Optical: systems and elements – Absorption filter
Reexamination Certificate
1999-08-06
2001-10-23
Chang, Audrey (Department: 2872)
Optical: systems and elements
Absorption filter
C359S577000, C359S578000, C359S199200, C359S586000, C359S247000, C385S031000
Reexamination Certificate
active
06307691
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical filter for correcting non-linear gain or attenuation in a wavelength band of the optical system by adding an insertion loss having a quadratic-shaped insertion loss curve in the wavelength band and having a maximum insertion loss at a center wavelength of the wavelength band.
2. Description of the Related Art
Optical fiber communications systems are advantageously and desirably used to transport data in the form of light pulses over long distances because they exhibit a very large capacity for carrying information, are light-weight, and are immune to electromagnetic interference. In some systems, several optical carriers of different wavelengths are used to simultaneously propagate the multiple wavelengths along the same fiber. These are referred to as Wavelength Division Multiplexed (WDM) systems and further increase the capacity of optical systems.
In a typical WDM system, optical fiber amplifiers are used to increase the power level of the optical signals and are typically arranged along a fiber path to keep the optical signal levels above the system noise. However, these optical amplifiers such, for example, as Erbium-doped fiber amplifiers (EDFA) typically exhibit wavelength dependent gains. The tendency is to have the gain at a maximum near the center of the wavelength band.
Furthermore, the attenuation rate of the transmitted signal in the optical fiber varies depending on the carrier wavelength, thereby causing some channels, i.e. wavelengths, to have a higher power level than others at the receiving end of the optical fiber. The difference in attenuation rates of different wavelengths within the fiber is known as tilt. To correct the effects of tilt—i.e. to flatten the gain over the wavelength band of the WDM system—power equalizers are introduced into the optical systems. One such power equalizer is described in commonly-owned U.S. patent application Ser. No. 09/217,710, filed Dec. 21, 1998, which discloses an active filter that compensates for gain tilt in WDM systems. However, it has been found that for overall wavelength ranges of 100 nm, this device still deviated by as much as 0.65 dB from linearity. This is because, similar to the optical amplifier, the effect of the power equalizer is not linear. Therefore, although the tilt effect is lessened, the equalized wavelength band is non-linear.
Accordingly, a problem in existing WDM optical systems, specifically those which include optical amplifiers and power equalizers, is that a non-linear effect exists across the wavelength band of the WDM system.
SUMMARY OF THE INVENTION
An optical filter according to the invention corrects the non-linearity in gain across a wavelength band by inserting an insertion loss at different levels for the wavelengths of the wavelength band so that the insertion loss curve across the wavelength band approximates a quadratic shape. According to an embodiment of the invention, the filter includes a first layer of silicon dioxide and a second layer of silicon nitride on a substrate. The first layer has a thickness that equals m&lgr;/(4n
SiO2
) and the second layer has a thickness equal to &lgr;/(4n
siNx
), where &lgr; is the center wavelength of the wavelength band, n
SiO2
is the refractive index of the first layer, n
siNx
is the refractive index of the second layer, and m is an odd integer. The amount of insertion loss through the filter at the center wavelength of the wavelength band is determined by the refractive index of the silicon nitride layer. The slope of the insertion loss curve at the edges of the wavelength band is determined by the thickness of the silicon dioxide layer, i.e. different values of the odd integer or multiple m. Accordingly, an insertion loss curve may be designed using the inventive optical filter for correcting a deviation from linearity centered around a central wavelength by controlling the refractive index of the silicon nitride layer to a value corresponding to the amount of insertion loss required at the center wavelength, and setting the thickness of the silicon dioxide layer such that the slope or shape of the insertion loss curve at the edges of the wavelength band adequately corrects the deviation from linearity.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.
REFERENCES:
patent: 5914804 (1999-06-01), Goossen
patent: 5940558 (1999-08-01), Bishop et al.
patent: 5943155 (1999-08-01), Goossen
Chang Audrey
Lucent Technologies - Inc.
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