Waveguide optical isolator based on Mach–Zehnder interferometer

J. Fujita, M. Levy, R. M. Osgood Jr. , L. Wilkens, and H. Dötsch

Citation: Applied Physics Letters 76, 2158 (2000); doi: 10.1063/1.126284

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 APPLIED PHYSICS LETTERS VOLUME 76, NUMBER 16 17 APRIL 2000

Waveguide optical isolator based on Mach–Zehnder interferometer

J. Fujita,a) M. Levy, and R. M. Osgood, Jr.
Microelectronics Sciences Laboratories, Columbia University, New York, New York 10027
L. Wilkens and H. Dötsch
Universität Osnabrück, 49069 Osnabrück, Germany
共Received 9 November 1999; accepted for publication 23 February 2000兲
A waveguide optical isolator based on nonreciprocal interference is demonstrated. Ridge
waveguides are fabricated in a Mach–Zehnder configuration on a single film of bismuth-, lutetium-,
neodymium-iron garnet. With this design, no polarizers are required to achieve extinction in the
backward propagation direction. This isolator exhibits a 19 dB extinction ratio at ␭⫽1.54 ␮m. A flat
wavelength dependence, to within 2 dB, has been observed in the range between 1.49 and 1.57 ␮m.
© 2000 American Institute of Physics. 关S0003-6951共00兲02416-5兴

Optical isolators are important components in optical inclusion of an element giving an additional reciprocal 90°
communication systems and are used, for example, to pre- phase shift results in constructive interference in the forward
vent destabilization of a laser source. At present, high quality direction and 180° 共destructive兲 interference in the backward
isolators are commercially available but only in bulk form. direction. The origin of the nonreciprocal phase difference
With the rapid development in optical fiber telecommunica- between counter propagating modes stems from the presence
tions, integration of optical isolators, e.g., at the source and of off-diagonal components in the dielectric tensor in a trans-
receiver ends of an optical link, has become desirable to verse magnetic field. Longitudinal and transverse optical
reduce cost and minimize size. Thus far, several groups have E-field components are coupled, resulting in a dispersion re-
demonstrated waveguide-based isolators, with the most com- lation which depends on the direction of propagation. A de-
mon approach being based on longitudinal field Faraday tailed description of the operational basis of this device can
rotation.1–3 Although these groups have obtained extinction be found in Refs. 4–6.
ratios of ⬃30 dB, fabrication of these devices is difficult Although this nonreciprocal effect has been observed in
because they require stringent modal phase-matching fabri- interferometric configurations before,7–10 no practical, fully
cation conditions,1–3 efficient mode conversion,1–3 and the integrated Mach–Zehnder isolator has yet been reported.
integration of auxiliary components such as polarizers, half- Here, we demonstrate efficient optical isolation in a Mach–
wave plates, etc.1,2 Zehnder configuration. The device works in TM mode and
However, the use of an interferometric design based on does not require phase matching between orthogonal polar-
the transverse magneto-optic nonreciprocal phase shift is less izations. Polarization-independent interferometric isolators
explored in device form, although it has the advantage of of this type are also possible and their design has been dis-
obviating these problems.4–6 In this approach, no phase cussed recently by Zhuromskyy et al.6 Such isolators utilize
matching is required as transverse-magnetic 共TM兲 and simultaneous nonreciprocal retardations for both TE and TM
transverse-electric 共TE兲 mode are handled separately in a modes in one Mach–Zehnder interferometer. The results pre-
single waveguide interferometer; no polarizers are needed to sented here handle one component of the device; the TE
achieve extinction, since the device relies on the destructive component is handled independently and requires no adjust-
interference of backward propagating waves; and no half- ments in TM retardation; the Zhuromskyy et al.6 approach is
wave plate needs to be inserted or patterned into the device based on splitting the nonreciprocal phase shift of either
to handle the transmission of forward propagating waves mode in both arms of the Mach–Zehnder separately. This
through an output polarizer. This type of isolator is typically approach eliminates the need for TE/TM phase matching and
monolithically integrated to minimize the overall insertion makes a polarization-independent device easier to fabricate.
loss, i.e., to avoid interconnection losses. A bismuth-, lutetium-, and neodymium-iron garnet film,
This interferometric approach utilizes the nonreciprocal (Bi,Lu,Nd兲3共Fe,Al兲5O12, is grown by liquid phase epitaxy on
phase shift that occurs in a waveguide when a magnetic field a 关111兴 oriented GGG substrate and is used in our experi-
is applied transversely to the beam axis through a magneti- ments. The film-substrate lattice mismatch is measured as
cally active material such as yttrium iron garnet. This field 0.001 nm, indicating minimum stress-induced anisotropy.
acts on the magnetic moment of the ferrite film and causes a This film has in-plane magnetization, and a refractive index
difference between the forward propagating, ␤ f , and back- of 2.2403 for the TM mode at ␭⫽1.55 ␮m. The thickness of
ward propagating, ␤ b , wave vectors ␦ ␤ ⫽ 兩 ␤ f 兩 ⫺ 兩 ␤ b 兩 . By ad- the film is 1.65 ␮m.
justing the path lengths of the two arms of a Mach–Zehnder The nonreciprocal properties of the as-grown film are
interferometer, a ⫾90° phase shift, with sign depending on characterized using a fiber interferometer setup.9 Before the
propagation direction, can be fabricated into the device. The rib waveguides are patterned, the films are thinned to opti-
mize the nonreciprocal response; previous measurements had
a兲
Electronic mail: juni@cumsl.ctr.columbia.edu shown that this optimum thickness is ⬃0.5 ␮m.7–10 Since
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 Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Fujita et al. 2159

Mach–Zehnder interferometer is then fabricated. The Mach–

Zehnder devices are patterned onto a single chip by a pho-
tolithographic direct laser writing system.11 The resist pat-
terns are made by focusing an Ar laser beam (␭⫽360 nm兲
directly onto a photoresist-coated sample with computer-
controlled XYZ translation stages and shutter. The total
length of the fabricated waveguide isolator is 8.0 mm, which
comprises 3.3-mm-long Mach–Zehnder arms, two 0.4-mm-
long Y-branches, and 3.9-mm-long input and output straight
waveguides. The arm separation between interferometer
arms is 24.4 ␮m. A previous study by the authors, using a
fiberoptic device, has shown that only small deviations
共⬍5%兲 in nonreciprocal phase shifts accrue as a result of arm
length inaccuracies expected here,9 so that efficient 共ⲏ20
dB兲 forward transmission and backward extinction are ex-
pected.
In order to study the dependence of isolator performance
on reciprocal retardation, a series of interferometers are fab-
ricated with reciprocal phase shifts ranging from 75° to 105°
with 5° increment while keeping the nonreciprocal phase
shift at 90° per arm between forward and backward propa-
gation. The reciprocal phase shifts are formed by creating
patterns for different interferometers on a single mask with
path-length differences of ⬃0.2 ␮m at 0.01 ␮m increments
between the two arms. The total length difference produces
FIG. 1. 共a兲 The fiber Mach–Zehnder interferometer setup used for nonre-
ciprocal phase shift measurements. 共b兲 Optical measurement setup to ob- less than 0.006° additional nonreciprocal phase shift and thus
serve performances of fabricated Mach–Zehnder isolator. The inset shows the unequal arm lengths do not otherwise affect the efficient
the isolator geometry. operation of the device.
The fabricated waveguide isolators are tested by focus-
thickness tuning improves the phase shift per length, proper ing a TM collimated beam into the sample, as shown in Fig.
tuning yields a shorter device, and hence reduces the total 1共b兲, and monitoring the light from the output facet with a Si
absorption loss in the isolator. Straight ridge waveguides are photodiode. An output spatial filtering with lens and aperture
then patterned onto the garnet film by standard photolitho- is used to couple light only through waveguide and, there-
graphic techniques. The ridge waveguides have a 2.0 ␮m fore, to eliminate any extraneous light before photodetection.
width, a 0.5 ␮m waveguide height, and a 0.07 ␮m rib height, Isolation measurements are made after applying oppos-
and are fabricated by photoresist patterning and phosphoric- ing magnetic fields to the arms of the Mach–Zehnder inter-
acid wet etching; the etch rate is 0.01 ␮m/min at 57 °C. ferometers, thus yielding an opposite sense of nonreciprocal
These waveguides are inserted into one arm of a fiber phase retardation shift between the arms. The electromagnets
Mach–Zehnder interferometer and the characteristic linear are placed on opposite sides of the BiLuNd-IG isolator
nonreciprocal phase shift at our waveguide structure is sample, as shown in Fig. 1共b兲, with a separation of 6 mm.
9 The electromagnets are mounted on XYZ translation stages
measured. Light is butt coupled into the film and collected
by a second fiber as shown in Fig. 1共a兲. An air gap is inserted for fine spatial adjustment of the field at the interferometer.
in the second arm both to balance the power between the two Specifically, their positions are adjusted to have exactly the
arms and to adjust the relative phase difference to ⬃90° to opposite magnetization between each arm, with the saturat-
achieve maximum modulation. ing field on each arm estimated at ⲏ50 Oe for this configu-
A 100 Hz ac transverse magnetic field with an amplitude ration of magnets. The backward propagation case is simu-
of ⬎10 Oe is then applied to the waveguide sample to induce lated by reversing the polarities of both electromagnets, and
a nonreciprocal phase retardation. The output signal is de- the ratio in output signals for both configuration is taken as
tected by a silicon photodiode and displayed on an oscillo- the isolation ratio.
scope. The nonreciprocal phase shift is obtained by measur- The measured extinction ratios of the fabricated Mach–
ing the amplitude of the modulated signal and comparing it Zehnder isolators at ␭⫽1.54 ␮m are shown in Fig. 2; the
to the absolute dc optical signal. Based on this measurement, measurements show a maximum extinction of 19 dB. The
the length to achieve a 90° nonreciprocal phase shift, or the fact that this maximum extinction is attained at a reciprocal
required arm length for an isolator, has been observed to be phase shift of ⬃90° verifies that the nonreciprocal phase shift
3.3 mm. The linear optical loss for the TM mode is also is ⬃90°, as expected. The solid line shows the calculated
measured for each of the waveguide samples; after subtrac- extinction ratios with 1.4% power imbalance between the
tion of the Fresnel reflection loss, an average absorption of two arms of interferometer. The good agreement between the
12⫾2 dB/cm is found for the samples. calculated and experimental results indicates that the mag-
After characterizing the nonreciprocal response in the netic optical circuit is functioning as anticipated, and that the
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BiLuNe-ionization gauge 共IG兲 waveguide, a fully integrated power imbalance of the Y junctions is low 共ⱗ1.4%兲. It is
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 2160 Appl. Phys. Lett., Vol. 76, No. 16, 17 April 2000 Fujita et al.

FIG. 2. Measured extinction ratios for various reciprocal phase shifts. Data
are based on the average of several measurements for each waveguide. FIG. 3. Measured extinction ratios and excess losses for various wave-
lengths. Data are based on the average of several measurements for each
waveguide.
believed that the anomalous data point in Fig. 2, correspond- Mach–Zehnder interferometer in BiLuNd-IG rib waveguide
ing to a 95° reciprocal phase shift is most likely caused by a is demonstrated. Extinction ratios up to 19 dB and excess
fabrication defect leading to power imbalance in the interfer- loss of 2 dB have been obtained with flat response at a wave-
ometer. This process is otherwise repeatable because the de- length range of 1.49–1.57 ␮m. In this experiment, the total
fect has not been observed in the six other interferometers. length of Mach–Zehnder interferometer is 8.0 mm. The use
The insertion loss for these waveguides is dominantly due to of higher Faraday rotation materials or double-layered gar-
material absorption 共10 dB兲, input coupling 共1.2 dB兲, reflec- net, as described in Ref. 7, can further reduce the device
tions at end facets 共1.4 dB兲, and the interferometer Y junc- length.
tions 共1.0 dB兲; these values are estimated based on previous
measurements or known formulae. The excess loss, or the The authors acknowledge helpful suggestions by
difference between the measured insertion loss and the loss Dr. K. H. Park on the mask fabrication processes. This work
contributions described above, at ␭⫽1.54 ␮m is 2 dB for the is supported by a joint MURI/DARPA contract with the Uni-
device. The largest contribution to the insertion loss comes versity of Minnesota 共No. F 49620-96-1-0111兲. Financial
from material absorption, which as mentioned above, is support by the Deutsche Forschungsgemeinschaft, Sonder-
12⫾2 dB/cm. This optical absorption is caused by nontriva- forschungsbereich 225, is gratefully acknowledged.
lent impurities incorporated into the film during the growth 1
M. Levy, R. M. Osgood, Jr., H. Hegde, F. J. Cadieu, R. Wolfe, and V. J.
process. These impurities are due to Pt4⫹ ions from the cru- Fratello, IEEE Photonics Technol. Lett. 8, 903 共1996兲.
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M. Levy, M. H. Ru, R. Scarmozzino, and R. M. Osgood Jr., IEEE Lasers
as low as 0.5 dB/cm.12 Work is presently underway to fab- and Electro-Optics Society 1996 Annual Meeting 2, 232 共1996兲.
5
ricate lower-loss Mach–Zehnder isolator with this material. T. Mizumoto, S. Mashimo, T. Ida, and Y. Naito, IEEE Trans. Magn. 29,
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6
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K. H. Park, H. Rao, J. Z. Huang, D. S. Levy, R. M. Osgood, Jr., D. H.
tially flat response. Woo, and S. H. Kim 共unpublished兲.
In summary, a waveguide optical isolator based on 12
L. Wilkens, O. Hagedorn, R. Gerhardt, and H. Dötsch 共unpublished兲.

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