Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
1999-08-12
2001-06-12
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S199200, C359S199200, C359S341430, C372S006000, C372S032000
Reexamination Certificate
active
06246511
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to optical communications. Specifically, this invention relates to controlling the variation of optical fiber amplifier gain due to changes in temperature.
2. Description of the Related Art
Optical fiber communications systems provide for low loss and very high information carrying capacity. Most advanced optical fiber communication systems now in place owe success and operating characteristics to optical fiber amplifiers such as the erbium-doped fiber amplifier (EDFA). The gain bandwidth of this amplifier is sufficient to permit simultaneous amplification of multiple channels and, for this reason may be used for dense wavelength division multiplexing (DVVDM).
In optical fiber amplifiers, like EDFAs, for DWDM systems the gain spectrum is required to be approximately uniform. However, as the temperature of the optical fiber varies, the shape of the gain spectrum changes significantly. It would be useful to control this variation of gain or to compensate within the amplifier for the variation.
Several solutions have been proposed or attempted. One of the attempted solutions deals with controlling the temperature of the optical fiber with a heater. This solution though causes high heat in the optical fiber which reduces the overall life and reliability of the optical fiber.
Another proposed solution involves gain flattening filters which have a transmission spectrum that varies with temperature in an appropriate manner related to the gain spectrum. However, this option has not been shown to be commercially viable.
SUMMARY OF THE INVENTION
The present invention provides a method and apparatus for controlling optical fiber amplifier gain due to temperature, and hence the output signal of the optical fiber amplifier.
According to one aspect of the present invention, an optical pump signal which has a certain wavelength is received by an optical fiber amplifier. The amplifier has a gain variation caused by temperature and a gain variation caused by the wavelength of the optical pump signal received. The gain variation due to temperature may be counteracted by the gain variation due to changes in optical pump wavelength. Moreover, an optical pump signal with a desired wavelength may be set by the present invention, and the set pump signal may be used to cause a gain which acts to partially or completely offset the optical fiber amplifier gain variation caused by temperature.
An optical fiber amplifier has an associated gain spectrum which is dependent upon temperature and upon optical pump signal wavelength. A change in either the temperature or the wavelength of the optical pump signal may cause a change in the gain spectrum.
FIG. 1
illustrates, in graph form, the difference in gain variation of the same optical fiber amplifier between the amplifier gain at a temperature of −5° C. and the amplifier gain at a temperature of 70° C.
FIG. 2
graphically illustrates the gain variation of an optical amplifier due to changes in the optical pump signal wavelength. Specifically,
FIG. 2
graphically illustrates the gain of an optical fiber amplifier at a wavelength of 972.5 nm and compares that with the gain of the same optical fiber amplifier at a wavelength of 980 nm.
The present invention may be used to minimize gain variation due to temperature (
FIG. 1
) by offsetting this change in gain with changes in optical amplifier gain from pump wavelength (FIG.
2
). For example, referring to
FIG. 1
, where the fiber temperature is 70° C., at the amplifier's spectrum range value of 1540 nm, there is a positive dB factor. The same optical fiber amplifier energized by a 980 nm pump laser, as seen in
FIG. 2
, at the amplifier's spectrum range value of 1540 nm, shows a negative dB factor. Accordingly, the positive gain at 1540 nm (
FIG. 1
) can be partially or completely offset by the negative gain of a 980 nm pump laser wavelength (FIG.
2
).
Various kinds of optical fiber amplifiers are available and each one has an associated gain variation for temperature and an associated gain variation for optical pump signal wavelength. Each different amplifier will have a unique gain curve which is different from the gain curves represented in
FIGS. 1 and 2
. Specifically,
FIGS. 1 and 2
represent the gain curves of a typical 1725-CBJA2 optical fiber amplifier. The concept of partially or completely offsetting the amplifier gain due to temperature would be the same for other optical fiber amplifiers.
Various methods and devices may be used to modify the optical pump signal wavelength to the extent required. These methods and devices include using a fiber grating device, control systems, and a ring reflector cavity system.
The fiber grating device can make use of high thermal variation glass or the use of an external Bragg reflector. According to another aspect of the invention, a Bragg reflector may be packaged such that any change in temperature applies stress to the Bragg reflector which changes the optical pump signal wavelength.
According to another aspect of the invention, a Fabry-Perot cavity may be used to create a phase shift. The phase shift can be used to set the optical pump wavelength characteristics as required to partially or completely offset optical amplifier gain due to temperature. The Fabry-Perot cavity can be a bulk Fabry-Perot cavity or make use of two Bragg reflectors as applied in a compound Bragg reflector Fabry-Perot cavity.
According to another aspect of the invention, a controller may be used to adjust the temperature of the optical pump signal source or to proportion the amount of optical pump signal portions combined to form an optical pumps signal with the desired wavelength characteristics.
According to another aspect of the invention, a ring reflector cavity system may be used to control amplifier gain due to temperature by altering the optical pump wavelength. The ring-reflector cavity systems may include the use of a long period grating filter, a dielectric filter, or an optical isolator.
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Agere Systems Optoelectronics Guardian Corp.
Dickstein , Shapiro, Morin & Oshinsky, LLP
Moskowitz Nelson
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