Abstract:
A ring cavity laser has at least two facets and a mechanism is provided to produce unidirectional propagation and light emission at a first wavelength. A source of laser light at a second wavelength is injected into the cavity to reverse the direction of propagation and to produce emission at the second wavelength.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates, in general, to a method and apparatus for providing a wavelength converter, and more particularly to a monolithic semiconductor ring laser assembly for converting a laser beam having a first wavelength to a corresponding laser beam having a second wavelength.  
         [0002]     Advances in current monolithic semiconductor integration technology have permitted solid state lasers of complex geometry to be fabricated, including, for example, ring lasers having a variety of cavity configurations. Examples of such configurations are illustrated in U.S. Pat. No. 5,132,983, the disclosure of which is hereby incorporated herein by reference. These advances have expanded the potential applications for integrated semiconductor lasers, have added the attractiveness of improved manufacturability and reduced cost, and have opened the opportunity to explore new and novel features that can be incorporated within and outside the laser cavity.  
         [0003]     Over the past few years, thanks mainly to the popularity of the Internet, the demand for increased bandwidth has experienced explosive growth. Some carrier companies and their suppliers have addressed this demand by installing wavelength division multiplexing (WDM) systems, which allow multiple wavelengths of light to be transmitted through a single strand of optical fiber. An important part of the enabling technology for this is the ability to convert optical signals having one wavelength of light to corresponding optical signals having another wavelength, and thus there is a growing need for effective, inexpensive wavelength converters.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention is directed to the provision of monolithic semiconductor wavelength converters that are capable of providing either predetermined or variable wavelength shifts in an optical signal.  
         [0005]     In one of its preferred forms, the invention includes a ring-type solid state laser having at least two facets. A first optical signal is supplied to the laser cavity input at a first facet, with this signal being in the form of a light beam at a wavelength λ 2  at a first angle to the first facet. This input signal results in laser propagation in a counter-clockwise (ccw) mode within the ring laser cavity to produce an output R of laser light at the wavelength λ 2  at the second, or output facet. In accordance with the invention, a second optical input signal A of laser light having a wavelength λ 1 , is directed into the laser cavity at a second angle to the first facet. If the second optical input is stronger than the first, and the first and second angles are symmetric about the perpendicular to the first facet, injection locking and light propagation in the clockwise (cw) mode is produced, substantially eliminating the output R. In this manner, the output signal R at wavelength λ 2  is switched on and off by the absence or presence, respectively, of an input signal at wavelength λ 1 , thereby converting the input signal at λ 1  to an inverted output signal at λ 2 .  
         [0006]     One use of the foregoing converter/inverter is in wavelength divisional multiplexing, where multiple input optical signals of a single wavelength, for example λ 1 , are to be transmitted through a single optical fiber. In such a case, each of the input signals may be supplied to a different, corresponding converter, each of which normally operates at a different wavelength λ 2 , λ 3 , etc. Supplying a first input signal at wavelength λ 1  to the first converter will change that first signal to a first inverted signal at λ 2 . Similarly, supplying a second input signal also at λ 1 , to a second converter will change that second signal to a second inverted signal at λ 3 , and so on for additional input signals. The inverted output signals λ 2 , λ 3 , etc. may then be transmitted through a single optical fiber (for example) and recovered at the opposite end of the transmission line and, if desired, converted back to the original wavelength λ 1  through corresponding converter/inverters.  
         [0007]     In accordance with the invention, the ring lasers may utilize straight waveguide sections and facets, but preferably will incorporate curved waveguide sections to eliminate unneeded facets. Although a variety of ring lasers can be used to form a wavelength converter, the preferred ring lasers are the solid state curved waveguide lasers disclosed in copending U.S. patent application Ser. No. 09/918,544 of Alex Behfar, filed Aug. 1, 2001 and entitled “Curved Waveguide Ring Laser”, the disclosure of which is hereby incorporated herein by reference. As is known, a ring laser can operate in clockwise (cw) or counter-clockwise (ccw) modes, and a number of ways are available in the art to force these lasers to propagate in one direction or the other, as described in copending U.S. Patent Application No. 09,918,548 of Alex Behfar, filed Aug. 1, 2001 and entitled “Unidirectional Curved Ring Lasers; the disclosure of which is hereby incorporated herein by reference.  
         [0008]     In a second embodiment of the invention, a wavelength tunable source provides the first optical input to the a ring laser wavelength converter described above to permit different wavelength shifts. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]     The foregoing, and additional objects, features, and advantages of the present invention will be apparent to those of skill in the art from the following detailed description of preferred embodiments thereof, taken in conjunction with the accompanying drawings, in which:  
         [0010]      FIG. 1  is a graphical illustration of power vs wavelength for the spectra of the optical output of a free-running ring laser, including the longitudinal modes visible at a bias current above its threshold current for lasing;  
         [0011]     FIGS.  2 ( a ) and  2 ( b ) are diagrammatic top plan views of two states of a first embodiment of a wavelength converting ring laser in accordance with the present invention;  
         [0012]     FIGS.  3 ( a ) and  3 ( b ) are diagrammatic top plan views of two states of a second embodiment of a wavelength converting ring laser, capable of converting an input optical signal to multiple wavelengths in accordance with the present invention,  
         [0013]      FIG. 4  is a graphical illustration of the power vs wavelength of the spectrum of the ring laser of  FIG. 2  with the introduction of a weak signal input λ 1 ; and  
         [0014]     FIGS.  5 ( a ) and  5 ( b ) are diagrammatic illustrations of two states of a two-stage ring laser wavelength converter, and the respective gain curves for each ring laser. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENT  
       [0015]     Turning now to a more detailed description of the present invention, as illustrated graphically in  FIG. 1  by the spectra  10  of a typical free-running solid state laser, a ring laser has certain allowed longitudinal modes, illustrated by spectral peaks  12  and  14  superimposed spectral peaks for both clockwise and counterclockwise modes, with peaks  12  corresponding to clockwise modes and peaks  14  corresponding to counterclockwise modes. As illustrated, the ccw and cw peaks occur at substantially the same wavelengths. For a free running bi-directional ring laser the optical output corresponding to the cw and ccw modes can be equal, but it has been experimentally observed that in many cases the outputs tend to be unequal. Typically, such a laser will oscillate in one or more of these allowed longitudinal modes.  FIG. 1  is a graph of actual longitudinal modes observed for a ring laser at a bias current above the threshold current for the laser. A number of modes are visible at spectral peak positions indicated by . . . λ m −2, λ m −1, λ m , λ m +1, λ m +2, λ m +3, . . . , and these peaks correspond to the allowed modes of the laser. The absolute position of these modes, i.e., the exact wavelength of each peak, can be controlled and adjusted in a variety of ways, including thermal and electrical, as is known in the art.  
         [0016]     FIGS.  2 ( a ) and  2 ( b ) are directed to a first embodiment of the present invention, wherein a wavelength converting ring laser  20  includes first and second curved semiconductor waveguides  22  and  24  integrally fabricated on a substrate  26  to form a laser cavity. The fabrication of such semiconductor lasers is known in the art, and is described, for example, in U.S. Pat. No. 4,851,368. As illustrated in FIGS.  2 ( a ) and  2 ( b ), the waveguides  22  and  24  of laser  20  are spaced apart and preferably are curved inwardly to meet at upper and lower junctures  28  and  30 . Facets  32  and  34  are formed at upper and lower junctures  28  and  30 , respectively, in known manner, with the curvature of the arms being selected to allow an external light beam, such as beam  36  directed at a critical angle toward facet  34  and having a wavelength λ 2  corresponding to one of the longitudinal modes of the laser cavity  20  to enter and to propagate around the interior of the ring laser when the laser is properly biased, in known manner. The light will propagate in a clockwise or in a counterclockwise direction within the laser cavity, depending on the angle of the impinging beam  36 ; in the illustrated example, beam  36  produces counterclockwise propagation of light in cavity  20 , as indicated by arrow  40 .  
         [0017]     Light beam  40  propagating in the laser cavity  20  is primarily reflected internally from the inner surfaces of facets  32  and  34 , with a selected portion of the light striking the inner surface of outlet facet  32  in  FIG. 2 ( a ), at an angle to produce an outlet beam  42  at wavelength λ 2 , corresponding to input beam  36 .  
         [0018]     It will be understood that if the angle of the input beam  36  with respect to facet  34  were shifted, so that the input to facet  34  is in the direction of beam  46  (shown in phantom in  FIG. 2 ( a )), the resulting propagation of light in cavity  20  would be in the clockwise direction, illustrated by phantom beam  48 , and the output from facet  32  would be in the direction illustrated by phantom beam  50 .  
         [0019]     The converter/inverter of the present invention is illustrated in FIGS.  2 ( a ) and  2 ( b ) as incorporating the ring laser  20  having a first optical input signal, or beam  36 . In the illustrated embodiment, the signal may be a laser beam of substantially constant amplitude generated by a suitable source  60  such as a laser at a wavelength λ 2  matching a longitudinal mode of the ring laser. The beam  36  impinges on facet  34  and enters the cavity of laser  20 , where it is propagated in a counterclockwise direction, under suitable bias, to emit output beam  42 , also at the wavelength  2 , which may be referred to as output “R”. In the “normal” state of the laser, the output R is present, so R=1.  
         [0020]     A second input location is available for facet  34  of laser  20 , as discussed above with respect to the phantom beam  46 . This second input, which may be referred to as input “A”, is symmetrical with input beam  36  about a line perpendicular to the surface of facet  34 , but in the device illustrated in  FIG. 2 ( a ) there is no input at this location, so A=0.  
         [0021]     As illustrated in  FIG. 2 ( b ) an incoming optical signal indicated at beam  62  from a source  64  may be supplied to impinge on facet  34  of ring laser  20  at input A of the laser simultaneously with input beam  36 . If input beam  62  is stronger than beam  36 , i.e., has a higher intensity, and if beam  62  is at a wavelength λ 1  that is different than the wavelength λ 2  (λ 1 ≠λ 2 ) but that corresponds to any of the longitudinal modes of the ring laser  20 , then the input A=1, the presence of this beam  62  will cause the ring laser to operate in a counterclockwise direction, indicated by arrow  66 . As a result, the output signal at output facet  32  will switch from beam  42  to a second output beam  68 , which will be at wavelength λ 1 , and the output at R will be cut off; i.e., R=0.  
         [0022]     The input at A, represented by beam  62 , may be a photonic data stream of 1&#39;s and 0&#39;s at wavelength λ 1  which modulates the output of ring laser  20 . Thus, the absence of a data bit (A=0) causes the laser  20  to produce a corresponding data bit output signal R=1 at wavelength λ 2 . The presence of a data bit (A=1) at wavelength λ 1  produces a corresponding data bit output signal R=0.  
         [0023]     The illustrated ring laser wavelength converter  20  thus operates in such a way that when input A=0, the output R=1, with the output having a wavelength λ 2 . When input A is present, i.e., when A=1, then output R=0. This is summarized in Table 1:  
                                   TABLE 1                                       Wavelength       Wavelength           Value of A   of A   Value of R   of R                           0   —   1   λ 2             1   λ 1     0   —                      
 
         [0024]     In addition to performing a wavelength conversion function from λ 1  to λ 2 , the ring laser wavelength converter performs an inverter function on the incoming photonic bit stream of input A, so that the corresponding output photonic bit stream R is inverted from that of A.  
         [0025]     FIGS.  3 ( a ) and  3 ( b ) illustrate a modified form of the embodiment described above, providing a wavelength converting ring laser  70 , capable of conversion to multiple wavelengths. Elements common to the embodiment of FIGS.  2 ( a ) and  2 ( b ) are commonly numbered. These figures schematically illustrate the operation of a ring laser wavelength converter  70  that is capable of converting the wavelength of an input signal at input B to multiple different wavelengths, λ v  and of inverting the input signal.  FIG. 3 ( a ) illustrates the operation of ring laser wavelength converter  70  with the input B in an off state, or B=0. The normal input to the ring laser  70  is a signal from a variable wavelength source  72 , which may be a tunable laser producing a variable wavelength optical signal λ v , represented by beam  74  and variable to select a beam wavelength which corresponds to any of the allowed longitudinal modes of the ring laser. This input at a selected wavelength λ v  causes the ring laser to operate in the ccw and results in a “normal” output signal S=1 represented by beam  76  at wavelength λ v .  
         [0026]      FIG. 3 ( b ) illustrates the situation where an incoming optical input B of wavelength λ n , represented by beam  78 , is supplied by a source  80  such as photonic data stream of pulses. If the signals of input B are is stronger in light intensity than the input  74  to the ring laser at wavelength λ v , the ring laser  70  will operate in the cw direction indicated by arrow  82 , so that when B=1, then S=0. The wavelength λ n  corresponds to any of the longitudinal modes of the ring laser, and wavelength conversion will occur when λ n ≠λ v , allowing the signal input B to modulate the corresponding output S.  
         [0027]     Following the same logic as for the ring laser of FIGS.  2 ( a ) and  2 ( b ), the behavior of the laser  70  can be summarized in Table 2:  
                                   TABLE 2                                       Wavelength       Wavelength           Value of B   of B   Value of S   of S                           0   —   1   λ v             1   λ n     0   —                      
 
         [0028]     In addition to performing a wavelength conversion function from λ n  to λ v , the ring laser wavelength converter  70  performs an inverter function on the incoming photonic bit stream of input B, so that the output photonic bit stream S is inverted from that of B.  
         [0029]      FIG. 4  illustrates the spectra  90  and the longitudinal modes of a ring laser corresponding to the state depicted in  FIG. 3 ( a ) where λ v =λ m , B=0 and S=1. The peak  92  corresponds to the longitudinal mode for wavelength λ m , and is caused by injection locking of the input wave. It is noted that the graph of  FIG. 4  includes spectra of both the cw mode (indicated at  94 ) and the ccw mode (indicated at  96 ), both of which are present in the ring laser. Because of the injection locking caused by input beam  74 , the counterclockwise mode (arrow  40  in  FIG. 3 ( a )) is dominant, and the clockwise mode is weak. When the stronger input signal at wavelength λ n  is injected (B=1), injection locking within the ring laser will cause the clockwise propagation mode to dominate, and under this condition the output S=0.  
         [0030]     Although the ring lasers illustrated in FIGS.  2 ( a ),  2 ( b ),  3 ( a ) and  3 ( b ) incorporate curved waveguides, it will be understood that straight waveguide segments having faceted junctures can be used. Such waveguides are used in the ring lasers illustrated in FIGS.  5 ( a ) and  5 ( b ), which lasers in turn may instead incorporate curved waveguides.  
         [0031]     The wavelength converters described above are able to efficiently convert one wavelength to another by an amount that is determined by the gain profile of the particular laser. A larger wavelength shift can be achieved by cascading two or more converters, in the manner illustrated at  98  in FIGS.  5 ( a ) and  5 ( b ), provided that the gain profile of the succeeding laser overlaps that of the prior laser, but extends beyond the gain curve of the prior laser.  
         [0032]     As diagrammatically illustrated in  FIG. 5 ( a ) a first ring laser  100  (labeled Laser A) is cascaded with a second ring laser  102  (labeled Laser B) so that the normal optical output from the first, or prior, laser  100  is the optical input injected into the second, or succeeding, laser  102 . The gain curve for laser  100  is illustrated at  104  on a graph  106  representing the spectrum of the laser. For simplicity, the positions of the longitudinal modes of the laser are illustrated by dashed lines  108 , with the gain curve  104  illustrating the power of the output from the laser when it operates at wavelengths corresponding to the various modes  108 . Similarly, the gain curve for laser  102  is illustrated at  114  on graph  116  representing the spectrum of laser  102 . For simplicity, the positions of the longitudinal modes are identical to those of laser  100 , and thus are also indicated by dashed lines  108 .  FIG. 5 ( b ) is similar to  FIG. 5 ( a ) but the two figures differ in that  FIG. 5 ( a ) illustrates the normal condition in the absence of a modulating signal to be converted, while  FIG. 5 ( b ) illustrates the modulated condition, where an input modulating signal to be converted is present.  
         [0033]     Lasers  100  and  102  both incorporate waveguide legs  120 - 123  which are joined at facets  126 - 129  to form a ring cavity, in known manner.  
         [0034]      FIG. 5 ( a ) and ( b ) schematically show the operation of the two-ring laser wavelength converter  98  as converting a wavelength λ a  at an input X (similar to input A of  FIG. 2 ( b )) to a corresponding wavelength λ c  at output Z. As illustrated in  FIG. 5 ( a ), the ring laser wavelength converter  98  initially has a modulating, or signal input X=0 to ring laser A, and also has a “normal” input  140  to ring laser A that is at wavelength λ b , where λ b  corresponds to any of the allowed longitudinal modes of both the ring lasers A and B. This input at wavelength λ b  causes the ring laser A to operate normally in the ccw and results in a normal output Y=1 at wavelength λ b , represented by beam  142 . Y serves as the modulating, or signal, input to ring laser B and is stronger in intensity than a second “normal” input  144  to ring laser B at wavelength λ c  Wavelength λ c  corresponds to a longitudinal mode of ring laser B, and produces ccw mode propagation in ring laser B in the absence of signal Y, in which case its output is Z=1. In the presence of Y, the ring laser B will operate in the cw mode, resulting in output Z=0.  
         [0035]      FIG. 5 ( b ) shows the situation for X=1 where an incoming optical modulating signal of wavelength λ a , represented by beam  146 , is stronger in light intensity than the input  140  at wavelength λ b . This causes the ring laser A to operate in the cw direction so that Y=0, because of the modelocking effect of input X=1. Since Y=0, the only input to ring laser B is the normal input at wavelength λ c , so ring laser B operates in the ccw mode and results in Z=1 at wavelength λ c , indicated at output beam  150 . Accordingly, the two-stage converter  98  receives input signals such as photonic pulsed signals at input  146  at a first wavelength λ a , and emits corresponding pulses at a second wavelength λ c , at output  150 , thus converting the data signal wavelength without inverting the signal.  
         [0036]     Ring lasers of different cavity lengths can be used for ring lasers A and B, the only requirement being that the longitudinal modes of the two lasers intersect at λ b . Additional ring lasers can be cascaded to laser B, to produce additional wavelength conversion or to invert the modulating signal.  
         [0037]     Although the present invention has been described and illustrated in terms of preferred embodiments, it will be apparent to those of skill in the art that numerous variations and modifications may be made without departing from the true spirit and scope of the invention, as set out in the following claims.