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A linearized dual parallel Mach-Zehnder modulator (DPMZM) based on electro-optic (EO) polymer was both fabricated, and experimentally used to suppress the third-order intermodulation distortion (IMD3) in a coherent analog fiber optic link. This optical transmitter design was based on a new EO chromophore called B10, which was synthesized for applications dealing with the fiber-optic communication systems. The chromophore was mixed with amorphous polycarbonate (APC) to form the waveguide’s core material. The DPMZM was configured with two MZMs, of different lengths in parallel, with unbalanced input and output couplers and a phase shifter in one arm. In this configuration each of the MZMs carried a different optical power, and imposed a different depth of optical modulation. When the two optical beams from the MZMs were combined to generate the transmitted signal it was possible to set the IMD3 produced by each modulator to be equal in amplitude but 180° out of phase from the other. Therefore, the resulting IMD3 of the DPMZM transmitter was effectively canceled out during two-tone experiments. A reduction of the IMD3 below the noise floor was observed while leaving fifth-order distortion (IMD5) as the dominant IMD product. This configuration has the capability of broadband operation and shot-noise limited operation simultaneously.
©2011 Optical Society of America
1. Introduction
Most techniques to modulate an optical carrier in analog fiber optic links composed of external Mach-Zehnder (MZ) modulators. The MZ modulators have intrinsically a nonlinear transfer function that distorts the transmitted signal, limiting the link’s dynamic range [1]. For external modulators, several approaches for improving the linearity of the devices have been suggested [2]. The two most common depend upon adjustment of the transfer function to fit a triangular function, or the elimination of the cubic term in the Taylor-expansion of the transfer function. Over the years, eliminating the cubical term has been shown to be the most successful. Linearization concepts using a DPMZM scheme have been explored in Ref [2], [3]. These approaches eliminate the cubic terms by using IMD3 cancellation along with an intensity modulation direct detection method.
In a recent paper [4], the optimization condition of the double-sideband suppressed-carrier (DSSC) coherent analog fiber optic link using a DPMZM transmitter to achieve the high spurious-free dynamic range (SFDR) was theoretically discussed. For a lossless link employing a transmitter laser with optical power of 1W, a SFDR as high as 152dB⋅Hz was derived by assuming the photo-conversion efficiency of 1A/W. In this letter, we demonstrate a linearized DPMZM transmitter that suppresses the nonlinear term in third order based on an amplitude modulation coherent detection scheme. In our experiment, a DPMZM transmitter was fabricated, using a new EO polymer material as a core layer that is both thermally and chemically stable. A two-tone test was performed on this complex structure which included three MZMs, five Y couplers, and a phase shifter. It was able to cancel the dominant IMD distortion in third order based on our dual parallel modulation scheme, resulting in a IMD reduction along with a modest signal decrease. We also discuss the advantages of the proposed linearization scheme, in a polymer photonic integrated device, which was confirmed by our experimental demonstration.
2. Polymeric materials, device fabrication and individual component test
The EO polymer structure, cross-section, and EO material properties of the device are illustrated in Fig. 1 . The device’s design parameters and procedures follow the previous publications [5,6]. The core material is a guest-host system which is a mixture of B10LC103 (or B10, Lumera co.) and amorphous polycarbonate [7]. The loading density for B10 is 30wt% and it has a refractive index of 1.6727 for TE and 1.6671 for TM polarizations. The B10 structure in APC has high thermal stability and exhibited 90% of the r33 retention after a thermal treatment of 85°C for 120h. The long term photo-stability of the B10 device when illuminating at 100mW continuous optical input has been achieved by not only engineered and more stable molecular structure but also packaging the modulators with a non-oxygen exposed environment [7]. The B10-APC layer is inserted between a lower (UV15LV) and upper cladding (UFC170A) as shown in Fig. 2 . To achieve a high poling efficiency in an EO polymer, the stacked layer was poled under a high electric field. The device poling for the B10-APC thin film was performed using contact poling with a push-pull poling geometry where a diluted APC solution covered the entire surface of the device (a 4-inch wafer) to prevent the dielectric breakdown. A poling electric field of 80V/μm at a temperature of 148°C was applied to the device for 15 min. After the poling process, the thin layer of APC was quickly removed using Acetone, and then a thin gold layer of the poling electrode was removed using a wet etching technique. The driving electrode was formed using Cr/Au deposition, and gold-electroplating. After etching of the photoresist and the seed layer of Cr and Au layers, the wafer was diced into individual devices to test. To reduce the RF signal losses the electrode thickness of the devices were increased by a pulsed gold electro-plating technique, and evaluated to be 2.0~2.5 μm thick.
Fig. 1 EO material and device structure. (a) chemical structure of guest material, B10LC103 (or B10), (b) device cross-section, and (c) material properties of B10-APC.
Fig. 2 Schematic diagram of the device fabrication process. A thin layer of a protection layer made of a diluted APC in TCE solution was applied to prevent from unwanted dielectric breakdown during the poling procedure. The electrode contact poling process (a circle) was done in a lab-made nitrogen purged box. The slab height of each layer such as lower cladding, core layer, and upper cladding was measured to be 2.5 μm thick, respectively. Different waveguide widths was chosen to be 3, 3.5, and 4 μm wide.
As shown in Fig. 3 , all the devices including MZMs, the directional coupler modulator, and phase shifter were tested separately to evaluate their operational characteristics after cutting out each component. The total device length was about 54 mm, and the total fiber-to-fiber insertion loss was estimated to be ~25dB. The MZs are intentionally different in length and the directional coupler modulator consists of a low frequency MZM with a coplanar output to give a variable splitting ratio. The output coupler was fixed in the experiment at 50:50 to simplify the fabrication. Ideally it would be for optimized at β as indicated below. In previous devices the B10 polymer optical materials were stable for over 100 days with 100mW of continuous laser illumination, and had been incorporated in modulators having high bandwidth and a low operating voltage, push-pull configuration [7]. Finally, a complete B10-APC based DPMZM transmitter was examined as shown in the next section.
Fig. 3 Half-wave voltage measurements of all the components that used to construct a DPMZM transmitter (as shown in Fig. 4 and 5). Each component was diced from the same internal chip. (a) a Mach-Zehnder modulator 1 (MZ1), Vπ = 3V with an interaction length of 2cm, (b) MZ2, Vπ = 2V with an interaction length of 2.5cm, (c) a directional coupler modulator, Vπ = 4.31V with an interaction length of 1.5cm, and (d) a phase shifter, Vπ = 10.5V with an interaction length of 0.5cm. The device measurements were performed at a low-frequency of 1KHz.
3. Linearized DPMZM transmitter, and characterization
Previously, a composite optical transmitter based on DPMZM structure was proposed and theoretically analyzed for double-sideband suppressed coherent analog optic fiber links [4]. In this study we fabricated and tested a DPMZM transmitter as depicted in Fig. 4 and Fig. 5 . Two MZMs are fed from a single optical source with a variable MZM optical coupler, and the outputs are combined to form a single transmitted signal. The two MZMs are biased at a null point such that the carrier and all even order modulation terms are suppressed [8]. The two control voltages on the directional coupler and the phase shifter are then chosen based upon the different modulation depths m and of the two MZMs. The voltage on the directional coupler permits the output of the two IMD3 terms to be made equal in magnitude. The phase shifter permits them to be made 180° out of phase with each other. The IMD3 terms are now suppressed using the optical fields that recombine destructively at the output port. Overall, this condition can be found experimentally to induce the third order portion of the signal to be almost cancelled completely [1–4].
Fig. 4 Schematic layout of (a) a linearized DPMZM transmitter, and (b) a test setup. It includes a DPMZM transmitter, the laser source of 1550nm, the erbium-doped fiber amplifier (EDFA), the local oscillator (LO), and the optical filter (FBG). The LO is generated using an optical modulator, operating at 12.25278 GHz along with an optical filter (FBG) to separate the sideband. The EDFA was used to develop sufficient optical power. Polarization controllers are utilized before and after the DPMZM and the LO. The LO is used to beat the generated DSSC signal to an IF frequency (7.25278GHz). To achieve the maximum beating current, the LO has been appropriately tuned, and the beating is done using a 3 dB coupler followed by an amplifier and either a photodetector or balanced photodetector for RIN suppression.
Fig. 5 Photograph of the polymeric DPMZM optical transmitter. The DPMZM is fiber-aligned in both sides, and the two MZM electrodes are contacted with GSG probes, respectively. For electrical contacts for the directional coupler, the phase shifter, and the ground contact, low-frequency pico-probes were utilized.
The calculated amplitude of the optical field at the DPMZM transmitter’s end [4] is
where mis the modulation depth of the applied analog signal, and the coefficient of α and β are related to the splitting ratios in the input and the output port. Expanding Eq. (1) to linearize the amplitude, one can find that the third order terms can be cancelled whenTherefore, the current generated by the balanced detector can be written as , which means that the fifth order distortion will be dominant factor after the cancellation of IMD3 component. When α is equal to β meaning that the input and output coupler are made identical, the maximum linearity can be reached as described in Ref. [4]. In this demonstration, a Y-junction output combiner is used for simplicity. Therefore, the obtainable maximum linearity is somewhat limited because β is set to a non-ideal condition. However, γ is still a function of α in Eq. (2) and much of the cancellation of the third order term can be achieved by adjusting the splitting ratio of the input coupler experimentally.Figure 4(b) shows the experimental measurement setup for the nonlinearity test. The two-tone RF signals of 5GHz and 5.00002GHz are first combined and divided equally, and then applied to the modulator electrodes via GSG probes (Cascade Microtech, Inc.). The DC bias voltages for the two MZMs were controlled separately. The modulated optical signals from each of the MZMs were sent to a combiner, and then a 3-dB fiber coupler for coherent detection. A local oscillator was also used to down-convert the generated signal via the 3-dB coupler. The local oscillator was generated from the input laser using a 12.25278 GHz. Optical modulator along with a FBG to isolate one part of the sidebands. After mixing in the photodetector, the RF fundamental and the nonlinear products are amplified, and then measured by the spectrum analyzer. The two IMD3 signals are added destructively when they are π out of phase. The reduction of more than 30dB brings the IMD3 signal below the noise floor.
The experimental setup for the high-frequency measurement is shown in Fig. 5. A light source from a CW laser operating at 1550 nm was launched via optical fiber into the waveguide, coupled-out via the output optical fiber, and subsequently mixed in balanced photodetector. The optical signal and the local oscillator (LO), generated via sideband modulation, used the same laser source to ensure phase coherency during the photodetection. A linear polarizer for the coupler and two DC bias controllers at the two MZMs were carefully used to control the polarization states of the DPMZMs and the LO while observing the detected output RF spectrum. Then the nonlinear distortion cancellation was realized as shown in Fig. 6 . Figure 6(a) mixing (at 12GHz-5GHz~7GHz) illustrates the measured RF electrical spectrum of the output signal when the internal phase shifter was used to make the fundamental maximum. Figure 6(b) plots the two-tone IMD result when the output linear polarizer and the bias controller of the phase shifter were adjusted for optimum IMD3 cancellation. On the other hand, while obtaining the maximum linearity, the fundamental output power was decreased by approximately 10dB. However, it is clear that the third-order IMD was significantly suppressed, (>30dB of IMD3 suppression along with a fundamental reduction of ~10dB) using the amplitude modulation and coherent detection of the polymer integrated DPMZM transmitter. The decrease of the IMD3 is a consequence of the phase cancellation of the coefficient of the IMD3 term as indicated in Eq. (8) in Ref. [4] and not a change in the optical power in either MZM.
Fig. 6 Experimental results for two-tone IMD test. (a) without and (b) with linearization. The IMD3 was suppressed by >30dB to the noise level, while the fundamental power was sacrificed by ~10dB. The improved linearity was achieved at the expense of a small decrease in the fundamental amplitude signal due to a partial cancellation when the dominant distortion from the MZMs is subtracted [3].
Figure 7 shows the fundamental and IMD3 power as a function of input power. Without linearization, the power of the fundamental and IMD3 signal have an increment of 1dB and 3dB for every 1dB rise in input power, indicating that the IMD3 is the limiting distortion product. When the input power reaches ~13.4dBm by controlling the biases of the directional coupler and the phase shifter, a partial reduction of IMD3 was performed as illustrated in Fig. 7. After further increasing the input power, the IMD5 power appeared as a measurable quantity. The increase of 5dB in IMD5 power for every 1dB rise reveals that the third-order IMD is being suppressed and the fifth-order IMD is now the limiting product, and the corresponding SFDR has estimated ~7dB enhancement in this experiment.
Fig. 7 Output power versus input RF power from the two-tone RF spectrum. (▪) with slope = 1 represents the fundamental signal without linearization, and (●) with slope = 3 represents the third order intermodulation distortion without linearization, (▲) with slope = 1 and (▼) with slope = 5 are the fundamental and IMD5 signal after linearization process. It is note that the suppression of IMD3 has not been optimized with β to obtain maximum distortion suppression. However, the SFDR enhancement of ~7dB in above experiment was determined.
4. Conclusion
In conclusion, we have fabricated and examined an EO polymeric DPMZM transmitter for a broadband linearized DSSC coherent analog fiber optic link. The linearized scheme used involves cancellation of the IMD3 signal to produce high linearity. By adjusting the DC bias of the components in the DPMZM transmitter, the observed IMD3 product of the MZMs was almost completely eliminated along with a 10dB reduction in the fundamental. This proof-of-concept experiment illustrates the validity of making a linearized modulator for a coherent system using an EO polymeric DPMZM transmitter. Because of the use of a balanced detector in this coherent link approach, the RIN noise in the system can be effectively removed, and the SFDR will improve directly with increasing laser power.
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