The current status of lithium-niobate external-modulator technology is reviewed with emphasis on design, fabrication, system requirements, performance, and reliability. The technology meets the performance and reliability requirements of current 2.5-, 10-, and 40-Gb/s digital communication systems, as well as CATV analog systems. The current trend in device topology is toward higher data rates and increased levels of integration. In particular, multiple high-speed modulation functions, such as 10-Gb/s return-to-zero pulse generation plus data modulation, have been achieved in a single device.

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A review of lithium niobate modulators for fiber-optic communications systems

Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Optical Modulation Formats

We give a tutorial review and brief summary of the present status of waveguide electrooptic modulators.

Waveguide Electrooptic Modulators

The influence of chromatic fiber-dispersion on the transmission distance of fiber-optic microwave and millimeter-wave links is analyzed and discussed in this paper. It is shown that dispersion significantly limits the transmission distance in intensity modulated direct detection links operating in the above 20 GHz frequency region by inducing a carrier to noise penalty on the transmitted signal. At 60 GHz, a 1 dB penalty is induced after less than 500 m transmission over standard single-mode fiber with a dispersion of 17 ps/km/spl middot/nm and the signal is completely extinct after 1 km. In remote heterodyne detection links, the dispersion induces both a carrier to noise penalty and a phase noise increase on the transmitted signal. It is shown, however, that the induced carrier to noise penalty is insignificant. At 60 GHz, the induced penalty is less than 0.3 dB after 100 km transmission. The phase noise increase proves more dominant. At 60 GHz, a 150 Mbit/s QPSK signal is limited to around 10 km of transmission.

Chromatic dispersion in fiber-optic microwave and millimeter-wave links

We theoretically prove that a conventional Mach-Zehnder modulator can generate an optical frequency comb with excellent spectral flatness. The modulator is asymmetrically dual driven by large amplitude sinusoidal signals with different amplitudes. The driving condition to obtain spectral flatness is analytically derived and optimized, yielding a simple formula. This formula also predicts the conversion efficiency and bandwidth of the generated frequency comb.

Asymptotic formalism for ultraflat optical frequency comb generation using a Mach-Zehnder modulator.

A novel light single-sideband modulator, or frequency shifter, of integrated optics structure is proposed. The operation was confirmed by constructing and testing the device using Ti indiffused LiNbO 3 waveguides. The first demonstration of frequency shifting was carried out at 2 GHz, and either the upper or lower single-sideband output without carrier was obtained. Conversion efficiency of -5 dB was achieved with input drive power of 730 mW.

Integrated optical SSB modulator/frequency shifter

A traveling-wave electrooptic phase modulator using a Ti-in-diffused LiNbO 3 optical waveguide was designed and constructed with emphasis placed on broad-band operation. As a microwave waveguide, a coplanar parallel stripline of aluminum with characteristic impedance of 48 Ω was fabricated on the crystal surface. The electrodes are 1.5 μm thick, 1 cm long, and 60 μm apart. The modulator was tested at 0.63 μm over a bandwidth of 7.5 GHz. For 250-mW drive power, the measured phase-modulation index was 1 rad up to about 3 GHz and reduced to 0.5 tad at 7.5 GHz.

Broad-band traveling-wave modulator using a LiNbO 3 optical waveguide

A broad-band traveling-wave electrooptic modulator of Ti-diffused LiNbO 3 optical waveguide was designed, constructed, and tested at 0.63 μm over a bandwidth of 10.5 GHz. Using a novel asymmetric electrode configuration as a microwave waveguide, practically smooth frequency response of the modulator was obtained. For 300 mW drive power, the phase modulation index was 1 rad at 2 GHz and 0.53 rad at 10 GHz, while the intensity modulation index was 64 and 34 percent, respectively.

10 GHz bandwidth traveling-wave LiNbO 3 optical waveguide modulator

Picosecond pulse operation of the guided-wave light modulator is reported in this paper. First, we analyze the time responses of phase and intensity modulators of the traveling-wave type, and the modulated output waveforms related to the modulating pulses are discussed. It is shown that there is an optimum interaction (or electrode) length of the modulator whereby the drive voltage is minimized for a desired output pulse width. The traveling-wave push-pull light intensity modulator of Ti-diffused LiNbO 3 waveguides was modulated by a pulse train of 1 GHz repetition frequency, and the modulated output was observed by the image tube streak camera modified for sinusoidal scan at the same frequency. The duration of the modulated light pulses was less than 45 ps, which shows good agreement with the predicted one.

M. Izutsu

Papers

Integrated optical SSB modulator/frequency shifter

Broad-band traveling-wave modulator using a LiNbO 3 optical waveguide

10 GHz bandwidth traveling-wave LiNbO 3 optical waveguide modulator

Picosecond pulse response of broad-band guided-wave interferometric light modulators