IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 9, SEPTEMBER 2000
`
`1195
`
`An Automatic 40-Wavelength Channelized Equalizer
`
`C. R. Doerr, Member, IEEE, L. W. Stulz, R. Pafchek, L. Gomez, M. Cappuzzo, A. Paunescu, E. Laskowski, L. Buhl,
`H. K. Kim, and S. Chandrasekhar
`
`Abstract—We demonstrate a wavelength equalizer in planar
`silica waveguides that can automatically control
`individual
`channel powers in a 40-channel 100-GHz-channel-spacing wave-
`length-division multiplexed system, yet gives no distortion to
`channels that already have the same power as their neighbors.
`6.8 dB insertion loss over 32-nm, 9–13-dB attenuation
`It has
`0.18 dB polarization/time-dependent loss.
`range, and
`Index Terms—Couplers, equalizers, gain control, glass mate-
`rials/devices, gratings, wavelength division multiplexing.
`
`I. INTRODUCTION
`
`I N LARGE channel-count wavelength-division multiplexed
`
`(WDM) networks, it is important to insure that none of
`the channels becomes significantly weaker than the others,
`otherwise its reduced optical signal-to-noise ratio could result
`in transmission penalties. Recently, several dynamic filters have
`been demonstrated that can equalize the gain of optical am-
`plifiers but cannot control individual channel powers [1]–[6].
`There have been some demonstrations of dynamic channelized
`filters [7]–[10], but these impose filtering on channels that do
`not need adjustment relative to their neighbors, limiting the
`allowable number of equalizers in the transmission line. Here,
`we present a dynamic filter that can adjust the transmissivity for
`each channel independently as well as provide distortion-free
`transmission when desired.
`
`II. DESIGN
`The equalizer is made in silica waveguides on a silicon sub-
`strate. The waveguide core index is
`0.65% higher than the
`cladding. The design (see Fig. 1) is a Mach–Zehnder interferom-
`eter with a grating-lens-grating cascade in one arm [5]. We will
`call this arm the filtered arm, and the other, the nonfiltered arm.
`The interferometer employs
`50/50 evanescent couplers and is
`in the cross-state (one fiber is glued to an upper input and the
`other to a lower) to reduce the wavelength dependence and fabri-
`cation uncertainty of the couplers. The gratings have 60 waveg-
`uides each, and there are 61 lens inlets per grating free-spectral
`range, the central 44 of which on each side are connected to each
`other by equal-length waveguides. Thus the optical spectrum
`is slightly oversampled, allowing the device to perfectly recon-
`struct the input spectrum over the band of interest, if so desired
`[5]. Each lens waveguide, as well as the nonfiltered arm, con-
`tains a thermooptic phase shifter, consisting of a chrome heater
`(3.8 mm 38 m) over the waveguide. The phase shifters are
`spaced by 100 m.
`
`Manuscript received March 29, 2000; revised May 3, 2000.
`The authors are with Bell Laboratories, Lucent Technologies, Holmdel, NJ
`07733 USA (e-mail: crdoerr@lucent.com).
`Publisher Item Identifier S 1041-1135(00)07456-5.
`
`Fig. 1. Channelized equalizer. PMF = polarization maintaining fiber,
`SMF = single-mode fiber, and PS = polarization splitter.
`
`Wafer real estate is precious for this long 44-channel device.
`A significant fraction of the physical size is the star couplers.
`The radiating waveguides from the star couplers must reach a
`certain spacing before they can bend in the gratings and lens. As
`explained in [11], the smaller the inlet center-to-center spacing
`at the star coupler edges, 10 m in the case here, the smaller the
`star coupler. A small spacing results in significant mutual cou-
`pling among the inlets. The mutual coupling results in a mainly
`periodic phase distortion in the star coupler, for which we com-
`pensate by appropriately lengthening the grating and lens outer
`arms. Fig. 2(a) shows the path-length correction, and Fig. 2(b)
`shows the calculated transmissivity from the input of one of
`the waveguide gratings to one of the central lens arms for with
`(solid line) and without (dashed line) the path-length correction.
`As one can see, the path-length correction keeps the passband
`narrow, despite the mutual coupling among the lens inlets, max-
`imizing the optical isolation between the equalizer controls.
`The circuit has high polarization-dependent loss (PDL). This
`is because of difference in strain and shape birefringence be-
`tween the filtered and nonfiltered arms and also strain birefrin-
`gence in the gratings. This PDL can be reduced through var-
`ious known techniques, but it is difficult to reach the
`0.1-dB
`PDL specification required for many long-haul systems. How-
`ever, by using a circulator, a polarization splitter, and polariza-
`tion-maintaining fiber between the device and the splitter ori-
`ented such that only one polarization exists on the chip, one has
`extremely low PDL without having to do anything to the circuit
`(see Fig. 1). For the device described here, all the light on the
`chip is transverse-magnetic polarized. Another advantage to this
`scheme is that it also eliminates the polarization-mode disper-
`sion (PMD) of the chip. A silica chip typically has a strain-in-
`duced birefringence of
`2
`10
`. Thus a 15-cm-path-length
`chip that does not use the above scheme would have 0.15 ps of
`PMD. Note that this method of PDL elimination is advantageous
`over the method of using a circulator on one side and a Faraday
`mirror on the other. This is because in this latter method, PDL
`in the chip will increase the insertion loss and/or reduce the dy-
`namic range, any fiber-coupling losses are paid for twice, and
`for a given dynamic range, a single-pass version of the single-fil-
`tered-arm interferometer can have lower loss than a double-pass
`
`1041–1135/00$10.00 © 2000 IEEE
`
`Exhibit 1011, Page 1
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`1196
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`IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 9, SEPTEMBER 2000
`
`(a)
`
`(b)
`(a) Path-length correction in the gratings and lens for mutual coupling
`Fig. 2.
`in the star-coupler inlets. (b) Calculated transmissivity from the input of the
`grating to a central lens arm for with (solid) and without (dashed) the path-length
`correction.
`
`version (via adjustment of the coupler ratios). On can verify this
`last point by considering the case of infinite dynamic range: in
`such a case the design will be the same whether it is single-pass
`or double-pass, yet the single-pass version will have lower loss.
`
`III. RESULTS
`The chip is 1.4 cm 11 cm. Its center is soldered to a copper
`block, and each grating has its own resistive heater and temper-
`ature sensor and is not in contact with anything but air. This is
`because the gratings shift by 0.01 nm/ C. Via a 50-pin SCSI
`connector, the device is connected to 45 12-bit computer-con-
`trolled voltage drivers. It takes 425 mW to shift a phase shifter
`by . The two gratings are wavelength-aligned when at the same
`temperature.
`The coupling ratios of the two evanescent couplers turned
`out to be
`60/40. However, because the interferometer is in
`the cross state, the net effect is 50/50 couplers, with 0.2-dB
`excess device loss. The insertion loss and attenuation range of
`the equalizer are shown in Fig. 3. The top and bottom smooth
`traces were obtained by adjusting all of the phase shifters for
`maximum and minimum transmissivity, respectively. One can
`see that the insertion loss is
`6.8 dB over 32 nm;
`2.8 dB of
`this is from the circulator and polarization splitter. The upper
`and lower curves with the dip and peak were obtained by ad-
`justing control no. 23 by
`and not adjusting any other phase
`shifter. As one can see, the thermal crosstalk between controls
`is small, although nonzero. One can also see that the optical
`crosstalk between controls is small, as explained in Section II.
`Also, the available attenuation range is less ( 9 dB) when a soli-
`tary control is changed compared to when its neighbors follow
`it ( 13 dB). This is due to the spectral overlap of the con-
`trol points. The polarization/time-dependent loss is
`0.18 dB,
`which was measured using a laser, polarization controller, and
`a power meter. The chromatic dispersion was not measured on
`
`Fig. 3. Measured transmissivity through the equalizer (including the circulator
`and polarization splitter) for various conditions of the phase shifters.
`
`Fig. 4. Measured result of automatic flattening of an amplified spontaneous
`emission spectrum. The upper is the spectrum before the equalizer, and the lower
`is after the equalizer.
`
`this particular chip, but was measured for other chips of the same
`general design and was found to be below
`0.6 ps/nm. It is so
`small because all of the path-lengths for all of the wavelengths
`are the same to within a wavelength. The response time of each
`control is
`2 ms, although if there is a sudden large change
`in total electrical driving power, there is a small settling on the
`order of a minute.
`Because we used the equal-straight-bend design for the lens
`waveguides [12], the lens-arm phases are well-aligned in the
`zero-power condition. Thus the power consumption is 5 W for
`relatively smooth desired filter shapes. The power consumption
`increases as the channel disorder increases. The highest power
`consumption condition is equalizing the case of 20 channels
`9–13 dB lower than the other 20. Ideally, the total power con-
`sumption for this would be
`8.5 W. In reality, we have seen
`cases up to
`12 W.
`We wrote an automatic equalization program using an optical
`spectrum analyzer (OSA) as the feedback source. The program
`
`Exhibit 1011, Page 2
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`

`
`DOERR et al.: AUTOMATIC CHANNELIZED EQUALIZER
`
`1197
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`(a)
`
`(b)
`
`(c)
`Fig. 5. Measured result of automatic equalization of 40 laser channels with
`unequal channel powers. (a) Before the equalizer. (b) After the equalizer. (c)
`Bit-error rate curve of an OC-192 signal, pattern length 2 1, before and
`after the equalizer.
`
`works as follows: after the device is packaged, it must be cal-
`ibrated. Using an optical noise source and an OSA, the max-
`imum transmissivity condition for all controls simultaneously is
`found. Then the program finds the differences between the con-
`trol and nonfiltered electrical powers to obtain a maximum and
`minimum for each control point and its wavelength center, and
`stores these values in a file. Then when used as an equalizer, it
`takes a scan from the OSA, looks in a wavelength band centered
`
`around each control point, and adjusts the voltage for that con-
`trol up or down appropriately, insuring that the electrical power
`difference between the control and the nonfiltered stays within
`the limits found in the calibration. Using this feedback system,
`we automatically equalized an amplified spontaneous emission
`spectrum (Fig. 4) and 40 laser peaks, with an initial deviation of
`up to 9 dB (Fig. 5) (only the central 42 controls were used). To
`verify that the filter has no transmission impairments, we mod-
`ulated one of the laser peaks at 10 Gb/s, and we observed no
`penalty, as shown in Fig. 5(c).
`
`ACKNOWLEDGMENT
`The authors thank M. Zirngibl, A. E. White, G. Bogert, W.
`Minford, P. Bernasconi, J. Fernandes, W. Shieh, and S. Patel.
`
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`Exhibit 1011, Page 3