Open Access

High-Power, High-Linearity, HeterogeneouslyIntegrated III–V on Si MZI Modulators for RFPhotonics SystemsVolume 11, Number 2, April 2019

Paul A. MortonMichael J. MortonChong ZhangJacob B. KhurginJon PetersChristopher D. MortonJohn E. Bowers

DOI: 10.1109/JPHOT.2019.29039791943-0655 © 2019 IEEE

IEEE Photonics Journal Si MZI Modulators for RF Photonics Systems

High-Power, High-Linearity,Heterogeneously Integrated III–V

on Si MZI Modulators for RFPhotonics Systems

Paul A. Morton ,1 Michael J. Morton,1 Chong Zhang,2Jacob B. Khurgin ,3 Jon Peters,2 Christopher D. Morton,1

and John E. Bowers 2

1Morton Photonics, West Friendship, MD 21794 USA2University of California Santa Barbara, Santa Barbara, CA 93106 USA

3Johns Hopkins University, Baltimore, MD 21218 USA

DOI:10.1109/JPHOT.2019.29039791943-0655 C© 2019 IEEE. Translations and content mining are permitted for academic research only.

Personal use is also permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received February 11, 2019; revised March 1, 2019; accepted March 5, 2019. Date ofpublication March 8, 2019; date of current version March 21, 2019. This work was supported in partby the Air Force SBIR program “Silicon Photonics Foundry-Ready Integrable High-Performance Ana-log Optical Modulators,” under Award FA8650-19-C-1007, and in part by the DARPA STTR program“Miniature Silicon WDM Modulators for Analog Fiber-Optics Links,” under contact W91CRB-10-C-0099.Corresponding author: Paul A. Morton (e-mail: pmorton@mortonphotonics.com).

Abstract: In this paper, heterogeneously integrated III–V on silicon (III-V/Si) Mach–Zehnderinterferometer (MZI) modulators providing record results for both linearity and high-poweroperation are described. Devices include hybrid III–V/Si phase modulation sections in aSi MZI, and with a single-drive push-pull operation provide a spurious-free dynamic range(SFDR) as high as 112 dB·Hz2/3 at 10 GHz, comparable to commercial lithium niobatemodulators. Optical power levels up to 100 mW into the modulator provide no degradationin device linearity, with modulators demonstrating typical SFDRs of 110 dB·Hz2/3. TheseIII–V/Si MZI modulators, using a 500-nm “thick” Si layer for III–V integration, demonstrateapplicability for high-SFDR analog fiber-optic links without need for an erbium-doped fiberamplifier.

Index Terms: Mach Zehnder interferometer (MZI) modulator, integrated modulator, hetero-geneous integration, silicon photonics, high power, high linearity, high SFDR, microwavephotonics.

1. IntroductionHigh-Performance commercial and DoD analog fiber-optic links [1], [2], and RF Photonics systemsincluding these links, used for antenna remoting, for mobile phone backhaul systems, and for morecomplex analog processing systems such as optical up- and down-converting links [3], and opticallyprocessed phased array antennas [4], plus a wide variety of other analog applications, require ahigh-performance optical modulator to translate the analog RF Input signal onto an optical carrier.A typical schematic for a high performance analog fiber-optic link is shown in Fig. 1, including thekey photonic components; a high performance laser [5], the high-performance optical modulator,and a high-speed, high-power photodetector [e.g., 6].

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IEEE Photonics Journal Si MZI Modulators for RF Photonics Systems

Fig. 1. Analog Fiber Optic Link.

Lithium Niobate (LiNbO3) Mach Zehnder interferometer (MZI) modulators have been used in RFPhotonics systems for decades [1], [2], however, their high cost, large size, weight and power dissi-pation (SWaP), and limited performance, have prevented their widespread adoption. Performanceof these discrete LiNbO3 MZI modulators is limited by their low efficiency, i.e., high Vπ (typically6 V)–which is due to the low electro-optic coefficient of LiNbO3, together with maximum opticalpower handling capability for most commercial devices of around 100 mW. A major goal in the RFPhotonics field has been to find an optical modulator with many of the attributes of LiNbO3 MZImodulators, however, with far higher efficiency, i.e., low Vπ (e.g., 1 V to 2 V), and much lower costand SWaP. Recent work to develop thin-film LiNbO3 MZI modulators has made excellent progress[7], [8], through smaller mode sizes, but still has not overcome the major performance limitationof LiNbO3, its large Voltage.Length product, an order of magnitude higher than in III-V based andpolymer [9]–[11] devices. Additionally, it is difficult to integrate LiNbO3 and polymer devices withother integrated photonics components.

Semiconductor based devices offer the chance to obtain highly linear performance together withsmall size and low cost. They also offer the key ability to be integrated with additional componentsto form a photonic integrated circuit (PIC). High performance III-V MZI modulators have beendeveloped based on GaAs [12] and InP [13], with the InP devices offering the opportunity tocombine the modulator with other components (e.g., laser, photodetector) to develop PICs [14] forcommonly used telecom bands. The use of photonic integration to create subsystems with highperformance and also significantly reduced cost and SWaP has been a driving force in recent years,and particularly, the use of Silicon Photonics-taking advantage of CMOS foundries, processes,and many billions of dollars of previous investments. The use of heterogeneous integration ofIII-V and other materials onto the Silicon Photonics platform allows for the creation of very highperformance optical components [15], e.g., using III-V materials for optical gain in lasers, for highefficiency phase modulation in MZI modulators, and for high-power, high-speed photodetectors.Heterogeneous integration on Si also allows the integration of magneto-optic (MO) materials, suchas Ce substituted YIG (Ce:YIG), which can be used for optical isolators and circulators on the SiliconPhotonics platform [16]. Additionally, PZT can be utilized to create ultra-low power tuning capabilitiesfor integrated photonics components such as microring resonators, and phase control elements [17].A major goal, which would be disruptive to the RF Photonics field, is the development of a high-performance Transmitter (Tx)-PIC, that includes a high-power, ultra-low RIN laser, integrated with ahigh-performance optical isolator (high-isolation, low loss) and a high-linearity, high-optical powercapable modulator. Such a Tx-PIC is the ultimate goal of this research, which can be fabricated athigh-volume and low-cost on a Silicon Photonics platform including heterogeneous integration [15].

All-Si MZI modulators [18] have shown significantly worse linearity than LiNbO3 MZI modulators[19], due mainly to the fact that index modulation in Si is achieved by a relatively weak mechanism,charge injection, which is also strongly nonlinear. A novel idea to utilize the super-linearity of aring-based phase modulator within an MZI modulator, the Ring Assisted MZI (RAMZI) modulator,to balance/reduce the sub-linearity of the MZI structure, proposed by Xie and Khurgin [20], wasdemonstrated in an all-Si structure [21]. It improved upon the performance of standard all-Si MZIdevices, however, the performance of the all-Si RAMZI modulator still did not approach that of astandard LiNbO3 MZI modulator.

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In previous work by the authors, high-linearity Silicon Photonics based MZI modulators using het-erogeneously integrated phase modulation sections fabricated from hybrid III-V multiple quantum-well (MQW)/Si waveguides were demonstrated [22]. The use of hybrid III-V/Si phase modulationsections was chosen due to the lower nonlinearity of the III-V MQWs, together with the potentialfor higher optical power level operation, and their far higher efficiency compared to all-Si based de-vices [23], [24]. Those devices provided record spurious free dynamic range (SFDR) performanceof 112 dB.Hz2/3 for operation at 10 GHz, similar to commercial LiNbO3 modulators. However, dueto high on-chip coupling losses (∼9 dB/facet), they could not be tested at high optical power levelsat the time, and so it was not confirmed whether they could operate at the optical power levelsrequired for high-performance analog links. This paper addresses high optical power operation ofthe heterogeneously integrated III-V/SI MZI modulators through extensive device testing using ahigh power (1 Watt), polarization maintaining (PM) erbium doped fiber amplifier (EDFA). Additionalprevious work by the authors demonstrated III-V/Si RAMZI modulators with demonstrated SFDR at10 GHz as high as 117 dB.Hz2/3 [25], and suggested a further improvement in device linearization,using a novel Grating Assisted Michelson interferometer (GAMI) modulator [26].

2. Analog Fiber Optic LinksIn order to assess the performance requirements of an optical modulator for use in a high-performance analog fiber-optic link, the complete fiber optic link must be carefully analyzed. Analogfiber optic links require a high dynamic range, or high SFDR, achieved in part through the useof highly linear components. A key element, and often the limiting component in these systemsregarding linearity, is the optical modulator; this transfers a microwave input signal, often from anantenna, onto an optical carrier signal. The linearity of the system is usually set by the linearityof the modulator. The other key element of system dynamic range is noise; the laser generatesrelative intensity noise (RIN), other noise components being the shot noise at the photodetector andthe thermal noise of electronic devices within the link. Optical loss within the system significantlyreduces the signal, a 1 dB increase in optical loss providing a 2 dB decrease in the RF output power,significantly reducing SFDR. The overall dynamic range of the system is well characterized by itsSFDR, which includes both linearity and noise in a practical measurement of the useful dynamicrange of a microwave system.

The analog fiber optic link shown in Fig. 1 includes;a) High-Power, low-RIN laser with an Optical Isolator to eliminate the effects of optical back-

reflections.b) RF Input, from antenna or other system, often with a low noise amplifier (LNA) to increase RF

signal power.c) High-Linearity MZI modulator to convert the electrical RF signal into an optical signal; with

High-Efficiency (low Vπ) to limit electrical to optical conversion loss, High-Bandwidth to supporthigh frequency system operation, and High-Power to enable high laser power and thereforeHigh System SFDR.

d) Low-Loss Optical Fiber Link.e) High-Speed, High-Power, High-Linearity, and High-Efficiency photodetector, to provide the

Output RF signal.Using an internal Morton Photonics simulation tool for analog fiber optic links, including param-

eters from high-performance optical components [e.g., a Morton Photonics hybrid laser providing100 mW output power and RIN �−165 dBc/Hz [5], an MZI modulator with Vπ of 2 V and 2 dBexcess loss, an optical fiber link with 1 dB loss, and photodetector with 0.8 A/W responsivity],example SFDR and noise figure (NF) values versus laser power curves were calculated, which areshown in Fig. 2. The simulated system in this case uses Intensity Modulation, plus it has a singleoptical fiber and single photodetector, i.e., no balanced photodetector for RIN reduction. The plotshows SFDR and NF as the laser power is increased from 10 to 20 dBm (10 mW to 100 mW), andtraces are shown for different laser RIN values, the RIN reduced from −155 dBc/Hz (good DFB) to−175 dBc/Hz (ultra-low RIN). In the ‘near shot noise limited’ case, with −175 dBc/Hz RIN, SFDR

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IEEE Photonics Journal Si MZI Modulators for RF Photonics Systems

Fig. 2. SFDR, NF vs. Laser Optical Power at 3 Laser RIN levels; −155, −165 and −175 dBc/Hz.

increases by 6 dB as the optical power is increased from 10 to 20 dBm, while NF is reduced by 9 dB.When RIN has a strong effect, i.e., at −165 dBc/Hz, or when RIN is dominant, i.e., at −155 dBc/Hz,the improvements obtained from increasing the laser power are much smaller. Very little systemimprovement is seen at all for the laser with −155 dBc/Hz RIN when increasing the laser powerfrom 10 to 20 dBm, because the system performance is already laser RIN limited.

Fig. 2 demonstrates the need for high laser power levels in a high SFDR analog optical link, withdiscrete systems often including an EDFA to boost the laser power in the link. It also demonstratesthat in order to take advantage of the high laser optical power, the laser must have (a) extremely lowRIN, or (b) RIN reduction through the inclusion of a balanced photodetector, or (c) an alternativemodulation format must be used that reduces the effect of laser RIN. Using a Tx-PIC device, noEDFA is possible to increase optical power levels in the link, and so a high-power laser, e.g.,20 dBm/100 mW or more should be used, however, the high-performance integrated modulatormust be able to support this high power level.

3. III–V on Silicon Heterogeneous MZI Modulator Device DesignA schematic of the heterogeneous III-V/Si MZI modulator used in this work [22] is shown in Fig. 3(a).Input light from the laser enters the waveguide at the bottom left of the device, is split at a 50/50coupler before entering the two arms of the MZI, which are combined at a second 50/50 couplerto provide two outputs for the device. The input and output waveguides are tapered to expand thewaveguide width from 600 nm within the device to 5 μm at a facets (not shown in schematic),allowing for easier alignment (in the X axis) and lower reflections, however, giving ∼9 dB in couplingloss per facet. This 9 dB input coupling loss, when using a tunable laser with +18 dBm maximumoutput provided ∼9 dBm (8 mW) into the MZI modulator itself [22]. While record results weredemonstrated, with SFDR as high as 112 dB.Hz2/3, the maximum optical power level at which thishigh SFDR could be achieved was not determined. In this paper a 1 Watt PM EDFA is used to allowthe power level at the MZI input to be increased to 100 mW, to measure the modulator performanceat a high power level; required for a high-performance analog optical link, and also the maximumpower level expected in a PIC device.

Fig. 4 shows a cross-section of the hybrid III-V/Si phase modulation section, made up of a lowerSi ridge waveguide which is 500 nm in thickness, with the III-V material wafer/die bonded onto theSi waveguide before the III-V layer is itself processed to provide the final structure. Fig. 4 includes

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IEEE Photonics Journal Si MZI Modulators for RF Photonics Systems

Fig. 3. Schematic of the heterogeneous III-V/Si MZI modulators.

Fig. 4. A cross-section of the device, with intensity contour of the fundamental TE mode.

Fig. 5. Mask Layout of two MZI modulator devices.

intensity contours to show the mode profile of the TE mode of the device, demonstrating that alarge proportion of the optical mode is in the III-V material, to provide a high efficiency modulator.The percentage of the optical mode in the III-V material can be chosen by design of the width ofthe Si ridge waveguide, the mode position moving up or down within this structure depending onthe ridge width; if required, the mode can be moved almost entirely into the III-V layers, effectivelyproducing an InP-based modulator. This modal design, coupled with the thick (500 nm) Si layer thatis required in these heterogeneous devices, compared to much thinner 220 nm Si used in a SiliconPhotonics Foundry, plus the use of an undoped/low loss Si waveguide, provides the much higherpower handling capabilities of these devices.

A large number of device designs were included in the fabrication run, from which the mask layoutfor two closely spaced MZI modulators is shown in Fig. 5. Two tapered, angled waveguides, onthe left of the mask layout show the two inputs for each device-these lead to a (thermally) tunable50/50 coupler in the top device, which can be tuned to achieve the optimum 50/50 splitting ratioeven as different optical wavelengths are used with the device. The two outputs from the tunable50/50 coupler pass through the active part of the device, the III-V/Si phase modulation sections,before being recombined at an output 50/50 coupler. The devices include phase control sections onboth arms of the MZI, made from heaters to thermally tune the phase of these waveguide sections,to allow easy bias of the modulator to a quadrature bias point for intensity modulation. The longwaveguide lengths and horizontal offset between adjacent devices in the mask layout allow for alarge number of devices to be placed in a single die, with inputs on the left and outputs on the right.

To assess modulator linearity, measurements of the SFDR of the device were carried out at awavelength of 1550 nm and at a modulation frequency of 10 GHz, as described in [19]. An EDFAwas used to overcome the loss of the two fiber couplings (∼18 dB total), plus to provide sufficientoptical power at the photodetector (PD) to support high SFDR operation. This 18 dB loss will

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Fig. 6. Measured SFDR for III-V/Si MZI modulator with a 100 μm phase modulation sections; 52 dB for1 GHz Bandwidth, i.e., 112 dB·Hz2/3.

not be present in the final device, and the link will not include an EDFA – which would increasenoise. The requirement to use an EDFA limited the wavelength range over which SFDR could bemeasured, limiting the possibility to further improve the performance by using a shorter wavelengthand increasing the Quantum Confined Stark Effect (QCSE).

For initial measurements, described in this section, a tunable laser with a +18 dBm maximumoptical output power was used to provide the optical power into the modulator device. High opticalpower measurements described in the following section, using this tunable laser followed by a PMEDFA with a maximum output power of 1 W, obtain a high (100 mW, +20 dBm) optical power directlyinto the Si MZI structure. Values for SFDR were measured versus variations in the input opticalpower level to the MZI, versus modulator p-n junction dc bias voltage, and for different photodetectorcurrents. A series of SFDR traces was taken, with the highest SFDR occurring for the highest opticalinput power level (+18 dBm laser output) and also highest photocurrent (15.6 mA), for a DC biasvoltage of −2 V. The measured response in Fig. 6 shows the highest measured SFDR; in thiscase the plot has an SFDR of 52 dB for 1 GHz bandwidth, which corresponds to an SFDR of112 dB·Hz2/3.

Additional results from [22] include a modulation efficiency Vπ.L of about 3 V.mm at 1550 nm,an order of magnitude smaller than typically seen in Lithium Niobate. The modulator used in thesemeasurements had relatively short phase modulation sections, each 100 μm in length, providing amodulation bandwidth of at least 20 GHz.

4. High Power OperationA photograph of the experimental setup used to measure the III-V/Si MZI modulator devices isshown in Fig. 7. The long modulator die, which is also shown on the right of Fig. 7, has manydevices on it, and is placed on top of a temperature controlled heatsink, at an angle to optimizethe coupling from the lensed optical fibers on the input and output of the device. A lensed PMoptical fiber is aligned for injection of TE polarized light into the modulator device, using the rotatingfiber holding mount on the left, whereas a lensed standard single mode fiber (SMF) is coupledto the output of the device on the right. The main probe shown on the left of the device providesmultiple DC biases for the device, plus a high-speed Ground:Signal:Ground (G:S:G) connection tothe modulator which provides push-pull drive [27]. The high-speed RF probe on the right was usedin some of the measurements to provide a 50 � load for the modulator. The device is on a mountthat is temperature controlled to 25 ˚C using a thermo-electric cooler (TEC), 10 k� thermistor anda TEC controller.

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Fig. 7. Experimental setup, showing high speed/multiple dc probes, additional RF probe. Photo ofmodulator die on right.

Fig. 8. Fiber coupled output power after modulator device versus in-fiber modulator input power fromthe booster EDFA.

For the high optical power measurements the tunable laser is set to 1550 nm, with +6 dBmoutput, the PM optical fiber output passed into a high power EDFA (Nuphoton EDFA-CW-PMHP-C0-PR-30-27-FCA+30 dBm) obtained for this experiment, which itself has a PM fiber output tomaintain control of polarization before coupling the laser light into the modulator. The modulatordevice output is amplified again to overcome the output coupling loss of 9 dB, and this signal isfiltered to reduce Amplified Spontaneous Emission (ASE) from the EDFA, before attenuating thesignal to provide ∼15 mA photodetector bias current for all of the measurements.

An exhaustive series of two tone measurements was carried out at 10 GHz in order to measureSFDR versus the optical power injected into one input of the MZI modulator. The optical inputpower connected to the lensed PM fiber was varied from 9 dBm up to 29 dBm, so that with an inputcoupling loss of 9 dB, the power entering the modulator input arm was varied from 0 dBm (1 mW)up to 20 dBm (100 mW). At each optical power level, and also across a range of modulator biasvoltages (from −1V to −3.5 V in 0.5 V steps), the SFDR was measured twice using two differentRF power ranges, +1 to +4 dBm per tone, and −3 to 0 dBm per tone (applied to modulator).Very similar results were obtained for both power ranges, however at some lower bias voltages thehigher drive power (+4 dBm) affected the results, due to the higher voltage swing increasing themodulator voltage closer to 0 V, where its response is less linear.

Significant care was required at the two highest optical power levels used in the experiment;29 dBm and 27.75 dBm, corresponding to 100 mW and 75 mW directly into the modulator, asthe optical power when optimally aligned to the input waveguide caused absorption within the

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Fig. 9. Phase Modulator bias current vs Modulator Bias Voltage and Optical Input Power to MZI.

waveguide that heated the device so that it physically moved, therefore changing the coupling,and the TEC loop controlling the mount temperature then oscillated on ∼30 second period. A veryslow alignment/peaking process, using a PZT controller, was required in order to optimize couplingand allow stable measurements to be taken over long periods of time – as is required for SFDRmeasurements.

A series of measurements was taken of the power coupled out of the modulator versus the outputpower of the booster PM EDFA that provides the input to the modulator, with points taken acrossthe range of modulator bias voltages at each EDFA power. These measurements show that overthe entire power range there is a linear variation of output power to input power, with about 22 dBdifference in power level; this includes ∼18 dB coupling losses, 3 dB for quadrature biasing, andaround 1 dB excess loss in the short modulator used in these experiments (phase modulators of100 μm length each [22]). The short modulator length would tend to accentuate any potential issuewith high optical power levels, as the whole modulator sees this high power level, whereas a longmodulator would see higher power at the input side and lower power on the output side due tooptical attenuation along the phase modulator section.

The bias current in the phase modulators was measured by the Keithly 2450 source measureunit (SMU) providing bias; set as a voltage source and measuring bias current. This bias currentis plotted in Fig. 9 versus the modulator bias voltage and also the optical power level entering themodulator input (after the 9 dB coupling loss). Of note is that the maximum current is less than1 mA, even for 100 mW input power and maximum reverse bias of −3.5 V, and so effects from highphoto-generated current as seen in some high-power photodetectors should not be an issue.

A large number of measurements of the SFDR versus both modulator bias voltage and theoptical power level into the modulator are shown in Figs. 10 and 11. The variation in SFDR versusmodulator bias voltage, shown in Fig. 10 for a large range of input optical power levels from 4 mWup to 100 mW, demonstrates high SDFR values over a very large range of these parameters, withSFDR >110 dB.Hz2/3 seen for bias voltages between −1.5 and −3 V. The highest SFDR valuesare seen between −1.5 and −2 V, which is why an additional series of tests was carried out atthe intermediary −1.75 V. Some spread was seen in this large number of SFDR measurements asfor each set of bias parameters the RF values were measured and SFDR calculated automatically.Over the large range of power levels there is very little change in the optimum SFDR achieved, andpeak results of 110 dB.Hz2/3 are seen at all power levels.

The measured SFDR values in Fig. 11 clearly demonstrate that the highest measured SFDRvalues are not modulator input power dependent, with peak values in all cases being above, orclose to for the 75 mW measurements, an measured SFDR of 110 dB.Hz2/3. The optimum biasvoltage does vary slightly with the optical power level, as shown in Fig. 10.

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Fig. 10. SFDR vs Modulator Bias Voltage and Modulator Input Power.

Fig. 11. SFDR vs Modulator Input Power (for a wide range of bias options).

5. ConclusionResults in this paper demonstrate that heterogeneously integrated III-V/Si MZI modulators canachieve high SFDR performance when operating at high optical power levels, up to 100 mW inputinto the integrated modulator. This is the maximum power level expected in a Tx-PIC device,and therefore this result demonstrates the applicability of these devices in high performance DoDand commercial applications. Over a wide range of input optical power levels the device providesan SFDR of 110 dB.Hz2/3 at a 10 GHz operating frequency, far higher than demonstrated in otherdevices with the capability to be part of a Tx-PIC device, i.e., to be integrable with a high performancelaser and isolator.

We believe that this is the first demonstration of an integrable high performance analog modulatorthat can provide high SFDR operation and operate at the required high operating power level to avoidthe need for an EDFA in an analog fiber optic link. This device can enable high performance, Tx-PIC based analog fiber optic links with very low cost and SWaP, enabling future DoD RF Photonicssystems as well as playing an important role in future 5G cellular networks.

AcknowledgmentThe authors would like to thank J. Morton, Z. Zhang, and Y. Wang for their useful discussions.

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