1440
`
`IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 10, OCTOBER 1998
`
`Dynamic Spectral Power Equalization
`Using Micro-Opto-Mechanics
`
`Joseph E. Ford and James A. Walker
`
`Abstract—We present a voltage-controlled spectral attenuator
`for gain shaping and power equalization in wavelength division
`multiplexed single-mode fiber systems. A micro-opto-mechanical
`modulator array, where electrostatic deflection of a silicon nitride
`quarter-wave dielectric layer suspended over a silicon substrate
`creates a column of variable reflectivity mirrors, is packaged
`using bulk optics and a diffraction grating to disperse the input
`spectrum across the device and collect the reflected light into a
`separate output fiber. The packaged component has 9-dB excess
`loss, 20-dB dynamic range and 10-s response. We demonstrate
`equalization of the amplified spontaneous emission spectrum from
`an erbium-doped fiber amplifier and of individual laser signals
`with 10-dB initial variation to less than 0.5-dB variation over a
`24-nm passband-free spectrum.
`Index Terms—Attenuators, gain control, level control, micro-
`electromechanical devices, optical components, optical equalizers,
`wavelength-divsion multiplexing.
`
`Fig. 1. MARS micromechanical modulator concept.
`
`NONUNIFORM signal
`
`I. INTRODUCTION
`intensity levels in wavelength-
`division-multiplexed (WDM) communication systems
`lead to transmission errors. Power equalization can be done
`by signal pre-emphasis and fixed fiber gratings. However,
`transmission link properties can change over time, and the
`network operator may not have precise, dynamic control
`over source powers. A passive optical component providing
`dynamic control over wavelength power levels could correct
`changing signal levels with a simple, local control algorithm
`and provide maximum flexibility to the network operator.
`The mechanical antireflection switch (MARS) microme-
`chanical modulator was originally developed for digital data
`transmission [1], and later used as a high-speed analog variable
`attenuator [2], [3]. The structure, shown in Fig. 1, is basically a
`quarter-wave dielectric antireflection coating suspended above
`a silicon substrate. A silicon nitride layer with
`optical
`thickness, separated from the silicon substrate by a fixed
`spacer, acts as a dielectric mirror with about 70% reflectivity.
`Voltage applied to electrodes on top of the membrane creates
`an electrostatic force and pulls the membrane closer to the
`substrate, while membrane tension provides a linear restoring
`force. When the membrane gap is reduced to /2, the layer
`becomes a antireflection coating with close to zero reflec-
`tivity. The 0.4- m vertical deflection is small compared to
`the 200–500- m-wide membrane. The mechanical resonance
`frequency of such devices is on the order of megahertz. Their
`Manuscript received May 1, 1998; revised June 17, 1998.
`The authors are with Bell Laboratories, Lucent Technologies, Holmdel, NJ
`07733 USA.
`Publisher Item Identifier S 1041-1135(98)07139-0.
`
`s, depending on
`optical response ranges from 0.1 to 10
`the surface geometry and material parameters. Mechanically,
`the device moves by elastic deformation, similar to a tuning
`fork. Electrically, the device behaves as a tiny capacitor, with
`zero-static power dissipation regardless of reflectivity state.
`Starting from the MARS modulator, we can create a WDM
`equalizer by simply fabricating a column of analog modulators
`and placing them into a fiber-coupled spectrometer that has
`been modified to collect the reflected and attenuated signals
`into a separate output fiber [4]. Fig. 2 shows the free-space
`optical system and the resulting optomechanical package.
`Light from the input fiber is collimated, diffracts from a gold-
`coated 600-lines/mm blazed grating, then focused by a 50-mm
`lens onto the device. The focus lens is vertically displaced so
`that the reflected light is spatially separated from the input.
`The reflected and attenuated signal is collected by a second
`pass through the grating, reflects from a small fold mirror,
`and focused into a separate output fiber. The grating insertion
`loss depends on polarization, and can vary from about 0.6 to
`1.1 dB at 1.5 m. We avoid polarization-dependent loss (PDL)
`by placing a quarter-wave plate in the collimated beam path to
`rotate the polarization of the reflected light for the second pass
`through the grating. Using a gold mirror at the device plane
`the total fiber-to-fiber insertion loss of the WDM package is
`4.6 dB, with 0.2-dB PDL.
`There are two approaches to making the equalizer device.
`A segmented array of mechanically and electrically discrete
`modulators offers arbitrary attenuation on each predefined
`wavelength channel. However, a mechanically continuous
`modulator surface actuated by individual electrodes offers a
`continuous operating wavelength spectrum with no passbands.
`The choice depends on system application. The segmented
`device can equalize amplifier inputs with widely varying
`levels as, for example, might be the case in a network with
`active WDM add–drop. The continuous device can equalize a
`transmission link despite changes in amplifier gain profiles
`and transmission loss, without restricting the network to a
`particular set of operating wavelengths.
`1041–1135/98$10.00 © 1998 IEEE
`
`Exhibit 1041, Page 1
`
`

`
`FORD AND WALKER: DYNAMIC SPECTRAL POWER EQUALIZATION USING MICROOPTOMECHANICS
`
`1441
`
`(a)
`
`Fig. 4. Dynamic attenuation response to 60-V square-wave drive.
`
`(b)
`(a) Optical design. (b) Optomechanical WDM package.
`
`Fig. 2.
`
`Fig. 5. Equalized erbium-doped fiber amplifier ASE.
`
`voltage is applied. This increases mechanical coupling between
`channels, but creates the best possible optical surface. The op-
`tically active region was a strip 40 m wide by approximately
`1 mm long, with 32 (connected) electrodes spaced at a 28- m
`pitch (corresponding to 0.8 nm in wavelength).
`
`II. EXPERIMENTAL RESULTS
`The total excess loss of the FC-connectorized equalizer was
`less than 9 dB with 0.1-dB ripple over an approximately 28-nm
`usable bandwidth. The center wavelength could be adjusted
`to lie anywhere within the 1.5- m band. Applying 0–60 V
`to any one electrode created a 3-nm-wide feature with an
`approximately Gaussian profile (on a logarithmic scale) and up
`to 22-dB additional attenuation. Intermediate regions between
`two nearby features were somewhat attenuated. For example,
`the midpoint between two 15-dB depressions separated by
`7 nm had 5-dB loss. In other words, the equalizer could
`create wavelength transmission functions that vary smoothly
`
`Exhibit 1041, Page 2
`
`Fig. 3. MARS variable-reflectivity strip mirror.
`
`In this letter, we demonstrate the passband-free WDM
`equalizer. Fig. 3 shows the device with a unbroken modulator
`strip with electrodes applying force at discrete steps along
`a continuous narrow membrane. The membrane deflects like
`a rubber sheet, with smooth, shallow depressions (of up to
`0.4 m across a 290- m suspended width) created wherever
`
`

`
`1442
`
`IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 10, OCTOBER 1998
`
`(a)
`
`(b)
`(a) Nonuniform laser input. (b) Flattened and amplified output.
`
`Fig. 6.
`
`compared to the 3-nm minimum feature made by a single
`charged electrode.
`Fig. 4 shows the equalizer time dynamics at one arbitrary
`wavelength. The MARS modulator has less than 10- s re-
`sponse to a voltage step, but displays a slow relaxation effect
`from static charge buildup on the dielectric membrane. This
`mechanism was also responsible for a slow (hours) drift in
`attenuation as a screening charge develops on the membrane.
`Closed-loop feedback can achieve a fast response and maintain
`a specified attenuation setting (with a 1–2 dB penalty in
`dynamic range). When the drive voltage used in Fig. 5 was
`increased, the response time to full (20-dB) attenuation was
`less than 10
`s.
`For the following experiments, the equalizer’s electronic
`control was simply a row of 32 manually-adjusted trim-
`potentiometers acting as voltage dividers for a common volt-
`age supply provided by seven 9-V batteries in series.
`Fig. 5 shows how the equalizer can flatten nonuniform
`amplifier gain. The upper curve is the original amplified
`spontaneous emission (ASE) output of an erbium-doped fiber
`amplifier with >12-dB dynamic range. The middle curve shows
`this signal transmitted through the packaged equalizer (with
`zero applied voltage). With an appropriate voltage setting,
`the ASE signal (in effect, the amplifier gain) was flattened to
`
`0.5 dB over a 23.5-nm bandwidth. Ripples in loss are due to
`scatter off the gold electrodes and small surface features in the
`membrane. Revising the equalizer design to widen the optical
`window and minimize stress concentration will significantly
`reduce the ripple.
`The continuous-strip equalizer was also used to demonstrate
`flattening of multiple laser inputs. Fig. 6(a) shows eight laser
`signals at 200-GHz pitch (transmitted though the switched-
`off equalizer) with signal levels that vary by an order of
`magnitude. When the equalizer is suitably adjusted (Fig. 6,
`bottom) the transmitted signal levels were equalized to
`0.4-
`dB variation. The smooth variation in the additional loss incurs
`an additional 8-dB loss below the original minimum power.
`However, the original signal level can be recovered using an
`amplifier. Fig. 6 also shows the output after amplification,
`where in fact the attenuation control was readjusted to compen-
`sate for amplifier nonuniformity, yielding 0.65-dB total level
`variation.
`
`III. CONCLUSION
`This letter reports the first demonstration of a micro-opto-
`mechanical WDM equalizer suitable for telecommunications
`networks. The packaged device has less than 9-dB excess
`loss, 20-dB dynamic range and 10- s response over a 24-nm
`passband-free optical spectrum. Such a component, used in
`conjunction with an amplifier and with active feedback from
`a WDM detector, can form a self-contained system capable of
`dynamic correction of signal levels in a WDM transmission
`system.
`
`ACKNOWLEDGMENT
`The authors thank Lucent colleagues K. Goossen for MARS
`modulator design, M. Nuss for originally suggesting the equal-
`izer application, C.-C. Chang for his lab expertise, C. Doerr
`for the multifrequency laser, and B. Ellard and F. Beisser for
`custom machining and electronics.
`
`REFERENCES
`
`[1] K. W. Goossen, J. A. Walker, and S. C. Arney, “Silicon modulator based
`on mechanically-active anti-reflection layer with 1 Mbit/s capability for
`fiber in the loop applications,” IEEE Photon. Technol. Lett., vol. 6, pp.
`1119–1121, Sept. 1994.
`[2] J. E. Ford, J. A. Walker, K. W. Goossen, and D. S. Greywall, “Mi-
`cromechanical fiber-optic attenuator with 3 microsecond response,” J.
`Lightwave Technol., submitted for publication.
`[3] J. E. Ford, J. A. Walker, and K. W. Goossen, “Fiber-coupled variable
`attenuator using a MARS modulator,” SPIE Proc., vol. 3226, pp. 86–93,
`1997.
`[4] J. E. Ford, J. A. Walker, M. C. Nuss, and D. A. B. Miller, “32 channel
`WDM graphic equalizer,” in Tech. Dig. IEEE/LEOS 1996 Summer
`Topical Meet. Broadband Optical Networks, Keystone, CO, Aug. 1996,
`pp. 26–27.
`
`Exhibit 1041, Page 3