Long-haul optical communications systems rely on a variety of electrical and optical components to send and receive high-rate data. Among these components, data converters and modulators represent important functions that are also technically difficult to integrate. Fortunately, iTerra Communications (Palo Alto, CA) has developed its model iT6130 (see table), a single surface-mount module that combines a non-return-to-zero (NRZ)/return-to-zero (RZ) converter and a modulator driver for long-haul applications to 11.5 Gb/s.
Modulation techniques that extend the distance between optical amplifiers can reduce hardware costs in long-haul optical communications systems. RZ modulation, the leading technique for long-haul optical transmissions, has traditionally been difficult to implement in practical systems. At present, optical RZ pulses can be generated by separate cascaded Mach-Zehnder modulators driven by an NRZ data stream for one section and a clock pulser for the second section. The approach requires precise control of amplitude and phase, as well as separate microwave amplifiers for the two sections.
A simpler approach which can reduce the number of components and optical loss would be to generate an electrical RZ signal and drive a single Mach-Zehnder optical modulator. Two of the fundamental components required for this technique are a converter that electrically transforms an NRZ signal, typically coming from a multiplexer, to an RZ signal, and a microwave amplifier that drives an optical modulator. These two devices are combined in the iT6130 NRZ-to-RZ converter/modulator driver (Fig. 1), sparing the optical system integrator component matching, tuning, and optimization.
Combining these two functions produces significant benefits for system integrators. Optical modulator drivers are notoriously sensitive to the input from the amplifier that drives them, and matching the two is extremely difficult if the signal is RZ rather than NRZ because of its different spectral shape. The modulator drivers themselves are equally sensitive to the signal conditions at their inputs. The iT6130 eliminates the device selection, design, and optimization required to accommodate these demands, because all required functions are contained within the module and optimized to work with each other. The result is a data converter/driver amplifier that is smaller and lighter than a unit built from discrete parts and typically delivers better performance. The system integrator need be concerned only with the digital interface at the iT6130's input and the output interface. The latter interface is simplified in the iT6130 by incorporating a DFF core in the digital RZ converter. A user need only adjust the NRZ data and clock phase margin which can be as broad as 320 deg.
The iT6130 is a complete subassembly for generating a high-level RZ signal from clock and NRZ data inputs. Its output is adjustable from 4 to 7 VDC p-p and tailored for use with lithium-niobate (LiNo) modulators. Maximum data rate is 11.5 Gb/s. The module consists of a single-chip GaAs monolithic converter and a hybrid traveling-wave FET amplifier plus ancillary components mounted on a substrate with high thermal conductivity.
The subassembly has internal DC regulators and filtering that eliminate the need for power-supply sequencing and accommodate even noisy, poorly regulated DC inputs. If interfacing agility or timing-dependent jitter optimization is required, the iT6130 provides the desired phase shift with active phase shifters, the only manufacturer yet to do so. This approach was chosen because cascaded passive phase shifters produce loss that must be compensated for with amplification, which increases cost and complexity.
RZ modulation enables link extension because of its inherently higher peak power for a given input signal. If the average optical power is constant, RZ modulation will produce twice the peak power of NRZ modulation (but require twice the bandwidth). Consequently, RZ modulation produces a theoretical 3-dB advantage in receiver sensitivity (reduced in practice by 0.5 dB because of increased shot noise caused by greater peak power in the receiver). While other factors complicate this rule, an RZ-coded signal can still allow much longer span lengths than NRZ.
However, implementing RZ modulation creates engineering problems. All components, from RZ converter to receive filter, must handle twice the bandwidth of an NRZ signal at the same data rate. "Off-the-shelf" RZ driver amplifiers are rare and expensive because they must accommodate a driving signal that forces their input stage to operate in a different region of their transistor current-voltage (I-V) characteristics than would an NRZ signal. This situation requires considerable engineering time for an optimal solution. The drop-in approach of the iT6130 should be welcome by all designers who have themselves wrestled with optimization and integration.
RZ signals were initially achieved with optical methods, in which electro-optical modulators were cascaded, and the pulse was generated in the optical domain using an optical filter. This approach results in a large, complex device that is expensive to produce, draws considerable current, and generates loss in the second optical modulator that requires a higher-power laser for compensation, but achieves very low jitter. Manufacturers have worked to enhance electrical RZ generation in recent years, since the method requires only a single optical modulator, modulator driver, and bias controller, to reduce system costs by $3000 to $5000. Only one driver amplifier is required for reduced power dissipation.
With "electrical RZ," the NRZ data from the multiplexer is converted to an RZ signal, amplified, and sent to the optical modulator as a 5-to-7-VDC signal. The output signal must have a flat zero-level (rail) and low jitter, which is a difficult challenge. Jitter performance of the RZ modulator integrated within the iT6130 represents some of the best jitter characteristics yet achieved, producing eye diagrams that are virtually indistinguishable from those produced by optical techniques (Fig. 2). Jitter at an output of 7 VDC is 3.5 ps p-p.
Two versions of the iT6130 are available: an SMA module and a two-chip surface-mount version. iTerra Communications, 2440-A Embarcadero Way, Palo Alto, CA 94303; (650) 424-1937, FAX: (650) 424-1938, e-mail: [email protected] rac.com, Internet: www.iterrac.com.