Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2000-02-01
2002-01-08
Schuberg, Darren (Department: 2872)
Optical waveguides
With optical coupler
Input/output coupler
C385S037000, C385S010000, C372S006000
Reexamination Certificate
active
06337939
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an optical amplifier and in particular to monitoring a pump power of an optical amplifier.
BACKGROUND OF THE INVENTION
Improvements in optical amplifiers have vastly enhanced the use of optical communication systems by increasing both data rates and the distances over which optical signals are transmitted. Optical amplifiers are used to intensify optical signals that are attenuated along the fiber-optic communication path. A significant advancement in such amplifiers is the development of optical amplifiers based on an optical fiber doped with a rare-earth element. These types of amplifiers have replaced cumbersome electrical repeaters in fiber-optic communication links allowing true all-fiber optical communications systems to be realized. Conventional repeaters require complex electronics to convert light into electric signals, amplify the signal, recover the data from the amplified signal, and then transform it back into light. In contrast, doped-fiber optical amplifiers do not interrupt the light signal, but merely add energy to it. The components in the optical amplifier system are comparatively simple. Similarly, optical fiber lasers have been proposed to generate an optical carrier for fiber-optic communications systems. These types of lasers can be externally modulated or mode locked, and in some cases are alternatives to diode lasers as sources of high-power light in fiber optic communications systems.
Both, fiber amplifiers and lasers operate on similar principles. The silica glass in the guided-wave portion of the optical fiber is doped with traces of ions of rare-earth elements, such as erbium, ytterbium, neodymium, or praseodymium. The energy structure of the erbium ions, for example, is such that signal light with wavelengths of approximately 1530-1565 nm can be amplified in the fiber if the population of the excited states of the erbium ions is such that a rate of stimulated emission exceeds that of spontaneous emission and absorption. In this event, light within the gain bandwidth entering the optical fiber will experience a net gain and exit the fiber with greater power. If a mechanism is established to re-circulate this amplified signal in the fiber, for example by placing appropriate reflectors at the ends of the fiber, then laser action occurs in the fiber if the net gain equals the loss of the light within some optical bandwidth. In either case, it is crucial to excite the erbium ions into the proper excited state for a gain to occur. This is accomplished by exciting/pumping the erbium ions with light of a suitable wavelength, for example 980 nm or 1480 nm, which is most conveniently provided by a commercially available high-power diode laser that is coupled into the guided wave portion of the optical fiber. Pumping the erbium ions at these wavelengths will configure the erbium ions to amplify signal light within a wavelength range of approximately 1535 nm to 1565 nm. The relatively small cross-sectional area of this portion helps to ensure a high intensity and therefore allows an appreciable gain of the signal wavelengths. However, those skilled in the art will appreciate that the properties of the signal produced by such an amplifier or laser will depend to a large extent on the properties of the diode laser used to pump the fiber itself.
FIG. 1
illustrates a block diagram of a prior art optical amplifier designated with reference numeral
110
. The block diagram is a simplified illustration of commercially available amplifiers, such as the FiberGain™ Module available from Corning Incorporated of Corning, N.Y., and identified as part number CL-10. Amplifier
110
includes an optical fiber
112
that is doped with an ion of a rare-earth element. In the preferred embodiment, the dopant is erbium. Other ions of rare-earth elements, such as neodymium, also have been used as dopants for the fiber, but erbium remains as the most prominent and successful. Optical fiber
112
provides an input
114
for receiving a light input signal, L
IN
, and an output
116
for providing an amplified light output signal, L
OUT1
.
Amplifier
110
further includes a light-sourcing device which is typically a laser diode
118
. The diode
118
couples power to the amplifier by “pumping” energy into optical fiber
112
and, hence, is also known as a pump laser diode. Specifically, the light provided by laser diode
118
is absorbed by the erbium ions in fiber
112
, pumping those ions to a high-energy level. When a weakened L
IN
signal enters fiber
112
, the excited erbium ions transfer their energy to the signal in a process known as stimulated emission. As a result, the fiber
112
provides the amplified light output signal, L
OUT1
. The anode of laser diode
118
provides an input
120
for receiving an amplification control current, I
c
. For ease of illustration, the cathode of laser diode
118
is shown as grounded. It should be understood, however, that alternative configurations may be implemented for activating and deactivating laser diode
118
.
Amplifier
110
also includes a pump power detector
122
, which is typically a photodiode. Power detector
122
is proximate the sourcing laser diode
118
and, hence, provides an electrical signal, I
INT1
, directly proportional to the light intensity L
PLD
of pump laser diode
118
. Signal I
INT1
is detectable at output
124
of amplifier
110
. In commercially available amplifiers, power detector
122
is often referred to as a “rear facet detector” due to its physical relationship to laser diode
118
. Specifically, a small portion of the light emitted by laser diode
118
is reflected “rearwardly” to the detector, thereby giving the detector its name. As known in the art, the photodiode converts the light to an electrical signal, i.e. I
INT1
, indicative of the intensity of the detected light.
While amplifier
110
of
FIG. 1
provides numerous advantages over repeaters, developmental efforts continue in an attempt to increase optical system performance, including device reliability. For example, it is known in the art to include a feedback circuit which adjusts I
c
to maintain a constant light output signal, L
OUT1
. Thus, as L
IN
changes in intensity or wavelength, I
c
is altered to maintain L
OUT
, at a fixed level. When L
IN
falls below a certain level, or is removed completely, the feedback system would try to greatly increase the magnitude of I
c
. However, above a certain level of optical output power, diode
118
will be damaged. Therefore, such a system will include a current limit for I
c
, limiting it to a value that does not produce a damaging level of optical output power. As diode
118
ages, its efficiency decreases, producing a lower level of optical output for a given I
c
. This reduces the amplifier performance and reduces its useful lifetime.
Thus, it is apparent that there is a need in accurately monitoring the pump power in an optical amplifier.
There are already various known constructions of optical waveguides, including optical fibers, that are provided with embedded gratings for being used either for inserting light into or for removing light from the respective optical waveguide at an intermediate location or at different intermediate locations of the waveguide. For example, U.S. Pat. No. 4,749,248 to Aberson, Jr. et al, issued on Jun. 7, 1988, discloses a device for tapping radiation from, or injecting radiation into, a single mode optical fiber. Aberson, Jr. et al disclose that it is possible to convert a guided mode in an optical fiber into a tunneling leaky mode or vice versa by forming a grating of appropriate periodicity at least in the core of the optical fiber, and either to remove the guided mode from the fiber core into the cladding by converting it into the leaky mode, and ultimately from the fiber altogether, or to insert light of an appropriate wavelength into the core to form a guided mode therein by directing light of a proper wavelength from the exterior of the fiber toward the grating to propagate in the fiber cladding and
Aspell Jennifer
Zimmerman Donald R.
Assaf Fayez
Greene Kevin E.
JDS Uniphase Inc.
Lacasse Randy W.
Lacasse & Associates
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