Abstract:
A semiconductor-based Raman ring amplifier is disclosed. A method according to aspects of the present invention includes directing a pump optical beam having a pump wavelength and an input pump power level from an optical waveguide into a ring resonator. The optical waveguide and ring resonator are comprised in semiconductor material. A signal optical beam having a signal encoded thereon at a signal wavelength is directed from the optical waveguide into the ring resonator. The pump optical beam is resonated within the ring resonator to increase a power level of the pump optical beam to a power level sufficient to amplify the signal optical beam via stimulated Raman scattering (SRS) within the ring resonator. A free carrier concentration in the optical waveguide and the ring resonator is reduced to reduce attenuation of the pump optical beam and the signal beam.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of invention relate generally to optical devices and, more specifically but not exclusively relate to amplifying optical power in an optical beam. 
     2. Background Information 
     The need for fast and efficient optical-based technologies is increasing as Internet data traffic growth rate is overtaking voice traffic pushing the need for fiber optical communications. Transmission of multiple optical channels over the same fiber in the dense wavelength-division multiplexing (DWDM) system provides a simple way to use the unprecedented capacity (signal bandwidth) offered by fiber optics. Commonly used optical components in the system include wavelength division multiplexed (WDM) transmitters and receivers, optical filter such as diffraction gratings, thin-film filters, fiber Bragg gratings, arrayed-waveguide gratings, optical add/drop multiplexers, optical amplifiers and optical attenuators. An optical amplifier is a device that can be used to increase the optical intensity or power of an optical beam. An optical amplifier can be useful to for example increase the intensity of an optical beam to compensate for power loss before or after being transmitted from a source to a destination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
         FIG. 1  is a block diagram illustrating generally an example optical system including an example of a semiconductor-based Raman optical ring amplifier in accordance with the teachings of the present invention. 
         FIG. 2  is a block diagram illustrating generally an example directional coupler included in an example of a semiconductor-based Raman optical ring amplifier in accordance with the teachings of the present invention. 
         FIG. 3  is a cross section view illustrating generally an example semiconductor-based optical waveguide including an example of a diode structure to control the free carrier concentration in the semiconductor waveguide in an example semiconductor-based Raman optical ring amplifier in accordance with the teachings of the present invention. 
         FIG. 4  is diagram illustrating generally a comparison in performance of an example semiconductor-based Raman optical ring amplifiers with a linear amplifier in accordance with the teachings of the present invention. 
         FIG. 5  is a block diagram illustrating generally an example of a semiconductor-based Raman optical ring amplifier with cascaded Raman ring amplifiers in accordance with the teachings of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Methods and apparatuses for amplifying optical beams are disclosed. In the following description numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale. 
       FIG. 1  illustrates generally a system including a signal optical beam source coupled to transmit a signal optical beam to an optical receiver through an example of an optical amplifier  107  in accordance with the teachings of the present invention. In particular,  FIG. 1  shows system  101  including signal optical beam source  103  directing a signal optical beam  105  to an optical amplifier  107 . In the illustrated example, optical amplifier  107  is a semiconductor-based Raman ring amplifier in accordance with the teachings of the present invention. In one example, signal optical abeam source  103  is coupled to receive a signal  135  and the signal  135  is encoded at a signal wavelength λ S  in signal optical beam  105 . In the example, the signal wavelength of λ S  includes infrared (IR) or near infrared (NIR) light such as for example light having wavelengths of 1310 nm or 1550 nm or the like. 
     As will be discussed, signal optical beam  105  is amplified such that the resultant amplified signal optical beam  105  is transmitted from optical amplifier  107  to an optical receiver  131 . In one example, the amplified signal optical beam  105  is transmitted through a standard optical fiber  129  from optical amplifier  107  to optical receiver  131 . For efficient operation, one of a variety of mode couplers such as for example tapers or the like in the semiconductor material  109  of optical amplifier  107  may be employed to optically couple optical amplifier  107  to optical fiber  129 . 
     As shown in  FIG. 1 , optical amplifier  107  includes an optical waveguide  111  coupled to a ring cavity or ring resonator  119  through a directional coupler  133 , all of which are integrated or included in the semiconductor material  109  that is optically coupled to receive optical beam  105 . In one example, optical waveguide  111 , ring resonator  119  and directional coupler  133  all include optical rib waveguides etched in the semiconductor material  109 , and are all are integrated with a diode structure  113  as shown to control free carrier concentrations in optical amplifier  107  in accordance with the teachings of the present invention. In one example, the diode structure  113  is a reverse biased PIN diode structure. As will be discussed, the free carrier concentrations in optical waveguide  111 , ring resonator  119  and directional coupler  133  are controlled for reduced attenuation or propagation loss to allow for efficient simulated Raman Scattering (SRS). In particular, the diode structure  113  is used in one example to control the removal of free carriers in accordance with the teachings of the present invention. 
     In operation, the example illustrated in  FIG. 1  shows that optical waveguide  111  is optically coupled to receive a pump optical beam  117  having an input pump power level from a pump optical beam source  115  and signal optical beam  105  from signal optical beam source  103 . In one example, the wavelengths used depend at least in part on the properties of the materials used in optical amplifier  107 . For instance, in an example in which silicon is used for semiconductor material  109 , pump optical beam  117  has a pump wavelength λ P  that is a Stokes shift less than the signal wavelength λ S  of signal optical beam  105 . 
     As illustrated in the example shown in  FIG. 1  propagating from right to left, the signal optical beam  105  and pump optical beam  117  are both directed from optical waveguide  111  into ring resonator  119  through directional coupler  133 . Since the Stokes shift in silicon is 15.6 THz, it is possible for directional coupler  133  to have different respective couplings for the pump wavelength λ P  and signal wavelength λ S  in accordance with the teachings of the present invention. In the illustrated example, directional coupler  133  is a wavelength-dependent coupler that results in ring resonator  119  having high Q, or low bandwidth, for pump optical beam  117  but low Q, or high bandwidth, for signal optical beam  105  in accordance with the teachings of the present invention. As a result, the pump optical beam  117  can be recirculated, reused or recycled within ring resonator  119 . In this way, the input power level of pump optical beam  117  is enhanced or magnified in ring resonator  119  to a much greater power than the input power level due to the cavity resonance effect. With the increased power level of pump optical beam  117  at the pump wavelength λ P , a fast signal  135  encoded in signal optical beam  105  is amplified at the signal wavelength λ S  inside ring resonator  119  with pump optical beam  117  via SRS, without sacrificing signal amplification bandwidth and/or speed in accordance with the teachings of the present invention. 
     To illustrate,  FIG. 2  is a block diagram illustrating generally increased detail of example directional coupler  133  included in an example of a semiconductor-based Raman optical ring amplifier in accordance with the teachings of the present invention. As shown in the illustration, pump optical beam  117  having the wavelength λ P  and signal optical beam  105  having the wavelength λ S  are directed along optical waveguide  111  to directional coupler  133 . In one example, directional coupler  133  includes a thin insulating region  210  between optical waveguide  111  and ring resonator  119  to provide an evanescent coupling through the semiconductor material  109  between optical waveguide  111  and ring resonator  119 . As a result, pump optical beam  117  and signal optical beam  105  are directed from optical waveguide  111  into ring resonator  119  through the evanescent coupling in directional coupler  133 . 
     In one example, the signal wavelength λ S  of signal optical beam  105  has close to 100% coupling in directional coupler  133 . As a result, the signal optical beam  105  propagates around ring resonator  119  and a majority or substantially all of signal optical beam  105  is directed back out of ring resonator  119  to optical waveguide  111  after one round trip in ring resonator  119 . In contrast, the pump wavelength λ P  of pump optical beam  117  is such that directional coupler  133  directs a majority or substantially all of pump optical beam  117  to remain or be recirculated within ring resonator  119  after each round trip, which results in the power level of pump optical beam  117  being enhanced by the cavity resonance effect within ring resonator  119 . Signal optical beam  105  is amplified within ring resonator  119  through SRS generated by the enhanced pump power of pump optical beam  117 . Since signal optical beam  105  has the close to 100% coupling in directional coupler  133 , signal optical beam  105  experiences practically no resonance effect and therefore amplification bandwidth is preserved in accordance with the teachings of the present invention. 
     In one example, since the majority of or substantially all of pump optical beam  117  is recirculated or reused within ring resonator  119  with directional coupler  133 , ring resonator  119  and/or directional coupler  133  also function as an on-chip filter to suppress the pump wavelength λ P . In one example, when substantially all of pump optical beam  117  is critically coupled into the ring resonator  119 , substantially all of the energy of pump optical beam  117  is consumed inside ring resonator  119 . 
     As shown in the example of  FIG. 2 , a resonance frequency controller  212  may also be included and is coupled to the ring resonator  119  in accordance with the teachings of the present invention. In various examples, the resonance frequency controller may include for example a temperature controller having for example an external thermoelectric cooler (TEC) or an on-chip heater electrode, current injection device or other suitable device to tune the resonance frequency of the ring resonator  119  in accordance with the teachings of the present invention. In other examples, such as for example multiple wavelength systems, the temperature controller could be used to adjust or tune the resonance frequency of the ring resonator to provide a tunable filter function to optical amplifier  107 . 
       FIG. 3  is a cross section view illustrating generally an example a diode structure  113 , which is integrated into optical waveguide  111 , to control the free carrier concentration in the semiconductor optical waveguide  111  in an example semiconductor-based Raman optical ring amplifier in accordance with the teachings of the present invention. It is noted that the example illustrated in  FIG. 3  shows signal optical beam  105  directed through the cross-section of optical waveguide  111 . However, in another similar diagram, it is appreciated that pump optical beam  117  could be illustrated being directed through a cross-section illustration of ring resonator  119  in accordance with the teachings of the present invention. 
     As shown in the depicted example, optical waveguide  111  is a rib waveguide including a rib region  339  and a slab region  341 . In the illustration, signal optical beam  105  is shown propagating through the rib optical waveguide  111 . As shown, the intensity distribution of the optical mode of optical beam  105  is such that the majority of the optical beam  105  propagates through a portion of the rib region  339  or a portion of the slab region  341  of optical waveguide  111  towards the interior of the rib optical waveguide  111 . As also shown with the optical mode of optical beam  105 , the intensity of the propagating optical mode of optical beam  105  is vanishingly small at the “upper corners” of rib region  339  as well as the “sides” of the slab region of optical waveguide  111 . 
     In the illustrated example, optical waveguide  111  is formed or etched in a silicon-on-insulator (SOI) wafer including the silicon of semiconductor material  109 , a silicon substrate layer  335  and a buried oxide layer  333  disposed between the silicon of semiconductor material  109  and silicon substrate layer  335 . 
     In the example shown in  FIG. 3 , the P region  121  and N region  123  of the diode structure  113  disposed in optical waveguide  111  are defined at opposite lateral sides of the slab region  341  in the optical waveguide  111 , outside of the optical mode of optical beam  105 . As shown in the example, diode structure  113  is a PIN diode structure, which includes P doped silicon in P region  121 , intrinsic silicon in semiconductor material  109  and N doped silicon in N region  123 . In the illustrated example, the optical mode of optical beam  105  propagates through the intrinsic silicon in semiconductor material  109  of the PIN diode structure  113 . 
     The example shown in  FIG. 3  shows voltage source  125  providing a bias voltage V-bias coupled between P region  121  and N region  123 . P region  121  is also grounded and voltage source  125  is coupled to apply the V-bias voltage between P region  121  and N region  123  to bias the diode structure  113  in accordance with the teachings of the present invention. For example, when reverse biasing the diode structure  113  as shown, an electric field is created between the P region  121  and N region  123  to sweep free carriers  337 , which may include electrons and/or holes, from the optical waveguide  111 . By sweeping out the free carriers  337 , as discussed, the free carrier concentrations, lifetimes or the removal of the free carriers  337  can be controlled in accordance with the teachings of the present invention. This enables control or reduction of attenuation of the power level of an optical beam propagating through the optical waveguide  111  in accordance with the teachings of the present invention. 
     In one example, realization of the wavelength-dependent directional coupler  133  in accordance with the teachings of the present invention is based on the dispersion and polarization dependence of the coupling coefficient. The Stokes wavelength has a larger mode profile, which increases the mode overlapping between coupled waveguides, and the TE field has stronger coupling compared to the TM field. The coupling ratio depends on the geometry of the rib, the coupler length and the spacing between the two waveguides of directional coupler  133 . 
     Referring briefly back to the example shown in  FIG. 1 , the evolution of the pump intensity I P  along the propagation direction z in the ring resonator can be described by 
                           ⅆ               ⅆ   z       ⁢       I   P     ⁡     (   z   )         =       [       -     α   P       -     β   ·       I   P     ⁡     (   z   )         -           σ   P     ·   β   ·     τ   eff         2   ·     E   P         ⁢       I   P   2     ⁡     (   z   )           ]     ·       I   P     ⁡     (   z   )           ,           (   1   )               
where α P  is the linear loss due to scattering and material absorption, which in one example waveguide is usually around 0.3 dB/cm, β=0.5 cm/GW is the TPA coefficient, τ eff  is the effective carrier lifetime and α p =1.45×10 −17  cm 2  is the FCA cross section at the pump wavelength λ P , and E P  is the photon energy. As shown in the example of  FIG. 1 , the integration of a reversed-biased diode structure  113  along optical waveguide and ring resonator  119  of optical amplifier  107  can reduce the effective carrier lifetime τ eff  to approximately 1 ns in accordance with the teachings of the present invention.
 
     In one example, the round trip gain in ring resonator  119  of signal optical beam  105  at the signal wavelength λ S , which is a Stokes shift from λ P , or is at Stokes wavelength, can be calculated by integrating the net gain, which is Raman gain minus loss, along the propagation path, 
                       G   s     =       ∮     L   ring       ⁢     ⅆ     z   ⁡     [         g   r     ·       I   P     ⁡     (   z   )         -     α   s     -     2   ⁢     β   ·       I   P     ⁡     (   z   )           -           σ   s     ·   β   ·     τ   eff         2   ·     E   P         ⁢       I   P   2     ⁡     (   z   )           ]             ,           (   2   )               
where g r =9.5 cm/GW is the Raman gain coefficient, α S =0.3 dB/cm is the linear loss and σ S =1.71×10 −17  cm 2  is the FCA cross section at Stokes wavelength. Using a finite element method, the evolution of the pump intensity and net gain inside ring resonator  119  can be numerically derived. At resonance, the relationship between the incident power I inc  and the power inside the cavity I p  can be described by
 
                       I   inc     =         I   P     ⁡     (   0   )       ·       (     1   -           A   ·     (     1   -   K     )       )     2           K         ,     A   =         I   P     ⁡     (   L   )           I   P     ⁡     (   0   )           ,           (   3   )               
where K is the power coupling ratio of the coupler and A is the round-trip loss coefficient. In one example, the strongest cavity enhancement occurs at critical coupling, where A+K=1.
 
       FIG. 4  is diagram illustrating generally a comparison in performance of an example semiconductor-based Raman optical ring amplifier with a straight or linear waveguide amplifier in accordance with the teachings of the present invention. In particular, based on Equations 1-3 above, the Raman amplification vs. pump power for an example ring resonator  119  that is 3 cm long and for a straight linear waveguide of the same length for comparison is illustrated. In the example, the performance for ring resonator  119  is illustrated with plot  443  while the performance for a straight linear waveguide is illustrated with plot  445 . In the example, 19% coupling is chosen at the pump wavelength λ P  to meet the critical coupling condition at the low input power level, 100% coupling at the signal wavelength λ S  is assumed so that the application bandwidth is not limited by the ring resonator  119 . As shown, 3 dB gain is achieved with a 3 cm ring resonator  119  with 0.3 dB/cm loss and a pump power of 335 mW, which is 2.5 times lower compared to a linear straight waveguide of the same length, which used a pump power of 850 mW. To achieve 1 dB gain, only 85 mW is required for the ring compared to 350 mW for the straight linear waveguide, which is 4 times lower. 
     It is appreciated that the relatively short device length and small footprint of an optical amplifier in accordance with the teachings of the present invention allow a greater number of devices to be produced on a single wafer. Since the design is compact, multi-stage amplification is possible on a single chip in accordance with the teachings of the present invention. To illustrate,  FIG. 5  is a block diagram illustrating generally an example of a semiconductor-based Raman optical ring amplifier  507  with cascaded Raman ring amplifiers in accordance with the teachings of the present invention. It is appreciated that optical amplifier  507  of  FIG. 5  shares similarities with optical amplifier  107  of  FIG. 1 . For instance, optical amplifier  507  includes an optical waveguide  111  coupled to ring resonator  119  through a directional coupler  133 . However, optical amplifier also includes an additional cascaded ring resonator  519  coupled to optical waveguide  111  through an additional directional coupler  533  in accordance with the teachings of the present invention. 
     In operation, pump optical beam  117  is directed into optical waveguide  111  at both ends as shown such that pump optical beam  117  is directed into both ring resonator  119  and ring resonator  519  through directional coupler  133  and directional coupler  533 , as illustrated. In the example, the coupling provided by directional couplers  133  and  533  is such that substantially all of pump optical beam  117  is recirculated or recycled within ring resonators  119  and  519  resulting in enhanced pump optical beams due to the cavity resonance effect as discussed above. 
     Continuing with the example, signal optical beam  105  propagates along optical waveguide  111  from right to left as shown in the  FIG. 5  example, and is directed into ring resonator  119  through directional coupler  133 . Signal optical beam  117  is amplified within ring resonator  119  due to SRS and then is directed back out from ring resonator  119  after one round trip into optical waveguide  111  through directional coupler  133 . The amplified signal optical beam  105  is then directed from directional coupler  133  along optical waveguide  111  to directional coupler  533 , where it is directed into ring resonator  519 . The amplified signal optical beam is then further amplified within ring resonator  519  due to SRS and this is directed back out from ring resonator  519  after one round trip into optical waveguide  111  through directional coupler  533 . 
     Therefore, with the cascaded ring resonators  119  and  519 , the signal optical beam  105  may be amplified with multiple stages in accordance with the teachings of the present invention. In one example using the two stage device as shown in  FIG. 5 , 3 dB amplification can be reached with a total pump power of 220 mW and the device size fits well with a 16 mm×2 mm footprint in accordance with the teachings of the present invention. 
     The above description of illustrated embodiments of the invention, including what is described in the abstract, is not intended to be exhaustive or to be limitation to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible, as those skilled in the relevant art will recognize. Indeed, it is appreciated that the specific wavelengths, dimensions, materials, times, voltages, power range values, etc., are provided for explanation purposes and that other values may also be employed in other embodiments in accordance with the teachings of the present invention. 
     These modifications can be made to embodiments of the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.