2×2 GaAs asymmetric Mach–Zehnder interferometer switchHao Feng, Xihua Li, Zhuoya Yang, and Minghua Wang Citation: Applied Physics Letters 60, 2843 (1992); doi: 10.1063/1.106842 View online: http://dx.doi.org/10.1063/1.106842 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/60/23?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mach–Zehnder interferometer as an instructional tool Am. J. Phys. 63, 39 (1995); 10.1119/1.17766 Highspeed alloptical switching experiment in Mach–Zehnder configuration using GaAs waveguide Appl. Phys. Lett. 62, 925 (1993); 10.1063/1.108521 Optically activated integrated optic Mach–Zehnder interferometer on GaAs Appl. Phys. Lett. 59, 2222 (1991); 10.1063/1.106076 MachZehnder Type of Ultrasonic Interferometer J. Acoust. Soc. Am. 36, 1040 (1964); 10.1121/1.2143323 Initial Adjustment of the MachZehnder Interferometer Rev. Sci. Instrum. 23, 162 (1952); 10.1063/1.1746214

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2x2 GaAs asymmetric Mach-Zehnder interferometer switch Hao Feng, Xihua Li, Zhuoya Yang, and Minghua Wang Department of Information and Electronic Engineering, Zhejiang University, Liu He Ta, Hangzhou 310008, People’s Republic of China

[Received 31 July 1991; accepted for publication 24 March 1992)

We report on the first 2 X2 GaAs asymmetric Mach-Zehnder interferential switch. The mechanism of this device is analyzed. Instead of using Y branches, two asymmetric X junctions have been utilized in the switch. The device is characterized by a switching voltage of 12.5 V and a crosstalk ratio of less than --22 dB at an operating wavelength of 1.15 pm. It is shown that this device could be applied to switching arrays.

The 2X2 waveguide switch is one of the elementary components in integrated optics. There exist four main cat- egories of switches, namely directional coupler (DC),’ switches that operate via total internal reflection (TIR),2 “bifurcation optique active” (BOA) structures,3’4 and Mach-Zehnder (MZ) interferometer switches.5-7 Com- pared with the other three, the advantages of asymmetric MZ interference switches are as follows: ( 1) This device can be used not only as an intensity modulator but also as a 2 x 2 switch because of its X-junction structure; (2) since a push-pull operation can be realized in the device, the switching voltage is reduced, (3) the technique is not com- plex and the coupler does not require a specific fabrication length or length range to operate; and (4) due to the char- acteristics of the asymmetric structure in the active region, coupling between two local normal modes in two asym- metric parallel waveguides can be neglected when the dis- tance between the two waveguides is decreased. As a result, the length of the device should not be quite so long al- though the angles of the X junctions are much smaller. It is evident that this kind of switch will find applications in switching arrays. The fabrication and measurement of a 2x2 asymmetric MZ interferential switch made of GaAs are presented in this letter.

In contrast with the traditional MZ interferometer, the 2 x 2 asymmetric MZ switch is shown in Fig. 1 (a). The X junction takes the place of a Y branch, and the waveguide widths in the central region are different. Suppose a light beam is incident upon one of the symmetric input waveguides of the left X junction. Even and odd normal modes are equally excited in the left branching section of the junction, and equal outputs are observed at the right side of the junction. Therefore, the left X junction acts as a 3-dB coupler. The two light beams from the 3-dB coupler propagating along the central asymmetric straight waveguides (the electro-optic effect active region) have different propagating constants fi= and /3$. If the adiabatic condition is satisfied, coupling between these two wave- guide modes will not occur. The two light beams from the active region interfere at the right X junction, and the fun- damental normal mode and the first higher-order normal mode are excited. The fundamental normal mode is ob- served at the upper output waveguide, and the first-order normal mode is found at the other. The X junction thus acts as an interferometer.

We detine A4 to be the relative phase difference of the

two normal modes, caused by the different effective indices of the two asymmetric waveguides and the electro-optic effect. In general, the two light beams will be simulta- neously observed at the output waveguides of the interfer- ometer. When A$=mr (m is an integer), the output light power will appear only at one of the output waveguides. Figures 1 (b) and 1 (c) show the distribution of optical fields in each section of the switch at the bias voltage 0 V and v, respectively, where v, stands for the bias voltage when A#=r. The output powers are given by

P,,=(Pi,/2)cos2(A~/2),

5, = (Pi& ) sin2 ( A$/2 1,

A# = A&, -I- A#,.

Here Pup and P,, are the powers from the upper and lower output waveguides of the right X junction in Fig. 1 (a) and A& is the initial phase difference at zero-bias voltage, while A$,, denotes the additional phase difference due to the electro-optic effect and is given by

A&= (~//2)r~~n~vIX/d,

where l? is overlap integral between the applied electric field and the optical field,’ d is the waveguide thickness, and L is the electrode length.

/ (a)

FIG. 1. Asymmetric MZ switch structure (a) and the optical distribution in each section with an applied voltage of 0 V (b) and U, (c).

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FIG. 2. The optic spot (a) and optic field distribution (b) of 3dB coupler.

Many novel GaAs high-speed electro-optic modulators and switches have been investigated in recent years.7’9.‘0 The bandwidth of these devices reached 8.4 GHz for lumped electrodes and 26.5 GHz for traveling-wave elec- trodes. The asymmetric switch will have high-speed mod- ulation capability due to the same operation mechanism, the electro-optic effect, as the above-mentioned device. Like other MZ switches, there are two main factors for limiting the high-speed performance. First, when the wave- length of the modulating wave is much larger than the length of electrode, the capacitance of electrode will re- strict the modulation bandwidth.’ The bandwidth is

Here C is the capacitance per unit length, L is the electrode length, and R, is the total shunt resistance seen by the capacitor.

Second, when the wavelength is smaller than the length of electrode, the bandwidth will be limited by ve- locity mismatch between the optical wave and modulating wave.’ The electrical bandwidth is

Here n, and n, are the optical and microwave effective refractive index values, respectively. L is the length, and c is the free-space velocity of light.

But in the case of pulsed operation, if the difference between the propagation constants fiO and f?, is large, the interferential intensity becomes weaker in the interferom- eter region because of the effect of pulse walkoff, which may deteriorate the performance of the device.

If the quality of the X junction is not very good, the powers from the two asymmetric waveguides of the 3-dB coupler are not equal and the crosstalk ratio of the switch will be large. We define A&,=A$/2 and A&-A&2, where A&, and A#,, are the phase differences of the two normal modes, respectively. When the two modes with energies I, and I2 propagate along the two asymmetric waveguides, the output state can be written as

Iup = 1 a/ &eiAgw - a/ JzeCiAglrI 2

= 1/2(1,+Iz) - ,i&&os(A@,

I,,= 1/2(1r +Iz) - $&OS+ A4).

2844 Appl. Phys. Lett., Vol. 60, No. 23, 8 June 1992

Using the above expressions, the crosstalk ratio will be given by

It is shown that zero crosstalk will be obtained when 1, =12. Therefore, to minimize crosstalk, high-quality perfor- mance of the 3-dB coupler is necessary. The structure of asymmetric X junctions has been extensively studied by many authors.“-14 It is pointed out that mode sorting is obtained if the branching angle 0 of an X junction satisfies

WWy.

Here AD is the difference between the propagation con- stants of the two individual output waveguides of the asymmetric region of the coupler and y is their transverse propagation constant in the cladding region.

(cl

(e)

(b)

(d)

FIG. 3. The optic spots and optic field distributions at (a), (b), 6 V (c), Cd), 8 V (e), (0, 12 V.

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0.00 0

Applied voltage (v)

FIG. 4. The energy at the end of the two branches vs applied voltage.

An effective-index approximation was used in the de- sign of the switch. The branching angles of Xjunction were chosen to be 1.2” for the symmetric side and 0.6” for the asymmetric side. The widths for the wide arm, the narrow arm, and the symmetric arm are 5.5, 4.5, and 5 pm, re- spectively. The distance between two waveguides is 14 ,um for the asymmetric parallel ones and 25 ,um for the input/ output ones. The electrode is 5 mm long, and the total length of the device is about 10 mm.

Though the angle of the asymmetric coupler is about a half that of the Y-branch coupler, the distance between the two asymmetric parallel waveguides is also reduced to about half, the length of the device, therefore, is about the same as that of the other MZ switches or modulators if they have the same length of electrode.

The device was made on a GaAs wafer consisting of a 2.2~pm-thick n- epitaxial layer (n--<lo” cmw3> grown on an n+ substrate (n+ 2 10” cm-“) oriented (100). An aluminum-Schottky electrode was fabricated by means of a liftoff technique, and a ridge waveguide with a height of 1.2 pm was made by wet etching. The ohmic contact was pro- duced with a Au-Ge-Ni electrode.

The performance of the switch was measured at /2- = 1.15 pm with a He-Ne laser. An output near-field pat- tern was observed with an IR camera, recorded by a mem- ory and displayed on an oscillograph. Figure 2 gives the (a) near-field pattern and the (b) optic field distribution of

the 3-dB coupler. A power division of -2.9 dB was achieved. The near-field patterns and the optic field distri- butions are shown in Fig. 3 for the different applied volt- ages. A crosstalk ratio of less than -22 dB and a switching voltage of 12.5 V were obtained. For the device with a Smm-long electrode, the energy variation with the applied voltage at the end of output waveguides are shown in Fig. 4. The 3-dB bandwidth of greater than 1.6 GHz for lumped electrode was estimated by the junction capacitance of 4.09 pF at V=O. The waveguide loss was less than 7dB/cm, which was measured by the absorption method. If the ab- sorption loss of the electrode, the scattering loss of the bending waveguides and branching point, and the Fresnel reflection loss are considered, the total loss of the device may be much larger than 10 dB. However, the loss should be further reduced if a GaAs/GaAlAs heterostructure is used.‘5,16

We investigated the first GaAs MZ 2x2 optic switch using two asymmetric X-junction couplers. A crosstalk ra- tio less than -22 dB and a switching voltage of 12.5 V were achieved. The result obtained in this letter will be valuable for the design of GaAs switching arrays.

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