U.S. patent application number 11/651824 was filed with the patent office on 2008-07-10 for compact optical modulator.
Invention is credited to Christopher Richard Doerr.
Application Number | 20080166083 11/651824 |
Document ID | / |
Family ID | 39594365 |
Filed Date | 2008-07-10 |
United States Patent
Application |
20080166083 |
Kind Code |
A1 |
Doerr; Christopher Richard |
July 10, 2008 |
COMPACT OPTICAL MODULATOR
Abstract
We present a novel design for an optical differential quadrature
phase shift keying modulator comprised of two intensity modulators
in a three-arm interferometer.
Inventors: |
Doerr; Christopher Richard;
(Middletown, NJ) |
Correspondence
Address: |
BROSEMER, KOLEFAS & ASSOCIATES, LLC - (LUCENT)
1 BETHANY ROAD, BUILDING 4 - SUITE # 58
HAZLET
NJ
07730
US
|
Family ID: |
39594365 |
Appl. No.: |
11/651824 |
Filed: |
January 10, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60879312 |
Jan 8, 2007 |
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Current U.S.
Class: |
385/3 |
Current CPC
Class: |
H04B 10/5561 20130101;
H04B 10/505 20130101 |
Class at
Publication: |
385/3 |
International
Class: |
G02F 1/035 20060101
G02F001/035 |
Claims
1. (canceled)
2. The optical modulator of claim 3, wherein one or more of the
intensity modulators is an electro-absorption modulator.
3. An optical modulator for generating a quadrature phase-shift
keyed signal comprising: an optical interferometer with at least
three arms: CHARACTERIZED IN THAT: at least two of the arms each
contain an intensity modulator; wherein the interferometer has
three arms, and the power ratios in the three arms are
substantially 37% for each of the two arms containing the
modulators and substantially 26% for the third arm.
4. An optical modulator for generating a quadrature phase-shift
keyed signal comprising: an optical interferometer with at least
three arms; CHARACTERIZED IN THAT: at least two of the arms each
contain an intensity modulator; wherein the interferometer has
three arms, and the relative phase between the two arms containing
the modulators is substantially +/-90 degrees, and the relative
phase between the third arm and those two arms is substantially
+/-135 degrees.
5. The optical modulator of claim 3, wherein the interferometer
includes a reflective surface for reflecting light therein.
6. (canceled)
7. The optical modulator of claim 4, wherein one or more of the
intensity modulators is an electro-absorption modulator.
8. (canceled)
9. (canceled)
10. The optical modulator of claim 4, wherein the interferometer
includes a reflective surface for reflecting light therein.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of Provisional Application
Ser. No. ______, filed on Jan. 9, 2007, which is incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates generally to the field optical
communications and in particular to an optical modulator for
creating a high-speed optical data signal.
BACKGROUND OF THE INVENTION
[0003] Next generation Ethernet is likely to have a data rate
around 100 Gb/s. One possibility to accomplish 100-Gb/s
transmission is the use of a multiplex of parallel lower-speed
channels. However, the parallel approach typically has a low
spectral efficiency, requires temporal de-skewing among channels,
and has a large footprint or consumes significant chip real-estate.
Another possibility is the use of a single 100-Gb/s serial channel.
With a multi-level modulation format, such as differential
quadrature phase-shift keying (DQPSK), in which the data is encoded
using four different phase levels, the serial approach can have a
high spectral efficiency, and there is no need for de-skewing. Many
existing 10-Gb/s systems use pluggable transceivers. It would be
highly desirable to make a 100-Gb/s multi-level modulator that is
small enough to fit in a pluggable transceiver.
[0004] DQPSK modulators demonstrated to date have been too large
for a pluggable transceiver because they employ phase modulators
based on GaAs or LiNbO.sub.3. A reported GaAs DQPSK modulator was
52 mm long, R. Griffin, R. Johnstone, R. Walker, S. Wadsworth, A.
Carter, and M. Wale, "Integrated DQPSK transmitter for
dispersion-tolerant and dispersion-managed DWDM transmission,"
Optical Fiber Communication Conference, paper FP6, 2003, and a
reported LiNbO.sub.3 DQPSK modulator was more than 43 mm long, K.
Higuma, S. Mori, T. Kawanishi, and M. Izutsu, "A bias condition
monitor technique for the nested Mach-Zehnder modulator," IEICE
Electronics Express, vol. 3, pp. 238-242, 2006. These modulators
use a traditional DQPSK modulator design consisting of a nested
pair of Mach-Zehnder modulators.
[0005] The existing modulators are so long because of the
relatively weak electro-optic effect in GaAs and LiNbO.sub.3. A
solution is to make this design in InP, which has a much stronger
electro-optic effect in the C-band by using the quantum-confined
Stark effect. However, despite an increased electro-refractive
effect, a phase shifter in InP with a reasonable V.sub..pi. is
still quite long, 0.5, L. Zhang, J. Sinsky, D. Van Thourhout, N.
Sauer, L. Stulz, A. Adamiecki, and S. Chandrasekhar, "Low-voltage
high-speed traveling wave InGaAsP--InP phase modulator," IEEE
Photon. Technol. Lett., vol. 16, pp. 1831-1833, August 2004 to 4
mm, H. N. Klein, H. Chen, D. Hoffmann, S. Staroske, A. G. Steffan,
and K.-O. Velthaus, "1.55 .mu.m Mach-Zehnder modulators on InP for
optical 40/80 Gbit/s transmission networks," Integrated Photonics
Research M, paper TuA2.4, 2006, and so requires a traveling-wave
structure. A traveling-wave structure in InP is highly demanding to
fabricate.
[0006] Thus there is a need for a new approach to making a DQPSK
modulator.
SUMMARY OF THE INVENTION
[0007] I have developed--according to the present invention--a
highly compact DQPSK modulator design. The design is so compact
because it uses the electro-absorption (EA) effect rather than the
electro-refraction effect, and because it requires only one
interferometer.
[0008] The modulator consists of an at least three-arm
interferometer with EA modulators (EAMs) in at least two of the at
least three arms. This device can be made fully integrated in a
semiconductor material such as InP.
BRIEF DESCRIPTION OF THE DRAWING
[0009] A more complete understanding of the present invention may
be realized by reference to the accompanying drawings in which:
[0010] FIG. 1 is a schematic of a prior art DQPSK modulator;
[0011] FIG. 2 is a schematic of an embodiment of the present
invention;
[0012] FIG. 3 is are schematics and photographs of an optical
device according to the present invention; FIG. 3a is a schematic
of the optical modulator, stretched vertically for clarity; FIG. 3b
is a cross-section of the waveguide structure in the EAM; FIG. 3c
is a photograph of the actual modulator (the size is 1.5
mm.times.0.25 mm); and FIG. 3d is a zoomed-in photograph of the
left-hand star coupler;
[0013] FIG. 4 is a series of diagrams explaining how the present
invention works;
[0014] FIG. 5 is a measured eye diagram of a demodulated DQPSK
signal generated by our invention at 107 Gb/s;
[0015] FIG. 6 is a reflective embodiment of the present invention;
and
[0016] FIG. 7 is another reflective embodiment of the present
invention.
DETAILED DESCRIPTION
[0017] A prior art DQPSK modulator design is shown in FIG. 1. It
consists of two small Mach-Zehnder interferometers (MZIs) contained
within a large MZI. Each small MZI constains phase modulators, 100.
This modulator is large in size because it has multiple stages and
because phase modulators are long. Also, high-speed phase
modulators are challenging to fabricate because of their
traveling-wave nature.
[0018] Our proposal to make a compact DQPSK modulator is to instead
use the electro-absorption (EA) effect in InP. An InP EA modulator
(EAM) can be as short as 100 .mu.m. Thus, up to modulation
bandwidths of 40-50 GHz, the EAM can be operated as a lumped
element instead of a traveling-wave structure, R. G. Walker,
"High-speed III-V semiconductor intensity modulators," IEEE J.
Quant. Electron, vol. 27, pp. 654-667, March 1991, and H.
Kawanishi, Y. Yamauchi, N. Mineo, Y. Shibuya, H. Murai, K. Yamada,
and H. Wada, "EAM-integrated DFB laser modules with than 40 GHz
bandwidth," IEEE Photon. Technol. Lett., vol. 13, pp. 954-956,
September 2001, greatly simplifying design and fabrication. Using
an EAM for a PSK format was first demonstrated in I. Kang,
"Interferometric operation of an electroabsorption modulator for
PSK modulation and OOK modulation with performance enhancements,"
European Conf. Opt. Comm., paper We3.P.59, 2006 by exciting both
polarizations of an EAM. In our design, two EAMs are used in a
three-arm interferometer to create the DQPSK signal. Because EAMs
have a steep response function of transmission vs. voltage, close
to digital phase modulation can be produced, as it is in the case
of the nested MZI design. Another advantage to using InP is the
monolithic integration potential with a laser and an optical gain
element.
[0019] The proposed DQPSK modulator is shown in FIG. 2. It consists
of two EAMs, 200 and 201, in a three-arm interferometer. The two
outer arms have a 90.degree. phase difference, and the center arm
has a 135.degree. phase difference from both outer arms. An
implemented version is shown in FIG. 3. The required power
splitting ratio of each 1.times.3 coupler is 37%, 26%, 37% (more
precisely, the ratio is {square root over (2)}:1: {square root over
(2)} all divided by 1+2 {square root over (2)}). If the EAMs have a
finite extinction ratio, then the optimum splitting ratio is
slightly different. In such a case, the phase of the center arm
must be adjusted to account for EAM chirp.
[0020] FIG. 4 explains how the four phase levels of DQPSK are
achieved. The four symbols produced by turning on and off the two
EAMs lie on the four corners of a square in the complex plane. The
center arm is responsible for placing the origin of the complex
plane in the middle of the square. For example, when both EAMs are
fully attenuating only the center arm transmits light, so the phase
is -135.degree. as shown in FIG. 4a. Setting either EAM to
transparency then moves the phasor up (b) or to the right (c).
Setting both to transparency moves the phasor to the upper right
point (d). The chirp of the EAMs causes the phasor to follow a
curved trajectory between the four dots. The inherent transmission
through the modulator is 1/(9+4 {square root over (2)})=-11.7 dB,
which could be compensated for by integrating a semiconductor
optical amplifier (SOA) in future designs. Note that the signal
from the modulator could be considered a DQPSK or a QPSK signal,
depending on how it is detected. Essentially, DQPSK is detected
using a one-bit-delay interferometer, whereas QPSK is detected by
interfering it with a local oscillator. For convenience, we will
refer to the modulator only as a DQPSK modulator, realizing that it
is a QPSK modulator, as well.
[0021] We now describe the fabrication and testing of an
implementation of the present invention. The modulator 300 contains
two EAMs and four static phase shifters (see FIGS. 3a and c). One
static phase shifter is on the upper arm 302, one on the lower arm
304, and two on the center arm 306. The static phase shifters can
be driven with a positive or negative voltage. With a negative
voltage, the shifter also attenuates. The desired 1.times.3 coupler
power splitting ratio is achieved by using a star coupler 308 with
a narrower center waveguide inlet than the inlets of the outer
waveguides, as shown in FIG. 3d. The layer stack is shown in FIG.
3b. The EAM waveguide width is 1.8 .mu.m (on the mask), and the
length is 115 .mu.m.
[0022] The device fabrication is as follows: on a regular 2-inch
n-doped InP wafer a 2-.mu.m thick n-doped InP layer is grown,
followed by 8 quantum wells (QWs) sandwiched between 10-nm
1.3-.mu.m-bandgap InGaAsP separate confinement layers, a 250-nm
undoped InP layer, a 1.4-.mu.m p-doped InP layer with gradually
increased doping, and finally a heavily p-doped InGaAs layer. The
QWs are 0.3% tensile strained with compressive strained barriers.
This layer structure is shown in FIG. 3b.
[0023] The first processing step is removing the heavily p-doped
InGaAs layer over the passive waveguides, mainly for electrical
isolation reasons. Then the waveguides are reactive-ion etched
using silica as a mask, to a depth of 2.2 .mu.m. Benzocyclobutene
(BCB) is then spun on and cured. Ground pads 310 are etched through
the BCB to the n-doped InP. Then small openings in the BCB are
etched over the modulators and static phase shifters. Then the
top-side metal is deposited, patterned via lift-off. Finally the
wafer is thinned and back-side metal is deposited. The EAMs and
passive waveguides both contain the same QWs. This greatly
simplified our fabrication, but also resulted in high loss for the
entire device. Future designs will have different bandgaps for the
passive waveguides and the EAMs.
[0024] The modulator chip was cleaved out and soldered to a metal
submount. Experiments were performed with the modulator at room
temperature. The modulator was accessed optically via lensed fibers
and electrically via two high-speed probes with internal 50-ohm
termination and four single-needle probes. The band edge of the QWs
is at .about.1540 nm. The waveguide loss at wavelengths much longer
than the band edge is .about.2 dB/mm. For the following experiments
we launched a CW wavelength of 1540.3 nm from an external cavity
laser into the modulator.
[0025] To generate DQPSK, two high speed probes were applied to the
two EAMs and were driven with two delayed and inverted copies of
the 53.5-Gb/s data stream. We adjusted the static phase shifters in
order to obtain the desired phases of the three arms to generate
DQPSK. Unfortunately, the coupling to the center arm was
significantly less than 26% (estimated to be 14%) due to larger
than expected waveguide undercut, as can be seen by the narrowness
of the center waveguide on the right in FIG. 3d. Because we have
only one static phase shifter on each outer arm, the best we could
do was attenuate one of the outer arms and phase shift the other.
This led to a higher insertion loss and a small eye opening. The
fiber-to-fiber insertion loss of the modulator was .about.40 dB at
1540 nm in the DQPSK condition: .about.12 dB is due to inherent
loss in the modulator design, .about.2 dB due to the center
waveguide having too low of coupling and so having to attenuate an
outer arm, .about.6 dB due to fiber coupling, .about.3 dB due to no
anti-reflection coatings, .about.4 dB due to star-coupler excess
loss, .about.3 dB due to waveguide scattering loss, and .about.10
dB due to absorption in the QWs in the passive sections. The last
contribution could be eliminated in the future by using a different
bandgap for the passive waveguide than the EAMs, and integrated
SOAs could compensate for the other losses. The modulator output
was amplified and sent through an 18.7-ps Mach-Zehnder delay
interferometer in a silica planar lightwave circuit and finally to
a balanced photodetector pair. The measured DQPSK eye diagram 500
is shown in FIG. 5 a 2.sup.7-1 PRBS.
[0026] We programmed the receiver of the bit-error rate (BER)
tester with the expected pattern for DQPSK and measured the BER.
With a 2.sup.7-1 PRBS, the best tributary exhibited a BER of
.about.6.times.10.sup.-4. With a 2.sup.15-1 PRBS and using a pulse
carver after the modulator, the best tributary exhibited a BER of
1.times.10.sup.-3. The BER is mainly limited because of
inter-symbol interference and the fact that we could not achieve
the desired amplitude and phase in all three arms, as mentioned
earlier.
[0027] We just described one of many possible implementations of
the present invention. Other possible implementations include using
hybrid integration instead of monolithic integration. For example,
the EAMs could be in InP, but the passive waveguides and couplers
could be silicon or silica. SOAs could be integrated into the
interferometer, anywhere in the center arm, but before the
modulators in the outer two arms, in order to avoid nonlinear
distortion in the SOAs, to provide a higher output power.
[0028] There are also reflective designs to consider. For example,
one could use the reflective design of FIG. 6, in which a mirror,
610, has been placed in the interferometer 600. The advantages to
this design include a smaller device and a shorter EAM, which gives
less capacitance and thus higher speed. An optical circulator is
needed to extract the reflected, modulated signal. FIG. 7 shows a
similar design for an interferometer 700 wherein a reflective
surface 710 is included therein, except that a 2.times.3 coupler is
used, instead of a 1.times.3 coupler. This eliminates the need for
an optical circulator.
[0029] While we have discussed and described our invention using
some specific examples, those skilled in the art will recognize
that our teachings are not so limited. Accordingly, our invention
should be only limited by the scope of the claims attached
hereto.
* * * * *