High-performance electro-optic modulators …specificpolymers.fr/medias/publications/PMMA_DR1_NLO_EN.pdfOrganic Photonic Materials and Devices XII, Proceedings of SPIE, Vol. 7599,

________________________________________________________________________________________________ Organic Photonic Materials and Devices XII, Proceedings of SPIE, Vol. 7599, 759901

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Invited Paper

High-performance electro-optic modulators realized with a commercial side-chain DR1-PMMA electro-optic copolymer

Sébastien Michela, Joseph Zyssa, Isabelle Ledoux-Raka, Chi Thanh Nguyena

a Laboratoire de Photonique Quantique et Moléculaire, UMR CNRS 8537, Institut d'Alembert, Ecole Normale Supérieure de Cachan, 61 avenue du Président Wilson, 94235 Cachan Cedex, France

ABSTRACT

Several high-performance polymeric electro-optic modulators have been demonstrated in the last decade. Most of them have been elaborated using specific high-performance electro-optic polymers designed for their exceptional electro-optic response and their thermal stability. In this paper we report the high performance of electro-optic modulators made of a commercial side-chain electro-optic copolymer DR1-PMMA as the active core material and of a passive epoxy polymer NOA73 as cladding material. The electro-optic polymer used in these modulators is a Disperse Red 1- poly-methyl-methacrylate (DR1-MMA) side-chain copolymer with relative molar concentrations of DR1-substituted (resp. MMA unsubstituted) groups equal to 30 % (resp. 70%). We have designed, elaborated and tested phase modulator and push-pull Mach-Zehnder modulators in order to optimize their figure of merit Vπ.L. A push-pull Mach-Zehnder modulator with 2 cm-long electrodes and an inter-electrode distance of 8.8 µm displays a half-wave voltage of 2.6 V at 1550 nm, corresponding to a figure of merit of 5.2 V.cm. This result was obtained with a moderate poling electric field of 75 V/µm applied to the core of the modulator waveguide. We report here the best figure of merit which has never been observed in a modulator realized with a commercial side-chain electro-optic polymer.

Keywords: Electro-optic polymer, Polymeric optical waveguide, Electro-optic modulator

1. INTRODUCTION High bit rate and high-capacity optical telecommunication networks require high-performance electro-optic modulators. The introduction of electro-optic polymers via a molecular engineering approach aiming at the optimization of second order optical nonlinear organic materials has triggered the emergence of a new generation of all-polymeric or hybrid electro-optic modulators1,2,3,4,5. Like other electro-optic materials used in optical telecommunications devices, polymers show specific advantages as compared to inorganic materials, not only in term of driving voltage5 and electrical bandwidth6 but also in fabrication technology7,8,9, which permits to envision a mass production of high-performance and low-cost electro-optic components. These specific high-performance electro-optic polymers have been designed, tailored and introduced from advanced researches of molecular engineering at the combine outcome of converging both theoretical10 and synthetic research efforts levels1. However, these materials are broadly not available as yet as fully commercial polymer products. As a consequence, however promising, such outstanding outcomes of molecular engineering cannot be used at the relevant scale that would be desirable towards the worldwide development of polymeric electro-optic modulators. On the other hand, the commercially available Disperse Red 1-methyl-methacrylate (DR1-MMA) side-chain electro-optic copolymer used as a core material and passive epoxy polymer NOA73 as cladding material can be easily purchased and are used in our electro-optic devices. This side-chain electro-optic copolymer is synthesized and commercialized by the company Specific Polymers11, while the passive epoxy polymer NOA73 is commercialized by the company Norland Products Incorporated12.

In this paper we report the design and realization of different types of polymeric electro-optic modulators as well as their performances measured at the 1550 nm telecommunication wavelength. We have conceived, fabricated and demonstrated various modulator architectures, such as phase, conventional Mach-Zehnder, push-pull Mach-Zehnder and loop structure push-pull Mach-Zehnder modulators. This last configuration permits to divide the half-wave voltage Vπ by a factor of four as compared to conventional Mach-Zehnder modulators13. In our study, we aim at the optimization of the Vπ.L modulator figure of merit, where L stands for the driving electrode length. A push-pull Mach-Zehnder modulator


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with 2 cm-long electrodes and an inter-electrode distance of 8.8 µm displayed a half-wave voltage of 2.6 V at 1550 nm, corresponding to a figure of merit of 5.2 V.cm. This result was obtained with a moderate poling electric field of 75 V/µm in the core of the modulator waveguide. We achieved the best figure of merit ever reported in a modulator realized with a commercial side-chain electro-optic polymer. The figure of merit of this polymeric modulator is better than that of lithium niobate electro-optic modulators14,15. This performance may question the values of the electro-optic coefficient r33 of the copolymer DR1-PMMA previously measured in previous studies, as in our case the corresponding r33 values must be significantly higher than those already published in the literature. We also tried to realize a high-frequency electro-optic modulator with the same configuration of polymeric core and cladding. The first results obtained with a phase modulator evidence an electrical bandwidth of 4 GHz. The bandwidth limitation is due essentially to the electrical connection between the electrodes and the output connectors of the modulator.

In the following section we will describe first the properties of the electro-optic copolymer DR1-PMMA used as core and the passive epoxy NOA73 used as cladding materials of the waveguide. We will then discuss the design and the fabrication of the different configurations chosen for our modulators as well as their properties. We will also show measurement results of low-frequency and high-speed modes of operations of these modulators. Finally, we will discuss the performances and perspectives of this family of polymer-based devices.

2. PROPERTIES OF ELECTRO-OPTIC COPOLYMER AND CLADDING MATERIALS 2.1 Side-chain electro-optic copolymer DR1-PMMA

The electro-optic polymer used in ours modulators is a Disperse Red 1- poly-methyl-methacrylate (DR1-MMA) side-chain copolymer. We studied two different kinds of DR1-PMMA copolymers: the first (resp. second) one corresponds to a 30/70 (resp. 50/50) molar ratio between DR1-functionalized and unfunctionalized MMA monomers. The chemical structure of the DR1-PMMA copolymer is shown in Figure 1. The synthesis of this electro-optic side-chain copolymer has been widely reported 16,17. It is well known that, contrary to DR1-PMMA guest-host polymer, this electro-optic side-chain copolymer displays a much better photo- and thermo-stability and can be considered as a suitable long-term stable polymer for photonic devices.

Figure 1. Chemical structure of the DR1-PMMA copolymer.

The optical refractive indices of these two types of DR1-PMMA copolymers were measured using spectroscopic ellipsometry. At 1550 nm, the refractive indices of DR1-PMMA (30/70) and DR1-PMMA (50/50) are 1.605 and 1.637, respectively. The Tg glass transition temperature for each copolymer was measured by using the Differential Scanning Calorimetry method. The Tg temperatures of DR1-PMMA (30/70) and DR1-PMMA (50/50) are 125°C and 132°C, respectively. DR1-PMMA copolymers are normally optically isotropic and statistically centrosymmetric at the relevant wavelength scale towards the manifestation of quadratic effects. In order to induce in this material any type of quadratic optical nonlinearity, such as for example the Pockels effect, it must be deprived of centrosymmetry. This can be achieved by thermo-assisted electrical poling of the active DR1 chromophore. We used parallel plate electrical poling to induce an


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electro-optic coefficient r33 in DR1-PMMA copolymers. Recent measurements17 reported on this electro-optic coefficient for different kinds of DR1-PMMA copolymers claimed relatively modest values for this material: at 1550 nm, considering an electric poling field of 120 V/µm, the r33 electro-optic coefficients of DR1-PMMA (30/70) and DR1-PMMA (50/50) are 8.6 pm/V and 12.6 pm/V, respectively.

2.2 Waveguide Claddings

We used the UV curable epoxy polymer NOA73 as lower and upper cladding layers of waveguides. Investigations of the physico-chemical properties of the core-claddings polymer couples has shown an excellent compatibility between the respective materials, then making the fabrication process much simpler wile maintaining a very good quality of the corresponding devices. This epoxy polymer is completely transparent at 1550 nm. Refractive index of cured NOA73 measured by spectroscopic ellipsometry at 1550 nm is 1.549. This epoxy displays a moderate viscosity of 130 cps at 25°C which facilitates direct deposition on inorganic materials or organic thin films by spin-coating. In fact, as the NOA73 epoxy can be used as a pure liquid (i.e. not dissolved in an organic solvent), it can be directly deposited on the DR1-PMMA copolymer thin films with the required thickness. This avoids possible interaction between the copolymer and the solvent that would otherwise be needed for cladding deposition. The surface state of NOA73 epoxy thin film after curing is very satisfactory in comparison with that of other UV curable epoxies such as NOA61 and NOA65. For these reasons, we chose NOA73 as a cladding material for the electro-optic waveguides.

2.3 Resistivities of electro-optic copolymers and claddings

The electro-optic response of poled polymers, represented by the value of electro-optic coefficients r33 and r13, depends on the poling electric field actually present at the core of the waveguide, namely core poling field. This poling field is chosen to be as large as possible and is carefully adjust to be weaker that the voltage corresponding to the dielectric breakdown of the polymer. To optimize the core poling field, the resistivity of the core at its glass transition temperature Tg must be much higher than that of claddings at the same temperature18.

Figure 2. Resistivities of some passive polymers and DR1-PMMA electro-optic copolymers as a function of applied

voltage. The DR1-PMMA (30/70) copolymer was measured at 120°C while the DR1-PMMA (50/50) and other passive polymers were measured at 130°C.

We measured the resistivity of the electro-optic side-chain copolymers DR1-PMMA and of some passive cladding polymers at a temperature near the glass transition temperature Tg of DR1-PMMA. In order to perform this measurement, the polymeric thin film sample is deposited between two gold electrodes as in the case of poling. This sample is set in a thermostatic container whose temperature is controlled by a computer. The measurement of applied voltage and electric current between two electrodes is ensured by a high resistance meter (Hewlett-Packard 4339B), also


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controlled by a computer. The measurement is carried out at the Tg temperature of polymer sample as a function of the values of applied electric field. The resistivities of the DR1-PMMA (50/50) copolymer and of different passive polymers which can be used as claddings for waveguides, have been measured at 130°C, as a function of the poling voltage and represented in Figure 2. The resistivity of the DR1-PMMA (30/70) has been measured at 120°C. We observe that, close to the Tg value of the DR1-PMMA (30/70) copolymer, the resistivity of the cladding polymer NOA73, under a poling field of 80V/µm, is at least one order of magnitude lower than that of the DR1-PMMA (30/70) copolymer in the same conditions. With a low poling field, the resistivity of the NOA73 polymer is two orders of magnitude lower than that of the DR1-PMMA (30/70) copolymer. On the other hand, we also observe that with the 80V/µm poling field, the resistivity of the NOA73 polymer is slightly lower than that of the DR1-PMMA (50/50) copolymer. Therefore the triple layers cladding/core/cladding of NOA73/DR1-PMMA (30/70)/NOA73 is expected to display a higher electro-optic response using parallel plate core poling than NOA73/DR1-PMMA (50/50)/NOA73. However, with a low poling field, because the resistivity of the NOA73 polymer is much lower than that of the DR1-PMMA (50/50) copolymer, the triple layer NOA73/DR1-PMMA (50/50)/NOA73 architecture could induce an acceptable electro-optic response of the core. But this case is not very interesting because the low core poling field cannot induce a high value of electro-optic response in the core. For this reason we have chosen the triple layers NOA73/DR1-PMMA (30/70)/NOA73 structure for our modulators.

Additionally, from this measurement we also observe that amongst the cladding polymers, the resistivity of NOA65 is the lowest. In fact, the resistivity of this cladding polymer is two orders of magnitude lower at high poling fields and three orders of magnitude lower at low poling fields than that of the DR1-PMMA (30/70) copolymer. In comparison with the resistivity of the DR1-PMMA (50/50) copolymer at a high poling field, the resistivity of NOA65 is at least one order of magnitude lower. Therefore, either the triple layer NOA65/DR1-PMMA (50/50)/NOA65 or NOA65/DR1-PMMA (30/70)/NOA65 architectures can be considered as the best configuration for optimizing the core poling field and consequently the electro-optic response of the core. We had investigated this configuration, but the surface quality of NOA65 cladding epoxy is very bad. The viscosity of this epoxy resin is 9 times higher than that of the NOA73 epoxy (the viscosity of NOA65 epoxy is 1200 cps at 25°C). It is then very hard to deposit directly by spin-coating a NOA65 cladding layer with an acceptable quality of surface. We have also tried to lower the viscosity of the NOA65 epoxy using different organic solvent but the obtained results were not better. Consequently, the configuration with NOA65 cladding has lead to a poor quality of waveguide and of the parallel plate electrodes for modulators.

2.4 Dielectric constants of electro-optic copolymers and claddings at low frequency

In dynamic operation of polymeric electro-optic modulators, its half-wave voltage Vπ depends on the voltage drop across the core layer for the three-layer waveguide structure. A greater voltage drop across the core layer induces a lower Vπ. In order to maximize the voltage drop across the core layer of the waveguide, the cladding materials must display a higher dielectric constant than that of the core material18. We measured at room temperature the dielectric constants of the core material (DR1-PMMA (30/70) copolymer) and of the cladding material (epoxy NOA73) by a set-up similar to that cited in [19]. We used the Hewlett-Packard 4192A-LF impedance analyzer to make the measurement within a frequency range from DC to 13 MHz. The dielectric constant values obtained from this measurement are 4.1 (resp.3.8) for the NOA73 epoxy (resp. for the DR1-PMMA (30/70) copolymer). We have then chosen an optimized core-cladding material couple for optimizing the half-voltage Vπ in dynamic operation of our modulators.

3. DESIGN AND FABRICATION OF POLYMERIC ELECTRO-OPTIC MODULATORS 3.1 Waveguide design and fabrication

In our modulators, we have chosen the rib waveguide configuration for wave-guided circuits. The rib geometry of waveguide shows several advantages to optimize the operation of electro-optic polymeric modulators such as single-mode operation control, determination of an optimum mode profile to match a specific input/output waveguide, reduction of the Vπ.L product and also the reduction of the waveguide bending loss 20. The structure of the rib waveguide used in polymeric electro-optic modulators is shown in Figure 3 below. Waveguides used in modulators were designed to be single-mode operation waveguides.

The thickness of lower and upper cladding must be optimized in order to avoid losses induced by plasmonic excitations from the evanescent waves of the core and also to reduce the interelectrode distance and then the half-wave voltage Vπ of modulators. The width of the upper electrode we is typical equal to 20 µm.


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In order to optimize and to select the best configuration of waveguides, a numerical simulation of mode propagation in waveguides with different structures was carried out. We made the simulation of propagation using the beam propagation method (BPM) and the finite-difference time-domain (FDTD) method by using the commercial softwares Optiwave BPM and Optiwave FDTD from the Optiwave. Results obtained from this numerical simulation clearly conclude to a single-mode operation and moreover permitting to choose a suitable structure for waveguide fabrication.

Figure 3. Structure of rib waveguide used in polymeric electro-optic phase modulators. Thickness of lower cladding dl (epoxy NOA of refractive index 1.549 @ 1550 nm) : 3.5 to 4 µm; total thickness of the core H (DR1-PMMA (30/70) electro-optic copolymer of refractive index 1.605 @1550 nm) : 1.7 to 2.2 µm; rib height h of the core : 0.4 to 0.9 µm; width wg of the rib waveguide : 6 µm to 7 µm; thickness of upper cladding du (epoxy NOA of refractive index 1.549 @ 1550 nm) : 3.5 to 4 µm.

The typical structure of modulators was elaborated on a silicon substrate (Si wafer), covered by a lower electrode deposited by evaporation. This titanium and gold electrode issued as an electrical ground plane in modulators. Its thickness ranges from 0.5 µm to 1 µm, depending on low or high frequency operation of modulators. On the lower electrode was then deposited by spin-coating the lower cladding of waveguide in NOA73 epoxy. After spin-coating this thin film was UV cured at 75°C about twenty minutes. Then it was baked at 120°C during two hours. The core of the waveguide in DR1-PMMA (30/70) copolymer was spin-coated on this lower cladding. The thin film was then baked at 120°C during two hours. The patterns of optical waveguide core were transferred to this thin film from an optical mask using conventional photolithography with a mask aligner. Then the geometry of waveguide core was etched by reactive ion etching with oxygen plasma. The upper cladding of NOA73 epoxy was then deposited on the DR1-PMMA (30/70) copolymer etched core by using the same process as for the lower cladding. Finally, an upper gold electrode was deposited by evaporation. Its thickness ranges from 0.2 µm to 2 µm, depending on the configuration of poling electrodes or drive electrodes. The gold upper electrode was patterned by photolithography and wet etched with potassium iodine (KI) solution.

3.2 Phase modulators

The structure of phase modulators is shown in Figure 3. This is the simplest possible structure for electro-optic modulators. Firstly, we had realized phase modulators in order to optimize the structure of optical waveguide and also to evaluate the electro-optic response of the core and consequently the figure of merit of device. At low frequency operation, when the travelling-wave configuration of the modulator is not taken into account, we can use the poling electrode of phase modulators as the drive electrode. In order to achieve the electrical poling of the waveguide core, we used thermo-assisted poling whose the diagram process is shown in Figure 4. To minimize poling-induced losses when the poling electrodes are in air and consequently induce a significant change in the mode confinement after poling21, device poling was made under a dry nitrogen flow. Firstly the device was preheated at 120°C during 20 minutes under a nitrogen flow in order to purge the oxygen dissolved in the polymer matrix and to avoid oxygen-current-induced degradation. After this step, the temperature was set at the Tg of the core DR1-PMMA (30/70) copolymer and the poling voltage was applied. The poling voltage was applied during 30 minutes at Tg, always under nitrogen flow. After this time, the device was cooling to the room temperature while the poling voltage was maintained until the end of poling process.

When phase modulators are used as intensity modulators, their half-wave voltage is expressed as


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( )333 13

dV = LΓn r - rπ

λ (1)

where λ is the working wavelength, d is the inter-electrode distance, L is the electrode length, Γ is the overlap integral between the modulating electric field and the confined optical field in the waveguide, n is the refractive index of the poled core, r33 is the electro-optic coefficient following the symmetry axis of the poled core and r13 is the transverse electro-optic coefficient following the direction orthogonal to the symmetry axis of the poled core.

Figure 4. Time diagram of electrical poling process.

3.3 Push-pull Mach-Zehnder modulators

Figure 5. Scheme of a polymer-based push-pull Mach-Zehnder modulator.

Mach-Zehnder modulators display the same waveguide structure as phase modulators. The total branching angle of the Y-branch of the interferometer was about 1° and the separation between the two straight waveguides in the interaction region was 50 µm. The width of the poling electrode was 20 µm. In order to study different modulator configurations, we elaborated various devices with 4 different lengths of poling electrodes, i.e. 12 mm, 17 mm, 20 mm and 22 mm respectively. The poling process was the same as that used for phase modulators. In the case of a conventional Mach-Zehnder intensity modulator, one arm of the Mach-Zehnder interferometer has to be poled (modulator operating with a single drive electrode). If the modulator operates in a quasi-TM mode, the half-wave voltage Vπ can be written as


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333

dV = LΓn rπλ

(2)

When a Mach-Zehnder modulator operates with a dual drive electrode set corresponding to a 180° phase shift of the modulating signal, it is called push-pull Mach-Zehnder modulator, and its half-wave voltage Vπ

(pp) is equal to that of a conventional Mach-Zehnder modulator divided by 2.

(pp)3

33

V dV = = 2 2LΓn rπ

π λ (3)

By measuring the half-voltage of Mach-Zehnder modulator we can then infer an estimate of the value of electro-optic coefficient r33 from equations (2) or (3).

Figure 6. Structure of push-pull Mach-Zehnder electro-optic modulators: (a) cross section of modulator with poling

electrodes; (b) overview of poling electrodes structure; (c) drive electrode structure.

In polymeric Mach-Zehnder modulator, we realized this 180° phase shift between the two arms of interferometer by orienting the NLO molecules (DR1 chromophore) in opposite direction (Figure 5). This was realized by push-pull poling, using two different upper poling electrodes, each situated on each arm of interferometer, by the same electric field, but in opposite field directions with respect to the ground (lower electrode). Figure 6(a) and 6(b) show respectively


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a cross section of modulator with poling electrodes and an overview of poling electrodes structure. The poling process was the same as that used for phase modulators.

In order to achieve push-pull operation of a polymeric Mach-Zehnder modulator, the same modulating voltage applied on two arms of the interferometer is required. Towards this driving operation, subsequent to the poling step, the upper poling electrodes of the modulator have to be cleared by wet etching prior to deposition and patterning of the drive electrode (Figure 6(c)). At low frequency operation, when the impedance matching of the drive electrode with the modulating signal generator is achieved over the whole electrode length, we were able to use poling electrodes as driving ones by connecting these two upper poling electrodes together.

3.4 Loop structure of the push-pull Mach-Zehnder modulators

(a)

(b)

Figure 7. Loop structure of push-pull Mach-Zehnder electro-optic modulators: (a) structure of the modulator ; (b) optical microscope images of parts of the modulator (bright lines are poling electrodes and grey lines are optical waveguides.

If the optical signal in a push-pull Mach-Zehnder is further designed to ensure that the carrier beam passes twice in each arm of the interferometer, the half-wave voltage Vπ

(lpp) of this device is expected to be divided by an additional factor of two, thus leading to an overall decrease of the half-wave voltage by a factor of four as compared to that of a conventional Mach-Zehnder modulator, namely. We have therefore:


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(lpp)3

33

V dV = = 4 4LΓn rπ

π λ (4)

Such a modulator configuration is called a loop structure push-pull Mach-Zehnder modulator13. We have designed and fabricated this loop structure according to the scheme depicted in Figure 7(a). Optical microscope images of various parts of the modulator (bright lines are poling electrodes and grey lines are optical waveguides) are shown in Figure 7(b).

The waveguide structure of the push-pull loop Mach-Zehnder modulator was the same as those used in other modulators. Because of the loop bending and waveguide crossing on one hand, (Figure 7(a) and (b)) and of a much longer optical propagation length (3.5 times greater than that of a conventional Mach-Zehnder modulator) on another hand, the waveguide structure of this modulator introduces additional propagation losses. The radius of curvature of the bending waveguide forming the loop was 200 µm. The distance between two parallel straight waveguides under the same electrode was 10 µm in order to avoid any directional coupling phenomena between these two waveguides. The separation between the two arms in the interaction region of modulator was 60 µm (Figure 7(a)). The width of the poling electrodes for each arm of the interferometer was 32 µm. The distance between two poling electrodes was 45 µm (Figure 7(b)). Several electrode lengths were used: 12 mm, 17 mm and 20 mm. The poling process for this modulator was also the same as that used for push-pull Mach-Zehnder modulators.

4. CHARACTERIZATION OF MODULATORS 4.1 Devices characterization set-up

We used Tunics-BT tunable laser source emitting within the 1500 nm-1600 nm spectral window towards modulators characterization in the favoured telecom spectral window. The polarization of the input beam was well controlled by a combination of a polarizer and a half-wave plate at 1550 nm for TE and TM polarizations. The input beam was injected into the waveguide using an IR microscope objective (Nachet x 20 or x 40 objectives) and the output beam was collected using another IR microscope objective (x 20). The transmittances in the 1500 nm-1600 nm window were 0.95 and 0.87 for x 20 and x 40 microscope objectives, respectively. The far-field spot of the output beam from the waveguide was visualized by an IR camera (Find-R-Scope). The photodetectors used for measurements were the Hamamatsu InGaAs photodiode and the Thorlabs S122B powermeter for low-frequency operation. For high-frequency operation we used a fast InGaAs photodetector with preamplifier (Da-LightCom 15 GHz PIN Preamp Receiver).

For modulation characterization at low-frequency operation we used a Hewlett-Packard 3225B synthesizer/function generator with a DC-20 MHz bandwidth and a digital oscilloscope (Tektronix TDS 3052B) with recordable digital data measurements. At high-frequency operation we used a high-frequency analog signal generator with a 250 kHz-20 GHz bandwidth (Agilent E8257D) and a high-frequency spectrum analyzer with a 3 Hz-44 GHz bandwidth (Agilent E4446A).

4.2 Optical properties of modulators

Typical dimensions for the geometry of the rib waveguide used for our modulators are : H = 1.9 µm; h = 0.9 µm; wg = 6 µm; dl = 3.9 µm; du = 3.9 µm (Figure 2). The numerical simulation of waveguide propagation over 1 mm gives a single mode profile. The far-field picture of the output mode of Mach-Zehnder modulator taken by an IR camera with an input optical power of 0.5 mW is shown in Figure 8. In our characterization measurements for Mach-Zehnder modulators the input laser beam polarization was set along the TM-polarization orientation.

Figure 8. Far-field picture of the output mode of Mach-Zehnder modulator taken by an IR camera


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We characterized the propagation loss of straight waveguides using the cutback method. From loss measurements for different waveguide propagation lengths, a propagation loss ranging from 1.5 dB/cm to 1.8 dB/cm at 1550 nm has been inferred. This confirms the good quality of the waveguides. Insertion losses resulting from using microscope objectives remain the major current limitation, however by no means an ultimate limit when pigtailing will be achieved. For Mach-Zehnder modulators, such losses range from 14 dB to 28 dB, depending on the type of modulator. In the case of push-pull Mach-Zehnder loop structures, the additional loss introduced by bending and crossing of waveguides as well as by the additional propagation length jointly contribute to about 5 dB increase as compared to a conventional Mach-Zehnder modulator.

4.3 Low-frequency modulation properties of modulators

We characterized our modulators at 1 kHz modulation frequency by using a triangular modulating voltage. The response of the modulator under test was measured by a digital oscilloscope from a photodetector and synchronized with the modulating signal. We collected acquired data on a digital oscilloscope.

4.3.1 Phase modulator

The input laser beam was polarized at an angle of 45° with respect to the two principal axes of the poled waveguide using a polarizer and a half-wave plate. The output beam was analyzed by an analyzer polarized perpendicularly to that of the input polarization. For phase modulators, we obtained the best value of half-voltage Vπ for a poling voltage value equal to 450 V, corresponding to a core poling field of 95 V/µm. To characterize the modulator at low frequency, we did not need to fabricate a high-frequency drive electrode, we used the poling electrode as the drive electrode of modulator. For a phase modulator, with a 2.5 cm-long electrode and an inter-electrode distance of 9 µm, the half-wave voltage was 3.15 volts, corresponding to a figure of merit of 7.8 V.cm. Figure 9 below shows the oscillogram of the polymer based phase modulator response at 1 kHz modulation frequency.

Figure 9. Oscillogram of the modulating signal and the response of the polymer-based phase modulator at 1 kHz

modulation frequency.

4.3.2 Loop structure push-pull Mach-Zehnder modulator

For the push-pull loop Mach-Zehnder modulators, we had attemped to apply the same poling field as in the case of the phase modulator, i.e. 95 V/µm, but the device was not able to withstand such a high poling field because of the proximity of the antisymmetric high voltage (± 450 V with respect to 0 V), with opposite field direction in the other arm of the interferometer. The overall high voltage difference of 900 V between two poling electrodes, led to breakdown processes in the device. In order to evaluate the response of the modulator, we tried then to apply a moderate poling antisymmetric high voltage of ± 260 V with respect to 0 V, i.e. a voltage difference of 520 V between the two poling electrodes. This resulted in a moderate core poling field of 60 V/µm. As in the case of phase modulators, for low-frequency characterization of the device we used the poling electrodes by connecting them together to obtain a single drive electrode at low frequencies. For a push-pull loop Mach-Zehnder modulator with a 1.2 cm-long electrode and an inter-electrode distance of 9 µm, we obtained a half-wave voltage of 5.4 volts, corresponding to a figure of merit of 6.48


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V.cm. As already pointed-out in § 3.4, in comparison with a conventional Mach-Zehnder modulator, this modulator displays additional losses due to waveguide bending and crossing as well as a longer optical propagation distance. However, we obtained a good output signal with a 1 mW optical input power. Figure 10 below shows the oscillogram of the polymeric loop structure push-pull Mach-Zehnder modulator response at 1 kHz modulation frequency.

Figure 10. Oscillogram of the modulating signal and low frequency response of the polymeric loop structure push-pull

Mach-Zehnder modulator at 1 kHz modulation frequency.

4.3.3 Push-pull Mach-Zehnder modulator

The best figure of merit was obtained with push-pull Mach-Zehnder modulators. As in the case of push-pull Mach-Zehnder loop modulators, we used the poling electrodes by connecting them together for obtaining a drive electrode at low frequencies. For poling, we applied a moderate poling antisymmetric high voltage of ± 280 V with respect to 0 V, i.e. a voltage difference of 560 V between two poling electrodes. These poling voltages correspond to a moderate core poling field of 75 V/µm.

Figure 11. Oscillogram of the modulating signal and low frequency response of the polymeric push-pull Mach-Zehnder

modulator at 1 kHz modulation frequency.


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With a 2 cm-long electrode and an inter-electrode distance of 8.8 µm, the push-pull Mach-Zehnder modulator displayed a half-wave voltage Vπ of 2.6 volts, corresponding to a figure of merit Vπ.L of 5.2 V.cm. Figure 11 shows the oscillogram of the polymeric push-pull Mach-Zehnder modulator response at 1 kHz modulation frequency.

4.4 High-frequency modulation characteristics of phase modulator

We also achieved travelling-wave polymeric phase modulators for high-frequency operation. In this configuration, the drive electrodes formed a micro-strip line having an impedance to be matched to a 50 ohms termination. Such a modulator displayed a high-frequency electromagnetic waveguide and ensured velocity matching between optical mode and modulating signal velocities. Numerical simulations on such a waveguide were carried out by the CST software. In order to assess their high-frequency response, we realized the phase modulators following a rectangular waveguide cross section made of the electro-optic copolymer DR1-PMMA (50/50). The lower Ti-Au electrode was 500 nm-thick while the upper gold electrode was 750 nm-thick and its width was 20 µm. Because the width of the optical waveguide was 6 µm, the much wider high-frequency drive electrode was able also to be used as a poling electrode. The phase modulator with a 19 mm-long electrode and an inter-electrode distance of 7 µm exhibited a measurable modulation frequency bandwidth up to 4 GHz. The frequency response of this phase modulator is shown in Figure 12. For this test modulator, we did not realize an optimized high-frequency connexion between the drive electrode and the SMA connector that would have to ensure the impedance matching of the line, due to the current absence of an adequate chip bonding technology at room temperature. To connect with driving electrodes, the gold wire of 50 µm-diameter were glued to the drive electrodes by an epoxy conductor (containing silver nano-particles) and then solder this gold wire to the SMA connectors as for low-frequency operation case, however ensuring now with a minimal length of gold wires so as to consequently reduce the inductance parasitic influence. The modulator response could not be completely offset at this stage. We think that a high-frequency adapted connexion could improve significantly the response of our modulators in the future.

Figure 12. Frequency response of the phase modulator realized with electro-optic copolymer DR1-PMMA (50/50).

5. RESULTS AND DISCUSSION 5.1 Estimation of the electro-optic coefficient r33 of the DR1-PMMA (30/70) copolymer

From equations (3) and (4) we can infer an estimate of the r33 electro-optic coefficient value of the DR1-PMMA (30/70) copolymer in our Mach-Zehnder modulators, based on a reasonable estimate for the overlap integral Г. If we assume that the overlap integral is equal to 1, we can infer the value of the electro-optic coefficient r33 in the case of the push-pull Mach-Zehnder loop modulator as well as that of the push-pull Mach-Zehnder modulator. However, we cannot make a comparison between the two r33 values obtained from the loop structure push-pull Mach-Zehnder modulator and from the push-pull Mach-Zehnder modulator because their respective core poling fields are different. The realization of


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modulators with the comparable core poling field distributions should be a prerequisite to any meaningful comparison of the performance of two configurations. Such a study will be undertaken in the next future. In the case of the Mach-Zehnder loop modulator, we obtained a r33 (loop push-pull) value of 12.7 pm/V at a moderate core poling field of 60 V/µm. This value was not very far from the published electro-optic coefficient r33 value17 for the DR1-PMMA (30/70) copolymer. As indicated in § 2.1, the published value extrapolated to a core poling field of 120 V/µm was 8.6 pm/V. For the case of the push-pull Mach-Zehnder modulator, we obtained a r33 value (push-pull) of 32 pm/V for a core poling field of 75 V/µm. This result was 3.69 times greater than that of the previously published value of electro-optic coefficient r33 of the DR1-PMMA (30/70) copolymer extrapolated at a core poling field of 120 V/µm. We cannot explain this difference of measured values for the electro-optic coefficient r33 between the published value17 and the estimated value from the performance of the push-pull Mach-Zehnder modulator. In order to explain this surprising high value of the electro-optic coefficient r33, we should make some new measurements of this new copolymer under the similar poling field conditions.

5.2 Performance of the modulators realized with commercial side-chain DR1-PMMA copolymer

With a 5.2 V.cm figure of merit obtained from the polymeric push-pull Mach-Zehnder modulator realized with a commercial side-chain DR1-PMMA (30/70) copolymer oriented with a moderate core poling field of 75 V/µm, we achieved the best performance ever observed in a modulator realized with a commercial electro-optic polymer. The figure of merit of this polymeric modulator is better than that of lithium niobate electro-optic modulators14,15. We should however develop the realization of high-frequency operation circuits to satisfy the impedance matching constraint between the electromagnetic waveguide and the external modulation system in order to further improve our current high-frequency performances. It is noteworthy that a similar kind of electro-optic polymer incorporated in a Mach-Zehnder modulator has exhibited the best performance so far in terms of high-frequency bandwidth6.

5.3 Perspectives

As commercial high-performance specific electro-optic polymers are still not available, we believe that this new high performance of the commercial side-chain DR1-PMMA copolymer could bring an adequate answer to study various configurations of electro-optic devices. For high-rate WDM applications, the resonant-structure modulators using electro-optic ring microresonators or electro-optic Fabry-Perot microresonators with Bragg reflectors should be an interesting solution. For applications like radio over fibre, an opto-hyperfrequency conversion based on polymeric electro-optic travelling wave devices might also present specific advantages22. In the domain of terahertz detection, electro-optic polymer-based devices23 will have to be studied for applications. Finally, a new emerging generation of active photonic devices using photonic crystal structures (2D and 3D) could play an interesting role in the domain of electro-optic polymers. By using this available commercial side-chain electro-optic copolymer we can build some prototypes in order to optimize the performance of photonic devices for possible future commercial high-performance electro-optic polymers.

ACKNOWLEGMENTS

The authors would like to acknowledge support from the EADS Enterprise Foundation research program under contract N° 050 – Type RP.

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