum well structure of the substrate layer is controlled to cause the desired peak wavelength amplification response to be offset from the desired perfect focus wavelength.
Further disclosed is a simultaneous optical splitter/amplifier comprising a semiconductor optical amplifier material formed on a substrate layer and a multi-mode interference splitter formed in the substrate layer and epitaxial layers with electrical contacts attached to the substrate and epitaxial layers and aligned to direct signals into and out of the splitter. The semiconductor optical material is such that its peak wavelength is offset from the perfect focus wavelength of the multi-mode interference splitter. In one embodiment, the semiconductor optical material's peak wavelength is offset approximately 15 nanometers from the perfect focus wavelength of the multi-mode interference splitter to give the simultaneous optical splitter/amplifier a bandwidth of approximately 55 nm.
A technical advantage of the invention is that wavelength equalization is achieved without compromising the optical amplification in the SOSA which is necessary to achieve 0 dB optical signal splitting, so that the output optical signals have the same intensity as the input optical signals.
Another technical advantage of the invention is the achievement of an optical splitter/amplifier useful in wavelength division multiplexed (WDM) systems. The performance of the equalized SOSA is superior to conventional SOSAs which, in turn, is superior to splitting followed by post-amplification.
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J. M. Heaton, et al., “Novel 1-to-N Way Integrated Optical Beam Splitters Using Symmetric Mode Mixing in GaAs/AlGaAs Multimode Waveguides”, Applied Ph
Kunkee Elizabeth T.
Leight James E.
Hughes Deandra
Tarcza Thomas H.
TRW Inc.
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