Abstract:
Systems and methods according to the present invention address this need and others by providing SLD devices and methods for generating optical energy that reduce internal reflections without the use of an absorber region. This can be accomplished by, among other things, adapting the waveguide geometry to dump reflections from the front facet out through the back facet of the device.

Description:
BACKGROUND  
       [0001]     The present invention relates generally to optical light sources and, more particularly, to superluminescent diodes (SLDs) having reduced internal reflectivity.  
         [0002]     Optical technologies associated with sensing, instrumentation, and communication have evolved significantly over the last several decades. For example, over the last two decades optical communication technologies have transitioned from laboratory curiosities to mainstream products which are the fundamental means for high speed/high bandwidth communications, advanced sensors, and high precision instruments. Various light sources can be used in these diverse applications. Lasers, for example, can be used to generate constant wave (CW) or pulsed optical signals suitable for use in communication devices. InGaAs/InP quantum well lasers with suitable guiding and gain regions can be driven to generate optical pulses for transmitting data in fiber optic communication networks. The optical output of a laser, however, may not be suitable for all optical applications. Low coherence interferometry and coherence domain reflectometry are examples of optical applications in which it is preferable to use light sources having a high power output over a much broader bandwidth than is generally available using lasers.  
         [0003]     Superluminescent diodes (SLDs), like lasers, use stimulated emission as a primary mechanism for generating light, but are not intended to exceed the threshold for laser oscillation. Even though lasing is not intended in SLDs, various internal reflections occur within conventional SLDs which may result in spectral output variations which are undesirable in a number of applications, including those mentioned above, and which may (under adverse conditions) result in lasing. To better understand these undesirable reflections, consider the exemplary SLD device  10  illustrated in FIGS.  1 ( a )-( b ).  
         [0004]     Therein, a generalized, side section of an SLD device  10  is shown in  FIG. 1 ( a ) including a contact region  12 , a first cladding layer  14 , an active region  16  which establishes a vertical waveguide and a second cladding layer  18 . Waveguide  16  confines the optical energy to the active region  16  in the vertical dimension in  FIG. 1 ( a ). Pumping current I is injected into the contact region  12  to pump active region  16  to generate light via spontaneous emission. Active region  16  can, for example, be fabricated as a multiple quantum well structure (e.g., alternating layers of GaInAs or GaInAsP) or a bulk active region (e.g., GaInAsP or AlInGaAs). The active region can also contain additional layers (e.g. GaInAsP or AlInGaAs) to form a separate confinement heterostructure (SCH) to tailor the optical waveguide properties of active region  16 . First and second cladding layers  14  and  18 , which can be fabricated from InP, operate to contain carriers vertically within the active region due to their higher bandgaps. Light generated within the waveguide created by the layers  14 - 18  is output via a front facet  20  in the direction indicated by the output arrow. Typically, the optical output energy would be coupled to, e.g., an optical fiber (not shown) to be guided to another device.  
         [0005]     Reflections can occur when light generated by injection luminescence strikes an interface between the front facet  20  of the SLD  10  and the outside environment (e.g., air). At normal incidence to the facet, the magnitude of the reflected optical energy will primarily depend upon the phase index n of waveguide  16  of the SLD device  10  relative to the phase index of the ambient. Taking for example the ambient to be air (n 1 =n p,air@STP@ 589 nm=1.00029) and the SLED waveguide index to be n 2 =n p,SLED =3.2, a typical value for the amount of power reflected at the normal incidence facet is approximately 27% (R=(n2−n1) 2 /(n2+n1) 2 ). These reflections, symbolized by the arrow R FF  in  FIG. 1 ( a ), will travel back along the waveguide created by layers  14 - 18  to the back facet  22  of the SLD, where they will again be reflected as indicated by the arrow R BF . These reflections may themselves be captured by the waveguide and transmitted as an unintended double-pass amplified output of SLD  10 . This results in various undesirable effects including, for example, the possibility that downstream devices will treat the reflections as optical “echoes” of the power that is intentionally transmitted by the SLD  10 . Another undesirable effect which may occur, particularly at high output powers, is that the reflections which are being generated back and forth between the front facet  20  and back facet  22  may result in a spectral modulation or “ripple” on the optical energy output from SLD  10 . This spectral ripple causes ghosting in low coherence interferometry applications, makes it difficult to provide power control over the optical power generated by SLD  10  and, in many cases, renders the SLD useless if the magnitude of the spectral ripple is too high. Many applications, for example, require the peak-to-peak spectral ripple to be less than 0.5 dB. The mean reflectivity at each facet (R) required for a SLD with internal single pass gain (G) to obtain a desired spectral ripple (ΔG) is given by R=G −1 (ΔG−1)/(ΔG+1). This means that for a high-power SLD, which can have a single-pass optical gain of 30 dB or more, the mean power reflected at each facet must be less than 0.006%. The need for this extremely low facet reflectivity represents one of the major challenges in producing high-performance, high-power SLDs.  
         [0006]     Various techniques have been employed in an attempt to reduce internal reflections in SLDs. One technique, seen in the  FIG. 1 ( a ) is to apply antireflective (AR) coatings  24  and  26  to the front facet  20  and back facet  22 , respectively. However, AR coatings providing less than 0.1% power reflectivity are extremely difficult to manufacture. AR coatings are also not broadband, i.e. they exhibit a wavelength dependent reflectance spectrum. Thus, while applying an AR coating can be effective within a certain narrow bandwidth, it may not be a viable solution for SLDs that generate wider bandwidth spectra.  
         [0007]     Another technique, described in an article by A. T. Seminov, V. R. Shidlovski and S. A. Safin, entitled “Wide spectrum single quantum well superluminesent diodes at 0.8 um with bent optical waveguide”, Electron. Lett, vol. 29, pp. 854-857, 1993), is to provide an angle θ at the interface between the lateral waveguide  17  and the front facet  20 . This technique is, for example, illustrated in  FIG. 1 ( b ) which figure is a top sectional view taken along section line A-A in  FIG. 1 ( a ). As used throughout this specification, such “angles” are referenced with respect to facet normal. The vertical waveguide  16  and lateral waveguide  17  form a 2-dimensional waveguide with a defined optical mode that propagates along the length of the chip. As one skilled in the art is aware, the lateral waveguide  17  can be formed by various means for example a ridge waveguide or a buried waveguide. Although the angled facet can reduce the reflections that propagate back through the waveguide to the back facet  22 , and has the advantage that the effective facet reflectivity is inherently broadband, nonetheless some reflections will still occur. The amount of reflection can be further reduced by using a combination of AR coatings and angled facet, as shown in  FIG. 1 ( b ). Yet even with this combination it can be difficult to obtain the required low facet reflectivity for high-power, high-performance broadband SLDs.  
         [0008]     Another mechanism that has been proposed for dealing with internal reflections is to dump the rearward traveling reflections into an absorbing region  17  indicated in  FIG. 1 ( b ). The absorbing region  17  is formed by not injecting current along the entire length of the active waveguide  16 , thereby creating an unpumped section  13  indicated in FIGS.  1 ( a ) and  1 ( b ). Since this section is not pumped, it absorbs light rather than emits light thereby attenuating the rearward traveling reflections and preventing them from reaching the back facet  22 . The main disadvantage of this approach is that it results in a longer chip which translates directly into higher production cost. This is particularly the case for quantum-well active regions which have a small confinement factor and are readily “bleached” at high optical power. As shown in the article by Song, J. H. et al., entitled “High-power broad-band superluminescent diode with low spectral modulation at 1.5-μm wavelength”, Photonics Technology Letters, IEEE, Volume: 12, Issue: 7, July 2000, Pages: 783-785, good spectral ripple can be obtained using an absorber, but at the expense of more than doubling the chip length.  
         [0009]     To reduce the length of the absorbing region, it has also been proposed to provide an active (reverse biased) absorption region to absorb reflections in the region proximate the back facet  22 , as shown in the side view in  FIG. 2 ( a ) (wherein the reference numerals are reused from  FIG. 1 ( a ) to denote similar structures). Therein, a second contact region  24  is placed over the portion of the SLD  26  proximate the back facet  22 . The contact region  22  is separated from the contact region  12  to create an absorption region and a gain region, respectively, within the SLD  26 . The contact region  22  is reverse biased relative to the contact region  12  using a voltage V B . This has the effect of biasing the absorber region which increases the amount of absorption and also increases the upper wavelength at which absorption occurs above that of the emission wavelengths of the gain region, thereby acting to further absorb reflections traveling along the waveguide/active region  16 . However, this solution suffers from the potential drawback that the interface between the absorber region and the gain region will itself cause reflections. Also, a second electrode that requires a separate bias voltage introduces additional complexity that increases the overall cost of operating the SLD. An example of a reverse-biased absorption region can be found in U.S. Pat. No. 5,252,839, the disclosure of which is incorporate here by reference.  
         [0010]     Yet another approach to reduce facet reflections is to fabricate so-called “window” sections adjacent to the facets. Using exemplary window sections  28  and  29  in  FIG. 2 ( a ), the optical mode is no longer confined and is allowed to freely diffract such that reflections from the facet do not couple as efficiently back into the waveguide as compared to SLDs wherein the waveguide extends all the way to the facet. However, these window regions are of limited utility because they cannot be made too long otherwise the freely diffracting beam can no longer be captured effectively with practical optical elements. Also, the window regions require complicated wafer fabrication processes to selectively remove the active region and replace it with epitaxial regrowth of the upper cladding material. The incremental reduction in facet reflectivity achieved with window structures often does not warrant the added fabrication complexity and associated yield loss/cost increase.  
         [0011]     Accordingly, it would be desirable to provide SLD techniques and devices that provide high power and high quality output optical energy by reducing internal reflections.  
       SUMMARY  
       [0012]     Systems and methods according to the present invention address this need and others by providing SLD devices and methods for generating optical energy that reduce internal reflections without the use of an absorber region. This can be accomplished by, among other things, adapting the waveguide geometry to dump reflections from the front facet out through the back facet of the device.  
         [0013]     According to one exemplary embodiment of the present invention, an optical light source includes a substrate having a front facet and a back facet, a gain section extending over a portion of the substrate proximate the front facet; and a waveguide extending from the front facet, through the gain section, to the back facet, wherein the waveguide interfaces with the front facet at a first angle and the waveguide interfaces with the back facet at a second angle, and further wherein a radius of curvature at substantially each point along the waveguide is greater than a predetermined minimum radius of curvature. Among other things, providing for a minimum radius of curvature prevents excessive losses of the guided mode to radiative slab modes within the laser crystal.  
         [0014]     According to another exemplary embodiment of the present invention, an optical light source includes a substrate having a front facet and a back facet, a gain section within the substrate which extends across substantially all of the substrate, a contact region on top of the substrate which extends across substantially all of the substrate, and a waveguide extending from the front facet, through the gain section, to the back facet, wherein the waveguide has a geometry which reduces spectral ripple in an output of the optical light source.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The accompanying drawings illustrate exemplary embodiments of the present invention, wherein:  
         [0016]      FIG. 1 ( a ) depicts a generalized side sectional view of a conventional SLD device;  
         [0017]      FIG. 1 ( b ) is a top sectional view taken along section lines A-A of the conventional SLD of  FIG. 1 ( a );  
         [0018]      FIG. 2 ( a ) depicts a generalized side sectional view of another conventional SLD device;  
         [0019]      FIG. 2 ( b ) is a top sectional view taken along section lines B-B of the conventional SLD device of  FIG. 2 ( a );  
         [0020]      FIG. 3 ( a ) depicts a generalized side sectional view of an SLD device according to an exemplary embodiment of the present invention;  
         [0021]      FIG. 3 ( b ) is a top section view taken along section lines C-C of the SLD device of  FIG. 3 ( a );  
         [0022]      FIG. 4  shows a waveguide geometry for an SLD device according to an exemplary embodiment of the present invention;  
         [0023]     FIGS.  5 ( a ) and  5 ( b ) depict a generalized side sectional view and top section view, respectively, for an SLD device according to another exemplary embodiment of the present invention; and  
         [0024]     FIGS.  6 ( a ) and  6 ( b ) show power output spectra for an SLD device according to an exemplary embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0025]     The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.  
         [0026]     Although it is desirable to minimize all internal reflections within an SLD device, of particular interest are so-called two-pass reflections. Two-pass reflections occur when optical energy is reflected from the front facet, makes a first pass through the gain section of an SLD wherein it is amplified once, is reflected from the back facet, and makes a second pass through the gain section prior to exiting the SLD device through the front facet. Two-pass reflections are particularly undesirable since they can have such large amplitudes due to experiencing, e.g., 40 dB, of gain from having passed through the device two times.  
         [0027]     These, and other, drawbacks associated with SLDs are overcome according to exemplary embodiments of the present invention in which SLD devices and methods are provided which dump reflected optical energy without using an absorber region. An exemplary embodiment of the present invention is illustrated in FIGS.  3 ( a ) and  3 ( b ). Therein, an exemplary device  30  has a vertical active waveguide  36  and curved lateral waveguide  37  that intersects the output or primary emission facet  40  at an angle θ 1  of, for example, 8 degrees. This angle reduces the modal reflectance by approximately 10 −4  for the mode field diameters obtained with exemplary ridge waveguides of 3.6 microns width and etch depths consistent with single mode operation at the mean emission wavelength of the active region. Mean wavelengths of 1310 and 1550 nm can be used since they are commonly employed due to their biological and telecommunincations and sensing relevance, however those skilled in the art will appreciate that any mean wavelength can be used that is compatible with semiconductor laser technology (˜0.3-10 μm). Angles for θ 1  less than about 8 degrees may provide too large a modal reflectance and angles greater than about 8 degrees may cause the beam exit angle, which is determined from Snell&#39;s law, to be excessive. Large beam exit angles complicate the design of the lens (not shown) positioned proximate the output region of the device to couple the output beam to a collection lens, subsequent lenses, and output fibers.  
         [0028]     According to one exemplary embodiment of the present invention, the rear facet (dump port) angle θ 2  can be 13 degrees or about 13 degrees. This angle should be larger than the waveguide-facet angle θ 1  of the output facet since it should have a lower modal reflectance. The angle θ 2  should not be much larger than 13 degrees, otherwise all or some of the mode propagating inside the waveguide  37  will be subject to total internal reflection (TIR). TIR will prevent efficient dumping of the undesired rear facet power and raises the risk that some energy will either be recaptured by the waveguide  37  and directed to the front facet  40  or will propagate to the front facet in slab waveguide modes or other modes within the device  30 .  
         [0029]     Thus, a value for θ 1  can be selected which provides a compromise between minimizing internal reflections from the front facet  40  and avoiding mechanical interference with other optics, e.g., 5-10 degrees. By way of contrast, the angle θ 2  provided between the waveguide  37  and the back facet  42  can have a value that is intended primarily to minimize reflections and, therefore, will be larger than the value of θ 1 . According to one exemplary embodiment, θ 1  equals 8 degrees and θ 2  equals 13 degrees, however those skilled in the art will appreciate that different angle values can be used (generally, θ 2 &gt;θ 1 ≧5 degrees). In this way, optical energy which is reflected from the front facet  40 , and which is also captured by waveguide  37 , will be dumped from the back facet  42  in the general direction of the arrow in  FIG. 3 ( b ) referring to exiting optical energy. Little, if any, energy is reflected from the back facet  42  and propagated back through the device  30  as a two-pass reflection. Some exemplary test results that show how well SLD devices according to the present invention reduce internal reflections are provided below.  
         [0030]     The epitaxial layer structure employed for SLDs typically involves high gain material with, for example, a gain coefficient of approximately 30 cm −1  and long chips (1 to 2 mm). An exemplary embodiment of the present invention therefore contains three, four or more quantum wells with four being exemplary. The SCH structure should be chosen to capture carriers efficiently so they can be transported into the quantum wells. The SCH region and quantum well stack together and constitute the “active region” described below.  
         [0031]     Referring again to FIGS.  3 ( a ) and  3 ( b ), the generalized, side section of an SLD device  30  according to an exemplary embodiment of the present invention further includes a contact region  32 , a first cladding layer  34 , an active region that acts as a vertical waveguide  36  and a second cladding layer  38 . Pumping current I is injected into the contact region  32  to pump active region  36  to generate light via spontaneous emission. Active region  36  can, for example, be fabricated either as an SCH region and a multiple quantum well structure (e.g., alternating layers of GaInAs or GaInAsP) or as a bulk active region. First and second cladding layers  34  and  38 , which can be fabricated from InP, operate to contain carriers within the active region due to their higher bandgap energy. Light generated within the waveguide created by the layers  34 - 38  is output via a front facet  40  in the general direction indicated by the output arrow. Typically, the optical output energy would be coupled to, e.g., an optical fiber (not shown) to be guided to another device. Unlike the SLD device of FIGS.  2 ( a ) and ( b ), the waveguide  36  extends across the entire substrate of SLD device  30 , i.e., from the front facet  40  of the SLD device  30  through the gain section to the back facet  42 . AR coatings can also be applied to front facet  40  and rear facet  42  of SLD device  30  to further reduce reflections.  
         [0032]     In addition to providing (different) angled interfaces at both the front and back facets to dump light reflected from the front facet  40  and minimize (or eliminate) captured reflections from the back facet  42 , SLD devices according to exemplary embodiments of the present invention also have minimum radius of curvature properties. More specifically, lateral waveguide  37  can be designed to, at all or substantially all points along the length of the substrate, have a minimum radius of curvature that is greater than a predetermined minimum value. In the example of FIGS.  3 ( a )-( b ), this is illustrated by forming the waveguide  37  as two curves concatenated together, the first curve having a radius of curvature of R OC1  and the second curve having a radius of curvature of R 0C2 . R OC1 , and R OC2  can be selected in order to provide the desired interface angles θ 1  and θ 2 , while at the same time adhering to a minimum radius curvature constraint.  
         [0033]     Maintaining a waveguide having a continuous and gradual curvature enables SLD devices according to exemplary embodiments of the present invention to minimize losses of modal energy since modal energy losses are a function of the radius of curvature of the waveguide. This provides design control over optical energy generated within the SLD device  30 , both in the forward path for energy directed out through the front facet  40  and in the rearward path for reflections directed out through the back facet  42 . Regarding this latter point, losses which can be created by less gradual curvature in waveguide  37  occur when the modal energy confined by waveguide  37  leaves the waveguide and becomes non-modal optical energy. This non-modal optical energy may then be reflected from the back facet  42  and be recaptured by the waveguide  37 . Accordingly, exemplary embodiments of the present invention seek to contain the modal energy reflected from the front facet  40  in order to dump a greater portion of that energy out through the back facet  42 .  
         [0034]     According to exemplary embodiments, the minimum radius of curvature will be on the order of several times the length of the substrate. According to one exemplary embodiment, the length of the substrate is 2 mm and the minimum radius of curvature is 9 mm, such that both R OC1  and R OC2  are greater than or equal to 9 mm. Although this exemplary embodiment of the present invention includes a waveguide  37  formed from two curves, i.e., having two radii of curvature, the present invention is not so limited. SLD devices according to the present invention can include curved waveguides having more than two radii of curvature. In fact, the radius of curvature may itself change along the length of the waveguide  37 . According to one exemplary embodiment of the present invention, the waveguide geometry can be determined using a third order polynomial y(x)=a 1 x+a 2 x 2 +a 3 x 3  and solving for a 1 , a 2 , and a 3 . The three unknown values a 1 , a 2 , and a 3  can be determined by setting up and solving three nonlinear simultaneous equations using three constraints. The constraints are the two values for the interface angles θ 1  and θ 2  and a minimum radius of curvature ROC min  at the rear (dump) facet. The ROC over the entire waveguide is then plotted to ensure that the minimum ROC is exceeded over the entire chip length. This prevents spurious solutions which may have unacceptably small ROCs in the middle of the waveguide or at the front (output) facet. According to one purely illustrative exemplary embodiment, this technique was used with θ 1  equal to 8 degrees, θ 2  equal to 13 degrees and ROC min  equal to 9 mm for a 2 mm substrate. This resulted in values of 0.141, −0.014 and 0.013 for a 1 , a 2 , and a 3 , respectively, with the resulting waveguide geometry being shown in  FIG. 4  as a function of the substrate width and length in millimeters.  
         [0035]     According to another exemplary embodiment of the present invention, shown in  FIG. 5  ( a ) and  FIG. 5 ( b ), the ROC of the SLD device  40  can be chosen to vary along the length of the waveguide  37  such that the minimum ROC is not exceeded over portions of the waveguide proximate to the back facet  42 . The radius of curvature in the region proximate to the back facet, ROC2, can be chosen to be small enough that optical energy radiates out of the lateral waveguide  37  as the optical mode traverses the bend. This creates, for example, a helical shape to the waveguide  37  toward the back portion of the device. The radius of curvature can be selected so that substantially most of the optical energy in waveguide  37  leaks out of the waveguide into region  48  prior to intercepting the back facet  42 . The optical energy that leaks out of the waveguide is incident on the back facet at an angle and is dumped out the back facet. There is no actual waveguide at the back facet reducing the amount of reflection that couples back into waveguide  37  from reflected optical energy at back facet  47 . The tail end  50  of waveguide  37  is oriented so that any modal optical energy in waveguide  37  that reaches the end of the waveguide propagates toward the back facet  47  rather than the front facet  40 . Anti-reflection coating can be applied to both front facet  40  and back facet  47  to further reduce reflections.  
         [0036]     As mentioned above, SLD devices according to exemplary embodiments of the present invention are able to reduce internal reflections without using an absorber region. This means that, for example, the SLD device  30  and SLD device  40  can be pumped across its entire length or substantially its entire length. Note in  FIG. 3 ( a ) that the contact region  32  extends across substantially the entire length of the substrate. Typically, SLD device  30  will be manufactured by fabricating a large number of such devices on a wafer. Since the contact region  32  is formed of a metallic material, it may be desirable for manufacturing reasons to provide a cleaving setback region  44  at each end of the device  30  that is free from metallic material so that when the wafer is cut into the individual devices, the cutting tool need not cut through metal. Since there is no need for an absorber region in SLD devices according to the present invention, the epitaxial contact cap layer portion (not shown) of the substrate under the cleaving setback regions  44 , which is heavily doped to provide pumping energy from the contact region  32  to the ends of the device  32 , can be retained. In this case, the cap layer will lie beneath an electrically insulating layer that also covers the noncontacted regions which are beneath the contact metallization. The cleaving setback regions  44  can be quite small, e.g., on the order of 10 microns, depending upon the technique used to cleave the wafer containing the SLD devices according to the present invention. These regions are normally rendered optically transparent by the intensity of the photons in the waveguide even though these regions are not electrically pumped quite as well as the fully contacted regions. Current spreading from the fully contacted regions, as described above, keeps these short uncontacted sections quite transparent optically. Pumping all or nearly all of the length of the chip allows the full gain of the chip to be used to amplify the spontaneous emission which seeds the ASE process. This increases the output power and electrical-to-optical conversion efficiency.  
         [0037]     An SLD device according to the present invention and substantially similar to that described above with respect to FIGS.  3 ( a )- 3 ( b ) and  4  has been tested to determine how well internal reflections are reduced by using the afore-described structure. The test results are provided below with respect to FIGS.  6 ( a ) and  6 ( b ). In  FIG. 6 ( a ) the amplified spontaneous emission (ASE) power spectra measured from a test device is plotted as a function of wavelength. Note that the power spectra in the peak wavelength range, e.g., approximately 1308-1318 nm, is wide and flat which indicates a lack of ripple which would be created by unwanted internal reflections within the SLD device fabricated in accordance with the present invention. To further illustrate this point, the peak portion of the spectra of  FIG. 6 ( a ) is magnified in  FIG. 6 ( b ), wherein the actual ripple can be seen and is at least an order of magnitude less than that which would be expected from a conventional SLD device at these power levels (approx 2 mW or greater) and with high resolution optical measurements (with optical filters&lt;0.1 nm or less than the free spectral range of the chip), indicating a significant reduction in internal reflections.  
         [0038]     The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items.