Towards heterogeneous integration of optical isolators and circulators with lasers on silicon [ Invited ]

Optical isolators and circulators are extremely valuable components to have in photonic integrated circuits, but their integration with lasers poses significant design and fabrication challenges. These challenges largely stem from the incompatibility of magnetooptic material with the silicon or III-V platforms commonly used today for photonic integration. Heterogeneous integration using wafer bonding can overcome many of these challenges, and provides a promising path towards integrating isolators with lasers on the same silicon chip. An optical isolator operating for TE mode with 25 dB of isolation, 6.5 dB of insertion loss, and tunability over 40nm is demonstrated and a path towards integrating this isolator with the heterogeneous silicon/III-V laser is described. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
Rapid development in photonic integrated circuit (PIC) technology using silicon waveguides has been made possible by an increasingly advanced library of photonic elements.By utilizing high quality silicon on insulator (SOI) wafers and borrowing decades of CMOS processing expertise, researchers have demonstrated low-loss waveguides, modulators, and photodetectors [1].These elements serve as building blocks for complex silicon photonic systems on chip for applications such as sensing [2,3], interconnects [4,5], and quantum optics [6].In most of these works, an external laser is used with a bulk optical isolator, which limits the size, power consumption, and cost of the product.The isolator is needed to block reflections from the PIC, such as those caused by the edge or grating coupler, from reaching the laser.A fully integrated solution in which the laser and isolator are on the same silicon chip has not realized to date.Since silicon does not emit light efficiently due to its indirect bandgap, Ge or III-V materials are required for electrically pumped lasers on silicon [7,8].One way to introduce III-V material into silicon photonics is through wafer bonding processes [9].This approach, termed "heterogeneous" integration, can be used to bond III-V material directly on top of the silicon chip prior to laser fabrication.This approach has tremendous benefit due to precise lithographic alignment between III-V and silicon, as well as increased scalability when compared to attaching pre-fabricated III-V chips to silicon dies [10].Since the first heterogeneous silicon laser was reported over a decade ago [11], increasingly complex PIC containing hundreds of elements have been demonstrated using heterogeneous integration for transceivers, interconnects as well as sensing applications [12][13][14][15].The technology has recently reached commercialization [16].
Despite the increasing maturity and complexity of heterogeneous silicon photonics, the lack of a readily available on-chip optical isolator limits the performance of these PICs, especially given the often-strict performance requirements for the integrated laser.Ideally, the isolator should be placed directly after the laser, to minimize the effect of undesired reflections from the rest of the PIC.Integrated circulators can separate counter-propagating light waves and can give rise to bidirectional transmission and optical amplification [17].Inclusion of isolators and circulators in the heterogeneous silicon photonic library is highly desirable, and crucial for some applications.The same wafer bonding technology used to realize lasers on silicon can be extended to bond magneto-optic materials for optical isolators and circulators.This highlights the ability to take the best material for each function, and heterogeneously integrate them together using silicon waveguides as a common denominator [15].This flexibility will give rise to multi-functional, high performance PIC, since compromises do not need to be made from a material standpoint.This paper addresses the challenges and solutions associated with such an approach.
This paper is organized as follows.First, we provide a brief overview of integrated optical isolators and circulators, their operating principles, and notable demonstrations to date.Then, we discuss the requirements and challenges associated with integrating such isolators with heterogeneous silicon/III-V active devices such as lasers, modulators, and photodiodes.Finally, we present our progress in this area, and demonstrate a widely tunable microring based optical isolator operating for transverse electric (TE) polarization.

Overview of integrated isolators and circulators
Optical isolators and circulators are nonreciprocal components.They are characterized by allowing the propagation of light in one specific direction, such that their scattering matrix is non-symmetric [18].The device symmetry can be effectively broken in three different ways: i) by spatiotemporal modulation (STM) of the refractive index; ii) exploiting nonlinear effects (NLE) and iii) using magneto-optical (MO) materials.
In STM case, the nonreciprocity of the device is induced by modulating the refractive index of the waveguide, usually with a microwave [19] or acoustic signal [20].For a given propagation direction, the modulating signal is used to couple the incident light with different modes or frequencies that can be filtered or radiated out of the device [21].This modulating signal has no effect on counter-propagating light.Integrated optical isolators have been demonstrated exploiting the electro-optic effects in a travelling wave III-V modulator [22] as well as a tandem phase modulator [23].Similar isolators were achieved in silicon [19,24].No additional materials are needed, making STM based isolators very attractive for integration with lasers.However, the operation of the isolators often requires complex, high-speed drive circuits that can consume large amounts of power.
In the second approach, a NLE is tailored to achieve nonreciprocal behavior.However, not all nonlinear effects can be used for this purpose, as some effects such as Kerr-like nonlinearities are subject to dynamic reciprocity [25].When a forward and backward propagating signal are simultaneously propagating through the device, the nonreciprocity of the system can break down, and the device cannot be used to perform isolation.Nonlinear effects suitable for isolation are Raman amplification [26], stimulated Brillouin scattering [27], and parametric amplification [28] among others.Like STM based isolators, the NLE isolators do not require materials outside of those commonly found in CMOS or III-V based foundries.However, a drawback of using nonlinear effects is the inherent dependence between optical isolation of the device and the optical power of the incident light.This is undesirable as the feedback to the laser should be minimized regardless of the output power.Furthermore, they also suffer from small isolation bandwidths since they generally rely on phase matching, meaning isolation is only performed at a specific wavelength.
The last approach uses MO material to break the reciprocity of the system when immersed in a magnetic field.If the light is propagating in the same direction of the external applied magnetic field as in Fig. 1(a), the plane of polarization rotates, otherwise known as Faraday rotation.This approach is widely used in free-space optical isolators, but challenging to implement in waveguides, which are highly birefringent [29,30].To overcome this, quasiphase matched approaches by periodically modulating the Faraday rotation along the length of a waveguide have been demonstrated [31,32].However, a drawback of these devices is the need for polarization manipulating components before and after the Faraday rotator, such as a 45-degree polarization rotator [33].These components are harder to implement in integrated optics compared to their bulk counterparts.along the pect to it engths is cerium or ignificant n [39] or and lattice promote otation of TIG) with mplify the ne garnet ve, lattice as high as 4500 deg/cm [51].While monolithic garnet approaches continue to develop at a rapid rate, a heterogeneous approach in which the Ce:YIG is first grown on native substrate, and then bonded onto silicon has shown the best results to date [52].Furthermore, heterogeneous integration of the garnet may be more suitable for laser integration from a design and fabrication standpoint, as we will discuss in the following sections.

Laser integration challenges and solutions
This section covers the requirements and challenges associated with integrating an optical isolator with a laser source on the same chip.The focus will be on heterogeneous integration on silicon, although many of the same arguments carry over to III-V PICs.

Integration with a heterogeneous silicon/III-V laser
The optical isolator should be placed directly after the laser for optimal performance.Thus, the process flow for the isolator and laser must be compatible with each other.From a fabrication standpoint, the challenges lie in managing the thermal budget of the process, as well as the simultaneous processing of vastly dissimilar materials (III-V, silicon, and garnet).While the lattice constant mismatch can be somewhat alleviated by wafer bonding, the thermal expansion coefficient mismatch between III-V and silicon provides a limited thermal budget for the process.Rapid thermal anneal performed at 420C for 2 minutes have degraded laser performance [53], and temperatures should be ideally kept below 300C.Monolithic approaches for garnet deposition may have a difficult time meeting this thermal budget.Studies have shown that 650C is required to crystallize YIG [54], and TIG films are annealed even hotter, at 900C [55].Therefore, if a monolithic approach is pursued, the garnet would have to be deposited near the beginning of the process, prior to any III-V bonding to preserve the thermal budget for the rest of the process.This could affect subsequent steps and may require a complete retooling of the heterogeneous silicon/III-V process.Alternatively, bonding of garnet is attractive as it can be added as a back-end process after laser fabrication.Since the garnet is already fully crystallized prior to bonding, the thermal anneal is not required.In fact, the highest temperature process in isolator fabrication is 200C [40], which will not negatively impact the laser performance.The inclusion of the isolator processing at the end also reduces the amount of overlap with laser fabrication, which simplifies the process greatly.
From a design standpoint, one of the main challenges is the mismatch of waveguide dimensions between the laser and the isolator.The cross-section of the heterogeneous silicon/III-V laser is shown in Fig. 2(a).It has a silicon waveguide height of 500nm, which is chosen to match the refractive index of the silicon slab with the thick InP gain region.Silicon waveguides thinner than 400nm will suffer from low coupling to the InP [56].However, the optimal silicon thickness for isolators is between 200 and 250nm, as seen in Fig. 2(b).It is possible to transition between the two silicon waveguides using a partial etch and taper structure [57], but this roughens the silicon surface, which complicates the bonding process.Furthermore, the taper could serve as a source of reflections.Since the main purpose of the isolator is to block reflections from reaching the laser, the isolator should not introduce significant reflection.A potential solution may be the local oxidation of silicon [59], which thins down a lithographically defined area of the wafer.Although this would be performed at the beginning of the process, prior to waveguide etching, it preserves the smooth surface for bonding III-V and MO material.Other design challenges are matching the operating polarization of the isolator with the laser, as well as biasing a magnetic field across the device in an efficient, compact, and integration friendly manner.These are further explored in the following sections.

Applying
The operation magnet is pac an integrated packaging.It europium dur However, the to be perpend the planar nat placed in groo One soluti footprint of t thermal tunin account for fa can be tuned expected.Fur radially orient compromising resonator [39] 3. (a) Schematic of c) linear taper und (e) are chosen ac band polarization ls of the PSR a μm long) for T vert from TE e polarization otator over pre olerance [63]  ional schematic o et.Multiple narro at the waveguide wing the increase s diminishing retur eld.NRPS increas can be made bstrate thickne process is adva ement for garn val could be "sm on implantatio antation is don llowing wafer p enough to avo distance to the the handling o cesses where her simplified ely controlled a planar thin-fil possess ferroma 5], and is pron r, the magnet be placed on to cays with dis This is also cr he distance to t distance is min nical polishing ed to reduce th uces the curren on based failu and thinner g f the magnet.F mA of current ckward propag ood agreement used in MZI, 3.6 mW of po of the Si wavegu owly spaced coils e and results in la in NRPS for thinn rns, as the outer co es linearly until t in the substr ess as thin as antageous for i nets without a d mart cut" [73] on, and then r ne prior to bond bonding and s oid roughening waveguide too of such a thin, the garnet is d, as the dist during claddin lm permanent agnetic propert ne to being de tic field must op of the wave stance, it is im rucial since po the waveguide nimized by rem technique.A he drive curren nt density in th ure mechanism garnet substrat For a 1 mm lo t is needed to gating light.Th t with the predi Δφ = 90 degr ower is consum uide with the bon s of the electroma arger NRPS for a nner SGGG and m oils do not contrib the magnetization rate removal p 1 micron sh increased unifo dedicated etch , in which a de released using ding, then a th subsequent rele g the surface p o much.The f brittle garnet s deposited in tance between ng deposition.magnet that is ties, its remnan emagnetized.In t be transvers guide, as show mportant to p ower dissipatio .For the heter moving the sub multicoil geom nt [71].While he electromagn ms [72].The c te result in si ong waveguide obtain a phase he NRPS satur icted value in F rees is needed med, given the

Heteroge
Taking the d integrated mi microring bas image of the the electroma 4(a).The TE before the iso here in order t The transm First, the perf through the r Si/Ce:YIG wa 0.8 dB of inse 1520 nm to 1 of the isolator is 25 dB, and characteristics insertion loss nm.The prim the transition length of the S measured by to the electro propagation d backward pro dB optical iso The isolat isolation band terial with hig he garnet [76]

Conclusio
In conclusion lasers on silic silicon contin incompatible laser is proce bandwidth [7 n be as large a [46].For MZI herwise the iso der to match th agnet can be us depicts the M avelength due t d then slightly lation wavelen lockwise inject tral range (FSR an be used to of optical isola , as depicted fo e deviation of t io.The bandw oncern for this Fig. 1 for lig polari TE an Fig. 2 wafer wide w 3.2 Polarizat Most semicon operate for T polarization d polarization, t the case of wa garnet integra been demonst challenge for It has bee silicon nitride NRPS.Howe of the trench.[61], while pr the magnetic plane.For Ce (>2kOe) wou fields needed attractive, the practicality fo Alternativ rotators befor isolator to TE isolation.Po characterized, should have l tolerant in fa requirements comprises of and TE1 mod be achieved u [67], or an ad linear taper to the TE1 and vertical symm cladding of th Fig. 3 and (c taper broad The detail coupler (100 long) to conv designed, the polarization r fabrication to simplicity [58 or silicon nitr nded Ce:YIG and agnet increase the given current.(b) multi-coils.Adding bute as much to the in the material is process Fig. 5 The m and co nm fo

Fig. 5
(a s-section of the ation rotator d →TE rotator cou e the insertion l um is measured he polarization plus rotator (T input) of the 1550 nm, and < PER of the rota ng the residual 0 dB to 30 dB rization rotato r is 6.5 dB com ons to the loss ed Ce:YIG reg s waveguide, w polarized light ping the orien ight.A split i bserved for 40 wn in Fig. 5(c).image of the mic sion spectra of th nce waveguides.( ed current showing th is shown in GHz.Cascadin on and coerciv er such as silic magnetized loc es.Ideally, only ains the field [7 minating any st formed to dete cal isolator fo e previous sec rating for TE E mode to the b a), in which a 3 e ring resonato described in th uld also be pla loss and PER o d with a tunabl rotator is char TE input) with same geometry <2 dB of loss a ator is measure TE light in th across the wh or is attributed mpared to a S is caused by t gion.This can which is 2 mm l into the devic ntation of the in the resonan mA of curren croring isolator wi e isolator with no (c) The transmissi g 25 dB of optical i n Fig.