Patent Application
20070110353
The invention relates to wavelength selective switches, and relates in particular to wavelength selective switches that may be used in a multiplexing switching system.
Wavelength selective switches include a variety of structures such as fiber grating couplers that permit all but a specified wavelength of light to pass through the coupler. For example, U.S. Pat. No. 4,737,007 discloses a wavelength selective contra-directional coupler for use in wavelength division multiplexing that includes a Bragg diffraction grating having a fixed period. U.S. Pat. No. 6,549,707 discloses a grating type optical filter that provides an apodized response by varying the distance between grating elements.
Superimposed multiple Bragg gratings are disclosed in A. Othonos, X. Lee and R. M. Measures, “Superimposed Multiple Bragg Gratings”, Electronics Letters, Vol. 30, No. 23, pp.1972-1974, (1994) in which more than one grating is inscribed at the same location on an optical fiber to provide a comb filter by varying the period of different grating elements. Serial inscription of a second grating, however, typically results in a reduction of reflectance of an first grating as disclosed in W. C. Swann, M. A. Hubbard and S. L. Gilbert, “Hybrid Multiple Wavelength Reference Using Fiber Grating and Molecular Absorption”, Proc. Bragg Gratings, Photosensitivity and Poling in Glass Wavelength, OSA Technical Digest (Optical Society of America), pp. 63-65 (1999). It has also been disclosed that performance may be degraded with an increase in the number of superimposed gratings in H. Ishii, H. Tanobe, F. Kano, Y. Tohmori, Y. Kondo, and Y. Yoshikuni, “Quasicontinuous Wavelength Tuning in Super-Structure-Gratings (SSG) DBR Lasers”, IEEE Journal of Quantum Electronics, Vol. 32, No. 3, pp. 433-441 (1996).
Multi-wavelength diffraction and apodization are disclosed using binary superimposed gratings in I. A. Avrutsky, M. Fay and J. H. Xu “Multiwavelength Diffraction and Apodizatrion Using Binary Superimposed Gratings”, IEEE Photonics Tech. Letters, Vol. 10, No. 6, pp. 839-841, (1998) in which the grating is segmented into sections of grouped segments permitting step-like apodization while maintaining a binary index and base segment size.
A sampled grating is disclosed in V. Jayaraman, Z. Chuang, and K. Coldren, “Theory, Design, and Performance of Extended Tuning Range Semiconductor Lasers With Sampled Gratings”, IEEE Journal of Quantum Electronics, Vol. 29, No. 6, pp. 1824-1834 (1993) in which grating elements are removed in a periodic fashion.
There is a need however, for a more efficient and economical wavelength selective switch that may be used in a multiplexing switching system, and that may be modified during use.
The invention provides a wavelength selective switch device in accordance with an embodiment that includes an elongated signal communication path extending from a first end of the device to a second end of the device. The signal communication path extends through a plurality of regions of varying indices of refraction, and the plurality of regions includes adjustable regions that are each coupled to an adjustable voltage source for changing a voltage potential across each of the adjustable regions such that the index of refraction of said adjustable regions may be changed by changing the voltage potential across each adjustable region.
The following description may be further understood with reference to the accompanying drawings in which:
The drawings are shown for illustrative purposes and are not to scale.
The invention provides a system for reconfiguring a fixed pitch grating in a programmable fashion to allow selective transmission or deflection of an arbitrarily chosen set of closely-spaced wavelengths of signals, such as optical signals. The grating may be constructed with a fixed periodicity, and provides for analog tuning of the refractive index of many of the individual grating elements. Either each element or every other element for example, may be independently addressed to provide one or more periodicities for wavelength selective responses.
The grating, therefore, may serve as a programmable add/drop element for a multiplexing system, and may be suitable for inclusion as an element in an N×N array of such gratings, each of which may be independently addressed. This may permit construction of a switch that provides fast, compact, non-blocking routing of many spatial channels, each containing many different wavelengths of signals carrying independent data.
As shown in
To reflect a single wavelength, the pitch of the grating is chosen such that λw=2
n(z)=
where Δn is the amplitude of the index of modulation along the grating. This equals:
n(z)=
As shown in
As shown in
If two different wavelengths are to be coupled in the Bragg grating, their design index contrast profiles may be added together to obtain:
n′(z)=
This may be expressed as:
n′(z)=
where 1λ′=(1/λ1+1/λ2), and where F(z) is a slowly varying quantity. This suggests that the pitch of the grating λg′=λ′/2
In further embodiments, a multiplexing circuit 100 may be provided as shown in
The slowly-varying modulation in the grating profile along its length F(z) is readily implemented as long as analog control of each adjustable element is obtained. This provides a slowly-varying index contrast function, whereas the fast-varying cosine term serves to achieve the desired average pitch. The pitch, therefore, may be kept fixed, the average index may need to be tuned, but only over a fractional range corresponding at most to the largest fractional separation among wavelengths. In a directional wavelength division multiplexing system, the fractional separation may be about 5%. The contrast term may also remain slowly-varying.
The following procedure may be followed to determine the design of such a grating. First an idealized (pre-sampled) Fourier component n(z) as discussed above. The sum of the refractive index profiles of the original single-component grating nj(z) should also be obtained. For each fixed-length period of the programmable grating, an average of n(z) over that period is obtained. This will be the value of
For a 100 GHz wavelength division multiplexed system, less than 0.3 nm selectivity may be obtained at −0.5 dB, less than 0.8 nm at −3 dB, and less than 1.0 nm at −25 dB.
Systems of the invention may be used, for example, as a single spatial port, and in a system for selectively dropping an arbitrary reconfigurable set of channels. This may be useful in creating networks that use light to reconfigure their connections. Protocols such as MPAS have been developed to utilize such components. The elements, therefore, may serve as one element in an array to enable construction of a compact integrated switch with N spatial inputs, N spatial outputs, and M wavelengths of each of these input/output fibers.
Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the invention.
