Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-03-07
2003-05-13
Chang, Audrey (Department: 2872)
Optical waveguides
With optical coupler
Input/output coupler
C385S126000, C385S127000, C385S128000, C359S199200
Reexamination Certificate
active
06563985
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an optical wavelength filter that is operable at ultra-high speeds in the nanosecond range and that is tunable over a wide range.
BACKGROUND OF THE INVENTION
Optical wavelength filters are useful for the advanced fiber optic links used, for example, in optical Wavelength Division Multiplexing (WDM) networks. It is well known that there is an exponentially growing demand on the data transmission bandwidth for both civilian and military applications. Fiber optic links and networks have become the backbones for data transmission with large bandwidth.
In terms of military applications, fiber optic link technology has the unique bandwidth capability, the immunity from electromagnetic interference (EMI) and crosstalk, the light weight and the electrical performance necessary to realize fast data rates and reduced signature multi-function antenna apertures. However, a major remaining issue associated with military fiber-optic systems is high cost. In commercial fiber optic systems, the transmission bandwidth has been enhanced without substantially increasing cost by using WDM networks. WDM technology also has great potential to reduce the cost for military fiber-optic systems by cutting down the number of fiber optic lines and connectors.
In addition, since analog and digital data transmission is dominant in military fiber-optic systems, it is very important to have packet-level and cell-level switching capability in the WDM system so that efficient data transmission can be achieved. To realize packet-level and cell-level switching capability, there is a need for optical filters that are capable of ultra-high-speed (nanosecond) manipulations and of very fast tuning speed.
On the other hand, it is also a very challenging task to develop such a ultra-high speed dynamic WDM network due to the fact that network functionality requires dynamic elements to perform signal processing manipulations at different levels of complexity for circuit as well as packet-level and cell-level switching networks. This functionality includes filtering, routing, add-drop multiplexing, wavelength conversion, optical cross-connects, header reading, and so on. Among these functions a key element is a tunable optical filter.
Since current commercially available dynamic elements, such as Fabry Perot (FP) tunable filters are relatively slow, current dynamic WDM technology relies on relatively low dynamics (i.e., up to millisecond speeds), which is most adequate for circuit switched applications. However, network functionalities such as packet-level or cell-level switching needs much faster speed (i.e., in the nanosecond range). Due to the lack of commercially available ultra-high speed dynamic elements such as tunable filters, currently, packet-level or cell-level switching still has to be implemented by electronics, which limits the huge bandwidth benefit of light signals, increases cost and weight, and reduces the robustness against EMI. In other words, the lack of optical packet-level and cell-level switching becomes a bottleneck for advanced fiber optic links in high speed dynamic WDM networks.
To meet the needs of WDM fiber optic networks, in recent years, a variety of tunable optical filters have been developed. These include Fabry Perot (FP) interferometer tunable filters, Ferroelectric liquid crystal FP filters, micro machined device filters, Mach-Zehnder interferometer (MZI) filters, Fiber Bragg grating (FBG) filters, acousto-optic tunable filters (AOTF), electro-optical tunable filters (EOTF), arrayed waveguide grating (AWG) filters, optical MEMs, active filters. Filter performance is evaluated by filter parameters that include insertion loss, bandwidth, sidelobe suppression, dynamic range, tuning speed, and cost.
Referring to
FIG. 1
, Table 1 summarizes the performance of the above types of filters. Among these filters, FBG and Fiber FP filters are most commercialized, mainly due to the fact that no medium transformation is required so that the filters are low cost, robust and easy to use. However, FBG and Fiber FP filters are inherently limited in tuning speed to the millisecond range due to their thermal or mechanical mechanisms. Thus, FBG and Fiber FP filters can not be used for packet-level or cell-level switching, in which nanosecond tuning speed is required. On the other hand, although tunable filters based on faster mechanisms such as electro-optic effect can have nanosecond tuning speed, they are still on the research stage. A major impediment to commercialization may be due to complexity and cost. Since this category of filters is not fiber based, medium transformation is required when connected in a fiber optic WDM network. This increases the complexity of the usage and cost.
ETOF filters have wide bandwidth and strong sidelobes. To reduce the bandwidth and strong sidelobes of EOTF, tunable narrow-band filters have been constructed with photorefractive LiNbO
3
fibers and bulk crystals. Bragg gratings are holographically written inside the LiNbO
3
materials so that narrow bandwidth with low sidelobe can be achieved. Since photorefractive LiNbO
3
materials are also electro-optic materials, the refractive index of the material can be fast tuned by the external electric field. The tuning speed can be in the nanosecond range, which is fast enough for the packet-level and cell-level switching. When the refractive index is changed, the effective Bragg grating period is also changed so that the wavelength response of the filter can be tuned. Although a very narrow bandwidth low sidelobe fast tuning speed filter can be synthesized, the tuning range of the filter is very limited. The wavelength tuning range of this Bragg grating filter can be estimated as
Δλ
≈
Δ
n
n
λ
,
(
1
)
where n is the refractive index of the material, &Dgr;n is the refractive index change, and &lgr; is the operating wavelength. Substituting typical values for LiNbO
3
materials, (i.e., n=2.3, &Dgr;n=10
−3
, and &lgr;=1500 nm) into Equation (1), one can obtain &Dgr;&lgr;<1 nm. Obviously, this tuning range is too small for practical use in a dense WDM network. In addition, medium transformation is also required in this type of filter, which further increases the difficulty in commercialization.
Long period gratings (LPG's) that are photoinduced fiber devices couple light from the core of a single-mode optical fiber into a fiber cladding at discrete wavelengths, producing one or more attenuation bands in the fiber transmission. The phase-matching condition of a LPG can be written as
&lgr;
p
=(
n
core
eff
−n
clad
eff
)&Lgr;, (2)
where &lgr;
p
is the wavelength of the pth-order resonance peak, &Lgr; is the period of the grating, and n
core
eff
and n
core
eff
are effective indices of core and cladding, respectively. Based on Equation (2), the wavelength tuning range &Dgr;&lgr; for the long period grating can be estimated as
Δλ
=
Δ
(
n
core
eff
-
n
clad
eff
)
n
core
eff
-
n
clad
eff
λ
p
,
(
3
)
where &Dgr;(n
core
eff
−n
core
eff
) is the difference of the effective refractive index change between the core and cladding. Since the effective refractive indices of core and cladding can be very close, i.e., n
core
eff
−n
clad
eff
<<1, a small change in the ambient refractive index can result in a big wavelength shift. Thus, a wide tuning range can be achieved. A 50 nm tuning range filter is described by A. Abramov, A. Hale, R. Windeler and T. Strausser in an article entitled
Widely Tunable Long
-
period Gratings
in Electrical Letters, vol. 35, pages 81, 82, 1999. Although wide tuning range was achieved, the tuning speed is very limited due to the use of low speed thermal tuning.
SUMMARY OF THE INVENTION
The optical filter of the present invention is tunable to different wavelengths in a portion of the spectrum, such as the infrared portion. The optical filter includes a core with a long period grating di
Kurtz Paul
Liu Hongyu
Reichard Karl
Yin Shizhuo
Zhang Qiming
Chang Audrey
Lavarias Arnel C.
Ohlandt Greeley Ruggiero & Perle L.L.P.
The Penn State Research Foundation
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