Abstract:
Segmented waveguide structures provide mode matching in planar lightwave circuits between waveguides and other waveguiding structures, e.g. slab waveguides and optical fibers. The present invention eliminates back reflections from the core segments by etching the leading and trailing faces of the core segments with a plurality of parallel facet sections, which are rearwardly offset in the transmission direction by an odd number of quarter wavelengths.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from U.S. Provisional Patent Application Nos. 61/073,152 and 61/073,045, both filed Jun. 17 2008, which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a segmented waveguide structure, and in particular to a segmented waveguide structure with controlled back reflections. 
     BACKGROUND OF THE INVENTION 
     Segmented structures are conventionally used in planar waveguide structures to act as fiber-to-waveguide couplers (FWC), Bragg gratings, or other such structure, whereby the geometry of the segmented structures is chosen to optimize some feature in transmission. For example, an FWC gradually enables an optical mode to expand or contract to match the mode of an optical fiber to the mode most conveniently carried within the planar waveguide structure. The reflection from the interface of any element of a segmented structure is generally very small; however, if many segmented structures are employed, the reflection from each interface of each segment will add to the reflection of other interfaces to produce a potentially large cumulative back reflection. In the case where the segments are “random”, or of no particular period, the individual segment reflections will accumulate to a relatively wavelength independent back reflection. In the case where the segments are periodic, the cumulative effect will show strong back reflections at specific wavelengths, and weaker reflections in between those wavelengths. Cumulative back reflections exceeding approximately −35 dB (approximately 0.03%) can be unacceptable in many waveguide applications, e.g. if the waveguide is receiving light from a laser.  FIG. 1  illustrates a conventional segmented waveguide structure  1 , in which the segments  2  are made of core material (dashed filled), and are surrounded by cladding material  3  for guiding light  4  between a continuous waveguide section  5  and an edge  6  of the structure  1 , wherein the segments  2  have progressively smaller widths. Examples of devices including segmented waveguides are illustrated in U.S. Pat. No. 5,745,618 issued Apr. 28, 1998 to Li; U.S. Pat. No. 6,892,004 issued May 10, 2005 to Yu; U.S. Pat. No. 7,006,729 issued Feb. 28, 2006 to Wang et al; U.S. Pat. No. 7,130,518 issued Oct. 31, 2006 to Yamazaki et al; and U.S. Pat. No. 7,212,709 issued May 1, 2007 to Hosai et al. 
     The segmented waveguides  2  are positioned in transition areas to provide mode expansion or mode contraction depending upon which direction the light  4  travels. The mode expansion and contractions are used to gradually match an optical field of an optical signal in the waveguide section  5  to optical fields of corresponding optical signals in the adjacent guiding structures optically coupled to the segmented waveguides  2 , e.g. optical fibers, slab waveguides etc, connected to the edge  6 . 
     Unfortunately, there is a reflection from each interface between the core segments  2  and the cladding  3 , which can combine coherently when the segments  2  are positioned periodically or quasi-periodically, e.g. spaced at a distance equal to the wavelength (λ) of the transmitted light or multiples thereof. In  FIG. 2 , a conventional method of reducing back reflections is demonstrated in a randomly offset, e.g. not periodic, segmented device  7  in which each of the aforementioned segments  2 , shown in solid outline, is moved in some random but small amount from its nominal location, resulting in repositioned segments  2 ′, shown in phantom outline. The feedback from randomly repositioned segments  2 ′ will likely not add together coherently after repositioning, thereby suppressing some back reflection; however, randomizing has limited benefits, and provides only from 10 dB to 20 dB of back reflection suppression. Alternatively, the widths of individual segments might vary to achieve the same randomization effect (not shown here). 
     An object of the present invention is to overcome the shortcomings of the prior art by providing a means for modifying and, when necessary, substantially suppressing cumulative back reflection from segmented planar waveguide structures. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention relates to a planar lightwave circuit (PLC) device comprising: 
     a waveguide structure including: 
     a core for transmitting optical signals in a light transmission direction, having a transition area at an end thereof for optically coupling to an adjacent optical element, wherein the transition area of the core includes core segments separated by cladding sections in the light transmission direction for mode matching the optical signals between the core and the adjacent optical element; and 
     cladding surrounding the core for guiding the optical signals substantially in the core; 
     wherein each core segment includes a first face and a second face through which the optical signals pass in the light transmission direction; 
     wherein the first face includes a first section and a second section; and 
     wherein the first section is rearwardly offset from the second section, whereby back reflections from the first section of the first face at least partially cancel back reflections from the second section of the first face. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
         FIG. 1  illustrates a conventional segmented waveguide structure; 
         FIG. 2  illustrates a conventional segmented waveguide structure with randomly repositioned segments; 
         FIG. 3  illustrates a planar waveguide device including offset segmented waveguides in accordance with the present invention; 
         FIG. 4  illustrates the offset segmented waveguides of  FIG. 3 ; 
         FIG. 5  illustrates a back-reflection spectrum from nearly periodic segment structure with no offset, and with quarter-wave fragmented offset segments; 
         FIG. 6  illustrates an offset randomized segmented waveguide in accordance with the present invention; 
         FIG. 7  illustrates a back-reflection spectrum from a nearly periodic segmented structure, a randomized segmented structure, and a randomized structure with the fragmented-offset technique of the present invention also applied; and 
         FIGS. 8   a ,  8   b , and  8   c  illustrate multi-faceted segments in accordance with the present invention. 
         FIG. 9  illustrates an alternative planar waveguide device including offset segmented waveguides in accordance with the present invention; 
         FIG. 10  illustrates an alternative planar waveguide device including offset segmented waveguides in accordance with the present invention; 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 3 and 4 , an exemplary planar waveguide (PLC) device  11 , in accordance with the present invention includes a laser  12  mounted on a substrate  13 , and optically coupled to an optical fiber  15  via a waveguide  16 . In accordance with the present invention, the transition portion  17  of the waveguide  16 , e.g. adjacent external optical fibers  15 , has been segmented, and includes a plurality of core segments  22 , e.g. five to fifty although only three shown, separated by portions of cladding  23 , for gradually enables the optical mode in the waveguide  16  from the laser  12  to expand or contract to match the mode of the optical fiber  15 . 
     The waveguide  16  is comprised of upper and lower cladding regions or layers of low refractive index, with one or more core regions of higher refractive index therebetween; however, in some embodiments, such as silicon-on-insulator (SOI), the upper cladding region may be air. Confined by the waveguiding structure, the input optical signal  25  expands horizontally in the core region, i.e. diverges in the horizontal plane. The PLC device  11  can be fabricated in silica on silicon, silica on quartz, silicon on insulator, or III-V materials, e.g. InP, GaAs or InGaAsP. 
     The core segments  22  have progressively smaller widths towards the ends of the waveguide  16  and/or the cladding portions  23  have progressively larger widths towards the ends of the waveguide  16 , as illustrated in  FIG. 4 . Each core segment  22 , in accordance with the present invention, includes stepped front and rear facets  26  and  27 , respectively. A first section  31  of the front facet  26 , having a length which is a fraction, e.g. one half, of the total length of each front facet  26 , has been rearwardly offset in the direction of light transmission from a second section  32  of each front facet  26  by an amount at or approximately equal to a quarter of a wavelength (λ/4) of the light  25  requiring suppression or any odd multiple of a quarter of a wavelength ((2n−1)λ/4) of the light  25  requiring reflection suppression. Similarly, a first section  41  of the rear facet  27 , having a length which is a fraction, e.g. one half, of the total length of each rear facet  27 , has been rearwardly offset in the light transmission direction from a second section  42  of each rear facet  27  by an amount at or approximately equal to a quarter of a wavelength (λ/4) or any odd multiple of a quarter of a wavelength ((2n−1)λ/4) of the light  25  requiring suppression. Therefore, the field reflected back from the first sections  31  and  41  of the front and rear facets  26  and  27 , respectively, will exactly cancel the fields reflected back from the second sections  32  and  42  of the front and rear facets  26  and  27 , respectively. 
     In the illustrated embodiment of  FIG. 4 , all of the first and second sections  31 ,  32 ,  41  and  42  are parallel, and the first sections  31  and  32  are the same distance apart as the second sections  41  and  42 ; however, for the purposes of anti-reflection, the first and second sections  31  and  32  of the front facet  26  are independent of the first and second sections of the rear facet  27 . For example, the front face  26  could have three faces for canceling or controlling back reflection, while the rear face  27  has only two. Moreover, the first sections  31  and  41  could be farther apart than the second sections  32  and  42 . Various segment shapes will be discussed hereinafter with reference to  FIGS. 9   a  to  9   c.    
     The edge of the device chip  11  has been cleaved at an acute angle, e.g. 7° to 10°, to cancel or adjust the back reflections occurring at the junction of the chip  11  and the external waveguide core  50 , e.g. core of optical fiber  15 . 
     With reference to  FIG. 5 , back-reflection spectrums from a periodically segmented structure and a structure using the fragmented-offset technique in accordance with the present invention are illustrated. Using periodically spaced segments with fragmented offsets, the back-reflection is suppressed by approximately 60 dB at the design wavelength (1.29 μm to 1.34 μm), while throughout the wavelength band shown the suppression exceeds approximately 25 dB. 
     To decrease the effect of back reflections even further, the offset core segments  22  can also be randomly distributed, as illustrated in phantom outline in  FIG. 6 . The back-reflection spectrum from the same periodic segmented structure as in  FIG. 5 , along with the periodic structure randomized, and a randomized structure with the fragmented-offset technique applied, is illustrated in  FIG. 7 . Even in the case of the randomized segmented structure, an additional 40 dB of suppression is provided by fragmenting the individual segments  22 . Therefore, this fragmenting technique can be used to substantially reduce back reflections in periodic or non-periodic structures. 
     The segment fragmenting technique, according to the present invention, works because the mode inside the waveguide  16  is defined by precision photo-lithography. Therefore, in the examples shown above, the mode distribution can be balanced quite well between the first and second sections, e.g.  31  and  41 , in each segment  22 . The precision construction also enables additional control over the back reflections, when total suppression of all back-reflections isn&#39;t desired. For instance, multiple sections, e.g. 3 or more, can be employed, e.g. cut the waveguide segment into 3 or more sections, each with their own controlled relative offset, instead of just the two equal fragments with a quarter-wave offset, as illustrated above. In this way, the back reflection can be altered in a known way, or eliminated if required. The sections can also be angled at an acute angle, e.g. 30 to 15°, preferably 70 to 10°, to a plane perpendicular to the direction of propagation of light, if it is desired to adjust the back-reflection spectrum and to send the back-reflected radiation to a separate location, not straight back into the original waveguide. 
       FIGS. 8   a ,  8   b  and  8   c  illustrate various examples of multi-faceted segments  71 ,  81  and  91 , respectively. The segment  71 , in  FIG. 8   a , includes a front facet  72  defined by a middle section  73  and side sections  74  and  75  on either side thereof. Similarly, rear face  76  include a middle section  77  with side sections  78  and  79  on either side thereof. As above the corresponding front and rear middle facet sections  73  and  77  are offset from the side facet sections  74  and  75 , and  78  and  79  by an amount at or approximately equal to a quarter of a wavelength (λ/4) or any odd multiple of a quarter of a wavelength ((2n−1)λ/4) of the light  25  requiring suppression. In the illustrated embodiment, the length of the middle section  73  (or  77 ) is equal to the sum of the lengths of the side sections  74 + 75  (or  78 + 79 ); however, the lengths of the middle section  73  (or  77 ) can vary depending upon the light intensity distribution of the optical beam traveling in the waveguide, and depending upon the amount of back reflection suppression desired. For total back reflection suppression the integral of the intensity of the middle section  73  (or  77 ) is equal to the sum of the integrals of the intensities of the side sections  74  and  75  (or  78  and  79 ), assuming a relative phase offset of 180°. The distance between the middle sections  73  and  77  can be the same as the distance between side sections  74  and  78  or, as illustrated in  FIG. 8   a , the distance between the middle section  73  and  77  can be λ/2 of the light  25  requiring suppression (or a multiple thereof) wider than the distance between the side sections  74  and  78 . 
     The multi-faceted segment  81  in  FIG. 8   b  includes front and rear faces  82  and  83  with four substantially equal facet sections  84   a  to  84   d  and  86   a  to  86   d , with adjacent facet sections offset from each other by an amount at or approximately equal to a quarter of a wavelength (λ/4) or any odd multiple of a quarter of a wavelength ((2n−1)λ/4) of the light  25  requiring suppression. Ideally, the middle sections  84   b  and  84   c  (or  86   b  and  86   c ) are equal in length to each other, and the end sections  84   a  and  84   d  (or  86   a  and  86   d ) are equal in length to each other; however, the middle sections  84   b  and  84  can have different lengths than the end sections  84   a  and  84   d . For total back reflection suppression, the sum of the integral of the intensity of the sections  84   a  and  84   c  is equal to the sum of the integral of the intensity of the sections  84   b  and  84   d , assuming a relative phase offset of 180°. 
     The offset multi-faceted segment  91 , illustrate in  FIG. 8   c , includes front and rear faces  94  and  96 , respectively, with curved, convex or arcuate portions, e.g. providing lensing surfaces with optical power. The front face  91  includes arcuate first and second side section  94   a  and  94   b  separated along a central axis by an offset section, which rearwardly offsets the first section  94   a  from the second section  94   b  by an amount at or approximately equal to a quarter of a wavelength (λ/4) or any odd multiple of a quarter of a wavelength ((2n−1)λ/4) of the light  25  requiring suppression. The rear face  96  includes arcuate first and second side section  96   a  and  96   b  separated along the central axis by an offset section, which rearwardly offsets the first section  96   a  from the second section  96   b  by an amount at or approximately equal to a quarter of a wavelength (λ/4) or any odd multiple of a quarter of a wavelength ((2n−1)λ/4) of the transmitted light  25  requiring suppression. 
     Ideally, the first and second sections  94   a  and  94   b  are mirror images of each other, but in practice the surfaces of the first and second section  94   a  and  94   b  can be substantially different, as long as the integral of the intensity of the first section  94   a  is equal to the integral of the intensity of the second section  94   b , assuming a relative phase offset of 180° and total back reflection suppression is desired. 
     With reference to  FIG. 9 , an exemplary planar waveguide device  111 , in accordance with the present invention includes one or more diffraction gratings  112  formed at an edge or edges of a slab waveguide region  113  for dispersing an input optical signal including a plurality of wavelength channels, as disclosed in U.S. Pat. No. 7,068,885, issued Jun. 27, 2006 to Bidnyk et al, and U.S. Pat. No. 7,151,635 issued Dec. 19, 2006 to Balakrishnan et al, and U.S. Pat. No. 7,149,387 issued Dec. 12, 2006 to Pearson et al, which are incorporated herein by reference. The input optical signal is launched from an external waveguide, e.g. optical fiber  115 , via an input/output port  114  along an input/output waveguide  116  to a slab inlet port of the slab waveguide  113 , wherein the diffraction grating(s)  112  disperses the wavelength channels according to wavelength to slab outlet ports  117 , which are positioned along a Rowland circle  118 . The separated wavelength channels propagate along output waveguides  119  to output ports  121 . The output ports  121  can be optically coupled to a photo-detector array  122 , to other optical devices or to optical fibers (not shown). Alternatively, the output waveguides  119  can transmit individual wavelength channels from the outlet ports  121  to the diffraction grating  112  for multiplexing onto the input/output waveguide  116  and outputting the input/output port  114 , as disclosed in U.S. Provisional Patent Application 61/073,152, filed Jun. 17, 2008, which is incorporated herein by reference. 
     When the device  111  is utilized for bi-directional transmission, a laser  126  mounted on an edge of the device  111 , emits an output optical signal along laser waveguide  127 , which gets filtered and redirected by the diffractions grating(s)  112  to the input/output waveguide  116  for output the input/output port  114 . 
     The slab waveguide  113  is defined by a waveguiding structure, which confines the light to predominantly two dimensions, while restricting the diffraction of the light in the third dimension. Typically, the slab waveguide  113 , like the waveguides  116 ,  119  and  127 , is comprised of upper and lower cladding regions or layers of low refractive index, with one or more core regions of higher refractive index therebetween; however, in some embodiments, such as silicon-on-insulator (SOI), the upper cladding region may be air. Confined by the waveguiding structure, the input optical signal expands horizontally in the core region, i.e. diverges in the horizontal plane. The circuit  111  can be fabricated in silica on silicon, silica on quartz, silicon on insulator, or III-V materials, e.g. InP, GaAs or InGaAsP. 
     For a diplexer or a triplexer the relevant passbands are 100 nm for the laser, and approximately 20 nm for the detector channels. Such a device would be impractical to implement with a single diffractive structure because the various channels would share a common physical dispersion. Assume that a spectrometer slab region has been chosen such that the smallest reasonable guiding waveguide widths handle the 20 nm passbands at the grating output. The waveguide width necessary for the 100 nm passband channel would be so wide as to support innumerable modes, creating a device with high sensitivity to fabrication tolerances if a reversible path is necessary for this channel. 
     With reference to  FIG. 10 , a two-stage optical filter according to the present invention includes a non-dispersive filter  131 , a dispersive filter  132 , a laser source  133 , and first and second photo-detectors  134  and  135  formed in or on a planar lightwave circuit (PLC) chip  136 . A single photo-detector  134  can be provided, when one of the detector channels is omitted. Preferably, the non-dispersive filter  131  is a wavelength selective directional coupler, i.e. two parallel waveguides of specific width, spacing and coupling length, which separates the receiver channels from the laser channel. Alternatively, the non-dispersive filter  131  can be a wavelength dependent modal interference (MMI) filter or a phase dependent wavelength splitter, e.g. a Mach Zehnder interferometer designed for splitting wavelength bands. Instead of a single-stage coupler, a multi-stage coupler or MMI can be used, which provides flatter passbands than those commonly produced by single-stage filters, which slightly improves the insertion loss at the outer edges of the channels, where the passbands from the single-stage filters begin to roll off. 
     The laser source  133  transmits the data channel along waveguide  141  to the non-dispersive filter  131 , which multiplexes the data channel onto output waveguide  142 . A system waveguide  143 , e.g. an optical fiber, is optically coupled to the output waveguide  142  at the edge of the PLC chip  136 . A monitor photodiode  146  can be positioned proximate the back facet of the laser source  133 ; however, the structure of the present invention enables the monitor photodiode  146  to be positioned upstream of the laser source  133  optically coupled thereto via a tap coupler  147 , which separates a small portion (2%) of the laser light. Back facet monitors measure the light produced by the laser, but not what is actually coupled to the waveguide  141 , i.e. into the PLC chip  136 ; however, the downstream photodiode  146  is able to directly measure what light has been coupled in the waveguide  141 . 
     The detector channels must pass through both stages of the filter, i.e. the non-dispersive filter  131  and the dispersive filter  132 , via waveguide  148 , and are processed by the grating-based dispersive filter  132 . Preferably, the dispersive filter  132  includes a concave reflective diffraction grating  150  with a focal line  156 , preferably defined by a Rowland circle. 
     Typical grating-based demultiplexers exhibit relatively sharp passbands that are difficult to make wide and flat, as required for the bi-directional transceiver application. Accordingly, the present invention incorporates multi-mode output waveguides  151  and  152  at output ports along the focal line  156 . The multi-mode waveguides  151  and  152  support an innumerable collection of modes, which serves to flatten the spectral response of the grating output. Alternatively, the first and second output waveguides  151  and  152  include a multimode section adjacent to the first and second ports, respectively, and a single mode section remote therefrom for providing the diffraction grating filter  150  with a flattened spectral response. The waveguides  151  and  152  direct the light from the output ports to the first and second photo-detectors  134  and  135 , respectively. 
     The present invention achieves the varying passbands for the detector and signal channels by incorporating a dual-stage filter, in which the laser channel is separated from the detector channels, which are further demultiplexed with a dispersive element of higher resolution. The passband of the laser channel is therefore determined by the first stage of the filter, e.g. the wavelength-selective directional coupler  131 , while the passband of the detector channels is determined predominantly by the second stage of the filter, e.g. grating-based dispersive element  132 . The directional coupler  131  can be designed to easily cover a passband of 100 nm, while the detector channels undergo further processing by the grating. 
     In accordance with the present invention, the transition portions of each waveguide  116 ,  119 ,  127 ,  141 ,  142 ,  148 ,  151  and  152 , e.g. adjacent external optical fibers  115  and  143  and/or slab waveguide regions  113  or  132 , and/or optical components  122 ,  126 ,  131 ,  134  and  135  can be segmented and include core segments  62 ,  72 ,  82  or  92  separated by portions of cladding, as illustrated in  FIGS. 4 ,  6 ,  8   a ,  8   b  and  8   c  for gradually enabling the optical mode in the waveguide to expand or contract to match the mode of the optical fiber neighboring structure.