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Tunable
MEMS Devices For Optical Networks Jill D. Berger and Doug Anthon
Early deployment of tunable lasers and filters in optical networks that use dense wavelength division multiplexing has been driven by the inventory advantages of universal line cards and multiwavelength sparing. But the greatest benefits can be achieved by using tunable devices for dynamic wavelength provisioning in reconfigurable transparent optical networks. A prerequisite for the realization of dynamic optical networks is the availability of tunable devices that meet the rigorous performance requirements of fixed-wavelength transceivers—at comparable manufacturing costs. Widely tunable external-cavity diode lasers and diffraction-grating filters based on silicon microelectromechanical actuators meet these criteria.
Widely tunable external-cavity diode laser, based on a silicon MEMS actuator, for optical network applications.
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TUNABLE MEMS DEVICES
S
ignificant advances in optical technology, particularly the development of optical amplifiers and dense wavelength division multiplexing (DWDM), have transformed the design of metro, regional, long-haul, and undersea telecommunications networks. Over the course of the past decade, the number of channels that can be carried on a single optical fiber has risen from one to more than 100, while data rates per channel have increased from 622 Mbits/s to 10 Gbits/s. Single-channel systems that once required optical-electrical-optical (OEO) regeneration at 40-km intervals have been supplanted by optically amplified DWDM systems with regeneration intervals of up to 3000 km. These advances tend to drive networks away from electronically switched “opaque” architectures, with frequent OEO conversions, and towards transparent, optically switched designs. Rapid technological progress, combined with the readily available capital in the late 1990s, generated an abundance of bandwidth that has yet to be fully employed. Although the marginal cost of providing bandwidth is rapidly falling, carriers are still having difficulty achieving profitability, in part because of the low prices being paid for data services. For carriers seeking to maximize revenue and minimize both capital investments and operating expenses, dynamic provisioning may be one solution. Opaque networks contain a number of costly redundancies that can be eliminated with appropriate network design. For example, it has been estimated that an agile transparent network, one that permits nonterminating channels to pass through nodes in optical form, can eliminate more than half the OEO conversions found in an opaque network.1 End-user bandwidth demand changes dynamically, because of both traffic growth and traffic churn. In fixed-wavelength networks, traffic churn results in stranded, unusable bandwidth. Provisioning time frames of up to several months—without benefit of preprovisioning to meet projected future traffic demands—make it difficult to respond to traffic growth. And when forecasting inaccuracies occur, the result is equipment that is both deployed and operat-
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(a)
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Figure 1. (a) Optical network based on widely tunC (b) able lasers and filters. At node A, signals from an array of tunable transmitters, each composed of a D A B tunable laser and modulator capable of transmitting E any one of 100 channels, are combined by a waveG length-insensitive power combiner and transmitted through an amplified fiber link with transparent Transparent Node F nodes B and C. Signals arriving at B and C may be locally terminated, or passed through to other nodes. Locally added or passed-through signals can be dynamically switched to any other transparent node. At node D, the signals are demultiplexed by a wavelength-insensitive power splitter and detected by an array of tunable receivers, each composed of a tunable filter and photoreceiver. (b) Dynamic traffic routing in an agile network can be accomplished by changing the tunable transmitter/ receiver wavelength or the optical switch configuration. Traffic traveling from A to C is rerouted from A to D by changing the transmitter wavelength at A. Traffic travelling from G to D is rerouted from G to E by changing the optical switch configuration at F.
ing, but not carrying revenue-generating traffic. In agile optical networks, such as the system depicted in Fig. 1, bandwidth can be remotely provisioned in only minutes instead of weeks or months, and subsequently reprovisioned as revenuegenerating traffic demands change. With appropriate network management, frequency contention problems can be minimized and optical routing becomes an efficient tool for dynamically reconfiguring networks.2 Tunable transmitters based on tunable lasers are analogous to fixed-wavelength transmitters. But in a transparent optical network, tunable filters can play several different roles. Single-channel tunable bandpass filters are used in amplified transmitters for the suppression of amplified spontaneous emission (ASE) and in receivers as adaptive prefilters for noise reduction. Tunable filters can reduce the costs of optical performance monitoring by allowing one monitor to select
between multiple channels. More generally, tunable filters can be used in the reconfigurable optical add-drop multiplexers (ROADM) that are the central switching element of transparent optical nodes. A ROADM can be constructed entirely from single channel tunable filters using a broadcast-and-select architecture. In this case, the incoming signal is amplified and split into N independent outputs that are divided into groups of pass-through channels, branched channels and received channels. Although the number of channels of each type is determined by the hardware, the use of tunable filters allows for control of the network configuration by making it possible to modify decisions over which signal is routed where. Because most optical networks are still opaque, with a set of point-to-point links connected through a mesh of electrical switches, full advantage cannot be taken of the benefits offered by tunability.
TUNABLE MEMS DEVICES Nonetheless, tunability offers a solution to the increasingly difficult problem of inventory stock and sparing for DWDM transmitters and receivers used in opaque networks capable of carrying more than 100 wavelengths per fiber. In case of hardware failure, a complete inventory of fixed-wavelength transmitters and receivers must be carried at numerous locations throughout the country so that spares can be deployed into the network. Widely tunable lasers and filters offer a cost-effective solution to this problem by allowing the creation of universal line cards that can be dynamically tuned to operate at any one of 100 wavelengths. Tunable transceivers have to meet or exceed the rigorous telecom performance demands made on fixed-wavelength transceivers—and at comparable costs. Today, DWDM channel spacing, which has already migrated from 100 GHz to 50 GHz, is moving toward 25 GHz. Longhaul systems are now being designed to take advantage of both the C-band (~1530-1565 nm) and the L-band (~1570-1610 nm) with 100 channels in each band. Long-distance, transparent transmission over distances greater than 3000 km requires tight specifications on laser parameters (linewidth, frequency stability and output power) and on filter parameters (pass bandwidth, loss and dispersion). Metro systems are now being designed with 32 wavelengths, a reach of several hundred kilometers, and data rates of 2.5 Gbits/s per channel. Although metro systems use very low-cost directly modulated lasers, they can still benefit from tunable lasers and filters that reduce inventory costs and enable agile optical networking to handle unstable demand patterns.3 Because network reconfigurations are infrequent on a 1-s timescale, acceptable network performance can be achieved by use of lasers and filters with tuning times in the tens of milliseconds range, within the requirements of SONET restoration. These tuning times are difficult for thermally tuned devices, but are achievable using MEMS devices. Telecom components must also meet Telcordia reliability criteria and be capable of operating in a variety of environmental conditions. The widely tunable laser technologies available for telecommunications appli-
cations include: MEMS tunable externalcavity diode lasers (MEMS-ECL)4,5; distributed Bragg reflectors (DBR)6-8; temperature-tuned distributed-feedback lasers (DFB)9,10; and MEMS tunable vertical-cavity surface-emitting lasers (VCSELs).11 The technologies employed for widely tunable filters include MEMS diffraction-grating filters12 and fiberBragg-grating,13 thin-film-dielectric14,15 and fiber-Fabry-Perot filters.16 Each of these technologies has advantages and disadvantages. Yet the fact that multiple vendors are bringing a variety of components to market has enabled the deployment of tunable devices to begin. MEMS-based external-cavity diode lasers and diffraction-grating filters offer optical performance and environmental stability rivaling that of high-performance fixed-wavelength DFB lasers and fiberBragg-grating or thin-film dielectric filters. These devices support full C- or L-band tuning at 25, 50 or 100 GHz channel spacings, with accurate wavelength control over the required environmental conditions. MEMS external-cavity diode lasers offer high power, high side mode suppression, narrow linewidth and low relative intensity noise (RIN); for use in cost-sensitive metro networks, they can also be directly modulated at 2.5 Gbits/s. MEMS diffraction-grating filters offer narrow bandwidths for high channel isolation, as well as low insertion loss, low
polarization dependent loss (PDL) and low dispersion. The manufacturing costs of MEMS-based agile optical components are on par with the scalable cost structures of their fixed-wavelength counterparts, so that direct replacement of fixed-wavelength devices can be justified purely on the basis of reduced inventory and sparing. This means that dynamic optical networking with photonic layer intelligence comes almost for free.
MEMS tunable external-cavity diode lasers The tunable laser shown in Fig. 2 is a Littman/Metcalf external-cavity diode laser17,18 tuned by a silicon MEMS actuator.4,5 The gain medium is based on a high-power, 1.55-m InGaAsP/InP multiple-quantum-well laser diode. Reflections at the intracavity facet of the laser diode are suppressed by use of a low reflectance (< 2 10-3) antireflective coating in combination with an angled facet; this makes possible an effective facet reflectance of less than 10-4. The low intracavity facet reflectance enabled by an angled facet is highly stable over the lifetime of the device. The laser output beam is collimated at the opposite diode facet, which serves as the output coupler of the resonator. Light emerging from the intracavity diode facet is collimated by a diffraction-limited, micro-optical lens
Glossary Agile optical network—An optical network in which data traffic can be dynamically routed by changing either the tunable transmitter or receiver wavelength or the optical switch configuration. Dynamic wavelength provisioning/routing—Remote provisioning or routing of traffic by changing the tunable transmitter or receiver wavelength in an agile optical network. Multiwavelength sparing—Use of a single tunable transceiver as a spare to replace many different fixed-wavelength transceivers in case of hardware failure. Opaque network—Optical links connected through a mesh of electrical switches, where the optical signals are detected, converted to electrical, and retransmitted as optical (OEO). Provisioning—Deployment/allocation of bandwidth into an area of the network. Traffic churn—Changing bandwidth demand patterns in a telecom network. Transparent network—Optical links connected through optically transparent nodes without OEO conversion. Universal line card—Hardware with transceivers that can be remotely controlled to operate at any one of many optical telecommunications wavelengths.
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TUNABLE MEMS DEVICES and then diffracted at grazing inciof a new class of devices for Lenses telecommunications systems.21 dence from a high-efficiency, freePM Fiber Many of the large arrays of optical space diffraction grating. The first Diffraction Pigtail Grating Laser switches have been built by use of diffracted order travels to an external Wavelength Diode Locker surface micromachining, the techmirror mounted out-of-plane on a Silicon nique whereby sensors and actuarotary silicon microactuator. Mirror MEMS tors are made from patterning thin Wavelength tuning is achieved by Actuator films, such as polysilicon, on the applying a voltage to the microactuShutter/ surface of a silicon substrate and ator, which rotates the mirror to Beamsplitter VOA sacrificially etching another mateallow a specific diffracted wavelength Isolator rial, such as silicon dioxide, to free to couple back into the laser diode. the mechanical structure. Most The actual wavelength of the laser accelerometers for deploying air output is determined by the gain bags are now made in this way. Figure 2. Schematic drawing of the packaged MEMSbandwidth of the diode, the grating Actuators made by use of this techECL-WLL, showing a scanning electron microscope dispersion and the external-cavity (SEM) image of the MEMS-ECL on the left and the isonique are poorly suited for translatmode structure. The laser-diode gain lator, WLL, shutter/VOA and fiber coupling on the right. ing or rotating relatively large, bandwidth is greater than 80 nm, externally fabricated elements— while the external-cavity mode spacsuch as mirrors or lenses—because ing is only 0.16 nm. For this reason, a their actuator force is low. For these purlarge number of external-cavity modes hops by allowing the cavity phase to poses, higher force actuators made by are supported by the diode-gain medium, change by several wavelengths across the deep-reactive-ion etching (DRIE) of a although the narrow spectral passband of full tuning range. In this case, adding an silicon substrate are preferred.19,20 the diffraction-grating filter introduces independent cavity-length control actuaAlthough thermal actuation and magmore than 1 dB loss at the adjacent side tor can optimize performance at any netic actuation are used for some devices, modes. This spectral filter works in comchannel by adjusting the laser frequency electrostatic actuation is still preferred. bination with gain-saturation mechawith respect to the grating-filter passDevices based on electrostatic actuators nisms to suppress the adjacent side modes band. The actuator voltage determines the can be accurately positioned and easily and preferentially select a single externallaser frequency with an open-loop accudriven with up to 150 V at very low cavity mode. Design criteria, including a racy of approximately 10 GHz, and the current. spectrally narrow grating-filter passband, frequency is then stabilized to ± 1.25 GHz The actuator fabrication process is rela high external-cavity feedback level, low by use of the error signal from a waveatively simple, requiring only five masks intracavity diode-facet reflectance and a length locker (WLL) etalon in a servo that (see Fig. 3). First, alignment marks are well-confined single-spatial-mode diode adjusts the mirror position. formed on the front and back surface of waveguide are all key to ensuring stable, The collimated output beam from the an oxidized carrier wafer. Shallow cavities single-mode laser operation.18 ECL, with optical power of up to 70 mW, are then plasma etched into the silicon on The laser is tuned by applying voltage passes through an isolator and is coupled the front surface. These cavities will to the comb elements of the MEMS actuinto a polarization-maintaining (PM) define what portions of the actuators will ator to produce an electrostatic force that fiber pigtail with 65% coupling efficiency. be free to move and which will be rotates the mirror about its virtual A beamsplitter reflects 5% of the light attached to the carrier. A second wafer is pivot.19,20 The angular range of the actuainto an integrated wavelength locker. The fusion bonded to this front surface and tor determines the tuning range; a variety MEMS shutter is used to block the output then ground and polished to the desired of 150-V actuators, with ranges of up to for dark tuning; with suitable external actuator device thickness, in this case ± 2.8 degrees, have been used to tune over feedback, it can also be used as a voltage85 m. The alignment marks from the a wavelength range of up to 42 nm. The controlled attenuator (VOA). The bottom surface are then transferred to mirror’s geometrical pivot can be MEMS-ECL-WLL is assembled on a the new top surface to allow alignment designed to adjust the cavity length ceramic substrate, bonded to a thermowith the now enclosed cavities in the because the laser tunes in a way that electric cooler (TEC), and packaged, as device. After oxidation, contact-hole etch, maintains constant cavity phase at all shown in Fig. 2, in a hermetic, 18-pin butand metallization deposition and patwavelengths across the tuning range. terfly package. The result is a compact, terning, the DRIE etch is performed The laser wavelength determined by the environmentally robust, high-perforthrough the thickness of the top wafer diffraction angle then scans synmance tunable laser that is suitable for until it intersects the cavity. Those porchronously with the grating-filter passnetwork applications. tions of the device that are above the cavband, and the laser tunes continuously ities are then suspended, typically by MEMS design and fabrication without mode hops. The pivot point can narrow flexural elements, to parts of the also be designed to provide for a limited device still fusion bonded to the carrier The availability of high-performance continuous tuning range between mode wafer. Electrical connections to the movMEMS actuators has led to the creation 46 Optics & Photonics News
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TUNABLE MEMS DEVICES ing elements are made through the conductive silicon flexures. Electrical insulation is provided by a combination of the oxide layer between the moving or fixed parts of the device and the carrier wafer, and by etched trenches that can surround device features.
Tunable laser assembly
(a)
peak-to-peak intensity modulation, for example, produces a linewidth of 200 MHz. The linewidth can also be reduced to near the instantaneous value by use of a current servo to lock the laser to an external etalon. The MEMS-ECL can be directly modulated with low chirp at data rates of up to 2.7 Gbits/s by use of an appropriate low-capacitance laser diode. Direct modulation at up to 2.7 Gbits/s provides a cost-effective tunable transmitter solution for metro applications. The laser can be tuned to another channel and locked within 15 ms. Figure 5 shows a sideby-side comparison of the MEMSECL and a high-performance, fixed-wavelength DFB in an externally modulated, 40-Gbits/s, 500-km amplified link. The MEMS-ECL and DFB exhibit nearly identical biterror ratio (BER) performance. Similar results have been obtained at 2.5 and 10 Gbits/s, and with amplified link lengths up to 3200 km.
The network tunable laser modules Etch shallow cavity in carrier wafer to create available today all use the same funfuture movable areas. damental package structure, comFusion bond device bining a tunable laser, free-space wafer to carrier, polish to isolator and wavelength locker on a final thickness. TEC in a butterfly package with a Oxidize; open contact PM fiber pigtail and drive electronholes; deposit and patics. Some tunable laser modules also tern pad metal. incorporate a semiconductor optical DRIE etch through amplifier (SOA) to reach 20-mW (b) device wafer. output power.6 Tunable laser transFigure 3. (a) Scanning electron microscope (SEM) mitters combine a tunable laser image and (b) cross-sectional view of the fabrication module with an external Machprocess for the DRIE-MEMS actuators used to tune Zhender or an integrated electroabthe lasers and filters. sorption (EA) modulator for 10-Gbits/s data rates, and use either an EA modulator or direct modulation of lasers currently being used in metro and Environmental the laser diode current for metro applicalong-haul systems. The MEMS-ECL performance and reliability tions at 2.5 Gbits/s. tunes over 42 nm in the C or L band, The MEMS-ECL takes advantage of with fiber-coupled output powers of up Thermal and vibrational sensitivities are dramatically reduced cost and complexity to 40 mW. Figure 4 shows the laser power important issues in a telecom tunable at the laser-diode level compared to other variation and frequency deviation for a laser. In the MEMS-ECL, they are tunable laser technologies. The MEMSlaser locked sequentially to 100 L-band addressed by a combination of thermoECL is based on a simple, high-power channels spaced by 50 GHz. Similar mechanical design and servo control. Fabry-Perot laser diode that is readily performance is obtained with channel Mounting the ECL and WLL components available from a variety of vendors. The spacings of 25, 50 or 100 GHz. The on a TEC eliminates the majority of therother cornerstone of the MEMS-ECL is MEMS-ECL has a side-mode suppression mal issues. Figure 4 shows how TEC therthe silicon MEMS actuator. With only ratio (SMSR) of more than 55 dB, a RIN mal control of the ECL and WLL, in five mask levels, standard silicon processof less than -155 dB/Hz from 10 MHz to combination with active servo control of ing, hundreds of devices per wafer and 22 GHz, a polarization extinction ratio the MEMS mirror position and cavity high yield, these actuators can be fabribetter than 20 dB and a spontaneous phase, stabilizes channel-locking perforcated at low cost. For this reason, the emission background of less than mance. The laser power stability (± 0.1 challenge of MEMS-ECL fabrication -50 dBc/nm. Due to its longer cavity dB) and frequency stability (± 0.5 GHz) resides not at the chip level but at the length, the MEMS-ECL has a narrower are shown as the laser is randomly locked assembly level, where high-level precision linewidth than a DFB, with a 3-dB 1500 times between 100 channels while automation is essential. The laser asseminstantaneous linewidth of 125 kHz the case temperature varies from -10 to bly process relies on automated robotic inferred from phase-noise spectral den70° C. Accelerated aging tests at 70° C vision systems and optical alignment syssity measurements. Submegahertz 1/f case temperature also show excellent tems for accurate component placement and actuator-motion-induced phase power and frequency stability while the and attachment, resulting in high noise produces a time-averaged effective laser is locked to a single channel for over throughput and yield, which translates linewidth of 2 MHz, measured by a 25-s 2000 hours. Coupling to external shock into low manufacturing costs. delayed homodyne measurement. As and vibration is minimized using a zerowith a DFB, this linewidth can be broadmoment balanced MEMS actuator. The Tunable laser performance ened for suppression of stimulated use of active servos to position the tuning Brillouin scattering (SBS) by adding a mirror allows the device to operate under The optical performance of the MEMSsinusoidal dither to the laser current. A rigorous shock and vibration conditions ECL matches or exceeds that of the fixed200-kHz current dither, giving a 3% far exceeding the combined European wavelength distributed feedback (DFB) March 2003
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TUNABLE MEMS DEVICES
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Figure 4. Laser power (upper curves a-c) and frequency (lower curves d-f) of the MEMS-ECL are stabilized by a combination of thermomechanical design and servo control. Power deviation of less than +/- 0.1 dB and frequency deviation of less than +/- 0.5 GHz are shown in (a,d) versus laser frequency as the laser is tuned and locked sequentially across 100 channels; in (b,e) versus case temperature from -10 to 70° C as the laser performs 1500 random channel switches; and in (c,f) over a 2000-h accelerated aging test at 70° C case temperature with the laser locked to a single channel.
Telecommunications Standards Institute (ETSI) and Telcordia specifications for earthquake, office and equipment rack environments, with a maximum frequency deviation of less than 2.5 GHz.5 Reliability of the MEMS-ECL has been proven through over 150,000 device hours in Telcordia testing.
MEMS tunable diffraction-grating filter The tunable MEMS-diffraction grating filter shown in Fig. 6 is analogous to the MEMS-ECL tunable laser, but without the gain medium. The tunable filter is a miniature monochromator based on a free-space Littrow diffraction grating, a beam expander and a silicon MEMS actuator. Optical devices using MEMS actuators work best with relatively lowmass moving parts, such as silicon mirrors, to optimize speed and range of motion. While traditional Littrow monochromators tune by grating rotation, the MEMS-grating filter—as in the case of the tunable laser—tunes by reflecting light from a rotating silicon micromirror to change the beam incidence angle at the grating. The optics are designed to maintain a constant filter bandwidth as the mirror rotates across the tuning range. High-efficiency diffraction gratings, optimized for a single 48 Optics & Photonics News
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polarization state, have inherently high polarization-dependent loss (PDL). The insertion loss and PDL of the MEMSgrating filter are minimized using a copackaged micro-optical circulator to impart light of a single polarization state on the grating. In the tunable filter schematic of Fig. 6, the collimated beam from a singlemode input fiber passes through a polarization recovery module (PRM), reflects from a mirror mounted on a MEMS actuator and diffracts from a 95% efficiency, Littrow-mounted diffraction grating. The PRM is a micro-optical circulator consisting of a polarizing beamsplitter, Faraday rotator and two half-wave plates. The PRM minimizes polarization-dependent loss (PDL) and polarization-mode dispersion (PMD) in the filter by converting the randomly polarized input light into p-polarized light incident at the grating, and then circulating the return light to the output fiber. The Gaussian output fiber mode is the wavelength-selective aperture, resulting in a Gaussian filter passband set by coupling loss versus lateral beam shift at the output fiber. The filter center wavelength is selected by the diffracted wavelength that returns to the center of the output fiber. The filter bandwidth is governed by the grating period and the pro-
Figure 5. BER versus received optical power and eye diagram for single-channel transmission at 40 Gbits/s in an amplified 500-km link. The MEMS-ECL is compared to a fixed-wavelength DFB. [Courtesy Lucent Technologies].
jected beam size, as determined by the grating incident angle and the prism magnification. Different bandwidths are obtained by changing prisms, and the angular dependence of the prism magnification is such that the beam size remains constant at the grating even as the incident angle changes with tuning. A constant filter bandwidth is maintained across the entire tuning range. The filter center wavelength is tuned over 40 nm by applying ± 140 V to the comb elements of the MEMS actuator to produce an electrostatic force that rotates the mirror ± 1.8 degrees about its center pivot.22 Similar to the tunable laser, the microoptical filter assembly and MEMS actuator are assembled on a ceramic substrate and packaged in a hermetic, 18-pin butterfly package with dual single-mode fiber pigtails. The tunable filter package design can be extended to create a tunable receiver module in which the output beam is coupled into an integrated spatial filter and detector, mounted on the same ceramic platform.
Tunable filter performance The MEMS-grating filter offers several performance advantages compared to other widely tunable filter technologies. The filter supports full C or L band tuning and narrow bandwidths for high
Output Fiber
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Figure 6. (a) Center wavelength of diffractiongrating filter selected by mirror angle, which determines the wavelength of diffracted light coupled to output fiber. (b) Micro-optics and silicon MEMS actuator are assembled on a common substrate mounted in a hermetically sealed butterfly package.
channel isolation at 25, 50, or 100 GHz channel spacing with a wide range of bandwidths achievable using the same optical components. The filter has low insertion loss and PDL, low dispersion (enabling use at 10-Gbits/s data rates) and accurate wavelength control. Figure 7(a) shows the Gaussian filter passband at a single C-band channel, with 32 GHz -3 dB bandwidth, 76 GHz -20 dB bandwidth and 1.3-dB insertion loss. The measured 50 GHz adjacent channel isolation is 30 dB, and the nonadjacent channel isolation is greater than 40 dB. Figure 7(b) shows the PDL of less than 0.2 dB within ± 20 GHz of the filter center frequency. Dispersion in the grating filter is governed by the free-space optical path difference between input polarization states (PMD) and wavelengths (CD). The grating tunable filter has PMD of less than 0.2 ps, chromatic dispersion (CD) of less than ± 20 ps/nm, and chromatic dispersion ripple of less than 10 ps/nm on all channels across the 40-nm tuning range. Figure 7(c) shows the filter tuning over 100 C-band channels spaced by 50 GHz. Figure 7(d) shows a constant 32 GHz -3dB filter bandwidth across the 40-nm tuning range. With appropriate selection of the prism beam expansion, -3 dB filter bandwidths ranging from 16
(a) 31.6 GHz (-3 dB BW)
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TUNABLE MEMS DEVICES
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Figure 7. (a) MEMS-diffraction-grating filter passband centered at 193.8 THz, with 32 GHz -3 dB bandwidth, 76 GHz -20 dB bandwidth and 1.3 dB insertion loss. A Gaussian fit to the data is indicated by the dashed line. (b) PDL of less than 0.2 dB within ± 20 GHz of the filter center frequency. (c) Tuning the filter to 100, 50 GHz ITU channels in the C band. (d) The 32-GHz, -3-dB-filter bandwidth is constant across the 40-nm tuning range.
to 64 GHz can be achieved to support 25, 50 and 100 GHz channel spacings using the same components. The Gaussian filter bandwidth can be similarly optimized to enable optically matched filtering for enhanced optical signal-to-noise ratio (OSNR)23 under various modulation formats and data rates, provided the filter center wavelength is accurately controlled. The actuator voltage determines the filter center wavelength, which can be stabilized to within ± 5 GHz by use of the error signal from a position sensor in a servo that adjusts mirror angle. High accuracy wavelength tracking, to within ± 1.25 GHz of the incoming signal wavelength, is possible by dithering the filter mirror position to maximize transmitted output power. Adaptive wavelength tracking to the incoming signal, in combination with a matched Gaussian filter passband as narrow as 16 GHz, enables improved system performance.23 The filter can be tuned to another channel and wavelength-stabilized in less than 15 ms.
As tunable devices approach cost parity with fixed-wavelength devices, the economic benefits of reduced inventory and sparing alone are sufficient to justify the replacement of fixed components with tunables in metro and long-haul networks. This conversion, which started with deployment of narrowly tunable DFBs, is expanding to widely tunable devices that enable universal line cards capable of dynamically tuning to any one of 100 channels. These are some of the first steps towards realizing all-optical networks that promise to increase carrier revenues and lower operating expenses by streamlining traffic patterns and allowing bandwidth to be dynamically reprovisioned as traffic patterns change.
Conclusion
References
Widely tunable ECLs and diffractiongrating filters based on silicon MEMS actuators offer the performance required in DWDM networks at costs comparable to equivalent fixed-wavelength modules.
Please see OPN Feature Article References, page 62.
Acknowledgments The authors gratefully acknowledge the Iolon development team, John D. Grade, Hal Jerman and Kevin Y. Yasumura, for the MEMS material, John McNulty for the reliability data, and Lucent Technologies for the BER test data.
Jill D. Berger (
[email protected]) and Doug Anthon (
[email protected]) are with Iolon, Inc., in San Jose, California.
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REFERENCES
Feature Article References Note: References in OPN feature articles are published exactly as submitted by the authors.
Novel Fiber Lasers and Applications
38
L.A. Zenteno and D.T. Walton
Tunable MEMS Devices for Optical Networks
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Jill D. Berger and Doug Anthon
1. L.A. Zenteno, J.D. Minelly, A. Liu, A.J.G. Ellison, S.G. Crigler, D.T. Walton, D.V. Kuksenkov and M.J. Dejneka, “1W single-transverse-mode Ybdoped double-clad fibre laser at 978 nm,” Electron. Lett. 37 (13) 819-20 (2001). 2. A. Liu, J.A. Garcia, M.D. Skeldon, W. Chen and L.A. Zenteno, “High power fiber laser prototype packaging and characterization,” unpublished report. 3. K. Oh, P.S. Westbrook, R.M. Atkins, P. Reyes, R.S. Windeler, W.A. Reed, T.E. Stockert, D. Brownlow and D. DiGiovanni, “UV photosensitive response in an antimony-doped optical fiber,” Opt. Lett. 27 (7) 488-90 (2002). 4. J.K. Sahu, C.C. Renaud, K. Furusawa, R. Selvas, J.A. Alvarez-Cordero, D.J. Richardson and J. Nilson; “Jacketed air-clad cladding-pumped Yb-doped fibre laser with wide tuning range,” Electron. Lett. 37 (18) 1116-7 (2001). 5. A. Liu, C.M. Truesdale, L.A. Zenteno, N. Venkataraman, M.T. Gallagher, G.E. Burke and W. Chen, “Holey double-clad fiber lasers,” unpublished report. 6. M.D. Mermelstein, C. Headley, J.C. Bouteiller, P. Steinvurzel, C. Horn, K. Feder and B.J. Eggleton, “Configurable three-wavelength Raman fiber laser for Raman amplification and dynamic gain flattening,” IEEE Photon. Technol. Lett. 13 (13) 1286-8 (2001). 7. D. Kuksenkov and K. Hoover, unpublished work
1. R. Nadon, “Wavelength provisioning with an agile photonic network control plane,” in National Fiber Optic Engineers Conference, Technical Proceedings, 2002, pp. 1377-1386. 2. S. Meyer, “Quantification of wavelength contention in photonic networks with reach variation” in Optical Fiber Communication Conference, OSA Technical Digest, (Optical Society of America, Washington, DC, 2002), paper TuG3. 3. J. Berthold, “WDM in the Metro” in European Conference on Optical Communication, Technical Digest, 2002, Tutorial Session 7. 4. D. Anthon, J. D. Berger, J. Drake, S. Dutta, A. Fennema, J. D. Grade, S. Hrinya, F. Ilkov, J. H. Jerman, D. King, H. Lee, A. Tselikov, and K. Yasumura, “External cavity diode lasers tuned with silicon MEMS” in Optical Fiber Communication Conference, OSA Technical Digest, (Optical Society of America, Washington, DC, 2002), paper TuO7. 5. D. Anthon, J.D. Berger, K.Cheung, A. Fennema, S. Hrinya, H. Lee, and A. Tselikov, “Frequency and mode control of tunable external cavity semiconductor lasers,” in Optical Fiber Communication Conference, OSA Technical Digest, (Optical Society of America, Washington, DC, 2003), paper MF61. 6. Y.A. Akulova, C. Schow, A. Karim, S. Nakagawa, P. Kozodoy, G.A. Fish, J. DeFranco, A. Dahl, M. Larson, T. Wipiejewski, D. Pavinski, T. Butrie, and L.A. Coldren, “Widely-Tunable Electroabsorption-Modulated Sampled Grating DBR Laser Integrated with Semiconductor Optical Amplifier,” in Optical Fiber Communication Conference, OSA Technical Digest, (Optical Society of America, Washington, DC, 2002), paper ThV1. 7. D. A. Ackerman, J. Johnson, L. Ketelsen, J.M. Geary, W.A. Asous, F.S. Walters, J.M. Freund, M. Hybertsen, K.G. Glogovsky, C. Lentz, C.L. Reynolds, R. Bylsma, E.J. Dean, and T. Koch, “Fully Functional Wavelength Selectable DWDM Transmitter,” in IEEE/LEOS Summer Topical Meeting on Advanced Semiconductor Lasers and Applications, Technical Digest, 2001, paper TuA2.3.
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8. G. Busico, N.D. Whitbread, P.J. Williams, D.J. Robbins, A.J. Ward, and D.C.J. Reid, “A widely tunable digital supermode DBR laser with high SMSR,” in European Conference on Optical Communication, Technical Digest, 2002, paper 3.3.2. 9. Y. Kotaki and K. Morito, “Wavelength Tunable DFB Laser Array for WDM Applications,” in European Conference on Optical Communication, Technical Digest, 2002, paper 3.3.1. 10. B. Pezeshki, E. Vail, J. Kubicky, G. Yoffe, S. Zou, P. Epp, S. Rishton, D. Ton, B. Faraji, M. Emanuel, R. King, X. Hong, M. Sherback, V. Agrawal, and T. Razazan, “12 element multiwavelength DFB arrays for widely tunable laser modules,” in Optical Fiber Communication
Conference, OSA Technical Digest, (Optical Society of America, Washington, DC, 2002), paper ThGG71. 11. P. Kner, D. Sun, J. Boucart, P. Floyd, R. Nabiev, D. Davis, W. Yuen, M. Jansen, and C.J. ChangHasnain, “VCSELs go the distance,” Optics and Photonics News, March 2002, pp. 44-47. 12. J.D. Berger, F. Ilkov, D. King, A. Tselikov, and D. Anthon, “Widely tunable, narrow optical bandpass Gaussian filter using a silicon microactuator,” in Optical Fiber Communication Conference, OSA Technical Digest, (Optical Society of America, Washington, DC, 2003), paper TuN2. 13. M.R. Fernald, T.J. Bailey, M.B. Miller, J.M. Sullivan, M.A. Davis, R.N. Brucato, M.A. Putnam, A.D. Kersey, and P.E. Sanders, “Compression-tuned Bragg grating and laser,” U.S. Patent No. US6363089, March 26, 2002. 14. JDS Uniphase product data sheet, “VoltageControlled Optical Filters, VCF050/100 series” from www.jdsu.com. 15. Santec product data sheet, “Optical Tunable Filter OTF-655,” from www.santec.com. 16. C.M. Miller and J.W. Miller, “Temperature compensated fiber Fabry-Perot filters,” U.S. Patent No. US5289552, February 22, 1994. 17. M. Littman and H. Metcalf, “Spectrally narrow pulsed dye laser without beam expander,” Appl. Opt. 17, 2224-2227 (1978). 18. P. Zorabedian, “Tunable External Cavity Semiconductor Lasers,” in “Tunable Laser Handbook, “ (F.J. Duarte Ed.), Academic Press, San Diego, 1995. 19. J.H. Jerman and J.D. Grade, “Balanced Microdevice and Rotary Electrostatic Microactuator for use therewith”, Int. Pub. No. WO 01/43268, June 2001. 20. H. Jerman and J.D. Grade, “A Mechanically balanced, DRIE rotary actuator for a highpower tunable laser,” Technical Digest 2002 Solid State Sensor and Actuator Workshop, Hilton Head, SC, June 2002, pp. 7-10. 21. E.L. Goldstein, L.Y. Lin, and J.A. Walker, “Lightwave Micromachines for Optical Networks,” Optics and Photonics News, March 2001, pp. 60 -65. 22. J.D. Grade and H. Jerman, “MEMS Electrostatic Actuators for Optical Switching Applications,” in Optical Fiber Communication Conference, OSA Technical Digest, (Optical Society of America, Washington, DC, 2001), paper WX2. 23. D.O. Caplan and W.A. Atia, “A quantum-limited optically-matched communication link,” in Optical Fiber Communication Conference, OSA Technical Digest, (Optical Society of America, Washington, DC, 2001), paper MM2.
