|Publication number||USRE40271 E1|
|Application number||US 11/180,027|
|Publication date||Apr 29, 2008|
|Filing date||Jul 12, 2005|
|Priority date||Sep 3, 1999|
|Publication number||11180027, 180027, US RE40271 E1, US RE40271E1, US-E1-RE40271, USRE40271 E1, USRE40271E1|
|Inventors||Andrew D. Sappey, Gerry Murphy|
|Original Assignee||Zolo Technologies, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (71), Non-Patent Citations (5), Referenced by (8), Classifications (4), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a reissue application of U.S. Pat. No. 6,647,182, issued Nov. 11, 2003, entitled “Echelle Grating Dense Wavelength Division Multiplexer/Demultiplexer,” which is a continuation-in-part application of U.S. patent application Ser. No. 09/628,774 now U.S. Pat. No. 6,415,080, filed Jul. 29, 2000, entitled, “Echelle Grating Dense Wavelength Division Multiplexer/Demultiplexer,” which claims priority from U.S. Provisional Patent Application Serial No. 60/209,018, filed Jun. 1, 2000, entitled “Lens-coupled Wavelength Division (De)multiplexing System Utilizing an Echelle Grating;” No. 60/152,218, filed Sep. 3, 1999, entitled “Method and Apparatus for Dense Wavelength Multiplexing and De-multiplexing Fiber Optic Signals Using an Echelle Grating Spectrograph;” No. 60/172,843, filed Dec. 20, 1999, entitled “Improved Method and Apparatus for Dense Wavelength Multiplexing and De-multiplexing Fiber Optic Signals Using an Echelle Grating Spectrograph;” and No. 60/172,885, filed Dec. 20, 1999, entitled “Method and Apparatus for Dense Wavelength Multiplexing and De-multiplexing Fiber Optic Signals from a Single or Many Individual Fibers Using a Single Echelle Grating Spectrograph,” each of which is incorporated herein in its entirety.
The present invention is directed toward optical communications, and more particularly toward a bulk optical echelle grating multiplexer/demultiplexer.
At the inception of fiber optic communications, typically a fiber was used to carry a single channel of data at a single wavelength. Dense wavelength division multiplexing (DWDM) enables multiple channels at distinct wavelengths within a given wavelength band to be sent over a single mode fiber, thus greatly expanding the volume of data that can be transmitted per optical fiber. The wavelength of each channel is selected so that the channels do not interfere with each other and the transmission losses to the fiber are minimized. Typical DWDM allows up to 40 channels to be simultaneously transmitted by a fiber.
The volume of data being transmitted by optical fibers is growing exponentially and the capacity for data transmission is rapidly being consumed. Burying additional fibers is not cost effective. Increasing the optical transmission rate is limited by the speed and economy of electronics surrounding the system as well as chromatic dispersion in the fibers. Thus, the most promising solution for increasing data carrying capacity is increasing the number of channels per a given bandwidth through DWDM.
DWDM requires two conceptually symmetric devices: a multiplexer and a demultiplexer. A multiplexer takes multiple beams or channels of light, each at a discrete wavelength and from a discrete source and combines the channels into a single multi-channel or polychromatic beam. The input typically is a linear array of waveguides such as a linear array of optical fibers, a linear array of laser diodes or some other optical source. The output is typically a single waveguide such as an optical fiber. A demultiplexer spacially separates a polychromatic beam into separate channels according to wavelength. Input is typically a single input fiber and the output is typically a linear array of waveguides such as optical fibers or a linear array of photodetectors.
In order to meet the requirements of DWDM, multiplexers and demultiplexers require certain inherent features. First, they must be able to provide for a high angular dispersion of closely spaced channels so that individual channels can be separated over relatively short distances sufficiently to couple with a linear array of outputs such as output fibers. Furthermore, the multiplexer/demultiplexer must be able to accommodate channels over a free spectral range commensurate with fiber optic communications bandwidth. Moreover, the devices must provide high resolution to minimize cross talk and must further be highly efficient to minimize signal loss. The ideal device would also be small, durable, thermally stable, inexpensive and scalable.
Much of the attention in DWDM devices has been directed to array waveguides. Array waveguides have a set of intermediate pathways, e.g., waveguides, that progressively vary in length to incline wavefronts of different wavelength signals within a free spectral range. Confocal couplers connect the common and individual pathways to opposite ends of the intermediate pathways. One illustrative example is disclosed in Lee, U.S. Pat. No. 5,706,377. Array waveguides suffer from the disadvantages of being expensive to design and manufacture, unable to provide high channel densities over broad wavelengths necessary for DWDM, thermal sensitivity and a lack of scalability and polarization dependent and high insertion losses.
Another family of DWDM devices use a network of filters and/or fiber Bragg gratings for channel separation. Pan, U.S. Pat. No. 5,748,350, is illustrative. However, the channel spacing of these devices, on the order of 0.8 or 1.6 nanometers (nm), limits the number of wavelengths that can be coupled into or out of a fibers. Further, these devices present significant issues of optical loss, cross talk, alignment difficulties and thermal sensitivity.
Various bulk optical DWDM devices have also been investigated in the prior art. Fu et al., U.S. Pat. No. 5,970,190, teaches a grating-in-etalon wavelength division multiplexing device using a Bragg diffraction grating. Fu requires either a tilt mechanism for fabrication of an etalon waveguide with reflective exposed faces having a Bragg grating written into the waveguide. This device has limited channel separation capacity and requires a tilt mechanism that can be difficult to control and is unreliable.
Dueck, U.S. Pat. No. 6,011,884, teaches a DWM device with a collimating optic and bulk grating in near-littrow configuration. Dueck is concerned with the use of a homogeneous boot lens to create a one-piece integrated device. This device is intended to be compact, robust and environmentally and thermally stable. However, the device taught by Dueck fails to address the need to provide many channels for DWDM, high efficiency and a short focal length to provide a compact device.
Lundgren, U.S. Pat. No. 6,018,603, like Dueck, teaches the use of a bulk diffraction grating for DWM. Specially, Lundgren teaches the use of an echellette grating in combination with a rod-like graded refractive index lens or imaging lens for correcting any offset in the focal length of a focusing lens. Lundgren also fails to teach a DWDM device capable of accommodating high channel density and providing a high angular dispersion of channels so as to minimize focal length and apparatus size.
Other examples of techniques for multiplexing and demultiplexing optical signals include the use of birefringement element, the use of optical band pass filters, the use of interference filters, the use of prisms and the use of sequences of cascaded gratings. However, none of these systems provide the combination of beneficial attributes necessary to meet the growing needs for DWDM.
The present invention is intended to overcome some of the problems discussed above and to provide a bulk optical echelle grating multiplexer/demultiplexer with many of the attributes necessary for cost-effective DWDM.
A dense wavelength multiplexer/demultiplexer (“DWDM”) for use in optical communication systems using optical signals in a select near infrared wavelength range and a select channel spacing includes at least two multiplex optical waveguides each propagating a distinct multiplexed optical signal comprising a plurality of channels. The multiplex optical waveguides are arranged in a linear array. A two dimensional array of single channel waveguides is arranged in linear rows perpendicular to the multiplexed linear array with each linear row corresponding to a multiplex optical waveguide. A reflective echelle grating is optically coupled to the multiplex optical waveguides and the single channel optical waveguides. The echelle grating has a groove spacing of between about 50-300 grooves/millimeter and a blaze angle of between about 51-53 degrees. The select near infrared wavelength range is preferably between about 1520-1610 nanometers and the select channel spacing is 0.8 nanometers or less. A collimating/focusing optic having a select focal length may be optically coupled between the multiplex and single channel waveguide arrays. The collimating/focusing optic preferably has a focal length less than 152.4 millimeters.
Another aspect of the present invention is an apparatus for use in optical communication systems to multiplex or demultiplex an optical signal comprising optical channel(s) of distinct wavelength(s) having a select channel spacing within a select wavelength range. The apparatus includes a plurality of optical waveguides aligned generally along the same optical axis with each having a propagating end. At least two of the optical waveguides each propagate a distinct multiplexed optical signal comprising a plurality of channels, with the multiplexed optical waveguides being arranged in a multiplex linear array. The others of the optical waveguides are single channel waveguides arranged in a two dimensional array with linear rows perpendicular to the multiplex linear array and with each linear row corresponding to a multiplex optical waveguide. A reflective echelle grating is optically coupled to the plurality of optical waveguides along the optical axis and receives an optical signal emitted from at least one of the optical waveguides and diffracts the optical signal(s) to at least one other of the optical waveguide(s). The echelle grating may have a groove spacing of between about 50-300 grooves/millimeter and a blaze angle of between about 51-53 degrees.
Another aspect of the present invention is a method of multiplexing or demultiplexing an optical signal in a optical communication systems. The optical signal comprises optical channels of a 0.8 nanometer or less channel spacing and different wavelengths within a wavelength range between 1520-1610 nanometers. The method includes providing a plurality of optical waveguides aligned generally along the same optical axis, at least two of the waveguides propagating a plurality of multiplexed channels, the at least two multiplexed waveguides being aligned in a multiplex linear array. The others of the optical waveguides propagate single channels. The single channel waveguides are aligned in a two-dimensional array having linear rows perpendicular to the multiplexed linear array with each multiplexed waveguide corresponding to a distinct linear row of single channel waveguides. An optical signal is directed from at least one of the optical waveguides to a reflective echelle grating optically coupled to the plurality of optical waveguides along the optical axis. The optical signal is diffracted generally along the optical axis and optically coupled into at least one other of the optical waveguides at a select focal length. The reflective echelle grating may have a blaze angle of between about 51-53 degrees and a groove spacing of between about 50-300 grooves/millimeter.
Yet another aspect of the invention is a bulk optic echelle grating for use in multiplexing and demultiplexing optical signals in optical communication systems operating in a near infrared wavelength range. The grating has a groove spacing of between about 50-300 grooves/millimeter and a blaze angle of between about 51-53 degrees.
A multiplexer/demultiplexer for use in optical communication systems 10 of the present invention is illustrated schematically in FIG. 1. It includes a pigtail harness 12 consisting of an input waveguide 14, a plurality of output waveguides 16 arranged in a linear array adjacent the input fiber, a collimating/focusing lens 18 and an echelle grating 20, each of which are optically coupled. In the present discussion the multiplexer/demultiplexer will be discussed in terms of a demultiplexer. The description applies equally to a multiplexer, only with the function of the input and output waveguides 14, 16 reversed. Also, for the sake of clarity, only seven output waveguides are illustrated (the center output waveguides underlies the input fiber in
As used herein, “optically coupled” or “optically communicates” means any connection, coupling, link or the like, by which optical signals carried by one optical element are imparted to the “coupled” or “communicating” element. Such “optically communicating” devices are not necessarily directly connected to one another, but may be separated by a space through which the optical signals traverse or by intermediate optical components or devices.
As illustrated in
The echelle grating 20, like other gratings such as echellette gratings, uses interference between light wavefronts reflected from various portions of its ruled surface or steps 22 to divide the incident beam consisting of a plurality of channels λ1-n having a select channel spacing within a select wavelength range λ1-n into separate channels of wavelength beams λ1-λ7 which are angularly dispersed by the grating into output waveguides some distance away. Referring to
Consideration of certain external and performance constraints point to the desirability of echelle gratings for DWDM. The external constraints include the following:
The performance constraints include:
The external constraints of ruggedness size and cost minimization as well as performance constraints of ease of alignment and high efficiency dictate a littrow configuration, which simplifies the system optimization analysis.
Examination of a number of constraining factors discussed above illustrate the utility of echelle gratings for DWDM.
1. Constraining Factors: f number (f) in range of 4-8 and resolution (“R”)>20,000.
2. Constraining Factors: Fl>124 mm and channel separation at least 80μ.
3. Constraining Factors: FSR (free spectral range)>150 Result:
4. Constraining Factors: Wish to provide a flat response over the bandwidth.
5. Constraining Factors: High efficiency. (>85°)
6. Constraining Factors: Limitations on m from 4, and 2. above.
7. Constraining Factors: For an echelle grating in littrow mode:
In designing a functioning multiplexer/demultiplexer, a number of design parameters were selected that were dictated by many of the external and performance constraints set forth above. An exemplary configuration is illustrated schematically in
1. Channel Characteristics
Currently optical communications utilize what is know as the “C” band of near infrared wavelengths, a wavelength band ranging from 1528-1565 nanometers (nm). This provides a bandwidth or free spectral range of 37 nm available for channel separation. Known prior art multiplexer/demultiplexers require a channel spacing of 0.8 nm or even 1.6 nm, resulting in a possibility of only between 48 and 24 channels. Because echelle gratings provide markedly superior channel dispersion, a much smaller channel spacing of 0.4 nm was chosen, resulting in a possibility of 93 channels over the C band. As the tuning range of semiconductor lasers increases and optical communications expand beyond the “C” band to include the “L” band (1566-1610 nm) and the “S” band (≈1490-1527 nm), a total bandwidth of about 120 nm or more is foreseeable, creating a possibility of the multiplexer/demultiplexer accommodating 300 channels or more per input fiber.
Current optical communications operate primarily at a channel frequency of 2.5 GHz, known as OC48. At OC48 the channel width λ48=0.02 nm. Optical communications are currently beginning to adopt a frequency of 10 GHz, know as OC192. At OC192 the channel width λ192=0.08 nm.
2. Fiber Dimensions
Standard single mode optical fiber used in optical communications typically have an outer diameter of 125 microns (μ) and a core diameter of 10μ. Optical fibers having an outer diameter of 80μ and core diameter of 8.3 i are available, model SM-1250 manufactured by Fibercore. In this example, both the input fiber 14 and the output fiber 16 are single mode and share the 80μ outer diameter. Assuming the output fibers 16 are abutted in parallel as illustrated in
3. Form Factor
The design was intended to provide a high channel density in a form factor consistent with or smaller than used in current multiplexer/demultiplexer devices. A total length of between 10-12 inches was the design target. To accommodate all the optics and harnesses, a maximum focal length of 5 inches (127 mm) was chosen. As discussed above, in light of the constraining factors of the f number between 4-8 and a resolution (R)>20,000, a focal length of 124 was ultimately dictated.
4. Dispersion Limitations
In order to prevent the loss of data, it was necessary that the dispersion of the echelle grating be constrained. The initial 0.4 μm channel spacing at the echelle grating was required to be about 80μ of separation at the output fibers (corresponding to the core spacing). On the other hand, the 0.08 μm channel width of OC192 frequencies could not disperse to much greater than the fiber core aperture over the focal length. Thus:
5. Grating Design
The variables affecting grating design are:
For design of the grating, 150 channels centered on 1550 nm was chosen. This results in a physical size of the spectral image of (number of channels)×(maximum separation, or 150×80μ=12,000μ. This desire to have 90% of the intensity contained in 12,000% constrains the size of b. The far field pattern of the diffraction grating is
N=number of lines illuminated,
Spread sheet calculations show that b≦5.5λ(or b≦8.5μ), is necessary to make the spectral image >12,000μ at its 90% intensity point. In littrow mode, the angular dispersion is:
However, for OC192, dispersion must be constrained to contain the 0.08 nm channel width in a 10μ core, so that m<3.34bμ.
Thus, 1.67b<m<3.34b (Condition B).
The desired resolution
Here, λ=1550 nm and Δλ=0.08 nm, yielding a required resolution R=19,375 or approximately 20,000. Assuming a beam size at the grating of 2.1 cm (based upon a fl=124 cm and 10° divergence):
To align the order m with the diffraction peak in littrow mode, we know
or a must have the values:
Only as θb increases to greater than 45° is it possible for conditions A and D to be satisfied. Assuming θb=60°, and m=5,
Selection of the precise groove density and blaze angle are also affected by the polarization dependent loss and manufacturing constraints. For the embodiment illustrated in
In the example of
The lens 18 could be a graded index (GRIN) optic with spherical surfaces or a compound lens with one or more surfaces that might not be spherical (aspheric). The use of lenses or a single lens to collimate the beam and focus the dispersed light limits spherical aberrations or coma resulting from the use of front surface reflectors that require the optical rays to transverse the system in a off-axis geometry. A first type of potential lens uses a radially graded refractive index to achieve near-diffraction limited imaging of off-axis rays. A second type of lens actually consists of at least two individual pieces cemented together (doublet). Another option uses three individual lens pieces (triplet). These pieces may individually have spherical surfaces, or if required for correction of certain types of aberration, aspheric surfaces can be utilized. In this case, the lens would be referred to as an aspheric doublet or triplet.
In the example illustrated in
In the pigtail 12 of
A second alternate embodiment 60 is illustrated in
A fifth alternate embodiment illustrated in
Various modifications can be provided to the basic echelle grating demultiplexer structures illustrated schematically in
By way of example, the operation of the apparatus for dividing broad band signals 110 will be discussed in terms of a demultiplexer. As with other embodiments of this invention, the apparatus may likewise function as a multiplexer simply by reversing the direction of light propagation. A multiplexed beam 124 emitted from the input fiber 112 is directed onto the high pass thin film filter 114. The high pass thin film filter has a design cut off wavelength that reflects the lower half of the wavelength range toward the first echelle grating DWDM 120. The upper half of the wavelength range passes through the filter 114 to the second echelle DWDM device 122. In this example, the input wavelength is in the range of 1460-1580 nm. The high pass thin film filter is designed to cut the band at 1520 nm. Thus, a wavelength range of 1460-1520 nm is directed toward the first echelle grating DWDM and a wavelength band of 1520-1580 nm is directed toward a second echelle grating DWDM device. The signal directed toward the first echelle grating DWDM is optically coupled to the first focusing lens 116 which directs the lower wavelength beam as an input to the first echelle grating DWDM. In a like manner, the upper wavelength beam 128 is optically coupled to the second focusing lens 118 which focuses the upper wavelength beam 128 as an input beam to the second echelle DWDM device 122.
The present example contemplates the use of a high pass thin film filter 114. However, other waveband dividing elements could be used instead, including devices using fiber Bragg gratings.
The first and second echelle grating DWDM devices 120, 122 of the present invention could have any of the configurations discussed above with regard to
The bulk optic echelle DWDM of the present invention is able to simultaneously demultiplex signals from a number of input fibers. In each of the echelle grating DWDM devices illustrated in
The echelle grating DWDM devices in accordance with the present invention provide for dense channel spacing (0.4 nm) over a given bandwidth, thereby maximizing the number of channels that can be carried by a single fiber for a given bandwidth. By careful selection of the echelle grating blaze angle and step spacing, the channels may be multiplexed/demultiplexed at high resolutions and high efficiencies. Further, use of the echelle grating enables a smaller form factor because the angular diffraction allows for shorter focal lengths between the focusing lens and the input/output fibers. The use of bulk optical elements provides a system which is easy to manufacture, highly reliable and scalable. Further embodiments of the invention including the use of a waveband dividing element such as a thin film high pass filter allows extremely broad bands of signals to be divided and simultaneously multiplexed or demultiplexed in parallel. Because the device disperses light in a single linear dimension, a plurality of input fibers can be stacked so that each bulk optic echelle grating DWDM device can accommodate multiple input fibers.
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|Nov 13, 2006||AS||Assignment|
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