Abstract:
A tunable Vertical Cavity Surface Emitting Laser (VCSEL) is disclosed that produces N frequencies of visible light. The VCSEL comprises an array of quantum wells (QWMs) which replaces one of the normally present Distributive Bragg Reflectors (DBR) at one end of a laser cavity of the VCSEL. These N independent QWMs produce N independent frequency in the laser cavity of the VCSEL device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to semiconductor lasers that emit light at visible wavelengths, and more particularly, to Vertical-Cavity Surface-Emitting Lasers (VCSELs) that produce N-frequencies of visible light in a single cavity by altering the optical length of the cavity through the use of a Quantum Well Mirror (QWM) replacing one of the Distributed Bragg Reflectors (DBRs) typically found in a VCSEL. 
     2. Description of the Prior Art 
     Semiconductor lasers, such as VCSELs, emit at a visible wavelength in the range of about 400 nm to 700 nm and are of particular interest for applications, such as optical scanning, image display, laser printing and xerography, optical data storage and readout, and plastic-fiber-base optical communications. The VCSELs are known in the art, some of which are described in U.S. Pat. Nos. 5,557,627 and 5,719,892, both of which are herein incorporated by reference. The prior art VCSEL devices commonly are comprised of a gain medium and at least two (DBRs) comprising alternate layers of semiconductive material such as GaAlAs/AlAs. It is desired to have a tunable VCSEL that produces N-light frequencies by adjusting the optical cavity length by applying an electric field to quantum wells devices, in the QWM, changing the refractive index of the well material. It is also desirable to have N of these structures fabricated into an array. In this manner N identical or different frequencies can be produced. 
     The frequency spectrum of the VCSEL devices is dependent, in part, on the optical cavity length of the laser cavity which, in turn, is dependent on the index of refraction of the alternating layers of the DBRs. One process known in the prior art is to have one VCSEL device produce multiple visible light frequencies in the spectrum by using the oxygenation of a layer or layers in the DBR to change the index of refraction. For example, if a multilayered DBR structure in which one of the alternating layers is composed in part of Al is fabricated and oxygenated after fabrication by introducing oxygen into the deposition chamber, a change in the refractive index of the layer containing Al will occur. The oxygen will migrate into the Al layer from the edge of the structure creating an index of refraction that changes its magnitude in a parabolic manner. This parabolic shape of the index of refraction will produce different visible light frequencies along the length of the AlAs layer. A second method for providing for multiple light frequencies developed by VCSEL devices is the heating of certain areas of the substrate of the DBR during the deposition thereof which will, in turn, creates different indices of refraction in the area being heated and hence creates different light frequencies to be developed by the VCSEL. These procedures are relatively complicated and it is desired to provide for a VCSEL, which can tune to N different light frequencies in a relatively simple manner. 
     OBJECTS OF THE INVENTION 
     It is a primary object of the present invention to provide for a VCSEL device that is tunable to N different light frequencies. 
     It is another object of the present invention to provide for a VCSEL device, wherein the fundamental frequency of its associated cavity can be established by the selection of a particular material composition. 
     It is a further object of the present invention to provide for a VCSEL array whose multiple light frequencies may be easily tuned and modulated. 
     It is a further object of the present invention to provide for a VCSEL device that may be fabricated of different semiconductor materials that provide a desired bandgap for fundamental light frequencies of interest. 
     In one embodiment, a tunable multi-frequency VCSEL is provided comprising a first DBR disposed on the surface of a substrate; a first electrode disposed on the DBR; a tuning region composed of quantum wells disposed on the first electrode; and a second electrode disposed on the tuning region. The VCSEL further preferably comprises an active region disposed on the second electrode and a second DBR disposed on the active region. 
     In another embodiment, an array of tunable multi-frequency VCSELs is fabricated on a substrate each of which has its own external connection to an external power supply. The array can provide N independent identical or different frequencies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features and advantages of the invention, as well as the invention itself, will become better understood by reference to the following description when considered in conjunction with the accompanying drawings, wherein like reference numbers designate identical or corresponding parts thereof and wherein: 
     FIG. 1 is a schematic diagram of a VCSEL in accordance with one embodiment of the present invention; 
     FIG. 2 is a schematic of a QWM used in the embodiments of the present invention; 
     FIG. 3 is a perspective view of an array of sixteen (16) QWM comprising an array of VCSELs that produces multiple visible light frequencies; and 
     FIG. 4 is a schematic diagram of another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the drawing, FIG. 1 illustrates a schematic diagram of a tunable multi-frequency VCSEL device  10  of the present invention. The VCSEL device  10  is tunable to N visible frequencies and comprises a QWM that replaces at least one of the DBRs commonly found in the prior art VCSEL devices. As will be further described hereinafter, the QWM element in the VCSEL is tunable so as to develop one of the visible light frequencies, by applying a specific voltage to its electrode, making up the multiple visible light spectrum developed by the this device  10 . 
     The VCSEL device  10  comprises a first DBR  12  disposed on a surface of the substrate (not shown in FIG. 1 because of the cross-sectional view thereof, but known in the art) and a first electrode  14  deposited on the DBR  12 . It is preferred that the VCSEL device  10  comprise an absentee layer  16  upon which is disposed on tuning region  18  provided by a QWM. A second electrode  20  is disposed on the quantum well mirror  18 . An active region  22  is disposed on the second electrode  20  and has first and second transitional layers  22 A and  22 B respectively. Disposed on the active region  22  is a second DBR  24 . The VCSEL device  10  may be further described with reference to FIG.  2 . 
     In general, the QWM device  18  has a multi-layer thin film semiconductor structure that has an effective plane of reflection. The effective plane of reflection exists within the structure and is defined to be the distance d, shown in FIG. 2 from the front surface of the structure. The position of distance d, relative to the front surface, is a function of the light absorption in the well layer  34  (to be described) of the QWM  18 . Because the position is a function of the absorption of the light, it is also a function of the index of refraction of the material making up the layers. Any change in the index of refraction of the material causes a shift in the position of the effective plane of reflectance. Therefore, in accordance with the practice of the present invention, by using the QWM structure with an applied and selectable electric field, the Quantum Confined Start Effect, known in the art, will shift the index of refraction of the well material changing the position of the effective plane of reflectance. 
     The QWM device  18  of FIG. 2 comprises top and bottom electrodes  26  and  28  respectively with the bottom electrode  28  having a surface upon which the alternating layers of the QWM device  18  rest. The QWM device  18  comprises n-layered pairs  30  of semiconductor material disposed on the surface of the bottom electrode  28 . The n-layered pairs  30  comprises barrier and well layers pairs  32  and  34 , respectively, which form at least one sequence of barrier ( 32 ) well ( 34 ), barrier ( 32 ), and well ( 34 ) layers. The n-layered pairs  30  terminate with the last of the sequence being a barrier layer  32  as shown in FIG.  2 . 
     The material comprising the well layer  34  serves as a confinement region for the electrons and as is the basis for changing the index of refraction of the present invention. This material is selected based on the central operation wavelength λ of the VCSEL device  10 . The wavelength defines the bandgap energy E g  through the relationship E g =hν, where h represents the Planks constant. The difference in energy between the conduction and valance bands of the well material making up the well layer  34  is E g . The material making up the barrier layer  32  is selected based on the well to barrier valance and conduction band offsets. The typical band energy offsets are 40% for the valance and 60% for the conduction band. The energy difference in the conduction band forms, in a manner known in the art, a well deep enough to trap electrons. The trapping in conjunction with the width of the well, typically &lt;100 Angstroms, restricts the movement of the electrons in the direction of the width of the well. If an electric field is applied to the quantum well (QW), the Quantum Confined Stark Effect (QCSE), known in the art, occurs. More particularly, if an electric field is applied across the electrodes  26  and  28  of the QWM device  18  then the QWM device  18  shifts its effective plane of reflection changing the frequency in the cavity. 
     As will be further described with reference to FIG. 3, the applied electric field shifts the bandgap energy levels which, in turn, changes the index of refraction which, in turn, changes the operating wavelength λ of the VCSEL device  10 . For the structure of QWM device  18  of FIG. 2, an apparent plane of reflection can be calculated based on the change of phase (∂φ) of the light with respect to the change in propagation constant (∂Δβ) described as        (       ∂   ϕ       ∂   Δβ       )                          
     which is evaluated at Δβ=o. This defines the effective cavity length (L eff   qw ) change for the QWM. This added to the effective path in the DBR (L eff   dbr ) provides the total path in the QWM, that is, L eff   qwm =L dbr +L qw  The total effective cavity length (L eff ) is then given by adding the length of the active gain media L ag  to this equation and then multiplying L eff   dbr  by 2 yielding the quantity L eff =2(L ag +L eff   qwm ) 
     With reference to FIG. 1, it is seen that a single QWM device  18  is used to replace one of the DBR of a typical VCSEL device, such as those disclosed in U.S. Pat. Nos. 5,557,627 and 5,719,892. The fundamental frequency within the cavity can be set by applying a DC voltage, having a possible value between 0 to 10 volts, across the electrodes  26  and  28  of the QWM device  18 . This will set a particular cavity length for the VCSEL device  10 . If modulation is required, an additional time varying signal can be applied across the electrodes  26  and  28 , in a manner to be described hereinafter with reference to FIG.  3 . 
     To produce a laser cavity, such as tuning region  18  shown in FIG. 1, which can shift between frequencies in a single cavity, in order to produce N simultaneous frequencies such as FIG. 3, an array of VCSEL&#39;s with QWM devices  18  needs to be fabricated, that is, to replace one of the typical DBR found in typical VCSEL devices with a QWM  18  device. One such array  38  of QWM devices  18  is shown in FIG.  3 . 
     The array  38  is composed of sixteen individual QWMs  18 . Each QWM  18  device can be set to a different fundamental frequency by applying a different DC voltage to each of the elements, that is, across its respective electrodes  26  and  28 . Modulation of the fundamental frequencies can be achieved through an additional AC signal applied to the electrodes  26  and  28  of each of the QWM devices  18 . The DC voltage and the AC signals may be generated by an excitation source  40  in a manner known in the art. The VCSEL device  44  containing the array  38  is shown in FIG.  4  and having some of the elements previously described with reference to FIG.  1 . 
     If different fundamental frequencies are desired, different fundamental (or central) wavelengths, one VCSEL at 850 nm and another VCSEL at 852 nm, may be used. The material composition of the material making up well layer  34  in the QWM device  18  can be altered. That is, for example, for Al x-1 Ga x As well material the concentration (X) at deposition for the material making up the well layer  34  can be fixed to a different value, for example, X=0.30 instead of the typical X=0.25. This will provide well layers  34  with different indices of refraction for each X value, and hence different cavity lengths. 
     The absentee layer  16 , shown in FIG. 1, is an optically inactive layer, of a prescribed thickness, and it can be added to the QWM  18  for phase adjustment. This optically inactive layer  16  can be placed between the top electrode  26  of the QWM  18  and the first barrier layer  32  that is in contact with electrode  26 . 
     It should now be appreciated that the practice of the present invention provides for a tunable multi-frequency VCSEL having a number of visible light frequencies. Each QWM device  18  within the VCSEL array  38  can produce a single frequency, which can be tuned through the use of electrodes  26  and  28  of the QWM device  18  and the excitation source  40  applied across the electrodes  26  and  28 . In addition, the individual fundamental frequency of the QWM device  18  can be set by the concentration of specific material in the well structure of the QWM device  18 , such as the material used for the well layers  34 . If a RF signal is applied, a modulation can be induced on the laser fundamental frequency of the selected QWM device  18 . Accordingly, the VCSEL device  10  can be easily tuned and modulated through easily controlled methods. 
     It should now be appreciated that the practice of the present invention provides for various embodiments of VCSEL devices, all of which are tunable to N frequency. 
     In the practice of the present invention a preliminary calculation was performed to determine the number of modes that can be supported in the cavity. In one embodiment, and assuming a typical gain, it is possible to switch between 8 to 16 modes. The result of the calculations were based on known values of existing multi-mode lasers, with the size of each QWM element being a 5 by 5 micron square. The size was selected for single mode operation of the QWM elements  18 . The dimension of the VCSEL devices  10  or  42  should include one-half of a two micron strip that would run between the QWM elements  18 , thereby, increasing the entire size of the surface of each QWM  18  element to be about a 6 by 6 micron square. 
     It is understood that the invention is not limited to the specific embodiments herein illustrated and described and may be otherwise without departing from the spirit and the scope of the invention.