Optical: systems and elements – Optical amplifier
Reexamination Certificate
2001-12-13
2004-04-13
Hellner, Mark (Department: 3663)
Optical: systems and elements
Optical amplifier
C359S341300
Reexamination Certificate
active
06721087
ABSTRACT:
FIELD
The described invention relates to the field of optical signal amplification. In particular, the invention relates to amplifying an optical signal using multiple pumping light sources.
BACKGROUND
A waveguide may serve as an optical amplifier by doping it with ions of a rare earth element such as Erbium. An optical signal propagating in the waveguide is amplified when a pumping light beam is introduced. For example, Erbium ions, excited to a higher energy state with a pumping light beam having a wavelength of approximately 980 nm or 1480 nm, will amplify an optical signal in a wide wavelength band around 1530-1600 nm as the Erbium ions fall down to a lower energy state. This technique is well-known in optical fiber amplification.
FIG. 1
is a schematic diagram showing one prior art method of amplifying an optical signal
10
in a planar waveguide
20
. The waveguide
20
is embedded in a substrate
30
and doped with Erbium ions. An optical signal
10
is directed into the waveguide
20
and propagates through the waveguide
20
. A laser
50
supplies pumping light beams into the waveguide
20
in a co-propagating direction, i.e., in substantially the same direction as the optical signal propagates. The signal
10
and the pump
50
are combined to the same waveguide
20
, for example, in an evanescent directional coupler. In one example, an optical signal
10
having wavelength of approximately 1550 nm is amplified as laser
50
supplies pumping light beams of approximately 980 nm or 1480 nm wavelength.
FIG. 2
is a schematic diagram showing another prior art method of amplifying an optical signal. In
FIG. 2
, a pump laser
50
is directed from the opposite end of the waveguide
20
to pump light in a counter-propagating direction, i.e., in a direction opposite to that of the optical signal. Similar to
FIG. 1
, the optical signal is amplified within the waveguide
20
and then exits the substrate
30
.
Modern optical networks use single-mode optical fibers for transmission over long distances. This avoids signal degradation coming from chromatic dispersion, i.e. dependence of the speed of the light on its wavelength. For efficient interfacing with single mode fibers, all optical components, including fiber or waveguide amplifiers, are effectively single-mode. Due to a general principle of optics, “brightness conservation theorem”, power of light in a single mode cannot be increased using just linear passive (not adding energy) optical elements. This results in a fact that the power of light with a certain wavelength from only one mode can be coupled to a single mode waveguide. For amplifiers, it translates that only one pump laser with a certain wavelength can supply pump light in each direction of propagation and each polarization.
The optical signal experiences gain in an optical amplifier provided that the intensity of the pump is higher than a certain threshold value dependent on the intensity of the optical signal and material properties of the optical amplifier. In order to achieve high enough gain, the intensity of the pump must be much higher than the threshold value. Consequently, a high power of a pump laser is typically required.
There are several disadvantages of the above methods compared to the invention described below. First, the relatively high power laser used in the described co-propagating and counter-propagating amplification is expensive. Second, high power lasers have a high power dissipation, which may cause thermal issues in their packaging. Third, the reliability of high power lasers is generally not as good as that of lower power lasers.
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Alduino Andrew C.
Funk David S.
Nikonov Dmitri E.
Scholz Christopher J.
Blakely , Sokoloff, Taylor & Zafman LLP
Hellner Mark
Intel Corporation
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