Transmitter IQ skew severely degrades the performance of coherent optical communication systems, especially for high baud-rate and higher order quadrature amplitude modulation (QAM) systems. Utilizing Nyquist pulse shaping can increase the spectral efficiency of WDM systems, while sharp roll-off of Nyquist pulse shaping further reduces the tolerance to residual IQ skew. Thus, there is a tradeoff between WDM system spectral efficiency and its tolerance to residual IQ skew. In this talk, we review the impact of Nyquist pulse shaping on the tolerance to residual IQ skew. The roll-off factor, channel spacing, receiver bandwidth and equalizer length are optimized given certain residual IQ skew. We further discussed the potential application scenarios and the methodologies to apply this technique. The results serve as guideline for high baud-rate coherent long-haul optical communication systems.
In this talk, we first review the current status for 400GBASE client-side optics standard and multi-source agreement (MSA). We then compare different form factors for 400GE modules, like CFP8, OSFP and QSFP-DD. The essential techniques to implement 400GE, like pulse amplitude modulation (PAM4), forward error correction (FEC) and continuous time-domain linear equalizer (CTLE), are discussed. A 400GE physical interface card (PIC) in Juniper’s PTX5000 platform has been developed, conforming to latest IEEE802.3bs standard. To validate the PIC’s performance, a commercial optical network tester (ONT) and the PIC are optically interconnected through two CFP8-LR8 modules. The CFP8-LR8 module utilizes 8 optical wavelengths through coarse wavelength division multiplexing (CWDM). Each wavelength carries 50Gb/s PAM4 signal. The signal transmits through 10km single mode fiber (SMF). The ONT generates framed 400GE signal and sends it to PIC through the first CFP8 module. The PIC recovers the signal, performs an internal loopback, and sends 400GE signal back to the ONT through the second CFP8 module. The optical spectrum, eye diagram, receiver sensitivity, long time soaking results, and internal digital diagnosis monitoring (DDM) result are fully characterized. The pre-FEC bit error rate (BER) is well below the KP4 FEC threshold of 2.2e-4. After KP4 FEC, error-free performance over 30km SMF is achieved. In this way, we demonstrate both the inter-operation between the PIC and the ONT, as well as the inter-operation between two CFP8 modules. This demonstration represents the successful implementation of 400GE interface in the core IP/MPLS router.
Coherent optical transponder is ubiquitous and dominant in long-haul optical network. Bit error rate (BER) versus optical signal to noise ratio (OSNR) determines the transmission distance for coherent optical transponder. However, a complicated setup is needed for this measurement, which limits this measurement to laboratory environment. We have developed an accurate model to predict BER versus OSNR at various receiver optical power (ROP) under assumption of additive white Gaussian noise (AWGN). The model has three parameters, which are related to noise floor, filter mismatching, and OSNR value without noise loading. We determined the first two parameters through curve fitting of BER vs. ROP curve. We determined the third parameters through design verification test (DVT). We validated the model over 100 channels within extended C band. Furthermore, we expanded the model to high modulation format 16- ary quadrature amplitude modulation (QAM). We investigated the influence of high baud rate, like 45G, 56G, 64G and 86G. The model works well for both high modulation format and high baud rate. The influence of baud rate on the fitting parameters are discussed. Since one can measure BER versus ROP using built-in components of coherent optical transponder, BER versus OSNR can be monitored during in-field deployment based on this accurate model. In addition, one can monitor OSNR based on BER reversely. No extra hardware or DSP processing algorithm is needed for this OSNR monitoring scheme. The monitoring accuracy is further improved with consideration of chromatic dispersion (CD), polarization mode dispersion (PMD) and nonlinearity impairment.
DP-QAM is one of the most promising paths towards 400-Gb/s and 1-Tb/s commercial optical communications systems. For DP-QAM transmitter, different tributary channel powers lead to IQ or XY power imbalance. Large uncompensated IQ or XY power imbalance can significantly degrade the performance in the coherent optical communications system. In this work, we propose and experimentally demonstrate a technique to detect and compensate DP-QAM transmitter power imbalances for tributary channels. By reconfigurably interfering de-skewed identical BPSK channels, the optical powers of any two tributaries can be balanced by minimizing the output power from their optical interference.
KEYWORDS: Polarization, Receivers, Transmitters, Laser sources, Optical character recognition, Digital signal processing, Multiplexing, Heterodyning, Analog electronics, Optical amplifiers
Intradyne coherent receiver (ICR) is an essential component for modern coherent systems. Critical parameters of ICR include frequency response, bandwidth, and gain imbalance. Conventionally one creates heterodyne beating between tunable laser source (TLS) and local oscillator, and sweeps frequency of TLS to measure those parameters. A complicated control loop aligns state of polarization (SoP) of TLS to 45/135 degree of principle axis of polarization beam splitter. Otherwise unequal amount of power will launch into two polarizations, leading to inaccurate result. To overcome this complexity, we present a novel scheme to characterize ICR using polarization-multiplexed laser source generated using internal components of coherent transmitter. The optical signals on two polarizations are modulated with sinusoidal signal at different frequencies. If frequency difference is larger than laser line width, the output at the coherent transmitter is a polarization-multiplexed laser source. This method allows on-board measurement of analog front-end of coherent receiver by connecting coherent transmitter with coherent receiver. Influence of automatic bias control and radio frequency amplifier is also discussed. Another method is to combine outputs of two separate TLSs with orthogonal polarization through a polarization beam combiner. Both methods lead to robust performance without active control of SoP. The measurement accuracy using polarization-multiplexed laser source is the same as that of the conventional method. The intrinsic resiliency to polarization change leads to simple setup and enables in-field characterization of ICR.
DP-QAM is one of the feasible paths towards 100Gbps, 400Gbps and 1Tbps optical communications systems. For DPQAM transmitter, the time mismatch between the XY tributary channels is known as the XY skew. Large uncompensated XY skew can significantly degrade the system performance. Sometimes, time-interleaved return-to-zero DP signal is preferred with lower nonlinear polarization scattering induced penalty. In this work, XY skew detection and alignment of dual-polarization optical quadrature amplitude transmitter using reconfigurable interference is experimentally demonstrated with >23-dB dynamic range. ~1.5-dB power change is achieved for 1-ps XY skew. Fast detecting scheme for arbitrary skew measurement is also experimentally verified. The scheme is compatible with different modulation formats, data sequences, and waveforms.
We demonstrated mutual injection locking and coherent beam combining of three individual Nd:YVO4 laser modules. A beam splitter couples three lasers as well as combines their outputs. In the free running state, the divergence of combined beams is large. Under mutual injection locking, the divergence of the combined beams becomes substantially smaller than that in the free-running state and is as small as that of the individual laser beam. Mutual injection locking was also realized without active stabilization with large individual laser cavity length difference and low individual laser Q-factor.
The crosstalk performance of an arrayed-waveguide grating (AWG) multi-/demultiplexer is
primarily caused by random optical phase errors introduced in the arrayed-waveguides. Since the layout of
waveguides on a wafer is patterned by photomask through photolithography process, the resolution of a
photomask has a direct influence on the phase errors of an AWG. This paper presents a theoretical analysis
on the phase error caused by photomask resolution along with other basic design parameters. Both
calculation and measurement results show that a high-resolution photomask (better than 25 nm) is a critical
requirement to produce low-crosstalk (less than -30 dB) AWG demultiplexers. We also investigated the
non-ideal power distribution in the array waveguides since it contributes considerable phase errors when
material impurity is not well controlled during wafer fabrication. Basic criteria of power profile truncation,
number of grating waveguides, and material index variation are summarized in this paper as well.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.