Optical filter

Details Optical filter Optical filter

1999-07-30

2003-03-04

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Light interference

Electrically or mechanically variable

C359S589000, C359S260000, C372S020000

Reexamination Certificate

active

06529328

ABSTRACT:

This invention relates to a variable reflectivity filter, and more particularly to an etalon having one or more end faces having a variable reflectance.
BACKGROUND OF THE INVENTION
Fabry-Perôt filters are widely used in optical systems where a dependence upon their periodic nature is required. For example, comb filters are now being developed wherein one or more etalons provide one or more output signals that are periodic in amplitude response. The free-spectral range, or period of a Fabry-Perot etalon is determined by a length of a gap between its two reflecting surfaces.
A multi-port tunable fiber-optic etalon filter (MTFET) having two spaced partially reflective mirrors is disclosed in U.S. Pat. No. 5,283,845 in the name of Ip assigned to JDS Fitel Inc. This filter has three or more ports, with at least two on one side and at least one on the other side of the etalon. A single signal can be filtered with the reflected signal being received, forming a wavelength division multiplexor, or a plurality of signals can be filtered, with or without the reflected signals being received. Ip provides a filter wherein the FSR is tunable within a range.
Another use for an etalon is described by Ip in U.S. Pat. No. 5,557,468 in the name of Ip assigned to JDS Fitel Inc., wherein an etalon is used a dispersion compensation device.
Yet another use for an etalon is described in U.S. Pat. No. 5,798,859 in the name of Colbourne et al. assigned to JDS Fitel Inc. and device for wavelength locking is provided, wherein an element having a wavelength dependent characteristic such as a Fabry-Perôt etalon is used to provide an output signal having an intensity that varies with wavelength. The intensity of a reference signal derived from an input signal is compared with an output from the Fabry-Perôt etalon to provide a feedback signal that corresponds to the frequency of the input signal. The system is calibrated before wavelength locking is performed to determine a ratio of intensities that determines a locked state or condition.
As can be seen from the above mentioned patents, Fabry-Perôt etalons have a variety of uses and are provided in a variety of forms.
Another type of etalon that differs from a fixed etalon, or even a tunable etalon as is described by Ip in U.S. Pat. No. 5,283,845 is one wherein the finesse or reflectivity of one or more of the reflectors is variable. Such a filter is described in U.S. Pat. No. 5,452,121 in the name of Hilgeman assigned to Northrop Grumman Corporation and is said to be useful as a spectral resolution agile filter.
The instant invention provides an etalon having a variable finesse over the length of at least one reflector.
One particular use for a filter of this type, is in the field of optical amplifiers and more particularly for gain tilt control.
Optical amplifiers and particularly erbium doped optical fiber amplifiers are nearly ubiquitous in optical transmission systems, particularly in the field of telecommunications. Erbium doped fiber amplifiers (EDFAs) have high polarization insensitive gain, low cross talk between signals of different wavelengths, good saturation output power, and a noise figure close to the fundamental quantum limit. The excellent noise characteristics allow hundreds of these amplifiers to be cascaded to cover spans of thousands of kilometers of optical fibre. EDFAs as opposed to electronic repeaters are also transparent to data rate, signal format, and wavelength over a limited range, making them useful for wavelength multiplexed (WDM) communication systems that simultaneously transmit a large number of signals using different wavelength bands for each signal.
Notwithstanding these generally excellent characteristics, a disadvantage associated with EDFAs is their narrow spectral width and uneven gain band. The useful telecommunications window of an EDFA is approximately 20-30 nm wide, while an ideal amplifier would have a flat spectral gain across the full spectrum which extends form approximately 1520 nm to 1570 nm. The peak wavelength of the erbium gain spectrum varies from about 1530 nm to about 1535 nm depending upon the host glass material.
FIG. 1
shows the characteristic gain spectrum of a particular conventional EDFA where it can be seen that the gain as a function of wavelength varies; this variation will be referred to hereinafter as gain ripple. Numerous techniques have been published for widening and flattening the gain spectrum (i.e. reducing the ripple) and include for example co-doping an erbium-doped silica glass fibre with Al
2
O
3
; changing the host glass material itself; using various forms of attenuating filters to reduce the gain at the emission peak; and, constructing hybrid devices having two or more different types of serially connected erbium doped fibre and actively adjusting pump conditions independently in each fibre section to compensate for the different gain slopes of each fibre.
In addition to the aforementioned problems and solutions associated with minimizing gain ripple, another significant problem exists to which there have been no simple, inexpensive, and practicable solutions. This other significant problem solved by this invention relates to improving dynamic gain tilt. The term dynamic gain tilt as used hereafter means the variation in gain at one wavelength as a result of changing the gain at any other wavelength via a change in input EDFA operating conditions. Although the techniques described above for minimizing gain ripple can provide a relatively flat spectrum in a specified wavelength band for a specific set of input optical powers and wavelength, the gain equalization performance degrades rapidly when the gain is changed (change in average population inversion levels) from the nominal conditions by changing the input power to the amplifier. One reported solution to this problem is allegedly achieved by a hybrid fibre device having cascaded amplifying stages with different gain spectra and an equal number of pump sources to allow the gain spectra of the individual stages to be effectively tuned independently so that when the total gain is changed, the relative contribution of each stage can be adjusted to arrive at the desired gain, with a resulting gain spectrum having a minimal amount of spectral distortion over the selected wavelength band. As an example, an erbium doped fibre having a positive gain slope may be combined with a different erbium doped fibre having a negative gain slope such that the hybrid device has a nearly flat gain at specific input power conditions. However, if the overall gain of the hybrid device must be changed, the gain slope of each of the constituent states will generally change at different rates when the pump power input to one of the stages is changed. In order to achieve good compensation at the new operating point, the relative gain of each of the constituent gain stages must be readjusted to make the gain slopes compensate each other. In implementing this type of amplifier, one skilled in the art would likely cascade two or more different erbium doped fibre compositions and provide a separate pump source for each amplifying stage at an end of each stage so as to minimize the number of splices and make it as convenient as possible to independently control the pump power to each stage. However, this technique for reducing or improving dynamic gain tilt requires a complex control scheme during operation in which the total power of multiple pump sources must be coordinated in order to realize gain slope compensation over a range of different gains (i.e. to change input power while maintaining a fixed target output power).
U.S. Pat. No. 5,764,406 in the name of Newhouse et al. entitled Hybrid Optical Amplifier Dynamic Gain Tilt incorporated herein by reference, describes a system wherein an erbium doped fibre amplifier device has a dynamic gain tilt that is less then the gain tilt of any of the constituent fibres. The hybrid device has at most one less pumping source than the number of constituent waveguides of the device. The hybrid device automatical

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