Optical: systems and elements – Optical modulator – Light wave directional modulation
Reexamination Certificate
2003-06-04
2004-11-23
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave directional modulation
C359S312000
Reexamination Certificate
active
06822785
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to apparatuses and method for providing tunable filter for optical fiber signal communication systems. More particularly, this invention relates to new configurations and methods for implementing a far off-axis, large Bragg's deflection angle, non-collinear Acousto-optical tunable filter having a miniaturized size and very narrow bandwidth suitable for DWDM applications with 25 Ghz, 50 Ghz and 100 Ghz channel spacing adapting no moving parts or thermal tuning to achieve high speed reliable tuning.
BACKGROUND OF THE INVENTION
Contemporary means of tunable filters for the optical telecommunications market use mechanical actuators such as piezzo electric devices, MEMS, step motors along with high-density gratings to achieve narrow bandwidth wavelength selection and tuning. Still others use thermal means on a grating device, which is either by direct heating/cooling or by indirect (such as current) of heating/cooling, to change the spacing of a thermally sensitive grating to achieve wavelength tuning. These methods, although feasible and commercially available, are very slow in tuning speed, e.g., tuning speed in the neighborhood of hundreds of milliseconds or seconds. Stability often becomes a problem too caused either by the slow response of sensing the temperature for thermally tuning the devices or caused by the high susceptibility to vibration and shock when a moving part is implemented for tuning the devices.
In contrast, tuning technologies implemented with Acousto-optical tunable filter (AOTF) using the birefringent crystals would have a higher likelihood to resolve the above-mentioned difficulties. However, tuning with conventional AOTF techniques is also faced with limitations of size and bandwidth. A typical AOTF with collinear configuration with TeO2 in share mode as that disclosed by Chang, et al as will be discussed below can achieve nanometer filter bandwidth but with very big sizes, e.g., a 2 nm bandwidth collinear AOTF would require a crystal 30 mm length. Similar constraints exist for non-collinear configuration with TeO2 in share mode and with low RF frequency applications also disclosed by Chang, et al.
Typical AOTF applying birefringent crystals are produced with specific configurations, namely, when a crystal is cut, the PZT electrodes platting surface is cut, instead of perpendicular to, but a mall angle (Qa) from the crystals [110] axis. All crystals have three principle axis [100], [010] & [001] as designated in [x y z] axes for optical interactions. For homogeneous crystals, all optical properties are the same along all three axes. For inhomogeneous crystals, optical properties along different principle axis may behave differently as that of birefringent crystals. However, acoustic waves may not necessarily travel along the optical axis. There are two types of acoustic traveling waves: Longitudinal and Share. The Longitudinal wave is a compression wave and the Share wave is simply to the light wave oscillating up/down while traveling horizontally for example. For “on-axis” share wave TeO2, it travels along [110], in Z plane and along the diagonal of X and Y-axes. For “Off-axis” share wave, it typically refers to what
FIG. 1A
depicts, a small angle off [110] and Z plane. For angles larger then 10 degrees, it is often referred to as “far off axis designs”. The off axis combination as shown may eliminate the need for tilted crystals. It may improve optical degeneracy, which happens often with symmetrical designs as that of the configuration shown in
FIG. 1A
as will be further discussed below wherein the crystal can be used with either surfaces functioning as the “front” surface.
When tuning with a radio frequency (RF) signal with a high frequency, the acoustic wave inside the crystal decays rapidly making the birefringent crystals, e.g., TeO2, almost unusable for RF signal with frequency higher than 200 MHz. For that reason, almost all non-collinear share-mode TeO2 designs are implemented with low RF frequency lower than 100 MHz. With an optical wavelength of 1.55 &mgr;m, the RF frequency is in a range of 20-50 MHz and the filter bandwidth is very wide. In terms of grating effect, its grating line density is too low to be, useful for use on tunable filters in a telecommunication system, in particular for a divisional wavelength demultiplexing (DWDM) system, with which the passband requirement is less than 0.2 nm. One way to narrow the filter bandwidth is to increase the crystal length along optical path direction, which results in a very long crystal. Typically for 2 nm wide Acousto-optical tunable filter (AOTF), the crystal length is more then 30 mm long. For modern telecommunication systems that required miniaturized devices, conventional technologies of applying acoustic-optical tuning techniques are not able to provide effective solutions for making a useful AOTF suitable for tunable laser implementations.
Chang disclosed in several United States patents, e.g., U.S. Pat. Nos. 5,329,397, 4,720,177, 4,582,397, 4,343,503, and 4,052,121, techniques of using collinear and non-collinear electrically tunable Acousto-optical filters. These filters are implemented with interactions between the acoustic and optical waves in the acoustically an-isotropic and optically birefringent crystals.
FIGS. 1A and 1B
are diagrams for showing the Optical Refractive Index Ellipsoid in a birefringent TeO2 crystal cut along particular axis and as a result shown in the figures that the X axis is along the diagonal of two other principle axes, the Y axis is shown as the principle axis. The symbol K designates wave vectors, e.g., the direction of incident light is projected along a direction shown by Ki and the direction of projection of the diffracted wave is shown in a direction of Kd and the direction of the acoustic wave is along a direction shown as Ka. For a share wave, the direction along [110] is the propagation direction. Therefore, in
FIG. 1
, the acoustic wave direction Ka is slightly non-parallel to [110] indicating the “off-axis” applications. The length of Ka measures the RF frequency (fa). In
FIG. 1A
, according to the prior art AO non-collinear filter of Chang, with TeO2 in Share-mode, in which all three waves are traveling along different directions satisfying Bragg's law of diffraction with Ki and Kd are on the same side of the quadrant thus resulting a low value of Ka, i.e., low RF frequency. At high frequency, the acoustic wave inside the crystal decays rapidly, especially in the rang of visible wavelength, the AO filter is almost unusable for an RF frequency fa greater than 200 Mhz. For this reason, almost all non-collinear Share-mode TeO2 Acousto-optical filters are with implemented with low RF frequency <100 Mhz. With 1.55 um optical wavelength, the RF frequency fa is in the range of 20-50 Mhz and the filter bandwidth becomes very wide. Hence, the grating line density is too low for application as tunable filters in a telecommunication system with a passband less than 0.2 nm. In order to narrow down the filter bandwidth, the length of the crystal has to increase along the direction of the optical path thus preventing further miniaturization of the AO filters implemented with such a technologies.
FIG. 1B
shows a similar configuration with collinear design with TeO2 in Share-mode, in which all three waves are traveling along the same direction also satisfying Bragg's law of diffraction. Again, for an optical wavelength of 1.55 um, the RF frequency (fa) is about 23 Mhz and the collinear filters encounter similar technical limitations as that discussed above for a non-collinear AO filters. Since the fiber optical signal transmissions are now more commonly implemented in the telecommunication and network systems, and as the tunable lasers using the Acousto-optical tunable filters are key and important devices for such systems, there is an ever-urgent demand to resolve these limitations
Chu Raymond R.
Jiang Qing
Acceeze, Inc.
Hanig Richard
Lin Bo-In
Sugarman Scott J.
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