Communication designers are under constant pressure to provide higher performance systems at ever increasingly lower cost. Equipment designers are evaluating new methods for reducing costs in their dense wave division multiplexing (DWDM) system designs while retaining the inherent bandwidth provided. Currently much of the signal processing is performed in the electrical domain, as bit rates and channel densities increase, electrical conversion and processing will become prohibitively expensive.
A better solution is to look at processing DWDM signals in the optical domain. By combining the capabilities of a digital signal processor (DSP) with a spatial light modulator, designers can build an optical signal processing solution that effectively grooms, filters, adds, drops, and blocks DWDM signals in metro and wide area networking (MAN and WAN) designs. Let's see how this proposed architecture works.
OSP—How it Works
Unlike traditional architectures, which require manual attention, an OSP approach enables dynamic and remote grooming and switching of the optical signal. Under the OSP approach, high-volume DSP and digital light processor technologies are combined, where both are in high volume production and are field-proven. For the purpose of this article, the digital light processor used offers a typical insertion loss of 1.5 dB at 1550 nm and systems based on the technology exhibit no additional power penalty beyond component insertion loss.
In the OSP approach described here (Figure 1), a Digital Light Processortm (DLP) switched blaze grating (SBG) mirror array is used as a spatial light modulator allowing modulation of the amplitude, direction, and phase of a beam of light within the active area. The SBG processor (a custom programmed DSP) is used for two primary functions. First, to calculate mirror patterns based on algorithms that accomplish the desired optical transforms or modulations of the optical signal. Second, to communicate with the network via custom or standard interfaces to report conditions, results of prior operations and receive new instruction.
Click here for Figure 1
Figure 1: Block diagram of optical signal processor using an SBG spatial light modulator together with a DSP.
As Figure 1 points out, to produce an OSP sub-system with the current offering, designers are provided with the SBG co-processor that accelerates the data load of the mirror array. An SBG reset chip is also provided to handle the reset signal used to execute mirror state changes. Future offerings will reduce chip count while simultaneously increasing available processing performance.
The SBG modulator is based on DLP technology. Each device is an array of 100's of thousands of micro-mirrors. Figure 2 shows a small 13x13 section of the mirror array. The SBG used in this application consists of 1024x768 individually addressable mirror pixels.
Click here for Figure 2
Figure 2: 13 x13 mosaic of mirror pixels in unpowered state. The SBG light modulator consists of 1024x768 individually addressable mirror pixels.
Drilling down further into the details of the SBG, Figure 3 is a graphical representation of two mirror-pixels on the SBG array. The SBG mirrors as shown in this figure represent a subset of larger class of modulators referred to as pixelated spatial light modulators (SLMs).
Click here for Figure 3
Figure 3: Graphic of two mirror-pixels on a typical SBG light modulator consisting of 1024 x 768 individually addressable mirror-pixels.
As the name implies, a spatial light modulator is a device capable of modulating the amplitude, direction, and phase of a beam of light within the active area of the modulator. A pixelated spatial light modulator is comprised of a mosaic of discrete elements on which a beam of light is spread and can be constructed as a transmissive or reflective device. In the case of the SBG, the discrete pixel elements are micrometer size mirrors, and hence are operated in reflection.
Each SBG's tilting micro-mirrors are mounted to a hidden yoke. A torsion-hinge structure connects the yoke to support posts. The hinges permit reliable mirror rotation to nominally a +9 deg. or --9 deg. state. Since each mirror is mounted atop a SRAM cell, a voltage can be applied to either one of the address electrodes, creating an electro-static attraction and causing the mirror to quickly rotate (15 microsecond transition time) until the landing tips make contact with the electrode layer. The mirror is electro-mechanically "latched" in its desired position (see Figure 3).
SBGs are manufactured using standard semiconductor process flows. All metals used for the mirror and mirror substructures are also standard to semiconductor processing.
The total integrated reflectivity of a mirror array (i.e. reflectivity into all output angles or into a hemispherical solid angle) is a function of the area of the mirrors constituting the array, the angle of incidence, and the reflectivity of the mirror material at a specific wavelength. (A consideration of second-order effects on the integrated reflectivity would include weak effects such as light rays scattered from the mirror gaps.)
To determine the power reflected into a small, well-defined, solid angle, a designer must know the pixel pitch or spacing in addition to the factors that control the integrated reflectivity (i.e., mirror area, angle of incidence and reflectivity). As a pixelated reflector, the SBG behaves like a diffraction grating with the maximum power reflected (diffracted) in a direction θr, relative to the surface normal, determined by the pixel period, d, the wavelength, λ, and the angle of incidence, θi.
Figure 4 depicts the optical layout in which the maxima in the reflectivity distribution function is governed by diffraction, d(sinθr + sinθi) = nλ, where n is the order of diffraction.
Click here for Figure 4
Figure 4: Incident light hitting SBG mirror-pixels under the blazed grating condition. Most of the diffracted radiation is concentrated in the second order, producing a highly efficient coherent light modulator.
The condition in which the direction of incidence and diffraction are identical (θi = θr) is referred to as the Littrow configuration, and the diffraction equation reduces to the well-known Bragg equation, 2d(sinθ) = nλ.
The tilt angle of the mirrors is also an effect that strongly controls the reflective power. The Fraunhofer diffraction in the Littrow case directs the light into a ray with an angle equal to the angle of incidence (θi = θr). When the angle of the Fraunhofer diffraction is equal to a diffractive order, the SBG is said to be blazed and explains the origin of our use of the term switched blaze grating. Greater than 90% of the diffracted energy can be coupled into a single diffraction order (see Figure 4). Using this blazed mirror approach, insertion losses of about 1.5 dB can be achieved for the SBG.
Intensity, Pixel Relationship
The relationship between intensity and the number of pixels turned "on" or "off" is one consideration designers must examine when using a pixelated modulator with a coherent monochromatic beam. In a typical single-mode fiber application, the Gaussian beam from the fiber is focused onto the SBG by means of a focusing lens. The light, which is reflected or transmitted by the modulator, is then collimated and focused back into a single-mode fiber. By turning "on" various pixels in the SBG, the amount of optical power coupled into the receiving fiber for each wavelength is varied.
The coupling of power into the output fiber, however, is not straightforward since it is dependent upon the power of the overlap integral between the modulated field and the mode of the output fiber.1 Thus, the coupled power is given by:
where Fs(x,y) and Ff(x,y) are the complex fields of the modulated signal and the fiber mode, respectively.
It is important to note that the efficiency of the fiber coupling depends not only on the amplitude of the two fields, but on how well they are matched in phase. It can be shown that a similar relationship can be derived at either the input to the fiber, at the collimated beam, or at the SBG.
Implementing the OSP
The SBG is suitable for applications where a series of parallel optical switches (e.g. 700,000 - 1x2 switches) are required. Figure 5 displays an OSP system for DWDM signals that employs an SBG.
Click here for Figure 5
Figure 5: SBG illuminated by several wavelength bands before and after attenuation.
An I/O medium (typically a fiber or array of fibers), a dispersive element (typically a grating), and the SBG chipset make up the OSP system. Attenuation functions in the system illustrated are achievable by switching pixels between +1 and --1 states to control the amount of light directed to the output coupler (e.g. with mirrors in +1 state). Monitoring can be achieved by detection of the light directed into the --1 state.
In systems configured like Figure 5, minimum system insertion loss of about 4 dB has been demonstrated with approximately 35-dB maximum attenuation. Systems with polarization dependent losses of less than 0.2 dB have also been demonstrated. Polarization dependent losses are limited by dispersive elements in these systems. With an approximate 15-microsecond switching speed at the mirror level and less the 1-millisecond to completely address and reconfigure the entire array, the SBG is well suited for dynamically balancing gain in optical networks.
Performance Capabilities
Figure 6 shows the intensity of an asynchronous spontaneous emission (ASE) source as transmitted through an OSP system similar to the one shown in Figure 5. The top trace is for all the mirrors in the SBG modulator on and the bottom trace is for all mirrors off. There is typically a 34 to 35 dB extinction ratio between the all-on versus all-off.
Click here for Figure 6
Figure 6: Diagram highlighting equalized ASE spectrum.
In the middle trace the ASE power is equalized to approximately +31 dBm. The spectrum was equalized manually in this case and shows approximately a +/- 0.2 dB variations across the band.
The system shown in Figure 6 only uses approximately 150 mirrors to carry a 100 GHz wideband, which limits the intensity resolution for equalization. Other designs using a few thousand mirrors per band are capable of much finer resolution.
The response function of a column of mirrors is shown in Figure 7. Note the entire abscissa of the graph is only 1.2 nm or 150 GHz. For this particular system approximately eight columns of mirrors are required to obtain full 35 dB of extinction. In addition, the use of phase manipulation algorithms can increase the extinction ratio an additional 3-5dB, if desired. The small dip on the short wavelength side of the spectrum is the result of phase effects in the system.
Click here for Figure 7
Figure 7: Diagram showing mirror column response.
An optical add-drop multiplexer (OADM) can be configured using an optical system similar to the one shown in Figure 5 by adding a second output coupler collecting the light corresponding to the --1 mirror state.
Figure 8 shows the performance of a system like Figure 5 configured in an optical add-drop function. Since the modulator is a highly parallel 1x2 switch, this implementation uses one mirror state as a pass (express) channel and the other position as a drop or add channel.
Click here for Figure 8
Figure 8: Performance of system configured as add drop.
One hundred gigahertz separates channels in the system displayed in Figure 8, which is limited by the dispersive element. An optical performance monitor can also be configured similarly by placing a detector at the position of the output fiber in Figure 5. In this case the SBG mirrors are switched between states to decode wavelength and intensity signals arriving at the detector.
Wrap Up
As a coherent light modulator, the SBG device can be used in DWDM optical networks to dynamically manipulate and shape optical signals. Systems exhibiting low insertion loss are achieved by designing mirror arrays to meet blaze conditions such that the mirror tilt angle coincides with a diffractive order determined by the mirror pitch. Digital spectral equalization, reconfigurable add-drop multiplexing, channel blocking and optical performance monitoring have been demonstrated with a common optical platform.
Editor's Note: This paper is based on a presentation made at the 2002 Communications Design Conference.
Author's Note: The authors would like to thank Lynn Endsley, Benjamin Lee, Don Powell, Paul Rancuret, and Bryce Sawyers for their contribution to this article.
Reference
1. R.E. Wagner, W.J. Tomlinson, "Coupling efficiency of optics in single-mode fiber components," Applied Optics, 21 (1982) 2671.
About the Authors
Walter M. Duncan is a senior member of the technical staff and system development manager for Optical Signal Processing Products in Texas Instruments' DLP Products Division. Since joining TI in 1979, Walter has been a technical contributor to the development of monolithic microwave integrated circuits, advanced silicon processing, photonic devices, and advanced characterization. Walter can be reached at wduncan@ti.com.
Wes Stalcup is the chief marketing officer Chief Marketing Officer for Optical Signal Processing Products in Texas Instruments' DLP Products Division. Joining TI in 1999, he has been a manager of applications, systems engineering, and marketing efforts for silicon solutions targeted at MAN/WAN infrastructure products since 1993. Wes can be reached at stalcup@ti.com.
