Improving optical 40-Gbit/sec signal generation
The conventional on-off keying method for the nonreturn to zero (NRZ) signal format is a cost-effective solution in optical systems. However, we anticipate that other signaling formats will emerge to further reduce overall system cost by offering longer transmission distances and more efficient use of the available optical bandwidth.
For example, return to zero (RZ) modulation is suited to long-haul transmission, whilst the compact spectral (CS) feature of CS-RZ offers the prospect of >0.4-bit/Hz/sec spectral efficiency in DWDM systems. Direct detection differential phase-shift keying (DPSK) is attracting renewed interest for its excellent resilience to cross-phase modulation, which is a key requirement for DWDM system implementation. It is also potentially rewarding that this approach could form the basic building block for doubling the transmission capacity without incurring extra penalty due to chromatic and polarisation-mode dispersion through the optical DPSK approach.
We have developed a versatile 40-Gbit/sec optical-transmitter platform that has shown delivers high-quality NRZ, RZ, CS-RZ, RZ-DPSK, and CS-RZ DPSK signal formats for a variety of inline passive-component testing and transmission system studies.
Figure 1 shows the basic architecture of this multiple-format optical-transmitter platform, which comprises a cascade of two chirp-free X-cut lithium niobate modulators—one for pulse generation, the other for data modulation. Both modulators have all-polarisation-maintaining fibres and only require single-ended RF drive. The minimum insertion loss of the cascaded structure is typically 7 dB. The electro-optical bandwidth is >30 GHz, and the drive voltage is <6 Vpp at 40 Gbit/sec.
A residual chirp of <±0.1 has been achieved with these modulators. The pulse modulator is driven by a high gain (34-dB) and high power (28 dBm) narrowband gallium arsenide RF amplifier with a passband from 18 to 27 GHz. The output of the clock driver exhibits a harmonic suppression of better than 30 dB below the clock signal when driven with a 20-GHz signal for RZ pulse generation. For RZ operation, the pulse modulator is biased to the maximum transmission point of the Mach-Zehnder (MZ) response curve, whilst the modulator bias was set to the null for CS-RZ pulse generation.
Several approaches exist for generating DPSK signals. These methods include using a phase modulator (driven at Vp), a dual-drive MZ modulator driven in a push-pull arrangement with two driver amplifiers, and a single-drive chirp-free MZ modulator driven at 2 Vp to achieve full p optical phase modulation. For simplicity, we have chosen the latter since it is identical to the intensity RZ and CS-RZ implementation, except for the data driver. Figure 2 shows the data-driver amplifier output waveforms for conventional intensity and phase modulations.
Figure 3 shows the optical intensity waveforms of typical NRZ, RZ, and CS-RZ signals. From these measurements, signal-to-noise ratios (SNRs) in excess of 18 and 23 are achieved for NRZ and RZ/CS-RZ data, respectively. A dynamic extinction ratio of better than 13 dB is typical. We keep the measured added peak-peak jitter of the NRZ data (total jitter—scope jitter) to <5 psec. In the case of the RZ/CS-RZ data, the very low peak-to-peak jitter below 2 psec can be set by the clock signal. Jitter transfer from the NRZ data is minimal.
In separate measurements using an optical sampling system, we have obtained optical pulse widths (FWHM) of typically 10 and 16 psec for RZ and CS-RZ, respectively, with the pulse generation modulator being driven at 13 Vpp. The measured RZ pulse shape is very close to transform-limited and therefore suited for soliton propagation studies. By varying the drive voltage level, the RZ and CS-RZ pulse widths can be reduced by up to 2 psec without affecting the overall performance.
For DPSK signal generation, the same data modulator was driven by a different broadband amplifier optimised for 40-Gbit/sec 2-Vp operation. The maximum output level was typically 12 Vpp. In this configuration, a variety of DPSK signal formats have been generated, including NRZ DPSK, RZ, and CS-RZ DPSK. Figure 4 shows the optical spectrum of the RZ DPSK signal at the output of the transmitter and the intensity-detected signal at the outputs of a fibre-based MZ interferometer (MZI) with an internal differential delay of about 25 psec.
To demonstrate the signal quality of the transmitter, a DPSK receiver utilising balanced detection was also constructed. A variable optical delay line is used to eliminate any path delay difference between the MZI decoder and balanced detectors. One example involves signals after balanced detection and at the output of the broadband preamplifier. An SNR >15 and a total RMS jitter of <1 psec can be obtained at the output of the receiver preamplifier. This detection scheme is anticipated to provide an extra 3-dB receiver sensitivity advantage compared to a single detector. Based on similar component-part and subsystem architecture, excellent DWDM transmission performance over transpacific distance has been successfully demonstrated.
W.S. Lee and V. Filsinger can be reached at SHF Communication Technologies AG (Berlin—www.shf-communication.de); S. Schmid at Corning OTI SpA (Milano, Italy—www.corning.com/photonictechnologies); P. Serbe at Huber+Suhner AG (Zurich—www.hubersuhner.ca); and G. Unterboersch at u2t Photonics AG (Berlin—www.u2t.de).