Abstract:
Back facet reflections are substantially minimized in a tilted, ridge wave-guide SLD. One end of the wave-guide terminates at the front facet and the other end terminates proximate, but not necessarily at, the back facet. The back facet termination includes a radiating structure causing light to dissipate prior to striking the rear facet.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional Application No. 60/414,277, filed on Sep. 27, 2002, entitled “Narrow Spectral Width Superluminescent Diodes Using Integrated Absorber,” and is related to U.S. application No. (SAR 14808) filed on ______, entitled “Narrow Spectral Width Light Emitting Devices.” 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to light emitting devices, and more particularly to diode devices having spontaneous emissions without lasing.  
         BACKGROUND  
         [0003]    Superluminescent diodes (SLDs) are optical devices that provide amplified spontaneous emission outputs confined to one spatial mode. The spatial distribution of the output light is similar to a laser while the spectral distribution is similar to an LED. SLDs are often specified for applications requiring high beam quality, but where the narrow linewidth of the laser is undesirable or detrimental. An SLD typically has a structure similar to that of a laser, with lasing being prevented by antireflection coatings formed on the end faces. One such device is described in U.S. Pat. No. 4,821,277, which is incorporated herein by reference and which is characterized by a tilted waveguide structure. The axis of symmetry of the waveguide is formed at an angle relative to the direction perpendicular to at least one of the end faces and the tangent of the angle is greater than or equal to the width of the effective optical beam path divided by the length of the body between the end faces.  
           [0004]    The SLD optical spectrum is just one measure of its performance. Another measure is its coherence spectrum, which consists of a narrow main peak and several other peaks of smaller amplitude. The smaller amplitude peaks are caused by reflections from the back facet and from imperfections in the waveguide. The largest peak besides the main peak is called the second coherence peak. Ideally, all peaks should be negligible in comparison to the main peak. But for certain applications, such as fiber optic gyroscopes and optical coherence tomography, the second coherence peak should be on the order of 30 dB below the main peak.  
         SUMMARY  
         [0005]    A light emitting device according to the principles of the invention substantially minimizes unwanted facet reflections. Light is dissipated or radiated away before the unwanted reflections occur. In one embodiment, back facet (also referred to as rear facet) reflections are substantially minimized in a tilted, ridge waveguide SLD, where the front facet is defined as the facet where the device emits light. One end of the waveguide terminates at the front facet, and the other end terminates proximate, but not necessarily at, the back facet. The back facet termination includes radiating structure, such as a curvature of a given radius or radii. This curvature causes light to dissipate prior to striking the rear facet. Further, the waveguide can include a pointed tip at the end proximate the rear facet and can terminate in an unpumped region of the device.  
           [0006]    A method according to the principles of the invention includes the step of dissipating light prior to an unwanted reflection, and can include providing structure for uncoupling the device waveguide from unwanted reflections. The dissipating step can include terminating the waveguide a distance from the back facet and can include terminating the device in an unpumped region of the device. In one embodiment, the device is operated in a single mode.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    In the Figures:  
         [0008]    [0008]FIGS. 1A and 1B show a cross-sectional view and a top view respectively of a device according to the principles of the invention; and  
         [0009]    [0009]FIG. 1C shows a detailed view of a portion of the device of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0010]    [0010]FIG. 1A shows a cross-sectional view of a light emitting device  1  according to the principles of the invention. In this example, the device  1  is a ridge waveguide SLD. The device  1  comprises a body  2  having opposed faces (not shown) and opposed sidewalls  6 . The body  2  includes a substrate  12  having a first cladding layer  14  thereon, an undoped active layer overlying the first cladding layer  14 , a second cladding layer  18  overlying the active layer  16 , and a capping layer  20  overlying the second cladding layer  18 . A dielectric layer  22  overlies the capping layer  20 , except in the area of the waveguide  24   a . An electrical contact  32  overlies the substrate and another electrical contact  30  overlies the dielectric layer  22 , except in the area of the waveguide  24   a  which waveguide does not comprise a dielectric layer. Channels  24   b ,  24   c  are adjacent to the waveguide  24   a.    
         [0011]    In one embodiment, the undoped active layer  16  overlies a cladding layer  14  of one conductivity type, such as n-type, and has a cladding layer  18  of the opposite conductivity type, such as p-type, overlying it  16 . These layers are deposited on the substrate  12 , which is of the first conductivity type. For example, the substrate  12  can be n doped semiconductor material. In the exemplary configuration, the electrical contact  32  overlying the dielectric  22  is heavily doped p-type material and the other contact  30  is n-type. The channel regions  24   b ,  24   c  are formed by etching. Various materials can be used to make a light emitting device according to the principles of the invention. For example, the substrate can be GaAs and the active region can be materials such as GaAs, AlGaAs, or InGaAs. The cladding layers can be doped AsGaAs. In another example, the substrate can be doped InP and the active and clad layers can be InGaAsP of appropriate composition. Of course other materials can be used, such as other Group III-V compounds.  
         [0012]    In a ridge waveguide configuration as described above, the effective refractive index in the channel regions is lower than that in the ridge by an amount which depends on the residual thickness of the p-clad material under the channels. Light is guided in the active layer under the ridge by virtue of the index difference between the ridge  24   a  and the channels  24   b ,  24   c . Upon application of a voltage across the metal contacts  30 ,  32 , current flows only through the region with the dielectric opening. At low current, the active layer  16  is absorbing, and the emitted light consists of spontaneous emission. Beyond a certain current, the spontaneous emission is amplified spontaneous emission. The light is guided along the ridge  24   a  and emitted at relatively high power. At these current values, the region with the current flow is called the pumped region. Current does not flow in the region of the semiconductor structure under the dielectric, and this region is the unpumped region. The unpumped region is absorbing.  
         [0013]    For single mode operation, the lateral index step is given by  
               Δ                 n     =         n   1     -     n   2       ≤         (     λ   W     )     2       4        (       n   1     +     n   2       )                   (   1   )                               
 
         [0014]    where λ is the wavelength of the light, as shown in H. Kogelnik, Integrated Optics, 2d ed., Chap. 2, Springer-Verlak, New York. In this equation, W is the ridge width and the effective refractive indices under the channels are n 1  and n 2 , respectively. λ is the wavelength of the light.  
         [0015]    [0015]FIG. 1B shows a top view of the light emitting device  1 . In this view, opposed facets  3 ,  4  are each coated with anti-reflecting coating  40  and  42  , respectively. The ridge waveguide  24   a  emits light at its  24   a  front facet  3  termination. The waveguide  24   a  forms an angle θ relative to the direction perpendicular to the front facet  3 . The angle θ 2  of the output light  39  is larger than the angle of the waveguide, by virtue of Snell&#39;s law. The tilt angle, θ 1 , can be any value below the critical angle at the front facet  3 , at which point the output angle is 90°, and light cannot be coupled out of the device  1 . In one embodiment, the tilt angle is 5° to 70°, which provides ease of coupling.  
         [0016]    The curved section  50  proximate the rear or back facet  4  is a light dissipating structure. The curvature causes light to dissipate or radiate from the waveguide  24   a  before it can reflect back into the waveguide  24   a  from another facet, such-as the back facet  4 . In this configuration, little or no light radiated from the waveguide  24   a  can reach the straight (amplification) section of the waveguide  24   a.    
         [0017]    A detailed view of the rear section of the waveguide  24   a  is shown in FIG. 1C, where the detail is projected from the top view of FIG. 1B and onto a cross-section of the device  1 . In this detail, it is shown that the curvature of the channels  24   b ,  24   c  follows the curvature of the waveguide  24   a  for less than the length of the waveguide  24   a . That is, the waveguide ridge  24   a  extends past the channels  24   b ,  24   c . A termination structure  52 , in this example a pointed or angular shaped tip, proceeds further than the channels  24   b,c . The tip  52  radiates all rearward propagating light into an unpumped region, where it is absorbed. In this arrangement, little or no reflects can couple back into the waveguide  24   a.    
         [0018]    The radiation from a bent waveguide (or fiber) is determined by the radius of curvture of the bend. Radiation is small if the bend radius is larger than some, critical value, Rc, and it is large if the radius is much smaller than Rc. The critical radius is given by  
                 R   c     =       3     2      π        2                  n   1           (     Δ                 n     )       3   /   2            λ       ,           (   2   )                               
 
         [0019]    where Δn is given by equation (1) above for a single-mode waveguide. See E. A. J. Marcatili, Bell System Tech Journal, p. 2103-2132, September 1969. Where a curved radiating structure is used, the radius of curvature should be chosen much less than Rc as limited by practical considerations. In one aspect, a first radius of curvature can be chosen closer to Rc, and, after some radiating effect, a second smaller radius of curvature can be chosen. Of course, the optimal radius or radii, can be found using trial and error or other techniques depending upon the precise light emitting device and application under consideration.  
         [0020]    A light emitting devices according to the principles of the invention substantially improve spectral modulation to less than 2 percent, and can achieve attenuation of second coherence peaks on the order of 30 dB or greater. Such devices can be used in a variety of applications, including FOGs, and communication devices.  
         [0021]    While the principles of the invention have been illustrated using a ridge waveguide SLD, it should be apparent that the invention is not limited to this application. Any radiating structure used to dissipate light prior to facet reflections can be used without departing from the principles of the invention. For example, merely terminating a waveguide in an unpumped region some distance from a back facet could achieve a decrease in back facet reflections. Similarly, the light dissipating structure should not be considered limited to curved structures. Other types of dissipating structures can be used without departing from the invention.