Abstract:
A periodically poled optical waveguide comprising a nonlinear optical crystalline material is provided having poled optical domains slanted with respect to direction of propagation of light within the waveguide. Light reflected from slanted poled optical domains does not couple efficiently back into the optical waveguide, which facilitates reduction of backreflection towards a semiconductor laser source coupled to the waveguide. Reduction of backreflections facilitates stable operation of the semiconductor laser source. A method of manufacturing of a periodically poled optical waveguide with slanted poled domains is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Patent application No. 61/418,225 filed Nov. 30, 2010, which is incorporated herein by reference for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to optical waveguides, and in particular to periodically poled optical waveguides for non-linear optical frequency conversion. 
       BACKGROUND OF THE INVENTION 
       [0003]    A nonlinear optical phenomenon of optical frequency conversion can be used to provide quasi-monochromatic visible and UV light sources based on inexpensive, efficient, and reliable laser diodes operating in a near infrared wavelength range. In these visible/UV sources, light emitted by a laser diode is directed through a nonlinear optical element, which converts the infrared emission of the laser diode into visible or UV light. 
         [0004]    A periodically poled waveguide formed in a nonlinear optical crystal is increasingly used as the nonlinear optical element for frequency conversion. Referring to  FIG. 1 , a waveguide  10  having a core  15  is formed in a nonlinear optical crystal  11 . Periodically disposed areas, or domains  12  of the nonlinear optical crystal  11  are “poled”, that is, a direction of a crystalline structure in these areas is reversed. The direction of the crystalline structure can be reversed, for example, by applying a localized strong electric field to the domains  12 . The poling period is selected so as to facilitate phase matching of light at the laser frequency, called fundamental frequency, and light at the converted frequency, called signal frequency or output frequency. 
         [0005]    Light at fundamental and converted frequencies can travel large distances in the waveguide while remaining highly concentrated. As a result, the periodically poled waveguide can have high enough conversion efficiency to provide a reasonable (50% or more) conversion even for continuous-wave (cw) infrared light of a moderate optical power, for example about 200 mW. Furthermore, sensitivity to optical misalignment, which has been a major disadvantage of previously used bulk nonlinear optical crystals, is considerably lessened in periodically poled crystalline waveguides. 
         [0006]    The excellent light guiding property of periodically poled waveguides, however, is inherently associated with a serious drawback. The periodic poling creates an optical waveguide grating that reflects light at some wavelengths back towards the laser source, creating a guided reflected wave, which destabilizes the laser source. For example, referring to  FIG. 2 , a typical reflection spectrum of the poled waveguide  10  of  FIG. 1  includes peaks at approximately 977, 985, and 989 nm. 
         [0007]    Theoretically, poling should not modify the refractive index of the waveguide  10 . However, unavoidable crystalline defects and dislocations at boundaries  14  of the poled domains  12  do create some refractive index modulation. Furthermore, periodic poling can corrugate the upper surface of the waveguide  10  as shown in  FIG. 3 , which is a cross-sectional side view of the waveguide  10  of  FIG. 1  taken along lines A-A. The poling-caused corrugation height can reach 10 nanometers at the waveguide depth of 3-4 micrometers. Over a length of the waveguide  10 , the refractive index modulation, corrugations, and other periodic perturbations can create a backreflection of up to tens of percent, which is more than sufficient to de-stabilize a reflection-sensitive laser diode. 
         [0008]    The problem of backreflection from a periodically poled waveguide into the laser is known. In U.S. Pat. No. 7,492,507 by Gollier, a wavelength conversion device having a reduced backreflection is disclosed. Referring to  FIG. 4 , a frequency doubled light source  40  includes a laser diode  41 , an optical coupler  42 , and a poled waveguide  43 . The crystallographic axis directions are denoted with arrows within the waveguide  43 . In operation, the laser diode  41  emits light  44 , which is focused by the optical coupler  42  onto the waveguide  43 . The waveguide  43  doubles the optical frequency of the laser light  44  through nonlinear optical effect known as second harmonic generation (SHG), and the emission at the doubled frequency exits the periodically poled waveguide  43  as shown at  45 . The waveguide  43  is a poled waveguide comprising domains of randomly varying widths. The domain widths are defined by an ideal poling period λ I  plus or minus a disruption value. The waveguide  43  includes “normal” domains  46 , “wide” domains  47 , and “narrow” domains  48 . “Wide” and “narrow” domains  47  and  48  reduce the coherence of reflected light, thus reducing the optical power of the backreflected light. 
         [0009]    In U.S. Pat. No. 7,414,778 by Gollier et al., a similar wavelength conversion device is disclosed, wherein the domain period is altered to shift reflection wavelengths away from the laser wavelength, thus reducing the optical power of backreflected light. 
         [0010]    In U.S. Pat. No. 7,177,340 by Lang et al., a tunable laser source is described wherein an optical isolator is inserted in front of a periodically poled waveguide to suppress reflections of light from the periodically poled crystal back into the laser. 
         [0011]    The prior art approaches to reducing the amount of backreflected light in poled waveguides require either separate optical isolators having a substantial insertion loss, or they require modifying the poling period, which considerably reduces optical conversion efficiency. Introduction of additional optical losses, or reduction of the optical conversion efficiency are undesirable because they lead to a reduction of output optical power and/or a reduction of wall plug efficiency of the prior-art light sources. 
         [0012]    It is a goal of the present invention to provide a periodically poled waveguide having a suppressed reflection of light at fundamental frequency, substantially without compromising the optical conversion efficiency. 
       SUMMARY OF THE INVENTION 
       [0013]    In accordance with the invention, there is provided a periodically poled optical waveguide comprising a nonlinear optical crystalline material, wherein poled domains of the optical waveguide are slanted with respect to an optical axis of the waveguide for reducing backreflection of light propagating therein. In a preferred embodiment, the slant angle is between 5 and 20 degrees. In other words, the angle between the poled domains and the optical axis or direction of propagation of light in the waveguide is away from perpendicular by 5 to 20 degrees. For ease of manufacturing of planar waveguides, it is preferable that the slant direction is in the plane of the waveguide. Light reflected by slanted poled domains does not couple back into the waveguide effectively, and as a result, the total backreflection by the poled domains is considerably reduced. 
         [0014]    In accordance with another aspect of the invention, there is further provided a light source comprising a semiconductor laser and the optical waveguide with slanted domains coupled to the semiconductor laser, whereby in operation, an emission frequency of the laser diode is converted by the optical waveguide to an output frequency different from the emission frequency. 
         [0015]    In accordance with another aspect of the invention, there is further provided a method of poling an optical waveguide formed on or in an optical crystal, comprising 
         [0000]    (a) providing a poling electrode having an array of slanted parallel fingers spaced apart along a first axis;
 
(b) applying the poling electrode to an outer surface of the optical waveguide; and
 
(c) energizing the poling electrode to form an array of slanted poled domains in the optical waveguide.
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Exemplary embodiments will now be described in conjunction with the drawings in which: 
           [0017]      FIG. 1  is a top view of a prior-art periodically poled waveguide; 
           [0018]      FIG. 2  is a typical reflection spectrum of the waveguide of  FIG. 1 ; 
           [0019]      FIG. 3  is a side cross-sectional view of the waveguide of  FIG. 1 , showing corrugation of an upper surface of the waveguide created in the poling process; 
           [0020]      FIG. 4  is a prior-art wavelength conversion device having a reduced backreflection; 
           [0021]      FIG. 5A  is a top view of a periodically poled waveguide of the invention; 
           [0022]      FIG. 5B  is a cross-sectional end view of the waveguide of  FIG. 5A  taken along lines B-B; 
           [0023]      FIG. 6A  is a top view of a poling apparatus of the invention; 
           [0024]      FIG. 6B  is a flow chart of a poling method of the invention; 
           [0025]      FIG. 7  is a diagram of a light source using the periodically poled waveguide of  FIGS. 5A and 5B ; 
           [0026]      FIG. 8  is a microphotograph of a front side of a poled waveguide prototype; 
           [0027]      FIGS. 9A to 9C  are results of simulation of light propagation in a waveguide having a non-slanted refractive index step; 
           [0028]      FIGS. 10A to 10C  are results of simulation of light propagation in a waveguide having a slanted refractive index step; 
           [0029]      FIG. 11  is a spectral plot of reflected and transmitted light propagating in a waveguide having non-slanted refractive index steps; and 
           [0030]      FIG. 12  is a spectral plot of reflected and transmitted light propagating in a poled waveguide having slanted refractive index steps. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. 
         [0032]    Referring to  FIGS. 5A and 5B , a periodically poled planar waveguide  50  ideally comprises a crystalline MgO:LiNbO 3  waveguide formed on a LiNbO 3  substrate  51 . The waveguide  50  includes an array of poled domains  52  slanted by a non-zero angle α with respect to an optical axis  59  of the waveguide  50 . In other words, the poled domains  52  are away from being perpendicular to the optical axis  59  by the angle α. The slant direction is in the plane of the waveguide  50 , that is, the poled domains  52  are tilted about an axis perpendicular to  FIG. 5A  and the plane of the waveguide  50  by the angle α. Referring to  FIG. 5B  specifically, the waveguide  50  is a ridge type waveguide including a core  53  surrounded by trenches  54 . Two oxide clad layers  55  are disposed on top and bottom of the MgO:LiNbO 3  waveguide  50 . The waveguide  50  is fixed to the LiNb substrate  51  by a thin adhesive layer  56 . 
         [0033]    In operation, light  57  at a fundamental frequency enters the waveguide core  53  as shown in  FIG. 5A . Frequency-converted light  58 , for example frequency-doubled light, exits the optical waveguide core  53  on the other side of the waveguide  50 . Since the domains  52  are slanted by the angle α with respect to the optical axis  59 , the reflections at domain borders occur at an angle  2 α with respect to the optical axis  59 , so the reflected light does not couple back into the waveguide core  53 . 
         [0034]    The invention can work with different types of waveguides, including ridge waveguides formed on a substrate as shown in  FIGS. 5A and 5B , buried waveguides formed in a substrate, non-planar waveguides, non-ridge waveguides, etc. In general, any poled crystalline waveguide used for nonlinear optical frequency conversion can benefit from slanted domains according to the present invention. The nonlinear optical crystalline materials of the waveguide can include LiNb, LiTa, KTP, or any other suitable nonlinear optical materials. 
         [0035]    The bigger the slant angle α of the poled domains  52 , the better is backreflection suppression, however the frequency conversion efficiency may drop. It has been found that a range of 5 to 20 degrees provides a useful backreflection suppression at a moderate conversion efficiency drop. A preferred range, within which the backreflection is well suppressed while the efficiency of frequency conversion drops negligibly, is between 6 and 12 degrees. The poled domains  52  are typically between 2 and 7 micrometers long. The domain length is measured in the direction of the optical axis  59  of the waveguide  50 . 
         [0036]    Although the direction of slant of domains  52  in  FIG. 5A  is in the plane of the waveguide  50 , that is, in the plane of  FIG. 5A , the invention will also work in cases where the slant direction is perpendicular to the plane of the waveguide  50 , or where it forms any other angle with the plane of the waveguide  50 . Furthermore, the invention will work with other waveguide types, for example with waveguides formed on or within the crystalline substrate  51 . An optical axis of the crystalline substrate  51 , not shown, is typically oriented in a pre-defined relationship to the optical axis  53  of the waveguide  50 , for example, the optical axis of the crystalline substrate  51  can be parallel to the optical axis  53  of the waveguide  50 . 
         [0037]    Referring now to  FIGS. 6A and 6B , an embodiment of a poling apparatus  60  of the invention includes top and bottom poling electrodes  61  and  62 , respectively. The top poling electrode  61  includes an array of slanted parallel fingers  63  spaced apart along an axis  64 . To make the poled waveguide  50 , an unpoled waveguide  65  is placed between the top and bottom poling electrodes  61  and  62  in a step  67 . In a step  68 , the top poling electrode  61  is applied to an upper surface  63  of the optical waveguide, so that an angle between the fingers  63  and an optical axis of the optical waveguide is away from perpendicular by 8 to 20 degrees. Then, in a step  69 , the poling electrodes  61  and  62  are energized by applying a high voltage  66  therebetween. Care is taken to prevent electrical sparks from forming between the top and bottom poling electrodes  61  and  62 , respectively, to avoid damaging the waveguide  65 . 
         [0038]    Turning to  FIG. 7 , a light source  70  includes a semiconductor laser  71  and the periodically poled optical waveguide  50  coupled to the semiconductor laser  71  by an optical fiber  72 . Due to low backreflection from the periodically poled waveguide  50 , an optical isolator needs not be placed between the semiconductor laser  71  and the periodically poled optical waveguide  50 . The semiconductor laser  71  is preferably a telecom grade laser diode operating in near-infrared wavelength range suitable for pumping erbium doped optical fibers. Such diode lasers are well developed and are quite reliable. 
         [0039]    In the embodiment of  FIG. 7 , the periodically poled optical waveguide  50  is optimized for second harmonic generation. Frequency doubled output  73  is in visible wavelength range, for example in green-blue range. By way of example, when the laser diode  71  operates at 976 nm, the frequency doubled output  73  is at 488 nm. Of course, other lasing wavelengths can be used. The material, waveguide dimensions, and poling period of the periodically poled waveguide  50  are all selected according to the laser diode and output beam specifications. Such selections are well within the scope of knowledge of a person skilled in the art. 
         [0040]    The non-linear frequency conversion can include second-harmonic generation (SHG); third-harmonic generation (THG); and generally any sum/differential frequency generation used in optical parametric oscillators (OPO). By way of another example, a 325 nm UV monochromatic light source can be constructed by coupling a 976 nm infrared semiconductor laser to a THG poled waveguide having slanted domains described above. Although a lens based free-space coupler can be employed to couple emission of the laser  71  to the periodically poled waveguide  50 , fiber coupling is preferable because it reduces alignment sensitivity and improves stability and reliability of the light source  70 . 
         [0041]    Referring to  FIG. 8 , a front end of a prototype of the periodically poled waveguide  50  for second harmonic generation from 976 nm to 488 nm has been photographed through a microscope. The waveguide core  53  is 4.5 micrometers wide. Trenches  81  are 2.1 microns deep. The periodically poled waveguide  50  is about 3.6 microns thick and 4.5 microns wide. The waveguide  50  is passivated with the oxide layers  55  on both sides. The epoxy layer  56  coalesces with the bottom oxide layer  55  in  FIG. 8  because of limited resolution of  FIG. 8 . The epoxy layer  56  fixes the waveguide  50  to the substrate  51 . 
         [0042]    The optical performance of the periodically poled waveguide  50  has been verified using two-dimensional Finite Difference Time Domain (FDTD) optical simulations. For comparison purposes, the optical simulations were performed for both a prior-art periodically poled waveguide having non-slanted poled domains and for a similar periodically poled waveguide having slanted poled domains. The simulated waveguides included a single refractive index step of a magnitude of 0.5, representing the poled domains in the waveguides. For non-slanted domains waveguide, the index step was perpendicular to the waveguide. For slanted domains waveguide, the index step was slanted by 8 degrees. Both simulated waveguides were 4.5 micrometers wide and had a refractive index of 2.14 at the wavelength of 976 nm. The cladding refractive index was taken to be 1.0. 
         [0043]    The non-slanted index step simulations will be described first. Referring to  FIG. 9A , a waveguide  90  has a core  90 A and a cladding  90 B. The waveguide core  90 A has an index step  91  (of the magnitude 0.5 as noted above) at an X-coordinate of approximately −11.3 micrometers. A reverse refractive index step of 0.5 in magnitude has also been added to the waveguide core  90 A at X=−8.7 micrometers (not shown in  FIG. 9A ) in the waveguide  90  of  FIG. 9A . The simulation included one light source  92  and “transmission” and “reflection” monitors  93  and  94  disposed at X=−9.5 micrometers and −14.5 micrometers, respectively. To simplify the simulation, the simulated light source  92  is disposed within the waveguide core  90 A. The light source  92  emits a planar wave propagating left to right, towards the index step  91 . The wave ridges and valleys are shown at  95 . A grayshades scale bar  80  represents a magnitude of the y-component of the electric field, E y , of the wave ridges and valleys  95 . 
         [0044]    Turning to  FIG. 9B , a time dependence of optical power detected by the transmission monitor  94  is shown. In  FIG. 9B , the optical power is plotted in linear units. It is normalized to the power of the light source  92 . The horizontal scale is in “cT” units, that is, time since turning “on” the light source  92  multiplied by speed of light in vacuum. For example, “10 micrometers” corresponds to the time it takes light to travel 10 micrometers in vacuum. The transmitted wave begins to arrive at the transmission monitor  93  disposed at −9.5 micrometers at cT=8.5 micrometers. Solid and grey lines  96  and  97  denote a total transmitted optical power level and a guided transmitted optical power level, respectively. The guided transmitted power  97  is obtained by calculating an overlap integral of the transmitted electric field E y  and a guided propagation mode of the waveguide  90 . 
         [0045]    Referring now to  FIG. 9C , a time dependence of optical power detected by the transmission and reflection monitors  93  and  94 , respectively, is shown on a common logarithmic graph in dB units. As in  FIG. 9B , the solid and the grey lines  96  and  97  denote a total transmitted optical power level and a guided transmitted optical power level, respectively. Solid and grey lines  98  and  99  denote a total reflected power and a guided reflected power, respectively. The guided reflected power  99  is obtained by calculating an overlap integral of the reflected electric field E y  and a reverse guided propagation mode of the waveguide  90 . The kinks in the transmitted and reflected power levels  96  to  99  at cT of less than 8 microns are artifacts of the simulation. At cT of between 4 and 8 microns, the total and guided transmitted power levels  96  and  97  are approximately −18 dB and −40 dB, respectively. At cT of between 4 and 9 microns, the total and guided reflected power levels  98  and  99  are approximately −22 dB and −41 dB, respectively. These power levels represent floor noise levels of the numerical simulation. At cT of approximately 8.5 micrometers, the total and guided transmitted power levels  96  and  97  go to the level of 0 dB and approximately −1 dB, respectively, corresponding to the linear power levels of 1.0 and 0.8 in  FIG. 9B . At cT of approximately 11.5 micrometers the total and guided reflected power levels  98  and  99  go to the level of −18 dB and −22 dB, respectively. These values correlate well with magnitude of Fresnel reflection from the refractive interface  91 . 
         [0046]    In  FIGS. 10A to 10C , the slanted index step simulation results are presented. Referring specifically to  FIG. 10A , a waveguide  100  is shown having the core  90 A, in which an index step  101  is slanted at 8 degrees with respect to light propagation direction. A reverse slanted refractive index step of 0.5 in magnitude (not shown in  FIG. 10A ) has also been added to the waveguide core  90 A at X=−8.7 micrometers in the waveguide  100  of  FIG. 10A . The rest of the simulation set-up is identical to that of  FIG. 9A . 
         [0047]    Turning to  FIG. 10B , solid and grey lines  106  and  107  denote a total transmitted optical power and a guided transmitted optical power, respectively, in the waveguide  100  having the slanted index step  101 . The guided transmitted power  107  is obtained by calculating an overlap integral of the transmitted electric field E y  and a guided propagation mode of the waveguide  100 . It is lower than the guided transmitted power  97  in  FIG. 9B  because the tilted index step  101  induces a slight angular misalignment of the propagating electromagnetic wave and the waveguide  100 . 
         [0048]    Referring now to  FIG. 10C , a time dependence of optical power detected by the transmission and reflection monitors  94  and  95 , respectively, is shown on a common logarithmic graph in dB units. Solid and grey lines  106  and  107  denote a total transmitted optical power and a guided transmitted optical power, respectively, in the waveguide  100  having the slanted index step  101 . Solid and grey lines  108  and  109  denote a total reflected power and a guided reflected power, respectively, of a light wave reflected from the tilted refractive index step  101  of the waveguide  100 . Again, the guided reflected power  109  is obtained by calculating an overlap integral of the reflected electric field E y  and a reverse guided propagation mode of the waveguide  100 . At cT of between 4 and 9 microns, the optical power levels  106  to  109  are almost identical to the corresponding optical power levels  96  to  99  of  FIG. 9C . At cT of approximately 8.5 micrometers, the total and guided transmitted power levels  106  and  107  of  FIG. 10C  go to the level of 0 dB and −1.5 dB, respectively, corresponding to the linear power levels of 1.0 and 0.7 in  FIG. 9B . At cT of approximately 11.5 micrometers the total and guided reflected power levels  108  and  109  go to the level of −17.7 dB and −37 dB, respectively. Note that the corresponding guided reflected optical power level goes to −22 dB for the waveguide  90  having a straight index step  91 . 
         [0049]    The guided reflected optical power  109  is about 15 dB lower than the guided reflected optical power  99  in  FIG. 9C . Therefore, tilting the refractive index step  101  by 8 degrees results in 15 dB drop in the reflected guided optical power. Accordingly, the simulations of  FIGS. 9A-9C  and  10 A- 10 C indicate that tilting poled domains in the waveguide  50  results in a backreflection suppression of the order of 15 dB. 
         [0050]    Similar calculations have been performed at the slant angles α 0  of the refractive index step  101  between 4 and 25 degrees. It has been determined that the guided backreflection is effectively suppressed at the slant angles α of at least 5 degrees. A drop in nonlinear conversion efficiency will depend on the slant angle α of the domains  52 . Generally, a larger slant angle α will decrease the conversion efficiency, so a tradeoff slant angle α needs to be found. It has been estimated that at slant angle α of over 20 degrees, the optical conversion efficiency for SHG drops by over 25%, while at slant angle α of 8 degrees it drops only by 10% or less. The conversion efficiency drop is moderate because in the present invention, the periodicity of poling of the optical waveguide  50  is preserved. Generally, the slant angle α of between 5 to 20 degrees has been found to be workable, and the range of between 6 and 12 degrees is preferable. Accordingly, the slant angle of the parallel fingers  63  of  FIG. 6A  of the electrode  61  is selected to be preferably between 5 to 20 degrees and most preferably between 6 and 12 degrees. 
         [0051]    A simulation of a steady-state optical power distribution in the waveguides  90  and  100  of  FIGS. 9A and 10A  has been performed. The structure used in the simulations is identical to that of  FIGS. 9A and 10A . A Fast Fourier Transform (FFT) was performed on the optical field time domain data as observed at the power monitors  93  and  94 . The wavelength of the light source was varied from 0.8 to 1.2 micrometers. 
         [0052]    Turning to  FIG. 11 , logarithmic spectral plots of simulated reflected and transmitted optical power are presented for the case of the straight waveguide domain. Lines  116 ,  117 ,  118 , and  119  represent wavelength dependence of total transmitted optical power, guided transmitted optical power, total reflected optical power, and guided reflected optical power, respectively, normalized to the input power value. The peak structure seen in all four spectra  116 ,  117 ,  118 , and  119  results from etalon-like effect observed between two straight refractive index steps. Peaks of the spectrum  119  of the guided reflected optical power are −3 to −6 dB down the input optical power value. 
         [0053]    Referring now to  FIG. 12 , logarithmic spectral plots of simulated reflected and transmitted optical power are presented for the case of the slanted waveguide domain. Lines  126 ,  127 ,  128 , and  129  represent wavelength dependence of total transmitted optical power, guided transmitted optical power, total reflected optical power, and guided reflected optical power, respectively, normalized to the input power value. One can see that peaks of the spectrum  129  of the guided reflected optical power are −19 to −22 dB down the input optical power value. Therefore, the 8-degree slanted domain reflects about 16 dB less light that is guided back into the waveguide  100 . 
         [0054]    Referring back to  FIG. 7 , reduction of light reflected back into the laser diode  71  by 15-16 dB considerably improves stability of the laser diode  71 , thus improving the power stability of the output optical signal  73  of the light source  70 . 
         [0055]    The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.