OPTICAL CONFIGURATIONS FOR ACHIEVING UNIFORM CHANNEL SPACING IN WDM TELECOMMUNICATIONS APPLICATIONS

Abstract

Optical diffraction configurations provide uniform physical channel spacing in dense wavelength division multiplexing (DWDM) applications. A grating has a dispersed side outputting (or receiving) or a plurality of spaced-apart optical frequencies or wavelengths to (or from) an image plane, and a prism is supported between the dispersed side of the grating and image plane improve the uniformity of the spacing between optical frequencies or wavelengths at the image plane. The diffraction grating may be a transmission or reflection grating. The diffraction grating is preferably a volume-phase holographic (VPH) grating. A second prism may be used such that the input and output beams have a substantially identical aperture.

Description

BRIEF DESCRIPTION OF TIE DRAWINGS

FIG. 1 shows a standard 940 line/mm grating geometry for C-band DWDM;

FIG. 2 shows a 100 GHz Channel Spacing with standard 940 grating and 100 mm EFL lens;

FIG. 3 shows a 940 l/mm grating with compensating 61.5° output prism;

FIG. 4 shows a 100 GHz Channel spacing with compensating 61.5° output prism;

FIG. 5 shows a 940 l/mm grating with input & output prisms; and

FIG. 6 shows a spacing-compensated 940 l/mm reflection grating.

DETAILED DESCRIPTION OF THE INVENTION

The present invention compensates this non-uniform channel spacing through the novel application of a grating/prism combination. In accordance with the invention, for a given grating geometry, a prism can be designed that balances the nonlinearity of the diffraction grating equation with the nonlinearity of the well-known Snell's law of refraction:

N1*Sin(θ1)=N2*Sin(θ2).

Where θ1 and θ2 are the input and output angles, and N1 and N2 are the refractive indices of the input and output media.

With respect to C-band telecommunications, such compensation is provided using a high-efficiency, substantially polarization-independent grating having approximately 940 lines/mm and a prism constructed of BK7 or similar glass having an input surface parallel to the grating and an output surface tilted at approximately 61.5 degrees with respect to the grating surface. This compensated geometry is shown in FIG. 3. The 100 GHz channel spacing nonuniformity across the C-band, shown in FIG. 4, has been reduced from a problematic 12.1 microns to less than 0.5 microns. Note also that the absolute dispersion has increased from a ˜110 micron average channel spacing, to ˜124 microns. This is generally desirable. Again, spacing of the focused frequencies can be adjusted up or down at will with the focal length of the lens.

The invention may be applied to other grating geometries, which will be similarly compensated at other prism angles. The same concept can also be applied with a similar prism on the input side of the grating, as shown in FIG. 5. This input prism has little impact on channel spacing, but may be desirable for device symmetry and/or to avoid anamorphic aperture stretching between the input and output beams, which may be required for efficient fiber coupling.

The invention may also be applied to a reflection grating, as shown in FIG. 6, where the output is a mirror image of the equivalent transmission grating geometry for the same grating frequency. The same equations and correcting prism angles apply.

The above describes a design that renders uniform optical channel spacing uniform as measured in optical frequency. Optical frequency, f, and optical wavelength, λ, are related by yet another nonlinear function:

f=c/λ,

where c is the speed of light, a constant. Because of this nonlinear relationship, a different prism angle is required to generate uniform wavelength spacing than would be used to generate uniform frequency spacing. The design concept is otherwise identical.

Claims

1. Optical apparatus, comprising: a diffraction grating having a dispersed side outputting (or receiving) a plurality of spaced-apart optical frequencies or wavelengths to (or from) an image plane; anda prism supported between the dispersed side of the grating and image plane to improve the uniformity of the spacing between optical frequencies or wavelengths at the image plane.

2. The optical apparatus of claim 1, wherein the prism is immediately adjacent to the dispersed side of the grating.

3. The optical apparatus of claim 1, wherein the diffraction grating is a transmission or reflection grating.

4. The optical apparatus of claim 1, wherein the diffraction grating is a holographic grating.

5. The optical apparatus of claim 1, wherein the diffraction grating is volume-phase holographic (VPH) grating.

6. The optical apparatus of claim 1, further including a second prism to render input and output beams having a substantially identical aperture.

7. The optical apparatus of claim 1, further including a plurality of spaced-apart optical fibers to deliver (or receive) the spaced-apart optical frequencies or wavelengths to (or from) the grating.

8. The optical apparatus of claim 1, wherein the dispersed side outputs a plurality of spaced-apart optical frequencies or wavelengths as part of a wavelength division demultiplexer in an optical telecommunications system.

9. The optical apparatus of claim 1, wherein the dispersed side receives a plurality of spaced-apart optical frequencies or wavelengths as part of a wavelength division multiplexer in an optical telecommunications system.

10. The optical apparatus of claim 1, wherein the dispersed side outputs a plurality of spaced-apart optical frequencies or wavelengths as part of a channel monitor or spectrograph with uniform detector spacing mapping to uniform channel spacing.

11. The optical apparatus of claim 1, wherein, the grating is a high-efficiency, substantially polarization-independent grating having approximately 940 lines/mm configured for use with C-band telecommunications; andthe prism is constructed of BK7 or similar glass having an input surface parallel to the grating and an output surface tilted at approximately 61.5 degrees with respect to the grating surface.

12. Optical apparatus, comprising a diffraction grating having a dispersed side outputting (or receiving) a plurality of spaced-apart optical frequencies or wavelengths to (or from) an image plane; anda prism mounted to the dispersed side of the grating at an angle such that the nonlinearity of Snell's law of refraction at the prism interface balances the nonlinearity of the diffraction grating angle versus wavelength or frequency, thereby improving the uniformity of the spacing between optical frequencies or wavelengths at the image plane.

13. The optical apparatus of claim 12, wherein the diffraction grating is a transmission or reflection grating.

14. The optical apparatus of claim 12, wherein the diffraction grating is holographic.

15. The optical apparatus of claim 12, wherein the diffraction grating is volume-phase holographic (VPH) grating.

16. The optical apparatus of claim 12, further including a second prism to render input and output beams having a substantially identical aperture.

17. The optical apparatus of claim 12, further including a plurality of spaced-apart optical fibers to deliver (or receive) the spaced-apart optical frequencies or wavelengths to (or from) the grating.

18. The optical apparatus of claim 12, wherein the dispersed side outputs a plurality of spaced-apart optical frequencies or wavelengths as part of a wavelength division demultiplexer in an optical telecommunications system.

19. The optical apparatus of claim 12, wherein the dispersed side receives a plurality of spaced-apart optical frequencies or wavelengths as part of a wavelength division multiplexer in an optical telecommunications system.

20. The optical apparatus of claim 12, wherein the dispersed side outputs a plurality of spaced-apart optical frequencies or wavelengths as part of a channel monitor or spectrograph with uniform detector spacing mapping to uniform channel spacing.

21. The optical apparatus of claim 12, wherein: the grating is a high-efficiency, substantially polarization-independent grating having approximately 940 lines/mm configured for use with C-band telecommunications; andthe prism is constructed of B7 or similar glass having an input surface parallel to the grating and an output surface tilted at approximately 61.5 degrees with respect to the grating surface.