# Patent

## Title
Planar waveguide reflective diffraction grating

## Abstract

The invention relates to a planar waveguide reflective diffraction grating for use in an optical device, such as a wavelength division multiplexer, providing an increased bandwidth over conventional planar waveguide reflection diffraction gratings while eliminating the polarization dependent loss (PDL) typically associated therewith. Accordingly, a low order (<3), high aspect ratio (>10) grating is provided with a very short side wall (less than the wavelength of the optical signal) for use with incident angles of less than 15°.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present invention claims priority from U.S. patent application Ser. No. 60/555,697 filed Mar. 24, 2004, which is incorporated herein by reference.


TECHNICAL FIELD
The present invention relates to a reflective diffraction grating for use in optical communication, and in particular to a planar waveguide reflective diffraction grating device with reduced polarization sensitivity and an increased free spectral range.
BACKGROUND OF THE INVENTION
In optics, a diffraction grating is an array of fine, parallel, equally spaced grooves (“rulings”) on a reflecting or transparent substrate, which grooves result in diffractive and mutual interference effects that concentrate reflected or transmitted electromagnetic energy in discrete directions, called “orders,” or “spectral orders.”
The groove dimensions and spacings are on the order of the wavelength in question. In the optical regime, in which the use of diffraction gratings is most common, there are many hundreds, or thousands, of grooves per millimeter.
Order zero corresponds to direct transmission or specular reflection. Higher orders result in deviation of the incident beam from the direction predicted by geometric (ray) optics. With a normal angle of incidence, the angle θ, the deviation of the diffracted ray from the direction predicted by geometric optics, is given by the following equation, where m is the spectral order, λ is the wavelength, and d is the spacing between corresponding parts of adjacent grooves:




  θ
  =
  
    ±
    
      
        sin
        
          -
          1
        
      
      ⁡
      
        (
        
          
            m
            ⁢
            
              
            
            ⁢
            λ
          
          d
        
        )
      
    
  




Because the angle of deviation of the diffracted beam is wavelength-dependent, a diffraction grating is dispersive , i.e. it separates the incident beam spatially into its constituent wavelength components, producing a spectrum.
The spectral orders produced by diffraction gratings may overlap, depending on the spectral content of the incident beam and the number of grooves per unit distance on the grating. The higher the spectral order, the greater the overlap into the next-lower order. Diffraction gratings are often used in monochromators and other optical instruments.
By controlling the cross-sectional shape of the grooves, it is possible to concentrate most of the diffracted energy in the order of interest. This technique is called “blazing.”
Originally high resolution diffraction gratings were ruled. The construction of high quality ruling engines was a large undertaking. A later photolithographic technique allows gratings to be created from a holographic interference pattern. Holographic gratings have sinusoidal grooves and so are not as bright, but are preferred in monochromators because they lead to a much lower stray light level than blazed gratings. A copying technique allows high quality replicas to be made from master gratings, this helps to lower costs of gratings.
A planar waveguide reflective diffraction grating includes an array of facets arranged in a regular sequence. The performance of a simple diffraction grating is illustrated with reference to FIG. 1. An optical beam 1, with a plurality of wavelength channels λ1, λ2, λ3 . . . , enters a diffraction grating 2, with grading pitch Λ and diffraction order m, at a particular angle of incidence θin. The optical beam is then angularly dispersed at an angle θout depending upon wavelength and the order, in accordance with the grating equation:

mλ=Λ(sin θin+sin θout)  (1)

From the grating equation (1), the condition for the formation of a diffracted order depends on the wavelength λN of the incident light. When considering the formation of a spectrum, it is necessary to know how the angle of diffraction θNout varies with the incident wavelength θin. Accordingly, by differentiating the equation (1) with respect to θNout, assuming that the angle of incidence θin is fixed, the following equation is derived:

∂θNout/∂λ=m/Λ cos θNout  (2)

The quantity dθNout/dλ is the change of the diffraction angle θNout corresponding to a small change of wavelength λ, which is known as the angular dispersion of the diffraction grating. The angular dispersion increases as the order m increases, as the grading pitch Λ decreases, and as the diffraction angle θNout increases. The linear dispersion of a diffraction grating is the product of this term and the effective focal length of the system.
Since light of different wavelengths λN are diffracted at different angles θNout, each order m is drawn out into a spectrum. The number of orders that can be produced by a given diffraction grating is limited by the grating pitch Λ, because θNout cannot exceed 90°. The highest order is given by Λ/□□. Consequently, a coarse grating (with large Λ) produces many orders while a fine grating may produce only one or two.
The free spectral range (FSR) of a diffraction grating is defined as the largest bandwidth in a given order which does not overlap the same bandwidth in an adjacent order. The order m is important in determining the free spectral range over which continuous dispersion is obtained. For a given input-grating-output configuration, with the grating operation at a preferred diffraction order m for a preferred wavelength λ, other wavelengths will follow the same path at other diffraction orders. The first overlap of orders occurs when

mλm=(m+1)λm+1  (3)





  
    
      
        
          λ
          
            m
            +
            1
          
        
        =
        
          
            m
            ⁢
            
              
            
            ⁢
            
              λ
              m
            
          
          
            (
            
              m
              +
              1
            
            )
          
        
      
    
    
      
        (
        4
        )
      
    
  
  
    
      
        Δλ
        =
        
          
            λ
            m
          
          
            m
            +
            1
          
        
      
    
    
      
        (
        5
        )
      
    
  




A blazed grating is one in which the grooves of the diffraction grating are controlled to form right triangles with a blaze angle w, as shown in FIG. 1. The selection of the blaze angle w offers an opportunity to optimize the overall efficiency profile of the diffraction grating, particularly for a given wavelength.
Planar waveguide diffraction based devices provide excellent performance in the near-IR (1550 nm) region for Dense Wavelength Division Multiplexing (DWDM). In particular, advancements in Echelle gratings, which usually operate at high diffraction orders (40 to 80), high angles of incidence (approx 60°) and large grading pitches, have lead to large phase differences between interfering paths. Because the size of grating facets scales with the diffraction order, it has long been considered that such large phase differences are a necessity for the reliable manufacturing of diffraction-based planar waveguide devices. Thus, existing devices are limited to operation over small wavelength ranges due to the high diffraction orders required (see equation 5).
Furthermore, for diffraction grating-based devices fabricated in a planar waveguide platform, a common problem encountered in the prior art is polarization dependent loss arising from field exclusion of one polarization caused by the presence of conducting metal S (a reflective coating) adjacent to the reflective facets F.
An optical signal propagating through an optical fiber has an indeterminate polarization state requiring that the (de)multiplexer be substantially polarization insensitive so as to minimize polarization dependent losses. In a reflection grating used near Littrow condition, and blazed near Littrow condition, light of both polarizations reflects equally well from the reflecting facets (F in FIG. 1). However, the metalized sidewall facet S introduces a boundary condition preventing light with polarization parallel to the surface (TM) from existing near the surface. Moreover, light of one polarization will be preferentially absorbed by the metal on the sidewall S, as compared to light of the other polarization. Ultimately, the presence of sidewall metal manifests itself in the device performance as polarization-dependent loss (PDL).
There are numerous methods and apparatus for reducing the polarization sensitivity of diffraction gratings. Chowdhury, in U.S. Pat. Nos. 5,966,483 and 6,097,863 describes a reduction of polarization sensitivity by choosing to reduce the difference between first and second diffraction efficiencies of a wavelength within the transmission bandwidth. This solution can be of limited utility because it requires limitations on election of blaze angles and blaze wavelength.
Sappey et al, in U.S. Pat. No. 6,400,509, teaches that polarization sensitivity can be reduced by including reflective step surfaces and transverse riser surfaces, separated by a flat. This solution is also of limited utility because it requires reflective coating on some of the surfaces but not the others, leading to additional manufacturing steps requiring selective treatment of the reflecting interfaces.
The free spectral range of gratings is proportional to the size of the grating facets. It has long been thought that gratings with a small diffraction order could not be formed reliably by means of photolithographic etching, because low order often implies steps smaller or comparable to the photolithographic resolution. The photolithographic resolution and subsequent processing steps blur and substantially degrade the grating performance. Therefore, the field of etched gratings has for practical reasons limited itself to reasonably large diffraction orders typically in excess of order 10. Devices with orders ranging close to order 1 have long been thought to be impractical to realize.
An object of the present invention is to overcome the shortcomings of the prior art by providing a planar waveguide reflective diffraction grating providing an increased bandwidth, due to operating at a relatively low order, with very little PDL, due to very small sidewall lengths.
SUMMARY OF THE INVENTION
Accordingly, the present invention relates to a reflective diffraction grating device on a planar waveguide platform for use in demultiplexing an optical signal, comprising:
an input port for launching a beam of light comprising a plurality of wavelength channels, defined by an average wavelength, at a diffraction grating incident angle;
a reflective diffraction grating for dispersing the wavelength channels at various angles according to wavelength, the reflective diffraction grating having a plurality of reflective walls defined by a facet length, and a plurality of sidewalls defined by a sidewall length; and
a plurality of output ports positioned to capture the wavelength channels;
wherein an aspect ratio, defined by the facet length divided by the sidewall length, is greater than 3.
Another aspect of the present invention relates to a reflective diffraction grating device on a planar waveguide platform for use in multiplexing or demultiplexing optical channels defined by an average wavelength, comprising:
a reflective diffraction grating, which includes a plurality of reflective walls defined by a facet length; and a plurality of non-reflective sidewalls defined by a sidewall length;
an input port for launching a beam of light comprising the optical channels at the diffraction grating at an incident angle;
a first output port for outputting one of the optical channels; and
a second output port for outputting another of the optical channels;
wherein the facet length and the incident angle are selected to ensure that the grating provides diffraction in an order with an absolute value of 7 or less.
Another feature of the present invention provides a reflective diffraction grating device on a planar waveguide platform for use in demultiplexing an optical signal, comprising:
an input port for launching a beam of light comprising a plurality of wavelength channels, defined by an average wavelength, at a diffraction grating incident angle;
a reflective diffraction grating for dispersing the wavelength channels at various angles according to wavelength, the reflective diffraction grating having a plurality of reflective walls defined by a facet length, and a plurality of sidewalls defined by a sidewall length; and
a plurality of output ports positioned to capture the wavelength channels;
wherein the sidewall length is less than or equal to twice the average wavelength.



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 reflective diffraction grating;
FIG. 2 illustrates a planar waveguide reflective diffraction grating according to the present invention; and
FIG. 3 illustrates an optical device incorporating the planar waveguide reflective diffraction grating according to the present invention.



DETAILED DESCRIPTION
One of the major concerns in the design of planar waveguide diffraction gratings is the manufacturability of the reflecting and sidewall facets F and S, respectively. Furthermore, a major limit to the manufacturability of the facets heretofore, has been the photolithographic resolution limitations. Typical photolithographic procedures are limited to resolutions in the range of 0.5 to 1.0 μm, so the minimal requirement to achieve reasonable performance from a grating is that the reflecting facet size F must be larger than this resolution, say 2.5 to 5 μm or more in size.
In FIG. 1, the light path is simplified by the assumption that the input and output angles θin and θNout, respectively are identical. This assumption is only to simplify the mathematical treatment of the facet geometry. Accordingly:

F≈Λ cos θin; and  (6)

Equation (1) simplifies to

mλ≈2Λ sin θin  (7)

Combining equations 6 and 7 yields




  
    
      
        F
        ≈
        
          
            m
            ⁢
            
              
            
            ⁢
            λ
          
          
            2
            ⁢
            
              
            
            ⁢
            tan
            ⁢
            
              
            
            ⁢
            
              θ
              in
            
          
        
      
    
    
      
        (
        8
        )
      
    
  




From FIG. 1:




  
    
      
        
          S
          F
        
        ≈
        
          tan
          ⁢
          
            
          
          ⁢
          
            θ
            in
          
        
      
    
    
      
        (
        9
        )
      
    
  




Historically, incidence and output angles of 45° to 65° have been used inevitably leading to grating facet aspect ratio of F/S to be about 1 (see FIG. 1 and Equation 9). At a wavelength of 1550 nm, one finds from equation (6) that facet sizes, for both reflecting F and non-reflecting surfaces S, of 10–17 μm are easily achievable in the prior art, for DWDM applications. This makes grating facets F manufacturable, but at the expense of large non-reflecting facets (or sidewalls) S contributing to the polarization dependent loss. In the prior art, facet size variation is also done by varying the diffraction order m, i.e. adjusting the numerator of equation (8).
Telecommunications networks have evolved from DWDM to CWDM and FTTH networks. The latter two network architectures have channels spanning large wavelength ranges, from  ̃1250 nm to  ̃1630 nm. These wide ranges cannot be served by a high-diffraction order device, and often require orders as low as 1. Practitioners of the prior art have not been aware of, or taken advantage of equation (8). At low diffraction orders m and operating angles θin and θout of 45° to 65° the resulting facet size F for a planar waveguide diffraction grating would be too small to be practically manufacturable. Existing planar waveguide diffraction based devices include AWGs and echelle gratings. Both rely on high diffraction orders; the AWGs need high order operation for guide routing reasons, the echelle technique employs high orders to maintain large facet sizes that are more easily manufactured. Hence, prior art has intrinsic limitations in addressing the CWDM or FTTH network architectures in a planar waveguide platform.
The present invention recognizes the importance of equation (8), in particular the fact that it is possible to increase the grating facet aspect ratio F/S through angular dependence of the denominator. As the diffraction angle is reduced, the facet size increases linearly with tan θin. Additionally, inventors recognize that the increase of the facet aspect ratio F/S yields devices with improved polarization dependent loss and larger free spectral range.
For example, in silica-on-silicon, a diffraction order of 5 or less (yielding the smallest practical free spectral range for CWDM or FITH networks), at a wavelength of 1550 nm, and size of reflecting facet F to exceed 5.0 μm, would require F/S to be increased to more than 3, which can be accomplished by lowering the diffraction angle to about 25°. Thus, the present invention encompasses all planar waveguide diffraction grating designs with the ratio of reflecting to non-reflecting facets (or sidewalls) of at least 3.
The amount of PDL is strongly dependent on the aspect ratio F/S and the length of the non-reflecting facet S. Conventional echelle designs have an aspect ratio of  ̃1, and are strongly subjected to sidewall dependent PDL; however, for F/S in excess of 3, the non-reflecting facets make substantially smaller contribution to the PDL. By further increasing F/S, it is possible to design manufacturable facets with the non-reflecting grating facet sizes S at or smaller than the wavelength of the reflected light, e.g. S≦3000 nm, preferably ≦2500 nm, even more preferably ≦2000 nm, and ultimately preferably ≦1550 nm. For such gratings, the interaction length of light with the metallized sidewall is so small that PDL-free operation of the device becomes possible.
Therefore, when we enter a regime in which tan(θ) is small, i.e. to achieve a 1⁄3 ratio or θ<25°, we can reduce sidewall dependent PDL.
From a manufacturability standpoint, if reflecting facets F are large, the facets themselves are reproduced faithfully despite photolithographic resolution limits. Small non-reflecting facets S will likely not be reproduced faithfully, and will be slightly rounded, but grating performance is not affected. Practitioners of prior art no doubt have realized that the pitch governs dispersion as per equation (1). However, it is quite common to equate the pitch of a grating to the normal distance between reflecting facets (the sidewall S in FIG. 1). With that thinking, a distortion to the sidewall S could be equated with a distortion to the pitch. This is a mistaken conception, and in fact the pitch is given by equation (6). Counter-intuitively, the pitch increases with F, not S. The present inventors recognize this fact and can increase the aspect ratio, i.e. decrease S/F, shown in equation (9) without risk of affecting the pitch. In fact, the fidelity of the grating reproduction is limited not by photolithography but by the accuracy of the features on the mask itself. This limit is several orders of magnitude (100-fold) smaller than the photolithographic resolution.
Combining equation (8) and (9), we find that:




  
    
      
        S
        ≈
        
          
            m
            ⁢
            
              
            
            ⁢
            λ
          
          2
        
      
    
    
      
        (
        10
        )
      
    
  




Thus, by choosing a small diffraction order (m=3, 2 or 1, if necessary) one can nearly eliminate PDL, because the sidewall size S becomes less than the wavelength.
In a preferred embodiment, illustrated in FIG. 3, a concave reflective diffraction grating 10 is formed at an edge of a slab waveguide 11 provided in chip 12. An input port is defined by an end of a waveguide 13, which extends from an edge of the chip 12 to the slab waveguide 11 for transmitting an input wavelength division multiplexed (WDM) signal, comprising a plurality of wavelength channels (λ1, λ2, λ3 . . . ), thereto. The diffraction grating 10, as defined above with reference to FIG. 2, has an aspect ratio (F/S) greater than 5, and a sidewall length S less than or equal to the average wavelength of the wavelength channels (λ1, λ2, λ3 . . . ). The input waveguide 13 is positioned to ensure that the incident angle θin is less than 30°, and the grating pitch Λ is selected to ensure that the grating 10 provides diffraction in an order of 5 or less. The diffraction grating 10 disperses the input signal into constituent wavelengths and focuses each wavelength channel on a separate output port in the form of an output waveguide 15, the ends of which are disposed along a focal line 16 of the grating 10 defined by a Rowland circle, for transmission back to the edge of the chip 12. The illustrated device could also be used to multiplex several wavelength channels, input the waveguides 15, into a single output signal transmitted out to the edge of the chip 12 via the input waveguide 13. The input and output ports represent positions on the slab waveguide 11 at which light can be launched or captured; however, the ports can be optically coupled with other transmitting devices or simply blocked off.
Specific examples for operating the aforementioned optical device are:


















θin = 5°
5°

5°

6°



m = 1
2

3

2


λavg = 1550 nm
1550
nm
1550
nm
1550
nm


Λ = 8892 nm
17784
nm
26676
nm
14828
nm


F = 8858 nm
17716
nm
26574
nm
14747
nm


S = 775 nm
1550
nm
2325
nm
1550
nm


F/S = 11.4
11.4

11.4

9.5

## Claims

1. A reflective diffraction grating device on a planar waveguide platform for use in demultiplexing an optical signal, comprising:
an input port for launching a beam of light comprising a plurality of wavelength channels, defined by an average wavelength, at a small diffraction grating incident angle;
a reflective diffraction grating for dispersing the wavelength channels at various angles according to wavelength, the reflective diffraction grating having a plurality of substantially triangular steps, each step comprising a reflective wall defined by a facet length, and a sidewall defined by a sidewall length; and
a plurality of output ports positioned to capture the wavelength channels;
wherein an aspect ratio, defined by the facet length divided by the sidewall length, is greater than 3.

2. The device according to claim 1, wherein the aspect ratio is greater than 5.

3. The device according to claim 1, wherein the aspect ratio is greater than 10.

4. The device according to claim 3, wherein the diffraction grating incident angle is less than 6°.

5. The device according to claim 3, wherein the sidewall length is less than or equal to two times the average wavelength.

6. The device according to claim 1, wherein the sidewall length is less than or equal to the average wavelength.

7. The device according to claim 1, wherein the incidence angle less than 30°.

8. The device according to claim 1, wherein the incidence angle is less than 15°.

9. The device according to claim 1, wherein the facet length is greater than 5 μm; and wherein the sidewall length is less than 3000 nm.

10. The device according to claim 1, wherein the sidewall length is less than 1550 nm.

11. The device according to claim 1, wherein the incidence angle is less than 45°.

12. A reflective diffraction grating device on a planar waveguide platform for use in multiplexing or demultiplexing optical channels defined by an average wavelength, comprising:
a reflective diffraction grating including a plurality of substantially triangular steps, each step comprising:
a reflective wall defined by a facet length; and
a non-reflecting sidewall defined by a sidewall length;

an input port for launching a beam of light comprising the optical channels at the diffraction grating at a small incident angle;
a first output port for outputting one of the optical channels; and
a second output port for outputting another of the optical channels;
wherein the facet length and the incident angle are selected to ensure that the grating provides diffraction in an order with an absolute value of 7 or less.

13. The device according to claim 12, wherein the order is 5 or less.

14. The device according to claim 12, wherein the order is 3 or less.

15. The device according to claim 14, wherein the diffraction grating incident angle is less than 6°.

16. The device according to claim 15, wherein the sidewall length is less than or equal to the average wavelength.

17. The device according to claim 12, wherein the incidence angle is less than 45°.

18. The device according to claim 12, wherein the incidence angle less than 30°.

19. The device according to claim 12, wherein the incidence angle is less than 15°.

20. The device according to claim 12, wherein the sidewall length is less than or equal to two times the average wavelength.

21. The device according to claim 12, wherein the sidewall length is less than or equal to the average wavelength.

22. The device according to claim 12, wherein the facet length is greater than 5 μm; and wherein the sidewall length is less than 3000 nm.

23. The device according to claim 12, wherein the sidewall length is less than 1550 nm.

24. A reflective diffraction grating device on a planar waveguide platform for use in demultiplexing an optical signal, comprising:
an input port for launching a beam of light comprising a plurality of wavelength channels, defined by an average wavelength, at a small diffraction grating incident angle;
a reflective diffraction grating for dispersing the wavelength channels at various angles according to wavelength, the reflective diffraction grating having a plurality of steps defining substantially triangular grooves, each step comprising a reflective wall defined by a facet length, and a sidewall defined by a sidewall length; and
a plurality of output ports positioned to capture the wavelength channels;
wherein the sidewall length is less than or equal to twice the average wavelength.

25. The device according to claim 24, wherein the sidewall length is less than or equal to the average wavelength.

26. The device according to claim 24, wherein the sidewall length is less than 3000 nm.

27. The device according to claim 24, wherein the incidence angle is less than 1500 nm.

28. The device according to claim 24, wherein the incidence angle less than 45°.

29. The device according to claim 24, wherein the incidence angle less than 30°.

30. The device according to claim 24, wherein the incidence angle is less than 15°.

31. A reflective diffraction grating device on a planar waveguide platform for use in demultiplexing an optical signal, comprising:
an input port for launching a beam of light comprising a plurality of wavelength channels, defined by an average wavelength, at a diffraction grating incident angle;
a reflective diffraction grating for dispersing the wavelength channels at various angles according to wavelength, the reflective diffraction grating having a plurality of steps defining substantially triangular grooves, each step comprising a reflective wall defined by a facet length, and a sidewall defined by a sidewall length; and
a plurality of output ports positioned to capture the wavelength channels; wherein the sidewall length is less than or equal to twice the average wavelength; and
wherein the facet length is greater than 5 μm.

32. The device according to claim 31, wherein the sidewall length is less than 1550 nm.

33. The device according to claim 31, wherein the incidence angle is less than 45°.