Optical waveguides – With optical coupler
Reexamination Certificate
1998-12-14
2002-02-26
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
C385S014000, C385S018000, C385S019000, C372S020000
Reexamination Certificate
active
06351577
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optical devices and more particularly to micromachined tunable optical filters and techniques for fabricating such filters.
BACKGROUND OF THE INVENTION
Tunable optical filters are useful devices for wavelength-division-multiplexing (WDM) systems, performing functions such as optical monitoring, channel selection in wavelength-based routing, noise filtering and coherent crosstalk reduction. As the number of wavelengths used in these systems grows, it is particularly desirable to have inexpensive tunable filters. Existing tunable filters have a relatively high unit cost, due to the labor-intensive fabrication and assembly processes which are used. Among these, tunable Fabry-Perot (FP) filters based on mechanical scanning of a FP cavity length are generally best suited to meet the high performance required in WDM systems, due to important optical properties such as, for example, low loss, polarization insensitivity, large tuning range and high bandwidth resolution. In addition to the problem of high cost, the tuning speed of bulk mechanical filters is typically rather slow, i.e., on the order of milliseconds, as the tuning process requires moving a relatively large mass.
A tunable FP filter is characterized by a cavity enclosed between two mirrors. The transmission function of a symmetric FP filter with identical mirrors is given by:
T
=
(
1
-
R
)
2
(
1
-
R
)
2
+
4
R
sin
2
(
δ
/
2
)
(
1
)
where R is the mirrors' power reflectivity and &dgr; is the accumulated phase a light wave acquires in each round-trip inside the cavity, given by:
δ
=
4
π
nL
λ
(
2
)
Here, n is the index of refraction of the material comprising the cavity (n=1 for air), L is the cavity length, and &lgr; is the operating wavelength. The resonant wavelengths of this filter are determined by the phase &dgr; given above, and the separation between the wavelengths, called the free-spectral-range (FSR), is given approximately by:
Δ
λ
FSR
=
λ
2
2
nL
(
3
)
The passband width of the resonant peak is determined by the filter finesse F, which is a measure of the overall cavity and mirror losses:
δλ
=
Δλ
FSR
F
(
4
)
For an ideal lossless filter, the finesse is given by F=&pgr;R/(1−R). The wavelengths that the filter transmits can be tuned, among other ways, by mechanically tuning the cavity length.
One type of conventional optimized tunable filter design approach sets the free spectral range to be about equal to the required tuning range. For WDM systems, a typical tuning range is in the range of 40-100 nm, and the center wavelength is approximately 1.55 &mgr;m. Using Eq. (3), this translates to a cavity length of 10-30 &mgr;m. If the WDM system uses 0.8 nm (100 GHz) channel spacing, a tunable filter used as a channel selector would require a filter bandwidth≦0.5 nm, and from Eq. (4), a finesse of 80-200. This means that the required mirror reflectivity would be close to 98-99%.
In order to reduce cost substantially and also to enable faster switching speeds, micromachined FP filters have been developed. Examples of micromachined FP filters are described in M. C. Larson, and J. S. Harris Jr., “Broadly-tunable resonant-cavity light-emitting diode,” IEEE Photon. Technol. Lett., Vol. 7, p. 1267, 1995; E. C. Vail et al., “GaAs micromachined widely tunable Fabry-Perot filters,” Electron. Lett., Vol. 31, p. 228-229, 1995; P. Tayebati et al., “Widely tunable Fabry-Perot filter using Ga(Al)As—AlO
x
deformable mirrors,” IEEE Photonic Technol. Lett., Vol. 10, pp. 394-396, 1998; J. Peerlings et al., “Long resonator micromachined tunable GaAs—AlAs Fabry-Perot filter,” IEEE Photonic Technol. Lett., Vol. 9, pp. 1235-1237, 1997; and A. Spisser et al., “Highly selective and widely tunable 1.55 &mgr;m InP/air-gap micromachined Fabry-Perot filter for optical communications,” IEEE Photonic Technol. Lett., Vol. 10, pp. 1259-1261, 1998.
These micromachined filters share a common design approach, which defines vertically the entire FP structure, including its cavity and mirrors, by a sequence of multi-layer thin-film depositions on a wafer substrate. In this design approach, both top and bottom cavity mirrors are typically comprised of several quarter-wave-thick layers with alternating high and low refractive indices, while the layer which is used to define the cavity is a sacrificial layer which is later etched away in one of the final processing steps. The etching process forms a membrane or a cantilever structure. Cavity tuning is obtained electrically by pulling the membrane or the cantilever toward the substrate with electrostatic force, which changes the cavity spacing between the mirrors.
The vertical design approach for micromachined filters restricts the initial cavity length to the thickness of the sacrificial layer, which is limited in most cases to only a few microns at the most. This results in a very large spacing between the periodic transmission peaks of the filter, and as a consequence, a very high mirror reflectivity is required to obtain a filter bandwidth which is narrow enough to meet dense WDM requirements. A typical cavity length in this type of micromachined filter may vary in the range of 2-5 &mgr;m, which translates to a FSR of 250-600 nm. To achieve a filter bandwidth of 0.5 nm, the required finesse and mirror reflectivity are 500-1200 and 99.5-99.8%, respectively.
Such high mirror reflectivities can be obtained with a large number of quarter wave mirrors, or alternatively by substantially increasing the index contrast between the layers, as described in, e.g., U.S. Pat. No. 5,739,945 issued to P. Tayebati and entitled “Electrically tunable optical filter utilizing a deformable multi-layer mirror,” and the above-cited A. Spisser et al. reference. Either of these approaches, however, unduly complicates the fabrication process. Furthermore, the filter is exposed to substantially higher throughput losses in the presence of any type of defect or deviation from an ideal FP structure, e.g., mirror curvature or tilt, and intracavity diffraction of a non-collimated illumination. As the channel spacing of WDM systems becomes even smaller, the requirements on the filter finesse and reflectivity become even more difficult to achieve with this short cavity design. It should also be noted that since the absolute change in cavity length required to shift the filter passband over a certain wavelength range is a linear function of the initial cavity length, shorter cavities are more sensitive to small fluctuations in cavity length. For typical micromachined filters with cavity length of only a few microns, a cavity length change on the order of 0.1 nm (1 Å) would shift the filter passband by 0.1 nm, making it difficult to stabilize the filter transmission wavelength.
As noted previously, a conventional non-micromachined design approach involves choosing the transmission peak spacing to be about equal to the required tuning range, such that a typical WDM requirement in the range of 40-100 nm translates to cavity spacing of 10-30 &mgr;m, and respective mirror reflectivities of 98-99%. In an attempt to achieve this type of cavity spacing for a micromachined tunable filter, a design disclosed in the above-cited J. Peerlings et al. reference patterns the top and bottom mirrors separately on two different substrates, and then assembles them together with a cavity spacing that is defined by fixed spacers between the substrates. In the disclosed device, the cavity length was 30 &mgr;m and the resulting filter periodicity was 56 nm. The obtained filter bandwidth was higher than expected from the mirror reflectivity, only 1.2 nm instead of about 0.5 nm, due to mirror tilt which probably developed during the assembly. However, this type of design also has a number of drawbacks. The additional assembly step adds to the device cost, since careful assembly is required to avoid degradation in dev
Aksyuk Vladimir Anatolyevich
Bishop David John
Sneh Anat
Font Frank G.
Lucent Technologies - Inc.
Punnoose Roy M.
Ryan & Mason & Lewis, LLP
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