Wavelength selective switching

Last updated

Wavelength selective switching components are used in WDM optical communications networks to route (switch) signals between optical fibres on a per-wavelength basis.

Wavelength-division multiplexing

In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity.

Contents

What is a WSS

A WSS comprises a switching array that operates on light that has been dispersed in wavelength without the requirement that the dispersed light be physically demultiplexed into separate ports. This is termed a ‘disperse and switch’ configuration. For example an 88 channel WDM system can be routed from a “common” fiber to any one of N fibers by employing 88 1 x N switches. This represents a significant simplification of a demux and switch and multiplex architecture that would require (in addition to N +1 mux/demux elements) a non-blocking switch for 88 N x N channels [1] which would test severely the manufacturability limits of large-scale optical cross-connects for even moderate fiber counts.

A more practical approach, and one adopted by the majority of WSS manufacturers is shown schematically in Figure 1 (to be uploaded). The various incoming channels of a common port are dispersed continuously onto a switching element which then directs and attenuates each of these channels independently to the N switch ports. The dispersive mechanism is generally based on holographic or ruled diffraction gratings similar to those used commonly in spectrometers. It can be advantageous, for achieving resolution and coupling efficiency, to employ a combination of a reflective or transmissive grating and a prism – known as a GRISM. The operation of the WSS can be bidirectional so the wavelengths can be multiplexed together from different ports onto a single common port. To date, the majority of deployments have used a fixed channel bandwidth of 50 or 100 GHz and 9 output ports are typically used.

Microelectromechanical Mirrors (MEMS)

The simplest and earliest commercial WSS were based on movable mirrors using Micro-Electro-Mechanical Systems (MEMS). [2] A schematic of a MEMS-based WSS is shown in Figure 2 [3] (to be uploaded). The incoming light is broken into a spectrum by a diffraction grating (shown at RHS of Figure) and each wavelength channel then focuses on a separate MEMS mirror. By tilting the mirror in one dimension, the channel can be directed back into any of the fibers in the array. A second tilting axis allows transient crosstalk to be minimised, otherwise switching (eg) from port 1 to port 3 will always involve passing the beam across port 2. The second axis provides a means to attenuate the signal without increasing the coupling into neighbouring fibers. This technology has the advantage of a single steering surface, not necessarily requiring polarization diversity optics. It works well in the presence of a continuous signal, allowing the mirror tracking circuits to dither the mirror and maximise coupling.

MEMS based WSS typically produce good extinction ratios, but poor open loop performance for setting a given attenuation level. The main limitations of the technology arise from the channelization that the mirrors naturally enforce. During manufacturing, the channels must be carefully aligned with the mirrors, complicating the manufacturing process. Post-manufacturing alignment adjustments have been mainly limited to adjusting the gas pressure within the hermetic enclosure. This enforced channelization has also proved, so far, an insurmountable obstacle to implementing flexible channel plans where different channel sizes are required within a network. Additionally the phase of light at the mirror edge is not well controlled in a physical mirror so artefacts can arise in the switching of light near the channel edge due to interference of the light from each channel.

Loop performance in control engineering indicates the performance of control loops, such as a regulatory PID loop. Performance refers to the accuracy of a control system's ability to track (output) the desired signals to regulate the plant process variables in the most beneficial and optimised way, without delay or overshoot.

Binary Liquid Crystal (LC)

Liquid crystal switching avoids both the high cost of small volume MEMS fabrication and potentially some of its fixed channel limitations. The concept is illustrated in Figure 3 (to be uploaded). [4] A diffraction grating breaks the incoming light into a spectrum. A software controlled binary liquid crystal stack, individually tilts each optical channel and a second grating (or a second pass of the first grating) is used to spectrally recombine the beams. The offsets created by the liquid crystal stack cause the resulting spectrally recombined beams to be spatially offset, and hence to focus, through a lens array, into separate fibers. Polarization diversity optics ensures low Polarization Dependent Losses (PDL).

This technology has the advantages of relatively low cost parts, simple electronic control and stable beam positions without active feedback. It is capable of configuring to a flexible grid spectrum by the use of a fine pixel grid. The inter-pixel gaps must be small compared to the beam size, to avoid perturbing the transmitted light significantly. Furthermore each grid must be replicated for each of the switching stages creating the requirement of individually controlling thousands of pixels on different substrates so the advantages of this technology in terms of simplicity are negated as the wavelength resolution becomes finer.

The main disadvantage of this technology arises from the thickness of the stacked switching elements. Keeping the optical beam tightly focused over this depth is difficult and has, so far, limited the ability of high port count WSS to achieve very fine (12.5 GHz or less) granularity.

Liquid Crystal on Silicon (LCoS)

Liquid Crystal on Silicon LCoS is particularly attractive as a switching mechanism in a WSS because of the near continuous addressing capability, enabling much new functionality. In particular the bands of wavelengths which are switched together (channels) need not be preconfigured in the optical hardware but can be programmed into the switch through the software control. Additionally, it is possible to take advantage of this ability to reconfigure channels while the device is operating. A schematic of an LCoS WSS is shown in Figure 4 (to be uploaded). [5]

LCoS technology has enabled the introduction of more flexible wavelength grids which help to unlock the full spectral capacity of optical fibers. Even more surprising features rely on the phase matrix nature of the LCoS switching element. Features in common use include such things as shaping the power levels within a channel or broadcasting the optical signal to more than one port.

LCoS-based WSS also permit dynamic control of channel centre frequency and bandwidth through on-the-fly modification of the pixel arrays via embedded software. The degree of control of channel parameters can be very fine-grained, with independent control of the centre frequency and either upper- or lower-band-edge of a channel with better than 1 GHz resolution possible. This is advantageous from a manufacturability perspective, with different channel plans being able to be created from a single platform and even different operating bands (such as C and L) being able to use an identical switch matrix. Products have been introduced allowing switching between 50 GHz channels and 100 GHz channels, or a mix of channels, without introducing any errors or “hits” to the existing traffic. More recently, this has been extended to support the whole concept of Flexible or Elastic networks under ITU G.654.2 through products such as Finisar's Flexgrid™ WSS.

For more detailed information on the applications of LCoS in telecommunications and, in particular, Wavelength Selective Switches, see chapter 16 in Optical Fiber Telecommunications VIA, edited by Kaminov, Li and Wilner, Academic Press ISBN   978-0-12-396958-3.

MEMS Arrays

A further array-based switch engine uses an array of individual reflective MEMS mirrors to perform the necessary beam steering (Figure 5 [6] (to be uploaded). These arrays are typically a derivative of the Texas Instruments DLP range of spatial light modulators. In this case, the angle of the MEMs mirrors is changed to deflect the beam. However, current implementations only allow the mirrors to have two possible states, giving two potential beam angles. This complicates the design of multi-port WSS and has limited their application to relatively low-port-count devices.

Future Developments

Dual WSS

It is likely that in future two WSS could use the same optical module utilizing different wavelength processing regions of a single matrix switch such as LCoS, [7] [8] provided that the issues associated with device isolation are able to be appropriately addressed. Channel selectivity ensures only wavelengths required to be dropped locally (up to the maximum number of transceivers in the bank) are presented to any mux/demux module through each fiber, which in turn reduces the filtering and extinction requirements on the mux/demux module.

Advanced Spatial Light Modulators

The technical maturity of spatial light modulators based on consumer driven applications has been highly advantageous to their adoption in the telecommunications arena. There are developments in MEMs phased arrays [9] and other electro-optic spatial light modulators that could be envisaged in the future to be applicable to telecom switching and wavelength processing, perhaps bringing faster switching or having an advantage in simplicity of optical design through polarisation-independent operation. For example, the design principles developed for LCoS could be applied to other phase-controllable arrays in a straightforward fashion if a suitable phase stroke (greater than 2π at 1550 nm) can be achieved. However the requirements for low electrical crosstalk and high fill factor over very small pixels required to allow switching in a compact form factor remain serious practical impediments to achieving these goals. [10]

Related Research Articles

Beam steering

Beam steering is about changing the direction of the main lobe of a radiation pattern.

An optical switch is a device that enables optical signals to be selectively switched-on and -off or switched from one channel to another. The former is known as an optical (time-domain) switch or an optical modulator, while the latter can be specifically called an optical space switch or an optical router. In its ways to be switched temporally or spatially, it can be seen as physical analogies to the one-way or two-way switches in electrical circuits. In general, optical modulators and routers can be made from each other.

Dispersion (optics) Dependence of phase velocity on frequency

In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency.

Liquid crystal on silicon is a miniaturized reflective active-matrix liquid-crystal display or "microdisplay" using a liquid crystal layer on top of a silicon backplane. It is also referred to as a spatial light modulator. LCoS was initially developed for projection televisions but is now used for wavelength selective switching, structured illumination, near-eye displays and optical pulse shaping. By way of comparison, some LCD projectors use transmissive LCD, allowing light to pass through the liquid crystal.

Monochromator optical device

A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is from the Greek roots mono-, "single", and chroma, "colour", and the Latin suffix -ator, denoting an agent.

Tunable laser

A tunable laser is a laser whose wavelength of operation can be altered in a controlled manner. While all laser gain media allow small shifts in output wavelength, only a few types of lasers allow continuous tuning over a significant wavelength range.

The grating light valve (GLV) is a "micro projection" technology which operates using a dynamically adjustable diffraction grating. It competes with other light valve technologies such as Digital Light Processing (DLP) and liquid crystal on silicon (LCoS) for implementation in video projector devices such as rear-projection televisions. The use of microelectromechanical systems (MEMS) in optical applications, which is known as optical MEMS or micro-opto-electro-mechanical structures (MOEMS), has enabled the possibility to combine the mechanical, electrical and optical components in very small scale.

Chirped pulse amplification technique for amplifying an ultrashort laser pulse

Chirped pulse amplification (CPA) is a technique for amplifying an ultrashort laser pulse up to the petawatt level with the laser pulse being stretched out temporally and spectrally prior to amplification.

Arrayed waveguide gratings (AWG) are commonly used as optical (de)multiplexers in wavelength division multiplexed (WDM) systems. These devices are capable of multiplexing a large number of wavelengths into a single optical fiber, thereby increasing the transmission capacity of optical networks considerably.

In fiber optics, a reconfigurable optical add-drop multiplexer (ROADM) is a form of optical add-drop multiplexer that adds the ability to remotely switch traffic from a wavelength-division multiplexing (WDM) system at the wavelength layer. This is achieved through the use of a wavelength selective switching module. This allows individual or multiple wavelengths carrying data channels to be added and/or dropped from a transport fiber without the need to convert the signals on all of the WDM channels to electronic signals and back again to optical signals.

Fiber Bragg grating

A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.

Optical vortex

An optical vortex is a zero of an optical field; a point of zero intensity. The term is also used to describe a beam of light that has such a zero in it. The study of these phenomena is known as singular optics.

Optical networking is a means of communication that uses signals encoded onto light to transmit information among various nodes of a telecommunications network. They operate from the limited range of a local-area network (LAN) or over a wide-area network (WAN), which can cross metropolitan and regional areas all the way to national, international and transoceanic distances. It is a form of optical communication that relies on optical amplifiers, lasers or LEDs and wave division multiplexing (WDM) to transmit large quantities of data, generally across fiber-optic cables. Because it is capable of achieving extremely high bandwidth, it is an enabling technology for today’s Internet and the communication networks that transmit the vast majority of all human and machine-to-machine information.

Optical add-drop multiplexer

An optical add-drop multiplexer (OADM) is a device used in wavelength-division multiplexing systems for multiplexing and routing different channels of light into or out of a single mode fiber (SMF). This is a type of optical node, which is generally used for the formation and the construction of optical telecommunications networks. "Add" and "drop" here refer to the capability of the device to add one or more new wavelength channels to an existing multi-wavelength WDM signal, and/or to drop (remove) one or more channels, passing those signals to another network path. An OADM may be considered to be a specific type of optical cross-connect.

Interferometric modulator display is a technology used in electronic visual displays that can create various colors via interference of reflected light. The color is selected with an electrically switched light modulator comprising a microscopic cavity that is switched on and off using driver integrated circuits similar to those used to address liquid crystal displays (LCD). An IMOD-based reflective flat panel display includes hundreds of thousands of individual IMOD elements each a microelectromechanical systems (MEMS)-based device.

Fourier-transform infrared spectroscopy

Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high-spectral-resolution data over a wide spectral range. This confers a significant advantage over a dispersive spectrometer, which measures intensity over a narrow range of wavelengths at a time.

An integral field spectrograph, or a spectrograph equipped with an integral field unit (IFU), is an optical instrument combining spectrographic and imaging capabilities, used to obtain spatially resolved spectra in astronomy and other fields of research such as bio-medical science and earth observation.

White light interferometry

As described here, white light interferometry is a non-contact optical method for surface height measurement on 3-D structures with surface profiles varying between tens of nanometers and a few centimeters. It is often used as an alternative name for coherence scanning interferometry in the context of areal surface topography instrumentation that relies on spectrally-broadband, visible-wavelength light.

In physics, a high contrast grating is a single layer near-wavelength grating physical structure where the grating material has a large contrast in index of refraction with its surroundings. The term near-wavelength refers to the grating period, which has a value between one optical wavelength in the grating material and that in its surrounding materials.

Virtually imaged phased array Dispersives, optisches Bauteil

A virtually imaged phased array (VIPA) is an angular dispersive device that, like a prism or a diffraction grating, splits light into its spectral components. It works almost independently of polarization of light. In contrast to prisms or regular diffraction gratings, it has a much higher angular dispersion but has a smaller free spectral range. This aspect is similar to that of an Echelle grating which is usually used in reflection, since high diffraction orders are also used there. The VIPA can be a compact optical component with high wavelength resolving power.

References

  1. D.J. Bishop, C.R. Giles, and G.P. Austin, “The Lucent LambdaRouter: MEMS Technology of the Future Here Today,” IEEE Communications Magazine 40, no. 3 (March 2002): 75 –79
  2. Robert Anderson, “US Patent 6.542,657: Binary Switch for an Optical Wavelength Router”, April 1, 2003.
  3. http://www.intechopen.com/books/advances-in-micro-nano-electromechanical-systems-and-fabrication-technologies/optical-mems-for-telecommunications-some-reliability-issues
  4. http://www.avanex.com/WSS_liquid_crystal.php
  5. Figure courtesy of Finisar Corporation
  6. Image courtesy of Nistica Corporation
  7. Steven James Frisken, “United States Patent: 7397980 - Dual-source Optical Wavelength Processor”, July 8, 2008
  8. P. Evans et al., “LCOS-based WSS with True Integrated Channel Monitor for Signal Quality Monitoring Applications in ROADMs,” in Conference on Optical Fiber communication/National Fiber Optic Engineers Conference, 2008. OFC/NFOEC 2008
  9. A. Gehner et al., “Recent Progress in CMOS Integrated MEMS AO Mirror Developments,” in Adaptive Optics for Industry and Medicine: Proceedings of the Sixth International Workshop, National University of Ireland, Ireland, 12–15 June 2007 (Imperial College Press, 2008), 53–58.
  10. Jonathan Dunayevsky, David Sinefeld, and Dan Marom, “Adaptive Spectral Phase and Amplitude Modulation Employing an Optimized MEMS Spatial Light Modulator,” in Optical Fiber Communication Conference, OSA Technical Digest (presented at the Optical Fiber Communication Conference, Optical Society of America, 2012), OM2J.5.