Abstract:
Integration of laser sources into optoelectronic integrated circuits requires that the laser do not operate using two cleaved end facets. Unfortunately, replacing of one of the end facets results by either a dry etched mirror or by a corner reflectors results in undesirable performance of the laser source since a gain coefficient for the laser source is lower than that for a dual cleaved end facet laser source. A modified waveguide is thus proposed which serves to reduce the undesirable effects found when a corner reflector is used by providing an improved waveguide region between the cleaved end facet and the corner reflector that facilitates excitation of a single optical mode within a laser cavity formed between the corner reflector and the cleaved end facet.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to optical waveguides and more particularly to optical waveguide structures having total internal reflecting mirrors.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fibre optic communication systems have gained widespread acceptance over the past few decades. With the advent of optical fibre, communication signals are transmitted as light propagating along a fibre supporting total internal reflection of the light propagating therein. Many communication systems rely on optical communications because they are less susceptible to noise induced by external sources and are capable of supporting very high speed carrier signals and increased bandwidth. It was found that single mode optical communications systems support a higher rate of data transfer over longer distances. Consequently, single mode optical fibre is now a standard medium for transferring optical signals. Unfortunately, optical fibre components are bulky and often require hand assembly resulting in lower yield and higher costs. One modern approach to automating manufacture in the field of communications is integration. Integrated electronic circuits (ICs) are well known and their widespread use in every field is a clear indication of their cost effectiveness and robustness.  
         [0003]     Presently, there is substantial promise in implementing waveguides and optical components within integrated waveguide material. These materials allow for integration of active and passive devices within a same physical substrate. These waveguides are typically formed in semiconductor material where they are often produced using layers of different material to provide a refractive index contrast between the waveguide core and its cladding. Alternatively, relative differences in dopant concentrations can provide small index differences that can be sufficient to provide guiding of an optical signal within a waveguide so formed.  
         [0004]     Amongst the active devices that are manufactured into a same physical substrate as optical waveguides are laser sources. These laser sources are manufactured within the same substrate as the waveguide and thus advantageously allow for direct coupling from the laser source to the waveguide. Unfortunately, difficulties arise when these laser sources are manufactured within a same substrate. One such difficulty is forming end facets with the necessary optical qualities. Typically, the end facets of the laser are cleaved which provides a very high quality surface. Unfortunately, cleaving the laser to provide high quality end facets defeats the advantages sought in producing an integrated semiconductor optoelectronic circuit.  
         [0005]     It would therefore be advantageous to provide a replacement for the cleaved end facet of the laser source to permit integration of the laser source within an optoelectronic substrate as well as to provide an improved reflection coefficient from the replaced end facet.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with the invention, there is provided a waveguide disposed on a substrate comprising: an input port for receiving an optical signal having a first optical mode.  
         [0007]     In accordance with an aspect of the invention, there is provided a first waveguide portion having a first optical length coupled to the input port for propagating the optical signal with the first optical mode; a second waveguide portion coupled to the first waveguide portion for receiving the optical signal having the first optical mode and for transforming the optical mode of the optical signal from the first optical mode to a second optical mode along a length of the second waveguide portion; and, a corner reflector optically coupled to the second waveguide portion for receiving the optical signal having the second optical mode from the second waveguide portion and for internally reflecting the second optical mode back into the second waveguide portion, where the optical signal having the second optical mode upon reflection from the corner reflector propagates along the second waveguide portion and therefrom to the first waveguide portion.  
         [0008]     In accordance with another aspect of the invention, there is provided a laser source for providing an optical signal having a first optical mode, said laser source disposed on a substrate, comprising: a first partially reflective optical component, the first partially reflective optical component for functioning as an output port; a first waveguide portion having a first optical length for propagating the optical signal with the first optical mode; a second waveguide portion having a second optical length coupled to the first waveguide portion for receiving the optical signal having the first optical mode and for transforming the optical mode of the optical signal from the first optical mode to a second optical mode along a length of the second waveguide portion; a corner reflector optically coupled to the second waveguide portion for receiving the optical signal having the second optical mode from the second waveguide portion and for internally reflecting the second optical mode back into the second waveguide portion, where the optical signal having the second optical mode upon reflection from the corner reflector propagates along the second waveguide portion and therefrom to the first waveguide portion; and, a gain medium, the gain medium disposed along an optical path between the corner reflector and the first partially reflective optical component forming a lasing cavity for the laser source for providing the optical signal having the first optical mode.  
         [0009]     In accordance with yet another aspect of the invention, there is provided a method of reflecting an optical signal using an integrated optical substrate comprising the steps of: providing a waveguide having a first waveguide portion and a second waveguide portion, the first and second waveguide portions in optical communication; receiving an optical signal having a first optical mode at an input port disposed at an end of the first waveguide portion; coupling the optical signal to the first waveguide portion for propagation therein in a lowest order single mode; coupling the optical signal in the second waveguide portion for propagation within a region therein in an other than lowest order single mode; reflecting the optical signal with a corner reflector.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Exemplary embodiments of the invention, will now be described, in conjunction with the drawings, in which:  
         [0011]     Prior Art  FIG. 1  illustrates three different types of prior art integrated laser sources;  
         [0012]     Prior Art  FIG. 2  illustrates the near field pattern in terms of position and optical intensity for light emitted from two prior art laser sources;  
         [0013]     Prior Art  FIG. 3   a  illustrates a single optical mode propagating within a waveguide strip region towards a prior art corner reflector;  
         [0014]     Prior Art  FIG. 3   b  illustrates a single optical mode propagating within a waveguide strip region away from the prior art corner reflector described with reference to  FIG. 3   a;    
         [0015]      FIG. 4  illustrates an embodiment of the invention, an improved waveguide design having a corner reflector that facilitates reflection of a multi mode optical signal therefrom;  
         [0016]      FIG. 5   a  illustrates an embodiment of the invention with demonstrates an optical signal propagating to a corner reflector;  
         [0017]      FIG. 5   b  illustrates the embodiment of the invention shown in  FIG. 5   a  with an optical signal propagating from a corner reflector;  
         [0018]      FIG. 6  another embodiment of the invention is shown where a corner reflector is used within a laser source manufactured on a semiconductor substrate;  
         [0019]      FIG. 7  illustrates dual laser sources utilized within an optoelectronic integrated circuit;  
         [0020]      FIG. 8  illustrates a variable optical attenuator; and,  
         [0021]      FIG. 9  is a top view of an embodiment of the invention featuring two corner reflectors. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]      FIG. 1  illustrates three different types of prior art integrated laser sources. A first laser source  101  has a first cleaved end facet  101   a  and a second cleaved end facet  101 b. These two end facets and a waveguide strip region form a lasing cavity for the laser source, where between these facets a gain medium is disposed within the waveguide strip region, which when provided with electrical energy causes lasing within the waveguide strip. The cleaved end facets provide an adequate reflection coefficient for the laser source to facilitate providing an output optical signal therefrom. Typically, one of the cleaved end facets has a lower reflectivity than the other and as a result light from the laser source is emitted from that facet. The emitted light from this facet typically follows an optical energy distribution that has an approximately Gaussian optical energy beam profile. The optical signal emitted from the laser source thus has a single transverse optical mode, or in terminology familiar to those of skill in the art, is “single mode”. Other optical modes, or transverse beam profiles of the optical signal, are also possible in dependence upon the waveguide strip geometry. For larger widths of the waveguide strip connecting the end facets, the optical signal tends to follow a multi mode distribution, where the optical mode typically has more than one peak in the transverse beam profile. In order to preserve signal integrity it would be beneficial to provide a more accurate reflection with minimal disruption of the optical signal as it is reflected.  
         [0023]     Unfortunately, laser sources with cleaved end facets are not easily integrated into an optoelectronic integrated circuit (OIEC). A second laser source  102  has a first cleaved end facet  102   a  and a second straight etched end facet  102   b  where these two end facets and a waveguide strip region  102   c  therebetween form a lasing cavity for the laser source.  
         [0024]     To those of skill in the art it is known that in order to eliminate cleaved end facets anisotropic dry etching techniques are used. Unfortunately, with the use of dry etching techniques the laser sources manufactured as a result thereof still generate an output optical signal at both ends of the laser, and they generally exhibit higher threshold currents than cleaved facet lasers due to surface roughness as a result of the dry etch. Thus having one of the end facets of the laser straight etched is not advantageous, although it permits integration of the laser source within the OEIC.  
         [0025]     In order to obtain single ended output from a laser source  103  manufactured within the OEIC, a corner reflector (CR)  103   b  typically replaces one of the straight etched end facets. A waveguide strip region  103   c  is disposed between a first cleaved end facet  103   a  and a second end facet  103   b  in the form of a CR. The CR typically has smooth sidewalls with a reasonably sharp corner with a corner radius of approximately 1.25 microns. Between the cleaved end facet  103   a  and the CR  103   b  the waveguide strip region acts a gain medium that has a gain coefficient that characterizes the laser source in terms of an amount of electrical energy that is converted into optical energy to form the optical signal emitted at the cleaved end facet. The corner reflector advantageously provides for increased reflection of the photons within the lasing cavity between the facets  103   a  and  103   b.    
         [0026]     In use the CR facilitates reflecting of a portion of the optical signal by an optical process of total internal reflection (TIR). For lasers manufactured containing GaAs materials, TIR is observed between the waveguide containing the gain medium and air interface for incidence angles of greater than 17 degrees.  
         [0027]     To those of skill in the art it is known that the facet reflectivity and scattering for the etched facet devices can be approximated using mathematical formulas. Thus, following from mathematical approximations, the straight-etched facets typically exhibit a reflectivity of 12% and a scattering loss of 63%, while etched CRs typically exhibit a reflectivity of 53% and a scattering loss of 44%. The decrease in scattering loss is typically attributed to the recapture of some of the scattered light by the etched CR. Although, cleaved end facets provide increased reflectivity, using a CR is a significant improvement over the straight etched end facet.  
         [0028]     To those of skill in the art it is known that the single optical mode  201  has a majority of its optical power located in a center peak of the optical mode. However, when the single optical mode reflects from the corner reflector, the single optical mode is transformed into a multi-mode optical signal because of the corner portion of the corner reflector created between two straight etched angled portions  104  and  105  making up the CR. The corner portion  106  scatters a portion of light from within the center peak of the optical mode upon reflection. Thus, reflecting from a corner reflector favours optical modes other than the desired lowest order single mode, especially when these modes are emitted from the laser source. As previously mentioned, it would be preferable to have a reflector that provides a reflected signal that is substantially equivalent to the signal that was incident on the reflector.  
         [0029]      FIG. 2  illustrates the near field pattern in terms of position and optical intensity for light emitted from the laser sources analogous to the laser sources  101  and  103  described with reference to  FIG. 1 . A beam profile of the optical signal emitted from the dual cleaved end facet laser source  101  is shown in trace  201  and a beam profile of the optical signal emitted from the laser source  103  having the CR is shown in trace  203 . From this graph it is evident that the near field pattern observed for the laser source having two cleaved end facets is approximately Gaussian in shape and having a majority of optical power from the laser source located approximately at a center of the optical signal. For the cleaved end facet and CR laser source the near field pattern is not as ideal for use with single mode waveguides. The etched CR unfortunately facilitates lasing of optical modes other than the desired lowest order single mode within the laser cavity waveguide strip region. Unfortunately, as a result of the design of this laser source  103 , in order to satisfy phase matching criteria at this pair of turning mirrors  104  and  105  making up the CR, and to account for optical loss due to emission at the corner of the CR  106 , the optical mode operating within the waveguide strip region is other than the desired lowest order single mode.  
         [0030]      FIG. 3   a  illustrates a single optical mode  301  propagating within the waveguide strip region  103  towards the CR  103   c.  Referring to  FIG. 3   b,  upon reflection of the optical mode from the CR  103   c,  the optical mode no longer has a single mode beam profile but now has a multi mode profile  302 , and in this case the optical mode is termed “odd ordered”. Unfortunately, odd ordered modes such as these do not easily couple into optical fibers or into other single mode devices. Thus having this odd ordered optical mode optical signal is not advantageous since it adds complications when the laser source is integrated with other single mode components within OEICs. It is known to those of skill in the art that single mode operation for optical device is preferable since a majority of the optical devices receive single optical modes and propagate single optical mode signals therefrom. Furthermore, operation of the laser source in this odd-ordered optical mode decreases the gain coefficient, thus offering inferior performance to an end facet laser source but providing for easy integration into an OEIC compliant package.  
         [0031]      FIG. 4 , illustrates an embodiment of the invention, an improved waveguide design  400  having a corner reflector  403  that facilitates reflection of a multi mode optical signal therefrom. The improved waveguide design has dual waveguide portions. A first waveguide portion  401  is designed to propagate a single optical mode along its length. This first waveguide portion  401  is extended into a second waveguide portion  402 . The second waveguide portion  402  is designed to propagate other than a single mode, and preferably an odd ordered transverse mode having dual optical power peaks. At an end of this second waveguide portion a CR  403  is disposed having two straight etched turning mirrors reflectors  404  and  405  and a corner  406  therebetween, similar to that shown in  FIG. 1 . The CR  403  is for reflecting of the other than a single optical mode therefrom. In  FIG. 5   a  and  FIG. 5   b,  the reflection of the optical mode is exemplified. Referring to  FIG. 5   a,  an optical signal having a single optical mode  501  a is shown propagating to a device according to the design described with reference to  FIG. 4 . The single optical mode  501   a  propagates through the first waveguide portion and in the second waveguide portion it is transformed into a multi mode  501   e  at an end of the second waveguide portion proximate the CR  403 . Optical modes  501   b,    501   c  and  501   d  are illustrative of the transformation from the single optical mode  501   a  to the multi mode  501   e.  Referring to  FIG. 5   b,  upon reflection form the CR  403 , the multimode  502   e  propagates through the second waveguide portion  402  and as it propagates along the optical mode is transformed by nature of the waveguide design back into a single mode  502   a.  Optical modes  502   d,    502   c  and  502   b  are illustrative of the transformation of the mode that occurs as the optical signal propagates within the waveguide device.  
         [0032]     By advantageously providing a multi mode optical mode having preferably two optical power distribution peaks  508  and  509  to the CR, optical losses associated with the corner  506  of the CR are significantly reduced. Since a majority of the optical power is found in these two peaks  508  and  509 , this optical power reflects from the first and second turning mirrors  504  and  505  and a significantly lower portion of the optical power found in the optical mode illuminates the corner  506  of the CR and hence a lesser portion of the optical power of the optical mode is lost as compared to a signal optical mode reflecting from the CR as taught in the prior art of  FIG. 3 .  
         [0033]     Referring to  FIG. 6  another embodiment of the invention is shown. In this embodiment the improved CR design is used within a laser source  600  manufactured on a semiconductor waver. A cleaved end facet,  602 , first and second waveguide portions  401  and  402 , as well as a CR  603  make up the laser cavity for the laser source. An optical path length between the cleaved end facet  602  and the CR  603  determine the wavelength of the optical signal output from the laser source. The first and second waveguide portions are doped in such a manner as to provide an optical gain to a portion of the optical signal in response to electrical input to permit lasing action within the laser source for facilitating the propagation of the output signal from the laser source. Advantageously, because a multi mode beam is provided to the CR, the optical mode output from the laser source follows a single mode Gaussian profile and is not multi mode as demonstrated in the prior art. Thus, the improved laser source shown in  FIG. 6  is easily integrated into optical devices that utilize single mode optical signals. Optionally, the end facet  602  is chemically deep etched when the corner reflector  603  is etched. Since chemical deep etching provides very accurate dimensional control, the length of the lasing cavity is precisely controlled. Additionally, the step of cleaving is avoided thereby reducing costs.  
         [0034]     In  FIG. 7 , dual laser sources  701  and  702  as shown in  FIG. 6 , are utilized within an OEIC  700 . In this case the laser sources  701  and  702  have two different optical lengths of the waveguide regions between the end facet and the CR. Thus each of these laser sources provides a different wavelength output signal at the end facet thereof. The OEIC device using the dual laser sources functions as a multiplexer, where the optical output signals from each of the laser sources illuminates an integrated wavelength dispersive element  703  in the form of an echelle grating. The wavelength dispersive element  703  combines the two optical signals from the laser sources into a single multiplexed output signal. This multiplexed output signal is provided to an output waveguide  704  in optical communication with the wavelength dispersive element and furthermore to an output port  705  on the OEIC  700 . Of course, a partially reflective end facet of the output waveguide along with the laser sources and the echelle grating, in addition with a gain medium, optionally form a multistripe array grating integrated cavity (MAGIC) laser.  
         [0035]     Advantageously, by providing an improved CR for use in, for example, a laser source, improved gain coefficients are realized for the laser sources since more optical power is provided from the source with respect to a same amount of current applied to the waveguide region acting as the optical gain medium disposed between the end facet and the CR. Whereas in the prior art a significant amount of light is lost from the optical signal because of the peak of the optical mode being centered on the corner of the CR. The improved laser source additionally facilitates easy integration into OEIC components because of not having dual cleaved end facets and hence is more easily integrated into the OEIC without many additional wafer processing steps.  
         [0036]     This embodiment is particularly advantageous because the echelle grating  703  is formed using a deep etching process. Thus, the echelle grating  703  and the corner reflectors are provided in a same product step further reducing costs.  
         [0037]     Referring to  FIG. 8 , a variable optical attenuator is shown. The attenuator includes an integrated substrate with an input port  801 , an arrayed waveguide grating  802 , a set of variable optical attenuators  803 , and a set of reflectors  804  according to the invention. Additionally, that attenuator features an optical circulator  810  having a first port  811 , a second port  812  and a third port  813  as well as a single mode optical fibre  820  for optically coupling the second port  812  of the circulator to the input port  801  of the integrated attenuator substrate. In operation, a wavelength multiplexed optical signal is received by the first port  811  of the optical circulator  810  and provided at the second port  812 . The wavelength multiplexed optical signal then propagates along the single mode fibre  820  and is received by the input port  801  of the integrated substrate. The wavelength multiplexed optical signal is separated into a variety of optical signals, each corresponding to a predetermined wavelength channel supported by the arrayed waveguide grating. Each of the optical signals corresponding to a predetermined wavelength channel is provided to one attenuator  803  and a reflector  804 . The attenuator  803  varies the optical power of the optical signal. The reflector then causes the optical signal to propagate back to the arrayed waveguide grating  802 . The arrayed waveguide grating combines the attenuated optical signals and provides a wavelength multiplexed optical signal at the input port  801 . The wavelength multiplexed optical signal propagates along the fibre  820  and is optically coupled to the second port  812  of the circulator  810 . The circulator then provides the optical signal to the third port  813 . Since there is an optical attenuator for each of the supported predetermined wavelength channels, the wavelength multiplexed optical signal provided by the arrayed waveguide grating  802  has an intensity profile that depends upon the amount of attenuation provided by each of the attenuators.  
         [0038]     Referring to  FIG. 9 a  top view of an alternative embodiment of the invention is shown. The illustrated device includes a single mode waveguide  901 , a multi-mode waveguide  902 , a set of reflective facets  903  all provided on a waveguide substrate  905 . In use, light propagating within the single mode waveguide  901  in provided to the multi-mode waveguide  902 . The propagation of the light within the multi-mode waveguide  902  causes excitation of the higher order modes. The length of the multi-mode waveguide  902  has been chosen to provide a good mode profile to the reflective facets. The reflective facets are well suited to reflecting optical signals incident on their face and less well suited to reflecting optical signals proximate the intersections of the facets. This particular embodiment of the invention is intended for exciting and reflecting a multiple of four modes. Clearly, other embodiments of the invention supporting other numbers of modes are easily envisioned by one of skill in the art of waveguide design. As is clearly demonstrated by the prior art of Jenkins et al. in U.S. Pat. No. 5,410,625 a shorter length of multi-mode waveguide will result in four peaks in comparison with the length associated with two peaks. This allows the reflector to be substantially shorter than an alternative embodiment of the invention that supports only two peaks. Additionally, as can be seen, the light enters the multi-mode waveguide  902  somewhat off-axis. This advantageously enhances excitation of the higher order modes, however it is likely to cause a higher level of attenuation when the optical signal is coupled back into the single mode waveguide. Thus, the length of the multi-mode waveguide  902  should be carefully chosen to provide the desired optical characteristics.  
         [0039]     One of skill in the art of optical component design will be aware that the invention is useful in a wide variety of applications in which integrated substrates incorporate reflectors and is not limited to the examples provided above. Clearly, the invention is useable with both buried waveguides and ridge waveguides although the processes used in creating the reflector will likely vary with the type of waveguide used.  
         [0040]     Numerous other embodiments can be envisaged without departing from the spirit or scope of the invention.