Mach-Zehnder Optical Modulator Research Articles

Low-repetition rate femtosecond laser writing of optical waveguides in KTP crystals: analysis of anisotropic refractive index changes.

We report on the direct low-repetition rate femtosecond pulse laser microfabrication of optical waveguides in KTP crystals and the characterization of refractive index changes after the thermal annealing of the sample, with the focus on studying the potential for direct laser fabricating Mach-Zehnder optical modulators. We have fabricated square cladding waveguides by means of stacking damage tracks, and found that the refractive index decrease is large for vertically polarized light (c-axis; TM polarized) but rather weak for horizontally polarized light (a-axis; TE polarized), this leading to good near-infrared light confinement for TM modes but poor for TE modes. However, after performing a sample thermal annealing we have found that the thermal process enables a refractive index increment of around 1.5x10(-3) for TE polarized light, while maintaining the negative index change of around -1x10(-2) for TM polarized light. In order to evaluate the local refractive index changes we have followed a multistep procedure: We have first characterized the waveguide cross-sections by means of Raman micro-mapping to access the lattice micro-modifications and their spatial extent. Secondly we have modeled the waveguides following the modified region sizes obtained by micro-Raman with finite element method software to obtain a best match between the experimental propagation modes and the simulated ones. Furthermore we also report the fabrication of Mach-Zehnder structures and the evaluation of propagation losses.

High-speed carrier-depletion silicon Mach-Zehnder optical modulators with lateral PN junctions

This paper presents new experimental data from a lateral PN junction silicon Mach-Zehnder optical modulator. Efficiencies in the 1.4V.cm to 1.9V.cm range are demonstrated for drive voltages between 0V and 6V. High speed operation up to 52Gbit/s is also presented. The performance of the device which has its PN junction positioned in the centre of the waveguide is then compared to previously reported data from a lateral PN junction device with the junction self-aligned to the edge of the waveguide rib. An improvement in modulation efficiency is demonstrated when the junction is positioned in the centre of the waveguide. Finally we propose schemes for achieving high modulation efficiency whilst retaining self-aligned formation of the PN junction.

Planar n-SI-n heterostructure athermal InP (110) optical modulator.

We report an athermal InP (110) optical modulator with a simple planar n-SI-n heterostructure. A symmetrical push-pull operation provides a high-extinction ratio of >25 dB over the entire C-band and zero-chirp modulation. A Mach-Zehnder optical modulator (MZM) exhibits a 3 dB-EO bandwidth of 30 GHz and 40-Gb/s NRZ high-speed modulation with wavelength and temperature insensitive operation. We also successfully demonstrate an MZM integrated twin-IQ modulator that exhibits 56-Gb/s × 2 athermal QPSK modulation at constant drive and bias voltages.

A 10 Gb/s, 6 V p-p , Digitally Controlled, Differential Distributed Amplifier MZM Driver

A 10 Gb/s, digitally controlled, differential distributed amplifier (DA) optical modulator driver is implemented in 0.18 μm SiGe-BiCMOS technology. The 2.87 mm 2 prototype integrates clock phase shifters, digital latches, limiting amplifiers, broadband n+/n-well back termination resistors and a substrate-shielded output line on chip. It produces 6 V p-p differential output swing across 50 Ω loads. The output edge speed is trimmable, with 20-80% rise/fall times ranging from <; 15 ps to 50 ps at 10 Gb/s. Minimum sensitivity of the ECL-compatible inputs is 65 mVp-p at 10 Gb/s single-ended, with negligible additive jitter. Measured output return loss is better than 10 dB below 35 GHz, sufficient to drive an external push-pull Mach-Zehnder optical modulator. Total power consumption is 2.13 W operating from -5.2 V and 5 V supplies. The fully-digital input interface supports scalability in the number of DA stages, output swing and to multiple output channels.

A method for identifi cation of static and dynamic characteristics of a non-zero chirp Mach-Zehnder optical intensity modulator for application in OOK fi ber optic communication line

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High-Speed Silicon Mach–Zehnder Optical Modulator With Large Optical Bandwidth

We report a carrier-depletion silicon Mach-Zehnder optical modulator with large optical bandwidth by adopting two symmetric arms. The fiber-to-fiber loss of the device is about 7.2-8 dB in the wavelength range from 1525 to 1565 nm. We have used the truncation method to accurately measure the loss of the optical splitter and combiner, and the on-chip loss is about 3.8 dB. The dynamic extinction ratios at the speed of 40 Gb/s are 4.9-6.4 dB in the wavelength range from 1529 to 1565 nm. By analyzing the dependence of the optical bandwidth on the optical path difference between the two arms, we find that there is an unexpected optical path difference of around 3.3 μm between the two arms, which is considered to originate from the nonuniform morphologies of the waveguide and the nonuniform doping profiles along the two arms and is responsible for the slight wavelength dependence of the static and dynamic response of the silicon Mach-Zehnder optical modulator.

Impact of interferometer delay time on the performance of ODSB-SC RoF system with wavelength interleaving

In this paper, we investigate the impact of interferometer delay time in a 5Gb/s optical double sideband-suppressed carrier (ODSB-SC) RoF system transmitting two wavelength interleaved radio frequency (RF) signals at 10 and 15GHz over an optical fiber. Here, an optical Mach–Zehnder modulator is used for both optical carrier suppression and signal modulation. At the receiver, delay interferometer is used for the separation of RF frequency signals. We analyze the performance of the RoF system by varying the value of delay time of interferometer from 0.02 to 0.14ns. The result shows that the RoF system performance is optimum for the time delay of 0.1ns. Further, the optical spectrums, RF spectrums and eye diagrams of two interleaved RF signals have been compared.

Heterogeneous lithium niobate photonics on silicon substrates

A platform for the realization of tightly-confined lithium niobate photonic devices and circuits on silicon substrates is reported based on wafer bonding and selective oxidation of refractory metals. The heterogeneous photonic platform is employed to demonstrate high-performance lithium niobate microring optical resonators and Mach-Zehnder optical modulators. A quality factor of ~7.2 × 10⁴ is measured in the microresonators, and a half-wave voltage-length product of 4 V.cm and an extinction ratio of 20 dB is measured in the modulators.

Optimization of efficiency-loss figure of merit in carrier-depletion silicon Mach-Zehnder optical modulator

In this paper we study the optimization of interleaved Mach-Zehnder silicon carrier depletion electro-optic modulator. Following the simulation results we demonstrate a phase shifter with the lowest figure of merit (modulation efficiency multiplied by the loss per unit length) 6.7 V-dB. This result was achieved by reducing the junction width to 200 nm along the phase-shifter and optimizing the doping levels of the PN junction for operation in nearly fully depleted mode. The demonstrated low FOM is the result of both low V(π)L of ~0.78 Vcm (at reverse bias of 1V), and low free carrier loss (~6.6 dB/cm for zero bias). Our simulation results indicate that additional improvement in performance may be achieved by further reducing the junction width followed by increasing the doping levels.

Static properties of planar Mach‐Zehnder optical modulator on (110) InP substrate

A single drive Mach-Zehnder optical modulator with a new simple planar structure was investigated by utilising a lateral electric field. To obtain the Pockels effect induced by the lateral electric field, a (110)-oriented InP substrate is introduced. The device exhibits an extinction ratio of > 25 dB over the entire C-band, a wide operating wavelength bandwidth of 80 nm and temperature insensitivity.

Electro-Optical Response Analysis of a 40 Gb/s Silicon Mach-Zehnder Optical Modulator

We demonstrate a 40 Gbit/s silicon Mach-Zehnder optical modulator driven by a differential voltage of 0.36 Vpp. The energy efficiency is as low as 32.4 fJ/bit which is near the power efficiency of the ring modulator. We analyze the relationship between the electrical bandwidth and the electro-optical (EO) bandwidth based on the electrical S parameter measurement. Because of the nonlinear response, the electro-optical bandwidths in the small-signal tests are is slightly different when the modulator is biased at different transmission points. But the EO response is much different when the optical phase change is large enough to cover the nonlinear and linear regions at the same time. The nonlinearity can greatly improve the EO response in large-signal test. In our experiment, the rise/fall (20%-80%) time decreases from 13 ps to 10 ps as the driving amplitude increases from 5 V to 6 V under the same reverse bias of 3 V.

Femtosecond pulse generation by actively modelocked fibre ring laser

Femtosecond optical pulses are generated by an actively modelocked fibre ring laser. The fibre ring cavity includes a dual-electrode Mach-Zehnder optical intensity modulator and an erbium-doped fibre amplifier. To generate femtosecond pulses, fast impulse modulation is employed. The modelocking generates femtosecond pulses with a pulse-width of about 500 fs.

12.5 Gb/s carrier-injection silicon Mach—Zehnder optical modulator

We demonstrate a 12.5 Gb/s carrier-injection silicon Mach—Zehnder optical modulator. Under a nonreturn-zero (NRZ) pre-emphasized electrical drive signal with voltage swing of 6.3 V and forward bias of 0.7 V, the eye is clearly opened with an extinction ratio of 8.4 dB. The device exhibits high modulation efficiency, with a figure of merit VπL of 0.036 V·mm.

Design and Fabrication of a Monolithic Optoelectronic Integrated Circuit Chip Based on CMOS Compatible Technology

A monolithic optoelectronic integrated circuit chip on a silicon-on-insulator is designed and fabricated based on complementary metal oxide semiconductor compatible technology. The chip integrates an optical Mach-Zehnder modulator (MZM) and a CMOS driving circuit with the amplification function. Test results show that the extinction ratio of the MZM is close to 20 dB and the small-signal gain of the CMOS driving circuit is about 26.9 dB. A 50 mV 10 MHz sine wave signal is amplified by the driving circuit, and then drives the MZM successfully.

75-km SMF Transmission of Optical 16 QAM Signal Generated by a Monolithic Quad-Parallel Mach–Zehnder Optical Modulator

We demonstrate 75-km single-mode fiber transmission of a 40-Gb/s optical 16-level quadrature amplitude modulation (16QAM) signal generated by a quad-parallel Mach-Zehnder optical modulator (QPMZM), where four Mach-Zehnder structures are embedded in parallel monolithically on a lithium niobate substrate. Owing to the optical 16QAM signal generation method based on the coherent superposition in the QPMZM and improved coherent receiver utilizing digital filters, improvement of receiver sensitivity has been achieved as well. The experimentally obtained receiver sensitivity for near the forward-error correction limit and power penalty due to transmission are experimentally evaluated as 16.3 dB and about 2-3 dB, respectively.

Photoassisted Corona Poled YLD-124/DR1-co-PMMA Electrooptic Device Using Photoisomerization

A photoassisted corona poling technique (PACP) was applied to a binary chromophore system that was composed of YLD-124 and DR1-co-PMMA to enhance the poling efficiency of an electrooptic (EO) polymer device. The DR1 chromophores as a photoaddressable host material were resonantly irradiated by a polarized pumping green laser beam at a wavelength of 532 nm which is within its molecular absorption band. This process caused the DR1 chromophore's reorientation via a cis-trans photoisomerization process. The coupled YLD-124 system's dipoles were then poled with the assistance of a static electrical field applied to the EO core layer. In this letter, we report the experimental demonstration of the photoassisted corona poling scheme in a binary chromophore system that improved Mach-Zehnder optical modulator performance by lowering its <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> . It was shown that the device's <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> was reduced by 21%-23% compared to a regular device that was only corona poled without the green light assistance.

Materials used for the optical section of an optoelectronic oscillator

The work presented here concerns the development of optoelectronic oscillators. Their high spectral purity is one of their most interesting qualities for photonics signal processing, communication or radio over fiber systems. The classical structure is based on a very long optical fiber loop acting as a delay line. Different techniques have been introduced, such as multiple loops, in order to get very narrow spectral lines and large mode spacing. Among the problems due to long fiber loops, there is the size and the requirement of temperature control. In order to go toward integrated solutions, it is also possible to introduce optical resonators instead of the delay line. Such resonators should present a very high quality factor in order to keep a low phase noise. Different solutions based on silica microspheres, on polymer materials or on silicon can be investigated. These resonators can be directly implemented with the associated Mach–Zehnder optical modulators in the case of both polymer and silicon materials.

16-level quadrature amplitude modulation by monolithic quad-parallel Mach-Zehnder optical modulator

A monolithically-integrated quad-parallel Mach-Zehnder optical modulator has been fabricated. 3 and 6 dB electro-optical bandwidths of Mach-Zehnder structures embedded in the modulator were evaluated as 22 and 36 GHz, respectively. By using the monolithic optical modulator, a 10 Gbaud 16-level quadrature amplitude modulation (QAM) signal was successfully obtained.

A theoretical model for sampled grating DBR laser integrated with SOA and MZ modulator

A theoretical model is presented for simulating the sampled grating distributed Bragg reflector (SGDBR) laser integrated with semiconductor optical amplifier (SOA) and Mach-Zehnder (MZ) modulator. In this model, the active and passive sections are processed separately. The active region of laser and the SOA section are modeled by time domain traveling wave (TDTW) method. While the spectral properties of the SG and the MZ modulator are firstly calculated by Transfer-Matrix Method (TMM) and Beam Propagation Method (BPM), respectively, and then transformed into time domain using digital filter approach. Furthermore, the nonuniform carrier-dependence of gain and refractive index are also incorporated via Effective Bloch Equations (EBE). Compared with the full time-domain method, our model would be more flexible and efficient. The static and modulation performances of device are successfully simulated. This indicates that it can be a powerful platform for investigating the complex Photonic Integrated Circuits (PICs).

High-modulation efficiency silicon Mach-Zehnder optical modulator based on carrier depletion in a PN Diode

We present a high phase-shift efficiency Mach-Zehnder silicon optical modulator based on the carrier-depletion effect in a highly-doped PN diode with a small waveguide cross-sectional area. The fabricated modulator show a V(pi)L(pi) of 1.8 V x cm and phase shifter loss of 4.4 dB/mm. A device using a 750 microm-long phase-shifter exhibits an eye opening at 12.5 Gbps with an extinction ratio of 3 dB. Also, an extinction ratio of 7 dB is achieved at 4 Gbps for a device with a 2 mm-long phase shifter. Further enhancement of the extinction ratio at higher operating speed can be achieved using a travelling-wave electrode design and the optimal doping.