Lithium niobate to rule 40G modulation
Transmission at 40 Gbits/sec initially bumped up against several technology limitations, but many of these limitations have been resolved. One of the key enabling factors in migrating to a 40-Gbit/sec DWDM network has been the advancement in optical signal generation.
A key component in optical signal generation is the modulator, which impresses the high-speed data stream onto the optical carrier signal in TDM and WDM applications. The output from a continuous wave (CW) laser passes through the modulator at the transmitter.
Over the years, lithium niobate (LiNbO3) has been and continues to be the material of choice for optical modulators at bit rates of 2.5 Gbits/sec and above, primarily due to its properties of enabling low-loss waveguides and its high electro-optic effect. LiNbO3 travelling-wave modulators, based on a Mach-Zehnder (MZ) waveguide structure, are the most widespread modulators in deployed systems.
LiNbO3 modulators typically are offered in X-cut and Z-cut versions, each format with its pros and cons; chirp-free operation leading to simplified transmitter design is the main advantage of the former, whereas lower driving voltage and higher bandwidth are the main advantages of the latter. A historical perception has been that X-cut modulators would not suit applications beyond 10 Gbits/sec because of the bandwidth and electro-optic coefficient limitations that X-cut modulators have compared with Z-cut modulators.
Despite this perception, the modulator group at Corning OTI (now Avanex) pursued an X-cut modulator design for 40-Gbit/sec applications. Along with system houses involved in designing high-bit-rate systems, we have found that single-drive X-cut LiNbO3 MZ modulators enable better and cheaper transport solutions at higher bit rates than other solutions based on LiNbO3 technology. X-cut modulators have been demonstrated in key 40-Gbit/sec transmission experiments by several systems houses, including Mintera Corp.
Mintera's 10,000-km, 40-Gbit/sec DWDM transmission demonstration at OFC 2003 last March used an X-cut modulator. Mintera noted that single-drive X-cut LiNbO3 MZ modulators suit applications that require chirp-free optical modulation such as ultra-long-haul (ULH) transmission based on differential phase-shift keying (DPSK) modulation and ultra-high spectral efficiency transmission based on duobinary modulation.
"Advanced modulation formats" have been implemented to overcome common 40-Gbit/sec system impairments. Simple and bandwidth-efficient nonreturn to zero (NRZ) is suitable for short-to-medium transmission distances, where 40-Gbit/sec systems will be deployed initially. Over longer distances, however, return to zero (RZ) transmission becomes more attractive due to the RZ format's higher robustness relative to fibre nonlinearities and polarization-mode dispersion (PMD). Specific RZ formats like carrier-suppressed-RZ (CS-RZ) help reduce the impact of the RZ format's inherently higher spectral bandwidth.
The RZ-DPSK modulation format further increases the transmission performance in ULH systems. The main advantage of this format is better receiver sensitivity (by 3 dB) than RZ and CS-RZ. The optimum choice of modulation format in a particular situation depends on the complete set of system parameters, including cost, transmission distance, bit rate, and PMD. As a result, component suppliers have developed versatile 40-Gbit/sec modulator product lines.
At 40 Gbits/sec, due to the large-bandwidth requirements, electrical RZ pulse generation is not feasible as it is at 10 Gbits/sec. Therefore, 40-Gbit/sec RZ signals are generated optically by cascading the NRZ data modulator and an optical gate, opened only for a fraction of the NRZ bit length. Optical RZ even reduces the electrical bandwidth requirements. Driving a 20-GHz modulator under certain conditions with a "half-bit rate" RF signal yields 40-Gbit/sec optical RZ pulses. The additional RZ-pulse stage enables a variety of sophisticated RZ-based modulation formats like CS-RZ. The characteristics of an MZ interferometer (MZI) also allow a simple and natural implementation of newly discussed RZ-DPSK transmission systems.
Only the external modulation of a CW-laser source is suitable for 40-Gbit/sec transmission due to its negligible impact on the spectral characteristics of the optical carrier (the CW-laser light). The best-performing industrial 40-Gbit/sec external modulators are LiNbO3-based, electro-optically controlled MZI modulators.
Many advanced modulation formats make use of phase modulation instead of amplitude modulation. MZI modulators are the key building block for chirp-free CS-RZ-DPSK modulation, where they act both as DPSK modulator and CS-RZ pulse shaper.
Semiconductor-based electro-absorption modulators (EAMs) are not suitable for generating such sophisticated formats. Their drawbacks include relatively strong chirp and relatively low output power. These factors limit the 40-Gbit/sec EAMs to "ultra-short-reach" applications.
The wavelength-independent LiNbO3 modulator also suits tunable transponders. The superior performance of the LiNbO3-based transmitter will make it the most cost-effective solution over time. Because of that, major system manufacturers are planning to replace the "conventional" EAM technology in the short-reach with LiNbO3 technology. The change will also have the advantage of at least 10 dB more of optical power available for higher unamplified reaches (>100 km).
Aside from eye-pattern characteristics (see the Figure on previous page), the so-called "chirp" is an important modulator parameter affecting system performance. Chirp is basically a phase/frequency modulation introduced during the intensity modulation. In most cases, chirp has a negative impact on the transmission quality. First of all, chirp causes a spectral broadening that limits the WDM channel separation and makes the signal more sensitive to chromatic dispersion.
Chirp-free modulators can be fabricated from a particular LiNbO3 crystal orientation (X-cut) that enables a completely symmetrical MZI configuration—the basic condition for chirp-free signal generation. The characteristic push-pull phase-shift of the same magnitude in both interferometer arms results in a constant output phase for all states of interference, hence no phase modulation takes place.
At 10 Gbits/sec, chirp can be beneficial in certain system configurations as it compensates for pulse-spreading in fibres with positive dispersion, extending the first span length from typical 80 km to 100 km (singlemode fibre). However, beyond 100 km (hence LH/ULH) the pulse width will increase more rapidly than for a non-chirped signal. Additionally, in complex meshed transmission systems, chirp-free signal generation allows for the most robust network operation under varying and unpredictable conditions.
Mintera found that X-cut modulators, because of their inherent chirp-free operation, are much more convenient from a manufacturability point of view. The traditional solution—a push-pull-driven Z-cut dual-drive MZ modulator—demands a very high level of symmetry of amplifiers, electrical waveguides, and the modulators themselves. Only LiNbO3 external modulators are fully adequate for 40-Gbit/sec LH and ULH transmission.
LiNbO3 is expected to remain the principal modulator technology for "high-end" applications. A qualified volume production requires the day-by-day experience of profound material understanding, design, testing, transfer into production, Telcordia qualification, secured logistics, and ramp-up to volume production.
A 40-Gbit/sec modulator is not merely an evolution of the 10-Gbit/sec devices. Technological "quantum leaps" were necessary to meet this design challenge. Developing and manufacturing 40-Gbit/sec modulators requires a combined expertise of designing for optical and microwave propagation as well as advanced packaging and assembly know-how.
The principal condition for the 30-plus-GHz bandwidth required for 40-Gbit/sec signal generation is the perfect velocity matching between the guided optical wave and the electrical millimeter wave. To achieve an efficient interaction up to the highest relevant frequency components (~60 GHz), the electrical pulse must travel along the electrodes at the same group velocity as the optical pulse (with <2% tolerance). A further condition for ultra-high bandwidths is maintaining the conversion efficiency along the entire interaction length. Therefore, the co-planar stripe waveguide structure must be designed for very-low-loss electrical waveguiding. The device housing contributes significantly to the overall electro-optic characteristics.
Some 40-Gbit/sec X-cut modulators can require a drive voltage of 5.4 V (Vamp) and exhibit a bandwidth of 33 GHz, allowing for the data-rate expansion required by forward error correction coding. The optical insertion loss is as low as 3 dB. The historical drive voltage advantage of single-drive Z-cut vs. X-cut modulators is almost negated and will decrease further. An integrated monitor PIN photodiode is already a standard for these modulators. Using commercial driver amplifiers, these 40-Gbit/sec X-cut modulators enable an extinction ratio of better than 13 dB, similar to what is the standard at only 10 Gbits/sec.
The choice of the 40-Gbit/sec modulation format will mainly be determined by the particular system application. Chirp-free X-cut LiNbO3 modulators are already the preferred choice for all modulation formats from classical NRZ over RZ or CS-RZ to very sophisticated formats such as RZ-DPSK for LH and ULH DWDM transmission.
With the upcoming "universal transponders," lithium niobate is likely to become the standard modulator solution because it is the single mature technology covering all possible applications from metro/NRZ to ULH/RZ.
Steffen Schmid is product-line engineer for lithium niobate components at Avanex (San Donato, Italy).