Optical waveguides – With optical coupler
Reexamination Certificate
2001-10-29
2003-04-22
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
C372S006000, C359S341100, C359S341300, C359S341400, C359S341410, C359S341420, C359S341330, C359S341440
Reexamination Certificate
active
06553159
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention generally relates to the field of electro-optics and more specifically to controlling the amplification of an optical communication signal.
BACKGROUND INFORMATION
In any communication system, ultimate reception of a given transmitted signal is governed by the overall signal-to-noise ratio (SNR) of the receiving system. The larger the SNR, the more reliable the signal detection process. A weak signal captured by a detector must compete with various internal noise sources produced by associated electronic and optical systems. External and background noise sources, typically encountered in free space communication links, provide additional noise components and are not a concern in closed fiber networks. Detection of the signal is accomplished when the signal level rises above the statistical summation of all contributory noise sources.
Much of the investigative work involving erbium preamplifiers has been targeted to laser radar and free space communication systems where issues of low signal levels, due to poor target reflectivity and inefficient signal coupling into receiver optics, are of paramount importance. The application of erbium preamplifiers to fiber optic systems would readily enhance weak signal detection.
FIG. 1
shows a typical optical receiver system
100
which includes a photo detector
105
whose electrical output is coupled to a transimpedance amplifier (TIA)
110
. The TIA serves to convert the photo detector signal current into an amplified voltage suitable for signal processing. The overall SNR of the receiver system
100
is statistically determined by the noise contributed by both the photo detector
105
and the TIA
110
. If the photo detector
105
is a positive-intrinsic-negative (PIN) photodiode, with a gain M of 1, both the TIA
110
and photo detector
105
will contribute to the overall receiver system noise figure. If the photo detector
105
is an APD (avalanche photo detector) photodiode, with a gain M of 10 to 20, the noise contributed by the TIA
110
will not prove as critical. The noise factor of a receiver system employing an APD detector may be described as follows:
F
total
=F
apd
+[(
F
tia
−1)*(
M
apd
)
−1
]
where,
F
total
is the total noise factor of the combined photo detector
105
and TIA
110
F
apd
is the noise factor of the APD photodiode (typical value of 5 or 6)
M
apd
is the gain of the photodiode, M=1 for a PIN and M=10 to 20 for an APD
T
tia
is the noise factor of the TIA
110
.
The high gain provided by the first stage, in this case an APD photodiode (M
apd
), reduces the effect of noise contributed by the TIA
110
. The key is to identify a first stage component device that provides high gain while contributing low noise.
The improving technology in erbium fiber amplifier technology has produced amplifier systems with relatively low 4 to 5 dB noise figures. Translated into a noise factor, the equation converts a 5 dB noise figure,
Noise Figure=10 Log [Noise Factor (
F
erbium-amp
)]
into a noise factor of approximately 3.16 which is better than a typical noise factor of 5 or 6 for a standalone APD detector. Additionally the erbium amplifier will provide power gains of 20 dB (Gain of 100) to 30 dB (Gain of 1000) with relatively modest pump power. The overall gain of the amplifier can be scaled either with a variation of fiber length or adjustment of pump power. It is this characteristic of high gain with relatively low noise factor that justifies the possible utilization of an erbium fiber system as a photodiode optical preamplifier stage.
The primary noise source in erbium receiver systems is amplified spontaneous emission (ASE) noise produced by the interaction of the pump energy source with the erbium doped fiber optic cable used in the system.
FIG. 2
(from Hewlett Packard's 1999 Lightwave Test and Measurement Catalog) shows that ASE noise is generally specified as a quantity of spectral noise power over a given optical bandwidth. The spectral noise power is distributed across the entire operational optical bandwidth of the erbium amplifier and can span a continuous wavelength region, which in this case is from 1500 nm to 1600 nm. A photo detector has a broad optical bandwidth response and will detect the ASE noise power across this entire band. Since an optical signal is centered at a particular wavelength, an optical transmission filter is utilized to block out the majority of the ASE power outside of the signal bandwidth. A detector following the filter will then be sensitive only to optical energy of wavelengths centered over the filter pass-band. The filter operational band-pass is chosen to accommodate the spectral content of the signal.
Conventional wavelength division multiplexing (WDM) systems efficiently use bandwidth in existing fiber-optic telecommunication infrastructures. WDM systems employ coupler technology with very narrow bandwidth transmission characteristics. This characteristic is utilized to select or insert signals at various wavelengths into the fiber optic transmission path. WDM couplers are naturally suited to erbium amplifier systems since the inherent narrow pass-band characteristic of these couplers automatically filters out the ASE noise power produced by an erbium amplifying medium and intercepted by a photo detector. Additionally, as shown in
FIG. 3
(from Hewlett Packard's 1999 Lightwave Test and Measurement Catalog), the overall ASE noise power is reduced as optical signals are amplified.
The overall gain of an amplification system will be the gain product of the individual components. Since an APD provides a gain of 10 to 20, the corresponding gain required from the erbium preamplifier can be reduced. For a given length of erbium fiber, gain reduction is obtained by adjusting the level of pump power. Reducing pump power will also reduce the amount of ASE noise. An operational configuration of an erbium amplifier with 10 dB of gain coupled to an APD with a gain of 10 will provide an overall system gain of 100 with noise levels below that obtained from a single APD, operating near its avalanching region, in an attempt to achieve a similar gain of 100. The overall system noise factor will therefore be
F
total
=F
erbium
+[(
F
apd
−1) (
G
ain-erbium
)
−1
]
The very high gain obtained from the erbium preamplifier will reduce the noise contributed by the APD photo detector. System issues will dictate the selection of component gain. A TIA following the ADP photo detector may not be required except to satisfy signal translation or interface issues.
SUMMARY OF THE INVENTION
The present invention insures that a control signal used to amplify a received optical signal has a sufficient output power level. Further, the present invention automatically optimizes system performance by maintaining a constant gain setting and adjusting the pump level for optimum pump depletion with a minimum production of ASE noise. The present invention improves the overall qualities of the amplified signal, such as broadband gain, pulse response, linearity and distortion characteristics. An optically pre-amplified detector component is incorporated in a receiver to provide long distance, high bandwidth, forward path link and distribution services. The present invention includes an enhanced optical erbium fiber amplifier used to pre-amplify an optical signal that is subsequently presented to an avalanche or PIN photodiode detector.
The present invention is an optical system that includes a first optical detector, a second optical detector, and an optical pump energy source which outputs an optical control signal at a particular output energy level. When an optical communication signal is received into the optical system, the communication signal is amplified using the optical control signal. The amplified communication signal is inputted into the first detector, the control signal is inputted into the second detector, and the energy level
Akin Gump Strauss Hauer & Feld, L.L.P
General Instrument Corporation
Lee John D.
Valencia Daniel
LandOfFree
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