Abstract:
Ultrafast directional beam switching is achieved using coupled VCSELs. This approach is demonstrated to achieve beam switching frequencies of 40 GHz and more and switching directions of about eight degrees. This switching scheme is likely to be useful for ultrafast optical networks at frequencies much higher than achievable with other approaches.

Description:
ORIGIN OF THE INVENTION 
   The invention described herein was jointly made by an employee of the United States Government and by a non-Government employee, whose contribution was made in the performance of work under a NASA contract and is subject to the provision of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457). 

   BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The present invention relates to information processing and transmission in optical communication systems, and more specifically, it relates to techniques for increasing the switching and modulation speed of vertical-cavity surface-emitting lasers (VCSELs). 
   2. Description of the Prior Art 
   The limit to switching and modulation speed of semiconductor lasers is a fundamental bottleneck for information processing and transmission in optical communication systems. This limit comes from the relatively slow carrier recombination lifetime in the III-V semiconductors used for optical applications. Though progress in pushing this limit in the past has been made, these improvements cannot meet the demand for higher speed in the long run. A new paradigm has to be explored to maintain the momentum of technology development. 
   The ability to steer or switch propagation direction of a laser beam in a controllable way is very important for many applications, and especially for optical interconnect networks. Beam scanning and steering in edge emitting lasers have been realized using thermal control (See Y. Sun, C. G. Fanning, S. A. Biellak, and A. E. Siegman, IEEE Photonics Technol. Lett. 7,26(1995)) and spatial phase controlling techniques (See J. P. Hohim, D. C. Craft, G. A. Vawter, and D. R. Myers, Appl. Phys. Lett. 58, 2886 (1991)). For optical interconnect applications, all the well-known advantageous attributes of vertical-cavity surface-emitting lasers (VCSELs) make them especially appealing elements. However, full advantage cannot be taken of compact two-dimensional (2D) VCSEL arrays if bulky external passive optical elements are used for routing and switching, For this reason, routers integrated together with VCSELs that can be controlled electronically are especially important to reduce the overall volume of an interconnect network. See L. Fan, M. C. Wu, and P. Gradzinski, Electron. Lett. 31, 729 (1995) and L. Fan, M. C. Wu, H. C. Lee, and P. Grodzinski, IEEE Photonics Technol. Left. 9, 505(1997). 
   Recently, it has been demonstrate that, by introducing a phase-shifter in part of the VCSEL cross-section, beam switching up to 2 gigahertz can be achieved. Another more conventional approach to VCSEL beam manipulation is to use coherently coupled VCSEL arrays. Indeed VCSEL arrays of various kinds have been quite extensively researched for tailoring and engineering near and far field patterns. See M. Orenstein and T. Fishman, “Supermodes of Hermite Tapered Arrays of Vertical-Cavity Semiconductor Lasers,” IEEE J. Quantum Electron 35 1062-1066, (1999). See D. Natan, M. Margalit and M. Orenstein, “Loca;lization immunity And Coherence Of Extended Two-Dimensional Semiconductor Vertical Cavity-Locked Laser Arrays,” J. Opt Soc. Am. B 14, 1501-1504, (1997). See T. Fishman and M. Orenstein, “Cyclic Vertical Cavity Semiconductor Laser Arrays With Odd Numbers Of Elements: Lasing Modes And Symmetry Breaking.” Opt. Lett. 21 600-602, (1996) 
   It is therefore desirable to provide techniques for increasing the switching and modulation speed of semiconductor lasers for use in applications such as information processing and transmission in optical communication systems. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to produce light output from two or more coupled VCSELs that are spatially dynamic at an extremely high frequency. The coupling is achieved by small inter-VCSEL separations and the VCSELs are all biased at the same steady current near threshold. 
   This and other objects will be apparent to those skilled in the art based on the disclosure herein. 
   A basic embodiment of the invention includes vertical-cavity surface-emitting lasers (VCSELs) spatially coupled together; and a current source connected to the VCSELs, with a steady current biased above threshold current, where the coupling of the VCSELs produces an output laser beam having a spatial oscillation. A switching device is shown that transfers the near laser field emitted by two coupled VCSELS to two receivers by using a micro lens system. Another embodiment is shown that produces a far field device where two VCSELs produce an output beam that is not directed through a micro lens system. 
   The invention is based on computer simulations of the light output of coupled VCSELs. The model equations that are solved on the computer are an approximation to the semi-conductor Maxwell-Blotch equations. 
   This disclosure illustrates that the resulting far field pattern has two lobes that are oscillating out of phase and are useful for beam switching. Dynamic beam switching of vertical-cavity surface-emitting lasers (VCSELs) has important applications for switching and routing in optical interconnect networks. 
   The time evolution of the spatial profiles of the laser and carrier density is obtained by solving the coupled partial differential Maxwell-Blotch equations that govern their evolution by a finite-difference algorithm. The algorithm is fairly general; it can handle devices with one or multiple active regions of any shapes, which can be either gain or index guided. The physical modeling includes the effects of nonlinear carrier dependence and dispersion with respect to wavelength on the optical gain and refractive index. The modeling of the optical susceptibility is based on first-principles and includes device details such as a quantum well structure and many-body effects. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A-1F  show a perspective view of the far field beam intensity over one and one-half cycles of beam switching. The switching speed is forty-two gigahertz and the beams are separated by about eight degrees. 
       FIGS. 2A-2F  show the top view of the far field beam intensity over one and one-half cycles of beam switching. As in  FIGS. 1A-1F , the switching speed is forty-two gigahertz and the beams are separated by about eight degrees. 
       FIG. 3  shows a device for coupling the near field laser output of two coupled VCSELs to two receivers by using a micro lens system. 
       FIG. 4  shows a device for coupling the far field laser output of two coupled VCSELs to two receivers. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A new method of directional beam switching is proposed that uses two or more coupled vertical-cavity surface emitting lasers (VCSELs). When two or more VCSELs are coupled by a small inter-VCSEL separation, and biased at the same steady current near threshold, then the resulting light output is spatially dynamic at an extremely high frequency. The invention is based on computer simulations of the light output of coupled VCSELs. The model equations that are solved on the computer are an approximation to the semi-conductor Maxwell Blotch equations. The first simulations are for two coupled VCSELs operating at 980 nm with circular current apertures of 5.6 μm in diameter and the resulting switching frequency is at forty-two gigahertz. The far field beams are separated by about eight degrees. The near field pattern also showed two spots that were oscillating in relative intensity. However, they were oscillating ninety degrees out of phase with respect to the oscillation in the far field pattern. Simulations that use two square VCSELs have been performed that increase the frequency to fifty gigahertz and the far field pattern remains similar. For three round VCSELs in a ring patter, the far field beam moved in a circular pattern at a frequency of thirty-one gigahertz. For four round VCSELs in a square pattern, two far field circularly shaped beams moved left and right at a frequency of fifty gigahertz. 
   As shown in  FIGS. 1A-1F  and  FIGS. 2A-2F , the resulting far field pattern has two lobes that are oscillating out of phase and are useful for beam switching. See Peter Goorjian and Cun-Zheng Ning: “Numerical Simulation of Ultrafast Directional Beam Switching in Coupled VCSELs,” Paper IWA4, IPR 2001, Integrated Photonics Research 2001, Optical Society of America, Monterey, Calif., Jun. 11-13, 2001, incorporated herein by reference and Peter Goorjian and CunZheng Ning: “Ultrafast Beam Self-Switching by Using Coupled VCSELs,” Paper 1 in Session Nonlinear Optics I, QEP-15/Photonics 2001 15th Institute of Physics Quantum Electronics and Photonics/QEP-15 2001 joint conference, University of Strathclyde, Glasgow, Scotland, Sep. 3-6, 2001. 
   For a discussion of the semiconductor Maxwell-Blotch equations upon which the present invention were modeled, see C. Z. Ning, R. A. Indik and J. V. Moloney, “Effective Blotch-Equations For Semiconductor Lasers And Amplifiers,” IEEE J. Quantum Electron. 33, 1543-1550, (1997). The time evolution of the spatial profiles of the laser and carrier density is obtained by solving the coupled partial differential equations by a finite-difference algorithm that governs the evolution of the spatial profiles. The algorithm is fairly general; it can handle devices with one or multiple active regions of any shapes, which can be either gain or index guided. See P. M. Goorjian, C. Z. Ning, and G. P. Agrawal, “Simulations of Spatial Dynamics in Vertical-Cavity Surface-Emitting Laser Arrays,” Paper ThB22, Nonlinear Optics: Materials, Fundamentals and Applications, OSA, Kauai-Lihue, Hi., Aug. 7-11, 2000. There is no a priori assumption about the type or number of modes. The physical modeling includes the effects of nonlinear carrier dependence and dispersion with respect to wavelength on the optical gain and refractive index. The modeling of the optical susceptibility is based on first-principles and includes device details such as a quantum well structure and many-body effects. See P. M. Goorjian and C. Z. Ning, “Transverse mode dynamics of VCSELs through space-time simulation,” Opt. Express 5, 55-62, (1999). See C. Z. Ning and P. M. Goorjian, “Microscope Modeling And Simulation Of Transverse Mode Dynamics Of Vertical-Cavity Surface-Emitting Lasers,” J. Opt. Soc. Am. B 16, 2072-2082, (1999). Temporal dynamics as fast as on a picosecond scale can be resolved. This bottom-up approach uses measured material parameters and quantum well structure parameters, with the number of free parameters minimized. The VCSELs given herein are based on InGaAs/GaAs quantum well structures. 
   Further calculations were made to show the effects of varying the inter VCSEL separation, the injection current and the VCSEL size on the frequency, relative brightness and angular separation of the two beams. Also similar studies were made of VCSELs arrayed in a ring pattern. 
     FIG. 3  shows a proposed switching device that would transfer the near laser field emitted by two coupled VCSELS to two receivers by using a micro lens system. In the figure, VCSELs  10  and  12  each have an output aperture diameter of 5 μm located at plane  14 . The output aperture of each VCSEL is located within a circular diameter of 12 μm. With an appropriate micro lens system  16 , as would be understood by those skilled in the art, the near field beam is directed to two receivers  18  and  20  located 25 μm from the output apertures of VCSELs  10  and  12 .  FIG. 4  illustrates a far field device where the same two VCSELs  10  and  12  produce an output beam that is not directed through a micro lens system. It is shown that the far field beam oscillates between plus and minus 4 degrees, so that at 4 mm from the VCSELs, the beam is oscillating between 2 receivers, (each 5 microns in diameter), that are separated by 0.6 mm. Receivers  22  and  24 , spaced apart by 0.6 mm, are located 4 mm from the output apertures of VCSELs  10  and  12 . 
   The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described to best explain the principles of the invention and its practical application. The scope of the invention is to be defined by the following claims.