We fabricated a waveguide optical isolator with a GaInAsP guiding layer integrated with spot size converters (SSCs) for efficient coupling to optical fibers. The isolator is constructed with a Mach–Zehnder interferometer (MZI), which is composed of multi-mode interference (MMI) couplers, as well as nonreciprocal and reciprocal phase shifters. The nonreciprocal phase shifter is constructed with a magneto-optical cladding layer directly bonded to a semiconductor guiding layer. The performance of the GaInAsP waveguide optical isolator was demonstrated with a maximum optical isolation of 28.3 dB at a wavelength of 1558 nm for the TM mode.
© 2013 OSA
Back reflections cause the instability of laser oscillation and the undesired oscillation of optical amplifiers. Much effort has been done to realize index matching at component interfaces for suppressing unwanted back reflections. An optical isolator allows light waves to propagate only in one direction and provides an essential solution by preventing unwanted back reflections from launching into optically active devices. Although the semiconductor lasers can be used as a light source without optical isolators in low bit rate systems, the optical isolator is usually used in a high speed and long-haul transmission system above 10 Gbps. Currently, practical optical isolators are composed of bulk optics such as Faraday rotators and polarization discriminating elements, which are not suitable for integration with optically active devices. For integrating them with optically active devices, waveguide optical isolators need to be developed.
Various waveguide optical isolators have been investigated thus far. In order to obtain a nonreciprocal effect, most of them use magneto-optical (MO) garnets such as yttrium iron garnet (YIG). The MO garnet exhibits a large first-order MO effect with practically low optical absorption. The first-order MO effect can be greatly enhanced by substituting the rare earth element of iron garnet with cerium. Although several optical isolators based on MO garnet waveguides have been proposed and demonstrated [1–6], integrating them with optically active devices constructed with III-V compound semiconductors is difficult.
Integration with optically active devices requires the development of an optical isolator composed of a III-V compound semiconductor waveguide. An optical isolator based on a nonreciprocal loss effect is one of such candidates, which is constructed with an SOA and a ferromagnetic cladding material [7, 8]. This isolator has an important advantage of high compatibility with optically active devices, and this is demonstrated with a maximum optical isolation of 14.7 dB/mm. The SOA, however, requires a continuous power supply to compensate for the large optical absorption of the ferromagnetic material. As a non-magneto-optical isolator, a traveling wave single sideband optical isolator is proposed, where a traveling electrical wave induces the direction-dependent transmission of a single sideband electro-optic modulator. Such a mechanism can be utilized to construct an optical isolator in a III-V compound semiconductor waveguide . This isolator, however, has an unavoidable insertion loss of 6 dB and requires RF driving power for modulation.
A nonreciprocal phase shift (NPS) waveguide isolator was demonstrated in a III-V semiconductor waveguide fabricated by directly bonding a cerium-substituted YIG (Ce:YIG) on a GaInAsP guiding layer through a hydrophilic bonding technique. The reported isolation was only 4.9 dB . In addition, NPS waveguide isolators were demonstrated in silicon-on-insulator (SOI) waveguides [11–15]. Our group reported an isolation of 21 dB in a SOI waveguide isolator, which was fabricated by directly bonding Ce:YIG through a surface activated direct bonding technique for achieving uniform and tight bonding of Ce:YIG .
In this paper, we describe a GaInAsP waveguide optical isolator employing NPS  integrated with spot size converters (SSCs) for efficient coupling to optical fibers. The optical isolator is composed of a GaInAsP Mach–Zehnder interferometer (MZI) waveguide on which a Ce:YIG cladding layer is directly bonded using a surface-activated direct bonding technique. A maximum optical isolation of 28.3 dB was obtained together with an insertion loss <15 dB at a wavelength of 1558 nm.
2. Device structure and design
The schematic illustration of an MZI isolator integrated with SSCs is shown in Fig. 1. The optical isolator is composed of multi-mode interference (MMI) couplers, a reciprocal phase shifter on one interferometer arm, and nonreciprocal phase shifters on two arms. The MMI couplers with port configurations of 2 × 3 and 1 × 2 perform as a 3-dB splitter and a combiner. The reciprocal phase shift is accomplished by the difference in optical path lengths between the two arms. The nonreciprocal phase shifters are composed of a Ce:YIG upper cladding layer magnetized transverse to the light propagation direction on film plane .
In order to obtain NPS in a push-pull manner, external magnetic fields are applied to the two interferometer arms in anti-parallel directions. As a consequence, the phase shift generated by the first-order MO effect becomes different in the two arms. In the forward propagation, the phase difference, measured in the upper arm with respect to the lower arm, is set to be -π/2 by proper adjustment of the length of the nonreciprocal phase shifters. When the reciprocal phase shifter gives a phase shift of π/2, the phase difference is cancelled. The light waves propagating in the two waveguide arms become in-phase and interfere constructively in the output 1 × 2 MMI coupler. The forward light wave is output from the central port of the MMI coupler. In the backward propagation, the phase difference given by the nonreciprocal phase shifters changes its sign. Namely, the phase difference of π/2 is given by the MO effect, whereas the reciprocal phase shift remains giving a π/2 phase difference. As a result, the total phase difference between the two arms becomes π. The light waves propagating in the two waveguide arms become anti-phase, and destructive interference occurs in the 2 × 3 MMI coupler. The light wave does not come out from the initial input port, but is radiated from the side waveguides. In order to suppress the reflection at the front edges of the side waveguides, a lateral taper is introduced at the ends of side waveguides. The light wave is radiated to the substrate effectively. Thus, the device exhibits direction-dependent and nonreciprocal transmission behavior and works as an optical isolator. Since the NPS is obtained only in TM mode, the isolator considered here operates only in TM mode. TE mode operation can be achieved by introducing TE-TM mode converters. Also, the polarization independent operation of NPS-based optical isolator is discussed in .
The device consists of a 0.45-μm-thick GaInAsP (n = 3.45, λg = 1420 nm ) guiding layer grown on three pairs of 0.42-μm-thick InP / 0.04-μm-thick GaInAsP (λg = 1420 nm) multilayers that have a total thickness of 1.38 μm. Such a layered structure is prepared for dual core SSCs that enable us to couple the GaInAsP waveguide to an optical fiber with low coupling loss . The isolator is constructed with a 1.7-μm-wide and 0.45-μm-thick GaInAsP channel waveguide. The multilayer waveguide used for coupling the optical fibers is 5 μm wide and 1.38 μm tall, where the over clad is air. Both the isolator waveguide and the coupling waveguide operate in a single mode.
The NPS is calculated for a layered structure consisting of the following: Ce:YIG, GaInAsP, three pairs of InP and GaInAsP, and InP. An upper cladding layer (Ce:YIG) is assumed to have a Faraday rotation coefficient of −4500°/cm and a refractive index of 2.20 at a wavelength of 1550 nm. The nonreciprocal phase shifters are designed using the perturbation theory with the optical field given by a mode solver based on the finite element method [5, 17]. Figure 2 shows the calculated NPS as a function of the thickness of the GaInAsP guiding layer. We observed that the NPS takes a maximum value when the waveguide is close to the cutoff. When the thickness of the GaInAsP guiding layer is 0.45 μm, the propagation distance needed for obtaining ± π/2 NPS was calculated to be 4.76 mm at a wavelength of 1550 nm. The reciprocal phase shifter, which provides a phase shift of π/2, is designed to L = 119.7 nm with a waveguide effective index of ne = 3.24. When the length of reciprocal phase shifter and the effective index of waveguide deviate from designed values by ΔL and Δne, respectively, the isolation wavelength at which the destructive interference occurs in the backward propagation and a maximum isolation is obtained deviates by 4*(L*Δne + ΔL*ne). Since some imperfections can be introduced in device fabrication even by using an electron beam lithography system, it is difficult to fabricate the device that has the isolation wavelength within a range used in the following measurement. Thus, in this study, in order to clearly observe the effect of NPS, the length of the reciprocal phase shifter, i.e. the length difference between the two MZI arms, is set to be longer than the design for the π/2 phase shift.
The length and width of the 2 × 3 MMI coupler are designed to be 109.9 μm and 10 μm, respectively, by using a simulation tool (Photon Design) based on an eigen mode expansion (EME) method as shown in Fig. 3(a). The design wavelength is 1550 nm. The distance between the side ports of 2 × 3 MMI coupler is 3.4 μm. Figures 3(b) and 3(c) illustrate the constructive (in-phase) and destructive (anti-phase) interferences, which correspond to the forward and backward propagation, respectively. In the forward propagation, the 2 × 3 MMI coupler performs as a 3 dB splitter as shown in Fig. 3(b). In the backward propagation, light waves launched in anti-phase to the MMI coupler are output through the side waveguides. Based on the same simulation tool, the length and width of 1 × 2 MMI coupler are designed to be 40.8 μm and 6 μm, respectively. The distance between the side ports of 1 × 2 MMI coupler is 1.3 μm.
We fabricated a GaInAsP waveguide optical isolator integrated with SSC as follows. A 100-μm-thick SiO2 layer used for an etching mask was deposited on a GaInAsP / multilayer / InP wafer using a plasma-assisted chemical vapor deposition technique. The waveguide pattern was drawn on ZEP520A resist with an EB lithography system with a dose of 100 μC/cm2. After the resist pattern was transferred to the SiO2 layer by CF4 reactive ion etching (RIE), the isolator waveguide was formed in the GaInAsP layer using the CH4 / H2 and O2 cyclic RIE with an etch rate of 90 nm/cycle. Similarly, the coupling waveguide of SSC was fabricated by etching the GaInAsP/InP multilayer with the CH4 / H2 and O2 cyclic RIE.
In this study, a surface-activated direct bonding technique  was employed to integrate Ce:YIG on the GaInAsP guiding layer. Before the direct bonding, a 0.5-μm-thick single crystalline Ce:YIG layer was grown on a (111)-oriented substituted gadolinium gallium garnet (GdCa)3(GaMgZr)5O12 (SGGG) substrate using an RF sputter epitaxial technique. A grown Ce:YIG wafer was cut into 1.5 × 5 mm2 dies. The surface roughness of Ce:YIG was less than 1 nm, which was defined by the average of root mean square values obtained by the atomic force microscope measurement. After the organic materials were removed from the patterned III-V semiconductor wafer and the Ce:YIG die, their surfaces were exposed to N2 RF plasma for surface activation. They were then brought into contact and pressed at an elevated temperature in a vacuum chamber.
We initially adopted the surface-activated direct bonding by exposing the surfaces of both wafers to plasma generated in N2 gas with an RF power of 500W. The N2 gas was supplied at a flow rate of 103 sccm with a pressure of 120 Pa, and the duration of the plasma exposure was 10 s. The samples are pressed at 5.2 MPa at a temperature of 200 °C for 30 min.
We observed cracks on the surface of the GaInAsP waveguide bonded in this condition, as shown in Fig. 4. The cracks, located on the edge of the bonded area along the short side of Ce:YIG die, caused a large propagation loss. The cause of these cracks can be attributed to the thermal stress brought about by the difference in the thermal expansion coefficient between the GaInAsP wafer and the Ce:YIG/SGGG die. The thermal expansion coefficients of Gd3Ga5O12 (GGG) and InP were 9.2 × 10−6 and 4.56 × 10−6 K−1, respectively. Such a large difference in thermal expansion coefficients causes a noticeable thermal stress in bonded samples while cooling to room temperature .
In order to prevent the cracks, we adopted the following two steps. First, we divided the bonding surface of GaInAsP into 100 × 100 μm2 sub-areas. The thermal stress induced in the GaInAsP layer was greatly reduced by dividing the large bonding area into small sub-areas . We then decreased the bonding temperature to 100 °C. By adopting these two steps, we succeeded in bonding Ce:YIG dies on GaInAsP waveguides without cracks, as shown in Fig. 5.
We then measured the bonding strength with a die shear test. Figure 6 shows the temperature dependence of the measured bonding strength. The blue circles correspond to the samples in which only an isolator waveguide was patterned in the GaInAsP layer; the red circles indicate samples with a Ce:YIG die bonded onto the GaInAsP layer patterned with 100 × 100 μm2 sub-areas together with an isolator waveguide. We see in Fig. 6 that the bonding strength is not significantly affected by dividing the bonding surface into sub-areas. Some samples that bonded at a temperature of 200 °C exhibited fracture in an InP substrate without de-bonding Ce:YIG. The maximum bonding strength of a sample bonded at a temperature of 100 °C was measured to be 4.1 MPa.
4. Characterization of isolator integrated with SSCs
An amplified spontaneous emission (ASE) light source was used to measure the transmittance of a fabricated device in a wavelength range of 1530–1580 nm. The output of the light source was polarized to TM mode and launched into an isolator through a high-△ optical fiber and an SSC located at the input port of the isolator as shown in Fig. 7(a). The spot size of the high-△ optical fiber was about 5 μm at a wavelength of 1550 nm. The light wave transmitted through the isolator and output from an SSC located at the output port of the isolator was coupled to another high-△ optical fiber. An optical spectrum analyzer was used to measure the transmitted spectrum. External magnetic fields were applied to the interferometer arms of MZI in anti-parallel directions using a pair of small permanent magnets with three reversed poles. The magnetization of Ce:YIG is saturated by applying a magnetic field of 20 Oe. The magnetic field can be generated by a thin film permanent magnet such as SmCo deposited on the SGGG substrate.
The measured transmittance spectra of an isolator are shown in Fig. 7(b). The transmission spectra without any external magnetic field are almost the same for both directions, as shown by the broken red and blue lines. A measured interference free spectral range (FSR) of 13.6 nm corresponds to a reciprocal phase shift of about 97π, which agrees well with a designed length difference of 46.5 μm between the two arms. The solid red and blue lines show the transmittance of forward and backward propagation, respectively, where magnetic fields are applied in an outward direction. The wavelength shift of the interference spectrum due to the MO effect is clearly observed. A maximum isolation of 28.3 dB was measured at a wavelength of 1558 nm.
The measured transmittance includes the coupling loss between the high-△ fiber and the SSC at two end facets as well as the insertion loss of the device. The insertion loss of the device is less than 15 dB at the wavelengths where the transmittance takes minimal values. Since the insertion loss (including the coupling loss) of SSC between the high-△ fiber and the SSC was measured to be 2.5 dB per facet based on a cut-back method , the insertion loss of the isolator was estimated to be 10 dB at most. The breakdown of the insertion loss of the isolator is as follows: The propagation loss of the GaInAsP waveguide was measured to be 0.6 dB/mm by using a cut-back method; the propagation loss of the 6 mm isolator waveguide is estimated to be 3 ~4 dB; and the bending loss of the waveguide was measured to be ~2 dB for four 90° bends by taking the difference with the linear reference waveguide having the same optical path length. The remaining factors of the insertion loss are the loss of MMI couplers, the absorption of Ce:YIG, and the loss associated with the interfaces between the Ce:YIG and air cladding regions as shown in Table 1. The insertion loss of two MMI couplers is simulated to be 1.0 dB by using the simulation tool based on the EME method. As the absorption of Ce:YIG waveguide is measured to be 6 dB/mm at a wavelength of 1550 nm in our group, the absorption of Ce:YIG is calculated by 6 dB/mm × 5 mm (NPS length) × 2.4% (confinement factor) = 0.72 dB. The reflection and scattering at the interface between the Ce:YIG and air cladding regions are simulated to be −59.6 dB and −23.9 dB, respectively, by using the simulation tool based on the EME method.
The measured transmittance spectra of the isolator equipped with an about 16π reciprocal phase shifter (the difference of arm length = 7.75 μm) are shown in Fig. 8. The FSR of interference is 95 nm. Instead of reversing the propagation direction of the light wave, the magnetic field directions are reversed by using SNS and NSN three pole magnets. The reversal of the magnetic field direction is equivalent to the reversal of the propagation direction. The broken black line shows the transmittance with no magnetic field applied. Blue and red lines correspond to the cases where inward and outward directed magnetic fields are applied, respectively. Compared with Fig. 7, the wavelength width for an isolation >20 dB increases to 4.04 nm.
We fabricated a GaInAsP MZI waveguide optical isolator integrated with SSCs for efficient coupling to optical fibers. A 1.5 × 5 mm2 Ce:YIG die is directly bonded to a GaInAsP MZI waveguide as a cladding layer. The thermal stress is reduced by dividing the bonding area into small sub-areas and decreasing the bonding temperature to 100°C, while obtaining a bonding strength of 4.1 MPa. We obtained a maximum optical isolation of 28.3 dB together with an insertion loss <15 dB at a wavelength of 1558 nm. A 20-dB isolation wavelength width of 4.04 nm was obtained by reducing the reciprocal phase shift to about 16π.
The authors would like to express their gratitude to Mr. Takanashi (Fujikura Ltd.) for providing high-△ fibers.
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