Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2001-12-27
2004-10-12
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S578000, C359S589000
Reexamination Certificate
active
06804059
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to tunable optical filters for optical communications systems, and more particularly to a tunable optical filter that utilizes electroholographic filter elements.
BACKGROUND OF THE INVENTION
The desire to transmit more information over optical fibers has led to the multiplexing of multiple optical carriers at different wavelengths into the same optical fiber (wavelength division multiplexing (WDM)). Improved optical sources have enabled the generation of optical carriers with narrower bandwidths, which in turn has allowed more optical carriers to be multiplexed into the same optical fiber. As the number of multiplexed optical carriers increases, there is a need for tunable optical filters that can be adjusted to isolate specific optical carriers from a WDM signal.
Some known tunable optical filters utilize microelectromechanical systems (MEMS) or diffraction gratings to selectively filter out a specific optical carrier. These known MEMS-based and grating-based tunable optical filters utilize mechanically moving parts to tune the filters over a range of wavelengths. Utilizing mechanically moving parts to tune filters makes the filters susceptible to vibration and mechanical failure.
An alternative technique for filtering an optical signal utilizes a volume hologram written into a photorefractive crystal to create a narrow-band optical filter. Utilizing a volume hologram written into a photorefractive crystal to create a narrow-band optical filter is described by George A. Rakuljic and Victor Leyva in “Volume Holographic narrow-band optical filter”,
Optics Letters
, vol. 18, No. 6 pp. 459-461 (1993). Although writing volume holograms into photorefractive crystals works well to create a filter with a fixed narrow band, a single fixed narrow-band filter does not fulfill the need for wide-band tunable optical filters for use in WDM-based optical communications systems.
Recently, voltage controlled volume holograms written into photorefractive crystals have been incorporated into optical communications devices such as optical switches. Optical switches that incorporate voltage controlled volume holograms written into photorefractive crystals are described in detail in international patent applications published under the Patent Cooperation Treaty (PCT) entitled “Electro-Holographic Optical Switch” (WO 00/02098) and “Electroholographic Wavelength Selective Photonic Switch For WDM Routing” (WO 01/07946).
In general, the principle of operation of photorefractive crystals involves writing a grating pattern into the crystal by establishing a periodic space-charge field. In the paraelectric region, the electro-optic effect is quadratic and is given by:
&Dgr;
n
=½
n
o
3
gP
2
(1)
where &Dgr;n is the birefringence, n
o
is the refractive index, g is the appropriate electro-optic coefficient, and P is the static polarization. In the linear region, P is given by P=∈
o
(∈−1)E, where ∈ is the dielectric constant (which when close to the phase transition follows ∈/∈
o
>>1) and where ∈
o
is the permittivity of a vacuum 8.854×10
−12
F/m. When the interference patterns are written into the crystal using two optical beams, the space-charge fields, E
sc
, that are created are spatially correlated to these patterns. These space-charge fields induce refractive index gratings in the presence of an external electric field, E
o
, that is given by:
&Dgr;
n
=½
n
o
3
g∈
o
2
(∈−1)
2
(2
E
o
E
sc
+E
sc
2
) (2)
where E
o
is the externally applied field. It is assumed that the polarization is in the linear region.
The mechanism described with reference to equation (2) provides the ability to selectively activate or de-activate the filtering capability of a photorefractive crystal based on the presence or absence of an external electric field.
FIG. 1
depicts a photorefractive crystal
102
with an electroholographic (EH) grating
104
that is activated through a voltage source. When the voltage source is in the “off” state (no voltage applied), the optical signal passes, undiffracted, through the electroholographic grating as indicated by dashed line
106
. However, when the voltage source is in the “on” state (voltage applied), the optical signal at the center wavelength of the gratings is diffracted by the grating as indicated by the solid line
108
. Because the grating is wavelength specific, only optical signals within the bandwidth of the grating are diffracted.
It is well known in the field of optics that for incident light of wavelength &lgr;, the response of a grating is given by:
d
sin (&thgr;)=
m&lgr;
o
/2
n
(3)
where, d is the spacing between the lines of a grating and &thgr; is the incident angle of light, m is an integer, n is the index of refraction of the crystal and &lgr;
o
is the wavelength of the incident light. For a grating with fixed d and &thgr;, any change in the index of refraction results in a variation of the center wavelength of the grating. As described above, the index of refraction of a photorefractive crystal and in turn the center wavelength of the EH grating changes as a function of the externally applied electrical field. For example, a grating written into a material such as strontium barium niobate (SBN):75 can experience as much as 0.5% variation in its index of refraction in the presence of a 330 volts/cm external electric field. For operation around 1,500 nm, a 0.5% variation in the index of refraction allows a grating to be tuned over a range of about 7.5 nm. Although a single electroholographic grating written into a photorefractive crystal can be tuned over a range of about 7.5 nm, WDM communications systems operate over a larger bandwidth, for example, 100 nm and therefore a single filter is not well suited for WDM applications.
In view of the need for tunable optical filters and the problems with mechanically tuned filters, there is a need for a robust tunable optical filter with a tuning range that is compatible with WDM communications systems.
SUMMARY OF THE INVENTION
Multiple electroholographic (EH) gratings with different center wavelengths are utilized to create a tunable optical filter that can be tuned over a wide wavelength range. The EH gratings are connected such that an input optical signal can pass through at least one of the EH gratings. The EH gratings are activated and tuned by electrode pairs that are controlled through a voltage controller. The tunable optical filter is coarse tuned by activating the EH gratings having a wavelength range that includes the center wavelength that is to be filtered and fine tuned by adjusting the voltage that is applied across the activated EH gratings. Because the tunable optical filter is tuned simply by the application and adjustment of voltage across EH gratings, the tunable optical filter can be accurately controlled and is less susceptible to vibration and mechanical failure. In addition, because the filter utilizes multiple EH gratings with different center wavelengths, the bandwidth of the filter can be extended beyond the bandwidth of any single EH grating.
An embodiment of a tunable optical filter includes multiple EH gratings with different center wavelengths. The EH gratings are optically connected such that an input optical signal can pass through at least one of the EH gratings. The EH gratings are activated to filter the input optical signal in response to an applied voltage.
In an embodiment, the EH gratings of the tunable optical filter are activated by electrode pairs that are associated with the EH gratings and the electrode pairs are controlled by a voltage controller. In an embodiment, EH gratings of the same center wavelength are controlled simultaneously by the voltage controller.
The tunable optical filter can be tuned over a range of wavelengths in response to adjustments in the applied voltage. In an embodiment, the tunable wavelength ranges of the EH gratings combine to form a continuous wavele
Baney Douglas M.
Depue Marshall T.
Miller Jeffrey N.
Motamedi Ali R.
Agilent Technologie,s Inc.
Amari Alessandro
Robinson Mark A.
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