Patent Application
20080100846
The present invention is directed generally to signal processing and, more particularly, to systems and methods for all-optical signal regeneration based on free space optics.
In communication systems, signals are often transmitted over very long distances. Transmission over such long distances causes signals to become degraded, for example, by attenuation, interference, and other impairments. Accordingly, some systems use signal repeaters or regenerators to receive a degraded signal and restore its original shape and amplitude.
Prior art fiber optics communication systems have used electrical signal repeaters that receive the light signal from the optical transmission medium, transform that optical signal into an electric signal, restore the electrical signal's shape and amplitude, and then transform the electrical signal back to light for transmission over another optical medium. This process, also called regeneration, can be further complemented by the conversion of the original optical wavelength to another optical wavelength.
Advances in fiber optics technology have allowed for the development of all-optical wavelength conversion, which performs the conversion without changing the light signal to an electric signal. However, the inventors hereof have recognized that prior art all-optical converters typically suffer from the disadvantages of using optical fibers to couple internal components. For example, optical fibers are susceptible to environmental changes, including temperature and pressure variations. Moreover, management and alignment of optical fibers require large workspaces, thus creating serious constraints with respect to the footprint (size) of the device. Furthermore, long optical fibers may induce chromatic and polarization dispersion to the converted signal, thus increasing the final cost of the optical system.
In one exemplary embodiment of the present invention, a method for regenerating an optical signal comprises counter-propagating an input signal and a regenerating signal within an all-optical signal regenerator based on free space optics, where the all-optical signal regenerator based on free space optics comprises a Sagnac loop interferometer, and extracting a regenerated output signal from the Sagnac loop interferometer. In another exemplary embodiment of the present invention, an all-optical signal regenerator based on free space optics comprises a Sagnac loop interferometer, an optical signal input path coupled to a semiconductor optical amplifier of the Sagnac loop interferometer, a regenerating optical signal path coupled to the semiconductor optical amplifier of the Sagnac loop interferometer, and a regenerated optical output path coupled to the Sagnac loop interferometer.
It is an object of the present invention to provide a device and method for an all-optical signal regenerator based on free space optics (FSO). FSO, also called free-space photonics, refers to the transmission and manipulation of light beams through free space to deliver high-speed, broadband communications. By using FSO and eliminating or reducing the use of optical fibers, embodiments of the present invention provide an optical signal processing device that is robust to vibrations, temperature, and pressure variation. Furthermore, the use of an FSO-based Sagnac loop greatly reduces or eliminates sensitivity to phase variations, and yield a robust interferometer as against thermal fluctuations without affecting polarization. Certain embodiments of the present invention also permit the miniaturization of an optical signal regenerator device due to the use of small free space components rather than long optical fiber spans.
It is a further object of the present invention to reduce the final cost of optical regeneration devices by using unpackaged components with significantly lower cost than their optical fiber-based counterparts. It is yet another object of the present invention to provide a regeneration device and method that avoids chromatic dispersion to the converted signal and that can support any wavelength.
The foregoing has outlined rather broadly certain features and technical advantages of the present invention so that the detailed description that follows may be better understood. Additional features and advantages are described hereinafter. As a person of ordinary skill in the art will readily recognize in light of this disclosure, specific embodiments disclosed herein may be utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. Several inventive features described herein will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, the figures are provided for the purpose of illustration and description only, and are not intended to limit the present invention.
For a more complete understanding of the present invention, reference is now made to the following drawings, in which:
In the following description, reference is made to the accompanying drawings that form a part hereof, and in which exemplary embodiments of the invention may be practiced by way of illustration. These embodiments are described in sufficient detail to enable a person of ordinary skill in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that changes may be made, without departing from the spirit of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined only by the appended claims.
Turning now to
In this embodiment, elements 105, 145, and 150 define a signal input optical path, whereas elements 110, 120, 125, 140, 130, 135, and 155 define a regenerating signal optical path, and elements 165, 170, and 115 define a regenerated output optical path. In addition, a combination of elements 145, 150, 160, 155, 135, and 140 create Sagnac loop interferometer (Sagnac loop) 185.
In operation, signal input SMF and collimator 105 introduce input optical signal 101 into Sagnac loop 185 to create cross-gain modulation (XGM), cross phase modulation (XPM), and/or cross polarization modulation (XPP) within Sagnac loop 185. Meanwhile, regenerating signal input PM fiber and collimator 110 counter-propagate regenerating signal 102 into Sagnac loop 185. In a preferred embodiment, regenerating signal input PM fiber and collimator 110 may preserve the polarization status of regenerating signal 102 as linear polarization. The XGM, XPM, and/or XPP modulation created above is transcribed onto regenerating signal 102 introduced by regenerating input PM fiber and collimator 110 within SOA 160. Output SMF fiber and collimator 115 enables regenerated signal 103 (output signal) to exit Sagnac loop 185, for example, into an external fiber pigtailed device (not shown).
External polarization controller 125 controls the polarization state outside of Sagnac loop 185. Internal polarization controller 135 controls the polarization of clockwise and counter-clockwise propagating light components within Sagnac loop 185. Preferably, both internal and external polarization controllers 135, 125 are mounted on internal and external TEC and thermistors 130, 120, respectively, which control the temperature of polarization controllers 135, 125 by measuring it during regular operation and locking on a target temperature.
Non-polarizing beam splitter 140 splits regenerating signal 102 into one counterclockwise portion and one clockwise portion, both portions circulating within Sagnac loop 185. In one embodiment, the beam splitter ratio of non-polarizing beam splitter 140 is 50%. However, other ratios may be used. Non-polarizing beam splitter 140 may also act as a polarization splitter, known as a Polarization Beam Splitter (PBS). Non-polarizing beam combiner 145 combines input signal 101 with regenerating signal components. In one embodiment, the mixing ratio of non-polarizing beam combiner 145 is 50%. However, other ratios may be used. For example, if more signal power is required, the ratio may be changed to 60%:40%, or any other value. Non-polarizing beam combiner 145 may also combine different polarization components.
First SOA arm 150 may be an SMF fiber, where the tip of the fiber contains a collimator. First SOA arm 150 collects and transmits the light that is combined by non-polarizing beam combiner 145 into SOA 120, and collects light from SOA 120 toward output SMF fiber 115. In one embodiment, first SOA arm 150 may move along its optical axis thereby setting a time delay within Sagnac loop 185, as disclosed in pending U.S. patent application Ser. No. 10/623,280, filed Jul. 18, 2003, entitled “ALL-OPTICAL, TUNABLE REGENERATOR, RESHAPER, AND WAVELENGTH CONVERTER,” and hereby incorporated by reference. In other embodiments, delay may be achieved by introducing a material with a higher refraction characteristics (e.g., glass, liquid crystal, bi-refringent crystal, or the like) rather that by moving first SOA arm 150. Similarly, second SOA arm 155 may also be an SMF fiber, where the tip of the fiber contains a collimator. Second SOA arm 155 collects and transmits light out of SOA 120 towards output SMF fiber 115, and introduces the counter clockwise propagating CW into SOA 120. In this embodiment, second SOA arm 155 does not require translation along the optical axis.
Polarizer 165 may be a linear polarizer and is positioned at the output port in order to improve the extinction ratio of the output signal. Free space isolator 170 may be used to prevent reflections of light from returning into Sagnac loop 185 and affecting the performance of SOA 120. Input/output pins 175 connect to the voltage and control electronics of the internal components, such as TEC controllers 120, 130, and SOA 120. Finally, sealed package 180 maintains regenerator 100 closed and sealed from humidity and dirt effects.
It will be readily appreciated by one of ordinary skill in the art that various deviations from this exemplary embodiment fall within the spirit and scope of the present invention. For example, components 165 and 170 may be combined to a single component, which is commercially available, thereby reducing the total number of FSO parts. Further, one may integrate a PC controller with TEC control between non-polarizing beam splitter 140 and polarizer 165 in order to optimize performance. Moreover, a tunable filter (not shown) may be integrated into the package between free space isolator 170 and output SMF fiber and collimator 115. The tunable filter may prevent input signal 101 from leaving regenerator 100 at the output port, thereby keeping only the new regenerating signal within regenerator 100.
In addition, as one of ordinary skill in art will readily recognize in light of this disclosure, it is possible to automate the manufacturing process of regenerator 100 by placing FSO components on a mechanical stage utilizing automated manufacturing tools to achieve sub-micron accuracy, thereby substantially reducing production costs.
The embodiment of
With respect to
In an alternative embodiment, a variable optical attenuator (VOA) (not shown) may be integrated between laser 310 and input PM fiber and collimator 110 in order to control the required input regenerating signal power to Sagnac loop 320. Alternatively, if a VOA is not used, the power of laser 310 may be controlled by external electronics. One of the many advantages of regenerator 300 is the ability to eliminate the cumbersome package of an external regenerating laser and to use it in a simpler form within package 180, thereby reducing cost and size, and simplifying the integration of regeneration 300 onto a standard electronic card.
With respect to
In an alternative embodiment, a low saturation power SOA 210 is used. In this case, it may be beneficial to inject optional regenerating laser 402 into SMF and collimator 105 and in parallel to input signal 101 regeneration performed through regenerating input PM fiber and collimator 110. Optional regenerating laser 402 may help balance gain variations within SOA 210 while it operates within Sagnac loop 420, thereby eliminating peaking effects and distortion of the signal due to non-linearities in SOA 120. Optional regenerating signal 402 at SMF and collimator 105 may be an idler signal of arbitrary wavelength.
With respect to
Accordingly, regenerator 500 utilizes SOA integrated with a multi-mode interference coupler (SOA/MMI) 515. Input fiber 505 connects input signal 101 directly to first input port 517 of SOA/MMI 515. Second input port 516 may be pigtailed with a fiber and collimator and maintains regeneration light circulating within Sagnac loop 520 in zero order mode. Corner reflecting prism 510 reflects regeneration light within Sagnac loop 520. In one exemplary embodiment, corner reflecting prism 510 provides total reflection of the regenerating signal, thereby reducing the power requirements for regenerating signal 102 and input signal 101.
In one embodiment, MMI/SOA 515 may be similar to the one disclosed in U.S. Pat. No. 5,933,554, issued on Aug. 3, 1999, entitled “COMPACT OPTICAL-OPTICAL SWITCHES AND WAVELENGTH CONVERTERS BY MEANS OF MULTIMODE INTERFERENCE MODE CONVERTERS,” and hereby incorporated by reference. An MMI may be a device based on an InP waveguide (not shown) that has 2 input ports and 2 output ports. The InP waveguide is designed so that a zero order mode laser light that enters in port 516 remains in zeroth mode at output port 518. Hence, this embodiment may provide a selective filter that prevents input signal 101 from circulating in Sagnac loop 520, thereby letting only regenerating light to circulate and interfere to create signal output. Furthermore, the MMI allows input signal 101 and regenerating signal 102 to be in the same wavelength without interfering in Sagnac loop 520.
Moreover, the input signal to MMI/SOA 515, which itself comprises two signals (the first being input signal 101 at the first transverse mode and the second being regenerating signal 102 at the second transverse mode), enters the SOA portion of MMI/SOA 515 in which a cross-gain process takes place. When the cross-gain signal exits the SOA and is coupled to a single mode fiber of output port 518, only the first-mode (zero order) signal can enter fiber 518. Thus, these two signals 101 and 102 may be distinguished even if they have the same wavelength, and having the MMI/SOA 515 to the single mode fiber of output port 518 provides a transversal mode filter, thus replaces a spectral filter.
An advantage of this exemplary embodiment is that it allows for input signal 101 and regenerating signal 102 to have the same exact wavelength, since these two signals are not in the same spatial mode when entering SOA 515. This allows regenerator 500 to regenerate a signal without the need to change its wavelength. Further, this embodiment may also block input signal 101 from leaving regenerator 500 without the use of optical filters such as, for example, a fixed wavelength or tunable filter at the output port of regenerator 500.
Turning to
With respect to
In this embodiment, since the SOA and MMI are integrated on the same chip, they are coupled to regenerator 700 via three (rather than two) lenses. Moreover, in this configuration, a single beam splitter 740 is needed. Beam direction may be controlled by Risley cells 705, which also couple the external fibers (carrying signals 101-103) to free-space regenerator 700. Dove prism 730 may be used to keep all the external ports on one end of regenerator 700, but is not essential to its proper operation. In one embodiment, due to the small dimensions of the chip that includes the MMI/SOA 700, it is more convenient that each input port be at a separate face of the chip.
Although certain embodiments of the present invention and their advantages have been described herein in detail, it should be understood that various changes, substitutions and alterations can be made without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present invention is not intended to be limited to the particular embodiments of the process, machine, manufacture, means, methods, and steps described herein. As a person of ordinary skill in the art will readily appreciate from this disclosure, other processes, machines, manufacture, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, means, methods, or steps.
