Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
1999-04-26
2002-02-05
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S199200, C359S337400
Reexamination Certificate
active
06344924
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical fiber amplifiers and in particular, to optical fiber amplifiers which flatten the power of optical signals which are amplified from the optical fiber amplifiers and will be output extending over a wide wavelength range and a wide optical input power range as well as a method that flattens the power of optical signals which will be output from the optical fiber amplifiers.
2. Description of Related Art
Optical fiber amplifiers include, for example, erbium-doped optical fiber amplifiers, praseodymium-doped optical fiber amplifiers, neodymium-doped optical fiber amplifiers and optical fiber amplifiers doped with other rare earth elements. For instance, production of erbium-doped optical fiber amplifiers, research into their characteristics and further development have advanced as described in the paper “HIKARIZOFUKUKI TO SONO OYO” (Ohm Corp. published May 30, 1992, pp. 99 to 123).
An erbium-doped optical fiber amplifier (EDFA) is an amplifier that utilizes an optical fiber with a glass fiber core doped with erbium ion (Er
+3
) (erbium-doped fiber). Pumping light forms an inverted population inside the EDF. When an optical signal is input as an optical input into an EDF having formed therein an inverted population, emissions occur inside the EDF amplifying these optical signals. The amplified optical signals are then output as an optical output.
FIG. 1
shows characteristics of the wavelengths and optical output power of an EDFA. In
FIG. 1
, the wavelength is shown in the horizontal axis (units: nm) and the optical output power is shown in the vertical axis (units: dBm)
FIG. 1
shows output characteristics when optical signals of eight different wavelengths each having an optical power of −26 [dBm/ch] are simultaneously input into an EDFA as optical inputs and then amplified. From the output characteristic data shown in
FIG. 1
, it is understood that there is a difference in the optical output power for each wavelength. In other words, within the plurality of wavelengths there is a wavelength dependency output power deviation &Dgr;G in the output between the maximum optical output power and the minimum optical output power. This deviation &Dgr;G is an output (or gain) deviation.
When this EDFA is provided in multistages in a transmission system having a long span as optical amplifier repeater, the output power deviation &Dgr;G accumulates sequentially. Because of this, that wavelength causes the optical output signal to experience S/N degradation, and in a worst case scenario, signal loss.
Further, the wavelength dependency output power deviation of a conventional EDFA changes in response to the input state.
FIG. 2
shows the relation between the wavelengths of an obtained optical output signal and optical output power when the optical input power changes to seven paths and is input into an EDFA for optical signals having eight different wavelengths. The wavelength is shown in the horizontal axis (units: nm) and the optical output power is shown in the vertical axis (units: dBm). Characteristic curves A, B, C, D, E, F and G correspond to and optical input power of −12, −14, −16, −18, −20, −22 and −24 (units: dBm/ch), respectively. As can be understood from the results shown in
FIG. 2
, the output deviations occurring in the optical output power of the EDFA differ in response to changes in the optical input power. For example, according to the results shown in
FIG. 2
, the optical output power close to 1532 nm is approximately 8 dBm and the output deviations of the optical output power are small for the input. However, if the wavelength expands to the 1544 nm side, the optical output power becomes smaller and is at its minimum close to 1540 nm. Then, the output deviation of the optical output power for the signal close to 1540 nm changes approximately 4 dB with respect to changes in the input power. This means that the slope of the spectrum of the optical output signal during batch amplification of multiple waveforms changes in response to changes in the input power.
Conventionally, the following two methods have been proposed as methods to reduce (compensate) the wavelength dependency output power deviation of an EDFA and flatten the output power.
The first method is a method wherein a gain equalizer comprising an interference filter and/or a fiber grating is inserted on the output side in a first stage EDFA or within or on the output side in a second or additional stage EDFA. This method cancels output deviations of EDFAs by means of providing the gain equalizer with characteristics opposite to the output characteristics which are wavelength dependent.
The second method is a method wherein improvements are made to the amplifier medium itself. For example, in this method the amplifier medium itself makes the output power flatten by adding phosphorus (P) to an EDF to create a hybrid EDF.
However, the first method that uses the above-mentioned gain equalizer has the following problems. At first, it is difficult to design a gain equalizer that has characteristics opposite to an EDFA. Next, in reality a gain equalizer only corresponds to a wavelength dependency output power deviation of one certain optical input power. In other words, it is not possible to use a gain equalizer to flatten the optical output power of a wavelength division multiplexing signal output from an optical fiber amplifier extending over the entire input power range and input wavelength region of a wavelength division multiplexing signal input to an optical fiber amplifier. For attaining the reduction of a wavelength dependency output power deviation over the entire input power range and input wavelength region by means of the first method, it would probably be necessary to use an active gain equalizer (optical component) with gain equalizing characteristics which depend on the optical input power. For example, the necessity of a movable portion to change the incident angle of the optical signal directed towards a filter for each power or the necessity of optical components to change the angle of the filter in response to an input signal make it difficult to practically use the first method reliably and with control.
The second method that utilizes an amplifier medium has the following problems. When this method is used, it is difficult to design the amplifier medium. Namely, the wavelength range in which the amplifier medium can handle amplification processing is limited. Therefore, it is difficult to provide a composition of an amplifier medium to achieve gain flattening of optical power of an optical output signal extending over a wide wavelength range. Further, the above-mentioned wavelength dependency output power deviation is dependent on the optical input power as previously described referring to
FIG. 2
for the hybrid EDF doped with phosphorus (P) and another amplifier medium. Because of this, the optical input power does not allow flattening of the optical output power. In other words, uniform gain cannot be achieved.
Thereupon, the inventors of this application carried out various research and experiments to solve the above-mentioned problems. As a result, the inventors were aiming at an EDFA with amplification characteristics of optical power which were dependent on the wavelength in addition to absorption characteristics of optical power which were dependent on the wavelength as well. The inventors considered that if amplification optical fibers which actuate by amplification characteristics and absorption optical fibers which actuate by absorption characteristics were connected in series together within one transmission path as well as absorption optical fibers having absorption characteristics which allow compensation of amplification characteristics, it would be possible to equalize (flatten) the gain, namely the optical power, of each optical multiplexed output signal from the amplification optical fibers.
SUMMARY OF THE INVENTION
Th
Shikii Shigeru
Suzuki Mikiya
Burdett James
Moskowitz Nelson
Oki Electric Industry Co. Ltd.
Venable
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