Abstract:
A dynamic channel power equalization arrangement compensates for uneven channel powers of a wavelength multiplexed optical signal using diffractive gratings and semiconductor attenuator. A novel optical design is used to provide power equalization function and optical spectrum analyzer function in one single optical arrangement, as compared to the prior art in which there is only the power equalization function in such devices. This invention provides a simple cost-effective means for a complete solution for managing individual channel powers of wavelength-division multiplexed (WDM) signals.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT  
         [0002]    Not applicable  
         REFERENCE TO SEQUENCE LISTING, A TABLE OR A COMPUTER PROGRAMM LISTING COMPACT DISK APPENDIX  
         [0003]    Not applicable  
         BACKGROUND OF THE INVENTION  
         [0004]    The present invention relates to method and apparatus for providing dynamic gain equalization (DGE) in high-speed optical transmission networks and systems.  
           [0005]    High capacity optical fiber transmission systems suffer from un-flat gain or loss during transmission, which strongly limits the transmission distances as well total capacity. There are four fundamental sources that cause uneven wavelength or frequency channel power levels. First, the optical transmission fiber has wavelength-dependent loss due to Rayleigh scattering. Second, optical amplifiers used to compensate for fiber loss have certain amount of gain tilt or ripples. Depending on the input optical power, optical amplifiers such as Erbium-doped fiber amplifiers (EDFAs) have different output gain profile called dynamic gain tilt. Third, optical components such as optical multiplexers/de-multiplexers, optical add-drop filters and other components can contribute to pass-band loss ripples. Finally, optical nonlinearities such as stimulated Raman scattering (SRS) and four-wave mixing (FWM) also induce tilt and ripples among channel powers. In optically amplified dense wavelength-division multiplexing (DWDM) systems, the un-flattened gain or loss will cause un-equal optical signal to noise ratio, which results in limited total bandwidth and transmission distance. Therefore, it is important to dynamically compensate for un-even power levels to increase system capacity and reach.  
           [0006]    There are a few different approaches that provide solutions for dynamic gain equalizations (DGEs). For example, one technique based upon acoustic-optical tunable filtering (AOTF) has been commercialized. A few AOTF filters combined together can be used to equalize power level of input optical DWDM channels to certain bandwidth resolution and residual power flatness. Due to the finite number of cascaded filters used, the resolution of AOTF-based dynamic gain equalizers is limited. Another drawback is the control algorithm is not trivial. Cascaded Mach-Zender filters can also be used to equalize optical power levels of a multi-wavelength DWDM signal. In addition to the slow thermal controls, the polarization dependence and difficult control mechanism make this type of device difficult to use in practical systems. Other types of DGEs use optical multiplexer and de-multiplexers to separate wavelength channels, and use variable optical attenuators to equalize channel power. The multiplexers and de-multiplexers can be based on bulk-optics or arrayed waveguide (AWGs), and the VOAs can be liquid-crystal based or microelectromechanical system (MEMs) based. The advantage is ease of control compared to optical filtering techniques, while the drawbacks are the pass-band narrowing due to the transfer function of MUX/DEMUX, and more importantly high cost. Although the above-mentioned methods all have their own pros and cons, they all share one important common drawback, that is, none of these methods provide any feedback control signals. In other words, the user have to use another external monitoring device to measure the flatness of the compensated signal and feed the error signal back to the DGE devices. Lack of integrated monitoring mechanism not only increase overall cost and size for completed dynamic gain equalization, also complicated DGE designs.  
           [0007]    It is important to provide a dynamic gain equalization mechanism which (a) dynamically equalizes power levels of all wavelength channels, (b) does not have effects of channel pass-band narrowing, (c) has a integrated channel power level monitoring.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    The present invention is directed to method and apparatus for providing dynamic gain equalization (DGE) using a novel technique based on diffractive gratings and integrated variable optical attenuator that simultaneously measures optical powers. Compared to prior art, the present invention has the advantages of (a) low cost, (b) compact size, (c) no pass-band narrowing, (d) integrated low-cost channel power analyzer.  
           [0009]    Viewed from one aspect, the present invention is directed to an optical arrangement for providing dynamic gain equalization to a received distorted input signal comprising N wavelength multiplexed channels. The optical arrangement comprises a pair of diffractive gratings, a quarter wave-plate, a variable optical attenuator, an integrated power monitor, a 90-degree optical prism, and two collimators. The two optical diffractive gratings are parallel to each other with a pre-determined distance between them. The input optical signal, consisting of N wavelength channels, is reshaped by the grating-pair after being collimated into free space in such a way that each wavelength channel is transformed into a parallel beam-let. Each wavelength beam-let is separated from each other by a predetermined amount. A variable optical attenuator is placed after the second grating so that it can attenuate each wavelength channel and provide power measurement of each channel. A 90-degree prism is placed after the variable optical attenuator to change the beam height and re-direct the beam to the 180 degrees direction with respect to input beam to the prism. The backward-propagating beam pass through the power-monitoring portion of the VOA, which measures the resulting power levels of all channels. A quarter wave-plate is inserted after the second grating to eliminate polarization dependence. The attenuator, made of semiconductor material, absorbs input optical beam. The amount of absorption is dependent upon the applied voltage. The electrodes on the attenuator are patterned in such a way that they form an array of independent unit to address a predetermined beam width. There is separate set of electrodes on the power-monitoring portion of the semiconductor attenuator for the returned beam. This set of electrodes have the same bias voltage so that the power levels of all channels can be monitored.  
           [0010]    The invention will be better understood from the following more detailed description taken with the accompanying drawings and claims.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0011]    [0011]FIG. 1 is a block diagram of a simultaneous channel power adjustment and measurement arrangement in accordance with a first embodiment of the present invention.  
         [0012]    [0012]FIG. 2 a  graphically shows the top view of the design of the semiconductor attenuator element used in the arrangement FIG. 1.  
         [0013]    [0013]FIG. 2 b  graphically shows the side view of the design of the semiconductor attenuator element used in the arrangement FIG. 1.  
         [0014]    [0014]FIG. 2 c  graphically shows the front view of the design of the semiconductor attenuator element used in the arrangement FIG. 1. 
     
    
       [0015]    The drawings are not necessarily to scale.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Referring now to FIG. 1, there is shown a block diagram of a dynamic gain equalization arrangement  10  (shown within a dashed line rectangle) in accordance with a first embodiment of the present invention. The dynamic gain equalization arrangement  10  comprises an input collimator  22 , two parallel optical gratings  24  and  25 , a quarter-wave plate  28 , a semiconductor attenuator  29  and a 90-degree prism  30 . The output of the collimator  22  is a collimated optical beam  23  in free space, which is aligned to the first grating  26 . Optical beam from collimator  22  propagates directly onto Grating  24  wit a predetermined incident angle. Grating  24  and  25  are parallel to each other. The diffracted optical beam from grating  24  propagates towards to the second grating  25 , which further diffracts the incoming beam  26  to an optical beam  27  that is parallel to beam  23 . A quarter-wave plate  28 , a semiconductor attenuator  29  and a 90-degree optical prism are serially placed in the path of beam  27 . The 90-degree optical prism  30  reflects the input optical beam towards 180 degrees direction with respect to the input beam, and simultaneously shifts the beam in vertical direction.  
         [0017]    In operation, a power level distorted optical input signal is received by the dynamic gain equalization arrangement  10  via the optical input fiber  21 , which is coupled to the input of the collimator  22 . The optical input signal comprises N wavelength multiplexed channels. The collimator  22  couples the optical signal from fiber  21  and collimates the output beam to a pre-determined beam width. The collimated beam  23  from the collimator  22  propagates onto the first grating  24  and is spatially dispersed into beam  26 . The second grating  25  is placed parallel to grating  24  with a predetermined angle with respect to beam  23 , it intersects the incoming beam  26 , and diffracts into a collimated beam  27 . The cross-section of beam  29  is elliptical due to grating diffraction. As a result of the mentioned double diffraction, the N wavelength-multiplexed signal is spatially de-multiplexed in such a way that lower wavelength channels are placed at the top of beam  27 , while longer wavelength channels are placed at the bottom of the beam  27 . The quarter wave-plate  28  is placed in such a way that the reflected beam has its polarization rotated 90 degrees after the second pass so that the polarization dependence of the optical setup, especially the gratings can be eliminated. The semiconductor attenuator  29  gives rise to a certain amount of attenuation to the transmitted optical beam, and there is a variation in attenuations depending upon the voltages provided for each electrode. The 90-degree optical prism  30  reflects the input optical beam towards 180 degrees direction with respect to the input beam, and simultaneously shifts the beam in vertical direction. The returned beam propagates onto the semiconductor attenuator at different height compared to the forward beam, and passes through a second set of transparent electrode array that provides channel power information. The returned beam further propagates through the quarter wave-plate  28 , grating  25  and  24 , and becomes a backward propagating beam  31 , which is shifted in height compared to input beam  23 . A mirror  32  with proper height is used to re-direct the returned beam to a second collimator  33 , which couples the input optical signal further to an output fiber  34 .  
         [0018]    The functional diagram of the semiconductor attenuator  31  is shown in FIG. 2. Referring to FIG. 2 a,  which is the top view of FIG. 1, the input beam  41  consists of N spatially separated beam lets with their wavelengths ordered across X direction. After propagating through the first electrode array on the semiconductor attenuator  40 , each beam let experiences different amount attenuation depending on the voltage applied to the transparent electrodes covering each beam let. An array of transparent electrodes  42  are placed in the front of the semiconductor attenuator  40 , and a common ground electrode  45  is placed at the back-side of semiconductor attenuator  40 . A 90-degree prism  44  shifts the optical beam in vertical direction, the direction of viewing, and turns the beam 180 degress with respect to the input direction so that the returned beam propagates onto the second set of electrode array  43  on the semiconductor attenuator  40 . Referring to FIG. 2 b,  the optical set-up is shown viewing from the side. The optical beam is shown to make a U-turn by the Prism  44 . The semiconductor attenuator  40  is divided into two parts, A and B, representing the optical attenuation and power monitoring, respectively. However, it does not mean semiconductor attenuator  40  has to be physically consists of two parts, it can be made of a single semiconductor material. FIG. 2 c  shows the front view of semiconductor attenuator  40 . The output beam  46  will have its intensity or power level modified when a set of voltage signal is applied to the electrode array  42 . The electric current (or photo-current) of electrode array  43  is dependent on the optical power received by each electrode element, and can be used to measure the average power of each beam let. The photo-current from electrode array  43  is proportional to the received optical power, and can be used as feedback signal to control the voltages applied to the electrode array  42  so that the final output power levels of all wavelength channels are set to desired values.  
         [0019]    Semiconductor attenuator  40  can be designed using a variety of opto-electrical properties of semiconductor materials. For example, free-carrier absorption (FCA) gives rise to enhanced attenuation by applying an external voltage across the active semiconductor regime. There are commercial variable optical attenuators (VOAs) utilizing FCA properties. Other potential useful properties that can be used in this Invention are the electro-absorption effects, or known as Franz-Keldysh and Stark effects. Both effects result in absorption of incident light with photon energies smaller than the band-gap with the application of an electric field.  
         [0020]    The present Invention simultaneously provides the optical power level adjustment and measurement capabilities in a single optical design, which results in cost-effectiveness and compactness of such devices.  
         [0021]    It is to be appreciated and understood that the specific embodiments of the invention described hereinabove are merely illustrative of the general principles of the invention. Various modifications may be made by those skilled in the art which are consistent with the principles set forth.